Phonon scattering changes the occupation or propagation of lattice-vibration normal modes through interactions with boundaries, defects or other phonons. Anharmonic terms permit phonon-phonon scattering. Different processes need not relax heat flow equally: normal processes conserve total crystal momentum within the first Brillouin zone, whereas Umklapp scattering changes its representative by a nonzero reciprocal lattice vector.
The phonon mean free path is the typical distance traveled by a phonon between the relevant scattering events. For heat transport, the relevant relaxation time must describe relaxation of heat current, not merely count all collisions. An isotropic kinetic estimate is , where is heat capacity per unit volume. Boundary-limited and the low-temperature Debye model give .
In a three-phonon Umklapp scattering process, the first-zone wavevectors obey the displayed equation for a nonzero reciprocal lattice vector , together with conservation of energy. The crystal momentum transferred to the lattice allows these processes to relax a phonon heat current. They are suppressed at sufficiently low temperature and often determine the approximate high-temperature thermal conductivity of a crystalline electrical insulator.
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Phonon scattering refers to the process by which phonons, which are quantized modes of vibrations occurring in a material (often modeled as sound waves or thermal vibrations), interact with impurities, defects, or other phonons in a solid. It plays a crucial role in determining the thermal and electrical properties of materials, especially in solids.