Microstructure and electrical properties of ZnO-based varistors prepared by high-energy ball milling

ZnO-based varistor ceramics were prepared at sintering temperatures ranging from 900 °C to 1,300 °C, by subjecting the mixed oxide powders to high-energy ball milling (HEBM) for 0, 5, 10 and 20 h, respectively. Varistor ceramics prepared by HEBM featured denser body, better electrical properties sintered at low-temperature than at traditional high-temperature. The high density is due to the refinement of the crystalline grains, the enhanced stored energy in the powders coming from lattice distortion and defects as well as the promotion of liquid-phase sintering. Good electrical properties is attributed to proper microstructure formed at low-temperature and improved grain boundary characteristics resulting from HEBM. With increasing sintering temperatures, the electrical properties and density became worse due to the decrease in amount of Bi-rich phase. Temperature increased up to 1,200 °C or above, the Bi-rich phase vanished and the ceramics exhibited very low nonlinear coefficient.     

Nanostructured constituents of ZnO-based varistors prepared by mechanical attrition

Nanostructured powders, with crystallite size of ZnO, spinel and γ-Bi₂O₃ phases below 100 nm, were obtained by the mechanical attrition of ZnO varistor powders prepared by method of directed synthesis of the constituent phases. Powders were further sintered and the resulting varistors showed excellent electrical characteristics with the non-linearity coefficients reaching 45, low values of the leakage current and density reaching 99% of the theoretical value.     

Importance of oxygen atmosphere to recover the ZnO-based varistors properties

A methodology to recover the non-ohmic properties of ZnO based varistors after degradation with long and short duration pulses was proposed in this work. The basic idea consists in submitt the degraded ceramics at different temperatures and oxygen flows. Thermal treatment at 900 °C for 2 h with oxygen flow of 15 l/h allowed to obtain better non-linear coefficient (α = 52.5) compared to the standard sample. Rietveld refinement showed that with the thermal treatment, the oxygen species and the β-Bi₂O₃ phase, lost in the degradation process, are recovered in the grain boundary.     

Effect of spark plasma sintering on microstructure and electrical properties of ZnO-based varistors

Conditions for the elaboration of varistors by spark plasma sintering (SPS) are investigated, using 70 nm zinc oxide nano-particles. For this purpose, the system constituted of zinc oxide, bismuth oxide and other metal oxide is used. Material sintering has been performed by SPS at various temperatures and dwell times. Determination of the microstructure and chemical composition of the as-prepared ceramics are characterized by scanning electron microscopy and X-ray diffraction analysis. Micro-structural analysis revealed the presence of ZnO, spinel and bismuth rich phases. ZnO based Varistor samples sintered within climb speeds 100 and 400 °C/min are compared. The nonlinear electrical characteristics, current–voltage, are measured. The breakdown voltage of the varistors strongly depends on grain sizes. The results show that the best varistors are obtained by SPS at sintering temperatures ranging from 900 to 1200 °C.     

氢化燃烧合成镁基储氢合金的电化学性能的研究

研究了氢化燃烧合成Mg2NiH4产物的电化学性能，并探索了机械球磨处理对产物电化学性能的影响。电化学测试表明，HCS产物不经任何处理，最大放电容量仅为45.13mAh／g；产物球磨后最大放电容量和高倍率放电能力得到提高，如产物经球磨1h后，最大放电容量增至259．24mAh／g，产物添加3％（质量分数）的石墨球磨5h，最大放电容量增加了10倍以上，达到481．50mAh/g。     

Magnetic properties of nanosized Sr2CrReO6 powders prepared by a combustion synthesis process

