Donor-doped BaTiO3 Ceramics Research Articles

Heavily donor-doped, optically translucent ferroelectric barium titanate ceramics through defect chemical engineering

The possibility of preparing translucent ferroelectric heavily donor-doped BaTiO3 ceramics rich in TiO2 has been demonstrated by conventional ceramic processing based on the synthesis via the solid state route and on subsequent sintering and annealing at high levels of oxygen partial pressure.

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This paper proposed a two-stage thermal processing method combined with the reduction-reoxidation procedure. Samples were firstly sintered in a reducing atmosphere by two stages and then reoxidized in air. Such two-stage thermal processing method was used to adjust density and grain size of ceramics separately. Finally, nanocrystalline semiconducting donor-doped BaTiO3 ceramics were first successfully prepared. Samples with average grain size of 340 nm exhibited unexpectedly low room resistivity of 136 Ω cm and a significant PTCR effect, with a resistance jump of 4 orders of magnitude. In addition, depletion layer thickness of 40 nm and surface acceptor-state densities of 9.52 × 1013 cm−2 were also calculated. Such good properties had not been reported in former publications. It also means that smaller components based on semiconducting BaTiO3 ceramics could be produced and more extensive application field could be proposed in the trend of miniaturization of electronic devices.

Nanocrystalline semiconducting donor-doped BaTiO3 ceramics for laminated PTC thermistor

This paper proposed a two-stage thermal processing method combined with the reduction-reoxidation procedure. Samples were firstly sintered in a reducing atmosphere by two stages and then reoxidized in air. Such two-stage thermal processing method was used to adjust density and grain size of ceramics separately. Finally, nanocrystalline semiconducting donor-doped BaTiO3 ceramics were first successfully prepared. Samples with average grain size of 340nm exhibited unexpectedly low room resistivity of 136Ωcm and a significant PTCR effect, with a resistance jump of 4 orders of magnitude. In addition, depletion layer thickness of 40nm and surface acceptor-state densities of 9.52×1013cm−2 were also calculated. Such good properties had not been reported in former publications. It also means that smaller components based on semiconducting BaTiO3 ceramics could be produced and more extensive application field could be proposed in the trend of miniaturization of electronic devices.

Investigation of Semiconducting Barium Titanate Ceramics by Oxygen Coulometry

Donor-doped BaTiO3 ceramics (0–0.6 mol% Nb) were investigated during the sintering process at an oxygen partial pressure of 2.4 Pa and at a maximum temperature of 1430∘C. The occurring oxygen exchange with the ambient atmosphere was monitored quantitatively by oxygen coulometry. The coincidence between the grain growth behavior and the occurrence of distinct oxygen release peaks during the anomalous grain growth reveals that significant amounts of the donor are incorporated and charge-compensated by electrons only under the conditions of the anomalous grain growth. Quantitative analysis of the coulometric data shows that at a doping level of 0.2 mol% nearly all donors are charge-compensated by electrons. With increasing Nb concentration also Ti vacancies gradually begin to serve for compensation. At 0.5 mol% Nb their influence reaches 70 % of the electronic compensation.

Semiconducting BaTiO3 ceramic prepared by low temperature liquid phase sintering

Donor-doped BaTiO3 ceramics were prepared by adding PbO B2O3 SiO2 as a sintering aid. Semiconducting BaTiO3 was obtained at a sintering temperature of 1100 °C. The sintered samples exhibit the Positive Temperature Coefficient of Resistivity (PTCR) effect, which depends on the amount of liquid phase, the concentration of the donor-dopant, and the sintering temperature. The cold resistivity of the samples decreases when the sintering temperature increases. The increase of the grain boundary resistivity and hence of the cold resistivity at lower sintering temperatures was explained by applying the diffusion grain boundary layer model.

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.

Compositional control of ferroelectric fatigue in perovskite ferroelectric ceramics and thin films

The mechanism and control of ferroelectric polarization fatigue (loss of polarization with cycling) in donor- and acceptor-doped BaTiO3 ceramics and Pb(Zr1−xTix)O3 (PZT) thin films has been investigated. Experimental results clearly demonstrate that fatigue behavior is related to the defects within the materials. Donor-doped BaTiO3 ceramics showed significantly improved fatigue characteristics when compared with acceptor-doped materials. A similar but reduced effect has been observed in donor-doped PZT thin films. The electric-field-assisted migration of charged species within ferroelectric materials may be responsible for the degradation/fatigue behavior. Results support the expectation that oxygen vacancies play an important role in fatigue that occurs as a result of polarization switching.

High-permittivity dielectric ceramics with high endurance

Abstract Donor-doped BaTiO3 ceramics show very high endurance under DC field stress and are there-fore suitable candidates for the manufacture of multilayer capacitors with thin dielectric layers. CeO2 additives were found to be excellent donor dopants. The donor effect results from Ce3+ ions incorporated on Ba sites of the perovskite lattice. Defect chemistry, solubility of CeO2 in BaTiO3 and the ferroelectric phase transition of Ce-modified BaTiO3 have been studied by several methods.

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 kinetics of hot-pressing for undoped and donor-doped BaTiO3 ceramics

The kinetics of hot-pressing for undoped and donor-doped BaTiO3 of high purity have been studied at 1050‡ C and at 2.3 to 27.6 MPa in an oxidizing atmosphere. The stress exponent and grain size exponent of the densification rate are indicative of control of densification by a grain boundary diffusion mechanism. The activation enthalpy (∼400 kJ mol−1) for the densification process has been measured over the range of 1050 to 1 200‡ C. No significant influence of the donor is observed on the densification of BaTiO3 until the late stages of sintering.

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 influence of kinetic processes on the electrical conductivity of donor-doped BaTiO3 ceramics

Barium titanate doped with donors is a good n-type semiconductor, the conductivity of which, however, is considerably influenced by intrinsic defects always present in these materials. Therefore not only the equilibrium behaviour of these defects but also the defect formation and the defect diffusion play an important role with respect to investigations concerning the semiconducting properties. It is shown that in many cases a heterogeneous distribution of frozen defects is formed at room temperature which leads to a corresponding inhomogeneity of the conductivity. A kinetic model is developed based on these considerations that gives a satisfactory explanation of the experimentally well known characteristic of conductivity vs. doping concentration. Donator-dotiertes BaTiO3 ist ein guter Elektronenhalbleiter, dessen Leitfahigkeit stark durch Eigenfehlstellen beeinflust wird, die in diesen Materialien stets vorhanden sind. Daher spielen nicht nur das Gleichgewichtsverhalten dieser Fehlstellen, sondern auch die Fehlstellenbildung und die Fehlstellendiffusion eine wesentliche Rolle fur Leitfahigkeitsuntersuchungen. Es wird gezeigt, das in vielen Fallen eine inhomogene Fehlstellenverteilung eingefroren wird, die zu entsprechenden Inhomogenitaten der Leitfahigkeit fuhrt. Aus diesen Uberlegungen wird ein kinetisches Modell entwickelt, mit dem die experimentell bekannte Dotierungsabhangigkeit der Leitfahigkeit befriedigend erklart werden kann.