Barocaloric Material
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Barocaloric materials are characterized by strong, reversible thermic responses to changes in pressure. Many involve solid-to-solid
phase changes In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic s ...
from disordered to ordered and rigid under increased pressure, releasing heat. Barocaloric solids undergo solid-to-solid phase change. One barocaloric material processes heat without a phase change:
natural rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Types of polyisoprene ...
.


Input energy

Barocaloric effects can be achieved at pressures above 200 MPa for
intermetallic An intermetallic (also called intermetallic compound, intermetallic alloy, ordered intermetallic alloy, long-range-ordered alloy) is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elements. Inte ...
s or about 100 MPa in
plastic crystal A plastic crystal is a crystal composed of weakly interacting molecules that possess some orientational or conformational degree of freedom. The name plastic crystal refers to the mechanical softness of such phases: they resemble waxes and are easil ...
s. However, changes phase at pressures of 80 MPa. The hybrid organic–inorganic layered perovskite (CH3–(CH2)''n''−1–NH3)2MnCl4 (''n'' = 9,10), shows reversible barocaloric entropy change of Δ''S''r ~ 218, 230 J kg−1 K−1 at 0.08 GPa at 294-311.5 K (transition temperature). 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

Barocaloric materials have potential use as
refrigerants A refrigerant is a working fluid used in the cooling, heating, or reverse cooling/heating cycles of air conditioning systems and heat pumps, where they undergo a repeated phase transition from a liquid to a gas and back again. Refrigerants are ...
in cooling systems instead of gases such as
hydrofluorocarbons Hydrofluorocarbons (HFCs) are synthetic organic compounds that contain fluorine and hydrogen atoms, and are the most common type of organofluorine compounds. Most are gases at room temperature and pressure. They are frequently used in air condit ...
. cycles, the pressure then drives a solid-to-solid phase change. A prototype
air conditioner Air conditioning, often abbreviated as A/C (US) or air con (UK), is the process of removing heat from an enclosed space to achieve a more comfortable interior temperature, and in some cases, also controlling the humidity of internal air. Air c ...
was made from a metal tube filled with a metal-halide
perovskite Perovskite (pronunciation: ) is a calcium titanium oxide mineral composed of calcium titanate (chemical formula ). Its name is also applied to the class of compounds which have the same type of crystal structure as , known as the perovskite (stru ...
(the refrigerant) and water or oil (heat/pressure transport material). A piston pressurizes the liquid. Another project used as the refrigerant. It achieved reversible entropy changes of \Delta _^ ~71 J K−1 kg−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 \left, \Delta _^/\Delta P\ ~1.78 J K−1 kg−1 MPa−1, and a temperature span of ~41 K under 80 MPa.
Neutron scattering Neutron scattering, the irregular dispersal of free neutrons by matter, can refer to either the naturally occurring physical process itself or to the man-made experimental techniques that use the natural process for investigating materials. Th ...
characterizations of
crystal structures In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat a ...
/atomic dynamics show that reorientation-vibration coupling is responsible for the pressure sensitivity.


See also

*
Thermoelectric effect The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...


References

{{Reflist Refrigerants Phase transitions