Heat transfer fluids and uses of such fluids

Abstract

The invention relates to heat transfer fluids, in particular in heating and cooling systems. In particular, the present invention relates to the use of surface-functionalized graphene particles in heat transfer fluids. The invention provides a use of surface functionalized graphene particles for improving the thermal performance of a heat transfer fluid, a heat transfer fluid comprising surface functionalized graphene particles dispersed in a base fluid, and a thermal management system comprising the heat transfer fluid, and a method of preparing a heat transfer fluid comprising dispersing surface functionalized graphene particles in a base fluid.

Description

Technical Field

[0001] The present invention relates to heat transfer fluids, and is particularly suitable for improving the efficiency of heating systems such as domestic central heating systems. Background Art

[0002] Central heating systems typically achieve heating by heating a heat transfer fluid (or "thermal fluid") and circulating it to one or more radiators. The heat transfer fluid is mainly water and sometimes contains additives to improve performance. For example, ethylene glycol is added to broaden the liquid temperature range of the fluid and reduce the freezing risk. In addition, inhibitors can be added to prevent mineral deposits (scale) and metal corrosion, and avoid the generation of particulate matter during long-term operation, which may cause system blockage and deposition. Generally speaking, such systems often operate for a long time, experiencing multiple heating and cooling cycles, but rarely or never replace or monitor the heat transfer fluid.

[0003] Heat transfer fluids are also widely used in other fields, including engine radiators, cooling of electronic devices (such as computer processors and solar panels), and cooling of industrial equipment, etc.

[0004] A specific heat transfer fluid disclosed in WO 2014 / 068367 uses a combination of monoethylene glycol, glycerol, and triethanolamine to achieve energy-saving effects.

[0005] In recent years, studies have proposed to improve the performance of heat transfer fluids by adding particulate additives. These particles can improve heat transfer efficiency by enhancing the fluid's heat absorption capacity and reducing heat loss. However, these particles may change the fluid viscosity, thereby increasing the energy required to pump the heat transfer fluid to the heating system, which may offset (at least partially) the energy-saving benefits brought by the improvement in heat transfer efficiency. Therefore, the choice of particulate additives needs to be carefully considered.

[0006] For example, US2011 / 001081 proposes using ceramic nanoparticles to enhance the thermal performance of the base fluid, arguing that it is an improvement compared to the earlier used metal nanoparticles (due to a lower surface oxidation rate and better chemical stability), and only moderately increases the fluid viscosity. Although this document generally refers to ceramic materials, it points out that such materials generally have a low thermal conductivity (see paragraph

[0013] thereof). Therefore, silicon carbide, which has the highest volume thermal conductivity among ceramics, is particularly recommended (see paragraph

[0040] ). However, in actual applications, silicon carbide will deposit on the inner wall of the system metal (see paragraph

[0063] ), which is actually not conducive to heat conduction.

[0007] In addition, WO 2020 / 035705 proposes using boron oxynitride as a filler for the heat transfer fluid.

[0008] In view of environmental, economic, and political factors driving the global reduction in the consumption of heating fuels (especially natural gas), there is an urgent need to develop alternative and improved heat transfer fluids. Summary of the Invention

[0009] The present invention relates to a technical solution for improving the performance of a heat transfer fluid by using surface-functionalized graphene particles. The term "heat transfer fluid" is a professional term in the field, referring to a fluid that serves as a medium in a heat exchange process, achieving cooling, transferring, and storing thermal energy on one side and heating on the other side. In the present invention, the fluid is a liquid and can thus also be referred to as a "heat transfer liquid". This heat transfer fluid can be applied to both heating and cooling systems simultaneously.

[0010] In a first aspect, the present invention provides the use of surface-functionalized graphene particles in improving the thermal performance of a heat transfer fluid.

[0011] Advantageously, graphene particles have excellent thermal conductivity. It has been measured that the in-plane thermal conductivity of graphene is generally in the range of 3000 - 5000 W / mK. In contrast, that of silicon carbide is 120 W / mK (see paragraph

[0040] of US2011 / 001081), and the surface thermal conductivity of boron nitride is 600 W / mK (see Table 1 on page 2 of WO 2020 / 035705). This enables the heat transfer fluid of the present invention to achieve excellent heat transfer performance in a thermal management system.

[0012] Furthermore, through surface-functionalization of graphene, these particles can be more easily dispersed in water during the production process and, more importantly, can maintain a stable dispersed state for a long time. In addition, the inventors of the present invention have found that the deposition amount of surface-functionalized graphene particles on the components of the heating / cooling system is extremely small (even none), which not only avoids material waste but also reduces the risks brought about by unnecessary particle agglomeration and deposition.

[0013] Graphene also has other beneficial properties. For example, the graphene particles described in the present invention have a lower tendency to wear the components of the thermal management system (such as pump impellers). At the same time, surface-functionalized graphene particles (especially oxygen-functionalized graphene particles) also exhibit antibacterial properties, which can effectively inhibit the growth of unnecessary microorganisms in the heat transfer fluid.

