Hong-Cun Chen

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.

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.

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.

Ferroelectric, Piezoelectric and Pyroelectric Properties of Sr1+xBi4-xTi4-xTaxO15 Ceramics (x=0–1)

The dielectric, ferroelectric, piezoelectric and pyroelectric properties of bismuth layer-structured ferroelectric (BLSF) Sr1+xBi4-xTi4-xTaxO15 (x=0–1) ceramics were investigated. The Curie temperature of this system is from 540°C to 30°C, and all samples show ferroelectricity at room temperature. From piezoelectric measurements, Sr1.1Bi3.9Ti3.9Ta0.1O15 was found to have the largest piezoelectric coefficient and electromechanical coupling factor of compounds in this system. The thermally stimulated current measurements showed that Sr1.1Bi3.9Ti3.9Ta0.1O15 also has excellent pyroelectric responses, and the enhancement of pyroelectric properties may partially result from a relatively large temperature–dependent internal electric field which was observed from the polarization–electric field (P–E) hysteresis loops.

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.

Effects of Cr2O3 on the properties of (Co, Nb)-doped SnO2 varistors

The effects of Cr2O3 on the properties of (Co, Nb)-doped SnO2 varistors were investigated. The samples with different Cr2O3 concentrations were sintered at 1350 °C for an hour. The properties of (Cr, Co, Nb)-doped SnO2 varistors were evaluated by determining their I–V and e–f relations, measuring their resistivities, scanning electron microscopy. It was found that the breakdown electrical field increases from 400 to 1000 V mm−1 and relative electrical permittivity decreases from 2022 to 147 with increasing Cr2O3 from 0.00 to 0.07 mol%. Nonlinear coefficient presents a peak of α=52 and grain boundary barrier becomes highest when 0.06 mol% Cr2O3 was added. Electrical permittivity and grain size decreases with increasing the content of Cr2O3. In order to illustrate the grain boundary barrier formation in this varistor system, an interface defect model was introduced.

Effects of In2O3 on the properties of (Co, Nb)-doped SnO2 varistors

The effects of In2O3 on the properties of (Co, Nb)-doped SnO2 varistors were investigated. It was found by characterizing the samples sintered at 1350°C that the nonlinear coefficient presents a peak of α = 20.4 for the concentration of 0.05 mol% In2O3, the average grain size decreases from 8.4 to 3.9 μm, the breakdown electrical field increases from 206 to 821 V mm−1 and relative electrical permittivity decreases from 2.3 to 0.16 k with increasing In2O3 from 0.00 to 0.10 mol%. The increase of the breakdown electrical field with increasing In2O3 concentration is mainly attributed to the decrease of the grain size. The reason why the permittivity decreases with increasing In2O3 concentration was originated from the ratio of the grain size to the barrier width. To illustrate the grain-boundary barrier formation of (In, Co, Nb) doped SnO2 varistors, a modified defect barrier model was introduced, in which the negatively charged acceptors substituting for Sn ions should not be located at the grain interfaces instead at SnO2 lattice sites of depletion layers.