Seed-Size Effect on the Growth and Superconducting Performance of

(1-3) Usually, REBCO superconductor bulks are prepared by a top-seeded ..... In comparison with SSS, LSS above the top plane of the sample covers a la...
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Seed-Size Effect on the Growth and Superconducting Performance of YBCO Single-Grain Bulks Hao-Chen Li,† Wen-Shuo Fan,† Bo-nan Peng,† Wei Wang,† Yu-Feng Zhuang,† Lin-Shan Guo,† Xin Yao,*,†,‡ and Hiroshi Ikuta§ †

State Key Lab for Metal Matrix Composites, Key Lab of Artificial Structures & Quantum Control (Ministry of Education), Department of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China ‡ Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China § Department of Crystalline Materials Science, Nagoya University, Nagoya 464-8603, Japan ABSTRACT: Exploiting their distinguished merits of commercial availability, large size, and extremely high thermal stability, a series of NdBCO film-seeds were employed to study their size effects on the YBCO bulks by melt-growth. First, our findings show that the nucleation range is almost the same with the variation of the seed size and that the effective contact area of the seed with the molten pellet is smaller than the seed area, which can be explained by the liquid’s wettability and its surface energy. Moreover, induced by the large-sized seed, the YBCO grain has the highest a-axis growth rate, Ra, because of a double mode of thermally driven plus seed-induced growth, leading to a larger c-GS (c-growth sector). Finally, the levitation force of bulks were proven to possess an ascending and subsequently descending tendency with increasing the seed size, which is clarified by the enlargement of c-GS in competition with enhancement of pore density of bulks. In short, the results from this work are helpful to understand the crystallization mechanism and to gain the optimal superconductivity property with a reasonable seed size.

1. INTRODUCTION High-temperature superconductors of REBa2Cu3O7−δ (REBCO or RE123, RE = rare earth elements) bulks with the c-axis orientation have considerable potential for engineering applications because of their distinguished properties, high critical current density, and trapped magnetic field.1−3 Usually, REBCO superconductor bulks are prepared by a top-seeded melt-growth (TSMG) method and characteristically possess two types of growth sectors: four a growth sectors (a-GS) and one c growth sector (c-GS), associated with grain sector boundaries (a/c-GSB and a/a-GSB), as illustrated in Figure 1. It is obvious that the large-sized seed (LSS) results in the larger

volume faction of c-GS than the small sized seed (SSS). Employing Sm123 LSS (14 × 14 mm2 on the a−b plane), Xu et al. succeeded in fabricating a large YBCO bulk (53 mm in diameter), demonstrating a higher levitation force than that induced by the bulk from SSS of 2 × 2 mm2.4 Linking the growth mode in Figure 1, this enhanced property of the YBCO bulk potentially relates to its larger c-GS within the sample. Thus, it is necessary to optimize the preparation of the REBCO bulk for enhancing its superconducting performance by exploring an effective method to obtain a single-domain bulk with the high volume fractions of c-GS. On the one hand, the control of the growth rate ratio, Ra/Rc (Ra, Rc: a- and c-axis growth rate, respectively), can be realized by adjusting either undercooling5 or the anisotropic temperature gradient of the furnace.6 A large Ra/Rc value leads to high volume fractions of c-GS.7On the other hand, the use of LSS to induce the growth of large-sized crystal with the high volume fractions of c-GS is commonly considered because of the following advantages.4 First, the use of LSS with large-area epitaxial growth results in the rapid full growth on the a−b plane surface, where self-nucleation is effectively suppressed. Second, the employment of LSS with a time-saving process inhibits the discontinuous growth of Y123 from two time-

Figure 1. Schematic illustration showing growth modes of c-axisoriented Y-123 crystals induced by a small and a large seed, respectively. The a growth sectors (a-GS) and c growth sectors (cGS) are represented as blue and green entities, respectively. © XXXX American Chemical Society

Received: December 3, 2014 Revised: February 14, 2015

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DOI: 10.1021/cg501754c Cryst. Growth Des. XXXX, XXX, XXX−XXX

