Cobalt Gradient Evolution in Sputtered TiNiCuCo Films for Elastocaloric Cooling

Abstract

Elastocaloric cooling demands for temperature changes larger than 30 K to become an alternative to classical vapour compression cooling systems. The principle of active regeneration allows to exceed the materials intrinsic adiabatic temperature change. The resulting temperature gradient of the cooling demonstrator leads to a change in the thermomechanical response along the regenerator bed, according to the Clausius-Clapeyron equation. A lowered efficiency as well as an increased probability of early breakdown of the device due to functional and especially structural fatigue are the result. These changes in the thermomechanical response are especially present in NiTi and NiTiCu-based systems with a coefficient @σ @T of about 7 and 10 MPa K À1 , respectively. To address these issues for future applications a change in the transformation temperature along the shape memory alloy film, adapted to the temperature gradient of the regenerator, is required. Cobalt is well known to reduce the transformation temperatures, while maintaining the functional stability of the TiNiCu-based films. In this study multilayer dcmagnetron sputtering is used to fabricate TiNiCuCo films with a cobalt concentration gradient along the samples, which can be precisely tuned by changing the sputter conditions. Transformation gradients of 0.3 K mm À1 are obtained, showing the same functional stability and adiabatic temperature changes of about À11 K as their counterparts without transformation gradient. Under isothermal conditions, a sloped transformation plateau is observed corresponding to a highly directed martensitic transformation.

Elastocaloric Cooling on the Miniature Scale: A Review on Materials and Device Engineering

“…The transition temperature of tailored elastocaloric films should follow the temperature gradient in the regeneration unit. In Section 3, we demonstrated that elastocaloric films can indeed be tailored by sputter deposition and a gradually changing chemical composition …”

“…Again al owered efficiency and the risk of failure would result.I nt his case samples with different transformation temperatures perpendicular to the flow direction of the cooling fluid can be used, or at ransformation gradient along the sample,w hich is placed along the flow direction of the regenerator. Both possibilities have been realized on the material site [49,50] and depictedi nF igure 6.…”

“…To increase the temperature span and eventually exceed the adiabatic temperature changeo ft he elastocaloric materiald evice (DT cool device DT ad ), the use of tailorede lastocaloric film materials is desirable.T he transition temperature of tailored elastocaloric films should follow the temperatureg radienti nt he regenerationu nit.I n Section 3, we demonstrated that elastocaloric films can indeed be tailored by sputter deposition and ag radually changing chemical composition. [50] 4.4 Benchmark Figure 11 gives ac ompilationo fp erformanced ata of elastocaloric cooling devicesf rom the literature.M ost devicesh ave been designed with macroscale dimensions revealing characteristic similarities and differences when compared with the miniature-scale devices presented here.Schmidt et al present ac ooling demonstrator based on changing mechanical contact between an SMAs pecimen and solid heat source/sink. In this case,t wo actuators are used, one for controlo ft he mechanical contact and one for tensilel oading of the SMA specimen.…”

Long-term stable compressive elastocaloric cooling system with latent heat transfer

“…For application in a commercial product, the ECM has to endure the reversible phase transformation process up to billions of times without failing 8,9 . Long-term stability is discussed as a major challenge for ECS [9][10][11][12][13] .…”

“…A comparison of the long-term stability of different nickeltitanium based ECM-samples is given in Fig. 1 [8][9][10][11]13,[25][26][27][28][29][30][31][32][33] . For bulk materials, compressive loading shows a significantly better long-term stability than tensile loading 6,9,13,34,35 .…”