Some aspects of semiconducting barium titanate

Mapping grain boundary heterogeneity at the nanoscale in a positive temperature coefficient of resistivity ceramic

“…Jonker modified Heywang's model by incorporating the ferroelectric nature of the material with the possibility that different grains have different polarisation orientations. 8 In fact, each grain itself can have different domains in it and thus a wide range of possibilities emerges, in terms of polar discontinuities present at the grain boundaries. Surface charges are generated when the net polarisation vector normal to grain boundaries is created.…”

“…4,5 It was identified quite early that the effect is dominant in polycrystalline samples, 6 with rich microstructures and an equally interesting domain structure embedded within grains, and several models have been proposed to explain the observed experimental behavior. [7][8][9][10] The most accepted among these models is the Heywang-Jonker model, which states that the presence of a potential barrier at the grain boundaries is responsible for enhanced resistivity of the boundary in comparison to the grain interior. According to the original Heywang model 7 (reviewed by Chen and Yang 4 ), there is a bi-dimensional layer of electron traps, i.e.…”

Barrier capacitance of nonstoichiometric barium titanate

“…The high resistivity of barium (24,25,26,27) and other titanates (27,28,29) can be reduced to 1 ohm-m or less by the controlled valency method (30,31,32) and/or by reduction (30,32) in a reducing atmos phere. The method used will determine the defect structure and conse quently the resistance and capacitance properties.…”

“…The excess charge introduced by the impurities or the oxygen vacancies may be compensated by trapped electrons on titanium ions (25), or by electrons in a narrow conduction band (24). In either case (33), a low carrier mobility may be expected.…”

“…Controlled valency semiconductors sintered in air (24,28,36) show a large increase in resistivity at the Curie point and extending to higher temperatures. This anomoly is known in the titanates as the positive temperature coefficient (PTC) effect.…”