WO2011039531A1

WO2011039531A1 - Graphitic body containing metallic inclusion

Info

Publication number: WO2011039531A1
Application number: PCT/GB2010/051612
Authority: WO
Inventors: Christopher Stirling
Original assignee: Morganite Electrical Carbon Limited
Priority date: 2009-09-29
Filing date: 2010-09-27
Publication date: 2011-04-07
Also published as: GB0917098D0; GB201000455D0

Abstract

A method of making a graphitic body comprising aligned graphite flakes with a binder and in corporating metallic inclusions, the method comprising the steps of : - providing, forming, or taking a mixture of a graphite powder having an average particle size of >200 [micro] m, and a binder; and - pressing the mixture to align the graphite powder to form a pressed graphite body characterised in that said mixture further comprises a metallic powder forming metallic inclusions in the pressed graphitic body. Such graphitic bodies have high thermal conductivity and anisotropy and may be used for thermal management.

Description

GRAPHITIC BODY CONTAINING METALLIC INCLUSION

This invention relates to carbon materials. The invention particularly relates to thermally anisotropic carbon materials.

Among their many applications, carbon materials are used for heat management purposes. This is particularly critical in electronics applications. More compact and sophisticated electronic devices and advances in semiconductor technology have resulted in rising transistor density and switching speed of microprocessors. This has been accompanied by drastic increases in heat production.

In response to this challenge, commercial companies are focusing on developing high performance, low cost, compact, and reliable schemes to handle these very large thermal loads.

The technologies proposed to date rely on the concepts of heat spreaders and heat sinks and various combinations of spreaders and sinks.

A heat spreader is an article that spreads heat quickly. A heat spreader requires high thermal conductivity and low heat capacity.

A heat sink is an article that absorbs heat quickly. It stores heat so it needs high thermal conductivity and high heat capacity.

Conventionally in electronics, heat is spread out by a heat spreader to a heat sink and then removed from the heat sink to the surroundings either by natural convection or by forced flow of a coolant (e.g. fan cooling). For more demanding applications other solutions such as heat pipes or liquid cooled systems may be required.

A carbon based material that has been proposed for use as a heat spreader is diamond. Diamond has the highest known thermal conductivity of any material. However it is currently expensive to make. Diamond is also isotropic in nature, the thermal conductivity in any one direction being the same as in any other direction. This means that as well as spreading heat, a diamond heat spreader allows heat to pass through it to the side remote the source of heat. This can be disadvantageous in applications where components are in close proximity.

A thermally anisotropic carbon material is graphite. The crystal structure of graphite comprises layers within which there is strong bonding, with weak bonding between the layers.

Additionally, within each layer there are delocalised electrons. This structure leads to a high degree of anisotropy, with thermal (and electrical) conductivity within the plane being very much higher than thermal (and electrical) conductivity through the plane.

To exploit this anisotropy, several proposals have been made for graphite based heat spreaders and sinks, for example among recent proposals are:-.

• US 6746768 which discloses a thermal interface material comprising a flexible graphite sheet containing oil which reduces thermal resistance when attached to a component.

• US 6771502 which discloses a finned heat sink constructed from the above resin- impregnated graphite sheets.

• US 6758263 which discloses an anisotropic laminated graphite heat sink made of the above resin-impregnated graphite sheets which has a cavity into which is inserted a thermally conductive material. The heat from a heat source can be conducted via the core and into the thickness of the heat sink and then out across the plane of the heat sink.

• US 6841250 which discloses heat sink designs using such laminated graphite sheets for conducting heat away from an electronic component and dissipating it through the heat sink.

• US 6777086 which discloses resin impregnated exfoliated graphite sheets which are resin-impregnated (5-35% wt) and calendered to 0.35-0.50 mm.

• US 6503626 which discloses that such resin impregnated graphite sheets may be

comminuted, pressed & cured to form a block which can then be machined into a desired finned shape.

• US 6844054 which discloses resin impregnated carbon fibre heat sinks of various

geometric designs (cones, pyramids, domes, etc.).

• US 6119573 which discloses the use of carbon fibre material as a thermally conductive interface between a missile housing and an electronics package to give a low weight high thermal conductivity heat sink.

• US 5837081 which discloses a composite produced from a mat of graphitised (to

2800°C) vapour grown fibre (i.e. highly oriented pyro lytic graphite - HOPG) which is densified by chemical vapour deposition (CVD) of pyro lytic carbon. • US 6514616 which proposes the use of highly oriented pyro lytic graphite encapsulated in polyimide, epoxy or other polymer.

• US 2006/0029805 [published after the earliest priority date of the present application] discloses the idea of hot pressing graphites with a mesophase pitch or phenolic resin as a binder and heat treating to graphitise the binder. US 2006/0029805 discloses that pressing preferably aligns the graphite perpendicular to the moulding direction and that such composite materials have high thermal conductivities (paragraph [0040] shows thermal conductivities of 204.4 W/mK and 76.8 W/mK in the in-plane and through- plane directions for a mesophase pitch binder material). US 2006/0029805 also discloses that mesophase pitch binder materials have a higher thermal conductivity than resin binder materials.

