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Influence of the maximum temperature during partial melt-processing of Bi-2212 thick films on microstructure and jc
Physica C 294 Ž1998. 7–16
Influence of the maximum temperature during partial melt-processing of Bi-2212 thick films on microstructure and jc Th. Lang ) , D. Buhl, L.J. Gauckler ETH Zurich, Department of Materials, Chair of Nonmetallic Materials, Swiss Federal Institute of Technology, Zurich CH-8092, Switzerland ¨ ¨ Received 20 March 1997; revised 10 July 1997; accepted 1 September 1997
Abstract The critical current density jc of Bi-2212 thick films Ž d s 60 mm. depends on the maximum processing temperature Tmax during the partial melting process. Processing at Tmax s 8938C leads to the highest jc of 3500 Arcm2 Ž77 K, 0 T.. A variation of Tmax by "5 K from the optimum temperature leads to a drop of jc below 80% of the maximum value. Processing below the optimum Tmax leads to insufficient consolidation. Partial melting at Tmax s 9048C leads to 5 vol% more residual peritectic phases with an average grain size about twice as large in the final microstructure compared to optimally processed samples. However, the drop of jc in samples processed at higher Tmax is attributed to inhomogeneities in the microstructure. These areas have a reduced film thickness and consist of second phase grains, pores, and misaligned 2212 platelets. The more frequent formation of these inhomogeneities is attributed to inhomogeneous nucleation of the Bi-2212 platelets upon slow cooling at a rate of 5 Krh from the partially molten state. In optimally processed thick films the nucleation of the Bi-2212 platelets occurs regularly in the whole cross section of the film. If processed at higher Tmax , fewer nucleation sites are present, leading to regions of high platelet density and regions containing no Bi-2212 grains. Due to the high capillary forces in the narrow gap between the Bi-2212 platelets, liquid is transported away from the platelet-free regions and stored between the Bi-2212 grains. Because of this phase separation, the solid peritectic grains persist in the areas where liquid was extracted. q 1998 Elsevier Science B.V. Keywords: Bi 2 Sr2 CaCu 2 O x ; High-temperature superconductor; Partial melt-processing; Maximum processing temperature; Critical current density
1. Introduction Bi 2 Sr2 CaCu 2 O x Ž Bi-2212 . combines several advantageous properties. It can be prepared weak-link free w1,2x by an easy partial melting process w3–5x, it contains no highly toxic elements, and is able to transport high currents in high external fields at low temperatures w6x. At elevated temperatures around 77 )
Corresponding author. Tel.: q41 1 632 56 51; Fax: q41 1 632 11 32; E-mail: [email protected].
K, however, pinning is rather weak compared to YBa 2 Cu 3 O x w7,8x. Nevertheless, polycrystalline Bi2212 can be prepared with reasonably high critical current densities exceeding 3 = 10 4 Arcm2 Ž77 K, 0 T. in thick films w9x and jc values of a few thousand Arcm2 in bulk components w10,11x. First industrial applications are being tested. In the fall of 1996 the Swiss–Swedish company Asea Brown Boveri ŽABB. installed a 1.2 MVA inductive fault current limiter working with large superconducting Bi-2212 rings with a diameter of 40 cm in a power plant w12x.
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 6 6 9 - 9
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The heat treatment used for the preparation of components of Bi-2212 is called partial melt processing. It consists of a partial melting step at a temperature Tmax which is above the solidus temperature of the Bi-2212 composition. The formation of a liquid phase facilitates the densification of the sample. After melting, the sample is cooled slowly at a rate of 5–10 Krh to a temperature below Tsolidus and annealed isothermally at that temperature in order to transform the multiphase microstructure back to Bi2212. We have shown earlier that each of the three mentioned processing steps is crucial on its own w13x. A further important processing step is the cooling period from the annealing temperature Že.g. 8508C. to room temperature w14,15x. The final properties as well as the microstructure depend on the cooling rate and the P ŽO 2 . during cooling. It was reported, that the Bi-2212 phase decomposes sluggishly below 6208C w16x. This can cause problems if the cooling rate is chosen too low. In addition, oxygen is taken up during cooling leading to an overdoped state of the superconductor and a reduced Tc . These facts must be considered when differently processed samples are compared. Nevertheless, a parameter of major importance is the maximum processing temperature Tmax . Shimoyama et al. w9x have observed a drastic drop of jc measured at 4.2 K in an external field of 10 T if their Bi-2212 thick films were processed above 8908C. They explained this behavior with the appearance of large amounts of the ŽSr,Ca. –Cu–O phase due to a large loss of Bi by vaporization at high temperatures. Grains of Bi-free phase were assumed to block the current flow and therefore reduce jc . Processing in presence of Bi 2 Al 4 O 9 reduced the volume fraction of the Bi-free phase in the final microstructure and better reproducibility of high jc values was achieved. However, jc still decreased if Tmax exceeded 9008C. This was attributed to areas of misoriented Bi-2212 grains between very large and well-aligned Bi-2212 platelets. The influence of the maximum processing temperature on jc and the microstructure of partial melt processed Bi-2212 tapes made by powder-in-tube was investigated by Endo et al. w17x. The highest jc values at 4.2 K in an external field of 1 T were obtained if processed at Tmax s 8858C. For Tmax 8858C the lack of liquid phase was responsible for
low jc values. At Tmax ) 8858C the authors observed an increased amount of Bi-free phase in the final microstructure. The final conclusion was that the higher volume fraction of second phases could not be solely responsible for the lower jc . The increased size of the second phase grains rather than the higher volume fraction reduced jc . Screen-printing and partial melting was used to prepare Bi-2212 thick films by Ko et al. w18x. Again a strong dependence of jc on the maximum processing temperature and the annealing time at Tmax was observed. Kanai et al. w19x observed that partial melting just above the incipient melting point gave the highest jc values in Bi-2212 wires. The formation and growth of voids inside the wire was assumed to be responsible for the drop of jc with rising maximum temperatures. Morgan et al. w20x even observed two maxima of the critical current density as a function of the maximum processing temperature in Bi-2212 tapes. The highest jc values were obtained in a narrow temperature window between 8758C and 8788C only. The problem of temperature homogeneity during processing of large samples of Bi-2212 Že.g. wound wires in a box furnace. was discussed by Haugan et al. w21x. The decrease of jc in their 90 m long wires compared to the 15 m wires was attributed to the sensitivity of jc to temperature nonuniformities in their box-type furnace. For future heat treatments a ring-type furnace with a very small temperature gradient is built. Polak et al. w22x explained the strong Tmax-dependence of jc at 4.2 K in their Bi-2212 tapes with the appearance of macropores, which appeared with a reduced density and hardness of the tape cores of the samples. No significant microstructural difference in phase content and grain size between optimally processed and low-jc samples was observed. We have reported previously w23,24x about the strong dependence of the maximum processing temperatures on the final superconducting properties in partial melt processed Bi-2212 thick films and bulk components. The critical current densities were higher in the whole temperature range from 4 K to 77 K in optimally processed samples compared to samples processed at Tmax exceeding the optimum range. This indicates that the current carrying cross
Th. Lang et al.r Physica C 294 (1998) 7–16
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section is reduced with increasing Tmax and excludes a different pinning behavior to be responsible for the drop of jc . A similar conclusion was drawn by Polak et al. w22x who found a correlation between the room-temperature resistivity and jc of their tapes. In this work we place special emphasis on the influence of Tmax on the final microstructure and the development of phases during the partial melt processing of Bi-2212 thick films in order to elucidate the dependence of jc on Tmax .
2. Experimental Powder with the stoichiometry Bi 2.2 Sr 2.05 Ca 0.95 Cu 2 O x was prepared by a standard calcination process, consisting of several firing steps at temperatures between 7508C and 8508C for a total of more than 200 h with several intermediate grindings. The chosen stoichiometry lies in the single phase region of Bi-2212 at temperatures above 8008C w25,26x. The powder was mixed with organic additives and tapecast onto glass as described elsewhere w28x. Samples with a diameter of 11 mm were cut from the dried tape, put on a silver substrate Ž dAg s 100 mm., and heat treated in a vertical furnace. A first set of samples was heated at a rate of 60 Krh to temperatures of 8868C, 8908C, 9008C, 9118C, 9228C, and 9308C, and quenched in oil from these temperatures immediately after they were reached Žsee Fig. 1Ža... Several other thick films were partial melt processed using the heat treatment schedule shown in Fig. 1Žb.. The samples were partially melted at Tmax between 8938C and 9048C, slowly cooled at a rate of 5 Krh to 8508C, and isothermally annealed at 8508C for 48 h. Some samples were quenched in oil from different temperatures during the heat treatment in order to investigate the microstructure of the high temperature state w27x. Samples used for jc measurements were not quenched but furnace cooled to room temperature. In order to adjust the oxygen content of these films and hence Tc , the specimens were subsequently annealed at 5508C for 30 h in flowing nitrogen Ž P ŽO 2 . s 0.001 atm.. Only samples that were furnace cooled to room temperature and subsequently annealed at 5508C for 30 h were used to measure the critical current density. The
Fig. 1. Žarb.: Heat treatment schedules used in this work. The schedule shown in Ža. was used to investigate the influence of temperature on the phase composition and microstructure of Bi2212 in the partially molten state Ž8868C -Tma x -9308C.. Schedule Žb. shows the heat treatment of a partial melting process used for the preparation of Bi-2212 thick films. The maximum processing temperature Tma x ranged from 8878C to 9048C. The doubleheaded arrows symbolize quenching procedures used to freeze the high temperature microstructures.
quenched samples were used for the microstructural investigations. The critical current density at 77 K in zero external field was measured in an AC-magnetometer at a frequency of 4–10 Hz. The experimental setup in combination with the sample size lead to a voltage criterion of E f 1 mVrcm. All heat treatments were performed in flowing oxygen, the final sample thickness was 60 mm. The quenched thick films were first analyzed by X-ray diffraction, then embedded in resin, ground and polished. The cross sections were analyzed using scanning electron microscopy ŽSEM., light microscopy, and energy dispersive X-ray spectroscopy
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ŽEDS.. The EDS data were corrected using standards with a known composition. The volume fractions and the average grain sizes of the phases appearing were measured by means of optical methods described elsewhere w29x. The thickness profiles of thick films were determined by light microscopy at 50 equidistant locations in each specimen.
3. Results 3.1. Influence of Tm a x on jc In order to verify the influence of the maximum processing temperature on the critical current density of Bi-2212 thick films under the given experimental conditions, several samples were partial melt processed with Tmax varying from 8878C to 9008C according to the heat treatment shown in Fig. 1Žb., except that the samples were not quenched after the isothermal annealing at 8508C, but furnace cooled to room temperature. After the partial melt processing, an additional annealing at lower temperature and lower P ŽO 2 . Ž5508C, 0.001 atm. was necessary to increase Tc above 90 K. We have shown earlier that such a reduction process is beneficial to increase Tc and the critical current density at 77 K w24x. Fig. 2
Fig. 3. Volume fractions of the phases in a Bi-2212 thick film which is put on a silver substrate and heated in oxygen below and above its solidus temperature at a rate of 60 Krh. The Cu-free phase has a composition of Bi 9 Sr11Ca 5 O x , the Bi-free phase belongs to the solid solution family 014x24 at temperatures below 9228C. Then the stoichiometry changes to 01 x1, 02 x1 and 01 x 0 w29x.
shows the normalized jc values as a function of the maximum processing temperature. The maximum jc was 3.500 Arcm2 Ž77 K, 0 T. in the sample processed at Tmax s 8938C. Small deviations from this temperature of "5 K lead to a strong decrease of jc to less than 80% of the maximum value. In the sample melt processed at 8878C, not enough liquid was produced to fully densify the structure, resulting in a low critical current density. The reason for the drop of jc with increasing maximum processing temperature was further investigated. Since the analyzed samples varied only in Tmax , this parameter must be responsible for the changes in jc . 3.2. Influence of Tm a x on the microstructure
Fig. 2. Influence of the maximum processing temperature Tma x on the normalized critical current density at 77 K in zero external field of fully processed Bi-2212 thick films Ž ds60 mm. prepared in the same furnace used for the quenching experiments. The maximum jc was 3.500 Arcm2 Ž77 K, 0 T..
The high-temperature microstructures of Bi-2212 thick films were investigated by quenching samples from different temperatures during heating at a rate of 60 Krh. When Bi-2212 is decomposed on a silver substrate in an oxygen atmosphere a liquid phase, two solid peritectic phases, and oxygen are formed. The solid phases are either Bi-free with an initial composition of Sr8.5 Ca 5.5 Cu 24 O x , or Cu-free with the stoichiometry Bi 9 Sr11Ca 5 O y w29x. Fig. 3 shows the vol-
Th. Lang et al.r Physica C 294 (1998) 7–16
Fig. 4. Comparison of two heat treatment schedules with different maximum temperatures. At the point marked with ‘ X ’ they coincide and from that point on both schedules are the same.
ume fractions of the solid and liquid phases as a function of the quenching temperature. Immediately after decomposition, the microstructure at 8908C consists of 70 vol% liquid, and 30 vol% solids. At 9118C the amount of liquid increased to 80 vol%. The composition of the Bi-free phase, which is 014x24 in the beginning Žthe abbreviation 014x24 stands for Bi 0 Sr14yx Ca x Cu 24 O y ., changes to 01 x1, 02 x1, and 01 x 0 at temperatures G 9228C as determined by EDS. However, in the temperature range between 8938C to 9048C, which are the maximum temperatures used for the partial melt processing
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described later, only small changes in the composition of the phases occur. The difference between two heat treatment schedules having two different maximum temperatures is small. Fig. 4 compares the heat treatment schedules of two samples which were processed at different maximum temperature Tmax,1 and Tmax,2 ) Tmax,1. At the point ‘ X ’ both schedules coincide and continue together. Only the history before both samples reached point ‘ X ’ was different. One sample was just heated to Tmax,1 where point ‘ X ’ was reached. The other was heated to Tmax,2 and cooled slowly to Tmax,1 to reach ‘ X ’. Assuming Tmax,2 to be 10 K higher than Tmax,1 , the full heat treatment of more than 60 h is prolonged by only 2 h. During this relatively short period, processes take place or are initiated, which reduce jc drastically in the fully processed samples. In order to compare the microstructures at the point ‘ X ’ from where the samples are submitted to the same heat treatment process, two samples were prepared: The first sample was just heated to 8908C and quenched from that temperature, the second was heated to 9048C and slowly cooled at 5 Krh to 8928C and then quenched. Fig. 5 compares the microstructure of these two samples. The left picture
Fig. 5. Polished cross sections of quenched Bi-2212 thick films. The left sample was heated to 8908C at a rate of 60 Krh and quenched immediately after this temperature was reached. The right sample was heated to 9048C, slowly cooled to 8928C, and quenched from 8928C. The Bi-free phase 014x24 appears black, the Cu-free phase 91150 is dark gray. Both phases float in a non-crystalline solidified matrix, which was the former liquid phase at high temperatures.
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Fig. 6. Volume fractions of the various phases during slow cooling at a rate of 5 Krh from two different maximum processing temperatures: 8938C Žgrey lines. and 9048C Žblack lines..
shows the quenched microstructure of the sample heated to 8908C and quenched from that temperature. The right image shows the thick film that was heated to 9048C and slowly cooled at a rate of 5 Krh to 8928C and then quenched. The phases that are present are the same in both samples: The Bi-free 014x24 phase appears black, the Cu-free 91150 phase is dark gray and the non-crystalline solidified matrix Žthe former liquid. appears in a lighter gray than the Cu-free phase. The most striking difference between the two microstructures is the size of the second phase grains. In the sample that was heated up to 9048C and slowly cooled to the quench temperature the second phase grains are 2–3 times larger than in the sample quenched from 8908C upon heating. The volume fraction of the phases is equal. The heat treatment of the partial melting process is not terminated at 8908C. The slow cooling ramp is continued to 8508C and the samples are annealed at 8508C for 48 h. Fig. 6 shows how the peritectic phases transform to the Bi-2212 phase during cooling and annealing and compares two samples that were melted at different Tmax Ž8938C and 9048C.. The formation of the Bi-2212 phase occurs at 10–15 K lower temperatures in the sample with the higher maximum temperature. At 8658C the sample processed at Tmax s 8938C showed more than 70 vol% of the Bi-2212 phase, whereas the sample processed at Tmax s 9048C still consisted of the peritectic phases and only a very small amount of Bi-2212. However,
after slow cooling to 8508C the amount of Bi-2212 and peritectic phases was almost equal in both samples and just about 5 vol% more peritectic phases were in the sample processed at the higher Tmax . In Fig. 7 the evolution of the average grain sizes of the solid peritectic phases is plotted against the temperature during the slow cooling ramp from Tmax to 8508C. Again two samples are compared. One processed at Tmax s 8938C, the other at Tmax s 9048C. The grain sizes in the samples with the higher Tmax are always larger than those in the lower melted ones. At 8508C the grains of the solid peritectic phases are twice as large in the higher melted sample compared to those processed at the lower Tmax . The volume fraction and the grain size of the solid peritectic phases in fully processed thick films increase slightly with increasing Tmax . However, the solidification experiments revealed an important difference between samples processed at different Tmax Ž8938C and 9048C.. In the thick film processed at Tmax s 8938C the Bi-2212 grains nucleated at T F 8758C homogeneously throughout the entire cross section of the sample. This was not observed in the sample processed at Tmax s 9048C. In this specimen the Bi-2212 platelets nucleated only at a few locations relatively far apart from each other. Fig. 8 shows a micrograph of the cross section of the sample that was partially melted at 9048C, slowly
Fig. 7. Average grain size of the solid peritectic phases ŽBi-free and Cu-free. during slow cooling at a rate of 5 Krh from two different maximum processing temperatures: 8938C Žgrey lines. and 9048C Žblack lines.. The circles belong to the Bi-free phase 014x24, the squares to the Cu-free phase 91150.
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Fig. 8. Polished cross section of Bi-2212 thick film that was partially melted at 8938C, slowly cooled at 5 Krh to 8708C, and quenched from 8708C.
cooled at 5 Krh to 8708C, and quenched from 8708C. There are no Bi-2212 platelets in the left part of the picture but many Bi-2212 grains nucleated and grew in the right section. The cross section of the
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thick film is divided into regions containing many Bi-2212 platelets and regions that are free of Bi-2212 at 8708C. The microstructures of two samples processed at two different maximum processing temperatures Ž8938C and 9048C., slowly cooled at a rate of 5 Krh to 8508C, and quenched from 8508C are shown in Fig. 9. The microstructures of both samples are very similar. They both consist mainly of well-aligned Bi-2212 phase. The amount of solid peritectic phases is slightly higher in the sample processed at Tmax s 9048C as shown in Fig. 6 and confirmed by XRD measurements. The thickness profiles of the two samples shown in Fig. 9 are plotted in Fig. 10. The sample processed at the higher Tmax Ž9048C. has a much rougher surface and non-homogeneous thickness profile, although the average thickness is the same as in the sample processed at Tmax s 8938C. Those areas where the cross section is most reduced are of special interest, as they can drastically lower the current carrying capacity of a thick film. Fig. 11 shows the microstructure of such an area of reduced thickness. The sample consists locally of many second phase grains ŽCu- and Bi-free phase., pores, and a small amount of misaligned Bi-2212 platelets. The rough thickness profile correlates also with regions of less
Fig. 9. Polished cross sections of Bi-2212 thick films. The left sample was partially melted at 8938C, slowly cooled at 5 Krh to 8508C, and quenched from 8508C. The right sample was partially melted at 9048C, slowly cooled at 5 Krh to 8508C, and quenched from 8508C.
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only two such misaligned regions were found in the entire cross section.
4. Discussion
Fig. 10. Thickness profiles of the samples processed at Tma x s 9048C Žtop. and 8938C Žbottom.. Both samples were cooled at a rate of 5 Krh from Tma x to 8508C and quenched from 8508C.
aligned grains. In the 10 mm long cross section of the thick film processed at Tmax s 9048C about ten regions were found with highly misaligned Bi-2212 platelets. In the sample processed at Tmax s 8938C
Fig. 11. Cross sectional area of reduced thickness in the sample that was partially melted at 9048C, slowly cooled at 5 Krh to 8508C, and quenched from 8508C. This area consists of many second phase grains, pores, and misaligned 2212 grains.
