Siloxane heat transfer fluid, apparatus, and methods

Abstract

Heat transfer fluids are described. In particular, siloxane heat transfer fluids that have a branched T or Q unit structure where each branch includes a siloxy D-unit group are described. These fluids may be particularly useful for demanding heat transfer applications and suitable for related heat transfer apparatuses and heat transfer methods.

Description

SILOXANE HEAT TRANSFER FLUID, APPARATUS, AND METHODSFIELD

[0001] The present description relates to heat transfer fluids, heat transfer apparatuses, and heat transfer methods.BACKGROUND

[0002] Heat transfer fluids facilitate the movement of heat between a heat source and a heat sink or spread heat concentrated in a small area to a larger volume. Associated apparatuses facilitate heat transfer through the use of a heat transfer fluid. Siloxanes include Si-O-Si linkages.SUMMARY

[0003] In one aspect, the present description relates to a method of transferring heat. The method includes providing a heat source, providing a heat sink, and providing a heat transfer fluid in fluid communication with both the heat source and the heat sink. The heat transfer fluid includes a branched siloxane including a T or Q unit, where each branch of the branched siloxane includes a siloxy D-unit group.

[0004] In another aspect, the present description relates to a heat transfer fluid. The heat transfer fluid includes at least one branched siloxane including a T or Q unit, each branch having at least one siloxy D- unit group. The heat transfer fluid has a kinematic viscosity at 23°C of less than 7 centistokes (cSt) and a flash point of greater than 100°C.

[0005] In yet another aspect, the present description relates to a method of transferring heat. The method includes providing a heat transfer fluid including at least one branched siloxane including a T or Q unit, where each branch of the branched siloxane includes a siloxy D- unit group. The heat transfer fluid has a kinematic viscosity at 23°C of less than 7 cSt and a flash point of greater than 100°C.

[0006] In another aspect, the present description relates to a heat transfer apparatus. The heat transfer apparatus includes a heat source, a heat sink and a heat transfer fluid in fluid communication with both the heat source and the heat sink. The heat transfer fluid includes a branched siloxane including a T or Q unit, each branch having a siloxy D- unit groups.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 is a schematic diagram of an exemplary heat transfer apparatus.

[0008] FIG. 2 is a schematic diagram of another exemplary heat transfer apparatus.DETAILED DESCRIPTION

[0009] Silicones are used in a wide variety of industrial and commercial applications at least in part because of their oxidation stability over a wide range of working temperatures, e.g., from -80°C to 250°C. Silicone oils are used as heat transfer fluids; however, conventional silicone heat transfer fluids arepredominantly based on linear polydimethylsiloxanes, which exhibit either a) low viscosity and low flash point, or b) high viscosity and high flash point. This trade-off is well-known and limiting in the application of these conventional silicones. For demanding heat transfer applications, both low viscosity (especially at low temperatures) and high flash point are required.

[0010] Branched siloxanes as described in this application may be particularly useful for heat transfer applications as they exhibit the surprising combination of low viscosity and a high flash point. This may allow for these siloxanes to be used in applications where previously only fluorinated chemistries were believed to be suitable. Heat transfer siloxanes described herein may be free of fluorine, free or chlorine, or free of all halogens.

[0011] Immersion cooling is one of the heat transfer applications where compositions having these properties may find particular utility. For example, large scale computer server systems perform significant workloads and draw a considerable amount of power. These servers are conventionally rackmounted and air-cooled via internal fans or fans attached to the back of the rack or elsewhere within the server ecosystem. As the need for higher density of computer components increases, more efficient conductive cooling mechanisms, such as immersion cooling, become increasingly attractive.

[0012] Other heat transfer applications and apparatuses may be suitable for the heat transfer fluids described herein. For example, branched siloxanes described herein may be used in a closed loop system, wherein the heat transfer fluid facilitates transfer of heat from a heat source to a heat sink, but is never directly in contact with the heat source. Instead, a heat-conductive medium is used to transfer heat between the heat source and the heat transfer fluid, including one or more of metals, thermal paste, and thermal interface materials.

[0013] Siloxane structural units are conventionally referred to as being a M, D, T, or Q siloxy unit. These correspond to mono-, di-, tri- and tetra- (quaternary-) functional structural units, with 1, 2, 3, or 4 silicon-oxy gen bonds, respectively. In some embodiments, siloxanes described herein may include T or Q units.