Other patents using graphite's anisotropic thermal conductivity for thermal management include US4878152, US5542471, US6208513, US5523260, US5766765, US6027807,

US6131651.

Potential problems with the use of graphite include strength and surface finish. WO00/03567 and WO2004/097934 disclose the coating of anisotropic graphite sheets with resins to improve strength and disclose methods for forming electrical structures using encapsulated anisotropic graphite sheets. The content of WO00/03567 and WO2004/097934 is incorporated in its entirety in the present application as indicating uses and structures to which the present invention can be put.

Encapsulated materials may have problems with delamination of the encapsulating material and such delamination [by creating voids between the encapsulant and the graphite] can inhibit heat transfer. To overcome this problem WO00/03567 suggests the use of a matrix of fine holes in the graphite that are filled during encapsulation, thereby providing mechanical keying of the encapsulant to the graphite. Forming such holes is both a time consuming process step and reduces the amount of graphite available to conduct heat.

Metal encapsulants have been proposed for use with pyro lytic graphite. US2008/0128067 discloses diffusion bonding of pyro lytic graphite within a metal matrix. This process was disclosed as a means of using "scrap" pyrolytic graphite as pyro lytic graphite is a very expensive material. Pyrolytic graphite exhibit negligible porosity and so the strength of bonding between the metal matrix and the pyrolytic graphite depends upon the extent of diffusion and the possible formation of aluminium carbide. Despite the widespread use of graphite the performance and cost of graphite based materials varies widely. This is because the degree of anisotropy depends upon the degree of orientation of the graphite. To obtain a highly oriented graphite is difficult and hence expensive.

In their patent application WO02/090291 the applicants proposed a method of forming a graphite material comprising the steps of:- a) forming under high shear a mouldable composition comprising :- i) graphite powder; and

ii) a binder; and

iii) a fluid carrier

b) working said mouldable composition under high shear to form an extruded shape c) forming bodies from said shape;

d) heat treating said bodies to stabilise the structure; and

e) machining the bodies to form features in their surfaces.

The high shear working of the compositions could be by rolling.

WO02/090291 concentrated on aqueous binders but did mention the possibility of using pitch based binders. The purpose behind WO02/090291 was to provide a highly graphitic body without the need for a high temperature graphitisation step

Pitch comes in many forms. One such form is mesophase pitch (sometimes called liquid crystal pitch). Mesophase pitch is a partially pyrolysed material containing highly linked aromatic groups and is in effect a pitch part way through conversion to graphite. Continued pyro lysis results in graphitisation. Mesophase pitch is sometimes used as a matrix material in carbon- carbon composites to bind carbon fibres.

The applicants realised that a highly oriented graphite material can be obtained by the process of-

• forming a mixture of a mesophase pitch with a graphite powder

• rolling the mixture to align the graphite powder and to form a body of graphite and pitch

• carbonising the body

and, optionally, • graphitising the body. The applicants further realised that:-

• the thermal conductivity depends upon the size of the graphite used and that large graphite particles provide a dramatically higher thermal conductivity than small graphite particles · density is also a determinant in thermal conductivity of such graphite composites

• purity of the graphite affects thermal conductivity of such graphite composites

• graphitisation is not always necessary to achieve a high thermal conductivity.

As a result in their earlier PCT application WO2007/063309 the applicants described and claimed methods for producing graphite composite materials having a high thermal

conductivity in which a carbonisation step was required. Carbonisation is the exposure of a material to a sufficient temperature that organic materials char to produce free carbon.

Typically temperatures of 900°C or more are required. The content of WO2007/063309 is incorporated in its entirety as exemplifying (although not limiting) the types of graphite and binders that can be used in the present invention.

The applicants have now realised that, including metallic inclusions in the material provides a product with improved mechanical propoerties and high thermal conductivity.

They have further realised that, unexpectedly, carbonisation is not necessary to achieve high thermal conductivities in anisotropic graphite composite materials.

They have further realised that by leaving uncarbonised binder present in their composite materials good strength is achieved, and that further the binder in the composite can provide good adhesion for a surface coating, with the increase in strength that that entails. As the binder has good adhesion to coatings the formation of insulating through holes in the graphite composite materials is eased, as the interior of the holes may be coated.

Such materials have a high thermal anisotropy and are suitable for use as heat spreaders.

The applicants have further realised that in pressing such materials the manner in which the materials are prepared for pressing has a significant effect on properties.

