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Vacuum distillation of metals I. Theory
VACUUM Classified A b s t r a c t s
I I I --
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v a l v e - p a r t s , t h e single p a r t s of t h e s y s t e m u n d e r g o a p r e - d e g a s s i n g process before a s s e m b l y . T h i s process cons i s t s of h e a t i n g t h e p a r t s in a n o v e n u n d e r h i g h v a c u u m or in a n a t m o s p h e r e of H 2. H 2 d e g a s s i n g is cheaper, q u i c k e r a n d p r o t e c t s p a r t s f r o m a i r - p e n e t r a t i o n d u r i n g storage. I t is widely u s e d for m a s s p r o d u c t i o n of r a d i o valves. However, v a c u u m - d e g a s s i n g is essential in m a n y cases, w h e n t h e presence of h y d r o g e n or of w a t e r or o x y g e n c o n t a m i n a n t s is undesirable. I n s o m e cases no p r e - d e g a s s i n g is desirable, e.g. in t h e case of w i r e - c a t h o d e s a n d cold stressed p a r t s of t h e valve. H i g h v a c u u m d e g a s s i n g is carried o u t a t ca. 10 -3 m m . H g a n d u p to 1,100°C. T h e f u r n a c e s are s u c h t h a t t h e whole process c a n be carried o u t w i t h i n 24 hours. R o t a r y ~)il p u m p s are u s u a l l y sufficient for t h e e v a c u a t i o n . F o r c o m p l e t e degassing, or for g r a p h i t e a n d t a n t a l u m , h i g h e r t e m p e r a t u r e s of 1,800-2,400°C are required. T h e d i s a d v a n t a g e of m o s t f u r n a c e s is t h e t i m e of t h e d e g a s s i n g process. T h i s is u s u a l l y a m i n i m u m of 20 h o u r s , of w h i c h 15 h o u r s m a y be cooling-down t i m e . A q u i c k process u s e d in E n g l i s h v a l v e factories, r e d u c e s t h i s t i m e b y u s i n g t r a n s p o r t a b l e q u a r t z t u b e s w h i c h are inserted i n t o t h e oven for u p to 2 hours, t h e n r e m o v e d a n d cooled in a n hour. Radiation-cooled h i g h - f r e q u e n c y f u r n a c e s achieve s h o r t e r d e g a s s i n g t i m e s for s m a l l q u a n t i t i e s of metal. P r e - d e g a s s i n g b y h e a t i n g in a h y d r o g e n c u r r e n t is t h e m o r e c o n v e n i e n t process a n d is m o r e suitable for c o n t i n u o u s m a n u f a c t u r e . H y d r o g e n f u r n a c e s w o r k i n g a t over 1,800°C are in use. T h e p u r i t y of t h e h y d r o g e n is of g r e a t i m p o r t a n c e . H y d r o g e n d e g a s s i n g is n o t to b e u s e d w i t h m e t a l s w h i c h deteriorate m a r k e d l y , e.g. Ta, Cu, Ti, Zr, etc. All pre-degassed p a r t s , w h e t h e r h y d r o g e n or v a c u u m - t r e a t e d , require careful h a n d l i n g , s t o r a g e a n d a s s e m b l y . H a n d l i n g w i t h tweezers or s u l p h u r - f r e e r u b b e r gloves is essential. Storage c a n be in bags, nitrogen-filled jars, e v a c u a t e d desiccators sealed t u b e s or b e s t of all, e v a c u a t e d b o t t l i n g jars.
