Grain growth in sintered Zn0 ceramics

Thesis (Ph. D.)--University of Washington, 1990The effects of the ZnO initial powder particle size and the spinel forming additives, Nb$\sb2$O$\sb5$ and Al$\sb2$O$\sb3$, on the ZnO grain growth processes and grain growth kinetics in pure ZnO and ZnO + 6 wt% Bi$\sb2$O$\sb3$ were studied. Two different initial ZnO particle sizes, 0.11 $\mu$m and 2.0 $\mu$m, were employed. Samples were fired at 900$\sp\circ$C, 1030$\sp\circ$C, 1192$\sp\circ$C, and 1400$\sp\circ$C. The levels of the Nb$\sb2$O$\sb5$ and Al$\sb2$O$\sb3$ additives varied from 0.05 wt% to 0.80 wt%.The presence of the Bi$\sb2$O$\sb3$-rich liquid phase promoted discontinuous grain growth when the initial ZnO particle size was 0.11 $\mu$m. Increasing the initial ZnO particle size, from 0.11 $\mu$m to 2.0 $\mu$m, suppressed the tendency for discontinuous grain growth. These observations were explained in terms of capillary forces excerted by the Bi$\sb2$O$\sb3$-rich liquid phase on the solid particles.Additions of Nb$\sb2$O$\sb5$ to the ZnO + 6 wt% Bi$\sb2$O$\sb3$ system enhances the ZnO discontinuous grain growth when the Nb$\sb2$O$\sb5$ content is less than about 0.40 wt%. However, the grain growth is inhibited and appears normal when the Nb$\sb2$O$\sb5$ content is increased to 0.80 wt%. It is suggested that the grain growth enhancement might be due to enhanced Zn$\sp{2+}$ diffusivity caused by the Nb$\sp{5+}$ ions in solid solution. Grain growth inhibition was attributed to spinel phase grain boundary drag mechanisms.The grain growth inhibition by the Al$\sb2$O$\sb3$ was explained in terms of segregation of the Al$\sp{3+}$ ions to the grain boundaries and by grain boundary drag mechanisms due to the ZnAl$\sb2$O$\sb4$ spinel phase.Higher values of the activation energies for grain growth, from 350 kJ/mol to 400 kJ/mol, are encountered for systems in which the grain growth process is normal, while lower values of the activation energy, from 150 kJ/mol to 35 kJ/mol, occur for those systems with discontinuous grain growth.The effects of the spinel forming additives, Nb$\sb2$O$\sb5$ and Al$\sb2$O$\sb3$, are expressed in terms of a general schematic diagram containing three regions. In the first region, grain growth is inhibited through solute segregation to the grain boundaries creating reduced grain boundary mobility through a solute drag mechanism. In the second region, additive solubility was accompanied by increased lattice vacancy concentration and enhanced cation diffusivity leading to an increased rate of grain growth. The third region is one of grain boundary pinning by spinel crystals at the grain boundaries, inhibiting grain growth. (Abstract shortened with permission of author.