Electron conductivity of refractory oxides in the MgO-Al2O3 system

Abstract

Refractory materials based on oxides of the MgO--AI203 system have become very widely used in recent years in vacuum metallurgy. We have studied the electron conductivity of these materials as a function of temperature and composition.Experimental specimens were prepared using the normal method of sintering a mechanical mixture of MgO and A1203 powders with a grain size of ~3 ~m at 2000OK. The results of the study of the electron conductivity of the specimens of the MgO--AI203 system (one of a series of measurements) are shown in Fig. i. In this particular system there is p-type impurity conductivity as is indicated by the thermo-emf data.It should be noted that there is a sharp increase in the electron conductivity and a marked rise in the activation energy of the impurity conductivity even with low concentrations of MgO in A1203 ~Fig. 2). This suggests that the refractory materials in the study are an essentially nonuniform system consisting of two phases with different conductivities,The conductivity in such a system, even with small concentrations of MgO, must be effected primarily in relation to the phase whose resistivity is significantly less than that of corundum.The MgO--AI=03 system is two-phase as is also indicated by the method of preparing the specimens. The sintering temperature was clearly low in relation to the melting of either component of the refractory but at 20006K mutual diffusion between MgO and A1203 is possible. These features of the structure and properties of the MgO--AI203 system allow us to make use of the methods of occurrence theory [I, 2] to explain the data in Figs. 1 and 2. The essence of this theory is that in a nonuniform system there is some critical parameter (in this partic ular case , the composition of the MgO--AI203 refractory) which can only be realized by a sharp change in the properties of the system, To do this, the temperature dependences log ~-(l/T) (Fig, I) were extrapolated to T + ~. The values of ~o obtained in this way depend only on the composition of the MgO--AI203 system in accordance with the expression M, I, Kalinin Leningrad Technological Institute.