- Email: [email protected]
palladium alloy
Matertals Letters North-Holland
12 ( 199 I ) 29 l-294
YBa,Cu,O,
processed with a silver/palladium
alloy
G.A. Risch, M.T. Lanagan, W. Wu, KC. Goretta Materials and ComponenntsTechnology l?rvision, Argonne National Laborator): Argonne, IL 604351. L’S.4
and B.M. Moon Swwceand Technology Received
19 June I99
Centerfor Superconductiwtv, Cfniwrsit~~ofIllinois at t’rbana-Champaign,
1Wmna. iL 6 IKOI. LSA
I ; in final form 5 August 199 I
YBa,Cu,O, powders were sintered in sealed Ag and Ag/ 10% Pd wraps and melted on Ag/ 10% Pd foils. The melting was performed at 980°C under an 0, pressure of lo3 Pa. Although YBazCu,O, was successfully melt-processed, some interdiffusion occurred between it and the Ag/Pd alloy. The superconducting transition temperature was 86-88 K for YBa>Cu,O, processed with Ag/Pd alloy and 89 K for YBazCu,O,y processed with pure Ag.
1, Intr~u~tion Very high critical current densities have been reported [ l-41 in bulk melt-textured YBa,Cu,O,X (YBCO). Bismuth-based high-?;r superconductors have been partially melted in Ag tubes or on Ag substrates to form long wires or tapes with good superconducting properties [ 5-71. YBCO has a higher melting temperature than do the B&based superconductors [ 8,9], however, and conventional melt-processing of Ag-clad YBCO wires has not proved successful. Some success has been reported for melting on Ag/Ni alloys [lo]. The melting temperature of YBCO decreases with O1 partial pressure [ 81, and the melting point of Ag increases with additions of Pd [ 111. For example, in lo/o 02, YBCO melts at z 970°C and 90% Ag/ 10% Pd melts at x 1000°C. An opportunity exists for melttexturing YBCO wires in Ag/Pd, but little information has been reported on this system. It is known that Pd removes oxygen from YBCO [12]; therefore. the superconducting transition temperature, T,, may be reduced for YBCO in contact with Ag/Pd alloys. Melting of YBCO doped directly with Pd has produced a 90 K superconductor, but the transition broadened substantially compared with undoped 0167-577x/9
I /$ 03.50 0 199
I Elsevier Science Publishers
YBCO [ 131. Extensive scanning electron microscopy (SEM ) has been performed on YBCO melted in air on 70% Ag/30% Pd and 30% Ag/70°h Pd substrates [ 141, It was found that Ba and Cu diffused into the substrates and that the 70% Ag/30% Pd substrate appeared to react more strongly with the YBCO than did the 30% Ag/70% Pd substrate. The few direct references to the YBCO-Ag/Pd system fail to provide a consistent understanding of the possibilities of coprocessing these constituents. This work was undertaken, therefore, to assess the feasibility of making bulk YBCO-Ag/Pd superconductors.
2. Experimental
methods
YBCO was synthesized by solid-state reaction of reagent-grade YzOj, BaCO,, and CuO powders. The powders were mixed for 16 h in polyethylene jars containing methanol and ZrO, grinding media and then dried. The mixture was placed in a shallow A1203 crucible and heated at 800°C for 4 h in flowing O2 at a reduced pressure of z 2.6~ i02 Pa. Grinding in a tungsten carbide rotary mill yielded a powder with an average particle diameter of =: 2 urn.
B.V. All rights reserved.
