WO2008087373A2

WO2008087373A2 - Structures with improved properties

Info

Publication number: WO2008087373A2
Application number: PCT/GB2008/000003
Authority: WO
Inventors: Antony Arthur Carter; Graham Alfred Beck
Original assignee: Queen Mary & Westfield College
Priority date: 2007-01-17
Filing date: 2008-01-02
Publication date: 2008-07-24
Also published as: WO2008087373B1; GB0700917D0; WO2008087373A3

Abstract

A method of producing a material for use as a thermally-conductive substrate or such like, the method comprising the steps of: obtaining a plurality of layers of an anisotropic constituent material, each layer of the constituent material having a first thermal conductivity in a first direction, and a through-thickness thermal conductivity, the first thermal conductivity being different from the through-thickness thermal conductivity; stacking and joining the said plurality of layers to form a stack; and cutting through the stack to produce a slice; wherein the orientation of the slice with respect to the layers in the stack is such that the through-thickness thermal conductivity of the slice is determined by the first thermal conductivity of the layers in the stack. Also provided is a structure comprising a plurality of strips, the structure being substantially planar, wherein the strips comprise an anisotropic constituent material, the constituent material having a first thermal conductivity in a first direction and a conventionally-through-thickness thermal conductivity, the first thermal conductivity being different from the conventionally-through-thickness thermal conductivity, and wherein the orientation of the strips within the structure is such that the through-thickness thermal conductivity of the structure is determined by the first thermal conductivity of the constituent material.

Description

STRUCTURES WITHIMPROVED PROPERTIES

This invention relates to material structures with improved properties - in particular (but not exclusively) improved properties of thermal conductivity, 5 which may be employed in applications such as thermally conductive substrates and thermal management devices for electronic components.

Background to the Invention

There is an ongoing desire in the microelectronics industry to increase the 0 processing capabilities of microprocessors, and to improve the performance of other components. As a consequence, thermal power densities in electronic devices continue to increase rapidly, with single-processor chip power now moving above 150 W/cm², and planned to double further in the next ten years.

On-die 'hot spots' can be ~1000 W/cm² over ~100 μm by -100 μm areas. 5 Such hot spots can lead to chip failure at worst if temperatures are not controlled, and are always prime candidates in determining the time-to-failure of a chip.

Thus, in modern microelectronic devices it is important to transfer heat as0 efficiently as possible from a chip or other heat-generating component to an external heat sink, by optimised interfacing, spreading and channelling. This task may be addressed through improving the packaging techniques with improved control of thermal interface material (TIM) bond line thickness and by enhancing the intrinsic material thermal conductances. The best current5 interface materials have thermal conductivity (K) of the order of a few W/mK and generally are isotropic. To be effective such 'packages' need to be extremely thin (of the order of tens of microns or less), as their direct transit temperature gradients are ~l-2°CAV/mm per cm² area, resulting in typical

810752V1 temperature differences of more than 5O⁰C over a 1 mm thickness for a 50W chip source.

The thermal demands of power electronics are even greater, with reduced 5 component size in devices such as integrated gate bipolar transistors (IGBTs), integrated gate commutated thyristors (IGCTs), metal oxide semiconductor field effect transistors (MOSFETs) and diodes, and their incorporation into power modules rated above 1 kV and for many 10s of amperes. Conventional solutions for their thermal management come from the use of directly copper 0 bonded (DCB) plaquettes with TIMs of thermal grease or solder onto finned heat sinks or cold plates. The device is typically fabricated from an alumina (Al₂O₃) or aluminium nitride (AlN) substrate (typically 0.25 to 1 mm thick with 0.1 to 0.3 mm copper foils directly bonded to each surface and ~20 mm by ~20 mm in area). The effectiveness of the so-called 'extraction surface' of the5 direct copper bonding is currently limited by its interface to aluminium or copper structures that provide the final heat sink.

For reference, examples of thermal conductivity values for some existing substrate and thermal interface materials will now be given. Alumina and0 aluminium nitride are both isotropic, and hence the thermal conductivity in each of the crystallographic directions is the same. The thermal conductivity of these materials may therefore be expressed as a single value, K(a,b,c), with a, b and c referring to the three crystallographic axes of the material, as is conventional and will be familiar to those skilled in the art. For alumina,5 K(a,b,c) is 28 W/mK. For aluminium nitride, K(a,b,c) is 180 W/mK.

Other materials which may be used in thermal management applications include pyrolytic graphites (PGs). PGs are made by different processes that include chemical vapour deposition (CVD) and the need to seed depositions

810752V1 onto substrates in furnaces at high temperatures. Further heating and pressure processes may be applied to the resulting graphite substrates to increase their intrinsic thermal conductivities.

5 Pyrolitic graphite is anisotropic, and hence the thermal conductivity is not the same in each of the crystallographic directions. By way of background, the thermal conductivity of pyrolytic graphite can be understood from considering the crystal structure of graphite. On an atomic level, graphite comprises parallel planar layers of carbon atoms extending in the a-b basal 0 crystallographic plane. That is to say, the planar layers extend parallel to the a and b crystallographic axes, and are normal to the c axis. In these planar layers, each carbon atom is covalently bonded to three other surrounding carbon atoms to form hexagonal structures. Due to the tight bonding between the carbon atoms within these layers, phonons can propagate very quickly 5 within the layers, and consequently the thermal conductivity within the layers, i.e. in the a and b directions, is very good. However, the planar layers are only loosely bonded to their neighbouring parallel layers by weak van der Waals forces, and so phonons are slower to travel from one layer to another. Consequently, in the c direction, the thermal conductivity of graphite is poor. 0

Thus, as a consequence of the crystal structure of graphite, PGs are very anisotropic in their thermal conductivies, with Kab (the in-plane thermal conductivity) being up to -1800 W/mK (potentially in the range of about 1000- 2400 W/mK), but with Kc (the through-thickness thermal conductivity) being5 only 8-10 W/mK.

Due to the layering processes, with mosaic ordering occurring in the a-b planes, and only van der Waals forces available to achieve adhesion in the c direction, such high thermal conductivity PG materials are made in planes, which can be

810752V1 metres by metres in area, but only tens of centimetres in thickness. The cooling-down process in the furnace results in temperature gradients across structures that lead to delamination in the c direction where the coefficient of thermal expansion (CTE) is ~25ppm/°C. Consequently, laminar high Kab PG is expensive, and high Kc material is even more so, and not available in large thicknesses.

PG materials may be encapsulated to improve their rigidity and structural integrity, as discussed in WO 00/03567, WO 2004/097934 and WO 2004/097936. In particular, WO 00/03567 discloses a fundamental technique which may be used to form encapsulated pyrolytic graphite material. Plates of anisotropic pyrolytic graphite or thermalised pyrolytic graphite are encapsulated with epoxy, acrylic or other polymers, including polyimides, using processes described in WO 00/03567. The initial bare plates have a highly ordered graphite structure in the plane of the plate (the a-b crystallographic plane), and the resulting encapsulated structure has its internal graphite planes unchanged, with the value of Kab remaining extremely high, and the orthogonal Kc remaining low in comparison.

Such encapsulated pyrolytic graphite material or encapsulated thermalised pyrolytic graphite material, having high Kab in the planar dimension, may be referred to herein as "TMS material" or "TMS structures" ("TMS" standing for "thermal management structure"). The relevant content of WO 00/03567 (our so-called "TMS patent") concerning the structure and method of manufacture of TMS materials, and their possible use, is incorporated by reference herein. By incorporating pyrolytic graphites with Kab values of the order of 1400 to 1800 W/mK (potentially in the range of about 1000-2400 W/mK), TMS structures maximise thermal spreading in-plane. However, their through- thickness thermal conductivity, Kc, being only 8-10 W/mK, is low in comparison.