The applicants have further realised that, prior to curing or carbonising, the press-formed graphite bodies retain sufficient flexibility that they may be formed into curved graphite bodies, which may then be cured or carbonised to harden the material in the desired geometry. Such curved graphite bodies also have a high thermal anisotropy and are suitable for use as heat spreaders, especially in specialist applications such as in tube furnaces, in concentrator photovoltaic systems, or in high brightness LEDs.

The applicants have still further realised that encapsulating graphite bodies within a metal coating can improve composites with improved mechanical properties in comparison with the unencapsulated anisotropic graphite composite materials, and that the process of encapsulation can be used to place the graphite composite materials in compression. The mechanical properties of the composite body may then primarily be determined by those of the encapsulant material.

The invention is as set out in the claims.

The invention is exemplified in the following illustrative description with reference to the drawings in which :-

Fig.1 shows schematically a method of die filling

Fig. 2 shows schematically a second method of die filling

Fig .3 shows schematically a third method of die filling.

The materials and mixing methods of WO2007/063309 are incorporated herein in their entirety as providing graphite bodies comprising aligned graphite flakes bonded with a binder, in which the graphite has an average particle size of >200μιη. However the processing methods of the present invention differ significantly.

In WO2007/063309 various pressing and rolling routes were disclosed all of which had a common feature of carbonising at temperatures of 900°C or more.

The applicant has realised that contrary to common prejudice carbonisation is not necessary to get high thermal conductivities.

In a series of tests a number of mixtures of graphite and resin binder were made.

The graphite used was V-RFL 99.5 +500, a 99.86% pure natural graphite available from Graphitwerke Kropfmuhl which has a flake size distribution [% by weight > than specified sieve size]: 30%>800μιη, 77%>630μιη, 95%>500um.

The resin used was 'Bake lite PF 0222 SP' produced by Hexion Specialty Chemicals GmbH

20g portions of mixture were used to fill a die for subsequent pressing.

As indicated in the drawings a die 1 can have an open mouth 2. Alternative die filling methods were used:-

• in a first method (hereinafter called "Direct filling") the mixture was poured into the die [mixture pile 3 in Fig. 1] and levelled with a thin rod before pressing

• in a second method (hereinafter called "Tamped filling") the mixture was poured into the die [mixture pile 3 in Fig. 1] and tamped flat [using a tamping tool 6 as shown in Fig. 2 approximately half the area of the die with successive strokes to ensure the whole surface was pressed flat]. [By "tamp" is meant to pack down by several gentle pressing strokes].

• in a third method (hereinafter called "Successive filling") the mixture was poured into the die in two portions, with levelling of the powder [e.g. by lightly tamping the mixture] between addition of the portions and at the end to assist in allowing the graphite flakes to lie flat prior to pressing. Fig 2 shows a first tamped layer 4 and a second layer 5 ready for tamping with a tamping tool 6.

Addition of the mixture at a slower rate, and more uniformly throughout the die cavity, also assists. This allows the material to fall more freely so that individual flakes will tend to land lying flat, with less flake-to-flake 'bridging' and formation of 'clumps' of flakes oriented transversely to the bulk of the graphite. Accordingly:-

• in a fourth method (hereinafter called "Progressive filling" the mixture was poured into the die slowly as a stream of mixture [7 in Fig. 3] with a movement of the stream of mixture across the mouth 2 of the die [in a to-and-fro and from side to side motion as indicated by arrow 8 - almost a "sprinkling" motion], so that successive portions of the mixture arrive at a given point in the die, with intervals between portions, so permitting large flakes from a first portion some time to partially settle before more mixture follows in a second portion as the stream moves over that point again.

• in a fifth method (hereinafter called "Successive Progressive filling" the mixture was poured into the die slowly [as with Progressive filling] in two steps [as with Successive filling].

In all cases the samples were cold pressed to 32 MPa before a subsequent hot pressing step.

Samples were pre-heated in an oven (at the pressing temperature) before hot pressing to 32 MPa in dies pre-heated (in an oven) to 100°C or 120°C. Some samples were "press cured" by pre-heating pre-formed samples to 120°C, pressing the samples to 32MPa in a die pre-heated to 140°C, then releasing pressure and re-applying to 10 MPa. The heated press platen was set at 160°C and the tooling was left under load overnight. The following morning the temperature of the outside of the die was measured at approximately 130°C, so it may be expected that for the duration of the pressure dwell the sample was at a temperature of at least 130 °C

This was to attempt a degree of further curing of the resin.

The results of these tests are summarised in Table 1 in which the compositions and processing conditions for a number of samples are indicated together with densities, and thermal conductivities .

It can readily be seen by comparison that:-

• Tamped filling results in a slight increase in in-plane thermal conductivity

• Successive filling results in an increase in in-plane thermal conductivity in comparison with direct filling, typically resulting in a 25-50% increase

• Progressive filling results in an increase in in-plane thermal conductivity in comparison with direct filling, typically resulting in a 20% increase

• Successive Progressive filling results in an increase similar to Successive filling.

