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Thermal Conductivity |
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In physics, thermal conductivity, k, is the property of a material that indicates its ability to conduct heat. It is used mostly in Fourier's Law for heat conduction. It is defined as the quantity of heat, transmitted during time, through a thickness, in a direction normal to a surface of area A, due to a temperature difference, 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)
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It can also be considered as a flux of heat (energy per unit area per unit time) divided by a temperature gradient (temperature difference per unit length).
Typical units are SI: W/(m·K) and English units: Btu·ft/(h·ft²·°F). To convert between the two, use the relation 1 Btu·ft/(h·ft²·°F) = 1.730735 W/(m·K). [Perry's Chemical Engineers' Handbook, 7th Edition, Table 1-4]
In metals, thermal conductivity approximately tracks electrical conductivity, as freely moving valence electrons transfer not only electric current but also heat energy. However, the general correlation between electrical and thermal conductance does not hold for other materials. As shown in the table below, highly electrically conductive silver is less thermally conductive than diamond, which is an electrical insulator. Thermal conductivity depends on many properties of a material, notably its structure and temperature. For instance, pure crystalline substances exhibit very different thermal conductivities along different crystal axes, due to differences in phonon coupling along a given crystal axis. Sapphire is a notable example of variable thermal conductivity based on orientation and temperature, for which the CRC Handbook reports a thermal conductivity of 2.6 W/(m·K) perpendicular to the c-axis at 373 K, but 6000 W/(m·K) at 36 degrees from the c-axis and 35 K (possible typo?).
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 expanded and extruded polystyrene (popularly referred to as "styrofoam") and silica aerogel.
Thermal conductivity is important in building insulation 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.
Here is a list of approximate values of thermal conductivity, k, for some common materials.
Air 0.025, Wood 0.04 – 0.4, Alcohol or oil 0.15, Soil 0.15, Rubber 0.16, Epoxy (unfilled) 0.19
Epoxy (silica-filled) 0.30, Water (liquid) 0.6, Thermal grease 0.7 – 3, Glass 1.1, Ice 2, Sandstone 2.4 Stainless steel 15, Lead 35.3, Aluminium 237, Gold 318, Copper 401, Silver 429, Diamond 900 – 2320
The reciprocal of thermal conductivity is thermal resistivity, measured in kelvin-metres 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. Other related terminology including infrared are :
Thermal conductivity steel, Thermal conductivity unit, Thermal conductivity air, Thermal conductivity of copper, Thermal conductivity measurement, Heat transfer coefficient, Thermal insulation, Heat conduction and Thermal drapes.
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