Article pubs.acs.org/crystal
Infiltration Growth and Crystallization Characterization of SingleGrain Y−Ba−Cu−O Bulk Superconductors Guo-Zheng Li,*,† De-Jun Li,† Xiang-Yun Deng,† Jian-Hua Deng,† and Wan-Min Yang‡ †
College of Physics and Electronic Information Science, Tianjin Normal University, Tianjin 300387, China College of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, China
‡
ABSTRACT: A series of single-grain Y−Ba−Cu−O (YBCO) bulk superconductors were fabricated using a modified infiltration growth (IG) technique, and the morphology feature associated with the inner crystallization structure of the samples was investigated in detail. Experimental observations indicated that YBCO single grains grew much larger with the increasing slow-cooling time. A bended a/c growth sector boundary was observed in the cross section of the YBCO sample grown for 48 h, which resulted from the variations in the Ra/Rc ratio during the slow-cooling growth process, where Ra and Rc represent the growth rate in the a axis and c axis direction, respectively. In addition, different side surface morphology (SSM) patterns were presented for the samples fabricated with different processings, which originated from the difference in the crystallization structure of the YBCO single grains and could be divided into four groups. On the other hand, for a defined inner-crystallization structure, the changes of sample size (diameter or thickness) could also lead to various SSM styles. Finally, it was concluded that the comparison relationship between the ratio of sample radius and thickness with the ratio of Ra/Rc determined the final SSM pattern of the sample.
1. INTRODUCTION During the past 20 years, bulk RE−Ba−Cu−O (REBCO, where RE is a rare-earth element such as Y, Nd, Sm, Eu, Gd, etc.) superconductors have been widely studied for their significant potentials in engineering applications such as magnetic levitation and trapped field magnets.1−3 It has been shown that, in these materials, any weak link (such as grain misalignments, chemical and physical inhomogeneities, congregation of unreacted nonsuperconducting phases at the grain boundary, etc.) will greatly limit the critical current densities (Jc) and yield dramatic deteriorations in their ability to trap magnetic field and associated levitation properties.4−6 Consequently, the REBCO bulk superconductors are required in the form of large, single grains without weak links for practical applications.7 For the fabrication of REBCO single grains, the top-seeded melt growth (MG) technique8−12 and the top-seeded infiltration growth (IG) technique13−21 are two of the most popular processes. The IG technique, which involves an infiltration process of Ba−Cu−O-rich liquid phase into a RE2BaCuO5 (RE-211) preform bulk at high temperatures and a subsequent reaction of RE-211 with the liquid to form REBa2Cu3O7−x (RE-123) superconducting phase during the slow-cooling process, has attracted considerable attention recently because of a number of advantages compared with the conventional MG technique, such as fewer pores and cracks, negligible shrinkage and distortion in the final products, and especially the ability to provide fine-sized spherical RE-211 inclusions in the RE-123 matrix.13−15 © 2013 American Chemical Society
During the heat treatment process, the employment of seed crystal is crucial for accomplishing the single-grain growth of RE-123. Indeed, no matter how the MG or the IG technique is used, the sample reaches a partial melting state composed of RE-211 solid phase and Ba−Cu−O liquid phase at high temperatures, and the seed crystal promotes the epitaxial, heterogeneous nucleation of RE-123 in the melt and subsequently induces the directional growth of the RE-123 individual grain to a large scale, and finally to the whole bulk size.9 Finally clear, 4-fold a-growth sectors (a-GS) are exhibited on the top surface of the as-grown sample,22−26 which is known as the typical morphology feature of top-seeded RE-123 single grains and usually named “X-type” pattern. Actually, in the directions of the a axis (100) and the c axis (001), the seed crystal initiates epitaxial growth of five sectors, including four aGS and one c-growth sector (c-GS). The boundaries of the four a-GS account for the “X-type” traces presented on the top surface of the single-grain RE-123 bulk, and the existence of boundaries (or interfaces) between a-GS and c-GS also leads to an appearance of complex traces on the side surface of the bulk sample, which has not been investigated systematically until now. In order to simplify the process flow and enhance the experimental stability of the IG process, in our previous studies for fabricating single-grain Gd−Ba−Cu−O (GdBCO) bulk Received: November 20, 2012 Revised: February 9, 2013 Published: February 14, 2013 1246
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superconductors, a modified IG technique has been developed through changing the used liquid source composition and the pellet configuration pattern.27−29 In this article, a series of single-grain Y−Ba−Cu−O (YBCO) bulk superconductors were successfully fabricated using our modified process, and the side surface morphology (SSM) associated with the inner crystallization structure of the produced bulk samples was investigated in detail.
