Study of metal--oxygen solid solutions utilizing solid electrolytic cells

Abstract

A study was made of the thermodynamic and kinetic behavior of oxygen in a number of Group V metals and alloys. Investigations were made with a ThO2Y2O3 electrolyte over the temperature range of 600 to 1150°to 1423°K). The activity of oxygen in three Nb--Ta alloys (nominally 25, 50, and 75 at. % Ta) was found to obey Henry's Law up to the solubility limit in each alloy. The standard entropy and enthalpy of solution of oxygen in the three Nb--Ta alloys were intermediate between those for pure niobium and pure tantalum, and varied almost linearly with Nb : Ta content. The diffusion coefficient of oxygen was measured in niobium and vanadium, and the values were found to be in excellent agreement with literature values obtained by a variety of techniques. The diffusion coefficient of oxygen in each of several dilute substitutional niobium alloys was measured and compared to the diffusion coefficient in pure niobium. The addition of 1 to 5% substitutional solutes resulted in as much as an order-of-magnitude decrease in the oxygen diffusivity. This decrease is believed to be due to trapping of oxygen by substitutional solute atoms. The substitutional-oxygen binding or ''trap'' energies (in eV) for several substitutional solutes in niobium were determined to be: Ta: 0.3 ± 0.1; V: 0.55 ± 0.05; Ti: 0.7 ± 0.1; Zr: 0.7 ± 0.05. The trap energy is rationalized as being the sum of a chemical interaction and an elastic interaction. 27 figures, 9 tables.