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Low-temperature synthesis of YBa2Cu3O7−x powders from citrate and nitrate precursors
PhysicaC 204 (1992) 135-146 North-Holland
Low-temperature synthesis of YBa2Cu307_x powders from citrate and nitrate precursors S.H. Shieh a n d W.J. T h o m s o n Department of Chemical Engineering, Washington State University, Pullman, WA 99164-2710, USA Received 21 August 1992
YBa2Cu307_xsuperconductorpowdershave been prepared fromcarbon-containingcitrate and flashnitrate precursors. In both cases, it was possible to decomposethe barium carbonate below 690°C and to form the 123 phase directlyfrom the precursor in an helium atmosphere. High-Te superconducting 123 phase with a sharp transition at 93 K was obtained after oxygenannealing. High-temperature, in situ, X-ray diffraction was used to followphase evolution and reaction sequences at several stages in the formationas well as in the decompositionof the 123 phase. We investigated the effectsof precursor homogeneityin the formation of the 123 phase during low-temperatureprocessing. The properties and phase purity of the low-temperatureprocessedsuperconductor ceramicsare stronglydependent on precursor homogeneityand morphology.Additionally, our results showthat the existence of low-temperature grownbinaryimpurities affectsthe successiveevolutionof entire phase assemblageduring further stages of heating till completedecompositionof the 123 phase.
1. Introduction Industrial production of superconductor ceramics requires a reaction pathway for efficient synthesis without compromising the material's superconducting properties. Low-temperature processing is a costeffective alternative to the traditional high-temperature processing, provided that sufficient superconducting properties can be achieved. Many studies have reported low-temperature processing of high quality orthorhombic 123 superconductor thin films at temperatures between 500-650°C, i.e. at temperatures just slightly higher than the tetragonal/orthorhombic phase transformation temperature, which is approximately 600°C [ 1-7] depending on the oxygen partial pressure [ 8 ]. The production of fine orthorhombic 123 superconductor powder, fine grained monolith, and superconducting fiber, on the other hand, has had very limited success in finding a lowtemperature pathway. Traditional solid-state powder synthesis from mixed oxide precursors requires a high processing temperature. Repeated calcination, grinding, and sintering for many hours are familiar experimental procedures reported in the literature. It is well known that a high processing
temperature encourages grain growth and often produces non-uniform large sintered particles [ 9 ]. In order to suppress the grain growth and to limit the particle size within submicron range, a low-temperature route for powder processing is very desirable. Several papers have dealt with the synthesis of superconducting ceramics via a sol-gel method [ 9-24 ]. Precursors are often prepared from solutions of gels of citrate, nitrate, oxalate, acetate and metal alkoxide. It is also possible to prepare precursors from formate and lactate by more elaborated methods [ 12 ]. Improved chemical homogeneity, greater control over particle size, better processing control, and lowered processing temperature are among the anticipated benefits in using this method. These new synthesis procedures allow superconductor ceramic powders to be prepared at processing temperatures 150200°C lower than the conventional calcination temperature. The majority of studies that started with solutionderived precursors for preparing the YBCO 123 superconductor used ambient atmosphere for processing. One major obstacle preventing traditional superconductor powder processing at lower temperatures is the decomposition of barium car-
0921-4534/92/$05.00 © 1992 ElsevierSciencePublishers B.V. All rights reserved.
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S.H. Shieh, W.J. Thomson ~Low-temperaturesynthesis of YBCOpowders
bonate in air. This has been described as the rate determining step during the formation of superconductor phase [21,25,26]. Barium carbonate is relatively stable in air up to 800°C. It undergoes a phase transformation from gamma to beta starting at 816 ° C [ 27 ]. It is not clear whether the beta barium carbonate is easier to decompose and easier to react with the other two oxides to form the 123 superconductor than the gamma phase. The decomposition of barium carbonate seems to be a common problem, and it has been reported that a complete elimination of carbonate during calcination cannot be achieved under 800°C, even with fine particle precursors [24]. If this is true, it may leave out any possibility for preparing a phase-pure superconductor product below the barium carbonate decomposition temperature under ambient conditions. A separate observation showed that a high percentage of the barium carbonate can be decomposed at 780°C, if calcined for a long enough time in air (60 h) [28]. Unfortunately, long heat treatment time is another undesirable factor that only promotes grain growth. It seems, in order to synthesize single phase 123 superconductor at temperatures substantially below 780°C, one has to avoid the formation of barium carbonate by either using a carbon-free precursor or by calcination in an atmosphere free of carbon dioxide, at the same time maintaining relatively fast reaction kinetics. The search for a low-temperature processing route is further plagued by the occurrence of the semiconducting tetragonal " X " phase [23,29-33], or the "low-temperature" tetragonal phase [ 34 ], while the "high-temperature" tetragonal phase is the superconductor phase. The X phase has a similar tetragonal structure to that of the superconductor phase but it has a high oxygen stoichiometry and therefore a shorter "c" cell [35 ]. This shorter "c" cell makes " c / 3 " very close to both " a " and "b" cell parameters, such that (110) and (013) peaks merge into one peak. This non-splitting peak character is unique to this X phase. The X phase does not transform to the superconducting orthorhombic phase at lower temperatures, even after extended annealing with oxygen [22 ]. Once the X phase forms during calcination in air, the simplest way to convert it to the proper superconductor tetragonal phase is to remove some oxygen out of it by reheating it in a higher pro-
cessing temperature. The oxygen content of the 123 phase is a strong function of temperature, and it becomes more reduced at higher temperatures [36,37]. In order to avoid the formation of X phase in air, a processing temperature exceeding 850°C is usually required [38]. A second method of removing oxygen from the X phase is to reprocess this material in a low-oxygen atmosphere. The inability to correctly recognize this X phase has led some studies to report a very low formation temperature for the superconductor phase, and yet measured its superconducting properties based on samples reheated to higher temperatures [ 17,21 ]. The possibility of low-temperature powder processing for the 123 superconductor was first reported by Rha et al. [ 39 ]. Using a helium atmosphere, they were able to sinter 123 superconductor at 850°C after 12 h from mixed oxide precursors. They observed an increased rate of sintering in an inert atmosphere and speculated that sintering in a depleted oxygen environment enhances ion mobility which lowers the superconductor formation temperature. Several other studies have reported similar success in the preparation of orthorhombic 123 superconductor in low oxygen atmosphere at temperatures below 800°C. Lay reported a formation temperature 750°C for the superconducting phase from mixed oxides and barium carbonate using 1 Torr vacuum [29 ]. Thomson et at. started with nanometer scale gel precursors and observed the high-temperature tetragonal phase start to form in helium at temperatures as low as 627°C [34]. Zheng et al. also prepared a sample at 780°C from metal alkoxides that showed resistivity at 57 K
[20]. So far, the only successful attempt of low-temperature powder synthesis of the orthorhombic 123 superconductor was the one conducted by Horowitz et al. [24]. They started with carbon-free organometallic precursors and used elaborated experimental procedures to prevent the formation of barium carbonate during heating. They found that it is possible to decompose the precursor and to form the hightemperature tetragonal phase in argon at 700°C for 12 h. This phase was annealed in oxygen for 12 h to make the superconducting orthorhombic phase. The sample they produced showed a broad transition re# o n beginning at 85 K. They also reported that the same processing technique can be employed to make
S.H. Shieh, W.J. Thomson~Low-temperaturesynthesis of YBCOpowders superconductor ceramics at 650°C. However, their X-ray data indicated the formation of a large amount of barium copper oxide contamination coexisting with the high-temperature tetragonal superconductor phase such that a subsequent high-temperature (900 ° C) calcination in oxygen became necessary to restore the phase purity. The objective of our study was to design a new pathway to prepare fine superconductor powder at low temperatures by using carbon-containing sol-gel precursors, and to look for a kinetically favorable route that does not require extended calcination time. A further objective of this study was to study the decomposition behavior of t h e 123 phase at higher temperatures in helium.
