Influence of additions on the conductivity of fused periclase

Abstract

Periclase, which is produced by fusion of various natural and artificial products, the chemical base of which is magnesium oxide, is a dielectric widely used as an electrical insulator in various types of electric heating equipment.Depending upon a number of technological factors (raw material quality, fusion conditions, cooling of the fused product, subsequent working of it) the electrical resistance of periclase may vary within significant limits. One of the methods of increasing the electrical resistance of periclase is additions [i].It is known that magnesium oxide, including fused, has a mixed conductivity (ionic and p-or n-type electron). Under normal conditions, magnesium oxide possesses primarily ionic conductivity [2, 3]. However, with a change in temperature and oxygen partial pressure or the addition of additions the ratio between the ionic, electron, and hole conductivities may change.In the presence of impurities with a charge greater than that of Mg 2+, such as those containing tri-and tetravalent cations, compensation of the positively charged substitutional defects must occur either as the result of formation of doubly charged vacancies of magnesium V~g or as the result of defects in the oxygen sublattice. Apparently, cationic vacancies predominate. This may be the result of the fact that the escape energy of an oxygen ion is greater than that of magnesium ion [4].Electron and hole conductivity are related to deviation from stoichiometry with respect to oxygen. The concentration of these defects in magnesium oxide is normally comparatively low but in view of the high mobility of electrons and holes the electron and hole conductivity may under known conditions determine its electrical conductivity.To study the nature of the conductivity and the influence of additions perielase fused from brucite was used, and as additions the flaky silicates hydroaluminum silicate and magnesium silicate (talc). These additions are used for improvement of the sliding of the crystals in compression of the powder in production of thermoelectric heaters.The conductivity was studied on samples of polycrystalline periclase of three compositions, without additions and with the addition of talc or hydroaluminum silicate.The additions were added in a quantity of i%. The chemical compositions of the periclase and the additions are given in Table I. The different contents of oxides with Me 3+ and Me 4+ in the original raw material and the additions may make it possible to reveal the influence of their content on the conductivity of the periclase.The samples were prepared from powder of fused periclase with a grain size of less than 0.5 mm in the form of 20-mm-diameter tablets pressed under a pressure of 150 MPa. The samples were fired at 1200~ and to their faces was applied a platinum paste (electrodes). The sample was placed in a quartz measuring cell and the cell with the sample was placed in a furnace with silicon carbide heating elements [5].The measurements were made in de with the use of an E6-4A megaohmmeter at 600-120...