Formation Mechanism of Nonohmic Zinc Oxide Ceramics

For the formation of nonohmic ZnO ceramics based on the system ZnO-Bi₂O₃-Sb₂O₃, the reactions on heating (→) and on slow cooling (←), 2PY+17ZnO\xleftarrow\xrightarrow950°∼1050°C3X_zso+3Bi₂O₃ [L] \ding"C0, and X_zso\xleftarrow\xrightarrow\gtrsim1300°CSP \ding"C1, play important roles, where PY is Zn₂Bi₃Sb₃O₁₄ pyrochlore, X_zso a polymorph of Zn₇Sb₂O₁₂ [SP] and L is a liquid. Oxide additives markedly influence these reactions and consequently the phases and microstructure of the ceramics, dissolving into the above phases and relating to composition and sintering conditions. In the case of Co, Mn and Cr oxides, the degree of influence is as follows : Co<Mn\lesssimCo+Mn\lesssimCr<Co+Mn+Cr \ding"C2. On heating, those reactions occur at lower temperatures in this order, and finally for Cr oxide the X_zso is not formed. On slow cooling after the SP is formed, the reaction \ding"C1 and then \ding"C0 partially occur only for Co oxide; for the others, the reaction, 3SP+3Bi₂O₃ [L]→2PY+17ZnO, occurs only partially and the degree of this reaction becomes smaller in that order \ding"C2; the nonreacting liquid crystallizes into Bi₂O₃-rich phases. On quenching, only the crystallization of the liquid occurs. The formation of various Bi₂O₃-rich phases is related to composition change of the liquid and cooling manner.