WO2019170894A1

WO2019170894A1 - Heat-sink formulation and method of manufacture thereof

Info

Publication number: WO2019170894A1
Application number: PCT/EP2019/055917
Authority: WO
Inventors: Gaute JULIUSSEN; Simon Broughton; Michael Smith
Original assignee: Graphitene Limited
Priority date: 2018-03-09
Filing date: 2019-03-08
Publication date: 2019-09-12
Also published as: GB2571791A; GB201803856D0; GB2571791B

Abstract

Disclosed is a formulation for conduction of heat between an electronic component and a heat sink, characterized in that the formulation comprises graphene having at least one structural layer and a base polymer, wherein the graphene is dispersed in the base polymer. Also disclosed is a method for manufacturing the formulation.

Description

HEAT-SINK FORMULATION AND METHOD OF MANUFACTURE

THEREOF

TECHNICAL FIELD

The present disclosure relates generally to a heat-sink formulation, especially for electronic devices, and to a method of manufacture thereof.

BACKGROUND

In recent times, there have been great advancements in electronic technologies which have enabled the electronic devices to be capable of executing high performance tasks at speeds which were unimaginable a few decades ago. Furthermore, due to the tremendous improvement in the field of microprocessors and microchips, the electronic devices have become increasingly compact, and generate greater heat intensity. Additionally, these electronic devices comprise components such as processors, display screens, battery packs and so forth which are packed in close proximity. Typically, in such an assembly, heat generation from electronic devices may be significant.

Conventionally, electronic devices employ a heat sink for cooling the electronic devices. Furthermore, a heat spreading film or heat-sink film is provided between the electronic device and the heat sink to accelerate the process of cooling. This is typically done by applying a heat-conducting paste to the electronic device and/or heat sink, and compressing the electronic device and heat sink together, e.g. by screw fixings when the heat sink is attached to the electronic device, squeezing the trapped paste into intimate thermal contact with the electronic device and the heat sink, hence forming a heat spreading film or heat-sink film therebetween. Fleat- conducting pastes are used by hobbyist computer enthusiasts, so it is preferable that they be easy to use. Essentially, the heat spreading film or heat-sink film transfers the unwanted heat generated by components of the electronic device to the heat sink, thereby increasing the efficiency of the heat sink. Further, the unwanted heat is dissipated away from the device into a fluid medium, wherein the fluid medium can be air or liquid coolant.

In a patent application document (publication number: W02005006403A2), a thermally conductive paste dispersed in a paste forming vehicle is disclosed. The thermally conductive paste includes porous agglomerates of carbon particles and is employed as a thermally conductive interface material between a heat or cold source and an object. The thermally conductive paste acts as a thermally conductive interface material between a heat source and a heat sink. However, such pastes are more difficult to apply than the available metal-based pastes.

Metal-based pastes have other disadvantages. Firstly, the conventional heat spreading layers are manufactured using metals and its constituents. However, with the growing environmental difficulties associated with the disposal of metals, the disposal of the heat spreading layers possess unintended consequences such as soil degradation, soil contamination and so on. Secondly, the heat spreading layers comprises precious metals such as silver which further increases the manufacturing cost of the heat sink device which makes it less cost-efficient. Furthermore, isolated operation of the heat sink (without heat spreading layer) does not provide good heat sink efficiency.

Therefore, in light of the foregoing discussion, there exist problems associated with conventional heat dissipation.

SUMMARY The present disclosure seeks to provide a heat-sink film for dissipation of heat.

The present disclosure also seeks to provide a formulation suitable to provide a heat-sink film and a method for manufacturing the formulation. The present disclosure seeks to provide a solution to the existing problems associated with the metallic heat spreading layers. An aim of the present disclosure is to provide a solution that overcomes at least partially the problems encountered in prior art, and to provide an improved heat-sink formulation, thereby facilitating a cost-effective and environment-friendly heat-sink film.

