Dielectric cooling fluid including nanoparticles

Abstract

A hybrid nanoparticle fluid having an hydrocarbon base liquid, a plurality of different types of nanoparticles and at least one dispersant to disperse the nanoparticles within the hydrocarbon base liquid. The present dielectric fluid is adapted for use as a dielectric immersion cooling liquid to transfer heat with energised electronic components such as GPUs, CPUs, motherboards etc.

Description

[0001] Dielectric Cooling Fluid Including Nanoparticles

[0002] Field of invention

[0003] The present disclosure relates to coolant fluid formulations and in particular, although not exclusively, to dielectric high-thermal-conductivity fluids for use in immersion cooling systems for electronics.

[0004] Background

[0005] Cooling electronics such as servers, motherboard, GPUs etc., has become a major technical challenge due to the need and rapid development of higher density, higher power capacity, faster and smaller components.

[0006] Datacenters typically contain thousands of computing / electronic devices and generate large amounts of heat. As the need for access to greater processing and digital storage continues to expand, the density of server systems continues to increase, alongside the resulting thermal challenges. Conventional fan-based cooling systems require large amounts of power and the cost of power required to drive such systems also increases with the increasing server densities.

[0007] Immersion cooling of IT hardware represents a suitably effective and efficient alternative to air cooled fan systems. Electrically insulating dielectric fluids may be divided into two main categories within liquid immersion technologies according to their chemical nature: i) hydrocarbon based (i.e. mineral, synthetic, or vegetable oils) and ii) fluorocarbon based. The use of dielectric fluids for cooling electronic devices is commonly known as immersion cooling, which in turn, can be divided into single-phase immersion cooling, and two-phase immersion cooling. Single-phase immersion cooling uses a fluid that is heated and cooled without relying on an evaporation process. When the cooling process relies on a phase change (liquid to vapor and vice versa), it is referred to as two-phase immersion cooling.

[0008] The two primary requirements of the dielectric fluid include film conductivity and very low electrical conductivity. It is known to enhance the thermal conductivity of fluids by incorporating nanoparticles. For example, US 2002 / 100578 Al describes nanocarbon materials for enhancing the thermal transfer properties of fluids. US 9,051,502 B2, describes nanofluids for use in cooling electronics in which nanoparticles are dispersed in a base liquid using a chemical dispersant.

[0009] However, optimising such fluids is problematic due to the additional related physical, electrical and chemical performance demands placed on these immersion cooling fluids. In particular, such fluids are typically required to be highly stable, to include sufficiently high breakdown voltages and be homogeneous with minimised sedimentation. Accordingly, there remains a need for improved dielectric fluids exhibiting enhanced thermal conductivity and maximised electrically insulating characteristics.

[0010] Summary of the Invention

[0011] One objective of the present concept is to provide a stable dielectric fluid suitable for immersion cooling of electronic / IT equipment and components offering enhanced thermal conductivity characteristics whilst being electrically insulating. It is a further objective to provide a thermally conductive dielectric nanofluid effective for the removal of heat energy from heat generating components (typically associated with operational IT hardware) with the fluid being a good electrical insulant to provide safe direct liquid-to- solid contact with energised electronic components.

[0012] The objectives are achieved via a hybrid nanofluid suspension in which nanoscale particles are suspended in a dielectric liquid. A hybrid immersion cooling fluid is provided that includes at least two types of suspended particles, with each particle optimised for selected characteristics such as enhanced thermal conductivity, liquid-solid stability, density, electrical conductivity and chemical reactivity.

[0013] Accordingly, the present inventors provide a hybrid dielectric fluid that includes at least a first and second types of small / fme particles suitable for suspending within a hydrocarbon base liquid, wherein at least one type of the particles has high thermal conductivity relative to at least a second type of the particles that are included in turn within the fluid to enhance the fluid’s electrical insulating characteristics. In combination, the different types of the particles form as a suspension with the base liquid to provide a hybrid nanoparticle liquid suspension with enhanced thermal conductivity (for effective heat removal from energised electronic components) and enhanced electrical insulating characteristics to maximise the breakdown voltage.

