Manabu Ikebe

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.

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.

Temperature and Frequency Dependence of the ESR Line Width in Two-Dimensional Ferromagnet K2CuF4

The temperature dependence of the ESR line widths of newly found two-dimensional ferromagnet K 2 CuF 4 (which has the K 2 NiF 4 type crystal structure and T c =6.25 K) has been observed at various frequencies. When the temperature is lowered from room temperature, the gradual narrowing of the line width occurs at first, reaching a minimum value at the temperature of several times as large as T c , and then it grows up and makes a peak near T c . Below the temperature where the peak appears, steep narrowing of the line width occurs with further decreasing temperature. At high temperatures, peculiar angular dependence of the line widths is observed in the a c -plane. This angular dependence has been explained by taking account of the effect of the anisotropic exchange interaction besides the magnetic dipolar broadening effect.

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.

Anomalous phonon-spin scattering in La1-xSrxMnO3

The temperature dependence of the phonon thermal conductivity κ( T ) of La 1- x Sr x MnO 3 (0.11≤ x ≤0.20) polycrystals was measured from 60 K to 360 K. For x =0.155 and 0.170, κ( T ) is markedly reduced over a wide temperature range near the ferromagnetic transition temperature T c . The analyses of κ( T ) have revealed the existence of anomalously strong phonon scattering caused by the coupling of spin fluctuations and the lattice instability in this double exchange coupled system.

Thermal and electrical properties of AgAu and AgCu alloy tapes for metal stabilizers of oxide superconductors

Abstract In order to develop new metal stabilizers with a low heat transport capacity for Bi-based oxide superconducting power current leads, the thermal conductivity and electrical resistivity of AgAu and AgCu alloy tapes heat treated in air were investigated from 15 to 260 K. The thermal conductivity of AgAu alloy tapes decreased drastically at low temperatures with increasing Au content after the heat treatment necessary to realize the superconducting wire. By contrast, the thermal conductivity of AgCu alloy tapes hardly decreased because of oxidation of Cu during the heat treatment.

Electron Spin Resonance in One Dimensional Antiferromagnet KCuF3

Magnetic properties of one dimensional antiferromagnet KCuF 3 were investigated by electron spin resonance using microwaves of 10∼70 GHz regions in the temperature range from 1.5 to 400°K. Two types of the resonances were discriminated corresponding to two crystal modifications (a) and (d). The Neel temperatures T N for these crystals were estimated to be 40±3°K and 20±3°K, respectively in accord with the data of neutron diffractions. Above T N , the line width showed a strong temperature dependence which could not be explained theoretically. The g -values obtained are, g a =2.28±0.01 and g c =2.16±0.01 for type (a) crystal and g a =2.25±0.01 and g c =2.16±0.01 for type (d), respectively. The resonance observed below T N can be explained by an antiferromagnetic resonance of orthorhombic symmetry except the presence of an extremely short and field dependent relaxation time which gives rise to quenching of the resonance below 30 GHz. In addition to a strong anisotropy along the spin hard c -axis, small anis...

Oscillation phenomenon of transition temperatures in Nb/Co and V/Co superconductor/ferromagnet multilayers

Abstract Tc oscillation phenomenon of Nb/Co and V/Co superconductor/ferromagnet multilayers has been observed as a function of Co layer thickness dCo. Spontaneous magnetization appeared for dCo≳7 A in both Nb/Co and V/Co systems. Tc exhibited double minimum oscillations with the first local minimum at ∼12 A for Nb/Co and at ∼18 A for V/Co and with the second local minimum at ∼32 A for both systems. The first minimum may be caused by a combined effect of magnetic spin-flip scattering, resonance scattering by Co virtual levels in the interfacial region and thickness dependent exchange field. The second minimum is found to be consistent with the appearance of so-called π-phase.