291
Volume
12, number
4
MATERIALS
The resulting YBCO powder was phase pure by Xray diffraction and differential thermal analysis (DTA) [ 151. The YBCO was processed with a 90% Ag/ 10% Pd foil in two ways. To examine effects of melting, YBCO powders were placed on the foil and melted in 1% O2 at 980°C. The furnace was cooled from 980 to 955°C in 24 h to allow adequate superconductor/ substrate interaction and to induce grain growth. The specimens were then annealed in flowing 0, at 450°C for 12 h. To examine the effects of annealing in O2 more closely, YBCO powder was placed in tightly sealed Ag and Ag/Pd foils, sintered in O2 at 910°C for 2 h, and annealed at 450°C for 36 h. This heat treatment did not produce a liquid phase and therefore little incorporation of either Ag or Pd into YBCO was expected. Possible effects on T, of Pd within the YBCO could therefore be separated from effects of oxygen loss from the YBCO into the Ag/Pd alloy. The melting temperature of high-T, superconductors can be affected by Ag because of the formation of eutectics [ 161. Therefore, DTA was performed on the YBCO powder and on mixtures of the YBCO and small pieces of the Ag and Ag/Pd foils. The heating rate for the measurements was 60”C/h, the atmosphere was air, and A&O3 was used as the reference. All of the specimens were examined by SEM. T, values were determined magnetically. The specimens were cooled in zero field, a field of 40 G was applied, and the magnetization was monitored during warming.
3. Results and discussion The DTA indicated that the YBCO powder used was of high phase-purity. Onset of melting in air was observed at z 1005°C (fig. la), which is typical of YBCO [ 81. No lower-temperature melts associated with BaCuOz-CuO eutectics [ 171 were observed. The YBCO-Ag mixture exhibited two melts: a sharp melt with an onset at z 944 oC and a broader, deeper melt with an onset at x 970’ C (fig. 1b). The first melt is associated with melting of Ag in the presence of 02; the onset is close to 93 1 “C, the invariant temperature for a three-phase mixture of Ag, 02, and a liquid [ 18,191. The measured onset at 944°C is slightly higher than that of 939-94 1 “C reported for 292
November
LETTERS
I
I
850
I
I
900
I
950
199
I
’
I
1000
-I-[“Cl Fig. I. DTA traces of (a) pure YBCO, (b) YBCO+Ag, YBCO+Ag/lO% I’d.
and (c)
YBCO/Ag mixtures in air [ 18,191. The observed discrepancy may be due to thermal inertia causing a slight lag in our DTA measurements or to failing to reach equilibrium because of the coarse mixture of components that we employed. The trace of the YBCO-Ag/Pd mixture consisted of a broad endotherm with an onset at ~978°C (fig. lc). Thus, the first melt in the YBCO-Ag/Pd mixture occurred about 34°C higher than that for the YBCO-Ag mixture. No separate lower-temperature endotherm associated with melting of the metal was observed, but a clear shoulder on the low-temperature side was present and it is inferred that the shoulder may be an overlap of an endotherm from melting of Ag/Pd/ O2 on the endotherm for melting of the YBCO. The relatively high melting temperature in the YBCO-Ag/Pd system should allow for the possibility of melt-texturing under reduced O2 pressures. Preliminary experiments conducted at 980°C in a 1% O2 environment indicated that melt-processing is possible. Large grains of YBCO were produced without gross melting of the Ag/ 10% Pd substrate (fig. 2). Some intrusions of superconductor into the substrate and small globules of the metal in the superconductor were observed. These intrusions extended approximately 200 urn into both host phases. In addition, Ba-Cu-rich regions, as revealed by Xray analysis in the SEM, were present at the superconductor/substrate interface. Large voids were also present along much of the interface (fig. 2 ). Similar phase development has been observed when YBCO
Volume
12.number 4
MATERIALS
November
LETTERS
I99 1
induced a decrease in T, of 3 K. YBCO melted on the Ag/Pd substrate had a T, onset of 88 K, but only a small fraction of the material exhibited a r, above 86 K (fig. 3; the small fraction of material with a I;: above 86 K is clearly visible when the scale is expanded). (The diamagnetic response of the meltprocessed YBCO was much larger than those of the sintered powders; thus, data for the melted sample are shown on a different scale. The differences in diamagnetic response are attributable to the much smaller grain sizes of the solid-state sintered specimens (a and b) and the relative amounts of total superconductor present in each sample.) These results agree with expectations from the literature. ‘4s reported by Wagener et al. [ 121. Pd will
Fig. 2. SEM micrograph of (a) elongated grains of YBCO grown from the partial melt and (b) interface between YBCO (top) and Ag/ 10% Pd (bottom); in (b), the dark gray region is rich in Ba and Cu and the black region is a void.