Double perovskite, Sr₂CrReO₆, nanoparticles in the size range 10–25 nm have been prepared by a combustion method. Its coercivity (H_c) at 5 and 300 K are determined to be 16.1 and 6.5 kOe, respectively. Using the law of approach to saturation, the obtained saturation magnetization σ_s (5 K) and σ_s (300 K) are 0.41 and 0.32 μB per formula unit (f.u.), respectively. The values of saturation magnetization are lower than the expected value 1 μB/f.u. considering the antiferromagnetic coupling between Cr³⁺ (3d³, S = 3 / 2) and Re⁵⁺ (5d², S = 1). At the same time high field irreversibility in M(H) curves and zero field cooled–field cooled (ZFC–FC) processes has also been found. A field-cooled (FC) hysteresis loop shift decreases with increasing temperature and vanishes near 350 K. It is suggested that the observed reduction of σ_s and shifted hysteresis loops are caused by the exchange interaction between the ferromagnetic core of the Sr₂CrReO₆ nanoparticles and its surface layer with disordered spins.     

Electrical properties and degradation phenomena of glass-doped ZnO chip varistors

Several glass additives with compositions in the PbO---B₂O₃---ZnO---SiO₂ system were applied to multilayer type ZnO ceramic chip varistors which had been manufactured using the ceramic green-sheet lamination technique. Measurements of capacitance-voltage (C-V), current voltage (I-V) and degradation behavior, as well as microstructural observations, were performed. It was found that specimens into which the above-mentioned glasses were incorporated had good surgewithstanding capability, with the exception of samples including Zn---B---Si glass. The electrical properties of speciments deteriorated gradually with increasing glass content. The addition of glasses improved the thermal stability of ZnO varistors except for G4 glass-doped sample (60 w/oPbO−12 w/oB₂O₃−605w/oZnO). Stable varistors with good nonlinearity parameters were obtained by combining the glass compositions and other additives.     

Microstructural and electrical properties of varistors prepared from coated ZnO nanopowders

This paper describes a solution-based technique for fabrication of varistor grade composite nanopowders. The method consists of coating major varistor dopants on the surface of the ZnO nanoparticles. As a result, a homogenous mixture of dopants and ZnO nanoparticles will be achieved. TEM results indicated that a composite layer of dopants with the average particle size of 9 nm on the surface of ZnO nanoparticles has been successfully prepared. Sintering of the coated powders was performed in temperatures as low as 850 °C and final specimens with average particle size of 900 nm and density of 98.5% were achieved. In comparison to conventional mixing, varistors prepared from coated nanopowders exhibited superior electrical properties and microstructure homogeneity. The improvement of electrical properties can be attributed to small grain size, homogenous distribution of dopants and elimination of large Bi-Pockets. In addition, the processing route of schottky barrier formation is quite different from what is generally considered as the method of barrier formation in ZnO grain boundaries.     

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 influences of bismuth antimony additives and cobalt manganese dopants on the electrical properties of ZnO-based varistors

This paper investigates the influences of additive contents and the additive ratio to dopants on the electrical characteristics of ZnO-based varistors. Bismuth and antimony are chosen as the additives while cobalt and manganese are selected as the dopants in this study. Our previous works discussed the influences of the initial additive content on the electrical characteristics of ZnO-based varistors without considering the dopant content and the weight loss during processing and sintering. Therefore, in this study, we fabricated varistors with same initial formula after sintering for 1, 3 and 5 h, respectively. The sintering temperatures were 950 and 1100 °C. After sintering, the additive content and dopant content of the varistors were measured using an inductively coupled plasma-atomic emission spectrometer (ICP). The experimental results showed that when the additive-content varies from 1.44 to 3.59 at % and the dopant/additive ratio changes from 0.16 to 0.69, the nonlinear coefficient, α, reaches up to 48 and the breakdown field E_bk is to 895 V mm⁻¹. The average grain size is 2.7 μm. The α value is higher with the higher additive-content and the lower dopant/additive ratio and vice versa. The breakdown field E_bk is increased with the additive-content increasing and sintering temperature lowering at a given dopant-content. The grain size is increased with the increase in sintering temperature while the bismuth concentration decreased. The observed effects are related to the quality of grain boundaries and the conductivity of grains.     

