GB2572635A

GB2572635A - Paste for polishing and method of manufacture thereof

Info

Publication number: GB2572635A
Application number: GB1805730.7A
Authority: GB
Inventors: Juliussen Gaute; Broughton Simon
Original assignee: Graphitene Ltd
Current assignee: Graphitene Ltd
Priority date: 2018-04-05
Filing date: 2018-04-05
Publication date: 2019-10-09
Anticipated expiration: 2038-04-05
Also published as: GB2572635B; GB201805730D0

Abstract

A paste for polishing a surface comprises particles of a derivative of a nanocarbon, particles of diamond and a liquid medium. The nanocabon derivative may be a functionalised nanocarbon, such as an oxide, aliphatic ester, aromatic ester, amine, epoxide, carbonyl, hydroxyl, siloxane or silane. The nanocarbon may be graphene, graphene nanoplatelets, nanocones, fullerenes, carbon nanotubes or multi-walled carbon nanotubes. The diamond particles may have a diameter of 10 nanometres to 20 micrometres, and the liquid medium may be water, vegetable oil, mineral oil or a surfactant. Also claimed is a method for polishing a surface by applying the paste on the surface by a chemical-mechanical technique and a method for manufacturing the paste. The paste may be used for polishing surfaces of central processing units or heat sink assemblies to improve heat transfer across such surfaces in computer hardware.

Description

PASTE FOR. POLISHING AND METHOD OF MANUFACTURE THEREOF

TECHNICAL FIELD

The present disclosure relates generally to surface polishing; and more specifically, to a paste for polishing a surface and a method of manufacturing the paste.

BACKGROUND

Generally, polishing plays an indispensable part in achieving even and smoother surfaces. This can be done to improve the appearance or the functional properties of an item, substrate or surface. Consequently, the role of polishing surfaces is significant. Compositions with abrasive materials made into pastes with oils and waxes, used for polishing applications, have been known for a very long time. More recently, the application of smooth surfaces may be found in computers. For example, smooth surfaces may enable in managing heat generated by processors in the computers. Typically, heat transfer can be enhanced when heat transfer surfaces are smoother and more even.

Conventionally, few techniques are employed for polishing surfaces, which include mechanical polishing and chemical polishing.

In mechanical polishing, the surface is grinded, buffed, and polished to improve the surface conditions of a product using a paste consisting of an abrasive material. In an example, the mechanical polishing utilizes minute particles of diamond or alumina. A relatively large amount of material may be removed quickly from the surface of an item using a coarse abrasive, but this leaves a rough surface. When finer abrasives are used, the amount removed can be considerably less and take considerably longer to achieve the desired effect.

In chemical polishing, a smooth and shiny surface is obtained by a chemical action. Typically, the chemical polishing obtains glossy surfaces by submerging the metals and alloys (to be polished) in a chemical bath namely acidic or alkaline solutions with salts added. However, such chemicals may give rise to small regions of uneven chemical reactions that cause holes of imperfections on the surface, sometimes referred to as pitting.

Another technique, called chemical mechanical polishing (CMP) is known, which combines these techniques. In this technique, a slurry of an abrasive grit and a chemical agent is applied while pressure is applied between an application surface and a surface of a substrate to be polished.

Nevertheless, there is a continuing need to provide smooth and even surfaces as quickly as possible.

SUMMARY

The present disclosure seeks to provide a paste for polishing a surface. The present disclosure also seeks to provide a method of manufacturing the paste. The present disclosure seeks to provide a solution to the existing problem associated with composition of paste used for polishing. An aim of the present disclosure is to provide a solution that overcomes at least partially the problems encountered in prior art, and provides costeffective and environment-friendly paste.

In one aspect, an embodiment of the present disclosure provides a paste for polishing a surface, characterized in that the paste comprises:

- particles of a derivative of a nanocarbon;

- particles of diamond; and

- a liquid medium.

In another aspect, an embodiment of the present disclosure provides a method for polishing a surface, in which a paste according to the previous aspect is applied on the surface by a chemical-mechanical polishing technique.

