Wang Chun-Ming

Nonlinear Electrical Behaviour of Composite SnO2–Zn2SnO4 System

We have found a new composite SnO2?Zn2SnO4 varistor material and the effect of different content ratios of SnO2 to Zn2SnO4 on this system is investigated. The lowest breakdown voltage (EB = 6.4?V/mm) and the highest relative dielectric constant (?r = 1.54?104) with higher nonlinear coefficient (? = 4.5) is obtained when the ratio of SnO2 to ZnO equals 100/35. The disappear of the Zn2SnO4 peak in x-ray diffraction (XRD) spectra of the SnO2+2.5?mol%ZnO sample and the electric insulation behaviour of the samples with 2.5?mol% ZnO and 200?mol% ZnO indicate that 2.5?mol% ZnO can be dissolved into SnO2 lattices and some reaction contributing to nonlinear electrical behaviour should occur between SnO2 and Zn2SnO4. The role of oxygen on the grain boundary is confirmed by the degradation of nonlinear electrical properties and the increase of relative dielectric constant of the samples annealed in nitrogen-rich atmosphere.

KNN Based Lead-Free Piezoceramics with Improved Thermal Stability

Lead-free piezoelectric ceramics (1 − x)(Na0.53 K0.404 Li0.066) Nb0.92 Sb0.08 O3+x SrTiO3 are fabricated by conventional solid-state sintering method, and their dielectric and piezoelectric characteristics are investigated. With the addition of SrTiO3, the growth of the grain size is restrained, meanwhile the phase transition temperature of orthorhombic-tetragonal is shifted below room temperature. It is found that the ceramics with x = 0.010 exhibit excellent piezoelectric properties (d33 = 220 pC/N, kp = 41%, kt = 39%) and improved thermal stability around room temperature. The results indicate that these materials are promising lead-free piezoceramics for practical operations.

Thermal Stability and Humidity Resistance of ScTaO4 Modified (K0.5Na0.5)NbO3 Ceramics

Lead-free (Na0.5K0.5)NbO3-xmol% ScTaO4 (x = 0–1.5) ceramics are prepared using the conventional solid-state reaction method and their properties are investigated in detail. The results indicate that the piezoelectric properties and density are improved by the introduction of ScTaO4. Due to the high orthorhombic-tetragonal phase transition temperature TO-T (around 200° C), stable piezoelectric properties against temperature are obtained. In a wide temperature range of 15–160° C, kp of the (Na0.5K0.5)NbO3–0.5 mol% ScTaO4 ceramic remains almost unchanged and d31 increases slightly from 59 pC/N to 71 pC/N. The deliquescent problem is effectively solved by the addition of ScTaO4. The piezoelectric properties of ScTaO4 modified (Na0.5K0.5)NbO3 ceramics show no obvious reduction and dielectric loss increases slightly after 120 h of immersion. From the analysis, it is suggested that the density is an important factor that improves the humidity resistance of the specimens.

Phase Transition and High Piezoactivity of Sb Doped (Na0.53K0.435Li0.035)Nb0. 94Ta0.06O3 Lead-Free Ceramics

Lead-free piezoelectric ceramics with the composition of (Na0.53K0.435Li0.035)Nb0.94-x SbxTa0.06O3 (NKLNST) are synthesized by a conventional solid-state sintering process. An MPB-like region between orthorhombic and pseudocubic phases is found in this system. The density, piezoelectric and dielectric properties are enhanced greatly in this region. A composition (Na0.53K0.435Li0.035) (Nb0.86Sb0.08Ta0.06)O3 is found to have excellent electrical properties: d33 = 320pC/N, kp = 49% and kt = 43%, as well as relatively low loss, tan δ = 4.2%, and high relative density higher than 96%, which indicate that this ceramics is a promising lead-free piezoceramics replacing for lead zirconate titanate.

Nonlinear Electrical Characteristics of Antimony and Copper Doped Tin Oxide Based Varistor Ceramics

The novel CuO-doped dense tin oxide varistor ceramics are investigated. The densification of tin oxide varistor ceramics could be greatly improved by doping copper oxide additives. The introduction of antimony additives into a SnO2.CuO ceramic system would make it possess excellent nonlinearity. The sample doped with 0.05 mol% Sb2O3 possesses the highest nonlinearity coefficient (α = 17.9) and the lowest leakage current density (JL = 52 μA cm−2) among all the samples. A modified defect barrier model is introduced to explain the formation of the grain-boundary barrier. The nonlinear behaviour of (Cu, Sb)-doped SnO2 varistor system could be explained by the barrier model.

Enhancement of Thermoelectric Performance of Sr0.9Ba0.1Ti0.8Nb0.2O3 Ceramics by A-Site Cation Nonstoichiometry*

Sr0.9Ba0.1-x Ti0.8Nb0.2 O3 ceramics (x=0, 0.01, 0.02 and 0.05) are prepared by solid state reaction, whose thermoelectric properties are investigated from 323 K to 1073 K. By introducing A-site nonstoichiometry, the absolute See-beck coefficient is enhanced, while the electrical resistivity is surprisingly reduced due to the significantly enhanced carrier mobility. These results are dramatic in thermoelectric materials, effectively enhancing the power factor. Moreover, the thermal conductivity is reduced, thus the thermoelectric performance of Sr0.9Ba0.1 Ti0.8Nb0.2O3 ceramic is significantly enhanced by A-site nonstoichiometry.