[0014] According to a first aspect of the present concept there is provided a dielectric fluid comprising: a hydrocarbon base liquid; a plurality of different types of nanoparticles; at least one dispersant to disperse the nanoparticles within the olefin base liquid; wherein the nanoparticles comprise a first type of nanoparticles having a first thermal conductivity and a first electrical insulating characteristic and a second type of nanoparticles having a second thermal conductivity greater than that of the first type of nanoparticles.

[0015] Reference within this specification to 'nanoparticles ’ encompasses particles having a particle size, typically a particle diameter, of the order of 0.1 to 1000 nanometres (nm). This terms also encompasses an average particle size, in the nanometre range. Reference within this specification to ‘an average particle size ’ encompasses mean, median and mode averages as will be appreciated by the skilled person. It is preferred the reference to average particle size refers to the mean average. Particle size and / or distribution may be determined using a nanoparticle analyser and / or a centrifugal nanoparticle analyser such as the ViewSizer 3000, the nanoPartica SZ-100V2 series or the Partica CENTRIFUGE available from Horiba Scientific. The particle sizes and particle size distributions referred to herein may be obtained / calculated by dynamic light scattering (DLS) and / or nanomaterial characterisation according to ISO DIS 19430 (en).

[0016] Reference within this specification to ‘thermal conductivity ’ of the nanoparticles and / or the fluid will be known to the skilled person and encompasses thermal conductivity techniques such as Transient Hot Wire (THW) technique in which the transient temperature rise of a thin vertical platinum wire is recorded when a step voltage is applied to it. Thermal qualities of the samples may be determined using a measured increase in temperature over time. Other suitable techniques include laser flash conductivity measurements typically undertaken using an Xenon flash lamp to heat a sample at one end via bursts of predetermined energy. Temperature increase is measured at the opposite end of the sample using an infrared detector. Further methods of measuring thermal conductivity include the transient hot wire method using thermal conductivity standard ASTM D7896-19.

[0017] Reference within this specification to electrical conductivity of the present liquid / hybrid fluid include available electrochemistry techniques such as those according to any one of standards ASTM D-1816, IEC 61620, IEC 60156, IEC 60247 and ASTM D 1169.

[0018] Reference within this specification to a ‘ratio ’ of the different types of suspended particle relative to one another, is based on a weight of each of the different types of particles, being the weight / mass of a first type of particles (the first part) relative to the weight / mass of a second type of particles (the second part) added to the present base liquid to form the hybrid fluid.

[0019] Reference within this specification to a ‘dispersant’ encompasses a compound / material configured to disperse another substance or material in a liquid and specially to reduce adhesion between the particles suspended within the base liquid to prevent flocculation or agglomeration.

[0020] Reference within this specification to a "density ’ of a material includes the density calculated and / or determined by international standard ISO 3369:2006. Reference within this specification to "density ’ of the base liquid may be determined using conventional techniques for density determination of esters and other ester-based materials. Such standard testing is undertaken at room temperature, 20°C or 25°C. Density may be obtained by reference to certified reference material such as international standard ISO 17034.

[0021] Preferably, the second type of nanoparticles comprise a second electrical insulating characteristic less than the first type of nanoparticles.

[0022] Optionally, the nanoparticles may comprise any one or a combination of the group of oxides, nitrides, carbides, a metal. Optionally, the oxides may comprise any one or a combination of aluminium oxide, silicon dioxide, quartz, titanium dioxide, iron oxide, copper oxide, zirconium dioxide, zinc oxide. Optionally, the nitrides may comprise any one or a combination of silicon nitride, aluminium nitride, boron nitride, titanium nitride. Optionally, the carbides may comprise any one or a combination of silicon carbide, titanium carbide, tungsten carbide, boron carbide. Optionally, the metal may comprise any one or a combination of silver, gold, copper, platinum, tin.

[0023] Optionally, the first type of nanoparticles may comprise an electrical conductivity of less than IxlO'5S / m, IxlO'7S / m, IxlO'9S / m, IxlO'11S / m, electrical conductivity in the range IxlO'13S / m to IxlO'5S / m. Optionally, the second type of nanoparticles may comprise a thermal conductivity of greater than 100 W / mK, 150 W / mK, 200 W / mK, 250 W / mK, 300 W / mK, a thermal conductivity in the range 100 to 450 W / mK.

