A Novel Test Method for Ultrafast Pulse Response Characteristics of ZnO Varistor Ceramics

A ZnO varistors in series connected with a semi‐insulating GaAs photoconductive switches (SI‐GaAs PCSS's), a test method for ultrafast pulse response characteristics of ZnO varistor ceramics with DC bias was presented for the first time. The DC voltage distribution of the PCSS's and the varistors was measured in a dark state and the pulse response characteristics of the ZnO varistor ceramics was examined by a nanosecond laser pulse illuminating the PCSS's. The results indicate that the electric pulse output from the varistors includes capacitive current and conduction current and there is a time delay between their peaks. It is revealed that ZnO varistors has a nonlinear conductivity for the nanosecond electric impulse excitation and the barrier capacitance decay constant of the ceramics sample is 105 ns, which is explained through the analysis of examining the material structure and the conductive mechanisms.     

Co2O3的理化性能对ZnO压敏陶瓷电性能的影响

本文详细研究了氧化钴的理化性能对ＺｎＯ压敏陶瓷性能的影响。论述了Ｃｏ２Ｏ３的热特性对压敏陶瓷烧成工艺产生的影响及控制措施，从理论上探讨了氧化 中所含杂质Ｆｅ、Ｃｕ，Ｎａ等对ＺｎＯ压敏陶瓷电性能的不利作用。提出了控制的上限，并试制出一种用于氧化锌压敏陶瓷的性能优良的Ｃｏ２Ｏ３粉体。     

Characteristics of Liquid Penetration into Undoped and Magnesia-Doped Alumina

Liquid-forming impurities (CaO + SiO₂) were placed on the top of undoped and MgO-doped Al₂O₃ powder compacts, and the microstructural evolution and the extent of liquid penetration during heat treatment were investigated. Compared with the undoped specimen, abnormal grain growth was limited and the impurities penetrated more deeply in the MgO-doped specimen. The homogenization of the microstructure and the enhanced liquid penetration in MgO-doped specimens could be explained in terms of an interface roughening or a modification of the liquid-(glass-) phase characteristics, such as viscosity.     

Origin of ZnO Varistor

The varistor properties were examined in porous ZnO. The sample used contained no dopants such as Bi₂O₃ or Pr₆O₁₁, which are usually considered to be useful in developing varistors. Comparison of I-V curves for the oxidized and unoxidized samples indicated that oxygen, which had chemisorbed and diffused into the grain boundary, played a role in developing varistor characteristics.     

Grain-Boundary Characteristics and Their Influence on the Electrical Resistance of Barium Titanate Ceramics

The integral and spectral cathodoluminescence (CL) properties of grain-boundary zones in positive-temperature-coefficient-type barium titanate ceramics were investigated with respect to typical sintering parameters in the spectral range 300 to 1800 nm at room temperature. Spatially resolved CL micrographs for the wavelength range 300 to 850 nm show dark grain-boundary zones and light grain interiors. Corresponding micrographs for the infrared wavelength range 800 to 1800 nm show just the opposite CL contrast. The CL properties of the grain-boundary zone can be correlated to doubly ionized barium vacancies. Therefore, grain-boundary zones which are visible in integral CL images of both wavelength ranges represent the theoretically predicted grain-boundary zone of high barium vacancy concentrations. The width of these grain-boundary zones varies characteristically with sintering time and cooling rate. Additional measurements of the electrical conductivity of the specimens confirmed the theoretically assumed correlation between grain-boundary-zone width and electrical resistance of the samples.     

掺杂Fe2O3对氧化锌压敏陶瓷压敏特性的影响

本文研究了分别掺杂微量Fe2O3杂质对ZnO压敏陶瓷的压敏特性的影响。研究结果表明:随Fe2O3掺杂量的增加,ZnO压敏陶瓷的压敏电压V1 mA和非线性系数先下降,后升高,最小值出现在Fe2O3摩尔掺杂量为1.00%;并从理论上详细地探讨了产生这些影响的原因。     

Gas Sensing Characteristics of Porous ZnO and Pt/ZnO Ceramics

Changes in resistivity and chemical changes in reducing gases were measured for porous zinc oxide ceramics with and without a platinum catalyst at 300° and 400°C to examine the gas sensing mechanism and the effect of platinum additions. Reducing gases were oxidized to CO₂ and Hz₂ on the sensor surfaces. Platinum addition promoted the oxidation of reducing gases but did not lead to an increase in the resistivity change at 400°C. The reaction sequences for the gas sensing process are proposed, taking into account partially oxidized intermediates of hydrocarbons and oxidation on platinum without an electron transfer process.     

