Barocaloric cooling or heating materials and methods

Abstract

The invention generally relates to composite materials for use as a barocaloric cooling or heating agent. The invention also provides methods of barocaloric cooling or heating, and an apparatus for barocaloric cooling or heating. The composite material comprises a barocaloric material and an additive that is dispersed within the barocaloric material. The method of barocaloric cooling or heating comprises comprising the steps of applying hydrostatic pressure to the composite material comprising a barocaloric material and particles of an additive dispersed in the barocaloric material, permitting heat flow from or to the barocaloric material; releasing the applied hydrostatic pressure; and permitting heat flow to or from the barocaloric material.

Description

[0001] BAROCALORIC COOLING OR HEATING MATERIALS AND METHODS

[0002] Related Application

[0003] The present case claims priority to, and the benefit of, GB 2500025.8 filed on 2 January 2025 (02.01.2025), the contents of which are incorporated by reference in their entirety.

[0004] The project leading to this application has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 680032 - BCOOL).

[0005] Technical Field

[0006] This invention relates to composite materials for use as a barocaloric cooling or heating agent. The invention also provides methods of barocaloric cooling or heating, and an apparatus for barocaloric cooling or heating.

[0007] Background

[0008] Foodstuffs, beverages, medical products and medical samples, electronics, and populated spaces such as offices, all require cooling. Heating systems are also needed, such as for buildings and homes. Current refrigeration units, air-conditioning units and heat pumps rely primarily on the compression and expansion of environmentally harmful fluids, and there is strong interest in developing cooling systems that avoid such fluids.

[0009] Caloric effects are currently under intense study due to the prospect of environmentally-friendly cooling applications. The work in this area is focused largely on developing large magnetocaloric effects and large electrocaloric effects, but the former require large magnetic fields that are challenging to generate economically, and the latter require large electric fields that can typically be applied without breakdown only in thin samples. The temperature changes that are displayed by these materials are typically small, and the amount of heat they are able to exchange with their surroundings is also relatively small. For these reasons, work has turned to mechanocaloric effects, such as elastocaloric effects, where the effects are driven by applied uniaxial stress, and barocaloric effects, which are driven by isotropic stress (hydrostatic pressure). These effects are attractive for the reason that mechanical stress is easy to generate and large mechanocaloric effects have been observed at relatively low applied stresses. However, the use of elastocaloric materials is limited, as these materials exhibit plastic flow which develops in the MPa range for metallic materials, and fracture. Barocaloric materials can overcome the limitations described above.

[0010] Large barocaloric effects (also referred to as BC effects) driven by hydrostatic pressure near phase transitions have been observed. Barocaloric materials are a class of caloric materials with the potential to provide cooling and / or heating via pressure changes, rather than

[0011] 008873440- PCT Specificationchanges in electric or magnetic fields. These materials possess benefits over present fluid refrigerants in that they do not pose the risk of release of potent greenhouse gases into the atmosphere, and that they may be able to achieve higher energy efficiencies than are possible with present fluid refrigerants.

[0012] The application of pressure to a conventional barocaloric material under isothermal conditions leads to entropy changes which generally cause a release of heat, and conversely an absorption of heat if the external pressure is subsequently removed. When the application of pressure is performed under adiabatic conditions, then a conventional barocaloric material displays an increase in temperature, and conversely a decrease in temperature if the external pressure is subsequently removed. Some barocaloric materials display “inverse barocaloric effects”, where under isothermal conditions they absorb heat upon the application of pressure and release heat on the release of the applied pressure, and where under adiabatic conditions they display a decrease in temperature upon the application of pressure and an increase in temperature on the release of the applied pressure.

[0013] Known barocaloric materials can have large hysteretic losses in a cooling or heating cycle, which makes their use less practical and / or efficient. High pressures are typically needed to drive barocaloric effects in a reversible manner in such materials.

[0014] There is a need for new improved materials for barocaloric cooling or heating applications, particularly those with reduced hysteretic losses, or that can be operated at lower pressures for use in cooling and heating.

[0015] Summary of the Invention

[0016] In a general aspect the invention relates to a composite material comprising a barocaloric material and an additive that is dispersed within the barocaloric material. The composite material is useful for barocaloric cooling or heating applications.

[0017] The additive alters the intermolecular bonding network, such as the hydrogen-bond network, within the barocaloric material. It is proposed that this results in the phase-transition hysteresis and hysteretic loss of material (during cooling or heating, or application or removal of pressure) being reduced. In this way, a barocaloric effect can be driven in the composite material in a reversible manner at a lower pressure compared to the barocaloric material in the absence of the additive.

[0018] In a first aspect, there is provided a method of barocaloric cooling or heating, the method comprising the steps of:

[0019] (i) applying hydrostatic pressure to a composite material comprising a barocaloric material and particles of an additive dispersed in the barocaloric material,

[0020] 008873440- PCT Specificationwherein the amount of the additive is from about 0.1% to about 3% by volume of the composite material;

[0021] (ii) permitting heat flow from or to the barocaloric material;

[0022] (iii) releasing the applied hydrostatic pressure; and

[0023] (iv) permitting heat flow to or from the barocaloric material.

[0024] The amount of the additive is at least 0.1 % by volume of the composite material, such as at least 0.6%, at least 0.7%, such as at least 0.8%. The amount of the additive in the composite material may be 2% or less by volume of the composite material, such as 1.5% or less, such as 1% or less. The amount of the additive may be in a range with upper and lower limits as described above, such as from 0.1% to 2% by volume of the composite material, such as from 0.8% to 2% by volume of the composite material.

[0025] In some preferred embodiments, the amount of the additive is about 1 % by volume of the composite material.

[0026] The amount of the additive by volume of the composite material may be determined using the masses of components to calculate associated volumes from ideal densities and volume fraction was calculated for the components. The volumes are determined at ambient temperature and pressure, as defined herein.

[0027] The additive is present in the composite material in the form of particles. The particles may be dispersed substantially uniformly throughout the barocaloric material, such as in a matrix of the barocaloric material. Preferably, the particles are not assembled in a percolated particle network. For example, the amount of the additive may be less than a percolation threshold. The percolation threshold for a particle can be determined as described in Bollobas and Riordan.

[0028] The composite material may have a phase transition (e.g. thermal) hysteresis that is no more than 90% of the phase transition (e.g. thermal) hysteresis of the barocaloric material in the absence of the additive, such as no more than 80%, such as no more than 70%, such as no more than 50%, such as no more than 30%, such as no more than 25%, such as no more than 10%, such as no more than 1%. The composite material may have a phase transition (e.g. thermal) hysteresis that is smaller than the phase transition (e.g. thermal) hysteresis of the barocaloric material in the absence of the additive by at least 0.5 K, such as 1 K, such as at least 2 K, such as at least 3 K, such as at least 5 K, such as at least 8 K, such as at least 10 K, such as at least 15 K. Phase transition (e.g. thermal) hysteresis may be determined by calorimetry as described herein.

[0029] The particles of the additive may have an average particle aspect ratio of up to 20, such as up to 10, such as up to 7.0, such as up to 5.0, such as up to 3.0, such as up to 2.5. The average particle aspect ratio may be 1.2 or more, such as 1.5 or more, such as 1.7 or more. The average particle aspect ratio may be in a range with upper and lower limits as described

[0030] 008873440- PCT Specificationabove, such as from 1.2 to 20, including from 1.5 to 5.0, such as from 1.5 to 3.0. These particles may further reduce hysteretic losses in the composite material.

[0031] The particles may have a particle size, such as an average particle size, of 10 nm or more, such as 50 nm or more, such as 60 nm or more. The particle size, such as an average particle size, may be 100 pm or less, such as 50 pm or less, such as 20 pm or less, such as 10 pm or less. Preferably, the average particle size is 60 nm or more. The average particle size may be in a range with upper and lower limits described above, such as from 10 nm to 100 pm, including from 60 nm to 100 pm, such as from 60 nm to 10 pm.

[0032] The additive may be an inorganic material, or the additive may be an organic material.

[0033] In some embodiments, the additive is an inorganic material. The additive may be selected from aluminium oxide, silicon dioxide, copper, gold and carbon, nitrides, perovskites or elemental solids, preferably aluminium oxide, silicon dioxide, copper, gold and carbon.

[0034] In some embodiments, the additive is an organic material. The additive may be a polymer, such as polyethylene, a molecular crystal such as neopentyl, adamantane or carborane and derivative (e.g. polymers) thereof. In some embodiments, the additive is a polymer, such as polyethylene.

[0035] The barocaloric material may have an intermolecular bonding network. The barocaloric material may be a molecular crystal having a hydrogen-bond network. The additive may alter the intermolecular bonding network of the barocaloric material, such as the hydrogenbond network, such as to allow the modification of vibrational and / or librational modes in the barocaloric material. Surface chemistry may also play a role.

[0036] The barocaloric material may be a plastic crystal. In some embodiments, the barocaloric material is a plastic crystal comprising an alkyl group or a cycloalkyl group.

[0037] The barocaloric material may be a liquid crystal. In some embodiments, the barocaloric material is a non-lyotropic liquid crystal, such as 4-(trans-4-pentylcyclohexyl)benzonitrile (PCH5); or a lyotropic material, such as a phospholipid.

[0038] The barocaloric material may be a hybrid organic-inorganic material. In some embodiments, the barocaloric material is a hybrid salt comprising an organic ion, such as an organic ion comprising an alkyl chain, such as a C2-20 alkyl chain. The barocaloric material may be selected from a hybrid salt, a hybrid perovskite or a hybrid zeolite.

[0039] The composite material, the barocaloric material, or both, may have a phase transition at a temperature within the range of 1 K to 450 K, such as 10 K to 450 K, such as 50 K to 450 K, such as 100 K to 450 K, such as 150 K to 450 K, such as 200 K to 450 K, such as 245 K to 340 K, such as 245 K to 315 K. This phase transition is driven by the applied hydrostatic

[0040] 008873440- PCT Specificationpressure in the method (step (i) of the method). Typically, the phase transition can be driven in a reversible manner i.e. is a readily reversible phase transition.

