Superconducting oxide formation in the YbBaCuO system

PHYSICA

Physica C 190 ( 1992} 581-596 No~h-Holland

Superconducting oxide formation in the Yb-Ba-Cu-O system A. O t t o , R. K o n t r a a n d J.B. V a n d e r S o n d e De vartment o f3later:a]~ Sc~ence and Eng~neer~ng. III~.55~tCliUJg.7"~¢ l ~ f ~ i u w o.f Techno]o~:. C'ambr~dg~. MA 10213~. US.{

Receaved 10 Augas'* ~991 Revised mamzscript received 14 October 1991

Condilions for forming YbBa,Cu40~. Yb,BaaCu;O,. ,sand "~"~BzrCu~O-_a ~*'ere&lerrnmeO m e'~.~gcna: l .Oaim i~ressureb? experiments ~ . ~ oxidized Yb--Ba~Cu-Ag altoys. Yb_~3a:Cu~O-_ ~formed m Jess titan 10 s wbe.n~sx~d~zz-dotto> ribbons *~.erebaked in lhe g 0 5 : C - 8 9 0 ' C range_ Large amounls of Ba,('u~Os f~rmed benny, g7,0 C. Yb2galCu,Or, ~,.and YbBa,Cu~O, then for*ned in the 10:s-a0 ~s range a= lempera~ares m ~he 8~¢ C-87~1 C and <840"C ranges, respect-:vek~.Ba:Cu=O~decomposmcm supphed (-u:" and t): for the ~p~d ~,'an~qnrmalion~f ~bBazCu~O- _,~grains~nlo Ybzga~CuvO~r~gram~ ~ i.merca!azmp. In~efcatartar ~as m~ch slo~'e:rm the absence ofBa:Cu~O5. AI~ Of Lhese ~ranfformation ~oduets under.vent coarsenm-g~ l h i~ereeging bake lime. Rapid YbBazCt~O~and "~o:Ba~C~-¢),~_~ formation_ cnmp~rett to fhe ~1o~'formation of Lhcse~,r~a~ ~g~eswhen szN ~'as subsIituled ~D5' Y, Is d~e Io lhe d~ffenng effects ~f 3' and " ~ o~ ferrnaIion k:mel,~'s Supercond~c',ing ox;de-si~er cornlm~e ribbons e.ommning rrzain~yYbBa:Ct~sO~_,. ~r~zga~Cu.O~:_~or ~%BarCu~O~can be ~rcpared (ram Yb~Ea-C a-Ag Mloy~,

1- Introduction T h e t-2-4 { n = I ) a n d 2-4-7 ( . = 2 1 phases { I - 8 ] in the R.Ba>~Cu3.+~O, series [2] were discovered by several groups f o I 1 o ~ n g investigations o f aperiodically placed double C u - O planes in t h e 1-2-3 I n = m ) structure with R = Y ( 3 - 5 . 9 . 1 0 ] a n d R = Y b | 2 . 6 . 7 ] . Early work with R - Y s~ggesled ~hat bulk -2-4 a n d 2-4-7 formed only at high oxygen pressures [ 7.11 ]. However. a n aI!o3 oxidation m e t h o d with R = Y b yielded b o t h 1-2-4 a n d 2-4-7 at a m b i e n t pressure t 5 . 6 ] . After further work i 12.13]. a proposed P - T - X phase diagram displayed the 1-2-4 s~ability field with R = Y b d o w a b o u t 840 =C for a wide range o f oxygen pressures including i_0 a t m a n d the 2 4 - 7 s~abilit3" field a b o v e a b o u t 10 arm oxygen pressure and 8 4 0 : C . with t-2-3 forming a b o v e 8 4 0 : C a n d below I0 a l m oxygen pressure [ t 4 . t 5 ] . Ionic d o p a n t s were a d d e d to increase f o r m a t i o n kinetics with R = Y a n d procedures were developed to m a k e ,I-2-4 at a m b i e n t pressure [ 16-18 ]. Ca substituted for Y in small a m o u n t s allowed m o r e rapid 1-2-4 a n d 2-4-7 f o r m a t i o n with slightly higher T~s ( t r a n s i t i o n t e m p e r a t u r e s ) t h a n the Tcs o f the p a r e phases l ! 9~21 ]. More recently, the f o r m a t i o n o f 1-2-4 at < 8 4 0 : C

a n d 2-4-7 in the 8 4 0 : C - 8 7 0 : C rar~ge ~vit~ R = Y at 1.0 a i m oxygen pressure has been reported { 22]. placing the 2-4-7:1--9-3 stabiliIy line it, the P - T - K diagram for R = Y { ~4] below L 0 arm oxygen pre~sure for the 840"-C-870~'C range. T h e smaller rare earth ions form I-2-4 a n d 24-.7 more readily a1 lower pressure [23.24 ]. SmaIler ions may' produce a chemical equivalent o f the physical pressure forming I-2-4 a n d 2-¢-7 with R = Y . T h e Yb 3+ ion is small a m o n g rare earths {25] a n d 2-47 f o r m a t i o n with R = Y b al 1.0 arm oxygen pressure via the oxidized alloy mute confirms that the 2-4-7: 12-3 stabitiD~ line for R = Y b is below 1.0 arm over a sig~ificarfl t e m p e r a t u r e range. However. the relation bev*,een ion si~e a n d 2 4 - 7 f o r m a t i o n may also arise from kinetic effects such as diffusiviues that d e p e n d o n ion size and atomic mass. o r nucleation on interfaces between the different phases formed for each rare earth, tn this regard. ~he chemistries o f Y b a n d Eu differ from other i a n t h a n ides in that the free energies o f forming Eu a n d Yb trivalem c o m p o u n d s are more positive {26.27]. R~Ba_..Cus.+ ~O~ nucleation a n d g r o ~ h musl occ u r in heterogeneous samples consisting o f nonsuperconducting oxides. However. in samptes containing 1-2-3 and o t h e r oxide phases, several 1-2-4 a n d

0921-4534/921505.00 © 1992 Elsevier Science Publishers B.V. All rig!its reserved.