[0024] Optionally, a (mean) average particle diameter of the second type of nanoparticles is in a range 80 to 120% of an average (mean) particle diameter of the first type of nanoparticle. Optionally, the average particle diameter of the first and second types of particles are approximately the same + / - 1 to 20%, 1 to 10%, 1 to 5%. Optionally, an average particle diameter of first and second types of nanoparticles is in a range 0.5 to 200 nm, 0.5 to 100 nm or 0.5 to 50 nm as determined by dynamic light scattering. The diameter and / or particle size may be determined using a dynamic image analyser in which large numbers of particles are analysed using a camera system in real-time according to established dynamic image analysis (DIA) techniques.

[0025] Optionally, the first and second types of nanoparticles are included at a ratio in a range 1 :5 to 5: 1; 1 :4 to 4: 1; 1 :3 to 3:1; 1 :2 to 2: 1 or 0.5: 1 to 1 :0.5.

[0026] Optionally, the dispersant may comprise any one or a combination of: a surfactant; at least a partially hydrophobic non-ionic surfactant; a fatty acid, optionally an oleic acid; an ester; a sorbitan, optionally sorbitan monolaurate, sorbitan monooleate, sorbitan trioleate; a polyester, polyethylene glycol, polyethylene glycol oleyl ether. Optionally, the dispersant may comprise hydrophobic and hydrophilic groups. Optionally, the dispersant may be lipophilic. Optionally, the dispersant may comprise a hydrophilic-lipophilic balance of less than 10.

[0027] The dispersant is selected to provide stable nanofluids that retain their thermal conductivity and electrically insulating characteristics for extended periods. The dispersant specifically prevents particle agglomeration and / or sedimentation (separation of the particles from the liquid suspension). Optionally, the dispersant is included at not more than 2 wt% or in a range 0.5 to 2 wt% based on a total wt% of the fluid. Optionally, the dispersant may comprise at least a first type and a second type included in the dielectric fluid in combination.

[0028] Optionally, the hydrocarbon base liquid comprises any one or a combination of: a hydrocarbon; an olefin, a polyalphaolefin (PAO), a PAO based dimer, a PAO based trimer, a PAO based tetramer, a C5-C20 olefin, a hydrogenated 1 -decene dimer. Preferably, the base liquid is single phase liquid. Optionally, the base liquid is a Fischer-Tropsch derived hydrocarbon base oil. Optionally, an average density of the first and / or second types nanoparticles is in a range less than 15, less than 10, less than 8, less than 6 or less than 4 g / cm3.

[0029] Optionally, the first and / or the second types of nanoparticles comprise any one or a combination of: SiC, TiC, AIN, Si3N4, SiO2, AI2O3, CuO, Ag, TiO2, BN, B4C, TiN. Optionally, the first type of nanoparticles comprise any one of SiC, BN or TiC and the second type of nanoparticles comprise any one of Si3N4, B4C or TiN. Optionally, the nanoparticles are included in combination at a concentration in a range less than 10 wt%, less than 8 wt%, 0.5 to 7 wt%, 0.1 to 6 wt% based on a total wt% of the fluid. Optionally, the first type of nanoparticles comprise SiC and the second type of nanoparticles comprise Si3N4, at a ratio in a range 1 : 1 to 1 :3. Optionally, the dispersant comprises a density in the range 0.4 to 1.4 g / cm3, 0.6 to 1.2 g / cm3or 0.8 to 1.2 g / cm3.

[0030] According to a further aspect of the present concept there is provided a method of making a dielectric fluid comprising: providing an hydrocarbon base liquid; dissolving at least one dispersant in the hydrocarbon base liquid to form a liquid mixture; adding a plurality of different types of nanoparticles to the liquid mixture, the nanoparticles comprising a first type of nanoparticles having a first thermal conductivity and a first electrical insulating characteristic and a second type of nanoparticles having a second thermal conductivity greater than that of the first type of nanoparticles; and suspending the nanoparticles within the liquid mixture to create a nanoparticle-liquid suspension.

