# Thermal conductivity of material

The thermal conductivity of a material is the rate of heat transfer from a unit thickness of material per unit area and unit temperature difference. For example, if the rate of heat transfer for a material is k with thickness 1 meter and the temperature difference of 1 Kelvin and areas of the 1 square-meter, then k is the *thermal conductivity* of that material.

**Thermal conductivity**

#### Here are some points related to thermal conductivity.

- The thermal conductivity of a substance is highest at solid state and lowest in the gas phase.
- Thermal conductivity is independent of pressure (Except vacuum)
- The thermal conductivity of the metal decrease with an increase in temperature (Except Mercury, Aluminium, Uranium)
- Thermal conductivity y of gas increase with an increase in temperature.
- In pure metal, electron dominates for thermal conductivity while for non-conductor and semiconductor, lattice vibration dominates for thermal conductivity.
- In the Crystalline non metal solids like the diamond, contribution of thermal conductivity by lattice vibration is quite large that it exceeds the value of conductivity of metals like aluminium which is one of the best metallic conductors.
- The thermal conductivity of the non-metallic liquid decrease with increase in temperature
- The thermal conductivity of liquids is insensitive to pressure (Except critical point)

#### Here is the list of thermal conductivity value for some materials.

- The thermal conductivity of air ( 0.026) > k of water (0.61) > k of ice (2.25)
- The thermal conductivity of solids > Liquids > Gas
- Diamond has the highest thermal conductivity. ( 2300)
- In metals, silver possed highest thermal conductivity (430) but due to the cost factor, the second alternative which is copper is widely used. (400)
- Value of thermal conductivity varies with temperature difference and medium in which heat transfer takes place. By default value of thermal conductivity at room temperature (25-degrees Celcius) is considered standard for calculations.

**Also read: Thermal conduction mechanism**