METHODS FOR PRODUCING BORON NITRIDE CONTAINING FLUIDS

Abstract

The present invention provides a method of producing a boron nitride containing fluid comprising the steps of: providing boron nitride; oxidising the boron nitride to functionalise the surface of the boron nitride; and dispersing the oxidised boron nitride in a base fluid to produce the boron nitride containing fluid. Associated boron nitride containing fluids and its use are also disclosed.

Claims

1. A method of producing a boron nitride containing fluid comprising the steps of: providing boron nitride; oxidising the boron nitride to functionalise the surface of the boron nitride; and dispersing the oxidised boron nitride in a base fluid to produce the boron nitride containing fluid.

2. The method according to claim 1 wherein oxidising the boron nitride comprises treating the boron nitride with plasma.

3. The method according to claim 2 wherein the boron nitride is treated with plasma in the presence of argon gas and oxygen.

4. The method according to claim 1 further comprising the step of sonicating the dispersion.

5. The method according to claim 4 wherein the boron nitride containing fluid is a nanofluid.

6. The method according to claim 5 wherein the boron nitride is provided in powder form.

7. The method according to claim 6 wherein the boron nitride is hexagonal boron nitride.

8. The method according to claim 7 wherein the hexagonal boron nitride is turbostratic.

9. The method according to claim 7 wherein the boron nitride is oxidised for 10 minutes to 5 hours.

10. The method according to claim 9 wherein the oxidised boron nitride is dispersed in water.

11. The method according to claim 10 wherein a mixer is used to aid dispersion of the oxidised boron nitride in the base fluid.

12. The method according to claim 11 wherein sonicating the dispersion comprises subjecting to ultrasound for a time period of from 10 minutes to 10 hours.

13. The method according to claim 12 further comprising the step of centrifuging the boron nitride containing fluid.

14. A boron nitride containing fluid prepared by the method of claim 13.

15. The boron nitride containing fluid of claim 14 wherein the fluid is a nanofluid.

16. A heat transfer fluid comprising a boron nitride containing fluid prepared by the method of claim 13.

17. Use of the heat transfer fluid according to claim 16 in electronics cooling system.

18. Use of the heat transfer fluid according to claim 16 in solar panels.

19. A dual function heat exchange and lubricity additive comprising a boron nitride containing fluid prepared by the method of claim 11.

20. A lubricant comprising a boron nitride containing fluid prepared by the method of claim 11.

21. Use of the dual function heat exchange and lubricity additive according to claim 19 as a cutting fluid.

22. Use of the lubricant according to claim 20 as a cutting fluid.

Description

DETAILED DESCRIPTION

[0038] Embodiments of the present invention will now be described with reference to the following, non-limiting examples and figures.

[0039] FIG. 1 illustrates the structure of hexagonal boron nitride;

[0040] FIG. 2 illustrates the top-down view of a single layer of hexagonal boron nitride;

[0041] FIG. 3 illustrates the turbostratic structure of a first commercially available form of boron nitride;

[0042] FIG. 4 shows an electron micrograph of the commercially available form of boron nitride of FIG. 3;

[0043] FIG. 5 shows a TEM micrograph of a second commercially available form of hexagonal boron nitride;

[0044] FIG. 6 illustrates functionalisation of a single layer of hexagonal boron nitride by oxidation;

[0045] FIG. 7 shows a typical RAMAN spectra plot for untreated hexagonal boron nitride;

[0046] FIG. 8 shows a RAMAN spectra plot for a hexagonal boron nitride containing fluid in accordance with a first example of the present invention;

[0047] FIG. 9 shows a transmittance spectrum for the accelerated dispersion test for the hexagonal boron nitride containing fluid in accordance with the first example of the present invention;

[0048] FIG. 10 shows a graphical representation of the sedimentation velocity profile for the hexagonal boron nitride containing fluid in accordance with the first example of the present invention;

[0049] FIG. 11 illustrates a Hot Disk thermal conductivity liquid cell diagram;

[0050] FIG. 12 shows a graphical representation of thermal conductivity of the hexagonal boron nitride containing fluid in accordance with the first example of the present invention and water; and

[0051] FIG. 13 show a graphical representation of thermal conductivity of the hexagonal boron nitride containing fluid in accordance with the first example of the present invention, hexagonal boron nitride containing fluid in accordance with a second example of the present invention and water.