[0041] The composite material may have a latent heat, | Qo| , at a phase transition that is at least 1 kJ kg-1, such as at least 5 kJ kg-1, such as at least 10 kJ kg-1, such as at least 25 kJ kg-1, such as at least 50 kJ kg-1, such as at least 100 kJ kg-1, such as at least 150 kJ kg-1, such as at least 200 kJ kg-1. The composite material may have an entropy change, |ASo|, at a phase transition that is at least 10 kJ kg-1, such as at least 50 kJ kg-1, such as at least 100 kJ kg-1, such as at least 150 kJ kg-1, such as at least 250 kJ kg-1, such as at least 400 kJ kg-1.

[0042] In some embodiments, the applied hydrostatic pressure is at most 1.0 GPa, such as at most 500 MPa, such as at most 250 MPa, such as at most 120 MPa, such as at most 100 MPa. The methods of the invention may be suitable for barocaloric cooling or heating at lower pressures than known barocaloric materials.

[0043] In steps (ii) and (iv), heat is permitted to flow from or to the barocaloric material. The heat may be permitted to flow from or to the composite material. That is, heat may be permitted to flow from or to the barocaloric material as part of the composite material. It will be understood that heat may also be permitted to flow from or to other components of the composite material, such as particles of the additive.

[0044] In a second aspect, there is provided a barocaloric cooling or heating apparatus comprising:

[0045] a composite material comprising a barocaloric material and particles of an additive dispersed in the barocaloric material, wherein the amount of the additive is from about 0.1% to about 3% by volume of the composite material; and

[0046] a hydrostatic pressuriser for pressurising and depressurising the composite material.

[0047] Preferences for the first aspect also apply to the second aspect.

[0048] The apparatus may comprise one or more heat-exchangers for connecting the composite material to a heat source or a heat sink.

[0049] In a third aspect, there is provided a composite material for barocaloric cooling or heating, wherein the composite material comprises a barocaloric material and particles of an additive dispersed in the barocaloric material, and wherein the amount of the additive is from about 0.1% to about 3% by volume of the composite material.

[0050] Preferences for the first and second aspects also apply to the third aspect.

[0051] The amount of the additive may be 0.1% or more by volume of the composite material. The particle size, such as the average particle size, may be 60 nm or more. The particle size, such as the average particle, size may be from 60 nm to 100 pm, such as from 60 nm to 10 pm.

[0052] 008873440- PCT SpecificationThe additive may be an inorganic material, such as a metal or an oxide, such as a metal, silicon dioxide or aluminium oxide. The additive may be selected from copper, gold, silicon dioxide (SiCh) and aluminium oxide (AI2O3).

[0053] In a fourth aspect, there is provided use of a composite material according to the third aspect as a barocaloric cooling or heating agent.

[0054] Preferences for the first to third aspects also apply to the fourth aspect.

[0055] In a fifth aspect, there is provided use of an apparatus according to the second aspect, for cooling or heating.

[0056] Preferences for the first to fourth aspect also apply to the fifth aspect.

[0057] Use of the composite, or use of the apparatus, may comprise applying hydrostatic pressure to the composite material, such as wherein the applied hydrostatic pressure is at most 1.0 GPa, such as at most 500 MPa, such as at most 250 MPa, such as at most 100 MPa.

[0058] Summary of the Figures

[0059] The present invention is described with reference to the figures listed below.

[0060] Figure 1 shows heat flow, dQ / |d T\ , on cooling and heating for a thermally driven transition in neopentylglycol (NPG, green lines, with peaks around 300 K and 320 K) and for a composite material comprising NPG and copper nanopowder (NPG + Cu NP, yellow lines, with peaks around 310 K and 320 K).

[0061] Figure 2 shows the phase-transition temperatures, To, for the thermally driven transition in neopentylglycol (NPG) and for a composite material comprising NPG and copper nanopowder, from the calorimetric data of Figure 1, where crosses represent peak values from in the calorimetric data, and bars indicate the width of the transition.

[0062] Figure 3 shows the change in entropy (AS, J K'1kg-1) as a function of temperature (7, K) for neopentylglycol (NPG, top panel) and for a composite material comprising NPG and copper nanopowder (bottom panel), with the application and release of hydrostatic pressure on the composite material. The lower portion of the graph (negative entropy change) shows data for the application of pressure from no applied pressure to the specified applied pressure (GPa), and the upper portion of the graph (positive entropy change) shows data for the release of pressure from the specified applied pressure (GPa) to no applied pressure.

[0063] Figure 4 shows inelastic neutron scattering data in neopentylglycol (NPG) and composite materials according to the invention. Panel (a) shows quasieleastic neutron scattering

[0064] 008873440- PCT Specification(QENS) data across the solid-solid phase transition in NPG above (330 K) and below (280 K, 300 K) the phase transition. The two vertical lines indicate the energies of the inelastic fixed-window scan (IFWS) used to collect the data shown in panel (b). Each line is 0.75 peV wide and signal between the vertical lines is not recorded. Panel (b) shows the IFWS intensity (measured at ± 3 peV) as a function of temperature T for NPG and composites for 0.59 A-1< Q < 1.56 A’1. Panel (c) shows DSC heat-flow data with the same temperature scale as in panel (b).

[0065] Figure 5, panel (a) shows the mean linewidth as a function of temperature T approaching the solid-solid phase transition in neopentylglycol (NPG) composite material. Error bars denote standard deviation of the mean linewidth for 1.06 A-1 < Q < 1.56 A-1. Panels (b) and (c) show examples of QENS fitting for NPG and composite materials at 300 K for Q = 1.18 A’1.

[0066] Figure 6 shows heat-flow data for NPG and other composite materials. Comparison of the data for NPG and the composite materials reveals change in phase transition hysteresis.

[0067] Detailed Description of the Invention

[0068] In a general aspect, the invention relates to a composite material comprising a barocaloric material and an additive that is dispersed within the barocaloric material. The composite material is useful for barocaloric cooling or heating applications. The additive alters the intermolecular bonding network, such as hydrogen-bonding network, within the barocaloric material such that the phase transition hysteresis and the hysteretic loss of the material is reduced and barocaloric effects can be driven in the composite material in a reversible manner at a lower pressure than the pure barocaloric material.

[0069] Barocaloric effects are thermal changes of a material as a result of volumetric phase transitions which occur when the material is subjected to an external hydrostatic pressure. The phenomenon forms the basis for barocaloric heating and cooling.

[0070] The use of barocaloric materials in cooling or heating applications is described, for example in WO 2018 / 069506 for organic materials, Matsunami etal. for MnsGaN, and Lloveras etal.

[0071] 2015 for ammonium sulfate. Colossal barocaloric effects near room temperature have been reported in neopentylglycol (NPG), for example (Lloveras et al., 2019). However, a large phase-transition hysteresis and the associated hysteretic loss in these materials mean that high hydrostatic pressures (typically >2,500 bar, or 250 MPa) are necessary in order to drive the barocaloric effects in a reversible manner which is necessary for cooling and heating applications and that such use has limited efficiency.

[0072] Liu et al. describe compositing neopentylglycol with graphene nanosheets to enhance thermal conductivity compared to pure neopentylglycol. The graphene nanosheets act as a filler, which is said to provide a three-dimensional network in the composite material that provides a path of lower resistance for heat transfer, which in turn improves thermal

[0073] 008873440- PCT Specificationconductivity in the composite material. The barocaloric effects in the composite material are greatly reduced in magnitude compared to pure neopentylglycol. The barocaloric effect is largely irreversible in the composite material, including at up to 1 ,000 bar of applied pressure, as demonstrated by minimal overlap in the pressure-driven isothermal entropy change AS curves with application and removal of hydrostatic pressure.

[0074] Wang et al. describe a material where neopentylglycol is dispersed in the porous network of an expanded graphite to increase the thermal conductivity of the composite for thermal energy storage-based heat sink applications. Parveen et al. describe a neopentylglycol / CuO composite solid-solid phase change material and its use in thermal energy storage-based heat sink applications. In each case, the authors study a thermally driven phase transition in the composite materials. There is no barocaloric effect described in such materials, and no reduction in hysteresis reported. The composite materials have decreased latent heat and corresponding entropy change compared to pure NPG.

[0075] The present inventors have found that composite materials comprising a barocaloric material and particles of an additive dispersed within the barocaloric material have reduced thermal hysteresis, which allows for more practical and efficient cooling and heating at lower pressures. This can be by introducing a limited amount of the additive, particularly in the form of particles, into the barocaloric material. Without wishing to be bound by theory, it is believed that the particles manipulate the hydrogen-bond network within the barocaloric material and allow for the modification of vibrational and / or librational modes in the barocaloric material, resulting in a significant decrease in phase-transition hysteresis and hysteretic loss. In the case of neopentylglycol, as an example, the composite material has a reversible phase transition that can be driven at around 1,000 bar (100 MPa) which is substantially lower than the around 2,500 bar (250 MPa) or higher pressure that is typically needed to drive the corresponding phase transition in the pure NPG material.

[0076] In some aspects, the invention provides a composite material for barocaloric cooling or heating, wherein the composite material comprises a barocaloric material and particles of an additive dispersed in the barocaloric material, and wherein the amount of the additive is from about 0.1% to about 3% by volume of the composite material.

[0077] In some aspects, the invention provides a method of barocaloric cooling or heating, the method comprising the steps of:

[0078] (i) applying hydrostatic pressure to a composite material comprising a barocaloric material and particles of an additive dispersed in the barocaloric material, wherein the amount of the additive is from about 0.1% to about 3% by volume of the composite material;

[0079] (ii) permitting heat flow from or to the barocaloric material;

[0080] (iii) releasing the applied hydrostatic pressure; and

[0081] (iv) permitting heat flow to or from the barocaloric material.