Change in the Fracture Mode of PLZT Ceramics by Chemically Induced Grain-Boundary Migration

The undulating grain-boundary structure formed by chemically induced grain-boundary migration in lanthanum-modified-lead-zirconium-titanium ceramics causes a change in the fracture mode and an increase in room-temperature fracture toughness. Specimens with normal and undulating grain-boundary structure exhibit intergranular and transgranular fracture, respectively. The grain-boundary undulation is suggested to be effective for improving high-temperature creep resistance.     

退火过程中Ti4+离子对低压ZnO陶瓷压敏性能的影响

本文对烧成后的掺ＴｉＯ２低压ＺｎＯ压敏陶瓷进行了退火处理。测量了不同退火温度下ＺｎＯ陶瓷的压敏性能,运用ＸＲＤ,ＳＥＭ等分析方法研究了退火过程中ＺｎＯ陶瓷结构的变化。研究结果表明,退火过程中Ｔｉ＾４＋离子取代Ｚｎ＾２＋离子而固溶进入ＺｎＯ晶粒表面,是引起ＺｎＯ陶瓷压敏性能变化的主要原因,在退火温度不超过４００℃时可获得较好的压敏性能。     

氧化锌压敏电阻粉体均匀性的电化学研究

粉体是ZnO压敏电阻制备的起点,粉料的混合均匀性直接影响压敏电阻的综合电性能。利用电位-电容法及Mott-Schottky分析并结合SEM扫描,探索ZnO压敏电阻粉体的均匀性与其电化学行为之间的关系。测试结果初步表明,ZnO压敏电阻粉体越均匀,电容越小,载流子浓度越小。     

Electrical-Impulse-Induced Fracture of Zinc Oxide Varistor Ceramics

When varistors are subject to very short (˜1 μ s), high-current pulses, they occasionally fracture. We attribute the occurrence of fracture to stress waves, created by inertial forces in response to the rapid Joule heating, causing the propagation of preexisting microstructural flaws. The amplitude of the stress oscillation is shown to be proportional to the second derivative of temperature with respect to time which, in turn, is directly related to the rate of increase of power. An upper limit for the magnitude of the peak stress is derived in terms of material parameters, varistor geometry, and the maximum electrical power. From the magnitude of the peak stress, a critical defect size is estimated which will cause fracture under given pulse conditions. Conclusions for the design and testing of varistors are drawn.     

Effects of Polarity on Grain-Boundary Migration in ZnO

The migration of boundaries between single crystal and polycrystalline ZnO was investigated using single crystals with well-defined crystallographic faces. The migration rate of the basal (0001) planes through polycrystalline ZnO depended on the crystallographic polarity of the basal plane. Grain boundary migration in the [0001] direction was much faster than in the [000[Onemacr]] direction. Migration rates of boundaries in nonpolar directions were somewhat slower than that in the [0001] direction. Slow growth in the [000[Onemacr]] direction and rapid growth in nonpolar directions found in the present study help to explain the anisotropic growth of the grains that contain head-to-head inversion boundaries in ZnO varistor ceramics.     

片式叠层压敏电阻器的发展和应用

本文论述了国外开发的几种片叠层压敏电阻的结构,特性及当前的发展方向,介绍了这类压敏电阻器在集成电路保护,汽车电路保护和ＣＭＯＳ器件保护等方面的应用。     

Optical and Structural Characteristics of Nanocrystalline ZnO:Cd Ceramics

Nanostructured cadmium (Cd) doped ZnO ceramic thin films were deposited on glass substrates by the sol-gel spin coating method and characterized by X-ray diffraction (XRD), scanning electron microscopy and UV-VIS-NIR spectrophotometry. The XRD analysis showed that thin films were crystalline with hexagonal wurtzite structure. The preferred orientation of un-doped and Cd-doped ZnO thin films lies along the (002) plane. FESEM images revealed that the morphology consists of spherical, non-spherical and partly cylindrical structures. The results of EDX showed that Cd was doped into the ZnO structure. The UV-Vis transmittance spectra showed that substitution of Cd into ZnO leads to bandgap narrowing. The Urbach energy increases with the increase in Cd concentration. The prepared nanostructured cadmium (Cd) doped ZnO ceramic thin films can be used in electronic device technologies.     