is melted on pure Ag [ 201. Work is now focused on improving the microstructure of the YBCO melted on Ag/Pd through the use of cooling in a controlled gradient. Two possibilities should be borne in mind when considering the effects of AgjPd on the T, value of Y BCO. The first is whether the alloy leached oxygen from the superconductor and thus depressed the T,. The second is whether the metals became incorporated into the YBCO and altered the Tc. As a first approximation, the solid-state sintering experiment was designed to address the first possibility and the melting experiment was designed to answer the second. In the sintering study, most of the YBCO was very close tothe Ag or the Ag/Pd. It was found that the onset of T, was 89 K for YBCO sintered with Ag and 86 K for YBCO sintered with Ag/Pd. Thus, the presence of the Pd in the Ag alloy appears to have
O-
l
l e*e
-
l
-10
0 l
(4
-20 -
l
0
-30
. 40
l l
I 50
’ 60
’ 70
’
’ 80
’
’ 90
100
T WI Fig. 3. Magnetization T, values of (a) YBCO sintered with Ag, (b) YBCO sintered with Ag/lO”~ Pd, and (c) YBCO melted on Ag/ 10% Pd.
293
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MATERIALS LETTERS
leach oxygen from YBCO. Hence the 3 K reduction of 7; for YBCO adjacent to the Pd-containing alloy was not surprising. As reported by Kammlott et al. [ 13 1, molten YBCO appears to incorporate little Pd, and thus little or no additional depression of T, was caused by the melting. A small amount of superconductor with a T, value of 88 K was present in the YBCO melted on the Ag/Pd. The much larger grains of the partially melted YBCO and their relatively poor proximity to the Ag/Pd may have allowed for the retention of some higher-T= material. It is not yet clear whether Ag/Pd alloys will be of use in melt-processing YBCO. It is important to examine alloys with higher Pd contents because they are more likely than the 10% Pd alloy to resist melting and intrusion into the superconductor layer. It must be determined, however, whether higher Pd concentrations will produce excessive reductions in I-,.
4. Summa~ YBCO was successfully melt-processed on a substrate of 90% Ag/ 10% Pd at 980°C in a 1% O2 atmosphere. The Pd addition raised the onset of melting temperature by ~34 K compared with the YBCO-Ag system. T, values were depressed by x 3 K by the Pd. It is recommended that alloys of higher Pd content be investigated for possible use.
Acknowledgement We are grateful to C.-T. Wu for rolling the Ag/Pd tape, to A.C. Biondo for providing the YBCO powder, and to Nan Chen for performing the DTA measurements. This work was supported by the U.S. Department of Energy (DOE), Conservation and Renewable Energy, as part of a DOE program to develop electric power technology (GAR, MTL, WW), and Basic Energy Sciences-Materials Sciences
294
November 199 1
(KCG) under Contract W-3 l-109-Eng-38; and by the National Science Foundation (DMR 8809854) through the Science and Technology Center for Superconductivity (BMM).
