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Title: Microstructural-property relationships of zinc oxide varistors
Author: Hng, H. H.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1999
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Characterisation of Bi2O3-, V2O5- and Pr6O11-doped ZnO varistor systems has been undertaken in this study. Bi2O3-, V2O5- and Pr6O11 are categorised as varistor formers, and without them ZnO will not exhibit varistor behaviour. The samples were prepared by conventional mixed-oxide ceramic technology. A variety of analytical techniques including X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used in this study. The electrical properties of the materials were characterised by their current-voltage behaviour. The investigation of each varistor system could be considered individually with its own objectives in order to gain a better understanding of each system. The research is also aimed at a comparison of the behaviour of the relatively new ZnO-V2O5 system and the less characterised ZnO-Pr6O11 system with the extensively studied ZnO-Bi2O3 system. The investigation of the ZnO-Bi2O3 system was based on the study of a multi-component system containing Sb2O3, MnO2 and Co3O4 cooled at different rates from a sintering temperature of 1200°C. The different cooling rates affected the electrical properties significantly. These could be explained in terms of the microstructure changes, particularly with respect to the different Bi-rich phases present. Fast cooling resulted in a decrease in the non-linear coefficient, α. In addition, the characterisation of the faceting at ZnO-δ-Bi2O3 interphase boundaries in the water-quenched specimens provided an insight to the anisotropic growth of ZnO during the sintering process. An attractive feature of the ZnO-V2O5 system is that the ceramic can be sintered at a relatively low temperature of ˜ 900°C. The effects of additive oxides (MnO2, Co3O4 and Sb2O3) commonly found in Bi2O3- doped ZnO varistor systems were studied in a ZnO - 0.25 mol% V2O5 system. The addition of MnO2 significantly improved the varistor behaviour. Although the system containing all the different additives produced the best varistor behaviour, the addition of Sb2O3 retarded the sintering process and a sintering temperature ≥ 1200°C had to be used. The varistor behaviour could also be improved by increasing the V2O5 content to 0.5 mol%. α-, β- and γ-Zn3(VO4)2 polymorphs as suggested by Brown and Hummel (1965) were detected in the samples as a secondary phase using XRPD.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available