• higher resin contents result in a slightly lower in-plane thermal conductivity

• "Stoving" [medium temperature treatment e.g. as 250°C] appears detrimental to thermal conductivity

• carbonising results in a return to (or slight increase in) the thermal conductivity of the unstoved materials

Thus, use of uncarbonised materials appears to be practical for achieving high thermal conductivities for example >400W.m^"1.K^"1.

Although stoving may be detrimental to thermal conductivity, it is nevertheless advisable for reasons of mechanical strength, as it provides additional curing and hence strength to the resin. A problem with stoving the most dense materials is that some blistering can occur with outgassing of volatiles from the resin, particularly as the strongly aligned nature of these materials may inhibit gas release and permit laminar failure. Slow approach to stoving temperature, or hot pressing at temperatures higher than those used in these tests (or a hot pressing schedule using conditions of temperature and pressure where a partial cure of the resin is effected), may alleviate this problem, which in any event was not shown in the samples hot pressed at 100°C and 120°C where good appearance was found for all samples.

Thse results show that the present invention is capable of providing a highly anisotropic material at relatively low cost in comparison with alternative methods of providing such anisotropy.

Although levelling of the material has been described using tamping, any method that results in an initial alignment of the graphite flakes may be applied [e.g. using a roller, or moderate vibration (excessive vibration can be counterproductive)].

The graphites usable in the invention are not limited to that stated above, but any graphite with an average particle size of >200μιη may be used. A natural or a synthetic graphite may be used, but natural gives better results as tending to be better aligned and having a higher inherent anisotropy than synthetic graphite.

Only two filling steps have been described in relation to the Successive filling method. It will be readily apparent to the skilled person that there is a balance between the number of filling steps used and processing time. While many small filling steps will give an improvement, the improvement over just two steps may not be worth the additional processing effort. The present invention covers the use of two or more steps for this purpose as an improvement over the direct filling method will be seen.

% Re_sn

Other incremental methods for filling the die are contemplated [e.g. addition of successive portions to different parts of the die mouth in successive portions rather than as a moving stream].

It should be noted that the described filling methods can be used for both carbonised and uncarbonised materials.

A further series of tests were performed to explore the option of forming a pressed graphite plate into a curved shape prior to curing or carbonisation. The graphite used in such tests was "RFL 99,5 O" grade from Graphitwerk Kropfmuhl, with a particle size distribution

approximately as follows: 35%>500μιη, 25%>425μιη, 30%>300μιη, 10%>300μιη. The resin binder was "Bakelite™ PF 0222 SP", produced by Hexion Speciality Chemicals GmbH. In some cases an expanded (flattened) copper mesh of 0.07 mm thickness, strand width 0.2 mm, and aperture dimensions of 2.6 mm (LW) by 1.2 mm (SW) was used to support the material.

The plates were pressed in an 85 x 35 mm die, using a powder addition of lOg to give a typical average cold-pressed thickness of about 1.7-1.75 mm, which after hot pressing would be reduced to about 1.6 mm. Throughout all pressing processes aluminium foil was placed between the plate and die faces to prevent adherence of the material to the die face. Both hot- pressed and cold-pressed materials were produced. For hot pressing the tooling was heated in an oven set for the required temperature. Cold-pressed plates were pre-heated prior to bending on the tooling.

Electrical resistivity of the flat plates was measured by applying a voltage across the plates and measuring the voltage drop across a 50mm gap centred across the centre of the plate.

Difficulties in applying this method to the curved plates led to a different method being used to measure the electrical resistivity of the curved plates, involving applying a voltage across the plate, measuring the voltage drop from end to end, and calculating using the total plate length. The in-plane thermal conductivity of the flat plates was measured by measuring the temperature difference between two surface-mounted thermocouples separated along the mid-section of the plate, with a heat source at one end and a heat sink at the other, this assembly being within a thermally-insulated container. Due to the practical difficulties of measuring the thermal conductivity of the curved plates in-plane thermal conductivity was usually estimated based on the electrical resistivity of the plates, since the relationship between thermal conductivity and electrical resistivity seemed to be the same for the curved plates as for the flat plates. A first set of experiments took place in which the plates were deformed on cylinders. An initial series of tests with a 5% resin-graphtite mixture pressed cold at 31 MPa, pre-heated at 120°C and pressed at 140°C for 1 minute was bent on a 102 mm diameter metal cylinder preheated to 140°C. From flat plates with a typical electrical resistivity of 1.95 μΩιη two curved plates were produced with resistivities of 2.9 and 2.6 μΩιη. From the resistivity/thermal conductivity relationship for the composition an assumed conductivity of the carbonised material was estimated at 300 W/mK, approximately 15% less than the typical value for the non-curved plates.

