Pressure dependence of phase transformation, thermal expansion and barocaloric property in a polycrystalline Ni54Mn23Ga23 alloy

An experimental investigation was conducted to understand the influence of hydrostatic pressure on phase transition, thermal expansion and barocaloric property of a polycrystalline Ni54Mn23Ga23 alloy. Two pressure ranges of 0-1.28GPa and 0-0.74GPa were selected for thermal expansion and barocaloric property measurements due to the different test limits of instrumentals. The pressure dependence of fourteen parameters was studied systematically. It was found that the sample experiences a martensitic transformation (MT) near room temperature and the response of MT peak temperature (TM) to pressure is about 15.89K/GPa. The phase transition strain (λtr), transformation width (ΔTtr) relative volume change (|ΔV/V|) and maximum adiabatic temperature change (ΔTad)max increase linearly with pressure and their shifting rates are about 0.071%/GPa, 7.5K/GPa, 0.211%/GPa and 12.2K/GPa, respectively. Simultaneously, applying hydrostatic pressure can also enhance the barocaloric property of the sample, leading to a large increase in the absolute value of the maximum entropy change (|ΔS|max), the refrigerating capacity (RC) and their reversible values. On cooling, the sensitivities of the reversible RC and its proportion to pressure are around 185.52J/kg GPa and 16.7%/GPa, respectively. Finally, the average linear expansion coefficient (αave) of both martensite and austenite phases keeps unchanged while in the MT region it decreases from -99.84×10–6/K to -86.04×10–6/K with increasing pressure from 0 to 1.28GPa. These findings indicate that applying hydrostatic pressure can effectively adjust the phase transformation, thermal expansion and barocaloric property of Ni-Mn-Ga alloys, which are meaningful for their application in solid-state refrigeration.