2. EXPERIMENTAL SECTION The Y2BaCuO5 (Y-211) and BaCuO2 single-phase precursor powders were prepared by the conventional solid-state reaction method in air using raw materials Y2O3 (99.99%), BaCO3 (99%) and CuO (99%). Then, the powders of Y2O3, CuO, and BaCuO 2 were weighed according to the molar ratio Y2O3:CuO:BaCuO2 = 1:6:1027 and mixed thoroughly using the ball milling machine, as the liquid source powder for the IG process. The Y-211 powder, in batches of 12 g, was uniaxially pressed into cylindrical pellets of diameter 20 mm, while the liquid source powder in batches of 20 g was pressed into cylindrical pellets of 30 mm in diameter (i.e., using liquid source pellets with bigger diameters to infiltrate and grow smaller YBCO samples), which can increase the support ability of the liquid source pellet and enhance the experimental stability.28 Finally, Yb2O3 powder in batches of 4.5 g was pressed into a plate of diameter 30 mm to support the liquid phase at elevated temperatures. The Y-211 pellet was placed on top of the liquid source pellet, which, in turn, was placed on the Yb2O3 support plate. A well-textured NdBCO seed crystal was placed on the top surface of the Y-211 green compact with the ab-plane parallel to the surface, as shown in Figure 1.
Figure 2. Surface morphology of YBCO samples grown with different slow-cooling times for (a) 32 h, (b) 48 h, and (c) 64 h in a temperature window of 998−982 °C.
the samples exhibit clear “X-type” a/a growth sector boundaries (a/a-GSB) on their top surface, indicating that they are grown in the form of a single grain. In addition, both the expanded growth area on the top surface and the prolonged GSB on the side surface indicate the enlarged volume of the single grain with the increased slow-cooling time. As can be seen in Figure 2c, the GSB lines on the side surface of the YBCO sample grown for 64 h have reached the bottom of the pellet, which demonstrates that the single grain has grown complete in its thickness (i.e., the whole bulk sample has been grown as a single grain). 3.2. SSM and Cross-Section View of YBCO Bulk Grown for 48 h. Figure 3 shows the SSM and the corresponding
Figure 3. Side surface and corresponding cross-section morphology along the {110} plane of the YBCO sample grown for 48 h.
cross-section view along the {110} plane of the YBCO sample grown for 48 h. As can be seen in the cross section, the c-GS exhibits many more pores trapped in the growth stage, which leads to lower crystal density compared with a-GS. Through the comparison between the SSM feature and the inner crystallization structure, we can see that point C represents the growth depth of the bulk toward the bottom, and the length AC is the thickness of the grown single grain. While along the direction of AC, the point B appears as a division of a-GS and cGS, which is actually an intersection point of the inner a/c growth sector boundary (a/c-GSB) (OB) with the side surface. From the side view, it can also be seen that point B is a triple junction of one a/a-GSB (AB) and two a/c-GSB (BD and BE); in other words, it is the triple junction (on the side surface) of two adjacent a-GS with one c-GS. Here, we name this specific point as Ptr for short. In the cross section of the sample, it is easily noted that the a/c-GSB OB appears to be bent, which should result from the variations in the Ra/Rc ratio during the slow-cooling growth process, where Ra and Rc represent the growth rate in the
Figure 1. Arrangement of the sample before infiltration and growth.
The sample was heated to 1045 °C and held for 1.5 h to ensure complete infiltration of liquid into the Y-211 preform. Next, it was cooled to 998 °C at a rate of 60 °C/h, and then slowly cooled to 982 °C for different time periods (32 h, 48 h, and 64 h) before being furnace-cooled to room temperature, corresponding to different cooling rates of 0.5 °C/h, 0.33 °C/h, and 0.25 °C/h, respectively.
