| dc.description.abstract | Shape memory alloys (SMAs) have recently been demonstrated as effective phase change materials for thermal energy storage owing to their ability to undergo thermally driven reversible martensitic transformations. NiTi SMAs show excellent performance in high heat flux and transient thermal energy storage as quantified by Lu’s figure of merit (FOM is equal to the product of density, thermal conductivity, and latent heat of transformation). However, NiTi SMAs are limited to transformation temperatures below 100°C, overall transformation ranges (OTR is equal to the difference between austenite finish (Af) and martensite finish (Mf) temperatures) above 50°C, and thermal conductivity under 18 W/m·K. In the current work, the transformation characteristics and thermophysical properties of NiTiHf, NiTiCu, and CuZnAl SMAs were measured and analyzed to determine their thermal energy storage performance, aiming to overcome the limitations of NiTi. Alloys of varying composition within the NiTiHf, NiTiCu, and CuZnAl systems were fabricated, and their transformation temperatures, transformation enthalpies, densities, and thermal conductivities were measured. NiTiHf SMAs showed more than double the FOM value (3217 106 J2K−1s−1m−4) of NiTi (1478 106 J2K−1s−1m−4) with transformation temperatures ranging from 0 to beyond 500°C. The transformation enthalpies of the alloys were found to be strongly correlated with composition, having a significant effect on FOM values. NiTiCu SMAs showed Af temperatures between -10 and 90 °C with FOM values up to 1048 106 J2K−1s−1m−4 and OTRs as low as 12°C. These properties make NiTiCu alloys excellent candidates for thermal energy storage applications requiring frequent cycling or narrow operation temperature ranges. The thermophysical properties of the NiTiCu alloys were strongly dependent on composition, and alloys near 50 at. % Ti showed the highest FOM values. CuZnAl SMAs achieved the highest FOM values (up to 3463 106 J2K−1s−1m−4) due to their high thermal conductivities, which ranged from 59 to 75 W/m·K. CuZnAl alloys also showed overall transformation ranges as low as 12°C. The NiTiHf, NiTiCu, and CuZnAl alloy systems showed excellent thermal energy storage performance, greatly improving upon the capabilities of NiTi shape memory alloys and traditional phase change materials. | |