In one aspect, an embodiment of the present disclosure provides a formulation for conduction of heat between an electronic component and a heat sink, characterized in that the formulation comprises graphene having at least one structural layer and a base polymer, wherein the graphene is dispersed in the base polymer.

Optionally, at least a portion of the graphene has less than 20 structural layers, more optionally, less than 10 structural layers, more optionally, less than 5 structural layers, yet more optionally, 1 to 3 structural layers. Optionally, the formulation further comprises a stabilizing material.

Optionally, the graphene consists of at least one of: doped graphene, reduced graphene oxide, functionalised graphene oxide.

Optionally, the base polymer is a High-Density Polysynthetic compound. More optionally, the High-Density Polysynthetic Compound is a polyol ester. Optionally, the base polymer is in a solution.

Optionally, the formulation further comprises at least one of: silicone oil, ceramics, metal particles, boron nitride.

Optionally, the formulation further comprises adding a non-metallic compound, wherein the non-metallic compound is hexagonal boron nitride (h-BN).

In another aspect, an embodiment of the present disclosure provides a heat sink assembly, characterized in that the heat sink assembly comprises a heat source, a heat sink, and a heat-sink film, the heat-sink film consisting of a formulation of the first aspect, arranged between the heat source and the heat sink.

In a further aspect, an embodiment of the present disclosure provides a method for manufacturing a formulation characterized in that the method comprises providing graphene having at least one structural layer and dispersing the graphene into a base polymer or monomer.

Optionally, the method further comprises exfoliating the graphene from ore into the base polymer or monomer. Optionally, the method further comprises adding a stabilizing material.

Optionally, the method comprises dispersing the graphene into monomer to provide a dispersion. More optionally, the method further comprises polymerizing the monomer to produce base polymer in the dispersion.

Optionally, the base polymer is a High-Density Polysynthetic Compound.

Optionally, the method further comprises adding at least one of: silicone oil, ceramics, metal particles, boron nitride.

Optionally, the method further comprises adding a non-metallic compound, wherein the non-metallic compound is hexagonal boron nitride (h-BN).

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

Additional aspects, advantages, features and objects of the present disclosure would be made apparent from the detailed description of the illustrative embodiments 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 formulation, which is applied as a heat sink film for dissipation of heat. The formulation comprises graphene that enhances the heat dissipation through a heat sink. Additionally, such formulation is eco-friendly and has no harmful effects when disposed of. Furthermore, the formulation further dispenses the use of precious metals such as silver and is therefore cost efficient. Moreover, the graphene is stable for wide range of temperature and is compatible with all types of heat sink devices. Beneficially, such formulations have lower density as compared to formulations having metals as a composition, which results in weight reduction. Additionally, weight of the electronic device can also be substantially reduced by employing the heat-sink film of the present disclosure. Furthermore, layers of graphene provide better weight to contact-area ratio as compared to graphite particles. For example, a small amount of graphene is employed for covering a larger surface area, thereby facilitating more contact with a surface of the heat source. This is particularly pronounced when there are fewer layers in the graphene structure. To this end, at least a portion of the graphene preferably has less than or equal to 20 structural layers. Optionally, at least a portion of the graphene has less than 10 layers, more optionally less than 5 layers. Most optionally, at least a portion of the graphene has 1 to 3 structural layers. Furthermore, such structural layers increase the efficiency of the heat sink devices, thereby increasing overall life and performance of the electronic devices.

Throughout the present disclosure, the term " heat-sink film " used herein relates to a thermal conductive film configured to absorb and transfer thermal energy from a heat source to a heat sink.

The term " formulation " is used interchangeably with the term " heat-sink film ", wherever appropriate i.e. whenever one such term is used it also encompasses the other term.

As mentioned previously, the formulation comprises graphene having at least one structural layer. It will be appreciated that the graphene exhibits strong thermal conductivity due to the fact that each the carbon atom in the graphene is bonded to three other carbon atoms in the two-dimensional plane, leaving one electron available in the third dimension for thermal conductivity.

