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Processing, properties and microstructure of melt-processed Bi-2212 thick films
PhysicaC 235-240 (1994)3399-3400 North-Holland
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Processing, properties and microstructure o f melt-processed Bi-2212 thick films D. Buhl, Th. Lang, B. Heeb and LJ. Gauckler Nonmetallic Materials, ETH Zurich, 8092 Zurich, Switzerland Bi-2212 thick films were produced by tape casting and partial melting on Ag-substrates. Highly aligned, almost single-phase microstructures throughout the oxide thickness of 20 grn were achieved. Optimizing the processing parameters to increase the critical current density Jc lead to current densities of 18'000 A/cm2 at 77K/0T and 350'000 A/cm 2 at 4K/0T (ll.tV/cm criterion, magnetically measured). In this paper the influence of the processing parameters on microstructure and properties of the thick films is discussed. 1. EXPERIMENTAL 1.1. Fabrication of tapes by tape casting Powder of the stoichiometry Bi2.2Sr2.o5Cao.95Cu20x was prepared by the standard calcination process. This stoichiometry proved to be in the center of the single-phase region of the doublelayer superconductor [1, 2]. The Bi-2212 powder was mixed with the organic formulation consisting of solvent (ethanol), dispersant (triolein), plasticizer (polyethylene glycol, phtalic acid ester) and binder (polyvinyl butyral) and milled for 4 hours. This resulted in a non-toxic slurry which was cast into tapes by the doctor blade tape casting process [3]. The green tapes were 1.5 m long and 15 cm wide. The thickness of the tape was 100 grn and was reduced to 50 l-tm during drying. Samples of the desired shapes were then cut, put on silver foils (50 I.trn thick) and subjected to the heat treatment. The thickness of the tapes after the heat treatment was 20 I.trn. 1.2. Heat treatment including partial melting The heat treatment consisted of 4 main steps. First the organic additives used for the tape casting process were burned out by heating slowly to 500 °C and holding there for 10 hours. The second, crucial step was the partial melting. The samples were heated at 40 °C/h to the maximum temperature (870 - 890 °C) and then cooled down with 5 °C/h to the annealing temperature of 850 °C. This part of the heat treatment was done in 0 2 or air. The third step was the annealing of the samples at 850 °C in O 2 .The last step after cooling down to room temperature was a reduction treatment at 550 °C for 20
hours in flowing N 2 (p(O2) < 10-3 atm).
2. RESULTS AND DISCUSSION 2.1. Influence of processing on microstructure Partial melting and slow cooling of the samples results in a dense, highly textured and almost singlephase microstructure. From the analysis of the cross-section of the films and the XP,D patterns the misalignment of the grains can be estimated to be smaller than 5.5 o. All samples were textured homogeniously over the whole cross-section. The main secondary phases are 2201, a Cu-free phase with the stoichiometry 3430 and the Bi-free phase 014x24. The Cu-free phase has the shape of small cubes or of stalks in the case of heating more than 15 °C above the onset of melting (874 °C on Ag substrate in 02) . The Bi-free phase always occurs in the form of needles, independent of the maximum heat treatment temperature. The amount of the secondary phases strongly depends on the maximum temperature of the partial melting step. Heating more than 10 °C above the melting temperature results in 2212-grains with a high density of 2201-intergrowths (determined by the shape of the (001) peaks [5]), 2201-grains and large amounts of the Bi-free and Cu-free phases. These two phases are believed to be the solid phases of the peritectic melting, reacting with the liquid on cooling to either 2201 or 2212 depending on temperature and cooling rate. Annealing at 850 °C in 02 is well known to be a crucial step to transform the 2201 into the 2212 phase by liquid/solid and solid/solid reactions in
0921-4534/94/$07.00 © 1994- ElsevierScienceB.V. All rightsreserved. SSD! 0921-4534(94)02265-8
3400
D. Buhl et al./Physica C 235 240 (1994) 3399- 3400
bulk samples [4]. However, in thick films this heat treatment seems to have only limited influence on the phase composition being determined to a great extent by the partial melting conditions (temperature, oxygen partial pressure). At a given p(O:z) high maximum temperatures (> 15 °C above the onset of incongruent melting) lead to a high amount of secondary phases such as 3430 and 0J.Ax24. Their volume fraction can hardly be reduced even by a post solidification heat treatment at 850 °C. The 2212 formation in thick films is much faster than in bulk samples due to large surface-volume ratio promoting oxygen diffusion. In addition, the diffusion distance for the cations is shorter. Therefore, the 2212 formation can be realized by controlling the maximum heat temperature between 5 and 10 °C above the onset of melting and slow cooling. Reducing the samples at 550 ÜC in flowing N a does not affect the microstructure but adjusts the oxygen content of the sample and thus the optimum carrier density [6] leading to hi~her Tc.
shape of the jc-T curve for T > 30K. Reducing at 550 °C for 20 hours in N 2 shifts the critical temperature Tc and the irreversibility temperature Tin to higher values and thus increases Jc at 77K. The highest values achieved so far are Tc = 94K, Titr = 89K, jc= 18'000 A/cm 2 (77K/0T) and 350'000 A/cm 2 (4K/0T) measured magnetically using the 1 pV/cm criterion. 2()O(XI
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2.2. Influence of processing on properties The maximum heat treatment temperature strongly influences the critical current density Jc. In riga the critical current densities determined at 77K/0T are shown as a function of the maximum temperature. Maximum Jc are obtained only in the narrow range of 876 + 3 °C as maximum temperature. The inset of fig.1 shows that different maximum temperatures shift the jc-T curve parallel to higher or lower values without a remarkable change in the shape of the curves. This indicates that Jc is controlled by the same parameter over the whole temperature range from 10 to 77K. We therefore suggest that this behavior can be attributed to the connectivity of the current leading paths rather than to pinning or granularity. In contrast to the maximum heat temperature annealing at 850 °C up to 75 hours did not influence the jc-T curve. The influence of oxygen partial pressure during melt processing on Jc was investigated. Samples processed in the temperature regime between 5 to 10 °C above the onset of melting ( Tonset(air) = 860 °C, TOnset (02) = 874 °C) showed no dependence of Jc from the processing atmosphere below 30K. At higher temperatures Jc of the samples processed in air are clearly below Jc for the samples processed in O a. Thus the processing atmosphere changes the
pV/cm crhcri,,n 875
~t) m [ i x i r h t J I l l leM/pcraTtlrc / " C
l~g5
Figure l'jc as a function of the maximum heat treatment temperature
3. CONCLUSIONS Highly aligned Bi-2212 thick films were prepared by tape casting and partial melting on Ag-substrates. The maximum heat treatment temperature was found to be the most important processing parameter for microstructure and properties. It has to be controlled within 5 to 10 °C above the onset of the peritectic melting to minimize the amount of secondary phases and to maximize the critical current density at 4 and 77K.
REFERENCES 1. R Majewski et al., MRS Proc.275, (1992) 627. 2. R. M~iller et al., Physica C, 203(3,4) (1992) 299. 3. J. Kase et al., IEEE, 27 (1991) 1254. 4. B. Hecb et al., J. Mater. Res., 8 (1993) 2170. 5. H. Heinrich et al., to be published. 6. Th. Schweizer, Diss. ETH No. 10167 (1993).
i ,~' (1
P H Y S ] C A (~
Processing, properties and microstructure o f melt-processed Bi-2212 thick films D. Buhl, Th. Lang, B. Heeb and LJ. Gauckler Nonmetallic Materials, ETH Zurich, 8092 Zurich, Switzerland Bi-2212 thick films were produced by tape casting and partial melting on Ag-substrates. Highly aligned, almost single-phase microstructures throughout the oxide thickness of 20 grn were achieved. Optimizing the processing parameters to increase the critical current density Jc lead to current densities of 18'000 A/cm2 at 77K/0T and 350'000 A/cm 2 at 4K/0T (ll.tV/cm criterion, magnetically measured). In this paper the influence of the processing parameters on microstructure and properties of the thick films is discussed. 1. EXPERIMENTAL 1.1. Fabrication of tapes by tape casting Powder of the stoichiometry Bi2.2Sr2.o5Cao.95Cu20x was prepared by the standard calcination process. This stoichiometry proved to be in the center of the single-phase region of the doublelayer superconductor [1, 2]. The Bi-2212 powder was mixed with the organic formulation consisting of solvent (ethanol), dispersant (triolein), plasticizer (polyethylene glycol, phtalic acid ester) and binder (polyvinyl butyral) and milled for 4 hours. This resulted in a non-toxic slurry which was cast into tapes by the doctor blade tape casting process [3]. The green tapes were 1.5 m long and 15 cm wide. The thickness of the tape was 100 grn and was reduced to 50 l-tm during drying. Samples of the desired shapes were then cut, put on silver foils (50 I.trn thick) and subjected to the heat treatment. The thickness of the tapes after the heat treatment was 20 I.trn. 1.2. Heat treatment including partial melting The heat treatment consisted of 4 main steps. First the organic additives used for the tape casting process were burned out by heating slowly to 500 °C and holding there for 10 hours. The second, crucial step was the partial melting. The samples were heated at 40 °C/h to the maximum temperature (870 - 890 °C) and then cooled down with 5 °C/h to the annealing temperature of 850 °C. This part of the heat treatment was done in 0 2 or air. The third step was the annealing of the samples at 850 °C in O 2 .The last step after cooling down to room temperature was a reduction treatment at 550 °C for 20
hours in flowing N 2 (p(O2) < 10-3 atm).
