THE MECHANISM OF ABNORMAL GRAIN-GROWTH IN SR0.5BA0.4NB2O6 CERAMICS

Strontium barium niobate, Sr0.6Ba0.4Nb2O6 (SBN40) ceramics were prepar ed by the mixed oxide route; specimens sintered at 1300 degrees C-1450 degrees C for 2-4 h had densities of similar to 95% theoretical. The use of ''conventional'' conditions (1300 degrees C; similar to 200 deg rees C below the melting temperature) gave rise to a duplex structure comprising a mixture of small grains and abnormally large grains (in e xcess of 100 mu m). Transmission electron microscopy studies revealed a Nb-rich, Ba-Door phase at the grain boundaries; the low melting temp erature of this phase caused localized liquid phase sintering. resulti ng in abnormal gain growth. Dual-stage sintering (sintering at 1250 de grees C for 4 h. followed by sintering at a temperature in the range 1 350 degrees C-1450 degrees C) inhibited abnormal grain growth. The use of a fast heating rate (600 degrees C/h) and dual-stage sintering (12 50 degrees C and 1450 degrees C) yielded samples with good transparenc y and preferential orientation along the c-direction of the tetragonal unit cell. The dielectric properties depended sensitively on microstr ucture. Specimens with the highest densities had the highest relative permittivities (2400-2750); specimens with more uniform distribution o f grain sizes had sharper dielectric maxima, A mechanism for abnormal grain growth is proposed. (C) 1997 American Institute of Physics.