A more detailed series of tests took place in which plates were bent on cylinders with a diameter of 65 (65.3) pre-heated at 140°C. Cold-pressed plates were pre-heated and then hot- pressed at 120°C immediately prior to shaping to minimise curing of the resin. It was found that reducing the pre-heat time and the time in the press meant that the prepared plates were easier to bend without fracturing. It was noted that it was possible for a helical form to be produced by these means if bending of the plate occurs at the correct angle. Similar experiments bending the plates on pre-heated cylinders with a diameter of 51 mm were also undertaken.

A two-step process was developed in which the plates were first bent on the 102 mm diameter preheated cylinder, and then on the 65 mm diameter preheated cylinder. It was found that this process caused less fracturing of the plate, and additionally produced plates with a more uniform curvature.

It was noted in the above experiments that, even when material had begun to fracture, pressure applied whilst forming the plate to the cylinder could "heal" some of the cracks to a certain extent.

Results of the above described experiments are tabulated below. The level of resistivity in flat plates produced via the pressing methods described and which not bent into a curved shape tends to be fairly consistent - plates with a 5% resin content had a resistivity after hot pressing of 4.5-5.0 μΩ/m, and a resistivity after curing of 5.1-6.5 μΩιη. It was noted that in general the resistivity of the curved plates was significantly higher than that of comparable flat plates, and a large variation in resistivity (from 5 to > 100 μΩιη) was observed. It was also noted that the resistivity tended to fall with curing of the curved plates, whereas the resistivity of the flat plates tended to rise with curing. Lastly, whilst the curved plates formed using a two-step process (first bending on one cylinder, then bending on a smaller cylinder) had less defects than the equivalent directly formed curved plates (which were simply bent on a cylinder of the desired diameter without any pre-bending), the directly formed plates had lower resistivities. % Preheating Preheating Hot press Pressing Cylinder Cylinder Hot- Cured

Resin time (min) temperature temperature time diameter temperature pressed resistivity

(°Q (°Q (seconds) (mm) resistivity (μΩηι)

(μΩηι)

5% 5 120 120 60 65 140 N/A* N/A*

5% 5 120 120 60 65 140 92.4 33.9

5% 2 120 120 30 65 140 5.1 5.0

5% 2 120 120 30 65 140 9.7 8.4

5% 2 120 120 30 65 140 20.4 16.2

5% 5 100 100 30 65 140 N/A* N/A*

5% 2 120 120 30 102 140 11.4 10.1

5% 2 120 120 30 50.8 140 22.9 17.0

5% 2 120 120 30 50.8 140 51.3 36.7

5% 0 N/A 120 10 50.8 140 55.5 38.5

5% 1 100 100 30 50.8 120 19.4 15.7

5% 2 120 120 30 2 step 140 28.5 20.7

102/65

5% 2 120 120 30 2 step 140 25.5 19.5

102/65

10% 2 120 120 30 50.8 140 102.0 95.6

10% 1 100 120 30 50.8 120 41.6 39.1

10% 2 120 120 30 2 step 140 120.8 104.8

102/65

- These samples broke during the bending process and so resistivity measurements were not taken.

Further tests suggested a number of additional features. It was found that hot-pressing a thin copper mesh onto the surface of the plates prior to bending could result in a reduction of splitting at the surface during the bending process. This mesh could be peeled away once no longer required. Another embodiment of this aspect of the invention comprises bending the plate utilising a system of mechanical rollers, such as have been devised for the bending of metal plates, in which a plate is "pinched" between two rollers and passed through them, and is bent as it does so through contact with a forming roller adjusted to provide the correct curvature. A copper mesh may be used to support the plate as in previous methods. The rolls may be warmed prior to the rolling process.

The mechanical properties of porous graphite composite materials can be improved by encapsulation in a metal when the encapsulation results in densification of the porous graphite composite materials. The porous graphite composite materials may be materials as disclosed in this application or in earlier application WO2007/063309, or may be materials such as flake, fibre, or foil graphite composite materials. The thickness of the metal may be greater or less than 0.25mm, or greater than 0.4mm. The thickness of the metal may be greater or less than 10%, 20%, 30% or 40% of the thickness of the anisotropic graphite composite material.

To permit the effect of metal encapsulation 2mm thick inserts were encapsulated in aluminium alloy 6061 and 6082 plates. Tests included providing a single insert or several inserts in a single composite body.

From a block of cast aluminium alloy pairs of plates were machined to contain the inserts, a 4mm thick plate with one or more 'pockets' to hold the inserts, and a 2mm thick cover plate. A clearance of 0.2 mm on all dimensions of the pocket was made, and the metal surface roughened to ensure ease of evacuation during sealing of the assembly of inserts and alloy. The inserts were pre-dried and the metal degreased before sealing.