M e t h o d s and Technique of Degassing Metals II. Czechoslovakia. A n i m p o r t a n t m e t h o d of o v e r c o m i n g t h e deleterious effect of gases c o n t a i n e d in m e t a l s used i n v a c u u m t u b e s is t h a t of d e g a s s i n g d u r i n g t h e p u m p i n g process. T h i s is n e c e s s i t a t e d b y t h e h i g h o p e r a t i n g t e m p e r a t u r e s of s o m e electrode parts, b y c o n t a m i n a t i o n d u r i n g a s s e m b l y , a n d b y t h e cost of h i g h - t e m p e r a t u r e p r e - d e g a s s i n g furnaces. T h e t e m p e r a t u r e a t w h i c h t h e electrode s y s t e m m a y be degassed is limited b y m e t a l v a p o u r deposition a n d b y m e c h a n i c a l stability. T e m p e r a t u r e control is therefore critical a n d is m o s t l y a c h i e v e d p y r o m e t r i c a l l y . F o u r different m e t h o d s of d e g a s s i n g d u r i n g t h e p u m p i n g process are available, n a m e l y (a) f u r n a c e heating, (b) resistance heating, (c) h i g h f r e q u e n c y h e a t i n g a n d (d) electron b o m b a r d m e n t . T h e s e m a y be c o m b i n e d in v a r i o u s ways. T h e e n e r g y s u p p l y n e c e s s a r y in t h e last t h r e e m e t h o d s will d e p e n d u p o n t h e electrode c o n s t a n t s . E x t e r n a l h e a r i n g d u r i n g p u m p i n g is carried o u t b y electric or g a s - h e a t e d b o x furnaces, or in a n a u t o m a t i c process, b y h e a t i n g t u n n e l s . T h e m e t a l p a r t s are less effectively degassed t h a n t h e glass surfaces. Noticeable d e g a s s i n g of m e t a l p a r t s n o t pre-degassed, in c o n t a c t w i t h glass surfaces, t a k e s place, h o w e v e r . E x t e r n a l l y accessible m e t a l p a r t s m a y be h e a t e d b y s e p a r a t e h e a t i n g coils. R e d u c t i o n of oxidised surfaces, e.g. Cu, in c o n t a c t w i t h glass, m a y be achieved b y p u m p i n g o u t at 550°C. T h e u s e of h a r d glass a n d a n e v a c u a t e d f u r n a c e avoids collapse of t h e tubes. D e g a s s i n g of filaments, c a t h o d e s etc. m a y be carried o u t before d e g a s s i n g of o t h e r p a r t s b y direct p a s s a g e of current. D e g a s s i n g b y h i g h - f r e q u e n c y h e a t i n g m a k e s use of t h e h e a t i n g of t h e electrodes b y t h e field of a n e x t e r n a l coil or winding. F r e q u e n c i e s u p to 8 × 105 c/s are u s e d . T h e source of field e n e r g y is chosen h a v i n g regard to t h e n a t u r e of t h e electrode, e.g. dimensions, specific r e s i s t a n c e a n d emissivity. Rise in gas pressure m a y be q u i t e rapid w i t h b.f. heating. S h o r t d e g a s s i n g t i m e s m a y be achieved b y ' s h o c k ' h e a t i n g , w h i c h also s a f e g u a r d s glass walls from softening. I n s o m e cases t h e h i g h t e m p e r a t u r e ' s h o c k ' h e a t i n g of specific p a r t s is undesirable, since t h e released gas m a y react or be a b s o r b e d b y cooler parts, as e.g. t a n t a l u m . A n indication of excessive g a s congestion is g i v e n b y t h e i n t e n s i t y of t h e glow discharge, w h i c h m u s t n e v e r be allowed to c h a n g e into a n arc discharge. M a x i m u m h.f. d e g a s s i n g t e m p e r a t u r e s are a r o u n d 850°C. H i g h f r e q u e n c y h e a t i n g of electrode s y s t e m s w i t h m e t a l envelopes is n o t possible. I n v a c u u m t u b e s w i t h a sufficiently s t r o n g e l e c t r o n - e m i t t i n g source, d e g a s s i n g b y electron b o m b a r d m e n t is feasible. A p o t e n t i a l is a p p l i e d to t h e electrode w h i c h is positive w i t h respect to t h e electron source. T h e electron c u r r e n t e n e r g y heats a n d degasses t h e electrode. T h e m e t h o d is v e r y efficient since it a p p r o a c h e s t h e conditions of t h e o p e r a t i n g state. Inaccessible electrode p a r t s c a n be h e a t e d in this way. B o m b a r d m e n t d e g a s s i n g is also u s e d for p r e - d e g a s s i n g s y s t e m s w i t h large m a s s e s of h i g h m . p . m e t a l s (e.g. W, Ta, Mo). T h i s is carried o u t in e v a c u a t e d m e t a l envelopes a n d t h u s t e m p e r a t u r e s u p to 2,500°C m a y be employed. Moreover, faults m a y be recognised before t h e final f u s i n g into t h e glass envelope. E l e c t r o n b o m b a r d m e n t in t h e r m i o n i c t u b e s m u s t be carried o u t below a certain pressure, otherwise d a m a g e b y ion b o m b a r d m e n t or arc f o r m a t i