582

A_ Otto et aL / Superconduoing oxMe fonnanon

2-4-7 formation mechanisms are possible in addition to nucleation and growth due to the structural sim~ ilarity between the superconducting oxides, in one mechanism, 1-2-3 is converted into 1-2-4 and 2-4-7 by the formation of additional C u - O layers beside some or all single C u - O layers [28]. This intercalation process begins in a I-2-3 grain by the nucleation of a double C u - O layer near the grain's edges in the a and b directions (the a - b perimeter) and its rapid growth through the grain via Cu -'÷ and O-'diffusion along the dislocation at the inner edge of the expanding Cu--O layer. The c directions of each grain fom,ed and its !-2-3 parent grain are then parat!el and the grains produced are the same size in the a and b directions as the parent grains at any stage of the transformation. Phases that remain in samples with overall 1-2-3 compositions despite long bakes (i.e. R:BaCuO~ and BaCuO~) are less likely to decompose and supply Cu-* and O ~- than phases 1hat are not generally found in samples. in the second mechanism. 1-2~ and 2-4-7 grains formed by intercalation grow beyond the parent 1-23 grain surface~ increasing the amount of superconducting oxides..MI elements diffuse to the expanding beundaD" from phases such as CuO. BaCuO~ and R:BaCuO~. !-2-4 and 2-4-7 may also d e c o m p o ~ via the reverse of each mechanism. However. decomposition is intrinsically faster than heterogeneous formation reactions that are limited by diffusion over dis~nc~-~3 set by the precursor microstructure. If phase formation and decomposition are understood, then specific structures can be formed for improved properties [29-31 ]. This paper describes experiments investigating the formation of I-2-4. 2-47 and I-2-3 compounds in the Yb..Ba~Cu_~... ~0. seties in an oxidized Y b - B a - C u - A g allo_v with an element ratio o f 1 : 2: 3:3 as well as the transformations of t-2-3 into 1-2-4 and 24-7, and 1-2-4 and 2-4-7 into !-2-3 [ !.21-

2, Experimental procednres Batches ( 1 g-2 g) of melt spun Y b B a z C u 3 A g 3 ribbon pieces and one meR spun YBazCu3Agt5 batch were oxidized slowly in a tube furnace by heating them at 0.01 K / s from 140=C to 420=C in 99.95%+

flowing oxygen at 1.0 arm pressure. Each batch was quenched by pulling it into the cooled reaction tube end. Thermogravimetric studies [31 ] showed that fully oxidized ribbons formed, consisting of fine grained ( < 0.5 lam ) elemental and binary oxides, silver metal and some 1 lain-4 lam Yb~O~ nodules. These oxidized ribbons retained their original shapes. A batch of YbBa2Cu~Ags alloy was also rapidly oxidized by heating it at 0.2 K / s until it ignited at about 175=C. the start of significant oxidation, and cornbusted with temperatures exceeding 1200°C. A long rod of fused, fine grained oxides resulted. Portions o f the slowly oxidized Yb alloy weighing -~0.t 5 g were then heated rapidly in a small thermal mass crucible by pulling the crucible into the preheated furnace. Each portion was quenched after baking in pure oxygen at one atmosphere pressure at one of seven temperatures in lhe 800~C-900"~C range for one o f seven time intervals in the l0 ° s - l . 3 × 10 "~ s range as described in table I. Some experiments were repealed with the combuslively oxidized $'~ alloy and slowly oxidized Y alloy. The rumple temperature was measured with a thermocouple positioned inside the crucible. The time at temperature measurement commenced as the thermocouple temperature in fig. t went within 8 =C of the bake temperature. Samples for the I s experiments were quenched as their temperatures reached Table ! 8(J5 : C - $ 9 ~ : C r~nge

Alloy

Oxada:mn Nominal NernmMbake t~me~ mode lempera:ures al each lem]~erature :C} {s)

'fbB~_,Cu~Ag~~ 0.01 K/s 806 {~to~~ 82,~ 835 g47 859 871 gsg YbBa,Cu>Ag~ combu~ed859 {fa.~ ~ 'trBa2Cu3Agl~~ 0.0i K/s 835 (slo~"t

I~ 120 360 t .080 4.200 ~ 22.000 a 130.000 as above asabove

~*ICP analyz..~lalloycompositionswere Ybo.s~Bazo~Cu3~g-_7~ and Yo.s3BaL~2Ca3.~gl~.~. b~Heated and quenched~s d~-'ribed in fig. I.

583

.4. Otto et aL / Superconducting atid¢" fi~rreation

~-8oc

900

PULL SAMPLE O U T

800 t(T| = 0

700 ttt ne

m O.

600 HEAT-UP

tu I"-4

T(t) 500 COOL-DDW74

o u o ~e

T(t} 4OO

Z kin .J O.

300

\

200

100

P~LL SAMPLE .IN o

1oo

200

309

~oo

500

600

T I M E (s)

Fig. ~. Samplethermoztmglehea~i~gand cooling~urves.

the set bake temperature minus 8 :C. Temperatures for longer bakes were within 1 2 . 0 : C of the set temperalures. Pieces of each sample were mounted .in epoxy. ground, diamond polished and examined by scanning electron microscopy. Compositions of micro* structures of sufficient size ( > !.5 ptm ) were determined by microprobe analysis with a Jeo] 733 Superprobe. Pox~der X-ray diffractograms of sample portions were obtained with a Rigaku 300 diffractomeler. Phase volume fractions were calculated f~om ~he heights of selected peaks in the diffmctograms [5] and compared Io the microscopy results. Calculated

composile and experimental diffractograms were •cernpared to verify ~he accuracy of the phase volume fractions. Portions of slowly oxidized Yb alloyed ribbons were then healed rapidly te ene cf Tw~ high temperatures where only YbBazCu~O~_,~ formed, fop lowed by rapid cooling to and baking at one of two "~o:Ba4Cu~O,~_ ~ ¢ormiag ie~pc~a;~r~s a~ dc~.fibed in table 2. Oxidized r~bbon batches were also baked al temperatures that formed mainly Y'bBazCu~O~ and YbaBa~Cu,-O~_~ in ~the isolhermal experiments. Portions were heated rapidly above 875:C as described in table 3 to temperatures forming Yb-

584

A. Otto et aL / Superconductmg o_~"ideformation

Table 2 Matrix of c~ditions employing a high temperature bake followed by a Io~er temperature hake in the 855"C-890C range Alloy

Oxidation High-low tem0era~.ure Nominal bake mode combinations times at each ( ~C ) temperature pa~r (s)

YbBa:Cu3Ag, 0.01 K/s 878. 868 (slo'a) 878, 857 888. 868 888_ 857

80

120 420 720 4.200 2 i.Ol~ | 30.0o0

Ba~CusOv_a- These samples were anal3~ed by the previously described methods.

