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It is defined as the quantity of heat, ''Q'', transmitted in time ''t'' through a thickness ''L'', in a direction normal to a surface of area ''A'', due to a temperature difference Δ''T'', under steady state conditions and when the heat transfer is dependent only on the temperature gradient. :: thermal conductivity = heat flow rate × distance / (area × temperature difference) :: EXAMPLES In Metal s, thermal conductivity approximately tracks Electrical Conductivity , as the freely moving valence electrons transfer not only electric current but also heat. However, this correlation does not apply to some materials, as shown in the table below, where highly electrically conductive Silver is shown to be less thermally conductive than Diamond , which is an electrical semiconductor. Thermal conductivity is not a simple property, and depends intimately on structure and temperature. For instance, pure, crystalline substances also exhibit highly variable thermal conductivities along different crystal axes. One particularly notable example is sapphire, for which the CRC Handbook reports a thermal conductivity perpendicular to the c-axis of 2.6 W·m-1·K-1 at 373 K, and 6000 W·m-1·K-1 at 35 K for an angle of 36 degrees to the c-axis. Air and other gases are generally good insulators, in the absence of Convection . Therefore, many insulating materials function simply by having a large number of gas-filled pockets which prevent large-scale convection. Examples of these include Polystyrene (styrofoam) and silica Aerogel . Thermal conductivity is clearly an important quantity for construction and related fields. However, materials used in such trades are rarely subjected to chemical purity standards. Several construction materials' ''k'' values are listed below. These should be considered approximate due to the uncertainties related to material definitions. The following table is meant as a small sample of data to illustrate the thermal conductivity of various types of substances. For more complete listings of measured ''k''-values, see the references. SOME TYPICAL THERMAL CONDUCTIVITIES (''K'' VALUES) References and additional sources of k values † CRC handbook of chemistry and physics †† Marble Institute ††† Soil Sci Journals MEASUREMENT For good conductors of heat, is an old data logging machine. An alternative traditional method using real thermometers is described at A brief review of relatively new class of dynamic methods that are measuring thermal conductivtiy, thermal diffusivity and specific heat within a single measurement is available at [http://thermophys.savba.sk/Methods.htm A thermal conductance tester, one of the instruments of Gemology , determines if Gems are genuine Diamond s using diamond's uniquely high thermal conductivity, which is higher still for natural blue diamond. RELATED TERMS The reciprocal of thermal conductivity is ''thermal resistivity'', measured in Kelvin - Metre s per Watt (K·m·W-1). When dealing with a known amount of material, its ''thermal conductance'' and the reciprocal property, ''thermal resistance'', can be described. Unfortunately there are differing definitions for these terms. First definition (general) For general scientific use, ''thermal conductance'' is the quantity of heat that passes in unit time through a plate of ''particular area and thickness'' when its opposite faces differ in temperature by one degree. For a plate of thermal conductivity ''k'', area ''A'' and thickness ''L'' this is ''kA/L'', measured in W·K-1. This matches the relationship between Electrical Conductivity (A·m-1·V-1) and Electrical Conductance (A·V-1). There is also a measure known as ''heat transfer coefficient'': the quantity of heat that passes in unit time through ''unit area'' of a plate of particular thickness when its opposite faces differ in temperature by one degree. The reciprocal is ''thermal insulance''. In summary:
The heat transfer coefficient is also known as ''thermal admittance'', but this term has other meanings. Second definition (buildings) When dealing with buildings, ''thermal resistance'' or '' R-value '' means what is described above as thermal insulance, and ''thermal conductance'' means the reciprocal. For materials in series, these thermal resistances (unlike conductances) can simply be added to give a thermal resistance for the whole. A third term, ''thermal transmittance'', incoporates the thermal conductance of a structure along with heat transfer due to Convection and Radiation . It is measured in the same units as thermal conductance and is sometimes known as the ''composite thermal conductance''. The term ''U-value'' is another synonym. The term ''K-value'' is a synonym for thermal conductivity. In summary, for a plate of thermal conductivity ''k'', area ''A'' and thickness ''L'':
TEXTILE INDUSTRY In textiles, a Tog value may be quoted instead of thermal resistance. MOLECULAR ORIGINS The thermal conductivity of a system is determined by how molecules comprising the system interact. There are no simple, correct expressions for thermal conductivity. The simplest exact expression employs one of the Green-Kubo Relations . Although this expression is exact, in order to calculate the thermal conductivity of a dense fluid or solid using this relation requires the use of molecular dynamics computer simulation . SEE ALSO
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REFERENCES 1 Halliday, David; Resnick, Robert; & Walker, Jearl(1997). ''Fundamentals of Physics'' (5th ed.). John Wiley and Sons, INC., NY ISBN 0-471-10558-9. TM 5-852-6 AFR 88-19, Volume 6 (Army Corp of Engineers publication) |