[0082] 008873440- PCT SpecificationIn some aspects, the invention provides a barocaloric cooling or heating apparatus comprising:

[0083] a composite material comprising a barocaloric material and particles of an additive dispersed in the barocaloric material, wherein the amount of the additive is from about 0.1% to about 3% by volume of the composite material; and

[0084] a hydrostatic pressuriser for pressurising and depressurising the composite material.

[0085] Composite Material

[0086] A composite material according to the invention comprises a barocaloric material and an additive. The additive is provided as particles, which particles are dispersed in the matrix of the barocaloric material.

[0087] Without wishing to be bound by theory, it is believed that the presence of the additive alters the intermolecular bond network, such as hydrogen-bond network, within the barocaloric material. This alteration may occur due to surface chemistry of the particle. This may allow for the manipulation of the intermolecular bond (e.g., hydrogen-bond) network within the barocaloric material and the modification of vibrational and / or librational modes in the barocaloric material, resulting in a significant decrease in phase-transition hysteresis and hysteretic loss.

[0088] The composite material displays barocaloric effects under an applied hydrostatic (or isotropic) stress. The composite material is suitable for barocaloric cooling or heating applications.

[0089] The composite material is preferably a solid, or it may have the characteristics of a mesophase. For example, the composite compound may be a plastic crystal, such as determined at ambient temperature, such as 270, 280 or 300 K, or at a lower temperature, such as 1, 10, 50, 100, 150, 200, or 250 K, or at a higher temperature such as 350, 400, or 450 K, and at ambient pressure, such as at 101.3 kPa.

[0090] A composite material for use in the present invention has a phase transition that occurs when the material is subjected to an external hydrostatic pressure. The composite material may be a material having a reversible phase transition at a temperature in the range of from about 1 K to about 450 K, such as from about 100 K to 450 K, such as about 200 K to about 450 K, such as at ambient pressure, such as at 101.3 kPa.

[0091] The entropy change, | ASo| , at the phase transition may be at least 10 J K'1kg-1, such as at least 20 J K'1kg-1, such as at least 50 J K'1kg-1, such as at least 100 J K'1kg-1, such as at least 150 J K1kg-1, such as at least 200 J K-1kg-1, such as at least 250 J K1kg-1, such as at least 300 J K'1kg-1, such as at least 400 J K'1kg-1, such as at least 450 J K'1kg-1, which is the magnitude of the change and the change may be negative or positive as appropriate during cooling and heating through the transition.

[0092] 008873440- PCT SpecificationThe entropy change may refer to the entropy change for the entire transition. Alternatively, where a transition includes a first-order transition optionally with other order transitions, the entropy change may refer to the entropy change for the first-order transition only. Where there is a first-order transition which is a part of the entire transition, the entropy change for the first-order transition may be at least 10%, at least 30%, at least 40% or at least 50% of the entropy change of the entire transition.

[0093] The entropy change may be expressed as the entropy change observed on heating, cooling, or as the average of the heating and cooling transitions. The entropy change at the transition may be determined by differential scanning calorimetry.

[0094] The phase transition is accompanied by a change in the unit cell volume of the material, for example the unit cell volume may increase on heating through the phase transition. This is the change observed for conventional barocaloric materials. A decrease in the unit cell volume on heating may be observed for inverse barocaloric materials.

[0095] The volume change at the transition may be at least 1.0 mm3g’1, such as at least 5.0 mm3g’1, such as at least 10 mm3g'1, such as at least 15 mm3g’1, such as at least 20 mm3g'1, such as at least 25 mm3g'1, such as at least 30 mm3g'1, such as at least 40 mm3g’1, such as at least 50 mm3g'1. The volume change at the transition may be a change of at least 0.1%, at least 0.2%, at least 0.5%, or at least 1.0%. The volume change values refer to the magnitude of the change, and the change may be positive or negative as appropriate during heating and cooling through the transition. For example, the volume change may refer to an increase (positive) change in the volume, as such might be observed on heating through the phase transition.

[0096] The volume change may refer to the volume change for the entire transition. Alternatively, where a transition includes a first-order transition optionally with other order transitions, the volume change may refer to the volume change for the first-order transition only. Where there is a first-order transition which is a part of the entire transition, the change in volume for the first-order transition may be at least 10%, at least 30%, at least 40% or at least 50% of the volume change for the entire transition.

[0097] The change in the volume at the transition may be determined from X-ray diffraction analysis of the composite material during a temperature sweep across the phase transition (where the transition typically occurs across a temperature range). Lloveras et al. 2015, for example, describe a typical experiment for determining the volume change at atmospheric pressure (see the Methods section, X-Ray Diffraction at page 5, together with Figure 1(d)). The change in the volume at the transition may be determined from volumetric thermal expansion measurements, or from volumetric isothermal compressibility measurements at various temperatures.

[0098] 008873440- PCT SpecificationThe latent heat, | Qo|, at the phase transition may be at least 1.0 kJ kg'1, such as at least 5.0 kJ kg-1, such as at least 10.0 kJ kg-1, such as at least 25.0 kJ kg-1, such as at least 50.0 kJ kg'1, such as at least 100.0 kJ kg-1, such as at least 150.0 kJ kg-1, such as at least 200 kJ kg-1.

[0099] Some organic materials, such as plastic crystals, have a phase transition that displays a latent heat, |Qo| , that is very high. Thus, in some embodiments, the organic material may have a latent heat, | Qo| , that is at least 25 kJ kg-1, such as at least 50 kJ kg-1, such as at least 75 kJ kg-1, such as at least 100 kJ kg-1, such as at least 150 kJ kg-1, such as at least 200 kJ kg-1.

[0100] The latent heat values may be determined from the differential scanning calorimetry analysis of the material.

[0101] The composite materials of the invention display large barocaloric effects under applied pressure. In particular, the composite material displays a strong pressure-induced shift in the phase transition temperature and reduced hysteresis. The shift in transition temperature with applied pressure dTo / dp, is relatively high for the organic components used herein, and the hysteresis is relatively low. Thus, small or moderate pressures may be used to drive relatively large barocaloric effects.

[0102] In some embodiments, the change in transition temperature is at least 1 K, such as at least 2 K, such as at least 3 K, such as at least 4 K, such as at least 5 K, such as at least 10 K, for example as measured for a change in applied pressure of 0.15 GPa. The figures given here relate to the magnitude of the change.

[0103] In one embodiment, the change is an increase in the transition temperature under the applied pressure. In one embodiment, the change is a decrease in the transition temperature under the applied pressure.

[0104] The temperature change refers to the adiabatic temperature change. The temperature change may be the temperature change derived from -cAT approximating to TAS = Q using the zero-pressure specific heat capacity c, the isothermal entropy change AS and the temperature of operation, T (Q is isothermal heat). It is appreciated that a large change in transition temperature with pressure does not always correspond to a large adiabatic temperature change, however in some instances it may do so.

[0105] The adiabatic temperature change can be calculated using the values of isothermal entropy change and heat capacity, which is a fundamental quantity known for most materials. The adiabatic temperature change may also be measured experimentally, for example using the experimental set up described by Rodriguez et al.

[0106] 008873440- PCT SpecificationThe composite material comprises a barocaloric material. The barocaloric material may have the properties as described herein for the composite material, such as properties of the transition temperature.

[0107] The composite material may comprise one or more barocaloric materials, such as a mixture of barocaloric materials, wherein the additive is dispersed in the one or more barocaloric materials. In some embodiments, the composite material comprises two or more barocaloric materials, such as more than two barocaloric materials, wherein the additive is dispersed in the two or more barocaloric materials. In these embodiments, the additive may be dispersed in a mixture of barocaloric materials. The additive may be a barocaloric material in particulate form.

[0108] In some embodiments, the composite material consists of one or more barocaloric materials and particles of one or more additives. In some embodiments, the composite material consists of a barocaloric material and particles of an additive dispersed in the barocaloric material.

[0109] Preferably, the composite material has a pressure-driven (nominally) reversible isothermal phase transition at a lower pressure than for the barocaloric material in the absence of the additive. The ‘nominally reversible transition refers to a transition that can be driven back and forth but in which there is hysteresis. The composite material may be suitable for use as a barocaloric cooling or heating agent at a lower pressure than a corresponding barocaloric material in the absence of the additive.

[0110] The composite material may have a pressure-driven reversible isothermal phase transition at a lower pressure than the barocaloric material in the absence of the additive, such as lower by at least 1 MPa, such as at least 10 MPa, such as at least 50 MPa, such as at least 100 MPa, such as at least 150 MPa. The isothermal phase transition may display an entropy change, such as where the isothermal phase transition is a first-order phase transition. The isothermal phase transition may be measured using variable-pressure calorimetry.

[0111] The composite material may have a phase transition temperature that is higher or lower than the phase transition temperature of the barocaloric material in the absence of the additive, such as lower by at least 1 K, such as at least 2K, such as at least 3 K, such as at least 5 K, such as at least 10 K, such as at ambient pressure, such as at 101.3 kPa.

[0112] References to a “phase transition temperature” may refer to an onset temperature or a peak temperature. An onset temperature is the temperature at which the phase transition begins. The phase transition temperature may be determined by calorimetry, such as differential scanning calorimetry by a method described herein. The peak temperature may be the temperature at which heat flow (dQ / d7) is highest.

[0113] 008873440- PCT SpecificationThe composite material may have a phase transition with an onset temperature that is higher or lower than that of the barocaloric material in the absence of the additive, such as lower by at least 1 K, such as at least 2K, such as at least 3 K, such as at least 5 K, such as at least 10 K, such as at ambient pressure, such as at 101.3 kPa.

