Cation self-diffusion in single-crystal MgO

Abstract

Cation self-diffusion coefficients have been obtained for MgO in an argon atomosphere over a temperature range 1000–2400°C from concentration gradients determined with the aid of mass spectrometry. The problems associated with use of the short-lived radioisotope 28Mg were avoided by employing the stable isotope 26Mg as a tracer. Samples were prepared utilizing vapor-exchange, thick-film, and semi-infinite source boundary conditions at high, intermediate, and low temperatures, respectively, and were protected from contamination and volatilization by encapsulation in cylinders of pressed MgO powder. No significant difference in transport behavior was noted between crystals of moderate quality and those of the highest purity presently available. The temperature dependence of the diffusion coefficients may be represented by an activation energy of 2.76±0.08 eV and D0 of 4.19 × 10−4 cm2/sec. These parameters are comparable to those previously reported for impurity ion diffusion. The activation energy obtained is interpreted as that for cation migration. Previous measurements of cation self-diffusion coefficients obtained with the short-lived radisotope 28Mg are an order of magnitude larger and had been interpreted as intrinsic diffusion. The discrepency is attributed to doping introduced by the 28Si decay product of 28Mg.

Effect of Ca and CaO on the High Temperature Oxidation of AZ91D Mg Alloys

“…Both CaO and MgO have Schottky defects, and cations diffuse faster than oxygen. [12][13][14] Hence, one can expect that the oxide scale would primarily form by the outward diffusion of Mg and Ca under normal conditions. In reality, since the Pilling-Bedworth ratios of MgO and CaO were 0.81 and 0.64, respectively, 15) these oxides could not form a compact layer and systematic fissuring occurred as oxidation progressed.…”

Time‐resolved study of charge generation and propagation in igneous rocks

“…This is reminescent of a situation encountered during a study of the model insulator MgO and its electrical conductivity as a function of temperature [Kathrein and Freund, 1983]. MgO had long been considered an excellent insulator up to temperatures above 700øC, the onset of ionic conductivity [Sempolinski and Kingery, 1980;Wuensch et al, 1973]. There were hints of an unusual orders of magnitude increase in the conductivity in the 400-700øC interval but nobody pursued these leads because it could be shown that this enhanced conductivity was confined to the surface and therefore looked like being caused by some kind of "dirt."…”

An ab initio study of the effect of charge localization on oxygen defect formation and migration energies in magnesium oxide

“…Early calculations were carried out using classical interatomic potentials (Catlow 1977;Mackrodt & Stewart 1979;Catlow & Stoneham 1983;Bush et al 1994) and the resulting defect formation energies gave reasonable agreement with experimental measurements (Oishi & Kingery 1960;Barr & Lidiard 1970;Hashimoto et al 1972;Wuensch et al 1973;Cohen & Gordon 1976). In recent years, the classical *Author for correspondence (d.duffy@ucl.ac.uk).…”