2. Thermal properties

Thermal properties are associated with the vibrations of atoms around their equilibrium position, the movements of electrons and so on. The amplitude of vibrations decreases with temperature; they can propagate at the speed of sound, and the propagation of these vibrations is treated like that of plane waves. In a material, the different energy levels due to atomic vibrations are called phonons. By extension, the term phonon is also given to vibration waves. This is how we describe the properties of insulating materials. In the case of conductive materials, this is supplemented by the displacement of electrons (negatively charged) or positive charges (called holes). Other effects can also contribute to thermal properties (magnetic characteristics, superconducting state, etc.)