There is therefore a desire for electronic substrate and thermal interface

5 materials in which heat may be transferred as efficiently as possible from a source device (e.g. a microprocessor or other heat-generating component) to a heat sink. In particular, it is desired to improve the thermal conductivity of such materials in the directions leading away from the device, i.e. both in the plane of the substrate and, importantly, in its through-thickness direction.0 There is also a need for thermal management materials which can readily be fabricated having a sufficient area or length.

Summary of the Invention

According to a first aspect of the present invention there is provided a method5 of producing a material for use as a thermally-conductive substrate or such like, as defined in Claim 1 of the appended claims. Thus, there is provided a method of producing a material for use as a thermally-conductive substrate or such like, the method comprising the steps of: obtaining a plurality of layers of an anisotropic constituent material, each layer of the constituent material0 having a first thermal conductivity in a first direction, and a through-thickness thermal conductivity, the first thermal conductivity being different from the through-thickness thermal conductivity; stacking and joining the said plurality of layers to form a stack; and cutting through the stack to produce a slice; wherein the orientation of the slice with respect to the layers in the stack is5 such that the through-thickness thermal conductivity of the slice is determined by the first thermal conductivity of the layers in the stack.

810752V1 The term "cutting" should be interpreted broadly, to encompass any process by which a slice may be produced from the stack, such as (but not limited to) using a diamond saw, a diamond wire or a laser; or cleaving.

5 In the phrase "the through-thickness thermal conductivity of the slice is determined by the first thermal conductivity of the layers in the stack", as used above, the expression "determined by" should also be interpreted broadly, to encompass instances in which the through-thickness thermal conductivity of the slice is solely determined by the first thermal conductivity of the layers in 0 the stack, and also instances in which the through-thickness thermal conductivity of the slice is not exclusively determined by the first thermal conductivity of the layers in the stack (i.e. where the through-thickness thermal conductivity of the slice results from the first thermal conductivity of the layers in the stack and the effect of some other factor which affects the resultant 5 through-thickness thermal conductivity of the slice.)

This technique provides the advantage that a slice having a high through- thickness thermal conductivity may be produced from a stack of layers of constituent material having a high thermal conductivity in the first direction.0 Such a slice may then be used to form a thermally-conductive substrate or such like having high through-thickness thermal conductivity. Additionally, the slice may have any desired thickness and planar area, the thickness being determined simply by the thickness of the slice cut, and the planar area being determined by the number of layers in the stack and the maximum planar5 dimension of the constituent layers. Thus, a range of sizes and thicknesses of thermal management materials may be produced using this technique.

Preferable, optional, features are defined in the dependent claims.

810752V1 Preferably the said first direction is an in-plane direction within the layers.

Preferably each layer of the constituent material has a second thermal conductivity in a second direction, and the orientation of the slice with respect 5 to the layers in the stack is such that an in-plane thermal conductivity of the slice is determined by the second thermal conductivity of the layers in the stack.

Preferably the said second direction is an in-plane direction within the layers. 0

Preferably the orientation of the slice is perpendicular to the orientation of the boundaries between the layers in the stack.

The constituent material may comprise any material offering the properties 5 desired for a particular application. For example, the material may be a carbon-carbon composite or a carbon fibre composite.

However, preferably the constituent material comprises graphite, particularly preferably pyrolytic graphite. Pyrolytic graphite possesses an excellent in- 0 plane thermal conductivity, thus providing the resulting slice with a similarly excellent through-thickness thermal conductivity. Thus, the first and second thermal conductivities of the layers in the stack may be determined by the thermal conductivity of graphite within its basal crystallographic planes, and the through-thickness thermal conductivity of the layers in the stack may be5 determined by the thermal conductivity of graphite perpendicular to its basal crystallographic planes.

According to the preferred embodiment, there is no need to individually encapsulate the layers in an encapsulating material prior to forming the stack.

810752V1 However, if desired, the layers may each be encapsulated in an encapsulating material prior to forming the stack. Encapsulating material may not only improve the structural integrity of the individual layers and thereby facilitate construction of the stack, but may also serve as a bonding agent to join adjacent 5 layers to form the stack, and can also enable the stack as a whole to be encapsulated due to spreading of the encapsulating material.

Regardless of whether or not the layers are individually encapsulated prior to forming the stack, the step of joining the layers to form the stack preferably 0 comprises using encapsulating material to bond the layers together. Preferably the step of joining the layers to form the stack further comprises the encapsulating material substantially covering the boundaries between the layers in the stack. Particularly preferably the step of joining the layers to form the stack further comprises the encapsulating material substantially completely 5 encapsulating the stack.

The encapsulating material may comprise epoxy resin, acrylic, polyimide, polyurethane, polyester, active solder, or any another suitable material as will be known to those skilled in the art, and the present disclosure is intended to0 apply to all suitable existing encapsulating materials and those which have yet to be invented or discovered.

Particularly preferably the encapsulating material further comprises an additive to enhance its thermal conductivity, such as particles of graphite, beryllia,5 boron nitride or alumina, or other ceramics, or metals such as silver. Other suitable additives may be known to those skilled in the art, and the present disclosure is intended to encompass all suitable existing additive materials and those which have yet to be invented or discovered.

810752v1 Preferably the step of joining the layers to form the stack comprises applying pressure to the layers, preferably under vacuum.

To aid interlayer adhesion, the method may further comprise a step of drilling 5 one or more holes through the layers in the stack prior to joining them. This advantageously enables encapsulating material to infuse through the holes which improves the integrity of the resulting structure.

According to a second aspect of the present invention there is provided a 0 structure as defined in Claim 28 of the appended claims. Thus, there is provided a structure comprising a plurality of strips, the structure being substantially planar, wherein the strips comprise an anisotropic constituent material, the constituent material having a first thermal conductivity in a first direction and a conventionally-through-thickness thermal conductivity, the first 5 thermal conductivity being different from the conventionally-through-thickness thermal conductivity, and wherein the orientation of the strips within the structure is such that the through-thickness thermal conductivity of the structure is determined by the first thermal conductivity of the constituent material. 0

The term "conventionally-through-thickness thermal conductivity" of the constituent material refers to the thermal conductivity property of the constituent material that, conventionally, is oriented in the through-thickness direction of the constituent material. 5

Thus, advantageously, a structure having a high through- thickness thermal conductivity may be produced from strips of constituent material having a high thermal conductivity in the first direction. Such a structure may then be used to

810752V1 form a thermally-conductive substrate or such like having high through- thickness thermal conductivity.

Preferably the constituent material has a second thermal conductivity in a 5 second direction, and the orientation of the strips within the structure is such that an in-plane thermal conductivity of the structure is determined by the second thermal conductivity of the constituent material. This second thermal conductivity may advantageously be used to provide a direction of high thermal conductivity within the plane of the structure. 0

As with the first aspect of the invention, preferably the constituent material comprises graphite, particularly preferably pyrolytic graphite.

According to a third aspect of the present invention there is provided a 5 thermally conductive structure as defined in Claim 47 of the appended claims.

According to a fourth aspect of the present invention there is provided apparatus for pressing and joining a plurality of layers of material to form a stack, as defined in Claim 53 of the appended claims. This apparatus0 comprises a base and a plurality of side walls arranged to form a cavity for receiving layers to be pressed and joined; means for applying pressure normal to the layers of the stack; and means for applying and adjusting the pressure exerted on the stack by the side walls. By providing means for applying and adjusting the pressure exerted on the stack by the side walls, this apparatus5 advantageously enables the layers to be joined using an encapsulating material which is initially provided around the outside of the stack, and which is forced in between the layers by the side walls.

810752V1 Preferably the means for applying pressure normal to the layers of the stack comprises a ram, piston or plunger.

Preferably the means for applying and adjusting the pressure exerted on the 5 stack by the side walls comprises adjustable bolts which fasten the side walls together. The adjustable bolts may be manually adjustable or motor driven, and the side walls may be moveably mounted on controlled racks.