MATERIAL SELECTION

[0052] Any hexagonal boron nitride may be used as the raw material in the process of the present invention. It is the use of oxidation to functionalise the boron nitride which enables more effective exfoliation and production of single layers. However, it has been found that the selection of the raw material boron nitride can further enhance the production of single layers in dispersion.

[0053] The ability to produce single sheets is dependent on the strength of the Van der Waals forces bonding the sheets together. The use of functionalising ions introduced by oxidation causes the distance between the sheets to swell slightly, but the selection of the raw material can further enhance this effect.

[0054] Two variants of hexagonal boron nitride were used in the examples as follows:

Turbostratic Hexagonal Boron Nitride

[0055] Turbostratic means the h-BN material has a crystal structure in which basal planes have slipped out of alignment. Accordingly, there is no guaranteed alignment of boron and nitrogen atoms layer-to-layer which will affect the inter-layer bond strength and produce different conditions for inter-layer positions for OH ions to attach. FIG. 3 graphically shows the turbostratic structure.

[0056] Momentive™ NX1 material was selected as this material is highly turbostratic and so the material properties afforded may allow for a more effective exfoliation.

[0057] FIG. 4 shows an electron micrograph of the NX1 material with a mean size of less than 1 micron and a discernible plate structure. There is not enough magnification to show the multiple layer structure.

Commercially Available Hexagonal Boron Nitride

[0058] Elinova® 2D Boron Nitride (available from Thomas Swan) contains an average of 7 to 10 layers of h-BN. This material undergoes exfoliation of hexagonal boron nitride to produce atomically thin nano-platelets and is manufactured by a proprietary Direct Liquid Exfoliation process which exfoliates hexagonal boron nitride to produce 2 dimensional nano-platelets of boron nitride or 2D boron nitride. FIG. 5 shows a TEM micrograph of the materials grade with an average particle size of 0.5 to 1.0 μm.

Oxidation

[0059] Haydale HDPlas® process was used to functionalise the 2D boron nitride material. In particular, oxygen (O.sub.2 gas) functionalization was used which results in 0 but mainly OH (hydrogen being adsorbed from the atmosphere by the highly reactive surface) functionalization of the material which has been pictorially represented in FIG. 6.

[0060] The material treatment was based on the following basic process for a 100 g sample: [0061] Material was first “cleaned” for 30 minutes using Argon gas. A plasma acceleration voltage of 0.5 kV was used with a 70 W energy input. 70 SCCM of Ar was used. [0062] Using the same plasma energy levels, O.sub.2 gas was then pumped into the chamber at 70 SCCM for 2 hours.

[0063] Where the process involves a liquid such as acetic acid being added to the plasma (in a stream of argon), CH.sub.3, COOH and OH species are formed which attach to the edges of the 2D material at available junctions functionalising the material by oxidation.

Ultrasonic/Centrifuge Treatment

[0064] The plasma treated material is subsequently subjected to ultrasonic treatment followed by optional centrifugation.

[0065] Prior to undergoing ultrasonic treatment, the plasma treated material is dispersed in deionised water. As a consequence of the functionalization of h-BN, the material wetted instantly. A Silverson L5M-A mixer was used to aid dispersion, at 500 rpm for 10 minutes.

[0066] The resulting dispersion is sonicated using a nano-lab QS1 system with a 0.5 inch tip. Energy exposure using a 30:30 second sonics ON:OFF profile. The temperature was limited to 40° C. using ice at 40% (max 125 W) power for safety reasons.

[0067] Subsequent optional centrifugation was carried out by techniques well known in the art.

Example 1

[0068] A 100 g sample of commercially available hexagonal boron nitride was subjected to treatment with the Haydale HDPlas® process by the basic process set out above.

[0069] From this initial experiment it was shown that this treatment allowed a dispersion of hexagonal boron nitride in water to be formed without the use of a surfactant. The dispersion shows sedimentation though due to the particles being multi-layer with a large degree of aggregation as well. However, the sediment is easily re-dispersible showing that the surface treatment is very effective.

Example 2—Variant 1—Turbostratic h-BN

[0070] Momentive™ NX1 was subjected to treatment with the Haydale HDPlas® process by the basic process set out above.

