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The effect of particle size distribution on the phase composition in YBa2Cu3O7−x as determined by DTA
Physica C 225 (1994) 374-380
ELSEVIER
The effect of particle size distribution on the phase composition in YBaECuaO7_x as determined by DTA A . M . M . B a r u s *, J . A . T . T a y l o r New York State College of Ceramics, Alfred University, Alfred, NY 14802, USA
Received 14 February 1994; revised manuscript received 15 March 1994
Abstract The effect of crystallite size on thermal events during beating of YBa2Cu3OT_xwas investigated. Differential thermal analysis up to 1100°C in air was performed on seven samples with different particle size distributions. The coarsest powder (d.,~ffi62 pm and 100%> 45 pro) has one endothermic peak at 1000°C, which is the 211 peritectic melting point of 123. The powders containing fine crystallites showed two peaks, at 940°C and at 1000°C. The 940°C peak disappears when the thermal analysis is done in oxygen, indicating that ambient atmosphere has a strong effect on the thermal decomposition of YBa2Cu3OT_x.
1. Introduction The peritectic melting point of YBa2Cu307_x (123) superconductors has been reported to occur at many different temperatures between 900°C and 1050°C [1-5]. The partial pressure of oxygen has been identified as an important variable. However, different values have been reported for the peritectic melting point of 123 using the same atmosphere. The difference in the reported value is probably due to experimental factors such as thermocouple error, various rates of heating, technique of measurement and powder characteristics including impurity and CO2content. Determination of the melting point of YBa2Cu3OT_x and the temperature at which the liquid phase starts to develop is important to optimize the processing condition during heat treatment of the samples, especially for melt processing. Differential thermal analysis ( D T A ) is one of the standard tech* Corresponding, author.
niques used to identify thermal events such as melting and crystallization. Identification of the products of the thermal events is accomplished by other characterization methods. This study focuses on the effect of particle size distribution (psd) of the precursor powder on the thermal events that occur during DTA analysis of YBa2Cu3OT-x superconductors.
2. Experimental procedure The phase decomposition of 123 as a function of precursor crystallite size was studied by DTA using seven different powders. The powders contained well characterized and reproducible amounts of fine and coarse size crystallites. Distributions a, b and c (Table 1 ) are coarse with more than 90% of the constituent crystallites larger than 5 pm. Distributions, d, e, f, and g (Table 1 ) are fine with more than 70°/0 of the crystaUites less than 5 pm. All DTA results were reproduced at least three times on different days. A
0921-4534/94/$07.00 © 1994 ElsevierScience B.V. All rights reserved SSD10921-4534 ( 94 )00171 -B
A.M.M. Barus, J.A.T. Taylor/Physica C 225 (1994) 374-380 Table I Panicle size analysis of powder used for DTA Size
Fraction retained (vol.%) for
(}tm) Very co- Coarse Coarse-l Tape Fine-2 Fine-I Fine arse (a) (b) (c) Cast (d) (e) (f) (g)
70-75 65-70 60-65 55-60 50-55 45-50 40-45 35-45 30-35 25-30 20-25 15-20 10-15 5-10 4-5 3-4 2-3 1-2 0-1
18.05 20.49 17.33 19.09 14.55 10.49 -
12.94 . 14.72 . 12.39 . 13.68 . 10.41 . 7.54 . 5.9 20.81 4.46 15.79 3.22 11.46 3.27 11.58 3.13 11.01 2.08 7.37 1.98 6.98 2.08 7.34 0.54 1 . 8 9 0.35 1 . 2 3 0.4 1.4 0.43 1 . 4 7 0.48 1.67
Days
62.4
56.8
31.2
. . . . . .
. . . . . .
. . . . . . . . . 32.62 28.5 11.85 9.1 1 5 . 8 7 22.2 23.09 20.3 12.59 11.7 3.59 8.2 4.04
3.44
-
-
. 14.9 4.9 4.7 15.4 25.8 34.3
1.6 12 19.7 37.3 29.3
375
500/o, this p o w d e r is coarser t h a n a n o r m a l distribution. This d i s t r i b u t i o n will be referred to as coarse ( T a b l e 1 ( b ) ) . Particle size d i s t r i b u t i o n o f the coarse p o w d e r was d e t e r m i n e d using the Sedigraph 5500L #~ a n d c o n v e r t e d to vol.% particle size d i s t r i b u t i o n so the d a t a could be c o m p a r e d with o t h e r analysis techniques. O n e fraction was p r e p a r e d b y regrinding coarse-1 p o w d e r in acetone with a m o r t a r a n d pestle a n d will be referred to as fine-2 p o w d e r ( T a b l e 1; d i s t r i b u t i o n e ) . T h e residue after e v a p o r a t i o n o f the acetone #2 is 0.001 wt.% [ 6 ] . S o m e o f t h e original m a s t e r b a t c h was b a l l m i l l e d by shaker agitation in a nalgene bottle with toluene a n d z i r c o n i a m e d i a for 12 h ( T a b l e l ( g ) ) a n d 24 h (Table 1 ( f ) ) , dried, a n d granulated through a 70 m e s h sieve. T h e shaker agitated p o w d e r a n d the reg r o u n d p o w d e r ( T a b l e 1 ( e ) ) were analyzed using a centrifugal s e d i m e n t a t i o n particle size analyzer #3.