Preferably the apparatus further comprises plates arranged to surround the 0 layers during pressing, the plates having a coating of encapsulating material. Preferably the coating is screen printed onto the plates.

Preferably the walls are demountable from the base, and preferably the walls and base have a non-adhesive coating. This facilitates removal of the finished 5 stack from the apparatus.

According to a fifth aspect of the present invention there is provided a method of producing a material as defined in Claim 62 of the appended claims. 0 According to a sixth aspect of the present invention there is provided a method of producing a material as defined in Claim 66 of the appended claims.

According to a seventh aspect of the present invention there is provided a method of producing a structure as defined in Claim 67 of the appended claims.5

With all the aspects of the invention, preferable, optional, features are defined in the dependent claims.

810752v1 Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which:

Figure 1 illustrates a stack formed from a plurality of layers (also referred to 5 herein as "plates") of Kab material;

Figure 2 illustrates the cutting of slices through the stack of Figure 1, to produce layers (as shown in Figure 2a) of high Kc' (also referred to herein as

"HKc" layers, planes or plates, etc.);

Figure 3 illustrates a cross-sectional side view through a stack, showing three 0 adjacent constituent Kab layers within the stack;

Figure 4 illustrates a side view of a structure formed by joining six adjacent

HKc plates;

Figures 5 and 6 illustrate perspective and plan views respectively of a box structure for use in forming a stack of Kab plates; 5 Figure 7 illustrates a side view of the box structure of Figures 5 and 6, in use with a plunger to press the Kab plates together;

Figure 8 illustrates a plan view of a box structure (similar to that of Figure 6) in more detail;

Figure 9a illustrates a cross-section view through one of the bolts shown in 0 Figure 8;

Figure 9b illustrates example wall sections of the box structure of Figure 8;

Figures 9c and 9d illustrate side views of two of the side walls of the box structure of Figure 8;

Figure 10 illustrates a side view of the box structure and plunger, in use in a5 first multi-layer processing technique ("MLTa");

Figure 11 illustrates a side view of the box structure and plunger, in use in a second multi-layer processing technique ("MLTb");

Figure 12 illustrates a plan view of HKc plates being used to form a thermal interface material (Example 1);

810752V1 Figures 13a and 13b illustrate plan and side views respectively of HKc plates being used to form a thermal spreader material (Example 2);

Figure 14 illustrates a plan view of HKc plates being used to form a thermal channelling material (Example 3); 5 Figure 15 illustrates a cross-section view of HKc plates being used in conjunction with high Kab material to form a multilayered thermal spreading structure (Example 4);

Figures 16a and 16b illustrate cross-section and plan view respectively of HKc plates being used for high power thermal spreading and heat extraction0 (Example 5);

Figure 17a illustrates a finite element simulation performed on the structure of

Figure 12, and Figure 17b illustrates a corresponding simulation performed on a copper pad;

Figure 18a illustrates a finite element simulation performed on the structure of5 Figures 13a and 13b, and Figure 18b illustrates a corresponding simulation performed on a copper plate;

Figure 19a illustrates a finite element simulation performed on the HKc channel of Figure 14, and Figure 19b illustrates a corresponding simulation performed on a copper channel; and 0 Figures 20a and 20b illustrate plan views of two further mosaic structures made of HKc plates.

The figures are not to scale. In the figures, like elements are indicated by like reference numerals throughout. 5

Detailed Description of Preferred Embodiments

The present embodiments represent the best ways known to the applicant of putting the invention into practice. However, they are not the only ways in which this can be achieved.

810752V1 Specific embodiments will be described in detail below, but by way of an initial overview, a basic embodiment will first be described, with reference to Figures 1, 2 and 2a, to illustrate the present concept. As shown in Figure 1, a 5 plurality of layers 12 of an anisotropic constituent material are first stacked and joined together to form a stack 10.

Each layer 12 of the constituent material has in-plane thermal conductivities Ka and Kb, and a through-thickness thermal conductivity Kc. In some materials, 0 Ka may be equal to Kb, and consequently the in-plane thermal conductivity may be referred to as Kab. The letters a, b and c refer to the crystallographic axes within the layers 12, as those skilled in the art will appreciate.

Next, as shown in Figure 2, the stack 10 is cut through, perpendicular to the 5 boundaries between the layers 12, to form one or more slices 14. The planes along which the cuts are made are indicated with dashed lines in Figure 2.

Figure 2a shows a slice 14 that has been cut from the stack 10 of Figure 2. The slice 14 is planar in form. The orientation of the slice 14 with respect to the 0 initial stack 10 is such that an in-plane direction (e.g. the b axis) within the initial layers 12 has become the through-thickness direction (denoted by c¹) of the slice 14. Similarly, the through-thickness direction (the c axis) of the initial layers 12 has become an in-plane direction (denoted by b¹) within the slice 14. The in-plane a direction within the initial layers 12 remains an in-plane5 direction (denoted by a') within the slice 14.

Thus, in this example, the through-thickness thermal conductivity of the slice 14, denoted by Kc', is determined by the in-plane thermal conductivity of the initial layers 12, denoted by Kb. The in-plane thermal conductivity of the slice

810752V1 14, parallel to the boundaries within the slice (the boundaries resulting from the boundaries between the individual layers in the stack 10), denoted by Ka', is determined by the in-plane thermal conductivity of the initial layers 12 in the same direction, denoted by Ka. The in-plane thermal conductivity of the slice 5 14, perpendicular to the boundaries within the slice, denoted by Kb', is determined by the through-thickness thermal conductivity of the initial layers 12, denoted by Kc.

It will be appreciated that the slice 14 in the above example has been formed by 0 cutting parallel to the a axis within the initial layers 12, and consequently the a axis within the initial layers 12 has remained an in-plane direction (a¹) within the slice 14. However, the slice could alternatively have be formed by cutting parallel to the b axis within the initial layers 12, perpendicular to the boundaries between the layers, in which case the a axis within the initial layers 5 12 would have become the through-thickness direction (c¹) of the slice, and the b and c axes within the initial layers would have respectively become the in- plane directions b' and a' within the slice, with b' being parallel to the boundaries within the slice, and a' being perpendicular to the boundaries. Thus, in this example, the thermal conductivity values within the plane of the slice,0 Ka' and Kb¹, would respectively be determined by Kc and Kb of the initial layers 12, and the through-thickness thermal conductivity of the slice, Kc', would be determined by Ka of the initial layers.

Instead of using in-plane thermal conductivities and a through-thickness5 thermal conductivity, the current principle may be regarded more generally, as follows. Layers of any suitable anisotropic constituent material may be used. Each layer of the constituent material has a first thermal conductivity in a first direction, and a through-thickness thermal conductivity, the first thermal conductivity being different from the through-thickness thermal conductivity.

810752V1 The layers are then stacked and joined to form a stack, and the stack is cut through to produce a slice. The orientation of the slice with respect to the layers in the stack is such that the through-thickness thermal conductivity of the slice is determined by the first thermal conductivity of the layers in the stack. 5 Moreover, the constituent material may have a second thermal conductivity in a second direction, and the orientation of the slice with respect to the layers in the stack may be such that an in-plane thermal conductivity of the slice is determined by the second thermal conductivity of the layers in the stack. The first and second directions need not be in the plane of the constituent layers in 0 the stack, although in preferred embodiments (as will be discussed below) the first and second directions are in-plane directions within the constituent layers.

