In ordinary metals, the same electrons which carry electrical current
are also responsible for most of the heat conduction. The reasons for
the variation in thermal conductivity are the same as the reasons for
the variation in electrical conductivity. Roughly speaking, there are
three factors which determine these conductivities:
1. the density of conduction electrons
2. the typical speeds of conduction electrons
3. the typical distances that the electrons travel before they bump into something and change directions.
In ordinary metals, the third factor is the most variable one. One
of the biggest reasons that it varies is that the electrons actually
travel as waves which don't bounce off the atoms in good crystals, the
same as light waves don't bounce off atoms as they travel through a
good transparent crystal. Some ordinary metals (say brass) are alloys
of different types of atoms, so the electrons bounce off the irregular
patterns of different types of atoms. Others, like copper, silver, and
aluminum, are pure elements. There will be a little electron scattering
even in perfect crystals of pure elements, since the atoms are always
jiggling a little out of position because of the energy connected with
temperature. This 'thermal' scattering varies substantially between
different metals. The scattering off alloys, defects, etc. can be much
larger.
mike w
(published on 10/22/2007)
