Solid-state thermal energy storage using reversible martensitic transformations

The identification and use of reversible Martensitic transformations, typically described as shape memory transformations, as a class of metallic solid-solid phase change materials are experimentally demonstrated here. Direct evidence of repeatable temperature leveling (9%-25% reduction in peak temperature rise) during transient heating and cooling using NiTi was obtained by cyclic Joule-heating in a simulated thermal energy storage application. Compared to standard solid-solid materials and solid-liquid paraffin, these experimental results show that shape memory alloys provide up to a two order of magnitude higher figure of merit (FOM). To calculate the material FOM and determine the crystal structure, direct measurements of latent heat, thermal conductivity, density, and diffraction were performed. Beyond these experimental results, a review of >75 binary NiTi and NiTi-based ternary and quaternary alloys in the literature shows that shape memory alloys can be tuned in a wide range of transformation temperatures (from - 50 to 500 degrees C), latent heats (up to 35.1 J/g), and thermal conductivities (from 15.6 to 28 W/m K). This can be accomplished by changing the Ni and Ti balance, introducing trace elements, leveraging intermediate R-phase transitions, and/or by thermomechanical processing. Combining excellent corrosion resistance, formability, high strength and ductility, high thermal performance, cyclic stability, and tunability, shape memory alloys represent a class of exceptional phase change materials that circumvent many of the scientific and engineering challenges hindering progress in this field.