Thermal-Conductivity-Temperature Table for Metals

TABLE 2-374 Thermal-Conductivity-Temperature Table for Metals ... 

The thermal conductivity of solids varies considerably (Table 15.2). Metals have a high thermal conductivity, with silver having the highest room-temperature thermal conductivity, at 430 W m K . Alloys have lower thermal conductivities than pure metals. Ceramics are even lower, especially porous porcelains or fired clay products (Figure 15.3). The lowest thermal conductivities are shown by plastic foams such as foamed polystyrene. As would be expected, the thermal conductivity of crystals varies with direction. For example, the thermal conductivity of the hexagonal metal cadmium Cd, (A3 structure), is 83Wm K parallel to the c axis and 104 W m parallel to the a axis. At 25 °C, the oxide quartz, which has a hexagonal unit cell, has a thermal conductivity parallel to the c axis of 11 W m K , and 6.5 W m K paraUel to the a axis. 

As a comparison, Table 4.1 lists the coefficients of thermal conductivity (at room temperature) for some metals employed in heat exchangers, together with some minerals commonly found in boiler deposits. 

The experiments were carried out in a Netzsch STA 409 C (Simultaneous Thermal Analysis - STA) in the TGA/DSC configuration. The STA has a vertical san le carrier with a reference and a sample crucible, and in order to account for buoyancy effects, a correction curve with empty crucibles was first conducted and then subtracted from the actual experiments. Platinum/Rhodium crucibles were used in order to get the best possible heat transfer. The thermocouple for each crucible was positioned Just below and in contact with the crucible. The ten rerature obtained from the measurement is the temperature in the reference side. This temperature is converted to the temperature in the sample side by using the DSC-signal in pV and a temperature-voltage table for the thermocouple. The product gases were swept away by lOO Nml/inin nitrogen which exited the top of the STA, The STA was calibrated for temperature and sensitivity (DSC) with metal standards at each heating rate. 

This equation offers a clear interpretation of the thermal diffusivity a and the heat conduction equation itself. According to (2.14) the change in the temperature with time d d/dt at each point in the conductive body is proportional to the thermal diffusivity. This material property, therefore, has an effect on how quickly the temperature changes. As Table 2.1 shows, metals do not only have high thermal conductivities, but also high values for the thermal diffusivity, which imply that temperatures change quickly in metals. 

The temperature dependence of thermal conductivity for liquids, metal alloys, and nonconducting solids is more complicated than those mentioned above. Because of these complexities, the temperature dependence of thermal conductivity for a number of materials, as illustrated in Fig. 1,11, does not show a uniform trend. Typical ranges for the thermal conductivity of these materials are given in Table 1.1, We now proceed to a discussion of the foundations of convective and radiative heat transfer. 

The thermal conductivity of thorium metal is given in Table 6.7. The electrical conductivity of thorium metal is very dependent on its impurity content. Chiotti [C3] found that at room temperature the resistivity of thorium metal containing 0.2 w/o (weight percent) carbon was 37 X 10 fl cm, and that of metal containing 0.03 w/o carbon was 18 X 10 fi cm. An extrapolated value for carbon-free thorium metal is 13 to 15 X 10 f2 cm. The temperature coefficient of resistivity is 3.6 to 4.0 X 10 per °C. 

In this section, we consider physical properties of the J-block metals (see cross references in Section 19.1 for further details) an extended discussion of properties of the heavier metals is given in Section 22.1. Nearly all the J-block metals are hard, ductile and malleable, with high electrical and thermal conductivities. With the exceptions of Mn, Zn, Cd and Hg, at room temperature, the metals possess one of the typical metal structures (see Table 5.2). The metallic radii (rjnetai) for 12-coordination Table 5.2 and Figure 19.1) are much smaller that those of the -block metals of comparable atomic number Figure 19.1 also illustrates that values of /-metal ... 

Several different materials have been studied. Metallic monoliths have been used extensively since their first application for automobile converters. They allow very thin walls and have a very high thermal conductivity. However, their thermal expansion gives rise to some problems when looking at the coating and stability of the washcoat on the metallic surface, compared with the ceramic monolith. Furthermore, their maximum operation temperature is limited to 1200-1400 C, cf. Table 1. Probably, the maximum temperature is somewhat lower for long-time exposure. However, several ceramic monoliths that can stand higher thermal conditions have been developed, as reported in Table 1. 

---