3. Results arm discuss'wns 3.1. Superconducting oxide formaeio~ in asoxidized adtol"s

The ,~-ariations o f cumulative v o l u m e percent phases formed ~ i t h respect to temperature over six t i m e ~.nten-a~s are presented in fig. 2 for the 0.01 K / s oxidized YbBa:Cuv~,g_~ alloy. The vertical distance bet-,~een neighboring lines at ans" temperature corresponds m the volume fraction o f the phase ide~tiffed by the pattern. The field width is the formation temperature range o f the phase at the specified bake time. T h e longest bakes in fig 2 yielded, o f the superconducting oxides, mainly !-2-4 below 84W-C, 2-4-

7 in the 840~C-870°C range and I-2-3 above 870:C, matching the temperatures at ~hich the same phases have been made with R = Y at t.0 atm [22]. In all cases, 1-2-3 was the superconducting phase formed firsl. That phase formed in seconds while the samples were heating to the set tempc,atmvs. By 6 rain, 1-2-4 and 2-4-7 were forming in substantial quantities. These kinetics greatly exceed the reported formation kinetics of the same phases with R = Y at the same pressure [22]. The variation of cumulative x~olume percent phases formed with respect to bake time at 835 "-C for 0.01 K / s oxidized YBazCu~Agt~ alloy in fig. 3 {a) shows that only BaCuOz. C u P and t-2-3 formed at bake times that formed mainly i-2-4 in the Yb system (835=C. fig. 2). Since !-2-4 and 2-4-7 can form with R = Y at 1 arm pressure [22]. the effect of Y and Yb on these phases" formation kinetics account for rapid I-2-4 and 2-4-7 formation in the oxidized Yb alloy and their lack o f formation in the oxidized Y alloy. The number o f nonsuperconducting phases and their quantities increased with decreasing temperatare in the 1 s bake ~ e l e . The eventual formation o f larger amounts o f superconducting oxides at the lower temperatures implies that superconducting oxide formation is thermally activated. However, the superconducting oxide contents had maxima in the 8 4 0 : C - 8 7 0 : C range at about 60 volume percent for longer bake times (fig. 2 ). The diminishing superconducting oxide content above 8 7 0 : C may be due to ;hermodynamic factors. BazCu~O~ formed below 871 =C in the I s bake ~'cle such that it ~-as the predominant phase below about 8 6 0 : C (fig_ 2). However_ the Ba~Cu~O, content decreased rapidly as 1-2-4 and 2 - ~ 7 formed. Other oxides p r e ~ n t in small amounts (YbzO~.

Table 3 Matrix of condidons emIfloyinga bake in the lo~er ~emcera!ure 2-¢-7 and 1-2-4 for~ir, g ranE-e~folloged b.~ a b~ghegler~pem~ure bake in the !-2-3 forming range Alloy

Ybl~.,CttsAg3

Oxida!mn mode

0.01 K/s (slow)

Lo~-high temperature bake comb/nations ( :C !

Nominal bake times of each temperalure pair (s) low

high

824. ~878, 888. 888) 835. (878. 888. 888 ) 847, (878. S88. 888)

30.000

(20. 120. 360)

,,1. Otto et al. / Superconducting oxide forty,orion

1 second

585

70 m t n u t ~

Q.

02 mtnu~es

i

8 ~murs

]

>o

40.

=1

8

minutes

311 h o u r s

4O

0 808

820

834

S4S

S82

8~8

808

820

834

848

882

8~6

RE~ic'rloN TEMPERATURE {oC) Fig. Z Varia,:~onsofc~mutalive volume per~m ~hases wi~h respect ~o temperature for the 0_01 K/s oxidized YbBazCu~.~3 alloy a~ six bake limes ~¢a~c~alaledfrom the X-ray diffrac~g:'arns L Yb~BzCuOs. YbBa~Cu3Og) also disappeared. The data at 835 =C a n d 847=C in fig. 2 is presented in fig. 4 as the v a r i m i o n o f phase volume percent with respect to bake t i m e io d~s~inguish the reactanl a n d

prodt~ct phases. Th_e Ba~Cu305 c o , t e n t decreases greal]y a n d the BaC~O2 c o n t e n t increases slightly as I-2-4 and 2~,-7 form~ The 1-2-3 content decreases stigkdy at 835 : C as 1-2-4 forms such that the total

A. O t t o

586

el aL

,: . ' h t l ~ e r : , m d u c t i n g

A 100" m ILl m

80"

-,.r n

60" 40-

ill

o rr a_

20" //

0.01 leds OXIDIZED / VBa2CuaAg~ s

0 B 100" 80"

>_ ,.,.,,I

BaCuO= v~a~CuOsy/~

40~ 20O,

(oo..,?o L

10 0

"

101

+

-

102

REACTION

~

- I

103

I

104

TIME

I0 s

(s)

Fig 3. Vanat,~o~ of cu~a-~L~ivevo~tm~ep~rc'~r p~_ses , ~ zesleet to baize time m fa~ 835-C f~r 00~ K/s o,ddfized YBazCu~a,gL~ alloy and (b} $59:C f~r c ~ m b u ~ e l y ox./d~z~-'~ YbBa,Cu~g~ atlo~-.

