Papers by Author: Zhi Wu Chen

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Authors: Zhen Ya Lu, Jian Qing Wu, Zhi Wu Chen
Abstract: The anti-reducing sintering of (Ca,Sr,Ba)TiO3-based dielectrics was achieved by the cooperating effect of Mn2+, Ca2+, Mg2+, Dy3+ and Yb3+. A small quantity of Zr4+ partly substitution for Ti4+ can improve the temperature dependence of dielectric constant. But ZrO2 will also reduce the solid solubility of BaTiO3-SrTiO3-CaTiO3 ternary system, and may cause phase separation. The separated ferroelectric phase increases the capacitance change under DC bias. Non-reducible (Ca,Sr,Ba)(Ti,Zr)O3-based dielectrics with single orthorhombic phase of perovskite structure was obtained. The dielectric and DC bias properties are: .C/C (DC bias: 1.5kV/mm) <0.05%, ε =250~300, tgδ < 0.5%.
Authors: Zhen Ya Lu, Yu Xiang Liu, Zhi Wu Chen, Jian Qing Wu
Abstract: The effect of Ho2O3 doping on the electrical properties and microstructure of ZnO base varistor was investigated. It was found that Ho2O3 is an effective dopant for increasing the breakdown electric filed. The Ho2O3 doping can also improve the nonlinear performance both in low and high current area. But excessive doping of Ho2O3 will decrease the withstanding surge current. With 0.8mol% Ho2O3 doping, the varistor samples exhibit a breakdown voltage of about 400V/mm, a nonlinear coefficient of 80 and the withstanding surge current of 8/20μs, waveshape is higher than 5kA. Ho2O3 dopant can hinder ZnO grain growth and make the crystal grains more uniform.
Authors: Zhi Wu Chen, Zhen Ya Lu, Xu Ming Chen, Ying Zhang, Xuan Cheng
Abstract: Electrical fatigue tests have been conducted on PLZT ferroelectric ceramics. It was found that the higher the applied electrical field magnitude, the faster the Pr decreases; the samples under the same electrical field magnitude but with square wave have higher fatigue rate than that of sine wave. It was observed that at low frequency of applied field (50Hz and 500Hz), the polarization decreased very quickly and dropped below 70% of the original value within 106.2 switching cycles, however, when fatigue test were conducted at high frequencies of 100kHz and 300kHz, the polarization did not change significantly even after 109 cycles. The SEM analysis indicated that the facture mode was mainly of trans-granular for the virgin non-fatigued sample, while of inter-granular for the fatigue sample. The quantitative information on the 90° domain switching was measured in situ using X-ray diffraction. The effects of electrical characters on electrical fatigue behavior were explained according to the mechanism of electrical fatigue in PLZT ferroelectric ceramics.
Authors: Zhi Wu Chen, Zhen Ya Lu, Jian Qiang Hu
Abstract: Lead-free piezoelectric ceramics (Bi0.5Na0.5)0.94(Ba1-xSrx)0.06TiO3 (abbreviated as BNBST-100x, with x ranges from 0.02 to 0.1) have been investigated. Effects of amount of Sr-substitution on the electrical properties and crystal structure of the ceramics were studied. The BNBST-100x ceramics sintered at 1200°C for 2h in air have high density around 5.69~5.75g/cm3. X-ray diffraction (XRD) analysis shows that all of the BNBST-100x ceramics have pure perovskite structure. At high amount level of Sr-substitution, the crystal structure of the samples changes from rhombohedral to tetragonal symmetry. Piezoelectric and dielectric measurements reveal that Sr-substitution amount within a certain range will lead to the increase of piezoelectric coefficient (d33), electromechanical coupling factor (kp), and relative dielectric constant (ε33 T/ε0). At 6 mol% Sr-substitution level, the d33 and kp of the ceramics reach maximum, with values of 168 pC/N and 34%, respectively.
Authors: Zhi Wu Chen, Zhen Ya Lu
Abstract: A lead-free piezoelectric ceramic ternary system based on (Bi0.5Na0.5)TiO3 (BNT)-(Bi0.5 K0.5)TiO3(BKT)-BaTiO3(BT) near the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases has been investigated. The samples were prepared by a conventional sintering technique. Piezoelectric properties, dielectric properties and microstructures of BNT-BKT-BT lead-free piezoelectric ceramic were studied. The X-ray diffraction (XRD) patterns show that the ceramics possess a single-phase perovskite structure. The measurements of piezoelectric properties reveal that this system has a relatively high piezoelectric performance: the piezoelectric constant d33 reached to 178pC/N and planar electromechanical coupling factor kp enhanced to 0.325. The BNT-BKT-BT ternary system ceramics could be well sintered at 11500C with a high-density ratio of more than 95% of the theoretical density.
Authors: Jian Qiang Hu, Zhi Wu Chen
Abstract: Bi2O3 doped (Bi0.5Na0.5)0.94Ba0.06TiO3 (BNBT6) lead-free piezoelectric ceramics were fabricated by a conventional sintering technique. The effects of Bi2O3 on the piezoelectric properties and microstructures of the doped BNBT6 were investigated. X-ray diffraction analysis showed that a solid solution was formed when Bi2O3 diffused into BNBT6 lattice and the crystal structure of the sintered hybrid changed from rhombohedral to tetragonal symmetry with increasing Bi2O3 amount. Piezoelectric and dielectric properties measurements revealed that doping Bi2O3 within a certain range enhanced the piezoelectric coefficient (d33), electromechanical coupling factor (kp), relative dielectric constant (ε33 T/ε0), and dielectric loss (tanδ). When 3mol% Bi2O3 was doped, both d33 and kp of the ceramics reached their maxima, 165pC/N and 24%, respectively.
Authors: Zhen Ya Lu, Zhi Wu Chen, Feng Jin Yang
Abstract: The voltage response of ZnO varistors to 8/20 µs surge current was investigated. The observed frontal spikes on the residual voltage waveforms are caused by the ignition gap, and no frontal spike was observed when a thyristor was used as the discharge trigger. The rear part of the waveform is determined by the damping coefficient of the RLC-circuit. Near the critical point, the residual voltage waveform changes from non-oscillating attenuation modes to distinct across zero oscillating modes along with the increase of the peak current, but there will be no oscillation happen when a thyristor is used as the discharge trigger. The residual voltage peak is not synchronized with the current peak, and the voltage peak is leading, implying that the ZnO varistor appears to be inductive. According to the experiment results, it can be reasonably explained that the voltage peak leading phenomenon is attributed to the transient skin effect of the varistor materials.
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