2. Experiments Two different precursors, citrate gel and flash nitrate, were used for this experiment. Nanometer scale precursors are ideal for low-temperature synthesis. The citrate precursor was prepared by dissolving stoichiometric amounts of yttrium, barium and copper acetates in a mixture of a weak hydroxycarboxylic acid such as citric acid and ethylene glycol. The solution was heated at 170°C to dissolve the acetates and then diluted with distilled water. Water was gradually removed by heating the solution at around 80 ° C until the solution turned blue and viscous. This viscous solution was dried in an oven at 70°C. It solidified into a greenish-white chalk after 48 h. The brittle solid chalk could be easily ground to little chips. Fragmented chips were again heated at 150200 °C in a tube furnace for 24 h in air. After heating, the precursor is ground to very fine, light brown color ash. The flash nitrate precursor was prepared by a "flashed" carbothermal reduction of the nitrate salts. The nitrates were reduced to yttrium oxide, copper oxide and barium carbonate from exposures to organic carbon at temperatures in excess of 600 ° C [34]. The main difference between these two precursors is that the flash nitrate precursor is a mixture of fine crystallites while the heat treated citrate precursor remains largely amorphous. Time resolved in situ high-temperature X R D was used to trace all the chemical reactions and phase evolutions in this study. A position-sensitive proportional counter permits each data scan over a range
137
of 50-degree two-theta to be completed within 2 min [ 40 ], thus allowing one to perform a precise monitoring of time critical changes during the formation of the superconductor. The precursor material was first wetted with de-ionized water and then a thin layer of sample approximately 100 microns thick was laid on the surface of a platinum heating ribbon. This heating platinum element is enclosed in a metal chamber of controlling the reaction atmosphere. All the temperatures reported in this work are measured by a thermal couple welded to the bottom of this heating ribbon and positioned at the X-ray focal point on the diffractometer. Cobalt radiation was used for all scans. Since the X-ray diffracts mostly from the upper portion of the sample while the thermal couple detects the sample temperature at the bottom, the true reaction temperature might be slightly lower than the temperature recorded by the thermal couple. We can use the thermal couple temperature as a upper limit for all the chemical reactions observed in this work.
3. Results 3.1. Optimized low-temperature synthesis We have found that it is possible to obtain orthorhombic 123 by first heating the dried citrate ashes in either oxygen or air to 580°C at 5 K / m i n , held for 20 min to allow for organic carbon to burn-out, then switching the calcining atmosphere from oxygen or air to helium and continuing heating it to 690°C at 100 K/min. The high-temperature tetragonal phase with well split (110) and (013) peaks was fully developed within 2 h at 690°C. This phase was then quenched to 500 ° C, and at that temperature, oxygen was reintroduced to replace helium. The sample was annealed in oxygen at 500°C for 12 h; however, the X-ray spectra showed little change after 2 h of annealing. Among several runs we have performed, two samples of citrate gel preheated in oxygen and calcined in helium were measured for their superconducting properties. Based on the magnetic susceptibility measurement, one sample showed diffused Tc although the X R D spectrum looked excellent, and it was suspected that some unreacted materials outside the hot zone were scraped in with the sample
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S.H. Shieh, W.J. Thomson / Low-temperature synthesis of YBCO powders
when the superconductor phase was collected from the heating ribbon. A second sample was collected exclusively from the central portion of the heating ribbon within the range of the X-rays. This sample showed a sharp superconducting transition starting at 93 K. In both cases, the measured flux exclusion is still weak, indicating the possible presence of other impurity phases, SEM micrographs taken from these citrate samples prepared at 690°C for 2 h already showed morphologies indicating some degrees of grain growth. Single particles 2 to 3 microns can be seen among fine particles that are in general less than 0.25 micron thick. Neck growth between minute fine particles is common.
3.2. Phase evolution during 123 formation The spectral data trailing the formation of tetragonal 123 from the citrate precursor is presented in figs. 1-4. The citrate gel remained X-ray amorphous up to about 200°C prior to complete drying; this can be seen in spectra A and B of fig. I. After drying, the precursor appeared slightly crystalline after oxidation and decomposition of some copper components in gel which formed copper oxides, CuO (spectrum C, fig. l ). Spectrum D through spectrum I in fig. 1 shows that copper oxide continued to precipitate
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from oxidation of copper metal in the largely amorphous citrate ash at lower temperatures, under 400°C. Barium carbonate began its nucleation around 470°C when the organic carbon in the precursor began to oxidize and generated enough carbon dioxide in the reaction chamber. After nucleation, the X-ray intensity of BaCO3 increased during the entire organic bum-out period indicating continued crystal growth. Similar reaction sequences were observed when the citrate precursor was heated in oxygen instead of in air, the only difference being that copper oxides formed at lower temperatures in oxygen. Following organic bum-out, the calcining atmosphere was changed from oxygen to helium at 580°C. During the change of reaction the atmosphere of the phase assemblage remained the same. After the atmosphere was switched, the precursor was heated rapidly to 690°C and within minutes 123 compound began to form at this temperature. The dynamic response of the system in growing the 123 phase is registered in fig. 2. An isothermal experiment at 690°C revealed that 123 phase is capable of growing as fast as the barium carbonate is decomposing. Figure 3 shows the phase evolution of a citrate sample during the isothermal experiment. The formation of 123 high-temperature tetragonal phase is nearly completed when the carbonate is nearly fully decomposed. The growth of the 123 phase is closely related to the decomposition of barium carbonate; this is shown in fig. 4. Once the barium carbonate is gone, the 123 tetragonal phase showed little change at longer soaking time, with the exception that the peak splitting between ( 110 ) and (013 ) appeared to be sharpening up with time. This seems to indicate a higher degree of deoxygenation at longer soaking time. During the isothermal experiment, initially, no impurity phase could be recognized in the X-ray spectra. Later on, a BaCuO2 peak appeared as a shoulder on the left fringe of the ( l 10) peak of the 123 phase, as shown in fig. 4, spectrum I. After the formation of tetragonal 123 phase reached maturity, the material was rapidly cooled from 690°C to 500°C at 20 K/min. The tetragonal phase remained stable in flowing helium down to the phase transformation region. This is shown in fig. 5, from spectra A to D. Then the reaction atmosphere was switched from helium back to oxygen at 500 ° C. The transformation from tetragonal to orthorhombic began almost im-
S.H. Shieh, W.J. Thomson ~Low-temperature synthesis of YBCO powders
139
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T i m e (min) T w o Thcta Fig. 3. XRD spectra of an isothermal experiment of a citrate precursor at 690°C in helium. The tetragonal 123 began to develop within minutes. The sample became nearly phase pure in 2 h.
mediately as soon as the oxygen was admitted into the reactor. This seems to be in good agreement with the reported swiftness of oxygen intake by the tetragonal phase at the phase transformation temper-
Fig. 4. The growth of the 123 phase is closely related to the decomposition of BaCO3 for both citrate and nitrate samples.
ature [ 37,41,49-51 ]. The spectra of orthorhombic 123 phase did not show significant change when it was cooled to room temperature. Hash nitrate precursor is a fine mixture of crystallites of oxides and barium carbonate. Sharp X R D peaks suggested that the size of these crystallites are
S.H. Shieh, W.J. Thomson / Low-temperature synthesis of YBCOpowders
140
10000
4 h at that temperature did not help the elimination of these impurities. On the contrary, intensities of impurity peaks grew steadily with soaking time. Figure 6 shows the nucleation and phase evolution of the 123 phase and impurities in a nitrate precursor at 690 ° C. In the presence of these impurities, the result of low-temperature synthesis based on flash nitrate is predictably not satisfactory.
o Oxygen • Tetragonal 123 Annealing o Orthorhombie123
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3.3. Reaction sequences during 123 decomposition
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T w o Theta Fig. 5. XRD spectra following the rapid cooling of the 123 phase from 690 °C to 500 ° C, the tetragonal/orthorhombic phase transformation, and oxygen annealing.