superconductingoxide comenl increases ~eatl_r. Both the i-2-3 and 1-2-4 contents increase to maxtma at the start of 2-4-7 formation at 847 :C. Howc-,*er. the 1-2-3 and I-2-4 contents then decreasegreatlv during 2-4-7 formation such that the total amount of superconducting oxide content stars constant. The Yb~BaCuOs c o n t e n t increases o r stays cons~Lantwhile the small Yb~Cu.O~, ~q~:O~ and YbBa4Cu~O~ contents decrease. The CuO content fluctuates about a mean that decreases as bake time increases. The phases formed at 859~C with the combustion oxidized YbBazCurAg~ alloy in fig. 3 (b) differ from the phases formed with the 0.01 K / s oxidized alloy at 859:C in fig, 2. Ba_,Cu30, and most other low

owde

fi~rmatiott

temperature phases did nol form. The 1-2-3. Yb203 and C u e contents were higher and the Yb~BaCuO~ content was lower after the I s bake cycle. As bake time increased, Yh,O~ content decreased and Yb2BaCuO, contem increased. 2-4-7 formed much later and !-2-4 did not form. The 1-2-3 content remained almost constant and 2-4-7 formed from the Yb20~. CuO and BaCuO2. decreasing their quantities. The delayed formation of 2-4-7 and lack of formation of 1-2-4 in the absence of BazCu30~ links this phase to the rapid 1-2-4 and 2-4-7 formation evident in fig. 2. The microstructures al some bake times for the 824:C. 847-C. 871:C and 888:C bake temperalures are presented in figs. 5-8. The microstructures coarsened with increasi.,ig bake time and lhe coarsening rates increased with temperatares. The oxide phases among the silver particles of the 824:C, 2 rain baked ~amplc in fig_ 5 were already p~atelike and I-z-_ ~as lhe p~edominant superconducting oxide formed ( fig. 2 }. By 70 rain. 1-2~ was ~he predominant superconducting oxide phase and microprobe composition analysis of sufficienlly large p_~ztes ( > 1.5 pm) in the 38 h sample yielded 1-2-4 (15 grains analyzed). The micrograph of the 38 h sample also displays a commonly seen rosette slructare ( upper left) consisting of many plalelets emanating from a node. This structure suggests multiple nacIeation at a point (or line exiending imp the micrograph ) |bllowed by growth outward along phase boundaries. The YbzO~ nodule evident in the micrograph of the i s. 847 :'C baked sample in fi~ 6 was formed during alloy oxidation 130]- A similar nodule in the t8 rain sample was shown by microprobe composition analysis to consist of a Yb_,O~ core and a YbzBaCuOs shell, demonstrating that ~q~,O~ nodules were converled rapidly to Yb2BaCuO~ by a shrinking core mechanism. C o n v e ~ o ~ of t h i s nodule could have been stopped after 18 rain by the quench, or sooner by the tack of Ba and Cu. Smaller Yb,O_~ particles can be converted in roach shorter times and Yb2BaCuO~ formation competes with superconducting oxide formation. 1-2-3 formation up to 6 rain in the 871 :C baked samples was followed by 2-4-7 formation (fig. 2). The micrographs of the 70 mino 6 h and 37 h samples in fig. 7 display both 2-4-7 (darker) and i-2-3

~. Otto et aL / Superconducting o_ride fi~rmatton

597

847 oC

835 oc Ba2Cu30 5

o BaCuO2 60.

40,

#

Cu30s

Ba2Cu305

~;z~

(

BaCu02

20-

(n ul ui

l

3: .r~

50-

IZ u] £3

40-

u3 o. l/J

I

I

¢

I

T

f

~

O 1-2-3 2-4-7 o 1-2-~

30" 20-

o

~ /\ ~,"'- ~--/~"-~

R

A

-i-2<

:E 4"1

o

1>

1

z

s

1

1

i

r"A

~

t

"r

0 YI~BaCuO~

CuO e Yb2Cu205

30'

(w°2Ba CuOs •

,

]

(Yb=lsaeuoz

f.'~b~c

,3 } 0

<3 C

10 -

?

Fcuo \

O'

• 100

~ t01

ir 102

¢=oo

~Vb-'---~.~Os •

i• ' 11 i 103 "]04

. 4 10 s 100

101

REACTION TIME

{s) "102

103

10 a

10 s

Fig. 4. Varz_a~ionsof volumepercen~phases~,ith respe~ Io bake time for the 835:C and 847 :C bakesin fig, 2. (lighter) plales. The 2-4-7 and 1-2-3 grains are similar in size at any time where 2-4-7 was forming. The orientations between a 2-4-7 grain and the I-2-3 grains on either side of the 24-7 grain (a sandwich structure ) or between a 2-4-7 grain and an adjacent 1-2-3 grain (a bic~,stal slructure ) are also frequently identical. Although orientation here refers to the shapes of the grains, these superconducting oxide g a i n s form plates with the c direction orthogonal to the large surfaces. The aligned shapes in fig. 7 there-

fore imp]y cr2;sta!lographic alignment with parallel ¢ directions. Both features are evidence for 1-2-3 transformazion into 2-4-7 by intercalation. The ] s sample o f t h e 888:C series in fig. 8 consisted of interconnected 1-2-3 platelets among sliver grains. ,&tier 2 rain the 1-2-3 plates are disconnected and scattered among BaCuO> suggesting t-2-3 decomposition. Alzhough d e 1-2-3 grains increased in size through ~o 70 rain. they remained separated by BaCuO,. After 6 h the t-2-3 g~ins increased in both

588

A. Otto et aL / Superconducting oxtdeformation

Fig. 5. Micrommcturesformed at g24~C in the 0_0t K/s oxidized YbBazC~Ag~alloy. The light structuresare mainlysilvec(a) and some "~orich oxides_The platesare supenoonductingoxides. YbBazCu~Os(b ) a~ 3~ h. The da~est slrucluregare CuO {c ~ and the next lighteslare barium-copperoxtde~suchas BaCuO.,(d) (backsc~tteredelectronimages)_ quantity and size. They also became more equiaxed and interconnected. The decrease in i~2-3 content up tc about 10-~s at 888=C followed by increases at longer bake times is also evident in fig. 2.

