Trapping Mode Controlled Continuous Growth of SmBCO Bulk

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Trapping Mode Controlled Continuous Growth of SmBCO Bulk Superconductors Bo-nan Peng,† Ling Cheng,† Yu-feng Zhuang,† Heng-heng Xu,† and Xin Yao*,†,‡ †

Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Department of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China ‡ State Key Laboratory for Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China ABSTRACT: In this work, we report a novel approach for a continuous growth of SmBa2Cu3Oy (Sm123) bulk superconductors. Under a conventional slow-cooling mode, it was found that the crystal growth slows down and terminates ahead of time due to a reduced effective supersaturation (σeff). Apart from commonly supposed two effects, RE2BaCuO5 (RE211, RE = rare earth element) coarsening in the melt and RE211 segregation at the growth front, we find that the noticeable reduction of t.c.s. (temperature coefficient of solubility) plays a significant role in the σeff declining phenomenon in the Sm−Ba−Cu−O system. In our new process, an accelerated cooling was applied to maintain a sufficient σeff so that a trapping mode controlled continuous growth is realized.

1. INTRODUCTION REBa2Cu3Oy (REBCO or RE123, RE = rare earth elements and Y) bulk high temperature superconductors have been widely investigated over decades, for their huge potential in various applications, such as flying wheel, motors and maglev transportation.1−3 All of these applications are premised on the character of the superconductors. Among all the REBCO systems, both SmBCO and NdBCO have higher merits than YBCO, such as higher Tc (superconducting critical transition temperature) and better Jc (critical current density).4,5 There are mature techniques in the growth of YBCO bulk, even in the batch process.6,7 For the SmBCO system, however, there are only a few reports on SmBCO bulk with large size and fullgrowth in both a and c axis.8−11 Using oxygen-controlled meltgrowth (OCMG), Ikuta et al. reported a high property SmBCO superconductor bulk with a world-record size of 36 mm in dia.9 On the other hand, several groups prepared SmBCO bulks under an air-processed condition. Sun et al. reported high Tc SmBCO bulks by the addition of novel Ba-rich phase of Sm242.10 However, it can be seen that the air-process approach resulted in distinctive demerit of incomplete-growth in the SmBCO system in comparison with OCMG. Furthermore, Xu et al. confirmed that self-nucleation easily occurs under a high cooling rate and concretely suggested that a low cooling rate of about 0.15 °C/h (evidently lower than one used in YBCO MT growth) should be applied to gain a full-growth SmBCO bulk.11 Continuous growth is a common objective for gaining any REBCO bulk superconductor. However, discontinuous growth seems to be an intrinsic problem in the SmBCO system, especially in air atmosphere. Conventionally, the segregation of © 2013 American Chemical Society

Sm2BaCuO5 (Sm211) particles was blamed for this result, as well as the coarsening of Sm211 particles.12−15 Imagawa et al.12 pointed out that a significant coarsening of Y2BaCuO5 (Y211) particles inevitably exists in the melts during the growth and it will inhibit the growth of YBa2Cu3O7‑δ (Y123) bulk. One expects the same in any REBCO system, on account of similar growth behaviors. On the other hand, based on the pushing/ trapping theories in the MT growth of YBCO by Endo et al.,14 there are always a part of Y211 particles that are pushed to the growth front. Thus, Y211 particles increasingly segregate at the growth front, becoming obstacles for the continuous growth. These two effects are commonly supposed to explain the reduced growth rate in the MT process of REBCO bulk. But, since they do not cause too much trouble in the growth of YBCO bulk, they should not be the main reasons for the discontinuous growth problem in the SmBCO system. There should be at least one more crucial factor. In this work, in comparison with characteristics of RE211Ba3Cu5Oz phase diagrams,16 we find that the t.c.s. (temperature coefficient of solubility) value reduces much more sharply in the SmBCO and NdBCO system than that in YBCO, leading to a noticeable reduction of the effective supersaturation (σeff) with deceasing growth temperature (equivalent to growth time). Thus, a constant and slow cooling rate can not sustain the continuous growth. In our new approach, an accelerated cooling rate was applied, resulting in a trapping mode Received: May 17, 2013 Revised: July 3, 2013 Published: July 9, 2013 3734

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controlled continuous growth. And finally, a SmBCO bulk fully grown in both the a- and c-axis was obtained. This method proved to be effective for SmBCO and should be universally valid also for other REBCO systems that possess the same nature of the phase diagram.

