Manufacture of Lamps and “Lamp Parts” Used in Lamps

246

MANUFACTUR

E OF LAMPS AND "LAMP PARTS" USED N I LAMPS

In thi s Chapter , we wil l firs t surve y th e manufacturin g protocol s use d o t prepar e wha t hav e generall y bee n calle d "Lamp-Parts" . Our discussio n wil l addres s tungste n wir e a s relate d o t lam p filaments , material s use d o t manufactur e lamps , an d ra w material s use d o t prepar e phosphors . We wil l the n discus s method s o f manufacturin g bot h incandescen t an d fluorescent lamps a s relate d o t th e material s neede d fo r suc h manufactur e (Not e tha t we have alread y discusse d optima l lam p desig n n i th e las t Chapter) . n I th e nex t Chapter , we wil l the n discus s phospho r manufacturin g practice s an d ho w phosphor s fo r us e n i fluorescent lamp s ar e made . The area s o t b e examine d n i thi s Chapte r encompass : 1. Tungste n wir e manufacture , includin g preparatio n o f tungste n compound s an d thei r chemistry , preparatio n an d fabricatio n o f tungste n meta l powde r int o ductil e wire , an d manufactur e o f coil s for incandescen t lamps . 2. Glas s use d n i lamp s an d cathode-ra y tub e includin g glas s form s prepare d fo r specifi c lamps .

manufacture ,

3. Manufactur e an d desig n o f incandescen t lamps . 4. Preparatio n o f ra w material s use d o t manufactur e phosphors , includin g phosphate s an d sulfides . 5. Manufactur e o f fluorescent lamps . 3.1TUNGSTE N

WIRE

MANUFACTURIN G

Up o t abou t 1975 , tungste n wir e intende d fo r us e a s a n incandescen t lam p filament was made b y a compaction-sintering-densifying-swaging-di e drawin g process . Afte r tha t time , som e manufacturer s change d th e proces s o t improv e method s whic h wer e regarde d a s labor-intensiv e an d pron e o t poo r qualit y predilections . n I orde r o t clarif y exactl y wha t thi s means , we wil l firs t discus s the olde r proces s an d the n presen t dat a concernin g th e newe r one . We do

247

thi s n i orde r o t establis h a fir m basi s o f knowledg e concernin g th e processe s originall y use d an d ho w th e modification s o t thes e processe s cam e about . As s i usual , th e change s wer e made partiall y becaus e o f monetar y considerations . Nonetheless , the y serv e o t establis h a greate r understandin g o n ou r par t o f the extremel y complicate d parameter s involve d n i th e manufacturin g o f tungste n wir e an d th e factor s whic h contribut e o t th e understandin g of ho w tungste n wir e perform s a s a n incandescen t filament . A . MANUFACTURIN G PROTOCOL S USED

UP TO ABOU T

197 5

This are a include s fiv e aspect s whic h nee d o t be discussed : 1 ) Tungste n chemistr y an d preparatio n o f chemical s use d o t make tungste n wire ; 2 ) Preparatio n o f tungste n form s suitabl e fo r wire-drawing ; 3) Densification ; 4) Method s o f formin g tungste n rod s b y swaging ; an d finall y 5 ) Wire-drawin g a s relate d o t formin g ductil e wir e suitabl e fo r manufacturin g coils . We wil l als o refe r o t molybdenu m chemistr y wher e t i s i directl y relate d o t tha t o f tungsten . A - l. Chemistr y of Tungste n an d Chemical s Use d o t Make Tungste n Wir e Tungste n ore s occu r n i natur e a s Wolframit e (a n isomorphi c mixtur e o f FeW04 and MnW04), Scheelit e - CaW04 , Stolzit e - PbW04t an d occasionall y a s Tungstit e -WO3 . Ore s suc h a s Reinit e - FeW04 an d Hubnerit e - MnWO i ar e als o known . Thes e ore s ar e generall y processe d b y fusio n wit h soda-as h a t s oxid e salts , viz hig h temperatur e o t for m Na2W0 4 plu 3.1.1. - (Fe,Mn)W0 + N a2 C 0 3 > 4

N a2 W 0

4

+ F e2 0 3 + M n2 0 3 + C 02 t I

The sodiu m tungstat e so-forme d s i solubl e n i wate r an d s i leache d fro m th e amalgamate d material . Hot HC1 s i the n adde d o t th e solutio n o t for m tungsti c aci d whic h precipitates : 3.1.2. -

N a2 W 0

4

+ HC1 > -

H2 W 0

4

i + H2 0

Heatin g tungsti c aci d serve s o t for m th e oxide , W Q 3 . However , th e oxid e so forme d s i no t pur e enoug h o t make lam p filament s an d need s o t be processe d further . Unlik e molybdenum , th e ammoniu m tungstat e sal t canno t b e use d fo r furthe r purificatio n step s becaus e o f th e tendenc y o f tungste n meta l o t reac t

248

wit h residua l NH3 o t for m a nonstoichiometri c nitrid e whe n th e oxid e s i reduce d o t metal . Therefore , NaOH s i use d o t dissolv e th e tungste n oxid e an d the resultin g solutio n s i filtere d o t remov e an y insolubl e hydroxid e precipitates . Organi c chelatin g reagent s may the n adde d o t th e solution , dependin g upo n th e impuritie s neede d o t b e removed . Afte r filterin g th e precipitate s formed , on e obtain s a solutio n fro m whic h tungsti c aci d ca n agai n be produce d b y th e additio n o f HC1. Thi s ste p als o ha s a purifyin g effec t o n th e H2WO 4 formed . Sinc e an y tungste n meta l intende d fo r th e wire-drawin g proces s mus t b e pur e an d substantiall y freeo f othe r element s (othe r tha n thos e deliberatel y added) , t i s i common o t recycl e th e dissolutio n an d precipitatio n step s mor e the m onc e o t obtai n a pur e tungsti c aci d whic h s i finall y fire d n i ai r o t for m WQ 3 Finally , th e WO 3 s i reduce d n i a hydroge n atmospher e o t tungste n metal , n i a manne r tha t produce s meta l substantiall y fre e o f oxygen . The reductio n ste p s i critica l sinc e tungste n meta l powde r ca n b e obtaine d n i severa l forms , dependin g upo n reductio n conditions . One suc h for m (wher e the particl e siz e s i to o small , i.e. - <1u ) s i pyrophoric . When WO 3 s i reduced , it goe s throug h severa l stage s whic h includ e formatio n of th e lowe r valenc e 6 5 state s o f tungsten . The mos t stabl e valenc e stat e o f tungste n s i W *, bu t W "*, 4 + 3 2 W , W "", an d W * ar e als o known . Durin g th e reductio n b y hydrogen , th e powder produce d become s sintere d int o friabl e masses , dependin g upo n a number o f factors , a s elucidate d below . Severa l lowe r valenc e oxide s ar e formed , i.e. 3.1.3. -

4WQ3 +

H2

~ W4O11 (viole t color ) + H2 0

W4O1 1

+ 3 H2

~ 4 W 0 2 (chocolat e brown ) + 3 H2 0

4W0 2

+ 8H2

~

WO 3 + 3 H2

4 W (blac k o t gray-black ) + 8 H2 0

~ W + 3 H2 0

In th e firs t stag e o f reduction , th e mixtur e o f viole t W4O11 an d yello w WO 3 form s a deep-blu e powder , easil y identifiabl e a s such . t I s i thi s mixtur e whic h can b e see n o n th e silic a tray s use d o t fir e tungsti c oxid e o t th e metalli c form . It s i essentia l o t thoroughl y remov e th e wate r forme d durin g th e reductio n stag e sinc e t i wil l preven t th e reactio n fro m goin g o t completion . Eve n a smal l

249

amount o f unreduce d oxid e n i th e tungste n powde r so-produce d s i extremel y detrimenta l o t propertie s require d fo r filamen t wire . Thus , a flowin g atmospher e n i th e reductio n furnac e s i necessar y o t flus h ou t al l o f th e wate r of reaction . Freedo m fro m impuritie s s i essential , a s s i a suitabl e physica l conditio n require d fo r furthe r processin g o f th e tungsten-powde r int o wire . The followin g Tabl e show s a maximu m impurit y leve l require d fo r tungste n wire , f it is i o t b e use d fo r lam p filaments . TABL E 31 Upper Leve l fo r Impuritie s Commonl y Foun d n i Tungste n Meta l Powde r Used o t Manufactur e Lamp Filament s Elemen t

Concentratio n n i ppm. Elemen t

Mo

300

Concentratio n n i ppm.

Fe

50

Si

90

Cu

30

Al

40

Oxygen

15

Κ

85

Nitroge n

1

Na

10

Hydroge n

1

Ca

80

Most o f thes e impuritie s come fro m th e origina l or e excep t Si whic h may originat e fro m th e silic a tray s use d durin g th e reductio n step . However , th e pure r tha t th e meta l powde r is , th e bette r ar e th e wir e propertie s a s relate d to filamen t failur e n i incandescen t lamps . Dependin g upo n reductio n conditions , on e ca n obtai n averag e meta l particl e size s u p o t 50 0 μ. Most method s o f obtainin g a n ingo t b y pressin g requir e a fin e tungste n powder , abou t 25 μ n i averag e size . The lowes t temperatur e a t which complet e reductio n s i practica l s i abou t 70 0 °C . In th e followin g Table , we summariz e th e effec t o f temperatur e upo n particl e siz e produced . n I examinin g thi s Table , t i s i eas y o t se e tha t th e reductio n temperatur e shoul d no t g o ove r abou t 85 0 °C . Reductio n a t constan t temperatur e produce s a coarse r grai n tha n s i obtaine d whe n th e temperatur e is raise d graduall y o t a fina l value . Many manufacturer s hav e a continuou s furnac e n i whic h th e temperatur e profil e s i controlle d s o tha t th e reductio n take s plac e n i suc h a manner . The tray s containin g WQ 3 o t be reduce d o t tungste n meta l ar e pushe d throug h th e temperatur e profil e

250

TABL E 32 Particl e Size s Produce d n i Tungste n Powder s a s a Functio n o f Reductio n Temperatur e Temperatur e

Produc t Obtaine d

600

W O 2

700

W

- 1. 0 μ

800

W

~ 2.0-3. 0μ

900

W

~ 3.0-4. 0 μ

1100

W

~

10. 0 μ

1500

W

~

300-40 0 μ (0. 3 mm)

+ w

Siz e Rang e n i Micron s 0.2 - 0. 5 (0.00 1 mm.)

so tha t eac h tra y experience s a n increasin g temperature . Generall y thi s profil e start s a t 550-65 0 °C , wit h a fina l temperatur e betwee n 78 0 an d 110 0 °C , dependin g upo n fina l particl e siz e require d fo r th e end-us e o f th e wir e so produced . W O 2 mixe d wit h W O 3 occur s a t th e lowes t temperatur e an d tungsten-meta l a t th e uppe r temperatur e stages . Some manufacturer s als o produc e th e meta l b y a n intermitten t proces s wherei n th e reduce d produc t s i removed , coole d an d the n subjecte d o t a 2n d stag e o f reductio n afte r certai n additive s suc h a s thori a hav e bee n made o t th e origina l reductio n product . n I suc h a process , th e temperatur e s i graduall y raise d fro m 50 0 o t 70 0 °C. n i abou t 2o t 3 hours . The n mor e WQ 3 s i mixe d n i and th e mi x s i furthe r reduce d o t tungsten-meta l n i a temperatur e profil e fro m 55 0 o t 75 0 o t 100 0 °C. ove r a perio d o f 3o t 4 hours . Particl e siz e o f tungste n meta l produce d ha s bee n foun d o t depen d upon : a) Firin g Time an d Temperature - increasin g th e tim e o r raisin g th e temperatur e rapidl y produce s a coarse r meta l powder . b) Oxid e Particl e Size - increasin g th e oxid e siz e change s th e W-meta l particl e size : Coars e oxid e give s a coars e metal-powde r (bu t a fin e oxid e can b e sintere d o t for m a coars e meta l powde rf i on e s i no t careful) . c) Firin g temperatur e use d fo r convertin g th e tungsti c aci d o t oxid e als o play s a n importan t rol e n i th e siz e o f oxid e particle s produced .

251

The majo r reaso n tha t on e wishe s o t obtai n a tungste n meta l powde r o f smal l but controlle d siz e s i tha t th e powde r s i o t b e use d o t for m a presse d ba r o f controlle d proportions . I f th e particl e siz e s i to o large , the n th e bar s produce d ar e to o fragil e an d canno t b e handle d a t all . The y als o wil l contai n considerabl e "voids" . f I th e particle s ar e to o small , the n th e produc t obtaine d is likel y o t b e pyrophoric . A s state d above , th e concentratio n o f wate r vapo r presen t n i th e hydroge n ga s has a pronounce d effec t o n W-meta l particl e size . I ts i likel y tha t th e followin g reactio n take s place : 3.1.4. -

W + H2 0 ~ WO 2 + 2 H2

Such a reactio n s i cycli c an d t i was fo r thi s reaso n tha t th e reaction s give n n i 3.1.3 . wer e writte n wit h a "double-arrow " sinc e wate r vapo r n i hydroge n ga s can revers e th e reaction s a s shown . Thi s mechanis m result s n i th e formatio n of larg e particle s o f W-meta l an d s i th e reaso n tha t friabl e masse s ar e forme d durin g reduction . I f on e wishe s o t make a fin e meta l powde r o f nearl y constan t size , t is i necessar y o t hav e a rapidl y flowin g H2 ga s strea m n i th e reductio n furnace . Dependin g upo n th e siz e of th e furnace , thi s flo w may b e a s hig h a s 3000-400 0 liters/hou r o f pre-drie d hydroge n gas , dependin g als o upon th e weigh t o f WQ 3 bein g reduce d pe r uni t volum e o f th e furnace-tub e per hour . Thi s paramete r als o mandate s th e us e o f a thi n laye r o f oxid e f i a fin e meta l powde r s i desired . Thus , anothe r consideratio n s i th e amoun t of oxid e presen t pe r cc .o f hydroge n ga s flowin g a t an y give n instant . The followin g dat a ar e relevan t o t th e "water-cycle " affecting W- size : TABL E 33 Siz e o f Tungsten-Meta l Powde r Produce d a s a Functio n o f Gas Flo w Maximum Temperatur e

r o f Tungste n Grams of WO 3 Presen t Diamete Metal Particle s per cc . o f H2 Gas n i Produce d Reductio n Tub e

800 °C .

0.0 5 gra m

0. 5 μ

830

0.5 0

2. 0 μ

900

1.0 0

4. 0 μ

252

It s i eas y o t se e tha t th e critica l factor s whic h contro l th e particl e siz e o f th e tungsten-meta l powde r produce d includ e th e dept h o f th e oxid e bein g reduce d an d it s relationshi p o t th e amoun t o f hydrogen-ga s presen t a t an y give n instant . Summarizing , th e factor s include : 3.1.5. - Factor s Controllin g Particl e Siz e of Tungste n Meta l Powde r 1. Particl e siz e o f WO 3 produce d 2. Reductio n temperatur e 3. Reductio n temperatur e profil e 4. Amount o f water-vapo r presen t a t an y give n instan t 5. Amount o f oxid e bein g reduce d pe r unit-volum e o f furnace-tube , i.e. - dept h o f oxid e laye r 6. Rati o o f flowin g hydroge n ga s o t oxid e bein g reduce d Many manufacturer s emplo y a rotatin g Kil n n i whic h bot h th e laye r thicknes s of th e oxid e s i controlle d b y continuou s feeding , th e temperatur e profil e tha t the oxid e experience s durin g reductio n s i controlled , an d th e hydroge n ga s flo w s i controlle d s o a s o t limi t th e effec t of wate r presen t an d remov e tha t produce d b y th e reductio n reaction . Befor e we leav e thi s subject , t i s i wel l o t not e tha t th e method s use d o t produc e tungsten-meta l powde r ar e applicabl e o t th e cas e o f molybdenu m meta l a s well . The majo r differenc e s i tha t a solubl e ammoniu m molybdat e s i usuall y employed . Becaus e of th e tendenc y o f Mo o t for m polymolybdates , on e usuall y obtains : (ΝΗ4)6Μθ7θ24· 4Η2<3 . Upon firin g thi s salt , th e oxide , M0Q3 t results . The us e o f th e sodiu m sal t ha s bee n tried , bu t a bette r meta l produc t result s fromth e us e o f th e ammoniu m salt . The sam e restriction s relevan t o t tungste n appl y fo r thi s cas e a s well . Becaus e Mo mete d s i generall y use d a s conductiv e lead-i n wire s an d foils , it s metallurgica l state , includin g grai n size s and grai n boundarie s withi n th e metal , s i no t s o critica l a s s i th e cas e fo r tungste n meta l whic h s i use d fo r incandescen t lam p filaments . One metho d tha t ha s bee n use d o t prepar e ammoniu m molybdat e is : 1. M0O3 s i dissolve d n i ammonia . 120 0 Kg. o f roaste d or e contain g 90% M0O3 s i place d n i a covere d rubber-line d tan k wit h sufficien t exces s NH3 o t completel y dissolv e th e M0O3 present . Gaseou s NH 3 may als o b e adde d a s required . The n ai r s i injecte d o t oxidiz e

253 2

3+

any iro n present , i.e. - Fe* F e ^ Fe(OH )3 I.H2O2 may als o b e added . The resultin g precipitat e s i the n allowe d o t settl e an d th e liqui d s i the n ru n throug h a filte r o t remov e al l o f th e precipitate . 2.Th e remainin g liqui d s i the n treate d wit h a solutio n o f 1: 4 HNO 3 and HC1 (2 5 mol%) o t a p H o f 1.5 . Thi s precipitate s th e ammonium paramolybdat e referre d o t above . The precipitat e s i now allowe d o t settl e an d the n s i brough t o t a filte r press . The 3 1 r lite r an d s i mothe r liquo r contain s 12 gram s o f Mo"" pe discarded . 3. I f th e p H s i to o high , to o much Μ ο "" wil l b e lost , an d f it is i to o low, H2M0O 4 results . 3 1

4. The crystal s ar e washe d wit h col d deionize d wate r an d

the n

wit h 2. 5 mol % NH4NO 3 solution , dilute d n i hal f wit h ethanol . 5. Finally , th e crystal s ar e remove d fro m th e filte r an d

vacuu m

drie d a t abou t 6 0 °C. fo r 2 4 hours . 6. Firin g a t 90 0 - 120 0 ° C n i ai r serve s o t for m th e M0Q3, whic h s i now read y fo r us e afte r t i cools . A-2. Preparatio n of Tungste n Forms - Pressin g an d Sinterin g Bar s Becaus e th e meltin g poin t o f W s i 368 0 °K. (340 7 ° C - a temperatur e no t readil y attainable) , onl y meta l particle s ca n b e forme d n i th e reductio n step . One therefor e take s thi s powde r an d presse s t in i a mol d o t for m a bar , usin g a wax binde r o f som e sort . Eac h tungsten-wir e manufacture r ha s hi s own preferre d binder , mos t usin g glycerine , paraffi n wax or th e like . The ingo t siz e s i usuall y abou t 0.5 0 inche s squar e an d abou t 2 4 inche s long . A typica l mold s i made from stainles s steel , an d ha s a simpl e desig n a s show n n i th e followin g diagram , give n o n th e nex t pag e a s 3.1.6 . 2

A pressur e o fa t leas t 12,00 0 lbs/in s i neede d o t compac t th e meta l powde r 2 wit h pressure s u p o t 50,00 0 lbs/in bein g mor e usual . I f th e meta l powde r use d s i fine, th e ingo t so-produce d may b e handled , wit h care .

254

3.1.6. Powder Pres s Use d n i Makin g W-Meta l Ingo t

I M l H I »U

Ml

Side Vie w

Durin g th e pressin g step , a n iner t atmospher e s i generall y use d sinc e th e operatio n tend s o t generat e hea t whic h migh t caus e th e formatio n o f a n oxid e fil m o n th e particles . The pressin g operatio n usuall y produce s a porou s ingo t havin g n o mor e tha n 30-40 % o f th e densit y o f pur e tungste n metal . Althoug h isostati c pressin g ha s bee n tried , th e ingo t so-produce d ha s bee n foun d o t b e no bette r n i strengt h tha n tha t made usin g a mold . f I th e particl e siz e o f th e meta l particle s s i smal l enough , th e presse d ingo t generall y ha s enoug h strengt h o t b e cautiousl y handled . I t s i the n place d o n a sla b o f W, Mo or graphit e an d sintere d n i a reducin g atmospher e o t fus e th e meta l particle s int o a soli d ba r o r ingot .I f a graphit e plat e s i use d o t sinte r th e presse d bars , the sinterin g temperatur e mus t b e kep t belo w 90 0 °C .o t avoi d th e formatio n l heatin g cycl e involve s heatin g a t of tungste n carbide , i.e. - W 2 C . The usua temperature s betwee n 900-105 0 °C . fo r abou t 3 0 minute s n i a flowin g hydrogen-ga s atmospher e an d the n coolin g n i th e sam e gaseou s atmosphere . Most manufacturer s us e a temperature-profil e furnac e an d pus h th e slab s throug h th e hot-zon e o f th e furnace . The sinterin g temperatur e s i lo w enoug h s o tha t a chang e n i surface-grai n growt h s i no t visible . An increas e n i strengt h o f th e sintere d ba r usuall y results , though t o t be du e o t th e reductio n o f a n oxid e surface-fil m o n th e meta l particles . The reduce d meta l so-forme d probabl y act s a s a cemen t

255

betwee n th e particle s thereb y increasin g th e strengt h o f th e sintere d bar . Nevertheless , th e ingot s so-obtaine d ar e porou s an d hav e a densit y o f abou t 60-65 % o f th e ultimat e tungste n value . As such , the y ar e to o fragil e o t withstan d an y workin g int o a wire-form . A-3. The "Treating-Bottle"A Temperatur e Densificatio n Ste p Becaus e o f th e hig h meltin g temperatur e o f tungsten , an y attemp t o t furthe r densit y th e sintere d bar s wil l requir e ver y hig h temperature s o t d o so . n I general , th e us e o f a furnac e di d no t appea r o t be practica l sinc e th e insulatio n presen t n i mos t furnace s o f tha t tim e coul d no t withstan d th e extremel y hig h temperature s needed . Thus , th e us e o f a so-calle d "treatin g bottle " evolved . This consiste d o fa n uninsulate d meta l enclosur e n i whic h th e to p o f th e ingo t was clamped , whil e th e botto m was submerge d n i a mercury-pool . Thi s s i shown n i th e followin g diagram : 3.1.7. A Typica l "Treatin g Bottle " Use d o t Densif y Tungste n Ingot s Hydroge n Gas

Clamp Tungste n Ingo t

Optica l uromet re Port

Y

Electrode s

Mercury ]

The purpos e o f th e mercur y poo l was o t b e abl e o t make electrica l contac t whil e allowin g on e en d o f th e ingo t o t be fre e o t move a s t i underwen t shrinkag e an d densification . The treatmen t too k plac e unde r a dry-hydroge n

256

atmosphere . Some manufacturer s use d a flowin g atmospher e whil e other s employe d a pressurize d atmosphere . In som e cases , a vacuu m was employe d withi n th e bottle . Heatin g too k plac e by a DC curren t whos e amplitud e depende d upo n th e siz e o f th e ingot . To hea t an 8-inc h ingo t clos e o t it s meltin g poin t require d abou t 150 0 ampere s a t 1 0 volts , whil e a2 4 inc h ba r require d abou t 450 0 amperes . Becaus e considerabl e shrinkag e occur s durin g densification , th e ba r tende d o t deform . I t s i fo r thi s reaso n tha t onl y th e to p o f th e ba r was clampe d withi n th e apparatus . Whil e one ca n monito r th e temperatur e o f th e ingo ta s t is i bein g heate d by us e o f a n externa l optica l pyrometer ,t i prove d easie r o t monito r th e curren t bein g use d to hea t th e bar . Grai n growt h begin s a t abou t 105 0 ° C an d shrinkag e accompanie s this . At th e same time , partia l eliminatio n o f void s normall y presen t n i th e ba r als o begins . These void s ar e a consequenc e o f th e pressin g ste p an d resul t becaus e o f interstitia l space s betwee n particle s an d void s n i th e meta l particle s normall y experience d durin g th e reductio n ste p fro m th e oxid e o t th e meta l stage . The usua l shrinkag e experience d s i abou t 17 % n i volume . The whol e operatio n take s abou t 30-4 5 minutes . The procedur e was o t rais e th e temperatur e o f th e ba r o t abou t 92 % o f th e fusio n temperature , i.e. - « 330 0 °C . Thi s temperatur e was hel d fo r abou t ΙΟΙ 5 minute s durin g whic h tim e grai n growt h continue d an d a coarsel y crystallin e microstructur e was obtained . Metallurgica l grai n siz e varie s considerabl y unde r differin g condition s o f densification , bu t grai n siz e ha s littl e effec t upo n subsequen t workin g propertie s o f th e meta l so-produced . Ingot s ca n b e worke d whos e averag e grai n siz e varie s fro m 1 0 μ o t ove r 10,00 0 μ The averag e grai n siz e produce d was usuall y abou t 1500-200 0 μ per squar e millimete r o f surface . Sinc e grai n growt h begin s a t 105 0 °C . an d continue s wit h time , ther e s i generall y a n equilibriu m tha t occur s betwee n densit y an d grai n growt h withi n 10-1 5 minute s a t an y give n temperatur e abov e 110 0 °C . One ca n observ e thi s growt h b y countin g th e grain s n i a n etche d cross-sectio n o f th e treate d bar . Sinc e tungste n meta l ha s a body-centere d structur e (a o = 3.15 5 ± 0.00 1 A) , etchin g take s plac e o n th e {112 } plane s (cleavag e occur s o n th e {100 } plane s and sli p occur s o n th e {112 } planes) . Generally , triangula r etch-pit s ar e seen .

257

(A goo d etchin g agen t s i 3 % H2O2 , bu t a 10 % NaOH + 30 % potassiu m ferrocyanid e solutio n may als o be used) . However , t i di d becom e apparen t tha t the rat e o f grain-growt h was modifie d by : 1 ) change s n i temperatur e an d time ; 2) th e siz e o f th e meta l particle s an d 3 ) impuritie s presen t n i th e metal . Thus , it s i possibl e o t densit y a n ingo ts o tha t th e grain s presen t var y fro m abou t 1o t 10 2 as hig h a s 1 0 grain s (crystals ) pe r mm . However , a s we said , th e norma l 2 figur e attaine d s i abou t 250 0 crystals/mm . The effec t o f temperatur e durin g th e heat-treatin g ste p s i complicate d indeed . First , th e surfac e s i coole r du e o t radiatio n los s ther e an d becaus e o f the presenc e o f coole r hydroge n gas . The surfac e temperatur e may diffe r fro m the interio r a s much a s 15 0 ° C an d th e cente r o f th e ba r may b e much hotte r tha n th e ends . Thes e difference s giv e ris e o t peculia r type s o f grai n growth . As the ba r s i heated , th e cente r may becom e melte d whil e th e outsid e remain s solid . I f thi s ba r s i coole d quickly , larg e radia l crystal s wil l form . But f i th e ba r is hel d a t thi s hig h temperatur e fo r longe r tha n abou t a minute , molte n meta l is likel y o t brea k through , leavin g a hollo w core . At lowe r temperatures , exaggerate d grai n growt h may occu r becaus e o f interna l temperatur e gradients . Grain s produce d n i th e cente r o f th e ba r may b e considerabl y large r tha n thos e produce d nea r th e surfac e an d edge s o f th e bar . Thi s exaggerate d growt h occur s ove r th e temperatur e rang e o f abou t 2500-280 0 °C . Belo w thi s range , on e obtain s a ver y fin e grai n (bu t lo w density ) whil e abov e thi s rang e unifor m grai n growt h occur s withou t a larg e contras t n i grai n siz e (an d wit h a relativel y hig h density) . A s show n n i th e followin g Table , give n o n th e nex t page , th e exaggerate d grai n growt h mechanis m ha s bee n foun d o t be a direc t functio n of temperature . At fairl y lo w temperatures , on e obtain s rathe r smal l grains . But , as on e increase s th e temperature , a temperature-rang e s i reache d wher e anomalou s grain-growt h occur s an d th e averag e grai n siz e increase s considerably . However , f i th e temperatur e s i raise d rapidl y almos t o t th e meltin g point , a "normal " grain-structur e s i obtained . It s i becaus e o f thes e vagarie s tha t th e us e o f th e "treatin g bottle " evolved , sinc e t i was th e onl y metho d known a t tha t tim e tha t coul d rais e th e temperatur e o f th e ba r clos e o t th e meltin g poin t of tungste n n i a shor t perio d of time .

258

TABL E 34 Exaggerate d Grai n Growt h Experience d a s a Functio n o f Meltin g Curren t Sinterin g % Meltin g Temp, n i °C . Curren t 2550

Grain s Produce d 2 per m m

65

1150

2850

75

2

2950

80

3

3200

88

5

3250

90

15

3300

92

1140

The particl e siz e o f th e meta l powde r an d th e pressur e use d fo r compactin g the meta l powde r particle s als o ha s a majo r effec t o n th e sinterin g propertie s of th e ba r produced . The followin g observation s hav e bee n made : a. The fine r th e particl e size , th e greate r s i th e cohesio n betwee n particles . Cohesio n ha s a marke d effec t o n grai n growt h a s well . Unde r some condition s o f tim e an d temperature , th e grai n siz e o f a n ingo t may be large r a s fine r powder s ar e used . (However , on e reache s a limi t n i tha t to o fin e a particl e siz e give s a pyrophori c powder) . b. t I become s ver y difficul to t hea t a n ingo t made fro m to o fin e a powde r throug h th e temperatur e rang e o f exaggerate d growt h rapidl y enoug h o t avoi d ver y larg e crysta l growth . c. Coars e powde r produce s a fine-graine d porou s ba r upo n heat-treatin g and wil l no t develo p larg e crystal s eve n unde r optima l grain-growt h conditions . d. n I practice , th e particl e siz e o f th e meta l powde r tha t s i use d varies . It s i bes to t obtai n a powde r whos e particl e distributio n s i narrow . Thi s is on e o f th e mor e importan t contro l parameter s use d o t maintai n hig h qualit y tungste n wir e production . The bes t powde r wil l hav e a mea n diamete r o f 3. 0 μ wit h a2 σ sprea d fro m 0. 5 o t 6. 0 μ n I manufacturing , thi s s i sometime s achieve d by blendin g powder s wit h differin g particl e distributions .

