Thermal conductivity:

Thermal conductivity:

Thermal conductivity is a physical property of the substance and it is defined as the ability of a substance to conduct heat. It is also defined as heat flux (W/m²) per unit temperature gradient (K/m). The unit of thermal conductivity becomes W/mK.

Thermal conductivity of a material depends upon its state and is a function of pressure, temperature, humidity and structure.

Pure metals have the highest value of thermal conductivities while gases and vapours have the lowest, insulating materials and inorganic liquids have thermal conductivity that lies between those of metals and gases.

For solids the transport of energy by conduction is due to drift of free electrons and lattice vibration waves. The thermal conductivity k for solids is the sum of electronic component (k_e) and the lattice component (k_l). In pure metals k_e is much larger than k_l and so they are good conductors of heat and electricity. The thermal conductivity for most pure metals except Aluminium and uranium decreases with increase in temperature. In alloys, k_e is less than that of pure metals and their values of thermal conductivity is less than that of pure metals. In non metallic solids k_e is very low but k_l is very high. The value of k_l increases with increasing temperature due to larger interactions between the atoms and lattice.

The thermal conductivity of insulating materials is very low. This is because many building and insulating materials have porous structure or have air trapped within. Since air is a bad conductor of heat so the thermal conductivity of air filled porous material is low.

The thermal conductivity of liquids and gases is smaller than that of solids because their intermolecular spacing is much larger and so there is less effective transport of energy. The thermal conductivity of gas increases with increasing temperature and decreasing molecular weight whereas it generally decreases with increasing temperature for non metallic liquids.

There are some materials which have a very high thermal conductivity at very low temperatures. These are called super conductors for example thermal conductivity of Aluminium at 10 K is of the order of 20000 W/mK which is more than 100 times its value at 20⁰C.