respectively. It is obvious that all the samples except Figure 2c are single domain crystal, indicating that they are induced from the film seed. In addition, the as-grown regions in all the samples are nearly the same. When comparing Figure 2a and c, one can easily find that the unexpected self-nucleation in Figure 2c is caused by the high initial cooling rate. For the SmBCO system, an initial cooling rate of 0.15 °C/h seems to be fine. From the as-grown sizes as shown in Figure 2a and b, it can be inferred that, with the cooling rate of 0.15 °C/h, 20 h are sufficiently long for the growth; an extra 20 h do not make any sense. Beyond that, the σeff must be diminishing during the first 20 h, since the growth terminates ahead of schedule, as shown in Figure 2b. In order to obtain a fully grown sample, a higher cooling rate must be used to obtain sufficient a σeff during the extra 20 h. However, the sample growth during the additional 20 h with a cooling rate of 0.4 °C/h does not show a distinct change in size as shown in Figure 2d, indicating that the second step with the high cooling rate of 0.4 °C/h does not work for facilitating the growth, either, and a much higher cooling rate may result in the self-nucleation again, posing a dilemma. 3.2. Origins of a Reduced Supersaturation in a Conventional Melt-growth of REBCO Bulk. According to pushing/trapping theories for the melt growth of YBCO bulk by Endo et al.,14 the critical radius (rc) of a particle trapped inside the RE123 matrix is roughly determined by the interfacial energy (Δσ0) and the critical growth rate (Rc) as follow:

2. EXPERIMENTAL SECTION SmBa2Cu3O7‑δ (Sm123) Sm2BaCuO5 (Sm211) and Sm2Ba4Cu4Oy (Sm242) single phase precursor powders were calcined through a solid state reaction method. Particularly, Sm242 must be prepared under oxygen free atmosphere. Then, the powders were mixed thoroughly in a molar ratio of Sm123/Sm211/Sm242 = 1:0.222:0.078, together with 1 wt % of CeO2. The mixed precursor powders were uniaxially pressed into pellets with 20 mm in diameter and into minipellets with 5 mm in diameter, which were put at the top of the big pellets for preventing contamination from the substrate. C-axis oriented NdBCO/YBCO/MgO thin film seeds were placed on the top surface of the mini-pellets. Subsequently, a series of heating profiles were employed. Figure 1 shows a typical optimized profile for the growth of the SmBCO bulk. Finally, the samples were cut along the a−c plane, and their macroscopic grown morphologies were analyzed.

Rc ∝

Δσ0 η × rc n

(1)

where Δ is the coefficient of melt viscosity (usually, it is a constant for a given system) and n is a power ranging from 1 to 2. Figure 3a illustrates the schematic graph presenting the correlation between Rc and rc, using eq 1 and supposing Rc × rc = constant (n = 1).14 Theoretically, since there are a part of trapped RE211 particles (assumed to be with a minimum radius of rc1) inside the as-grown RE123 matrix, the bulk must be grown quicker than the corresponding critical growth rate (Rc1). If the bulk grows at a lower rate of Rc2, then minimum size of the trapped RE211 particles will be rc2 > rc1, and more RE211 particles will be pushed out of the RE123 matrix. On the other hand, Figure 3b shows a schematic diagram of the three classical growth mechanisms in the solution,17 which are termed as the continuous growth, helical dislocation growth

Figure 1. Typical optimized profile for the continuous growth of the SmBCO bulk. During the growth period, an accelerated cooling rate was applied.

3. RESULTS AND DISCUSSION 3.1. Reduced Growth Rate in a Conventional Meltgrowth of SmBCO Bulk. Figure 2 shows macroscopic morphologies cut along the a−c plane of SmBCO bulks grown under various cooling rates and growth times, which are (a) 0.15 °C/h for 20 h, (b) 0.15 °C/h for 40 h, (c) 0.3 °C/h for 20 h, and (d) 0.15 °C/h for 20 h and 0.4 °C/h for 20 h,

Figure 2. As-grown area cut along the a−c plane of SmBCO bulks using various cooling rates and growth times: (a) 0.15 °C/h for 20 h, (b) 0.15 °C/ h for 40 h, (c) 0.3 °C/h for 20 h, (d) 0.15 °C/h for 20 h and 0.4 °C/h for 20 h. The single domain areas are inside the red lines. 3735

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Figure 3. (a) Schematic graph of the pushing/trapping theories in the melt growth of REBCO bulk; (b) schematic diagram of the three classical growth mechanisms in solution.14,17