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for 1.5h, and fast-cooled down to 1006 °C (the peritectic temperature (Tp)) within 25 min. Then, all molten pellets were cooled at a rate of 0.5 °C/h from Tp to Tend, which is 996 °C for 20 mm diameter samples and 986 °C for 30 mm diameter ones, respectively. Finally, these crystallized pellets were fast-cooled to room temperature within 4 h. We gained small-sized grown bulks (16 mm diameter and 24 mm diameter, respectively) because of solidification shrinkage. The levitation forces were measured in the zero-field-cooled state at 77 K by using a NdFeB magnet 50 mm in diameter and with a surface magnetic field of 0.5 T for a series of YBCO bulk samples grown by using differently sized seeds. Details of the levitation force measurement are given below. The measurements of trapped magnetic field were carried out by magnetizing the bulk samples in a 10 T superconducting magnet. The bulk samples were cooled to liquid nitrogen temperature in a magnetic field of 1.5 T applied parallel to the c axis and kept there for 15 min. After switching off the external field, the profile of the trapped magnetic flux density was measured by scanning a Hall probe sensor. The total gap between the top surface of the sample and the active area of the Hall sensor was 1.2 mm, including the sensor mold thickness of 0.7 mm. After mapping the field distribution, the Hall sensor was moved to the peak position of the profile and was lowered to make a contact with the sample to measure the axial component of the trapped magnetic flux density (Bzmax) at the height of the active area of the Hall sensor.

involved problems: Y211 coarsening8 in the melt and Y211 segregation at the growth front.9 Finally and most importantly, following a rapid completion of the growth of four small a-GS by applying LSS, it is possible to achieve the YBCO crystal with large c-GS. In this regard, LSSs have been reported for growing large YBCO bulk superconductors, YBCO seeds by Scruggs et al.,10 GdBCO elongated seeds by Nizhelskiy et al.,11 and SmBCO seeds by Xu et al.4 In their processing methods, either a complex hot-seeding technique or a low maximum processing temperature (Tmax) had to be adopted. For the latter case, using GdBCO LSS, a Tmax of 1025 °C11 is insufficient for growing the large-sized bulk because of its small growth window; whereas applying SmBCO LSS, Tmax of 1045 °C4 is inadequate to grow high-peritectic-temperature REBCO. In addition, the preparation of REBCO LSSs requires a complex and time-consuming route: growing single-domain bulks in an optimized procedure, cutting carefully down along the a−b plane, and polishing the surface. In this work, taking advantage of their commercial availability, large size, and superior thermal stability (having a tolerance of Tmax up to 1120 °C)12 and using different sized NdBCO film-seeds, we succeeded in fabricating a series of YBCO bulks. Their growth characteristics in both macrostructure and microstructure were investigated and associated with the volume changes of c-GS and the a-axis growth rate (Ra). Furthermore, the relationship between the levitation force of YBCO bulks and the size of film-seeds was constructed and explained by the competitive effect between enlargement of cGS and enhancement of pore density with the size of seeds.

3. RESULTS AND DISCUSSION 3.1. Effect of Seed-Size on the Growth of YBCO bulk. First, we succeeded in fabricating a series of YBCO samples by using NdBCO film-seeds with sizes of 2 × 2, 4 × 4, 7 × 7, and 9 × 9 mm2. As shown in Figure 3, the top views of all samples

2. EXPERIMENTAL SECTION YBCO single-grain bulks were grown by adapting a typical TSMG method. The Y123 and Y211 precursor powders were obtained through a solid-state reaction by mixing Y2O3, BaCO3, and CuO powders in a stoichiometric ratio. The mixture of raw powders (Y2O3, BaCO3, and CuO) was calcined at 900 °C for 48h, and the process was repeated three times. The precursor powder was obtained from a mixture of Y123 and 30 wt % Y211 with an addition of 1 wt % CeO2. Then, the precursor powder was pressed into two kinds of bulks: 20 mm in diameter with 7 mm thickness and 30 mm in diameter with 10 mm thickness. The c-axis-oriented YBCO-buffered NdBCO-thin-film seeds (shortly, NdBCO film-seeds) were used with sizes of 2 × 2, 4 × 4, 7 × 7, and 9 × 9 mm2. The detailed temperature procedure used for the growth of YBCO samples is shown in Figure 2. The pellets were heated up to 1080 °C (the maximum processing temperature (Tmax)) within 8 h, kept there

Figure 3. Top (left) and side (right) views of c-axis-oriented as-grown YBCO bulks induced by NdBCO film-seeds with different sizes: (a) 2 × 2, (b) 4 × 4, (c) 7 × 7, and (d) 9 × 9 mm2.