2. RESULTS AND DISCUSSION 2.1. Influence of processing on microstructure Partial melting and slow cooling of the samples results in a dense, highly textured and almost singlephase microstructure. From the analysis of the cross-section of the films and the XP,D patterns the misalignment of the grains can be estimated to be smaller than 5.5 o. All samples were textured homogeniously over the whole cross-section. The main secondary phases are 2201, a Cu-free phase with the stoichiometry 3430 and the Bi-free phase 014x24. The Cu-free phase has the shape of small cubes or of stalks in the case of heating more than 15 °C above the onset of melting (874 °C on Ag substrate in 02) . The Bi-free phase always occurs in the form of needles, independent of the maximum heat treatment temperature. The amount of the secondary phases strongly depends on the maximum temperature of the partial melting step. Heating more than 10 °C above the melting temperature results in 2212-grains with a high density of 2201-intergrowths (determined by the shape of the (001) peaks [5]), 2201-grains and large amounts of the Bi-free and Cu-free phases. These two phases are believed to be the solid phases of the peritectic melting, reacting with the liquid on cooling to either 2201 or 2212 depending on temperature and cooling rate. Annealing at 850 °C in 02 is well known to be a crucial step to transform the 2201 into the 2212 phase by liquid/solid and solid/solid reactions in
0921-4534/94/$07.00 © 1994- ElsevierScienceB.V. All rightsreserved. SSD! 0921-4534(94)02265-8
3400
D. Buhl et al./Physica C 235 240 (1994) 3399- 3400
bulk samples [4]. However, in thick films this heat treatment seems to have only limited influence on the phase composition being determined to a great extent by the partial melting conditions (temperature, oxygen partial pressure). At a given p(O:z) high maximum temperatures (> 15 °C above the onset of incongruent melting) lead to a high amount of secondary phases such as 3430 and 0J.Ax24. Their volume fraction can hardly be reduced even by a post solidification heat treatment at 850 °C. The 2212 formation in thick films is much faster than in bulk samples due to large surface-volume ratio promoting oxygen diffusion. In addition, the diffusion distance for the cations is shorter. Therefore, the 2212 formation can be realized by controlling the maximum heat temperature between 5 and 10 °C above the onset of melting and slow cooling. Reducing the samples at 550 ÜC in flowing N a does not affect the microstructure but adjusts the oxygen content of the sample and thus the optimum carrier density [6] leading to hi~her Tc.
shape of the jc-T curve for T > 30K. Reducing at 550 °C for 20 hours in N 2 shifts the critical temperature Tc and the irreversibility temperature Tin to higher values and thus increases Jc at 77K. The highest values achieved so far are Tc = 94K, Titr = 89K, jc= 18'000 A/cm 2 (77K/0T) and 350'000 A/cm 2 (4K/0T) measured magnetically using the 1 pV/cm criterion. 2()O(XI
E ,<
I 0(100
I
'
i
g ,'(I
2.2. Influence of processing on properties The maximum heat treatment temperature strongly influences the critical current density Jc. In riga the critical current densities determined at 77K/0T are shown as a function of the maximum temperature. Maximum Jc are obtained only in the narrow range of 876 + 3 °C as maximum temperature. The inset of fig.1 shows that different maximum temperatures shift the jc-T curve parallel to higher or lower values without a remarkable change in the shape of the curves. This indicates that Jc is controlled by the same parameter over the whole temperature range from 10 to 77K. We therefore suggest that this behavior can be attributed to the connectivity of the current leading paths rather than to pinning or granularity. In contrast to the maximum heat temperature annealing at 850 °C up to 75 hours did not influence the jc-T curve. The influence of oxygen partial pressure during melt processing on Jc was investigated. Samples processed in the temperature regime between 5 to 10 °C above the onset of melting ( Tonset(air) = 860 °C, TOnset (02) = 874 °C) showed no dependence of Jc from the processing atmosphere below 30K. At higher temperatures Jc of the samples processed in air are clearly below Jc for the samples processed in O a. Thus the processing atmosphere changes the
pV/cm crhcri,,n 875
~t) m [ i x i r h t J I l l leM/pcraTtlrc / " C
l~g5
Figure l'jc as a function of the maximum heat treatment temperature
3. CONCLUSIONS Highly aligned Bi-2212 thick films were prepared by tape casting and partial melting on Ag-substrates. The maximum heat treatment temperature was found to be the most important processing parameter for microstructure and properties. It has to be controlled within 5 to 10 °C above the onset of the peritectic melting to minimize the amount of secondary phases and to maximize the critical current density at 4 and 77K.
REFERENCES 1. R Majewski et al., MRS Proc.275, (1992) 627. 2. R. M~iller et al., Physica C, 203(3,4) (1992) 299. 3. J. Kase et al., IEEE, 27 (1991) 1254. 4. B. Hecb et al., J. Mater. Res., 8 (1993) 2170. 5. H. Heinrich et al., to be published. 6. Th. Schweizer, Diss. ETH No. 10167 (1993).
i ,~' (1