significantly large and SEM studies showed round oxide aggregates in the submicron range mixed with similar size balls of barium carbonate dendrites. Citrate ash, on the other hand, does not have any specific particle microtexture. It appeared as fragmented glassy chips even under the SEM. Therefore, kinetically at least, the flash nitrate precursor is not as favorable as the citrate for low-temperatures synthesis. The X-ray spectra did not change when the flash nitrate precursor was heated from room temperature to 600 ° C, although, the sharpening of X R D peaks with increasing temperature does indicate some crystal growth as a result of heating. When flash nitrate precursor is being heated to 690 ° C, diffraction data clearly show that oxides and carbonates began to react, leading to the nucleation of high-temperature tetragonal 123 as well as other impurity phases such as BaCuO2, Y 2 C u 2 0 5 and Y2BaCuO5 at about the same time. We are unable to reconstruct the exact nucleation sequences of the appearance of these phases. Following nucleation, the tetragonal 123 continued to increase during soaking, and peak splitting between (110) and (013) improved with time. For those impurity phases that formed simultaneously with the 123 phase at 690°C, they seemed quite stable at that temperature. Extended calcination for
In our experiments, 690°C seems to be the lowest temperature where the 123 phase can be grown at a reasonably fast rate. Synthesis rate of the 123 phase increased with soaking temperature. At temperatures just higher than 690 ° C, BaCuO2 formed during the previous isothermal treatment, began to decrease with increasing temperature while the 123 phase continued to increase at the same time. This seems to indicate that binary impurities began to decompose to form the 123 phase when the calcination temperature increased. We also noted that BaCuO2 became more reduced in helium at higher temperatures and decreased when the stability field of B a C u 2 0 2 is reached. Heating in helium eventually destabilized the 123 phase which decomposed to form Y2BaCuO5 (211), YBa2CuEO7_x (132),
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S.H. Shieh, W.J. Thomson ~Low-temperaturesynthesis of YBCO powders
141
maximum at that temperature. This decomposition sequencing can be deduced from the XRD integrated intensities as shown in fig. 8. In the nitrate sample, barium copper oxide coexisted with the 123 phase during subsequent heating beyond 690°C. Barium copper oxides remained in the sample throughout the entire heating rang e . The 123 phase gradually increased during heating and reached maximum at 800°C. This maturing temperature is nearly 90 degrees higher than what was observed for the citrate sample. Figure 9 shows the 123 phase began to decompose at approximately 810°C; as a result, BaCu202 increased. Following the growth of BaCu202, 132 and 211 were soon detected at 825 ° C. The decomposition temperature of the 123 phase in nitrate was significantly lower than that of the 123 phase in citrate. Presumably, since barium copper oxides were not removed or reacted away after their formation in nitrate, the presence of these impurity phases may act as nucleation sites for promoting the 123 phase to decompose at lower temperatures. A comparison with the citrate sample showed that a lot more BaCu202 was produced in the nitrate sample after complete decomposition of the 123 phase. This is revealed in the much higher integrated
BaCu202 and possibly other phases that were not readily identified in our diffraction data. A 3D plot showing the phase evolution during the 123 decomposition is presented in fig. 7. Again, the citrate sample behaved differently from the nitrate sample during decomposition. Above 690°C, impurity phases in citrate decreased, the sample appeared nearly phase pure, consisting mainly of the 123 phase and perhaps a very small amount of barium copper oxides. Further heating of this sample to 750°C revealed little change in X-ray spectra indicating that phase assemblage and relative intensities of these phases remained similar. This seems to suggest that the 123 phase was growing to approach maturity within this temperature range. In this citrate sample, BaCu202 started to form at 750°C. However, after its nucleation, the growth of BaCu202 was constrained by the slow kinetics. The X-ray intensity of BaCu202 showed an unaltered behavior versus temperature before reaching 850 ° C. A minor increase of the 123 phase continued till 850 ° C, then it began to decompose. Meanwhile, 132, 21 l and BaCu202 sharply increased as the 123 phase disappeared. In helium, the 123 phase completely decomposed at 900°C. Correspondingly, the peak intensities of 132, 211 and BaCu202 all reached
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142
S.H. Shieh, W.J. Thomson/Low-temperaturesynthesis of YBCOpowders
to the persistence and growth of the barium copper oxide phases during the decomposition sequence. •~;~
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4. Discussion
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Temperature (C) Fig. 9. Successivephase evolution during the decompositionof the 123 phase in a nitrate sample. X-ray intensity observed for BaCu202 in the nitrate sample. Because a large quantity of BaCu202 was produced in the phase assemblage from the decomposition of the 123 phase, more yttrium ions will be available for forming other decomposition products that have high yttrium concentrations. This assumption is in good agreement with the observation that the peak intensity ratio between 211 and 132 increased from approximately 1 : 2 in the citrate sample to above 1 : 1 in the nitrate sample, suggesting that after the decomposition of the 123 phase, more 211 was formed in the nitrate sample than in the citrate sample. This enrichment of 211 was closely related
One of the key issues that critically affects the quality of low-temperature prepared superconductors is the phase purity of the product. While it would be ideal to be able to synthesize a phase-pure superconducting powder, contaminations from binary oxides and other minor impurities have been universally observed in many of the previous attempts on low-temperatures synthesis [ 17,19-21,24,29,34]. Despite their differences in using various starting precursors, many X-ray spectra showing low-temperature, i.e. below 750°C, synthesized 123 compound have a striking similarity in terms of the accompanying impurity peaks. These impurity peaks are usually weak and are difficult to identify due to overlaps among several coexisting phases. The published data on phase identification of these impurities are often subjective with different authors concluding in favor of different phases. We have assigned the impurity peaks that we have observed in flash nitrate during isothermal processing to BaCuO2, Y2Cu205 and Y2BaCuO5. We did not identify CuO among the impurities although it might exist. In the citrate sample, it was rather difficult to identify any impurity phase other than BaCuO2. BaCuO2, has been a well known byproduct in 123 superconductor synthesis. It was also reported as a decomposition product of 123 at higher temperatures in air [42-46 ]. It has been shown that the amount of BaCuO2 contamination in the 123 sample is out of proportion with their peak intensity ratios. The amount of contamination is not negligible even if the relative intensity of its strongest diffraction peak is negligibly weak as compared to peaks of the 123 phase [47]. Achieving a low-temperature orthorhombic 123 synthesis with good phase purity proved a considerable challenge. Between the two precursors that we have studied, the impurity contamination in the citrate-derived sample was much less severe as compared to the impurity abundant sample grown from flash nitrate. Since both precursors were processed under identical conditions, the only difference that explains the
S.H. Shieh, W.J. Thomson / Low-temperaturesynthesis of YBCOpowders
discrepancies in observed phase purity and the growth rate of 123 phase seems to be the difference of the crystallite size of the starting material. Larger crystallites from the flash nitrate precursor seem to slow down the formation of 123 phase and enhance the growth of impurities. There are several important implications for the presence of these impurity phases. More impurities from a larger crystaUite precursor is suggesting that a larger size precursor slows the reaction kinetics for the ternary phase and favors the binary phases. If we follow this line of argument, it can be predicted that by lowering the processing temperature, which drops the ternary formation kinetics even lower, the calcination should produce more binary phase relative to the ternary. This is in good agreement with the XRD data presented by Zheng et al. [ 19 ], and by Horowitz et al. [ 24 ]. Both studies show that samples calcined at 650°C contained more binary impurities than samples prepared at and above 700°C. It was also unanimously observed that these low-temperature grown impurities decompose as the calcination temperature is raised higher. Evidently, the formation of these impurity phases were also assisted by the inert atmosphere. They grow when the reaction kinetics for the growth of ternary 123 phase is not fast enough to dominate the whole phase assemblage. Another possible important factor that contributes to the impurity formation is the slow decomposition of barium carbonate. We have observed the flash nitrate precursor carrying barium carbonate during isothermal experiments for more than 240 min. This resilient carbonate narrowed the supply of free barium from entering the ternary reaction, which is turn caused the chemistry of the whole batch of material to drift away from the originally well-mixed 123 stoichiometry. This can create a more advantageous condition for binary impurity formation. If favorable growth conditions for ternary reaction are provided, such as higher processing temperatures and finer size precursors, it is then possible to achieve faster rates of carbonate decomposition and 123 formation. Under this circumstance, the impurity formation is not favored and some even decompose to form 123 phase. A precursor such as flash nitrate that consists of a large size crystallite requires a higher calcination temperature than that for citrates, if the impurity formation is to be avoided. Longer calci-
143
nation time by itself will make little difference without raising the processing temperature to improve the reaction kinetics. It is interesting to note that, so far, BaCu202 has not been found as an impurity product during our isothermal experiment. Although we have observed its formation as a decomposition product of 123 phase in inert atmosphere at higher temperatures and confirming observations made by other studies [ 34,45,48,52]. This is suggesting that the low temperature thermodynamic stability of 123 phase and the quaternary phase relations near this phase is quite different from that of higher temperatures. According to the quaternary phase relations determined by Ahn et al. [ 53 ], the 123 phase decomposes to form BaCu202, 211 and 132 at an oxygen partial pressure of 4 × 10 -4 atm at 850°C. This reaction condition corresponded exactly to what we have observed in our experiments, which is a good indication that the partial oxygen pressure in our reaction chamber was at most 4 × l0 -4. The fact that we have found only BaCuO2 coexisting with 123 in our isothermal experiments only means that the phase assemblage formed at 690°C is less reduced, compared with a high-temperature one under similar depleted oxygen environment. When the 123 phase and impurities are quenched to 500°C for annealing in oxygen, the intensity of BaCuO2 peaks rapidly decreased within minutes of oxygen induction, and became nearly undetectable by XRD. This reaction bears an interesting resemblance to the disappearance of BaCu202 at high temperatures when the environment is switched from low to high oxygen atmosphere. Ruckenstein et al. reported that BaCuO2 reacts rapidly with YECU205 or with Y203 and CuO, and fast 123 formation can be achieved by using BaCuO2 as a precursor instead of barium carbonate for the source of barium [25]. Therefore, ideally, the precursor can react according to reaction eq. ( 1 ) and (2) as reported by Selvaduray et al. [26]: BaCO3 + CuO = CO2(s) + BaCuO2,
( 1)
2BaCuO2 + ½Y203 +CuO=YBa2Cu307_x.
