Thermal conductivity, of various materials

For other materials divide factor number by the thermal conductivities of various materials ... 

FIG. 17.5 Thermal conductivities of various materials in comparison with polymers. 

Nanofillers have superb thermal and electrical properties. All nanotubes are expected to be very good thermal conductors along the tube axis, exhibiting a property known as ballistic conduction, but good insulators laterally to the tube axis. It has been reported that single-wall carbon nanotubes exhibit thermal conductivity (TC) values as high as 2000-6000 W mK [4] under ideal circumstances. The temperature stability of carbon nanotubes is estimated to be up to 2800 °C in a vacuum, and about 750 °C in air. By comparison, metals have TC values of several hundred W mK , and water and oil have TC values of only 0.6 W mK and 0.2 W mK respectively. Table 19.1 lists the thermal conductivities of various materials, including nanofillers (nanotubes), metals, and oils. 

Differences in thermal conductivity of various materials may play a role in the choice of a heater and the choice of the material of a heating vessel as well. A high thermal conductivity coefficient means a high conductivity, see Table 28.2. 

The thermal conductivities of various materials differ widely, as shown below (see Ref. 7 and... 

Relative Thermal Conductivity of Various Materials As a Percentage of the Thermal ... 

TABLE 11-9 Thermal Properties of Various Materials as Affecting Conductive Heat Transfer... 

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... 

The thermal conductivities of various commercial insulating materials are given in Table 10.7. Plastic foams containing a captive blowing agent have much lower thermal conductivities than other insulating materials. 

Consideration of the history (to calculate quantities like ignition times) necessitates retention of time derivatives in the conservation equations. Just as in the previous section, to achieve the greatest simplicity we adopt a thermal theory, although in various applications that have been cited the full set of conservation equations has been considered. Let a reactive material occupy the region x > 0, and to avoid complications assume that the material remains at rest and has a constant density p, although coordinate transformations readily enable this assumption to be removed. Let the material, initially at temperature Tq, be exposed to a constant heat flux q — — A 5T/5x at X = 0 for all time t > 0, where A is the constant thermal conductivity of the material. The time-dependent equation for conservation of energy for the material, analogous to equation (9), is... 

The thermal conductivities of materials vary with temperature (Table 1-3). The variation of thermal conductivity over certain temperature ranges is negligible for some materials, but significant for others, as shown in Fig. 1-29. Tlie thermal conductivities of certain solids cxliibit dramatic increases at temperatures near absolute zero, when these solids become superconductors. For example, the conductivity of copper reaches a maximum value of about 20,000 W/m C at 20 K. which is about 50 limes the conductivity at room temperature. The Ihermal conductivities and other thermal properties of various materials are given in Tables A-3 to A 16. 

FIG. 213. Thermal conductivity of various types of materials versus temperature (schematic diagram) (a) a dense polycrystalline substance, (b) a material with large pores, (c) a material with small pores. 

APPENDIX 11 THERMAL CONDUCTIVITIES OF VARIOUS SOLIDS AND INSULATING MATERIALS 1099... 

The thermal conductivities of various metals, alloys, and ceramic materials used in electronic packaging are given in Tables 2.5 and 2.6. 

The distribution of electromagnetic field within the materials to be processed is determined by various factors, such as effective dielectric and magnetic parameters of the materials that are dependent on temperature and microstracture, dimensions and properties of the applicator, and the degree of matching between the microwave source, transmission line, and applicator. Consequently, the distribution of the electromagnetic field will affect the distribution of temperature within the sintered object. Of course, other factors, such as effective absorption properties, heat capacity, and thermal conductivity of the materials, as well as heat transportation characteristics of the materials and the furnaces, wiU also have their specific effects. Ultimately, all these will be reflected by the properties of the final sintered products. While detailed theoretical elaboration of microwave sintering can be found in Ref. [39], a brief description will be presented in this section. 

As shown in Table 7.1, an Increase in the thermal conductivity of the barrel material reduces the melt temperature. Considering that the thermal conductivity of various metals varies substantially, it makes sense to consider using a highly conductive barrel material If low melt temperatures are required. The thermal conductivity of various metals Is shown in Table 7.2. Corrosion-resistant metals tend to have a low conductivity, while copper-containing alloys have a conductivity about three to five times higher than carbon steel. 

The thermal conductivity of a material is measured by putting a sheet of it between the slabs made of highly conductive material kept at constant temperatures, either by measuring the heat flux under stationary conditions, or by measuring the temperature history at various places under transient conditions. 

Thermal Conductivity of LTCC Materials from Various Manufacturers at Room Temperature... 

Diamonds found in nature in the earth s crust are called natural diamonds. Some of them (familiar as the precious stones used for jewelry) reach gem-quaHty standards for clarity, color and size others may be used as industrial diamonds to make various tools for abrasive machining of hard materials, etc. (Figure M51). The main part of the latter, however, are synthetic diamonds, manufactured from graphite, and used for many purposes in industry. Diamond is the hardest material known and has the highest thermal conductivity of aU materials at room temperature. 

The so-called apparent thermal conductivity of insulating materials depends upon four modes of heat transfer gas conduction and convection, radiation, and solid conduction. The principles of these four mechanisms of heat transfer are fairly well understood individually but their combined effect on heat transfer in insulating materials is complicated. Nevertheless, because of the additive nature of the heat transferred by these mechanisms, the conductivities assigned to each mechanism are additive. Thus, if each of these conductivities can be evaluated under various conditions of temperature and pressure, their sum stated as an apparent conductivity may be estimated. 

The measurement of very low thermal conductivities is done directly by equilibrium methods, where typically a constant heat flux is measured to maintain a given temperature difference between a hot and a cold side. Dynamic methods rely on a transient heat pulse or wave that is sent from a material interface and travels over a known distance to reach a detector. Indirect methods then rely on physical models to calculate the thermal conductivity based on heat diffusion equations. A detailed review on the physics of heat transport in aerogels was given by Ebert [203] in the aerogels handbook. Various theoretical models exist, which allow one to determine the effective thermal conductivity of superinsulation materials based on dynamic measurement methods. 

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