Development and Scaling up of Oxide Buffer Architectures on Flexible Substrates for Efficient and Application-Derived Growth of REBCO Thin Films
High-performance REBCO film growth on low-cost, flexible, and scalable metallic substrates is sought for many applications requiring high critical current densities (Jc) with operability over a wide range of magnetic fields of 0–20 T and temperatures of 4.2–77 K. In addition, the growth of high-quality REBCO films on low-cost, flexible, dielectric, and nonmetallic substrates is needed for use in high-frequency applications because of their low surface resistivity, high-quality factor (Q-factor), and low signal-to-noise ratio (SNR), which are essential for ensuring high-quality signal transmission. This study aims to improve the Jc and processing efficiency of REBCO to support the fabrication of 5-μm-thick film REBCO tapes using advanced metal organic chemical vapor deposition (A-MOCVD), with three times the critical current of commercial REBCO tapes in high magnetic fields at 4.2–20 K, and more than four times the precursor-to-film conversion efficiency of conventional MOCVD (approximately 45%). Single crystalline-like REBCO thin films are enabled through the growth of biaxially-textured oxide buffer films based on the state-of-the-art ion beam-assisted deposition (IBAD) of thin MgO templates. In this study, a novel approach was developed to grow double-sided oxide buffer to facilitate the growth of double-sided REBCO films thicker than 3 μm on each side so as to improve the engineering current density (Je). The double-sided buffer stack was grown on 50-μm-thick in-house electropolished Hastelloy tape. This tape, which has an average surface roughness (Ra) of <1 nm, was selected for its good thermal and chemical compatibility. The buffer stack consisted of a 240-nm-thick amorphous alumina (diffusion barrier) deposited by magnetron reactive sputtering, followed by an 8-nm-thick amorphous yttria (seed layer) and a 10-nm-thick IBAD MgO using the IBAD process. Subsequently, Homo-epi MgO and LaMnO3 were optimized using reactive mid- and radio frequency (RF) magnetron sputtering to achieve strictly (00l) growth and good out-of-plane and in-plane textures. Moreover, 13 double-sided tapes were manufactured, with an average out-of-plane texture spread Δω of LMO (002) between 3.22° and 3.24° for sides 1 and 2, respectively and an average in-plane texture spread Δɸ of LMO (110) between 7.6° and 7.64° for sides 1 and 2, respectively. These values are comparable with the texture of single-sided commercial oxide buffer tapes, which have Δω of –3° and Δɸ of 6°–8°. Consequently, the growth of double-sided REBCO films has yielded high self-field critical current at 77 K and 0 T and in-field critical current exceeding 3.5 times that of commercial tapes at 20 K and 20 T. Flexible, polycrystalline yttria-stabilized zirconia (YSZ) with a thickness of 40 μm was selected for REBCO films for microwave and RF applications. This material offers low dielectric constant, low thermal conductivity, and compatible thermal and chemical properties. The YSZ samples were surface planarized to Ra <2 nm over both 5-cm- and 15-cm-long samples. A buffer architecture consisting of 8 nm yttria, 10 nm IBAD MgO, 130 nm HE MgO, and 120 nm LMO was optimized and grown with average FWHM values of 3.5° Δω for LMO (002) and 8.2° Δɸ for LMO (110), on 5 cm YSZ samples. 15 cm samples displayed Δω of 3.1°–5.3° and Δɸ of 6.9°. 350-nm-thick A-MOCVD and conventional MOCVD REBCO layers grown on 5 and 15 cm samples recorded Jc > 2 and 0.8 MA/cm2, respectively, at 77 K and 0 T. RF characterization of surface resistivity revealed a negative correlation with Jc. A Q-factor of 35,000 at 20 K, 0 T, 8–8.3 GHz was achieved in REBCO film on YSZ, which is superior to Q-factor measured on REBCO film on single-crystalline substrates.
REBCO, Double-sided, Oxide buffer layers, Flexible YSZ.
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