259

e. The pressur e use d o t for m th e ingo t s i importan t onl y n i tha t t i affect s inter-particl e contacts . The temperatur e a t whic h grai n growt h begin s depend s upo n th e particl e siz e o f th e meta l powde r an d th e compactin g pressur e used . Fo r a 0. 6 μ powder , thi s temperatur e s i 7 0 °C. lowe r tha n tha t fo r a 3. 0 μ powder . n I th e following , we sho w th e minimu m temperatur e a t whic h grai n growt h begin s a s a functio n o f pressur e use d o t compac t th e ba r (Obviously , pressur e affect s ho w clos e the individua l particle s touch) : 2

Pressur e (tons/i n ) 10 25 40

Initia l Grai n Growt h 120 0 ° C 105 0 90 0

f. Impuritie s hav e a n effec t o n th e qualit y o f grai n growth . Most ar e volatil e a t relativel y lo w temperature s an d escap e fro m th e porou s ba r befor e t i compact s an d shrinks . Thos e tha t volatiliz e a t highe r temperature s may forc e grain s apar t an d preven t norma l grai n size s fro m forming . The y may eve n spli t th e bar . Therefore , intentiona l addition s o f impuritie s lik e alkalin e oxides , silic a or bori c oxide , whic h volatiliz e betwee n 1000-200 0 °C . facilitate s th e escap e o f les s volatil e impurities , o r leav e channel s fo r thei r escape . t I shoul d be note d tha t thi s purifyin g effect , obtaine d durin g th e densificatio n ste p b y heat treating , ha s a n importan t influenc e o n subsequen t recrystallizatio n an d ductilit y o f th e worke d metal . g. Certai n impuritie s ar e relativel y non-volatile . Thes e includ e oxide s of thorium , uranium , calciu m an d magnesium . Sinc e the y ar e insolubl e n i tungsten , suc h particle s obstruc t grai n growt h n i proportio n o t th e number presen t an d thei r relativ e size . Thoria , i.e. - TI1O2 . s i usuall y adde d o t th e tungste n powde r befor e th e pressin g ste p o t contro l grai n siz e and/o r recrystallization . The optimu m valu e fo r thori a o t achiev e thi s effec t s i abou t 3-4 % by weigh to f powder . Othe r non-volatil e impuritie s hav e concentration , viz -

a n

effec t proportiona l o t thei r

260

Fe & Ca Concentratio n Number o f Grain s Produce d n i Heat-Treate d Ba r in Weigh t% 2

0

0.5 4 /mm

0.00 2

1.3 0

0.0 1

59

0.0 5

760

h. I ts i possibl e o t prepar e wha t s i substantiall y single-crysta l tungste n wire b y firs t formin g a smal l ro d b y "swaging " an d the n passin g th e ro d throug h a furnace , n i a reducin g atmospher e o f hydroge n gas , a t temperature s betwee n 2400-260 0 °C . a t rat e slo w enoug h tha t th e "exaggerated " growth , mentione d above , ca n tak e place . Singl e crysta l wire s wit h length s u p o t severa l meter s n i lengt h hav e bee n produce d by thi s method . I t s i no t know n exactl y why thori a promote s th e formatio n o f tungste n singl e crystals , bu t t i ha s bee n show n tha t som e o f the thori a s i converte d o t thoriu m meta l durin g th e heatin g operation . Once treated , th e densit y o f th e ba r approache s 18. 5 g m / c,c i.e. - abou t 95 % of theoretica l density . Workin g propertie s wil l depen d upo n th e compactnes s of th e bar , a s indicate d b y th e degre e o f shrinkag e whic h occur s durin g densification , an d mor e importantly , b y freedo m fro m certai n impuritie s suc h as silicon . The treate d ingo t emerge s fro m th e "treatin g bottle " wit h considerabl e strength , bu t t is i ver y brittl e a t roo m temperature . Fortunately , at highe r temperatures , tungste n become s mor e ductil e an d a t abou t 130 0 °C , it ca n b e hammere d o r rolled . Reiterating , th e mos t importan t paramete r of contro ls i th e amoun t o f shrinkag e tha t occurs . Usually ,f i sufficien t shrinkag e does no t occu r durin g heat-treating , th e ba r canno t b e worke d b y swaging . A-4. Swagin g o f th e Heat-Treate d Ingot . Tungste n meta l s i uniqu e n i tha t t i s i on e of th e fe w metal s tha t become s "work-hardened " a t lo w temperatures . Accordin g o t Valentin e an d Hul l (1) , the onse t o f th e ductile-brittl e transitio n o f polycrystallin e tungste n meta l havin g a n averag e grai n siz e o f abou t 30-30 0 μ occur s a t 35 0 °K, i.e. - 7 7 °C . Therefore , t is i necessar y o t subjec t th e ba r o t sever e workin g a t a relativel y low temperatur e o t brea k u p th e larg e crystal s whic h for m whe n th e meta l s i heate d an d th e resultin g grai n growt h render s t i brittle . To d o this , a

261

specially-designe d apparatu s ha s bee n used , calle d a "swaging " machine . I t s i actuall y a rotatin g hammer , consistin g o f opposin g hammer s usuall y made of tungste n carbide , i.e. - W2C . Thei r numbe r s i generall y fou r o t si x an d tw o actuall y strik e th e ba r simultaneousl y a s th e swag e revolves . The hea d o f suc h a machin e s i show n a s follows . Not e tha t th e individua l race s rotat e n i opposit e directions . 3.1.8. Rotatin g Hammer Swag e

Durin g swaging , th e hea d o f th e machin e become s ho t an d som e surfac e oxid e form s o n th e ba r bein g swaged . However , sinc e th e ingo to r ro d bein g reduce d in diamete r need s o t be "retreated " by reheatin g o t abou t 300 0 °C . n i a reducin g atmosphere , an y WQ 3 forme d s i agai n reduce d o t th e metalli c state . In general , th e hammer s ar e actuate d by insertio n o f a ba r o t be swage d an d the proces s s i manual , tha t is - a worke r "swages " on e tungste n ingo t a t a time . Each pas s o f th e ba r throug h th e swaging-machin e reduce s th e diamete r b y abou t8 % wit h eac h passage . Eac h ingo ts i heate d o t abou t 1500-160 0 °C .n i a furnac e havin g a flowing hydrogen-ga s atmosphere . As th e ingo ts i withdrawn , it cool s o t abou t 135 0 °C a s t is i bein g worked . Originally , th e ba r was abou t 0.5 0 inche s squar e bu t ha d bee n reduce d o t abou t 0.37 5 inche s b y sinterin g and densifying , i.e. - 9.52 5 mm., bu t ca n be a s smal l a s 0.35 0 inche s square , i.e. - 8.8 9 mm., dependin g upo n th e Manufacturer . Obviously , mor e meta l ca n

262

be swage d int o a longe r wir e 4 ( time s n i th e abov e exampl e fo r th e large r ingot) . The appropriat e Swagin g Temperatur e s i a functio n of th e reduce d rod-diamete r a s show n n i th e following : 3.1.9. -

Rod o r Wir e Diamete r Swagin g Temperatur e 6-1 1 mm.

1350 °C .

5.5-2. 5

1250

2.5-1. 0

1175

1.0-0.0 5

800 - 55 0

Afte r eac h pas s throug h th e Swager , th e ro d mus t be re-treate d b y heatin g t i to 300 0 ° C n i a reducin g atmospher e fo r 2. 0 minutes . Therefore , t i s i necessar y o t hav e a "Retrea t Furnace " nex to t th e swagin g machine . Retreatin g lessen s th e tendenc y o f th e ro d o t spli t an d give s a softe r material . Car e mus t be take n no t o t wor k th e meta l afte r coolin g belo w it s prope r swagin g temperatur e o r crack s an d split s wil l develop . The worke r learn s ho w o t judg e temperatur e fro m it s color , an d fo r thi s reason , "swaging " s i a learne d art . Flexibilit y o f th e swage d ro d increase s a s th e ro d diamete r s i reduced . However , th e meta l doe s no t reall y becom e ductil e unti l size s belo w abou t 1. 0 mm . ar e reached . Large r sizes , i.e. - 23 mm., ca n attai n ductilit y b y heatin g o t a dull-re d hea t an d the n hammerin g or bendin g th e metal , a s n i swaging . Not e tha t th e temperatur e of workin g s i alway s belo w tha t temperatur e n i whic h visibl e crystallizatio n occurs , i.e. - metallurgicall y speaking , tungste n meta l s i alway s "cold-worked" . Thus , low-diamete r ductilit y s i characteristi c o f cold worked tungste n metal . However , th e meta l ro d o r wir e mus t be reheate d afte r eac h operatio n n i orde r o t reduc e stresse s incurre d withi n th e ro d o r wire becaus e o f th e cold-workin g process . The recrystallizatio n temperatur e drop s progressivel y a s th e ro d o r wir e diamete r s i reduced , an d th e swagin g temperature s use d tak e thi s int o account . f I th e meta l s i heate d abov e th e recrystallizatio n temperatur e a t an y time , th e ductilit y s i immediatel y lost , an d th e meta l the n ca n onl y b e worke d at th e hig h temperatur e use d n i th e firs t stage s o f swaging . Thus , f i th e meta l is worke d to o ho t n i late r stage s o f swaging , th e recrystallizatio n temperatur e may be exceede d an d th e wir e become s brittle . Therefore , th e primar y ai m o f

263

the swagin g proces s s io t brea k u p th e origina l equiaxia l grain s o f th e forme d ingo t an d o t reorien t the m an d elongat e the m n i th e directio n of th e wir e axi s so tha t the y ultimatel y becom e lik e a bundl e o f fibers . Deformatio n take s plac e by a serie s o f slip s o n th e {112 } plane . As a result , th e crystallin e grain s ar e broke n u p n i "cold-working" . n I som e cases , thes e grain s ar e visibl e bu t eventuall y the y ar e broke n int o sub-microscopi c crysta l fragments . Thes e fragment s ten d o t assum e a n equiaxia l orientatio n wit h respec t o t th e directio n o f working . Nevertheless , heatin g th e wir e to o ho t cause s replacemen t of th e fiber s wit h crystals , whereupo n ductilit y s i los t an d th e wire become s brittle . The n t is i necessar y o t star t ove r agai n a t th e beginnin g of th e swagin g process . The lowes t temperatur e a t whic h recrystallizatio n ha s been observe d s i abou t 100 0 ° C fo r severel y worke d wire s o f pur e tungsten . However , th e presenc e o f alumin a o r thori a ca n rais e th e recrystallizatio n temperatur e abov e 200 0 °C .I t s i primaril y fo r thi s reaso n tha t mos t tungste n wire manufacturer s us e alumin a addition s o t th e tungste n trioxid e (whic h s i reduce d o t th e metal , compacte d int o bars , heat-treated , an d densifie d int o ingots ) befor e beginnin g th e wir e manufacturin g process . I t s i wel l o t not e tha t th e structur e of draw n tungste n wir e s i practicall y unaffecte d b y annealin g a t temperature s belo w 550-60 0 °C . Thus , thi s represent s a temperatur e limi t fo r reducin g strai n n i th e wir e induce d by th e wire-drawin g process . A-5. The Wire-Drawin g Proces s Whe n th e origina l ingo t ha s bee n reduce d o t a diamete r o f abou t 1. 0 mm. (th e rod so-produce d fro m a 0.5 0 inc h χ 24 inc h ingo t s i no w abou t 60 fee t long) , one change s fro m swagin g o t wire-drawing . A typica l setu p s i give n n i th e followin g diagram , show n a s 3.1.10 . o n th e nex t page . In thi s process , th e ro d s i initiall y draw n throug h a di e made of diamon d o r othe r har d materia l lik e W 2C The di e itsel fs i large r o n on e en d tha n th e exi t end s o tha t t i ca n accep t th e large r siz e an d s o tha t th e wir e bein g draw n become s smalle r n i diamete r a s t i passe s throug h th e die . I t s i common o t etc h on e en d electrolyticall y n i NaOH solutio n s o tha t t i ca n be introduce d int o th e die .

264

3.1.10. Tungste n Wir e Bein g Draw n Throug h Di eo t Reduc e it s Diamete r

The wire-drawin g proces s s i continuou s an d consist s o f th e followin g steps : a. Rod s ar e generall y reduce d n i diamete r fro m abou t 1. 0 mm. o t 0. 5 mm . n i on e ste p o t produc e "heavy-wire" . Thi s produc t the n s i furthe r reduce d o t abou t 0.2 5 mm. afte r th e heavy-wir e ha s bee n heat-treate d a t abou t 55 0 °C . o t reliev e strain . Thes e step s constitut e "heavy-wir e drawing" . b. The 0.2 5 mm. diamete r wir e s i the n lubricate d b y passin g t i throug h a bat h containin g colloida l graphite , i.e. - "Aguadag™" , heate d jus t befor e t i passe s throug h th e die , an d the n s i woun d upo n a drum . Heatin g bake s on th e lubricant , lower s bot h th e forc e neede d o t pas s th e wir e throug h the die , an d lower s th e stresse s induce d n i th e wir e durin g th e drawin g pas s throug h eac h die . Ductilit y o f th e wir e s i thu s improve d an d maintained . Betwee n eac h pas s throug h a give n die , th e wir e s i heat treate d o t reliev e strain . c. To measur e th e diamete r o f an y give n wire , on e measure s a standar d lengt h o f exactl y 20.0 0 cm. an d weigh s it . The followin g Table , presente d o n th e nex t page , give s wir e diameter , weigh t o f 2 0 cm. , an d nomina l curren t ratin g fo r variou s diameter s o f tungste n wire , slate d fo r use a s filament s n i incandescen t lamps .

265

TABL E 35 Wire Diamete r

Mils

Weight o f 2 0 c m INomina l Curren t

0.18 0 mm.

7.0 8

95.6 5 mg.

3.4 0

0.01 7

6.6 9

85.4 7

3.1 2

0.01 6

6.3 0

75.9 9

2.8 6

0.01 5

5.9 0

66.2 0

2.5 8

0.01 4

5.5 1

58.1 2

2.3 4

0.01 3

5.1 2

50.3 1

2.1 0

0.01 2

4.7 2

42.8 0

1.86 5

0.01 1

4.3 3

35.6 0

1.62 5

0.10 0

3.9 4

29.3 0

1.40 0

0.09 0

3.5 4

23.8 0

1.19 5

0.08 0

3.1 5

19.2 0

1.01 5

0.07 0

2.7 6

14.5 0

0.82 7

0.06 5

2.5 6

12.4 0

0.73 2

0.06 0

2.3 6

10.6 0

0.65 1

0.05 5

2.1 7

8.9 0

0.56 9

0.05 0

1.9 7

7.3 0

0.48 8

0.04 5

1.7 7

5.9 0

0.41 6

0.04 0

1.5 7

4.7 0

0.35 3

0.03 5

1.3 8

3.6 0

0.29 0

0.03 0

1.1 8

2.6 5

0.23 1

0.02 5

0.9 8

1.8 2

0.17 2

0.02 0

0.7 9

1.1 5

0.12 2

0.01 5

0.5 9

0.6 5

0.08 0

0.01 1

0.4 3

0.3 8

0.05 0

ampere s

d. I ts i impractica lo t dra w wir e finer tha n abou t 0.01 1 mm. n i diamete r (0.01 1 mm. = 0.4 3 mil s = 0.0004 3 inches) . However , on e ca n make finerwir e b y etching . But , th e etchin g proces s mus t be don e rapidl y o t obtai n a unifor m cross-sectio n n i th e wire . The bes t etchin g materia l s i a fuse d mixtur e o f NaNU3 an d NaNQ2 hel d a t 34 0 °C . Fo r example , a 0.01 4 mm wir e s i ru n throug h thi s bath , wit h a 45 secon d immersio n o t produc e a uniforml y etche d wir e of 0.00 7 mm. diameter .

266

e. An alternativ e metho d use d consist s o f a multiple-di e setu p show n a s follows . n I thi s unit , a wir e a t 20 0 micron s (0.20 0 mm.) s i draw n continuousl y o t 5 0μ n i si x passes . Drawin g speed s ar e 100-20 0 meter s per minute . Thi s metho d s i use d fo r wire s use d n i hig h volum e coil s fo r incandescen t lamps . 3.1.11 . Multipl e Di e Drawin g Uni t (To p View)

+

Γ

Capstan s

Dies

Electri c Furnac e f. Onc e th e wir e diamete r sough t s i achieved , th e wir e mus t be cleane d befor e furthe r use . Fo r size s large r tha n abou t 0.2 0 mm., th e "heavy wire " s i woun d upo n a larg e dru m an d the n hel d n i a solutio n o f boilin g KO H o r NaOH fo r abou t 3 0 minutes . The wir e s i the n washe d b y passin g it throug h a va t o f water , passin g t i throug h fel t pad s o t remov e graphite , an d the n rewindin g t i o n anothe r drum . Fo r fine r wires , th e cleanin g proces s s i o t spoo l th e wir e o t another , passin g t i throug h boilin g 40 % NaOH solution , cleanin g water-jets , an d the n felt-pad s o t remove an y clingin g graphite . Once th e wire-drawin g proces s s i complete ,t is i common o t joi n wire s o f th e same diamete r togethe r befor e processin g further . Sinc e tungste n meta l oxidize s s o easily , a specia l techniqu e s i required . Tungste n o t tungste n joint s are to o brittl e whe n joine d b y welding . But f i nicke l meta l s i plate d o n th e ends o f th e wire , weldin g serve s o t make a goo d joint , bot h flexible an d strong . Tungste n alloy s wit h bot h Ni an d Mo metals . Thus , stron g joint s ca n be made usin g eithe r metal . But Cu an d W d o no t alloy .t I ha s bee n foun d tha t an y exces s meta l use d o t joi n tungste n wil l volatiliz e s o tha t it s presenc e s i no t

267

detrimenta l o t operatio n a s a n incandescen t filamen t (Mo melt s a t 289 0 ° K (261 7 °C) wherea s Ni melt s a t 172 6 ° K (145 3 °C. ) A-6. The Coil-Windin g Proces s The primar y us e o f tungsten-wir e s i a s th e filamen t n i incandescen t lamps . A number o f method s o f producin g coil s hav e evolved , dependin g upo n th e manufacturer . Fo r th e mos t part , th e basi c metho d ha s remaine d th e same , tha t of windin g th e wir e aroun d a mandrel , usuall y a molybdenu m wire , o n a hig h spee d automati c coilin g machine . Thi s provide s a mor e unifor m produc t wit h close r tolerance s tha n coul d b e reache d b y manua l assembly . n I thi s process , th e cleane d wire , havin g bee n anneale d o t reliev e strain , s i wrappe d aroun d th e mandre l n i a tigh t spira l a s a continuou s singl e coi l whos e lengt h is generall y severa l hundre d meter s ormore . Thes e coil s ar e the n anneale d n i a hydrogen-furnac e o t remov e th e lubrican t an d o t reliev e strain s induce d durin g coiling . The initia l coilin g proces s s i show n n i th e followin g diagram : 3.1.12. Tungste n Filamen t Coi l Bein g Forme d o n Coi l Windin g Machin e

The wir e wit h th e mandre l s i the n cu t o t leav e shor t "leads" , whic h ar e the n bent o t for m th e fina l shap e o f th e filament . Then , th e mandre l s i dissolve d n i an aci d bath , leavin g th e finishe d coi l behind . The majo r reaso n fo r producin g a coi l fo r us e a s a filamen tn i a lam p is , a s we hav e alread y stated ,o t lengthe n the amoun t o f fin e tungste n wir e presen t n i an y give n lamp . Ther e ar e a number o f machine s availabl e o t d o thi s operation . Some manufacturer s hav e develope d thei r own proprietar y machines , whil e other s hav e purchase d suc h a machin e o n th e ope n market . Onc e th e coi l ha s bee n forme d an d ha s bee n

268

cut alon g wit h th e mandrel , the n anothe r Mo-mandre l wir e advance s an d operatio n begin s again .

th e

If a "coiled-coil " s i o t b e made , a lon g coi l s i firs t made o n a molybdenu m wir e mandre l b y th e windin g mechanism . Then , th e coi l s i reshape d aroun d a secon d mandrel , usuall y o f steel , th e coiled-coi l s i cu t o t lengt h alon g wit h th e initia l molybdenu m mandrel , an d the n shape d int o a fina l form . The secon d mandre l the n advances , an d th e operatio n begin s again . The coiled-coil s ar e the n anneale d a s befor e an d th e molybdenu m mandre l s i dissolved , leavin g the coiled-coi ln i a finishe d state . An exampl e o f a coiled-coi l s i a s follows : 3.1.13. |A Coile d ^Cou l

Coiled-coil s ar e use d n i incandescen t lamp s becaus e the y concentrat e th e volume o f hea t generate d withi n a smalle r spac e s o tha t losse s by convectio n are minimized . Thes e lamp s als o hav e mor e resistanc e o t vibrationa l shock . Most manufacturer s us e a continuou s coilin g machin e o t produc e coile d coils . The followin g diagra m illustrate s a vertica l coilin g machin e use d o t produc e the firs t coil : 3.1.14. -

269

Ther e ar e si x head s pe r machine . The secondar y doubl e mandre l run s a t abou t 1500-200 0 rpm. , dependin g upo n coil s dimensions . The y ar e usuall y four head skip-windin g affair s wit h a constan t mandre l spee d wher e th e primar y coi l s i brake d o t 2/ 3 o f th e operatin g spee d a s th e coiled-coi l near s completion . The sto p s i applie d by a magneti c brake , followe d by th e cut-off , and th e coiled-coil s ar e produce d a t ± 1. 0 mm. lengt h tolerance . In th e followin g diagra m s i show n som e o f th e design s use d fo r filaments , includin g tha t fo r so-calle d "rough-service " lamps : 3.1.15. Variou s Coiled-Coi l Filamen t Mount s forIncandescen t Lamps

B. MANUFACTURIN G PROTOCOL S DEVELOPED AFTER

197 5

Havin g describe d th e step s use d o t manufactur e tungste n wir e u p o t abou t 1975, we ar e no w n i a positio n o t examin e "improvements " made n i th e process , havin g a fir m basi s o f knowledg e concernin g th e step s require d o t make tungste n wire . Reiterating , th e step s involved : 1. Firin g o f WQ 3 powde ro t obtai n tungste n meta l powde r

270

2. Compactio n an d sinterin g o f tungste n meta l powde r int o a ba r 3. Densifyin g th e ba r vi a th e "Treatin g Bottle " 4. "Swaging " th e ba r int o a lon g rod(heat-treatin g betwee n passes ) 5. Initia l wire-drawin g int o "heav y wire" , includin g annealin g 6. Wir e drawin g int o size s a s lo w a s 0.01 1 mm

= 0.4 3 mil s

W e wil l no w examin e eac h o f thes e area s mor e thoroughl y n i orde r o t clarif y certai n technologica l area s whic h hav e remaine d obscur e o t thi s point . B-l Firin g of W O i Powde ro t Obtai n Tungste n Meta l Powde r A s a resul t o f considerabl e explorator y work , t i becam e obviou s tha t th e reductio n o f WO 3 o t th e metalli c for m was mor e complicate d tha n originall y believed . Furthermore , th e rol e o f additive s (an d impurities ) was observe d o t have a greate r influenc e n i th e performanc e o f tungsten-wir e filament s the m formerl y believed . The recognitio n cam e abou t becaus e som e manufacturer s use d a potassiu m silicate , i.e. - K2S12O9 , fo r a binde r n i pressin g ingot s befor e sinterin g an d Al2Q3andTh0 2 a s additive s o t contro l grain-siz e growt h durin g sintering . Othe r manufacturer s als o adde d KC1, AICI 3 an d othe r compound s fo r the sam e purpose . 1. Reductio n Reaction s o t for m Tungste n Meta l In th e followin g Table , give n o n th e nex t pag e a s Tabl e 3-6 , we sho w th e reaction s involve d n i th e reductio n o f WQ 3 o t W-meta l an d th e equilibriu m constant s determine d a s a functio n o f temperature . n I thi s Table , 1 4 reaction s to for m th e variou s gaseous , liqui d an d soli d phase s o f tungste n an d tungste n oxide s ar e given . Thes e equation s o f formatio n ar e show n alon g wit h th e correspondin g equilibriu m constants . Two temperatur e region s ar e o f interest . The first s i th e regio n o f 100 0 o t 140 0 ° K wher e th e reductio n o f WQ3(s) o t W(s ) take s place . The secon d s i th e hig h temperatur e regio n o f 2000 o t 300 0 ° K wher e th e final sinterin g an d reductio n occurs .

271

TABL E 36 Thermochemica l Dat a fo r th e Tungste n - Oxyge n Syste m log Kr a t Differen t Temperatures . ° K 1000 Equilibriu m Constan t

Reactio n

2200

3000

W(s) +1/ 2 0 2 ~ W O ( g)

K p = p w o/ i p c ^ )

6 -16.6 1 -4.82 5 -2.29

W(s) + 0 2 ^ W 0 2( S )*

Kp = 1 /PQ2

1 21.28 1 4.96

1.51 2

W(s) + 0 2 - W 0 2 ()g

K p = PW0 2/P02

2 -1.70 0 0.24

0.58 5

28.44 4 6.78 8

2.14 8

29.79 2 7.05 6

2.17 8

30.33 6 7.18 8

2.2 0

30.59 6 7.23 1

2.21 3

1 29.21 6 7.71

3.23 5

12.42 4 4.03 3

2.12 0

1 72

( s) K p = W(s) +1.3 6 02~ W 0 2 . 7 2 W(s) +1.4 5 O2^WQ2.90(s ) Kp

1

36

l/tpoa -) 1 /2

= PWO/(P02)

W(s) +1.4 8 02~ W 0 2 . 9 6 ()s K p = 1/ ( P 0 2 ) 184 15

W(s) +3/ 2 02 < - W Q 3 ( s )* * Kp =

1/ ( P Q 2 )

W(s)+3/ 2 O 2 - W 0 3( D * ** Kp =

ι/ίροζ) ·

W(s)+3/ 2 02 - W 0 3 ( g) 2W(s)+ 3 02 ~ ( W Q 3 )2 ( g)

Kp =

1

5

3 /2

P W 0 3/ ( P Q 2 )

4 7.90 2 47.59 7 15.09

3

Kp = PiWOs^/iPCk)

79.63 7 23.08 5 10.61 6

/2 3W(s)+9/ 2 02<=>(W03)3(g) Kp = P ( W 0 3)3 / ( P 0 29 )

4W(s)+ 6 02 ~ 3W(s) + 4 02 ~ W(s) ~ W (g )

(WQ3)4(g)

K p = P(W0 3 ) 4/ ( P 0 2 )

6 108.4 4 30.24 2 13.03

W 3 0 8 ) ( g)

Kp =Ρν^3θ8/(Ρθ2)

9 0 9.52 67.32 3 20.10

6

4

4 -37.0 5 -12.5 6 -7.10

Kp = pWg

* 3 WU 2 <=> W(s) + 2 W 03( s) a t Td = 199 7 ° K an d highe r ** *** Log

0 °K, Tm = 174 5 ° K Tt = 105 0 ° K Tb = 211

Kp value s ar e give n n i thi s tabl e fo r eac h o f th e reaction s tha t occur . I t

shoul d b e

clea r tha t th e reduction-chemistr y s i mor e comple x tha n was

d eviden t previously . Durin g thes e reactions , we assum e tha t th e K2Si2O g adde as a compactio n agen t s i reduce d o t K2 0 an d S i20 , an d

tha t th e alumin a

presen t s i no t affecte d belo w 180 0 °K. I t s i wel l o t observ e her e tha t thoria , i.e. - T h 02 , s i sometime s adde d o t contro l grai n siz e durin g sintering . The partia l pressur e o f oxyge n associate d wit h th e ga s mixtur e durin g reductio n can b e evaluate d fro m th e equilibriu m constan t fo r water , viz 3.1.16 -

H2( g) + l /2 02( g)

- H20(g) ; PQ2

2

= K " H 2O ( P H 2/ P H 2o )

2

272

Here, PH2/PH2OI S th e partia l pressur e rati o o f hydroge n o t wate r vapo r o f th e gas mixture . We shal l cal l thi s ratio , "R" . Not e tha t larg e value s o f R indicat e a lower rati o o f wate r vapo ro t H2 ga s present . Becaus e th e reductio n o f WQ3U ) to W(s) produce s wate r vapor , we mus t evaluat e th e effec t o f th e wate r vapo r oxidation-reductio n potentia l o n th e equilibriu m constant s n i th e Table . Thi s can be don e b y usin g th e equation s give n abov e an d substitutin g P02 n i eac h o f the equilibriu m equations . However ,n i tabula r form , thes e dat a ar e difficul t o t apply . Two feature s ca n

b e

use d o t

hel p visualiz e th e variou s chemica l

processe s occurring . First , oxyge n s i involve d n i many o f th e equations , an d secondly , volatil e gase s form . A plo t o f th e volatilit y o f th e tungsten-oxyge n specie s vs : th e oxyge n pressur e allow s on e o t evaluat e th e relativ e importanc e o f eac h o f th e reactions . Suc h a plo t s i show n n i th e following : 3.1.17. Thermochemica l Dat a forth e Tungsten-Oxyge n Syste m a t 100 0 ° K log p0 (arm. ) -32 -3 0 -2 8 -2 6 -2 4 -2 2 -2 0 -1 8 -1 6 -1 4 -1 2

6

5

4

3

2

1 l 0g

0

1

P

H

/ P 2

H

2 2

0

3 -

4 -

S -

-1 0

6 -

273

The

vertica l dotte d line s show s

th e

region s o f existenc e o f th e

tungste n oxide s involved , whil e th e angula r an d

severa l

horizonta l line s giv e th e

volatilit y o f thes e oxide s a s a functio n o f th e oxyge n pressur e (top ) an d rati o o f hydroge n o t wate r vapo r pressur e (bottom) . Thes e dat a sho w avoi d losse s o f tungsten , th e initia l reductio n o f WO 3

th e

tha t o t

shoul d b e carrie d ou t a t

temperature s belo w 140 0 °K. At 100 0 °K, lo g R > 0. 6 fo r reductio n whil e a t 1400, lo g R >0 . Fo r reductio n o f WQ3(s) o t W (s ) a t leas t 1.0 % o f wate r vapo r can b e present , i.e. - lo g R ca n b e 2 withou t greatl y affectin g th e efficiency. But

a t

220 0

°K. , much

pure r hydroge n s i

reductio n

require d sinc e

considerabl e los s o f W ca n occu r throug h formatio n o f volatil e oxide s f i lo g Rs i not sufficientl y large . The amoun t o f volatil e oxide s produce d wil l depen d upo n th e oxyge n ga s an d wate r vapo r conten t o f th e hydroge n ga s used . Volatilizatio n o f tungste n oxide s is smal l a t 2200-260 0 ° K f i lo g R > 2.0 . Fo r remova l o f S1O2 an d A1203, th e reductio n temperatur e an d

th e compositio n o f th e hydrogen-wate r vapo r ga s

mixtur e s i ver y important . SiO(g ) ca n b e forme d fro m Si02 (s) a t 140 0 ° K a t a n appreciabl e rat e f i lo g R > 4.0 . Abov e 180 0 °K, Si O (g ) s i forme d ove r a wid e rang e o f lo g R. AI2O3 s i inactiv e a t 140 0 ° K n i al l ga s mixture s belo w lo g R = 8.0 , bu t react s a t 180 0 ° K an d

highe r o t

for m Al(g ) an d

Al20(g) . Los s o f

aluminu m occur s b y transfe r o f Al (s) , volatil e oxide , hydride s an d oxyhydride s of aluminum . The bes t procedur e fo r preparin g tungste n ingot s s i o t

reduc e

the tungste n trioxid e o t tungste n meta l a t temperature s belo w 140 0 °K. 2. Los s o f Additive s Durin g Reductio n Aluminu m

an d

silicon-containin g impuritie s ca n be remove d n i a controlle d

manner a t highe r temperature s n i purifie d H2 gas . A lo g R > 2. 0 s i neede d o t remove SiO(g ) a t 180 0 °K. The optimu m valu e s i foun d a t lo g R = 4.25 . To optimiz e th e volatilit y o f Al, lo g R shoul d b e increase d o t 6.25 . Not e tha t we are

speakin g o f remova l o f impuritie s b y

a therma l treatmen t prio r o t

sinterin g o f th e ingot . Thi s aspec t s i show n n i th e followin g diagram , show n a s 3.1.18 . o n th e nex t page . In thi s diagram , we sho w a thermogravimetri c analysi s o f a "doped " tungste n trioxid e an d th e losse s observe d a s a functio n o f temperatur e o f reductio n b y hydrogen-gas . I t s i apparen t tha t par t o f th e S1O2 an d AI2O3 reacte d o t for m "Mullite" , i.e. - 3 AI2O3

+ 2 Si0 2 = A l6 S i2 O i 3.

274

3.1.18. Loss of Impuritie s a s Determine d by Thermogravimetri c Mean s

ι — ι — ι — ι — ι — ι — ι — ι —

1500

170 0

190 0

210 0

r

230 0

Furnac e Temperatur e n i °C . Note als o tha t par t o f th e silic a an d alumin a volatilize d befor e th e mullit e finall y decompose d T(M =185 0 °C. ) an d was lost . Thi s behavio r occurre d regardles s o f whethe r th e tw o oxide s wer e pre-reacte d o r not . I t therefor e become s apparen t tha t th e initia l reductio n o f WO 3 o t W-meta l b y hydroge n gas a t hig h temperature s serve s a s a purificatio n ste p a s wel l (i f t i s i don e properly) . The pronounce d "bump " n i 3.1.18 . nea r o t 195 0 °C .s i believe d o t be du e th e reaction : 3.1.19. -

3 { A l 6 S2 Oii3 } + 2 W -*

6 SiOl t + 9A1203 + 2 W 03 t

Overall , thes e reaction s may be summarize d a s give n n i 3.1.20 . o n th e nex t page .