[0014] Branched siloxanes suitable as heat transfer fluids as described herein may include at least one siloxy D-unit group. In some embodiments, the branched siloxanes may include at least two siloxy D- unit groups. In some embodiments, each of the branches of the branched siloxane includes at least one siloxy D-unit group. In some embodiments, each of the branches of the branched siloxane includes at least two siloxy D-unit groups. In some embodiments, the branched siloxanes are symmetric - meaning that each of their branches is the same. In some embodiments, the branched siloxanes may have a high molecular weight of at least 450 g / mol. In some embodiments, the branched siloxanes may include three branches. In some embodiments, the branched siloxanes may include four branches.

[0015] In some embodiments, a heat transfer fluid including branched siloxanes as described herein may exhibit flash points of greater than 100°C, of greater than 110°C, of greater than 120°C, of greater than 130°C, of greater than 140°C, or of greater than 150°C. In some embodiments, a heat transfer fluid including branches siloxanes as described herein may also exhibit kinematic viscosity of less than 20 cStat 23°C, of less than 15 cSt at 23°C, of less than 10 cSt at 23°C, of less than 7 cSt at 23°C, of less than5 cSt at 23°C, of less than 3 cSt at 23°C, or of less than 2 cSt at 23°C. In certain applications, the kinematic viscosity at -60°C may be particularly relevant. Heat transfer fluid described herein including branched siloxanes may also or alternatively exhibit kinematic viscosity of less than 100 cSt at -60°C, of less than 30 cSt at -60°C, or of less than 10 cSt at -60°C.

[0016] In some embodiments, the branched siloxanes may be of one of the forms shown in Formula I or Formula II.Formula I.Formula II.

[0017] Synthesis of these branched siloxanes may be accomplished by known techniques, such as the Pirers-Rubinsztajn reaction (Brook, M. A. Chem. Eur. J. 2018, 24, 8458-8469) from an alkoxysilane and a silyl hydride or via reaction between silanols or silanolates and chlorosilanes (Katarzhnova, E. Y. et al. Mendeleev Commun. 2021, 31, 393-396).

[0018] Synthesis of these branched siloxanes may be alternatively accomplished through the following exemplary process. First, equilibrate a mixture of hexamethyldisiloxane and tetramethyldisiloxane using a solid-supported acid catalyst in a stoichiometric ratio that produces mostly pentamethyldisiloxane (with very little tetramethyldisiloxane remaining). Then, carry out boron Lewis acid mediated coupling. After distilling the volatile products and purifying the branched product, addmore tetramethyldisiloxane to the volatile fraction, equilibrate as before, and re-use in the coupling chemistry. This can also be conducted using polymethylhydrosiloxane as the hydride source. Examples of the synthesis pathway are shown below:C H3CH3CH3B(C F ) CH3CH3CH3C H3CH3EtO-Si-OEt H- Si-O-Si-CH3- ► H3C-Si-O-Si-O-Si-O-Si-O-Si-CH3OEt CH3CH3-3 C2H6CH3CH36 C H3CH3H3C-Si-C H3OH3C-Si-C H3CH3

[0019] In some embodiments, these heat transfer fluids may be incorporated into a heat transfer apparatus. FIG. 1 is a schematic of an exemplary heat transfer apparatus. Heat transfer apparatus includes heat source 110, heat transfer channel 120 including heat transfer fluid 122, and heat sink 130. Heat source 110 may be any suitable heat source, including electronic devices such as a computer or server. Heat source 110 may reach normal operating temperatures (in the absence of a cooling system) of 40°C, 50°C, 60°C, 70°C, 80°C, or higher. Heat transfer channel 120 may take any suitable form or be made from any suitable material. For example, heat transfer channel 120 may be a pipe or cable filled with heat transfer fluid 122. In some embodiments, heat transfer channel 120 is directly attached to heat source 110. In some embodiments, heat transfer channel 120 is attached to heat source 110 via a thermal adhesive, a thermal paste, or a metal joint (such as solder). In some embodiments, heat transfer channel 120 is similarly attached to heat sink 130. Heat transfer fluid 122 is in fluid communication with both the heat source and the heat sink. In some embodiments, heat transfer fluid 122 is circulated without the aid of a pump or other mechanical forcing. In some embodiments, heat transfer fluid 122 is circulated with the assistance of a pump. Heat transfer fluid 122 includes at least one branched siloxane as described herein. Heat sink 130 is configured to release heat transferred from heat transfer fluid 122 to an external environment. In some embodiments, this external environment is air. Heat sink 130 may be configured with fins or another design to provide a high ratio between surface area and volume. This high ratio mayassist in allowing the maximum heat energy to transfer between the heat sink and the external environment.

