Field

[0001 ] The invention relates to phase change materials containing thermally conductive platelet materials.

Priority

[0002] This application claims priority from Australian Provisional Patent Application Nos. 2015904220 and 2015904217, the entire contents of both of which are incorporated herein by cross-reference.

Background

[0003] Phase change materials (PCMs) are materials that can be used to store and release energy by changing from one phase to another. The most common phase change for this purpose is from solid to liquid (for storage of energy) and liquid to solid (for release of energy). In order to be useful for such purposes, a phase change material should have a high heat of fusion, so that a large amount of energy may be stored and/or released using a relatively small quantity of the material.

[0004] Phase change materials are used in many applications in order to passively manage excessive temperature fluctuations. Typical PCMs have very low thermal conductivities, thereby inhibiting transfer of heat within the PCM . This can lead to "hot spots" in use, i.e. localised areas of disproportionate heating. This leads to inefficiencies in storage and release of energy.

[0005] It is an aim of the present invention to at least partially overcome the above problem

Summary of Invention

[0006] In a first aspect of the invention there is provided a phase change material comprising, optionally consisting essentially of, a thermally conductive platelet material dispersed in a matrix. The matrix may have a melting point between about 0 and about 200°C. It may have a heat of fusion greater than about 150 kJ/kg. It may be a phase change matrix. [0007] The following options may be used in conjunction with the first aspect, either individually or in any suitable combination.

[0008] The conductive platelet material may be graphene or it may be exfoliated boron nitride, or it may be a mixture of these.

[0009] The ratio of matrix to platelet material may be between about 20: 1 and about 1 : 1 on a w/w basis.

[00010] The phase change material may have a heat of fusion greater than about 100 kJ/kg.

[0001 1 ] The matrix may comprise, optionally consist essentially of, a polymeric non-ionic surfactant. The non-ionic surfactant may be an ethylene oxide-propylene oxide copolymer, for example an ethylene oxide-propylene oxide-ethylene oxide triblock copolymer.

[00012] The phase change material may comprise a monomeric surfactant, for example an ionic surfactant. Alternatively it may comprise no monomeric surfactant.

[00013] In one embodiment of the invention, there is provided a phase change material comprising a thermally conductive platelet material dispersed in a matrix which comprises, or consists essentially of, a polymeric non-ionic surfactant having a melting point between about 0 and about 200°C and a heat of fusion greater than about 150 kJ/kg.

[00014] In another embodiment, there is provided a phase change material comprising, optionally consisting essentially of, a thermally conductive platelet material dispersed in a matrix having a melting point between about 0 and about 200°C and a heat of fusion greater than about 150 kJ/kg, and additionally a monomeric surfactant.

[00015] In a second aspect of the invention there is provided a process for preparing a phase change material comprising:

a) exfoliating a thermally conducting laminar material in water in the presence of a surfactant so as to generate a dispersion of a thermally conductive platelet material in water;

b) optionally dispersing or dissolving a phase change matrix in the dispersion; and c) removing substantially all of the water from said dispersion; wherein the ratio of surfactant plus phase change matrix (if present) to conductive platelet material is between about 20: 1 and about 2: 1 on a w/w basis.

[00016] The following options may be used in conjunction with the second aspect, either individually or in any suitable combination.

[00017] The nature of any one or more of the components used in the process may be as described above for the first aspect.

[00018] Step b) may be performed. In this instance, the process comprises:

• exfoliating a thermally conducting laminar material in water in the presence of a surfactant so as to generate a dispersion of a thermally conductive platelet material in water;

• dispersing or dissolving a phase change matrix in the dispersion; and

• removing substantially all of the water from said dispersion;

wherein the ratio of surfactant plus phase change matrix to conductive platelet material is between about 20: 1 and about 2: 1 on a w/w basis.

[00019] The surfactant may be monomelic. In this instance, the process may additionally comprise dialysing the dispersion following step b) so as to remove at least 80% of the surfactant.

[00020] The phase change matrix may be the same as the surfactant of step a), or it may be different. It may be a polyethylene oxide polymer or a polyethylene oxide-polypropylene oxide copolymer or may be a water dispersible or water soluble acrylic polymer or may be a mixture of these.

[00021 ] Step a) may comprise preparing a dispersion of the thermally conducting laminar material in water and sonicating said dispersion whilst adding the surfactant at a rate sufficient to maintain the surface tension of the dispersion in the range of about 35 to about 45mJ.m ^"" .

[00022] Step c) may comprise freeze-drying the dispersion.

[00023] In one aspect of the invention there is provided a process for preparing a phase change material comprising: • exfoliating a thermally conducting laminar material in water in the presence of a surfactant so as to generate a dispersion of a thermally conductive platelet material in water, said surfactant being a polymeric non-ionic surfactant which is a phase change matrix;

• removing substantially all of the water from said dispersion;

wherein the ratio of surfactant to conductive platelet material is between about 20: 1 and about 2: 1 on a w/w basis.

[00024] In another aspect there is provided a process for preparing a phase change material comprising:

• preparing a dispersion of a thermally conducting laminar material in water and sonicating said dispersion whilst adding a surfactant at a rate sufficient to maintain the surface tension of the dispersion in the range of about 35 to about 45mJ.m ^"~ , so as to generate a dispersion of a thermally conductive platelet material in water;

• dispersing or dissolving a phase change matrix in the dispersion; and

• freeze-drying the dispersion so as to remove substantially all of the water from said dispersion;

wherein the ratio of surfactant plus phase change matrix to conductive platelet material is between about 20: 1 and about 2: 1 on a w/w basis.

[00025] The phase change material of the first aspect may be made by the process of the second aspect. The process of the second aspect may be for making the phase change material of the first aspect.

