Effects of Heat-Treatment on Crystal Phases, Microstructure and Electrical Properties of Nonohmic Zinc Oxide Ceramics

Abstract

An investigation was carried out of the effects of heat-treatment (h.t.) on the crystal phases, microstructure, and nonohmic properties of ZnO ceramics consisting of ZnO containing dissolved Co and Mn; Zn7Sb2O12 spinel (SP) containing dissolved Co, Mn, and Cr; and β, δ-Bi2O3 containing dissolved Zn and Sb. The properties change with h.t. temperature, and the changes are closely related to each other. The nonohmic exponent α decreases greatly at 700° to 800°C, where the intergranular (i.g.) layer is coarsened by the phase transformation of β, δ-Bi2O3 into γ-Bi2O3. The α-value again changes abruptly and almost recovers at 800° to 900°C, where the dense i.g. layer is reconstructed from the Bi2O3-rich liquid (B.r.l.) formed by the melting of γ-Bi2O3. The α-value gradually recovers at 900° to 1100°C with the decrease of Cr in δ-Bi2O3 solid solution (δ-S.S.) and the decrease of Zn2Bi3Sb3O14 pyrochlore (PY) by reaction with ZnO. The formation of δ-S.S. and the decrease of Cr in it are brought about by the movement of Cr from the SP to the B.r.l. around 900°C and from the B.r.l. to the SP above 900°C, respectively. The PY is formed by partial reaction between the SP and the B.r.l.around 900°C. The phases, microstructure, and α-value recover completely around 1100°C.