[0014] To achieve these effects, it is preferred to have oxygen-containing functional groups or surfactant molecules on the surface of the graphene particles, preferably connected by covalent bonds.

[0015] In a second aspect, the present invention provides the use of a dispersion of surface-functionalized graphene particles in improving the thermal performance of a thermal management system. The thermal management system can be, for example, a central heating system, such as a domestic central heating system.

[0016] In a third aspect, the present invention provides a heat transfer fluid comprising graphene particles dispersed in a base fluid, wherein the graphene particles are covalently bonded with oxygen-containing functional groups on the surface. A suitable surface oxygen content is 1-20 atomic %, most preferably 3-9 atomic %. The oxygen-containing functional groups are preferably one or more of phenolic hydroxyl groups, hydroxyl groups, epoxy groups, and / or carboxylate groups.

[0017] In a fourth aspect, the present invention provides a heat transfer fluid comprising graphene particles dispersed in a base fluid, wherein the graphene particles are bonded (preferably covalently bonded) with surfactants on the surface. The surfactants can be anionic, cationic, or non-ionic surfactants.

[0018] In a fifth aspect, the present invention provides a thermal management system comprising a heat transfer fluid containing surface-functionalized graphene particles dispersed in a base fluid. The thermal management system can adopt a closed-loop operating system, such as a closed-loop system having a heater connected to one or more radiators, wherein the closed-loop system is filled with the heat transfer fluid of the present invention. The thermal management system can be used for heating, or alternatively, the thermal management system can be used for cooling. The thermal management system can be, for example, a central heating system (such as a domestic central heating system). Preferably, the surface-functionalized graphene particles described in the above third or fourth aspect are used.

[0019] Brief Description of the Drawings

[0020] The technical solution of the present invention will be further described in conjunction with the accompanying drawings:

[0021] Figure 1 The figure shows a schematic diagram of oxygen-containing functional groups on the surface of graphene sheets. Detailed Embodiments

[0022] Unless otherwise defined, scientific and technical terms used herein have the meanings commonly understood by those of ordinary skill in the art to which the present invention pertains. Although any methods and materials similar or equivalent to those described herein can be used to implement the tests of the present invention, the preferred materials and methods have been detailed herein. When describing and claiming the present invention, the following defined terms will be used. Unless otherwise specified, the use of expressions such as "a", "an", etc. means one or more.

[0023] Surface-functionalized graphene particles

[0024] The present invention is based on the following discovery: By precisely regulating the surface chemistry of graphene particles to improve their dispersibility (solving the problem of poor water dispersibility of non-functionalized graphene), while maintaining good thermal conductivity, graphene particles can be used to enhance the performance of heat transfer fluids.

[0025] Surface-functionalized graphene particles refer to graphene particles with functional groups introduced on the surface (including the basal plane and edges). Unless otherwise required by the context, "graphene particles" and "surface-functionalized graphene particles" have the same meaning in this specification.

[0026] The graphene particles can be single-layer graphene (a single carbon atom layer) or multi-layer graphene (particles composed of multiple stacked graphene layers). The multi-layer graphene particles can contain an average (mean or median) of 2 - 100 graphene layers / particle. When the graphene particles contain 2 - 5 graphene layers / particle, they can be called "few-layer graphene" particles.

[0027] The number of layers of graphene particles can be determined by directly counting in a transmission electron microscope (TEM) image; or by Raman spectroscopy, by comparing the intensity ratio of the 2D peak to the G peak. These two methods are described in Kumar et al., "Estimation of Number of Graphene Layers Using Different Methods: A Focused Review" (Materials 2021, 14, 4590).

[0028] Preferably, the median number of layers of the surface-functionalized graphene particles is 1 - 10 layers, more preferably 1 - 5 layers. In the most preferred embodiment, the graphene particles are mainly (at least 50%, at least 60%, at least 70%, at least 80%, more preferably at least 90%) single-layer graphene particles.

[0029] The form of the surface-functionalized graphene particles can be sheet-like, flaky, lamellar, and / or ribbon-like structures of multi-layer graphene materials, collectively referred to herein as "graphene nanosheets" (the "nano" prefix characterizes the thickness feature rather than the lateral dimension).

[0030] The surface-functionalized graphene particles can be in the form of flakes with a thickness of less than 100 nm, having a major dimension (length or width) perpendicular to the thickness. The major dimension can be determined by TEM. The flake thickness is preferably less than 20 nm, more preferably less than 10 nm, and even more preferably less than 5 nm (based on the proportion of particles satisfying this property being > 90%, measured using Mastersizer light scattering). The major dimension is preferably at least 10 times the thickness, more preferably at least 100 times, even more preferably at least 1,000 times, and even more preferably at least 10,000 times the thickness. A particularly preferred combination is to use few-layer graphene particles with a major dimension that is at least 500 times the thickness, preferably at least 1,000 times. The aspect ratio can be calculated based on the median thickness of a representative sample of graphene particles measured by TEM and the median length of a representative sample of graphene particles measured by TEM. The advantage of using graphene particles with a high aspect ratio is that they can be added in a lower amount, which means that the thermal conductivity can be significantly improved without causing a significant change in the viscosity of the base fluid.