[00026] In a third aspect of the invention there is provided use of a phase change material as defined in the first aspect, or as prepared by the process of the second aspect, as a phase change material.

[00027] In a fourth aspect of the invention there is provided a method for maintaining an object at a substantially constant temperature. This method comprises maintaining said object in thermal contact with a phase change material as defined in the first aspect, or as prepared by the process of the second aspect, the melting point of said phase change materi al being said constant temperature. In this way, an increase in ambient temperature causes the phase change material to at least partially melt and a decrease in ambient temperature causes the phase change material to at least partially solidify. In this context, the melting point "being" said constant temperature should be considered to indicate that there is a difference in temperature between the melting point and the constant temperature of less than about 5°C, or less than about 4, 3, 2 or 1 °C.

Brief Description of the Figures

[00028] Figure 1 : a) Image of aqueous graphene suspension stabilized with F108 surfactant, b) UV-Visible spectrum of a diluted aqueous graphene suspension, c) TEM image of exfoliated graphene sheets (image width 1 μπι). d) Raman spectrum of graphene particles using laser irradiation with a 532 nm laser.

[00029] Figure 2: TGA of as prepared freeze-dried Pluronic® F108-graphene mixture. TGA was conducted under a nitrogen atmosphere using a temperature gradient of 10°C/minute.

[00030] Figure 3: a) Image of Pluronic® F108-graphene PCM material, b) DSC scan of PCM material with 3% w/w graphene loading. DSC was conducted with a N ₂ flow rate of 50 mL/min with a heating rate of 5 °C/min. c) Example LFA (laser flash analysis) data showing the change in temperature with time after the laser flash for the calibration reference, the neat PCM and PCM with 3% w/w graphene.

[00030a] Figure 4: Graph of thermal conductivity of PCMs as a function of graphene

concentration.

Description of Embodiments

[00031 ] In the present specification the following definitions and descriptions apply.

[00032] Phase change material - a material that can be used to absorb heat energy by converting from a first phase (e.g. solid phase) to a second phase (e.g. liquid phase) and release energy by reverting from the second phase to the first phase. Typically phase change materials have a melting point between about 0 and about 200°C. They may have a melting point between about 0 and 150, 0 and 100, 0 and 50, 0 and 20, 0 and 10, 10 and 200, 20 and 200, 50 and 200, 100 and 200, 10 and 100, 20 and 100, 50 and 100, 10 and 50, 10 and 20 or 20 and 50°C, e.g. about 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 1 10, 120, 130, 140, 150, 160, 170, 180, 190 or 200°C. They may have a moderately broad melting range, e.g. about 1 to about 5°C, or about 1 to 3, 3 to 5 or 2 to 4°C. The above melting points may therefore be taken to represent the midpoint of the melting range. The phase change materials typically have high phase change energy. They may have a heat of fusion of at least about l OOkJ/kg, or at least about 1 10, 120, 130, 140, 150, 160, 170, 180, 190, 200 or 250 or 300kJ/kg, or from about 100 to about

250kJ/kg, or from about 100 to 200, 200 to 250 or 150 to 200kJ/kg, e.g. about 100, 150, 200 or 250kJ/kg. They may have a specific heat capacity of at least about lkJ/kg.K, or at least about 1.2, 1.4, 1.6, 1.8, 2, 2.2 or 2.4kj/kg.K or from about 1 to about 3kj/kg. , or from about 1 to 2, 2 to 3 or 1.5 to 2.5kJ kg.K, e.g. about 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1 .7, 1 .8, 1.9, 2, 2.1, 2.2, 2.3, 2.4 or 2.5kj/kg.K.

[00033] Thermally conductive - capable of conducting heat. Typically the thermal conductivity of a thermally conductive material according to the present invention will be at least about 500W/m.K, or at least about 1000, 1500, 2000, 2500 or 3000W/m.K, or from about 500 to about 4000W/m.K, or from about 500 to 3000, 500 to 2000, 1000 to 4000, 2000 to 5000, 1000 to 3000, 1000 to 2000 or 2000 to 3000W/m. , e.g. about 500, 1000, 1500, 2000, 2500, 3000, 3500 or 4000W/m.K. The phase change material of the invention will have considerably lower conductivity. It may have a conductivity of at least about 0.2W/m.K, or at least about 0.25, 0.3, 0.35 or 0.4W/m.K, or about 0.2 to about 0.5W/m.K, or about 0.2 to 0.4, 0.3 to 0.5 or 0.3 to 0.4W/m.K, e.g. about 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5W/m.K. In some instances however it may have a higher thermal conductivity. It should be noted that in the present specification, unless indicated otherwise, reference to "conductivity" should be taken as referring to thermal conductivity.

[00034] Platelet material - a particulate substance composed of platelets of from 1 to 10 molecular layers thick. Examples of platelet materials include graphene (exfoliated graphite) and exfoliated boron nitride. In this context, boron nitride refers specifically to the laminar form of the material (also referred to as "graphitic boron nitride", "a-boron nitride" or "h exagonal boron nitride") or to a platelet material obtained from it. The platelet material may comprise platelets with different numbers of molecular layers. The mean number of molecular layers per platelet may be from 1 to 10, or from 1 to 5, 1 to 3, 2 to 10, 5 to 10 or 2 to 5, e.g. about 1 , 2, 3, 4, 5, 6, 7, 8, 9 or 10. The maximum number of molecular layers per platelet may be from 1 to 10, or from 1 to 5, 1 to 3, 2 to 10, 5 to 10 or 2 to 5, e.g. about 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

[00035] Phase change matrix - a phase change material (as described above) which does not include platelet materials (although it may have platelet materials dispersed therethrough). It may not comprise any particulate materials. Thus a phase change material as defined herein may comprise a phase change matrix (i.e. having no platelet material) or may comprise a phase change matrix with a platelet material dispersed therethrough. The melting points and heats of fusion described above for phase change materials are also suitable for a phase change matrix.

[00036] Exfoliated - an "exfoliated" material as used in this specification refers to a platelet material which is obtainable by exfoliation of a laminar material. Thus graphene may be regarded as exfoliated graphite, regardless of whether it was in fact made from graphite. An exfoliated material is commonly obtained by exfoliation of a laminar material.

[00037] Exfoliate/exfoliating/exfoliation - these terms refer to the process of separating the laminae of a laminar material into an exfoliated material. The process of exfoliation may be complete, i.e. may result in only single molecular thickness exfoliated materials, or may be partially complete, i.e. may result in exfoliated materials having at least some particles with more than 1 molecular layer.

[00038] Laminar material - a material composed of large numbers (typically greater than about

10 ) of substantially parallel layers each of which is substantially planar. The layers are typically held together by van der Waals forces. Typical laminar materials include graphite and graphitic boron nitride.

[00039] Comprise - this and related terms indicate the presence of the specified integer(s) but allow for the possibility of other integers, unspecified. This term does not require that the specified integers are in the majority.

[00040] Consist essentially of - this and related terms indicates that the specified integer(s) represent the majority components and that any other integers present are not intentional.

[00041 ] The inventor has found that the conductivity of some PCMs, such as those based on copolymers of polyethylene oxide and polypropylene oxide, may be been increased through the incorporation of graphene or other conductive platelet materials. In this way, such PCMs may be used with heat exchangers to efficiently transfer waste heat, allowing a reduction in mass of the required PCM for a given energy input. This is of particular relevance for the casings of Li ion batteries where thermal runaway has the potential for catastrophic failure or significantly reduce battery lifetime. In a representative process, graphene was prepared using an aqueous based liquid phase exfoliation procedure with a polymeric surfactant employed to stabilise the particles against re- aggregation, as described in International Patent Application no. PCT/AU2012/000847, published as WO2013/01021 1, the entire contents of which are incorporated herein by cross- reference. The exfoliated graphene was subsequently centrifuged to remove large particles. A pre-concentrating step was undertaking to remove excess water before subsequent freeze-drying. The pre-concentrating step may for example be by evaporation of the water. It may be at reduced pressure and/or at elevated temperature. The reduced pressure may be for example at around 10 to about 200mbar, or about 10 to 100, 10 to 50, 50 to 200, 100 to 200 or 20 to lOOmbar, e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150 or 200mbar. The elevated temperature maybe from about 40 to about 90°C, or from about 40 to 70, 50 to 80, 60 to 90, 50 to 80, 70 to 80 or 75 to 80°C, e.g. about 40, 45, 50, 55, 60, 65, 70, 75, 80, 85 or 90°C. The dried graphene-polymeric surfactant was then melted and cast into shape upon cooling. The thermal properties of the phase change material were found to depend on the properties of the surfactant and the relative rati o of surfactant to graphene. Additional surfactant of the same type may be added to reduce the thermal conductivity if required. Furthermore, other types of surfactants or non-amphiphilic polymers, such as PEO, may be blended in so as to adjust the melting point. It will be understood that the blended surfactants or non-amphiphilic polymer(s) should be miscible with the polymeric surfactant matrix. More generally, the phase change material of the invention may comprise a melting point adjustment additive. This additive should be miscible with the matrix of the phase change material in the proportion in which it is used. It may be used in a ratio of from about 0.1 to about 20% by weight or volume relative to the the matrix, or from about 0.1 to 10, 0.1 to 5, 0.1 to 1 , 1 to 20, 5 to 10, 10 to 20, 1 to 10, 1 to 5 or 5 to 10%, e.g. about 0.1 , 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 , 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 6, 7, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19 or 20%. The proportion will depend in part on the degree of melting point adjustment required. In general addition of a melting point adjustment additive will reduce the melting point of the phase change material, and a higher amount of the additive will result in a larger change in melting point of the phase change material. The nature of the additive may depend on the nature of the matrix, due to the requirement for miscibility as discussed above.

[00042] Polymeric surfactants used in the present invention include polyethylene oxide- polypropylene oxide (PEO-PPO) copolymers. In particular, PEO-PPO-PEO triblock

copolymers, which are marketed under the trade name Pluronic® and are commonly known as "poloxamers", have been found to be particularly suitable due to their excellent stability in water. Other polymeric surfactants that may be used are polyetheramines, for example aminofunctional PEO-PPO copolymers, which are marketed under the trade name Jeffamine®. Additionally, hydrophilic polymers, such as hydrophilic acrylic polymers and hydrophilic polymers of N-vinylpyrrolidone, may be used. Hydrophilic acrylic polymers that may be used include those based at least partially on aciylic acid, methacrylic acid, acrylamide,

methacrylamide, hydroxyalkyl acrylate, hydroxyethyl raethacrylate, N-alkyl acrylamide, N-alkyl methacrylamide and mixtures and copolymers thereof. Hydrophilic polymers of N- vinylpyrrolidone that may be used include polyvinylpyrrolidone and hydrophilic copolymers of N-vinylpyrrolidone. The surfactant may be a copolymer. It may be an ethylene oxide-propylene oxide copolymer. It may have other comonomers or may have no other comonomers. It may be an amine having one or more (optionally 3) ethylene oxide-propylene oxide copolymer substituents on the nitrogen atom. It may be a block copolymer. It may be a triblock copolymer. It may be an ethylene oxide-propylene oxide block copolymer. It may be a poloxamer. It may be an ethylene oxide-propylene oxide-ethylene oxide triblock copolymer. The two ethylene oxide blocks may be the same length or may be different lengths. The proportion of ethylene oxide in the polymer may be about 10 to about 90% by weight or mole, or about 10 to 50, 10 to 30, 50 to 90, 70 to 90, 20 to 80, 20 to 50, 50 to 80, 20 to 40 or 60 to 80%, e.g. about 10, 20, 30, 40, 50, 60, 70, 80 or 90%.

[00043] The surfactant may have an HLB (hydrophilic/lipophilic balance) of greater than about 6, or greater than about 7, 8, 10, 12, 15 or 20, or of about 6, 7, 8, 9, 10, 1 1, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24 or greater than 24. Suitable surfactants which may be used in the present invention include Pluronic® PI 23 (nominally

HO(CH ₂ CH ₂ 0) ₂ o(CH ₂ CH(CH ₃ )0)7o(CH ₂ CH ₂ 0) ₂ oH: HLB about 7), Pluronic® L31 (nominally HO(CH ₂ CH ₂ 0)2(CH ₂ CH(CH ₃ )0)i6(CH ₂ CH ₂ 0) ₂ H: HLB about 1-7), Pluronic® F 127 (nominally HO(CH ₂ CH ₂ 0) ₁ oi(CH ₂ CH(CH ₃ )0) ₅ 6(CH ₂ CH ₂ 0) ₁ ()iH: HLB about 22) and Pluronic® F108 HO(C ₂ H ₄ 0) ₁₄₁ (C ₃ H ₆ 0) ₄₄ (C ₂ H ₄ 0) _]41 H: (nominally HLB >24) and aminofunctional polyethers (for example those sold under the trade name Jeffamine®). In general, surfactants having higher HLB also have higher cloud point. Commonly surfactants with HLB over about 12 have a cloud point over about 100°C. In some embodiments therefore, the HLB of the surfactant may be over 12. The surfactant may have a cloud point over 100°C, or over about 110, 120, 130, 140 or 150°C. In general, a higher HLB is preferable so as to better stabilise the dispersion. The surfactant may be a non-foaming surfactant. [00044] A polymeric surfactant used herein may have a molecular weight (number average or weight average) of about 10 to about lOOOkDa, or about 10 to 500, 10 to 200, 10 to 100, 10 to 50, 10 to 20, 20 to 1000, 50 to 1000, 100 to 1000, 200 to 1000, 500 to 1000, 100 to 500, 500 to 800 or 400 to 800kDa, e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950 or lOOOkDa. In some instances the polymeric surfactant may have a lower molecular weight, e.g. about 2 to about 10 or 2 to 5 or 5 to 1 OkDa, e.g. about 2, 3, 4, 5, 6, 7, 8 or 9 kDa. It may have a narrow molecular weight range or a broad molecular weight range. The ratio Mw/Mn may be greater than about 1.1 , or greater than about 1.2, 1.3, 1.4, 1.5, 2, 3, 4 or 5, or it may be less than about 5, or less than about 4, 3, 2, 1.5 or 1.2. It may for example be about 1.1 , 1.2, 1.3, 1.4, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5. It may have a degree of polymerisation of about 10 to about 1000, or about 10 to 500, 10 to 200, 10 to 100, 10 to 50, 20 to 1000, 50 to 1000, 100 to 1000, 500 to 1000, 20 to 200, 20 to 100 or 100 to 200, e.g. about 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900 or 1000. Mixtures of surfactants may be used. In this case, at least one of the surfactants, optionally all, of the surfactants may be as described above.

[00045] As discussed above, the platelet material may be obtained by exfoliation, in particular in water. This may comprise ultrasonicating a precursor laminar material (e.g. graphite, graphitic boron nitride etc.) in water. This is preferably conducted in the presence of surfactant, in such a way that at all stages of the ultrasonication the concentration of surfactant is sufficient to form a complete monolayer on the various particles (laminar and exfoliated platelet-like particles) in the dispersion. This process is discussed in detail in WO2013/01021 1. In one option, the initial surfactant concentration in the mixture is sufficient that, once the laminar material is exfoliated, it is sufficient to form a monolayer on the exfoliated particles formed during the subsequent ultrasonication. In another option, the initial surfactant concentration in the mixture is sufficient to form a monolayer on the initial laminar particles (or alternatively no surfactant may be present initially) and further surfactant is added, either continuously or batchwise, during the sonication in order to ensure that at all times during the ultrasonication the surfactant level is sufficient to form a complete monolayer on all particles in the dispersion. It should be noted that as exfoliation proceeds, the total surface area increases and therefore more surfactant is required in order to form a monolayer on all particles in the dispersion.

[00046] In the first option, described above, the initial surfactant concentration may be readily determined from the calculated surface area of the exfoli ated platelet-like particles and the known area per molecule of the surfactant. The latter may be obtained from readily available literature sources or may be measured experimentally for example using Langmuir-Blodgett apparatus. In the second option, described above, a suitable method for determining the rate of addition of surfactant is as follows.

1. The surface tension of the liquid phase (water) is measured as a function of concentration of surfactant and the concentration region identified corresponding to the surface tension of between a lower value (C I) and an expected threshold value (C2e, commonly corresponding to surface tension above about 48-50mJ/m ^" ).

2. Surfactant is first added to a dispersion of laminar material to produce a liquid of about concentration C 1.

3. Sonication of the dispersion is commenced and samples are removed at regular time intervals. The surface tension of the liquid phase is determined as a function of time from commencement of sonication.

4. A calibration curve (see for example Fig. 3) is produced form the data obtained in step 3, which shows the surface tension of the solution as a result of surfactant consumed through adsorption to the exfoliated material as a function of time.

5. The time (Tl) at which exfoliation ceases can be determined by observing plateauing of the surface tension/time curve from step 4. The concentration at that time is the threshold value C2.

6. Surfactant is replaced at the minimum rate of consumption. (C 1 -C2)/T 1.

[00047] Commonly the lower value C I is less than about 45 mJ/πΓ, or less than about 44, 43, 42, 41 or 4045 mJ/m ² , or about 35 to about 45 mJ/m ² , or about 38 to 45, 40 to 45, 35 to 43, 35 to 40, 38 to 42 or 40 to 42 mJ/m ² , e.g. about 35, 36, 37, 38, 39, 40, 41, 42, 43, 44 or 45 mJ/m ² . The threshold value (C2e, C2) is commonly above 45 mJ/m ^" , or above 46, 47, 48, 49 or 50, or between about 45 and 55, or about 45 to 50, 50 to 55, 48 to 52 to 47 to 40, e.g. about 45, 46, 47, 48, 49, 50, 51, 52, 53 ,54 or 55 mJ/m ² .

[00048] The ultrasonication may have a power of greater than about l OW, or greater than about 20, 50, 100, 200, 500, 1000, 2000, 3000 or 4000W, or may be about 10 to about 1000W, or about 10 to 500, 10 to 200, 10 to 100, 10 to 50, 50 to 1000, 50 to 100, 100 to 1000, 200 to 1000, 500 to 1000, 1000 to 5000, 1000 to 4000, 200 to 5000, 100 to 500, 300 to 700 or 500 to 800W, e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500 or 5000W. It may have a frequency of greater than about 2kHz, or greater than about 5, 10, 20, 50, 100, 150 or 200kHz, or about 2 to about 200kHz, or about 2 to 100, 2 to 50, 2 to 20, 2 to 10, 10 to 200, 20 to 200, 50 to 200, 100 to 200, 10 to 100, 50 to 100 or 10 to 50kHz, e.g. about 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180 or 200kHz. A suitable ultrasonication condition may be for example about 50-100W at about 10 to 50kHz. The ultrasonication may be continued for sufficient time to achieve the desired degree of exfoliation. A suitable time may be for example at least about 0.5 minutes, or at least about 1 , 2, 5, 10, 15, 20, 30, 40, 50 to 60 minutes, or about 0.5 to about 60 minutes, or about 0.5 to 30, 0.5 to 10, 0.5 to 2, 0.5 to 1 , 1 to 60, 2 to 60, 5 to 60, 10 to 60, 30 to 60, 1 to 30, 1 to 10, 1 to 5, 5 to 30, 10 to 30, 10 to 20 or 5 to 15 minutes, e.g. about 0.5, 1 , 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 or 60 minutes. It may be less than about 30 minutes, or less than about 25, 20 or 15 minutes. In some instances ultrasonication itself may provide the agitation required to prepare a dispersion and no separate agitation may be required.

[00049] The dispersion of the platelet material in water may have a content of platelet material of at least about 0.01%, or of at least about 0.02, 0.05, 0.1 0.2, 0.5, 1, 2, 5 or 10%, or fi-om about 0.01 to about 20%, or about 0.01 to 10, 0.01 to 1, 0.05 to 20, 0.05 to 10, 0.05 to 1, 0.1 to 20, 0.1 to 10, 0.1 to 5, 0.1 to 1 , 0.1 to 1 , 0.1 to 0.5, 0.5 to 20, 1 to 20, 2 to 20, 5 to 20, 10 to 20, 0.5 to 5, 1 to 10, 1 to 5 or 5 to 10%, e.g. about 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 , 0.2, 0.3, 0.4, 0.5, 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1, 12, 13, 14, 15, 16, 17, 18, 19 or 20%.

[00050] There are two different forms of the process for making the phase change material of the present invention. In the first, a monomeric surfactant is used in the exfoliation of a laminar material dispersed in water so as to prepare a dispersion of the corresponding platelet material in water, stabilised by the monomeric surfactant. A phase change matrix is then dispersed in the dispersion so as to provide a dispersion containing stabilised particles of the platelet material and phase change matrix. If desired, the monomeric surfactant may be removed by dialysis. It is thought that in this process, polymeric surfactant replaces the monomeric surfactant on the surface of the platelet material particles so as to maintain their stability. The resulting dispersion (which may or may not contain monomeric surfactant) is then dried, commonly by freeze- drying, so as to afford the phase change material. In a second form, a polymeric surfactant is used both as a stabiliser for the platelet material and as phase change matrix. In this option, the exfoliation of the laminar material in water is conducted using the polymeric surfactant. In this instance, further polymeric surfactant, or a separate phase change matrix material (either surfactant or other) may, if required, be added in order to achieve the desired concentration of platelet material prior to drying as described above. A further form is a blend of the above two. Thus a first dispersion, made as described above using a monomeric surfactant, and a second dispersion, made as described above using a polymeric surfactant, may be blended to achieve a desired concentration of platelet material and the resultant blend dried as described above. It will be understood that in either or both exfoliation steps, the laminar material, and the

corresponding platelet material, may be mixtures of suitable such materials. Also in the process described above in which two separate dispersions are prepared, they may have the same platelet material or may have different platelet materials.

[00051 ] As discussed elsewhere, and as discussed in detail in WO2013/01021 1 , the exfoliation step may be conducted by adding sufficient surfactant (either monomeric or polymeric) to stabilise the ultimate dispersion of platelet materials (in particular to form a monolayer on the surfaces of the exfoliated platelets) or surfactant may be added during the exfoliation, either batchwise or continuously, at such a rate that at each stage there is sufficient surfactant to stabilise the platelet materials present in the dispersion. In either case, once exfoliation has been conducted, it may be useful to remove unexfoliated laminar materials. This may be achieved by settling, centrifuging or other suitable method.

[00052] In an example, Pluronic® F127 surfactant was added continuously to a suspension of graphite in water during sonication in order to keep the surface tension in an optimum range of about 40 to about 42mJ/m ^~ thereby increasing the amount of graphene produced. Typically the ratio of Pluronic® F 127 to single and few layer graphene in the aqueous suspensions is of the order of 4-5: 1 on a weight basis. This ratio is highly dependent on the centrifugation regime that influences the distribution of graphene particle thickness. Lower speeds/shorter times of centrifugation result in many-layered graphene (could include nanographite) thereby reducing the overall ratio of surfactant to graphene. Conversely, high-speed centrifugation for extended periods results in the enrichment of single and few layer graphene leading to increased ratios of surfactant to graphene. For the samples with the greatest level of exfoliation, the ratio of Pluronic® F 127 to graphene can be as high as 12: 1. The rate and time of centrifugation influences the observed particle size distribution. Thus longer centrifugation times and higher rates lead to thinner particles on average however some of the thicker graphene is lost. Accordingly, the specific surface area increases with increasing centrifugation because there are more single layer sheets however this is at the expense of reduced yield of graphene.

[00053] In a related process, monomeric surfactants may also be used in the exfoliation of graphite to graphene. Suitable monomeric surfactants include ionic surfactants, particularly anionic surfactants. Suitable surfactant types include alkyl sulfonates and sulfates, alkylaryl sulfonates and sulfates and quaternary ammonium salts. Typically the ratio of surfactant to graphene with monomeric surfactants such as CTAB (cetyltrimethylammonium bromide), SDS (sodium dodecylsulfonate) or SDBS (sodium dodecylbenzenesulfate) is much lower, approaching 1 :1 on a weight basis. Suspensions of graphene with CTAB may be blended with those stabilised with polymeric surfactants thereby enriching the concentration of graphene relative to the polymeric phase change matrix. If required, the low molecular weight surfactant may then be partially or fully removed via dialysis prior to freeze-drying. The dialysis preferably uses a membrane having a molecular weight cut-off between that of the monomeric surfactant and the polymeric surfactant or phase change matrix. Suitable cut-offs are from about 500 to about 2000 Daltons, or from about 500 to 1000 or 1000 to 2000 Daltons, e.g. about 500, 1000, 1500 or 2000 Daltons, depending on the nature of the monomeric surfactant and the polymeric surfactant or phase change matrix. The dialysi s may remove at least about 70% of the monomeric surfactant, or at least about 80, 90 or 95%, thereof.

[00054] The heat of fusion for the novel PCMs described herein is somewhat reduced from that of the neat surfactant due to the incorporation of an amount of graphene, if normalised by the total mass (kJ/kg). However if the known mass of graphene is taken into consideration, the latent heat of fusion for the surfactant is essentially unchanged by the presence of graphene. Typical values of the heat of fusion for these surfactants, surfactant blends and polymers are in the range of 140-180 kJ/kg. The incorporation of around 10% by mass of graphene results in an almost stoichiometrically reduced heat of fusion as the graphene does not contribute to the transition in any substantial way.

[00055] The Pluronic® F 127-graphene material once completely dried has a melting point at 56-57 °C that is essentially unchanged from that of the neat surfactant. Similarly Pluronic® F108-graphene has a melting point of 58 °C, Pluronic® F68-graphene 52 °C and Pluronic® L64-graphene 16 °C. There is a range of ethylene oxide-propylene oxide copolymer surfactants with varying melting (or pour) points which may be employed in the production process leading to a wide range of available melting points for thermally conducting PCMs. The melting point may be broadened by blending with small amounts of other Pluronic® surfactants, PEG or PPG of varying molecular weight or other non-ionic polymeric surfactants. Alternately, Pluronic® F127-graphene may be added as a minor component to neat surfactants or polymers in order to increase the thennal conductivity of these materials without substantially changing their melting points or heats of fusion.

[00056] The focus in the above discussion has largely been on graphene due to its known high thennal conductivity. The process for exfoliating boron nitride has also been investigated. Boron nitride has a lower thermal conductivity than graphene but much higher than many other materials. Furthermore, boron nitride has very low electrical conductivity and hence may provide suitable properties in certain applications in which elevated thermal conductivity but low electrical conductivity is desired.

[00057] Another method for producing the graphene loaded PCM was also undertaken. In this case, the graphene was produced using the method described by WO2013/01021 1 however using a monomeric surfactant (preferably an anionic surfactant such as SDS or SDBS). Prior to freeze drying a higher molecular weight polymer of for example PEO could be added to the aqueous graphene suspension. This results in a much wider range of potential melting points as this is related to molecular weight. In a similar manner, polyacrylic acid could also be added to the aqueous graphene suspension to produce a phase change material on drying. Solution blending is hence a convenient method for tuning thennal properties of the resultant PCM, in particular, the melting point, density and the heat of fusion. The monomeric surfactant may be dialysed out prior to drying.

[00058] High thermal conductivity phase change materials are particularly appropriate in thermal management applications including: Li Ion Batteries, textiles, energy storage and release, transport of temperature sensitive goods, construction materi als etc .

Examples

[00059] An aqueous solution of Pluronic® F108 (MW 14 kDa, HLB > 24, m.p. 59 °C) was used to produce graphene as a dispersion in water, according to the method of International Patent application WO2013/01021 1. Pluronic® F108 is a polyethylene glycol-polypropylene glycol- polyethylene glycol triblock copolymer in which each polyethylene glycol block is approximately 141 units in length and the polypropylene glycol block is about 44 units in length.

[00060] 20 g of flake graphite with a mean particle size of approximately 2 mm was added to l L water to give a concentration of 2% w/v. To this suspension, 1 g of solid Pluronic® F108 was added. The combined suspension was sonicated continuously using a Qsonica© Q700 sonicator with a solid sonitrode probe at a power of 160W. The suspension chamber was maintained at a temperature of 25 °C throughout using a chilling recirculation unit. After an initial 10 minutes of sonication, an aqueous Pluronic® F108 solution with a concentration of 10% w/w was added dropwise at a rate of 100 mL/hour to the suspension. The sonication was ceased after 10 hours upon which the suspension volume was reduced to 500 mL (a factor of 4) by heating at 80 °C. Once reduced, the suspension was cooled and transferred to centrifuge tubes. Any large and non-exfoliated particles were sedimented by centrifugation at 3000 rpm for 15 minutes. The supernatant was subsequently collected and is shown in Figure 1 a. The suspension concentration and graphene particle properties were hence characterized prior to use.

[00061 ] The concentration of graphene in the suspension was detennined to be 10.4 mg/mL by UV-Vis spectrophotometry at a wavelength of 660 nm using an extinction coefficient of 2460 L g ^"1 m ^"1 . The UV-Vis spectrum of a diluted sample of graphene in water is shown in Figure lb. This spectrum shows broad absorption of light across the wavelengths measured as well as a peak at 269 nm indicative of the absence of any significant basal plane defects.

[00062] Figure lc shows an example transmission electron microscopy (TEM) image of a graphene sheet produced using the described method. A drop of graphene suspension was placed on a holey C TEM grid and left to dry for 24 hours. The graphene sample was imaged using a JEOL 21 OOF transmission electron microscope. The lateral particle size of the particles typically ranged between 0.1 μιη and 2 μιη. The TEM image in Figure lb also shows that the particles were well exfoliated and consisted of predominantly single and few layers.

[00063] The graphene sheets were also characterized using Raman spectroscopy. Graphene suspension was deposited on an oxidized silicon wafer and allowed to dry in ambient laboratory conditions. Figure Id shows a Raman spectrum of graphene produced using the method described measured using an incident laser with a wavelength of 532 nm. Three peaks are observable at 1350 cm ^"1 , 1580 cm ^"1 and 2685 cm ^"1 corresponding to the D, G and 2D peaks commonly reported for graphene. The D peak relates to edge defects while the 2D peak indicates the number of graphene layers. It is clear that the shift in frequency of the 2D peak as well as the shape is significantly different to that of natural graphite and that the sample consists of single or few layer material. It should be noted that the laser spot size spans more than one particle.

[00064] After characterization, the highly concentrated aqueous graphene suspension was frozen at -80 °C and then transferred to a freeze drying apparatus. The pressure inside was reduced to < 1 millibar and the temperature set to -52 °C and the suspension freeze dried over a period of at least 48 hours to remove all water, leaving a solid mixture of graphene and

Pluronic® F108 surfactant, which is useful as a PCM. The ratio of surfactant to graphene was determined using thermogravimetric analysis (TGA) to be approximately 6: 1 as shown in Figure 2.

[00065] Aside from this, additional Pluronic® F108 surfactant was added to the suspension in order to demonstrate that the improved thermal properties of the surfactant with a much lower concentration of graphene. The example used here yielded a final ratio of 30: 1 (graphene concentration of 3% w/w). The suspension with additional surfactant was similarly freeze dried to remove water giving the phase change material as shown in Figure 3a.

[00066] Differential scanning caloritnetry (DSC) was used to determine the melting point and the latent heat of fusion for the PCM. Figure 3b shows the heat flow curve as a function of temperature indicating that the melting transition begins at about 53 °C with a defined melting point of 59 °C similar to the neat Pluronic® F108 surfactant. The area under the heat flow curve was used to determine the heat of fusion of 155 kg/kJ. This is almost a stoichiometric reduction from the neat surfactant of 165 kg/kJ due to the incorporation of 3% w/w graphene.

[00067] The density of the PCM with and without the addition of 3% w/w graphene was measured using a MicroMeritics® He Pycnometer. The densi ty of the PC M with graphene increased to 1.278 kg/nr as compared to neat Pluronic® F 108 of 1.23 kg/nr .

[00068] A Linseis® LFA1000 Laser Flash Analyser (LFA) was used to determine the thermal conductivity of the PCM with and without added graphene according to the ASTM E1416-13 method. The PCM was pressed into a 13 mm diameter pellet with a thickness of 250 μιη at a pressure of 10 t for 10 minutes prior to mounting in the graphite cell. Alternatively, the PCM was melted in a mould of similar dimensions followed by cooling and removal. Figure 3c shows a typical data set for the LFA experiments performed at 25 °C. The sample is irradiated with a laser flash (Nd:YAG laser with wavelength of 1064 nm and power output of 25 J/pulse) at time 0 ms. The heat from the laser pulse is transferred through the sample and the overall temperature increase measured. The rate at which the heat transfers is directly related to the thermal conductivity if the density of the material and the specific heat capacity is known. The specific heat capacity can also be determined from LFA experiments by comparison with a graphite reference. Figure 3c shows that the temperature increases more rapidly with time for the PCM with 3% w/w added graphene. The thermal properties of the graphene- Pluronic® F108 PCM are summarized in Table 1. Hence for a very low loading of graphene a greater than 250% improvement in the thermal conductivity was observed.

