In conduction, thermal rectification occurs when a material or structure transfers heat asymmetrically. In this work, it is shown that, in the bulk, a necessary condition for thermal rectification is that the thermal conductivity of the material or structure be a function of both space and temperature, and that this function be nonseparable. Practically, this can be achieved using a composite structure consisting of many materials although it could also be achieved with a properly developed single material.

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