Key Engineering Materials Vols. 602-603

Abstract: (1-x)BiScO₃-xPbTiO₃ (BSPT) ceramics with grain size range of 0.5~2.5 μm at morphotropic phase boundary (MPB) were prepared by conventional sintering method. The grain sizes of the ceramics were determined by scanning electron microscopy (SEM). The grain size effects on the structure of BPST ceramics were analyzed by X-ray diffraction (XRD). The ferroelectric hysteresis response under various uniaxial compressive stress of up to 150 MPa was measured. With increasing the mechanical uniaxial stress, the remnant polarization (P_r) and the coercive field (E_c) were significantly reduced. The direct current (DC) bias dependence of capacitance of MPB-BSPT ceramics with different grain sizes was compared. The results indicated that the domain switching was prevented by the compressive stress. The domain size and domain structure were influenced by the grain size. The domain switching under uniaxial stress and electric field became more difficult due to the decrease of grain sizes. The grain size as well as the domain size played a key role in the properties under external uniaxial compressive stress and electric field.

Abstract: The effects of CuO-B₂O₃ (CBO) additive on densification, microstructure, and electrical properties of (Ba_0.98Ca_0.02)(Sn_0.04Ti_0.96)O₃ (BCST) cermaics were investigated. The phase compositions, microstructure, dielectric, ferroelectric and piezoelectric properties of BCST ceramics were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), precision impedance analyzer, ferroelectric tester and quasi-static piezoelectric constant testing meter, respectively. The results show that CuO-B₂O₃ (CBO) frit can improve the densification and promote low-temperature sintering of BCST ceramics prepared by a conventional solid-state reaction method. Dense BCST ceramics with CBO can be sintered at temperatures as low as 1175 °C, which is approximately 275 °C less than the sintering temperature of pure BCST. When sintered at 1200 °C, the optimized properties of the BCST ceramics with 0.5 wt% CBO were obtain as ε = 1206, d₃₃ = 346 pC/N, k_P = 0.39, tgδ = 0.009.

Abstract: Uniaxial compressive stress was applied during fatigue process of soft lead zirconate titanate piezoelectric ceramics and their fatigue resistance was improved when the stress was larger than 20MPa. Before fatigue, compressive stress had a strong depolarization effect and restricted domains switching behavior under large electric field and domain walls motion under small electric field. However, in a partially fatigued state, while domains switching behavior was still restricted by compressive stress, domain walls motion was enhanced. Removal of the applied stress after partial fatigue induced the remnant polarization restored significantly.

Abstract: Lead-free piezoelectric 0.97 K_0.5Na_0.5NbO₃-0.03 AlFeO₃ (KNN-AF) ceramics were prepared at low temperature of 980 °C to 1020 °C by the conventional ceramic process. The effect of sintering temperature on the crystal structure, density and electrical properties of the ceramics was investigated. The results indicate that KNN-AF ceramics sintered at an low temperature of 1000 °C exhibit high electrical and piezoelectric properties, with piezoelectric constant d₃₃=116ρC/N, and electromechanical coupling factor k_p = 32.9%, polarization (P_r) was P_r =21.8 μC/cm² and curie temperature T_C=382°C. This also indicates that KNN-AF ceramics are promising candidate materials for lead-free piezoelectric applications.

Abstract: In this paper, the KNN powders with Li⁺ substituted (K_0.5Na_0.5)⁺ were prepared by the sol-gel method in order to make powders pure, (Na_0.50+xK_0.50-2xL_ix) -NbO₃ (x=0, 0.04, 0.06, 0.08) ceramics were prepared by pressureless sintering. The phase composition of powders was determined by XRD. The effects of the Li⁺ substitution on piezoelectric and ferroelectric properties of ceramics were systematically studied for determining optimal Li⁺ content. Li⁺ can significantly increase the piezoelectric constant of ceramic. When Li⁺ content was 0.04, piezoelectric properties of ceramics were the best, d₃₃=115 pC/N, P_r=6.35 μC/cm², E_c=14.24 kV/cm.

Abstract: Through a conventional ceramic process, Y₂O₃ and Sb₂O₃ co-doped ZnO-based varistors were prepared. The microstructure and electrical properties of the as-prepared varistors were investigated. Y₂O₃ could act as an inhibitor to the growth of ZnO grains when working with Sb₂O₃. The mean size of ZnO grains in the Sb₂O₃ and Y₂O₃ co-doped samples was smaller than those of the samples only added with Y₂O₃ or Sb₂O₃. And with appropriately increased ratio of Y:Sb, it would result in increased sample densification. When the doping level of Y₂O₃ was small, the nonlinear coefficient and breakdown voltage of the varistors would increase with increasing doping amounts of Y₂O₃, and the leakage current would decrease. However, when Y₂O₃ was doped without Sb₂O₃, both the nonlinear coefficient and breakdown voltage of the varistors would decrease sharply, thus the leakage current increase dramatically. The electrical properties of the Sb₂O₃ and Y₂O₃ co-doped varistors would be better than those of the samples only added with Y₂O₃ or Sb₂O₃, and when the Y:Sb atom ratio was 5, the nonlinear coefficients, breakdown voltages and leakage current of the varistors reached their optimum values of 777 V/mm, 23 and 0.17 mA/cm², respectively.

