The thermal conductivity , and the coefficient of thermal conductivity is a material property that the heat flow through a material because of the heat conduction determined. The thermal conductivity shows how well a material conducts heat or how well it is suitable for thermal insulation . The lower the value of the thermal conductivity, the better the thermal insulation. In the SI system, the thermal conductivity has the unit watt per meter and Kelvin.
The thermal conductivity of most materials increases slightly with increasing temperature. At a phase transition or physical state transition (. Eg fixed <> liquid <> gas), the conductivity changes, however, usually strongly and abruptly.
To define the “thermal conductivity” parameter, imagine two heat reservoirs that have the temperatures and (it applies ), and are separated from each other by a flat wall of a certain material. The properties of the material are the same at every location inside and have no preferred direction; the material is therefore homogeneous and isotropic . The wall has a thickness and is infinitely extensive. (In practice it is sufficient that the wall is much wider and higher than it is thick.) There is a constant flow of heat between the two reservoirs. The heat flow then flows through any part of the wall with the surface .
- the area
- the temperature difference
- and inversely proportional to the wall thickness
and otherwise only depends on the thermal conductivity of the medium (wall material). This gives the definition equation for the thermal conductivity:
This connection is also called Fourier's law . The unit of thermal conductivity immediately follows from the definition:
In this equation is the (vectorial) heat flux density . The negative sign is due to the fact that heat always flows along the temperature gradient, i.e. against the temperature gradient.
In the general anisotropic case, the thermal conductivity is a tensor of the second order . B. described by a 3 × 3 matrix . So lead z. B. Wood and slate in the grain direction and a quartz crystal in the direction of the c-axis heat better than across it. If the temperature gradient runs at an angle to the material axes, the direction of the heat flow deviates from that of the gradient.
- Dry pine wood with a density of 0.45 g / cm³ has a thermal conductivity of 0.26 W / (m · K) parallel to the fiber and 0.11 W / (m · K) perpendicular to it. If you choose the fiber direction as the z-axis and the x- and y-axes perpendicular to it, you can write the tensor of the thermal conductivity as a diagonal 3 × 3 matrix:
Mechanisms of heat conduction
In addition to thermal conduction, thermal energy can also be transmitted through thermal radiation and convection . In the case of substances with high thermal conductivity, these mechanisms can be neglected in some cases.
In a vacuum there is no heat conduction and no convection, only heat radiation. High vacuum is therefore the best insulator against heat flows.
In metals , the conduction electrons can transport not only charge (= electrical current ) but also thermal energy, see Wiedemann-Franz's law . Therefore, metals with high electrical conductivity usually also have good thermal conductivity. One example is silver, which of all pure metals is both the best electrical conductor and the best thermal conductor.
Measuring devices for determining the thermal conductivity of thermal insulation materials , so-called heat flow meters and other heat flow calorimeters , measure the electrical power of a heating element corresponding to the heat flow , the thickness of a sample and the temperature difference on a defined measuring surface ( Peltier element ). Furthermore, so-called heat flow sensors enable the non-invasive measurement of heat flows due to the Seebeck effect . Measured quantities are the heat flow and the absolute temperature. On the basis of these measurement principles, the thermal radiation of materials that are transparent to thermal radiation and the thermal convection due to gases trapped in the insulation material are also determined. The result is therefore the sum of the heat flows of the three types of heat transfer and not just a heat flow due to heat conduction .
The thermal conductivity of a substance can be determined using thermal conduction or Fourier's law .
Thermal conductivity in construction
In the construction industry, since the introduction of the European Construction Products Regulation in 2013, three different sizes have been used in parallel to identify thermal insulation materials and for calculation.
- , Nominal value of thermal conductivity according to CE marking
- , Rated value of thermal conductivity according to DIN 4108-4
- , Limit value of the thermal conductivity according to the general building authority approval (ABZ) of a building product
They differ from one another in the way they are identified and used. Only the rated value of the thermal conductivity according to DIN 4108-4 can be used directly to verify the physical properties of building components; the other thermal conductivity values require a safety margin.
- DIN 4108-4 Thermal insulation and energy savings in buildings - Part 4: Thermal and moisture-proof rated values
- ÖNORM B 8110-7 Thermal insulation in building construction - Part 7: Tabulated thermal insulation design values
The thermal conductivity values of various substances can vary by many orders of magnitude. For example, high values are required for heat sinks , which are supposed to dissipate heat well , whereas thermal insulation materials should have low values.
The thermal conductivity is a material constant at a defined ambient conditions ( temperature and humidity ) and therefore is partially provided with an index: , or . Unless otherwise stated, the following numerical values apply to 0 ° C. A higher thermal conductivity means a greater heat transfer per period.
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