Barium titanate containing glass-ceramics - The effect of phase composition and microstructure on dielectric properties

From a glass with the mol% composition 20.1 Na2O-3 Al2O3-23.1 BaO-23 TiO2-7.6 B2O3-17.4 SiO2-5.8 Fe2O3 cubic barium titanate was crystallized. Scanning and transmission electron microscopy micrographs of the prepared glass-ceramics show fine crystals agglomerated to globular particles. With increasing crystallization time, their volume concentration increases and the particles form interconnected aggregates. From electrical impedance measurements of selected glass ceramics, the electrical conductivity and the dielectric permittivity of the prepared materials were determined for temperatures from -170 degrees C to 97 degrees C, and frequencies from 100 Hz to 100 kHz. The typical values of the resistance are in the order of 10(8) Omega and the dielectric constants in the order of 40 at 100 kHz at room temperature. The two diffuse relaxor type phase transitions observed from -123 to -73 degrees C and from -33 to 0 degrees C are attributed to the transformation rhomboherdral-orthorhombic and orthorhombic-cubic BaTiO3, respectively.