Wen-Bin Su

Perovskite (Na0.5K0.5)1−x(LiSb)xNb1−xO3 lead-free piezoceramics

Lead-free potassium sodium niobate piezoelectric ceramics substituted with lithium and antimony (Na0.5K0.5)1−x(LiSb)xNb1−xO3 have been synthesized by conventional solid state sintering method. Compositionally engineered around the orthorhombic-tetragonal polymorphic phase transition, the dielectric and piezoelectric properties were further enhanced with the addition of lithium and antimony substituted into the perovskite structure. The combined effects of lithium and antimony additions resulted in a downward shift in the orthorhombic-tetragonal (TO-T) without significantly reducing TC. The dielectric, piezoelectric, and electromechanical properties were found to be e∕e0>1300, d33>260pC∕N, and kp>50%, while maintaining low dielectric loss. The enhanced polarizability associated with the polymorphic TO-T transition and high TC transition (∼390°C) should provide a wide range of temperature operation.

Enhancement of thermoelectric efficiency in oxygen-deficient Sr1−xLaxTiO3−δ ceramics

We report that the Seebeck coefficient (S) is remarkably enhanced in oxygen-deficient Sr1−xLaxTiO3−δ ceramics. The S values of all oxygen-deficient samples are larger than those of the near-stoichiometric ones and are temperature-independent at high temperatures, showing a narrow band behavior. This indicates that the introduction of oxygen vacancy changes the density of electronic states around the Fermi energy. The maximum for the figure of merit (ZT) of Sr0.9La0.1TiO3−δ ceramic reaches 0.21 at about 750 K, demonstrating enhancement by a factor of more than 1.3 over that of the near-stoichiometric materials.

Nonlinear electrical properties of TiO2–Y2O3–Nb2O5 capacitor-varistor ceramics

Abstract The nonlinear electrical properties of TiO2–Y2O3–Nb2O5 ceramics were investigated as a new varistor material. It was found that an optimal doping composition of 99.75%TiO2–0.60%Y2O5–0.10% Nb2O5 was obtained with low breakdown voltage of 8.8 V mm−1, high nonlinear constant of 7.0 and ultrahigh relative dielectric constant of 7.6×104, which is consistent with the highest and narrowest grain boundary barriers in the composition. Samples doped with 0.10 mol.% Nb2O5 exhibit the highest permittivitty and resistivity at low frequencies and comparatively lower values at high frequencies in comparison with other samples studied. In view of these electrical characteristics, the ceramics of 99.75%TiO2–0.60%Y2O3–0.10%Nb2O5 is a viable candidate for capacitor-varistor functional devices. The performance of the ceramics as a function of Nb-doping depends primarily on the extent of substitution of Ti4+ with Nb5+. In order to illustrate the role of grain boundary barriers for high Nb-doping co-concentrations in TiO2–Y2O3–Nb2O5 varistors, a grain-boundary defect barrier model was introduced.

Novel (Zn, Nb)-doped SnO2 varistors

Abstract The effect of ZnO on the Nb-doped SnO 2 varistors was investigated. The grain boundary barrier height (Φ B ) of the varistors were determined by use of Schottky type of conduction mechanism. The grain boundary impedance measurements of the varistors were performed using the impedance spectroscopy technique. It was found that ZnO had a significant effect on the varistor properties of Nb-doped SnO 2 ceramics. An optimal doping composition of 98.95SnO 2 –1.00ZnO–0.05 mol% Nb 2 O 5 with the highest nonlinearity with α =12.3, the highest Φ B =0.73 eV and the largest grain boundary resistance R GB =3.43×10 6 Ω cm was obtained. A defect barrier model was introduced to illustrate the formation of the grain boundary barrier for SnO 2 –ZnO–Nb 2 O 5 ceramic varistors. The key element of this model is that the depletion layers formed at the near-grain-boundary region due to the introduction of defects in the crystal lattice, are responsible for the formation of Schottky type potential barriers at the grain boundaries.

Influence of rare earth doping on thermoelectric properties of SrTiO3 ceramics

Thermoelectric properties of SrTiO3 ceramics, doped with different rare earth elements, were investigated in this work. Its found that the ionic radius of doping elements plays an important role on thermoelectric properties: SrTiO3 ceramics doped with large rare earth ions (such as La, Nd, and Sm) exhibit large power factors, and those doped with small ions (such as Gd, Dy, Er, and Y) exhibit low thermal conductivities. Therefore, a simple approach for enhancing the thermoelectric performance of SrTiO3 ceramics is proposed: mainly doped with large ions to obtain a large power factor and, simultaneously, slightly co-doped with small ions to obtain a low thermal conductivity. Based on this rule, Sr0.8La0.18Yb0.02TiO3 ceramics were prepared, whose ZT value at 1 023 K reaches 0.31, increasing by a factor of 19% compared with the single-doped counterpart Sr0.8La0.2TiO3 (ZT = 0.26).