3. RESULTS AND DISCUSSION 3.1. Surface Morphology of YBCO Bulk Samples Grown with Different Slow-Cooling Times. Figure 2 shows surface morphology of the YBCO bulk samples grown with different slow-cooling times of (a) 32 h, (b) 48h, and (c) 64 h in a temperature window of 998−982 °C. It is clear that all 1247
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Figure 4. (a) Three-dimensional (3D) crystal growth model of Y-123 single grain. (b and c) Schematic illustration of the top surface and cross section of the YBCO sample shown in Figure 3.
direction of the a axis and c axis, respectively. As described above, the grown Y-123 single grain consists of five regions, i.e. four a-GS and one c-GS (Figure 4a). The growth rate along the a/a-GSB on the top surface, which is the largest growth rate in the ab-plane of the crystal, is determined as R max(ab) =
2 Ra
(1)
as shown in Figure 4b, and then the angle of the observed a/cGSB OB with the c axis in cross section θ, (Figure 4c), is determined as30 tan θ =
R max(ab) = Rc
2 Ra Rc
Figure 5. A complex SSM pattern of an YBCO bulk sample from different visual angles.
from different visual angles of an YBCO bulk sample which was grown with the same heat treatment procedure as the sample shown in Figure 2c (i.e., slow-cooled for 64 h) but processed in another tube furnace. It is clear that the two samples exhibit different patterns of SSM, which should result from the different axial temperature distributions (or gradients) in the two furnaces. As is known, the axial temperature gradient affects the real undercooling degree (closely associated with the growth rate) of the growth interface at different depths of the sample and influences the variations in the Ra/Rc ratio as well as the slope of a/c-GSB within the sample and finally leads to an exhibition of different morphology patterns on the side surface. More detailed investigations on the effects of the axial temperature gradient may be reported in our future work. To demonstrate the origination of the complex morphology shown in Figure 5, we give schematic illustrations showing 3D growth of the Y-123 crystal within a cylindrical sample in Figure 6. First, in Figure 6a, we exhibit a virtual growth model of the cuboid Y-123 crystal which extends out of the cylindrical sample. For simplification, the a/c-GSB (marked as a green line) is supposed to be a direct line (i.e., the Ra/Rc ratio is constant). The c-GS which actually initiates from the bottom surface of the seed is recognized as a rectangular pyramid (i.e., the seed crystal is assumed to be small enough). By combining the virtual growth model expanded in space with the real cylinder entity (i.e., the bulk sample), the tridimensional crystallization structure of a cylindrical Y-123 grain is obtained (Figure 6b). As is shown, the grain is divided into five parts by the growth sector boundaries (or interfaces), and the blue region represents a-GS, while the yellow region is c-GS. Figure 6 (panels c and d) shows schematic patterns of the SSM from different visual angles for the as-grown cylindrical sample indicated in Figure 6b. Clearly, they are very consistent with that shown in Figure 5. In Figure 6e, we exhibit the view of the sample’s inner structure by cutting off one a-GS for easy understanding and the separated a-GS cut from the sample is shown in Figure 6f. 3.4. SSM Comparison of YBCO Single Grains Fabricated with Different Processings. In accordance with the discussion in section 3.2, the Ra/Rc ratio changes
(2)
From the illustration, we can easily understand that the growth speed along the a/c-GSB OB is the maximum growth rate of the 3D crystal, which can be obtained by the following equation:31 R max =
Rc cos θ
(3)
From the cross-section view of the sample (Figure 3), it is easy to observe that the angle of the a/c-GSB OB with the c axis increases gradually during the crystal growth process (i.e., from point O to point B). In accordance with eq 2, it is easily understood that the increasing θ values are attributed to the changes in the Ra/Rc ratio. Previous studies have indicated that the growth rate of the Y-123 crystal is proportional to the undercooling (ΔT),32 which is defined as the difference between the peritectic temperature (Tp = 1010 °C for Y-123) and the growth temperature (Tg) (i.e., ΔT = Tp − Tg). Accordingly, during the slow-cooling process, both Ra and Rc increase with the increasing undercooling. At the initial growth stage from seed, it is revealed that the growth speed in the c axis direction is larger than that along the a axis (i.e., Ra < Rc), while with the advance of the slow-cooling growth, this anisotropy in growth rates reverses (i.e., changing to Ra > Rc) when undercooling reaches a certain value, such as 17 K for MGprocessed Y-123 bulk with a composition of Y-123 + 0.6 Y-211 + 0.5 wt % Pt.33 This phenomenon implies a larger increasing rate of Ra compared with Rc, which leads to the increase in the Ra/Rc ratio, as well as the values of tan θ and θ and finally accounts for the observed bending of the a/c-GSB OB. Also considered is the anisotropy of the Y-211 particle pushing toward different directions affects the changes in the Ra/Rc ratio because the Y-211 particles, pushed mainly by the cGS, may cause retardation of the Y-123 crystal growth in the c axis direction, and a decreasing a/c-GSB slope may be observed on the a/c cross section even during an isothermal period (i.e., at a constant undercooling stage).34 3.3. A Complex SSM pattern of YBCO Bulk and Its Origination. Figure 5 shows one type of complex SSM pattern 1248
