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.
Barium titanate ceramics exhibiting the positive temperature coefficient of resistance (PTCR) effect were prepared with donor concentrations ranging from 0.05-1.8 at.%. The resistance of the various samples (normalised to give the specific resistance of a unit area of grain boundary rho L in order to eliminate the effect of the observed reduction in grain size with increased donor concentration) was measured in the vicinity of the ferroelectric transition temperature Tc. Arrhenius plots of rho L against (T- theta )/T, where T is the absolute temperature and theta is the Curie point, were then obtained for T>Tc. The plots were found to be linear up to a few degrees below the temperature corresponding to the maximum value of rho L, in agreement with the well known Heywang model. The potential barrier heights, calculated from the slopes of the Arrhenius plots, were found to be similar for samples containing 0.1-1.4 at.% ( approximately 0.2 eV at 150 degrees C) and greater in more heavily doped specimens (0.4 eV at the same temperature). However, the Arrhenius plots belonging to samples containing 0.05 at.% were linear for a small temperature range since the PTCR jump was very small and consequently it was not possible to calculate the potential barrier height with any accuracy.
Positive temperature coefficient of resistance BaTiO3 specimens containing different donor dopant concentrations of Ho ranging from 0.05 to 1.8 at. % were investigated. The intergranular barrier layer capacitance per unit area, C′L, measured at a constant frequency of 30 kHz at both 40 and 160 °C was found to be proportional to the donor concentration up to 0.55 at. %, but then began to decrease as the donor concentration was increased beyond this. This indicated that both the density of acceptor states at the grain surfaces, Ns, and the relative permittivity εL of the material within the barrier layer were not affected by donor impurity concentrations below 0.55 at. % Ho. However, above this level of Ho concentration, the decrease in C′L appears to be related mainly to an increase in the value of Ns although it is possible that there were changes in εL. Initially both the maximum resistance and the room-temperature resistance (normalized per grain boundary per unit area), ρ′max and ρcold, respectively, were found to decrease sharply with donor concentration towards a broad minimum between ∼0.5 and ∼1.5 at. %, followed thereafter by a gradual increase. The temperature Tmax at which ρ′max occurred was also affected by the donor concentration; initially Tmax was found to increase with donor concentration followed by a reduction forming a broad maximum between about the same donor concentration limits corresponding to the minima in ρcold and ρ′max. These results are interpreted in terms of the well-established Heywang model.
X-ray photoelectron spectroscopy (XPS) has been employed to investigate the chemical nature of samples of dicadmium stannate (CdzSn04) in the as-fired, electrochemically reduced, and reoxidized states. The reduction of Cd2Sn04 was found to be associated with a dramatic color change from bright yellow to dark green, a phenomenon commonly known as the electrochromic effect. Both quantitative XPS results and binding energy measurements proved that, upon exposure of the reduced ceramic bodies to air, the SnZ+ to Sn4+ transition readily took place to produce the intermediate compound, Cd2Sn03 with divalent tin. Prolonged exposure to the atmosphere did not result in further progress of reoxidation extending to monovalent cadmium. However, complete reoxidation of the reduced samples was possible by annealing in air at 350°C for a short period of time, e.g., 3 h, by which the original features of the as-fired state such as color and electrical conductivity were restored. The results also showed that reoxidized samples at high temperature assume the same XPS characteristics as those of as-fired ceramics. [
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