Factors Affecting Thermal Conductivity

Polymers and foamed polymers. Temperature, pressure, density of polymer, orientation of chain segments, crystal structures, degree of crystallinity, and many other factors may significantly affect thermal conductivity of polymers [9—18]. Therefore the thermal conductivity values can be varied in literatures for same polymers. In addition, discrepancies also occur for thermal conductivity values obtained using different test methods [19]. The data in the tables in this chapter may be the representative values of thermal conductivity and are not necessarily the absolute ones. 

Since the results of zone refining depend on the interaction of momentum, heat and mass transfer in the system, all the basic factors affecting these three processes, both molecular and convective, have to be taken into consideration. These basic factors are concentration W, Density f, viscosity /i, heat capacity Cp, temperature den-sification coefficient, thermal conductivity k, molecular dif-fusivity D, zone diameter d, zone length L, zone travel speed u, temperature difference in zone A T and acceleration g. The concentration W may affect, JJi, Cp,, k, and D as well as the properties of the P.S.Z. (mushy region). Aside from the concentration W, all... 

Irradiation by fast neutrons causes a densification of vitreous silica that reaches a maximum value of 2.26 g/cm3, ie, an increase of approximately 3%, after a dose of 1 x 1020 neutrons per square centimeter. Doses of up to 2 x 1020 n/cm2 do not further affect this density value (190). Quartz, tridymite, and cristobalite attain the same density after heavy neutron irradiation, which means a density decrease of 14.7% for quartz and 0.26% for cristobalite (191). The resulting glass-like material is the same in each case, and shows no x-ray diffraction pattern but has identical density, thermal expansion (192), and elastic properties (193). Other properties are also affected, ie, the heat capacity is lower than that of vitreous silica (194), the thermal conductivity increases by a factor of two (195), and the refractive index, increases to 1.4690 (196). The new phase is called amorphous silica M, after metamict, a word used to designate mineral disordered by radiation in the geological past (197). 

The material properties of solids are affected by a number of complex factors. In a gas-solid flow, the particles are subjected to adsorption, electrification, various types of deformation (elastic, plastic, elastoplastic, or fracture), thermal conduction and radiation, and stresses induced by gas-solid interactions and solid-solid collisions. In addition, the particles may also be subjected to various field forces such as magnetic, electrostatic, and gravitational forces, as well as short-range forces such as van der Waals forces, which may affect the motion of particles. 

Incidentally, thermocouples can be affected by CFCs and display a reading three to five times greater than the real pressure. The gauge itself is not affected because the difference is due to the greater thermal conductivity of a CFCs to air. The user needs to divide the thermocouple reading by a factor of three to five to obtain actual pressure. 

The maximum speed for using solid lubricants is determined mainly by heat build-up, and this in turn is affected by friction, thermal conductivity and other forms of heat loss. The maximum specific load is determined by three principal factors. One is the ability of the particular lubricant to withstand the applied stresses. The second is the effect of load on friction and heat generation. The third is the effect on wear rate. 

---