With reference to Figures 1, 2 and 2a, these so-called first and second directions are in-plane directions within the constituent layers 12. The thermal 5 conductivity in the first direction is Kb, which determines the through- thickness thermal conductivity Kc' of the slice 14. The thermal conductivity in the second direction is Ka, which determines the in-plane thermal conductivity Ka' of the slice. 0 New orthotropic material with excellent through-thickness thermal conductivity Using the above principle, as illustrated in Figures 1, 2 and 2a, a new orthotropic material has been developed having excellent through-thickness thermal conductivity. The new material is formed from layers 12 of high Kab pyrolytic graphite, which are stacked and joined to form a stack 10. The stack5 10 is then cut to produce slices 14 of the new material. The new material will be referred to herein as "HKc" pyrolytic graphite ("HKc" being short for "high Kc"), and has through-thickness thermal conductivity values, Kc¹, of typically 1400 to 1800 W/mK (potentially in the range of about 1000-2400 W/mK) for 'through' heat transmission, whilst also providing one planar degree of

810752V1 freedom, e.g. Ka', for in-plane heat spreading parallel to the internal boundaries. The thermal conductivity along the third axis, e.g. Kb', perpendicular to the internal boundaries, is determined by the thermal conductivity in the c direction of the original pyrolytic graphite layers, and is 5 typically 10 W/mK. In this direction there is also a thermal interface resistance, determined by the thermal properties of the interface material and its thickness, that can be of comparable thermal resistance to the intrinsic Kb¹ value, or less than Kb'. 0 The Kc' value of the HKc material (i.e. typically 1400 to 1800 W/mK) should be contrasted with the typical Kc values of 28 W/mK for alumina, 180 W/mK for aluminium nitride, and 10 W/mK for conventional graphite materials. Thus, compared to these existing materials, an improvement in through- thickness thermal conductivity of the order of 10-180 times has been achieved. 5

It is also instructive to compare the Kc' and density values of the HKc material with those of copper. The Kc' value of the HKc material is about four times that of copper, whilst the density of the HKc material is about one quarter that of copper. Using a figure of merit determined by Kc' divided by density, this 0 leads to the HKc material providing a 16-fold improvement over the properties of copper.

Using micron-level encapsulation by epoxy, acrylic or other polymers (optionally enhancing the thermal conductivity of the encapsulation by adding5 additives such as graphite, boron nitride, beryllia, alumina, or other ceramics, or metals such as silver), or using direct metallic encapsulation, thin plates of HKc material (typically ~1 mm thick) can provide thermal interface resistance down to ~0.1⁰CAV per cm² area, a value dominated by the properties of the encapsulant, and where resulting typical temperature differences are in the

810752V1 range 2°C to 10°C for a 50 W chip, dependent upon the thickness and K value of the encapsulant, for HKc thicknesses in the millimetre range.

In overall thermal management design at the board level the new HKc devices 5 can be used as custom TIMs for device interfacing and/or incorporated with TMS planar devices into spreader boards (or circuit boards) to spread, channel, or transfer heat within customised geometries.

As mentioned above, the effectiveness of the 'extraction surface' of direct0 copper bonding (DCB) is currently limited by its interface to aluminium or copper structures that form extraction heat sinks. It could in future be interfaced to HKc structures, or HKc structures may themselves be directly encapsulated in copper and processed to provide alternative devices that could replace DCB technology, which currently has difficulties to overcome internal5 CTE mismatches that lead to premature mechanical plaquette failures.

In summary, HKc devices use high orthotropic thermal conductivity, K(c',a') or K(c',b'), for much enhanced heat transit orthogonal to the fabricated graphite planes in addition to excellent planar spreading, and are complementary to0 TMS devices where the high in-plane Kab spreads heat much more effectively than other more conventional materials. Optimised thermal designs may best be achieved by being able to combine both technologies to meet the needs of each particular configuration. 5 The present proposal provides high Kc¹ planes of potentially large area (say 200mm by 200mm) that would meet the normal geometrical requirements of the microelectronics and power electronics industries, and have exceptionally high K(c',a') or K(c',b') values of the order of 1400 to 1800 W/mK (potentially

810752V1 in the range of about 1000-2400 W/mK) in the through-thickness (c¹) direction and in an in-plane direction.

Procedure for producing planar high Kc ("HKc") pyrolytic graphite material 5 by infusion and encapsulation

This technique uses high Kab planar PG structures, from which the HKc materials are fabricated in a novel, straightforward and efficient process, producing large areas of custom thickness. The procedure to convert these plates directly through encapsulation into thermal interface materials (TIMs) 0 follows below. By custom encapsulation, these TIMs may be incorporated into more complex thermal management systems. Possible encapsulating materials include epoxy resin, acrylic, polyimide, polyurethane, polyester or another suitable polymer, or active (fluxless) solder. The thermal conductivity of the encapsulating material may be increased by adding an additive such as 5 graphite, beryllia, boron nitride, alumina, or other ceramics, or metals such as silver.

1. Prepare planar Kab PG materials 12 as described in the TMS patent WO 00/03567, clean, degrease, etc. Each planar layer 12 comprises pyrolytic 0 graphite. The initial individual PG layers 12 can be of any initial thickness, but are each preferably from 0.5mm to a few mm thick. There is no need for the initial individual PG layers 12 to be individually encapsulated prior to forming a stack. 5 2. The planar Kab PG materials 12 are used to form a stack 10 as shown in Figure 1, where the horizontal side dimensions can be from a few cm to 10s of cm, and the number of plates 12 in the stack 10 can be from a few to a few hundred or more.

810752V1 3. At this stage it is possible to drill fine (e.g. 100 μm diameter) penetration holes through the layers 12, orthogonal to the a-b plane, if required, to aid interlayer adhesion.

5 4. The stack 10 is then introduced into jigging to allow infusion of epoxy (or another suitable encapsulating material) between the plate layers 12 and also to completely encapsulate the stack 10. The encapsulating material may initially be provided around the outside of the stack 10, and may be forced in between the layers 12 by the jigging. 0

5. The processing is done preferably under conditions of vacuum (to remove air) and under interlayer pressure to provide a minimum but complete (few micron) epoxy interface between each adjacent layer 12 in the stack 10. 5 6. After the encapsulation has polymerised, the stack 10 is removed from the jigging. Figure 3 shows a cross-sectional view through the stack 10, showing the structure of three adjacent layers 12. Adjacent layers 12 are joined by encapsulating material 16, which covers the boundaries between adjacent layers and also surrounds the stack 10 as a whole. The stack 10 is then cut as 0 required along the planes parallel to the c-direction to form one or more slices 14. The slices 14 may be any desired thickness, but are typically of the order of a millimetre thick. Thus, this procedure produces layers 14 of high K(c',a') material of predetermined thickness with minimal added thermal impedance across the c-direction boundaries, and hence having a thermal conduction5 ability very comparable to that of a 'pure' K(c',a') material.

7. The cutting of the stack 10 (now a block) can be done for example by laser or by diamond saw, to produce HKc plates 14 with micron-smooth surfaces. There need be minimal loss of material (already achieved down to 0.3 mm for

810752V1 each cut, and this can be reduced further with thinner blades or by laser slicing). When the HKc plates are cut from the stack, it will be appreciated that they will not be completely encapsulated, due to the freshly cut surfaces being exposed. The HKc plates may be left in this form if desired, or may be 5 completely encapsulated by a further encapsulation procedure (see steps 8 and 9 below).

8. HKc planes 14 can be further made into larger mosaic structures by epoxy (or other) encapsulation using the jigging, pressure, etc techniques of the TMS 0 patent. A linearised example is shown herein in Figure 4, in which a series of individual HKc plaquettes 14 have been integrated to form a larger HKc structure 18 using TMS fabrication techniques, using encapsulant, pressure and vacuum. Alternatively HKc planes can, by the same techniques, be combined with high Kab (i.e. TMS-like) material to form transit and transfer channels, 5 and spreading, as required for any given application.

9. HKc plates 14 can be individually surface-encapsulated to make TIMs of tailored thickness, with only a few microns of encapsulation. This is a unique feature of our processing for both TMS and HKc to minimise thermal interface0 resistance down to values below ~0.1⁰CAV, to make them attractive for processor chip interfaces.