[0071] The plasma treated Momentive™ NX1 material was processed under the following conditions to render a suitable nanofluid: [0072] 500 ml of a 2.5% (wt/wt) dispersion of the functionalised h-BN in DI water was produced. The material wetted instantly. A silverson L5M-A mixer was used to aid dispersion, 500 rpm for 10 minutes. [0073] The resultant dispersion was then sonicated using a nano-lab QS1 system with 0.5 inch tip for 2 hours. Energy exposure using a 30:30 second sonics ON:OFF profile. Temperature was limited to 40° C. using ice at 40% (max 125 W) power. [0074] The sonicated material was centrifuged for 2 hours at 200 rpm. [0075] 400 ml of fluid was produced. Tests showed a 0.6% wt/wt h-BN content.

Characterisation of Fluid

[0076] The determination of the degree of exfoliation is a fundamental measurement. This is done via RAMAN spectroscopy. RAMAN spectroscopy excites the sample using radiation supplied by a very high power monochromatic laser. This excitement causes an interaction with the sample which may be reflected, absorbed or scattered in some manner. It is the scattering of the radiation that occurs which can tell the Raman spectroscopist something of the samples molecular structure. The light is collected from the sample and analysed

[0077] Some of the scattered light is detected with a small change in wavelength (colour), this is known as the RAMAN scattered component. This scattering gives information on the bonds between atoms and as such can be used to identify molecules in the sample as the positioning of the bonds dictates the structure of the molecule.

[0078] The output from the spectrometer is a plot of the intensity against the wavelength shift caused by the RAMAN scattering (reported as cm.sup.−1). FIG. 7 shows an example of a typical plot for untreated hexagonal boron nitride. The area of interest for hexagonal boron nitride is around 1300 (cm.sup.−1) where the Van der Waals bonds are detected so the height of this peak indicates the number of these bonds and thus how many layers are present in the sample.

[0079] As the material was processed, each stage was analysed using a RAMAN spectrometer. FIG. 8 shows the RAMAN trace (concentrated around the 1350 cm.sup.−1 region) for each stage, i.e. powder sample (i.e. the raw material), sonicated sample and centrifuged sample. It can be seen that processing from the powder to sonicated material there is a drop in the intensity. As these plots have been normalized, this drop in intensity is due only to the change in structure of the hexagonal boron nitride, i.e. the number of layers have been reduced significantly. The plots are normalized by shifting the RAMAN peak upwards in monolayers and downwards in bilayers with respect to its position in bulk h-BN.

[0080] The RAMAN spectra of the h-BN containing fluid clearly shows that there has been a high level of exfoliation in the material, i.e. there are significant levels of single layer material and many double layers.

[0081] To assess the stability of the manufactured nanofluid, an accelerated test was performed using a LUMiFuge® instrument (available from LUM GmbH). A spectrum was obtained over 50 minutes at 10 g with a 10 second read interval. Area of interest on profile 115 to 125 mm (Area 105 to 115 is affected by sample tube geometry and sample meniscus, and 125 to 130 is the bottom of the tube). A steady drop in the transmission can be seen which is indicative of some settling in the sample—although the transmission falls from 18% to 15% within this area. This shows that the higher yield single layer BN achieved by the method of the present invention results in a more stable dispersion than is achieved by the prior art methods.

[0082] FIG. 9 shows the spectra obtained from the LUMiFuge®. The spectrum shows some settling indicating that there are probably some multiple (2-3) layer particles, but the majority are single layer.

[0083] FIG. 10 shows the sedimentation velocity profile for the material from the LUMiFuge®. This shows no aggregation and a very slow sedimentation velocity 900 nm/second at 10 g acceleration. There is no sedimentation of the sample either.

Thermal Properties

[0084] The fluids were tested for thermal conductivity across a wide range of temperatures (20 to 80° C.). The instrument used to measure the thermal conductivity was a Hot Disk TPS3500. FIG. 11 shows the general geometry of the measurement cell used.

[0085] Ten measurements of the sample were taken at each temperature, with five minutes between each measurement to allow the sample temperature to reach equilibrium again. Measurement conditions were 50 nW of energy for 10 seconds giving a 2 to 5 kelvin raise in temperature.

