1986 Volume 94 Issue 1095 Pages 1131-1135
Hydrothermal reaction sintering is one of the methods to prepare oxides ceramics. Metals, intermetallic compounds or alloys are sealed in a Pt capsule with water. On heating they react with high pressure high temperature water, and produce metal oxides and hydrogen gas. At high temperatures, hydrogen gas diffuses out of the capsule, then the capsule containing oxide powders is compressed by outside gas pressure. The oxide powders, therefore, can be sintered in the capsule as in HIP (hot isostatic pressing). By this method, we can obtain a metal-oxide composite by the control of water to metal or alloy ratio in the capsule. The known quantities of alloy powders; water-atomized permalloy powder (Fe-47wt% Ni<#100, and Fe-36wt% Ni<#100) and redistilled water were hermetically sealed with an electric arc in a platinum capsule 2.7mm inside diameter, 0.15mm thick and 35mm long. This capsule was treated at 100MPa, 800°-1000°C, for 10-240min in a high pressure high temperature apparatus using Ar gas as pressurizing medium. The produced sintered ceramics were examined by X-ray diffraction, optical and scanning electron microscopy, and energy dispersive spectroscopy (EDS). The products are composed of oxide matrix and disperse alloy phase. According to the X-ray diffraction and EDS, the oxide phase contained only iron as cation. Selective oxidation of iron, therefore, was occurred under the hydrothermal conditions as well as in air. Magnetite and wüstite were observed as iron oxide phase. The ratio of magnetite and wüstite is dependent on temperature, quantity of H2O, and time. When a specimen is treated at high temperatures, for a long time, or with a little quantity of H2O, wüstite phase is a main phase. When the opposite conditions, magnetite is a main phase. This tendency results from both the kinetics and the thermodynamics. The composites showed superposed magnetic properties of Fe-Ni alloy and iron oxides, magnetite and/or wüstite.