The assemblies were electron beam welded and hot isostatically pressed for 2 hours at 450°C and 185 MPa. The pressed assemblies were then machined on both sides to down to 3.6mm thickness for thermal conductivity testing. The inserts were either graphitised inserts using the "O" grade graphite mentioned above

[indicated as "O" in the following table], or were aluminium metal containing materials

[indicated as KAl in the following table]. The KAl materials comprised the "O" grade graphite mentioned above, 5wt%> resin binder, and 6wt%> aluminium powder.

To maintain the alignment and contact of the graphite flakes, an available aluminium powder with a small particle size ('Aluminium powder, spherical, 10-14 microns', from Alfa Aesar) was used. The aluminium powder was mixed in with pre-mixed resin-graphite, and the powder was processed to produce stoved plates as described above. These plates were then high- temperature heat-treated to 750°C, while packed in coke dust, in a reducing atmosphere kiln.

It should be noted that such metal containing materials may be of use in thermal management independently of encapsulation. The applicants have tested materials with aluminium contents of up to 12wt% and the metal content may go higher. Increased metal content results in a lower in-plane thermal conductivity but higher through-plane thermal conductivity.

Providing metal inclusions within the core assists bonding of the core to the metal coating. This effect is improved where the metal inclusions comprise a metal in common with the metal coating. However, the use of alloying inclusions is not precluded.

The following table shows results for tests. This shows calculated values for the hybrid [insert plus metal coating] thermal conductivity. These values were calculated by using the mass of the encapsulated insert, the density of the hybrid, and the density of the metal to calculate the volume and density of the insert on the assumption that there is no change in carbon mass; and then using the volume of the insert, assuming no change in thermal conductivity of the insert, and the volume of the metal to calculate the hybrid thermal conductivity.

As can be seen, the measured thermal conductivities exceed those calculated from the amounts and properties of the original inserts and alloys. The applicants suspect that the hot isostatic pressing results in additional densification of the insert and this suspicion is supported by microstructural observations.

The difference in thermal coefficient of expansion between the insert and the encapsulating metal is suspected to place the insert in compression, with improved mechanical properties over, for example, a graphite body alone or a graphite body with metal facings of equivalent thickness.

It will be apparent that the thermal conductivity of encapsulated products will depend upon the relative amounts of core and encapsulant but as can be seen thermal conductivities of above 400W.m¹.K¹ (> copper) can be achieved at a much lower density (copper 8.94 g/cc).

In these tests hot isostatic pressing was used.

The temperature of pressing is desirably above 400°C although lower temperatures may suffice. The pressure applied should at least be sufficient to provide good thermal and mechanical contact between the core and encapsulant and induce the further densification of the core.

Although pressures as low as lOMPa may be sufficient to provide good thermal and mechanical contact, the pressure at which densification occurs will depend upon the nature of the core, and so the pressures may be higher still (>20MPa, or > 30MPa, or >50MPa, or >70MPa, or

>90MPa, or > 1 lOMPa, or > 130MPa, or >150MPa).

The composites may further comprise isotropic high thermal conductivity materials [e.g.

diamond or cubic boron nitride] encapsulated within the metal.

Within the composites the core anisotropic graphite materials may be:-

• aligned parallel to the composite surface to provide good in-plane thermal conductivity;

• aligned transverse to the composite surface to provide good through-plane thermal

conductivity;

• aligned at one or more angles to the composite surface;

• aligned in varying orientation to the composite surface to provide regions of the

composite having different relationships of through and in-plane thermal conductivity [e.g. having high through plane conductivity in some regions and high in-plane thermal conductivity in others]; • randomly distributed;

• or otherwise distributed.

To maintain orientation (where desired) isostatic pressing is preferred. The applicants believe this to be superior in effect over other hot pressing methods, but the invention does not exclude such methods as uniaxial pressing.

The composites may comprise a plurality of layers of cores within the composite, To make such composites, layers of metal and cores can be assembled and hot pressed to form the multi-layer composite.

In addition to the diffusion bonding and possible aluminium carbide formation mechanisms of US2008/0128067, because the core materials are porous, infiltration of metal into the core surface may provide additional bonding.

The present specification discloses a number of different techniques and products. These techniques and products may be combined where appropriate.

The following list of features may be used in addition to or in place of the claimed invention, and is filed to provide basis for amendment and/or division as appropriate: In those jurisdictions where appropriate, these features may be claimed in divisional applications with disclaimers to the matter claimed in this or any divisional application as may prove required to avoid double patenting.

Features

1 : A method of making a composite article, comprising the steps of:- providing at least one porous graphite body encapsulating the at least one porous graphite body in a metal coating to form an assembly of at least one graphite body and metal coating hot pressing the assembly at a pressure sufficient to further densify the porous graphite body.

2. A method as described in Feature 1 in which the at least one porous graphite body

comprises a graphite body comprising aligned graphite flakes bonded with a binder, in which the graphite has an average particle size of >200μιη. A method as described in Feature 1 or Feature 2, in which the step of hot pressing comprises exposure to a pressure in excess of lOMPa.

