Resistance relaxation studies of gas/metal reactions leading to simultaneous dissolution and gasification. The dissociated oxygen/tantalum system above 2000 K. Part 1.—Methodology and the role of atomic oxygen

Abstract

By measuring the electrical resistance of tantalum filaments (i) during isothermal reaction with oxygen, (ii) after isothermal degassing, and (iii) after rapid temperature quench, we infer both the instantaneous dissolved oxygen concentration and the metal " gasification " rate. For the conditions : 1.6 x lo-' Q p(oxygen) Q 0.67 Pa, 2400 K < Tw < 2800 I ( , the following mechanistic conclusions are drawn concerning the production/desorption of metal oxides [TaO(g), TaO,(g)] and the cause of the observed enhanced reactivity of microwave discharge-produced atomic oxygen : (a) under conditions of steady-state O(abs)-concentration and oxide desorption rate, the mechanisms of metal oxide production/desorption in dissociated and undissociated gaseous oxygen are identical ; (b) enhanced chemisorption probability accounts for increased oxygen coverage and tantalum volatilization. rate in atomic oxygen. Part 2 includes the extraction of elementary rate constants for interface penetration and oxide desorption based on transient electrical resistance data following the isothermal exposure of tantalum to step-function increases in oxygen pressure. * This neglects the small dependence that & T ~ might have on the solute concentration c. When c < 1 and the corresponding adatom coverage 8 is small, this dependence is indeed negligible; cf. Section 5.