Wang Jin-Feng

Nonlinear Electrical Behaviour of the TiO2?Sb2O3 System

The Sb2O3-doped TiO2 varistors were obtained by conventional ceramic processing. The 0.75 mol% Sb2O399.25 mol% TiO2 varistor has a nonlinear coefficient α = 7 and a minimum breakdown electrical field of 6 V/mm. The nonlinear electrical behaviour of TiO2Sb2O3 ceramics was explained by the introduction of defects in the crystal lattice that are responsible for the formation of Schottky potential barriers at grain boundaries.

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.

(Na0.52K0.44Li0.04)Nb0.9-xSbxTa0.1O3 Lead-Free Piezoelectric Ceramics with High Performance and High Curie Temperature

(Na0.52K0.44Li0.04)Nb0.9-xSbxTa0.1O3 lead-free piezoelectric ceramics are prepared by a solid-state reaction method. With increasing Sb content, the transition temperature from orthorhombic to tetragonal polymorphic phase decreased. A composition (Na0.52K0.44Li0.04)Nb0.863Sb0.037Ta0.1O3 is found to possess excellent piezo-electric and electromechanical performances (d33 = 306 pC/N, kp = 48%, and kt = 50%), and high Curie temperature (TC = 320°C). These results indicate that (Na0.52K0.44Li0.04)Nb0.863Sb0.037Ta0.1O3 is a promising lead-free piezoceramics replacement for lead zirconate titanate.

Grain-Size Effect on the Dielectric Properties of Bi0.5Na0.5TiO3

We prepared bismuth sodium titanate (Bi0.5Na0.5TiO3) ultrafine powders by the sol-gel method. The dielectric properties of the pressed pellets and fired ceramics with different grain sizes as a function of temperature at various frequencies were studied. With decreasing grain size, the dielectric anomaly around 200°C increases, while the dielectric thermal hysteresis decreases. All the samples with grain sizes larger than 100 nm show dielectric peaks at temperature of about 350°C. The very little change in Tm observed down to the critical size indicates that Bi0.5Na0.5TiO3 is an order-disorder system above 200°C. In addition, the dielectric peak becomes lower with decreasing grain size and the ferroelectric critical size of Bi0.5Na0.5TiO3 was eventually determined to be about 100 nm according to the disappearance of dielectric peak.

Nonlinear Electrical Behaviour of the TiO2?Y2O3?Nb2O5 System

The nonlinear and dielectric properties of a new TiO2-based varistor system, TiO2Y2O3Nb2O5, have been investigated. The (Nb, Y)-doped TiO2 ceramics have nonlinear coefficients of α = 5-8 and a lower breakdown electrical field of 6-12 Vmm-1, in which the varistor of (99.3 mol%)TiO2(0.6 mol%)Y2O3(0.1 mol%)Nb2O5 composite sintered at 1400°C has a nonlinear coefficient of α = 7.8 and a maximum breakdown electrical field of 12 Vmm-1 at 1 mAcm-2. Also, the relative dielectric constants of the composite reach more than 85 000, which is almost unchanged within the temperature range from -20 to 280°C and the frequency range of 0.5 K-2 MHz. Due to these properties the (Nb, Y)-doped TiO2 varistor has varistor-capacitance bifunctional components, which are quite useful in the situation that the voltage protection and high-frequency noise absorption are meanwhile required.

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.

Effects of Li Substitution and Sintering Temperature on Properties of Bi0.5(Na, K)0.5 TiO3 Lead-Free Piezoelectric Ceramics

Bi0.5(Na0.72 K0.28-xLix)0.5TiO3 (BNKLT-1OOx) lead-free piezoelectric ceramics are synthesized by conventional solid state sintering techniques. The dielectric and piezoelectric properties of the BNKLT-1OOx ceramics as a function of Li content are systematically investigated. It is found that not only Li content but also the sintering temperature has a strong effect on the piezoelectric properties of BNKLT. The piezoelectric constant d33 of BNKLT varies from 120 to 252pC/N in the Li content range from 0.03 to 0.16. In the sintering temperature range from 1080 to 1130° C, the d33 value of BNKLT-6 changes from 200pC/N to 252pC/N. The BNKLT-6 sample sintered at 1100° C has the highest piezoelectric constant d33 of 252pC/N, with the electromechanical coupling factors kp of 0.32 and kt of 0.44.

Phase Transition and High Piezoactivity of Sr Doped Lead-Free (Na0.53.K0.422Li0.048) (Nb0.89Sb0.06Ta0.05)O3 Ceramics

Lead-free piezoelectric ceramics (Na0.53K0.422Li0.048) (Nb0.89Sb0.06Ta0.05)O3(NKLNST) + x mol SrCO3 are prepared by conventional solid state sintering method. The specimens with pure perovskite structure show tetragonal phase at x ≤ 0.01, and become pseudo-cubic phase at x≥ 0.02. A lattice parameter discontinuity is found in the specimens with 0.004 ≤ x ≤ 0.0075, along with a great improvement in piezoactivity. The 0.004mol SrCO3 added NKLNST ceramics possesses outstanding performances of kp = 0.53, kt = 0.26, and d33 = 309 pC/N. Moreover, the Sr2+ modification inhibits the grain growth, decreases the Curie temperature, and induces a diffuse phase transition.