How: Thermally conductive yet electrically resistive.

[forrealfyziks]

Today I had a materials lab where we looked through various properties of materials. One of the materials we had was alumina, and we had to describe how it was both thermally conductive and electrically resistive. I couldn't figure out the answer on my own. Could anyone describe the reasoning behind this?

Note: This isn't for homework or a class I'm just curious. Thanks!

 

[NobodySpecial]

Thermal conduction is by bonds vibrating - so most crystals with stiff bonds are good conductors.
Electrical conduction is by free electrons, so good resistors are those with strongly held electrons - again crystals are good.

Diamond is the best ratio of thermal conduction to electrical resistance.

 

[JoaoPais]

I don’t know if it’s still relevant, but some of the thermal conduction is also made by the free valence electrons in conducting metals, that’s why a good electrical conductor is in general also a good thermal conductor =)