[0031] Optionally, the step of suspending the nanoparticles within the liquid mixture comprises sonicating the nanoparticle-liquid suspension. Optionally, the step of sonicating the nanoparticle-liquid suspension comprises applying sound energy at a frequency in the range 15 to 35 kHz, 20 to 30 kHz or 22 to 26 kHz. Optionally, the step of sonicating the nanoparticle-liquid suspension comprises applying sound energy at vibrational amplitudes between 45 and 95 pm. Optionally, the step of sonicating the nanoparticle-liquid suspension comprises exposing the mixture to ultrasound energy at a power in the range 50 to 200W, 80 to 160W or 100 to 140W. Optionally, the step of sonicating the nanoparticleliquid suspension comprises applying continuously the sound energy for 5 to 10 minutes. Detailed description of preferred embodiment of the invention

[0032] The inventors have identified that the combination of electrically insulating nanoparticles and high thermal conductive nanoparticles provide a fluid with better heat transfer properties than formulations with just one type of particles. The present dielectric cooling fluid is suitable for use with immersion cooling in which IT components and / or electronics may be submerged directly in the thermally conducting dielectric fluid. The present fluid is effective for removal of heat energy from the electronic components via a circulation of the fluid through a suitable heat exchange unit as described in EP 3941171 Al the details of which are incorporated herein by reference. The present fluids are configured specifically for enhanced thermal conductivity whilst also comprising very good to excellent electrically insulating characteristics so at to ensure the present fluids are suitable for direct contact with energized electronic components. The present fluids comprise a base liquid, a dispersant and a nanoparticle mixture comprising at least two different types of nanoparticles having characteristics to enable them to be suspended within the base liquid to form a hybrid nanoparticle-fluid suspension.

[0033] The formulation

[0034] The base liquid is preferably a low viscosity base liquid being a hydrocarbon and optionally an olefin such as a polyalphaolefin (PAO). Hydrogenated 1 -decene dimers are suitable candidate liquids. Such liquids with a kinematic viscosity of 5.1 cSt at 40 °C, density of 0.79 g / L and a flash point greater than 159°C may be used. Other suitable base fluids include the ‘Immersion Cooling Fluid S5 X’ range available from Shell™.

[0035] The present fluid comprises a dispersant that stabilizes the nanoparticles and avoids agglomeration and sedimentation. Preferred examples of suitable dispersants include surfactants that contain both hydrophobic and hydrophilic groups. An example surfactant is the hydrophobic non-ionic surfactant Span™ 85 (sorbitan trioleate) being a triester of oleic acid and hexitol anhydrides derived from sorbitol. Such a surfactant may have a hydrophilic-lithophilic balance (HLB) number of 1.8, which has been found to enhance stabilization of the hydrophobic mixtures. Mixture homogenization of the base liquid and dispersant may be provided by magnetic stirring at 500 rpm for 5 minutes. The present nanofluid comprises at least two types of nanoparticles, with at least a first type of particles having enhanced thermal conductivity relative to a second type, with the second type comprising good to excellent electrical insulation characteristics. Dispersion of the thermally conductive nanoparticle(s) and the insulating nanoparticles may be achieved by ultrasonication. Such ultrasonication may be undertaken at a fixed frequency of 24 kHz and vibrational amplitudes between 45 and 95 micrometers for 5 to 10 minutes in continuous mode.

[0036] Suitable thermally conductive materials include carbides, nitrides, and oxides of silicon, titanium, aluminum, and boron. These materials are resistant to decomposition induced by heat, pressure, or chemical attacks, and retain strength and form at high temperatures. More specific suitable thermally conductive materials include silicon carbide, titanium carbide, boron carbide, silicon nitride, titanium nitride, boron nitride, titanium dioxide, silicon dioxide, alumina, aluminum nitride, and combinations thereof. The nanoparticle diameter is preferably not be greater than 100 nm, and more preferably is less than 50 nm.

[0037] The present fluid may comprise an antioxidant to enhance longevity with exposure to UV or other energy sources or chemical exposure that may otherwise induce oxidation of the fluid. Antioxidants inhibit oxidation by reacting with oxygen that would otherwise degrade the base oil. Preferably, the present fluid composition comprises a blend of antioxidants. Optionally, the fluid composition comprises at least two, three or four antioxidants that differ in a chemical composition and optionally concentration within the dielectric fluid composition.