3-2. ) ~,:Ba,Cu~3r:_~ ~rmatW,, in samples p r c g ~ e d at ]'bBa:Cu~O~_a fertms~g temperatures The high-low temperature bake combinations described in table 2 suppressed 2-4-7 formation as s h o ~ by comlmring the 868~C and 857-~C lower temperature bake data in fig. 9 to the 871 "~C and 859"C dam in fig 2. The higher temperature prebake eliminated the formation of Ba;Cll~Os and other lower temperature ohases evident in fig. 2, Only the 1-2-3, Y'ozBaCuO~, BaCuO_, and CuO contents were significant 24-7 did not form at 86g~c following either preimke, e~,en though 868:C is in the 2-4-7 phase forming field in fig. 2. However. I-2-3 content increased ~4th bake time and YbzBaCuO~, CuO and BaCuO2 contents decreased. For the 857 ~C bakes, 24-7 formed later in the 888 ~'C prebaked samples than in the g78~C prebaked samples, implying that the absence of Ba2Cu~O~ and higher prebake temperat u r ~ impeded 2-4-7 formation. The interconnected I-2-3 plates in the 6 rain, 878°C/857°C sample o f f i g !0 display many rosette structures. At 6 h, 24.7/1-2-3 sandwich and bicD~shal structures similar to those in fig. 7 are also evident (upper left). Sample central regions enriched in CuO, BaCu02 and Yb2BaCuO~ with structures similar to those ofthe 6 h sample were also observed in

other samples. This microstructure may arise from the minor element segregation that can occur during oxidation [ 30 ]. The 888"C/857~C [lake after 2 rain in fig. !! yielded interconnected t-2-~ at an early stage ofdecemposilion similar io the structure in fig. 8 for the 88~. ~C sedes. Ho~'ever. large I-2-3 structures formed quickly at 857~C and the amount of I-2-3 increased. After 39 h the t~o micrographs in fig. l 1 i|lustrate 24-7 formation, extensive grain growth and the mi~ crostructural variation possible in a sample. Sandwich and bicrystal stractures between adjacent |-23 and 24-7 grains evident in the second 39 h sample also support the intercalation mechanism. However. conversion by intercalation occurred much slower than in samples containing BazCu~O~. } ~ b ~ * f t ~ ~ ~: l ~ and YbBa_,Cu¢O~ decomposition at YbBa:Cu X)~_:.formi~g lenlperatilres 3~ £

Both b2-4 and 2-4-7 decomposed_ rapidly (in seconds) at i-2-3 forming temperatures into mainly BaCuO_~, Yb2BaCuO~ and CuO (fig. t2) yielding small amoums of highly aspected I-2-3 grains (fig. 13). This trend is similar to the short time decrease in 1-2-3 content during the 888-~C isothermal bakes ( figs_ 2 and 8). The 1-2-3 content may increase again as at 888--C (fig. 2). The sample heat up through 24-7 forming temperatures may account for the superconducting oxide content increase at 20 s or, de-

A_ Otto eI aL /Superconducting oxide fi,rmation

589

composition al temperature may be preceded by rapid growth. 3.4. STnlering

Contacting ribbon pieces progressively sintered more as bake time and temperature increased above 835 ~C. Beyond 87t "C. the pieces .deformed readily in response t_o internal stresses and grayly,, with the deformation amount increasing as hake time increased. This suggests that dense parts can be made by pressing oxidized ribbons together ai low pressures during a brief superconducting .oxide forming bakt This sinlering may also be used to make ribbon4o-dhbon joints. 3.5. Reactions a n d mechani~_,~s

Reactions for the rapid 1-2-3 formation in as-oxidized alloys, the formation of 1-2-4 and 2-4-7 f~am the t-2-3 and other oxide phases are iisled betow: Yb ~ +2Ba :~ +3Cu:" + 6 . 5 0 -~+ ( 0 . 2 5 - 0.5~)O: -, YbBa,Cu ~O~_~,

( 1)

( u + l )CuO+ ~/~aCuO.z + ~ Yb, l~aC,,O~

\

2 ] O'-+n'~°Ba:cu~OT-e

Ba~_Cu_~O_~--,CuO+ 2BaCuO,.

(4)

zt--t -, Yb,,Ba,,~Cu~,~ ~O~ 7_o~+ ~+ 2BaCuO.,.

(5)

~.5. L YbBaeCu,.O~ ~ f o n n a t i o n in the as-oxidized dlojo

Fi~ 6. Microstracturesformed al $47~C in the 0.O! K/s oxidizedYbBaaCu:.gvalloy.Y'b_~Oa(a) reacts Io form Yb_.BaCuO~ (bL The matrix is superconductingoxide ~back.scalteredelectron images~.

Given that all the ions must be presem locally to :-orm superconducting oxides, and that as-oxidized altoys consisl of finely divided oxide phases, I-2-3 nucleates rapidly via reaction (1) on oxide triple junction lines as depicted in fig. 14 or on oxide june-

