Preparation and characterization of dielectric glass-ceramics in Na ₂O-PbO-Nb₂O₅-SiO₂ system

Glass-ceramics in the Na₂O-PbO-Nb₂O ₅-SiO₂ system have been synthesized to produce bulk materials with nanometer-sized crystals grown in a glass phase via roll-quenching followed by controlled crystallization. X-ray diffraction studies indicate that Pb₂Nb₂O₇, NaNbO₃ and PbNb₂O₆ phases are formed from the as-quenched glass in temperature range from 750 to 900 °C. Pb₂Nb₂O ₇ crystallizes at 750 °C and disappears at 850 °C, NaNbO ₃ is the primary phase at 850 °C, while PbNb₂O ₆ is formed at a higher temperature of 900 °C. The dielectric properties of the glass-ceramics formed through controlled crystallization have a strong dependence on the phase assemblages that are developed during heat treatment. The highest dielectric constants (> 600) were found in samples annealed at 850 °C for 3 h. Microstructural observation shows that randomly oriented, nanometer-sized crystalline are found with residual glass concentrated at crystallite boundaries. Further studies by scanning tunneling electron microscopy (STEM) in conjunction with energy dispersive spectroscopy (EDS) reveals inhomogeneous distribution of NaNbO₃ and PbNb ₂O₆ in the sample annealed at 850 °C for 3 h and these phases contribute to the high dielectric constant.