The strong dependence of the critical current density on the maximum processing temperature has been recognized and thoroughly discussed in literature. The explanation most often given is the appearance of larger amounts of the Bi-free phase in samples processed at higher Tmax . They lead to local misalignment of the Bi-2212 platelets and interruptions of the current path. The occurrence of the Bi-free phases is attributed to the vaporization of Bi during the high-temperature treatment. Wu et al. w30x have observed a reduction of the Bi-content in Lidoped Bi-2212 from Bi s 2.0 to 1.75 during melt processing, corresponding to a Bi-loss of 13%. In contrast to that result, Shimoyama et al. w9x reported that approximately 1.5% of the total Bi in their thick films vaporized during the high-temperature heat treatment above 8008C. Sata et al. w32x measured the vapor pressure of Bi, Pb and Cu components in Bi-2212 and Bi-2223 ceramics and pointed out the volatility of these elements at elevated temperatures. The initial composition of our Bi-2212 powder was Bi 2.2 Sr2.05 Ca 0.95 Cu 2 O x . According to Majewski et al. w26x the Bi-content can be reduced to 2.1 without leaving the single phase region of Bi-2212 at 8508C. De Biasi et al. w31x reported that substantial amounts of Bi were lost after annealing at 8808C for more than 48 h. We have reported previously about the coarsening of the solid peritectic phases at temperatures above Tsolidus of Bi-2212 w29x. Zhang et al. w33x observed in their melt processed Bi-2212 tapes that the size of the Cu-free phase increased with temperature, whereas the Bi-free phase 01 x1 rather grew with time. In the microstructure of fully processed thick films melted at 20–30 K above the optimum Tmax w34x they observed very large Cu-free grains, interrupting the well-aligned Bi-2212 platelets. However, the influence of the maximum processing temperature on jc is already very strong if the optimum temperature is only exceeded by 5–10 K. We have shown in this work that there are differences in size and amount of the solid peritectic phases left in the
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Fig. 12. Schematic drawing of the mechanism leading to areas of reduced thickness and homogeneity as shown in Fig. 11. In samples processed at Tma x s9048C the Bi-2212 platelets nucleate only in a few areas which are relatively far apart from each other. During growth, liquid and second phases are consumed, some of the liquid is stored between the Bi-2212 grains by capillary forces. When the growing Bi-2212 platelets collide, there is not enough liquid for a full transformation to Bi-2212 and an area of reduced thickness with many second phase grains remains in the structure.
microstructure between an optimally processed sample and a sample that was melted at a temperature exceeding the optimum range. At the end of the heat treatment, these differences are to small to be solely responsible for the decrease of jc . The drop of jc in samples processed above the optimum Tmax is attributed to the occurrence of areas of reduced thickness and homogeneity. These areas reduce the current carrying cross section. Earlier we have shown w24x that the critical current density of partial melt processed Bi-2212 thick films is reduced in the entire temperature range from 4 K to 77 K if the samples are processed at Tmax ) Toptimum . This is a strong indication that the number andror size of percolating current paths is reduced by too high processing temperatures. As shown in Fig. 10, the sample processed at Tmax s 9048C was much rougher than the sample processed at Tmax s 8938C, containing several areas consisting of large amounts of second phases, pores, and misaligned Bi-2212 grains. The mechanism leading to these regions is drawn in Fig. 12. They are formed because the Bi-2212 platelets nucleated only in few areas during slow cooling from Tmax if the optimum temperature is exceeded. The concentration of nuclei is reduced due to the higher Tmax . Liquid is stored in the gap between the Bi-2212 platelets by
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capillary forces. The liquid travels from regions containing no Bi-2212 to regions with many Bi-2212 platelets where the capillary forces are strongest. This phase separation leads to an incomplete transformation of the microstructure to Bi-2212. The regions that were separated from the liquid remain porous and contain the Bi- and Cu-free solid peritectic phases. However, in samples processed at the optimum Tmax the density of nuclei is higher and they are distributed homogeneously in the entire cross section of the thick film. Therefore, the described local phase separation does not occur and the microstructure is more homogeneous at the end of the processes. In samples processed at temperatures exceeding 9048C other mechanisms, such as the loss of Bi, the extreme growth of second phase grains not dissolving during cooling, or even the loss of liquid phase through grain boundaries of the silver sheath occur simultaneously leading to non-homogeneous microstructures with reduced jc . Processing at Tmax exceeding the optimum temperature also suppresses jc of bulk components with a thickness of 1 mm w23x. However, the influence of Tmax is not as drastic as in the thick films with a thickness of 10–60 mm. Processing of bulk samples requires a temperature homogeneity of about "8 K, whereas the 10 mm thick films need a temperature control better than "2 K in order to maximize jc . It is still an open question, whether the mechanisms suppressing jc in bulk samples are the same as in thick films.
5. Summary We have investigated the microstructural differences between Bi-2212 thick films processed in oxygen at an optimized maximum processing temperature Tmax and samples that were processed at Tmax 11 K above the optimum range. Partial melting at Tmax s 9048C instead of 8938C leads to an increased amount of solid peritectic phases with a grain size twice as large in the fully processed samples. The most striking difference between the two sample sets is the number and extension of inhomogeneities in the microstructure. The samples processed at Tmax s 9048C showed a very rough thickness profile but the same average thickness as the sample processed at
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8938C. The areas of reduced thickness in the rough samples consisted of many second phase grains, pores, and misaligned Bi-2212 platelets. These local inhomogeneities in the otherwise well-aligned microstructure were often found in samples processed at Tmax ) Toptimum . The suppression of jc in such samples correlate with these local inhomogeneities, which reduce the current carrying cross section and impede the percolative current flow. The formation of these areas is due to inhomogeneous nucleation of the Bi-2212 platelets upon slow cooling from the partially molten state at a rate of 5 Krh. In optimally processed samples many more Bi-2212 grains nucleate in the cross section of the sample, whereas in films processed at Tmax ) Toptimum , the Bi-2212 platelets are formed only in a few areas.
Acknowledgements The authors gratefully acknowledge the financial support of the Swiss National Science Foundation ŽNFP 30. and the Swiss Priority Program on Materials Research ŽPPM..
