Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using beta-SiC powder with the addition of 6 wt% Al2O3 and 4 wt% Y2O3. The relationships between sintering conditions, microstructural ...
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Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using beta-SiC powder with the addition of 6 wt% Al2O3 and 4 wt% Y2O3. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the beta-SiC powder compact, the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the beta --> alpha phase transformation. With increasing holding time, the fraction of beta --> alpha phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m(1.2) to 8.3 MPa m(1.2) with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.
Cooling-rate effects on the depletion layer of BaTiO3 positive temperature coefficient of resistance (PTCR) were analysed using d.c. resistivity and a.c. complex impedance spectroscopy. As the cooling rates decreased,...
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Cooling-rate effects on the depletion layer of BaTiO3 positive temperature coefficient of resistance (PTCR) were analysed using d.c. resistivity and a.c. complex impedance spectroscopy. As the cooling rates decreased, the d.c. resistivity of the sample increased. A.c. complex impedance spectra showed that the observed d.c. resistivity increase was mainly due to the grain-boundary resistivity increase. The grain resistivity also exhibited relatively weak PTCR behaviour. The built-in potential and the depletion-layer width were analysed using the a.c. complex impedance data. Slow cooling rate produced higher built-in potential and large depletion-layer width. The relationship between the built-in potential and the depletion-layer width at different temperatures was analysed and used to explain the observed grain-boundary resistivity change. The resistivity decrease in the low-temperature region was due to the depletion-layer width decrease. The resistivity jump over the temperature was due to both the built-in potential and the depletion-layer width changes.
In the present work, the effects of sedimentation treatment on the mechanical properties of SiC whisker-reinforced Si3N4 composites were investigated. The sedimentation treatment was designed to make a difference to t...
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In the present work, the effects of sedimentation treatment on the mechanical properties of SiC whisker-reinforced Si3N4 composites were investigated. The sedimentation treatment was designed to make a difference to the raw materials, especially to remove large agglomerates and impurities. The treatment improved the fracture toughness of the whisker reinforced composites.
Monodispersed spherical SiC gel powders were synthesized by hydrolysis and condensation of phenyltrimethoxysilane (PTMS) or a mixture of PTMS and tetraethylorthosilicate (TEOS) from a system of silane-H2O-catalyst to ...
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Monodispersed spherical SiC gel powders were synthesized by hydrolysis and condensation of phenyltrimethoxysilane (PTMS) or a mixture of PTMS and tetraethylorthosilicate (TEOS) from a system of silane-H2O-catalyst to which no co-solvent such as alcohol was added. The experiments were conducted through two routes;base catalysed and acid base catalysed routes. In each process, excess water was used as a dispersing medium in addition to a hydrolysing agent. In the base-catalysed route, monodispersed spherical gel powders were obtained regardless of NH4OH content used in this experiment when the concentration of silane and the molar ratio of TEOS to PTMS were less than 0.5 mol l-1.and 0.5, respectively. In the acid base catalysed route, polydispersed powders were produced when the concentration of silane exceeded 0.25 mol l-1. When heated above 1.00-degrees-C, the initially monodispersed powders sustained their shape in the PTMS TEOS system, but bulk and fibre phase were produced in the PTMS system.
Electrophoretic deposition has been employed to synthesize yttria-stabilized zirconia (YSZ)/alumina, laminar microcomposites with a total of 80 layers, > 2 mm. 1. wt% solids, ethanol suspensions of YSZ or Al2O3 pow...
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Electrophoretic deposition has been employed to synthesize yttria-stabilized zirconia (YSZ)/alumina, laminar microcomposites with a total of 80 layers, > 2 mm. 1. wt% solids, ethanol suspensions of YSZ or Al2O3 powders were deposited layer by layer. The deposited samples had a green density, approximately 60% theoretical. The deposition process was characterized by the rate-of-deposition as a function of voltage and the microstructure of the sintered, theoretically dense samples was characterized by optical and electron microscopy. Microindentation was used to explore the mechanical properties of the laminates.
The effects of alumina on the densification of boron carbide and related reaction phenomena in alumina-doped B4C were studied. Pressureless sintering was conducted at various temperatures for 1. min in a flowing Ar at...
