Learn about the 3 most common types of phonon interactions and how they affect the transport properties of materials. Read now to optimize your material design.
3 Most Common Types of Phonon Interactions
Phonons are quanta of lattice vibrations, and they play a crucial role in various physical phenomena, including thermal conductivity, thermal expansion, and superconductivity. Phonon interactions refer to the scattering of phonons by various types of defects, such as impurities, dislocations, and phonon boundaries. Understanding the different types of phonon interactions is essential to control the transport properties of materials, which is important for various technological applications. In this article, we will discuss the three most common types of phonon interactions.
Umklapp Scattering
Umklapp scattering is a type of phonon-phonon interaction that occurs when a phonon with a certain wavevector collides with another phonon and creates a new phonon with a different wavevector. This process violates the crystal symmetry and results in a net momentum transfer to the lattice. As a result, umklapp scattering is the dominant mechanism for phonon scattering at high temperatures, where phonons have large amplitudes and frequent collisions.
The rate of umklapp scattering is proportional to the phonon population and the phonon mean free path, which is the average distance a phonon travels before scattering. Therefore, materials with a high phonon population and short phonon mean free path have a higher rate of umklapp scattering, resulting in lower thermal conductivity.
Boundary Scattering
Boundary scattering is a type of phonon scattering that occurs when phonons collide with defects, such as grain boundaries or surfaces. When phonons encounter a boundary, they can reflect, transmit, or be absorbed by the boundary, depending on the boundary properties and the phonon energy. The reflection of phonons results in a change in the phonon direction, while the transmission of phonons allows them to travel through the boundary and continue to transport energy.
The rate of boundary scattering depends on the boundary properties, such as the roughness and the crystallographic orientation, as well as the phonon energy. Generally, higher energy phonons are more likely to transmit through the boundary, while lower energy phonons are more likely to be reflected or absorbed. Therefore, materials with fewer and smoother boundaries have a lower rate of boundary scattering and higher thermal conductivity.
