Naoki Igawa

Synchrotron radiation study on the high-pressure and high-temperature phase relations of KAlSi3O8

In situ X-ray diffraction study on KAlSi3O8 has been performed using the cubic type high pressure apparatus, MAX90, combined with synchrotron radiation. We determined the phase relations of sanidine, the wadeite-type K2Si4O9+kyanite (Al2SiO5)+coesite (SiO2) assemblage, and hollandite-type KAlSi3O8, including melting temperatures of potassic phases, up to 11 GPa. Our data on subsolidus phase boundaries are close to the recent data of Yagi and Akaogi (1991). Melting relations of sanidine are consistent with the low pressure data of Lindsley (1966). The breakdown of sanidine into three phases reduces melting temperature, and wadeite-type K2Si4O9 melts first around 1500° C in three phase coexisting region. Melting point of hollandite-type KAlSi3O8 is between 1700° C and 1800° C at 11 GPa. If these potassic phases host potassium in the earths mantle, the true mantle solidus temperature will be much lower than the reported dry solidus temperature of peridotite.

Crystal and magnetic structures and their temperature dependence of Co2Z-type hexaferrite (Ba,Sr)3Co2Fe24O41 by high-temperature neutron diffraction

We have prepared nonoriented and magnetically oriented specimens of Co2Z-type Ba ferrite Ba3Co2Fe24O41 (Ba3Z) and those with Sr2+ substitution for Ba2+, i.e., Ba1.5Sr1.5Co2Fe24O41 (Ba1.5Sr1.5Z) and Sr3Co2Fe24O41 (Sr3Z) with the conventional solid-state reaction method. Permeability measurements of nonoriented specimens have shown that this substitution improves the frequency characteristic of permeability, though the permeability in Sr3Z significantly decreases. X-ray diffraction (XRD) and magnetization measurements of magnetically oriented specimens have shown that the magnetic moments of iron and cobalt ions in Ba3Z and Ba1.5Sr1.5Z lie in the c plane, but that those in Sr3Z deviate from the c plane. We have studied the substitution effect of Sr2+ for Ba2+ on the crystal structures and the effective sizes and directions of magnetic moments and their temperature dependences with high-temperature neutron diffraction technique. This substitution induces the change in the distribution of cobalt ions and mome...

Mechanical and thermal properties of 2D and 3D SiC/SiC composites

SiC fiber-reinforced SiC composites (SiC/SiC), whose preforms had 3D satin weave or 2D non-woven fabric, were fabricated by chemical vapor infiltration (CVI) or polymer impregnation and pyrolysis (PIP). The 3D satin texture made from Hi-Nicalon Type S fiber was successfully woven although the fiber has high elastic modulus. Both CVI and PIP SiC/SiC composites of Type S fiber had higher thermal conductivity than those of Hi-Nicalon fiber. These results can be ascribed to higher thermal conductivity of Type S fiber. The thermal conductivity of the 3D PIP SiC/SiC composites was increased after annealing at 1400°C in vacuum. The bend strength of the 2D CVI SiC/SiC composites of non-coated Hi-Nicalon fiber was higher than that of non-coated Type S fiber, indicating that interfacial modification for Type S fiber is needed to obtain good mechanical properties.

Highly thermal conductive, sintered SiC fiber-reinforced 3D-SiC/SiC composites: experiments and finite-element analysis of the thermal diffusivity/conductivity

Chemical vapor infiltrated (CVI) and polymer impregnated and pyrolized (PIP) SiC/SiC composites were fabricated by using highly thermal conductive, sintered SiC fiber. These composites had relatively high thermal diffusivity/conductivity values, which were two or three times larger than those reinforced with Hi-Nicalon or Hi-Nicalon Type S fibers. The improvement of thermal diffusivity by using this fiber was more noticeable for PIP composites than for CVI composites. A 2D finite element thermal analysis supported the experimental results, and revealed that highly thermal conductive SiC fiber was much effective for PIP composites and that the increase of fiber volume worsened CVI composite thermal diffusivity if low thermal conductive SiC fibers were used.

