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

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 BaAl6 TiO12, BaB6TiO12 or BaTiSi3O9 at the grain boundaries. Segregation of bariu m aluminotitanates resulted in broad PTCR curves, whereas B2O3 addition gav e rise to steeper jumps and SiO2 addition did not result in much broadening compared with donor-only doped samples. Microstructural studies clearly sh ow the formation of a structurally coherent epitaxial second phase layer of barium aluminotitanate surrounding the BaTiO3 grains. Electron paramagneti c resonance investigations indicated barium vacancies, V-Ba, as the major e lectron trap centres which are activated across the tetragonal-to-cubic pha se transition according to the process V-Ba(X) + e' reversible arrow V-Ba'. The grain size dependence of the intensity of the V-Ba' signal indicated t he concentration of these trap centers in the grain-boundary layer (GBL) re g ions. Furth er, the charge occupancy of these centres is modified by the secondary phases formed through grain-boundary segregation layers. BaAl6TiO 12 gave rise to Al-O- hole centres whereas no paramagnetic centres correspo nding to boron could be detected on B2O3 addition. Such secondary phases, f orming epitaxial layers over the BaTiO3 grains, modify the GEL region, rich in electron traps, surrounding the grain core. The complex impedance ana l yses support this three-layer structure, showing the corresponding contribu tions to the total resistance which can be assigned as R-g, R-gb and R-seco ndary (phase). The epitaxial second phase layers bring about inhomogeneity in the spatial distribution of acceptor states between the grain boundary a nd the grain bulk resulting in extended diffuse phase transition characteri stics for the GEL regions in n-BaTiO3 ceramics. This can cause the GBL regi ons to have different transition temperatures from the grain bulk and a spr ead in energy levels of the associated GEL trap states, thus modifying the PTCR curves. An attempt has been made to explain the results based on the v ibronic interactions applied to the mid-band-gap states in n-BaTiO3. (C) 19 98 Kluwer Academic Publishers.