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triple junction, Ptr) will be exhibited at a position relatively low on the side surface. Figure 7 shows the SSM patterns of the YBCO single grains fabricated with different processings; here for comparison, the two samples grown using the procedure P1 but in different furnaces are also presented (Figure 7, panels a and b). Figure 7 (panels c and d) depicts the SSM patterns of the samples processed with the procedures P2 and P3, respectively. As is shown, first, the samples processed with the new procedures also have been grown completely. Second, they exhibit an increasingly lower position of the triple junction, Ptr, on the side surface compared with the samples fabricated by the procedure P1, which is in agreement with the above analysis. Moreover, from Figure 7, it can also be seen that the angle of the two a/c-GSB on the side surface becomes smaller with the lowering of Ptr. For explaining this phenomenon, we exhibit schematic illustrations simulating different crystallization structures of the samples (Figure 8). Here, for simplification, the c-GS is also assumed to be a rectangular pyramid from the seed (i.e., the Ra/Rc ratio is constant for each sample) but is different between different samples. As shown in Figure 8, with the decreasing vertex angle, the c-GS intersects with the side surface in various ways and results in different SSM styles. Figure 8a shows an instance where all the a/c-GSB of the Y-123 single grain intersect only with the side surface when the position of Ptr is relatively high and leads to one type of SSM similar with the sample shown in Figure 7a. In Figure 8b, with the decreased vertex angle of the c-GS, the a/c-GSB begins to intersect with the bottom surface of the sample and produces one SSM pattern approximately corresponding to that shown in Figure 7 (panels b and c). Figure 8c shows one type of SSM style alike with that shown in Figure 7d. As we can see, only a rather small part of c-GS is exhibited in the side surface. From the right panels of Figure 8 (panels a, b, and c), it can easily be understood that the a/c-GSB on the side surface exhibit smaller angles with the lowering position of Ptr. If the vertex angle of c-GS is further reduced (corresponding to an even further smaller Ra/Rc ratio), the c-GS will not intersect with the side surface any more, as shown in Figure 8d. In this case, the cuboid Y-123 crystal propagates to the bottom of the pellet first, while the triple junction, Ptr, will appear on the bottom surface, as shown in the middle panel of Figure 8d. On the side surface, no c-GS can be observed and the sample exhibits one type of the simplest SSM pattern because only a/aGSB is exhibited on the side surface, as shown in the right panel of Figure 8d. 3.5. Effects of Sample Size on the SSM Patterns. As described in section 3.4, for a certain-sized sample, the variations of the inner crystallization structure of the Y-123 grain result in different SSM patterns. On the other hand, for a defined inner crystallization structure, the changes of the sample size (diameter or thickness) also lead to various SSM styles. Figure 9 schematically illustrates the SSM changes with the increasing diameter of the sample, where the sample thickness is fixed. As is shown, the c-GS is defined with a certain vertex angle, which represents a defined crystallization structure of the Y-123 grain. With the enlargement of diameter, the sample exhibits four different SSM patterns, just like that shown in Figure 8. If we fix the sample diameter, the variations of thickness will also make similar influences on the SSM pattern of the sample, as shown in Figure 10. From the above discussions, it can be concluded that the comparison relationship between the ratio of the sample radius
Figure 6. Schematic illustrations showing (a) a virtual growth model of cuboid Y-123 crystal which extends out of the cylindrical sample, (b) tridimensional crystallization structure of the as-grown cylindrical Y-123 grain, (c and d) SSM of the cylindrical sample from different visual angles, (e and f) view of the sample’s inner structure after cutting off one a-GS and the separated a-GS cut from the sample. The a-GS and c-GS are represented as blue and yellow entities, respectively.
