Question

Why thermal conductivity of gases and solid decreses with temperature?

Answer 1

This is from my 1970s heat transfer textbooks. In the classical theory of heat transfer in solids, the mechanism of heat “movement” is via phonons. Phonons can be thought of as sound waves, but instead of being transfered by collisions of atoms or molecules (as in a gas), they are transferred by collisions of electrons. These phonons are directional, as the local vector follows the maximum gradient in temperature. They also have to compete with random collisions of electrons. As the solid heats up there are more random collisions of electrons and these random collisions hinder the passage of the phonons. It is essentially the same mechanism that causes electrical resistance to increase with temperature in metals.