Initial oxidation rate of nickel and effect of the curie temperature

Abstract

Nickel foil was oxidized for 24 hr periods within the temperature range 307° to 442°C producing NiO films up to 3400 Å thick maximum. A discontinuity in oxide growth is observed at the Curie temperature (353°C). Data obtained at 390°, 394° and 405°C as a function of time show that a two-stage oxide growth rate occurs, a higher rate following an initially lower rate. Both stages are represented by a linear relation between oxide thickness and logarithm of time. Many initial oxidation rate data obtained by other investigators, for Ni and a variety of metals, also follow the two-stage logarithmic equation. Based on control of the initial oxidation rate by electron transfer from metal to oxide, the discontinuity of oxide thickness at the Curie temperature is explained by an observed work function of nickel slightly higher above the Curie temperature than below. The change of work function also explains a slightly higher activation energy for oxidation above the Curie temperature than below. Calculations show that the density of trapped electrons in a constant density space charge next to the metal surface is 1.3 × 10 16 at 390°C. Density of available sites for trapped charge in a diffuse charge layer, which grows on top of the constant density layer, is the same order of magnitude (5.5 × 10 15). A higher density of sites for NiO than for Cu 2O, plus a higher work function for Ni than for Cu, are in line with a lower oxidation rate of Ni compared to Cu. It is calculated that contact of oxide with Ni or Cu lowers the work function at the metal-oxide interface by about 0.6 V. This value agrees, in the case of Cu 2O, with the energy required to excite an electron from the filled band to the conduction band.