Initial oxidation kinetics near the Curie temperature of nickel

Abstract

The oxidation rate of nickel has been measured in the vicinity of its Curie temperature ${\Theta }_C=631\mathrm{}$ K in flowing oxygen at a pressure of one atmosphere by means of a gravimetric technique. The average oxide thickness $x$ versus time $t$ isotherms at each temperature $T$ could be described by a simple power law: $x\left(t\right)=A\left(T\right)\mathrm{}{t}^{\alpha \mathrm{}\left(T\right)\mathrm{}}$. The dependence of the factor $A\left(T\right)\mathrm{}$ on $T$ is of the form $A\left(T\right)=A_0\exp (-\frac{\Delta }{T})$ with $A_0=2.1\mathrm{}\times {10}^5$ Å and $\Delta =5.1\mathrm{}\times {10}^3$ K and apparently reflects an activated process, while the exponent $\alpha \mathrm{}\left(T\right)\mathrm{}$ increases monotonically with temperature from 0.17 at 570 K to 0.44 at 700 K. Oxidation rates were determined by differentiating the $x$ vs $t$ isotherms at a fixed oxide thickness $x_0$. For $x_0\simeq 115$ Å, the oxidation rate follows two Arrhenius laws. For temperatures greater than ${\Theta }_C$, the bulk Curie temperature of nickel, the activation energy is 36.8±1 kcal/mole, while for temperatures smaller than ${\Theta }_C$ the activation energy is 61±1 kcal/mole. In addition to the break in slope at ${\Theta }_C$, there appears to be a small cusp-like feature near ${\Theta }_C$. The observed modifications of the nickel oxidation kinetics are qualitatively consistent with recent theoretical predictions by Suhl.