  The idea that one can build a solid-state device that lets heat flow more
easily in one way than in the other, forming a heat valve, is
counter-intuitive. However the design of a thermal rectifier can be easily
understood from the basic laws of heat conduction. Here we show how it can be
done. This analysis exhibits several ideas that could in principle be
implemented to design a thermal rectifier, by selecting materials with the
proper properties. In order to show the feasibility of the concept, we complete
this study by introducing a simple model system that meets the requirements of
the design.
