Effect of secondary-phase segregation on the positive temperature coefficient in resistance characteristics of n-BaTiO3 ceramics

Abstract

Modifications in the positive temperature coefficient in resistance (PTCR) of n-BaTiO3 ceramics are brought about by specific additives such as Al2O3, B2O3 or SiO2, leading to the segregation of secondary phases such as BaAl6TiO12, BaB6TiO12 or BaTiSi3O9 at the grain boundaries. Segregation of barium aluminotitanates resulted in broad PTCR curves, whereas B2O3 addition gave rise to steeper jumps and SiO2 addition did not result in much broadening compared with donor-only doped samples. Microstructural studies clearly show the formation of a structurally coherent expitaxial second phase layer of barium aluminotitanate surrounding the BaTiO3 grains. Electron paramagnetic resonance investigations indicated barium vacancies, VBa, as the major electron trap centres which are activated across the tetragonal-to-cubic phase transition according to the process VXBa + e′ ⇋ V′Ba. The grain size dependence of the intensity of the V′Ba signal indicated the concentration of these trap centers in the grain-boundary layer (GBL) regions. Further, the charge occupancy of these centres is modified by the secondary phases formed through grain-boundary segregation layers. BaAl6TiO12 gave rise to Al-O− hole centres whereas no paramagnetic centres corresponding to boron could be detected on B2O3 addition. Such secondary phases, forming epitaxial layers over the BaTiO3 grains, modify the GBL region, rich in electron traps, surrounding the grain core. The complex impedance analyses support this three-layer structure, showing the corresponding contributions to the total resistance which can be assigned as Rg, Rgb and Rsecondary phase. The epitaxial second phase layers bring about inhomogeneity in the spatial distribution of acceptor states between the grain boundary and the grain bulk resulting in extended diffuse phase transition characteristics for the GBL regions in n-BaTiO3 ceramics. This can cause the GBL regions to have different transition temperatures from the grain bulk and a spread in energy levels of the associated GBL trap states, thus modifying the PTCR curves. An attempt has been made to explain the results based on the vibronic interactions applied to the mid-band-gap states in n-BaTiO3.