without an appearance of self-nucleated/grown grain present clear X-type a/a growth sector boundaries (a/a GSB), indicating that film-seed-induced crystals have four a-GS with c-axis orientations. Most importantly, as shown in Figure 1, the triangle region in the side views (representing c-GS) becomes increasingly larger with increasing seed size, verifying that the grown YBCO bulk with LSS possesses a larger volume fraction of c-GS, which tends to be beneficial for achieving a stronger capacity for trapping magnetic fields for engineering applications. To study inner crystallization structure along the (100) plane, we cut two as-grown samples that were induced by filmseeds with a size of 2 × 2 and 5 × 5 mm2. After polishing, the cross sections of samples present one c-GS, two adjoining aGSs, and a corresponding a/c GSB. First, it is noticeable that

Figure 2. Temperature−time profile for the growth of YBCO bulks by TSMG. B

DOI: 10.1021/cg501754c Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Crystal Growth & Design different seed sizes lead to different slopes of a/c GSBs in the samples. The slope of a/c GSB with the 5 × 5 mm2 film-seed is smaller relative to that of the 2 × 2 mm2 one, which means that the sample took less time to complete the growth of four aGSs, i.e., it has a higher a-axis growth rate (Ra), leading to larger c-GS. Surprisingly, from further observation we found that the top length of c-GS (TL-c-GS) is less than the side length of the film-seed (SL-FS) for both cases, as shown in Figure 4. TL-c-

GS is less than SL-FS of 2 mm in Figure 4a, whereas it is about 3 mm less than SL-FS of 5 mm in Figure 4b. This distinctive macrostructure indicates that the nucleation area induced by the seed is smaller than the area of the film-seed, which is certainly detrimental to gaining a large volume fraction of c-GS. To illustrate the discordance between TL-c-GS and SL-FS, we drew an actual schematic model and an expected schematic model, as shown in Figure 5. There are two potential reasons for this unpredicted phenomenon. First, the Ba−Cu−O liquid in the molten pellet does not effectively wet Nd123 seed materials,13 and there is a contact angle between the film-seed and the molten pellet. The second possible reason is that the molten pellet has a convex contour at the top because of surface-energy-caused stress. Owing to these two reasons, the effective contact area between the film-seed and the molten pellet becomes small in the initial stage of crystallization, and the region resulting from the film-induced nucleation is smaller than the size of the seed. In addition, because of its small contact area with the molten pellet, the film-seed experiences a long exposure time in air at its edge part, where dissolution or decomposition of the NdBCO film easily takes place, suppressing the effect of inducing growth from the film-seed. In short, all those unfavorable origins result in the actual c-GS being smaller than the expected c-GS. To study the seed-size effect on the a-axis growth rate, we fabricated a series of incompletely grown YBCO samples by batch production using NdBCO film-seeds of various sizes. In that case, all the samples experienced the same processing, terminating growth by cooling down to a temperature of 1000 °C from Tp (∼1006 °C) at a rate of 0.5 °C/h. Because all the samples did not grow completely to the edge, we can easily measure the growth length of the samples in the a-axis direction. Because of the poor wettability of liquid with the NdBCO film-seed and the convex surface of the molten pellet, we can consider that whatever the size of the film-seed is that the initially induced area is approximately equal. In this point of view, we determine the growth length starting from the initial nucleation site, which is the central point of the a−b plane of samples. Compared with the result,14 measuring the growth length starting from the edge of the seed, our method should be more reasonable. Figure 6 shows the correlation between seed size and Ra. As it can be seen, Ra increases with the increasing size of the seed, and the YBCO grain induced by LSS has a higher Ra than those induced by SSS. Thus, the growth area induced by LSS is larger than that induced by SSS.

Figure 4. Cross-sectional macrostructures along the (100) plane of the as-grown YBCO bulks induced by a film-seed with a size of (a) 2 × 2 and (b) 5 × 5 mm2, showing a/c grain sector boundaries (a/c GSB).

Figure 5. (a) Expected and (b) actual schematic models at the cross section along the (100) plane of growing bulks with film-seeds. In the later case, the top length of c-GS (TL-c-GS) is less than the side length of the film-seed (SL-FS). C

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Figure 6. Relationship between the a-axis growth rate, Ra, and SL-FS and schematic illustrations showing the seed-size effect on the growth mode at the initial stage of Y123 crystsallization.

Figure 7. Relationship between the levitation force of YBCO bulks (16 mm in diameter) and the size of film-seed. The inset drawing represents the schematic illustration of the factors affecting the levitation force with the increase of seed size. The y axis represents the influential effect strength, and the x axis represents the seed size.