References [ 1] S. Jin, T.H. Tiefel, R.C. Sherwood, M.E. Davis, R.B. van Dover, G.W. KammIott, R.A. Fastnacht and H.D. Keith, Appl. Phys. Letters 52 ( 1988) 2074. f2] K. Salama, V. Selvamani~kam, L. Gao and K. Sun, Appl. Phys. Letters 54 ( 1989) 2352. [ 31 M. Murakami, M. Morita, K. Doi and K. Miyamoto, Japan. J. Appl. Phys. 28 ( 1989) 1189. [4] D. Shi, M.M. Fang, J. Akujieze, M. Xu, J.G. Chen and C. Segre, Appl. Phys. Letters 57 (1990) 2606. [ 51 S. Jin and J.E. Graebner, Mater. Sci. Eng. B 7 ( 1991) 243. [6] K. Heine, J. Tenbrink and M. Thoner, Appl. Phys. Letters 55 (1989) 2441. [7] R.D. Ray II and E.E. Hellstrom, Appl. Phys. Letters 57 ( 1990) 2948. [ 81 N. Chen, D. Shi and KC. Goretta, J. Appl. Phys. 66 ( 1989) 2485. [9] EA. Karbarz, O.D. Lacy, KC. Goretta, U. Balachandmn, D. Shi, J.G. Chen, M. Xu and M.C. Hash, Mater. Res. Bull. 25 (1990) 251. [IO] C. Zhao, Y. Huang, R. Wang, Y. Wang and F. Deng, Supercond. Sci. Technol. 3 (1990) 587. [ 111R. Ruer, Z. Anorg. Chem. 5 1 ( 1906) 315. [ 121 T.J. Wagener, Y. Gao, I.M. Vitomirov, CM. Aldao, J.J. Joyce, C. Capasso, J.H. Weaver and D.W. Capone II, Phys. Rev. B 38 (1988) 232. [ 131 G.W. Kammlott, T.H. Tiefel and S. Jin, Appl. Phys. Letters 56 ( 1990) 2459. [ 141 J.L. Porter, T.K. Vethanayagam, R.L. Snyder and J.A.T. Taylor, J. Am. Ceram. Sot. 73 ( 1990) 1760. [ 151 U. Balachandran, R.B. Poeppel, J.E. Emerson, S.A. Johnson, M.T. Lanagan, C.A. Youngdahl, D. Shi, K.C. Goretta and N.G. Eror, Mater. Letters 8 (1989) 454. [ I6 ] R.E. Loehman, A.P. Tomsia, J.A. Pask and A.H. Carim, PhysicaC 170 (1990) I. [ 171 T. Asalage and K. Keefer, J. Mater. Res. 3 (1988) 1279. [I81 J.C. Chaston, in: Metals handbook, Vol. 8, 8th Ed. (Am. Sot. Met., Metals Park, OH, 1973) pp. 254, 340. [ 191 Y.-L. Jeng, Northwestern University, private communication (1991 ). [ZO] W. Wu, Argonne National Laboratory, unpublished results (1991).
12 ( 199 I ) 29 l-294
YBa,Cu,O,
processed with a silver/palladium
alloy
G.A. Risch, M.T. Lanagan, W. Wu, KC. Goretta Materials and ComponenntsTechnology l?rvision, Argonne National Laborator): Argonne, IL 604351. L’S.4
and B.M. Moon Swwceand Technology Received
19 June I99
Centerfor Superconductiwtv, Cfniwrsit~~ofIllinois at t’rbana-Champaign,
1Wmna. iL 6 IKOI. LSA
I ; in final form 5 August 199 I
YBa,Cu,O, powders were sintered in sealed Ag and Ag/ 10% Pd wraps and melted on Ag/ 10% Pd foils. The melting was performed at 980°C under an 0, pressure of lo3 Pa. Although YBazCu,O, was successfully melt-processed, some interdiffusion occurred between it and the Ag/Pd alloy. The superconducting transition temperature was 86-88 K for YBa>Cu,O, processed with Ag/Pd alloy and 89 K for YBazCu,O,y processed with pure Ag.
1, Intr~u~tion Very high critical current densities have been reported [ l-41 in bulk melt-textured YBa,Cu,O,X (YBCO). Bismuth-based high-?;r superconductors have been partially melted in Ag tubes or on Ag substrates to form long wires or tapes with good superconducting properties [ 5-71. YBCO has a higher melting temperature than do the B&based superconductors [ 8,9], however, and conventional melt-processing of Ag-clad YBCO wires has not proved successful. Some success has been reported for melting on Ag/Ni alloys [lo]. The melting temperature of YBCO decreases with O1 partial pressure [ 81, and the melting point of Ag increases with additions of Pd [ 111. For example, in lo/o 02, YBCO melts at z 970°C and 90% Ag/ 10% Pd melts at x 1000°C. An opportunity exists for melttexturing YBCO wires in Ag/Pd, but little information has been reported on this system. It is known that Pd removes oxygen from YBCO [12]; therefore. the superconducting transition temperature, T,, may be reduced for YBCO in contact with Ag/Pd alloys. Melting of YBCO doped directly with Pd has produced a 90 K superconductor, but the transition broadened substantially compared with undoped 0167-577x/9
I /$ 03.50 0 199
I Elsevier Science Publishers
YBCO [ 131. Extensive scanning electron microscopy (SEM ) has been performed on YBCO melted in air on 70% Ag/30% Pd and 30% Ag/70°h Pd substrates [ 141, It was found that Ba and Cu diffused into the substrates and that the 70% Ag/30% Pd substrate appeared to react more strongly with the YBCO than did the 30% Ag/70% Pd substrate. The few direct references to the YBCO-Ag/Pd system fail to provide a consistent understanding of the possibilities of coprocessing these constituents. This work was undertaken, therefore, to assess the feasibility of making bulk YBCO-Ag/Pd superconductors.