Electrical and thermomechanical properties of materials based on nonstoichiometric titanium carbide prepared by self-propagating high-temperature synthesis

This paper examines the effect of the chemical and phase compositions of nonstoichiometric titanium carbide prepared by self-propagating high-temperature synthesis on the electrical and thermophysical properties of polymer compounds. Doping of titanium carbide with trace amounts of nitrogen raises the conductivity of the polymer compounds. The addition of epoxy to the polymer matrix markedly reduces the thermal expansion of the material. The proposed materials can be used at temperatures of up to 300–350°C.     

激光燃烧合成掺杂钨精矿Fe-Al复合材料组织及性能

通过对掺杂不同含量钨精矿粉末Fe-Al系粉末压坯进行激光点燃自蔓延烧结（SHS）,利用XRD、SEM、硬度测试、磨损测试表征手段,分析研究了钨精矿粉含量对烧结合金微观组织结构及宏观性能的影响。实验结果表明：在烧结过程中,压坯实现了自蔓延燃烧。合金组织为针状组织,且随着钨精矿粉含量的增加,针状组织长大。产物物相主要为AlFe、AlFe3、WO3以及Fe7W6。当钨精矿粉添加量为1%时,烧结合金显微硬度最大,达到HK956,烧结合金的密度最大,为4.27g/cm3,孔隙率最小,为12.2%,烧结合金的相对磨损率最低,达到0.05%。     

脉冲激光沉积氮化铝薄膜的电学性能研究

采用脉冲激光沉积(PLD)技术在硅片上合成了AlN薄膜.X射线衍射(XRD)结果证实制备的AlN薄膜具有(002)择优取向的六方纤锌矿晶体结构,并且结晶质量随Si衬底温度的提高而改善.电流-电压(I-V)、电容-电压(C-V)、极化曲线结果表明室温生长的AlN薄膜的击穿场强约2.5MV/cm,同时呈现明显的极化现象(类铁电),对应矫顽场强为150kV/cm,剩余极化为0.002C/m2.晶态AlN存在较强的自发极化,薄膜中可动电荷密度高,据此提出了动态电荷模型,指出较大的AlN薄膜极化回线是由于可动电荷在电场中的再分布形成的,因而有别于铁电材料.     

超重力场燃烧合成TiCTiB2凝固陶瓷组织与性能

通过调整反应体系中Ti、 C及B之间的原子摩尔比, 采用超重力下燃烧合成工艺, 制备出TiB₂系列摩尔分数的TiC-TiB₂复合陶瓷。利用场发射扫描电镜(FESEM)观察了复合陶瓷微观组织, 研究了TiB₂成分对复合陶瓷力学性能的影响。结果表明: 随着TiB₂摩尔含量增加, 陶瓷基体逐渐从TiC球晶组织转化为TiB₂片晶组织, 在TiB₂摩尔分数为50%时, 可获得细晶乃至超细晶TiC-TiB₂复合陶瓷, 而且残留于基体上的α-Al₂O₃夹杂量也最低。陶瓷相对密度、 Vickers硬度与弯曲强度均在50%TiB₂(摩尔分数, 下同)时呈现最大值, 而陶瓷断裂韧性则在66.7% TiB₂时出现最高值。陶瓷断裂模式为TiC穿晶断裂与TiB₂沿晶断裂的混合模式, 且随TiB₂摩尔分数增加至66.7%, TiC穿晶断裂倾向显著减弱而TiB₂沿晶断裂倾向明显增强。TiC-TiB₂细晶及超细晶凝固组织的获得使TiC-50%TiB₂复合陶瓷在小尺寸TiB₂片晶诱发的裂纹偏转、 裂纹桥接及片晶拔出增韧机制作用下, 具有最高的弯曲强度及较高的断裂韧性。     

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.     