[0020] FIG. 2 is a schematic of another exemplary heat transfer apparatus. Heat transfer apparatus 200 is similar to heat transfer apparatus 100 of FIG. 1 except heat transfer fluid 222 is not only in fluid communication with, but is also in direct contact with heat source 210. Heat transfer channel 220 provides a volume that surrounds heat source 210. Heat transfer fluid 222 is also in fluid communication with heat sink 230. Similar to heat transfer apparatus 100 in FIG. 1, a pump or other mechanism may be used to circulate heat transfer fluid 222. Heat transfer fluid 222 includes at least one branched siloxane as described herein. FIG. 2 illustrates an alternative exemplary approach wherein the heat source is immersed, i.e., is in direct contact with heat transfer fluid 222.

[0021] Modifications and enhancements to the general functional form shown in FIGS. 1 and 2 are possible; for example, access doors, support mechanisms, electronic cabling and components, monitoring sensors and hardware, piping and / or tubing, coatings, filters, and other mechanisms can be utilized as necessary or as suited to the particular application.EXAMPLESPreparative Example 1: Preparation of a statistical mixture of tetramethyl-, pentamethyl, and hexamethyldisiloxane

[0022] To a 4 L polypropylene bottle were added 2458 g (15.14 mol, 3217 mL) of hexamethyldisiloxane (Wacker Chemie AG, Munich, Germany), 249.79 g (1.86 mol, 329 mL) of 1, 1,3,3- tetramethyldisiloxane (Gelest, Inc., Morrisville, Pa.), 13.25 g of DARCO G60 (Thermo Fisher Scientific, Inc., Waltham, Mass.), and 2.42 g of concentrated sulfuric acid. The bottle was sealed and then placed on a shaker overnight. The following day, the mixture was checked by gas chromatography for the formation of a new species: 1, 1,3,3,3-pentamethyldisiloxane. The mixture was then filtered through a pad of CELITE 545 (Sigma Aldrich, Burlington, Mass.), in a glass fritted funnel to remove the catalyst. The mixture was then collected and used as-is.Preparation of tris[[dimethyl(trimethylsilyloxy)silyl]oxy]-methyl-silane (Example 1)

[0023] To a 12 L glass reactor equipped with a thermocouple driven by a controller (available from J- KEM Scientific, St. Louis, Mo.), heating mantle, mechanical stirrer, glass stir shift, polytetrafluoroethylene paddle, reflux condenser with a nitrogen inlet / outlet with a bubbler, septum, and a glass sparge tube with a PTFE 24 / 40 adapter were added 220.74 g (1234 mmol, 1.00 equiv.) of triethoxymethylsilane (TCI America Chemical, Portland, Ore.), and 3460.46 g of a statistical mixture of tetramethyldisiloxane, pentamethyldisiloxane, and hexamethyldisiloxane (about 20 wt% pentamethyldisiloxane), as detailed in Preparative Example 1. The reactor was sparged with nitrogen gently overnight. The next day, the internal temperature was set to 60 °C, the sparging was stopped, and the nitrogen was set to flow through the condenser and the bubbler.

[0024] In a nitrogen-filled glovebox, 100 mg of tris(pentafluorophenyl)borane (TCI America Chemical, Portland, Ore.) was dissolved in 10 mL of toluene, and the solution was removed from the glovebox in a syringe with a long stainless-steel needle. The catalyst was added slowly through the septum until bubbling was observed, accompanied by an exotherm. The internal temperature was maintained below 80 °C. Once little bubbling was observed, even after additional catalyst was added, an aliquot was taken for H-NMR analysis, which indicated the presence of ethoxysilane. After about 6.5 hours, the reaction was not yet complete. The internal temperature was set to 80 °C and stirred overnight. Another 70 mg of catalyst was dosed in over 7 hours, after which the reaction was essentially complete by H-NMR and GC analysis. The reaction was cooled to room temperature, filtered through a pad of neutral alumina in a new 12 L glass reactor, and the original reactor was rinsed with two 300 mL portions of heptane. The volatiles were then distilled under vacuum.