In a further aspect, an embodiment of the present disclosure provides a method for manufacturing a paste for polishing a surface, the method comprising:

dispersing particles of a derivative of a nanocarbon in a liquid medium to form a dispersion; and mixing the dispersion with particles of diamond.

Embodiments of the present disclosure substantially eliminate or at least partially address the aforementioned problems in the prior art, and enables efficient, cost-effective and eco-friendly paste.

Additional aspects, advantages, features and objects of the present disclosure would be made apparent from the detailed description construed in conjunction with the appended claims that follow.

It will be appreciated that features of the present disclosure are susceptible to being combined in various combinations without departing from the scope of the present disclosure as defined by the appended claims.

DETAILED DESCRIPTION OF EMBODIMENTS

The following detailed description illustrates embodiments of the present disclosure and ways in which they can be implemented. Although some modes of carrying out the present disclosure have been disclosed, those skilled in the art would recognize that other embodiments for carrying out or practicing the present disclosure are also possible.

The present disclosure provides a paste for polishing a surface. The paste comprises particles of a derivative of a nanocarbon, particles of diamond and a liquid medium. Such a derivative of a nanocarbon may be a functionalized nanocarbon or an oxide of the nanocarbon. The particles of a derivative of a nanocarbon may take any shape depending on the nanocarbon, e.g. plates, sheets, tubes, spheres, irregular solids.

It will be appreciated that the term nanocarbon used herein relates to nanostructures of carbon.

Optionally, the nanocarbon is at least one of the group: graphene, graphene nanoplatelets, nanocones, fullerenes, carbon nanotubes, multiwalled carbon nanotubes.

Optionally, the derivative of a nanocarbon may include functionalized graphene, reduced graphene oxide, nanoplatelets, functionalized reduced graphene oxide, functionalized fullerenes, functionalized nanocones, and functionalized multi-walled carbon nanotubes. Such derivatives of a nanocarbon provide higher adhesion to the substrate and effectiveness to the paste, and are unlikely to cause pitting. The functionalization may be as an aliphatic ester, aromatic ester, amine, epoxide, carboxyl, hydroxyl, siloxanes or silanes. More optionally the functionalization is with hydroxyl or epoxide groups. Further optionally, the nanocarbon is graphene, i.e. as graphene oxide. Further optionally still, the graphene oxide comprises single structural sheets oxidised or functionalized on both sides of the sheet.

Optionally, the particles of diamond consist of at least one of: natural monocrystalline, synthetic monocrystalline, and synthetic polycrystalline. The natural monocrystalline particles of diamond have a regular shape and a smooth surface, whilst the synthetic monocrystalline particles have an irregular shape with a rough surface. Furthermore, the synthetic polycrystalline particles have a uniform shape with a rough surface.

Optionally, in such a case, precision micro-fractures are built into each particle of diamond that provides uniform and consistent fracturing under stress.

Particles of diamond are typically industrial diamonds. The particles may be provided in different particle size ranges depending on the degree of smoothness or cut to be achieved. The particle size (Mass median diameter) may be in the range 10 nanometres to 20 microns, optionally 250 nanometres to 15 microns. Optionally, a set of pastes may be prepared with different particle size, e.g. with mass median diameters of 10 microns, 5 microns, 2 microns 1 micron, 0.5 microns and 0.2 microns.

Particles of other hard materials of similar particle size may be included in the paste, such as boron nitride, silicon nitride, silicon carbide, alumina, or silicates.

The liquid medium may comprise water or a vegetable oil, a mineral oil. The liquid medium may comprise a surfactant, such as a non-ionic surfactant, an anionic surfactant or cationic surfactant. Such surfactants serve to help disperse abraded matter enabling it to be washed away more easily.

