Crystal Phases of Nonohmic Zinc Oxide Ceramics

The crystal phases of nonohmic ZnO ceramics, having Bi2O3, Sb2O3, Co2O3, MnO2 and Cr2O3 as additives, have been investigated by classifying and clarifying them as a function of the amounts of the additives and sintering temperature, and synthesizing their single phases. These ceramics consist of ZnO phase dissolving Co and Mn components, spinel phase of Zn7Sb2O12 dissolving Co, Mn and Cr components, and Bi-rich phases (B, C, D or B'). The B-phase is a δ-Bi2O3 type phase (a=5.59 Å) having a fundamental composition of 12Bi2O3·Cr2O3. The C-phase is a tetragonal phase (a=7.76 Å, c=5.75 Å) having a fundamental composition of 14Bi2O3·Cr2O3. The D-phase is a β-Bi2O3 type tetragonal phase (a=10.93 Å, c=5.62 Å) dissolving a large amount of Zn component and a small amount of Sb component. The B'-phase is also a δ-Bi2O3 type phase (a=5.48 Å) formed in the system Bi2O3–ZnO–Sb2O3. A solid solution between B and B'-is formed. The nonohmic property of ZnO ceramics is improved with the changes of phases, B→C→D or D+B', by raising the sintering temperature. The extinction of the nonohmic property caused by the change in the amounts of additives and sintering temperature corresponds to that of ZnO or Bi-rich phases in the ceramics.