DEVELOPMENT OF HIGH PERFORMANCE STRESS SENSOR USING SEMICONDUCTING BARIUM TITANATE THIN WIRES WITH GIANT PIEZORESISTIVE EFFECTS

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The objective of this study is to clear the mechanism of the piezoresistive effect in semiconducting barium titanate ceramics and to establish a procedure for producing high-peformance stress sensors using barium titanate thin wires with a single grain boundary structure, which exhibit a significantly large piezoresistive effect at room temperature.The obtained results are summarized as follows :1) Preparation of semiconducting barium titanate thin ceramic wires : We succeeded in producing semiconducting barium titanate thin ceramic wires, with a diameter of 10-20 μm, consisting of single grains joined together in series by drawing green fibers from slurry, followed by sintering at 1370℃ in air.2) Observation of giant piezoresistive effects : An extremely large piezoresistive effect of more than 10^7 in gauge factor (defined as the resistance change ratio per strain) was observed for some of the single grain boundaries formed in semiconducting barium titanate thin ceramic wires produce … More d.3) Analysis of the 90゜ domain wal motion under mechanical stresses : We directly observed how 90゜ domain walls move in a BaTiO_3 ceramic wire under tensile and compressive stresses with a polarizing microscope. In this experiment, we found that just as a 90゜ domain wall arrived at a grain boundary, associated with an instantaneous change of the domain morphology in the vicinity of the grain boundary, the grain boundary resistivity drastically changed. It has been for the first time confirmed that the piezoresistivity in semiconducting barium titanate ceramics is directly connected with the change of the domain morphology in the vicinity of grain boundaries.4) Analysis of the piezoresistivity mechanism : We were able to successfully determine the direction of spontaneous polarization in the two ferroelectric domains adjacent to a 90゜ domain wall moving in a single grain under mechanical stresses. From the obtained results it has been found that the meachanism of the piezoresistivity in semiconducting barium titanate ceramics can be interpreted by a surface charge compensation model by spontaneous polarization at grain boundaries.5) Est ablishment of a procedure for producing high-performance stress sensor devices : We made prototype stress sensor devices using semiconducting barium titanate thin ceramic wires, which exhibit a large piezoresistive effect at room temperature, and confirmed their stress sensor function working as expected though some problems such as reproducibility remain. Less