If desired, an additional intermediate step may be included, after the formation of the stack 10, and before the stack is cut to produce the HKc slices 14. This5 additional intermediate step comprises cutting through the stack 10, and optionally reshaping or reforming the stack or otherwise changing its geometry, before the stack is cut to produce the HKc slices 14. This intermediate step facilitates the production of a plurality of HKc slices 14 having a desired geometry.

810752V1 Practical techniques for producing planar high Kc ("HKc") pyrolytic graphite material by infusion and encapsulation

1. Jigging: A box structure 20 such as that shown in Figures 5 and 6, Figure 7, 5 or Figures 8 and 9a-c may be used. The box structure has a base 30, and side panels 22, 24, 26, 28 each several mm thick. All (Figs 5 and 6) or the majority (Fig 8) of the box sides 22, 24, 26, 28 are demountable and coated to have non- adhesive surfaces. 0 2. The sides 22, 24, 26, 28 can be fixed together with screws or preferably on controlled racks and pulled into place with bolts 36, and similarly can be screwed to the base plate 30 from below or attached in grooves. This process causes the infusion of epoxy (or other encapsulating material) into the planar assembly from initially around the outside of the stack. Figure 9a shows a 5 cross-section through such a bolt 36, having a threaded end section, and a spring 37 in a cavity between the two walls being joined. Figure 9c shows an arrangement of four such bolts 36 on side 26 of the box structure 20. As shown in Figure 9d, side 28 may include an extra bolt 36a for use in drawing sides 26 and 28 together, to apply additional overall pressure to the stack, to aid infusion0 of the encapsulating material. The bolts may have an initial degree of freedom to aid the initial fit-up and drawing-together of the walls 22, 24, 26, 28.

3. As shown in Figure 8, it is preferable to use thin (~250μm thick) metal plates 34 (with non-adhesive coatings) that are screen printed with the epoxy5 for infusion and encapsulation, and that fit into groves in the side panels 22, 24, 26, 28 of the box structure 20. A similar plate is set into the base area 30 and, as shown in Figure 9b, another such plate 35 is placed above the top element of the stack 10 prior to pressing. This allows the correct application of the epoxy and ease of dismounting the final stack/block 10 after pressure and vacuum

810752V1 have been applied and curing is complete. As shown in Figure 9b, mounting pins 38 can be used to hold the printed plates 34 in position on the side panels before and during insertion of the Kab plates. The mounting pins 38 are removed prior to compression of the Kab plates. 5

4. As shown in Figure 7, to press the Kab plates together a ram or plunger 32 is inserted into the open area of the box structure 20, above the planar stack 10. The bottom surface of the plunger has a non-adhesive coating and fits, with a ~50 μm perimeter free spacing, inside the vertical cavity within the box 0 structure 20. The side walls 22, 24, 26, 28 of the box structure are tightened together to cause the infusion of epoxy into the planar assembly from the surface of the plates 34.

5. The stack inside the support jigging is then placed inside an overall 5 'pressing frame' as for TMS production and placed in a press that can be heated if required. Air is removed from the encapsulant through evacuation, and pressure is applied to the stack 10 via the plunger 32, and the whole is left for curing at the required temperature. 0 6. The resultant block 10 of PG material is then accessed by dismantling the surrounding walls 22, 24, 26, 28. Once cured, the thickness of the interlayer epoxy between the initial layers 12 may be of the order of a few microns to a few tens of microns, typically around 10 μm, but for some purposes may be as low as a few microns (i.e. typically 5 μm or less) in order to optimise the5 thermal conductivity between adjacent layers 12. The thermal conductivity between adjacent layers 12 may also be enhanced by adding additives to the encapsulating material, such as graphite, boron nitride, beryllia, alumina, or other ceramics, or metals such as silver.

810752V1 7. The resultant block 10 can then be cut in one of two alternative planes as shown in Figure 2. In one case the HKc sheet 14 has a high K(c',a'), and in the other it has a high K(c',b').

5 8. The HKc planes 14 can be epoxy coated (as in the TMS patent) to provide TIMs, or if sub-micron coatings (or coatings up to a few microns) are required a conformal parylene (paralene) coating can be made. Alternatively the HKc planes can be made into larger mosaic HKc plates with thin epoxy coatings through encapsulating again under vacuum and pressure. They can also be 0 directly coated with metal (e.g. copper) by chemical and electrolytic plating to use as basic elements for thermal transmission/spreading for power electronics.

Multi-layer processing techniques (MLT) for Kab pryolytic graphite structures WO 00/03567, WO 2004/097934 and WO 2004/097936 cover various 5 encapsulation and processing techniques for providing new forms of thermal management structures and devices for electronic systems and power sources. The MLT process is a novel technique for making 3D structures using TMS patented techniques, and can be combined with the present HKc devices to make extended novel 3D structures that optimise the full 3D thermal 0 management potential of pyrolytic graphite materials.

MLT allows individual high Kab pyrolytic plates to be combined into thicker plates, so providing a straightforward method of graphite substrate production (and with high yield), and with the ability to castellate such structures into 3D5 forms. The resulting thermal conductivity properties of the 'extended thickness' MLT plates have been shown to be consistent with those provided by substrates of the same thickness that have come from single plates extracted directly in the initial CVD processing; this confirms that the MLT techniques provide excellent Kab interlayer adherence.

810752V1 The process is as follows:

1. Prepare sub-plates of Kab material as for TMS processing (degrease and 5 clean).

2. MLTa (Figure 10): If the 'unified' structure is to be used as single unit with n-layers of thickness d becoming a new layer of thickness n*d it can be prepared simply by placing in a mould 50 of appropriate depth, with an epoxy 0 coated base plate 52 (itself coated with a non-adhesive coating such as Teflon (RTM)), and a similarly prepared top plate 54. In the example illustrated, there are four layers of Kab plates 58. A plunger 56 is then inserted into the jig mould 50 and the system is put under pressure and vacuum. After the epoxy curing the n*d thick MLT plate is removed from the jig, and the edge coating 5 of epoxy 59 provides the necessary forces to ensure the integrity of the complete structure. This is maintained even if the encapsulating top and bottom epoxy layers are removed.

The MLTa process provides a method of producing a structure comprising a 0 plurality of layers of a constituent material (e.g Kab plates), each layer of the constituent material having an in-plane thermal conductivity significantly greater than the thermal conductivity in the through-thickness direction. The method comprises stacking and joining the said plurality of layers to form a stack, wherein the layers are sufficiently compressed in order to provide5 interlayer forces comparable with the forces present in a single crystal or monolithic object formed of the same constituent material, such that the thermal conductivity properties of the stack are essentially those of a single crystal or monolithic object formed of the same constituent material and having

810752V1 the same dimensions and internal crystallographic planar orientations as the stack.

3. MLTb (Figure 11): If a 3D structured surface is required it may be 5 necessary to provide additional interlayer adherence by introducing small holes 60 into the plates, by drilling orthogonal to the a-b plane. Such holes 60 can be from 10s of microns to a few hundred micron in diameter, and be on a matrix with centimetre spacing if required. Epoxy 59 can infuse through the holes 60, which improves the integrity and resultant thermal conductivity of the structure0 as a whole. The infused epoxy 59 in the holes 60 may typically be designed to occupy no more than about 1% of the area of the Kab plane, and causes negligible decrease to the Kab (typically of the level of 0.5-1% or less). The example shown has 4 layers 58, with a layered template 62 being used to form a castellation in the top layer 58a, and an HKc insert 64 in the second layer5 58b, and shows a schematic representation of the epoxy-filled microholes 60.