590

A. Otto et aL / Supemonducting oxideJbrma~ion

F g 7. Mk'tostractun~formed~llgTl °C in the 0.81 K/s oxidized YbBa:Ca~-~g_~a|loy. "fineligltt ~ia'ucturesare mainlysqver (a) a~d Yb2B~CuO~ gbL T ~ s u ~ u c d n g o x i d e is nminlyY'bBa:Cu~Or_., (c~ up to 70 nain~'i~hincreasingYb:Ba~C~O~s_~,*~~quantities

tip Io 37 h. CttO ~e Dand B,Ctd32 (f) are atso i ~ n t (hackscmterede'|ectmn~m2g~L lion planes where the abutting oxides contain all the elements. 1-2-3 forms at these junctions by short range diffusion. Both junction t~TJes can be interconnected like the films and film junction lines in soap foam. resulting in interconnected i-2-3 networks at low voiume fractions. Growth is along junctiQn planes and into the bulk phases (via for example reaction (2} for n = ~ } folloving nucleation. Rosette miccostructures then arise from 1-2-3 nucleation on junction lines followed by growth along the local phase boundaries and beyond such that the initial gro,~h geomet~, is preservecL Silver from small particles diffuses rapidly to Larger

particles resulting in the observed rapid coarsening. The angular silver particle shapes indicate that silver accommodated the g~owlh geometries and directions o f the superconducting oxide grains. 1-2-3 formation in oxidized alloys is therefore rapid because of the many nucleation sites in th~ finely divided, mixed phase microsm~cture and the associaled short diffusion distances. Oxidation in pure ox3fen is also likely to form boundaries free of contaminants that could impede superconducting oxide formation. The initial formation of I-2-3 at !_7-4 and 2-4-7 forming temperatures is probably due to the slower rates of 1-2-4 and 2--4-7 fon'nation.

A- Otlo et al. / Supemonducling oxide formation

59 t

Fig= 8. Microstruetures formed at ,$g~C in ihe 0_0t K t s oxidized Yb~zrCu~Ag.~ all~,_ The light 9.ructures are ~;ainly siJ~'er Ca ) and Yb~BaCuO, ~b ) The # a l e s are Y~ ~.a2Co~O?_~ (c)_ CuO t d ) z~d 8aCu02 ~e ~ are also p r e e n ! (back~cauerod e~e~zr~ images )_ LEG E.I~D -

CoO

888oc~868oC

!

878oC1868~C

888 o c

!] =878 oC I 857oC

o

1-~-~

~

Yb_~BICuO$

24-T

o

~a,CoO2

7q 100 W 3= Q. Z 0 W

ttl

8o' 604O 2O

0

-J

O nl i-

O

~ 857 oC

100 t 806040200tO'tow) = 0

10;

102

103

104

108 t = 0

101

102

113

104

10 5

REACTION TIME (s) Fig. 9. Variations o f cmmulative volume percenZ p ~ ' x i t h combinalion in table 2.

respect ~o bake time at the lower lemperalure for each high-low temperature

A. Otto et al- /S~percondttct#~g o:cfdeforn~ation

592

3.5.2. }'bBt12Cl140s and Yb:Ba¢Cu zOt_~_~formation from YbBaeCu3Oz_~ and other phases

;?-

"23:

":'-+

"

6 minutes .~tm

Chemical reaction ( 5 ). consisting of reactions (3) and (4). describes the rapid transformation of !-23 into 2-4-7 given that 2-4-7 fermalion follows 1-23 fo~-mation (fig. 2) in as-oxidized samples and that 1-2-3 and Ba~Cu~O~ contents decrease as 2-4-7 and BaCuO, contents increase (fig. 4). 2-4-7 formation is much slower (figs. 3 and 9) when Ba~Cu~O, is absent. Given that many 2-4-7 ~,'ams were adjacent to, or sandwiched bctwcca. I-2-3 g:'ains and lhal they had identical c direction orientations and a and b d;reclion lengths (figs. 6. 7. 10 and i 1 ). 1-2-3 was convened to 2-4-7 primarily by intercalation as illustrated in fig. 15. In this mechanism, a Cu-O layer nucleates adjacent to a Cu-O layer at the 1-2-3 gcain's a-b perimeter. The Cu-O layer grows rapidly through the !-2-3 grain by Cu t * and O"- diffusion down the dislocalion at the inner limit of the layer. C u - O faye ~ then successively nucleate and grow through the 1-2-3 grain at the next nearest Cu--O layer on eilher side of the first Cu-O double layer for 2-4-7 growth into the 1-2-3 grain along their aligned c directions. Fig. I0. Micr~-~Facture~ formed a1857:C follo~ng an 80 5 b-~ke el $75"C in the 0.01 K/s oxidized "lr'DBa2Cu3Ag~alloy. "lqle|[gh[ s'~cle~es a ~ sils~et (a) and Y't),BaCuO~ (b). The superconducting oxide is YbBa~Cu~O._~ (c) m 6 rain -~ith ~-o_~Ba=C~_,._z ~d) subsequently formed. CuO (e) and BaC~O: ( f ) are ~lso present ( b,acksc~tlered e!~t~On images ).

7

2..

~

~--

6,

-

~

~

a

di g

ours #2

Fig. I I. Mict'ostruetures formed at 857~C foUowi~g an 80 s bake at 888=C in the 0.0t K / s oxidized YbBa~CusAg3 alloy. The light stl~ClUl~are silver (a) and Yb2BaClltOs (b). The supcrc~ducting oxide is mainly YbBazCu30~_~ (c) up to 8 h with Yb2Ba4Cu~O=~_a ( d ) prmeat at 39 It. CuO (e) and BaCuOz ( f ) are also presem (backscattered electron images).

A 011e et al_ / Superconducting oxtdefortna~on

o

x.;~..1

O

Y~laCuO|

cuo

vbllqca~o~d

E~14.4 84"1nC : 'g~ oC

6oi

¢n <

20 i

83S o C : ~[~! QC 10o '

8o6Q;-

593

tions inside 1-2-3 grains have been observed [33]. The more aspecled shapes of 2-4-7 grains after comple!e conversion, their large sizes and the slight increase in superconducting phase contem indicate that the 24-7 grains formed by intercalation grew be3ond the parem 1-2-3 grain surfaces by the :diffusion of all ions to the growtJ~ surfaces as described generally by reaction ( t ) and by reaction (2) in lerms of specific reactants. Reacaion (2) is supported by the inappropriately small increase in BaCuO, expected from the amount of Ba:Cu~Os decomposed and the disappearance of phases such as Yb2Ba20~ and YbBa4Cu.~09. The large superconducting oxide content increase and marginal I-2-3 contem decrease during 1-2-4 formation (figs. 2 and 4) indicates thai i-2-4 formed from other oxides (i.e. by reaction ~2 ) ) i.n addition to t-2-3. Most of the 1-24 may then have formed by nudealion z~d growth, possibly at 1-2-3 grain surfaces, ~4ga the decomposition of phases such as YbzBa20.~ BazCu~O~ and VbBa,Cu~O~ contributing the required ions (reaclion {2) wah n = l and other reactant phases ).

