The influence of stoichiometry (Ba/Ti ratio) on the densification, grain growth and its dielectric properties of $BaTiO_3$ were investigated. The $BaTiO_3$ powders were prepared by conventional calcination of $BaCO_3$and $TiO_2$. The stoichiometric ratios(Ba/Ti) were in the range 0.970 to 1.005.
The sintering behavior of $BaTiO_3$ has been studied extensively but an understanding of stoichiometric effects on densification is still incomplete. The excess of $TiO_2$ lowered the onset temperature of sintering (initial state of sintering;3% shrinkage). These results indicate that the stoichiometry variation of $BaTiO_3$ affect the initial state of sintering. The rate of densification for Ti-rich samples were considerably faster than those for stoichiometric and Ba-rich sample. It has been so-called “activated” sintering. The $TiO_2$ excess reacts with $BaTiO_3$ to $Ba_6Ti_{17}O_{40}$ which forms with $BaTiO_3$ a eutectic melt at 1320℃. The liquid phase, however, enhanced grain growth, not densification.
Below the eutectic temperature (1320℃), grain growth was strongly inhibited with increasing amounts of $TiO_2$ excess. The inhibition of grain growth caused abnormal grain growth due to inhomogeneous distribution of a Ti-rich second phase. With increasing $TiO_2$ excess, the matrix grain sizes were decreased and the onset temperature of grain growth was elevated. And also abnormally grown grain shapes were changed with $TiO_2$ excess. These results mean that the Ti-rich second phase strongly inhibits grain growth.
Many researchers have investigated the grain size effects on dielectric properties with average grain size neglecting bimodal grain size distribution. The study on the ferroelectric phase transition of this bimodal grain size distribution effect has not been at all thorough, perhaps, because nobody would expect this bimodal grain size distribution to behave differently from better known average grain size effects.
As a result of precisely observation of specimens which have bimodal grain size distribution, double Curie peak which represent mixing of broad and sharp peak was observed in $BaTiO_3$ ceramics. The double Curie peaks varied with the volume fraction of large grains and with the size of small matrix grains. Also the dielectric loss peaks clearly represented double step. These double peaks mean that the Curie peaks which correspond to the large grains and small grains are mixed.
In spite of some contradictory result on Tc shift with grain size variation, double Curie peak clearly represents that the Tc is lowered with decrease of grain sizes. These results could be explained by internal stress model. Therefore Tc lowering, $\epsilon_{max}$ suppression and Curie peak broadening as a decreasing grain size also can be explained by our suggestion (Tc distribution of each grains are more broadened and compressive stresses increase with decreasing grain size).
