Differential Negative Resistance and Piezoresistivity in Thin Semiconducting BaTiO3 Ceramic Bars

Thin, semiconducting barium titanate (BaTiO₃) ceramic bars, with a diameter of 10 to 20 μm, consisting of single grains joined together in series have been prepared to investigate the piezoresistivity in the materials, which was evaluated from their current ( I )-voltage ( V ) characteristics under the loading condition of various bending stresses. I-V characteristics of single grain boundaries in some of the materials were found to exhibit distinct differential negative resistance (DNR) at room temperature with its feature changing with stress. The DNR appeared on the I-V curves at an electric field of several volts per one grain, and has been confirmed to be connected with the transition of current between two conduction states in the grain boundary region. The obtained results indicate that this phenomenon cannot be interpreted by a rise in the temperature of the materials up to their positive temperature coefficient of resistivity (PTCR) region above the Curie point by Joule heating due to current flow, that is their self-heating effect. This newly observed DNR phenomenon has thus been tentatively interpreted by the morphological change in the ferroelectric domain structure in the vicinity of grain boundaries under mechanical and electric stresses, on an assumption that different configurations of ferroelectric domains yield different conduction states in the grain boundary due to a difference in the degree of surface acceptor charge compensation or the anisotropic carrier mobilities in the crystal.