[0114] The composite material may have a phase transition temperature, such as an onset temperature or a peak temperature, that is higher than the phase transition temperature of the barocaloric material in the absence of the additive, such as higher by at least 1 K, such as at least 2K, such as at least 3 K, such as at least 5 K, such as at least 10 K, such as at ambient pressure, such as at 101.3 kPa. The composite material may have a phase transition with an onset temperature that is higher than that of the barocaloric material in the absence of the additive, such as lower by at least 1 K, such as at least 2 K, such as at least 3 K, such as at least 5 K, such as at least 10 K, such as at ambient pressure, such as at 101.3 kPa.

[0115] The composite material may have a phase transition temperature, such as an onset temperature or a peak temperature, that is lower than the phase transition temperature of the barocaloric material in the absence of the additive, such as lower by at least 1 K, such as at least 2K, such as at least 3 K, such as at least 5 K, such as at least 10 K, such as at ambient pressure, such as at 101.3 kPa. The composite material may have a phase transition with an onset temperature that is lower than that of the barocaloric material in the absence of the additive, such as lower by at least 1 K, such as at least 2 K, such as at least 3 K, such as at least 5 K, such as at least 10 K, such as at ambient pressure, such as at 101.3 kPa.

[0116] In the composite material of the invention, the two peaks from the cooling and heating transition are closer to each other than the two peaks from cooling and heating in the barocaloric material in the absence of an additive. In this way, the phase-transition hysteresis is smaller. The absolute changes in transition temperature may be positive or negative.

[0117] In some embodiments, the composite material comprising the additive has a reduced thermal hysteresis compared to the one or more barocaloric material in the absence of the additive. The thermal hysteresis may be determined as the difference in the phase transition temperature on cooling and heating, such as the onset temperature or the peak temperature. Thermal hysteresis may be measured by calorimetry, such as by differential scanning calorimetry (DSC), such as a DSC method as described herein. The difference between the thermal hysteresis of the composite material and the barocaloric material may be determined by subtracting the temperature of the peaks in the calorimeter data measured on heating and on cooling (such as the peak transition temperature, for example as shown in Figure 2 and described at Example 1).

[0118] 008873440- PCT SpecificationThere will be an equivalent reduction in pressure hysteresis when driving the transition with pressure instead of temperature.

[0119] Preferably, the composite material has a thermal hysteresis that is no more than 90% of the thermal hysteresis of the barocaloric material in the absence of the additive, such as no more than 80%, such as no more than 70%, such as no more than 50%, such as no more than 30%, such as no more than 25% such as no more than 10% such as no more than 1%.

[0120] The composite material may have a thermal hysteresis that is smaller than the thermal hysteresis of the barocaloric material in the absence of the additive by at least 0.5 K, such as 1 K, such as at least 2 K, such as at least 3 K, such as at least 5 K, such as at least 8 K, such as at least 10 K, such as at least 15 K.

[0121] Barocaloric Material

[0122] A barocaloric material as described herein is a material suitable for use as a barocaloric cooling or heating agent.

[0123] A barocaloric material has a phase transition that can be driven and undriven with the application and removal of pressure, and the phase transition involves heat exchange. The phase transition may be a first order phase transition. Without wishing to be bound by theory, a first-order phase transition displays a shift of transition temperature with pressure due to a change in volume at the transition, which results in a substantial barocaloric effect. A second-order phase transition may also give rise to a barocaloric effect, but generally does not display a change in volume or a change in entropy and there is no shift of transition temperature with pressure. A first-order phase transition may release or absorb latent heat. Therefore, a first-order phase transition may be preferred for use in barocaloric cooling or heating.

[0124] In some embodiments, the barocaloric material has a first-order phase transition that can be driven by the application or removal of hydrostatic pressure. In some embodiments, the barocaloric material has a second-order phase transition.

[0125] It is necessary for a barocaloric material to have a reversible phase transition in order to be used as a barocaloric cooling or heating agent. In this way, the barocaloric material can cycle between two phases, such as over two or more cycles, such as three or more cycles. An irreversible phase transition is typically not suitable for use in barocaloric cooling or heating agent.

[0126] The presence of a phase transition in the material may be established from the analysis of the calorimetric data of the material across a temperature range including the temperatures mentioned above. For example, the measurements of heat flow may be made by differential scanning calorimetry, such as using a temperature scan rate of 10 K min-1. Lloveras etal.,

[0127] 008873440- PCT Specificationfor example, describe a typical experiment for determining the transition temperature at atmospheric pressure (see the Methods section, Calorimetry at Atmospheric Pressure at page 5, together with Figure 1(b)).

[0128] A barocaloric material according to the invention may be capable of releasing heat upon application of hydrostatic pressure, and absorbing heating upon release of the applied pressure. These materials may be referred to as conventional barocaloric materials. A barocaloric material may be capable of absorbing heat upon application of hydrostatic pressure and releasing heat upon release of the applied pressure, and these materials may be referred to as inverse barocaloric materials. A barocaloric material for use according to the invention may be a conventional or an inverse barocaloric material.

[0129] Preferably, the barocaloric material comprises a first-order phase transition where an applied hydrostatic pressure at or near the first-order phase transition temperature causes a reduction in volume and release of heat (conventional barocaloric effect), or a reduction in volume and absorption of heat (inverse barocaloric effect).

[0130] Suitable barocaloric materials are known in the art. Examples include organic materials, inorganic materials and organic-inorganic hybrid materials.

[0131] In some preferred embodiments, the organic material is or comprises an alkane. The organic material may be an alkane, such as a linear or branched alkane, or the organic material may be an organic-inorganic hybrid material comprising an alkyl chain, such as a hybrid perovskite comprising an alkyl chain.

[0132] Organic barocaloric materials and their use as barocaloric cooling or heating agents are described in WO 2018 / 069506, which is incorporated by reference herein. Neopentylglycol is described in Lloveras et al., 2019, Liu etal., 2022, Qian et al., 2024 and Dai et al., 2024, the contents of which are incorporated by reference herein.

[0133] An organic barocaloric material may be an organic compound or a salt thereof. References to an organic compound may include a reference to the ionic form of the compound, as appropriate. An organic material as described herein typically has a molecular weight that is 2,000 or less. Thus, an organic materials for use as a barocaloric agent is preferably a small molecular weight compound, or a salt thereof.

[0134] An organic compound, or the ionic form of the compound, has one or more carbon atoms, and more preferably two or more carbon atoms. The organic compound will usually possess one or more carbon-carbon bonds, such as two or more carbon-carbon bonds.

[0135] The organic compound, or an ionic form thereof, will typically also contain hydrogen, and the material will usually have one or more carbon-hydrogen bonds, such as two or more carbon-hydrogen bonds.

[0136] 008873440- PCT SpecificationThe organic compound may additionally contain heteroatoms, such as nitrogen, oxygen, sulfur and / or fluorine atoms. The organic compound may therefore contain one or more carbon-heteroatom bonds, such as carbon-oxygen bonds.

[0137] The organic compound may contain one or more groups selected from alkyl, alkenyl, alkynyl, cycloalkyl, heterocyclyl, aryl including carboaryl and heteroaryl. Multiple groups of the same type may be present in the organic compound. For example, an organic material may contain a plurality of cycloalkyl, heterocyclyl, or aryl groups.

[0138] The organic compound may comprise a ring system, such as a cycloalkyl, heterocyclyl, or aryl group. A ring system may be a fused ring systems. For example, an organic material may be adamantane or a derivative thereof, which may be regarded as having fused cycloalkyl groups.

[0139] The organic compound may contain one or more groups selected from halo, hydroxyl, carboxyl, oxo such as keto and acyl, ester, carbonate, amino, amido, carbamate, carbamide (urea), cyano, nitrile and nitro.

[0140] A carboxyl group may be in ionic form as a carboxylate together with an appropriate counter cation.

[0141] An amino group may be in ionic form as a quaternary amine together with an appropriate counter anion.

[0142] The organic compound may contain an alkyl group, such as a C1-12 alkyl group.

[0143] The organic compound may be a hydrocarbon. The organic compound may be an alkane, including linear and branched alkanes, or a cycloalkane, such as those having fused carbocyclic rings, such as an alkane or cycloalkane having 4 or more, such as 5 or more, such as 10 or more, carbon atoms. For example, 1,3-dimethyladamantane may be used as an organic material in the methods of the present case.

[0144] The organic compound may contain an aryl group, such as a carboaryl or a heteroaryl group. The carboaryl group may be phenyl or naphthyl.

[0145] The organic compound may comprise a halo group, such as a fluoro, chloro, bromo or iodo group. In some embodiments the organic compound is 2-bromoadamantane.

[0146] In some embodiments, the organic compound is not a salt. In some embodiments, the organic material is a salt of an organic compound, such as where the counter ion to the ionic form of the organic compound is a metal ion or an organic ion.

[0147] 008873440- PCT SpecificationAlthough a metal counter ion may be present, the material may nevertheless be referred to as an organic material owing to the organic character of the ionic form of the organic compound. A metal ion may be a mixed metal ion.

[0148] In some embodiments, the organic material does not contain a metal. In some embodiments, the organic material does not contain a metal selected from the group consisting of Group 3 to 16 metals, such as Group 3 to 12 metals.

[0149] In other embodiments, the organic material does contain a metal. Typically, where a metal is present it is a counter ion to an ionic form of the organic compound. Thus, the metal is typically a cationic species. Where a metal is present it may be a Group 1 and / or a Group 2 metal. The metal may be a Group 2 metal cation, such as barium, calcium and strontium cations.

[0150] In some embodiments, the barocaloric material is selected from:

[0151] plastic crystals, such as neopentane and adamantane derivatives, such as neopentylglycol, 1-adamantanol and 2-bromoadamantane.

[0152] liquid crystals, such as a non-lyotropic liquid crystal including 4-(trans-4-pentylcyclohexyl)benzonitrile (PCH5), or a lyotropic material, including phospholipids.

