Thermal Conductivity and Diffusivity
Thermal conductivity is a measure of how well a given material conducts heat. While the related property of thermal diffusivity takes the specific heat and specific gravity of the material into account and is a better guide to how well heat moves through it, most composite literature uses the term thermal conductivity. Many fillers are minerals and they have much higher volume thermal conductivities than polymers. This helps in processing because one can heat and cool the melt more quickly. Parts can be made faster and therefore more cheaply. Specialty fillers are used when high thermal conductivity is paramount, for example, in heat sinks for laptop computers where enormous amounts of heat must be managed in a confined space. One would imagine that the higher the thermal conductivity of the filler, the more effective it would be at increasing the thermal conductivity of the composite. This, however, turns out not to be the only factor; the elastic modulus of a composite also has a significant effect, and fillers that significantly increase this can perform better than expected.