Microstructure, electrical properties, and degradation behavior of praseodymium oxides-based zinc oxide varistors doped with Y2O3

The microstructure, electrical properties, and degradation behavior of Pr-based zinc oxide varistors, which are composed of Zn-Pr-Co-Cr-Y oxides were investigated according to Y₂O₃ additive content in the range 0.5–4.0 mol%. The majority of the Sadded Y₂O₃ were segregated at the multiple ZnO grain junctions and grain boundaries. The average grain size was markedly decreased in the range 27.3–8.6 m with increasing Y₂O₃ additive content. Y₂O₃ acted as an inhibitor of grain growth. Additions of Y₂O₃ increased the varistor voltage in the range 36.90–686.58 V/mm, increased the nonlinear exponent in the range 3.75–87.42, decreased the leakage current in the range 115.48–0.047A, increased the barrier height in the range 1.06–2.16 eV, and decreased the donor concentration in the rang 1.87 × 10¹⁸–0.19 × 10¹⁸ cm^–3. Y₂O₃ acted as an acceptor, as a result of the decrease of donor concentration. All Pr-based ZnO varistors doped with Y₂O₃ exhibited very predominant degradation characteristics, which show a nearly symmetric I-V after the stress. In particular, since 4.0 mol% Y₂O₃-added ZnO varistor has not only very excellent non-ohmicity, but also very stable degradation behavior, it is estimated to be sufficiently used to various application fields.     

Temperature dependence of electrophysical characteristics of zinc oxide based varistors

ZnO-based varistors were fabricated by sintering zinc oxide micro crystals with several additives of metal oxides. To avoid the shortage of oxygen, the samples were annealed in a continuously oxygen injected oven at 400 °C for 3-6 h. Temperature dependence of current-voltage characteristics (CVC) and the electrical conductivity of these samples were investigated in the temperature range 25-190 °C. They exhibited high nonlinearity in their I-V characteristics. The nonlinear coefficient varied between 40 and 60 but the highest nonlinearity coefficient for an individual sample was β = 100. At high temperatures, nonlinearity in the I-V characteristics of samples disappeared and the conductivity became ohmic in nature. The observed nonlinearity, an exponential dependence of electric conductivity, and also reduction of samples opening voltage are related with zinc oxide grain boundaries. In all temperature intervals, the hysteresis on curves I = f (U) was observed.     

Microstructure, electrical properties, and dc aging characteristics of Tb4O7-doped ZnO-based varistors

The microstructure, electrical properties, and dc-accelerated aging characteristics of Tb₄O₇-doped ZnO-based varistors were investigated for different Tb₄O₇ amounts and sintering temperatures. The sintered density increased with increasing Tb₄O₇ amount and sintering temperature. The average grain size decreased with increasing Tb₄O₇ amount and increased with increasing sintering temperature. The varistor voltage and nonlinear coefficient increased with increasing Tb₄O₇ amount and decreased with increasing sintering temperature. The stability was worse with increasing Tb₄O₇ amount for the varistors sintered at 1,300 °C. The 0.5 mol% Tb₄O₇-doped varistors sintered at 1,350 °C exhibited a good stability for dc-accelerated aging stress of 0.95 V _{1 mA}/150 °C/24 h.     

The influence of heat-treatment on the current-voltage behavior of the ZnO-Bi2O3 based varistors

The post-sintering heat treatment on the current-voltage behavior of the ZnO-Bi₂O₃ based varistors is reported. It is suggested that only phase transition of the Bi₂O₃ is not alone responsible for the resulting electrical performance by these devices. The ambient condition influences the performance parameters. Thus, incorporating the role of oxygen and zinc interstitial in the total device during the post-sintering heat-treatment is accounted as responsible parameters for the ultimate performance. Two recipes are used in demonstrating the effect of heat-treatment on the functional behavior of the devices.     

Microstructure development in Sb2O3-doped ZnO

The microstructure development in Sb₂O₃-doped ZnO was studied at doping levels up to 2.0 mol%. Dopant Sb₂O₃ reacted with ZnO to form inclusion particles,α-Zn₇Sb₂O₁₂, and inhibited the grain growth of ZnO. With increasing doping level of Sb₂O₃, the growth rate of ZnO decreased whereas that of inclusion particles increased. Some inclusion particles were trapped in ZnO grains at low doping levels of Sb₂O₃, but the volume fraction of trapped inclusion particles decreased with increasing doping level. Stereological analysis of the size and number ratios of ZnO grains and inclusion particles indicated that a compatible assumption is needed to evaluate Zener effect in two-phase sintering.     

