Thermal Conductivity

The amount of heat passing through unit cross-section of a solid material in unit time for unit thermal gradient is called as thermal conductivity. The units of thermal conductivity are JH/m/s/K or W/m/K. The heat conduction takes place from higher temperature to lower temperature that means opposite to direction of thermal gradient.

The thermal conductivity in solid material is because of

1.    elastic vibration of atoms or molecules whereby energy is transmitted in from of kinetic energy of vibrating units, and
2.    movement of electrons.

The materials in which electrons may freely move are capable of transmitting large amount of heat & are good thermal conductor. Metals are such solids in which free electrons are available in big number and therefore they are good conductor.

Alternatively, these materials which do not have free electrons conduct heat by built of elastic vibration of atoms & molecules and therefore are not good thermal conductors but insulators. Thermal conductivity of pure metals on a moderate temperature as aluminium silver, copper, and brass show variation & peak. Apparently pure metals & single crystals show high conductivity because electrons do not scatter at impurities & grain boundaries. Figure illustrates the variation of thermal conductivity with temperature for metallic materials.