Q:

Heat switch material?

Sort of a more hypothetical/general question here.  Does there exist or might there exist a sort of thermal switching material?  Like a material that could be switched from a thermal insulator (maybe more of a thermal insulator than air) to a thermal conductor?  Kind of like a thermal semiconductor?  It would be a useful material if you wanted a device to remain hot or cool off quickly, without using a fan.

A:

Vanadium dioxide might be what you're looking for. Below a certain temperature it's an insulator, but changes to a conductor above that temperature. The temperature threshold of the material can be changed by doping it with other elements.
Other materials have this property but most switch far below freezing. Vanadium dioxide can switch somewhere between the freezing and boiling points of water, as well as being tuned for the exact temperature, making it potentially useful for household applications.

phys.org: For this metal, electricity flows, but not the heat