[0025] Upon concentration, a small amount of ethoxysilane was observed in the H-NMR spectrum. A three neck 2L round bottom flask equipped with a thermocouple, rubber septum, Claisen adapter with a nitrogen sparge tube and T-joint with a nitrogen inlet / outlet, and stir bar was then set up with 12.31 g of additional pentamethyldisiloxane. The internal temperature was set to 80 °C. 1.3 mL of a 9.5 mg / mL solution of tris(pentafluorophenyl)borane in toluene was added to the solution, and a small amount of bubbling was observed. Another 0.3 mL was added but no bubbling occurred. An aliquot showed that all the ethoxy silane had been consumed. The reaction was cooled to room temperature, filtered through neutral alumina, rinsed with heptane, and then the volatiles were stripped under vacuum at about 60 °C.Distillation of tris[[dimethyl(trimethylsilyloxy)silyl]oxy]-methyl-silane (Example 1)

[0026] 278.38 g of crude tris[[dimethyl(trimethylsilyloxy)silyl]oxy]-methyl-silane was added to a single neck 500 mL distillation pot with a thermal well. A short 24 / 40-24 / 40 extender, short path distillation head, and a heating mantle with a Variac were added. The receiver was cooled with a dry ice / isopropanol bath in a Dewar. The vacuum source was a Schlenk line at 4.31 mtorr. When open to the system, the vacuum was about 42 mtorr. A forecut was removed once the head temperature reached 70 °C. The main cut distilled at about 18 mtorr and at a head temperature of 84 °C. 201.95 g was collected as a clear, colorless liquid in 72.5% mass recovery.2H NMR (500 MHz, chloroform-;- / ) 5 ppm 0.07 (s, 21 H) 0.10 (s, 27 H).29Si NMR (99 MHz, chloroform-^ / ) 5 ppm -67.51 (s, 1 Si) -21.61 (s, 3 Si) 7.29 (s, 3 Si). Quantitative13C NMR (126 MHz, chloroform-;- ) 5 ppm -2.49 (s, 1 C) 0.78 (s, 6 C) 1.53 (s, 9 C).Preparation and distillation of tetrakis[dimethyl(trimethylsilyloxy) silyl] silicate (Example 2)

[0027] In a nitrogen-filled glovebox, a 500 mL round bottom flask equipped with a Vigreaux reflux condenser was charged 8.99 g (9.646 mL, 43.2 mmol) of tetraethoxyorthosilicate (Sigma Aldrich, Burlington, Mass.). Then, 31.5463 g of pentamethyldisiloxane (42.163 mL, 216 mmol, Gelest, Inc., Morrisville, Pa.) was added via syringe. This reaction can also be performed with the statistical mixture of tetramethyldisiloxane, pentamethyldisiloxane, and hexamethyldisiloxane, as detailed in Preparative Example 1. The two reactants were diluted with 95 g of hexane (Sigma Aldrich, SureSeal bottle). The mixture was placed in a heating block set to 55 °C. Meanwhile, 50 mg of B(C6F5)3 (TCI America) wasadded to a 20 mL vial and dissolved in 5 mL of toluene. 250 pL of the catalyst solution was added to the reaction, upon which the reaction bubbled and refluxed vigorously. 10 minutes later, another 200 pL of the catalyst solution was added, upon which more bubbling and refluxing occurred. After two hours, an aliquot was removed for H-NMR analysis, which indicated the total consumption of alkoxysilane. The crude mixture was cooled, removed from the glovebox, and then filtered through a pad of neutral alumina. The volatiles were removed on a rotary evaporator. The product was then distilled on a Kugelrohr in two batches. The product distilled at 155 mtorr and at 235 °C in the tube furnace. A total of 26.83 g was collected in 91% yield as a clear, colorless liquid.2H NMR (500 MHz, chlorofonn-c / ) 5 ppm 0.08 (s, 24 H), 0.10 (s, 36 H).29Si NMR (99 MHz, chloroform -d) 5 ppm -109.68 (s, 1 Si) -21.19 (s, 4 Si) 7.37 (s, 4 Si). Quantitative13C NMR (126 MHz, chloroform -d) 5 ppm 0.75 (s, 8 C) 1.58 (s, 12 C).Measurement of physical properties and kinematic viscosity

[0028] Flash point was analyzed for Closed Cup Flash Point using ASTM D-3278-20 “Flash Point of Liquids by SETAFLASH SERIES 8 ‘ACTIVECOOL’ Small Scale Closed-Cup Apparatus.”

[0029] Kinematic viscosity was measured using a SCHOTT GERATE AVS 350 Viscosity Timer (available from Xylem Analytics Germany Sales GmbH & Go. KG, Mainz, Germany). The viscometer was corrected using the Hagenbach correction factor.

[0030] Pour point was measured by placing a sealed glass vial containing 1 mL of the fluid into a stirred Dewar flask containing cold iso-pentane bath fluid. The vial was attached directly to the thermocouple probe. The bath fluid was chilled by placing a plastic beaker of liquid nitrogen in contact with the bath fluid, cooling until the sample did not pour. Temperature was increased in 1 degree Celsius increments until it poured. Pouring is defined as visible movement of the material during a five-second count. This criterion is specified in ASTM D97.

