Thermal conductivity
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Surname  Thermal conductivity  
Formula symbol  

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.
From the thermal conductivity can by dividing the volume heat capacity , the thermal diffusivity are calculated. The reciprocal of thermal conductivity is the (specific) thermal resistance .
definition
Thermal conduction is understood to mean the transport of heat in a medium without material transport (such as with convection ).
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 .
Under the conditions mentioned, the temperature gradient is constant over the entire thickness of the wall. The heat flow is then proportional to
 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 the general case it is not enough to consider only one dimension . In particular, the temperature profile is linear only in exceptional cases . The more general formulation is therefore:
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.
Tensor representation
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 caxis 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.
 example
 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 zaxis and the x and yaxes 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 WiedemannFranz'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.
Measurement
Measuring devices for determining the thermal conductivity of thermal insulation materials , socalled 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, socalled heat flow sensors enable the noninvasive 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 41084
 , 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 41084 can be used directly to verify the physical properties of building components; the other thermal conductivity values require a safety margin.
Norms
 DIN 41084 Thermal insulation and energy savings in buildings  Part 4: Thermal and moistureproof rated values
 ÖNORM B 81107 Thermal insulation in building construction  Part 7: Tabulated thermal insulation design values
Sample 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.






literature
 LandoltBörnstein  database for almost all physical properties, including thermal conductivity values
Web links
 Thermal conductivity of the elements
 Search in the Dortmund database for the thermal conductivity of pure substances
Individual evidence
 ↑ ^{a } ^{b} David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 87th edition. (Internet version: 2006–2007), CRC Press / Taylor and Francis, Boca Raton, FL, Properties of Solids, pp. 12204 ( limited preview in Google Book Search).
 ^ Walter J. Moore: Physical chemistry. Walter de Gruyter, 1986, ISBN 9783110109795 , p. 47 ( limited preview in the Google book search).
 ↑ Confusion about thermal conductivity . In: Deutsches Architektenblatt , October 1, 2013.
 ↑ Handbook of concrete protection through coatings, Expert Verlag 1992, page 413
 ^ Sven Fuchs, Andrea Förster: Rock thermal conductivity of Mesozoic geothermal aquifers in the Northeast German Basin . In: Chemistry of the Earth  Geochemistry . tape 70 , Supplement 3, August 2010, p. 13–22 , doi : 10.1016 / j.chemer.2010.05.010 ( edoc.gfzpotsdam.de [PDF]).
 ↑ Leaflet 821 (PDF; 877 kB); Stainless Steel  Properties; Publisher: Informationsstelle Edelstahl Rostfrei Table 9; Status: 2014.
 ↑ Data sheets Trocellen PE insulation materials, accessed on July 30, 2010 ( Memento from August 21, 2010 in the Internet Archive )
 ↑ ^{a } ^{b } ^{c } ^{d } ^{e } ^{f } ^{g } ^{h} Guidelines for ecological insulation materials (PDF) from BENZ GmbH & Co. KG Baustoffe, accessed on March 1, 2017.
 ↑ Product information Thermosafehomogen® from GUTEX Holzfaserplattenwerk H. Henselmann GmbH & CO. KG, accessed on May 29, 2016.
 ↑ Product information THERMO HEMP PREMIUM from THERMO NATUR GmbH & Co. KG, accessed on February 22, 2020.
 ^ HansJürgen Bargel, Hermann Hilbrans: Material science . Springer, 2008, ISBN 9783540792963 , pp. 275 ( limited preview in Google Book Search).
 ↑ Material properties of cast alloys (PDF) and pipe materials (PDF) from WielandWerke AG, accessed in August 2014.
 ↑ Thermal conductivity . ( Memento from March 11, 2016 in the Internet Archive )
 ↑ ^{a } ^{b } ^{c } ^{d } ^{e } ^{f } ^{g } ^{h } ^{i} David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Fluid Properties, pp. 6184. Values apply at 300 K.
 ↑ schweizerfn.de
 ↑ ^{a } ^{b } ^{c } ^{d } ^{e } ^{f } ^{g } ^{h } ^{i } ^{j } ^{k} Horst Czichos (ed.): The basics of engineering, D materials, thermal conductivity of materials . 31st edition. Springer, 2000, ISBN 3540668829 , pp. D 54 .
 ↑ ^{a } ^{b} Data sheets for engineering plastics and their properties, accessed on November 23, 2010 .
 ↑ entry in makeitfrom.com
 ↑ ^{a } ^{b } ^{c } ^{d} schweizerfn.de
 ↑ Material data water . Wikibooks
 ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Fluid Properties, pp. 6220.
 ↑ Lecture documents hydroscript.  PTB Braunschweig ( memento from September 24, 2015 in the Internet Archive ).
 ↑ geizhals.eu
 ↑ oskarmoser.de: Technical data for synthetic sapphire