Diffusion enthalpies and entropies in thermally forming NiO and (Ni, Fe)O from 800–1200°C

Abstract

The oxidation of pure Ni and a Ni-1.98 wt.% Fe alloy in air between 800–1200°C has been investigated gravimetrically using a thermobalance. Parabolic reaction kinetics were observed and analysed by the Arrhenius–Zener and Eyring theories. The enthalpies and entropies of activation for the Ni were 174.4 ± 13.6 kJ mol^–1 and –28.4 ± 10.9 J mol^–1 K^–1, and 164.1 ± 13.6 kJ mol^–1 and –46.0 ± 10.9 J mol^–1 K^–1 for the Zener and Eyring theories respectively. For the alloy the enthalpies and entropies of activation were 206.8 ± 8.1 kJ mol^–1 and 36.8 ± 6.4 J mol^–1 K^–1, and 196.3 ± 8.0 kJ mol^–1 and 19.1 ± 6.3 J mol^–1 K^–1 for the Zener and Eyring theories respectively. Enthalpies calculated by the two theories are similar but the difference in entropies is large. The enthalpies account quantitatively for the diffusion processes occurring during oxidation of Ni. Zener's theory provides a physical basis for understanding entropy changes during diffusion and the entropies are consistent with the theory.