The effects of donor dopant concentration on the grain boundary layer characteristics in n-doped BaTiO3 ceramics

Abstract

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.

Annealing effects on the characteristics of high Tc lead-free barium titanate-based positive temperature coefficient of resistivity ceramics

“…21 Since the barrier layer is considered as a parallel-plate capacitance with an area d 2 and the same thickness as the width of the barrier layer b, we can have Ј = C L b / 0 d 2 and then 21 Since the barrier layer is considered as a parallel-plate capacitance with an area d 2 and the same thickness as the width of the barrier layer b, we can have Ј = C L b / 0 d 2 and then…”

“…21 With g determined from the impedance spectra in Table I, N d of annealed samples can be evaluated. Assuming that is not affected by the ͑Bi 1/2 K 1/2 ͒TiO 3 addition, the value of equals 0.5 cm 2 V −1 s −1 .…”

Donor and Acceptor Levels in Undoped β-FeSi2 Films Grown on Si (001) Substrates

“…However, it reverts to its insulating behavior and its grain size becomes very small above certain doping concentrations (about 0.4-0.5 mol%). Several researchers (like Jonker, 1) Wernicke, 2) Al-Allak et al 3) and Drofenik 4) ) have reported and explained this phenomenon by assuming a uniform distribution of dopants in the materials. A single crystal however, is semiconducting above a doping concentration of 2 mol%.…”

Barium titanate-based thermistors: Past achievements, state of the art, and future perspectives

“…Hence, during cooling to room temperature, the material will transition from a semiconductor to a heterogeneous PTCR material, and eventually to an insulator. 183,184 Kahn confirmed this for Nb-doped BaTiO 3 cooled under different oxygen partial pressures and cooling rates. 29 This was also investigated by Sinclair's using impedance spectroscopy, which also confirmed that slower cooling resulted in a sharper and larger PTCR intensity, but also larger room temperature resisitivities.…”

“…% holmium donor-doped sample in air. 184,203 According to Drofenik's model, this should be an oxidized, insulating sample. However, Huybrechts et al pointed out that the sintering temperature Al-Allak et al used was much higher 1420-1460 C compared to Drofenik's 1340 C. 12 Therefore, it is possible that the donor dopant could be incorporated without recrystallization and the reduced form was produced by equilibration at high temperature.…”