Giant dielectrics from modified boundary layers in $n-BaTiO_3$ ceramics involving selective melting reactions of silver/glass composites at the grain boundaries

High permittivity ceramics with $\epsilon_{eff} > 10^5$ can be realized from semiconducting $BaTiO_3$ by the two-step processing namely, sintering the donor doped samples in static air followed by electroding with the fired-on silver/glass composites. Doping with $Sb^5^+$ and $Bi^3^+$ not only enhances the grain conductivity but also increases the grain size $(10–60 \mu m)$, when sintered at $1370 ^oC$ in static air. The ceramic samples are electroded with the paste containing nanometer particles of silver dispersed in varied amounts of low melting $(600-900 ^oC)$ glass compositions $PbO + Bi_2O_3 + B_2O_3\pm SiO_2 \pm CuO$. High permittivities are obtained for these capacitors stable over a wider range of temperature and over a broad frequency range. The grain boundary layer effect superimposed with the contributions from the barrier layers formed during electroding, related to ceramic microstructure is proposed to be responsible for the unusual high permittivity in semiconducting $BaTiO_3$. The energy dispersive X-ray analyses indicate selective melting reactions at the grain boundary layers with higher concentrations of the low melting oxides at the grain boundaries near to the electrodes. Impedance spectroscopy on $BaTiO_3$ ceramics demonstrates that they are electrically heterogeneous with insulating grain boundaries together with the ceramic/electrode interface acting as barrier layers. On the basis of the symmetrical Schottky-barrier model of the grain boundary region, the barrier height $\phi$ and donor concentration $N_d$ of the grains were obtained by the modified $1/C^2-V$ plot. These modified boundary layer capacitors having high field strength withstandability can be used in a wide range of frequencies