Low-Temperature Fabrication of High-T_c NdBa₂Cu₃O_y Films by Oxygen Partial Pressure Controlled KOH Flux Method

To apply coated conductors operated at boiling temperature of liquid nitrogen (77.3 K) to various fields, an improvement of the superconducting properties is required. In the case of REBa₂Cu₃O<i>_y</i> (RE123; RE: rare earth elements) coated conductors, a biaxial orientation of RE123 crystals on substrate and the improvement of a critical current density (<i>J</i>_c) have been greatly conducted. However, this RE123 coated conductor needs a high growth temperature during film fabrication that causes degradation of superconducting properties due to impurity diffusion from metallic tape substrate. Moreover, for the achievement of high critical current (<i>I</i>_c), a particular technique for the fabrication of thick RE123 film is needed. A single crystalline REBa₂Cu₄O₈ (RE124) has been fabricated by a low-temperature liquid-phase growth process using molten alkali hydroxide (KOH) in an ambient atmosphere [1–3]. By this approach, we synthesized biaxial oriented RE124 epitaxial films on NdGaO₃ (001) single crystalline substrate at low-temperature of ∼650°C [4]. Furthermore, by controlling the oxygen partial pressure (<i>p</i>O₂), we fabricated biaxial oriented Y123 epitaxial films [5]. The critical temperature (<i>T</i>_c) of Y123 film fabricated at 650°C was ∼90 K, comparable to that of conventional Y123 films. Recently, we have focused on light RE123 (<i>L</i>RE123), which has potentially high <i>T</i>_c, and prepared the Nd123 on SrTiO₃ (100) single crystalline substrate with high <i>T</i>_c by a low-temperature crystal growth process using KOH [6]. We succeeded in obtaining biaxially oriented Nd123 films even at a low temperature of 425°C. However, the <i>T</i>_c was dramatically reduced due to the increase of Nd/Ba substitution as the deposition temperature was lowered. In contrast, by increasing the Ba/Cu composition ratio of the flux to Ba-rich, the <i>T</i>_c of the Nd123 film was slightly improved, but was much lower than the conventional <i>T</i>_c[7]. In addition to Ba enrichment of the flux composition, a low <i>p</i>O₂ is also reported to be effective in suppressing Nd/Ba substitution in Nd123 [8]. In this investigation, to establish the fabrication method of high performance <i>L</i>RE123 films by feasible simple process, we endeavored to fabricate the <i>L</i>RE123 films on single crystalline substrate at low temperatures by liquid phase epitaxial growth using KOH flux with controlled <i>p</i>O₂.<br/>Obtained films of <i>L</i>RE123 (<i>L</i>RE=Gd, Sm, Nd, La) fabricated at 700°C in various <i>p</i>O₂ of 2x10^–1, 1x10^–2, 1x10^–3 atm showed biaxial orientation of 123 single phase completely, and <i>c</i>-axis length was elongated to the stoichiometric value with decreasing <i>p</i>O₂. Moreover, Nd123 films fabricated at 700°C in <i>p</i>O₂≦1x10^–2 atm showed high-<i>T</i>_c^onset of ~90 K. From these facts, we succeeded in the fabrication of high-<i>T</i>_c (&gt; 90 K) Nd123 film using KOH flux method at low temperature by controlling of <i>p</i>O₂.<br/><br/>References<br/>[1] D. Sandford, L. N. Marquez, and A.M. Stacy, Appl. Phys. Lett., vol. 67, no. 3, pp. 422–423, May 1995<br/>[2] Y. T. Song, J. B. Peng, X. Wang, G. L. Sun, and C. T. Lin, J. Cryst. Growth, vol. 300, no. 2, pp. 263–266, Mar. 2007<br/>[3] Y. Nagira, T. Hara, Y. Yamada, K. Kuroda, and S. Kubo, Trans. Mater. Res. Soc. Jpn., vol. 35, no. 1, pp. 11–13, Apr. 2010<br/>[4] S. Funaki, F. Nakayama, and Y. Yamada, Phys. Procedia, vol. 27, pp. 284–287, Apr. 2012<br/>[5] S. Funaki, Y. Yamada, Y. Miyachi, and R. Okunishi, Jpn. J. Appl. Phys., vol. 55, Mar. 2016, Art. no. 04EJ13<br/>[6] S. Funaki, Y. Yamada, Y. Miyachi, and R. Okunishi, Phys. Procedia, vol. 65, pp. 125–128, Jun. 2015<br/>[7] S. Funaki, Y. Yamada, R. Okunishi, and Y. Miyachi, IEEE Trass. Appl. Supercond, vol. 26, no. 3, pp. 7201404, Apr. 2016<br/>[8] M. Murakami, N. Sakai, T. Higuchi, and S. I. Yoo, Supercond. Sci. Technol., vol. 9, no. 12, pp. 1015–1032, Jun. 1996