The relationship between microstructure and dielectric constant of $0.8Pb(Zn_{1/3}Nb_{2/3})O_3-0.1BaTiO_3-0.1PbTiO_3$ (mol) has been investigated. On varing PbO content, the microstructure and dielectric constant of specimens changed considerably. A perovskite single phase was formed in a specimen with 0.5 wt% PbO deficiency. When the PbO content was increased from this value, a liquid phase formed and was present between perovskite grains. With increasing PbO deficiency in the specimens, on the other hand, a pyrochlore phase and a ZnO phase were formed, as second phases. The size of pyrochlore grains was much larger than that of perovskite grains. The densification was much suppressed with decreasing PbO content. The dielectric constant showed the maximum value at -0.5 wt% PbO content. The presence of a second phase and the reduction in sintered density reduced considerably the dielectric constant. The liquid phase, however, was found to be worse than the pyrochlore phase for dielectric constant of the material.
The effect of heat-treatment of sintered specimens was also studied. After sintering of the specimens with 0.5 wt% PbO deficiency at 1050℃ for 1h, the sintered specimens were heat-treated at 900℃, 950℃, 1000℃, 1025℃, or 1100℃ for 7h. The crystal structure and the dielectric constant of the sintered specimen changed significantly with heat-treatment temperature. With increasing heat-treatment temperature above 1000℃, the tetragonality of the specimen increased. With the tetragonality increase, the dielectric constant also increased from 22,000 (1000℃) to 26,000 (1100℃). In contrast to these changes in tetragonality and dielectric constant, the average grain size and the sintering density were not changed up to the heat-treatment temperature of 1025℃. It appears therefore that the main cause of the increase in dielectric constant by heat-treatment was the tetragonality increase.
The increase in tetragonality during the heat treatment was studied by convergent electron beam diffraction (CBED). It was found that a grain consisted of a nucleus, a paraelectric region, and a ferroelectric region. The nucleus contained more Nb than the other regions. The nucleus had a tetragonal structure with a(=b)/ c ratio of 1/ 0.9755. The ferroelectric region had also a tetragonal structure with a(=b)/c ratio of 1/1.0219. During heat-treatment of the sintered specimens, low permittivity phases (nucleus and paraelectric region) became smaller, and high permittivity ferroelectric region grew in the grains. The increase in the ferroelectric region which had a tetragonal structure with one axis larger than the other two led to increase in tetragonality and dielectric constant. By a heat-treatment at 1100℃ for 7h, the nucleus and the paraelectric region disappeared completely in the grains; the dielectric constant increased by about 25%.