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 at least one layer of 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 at least one layer of graphene by synthesizing graphene oxide and subsequently reducing with hydrazine. The term " graphene having at least one structural layer " is used interchangeably with the term "at least one layer of graphene" , wherever appropriate i.e. whenever one such term is used it also encompasses the other term(s). Optionally, the properties and structure of the at least one layer of graphene may depend on the technique employed for the synthesis of the at least one layer of graphene. Specifically, the synthesised at least one layer of graphene may be a two-dimensional structure with hexagonal lattices. More specifically, the synthesised at least one layer of 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 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 base polymer. Examples of the element may include, but are not limited to, boron, sulphur, nitrogen, silicon. Optionally, graphene may be doped by employing a doping technique such as hetero atom doping, chemical modification, arc discharge and so forth. Optionally, the at least one layer of graphene consists of at least one of: doped graphene, reduced graphene oxide, functionalised graphene oxide. In such a case, the at least one layer of graphene may be doped with nitrogen by employing the chemical modification technique. Specifically, the at least one layer of graphene may be chemically modified by nitrogen- containing compounds such as nitrogen dioxide, ammonia and so forth. More optionally, the at least one layer 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 base polymer in comparison with the synthesised graphene solely comprising carbon atoms. Furthermore, the element introduced in the two-dimensional structure of graphene may interact with the base polymer. Examples of the interaction may include Van der Waals interactions, Pi-interactions and so forth. Subsequently, such interactions may enhance adhesion of the at least one layer of doped graphene with the base polymer.

Optionally, the 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. Specifically, 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. In contrast, graphite-based formulations of the prior art have the disadvantage, when used with electronic devices, that they produce electrically conducting films.

Optionally, the at least one layer 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 base polymer and improved thermal spreading performance. In one example, the ratio between graphene and graphene oxide within the at least one layer of graphene could be 1 : 1.

In one embodiment, reduction of the graphene oxide may be arranged 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 base polymer. Consequently, thermal reduction of the graphene oxide may increase mechanical stability and intractability thereof with the base polymer. 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.

In another embodiment, the graphene oxide may be reacted with a suitable compound to obtain a 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 nano-composites 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. In an example, a functionalised graphene may include functional groups, 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.

Optionally, the at least one layer of graphene may include exfoliated graphite nano-platelets. Such exfoliated graphite nano-platelets provides higher thermal conductivity and strength to the heat-sink film. As mentioned previously, the formulation comprises a base polymer. The base polymer may be provided by admixture with other components of the formulation, of the base polymer may be provided by admixture of a corresponding monomer, followed by later polymerization to result in base polymer. The use of a monomer in this way facilitates the dispersion of solid formulation components because the monomer is much less viscous than the corresponding polymer. Dispersion can also be facilitated by incorporating a solvent into the formulation, thereby providing a base polymer solution. In this regard, graphene and base polymer solution may be heated at a predefined temperature, thereby leading to evaporation of the solvent from the solution. Once the solvent is evaporated, the graphene is finely dispersed in the base polymer solution.

Optionally, the base polymer solution includes an elastomer. It will be appreciated that the elastomer relates to an amorphous polymer existing above their glass transition temperature, so that considerable segmental motion is possible. In other words, the elastomer is a polymer with viscoelasticity (having both viscosity and elasticity) and very weak inter- molecular forces. Moreover, the elastomers have low Young's modulus and thereby allow high fracture strength as compared with other materials. Beneficially, such properties of the elastomers allow the heat-sink film to maintain its shape when exposed to heat. More optionally, the elastomer comprises at least thermoplastic polyurethane, natural rubber, or neoprene rubber.

Optionally, the base polymer is a High-Density Polysynthetic Compound. Beneficially, the High-Density Polysynthetic Compound provides consistency and stability to the heat-sink film. Furthermore, optionally, the High-Density Polysynthetic Compound provides a non-sticky and a flexible structure to the heat-sink film. More optionally, the High-Density Polysynthetic Compound is a polyol ester. More optionally, the base polymer is a synthetic compound with similar properties to polyol ester, which is selected from a group of synthetic polymers which do not harden nor melts when heat is applied within operation. That is to say, the compound is stable when heat is applied within practical limits of the application. For example, such practical limit could be defined by the temperature of operation of a typical CPU, which in turn can be 50 or 60 degrees Celsius.