(2)
We are unable to deduce whether this rapid reaction pathway designed for the 900-950°C range agrees well with our low-temperature synthesis data under a reduced atmosphere. If this rapid reaction path also
144
S.H. Shieh, W.J. Thomson ~Low-temperaturesynthesis of YBCOpowders
applies to low-temperature processing in helium, one would observe initially the formation of a significant amount of BaCuO2 which decreased with time while the 123 phase increased. We did not observe significant binary phase formation prior to the formation of the 123 phase in the citrate precursor. Our X-ray data showed that significant amount of the 123 phase seemed to form directly from the precursor before any BaCuO2 can be detected in the spectra. In the case of processing the nitrate, once BaCuO2 formed, it is actually functioning as an annoying impurity rather than a good precursor phase. It does not react to form the 123 phase at longer time. However, we do not rule out the possibility that this rapid reaction path took place so fast that most BaCuO2 formed from the decomposition of BaCO3 reacted instantaneously to form the 123 phase through fast diffusion paths as provided by the finely mixed nature of the precursor. Extra BaCuO2 appeared in the spectra only after a significant amount of the 123 phase has been grown. If this is true, one could also observe ternary 123 phase appeared forming directly from the precursor under favorable conditions while BaCuO2 was detected only after the 123 phase has grown to a certain extent. Once these rapid diffusion paths were exhausted in the precursor, it became difficult to react to form the ternary phase and subsequently formed coexisting binary phases. This may explain why at low temperatures binary impurities increased with soaking time but do not react with one another to form the ternary phase unless a higher processing temperature is used. Between 690°C and 750°C, within this temperature range, BaCuO2 decreased and was reduced to form BaCu202 as described in eq. (3) as reported by Ahn et al. [53]: BaCuO2 + CuO = BaCu202 + ½02(s) •
(3)
With further heating in helium, the 123 phase started to decompose to form 211 and BaCu202 according to eq. (4). As a result, BaCu202 increased rapidly as shown in fig. 6. 2YBa2Cu306.5+x + ½Cu20 =Y2BaCuO5 + 3BaCu202 + ( 1 q-x)O2~g).
(4)
A similar reaction was also reported by Alan et al. [48] and by Zhang et al. [45 ]. This decomposition
reaction may take place in nitrate at slightly lower temperatures than that of the citrate sample. When heated above 830°C, the 123 phase was no longer a stable phase under a low-oxygen condition; it decomposed to produce 132 along with 211 and BaCu202 as expressed by eq. (5) according to Ahn et al. [53]: 7YBa2 Cu3 06 = 3Y2 BaCuO5 + YBa3 Cu2 O6+x + 8BaCu202 + ( 5 - x ) / 2 0 2 .
(5)
For both citrate and nitrate samples, the 123 phase completely decomposed at 900°C indicating thermodynamic phase equilibria having been established at this temperature under depleted oxygen environment. Because the decomposition reaction according to eq. (4) occurred earlier in nitrate, relatively more 211 and BaCu202 were produced in the final assemblage. This explains why the 211 phase was enriched with respect to the 132 phase in the decomposed nitrate sample. For the citrate sample, rapid decomposition took place mainly according to eq. (5), consequently, more 132 formed at elevated decomposition temperatures. Therefore, although the decomposition reaction of the 123 phase was complete at the same temperature for both citrate-derived and nitrate-derived samples, their differentiation paths for forming the final phase assemblages were not similar.
5. Conclusion
We report a low-temperature procedure for the 123 superconductor synthesis using carbon-containing citrate and nitrate precursors at formation temperatures below 700°C. In both cases, it was possible to decompose the barium carbonate below 700°C. Between the two precursors, the flash nitrate precursor is a less favorable candidate for low-temperature synthesis due to its large size crystallites which inflict a higher kinetic barrier for low-temperature ternary reactions, The amorphous citrate precursor, on the other hand, proved more successful for the formation of superconducting 123 phase at low temperature. Synthesis using the amorphous citrate precursor results in a finer mixing and better chemical homogeneity on the molecular scale. The difference
S.H. Shieh, W.J. Thomson ~Low-temperature synthesis of YBCO powders in o b s e r v e d kinetics b e t w e e n different precursors reflects correctly their i n h e r e n t characteristics in b o t h c h e m i c a l a n d textural h o m o g e n e i t i e s . Slow t e r n a r y reactions lead to the f o r m a t i o n o f a c o n s i d e r a b l e a m o u n t o f c o n t a m i n a t i o n i n the f o r m o f b i n a r y c o m p o u n d s . Therefore, r e a s o n a b l e phase p u r i t y a p p e a r s to be difficult to achieve via l o w - t e m p e r a t u r e processing e v e n with precursors that are p r e p a r e d with a m i x i n g o n the a t o m i c scale. As a result, a l t h o u g h l o w - t e m p e r a t u r e c a l c i n a t i o n seems to p r o v i d e a good a l t e r n a t i v e in p r o d u c i n g the 123 s u p e r c o n d u c t o r , the practical v a l u e o f this m e t h o d is l i m i t e d w h e n scaled for mass p r o d u c t i o n .
Acknowledgements We express o u r a p p r e c i a t i o n to M. Strasik o f B o e i n g M a t e r i a l T e c h n o l o g y w h o p e r f o r m e d the m a g n e t i c susceptibility m e a s u r e m e n t s o f the superc o n d u c t o r , to K.A. Y o u n g d a h l o f B o e i n g Aerospace a n d Electronics for p r e p a r a t i o n o f citrate gel, a n d to C. H a n o f the U n i v e r s i t y o f W a s h i n g t o n for the prepa r a t i o n o f flash nitrate.
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145
[ 13 ] M.K. Agarwala, D.L. Bourell and C. Persad, J. Am. Ceram. Soc. 75 (1992) 1975. [ 14] IC Koyama, A. Jundo, T. Grap, G. Triscone and J. Muller, Physica C 185-189 (1991) 461. [15]V.R. Palkar, P. Gaptasarma, M.S. Multani and R. Vijayaraghavan, Physica C 185-189 ( 1991 ) 479. [ 16] R.V. Kamat, T.V. Rao, K.T. Pillai, V.N. Vaidya and D.D. Sood, Physica C 181 ( 1991 ) 245. [ 17 ] P. Ravindranathan, S. Komarneni, A. Bhalla, R. Roy and L.E. Cross, J. Mater. Res. 3 (1988) 810. [ 18 ] M. Kakihana, L. Borjesson, S. Erikson, P. Svedlindh and P. Norlin8, Physica C 162-164 (1989) 931. [ 19] S.C. Zhang, G.L. Messing, W. Huebner and M.M. Coleman, J. Mater. Res. 5 (1990) 1806. [20 ] H. Zheng and J.D. MacKenzie, Mater. Lett. 7 (1987) 182. [21 ] M.J. Cima, R. Chiu and W.E. Rhine, Mater. Res. Soc. Symp. Proc. 99 (1988) 241. [22] X.Z. Wang M. Henry, J. Livage and I. Rosenman, Solid State Commun. 64 (1987) 881. [23] C.T. Chu and B. Dunn, J. Am. Ceram. Soc. 70 (1987) C375. [24] H.S. Horowitz, S.J. McLain, A.W. Sleight, J.D. Druliner, P.L. Gai, M.J. VanKavelaar, J.L. Wagner, B.D. Biggs and S.J. Poon, Science 234 (1989) 66. [25] E. Ruckenstein, S. Narain and N.L. Wu, J. Mater. Res. 4 (1989) 267. [26] G. Selvaduray, C. Zhang, U. Balachandran, Y. Gao, K.L. Merlde, H. Shi and R.B. Poeppel, J. Mater. Res. 7 (1992) 283. [27 ] Crystal structure data of inorganic compound vol. 7, eds. ICH. Hellwege and A.M. Hellwege (Springer, Berlin, 1979) p. 109. [28 ] A. Manthiram and J.B. Goodenough, Nature (London) 329 (1987) 701. [29] K.W. Lay, J. Am. Ceram. Soc. 72 (1989) 696. [30 ] K. Kinoshita, A. Matsuda, T. Ishii, N. Suzuki, H. Shibata, T. Watanaba and T. Yamada, Jpn. J. Appl. Phys. 27 ( 1988 ) L795. [31] S. Sato, I. Nakada, T. Kohara and Y. Oda, Jpn. J. Appl. Phys. 26 (1987) L663. [32] F.P. Okamura, S. Sueno, I. Nakai and A. Ono, Mater. Res. Bull. 22 (1987) 1081. [33] V. Caignaert, M. Hervieu, J. Wang, G. Desgardin and B. Raveau, Physica C 170 (1990) 139. [34]W.J. Thomson, H. Wang, D.B. Parkman, D.X. Li, M. Strasik, T.S. Luhman, C. Han and I.A. Aksay, J. Am. Ceram. Soc. 72 (1989) 1977. [ 35 ] Y. Nakazawa, M. Ishikawa, T. Takabatake, K. Koga and K. Terakura, Jpn. J. Appl. Phys. 26 ( 1987 ) L796. [ 36 ] H.I. Yoo, J. Mater. Res. 4 (1989) 23. [37] K.N. Tu, S.I. Park and C.C. Tsuei, Appl. Phys. Lett. 5 (1987) 2158. [38] C.C. Torardi, E.M. McCarrun, M.A. Subramanian, H.S. Horowitz, J.B. Michel and A.W. Sleight, Am. Chem. Soc. Sym. 351 (1987) 152. [39] J.J. Rha, K.J. Yoon, S.J.L. Kang and D.N. Yoon, J. Am. Ceram. Soc. 71 (1988) C 328. [ 40 ] W.J. Thomson, Ceram. Trans. 5 (1989) 131.