[0038] Optionally, the at least one antioxidant comprises at least one phenolic antioxidants and at least one aminic antioxidants. Optionally, the at least one antioxidant comprises phenolic antioxidants and aminic antioxidants. Examples of suitable phenolic antioxidants include butylated hydroxytoluene (BHT), high molecular weight phenolic antioxidants, hindered bis-phenolic antioxidant, di-alpha-tocopherol and di-tert-butylphenol. Examples of suitable aminic antioxidants include n-phenylnaphthylamine (NAPA) and octyl ated / butylated diphenylamine. It has been identified by the inventors that a combination of phenolic and aminic antioxidants provides a synergistic effect whereby the phenolic antioxidant is regenerated by the aminic one. The small addition of at least one antioxidant to the dielectric fluid composition enhances the oxidation resistance as well as the overall stability of the fluid.

[0039] Suitable oxide, carbide, nitride and metal nanoparticles envisaged are suitable for cooperation within the present fluid as detailed in table 1 together with their density, thermal conductivity and electrical conductivity.

[0040] Table 1. Suitable oxide, carbide, nitride and metal nanoparticles with corresponding density, thermal conductivity and electrical conductivity.

[0041] Formula Density Thermal Electrical

[0042] (g / cm3) conductivity conductivity (S / m)

[0043] (W / mK)

[0044] Oxides

[0045] Aluminium AI2O3 3.96 35 1x1 O'5oxide Silicon dioxide SiO22.2 1.4 1.4xl0'9

[0046] Quartz SiO22.65 11 1.3xl0'18

[0047] Titanium TiO24.23 8.5 IxlO'11dioxide

[0048] Iron oxide FesO4 5.17 6 IxlO5

[0049] Copper oxide CuO 6.51 77 l.OxlO'1

[0050] Zirconium ZrO25.68 2.5 3.8xl0'4dioxide Zinc oxide ZnO 5.61 25 IxlO3

[0051] Carbides

[0052] Silicon carbide SiC 3.21 270 IxlO2

[0053] Titanium carbide TiC 4.93 300 2xl02

[0054] Tungsten WC 15.63 110 5xl06carbide

[0055] Boron carbide B4C 2.5 17 2xl02

[0056] Nitrides

[0057] Silicon nitride SisN4 3.2 30 IxlO'11

[0058] Aluminium AIN 3.26 321 IxlO'11nitride Boron nitride BN 2.1 27 5.9xl0'12

[0059] Titanium nitride TiN 5.4 29 IxlO'8

[0060] Metals

[0061] Silver Ag 10.49 424 6.3xl07

[0062] Gold Au 19.3 315 4. IxlO7

[0063] Copper Cu 8.96 401 6xl07

[0064] Platinum Pt 21.45 72 9.43xl06

[0065] Tin Sn 6.99 67 9.17xl06 The hybrid nanofluid, to be suitable as an immersion cooling nanofluid, must satisfy three important requirements including specifically:

[0066] 1. Stability - the hybrid fluid should have a homogeneous appearance and exhibit no significant sedimentation at the bottom of the container.

[0067] 2. Thermal conductivity - this should be statistically greater than the base liquid (no nano particles), and ideally represent at least a 3% improvement.

[0068] 3. Dielectric breakdown voltage - this should be greater than 15 kV and preferably is greater than 20 kV.

[0069] The present hybrid nanoparticle fluid must be stable as a suspension over short, medium and long-term periods. The inventors have identified that stability of the present nanofluids is influenced by: (1) dispersant type (2) dispersant concentration, (3) nanoparticle density, (4) nanoparticle concentration, and (5) dispersion method.

[0070] The Dispersant

[0071] Dispersant were selected based on the following criteria:

[0072] 1. Soluble in the base liquid (e.g. PAO) 2 cSt up to 2 wt%.

[0073] 2. Negligible impact on the nanofluid properties, especially the dielectric properties (as measured by dissipation factor and the dielectric strength), related to requirement 3 above.

[0074] 3. Increased nanofluid stability, related to requirement 1 above.

[0075] Suitable surfactant dispersions include: Span™ 20 (Sorbitan monolaurate - being the mixture of an ester formed from the fatty acid lauric acid and polyois derived sorbitol including sorbitan and isosorbide); Span™ 80 (Sorbitan monooleate, (Z)-Sorbitan mono-9- octadecenoate. CAS 1338-43-8, molar mass 428.60 g / mol.); Span™ 85 (Sorbitane trioleate); Brij 93 (Polyethylene glycol oleyl ether, Polyoxyethylene or oleyl ether). The dispersants (chemical name and structure) are detailed in table 2 Table 2. Dispersant commercial name, chemical name and molecular structure. Based on factor analysis over a range of surfactant concentrations (wt%) within the present fluid, the dispersant is included at 0.5 to 2 wt% based on a total wt% of the fluid. One preferred dispersant is the non-ionic surfactant Span™ 85 (Sobitan trioleate) with a HLB number of 1.8.