3.5.3. Yb2Ba,CueOl_~_~and YbBa2Cugg~ decomposition at YbBazCug9 ~_~forming lo~lp~r~tt~res

824oC

, 888oC

80

40 20 0

o

tol

102

REACTION TIME

{$)

Fig. 12. Varialioasofcumalative volume percent phases with respect l o bake time at YbBa:CusO~ ~ forming temperatures following Yb:Ba~CuTO~_~ and YbBa2Cu,Os formation described in table 3.

Both Cu 2+ and 0 ~-- diffuse from the Ba2Cu305 particles as they decompose into BaCuO> Partially formed double Cu-O layers terminating at disloca-

The rapid 2-4-7 and 1-2--4decomposition beyond 20 s at t-2-3 forra~g temperatures (fig_ 12k occ~lrred by the reverse of :reaction (2) (with n = L 2 ). Yb:BaCuOs, CuO and BaCuOz formed as 1-2-4 and 24-7 decompose& Ieaving small amounts of hig!aly aspected !-2-3 grains and cc~arsened silver {fig. 13 ). Decomposition occurred primarily at the 1-2-4 and 2-~7 grain surfaces even if they were starling to revert to 1-2-3 by the reverse o f ~ e intercalation mechan: m . Due to the short bake ~irnes,some I-2-3 grains let: after decomposition may be the 1-2-3 grains presenl initially among the I-2-4 and 2-4-7 grains. Their aspected shapes then arise from the intercalation mechanism that formed 2-4-7 by growth along the c direction of the !-2-3 grains, thinning them wilhoul changing their other dimensions~ Bener control over the reactions and micxostructures may be achieved by short bakes of 1-2-4 marginally below 870°C (the 2-4-7/1-2-3 lxmnclaD') and of 2-4-7 marginally above 870:C.

594

.4. Otto et al. ~Superconducting oxide_formation

Fig. 13_ Microstrac~ures formed at T> 877~C following a 9.5 :a bake =; 835"C in the 0.O! K/s oxidized YbBa:CurAg3 alloy. The ligh~ structures are silver (a) and YbzBaCuO~ (bl. The superconducting oxide is YbBa:Cu40~ (ct initially and YbBa.,Cu~O~_.5(d) beyond 20 s. CuO (el and mixlur~s ofbinaD" bafium-ce~per oxides (f I are also presenl ( backscattered dectron images )_

NUCLEATION ON TR!PLE JUNCTION Lit;E$ A five grein piece of oxidized Yb-Ba*Cu-Ag

Y

b

alloy

~

BaO c

~Superconducting ~/~:~._. oxide nuclei on ~ the triple junction

~~ *

~

hnes

R3pid growth along junction lines

S

After the nuclei grow along all

Relevant triple junction line segment

,/,,, j u n c t i o n

lines

Yb203 -CuO ~ - ~ J J J

to form an interconnected network, ~,_~j ~2=:::f=~===~growth proceeds

v,.,-, -,~v3- o=v~ ~

Yb203 - SaP

/--~

l

along binary phase intersection planes

Some nuclei are better oriented for this growth

Ftg. 14_ [Uustmtion of I-2-3 nucleation at triple junction lines. 4. Conclnsions As-oxidized YbBa:Cu~Ag3 forms the 1-2-4 phase below 840=(?, the 2-4-7 phase in the 840-~C-870=C range a n d t h e t-2-3 p h a s e a b o v e 870°C, m a t c h i n g t e m p e r a t u r e s for t h e i r f o r m a t i o n in t h e Y - B a - C u - O

system at 1.0 a t e oxygen pressure. !-2-3 forms rapidly b y nucleation a n d growth o n the j u n c t i o n lines a n d planes o f the finely d i v i d e d e l e m e n t a l a n d bin a r y oxides o f the as~x~dJzed precursor alloy. Int e r c o n n e c t e d 1-2-3 structures f o r m at low v o l u m e fractions because t h e j u n c t i o n lines a n d planes be-

595

A. Ozt( et al. / Xuperconducting oxide farmation

c-direction c-direction

- .... .....

2-4-7

Cu-O Yb-O Ba-O

Ba2Cu30:

dislocation c-direction

Ag

/

~ ~ . . ~

overall 2-4.7 growth direction

la2Cu3Os

diffusing

Cu2÷, O2-

Ea2Cu30 5 -,-t. Cu2* ÷

02-

+

2BaCu02

F~g. ! 5~Pdustr-atio~of 1-2-3to 2-4-7transformalion by i~'calation. tween the precursor oxide phase particles are interconnected. 2-4-7 then forms rapidly from 1-2-3 at the above temperatures by an intercalation mechanism. Ba_-Cu3Os decomposition supplies Cu -'+ and O z-. Slower gro,~xh beyond the t-2-3 grain surfaces increases the amount o f superconducting oxide, tntercalalion without Ba,Cu~O5 is much slower. 1-2-4 forms rapidly from nonsuperconducting oxide phases by a nucleation and gromah mechanism with the 12-3 content only decreasing slightly. The rapid 1-24 and 2-4-7 formation with Yb compared to their slow formation with Y is due to differences between these dements, not to the precursor alloy method. Rapid 1-2-4 and 2-4-7 decomposition into mamb" YbzBaCuO~, CuO and BaCuO2 at i-2-3 forming

temperatures yields large product phase particles and highly aspected, isolated 1-2-3 grains. Some of these 1-2-3 grains are remnants from the prior formation o~ 2-4-7 by intercalation.

AelmowleSgements This work was supported by the US DOE through DE-FCr02-85ER45 ! 79A005. The a~.hors thank Steve Recca for assistance with the microprobe work and American Superconductor Corporation for supplying some o f the alloys.