[0086] FIG. 12 shows the initial results for the thermal conductivity of the h-BN containing nanofluid. The bottom graph of the two shows a flat (lower) curve which is the thermal conductivity of water alone and the exponentially rising (upper) curve being the thermal conductivity of the h-BN containing fluid. The upper of the two graphs is a representation of the % increase in thermal conductivity compared to water.

[0087] At the typical operating temperatures of computer central processing units (60-85° C.), there is a rapid rise in efficiency from 35% at the lower limit and 180% at the higher limit, making this fluid an extremely good candidate for a thermal fluid.

Example 3—Variant 2—Commercially Available h-BN

[0088] Variant 2 h-BN material has differing requirements to Variant 1. Variant 2 material can be used as a metalworking (cutting) fluid which requires not only good thermal properties, but also provides lubrication in high pressure areas such as the cutting tip at the metal interface.

[0089] To enhance the lubricity of the fluid, Variant 2 h-BN material was sourced and processed differently.

[0090] Utilising commercially available h-BN allows the processing of the material to be much simpler to make Variant 2 h-BN. The material had already undergone liquid exfoliation carried out by the manufacturer to reduce the number of stacks of h-BN. Therefore, following plasma processing the material was simply mixed into an aqueous dispersion and limited ultrasonic energy applied just to start the break-up of the agglomerates. [0091] Material was first “cleaned” for 30 minutes using Argon gas. A plasma acceleration voltage of 0.5 kV was used with a 70 W energy input. 70 SCCM of Ar was used. [0092] Using the same plasma energy levels, O.sub.2 gas was then pumped into the chamber at 70 SCCM for 2 hours. [0093] 40 litres of a 0.5% (wt/wt) dispersion of the functionalised h-BN in DI water was produced. The material wetted instantly. A silverson L5M-A mixer was used to aid dispersion, 500 rpm for 10 minutes. [0094] The resultant dispersion (in 5 litre aliquots) was then sonicated using a nano-lab QS1 system with 0.5 inch tip for 30 minutes. Energy exposure using a 30:30 second sonics ON:OFF profile. Temperature was limited to 40° C. using ice at 40% (max 125 W) power. [0095] No centrifugation was applied.

[0096] Visible settling over a few days was observed indicating both aggregation and multi-layer morphology. The material was tested for thermal conductivity.

[0097] FIG. 13 shows the thermal conductivity measurements over 20 to 80° C. for a 0.6 wt % sample of the variant 1 material (blue line) and a 0.5 wt % sample of the variant 2 material (black solid line). The variant 1 material was concentrated to a 1.8 wt % sample (green line) by centrifugation to remove some of the liquid by techniques known in the art. The red line (the lowest line) represents the thermal conductivity of water and the black dashed line represents the % improvement of the variant 2 material over water.

[0098] As expected, water has the lowest thermal conductivity which remains relatively constant as temperature increases. The variant 2 material shows an improvement over water (as illustrated by the dashed line) with the thermal conductivity steadily increasing as the temperature increases. The thermal conductivity of the 0.6 wt % sample of the variant 1 material increases steadily to a temperature of 60° C., at which point it undergoes more rapid increases from 60° C. to 70° C. and from 70° to 80° C. The 1.8 wt % sample of the variant 1 material shows the high thermal conductivity across the temperature range.

[0099] The Variant 2 h-BN material is not as efficient at heat extraction at the 60 to 80° C. operating temperature. This efficiency is probably due to the Variant 2 material being multi-layer in its morphology. However, the Variant 2 h-BN material is better suited for use as a metalworking cutting fluid as the multilayer morphology enhances lubricity and, in use under shear, the layers easily separate due to the processing condition, thus enhancing the thermal properties.

[0100] In particular, while the Variant 2 h-BN may undergo settling of the material in use, early tests in a very high power liquid cooled PC system built to generate significant amount of heat from CPU and GPU have shown that the heat energy removed from the system increases from 94 Watts (joules per second) for prior art systems to 178 Watts for the Variant 2 h-BN containing fluid.

[0101] Accordingly, boron nitride containing fluids prepared by the method of the present invention provide better dispersions with weaker Van der Waals interactions between layers of the boron nitride and/or increased yields of single layer boron nitride. The boron nitride containing fluids thus provide improved properties including enhanced heat exchange/transfer and lubricity, thus lending themselves to use in heat transfer fluids for various applications such as electronics cooling or solar panels or as cutting fluids for metalworking processes.