More optionally, the formulation includes at least one of: silicone oil, ceramics, metal particles, boron nitride. More optionally, the High-Density Polysynthetic Compound provides durability to the heat sink-film.

The High-Density Polyethylene compound provides durability and strength to the heat-sink film. Beneficially, such base polymer withstands the heat absorbed by the heat sink and thereby, does not melt away when exposed to high temperatures.

Optionally, the formulation further comprises a stabilizing material. The heat-sink film is formed by mixing base polymer and the graphene with a stabilizing material. It will be appreciated that since base polymer is made from the monomer, the stabilizing material may terminate the polymerization process at a desired point. More optionally, the stabilizing material may enhance the dispersion properties of the graphene with the base polymer. More optionally, the stabilizing material may enhance the thermal stability of the formulation.

Furthermore, optionally, the stabilizing material may be added prior to dispersion of the graphene and a monomer. In such a case, the polymerization process takes place once the stabilizing material is mixed with the graphene and the monomer. Additionally, nature and/or functional group of the stabilizing material may be compatible and/or similar to the graphene.

Optionally, the stabilizing material includes an organic compound. More optionally, the stabilizing material may facilitate bond formation between an organic solvent and the graphene. Examples of the stabilizing material may include but, are not limited to, natural and synthetic rubbers, alkane polymers, alkene polymers, polyamides, polyurethanes and so forth.

Optionally, the base polymer is in a solution. In such a case, the graphene is dispersed in a solution, wherein the solution comprises a solvent and a plurality of monomers (linking together to form the polymer). In one embodiment, the solution is polymerized by continuous stirring of the solution for a predefined duration. In another embodiment, the solution is polymerized by continuous stirring of the solution after the addition of the stabilizing material.

Optionally, the graphene is exfoliated directly from ore into a solution to facilitate dispersion in the base polymer.

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 graphene. Additionally, optionally, the nature of the solvent may determine the dispersion characteristics of the mixture of the graphene and the base polymer solution. In an example, the solvent employed may be the polar solvent such as water or ethanol. In such example, the 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.

Optionally, the formulation for conduction of heat comprises adding a non- metallic compound, wherein the non-metallic compound is hexagonal boron nitride (h-BN). Beneficially, hexagonal boron nitride is electrically insulating, thus formulation using this form of non-metallic compound do not conduct electricity. Furthermore, the hexagonal boron nitride (h-BN) has a high thermal conductivity (600 W/mK). Furthermore, the hexagonal boron nitride (h-BN) is structurally similar to graphene. Furthermore, the hexagonal boron nitride (h-BN) is soft, lubricating and stable. Such formulation allows for better conduction of heat while being cost efficient as compared with other nanomaterials (for example such as hexagonal boron nitride nano-ribbons (BNNRs).

Additionally, a heat sink assembly is provided, which comprises a heat source, a heat sink, and a heat-sink film. The heat-sink film is arranged between the heat source and the heat sink. The heat source relates to an electrical or mechanical device that generates thermal energy. For example, the heat source may include heat generating chip like graphics chip, various fast memory cards, processors and so forth.