2.1. Chemical analysis reports Zr concentration as a trace only
1.61 1.55
(~m)
precursor p o w d e r o f YBa2Cu307_x, ( 123 ) was synthesized by solid state reaction from reagent grade Y203, BaCO3 a n d C u O powders. T h e raw m a t e r i a l s were baUmilled in a nalgene bottle with distilled water a n d zirconia m e d i a for 24 h, d r i e d at 110°C, granulated using a 40 m e s h sieve, calcined in air at 925 ° C for 18 h, cooled, then r e g r o u n d with m o r t a r a n d pestle. T h e p o w d e r was checked for phase p u r i t y using X-ray diffraction a n d recalcined i f necessary. The p o w d e r was then p r e p a r e d as seven different particle size distributions, as d e s c r i b e d in Table 1. O n e fraction was r e g r o u n d with a m o r t a r a n d pestle a n d s e p a r a t e d using 200 a n d 325 m e s h screens. T h e p o w d e r r e t a i n e d on 325 mesh (44 t t m ) , which is called very coarse p o w d e r ( T a b l e 1 ( a ) ) , a n d the fraction that passed through 325 mesh, which is called coarse-1 p o w d e r ( T a b l e 1 ( c ) ) , were collected separately. O n e fraction was p r e p a r e d b y m i x i n g 50% o f powders that passed 200 mesh (74 t t m ) b u t were ret a i n e d on 325 m e s h a n d 50% o f p o w d e r which passed through 325 mesh. Since the fraction o f particles larger t h a n 325 m e s h w o u l d n o r m a l l y be less t h a n
T h e size o f the p o w d e r s p r e p a r e d b y this m e t h o d ranges f r o m 0.5 to 5 ttm when d i s p e r s e d with 0.2% M e n h a d e n fish oil d u r i n g grinding ( T a b l e 1 (g) ) a n d f r o m 0.5 to 10 ttm when g r o u n d w i t h o u t any dispersant ( T a b l e 1 ( f ) ) . Both toluene a n d acetone have been r e p o r t e d not to react with 123 [ 7 ] . T h e n o m e n clature a n d the particle size d i s t r i b u t i o n o f all samples can be seen in Table 1. The powders were stored
in an oven maintained at 100 + 5 ° C for short periods. Great effort was expanded to use only fresh powders for the DTA analyses to avoid possibility o f corrosion. X - r a y diffraction p a t t e r n s o f the p o w d e r s before a n d after grinding showed t h e m to be identical at a count t i m e o f 1 s, which allows resolution o f a b o u t 1 to 2 vol.%. SEM m i c r o g r a p h s o f the fine a n d the coarse p o w d e r s i n d i c a t e d the particles are crystallites, not agglomerates, as shown in Fig. 1. T h e D T A analysis was c o n d u c t e d in air to 1200°C at a rate o f 1 0 ° C / m i n on a H a r r o p D T A #4. ~i Micromeretics, Norcross, Georgia. ~2 Fisher Scientific Company, PittsburlB PA. ~s CAPA-500, Horiba, Japan. Harrop Industries, Columbus, Ohio with a modification by Innovative Thermal Systems, Alfred, N.Y.
376
A.M.M. Barus, J.A.T. Taylor/Physica C 225 (1994) 374-380
•
56.8
um
31.2
urn
u
"o
,
200
400
600
Temperature.
I
800
i
t000
t200
°C
Fig. 2. Differentialthermal analysisof (a) very coarse (b) coarse (c) coarse-1 (d) tape cast (e) fine-2 (f) fine-l, and (g) very t'mepowders showingthe developmentof 940 °C peak as particle size decreases.
Fig. 1. SEM microstructure of (a) fine and (b) coarse precursor powders showing 123 crystallite size. The rod is a fiber from the paper under the fine crystallite. Note the difference in magnification. 3. Results Fig. 2 shows the DTA graphs for all samples. Fig. 2 (a) is the DTA for a sample consisting only of very coarse powder with a mean diameter of 62.4 ~tm (Table 1; a). One endothermic peak which corresponds to peritectic melting of 123 superconductors is visible at To.~t = 1000 ° C, as expected based on work re-
ported in the literature [2,8-10]. The transformation from orthorhombic to tetragonal at about 600 ° C is not detected in this experiment because the transformation involves only a small energy change [ 1 ]. Fig. 2(g) shows the DTA for samples consisting of fine powder only with a mean diameter of 1.5 ~m. No particles in this specimen are larger than 5 pm (Table I (g)). A large exothermic reaction occurs at low temperature, marking organic burn-out. At higher temperatures there are two endothermic peaks that occur at To.set=940°C and at Tonm= 1000°C. The second peak is due to peritectic melting at about 1000°C, as would be expected for 123 heated in air. Both 940 ° and 1000°C peaks appear in thermograms for the other five powders. The size of the 940 °C peak increases as the fraction of distribution less then 5 ttm increases. From these results it is apparent that some variable related to particle size influences the thermal events during heating of 123 in air. Powders containing a
A.M.M. Barus, J.A.T. Taylor/PhysicaC 225 (1994) 374-380 fine fraction display a 940 ° C reaction while the c o a r s e powder does not. The amount of organic material seems to have no effect on the thermal events as shown in Figs. 2(d), 2(e) and 2 ( f ) . Fig. 2 (d) shows the DTA pattern of tape cast 123. This pattern is similar to Fig. 2 (e) and 2 ( f ) which contain very little or no organic, material. The mean diameter of the mixture was 4.04 I~m (Table 1 ( d ) ) . The weight fraction of organic material is about 9% after drying. This graph shows one exothermic peak which corresponds to binder burnout and two endothermic peaks which correspond to the 940°C reaction and the 211 peritectic melting. Apparently the partial pressure of CO2 and H20 at binder burn-out temperatures do not affect the thermal events at 940 ° C and 1000 ° C. For additional information about the 940 °C reaction. DTA was performed in oxygen. Oxygen was pulled out through tube packed with anhydrous calcium sulfate to dry it. This data show only one endothermic event corresponding to the peritectic decomposition of 123 at about 1030°C as reported in the literature (Fig. 3). This experiment was performed three times and the result was the same. The cause of the 940°C peak is therefore related to an interaction between atmosphere and 123 particles during DTA analysis. For further information, weight loss as a function of temperature was measured in air using simultaneous thermal analysis (STA). Fig. 4 shows the thermal gravimetric analysis ( T G ) for fine-1 powder which has a mean diameter of 1.6 ~tm (Table 1 (f). Also
~..8:L
i
I
200
i
I 400
.
I 600
Tef~oePature,
um
in
i
I 800
O=
i
i ~.000
i
1200
°C
Fig. 3. DTA of fine-I powder in oxygen;showingthe disappearance of 940°C peak and the shift of211 peritectic from 1000°C (air) to 1030°C (oxygen).
3.00
r
,
,
,
377
r
,
,
.
:
,
,
'.. 25
,
440
× ~
-0.50
430
-2.25
420
I 200
i 400
i
i
r
800
800
Tem~er2tur'e.
a
i
tooo
,4t0
:1300
°C
Fig. 4. Simultaneous thermal analysis of fine-I powder performed in air, showingthe weightloss; (a) TG and (b) DTG. plotted is the change in slope of the TGA (derivative thermogravimetric, D T G ) . Weight loss as determined by this analysis was 3.97_ 0.82% as shown in Fig. 4 (b). A theoretical weight loss of 3% is predicted during the transformation from 123 orthorhombic to 123 tetragonal based on the assumption that the initial powder was all YBa2Cu307 and that all Cu was converted to Cu +. An extra peak is clearly defined in the D T G just below the peritectic which is shown only as a shoulder in TG. Further research to determine the cause ot this shoulder is necessary. A quench study was performed to look for confirming evidence that no significant change occurs in phase composition below the peritectic melting point, when fine 123 is fired in oxygen. For the quench study samples made from fine-2 in the form of a bar were fired in oxygen to several temperatures, held at those temperatures for several hours and then air quenched. The fh-ing temperature and holding time were 1000 ° C for 6 h, 1010°C for 5 h and 1020°C for 9 h. At the end of the holding time the samples (on a setter) were pulled out fiom thefurnace and placed on a metal plate in open air. The cooling rate is estimated to be 200 ° C/ min. Fig. 5 shows the X-ray diffraction of quenched samples. From this figure it can be seen that the specimens fired at 1000°C, 1010°C and 1020°C consist mostly of 123 and a small peak of BaCuO2. This quench study shows that no substantial change has occurred in the crystalline phase assembly during thermal treatment in oxygen. The firing cycle is similar in range but not heating rate to the DTA cycle that showed a significant endothermic event at 940°C in air, but not in oxygen. This reaction should have
378
A.M.M. Barus, J.A.T. Taylor/Physica C 2 2 ~ (1994) 374-380 BOO
e . saa~ 6OO .'2.
