Doped BaTiO3 Ceramics Research Articles

Spectral analysis of switching currents in doped barium titanate ceramics and nonlinear compensation

Abstract The experimental switching currents obtained in (Nb, Co)-doped BaTiO3 ceramics at various field amplitudes and frequencies are compared to those computed from the extended Kolmogorov-Avrami theory, using the method of Fourier spectral analysis. For improving the agreement between data and model, a nonlinear correction in the manner proposed by Hatano et al. was used. The pure switching currents were numerically computed on the basis of equivalent circuit, in which the field dependences of capacity and resistance of ceramic C(E) and R(E) are experimentally determined.

Microscopic origin of ptc effect in doped BaTiO3 ceramics

Abstract The PTC effect in doped BaTiO3 is usually ascribed to specific electrical properties of grain boundaries only. From microscopic four-probe electrical resistivity measurements, it is demonstrated that the bulk is also involved in this effect. To explain these new results, which conflict with the usual PTC theory, a model based on dipolar interaction is used.

Study on the electrical properties of single grain boundaries in BaTiO3 ceramics

In the article, the electrical properties of single grains and single grain boundaries in donor doped BaTiO3 ceramics have been investigated. The results show that the grains have no positive temperature coefficient of resistance effect (PTCR effect) if the influence of electrode was neglected. The results also show that different grain boundaries have various PTCR effect. The characteristics of current-voltage do not follow the Heywang model and accord with our modified model. The maximum barrier heights of single grain boundaries are also deduced experimentally.

Temperature dependence of polarization reversal in (Nb, Co)-DOPED BaTiO3ceramics

Abstract The switching properties of (Nb, Co)-doped BaTiO3 ceramics were studied in the range of temperatures between 0° and 140°C. Experimental dependences of switching current density j(t) and electric displacement D(t) are compared to those predicted by the extended Avrami theory.1–2 The effective dimension of nucleation is noninteger d∈(2,3), irrespective of the temperatures and amplitudes of field strength.

Grain-boundary decorated titanate ceramics: Preparation and processing

A new ceramic technology is presented which allows (1) to introduce controlled quantities of grain boundary dopants and (2) to tailor the microstructure of the ceramic. The technology is applied to doped BaTiO3 and SrTiO3 ceramics and compared to conventionally sintered specimen.

PTC effect in doped BaTiO3 ceramics: evidence for a bulk contribution

Using photolithographic techniques, four-probe electrical resistivity measurements of Nb-doped BaTiO3 ceramics, at the microscopic scale, are presented from room temperature up to 180 degrees C. The PTC effect displayed by several grains exhibits an abrupt jump at the Curie temperature, followed by a monotonic quasi linear increase of the resistance. This behaviour looks quite different from the usual PTC effect observed on macroscopic ceramics. The jump is clearly due to a change of resistance within the bulk. It appears, from the microscopical results, that the bulk itself must be partially involved in the increase of resistance above Tc contrary to the generally accepted theory.

Properties of mechanochemically pretreated precursors of doped BaTiO3 ceramics

The properties of doped BaTiO3 ceramics prepared from mechanically activated precursors (BaCO3, TiO2, PbO), dopant (Sb) and admixtures (SiO2, Al2O3) were investigated. Mechanical activation of the mixture 1.00 BaCO3 + 1.15 TiO2 + 0.10 PbO brings about an increase in its specific surface and partial amorphization of BaCO3; Ba, O and C occur in the surface of this mixture in at least two chemically different states. BaTiO3 obtained from the above mentioned mechanically activated precursors by high-temperature solid-state synthesis at 1100 ° C consists of homogeneous particles with a narrow particle size distribution.