Examples of applications of the HKc material

Some possible applications of the HKc material will now be discussed, by way of example. 0

Example 1: Thermal Interface Pad (TIP) (Figure 12)

Figure 12 illustrates a plan view of a thermal interface pad or plate 70, made up of four quadrant sections of HKc material. The HKc material has high through-thickness thermal conductivity (Kc') and, in each quadrant, high in-5 plane thermal conductivity (Ka') in the direction shown by the large arrows.

The dashed lines indicate the possible outline edges of a chip 76 mounted beneath the HKc plate 70.

810752V1 The HKc thermal interface pad 70 provides good thermal spreading laterally AND direct upwards heat transfer from the chip 76 below, and spreading decreases losses at subsequent thermal interfaces by lowering heat flux density. The lateral spreading decreases the heat flux density at the upper pad surface 5 and hence decreases temperature gradients across the subsequent thermal interface.

Each or all of the four outer edges of the pad 70 may be located at or near boundary cooled areas. 0

The thickness of the pad 70 can be customised as required, preferably being 500μm to few mm.

The pad 70 can have encapsulated top/bottom/edges. Indeed, encapsulation 5 can be applied to any HKc device if required. Encapsulating materials can include polyimides or epoxy resins or acrylics or polyurethanes or polyesters or any other suitable polymers. The thermal conductivity of the encapsulating material may be increased by adding an additive such as graphite, beryllia, boron nitride, alumina, or other ceramics, or metals such as silver. The 0 encapsulant can be from microns to tens or hundreds of microns thick, dependent upon thermal and thermal-mechanical requirements. The encapsulant can also be a paint or parylene (paralene) coated with thickness from ~lμm to tens of microns, or the HKc device can be directly metalised with, for example, deposited copper in thicknesses from ~lμm to many tens of5 microns.

Assembly process: First produce a HKc material 'block' 10, then cut planes 14 to the required thickness for the thermal interface pad 70, and then cut the planes 14 into 'quadrant' (or other) forms. Clean and degrease all surfaces.

810752V1 Assemble HKc subsections into non-adhesive coated template moulds of an appropriate thickness. (Screen) print top and bottom pressure plates of mould with epoxy or other suitable adhesive and encapsulate the complete structure under pressure and vacuum. This can be done to produce a final TIP 70 with 5 required encapsulant thickness, or alternatively this can provide the structured planar form that can be skimmed on its upper and lower surfaces to provide access to the bare graphite surface, that can itself then be coated with for example parylene, epoxy, etc, or with a compliant thermally conducting compound, or grease, or with a combination of such materials. This produces a0 device of extremely high thermal conductivity normal to the interface plane and with excellent lateral spreading, and with potential for low thermal interface resistance.

Figures 17a and 17b illustrates finite element simulations of a square plate or5 pad 70 of thermally conductive material, as illustrated in Figure 12, with a heat source 76 directly attached on its underside. Figure 17a illustrates the simulation performed on a HKc pad 70. The dashed lines 100 indicate the boundaries between the HKc quadrant sections 71, 72, 74, corresponding to the diagonal lines in Figure 12. By way of comparison, Figure 17b illustrates the0 same simulation performed on a same-dimensioned copper pad 102. In each case, equi-spaced temperature contours are illustrated, with spacing ΔT.

In the simulation, boundary conditions hold the perimeter edge of each section 71, 72, 74 at the same fixed temperature, and each has the same power supplied5 by its underside heat source 76. The temperature contours shown are equi- spaced in temperature difference and have the same scale in each case. The plots show 3D views of half of each structure (which is symmetric front to back), showing temperature contours through the centre (nearest to the viewer) and on the top surface.

810752V1 The upper plot (Figure 17a) is for a pad of HKc material 70 assembled from four quadrants 71, 72, 73, 74, and the lower plot (Figure 17b) has the same thickness and area of pad, but with copper 102 as the material. Hence there is 5 four times the mass in the copper pad than in the HKc pad, but even so for the copper pad the temperature difference between the heat source 76 and the pad boundary is more than twice that for the HKc case (-14 ΔT units for copper, compared to ~6 for HKc). 0 In practice, in such a configuration, a power source of 5OW would be lower in temperature by about 15°C for a HKc pad, compared to using the same thickness of copper pad.

Example 2: Thermal Spreader Material (TSM) (Figures 13a and 13b) 5 Figures 13a and 13b illustrate plan and side views respectively of a thermal spreader structure 77 employing both HKc material 78 and TMS material 80.

By providing a HKc insert 78 adjacent the heat source 76, within TMS material, this configuration allows a TMS device 80 with excellent Kab thermal in-plane conductivity to access the heat as efficiently as possible from a0 powered semiconductor device 76 below it. The HKc material 78 can be fused into a structure as shown. This can be done using micron level adhesive interface materials at the HKc/TMS boundaries with high thermal conductivites. Normally in such usage the heat is spread to cooled edges of the

Kab material, or to surfaces with convection cooling for extracting the heat. In5 this example the front and rear edges 84 are best suited as the cooled edges.

Figure 18a illustrates a finite element simulation of a plate with an insert of HKc thermally conductive material 78 in a TMS plate 80. By way of comparison, Figure 18b illustrates the simulation performed on the same

810752V1 thickness and area of plate with copper 104 as the material. In each case there is a directly attached heat source 76 on the underside.

In Figure 18a, the dashed lines 82 indicate the orientation of the high 5 conductivity planes of the HKc insert material 78.

In each case, equi-spaced temperature contours are illustrated, with spacing ΔT along the direction of the arrow. The heat source 76 is at the centre of the plate. The boundary condition is a fixed sink temperature along two opposite 0 plate edges 84, and each plate has the same power supplied by its underside heat source 76. The arrangement is symmetric (front-back), and the plots in Figures 18a and 18b show temperature contours for the back half only.

In Figure 18a the block of HKc material 78 directly contacts the source 76, and 5 is oriented with its high conductivity direction aligned front to back (i.e. towards the heat sink 84). It is embedded in a larger plate of TMS 80. Contact between the HKc material 78 and the TMS material 80 is assumed to be a high thermal conductivity compound (grease or adhesive). 0 Figure 18b shows the result, on the same scale, when the HKcATMS structure 77 is replaced by a single copper plate 104 of identical dimensions. Hence there is four times the mass in the copper structure 104 than in the HKc/TMS structure 77, but even so with the copper plate 104 the maximum temperature of the source is -19 ΔT units above the sink, while for the HKc/TMS structure5 77 the temperature of the source is only ~7 ΔT units above the sink; a difference of 12 ΔT units.

In practice, the plot in Figure 18a corresponds to a 10 mm x 10 mm chip 76, dissipating 50W, in ideal contact with a 1 mm thick HKc block 78. This HKc

810752V1 block 78 is embedded in a TMS block 80, 30 nun wide and 50 mm from front to back, also 1 mm thick. The heat sink is at 3O⁰C. With the HKc/TMS structure 77, the temperature of the source 76 is about 21⁰C above that of the sink 84. However, in the plot in Figure 18b, where the HKc/TMS structure 77 5 has been replaced by copper 104, the temperature of the source 76 is about 57⁰C above that of the sink - i.e. about 36°C hotter than in the HKc/TMS case.

Example 3: Thermal Channelling Material (TCM) (Figure 14)

Figure 14 illustrates that, within a thermal spreader plate 86 servicing several 0 powered devices 88, 89, 90, it is possible to insert (using process steps as for Examples 1 and/or 2 above) HKc channels 92 for preferential heat channelling to cooled points 94 or areas for heat extraction. In the illustration, device 90 could be high power. Devices 88 and 89 could be lower power, i.e. not requiring preferential heat channelling. The channelling not only removes heat 5 from device 90, but also shields devices 88 and 89 from being heated by device 90.

