Studying the adhesion and electric conductivity of alumina and magnesia

Abstract

Previously [i] investigators showed that it is possible to regulate the chemical and adhesive reactions at the contact point of metallic melt and refractory oxide in a systematic way by alloying the latter with various additives.In particular, additions made to the corundum transition metals, and also zircon and titanium, leadto a reduction in the energy of adhesion in a certain range of concentration, and correspondingly produce a reduction in the mass exchange between the lining and the metal.And, on the other hand, dilution of corundum batch with powdered oxides of the alkaline-earth metals increases the chemical and adhesive reaction, which may find extensive use in cermet production.Theoretical predictions of the possibility of regulating the catalytic activity of oxides were based on an approach to the physical nature of this interaction from the viewpoint of the electron--ionic energy structure of the refractory and its dependence on the extraneous additives, the perfection of the structure, and the material's previous history.The electron structure of the material governs its electrophysical, optical, and other properties. The correlation obtained later between the experimental data for electric conductivity and setting edge angle of the substratum of droplets of steel [2] stimulated further investigation toward the detailed development of an electron--ionic model of interaction on the boundary of oxide with melt (adhesion, mass exchange, diffusion, reaction capacity, phase inversion, etc.) the full picture of which remains unclear.The present authors studied the volume electric conductivity of single-crystal substrate of sapphire (pure corundum), ruby (corundum with an addition of chromium in amounts of about 1%),* and pure magnesium oxide, and also ceramic substrata of the same composition, using the triple-electrode method (with a guard ring for the dc current). The results were obtained for the range 900-1800~ in vacuum (0.0133 Pa). After repeated measurements of the electric conductivity without breaking the vacuum on the specimens during heating and cooling (until stable results were obtained) the substrata with the metallic tablets were placed in equipment for measuring the wetting angle, in which they were held under the droplet of melt for up to 8 min.In this case we noted intense deoxidation of the metal stimulated by the substratum, masked sticking of the tablet to some specimens, and a change in the color of the contact site. Then the remaining, undamaged, substratum was again subjected to careful testing for electric conductivity.The results of y before and after the action of the droplet of melt 80N are shown in the form of graphs log y = f(lO4/T) in Fig. i. The concept accepted in semiconductor physics of a temperature relationships for electric conductivity in the form log y = f(104/T) enables us to calculate the value of the energy of formation of free discharges (energy of activation of the current carrier), ensuring electric conductivity.Bearing in mind the zone theory of activation Ea,...