Introduction
The improvement of the critical current I_c is indispensable for the wide use of REBa₂Cu₃O_y (REBCO, RE = rare earth) Coated Conductors (CCs). To achieve an increased I_c with high deposition rate, we have introduced the vapor-liquid-solid (VLS) growth technique combined PLD and liquid phase epitaxy (LPE) [1]-[3]. Since VLS growth technique realizes crystal growth via liquid layer, it is possible to fabricate REBCO CCs with a high production speed and high I_c. We reported that a critical current density J_c of 1.7 MA/cm² and deposition rate of 26 nm/s for BaHfO₃(BHO)-doped YBCO (YBCO+BHO) CCs were achieved by using VLS growth technique [4]. Furthermore, J_c in magnetic fields of the YBCO+BHO CCs with the thickness of 1.4 mm was improved at various temperatures compared with a sample fabricated by PLD method [5]. While we have attempted to fabricate thick YBCO+BHO CCs using VLS growth technique, it was proved to be difficult to control the amount of the liquid layer. For example, when a small amount of liquid layer is supplied, there is a shortage of liquid layer during film growth. On the other hands, when a large amount of liquid layer is supplied, melt back of the seed layer occurs. Therefore, we consider that control the optimum amount of liquid layer, especially the duration time and timing for the supply. In this study, we fabricated and evaluated YBCO+BHO CCs with ultra-thick films using VLS growth technique with the optimal method to supply liquid layer.

Experimental method
First of all, we fabricate YBCO+BHO CCs by using conventional VLS growth technique. The VLS growth technique consists of three steps as follows. The first step is to fabricate a YBCO solid layer with the thickness of 200 nm on the IBAD-MgO tapes. The second step is to fabricate a liquid layer of Ag₂O-doped Ba₃Cu₇O₁₀ on the YBCO layer with the thickness of 50 nm. The next step is to fabricate 3vol.% BHO-doped YBCO thickness of 2.8 µm via the liquid layer for each thickness 2.8 µm. It was changed from 10 to 30 second duration time of liquid layer on YBCO+BHO thicker films. The J_c in a magnetic field were measured using a Physical Properties Measurement System (PPMS). For the J_c measurement, the CCs were patterned into a bridge shape with the width of 40 µm and the length of 1 mm for the DC four-probe method. The search for the optimal liquid layer duration time was done with reference to J_c^min. Here, J_c^min was defined as the minimum value in the J_c-θ curve.

Experimental results and discussion
Figure 1 shows the highest value of J_c ^min was obtained in the case the duration time of the second liquid layer is 15 seconds so that we determined that 15 seconds was the best time to supply the liquid layer. Hence, we also fabricated YBCO+BHO CCs with a thickness of 8.4 mm in which the third liquid layer was also supplied for 15 seconds. Figure 2 shows the dependence of the I_c in the magnetic field on the thickness of the YBCO+BHO CCs. As the result, a high I_c with 231.6 A/cm-width at 70 K, 2.5 T with a thickness of 8.4 µm was obtained. Furthermore, it is confirmed that J_c maintained even with thicker films. The reason for these result is that we could control the optimal amount of liquid layer for VLS growth technique.

References
[1] R. S. Wagner and W. C. Ellis, “Vapor-liquid-solid mechanism of single crystal growth,” Appl. Phys. Lett., vol. 4, no. 5, pp. 89–90, Mar. 1964.

[2] Y. Yoshida, Y. Ito, I. Hirabayashi, H. Nagai, and Y. Takai, “Surface morphology and growth mechanism of YBa₂Cu₃O₇-y films by metalorganic chemical vapor deposition using liquid sources,” Appl. Phys. Lett., vol. 69, no. 6, pp. 845–847, Aug. 1996.

[3] K. S. Yun et al., “Vapor–liquid–solid tri-phase pulsed-laser epitaxy of RBa₂Cu₃O₇−y single-crystal films,” Appl. Phys. Lett., vol. 80, no. 1, pp. 61 –63, Jan. 2002.

[4] Tomohiro Ito et al., “Enhancement of I_c of BaHfO₃ -Doped REBCO Thick Coated Conductor Using Vapor-Liquid-Solid Growth Technique” IEEE. Trans. Appl. Supercond., vol. 31, no. 5, Aug. 2021, Art. No. 6601304.

[5] Tomohiro Ito et al., “Effect of Surface Liquid Layer during Film Growth On Morphology of BaHfO₃ in YBa₂Cu₃O_y Coated Conductors Fabricated by Pulsed Laser Deposition” IEEE. Trans. Appl. Supercond., vol. 31, no. 5, Aug. 2021, Art. No. 6601205.

Acknowledgements
This work was partly supported by JSPS-KAKENHI (19K22154, 20H0268, 20K15217, 21H01872) and NEDO. The IBAD-MgO substrates were provided from Dr. Izumi, Dr. Ibi and Dr. Machi of AIST.

Keywords: Thick film, Vapor Liquid Solid, Critical current, Flux pinning