J Supercond Nov Magn (2015) 28:1725–1728 DOI 10.1007/s10948-015-3008-7
ORIGINAL PAPER
High-Efficient Method for Introducing Nanometer Y2Ba4CuNbOx Flux Pinning Centers in Single-Domain YBa2Cu3O7−δ Bulk Superconductors by TSIG Process Jiawei Li · Wanmin Yang · Miao Wang · Yuxia Guo · Zhongling Feng
Received: 11 December 2014 / Accepted: 24 January 2015 / Published online: 4 February 2015 © Springer Science+Business Media New York 2015
Abstract Single-domain YBa2 Cu3 O7−δ (YBCO) superconductors with different additions of Nb2 O5 have been fabricated by a modified top seeded infiltration growth (TSIG) process. The growth morphology, microstructure and levitation force were investigated in details. The experimental results reveal that Nb2 O5 can react with precursor powders in the TSIG process and finally form nanometer Y2 Ba4 CuNbOx (YNb2411) particles which can act as effective flux pinning centers. The content addition of Nb2 O5 has sensitive effect on microstructure and superconductive property of YBCO bulks The levitation force increases firstly from 30 to 36 N and then reduces down to 21 N with the increasing of Nb2 O5 additions from 0.1 to 0.9 wt% and 1.5 wt%. These results indicate a high-efficient method to improve the properties of YBCO bulks. Keywords High-temperature superconductors · TSIG · Flux pinning center · Single-domain YBCO bulk
1 Introduction In the past years, increasing interest has been focused on bulk high-temperature superconductors, for example YBa2 Cu3 O7−δ (Y123) bulk. This is mainly due to their abilities of trapping magnetic flux and high critical current density (Jc ) [1], which allows practical applications such as permanent magnets, magnetic levitation, magnetic J. Li · W. Yang () · M. Wang · Y. Guo · Z. Feng School of Physics and Information Technology, Shaanxi Normal University, Xi’an, Shaanxi 710062, China e-mail:
[email protected] J. Li e-mail:
[email protected]
bearings, and flywheels [2–6]. However, the supercurrent of YBCO bulk is limited by the weak links and flux creep, which caused by grain boundaries and thermal activation, respectively. Weak links can be resolved by fabricating single-domain samples with texture microstructure. To prevent vortex motions, it is necessary to introduce artificial defect as flux pinning centers in superconductors, which can be done by high-energy ion irradiation [7, 8] and chemical doping. According to superconducting theory, the effective flux pinning force can be realized when the size of defects is close to the coherent length. Due to short coherence length of YBCO, several nanometers, the second phase particles in nanometer scale can act as effective flux pinning centers. Recently, Weinstein et al. [9] and Hari Babu et al. [10, 11] found that RE2 Ba4 CuMOy (REM2411, M = U, Mo, Zr, Bi, Nb, W, ...) particles could form nanoscale inclusions in the superconducting phase matrix both during and post-melt processing and result in obvious enhancement of the Jc of YBCO bulks. Therefore doping REM2411 in YBCO bulks became an important method to improve superconducting properties, such asJc [11–13]. It is found that the flux pinning centers are mainly introduced by direct addition of REM2411 powders in precursor pellets [12–14]. But fabrication of REM2411 powders usually need several times sintered at high temperature, this is not economical and time costing. In order to improve the efficiency and reduce the cost, a new method, without fabrication YNb2411 precursor powders, for introducing nanometer YNb2411 particles as flux pinning centers into singledomain YBCO bulks is proposed in this paper. And the effect of YNb2411 nanoparticles on the growth morphology, microstructure and levitation force of YBCO bulks has been investigated in details.
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5 mm). It can clearly be observed that there are two different kinds of particles distributed in the YBCO matrix. The small particles are YNb2411, while the other particles are Y211 phase. The YNb2411 particle is generated by the chemical reaction within the Y211, Y011, and Nb2 O5 during the melt growth process, as the following reaction: 2Y2 BaCuO5 + 6BaCuO2 + Nb2 O5 → 2Y2 Ba4 CuNbO10.5−y + 6CuO + yO2 ↑
Fig. 1 Arrangement of the sample before infiltration and growth
2 Experimental The Y2 BaCuOy (Y211) and BaCuO2 (Y011) single-phase precursor powders were prepared by the traditional solidstate sintering method in air with the raw materials of Y2 O3 , Ba2 CO3 and CuO (99.9 % purity). The powders of Y211 and Nb2 O5 were weighed according to the weight ratio of Nb2 O5 :Y211 = x: (100-x) (x = 0.1, 0.3, 0.5, 0.9, 1.2 and 1.5) and well mixed and pressed uniaxially into cylindrical pellet of diameter 20 mm as the solid phases for the top seeded infiltration growth (TSIG) process. The powers of Y211, Y011 and CuO were weighed according to the molar ratio Y211:Y011:CuO = 1:9:6 and well mixed and pressed pellet as the liquid source for the TSIG process. This method is different compare to the conventional TSIG process, where the conventional liquid source is composed with the mixture of (Y123 + 3BaCuO2 + 2CuO). The Y123 precursor powder is not involved during this whole process, hence, to simplified the experiment procedure and directly improve the efficiency of preparation process and become more advanced compared to the conventional TSIG process The layout of the pellets is shown in Fig. 1. All of the samples in batches were heated and annealed with a heating profile which is in agreement with our previous work [15]. The microstructure of the samples were observed using the scanning electron microscope (SEM, Quanta 200) and the levitation forces were measured with a homemade system at liquid nitrogen temperature [16].