2. Experimental
methods
YBCO was synthesized by solid-state reaction of reagent-grade YzOj, BaCO,, and CuO powders. The powders were mixed for 16 h in polyethylene jars containing methanol and ZrO, grinding media and then dried. The mixture was placed in a shallow A1203 crucible and heated at 800°C for 4 h in flowing O2 at a reduced pressure of z 2.6~ i02 Pa. Grinding in a tungsten carbide rotary mill yielded a powder with an average particle diameter of =: 2 urn.
B.V. All rights reserved.
291
Volume
12, number
4
MATERIALS
The resulting YBCO powder was phase pure by Xray diffraction and differential thermal analysis (DTA) [ 151. The YBCO was processed with a 90% Ag/ 10% Pd foil in two ways. To examine effects of melting, YBCO powders were placed on the foil and melted in 1% O2 at 980°C. The furnace was cooled from 980 to 955°C in 24 h to allow adequate superconductor/ substrate interaction and to induce grain growth. The specimens were then annealed in flowing 0, at 450°C for 12 h. To examine the effects of annealing in O2 more closely, YBCO powder was placed in tightly sealed Ag and Ag/Pd foils, sintered in O2 at 910°C for 2 h, and annealed at 450°C for 36 h. This heat treatment did not produce a liquid phase and therefore little incorporation of either Ag or Pd into YBCO was expected. Possible effects on T, of Pd within the YBCO could therefore be separated from effects of oxygen loss from the YBCO into the Ag/Pd alloy. The melting temperature of high-T, superconductors can be affected by Ag because of the formation of eutectics [ 161. Therefore, DTA was performed on the YBCO powder and on mixtures of the YBCO and small pieces of the Ag and Ag/Pd foils. The heating rate for the measurements was 60”C/h, the atmosphere was air, and A&O3 was used as the reference. All of the specimens were examined by SEM. T, values were determined magnetically. The specimens were cooled in zero field, a field of 40 G was applied, and the magnetization was monitored during warming.
3. Results and discussion The DTA indicated that the YBCO powder used was of high phase-purity. Onset of melting in air was observed at z 1005°C (fig. la), which is typical of YBCO [ 81. No lower-temperature melts associated with BaCuOz-CuO eutectics [ 171 were observed. The YBCO-Ag mixture exhibited two melts: a sharp melt with an onset at z 944 oC and a broader, deeper melt with an onset at x 970’ C (fig. 1b). The first melt is associated with melting of Ag in the presence of 02; the onset is close to 93 1 “C, the invariant temperature for a three-phase mixture of Ag, 02, and a liquid [ 18,191. The measured onset at 944°C is slightly higher than that of 939-94 1 “C reported for 292
November
LETTERS
I
I
850
I
I
900
I
950
199
I
’
I
1000
-I-[“Cl Fig. I. DTA traces of (a) pure YBCO, (b) YBCO+Ag, YBCO+Ag/lO% I’d.
