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 appears primarily in Fourier's Law for heat conduction.
First, we define heat conduction by the formula:

where is the rate of heat flow, k is the thermal conductivity, A is the total cross sectional area of conducting surface, ?T is temperature difference and x is the thickness of conducting surface separating the 2 temperatures.
Thus, rearranging the equation gives thermal conductivity,

(Note: is the temperature gradient)
In other words, it is defined as the quantity of heat, ?Q, transmitted during time ?t through a thickness x, in a direction normal to a surface of area A, due to a temperature difference ?T, under steady state conditions and when the heat transfer is dependent only on the temperature gradient.
Alternatively, it can be thought of 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绋�) and English units: Btu/(hr绌巘閽扚). To convert between the two, use the relation 1 Btu/(hr绌巘閽扚) = 1.730735 W/(m绋�). [Perry's Chemical Engineers' Handbook, 7th Edition, Table 1-4]
Contents
1 Examples
2 List of thermal conductivity values
3 Measurement
3.1 Standard Measurement Techniques
4 Difference between US and European notation
5 Related terms
5.1 Thermal conductance
5.2 Thermal Resistance
5.3 Alternate definition (buildings)
6 Textile industry
7 Origins
8 See also
9 References
10 External links
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Examples
In metals, thermal conductivity approximately tracks electrical conductivity according to the Wiedemann-Franz law, 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, due to the increased importance of phonon carriers for heat in non-metals. As shown in the table below, highly electrically conductive silver is less thermally conductive than diamond, which is a 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, with 35 W/(m绋�) along the c-axis and 32 W/(m绋�) along the a-axis.[1]
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. Natural, biological insulators such as fur and feathers achieve similar effects by dramatically inhibiting convection of air or water near an animal's skin.
Light gases, such as hydrogen and helium typically have high thermal conductivity. Dense gases such as xenon and dichlorodifluoromethane have low thermal conductivity. An exception, sulfur hexafluoride, a dense gas, has a relatively high thermal conductivity due to its high heat capacity. Argon, a gas denser than air, is often used in insulated glazing (double paned windows) to improve their insulation characteristics.
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.
List of thermal conductivity values
Main article: List of thermal conductivities
This is a list of approximate values of thermal conductivity, k, for some common materials. Please consult the list of thermal conductivities for more accurate values, references and detailed information.
Material
Thermal conductivityW/(m绋�)
Silica Aerogel
0.004 - 0.04
Air
0.025
Wood
0.04 - 0.4
Hollow Fill Fibre Insulation Polartherm
0.042
Alcohols and oils
0.1 - 0.21
Polypropene
0.12 [1]
Mineral oil
0.138
Rubber
0.16
LPG
0.23 - 0.26
...(and so on)

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