The mechanism of abnormal grain growth in Sr0.6Ba0.4Nb2O6 ceramics

Abstract

Strontium barium niobate, Sr0.6Ba0.4Nb2O6 (SBN40) ceramics were prepared by the mixed oxide route; specimens sintered at 1300 °C–1450 °C for 2–4 h had densities of ∼95% theoretical. The use of “conventional’’ conditions (1300 °C; ∼200 °C below the melting temperature) gave rise to a duplex structure comprising a mixture of small grains and abnormally large grains (in excess of 100 μm). Transmission electron microscopy studies revealed a Nb-rich, Ba-poor phase at the grain boundaries; the low melting temperature of this phase caused localized liquid phase sintering, resulting in abnormal grain growth. Dual-stage sintering (sintering at 1250 °C for 4 h, followed by sintering at a temperature in the range 1350 °C–1450 °C) inhibited abnormal grain growth. The use of a fast heating rate (600 °C/h) and dual-stage sintering (1250 °C and 1450 °C) yielded samples with good transparency and preferential orientation along the c-direction of the tetragonal unit cell. The dielectric properties depended sensitively on microstructure. Specimens with the highest densities had the highest relative permittivities (2400–2750); specimens with more uniform distribution of grain sizes had sharper dielectric maxima. A mechanism for abnormal grain growth is proposed.