275

3.1.20. a) 2 S 1 O2 2 Si O t + 02 b) 02 + W =• WO 3 (vacuum ) c) W O 3 + 3 H2 = > W +3 H 2O

(reducin g atmosphere )

It s i fo r thi s reaso n tha t tungste n meta l powde r s i made usin g a reducin g atmospher e rathe r tha n n i vacuu m sinc e an y oxyge n presen t wil l for m th e trioxid e unles s t is i reacte d upo n by hydroge n o t for m wate r vapor . 3. Siz e an d Morpholog y of Tungste n Meta l Powder s Produce d W e hav e alread y mentione d th e fac t tha t tungste n meta l powde r need s o t b e prepare d wit h a rathe r smal l particl e siz e an d wit h a smal l rang e o f particl e sizes . A typica l rang e o f size s s i show n n i th e followin g diagram : 3.1.21. Particl e Siz e Distributio n of Typica l Tungste n Powde r 30

0

2 4 6 Particl e Siz e n i Micron s(μ)

8

However ,t i ha s bee n foun d tha t additives , i.e. - "doping' , hav e a profoun d effec t upon bot h th e siz e an d morpholog y o f th e tungste n meta l particle s produce d (Not e tha t dopant s ar e usuall y adde d o t promot e improve d pressabilit y an d sinterability) .

276

A s show n n i th e followin g Table , th e morpholog y ca n rang e fro m cube s o t octahedr a an d othe r polyhedrons . The standar d dopin g includes : 2 % KC1, 2% A1C13 an d 1 % K2S12O 9 b y weight . TABL E 37 Electro n Microscop e Examinatio n o f Tungste n Particl e Morpholog y Dopant

Siz e

Morpholog y Surfac e Appearanc e Number/

Range* Standar d 1- 2. 5μ -se e abov e 3 Time s

1 - 5μ

Standar d A 1 ( N 03 )3 - 2%

Siz e

W-particl e of Occ 0. 1 μ

Round Cube s Smooth-fe w irreg . 5 protrusion s Cubes &

Smooth

>

Covere d wit h many

none

1 0

0. 3 μ

Polyhedr a 1 - 5μ

Polyhedr a

0.1 μ cub e shape d protrusion s

KCl-2%

0.5-2. 0μ

Standar d 1 - 3μ withou t

Polyhedr a

Smooth-som e irreg . none protrusion s

Rounde d Cubes

Covere d wit h

AICI3

0. 1 μ

5

Geometricall y Shape d

Protrusion s

Standard - 0. 2 1 -μ Reduce d 2nd tim e

Cubes & Octahedr a

none Small Protrusion s

Thoriate d 0. 5 - 3μ Only

Cubes &

Numerous Ver y

Octahedr a

Small Protrusion s

Unwashe d 0. 5 - 3μ Powder

Rounde d Cubes

Smooth - Fe w Smal l > Protrusion s

Not dope d 0.0 5 μ

Sphere s &

Smooth Surfac e

Smooth - many

1000 1 0

<

.01 μ

0.1 1. 0

none

Bean-shape d In al l cases , t is i possibl e o t dissolv e th e tungste n particle s n i 30 % hydroge n peroxid e n i orde r o t determin e th e iner t particle s produce d durin g sintering . (Not e tha t we ar e no t speakin g o f compactio n an d densification , bu t th e initia l proces s o f sinterin g o t obtai n a n ingo t capabl e o f bein g handled) .

277

It shoul d be clea r tha t dopin g an d proces s variation s hav e a significan t influenc e upo n th e morphology , surfac e topograph y an d siz e o f th e tungste n particle s bein g produced . Thi s obviousl y ha s a n importan t effec t o n th e "voids " presen t whe n th e powde rs i compacte d int o a bar . Note n i th e abov e Tabl e tha t certai n additive s di d no t produc e inclusion s withi n th e tungste n particle . Most inclusion s wer e identifie d a s oxide s o f th e variou s element s adde d b y mean s o f x-ra y analysis . It ha s bee n observe d tha t certai n volatil e dopant s adde d (though t mainl y o t be potassium) , bein g insolubl e n i tungsten , exis t n i th e vapo r phas e durin g reductio n an d sinterin g a t 300 0 °C . The vapo r s i trappe d withi n th e pore s which exis t becaus e o f th e interstitia l spac e betwee n packe d particles . Upon workin g th e sintere d ingo to t a fin e wir e size , th e pore s ar e close d by plasti c flo w an d th e dopan t s i smeare d ou t int o row s o f closel y space d submicroscopi c particle s aligne d paralle l o t th e workin g direction . Upon furthe r annealing , th e dope-particle s volatilized , leavin g bubble s behind . The observatio n of th e productio n of bubble s s i perhap s th e mos t importan t on e made n i recen t times . Thes e bubble s hav e bee n observe d o t be essentia l o t produc e sag-resistanc e n i filamen t wire , a s wel l a s producin g a n anomalousl y hig h recrystallizatio n temperature . B-2. Rol e of Annealin g n i th e Preparatio n of Tungste n Wir e fromIngot s To establis h certai n fact s regardin g th e rol e of annealin g durin g processin g of tungste n int o wire , fou r type s of swage d & draw n wir e wer e examine d by transmissio n electron-microscop y n i th e as-worke d condition . Simila r examination s wer e als o carrie d ou t o n specimen s o f th e sam e type s afte r annealin g a t 220 0 °C. fo r 1 5 minutes . The sample s encompasse d thos e show n in 3.1.22 . Afte r workin g an d annealin g procedure s wer e carrie d out , pore s were observe d o t hav e forme d n i eac h of thes e samples . 3.1.22. - Type s o f Tungste n Examine d by Electron-Microscop e Microscop y a. b. c. d.

4.5 0 2.2 0 2.0 0 1.3 0

mm. mm. mm mm.

- swage d ro d - swage d ro d - swage d & draw n ro d - swage d & draw n ro d

278

The pore s n i th e swage d rod s range d fro m « 0.0 5 μ o t 1.0 0 μ n i siz e an d wer e randoml y distributed . The large r pore s wer e slightl y elongate d n i th e workin g direction . However , th e numbe r o f pore s an d thei r volum e wer e significantl y reduce d b y drawin g fro m 2.2 0 mm. o t 2.0 0 mm. The remainin g pore s wer e the n needle shaped . The effec t o f deformatio n (swage d & draw n rod ) o n th e densit y an d distributio n o f th e bubble s produce d was ver y eviden t a s show n b y th e followin g diagram , give n a s 3.1.23 . o n th e nex t page . Annealin g cause s th e large , elongate d pore s n i swage d rod s o t becom e spherica l o r spheroida l n i shap e du e o t anisotrop y o f surfac e energ y an d enlarge s th e smalle r irregula r pore s du e o t th e volatilizatio n o f th e dopant . n I the annealed , draw n wires , bubble s aligne d themselve s n i row s paralle l o t th e workin g direction . The tota l bubbl e density , th e degre e o f alignment , an d th e number o f bubble s n i eac h ro w wer e observe d o t increas e a s th e siz e o f th e wire decreased , wit h eac h succeedin g drawin g operation . These observation s no t onl y confirme d tha t workin g o f th e tungste n wir e smeare d ou t th e dopan t int o row s o f particles , bu t als o explaine d th e necessit y o f sufficien t wir e drawin g o t creat e a larg e numbe r o f smal l bubble s neede d o t creat e sag-resistanc e o f wir e forme d int o filaments . I t als o show s tha t wire-drawin g s i fa r mor e effectiv e tha n swagin g n i closin g u p th e pores , or voids . A s a result ,t i was conclude d that : a. Annealin g orretreatin g a ro d (o r wire ) whic h s i no t completel y densifie d s i detrimenta l o t th e developmen t o f non-sa g properties , becaus e enlargemen t an d spheroidizatio n of th e elongate d pore s partiall y destro y th e wor k don e b y wire-drawin g in smearin g ou t th e "dope" . b. As a resul t o f thi s work , t i becam e mor e obviou s tha t a n alternativ e o t swagin g neede d o t be sought . A rollin g proces s might be mor e effectiv e n i producin g an d preservin g bubbles , especiall y f i a rol l wit h circula r groove s wer e o t b e use d fo r reductio n o f rod s and/o r ingot s prio r o t wire-drawing .

279

3.1.23. Transmissio n Electron-Micrograph s of Tungste n Rod s

1. As Swage d - 4.S 0 mm. Rod

3. As Draw n - 1. 3 mm. Rod

2. As Swage d - 2. 2 mm. Rod

4. Swage d & Anneale d - 220 0 °C . for I S Minute s - 4.5 0 mm. Rod

S. Draw n & Anneale d - 220 0 °C . for I S Minute s - 2.2 0 mm. Rod

6. Draw n & Anneale d - 220 0 °C . for I S Minute s - 1.3 0 mm. Rod

280

However , le t u s furthe r pursu e th e questio n o f bubble-formatio n befor e we tackl e th e proble m o f advance d technique s o f processin g o f tungste n bar s int o fin e wir e wit h improve d "sag-resistance" . 1. Dopin g o f th e Tungste n Powde r an d Bubbl e Formatio n The nex t questio n tha t neede d o t be answere d was tha t concernin g th e minimu m numbe r o f aligned-bubble s require d o t produc e a "sag-proo f wire . The answe r was obtaine d b y preparin g a serie s o f doped - tungste n oxide s an d reducin g the m accordin g o t a "standard " method , viz 3.1.24. -

Standar d Metho d fo r Reductio n an d Sinterin g a. Reductio n of WO 3 o t W : 3 hour s @ 125 0 °C . b. Wash W powde rn i HF c. Sinterin g W Ingo t afte r pressing : 4 5 minute s @ 300 0 °C . d. Swag e an d dra w int o ro d (2. 1 mm.) e. Annea l @ 210 0 °C. fo r 3 0 minute s

The oxide s use d wer e "doped " a s show n abov e n i Tabl e 37 an d the n reduce d to tungste n powder . Measurement s wer e take n concernin g th e residu e lef t n i the powde r (a s determine d b y H2Q2 digestion) , th e compositio n o f th e residu e an d th e surfac e are a o f th e particle s (whic h measure s bot h th e apparen t siz e o f th e particle s an d thei r porosity) . Thi s s i show n n i Tabl e 3-8. , as give n o n th e nex t page . The nex t ste p was o t compac t th e tungste n powder , swag e an d dra w t i int o wire , wit h appropriat e sinterin g an d annealing . Finally , a coun t o f th e numbe r 2 of bubble s presen t pe r 10 0 u was made an d als o a n averag e valu e fo r grain siz e presen t was determined . Sinc e most , f i no t all , o f th e grain s wer e observe d o t b e elongated , a paramete r calle d "M-value " was established . Thi s valu e s i a combinatio n o f grain-are a an d grain-diamete r estimations .

281

TABL E 38 Analysi s o f Experimenta l Tungste n Powder s Compositio n o f Residu e Residu e n i DDIH %Si %A 1 %K Dopin g Histor y 1

Surfac e Are a 2 n i m /gm .

Standard-se e abov e

622

30

4

2

0.2 0

3 Time s Standar d

8,49 2

30

2

1

6.5 0

518

100

2.9 0

KCl-2%

48

100

7.3 0

Standar d withou t AICI3

305

A 1 ( N 03) 3~

2 %

Std - Reduce d 2n d tim e 209

30

0.4

0.4

0.6 0

30

4

2

0.1 0

1

0.2 0

Unwashe d Powde r

2,09 0

30

3

Not dope d

12

3

2

0.2 0

Whe n Μ was plotte d agains t bubble-density , a stron g linea r correlatio n was seen , a s show n n i th e followin g diagram : 3.1.25. Correlatio n ofΜ-Value an d Bubbl e Densit y

100

20 0

30 0

40 0

50 0

Bubbl e Densit y pe r 10 0 μ2 Are a

60 0

282

A n examinatio n o f thi s diagra m make s t i clea r tha t grain-siz e s i linearl y relate d o t bubbl e density . n I thi s manner , th e recrystallize d grai n siz e s i 2 clearl y relate d o t bubbl e density . A bubbl e densit y o f > 40 0 bubbles / 10 0 μ are a apparentl y s i require d o t achiev e a hig h M-valu e (an d satisfactor y sag resistance) . To furthe r illustrat e thi s aspect , a numbe r o f thes e specimen-rod s wer e anneale d n i th e rang e o f 1600-242 5 ° C fo r time s rangin g from 1 0 o t 10,00 0 seconds , an d bot h bubble-densitie s an d grainsiz e wer e determine d fo r eac h o f the specimens . Self-diffusio n distance s o f som e o f thes e bubble s wer e calculate d n i micron s fro m th e diffusio n equation : 3.1.26. -

D = D0 e xp ( -Ε /kfc>T )

which s i relate d o t Fick' s Law: JN = - D gra d N, wher e JN s i th e numbe r o f atoms crossin g a uni t are a n i uni t tim e an d D s i th e diffusio n constan t wit h 2 unit s o f c m/ s e c Ε s i th e activatio n energ y fo r th e give n temperature . The followin g diagram , give n a s 3.1.27 . o n th e nex t page , show s th e resul t o f the variou s annealin g schedule s employed .

s bee n calculate d a s wel l a s Q, th e activatio n energ y fo r Note tha t D0 ha diffusion . The dat a wer e foun d o t fital l ont o on e curv e irrespectiv e o f th e annealin g temperatur e an d time . Thi s s i stron g evidenc e tha t bubbl e formatio n occur s b y vacanc y diffusion . Sinc e th e bubbl e densit y become s mor e or les s constan t abov e abou t 0. 5 μ , whic h s i approximatel y 1/ 2 o f th e grai n size , t i appear s tha t th e grai n sub-boundarie s ar e th e sourc e o f vacancies . Moreover , thi s resul t confirm s th e hypothesi s tha t a minimu m bubbl e densit y is require d n i anneale d wir e fo r sag-resistanc e whe n use d a s a n incandescen t filament . (We shal l discus s thi s n i mor e detai l below) . Thermodynami c consideration s concernin g th e nucleatio n o f a bubbl e b y volatilizatio n o f a n insolubl e particl e hav e predicte d (2 ) tha t th e minimu m annealin g temperatur e require d fo r nucleatio n depend s upo n th e siz e o f th e particle . However , th e abov e experimenta l result s hav e no t supporte d thi s vie w sinc e th e bubbl e densit y appear s o t depen d upo n th e diffusio n distanc e rathe r tha n upo n th e annealin g temperature .

283

3.1.27. Densit y of Bubble s n i Anneale d -Dope d Tungste n vs : Distanc e of Self-Diffusio n

4oo

ι,

Ο

ο

ω

Q = 15 3 Κ cal./mol e

300 r t

D ω Ω ω

-Ω -Q 3 CD

= 43 cm2 /sec .

200 ΓΓ

100

J 0.30

I

I

0.6 0 0.9 0

I

I

L

1.2 0 1.5 0 1.8 0 2.1 0

Diffusio n Distanc e n i Micron s This suggest s tha t nucleatio n doe s no t pla y a significan t rol e fo r th e annealin g condition s used . The fac t tha t th e bubble s di d no t var y appreciabl y n i siz e als o support s thi s view . Thus, much evidenc e ha s bee n foun d tha t th e submicroscopi c bubble s whic h for m a t temperature s a s lo w a s 140 0 °C .n i dope d tungste n particle s ar e primaril y responsibl e fo r th e hig h temperatur e strengt h an d sag-resistanc e o f tungste n filament s n i lamps . Moreover , t i ha s bee n establishe d clearl y tha t change s n i grain-morpholog y occu r a s a consequenc e of thes e bubbles . The followin g diagram , give n a s 3.1.28 . on th e nex t page , show s th e grain morpholog y o f a dope d vs : a n undope d tungste n wire , bot h anneale d unde r the sam e conditions .

284

3.1.28. Microstructur e of S mi l Tungste n Wire Anneale d for4 5 mi n @ 200 0 ° C

Undoped Wir e @ 30 0χ

Doped Wir e @ 30 0χ The bubbl e densit y n i th e doped-wir e was determine d b y transmissio n 2 electro n microscop y o t be abou t 40 0 bubble s pe r 10 0 u . No bubble s wer e detecte d n i th e undope d wire . To confir m th e postulat e tha t bubbl e densit y s i relate d o t volatilizatio n o f inclusion s remainin g fro m adde d "dope" , formatio n o f bubble s b y anothe r metho d was sought . A ver y convenien t way o f introducin g iner t ga s bubble s int o undope d tungste n s i bombardmen t b y alpha-particle s o f sufficien t energy , couple d wit h subsequen t annealin g o t precipitat e heliu m gas . Thi s metho d ha s the advantag e o f havin g relativel y shor t irradiatio n time s an d shor t half-live s o f radioactiv e product s s o tha t th e specimen s ca n be handle d afte r withi n 23 days . However , du e o t th e natur e of alpha-particles , th e heliu m bubble s precipitat e n i a relativel y narro w surfac e layer , a s previousl y reporte d n i alpha-irradiatio n experiment s wit h berylliu m (3 ) an d aluminu m (4) . Whe n thi s was done , bubbl e densit y an d bubbl e siz e wer e foun d o t be dependan t upo n th e temperatur e o f annealing . Fo r example , th e firs t bubble s were see n unde r electron - transmissio n microscop y a t abou t 160 0 °C. (usin g a magnificatio n o f 150 0 x) . However , a t magnification s of 100,00 0 χ or higher , ver y smal l bubble s approximatel y 5 0 A n i siz e wer e found . Upon furthe r annealin g a t 220 0 ° C fo r 4 5 minutes , th e bubbles , no w grow n o t 0. 5 o t 1. 0 pn i size , coul d be see n b y a ligh t microscop e a t 100 0 x . Afte r a n annea l o f 4 5 minute s a t 240 0 ° C, th e bubbl e siz e ha d increase d o t 2.0-4. 0 μ , bu t th e

285

number o f bubble s ha d decrease d considerably . However , th e bubble s wer e mostl y non-spherica l an d wer e locate d a t o r nea r o t th e grai n boundaries . Nevertheless , thi s wor k furthe r establishe d th e rol e o f bubble s n i th e annealin g process . A definit e chang e n i grai n morpholog y was note d wit h th e productio n o f mor e elongate d grain s becomin g mor e prevalent , jus t a s n i "doped " tungste n wire . However , t i s i clea r tha t sinc e recrystallizatio n s i a therma l process , th e numbe r o f bubble s require d o t retar d thi s recrystallizatio n proces s increase s wit h increasin g temperature . It s i possibl e o t explai n th e dependenc e o f recrystallize d grai n siz e upo n bubbl e density . t I s i foun d tha t th e recrystallize d grai n siz e s i determine d by the ratio , G / N, wher e G s i th e rat e o f growt h of a recrystallizin g grai n n i th e unrecrystallize d matri x an d Ν s i th e rat e o f nucleatio n o f th e primar y grains . Whe n th e bubbl e densit y s i low , i.e. - hig h inter-bubbl e spacing , Gs i retarde d due o t Zene r drag , bu tΝ s i unaffected . I f th e bubbl e densit y s i highe r tha n a certai n critica l density , th e bubble s (o r a disperse d secondar y phase ) hav e a far greate r effec t o n reducin g Ν tha n G b y increasin g th e critica l nucleu s size . The followin g equatio n clarifie s thi s relationshi p further : 3.1.29. -

Rc = 2σ / (Em- ER

- nF )

Ν - e( /3 π σ ΐ ^ ) / 4

^

where Rc s i th e radiu s of th e critica l nucleus ,σ s i th e grain-boundar y energ y of th e recrystallizin g grain , EM s i th e dislocatio n energ y densit y of th e unrecrystallize d matrix , ER s i tha t o f th e recrystallizin g nucleus ,η s i th e number o f bubble s interactin g wit h th e grai n boundar y pe r uni t area , Fs i th e Zener-dra g pe r bubble , an dΤ s i th e temperatur e n i °K. If G s i assume d o t b e inversel y proportiona l o t th e bubbl e density , a plo to f G /N vs : bubbl e siz e wil l predic t th e dependenc e o f grai n siz e o n bubbl e density . Suc h a plo t s i give n n i th e followin g diagra m presente d a s 3.1.30 . o n the nex t page . Becaus e ther e s i a subsequentl y observe d increas e n i strengt h o f dope d wire , the chang e ca n b e attribute d o t th e simultaneou s formatio n o f submicroscopi c bubble s whic h result s bot h n i th e strengthenin g of th e matri x an d inhibitio n of recrystallizatio n durin g annealing .

286

3.1.30. Critica l Nucleus , Rc , Nucleatio n Frequency ,Ν and Growt h Rate , G ,vs : Bubbl e Densit y

0

20 0

40 0

Bubbl e Densit y / 10 0 μ

60 0 2

Indeed , measurement s o f tungste n wir e a s a functio n o f annealin g tim e hav e shown a dro p o f tensil e strengt h fro m a n origina l valu e o f 380,00 0 psi . fo r a 9. 0 mi l wir e o t 140,00 0 psi . afte r annealin g fo r 1 5 second s a t 200 0 °C . (a dro p o f ove r 60%) . To furthe r substantiat e this , a simila r plo t o f th e material s show n n i 3.1.22 . and 3.1.23 .s i illustrate d a s follow s n i 3.1.31. , give n o n th e nex t page . To furthe r clarif y thi s point , thre e type s o f 9. 0 mi l wir e wer e prepare d from: a) b) c) d)

Fully-dope d tungste n Doped , bu t withou t AICI3 2 % KC1 Undope d tungsten .

The annealin g time s range d fro m 1 5 second s o t 8 minute s a t temperature s fro m 1500-240 0 °C To establis h th e reproducibilit y o f th e results , fiv e 4-inc h specimen s wer e anneale d simultaneousl y a t eac h specifie d tim e an d

287

3.1.31. G/N Rati o an d Mean Grai n Diamete r vs : Bubbl e Densit y 1.2 mm

_

0

20 0 40 0 2 Bubbl e Densit y / 10 0 μ

60 0

temperature . The specimen s wer e the n tensile-teste d a t roo m temperatur e a t a strai n rat e o f 0.05/minute . Result s showe d tha t th e ultimat e loa d (fo r specimen s exhibitin g brittl e fracture : breakin g loa d = fractur e load ) o f thes e material s a s a functio n o f annealin g tim e showe d considerabl e deviation . The followin g diagram , show n a s 3.1.32 . o n th e nex t page , show s result s obtaine d a t 200 0 ° C whic h ar e typica l o f al l measurement s made (excep t tha t the highe r temperature s promot e large r bubbl e siz e formation) .

288

3.1.32. Measure d Tensil e Strengt h vs : Annealin g Time for9. 0 Mil Wir e Diffusio n Distanc e n i Micron s 400 3 70

0.3 2

0.1 6

"~Γ~

0.4 8

Τ

Τ

0.9 6

0.6 4

1 400

h

Full y Dope d Tungste n 340 g

Ο

31 0

300

iH

*

28 0

w a

a

KC1

2%

200

25 0 2 10

Doped, taut Withou t AICI3 180

—1 100

ISO

Undoped Tungste n

120

1 0

2

•

4

1

Η ο

6

Annealin g Time n i Minute s In thi s diagram , th e initia l rapi d dro p n i strengt h s i a consequenc e o f eithe r recover y fro m strai n induce d by drawin g o r recrystallizatio n of th e wire . The temperatur e a t whic h th e latte r occur s depend s upo n th e recrystallizatio n temperatur e o f th e material . Photomicrograph s o f th e wire s anneale d fo r 1 5 second s a t 200 0 ° C showe d tha t th e undope d wir e s i full y recrystallize d wherea s bot h of th e othe r tw o showe d onl y partia l recrystallization . Onl y th e fully-dope d tungste n showe d n o recrystallizatio n a t all , an d it s strengt h increase d a s th e annealin g tim e was increased . Not e tha t a t time s les s tha n abou t 1 minut e o f annealin g time , al l specimen s showe d failur e b y brittl e fractur e du e o t lac k o f sufficien t bubbl e formation . The scatte r o f fractur e dat a at a give n se t of condition s als o decrease d a t th e longe r annealin g times .

289

The subsequen t increas e n i strengt h fo r al l wire s excep t th e undope d wir e s i clearl y attributabl e o t th e formatio n o f submicroscopi c bubble s whic h result s in strengthenin g an d inhibitio n o f recrystallizatio n o f th e matrix . Include d n i 3.1.32 . a t th e to p s i th e calculate d curv e whic h relate s th e "self-diffusion " distanc e o f tungste n atom s o t th e bubbl e densitie s (a s determine d b y electro n transmissio n micrographs , a s calculate d from th e equation : 3.1.33. -

D = D0

ex p (Q / kb T an d χ =2 ( Dt )°5

where th e sam e value s a s n i 3.1.2 5 ar e used . Thi s correlatio n o f self-diffusio n distance s an d bubbl e densit y ha s establishe d beyon d an y doub t tha t fo r an y combinatio n o f annealin g time s an d temperature s whic h resul t n i a diffusio n distanc e large r tha n approximatel y 0. 4 μ, th e densit y o f submicroscopi c bubble s do e no t furthe r increase . If , therefor e th e increas e n i strengt h s i indee d cause d b y submicroscopi c bubbles , th e maximu m n i thi s strengt h shoul d occu r whe n th e formatio n o f bubble s s i complete .I t was fo r thi s reaso n tha t th e tim e scal e an d th e "self diffusion " distanc e scal e wer e chose n s o tha t the y coul d b e compared . I t wil l be note d tha t fo r thi s particula r se t o f materials , a t leas t 2. 0 minute s o f annealin g tim e ar e require d o t achiev e maximu m strengt h an d tha t thi s correspond s exactl y o t th e tim e whe n bubbl e densit y achieve s it s saturation . Thus, difference s n i strengt h ca n b e attribute d o t difference s n i bubbl e density . However , whe n annealin g s i carrie d ou t a t successivel y highe r temperatures , the material s becom e mor e brittle , resultin g n i lowe r brittle-breakag e loads . This indicate s tha t th e embrittlemen t du e o t progressin g recrystallizatio n exceed s th e effec t o f bubbl e strengthening , du e perhap s o t bubbl e enlargemen t observe d a t th e highe r temperatures . In orde r o t clarif y thi s point , a serie s o f test s wer e don e n i whic h th e fully dope d wir e was anneale d fo r 4 minute s an d tensile-teste d a t a strain-rat e o f 0.0 5 m i n.i Two sample s o f 9 mi l wir e wer e used , on e whic h ha d bee n bottle sintered , an d th e othe r whic h was furnace-sintered . A ( vacuu m furnac e was use d havin g molybdenu m silicid e heatin g elements , insulate d wit h zirconi a fel t an d presse d slab s o f zirconia) . The result s ar e show n n i th e followin g diagram :

290

3.1.34. Tensil e Strengt h of 9 mi l Wir e vs.Annealin g Temperatur e ο ο ο

450

Bottle-Sintere d

400 350

Furnace-Sintere d

Ν .

•ι—I

3 ι—Η •ΓΗ

300

4 Minute s Annealin g Time

250 200

JL ι 1000 1200

1

1

1

1400 1600 1800 2000 Annealin g Temperature , °C.

1

χ

2200

1 2400

It shoul d be clea r fro m thi s diagra m tha t annealin g temperature s betwee n 1700 an d 200 0 °C . ar e neede d o t promot e th e stronges t wire , an d o t promot e maximum growt h o f bubbles . However , ther e s i a balanc e whic h mus t be maintained , sinc e th e formatio n o f bubble s s i relate d o t void s lef t behin d durin g th e siz e reductio n o f rod s o t wir e b y drawing . Too many void s wil l lea d to a filamen t whic h undergoe s prematur e failure . 2. Voi d Migratio n an d Filamen t Failur e The law s governin g th e motio n o f void s n i tungste n hav e bee n bee n formulate d mathematicall y n i term s o f know n transpor t coefficients . What we mean by void s ar e thos e forme d eithe r by interstitial s betwee n particle s o f tungste n meta l whe n compactio n an d sinterin g occurs . Voi d velocitie s ca n be calculate d a s a functio n o f siz e an d temperature . I t wil l be show n tha t fo r void s les s tha n abou t 5. 0 u , an d temperature s les s tha n 300 0 ° C, th e transpor t mechanis m s i alway s tha to f surfac e diffusion , whic h dominate s tha t o f volum e diffusio n an d vapo r transport . The velocit y o f a voi d unde r th e influenc e o f a force , 4 .o n a n individua l ato m (o r th e force , F, o n th e entir e void ) ca n be foun d fromth e equation : 3.1.35. -

/ r kT v = -2DS V Of a

= 3/2 π (Ds/kT)( V

4

Ω2/Γ )

F

291

where

Ds

s i th e surfac e diffusio n coefficient , V s i th e numbe r o f diffusin g

tungste n atom s pe r uni t are a o f surface , ks i Boltzmann' s constant ,Τ s i n i °K, Ω s i th e volum e pe r tungste n ato m n i th e soli d an d rs i th e void-radius . Unde r the influenc e o f therma l agitatio n (o r rando m walk ) th e motio n o f a voi d ca n be calculate d b y mean s o f diffusio n theor y provide d tha t on e

employ s a n

effectiv e diffusio n coefficient , i.e. 3.1.36. -

Dvoid = 3/ 2 π

V( aVr) 4

Ds

Numerica l value s o f Dvoi d hav e bee n calculate d an d ar e give n n i th e followin g Table : Tabl e 39 Effectiv e Voi d Diffusio n Coefficient s fo r Voi d Migratio n vi a Surfac e Diffusio n 2

(Dvoi d n i cm/sec. ) Value s o f rn i Angstrom s Τ n i ° K 1500

2.35x10 -15

r= 2 5 A

2000

1.59x10 -12

25 0 250 0 -23 2.35x10 -19 2.35x10 1.59x10 -2 0 1.59x10 -16

25,00 0 2.35x10 -27

250,00 0 2.35x10 -31

1.59x10 -24

1.59x10 -28

2500

7.93xlO -H

19 7.93x10 -23 7.93xl0-! 5 7.93X10"

7.93x10 -27

3000

l.lOxlO

9

1.10x10 -13

3300

3.60x10 -9

3.60x10 -13

17 -25 -21 1.10x10 l . l O x l O 1.10x10 3.60x10 -25 3.60x10 -17 3.60x10 -21 ^

Note tha t thes e number s indicat e a rathe r lo w

velocit y o f voi d

migration .

Nevertheless , t i doe s occur . The velocit y o f a voi d unde r th e influenc e o f a temperatur e gradient , dT/dx , is : give n by : /Ω) (q*/kT ) D v0 i d ( l /)T d T /xd 3.1.37. ν =2 π (r 3

= 3( I V Ω ) / Γ ] [q*/kT ] [1/T ] dt/d x where q * s i a n energ y of transpor t associate d wit h therma l diffusion . Fo r al l excep t th e smalle r void s considere d n i Tabl e 3-9 , a temperatur e gradien t s i the mor e effectiv e drivin g forc e tha n rando m walk . Numerica l value s ar e give n in Tabl e 3-1 0 (show n on th e nex t page) , fo r th e cas e where : ( l / T ) ( d T)/ d= x 1

0. 1 cm .

292

Tabl e 3-1 0 Void Velocit y (cm./sec/ ) n i a Temperatur e Gradien t Accordin g o t 2

the Surfac e Diffusio n Mechanis m (Dvoi d n i cm/sec. ) Value s o f rn i Angstrom s T in

° K

r = 25 A

25 0

250 0

25,00 0

250,00 0 16 4.88xl0-4.88xl0" 4 . 8 8 x l 0 4 . 8 8 x l 0 13

14

10

11

15

1500

4.88x10 -12

2000

2.48xl0" 2 . 4 8 x l 0 - 2. 4 8 X 1 0 "

2500

9.92xl0" 9. 9 2 X 1 0 -

3000

1.14xl0- 1 . 1 4 x l 0 1.14x10~ 1.14xl0" 1.14xl06 l 3.41xl0 ;9 3.41x10-! ° 3.41x10 -8 3.41xl0- 3 . 4 1 x£7

9

3300

8

9

6

7

2.48x10 -12 11

9.92x10-! ° 9. 9 2 X 1 0 "

3

2.48xl0-l 12

9.92xl0

9

8

10

The facttha t void s migrat e up a temperatur e gradien t suggest s tha t voi d migratio n s i probabl y on e o f th e mor e importan t factor s n i

determinin g

filamen t lifetime . Indeed , th e presenc e o f exces s void s n i variou s region s o f th e filamen t cut s down

th e cross-sectiona l are a a t thos e region s an d

resistance . The curren t draw n b y th e filamen t fro m

increase s th e

loca l

th e lin e is , however ,

determine d b y th e remainde r o f th e filamen t an d is , therefore , essentiall y th e same a s a homogeneou s wire . As thi s curren t s i "forced " throug h th e region s of relativel y hig h resistance , ther e s i loca l overheatin g an d

concomitan t

temperatur e gradient s directe d towar d them . Thi s attract s mor e void s o t thes e regions , makin g the m

eve n hotte r an d

henc e attractin g eve n mor e

voids , etc . Suc h a proces s s i potentiall y catastrophi c f i ther e ar e enoug h mobil e void s presen t n i th e wire . 3. Bubble s an d Filamen t "Sag " On

th e othe r hand , th e presenc e o f bubble s s i aki n o t voids , wit h on e

importan t difference . Bubble s hav e bee n foun d o t b e "pinned " nex t o t grai n boundarie s an d

th e

d o no t ordinaril y migrate . Thus , the y serv e a s "void -

reservoirs" . However , an y movemen t o f a bubbl e wil l caus e th e formatio n o f a voi d n i th e meta l structur e itself . Not e als o tha t we hav e sai d tha t sinc e th e bubbl e densit y become s mor e or les s constan t abov e abou t a 0. 5 μ diffusio n distance , whic h s i approximatel y 1/ 2 o f th e grai n size , t i appear s tha t th e grai n sub-boundarie s Eir e th e sourc e of vacancies . Any los s o f bubble s therefor e

293

manifest s itsel f a s th e appearanc e o f void s whic h contribut e o t filamen t failure .

prematur e

Moreover , a s a filamen t operate s a t hig h temperature , grai n growt h occurs , albei t a t a much slowe r rat e whe n bubble s ar e present . Suc h growt h s i th e resul t o f movemen t o f void s an d change s th e grain-boundar y positio n an d structure . Eventually , voi d movemen t overcome s th e steadyin g influenc e o f bubbles , slip s alon g th e {112 } plan e n i th e meta l structur e occu r leadin g o t "hot spots " an d filamen t failure . Thi s ca n be see n by th e amoun t o f "sag " whic h occur s n i th e operatin g filamen t a s a functio n o f time . As a n exampl e o f difference s n i "sag " o f filament s made fro m differin g type s o f wire , we sho w the followin g diagram : 3.1.38. I Amoun t of Sag Occurin g fo r Variou s Filamen t W i r es Afte r 50 Minute s of Burnin g |

Undoped tungste n W i re

Doped Without A1C13

Standar d Doped Tungste n Wi r e

It shoul d be clea r tha t th e amoun t of "sag " experience d s i directl y relate d o t bubbl e formatio n n i th e finishe d wir e use d fo r filaments . The sag-tes t s i mos t usefu l o f thos e use d n i determinin g th e qualit y o f filament s processe d from differen t materials . However , t i s i no t sensitiv e enoug h o t determin e difference s whic h may exis tn i th e sam e typ e o f material . In conclusion , th e abov e consideration s o n th e rol e o f voi d migratio n n i filamen t failur e hav e bee n examine d mathematically . The existenc e o f th e potentiall y catastrophi c instabilit y give n abov e ha s als o bee n rigorousl y demonstrate d n i tha t th e tim e fo r voi d migratio n o t tak e plac e s i a n importan t par t o f filamen t lifetime . Thus , th e measure d lifetim e o f a give n typ e o f a filamen t shoul d displa y a dependenc e o f th e form :

294

3.1.39. -

τ

c * 3e 9 , 2 0 0 KT/

O n th e othe r hand , f i th e tim e fo r evaporatio n (voi d formation ) dominate s filamen t lifetime , th e correspondin g dependenc e o n temperatur e woul d be : 3.1.40. -

τ

ex . e 94,34 0 κ ° / T

Unfortunately , eve n today , thi s facto r s i no t know n rigorously. 4. Presen t Statu s o f Knowledg e Regardin g Tungste n Filamen t Lif e To summariz e wha t s i know n concernin g failur e o f a n filament , th e followin g s i relevant :

incandescen t lam p

1. t I s i generall y agree d tha t undope d tungste n make s a poo r lam p filament . Suitabl y dope d tungsten , o n th e othe r hand , furnishe s a satisfactor y filament , long-live d an d non-sagging . The exac t rol e o f th e dopant s ha s ye t o t be completel y identified , an d 8 0 year s o f lam p production , i.e. - 191 0 o t 1992 , hav e onl y serve d o t clarif y th e proble m but onl y hinte d a t it s solution . Still ,t is i know n tha t lamp s operatin g a t temperature s o f abou t 260 0 °K. ar e fa r abov e th e recrystallizatio n temperatur e o f pur e tungsten . Therefore , glowin g filament s o f tungste n sag unde r th e influenc e o f gravity , wit h a slidin g a t th e grai n boundarie s known a s offsettin g (5) . The commercia l answe r o t thi s ha s bee n o t fabricat e a wir e whos e grai n boundarie s for m ver y acut e angle s wit h th e longitudina l axi s o f th e wire , so-calle d "overlappin g grains" . The larg e are a betwee n suc h grain s thu s reduce s offsettin g o t a minimum . 2. P.E . Wretblad , writin g n i 194 1 (6) , attribute d th e overlappin g grai n structure s o t th e presenc e o f bot h volatil e alkalie s an d non-volatil e additive s n i th e startin g powder . The functio n o f thes e dopant s s i no t o t inhibi t grai n growth , bu t rathe r o t ensur e correc t orientatio n o f th e grai n boundarie s wit h respec t o t th e wir e axis , a s show n n i th e followin g diagram , give n o n th e nex t pag e a s 3.1.41 . Actually , durin g drawin g a t th e processin g temperatures , a s argue d b y Professo r Wretblad , exaggerate d grai n growt h occurs , bu t n i a preferre d

295

3.1.41. Morpholog y of Grai n Structure s n i Tungste n Wir e a s Processe d ksks?

K/1

Coarse Offsettin g Monocrystal s Fibrou s RecrystalliEe d Structur e with Cros s I Structur e Pure Tungste n Crystal s Afte r Boundary In i Drawn Wire Annealin g Wire for a Long Time

Long Overlappin g Crystal s with Longitudina l Boundarie s

orientation . I f thi s pictur e s i accurate , the n processin g of th e tungste n wire s i foremos t n i determinin g ho w th e wir e wil l perfor m a s a n incandescen t filament . Nevertheless , th e rol e of additive s prio r o t reductio n an d th e exac t effec t of processin g variable s hav e remaine d obscur e n i severa l aspects . The rol e of vacancie s generate d durin g operatio n of th e filamen t an d th e generatio n an d migratio n of void s fro m grai n boundar y location s ha s stil l no t bee n clarifie d completel y an d totally . B-3. Developmen t of Advance d Method s of Compactio n an d Densificatio n Althoug h th e prio r method s of compactio n an d densificatio n worked , som e manufacturer s sough t eve n bette r methods . Thi s cam e abou t n i th e Unite d State s becaus e o f th e the n ne w environmenta l law s passe d by Congres s n i th e lat e 1970s . 1. Compactio n an d Densificatio n Originally , a rolling-compactio n was trie d n i whic h th e tungste n meta l powde r was compacte d by a roller . Suc h a n apparatu s s i show n n i th e following :

296

The roller-compacto r show n her e consiste d o f a 1 4 inc h diamete r roller segmen t o f approximatel y 9 0 ° ar c an d a 3/ 8 inc h serrate d mold . Thi s apparatu s produce d tungste n ingot s 3/ 8 inc h squar e an d 1 5 inche s long . Eac h ingo t coul d be handle s withou t difficult y bu t ha d a sligh t bo w characteristi c resultin g fromrollin g a roun d objec t ove r a flat surface . Ingo t densitie s range d fro m 11. 9 o t 12. 3 gm./cc . However , whe n t i was attempte d o t densif y thes e ingot s n i a hig h temperatur e furnace , th e ingot s cracke d a s t i was pushe d int o the ho t zon e a t 220 0 °C . On th e othe r hand , f i th e ingot s wer e firs t pre sintere d a t 110 0 ° C, the y coul d be densifie d a t th e highe r temperatur e o t produc e ingot s wit h densitie s rangin g fro m 17. 3 o t 17. 4 g m . / c,c i.e. - 89 % o f theoretica l density . Nonetheless , thi s metho d was no t a n improvemen t ove r prio r ones . Therefore , anothe r typ e o f compactio n was investigated , tha t o f a "rockin g compactor" . I t consiste d o f a rocke r wit h a twelv e foo t radiu s of curvatur e wit h a mol d 3/ 8 inche s wid e an d 3 0 inche s long . Suc h a n apparatu s is show n n i th e followin g diagram , give n a s 3.1.43 . o n th e nex t page . The rocke r blad e was 3/ 8 inche s wid e an d operatio n require d tha t pressur e in Cylinde r A an d Cylinde rΒ be programmed . The rat e a t whic h Cylinde r A s i pressurize d s i th e rat e tha t Cylinde rΒ s i decreased , an d vice-versa , s o tha t the pressur e o n th e powde r a t an y poin t alon g th e rocker' s trave l wil l alway s be th e same . Thi s apparatu s produce d ingot s havin g densitie s o f 10. 9 o t 11. 4 g m . / c,c i.e. - 60.1 % o f th e theoretica l densit y o f tungste n metal . However , thi s metho d was faste r tha n direc t mol d pressin g o r th e abov e roller-compactor , and n i additio n produce d ingot s tha t wer e no t bowed .

297

3.1.43. Rockin g Compacto r forProducin g Tungste n Ingot s

This le d o t th e us e o f a continuou s rollin g compacto r n i whic h th e powde r was progressivel y compacte d withou t th e us e o f th e to p o f th e die , show n n i 3.1.44 . o n th e nex t pag e a s follows . This apparatu s consiste d o f a se t o f four18-inc h roller s wit h th e di e havin g serrate d edges , eac h rolle r havin g progressivel y increasin g monostati c pressure , fro m 400 0 ps io t 10,00 0 psi . Eac h rolle r s i 3/ 8 inc h wid e s o tha t t i fit s int o th e botto m par t o f th e die . Tungste n powde r s i continuousl y adde d o t th e formin g ba r jus t befor e t i encounter s eac h roller . However , a binde r and/o r lubrican t s i require d n i th e process , usuall y campho r o r methacrylate . A n end-o n vie w s i show n n i th e diagram . The di e s i abou t 1 0 fee t lon g an d produce s a ba r abou t 1 0 fee t lon g an d 3/ 8 b y 3/ 8 inche s square . The ba r s i supporte d a t th e botto m b y a stri p o f molybdenu m meta l whic h s i remove d jus t befor e th e ba r enter s a pre-sinterin g furnac e a t 140 0 °C , an d the n enter s

298

3.1.44. Progressiv e Compactio n Apparatu s

a sinterin g furnac e wher e heatin g a t 220 0 ° C produce s th e ingot . Densitie s u p to abou t 17. 8 gm/c c (93 % o f theoretical ) hav e bee n achieved . This resul t le d o t desig n o f a n apparatu s o t replac e th e swagin g operatio n which ha d alway s bee n a sourc e o f frustratio n becaus e o f th e hig h degre e o f non-contro l ove r th e processin g variables , i.e. - som e operator s coul d "swage " properl y an d other s coul d not . 2. Advance d Swagin g Technique s To replac e th e swagin g metho d o f heavy-wir e productio n require d a completel y differen t approach . A four-stan d tandem-mil l was develope d o t rol l the tungste n ingo t sequentiall y o t a ro d whic h coul d the n be pu t throug h th e wire-drawin g process . Thi s apparatu s consist s o f sequentia l roll s havin g a groov e insid e o f th e rol l whic h s i tapere d opposit e o t th e directio n o f rollin g so a s o t reduc e th e diamete r of th e ro d wit h eac h patS s throug h a specific size d roll , a s show n n i th e followin g diagram :

299

3.1.45. Tandem Rollin g Mil l fo r Tungste n Ingot s (One of Four )

Upper Roll

Lower Roll

Ingot Being Reduced in Siz e Side Vie w

The roll s themselve s wer e made o f nodula r iro n an d ar e 8 by 1 2 inche s n i size , drive n by 30 HP electri c motors . The insid e surfac e of th e groove s n i eac h rol l ha s a diamond-shape d profil e o t hel p "cold-roll " th e rod . A heat treatin g furnac e s i neede d betwee n eac h stan d o t sinte r th e ro d a t 220 0 ° C n i flowin g hydroge n gas . At first , a tungsten-ro d elemen t furnac e insulate d wit h zirconi a was used , bu t late r on , inductio n prehea t furnace s wer e used . The furnace-stan d combinatio n s i space d s o tha t th e ro d wil l coo l o t abou t 135 0 ° C, jus t a s n i th e swagin g operation . Roilin g occur s a t speed s n i exces s o f 30 0 fee t pe r minute . Closel y fitte d stee l guide s ar e fitte d a t entranc e an d exi t betwee n eac h mil l an d furnace . Inter-stan d tensio n (o r compression ) ca n aris e if speed s ar e no t precisel y set . The ingo t progresse s from squar e o t ova l o t roun d a t th e en d o f th e size-reductio n durin g rolling . The followin g table , give n on th e nex t page , present s a n exampl e o f som e of th e parameter s of rolling . The mos t sever e problem s encountere d n i th e rollin g proces s are : 1) Rol l wea r 2) Prehea t time . The firs t s i responsibl e fo r jam-u p durin g processin g wherea s th e secon d

300

Pass No.

TABL E 3-1 1 Rollin g Parameter s fo r Tande m Rollin g o f Tungste n Ingot s Outpu t Siz e Rol l Gap finches ) Mil l Spee d ffpm ) n i Inche s Mil l 1 Mil l 2 Mil l 1 Mil l 2

1

0.23 8 Sq .

0.01 2

0.00 2

350

435

2

0.19 1 Sq .

0.00 6

0.00 3

480

600

3

0.15 5 Sq .

0.00 1

0.00 8

350

445

4

0.12 7 Sq .

0.00 3

0.01 2

525

600

5

0.11 0 Ov.

0.00 1

0.00 7

375

450

6

0.10 2 Rd.

0.00 2

0.00 8

500

600

determine s th e rat e o f through-pu t o f th e tungste n ro d processin g a s t i progresse s o t wire-drawin g sizes . Althoug h continuou s compactio n o t for m a lon g ingo t capabl e o f bein g processe d by pre-sinterin g ha s bee n successful, some manufacturer s hav e chose n o t sta y wit h th e rocker-compacte d bar s becaus e o f cost-savings . The majo r proble m observe d wit h tungste n wir e made fro m rolle d ro d was tha t o f hig h iro n contaminatio n whic h was believe d o t hav e cause d hig h spli t levels . Thi s mandate d a n electrolyti c cleanin g proces s whic h solve d bot h problems . The wir e thu s produce d was entirel y equa l o t wir e made by th e olde r method . Split s n i 9 mi l wir e wer e the n foun d o t be almos t nonexistent . In som e cases , wir e made fro m rolle d rod s was superio r n i tha t wire-break s durin g coilin g wer e almos t completel y eliminated . The Preferre d Proces s turne d ou t o t be : 1) electrolyti c cleanin g a t finishe d rol l siz e (« 8 %

loss )

2) annealin g of wir e a t 29 mi l diameter . It shoul d b e note d tha t molybdenu m meta l s i no w bein g produce d b y th e sam e compaction-ho t rollin g procedure s a s outline d above . Mo s i use d n i support s for tungste n filament s n i incandescen t lamp s an d a s electrica l lead s n i othe r type s of lamps .

301

3.2. - GLASS USED I N MANUFACTUR E OF LAMPS Severa l type s o f glas s ar e use d o t make glass-form s fo r lamps . Thes e includ e bot h larg e an d smal l incandescen t lamps , fluorescen t lamp s o f variou s sizes , oute r bulb s fo r HPMV an d fo r sodium-vapo r lamps . Accordin g o t th e 198 6 CERAMIC are use d fo r lamps :

SOURCE

(7) , th e followin g glas s formula s

TABL E 3-1 2 Glas s composition s Use d fo r Lamps 1 Use Exhaus t

S I1O2 jAI2O31B2O 3 1N a2Q |K2 Q 61. 8

2. 2

7. 0

jMgO \Ca O

7. 3

\Ba O

\Pb O

21. 5

0. 2

Flar e

As203

Fluorescen t 73. 6

1. 4

16. 2

0. 4

3. 4

4. 8

0. 2

Tube Sign

SO3 70. 5

2. 0

2. 6

12. 2

78. 1

2. 0

14. 9

4. 9

Fluorescen t 71. 4

2. 2

15. 0

1. 1

17. 0

5. 3

3. 0

4. 2

0. 2

Tubin g Seale d

|Other |

SQ3 0. 1 CI

Beam 1. 7

4. 0

4. 6

4. 1

5. 1

0. 8

0. 2

Tube

SQ3

Lamp Bulb s 73. 1 Neon Sign s 67. 0

10. 0

9. 0

7. 0

12. 0 ZnO

TV

Tube s

50. 3

4. 7

6. 1

8. 4

2. 9

4. 3

0. 2

22. 5

0.1 F

Most o f thes e formulation s ar e classifie d a s "Soda-Lime " glasses . The step s generall y use d n i manufacturin g glas s fo r lamp s ar e give n n i

th e

followin g diagram , show n a s 3.2.1 .o n th e nex t page . The glas s component s ar e loade d int o th e fron t en d

o f a larg e continuou s

furnace . Onc e a mel t ha s bee n started , additio n o f component s s i continuou s and glas s s i take n of f th e othe r en d o f th e furnace . A s th e molte n glas s accumulate s n i th e holdin g area , t i s i draw n progressivel y eithe r int o a lon g continuou s roun d tub e (an d cu t o t th e desire d length ) o r t i

302

3.2.1. Step s n i th e Manufactur e of Glas s SodaA3 h

Glass San d SiO >99 % crushe d & -washe d

Limestone to yiel d CaO + someMg O -pulverize d

N a 2C 0 3 to yiel d N a 20

Feldspar-t o yiel d alumin a silica , NazO andK2 0 -pulverize d

^ 3

Batc h Mixin g

Jlllllllllllllllllffffif f

Initia l Melting-lSO O

.

^ICoolingT =130Q»CJ>

FORMIN G - ho t viscou s glas s shapedb y blowin g I Finishin g

Gullet (broke n qlass ) -same compositio n

1

-4-

—

Annealin g and Coolin g -] Finishin g

|

Inspectio n & Shippin g

is blow n int o bul b shape s continuousl y b y blow-molding . n I eithe r case , million s o f item s ar e made eac h month . Some lam p manufacturer s hav e thei r own glas s furnace s whil e other s bu y th e glas s form s o t make int o lamps . We have no t show n th e glas s moldin g equipmen t her e whic h consist s generall y o f a mandre l ove r whic h th e molte n glas s s i draw n an d coole d (fo r tubing ) or th e moldin g equipmen t use d fo r bul b making . Some o f the ' variou s glas s form s use d fo r manufacturin g lamp s ar e show n n i th e followin g diagram , give n a s 3.2.2 . o n th e nex t page .

303

3.2.2. Types of Glas s Form s Use d forLamp Manufactur e Fluorescen t Tube s Variou s Size s 8 foot- 4 foot- 2 footLength s (Diameter s o t matc h length )

Variou s Size s

3 ["Compac t Fluorescent s1

l A Q ii Incandescen t Bulb s

Oute r Protectiv e Bulb s

304

Note als o tha t we hav e no t provide d an y detai l concernin g th e comple x step s require d n i glas s manufacture . Thi s include s th e formin g operation s n i whic h the glas s form s ar e manufactured . Variou s form s use d fo r bot h fluorescent an d incandescen t bulb s ar e show n n i th e abov e diagra m a s wel l a s thei r relativ e size . Additionally , protectiv e shield s fo r HPMV an d high-pressur e sodiu m discharg e lamp s ar e shown . The form s show n n i 3.2.2 . represen t mos t of th e glas s form s use d fo r hig h volum e manufacturin g of lamp s intende d fo r bot h residentia l an d stree t lightin g n i th e U.S . Additionally , blank s suitabl e fo r th e manufactur e o f automobil e headlamp s ar e made , althoug h we hav e no t show n the m n i thi s diagra m becaus e o f lac k o f space . Most ar e made a t hig h spee d b y blowin g molte n glas s "blobs " int o th e desire d shape . Immediatel y followin g th e formin g operatio n s i a furnac e wher e th e glas s form s ar e anneale d an d the n cooled . The y ar e the n shippe d o t th e user . Sinc e ther e ar e abou t 70 0 differen t type s o f lamp s n i usag e today , we hav e onl y show n a smal l fractio n o f th e actua l numbe r o f glas s shape s tha t are , o r have been , manufacture d n i th e diagra m give n above . However , thos e give n represen t a majorit y o f th e glas s form s use d n i th e manufactur e o f lamps . 3.3. - MANUFACTUR E OF LEAD-I N WIRES In orde r o t manufactur e incandescen t lamps , on e need s o t hav e a suppl y o f variou s part s o n hand , includin g thos e o f lead-i n wires . Thi s sectio n wil l describ e som e o f thes e part s an d thei r manufacture , includin g tha t o f "Dumet™" wire . A . Manufactur e o f Dumet Wire . The manufactur e o f Dumet ™ wir e s i basicall y simple . As we sai d before , Dumet wir e was invente d a t G.E . b y Coli n G. Fin k n i 1912 . The standar d metho d involve s th e us e o f a cor e ro d o f nickel-iron , aroun d whic h s i woun d a thi n stri p o f bras s o t for m a tube , thi s assembl y s i the n place d withi n a coppe r tube , an d th e whol e s i heate d o t braz e t i together . Thi s composit e ro d s i the n drawn int o wire , usin g th e sam e technique s use d fo r tungste n wire . A newe r proces s involve s formin g th e sam e assembl y by drawin g th e nickel-iro n ro d throug h a molte n bat h o f ver y pur e copper . The proces s s i show n n i 3.3.1 . give n o n th e nex t page .

305

Once th e composit e ro d s i obtained , t i s i the n draw n int o finewire , whic h the n ca n b e processe d o t for m variou s type s o f lead-i n wire s an d sealin g wir e lead-ins . B. Manufactur e o f Lead-i n Wire s The manufactur e o f lead-i n wire s involve s th e us e o f severa l differen t metals . In general , ther e ar e thre e section s o t lead-i n wires , th e inne r section , th e pres s sectio n an d th e oute r section . Thi s s i show n n i th e followin g diagram : 3.3.2. Lead-I n Wir e Weld-

Inne r Sectio n

Pres s Sectio n

Oute r Sectio n

The procedur e use d is : 1. Inner-Sectio n - Thi s s i th e portio n extendin g int o th e insid e of the lamp . I t conduct s curren t an d support s th e filament an d othe r parts . Standar d material s use d are :

306

Copper , coppercla d steel , nickel , molybdenum , tungsten , nickel-plate d iron , nickel-plate d copper , chrome-coppe r alloy , an d zirconium-coppe r alloy . The tempe r ca n rang e fro m sof t o t full-hard . Inne r section s ar e manufacture d a s straight , o r wit h hook s o t whic h th e filamen t s i crimped . 2. Press-Section : Thi s portio n s i containe d withi n th e press , or lead-seal . I ts i use d o t sea l th e glas s fo r a vacuum-tigh t seal . Thi s s i possibl e becaus e it s coefficien t o f expansio n closel y matche s tha t of th e glas s used . By fa r th e mos t common wir e use d fo r thi s protio n s i th e Dumet composition . However , tungsten , molybdenu m an d molbdenu m foil , an d nickel-iron-cobal t allo y wire hav e als o bee n used . Thi s sectio n mus t als o be conductive . 3. Outer-Sectio n - Thi s sectio n o f th e lead-i n extend s fro m th e press-sea l o t th e bas e o f th e lamp . Metal s use d fo r thi s portio n have included : copper , nickel-plate d copper , coppercal d steel , manganese-nicke l alloy , an d nickel-plate d iron . 4. Othe r Design s -The for m show n abov e s i a one-par t lead . Other s tha t hav e bee n manufacture d (deopendin g upo n th e typ e o f lam p bein g made ) hav e included : one-par t lea d - a straigh t wir e two-par t lea d - welde d wire s fo r direc t connectio n three-par t lea d -welde d wire s a s show n abov e four-par t lea d five-part-lead . The other s ar e use d wher e 4 or 5 wire s ar e welde d togethe r o t contro l th e individua l filament s use d n i three-wa y ligh t bulb s use d for residentia l lighting . In general , a grea t variet y o f lead-i n wire s hav e bee n use d o t sui t th e purpose .

307

3.4. - MANUFACTUR E OF INCANDESCEN T LAMPS W e hav e alread y give n a brie f descriptio n n i Chapte r 2 o f ho w incandescen t lamps ar e manufacture d (e.g. - se e 2.1.1.) . n I th e followin g diagram , we sho w a complete d incandescen t lam p an d it s part s a s wel l a s common shape s o f incandescen t lamp s made : 3.4.1 . Interna l Constructio n of a n Incandescen t Lamp Bulb l Filamen t

Lead-i n and Suppor t I Exhaus t Hole Exhaus t Tub e

Shape s of Incandescen t Lamps Commonl y Manufacture d

S

F

S = straigh t sid e

G

A F=

Τ

PS

PAR

R

flame G = globula r A = arbitrar y

Τ = tubula r PAR = paraboli c R = reflecto r PS = pea r shap e Sinc e ther e ar e ove r 70 0 differen t type s o f incandescen t lamp s made , whic h can b e expande d o t severa l thousan d becaus e o f difference s n i voltage , finis h and base , t is i impractica lo t describ e ho w al l lam p type s ar e manufactured . I t is fo r thi s reaso n tha t we wil l describ e th e manufactur e o f a n incandescen t

308

lamp base d o n thel2 0 vol t Α-type for m (se e 3.4.1.) , whic h s i a larg e volum e item . Our nex t objectiv e s i o t describ e n i som e detai l al l of th e intricat e step s use d o t manufactur e incandescen t lamps . n I thi s case , hig h spee d machine s are use d o t accomplis h eac h o f th e individua l step s n i sequence , whil e maintainin g a n outpu t o f severa l thousan d lamp s pe r hour . A . Step s Involve d n i Manufactur e o f Incandescen t Lamps The step s require d o t manufactur e suc h lamp s involve : 1. Filamen t Mount Assembl y Operation : The stem-moun t machin e consist s o f a conveye r whic h hold s th e moun t part s n i a desire d positio n and carrie s the m throug h a serie s o f operation s n i whic h th e glas s s i heate d an d formed . The sequenc e o f operatio n is : a. A piec e o f glas s tubin g s i flared o n one-end . b. I ts i the n positione d wit h th e flared en d up . c. Two lead-i n wire s a s fe d throug h cylinder s ar e place d insid e th e flared-glass an d come o t res tn i positionin g guides . d. A piec e o f exhaus t tubin g (abou t 3/1 6 inc h diameter ) s i als o place d insid e th e flared glas s an d force d o t a sto p a t abou t on e hal f of it s length . e. Filamen t suppor t wire s ar e the n inserte d int o thi s assembl y which s i the n heate d o t softe n th e glass . Thereupon , clamp s move in an d for m a flat soli d sectio n calle d th e "press " whic h no w contain s th e support s an d lead-i n wire s seale d int o th e glass . f. Jus t befor e th e "press " solidifies , an d a puf f o f ai r throug h th e exhaus t tub e provide s a hol e n i th e "press " throug h whic h th e lamp ca n b e evacuate d of ai r a t a late r stag e o f manufactur e o f th e incandescen t lamp . g. Followin g thi s operation , th e en d o f th e exhaus t tub e s i heate d and shape d int o a butto n o f glas s abou t 1/ 4 inc h n i diamete r an d

309

1/8 inc h thick , an d a molybdenu m suppor t wir e s i inserte d int o the butto n befor e t i cools . h. The to p end s o f th e tw o lead-i n wire s ar e shape d an d separate d at a distanc e o f abou t a millimete r les s tha n th e lengt h o f th e tungste n filamen t coil . i. The coi l s i lifte d int o positio n jus t a s th e glass-lea d wir e assembl y arrive s opposit e o t t i an d th e coi l s i mechanicall y clampe d o t th e tw o lead-i n wire s a t bot h th e to p an d botto m an d the othe r filamen t support s ar e attache d o t th e filament . This complete s th e filamen t moun t assembl y operation . Thes e operation s ar e accomplishe d o n a highl y automate d machin e whic h complete s mount s a t a rat e o f severa l thousan d pe r hour . Many manufacturer s hav e designe d thei r own versio n o f suc h machine s whic h are regarde d a s proprietar y an d whos e design s ar e no t readil y available . 2. Bul b Washing :n I general , th e glas s bulb s nee d o t be washe d befor e use . Thi s s i don e by severa l means , dependin g upo n th e Manufacturer . Mostly , t i consist s o f mountin g th e bulb s verticall y n i a movin g rac k which move s th e bulb s ove r a serie s of water-jet s sprayin g upward s s o a s to was h th e insid e o f th e glass . A detergen t may be employe d a s may a n acidi c solutio n (usuall y a mixtur e o f HC1 an d HF). A fina l rinse complete s the operation.Th e bulb s ar e the n drie d n i a Leh r equippe d wit h infra red heatin g element s (sometime s jus t larg e high-intensit y lamps) , moving throug h a t a rat e sufficien t o t thoroughl y dr y th e surface s thereof . Ai r jet s durin g thi s phas e may als o be employed . Al l o f thes e operation s ar e accomplishe d o n a larg e machin e havin g station s equippe d fo r eac h individua l ste p o f th e proces s s o a s o t produc e severa l thousan d washe d (and/o r coated ) glas s bulb s pe r hour . 3. Bul b Coating s f I th e bul b s i o t be coate d (an d mos t are) , th e coatin g s i applie d eithe r a s a n electrostati c coatin g a ( "puff ' proces s fo r silic a coating ) o r a s a frosting. Fo r th e former , th e silic a particle s ar e charge d by passin g the m throug h a n electri c fiel d n i a n ai r strea m s o tha t the y wil l adher e o t th e freshly prepare d glas s surface . The bulb s the n pas s throug h o t a Lehrin g proces s wher e applie d hea t make s th e coatin g

310

more adherent . f I th e glas s s i o t be frosted , a stron g HF solutio n s i applie d fo r severa l second s s o a s o t "frost " th e glas s b y dissolvin g par to f the surface , i.e. 3.4.2. -

S1Q2 (glass ) + 6 HFaqueou s

= >

H2S i F6 (aqueous ) + 2 H20

A fina l rins e finishe s thi s operation , an d th e bul b proceed s o t th e dryin g station . I f a colore d coatin g s i desired , a lacque r compose d of eithe r nitrocelluos e o r ethy l cellulos e dissolve d n i xylo l s i used , n i whic h silic a plu s a n inorgani c pigmen t s i suspended . The lacque r s i prepare d by millin g th e vehicl e (whic h contain s dispersin g agent s plu s xylol ) wit h bot h pigment s fo r severa l hours . The lacque r s i spraye d ont o th e glas s surfac e an d the n dried . The organi c par t mus t the n b e remove d b y passin g th e coate d bulb s throug h a Leh r whos e temperatur e s i sufficien t to bur n of f th e organic s presen t withou t leavin g an y carbo n residues . The bulb s ar e the n allowe d o t coo l befor e encounterin g th e sealing-i n stage s o f th e operation . 4. Sealin g n I Operatio n :The sealin g an d exhaustin g proces s s i generall y accomplishe d n i tw o operation s o n a machin e consistin g o f a rotatin g turre t havin g tw o circula r receivin g mechanisms , on e o n th e to p o f th e othe r an d separate d b y abou t 10-1 2 inches , wit h th e to p on e bein g smalle r n i diamete r tha n th e botto m one . a. The moun t s i automaticall y positione d n i th e to p sectio n n i a hol e slightl y large r tha n th e exhaus t tub e (se e 2.2.1. ) an d hel d fairl y rigid. b. A lam p bul b glas s blan k s i droppe d down ove r th e mount . Not e tha t th e inne r surfac e o f th e bul b may hav e alread y bee n coated . c A flam e s i applie d a t th e nec k o f th e bul b n i th e regio n o f th e mount flar e an d th e flare-bul b nec k ar e fuse d togethe r int o on e piece . d. At th e en d o f th e sealing-i n process , th e lowe r en d o f th e bul b is reshape d n i a mol d whil e stil l ho t o t facilitat e a bette r fi t wit h the bas e late r on .

