Hiroyuki Fujishiro

Simultaneous Measurement of Thermal Diffusivity and Conductivity Applied to Bi-2223 Ceramic Superconductors

A method to measure the thermal diffusivity α and the conductivity κ under an identical experimental setup has been developed and α and κ of Bi-2223 oxide superconductor have been measured quasi-simultaneously. The results are analyzed on the basis of the BRT and Tewordt-Wolkhausen theory. The simultaneous measurement makes it possible to estimate the specific heat C and the Debye temperature Θ D , as well as to separate the electron and phonon contributions to the diffusivity. The simultaneous measurement also provides a useful check on the reliability and the consistency of the analyses.

Thermal Conductivity and Diffusivity of High-Strength Polymer Fibers.

The thermal conductivity κ and diffusivity α of high-strength and high-modulus crystalline polymer fibers (polybenzobisoxazole (Zylon\circR) and polyethylene (Dyneema\circR)) and their fiber-reinforced plastics (FRPs) were measured in directions parallel and perpendicular to the molecular chain axis of the fibers. The main contribution to thermal conductivity was from phonon conduction along the molecular chains in both fibers and the phonon conduction was limited by boundary-like scattering over the temperature range of 10–260 K. From the analyses using a phenomenological model, the thermal conductivity anisotropy ratio (κ// fiber/κ⊥ fiber) of Zylon fiber was estimated to be 80 at 100 K, which was about two or three times larger than that of Dyneema fiber.

Modelling of bulk superconductor magnetization

This paper presents a topical review of the current state of the art in modelling the magnetization of bulk superconductors, including both (RE)BCO (where RE?=?rare earth or Y) and MgB2 materials. Such modelling is a powerful tool to understand the physical mechanisms of their magnetization, to assist in interpretation of experimental results, and to predict the performance of practical bulk superconductor-based devices, which is particularly important as many superconducting applications head towards the commercialization stage of their development in the coming years. In addition to the analytical and numerical techniques currently used by researchers for modelling such materials, the commonly used practical techniques to magnetize bulk superconductors are summarized with a particular focus on pulsed field magnetization (PFM), which is promising as a compact, mobile and relatively inexpensive magnetizing technique. A number of numerical models developed to analyse the issues related to PFM and optimise the technique are described in detail, including understanding the dynamics of the magnetic flux penetration and the influence of material inhomogeneities, thermal properties, pulse duration, magnitude and shape, and the shape of the magnetization coil(s). The effect of externally applied magnetic fields in different configurations on the attenuation of the trapped field is also discussed. A number of novel and hybrid bulk superconductor structures are described, including improved thermal conductivity structures and ferromagnet?superconductor structures, which have been designed to overcome some of the issues related to bulk superconductors and their magnetization and enhance the intrinsic properties of bulk superconductors acting as trapped field magnets. Finally, the use of hollow bulk cylinders/tubes for shielding is analysed.

Charge Ordering and Sound Velocity Anomaly in La1-XSrXMnO3(X≥0.5)

The electrical resistivity, magnetization, dilatation and sound velocity have been measured for La 1- X Sr X MnO 3 (0.48≤ X ≤0.90) polycrystals. Observed anomalies in the sound velocity and the dilatation strongly suggest the occurrence of the charge ordering within the Sr concentration range 0.48≤ X ≤0.82. A phase diagram of the charge-ordered state in La 1- X Sr X MnO 3 as a function of temperature T and the Sr concentration X is proposed.

Anisotropic Thermal Diffusivity and Conductivity of YBCO(123) and YBCO(211) Mixed Crystals. II

The anisotropic thermal diffusivity α of the highly c-axis-oriented Y–Ba–Cu–O bulk superconducting crystals has been measured quasi-simultaneously with the thermal conductivity κ. The estimated values of the specific heat C by use of α and κ values parallel and perpendicular to the c-direction agreed with each other. In these crystals prepared by the modified melt texture growth (MMTG) method, fine Y2BaCuO5 particles are dispersed in the YBa2Cu3O7-x superconducting matrix phase. Based on a simple model, the thermal diffusivity and the specific heat of the YBa2Cu3O7-x matrix phase are separated using independently measured data of a Y2BaCuO5 polycrystal. The influence of Y2BaCuO5 particles on the thermal properties of the mixed crystals is discussed.