Optionally, the diamond content is, preferably, present in the form of diamond particles in the range of approximately 0.5 wt.% to approximately 6 wt.%. More preferably, the diamond content in the paste is present in the range of approximately 0.5 wt.% to approximately 2 wt.%. Optionally, the polishing composition may contain, in addition to the diamond particles in the range of approximately 2-6 wt.%, diamond particles with smaller average particle size. For example, the polishing composition of the present invention may comprise a group of diamond particles having an average particle size of approximately 1 micron, and a second group of diamond particles with an average particle size of less than about 0.1 micron.

Optionally, the concentration of particles of a derivative of nanocarbon mixed with the dispersed particles of diamond may be in a range of 1% weight/volume to 5% weight/volume.

Such compositions facilitate high dispersibility and processibility in with an aqueous solution. Such derivative of nanocarbon provides higher adhesion and strength to the paste. Furthermore, the addition of derivative of nanocarbon shifts the polishing mechanism from mechanical polishing to chemical-mechanical polishing mechanism through which the surface is polished. Additionally, the aforesaid paste provides a sharp and even polishing to the surface. Moreover, the paste improves material removal rate of the surface. Such removal rate can be achieved without compromising the smoothness or evenness of the surface achieved. Moreover, when graphene oxide is used, the surface roughness improves (decreases) when the lateral size of the graphene oxide particles is smaller.

Optionally, the derivative of nanocarbon comprises a derivative of graphene. It will be appreciated that the derivative of graphene relates to a structure or carbon that is obtained by modifying/altering a structure of graphene.

Optionally, the graphene may be synthesised by one of the synthesis techniques: mechanical cleaving, chemical exfoliation, chemical synthesis or chemical vapour deposition. In an example, the synthesis technique employed to synthesise the graphene may be mechanical cleaving. In such example, graphite or graphite oxide is mechanically cleaved to obtain graphene sheets. In another example, the graphene may be synthesized by chemical vapour deposition. In such example, methane and hydrogen are made to react on a metal surface at high temperatures to deposit sheets of graphene thereon. In yet another example, chemical synthesis may be employed to obtain the derivative of graphene by synthesizing graphene oxide and subsequently reducing it with hyd razine.

Optionally, the properties and structure of the graphene may depend on the technique employed for the synthesis of the derivative of graphene. Specifically, the synthesised graphene may be a two-dimensional structure with hexagonal lattices. More specifically, the synthesised graphene may comprise carbon atoms on the vertices of the hexagonal lattice. Additionally, the chemical vapour deposition technique may be employed to obtain graphene sheets with least amount of impurities.

Optionally, the derivative of graphene consists of a doped graphene. Specifically, the synthesised graphene may be doped with an element to enhance the properties of the synthesised graphene and improve the intractability of the synthesized graphene with the particles of diamond. Examples of such element may include, but are not limited to, boron, sulphur, nitrogen, silicon. Optionally, graphene may be doped by employing a doping technique such as heteroatom doping, chemical modification, arc discharge and so forth. More optionally, the derivative of graphene may be doped with nitrogen by employing the chemical modification technique. In such a case, the derivative of graphene may be chemically modified by nitrogen-containing compounds such as nitrogen dioxide, ammonia and so forth. More optionally, the derivative of graphene doped with an element (for example, such as boron, nitrogen and so forth) may be obtained by employing the arc discharge of graphite electrodes in presence of a gas and a compound containing the element to be doped. In an example, boron doped graphene may be obtained by the arc discharge of graphite electrodes in presence of a gas such as hydrogen or helium, and a compound containing boron such as diborane. In another example, nitrogen doped graphene may be obtained by the arc discharge of graphite electrodes in presence of a gas such as hydrogen or helium, and a compound containing nitrogen such as ammonia or pyridine.

It will be appreciated that the doped graphene may exhibit superior dispersion properties with the particles of diamond in comparison with the graphene solely comprising carbon atoms. Furthermore, the element introduced in the two-dimensional structure of graphene may interact with the particles of diamond. Examples of the interaction may include Van der Waals interactions, Pi-interactions and so forth. Subsequently, such interactions may enhance adhesion of the doped graphene with the Particles of diamond.

Optionally, the derivative of graphene may consist of reduced graphene oxide. Specifically, graphene oxide may be arranged by the exfoliation of graphite oxide. More specifically, the graphite may be oxidized by reaction with strong oxidising agents such as sulphuric acid, potassium permanganate and sodium nitrate. Subsequently, the oxidised graphite may be dispersed in a solution such as water. In such a case, the oxidised graphite may be dispersed in water to further increase inter-planar spacing between the layers of graphene in graphite oxide. Since exfoliated graphene oxide is electrically insulating, formulation using this form of graphene do not conduct electricity.

Optionally, the derivative of graphene may be a combination of oxidised and non-oxidised graphene flakes. In one example, exfoliated graphene flakes may be mixed with the graphene oxide flakes, to provide dispersibility in water-based solution, improved adhesion to the particles of diamond and improved adhesive performance of the paste. In one example, the ratio between graphene and graphene oxide within the derivative of graphene could be 1:1.

In one embodiment, reduction of the graphene oxide may be obtained by employing chemical, electrochemical or thermal means. In an example, the reduction of the graphene oxide may be arranged by employing chemical means. In such example, the graphene oxide may be heated in distilled water at high temperatures. Alternatively, in such example, the graphene oxide may be reacted with a reducing agent such as urea, hydrazine, ascorbic acid or others. In another example, the reduction of graphene oxide may be arranged by employing thermal means. In such example, the graphene oxide may be reduced at high temperatures in a range of 1000° - 1200° Celsius. In another example, the graphene oxide may be partially reduced upon heating at mild temperatures in a range of 150° - 300° Celsius. In an example, the reduction could occur in controlled atmosphere, either in vacuum or in an inert gas. Furthermore, reduction of the graphene oxide may be arranged after deposition of the graphene oxide on particles of diamond. Consequently, thermal reduction of the graphene oxide may increase mechanical stability and intractability thereof with the particles of diamond.

In an exemplary embodiment, the reduction of the graphene oxide is carried out in a plasma zone. In another exemplary embodiment, the reduction of the graphene oxide is employed within an oxygen-evacuated high temperature furnace. In such an embodiment, the furnace comprises compound containing dopant such as ammonia or pyridine, wherein the furnace is flooded with Hydrogen gas or Helium gas. In such a case, the furnace may be a tube furnace or a vacuum furnace.

Optionally, the derivative of graphene includes functionalised graphene. Optionally, the graphene oxide could be non-covalently functionalised by mixing graphene with organic molecules such as polymers. In an example, a water solution processing method can be used for the preparation of polyvinyl alcohol (PVA) and nanocomposites with graphene oxide (GO).

Optionally, each of the carbon atoms in the graphene oxide comprises a delocalised electron. Consequently, a functional group may react with the carbon atoms thereof. More optionally, the functionalised graphene includes at least one of a functional group: aliphatic ester, aromatic ester, amine, epoxide, carboxyl, hydroxyl, siloxanes, silanes. In addition, the functional groups of the functionalized graphene may influence the properties thereof. Furthermore, the functional groups of the functionalized graphene oxide may enhance thermal conductivity, when exposed to high temperature in comparison with the synthesized graphene solely comprising carbon atoms.

In an example, the derivative of graphene may be a single atomic layer of graphene having a high density of epoxide, wherein hydroxyl functional groups (functionalities) are present on both sides of the basal carbon plane. In such an example, the single atomic layer of graphene further has carboxyl groups around their edges. Beneficially, such structure allows high dispersibility and processibility in with an aqueous solution (for example, the dispersed particles of diamond in the solvent).

Optionally, the particles of diamond are dispersed in the solvent, wherein the derivative of nanocarbon is mixed with the dispersed particles of diamond. It will be appreciated that the particles of diamond have a hard and crystal structure, thereby providing toughness and stability to the paste. Furthermore, the particles of diamond are thermally conductive and therefore provide thermal stability at high temperatures.

The present disclosure also relates to a method of manufacturing the aforementioned paste.

To assist with the mixing of the derivative of the nanocarbon, a dispersion of the derivative of the nanocarbon may be dispersed in a suitable liquid medium prior to addition of particulate diamond. The suitable liquid medium may be a solvent. This process helps the mixing, particularly when the derivative of the nanocarbon is in the form of very small tenacious particles. It may be advantageous to use the liquid medium from an exfoliation process in which nanocarbon particles are isolated to transfer them into the paste.

Alternatively or in addition to the above process, the particles of diamond may be dispersed in a liquid medium, which may be a solvent. This may be done prior to addition of derivative of nanocarbon. In one embodiment, the particles of diamond are dispersed by continuous stirring of the solution for a predefined duration of time. Furthermore, the solution is heated at a predefined temperature to obtain a homogeneous mixture of particles of diamond and the solvent. In another embodiment, the particles of diamond are dispersed by continuous stirring of the solution after the addition of the derivative of graphene. Examples of solvents may include but are not limited to polyethylene glycol ether, castor oil, vegetable wax and water.

Optionally, the solvent may include one of: a polar solvent, a non-polar solvent. More optionally, the solvent employed may be based on the type of the particles of diamond and derivative of graphene. Additionally, optionally, the nature of the solvent may determine the characteristics of the mixture of the derivative of graphene and the particles of diamond. In an example, the solvent employed may be the polar solvent such as water or ethanol. In such example, the derivative of graphene may include functional group of functionalized graphene, wherein the functionalized graphene may be polar functional group such as carboxyl or amine.

In another example, the solvent employed may be the non-polar solvent such as benzene or diethyl ether. In such example, the functional group of the functionalized graphene may be a non-polar functional group such as an aliphatic hydrocarbon.

Furthermore, the derivative of nanocarbon is mixed with the dispersed particles of diamond to form the paste for polishing. Beneficially, such mixing of derivative of nanocarbon synergistically act with the dispersed particles of diamond. The synergistic effect of both forms of carbon atoms (carbon atoms of derivative of graphene and diamonds) acting together, enabled by the functional groups, shifts the mechanism of polishing from pure mechanical to chemical-mechanical, and result in a synergistic improvement in polishing. In other words, the addition of derivative of nanocarbon shifts the polishing mechanism from mechanical polishing to chemical-mechanical polishing mechanism through which the surface is polished. Furthermore, the formed paste improves material removal rate of the surface.

Optionally, the derivative of nanocarbon is poured into the mixture of diamond paste and solvent. The obtained mixture is stirred for predefined time duration at room temperature. Thereafter, the mixture is heated at a predefined temperature and is simultaneously stirred to obtain a mixture having particles of diamond and derivative of nanocarbon uniformly distributed within the entire volume thereof.

Optionally, the derivative of nanocarbon is poured into the mixture of diamond paste and solvent, wherein the obtained mixture is stirred for predefined time duration. Optionally, the obtained mixture may be heated (or cooled) to a predefined temperature while the obtained mixture is simultaneously stirred for obtaining a mixture having particles of diamond and derivative of nanocarbon particles uniformly distributed within the entire volume thereof. In an example, the mixture is heated at a temperature belonging to a temperature range of 30-60 degrees Celsius. Optionally the obtained mixture is stirred is held in the range of 20-80 degrees Celsius during stirring. More optionally, the obtained mixture is stirred at room temperature.

Optionally, the paste is applied on the surface by chemical-mechanical polishing mechanism. In such a case, the chemical action weakens the atomic bonding of the surface and the rotatory mechanical action assists in the material removal therefrom. Specifically, the chemical reaction of the surface and the paste (having derivative of graphene) weakens the atomic bonding of the surface and the particles of diamond act as abrasive to assist the material removal from the surface.