311

e. Subsequentl y an d automatically , th e assemble d mount-bul b s i transferre d o t th e botto m circula r mechanis m wher e th e exhaus t tub e s i positione d o n a vertica l rubbe r compressio n fitting, whic h in tur n s i connecte d o t th e exhaus t manifol d an d fill-ga s tank s throug h a serie s o f valves . f. First , th e interna l ai r withi n th e bulb-moun t s i remove d down o t abou t 1. 0 μ pressure . At thi s point , th e "getter " (usuall y a mixtur e of cryolit e an d red-phosphorous ) s i introduce d int o th e bul b whil e it s i bein g flushe d ou t wit h gas , usuall y nitrogen . Thi s s i don e b y squirtin g a smal l amoun t o f a lacque r n i whic h th e "gette r s i suspended . The bul b s i the n re-exhausted , repeatin g thi s cycl e severa l times . Finally , th e bul b s i refille d wit h th e fina l fill-ga s a t a pressur e o f abou t 23 mm. an d the n "sealed-off . g. Thre e gase s ar e use d n i incandescen t lamps . Of these , argo n s i the mos t important . I ts i use d wit h a trac e o f nitroge n n i perhap s 98% o f al l gas-fille d lamps . The majo r characteristic s tha t make t i desirabl e ar e it s relativel y hig h molecula r weigh t an d it s lo w hea t conductivity , whic h resul tn i a slowe r rat e o f tungste n evaporatio n and n i smalle r hea t losse s fro m th e lamp . The trac e o f nitroge n makes th e mixtur e les s susceptibl e o t short-circuitin g an d arcing . Nitroge n s i stil l use d fo r lamp s o f hig h wattag e suc h a s projectio n lamps an d wher e distance s betwee n filamen t section s o r lead-i n wire s s i small . Krypto n ga s s i als o used , an d woul d be use d mor e widel y f i it s cost s wer e no t s o high . Recen t advance s n i th e isolatio n o f Krypto n a t lowe r cos t fro m ai r hav e made t i on e o f th e major gase s fo r productio n o f hig h intensit y incandescen t lamps . Krypto n no t onl y increase s outpu t bu t ha s a beneficia l effec t o n lifetim e o f th e lam p a s well . h. Sealing-of f consist s o f heatin g th e exhaus t tub e unti l t i collapse s and s i fuse d an d closed , meanwhil e maintainin g th e prope r ga s pressur e withi n th e bulb . The assembl y ha s no w becom e a Tamp" , but withou t a base . The lam p s i carrie d o t th e basin g ree l b y conveyer .

312

5. The Basin g Operation : Meta l base s (mainl y brass ) ar e usuall y purchased , althoug h som e manufacturer s make thei r own. The basin g operatio n includes : a. Base s ar e filled automaticall y wit h a thermosettin g cemen t o n a circula r turret . The cement , viscou s n i form , s i force d unde r pressur e throug h a n orific e tha t deposit s t i aroun d th e inne r peripher y o f th e bas e jus t belo w th e star t o f th e threade d section . b. The basin g ree l consist s o f a larg e circula r rotatabl e turre t wit h an automati c vertica l positionin g mechanis m equall y space d aroun d th e circumference . Directl y abov e thes e s i a mechanis m for holdin g th e base-bul b assembl y n i a base-u p position . c. The threadin g o f on e lea d wir e throug h th e cente r o f th e base , and th e othe r aroun d th e botto m o f th e bul b s i nex t accomplishe d by th e machin e operation . d. Flame s ar e positione d s o a s o t provid e varyin g degree s o f heatin g a s th e turre t rotate s s o a s o t cur e th e cement . e. The turre t indexe s circumferentiall y s o tha t severa l sequentia l operation s occur . Thes e consis t o f cuttin g of f exces s of th e lea d wires , solderin g o r weldin g th e sid e lea d wir e o t th e sid e o f th e bas e a t th e poin t wher e th e bul b nec k an d bas e mee t an d solderin g th e othe r lea d wir e o t th e bas e eyele t a t th e botto m o f the base . f. The las t fe w indexin g position s ar e use d o t "flash " th e lamp . Flashin g a vacuu m lam p s i don e o t ignit e th e reducin g chemical , i.e. - "getter " o t combin e wit h th e remainin g oxyge n n i th e lamp . For filled-gas type s o f lamps , th e filament s i lighte d b y a voltag e considerabl y les s tha n ful l voltage , n i a repeate d fashion , n i orde r to conditio n t i fo r us e later . The operation s describe d abov e complet e th e manufacturin g operatio n fo r incandescen t lamps . Most manufacturer s hav e develope d hig h spee d machine s n i whic h th e part s ar e loade d an d n i whic h th e variou s operation s

313

are carrie d out . Not e tha t tw o separat e piece s o f glas s ar e use d o t for m th e "mount" . Thes e ar e seale d togethe r o t for m a glas s ro d suppor t fo r th e filament . Suppor t wire s ar e adde d togethe r wit h electrica l lead-i n wires , an d al l of thes e ar e seale d int o th e glass . Al lo f thes e step s ar e accomplishe d o n high-spee d machine s whic h ca n make u p o t on e lam p ever y 5 second s o f operatin g time . Not e als o tha t mos t lamp s ar e "frosted " o n th e inne r surfac e o f the glas s bulb . Thi s proces s involve s eithe r acid-etchin g of th e surfac e by HF, or coatin g th e insid e surface.wit h a silic a coatin g o t produc e a diffusin g effec t on th e ligh t generate d by th e lamp . Ver y few ,f i any , lamp s ar e manufacture d toda y havin g a bar e glas s bul b n i whic h th e ho t filamen t s i readil y visible . Most consumer s prefe r a lam p whos e ligh t s i diffuse d by som e sor t o f interna l coatin g o n th e insid e o f th e glas s bulb . The silic a coatin g may eve n be colored , as describe d above . The majo r problem s encountere d n i th e manufactur e o f incandescen t lamp s are a s follows : 1. Glas s Bul b Preparation : i. - Bul b Washing - wate r temperature , tim e & rins e ii. - Drying - us e o fI R hea to r filtere d ai r blas t plu s hea t iii. - Bul b Handling - smal l househol d lamp s ge t a n air-blas t prio r o t applyin g silic a coatin g o n machine . Reflecto r an d Larg e Lamp type s d o not . 2. Coatings : i. Pigmen t lacquers - viscosit y problem s aris e cause d by hea t and weather , dryin g an d lehrin g problems , furnac e tim e an d temperatures , air-blas t impuritie s introduce d an d bul b surfac e impurities . ii . Silic a coatings - particl e siz e o f silic a an d it s moistur e content , air-pressur e o t be use d o n "puf f (electrostatic ) process , moistur e conten t o f compresse d ai r used , Lewhrin g temeperatur e an d inde x time , pressur e of air-flus h used .

314

3. Filament s an d Getters : i. Getters - maintainin g prope r cryolit e an d red-phosphorou s ratios , viscositie s o f lacque r containin g "getter" , effect s o f basing , soldering , flashin g an d seasonin g o n "getter " function . ii . Filaments - operatin g temperature , effect s of ste m insertion , exhausting , basin g solderin g o n filamen t seasoning , vibratio n chec k fo r filamen t failur e (i.e. - th e "drop-test" , oxidize d Dumet obtaine d o n press , ba d filamen t lea d clamps , poorl y positione d filamen t "stand-offs " 4. Miscellaneou s Problems : i. Reflecto r Type - contro l o f san d fil l prio r o t flashin g o f aluminu m coating , surfac e cleanlines s du e o t sittin g prio r o t coatin g o f th e glas s bul b (mos t o f thes e type s o f lamp s ar e not made o n hig h spee d machines . The cleane d bulb s ar e place d withi n a vacuum-coater , fille d wit h san d o t limi t th e reflectiv e coating , "flashed " o t vaporiz e th e aluminu m a s a coatin g an d the n ar e remove d an d place d o n th e seal-i n machine s a t a late r date) , purit y o f reflectiv e coatin g an d it s adherenc e o t glass , "flashing " tim e an d voltage , an d tim e elapse d befor e actua l manufactur e int o lamp s (th e glas s an d coatin g may furthe r ozidiz e an d pic k u p moistur e from th e ambien t air) . The "sand-fiH " als o cause s problem s n i tha t th e reflecto r glas s mus t b e washe d wit h war m wate r prio r o t puttin g the m o n th e manufacturin g lin e o f th e lam p manufacturin g machine . B. Type s o f Incandescen t Lamps Manufacture d The majo r consideratio n o f ever y Lamp Manufacture r is , o f course , th e cost s of makin g suc h lamps . Lamp sale s hav e becom e s o competitiv e tha t ever y fractio n o f a cen t save d pe r lam p translate s int o thousand s o f dollar s o f profi t per year .

315

The type s o f lamp s tha t hav e bee n manufacture d include : 1. Residentia l Incandescen t Lamps : Wattage s o f - 25 , 40 , 60 , 75 , 100 , 200 , 500 , 100 0 & 1500 . 2. Photofloo d & Reflecto r Lamps : Wattage s o f - 75 , 150 , 200 , 300 , 500 , 1000 , 250 0 & 5000 . 3. Subminiatur e Lamps : 4. Automobil e Headlamps : 5. Specialt y Incandescen t Lamps : 1. Incandescen t Lamps fo r Home Use - th e majo r difference s betwee n thes e lamps lie s n i th e siz e an d shap e o f th e filament s used . As state d above , a wid e variet y o f bul b shape s an d coil-shape s hav e bee n use d an d continu e o t b e use d for a wid e variet y o f lightin g purposes . Most of th e lamp s sol d ar e o f th e "frosted " variety , i.e. - th e glas s surfac e s i etche d wit h hydrofluori c acid , bu t some hav e a pink-tinte d interna l coatin g o f silica . 2. Photofloo d an d Reflecto r Lamps : Thes e lamp s ar e n i genera l large r n i wattag e tha n thos e use d fo r genera l lightin g an d requir e large r size s o f tungste n wir e fo r operation . As such , lifetim e s i les s importan t tha n tha t of ligh t output . Many lamp s hav e interna l reflector s o f sputtere d aluminu m meta l to totall y reflec t interna l ligh t generate d b y th e filamen t n i a forwar d direction . 3. Sub-Miniatur e Lamps : These lamp s hav e bee n use d n i many area s includin g indicato r lamp s an d lamps arrange d o t for m sign s an d letter s fo r advertising . Becaus e th e lamp s are ver y small , the y ar e made n i a slightl y differen t process . Most filament s consis t o f a a singl e coi l an d many ar e a smal l piec e o f straigh t tungste n wir e which operate s o f milliwatt s o f power . The followin g diagram , give n a s constructio n o f thes e lamps .

3.4.3 . o n

th e nex t page , show s typica l

316

Butt Seale d L a m

P

B

e da

S

e

a

ld e

Lamp

S

tm e

S

e

a

ld e

Lamp

Note th e ver y smal l siz e o f th e lamps . Eve n smalle r lamp s hav e bee n made fo r use n i digita l quart z watches . The electrica l characteristic s o f thes e lamp s ar e presente d n i th e followin g Table . The lon g lif e s i a resultan t of th e lo w filamen t temperatures . TABL E 3 - 1 3 Electrica l Characteristic s o f Subminiatur e Lamps Lamp No.

Volt s

Amperes

Candl e Averag e Filamen t Power- Ave. Lif ( K) e (Hours ) Temp. °

680

5. 0

0.06 0

0.03 0

100,00 0

1850

683

5. 0

0.06 0

0.05 0

100,00 0

1950

713

5. 0

0.07 5

0.08 8

25,00 0

2100

328

5. 0

0.11 5

0.15 0

40,00 0

2125

327

28. 0

0.18 0

0.34 0

3,00 0

2275

0.34 0

0.34 0

7,00 0

2200

In additio n o t th e bayone t terminal s show n n i 3.4.3. , thes e lamp s Eir e als o availabl e wit h screw-i n bases . 4. Automobil e Headlamp s : All automobil e manufacturer s hav e provide d headlamp s fo r drivin g a t nigh t sinc e th e inceptio n o f th e automobile . The firs t ligh t source s wer e ver y crud e affairs , bein g oi l lamp s adapte d fo r th e purpose . At tha t time , th e

317

incandescen t lam p was stil l unde r development , althoug h th e hors e an d bugg y mode o f transportatio n continue d o t us e suc h lamps . However , th e greate r spee d o f th e automobil e made thes e lamp s nearl y useles s becaus e th e win d usuall y ble w ou t th e flame , eve n thoug h a glas s chimne y was n i place . As shown n i th e followin g diagram , give n a s 3.4.4 . o n th e nex t page , acetylen e lamps wer e nex t o n th e list . This sourc e was somewha t bette r bu t th e amoun t o f ligh t produce d was stil l nearl y non-existent . I t was no t unti l abou t 191 0 tha t incandescen t tungste n filamen t lamp s ha d sufficientl y improve d n i qualit y s o a s o t provid e sufficien t r ligh t fo r nigh t driving . One facto r was th e fac t tha t earl y cas ha d magneto s o t provid e th e spar k fo r ignitio n an d batterie s wer e no t use d a t all . Most o f th e incandescen t bulb s operate d a t 11 0 VAC, no t 6 VDC. Batterie s wer e installe d in car s abou t 191 2 an d lo w voltag e incandescen t lamp s bega n o t be use d a s headlamps . The first lamp s use d wer e vacuu m lamp s whic h provide d abou t 30,00 0 candlepowe r n i th e forwar d direction . However , th e ligh t bea m was diffus e and no t wel l directed . n I 1915 , th e us e o f gas-fille d lamp s began , du e o t th e facttha t the y ha d shorte r coile d filament s and , mor e importantly , wer e use d wit h a glas s len s ove r th e fron t o f th e headlight . Thi s us e continue d unti l 192 4 when th e firs t two-filamen t lamp s fo r automotiv e us e wer e introduced . Thes e lamps ha d th e advantag e tha t a n uppe r headligh t bea m coul d be use d fo r genera l drivin g a t night , bu t th e ligh t bea m coul d be shifte d o t a lowe r bea m so a s no to t blin d oncomin g traffic . However , thi s typ e o f headligh t ha d th e tendenc y o t shif t th e ligh t bea m ou t o f initia l focus , a s vibration s affecte d th e lamp positio n s o a s o t make th e bea m ver y diffuse . It was no t unti l 192 8 tha t a fixe d focu s headligh t was introduced . I n thi s headlamp , th e bul b was place d a t a poin t wher e al lo f th e ligh t was collimated . By 1934 , a pre-focuse d lam p assembl y bega n o t use d n i whic h th e lam p ha d a bayone t socke t whic h turne d int o a socke t an d was hel d n i positio n a t th e exac t focu s o f th e reflecto r a t th e bac k o f th e assembly . Thi s was th e pre focuse d headlamp , a s show n n i th e followin g diagram , give n o n pag e 31 9 a s 3.3.5 . However , t i was no t unti l 193 9 tha t th e pre-focuse d lamp - reflecto r uni t was replaced , thi s tim e b y a n entirel y ne w typ e of headlight , th e Seale d Bea m headlight . Thi s lam p was manufacture d quit e differentl y n i tha t th e dua l

318

e Headlamp 3.4.4 . Automotiv

Histor y

Beam Sprea d ni Degree s 20

15

10

5

0

5

10

15

20

•10 -20 •30 •40 •50 -60 •70 •80 -90 ΊΟ "20 •30 •40 •50 "60 •70 "80 -10 -20 •30 •40 -50 -60 -70 -80 "10 "20 "30 •40 "50 '60 •70 "80 -10 -20 -30 -40 50 •60 -70 -80 "10 "20 "30 -40 •50 "60 •70 •80

319

3.4.5. -

Automotiv e Headlamp Histor y

Beam Sprea d n i Degree s 20

1 5

1 05

0

5

101 5

Prefocu3e d

-10 -20 -30 . 40 ^50 -60 -70 -80

1934-193 9

Sealed Beam a mi l

Sealed Beam- Filamen t Cap Better V i s i b i y lii t n fo g ^ Rai n

Four Lamp Headligh t Syste m

Lens

Filament s ar e seale d directl y int o glas s and len s cap i s seale d i n; Len 3 is seale d vi a f rt i t o f om r integra l unit . 1 9 5 5 - 1 9 75

-10 -20 -30 -40 •50 -60 -70 -80

diamete r Accuratel y focuse d

Sealed Beam Headlam p Filament & Lens Cap

"10 "20 ^30 -40 -50 r 60 "70 "80

7" Diamete r Accuratel y Focus t %

1939-195 5

20

High Beam

Low Beam

7 inc h PAR

Low Beam

High Beam

5.5 7 inc h Par

1 9 65

- 1 9 92

filament s wer e mounte d directl y int o th e glas s o f th e reflecto r bas e whic h was the n seale d o t a fixe d glas s lens . Onl y the n di d th e ligh t outpu t o f theseheadlamp s excee d 50,00 0 candlepower . By 1955 , a ca p ha d bee n adde d over th e lo w bea m filamen t o t improv e th e seein g distanc e whe n fo g or rai n was present . Dual headlamp s wer e use d an d aiming-pad s wer e molde d int o th e oute r surface o f eac h glas s lam p o t facilitat e mechanica l aimin g o f th e ligh t beams . In 1965 , a four-lam p syste m was introduce d whic h use d tw o hig h bea m (7 5

320

watt filament - 7 " diameter ) seale d bea m lamp s an d tw o lo w bea m (6 0 watt - 5 3/4 " diameter ) lamp s arrange d n i a horizonta l row . The highe r wattag e use d allowe d th e generatio n o f a s much ligh tn i th e lo w bea m configuratio n a s was availabl e formerl y n i th e hig h bea m mode . However , t i was no t unti l abou t 197 5 tha t quartz-iodin e seale d bea m headlamp s wer e introduced . Thi s was don e o t facilitat e th e introductio n o f even brighte r automotiv e headlamp s a t abou t th e sam e wattage . The majo r differenc e was , o f course , th e loadin g tha t coul d be achieve d a t littl e increas e in cost . The increas e n i candlepowe r approache d 150 % tha t o f th e ol d sealed beam lamps , particularl y whe n krypto n ga s replace d th e olde r ga s mixtur e formerl y used . n I thi s case , th e quart z tub e was mounte d withi n th e botto m hal f o f th e sealed-bea m lamp , replacin g th e filament s an d th e len s par t was the n seale d o t for m th e integra l unit . 5. Specialt y Incandescen t Lamps These lamp s includ e bot h th e so-calle d "lumiline " lamp s an d "showcase " lamps . Thes e lamp s wer e merel y lon g incandescen t lamp s designe d o t fi t int o smal l spaces : 3.4.6. Showcase Lamps

Lumi e Lamp Lumilin

Also include d n i thi s categor y ar e th e quartz-iodin e lamp s whic h hav e o f lat e begun o t fin d thei r way int o al l type s o f incandescen t lam p bulbs . Thi s s i shown n i th e followin g diagram , give n a s 3.4.7 . o n th e nex t page . Althoug h th e quartz-iodin e lam p run s hotter , t i doe s no t consum e mor e power , it s lume n outpu t s i nearl y 50 % highe r tha n correspondin g incandescen t filaments , an d it s lif e s i considerabl y longer . Se e Chapte r 2page s 84-8 7 fo r a mor e complet e discussio n o f lam p design .

321

3.4.7. Bulb Shapes wit h Quartz-Iodin e Lamps

In general , manufactur e o f suc h lamp s start s wit h a quart z tubin g of selecte d lengt h an d diameter . The filamen ts i made u p a s give n abov e fo r incandescen t lamps excep t tha t fla t ribbons o f molybdenu m ar e used . An exhaus t tub e s i attache d o t th e mai n bod y o f th e quart z tub e an d th e filamen t assembl y s i place d withi n th e quart z tubing . The tub e s i the n heate d o n eac h en d an d a "press " s i made o n eac h end . The fla t ribbons o f molybdenu m for m a n excellen t sea l wit h th e quart z wit h no othe r additive s required . The seale d tubin g s i the n evacuate d vi a th e exhaus t tube , abou t 56 mg. o f iodin e (I2 ) s i adde d o t th e lam p an d th e tubin g s i backfille d wit h abou t 5 mm. o f gas , typicall y nitoge n an d argon . More recently , krypto n ga s ha s bee n use d o t produc e a lam p superio r n i bot h outpu t an d lifetime , th e exhaus t tub e s i the n seale d of f an d th e complete d quartz-iodin e lam p s i the n mounte d n i it s fina l oute r protectiv e glas s bulb . Becaus e thi s typ e o f lam p run s hotte r tha n th e usua l incandescen t lamp , th e oute r bul b s i necessar y o t protec t th e quart z tub e from therma l gradient s tha t may be presen t n i th e ambien t atmosphere . 3.5. - PREPARATION

OF RAW

MATERIALS FOR PHOSPHOR S

In thi s section , we wil l describ e method s o f manufactur e o f ra w material s use d o t fabricat e bot h fluorescen t lam p an d cathode-ra y tub e phosphors . Suc h material s include :

322

3.5.1. -

CaHP04 BaHP04

Alkalin e Eart h Carbonate s SrHP0 ZnS 4 ZnCdS 2 CdNH4P04 · 2 H20 M n N H 4 P 04 · H2 0

O f these , CaHP04 s i th e mos t importan t materia l fo r manufactur e o f fluorescent lam p phosphor s an d ZnS s i th e primar y on e use d fo r preparin g cathode-ra y tub e phosphors . The genera l metho d o f preparatio n o f suc h material s involve s precipitatio n fro m solution . Specifically , on e prepare s a solubl e solutio n o f bot h th e cationi c and anioni c part s o f th e materia l desire d an d the n pump s on e int o th e othe r to caus e a n insolubl e precipitat e o t form . n I Industry , a 200 0 gallo n glass line d tan k s i mos t ofte n used , wit h smalle r size d tank s o f th e sam e clas s use d for holdin g an d storag e o f purifie d solutions . Probabl y th e mos t importan t criterio n fo r al l ra w material s use d o t prepar e phosphor s s i tha t the y mus t b e as pur e a s possible , an d certainl y mus t no t contai n impuritie s whos e tota l concentratio n exceed s abou t 1. 0 par t pe r millio n (ppm) . n I a prio r wor k ( 8,) the natur e o f suc h impuritie s whic h ar e detrimenta l o t phospho r qualit y wer e define d a s "killers" . Thes e include d th e followin g cations : 3.5.2. - Cationi c Quencher s o f Luminescenc e Divalen t Ni Cu

Ti

Trivalen t V Cr Ru

Pd

Co

Zr

Nb

Mo

Pt

Fe

Hf

Ta

W

Rh

Quadrivalen t Os Ir Re

Most o f thes e ar e transitio n element s wit h d-electron s an d unpaire d spins . However , the y for m insolubl e precipitate s wit h eithe r sulfid e or many organi c chelatin g compounds , wherea s th e alkalin e eart h sulfide s and/o r chelate s ar e soluble . Thus , thes e analytica l reagent s ca n be use d fo r solution-purifying . A genera l metho d o f bot h solutio n purificatio n an d formatio n o f insolubl e precipitates , includin g thos e o f alkalin e eart h orthophosphates , s i show n n i the diagra m give n o n th e nex t pag e a s 3.4.3. , alon g wit h th e equipmen t neede d n i th e process . n I general , on e dissolve s a chlorid e o r nitrat e o f th e appropriat e catio n n i on e processing-tan k an d use s a solutio n o f a n ammonium sal t o f th e anio n desire d n i th e other .

323

3.5.3. -

Sti rre r

Equipment fo r General Proces s of Precipitatio n

Initia l Processin g Tank

Filte r

I Rav Materia l |

Tank Stirrin g Motor

v-i

Pump

Stirre r

Initia l Processin g Tank

Precipitatio n Tank

Tank Baffle s

Filte r

u y yw UVV W

324

Althoug h ammoniu m polysulfid e solution , i.e. - ( N r L ^ S,x s i generall y use d o t for m insolubl e transitio n meta l sulfides , precipitatio n o f th e impuritie s b y additio n o f certai n organi c chelatin g agent s ca n als o b e used . Bot h solution s are firs t stirre d afte r th e precipitatin g agen t s i added , an d the n allowe d o t remai n quiescen t s o tha t th e precipitat e forme d settle s o t th e botto m o f th e tank . The n bot h Tl an d T2 valve s ar e opene d an d eac h solutio n s i circulate d throug h it s correspondin g filte r o t remov e th e precipitate d impuritie s fro m the solution . At th e sam e time , th e pump circulate s th e solutio n bac k int o eac h correspondin g tank , valve s V - ,l V-2 , V3 & V4 havin g bee n turne d s o a s to b e abl e o t facilitat e thi s operation . Thi s continue s unti l th e solutio n s i fre e of th e precipitate . At th e sam e time , sample s o f th e solutio n ar e analyze d o t determin e whe n o t sto p th e recirculatin g cycle . The followin g list s th e organi c chelatin g agent s tha t ar e generall y employe d n i thi s process , viz 3.5.4. -

Organi c Agent s Use d fo r Solutio n Purificatio n Ammonium 1-Pyrrolidinedithi o Carbamat e Ammonium Nitrosopheny l Hydroxylamin e 8-Hydroxyquinolin e Dimethy l Glyoxim e Sodiu m orAmmoniu m Polysulfid e

The nex t par t o f th e proces s s i th e critica l one . As ha s bee n detaile d n i a prio r work (7) , an y precipitat e o t b e use d o t prepar e phosphor s mus t be a s clos e o t exac t stoichiometr y a s possible . I f on e add s on e solutio n o t th e othe r o t caus e precipitatio n o t tak e place ,t is i necessar y o t ad d a sligh t exces s o f th e on e o t caus e a complet e precipitation . Sinc e t i s i know n (7 ) tha t th e presenc e o f a n exces s o f th e cation , eve r s o slight , s i detrimenta l o t phospho r quality , on e alway s add s o t exces s th e anioni c solutio n o t th e cationi c one . To illustrat e thi s fact , conside r th e following . The solubilit y produc t o f th e hypothetica l compound , MX , s i give n by : 3.5.5. -

Ksp

=[Μ]

[X ]

325

where [Ml an d [X I ar e th e mola r concentration s o f th e catio n an d anio n respectively . When "X" s i adde d o t sligh t excess , the n al lo f th e catio n possibl e is precipitate d a s th e compound , MX (Not e tha t al l o f th e solvate d ion s canno t be completel y precipitate d sinc e th e Ksρ neve r equal s zero) . However , sinc e the precipitatio n proces s s i a phase-change , i.e. - a chang e from solvate d ion s in a liqui d o t a soli d suspension , a surfac e charg e calle d th e "zeta-potential " alway s prevails . Therefore , f i on e io n s i n i exces s n i solutio n afte r th e precipitat e ha s formed , t i wil l adsor b o n th e surfac e o f th e precipitat e particles , du e o t attractio n b y th e surfac e charge . I ts i fo r thi s reaso n tha t t is i bette r o t hav e a n anioni c exces s durin g precipitatio n sinc e anion s ar e usuall y opticall y transparen t an d cation s ar e no t (7) .I t s i als o fo r thi s reaso n tha t ammonium salt s o f th e anio n ar e use d sinc e ammoni a s i volatil e a t rathe r lo w temperatures , wherea s th e us e o f a solubl e meta l sal t o f th e anio n desire d would resul t n i surface-contaminatio n o f th e particle s b y bot h th e meta l io n and th e anion . In general , th e solutio n s i stirre d a s th e precipitatio n take s place . Particl e siz e produce d s i a functio n o f bot h solutio n concentratio n an d temperature . Precipitatio n a t roo m temperatur e usuall y produce s smal l particles . Thi s s i due o t th e fac t tha t th e solubilit y produc t s i neve r zer o an d a smal l amoun t o f bot h cation s an d anion s wil l alway s b e presen t n i solutio n n i equilibriu m wit h the precipitate . Eve n th e mos t insolubl e compoun d known , i.e. - ZnS , ha s a - 23 solubilit y produc t o f 1 0 (whic h s i no t zero) . Thi s result s n i solubl e cation s and anion s lef t n i th e solution . However , a t highe r temperature s n i solution , the solubilit y produc t usuall y increase s slightly . Thi s fac t s i use d o t gro w large r particles , sinc e a dynami c equilibriu m betwee n solubl e ion s an d th e soli d phas e exist s n i al l case s wher e a precipitat e ha s formed . Most ammoniu m sal t solution s ar e subjec t o t los s o f volatil e NH3 whe n heate d to elevate d temperatures . Thus , th e ammoniu m anio n solutio n canno t be heate d abov e abou t 40-4 5 ° C withou t loosin g appreciabl e amount s o f ammoni a as vapor . n I case s wher e th e p H s i importan t durin g precipitation , th e ammonia solutio n s i kep t belo w 4 5 °C. whil e th e catio n solutio n ca n be as . hig h as 9 5 °C . Onc e th e additio n o f anio n s i complete , th e solutio n wit h it s suspensio n o f forme d particle s s i stirre d o t promot e growt h o f large r particles . The fina l ste p n i th e precipitatio n proces s involve s pumpin g th e suspensio n int o a dru m filte r wher e th e so-calle d "mother-liquor " s i separate d fro m th e precipitate . The powde r s i the n pu t int o a n ove n a t 110-12 5 °C .o t

326

dry an d

the n s i place d n i a ribbon-blender o t brea k u p an y lump s tha t may

have forme d durin g th e dryin g process . In th e followin g sections , th e specifi c condition s o t b e use d n i

manufacturin g

the variou s product s ar e detailed . Thes e include : , BaHP04, SrHP0 4 CaHP04, CdNH4P04 · 2 H2 O, and alkalin e eart h carbonates . Α.- MANUFACTUR E OF BaHP04 BY

PRECIPITATIO N

1. Bariu m Nitrat e Solutio n = 261.3 4 MW - Concentratio n neede d is 2.4 3 mola r o r 1.4 0 lbs/gallon . 11 2 lbs . o f Ba(N03 )2 produce s 100 lbs . o f BaHP04. Usin g 80 % efficienc y (actua l measure d s i 87%), we nee d 140. 0 lbs . o f Ba(N0 , o r 24 3 mols . The 3 )2 procedur e s i a s follows : a. Fil l 30 0 gallo n glass-line d tan k wit h 20 0 gallo n o f 7 0 °C . deionize d water . Star t stirrer . b. Add 14 0 lbs . o f approve d lo t o f Ba(NOs )2 o t tan k an d sti r unti l materia l dissolves . c. Add 40 0 ml . o f ( N H4 )2 S X o t tan k wit h stirring . Sti r 1 5 minutes . Allo w o t stan d 2 hour s a t 6 5 °C .o t diges t an d for m large r particles . d. Fil l 5 gallo n pai l wit h Solka-Floc ™ an d slurr y wit h ho t deionize d water . Pou r slurr y int o filter-pres s an d was h flo e unti l wash-wate r s i clear . e. Filte r th e Ba(NOs )2 solutio n b y recirculatio n unti l th e solutio n s i clear . f I th e solutio n stil l ha s a greenis h cast , ad d more Solka-floc ™ an d continu e recirculatio n unti l a pal e yello w solutio n s i obtained . Flus h line s wit h approximatel y 10 gallon s o f ho t deionize d wate r o t clea r lines .