Modelling and comparison of trapped fields in (RE)BCO bulk superconductors for activation using pulsed field magnetization

The ability to generate a permanent, stable magnetic field unsupported by an electromotive force is fundamental to a variety of engineering applications. Bulk high temperature superconducting (HTS) materials can trap magnetic fields of magnitude over ten times higher than the maximum field produced by conventional magnets, which is limited practically to rather less than 2 T. In this paper, two large c-axis oriented, single-grain YBCO and GdBCO bulk superconductors are magnetized by the pulsed field magnetization (PFM) technique at temperatures of 40 and 65 K and the characteristics of the resulting trapped field profile are investigated with a view of magnetizing such samples as trapped field magnets (TFMs) in situ inside a trapped flux-type superconducting electric machine. A comparison is made between the temperatures at which the pulsed magnetic field is applied and the results have strong implications for the optimum operating temperature for TFMs in trapped flux-type superconducting electric machines. The effects of inhomogeneities, which occur during the growth process of single-grain bulk superconductors, on the trapped field and maximum temperature rise in the sample are modelled numerically using a 3D finite-element model based on the H-formulation and implemented in Comsol Multiphysics 4.3a. The results agree qualitatively with the observed experimental results, in that inhomogeneities act to distort the trapped field profile and reduce the magnitude of the trapped field due to localized heating within the sample and preferential movement and pinning of flux lines around the growth section regions (GSRs) and growth sector boundaries (GSBs), respectively. The modelling framework will allow further investigation of various inhomogeneities that arise during the processing of (RE)BCO bulk superconductors, including inhomogeneous Jc distributions and the presence of current-limiting grain boundaries and cracks, and it can be used to assist optimization of processing and PFM techniques for practical bulk superconductor applications.

Low thermal conductive Bi-2223 tapes sheathed with Ag-Au alloys

With the view of applying to power current leads for superconducting magnet systems and for other cryogenic power handling systems, low thermal conductive Bi-2223 superconducting tapes sheathed with Ag-Au alloy were fabricated and their thermal conductivities were measured from 12 to 260 K. The critical current density (overall-J/sub c/) was about 1700 A/cm/sup 2/ at 77 K, 0 T and remained nearly constant irrespective of Au concentration up to 11 at.%. The tape sheathed with Ag+11 at.%Au alloy, of which the superconductor cross-section ratio f/sub sc/ was 0.65, had a thermal conductivity value about 0.2 W/cmK at 77 K this value is as low as that of Cu-Zn. It was found that the thermal conductivity of the tape was close to the calculated one based on f/sub sc/ and the independently measured thermal conductivities of the Ag-Au alloy acid the Bi-2223 superconductor. The superconducting tapes sheathed with the alloy were confirmed to be suitable for the application as power current leads. >

CHARACTERISTICS OF AG-AU ALLOY SHEATHED BI-PB-SR-CA-CU-O SUPERCONDUCTING TAPES FOR CURRENT LEADS

Oxide superconductor tape with Ag‐Au sheath was studied for current leads. A 100‐cm‐long current lead of 1000 A with 0.2 W/kA heat leakage was designed using Ag‐11 at. % Au alloy sheath with low thermal conductivity. The overall current density of the superconducting tape is required to be 350–770 A/cm2 at 77 K when the superconductor cross‐section ratio in the tape is 0.35–0.7. Bi‐Pb‐Sr‐Ca‐Cu‐O superconducting tape with Ag‐11 at. % Au alloy sheath was fabricated and the critical current density Jc was measured. The overall‐Jc was 1700 A/cm2 at 77 K under 0 T. The possibility of the current lead of 1000 A under 0.28 T with 0.2 W/kA was made clear.

Record-High Trapped Magnetic Field by Pulse Field Magnetization Using GdBaCuO Bulk Superconductor

A trapped magnetic field BTP as high as 4.47 T, which is the highest reported using pulse field magnetization to date, has been realized on the surface of a GdBaCuO bulk superconductor by a modified multi pulse technique combined with stepwise cooling. Following an introduction of a small amount magnetic flux into the bulk center by applying lower pulse fields Bex=4.5–4.6 T twice at a higher starting temperature Ts=45–48 K, higher fields of Bex=6.6–6.7 T are applied three times at a lower Ts=28–29 K. The reduction in the temperature rise due to the already existing trapped flux, in addition to the optimization of the higher Bex value at the lower Ts, is a key point in enhancing BTP.

Time evolution and spatial distribution of temperature in YBCO bulk superconductor after pulse field magnetizing

The time evolutions of temperature rise ?T(t) after pulse field magnetizing have been measured on the surface of a cryo-cooled YBaCuO bulk superconductor. The temperature rise ?T increases with increasing pulse field strength, and a quite large temperature rise (?Tmax ~ 18.8 K) is observed just before the trapped magnetic flux reaches the central region of the bulk. The epoxy resin, impregnated and coated to the bulk crystal for mechanical reinforcement, prevents the heat exhaust towards the cold stage of the refrigerator and it needs much time to recover its initial temperature. The spatial distribution of ?T(t) has indicated that the magnetic fluxes tend to enter into the bulk, avoiding the 4-fold growth sector boundaries of the bulk crystal.