Metals thermal conductivity values

Some applications, however, must conduct heat but not electricity. In these applications the adhesive must permit high transfer of heat plus a degree of electrical insulation. Fillers used for achieving thermal conductivity alone include aluminum oxide, beryllium oxide, boron nitride, and silica. Table 9.9 lists thermal conductivity values for several metals as well as for beryllium oxide, aluminum oxide, and several filled and unfilled resins. 

Thermal conductivity values of polymers are lower than those of metals and ceramics in table 1 the range of thermal conductivity is of the order of 0.1-0.5 W/m.K. As a general rule, crystalline polymers have higher conductivities than amorphous polymers [7], Price et al find that the thermal conductivity of semi-crystalline polymers (PTEF) tend to increase linearly with crystallinity at 232°C (Figure 1) [8],... 

Typical thermal conductivity values (all in W/mK) are 0.2 to 0.3 for pol5Tners, 1 to 2 for carbon black, 10 to 20 for polyacrylonitrile (PAN) based carbon fiber, 100 to 800 (depends on the heat treatment temperature) for petrolaim pitch-based carbon fiber, 400 for copper, and 600 for graphite. Electrical resistivity (1/electrical conductivity) values (all in ohm-cm) for various materials are lO to 10 for polymers, 10" for electrically conductive carbon black, 10 for PAN-based carbon fiber, 10" for pitch based carbon fiber, 10 for graphite, and lO" for metals such as aluminum and copper. One proach to inqjroving the conductivity of a polymer is through the addition of conductive filler, such as carbon and metal [1, 2]. Typically for a bipolar plate, the desired thermal and electricM conductivity are 20 W/mK and 50 S/cm (0.02 ohm-cm). 

A guarded hot-plate method, ASTM D1518, is used to measure the rate of heat transfer over time from a warm metal plate. The fabric is placed on the constant temperature plate and covered by a second metal plate. After the temperature of the second plate has been allowed to equiUbrate, the thermal transmittance is calculated based on the temperature difference between the two plates and the energy required to maintain the temperature of the bottom plate. The units for thermal transmittance are W/m -K. Thermal resistance is the reciprocal of thermal conductivity (or transmittance). Thermal resistance is often reported as a do value, defined as the insulation required to keep a resting person comfortable at 21°C with air movement of 0.1 m/s. Thermal resistance in m -K/W can be converted to do by multiplying by 0.1548 (121). 

Electrical conductivity is comparatively easy to measure, whereas thermal conductivity is not. Electrical conductivity values for the important cast alloys are Hsted in Table 2. Eigure 1 schematically shows the electrical conductivity of cast copper-base alloys compared with various other cast metals and alloys. The equation Y = 4.184 + 3.93a gives an approximation of thermal conductivity in relation to electrical conductivity, where Tis in W/(m-K) at 20°C and X is the % lACS at 20°C. 

Below -10°C, heat is conducted away too quickly to allow this melting - and because their thermal conductivity is high, skis with exposed metal (aluminium or steel edges) are slower at low temperatures than those without. At these low temperatures, the mechanism of friction is the same as that of metals ice asperities adhere to the ski and must be sheared when it slides. The value of jl (0.4) is close to that calculated from the shearing model in Chapter 25. This is a large value of the coefficient of friction - enough... 

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. 

The thermal conductivity of solids has a wide range of numerical values, depending upon whether the solid is a relatively good conductor of heat, such as metal, or a poor conductor, such as glass-fiber or calcium silicalc. The laUer serves as insulation. 

The values of /"fh and ffc (the film resistances for the hot and cold fluids, respectively) can be calculated from the Dittus-Boelter equations previously described and the wall metal resistance / from the average metal thickness and thermal conductivity. The fouling resistances of the hot and cold fluids /"dh and are often based on experience, but a more detailed discussion of this will be presented later in this chapter. 

All these alloys are characterised by high hardness values and low resistance to impact. In this they are probably more similar to stoneware than to other metals but they are superior to stoneware in thermal conductivity and in their resistance to thermal shock, which, however, is poor compared with that of other metals. Moreover, it is usually easier to make castings of silicon iron than to fabricate required parts from stoneware. 

For certain products, skill is required to estimate a product s performance under steady-state heat-flow conditions, especially those made of RPs (Fig. 7-19). The method and repeatability of the processing technique can have a significant effect. In general, thermal conductivity is low for plastics and the plastic s structure does not alter its value significantly. To increase it the usual approach is to add metallic fillers, glass fibers, or electrically insulating fillers such as alumina. Foaming can be used to decrease thermal conductivity. 

Whereas heat capacity is a measure of energy, thermal diffusivity is a measure of the rate at which energy is transmitted through a given plastic. It relates directly to processability. In contrast, metals have values hundreds of times larger than those of plastics. Thermal diffusivity determines plastics rate of change with time. Although this function depends on thermal conductivity, specific heat at constant pressure, and density, all of which vary with temperature, thermal diffusivity is relatively constant. 

The value of the coefficient will depend on the mechanism by which heat is transferred, on the fluid dynamics of both the heated and the cooled fluids, on the properties of the materials through which the heat must pass, and on the geometry of the fluid paths. In solids, heat is normally transferred by conduction some materials such as metals have a high thermal conductivity, whilst others such as ceramics have a low conductivity. Transparent solids like glass also transmit radiant energy particularly in the visible part of the spectrum. 

Special correlations have also been developed for liquid metals, used in recent years in the nuclear industry with the aim of reducing the volume of fluid in the heat transfer circuits. Such fluids have high thermal conductivities, though in terms of heat capacity per unit volume, liquid sodium, for example, which finds relatively widespread application, has a value of Cpp of only 1275 k.l/ni1 K. 

An organic liquid is boiling at 340 K on the inside of a metal surface of thermal conductivity 42 W/m K and thickness 3 mm. The outside of the surface is heated by condensing steam. Assuming that the heat transfer coefficient from steam to the outer metal surface is constant at 11 kW/m2 K, irrespective of the steam temperature, find the value of the steam temperature to give a maximum rate of evaporation. 

Currently, there is a trend of low dielectric constant (low-k) interlevel dielectrics materials to replace Si02 for better mechanical character, thermal stability, and thermal conductivity [37,63,64]. The lower the k value is, the softer the material is, and therefore, there will be a big difference between the elastic modulus of metal and that of the low-k material. The dehiscence between the surfaces of copper and low-k material, the deformation and the rupture of copper wire will take place during CMP as shown in Fig. 28 [65]. 

Effect of physical properties Physical properties of liquid metals that have significant effects on CHF values are thermal conductivity, latent heat of vaporization, and surf ace tension. 

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