o n m a y result. C u r r e n t - l i m i t i n g resistances in t h e form of t u n g s t e n f i l a m e n t l a m p c o m b i n a t i o n s are therefore included in t h e discharge circuit. B o m b a r d m e n t d e g a s s i n g is i n d i s p e n s a b l e for larger t h e r m i o n i c tubes, a n d w h e r e oxide c a t h o d e s are e m p l o y e d t h e m e t h o d serves to a c t i v a t e t h e c a t h o d e layer. I t is no longer u s u a l to d e g a s t h e electrodes of s m a l l e r o x i d e - c a t h o d e tubes, b y electron b o m b a r d m e n t , a n d a n y gas liberated after sealing off m u s t be a b s o r b e d b y t h e getter. V a c u u m Distillation of Metals I. T h e o r y United Kingdom. T h e distillation of m e t a l proceeds at a rate d e p e n d e n t on its v a p o u r pressure, t h e pressure of t h e residual gas a n d t h e c o n c e n t r a t i o n g r a d i e n t of t h e e v a p o r a t e d m e t a l a t o m s b e t w e e n t h e surface of t h e evapor a t i n g liquid a n d t h e condenser. D u e to t h e low t e m p e r a t u r e of t h e c o n d e n s e r r e - e v a p o r a t i o n is limited a n d t h i s r e s u l t s in a p r e s s u r e g r a d i e n t in t h a t space w h i c h d e t e r m i n e s t h e n e t distillation rate. W h e r e t h e residual g a s p r e s s u r e is so low t h a t o n l y relatively few collisions occur b e t w e e n t h e v a p o u r a t o m s a n d t h e g a s molecules t h e conditions are k n o w n as m o l e c u l a r distillation. However, n o r m a l l y distillations of m e t a l are carried o u t in a p r e s s u r e r a n g e s o m e w h a t a b o v e t h a t of t h e m o l e c u l a r range. T h e a d v a n t a g e s of v a c u u m versus a t m o s p h e r i c e v a p o r a t i o n are: (a) t h e n e t e v a p o r a t i o n rate is increased; (b) a b e t t e r s e p a r a t i o n of volatile c o m p o n e n t s is achieved; (c) c o n t a m i n a t i o n b y o x i d a t i o n is r e d u c e d a n d (d) r e d u c t i o n processes i n v o l v i n g a volatile phase can b e o p e r a t e d a t lower t e m p e r a t u r e s . T h e a u t h o r proceeds to discuss v a p o u r p r e s s u r e a n d associated w i t h it the
October, 1956
Vacuum Vol. VI
Abstract No. anO Referenoo8
Article by W. Espe Vakuum. Tech. 3, May 1956 39-53 122/III
Article by W. Espe Vakuum- Tech. June 1956 69-82 123/III
283
VACUUM Classified A b s t r a c t s Almtraet No.
at:d References
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C l a u s i u s - C l a p e y r o n e q u a t i o n . R a o u h ' s law a nd H e n r v ' s l a w . If in at b i n a r y m i x t u r e one c o m p o n e n t present in s m a l l a m o u n t s e x e r t s a higher pressure t h a n would be p r e d i c t e d by R a oul {' s law t h e n t he pressure of t he o t h e r c o m p o n e n t will also be higher. This m a y re s ul t in the f o r m a t i o n ¢;f a a z e o t r o p i c m i x t u r e , i.e. a m i x t u r e h a v i n g a m i n i m u m b o i l i n g point. Such m i x t u r e s can ne ve r be s e p a r a t e d c o m p l e t e l y but as the c o m p o s i t i o n of an a z e o t r o p e g e n e r a l l y a l t e r s with v a r i a t i o n in pre s s ure an a l t e r a t i o n of t he p r e s s u r e - t e m p e r a t u r e c o n d i t i o n s m a y m a k e s e p a r a t i o n possible, l : n d e r m o l e c u l a r d i s t i l l a t i o n c o n d i t i o n s a z e o t r o p e s c a n n o t occur. It followb a discussion of the t e r m ' R e l a t i v e v o l a t i l i t y ' . In a tattle r e p r o d u c e d below t he v a p o u r pressure ra t i os e x i s t i n g l ' a p o u r P r e s s u r e Nalios I'olatility Ratio, .l[etal X / A l u m i n i u m at 81111"(' (A.*mtmintg Raoult's Law1
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b e t w e e n v a r i o u s m e t a l s at ~ 0 0 C are shown, t a k i n g t he v.p. of a l u n f i n i u n l as uni t y. N ~ r m a l l v t he c o u d e n s e ! t e m p e r a t u r e is below t h a t of the m e l t i n g p o i n t of t h e d i s t i l l a t e . B ut t h e r e arc cases where p a r t i a l c o n d e n s a t i o n is a p p l i e d in order to assist s e p a r a t i o n of two or more c o m p o n e n t s . Thi s is e'ttected by k e e p i n g t h e condenser t e m p e r a t u r e at t h e e v a p o r a t i o n t e m p e r a t u r e of t he more v o l a t i l e c o m p o m ' n t t h u s l )r~ mot i ng re-evaporati~,n of the same. while the other is being d e p o s i t e d on the condenser.
124/III
V a c u u m Distillation of Metals II. Practical
Artieh" by" A. J. Martin Metal Industr.
