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M I N D M A P 5 - Coggle Diagram
M I N D M A P 5
THERMAL CONDUCTIVITY
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Generally, heat can be transmitted through a crystal by phonons, photons, free electrons or holes, and electron-hole pairs
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A single phonon cannot be used to describe a deviation from equilibrium in one region of the crystal
The deviation allows the phonon to be localized in space within a distance 
A temperature variation localized in space within a small region compared to sample dimensions but larger compared to a unit cell dimension
In a perfectly harmonic crystal, a wave packet of phonons travels unaltered and thus, the thermal conductivity is infinite
In real crystal, there is phonon scattering
PHONON MEAN FREE PATH
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The thermal conductivity, k is the constant or proportionality between a temperature gradient and the rate of energy flow per unit area, Q
The phonons is diffused through the sample, then suffer frequent collisions, from a higher temperature to a lower temperature end
For any distribution of phonons, a nominal mean free path is defined
ANHARMONIC EFFECTS
Forces that are only between nearest neighbors are considered
For 3D crystals, Hooke's law appears complicated because there are non-negligible "springs" between any atom and its nearest, and non-nearest neighbors
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In a perfect crystal, the thermal conductivity would be infinite
The quadratic term is the first nonzero term in a Taylor expansion of the potential energy about the equilibrium position
These anharmonic terms couple one phonon to another, causing phonon "collisions" and a finite mean free path and, by several different processes, establishing equilibrium
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