Thermal conductivity is the constant of proportionality for heat transfer in response to a temperature gradient. Thermal conductivity can be accomplished by contributions from electrons, phonons, radiation, and convection. Metals have high thermal conduction due to the many free electrons, ceramics rely on phonons and in glassy materials or those with many defects these are greatly reduced. Thermal conductivity can be calculated as the product of thermal diffusivity, density, and heat capacity. Polymers have the lowest thermal conductivity since they are rarely conductivity and the chains scatter most phonons. Introducing tiny air or vacuum void pockets or bubbles can further reduce thermal conductivity since these rely on radiation if they are small.