88, 8.6.1956
Article by A. J. Martin Metal lndutttr. 15.6.1956 495-498
125/III
U n i t e d K i n g d o m . V a c u u m d i s t i l l a t i o n m a y l)e d i v i d e d i nt o t hre e processes: (a) I~ectuctious. (b) P uri t i c a t i ons. to: Miscellaneous A p p l i c a t i o n s . These are c(msidered in turn. (a) H i gh costs h a v e l i m i t e d c o m m e r c i a l a p p l i c a t i o n of v a c u u m in m a n y processes b u t v a c u u m a s s i s t e d e v a p o r a t i o n of zinc in a e ( m t i n u o u s process has been used for m a n 5" y e a r s and r e d u c t i o n of m a g n e s i u m by silicon u n d e r v a c u u m by t h e l-'idgeon process e n a b l e s t he r e d u c t i o n t e m p e r a t u r e to be lowered from 2,300°(; to 1,100'C a nd di re c t reducti(m of d o l o m i t i c ()re can be c a rri e d out. T h e a r t i c l e i l l u s t r a t e s the t y p e of r e t o r t in use for t hi s o p e r a t i o n . 75°o of t he m a g n e s i u m can t h e n be collected p r o v i d e d t h a t the c o l l e c t i n g t e m p e r a t u r e is sufficiently high. A s i m i l a r m e t h o d can be used for p r o d n c i n ~ l i t h i u m from s p o d u m e n e ore, a large excess of lime being a d d e d d u r i n g t he process. ,90°'~, of t he t o t a l l i t h i u m c o n t e n t is t h e n collected a t a t e m p e r a t u r e of ISWC. B a r i u m has been p r o d u c e d [)y t he M a t i g n o n t e c h n i q u e . Fine b a r i u m o x i d e and a l u m i n i u m p o w d e r s are c o m p r e s s e d into pellets. R e d u c t i o n occurs a t 600~C a nd 0.5 ram. t t g p r e s s u r e . C a l c i u m can he p r o d u c e d from its o x i d e bv v a c u u m r e d u c t i o n w i t h a l n m i n i u m . (b) V a c u u m d i s t i l l a t i o n is used more f r e q u e n t l y for p u r i f i c a t i o n p u r p o s e s t h a n for m e t a l p r o d u c t i o n . The process is onh" w o r t h w h i l e where a t l e a s t one of the folh)wing a d v a n t a g e s accrue: (1) C o m p l e t e s e p a r a t i o n of t w o c o m p o n e n t s w i t h o u t f u r t h e r processing, (2) S e p a r a t i o n of i m p u r i t i e s from a h i g h l y e l e c t r o n e g a t i v e m a t e r i a l wtfich would o t h e r w i s e r e q u i r e e x p e n s i v e refining t e c h n i q u e s . (3) P r e v e n t i o n of o x i d a t i o n . (4) P r e v e n t i o n of m e t a l losses. (1) a p p l i e s to t h e process, d e v e l o p e d bv B roke n Hill Associated Smelters, of r e c o v e r i n g 9 0 % of t he Zn from Pb. (2) was the m a i n reason d u r i n g the las t war, for p u r i f i c a t i o n of a l u m i n i u m s c ra p b y d i s t i l l a t i o n . The m e r c u r y process a n d t h e Beck process were used a n d a re d e s c r i b e d in detail, (3) Is t he reason for t he r e f i n e m e n t ot t i t a n i u m , zirconium, h a f n i u m a n d t a n t a l u m by t he Kroll process, a n d (4) a p p l i e s to the re c ove ry of s i l ve r from zinc, lead, silver c r u s t s etfected in the l ' a r k e s process.
High V a c u u m Distillation Apparatus See A b s t r a c t No.: 134 I I I
126,'III
The E v a p o r a t i o n of Impurities from Silicon See A b s t r a c t No.: 136.'I
127;III
I n v e s t m e n t Casting of V a c u u m Alloys U n i t e d 5;tales. l ) e v e l o p m e n t s t o w a r d s i n v e s t l n e n t c a s t i n g of vacuum-n~elted a l l o y s ~m a e~,mmereial scale are described. I n v e s t m e n t c a s t i n g is the o n l y m a j o r c a s t m g m e t h o d w hi c h can be used w i t h o u t basic: revisions f(~r v a c u u m - c a s t i n g iron, a n d nickel a n d c o b a l t alloys. The a l l oy m a y be p r e p a r e d a n d cast in one v a c u u m uni t or a p r e v i o u s l y p r e p a r e d a l l o y m a y be r e - m e l t e d a nd poure d e i t h e r in an i ne rt gas or u n d e r v a c u u m . The inert-~a~ m e t h ~ l can s o m e t i m e s fw used with e x i s t i n ~ e q u i p m e n t a n d can ~ixe high p n M u c t i o n rates. \ \ ' i t h high-toni-
2,'g4
Vacuum Vr~l I "[
October, 19,56
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v a l v e - p a r t s , t h e single p a r t s of t h e s y s t e m u n d e r g o a p r e - d e g a s s i n g process before a s s e m b l y . T h i s process cons i s t s of h e a t i n g t h e p a r t s in a n o v e n u n d e r h i g h v a c u u m or in a n a t m o s p h e r e of H 2. H 2 d e g a s s i n g is cheaper, q u i c k e r a n d p r o t e c t s p a r t s f r o m a i r - p e n e t r a t i o n d u r i n g storage. I t is widely u s e d for m a s s p r o d u c t i o n of r a d i o valves. However, v a c u u m - d e g a s s i n g is essential in m a n y cases, w h e n t h e presence of h y d r o g e n or of w a t e r or o x y g e n c o n t a m i n a n t s is undesirable. I n s o m e cases no p r e - d e g a s s i n g is desirable, e.g. in t h e case of w i r e - c a t h o d e s a n d cold stressed p a r t s of t h e valve. H i g h v a c u u m d e g a s s i n g is carried o u t a t ca. 10 -3 m m . H g a n d u p to 1,100°C. T h e f u r n a c e s are s u c h t h a t t h e whole process c a n be carried o u t w i t h i n 24 hours. R o t a r y ~)il p u m p s are u s u a l l y sufficient for t h e e v a c u a t i o n . F o r c o m p l e t e degassing, or for g r a p h i t e a n d t a n t a l u m , h i g h e r t e m p e r a t u r e s of 1,800-2,400°C are required. T h e d i s a d v a n t a g e of m o s t f u r n a c e s is t h e t i m e of t h e d e g a s s i n g process. T h i s is u s u a l l y a m i n i m u m of 20 h o u r s , of w h i c h 15 h o u r s m a y be cooling-down t i m e . A q u i c k process u s e d in E n g l i s h v a l v e factories, r e d u c e s t h i s t i m e b y u s i n g t r a n s p o r t a b l e q u a r t z t u b e s w h i c h are inserted i n t o t h e oven for u p to 2 hours, t h e n r e m o v e d a n d cooled in a n hour. Radiation-cooled h i g h - f r e q u e n c y f u r n a c e s achieve s h o r t e r d e g a s s i n g t i m e s for s m a l l q u a n t i t i e s of metal. P r e - d e g a s s i n g b y h e a t i n g in a h y d r o g e n c u r r e n t is t h e m o r e c o n v e n i e n t process a n d is m o r e suitable for c o n t i n u o u s m a n u f a c t u r e . H y d r o g e n f u r n a c e s w o r k i n g a t over 1,800°C are in use. T h e p u r i t y of t h e h y d r o g e n is of g r e a t i m p o r t a n c e . H y d r o g e n d e g a s s i n g is n o t to b e u s e d w i t h m e t a l s w h i c h deteriorate m a r k e d l y , e.g. Ta, Cu, Ti, Zr, etc. All pre-degassed p a r t s , w h e t h e r h y d r o g e n or v a c u u m - t r e a t e d , require careful h a n d l i n g , s t o r a g e a n d a s s e m b l y . H a n d l i n g w i t h tweezers or s u l p h u r - f r e e r u b b e r gloves is essential. Storage c a n be in bags, nitrogen-filled jars, e v a c u a t e d desiccators sealed t u b e s or b e s t of all, e v a c u a t e d b o t t l i n g jars.