with the increasing undercooling, resulting in different slopes of a/c-GSB (i.e., different θ values). Thus, it can be considered that different heat treatment procedures will conduce to different SSM patterns. On the basis of the used procedure from 998 to 982 °C at a rate of 0.25 °C/h (labeled as P1), two new slow-cooling procedures with higher initial cooling temperatures of 1004 and 1010 °C (labeled as P2 and P3, respectively) were employed to grow single-grain YBCO bulks. The details of each procedure are shown in Table 1. As Table 1. Slow-Cooling Procedures from Different Initial Temperatures slow-cooling procedure
initial temperature (°C)
cooling rate (°C h−1)
ending temperature (°C)
cooling time (h)
P1 P2 P3
998 1004 1010
0.25 0.25 0.25
982 982 982
64 88 112
described above, the c axis growth speed of the Y-123 crystal is larger than that in the a axis at lower undercoolings, thus when the sample initially grows from higher temperatures at a constant cooling rate, it will undergo longer periods of Ra < Rc, corresponding to smaller θ values. Accordingly, when the single grain expands over the diameter of the sample, it will propagate to a longer distance toward the sample bottom, so the intersection point of the a/c-GSB with the side surface (i.e., the 1249
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Figure 7. SSM patterns of YBCO single grains fabricated with (a and b) the procedure P1 but in different furnaces and (c and d) the procedures P2 and P3.
(r) and thickness (th) (i.e., r/th), with the ratio of Ra/Rc, determines the final SSM pattern of the sample. First, when (Ra/Rc) = (r/th), the sample will exhibit one type of critical SSM pattern as that shown in Figure 8b. While if (Ra/Rc) > (r/ th), the SSM pattern shown in Figure 8a will be exhibited. When (Ra/Rc) < (r/th), but (√2Ra/Rc) > (r/th) [i.e., (r/ √2th) < (Ra/Rc) < (r/th)], the instance corresponding to Figure 8c can be presented. If (√2Ra/Rc) ≤ (r/th) [i.e., (Ra/ Rc) ≤ (r/√2th)], the sample will exhibit the simplest SSM pattern as shown in Figure 8d. It is noted that these formulas are applicable only in the case when the Ra/Rc ratio is constant during the growth process of Y-123 single grain. For the continuously changing Ra/Rc ratios, this item should be replaced by the growth length ratio along the a axis and c axis directions: ∫ t0rRa dt/∫ t0rRc dt, while tr represents the growth time when the cuboid Y-123 growth region reaches the edge (side or bottom) of the cylindrical sample, not the time for the complete growth of the whole bulk.
4. CONCLUSIONS Single-grain YBCO bulk superconductors have been fabricated using a modified IG technique. The morphology feature of the produced samples was investigated systematically and associated with the inner crystallization structure. The results indicated that YBCO single grains grew much larger with the increasing slow-cooling time and finally up to the whole sample size at a slow-cooling time of 64 h. A bended a/c-GSB was observed in the cross section of the YBCO sample grown for 48 h, which resulted from the variations in the Ra/Rc ratio during the slow-cooling growth process. In addition, the samples fabricated with different processings exhibited different SSM patterns, which originated from the differences in the crystallization structure of the YBCO single grains and could be divided into four groups. Moreover, the changes in the sample size could also lead to various SSM styles. It was concluded that the comparison relationship between the ratio of the sample radius and thickness with the ratio of Ra/Rc determined the final SSM pattern of the sample. The results are helpful for understanding the fundamental crystallization behavior of REBCO single-grain superconductors.
Figure 8. (a−c) Schematic illustrations simulating different crystallization structure of the samples shown in Figure 7. (d) An instance where no c-GS can be observed on the side surface of the sample when the vertex angle of c-GS is small enough, leading to one type of the simplest SSM pattern.
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Figure 9. SSM changes with the increasing diameter of the sample.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel/Fax: +86 22 23766519. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 11075116), the Natural Science
Figure 10. SSM changes with the decreasing thickness of the sample.
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(32) Endo, A.; Chauhan, H. S.; Shiohara, Y. Phys. C 1996, 273, 107− 119. (33) Endo, A.; Chauhan, H. S.; Egi, T.; Shiohara, Y. J. Mater. Res. 1996, 11, 795−803. (34) Volochová, D.; Diko, P.; Radǔsovská, M.; Antal, V.; Piovarči, S.; Zmorayová, K.; Šefčiková, M. J. Cryst. Growth 2012, 353, 31−34.
Funds of Tianjin Normal University (Grant 5RL118), the National High Technology Research and Development Program of China (863 Project, Grant 2012AA03A610), and the Key grant Project of Chinese Ministry of Education (Grant 311033).
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