Effect of undercooling temperature on the solidification kinetics and morphology of Y-Ba-Cu-O during melt texturing

Physica C 217 (1993) 367-375 North-Holland

Effect of undercooling temperature on the solidification kinetics and morphology of Y-Ba-Cu-O during melt texturing B.J. Chen, M.A. Rodriguez 1, S.T. M i s t u r e a n d R.L. Snyder Institute for Ceramic Superconductivity, New York State College of Ceramics at Alfred University, Alfred, N Y 14802, USA

Received 5 August 1993 Revised manuscript received 9 September 1993

The phase content of a series of YBa2CusO,_o ( 123 ) bars was determined as a function of the holding time during the postmelting solidificationprocessat different undercoolingtemperatures. Quantitative XRD analysisof these samplesshowsthat 123 recrystallization occurs in three distinct stagesafter coolingfrom the meltingtemperature. The first stage is an incubation period during which no 123 forms, followedby a rapid liquid-assisted reaction stage, then finally a slow solid-state sintering stage. Nucleation of 123 from the partial melt which contains 211 +BaCuO2+liquid is a low-probabilityprocess which results in only a limited number of nuclei forming during the texturing process. The undercoolingtemperature is a critical parameter which controls the final morphologyof 123 in the case of non-directional solidification. Large amounts of undercooling results in a microstructure characterized by randomly-orienteddendritic needles or plates, while a low degree of undercooling yields large, blocky domains of oriented 123.

1. Introduction The melt-processing [ 1-5 ] and gradient firing [68 ] techniques have thus far shown the most promise for producing high-Jc bulk superconducting products in the Y - B a - C u - O system. Both techniques involve liquid-phase-assisted reformation of the YBa2Cu307_~ (123) phase which requires heating the 123 above the pcritectic point to form a liquid phase, then cooling to a lower temperature and holding or slow cooling to allow reformation and growth of oriented 123 crystals to occur. The reformation reaction of the 123 phase can be expressed by the equation Y2BaCuOs(211 ) + liquid --, 123. The kinetics and mechanism of solidification of the 123 phase are of fundamental importance for optimizing the melt-texturing process. Currently, there are two theories for the formation mechanism of 123 when cooled from 211 + liquid. The original theory suggests that 123 crystals heterogeneously nucleate on, and grow from, 211 crys1

Present address: Department of Geological and Geophysical Science,Princeton University,Princeton,NJ 08544, USA.

tals [9,10]. The other, more recent, model involves 211 dissolution into the yttrium-deficient liquid to adjust the liquid stoichiometry and allow nucleation and growth of 123 [ 11 ]. Several investigations have provided evidence for the dissolution of 211 and the existence of a yttrium gradient [ 12-16 ]. Physically, the 211 dissolution mechanism is more likely, given the rapid 123 growth rates and the presence of a liquid phase to aid in fast transport. If 211 crystals act as nucleation sites, 123 growth would he limited by the transport of cations from the 211 core, through a growing layer of 123, to the surface of the 123 layer and a slow growth rate would be expected. The authors have reported a detailed discussion of 123 growth and formation kinetics using real-time analysis techniques showing that the growth rates of 123 at different undercooling temperatures are relatively fast [ 17 ]. The purpose of this study is to further clarify the nucleation and growth mechanism and the reaction kinetics during the melt-texture process by quenching bulk samples at various undercooling temperatures. Quantitative X R D using the intensity-ratio [ 18 ] and internal-standard [ 19 ] methods

0921-4534/93/$06.00 © 1993 ElsevierSciencePublishers B.V. All fights reserved.

368

B.J. Chen et aL / Effect of undercooling temperature

was employed to determine the relative amounts of the phases present.