M e t h o d s and Technique of Degassing Metals II. Czechoslovakia. A n i m p o r t a n t m e t h o d of o v e r c o m i n g t h e deleterious effect of gases c o n t a i n e d in m e t a l s used i n v a c u u m t u b e s is t h a t of d e g a s s i n g d u r i n g t h e p u m p i n g process. T h i s is n e c e s s i t a t e d b y t h e h i g h o p e r a t i n g t e m p e r a t u r e s of s o m e electrode parts, b y c o n t a m i n a t i o n d u r i n g a s s e m b l y , a n d b y t h e cost of h i g h - t e m p e r a t u r e p r e - d e g a s s i n g furnaces. T h e t e m p e r a t u r e a t w h i c h t h e electrode s y s t e m m a y be degassed is limited b y m e t a l v a p o u r deposition a n d b y m e c h a n i c a l stability. T e m p e r a t u r e control is therefore critical a n d is m o s t l y a c h i e v e d p y r o m e t r i c a l l y . F o u r different m e t h o d s of d e g a s s i n g d u r i n g t h e p u m p i n g process are available, n a m e l y (a) f u r n a c e heating, (b) resistance heating, (c) h i g h f r e q u e n c y h e a t i n g a n d (d) electron b o m b a r d m e n t . T h e s e m a y be c o m b i n e d in v a r i o u s ways. T h e e n e r g y s u p p l y n e c e s s a r y in t h e last t h r e e m e t h o d s will d e p e n d u p o n t h e electrode c o n s t a n t s . E x t e r n a l h e a r i n g d u r i n g p u m p i n g is carried o u t b y electric or g a s - h e a t e d b o x furnaces, or in a n a u t o m a t i c process, b y h e a t i n g t u n n e l s . T h e m e t a l p a r t s are less effectively degassed t h a n t h e glass surfaces. Noticeable d e g a s s i n g of m e t a l p a r t s n o t pre-degassed, in c o n t a c t w i t h glass surfaces, t a k e s place, h o w e v e r . E x t e r n a l l y accessible m e t a l p a r t s m a y be h e a t e d b y s e p a r a t e h e a t i n g coils. R e d u c t i o n of oxidised surfaces, e.g. Cu, in c o n t a c t w i t h glass, m a y be achieved b y p u m p i n g o u t at 550°C. T h e u s e of h a r d glass a n d a n e v a c u a t e d f u r n a c e avoids collapse of t h e tubes. D e g a s s i n g of filaments, c a t h o d e s etc. m a y be carried o u t before d e g a s s i n g of o t h e r p a r t s b y direct p a s s a g e of current. D e g a s s i n g b y h i g h - f r e q u e n c y h e a t i n g m a k e s use of t h e h e a t i n g of t h e electrodes b y t h e field of a n e x t e r n a l coil or winding. F r e q u e n c i e s u p to 8 × 105 c/s are u s e d . T h e source of field e n e r g y is chosen h a v i n g regard to t h e n a t u r e of t h e electrode, e.g. dimensions, specific r e s i s t a n c e a n d emissivity. Rise in gas pressure m a y be q u i t e rapid w i t h b.f. heating. S h o r t d e g a s s i n g t i m e s m a y be achieved b y ' s h o c k ' h e a t i n g , w h i c h also s a f e g u a r d s glass walls from softening. I n s o m e cases t h e h i g h t e m p e r a t u r e ' s h o c k ' h e a t i n g of specific p a r t s is undesirable, since t h e released gas m a y react or be a b s o r b e d b y cooler parts, as e.g. t a n t a l u m . A n indication of excessive g a s congestion is g i v e n b y t h e i n t e n s i t y of t h e glow discharge, w h i c h m u s t n e v e r be allowed to c h a n g e into a n arc discharge. M a x i m u m h.f. d e g a s s i n g t e m p e r a t u r e s are a r o u n d 850°C. H i g h f r e q u e n c y h e a t i n g of electrode s y s t e m s w i t h m e t a l envelopes is n o t possible. I n v a c u u m t u b e s w i t h a sufficiently s t r o n g e l e c t r o n - e m i t t i n g source, d e g a s s i n g b y electron b o m b a r d m e n t is feasible. A p o t e n t i a l is a p p l i e d to t h e electrode w h i c h is positive w i t h respect to t h e electron source. T h e electron c u r r e n t e n e r g y heats a n d degasses t h e electrode. T h e m e t h o d is v e r y efficient since it a p p r o a c h e s t h e conditions of t h e o p e r a t i n g state. Inaccessible electrode p a r t s c a n be h e a t e d in this way. B o m b a r d m e n t d e g a s s i n g is also u s e d for p r e - d e g a s s i n g s y s t e m s w i t h large m a s s e s of h i g h m . p . m e t a l s (e.g. W, Ta, Mo). T h i s is carried o u t in e v a c u a t e d m e t a l envelopes a n d t h u s t e m p e r a t u r e s u p to 2,500°C m a y be employed. Moreover, faults m a y be recognised before t h e final f u s i n g into t h e glass envelope. E l e c t r o n b o m b a r d m e n t in t h e r m i o n i c t u b e s m u s t be carried o u t below a certain pressure, otherwise d a m a g e b y ion b o m b a r d m e n t or arc f o r m a t i o n m a y result. C u r r e n t - l i m i t i n g resistances