Effect of commercial anatase-TiO2 raw materials on the electrical characteristics of ceramics with positive temperature coefficient of resistivity

Cause and effect of commercially available anatase-TiO2 on the positive temperature coefficient of resistivity (PTCR) characteristics of donor- and acceptor-codoped (Ba,Ca)TiO3-ceramics have been investigated. Compositions containing several anatase raw materials have been studied by means of scanning electron microscopy, in particular EDXS/WDXS-techniques, as well as electron backscattering diffraction (EBSD) and impedance spectroscopy. It is clearly shown that the crystallographic modifications rutile and anatase are replaceable without any significant effect on the PTCR-characteristics, if these raw materials have similar physical and chemical properties such as grain size distribution, specific surface area and impurity content. Further it is demonstrated that the impurities of commercial anatase powders, which are a result of preparation conditions, strongly affect the electrical properties. By use of impedance analysis and electron backscattering diffraction we conclude that the differences in PTCR-characteristics can be attributed to the grain boundary region due to dopant distribution and segregative additive effects and not to microstructural modifications.