Supercompatibility and the direct conversion of heat to electricity

Big first order phase transformations in crystalline solids can be highly reversible, if the lattice parameters are “tuned” to satisfy certain relations that promote the compatibility between phases. We outline the basic theory behind this tuning and give examples of recently discovered alloys. Some of these alloys have thermal hysteresis as low as 0.2 C despite having a 7% transformation strain. The lowered hysteresis correlates with the reversibility of the transformation under repeated cycling. Magnetoelectric properties of solids are often sensitive to lattice parameters, so they can be switched at a phase transformation: briefly, multiferroism by reversible phase transformation. This switching can be combined with induction in the ferromagnetic case, or capacitance in the ferroelectric case, to yield devices that convert heat directly to electricity, without a separate electrical generator. The resulting multiferroics provide interesting possible ways to recover the vast amounts of energy stored on earth at small temperature difference.