RE211 particles in the liquid, less and less RE solute is dissolved from the RE211 particles. It means the two reactants become totally out of the peritectic reaction ratio. In other words, under a fixed cooling rate, RE211 particles coarse at the growth front with time. One of the peritectic reactant (RE211 particles) becomes stable, resulting in difficulty in the peritectic solidification. Δcr is increasing gradually with the growth. Based on eq 2, Δce at the growth front is decreasing, leading to a reduction of the growth rate. The two phenomena discussed above inevitably exist in a traditional melt growth, but they do not cause too much difficulty in the growth of the YBCO bulk. Apart from them, there should be one more crucial factor relevant for the Sm123 growth. 3). Effect of Temperature Coefficient of Solubility (t.c.s.). On the basis of the report by Krauns et al., Figure 4 shows the

and two-dimensional growth, respectively. Melt growth of REBCO bulk is the typical helical dislocation growth.17,18 It is obvious that, in order to obtain the critical growth rate (Rc1), the melt (corresponding to solution in the solution growth) needs to supply a sufficiently large driving force, persistently, that is a sufficiently large degree of the effective supersaturation (Δce1). Usually, the degree of the effective supersaturation (Δce) is obtained by applying a slow cooling profile. However, in a practical melt growth experiment, Δce is not the one calculated directly from the phase diagram associated with the degree of supercooling. It is determined as: Δce = Δc − Δcr

(2)

where Δc represents the degree of supersaturation calculated directly from the phase diagram associated with the degree of supercooling, and Δcr represents the supersaturation term related to growth resistance of the melt. If Δce reduces from Δce1 to Δce2, then the growth rate will reduce from Rc1 to Rc2, too. There are several factors that reduce the Δce. 1). Segregation Effect of RE211 Particles at the Growth Front. In the melt, size of RE211 ranges variously. Thus, there must be a certain amount of RE211 particles whose sizes are less than the critical one at a given growth condition. According to the pushing/trapping theories, they are continuously being pushed, gathering at the growth front, resulting in a barrier for the diffusion of the solute. Thereby, the growth rate gradually decreases and then the growth terminates. Therefore, with a fixed cooling rate, the small RE211 particles segregate at the growth front with time. The segregation of RE211 particles impedes the diffusion of the solute, which means that Δcr is increasing gradually during the growth. According to eq 2, Δce at in the growth front is decreasing, resulting in a reduction of the growth rate. 2). Coarsening Effect of RE211 Particles in the Liquid. A significant coarsening of Y211 particles has been reported to exist during the growth of YBCO,8 so it is expected in any REBCO system, due to the similar growth behavior. The coarsened RE211 particles are characterized by a large size and small curvature that are beneficial to the stability of RE211 particles in the solution, while they become obstacles to the melt growth of the REBCO bulk. It is known that the melt growth of REBCO bulk is based on the peritectic solidification of RE211 (s) + Ba3Cu5Oz (l) → RE123 (s). In order to obtain a sustained growth, there must be a sufficient amount of active RE211 particles and Ba3Cu5Oz melts at the growth front. In fact, due to the coarsening effect of

Figure 4. Liquidus line of RE elements in the Ba3Cu5O8 melt under an air atmosphere. RE = Y, Sm, and Nd.16

liquidus line of RE elements in the Ba3Cu5O8 melt in the air atmosphere, where RE = Y, Sm and Nd.16 Essentially, Δc is connected with (∂c)/(∂T)T (the t.c.s value) and the degree of supercooling (ΔT), as is shown in eq 3 Δc =

⎛ ∂c ⎞ ⎜ ⎟ × ΔT ⎝ ∂T ⎠T

(3)

When in our experiment, the bulk grows with the slow cooling process, the equation above should be divided by time (Δt) on the both side. Then the left side Δc/Δt becomes J, which represents the solute flux for the crystal growth. Accordingly, on the right side, ΔT/Δt becomes R, which represents the cooling rate. Apparently, as is illustrated in eq 4, 3736

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Figure 5. As-grown area along the a-c plane of SmBCO bulks grown with gradually optimized cooling rates and times: (a) 0.15 °C/h for 6.7 h, (b) 0.15 °C/h for 6.7 h and 0.4 °C/h for 20 h, (c) 0.15 °C/h for 6.7 h and 0.4 °C/h for 7.5 h, and (d) 0.15 °C/h for 6.7 h, 0.4 °C/h for 7.5 h, and 0.8 °C/h for 7.5 h. The single domain areas are inside the red lines.

