Mechanism of internal reduction in MgO-NiO solid solutions

Kirk Alan Rogers, Purdue University

Abstract

Internal reduction of $\rm (Mg\sb{1-x},Ni\sb{x})O$ solid solutions results in a variety of microstructures that may be useful in enhancing the toughness of MgO, in making MgO-bonded Ni catalysts or optical devices. In this study, (Mg,Ni)O powders were produced by co-precipitation from the carbonates, then sintered to nearly theoretical density. Specimens were then reduced over a wide range of temperatures, from 800 to 1350$\sp\circ$C and microstructure and kinetics were observed. The variety of microstructure extends from submicrometer Ni particles to a micrometer-scale, long-range interpenetrating vermiculite structure. Reduction above one-half of the homologous temperature results in uniformly distributed Ni particles, for x $<$ 0.30 and in interpenetrating networks for x $\ge$ 0.30. Reduction below that temperature results in Ni particle formation primarily at the grain boundaries. The kinetics of reduction observed decrease with increasing x, and are an order of magnitude faster than observed previously with single crystals. The rate decrease with composition appears counter intuitive, but may arise from a fixed defect flux from the surface to the reaction front. The density changes and kinetics cannot be realized with the Wagner mechanism, so a mechanism based on Schottky defect formation has been proposed.

Degree

Ph.D.

Advisors

Trumble, Purdue University.

Subject Area

Materials science|Metallurgy

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