and (c)
YBCO/Ag mixtures in air [ 18,191. The observed discrepancy may be due to thermal inertia causing a slight lag in our DTA measurements or to failing to reach equilibrium because of the coarse mixture of components that we employed. The trace of the YBCO-Ag/Pd mixture consisted of a broad endotherm with an onset at ~978°C (fig. lc). Thus, the first melt in the YBCO-Ag/Pd mixture occurred about 34°C higher than that for the YBCO-Ag mixture. No separate lower-temperature endotherm associated with melting of the metal was observed, but a clear shoulder on the low-temperature side was present and it is inferred that the shoulder may be an overlap of an endotherm from melting of Ag/Pd/ O2 on the endotherm for melting of the YBCO. The relatively high melting temperature in the YBCO-Ag/Pd system should allow for the possibility of melt-texturing under reduced O2 pressures. Preliminary experiments conducted at 980°C in a 1% O2 environment indicated that melt-processing is possible. Large grains of YBCO were produced without gross melting of the Ag/ 10% Pd substrate (fig. 2). Some intrusions of superconductor into the substrate and small globules of the metal in the superconductor were observed. These intrusions extended approximately 200 urn into both host phases. In addition, Ba-Cu-rich regions, as revealed by Xray analysis in the SEM, were present at the superconductor/substrate interface. Large voids were also present along much of the interface (fig. 2 ). Similar phase development has been observed when YBCO
Volume
12.number 4
MATERIALS
November
LETTERS
I99 1
induced a decrease in T, of 3 K. YBCO melted on the Ag/Pd substrate had a T, onset of 88 K, but only a small fraction of the material exhibited a r, above 86 K (fig. 3; the small fraction of material with a I;: above 86 K is clearly visible when the scale is expanded). (The diamagnetic response of the meltprocessed YBCO was much larger than those of the sintered powders; thus, data for the melted sample are shown on a different scale. The differences in diamagnetic response are attributable to the much smaller grain sizes of the solid-state sintered specimens (a and b) and the relative amounts of total superconductor present in each sample.) These results agree with expectations from the literature. ‘4s reported by Wagener et al. [ 121. Pd will
Fig. 2. SEM micrograph of (a) elongated grains of YBCO grown from the partial melt and (b) interface between YBCO (top) and Ag/ 10% Pd (bottom); in (b), the dark gray region is rich in Ba and Cu and the black region is a void.
is melted on pure Ag [ 201. Work is now focused on improving the microstructure of the YBCO melted on Ag/Pd through the use of cooling in a controlled gradient. Two possibilities should be borne in mind when considering the effects of AgjPd on the T, value of Y BCO. The first is whether the alloy leached oxygen from the superconductor and thus depressed the T,. The second is whether the metals became incorporated into the YBCO and altered the Tc. As a first approximation, the solid-state sintering experiment was designed to address the first possibility and the melting experiment was designed to answer the second. In the sintering study, most of the YBCO was very close tothe Ag or the Ag/Pd. It was found that the onset of T, was 89 K for YBCO sintered with Ag and 86 K for YBCO sintered with Ag/Pd. Thus, the presence of the Pd in the Ag alloy appears to have
O-
l
l e*e
-
l
-10
0 l
(4
-20 -
l
0
-30
. 40
l l
I 50
’ 60
’ 70
’
’ 80
’
’ 90
100
T WI Fig. 3. Magnetization T, values of (a) YBCO sintered with Ag, (b) YBCO sintered with Ag/lO”~ Pd, and (c) YBCO melted on Ag/ 10% Pd.
293
Volume 12, number 4
MATERIALS LETTERS
leach oxygen from YBCO. Hence the 3 K reduction of 7; for YBCO adjacent to the Pd-containing alloy was not surprising. As reported by Kammlott et al. [ 13 1, molten YBCO appears to incorporate little Pd, and thus little or no additional depression of T, was caused by the melting. A small amount of superconductor with a T, value of 88 K was present in the YBCO melted on the Ag/Pd. The much larger grains of the partially melted YBCO and their relatively poor proximity to the Ag/Pd may have allowed for the retention of some higher-T= material. It is not yet clear whether Ag/Pd alloys will be of use in melt-processing YBCO. It is important to examine alloys with higher Pd contents because they are more likely than the 10% Pd alloy to resist melting and intrusion into the superconductor layer. It must be determined, however, whether higher Pd concentrations will produce excessive reductions in I-,.