tn b-
400
Z
200
~o
24
2e
a2 as 40 44 ,e .2 TW0-THeTA tOEGa~ES)
ae
eo
Fig. 5. X-ray diffraction o f fine-1 powder quenched from (a) 1000°C, (b) 1010°C, and (c) 1020°C showing the appearance o f BaCuO2 peaks. The specimens were held for 5, 9 and 6 h at each temperature before quenching.
led to a detectible change in phase assemblage if it occurred. The difference in heating rate could lead to a difference in the onset of a thermally driven reaction but not in suppression of the reaction.
4. Discussion The discovery that the 940°C reaction seen in DTA is related to crystallite size and atmosphere must be explained. The size of the thermal event at 940 °C depends on the amount of fines in the powder, as can be seen in Fig. 2. The 940°C peak in the DTA data has been reported by many different research teams working on 123 [ 5,11-14 ]. The relationship between this peak, particle size distribution and atmosphere has not been previously noted. The fact that no 940°C reaction is detected during DTA in oxygen means that reaction between fine particles and active gases in ambient air are related to this endothermic event. The reaction sequence has not been defined by this work. Reports in the literature of the 940°C peak ascribe the reaction to many different events. Rodriguez et al. [5] report a 940°C reaction during high-temperature X-ray diffraction analysis is supplemented with
optical microscopy during heating in air. They report the development of a liquid phase and crystalline BaCuO2 coexisting with 123 between 950 ° C and the peritectic melting of 123. The BaCuO2 phase was seen to increase in volume slowly according to the X-ray data. They concluded that the 940°C DTA event could be explained as a decomposition reaction of 123 which produces Y2Cu205 and BaCuO2. Such a reaction, they reason, would result from the metastable nature of the 123 phase, as described by Zhou [ 12]. No particle size measurements were made and the air was not cleaned of CO2 or water during processing, so comparisons with the DTA from this study are not possible. Sue et at. [ 13] reported the 940°C event on their DTA. They used powder calcined at 800°C which contained a detectable amount of BaCuO2. They suggested that the 940°C peak is due to entectic of BaCuO2-YBa2Cu3OT_x. This entectic has been reported by Roth et at. [ 14 ]. However, when the DTA was run using single-phase material ( 123 ) calcined at 910 ° or 970°C they saw only one thermal event at 1000 ° C. The particle size reported was in the range of 2-3 ttm. Since the atmosphere during analysis is not mentioned, comparison with this work is not possible. Aselage and Keefer [ 15 ] reported the appearance of a single peak at 940°C using DTA in flowing air at a rate of 10°C/rain when the sample was heated to 974°C or 994°C. No particle size was reported. The initial powder contains 123 with 21 l, BaCuO2 and CuO impurities. However they reported that postmortem analysis of the product showed pure 123. They suggested that the 940°C reaction is the peritectic reaction between 123 and CuO as follows: 123 + CuO-~211 + l i q u i d .
( 1)
Nevriva et at. [ 16 ] reported that within the range 950-1050°C there were two endothermic effects, at 911 ° C and 977 ° C using air atmosphere and at 957 ° C and 990°C in oxygen. No particle size information nor the purity of powder used were reported. They proposed that the first peak was a ternary entectic in the YOLs-BaO-CuO system. This deduction was based on a comparison of their data with DTA of known single phases; BaCuO2, YCuO2.s and YeBaCuOs. A melting temperature of a 123 superconductor
A.M.M. Barus, J.A.T. Taylor/Physica C 225 (1994) 374-380
significantly lower than expected has been reported by Ono and Tanaka [ 3 ]. They observed that in air 123 started to melt at 925°C which is about 15°C lower than the temperature of the endothermic event reported for the powder containing fine crystallites in this experiment. No particle size distribution was reported. Their results are probably not the peritectic melting of 123 as they believed but the same reaction seen at 940°C in this work. None of the previous work identified a relationship between particle size and atmosphere. To isolate the factors that could be causative in the 940 °C reaction, some additional DTA work was carded out by Lindemer at Oak Ridge National Lab, using three of the powders prepared for this research [ 11 ]. The first powders were the very coarse (Table 1 (a)), the fine-1 powder (Table 1( f ) ) and the fine powder (Table 1 (g)). He observed that in flowing oxygen or air from which the carbon dioxide and water had been removed, there is only one thermal event at about 1000 ° C. These results imply that CO2 and H20 contained in ambient air may be responsible for the 940°C peak. He then exposed samples of both fine and coarse powders to ambient conditions for about a week and reanalyzed the powders with DTA in oxygen. He observed that for both fine powders there were two thermal events, one at 940°C and another at about 1000°C. Apparently, corrosion of fine 123 powder occurs during exposure to ambient conditions. The coarse powders did not show any change in a DTA trace as a result of the ambient exposure. It has been reported that 123 reacts with ambient atmosphere, especially moisture and CO2 [ 17-19 ]. Naito et al. [ 17] reported that when 123 powder is placed in a chamber with 78% relative humidity, 75% of the powder decomposed to BaCO3, CuO and 21 l (based on XRD) in a week, while at 5 I% relative humidity single-phase orthorhombic perovskite phase is still present after six months. Particle size was reported as in the range of a few microns. BaCO3, formed when Ba(OH)2, which was produced during hydrolysis, reacted with CO2 from the air. Fitch and Burdick [20] observed that when 123 powder was exposed to 100% relative humidity atmosphere at 80°C it decomposed into 211, CuO, BaCO3, Cu(OH)2 and possibly BaO and Y(OH)3. Particle size was not reported.
