Colossal Elastocaloric Effect in Ferroelastic Ni-Mn-Ti Alloys

Energy-efficient and environment-friendly elastocaloric refrigeration, which is a promising replacement of the conventional vapor-compression refrigeration, requires extraordinary elastocaloric properties. Hitherto the largest elastocaloric effect is obtained in small-size films and wires of the prototype NiTi system. Here, we report a colossal elastocaloric effect, well exceeding that of NiTi alloys, in a class of bulk polycrystalline NiMn-based materials designed with the criterion of simultaneously having large volume change across phase transition and good mechanical properties. The reversible adiabatic temperature change reaches a strikingly high value of 31.5 K and the isothermal entropy change is as large as $45\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$. The achievement of such a colossal elastocaloric effect in bulk polycrystalline materials should push a significant step forward towards large-scale elastocaloric refrigeration applications. Moreover, our design strategy may inspire the discovery of giant caloric effects in a broad range of ferroelastic materials.

Figure 1

Guide for the design of high-performance elastocaloric materials. (a) Correlation between transformation entropy change $\mathrm{\Delta }{S}_{\mathrm{tr}}$ and unit cell volume change $\mathrm{\Delta }V/V_0$ across the transformation in NiMn-based Heusler alloys. The data are taken from the present work and literature (see Supplemental Material, Note 1 [28] for details). For “This work” the upper symbol is for ${({\mathrm{Ni}}_{50}{\mathrm{Mn}}_{31.5}{\mathrm{Ti}}_{18.5})}_{99.8}{\mathrm{B}}_{0.2}$ and the lower one for ${({\mathrm{Ni}}_{50}{\mathrm{Mn}}_{32}{\mathrm{Ti}}_{18})}_{99.8}{\mathrm{B}}_{0.2}$. All the data presented in this figure are taken from the alloys in which the martensitic transformation occurs above the Curie transition of austenite, namely, the magnetic contribution to $\mathrm{\Delta }{S}_{\mathrm{tr}}$ is negligible. (b) Ratio of bulk modulus $B$ to shear modulus $G$, $B/G$ and the Cauchy pressure $C_{12}–C_{44}$, obtained from ab initio calculations, plotted as a function of $\mathrm{\Delta }V/V_0$ for NiMn-based Heusler alloys (see Supplemental Material, Note 1 [28] for detailed data).

Figure 2

Colossal adiabatic temperature change in ${({\mathrm{Ni}}_{50}{\mathrm{Mn}}_{31.5}{\mathrm{Ti}}_{18.5})}_{99.8}{\mathrm{B}}_{0.2}$. (a) Temperature variation during loading, holding, and unloading, shown as a function of time. The maximum applied stress is 700 MPa, and the strain rate $\dot{ϵ}$ for loading and unloading is displayed in the figure. (b) Adiabatic temperature change as a function of strain rate $\dot{ϵ}$ (the same $\dot{ϵ}$ is applied for both loading and unloading), with the maximum applied stress of 700 MPa. (c) Adiabatic temperature change as a function of maximum applied stress, with loading and unloading rate $\dot{ϵ}$ of 0.16 and $5.33{\mathrm{s}}^{-1}$, respectively.

Figure 3

Stress-induced isothermal entropy change in ${({\mathrm{Ni}}_{50}{\mathrm{Mn}}_{31.5}{\mathrm{Ti}}_{18.5})}_{99.8}{\mathrm{B}}_{0.2}$. (a) Compressive stress-strain curves measured with a low strain rate of $1.3\times {10}^{-4}{\mathrm{s}}^{-1}$ at different temperatures. For clarity, only the curves recorded during loading are displayed. The inset shows a full stress-strain loop measured at 288 K, as an example. (b) Isothermal entropy change for different strain levels, shown as a function of temperature. The data are derived from the stress-strain curves displayed in (a).

Figure 4

Crystal structure evolution during stress-induced transformation in ${({\mathrm{Ni}}_{50}{\mathrm{Mn}}_{31.5}{\mathrm{Ti}}_{18.5})}_{99.8}{\mathrm{B}}_{0.2}$. (a) 1D HEXRD patterns at different stress levels during loading and unloading at 295 K. (b) Representative zone of the 2D HEXRD patterns collected before loading (2 MPa), at the maximum stress (490 MPa), and after unloading (1 MPa) at 295 K.