Electrical Resistivity and Electron-microscopic Observation of Sintered Chromium Dioxide

Abstract

Since an establishment of the preparation method of the CrO2 powder, various efforts have been made to obtain a compact body of the oxide to measure its electrical and magnetic properties. Since the oxide decomposes to nonmagnetic Cr2O3 at temperatures above 400°-450°C in the atmospheric pressure, a sintered body could not be obtained by the usual technique.The present paper describes the preparation method of the highly dense sintered bodies of CrO2 by hot-pressing at mechanical pressure of 30000kg/cm2 and at 450°C. A high pressure apparatus used in this experiment is of a modified internal heating type. A starting powder of CrO2 was obtained by the thermal decomposition of CrO3 under an oxygen pressure of about 350kg/cm2 at 400°C. The obtained powder was coldpressed into a disk having the diameter of 6mm and the thickness of 3mm. The pressed body was heated at 450°C for 2hours at various pressures ranging from 10000 to 30000kg/cm2 in the pressing apparatus.An electronmicroscopic observation of fractured surfaces of the samples hot-pressed at pressures less than 15000kg/cm2 showed an intergranular fracture and the original shape of each grain remained unchanged. In the samples pressed at above 25000kg/cm2 a transgranular fracture was observed and therefore the samples conclusively exhibited a strong bonding between grains. The applied pressures higher than 30000kg/cm2 remarkably promoted the sintering of CrO2 even at low temperature as 450°C.The bulk density of the samples pressed at 30000kg/cm2 was higher than 99% of the X-ray density. The electrical properties were quite different from the result obtained by CrO2 powder. The electrical resistivity showed a metal-like behavior, i.e. it increased monotonously with increasing temperature in the range -196°C to 300°C and there existed a small change in a slope of temperature dependence of resestivity at the Curie temperature of CrO2. The result is consistent with the metallic behavior of CrO2 as predicted by a Goodenough model.