High-temperature tensile strength of near-stoichiometric SiC/SiC composites

Abstract In an attempt to characterize mechanical properties of near-stoichiometric SiC/SiC composites, tensile tests were conducted at room temperature in air and at elevated temperature under mild oxidizing gases atmosphere. SiC/SiC composites were fabricated by forced-flow chemical vapor infiltration method using two-dimensional fabrics of carbon coated near-stoichiometric Tyranno™SA fibers. Tensile tests were conducted on composites with two types of lay-up schemes using edge-loading small specimens. The effect of lay-up orientation on the mechanical properties and fracture behavior of composites were also examined. Tensile strength of composite was slightly decreased at 1573 K, while it retained approximately 80% of the strength at room temperature. Porosity dependence on elastic modulus was clearly exhibited.

Effect of simultaneous ion irradiation on microstructural change of SiC/SiC composites at high temperature

Abstract The effect of simultaneous triple ion irradiation of He, H and Si on microstructural evolution of two kinds of SiC/SiC composites (HNS composite (using Hi-Nicalon type S SiC fiber) and TSA composite (using Tyranno SA SiC fiber)) at 1000 °C has been investigated. The microstructure observations of SiC/SiC composites irradiated to 10 dpa were examined by transmission electron microscopy. He bubbles were hardly formed in matrix of TSA composite, but many helium bubbles and some cracks were observed at grain boundaries of matrix of HNS composite. He bubbles and cracks were not, on the other hand, observed in the both fiber fabrics of HNS and TSA composites. Debonding between fiber and carbon layer following irradiation region was not observed in the both composites. Under these irradiation conditions, TSA composite showed the better microstructural stability against ion beams irradiation than one of HNS composite.

Optimizing the fabrication process for superior mechanical properties in the FCVI SiC matrix/stoichiometric SiC fiber composite system

Abstract The optimization of the fabrication of SiC composites with stoichiometric SiC fibers (Hi-Nicalon Type S and Tyranno SA) was carried out by the forced thermal-gradient chemical vapor infiltration (FCVI) process. These SiC/SiC composites had a low porosity (11%) with uniform pore distribution and uniform thickness of carbon interphase between advanced SiC fibers and SiC matrix. The tensile strength was slightly increased with the thickness of the carbon interphase in the range of 75–300 nm. The effectiveness of the carbon interphase for the excellent mechanical properties was confirmed by scanning electron microscopy observation.

Synergistic Effect of Displacement Damage, Helium and Hydrogen of Silicon Carbide Composite

ABSTRACT The mechanical properties of advanced SiC/SiC composite and polycrystalline, monolithic β-SiC under dual- and triple-ions irradiation to 1 and 10 dpa at 800°C, 1000°C, and 1300°C were investigated by a Nano-indentation test. Preliminary microstructural analysis by transmission electron microscopy was performed. Hardness and elastic modulus changes in response to ion irradiation were observed, but synergistic effects on these mechanical properties were not significant. In contrast, microstructural observation of the composites after 10 dpa at 1000°C showed that cavity formation behavior was dependent on the material and the helium or hydrogen implanted mode. The effect of gas elements on cavity formation and the mechanical properties are discussed.

Phase identification and electrical conductivity of Li2WO4

Abstract This paper describes the phase identification by X-ray diffraction and Raman spectroscopy as well as the electrical conductivity measurement by a.c. two terminal method for Li2WO4. The results indicate that the high-temperature superionic polymorph which has been claimed by a few groups does not exist.

Effect of gamma-ray irradiation on in-situ electrical conductivity of ZrO2-10 mol% Gd2O3 single crystal at elevated temperatures

Abstract Change of electrical conductivity with irradiation of 60 Co gamma rays was investigated on ZrO 2 -10 mol% Gd 2 O 3 single crystal at 623, 723, and 823 K in vacuum. Conductivity increased gradually with irradiation time, while activation energy remained invariant. The increment of in-situ conductivity was attributed to the creation of oxygen vacancies introduced by the formation of Frenkel defects that were produced by the bombardments of Compton electrons. These increased oxygen vacancies were responsible for the radiation induced electrical conductivity (RIC) of the material and their contribution to RIC continued until they recombine with oxygen interstitials.