Co2O3掺杂对SnO2-Ni2O3-Nb2O5系压敏材料性能的影响

研究并分析了Ni^2＋掺杂和Co^2＋掺杂对SnO2压敏电阻致密度和电学非线性性能的影响。研究了掺Co^3＋对Sno2-Ni2O3-Nb2O5压敏材料性能的影响。实验结果表明，Co2O3在高温下可转变为CoO。Co^2＋的掺入不仅能够增大Sno2-Ni2O3-Nb2O5材料的质量密度，而且在线性系数，在较大程度上提高了Sno2-Ni2O3-Nb2O5压敏材料的性能。     

Microstructural and electrical characteristics of SiO2 doped ZnO-Bi2O3 varistors

Varistors in the new system ZnO-Bi₂O₃-SiO₂ were prepared through conventional ceramic processing route. The effect of sintering temperature and time (0·5 h to 2 h between 1000° and 1250°C) on the microstructure and current/voltage characteristics of the varistors of the new system were investigated and the results were compared with those of ZnO-Bi₂O₃ system varistors prepared. An increase in nonlinear coefficient (α) value was observed in the SiO₂ added varistors. The microstructure and the phase of the varistors were examined by means of SEM and XRD. The Zn₂SiO₄ spinel phase was found to be present in the intergranular region. The grain growth exponent was determined to be 2·5±0·2 and the activation energy for the ZnO grain growth was estimated to be 251±11 kJ/mol. These values were compared with those estimated for ZnO-Bi₂O₃ system varistors.     

Sintering and characterization of bismuth-oxide-containing zinc oxide varistors

Zinc oxide with different contents of Bi₂O₃ was prepared via a solid-state reaction to be used as varistors. Sintering was performed at 1200 and 1300 °C. Densification was achieved through liquid-phase sintering. A zincite phase, together with bismuth zinc oxide (Bi_7.65Zn_0.35O_11.83) and zinc bismuth oxide (Bi_25.33Zn_0.667O₄₀) phases, was formed at 1200 °C. A free Bi₂O₃ phase was still present at 1200 °C. Only the zincite phase was developed, and the other phases disappeared at 1300 °C. I–V characteristics show nonlinear behavior in all samples. The behavior was expected to be a result of the development of intergranular phases that crystallized from the Bi-rich liquid phase.     

Cr2O3对(Co,Ta)掺杂的SnO2压敏电阻电学特性的影响

研究了Cr对(Co,Ta)掺杂的SnO2压敏材料电学性质的影响.当Cr2O3的含量从0增加到0.15mol%时,(Co,Ta)掺杂SnO2压敏电阻的击穿电压从206V/mm增加到493V/mm;1kHz时的相对介电常数从1968猛降至498;晶界势垒高度分析表明,SnO2晶粒尺寸的迅速减小是样品击穿电压增高、相对介电常数急剧降低和电阻率迅速增大的主要原因.对Cr含量增加引起SnO2晶粒减小的原因进行了解释.掺杂0.15mol% Cr2O3的SnO2压敏电阻非线性系数为24,击穿电压达498V/mm,在高压保护领域有很好的应用前景.     

Phase separation in tellurite glass-forming systems containing B2O3,GeO2,Fe2O3,MnO,CoO,NiO and CdO

The boundaries between the regions of single-phase and two-phase glasses were established in tellurite glass-forming systems containing B₂O₃ and one of the following oxides: GeO₂, Fe₂O₃,CoO, NiO, MnO and CdO. The character of the microstructures inside and outside the regions of stable phase separation were determined by electron microscopy. It was shown that the existing microheterogeneities may either result from incomplete liquid immiscibility during melting and supercooling or be due to typical metastable separation.     

Effect of silver oxide doping on surface and catalytic properties of Co3O4/Al2O3 system

Cobaltic oxide catalyst supported on gamma-alumina having the formula 0.1 Co₃O₄/Al₂O₃ was prepared by wet impregnation method using finely powdered Al(OH)₃ and cobalt nitrate dissolved in the least amount of distilled water. Four silver oxide-doped samples were prepared by impregnating Al(OH)₃ solid with calculated amounts of silver nitrate dissolved also in the least amount of distilled water prior to impregnation with cobalt nitrate solution. The amounts of Ag₂O were 0.2, 0.4, 0.8 and 1.6 wt.%. Pure and doped solids were subjected to heat treatment at 400, 600 and 800 °C. Diffraction lines of Co₃O₄ phase were detected in the XRD patterns of pure and doped solids precalcined at different temperatures. However, the doping process conducted at different temperatures brought about a progressive significant increase in the particle size and degree of ordering of Co₃O₄ phase. Ag₂O-doping of the investigated system affected a measurable decrease in its specific surface areas. The catalytic activities of different solids, in CO-oxidation by O₂ conducted at 125–225 °C, were found to increase significantly by increasing the amount of dopant added reaching to a maximum limit at 0.8 wt.% Ag₂O. The maximum increase in the catalytic activity expressed as reaction rate constant k measured at 150 °C over the solids precalcined at 400 °C and at 175 °C for the solids precalcined at 600 and 800 °C attained 337%, 118% and 219%, respectively. The doping process did not modify the activation energy (E_a) of the catalyzed reaction, and the observed apparent changes in E_a values resulted from a compensation effect as evidenced from almost same changes in the values of pre-exponential factor of Arrhenius equation. Therefore, the observed increase in the catalytic activity of the investigated system due to Ag₂O treatment, which did not change the mechanism of the catalyzed reaction, could be attributed to an effective increase in the concentration of active sites contributing in chemisorption and catalysis of CO oxidation by O₂.     

