Kinetics of evaporation/oxidation of iridium

Abstract

Iridium wires self-resistance-heated to temperatures in the range of 1675 to 2260°C (1948 to 2533 K) were oxidized in naturally convected oxygen and air at pressures in the range of 9.9×10−8 to 1.32 atm (10−2 to 1.34×105 Pa) and in force convected oxygen and air at pressures in the range of 4.6×10−3 to 0.84 atm (470 to 8.5×104 Pa). The experimental results were closely correlated by a theoretical rate equation based on control of the oxidation rates by the rates of evaporation of Ir(g), IrO2(g) and IrO3(g) and by the rates of their subsequent diffusion through the gaseous boundary layer. Apparent values were obtained for the standard state free energies of formation of IrO2(g) and IrO3(g) and were correlated linearly with temperature.