# Thermal conductivity

1. ### aaaa202

For a nonmetal what determines thermal conductivity is the propagation of lattice vibrations. As T increases these lattice vibrations collide with each other more often. Does this mean that the thermal conductivity will de- or increase?

2. ### DrDu

4,249
What would you guess?

3. ### aaaa202

Im guessing the phonon collisions act as a sort of resistance, so the conductivity drops with increasing temperature. But on the other hand, what if there were no collisions? What would then transmit heat?

4. ### DrDu

4,249
If there are no collisions, e.g. at very low temperatures, you have ballistic transport which is very rapid.
I think that only so-called Umklapp scattering processes actually can reduce the heat transport and this requires the sum of the crystal momenta of the two phonons to be larger than a reciprocal lattice vector. So it is only important at relatively high energies ~ Debye energy. Whether this includes room temperature depends on the material.

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