Kenichiro Takahashi

Performance of an over-pressure processed Bi-2223/Ag/SUS tape and a small layer-winding coil

We have successfully demonstrated the utility and potential of Bi-2223/Ag tapes fabricated by a CT-OP (controlled-overpressure) process. In this paper, we report the Ic–B–T performance of a short tape with stainless steel lamination and energizing-test results of a small layer-winding coil in an external magnetic field ranging from 0 to 12xa0T and at temperatures ranging from 4.2 to 90xa0K. The coil is a solenoid layer-winding of a 135xa0m piece of Bi-2223/Ag tape with stainless steel lamination on both sides. The dimensions of the conductor are 2.7xa0mm in width and 0.25xa0mm in thickness, including a 0.02xa0mm thick stainless steel lamination on both sides. A short sample cut from the same tape has critical currents exceeding 76xa0A, 307xa0A and 438xa0A at 77xa0K, 30xa0K and 4.2xa0K, respectively, in self-field. The winding was performed using a react & wind technique and the resultant coil had 597 turns and dimensions of 65xa0mm (inner diameter), 83xa0mm (outer diameter) and 60xa0mm (winding height). In 12xa0T, 4.2xa0K, the coil-Ic is 191xa0A, which adds 1.42xa0T to the external field. In the present coil, the coil-Ic is controlled by the field component in the coils radial direction generated by the coil itself. The Ic values of the coil agree very well with those estimated from the short samples performance and field distribution. The results indicate that the homogeneity of the Bi-2223 conductor is very good throughout its whole 135xa0m length.

MgB2 Thin Films Fabricated by a Precursor and Post-annealing Method Have a High Jc in High Magnetic Fields

The critical current densities (Jc) of MgB2 thin films deposited on α-Al2O3 fabricated by a precursor and post-annealing process have been investigated in high magnetic fields of up to 30 T. Jc values of 2.8×105, 3.0×104, and 3.7×103 A/cm2 were obtained at 4.2 K in external magnetic fields of 10, 20, and 26 T respectively, applied parallel to the film surface. These values were comparable to those seen for Nb3Sn in fields up to 18 T, and surpassed them in fields over 18 T. The superconducting transition temperature was Tconset=29.6 K and Tczero=28.5 K. Grain connectivity was estimated to be 38.4%. The Hc2(0) estimated from the resistivity measurements was 45 T. Transmission electron microscope (TEM) observations of the microstructures suggested that grains 10–20 nm in size exist that show no epitaxial growth relationship to the sapphire substrate. Jc enhancements in fields up to ~26 T are due to the strong grain boundary pinning associated with the small grains and the small size of MgO precipitates.

Monotonic decrease of TcS with thinning of the superconducting MgB2 layer in MgB2/Ni and MgB2/B alternately-layered thin films

We prepared MgB2/Ni and MgB2/B alternately-layered films using electron beam evaporation and coaxial vacuum arc evaporation techniques without any post-annealing. The thickness of each MgB2 layer was designed to be 42, 24 or 15 nm for both MgB2/Ni and MgB2/B alternately-layered films. We confirmed that the layered structure was successfully obtained from the scanning transmission electron microscopic cross-sectional image of the MgB2/Ni alternately-layered film. The critical temperature, Tc, of the alternately-layered film decreased as the MgB2 layer became thinner, for both MgB2/Ni and MgB2/B films. Moreover, the Tc was affected by only the MgB2 layer thickness, and was independent of the inserted layer materials.

Epitaxial growth of nonpolar ZnO and n-ZnO/i-ZnO/p-GaN heterostructure on Si(001) for ultraviolet light emitting diodes

Nonpolar a-plane ZnO-film and n-ZnO/i-ZnO/p-GaN heterostructure LEDs were grown epitaxially by pulsed laser deposition and metal?organic chemical vapor deposition on Si(001) using AlN and MnS as buffer layers. X-ray diffraction pole figures showed an epitaxial relationship of ZnO() ? AlN() ? MnS(001) ? Si(001). A near band-edge emission from ZnO was observed at 378 nm in photoluminescence measurements. Electroluminescence of nonpolar n-ZnO/i-ZnO/p-GaN LEDs displayed UV emission at 390 nm under forward and reverse bias. Successful growth of nonpolar n-ZnO/i-ZnO/p-GaN heteroepitaxial on Si provides an attractive solution for integrating nonpolar ZnO-based optoelectronic devices with Si substrates for various applications.

Artificial pinning enhancement by multilayer nanostructures in MgB2/Ni thin films

We prepared MgB2∕Ni multilayer films on a polished Si (100) single crystal substrate by the growth of alternating layers of MgB2 (15nm) and Ni (∼1nm) layers. We then compared the magnetic field dependence of the critical current density Jc and the global pinning force Fp between films with and without the multilayer nanostructure. In the multilayer MgB2∕Ni film, Jc was higher in fields parallel to the substrate than in perpendicular fields, reflecting the layered nanostructure parallel to the substrate. The Jc-B curve has a plateau in parallel fields ranging between 1–5T, and Jc exceeds 1MA∕cm2, even at 6T (at 4.2K). The results indicate that the multilayer nanostructure is effective to enhance flux-pinning performance in parallel fields.