As observed and discussed above, the difference among the initial nucleation area is negligible with the variation of the seed size, i.e., all samples started their growth from approximately the same sites after the nucleation stage. One question may be raised: why does the YBCO grain induced by LSS have a higher Ra than those by SSS? To answer that question, the growth modes for both SSS and LSS were constructed, as illustrated in the insets of Figure 6. At initial crystallization of YBCO after the first step of nucleation at the central site, there are two types of growth modes for facilitating grain growth: thermally driven growth and seed-induced growth. Thermally driven growth is the epitaxial growth of the crystal after nucleation. Seed-induced growth is the growth of the crystal with the induction of seed after nucleation. In SSS cases, the YBCO grain proceeds conventionally by thermally driven growth. However, for the LSS case, apart from the initial nucleation area, the remaining part of the film-seed could effectively induce YBCO growth. Thus, the YBCO grain proceeds by a combined mode of thermally driven plus seed-induced growth. It is the seed-induced growth of LSS that plays a significantly role in facilitating Ra. In brief, LSS results in a higher Ra in melt-growth of YBCO and less time to accomplish the growth of four a-GSs, leading to the low slope of a/c GSB and the high volume fraction of cGS. 3.2. Effect of Seed-Size on the Performance of YBCO Superconductors. The effect of seed size on the superconductivity performance of as-grown YBCO bulks was also investigated. The levitation forces of 16 mm diameter bulks induced by different seed sizes were measured at 77 K under a zero-field-cooling state. It can be seen that the levitation force initially increases with increasing seed size and slightly decreases with further increasing seed size, as shown in Figure 7. The largest levitation force is obtained in the bulk with a seed size of 5 × 5 mm2. To confirm this tendency, a second batch experiment was conducted that presents a similar trend, as shown in Figure 7. To further confirm this relationship, a series of 24 mm diameter YBCO bulks induced by differently sized seeds were fabricated, and their trapped fields were measured, as shown in Figure 8. The trapped field increases from 0.53 to 0.56 T and then deceases to 0.52 T, as the used seed size increases from 2 × 2 mm2 to 5 × 5 mm2 and then to 9 × 9 mm2. All the above results show that the capabilities of magnetic levitation of bulks have a consistent correlation with seed size, presenting an initially upward and finally downward tendency. Generally, with increasing seed size, the c-GS volume fraction of the grown bulk

gradually becomes larger along with a smaller area of a/c-GSB. However, a large YBCO bulk (53 mm in diameter) induced by a Sm123 LSS (14 × 14 mm2) has been reported to possess a higher levitation force than that induced by a bulk induced by a SSS of 2 × 2 mm2.4 Considering these two events, it is reasonable to deduce that the increased c-GS volume has a strengthening effect on the magnetic levitation. Thus, the levitation force of the bulk should increase with its seed size. At the initial ascending stage, the levitation force obeys this extrapolated rule; however, as shown in Figure 7, it does not in the descending stage. To study the reason why the capability of magnetic levitation (CML) presents a descending trend at the final stage, the crosssectional microstructure along the (100) plane of bulks was investigated. Figure 9a,b shows micrographs with the low magnification of two different samples taken at the almost same location so that we can observe as large an area as possible. Interestingly, we found that there are a greater number of and larger sized pores in the bulks induced by LSS than those induced by SSS. It is well-known that such defects are inevitable during the heating and cooling step of the process, which can strongly affect the superconducting properties. 2Y123 → Y211(s) + Ba3Cu5O8(l) + 0.42O2 (g)

At a temperature above Tp, the Y123 phase decomposes into Y211 solid, Ba−Cu−O liquid, and oxygen gas, as shown in the reaction above.15 During this incongruent melting, the oxygen gas produced forms spherical pores in the liquid and gradually is released into the air. In the process of oxygen evolution, the seed covering the top plane of the sample has an inhibitory effect. In comparison with SSS, LSS above the top plane of the sample covers a larger area and leaves a smaller area exposed. The release of oxygen gas becomes increasingly difficult with increasing seed size because of the reduced exposed area at the sample surface. Thus, the redundant oxygen gas inside the sample results in a high pore density, which will certainly have a negative effect on CML. As discussed above, there are mainly two kinds of crystalline features of YBCO bulks: the c-GS volume fraction in macrostructural scale and the pore density in microstructural scale, both of which change with the size of film-seeds used and significantly influence CML. As illustrated in the inset of Figure 7, these two structure effects on magnetic levitation become D

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Figure 8. Trapped field of YBCO bulks (24 mm in diameter) induced by a film-seed with a size of (a) 2 × 2, (b) 5 × 5, and (c) 9 × 9 mm2.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support from the MOST of China (grant no. 2012CB821404), NSFC (grant no. 51172143), and the Minhang Government.