Furthermore, optionally, the heat sink relates to a device that transfers the heat generated by the heat source via air or a liquid coolant, from where the heat is dissipated away from the heat source, thereby allowing regulation of the heat source temperature to an optimal level. Furthermore, the heat sink increases the surface area, using fins or pins, and maximizes dissipation of heat by providing airflow using fans to dissipate heat from the heat source into the surrounding air. Beneficially, the heat sink is designed to maximize the surface area in contact with the cooling medium surrounding it, such as the air. The heat sink is made of copper, aluminium, and other thermal conductive elements. Copper and aluminium are preferred as they have high thermal efficiency and are more durable. Optionally, the heat-sink film is arranged between the heat source and the heat sink. Beneficially, the heat-sink film improves the cooling performance of the heat sink. Furthermore, the heat source is cooled via the heat-sink film instead of being cooled in direct contact with the cooling fluid. Furthermore, the heat-sink film significantly improves heat transfer between the heat source and the heat sink by way of conducting the heat current in an optimal manner. Furthermore, the heat-sink film is sandwiched between the heat source and the heat sink. It will be appreciated that the heat-sink film may be interchangeably used with the different heat sources and heat sinks. Without wishing to be bound by any particular theory, it is considered that the normal application of formulations containing graphene to electronic components/heat sinks may cause alignment of the graphene structural layers with surfaces to which the formulation is applied. Such alignment would greatly increase the surface exposure of the graphene content and hence account for high heat conduction and dissipation through the film.

The present disclosure also relates to the method as described above. Various embodiments and variants disclosed above apply mutatis mutandis to the method. Optionally, the method comprises dispersing the graphene into monomer to provide a dispersion, and which further comprises polymerizing the monomer to produce base polymer in the dispersion.

Modifications to embodiments of the present disclosure described in the foregoing are possible without departing from the scope of the present 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 non-exclusive manner, namely allowing for items, components or elements not explicitly described also to be present. Reference to the singular is also to be construed to relate to the plural.

Claims

1. A formulation for conduction of heat between an electronic component and a heat sink, characterized in that the formulation comprises:

-graphene having at least one structural layer; and

-a base polymer, wherein the graphene is dispersed in the base polymer.

2. A formulation according to claim 1 in which at least a portion of the graphene has less than 20 structural layers, preferably less than 10 structural layers, more preferably less than 5 structural layers, most preferably 1 to 3 structural layers.

3. A formulation according to claim 1 or claim 2, further comprising a stabilizing material.

4. A formulation according to any one of the preceding claims, characterized in that the graphene consists of at least one of: doped graphene, reduced graphene oxide, functionalised graphene oxide.

5. A formulation according to any one of the preceding claims, characterized in that the base polymer is a High-Density Polysynthetic Compound.

6. A formulation according to claim 5, in which the High-Density

Polysynthetic Compound is a polyol ester.

7. A formulation according to any one of the preceding claims, characterized in that the base polymer is in a solution.

8. A formulation according to any one of the preceding claims, further comprising at least one of: silicone oil, ceramics, metal particles, boron nitride.

9. A formulation according to any one of the preceding claims, further comprising adding a non-metallic compound, wherein the non-metallic compound is hexagonal boron nitride (h-BN).

10. A heat sink assembly, characterized in that the heat sink assembly comprises:

-a heat source;

-a heat sink;

-a heat-sink film, consisting of the formulation of any one of the claims 1-9, arranged between the heat source and the heat sink.

11 A method for manufacturing a formulation, characterized in that the method comprises:

-providing graphene having at least one structural layer; and

-dispersing the graphene into a base polymer or monomer.

12. A method according to claim 11, which further comprises exfoliating the graphene from ore into the base polymer or monomer.

13. A method according to claim 11 or claim 12, which further comprises adding a stabilizing material.

14. A method according to any one of claims 11 to 13, which comprises dispersing the graphene into monomer to provide a dispersion, and which further comprises polymerizing the monomer to produce base polymer in the dispersion.

15. A method according to any one of claims 11 to 14, characterized in that the graphene consists of at least one of: doped graphene, reduced graphene oxide, functionalised graphene oxide.

16. A method according to any one of claims 11 to 15, characterized in that the base polymer is a High-Density Polysynthetic Compound.

17. A method according to any one of claims 11 to 16 which further comprises adding at least one of: silicone oil, ceramics, metal particles, boron nitride.

18. A method according to any one of the preceding claims, further comprises adding a non-metallic compound, wherein the non-metallic compound is hexagonal boron nitride (h-BN).