327

f. Add 17 0 ml .o f 30 % hydroge n peroxid e wit h stirring . The solutio n shoul d tur n white . g. Clea n filte r press , an d re-coa t wit h Solka-Floc ™ a s before . h. Filte r B a( N Q 3 )2

solutio n whe n t i turn s fro m a yellowis h

cas to t whitis h cast , recirculatin g o t obtai n a clea r solution . i. When clear , pump purifie d B a( N Q 3 )2 gallo n precipitatin g tank .

solutio n int o 70 0

j. Tes t 5 0 ml . o f purifie d solutio n b y addin g 5 ml . o f ammonium sulfid e o t solution . f I solutio n turn s darkish , t i must be reprocesse d befor e furthe r use . k. Clea n Filte r Pres s fo r furthe r use . 2. Diammoniu m Phosphat e Solutio n = 132.05 6 MW :

Fo r 24 3 mol s

2+

of Ba solution , a 1.05:1.0 0 rati o s i required . Thi s s i 25 5 mols , o r 74. 3 lbs . o f (ΝΗ4) 2ΗΡθ4. n I 6 0

gallon s o f deionize d water , th e

concentratio n s i 3.0 5 molar . a. Fil l a 90-gallo n glass-line d tan k wit h 6 0 gallo n o f 60°C . deionize d water . Star t stirrer . b. Add 75. 0 lb . o f approve d lo t o f ( N H 4) 2 H P 04 o t tan k wit h stirring . Maintai n a t 40-4 5 ° C whil e stirrin g o t dissolv e salt . c. Add 41 0 ml . o f ammoniu m

sulfid e an d

abou t 1 0 minutes . Sto p stirre r an d

continu e o t sti r

allo w o t

diges t fo r 6

hours , maintainin g temperatur e betwee n 35-4 0 C. d. Re-coa t filte r pres s wit h Solka-Floc ™ a s give n above . Wash th e Flo e befor e attemptin g o t us e coate d filter-press . e. Filte r th e (NH4) 2 HPO4 solutio n o t remov e th e blackis h precipitate . Continu e o t recirculat e th e solutio n unti l t i turn s o t a clea r pal e yello w solution . I ft i remain s greenish , re-coa t th e filter-pres s an d refilte r th e solution .

f. Clea n filte r pres s an d flus h line s bac k int o tank . Re-coa t filte r pres s wit h Solka-Floc ™ a s before . g. Add 16 5 ml . o f 30 % hydroge n peroxid e o t oxidiz e exces s sulfid e o t sulfur . The solutio n wil l tur n whitish , or t i may become almos t clear . Remove th e whitis h precipitat e b y recirculatio n throug h th e filter-pres s an d continu e unti l th e solutio n s i completel y clear . Maintai n temperatur e a t 3 5 40 °C. Chec k purit y b y analysis . h. Chec k p H o f solution . f I lowe r tha n 7.80 , adjus t o t pH > 8. 2 wit h NH4OH , addin g reagen t slowly . i. Clea n filter-pres s fo r nex t use . 3. Precipitatio n o f BaHPO 4 a. Star t th e stirre r n i th e purifie d Ba(NC >3)2 and hea t o t 70-7 5 °C .

solutio n tan k

b. Tes t th e p H o f th e (NH4) HPO4 solution . t I shoul d b e 2 abou t p H = 8.3 . f It is i lowe r tha n abou t 8.0 , ad d NH4OH o t adjus t p H o t 8. 0 - 8.5 . Maintai n temperatur e betwee n 35-4 0 °C. c. Add th e ( Ν Η 4 )2 Η Ρ θ 4 solutio n a t a rat e o f abou t 6.0-10. 0 gallo n pe r minute , whil e maintainin g it s temperatur e betwee n 35-4 0 °C . Thi s wil l tak e abou t 6-1 0 minute s o t complet e th e precipitation . d. I t s i importan t o t direc t th e (ΝΗ4)2ΗΡθ4 solutio n a t a poin t n i th e precipitatio n tan k wher e t i s i instantl y disperse d withi n th e Ba(NOs )2 solution . f It i encounter s th e surfac e o f th e solution , a n immediat e precipitat e form s an d float s upo n th e surfac e o f th e solution . Thi s result s n i a precipitat e o f indeterminat e particl e size . The (ΝΗ4)2ΗΡθ4 solutio n shoul d b e directe d agains t on e o f th e tan k baffle s where t i ca n be instantl y dispersed . Increas e stirrin g spee d

329

if necessar y o t accomplis h thi s action . When th e (NH4) 2 HPO 4 solutio n ha s bee n completel y added , was h th e tan k wit h ho t deionize d wate r an d ad d o t precipitatio n tan k whil e stirring . e. Allo w o t sti r fo r 1 5 mor e minutes . Tes t clea r liqui d fo r exces s phosphat e wit h previousl y prepare d bariu m nitrat e solution . A precipitat e wil l for m f i exces s s i present . f. Allo w BaHPU4 precipitat e o t settl e an d sipho n of f supernaten t liquid . Add enoug h ho t (85-9 0 °C ) deionize d wate r o t re-suspen d precipitate . Allo w precipitat e o t settl e and remov e supernaten t water . g. Repea t thi s washin g operatio n fo r 4 mor e time s fo r a tota l of 5 washes . h. Usin g drum-filter , remov e precipitat e an d plac e n i stainles s stee l trays . Dr y n i ove n a t 12 5 °C. fo r 1 0 hours . i. Remove tray s fro m oven , allo w o t coo l an d plastic-line d drums .

plac e n i tare d

j. Weig h complet e produc t an d recor d weigh t plu s yield . This complete s th e productio n o f th e BaHPU4 precipitat e whic h s i the n read y for use . Durin g precipitation , th e p H n i th e Ba(NU3)2 solutio n change s fro m abou t 3. 0 o t abou t p H 7. 0 durin g th e precipitation , a s show n n i th e followin g diagram , give n a s 3.5.6 . o n th e nex t page . The critica l part s o f th e procedur e hav e bee n determine d o t be : 1. p H o f th e (NH4)2 HPO4 solutio n 2. Stirrin g afte r additio n o f (NRj^S x shoul d b e minima l 3. The (ΝΗ4)2ΗΡθ4 solutio n shoul d alway s b e adde d o t th e Ba(NC>3)2 solution , no t vice-versa . The amoun t o f exces s

330

3.5.6. pH of Solutio n Durin g Precipitatio n of BaHPU4

20

4 0 %

6 0

8 0

10 0

n Adde d of Tota l B a ( W 03 )2 Solutio

phosphat e s i no t critica l an d ca n b e a s hig h a s 1:1.25 . However , th e precipitat e mus t b e thoroughl y washe d o t remove a s much o f exces s phosphat e presen t a s possible . 4. The rat e o f additio n o f th e ( N H 4 )2 HPO4 solutio n ha s some effec t o n particl e size . Faste r rate s ten d o t for m smalle r averag e size s an d vice-versa . A averag e siz e o f abou t 5. 6 μ s i considere d normal . B. PRECIPITATIO N OF S r H P 0 4 The preparatio n o f SrHP0 4 s i quit e simila r o t tha t o f BaHPOj excep t fo r one factor . Wherea s th e latte r exist s n i on e form , th e forme r exist s n i one o f tw o forms , dependin g upo n th e temperatur e o f precipitatio n (9) . A t temperature s belo w abou t 25 °C, β- SrHP0 4 s i obtaine d an d abov e 4 0 ° C, cc-SrHP0 4 s i formed . The particl e siz e o f th e precipitat e als o s i more temperatur e dependan t tha n fo r BaHP0 4 ,a s wil l b e show n below . 1. Strontiu m Nitrat e Solutio n = 211.6 3 MW - Solubilit y @ 6 0 °C .s i 938 gm/lite r or 7.8 2 gm. /gallo n o f water . 115. 0 lb . wil l produc e

331

100. 0 lb . o f S1-HPO4 . Usin g 80 % efficiency , 14 4 lbs . ar e required . This s i 308. 6 mols , or0.4 1 mola r f i 20 0 gallon s ar e used . M

a. Step s "a " throug h k" ar e followe d a s fo r BaHPC>4, excep t tha t 14 4 lbs . o f S r ( N30)2 ar e used . 2. Diammoniu m Phosphat e Solutio n = 132.05 6 MW : Fo r 308. 6 2+ mols o f Srsolution , a 1.05:1.0 0 rati o s i required . Thi s s i 324. 1 mols, or 94. 3 lbs . n I 60 gallon s o f deionize d water , th e concentratio n s i 1.4 3 molar . a. Step s "a " throug h "k " ar e followe d a s

fo r th e BaHPU4

e used . procedur e excep t tha t 94. 3 lbs . of (NH4) 2HPC >4 ar 3. Precipitatio n o f SrHP O

4

a. The critica l par t o f th e procedur e involve s th e decisio n concernin g whic h for m o f SrHPU 4 s i desired . The temperatur e s i controlle d eithe r a t < 2 5 °C .o t produc e βS r H P 04 or a t > 40 °C . o t produc e « - S r H P4 . 0 n I general , step s "a " throug h "i " ar e followed , excep t fo r temperatur e contro l o f th e solutions . i forme d an d abov e 40 °C . a b. Belo w 2 5 °C , p - S r H P4 0s SrHPU4 obtains . Betwee n 25-4 0 ° C, a mixtur e o f th e tw o results . The majo r differenc e betwee n th e tw o allotropi c form s s i th e particl e siz e produced . Thi s s i show n n i th e followin g table , viz Physica l Propertie s o f th e Two Form s o f SrHP0 4 Form

Aver. Particl e Siz e Surfac e Are a (BET )

p - S r H P 40

19. 1 μ

oc-SrHP0 4

8. 2 μ

„ „_

2

20. 6 m / g m. 2

5. 4 m /gm .

.

It shoul d b e clea r tha t th e majo r differenc e betwee n th e tw o form s s i tha to f surfac e area . Wherea s 3~SrHP0 4 ha s a large r averag e siz e tha n tha t o f a -SrHP0 4 ,t i s i compose d o f much

332 smalle r particle s agglomerate d int o on e large r one . Thi s account s fo r th e surfacearea . The large r surfac e are a particl e produce s a brighte r phosphor , n i general , tha n doe s the oc-SrHP0 4 , whe n use d n i a soli d stat e reactio n o t produc e th e phosphor . Onl y whe n th e phosphor , Sr : 2P 207 Sn, s i o t b e prepare d doe s th e a-SrHP0 4 produc e a superio r product . c. The effec t o f precipitatio n temperatur e o n particl e siz e s i considerable , a s show n n i th e following : Precipitatio n Allotropi c Aver. Particl e σ of Particl e Form Temperatur e Siz e Distributio n 95 °C.

a - S r H P 40 3. 9 μ

1.5 6

80

7 a - S r H P 40 9.

1.59 .

60

0 t- S r H P O4

13. 8

1.6 2

20

6 p - S r H P 40 17.

1.5 6

Thus, th e precipitatio n temperatur e o t produc e a particl e siz e o f 56 μ shoul d b e abou t 8 5 °C . 2+

d. However , f i th e S r solutio n s i adde d o t th e phosphate , i.e. - th e revers e addition , th e pur e SrHP0 4 doe s no t result . At 8 0 ° C, 12 % o f th e produc t s i th e hydroxyapatite , i.e. Sr5OH(P04)3 an d 88 % s i SrHP0 95 ° C, th e produc t 4. At consist s o f 15 % hydroxyapatit e an d 85 % SrHP0 4 .Thus , th e revers e additio n shoul d neve r b e use d n i th e preparatio n of S r H P 04 . e. The final p H o f th e solutio n shoul d b e adjuste d o t abou t pH = 6.8-6. 9 wit h ammonia , a s necessary , stirrin g abou t 1 5 minute s befor e th e precipitat e s i allowe d o t settl e an d befor e th e mothe r liquo r s i withdraw n an d washin g o f th e precipitat e begins .

333

C. MANUFACTUR E OF Μ11ΝΗ4ΡΟ4· H2 0 (1 0 gallo n batc h size ) Sinc e thi s compoun d s i use d a s a sourc e o f activato r fo r som e phosphors , it s preparatio n wil l b e described . Obviously , larg e amount s are no t usuall y require d an d precipitatio n take s plac e n i a smalle r glass line d vessel . 1. MnCl9 Solution - MW = 161.86 : 1 4 liter s of de-ionize d wate r ar e adde d o t a 1 0 gallo n tan k an d heate d o t nea r 10 0 °C . Add 40. 0 h stirring . Maintai n mols (171b s 7 oz. ) of MnCl2 · 4 H2 0 wit 2+ solutio n canno t be temperatur e nea r o t 10 0 °C . Sinc e th e Mn purifie d b y th e usua l reagents , t is i use d a s th e analytica l reagen t grad e purity . 2. Diammoniu m Phosphat e Solutio n = 132.05 6 MW : Fo r 40. 0 mol s 2+ of M n solution , a 1.10:1.0 0 rati o o f P 04 / Mn s i required . Thi s s i h s i dissolve d n i 17. 2 45 mol s or 1 3 lbs . 1 oz .o f ( N H4 )2 H P 0 4 whic liter s o f war m ( « 45 °C. ) deionize d water . The solutio n s i purifie d as before , usin g step s "a " throug h "c " give n above , excep t tha t onl y 30 ml o f ammoniu m sulfid e s i used . I t s i importan t tha t all of th e sulfid e be remove d b y oxidatio n wit h hydroge n peroxid e , an d th e sulfu r precipitat e s i filtere d ou t befor e thi s solutio n s i used . Therefore , th e solutio n s i coole d o t abou t 25 °C. befor e attemptin g to oxidiz e th e sulfid e o t a sulfu r precipitate . The solutio n s i filtere d throug h a 0.4 5 μ filte r befor e use . 3. Precipitatio n :The phosphat e solutio n s i heate d o t 5 5 ° C an d s i 2+ adde d slowl y o t th e M n solutio n whic h ha s bee n maintaine d nea r to 10 0 °C . Additio n tim e shoul d be abou t 1 5 minute s or abou t a rat e o f additio n o f 1. 2 liters/minute . Afte r additio n o f th e phosphat e solutio n s i complete , continu e stirrin g whil e addin g ammonia o t rais e th e p H o t abou t 6.8 . The precipitat e s i the n allowe d o t settl e befor e th e mothe r liquo r s i draw n off . 4. Washing : Wash th e precipitat e wit h war m deionize d wate r by fillin g th e tan k nea r o t th e to p a s stirrin g continues . Allo w o t settl e an d repea t th e procedure . Tes t th e was h wate r fo r chlorid e by addin g silve r nitrat e solutio n an d continu e washin g unti l th e

334

tes t s i negative . Allo w o t settl e fo r th e fina l time , an d filte r int o a prepare d larg e Buchne r filter . 5. Drying : Dr y th e precipitat e fo r a t leas t 1 6 hour s a t a temperatur e n o highe r tha n 11 0 °C . s o a s o t preven t los s o f th e ammonium par t o f th e prepare d salt . Too hig h a temperatur e result s n i los s o f ammoni a an d los s o f certai n physica l propertie s importan t n i subsequen t phospho r preparation . D . MANUFACTUR E OF CdNrL^PO ^ H2 0 (2 0 gallo n batc h size ) This compoun d s i als o use d a s a n additiv e fo r som e phospho r preparation s a s wel l a s a bas e fo r C d s C l f P O ^nM phosphor . Sinc e th e 2+ Cd solutio n canno t b e purifie d b y th e usua l method s give n above , t is i necessar y o t resor t o t othe r means . t I s i wel l o t not e her e tha t thi s metho d ca n als o b e use d fo r manufactur e o fΜηΝ H4 PO4 ·H2O. 1. C d f N 0 Solution - MW = 308.4 6 : Dissolv e 7934. 6 gm. (1 7 lbs . 3 )2 i approximatel y 3. 0 8 oz . = 25.7 2 mols ) o f Cd(NOs )2 · 4 H2 0 n gallo n o f col d deionize d water . Thi s solutio n s i the n gasse d wit h NH 3 unti l th e solutio n s i clea r (Th e solutio n firs t turn s whitis h due o t formatio n o f Cd(OH) an d the n change s o t a clea r solutio n 2 2+ of Cd (NU4)4 ion , du e o t th e reaction : 2+

N H 3 + 2 H 2 0 =* NH4++ OH- + C d

2+

= > Cd(OH) l + NH3 = > Cd(NH4 )4 2l

Whe n thi s catio n precipitate s a s th e phosphate , th e actua l allotropi c compoun d ha s th e stoichiometry : (CdHP04) 3 · 3 NH3 · 3 H 2 0 (9) . The fina l p H wil l the n b e abou t 8.5. Add 3 9 gram s o f tanni c aci d o t solutio n an d le t si t fo r 1 8 hours . Filte r thi s solutio n throug h a n Erte l pa d an d the n throug h a 0.4 5 μ filte r o t remov e al l trace s o f th e precipitate . 2. Diammoniu m Phosphat e Solutio n = 132.05 6 MW : A rati o o f 1. 4 PO4: 1 Cd s i needed . Thi s s i 36. 0 mol s or 4755. 7 gm. (1 0 lbs . 8 oz.) . Add thi s weigh t o f (NH4) HPO4 o t 13. 8 gallo n o f deionize d 2 wate r whic h ha s bee n heate d o t 4 0 °C . wit h stirrin g o t for m a 2.6 1 molar solution . Add 55. 0 gm. o f tanni c aci d whil e stirring , sto p th e

335

stirrin g an d allo w solutio n o t si t fo r 1 6 hours . The solutio n wil l cool whil e th e tannic-aci d precipitat e o f impuritie s s i formin g an d settling . At th e en d o f th e settlin g time , filte r th e solutio n throug h an Ertel ™ pa d an d the n throug h a 0.4 5 μ filte r o t remov e al l trace s o f th e precipitate . 3. Degassin g th e Cadmiu m Solution : n I orde r o t obtai n a stoichiometri c compound , i.e. - (CdHPU4 )3 · 3 NH3 · 3 H20 o r CdNH4P04 · H2 0, th e exces s ammoni a presen t (whic h was adde d in orde r o t caus e th e tannic-aci d precipitat e o t for m a t p H = 8.5 ) must be remove d befor e final precipitatio n take s place . Therefore , the cadmiu m solutio n s i heate d o t 85-9 5 ° C wher e heatin g cause s NH 3 ga s o t boi l of f a s th e solutio n s i stirred . Thi s proces s continue s unti l a fain t trac e o f a whit e precipitat e [Cd(OH) 1 2 appears . The solutio n s i the n transferre d o t a2 0 gallo n glass-line d tan k an d dilute d o t a 6. 0 gallo n tota l volume . 4. Precipitatio n o f CdNHdPOa · H 9O : The degasse d cadmiu m solutio n s i heate d o t 75-8 0 ° C wit h stirrin g whil e th e phosphat e solutio n s i bein g heate d o t 40 °C . The latte r s i the n pumpe d int o 2+ n a t a rat e o f 1. 0 lite r pe r minute . The solutio n the C d solutio must be well-stirre d durin g thi s tim e o t brea k u p an y clump s o f precipitat e tha t may form , an d th e phosphat e solutio n s i directe d at th e baffle s presen t n i th e tan k o t facilitat e dispersio n of th e phosphat e solutio n durin g addition . Afte r th e precipitatio n s i complete , tes t th e solutio n fo r pH an d adjus t th e p H wit h NH4OH to abou t p H = 6.8 , continuin g o t sti r th e solutio n durin g thi s time . Allo w o t sti r fo r a t leas t 1 5 minute s an d the n le t th e precipitat e o t settle . 5. Washin g o f th e Precipitate : Sipho n of f th e mothe r liquo r an d wash th e precipitat e b y re-suspensio n wit h 40 ° C deionize d water , fillin g th e tan k o t th e brim . Allo w th e precipitat e o t settl e an d repea t th e sequenc e fo r fiv e times . 6. Dryin g o f th e Precipitate : Filte r of f th e precipitat e an d ove n dr y for 1 6 hour s a t 11 0 °C . Do no t le t temperatur e rise abov e thi s limi t sinc e los s o f ammoni a fro m th e sal t wil l occur . At th e en d o f thi s

336

time , remov e fro m oven , le t cool , recor d weigh t an d

yield , an d

plac e n i tare d plastic-line d drums . E. Manufactur e o f Alkalin e Eart h Carbonate s These material s ar e use d n i th e manufactur e o f phosphor s an d a s emissio n material s fo r electroni c filament s n i cathode-ra y tube s an d th e like . Thei r preparatio n follow s th e genera l metho d give n abov e n i tha t purificatio n s i achieve d b y eithe r sulfid e addition s (wit h subsequen t oxidatio n an d remova lo f exces s sulfur ) o r b y us e o f organi c precipitants , as state d above . n I general , eithe r nitrat e o r chlorid e salt s ca n be used . The latte r hav e th e advantag e tha t washin g ca n be controlle d b y testin g for chlorid e a s t i proceeds . 1. Alkalin e Eart h Solution : BaCl o •2H9O -MW =244.31 : CaCb -MW = 110.99 : SrC l
Weight so t be adde d 19,244.3 0 gm. 42 lb . 7 oz . 8 ,742 .6 8 1 9 lb . 4 oz . 21,003.2 3

Carbonat e Yiel d @ 95 %

32 lb . 9 oz . 16 lb . 8 oz . 4 6 lb . 4. 5 oz . 24 lb . 6 oz .

Obviously , f i a mixtur e s i needed , e.g. - lBa:2Sr:6Ca , on e woul d multipl y th e abov e weight s b y 1/9 , 2/ 9 an d 6/ 9 respectively . A large r batc h suc h a s tha t require d fo r a 200 0 gallo n tan k woul d follo w th e sam e procedure , i.e. - multipl y b y 100.0 . More concentrate d solution s ca n be used , bu t th e abov e molarit y ha s been foun d o t produc e carbonate s wit h th e require d particl e siz e when t i (they ) s i (are ) use d o t prepar e phosphor s and/o r emissio n mixture s (whic h ar e mille d wit h othe r additive s suc h a s titania , T1O2, and/o r silica , S1O2) .

337

a. Purificatio n o f Solution : - Step s "a " throug h "k " ar e followe d a s give n fo r th e BaHPU4 manufactur e excep t tha t 40 ml . o f (NH4)2S x an d 1 7 ml o f 30 % hydroge n peroxid e s i use d durin g thi s stage . The solutio n s i filtere d throug h a n Ertel ™ pa d an d the n throug h a 0.4 5 μ filte r o t remov e al l remainin g impurities . Solka-floc ™ ca n b e adde d jus t prio r to filtratio n o t preven t th e precipitate d sulfide s fro m deglommerizin g an d passin g throug h th e filter . The solutio n temperatur e s i maintaine d a t 7 0 °C . 2. Ammoniu m Carbonat e Solution - MW = 157.1 1 : The usua l 2 commercia l for m is : NH4HCQ 3 · NH2CO2NH 4 .A 1. 4 CQ3 : 1 M * rati o s i require d sinc e th e ammoniu m carbonat e n i solutio n a t 50 60 °C. s i no t stable , an d tend s o t decompos e b y losin g ga s a s NH3 and CO2 o t th e ope n air . However , becaus e o f th e solubilit y limit , i.e. - NH4HC03* NH2C02NH 4 s i solubl e onl y o t th e exten t o f 51 0 gm . pe r lite r a t 5 0 °C , highe r temperature s ar e use d o t promot e highe r yield s o f carbonate , eve n thoug h som e o f th e carbonat e n i solutio n s i los t befor e th e precipitatio n ca n tak e place . The followin g amount s of ammoniu m carbonat e ar e adde d o t 9 gallon s of deionize d wate r a t 5 0 ° C n i a separat e 1 0 gallo n glass-line d tank . Mols Carbonat e Weigh t o f Carbonat e 110.2 7 17,32 5 gm 3 8 lb . 3 oz . a. Purificatio n o f Solution : - Step s "a " throug h "k " ar e followe d excep t tha t 4 1 ml . o f ammoniu m sulfid e an d 1 7 ml of 3 0 % Η2θ2 s i used . Filterin g o f th e impurit y precipitate s is accomplishe d wit h a n Ertel ™ pa d an d a 0.4 5 μ filte r a s before , takin g car e tha t th e precipitate d sulfide s d o no t deagglomerate . Solka-floc ™ may be use d a s a filte r aid . b. Adjustmen t o f pH: The p H s i measure d an d adjuste d o t abou t 7. 5 b y additio n o f ammoniu m hydroxid e solution . 3. Precipitatio n o f Carbonates : The purifie d alkalin e eart h solutio n is heate d o t 7 0 °C . wit h stirring . The purifie d carbonat e solutio n s i heate d o t 5 0 °C . wit h stirrin g an d the n pumpe d int o th e alkalin e

338

eart h chlorid e solutio n a t a rat e o f 28 0 ml. / minut e s o tha t th e overal l precipitatio n take s abou t 2 hours . Additio n s i made s o tha t clump s o f precipitat e d o no t for m an d s o tha t dispersio n o f th e carbonat e solutio n occur s quickly . At th e en d o f th e addition , th e line s ar e flushe d wit h deionize d wate r an d th e precipitat e s i allowe d o t settle . 4. Washing : Wash th e precipitat e wit h war m deionize d wate r b y fillin g th e tan k nea r o t th e to p a s stirrin g continues . Allo w o t settl e an d repea t th e procedure . Tes t th e was h wate r fo r chlorid e by addin g silve r nitrat e solutio n an d continu e washin g unti l th e tes ts i negative . Allo w o t settl e fo r th e fina l time , an d filte r int o a prepare d larg e Buchne r filter . 5. Drying : Dr y th e precipitat e fo r a t leas t 1 6 hour s a t a temperatur e n o highe r tha n 11 0 °C . s o a s o t preven t los s o f th e carbonat e par t o f th e prepare d salt . Too hig h a temperatur e result s n i los s o f carbo n dioxid e an d los s o f certai n physica l propertie s importan t n i subsequen t phospho r preparation . 6. At th e en d o f thi s time , remov e fro m oven , le t cool , recor d weigh t an d yield , an d plac e n i tare d plastic-line d drums . Mixed alkaline-eart h carbonate s ar e use d a s emissio n material s o n tungste n filament s fo r bot h variou s vapo r lamp s an d cathode-ra y tubes . They ar e generall y applie d a s th e carbonate s fro m a slurr y (non aqueaous ) directl y o t th e filament . When th e filamen t s i heated , th e carbonate s decompos e o t th e oxides , an d th e oxide s thereb y greatl y increas e th e emissivit y o f th e meta l filament . The mixtur e ca n b e prepare d b y millin g th e separat e carbonate s togethe r fo r severa l hours , or b y co=precipitation . The fina l mixtur e use d depend s upo n th e individua l manufacturer , eac h havin g hi s own proprietar y composition . However ,t is i usuall y compose d of :

56%BaC03 b y weigh t 13% C a C 03 32% SrCQ3

Mols/ Mol o f Carbonat e 0.45 0 0.20 6 0.34 4

339

If coprecipitate d carbonate s ar e desired , the n on e coprecipitate s th e mixtur e accordin g o t th e mol ratio s give n above . Thus , on e add s th e followin g weight s o t 10. 0 gallon s o f deionize d water : Materia l BaCl2 · 2 H2 0 CaCl2 S r C l2 * 6 H 2 0

Weight s o t be adde d Weight .3 0 gm. 1 9 lb . 1 oz . 8,655.3 0 ;. 3 lb . 1 5 oz . 1,798. 0 i. l 1 5 lb . 1 5 oz . 7,226. 1

The sam e procedur e s i followe d a s give n abov e o t produc e th e co precipitate d carbonate s whic h ar e the n drie d befor e use . F. Manufactur e of CaHP04 This compoun d s i use d o t manufactur e th e halophosphat e phospho r use d n i fluorescen t lamp s an d s i made n i tonnag e quantitie s eac h month . CaHP0 4 s i polymorphi c lik e th e strontiu m orthophosphate s n i tha t t i form s "brushite" , i.e. - CaHP0 4 · 2 H2 0 a t precipitatio n temperature s belo w abou t 3 5 °C . an d "monetite " , i.e. - CaHP04 , abov e abou t 65-7 0 °C. More importantly ,t is i possibl e (10 )o t conver t brushit e to monetit e fro m a slurr y suspensio n b y merel y reheatin g th e 1 4 suspension . Sinc e th e solubilit y produc t o f CaHP04 s i Ks p = 3. 4 χ ΙΟ , -7 2+ it s i eviden t tha t abou t 10 mols/lite r o f bot h C a an d HPU4= wil l be presen t n i a saturate d solutio n containin g bot h o f thes e ions . The solutio n equilibriu m s i a dynami c one : 3.5.7 -

2+

Ca

+ HP04=

~ CaHP04 1

Brushit e precipitate s a s ver y smal l crystal s o f poo r crystallin e quality . Monetit e prepare d directl y fro m solutio n a t 75-8 0 ° C s i somewha t mor e crystalline . However , f i on e firs t prepare s brushite , on e ca n caus e th e brushit e o t redissolv e an d for m monetit e o f hig h crystallin e quality . Thi s can b e don e eithe r o n a continuou s basi s o r a batc h basis . The forme r metho d remove s th e mothe r liquor , an d resuspend s th e precipitat e o t for m a slurry , wherea s th e latte r metho d accomplishe s th e conversio n directl y n i th e mothe r liquor .

340

The followin g diagra m show s th e variou s type s o f CaHP0 4 possibl e o t obtai n b y suc h a conversio n process :

tha t ar e

3.5.8. Types of CaHPU4 Crystal s Precipitate d fro m Solutio n

•

#

Brushit e @ 25 ° C

Diamond-Shape d Crystal s of Monetit e Converte d fro m Brushit e @ 95 ° CHigh Slurr y Conten t

Monetit e @ 70 °C.

Square Monetit e Crgstal s Converte d fro m Brushit e @ 85 °C. - Low Slurr y Conten t

Obviously , a mixtur e o f thes e tw o type s o f crystal s ca n be obtaine d unde r intermediat e condition s b y controllin g th e slurr y proportions . Becaus e th e manufactur e o f CaHP04 s i a hig h volum e procedure , som e manufacturer s hav e use d a continuou s manufacturin g procedure . Suc h a syste m s i show n n i th e followin g diagram , a s give n a s 3.5.9 . on th e nex t page . In thi s process , a serie s o f processin g an d holdin g tank s ar e use d o t purif y bot h th e CaCf e an d th e (ΝΗ4)2ΗΡθ4 solutions , whereupo n eac h solutio n s i store d read y fo r use . Sulfid e s i adde d o n a continuou s basi s

341

342

and sinc e th e solution s ar e no t allowe d o t si t s o a s o t coagulat e th e impurit y sulfides , th e solutio n s i pumpe d o n a continuou s basi s throug h a filte r pres s an d 0.4 5 μ filter s unti l analysi s determine s tha t al l o f th e impuritie s hav e bee n removed . Eac h pur e solutio n s i the n pumpe d o t a storag e tank . These solution s ar e the n pumpe d int o a precipitatio n tan k o t for m

th e

CaHP04 · 2 Η20 o n a continuou s basis . Then , th e slurr y s i fe d o t a dru m filte r wher e th e mothe r liquo r s i separated . The wet cak e s i the n re slurrie d a t a specifi c rati o o f soli d o t wate r (sometime s containin g a smal l amoun t o f solubl e phosphate ) an d th e p H o f th e slurr y s i adjusted . Thereupon , t i s i pumpe d

o t

a conversio n tan k wher e th e particle s

underg o a chang e fro m brushit e o t monetit e o f predetermine d siz e an d proportions . As state d above , a thic k slurr y wil l for m

th e diamond -

shape d crystals , wherea s a thinne r slurr y form s th e squar e crystals . The procedur e o t b e use d s i a s follows : 1. Calciu m Chlorid e Solutio n = 110.9 9 MW : The molarit y o f th e solutio n desire d s i 2.4 3 mols/lite r o r 2.2 5 lbs/gallo n o f water . The 300 gallo n tan k s i fille d wit h 20 0 gallon s o f 20-25°C . wate r an d ar e adde d wit h stirring . 45 0 ml o f (NH4) s i 450 lbs . o f CaCl 2 2Sx adde d o t tan k wit h stirrin g an d th e solutio n s i pumpe d throug h the filte r pres s an d 0.4 5 μ filte r int o th e holdin g tan k wher e t i s i allowe d o t stan d fo r 4 hours . Then , th e solutio n s i re-circulate d throug h th e filte r pres s an d th e 0.4 5 μ filte r unti l analysi s indicate s tha t th e solutio n contain s les s tha n 1. 0 ppm. o f tota l impurities . The pur e solutio n s i the n pumpe d int o th e CaCl 2 storag e tank . Meanwhile , anothe r batc h o f CaCl solutio n ha s bee n 2 starte d an d s i bein g processed . Onc e th e storag e tan k s i full , th e feede r tank s ar e filled . 2. Diammoniu m Phosphat e Solutio n = 132.05 6 MW : Fo r a 2.4 3 2+ molar solutio n o f C a , th e phosphat e solutio n need s o t be 2.5 6 molar , a rati o o f 1.05:1.00 . Therefore , a 20 0 gallo n batc h require s 2.8 2 lbs/gallo n fo r a tota l o f 563. 9 lb s o f (NH4) HPO4. The 2 procedur e s i a s give n abov e n i 1 . excep t tha t th e batc h ha s 40 0 ml s i adde d o t th e tan k wit h stirring . Onc e th e solutio n of (NH4) 2Sx

343

has bee n analyze d o t be o f sufficien t purity , t i s i pumpe d o t it s storag e tank . Meanwhile , anothe r batc h s i bein g processed . Onc e the storag e tan k s i full , th e feeder-tank s ar e fille d a s well . 3. Initia l Precipitatio n o f Brushite : The precipitatio n tan k ha s a tota l volum e o f abou t 50 0 gallons . Two o f th e feede r tank s ar e selecte d an d thei r pump s ar e adjuste d o t a rat e of 1.5 0 gallons/minute . The line-heater s ar e adjuste d s o tha t th e solution s are heate d o t 25-30°C . an d th e tank-heate r s i adjuste d o t maintai n thi s temperature . Onc e th e precipitatio n tan k s i full , th e pump a t the outle t o f thi s tan k s i adjuste d o t 1.5 0 gallons/minut e o t dra w off th e precipitat e an d mothe r liquo r fro m th e botto m o f th e tan k int o th e dru m filter . The wet cak e s i washe d o n th e dru m filte r and s i fe d int o th e reslurryin g tan k (show n o n th e lowe r righ t hand sid e o f 3.5.9.) . 4. Conversio n o f Brushit e o t Monetit e Crystals : A slurr y o f abou t 3.7 5 lbs./gallo n s i made u p b y adjustin g th e flo w o f ho t (8 5 °C ) wate r int o th e slurry-tank . Hydrochlori c aci d s i use d o t adjus t th e pH o f th e slurr y o n a continuou s basi s o t abou t 3.6-3. 8 pH. whil e heatin g th e solutio n o t nea r boiling . The slurr y s i the n pumpe d int o th e conversio n tan k whic h s i stirre d slowl y whil e maintainin g the slurr y temperatur e nea r o t boilin g s o tha t th e slurr y reside s ther e abou t 5-1 0 minutes . A characteristi c dro p n i pH wil l occu r wherei n th e pH wil l dro p o t abou t 2.6-2. 8 indicatin g a conversio n fro m brushit e o t monetite . The slurr y s i the n pumpe d int o th e fina l dru m filte r wher e th e wet cak e s i washe d fre e o f residual s and finall y s i drie d n i a n ove n a t 12 5 °C. fo r 1 6 hours . It s i importan t o t observe , o n a continuou s basis , th e typ e o f crystal s bein g forme d durin g conversion . Increasin g th e solid s conten t o f th e slurr y wil l produc e diamond-shape d crystal s whil e decreasin g t i wil l ten d o t for m elongate d cube s rathe r tha n fla t squar e crystals . The particl e siz e shoul d be abou t 8. 0μ o n th e averag e o t produc e a phospho r whose coverin g powe r insid e a fluorescen t lam p s i suc h tha t al l o f th e ultraviole t radiatio n s i absorbe d b y th e phosphor . Thi s coverin g powe r s i a functio n o f bot h particl e siz e an d shap e a s wel l a s ho w th e particle s "fit " togethe r o t for m a laye r withi n th e lamp .