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S.H. Shieh, W.J. Thomson ~Low-temperaturesynthesis of YBCOpowders
[ 41 ] H.M. O'Bryan and P.K. Gailagher, J. Mater. Res. 3 ( 1988 ) 619. [42] T. Aselage and IC Keefer, J. Mater. Res. 3 (1988) 1279. [43] K.G. Frase, E.G. Liniger and D.R. Clarke, J. Am. Ceram. Soc. 70 (1987) C204. [44] R.S. Ruth, K.L. Davis and J.R. Dennis, Adv. Ceram. Mater. 2 (1987) 303. [ 45 ] W. Zhang and K. Osamura, Physica C 190 (1992) 396. [46 ] T. Izumi, Y. Nakamura, T. Sung and Y. Shiohara, J. Mater. Res. 7 (1992) 801. [47] T. Itoh, M. Uzawa and H. Uchikawa, J. Am. Ceram. Soc. 71 (1988) C188.
[ 48 ] B.T. Ahn, V.Y. Lee, R. Beyers, T.M. Gur and R.A. Hu88ins, Physica C 162-164 (1989) 883. [49] C. Nobili, G. Ottaviani and M.C. Rossi, Physica C 168 (1990) 549. [50] H. Yoo, J. Mater. Res. 4 (1989) 23. [51 ] D.J. Holocomb and M.J. Mayo, J. Mater. Res. 5 (1990) 1827. [52] M. Murakami, Mod. Phys. Lett. B 4 (1990) 163. [ 53 ] B.T. Ahn, V.Y. Lee, R. Beyers, T.M. Gur and R.A. Huggins, Physica C 167 (1990) 529.
Low-temperature synthesis of YBa2Cu307_x powders from citrate and nitrate precursors S.H. Shieh a n d W.J. T h o m s o n Department of Chemical Engineering, Washington State University, Pullman, WA 99164-2710, USA Received 21 August 1992
YBa2Cu307_xsuperconductorpowdershave been prepared fromcarbon-containingcitrate and flashnitrate precursors. In both cases, it was possible to decomposethe barium carbonate below 690°C and to form the 123 phase directlyfrom the precursor in an helium atmosphere. High-Te superconducting 123 phase with a sharp transition at 93 K was obtained after oxygenannealing. High-temperature, in situ, X-ray diffraction was used to followphase evolution and reaction sequences at several stages in the formationas well as in the decompositionof the 123 phase. We investigated the effectsof precursor homogeneityin the formation of the 123 phase during low-temperatureprocessing. The properties and phase purity of the low-temperatureprocessedsuperconductor ceramicsare stronglydependent on precursor homogeneityand morphology.Additionally, our results showthat the existence of low-temperature grownbinaryimpurities affectsthe successiveevolutionof entire phase assemblageduring further stages of heating till completedecompositionof the 123 phase.
1. Introduction Industrial production of superconductor ceramics requires a reaction pathway for efficient synthesis without compromising the material's superconducting properties. Low-temperature processing is a costeffective alternative to the traditional high-temperature processing, provided that sufficient superconducting properties can be achieved. Many studies have reported low-temperature processing of high quality orthorhombic 123 superconductor thin films at temperatures between 500-650°C, i.e. at temperatures just slightly higher than the tetragonal/orthorhombic phase transformation temperature, which is approximately 600°C [ 1-7] depending on the oxygen partial pressure [ 8 ]. The production of fine orthorhombic 123 superconductor powder, fine grained monolith, and superconducting fiber, on the other hand, has had very limited success in finding a lowtemperature pathway. Traditional solid-state powder synthesis from mixed oxide precursors requires a high processing temperature. Repeated calcination, grinding, and sintering for many hours are familiar experimental procedures reported in the literature. It is well known that a high processing
temperature encourages grain growth and often produces non-uniform large sintered particles [ 9 ]. In order to suppress the grain growth and to limit the particle size within submicron range, a low-temperature route for powder processing is very desirable. Several papers have dealt with the synthesis of superconducting ceramics via a sol-gel method [ 9-24 ]. Precursors are often prepared from solutions of gels of citrate, nitrate, oxalate, acetate and metal alkoxide. It is also possible to prepare precursors from formate and lactate by more elaborated methods [ 12 ]. Improved chemical homogeneity, greater control over particle size, better processing control, and lowered processing temperature are among the anticipated benefits in using this method. These new synthesis procedures allow superconductor ceramic powders to be prepared at processing temperatures 150200°C lower than the conventional calcination temperature. The majority of studies that started with solutionderived precursors for preparing the YBCO 123 superconductor used ambient atmosphere for processing. One major obstacle preventing traditional superconductor powder processing at lower temperatures is the decomposition of barium car-
0921-4534/92/$05.00 © 1992 ElsevierSciencePublishers B.V. All rights reserved.
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S.H. Shieh, W.J. Thomson ~Low-temperaturesynthesis of YBCOpowders
bonate in air. This has been described as the rate determining step during the formation of superconductor phase [21,25,26]. Barium carbonate is relatively stable in air up to 800°C. It undergoes a phase transformation from gamma to beta starting at 816 ° C [ 27 ]. It is not clear whether the beta barium carbonate is easier to decompose and easier to react with the other two oxides to form the 123 superconductor than the gamma phase. The decomposition of barium carbonate seems to be a common problem, and it has been reported that a complete elimination of carbonate during calcination cannot be achieved under 800°C, even with fine particle precursors [24]. If this is true, it may leave out any possibility for preparing a phase-pure superconductor product below the barium carbonate decomposition temperature under ambient conditions. A separate observation showed that a high percentage of the barium carbonate can be decomposed at 780°C, if calcined for a long enough time in air (60 h) [28]. Unfortunately, long heat treatment time is another undesirable factor that only promotes grain growth. It seems, in order to synthesize single phase 123 superconductor at temperatures substantially below 780°C, one has to avoid the formation of barium carbonate by either using a carbon-free precursor or by calcination in an atmosphere free of carbon dioxide, at the same time maintaining relatively fast reaction kinetics. The search for a low-temperature processing route is further plagued by the occurrence of the semiconducting tetragonal " X " phase [23,29-33], or the "low-temperature" tetragonal phase [ 34 ], while the "high-temperature" tetragonal phase is the superconductor phase. The X phase has a similar tetragonal structure to that of the superconductor phase but it has a high oxygen stoichiometry and therefore a shorter "c" cell [35 ]. This shorter "c" cell makes " c / 3 " very close to both " a " and "b" cell parameters, such that (110) and (013) peaks merge into one peak. This non-splitting peak character is unique to this X phase. The X phase does not transform to the superconducting orthorhombic phase at lower temperatures, even after extended annealing with oxygen [22 ]. Once the X phase forms during calcination in air, the simplest way to convert it to the proper superconductor tetragonal phase is to remove some oxygen out of it by reheating it in a higher pro-
cessing temperature. The oxygen content of the 123 phase is a strong function of temperature, and it becomes more reduced at higher temperatures [36,37]. In order to avoid the formation of X phase in air, a processing temperature exceeding 850°C is usually required [38]. A second method of removing oxygen from the X phase is to reprocess this material in a low-oxygen atmosphere. The inability to correctly recognize this X phase has led some studies to report a very low formation temperature for the superconductor phase, and yet measured its superconducting properties based on samples reheated to higher temperatures [ 17,21 ]. The possibility of low-temperature powder processing for the 123 superconductor was first reported by Rha et al. [ 39 ]. Using a helium atmosphere, they were able to sinter 123 superconductor at 850°C after 12 h from mixed oxide precursors. They observed an increased rate of sintering in an inert atmosphere and speculated that sintering in a depleted oxygen environment enhances ion mobility which lowers the superconductor formation temperature. Several other studies have reported similar success in the preparation of orthorhombic 123 superconductor in low oxygen atmosphere at temperatures below 800°C. Lay reported a formation temperature 750°C for the superconducting phase from mixed oxides and barium carbonate using 1 Torr vacuum [29 ]. Thomson et at. started with nanometer scale gel precursors and observed the high-temperature tetragonal phase start to form in helium at temperatures as low as 627°C [34]. Zheng et al. also prepared a sample at 780°C from metal alkoxides that showed resistivity at 57 K
[20]. So far, the only successful attempt of low-temperature powder synthesis of the orthorhombic 123 superconductor was the one conducted by Horowitz et al. [24]. They started with carbon-free organometallic precursors and used elaborated experimental procedures to prevent the formation of barium carbonate during heating. They found that it is possible to decompose the precursor and to form the hightemperature tetragonal phase in argon at 700°C for 12 h. This phase was annealed in oxygen for 12 h to make the superconducting orthorhombic phase. The sample they produced showed a broad transition re# o n beginning at 85 K. They also reported that the same processing technique can be employed to make
S.H. Shieh, W.J. Thomson~Low-temperaturesynthesis of YBCOpowders superconductor ceramics at 650°C. However, their X-ray data indicated the formation of a large amount of barium copper oxide contamination coexisting with the high-temperature tetragonal superconductor phase such that a subsequent high-temperature (900 ° C) calcination in oxygen became necessary to restore the phase purity. The objective of our study was to design a new pathway to prepare fine superconductor powder at low temperatures by using carbon-containing sol-gel precursors, and to look for a kinetically favorable route that does not require extended calcination time. A further objective of this study was to study the decomposition behavior of t h e 123 phase at higher temperatures in helium.