379
Horowitz et al. [ 18] exposed 123 powder to ambient atmosphere. No particle size was reported for this powder which was prepared by grinding calcined material. After 48 h exposure, the X-ray pattern showed the presence of BaCO3 which did not increase from 48-672 h of exposure. They also reported that calcined unground material exposed to ambient atmosphere for the same period shows no evidence of surface reaction. Yan et al. [ 19 ] proposed the reaction sequence between 123 and moisture and CO2 as: 2YBa2 Cu307 + 3H2 O--, Y2BaCuO5 + 3Ba(OH)2 + 5CUO+0.502,
(2)
Ba(OH)2 +CO2 ~,BaCO3 + H 2 0 .
(3)
Yah et al. [ 19] also reported that Cu(OH)2 X-ray reflections appeared when the sample was exposed to air at 85 °C and 85% relative humidity. Particle size was not reported. Bansal and Sandkuhl [ 21 ] exposed a bar made of 123 powder to 100% relative humidity at room temperature over eight weeks. They reported that the main corrosion product was BaCO3. The powder was reported as 100% less than 325 mesh (45 ~tm). While the reports in the literature vary, all predict decomposition of 123 in an ambient environment as a function of humidity and CO2 content. There is a possibility that the compounds present in 123 after exposure to ambient atmosphere may promote the 940 °C reaction. This hypothesis would explain the disappearance of the reaction when the experiment is performed in oxygen, where no CO2 nor H20 is present. No conclusive evidence of liquid phase developing below 1000 ° C in oxygen has been found in the microstructure of quenched samples. The presence of liquid phase at a lower temperature is useful for melt processing. Being able to perform liquid-phase sintering at about 950 °C might allow the development of the oriented microstructure necessary for high current density, using the correct stoichiometry for 123. Liquid phase sintering at low temperature is usually achieved by the addition of dopants or use of off-stoichiometry compositions which leads to non-superconducting phases at grain boundaries. The fine powder has more surface area than the coarse powder. Calculations based on panicle size distribution indicate that the fine powder
380
A.M.M. Barus, J.A. T. Taylor/Physica C 225 (1994) 374-380
( T a b l e 2 ( g ) ) has a b o u t 57 times as m u c h specific surface are a as coarse p o w d e r ( T a b l e 1 ( a ) ) . This estimate is based on the the following e q u a t i o n [ 22 ]:
= Z ¢i
(4)
where S M = s u r f a c e area in m2/kg, ¢ = v o l u m e fraction, ¢ , = s h a p e factor, a v = a v e r a g e size in m, D, = density in k g / m 3. The shape factor a n d density were a s s u m e d to be the same for b o t h powders. This surface area is the only difference between the fine a n d coarse powders. C o n v e n t i o n a l t h e o r y predicts no significant difference in surface energy in these crystallite size ranges. However, if the fines are corroding in a m b i e n t air at r o o m t e m p e r a t u r e while the coarse p o w d e r does not, either an u n i d e n t i f i e d factor is present o r the difference in surface area between coarse a n d fine powders is m o r e significant than expected.
5. Conclusions D u r i n g D T A o f 123 two e n d o t h e r m i c events were observed, one at T o n ~ t = 9 4 0 ° C a n d a n o t h e r at Tnn~t= 1000 ° C for p o w d e r s containing a fine fraction, while only one e n d o t h e r m i c event at Ton. set = 1000°C was o b s e r v e d for coarse powders. The first p e a k d i s a p p e a r s when the e x p e r i m e n t is perf o r m e d in oxygen atmosphere. T h e usual depression o f the 211 peritectic melting p o i n t as related to amb i e n t a t m o s p h e r e a n d due to the e q u i l i b r i u m that is affected by oxygen c o n c e n t r a t i o n is o b s e r v e d in this work.
References [ 1] P.K. Gallagher, Adv. Ceram. Mater. 2 (1987) 632. [2] R.S. Roth, C.J. Rawn, F. Beech, J.D. Whirler and J.O. Anderson, in: Research Update, 1988 - Ceramic Superconductors II, ed. M.F. "fan (Am. Ceram. So¢., Westerville, OH, 1988)pp. 13-26. [ 3 ] A. Ono and T. Tanaka, Jpn. J. Appl. Phys. 26 (1987) L825. [4] ICW. Lay and G.M. Ren]und, J. Am. Ceram. Soc. 73 (1990) 1208. [5] M. Rodriguez, R.L. Snyder, B.J. Chen, D.P. Matheis, S.T. Misture and V.D. Frechette, Physica C 206 (1993) 43. [6] The Fisher Catalog, Fisher Scientific, Springfield, NJ (1991). [7] S.E. Troller, S.D. Atkinson, P.A. Fuiercr, J.H. Adair and R.E. Newnham, Ceram. Bull 67 (1988) 759. [8]T.B. Liodemer, F.A. Washburn, C.S. Mac DougaU, R. Feenstra and O.B. Cavin, Physica C 178 ( 1991 ) 93. [9 ] J.E. Ullman, R.W. McCallum and J.D. Verhoeven, J. Mater. Res. 4 (1989) 752. [ 10] J. Takada, H. Kitaguchi, A. Osaka, Y. Miura, K. Takahashi, M. Takano, Y. Ikeda, Y. Bando, N. Yamamoto, Y. Oka and Y. Tomii, Jpn. J. Appl. Phys. 26 (1987) L 1707. [ 11 ] T. Lindemer, private communication (1992). [ 12 ] Z. Zhou and A. Navrotsky, J. Mater. Res. 7 (1992) 1. [ 13 ] S.R. Sue, M. O'Connor and M. Levinson, J. Mater. Res. 6 (1991) 245. [ 14] R.S. Roth, K.L. Davis and J.R. Dennis, Adv. Ceram. Mater. 2 (1988) 303. [ 15 ] T. Aselage and K. Keefer, J. Mater. Res. 3 (1988) 1279. [ 16 ] M. Nevriva, E. Poilert, J. Sestak and A. Triska, Thermochim. Acta 127 (1988) 395. [ 17 ] N. Naito, J. Kafalas, L. Jachim and M. Downey, J. Appl. Phys. 67 (1990) 3521. [ 18 ] H.S. Horowitz, R.K. Bordia, R.B. Flippen and R.E. Johnson, Mater. Res. Soc. Syrup. Proc. 99 (1988) 903. [ 19] M.F. Yan, R.L. Barns, H.M. O'Brian Jr., P.K. Gallagher, R.C. Sherwood and S. Jin, Appl. Phys. Lett. 51 ( 1987 ) 532. [ 20 ] L.D. Fitch and V.L Burdick, J. Am. Ceram. Soc. 72 (1989) 2020. [21 ] N.P. Bansal and A.L Sandkuhl, Appl. Phys. Lett. 52 (1988) 323. [22] J. Zheng, P.F. Johnson and J.S. Reed, J. Am. Ceram. Soc. 73 (1990) 1392.
