Resistance relaxation studies of gas/metal reactions leading to simultaneous dissolution and gasification. The dissociated oxygen/tantalum system above 2000 K

The experimental methods of Part 1 (Rosner et al., JCS Faraday I; 72:842(1976)) are exploited to obtain both instantaneous 0(abs) concentrations and metal oxide desorption rates for Ta(s) filaments at Tsub(w) > 2100 K exposed to step-function oxygen pressures between 1.6 x 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 0-atom passage across the gas/metal interface and the formation/desorption of each of the reaction products: Ta0(g) and Ta02(g). These kinetic data, together with the 0, 02 chemisorption probabilities reported in Part 1, lead to the following energetics of the oxygen/tantalum system above 2000 K: (i) 0(ad) → 0(abs) experiences an energy barrier of about 290 kJ mol-1, and (ii) the exothermicity of the chemisorption reaction 0(g) → 0(ad) is about 720 kJ mol-1. (author)