Resistance relaxation studies of gas/metal reactions leading to simultaneous dissolution and gasification. The dissociated oxygen/tantalum system above 2000 K. Part 2.—Mechanism, kinetics and energetics of chemisorption, interface crossing and product desorption

Abstract

The experimental methods of Part 1 are exploited to obtain both instantaneous O(abs) concentrations and metal oxide desorption rates for Ta(s) filaments at T_w > 2100 K exposed to step-function oxygen pressures between 1.6 × 10^–2 and 0.67 Pa. The observed transient engassing rates imply a reaction mechanism involving oxygen adatom intermediates, from which are derived elementary rate constants for O-atom passage across the gas/metal interface and the formation/desorption of each of the reaction products: TaO(g) and TaO₂(g). These kinetic data, together with the O, O₂ chemisorption probabilities reported in Part 1, lead to the following energetics of the oxygen/tantalum system above 2000 K: (i) O(ad)→ O(abs) experiences an energy barrier of about 290 kJ mol^–1, and (ii) the exothermicity of the chemisorption reaction O(g)→ O(ad) is about 720 kJ mol^–1.