344

It s i fo r th e latte r reaso n tha t mos t manufacturer s hav e chose n o t prepar e flat squar e plat y crystal s o t for m th e particulat e film.Thi s s i referre d o t n i th e Trad e a s "coverin g power" .t I s i no w wel l establishe d tha t particle s les s tha n abou t 3. 0 μ an d greate r tha n 35-4 0 scatte r th e ultraviole t an d generate d visibl e ligh t withi n th e fil m o t suc h a degre e tha t a los s o f outpu t o f th e fluorescent lam p results . Thus , th e optimu m particl e siz e ha s bee n determine d o t be : 3.5.10. -

3. 0 μ < optima l particl e siz e < 35-4 0μ

It migh t see m incongruou s o t discus s particl e siz e an d shap e n i relatio n to preparatio n o f a ra w materia l use d o t prepar e a phosphor . Nonetheless , t i ha s bee n establishe d tha t th e Particl e Habi t o f th e CaHPU 4 use d o t for m th e phospho r determine s th e final Particl e Habi t of th e phospho r s o produced . What thi s mean s s i tha t f i flat squar e crystal s ar e used , th e final shap e o f th e phospho r particle s wil l b e flat squares . Thi s fact ha s bee n firmly establishe d b y observin g th e particl e habi t produce d a s a functio n o f th e soli d stat e reaction s ocurring . For th e halophosphat e phospho r whic h s i universall y use d n i fluorescent lamps , th e soli d stat e reaction s involve :

al l

3.5.11. - Soli d Stat e Reaction s Leadin g o t Formatio n o f Halophosphat e Overal l Reaction : 2 CaHP04 + CaCQ3 + CaF + CaCl + M n2 0 3 + SboAi 2 2 CasiF , C1)(P0 : Mn (Halophosphate ) 4)3: Sb Partia l Reaction s a. 2 C a H P 0 =^£-Ca ) + H2 0 t 4 2P 207 (monoclinic b. CaC0 3 = > Ca O + C 02 Ϊ1 c. C a2 P 2 0 7 + Ca O => β -C a 3( P 04 )2 (triclinic ) d. & -C a 3 ( P40 )2 + CaF + CaCl = > 2 2

Ca5(F,Cl ) (P0 (hexagonal ) 4 )3

In th e abov e reactions , we hav e lef t ou t th e sid e reaction s tha t occu r 2+ 3+ which involv e th e tw o activators , Mn an d S b . A mor e detaile d

345

discussio n o f th e SrsfF , C1)(P04 )3*. Sb : Mn phospho r was presente d n i a prio r wor k (8) . A s show n n i th e followin g diagram , electro n micrograph s take n of th e reactio n produc t betwee n th e fla t squar e platelet s of CaHP04 an d variou s component s o f th e abov e reaction s sho w tha t th e particl e habi t of th e CaHP04 s i retaine d n i fire d produc t o f th e soli d stat e reaction . 3.5.12. Crysta l Habi t of CaHP04 and Some of It s Reactio n Product s at 10,00 0 X

if f

CaHP0

CaHP0 4 + S b 2 03

S b 2 03

C a2 P 2 0 7

4

CaHP04

M n C 03

2 C a H P 04

+ MnCD 3

C a C 03

+ CaCQ 3

C a5 F ( P 04 )3

CaF2

The reaso n fo r thi s behavio r lie s n i th e relativ e reactio n temperature s of the variou s components . Accordin g o t Wanmake r e t a l (11) , Reactio n "a. "

346

in

3.5.11 . start s as

lo w

as

38 0

°C. Sinc e t i

s i

merel y

a

"lattice -

rearrangement " wit h los s o f wate r vapor - tha t is , th e basi c structur e of the

P04-tetrahedr a shift s slightly - th e

overal l crysta l habi t o f

th e

Ca2P207 doe s no t chang e much fro m th e flat-plat y for m of th e origina l CaHPU 4

crystal . However , grai n boundarie s ca n be note d n i

product . I f CaCQ3

s i present , th e reactio n s i tha t o f "c. " o t

th e

fire d

for m

tribasi c calciu m phosphate , i.e . 3 - Cas(Ρθ4)2 . Thi s compoun d

th e was

identifie d by x-ra y analysis . However , th e "a. " reactio n take s plac e firs t becaus e CaCCf e doe s no t reac t unti l a temperatur e abov e abou t 65 0 reached (12) . By tha t time , th e

$- Ca2P20 7

°C. s i

crysta l habi t has alread y

become stable . Thus , th e C aO forme d n i Reactio n "b. " diffuse s int o th e structur e and change s th e lattic e structure , but no t th e

oute r crysta l

habi t form . Not e als o tha t th e crystal s o f th e othe r component s ar e not the same as th e CaHP04

and th e reactio n betwee n eac h on e has littl e

effec t on th e fina l crysta l habi t of th e fire d product . This

therma l behavio r of

th e

reactin g component s has

importan t

consequence s concernin g halophosphat e phospho r preparatio n n i the crysta l habi t and for m of th e firedproduc t ca n

tha t

be controlle d o t

a

significan t extent . As mentione d above , th e propertie s of th e particulat e fil m on th e inne r surfac e of th e glas s tub e of th e fluorescen t lam p play s a significan t rol e n i how

th e lam p performs , bot h n i initia l brightnes s and

maintenanc e o f tha t brightnes s ove r th e lif e of th e

lamp . Indeed , by

eliminatin g particle s smalle r tha n abou t 3. 0 u, th e initia l brightnes s was increase d by ove r 10 %

and

th e

outpu t maintenenc e was

significantl y

improved . Lamps ar e coate d accordin g o t a "coatin g weight " whic h

s i

2

specifie d n i mg./cm o f surface . What s i require d s i o t cove r th e surfac e of th e glas s o t for m a particulat e fil m of a particula r depth . How particle s interloc k on

th e

th e surfac e s i critica l sinc e to o larg e a particl e

wil l leav e ope n space s betwee n particles . A coatin g weigh t of abou t 6. 5 2

m g . / c mis usual , but thi s depend s upo n th e densit y o f th e crystal s as well as thei r crysta l habit . Althoug h th e diamond-shape d crystal s wil l interloc k o t for m a dens e coating , th e platy - squar e crystal s hav e bee n observe d o t for m a coatin g whic h has a bette r maintenenc e durin g th e lif e o f th e

lamp . I f th e

particle s ar e

to o

small , th e

scatterin g o f

ultraviole t ligh t generate d by th e mercur y discharg e increase s radicall y and result s n i a lowe r brighnes s lamp . Thi s s i th e reaso n fo r preparin g crystal s betwee n th e siz e limit s give n abov e n i 3.5.10 .

347

The

abov e descriptio n ha s presente d a metho d fo r producin g CaHP0 4 o n a

continuou s basis .f I a batc h metho d s i required , a somewha t differen t metho d is used . n I thi s process , th e concentration s o f bot h C a a nd Ρθ4^- ar e lowe r 2+

and th e conversio n take s plac e directl y n i th e mothe r liquor . Produc t yield s rang e fro m

3 0 y o 95 %

o f theoretica l yield , base d o n

calcium , dependin g

mainl y o n th e p H o t whic h th e dihydrat e suspensio n s i adjuste d (conversio n pH) befor e reheatin g th e suspensio n o t nea r boiling . I t s i possibl e o t produc e ver y highl y crystalline , closely-sized , luminescen t grad e CaHP0 4

possessin g

characteristi c for m an d habit . 1. Calciu m Chlorid e Solutio n = 110.9 9 MW : The molarit y o f th e solutio n use d n i thi s cas e s i 0.33 3 mols/lite r o r 0.30 9 lbs/gallo n o f water . A 200 0 gallo n tan k s i filled wit h 140 0 gallon s o f 20°C . wate r an d 43 2 lbs . o f CaCf e ar e adde d wit h stirring . 45 0 ml o f ( N H 4) 2 Sx s i adde d o t tan k wit h stirrin g an d th e solutio n s i pumped throug h th e filte r pres s an d 0.4 5 μ filter int o th e holdin g tan k wher e t is i allowe d o t stan d fo r 4 hours . Then , th e solutio n s i re-circulate d throug h th e filter pres s an d th e 0.4 5 μ filte r unti l analysi s indicate s tha t th e solutio n contain s les s tha n 1. 0 ppm. o f tota l impurities . The pur e solutio n s i the n pumpe d int o a CaCl 2 precipitatio n tank . Alternately , CaNQ3 may b e use d a t 0.5 0 mols / lite r concentration . 2. Diammoniu m Phosphat e Solutio n = 132.05 6 MW : Fora 0.33 3 2+ e phosphat e solutio n need s o t b e 0.33 4 molar solutio n o f C a ,th molar , a rati o o f 1.003:1.000 . Therefore , a 200 0 gallo n batc h require s 30 0 gallon s o f 1.7 2 lbs/gallo n fo r a tota l o f 515. 5 lb s o f ( Ν Η 4 )2 Η Ρ θ 4 .The procedur e s i a s give n abov e n i 1 . excep t tha t the batc h ha s 40 0 ml o f ( N H 4 )2 S x s i adde d o t th e tan k wit h stirring . Onc e th e solutio n ha s bee n analyze d o t b e o f sufficien t purity , t is i hel d prio r o t precipitation . 3. Initia l Precipitatio n o f Brushite : The ( Ν Η 4 )2Η Ρ θ 4

solutio n s i

pumped int o th e CaCl solutio n a t 2 0 °C , wit h stirrin g a t a rat e o f 2 abou t 1.5 0 gallons/minute . The line-heater s ar e adjuste d s o tha t the solution s ar e heate d o t 25°C . an d th e tank-heate r s i adjuste d to maintai n thi s temperature . Onc e th e precipitatio n tan k s i full , stirrin g continues .

348

4. Conversio n o f Brushit e o t Monetit e Crystals : Hydrochlori c aci d is use d o t adjus t th e p H o f th e slurr y o t abou t 3. 1 pH wit h stirring . The

slurr y s i the n heate d o t nea r boilin g ~ ( 10 4

°C ) fo r 10-1 5

minutes . The conversio n ca n b e judge d o t b e complet e whe n pH

th e

o f th e slurr y drop s o t a valu e lowe r tha n th e origina l p H

adjustment . Usually , thi s wil l b e n i th e rang e o f 2.4 - 2. 8 pH. The slurr y s i the n pumpe d int o th e final dru m

filte r wher e th e wet

cake s i washe d fre e o f residual s an d finally s i drie d n i a n ove n a t 125 °C . fo r 1 6 hours . The yiel d wil l b e n i th e rang e o f 75-85 % o f the theoretica l yield . 5. Yield s an d Concentratio n o f Solution s :t I s i possibl e o t increas e the initia l concentration s o f th e solution s use d fo r precipitation . For example , 148 0 lb .o f purifie d CaCl2 s i adde d o t 140 0 gallon s o f pur e

wate r an d

th e volum e s i adjuste d o t

volume . One ca n the n ad d soli d (ΝΗ4)2ΗΡθ4

160 0

gallon s tota l

directl y o t thi s solutio n 175 7 lb s o f

whic h dissolve s an d

the n cause s brushit e o t

for m n i solution . The temperatur e n i thi s cas e s i bes t hel d a t abou t 30-3 5 °C . Onc e th e precipitatio n s i judge d o t b e complet e (th e p H wil l rise abov e 7.00) , th e slurr y continue s o t b e stirre d whil e th e pH

s i adjuste d o t p H = 3. 1 wit h HC1. (I f Ca(N0 s i used , the n on e 3 )2

use s HNQ 3

fo r p H

adjustment) . Abou t 400 0 ml . o f HC1 wil l b e

required . The tan k temperatur e (an d th e slurry ) s i the n increase d to

a boilin g stag e an d

hel d fo r abou t 1. 0 hour , o r unti l a

characteristi c dro p n i p H

o t 2.5-2. 6 s i observed . The tota l tim e

require d fo r heatin g an d

conversio n s i abou t 4. 0 hours . T he

produc t n i thi s cas e wil l consis t o f flat, plat y diamond-lik e crystal s of abou t 79μ n i size , i.e. -* ^ Β π £ ^

h

. Yiel d expecte d s i abou t 159 0

lbs . o r 87.7 % o f theoretical . f I a trul y diamond-shape d crysta l s i

desired , i.e. - \f

th e

procedur e s i modifie d a s

follows . The

precipitaio n o f brushit e proceed s a s give n abov e an d continue d fo r

1/ 2

hou r

stirrin g s i

befor e conversion . The

brushit e

precipitat e s i the n allowe d o t settl e n i it s own mothe r liquo r fo r abou t 2. 0 hour s or unti l th e mothe r liquo r abov e th e

settle d

presipitt e s i clear , meanwhil e maintainin g th e temperatur e withi n the tan k betwee n 3 0 an d

3 5 °C . The n approximatel y one-hal f o f

349

the mothe r liquo r s i remove d fro m th e tank , takin g car e tha t non e of th e precipitat e s i los t durin g thi s process . The

nex t ste p

require s tha t th e precipitat e s i resuspende d b y stirrin g an d

th e

conversio n proceed s a s give n above . Obviously , les s aci d wil l b e require d o t adjus t th e p H

an d

th e tim e neede d o t

reac h th e

boilin g stag e wil l als o b e less . G . Manufactur e of Cd0 2 an d Ζηθ2 These material s ar e use d n i phospho r manufactur e an d ar e made b y th e followin g method . Eithe r o r bot h th e cadmiu m an d

zin c salt s ca n b e

use d n i thi s procedure . 1. C d i N 0 3 )2 8

Solution - MW

oz . = 25.7 2 mols ) o f

= 308.4 6 :Dissolv e 7934. 6 gm. (1 7 lbs . Cd(NQ3) 2 · 4 H2 0 n i approximatel y 3. 0

gallo n o f col d deionize d water . Thi s solutio n s i the n gasse d wit h NH 3 unti l th e solutio n s i clea r (Th e solutio n firs t turn s whitis h 2+

an d the n change s o t th e Cd (NH4)4 due o t formatio n o f Cd(OH) 2

ion , du e o t th e reactio n a s give n above . Add 3 9 gram s o f tanni c aci d o t

solutio n an d

le t si t fo r 1 8

hours . Filte r thi s solutio n

throug h a n Erte l pa d an d the n throug h a 0.4 5 μ filte r o t remov e all trace s o f th e precipitate . 2. Precipitatio n o f CdO In

2

Usin g Hydroge n Peroxid e 2+

thi s case , th e Cd(NH4)4

preparato n o f th e

cadmiu m

solutio n s i no t degasse d a s n i th e ammoniu m

phosphat e procedur e

give n above . The solutio n s i transferre d o t a2 0 gallo n glass-line d tan k an d dilute d o t a 6. 0 gallo n tota l volume . t I s i the n coole d o t a temperatur e belo w 1 5 ° C an d th e 30 %

H2 0 2 is pumpe d n i a t a rat e

of abou t 12 5 ml . / minut e unti l 5. 0 gallo n o f peroxid e ha s

bee n

added . A ver y fin e precipitat e wil l result . The solutio n mixtur e s i allowe d o t stan d overnigh t an d th e temperatur e wil l graduall y ris e to

85-9 0 °C . du e o t

decompositio n o f th e exces s peroxide . n I

addition , th e particl e siz e o f th e

C d 02 wil l increas e du e

o t

recrystallizatio n o t th e poin t wher e t i ca n b e easil y filtered . Some foa m

wil l b e observed .

350

3. Washin g o f th e Precipitate : Sipho n of f th e mothe r liquo r an d wash th e precipitat e b y re-suspensio n wit h 85-9 5 ° C deionize d water , fillin g th e tan k o t th e brim . Allo w th e precipitat e o t settl e and repea t th e sequenc e fo r fivetimes . 4. Dryin g o f th e Precipitate : Filte r of f th e precipitat e an d ove n dr y for 1 6 hour s a t 10 0 °C. Do no t le t temperatur e rise abov e thi s limi t sinc e los s o f oxyge n fro m th e sal t wil l occur . At th e en d o f thi s time , remov e fro m oven , le t cool , recor d weigh t an d

yield , an d

plac e n i tare d plastic-line d drums . The zin c sal t may als o b e made b y th e sam e method s excep t tha t 5902.2 0 gms (1 3 lbs . 1 oz. ) ar e use d sinc e th e molecula r weigh t o f the zin c sal t is : Zn(NO£) ·4 H2 0 -MW 2

= 229.4 5

H . Manufactur e o f ZnS an d Zni _xCdx S2 These material s ar e use d primaril y n i th e manifactur e o f cathode-ra y tub e screens , includin g color-television . Two method s ar e commonl y use d o t manufactur e thes e materials , tha t o f th e chlorid e proces s an d tha t o f th e sulfat e process . We wil l addres s th e latte r method , althoug h on e shoul d kee p in min d tha t th e othe r proces s s i als o bein g used . Althoug h zin c sulfid e phosphor s wer e trie d earl y o n n i fluorescent lamps , the y wer e foun d o t decompos e unde r th e influenc e o f th e 252 7 A radiatio n presen t n i th e lo w pressur e mercur y discharge . Eve n whe n use d a s a coatin g o n th e oute r bul b o f a hig h pressur e mercur y discharg e lam p havin g 365 0 A radiatio n (an d no t directl y expose d o t th e mercur y discharge) , thes e phosphor s deteriorat e fairl y rapidl y durin g th e initia l lif e o f th e lamp . Thus , thei r mai n us e ha s bee n as cathod e ra y phosphors , an d mor e recentl y a s optica l component s fo r th e infra-re d regio n o f th e spectrum . Typically , th e followin g ra w materials , a s give n n i th e followin g table , ar e require d fo r manufactur e o f bot h o f thes e materials , viz -

351

Raw Material s Require d fo r Manufactur e o f Zn & Cd Sulfide s NH4O H ZnO NaSQ3 N a Diethyl-Dithiocarbamat e HC 1 CaO DiMethylglyoxim e CdO N a 2 S 2 03 H 2S AgN03 CaCl2 Cupferro n Ludox™ HNO3 H2SO 4 NaCl CaCl2 C u ( N 03 )2 BaCl2 HNQ 3 Ludox™ N a 2 S 2 03 MgCl2 In general , bot h ZnS an d CdS ar e made b y th e sam e method s an d us e th e same equipmen t give n abov e n i 3.5.3 . Some manufacturer s prefe r o t us e a ZnCl2 solutio n sinc e th e presenc e of CI s i know n o t be necessar y n i th e fina l phospho r compositio n o t obtai n brigh t an d efficien t cathode-ra y phosphors . Other s prefe r o t us e ZnS0 4 solution s n i orde r o t be abl e o t contro l th e amoun t of chlorid e io n adde d o t for m th e fina l phosphor . A s show n n i th e followin g diagram , give n a s 3.5.13 . o n th e nex t page , the procedur e involve s us e o f ZnO an d sulfuri c acid . The step s involve d are : 1. Dissolv e ZnO n i H2SO4 o t for m solutio n of ZnS0 4 4 ( hrs) . 2. Purif y solutio n b y additio n of Dimethylglyoxim e -le t si t 8 hours , filte r ou t sludg e 8 ( hrs.) . 3. Repea t purificatio n wit h Cupferron - filte r ou t sludg e (16-2 0 hrs.) . 4. Repea t purificatio n wit h Sodiu m Diethyl-Dithiocarbamat e - filte r out sludg e (6-2 0 hrs.) . 5. Analyz e solutio n fo r approva l 4 ( hrs.) . 6. Precipitat e ZnS b y H2S gassin g fo r 8 hour s unde r pressur e ( « 15 psi. ) whil e stirrin g slowly . 7. Le t settl e an d decan t mothe r liquo r 2 ( hrs) .

352

s 3.5.13 . Proces

Hot& Cold Water

For Precipitatio n of ZnS

6 00 Lb. of ZnO 3 0 0 1b

D i Methylg l yoxi me + ΝΗ^ΟΗ

Cupferron

Activate d Carbon + Na Di Ethy l DithioCarbamat e

3 50 Gallon

1 2 0 l b s . Z n S / dya

353

8. Wash precipitat e 45 time s wit h ho t deionize d water , lettin g settl e n i betwee n washe s an d decantin g supernaten t liqui d (8-1 2 hrs.) . 9. Trea t al l mothe r liquo r an d was h wate r wit h sodiu m sulfit e o t oxidiz e sulfid e o t thiosulfate . 10. Pump sulfid e precipitat e slurr y int o dru m filte r o t separate . 11. Filte r an d dr y th e wet cak e obtaine d fo r 1 6 hour s @ 17 5 °C . 12. The sam e procedur e applie s fo r CdS preparation . Note tha t th e origina l volum e o f th e purifie d solutio n s i abou t 36 0 gallon s an d tha t 1/ 6 o f thi s s i dilute d o t 48 0 gallon s befor e sulfid e precipitatio n take s place . Thi s mean s tha t 6 sulfid e batche s ca n b e made fro m th e origina l purifie d batc h an d tha t anothe r batc h ca n b e starte d and purifie d whil e precipitatio n s i takin g place . Time s require d o t complet e eac h ste p ar e als o give n n i th e above . Abou t 12 0 lb s o f ZnS ca n be prepare d eac h workin g day . The sludge s obtaine d durin g eac h separat e purificatio n ste p ar e show n in th e following : Ion s Precipitate d

Reagen t Use d DiMethylglyoxim e Cupferro n

2+

Ni

3+

, Ni

2

Ol + 2+

2+

2+

N a Diethyl-Dithiocarbamat e p e3+ C o , P b , M n

The abov e procedur e represent s bu t on e way o t prepar e ZnS an d CdS. 2+ Some manufacturer s prefe r o t make variou s combination s o f Zn an d 2+ solution s an d coprecipitat e Zni_xCd Other s prefe r o t make Cd xS2. eac h separatel y an d the n combin e the m befor e firing o t for m th e phosphor . Stil l other s wil l ad d A g N 03 an d co-precipitat e al l thre e a s Zni_ S2 : A gy (wher e Agy s i th e activator) . t I shoul d b e clear , then , x_y C d x tha t eac h manufacture r ha s hi s own specia l metho d o f manufactur e bu t tha t th e product s ar e ver y similar . Althoug h 3.5.13 . show s a tan k fo r eac h separat e step , thi s s i no t necessary .

354

In th e followin g diagram , we sho w a typica l equipmen t arrangemen t which s i sufficien t fo r manufactur e o f bot h ZnS an d CdS, viz 3.5 . 14. General Equipmen t Layou t fo r Sulfid e Manufactur e

The actua l piping-hooku p s i no t show n here . Ther e ar e thre e glass-line d tank s whic h ar e use d fo r purificatio n of th e solution s an d precipitatio n of th e sulfid e an d on e rubber-line d tan k whic h s i use d fo r aci d dissolutio n steps . Sometimes , th e rubber-line d tan k s i als o use d fo r sulfid e precipitation . Usually , a dru m filte r s i use d fo r th e produc t an d the filte r presse s ar e reserve d fo r th e impurit y sludge s produced . This complete s th e descriptio n o f manufactur e o f ra w material s use d o t prepar e phosphor s fo r us e n i bot h fluorescen t lamp s an d cathode-ra y tubes . 3+ s a ra w materia l by Althoug h we coul d discus s th e preparatio n o f YVO4: E u a precipitatio n methods , we prefe r o t wai t unti l th e nex t chapte r s o a s o t be abl e o t compar e th e produc t o f th e soli d stat e metho d o t tha to f th e precipitatio n method .

355

3.6 . -MANUFACTUR E OF FLUORESCEN T LAMPS Befor e we tur n o t th e manufactur e o f phosphor s n i th e nex t Chapter , we nee d to discus s briefl y ho w fluorescen t lamp s ar e made . Ther e ar e many varietie s o f suc h lamps , includin g difference s n i size , wattag e an d shape . The manufactur e of al l o f thes e lamp s take s plac e o n highl y automate d machine s whic h hav e a through-pu t o f thousand s o f individua l lamp s o n a dail y basis . We d o no t inten d to discus s th e desig n o f suc h machine s sinc e thi s coul d b e th e subjec t o f a separat e an d length y monograph . Most fluorescen t lam p manufacturer s produc e betwee n tw o o t si x millio n lamp s pe r month , or abou t 90,00 0 o t 272,00 0 lamp s daily . Thi s rat e s i abou t 4,00 0 o t 11,00 0 lamp s pe r hou r o n a 24 hou r basis , or 1 o t 3 pe r secon d o n a continuou s basis . Suc h productivit y s i obviousl y accomplishe d throug h th e us e o f mor e tha n on e manufacturin g an d finishin g line . The step s involve d n i th e manufactur e o f fluorescen t lamp s ca n b e delineate d as follows : A . Washin g Fluorescen t Lamp Bulb s B. Preparatio n o f "Phospho r Paint " C. Bul b Coatin g an d Dryin g D . Lehrin g E. Manufactur e o f Flare d Stem s F. Ste m an d Flar e Mountin g G . Exhaustin g o f Lamps H . Mercur y Dosin g an d "TipofT . I. Bas e Mountin g J. Lamp Testin g Each o f thes e step s requir e a particula r protoco l whic h ha s bee n develope d b y eac h Manufacture r accordin g o t hi s own need s an d desires . Our descriptio n wil l asses s thes e step s n i a genera l manne r withou t tryin g o t includ e al l o f th e variou s approache s tha t hav e bee n made fo r eac h individua l process . The followin g diagram , give n a s 3.6.1 .o n th e nex t page , show s a flow-char to f eac h o f thes e individua l step s n i th e manufactur e o f fluorescen t lamps . Again , as we hav e stated , thi s represent s jus t on e way tha t suc h lamp s ca n b e manufactured .

356

3.6.1. Manufactur e of Fluorescen t Lamps -Schemati c y x y y y x x x> x Bulb Washing^ ; Glass Inspectio n

Spray Heads - ·

mm

Bulb Coatin g Caatin-guead s

Coate d Bulbs

357

A . Washin g Fluorescen t Lamp Bulb s In general , th e glas s bulb s ar e visuall y inspecte d o t determin e amoun t o f breakag e present , f i any . Any bulb s wit h end-crack s ar e obviousl y rejected . The qualit y o f glas s ca n b e easil y checke d b y runnin g a bar e bul b throug h th e Lehr , alon g wit h th e coate d bulbs . The bar e bul b s i the n retrieve d an d s i washe d internall y wit h a dilut e solutio n o f HC1. The was h solutio n s i the n + analyze d fo r Na content . f I a n inferio r glas s batc h ha s bee n use d o t for m th e tubing , sodiu m ion s interna l o t th e glas s structur e wil l diffus e ou t fro m th e interio r o t th e surfac e whe n th e glas s tub e s i subjecte d o t th e Leh r + temperatur e o f abou t 65 0 °C . By analyzin g th e amoun t o f Na recovered , on e can obtai n a n empirica l resul t concernin g th e relativ e qualit y o f th e glass . The + 2 d n i th e was h wate r shoul d no t excee d abou t 0.0 5 mg/cm amount o f Na foun of glas s surface . By fa r th e greates t volum e o f th e fluorescen t lamp s manufacture d involv e eithe r 48 inc h or 9 6 inc h lon g glas s tubes . The y ar e generall y receive d wit h "shoulder-formed " ends , viz Typica l Fluorescen t Lamp Tubin g

v. They ar e manuall y place d n i a vertica l positio n withi n a "washing " machin e where the y progres s fro m statio n o t statio n whil e ho t wate r s i spraye d downwar d internall y o t was h th e surfac e clea n o f an y particle s present . Thi s s i followe d b y a n upwar d ho t wate r spra y whic h allow s th e was h wate r o t dri p fro m th e botto m o f th e tubing . Sometimes , a n acidi c was h s i use d a s a nex t ste p n i whic h abou t 2 % b y weigh t o f HC1 an d 1 % HF b y weigh ts i presen t n i the was h water . A fina l rinse s i the n necessary . Hot ai r jet s ar e the n use d o t dry th e bulb s befor e the y emerg e fro m th e washe r o t be coole d o t roo m temperatur e b y a blower . All thes e operation s tak e plac e withi n a close d spac e so tha t outsid e contaminatio n b y dus t orothe r particle s canno t occur . It s i importan t o t us e clean , filtere d wate r a s wel l a s filtere d ai r fo r th e washin g an d dryin g processes . The temperatur e o f th e was h wate r need s o t be

358

controlle d a t abou t 18 0 °F . an d th e presenc e o f alga e s i discourage d b y th e us e of a n anioni c detergen t presen t n i th e was h water , whic h s i filtered an d the n recirculate d throug h th e washin g machin e o n a continuou s basis . Onc e th e clea n bulb s exi t th e washin g machine , the y ar e read y o t b e coated , or "painted" . n I thi s process , a "paint " s i firs t prepare d b y formin g a suspensio n of phospho r n i on e o f severa l vehicles . B. Preparatio n o f Phospho r Pain t A typica l organic-base d "paint " consist s o fa n ethylcellulos e bas e dissolve d n i a mixtur e o f xylo l an d butanol . t I s i prepare d b y mixin g th e followin g ingredients : 1. Organi c Solven t Base d Paint s : a. - A volum e o f n-butano l an d xylo l (xylo l s i a mixtur e o f xylenes , i.e. para, meta - an d ortho - dimethylbenzenes ) s i prepare d consistin g o f abou t 60 % butano l an d 40 % xylol . To thi s s i adde d abou t 5.0 % b y weigh t of ethylcellulos e an d abou t 2.5 % b y weigh t o f dibuty l phthalat e (th e latte r s i a plasticizer) . Thi s mixtur e s i stirre d an d rolle d abou t 2 4 hour s unti l th e lacque r s i smoot h an d al l o f th e solid s hav e dissolved . The lacque r s i the n filtere d o t remov e an y remainin g particles . Thi s lacque r can als o b e purchase d from : Pierc e & Steven s Chemica l Corp . 724 Ohi o St . Buffalo , NY 1420 3 or

Raff i & Swanso n Wilmington , Mass . 0188 7

b. The "paint " s i prepare d b y placin g 1. 0 gallo n o f th e ethylcellulos e lacque r n i a 4-gallo n mil l containin g abou t 2 8 lbs . o f 3/ 4 inc h "French flint " pebble s (contac t Pau l O. Abbe , Littl e Falls , NJ fo r details) . To thi s s i adde d 50. 0 ml . o f Armee n CD™ (whic h s i a primar y aliphati c amin e which act s a s a dispersin g agen t an d s i availabl e fro m th e Armou r Co. , Chicago , 111.) . Add 12. 0 lbs . o f phospho r an d ru n mil l fo r 1. 0 hour . Chec k suspensio n fo r textur e an d adherenc e b y dip-coatin g a glas s slid e an d

359

lettin g t i dr y a t roo m temperature . Mil l fo r 1 5 minute s longe r f i necessary , a s judge d b y phospho r particle s whic h may protrud e fro m the surfac e o f th e film . Ultrasoni c dispersio n s i als o a n accepte d practice . c. The viscosit y an d specifi c gravit y o f th e pain t mus t be adjuste d o t obtai n th e desire d densit y o f coatin g a s wel l a s uniformit y o f densit y fro m en d o t end . Lacque r lower s specifi c gravit y bu t raise s viscosity , whil e xylo l lower s bot h viscosit y an d specifi c gravity . Viscosit y s i measure d b y a Zahn-eu p (usuall y #3) . specifi c gravit y b y us e o f hydrometers , whil e coatin g densit y s i measure d b y ligh t transmissio n orthogana l o t th e axi s o f th e glas s tube . Usually , th e pain t mus t b e adjuste d s o a s o t doubl e th e origina l mille d volum e o t giv e a viscosit y o f abou t 15-3 0 Zahn-seconds , a t a specifi c gravit y o f abou t 1.100-1.400 . These value s may depen d somewha t upo n th e ambien t condition s o f humidit y an d temperature . A n optimu m valu e usefu l fo r bot h Coo l Whit e an d othe r color s ha s bee n foun d o t be : 28 Zahn-second s 1.32 0 Sp . G. These ar e value s fo r th e physica l parameter s o f th e pain t use d o t coa t bot h 40T1 2 an d 96T1 2 lamp s (Suc h lam p designation s ar e base d o n firs t th e wattage , configuratio n an d diamete r o f th e lamps , i.e .4 0 = 40 watts ,Τ = tubular , & 1 2 = 12/ 8 inc h n i diameter . The actua l length s neede d fo r thes e wattage s are : 4 8 inche s an d 9 8 inches , respectively) . A 2 phospho r coatin g weigh t o f abou t 7. 5 m g / c m wil l resul t afte r th e lam p has bee n lehred . d. I t s i importan t o t minimiz e evaporatio n losse s an d o t avoi d operato r exposur e o t fume s wheneve r possible . An exhaus t syste m s i o t be use d a t al l time s n i th e pain t preparatio n process . Armee n CD s i corrosiv e o t the ski n an d mus t be washe d of f wit h vinega r immediately . Sinc e th e pain t s i ver y flammable , al l source s o f flame an d spark s mus t be carefull y avoided . Al l source s o f dirt-contaminatio n mus t als o b e avoide d tha t might caus e damag e o t th e phospho r coatin g a s applicatio n o f th e pain t proceeds .