[0076] The Nanoparticles

[0077] Based on nanoparticle material diameter, density, thermal conductivity and electrical conductivity, twelve different nanoparticles materials were considered preferable for incorporation within the present fluid as detailed in table 3.

[0078] Table 3. A list of suitable nanoparticles, their diameter (nm), density (g / cm3), thermal conductivity (W / mK) and electrical conductivity (S / m) Specific combinations of nanoparticles considered preferable within the present fluid are detailed in table 4.

[0079] Table 4. Nanoparticle combinations selected for testing. O = selected

[0080] The inventors identified through analysis and testing that the present fluid preferably comprises nanoparticles having a density as close as possible to the density of the base liquid, for example PAO having a density (0.8 g / cm3). Consideration of this density provides a more stable nanofluid with regard to preventing agglomeration and sedimentation. Preferably, density of the nanoparticles should be less than 4 g / cm3. Hybrid nanofluids (comprising of combinations of nanoparticles) consisting of higher density nanoparticles could be stabilized in some cases by combining the higher density nanoparticle with low density nanoparticles.

[0081] The nanoparticle combinations of table 4 were analysed over a range of concentrations to assess stability. Example concentrations of the nanoparticles were assessed by the inventors as detailed in table 5. Table 5. Maximum concentrations of nanoparticles at which the hybrid nanofluids exhibit good stability.

[0082] Data was obtained for 50:50 concentration combinations of the two types of nanoparticles.. It was noted that increasing the concentration of the nanoparticles decreased stability of the nanofluid. However, increasing the concentration of thermally conductive nanoparticles provides a fluid that is more effective for heat transfer with energized electronic components. Accordingly, the present fluid represents a balance between various and opposing physical, mechanical, electrical and chemical characteristics as determined by experimental analysis undertaken by the present inventors.

[0083] Dispersion method

[0084] Amongst the different dispersion methods tested, the following method was found to be effective at stabilizing the present nanofluids:

[0085] 1. Mixing the dispersant (Span 85) and the base (PAO 2) in a glass beaker with a magnetic stirrer at 500 rpm for 5 min.

[0086] 2. Transferring the liquid mixture to a plastic bottle.

[0087] 3. Weighing the required amount of the first type of nanoparticles and adding these to the liquid mixture. 4. Weighing the required amount of the second type of nanoparticles and adding these to the liquid mixture.

[0088] 5. Using an ultrasonic probe with a frequency of 24 kHz and vibrational amplitude of 66 pm for sonication for 5 minutes in continuous mode.

[0089] Thermal conductivity

[0090] Experimental thermal conductivity measurements were undertaken. Various binary nanoparticle combinations exhibited enhanced performance with regard to thermal conductivity and electrical insulation. In particular, the first type of nanoparticle may comprise any one or a combination of SiC, BN or TiC and the second type of nanoparticles may comprise any one or a combination of SisN4, B4C or TiN. According to the testing, the nanoparticles including the first type and second type in combination are preferably included at a concentration range less than 10 wt% and preferably between 0.5 to 7 wt% and 0.1 to 6 wt% based on a total wt% of the fluid.

[0091] Such combinations provide an increased thermal conductivity whilst maintaining electrically insulating characteristics relative to the fluid comprising a single type of nanoparticles and also the base liquid including a dispersant without any nanoparticles.

[0092] Dielectric breakdown voltage

[0093] The Open Compute Project Immersion Requirements Revision 2.1 establishes a minimum breakdown voltage of 15 kV / 2.5 mm for fluids of the present type. The breakdown voltage of the nanofluids were measured and compared with this minimum. The inventors identified at least combinations of nanoparticles detailed in table 3 above that satisfied the required breakdown voltage minimum at the preferred nanoparticle concentrations detailed herein.

