Shin-ichi Kinouchi

Irreversibility line in superconducting Bi-2212 single grain with fine normal particles

Abstract The irreversibility line was obtained in terms of the imaginary a.c. susceptibility measurement for a melt-processed superconducting Bi-2212 single grain with fine non-superconducting particles. This line showed different temperature dependences in high and low temperature regions. The one in the low temperature region agrees approximately with the prediction from the flux creep theory with experimentally observed pinning parameters. On the other hand, the one in the high temperature region is close to the well known characteristics in Y-based superconductors, suggesting flux pinning by boundaries such as the surfaces of non-superconducting particles. A quantitative discussion is also given on flux bundle size.

Isotropic Jc‐B properties of YBa2Cu3O7−x thin films containing fine precipitates by metalorganic chemical vapor deposition

Y1Ba2Cu3O7−x superconducting thin films which have isotropic Jc‐B properties were prepared onto SrTiO3 (100) substrates at 700u2009°C by metalorganic chemical vapor deposition. The Jc did not depend on directions of the field either perpendicular or parallel to the c axis of the films and the films had a high Jc of 6.1×104 and 7.1×104 A/cm2 at 77 K and 5 T perpendicular and parallel to the c axis, respectively. Transmission electron microscopy observation and x‐ray energy dispersive spectroscopy analysis indicated that the films contained many fine Y2BaCuO5‐like precipitates with diameter of ∼10 nm and they were distributed uniformly with the density Np as high as 1023/m3.

A fast short-circuit protection method using gate charge characteristics of SiC MOSFETs

This paper describes a fast protection circuit for silicon carbide metal-oxide semiconductor field-effect transistors (SiC MOSFETs) subjected to hard-switching faults (HSFs). In terms of reliability of power converters, the protection of power semiconductors against short-circuit failures is of great concern. The reverse transfer capacitance increases with decreasing drain-source voltage during normal turn-on transient. Under HSF conditions, on the other hand, it hardly changes because the drain-source voltage remains high. As a consequence, quite a significant difference appears in gate charge characteristics between under HSF conditions and normal turn-on operation. Hence, an HSF can be detected by monitoring both the gate- source voltage and the amount of gate charge. The proposed protection circuit has high noise tolerance because it monitors not only the gate-source voltage but also the amount of gate charge. The validity of the protection circuit is verified by experiment. The proposed protection circuit can detect the HSF within only one microsecond.

Characterization and Modeling of a 1.2-kV 30-A Silicon-Carbide MOSFET

This paper describes a novel compact model for a SiC-MOSFET. The model is useful to achieve accurate simulation of output characteristics from a linear region to a saturation region, selecting both gate-source voltage and temperature as parameters. In order to construct the model systematically, attention is paid to a physics-based modeling procedure with channel mobility as an adjustable parameter. The model also features characterization and modeling of an internal drain-gate capacitor. The model shows fairly good agreement in the output characteristics and the dynamic behavior of both gate drive circuit and main power circuits between the experimental and simulated results. This successful validation indicates that this model offers a promising circuit-based simulation tool for designing whole power conversion systems using SiC-MOSFETs.

Introduction of pinning center in Bi-based oxides prepared using melt process

Abstract To strengthen the pinning force by introducing extrinsic pinning centers, Bi 2 Sr 2 Ca 1+2n Cu 2+n O y superconductors which contained dispersed precipitates with high density were synthesized by using the melt process. (Ca, Sr) 2 Cu 1 O y and Ptue5f8Srue5f8O seemed to be candidates for the pinning centers through XRD and EPMA observations. Critical currents flowing perpendicular to the c-axis were evaluated through magnetization measurements. Those at 4.2K were scarcely influenced by the existence of such precipitates. But, with the n =0.15 those at 20K were five times higher in the magnetic field below 0.1 teslas, and three hundred times higher above 3 teslas, and the degradation by increasing the magnetic field from 0.1 to 3 teslas was only one unit.

Flux creep in the melt‐processed Bi‐based oxide superconductors including normal particles

The Bi‐based (2212) single grains including a large number of normal particles, (Ca,Sr)2Cu1O3, were synthesized by a melt process. To characterize the flux creep behavior in such a superconductor, magnetization measurements were performed at various temperatures and magnetic fields. The flux creep rate exhibited a maximum at 20 K in both magnetic fields parallel and normal to c axis. The magnetic field dependence of the pinning potential, U0, evaluated from the residual magnetization was not represented by the expression, B−α, in Y1Ba2Cu3O7 superconductors. The relation between U0 and the critical current density is discussed.

A Study of a High-Tc Superconducting Fault Current Limiter (No. 2): Development of an Element for 100-V Class Power Systems

The application of a high critical temperature (Tc) superconductor with a high critical current density (Jc) to a fault current limiter (FCL) was investigated to obtain a FCL which exhibits superior current limiting performance. When the superconductor changes to normal conduction, it produces a resistance that can limit fault currents. A new current limiting element consisting of a 0.4 μm — thick YBaCuO film coated with a 0.12 μm — thick protective silver film was developed. The element has a current limiting section 1 mm wide and 40 mm long. The critical current density and critical current of the superconducting film were 1.6 × 106 A/cm 2 and 7 A, respectively. For a half cycle, an alternating voltage (50 Hz) was applied to the FCL element immersed in 77K liquid nitrogen, and a typical current limiting waveform was measured. At an applied voltage of 134 Vrms, a fault current with a crest of 320 A was limited to a peak let-through current of 17 A. After the current peaked, it rapidly decreased to less than 7.5 A. By using the protective silver film, a superconducting FCL element for 100-V class power systems was developed.

Flux Pinning Characteristics in Superconducting Bi-(2212) Single Grains with Normal Particles

The temperature and magnetic field dependence of the critical current density, Jc, in the melt processed Bi2Sr2Ca1Cu2Oy superconductors, (2212), with normal particles was measured. Although Jc at 4.2K was about one order of magnitude larger than in the polycrystalline (Bi,Pb)2Sr2Ca2Cu3Oz, its temperature dependence seemed to be enhanced by the existance of weak links. The irreversibility magnetic fields were represented as [1-(T/Tc)2]n with the exponent n=5.44 at low temperatures and n=1.5 in high temperatures. Discussion is given on the dominant pinning centers by comparing the results and the theoretical predictions based on the flux creep.

A Study on a High-T c Superconducting Fault Current Limiter

The application of a high-Tc superconductor with a high critical current density (Jc) to a fault current limiter has been investigated. When the superconductor changes to the normal conductor, it has a resistance which can limit fault currents. Experiments were made using a current limiting element of a YBaCuO film 0. 2 5 μ m thick. The element has a current limiting portion with 1mm wide and 2mm long to limit fault currents. For a half cycle, an alternating voltage (10 Vp, 5 0 H z) was applied to the element. A typical current limiting waveform was obtained in the experiment. The fault current, with a crest value of 420A, was limited to the crest value of 8A. After that, the current was rapidly decreased and was less than 1.6A.

Uniform Distribution of Precipitates in Bi-Base Low Tc Single Crystal-like Samples

Fine precipitates may be one of candidates of pinning centers in oxide superconductors. The process of the crystal growth giving fine distributions of precipitates in Bi2Sr2CaCu2Oy (2212) crystals was investigated. Pt powders were mixed with glassy oxide powders and the mixtures were heated into partially melting state and slowly cooled down for crystallization. This procedure has made the precipitate distribution more uniformly and the size of precpitates smaller than in the case of no Pt addition. To clear the role of Pt elements,the grain structures in the cool down process were observed and analyzed.