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w10x S. Elschner, J. Bock, G. Brommer, P.F. Herrmann, IEEE Trans. Magn. 32 Ž1996. 2724. w11x K. Hayashi, H. Nonoyama, M. Nagata, K. Takahashi, in: K. Kajimura, H. Hayakawa ŽEds.., Proceedings of the 3rd International Symposium on Superconductivity ŽISS ’90., Sendai, Japan, November 6–9, 1990; Advances in Superconductivity III, Springer-Verlag, Tokyo, p. 671. w12x ABB Press Release, Switzerland, November 21, 1996. w13x D. Buhl, Th. Lang, M. Cantoni, D. Risold, B. Hallstedt, L.J. Gauckler, Physica C 257 Ž1996. 151. w14x J. Shimoyama, J. Kase, T. Morimoto, H. Kitaguchi, H. Kumakura, K. Togano, H. Maeda, Jpn. J. Appl. Phys. 31 Ž1992. L1167. w15x H. Noji, W. Zhou, B.A. Glowacki, A. Oota, Physica C 205 Ž1993. 397. w16x J. Shimoyama, J. Kase, T. Morimoto, J. Mizusaki, H. Tagawa, Physica C 185–189 Ž1991. 931. w17x A. Endo, S. Nishikida, IEEE Trans. Appl. Supercond. 3 Ž1993. 931. w18x J.-W. Ko, S.-Y. Lee, H.-D. Kim, K.-H. Ha, J.-H. Ahn, H.-S. Chung, K. Togano, H. Maeda, J. Mater. Res. 29 Ž1994. 4638. w19x T. Kanai, T. Kamo, J. Mater. Res. 9 Ž1994. 1363. w20x C.G. Morgan, M. Priestnall, N.C. Hyatt, C.R.M. Grovenor, Inst. Phys. Conf. Ser. No. 148 331-334, Paper presented at Applied Superconductivity, Edinburgh, 3–6 July, 1995. w21x T. Haugan, S. Patel, M. Pitsakis, F. Wong, S.J. Chen, D.T. Shaw, J. Electron. Mat. 24 Ž1995. 1811. w22x M. Polak, W. Zhang, A. Polyanskii, A. Pashitski, E.E. Hellstrom, D.C. Larbalestier, Paper presented at Applied Superconductivity Conference ŽASC ’96., to be published in: IEEE Trans. Appl. Supercond. w23x Th. Lang, D. Buhl, M. Cantoni, Z. Wu, L.J. Gauckler, in: Proceedings for the 1995 International Workshop on Superconductivity, Co-sponsored by ISTEC and MRS, June 18–21, Maui HI, USA, 1995. w24x D. Buhl, Th. Lang, L.J. Gauckler, Supercond. Sci. Technol. 10 Ž1997. 32. w25x R. Muller, Th. Schweizer, P. Bohac, R.O. Suzuki, L.J. ¨ Gauckler, Physica C 203 Ž1992. 299. w26x P. Majewski, Adv. Mater. 6 Ž1994. 460. w27x R.D. Ray II, E.E. Hellstrom, Physica C 175 Ž1991. 255. w28x Th. Lang, D. Buhl, S. Al-Wakeel, L.J. Gauckler, J. Electroceram. 1 Ž1997. 133. w29x Th. Lang, D. Buhl, S. Al-Wakeel, D. Schneider, L.J. Gauckler, Physica C 281 Ž1997. 283. w30x S. Wu, J. Schwartz, G.W. Raban Jr., Physica C 246 Ž1995. 297. w31x R.S. De Biasi, S.M.V. Araujo, J. Magn. Magn. Mater. 104– 107 Ž1992. 471. w32x T. Sata, K. Sakai, S. Tashiro, J. Am. Ceram. Soc. 75 Ž1992. 805. w33x W. Zhang, R.D. Ray II, E.E. Hellstrom, in: R.P. Reed et al. ŽEds.., Adv. Cryog. Eng., vol. 40, Plenum Press, New York, 1994. w34x W. Zhang, E.E. Hellstrom, Physica C 218 Ž1993. 141.
Influence of the maximum temperature during partial melt-processing of Bi-2212 thick films on microstructure and jc Th. Lang ) , D. Buhl, L.J. Gauckler ETH Zurich, Department of Materials, Chair of Nonmetallic Materials, Swiss Federal Institute of Technology, Zurich CH-8092, Switzerland ¨ ¨ Received 20 March 1997; revised 10 July 1997; accepted 1 September 1997
Abstract The critical current density jc of Bi-2212 thick films Ž d s 60 mm. depends on the maximum processing temperature Tmax during the partial melting process. Processing at Tmax s 8938C leads to the highest jc of 3500 Arcm2 Ž77 K, 0 T.. A variation of Tmax by "5 K from the optimum temperature leads to a drop of jc below 80% of the maximum value. Processing below the optimum Tmax leads to insufficient consolidation. Partial melting at Tmax s 9048C leads to 5 vol% more residual peritectic phases with an average grain size about twice as large in the final microstructure compared to optimally processed samples. However, the drop of jc in samples processed at higher Tmax is attributed to inhomogeneities in the microstructure. These areas have a reduced film thickness and consist of second phase grains, pores, and misaligned 2212 platelets. The more frequent formation of these inhomogeneities is attributed to inhomogeneous nucleation of the Bi-2212 platelets upon slow cooling at a rate of 5 Krh from the partially molten state. In optimally processed thick films the nucleation of the Bi-2212 platelets occurs regularly in the whole cross section of the film. If processed at higher Tmax , fewer nucleation sites are present, leading to regions of high platelet density and regions containing no Bi-2212 grains. Due to the high capillary forces in the narrow gap between the Bi-2212 platelets, liquid is transported away from the platelet-free regions and stored between the Bi-2212 grains. Because of this phase separation, the solid peritectic grains persist in the areas where liquid was extracted. q 1998 Elsevier Science B.V. Keywords: Bi 2 Sr2 CaCu 2 O x ; High-temperature superconductor; Partial melt-processing; Maximum processing temperature; Critical current density
1. Introduction Bi 2 Sr2 CaCu 2 O x Ž Bi-2212 . combines several advantageous properties. It can be prepared weak-link free w1,2x by an easy partial melting process w3–5x, it contains no highly toxic elements, and is able to transport high currents in high external fields at low temperatures w6x. At elevated temperatures around 77 )
Corresponding author. Tel.: q41 1 632 56 51; Fax: q41 1 632 11 32; E-mail: [email protected].
K, however, pinning is rather weak compared to YBa 2 Cu 3 O x w7,8x. Nevertheless, polycrystalline Bi2212 can be prepared with reasonably high critical current densities exceeding 3 = 10 4 Arcm2 Ž77 K, 0 T. in thick films w9x and jc values of a few thousand Arcm2 in bulk components w10,11x. First industrial applications are being tested. In the fall of 1996 the Swiss–Swedish company Asea Brown Boveri ŽABB. installed a 1.2 MVA inductive fault current limiter working with large superconducting Bi-2212 rings with a diameter of 40 cm in a power plant w12x.