[0031] Properties of Comparative Examples (designated as CE) come from Gelest Silanes and Silicones Handbook 5000-A (available from Gelest, Inc., Morrisville, Pa.).

[0032] Properties of Examples and Comparative Examples

[0033] As can be seen from the testing of the Examples and the reported properties of the Comparative Examples, the Examples exhibit a surprising combination of both a high flash point and a low viscosity at low temperatures.

[0034] Given the densities near 1 for these materials, the kinematic viscosity and dynamic viscosity are in general agreement (in magnitude) and the reported room temperature dynamic viscosity for the Comparative Examples suggests the kinematic viscosity would be similar (though of course in cSt versus cP).

[0035] The very low pour point measured for the Examples also contrasts with the Comparative Examples. As the viscosity would be expected to increase sharply to the point where it is unmeasurable below the pour point, unacceptable low-temperature viscosity may be reasonably inferred by the reported pour point and room temperature viscosity of the Comparative Examples. In other cases, the viscosity at room temperature (23 °C) was already too high, and would be expected to only increase with decreasing temperature.

Claims

What is claimed is:

1. A method of transferring heat, comprising: providing a heat source; providing a heat sink; providing a heat transfer fluid in fluid communication with both the heat source and the heat sink; wherein the heat transfer fluid includes a branched siloxane wherein each branch of the branched siloxane includes a siloxy D- unit group.

2. The method of claim 1, wherein the branched siloxane has a form according to Formula I.Formula I.

3. The method of claim 1, wherein the branched siloxane has a form according to Formula II.Formula II.

4. The heat transfer fluid of claim 1, wherein the heat transfer fluid has as kinematic viscosity at 23°C of less than 10 cSt.

5. The heat transfer fluid of claim 1, wherein the heat transfer fluid has as kinematic viscosity at -60°C of less than 100 cSt.

6. The heat transfer fluid of claim 1, wherein the heat transfer fluid has a pour point of less than -100°C.

7. The heat transfer fluid of claim 1, wherein the heat transfer fluid is free of halogens.

8. A heat transfer fluid, comprising: at least one branched siloxane including a T or Q unit, each branch having at least one siloxy D- unit group; wherein the heat transfer fluid has a kinematic viscosity at 23 °C of less than 7 cSt and a flash point of greater than 120°C.

9. The heat transfer fluid of claim 8, wherein the heat transfer fluid has as kinematic viscosity at -60°C of less than 100 cSt.

10. The heat transfer fluid of claim 8, wherein the heat transfer fluid has as kinematic viscosity at -60°C of less than 50 cSt.

11. The heat transfer fluid of claim 8, wherein the heat transfer fluid has as kinematic viscosity at -60°C of less than 30 cSt.

12. The heat transfer fluid of claim 8, wherein the heat transfer fluid has a pour point of less than -100°C.

13. The heat transfer fluid of claim 8, wherein the heat transfer fluid is free of halogens.

14. The heat transfer fluid of claim 8, wherein the branched siloxane has a form according to Formula IFormula I.

15. The heat transfer fluid of claim 8, wherein the branched siloxane has a form according to Formula ii.Formula II.

16. A method of transferring heat, comprising: providing a heat transfer fluid comprising at least one branched siloxane including a T or Q unit, each branch having a siloxy D- unit group, and having a kinematic viscosity at 23°C of less than 7 cSt; providing at least one surface from which or to which to conduct heat; providing the heat transfer fluid such that the heat transfer fluid is in fluid communication with the surface; wherein the surface is at least 70°C.

17. The heat transfer fluid of claim 16, wherein the heat transfer fluid is free of halogens.

18. A heat transfer apparatus, comprising : a heat source; a heat sink; a heat transfer fluid in fluid communication with both the heat source and the heat sink; wherein the heat transfer fluid includes a branched siloxane including a T or Q unit, each branch having a siloxy D- unit group.

19. The heat transfer fluid of claim 18, wherein the heat transfer fluid has as kinematic viscosity at -60°C of less than 100 cSt.

20. The heat transfer fluid of claim 18, wherein the heat transfer fluid has as kinematic viscosity at -60°C of less than 50 cSt.

21. The heat transfer fluid of claim 18, wherein the heat transfer fluid has a pour point of less than -100°C.

22. The heat transfer fluid of claim 18, wherein the heat transfer fluid is free of halogens.

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