The Electrical Properties of Grain Boundaries in Nickel-Oxide and Magnesium-Oxide

CARLTON MORRIS OSBURN, Purdue University

Abstract

The study of the effect of grain boundaries on the electrical properties of materials has been expanded to include oxide materials. Single crystals, bicrystals, and polycrystals of nickel oxide and magnesium oxide were studied from room temperature to 1600°C over the oxygen pressure range of their phase field. Conductivity, di- electric constant, Seebeck coefficient, and thermogravimetric measurements were taken.The predominant defect in pure Nio is a doubly- ionized nickel vacancy. The activation energies for conduction and weight change are the same 18.6 kcal/mole. The hole mobility at high temperatures is 0.53 cm²/V-sec. and is almost temperature independent. The density of states in Nio at high temperatures varies as T3/2 supporting a wide band conduction model; however, both the magnitude and temperature independence of the mobility point to a model between a polaron band and a narrow band conductor.Below the temperature of equilibration with the atmosphere, the conductivity was almost temperature independent in pure material. Most of the samples, however, contained donor impurities, presumably iron, which increased the resistivity by up to six orders of magnitude and increased the temperature dependence of the resistivity by two orders of magnitude. A computer solution for the concentrations of seven species in a doped material was obtained using a neutrality condition containing five charged species. The hole concentrations obtained in this fashion agreed very well with the experimentally- measured resistivity.Changes (of over two orders of magnitude) in the grain boundary resistance of Ni0 relative to the crystal resistance were observed. The effect was a function of temperature and oxygen pressure but not a function of grain boundary orientation or direction of transport. The effective width of the grain boundary was of the order of ten mils and was explained in terms of impurity segregation and increased grain boundary diffusion. Computer calculations agreed with experimental results.In magnesium oxide at high temperatures, the high oxygen pressure conductivity varies as the one-fourth power of the oxygen partial pressure with a 3 ev activation energy; the low pressure conductivity varies as the minus one-sixth power of the pressure with a 4.4 ev activation energy. At high pressure the predominant defects are presumed to be holes and singly-ionized magnesium vacancies; at low pressures, electrons and doubly- ionized oxygen vacancies are believed to be predominant. The low pressure conductivity is electronic in nature while the high pressure conductivity is mixed. In this study increased temperature and pressure favor ionic conduction. Diffusion of cations in MgO takes place via singly-ionized magnesium vacancies. Impurities tend to increase the conductivity and decrease the oxygen pressure dependence of the conductivity; they can either increase or decrease the ionic conductivity. The possible effect of impurities on earlier studies is noted; however, a basic disagreement still exists on the ionic transference numbers. A large frequency dispersion of the conductivity is observed at low temperature; dispersion of the dielectric constant is observed at high temperatures.Changes (of over 101) in the grain boundary conduc. tance of MgO relative to the crystal conductance were ob served. As with Nio the effect was a function of temperature and pressure but not of grain boundary orientation or direction of transport. The magnitude of the effect in MgO was not affected by the total impurity content. An increased high pressure ionic conductivity was observed in polycrystalline material offering possible evidence for increased grain boundary diffusion.

Degree

Ph.D.

Subject Area

Electrical engineering

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