Influencing factors of thermal conductivity-5

- Jun 20, 2018-

Magnetic field

The influence of magnetic fields on thermal conductivity is known as the Righi-Leduc effect.

Gaseous Phases

Exhaust system components with ceramic coatings having a low thermal conductivity reduce heating of nearby sensitive components

Air and other gases are generally good insulators, in the absence of convection. Therefore, many insulating materials function simply by having a large number of gas-filled pockets which obstruct heat conduction pathways. Examples of these include expanded and extruded polystyrene (popularly referred to as "styrofoam") and silica aerogel, as well as warm clothes. Natural, biological insulators such as fur and feathers achieve similar effects by trapping air in pores, pockets or voids, thus dramatically inhibiting convection of air or water near an animal's skin.

Light gases, such as hydrogen and helium typically have high thermal conductivity. Dense gases such as xenon and dichlorodifluoromethane have low thermal conductivity. An exception, sulfur hexafluoride, a dense gas, has a relatively high thermal conductivity due to its high heat capacityArgon and krypton, gases denser than air, are often used in insulated glazing (double paned windows) to improve their insulation characteristics.

The thermal conductivity through bulk materials in porous or granular form is governed by the type of gas in the gaseous phase, and its pressure. At lower pressures, the thermal conductivity of a gaseous phase is reduced, with this behaviour governed by the Knudsen number, defined as Kn=l/d. where l is the mean free path of gas molecules and d is the typical gap size of the space filled by the gas. in a granular material d corresponds to the characteristic size of the gaseous phase in the pores or intergranular spaces.

Isotopic purity

Isotopically pure diamond can have a significantly higher thermal conductivity.[8] eg. 41,000 W·m−1·K−1 [9] (99.999% 12C calc.200,000[9])