A composite article having at least one graphite body core encapsulated within a metal coating, the at least one graphite body core comprising a graphite body densified during encapsulation in the metal coating.

A composite article as described in Feature 4, in with the graphite body core comprises aligned graphite flakes bonded with a binder, in which the graphite has an average particle size of >200μιη.

A composite article as described in Feature 5, in which the graphite body core further comprises metal inclusions.

A composite article as described in Feature 6 in which the metal inclusions comprise an element in common with the metal coating.

A composite article as described in Feature 7, in which the metal coating is an aluminium alloy and the metal inclusions comprise aluminium.

A composite article as described in any one of Features 4 to 8 further comprising isotropic high thermal conductivity materials encapsulated within the metal coating.

A method of making a curved graphite plate comprising aligned graphite particles bonded with a resin binder material, the method comprising the steps of:- a) providing a pressed graphite plate in a flexible state, said plate comprising aligned graphite particles bonded with a resin binder material.

b) bending the pressed plate into a curved form by forming on a heated cylinder and/or by passing the plate through a system of rollers. c) subjecting the curved plate to a curing process, such that it is no longer flexible. : A method as in Feature 10, in which the pressed graphite plate in a) is provided directly from a heated pressing process. : A method as in Feature 10, in which the pressed graphite plate in a) is reheated after cooling from a prior heated pressing process. : A method as in any of Features 10-12, in which the curved graphite plate is passed

through a system of at least three rollers. : A method as in Feature 13, in which the curved graphite plate is pinched between two of the rolls and bent through contact with a forming roll. : A method as in Feature 14, in which the position of at least one of the pinching rolls is adjustable. : A method as in Features 14 or 15, in which the position of the forming roll is adjustable. : A method as in any of Features 10-16, in which the curved plate is passed through a system of rollers which are preheated. : A method as in any of Features 10-17 in which the curved plate is passed through a system of rollers more than once. : A method as in any of Features 10-12, in which the curved graphite plate is formed on a heated cylinder of the desired diameter. 20: A method as in Feature 19, in which the curved graphite plate is first bent on at least one heated cylinder of a diameter larger than that which is desired prior to being formed on the heated cylinder of the desired diameter.

21 : A method as in any of Features 10 to 20, in which the curing process after bending

comprises a heat treatment.

22: A method as in Feature 21, in which the heat treatment comprises carbonisation of the curved graphite plate.

23: A method as in any of Features 10 to 22, in which the pressed graphite plate is

supported by a layer, film or mesh.

24: A method as in Feature 23, in which the layer, film or mesh is removed subsequent to the bending of the plate.

25: A method as in Feature 23 or 24, in which the layer, film or mesh is a copper mesh.

26: A method as in any of Features 10 to 25, in which the graphite particles the body is formed from have an average particle size of >200μιη.

27: A method as in any prior Feature, in which the pressed graphite body is formed

according to the methods in any of Features 33-49.

28: A curved graphite plate comprising aligned graphite particles bonded with a resin binder material.

29: A curved graphite plate as in Feature 28 wherein the plate is carbonised. : A curved graphite plate as in any of Features 27 to 29, in which the plate is supported by a layer, film or mesh. : A curved graphite plate as in Feature 30, in which the layer, film or mesh may be

removed. : A curved graphite plate as in Features 30 or 31, in which the layer, film or mesh is copper mesh. : A method of making a graphite body comprising aligned graphite flakes bonded with a binder, in which the graphite has an average particle size of >200μιη, the method comprising the steps of:- a) providing, forming, or taking a mixture of a graphite powder and a binder; b) incrementally filling a die with the mixture

c) pressing the mixture to align the graphite powder and to form a body of graphite and binder.

: A method as described in Feature 33, in which step b) comprises:- bl) partially filling a die with the mixture

b2) levelling the mixture to form a levelled layer of the mixture

b3) repeating steps bl) - b2) at least once until the amount of mixture in the die reaches an adequate thickness

: A method as described in Feature 33, in which step b) comprises :- b4) pouring the mixture into a die in a manner such that at a given position in the die successive portions of mixture arrive, with intervals between the portions permitting graphite flakes in a first portion some time to partially settle before a following portion arrives.

: A method, as described in Feature 35, in which the mixture is delivered to the die as a stream of mixture, with the stream being moved across the mouth of the die and passing repeatedly over said given position. : A method, as described in any one of Features 33 to 36, in which the pressing is at a temperature at which the binder is not carbonised.

: A method, as described in any one of Features 33 to 37, in which a pressing step at a first temperature precedes a further higher temperature pressing step..

: A method of making a graphite body comprising aligned graphite flakes bonded with a binder, in which the graphite has an average particle size of >200μιη, comprising the steps of:- i) providing, forming, or taking a mixture of a graphite powder and a binder; ii) pressing the mixture at a temperature at which the binder is not carbonised to align the graphite powder and to form a body of graphite and binder iii) encapsulating the body of graphite and binder in an encapsulant material that chemically bonds to the binder.