The paste of this disclosure may be used to provide sharp and even polishing on surfaces of central processor unit (CPU), Graphics processor units (GPU) or heat sink assemblies, particularly with the objective of providing for better heat transfer across such surfaces in computer hardware. The aforesaid paste has many applications and can be used for polishing different surfaces. Example of such surfaces include but are not limited to Sapphire, Stainless Steel, Inconel, Germanium, Various Tool Steel, Optical Glasses, Sintered Material, Silicon Carbide, Tungsten Carbide, Yttrium aluminium garnet Materials, Aluminium, Ceramics, Titanium, Zinc Selenide, and Copper.

In use, the paste may be applied in a slurry, optionally with dilution in further liquid medium to an application surface in the form of a rotating smooth wheel, while the item to be polished is lowered in a clamp onto the wheel. Alternatively, the paste can be applied using a polishing cloth.

A fine polishing may be achieved by carrying out a succession of the above polishing processes on a substrate, using pastes with successively finer particle size of diamond.

EXAMPLE

A paste (Example 1) was prepared containing 2% w/w Graphene oxide particles, diamond particles and carrier. A control paste (control) was prepared without Graphene oxide articles, but with the same content of diamond particles in the same carrier as example 1.

An apparatus was set up to apply a controlled load at a prescribed rate of area swept between an application surface and a sample surface, while applying a prescribed amount of paste to the sample surface.

The material removal rate recorded for the sample was up to 220% higher 5 for example 1 than for the control, in replicate tests.

Various embodiments and variants disclosed above apply mutatis mutandis to the method.

Modifications to embodiments of the present disclosure described in the foregoing are possible without departing from the scope of the present 10 disclosure as defined by the accompanying claims. Expressions such as including, comprising, incorporating, have, is used to describe and claim the present disclosure are intended to be construed in a nonexclusive manner, namely allowing for items, components or elements not explicitly described also to be present. Reference to the singular is 15 also to be construed to relate to the plural.

Claims

1. A paste for polishing a surface, characterized in that the paste comprises:

- particles of a derivative of a nanocarbon;

- particles of diamond; and

- a liquid medium.

2. A paste according to claim 1, characterized in that the derivative of the nanocarbon is a functionalized nanocarbon or an oxide of the nanocarbon.

3. A paste according to claim 2, characterized in that the functionalised nanocarbon includes at least one of a functional group: aliphatic ester, aromatic ester, amine, epoxide, carboxyl, hydroxyl, siloxanes, silanes.

4. A paste according to claim 1 or claim 2, characterized in that the nanocarbon is at least one of the group: graphene, graphene nanoplatelets, nanocones, fullerenes, carbon nanotubes, multi-walled carbon nanotubes.

5. A paste according to any one of the preceding claims, characterized in that the particles of diamond have a mass median diameter in the range 10 nanometres to 20 microns.

6. A paste according to any one of the preceding claims, characterized in that the liquid medium comprises water, a vegetable oil, a mineral oil, and/or a surfactant.

7. A method for polishing a surface, characterized in that the paste according to any one of the preceding claims is applied on the surface by a chemical-mechanical polishing technique.

8. A method for manufacturing a paste for polishing a surface, the method comprising:

- dispersing particles of a derivative of a nanocarbon in a liquid medium to form a dispersion; and

- mixing the dispersion with particles of diamond.

9. A method according to claim 8, characterized in that the derivative of the nanocarbon is a functionalized nanocarbon or an oxide of the nanocarbon.

10. A method according to claim 9, characterized in that the functionalised nanocarbon includes at least one of a functional group: aliphatic ester, aromatic ester, amine, epoxide, carboxyl, hydroxyl, siloxanes, silanes.

11. A method according to claims 8 or 9, characterized in that the nanocarbon is at least one of the group: graphene, graphene nanoplatelets, nanocones, fullerenes, carbon nanotubes, multi-walled carbon nanotubes.

12. A method according to any one of claims 8 to 11, characterized in that the particles of diamond have a mass median diameter in the range 10 nanometres to 20 microns.

13. A method according to any one of claims 8 to 12, characterized in that the liquid medium comprises water, a vegetable oil, a mineral oil, and/or a surfactant.