Effect of Mn2+ on the electrical nonlinearity of (Ni, Nb)-doped SnO2 varistors

The reason that the (Ni, Nb)-doped SnO2 varistors exhibit poorer densification and electrical nonlinearity than the (Co, Nb)-doped SnO2 varistors is explained. The effect of Mn2+ on the electrical nonlinear properties of SnO2 based ceramics were investigated. The sample doped with 0.10 mol% MnCO3 exhibits the highest reference electrical field of 686.89 V/mm, the highest electrical nonlinear coefficient of 12.9, which is consistent with the highest grain-boundary defect barriers. It can be explained by the effect of the substitution of Sn4+ for Mn2+, which facilitate the formation of the defect barriers, and the maximum of the substitution. The shrinkage rates increase with the doping of MnCO3, although the sample doped with 0.5 mol% MnCO3 appears the highest density (ρ=6.87 g/cm3). In order to illustrate the grain boundary barriers formation in SnO2.Ni2O3.Nb2O5.MnCO3 varistors, a grain-boundary defect barrier model was introduced.

Nonlinear electrical behavior of the TiO2⋅WO3 varistor

The nonlinear electrical behavior and dielectric properties of TiO2-based ceramics with various WO3 contents have been investigated. It was found that 0.25% WO3+99.75% TiO2 has an optimal nonlinear coefficient of α=9.6, a breakdown electrical field of 44.5 V/mm, and ultrahigh relative dielectric constant of 7.41×104 (measured at 1 kHz). The theory of defects in the crystal lattice was introduced to explain the nonlinear electrical behavior of the TiO2⋅WO3 system. Schottky potential barriers at the grain boundaries that are analogies to the grain boundary defect model for ZnO varistors are introduced in the article. From this point, the nonlinear electrical behavior of the TiO2 system is explained.

Nonlinear electrical characteristics of SnO2∙CuO ceramics with different donors

The effects of various donors such as Nb, Sb, Ta, and V on the densification and nonlinear current-density J–electrical-field E relations of tin oxide ceramics are investigated. The room-temperature resistivity ρ and the three vital varistor parameters, the nonlinear coefficient α, breakdown electrical field EB, and leakage current density JL, are studied as a function of donor concentration. Minor donors make highly resistive SnO2∙CuO ceramics nonlinear or conductive. The optimum doping samples with tantalum or niobium show promising properties for high-voltage varistor application.

Effects of Ta2O5 on the grain size and electrical properties of SnO2-based varistors

The effects of Ta2O5 on SnO2-based varistors were investigated. It was found that Ta2O5 significantly affects the grain size and the electrical properties. The average grain size decreases from 9.3 to 3.8 µm, the breakdown electrical field increases from 246 to 1412 V mm−1 and relative electrical permittivity decreases from 1.9 to 0.42 k with an increase in Ta2O5 concentration from 0.10 to 1.00 mol%. The sample with 1.00 mol% Ta2O5 has the best nonlinear electrical property and the highest nonlinear coefficient (α = 52.6) among all samples. The reason for grain size decrease with increasing Ta2O5 concentration is explained. To illustrate the grain–boundary barrier formation of (Co, Ta)-doped SnO2 varistors, a modified defect barrier model is introduced.

Effect of Co2O3 on the microstructure and electrical properties of Ta-doped SnO2 varistors

The effect of Co2O3 on the microstructure and electrical properties of Ta-doped SnO2 varistors was investigated. It was found that a sample doped with 0.1 mol% Co2O3 had the highest nonlinear coefficient α = 33, the highest breakdown electrical field EB = 872 V mm−1 and the lowest relative dielectrical constant er = 598 (measured at 1 kHz). However, 0.1 mol% Co2O3 is not sufficient for densification of SnO2 ceramics, and the relative density of the sample doped with 0.1 mol% Co2O3 (85.8%) is much lower than that of the samples doped with 0.3, 0.5, 0.8 and 1.2 mol% Co2O3 (about 98%). The highest breakdown electrical field and lowest relative dielectric constant of the sample doped with 0.1 mol% Co2O3 are mainly the result of the loose microstructure and the smallest average grain size. The measurements of grain boundary barrier height, ΦB, and grain boundary resistance, RGB, indicate that CoSn× should be located at the depletion layer and is important to the formation of the grain boundary barrier.