Composite microstructures, thermal conductivity

As can be seen from this figure, the heat-resistance was remarkably improved by the drastic changes in the microstructure from amorphous to polycrystalline structure. Another type of SiC-based fiber, SA fiber (2), has a sintered SiC polycrystalline structure and includes very small amounts of aluminum. This fiber exhibits outstanding high temperature strength, coupled with much improved thermal conductivity and thermal stability compared with the Nicalon and Hi-Nicalon fibers. The fabrication cost of the SA fiber is also reduced to near half of that of the Hi-Nicalon Type S [ 17]. The SA fiber makes SiC/SiC composites even more attractive to the many applications [18]. In the next section, the production process, microstructure and physical properties of the SA fiber are explained in detail. 

Apart from their thermal stability a number of other factors are important in the choice of refractory materials e.g. thermal expansion coefficient, thermal shock resistance, chemical resistance, thermal conductivity and abrasion resistance. These properties depend upon both the microstructure and composition of the ceramic material. 

A consequence of the complex interplay of the dielectric and thermal properties with the imposed microwave field is that both Maxwell s equations and the Fourier heat equation are mathematically nonlinear (i.e., they are in general nonlinear partial differential equations). Although analytical solutions have been proposed under particular assumptions, most often microwave heating is modeled numerically via methods such as finite difference time domain (FDTD) techniques. Both the analytical and the numerical solutions presume that the numerical values of the dielectric constants and the thermal conductivity are known over the temperature, microstructural, and chemical composition range of interest, but it is rare in practice to have such complete databases on the pertinent material properties. 

Combination of several properties is becoming increasingly important in modem industry. One example may be taken from electronics, where in addition to mechanical properties and electric resistance, themial stability and conductivity are important requirements. It was estimated that the increase of temperature by 10°C reduces time to failure by the factor of two." A finite analysis model was developed which accounts for the following properties of filled composites microstructure, effect of particle shape, formation of conductive chains, effect of filler aspect ratio, and interfacial thermal resistance. The predictions of the model indicate the most... 

We report on the powder metallurgical fabrication of bismuth-antimony solid solution and the thermoelectric properties of the fabricated composites. The solid solution powders were prepared by mechanical alloying (MA) aiming at large reduction of the thermal conductivity with the very fine microstructures obtained through MA process. The prepared bismuth-antimony powders (Bi-7.5at%Sb) have been sintered by hot pressing. 

Trade Name Manufacturer Composition (wt%) Density (g/cm ) Grain Size (pm)- Microstructure Diameter (pm) Fibers- Tow Process Elastic Modulus (GPa) Strength (GPa) CTE c (ppm/ °C) Thermal Conductivity Electrical Conductivity Comments... 

The poor corrosion resistance of borides may be partially overcome in composites with SiC if the microstructure allows passive oxidation kinetics. In combination with SiC the borides retain their high electrical and thermal conductivity and thus suitable thermal shock resistance. SiC-TiB2 composites have been extensively developed for wear parts in machinery such as sliding rings, valves, valve seats, roller and ball bearings, plungers, and rocker arm pads. 

The interest in development of SiC/Si3N4 composites stems from the fact that (a) SiC and Si3N4 are thermodynamically compatible and stable at temperatures to 1700°C, (b) silicon nitride processing temperature can be tailored to avoid fiber degradation, (c) silicon nitride matrix microstructure can be controlled to inprove composite properties such as matrix cracking strength and thermal conductivity. 

The aim of our investigation was to study this very interesting phase, in the course of which we considered the dependences of the electrical conductivity, thermoelectric power, thermal conductivity, microhardness, coefficient of linear expansion, carrier mobility, and carrier density on composition in the most interesting concentration region (45 to 51 wt.% Si). Microstructure, x-ray structure, and chemical analysis were carried out on the most interesting alloys the dependence of a number of electrical properties on temperature was studied x-ray structure studies were carried out on single crystals. 

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