2. Experimental procedure High-purity orthorhombic 123 powder was synthesized by the nitrate method as outlined elsewhere [20,21 ]. XRD analysis of the resulting powder indicated a high-quality orthorhombic 123 phase with no detectable impurity phases. About 0.4 g were pressed into 13.0 × 3.3 × 2.4 mm bars. The bars were individually heated in a vertical tube furnace to perform the quenching study. All samples were heated at 10 K / m i n to 1100°C, held for 30 min, cooled at about 13 K/rain to 940 or 1000°C, and held for various times before quenching. Choosing 940°C is based on previous real-time studies which showed that 940 ° C is the optimum temperature for both reaction rate and orientation [17]. 1000°C is picked since it is close to the peritectic temperature of 123 which gives a low degree of undercooling compared with 940 oC. A schematic diagram of the quench furnace is shown in fig. 1. Samples were quenched by sending AC current through the fuse wire from which the sample was suspended, causing the sample to drop on to a steel plate at room temperature. To prevent the loss of liquid phase during the melt-texturing

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process (which would change the overall stoichiometry of the sample), samples were placed on single-crystal MgO substrates. No contamination of the MgO crystals was apparent, indicating that the liquid phase did not flow out of the sample bars. Powder X-ray data were collected on a Siemens D500 diffractometer with Cu Ka radiation and a diffracted-beam graphite monochromator. High-quality data were collected using a step scan with a count time of 5 s and a step size of 0.01 °20. Samples were ground into fine powder and mixed with 50 weight percent A1203 powder, which was used as an internal standard for the quantitative XRD analysis. Standard 123, 211, and BaCuO2 samples were obtained by quenching phase-pure 123, 211, and BaCuO2 bars from 940°C. The strongest peaks from each phase were chosen for quantitative XRD analysis, and are listed in table 1. The integrated area of non-overlapping peaks was determined by fitting an appropriate profile after background subtraction. Overlapping peaks were deconvolved from each other to give the intensity of the individual reflections [22]. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses were performed on an AMRAY scanning electron microscope to identify the phases. Samples were cross-sectioned, polished, and some of the samples were etched using a solution of 0.5 vo1.% acetic acid in ethanol in order to discern the individual grains. The microstructures were examined using a Versamet optical microscope with polarized light.

3. Results and discussion Figure 2 presents the XRD patterns obtained from separate samples quenched after melting, cooling, and soaking at 940 ° C, for the length of time indicated. Note that 940°C is undercooled by about 75 IC This figure qualitatively illustrates the development of the 123 phase from the 211, BaCuO2, and liquid phases which are initially present at 940°C. Figure 3 shows the results of quantitative X-ray analysis based on the patterns in fig. 2. The relative concentrations of each phase are expressed in weight percentage. Figure 3 allows the recrystallization process to be divided into three stages. No 123 forms in the first 3 rain at 940°C, which is the initial stage,

B.J. Chen et al. / Effect of undercooling temperature

369

Table 1 A list of peaks chosen for quantitative XRD analysis Phase

123 (T)

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Peak position (20 degrees) Relative intensity (l,~t)

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Two-Theta (degrees) Fig. 2. 3D XRD plots showingthe development of 123 with time. The sampleswere heated to 1100°C, held for 30 rain, cooled to 940°C and held for various quench times. but small concentrations of 123 are detected at 3.2 min. Also in the first 3 min, the 211 concentration decreases slightly and the BaCuO2 concentration increases slightly. It appears that the 3 rain time lag in forming 123 results from the time required to allow sufficient dissolution to adjust the liquid stoichiometry in localized regions. The slow dissolution of the 211 contributes to the slow nucleation of the 123. The increase in BaCuO2 concentration in the first stage can be attributed to two factors; the slight decomposition of 211 forms small amounts of BaCuO2 and C u e , and, more importantly, the liquid present

at 940 ° C crystallizes during the quench to form large amounts of BaCuO2. During the second stage, from 3.2 to ~ 12 rain, rapid 123 growth occurs, accompanied by the decomposition of BaCuO2 and 211. Figure 3 clearly shows a sharp decrease in the 211 and BaCuO2 concentrations, while the amount of 123 increases to ~ 66%. From ~ 12 rain, the third stage, which is a slow solid-state sintering stage, is observed. Most of the unreacted impurity phases are segregated at the grain boundaries and voids, with some embedded in 123

