Abstract Body

REBa₂Cu₃O_y(REBCO) has a higher critical temperature (T_c), critical current density (J_c), and critical magnetic field (H_c) compared to other superconductors. Therefore, REBCO is expected to be applied to transmission cables, magnets, motors, and microwave devices (filters and NMR pick-up coil). Many research groups have attempted to improve the performance and practical application of these superconducting devices [1-4]. However, research and development of superconducting devices using REBCO is mainly focused on DC or AC devices such as power transmission cables, magnets, and motors.

 On the other hand, it is known that the surface resistance of REBCO film is more than three orders of magnitude lower than that of copper in the high frequency above kHz. Therefore, REBCO film is a promising candidate material for high performance microwave devices. Microwave receive filters using REBCO film have excellent performance with low loss and sharp skirt characteristics and are already used in the receiving systems of mobile communication systems [5, 6]. However, practical applications of microwave devices using REBCO films are limited to receive filters. Research and development on superconducting microwave devices is mainly focused on device structure such as shape and size, and is not focused on superconducting materials. Attempts to develop superconducting microwave devices (transmit filter and NMR pick-up coil) have not yet been successful, due to low properties of superconducting films. In this study, we proposed the preparation and properties of trifluoroacetates metal organic deposition (TFA-MOD) derived REBCO films for microwave devices.

 A CeO₂-buffered R-Al₂O₃substrate (25*25 mm) was annealed at 1000°C in a tubular furnace with a flow of pure oxygen gas. The (Y,Gd)BCO thin film was deposited on the annealed CeO₂-buffered R-Al₂O₃substrate from a metal organic solution containing Y-, Gd-, and Ba-trifluoroacetates, and Cu-octylic acid salt with a cation ratio of 0.77:0.23:1.5:3.0. The metal organic solution was coated by a spin-coating method at a rotation speed of 6000 rpm. The coated film was calcined in a humid oxygen atmosphere to form an amorphous precursor film by increasing the temperature to 500°C at 5°C/min. For conversion to the (Y,Gd)BCO phase, the amorphous precursor film was heated to 720°C at 20°C/min in a humidified mixed atmosphere of argon and oxygen.

 Figure 1 (a) shows a photograph of the 25*25 mm (Y,Gd)BCO film. A black film was formed over the entire substrate surface. For an XRD θ-2θ scan, (Y,Gd)BCO (001)-orientation peaks were evident. Δω and ΔΦ were estimated from the (Y,Gd)BCO (005) peak in the ω scan and the (Y,Gd)BCO (103) peaks in the Φ scan, respectively. The (Y,Gd)BCO film showed high crystallinity (Δω ≈ 0.18° and ΔΦ ≈ 0.85°), which is consistent with our previous report. The T_cfor the (Y,Gd)BCO film only varied from 90.8 to 91.2 K. The average value of self-field J_c (J_c^sf) at 77 K for the (Y,Gd)BCO film was 8.9 MA/cm², which is approximately 3 times higher than that for a co-evaporated YBCO thin film grown on CeO₂buffered R-Al₂O₃ substrate fabricated by Ceraco GmbH (about 3.0 MA/cm2). Figure 1 (b) shows J_c^sf / J_c^sf _max maps at 77 K for the(Y,Gd)BCO film. The fluctuation of J_c^sf was within ±~10% of the average. The fluctuation of J_c^sf were small, which confirms the uniformity of the film. The surface resistance (R_s) at 21.8 GHz and 77 K for the (Y,Gd)BCO film was 2.3 mΩ, which is lower than that for the co-evaporated YBCO thin film. In the presentation, we will describe the microwave properties of (Y,Gd)BCO film and the filters using TFA-MOD derived (Y,Gd)BCO film.

References

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Acknowledgment

This work was supported by JKA and its promotion funds from KEIRIN RACE. A part of this work was supported by JSPS KAKENHI (23K13365), CASIO Science Promotion Foundation, Iketani Science and Technology Foundation, and Fujikura Foundation.

Figure 1. (a) A photograph of the 25*25 mm size (Y,Gd)BCO thin film. (b) The J_c^sf at 77 K maps of the (Y,Gd)BCO thin film.