[0031] The graphene particles have a high aspect ratio, for example, the length is at least 1 time, at least 2 times, at least 3 times, at least 5 times, or at least 10 times the width.

[0032] The d90 of the particles can be 5 μm or less, 4 μm or less, 3 μm or less, 2 μm or less, preferably 1 μm or less, and more preferably 500 nm or less. The d90 is measured by Mastersizer light scattering.

[0033] Optionally, the graphene particles can have a multimodal particle size distribution. This can be achieved by mixing two samples with different particle sizes. For example, the graphene particles have a first peak in the particle size distribution in the range of 0.1 - 1 μm and a second peak at greater than 1 μm (specifically, the first peak is about 0.3 - 0.6 μm and the second peak is about 1.5 - 2 μm), as measured by light scattering.

[0034] Oxygen functionalization treatment

[0035] Preferably, oxygen-containing functional groups are attached to the surface of the surface-functionalized graphene particles (referred to as "oxygen-functionalized graphene particles"). In the most preferred embodiment, the surface functional groups of graphene are phenolic hydroxyl groups, hydroxyl groups, epoxy groups, and / or carboxylate groups. Figure 1 For the schematic diagram showing the graphene sheet with oxygen-containing functional groups, including: epoxy group 1, hydroxyl group 2, carboxyl group 3, and phenoxy group 4.

[0036] Advantageously, the inventors have found that the oxygen functional groups on the surface can significantly improve the dispersibility (both in the initial preparation stage and during long-term use) and effectively reduce the deposition tendency of graphene on the surface of components in the thermal management system. The oxygen functional groups also exhibit antibacterial properties.

[0037] Optionally, the oxygen groups are covalently attached to the graphene surface via an organic linking group (such as a hydrocarbon group). However, the preferred embodiment is that the oxygen-containing functional groups are directly covalently bonded to the surface of the graphene particles. For example, Figure 1 phenolic hydroxyl groups, hydroxyl groups, epoxy groups, and / or carboxylate groups directly bonded to the surface of the graphene particles as shown.

[0038] The surface functional group coverage of the functionalized graphene particles can be obtained by using X-ray photoelectron spectroscopy (XPS) to detect the atomic weight percentage of the added functional groups compared to the unfunctionalized material. The total surface area of the graphene particles is measured by the BET isotherm method (gas adsorption).

[0039] It has been found that too high an oxygen functionalization degree will reduce the thermal conductivity of the graphene particles (the thermal conductivity of graphene oxide is significantly lower than that of graphene). However, if the oxygen functionalization degree is too low, it will lead to poor dispersibility in water, resulting in more / faster precipitation. Therefore, the preferred embodiment is that the oxygen functionalization level of the surface-functionalized graphene particles is 1-20 atomic%, preferably 1.5-15 atomic%, more preferably 2-10 atomic%, most preferably 3-9 atomic% (such as 4-8 atomic%), as determined by XPS. In particular, a level of 3-9 atomic% can achieve a good balance between thermal conductivity and dispersibility. This is especially applicable to the case where the oxygen-containing functional groups are directly covalently bonded to the surface of the graphene particles, for example corresponding to phenolic, hydroxyl, epoxy, and / or carboxylate groups directly bonded to the surface of the graphene particles.

[0040] Some surface groups may not be applicable because they reduce the dispersibility of the surface-functionalized graphene particles in water or have a negative impact on thermal conductivity. Therefore, preferably, the proportion of carbon and oxygen atoms in the oxygen-functionalized graphene particles is at least 95 atomic%, more preferably at least 96 atomic%, more preferably at least 97 atomic%, more preferably at least 98 atomic%, and most preferably at least 99 atomic%.

[0041] Any suitable functionalization method can be used to achieve the target oxygen functionalization. However, the preferred embodiment is to use an oxygen-containing plasma feedstock (such as oxygen (O2) gas) to achieve it by plasma treating a suitable precursor (such as graphite particles), as described below. Importantly, these functional groups are only present on the graphene surface (such as the surface layer of the graphene particles), and not in the bulk of the material. Theoretical speculation (not limited thereto): For multi-layer graphene, the functionalization only occurs within the top 1-2 layers of the surface-functionalized graphene particles.

[0042] Preferably, the surface-functionalized graphene particles are plasma-functionalized graphene particles (i.e., graphene functionalized by the plasma treatment method). The advantage of plasma functionalization is that it can achieve a high degree and uniform functionalization, while restricting or avoiding damage to the graphene sheet structure and the introduction of unwanted impurities.