ELSEVIER
The effect of particle size distribution on the phase composition in YBaECuaO7_x as determined by DTA A . M . M . B a r u s *, J . A . T . T a y l o r New York State College of Ceramics, Alfred University, Alfred, NY 14802, USA
Received 14 February 1994; revised manuscript received 15 March 1994
Abstract The effect of crystallite size on thermal events during beating of YBa2Cu3OT_xwas investigated. Differential thermal analysis up to 1100°C in air was performed on seven samples with different particle size distributions. The coarsest powder (d.,~ffi62 pm and 100%> 45 pro) has one endothermic peak at 1000°C, which is the 211 peritectic melting point of 123. The powders containing fine crystallites showed two peaks, at 940°C and at 1000°C. The 940°C peak disappears when the thermal analysis is done in oxygen, indicating that ambient atmosphere has a strong effect on the thermal decomposition of YBa2Cu3OT_x.
1. Introduction The peritectic melting point of YBa2Cu307_x (123) superconductors has been reported to occur at many different temperatures between 900°C and 1050°C [1-5]. The partial pressure of oxygen has been identified as an important variable. However, different values have been reported for the peritectic melting point of 123 using the same atmosphere. The difference in the reported value is probably due to experimental factors such as thermocouple error, various rates of heating, technique of measurement and powder characteristics including impurity and CO2content. Determination of the melting point of YBa2Cu3OT_x and the temperature at which the liquid phase starts to develop is important to optimize the processing condition during heat treatment of the samples, especially for melt processing. Differential thermal analysis ( D T A ) is one of the standard tech* Corresponding, author.
niques used to identify thermal events such as melting and crystallization. Identification of the products of the thermal events is accomplished by other characterization methods. This study focuses on the effect of particle size distribution (psd) of the precursor powder on the thermal events that occur during DTA analysis of YBa2Cu3OT-x superconductors.
2. Experimental procedure The phase decomposition of 123 as a function of precursor crystallite size was studied by DTA using seven different powders. The powders contained well characterized and reproducible amounts of fine and coarse size crystallites. Distributions a, b and c (Table 1 ) are coarse with more than 90% of the constituent crystallites larger than 5 pm. Distributions, d, e, f, and g (Table 1 ) are fine with more than 70°/0 of the crystaUites less than 5 pm. All DTA results were reproduced at least three times on different days. A
0921-4534/94/$07.00 © 1994 ElsevierScience B.V. All rights reserved SSD10921-4534 ( 94 )00171 -B
A.M.M. Barus, J.A.T. Taylor/Physica C 225 (1994) 374-380 Table I Panicle size analysis of powder used for DTA Size
Fraction retained (vol.%) for
(}tm) Very co- Coarse Coarse-l Tape Fine-2 Fine-I Fine arse (a) (b) (c) Cast (d) (e) (f) (g)
70-75 65-70 60-65 55-60 50-55 45-50 40-45 35-45 30-35 25-30 20-25 15-20 10-15 5-10 4-5 3-4 2-3 1-2 0-1
18.05 20.49 17.33 19.09 14.55 10.49 -
12.94 . 14.72 . 12.39 . 13.68 . 10.41 . 7.54 . 5.9 20.81 4.46 15.79 3.22 11.46 3.27 11.58 3.13 11.01 2.08 7.37 1.98 6.98 2.08 7.34 0.54 1 . 8 9 0.35 1 . 2 3 0.4 1.4 0.43 1 . 4 7 0.48 1.67
Days
62.4
56.8
31.2
. . . . . .
. . . . . .
. . . . . . . . . 32.62 28.5 11.85 9.1 1 5 . 8 7 22.2 23.09 20.3 12.59 11.7 3.59 8.2 4.04
3.44
-
-
. 14.9 4.9 4.7 15.4 25.8 34.3
1.6 12 19.7 37.3 29.3
375
500/o, this p o w d e r is coarser t h a n a n o r m a l distribution. This d i s t r i b u t i o n will be referred to as coarse ( T a b l e 1 ( b ) ) . Particle size d i s t r i b u t i o n o f the coarse p o w d e r was d e t e r m i n e d using the Sedigraph 5500L #~ a n d c o n v e r t e d to vol.% particle size d i s t r i b u t i o n so the d a t a could be c o m p a r e d with o t h e r analysis techniques. O n e fraction was p r e p a r e d b y regrinding coarse-1 p o w d e r in acetone with a m o r t a r a n d pestle a n d will be referred to as fine-2 p o w d e r ( T a b l e 1; d i s t r i b u t i o n e ) . T h e residue after e v a p o r a t i o n o f the acetone #2 is 0.001 wt.% [ 6 ] . S o m e o f t h e original m a s t e r b a t c h was b a l l m i l l e d by shaker agitation in a nalgene bottle with toluene a n d z i r c o n i a m e d i a for 12 h ( T a b l e l ( g ) ) a n d 24 h (Table 1 ( f ) ) , dried, a n d granulated through a 70 m e s h sieve. T h e shaker agitated p o w d e r a n d the reg r o u n d p o w d e r ( T a b l e 1 ( e ) ) were analyzed using a centrifugal s e d i m e n t a t i o n particle size analyzer #3.