Wetting of Ceramics by Liquid Metals

A linear relation is established between the cosine of the contact angle and the temperature of a liquid metal sessile drop resting on a ceramic substrate. This relation is demonstrated for many liquid metal/ceramic systems. The work of adhesion and the effects of surface roughness, porosity, chemical reactions, and structural transformations in the substrate surface are also discussed.     

Wetting of Ceramics by Liquid Aluminum

The wettability of AlN, TiB₂, TiN, and TiC by liquid aluminum was investigated by the sessile drop technique at 2×10^-7 torr or less. The contact angle decreased linearly with increasing temperature in all cases. Variation of the contact angle with temperature is correlated with the surface tension of the drop, and the Zisman-type relation is established. The critical surface tensions (Rγ_c) of AlN, TiB₂, TiN, and TiC are 664, 700, 713, and 725 dynes/cm, respectively. These values are correlated with the melting point of the ceramics. The Zisman relation is discussed and applied to calculation of the work of adhesion. The surface tension of liquid aluminum varies linearly with temperature: γ_Lv (dynes/cm) = 948–0.202 T (°K).     

Grain-Boundary Sliding and Grain Interlocking in the Creep Deformation of Two-Phase Ceramics

Grain-boundary sliding and grain interlocking of two-phase ceramics during creep are examined on the basis of the Dryden–Kucerovsky–Wilkinson–Watt theory. That theory is extended to the plane–strain creep deformation of model arrays of square and hexagonal grains embedded in a continuous grain-boundary melt phase to develop consti-tutive equations for their universal creep. Superposition is derived for the "time-applied stress" and the "time–temperature" relations during creep. The "shift factor" for the time–temperature superposition characterizes the temperature dependence of the apparent viscosity of the ceramic. A two-phase ceramic during creep acts as a non-Newtonian fluid, the viscosity of which is dependent on the creep strain. The constitutive relation between the creep strain rate and the creep strain is then utilized to estimate the viscosity and the volume fraction of the intergranular melt phase. The equilibrium creep strain to achieve grain interlocking is considered through the rotational motion of grains. Creep tests for a β-spodumene glass-ceramic are conducted under simple shear to generate experimental results for scrutinizing the theoretical predictions. Satisfactory agreement is observed, giving important rheological information on the creep process.     

Mechanical Strength and Microstructure of Zinc Oxide Varistor Ceramics

ZnO-based varistor ceramics were sintered under various conditions to optimize their mechanical strength. For highest strength, the optimum sintering temperature was 1070°C or below. At higher maximum temperature, the strength decreased because of grain coarsening and the increasingly inhomogeneous distribution of secondary phases thereby induced. Fracture typically started from holes associated with hollow or poorly compacted sprayed granules. All series contained the same type of critical flaws, but, depending on the sintering temperature, the fracture toughness changed, which led to different strengths. At sintering temperatures above 1050°C, the density started to decrease slightly because of swelling attributed to the pressure of gas entrapped in closed pores.     

Inhomogeneous Conductivity in Varistor Ceramics: Methods of Investigation

The grain boundaries in zinc oxide ceramics exhibit different electrical behaviors. This results in separate paths of current flux through the microstructure and also in different breakdown voltages of each path. The paper describes some new methods for characterization of these paths. A galvanic determination is able to show the number of paths and their distribution across the varistor surface. The differences in breakdown voltage are visible using a line scan method. Current images in SEM can detect the paths of current along a varistor surface. Possible reasons for inhomogeneous current flux are inhomogeneous distribution of dopants, insufficient binder burnout, and pressing faults.