Calculation of Ultraslow Dielectric Relaxation of Doped BaTiO3 Ceramics

Using a linear voltage ramp method, ultraslow dielectric relaxation processes of La-doped BaTiO3 boundary layer (BL) capacitors are measured at room temperature. For the electric polarization model of a double-layer dielectric, a procedure to calculate the electrical parameters of grain and grain boundary has been developed. Calculated results suggest that there is a shell on the surface of the grain. TEM images of the BL capacitor also show a lot of microregions with minor structural modifications. Relaxation times are estimated from the equivalent circuit and are in agreement with fitted results.

Dielectric properties of BaTiO3 and La doped BaTiO3 ceramics

The dielectric constant and loss tangent of barium titanate and barium titanate doped with 0.4% (mole) La ceramics have been studied in the frequency range 22 KHz-70 MHz at room temperature and in the temperature range 20°C–170°C at 22 KHz using aQ-meter. It is observed that the dielectric constant at room temperature of the samples increases nearly a hundred fold on doping. The maximum of the dielectric constant broadens and shifts to a lower temperature in the La doped samples. The loss tangent is also relatively high in the doped sample. The dielectric constant and loss tangent of the doped samples decrease faster with frequency than that of the undoped samples.

Studies on some BaTiO3based commercial electroceramics

Abstract Rapid firing studies on commercial electroceramics is briefly reviewed with particular emphasis on capacitor dielectrics. In addition to better throughput, rapid firing improves the properties of most of the commercial electroceramics excepting Type II dielectrics. In the case of Type II dielectrics, there is a decrease in the volumetric efficiency and an improvement in the temperature characteristics on rapid firing. This behaviour is attributed largely to the grain size effects on the Curie peak. The effect of acceptor dopants and additives on the electrical properties and grain size variation of donor doped BaTiO3 ceramics is briefly reviewed. Results of studies on the effect of addition of Al2O3, SiO2, TiO2(AST) on the Sb and Mn codoped BaTiO3 ceramics are presented. AST addition reduces the sharp variation of conductivity introduced by Mn and increases the effective semiconductivity range of the donor dopant.

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

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.

The effect of Mn on the positive temperature coefficient of resistance characteristics of donor doped BaTiO3 ceramics

BaTiO3 positive temperature coefficient of resistance (PTCR) specimens were made from commercial BaTiO3 which was mixed with 0.35 mol. % of Ho2O3 to make it semiconducting and 0.07 mol. % of MnCO3. After sintering at 1320 °C the samples were annealed in batches at 1220 °C for various periods between 0 and 5 h. The effect of Mn was studied by making direct comparison with previous results obtained from Mn free but otherwise identical specimens. Room-temperature dielectric measurements in the audio and radio frequency ranges revealed that Mn had a negligible effect on the grain bulk resistance. Mn was found to result in an increase in the minimum and maximum values of the resistivity, the temperature at which the resistivity is a maximum and the slope of the resistivity-temperature characteristic in the transition region, all of which were attributed to an enhancement of the potential barrier at the grain boundaries. The acceptor state energy of the Mn-doped samples was found to be ∼1.4 eV, while a lower value was obtained for Mn-free material (∼1.12 eV). The effective concentration of Mn acceptors was observed to increase with the annealing time. To explain this behavior a thin Mn-rich boundary layer was assumed to exist, within which there was partial compensation of the Mn ions due to the formation of ionized oxygen vacancies. During annealing in air at 1220 °C, oxidation takes place and more of these vacancies are filled resulting in an increase in the effective Mn concentration. This model is also capable of explaining other phenomena such as the disappearance of the PTCR effect in reduced samples and the effects of cooling rate and quenching on the PTCR behavior.

Release of Oxygen During the Sintering of Doped BaTiO3 Ceramics

The release of oxygen during the sintering of Sb2O3-doped BaTiO3 ceramics containing excess TiO2 was measured using a mass spectrometer. The amount of oxygen released is proportional to the dopant concentration in the product phase. The evolution of oxygen during sintering was attributed to dissolution of the oxidized form of doped BaTiO3 in the reacting mixture and simultaneous re-crystallization of the- reduced form.