2.1. Chemical analysis reports Zr concentration as a trace only
1.61 1.55
(~m)
precursor p o w d e r o f YBa2Cu307_x, ( 123 ) was synthesized by solid state reaction from reagent grade Y203, BaCO3 a n d C u O powders. T h e raw m a t e r i a l s were baUmilled in a nalgene bottle with distilled water a n d zirconia m e d i a for 24 h, d r i e d at 110°C, granulated using a 40 m e s h sieve, calcined in air at 925 ° C for 18 h, cooled, then r e g r o u n d with m o r t a r a n d pestle. T h e p o w d e r was checked for phase p u r i t y using X-ray diffraction a n d recalcined i f necessary. The p o w d e r was then p r e p a r e d as seven different particle size distributions, as d e s c r i b e d in Table 1. O n e fraction was r e g r o u n d with a m o r t a r a n d pestle a n d s e p a r a t e d using 200 a n d 325 m e s h screens. T h e p o w d e r r e t a i n e d on 325 mesh (44 t t m ) , which is called very coarse p o w d e r ( T a b l e 1 ( a ) ) , a n d the fraction that passed through 325 mesh, which is called coarse-1 p o w d e r ( T a b l e 1 ( c ) ) , were collected separately. O n e fraction was p r e p a r e d b y m i x i n g 50% o f powders that passed 200 mesh (74 t t m ) b u t were ret a i n e d on 325 m e s h a n d 50% o f p o w d e r which passed through 325 mesh. Since the fraction o f particles larger t h a n 325 m e s h w o u l d n o r m a l l y be less t h a n
T h e size o f the p o w d e r s p r e p a r e d b y this m e t h o d ranges f r o m 0.5 to 5 ttm when d i s p e r s e d with 0.2% M e n h a d e n fish oil d u r i n g grinding ( T a b l e 1 (g) ) a n d f r o m 0.5 to 10 ttm when g r o u n d w i t h o u t any dispersant ( T a b l e 1 ( f ) ) . Both toluene a n d acetone have been r e p o r t e d not to react with 123 [ 7 ] . T h e n o m e n clature a n d the particle size d i s t r i b u t i o n o f all samples can be seen in Table 1. The powders were stored
in an oven maintained at 100 + 5 ° C for short periods. Great effort was expanded to use only fresh powders for the DTA analyses to avoid possibility o f corrosion. X - r a y diffraction p a t t e r n s o f the p o w d e r s before a n d after grinding showed t h e m to be identical at a count t i m e o f 1 s, which allows resolution o f a b o u t 1 to 2 vol.%. SEM m i c r o g r a p h s o f the fine a n d the coarse p o w d e r s i n d i c a t e d the particles are crystallites, not agglomerates, as shown in Fig. 1. T h e D T A analysis was c o n d u c t e d in air to 1200°C at a rate o f 1 0 ° C / m i n on a H a r r o p D T A #4. ~i Micromeretics, Norcross, Georgia. ~2 Fisher Scientific Company, PittsburlB PA. ~s CAPA-500, Horiba, Japan. Harrop Industries, Columbus, Ohio with a modification by Innovative Thermal Systems, Alfred, N.Y.
376
A.M.M. Barus, J.A.T. Taylor/Physica C 225 (1994) 374-380
•
56.8
um
31.2
urn
u
"o
,
200
400
600
Temperature.
I
800
i
t000
t200
°C
Fig. 2. Differentialthermal analysisof (a) very coarse (b) coarse (c) coarse-1 (d) tape cast (e) fine-2 (f) fine-l, and (g) very t'mepowders showingthe developmentof 940 °C peak as particle size decreases.
Fig. 1. SEM microstructure of (a) fine and (b) coarse precursor powders showing 123 crystallite size. The rod is a fiber from the paper under the fine crystallite. Note the difference in magnification. 3. Results Fig. 2 shows the DTA graphs for all samples. Fig. 2 (a) is the DTA for a sample consisting only of very coarse powder with a mean diameter of 62.4 ~tm (Table 1; a). One endothermic peak which corresponds to peritectic melting of 123 superconductors is visible at To.~t = 1000 ° C, as expected based on work re-
ported in the literature [2,8-10]. The transformation from orthorhombic to tetragonal at about 600 ° C is not detected in this experiment because the transformation involves only a small energy change [ 1 ]. Fig. 2(g) shows the DTA for samples consisting of fine powder only with a mean diameter of 1.5 ~m. No particles in this specimen are larger than 5 pm (Table I (g)). A large exothermic reaction occurs at low temperature, marking organic burn-out. At higher temperatures there are two endothermic peaks that occur at To.set=940°C and at Tonm= 1000°C. The second peak is due to peritectic melting at about 1000°C, as would be expected for 123 heated in air. Both 940 ° and 1000°C peaks appear in thermograms for the other five powders. The size of the 940 °C peak increases as the fraction of distribution less then 5 ttm increases. From these results it is apparent that some variable related to particle size influences the thermal events during heating of 123 in air. Powders containing a
A.M.M. Barus, J.A.T. Taylor/PhysicaC 225 (1994) 374-380 fine fraction display a 940 ° C reaction while the c o a r s e powder does not. The amount of organic material seems to have no effect on the thermal events as shown in Figs. 2(d), 2(e) and 2 ( f ) . Fig. 2 (d) shows the DTA pattern of tape cast 123. This pattern is similar to Fig. 2 (e) and 2 ( f ) which contain very little or no organic, material. The mean diameter of the mixture was 4.04 I~m (Table 1 ( d ) ) . The weight fraction of organic material is about 9% after drying. This graph shows one exothermic peak which corresponds to binder burnout and two endothermic peaks which correspond to the 940°C reaction and the 211 peritectic melting. Apparently the partial pressure of CO2 and H20 at binder burn-out temperatures do not affect the thermal events at 940 ° C and 1000 ° C. For additional information about the 940 °C reaction. DTA was performed in oxygen. Oxygen was pulled out through tube packed with anhydrous calcium sulfate to dry it. This data show only one endothermic event corresponding to the peritectic decomposition of 123 at about 1030°C as reported in the literature (Fig. 3). This experiment was performed three times and the result was the same. The cause of the 940°C peak is therefore related to an interaction between atmosphere and 123 particles during DTA analysis. For further information, weight loss as a function of temperature was measured in air using simultaneous thermal analysis (STA). Fig. 4 shows the thermal gravimetric analysis ( T G ) for fine-1 powder which has a mean diameter of 1.6 ~tm (Table 1 (f). Also
~..8:L
i
I
200
i
I 400
.
I 600
Tef~oePature,
um
in
i
I 800
O=
i
i ~.000
i
1200
°C
Fig. 3. DTA of fine-I powder in oxygen;showingthe disappearance of 940°C peak and the shift of211 peritectic from 1000°C (air) to 1030°C (oxygen).
3.00
r
,
,
,
377
r
,
,
.
:
,
,
'.. 25
,
440
× ~
-0.50
430
-2.25
420
I 200
i 400
i
i
r
800
800
Tem~er2tur'e.
a
i
tooo
,4t0
:1300
°C
Fig. 4. Simultaneous thermal analysis of fine-I powder performed in air, showingthe weightloss; (a) TG and (b) DTG. plotted is the change in slope of the TGA (derivative thermogravimetric, D T G ) . Weight loss as determined by this analysis was 3.97_ 0.82% as shown in Fig. 4 (b). A theoretical weight loss of 3% is predicted during the transformation from 123 orthorhombic to 123 tetragonal based on the assumption that the initial powder was all YBa2Cu307 and that all Cu was converted to Cu +. An extra peak is clearly defined in the D T G just below the peritectic which is shown only as a shoulder in TG. Further research to determine the cause ot this shoulder is necessary. A quench study was performed to look for confirming evidence that no significant change occurs in phase composition below the peritectic melting point, when fine 123 is fired in oxygen. For the quench study samples made from fine-2 in the form of a bar were fired in oxygen to several temperatures, held at those temperatures for several hours and then air quenched. The fh-ing temperature and holding time were 1000 ° C for 6 h, 1010°C for 5 h and 1020°C for 9 h. At the end of the holding time the samples (on a setter) were pulled out fiom thefurnace and placed on a metal plate in open air. The cooling rate is estimated to be 200 ° C/ min. Fig. 5 shows the X-ray diffraction of quenched samples. From this figure it can be seen that the specimens fired at 1000°C, 1010°C and 1020°C consist mostly of 123 and a small peak of BaCuO2. This quench study shows that no substantial change has occurred in the crystalline phase assembly during thermal treatment in oxygen. The firing cycle is similar in range but not heating rate to the DTA cycle that showed a significant endothermic event at 940°C in air, but not in oxygen. This reaction should have
378
A.M.M. Barus, J.A.T. Taylor/Physica C 2 2 ~ (1994) 374-380 BOO
e . saa~ 6OO .'2.