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 6 6 9 - 9
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The heat treatment used for the preparation of components of Bi-2212 is called partial melt processing. It consists of a partial melting step at a temperature Tmax which is above the solidus temperature of the Bi-2212 composition. The formation of a liquid phase facilitates the densification of the sample. After melting, the sample is cooled slowly at a rate of 5–10 Krh to a temperature below Tsolidus and annealed isothermally at that temperature in order to transform the multiphase microstructure back to Bi2212. We have shown earlier that each of the three mentioned processing steps is crucial on its own w13x. A further important processing step is the cooling period from the annealing temperature Že.g. 8508C. to room temperature w14,15x. The final properties as well as the microstructure depend on the cooling rate and the P ŽO 2 . during cooling. It was reported, that the Bi-2212 phase decomposes sluggishly below 6208C w16x. This can cause problems if the cooling rate is chosen too low. In addition, oxygen is taken up during cooling leading to an overdoped state of the superconductor and a reduced Tc . These facts must be considered when differently processed samples are compared. Nevertheless, a parameter of major importance is the maximum processing temperature Tmax . Shimoyama et al. w9x have observed a drastic drop of jc measured at 4.2 K in an external field of 10 T if their Bi-2212 thick films were processed above 8908C. They explained this behavior with the appearance of large amounts of the ŽSr,Ca. –Cu–O phase due to a large loss of Bi by vaporization at high temperatures. Grains of Bi-free phase were assumed to block the current flow and therefore reduce jc . Processing in presence of Bi 2 Al 4 O 9 reduced the volume fraction of the Bi-free phase in the final microstructure and better reproducibility of high jc values was achieved. However, jc still decreased if Tmax exceeded 9008C. This was attributed to areas of misoriented Bi-2212 grains between very large and well-aligned Bi-2212 platelets. The influence of the maximum processing temperature on jc and the microstructure of partial melt processed Bi-2212 tapes made by powder-in-tube was investigated by Endo et al. w17x. The highest jc values at 4.2 K in an external field of 1 T were obtained if processed at Tmax s 8858C. For Tmax 8858C the lack of liquid phase was responsible for
low jc values. At Tmax ) 8858C the authors observed an increased amount of Bi-free phase in the final microstructure. The final conclusion was that the higher volume fraction of second phases could not be solely responsible for the lower jc . The increased size of the second phase grains rather than the higher volume fraction reduced jc . Screen-printing and partial melting was used to prepare Bi-2212 thick films by Ko et al. w18x. Again a strong dependence of jc on the maximum processing temperature and the annealing time at Tmax was observed. Kanai et al. w19x observed that partial melting just above the incipient melting point gave the highest jc values in Bi-2212 wires. The formation and growth of voids inside the wire was assumed to be responsible for the drop of jc with rising maximum temperatures. Morgan et al. w20x even observed two maxima of the critical current density as a function of the maximum processing temperature in Bi-2212 tapes. The highest jc values were obtained in a narrow temperature window between 8758C and 8788C only. The problem of temperature homogeneity during processing of large samples of Bi-2212 Že.g. wound wires in a box furnace. was discussed by Haugan et al. w21x. The decrease of jc in their 90 m long wires compared to the 15 m wires was attributed to the sensitivity of jc to temperature nonuniformities in their box-type furnace. For future heat treatments a ring-type furnace with a very small temperature gradient is built. Polak et al. w22x explained the strong Tmax-dependence of jc at 4.2 K in their Bi-2212 tapes with the appearance of macropores, which appeared with a reduced density and hardness of the tape cores of the samples. No significant microstructural difference in phase content and grain size between optimally processed and low-jc samples was observed. We have reported previously w23,24x about the strong dependence of the maximum processing temperatures on the final superconducting properties in partial melt processed Bi-2212 thick films and bulk components. The critical current densities were higher in the whole temperature range from 4 K to 77 K in optimally processed samples compared to samples processed at Tmax exceeding the optimum range. This indicates that the current carrying cross
Th. Lang et al.r Physica C 294 (1998) 7–16
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section is reduced with increasing Tmax and excludes a different pinning behavior to be responsible for the drop of jc . A similar conclusion was drawn by Polak et al. w22x who found a correlation between the room-temperature resistivity and jc of their tapes. In this work we place special emphasis on the influence of Tmax on the final microstructure and the development of phases during the partial melt processing of Bi-2212 thick films in order to elucidate the dependence of jc on Tmax .
2. Experimental Powder with the stoichiometry Bi 2.2 Sr 2.05 Ca 0.95 Cu 2 O x was prepared by a standard calcination process, consisting of several firing steps at temperatures between 7508C and 8508C for a total of more than 200 h with several intermediate grindings. The chosen stoichiometry lies in the single phase region of Bi-2212 at temperatures above 8008C w25,26x. The powder was mixed with organic additives and tapecast onto glass as described elsewhere w28x. Samples with a diameter of 11 mm were cut from the dried tape, put on a silver substrate Ž dAg s 100 mm., and heat treated in a vertical furnace. A first set of samples was heated at a rate of 60 Krh to temperatures of 8868C, 8908C, 9008C, 9118C, 9228C, and 9308C, and quenched in oil from these temperatures immediately after they were reached Žsee Fig. 1Ža... Several other thick films were partial melt processed using the heat treatment schedule shown in Fig. 1Žb.. The samples were partially melted at Tmax between 8938C and 9048C, slowly cooled at a rate of 5 Krh to 8508C, and isothermally annealed at 8508C for 48 h. Some samples were quenched in oil from different temperatures during the heat treatment in order to investigate the microstructure of the high temperature state w27x. Samples used for jc measurements were not quenched but furnace cooled to room temperature. In order to adjust the oxygen content of these films and hence Tc , the specimens were subsequently annealed at 5508C for 30 h in flowing nitrogen Ž P ŽO 2 . s 0.001 atm.. Only samples that were furnace cooled to room temperature and subsequently annealed at 5508C for 30 h were used to measure the critical current density. The
Fig. 1. Žarb.: Heat treatment schedules used in this work. The schedule shown in Ža. was used to investigate the influence of temperature on the phase composition and microstructure of Bi2212 in the partially molten state Ž8868C -Tma x -9308C.. Schedule Žb. shows the heat treatment of a partial melting process used for the preparation of Bi-2212 thick films. The maximum processing temperature Tma x ranged from 8878C to 9048C. The doubleheaded arrows symbolize quenching procedures used to freeze the high temperature microstructures.
quenched samples were used for the microstructural investigations. The critical current density at 77 K in zero external field was measured in an AC-magnetometer at a frequency of 4–10 Hz. The experimental setup in combination with the sample size lead to a voltage criterion of E f 1 mVrcm. All heat treatments were performed in flowing oxygen, the final sample thickness was 60 mm. The quenched thick films were first analyzed by X-ray diffraction, then embedded in resin, ground and polished. The cross sections were analyzed using scanning electron microscopy ŽSEM., light microscopy, and energy dispersive X-ray spectroscopy
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ŽEDS.. The EDS data were corrected using standards with a known composition. The volume fractions and the average grain sizes of the phases appearing were measured by means of optical methods described elsewhere w29x. The thickness profiles of thick films were determined by light microscopy at 50 equidistant locations in each specimen.
3. Results 3.1. Influence of Tm a x on jc In order to verify the influence of the maximum processing temperature on the critical current density of Bi-2212 thick films under the given experimental conditions, several samples were partial melt processed with Tmax varying from 8878C to 9008C according to the heat treatment shown in Fig. 1Žb., except that the samples were not quenched after the isothermal annealing at 8508C, but furnace cooled to room temperature. After the partial melt processing, an additional annealing at lower temperature and lower P ŽO 2 . Ž5508C, 0.001 atm. was necessary to increase Tc above 90 K. We have shown earlier that such a reduction process is beneficial to increase Tc and the critical current density at 77 K w24x. Fig. 2
Fig. 3. Volume fractions of the phases in a Bi-2212 thick film which is put on a silver substrate and heated in oxygen below and above its solidus temperature at a rate of 60 Krh. The Cu-free phase has a composition of Bi 9 Sr11Ca 5 O x , the Bi-free phase belongs to the solid solution family 014x24 at temperatures below 9228C. Then the stoichiometry changes to 01 x1, 02 x1 and 01 x 0 w29x.
shows the normalized jc values as a function of the maximum processing temperature. The maximum jc was 3.500 Arcm2 Ž77 K, 0 T. in the sample processed at Tmax s 8938C. Small deviations from this temperature of "5 K lead to a strong decrease of jc to less than 80% of the maximum value. In the sample melt processed at 8878C, not enough liquid was produced to fully densify the structure, resulting in a low critical current density. The reason for the drop of jc with increasing maximum processing temperature was further investigated. Since the analyzed samples varied only in Tmax , this parameter must be responsible for the changes in jc . 3.2. Influence of Tm a x on the microstructure
Fig. 2. Influence of the maximum processing temperature Tma x on the normalized critical current density at 77 K in zero external field of fully processed Bi-2212 thick films Ž ds60 mm. prepared in the same furnace used for the quenching experiments. The maximum jc was 3.500 Arcm2 Ž77 K, 0 T..
