- In-situ measurements of the Liquid-Phase Sintering of Zinc Oxide (2009)
- Polycrystalline ceramic semiconductor devices based on ZnO and several additive oxides show highly non-ohmic current-voltage behavior similar to the Zener diodes. The devices act as an insulator up to a certain electrical field called the breakdown field (EBR) but change into a highly conducting one just above it. Below and above the breakdown field they behave perfectly ohmic. This overall non- linear current-voltage characteristic together with the ability to withstand repeated high power pulses has made metal oxide varistors popular as "surge-arrestors" in electrical circuitry. ZnO doped with Bi2O3 and Sb2O3 (ZBS), is the basic system for ceramic varistors. Phase formation during sintering of ZBS was measured in situ, using 1 mm thick samples and synchrotron X-rays. Thermodynamic calculations were performed to explain phase formation, composition, stability of additive oxides and influence of the oxygen fugacity on sintering. Sb2O4, pyrochlore, trirutile and spinel were formed at temperatures of 500°C to 800°C. The oxidation of antimony was controlled by the oxygen partial pressure and affected both, phase formation and sintering kinetics, in the ZBS system. There are three well defined phases in the final microstructure e.g. the ZnO-grains, Pyrochlore and Spinel phases. The evolution of these phases with temperature and time are important facts to the understanding of the basic functionality of the ZnO varistor system. Liquid phase sintering kinetics in the system ZnO-Bi2O3-Sb2O3 was studied using closed crucibles and an optical dilatometer. The kinetic field technique was modified to compare densification rates with liquid phase sintering models. Grain growths data were derived directly from the kinetic field diagram and compared to microstructure analysis of quenched samples. A reasonable agreement was obtained between both techniques - demonstrating that the modified kinetic field technique is an efficient tool for process optimization.