Figures 19a and 19b illustrate finite element simulations of a channel of thermally conductive material with a heat source 76 directly attached on its0 underside. Figure 19a illustrates the simulation performed on a HKc channel 92. By way of comparison, Figure 19b illustrates the same simulation performed on a same-dimensioned copper channel 106. In each case, equi- spaced temperature contours are illustrated, with spacing ΔT along the direction of the arrow. 5

In both Figures 19a and 19b, the boundary condition is a fixed sink temperature at the far end 108 of channel, and each channel 92, 106 has the same power supplied by its underside heat source 76 (e.g. an electronic chip). Heat is conducted from the source 76 to the remote heat sink 108 by a strip of

810752V1 thermally conductive material of identical width to the source 76. The strip length is five times that of the source 76. The left hand edge of the strip 108 is in contact with the heat sink, and heat is generated uniformly across the contacting area of the source 76. The geometry and the scale of the 5 temperature contours are identical for both simulations.

The upper plot (Figure 19a) is for a channel of HKc material 92, and the lower plot (Figure 19b) has the same thickness and length of channel, but with copper 106 as material. Hence there is four times the mass in the copper channel 106 0 than in the HKc channel 92, but even so for the copper channel the temperature difference between the heat source 76 and the sink 108 is approximately four times that for the HKc case (~20 ΔT units for copper, compared to ~5 ΔT units for HKc). 5 In practice, this simulation corresponds to a 10 mm x 10 mm chip 76, dissipating 2OW, in ideal contact with a 50 mm x 10 mm strip of material attached to a heat sink at 3O⁰C. With the HKc channel 92 the power source 76 would be 53⁰C above the sink. However, with the copper channel 106 of the same dimensions the source 76 would be 221⁰C above the sink, which is 168°C 0 hotter.

Example 4: Multilayered Thermal Spreading (Figure 15) Figure 15 illustrates that, using the Kab mutilayering techniques described above as 'MLT processing', multilayer 3D structures can be produced with5 HKc inserts 78 within a high Kab structure 80.

Example 5: High Power Thermal Spreading (Figures 16a and 16b)

Figures 16a and 16b illustrate that HKc material 96 can be directly metal coated (e.g. by a copper deposit coating 97) and attached to fins 98 for

810752v1 convective heat extraction, or alternatively to a heat sink block for conductive heat extraction. High power devices can be attached directly (e.g. brazed) to the upper surface of the structure to provide excellent direct heat transfer. As the double-headed arrow in Figure 16a shows, the HKc material 96 provides 5 high through-thickness transit thermal conductivity (Kc') for heat extraction to the sink. Similarly, as the double-headed arrow in Figure 16b shows, the HKc material also provides high planar thermal conductivity (Ka¹) for heat spreading. 0 Also thin (e.g. 100 micron) ceramic coatings of, for example, alumina can be applied to the copper-coated HKc material to provide electrical insulation, and subsequent metal processing can be made on the ceramic surface to provide power and control line connectivity for the power devices. 5 Example 6: Further mosaic structures for thermal spreading (Figures 20a and 20b)

Figures 20a and 20b illustrate plan views of two further mosaic structures made of HKc plates. Both structures may be used with a centrally-mounted heat source (not shown). The structure shown in Figure 20a may be used if all the 0 edges (A, B, C and D) are cooled to serve as heat sinks. On the other hand, due to the preferential thermal conduction of the Kc material in the direction parallel to the boundaries between the initial Kab layers 12, the structure shown in Figure 20b is suitable if only sides B and D are cooled to serve as heat sinks. 5 Other materials

The principles described above may be carried out using any constituent material having anisotropic properties of thermal conductivity properties- for example, carbon-carbon composites or carbon fibre composites.

810752v1 Other material properties

It will be appreciated that the examples, materials and techniques described thus far have all been in connection with the property of thermal conductivity. However, to the extent that other physical properties change anisotropically in 5 a similar manner, then the technique of stacking and slicing layers as described herein may be applied to preferentially obtain (or re-orientate) other material properties.

Examples of such properties may include magnetic susceptibility, magnetic 0 permeability, electrical conductivity, dielectric properties, electrical permittivity, optical properties, piezoelectric constants, hardness, Young's modulus, shear modulus, Poisson ratio, components of the stiffness tensor, etc.

The electrochemical characteristics of anisotropic materials (e.g. carbon fibres) 5 may be transformed by the present process. Thermo-physical properties, such as the coefficient of thermal expansion (volume and linear) and diffusivity may be transformed in materials such as fine-grained graphites, anisotropic carbon or carbon fibre composites, or woven fibres in a graphitised pitch matrix.

810752V1

Claims

1. A method of producing a material for use as a thermally-conductive substrate or such like, the method comprising the steps of:

5 obtaining a plurality of layers of an anisotropic constituent material, each layer of the constituent material having a first thermal conductivity in a first direction, and a through-thickness thermal conductivity, the first thermal conductivity being different from the through-thickness thermal conductivity; 0 stacking and joining the said plurality of layers to form a stack; and cutting through the stack to produce a slice; wherein the orientation of the slice with respect to the layers in the stack is such that the through-thickness thermal conductivity of the slice is determined by the first thermal conductivity of the layers in the stack. 5

2. A method as claimed in Claim 1, wherein the said first direction is an in- plane direction within the layers.

3. A method as claimed in Claim 1 or Claim 2, wherein each layer of the 0 constituent material has a second thermal conductivity in a second direction, and wherein the orientation of the slice with respect to the layers in the stack is such that an in-plane thermal conductivity of the slice is determined by the second thermal conductivity of the layers in the stack. 5

4. A method as claimed in Claim 3, wherein the said second direction is an in-plane direction within the layers.

810752V1

5. A method as claimed in any preceding claim, wherein the orientation of the slice is perpendicular to the orientation of the boundaries between the layers in the stack.

5 6. A method as claimed in any preceding claim, wherein the constituent material comprises a carbon-carbon composite or a carbon fibre composite.

7. A method as claimed in any of Claims 1 to 5, wherein the constituent 0 material comprises graphite.

8. A method as claimed in Claim 7, wherein the constituent material comprises pyrolytic graphite. 5

9. A method as claimed in Claim 7 or Claim 8, wherein the first and second thermal conductivities of the layers in the stack are determined by the thermal conductivity of graphite within its basal crystallographic planes, and the through-thickness thermal conductivity of the layers in the stack is determined by the thermal conductivity of graphite 0 perpendicular to its basal crystallographic planes.

10. A method as claimed in any of Claims 1 to 9, wherein the layers are each encapsulated in an encapsulating material prior to forming the stack. 5

11. A method as claimed in any preceding claim, wherein the step of joining the layers to form the stack comprises using encapsulating material to bond the layers together.

810752V1

12. A method as claimed in Claim 11, wherein the step of joining the layers to form the stack further comprises the encapsulating material substantially covering the boundaries between the layers in the stack.

5 13. A method as claimed in Claim 11 or Claim 12, wherein the step of joining the layers to form the stack further comprises causing the encapsulating material to substantially completely encapsulate the stack.

14. A method as claimed in any of Claims 11 to 13, wherein the 0 encapsulating material comprises epoxy resin, acrylic, polyimide, polyurethane, polyester or another suitable polymer, or active solder.

15. A method as claimed in any of Claims 10 to 14, wherein the encapsulating material further comprises an additive to enhance its 5 thermal conductivity.

16. A method as claimed in Claim 15, wherein the additive comprises graphite, beryllia, boron nitride, alumina or silver. 0

17. A method as claimed in any preceding claim, wherein the step of joining the layers to form the stack comprises applying pressure to the layers.

18. A method as claimed in any preceding claim, wherein the step of joining the layers to form the stack is performed under vacuum. 5

19. A method as claimed in any of Claims 11 to 18, further comprising a step of drilling one or more holes through the layers in the stack prior to joining them.

810752V1

20. A method as claimed in Claim 19, wherein the diameter of the hole(s) is of the order of 100 μm.

21. A method as claimed in any preceding claim, wherein the step of cutting 5 is performed using a diamond saw or diamond wire or a laser, or by cleaving.

