Coarsening Behavior of Round-Edged Cubic Grains in the Na1/2Bi1/2TiO3-BaTiO3 System

The coarsening behavior of 0.95Na(1/2)Bi(1/2)TiO(3)-0.05BaTiO(3) (mole fraction, NBT-5BT) grains has been investigated with respect to the grain shape and compared with a calculation based on crystal growth theories. The powder compacts of NBT-5BT were sintered at 1200 degrees C for various times up to 96 h. The shape of growing grains was a cube with flat {100} faces and rounded edges. The coarsening behavior during sintering up to a few hours was quite normal. However, with an increased sintering time, the relative grain size distribution broadened and apparently abnormal grains appeared. The abnormal grain coarsening behavior intensified and then decreased during extended sintering. This result shows that the coarsening behavior of partially faceted grains changes continuously with sintering time. Grain size distributions were also calculated under the assumption that the growth of grains was governed by the growth of facet planes with a low step free energy. The calculation showed the broadening of relative size distribution, the formation of abnormal grains, and then eventually narrowing of the distribution. This agreement between the measured and calculated distributions suggests the growth of round-edged polyhedral grains to be governed by the growth of facet planes, which can occur via step formation and growth on the facets.