Oxidation kinetics of large nickel particles

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Oxidation kinetics of large nickel particles Richard Karmhag and Gunnar A. Niklasson Department of Materials Science, Uppsala University P.O. Box 534, S-751 21 Uppsala, Sweden

Mats Nygren Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University S-10691 Stockholm, Sweden (Received 20 July 1998; accepted 15 March 1999)

Oxidation of polycrystalline nickel particles with an approximate median diameter of 158 ␮m has been studied by thermogravimetric measurements in the temperature range 773–1473 K. The oxidation was found to be thermally activated with an apparent activation energy of about 1.9 eV at temperatures below 1073 K and 1.2 eV at higher temperatures. Our data showed the qualitative features expected for oxidation of spherical metal particles. The kinetics was compared with a homogenous field coupled-currents theory for oxidation of spherical metal particles. Calculations using a median particle size or a distribution of particle sizes could only give a satisfactory fit to part of the experimental data. Possible explanations for the deviations in terms of space charge, grain boundary diffusion, grain growth, and sintering are discussed in this paper. Scanning electron microscopy studies of the particles after oxidation showed that a large difference exists in the surface structure and the degree of sintering between the particles oxidized at low and high temperatures.

I. INTRODUCTION

II. EXPERIMENTS

Thermal oxidation of bulk metals and thin films has been studied for a long time. The research has been summarized in several books and review articles.1–10 To characterize the oxidation process in detail, measurements over a large temperature range and covering different time scales are needed. The functional dependence of the oxidation kinetics has to be accurately determined to be able to compare with theoretical expressions. In this article we present a detailed study of the oxidation kinetics of nickel particles of an average diameter of 158 ␮m. Only a few studies on oxidation of metal particles exist. Theories based on diffusion11 and on the coupledcurrents approach12 have been suggested. A previous experimental work on the oxidation kinetics of nickel particles with a diameter of 149 ␮m by Carter11 will be compared with our data. This work, however, covers a much wider range of temperatures (i.e., from 773–1473 K). We superimpose all data in a single master plot as in an earlier work of ours to reveal changes in the kinetic exponent.13 In the following discussion we describe our experimental setup and the characterization of our samples. The results are reported in the results section, and a discussion of the applicability of existing theories11,12 follows the results section.

A. Sample characterization

J. Mater. Res., Vol. 14, No. 7, Jul 1999

http://journals.cambridge.org

Downloaded: 13 Mar 2015

The nickel particles used in the experiments were provided by INCO and were >99.9% pure. An impurity analysis14 for a number of elements is