tn b-
400
Z
200
~o
24
2e
a2 as 40 44 ,e .2 TW0-THeTA tOEGa~ES)
ae
eo
Fig. 5. X-ray diffraction o f fine-1 powder quenched from (a) 1000°C, (b) 1010°C, and (c) 1020°C showing the appearance o f BaCuO2 peaks. The specimens were held for 5, 9 and 6 h at each temperature before quenching.
led to a detectible change in phase assemblage if it occurred. The difference in heating rate could lead to a difference in the onset of a thermally driven reaction but not in suppression of the reaction.
4. Discussion The discovery that the 940°C reaction seen in DTA is related to crystallite size and atmosphere must be explained. The size of the thermal event at 940 °C depends on the amount of fines in the powder, as can be seen in Fig. 2. The 940°C peak in the DTA data has been reported by many different research teams working on 123 [ 5,11-14 ]. The relationship between this peak, particle size distribution and atmosphere has not been previously noted. The fact that no 940°C reaction is detected during DTA in oxygen means that reaction between fine particles and active gases in ambient air are related to this endothermic event. The reaction sequence has not been defined by this work. Reports in the literature of the 940°C peak ascribe the reaction to many different events. Rodriguez et al. [5] report a 940°C reaction during high-temperature X-ray diffraction analysis is supplemented with
optical microscopy during heating in air. They report the development of a liquid phase and crystalline BaCuO2 coexisting with 123 between 950 ° C and the peritectic melting of 123. The BaCuO2 phase was seen to increase in volume slowly according to the X-ray data. They concluded that the 940°C DTA event could be explained as a decomposition reaction of 123 which produces Y2Cu205 and BaCuO2. Such a reaction, they reason, would result from the metastable nature of the 123 phase, as described by Zhou [ 12]. No particle size measurements were made and the air was not cleaned of CO2 or water during processing, so comparisons with the DTA from this study are not possible. Sue et at. [ 13] reported the 940°C event on their DTA. They used powder calcined at 800°C which contained a detectable amount of BaCuO2. They suggested that the 940°C peak is due to entectic of BaCuO2-YBa2Cu3OT_x. This entectic has been reported by Roth et at. [ 14 ]. However, when the DTA was run using single-phase material ( 123 ) calcined at 910 ° or 970°C they saw only one thermal event at 1000 ° C. The particle size reported was in the range of 2-3 ttm. Since the atmosphere during analysis is not mentioned, comparison with this work is not possible. Aselage and Keefer [ 15 ] reported the appearance of a single peak at 940°C using DTA in flowing air at a rate of 10°C/rain when the sample was heated to 974°C or 994°C. No particle size was reported. The initial powder contains 123 with 21 l, BaCuO2 and CuO impurities. However they reported that postmortem analysis of the product showed pure 123. They suggested that the 940°C reaction is the peritectic reaction between 123 and CuO as follows: 123 + CuO-~211 + l i q u i d .
( 1)
Nevriva et at. [ 16 ] reported that within the range 950-1050°C there were two endothermic effects, at 911 ° C and 977 ° C using air atmosphere and at 957 ° C and 990°C in oxygen. No particle size information nor the purity of powder used were reported. They proposed that the first peak was a ternary entectic in the YOLs-BaO-CuO system. This deduction was based on a comparison of their data with DTA of known single phases; BaCuO2, YCuO2.s and YeBaCuOs. A melting temperature of a 123 superconductor
A.M.M. Barus, J.A.T. Taylor/Physica C 225 (1994) 374-380
significantly lower than expected has been reported by Ono and Tanaka [ 3 ]. They observed that in air 123 started to melt at 925°C which is about 15°C lower than the temperature of the endothermic event reported for the powder containing fine crystallites in this experiment. No particle size distribution was reported. Their results are probably not the peritectic melting of 123 as they believed but the same reaction seen at 940°C in this work. None of the previous work identified a relationship between particle size and atmosphere. To isolate the factors that could be causative in the 940 °C reaction, some additional DTA work was carded out by Lindemer at Oak Ridge National Lab, using three of the powders prepared for this research [ 11 ]. The first powders were the very coarse (Table 1 (a)), the fine-1 powder (Table 1( f ) ) and the fine powder (Table 1 (g)). He observed that in flowing oxygen or air from which the carbon dioxide and water had been removed, there is only one thermal event at about 1000 ° C. These results imply that CO2 and H20 contained in ambient air may be responsible for the 940°C peak. He then exposed samples of both fine and coarse powders to ambient conditions for about a week and reanalyzed the powders with DTA in oxygen. He observed that for both fine powders there were two thermal events, one at 940°C and another at about 1000°C. Apparently, corrosion of fine 123 powder occurs during exposure to ambient conditions. The coarse powders did not show any change in a DTA trace as a result of the ambient exposure. It has been reported that 123 reacts with ambient atmosphere, especially moisture and CO2 [ 17-19 ]. Naito et al. [ 17] reported that when 123 powder is placed in a chamber with 78% relative humidity, 75% of the powder decomposed to BaCO3, CuO and 21 l (based on XRD) in a week, while at 5 I% relative humidity single-phase orthorhombic perovskite phase is still present after six months. Particle size was reported as in the range of a few microns. BaCO3, formed when Ba(OH)2, which was produced during hydrolysis, reacted with CO2 from the air. Fitch and Burdick [20] observed that when 123 powder was exposed to 100% relative humidity atmosphere at 80°C it decomposed into 211, CuO, BaCO3, Cu(OH)2 and possibly BaO and Y(OH)3. Particle size was not reported.