[0153] alkyl carboxylic acids or alkyl carboxylate salts, such as barium dicalcium propionate and barium dicalcium butyrate;

[0154] alkanes, such as linear alkanes (n-alkanes) or branched alkanes, such as

[0155] C3-20 alkane, such as C3-20 n-alkane, such as n-butane, n-pentane or C9-18 n-alkane.

[0156] A plastic crystal typically has long-range translational order, which may be seen, for example, in the X-ray diffraction pattern for the material, where sharp Bragg peaks are visible, but there is no orientational order. An organic material having a plastic crystal phase, where there is no orientational order of the crystals, may be converted to or from a phase where there is orientational order, for example with a change in the temperature. Such a phase transition may be used in the methods of the invention.

[0157] A description of organic materials having plastic crystal character is provided by Timmermans, the contents of which are hereby incorporated by reference. A description of organic materials having plastic crystal or liquid crystal character is provided by Wunderlich et al., the contents of which are hereby incorporated by reference.

[0158] In some embodiments, the plastic crystal may comprise an alkyl group or a cycloalkyl group. The cycloalkyl group may be an adamantyl group. The adamantyl group may be optionally substituted with one or more groups selected from the group consisting of alkyl, halo, hydroxyl and oxo. The alkyl group may be a C4-20 alkyl group, such as a C4 alkyl group. The alkyl group is optionally substituted with one or more groups selected from the group consisting of hydroxyl, amino and nitro.

[0159] 008873440- PCT SpecificationThe plastic crystal may be selected from neopentylglycol and adamantane derivatives, such as neopentylglycol and a substituted adamantyl compound, such as neopentylglycol and an adamantyl compound substituted with a hydroxyl or halo group. In some embodiments, the plastic crystal is neopentylglycol.

[0160] A liquid crystal has little or no long-range translational order, but orientational order is present. Thus, Bragg peaks are not visible in the X-ray diffraction pattern of the material. Further, plastic crystals have typically globular motifs, whereas liquids crystals have typically non-globular motifs.

[0161] The liquid crystal may comprise one or more groups selected from alkyl, cycloalkyl and aryl. The liquid crystal may comprise a cycloalkyl group, such as a cyclohexyl group. The liquid crystal may comprise an aryl group, such as a phenyl group. The liquid crystal may comprise a nitrile group. In some embodiments, the liquid crystal has an alkyl group, a cycloalkyl group and an aryl group. In some embodiments, the liquid crystal has an alkyl group, a cycloalkyl group, an aryl group and a nitrile group. In some embodiments, the organic material is not lyotropic. Thus, the organic material may not be a phospholipid. Typically, the organic material is not provided, such as dispersed, within a solvent, such as water.

[0162] The liquid crystal may be 4-(trans-4-pentylcyclohexyl)benzonitrile (PCH5).

[0163] The barocaloric material may be an alkyl carboxylic acid or an alkyl carboxylate salt. In some embodiments, the barocaloric material is an alkyl carboxylate. The alkyl group may have three or more carbon atoms, such as 3 or 4 carbon atoms (including the carbon atom provided by the acid group).

[0164] In some embodiments, the barocaloric material may be an alkyl carboxylic acid or an alkyl carboxylate salt where one or more, such as one or two, hydrogen atoms may be substituted with fluorine. The barocaloric material may be selected from barium dicalcium propionate (BDP) and barium dicalcium butyrate (BDB). Fluorinated versions of the alkyl carboxylates may also be used as the barocaloric material. For example, Yano et al. describe the phase transitions of alkyl carboxylates and fluorinated alkyl carboxylates.

[0165] In some embodiments, the barocaloric material is not an aminocarboxylic acid salt such as an amino acid salt, for example, a triglycine salt. In particular, the barocaloric material is not an amino acid sulfate.

[0166] A barocaloric material may contain one or more different organic compounds or salts thereof, and each of these compounds or salts may be as described herein. The individual compounds or their salts may have different transition temperatures. Such composites can be operated in a parallel or a cascade mode in order to enhance the temperature span of

[0167] 008873440- PCT Specificationoperation.

[0168] The barocaloric material may be an organic-inorganic hybrid material. Examples include metal organic frameworks (MOFs), covalent organic frameworks (COFs), hybrid perovskites, and hybrid zeolites. The barocaloric material may be an organic-inorganic hybrid material comprising an alkyl chain, such as a C2-20 alkyl chain.

[0169] The barocaloric material may be a two-dimensional perovskite, such as described in Seo et al., the contents of which is incorporated by reference herein.

[0170] Preferably, the barocaloric material has a molecular weight that is at least 50 g / mol, such as at least 65 g / mol, such as at least 70 g / mol, such as at least 100 g / mol, such as at least 150 such as. The barocaloric material may have a molecular weight that is at most 500 g / mol, such as at most 700 g / mol, such as at most 1,000 g / mol, such as at most 2,000 g / mol, such as at most 5,000 g / mol, such as at most 10,000 g / mol. The organic material may have a molecular weight that is in a range selected from the lower and upper limits given above. For example, an organic material may have a molecular weight that is in the range 50 to 1,000 g / mol, such as 100 to 500 g / mol.

[0171] Preferably, the barocaloric material is not a high molecular weight compound, such as a polymer.

[0172] The barocaloric material in the composite material may be provided as a mixture of two or more materials displaying barocaloric effects. Thus, the composite material may comprise a plurality of different barocaloric materials, in which the additive is dispersed. The composite material may comprise a mixture of two or more barocaloric materials, such as a mixture of two barocaloric materials or a mixture of three barocaloric materials. In these embodiments, the additive may be dispersed throughout the mixture of barocaloric materials.

[0173] The barocaloric material may have one or more features described herein for the composite material. For example, the barocaloric material may have a phase transition temperature; an entropy change, | ASo| , at the phase transition; a volume change at the phase transition; a latent heat, |Qo| , at the phase transition; or a change in transition temperature as described herein for the composite material.

[0174] In some embodiments, the composite material comprising the additive has a latent heat, | Qo| , at a phase transition that is equal to or higher than the barocaloric material in the absence of the additive. In some embodiments, the composite material comprising the additive has an entropy change, |ASo|, at a phase transition that is equal to or higher than the barocaloric material in the absence of the additive. In these embodiments, the additive may be a material displaying barocaloric effects.

[0175] Additive

[0176] 008873440- PCT SpecificationThe composite materials of the invention comprise an additive. The additive is provided as a particulate species dispersed within the barocaloric material, which may be an organic, inorganic, or hybrid organic-inorganic matrix described herein. The additive itself may be a barocaloric material in particulate form. The additive may reduce the pressure needed to drive a phase transition in the barocaloric material in a reversible manner.

[0177] The composite materials described herein comprises particles of an additive. By “particles of an additive” it is meant a population of particles of the additive described herein. Within the population, individual particles of the additive may have differing sizes or dimensions to each other such that there is a distribution of particle sizes or dimensions present within the population.

[0178] The average particle size typically refers to the average size of additive particles that are present in the composite material.

[0179] The “average particle size” may refer to the mean value or the median value of particle sizes across a population of particles.

[0180] A mean value may refer to the number average or the mass average.

[0181] Preferably, the average particle size refers to the median particle size of a population. The population of particles may be a typical amount that can be analysed together, such as by microscopy or by spectroscopic methods. Suitable methods include scanning electron microscopy (SEM), dynamic light scattering (DLS) and scanning probe microscopy. The number of particles within a population may be at least a typical amount that can be analysed together, such as at least 50 particles, such as at least 100 particles.

[0182] A particle may be characterized by its largest dimension. By “largest dimension”, it is meant the maximal diameter in the largest cross-section of the particle, such as the diameter of the particle.

[0183] A population of particles may be defined by an average largest dimension. By “average largest dimension”, it is meant the mean value or the median value, for example, of the largest dimension of individual particles in a collection of particles, where the largest dimension is as defined above, such as the diameter. The population of particles may be a typical amount that can be analysed together such as by microscopy, such as at least 50 particles, such as at least 100 particles.

[0184] The particles may be spherical in shape, or substantially spherical.

[0185] The particles may be non-spherical, such as elongate or ellipsoidal.

[0186] 008873440- PCT SpecificationWhen a particle is irregular in shape, the particle size may refer to the geometric average of two or more lengths measured across the particle. For example, when a particle is ellipsoidal, the particle size preferably refers to the geometric average of the length of the two main axes that define the ellipsoid.

[0187] The additive may have an average particle size (which may be a mean, median or D50 value) of 10 nm or more, such as 20 nm or more, such as 50 nm or more, such as 60 nm or more, such as 70 nm or more. The average particle size may be up to 100 pm, such as up to 50 pm, such as up to 20 pm, such as up to 10 pm, such as up to 5 pm, such as up to 1 pm, such as up to 750 nm, such as up to 500 nm, such as up to 250 nm, such as up to 100 nm, such as up to 80 nm. The particles may have an average particle size in a range with upper and lower limits described above, such as from 10 nm to 100 pm, including from 50 nm to 50 pm, such as from 60 nm to 20 pm, such as about 60 nm to 10 pm.

[0188] In some embodiments, the particles have an average particle size of 60 nm or more, such as from 60 nm to 10 pm, such as from 60 nm to 100 nm, such as from 60 nm to 80 nm.

[0189] In some embodiments, the additive is selected from:

[0190] silicon dioxide (SiCh) particles, optionally having an average particle size of from about 1 pm to about 20 pm, such as from about 1 pm to about 10 pm.

[0191] aluminium oxide (AI2O3) particles, optionally having an average particle size of from about 1 pm to about 20 pm, such as from about 1 pm to about 10 pm.

[0192] copper particles, optionally having an average particle size of from about 1 pm to about 5 pm, such as about 3 pm; or an average particle size of from about 50 nm to about 100 nm, such as from about 60 nm to about 80 nm.

