Thermal conductivity
The (specific ) thermal conductivity, and thermal conductivity ( λ, k or κ ) is a material property to calculate the heat flux from the temperature gradient:
The thermal conductivity has the SI units of watts per meter per Kelvin. It is temperature dependent. Its reciprocal value is the specific heat resistance.
In practical terms, the heat conductivity, the heat quantity ( WS) which flows S in Figure 1 by a 1 m thick layer of the surface material 1 m2, when the temperature difference is 1 K. The full unit is thus watt-second times meters per square meter Kelvin and second.
Measuring devices for determining the thermal conductivity of thermal insulating materials, so-called thermal flow meter, and other heat flow calorimeter to measure the heat flow equivalent electric power of a heating element, the thickness of a sample, and the temperature difference at a predefined measurement area (Peltier element). Based on this measurement principle this heat radiation with thermal radiation transparent fabrics and heat convection are influenced due to trapped gases in the insulation material. The result is therefore the sum of the heat flows of the three kinds of heat transfer and not just a heat flux due to thermal conduction.
In non-stationary temperature fields (see heat equation ) is sometimes used instead of the thermal diffusivity of the thermal conductivity, which differs from it by the volume-based heat capacity.
In general anisotropic case, the thermal conductivity is a tensor of rank. Diverting wood in the fiber direction, the heat better than across it. Extends the temperature gradient angle to the material axes, the direction of heat flow from the gradient deviates from.
The values of the thermal conductivity for various materials vary by many orders of magnitude. High values are required for the heat sink, the heat should derive good heat insulation materials should have low values.
Calculation example
The length of a rectangular parallelepiped with the cross-sectional area there is a temperature difference. By the side surfaces flowed the heat ( they were isolated ), the material is isotropic ( eg, copper) and the state hospital. The temperature gradient is then everywhere and the density of from hot to cold -related heat flow. Over the cross section of the heat flow that is flowing
With the values = 1.5 mm2 = 1.5 · 10-6 m2, = 3 cm = 0.03 m, = 200 K and = 350 W / ( m · K ) results in a heat flow of
For the thermal conductivity in a compact, non-metallic solids and liquids usually mainly contributes to the transport of vibrational energy by mechanical coupling of adjacent atoms. In some crystals, especially in isotopically pure diamond, vibrational excitations can be as undisturbed spread (large mean free path ), the heat conduction equation is no longer valid at small dimensions.
In metals, the conduction electrons do not only transport cargo but also energy over a larger distance, see Wiedemann - Franz law.
In gases, the molecules are the carriers of energy. The kinetic theory of gases explains the high degree of independence from pressure by compensation: The number of particles increases, the mean free path from. The slight increase of thermal conductivity with temperature is due to the increasing particle velocity, the decrease in the scattering cross sections for tougher shocks and on increasing participation of vibrational degrees of freedom. Heavy molecules move slower than light, which explains most of the difference in heat transfer between hydrogen and air (factor 7). The atoms of noble gases do not transport molecules as well as vibrational and rotational energy, but only kinetic, argon why only 2/3 has the thermal conductivity of air.
The low thermal conductivity of gases, however, can only be used when the concurrent heat transfer by convection and radiation is limited, see Insulating glass units.
While in gases, the heat radiation covers a large area and therefore must be considered separately, is used as insulating materials (eg foam or fibrous material ), the heat transfer by radiation locally within the sample and take is contained in the coefficient of thermal conductivity. The larger the range of the radiation, the more can contribute to the radiation heat conduction. With increasing temperature, the radiation component and thus the thermal conductivity of insulating materials is increasing.
Numerical values
Thermal conductivity is a material constant at a defined ambient conditions (temperature and humidity) and is therefore partially indexed: , or
The numerical values apply, unless otherwise specified, is 0 ° C. A higher thermal conductivity is a greater heat transfer per unit time.
Particularly at very low levels of substances such as, for example, xenon is to be noted that in addition to thermal conduction thermal energy can be transferred by heat in addition to radiation and convection; However, in vacuum only by thermal radiation.