MgO-Al2O3-Cr2O3系の真空蒸発

The vaporization mechanism of the system MgO-Al2O3-Cr2O3 has been studied in vacuum <10-5 Torr over the temperature range 1500 to 1700°C using the Langmuir method.It was found that the rate of vaporization for the system MgO-Al2O3-Cr2O3 was controlled by diffusion through the alumina and alumina-rich spinel layers remaining on the sample surface due to the incongruent vaporization, and that the rate of vaporization VL(g/cm2·s) for the systems MgO-Al2O3 and MgAl2O4-MgCr2O4 could be expressed as a function of the mole fraction X of Al2O3 or MgAl2O4 and a constant β as given by a general equationVL=VL0exp(-βX).The obtained activation enthalpies of vaporization showed that the vaporization in both the systems MgO-Al2O3 and MgAl2O4-MgCr2O4 is rate-controlled due to the diffusion of the vaporizing species, probably oxygen ion through the alumina and alumina-rich spinel layers accompanied by enhancement of diffusion along grain boundaries.The applicability of the additivity rule to the Langmuir vaporization rates of the system MgO-Al2O3 and the composition changes of surface layer of the MgAlCrO4 specimen along the depth from the surface to the inside bulk were observed.