Observation of the Grain Boundary Formation Process in Barium Titanate Ceramics and the Mechanism of the Grain-Boundary Nonlinear Electronic Properties

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The process of grain growth and grain boundary formation in semiconducting barium titanate ceramics (prepared in the form of a thick film) has been examined by observing repeatedly under an optical microscope exactly the same position of the ceramic sample after each repeated firing at six temperatures between 1300ﾟC and 1350ﾟC raised by every 10ﾟC.After this examination, the PTCR (positive temperature coefficient of resistivity) characteristics (one of the most representative nonlinear electronic properties in this material) of single grain boundaries in the materials have been investigated as the main part of this study. Ceramic strings of semiconducting barium titanate were used for the PTCR measurements, with dimensions of 50 mum wide and 40 mum thick in average. The samples were obtained by firing strings of barium titanate powder, doped with 0.1at% La as donor and 0.5 mole% excess TiO_2 for enahancing grain growth, at 1350ﾟC in air, which were prepard by cutting from a tape of th … More e ceramic powder produced by a doctor-blade method. Sintering was carried out to allow constituent grains grow up to have an average size greater than 50 mum, so that barium titanate ceramic strings had a grain structure consisting single gains sintered in line, that is, with as many cross-sectional grain boundaries as the constituent grains. In-Gamicroelectrodes were put on the adjacent grains of a single grain boundary to measure the PTCR characteristic of the single grain boundary. Such measurements were conducted for many single grain boundaries foundaries formed between grains with various morphologies of growth patterns on their surfaces.The present experiment revealed that there existed grain boundaries exhibiting at least four different PTCR characteristics in the obtained ceramic samples. These characteristics can be described as follows : (1)similar to usual PTCR chracteristis with a large resistivity jump above the Curie point, (2)an abrupt jump of resistivity (of more than two orders of magnitude) immediately followed by a monotonous decrease of it, (3)an abrupt resistivity jump (of less than one order of magnitude) followed by small successive resistivity changes, resulting in only a small resitivity jump as a whole, and (4)little or substantially no resistivty jump at the Curie point. The formation of grain boundaries which exhibit these different PTCR characteristics is considerrd to be closely connected with the growth process of their adjacent grains, associated with the appearance of various types of growth patterns on their surfaces. Less