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Date of Award

6-1972

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Metallurgy and Materials Science

Supervisor

W.W. Smeltzer

Abstract

In this thesis, the growth and structure of nickel oxide on three single crystal faces of nickel, viz the (100) (110), and (111), in the temperature range from 500 to 800°C have been investigated.

A study has been made on the morphological development of the oxide and its structures by electron microscopy, X-ray diffraction and scanning electron microscopy in order to correlate these structures with the kinetics. Measurements of the kinetics for oxide growth on the different nickel crystal faces have shown non-parabolic behaviour and marked anisotropy in reaction rates due to the differences in crystallographic orientations. The deviation from parabolic behaviour and anisotropy in reaction rates are shown to be associated with the structural defects arising in the oxide layer during the growth process, epitaxial relationships between the metal and oxide, and orientation relationships between the oxide grains in the scale. The development of epitaxy can be justified in terms of the coincidence of a close packed direction between the metal and the oxide. Preferred growth leads to the formation of a duplex scale.

A model for growth of the nickel oxide films and scales based on the simultaneous diffusion of nickel across the films via lattice defects and short-circuit paths is advanced to correlate the oxidation kinetics with the oxide structure. It is assumed in this model that the decrease in the density of the short-circuit diffusion sites at nickel oxide grain boundaries is determined by oxide grain growth processes. This model is shown to account satisfactorily for the observed oxidation kinetics of the (100) and (111) nickel faces. Also, the values for nickel grain boundary diffusion coefficients have been calculated for the first time for oxide on these two faces.

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