Effect of Interface Morphology on Kinetics of Microstructural Evolution in $BaTiO_3$

Abstract

Barium titanate is a typical material exhibiting abnormal grain growth during processing for such electronic components, for example, as chip capacitors and positive-temperature-coefficient (PTC) resistors. Since grain size and distribution considerably affect electrical properties, investigators have been trying to control microstructural development. The control of microstructure in materials ranging from ultrafine, moderately sized, duplex to single crystalline, has long been a challenging subject to material scientists. A basic means to achieve this goal is related to the control of grain growth. Taking $BaTiO_3$ as a model system, this investigation shows that control of interface structure between rough and faceted allow us to achieve the goal. When the interface is rough, normal grain growth occurs with a moderate rate. On the other hand, for faceted interface, either abnormal grain growth or grain growth inhibition occurs resulting in a duplex grain structure or fine-grained structure, respectively. Growth of single crystals is also possible when the boundary is faceted. During crystal growth amorphous films can form and thicken at dry grain boundaries above the eutectic temperature. In chapter III, the roughening transition of the grain boundary was investigated from the theoretical and experimental point of view. When $BaTiO_3$ powder compacts with 0.2 mol% excess $TiO_2$ were sintered in air at 1250℃, below the eutectic temperature, some grains grow abnormally to very large sizes with double twins in the <111> directions at their centers. The abnormal grains, elongated in the <111> directions, formed mostly straight grain boundaries lying on their {111} planes with the fine matrix grains. Some grain boundaries between the nearly (111) face of an abnormal grain and the matrix grains were faceted with some facet planes lying on either the (111) plane of the abnormal grain or (210) plane of the matrix grain. These flat grain boundaries, as observed by ...