for a given period of time, J is determined by both (∂c)/(∂T)T and R. R is constant in the conventional growth of REBCO bulk. However, according to Figure 4, for the SmBCO or NdBCO systems, (∂c)/(∂T)T is large around the Tp (the peritectic temperature). If we set a high cooling rate at the beginning, there will be a large J causing the self-nucleation. This explains the result shown in Figure 2c obviously. On the other hand, (∂c)/(∂T)T clearly decreases with the decreasing temperature (T). Thus, J is decreasing markedly with time (t), so are Δc and Δce. It is this noticeable reduced t.c.s value that plays a significant role in the σeff declining phenomenon in the Sm−Ba−Cu−O system. J=

⎛ ∂c ⎞ ⎛ ∂c ⎞ Δc ΔT =⎜ ⎟ × =⎜ ⎟ ×R ⎝ ⎠ ⎝ ∂T ⎠T Δt ∂T T Δt

otherwise, it would not work on facilitating the growth as shown in Figure 2d. As long as the accelerated cooling rate is set at the right time (the right time means some time before the growth ceases), the bulk can obtain continuous sufficient driving force and realize a sustaining trapping mode growth to the edge of the sample. On the other hand, with the growth continuing, associated with such an accelerated cooling, segregation and coarsening of RE211 particles are partially remitted. This in turn is beneficial for the continuous growth. On the basis of the theoretical consideration above, the accelerated cooling was applied during the SmBCO growth in this work, and the experimental evidence confirmed the hypothesis. Figure 5 shows macroscopic morphologies cut along the a−c plane of SmBCO bulks grown under various cooling modes for process optimization, which are (a) 0.15 °C/ h for 6.7 h, (b) 0.15 °C/h for 6.7 h and 0.4 °C/h for 20 h, (c) 0.15 °C/h for 6.7 h and 0.4 °C/h for 7.5 h, and (d) 0.15 °C/h for 6.7 h, 0.4 °C/h for 7.5 h, and 0.8 °C/h for 7.5 h, respectively. For each attempt, a small initial cooling rate of 0.15 °C/h (based on our former results, this cooling rate has been confirmed to be fine) is adopted to avoid the self-nucleation. In Figure 5a, the as-grown length in the c-axis direction is nearly the same as that in Figure 2a, indicating that at the cooling rate of 0.15 °C/h, 6.7 h is enough for the growth, while the other 13.3 h is ineffective. The accelerated cooling rate should therefore be applied at that point. In Figure 5b, applying an accelerated cooling to the as-grown length in the c-axis direction almost doubles, implying that the accelerated cooling rate is indeed implemented at the right time. When comparing the results in Figure 5b and c, one can easily conclude that 7.5 h of the cooling rate of 0.4 °C/h is adequate, whereas the other 12.5 h is useless for the growth. Analogously, another accelerated cooling rate of 0.8 °C/h for 7.5 h is employed. Finally, fully grown bulks are obtained, as is shown in Figure 5d and Figure 6. It is obvious that the new approach valid for the continuous growth of SmBCO bulk in this work should be suitable for the continuous growth of NdBCO too. Most importantly, more attention should be paid when an evident reduction of t.c.s. (temperature coefficient of solubility) appears in the phase diagram for crystal growth in the solution. In such case, an

(4)

All the three factors discussed above contribute to the reduction of Δce in the melt growth of REBCO systems and the results shown in Figure 2 can be easily interpreted. Figure 2b and d further demonstrates that once the growth ceased, it is extremely hard to resume growing by prolonging the growth time even under a larger cooling rate. Contrary for the YBCO system, (∂c)/(∂T)T is much smaller around the Tp in comparison with the SmBCO system and has an insignificant change with temperature. Therefore, it is very easy to realize a continuous growth without a self-nucleation under a relatively higher cooling rate. 3.3. Approaches for the Trapping Mode Controlled Continuous Growth of REBCO Bulk. On the basis of eq 2, there should be two kinds of solutions to maintain a sufficient Δce. The first is to lessen Δcr, and the second is to augment Δc. According to the analysis above, it is nearly impossible to decrease Δcr, since the segregation and coarsening of RE211 particles unavoidably occur during the growth. The only convenient way is to augment Δc (or J) persistently during the growth. J is composed of (∂c)/(∂T)T and R. For a given system, (∂c)/(∂T)T is the intrinsic value at a certain temperature. Taking the SmBCO system as an example, (∂c)/(∂T)T decreases gradually with deceasing temperature. In order to augment J, R must be increased gradually, that is, enforcing an accelerated cooling rate (R). And most importantly, the accelerated cooling rate should be set at a proper time; 3737