[0094] Example 1

[0095] 1 L of the PAO was mixed with 8 g of sorbitan trioleate to prepare the dispersion mixture. This mixture was stirred using a magnetic stirring rod at 500 rpm at room temperature until the stabilizing agent was fully dissolved in the PAO. 8 g of SisN4 and 8 g of SiC were added to the mixture to obtain a nanoparticle concentration in suspension of approximately 2 wt%. A Hielscher UP400St ultrasonic titanium tip-sonicator (400 W, 24 kHz) was used to disperse the nanoparticles in the PAO. A stable nanofluid was obtained after exposing the mixture to 120 W of ultrasound energy from a titanium ultrasound probe with peak amplitude of 65 pm and energy density of 55W / cm2for 10 minutes. Visual inspection confirmed the nanoparticles stayed in suspension with little sign of sedimentation for three months after preparation (within the available time limits of the experiment).

[0096] Unless defined otherwise all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the presently described subject matter pertains.

[0097] Unless otherwise indicated, any reference to “wt%” refers to the mass fraction of the component relative to the total mass of the fluid composition.

[0098] Where a range of values is provided, for example, concentration ranges, percentage range or ratio ranges, it is understood that each intervening value to the tenth of the unit of the lower limit, unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the described subject matter. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges, and such embodiments are also encompassed within the described subject matter, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both limits, ranges excluding either of both of those included limits are also included in the described subject matter.

[0099] It should be understood the terms “a” and “an” as used above and elsewhere herein refer to “one or more” of the enumerated components. It will be clear to one of ordinary skill in the art that the use of the singular includes the plural unless specifically stated otherwise. Therefore, the terms “a”, “an” and “at least one” are used interchangeably in this application. Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as size, weight, reaction conditions and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present subject matter. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0100] Throughout the application, descriptions of various embodiments use “comprising” language; however, it will be understood by one of skill in the art that, in some instances, an embodiment can alternatively be described using the language “consisting essentially of’ or “consisting of’.

[0101] The present subject matter being thus described, it will be apparent that the same may be modified or varied in many ways. Such modifications and variations are not to be regarded as a departure from the spirit and scope of the present subject matter, and all such modifications and variations are intended to be included within the scope of the following claims.

Claims

Claims1. A dielectric fluid comprising: an hydrocarbon base liquid; a plurality of different types of nanoparticles; at least one dispersant to disperse the nanoparticles within the hydrocarbon base liquid; wherein the nanoparticles comprise a first type of nanoparticles having a first thermal conductivity and a first electrical insulating characteristic and a second type of nanoparticles having a second thermal conductivity greater than that of the first type of nanoparticles.

2. The fluid as claimed in claim 1 wherein the second type of nanoparticles comprise a second electrical insulating characteristic less than the first type of nanoparticles.

3. The fluid as claimed in claims 1 or 2 wherein the nanoparticles comprise any one or a combination of the group of oxides, nitrides, carbides, a metal.

4. The fluid as claimed in claim 3 wherein the oxides comprise any one or a combination of aluminium oxide, silicon dioxide, quartz, titanium dioxide, iron oxide, copper oxide, zirconium dioxide, zinc oxide.

5. The fluid as claimed in claim 3 or 4 wherein the nitrides comprise any one or a combination of silicon nitride, aluminium nitride, boron nitride, titanium nitride.

6. The fluid as claimed in any one of claims 3 to 5 wherein the carbides comprise any one or a combination of silicon carbide, titanium carbide, tungsten carbide, boron carbide.

7. The fluid as claimed in any one of claims 3 to 6 wherein the metal comprises any one or a combination of silver, gold, copper, platinum, tin.

8. The fluid as claimed in any preceding claim wherein the first type of nanoparticles comprise an electrical conductivity of less than 1x1 O'5S / m.

9. The fluid as claimed in any preceding claim wherein the second type of nanoparticles comprise a thermal conductivity of greater than 100 W / mK.

10. The fluid as claimed in any preceding claim wherein an average particle diameter of the second type of nanoparticles is in a range 80 to 120% of an average particle diameter of the first type of nanoparticle.

11. The fluid as claimed in any preceding claim wherein an average particle diameter of first and second types of nanoparticles is in a range 0.5 to 200 nm, 0.5 to 100 nm or 0.5 to 50 nm as determined by dynamic light scattering.

12. The fluid as claimed in any preceding claim wherein the first and second types of nanoparticles are included at a ratio in a range 1 :5 to 5: 1; 1 :4 to 4: 1; 1 :3 to 3: 1; 1 :2 to 2: 1 or 0.5: 1 to 1 :0.5.