2. Experiments Two different precursors, citrate gel and flash nitrate, were used for this experiment. Nanometer scale precursors are ideal for low-temperature synthesis. The citrate precursor was prepared by dissolving stoichiometric amounts of yttrium, barium and copper acetates in a mixture of a weak hydroxycarboxylic acid such as citric acid and ethylene glycol. The solution was heated at 170°C to dissolve the acetates and then diluted with distilled water. Water was gradually removed by heating the solution at around 80 ° C until the solution turned blue and viscous. This viscous solution was dried in an oven at 70°C. It solidified into a greenish-white chalk after 48 h. The brittle solid chalk could be easily ground to little chips. Fragmented chips were again heated at 150200 °C in a tube furnace for 24 h in air. After heating, the precursor is ground to very fine, light brown color ash. The flash nitrate precursor was prepared by a "flashed" carbothermal reduction of the nitrate salts. The nitrates were reduced to yttrium oxide, copper oxide and barium carbonate from exposures to organic carbon at temperatures in excess of 600 ° C [34]. The main difference between these two precursors is that the flash nitrate precursor is a mixture of fine crystallites while the heat treated citrate precursor remains largely amorphous. Time resolved in situ high-temperature X R D was used to trace all the chemical reactions and phase evolutions in this study. A position-sensitive proportional counter permits each data scan over a range
137
of 50-degree two-theta to be completed within 2 min [ 40 ], thus allowing one to perform a precise monitoring of time critical changes during the formation of the superconductor. The precursor material was first wetted with de-ionized water and then a thin layer of sample approximately 100 microns thick was laid on the surface of a platinum heating ribbon. This heating platinum element is enclosed in a metal chamber of controlling the reaction atmosphere. All the temperatures reported in this work are measured by a thermal couple welded to the bottom of this heating ribbon and positioned at the X-ray focal point on the diffractometer. Cobalt radiation was used for all scans. Since the X-ray diffracts mostly from the upper portion of the sample while the thermal couple detects the sample temperature at the bottom, the true reaction temperature might be slightly lower than the temperature recorded by the thermal couple. We can use the thermal couple temperature as a upper limit for all the chemical reactions observed in this work.
3. Results 3.1. Optimized low-temperature synthesis We have found that it is possible to obtain orthorhombic 123 by first heating the dried citrate ashes in either oxygen or air to 580°C at 5 K / m i n , held for 20 min to allow for organic carbon to burn-out, then switching the calcining atmosphere from oxygen or air to helium and continuing heating it to 690°C at 100 K/min. The high-temperature tetragonal phase with well split (110) and (013) peaks was fully developed within 2 h at 690°C. This phase was then quenched to 500 ° C, and at that temperature, oxygen was reintroduced to replace helium. The sample was annealed in oxygen at 500°C for 12 h; however, the X-ray spectra showed little change after 2 h of annealing. Among several runs we have performed, two samples of citrate gel preheated in oxygen and calcined in helium were measured for their superconducting properties. Based on the magnetic susceptibility measurement, one sample showed diffused Tc although the X R D spectrum looked excellent, and it was suspected that some unreacted materials outside the hot zone were scraped in with the sample
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S.H. Shieh, W.J. Thomson / Low-temperature synthesis of YBCO powders
when the superconductor phase was collected from the heating ribbon. A second sample was collected exclusively from the central portion of the heating ribbon within the range of the X-rays. This sample showed a sharp superconducting transition starting at 93 K. In both cases, the measured flux exclusion is still weak, indicating the possible presence of other impurity phases, SEM micrographs taken from these citrate samples prepared at 690°C for 2 h already showed morphologies indicating some degrees of grain growth. Single particles 2 to 3 microns can be seen among fine particles that are in general less than 0.25 micron thick. Neck growth between minute fine particles is common.
3.2. Phase evolution during 123 formation The spectral data trailing the formation of tetragonal 123 from the citrate precursor is presented in figs. 1-4. The citrate gel remained X-ray amorphous up to about 200°C prior to complete drying; this can be seen in spectra A and B of fig. I. After drying, the precursor appeared slightly crystalline after oxidation and decomposition of some copper components in gel which formed copper oxides, CuO (spectrum C, fig. l ). Spectrum D through spectrum I in fig. 1 shows that copper oxide continued to precipitate
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from oxidation of copper metal in the largely amorphous citrate ash at lower temperatures, under 400°C. Barium carbonate began its nucleation around 470°C when the organic carbon in the precursor began to oxidize and generated enough carbon dioxide in the reaction chamber. After nucleation, the X-ray intensity of BaCO3 increased during the entire organic bum-out period indicating continued crystal growth. Similar reaction sequences were observed when the citrate precursor was heated in oxygen instead of in air, the only difference being that copper oxides formed at lower temperatures in oxygen. Following organic bum-out, the calcining atmosphere was changed from oxygen to helium at 580°C. During the change of reaction the atmosphere of the phase assemblage remained the same. After the atmosphere was switched, the precursor was heated rapidly to 690°C and within minutes 123 compound began to form at this temperature. The dynamic response of the system in growing the 123 phase is registered in fig. 2. An isothermal experiment at 690°C revealed that 123 phase is capable of growing as fast as the barium carbonate is decomposing. Figure 3 shows the phase evolution of a citrate sample during the isothermal experiment. The formation of 123 high-temperature tetragonal phase is nearly completed when the carbonate is nearly fully decomposed. The growth of the 123 phase is closely related to the decomposition of barium carbonate; this is shown in fig. 4. Once the barium carbonate is gone, the 123 tetragonal phase showed little change at longer soaking time, with the exception that the peak splitting between ( 110 ) and (013 ) appeared to be sharpening up with time. This seems to indicate a higher degree of deoxygenation at longer soaking time. During the isothermal experiment, initially, no impurity phase could be recognized in the X-ray spectra. Later on, a BaCuO2 peak appeared as a shoulder on the left fringe of the ( l 10) peak of the 123 phase, as shown in fig. 4, spectrum I. After the formation of tetragonal 123 phase reached maturity, the material was rapidly cooled from 690°C to 500°C at 20 K/min. The tetragonal phase remained stable in flowing helium down to the phase transformation region. This is shown in fig. 5, from spectra A to D. Then the reaction atmosphere was switched from helium back to oxygen at 500 ° C. The transformation from tetragonal to orthorhombic began almost im-
S.H. Shieh, W.J. Thomson ~Low-temperature synthesis of YBCO powders
139
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mediately as soon as the oxygen was admitted into the reactor. This seems to be in good agreement with the reported swiftness of oxygen intake by the tetragonal phase at the phase transformation temper-
Fig. 4. The growth of the 123 phase is closely related to the decomposition of BaCO3 for both citrate and nitrate samples.