[0043] The plasma functionalization treatment of graphene particles can be achieved by the methods disclosed in the applicant's early patents (WO2010 / 142953, WO2012 / 076853, WO2022 / 058542, WO2022 / 058546 or WO2022 / 058218). For example, plasma functionalization can be achieved by placing the initial carbon material (such as graphite particles) in a treatment chamber, and the particles to be treated are subjected to plasma treatment and stirring in the treatment chamber, most preferably by glow discharge plasma. The treatment chamber is preferably a rotating container or drum. Preferably, the treatment chamber contains or consists of multiple groups of conductive solid contact bodies or contact layers, and the particles are stirred together with the contact bodies or contact layers and come into contact with the plasma in the contact chamber.

[0044] Preferably, the contact bodies can move freely in the treatment chamber. The treatment chamber can be a drum type, preferably a rotatable drum, so that multiple contact bodies tumble and mix or stir with the particles to be treated. The wall of the treatment container can be conductive and form a counter electrode system with the electrode extending into the internal space of the treatment chamber.

[0045] The plasma treatment can be glow discharge plasma treatment. When using contact bodies or contact layers, the glow discharge plasma preferentially forms on the surface of the contact bodies or contact layers.

[0046] The pressure in the treatment container is usually below 500 Pa. Ideally, a plasma-forming raw material (gas or liquid) is introduced into the treatment chamber during the treatment process, and the gas is discharged through a filter. That is to say, if necessary, it is introduced to maintain the required chemical composition and / or prevent the accumulation of pollutants.

[0047] The obtained graphene material after treatment (i.e., particles or decomposed, aggregated or exfoliated components) can undergo a chemical functionalization reaction with the plasma-forming gas components to form functional groups such as carboxyl, carbonyl, and hydroxyl groups on its surface. Other plasma-forming gases used in the treatment chamber can be or can include, for example, any one of oxygen, water, hydrogen peroxide, alcohol (such as ethanol), and preferably oxygen is used as the gas source to obtain oxygen-functionalized graphene particles.

[0048] The plasma functionalization process can precisely control the degree of functionalization on the surface of graphene, with low energy consumption and no need to use harmful chemical reagents. The plasma functionalization carried out by the above method is also relatively mild and can avoid introducing unnecessary defects in the graphene particles.

[0049] The plasma functionalization process can precisely control the degree of functionalization on the surface of graphene, and at the same time can avoid a large amount of other impurities remaining in the surface-functionalized graphene particles. These impurities include, for example, sulfur, NO x(Nitrogen oxides in various forms, including NO, NO2, N2O) and manganese.

[0050] When using wet chemical methods such as the Hummers method for graphene functionalization, it often leads to graphene being contaminated by impurities, especially sulfur residues. It is very difficult to remove the acidic residues on the surface-functionalized graphene particles, and even if it can be removed, it requires time and a large amount of water, and generates a large amount of acidic waste liquid.

[0051] Preferably, based on the total weight of the surface-functionalized graphene particles, the sulfur (determined by XPS) in the surface-functionalized graphene particles is less than 0.2 wt%, preferably less than 0.15 wt%. The total amount of sulfur impurities is preferably less than 1 wt%, preferably less than 0.5 wt%, and more preferably less than 0.2 wt% (determined by XPS). In contrast, the sulfur content in graphene oxide obtained by graphene functionalization using wet chemical methods such as the Hummers method can be as high as 5 wt%. As is well known, sulfuric acid is harmful to common metal components in heating systems using heat transfer fluids and may cause / corrode.

[0052] The plasma functionalization process can also avoid the unnecessary nitrogen impurities in the form of nitric acid and NO x (Nitrogen oxides in various forms, including NO, NO2, N2O).

[0053] Preferably, based on the total amount of the surface-functionalized graphene particles, the nitric acid content in the surface-functionalized graphene particles is less than 10 ppm, preferably less than 5 ppm, and most preferably less than 1 ppm. The amount of NO x is less than 10 ppm, preferably less than 5 ppm, and most preferably less than 1 ppm. The amounts of these nitric acid and NO x are determined by XPS. Impurities such as sulfur, nitric acid, and NO x are residues in the wet chemical process and are not found in the surface-functionalized graphene particles obtained by plasma treatment.

[0054] Compared with the materials prepared by functionalization using the Hummers method, plasma functionalization also significantly reduces (or eliminates) the manganese pollutant content. Preferably, based on the total amount of the surface-functionalized graphene particles, the manganese content in the surface-functionalized graphene particles is less than 10 ppm, preferably less than 5 ppm, and most preferably less than 1 ppm (determined by XPS). Preferably, based on the total amount of the surface-functionalized graphene particles used for the heat transfer fluid, the total amount of manganese impurities is less than 0.1 wt%, preferably less than 10 ppm, more preferably less than 5 ppm, and most preferably less than 1 ppm (XPS determination).

[0055] Surfactant-based functionalization

[0056] In addition to, or as an alternative to, surface functionalization with oxygen-containing functional groups, graphene particles can also be functionalized with surfactant molecules.

[0057] The surfactant can be selected from anionic, cationic, or non-ionic surfactants.