596

A+ Otto, et aL / SuperconducHng oxtde Ibrmation

References

It I C w . Chu, P.H. |'let. RL. Meng, L Gao and Z.J. Huang. Science 235 ( 1987 ) 567. 121 18. Kogure+ A_J+Olin and J_B. Vander Sande~ Physlca C t 57 (1989) 159. [ 3 ] A. Ourmazd+ J.A. Rensch|er. J.C+H. Spence. M.R. Graham. D.W. Johnson J~. and W_W_ R h o d e . Nature 327 (19871

308. [41 H.W. Zandbergen. R. Grensk3_-. K. Wang and G. Thomas. Nature 331 (1988) 596. |51A.F. MarshalL KV¢~ Ba~nn, K_ Char. A_ Kap~tulnik. D Oh. R.H. Hammond and S.S. 1.2dermen. PhyK Rev. B 37 ~ ! 988 } 9353. I6 ] 3". Kogure. R KOnl~ and J.R Vander Sande. Physica C 156 ~ 9 8 8 ) 35. 171T. Kogme. R. K~nU'~. G J . Yurek am~ J.B. Vmider Sande. Physic~ C t 56 (~988) 45_ [8 i P. Bordel, C_ ChadlOuL J_ Chermvas. J_L Hode~u. M_ Marezio. J. K.z~in:iia and E. KaJd~s. Natnre 334 < ~988~ 596lgl D J_ I_i. H S h d ~ m . J.P+ Z~n_g. LiD_ Marks, H.O. Marc? and S. So~g. Ph~5~-'~C 156 ( | 988 } 20 I_ [101M.L ,Mat~dich el ~_. Phys. Re~'. B 38 <|955) 5030. [ I l ]J- Kt~'o et aL. AppL P h ~ . LetL 52 ~ 1988.~ 1625+ [ ! 2 ] J+ K2~inskL E_ K~ldls~ E J~¢L S. R,~siecla an~ B Bucker Natm'e 336 ( 1988 ) &50. [ i 3 ] J+ Karpim,ki. ~ KaldLs. S. R ~ e c ~ and E J~ek. J. Les~ Common Met_ i 50 ( i989 ~ ~29_ [ 14] D+F_..Morals. N.G. Am"~aLJ_H+N~ckeL R_L S ~ JA'+T+ We+ •m d J _ ~ Po~_ Ph,3_~k~C ~59 ~H989~ 287_ [ I $ l L Kar~im,~i. S- Rt~t~k/o F~ Ke3dis. B. Bucher a~d 1L J~ie~.'= Ph3~ca C 160 < 1989} 449_ [ t 6 | J+ Karoimld, S. Res~ecki+ B_ Buch~.. E K~d~s a~d E_ .~~eL ~ y s i ~ C 161 11989) 618+ [ I"P] RJ+ Ca.-.m J J . K~je~,kL W.E P e ~ ,Jr.. B. Ba~m~_. LW. R a l ~ Jro. R.M. Fleming..:LC_W.P_ James and P. M a s k N a m ~ 338 K t989'.~ 328_

[ 181 S. Jm. H.MJ O'B~'an. PK. GMlager, T H . TiefeL R J . Cava, R~,. Faslnacht and G.W Kammlott_ Physica C 165 ( I t~913 415. I t9] U. Balachandran, M.E_ Biznek, G.W. Tom,ins. B.W. "*Zeal and R.B+ PoeppeL Physica C 165 ( ~9901 335. [ 20 ] T. Miyatake. S. Gotch. N. l(oshizuka and S_ Tanaka. Nature 341 { 1989} 41. [21 ] R+G+Buckley. L L Tal~on, D.M. Pooke and M.R. Presland. Physica C t65 < 1990} 391_ [22] D_E_ Morris. D_K+ Na.,~nkar and A.P.B. Sinha. Phymca C ]69 ( 1990} 7+ [23] D.M. Pooke. R.G. BucMey+ M_R Presland and J.L Tz.~!en. Ph_~s_Re~_ B 14 <1990~ 6616. [24[ D_E Mo~s_ J_H_ N~ckeL J.Y.T. Wei. N.G. Asmar. JS_ Sc¢ti. U M. Sc~e~n. C_T. Huhgren. A.G. Markelz. J.E. PosL PJ. Heane% D.R. Veb]en and R_M. Hazen. Phys. Rev. B 39 ( i989~ 7347. ~25] D_F. ,Mom~ N_G_ Asmar. J_Y+T_WeL L H N~ckeL R.L Si~l. LS+ Scott and J.E. Post+ Ph~x_ Rev_ B 40 < 1989~ 11406_ [ 26 } R.C- Weast. ed.. Handbook of C h e m i ~ ' and Physics, The Chera~cal Rubber Co_. 1968, pp. ,F- 152. Fd 53. 27 ] K~A~G_~hnetder Jr+ J. L ~ C f , m r o o n Mel. 17 ( ~969 ) 13. ~28] F.R. DeBoer. W . H D.~jkman. W C_M_ Mz~lens and A.R. M~cdema. J. Less..Comm.oa Met 64 ~ ~979 ) 241_ {29] M+ Fe~doff. CP. Burmeslcr. L T . W~]te a~d It, Gronxk_v. J. ~emmon Me'+- 164-165 ( t 9 9 ~ 84. 3,O] D.E_ ?,|errs+ .*G. M ~ e l L B. Fayn and JH_ Nickel Physica C t6~ ~]99~} 153. T31] A_ O'_m+ " ~ e exudation formatizn and properlies of silver ~.'~mp,~ites. Ph.D. Thems. Mas~chusells lnslff~le of Techn~lo~;. February:. ~991. [ 32 ] M_L Kr-amer, L_S_Curabley. KW. McCattum. W38 Nelt~s. S. ~a-~r and E_P_ Kvam. P h s x ~ C 166 { 1990~ 155. [ 33 ] R. K~nh--~.A. Otto a~d J. Vander -Sr~de~ unpubhshed_