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Figure 9. Optical micrographs of YBCO grown bulks induced by a film-seed with a size of (a) 2 × 2 and (b) 9 × 9 mm2.

REFERENCES

(1) Campbell, A. M.; Cardwell, D. A. Cryogenics 1997, 37, 567−575. (2) Werfel, F. N.; Floegel-Delor, U.; Rothfeld, R.; Riedel, T.; Goebel, B.; Wippich, D.; Schirrmeister, P. Supercond. Sci. Technol. 2012, 25, 014007. (3) Headings, N. S.; Hayden, S. M.; Kulda, J.; Babu, N. H.; Cardwell, D. A. Phys. Rev. B 2011, 84, 104513. (4) Wu, X.-d.; Xu, K.-X.; Fang, H.; Jiao, Y.-L.; Xiao, L.; Zheng, M.-H. Supercond. Sci. Technol. 2009, 22, 125003. (5) Volochova, D.; Diko, P.; Radusovska, M.; Antal, V.; Piovarci, S.; Zmorayova, K.; Sefcikova, M. J. Cryst. Growth 2012, 353, 31−34. (6) Lo, W.; Cardwell, D. A. Mater. Sci. Eng., B 1998, 1−2, 45−53. (7) Guo-Zheng, L.; De-Jun, L. Cryst. Growth Des. 2013, 3, 1246− 1251. (8) Oda, M.; Yao, X.; Yoshida, Y.; Ikuta, H. Supercond. Sci. Technol. 2009, 22, 075012. (9) Endo, A.; Chauhan, H. S.; Egi, T.; Shiohara, Y. J. Mater. Res. 1996, 11, 795−803. (10) Scruggs, S. J.; Putman, P. T.; Zhou, Y. X.; Fang, H.; Salama, K. Supercond. Sci. Technol. 2006, S451, 19. (11) Nizhelskiy, N. A.; Poluschenko, O. L.; Matveev, V. A. Supercond. Sci. Technol. 2007, 81, 20. (12) Xu, H. H.; Chen, Y. Y.; Cheng, L.; Yan, S. B.; Yu, D. J.; Guo, L. S.; Yao, X. J. Supercond. Novel Magn. 2013, 26, 919−922. (13) Yao, X. J. Phys.: Condens. Matter 2004, 16, 3819−3826. (14) Sudhakar Reddy, E.; Hari Babu, N.; Iida, K.; Withnell, T. D.; Shi, Y.; Cardwell, D. A. Supercond. Sci. Technol. 2005, 18, 64−72. (15) Cardwell, D. A. Mater. Sci. Eng. 1998, B53, 1−10.

increasingly strong with increasing film-seed size. More precisely, enlargement of c-GS plays a positive role, whereas enhancement of pore density has a negative effect on CML. In the beginning, with increasing seed size, the effect of c-GS enlargement is predominant; thus, the levitation force exhibits an ascending trend. However, with further increase in seed size, the enhancement of pore density overwhelms the enlargement of c-GS with regard to their influence on the levitation force, leading to its detrimental effect. Thus, the relationship between CML and seed size, initially showing an ascending effect and finally a descending effect, can be explained by the competitive effect between enlargement of c-GS and enhancement of pore density.

4. CONCLUSIONS Making use of their merits (being commercially available and highly thermal stable), NdBCO film-seeds were used to study their size effects on YBCO bulks by top-seeded melt-growth. The distinctive features were observed from as-grown bulks. First, the triangle regions in side views, representing the c-GS, becomes increasingly large with seed size, which tends to be beneficial for achieving a stronger capacity to trap magnetic fields for engineering applications. Second, the a/c GSBs, starting from the sites near the seed center in the cross section, suggest a small effective contact area between the seed and the molten pellet. The origin of this unexpected growth phenomenon is attributed to both the poor wettability of liquid with seeds and the surface-energy-caused convex pellet shape. More importantly, the levitation force of YBCO bulks was found to have an initially ascending trend and finally descending trend with increasing seed size, which is explained by the competitive effect between the enlargement of c-GS and enhancement of pore density of bulks that correlate with increasing seed size. E

DOI: 10.1021/cg501754c Cryst. Growth Des. XXXX, XXX, XXX−XXX