[0058] Suitable surfactants include, for example, poloxamers (copolymers consisting of a central hydrophobic polyoxypropylene (PPG) chain and two hydrophilic polyoxyethylene (PEG) chains), such as poloxamer 407 (trade name Pluronic TM F-127). Other suitable surfactants include, for example, Rheobyk 7420ES produced by BYK.

[0059] Preferably, the surface of the graphene particles contains surfactant groups covalently bonded. To achieve covalent attachment of the surfactant to the graphene particles, the graphene can be pre-treated to introduce reactive groups, and then the surfactant reacts with these surface groups.

[0060] For example, these reactive groups can be, for example, amino-reactive groups (such as amine or amide groups) or oxy-reactive groups (hydroxyl, carboxyl, or carbonyl groups). These types of reactive groups can be introduced by a plasma functionalization process. Oxy-reactive groups can be introduced by the plasma treatment method in the aforementioned oxygen-functionalized graphene particles. Amino-reactive groups can be introduced by plasma treating the graphene particles using ammonia or nitrogen as the plasma-forming feedstock.

[0061] The surfactant can be directly bonded to the reactive groups. Additionally, a coupling agent can first react with the reactive groups, and then the surfactant reacts with the coupling agent.

[0062] Base fluid

[0063] The suitable base fluid contains water. Optionally, the water is deionized water to reduce the risk of forming mineral deposits in the system.

[0064] Optionally, the base fluid further contains a diol. The diol can be, for example, ethylene glycol or propylene glycol. When present, the diol in the heat transfer fluid is preferably less than 50 vol% (volume percentage based on the total volume of the heat transfer fluid). The amount of the diol can be, for example, 10 to 50 vol%, 20 to 50 vol%, or 20 to 40 vol%.

[0065] Other additives

[0066] The heat transfer fluid can also contain one or more other additives.

[0067] The other additives may be present in relatively small amounts. For example, each other additive may comprise less than 5 wt%, less than 4 wt%, less than 3 wt%, less than 2 wt%, or less than 1 wt% of the heat transfer fluid. The total amount of the other additives may be, for example, less than 5 wt%, less than 4 wt%, less than 3 wt%, less than 2 wt%, or less than 1 wt% of the heat transfer fluid. When present, the other additives may be at least 0.1 wt% or at least 0.5 wt% of the heat transfer fluid.

[0068] The other additives may be corrosion inhibitors. Corrosion (such as the formation of rust) can lead to the accumulation of fouling in the heat transfer system, which may cause blockages and significantly reduce the thermal performance (since the thermal conductivity of corrosion products is much lower than that of surface-functionalized graphene and is generally much lower than the materials used to fabricate the heat transfer system). Over time, corrosion can weaken the heat transfer system to the point of leakage.

[0069] The corrosion inhibitor may be selected from the group consisting of inhibitors for preventing the corrosion of iron, zinc, aluminum, copper, and combinations thereof. The corrosion inhibitor may be, for example, hydrazine, amines (such as hexamine, phenylenediamine, and dimethylethanolamine, and their derivatives), or antioxidants such as sulfites and ascorbic acid.

[0070] The other additives may be stabilizers. The stabilizers are used to reduce the settling of the heat transfer fluid components (including surface-functionalized graphene particles) over time. Thus, the stabilizers may also be referred to as settling inhibitors. The stabilizer may be, for example, a surfactant. In the case where surfactant molecules are included on the surface of the graphene particles, the surfactant used as a stabilizer may be referred to as a "free surfactant" (i.e., not bound to the surface). Preferably, the stabilizer is less than 5 wt%, less than 3 wt%, less than 2 wt%, less than 1 wt%, or preferably less than 0.5 wt% (wt% is defined based on the total weight of the heat transfer fluid). In a preferred embodiment, the heat transfer fluid is substantially free of stabilizers (e.g., the content is less than 0.1 wt% or less than 0.01 wt%).

[0071] pH

[0072] Generally, the settling of particulate components (including surface-functionalized graphene particles) is sensitive to the pH value. Therefore, it is advantageous for the pH value of the heat transfer fluid to be maintained within a range that minimizes settling.

[0073] In fact, the pH value of the heat transfer fluid is typically maintained within the range of 6.5 to 10. When the pH value of the heat transfer fluid is below 6.5, the dispersion stability tends to decrease. On the other hand, when the pH value of the heat transfer fluid is above 10, the composition tends to become too viscous to be used. Therefore, a pH value range of 7.0 to 10.0 is preferred, more preferably 7.0 to 9.0, and most preferably 7.0 to 8.0, which can minimize sedimentation and avoid excessive viscosity.

[0074] If it is necessary to adjust the pH value of the heat transfer fluid to be more alkaline (i.e., higher), an alkali can be added. Suitable alkalis include organic amines or ammonia. The neutralizing agent can be selected from, for example, ammonia, hydroxylamine, dimethylethanolamine (DMAE), ethylamine, diethylamine, and triethylamine.