379
Horowitz et al. [ 18] exposed 123 powder to ambient atmosphere. No particle size was reported for this powder which was prepared by grinding calcined material. After 48 h exposure, the X-ray pattern showed the presence of BaCO3 which did not increase from 48-672 h of exposure. They also reported that calcined unground material exposed to ambient atmosphere for the same period shows no evidence of surface reaction. Yan et al. [ 19 ] proposed the reaction sequence between 123 and moisture and CO2 as: 2YBa2 Cu307 + 3H2 O--, Y2BaCuO5 + 3Ba(OH)2 + 5CUO+0.502,
(2)
Ba(OH)2 +CO2 ~,BaCO3 + H 2 0 .
(3)
Yah et al. [ 19] also reported that Cu(OH)2 X-ray reflections appeared when the sample was exposed to air at 85 °C and 85% relative humidity. Particle size was not reported. Bansal and Sandkuhl [ 21 ] exposed a bar made of 123 powder to 100% relative humidity at room temperature over eight weeks. They reported that the main corrosion product was BaCO3. The powder was reported as 100% less than 325 mesh (45 ~tm). While the reports in the literature vary, all predict decomposition of 123 in an ambient environment as a function of humidity and CO2 content. There is a possibility that the compounds present in 123 after exposure to ambient atmosphere may promote the 940 °C reaction. This hypothesis would explain the disappearance of the reaction when the experiment is performed in oxygen, where no CO2 nor H20 is present. No conclusive evidence of liquid phase developing below 1000 ° C in oxygen has been found in the microstructure of quenched samples. The presence of liquid phase at a lower temperature is useful for melt processing. Being able to perform liquid-phase sintering at about 950 °C might allow the development of the oriented microstructure necessary for high current density, using the correct stoichiometry for 123. Liquid phase sintering at low temperature is usually achieved by the addition of dopants or use of off-stoichiometry compositions which leads to non-superconducting phases at grain boundaries. The fine powder has more surface area than the coarse powder. Calculations based on panicle size distribution indicate that the fine powder
380
A.M.M. Barus, J.A. T. Taylor/Physica C 225 (1994) 374-380
( T a b l e 2 ( g ) ) has a b o u t 57 times as m u c h specific surface are a as coarse p o w d e r ( T a b l e 1 ( a ) ) . This estimate is based on the the following e q u a t i o n [ 22 ]:
= Z ¢i
(4)
where S M = s u r f a c e area in m2/kg, ¢ = v o l u m e fraction, ¢ , = s h a p e factor, a v = a v e r a g e size in m, D, = density in k g / m 3. The shape factor a n d density were a s s u m e d to be the same for b o t h powders. This surface area is the only difference between the fine a n d coarse powders. C o n v e n t i o n a l t h e o r y predicts no significant difference in surface energy in these crystallite size ranges. However, if the fines are corroding in a m b i e n t air at r o o m t e m p e r a t u r e while the coarse p o w d e r does not, either an u n i d e n t i f i e d factor is present o r the difference in surface area between coarse a n d fine powders is m o r e significant than expected.
5. Conclusions D u r i n g D T A o f 123 two e n d o t h e r m i c events were observed, one at T o n ~ t = 9 4 0 ° C a n d a n o t h e r at Tnn~t= 1000 ° C for p o w d e r s containing a fine fraction, while only one e n d o t h e r m i c event at Ton. set = 1000°C was o b s e r v e d for coarse powders. The first p e a k d i s a p p e a r s when the e x p e r i m e n t is perf o r m e d in oxygen atmosphere. T h e usual depression o f the 211 peritectic melting p o i n t as related to amb i e n t a t m o s p h e r e a n d due to the e q u i l i b r i u m that is affected by oxygen c o n c e n t r a t i o n is o b s e r v e d in this work.
References [ 1] P.K. Gallagher, Adv. Ceram. Mater. 2 (1987) 632. [2] R.S. Roth, C.J. Rawn, F. Beech, J.D. Whirler and J.O. Anderson, in: Research Update, 1988 - Ceramic Superconductors II, ed. M.F. "fan (Am. Ceram. So¢., Westerville, OH, 1988)pp. 13-26. [ 3 ] A. Ono and T. Tanaka, Jpn. J. Appl. Phys. 26 (1987) L825. [4] ICW. Lay and G.M. Ren]und, J. Am. Ceram. Soc. 73 (1990) 1208. [5] M. Rodriguez, R.L. Snyder, B.J. Chen, D.P. Matheis, S.T. Misture and V.D. Frechette, Physica C 206 (1993) 43. [6] The Fisher Catalog, Fisher Scientific, Springfield, NJ (1991). [7] S.E. Troller, S.D. Atkinson, P.A. Fuiercr, J.H. Adair and R.E. Newnham, Ceram. Bull 67 (1988) 759. [8]T.B. Liodemer, F.A. Washburn, C.S. Mac DougaU, R. Feenstra and O.B. Cavin, Physica C 178 ( 1991 ) 93. [9 ] J.E. Ullman, R.W. McCallum and J.D. Verhoeven, J. Mater. Res. 4 (1989) 752. [ 10] J. Takada, H. Kitaguchi, A. Osaka, Y. Miura, K. Takahashi, M. Takano, Y. Ikeda, Y. Bando, N. Yamamoto, Y. Oka and Y. Tomii, Jpn. J. Appl. Phys. 26 (1987) L 1707. [ 11 ] T. Lindemer, private communication (1992). [ 12 ] Z. Zhou and A. Navrotsky, J. Mater. Res. 7 (1992) 1. [ 13 ] S.R. Sue, M. O'Connor and M. Levinson, J. Mater. Res. 6 (1991) 245. [ 14] R.S. Roth, K.L. Davis and J.R. Dennis, Adv. Ceram. Mater. 2 (1988) 303. [ 15 ] T. Aselage and K. Keefer, J. Mater. Res. 3 (1988) 1279. [ 16 ] M. Nevriva, E. Poilert, J. Sestak and A. Triska, Thermochim. Acta 127 (1988) 395. [ 17 ] N. Naito, J. Kafalas, L. Jachim and M. Downey, J. Appl. Phys. 67 (1990) 3521. [ 18 ] H.S. Horowitz, R.K. Bordia, R.B. Flippen and R.E. Johnson, Mater. Res. Soc. Syrup. Proc. 99 (1988) 903. [ 19] M.F. Yan, R.L. Barns, H.M. O'Brian Jr., P.K. Gallagher, R.C. Sherwood and S. Jin, Appl. Phys. Lett. 51 ( 1987 ) 532. [ 20 ] L.D. Fitch and V.L Burdick, J. Am. Ceram. Soc. 72 (1989) 2020. [21 ] N.P. Bansal and A.L Sandkuhl, Appl. Phys. Lett. 52 (1988) 323. [22] J. Zheng, P.F. Johnson and J.S. Reed, J. Am. Ceram. Soc. 73 (1990) 1392.