掺杂SnS薄膜的制备及电学性能

硫化锡（SnS）具有很高的光吸收系数和合适的禁带宽度，又无毒性，因此在太阳电池等光电器件中具有潜在应用价值。本文用真空蒸发法制备掺杂的SnS薄膜，掺杂源有Sb、Sb：O3、Se、Te、In、In2O3、Se和In2O3的混合物。对各种掺杂SnS薄膜的厚度、电流-电压（Ⅰ—Ⅴ）特性等进行了表征，并计算了其电阻率和光电导与暗电导的比值（Gphoto／Gdark）。结果表明较有效的掺杂源是Sb，Sb掺杂的薄膜电阻率比纯薄膜的电阻率降低四个数量级，Gphoto／Gdark增加约一倍。同时，研究了Sb掺杂量对SnS薄膜电学性能的影响，表明Sb的最佳掺入量约为1．3wt％～1．5wt％。     

Electrical properties of silver selenide thin films prepared by reactive evaporation

The electrical properties of silver selenide thin films prepared by reactive evaporation have been studied. Samples show a polymorphic phase transition at a temperature of 403 ± 2 K. Hall effect study shows that it has a mobility of 2000 cm²V^?1s^?1 and carrier concentration of 10¹⁸ cm^?3 at room temperature. The carriers are ofn-type. X-ray diffraction study indicates that the as-prepared films are polycrystalline in nature. The lattice parameters were found to bea= 4.353 Å,b= 6.929 Å andc = 7.805 Å.     

TiC-xNi金属陶瓷的燃烧合成及其热物理性能

利用自蔓延高温燃烧民结合机械厂压力方法制备了TiC-Ni金属陶瓷,研究了ＴiC-Ni金属陶瓷材料的密度,比热度、比热容、热传导以及热膨胀性质随温度和组成的变化关系,结果表明：ＴiC-Ni材料的密度在w(Ni)为２０％时致密性达到最高,材料的比定压热容在浊试温度区内呈线性增加;ＴiC-Ni材料的热扩散率随度的升高基本呈增加趋势,热扩散率随Ｎi含量的增加而增加,热导率的变化与热扩散率的变化有相似的规律,材料的热膨胀系数随温度的升高单调递增,温度上升相同情况下,材料的热膨胀系数随Ｎi含量的增加而增加。     

超重力下燃烧合成TiB_2-TiC共晶复合陶瓷

采用超重力下燃烧合成技术,制备出TiB2-TiC共晶复合陶瓷。XRD、SEM与EDS结果表明,复合陶瓷主要由大量细小的TiB2片晶均匀分布于TiC基体上的共晶组织构成,而富钛ε碳化物(Ti,Cr)C1-x则断续分布于TiC基体间,同时在基体中还孤立分布着少量的、形态不规则的α-Al2O3晶粒或Al2O3-ZrO2共晶团组织。高温化学反应使所有产物均呈液态,且超重力的引入诱发熔体内部Stocks流,从而获得液态Ti-Cr-C-B与液态氧化物的分层熔体,液态Ti-Cr-C-B在远离平衡态下发生共晶反应生成TiB2-TiC共晶复合陶瓷。性能测试表明,随着B4C+Ti+C在燃烧体系中质量分数增加,TiB2-TiC共晶复合陶瓷相对密度和断裂韧性变化不大,分别为97%~99%与6.5~7.1 MPa.m1/2,而维氏硬度与弯曲强度则逐渐增加,最高可达28.6 GPa与615 MPa。     

pH值对溶液燃烧法制备纳米LaFeO3的影响

以硝酸镧和硝酸铁为氧化剂,柠檬酸为燃料,采用溶液燃烧法制备了钙钛矿型LaFeO3.通过XRD和SEM研究了分别以氨水和NaOH溶液为pH调节剂,前驱物溶液pH值对制备的纳米LaFe03粉体的晶相组成和微观形貌的影响.结果表明不同的pH调节剂和前驱物溶液pH值对燃烧产物的晶相组成、晶粒大小和微观相貌都有显著的影响.氨水调...