It can be seen from Fig. 2 that the granular Y211 particles, with a size in the range of 1–2 .μm, are homogeneously distributed in the Y123 matrix. It is very interesting that the Y211 particles in our samples are smaller than the particle size obtained by melt-textured growth process (MTG), [17], which with the particle size in the range of 1–10 .μm. The results indicate the superiority of the TSIG technique. The YNb2411 particles, with the sizes about 100 nm, are not obvious segregation or cluster. It is quite different from the results by direct addition of YNb2411 mentioned in the refs. [15, 18, 19]. Therefore, it can be concluded that the segregation of YNb2411 can be effectively suppressed by addition of Nb2 O5 In addition, it can be seen that there are a clearly increasing trend of the pores density in the YBCO matrix with the increase of Nb2 O5 amount. The generation of pores may attributing to the release of oxygen during the chemical reaction for synthesis of YNb2411. 3.2 Surface Morphology Figure 3 shows the top surface morphology of YBCO bulk superconductors with different Nb2 O5 additions fabricated in air by the TSIG process. It can be seen that all the samples exhibit fourfold growth sector boundaries on their top surfaces, which indicates the formation of a single domain. Periodic macrosegregated bands are observed on the surface of single grains in the vicinity of seeds, whereas the surface becomes smooth far away seed regions. This result is in well agreement with the previous reports [15, 20]. More important, no spontaneous nucleation grains were observed in these samples. According to ref. [15], more than 10 wt% additions of YNb2411 in the YBCO bulks will result in the serious random nucleation during the growth process. The result suggests that the growth of singledomain YBCO bulks, with the secondphase particle YNb2411, is more steady by addition of Nb2 O5 rather than direct addition of YNb2411.
3 Results and Discussion
3.3 Levitation Force
3.1 Microstructure
The permanent magnet (20 mm in diameter) made of NdFeB of which the magnetic field is about 0.5 T was used for the levitation force measurement The samples were kept in liquid nitrogen during the whole process of cooling
Figure 2 are the SEM images shown the microstructure of different samples, and the scanning area is approximately same for each sample (2 mm beneath the seed at a radius of
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Fig. 2 SEM micrographs of the YBCO samples with different Nb2 O5 additions. a 0.1 wt%, b 0.3 wt%, c 0.6 wt%, d 0.9 wt%, e 1.2 wt%, f 1.5 wt%
and measurement. Figure 4 presents the curves of levitation force values versus the distance between the sample and the magnet. It can be seen from this figure the levitation force is much different for the YBCO bulks with different Nb2 O5 additions. The maximum levitation force
of each sample was achieved at the smallest separation of 0.1 mm and plotted with respect to the content of Nb2 O5 addition in the inset of Fig. 4. The maximum levitation force increases firstly from 30 to 36 N as the Nb2 O5 content increases from 0.1 to 0.9 wt%, and then decreases from
Fig. 3 Top surface macrographs of YBCO samples with different additions of Nb2 O5 . a x = 0.1 wt%, b x = 0.3 wt%, c x = 0.6 wt%, d x = 0.9 wt%, e x = 1.2 wt%, f x = 1.5 wt%
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can greatly improve the levitation force of YBCO superconductor. The maximum levitation force is 36 N, as exhibited in the sample with 0.9 wt% Nb2 O5 . The results are helpful to improve the superconducting properties of YBCO bulk superconductors. Acknowledgments The authors acknowledge the funding support of the National Natural Science Foundation of China (Nos. 51342001, 51167016), the Keygrant Project of Chinese Ministry of Education (No. 311033), Research Fund for the Doctoral Program of Higher Education of China (No. 20120202110003), the Fundamental Research Funds for the Central Universities (Nos. GK201305014, GK201503023), and the Outstanding Doctoral Thesis Foundation Project of Shaanxi Normal University (Nos. X2011YB08, X2012YB05).