[0193] The additive may have an average largest dimension of 10 nm or more, such as 20 nm or more, such as 50 nm or more, such as 60 nm or more, such as 70 nm or more. The average largest dimension size may be up to about 100 pm, such as up to about 50 pm, such as up to about 20 pm, such as up to about 10 pm, such as up to about 5 pm, such as up to about 1 pm, such as up to about 750 nm, such as up to about 500 nm, such as up to about 250 nm, such as up to about 100 nm, such as up to about 80 nm. The particles may have an average largest dimension in a range with upper and lower limits described above, such as from about 10 nm to about 100 pm, including from about 50 nm to about 50 pm, such as about 60 nm to about 20 pm, such as about 60 nm to 10 pm.

[0194] The particles may be defined by a particle aspect ratio. The particle aspect ratio refers to the ratio between the longest diameter of a particle to the shortest perpendicular diameter. The particle aspect ratio, which may be an average particle aspect ratio for a population of particles, may be up to 20, such as 10, such as 8.0, such as 5.0, such as 4.0, such as 3.0, such as 2.5, such as 2.0. The particle aspect ratio may be 1.1 or more, such as 1.2 or more, such as 1.5 or more, such as 1.7 or more, such as 1.8 or more. The particle aspect ratio, or average particle aspect ratio, may be in a range with upper and lower limits described

[0195] 008873440- PCT Specificationabove, such as from about 1.1 to about 20, including from about 1.1 to about 10, such as from about 1.5 to about 5.0, such as from 1.5 to 3.0. The particle aspect ratio may be determined by scanning electron microscopy.

[0196] The particles are preferably dispersed substantially uniformly throughout the composite material. Preferably, the particles are not in contact with each other in the composite material. In some embodiments, at least 25% of the particles are not in contact with another particle, such as at least 50%, such as at least 75%. Preferably, the particles do not aggregate to form a heat transfer path. For example, the amount of the additive in the composite material may be below a percolation threshold. The percolation threshold is a threshold amount where the thermal conductivity of the composite material substantially increased, such as described in Bollobas and Riordan.

[0197] The additive may show (or may display) barocaloric effects. In some embodiments, the additive is provided as particles of a barocaloric material as described herein. Preferably, where the additive is of a material that itself displays barocaloric effects, the additive is not the same material as another barocaloric material provided in the composite material.

[0198] Preferably, the one or more barocaloric materials and the additive provided in the composite materials are each different materials.

[0199] The additive may be a barocaloric material such as an organic barocaloric material, or an inorganic barocaloric material. The additive may display conventional barocaloric effects, or the additive may display inverse barocaloric effects. Suitable additive organic barocaloric materials are as described herein. Suitable additive inorganic barocaloric materials include particles of MnsGaN, Fe-Rh, Heusler alloys, or ammonium sulfate.

[0200] The additive may be a thermal insulator or a thermal conductor. In some embodiments, the additive is a thermal insulator. The additive may be a thermal conductor. In some embodiments, the additive is not a thermal conductor.

[0201] The additive may be an inorganic material or an organic material, or a mixture thereof.

[0202] The additive may be an inorganic material, which may be selected from:

[0203] metals, such as copper or gold;

[0204] oxides, including metal oxides, such as aluminium oxide and copper oxide, or silicon dioxide; and

[0205] carbon, such as carbon nanoparticles, graphene particles or graphite particles.

[0206] In some preferred embodiments, the additive is a metal or an oxide, such as a metal or a metal oxide.

[0207] 008873440- PCT SpecificationThe additive may be an organic material, such as a polymer. Suitable examples include polyethylene. The additive may be a polymer, such as polyethylene, a molecular crystal such as neopentyl, adamantane or carborane and derivative (e.g. polymers) thereof.

[0208] In some embodiments, the additive is not in the form of nanosheets. Preferably, the additive is not graphene nanosheets. In some embodiments, the composite is substantially free from nanosheets.

[0209] In some embodiments, the additive is graphene, such as graphene particles. In other embodiments, the additive is not graphene, such as not graphene nanoparticles.

[0210] The amount of the additive may be up to 3% by volume of the composite material, such as up to 2%, such as up to 1.5%, such as up to 1.2%. The amount of the additive in the composite material may be at least 0.1%, such as at least 0.2%, such as at least 0.3%, such as at least 0.5%, such as at least 0.6%, such as at least 0.8%, such as at least 0.9%, such as at least 1%. The amount of the additive in the composite material may be in a range with upper and lower limits as described above, such as from 0.1% to 2% by volume, including from 0.1% to 1.5% by volume, such as from 0.3% to 3% by volume, such as 0.5% to 3% by volume, such as from 0.6% to 2% by volume, such as from 0.6% to 1.5% by volume. In some preferred embodiments, the amount of the additive is from 0.6% to 3.0% by volume of the composite material, such as from 0.6% to 2.0% by volume, such as from 0.6% to 1.5% by volume.

[0211] In some preferred embodiments, the amount of the additive is about 1 % by volume of the composite material, such as about 1.0% by volume. The volumes are determined at ambient temperature and pressure, as defined herein.

[0212] The amount of the additive in the composite material may be up to about 15% by weight, such as up to 10%, such as up to 9%, such as up to 8%, such as up to 7%, such as up to 6%. The amount of the additive in the composite material may be at least 1% by weight, such as at least 2%, such as at least 3%, such as at least 4%, such as at least 5%, such as at least 6%. The amount of the additive in the composite material may be in a range with upper and lower limits as described above, such as from 1% to 15% by weight, including from 2% to 10% by weight, such as from 2% to 8% by weight, such as from 4% to 8% by weight.

[0213] 008873440- PCT SpecificationMethods

[0214] The present invention provides a method of cooling or heating using the barocaloric effects of the composite materials described herein. The composite materials find use as cooling or heating agents, for example within a cooling or heating apparatus.

[0215] Thus, the invention provides a method of barocaloric cooling or heating, the method comprising the steps of:

[0216] (i) applying hydrostatic pressure to a composite material comprising a barocaloric material and particles of an additive dispersed in the barocaloric material, wherein the amount of the additive is from about 0.1% to about 3% by volume of the composite material; and

[0217] (ii) permitting heat flow from or to the barocaloric material;

[0218] (iii) releasing the applied hydrostatic pressure; and

[0219] (iv) permitting heat flow to or from the barocaloric material.

[0220] The methods of the invention include a cycling step of applying hydrostatic pressure to a composite material and releasing the hydrostatic pressure. This provides for a complete heating and cooling cycle (or cooling and heating cycle).

[0221] In steps (ii) and (iv), heat is permitted to flow from or to the barocaloric material. The heat may be permitted to flow from or to the composite material. That is, heat may be permitted to flow from or to the barocaloric material as part of the composite material. It will be understood that heat may also be permitted to flow from or to other components of the composite material, such as particles of the additive.

[0222] Where a composite material displays a conventional barocaloric effect upon release of hydrostatic pressure, step (iv) provides for heat to flow to the composite material. Where a composite material displays an inverse barocaloric effect, step (iv) provides for heat to flow from the composite material.

[0223] Steps (i) and (iv) may be performed in order.

[0224] Step (i) may further comprise providing an apparatus as described herein. The application of hydrostatic pressure during step (i) may comprise use of a hydrostatic pressuriser as described herein.

[0225] The composite material may be used within a cycling step within a Brayton cycle, an Ericsson cycle or a Carnot cycle, to provide cooling or heating as required. The composite material may be used in an alternative thermodynamic cycle to provide cooling or heating.

[0226] Where the cycling step is a part of a Brayton cycle, for example, the pressure may be applied adiabatically, with, for example, a consequential heating of the composite material.

[0227] 008873440- PCT SpecificationSubsequent heat flow allows the composite material to return to its original temperature. In a further part of the cycling step, pressure may be released adiabatically with, for example, a consequential cooling of the composite material, and subsequent heat flow allows the composite material to return to its original temperature.

[0228] Where the cycling step is a part of the Ericsson cycle, for example, the pressurisation and depressurisation steps are performed isothermally. Thus, the composite material is maintained at a substantially constant temperature, with heat transfer occurring, and not subsequent to, the pressurisation and depressurisation steps.

[0229] The methods may be used within a heat pump cycle or a refrigeration cycle to provide heating or cooling, respectively.

[0230] The method of the invention may be performed at a temperature from 1 K to 500 K, such as from 1 K to 450 K, such as from 50 K to 450 K, such as from 100 K to 450 K. The method of the invention may be performed in an environment at around ambient temperature, such as a temperature from -20 °C to 50 °C, such as from 0 °C to 35 °C, such as 5 °C to 35 °C, such as 10 °C to 30 °C (corresponding to 253.15 K to 323.15 K, such as 273.15 to 308.15 K, such as 278.15 to 308.15 K, such as 283.15 to 303.15 K). The methods of the invention allow for the generation of cooler and warmer localised regions within the environment as part of the barocaloric cooling process. For example, the composite material may become warmer or cooler than the ambient environment in the methods of the invention. The methods of the invention allow for cryogenic applications, or for heating applications, such as in a heat pump.

[0231] The pressurising force is the change in pressure applied to the composite material.

[0232] Typically, the change in pressure is to or from ambient (atmospheric) pressure, such as to or from about 101 kPa.

[0233] In some embodiments, a sufficient pressuring force is applied to move the transition temperature of the composite material to a temperature that is in an ambient temperature, such as a temperature from -20 °C to 50 °C, such as 0 °C to 35 °C, such as 5 °C to 35 °C, such as 10 °C to 30 °C.

[0234] In the methods of the invention, the hydrostatic pressure applied in step (i) is typically near a transition temperature of the composite material. For example, the hydrostatic pressure is applied at a temperature that is within 150 K, within 100 K, within 50 K, within 20 K, within 15 K, within 10 K, such as within 5 K, such as within 2 K, such as within 1 K, such as within 0.5 K of the transition temperature, which is the transition temperature of the composite material absent the applied hydrostatic pressure (for example, the transition temperature under ambient pressure).

[0235] 008873440- PCT SpecificationThe hydrostatic pressure may be applied at a level sufficient to induce the phase transition. The hydrostatic pressure may be at most 1.0 GPa, such as at most 500 MPa, such as at most 250 MPa, such as at most 200 MPa, such as at most 150 MPa, such as at most 120 MPa, such as at most 100 MPa, such as at most 80 MPa, such as at most 50 MPa. The hydrostatic pressure may be at least 0.1 MPa, such as at least 0.5 MPa, such as at least 1.0 MPa, such as at least 5.0 MPa, such as at least 10 MPa, such as at least 25 MPa, such as at least 50 MPa, such as at least 100 MPa. The pressure applied may be a hydrostatic pressure in a range with the lower and upper limits as described above. For example, the hydrostatic pressure may be a pressure in the range of from 0.1 MPa to 1.0 GPa, including from 10 MPa to 1.0 GPa, such as from 10 MPa to 200 MPa.

[0236] In some embodiments, the hydrostatic pressure is at most 500 MPa, such as at most 250 MPa, such as at most 100 MPa.

[0237] Apparatus

[0238] The present invention provides a cooling or heating apparatus containing a composite material as described herein. The cooling apparatus is adapted to provide barocaloric cooling or heating using the composite material.

[0239] The apparatus comprises the composite material. The composite material may be provided within a pressure cell. A pressure cell is a housing for the composite material. The composite material may be in direct contact with one or more walls of the pressure cell, or the composite material may be contained within another structure within the pressure cell, such as to separate the composite material from the pressure cell. A pressure cell is not particularly limited in size or dimension as long as it suitably contains the composite material and is capable of transmitting hydrostatic pressure.

[0240] A pressure cell may comprise a piston, and the composite material may be provided within the piston headspace.

[0241] The composite material may be provided in the presence of a pressure transmitting medium. A pressure-transmitting medium is any suitable medium for transmitting an applied hydrostatic pressure to the composite material. The pressure-transmitting medium may be a pressure-transmitting fluid, which may be contained within a piston head space that is in contact with the composite material. The composite material may be provided within the pressure-transmitting medium, such as where the composite material is surrounded by the pressure-transmitting medium.

[0242] The pressure transmitting medium is not essential. In some embodiments, pressure may be applied directly to the composite material, such as where the composite material is in a liquid phase.

[0243] 008873440- PCT SpecificationPressure-transmitting media are well known in the art, including those described in WO 2018 / 069506, the contents of which are incorporated by reference. A pressuretransmitting medium may be a liquid or a solid.

[0244] Examples of a pressure-transmitting liquid include alkoxy silane materials, such as

[0245] DW-Therm, available from Huber Kaltemaschinenbau GmbH.

[0246] An example of a pressure-transmitting solid is alumina powder.

[0247] In some embodiments, the pressure-transmitting medium is not water.

[0248] The apparatus comprises a hydrostatic pressuriser for applying hydrostatic pressure to the composite material.

[0249] A hydrostatic pressuriser may be any means of applying hydrostatic pressure to the composite material. A hydrostatic pressuriser is suitable for pressurising and depressurising the composite material to reversibly drive a phase transition.

[0250] Where the composite material is to be provided in the piston headspace of a pressure cell, the piston may be operably connected to a hydrostatic pressuriser such that in use, hydrostatic pressure can be applied and released from the composite material.

[0251] The hydrostatic pressuriser may be configured to apply, or may be suitable for applying, hydrostatic pressure at a level sufficient to induce a phase transition in the composite material. The hydrostatic pressure may be at most 1.0 GPa, such as at most 500 MPa, such as at most 250 MPa, such as at most 200 MPa, such as at most 150 MPa, such as at most 120 MPa, such as at most 100 MPa, such as at most 80 MPa, such as at most 50 MPa. The hydrostatic pressure may be at least 0.1 MPa, such as at least 0.5 MPa, such as at least 1.0 MPa, such as at least 5.0 MPa, such as at least 10 MPa, such as at least 25 MPa, such as at least 50 MPa, such as at least 100 MPa. The pressure applied may be a hydrostatic pressure in a range with the lower and upper limits as described above. For example, the hydrostatic pressure may be a pressure in the range of from 0.1 MPa to 1.0 GPa, including from 10 MPa to 1.0 GPa, such as from 10 MPa to 200 MPa.

[0252] The hydrostatic pressuriser may be configured to apply, or may be suitable for applying, pressure to each pressure cell to move the transition temperature of the composite material to a temperature that is in an ambient temperature, such as a temperature from -20 °C to 50 °C, such as 0 °C to 35 °C, such as 5 °C to 35 °C, such as 10 °C to 30 °C (corresponding to 253.15 K to 323.15 K, such as 273.15 to 308.15 K, such as 278.15 to 308.15 K, such as 283.15 to 303.15 K).

[0253] The apparatus may comprise a heat exchanger for connecting the composite material with a heat source or a heat sink. The apparatus may comprise a set of heat exchangers, such as

[0254] 008873440- PCT Specificationa first heat exchanger for connecting the composite material to a heat source and a second heat exchanger for connecting the composite material to a heat sink in use.

[0255] Uses

[0256] The invention also provides use of a cooling or heating apparatus as described herein for cooling or heating.

[0257] The methods of the invention may be for use in cooling foodstuffs or beverages.

[0258] The methods of the invention may be for use in cooling medicines.

[0259] The methods of the invention may be for use in cooling biological samples, such as tissues.

[0260] The methods of the invention may be for use in cooling electronic devices, such as devices for analytical measurements, and data centres.

[0261] The methods of the invention may be used to cool air, such as air within building and vehicles.

[0262] Definitions

[0263] A “barocaloric material” is a material suitable for use as a barocaloric cooling or heating agent. A barocaloric material preferably has a first-order phase transition that can be driven and undriven by the application and release of hydrostatic pressure. A barocaloric material exhibits a temperature change in response to an applied hydrostatic pressure.

[0264] A “conventional barocaloric material” is a barocaloric material that releases heat upon application of hydrostatic pressure, and absorbs heat upon release of the applied pressure.

[0265] An “inverse barocaloric material” is a barocaloric material that absorbs heat upon application of hydrostatic pressure, and releases heat upon release of the applied pressure.

[0266] An “additive” is an agent added to a barocaloric material to produce a composite material as described herein. The additive is typically provided in particulate form.

[0267] A “particle” may be a microparticle or a nanoparticle. A particle may be spherical in shape, or substantially spherical, or a particle may be irregular in shape such as non-spherical, such as elongate or ellipsoidal. Typically, a particle is not a nanosheet or a nanotube. A particle is typically provided in a population of particles, and the population of particles may be defined by a particle size, such as an average particle size, or a largest dimension.

[0268] 008873440- PCT SpecificationAn “average” value as defined herein may be a mean value or a median value, unless stated otherwise.

[0269] An “average particle size” may refer to the mean value or median value of particle sizes in a population of particles. A median particle size may be a D50 value. The average particle size may be determined by small angle X-ray scattering, scanning electron microscopy (SEM) or dynamic light scattering.

[0270] “Hydrostatic pressure” may also be described as isotropic stress. Hydrostatic pressure differs from uniaxial stresses that are applied to materials for use in elastocaloric cooling, for example. Unless stated otherwise, references herein to “pressure” refer to hydrostatic pressure, and references to “pressurisation” refer to the application of a hydrostatic pressure.

[0271] A “heat-exchanger” is a means for transferring heat from one medium to another. A heatexchanger may comprise a heat-exchange fluid, which may be capable of being circulated to a heat source or heat sink, such that in use, heat is extracted from or supplied to a composite material during pressurisation or depressurisation. A composite material as described herein, when provided within an apparatus, is preferably connected to a set of heat-exchangers, with one heat-exchanger configured to connect the composite material to a heat source and another heat-exchanger configured to connect the composite material to a heat sink. Each set of heat-exchangers may be a single unit, or two units for example.

[0272] Other Embodiments

[0273] Each and every compatible combination of the embodiments described above is explicitly disclosed herein, as if each and every combination was individually and explicitly recited. Various further aspects and embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure.

[0274] “and / or” where used herein is to be taken as specific disclosure of each of the two specified features or components with or without the other. For example “A and / or B” is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if each is set out individually herein.

[0275] Unless context dictates otherwise, the descriptions and definitions of the features set out above are not limited to any particular aspect or embodiment of the invention and apply equally to all aspects and embodiments which are described.

[0276] Certain aspects and embodiments of the invention will now be illustrated by way of example and with reference to the figures described above.

[0277] Examples

[0278] 008873440- PCT SpecificationMaterial Preparation - General Protocol

[0279] Composite samples were prepared by thoroughly mixing neopentylglycol (NPG) and powder inclusions in the solid state. The components were normalised by volume, and as such, they are presented as a volume fraction of the total pellet volume. All samples were made with a volume fraction of 1% of powder inclusions unless otherwise stated. The mixture was transferred to a pellet press, and pellets were formed under an applied uniaxial pressure of 15 MPa. All pellets had a standard geometry of 8 mm diameter and 0.8 mm thickness.

[0280] The inorganic inclusions used in the following examples include SiC>2 (<10 pm particle size), AI2O3 (~10 pm particle size), Cu (~3 pm particle size), and Cu nanopowder (60 - 80 nm particle size).

[0281] Example 1 - Thermally driven transition in a neopentylglycol / copper nanopowder composite material

[0282] A composite material comprising neopentylglycol (NPG) and 1 volume% copper nanopowder (60-80 nm particle size) was prepared according to the general protocol.

[0283] A thermally driven phase transition in the composite material was studied using calorimetry, and the results are shown in Figures 1 and 2.

[0284] Figure 1 shows dQ / dT curves measured by differential scanning calorimetry at 0 bar applied pressure. Figure 2 shows the transition temperature for the heating and cooling processes, respectively, corresponding to the peak dQ / dT values from Figure 1.

[0285] The results show a reduction in phase transition hysteresis for the NPG and Cu nanopowder composite material compared to pure NPG, with an overlap in the transition temperature for cooling and heating for the composite material (Figure 2).

[0286] Example 2 - Barocaloric effects in a neopentylglycol / copper nanopowder composite material

[0287] Next, barocaloric effects were studied in the NPG + Cu nanopowder composite material and compared to pure NPG. |AS| was determined by differential scanning calorimetry and the results are shown in Figure 3.

[0288] Figure 3 shows that pure NPG displays a large hysteresis such that reversible barocaloric effects are not practical at up to 1 ,200 bar pressure. By contrast, in the composite material according to an embodiment of the invention, a reversible barocaloric effect can be driven using pressure at and below 1,200 bar. Accordingly, the composite material displays reversible barocaloric effects at low pressures where the pure NPG material displays

[0289] 008873440- PCT Specificationirreversible barocaloric effects. The composite materials of the invention are therefore beneficial for barocaloric cooling or heating applications.

[0290] Example 3 - Quasielastic Neutron Scattering Studies

[0291] Quasielastic neutron scattering was carried out to analyse pure NPG and composite materials comprising 1% or 5% copper nanopowder.

[0292] Figure 4a shows QENS data across the solid-solid phase transition of the materials, highlighting the difference in quasielastic signal above (330 K) and below (280 K, 300 K) the phase transition. A substantial increase in quasielastic intensity is seen from 300 K to 330 K, which demonstrates the emergence of additional rotational degrees of freedom in the NPG plastic crystal phase in the composite material compared to pure NPG. At 300 K, the increase in quasielastic signal in the composites compared to pure NPG suggest a difference in molecular modes.

[0293] Figure 4b shows the inelastic fixed-window scan (IFWS) intensity (measured at ± 3 peV) as a function of Tfor NPG and composites for 0.59 A-1< Q < 1.56 A’1. A significant divergence of IFWS intensity is observed approaching the phase transition in the composite materials compared to pure NPG, which suggests molecular mode(s) appearing before the phase transition. The inset plot shows the full data across this Q range, with increased intensity in yellow. The intensity rises at a lower temperature for the composite material than for NPG, and in both cases, and the Q-independence indicates a localised rotational process.

[0294] Figure 4c shows ambient pressure differential scanning calorimetry (DSC) heat-flow data, demonstrating an earlier onset of the solid-solid phase transition in the composite materials as compared to pure NPG. These results match the behaviour seen in the IFWS data in Figure 4b.

[0295] Figure 5a shows the mean linewidth as a function of T approaching the solid-solid phase transition. For the plastic crystal phase, mode 1 (methyl rotation) appears in all of the materials studied, whereas Mode 2 (hydroxymethyl rotation) is present in only the composite materials at 280 K and 300 K. Molecular schematics with annotated arrows illustrate the assigned molecular rotations responsible for each mode; in both cases, rotation is around the bond to the central carbon.

[0296] Figure 5b shows an example of QENS fitting for NPG and a composite material at 300 K for Q = 1.18 A-1, showing Mode 1 (L1) in all materials and Mode 2 (L2) present in the composites only.

[0297] 008873440- PCT SpecificationReferences

[0298] A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.

[0299] Bollobas and Riordan, Percolation, 2006, doi: 10.1017 / CBO9781139167383

[0300] Dai etal., Energy 2024, 294, 130800

[0301] Liu et al., Materials Research Letters, 2022, 10 (10), 675-681

[0302] Lloveras etal. Nature Commun. 2015, 6, Article no. 8801

[0303] Lloveras etal., Nature Commun. 2019, 10, Article no. 1803

[0304] Matsunami etal. Nature Materials 2015, 14, 73

[0305] Parveen et al., Eng. Sci. Technol. Int J. 2018, 21, 1086-1094

[0306] Qian etal., Cell Rep. Phys. Sci. 2024, 5, 101981

[0307] Rodriguez et al. J. Appl. Phys. Chem. 1982, 53, 6536

[0308] Seo etal. Nature Commun. 2022, 13, Article number. 2536

[0309] Timmermans J. Phys. Chem. Solids 1961, 18, 1

[0310] Wang et al., Renew. Energy 2013, 51, 241-246

[0311] Wunderlich et al. Advances in PolymerScience 1984, 60 / 61, Springer-Verlag, Berlin Yano et al. J. Phys. Soc. Japan. 1989, 58, 577

[0312] WO 2018 / 069506

[0313] 008873440- PCT Specification

Claims

- 33 -Claims:

1. A method of barocaloric cooling or heating, the method comprising the steps of:(i) applying hydrostatic pressure to a composite material comprising a barocaloric material and particles of an additive dispersed in the barocaloric material, wherein the amount of the additive is from about 0.1% to about 3% by volume of the composite material;(ii) permitting heat flow from or to the barocaloric material;(iii) releasing the applied hydrostatic pressure; and(iv) permitting heat flow to or from the barocaloric material.

2. A barocaloric cooling or heating apparatus comprising:a composite material comprising a barocaloric material and particles of an additive dispersed in the barocaloric material, wherein the amount of the additive is from about 0.1% to about 3% by volume of the composite material; anda hydrostatic pressuriser for pressurising and depressurising the composite material.

3. A composite material for barocaloric cooling or heating, wherein the composite material comprises a barocaloric material and particles of an additive dispersed in the barocaloric material, and wherein the amount of the additive is from about 0.1% to about 3% by volume of the composite material.

4. The method, apparatus or composite material of any of claims 1 to 3, wherein the amount of the additive is about 0.6% or more by volume of the composite material, such as about 0.7% or more by volume of the composite material, such as about 0.8% or more by volume of the composite material.

5. The method, apparatus or composite material of any of claims 1 to 4, wherein the amount of the additive is about 2% or less by volume of the composite material, such as about 1% or less by volume of the composite material.

6. The method, apparatus or composite material of any of claims 1 to 5, wherein the amount of the additive is about 1% by volume of the composite material.

7. The method, apparatus or composite material of any of claims 1 to 6, wherein the particles have an average particle size from about 20 nm to about 100 pm, such as from about 60 nm to about 10 pm.

8. The method, apparatus or composite material of any of claims 1 to 7, wherein the additive has an average particle aspect ratio of up to about 20, such as up to about 10, such as up to about 5.008873440- PCT Specification- 34 -9. The method, apparatus or composite material of any of claims 1 to 8, wherein the composite material has a thermal hysteresis that is no more than 90% of the thermal hysteresis of the barocaloric material, such as no more than 80%, such as no more than 70%, such as no more than 50%, such as no more than 30%, such as no more than 25% such as no more than 10%, such as no more than 1%.

10. The method, apparatus or composite material of any of claims 1 to 9, wherein the additive is an inorganic material.

11. The method, apparatus or composite material of any of claims 1 to 10, wherein the additive is a metal or an oxide, such as wherein the additive is selected from copper, aluminium oxide, silicon dioxide, and gold.

12. The method, apparatus or composite material of any of claims 1 to 9, wherein the additive is an organic material.

13. The method, apparatus or composite material of any of claims 1 to 9, wherein the additive is a polymer, such as polyethylene or a molecular crystal, such as neopentyl, adamantane or carborane, and derivative thereof.

14. The method, apparatus or composite material of any of claims 1 to 9, wherein the barocaloric material is a material having a hydrogen-bond network.

15. The method, apparatus or composite material of any of claims 1 to 9, wherein the barocaloric material is an alkane, or comprises an alkyl chain, such as where the barocaloric material is a C2-20 alkane or comprises an C2-20 alkyl chain.

16. The method, apparatus or composite material of any of claims 1 to 15, wherein the barocaloric material is a plastic crystal, optionally wherein the plastic crystal comprises an alkyl group or a cycloalkyl group.

17. The method, apparatus or composite material of any of claims 1 to 15, wherein the barocaloric material is a liquid crystal.

18. The method, apparatus or composite material of claims 1 to 17, wherein the barocaloric material is a hybrid organic-inorganic material.

19. The method, apparatus or composite material of any of claims 1 to 18, wherein the composite material has a phase transition at a temperature within the range of 1 K to 450 K, such as 200 K to 450 K, such as 245 to 340 K, such as 245 to 315 K.

20. The method, apparatus or composite material of any of claims 1 to 19, wherein the composite material has a latent heat, |Qo|, at a phase transition that is at least 1 kJ kg-1, such008873440- PCT Specificationas at least 5 kJ kg-1, such as at least 10 kJ kg-1, such as at least 25 kJ kg-1, such as at least 50 kJ kg-1, such as at least 100 kJ kg-1, such as at least 150 kJ kg-1, such as at least 200 kJ kg-1.

21. The method, apparatus or composite material of any of claims 1 to 20, wherein the composite material has an entropy change at a phase transition, |ASo| of at least 10 kJ kg-1, such as at least 50 kJ kg-1, such as at least 100 kJ kg-1, such as at least 150 kJ kg-1, such as at least 250 kJ kg-1, such as at least 300 kJ kg-1, such as at least 400 kJ kg-1.

22. The method of any of claims 1 and 4 to 21, wherein the applied hydrostatic pressure is at most 1.0 GPa, such as at most 500 MPa, such as at most 250 MPa, such as at most 100 MPa.

23. Use of a composite material according to any of claim 3 to 21 as a barocaloric cooling or heating agent.

24. Use of an apparatus according to any of claim 2 and 4 to 21 , for cooling or heating.

25. The use of claim 23 or claim 24, comprising applying hydrostatic pressure to the composite material, wherein the applied hydrostatic pressure is at most 1.0 GPa, such as at most 500 MPa, such as at most 250 MPa, such as at most 100 MPa.008873440- PCT Specification

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