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Fig. 3. Results of quantitative XRD analysis based on the patterns in fig. 2. grains. The main activity during this third stage is to clean up the impurities at the grain boundaries. The grain-boundary cleaning process is thought to be controlled by the diffusion of yttrium from 211 at the grain boundaries to isolated regions rich in barium and copper. It is thought that these regions result from crystallization of yttrium-poor liquid left by the rapid passing of the 123 growth front during stage two. Detailed morphologies from this stage will be discussed later. The reaction rate constants of the second and the third stage can be obtained using the reaction rate law d[A] = k [ A ] dt or [A], In [--~o = k t , where t is time, k is the rate constant, and [A] is the percentage (amount) of 123 formed. Figure 4 displays a plot of In ( [ A]t / [ A ] 0) versus time. The percentage of 123 formed after 192 s (3.2 min) of holding was chosen as [A]o since it is the first non-zero value of 123. Figure 4 shows the linearity of the second and the third reaction stages. The rate constants, k, for the second and third growth stages correspond to the slopes of their best-fit lines, and are calculated as 0.2597 X 10 -1 and 0.1020X 10 -4 s -l, respectively. Figure 5 includes four micrographs illustrating the

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Fig. 4. A plot of In ( [A ]t/[ A]o) versustime showingthe linearity of the second and the third reaction stages. The slopes of their least-squares-fitlines for the two stagesare consideredto be their rate constants (k). development of the morphology during the solidification process at 940°C. Figure 5(a) shows a crosssection of the sample quenched after holding at 940°C for 3.5 min. EDS analysis of this sample indicates only one small region where 123 has formed, which is marked on the micrograph. The remaining area contains only 211 and BaCuO2 from the liquid matrix. There are only two areas containing localized 123 in the entire cross-section of this sample. The quantitative XRD results show that only 1.03 wt.% 123 formed in this sample, while the micrographs indicate that 123 crystals grow in well-separated regions. Since only few, widely spaced crystals are observed in this early stage of growth, the nucleation process can be attributed to a low-probability nucleation event, which agrees with the postulate of Goyal et al. that there is a large nucleation barrier [23], and not to nucleation on existing 211 crystals. Further support for this argument lies in the fact that heterogeneous nucleation on existing 211 crystals would result in a finely dispersed, uniform 123 crystallite distribution, which is clearly not observed. The second stage begins as the holding time at 940°C is increased to 3.2 min. At this time and for longer times at 940°C, the 123 phase grows rapidly and in a dendritic spherical cluster from the individual nuclei formed in the first stage. Figure 5 (b) shows the cross-section of a sample quenched after

B.£ Chen et al. / Effect of undercooling temperature

371

Fig. 5. Polarized optical (a), (b) and (c), and SEM (d) micrographs illustrating the development of 123 from the 211, BaCuO2, and liquid matrix during the melt-texturingprocess. All samples were quenched from 940°C. (a) 3.5 min hold, (b) 4.5 rain, (c) 5.4 rain, (d) 12 rain.

4.5 min. As the holding time is extended to 5.4 min, as shown in fig. 5 (c), the 123-rich spherical clusters grow so large that they impinge on each other. Although at this point 123 is the major phase, areas containing 211 and BaCuO2 can still be distinguished. Figure 5(d) shows that after 12 min. 123 crystals have formed a continuous matrix, with interspersed impurity phases. Small amounts of 211 and BaCuO2 phase can be found embedded in 123 grains, voids, and at grain boundaries. At this point, the third stage of the solidification process begins. Figure 6 presents the X R D patterns obtained from separate samples quenched after melting, cooling, and

soaking at 1000 ° C, which gives an undercooling of only 15 K. This figure qualitatively illustrates the development of the 123 phase at 1000 ° C after cooling from 1100 ° C. The reaction sequence when soaking at 1000°C is similar to that found when soaking at 940 ° C. The only difference is that there is Ba2Cu305.9 phase present in the first two patterns and only very small amounts of BaCuO2 present at up to 40 min of soaking. At 1000 ° C, which is undercooled by only 15 K, the 123 formation process is much slower than at 940°C. It can also be seen from this figure that the 211 dissolves slower at 1000°C than at 940°C. Figure 7 displays the results of quantitative X-ray

B.J. Chen et al. / Effect of undercooling temperature

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after melting, cooling,and soaking at 1000°C for the length of time indicated. analysis based on the patterns in fig. 6. Relative concentrations of each phase are expressed in weight percentage. Figure 7 shows that the recrystallization process can also be divided into three stages at 1000°C. The first stage in which no 123 forms lasts about 18 rain, which is much longer than that at 940°C, indicating that nucleation of 123 when un-

dercooled by 15 K is less favorable than when undercooled by 75 IC The fast formation period of 123 at 1000°C, which belongs to the second stage, takes about 2.5 h in contrast with about 10 min in the case of soaking at 940°C. The third stage is also a slowreaction stage. The percentage of converted 123 phase in the third stage at 1000°C is smaller than that at 940°C. The inset in fig. 7 displays a comparison of the 123 formation processes at the two soaking temperatures. Figure 8 is a plot of I n ( [ A ] , / [ A ] o ) versus time which is calculated from the data in fig. 7. The percentage of 123 formed after 1200 s (20 rain) of holding was chosen as [A]o. Figure 8 also shows the linearity of the second and the third reaction stages. Table 2 lists the calculated rate constants, k, for the second and third growth stages for both the 1000 and 940°C cases. Figure 9 displays four optical micrographs illustrating the nucleation and growth characteristics of 123 from the matrix of211 and liquid during the solidification process at 1000°C. Figure 9(a) is a portion of the cross-section of the sample quenched after holding at 1000°C for 40 rain. This micrograph

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Second stage

Third stage

1000 940

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demonstrates the early stage of solidification of 123 at 1000°C. There is only one 123 crystal in the entire cross-section. The remaining area contains only 211 and BaCuO2 from the liquid matrix. The cross-section of this 123 crystal has already reached 200 × 300 ~tm and is equiaxed with sharp edges (growth fronts). Figure 9(b) shows a cross-section of the sample quenched after 1.1 h, and shows that there is also only one 123 crystal in this cross-section of the sample. The size of this crystal is much larger than that in fig. 9(a) with the same kind of morphology. This also reflects that the nucleation of 123 during solidification is a low-probability process and the growth rate of 123 is faster than its nucleation rate. The crystal is grown from a single nucleus. Figure 9(c) is a cross-section of the sample quenched after 1.5 h holding which is still in the second stage of the solidification process. There are six large, interconnected 123 crystals in this cross-section, but the 211 plus BaCuO2 matrix can still be clearly distin-

373

guished. Figure 9 (d) shows the cross-section of samples in the third stage of the solidification process. All 123 crystals are interconnected, leaving 211 and BaCuO2 at the grain boundaries. In this stage, the common large domain microstructure for melt-processed YBa2Cu307_6 has already formed. Observations reveal that the 211-dissolution rate is the rate-limiting step in the 123 recrystallization process, for both nucleation and growth. A lower degree of tmdercooling has a lower 211-dissolution rate and there results a slower reaction rate. Vice versa, a higher degree of undercooling gives a higher 211dissolution rate and produces a higher reaction rate. But there is a limit to the undercooling: if the undercooling is too high, the liquid phase solidifies too fast causing the 211 to be frozen-in and preventing further dissolution [ 17 ]. The 123-formation process then becomes a solid-state reaction or sintering process. In the case of directional solidification of 123 during the melt-texturing process, Cima et al. [ 12 ] have indicated that the growth rate (sample motion rate) and temperature gradient are the most important parameters influencing the final morphology. These two parameters, in fact, yield an undercooling temperature at the growth front. In the case of the non-directional solidification process used in this study, the undercooling temperature becomes the only important parameter controlling the final morphology.

4. Conclusions Three stages can be identified in the solidification of Y - B a - C u - O semimelt during melt texturing in both high and low undercooling temperature cases. The initial stage includes a time lag, where no 123 forms. During this time the 211 dissolves releasing yttrium ions to diffuse into the liquid providing the appropriate concentration for nucleation of 123. In this stage, the concentration of the 211 phase decreases slightly and the concentration of BaCuO2 increases. The nucleation of 123 from the 211 +BaCuO2+liquid melt is a low-probability event; no evidence for heterogeneous nucleation on existing 211 crystals was found. During the second stage a rapid liquid-assisted reaction dominates, which leads to a rapid increase in