13. The fluid as claimed in any preceding claim wherein the dispersant comprises any one or a combination of:• a surfactant;• at least a partially hydrophobic non-ionic surfactant• a fatty acid, optionally an olaic acid;• an ester;• a sorbitan, optionally sorbitan monolaurate, sorbitan monooleate, sorbitan trioleate;• a polyester,• polyethylene glycol,• polyethylene glycol oleyl ether.

14. The fluid as claimed in any preceding claim wherein the dispersant comprises hydrophobic and hydrophilic groups.

15. The fluid as claimed in any preceding claim wherein the dispersant is lipophilic.

16. The fluid as claimed in any preceding claim wherein the dispersant comprises a hydrophilic-lipophilic balance of less than 10.

17. The fluid as claimed in any preceding claim wherein the hydrocarbon base liquid comprises any one or a combination of: a hydrocarbon; an olefin, a polyalphaolefin, a polyalphaolefin based dimer, a polyalphaolefin based trimer, a polyalphaolefin based tetramer, a C5-C20 olefin, a hydrogenated 1 -decene dimer.

18. The fluid as claimed in any preceding claim wherein the dispersant is included at not more than 2 wt% or in a range 0.5 to 2 wt% based on a total wt% of the fluid.

19. The fluid as claimed in any preceding claim wherein the dispersant comprises at least a first type and a second type included in the dielectric fluid in combination.

20. The fluid as claimed in any preceding claim wherein an average density of the first and / or second types nanoparticles is in a range less than 15, less than 10, less than 8, less than 6 or less than 4 g / cm3.

21. The fluid as claimed in any preceding claim wherein the first and / or the second types of nanoparticles comprise any one or a combination of: SiC, TiC, AIN, Si3N4, SiCh, AI2O3, CuO, Ag, TiO2, BN, B4C, TiN.

22. The fluid as claimed in claim 21 wherein the first type of nanoparticles comprises any one of SiC, BN or TiC and the second type of nanoparticles comprise any one of Si3N4, B4C or TiN.

23. The fluid as claimed in any preceding claim wherein the nanoparticles are included in combination at a concentration in a range less than 10 wt%, less than 8 wt%, 0.5 to 7 wt%, 0.1 to 6 wt% based on a total wt% of the fluid.

24. The fluid as claimed in any one of claims 1 to 11 wherein the first type of nanoparticles comprise SiC and the second type of nanoparticles comprise SisN4, at a ratio in a range 1 : 1 to 1 :3.

25. The fluid as claimed in any preceding claim wherein the dispersant comprises a density in the range 0.4 to 1.4 g / cm3, 0.6 to 1.2 g / cm3or 0.8 to 1.2 g / cm3.

26. A method of making a dielectric fluid comprising: providing an hydrocarbon base liquid; dissolving at least one dispersant in the hydrocarbon base liquid to form a liquid mixture; adding a plurality of different types of nanoparticles to the liquid mixture, the nanoparticles comprising a first type of nanoparticles having a first thermal conductivity and a first electrical insulating characteristic and a second type of nanoparticles having a second thermal conductivity greater than that of the first type of nanoparticles; and suspending the nanoparticles within the liquid mixture to create a nanoparticleliquid suspension.

27. The method as claimed in claim 20 wherein the step of suspending the nanoparticles within the liquid mixture comprises sonicating the nanoparticle-liquid suspension.

28. The method as claimed in claim 27 wherein the step of sonicating the nanoparticle-liquid suspension comprises applying sound energy at a frequency in the range 15 to 35 kHz, 20 to 30 kHz or 22 to 26 kHz.

29. The method as claimed in claim 27 or 28 wherein the step of sonicating the nanoparticle-liquid suspension comprises applying sound energy at vibrational amplitudes between 45 and 95 pm.

30. The method as claimed in any one of claims 26 to 29 wherein the step of sonicating the nanoparticle-liquid suspension comprises exposing the mixture to ultrasound energy at a power in the range 50 to 200W, 80 to 160W or 100 to 140W.

31. The method as claimed in any one of claims 28 to 30 wherein the step of sonicating the nanoparticle-liquid suspension comprises applying continuously the sound energy for 5 to 10 minutes.

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