Fig. 4 The levitation force of the samples with the different additions of Nb2 O5 . a x = 0.1 wt%, b x = 0.3 wt%, c x = 0.6 wt%, d x = 0.9 wt%, e x = 1.2 wt%, f x = 1.5 wt%
36 to 21 N when the Nb2 O5 content increases from 0.9 to 1.5 wt%. The results indicate that the levitation force of the sample is closely related to the addition amount of Nb2 O5 . The density of nanoscale YNb2411 particle will increase as the Nb2 O5 increases from 0.1 to 0.9 wt%, which means that the interface area of YNb2411/Y123 will increase, and finally result in an enhancement of the levitation force because the defects at the interface can act as flux pinning centers to improve the flux pinning force. However, with Nb2 O5 additions of more than 0.9 wt%, the amount of pores increases rapidly (as shown in Fig. 2e, f), which suppressed the flow of supercurrent. So the levitation force decreases as the increasing of amount of pores in YBCO when the addition of Nb2 O5 is excessive, even though the flux pinning force maybe increase in this situation. The results indicate that appropriate Nb2 O5 additions are helpful to promote the levitation force of the YBCO bulks.
4 Conclusion In summary, single-domain YBCO bulk superconductors with different Nb2 O5 additions have been fabricated by modified TSIG process. Nanoscale YNb2411 phase inclusions have been successfully introduced into the YBCO bulk by doping Nb2 O5 rather than direct doping YNb2411. Nb2 O5 can be reacted with Y211 and liquid phase and finally form YNb2411 nanoparticles, which can act as effective flux pinning centers. An appropriate addition of Nb2 O5
References 1. Tomita, M., Murakami, M.: Nature 421, 517 (2003) 2. Tomita, M., Fukumoto, Y., Suzuki, K., Ishihara, A., Muralidhar, M.: J. Appl. Phys. 109, 023912 (2011) 3. Cloots, R., Koutzarova, T., Mathieu, J.P.: Supercond. Sci. Techno. 18, R9 (2005) 4. Strasik, M., Hull, J.R., Mittleider, J.A., Gonder, J.F., Johnson, P.E., McCrary, K.E., McIver, C.R.: Supercond. Sci. Technol. 23, 034021 (2010) 5. Werfel, F.N., Floegel-Delor, U., Rothfeld, R., Riedel, T., Goebel, B., Wippich, D., Schirrmeister, P.: Supercond. Sci. Technol. 25, 014007 (2012) 6. Del-Valle, N., Sanchez, A., Navau, C., Chen, D.-X.: J. Low Temp. Phys. 62, 162 (2011) 7. Petrean, A., Paulius, L., Tobos, V., Cronk, H., Kwok, W.-K.: Physica C 505, 65 (2014) 8. Jia, Y., LeRoux, M., Miller, D.J.: Appl. Phys. Lett. 103, 122601 (2013) 9. Weinstein, R., Sawh, R., Ren, Y., Parks, D.: Mater. Sci. Eng. B 53, 38 (1998) 10. Hari Babu, N., Reddy, E.S., Cardwell, D.A., Campbell, A.M.: Supercond. Sci. Tech. 16, L44 (2003) 11. Hari Babu, N., Reddy, E.S., Cardwell, D.A., Campbell, A.M., Tarrant, C.D., Schneider, K.R.: Appl. Phys. Lett. 83, 4806 (2003) 12. Zhou, M., Sun, A., Sun, L., Han, P.: Supercond. Sci. Technol. 26, 105006 (2013) 13. Muralidhar, M., Tomita, M., Jirsa, M., Sakai, N., Murakami, M., Hirabayashi, I.: Physica C 469, 1196 (2009) 14. Shi, Y., Dennis, A.R., Cardwell, D.A.: IEEE Trans. Appl. Supercond. 21, 1576 (2011) 15. Li, J.W., Yang, W.M., Wang, M., Yang, P.T., Ma, J.: J. Supercond. Nov. Magn. 27, 2487 (2014) 16. Yang, W.M., Chao, X.X., Shu, Z.B., Zhu, S.H., Wu, X.L., Bian, X.B., Liu, P.: Physica C 445–448, 347 (2006) 17. Cardwell, D.A., Hari Babu, N., Iida, K.: Nanopages 1, 165 (2006) 18. Wang, M., Yang, W.M., Fan, J.: J. Supercond. Nov. Magn. 25, 867 (2012) 19. Pathak, S.K., Yeoh, W.K., Babu, N.H.: Physica C 469, 1173 (2009) 20. Pathak, S.K., Babu, N.H., Shi, Y.H., Dennis, A.R., Strasik, M., Cardwell, D.A.: J. Mater. Res. 25, 1243 (2010)
