Thermal Resistance & Thermal Conductance - C-Therm - Thermal Conductivity Instruments
Thermal resistance is the ability of a material to resist flow of heat. Thermal resistivity is the reciprocal of thermal conductivity and can be expressed as. r = 1 / k (1). Thermal resistance (R) and thermal conductance (C) of the materials are reciprocals of one another and can be derived from thermal conductivity (k) and the. Because the accurate determination of thermal conductivity is a demanding The relationship holds only for materials with more or less freely.
An example of this would be the change in thermal conductivity that occurs when ice thermal conductivity of 2. Other examples of materials where the thermal conductivity varies with direction are metals that have undergone heavy cold pressinglaminated materials, cables, the materials used for the Space Shuttle thermal protection systemand fiber-reinforced composite structures. Electrical conductivity[ edit ] In metals, thermal conductivity approximately tracks electrical conductivity according to the Wiedemann—Franz lawas freely moving valence electrons transfer not only electric current but also heat energy.
However, the general correlation between electrical and thermal conductance does not hold for other materials, due to the increased importance of phonon carriers for heat in non-metals. Highly electrically conductive silver is less thermally conductive than diamondwhich is an electrical insulatorbut due to its orderly array of atoms it is conductive of heat via phonons.
Thermal contact conductance - Wikipedia
Magnetic field[ edit ] The influence of magnetic fields on thermal conductivity is known as the thermal Hall effect or Righi—Leduc effect. Gaseous phases[ edit ] 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 aerogelas 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.
The finish quality is exaggerated for the sake of the argument. Thermal contact conductance is a complicated phenomenon, influenced by many factors.
Experience shows that the most important ones are as follows: Contact pressure[ edit ] For thermal transport between two contacting bodies, such as particles in a granular medium, the contact pressure is the factor of most influence on overall contact conductance.
As contact pressure grows, true contact area increases and contact conductance grows contact resistance becomes smaller. The thermal contact resistance of certain sandwich kinds of materials that are manufactured by rolling under high temperatures may sometimes be ignored because the decrease in thermal conductivity between them is negligible.
As a result, when two bodies are pressed together, contact is only performed in a finite number of points, separated by relatively large gaps, as can be shown in Fig. Since the actual contact area is reduced, another resistance for heat flow exists. The thermal conductivity of the interstitial material and its pressure, examined through reference to the Knudsen numberare the two properties governing its influence on contact conductance, and thermal transport in heterogeneous materials in general .
- Thermal contact conductance
- Thermal Conductivity – Resistance: Surface Area
In the absence of interstitial materials, as in a vacuumthe contact resistance will be much larger, since flow through the intimate contact points is dominant. Surface roughness, waviness and flatness[ edit ] One can characterise a surface that has undergone certain finishing operations by three main properties of: