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Barocaloric material

Barocaloric effects can be achieved at pressures above 200 MPa for intermetallics or about 100 MPa in plastic crystals. However, NH₄I changes phase at pressures of 80 MPa.^[3] The hybrid organic–inorganic layered perovskite (CH₃–(CH₂)n−1–NH₃)₂MnCl₄ (n = 9,10), shows reversible barocaloric entropy change of ΔSr ~ 218, 230 J kg−1 K−1 at 0.08 GPa at 294-311.5 K (transition temperature).^[4]

Barocaloric materials are one of several classes of materials that undergo caloric phase transitions. The others are magnetocaloric, electrocaloric, and elastocaloric. Magnetocaloric effects typically require field strengths larger than 2 T, while electrocaloric materials require field strengths in the kV to MV/m range. Elastocaloric materials may require force levels as large as 700 MPa.

Potential applications[edit]

Barocaloric materials have potential use as refrigerants in cooling systems instead of gases such as hydrofluorocarbons. cycles, the pressure then drives a solid-to-solid phase change.^[5] A prototype air conditioner was made from a metal tube filled with a metal-halide perovskite (the refrigerant) and water or oil (heat/pressure transport material). A piston pressurizes the liquid.^[1][6]

Another project used NH₄I as the refrigerant. It achieved reversible entropy changes of ${\displaystyle \Delta {S}_{{P}_{0}\to P}^{\max }}$ ~71 J K⁻¹kg⁻¹ at ambient temperature. The phase transition temperature is a function of pressure, varying at a rate of ~0.79 K MPa−1. The accompanying saturation driving pressure is ~40 MPa, a barocaloric strength of ${\displaystyle \left|\Delta {S}_{{P}_{0}\to P}^{\max }/\Delta P\right|}$ ~1.78 J K−1 kg−1 MPa−1, and a temperature span of ~41 K under 80 MPa. Neutron scattering characterizations of crystal structures/atomic dynamics show that reorientation-vibration coupling is responsible for the pressure sensitivity.^[3]

See also[edit]

• Thermoelectric effect

References[edit]