ature [ 37,41,49-51 ]. The spectra of orthorhombic 123 phase did not show significant change when it was cooled to room temperature. Hash nitrate precursor is a fine mixture of crystallites of oxides and barium carbonate. Sharp X R D peaks suggested that the size of these crystallites are
S.H. Shieh, W.J. Thomson / Low-temperature synthesis of YBCOpowders
140
10000
4 h at that temperature did not help the elimination of these impurities. On the contrary, intensities of impurity peaks grew steadily with soaking time. Figure 6 shows the nucleation and phase evolution of the 123 phase and impurities in a nitrate precursor at 690 ° C. In the presence of these impurities, the result of low-temperature synthesis based on flash nitrate is predictably not satisfactory.
o Oxygen • Tetragonal 123 Annealing o Orthorhombie123
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3.3. Reaction sequences during 123 decomposition
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T w o Theta Fig. 5. XRD spectra following the rapid cooling of the 123 phase from 690 °C to 500 ° C, the tetragonal/orthorhombic phase transformation, and oxygen annealing.
significantly large and SEM studies showed round oxide aggregates in the submicron range mixed with similar size balls of barium carbonate dendrites. Citrate ash, on the other hand, does not have any specific particle microtexture. It appeared as fragmented glassy chips even under the SEM. Therefore, kinetically at least, the flash nitrate precursor is not as favorable as the citrate for low-temperatures synthesis. The X-ray spectra did not change when the flash nitrate precursor was heated from room temperature to 600 ° C, although, the sharpening of X R D peaks with increasing temperature does indicate some crystal growth as a result of heating. When flash nitrate precursor is being heated to 690 ° C, diffraction data clearly show that oxides and carbonates began to react, leading to the nucleation of high-temperature tetragonal 123 as well as other impurity phases such as BaCuO2, Y 2 C u 2 0 5 and Y2BaCuO5 at about the same time. We are unable to reconstruct the exact nucleation sequences of the appearance of these phases. Following nucleation, the tetragonal 123 continued to increase during soaking, and peak splitting between (110) and (013) improved with time. For those impurity phases that formed simultaneously with the 123 phase at 690°C, they seemed quite stable at that temperature. Extended calcination for
In our experiments, 690°C seems to be the lowest temperature where the 123 phase can be grown at a reasonably fast rate. Synthesis rate of the 123 phase increased with soaking temperature. At temperatures just higher than 690 ° C, BaCuO2 formed during the previous isothermal treatment, began to decrease with increasing temperature while the 123 phase continued to increase at the same time. This seems to indicate that binary impurities began to decompose to form the 123 phase when the calcination temperature increased. We also noted that BaCuO2 became more reduced in helium at higher temperatures and decreased when the stability field of B a C u 2 0 2 is reached. Heating in helium eventually destabilized the 123 phase which decomposed to form Y2BaCuO5 (211), YBa2CuEO7_x (132),
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S.H. Shieh, W.J. Thomson ~Low-temperaturesynthesis of YBCO powders
141
maximum at that temperature. This decomposition sequencing can be deduced from the XRD integrated intensities as shown in fig. 8. In the nitrate sample, barium copper oxide coexisted with the 123 phase during subsequent heating beyond 690°C. Barium copper oxides remained in the sample throughout the entire heating rang e . The 123 phase gradually increased during heating and reached maximum at 800°C. This maturing temperature is nearly 90 degrees higher than what was observed for the citrate sample. Figure 9 shows the 123 phase began to decompose at approximately 810°C; as a result, BaCu202 increased. Following the growth of BaCu202, 132 and 211 were soon detected at 825 ° C. The decomposition temperature of the 123 phase in nitrate was significantly lower than that of the 123 phase in citrate. Presumably, since barium copper oxides were not removed or reacted away after their formation in nitrate, the presence of these impurity phases may act as nucleation sites for promoting the 123 phase to decompose at lower temperatures. A comparison with the citrate sample showed that a lot more BaCu202 was produced in the nitrate sample after complete decomposition of the 123 phase. This is revealed in the much higher integrated
BaCu202 and possibly other phases that were not readily identified in our diffraction data. A 3D plot showing the phase evolution during the 123 decomposition is presented in fig. 7. Again, the citrate sample behaved differently from the nitrate sample during decomposition. Above 690°C, impurity phases in citrate decreased, the sample appeared nearly phase pure, consisting mainly of the 123 phase and perhaps a very small amount of barium copper oxides. Further heating of this sample to 750°C revealed little change in X-ray spectra indicating that phase assemblage and relative intensities of these phases remained similar. This seems to suggest that the 123 phase was growing to approach maturity within this temperature range. In this citrate sample, BaCu202 started to form at 750°C. However, after its nucleation, the growth of BaCu202 was constrained by the slow kinetics. The X-ray intensity of BaCu202 showed an unaltered behavior versus temperature before reaching 850 ° C. A minor increase of the 123 phase continued till 850 ° C, then it began to decompose. Meanwhile, 132, 21 l and BaCu202 sharply increased as the 123 phase disappeared. In helium, the 123 phase completely decomposed at 900°C. Correspondingly, the peak intensities of 132, 211 and BaCu202 all reached
• YBa2Cu307_ x D BaCu202 • Y2BaCuO5 o YBa3Cu207. x Pt Substrate
° .~
0
%
pt •
a
• 0
D
¢1
h
,A
pt
•
2468
a
.~
900
0 "/'3.0
62.5 62.5
~
TWo Theta
"°
880
•
860
~.~
4~~'~
~.o
Fig. 7. A 3D plot of XRD spectraof a citrate sample showingthe decompositionof the 123 phase in helium.
142
S.H. Shieh, W.J. Thomson/Low-temperaturesynthesis of YBCOpowders
to the persistence and growth of the barium copper oxide phases during the decomposition sequence. •~;~
0.8
YBa2Cu307"x
4. Discussion
i
i
i
Temperature (C) Fig. 8. A plot of XRD integrated intensities showing successive phase evolutionduring the decompositionof the 123 phase in a citrate sample.
0.2
211
0.0
Temperature (C) Fig. 9. Successivephase evolution during the decompositionof the 123 phase in a nitrate sample. X-ray intensity observed for BaCu202 in the nitrate sample. Because a large quantity of BaCu202 was produced in the phase assemblage from the decomposition of the 123 phase, more yttrium ions will be available for forming other decomposition products that have high yttrium concentrations. This assumption is in good agreement with the observation that the peak intensity ratio between 211 and 132 increased from approximately 1 : 2 in the citrate sample to above 1 : 1 in the nitrate sample, suggesting that after the decomposition of the 123 phase, more 211 was formed in the nitrate sample than in the citrate sample. This enrichment of 211 was closely related
One of the key issues that critically affects the quality of low-temperature prepared superconductors is the phase purity of the product. While it would be ideal to be able to synthesize a phase-pure superconducting powder, contaminations from binary oxides and other minor impurities have been universally observed in many of the previous attempts on low-temperatures synthesis [ 17,19-21,24,29,34]. Despite their differences in using various starting precursors, many X-ray spectra showing low-temperature, i.e. below 750°C, synthesized 123 compound have a striking similarity in terms of the accompanying impurity peaks. These impurity peaks are usually weak and are difficult to identify due to overlaps among several coexisting phases. The published data on phase identification of these impurities are often subjective with different authors concluding in favor of different phases. We have assigned the impurity peaks that we have observed in flash nitrate during isothermal processing to BaCuO2, Y2Cu205 and Y2BaCuO5. We did not identify CuO among the impurities although it might exist. In the citrate sample, it was rather difficult to identify any impurity phase other than BaCuO2. BaCuO2, has been a well known byproduct in 123 superconductor synthesis. It was also reported as a decomposition product of 123 at higher temperatures in air [42-46 ]. It has been shown that the amount of BaCuO2 contamination in the 123 sample is out of proportion with their peak intensity ratios. The amount of contamination is not negligible even if the relative intensity of its strongest diffraction peak is negligibly weak as compared to peaks of the 123 phase [47]. Achieving a low-temperature orthorhombic 123 synthesis with good phase purity proved a considerable challenge. Between the two precursors that we have studied, the impurity contamination in the citrate-derived sample was much less severe as compared to the impurity abundant sample grown from flash nitrate. Since both precursors were processed under identical conditions, the only difference that explains the
S.H. Shieh, W.J. Thomson / Low-temperaturesynthesis of YBCOpowders
