Novel Experimental Approach to Determine Elastocaloric Latent Heat

One of the most important parameters of superelastic shape memory alloys to be used in elastocaloric cooling and heating processes is their specific latent heat. Usually, the latent heat of a material is determined by differential scanning calorimetry (DSC) where the material phase transformation is induced thermally under zero stress. During elastocaloric processes however, the latent heat becomes accessible by stress-induced transformation under tensile or compression loading and unloading of the sample. In recent elastocaloric experiments, we observed drastic differences between latent heat values determined in DSC experiments and the ΔT values observed in nearly adiabatic elastocaloric cycles, which reflect the latent heat; in fact, the DSC experiments predicted rather pessimistic values and thus poor cooling performance. Based on these observations we developed and tested a novel experimental approach to determine the latent heat of superelastic materials directly during the elastocaloric process. By comparing or combining direct Joule heating with the strain based process we are able to accurately determine the latent heat for both tensile loading and unloading for any elastocaloric heating or cooling process. Furthermore, the influence of applied mechanical parameters as well as material conditioning on the latent heat can be observed in the elastocaloric experiment.