[0075] Generally, ammonia and DMAE are provided in the form of aqueous solutions with a concentration of 25% to 50% (w / w). Therefore, adding 0.1 wt% of aqueous ammonia (50 wt% concentration) is equivalent to adding 0.05 wt% of anhydrous ammonia. Similarly, adding 0.1 wt% of an aqueous DMAE solution (25 wt% concentration) is equivalent to adding 0.025 wt% of anhydrous DMAE.

[0076] When it is necessary to lower the pH value of the solution, an acid can be used, such as an organic acid or an inorganic acid.

[0077] Component content

[0078] If the content of surface-functionalized graphene particles is too high, it will increase the fluid viscosity, thereby increasing the energy consumption required for the circulation of the heat transfer fluid in the thermal system. Therefore, the content of surface-functionalized graphene particles in the heat transfer fluid is preferably not more than 10 wt%, such as not more than 8 wt%, not more than 5 wt%, or not more than 2 wt%.

[0079] If the content of surface-functionalized graphene particles is too low, the improvement effect of heat transfer performance will be limited. Therefore, the particle content in the heat transfer fluid is at least 0.1 wt%, at least 0.5 wt%, or at least 1 wt%, based on the total weight of the heat transfer composition.

[0080] The preferred content range of surface-functionalized graphene particles in the heat transfer fluid can be, for example, 0.1 - 10 wt%, 0.1 - 8 wt%, 0.1 - 5 wt%, preferably 0.1 - 4 wt%.

[0081] In terms of vol%, the content of surface-functionalized graphene particles in the heat transfer fluid can be, for example, not more than 5 vol%, not more than 4 vol%, not more than 3 vol% or not more than 2 vol%. The lower limit of the content of surface-functionalized graphene particles can be, for example, at least 0.05 vol%, at least 0.1 vol%, at least 0.2 vol% or at least 1 vol%. The range can be, for example, 0.1 - 5 vol%, 0.1 - 4 vol%, 0.1 - 3 vol%, optionally 0.1 - 2 vol% or 0.1 - 0.5 vol%.

[0082] Optionally, the heat transfer fluid can consist essentially of (or entirely of) the surface-functionalized graphene particles and water. In other words, the heat transfer fluid does not include any components other than the surface-functionalized graphene particles and water. For example, the heat transfer fluid can consist of an aqueous solution of 0.1 - 0.5 vol% of surface-functionalized graphene particles.

[0083] In the heat transfer fluid of the present invention, the balance of the remaining components is usually made up to 100% by water.

[0084] The viscosity of the heat transfer fluid can be, for example, not more than 30 mPa s -1 or not more than 20 mPa s -1 . If the viscosity is too high, it will increase the energy consumption required for the system to pump the heat transfer fluid. Preferably, the viscosity value of the heat transfer fluid composition is greater than 0.1 mPa s -1 , or greater than 0.5 mPa s -1 . For example, the viscosity range of the heat transfer fluid can be 0.1 to 20 mPa s -1 , such as 0.5 to 5 mPa s -1 , preferably 0.5 to 1.5 mPa s -1 . Unless otherwise specified, all viscosity values refer to the values measured at 20 °C using a dynamic shear rheometer (such as the Kinexus DSR from Nesus Analytic).

[0085] Concentrated form

[0086] The heat transfer fluid may be provided in concentrated form for subsequent dilution and use in a heating system. When in concentrated form, the content of surface-functionalized graphene particles in the heat transfer fluid may be, for example, at least 5 wt%, at least 10 wt%, at least 15 wt%, at least 20 wt% or at least 25 wt%. The upper limit of the amount of surface-functionalized graphene particles in concentrated form may be, for example, 30 wt% or 40 wt%. Advantageously, increasing the content of surface-functionalized graphene particles in the concentrate can reduce the amount of fluid that must be added to the thermal management system. However, if the content of surface-functionalized graphene particles is too high, the viscosity of the concentrate is too large, and it is not easy to handle and mix when forming the final heat transfer liquid.

[0087] Preferably, the concentrate is formed by dispersing surface-functionalized graphene particles in water (most preferably deionized water), and a stabilizer may be selectively added. The concentrate can be diluted and used with water and other added components (such as glycols, corrosion inhibitors, etc.).

[0088] Use

[0089] The present invention also provides a use of surface-functionalized graphene particles for improving the thermal performance of a heat transfer fluid.

[0090] The present invention also provides a use of a dispersion of surface-functionalized graphene particles for improving the thermal performance of a thermal management system. The thermal management system may be, for example, a central heating system (such as a domestic central heating based on a gas boiler or a heat pump system), an automotive cooling system, a plate heat exchanger system, an electronic cooling system, a refrigeration system or an air conditioning system. Preferably, the thermal management system is a central heating system, such as a gas central heating system.

[0091] Thermal management system

[0092] In another aspect, the present invention provides a thermal management system comprising the heat transfer fluid of the present invention. The system may include a closed-loop circuit having a heater connected to one or more radiators or plate heat exchangers, wherein the closed-loop circuit is filled with the heat transfer fluid described in the first aspect of the present invention. The thermal management system may be, for example, a central heating system, such as a domestic central heating system.

