must have high latent heat density (>0.3 MJ/kg) and sufficiently high thermal conductivity for
enhanced thermal energy charge/discharge processes. Encapsulation of “pockets” of PCM is a
possible approach to improve thermal energy transport, while maintaining the chemical and
mechanical stability of the material.
Recent developments in nanocrystal polymer composites can be a key to finding a stable cycling
solution for thermal storage.
The unique characteristics of solid-solid structural transformations in nanocrystals can lead to a
new generation of thermal storage materials. Present materials are limited by the lack of
reversibility of structural transformations in extended solids. In contrast, nanocrystals embedded
in a “soft matrix” can reversibly undergo structural transitions involving a large volume change
per unit cell. This is because a structural transition in a nanocrystal may proceed through a single
nucleation event per particle. Further, a nanocrystal can change shape and volume without
fracturing or undergoing plastic deformation. Because the barrier to a structural transition
depends strongly on the size of the nanocrystals, the hysteresis and kinetics of the structural
transition can be controlled. Much of the prior work on structural transitions in nanocrystals has
focused on pressure-induced transitions, or transitions that occur at modest temperatures (a few
hundred °C), so exploratory work must be performed to find materials and transitions that allow
thermal storage under the appropriate conditions for solar energy.
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