374

B.J. Chen et al. / Effect of undercooling temperature

Fig. 9. Polarized optical micrographs illustrating the nucleation and growth characteristics of 123 from the matrix of 211 and liquid during the solidificationprocessat 1000°C. All sampleswere quenched from 1000°C. (a) 40 rain hold, (b) 1.1 h, (c) 1.5 h, (d) 8.0 h. 123 concentration with a corresponding rapid decrease in 211 and BaCuO2 content. The third stage is a solid-state, diffusion-controlled sintering stage during which remaining impurities slowly react at the grain boundaries to form 123. With a high degree of undercooling, 123 grows dendritically with a faster rate than with a low degree of undercooling and produces a spherical growth front giving a morphology of random oriented needles or plates. On the other hand, 123 grows with a straight and sharp growth front and forms equiaxed blocky crystallites, in the case of small undercooling, to give domains of textured 123. In both cases, the 123 nucleation process is slow and has a low prob-

ability so that only a few nuclei originate in each sample. A faster melt-texturing schedule can be proposed based on the knowledge obtained so far. The key point of the melt-texturing process is the second stage which dominates the final morphology of the sample. After cooling from above the pedtectic temperature, the sample should be held or very slowly cooled with a very low degree of undercooling for a few hours to pass the second stage of solidification. The smaller the undercooling temperature, the higher is the nucleation barrier, causing only a limited number of nuclei to form. The few nuclei will grow larger due to the small amount of competition for the yttrium

B.J. Chen et al. / Effect of undercooling temperature

being released by the decomposition of 211. After the second stage is finished an increase in the degree of undercooling, to perhaps 940 ° C, will speed up the third-stage reaction and will greatly reduce the melttexturing process time.

Acknowledgements We would like to thank the New York State Science and Technology Foundation and their Center for Advanced Ceramic Technology along with the New York State College of Ceramics and the New York State Institute on Superconductivity for their sponsorship for this work.

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[7] J. Kase, J. Shimoyama, E. Yanagisawa, S. Kondoh, T. Matsubara, T. Morimoto and M. Suzuki, Jpn. J. Appl. Phys. 29 (1990) 277. [8] R.L. Meng, C. Kinalidis, Y.Y. Sun, L. Gao, Y.]C Tao, P.H. Hor and C.W. Chu, Nature (London) 345 (1990) 326. [9] S. Jin, G.W. Kammlott, T.H. Tiefel, T.T. Kodas, T.L. Ward and D.M. Kroeger, Physica C 181 ( 1991 ) 57. [ 10 ] P. McCfinn, N. Zhu, W. Chen, S. Sengupta and T. Li, Physica C 176 (1991) 203. [ 11 ] M.A. Rodriguez, B.J. Chen and R.L. Snyder, Physica C 195 (1992) 185. [ 12] M.J. Cima, M.C. Fleming,s, A.M. Fgueredo, M. Nakade, H. Ishii, H.D. Brody and J.S. Haggerty, J. Appl. Phys. 72 (1992) 179. [ 13] C.A. Bateman, L. 7_&ang, H.M. Chan and M.P. Harmer, J. Am. Ceram. Soc. 75 (1992) 1281. [14] H. Hojaji, A. Barkatt, FLA. Michael, S. Hu, A.N. Thorpe, M.F. Ware, I.G. Talmy, D.A. Haught and S. Alterescu, J. Mater. Res. 5 (1990) 721. [15]M. Murakami, T. Oyama, H. Fujimoto, S. Gotoh, ]C Yamaguchi, Y. Shiohara, N. Koshizuaka and S. Tanaka, IEEE Trans. Mag. 27 ( 1991 ) 1479. [16] 1". Izumi, Y. Nakamura and Y. Shiohara, J. Mater Res. 7 (1992) 1621. [ 17] B.J. Chen, M.A. Rodriguez, S.T. Misture and R.L. Snyder, Physica C 198 (1992) 118. [ 18 ] R.L. Snyder, Powder Diffraction 7 (1992) 186. [ 19] R.L. Snyder and D. Bish, in: Modern Powder Diffraction, Chapter 5, ed. D.L. Bish and J.E. Post, Reviews in Mineralogy, vol. 20, Mineralogical Society of America, Washington DC (1989) p. 101. [20] M.A. Rodriguez, R.L. Snyder, B.J. Chen, D.P. Matheis, S.T. Misture, V.D. Frechette, G. Zorn, H.E. G6bel and B. Seebacher, Physica C 206 (1993) 43. [21 ] E.C. Berhman et al., Adv. Cer. Mat. 2 (1987) 539. [22] S.A. Howard and R.L Snyder, J. Appl. Crystallogr. 22 (1989) 238. [23 ] A. Goyal, K.B. Alexander, D.M. Kroeger, P.D. Funkenbusch and S.J. Burns, Physica C 210 ( 1993 ) 197.