discrepancies in observed phase purity and the growth rate of 123 phase seems to be the difference of the crystallite size of the starting material. Larger crystallites from the flash nitrate precursor seem to slow down the formation of 123 phase and enhance the growth of impurities. There are several important implications for the presence of these impurity phases. More impurities from a larger crystaUite precursor is suggesting that a larger size precursor slows the reaction kinetics for the ternary phase and favors the binary phases. If we follow this line of argument, it can be predicted that by lowering the processing temperature, which drops the ternary formation kinetics even lower, the calcination should produce more binary phase relative to the ternary. This is in good agreement with the XRD data presented by Zheng et al. [ 19 ], and by Horowitz et al. [ 24 ]. Both studies show that samples calcined at 650°C contained more binary impurities than samples prepared at and above 700°C. It was also unanimously observed that these low-temperature grown impurities decompose as the calcination temperature is raised higher. Evidently, the formation of these impurity phases were also assisted by the inert atmosphere. They grow when the reaction kinetics for the growth of ternary 123 phase is not fast enough to dominate the whole phase assemblage. Another possible important factor that contributes to the impurity formation is the slow decomposition of barium carbonate. We have observed the flash nitrate precursor carrying barium carbonate during isothermal experiments for more than 240 min. This resilient carbonate narrowed the supply of free barium from entering the ternary reaction, which is turn caused the chemistry of the whole batch of material to drift away from the originally well-mixed 123 stoichiometry. This can create a more advantageous condition for binary impurity formation. If favorable growth conditions for ternary reaction are provided, such as higher processing temperatures and finer size precursors, it is then possible to achieve faster rates of carbonate decomposition and 123 formation. Under this circumstance, the impurity formation is not favored and some even decompose to form 123 phase. A precursor such as flash nitrate that consists of a large size crystallite requires a higher calcination temperature than that for citrates, if the impurity formation is to be avoided. Longer calci-
143
nation time by itself will make little difference without raising the processing temperature to improve the reaction kinetics. It is interesting to note that, so far, BaCu202 has not been found as an impurity product during our isothermal experiment. Although we have observed its formation as a decomposition product of 123 phase in inert atmosphere at higher temperatures and confirming observations made by other studies [ 34,45,48,52]. This is suggesting that the low temperature thermodynamic stability of 123 phase and the quaternary phase relations near this phase is quite different from that of higher temperatures. According to the quaternary phase relations determined by Ahn et al. [ 53 ], the 123 phase decomposes to form BaCu202, 211 and 132 at an oxygen partial pressure of 4 × 10 -4 atm at 850°C. This reaction condition corresponded exactly to what we have observed in our experiments, which is a good indication that the partial oxygen pressure in our reaction chamber was at most 4 × l0 -4. The fact that we have found only BaCuO2 coexisting with 123 in our isothermal experiments only means that the phase assemblage formed at 690°C is less reduced, compared with a high-temperature one under similar depleted oxygen environment. When the 123 phase and impurities are quenched to 500°C for annealing in oxygen, the intensity of BaCuO2 peaks rapidly decreased within minutes of oxygen induction, and became nearly undetectable by XRD. This reaction bears an interesting resemblance to the disappearance of BaCu202 at high temperatures when the environment is switched from low to high oxygen atmosphere. Ruckenstein et al. reported that BaCuO2 reacts rapidly with YECU205 or with Y203 and CuO, and fast 123 formation can be achieved by using BaCuO2 as a precursor instead of barium carbonate for the source of barium [25]. Therefore, ideally, the precursor can react according to reaction eq. ( 1 ) and (2) as reported by Selvaduray et al. [26]: BaCO3 + CuO = CO2(s) + BaCuO2,
( 1)
2BaCuO2 + ½Y203 +CuO=YBa2Cu307_x.
(2)
We are unable to deduce whether this rapid reaction pathway designed for the 900-950°C range agrees well with our low-temperature synthesis data under a reduced atmosphere. If this rapid reaction path also
144
S.H. Shieh, W.J. Thomson ~Low-temperaturesynthesis of YBCOpowders
applies to low-temperature processing in helium, one would observe initially the formation of a significant amount of BaCuO2 which decreased with time while the 123 phase increased. We did not observe significant binary phase formation prior to the formation of the 123 phase in the citrate precursor. Our X-ray data showed that significant amount of the 123 phase seemed to form directly from the precursor before any BaCuO2 can be detected in the spectra. In the case of processing the nitrate, once BaCuO2 formed, it is actually functioning as an annoying impurity rather than a good precursor phase. It does not react to form the 123 phase at longer time. However, we do not rule out the possibility that this rapid reaction path took place so fast that most BaCuO2 formed from the decomposition of BaCO3 reacted instantaneously to form the 123 phase through fast diffusion paths as provided by the finely mixed nature of the precursor. Extra BaCuO2 appeared in the spectra only after a significant amount of the 123 phase has been grown. If this is true, one could also observe ternary 123 phase appeared forming directly from the precursor under favorable conditions while BaCuO2 was detected only after the 123 phase has grown to a certain extent. Once these rapid diffusion paths were exhausted in the precursor, it became difficult to react to form the ternary phase and subsequently formed coexisting binary phases. This may explain why at low temperatures binary impurities increased with soaking time but do not react with one another to form the ternary phase unless a higher processing temperature is used. Between 690°C and 750°C, within this temperature range, BaCuO2 decreased and was reduced to form BaCu202 as described in eq. (3) as reported by Ahn et al. [53]: BaCuO2 + CuO = BaCu202 + ½02(s) •
(3)
With further heating in helium, the 123 phase started to decompose to form 211 and BaCu202 according to eq. (4). As a result, BaCu202 increased rapidly as shown in fig. 6. 2YBa2Cu306.5+x + ½Cu20 =Y2BaCuO5 + 3BaCu202 + ( 1 q-x)O2~g).
(4)
A similar reaction was also reported by Alan et al. [48] and by Zhang et al. [45 ]. This decomposition
reaction may take place in nitrate at slightly lower temperatures than that of the citrate sample. When heated above 830°C, the 123 phase was no longer a stable phase under a low-oxygen condition; it decomposed to produce 132 along with 211 and BaCu202 as expressed by eq. (5) according to Ahn et al. [53]: 7YBa2 Cu3 06 = 3Y2 BaCuO5 + YBa3 Cu2 O6+x + 8BaCu202 + ( 5 - x ) / 2 0 2 .
(5)
For both citrate and nitrate samples, the 123 phase completely decomposed at 900°C indicating thermodynamic phase equilibria having been established at this temperature under depleted oxygen environment. Because the decomposition reaction according to eq. (4) occurred earlier in nitrate, relatively more 211 and BaCu202 were produced in the final assemblage. This explains why the 211 phase was enriched with respect to the 132 phase in the decomposed nitrate sample. For the citrate sample, rapid decomposition took place mainly according to eq. (5), consequently, more 132 formed at elevated decomposition temperatures. Therefore, although the decomposition reaction of the 123 phase was complete at the same temperature for both citrate-derived and nitrate-derived samples, their differentiation paths for forming the final phase assemblages were not similar.
5. Conclusion
We report a low-temperature procedure for the 123 superconductor synthesis using carbon-containing citrate and nitrate precursors at formation temperatures below 700°C. In both cases, it was possible to decompose the barium carbonate below 700°C. Between the two precursors, the flash nitrate precursor is a less favorable candidate for low-temperature synthesis due to its large size crystallites which inflict a higher kinetic barrier for low-temperature ternary reactions, The amorphous citrate precursor, on the other hand, proved more successful for the formation of superconducting 123 phase at low temperature. Synthesis using the amorphous citrate precursor results in a finer mixing and better chemical homogeneity on the molecular scale. The difference
S.H. Shieh, W.J. Thomson ~Low-temperature synthesis of YBCO powders in o b s e r v e d kinetics b e t w e e n different precursors reflects correctly their i n h e r e n t characteristics in b o t h c h e m i c a l a n d textural h o m o g e n e i t i e s . Slow t e r n a r y reactions lead to the f o r m a t i o n o f a c o n s i d e r a b l e a m o u n t o f c o n t a m i n a t i o n i n the f o r m o f b i n a r y c o m p o u n d s . Therefore, r e a s o n a b l e phase p u r i t y a p p e a r s to be difficult to achieve via l o w - t e m p e r a t u r e processing e v e n with precursors that are p r e p a r e d with a m i x i n g o n the a t o m i c scale. As a result, a l t h o u g h l o w - t e m p e r a t u r e c a l c i n a t i o n seems to p r o v i d e a good a l t e r n a t i v e in p r o d u c i n g the 123 s u p e r c o n d u c t o r , the practical v a l u e o f this m e t h o d is l i m i t e d w h e n scaled for mass p r o d u c t i o n .
Acknowledgements We express o u r a p p r e c i a t i o n to M. Strasik o f B o e i n g M a t e r i a l T e c h n o l o g y w h o p e r f o r m e d the m a g n e t i c susceptibility m e a s u r e m e n t s o f the superc o n d u c t o r , to K.A. Y o u n g d a h l o f B o e i n g Aerospace a n d Electronics for p r e p a r a t i o n o f citrate gel, a n d to C. H a n o f the U n i v e r s i t y o f W a s h i n g t o n for the prepa r a t i o n o f flash nitrate.
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