4. Summa~ YBCO was successfully melt-processed on a substrate of 90% Ag/ 10% Pd at 980°C in a 1% O2 atmosphere. The Pd addition raised the onset of melting temperature by ~34 K compared with the YBCO-Ag system. T, values were depressed by x 3 K by the Pd. It is recommended that alloys of higher Pd content be investigated for possible use.
Acknowledgement We are grateful to C.-T. Wu for rolling the Ag/Pd tape, to A.C. Biondo for providing the YBCO powder, and to Nan Chen for performing the DTA measurements. This work was supported by the U.S. Department of Energy (DOE), Conservation and Renewable Energy, as part of a DOE program to develop electric power technology (GAR, MTL, WW), and Basic Energy Sciences-Materials Sciences
294
November 199 1
(KCG) under Contract W-3 l-109-Eng-38; and by the National Science Foundation (DMR 8809854) through the Science and Technology Center for Superconductivity (BMM).
References [ 1] S. Jin, T.H. Tiefel, R.C. Sherwood, M.E. Davis, R.B. van Dover, G.W. KammIott, R.A. Fastnacht and H.D. Keith, Appl. Phys. Letters 52 ( 1988) 2074. f2] K. Salama, V. Selvamani~kam, L. Gao and K. Sun, Appl. Phys. Letters 54 ( 1989) 2352. [ 31 M. Murakami, M. Morita, K. Doi and K. Miyamoto, Japan. J. Appl. Phys. 28 ( 1989) 1189. [4] D. Shi, M.M. Fang, J. Akujieze, M. Xu, J.G. Chen and C. Segre, Appl. Phys. Letters 57 (1990) 2606. [ 51 S. Jin and J.E. Graebner, Mater. Sci. Eng. B 7 ( 1991) 243. [6] K. Heine, J. Tenbrink and M. Thoner, Appl. Phys. Letters 55 (1989) 2441. [7] R.D. Ray II and E.E. Hellstrom, Appl. Phys. Letters 57 ( 1990) 2948. [ 81 N. Chen, D. Shi and KC. Goretta, J. Appl. Phys. 66 ( 1989) 2485. [9] EA. Karbarz, O.D. Lacy, KC. Goretta, U. Balachandmn, D. Shi, J.G. Chen, M. Xu and M.C. Hash, Mater. Res. Bull. 25 (1990) 251. [IO] C. Zhao, Y. Huang, R. Wang, Y. Wang and F. Deng, Supercond. Sci. Technol. 3 (1990) 587. [ 111R. Ruer, Z. Anorg. Chem. 5 1 ( 1906) 315. [ 121 T.J. Wagener, Y. Gao, I.M. Vitomirov, CM. Aldao, J.J. Joyce, C. Capasso, J.H. Weaver and D.W. Capone II, Phys. Rev. B 38 (1988) 232. [ 131 G.W. Kammlott, T.H. Tiefel and S. Jin, Appl. Phys. Letters 56 ( 1990) 2459. [ 141 J.L. Porter, T.K. Vethanayagam, R.L. Snyder and J.A.T. Taylor, J. Am. Ceram. Sot. 73 ( 1990) 1760. [ 151 U. Balachandran, R.B. Poeppel, J.E. Emerson, S.A. Johnson, M.T. Lanagan, C.A. Youngdahl, D. Shi, K.C. Goretta and N.G. Eror, Mater. Letters 8 (1989) 454. [ I6 ] R.E. Loehman, A.P. Tomsia, J.A. Pask and A.H. Carim, PhysicaC 170 (1990) I. [ 171 T. Asalage and K. Keefer, J. Mater. Res. 3 (1988) 1279. [I81 J.C. Chaston, in: Metals handbook, Vol. 8, 8th Ed. (Am. Sot. Met., Metals Park, OH, 1973) pp. 254, 340. [ 191 Y.-L. Jeng, Northwestern University, private communication (1991 ). [ZO] W. Wu, Argonne National Laboratory, unpublished results (1991).