Abstract: Barium strontium titanate glass-ceramics with different La₂O₃ concentrations have been prepared by a melting-annealing technique. The effect of La₂O₃ concentration on phase evolution, microstructure and electrical properties of (Ba,Sr)TiO₃-Al₂O₃-SiO₂ glass-ceramics has been investigated. The X-ray diffraction results showed the volume fraction of the major perovskite (Ba,Sr)TiO₃ crystalline phase increased with increasing La₂O₃ concentration. In addition, the scanning electron microscopy observations revealed that small amounts of La₂O₃ could accelerate the growth of crystals. When the La₂O₃ concentration was increased to 1.5 mol%, the microstructure of these samples displayed nonuniform distribution of the crystals. The dc conductivity measurements showed an increasing temperature dependence of conductivity with La₂O₃ doping at high temperature range above 400 °C. Impedance spectroscopy analysis indicated that the additive of La₂O₃ changed the major polarization mechanism to the relaxation polarization and the relaxation time for the La₂O₃ doped samples was much shorter than the undoped ones. The frequency dependence of dissipation factor curve displayed a peak at the range of 1 to 10 kHz due to the La₂O₃ addition. And the peak value decreased with increasing La₂O₃ concentration.

Abstract: Abstract. This article attempts to use a new sensing electrode material _{La5/3Sr1/3NiO4} for mixed-potential-type NOx sensors as they are high ionic conductivity, close thermal expansion coefficients to electrolytes (YSZ), low-cost and stable even at above 1000 °C. The nanostructured La_5/3Sr_1/3NiO₄ was synthesized as a sensing electrode (SE) for the mixed-potential-type yttria stabilized zirconia (YSZ)-based NO sensor by a sol-gel method. The synthesized La_5/3Sr_1/3NiO₄ powders were characterized using XRD, BET and FESEM. The sensing characteristics were examined in the temperature range of 400-700 °C. At 450 °C, the sensor exhibited the biggest response (about 20.3 mV for 1000 ppm NO). Moreover, the sensor response and recovery were generally rapid at all the temperatures. This work demonstrated that the La_5/3Sr_1/3NiO₄ can be an effective sensing electrode candidate of mixed-potential-type NO sensors.

Abstract: The powder of (La_0.8Sr_0.2)₂FeNiO_6-δ (LSFN) oxide with double-perovskite structure was synthesized by polymeric precursor method. Then the YSZ-based NO sensors with LSFN sintered at different temperatures (1000, 1100, 1200 and 1300 °C) as sensitive electrode (SE) were fabricated. All samples were characterized by XRD. The morphologies of the LSFN-SEs were observed with ESEM. The NO sensing properties of the sensors were investigated in the operating temperature range of 350-650 °C in 10 vol. % O₂. Results demonstrated that the sensor with LSFN-SE sintered at 1300 °C exhibited highest response to 500 ppm NO at 400 °C, which was about 85 mV. A linear relationship was obtained between the emf and the logarithm of NO concentration from 500 to 800 ppm at 400 and 500 °C. Moreover, both magnitude and slope to NO response decreased as operating temperature increased. And both the response time and recovery time shortened as temperature increased. But the recovery rate was slower than the response rate, especially at and below 450 °C. The optimal sensor response was obtained at 500-550 °C.

Abstract: Mg₂Cu_xFe₁O_3.5+x mixed metal oxides were found to be active catalysts for the selective oxidation of ammonia. In this paper, Mg₂Cu_xFe₁O_3.5+x mixed metal oxides were prepared by co-precipitation method and calcined at 600°C for 12 h in an air atmosphere, then ammonia sensors have been made by screen-printed Mg₂Cu_xFe₁O_3.5+x electrode on the electrolyte (8moleYSZ) surface and sintered at 1126°C for 1h in an air atmosphere. The samples were characterized by XRD and ESEM. The performance of ammonia sensors were tested in different concentration of ammonia. It can be found that Mg₂Cu_0.25Fe₁O_3.75 electrode showed a higher response value and good sensitivity to ammonia at 350°C, the value of sensitivity is 226.6mV/decsde. The influences of Cu content on the ammonia response performance of Mg₂Cu_xFe₁O_3.5+x mixed metal oxides electrodes are discussed.