Latent heat Thermal Specific heat

Melting Density

Sample of fusion Conductivity capacity

point (°C) (kg/m ³ )

(kJ/kg) (W/m.K) (kJ/kg.K)

PCM

without 59 165 1.2283 0.171656434 2.78 graphene

PCM with

3% 59 155.1 1.2778 0.417885071 2.3755 graphene

Table 1. Critical Parameters for Technical Data Packages of PCMs

Experimental Details for Graphene Phase Change Materials (PCM): Method 2

[00069] Graphene was exfoliated from graphite using the method described in

WO2013/01021 1. 10 g of ftake graphite with a particle size of approximately 2 mm was added to 0.5 L of water to give a concentration of 2% w/v. To this suspension, 1 g of the solid anionic surfactant sodium dodecylsulfate (SDS) was added to give a surfactant concentration of 7 mM corresponding to a surface tension of 42 mNm ^"1 . The combined suspension was sonicated continuously using a Qsonica® Q700 sonicator with a solid sonitrode probe at a power of 160W. The suspension chamber was maintained at a temperature of 25 °C throughout using a chilling recirculation unit. After an initial 10 minutes of sonication, an aqueous SDS solution with a concentration of 100 g/L was added dropwise at a rate of 200 mL/hour for 2.5 hours to the suspension.

[00070] The suspension was partially dialysed against water to remove some SDS using a cellulose ester dialysis membrane with a 12kDa cut-off for about 6 hours. The volume of the suspension was reduced from 1L to 0.2L through evaporation and then centrifuged at 2500 rpm for 10 minutes. If no SDS was removed prior to concentration, the suspension could irreversibly aggregate due to the high ionic strength of the aqueous phase. The supernatant was removed and further dialysed. The water was exchanged every 12 hours for 2 days with the conductivity of the dialysate monitored. Once the conductivity of the aqueous suspension of exfoliated graphene reached that of pure water, the suspension was removed from the dialysis tubing.

[00071 ] The graphene particles produced using this method were characterized using standard techniques such as UV-Vis spectrometry, TEM imaging and Raman spectroscopy. The graphene concentration in the suspension was determined to be 2.25 mg/mL. The zeta potential of the particulate suspension was also measured before, during and after the dialysis. The zeta potential of the particles of the as prepared suspension directly after sonication was -65 mV but became less negative as SDS was removed from the suspension to be -28 mV for the fully dialysed sample, high enough for at least stability of the suspension for 6-12 hours. This is significant as using a cationic surfactant results in a transition from positive to negative potential and hence the very low net charge from the edge defects of the graphene combined with that of the surfactant causes aggregation of the particles prior to all surfactant removal.

[00072] Pluronic® F108 surfactant was then added to the dialysed graphene suspension. The ratio of Pluronic® F 108 to graphene could easily be selected but typically was of the order of 10: 1 to 40: 1 , although much lower ratios are practicable. For example, in 100 mL of graphene suspension, 22.5 mg of Pluronic® F108 was added to give a ratio of 10: 1. The suspension was then frozen at -80 °C and transferred to a freeze drier. The pressure was set to < 1 mbar and the temperature maintained at -52 °C for 48 hours or until all of the water was removed.

[00073] The freeze dried F108-graphene mixture could then be melted in a mould to the desired shape for subsequent characterization of the thermal properties. Graphene with Pluronic® F68 (MW 8.4 kDa, HLB > 24, m. p. 52°C)

[00074] Graphene was exfoliated from graphite using the method described in

WO2013/01021 1. 20 g of flake graphite with a particle size of approximately 2 mm was added to 1L water to give a concentration of 2% w/w. To this suspension, 1 g of solid Pluronic® F68 was added. The combined suspension was sonicated continuously using a Qsonica® Q700 sonicator with a solid sonitrode probe at a power of 160W. The suspension chamber was maintained at a temperature of 25 °C throughout using a chilling recirculation unit. After an initial 10 minutes of sonication, an aqueous Pluronic® F68 solution with a concentration of 10% w/w was added dropwise at a rate of 100 mL/hour to the suspension. The sonication was ceased after 10 hours upon which the suspension volume was reduced to 500 mL (a factor of 4) through heating at 80 °C. Once reduced, the suspension was cooled and transferred to centrifuge tubes. Any large and non-exfoliated particles were sedimented through centrifugation at 3000 rpm for 15 minutes. The supernatant was subsequently collected. The suspension concentration and graphene particle properties were hence characterized prior to use. The concentration of graphene in the suspension was determined to be 1.1 mg/mL by UV-Vis spectrophotometry at a wavelength of 660 nm using an extinction coefficient of 2460 L g-1 m-1. The graphene suspension was frozen at -80 °C and then transferred to a freeze drying apparatus. The pressure inside was reduced to < 1 millibar and the temperature set to -52 °C and the suspension dried (sublimation from ice to vapour) over a period of at least 48 hours to remove all water leaving a solid mixture of graphene and Pluronic® F68 surfactant. The ratio of surfactant to graphene was determined 1.5:98.5. The melting point of the graphene - Pluronic® F68 composite was 52 °C and heat capacity of 3.2 kJ/kg.K were determined from DSC. The density was 1255 kg/m3 determined using He picnometry. The thermal conductivity was calculated from the density, heat capacity and thermal diffusivity for the 1.5% w/w graphene in Pluronic® F68 PCM to be 0.651 1 W/m.K. This is somewhat higher than the equivalent mass loading of graphene for F108 reflecting the shorter chains of the surfactant allowing closer contact between sheets.

Graphene - Pluronic® F108 varying concentration

[00075] The thermal conductivity of the PCMs prepared using Pluronic® F108 was determined as a function of graphene concentration as shown in Figure 4.

Graphene - Pluronic® F108 varying centrifugation rate [00076] The same suspension as previously described in the first example described above was subjected to varying centrifugation rate to probe the influence of the level of exfoliation on the observed thermal conductivity of the PCM. Under the conditions already described, there is already a high proportion of single and few layer graphene however this can be improved by centrifuging at higher rates. At 4000 rpm for 15 minutes, the observed thermal conductivity was slightly enhanced to 0.356 W/m.K in comparison to 0.32 W/m. at the equivalent concentration of graphene of 4% w/w.