Fabrication and Dielectric Properties of (Ba_0.7Sr_0.3)TiO₃--Glass Composites

Bulk and thick-film (Ba_0.7Sr_0.3)TiO₃ (BST)--glass composites with different dielectric properties were fabricated at 1000 °C from powder mixtures of a sol--gel-derived BST and a commercial glass (GP). Increasing the calcination temperature of the BST gel powder from 850 to 1000 °C resulted in an increase in the sintered density of the composites with simultaneous increases in dielectric permittivity ($\varepsilon_{\text{r}}$) and its temperature coefficient. Similar changes were observed in composites with different GP contents. A substantial decrease in $\varepsilon_{\text{r}}$ with a broadened $\varepsilon_{\text{r}}$--temperature curve for the 12% GP composite could be attributed to the inhibited densification and enhanced formation of the secondary fresnoite phase. The dielectric properties and dielectric tunability of the composites were examined in relation to their relative density ($d_{\text{rel}}$), crystallinity ($L_{\text{app}}$), and relative fraction of the BST phase ($f_{\text{BST}}$). Although changes in $\varepsilon_{\text{r}}$, its temperature dependence, and tunability were nominally correlated with the $d_{\text{rel}}$ change of the sintered composites, the result that the $d_{\text{rel}}$ of the composites was basically an increasing function of $L_{\text{app}}$ and $f_{\text{BST}}$ suggested the substantial contributions of $L_{\text{app}}$ and $f_{\text{BST}}$ to those changes.