Experimental Determination of elastocaloric Latent Heat

So far shape memory alloys (SMAs) have been used in medical applications as stents due to the superelastic properties and as actuators due to their high energy density combined with self-sensing properties. Another field of application recently gaining interest is elastocaloric cooling, which has also been investigated as a part of the German Science Foundation (DFG) priority program SPP 1599 Ferroic Cooling.

Large latent heats and a small work input during the application of elastocaloric materials (mostly Ni-Ti based SMAs) result in an efficient cooling process. The latent heats of the material become accessible under tensile loading and unloading of the SMA sample. Adiabatic loading with a high strain rate leads to a phase transformation and a temperature increase of the material whereas fast unloading leads to a temperature decrease.

To evaluate the elastocaloric potential of new materials, the latent heats are usually measured with the help of differential scanning calorimetry (DSC). In recent experiments, we discovered a large difference in the expectable ΔT values resulting from the DSC measurement and the ΔT values achieved in the elastocaloric cycle, where the DSC values showed rather pessimistic results. Based on these observations we implemented a new experimental approach to determine the latent heats of the SMA sample, which is presented in this contribution. By comparing and combining direct joule heating with the strain based process we are able to measure the latent heat of the elastocaloric heating and cooling process accurately.