in t h e form of t u n g s t e n f i l a m e n t l a m p c o m b i n a t i o n s are therefore included in t h e discharge circuit. B o m b a r d m e n t d e g a s s i n g is i n d i s p e n s a b l e for larger t h e r m i o n i c tubes, a n d w h e r e oxide c a t h o d e s are e m p l o y e d t h e m e t h o d serves to a c t i v a t e t h e c a t h o d e layer. I t is no longer u s u a l to d e g a s t h e electrodes of s m a l l e r o x i d e - c a t h o d e tubes, b y electron b o m b a r d m e n t , a n d a n y gas liberated after sealing off m u s t be a b s o r b e d b y t h e getter. V a c u u m Distillation of Metals I. T h e o r y United Kingdom. T h e distillation of m e t a l proceeds at a rate d e p e n d e n t on its v a p o u r pressure, t h e pressure of t h e residual gas a n d t h e c o n c e n t r a t i o n g r a d i e n t of t h e e v a p o r a t e d m e t a l a t o m s b e t w e e n t h e surface of t h e evapor a t i n g liquid a n d t h e condenser. D u e to t h e low t e m p e r a t u r e of t h e c o n d e n s e r r e - e v a p o r a t i o n is limited a n d t h i s r e s u l t s in a p r e s s u r e g r a d i e n t in t h a t space w h i c h d e t e r m i n e s t h e n e t distillation rate. W h e r e t h e residual g a s p r e s s u r e is so low t h a t o n l y relatively few collisions occur b e t w e e n t h e v a p o u r a t o m s a n d t h e g a s molecules t h e conditions are k n o w n as m o l e c u l a r distillation. However, n o r m a l l y distillations of m e t a l are carried o u t in a p r e s s u r e r a n g e s o m e w h a t a b o v e t h a t of t h e m o l e c u l a r range. T h e a d v a n t a g e s of v a c u u m versus a t m o s p h e r i c e v a p o r a t i o n are: (a) t h e n e t e v a p o r a t i o n rate is increased; (b) a b e t t e r s e p a r a t i o n of volatile c o m p o n e n t s is achieved; (c) c o n t a m i n a t i o n b y o x i d a t i o n is r e d u c e d a n d (d) r e d u c t i o n processes i n v o l v i n g a volatile phase can b e o p e r a t e d a t lower t e m p e r a t u r e s . T h e a u t h o r proceeds to discuss v a p o u r p r e s s u r e a n d associated w i t h it the
October, 1956
Vacuum Vol. VI
Abstract No. anO Referenoo8
Article by W. Espe Vakuum. Tech. 3, May 1956 39-53 122/III
Article by W. Espe Vakuum- Tech. June 1956 69-82 123/III
283
VACUUM Classified A b s t r a c t s Almtraet No.
at:d References
III
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Vacuum
Processing Contd.
Techniques
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llI
C l a u s i u s - C l a p e y r o n e q u a t i o n . R a o u h ' s law a nd H e n r v ' s l a w . If in at b i n a r y m i x t u r e one c o m p o n e n t present in s m a l l a m o u n t s e x e r t s a higher pressure t h a n would be p r e d i c t e d by R a oul {' s law t h e n t he pressure of t he o t h e r c o m p o n e n t will also be higher. This m a y re s ul t in the f o r m a t i o n ¢;f a a z e o t r o p i c m i x t u r e , i.e. a m i x t u r e h a v i n g a m i n i m u m b o i l i n g point. Such m i x t u r e s can ne ve r be s e p a r a t e d c o m p l e t e l y but as the c o m p o s i t i o n of an a z e o t r o p e g e n e r a l l y a l t e r s with v a r i a t i o n in pre s s ure an a l t e r a t i o n of t he p r e s s u r e - t e m p e r a t u r e c o n d i t i o n s m a y m a k e s e p a r a t i o n possible, l : n d e r m o l e c u l a r d i s t i l l a t i o n c o n d i t i o n s a z e o t r o p e s c a n n o t occur. It followb a discussion of the t e r m ' R e l a t i v e v o l a t i l i t y ' . In a tattle r e p r o d u c e d below t he v a p o u r pressure ra t i os e x i s t i n g l ' a p o u r P r e s s u r e Nalios I'olatility Ratio, .l[etal X / A l u m i n i u m at 81111"(' (A.*mtmintg Raoult's Law1
Hg: Na: Mg:
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473-477
b e t w e e n v a r i o u s m e t a l s at ~ 0 0 C are shown, t a k i n g t he v.p. of a l u n f i n i u n l as uni t y. N ~ r m a l l v t he c o u d e n s e ! t e m p e r a t u r e is below t h a t of the m e l t i n g p o i n t of t h e d i s t i l l a t e . B ut t h e r e arc cases where p a r t i a l c o n d e n s a t i o n is a p p l i e d in order to assist s e p a r a t i o n of two or more c o m p o n e n t s . Thi s is e'ttected by k e e p i n g t h e condenser t e m p e r a t u r e at t h e e v a p o r a t i o n t e m p e r a t u r e of t he more v o l a t i l e c o m p o m ' n t t h u s l )r~ mot i ng re-evaporati~,n of the same. while the other is being d e p o s i t e d on the condenser.
124/III
V a c u u m Distillation of Metals II. Practical
Artieh" by" A. J. Martin Metal Industr.
