GRAIN-GROWTH OF ZNO IN ZNO-BI2O3 CERAMICS WITH TIO2, ADDITIONS

Grain growth of ZnO in liquid-phase-sintered ZnO-Bi2O3 ceramics contai ning systematic additions of TiO2 to the 0.55 wt% level has been studi ed for firing in air from 900 degrees to 1400 degrees C. The Bi2O3 mel ts during firing, and then the TiO2 dissolves into the Bi2O3-rich liqu id. The TiO2 initially reacts with the Bi2O3 to form the compound Bi4T i3O12. Above similar to-1050 degrees C, the Bi4Ti3O12 reacts with ZnO to form the Zn2TiO4 spinel phase. This results in two distinct regions of ZnO grain growth in this system: one below 1050 degrees C, where t he Bi4Ti3O12 affects the process, and the other above 1050 degrees C, where the Zn2TiO4 spinel dominates. The exponent for ZnO grain growth decreases from 5 for TiO2-free ZnO-Bi2O3 to 3 for ZnO-Bi2O3-TiO2 at te mperatures below 1050 degrees C, but increases to 6 at temperatures ab ove 1050 degrees C. The exponents are related to the presence of Bi4Ti 3O12 in the Bi2O3-rich liquid phase below 1050 degrees C and the Zn2Ti O4 particles that are present above 1050 degrees C. The activation ene rgy for grain growth of the ZnO is similar to 300 kJ/mol below 1050 de grees C, but 360 kJ/mol above 1050 degrees C. It is suggested that dif fusion of ZnO through the Bi2O3-rich liquid phase is the rate-controll ing mechanism for ZnO grain growth below 1050 degrees C and that grain -boundary drag by the pores and Zn2TiO4 spinel particles is the rate-c ontrolling mechanism above 1050 degrees C.