摘要:

This paper deals with the application of ZnO varistors—an area which has not been treated in a systematic way in the literature. The paper starts with a brief description of the fundamental properties comprising the electrical behavior as well as the physics, chemistry, and microstructure of the varistor. These properties then form the basis for defining the application parameters that are directly related to the nonlinear current‐voltage characteristics of the varistor. This paper provides a detailed description of these parameters and their relation to microstructure and the processing of the varistor. Finally, a discussion is presented on the reliability of the varistor by considering a grain‐boundary defect model which explains both the instability and the stability under use condi

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05232.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The paper shows that a previously proposed size effect law can be used to identify nonlinear fracture properties solely from measured maximum loads of geometrically similar ceramic fracture specimens of sufficiently different sizes. This law represents a first‐order global approximation of the deviations from linear elastic fracture mechanics, independent of the type of the toughening mechanism in the fracture process zone. It provides a simple and unambiguous way to determine the size‐ and shape‐independent values of the fracture energy the effective length of the process zone, and the effective crack‐tip opening displacement. It also yields theRcurve, which is geometry (shape) dependent. The proximity of response to linear elastic fracture mechanics is characterized by a brittleness number, which is shape inde

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05233.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

A simple model of substituted perovskites of the PZT type involving the presence of centers which can act both as donors and acceptors provides an explanation of the cancellation of space charge density (ρ=−div ) in polarized ceramics. A cause of instability can be avoided by incorporating substitutions in the cation lattice to form these centers and lock them in position. These substitutions insure the nonconductive state of the material although they allow a short‐range electron transfer between the cen

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05234.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

A mechanism is proposed to describe the influence of redox reactions, which involve variable‐valence ions, on the diffusional mass transport of molecular dissolved oxygen in glassmelts. The concentration profile for molecular dissolved oxygen in a melt with diffusional transport in one direction in a Cartesian coordinate system is described by a differential equation: ∂(O2/∂t=Dc(∂2[O2]/∂x2), whereDcdepends on the actual value of the diffusivityDof physically dissolved oxygen, the local molecular oxygen concentration, the concentration of variable‐valence ions, and the equilibrium constant for the relevant redox reaction. This equation is applicable for diffusion into or outwards from the melt from or into the furnace atmosphere. It is shown here that the value ofDcis always less thanD. Concentration profiles for diffusing oxygen in glassmelts with refining agents (containing variable‐valence ions) are thus steeper than profiles in melts without refining agents. A decrease in temperature of a glassmelt with variable‐valence ions generally leads to diffusion of oxygen into the melt. The flux of oxygen gas into (oxidation) or out (reduction) of the melt is proportional to (D/Dc)1/2and is enhanced by the effect of variable‐valence ions diss

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05235.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Submicrometer powders of various Pb‐based perovskites, including PbTiO3, PbZrO3, Pb(Zr0.53Ti0.47)O3, and Pb(Mg1/3Nb2/3)O3were prepared by a reactive calcination process. Using only reagent‐grade raw materials and conventional processing techniques, highly reactive powders were produced by reacting the materials near the temperature of maximum volumetric expansion. At this point, the morphological development results in a skeletal‐type structure consisting of ultrafine particulates that can be readily broken down further by milling. Powder sizes less than 0.3 μm and as small as 70 nm generally only achievable using chemical processing techniques were achieved. The highly reactive powders allowed densification to occur at temperatures as low as ∼900°C with correspondingly small grain sizes. A model describing the physiochemical behavior and associated morphological development of Pb‐based perovskites was here

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05236.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The effects of oxygen partial pressure (Po2) on the oxidation behavior and room‐temperature flexural strength of sintered α‐SiC were investigated. Groups of flexure bars were exposed at 1400°C to flowing Ar containing various levels of oxygen (Po2ranging from 7.5 × 10−7to 1.5 × 10−4MPa). The changes in weight, flexural strength, and surface morphology of the samples were strongly influenced by thePo2level. When thePo2was higher than 3 × 10−5MPa, SiO2was formed on the surface (i.e., passive oxidation occurred) and the strengths of the samples were not significantly affected. However, when thePo2was lower than 2 × 10−5MPa, material loss occurred (active oxidation), decreasing the weight and strength of the samples. Both the reduction in strength and the weight loss resulting from active oxidation were proportional to thePo2. An approximately 50% reduction in strength was observed in the SiC after oxidation for 20 h at aPo2of 1.5 × 10−5MPa, a level that is slightly lower than thePo2at which the transition from active to passive oxidation occurs. Large pits formed during exposure were responsible for the r

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05237.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The fracture properties of three polycrystalline Y2O3materials: one fully cubic phase, one containing an Al2O3grain‐boundary phase, and one containing hexagonal phase, were examined by indentation over a wide range of contact loads. The two former microstructures displayed tendencies at large indentation loads to radial crack lengths shorter than those extrapolated from the ideal response at low loads. The deviations correlated with the development of lateral cracks at the larger contacts, rather than with any observable change in the interaction between the cracks and the microstructure. After taking the lateral crack influence into account, the toughness of all three materials was estimated to be constant over the range of crack lengths studied, in contrast to the phenomena observed in similar grain size noncubic materials and inferred from earlier fractographic studies. The toughness of the partially hexagonal material was estimated to be 50% greater than the cubic materials. The general phenomenon of partitioning energy into lateral cracks at the expense of radial cracks at large indentation loads has been characterized by a lateral crack development parameter,LD, which varies from 0 to 1 as lateral cracks progressively develop and remove materia

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05238.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Possible mechanisms for the hysteresis in stress‐strain response observed during fatigue of fiber‐reinforced ceramics are examined analytically. In the model developed, the microstructure of a unidirectional composite is divided into adjacent cells, each containing a single fiber. The compliance of each cell is modeled by a series of springs, with frictional sliding of fibers represented by sliding blocks. Fatigue damage is modeled by allowing fibers to debond and fracture on a random cycle‐by‐cycle basis. The magnitude of the interfacial shear between the fibers and matrix is shown to play a significant role in determining the extent of hysteresis observed during fatigue loading of unidirectional composites. Practical considerations, such as the influence of fiber volume fraction on macroscopic fatigue behavior, are also di

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05239.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Pressureless sintering of SiC‐whisker‐reinforced Al2O3composites was investigated. In Part I of the study, the effect of the matrix (Al2O3) powder surface area on densification behavior and microstructure development is reported. Compacts prepared with higher surface area Al2O3powder showed enhanced densification at lower whisker concentrations (5 and 15 vol%). Samples with 15 vol% whiskers could be pressureless sintered to ∼97% relative density with zero open porosity and ∼1.6‐μm matrix average grain inte

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05240.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Pressureless sintering of SiC‐whisker‐reinforced Al2O3composites was investigated. In Part II of the study, the effects of Y2O3/MgO sintering additives and green body infiltration on densification behavior and microstructure development are reported. Both sintering additives and green body infiltration resulted in enhanced densification. However, the infiltration approach was more effective for samples with high SiC whisker concentrations. Samples with 27 vol% whiskers could be pressureless sintered to ∼93% relative density and ∼3% open porosity. Fracture toughness values and microstructural features (e.g., grain size) for the infiltrated samples remained approximately the same as observed in the uninfiltrated

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05241.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY