Thermal conductivity selected materials

C. Y. Ho, R. W. Powell, and P. E. Liley, Standard Reference Data on the Thermal Conductivity of Selected Materials, part 3, final report NBS-NSR05 Contr. CST-1346, Purdue University, Lafayette, Ind., Sept. 1968. 

When experiments are carried out to select a suitable dryer and to obtain design data, the effect of changes in various extern variables is studied. These experiments should be conducted in an experimental unit that simulates the large-scale diyer from both the thermal and the material-handling aspects, and only material which is truly representative of full-scale production should be used. 

Fig. 7. Irradiation induced thermal conductivity degradation of selected graphite materials.

The other principal thermal properties of plastics which are relevant to design are thermal conductivity and coefficient of thermal expansion. Compared with most materials, plastics offer very low values of thermal conductivity, particularly if they are foamed. Fig. 1.10 shows comparisons between the thermal conductivity of a selection of metals, plastics and building materials. In contrast to their low conductivity, plastics have high coefficients of expansion when compared with metals. This is illustrated in Fig. 1.11 and Table 1.8 gives fuller information on the thermal properties of pl tics and metals. 

GP 1[ [R 1[ A change from aluminum to platinum as construction material results in reduced micro-reactor performance concerning oxidation of ammonia, decreasing N2O selectivity by 20% [28]. This is explained by the lower thermal conductivity of platinum, which causes larger temperature differences (hot spots) within the micro channels, i.e. at the catalyst site, e.g. due to insufficient heat removal from the channels or also by non-uniform temperature spread of the furnace heating. 

A first selection of materials is usually done with respect to phase change temperature, enthalpy and reproducible phase change. The state of the art with respect to that selection is discussed in the following section Classes of materials . Usually a material is not able to fulfill all the above mentioned requirements. For example the thermal conductivity is generally small and an encapsulation is always needed. Therefore strategies and approaches have been developed to cope with these problems. These are discussed after the section Classes of materials in Approaches to solve material problems . 

Table 2 Thermal Conductivity Values for Selected Materials... 

The problems associated with direct reaction calorimetry are mainly associated with (1) the temperature at which reaction can occur (2) reaction of the sample with its surroundings and (3) the rate of reaction which usually takes place in an uncontrolled matmer. For low melting elements such as Zn, Pb, etc., reaction may take place quite readily below S00°C. Therefore, the materials used to construct the calorimeter are not subjected to particularly high temperatures and it is easy to select a suitably non-reactive metal to encase the sample. However, for materials such as carbides, borides and many intermetallic compounds these temperatures are insufficient to instigate reaction between the components of the compound and the materials of construction must be able to withstand high temperatures. It seems simple to construct the calorimeter from some refractory material. However, problems may arise if its thermal conductivity is very low. It is then difficult to control the heat flow within the calorimeter if some form of adiabatic or isothermal condition needs to be maintained, which is further exacerbated if the reaction rates are fast. 

Table 13.2 contains a listing of the thermal conductivities of selected materials. 

Of the three general categories of transport processes, heat transport gets the most attention for several reasons. First, unlike momentum transfer, it occurs in both the liquid and solid states of a material. Second, it is important not only in the processing and production of materials, but in their application and use. Ultimately, the thermal properties of a material may be the most influential design parameters in selecting a material for a specific application. In the description of heat transport properties, let us limit ourselves to conduction as the primary means of transfer, while recognizing that for some processes, convection or radiation may play a more important role. Finally, we will limit the discussion here to theoretical and empirical correlations and trends in heat transport properties. Tabulated values of thermal conductivities for a variety of materials can be found in Appendix 5. 

The introduction of heat capacity into the relationships for thermal conductivity and the Prandtl number gives us an opportunity to make a clarification regarding these two quantities. Thermal conductivity is a true heat transport property it describes the ability of a material to transport heat via conduction. Heat capacity, on the other hand, is a thermodynamic quantity and describes the ability of a material to store heat as energy. The latter, while not technically a transport property, will nonetheless be described in this chapter for the various materials types, due in part to its theoretical relationship to thermal conductivity, as given by Eq. (4.35) and (4.36), and, more practically, because it is often used in combination with thermal conductivity as a design parameter in materials selection. 

Figure 8.17 A materials selection bar chart showing ranges of thermal conductivity values for three of the material classes. Reprinted, by permission, from M. F. Ashby, Materials Selection in Mechanical Design, p. 33, 2nd ed. Copyright 1999 by Michael F. Ashby.

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