For steady-state heat flow, the proportionality constant between the heat flux and the temperature gradient. Also, a parametre characterizing the ability of a material to conduct heat.
Thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction. Heat transfer across materials of high thermal conductivity occurs at a faster rate than across materials of low thermal conductivity. Correspondingly materials of high thermal conductivity are widely used in heat sink applications and materials of low thermal conductivity are used as thermal insulation. Thermal conductivity of materials is temperature dependent. In general, materials become more conductive to heat as the average temperature increases. The reciprocal of thermal conductivity is thermal resistivity.
In the International System of Units (SI), thermal conductivity is measured in watts per meter kelvin (W/(m·K)). In the imperial system of measurement thermal conductivity is measured in Btu/(hr·ft⋅F) where 1 Btu/(hr·ft⋅F) = 1.730735 W/(m·K). Other units which are closely related to the thermal conductivity are in common use in the construction and textile industries. The construction industry makes use of units such as the R-Value (resistance value) and the U-Value (thermal transmittance). Although related to the thermal conductivity of a product R and U-values are dependent on the thickness of a product.
There are a number of ways to measure thermal conductivity. Each of these is suitable for a limited range of materials, depending on the thermal properties and the medium temperature. There is a distinction between steady-state and transient techniques.
In general, steady-state techniques are useful when the temperature of the material does not change with time. This makes the signal analysis straightforward (steady state implies constant signals). The disadvantage is that a well-engineered experimental setup is usually needed. The Divided Bar (various types) is the most common device used for consolidated rock samples.
The transient techniques perform a measurement during the process of heating up. Their advantage is quicker measurements. Transient methods are usually carried out by needle probes. A method described by Angstrom involves rapidly cycling the temperature from hot to cold and back and measuring the temperature change as the heat propagates along a thin strip of material in a vacuum.