Determination of thermodynamic stability of the Fe 2Ta intermetallic phase by non-stationary e.m.f. measurements on solid galvanic chains

Abstract

The open circuit voltage of the all-solid galvanic chain Pt| Fe + Fe 2 Ta + Ta 2 O 5| ThO 2(+ LaO 1.5| Fe + Fe 0.95 O| Pt and its decay after constant current polarization has been measured and recorded. The decay curves are step-shaped, each rest-potential corresponding to well defined values of the oxygen chemical potential difference across the electrode/electrolyte interfaces. The trend of these curves has been interpreted in terms of the rates of the following processes: 1. (1) interdiffusion of iron and tantalum between bulk and electrode interface 2. (2) reaction of tantalum with oxygen impurities of the gas phase. On this basis, it is possible to define and determine the oxygen chemical potential in equilibrium with the ternary system Fe + Fe 2Ta + Ta 2O 5, corresponding to a rest-potential which is constant over a period of time and is reproducible. Taking into account analogous non-stationary experiments, carried out on a similar cell, in which the ternary system was replaced by the binary Ta + Ta 2O 5 system, the value of the standard molar free energy of formation of the iron-rich boundary intermetallic phase has been calculated. Between 1070 ° and 130O °K the following relationship applies: ΔG ° f, Fe 2 Ta = − 15.21 + 1.06 × 10 −3 T (°K) kcal/mole.