Investigation of the dopant segregation phenomenon in Nb-doped BaTiO3 positive-temperature-coefficient-resistor by impedance analysis

We sought evidence of the dopant segregation phenomenon and the formation o f a dopant-segregation-induced insulating layer in Nb-doped BaTiO3, by impe dance and modulus analyses. In a 0.05 mol% Nb-doped specimen in which dopan t segregation is not expected to occur, impedance and modulus data over the range of 30-400 degrees C can be explained by the conventional positive-te mperature-coefficient-resistor (PTCR) equivalent circuit of grain resistanc e (R) and grain boundary resistance and capacitance (C). In a 0.4 mol% Nb-d oped specimen, however, impedance and modulus data can be described by intr oducing another third RC component in addition to the grain and grain bound ary component. This RC component is assumed to have originated from the dop ant segregation layer. Above 350 degrees C, this RC component associated wi th segregation layer begins to disappear. This phenomenon can be explained by the Fermi level change and the valence change of the titanium vacancy in the dopant segregation layer with change in temperature.