Temperature Coefficient Of Resistivity Ceramics Research Articles

The influence of Mn on the grain-boundary potential barrier characteristics of donor-doped BaTiO3 ceramics

Two compositions of BaTiO3 positive temperature coefficient of resistance ceramics, prepared identically except for the fact that a small addition of Mn (0.04 at. %) was made to one of them, were studied. The samples were sintered simultaneously in air at 1400 °C for 1 h and then annealed at 1200 ° for 5 h, using a muffle furnace. Room-temperature dielectric measurements in the audio- and radio-frequency ranges confirmed that Mn has a negligible effect on the bulk resistance. Arrhenius plots of resistivity vs 1/[Tε′m(T)] were found to give straight lines for Tc<T<Tmax (where ε′m is the relative permittivity of the specimen measured at a constant frequency of 30 kHz, Tc is the ferroelectric transition temperature, Tmax is the temperature corresponding to the maximum in resistivity, and T is the absolute temperature), in accordance with the well-known Heywang model. The height of the potential barriers at different temperatures, as calculated from the slopes of these plots, were found to increase by about 40% (from ∼0.34 to ∼0.50 eV) by the addition of Mn. A small increase in the acceptor-state density at the grain surfaces, which was again obtained from these plots, was observed in Mn-doped specimens (3.9×1013 cm−2 as compared to 2.7×1013 for Mn-free specimens). It was also found that the inclusion of Mn had a negligible effect on ε′m above Tc.

The effect of aluminium on the electrical and mechanical properties of BaTiO3 ceramics as a function of sintering temperature

The effect of aluminium additions on the mechanical behaviour of BaTiO3 positive temperature coefficient of resistance ceramics sintered in air at temperatures ranging between 1220 and 1400° C has been investigated. Tensile strength has been measured indirectly by the diametral compression of lapped discs using concave loading anvils. Values of ∼ 85 and ∼ 110 MPa for samples fired near their optimum sintering temperature were determined for two batches of material, the latter of which contained additions of Al2O3 (0.55 mol%). Strength did not vary systematically with grain size and appeared to be controlled by near surface defects. The size of these cavities, which were generally crescent shaped, was consistent with the material having a bulk fracture toughness of ∼1.3 MPam1/2. The higher mechanical strength of samples which contained Al2O3 additions was attributed to the enhanced “healing up” of these cavities by the liquid phase giving a smaller inherent critical defect size rather than by increasing the bulk toughness of the ceramic.