BaTiO3 based relaxor ferroelectric epitaxial thin-films for high-temperature operational capacitors

The epitaxial growth of 0.6[BaTiO3]–0.4[Bi(Mg2/3Nb1/3)O3] (BT–BMN) relaxor ferroelectric thin-films on (100) Nb doped SrTiO3 substrates has been achieved and the structure is investigated for high-temperature capacitor applications. The post growth annealing decreases the oxygen vacancy and other defects in BT–BMN films, resulting in the enhancement of dielectric constant. An insertion of intermediate SrRuO3 layers as an electrode instead of Pt, sandwiching the film, is found to be more effective in enhancing the dielectric constant. A very high dielectric constant exceeding 400 was achieved from high-temperature annealed film and the film showed an excellent dielectric constant stability of below 11% in the temperature range of 80–400 °C. This will enable smaller, high-temperature tolerant, monolithically integrated thin-film capacitors on power semiconductor devices.

The effect of MgB2 layer thickness on superconducting properties of MgB2/Ni multilayer thin films

We have investigated the influence of MgB2 layer thickness on the superconducting properties of MgB2 /Ni multilayer thin films. Multilayer thin films were prepared using an alternate growth of MgB2 (31, 22, and 15 nm in thickness) and Ni (~1 nm) layers. The critical temperature, Tc, upper critical field, Bc2, and magnetic irreversibility field, Birr, values decreased as the MgB2 layer become thinner. In the multilayer thin films, the critical current density, Jc, was higher in fields parallel to the substrate than in perpendicular fields, reflecting the layered nanostructure parallel to the substrate. On decreasing the MgB2 layer thickness, the peak position of the global pinning force, Fp, curves moved to higher magnetic field. These peak shifts could be explained by a matching effect of the flux lines with the periodic multilayer nanostructure.

Combinatorial synthesis of BaTiO3–Bi(Mg2/3Nb1/3)O3 thin-films for high-temperature capacitors

Combinatorial thin-film of (1−x)[BaTiO3]–x[Bi(Mg2/3Nb1/3)O3] — (BT–BMN) was grown on Pt/Ti/SiO2/Si, using pulse laser deposition (PLD) method, by ablating stoichiometric and Bi-10 wt % enriched targets to optimize the Bi content. X-ray photoelectron spectroscopy analysis revealed a linear Bi composition spread. As-deposited films post-annealed at high-temperatures, under oxygen atmosphere, turned into crystalline state. The crystallinity, characterized by X-ray diffraction, is better towards Bi-enriched region. The dielectric constant showed a strong dependency of Bi composition and saturated over Bi-7 wt %. The scanning nonlinear dielectric microscopic investigation revealed ferroelectric phase distribution is better around Bi-7 wt % region where the measured leakage current is also minimum. Dielectric constant over 240 and dielectric constant stability below 13% (25–400 °C range) were obtained.

Interface stability of electrode/Bi-containing relaxor ferroelectric oxide for high-temperature operational capacitor

The interface stability between electrodes (Pt, TaC, TiC, and RuO2) and a Bi-containing relaxor ferroelectric oxide, BaTiO3–Bi(Mg2/3Nb1/3)O3 (BT–BMN), applied to a high-temperature operational capacitor was investigated by hard X-ray photoelectron spectroscopy. All the electrodes showed electron filling at the Fermi level after annealing at 400 °C. However, Pt and TaC indicated electrical property degradations due to the thick intermediate layer formation and defect formation of the BT–BMN layer relating to the Bi diffusion into the electrodes. In contrast, TiC inhibited the Bi diffusion and did not show any change in the band alignment after annealing. Furthermore, RuO2 eliminated the defect formation in BT–BMN and showed no change in the band alignment although the Bi diffusion was also observed. These results suggest that the TiC/RuO2/BT–BMN stack structure is a potential candidate for the high-temperature operational capacitor.

Heteroepitaxial growth of nonpolar Cu-doped ZnO thin film on MnS-buffered (100) Si substrate

The preparation of nonpolar ZnO and Cu-doped ZnO thin films on Si substrates was studied for the application to the fabrication of green-light-emitting diodes. The use of rocksalt MnS and wurtzite AlN as buffer layers is a key technology for achieving the heteroepitaxial growth of nonpolar ZnO thin film on a (100) Si substrate. X-ray diffraction and photoluminescence measurements revealed that deposition under a high oxygen partial pressure (~1 Torr) can enhance the nonpolar crystallization of undoped ZnO, and can simultaneously suppress the formation of defects such as oxygen vacancies. These techniques can be also applied to the growth of Cu-doped ZnO. A room-temperature photoluminescence study revealed that nonpolar -oriented Cu-doped ZnO film exhibits enhanced green emission owing to the doped Cu ions.