Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Materials Science and Engineering


Electrical switching of some polycrystalline ZnO based ceramics was investigated. The electrical characteristics of multicomponent, two-phase, and single-phase materials were related to the microstructures and defect structures of those materials;Subsequent to the study of the multiphase microstructure of multicomponent switching materials, a simplified two-phase threshold switching material was produced consisting of Co-doped ZnO and Zn-doped PrCoO(,3) phases. The roles of individual grains and interphase boundaries in the switching behavior of the two-phase threshold switching material were identified using microelectrodes deposited photolithographically on a thin section of the material. Pristine Co-doped ZnO grains showed high 'off' state resistance and a high 'threshold' voltage drop for the initial voltage sweep and were permanently conductive for subsequent sweeps. Zn-doped PrCoO(,3) grains exhibited stable and reproducible threshold switching behavior. Through the study of the electrical characteristics of single-phase polycrystalline undoped and Co-doped ZnO materials and undoped and Zn-doped PrCoO(,3) materials, the microelectrode observations of the switching behavior of the two-phase material were confirmed. Undoped ZnO showed n-type conductivity with resistivity of around 1 (OMEGA)-cm. Doping ZnO with Co increased the resistivity. It was established through a series of studies that the resistivity increase in ZnO accompanying doping with Co was not due to blocking contacts. A model was proposed for this resistivity increase. Undoped and Zn-doped PrCoO(,3) showed p-type conductivity with a resistivity of around 100 (OMEGA)-cm. Doping PrCoO(,3) with Zn decreased the resistivity. A blocking contact was observed between PrCoO(,3) and electrode metals having low work functions;The Hunter model for switching behavior of ZnO-based materials was tested on the basis of the observed microstructure in the two-phase material, and was found not to be appropriate. A microscopic model of the switching behavior of the two-phase materials was proposed.



Digital Repository @ Iowa State University,

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Kwangsoo No



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205 pages