High temperature low pressure reactions of oxygen with tantalum as studied by Auger electron spectroscopy and line-of-sight mass spectrometry

Abstract

The oxidation of a polycrystalline tantalum ribbon has been studied at low oxygen pressures (⩽ 10^–4 Pa) from 1300 to 2300 K using Auger electron spectroscopy A.E.S. to determine the surface oxygen coverage θ, line-of-sight mass spectrometry to measure the oxide evaporation fluxes of TaO and TaO₂, and electrical resistivity to measure the dissolved oxygen concentration. For a 10^–4 Pa oxygen pressure, three temperature regions are observed by A.E.S.: (i) < 1500 K the precipitation of Ta₂O₅, (ii) from 1500 to 1750 K the formation of TaO on the surface, and (iii) > 1800 K the adsorption of oxygen on the tantalum surface above the dissolved layer. In this latter temperature region, A.E.S. provides a direct measure of the oxygen surface coverage. In this high temperature region the activation energy for TaO evaporation was found to be 710–105 θ kJ mol^–1, that for TaO₂ evaporation 822–209 θ kJ mol^–1, and the endothermic heat of reaction passing from an adsorbed to a dissolved oxygen atom was determined to be 121–59 θ kJ mol^–1.