[0093] Preparation method of heat transfer fluid

[0094] The heat transfer fluid can be prepared by forming a dispersion by dispersing graphene particles in a base fluid.

[0095] The method may include subjecting the dispersion to sonication. Preferably, the sonication is relatively high-intensity sonication. This can be achieved, for example, by using a cascaded ultrasonic vibration head (sometimes referred to as a cascatrode). Advantageously, sonication also helps to break the sheets of (turbostratic) surface-functionalized graphene particles into smaller flakes, which helps to reduce their thickness and thus increase the aspect ratio of the particles.

[0096] Optionally, the method may include a separation step to remove any undispersed filler and / or break the filler into smaller parts, for example by centrifugation. Such steps are known in other heat transfer fluids and are generally required to remove inappropriately dispersed filler. However, advantageously, the present inventors have found that the surface-functionalized graphene used in the present invention can be dispersed sufficiently effectively such that a separation step is not required. Thus, the method may not include a separation step (e.g., not include a separation step before / after the sonication step).

[0097] Preferably, the method comprises:

[0098] 1. Treating a graphene precursor (such as graphite particles) with plasma to form oxygen-functionalized graphene particles;

[0099] 2. Dispersing the oxygen-functionalized graphene particles in a base fluid to form a dispersion; and

[0100] 3. Preferably, subjecting the dispersion to sonication.

[0101] Any other additives may be added at any suitable stage, for example added to the base fluid before step 2), added to the dispersion after step 2), or added after step 3).

[0102] For surfactant-functionalized particles, the method may include:

[0103] A. Treating a graphene precursor (such as graphite particles) with plasma to form surface-functionalized graphene particles having reactive groups on their surface;

[0104] B. Reacting the reactive groups of the surface-functionalized graphene particles so as to attach a surfactant to form surfactant-functionalized graphene particles;

[0105] C. Dispersing the surfactant-functionalized graphene particles in a base fluid to form a dispersion; and

[0106] D. Preferably, subjecting the dispersion to sonication.

[0107] The surfactant can directly react with the reactive group. Alternatively, after step (A), the method further includes step (A2): reacting the reactive group with a coupling agent, and then reacting the coupling agent with the surfactant to form surfactant-functionalized graphene particles.

[0108] Any other additives can be added at any suitable stage, for example, added to the base fluid before step (C), added to the dispersion after step (C), or added after step (D).

[0109] Examples

[0110] The thermal conductivity of various heat transfer fluids was tested in the following examples.

[0111] Test devices and conditions

[0112] Use Hot Instrument TPS 3500 to measure the thermal conductivity of the sample, where the sensor switch is a 4-port switch, and the sensor used is 7577F1 Kapton (radius 2 mm). Use Carbolite LHT4 / 30 as the fan furnace for heating the sample.

[0113] Each sample was heated to 80 °C as the target temperature. Five measurements were made on each sample using the isotropic standard module of the TPS instrument. The time for each measurement was 5 seconds, and the heating power during each measurement was 0.04 W. Each sample was ultrasonically treated before measuring its thermal conductivity and re-stirred immediately before the measurement.

[0114] Surface functionalization of the material was carried out using the apparatus and conditions taught in WO2012 / 076853, forming a plasma in a rotatable plasma chamber using an oxygen feedstock, with steel ball bearings present in the chamber. Unless otherwise stated, the surface oxygen content of the functionalized graphene under these conditions is between approximately 4 atomic % and approximately 8 atomic % (i.e., 4 - 8 atomic %). By providing a surface oxygen content of 4 - 8 atomic %, it was found that the samples were generally stable and exhibited ideal thermal conductivity. The surface oxygen content can be measured by, for example, XPS as described herein.

[0115] Test samples

[0116] Table 1 relates to the following materials:

[0117] FLG 5μm: few-layer graphene particles (sheet-like), with an average major dimension of 5μm, functionalized to a surface oxygen content of 4 - 8 atomic % using the above scheme.

[0118] FLG 7μm: Few-layer graphene particles (flake-shaped), with an average major size of 7μm, functionalized to a surface oxygen content of 4 - 8 atomic % using the above scheme.

[0119] POGO: Plasma-oxidized graphene oxide, with an average major size of less than 2μm and a surface oxygen content of approximately 28 atomic %.

[0120] Unless otherwise stated, the base fluid used is deionized water (DI water).

[0121] Results

[0122] The results of the above measurements are shown in Table 1.

[0123] Table 1

[0124] Samples Particles Distribution amount (wt%) Thermal conductivity (W / mK) 1 None 0 0.43 2 FLG 5μm 3.3 0.78 3 FLG 5μm 7.5 0.95 4 FLG 7μm 1.5 0.55 5 POGO 0.5 0.46

[0125] Sample 1 is deionized water without any additives or particles. As shown in the results of Table 1, the thermal conductivity of the heat transfer fluids containing graphene particles with a surface oxygen content of 4 - 8 atomic % (Samples 2, 3, and 4) is significantly improved. The best thermal conductivity is found in the heat transfer fluid containing 7.5 wt% FLG 5μm (4 - 8 atomic % surface oxygen content), but even with the addition of only 3.3 wt% FLG 5μm (4 - 8 atomic % surface oxygen content) or 1.5 wt% FLG 7μm (4 - 8 atomic % surface oxygen content), the thermal conductivity is significantly improved compared to deionized water.