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(6) Ren, H. T.; Xiao, L.; Jiao, Y. L.; Zheng, M. H. Phys. C 2004, 412− 414, 597−601. (7) Gawalek, W.; Habisreuther, T.; Zeisberger, M.; Litzkendorf, D.; Surzhenko, O.; Kracunovska, S.; Prikhna, T. A.; Oswald, B.; Kovalev, L. K.; Canders, W. Supercond. Sci. Technol. 2004, 17, 1185−1188. (8) Oda, M.; Yao, X.; Yoshida, Y.; Ikuta, H. Supercond. Sci. Technol. 2009, 22, 075012. (9) Ikuta, H.; Mase, A.; Yanagi, Y.; Yoshikawa, M.; Itoh, Y.; Oka, T.; Mizutani, U. Supercond. Sci. Technol. 1998, 11, 1345−1347. (10) Sun, L. J.; Li, W.; Liu, S. F.; Mertelj, T.; Yao, X. Supercond. Sci. Technol. 2009, 22, 125008. (11) Xu, K. X.; Zuo, P. X.; Cao, Y.; Hu, S. B.; Lian, B. W.; Jiao, Y. L.; Xiao, L.; Zheng, M. H.; Wu, X. D. Supercond. Sci. Technol. 2012, 25, 075005. (12) Imagawa, Y.; Shiohara, Y. Phys. C 1996, 268, 61−70. (13) Vanranasi, C.; Black, M. A.; McGinn, P. J. J. Mater. Res. 1996, 11, 565−571. (14) Endo, A.; Chauhan, H. S.; Egi, T.; Shiohara, Y. J. Mater. Res. 1996, 11, 795−803. (15) Izumi, T.; Nakamura, Y.; Shiohara, Y. J. Mater. Res. 1993, 8, 1240−1246. (16) Krauns, C.; Sumida, M.; Tagami, M.; Yamada, Y.; Shiohara, Y. Z. Phys. B 1994, 96, 207−212. (17) Shiohara, Y.; Endo, A. Mater. Sci. Eng. 1997, R19, 1−86. (18) Wolf, T. J. Cryst. Growth 1996, 166, 810−815.

Figure 6. Top-view and side-view of fully grown SmBCO bulks under an accelerated cooling profile of 0.15 °C/h for 6.7 h, 0.4 °C/h for 7.5 h and 0.8 °C/h for 7.5 h.

accelerated cooling can be effectively used to compensate the reduced effective supersaturation.

4. CONCLUSION A single cooling rate in the air process of SmBCO bulk may result in either self-nucleation or discontinuous growth. In our new approach, an accelerated cooling was applied to maintain a sufficient σeff so that a trapping mode controlled continuous growth is realized. Thus, SmBCO bulks fully grown in both the a- and c-axis were successfully obtained; meanwhile, selfnucleation was effectively avoided. The present study has demonstrated the effectiveness of the novel method in an ordinary SmBCO system. But its applicability should be far from that. First, accelerated cooling is capable of maintaining continuous and large-sized growth because of the suppression of the σeff declining effect. Second, the new process enables one to grow difficult RE123 bulk superconductors (RE = Nd, Sm), including doped REBCO systems, which are characterized by a noticeable reduction of t.c.s. with decreasing temperature. Finally, this method is also a cost-effective method since the ineffective processing time is avoided.

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86 021 54745772. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the MOST of China (Grant No. 2012CB821404) and the NSFC (Grant No. 51172143 and 51072115). We also thank Tomaz M. for comments and critical reading the manuscript within the bilateral project BI-CN/11-13-014.

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REFERENCES

(1) Campbell, A. M.; Cardwell, D. A. Cryogenics 1997, 37, 567−575. (2) Murakami, M. Int. J. Appl. Ceram. Technol. 2007, 4, 225−241. (3) Werfel, F. N.; Floegel-Delor, U.; Rothfeld, R.; Riedel, T.; Goebel, B.; Wippich, D.; Schirrmeister, P. Supercond. Sci. Technol. 2012, 25, 014007. (4) Babu, N. H.; Lo, W.; Cardwell, D. A.; Campbell, A. M. Appl. Phys. Lett. 1999, 75, 2981−2983. (5) Muralidhar, M.; Sakai, N.; Chikumoto, N.; Jirsa, M.; Machi, T.; Nishiyama, M.; Wu, Y.; Murakami, M. Phys. Rev. Lett. 2002, 89, 237001 (4pp). 3738

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