The purpose of this research is to develop fundamental technologies for innovative superconducting power networks that enable the large-scale use of renewable energy. Considering the depletion of fossil fuels, global environmental concerns, and Japan's extremely low energy self-sufficiency rate, the introduction and expansion of renewable energies such as solar and wind power is an urgent issue. However, the power generated by these sources fluctuates significantly due to weather conditions, and the current power system has a very limited capacity to accept such power due to the problems with installed capacity and frequency fluctuations. From this perspective, the establishment of power storage technology that can absorb such output power fluctuations is an extremely important issue. This research aims to develop an innovative energy network infrastructure technology to realize a society that massively utilizes renewable energy, based on the new concept of giving the power grid itself an energy storage function by using superconducting cables.

We proposed a superconducting diode as a novel superconducting device for the power receiver of wireless power transmission in the space solar power system (SSPS), which is expected to be a renewable energy source in the future. In order to realize efficient power reception and rectification for wireless power transmission, we investigated the deposition method of MgB₂ film as a suitable material for superconducting diodes, the surface planarization process to improve rectification efficiency, and an evaluation method for rectification in superconducting diodes at high frequency in the microwave range. We present the basic concept of the superconducting diodes after explaining the theory on surface barrier to the flux quantum in superconductors. Earlier studies of various superconducting diodes are also reviewed.

As results of this project, MgB₂ films were prepared by the pulsed laser deposition using a Nd:YAG laser with in-situ post annealing process. The film shows T_c^onset = 33.6 K and J_c = 0.9×10⁶ A/cm² at 20 K and under self-field. The surface planarization of REBa₂Cu₃O_y films increased the rectification rate up to 145 % compared to the untreated sample due to the enhancement of the surface barrier. Wet polishing with loose abrasives is promising for the surface planarization of the films. We established a test facility of superconducting diode to explore for a wide range of frequencies from 60 Hz to 80 MHz and confirmed the rectification at frequencies up to 80 MHz in the REBa₂Cu₃O_y films. In the future, it is expected that we will fabricate superconducting diodes using the MgB₂ films, develop a polishing technique for mass production, and develop a rectification test method at a microwave frequency of 5.8 GHz. Furthermore, it is desirable to fabricate a rectenna with a superconducting diode built into the antenna, and to develop a device with the functions of high-efficiency power reception, rectification, and storage of microwaves for wireless power transmission including the SSPS.

Magnesium diboride (MgB₂) is a promising candidate for superconducting diodes due to its high lower critical field (H_c1). In this study, we fabricate MgB₂ thin films by a precursor and in-situ post-annealing process as a candidate material for superconducting diodes. The precursor is obtained by pulsed laser deposition (PLD) method using a Nd:YAG laser. The MgB₂ thin films prepared by a high-density Mg-rich target with a ratio of Mg : MgB₂ = 2 : 1 show the highest superconducting transition at T_c^onset = 33.6 K and T_c^zero = 31.7 K. The critical current density J_c(0 T) calculated from magnetization measurements reaches 0.9×10⁶ A/cm² at 20 K. The X-ray diffraction patterns indicate that the MgB₂ phase would be oriented with c-axis perpendicular to the substrate surface and/or would have an extremely fine grain size. These results demonstrate that the precursor and in-situ post-annealing process by Nd:YAG-PLD method is favorable for fabricating superconducting diodes with MgB₂ material.

Planarization of the REBa₂Cu₃O_y (RE: rare earth element) film surface is expected to improve the surface barrier against transverse magnetic field and enhance the asymmetricity of the critical current, i.e., the superconducting diode’s performance. The reactive ion etching (RIE), dry polishing with fixed abrasives, and wet polishing with loose abrasives are investigated as a surface treatment process for planarization of the REBa₂Cu₃O_y films. The RIE with various reactive gases, power, and treatment times resulted in localized etching pinholes on the film surface without change in flatness. In addition, the critical current was significantly reduced due to the degradation of the film during the RIE process. The dry polishing with fixed abrasives was simple and easy for planarization of the film surface. The root-mean-square roughness (RMS) value decreased from 10.5 nm of the untreated sample to 1.62 nm after the dry polishing process. However, numerous large scratches, which were caused in the process by coarse abrasive grains, blocked the current path and significantly reduced the critical current. The wet polishing with loose abrasives flattened the film surface to an RMS value of 0.82 nm without scratches. After the wet polishing process, the superconducting properties, such as the critical temperature and critical current, maintained the values they had before the process. The samples subjected to the wet polishing process show higher critical current asymmetricity at 77 K than the untreated sample possibly due to the enhanced surface barrier with the flatter surface. The critical current asymmetricity increases up to 145 % compared to the untreated sample with decreasing RMS value. The wet polishing with loose abrasives is a promising process for surface planarization of REBa₂Cu₃O_y film, and further surface planarization is expected to further enhance the asymmetricity of the critical current.

In superconducting coil applications, magnetic fields are applied to superconducting material from various directions. YBa₂Cu₃O_y (YBCO) coated conductor containing BaHO3 (BHO) flux pinning centers is expected to be used as coil application. In this study, BHO-doped YBCO films were prepared by changing the substrate temperature (T_s) and thickness per pulse (t_pulse) using the pulsed laser deposition method. As a result, the isotropy of the critical current density vs field angle (J_c-θ) curve was improved in the BHO-doped YBCO film fabricated under the condition of low T_s and high tpulse. In the cross-sectional transmission electron microscopic observation, it was confirmed that BHO nanorods with gradually increasing tilt angles grew in BHO-doped YBCO film. The surface atomic force microscope images also confirmed that YBCO grains grew in three-dimensional island growth. The shape of BHO is determined by the growth rate of YBCO and BHO, which affects the tilt angle of the BHO nanorods.