: A method, as described in Feature 39, in which the encapsulant material and the binder are both resins.

A method, as described in Feature 40 in which the encapsulant material and the binder are the same resin.

A method, as described in any one of Features 39 to 41 in which steps i) - ii) comprise steps a) - c) of any one of Features 33 to 36.

: A method of dissipating heat from an article, comprising placing it in contact with a graphite body comprising aligned graphite flakes bonded with a binder, in which the graphite has an average particle size of >200μιη, the method comprising the steps of:-

A) providing, forming, or taking a mixture of a graphite powder and a binder;

B) pressing the mixture at a temperature at which the binder is not carbonised to align the graphite powder and to form a body of graphite and binder

C) without exposing the body of graphite and binder to temperatures in excess of 500°C placing the body of graphite and binder in thermal contact with the heat generating device.

: A method as described in Feature 43, in which steps A) - B) comprise steps a) - c) of any one of Features 33 to 36. 45 : A heat management device comprising a graphite body comprising aligned graphite flakes bonded with an uncarbonised binder, in which the graphite has an average particle size of >200μιη.

46: A heat management device, as described in Feature 45, in which the graphite powder has a flake size distribution in which >50% by weight of the graphite has a flake size >200μιη.

47: A heat management device, as described in Feature 46, in which the graphite powder has a flake size distribution in which >95% by weight of the graphite has a flake size >500μιη.

48: A heat management device, as described in any one of Features 45 to 47, in which the body has an in-plane thermal conductivity of > 400 WmE ¹.

49: A heat management device, as described in Feature 48, in which the body has a through plane thermal conductivity of < 30 WmK^"1.

Claims

1 : A method of making a graphitic body comprising aligned graphite flakes bonded with a binder, the method comprising the steps of:- a) providing, forming, or taking a mixture of a graphite powder having an average particle size of >200μιη, and a binder; and

c) pressing the mixture to align the graphite powder and to form a pressed graphitic body

characterised in that

said mixture further comprises a metallic powder forming metallic inclusions in the pressed graphitic body.

2. A method as claimed in Claim 1, in which the pressed graphitic body is heat treated during or subsequent to pressing.

3. A method as claimed in Claim 2, in which the heat treatment is at a temperature below a melting point of the metallic powder.

4. A method as claimed in any one of Claims 1 to 3, in which the graphitic body or a

smaller graphitic body machined there from, is further encapsulated in a metal coating to form an assembly of at least one graphitic body and metal coating.

5. A method as claimed in Claim 4, in which the assembly is pressed at a pressure

sufficient to further densify the graphitic body.

6. A method as claimed in Claim 5, in which the pressing takes place at a temperature above room temperature, and below a melting point of the metal coating.

A method as claimed in any one of Claims 1 to 6, in which:-

• the pressed graphitic body is flexible;

• while in a flexible state the pressed graphitic body is bent into a curved graphitic body by forming on a heated cylinder and/or by passing the plate through a system of rollers.

• the curved graphitic body is subjected to a curing process, such that it is no longer flexible.

A method as claimed in any one of claims 1 to 7 comprising, after step a) of Claim 1 , the step b) of incrementally filling a die with the mixture preparatory to pressing the mixture.

A method as claimed in Claim 8, in which step b) comprises:- bl) partially filling a die with the mixture

b2) levelling the mixture to form a levelled layer of the mixture

b3) repeating steps bl) - b2) at least once until the amount of mixture in the die reaches an adequate thickness.

A method as claimed in Claim 8, in which step b) comprises:- b4) pouring the mixture into a die in a manner such that at a given position in the die successive portions of mixture arrive, with intervals between the portions permitting graphite flakes in a first portion some time to partially settle before a following portion arrives.

A method, as claimed in Claim 8, in which the mixture is delivered to the die as a stream of mixture, with the stream being moved across the mouth of the die and passing repeatedly over said given position.

12 A graphite body comprising: aligned graphite flakes of average particle size of >200μιη; a binder; and metallic inclusions

13. A composite article having at least one graphite body as claimed in Claim 12

encapsulated within a metal coating.

14. A composite article as claimed in Claim 13 in which the metal inclusions comprise an element in common with the metal coating.

15. A composite article as claimed in Claim 14, in which the metal coating is an aluminium alloy and the metal inclusions comprise aluminium.

16. A composite article as claimed in any one of Claims 13 to 15 further comprising

isotropic high thermal conductivity materials encapsulated within the metal coating.

17: A method of dissipating heat from an article, comprising placing it in contact with a graphitic body or composite article as claimed in any one of Claims 12 to 16.

18: A method as claimed in Claim 17, in which the graphitic body or composite article has an in-plane thermal conductivity of > 400 WmK^"1.