Which liquid has the highest thermal conductivity

Thermal conductivity

Lexicon> Letter W> Thermal conductivity

Definition: a measure of a material's ability to conduct thermal energy

English: heat conductivity, thermal conductivity

Categories: Basic Terms, Physical Basics, Heat and Cold

Formula symbol: λ

Unit: W / (m K)

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 08/26/2010; last change: 03/26/2020

URL: https://www.energie-lexikon.info/waermeleitfaehigkeit.html

The specific thermal conductivity or Thermal conductivityλ of a material is a quantitative measure of its ability to transmit energy in the form of heat. It is given in the unit W / (m K) (watts per meter and Kelvin). It is the reciprocal of the specific heat resistance.

The information can be understood as follows. Assume that you have a cuboid made of the material in which one side surface is kept at a certain temperature, while the opposite side surface has a temperature that is 1 K higher. The other side surfaces are thermally insulated so that no heat can escape or penetrate there. Then, as soon as an equilibrium (steady state) has been established, there is a heat flow through the cuboid, its powerP according to the formula

can be calculated. Here is d the thickness of the cuboid, d. H. the distance between the two first mentioned side surfaces, ΔT the temperature difference between and A. their area. The heat flow becomes stronger the greater the coefficient of thermal conductivity λ, the area and the temperature difference, and the thinner the cuboid.

It is easy to see that the numerical value of λ is the flowing heat output in watts if you make a cube with an edge length of 1 m from the material and two opposite side surfaces have a temperature difference of 1 K.

In the construction sector, one often speaks of Thermal conductivity groups (WLG). For example, WLG 040 means a thermal conductivity of 0.040 W / (m K) = 40 mW / (m K).

Relationship with the heat transfer coefficient

The heat transfer coefficient (U-value) of a plate made of a homogeneous material is calculated as the λ value divided by the thickness. For example, a 0.5 m thick wall made of limestone (λ = 2.2 W / (m K)) has a U-value of 2.2 W / (m K) / 0.5 m = 4.4 W / ( m2 K). It can be seen that a good insulating effect (a small heat transfer coefficient) is achieved by choosing a thick layer of a material with a low λ value, with a lower thickness being sufficient for a low λ value. For example, 20 cm polyurethane or polystyrene insulate much better than even very thick walls.

Thermal conductivity of selected materials

The following table shows the thermal conductivity of some materials that are particularly important in connection with energy in buildings.

materialλ-Value in W / (m K)
compact concrete2,1
Aerated concrete (aerated concrete)z. B. 0.2
Brickwork0.5 to 1.4
Bricks with fine pores or insulation material in cavitiese.g. 0.1
Sand-lime brick0.5 to 1.3
limestone2.2 to 2.5
Window glass (pure glass, not double glazing or similar)0,75
steelapprox. 15 to 60
solid wood0,1 – 0,2
Fibreboard0,04 – 0,05
Pulp flakes0,04
Glass wool0,032 – 0,050
expanded polystyrene (EPS)0,035 – 0,050
Polyurethane (PUR)0,024 – 0,035
Aerogels0,015 – 0,02

Table 1:λ-values ​​of different materials.

Please note that the actual values ​​for many substances can vary significantly depending on the exact composition and density. For example, pure metals usually have a very high thermal conductivity, which, however, can be greatly reduced when alloying with other metals or by impurities. Stainless steel, for example, consists largely of iron and still has a much lower thermal conductivity than pure iron. Another example is aerated concrete: Here, a higher volume fraction of the pores leads to a lower density and a lower thermal conductivity, but also a lower mechanical stability.

Moisture can greatly increase the thermal conductivity of building materials!

In the case of many building materials (such as masonry), the thermal conductivity can increase significantly when moisture occurs. The values ​​in the tables as above naturally apply to the dry state and are no longer valid. If, for example, an outside wall of a non-insulated house becomes damp on the inside because the dew point is not reached there, the heat losses increase further and the formation of moisture is increased again. This situation must be avoided at all costs, if only because of the risk of mold growth.

Materials with high electrical conductivity also tend to have high thermal conductivity and vice versa. (This is because certain electronic properties are relevant for both conduction phenomena.) It is therefore difficult to find materials for poorly thermally conductive power cables or for good thermally conductive electrical insulators. There are exceptions, however - for example, diamond is a very good conductor of heat and still an excellent electrical insulator.

Why is a thick layer of air or water unsuitable for thermal insulation despite its low thermal conductivity?

In the case of gases and liquids, the values ​​apply under the assumption that nothing moves - which may be completely unrealistic in practice. For example, one would greatly overestimate the insulating effect of the air layer between the two panes of double glazing if it were calculated using the λ value of air, as convection (circulation) of the air occurs as soon as there is a temperature difference between the panes. The heat is then not primarily transferred by conduction, but rather by transporting the heated air. The function of many thermal insulation materials is based on the fact that they contain a lot of air, but that it is prevented from moving in the material by being trapped in small bubbles or pores.

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See also: thermal conduction, heat, heat transfer coefficient, thermal resistance, thermal insulation, thermal insulation material
as well as other articles in the categories basic concepts, physical principles, heat and cold