The effects of alumina on the densification of boron carbide and related reaction phenomena in alumina-doped B4C were studied. Pressureless sintering was conducted at various temperatures for 1. min in a flowing Ar atmosphere. The addition of alumina improved the densification of boron carbide. Maximum density of 96% theoretical was obtained with the 3 wt% alumina-doped B4C sintered at 21.0-degrees-C. Abnormal (or exaggerated) grain growth was observed in the specimen containing more than 4 wt% alumina. In the B4C-Al2O3 reaction couples, good wetting of the liquid phase on the boron carbide grains was observed. X-ray diffraction and Auger electron spectra showed that the AlB1.C2 phase was formed by the reaction between boron carbide and alumina. It is suggested that these phenomena promote the densification of boron carbide.
The infrared reflectance and transmission spectra of amorphous CdGe(x)As2 were investigated to study the amorphous structure with varying Ge content. The mid-infrared optical absorption edges of amorphous CdGe(x)As2 w...
The infrared reflectance and transmission spectra of amorphous CdGe(x)As2 were investigated to study the amorphous structure with varying Ge content. The mid-infrared optical absorption edges of amorphous CdGe(x)As2 with 0 < x < 1.2, were observed in the range 1.6-1.7-mu-m, indicating a structural similarity in short-range order throughout. Comparing the reflectance and transmission spectra of amorphous and devitrified crystalline phases established that the structures of amorphous CdGe(x)As2 transformed from the CdAs2 basic structure to the chalcopyrite (CdGeAs2) structure, as one progressed from x = 0 to x = 1.2. The infrared spectra were interpreted interactively with radial distribution function, magnetic susceptibility, density measurements, and crystallization studies. The crystallization behaviour of amorphous CdGe(x)As2 that was heat treated at different temperatures was investigated using far-infrared transmission spectra.
An analysis of the influence of crystalline microstructure on fatigue crack propagation in polyoxymethylene is presented. A series of test specimens containing a variety of diverse microstructures was prepared through...
An analysis of the influence of crystalline microstructure on fatigue crack propagation in polyoxymethylene is presented. A series of test specimens containing a variety of diverse microstructures was prepared through controlled thermal treatments of plaques from four different lots of polyoxymethylene. Extensive characterization of the crystalline microstructure was carried out in order to permit a direct comparison between the fatigue behaviour and crystalline microstructure. The degree of crystallinity and tie molecule density were both found to have a significant affect on fatigue crack propagation rate while average spherulite size did not appear to influence fatigue behaviour. Additionally, the fatigue fracture surfaces of many of the test specimens were examined. Three distinct surface topographies were observed and found to correlate with different stages of crack growth. In the region near the end of fatigue crack propagation, closely spaced surface markings that resemble fatigue striations were observed.
In much of the San Joaquin Valley, California, USA. inadequate drainage of applied irrigation water and accumulating salts in the soil have necessitated the installation of subsurface tile drainage systems to preserve...
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In much of the San Joaquin Valley, California, USA. inadequate drainage of applied irrigation water and accumulating salts in the soil have necessitated the installation of subsurface tile drainage systems to preserve crop productivity. At present, these subsurface drainage waters are disposed of by means of evaporation ponds or discharges into the San Joaquin River. Unfortunately, most of these agricultural drainage waters contain high concentrations of salts and naturally occurring trace elements, such as selenium, and recent evidence indicates that substantial numbers of waterbirds are exposed to contamination by selenium in the evaporation ponds. In order to avoid, minimize, or mitigate the adverse impacts on wildlife using the ponds, alternative pond management methods must be identified and evaluated for implementation. A number of methods have the potential to be cost-effective in significantly reducing the contamination hazard to birds using agricultural evaporation ponds. Twenty general methods were evaluated in this study, and four methods are recommended for implementation: remove levee vegetation. remove windbreaks, deepen the ponds, and haze birds. A number of other methods are recommended for further consideration because they appear to have good prospects for reducing the contamination hazard: steepen interior levee slopes, apply herbicides and insecticides, place netting on pond shorelines, and provide freshwater habitat adjacent to evaporation ponds. It may be necessary to use a combination of methods to effectively control selenium contamination of aquatic birds because it is unlikely that a single affordable pond management method will be able to entirely eliminate the contamination hazard.
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