The high-temperature microstructures of Bi-2212 thick films were investigated by quenching samples from different temperatures during heating at a rate of 60 Krh. When Bi-2212 is decomposed on a silver substrate in an oxygen atmosphere a liquid phase, two solid peritectic phases, and oxygen are formed. The solid phases are either Bi-free with an initial composition of Sr8.5 Ca 5.5 Cu 24 O x , or Cu-free with the stoichiometry Bi 9 Sr11Ca 5 O y w29x. Fig. 3 shows the vol-
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Fig. 4. Comparison of two heat treatment schedules with different maximum temperatures. At the point marked with ‘ X ’ they coincide and from that point on both schedules are the same.
ume fractions of the solid and liquid phases as a function of the quenching temperature. Immediately after decomposition, the microstructure at 8908C consists of 70 vol% liquid, and 30 vol% solids. At 9118C the amount of liquid increased to 80 vol%. The composition of the Bi-free phase, which is 014x24 in the beginning Žthe abbreviation 014x24 stands for Bi 0 Sr14yx Ca x Cu 24 O y ., changes to 01 x1, 02 x1, and 01 x 0 at temperatures G 9228C as determined by EDS. However, in the temperature range between 8938C to 9048C, which are the maximum temperatures used for the partial melt processing
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described later, only small changes in the composition of the phases occur. The difference between two heat treatment schedules having two different maximum temperatures is small. Fig. 4 compares the heat treatment schedules of two samples which were processed at different maximum temperature Tmax,1 and Tmax,2 ) Tmax,1. At the point ‘ X ’ both schedules coincide and continue together. Only the history before both samples reached point ‘ X ’ was different. One sample was just heated to Tmax,1 where point ‘ X ’ was reached. The other was heated to Tmax,2 and cooled slowly to Tmax,1 to reach ‘ X ’. Assuming Tmax,2 to be 10 K higher than Tmax,1 , the full heat treatment of more than 60 h is prolonged by only 2 h. During this relatively short period, processes take place or are initiated, which reduce jc drastically in the fully processed samples. In order to compare the microstructures at the point ‘ X ’ from where the samples are submitted to the same heat treatment process, two samples were prepared: The first sample was just heated to 8908C and quenched from that temperature, the second was heated to 9048C and slowly cooled at 5 Krh to 8928C and then quenched. Fig. 5 compares the microstructure of these two samples. The left picture
Fig. 5. Polished cross sections of quenched Bi-2212 thick films. The left sample was heated to 8908C at a rate of 60 Krh and quenched immediately after this temperature was reached. The right sample was heated to 9048C, slowly cooled to 8928C, and quenched from 8928C. The Bi-free phase 014x24 appears black, the Cu-free phase 91150 is dark gray. Both phases float in a non-crystalline solidified matrix, which was the former liquid phase at high temperatures.
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Fig. 6. Volume fractions of the various phases during slow cooling at a rate of 5 Krh from two different maximum processing temperatures: 8938C Žgrey lines. and 9048C Žblack lines..
shows the quenched microstructure of the sample heated to 8908C and quenched from that temperature. The right image shows the thick film that was heated to 9048C and slowly cooled at a rate of 5 Krh to 8928C and then quenched. The phases that are present are the same in both samples: The Bi-free 014x24 phase appears black, the Cu-free 91150 phase is dark gray and the non-crystalline solidified matrix Žthe former liquid. appears in a lighter gray than the Cu-free phase. The most striking difference between the two microstructures is the size of the second phase grains. In the sample that was heated up to 9048C and slowly cooled to the quench temperature the second phase grains are 2–3 times larger than in the sample quenched from 8908C upon heating. The volume fraction of the phases is equal. The heat treatment of the partial melting process is not terminated at 8908C. The slow cooling ramp is continued to 8508C and the samples are annealed at 8508C for 48 h. Fig. 6 shows how the peritectic phases transform to the Bi-2212 phase during cooling and annealing and compares two samples that were melted at different Tmax Ž8938C and 9048C.. The formation of the Bi-2212 phase occurs at 10–15 K lower temperatures in the sample with the higher maximum temperature. At 8658C the sample processed at Tmax s 8938C showed more than 70 vol% of the Bi-2212 phase, whereas the sample processed at Tmax s 9048C still consisted of the peritectic phases and only a very small amount of Bi-2212. However,
after slow cooling to 8508C the amount of Bi-2212 and peritectic phases was almost equal in both samples and just about 5 vol% more peritectic phases were in the sample processed at the higher Tmax . In Fig. 7 the evolution of the average grain sizes of the solid peritectic phases is plotted against the temperature during the slow cooling ramp from Tmax to 8508C. Again two samples are compared. One processed at Tmax s 8938C, the other at Tmax s 9048C. The grain sizes in the samples with the higher Tmax are always larger than those in the lower melted ones. At 8508C the grains of the solid peritectic phases are twice as large in the higher melted sample compared to those processed at the lower Tmax . The volume fraction and the grain size of the solid peritectic phases in fully processed thick films increase slightly with increasing Tmax . However, the solidification experiments revealed an important difference between samples processed at different Tmax Ž8938C and 9048C.. In the thick film processed at Tmax s 8938C the Bi-2212 grains nucleated at T F 8758C homogeneously throughout the entire cross section of the sample. This was not observed in the sample processed at Tmax s 9048C. In this specimen the Bi-2212 platelets nucleated only at a few locations relatively far apart from each other. Fig. 8 shows a micrograph of the cross section of the sample that was partially melted at 9048C, slowly
Fig. 7. Average grain size of the solid peritectic phases ŽBi-free and Cu-free. during slow cooling at a rate of 5 Krh from two different maximum processing temperatures: 8938C Žgrey lines. and 9048C Žblack lines.. The circles belong to the Bi-free phase 014x24, the squares to the Cu-free phase 91150.
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Fig. 8. Polished cross section of Bi-2212 thick film that was partially melted at 8938C, slowly cooled at 5 Krh to 8708C, and quenched from 8708C.
cooled at 5 Krh to 8708C, and quenched from 8708C. There are no Bi-2212 platelets in the left part of the picture but many Bi-2212 grains nucleated and grew in the right section. The cross section of the
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thick film is divided into regions containing many Bi-2212 platelets and regions that are free of Bi-2212 at 8708C. The microstructures of two samples processed at two different maximum processing temperatures Ž8938C and 9048C., slowly cooled at a rate of 5 Krh to 8508C, and quenched from 8508C are shown in Fig. 9. The microstructures of both samples are very similar. They both consist mainly of well-aligned Bi-2212 phase. The amount of solid peritectic phases is slightly higher in the sample processed at Tmax s 9048C as shown in Fig. 6 and confirmed by XRD measurements. The thickness profiles of the two samples shown in Fig. 9 are plotted in Fig. 10. The sample processed at the higher Tmax Ž9048C. has a much rougher surface and non-homogeneous thickness profile, although the average thickness is the same as in the sample processed at Tmax s 8938C. Those areas where the cross section is most reduced are of special interest, as they can drastically lower the current carrying capacity of a thick film. Fig. 11 shows the microstructure of such an area of reduced thickness. The sample consists locally of many second phase grains ŽCu- and Bi-free phase., pores, and a small amount of misaligned Bi-2212 platelets. The rough thickness profile correlates also with regions of less
Fig. 9. Polished cross sections of Bi-2212 thick films. The left sample was partially melted at 8938C, slowly cooled at 5 Krh to 8508C, and quenched from 8508C. The right sample was partially melted at 9048C, slowly cooled at 5 Krh to 8508C, and quenched from 8508C.
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only two such misaligned regions were found in the entire cross section.
4. Discussion
Fig. 10. Thickness profiles of the samples processed at Tma x s 9048C Žtop. and 8938C Žbottom.. Both samples were cooled at a rate of 5 Krh from Tma x to 8508C and quenched from 8508C.
aligned grains. In the 10 mm long cross section of the thick film processed at Tmax s 9048C about ten regions were found with highly misaligned Bi-2212 platelets. In the sample processed at Tmax s 8938C
Fig. 11. Cross sectional area of reduced thickness in the sample that was partially melted at 9048C, slowly cooled at 5 Krh to 8508C, and quenched from 8508C. This area consists of many second phase grains, pores, and misaligned 2212 grains.
The strong dependence of the critical current density on the maximum processing temperature has been recognized and thoroughly discussed in literature. The explanation most often given is the appearance of larger amounts of the Bi-free phase in samples processed at higher Tmax . They lead to local misalignment of the Bi-2212 platelets and interruptions of the current path. The occurrence of the Bi-free phases is attributed to the vaporization of Bi during the high-temperature treatment. Wu et al. w30x have observed a reduction of the Bi-content in Lidoped Bi-2212 from Bi s 2.0 to 1.75 during melt processing, corresponding to a Bi-loss of 13%. In contrast to that result, Shimoyama et al. w9x reported that approximately 1.5% of the total Bi in their thick films vaporized during the high-temperature heat treatment above 8008C. Sata et al. w32x measured the vapor pressure of Bi, Pb and Cu components in Bi-2212 and Bi-2223 ceramics and pointed out the volatility of these elements at elevated temperatures. The initial composition of our Bi-2212 powder was Bi 2.2 Sr2.05 Ca 0.95 Cu 2 O x . According to Majewski et al. w26x the Bi-content can be reduced to 2.1 without leaving the single phase region of Bi-2212 at 8508C. De Biasi et al. w31x reported that substantial amounts of Bi were lost after annealing at 8808C for more than 48 h. We have reported previously about the coarsening of the solid peritectic phases at temperatures above Tsolidus of Bi-2212 w29x. Zhang et al. w33x observed in their melt processed Bi-2212 tapes that the size of the Cu-free phase increased with temperature, whereas the Bi-free phase 01 x1 rather grew with time. In the microstructure of fully processed thick films melted at 20–30 K above the optimum Tmax w34x they observed very large Cu-free grains, interrupting the well-aligned Bi-2212 platelets. However, the influence of the maximum processing temperature on jc is already very strong if the optimum temperature is only exceeded by 5–10 K. We have shown in this work that there are differences in size and amount of the solid peritectic phases left in the
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Fig. 12. Schematic drawing of the mechanism leading to areas of reduced thickness and homogeneity as shown in Fig. 11. In samples processed at Tma x s9048C the Bi-2212 platelets nucleate only in a few areas which are relatively far apart from each other. During growth, liquid and second phases are consumed, some of the liquid is stored between the Bi-2212 grains by capillary forces. When the growing Bi-2212 platelets collide, there is not enough liquid for a full transformation to Bi-2212 and an area of reduced thickness with many second phase grains remains in the structure.
microstructure between an optimally processed sample and a sample that was melted at a temperature exceeding the optimum range. At the end of the heat treatment, these differences are to small to be solely responsible for the decrease of jc . The drop of jc in samples processed above the optimum Tmax is attributed to the occurrence of areas of reduced thickness and homogeneity. These areas reduce the current carrying cross section. Earlier we have shown w24x that the critical current density of partial melt processed Bi-2212 thick films is reduced in the entire temperature range from 4 K to 77 K if the samples are processed at Tmax ) Toptimum . This is a strong indication that the number andror size of percolating current paths is reduced by too high processing temperatures. As shown in Fig. 10, the sample processed at Tmax s 9048C was much rougher than the sample processed at Tmax s 8938C, containing several areas consisting of large amounts of second phases, pores, and misaligned Bi-2212 grains. The mechanism leading to these regions is drawn in Fig. 12. They are formed because the Bi-2212 platelets nucleated only in few areas during slow cooling from Tmax if the optimum temperature is exceeded. The concentration of nuclei is reduced due to the higher Tmax . Liquid is stored in the gap between the Bi-2212 platelets by
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capillary forces. The liquid travels from regions containing no Bi-2212 to regions with many Bi-2212 platelets where the capillary forces are strongest. This phase separation leads to an incomplete transformation of the microstructure to Bi-2212. The regions that were separated from the liquid remain porous and contain the Bi- and Cu-free solid peritectic phases. However, in samples processed at the optimum Tmax the density of nuclei is higher and they are distributed homogeneously in the entire cross section of the thick film. Therefore, the described local phase separation does not occur and the microstructure is more homogeneous at the end of the processes. In samples processed at temperatures exceeding 9048C other mechanisms, such as the loss of Bi, the extreme growth of second phase grains not dissolving during cooling, or even the loss of liquid phase through grain boundaries of the silver sheath occur simultaneously leading to non-homogeneous microstructures with reduced jc . Processing at Tmax exceeding the optimum temperature also suppresses jc of bulk components with a thickness of 1 mm w23x. However, the influence of Tmax is not as drastic as in the thick films with a thickness of 10–60 mm. Processing of bulk samples requires a temperature homogeneity of about "8 K, whereas the 10 mm thick films need a temperature control better than "2 K in order to maximize jc . It is still an open question, whether the mechanisms suppressing jc in bulk samples are the same as in thick films.
5. Summary We have investigated the microstructural differences between Bi-2212 thick films processed in oxygen at an optimized maximum processing temperature Tmax and samples that were processed at Tmax 11 K above the optimum range. Partial melting at Tmax s 9048C instead of 8938C leads to an increased amount of solid peritectic phases with a grain size twice as large in the fully processed samples. The most striking difference between the two sample sets is the number and extension of inhomogeneities in the microstructure. The samples processed at Tmax s 9048C showed a very rough thickness profile but the same average thickness as the sample processed at
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Th. Lang et al.r Physica C 294 (1998) 7–16
8938C. The areas of reduced thickness in the rough samples consisted of many second phase grains, pores, and misaligned Bi-2212 platelets. These local inhomogeneities in the otherwise well-aligned microstructure were often found in samples processed at Tmax ) Toptimum . The suppression of jc in such samples correlate with these local inhomogeneities, which reduce the current carrying cross section and impede the percolative current flow. The formation of these areas is due to inhomogeneous nucleation of the Bi-2212 platelets upon slow cooling from the partially molten state at a rate of 5 Krh. In optimally processed samples many more Bi-2212 grains nucleate in the cross section of the sample, whereas in films processed at Tmax ) Toptimum , the Bi-2212 platelets are formed only in a few areas.
Acknowledgements The authors gratefully acknowledge the financial support of the Swiss National Science Foundation ŽNFP 30. and the Swiss Priority Program on Materials Research ŽPPM..
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