Thermal Conductivities of Insulating Materials at High Temperatures

TABLE 2-378 Thermal Conductivities of Insulating Materials at High Temperatures ... 

The most important properties of refractory fibers are thermal conductivity, resistance to thermal and physical degradation at high temperatures, tensile strength, and elastic modulus. Thermal conductivity is affected by the material s bulk density, its fiber diameter, the amount of unfiberized material in the product, and the mean temperature of the insulation. Products fabricated from fine fibers with few unfiberized additions have the lowest thermal conductivities at high temperatures. A plot of thermal conductivity versus mean temperature for three oxide fibers having equal bulk densities is shown in Figure 2. 

The thermal conductivities of various insulating materials are also given in Appendix A. Some typical values are 0.038 W/m °C for glass wool and 0.78 W/m °C for window glass. At high temperatures, the energy transfer through... 

Relaxation in Insulating Crystals. The low-frequency relaxation of ionic materials consists typically of a set of simple decays arising from the motion of defects. - Some of these have freedom of extensive motion and contribute to the conduction current others are constrained to a few ndghbouring sites. Except at high temperature, the defects are remnants of the previous thermal and mechanical history of the sample. Their movement is a thermally activated process and relaxation times normally vary with temperature as exp AjkT). The e qierimental picture may be much complicated by the motion of electrons loosely bound to crystal defects. 

The linings of industrial furnaces require stable materials that retain their strength at high temperatures, have resistance to abrasion and to furnace gases, and have poor thermal conductivity (good heat-insulating capability). 

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