[0126] Therefore, using the graphene material of the present invention can significantly improve the thermal conductivity even at low addition amounts, thus avoiding additional costs and minimizing the flow performance of the heat transfer fluid, relative to deionized water.

[0127] Sample 5 contains functionalized graphene oxide particles with a surface oxygen content of 28 atomic %. When the surface oxygen content of the particles is relatively high, such as this, although the thermal conductivity is still improved to a certain extent compared to water, the degree of improvement is lower than that of particles with a surface oxygen content of 4 - 8 atomic %. Without being bound by theory, we believe that when there is an excessive degree of surface functionalization, the inherent thermal conductivity of the graphene particles will be affected, resulting in limited improvement of the heat transfer fluid.

[0128] During the tests, attempts were also made to use pristine (i.e., non-functionalized) graphene particles dispersed in deionized water as the heat transfer fluid, but a stable dispersion could not be obtained. Without being bound by theory, we believe that the lack of surface functionalization results in ineffective dispersion of the particles. Therefore, the measurement data for this sample are not shown in the results, but its thermal conductivity is actually the same as that of deionized water (Sample 1) because the particles simply settle at the bottom of the container. Such a sample cannot be used as a practical heat transfer fluid because it causes unnecessary agglomeration and deposition in the heating / cooling system.

[0129] Therefore, the above test results clearly demonstrate that improved heat transfer fluids with excellent thermal conductivity can be prepared using surface-functionalized graphene particles, for example, suitable for various heating / cooling systems.

Claims

1. Use of surface-functionalized graphene particles in enhancing the thermal performance of heat transfer fluids.

2. The use according to claim 1, characterized in that, The surface of the surface-functionalized graphene particles is connected with oxygen-containing functional groups.

3. The use according to claim 2, wherein The surface oxygen content is 1 to 20 atomic %.

4. The use according to claim 2, wherein The surface oxygen content is 3 to 9 atomic %.

5. The use according to claim 1, characterized in that, The surface of the surface-functionalized graphene particles is connected with a surfactant.

6. The use according to claim 5, wherein The surfactant is covalently connected to the graphene particles.

7. Use according to any one of the preceding claims, characterized in that, The median number of layers of the surface-functionalized graphene particles is 1 to 5 layers.

8. The use according to any one of the preceding claims, characterized in that, The main dimension of the surface-functionalized graphene particles is at least 500 times its thickness.

9. Use of a surface-functionalized graphene particle dispersion in enhancing the thermal performance of a thermal management system.

10. The use according to claim 9, characterized in that, The thermal management system is a central heating system.

11. A heat transfer fluid comprising graphene particles dispersed in a base fluid, characterized in that, The surface oxygen content of the graphene particles is 3 to 9 atomic %.

12. A heat transfer fluid comprising graphene particles dispersed in a base fluid, characterized in that, The surface of the graphene particles is bound with a surfactant.

13. The heat transfer fluid according to claim 12, characterized in that, The surfactant is covalently bound to the particle surface.

14. The heat transfer fluid according to any one of claims 11 to 13, characterized in that, The base fluid is water.

15. The heat transfer fluid according to any one of claims 11 to 14, characterized in that, The base fluid contains a mixture of water and glycol.

16. The heat transfer fluid according to any one of claims 11 to 15, further comprising a corrosion inhibitor.

17. The heat transfer fluid according to any one of claims 11 to 16, further comprising a stabilizer.

18. The heat transfer fluid according to claim 17, wherein The stabilizer is a surfactant.

19. The heat transfer fluid according to any one of claims 11 to 13, characterized in that, It is composed of the graphene particles and water.

20. A thermal management system comprising the heat transfer fluid according to any one of claims 11 to 19.

21. The thermal management system according to claim 20, characterized in that, The thermal management system is a central heating system.

22. A method for preparing a heat transfer fluid, the heat transfer fluid comprising a dispersion of oxygen-functionalized graphene particles in a base fluid; the method includes:

1. Treating a graphene precursor with plasma to form oxygen-functionalized graphene particles; and 2. Dispersing the oxygen-functionalized graphene particles in the base fluid to form the dispersion.

23. A method for preparing a heat transfer fluid, the heat transfer fluid comprising a dispersion of surfactant-functionalized graphene particles in a base fluid; the method includes: A. Treating a graphene precursor with plasma to form surface-functionalized graphene particles having reactive groups on the surface; B. Reacting the reactive groups of the surface-functionalized graphene particles to connect a surfactant to form surfactant-functionalized graphene particles; C. Dispersing the surfactant-functionalized graphene particles in the base fluid to form a dispersion; and D. Preferably, subjecting the dispersion to ultrasonic treatment.

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