88, 8.6.1956
Article by A. J. Martin Metal lndutttr. 15.6.1956 495-498
125/III
U n i t e d K i n g d o m . V a c u u m d i s t i l l a t i o n m a y l)e d i v i d e d i nt o t hre e processes: (a) I~ectuctious. (b) P uri t i c a t i ons. to: Miscellaneous A p p l i c a t i o n s . These are c(msidered in turn. (a) H i gh costs h a v e l i m i t e d c o m m e r c i a l a p p l i c a t i o n of v a c u u m in m a n y processes b u t v a c u u m a s s i s t e d e v a p o r a t i o n of zinc in a e ( m t i n u o u s process has been used for m a n 5" y e a r s and r e d u c t i o n of m a g n e s i u m by silicon u n d e r v a c u u m by t h e l-'idgeon process e n a b l e s t he r e d u c t i o n t e m p e r a t u r e to be lowered from 2,300°(; to 1,100'C a nd di re c t reducti(m of d o l o m i t i c ()re can be c a rri e d out. T h e a r t i c l e i l l u s t r a t e s the t y p e of r e t o r t in use for t hi s o p e r a t i o n . 75°o of t he m a g n e s i u m can t h e n be collected p r o v i d e d t h a t the c o l l e c t i n g t e m p e r a t u r e is sufficiently high. A s i m i l a r m e t h o d can be used for p r o d n c i n ~ l i t h i u m from s p o d u m e n e ore, a large excess of lime being a d d e d d u r i n g t he process. ,90°'~, of t he t o t a l l i t h i u m c o n t e n t is t h e n collected a t a t e m p e r a t u r e of ISWC. B a r i u m has been p r o d u c e d [)y t he M a t i g n o n t e c h n i q u e . Fine b a r i u m o x i d e and a l u m i n i u m p o w d e r s are c o m p r e s s e d into pellets. R e d u c t i o n occurs a t 600~C a nd 0.5 ram. t t g p r e s s u r e . C a l c i u m can he p r o d u c e d from its o x i d e bv v a c u u m r e d u c t i o n w i t h a l n m i n i u m . (b) V a c u u m d i s t i l l a t i o n is used more f r e q u e n t l y for p u r i f i c a t i o n p u r p o s e s t h a n for m e t a l p r o d u c t i o n . The process is onh" w o r t h w h i l e where a t l e a s t one of the folh)wing a d v a n t a g e s accrue: (1) C o m p l e t e s e p a r a t i o n of t w o c o m p o n e n t s w i t h o u t f u r t h e r processing, (2) S e p a r a t i o n of i m p u r i t i e s from a h i g h l y e l e c t r o n e g a t i v e m a t e r i a l wtfich would o t h e r w i s e r e q u i r e e x p e n s i v e refining t e c h n i q u e s . (3) P r e v e n t i o n of o x i d a t i o n . (4) P r e v e n t i o n of m e t a l losses. (1) a p p l i e s to t h e process, d e v e l o p e d bv B roke n Hill Associated Smelters, of r e c o v e r i n g 9 0 % of t he Zn from Pb. (2) was the m a i n reason d u r i n g the las t war, for p u r i f i c a t i o n of a l u m i n i u m s c ra p b y d i s t i l l a t i o n . The m e r c u r y process a n d t h e Beck process were used a n d a re d e s c r i b e d in detail, (3) Is t he reason for t he r e f i n e m e n t ot t i t a n i u m , zirconium, h a f n i u m a n d t a n t a l u m by t he Kroll process, a n d (4) a p p l i e s to the re c ove ry of s i l ve r from zinc, lead, silver c r u s t s etfected in the l ' a r k e s process.
High V a c u u m Distillation Apparatus See A b s t r a c t No.: 134 I I I
126,'III
The E v a p o r a t i o n of Impurities from Silicon See A b s t r a c t No.: 136.'I
127;III
I n v e s t m e n t Casting of V a c u u m Alloys U n i t e d 5;tales. l ) e v e l o p m e n t s t o w a r d s i n v e s t l n e n t c a s t i n g of vacuum-n~elted a l l o y s ~m a e~,mmereial scale are described. I n v e s t m e n t c a s t i n g is the o n l y m a j o r c a s t m g m e t h o d w hi c h can be used w i t h o u t basic: revisions f(~r v a c u u m - c a s t i n g iron, a n d nickel a n d c o b a l t alloys. The a l l oy m a y be p r e p a r e d a n d cast in one v a c u u m uni t or a p r e v i o u s l y p r e p a r e d a l l o y m a y be r e - m e l t e d a nd poure d e i t h e r in an i ne rt gas or u n d e r v a c u u m . The inert-~a~ m e t h ~ l can s o m e t i m e s fw used with e x i s t i n ~ e q u i p m e n t a n d can ~ixe high p n M u c t i o n rates. \ \ ' i t h high-toni-
2,'g4
Vacuum Vr~l I "[
October, 19,56