Bi4Ti3O12 modified low voltage ZnO-based varistors: microstructure, electrical properties and aging characteristics

The effects of Bi₄Ti₃O₁₂ addition on the microstructure, electrical properties and aging characteristics of ZnO-based varistors were investigated. The addition of Bi₄Ti₃O₁₂ can reduce the formation of Zn₂TiO₄ spinel phase and promote the growth of ZnO grain. Bi₄Ti₃O₁₂ doped ZnO-based varistors sintered at 1,130 °C possessed excellent performance of I _L = 2.9 μA, E _1mA = 29.7 V/mm, α = 30.2, and good surge absorption capability with %△E _1mA = ?3.5 % for 500 A 8/20 μs impulse current. In addition, the varistors exhibited the most stable accelerated aging characteristics with %△E _1mA = ?0.3 % for DC accelerated aging stress of 125 °C/7 h. These results revealed that Bi₄Ti₃O₁₂ addition is highly beneficial to attain enhanced varistor properties of ZnO ceramics.     

AC ageing characteristics of Y2O3-doped ZnO varistors with high voltage gradient

In this paper, AC ageing characteristics of high voltage gradient ZnO varistors doped with different Y₂O₃ concentration were investigated. The voltage gradient of these samples is markedly improved by increasing Y₂O₃ content, however, the nonlinear coefficient decreases and the leakage current increases at the same time. Y₂O₃-doped ZnO samples exhibit lower stability under accelerated AC ageing stress of 0.85 V_1mA/135 °C/168 h, compared with samples without Y₂O₃. Double-Schottky barrier(DSB) parameters before and after ageing tests indicate that the decrease of barrier height for traditional sample is less than that for high voltage gradient sample, which should be ascribed to its slight variation in the interface state density.     

Microstructure and Thermoelectric Properties of Bi- and Cu-Substituted Ca3Co4O9 Oxides

Bi- and Cu-substituted Ca3Co4O9 samples were prepared by conventional solid-state reaction method and the effect of element substitution on the microstructures and thermoelectric properties was investigated. Partial substitution of Cu for Co leads to an increase in electrical conductivity and a decrease in Seebeck coefficient due to the rise of hole concentration. The microstructure of Cu-substituted sample is almost unchanged compared with undoped Ca3Co4O9. On the other hand, partial substitution of Bi for Ca gives rise to a significant increase in the grain size, and c-axis-oriented structure can be formed in Ca2.7Bi0.3Co4O9, resulting in an obvious increase in electrical conductivity. Cu and Bi co-substitution further increases the grain growth and the electrical conductivity of Ca2.7Bi0.3Co3.7Cu0.3O9. Thus, Cu and Bi co-substitution samples possess the optimal thermoelectric performance at high temperature and the highest value of power factor can reach 3.1×10-4 Wm-1·K-2 at 1000 K.     

Vapor phase synthesis, characterization and gas sensing performances of Co3O4 and Au/Co3O4 nanosystems

Al2O3-supported Co3O4 nanosystems were grown by a Chemical Vapor Deposition route under O2 + H2O atmospheres at 500 degrees C. Subsequently, the preparation of Au/Co3O4 composites was attained by Radio Frequency-Sputtering of gold onto the previous Co3O4 nanodeposits. Important data on the system structure, morphology and chemical composition were obtained by the combined use of complementary techniques, namely Glancing Incidence X-ray Diffraction, Field Emission-Scanning Electron Microscopy, Atomic Force Microscopy, Energy Dispersive X-ray Spectroscopy, X-ray Photoelectron Spectroscopy and Secondary Ion Mass Spectrometry. Finally, the gas sensing properties of the synthesized systems were probed in the detection of ethanol and hydrogen. The obtained results revealed significant responses already at moderate temperatures, which could be further enhanced by Co3O4 functionalization with Au nanoparticles.