360

e. n I anothe r ethylcellulos e pain t variation , th e followin g ingredient s have bee n used : Xylol = 78. 0 gallon s Ethylcellulos e Lacque r = 28. 0 gallon s Butano l = 24. 0 gallon s Armeen CD = 2.5 0 gallon s AlonC =15. 5 lb s Phospho r = 150 0 lbs . This mixtur e s i mille d fo r 8. 0 hour s an d th e viscosit y s i the n adjuste d a s give n above . Alternately , ultrasoni c mixin g an d dispersio n may b e use d n i plac e o f the millin g step . Becaus e o f th e flammability an d possibl e damag e o t huma n respirator y systems , water-base d paint s hav e bee n mor e recentl y used . Preparatio n o f suc h paint s involve s th e following : 2. Wate r Base d Paint s a. Polymethacrvlat e Lacque r i. To make thi s lacquer , 30 0 liter s o f pur e wate r an d 15. 0 liter s o f a n ammonium polymethacrvlat e solutio n ~ ( 7 % solid s b y weight) , Vulcastab-T ™ (Cod e 0597 1 fro m IC I Industries , 3 3 Brazennoz e St. , Manchester , England ) ar e fe d int o a reactio n tank . The basi c solutio n s i steam-heate d unti l abou t 15 0 liter s ar e lef t n i th e tan k and/o r th e pH reache s p H = 7.00 . Boilin g s i continue d longe r f i th e pH remain s abov e thi s limit . Wate r s i the n adde d o t make a tota l o f 30 0 liter s o f solution , which s i the n pumpe d throug h a filte r o t remov e insoluble s an d the n int o a storag e tank . The reactio n tan k s i cleane d b y stirrin g 50. 0 liter s of was h wate r o t clea n it , an d thi s volum e s i adde d o t th e storag e tenk . Once th e solutio n s i cool , it s viscosit y s i measure d an d wate r s i adde d o t adjus t t i int o a n acceptabl e range . Thi s procedur e usuall y result s n i a tota l volum e o f aroun d 425-45 0 liter s o f premix . To make th e paint , 4 0 liter s o f premi x s i mixe d wit h 80. 0 kilogra m o f phospho r n i a n 8 0 lite r tan k fo r 1 5 minutes . Thi s volum e s i the n move d o t a 40 0 lite r ball-mil l containin g 12 0 kilogram s o f flin t stones , an d th e mil l s i rolle d a t 18. 5

361

rpm. fo r LO hour . 40. 0 liter s o f wate r ar e the n adde d an d millin g s i continue d fo r 2. 0 minutes . The pain t s i the n pumpe d int o a reservoi r and s i read y fo r use . A wettin g agent , calle d Lissapo l NXA™ (Cod e 06544 - availabl e fro m IC I Industries , England )s i automaticall y adde d a s 1. 0 dro p pe r 5.0 0 liter s o f dispense d pain t a s t i s i draw n from th e pain t tank . ii . Anothe r metho d fo r preparin g th e sam e typ e o f water-base d pain t involve s th e followin g procedure . A 7 % solutio n o f Vulcastab-T ™ (Cod e 05971 ) s i mixe d wit h pur e wate r a t a rat e o f 6.2 5 gallon s an d dilute d a t a 1: 7 rati o wit h wate r o t obtai n a 1.0 % solution . At thi s point , th e viscosit y s i abou t 5 0 second s usin g a DuPon t #7 0 cup . Abou t 1. 0 cc . o f a wettin g agent , Suproni c B75/2 5 (availabl e fromGlover s Chemica l Ltd. , Wortle y Lo w Mills , Whirehal l Road , Leeds , England ) s i adde d (havin g been dilute d 150/ 1 wit h wate r befor e us e o t abou t a 0.67 % concentration) . Thi s premi x solutio n s i the n read y fo r use . A phospho r slurr y s i prepare d b y placin g 50. 0 kilogra m o f phospho r n i a 50 gallo n tan k alon g wit h 7. 5 kilogra m o f citri c aci d crystals , 1 5 liter s o f a 10 % hydroge n peroxid e an d enoug h wate r o t fil l th e tank . Thi s mixtur e s i stirre d o t dispers e th e phospho r an d the n s i filtere d o t remove th e phospho r whic h remain s a s a wet cak e (whic h ha s bee n acid-washed) . The filtratio n produc t s i resuspende d a t a concentratio n of 1.0 0 gra m pe r 1.0 0 ml . o f wate r o t for m th e slurry , alon g wit h a n adherenc e agen t suc h a s bariu m caliu m borat e (abou t 1 0 gram s pe r lite r of fina l volume) . Thi s suspensio n s i the n mixe d a t a n equa l volum e rati o wit h th e premi x solution . t I s i thi s fina l pain t suspensio n whic h s i the n use d fo r coatin g o f th e lamps . iii . Not e tha tn i th e tw o method s give n abov e fo r us e o f polymethacrvlat e lacquer , th e firs t neutralize s th e syste m b y boilin g of f th e ammoni a present , whil e th e secon d metho d use s a n aci d was h o f th e phospho r o t presen t a n acidi c surfac e o t th e basi c ammoniu m polymethacrvlat e n i orde r o t obtai n a neutra l pain t suspension . iv. n I mos t o f thes e paints , adherenc e agent s ar e usuall y adde d o t improv e th e coherenc e an d stickin g o f th e phospho r laye r o t th e glas s

362

afte r lehring . Thes e agent s hav e generall y consiste d o f on e following :

o f th e

AlonC (whic h s i a 30 0 A -size d alumina , AI2O3) Bariu m calciu m borat e Titania , T1O2 (micron-sized ) Diammoniu m phosphate , (ΝΗ4)2ΗΡθ4 (whic h solubl e n i water-base d paint )

s i

One o f thes e agent s s i usuall y adde d prio r o t th e fina l millin g o f th e paint . Ultrasoni c dispersio n s i als o use d b y som e Manufacturers . b. Hydroxypropylmethylcellulos e Pain t Anothe r pain t use d ha s bee n base d upo n Methocel™ - HG grade , a modifie d cellulos e availabl e fro m Dow Chemica l Co. The pain t s i prepare d b y makin g a stoc k solutio n consistin g o f 42 0 gms . o f Methocel-H G o t 10 0 liter s o f pur e water . Thi s initia l solutio n s i rolle d for abou t 12 hour s unti l a clea r solutio n s i obtained . Thi s s i the n filtere d o t remov e an y ge l particle s tha t may hav e formed . Then , 550 0 ml. o f th e solutio n s i dilute d wit h 450 0 ml . of wate r o t whic h ha s bee n adde d 1 0 cc . o f Igepol ™ CO610 , a dispersin g agen t (als o availabl e from Dow Chemica l Co. ) an d 2 0 gra m o f an y dy e o t be adde d (I t s i quit e common o t identif y variou s phospho r color s n i specifi c lamp s b y addin g a solubl e organi c dye , whic h remain s n i th e pain t unti l lehrin g s i don e to bakeou t th e organic s n i th e paint) , o t for m th e fina l compositio n o f the premix . i. To th e 1 0 liter s o f premi x s i adde d 7.0 0 kilogram s o f phosphor , 10 0 gm. of Alon-C , an d 10. 0 ml . o f Igepo l CO610 . Thi s suspensio n s i stirre d and the n rolle d fo r abou t 9 0 minutes . Followin g this , th e suspensio n s i the n read y fo r us e a s a paint . ii . n I anothe r variatio n o f thi s procedure , Carboxymethylcellulos e s i substitute d fo r th e Hydroxypropylmethylcellulos e use d n i th e pain t makin g process .

363

C. Coatin g o f Lamps The objectiv e n i th e coatin g operatio n s i o t obtai n a unifor m interna l coatin g of th e pain t containin g th e phospho r particle s b y "squirting" a predetermine d volume o f pain t ont o th e inne r surfac e o f th e verticl e glas s tub e an d allowin g t i to ru n down th e sides . t Is i essentia l tha t th e coatin g b e unifor m fro m th e to p to th e botto m o f th e glas s tubing . Also , th e "densit y o f coating " need s o t b e unifor m fro m en d o t en d o f th e finishe d lamp . t I s i th e densit y o f coating , or the

"powde r weight " whic h determine s th e fina l qualit y o f th e lam p a s t i

operate s throughou t it s rate d lifetim e o f operation . Most manufacturer s striv e to obtai n a powde r weigh t fo r Coo l Whit e Halophosphat e phospho r betwee n 6.0 0 o t 8.0 0 milligram s o f powde r pe r squar e centimete r o f interna l glas s 2

surface , wit h 6. 8 o t 7. 5 m g / c m bein g th e average . This , o f course , depend s upon th e crysta l densit y o f th e phospho r bein g used , a s note d elsewher e (e.g. see "Manufactur e o f Cd5Cl(P04 )3 :Mn Phosphor " n i th e nex t chapter) . The coatin g chambe r consist s o f a continuou s movin g rac k n i whic h th e clean , vertica l glas s tube s move throug h th e chamber . One o f th e firs t position s heat s the tubin g o t abou t 75-8 0 ° F b y us e o f a heate d ai r strea m (abou t 100 0 cfm . fo r a 9 6 inc h tubing) ,a t a controlle d humidit y inde x leve l s o a s o t dry , o r kee p dry , th e interna l surfac e o f th e glass . A dehumidifie r s i generall y use d n i thi s room. At th e nex t position , th e pain t s i squirte d a t th e exac t to p o f th e tubin g and s i allowe d o t ru n down th e side s o f th e tubin g s o a s o t completel y cove r the insid e o f th e tubin g wit h th e paint . The exces s drip s fro m th e en d o f th e tub e an d s i recapture d an d sen t bac k o t th e pain t position . One o f th e bigges t problem s encountere d s i th e productio n o f "thin-ends " a t th e botto m o f th e tubing , especiall y f i 9 6 inc h tubin g s i bein g coated . Many manufacturer s hav e solve d thi s proble m b y "top-coating " usin g a specifi c volum e o f pain t whic h run s downward s an d als o "bottom-coating " b y squirtin g a smalle r volum e o f pain t upward s fro m th e botto m s o tha t th e tw o volume s meet . Durin g coating , it s i essentia l o t kee p th e temperatur e o f th e lacque r constan t (eve n thoug h t i is bein g stirre d constantl y an d recirculate d fro m dripping s o f th e tubes ) s o a s to obtai n reproducibl e coatings . The nex t positio n s i calle d a "cold-set " positio n sinc e th e ai r temperatur e s i lowere d o t abou t 6 0 ° F s o a s o t sto p mos t o f th e pain t flo w a t thi s point . At thi s point , th e coate d tube s exi t th e coatin g chambe r wher e the y procee d o t th e Lehr .

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D . Lehrin g o f Lamps The coated-bulb s nex t procee d o t th e entranc e o f th e Leh r (whic h s i nothin g more tha n a lo w temperatur e furnac e equippe d o t tak e glas s tubing . The tube s rotat e aroun d o t a horizonta l positio n jus t befor e the y ente r th e Lehr , an d ar e caugh t o n eac h en d b y a serie s o f roller s whic h suppor t an d spi n th e glas s tube s a s the y pas s throug h th e Lehr . One end-rolle r ha s a n air-je t positione d whic h blow s ai r a t a se t velocit y throug h th e tub e a s th e organi c binde r get s hot enoug h o t bur n out , leavin g behin d jus t a n adheren t coatin g o f phosphor . A t th e fron t en d o f th e Lehr , th e tube s ar e kep t apar t a s the y pas s int o th e interio r o f th e Lehr . The maximu m temperatur e tha t th e coate d glas s tube s reac h s i abou t 65 0 °C. , a temperatur e hig h enoug h o t caus e th e glas s o t sa g f i the y wer e no t rotatin g continuously . In som e Leh r designs , th e tube s ar e allowe d o t roll , caugh t by thei r ends , down a sligh t inclin e buil t withi n th e lehr , kep t apar t b y mean s o f asbesto s o r othe r cerami c flexibl e strip s s o tha t th e tube s d o no t touc h eac h othe r (i f the y did happe n o t touc h on e another , "microcracks " woul d develo p n i th e glas s wall o f th e tub e an d th e tub e woul d likel y brea k somewher e furthe r n i th e process) . The air-je t velocit y start s a t abou t 60 0 cfm . an d advance s o t abou t 2000 cfm . a t th e poin t wher e th e burnin g proces s s i mos t vigorous . n I th e cas e wher e roller s ar e use d n i th e Lehr , th e air-jet s move wit h th e tubes , wherea s n i th e rollin g tub e version , th e air-jet s ar e stationary . The lehrin g operatio n cause s complet e "burn-out " o f th e organi c binde r from the phospho r whe n th e insid e temperatur e reache s abou t 60 0 °C . I t s i importan t tha t n o carbo n particle s ar e lef t behin d durin g burn-out . I t s i fo r thi s reaso n tha t th e air-je t velocit y s i varie d o t remov e th e combustio n product s fro m th e tubes . The "smoke " an d fume s mus t be directe d o t a n air scrubbe r o t protec t operatin g personnel . Fo r a complet e burn-out , th e temperatur e profil e insid e th e Leh r mus t be carefull y monitore d an d controlled . Thi s s i usuall y accomplishe d b y a set-poin t recorde r whic h continuousl y monitor s temperatur e throug h a serie s o f thermocouple s strategicall y place d withi n th e Lehr . Once th e coate d bulb s reac h th e en d o f th e Lehr , the y exi t an d ar e allowe d o t cool whil e movin g o n a carrier-bel t o t th e nex t ste p o f th e process .

365

Ε. Sealin g Machin e Operatio n Once th e coate d bulb s exi t th e Lehr , air-jet s ar e playe d acros s th e oute r surface s o t coo l th e glas s bulbs . The bulb s ar e picke d u p b y a serie s o f roller s whil e stil l n i a horizonta l positio n an d fe d int o a n "end-sealing " machine . At the firs t statio n o f thi s high-spee d machine , tw o brushes , on e o n eac h end , are use d o t remov e exces s phospho r abou t 1/ 4 inc h fro m th e en d s o tha t th e ends ca n b e seale d o t for m th e basi c lam p (I ts i importan t o t us e a vacuu m o t pic k u p an y exces s powde r tha t result s fro m thi s operatio n sinc e th e exces s may caus e damag e o t th e phospho r coatin g durin g subsequen t operation s f i lef t insid e o f th e lamp) . Onc e thi s s i done , th e bulb s ca n hav e a ste m attache d to eac h end ,o t for m th e basi c lamp . t Is i importan t o t hav e th e vacuu m syste m to pic k u p th e powde r remove d b y brushin g sinc e many fina l reject s ca n resul tf i thi s operatio n s i no t don e properly . 1. Ste m Manufactur e Fluorescen t lam p stem s ar e comprise d o f a flar e wit h a pressed-ste m seede d within , an d contai n a coiled-coi l filamen t ver y simila r o t tha t use d n i incandescen t lamps . Indeed , thes e stem s ar e made n i a lik e manner . The procedur e s i a s follows : a. A piec e o f glas s tubin g s i flare d o n one-end . The flar e mus t be jus t slightl y large r tha n th e en d o f th e glas s tubin g use d o t for m the fluorescen t lamp . b. t I s i the n positione d wit h th e flare d en d u p an d tw o lead-i n wires , a s fe d throug h placement-cylinders , ar e aligne d insid e th e flared-glas s an d come o t res tn i positionin g guides . c. A piec e o f exhaus t tubin g (abou t 3/1 6 inc h diameter ) s i als o place d insid e th e flare d glas s an d force d o t a sto p a t abou t on e hal f of it s length . Thi s s i don e fo r onl y on e o f th e stem s use d n i th e fluorescen t lamp , i.e. - fo r 1/ 2 o f th e stems . d. Filamen t suppor t wire s wit h properl y place d "Dumet " connector s ar e the n inserte d int o thi s assembl y whic h s i the n heate d o t softe n th e glass . Thereupon , clamp s move n i an d for m a

366

fla t soli d sectio n calle d th e "press " whic h no w support s an d lead-i n wire s seale d int o th e glass .

contain s th e

e. Jus t befor e th e "press " solidifies , an d a puf f o f ai r throug h th e exhaus t tub e provide s a hol e n i th e "press " throug h whic h th e lamp ca n b e evacuate d o f ai r a t a late r stag e o f manufactur e o f th e fluorescen t lamp . f. The to p end s o f th e tw o molybdenu m lead-i n wire s ar e shape d and separate d a t a distanc e o f abou t a millimete r les s tha n th e lengt h o f th e tungste n filamen t coil . g. The coi l s i lifte d int o positio n jus t a s th e glass-lea d wir e assembl y arrive s opposit e o t t i an d th e coi l s i mechanicall y crimpe d o t th e tw o lead-i n wire s a t eac h en d o f th e filament . The coil s wil l hav e alread y bee n coate d wit h a n emissio n mixtur e o f carbonates . h. To coa t th e filaments , t i s i necessar y o t make u p a lacque r containin g th e emissio n mixture . The mos t usua l electrod e coatin g compositio n is : 45.60 % B a C 03 9.025 % C a C 03 40.375 % SrCC b 5.0 0 % Zr0 2 The lacque r s i made u p b y mixin g 5. 0 gallon s o f amyl acetat e wit h 2. 0 gallon s o f ethy l acetat e an d 1. 0 gallo n o f a nitrocellulos e binder , S-166 7 whic h contain s 5.7 % solid s (availabl e fromRaff i & Swanson) . Thi s mixtur e s i stirre d togethe r an d filtere d o t remov e any ge l particles . To thi s volum e s i adde d abou t 16. 0 lb s o f th e above mixtur e o f carbonates , an d th e mi z s i mille d fo r 2 4 hour s a t abou t 17 0 rpm . n i a 1 0 gallo n mil l containin g 1/ 2 inc h flin t pebbles .

367

i. The emissio n mixtur e lacque r s i the n adjuste d wit h amyl acetat e to a viscosit y an d specfi c gravit y between : 20-3 0 centipoise s an d 1.70 0 o t 1.78 0 Sp.G . dependin g upo n th e typ e o f fluorescent lam p bein g manufactured . 2. Ste m Mountin g Tw o version s o f th e complete d ste m assembl y ar e show n n i th e followin g diagram . One ste m s i use d a s th e exhaus t por t whil e th e othe r form s a sea l a t th e othe r en d o f th e tube , viz 3.6.2. - Ste m Design s use d fo r Mountin g o t For m Fluorescen t Lamps Stems forMountin g Coate d Filamen t

End of Fluorescen t Tub e

Jus ta s n i incandescen t lam p manufacture , th e end s o f th e coate d tube s (now cleane d o f powde r lef t fro m th e coatin g operation ) ar e heate d unti l the glas s s i softene d (abou t 65 0 °C. ) Stem s ar e fe d int o th e sealin g machin e from eac h end , on e havin g a n exhaus t tubin g alread y attached , and th e othe r without . Eac h ste m s i belt-mounte d an d auto-indexe d o t co-incid e wit h th e en d o f eac h coate d tub e a s t i progresse s throug h th e stem-sealin g machine . The flare o f eac h ste m come s int o contac t wit h the tub e en d whil e th e tub e s i bein g heate d firs t b y a flame . The n th e flar e s i heate d (sinc e t i ha s thinne r wall s tha n th e tubing) . As bot h become ho t an d hav e softened , the y ar e presse d togethe r b y machin e actio n o t for m th e seale d glas s joint . The flame-sealing operatio n s i the n

368

adjuste d o t a lowe r temperatur e s o a s o t giv e a n annealin g effec t o t th e glas s joint . Thi s operatio n s i critica l sinc e "leakers " ca n resul t f i th e glass-sea l s i no t perfect . The seal s ar e the n allowe d o t come o t roo m temperatur e whil e eac h lam p proceed s towar d th e Exhaus t machine , meanwhil e bein g rotate d a s th e seal s cool . F. Exhaus t Machin e Operatio n A t thi s poin t th e coate d tubin g no w begin s o t loo k lik e a fluorescen t lamp . At the exhaus t machine , eac h lam p s i exhauste d down o t a 0.0 5 micro n vacuum . The lam p s i the n refille d wit h argo n o r nitroge n ga s o t atmospheri c pressur e and thi s cycl e s i usuall y repeate d a t leas t twice . On th e thir d pump-down , th e lamp enter s a n ove n se t a t 350-40 0 ° C wher e degassin g o f th e lam p occurs . Anothe r pumpdow n the n occur s an d mercur y s i introduce d int o th e lamp . The amoun t s i usuall y abou t 70. 0 milligram s fo r a 40T1 2 lam p an d varie s mor e or less , dependin g upo n th e lam p size . The lam p the n exit s th e ove n an d s i subjecte d o t a n increasin g serie s o f voltages , applie d throug h th e electrica l leads . Thes e voltage s star ta t 0.5 0 volt s AC an d ar e increase d b y 1. 5 vol t stage s so a s o t brea k down th e emissio n mixtur e whe n th e filamen t become s heate d above 120 0 °C. All o f thes e step s tak e plac e a s th e lam p progresse s fro m stag e to stage . Nea r th e en d o f th e exhaus t cycle , th e lam p s i agai n fille d wit h argo n to atmospheri c pressure . Al l o f thes e evacuation s tak e plac e throug h th e exhaus t par t o f th e stem . Finally , th e interna l pressur e o f th e lam p s i adjuste d to abou t 2.3-2. 5 mm. o f argon , befor e th e exhaus t tub e s i tippe d of fb y heatin g in a flame . The lamp s no w procee d throug h a basin g machin e cycle . G . Basin g an d Seasonin g Machin e Operatio n This fina l par t o f th e operatio n s i ver y simila r o t tha t o f th e incandescen t lam p basin g operation . 1. Base s ar e fille d automaticall y wit h a thermosettin g cemen t o n a circula r turret . The cement , viscou s n i form , s i force d unde r pressur e throug h a n orific e tha t deposit s t i aroun d th e inne r peripher y o f th e bas e jus t belo w th e star t o f th e threade d section . 2. The basin g ree l consist s o f a larg e circula r rotatabl e turre t wit h an automati c vertica l positionin g mechanis m equall y space d

369

aroun d th e circumference . Directl y abov e thes e s i a mechanis m for holdin g th e base-bul b assembl y n i a horizonta l position . 3. The threadin g o f on e lea d wir e throug h on e pi n o f th e bas e an d the othe r aroun d th e othe r pi n s i nex t accomplishe d b y th e machin e operation . 4. Flame s ar e positione d s o a s o t provid e varyin g degree s o f heatin g a s th e lam p rotate s s o a s o t cur e th e cement . 5. The lam p indexe s circumferentiall y s o tha t severa l sequentia l operation s occur . Thes e consis t o f cuttin g of f exces s o f th e lea d wires , an d solderin g or weldin g th e lea d wire s o t th e sid e o f th e bas e b y applicatio n o f solde r o t th e joint . Once thi s s i done , th e lamp s procee d o t a "starting " turre t wher e the y procee d aroun d th e turre t whil e a voltag e s i applie d o t star t th e lamp . Thi s usuall y require s a n overvoltag e o t d o s o o n th e firs t start . The lam p operate s for abou t 6 0 seconds , s i cu t off , an d the n restarted . Afte r thi s cycl e repeat s itsel f abou t thre e times , th e lam p s i allowe d o t operat e abou t 5 minute s befor e it s i stopped , finally , th e lam p proceed s o t a storag e are a wher e t i s i boxe d fo r shipment . Ther e ar e severa l cause s fo r rejectio n of th e finishe d lamps . Thes e include : "Non-starters " "Leakers " Coatin g Defect s Breakag e du e o t handlin g afte r manufactur e Sometimes , jam-up s ca n occu r n i th e Factory-Lin e an d thi s to o cause s breakag e an d los s o f production . Qualit y check s involv e bot h visua l an d photometri c evaluation . To d o th e latter , on e require s a photometri c spher e in whic h th e lam p s i place d an d th e tota l luminou s outpu t s i directl y measure d afte r 1. 0 hou r o f operation . Thi s s i calle d "zero - hour " lumens . The tes t lam p s i the n operate d fo r 10 0 hour s longe r o t obtai n a n outpu t n i lumens . Thi s luminosit y s i generall y abou t 95 % o f th e initia l zero-hou r value .

370

Η . Type s o f Fluorescen t Lamps Manufacture d The lis t o f fluorescen t lamp s tha t hav e bee n manufacture d s i lon g indeed . We wil l delineat e som e o f thes e n i orde r o t separat e the m int o lam p type s an d thei r electrica l operatin g circuits . Suc h lamp s include : Prehea t lamp s Rapi d star t lamp s Instan t star t lamp s High outpu t lamp s Very hig h outpu t lamp s The difference s n i thes e lamp s lie s primaril y n i th e electrica l contact s an d the powe r use d n i th e lamp . The followin g diagram , give n a s 3.6.3 . o n th e nex t page , show s som e o f th e electrica l connection s o f ballast s use d fo r variou s type s o f fluorescen t lamps . Note tha t th e differenc e lie s n i ho w th e ballas t s i connecte d o t th e powe r Line , a s wel l a s th e numbe r o f individua l choke s an d coils . In th e Prehea t Start , th e filament s ar e heate d o t ioniz e th e mercur y an d the n the voltag e s i switche d acros s th e lam p o t star t th e lam p an d it s attenden t mercur y vapo r arc-stream . In a Rapi d Star t lamp , th e electrode s ar e als o preheate d o t star t th e arc , an d two contact s a t eac h en d ar e require d fo r th e heatin g circuit . However , ther e is n o switc h an d a smal l heatin g curren t flow s throug h th e elctrode s whil e th e lamp s i operating . Instan t Star t lamp s ar e starte d directl y b y applicatio n o f sufficien t high voltag e o t strik e th e ar c withou t an y preheatin g o f th e electrodes . Thus , Instar t Star t lamp s requir e onl y on e contac t o n eac h en d o t star t an d operat e the lam p (se e 3.6.3 . o n th e nex t page) .

371

3.6.3. Circui t forPrehea t Fluorescen t Lamps

IE Line

51

Lam p Bimetalli c

Ballas t

/ΠΠΜΙ

Starte r Condense r

Circui t forRapi d Star t Fluorescen t Lamps ! Lam p Lam p

Circui t forInstan t Star t Fluorescen t Lamps Lam p

Lam p

Line

1 High Outpu t (H O & VHO) lamp s involv e a specia l electrod e desig n s o tha t more powe r ca n be use d o t operat e th e lamp . Wherea s th e ordinar y 96T1 2 lamp use s 97. 7 watt s (6 3 VAC an d 1.55 0 amps.) , th e 96T12H O lam p use s 12 0 watt s (15 0 VAC an d 0.80 0 amps ) . The 96T12VH O lam p use s 25 8 watt s (17 2 VA C an d 1.50 0 amps) . Thi s s i achieve d by us e of th e followin g typ e of filamen t mount (tw o o t a lamp) , viz -

372

Electrod e Desig n forVHO Lamps

Note tha t a "coolin g chamber " s i presen t behin d th e filamen t electrode . Thi s prevent s th e mercur y vapo r pressur e fro m becomin g to o hig h durin g operatio n a t thes e hig h wattages . Also , a specia l mixtur e of rar e gase s s i use d to provid e lon g lif e fo r th e electrode s an d o t giv e goo d luminou s output . A compariso n of thes e lamp s s i show n n i th e following : Lamp Typ e

Wattag e 97. 7

5,56 0

96T12 HO

120. 0

8,50 0

96T12 VHO

258. 0

15,00 0

96T12

_

Rate d Lumen s

_

These value s ar e typica l of thes e type s of lamps . Most o f th e VHO typ e of lamp s areuse d fo r industria l lightin g or n i Supermarket s an d th e like . 40T1 2 lamp s ar e use d n i Restaurant s an d privat e homes. n I general , no VHO varietie s ar e made n i th e 40 wat t rang e of fluorescen t lamp . REFERENCE S CITE D L A P Valentin e an d D Hull , J.LessCommon Metals, 1 7 353-36 1 (1969) . 2. R.A. Swalin , Privat e Communicatio n (1967) . 3. CE Ell s an d W Evans , "Th e Effec t of Temperatur e Durin g Irradiatio n o n th e Behavio r o f Heliu m o n Beryllium" , AECL Report#898,Chal k River , Ontario , Canada (Oct. - 1959) . 4. CE Ells , ActaMet 1 1 87-9 6 (1963) .

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5. C Agf e & J Vacek , Tungsten and Molybdenum,NASA Transl . - Washington , D C (1963) . 6. P.E . Wretblad , Powder Metallury, Publ. - Amer. Soc . MetalsJ Wulf f -Ed. , p. 433 (1942) . 7. Publishe d n i CeramicSource'8 6 Vol . 1 p.303 - compile d by th e Amer. Cer . S o c, Inc . (1986 ) 8. R C Ropp , "Luminescence and the SolidState", Elsevie r Sci . Publ. , New York & Amsterdam , (Apri l 1991 ) 9. MA Aia , USP 3,113,83 5 (1963) . 10. "X-ray Powde r Diffractio n Pattern s fo r Some Cadmiu m Phosphates" , by RC. Ropp, R.W. Moone y an d C.W.W. Hoffman , Ana L Chem.,33 , 124 0 (1961) . 11. WL Wanmaker , AB Hoekstra , an d MG Tak , Philips Res.Rpts. 1 0 1 1 (1955) . 12. "Phospho r Industr y an d X-ra y Diffractio n Analysis " by C.W.W. Hoffma n an d R. C. Ropp , Encyclopedia of X-Raysand Gamma Rays - Ed. by H. Clark, Reinhol d Publ . Co. , New Yor k (1963) .