22. A method as claimed in any preceding claim, further comprising cutting a plurality of such slices and then joining them to form a structure. 0

23. A method as claimed in any preceding claim, wherein the thickness of each layer used to form the stack is in the range of 100 μm to a few millimetres or tens or millimetres. 5

24. A method as claimed in any preceding claim, wherein the side dimension of the layers used to form the stack is in the range of a few centimetres to tens of centimetres.

25. A method as claimed in any preceding claim, wherein the number of0 layers used to form the stack is in the range from a few to a few hundred.

26. A method as claimed in any preceding claim, wherein the thickness of the or each slice is of the order of a millimetre. 5

27. A method as claimed in any preceding claim, further comprising an intermediate step, after the formation of the stack, and before the stack is cut to produce the slice, of cutting through the stack, and optionally reshaping or reforming the stack or otherwise changing its geometry.

810752V1

28. A structure comprising a plurality of strips, the structure being substantially planar, wherein the strips comprise an anisotropic constituent material, the constituent material having a first thermal

5 conductivity in a first direction and a conventionally-through-thickness thermal conductivity, the first thermal conductivity being different from the conventionally-through-thickness thermal conductivity, and wherein the orientation of the strips within the structure is such that the through- thickness thermal conductivity of the structure is determined by the first 0 thermal conductivity of the constituent material.

29. A structure as claimed in Claim 28, wherein the constituent material has a second thermal conductivity in a second direction, and wherein the orientation of the strips within the structure is such that an in-plane 5 thermal conductivity of the structure is determined by the second thermal conductivity of the constituent material.

30. A structure as claimed in Claim 28 or Claim 29, wherein the constituent material comprises a carbon-carbon composite or a carbon fibre 0 composite

31. A structure as claimed in Claim 28 or Claim 29, wherein the constituent material comprises graphite. 5

32. A structure as claimed in Claim 31, wherein the constituent material comprises pyrolytic graphite.

33. A structure as claimed in Claim 31 or Claim 32, wherein the first and second thermal conductivities of the strips are determined by the thermal

810752V1 conductivity of graphite within its basal crystallographic planes, and the conventionally-through-thickness thermal conductivity of the strips is determined by the thermal conductivity of graphite perpendicular to its basal crystallographic planes. 5

34. A structure as claimed in any of Claims 28 to 33, further comprising a bonding material between adjacent strips, bonding the strips together.

35. A structure as claimed in Claim 34, wherein the thickness of the 0 bonding material is of the order of a few microns to a few tens of microns.

36. A structure as claimed in Claim 35, wherein the thickness of the bonding material is about 10 μm. 5

37. A structure as claimed in Claim 35, wherein the thickness of the bonding material is about 5 μm or less.

38. A structure as claimed in any of Claims 34 to 37, wherein the bonding0 material comprises epoxy resin, acrylic, polyimide, polyurethane, polyester or another suitable polymer.

39. A structure as claimed in any of Claims 34 to 38, wherein the bonding material further comprises an additive to enhance its thermal5 conductivity.

40. A structure as claimed in Claim 39, wherein the additive comprises graphite, beryllia, boron nitride, alumina or silver.

810752v1

41. A structure as claimed in any of Claims 38 to 40, further comprising one or more holes through the structure, the hole(s) being normal to the boundaries between adjacent strips.

5 42. A structure as claimed in Claim 41, wherein the diameter of the hole(s) is of the order of 100 μm.

43. A structure as claimed in any of Claims 28 to 42, wherein the width of each strip is in the range of 100 μm to a few millimetres or tens of 0 millimetres.

44. A structure as claimed in any of Claims 28 to 43, wherein the length of each strip is in the range of a few centimetres to tens of centimetres. 5

45. A structure as claimed in any of Claims 28 to 44, wherein the number of strips is in the range from a few to a few hundred.

46. A structure as claimed in any of Claims 28 to 45, wherein the thickness of the structure is of the order of a millimetre. 0

47. A thermally conductive structure comprising one or more constituent structures, the or each constituent structure being a structure as claimed in any of Claims 28 to 46. 5 48. A thermally conductive structure as claimed in Claim 47, wherein the or each constituent structure is arranged such that the through-thickness thermal conductivity of the thermally conductive structure is determined by the through-thickness thermal conductivity of the constituent structure(s), and a first in-plane thermal conductivity of the thermally

810752V1 conductive structure is determined by the second thermal conductivity of the constituent material.

49. A thermally conductive structure as claimed in Claim 48, wherein the or 5 each constituent structure is arranged such that the direction of the first in-plane thermal conductivity of the constituent structure(s) is oriented longitudinally along the thermally conductive structure.

50. A thermally conductive structure as claimed in Claim 48, wherein a 0 plurality of constituent structures are arranged in a plurality of directions.

51. A thermally conductive structure as claimed in Claim 50, wherein the plurality of constituent structures form a substantially radial 5 arrangement within the thermally conductive structure.

52. A thermally conductive structure as claimed in any of Claims 47 to 51, further comprising one or more further components to conduct or dissipate heat, selected from a group comprising: graphite, pyrolytic 0 graphite, a composite containing graphite, a composite containing pyrolytic graphite, a carbon fibre composite, a carbon-carbon composite, one or more cooling fins, a base plate, a heat sink.

53. Apparatus for pressing and joining a plurality of layers of material to5 form a stack, the apparatus comprising: a base and a plurality of side walls arranged to form a cavity for receiving layers to be pressed and joined; means for applying pressure normal to the layers of the stack; and

810752V1 means for applying and adjusting the pressure exerted on the stack by the side walls.

54. Apparatus as claimed in Claim 53, wherein the means for applying 5 pressure normal to the layers of the stack comprises a ram, piston or plunger.

55. Apparatus as claimed in Claim 53 or Claim 54, wherein the means for applying and adjusting the pressure exerted on the stack by the side 0 walls comprises adjustable bolts which fasten the side walls together.

56. Apparatus as claimed in Claim 55, wherein the adjustable bolts are manually adjustable. 5

57. Apparatus as claimed in Claim 55, wherein the adjustable bolts are motor driven.

58. Apparatus as claimed in any of Claims 53 to 57, wherein the side walls are moveably mounted on controlled racks. 0

59. Apparatus as claimed in any of Claims 53 to 58, further comprising plates arranged to surround the layers during pressing, the plates having a coating of encapsulating material. 5

60. Apparatus as claimed in any of Claims 53 to 59, wherein the walls are demountable from the base.

61. Apparatus as claimed in any of Claims 53 to 60, wherein the walls and base have a non-adhesive coating.

810752V1

62. A method of producing a material comprising the steps of: obtaining a plurality of layers of an anisotropic constituent material, each layer of the constituent material having a first value of a material 5 property in a first direction, and a through-thickness value of the material property, the first value being different from the through- thickness value; stacking and joining the said plurality of layers to form a stack; and cutting through the stack to produce a slice; 0 wherein the orientation of the slice with respect to the layers in the stack is such that the through-thickness value of the material property of the slice is determined by the first value of the material property of the layers in the stack. 5

63. A method as claimed in Claim 62, wherein the said first direction is an in-plane direction within the layers.

64. A method as claimed in Claim 62 or Claim 63, wherein each layer of the constituent material has a second value of the material property in a 0 second direction, and wherein the orientation of the slice with respect to the layers in the stack is such that an in-plane material property of the slice is determined by the second value of the material property of the layers in the stack. 5

65. A method as claimed in Claim 64, wherein the said second direction is an in-plane direction within the layers.

66. A method of producing a structure comprising a plurality of layers of a constituent material, each layer of the constituent material having an in-

810752V1 plane thermal conductivity significantly greater than the thermal conductivity in the through-thickness direction, the method comprising stacking and joining the said plurality of layers to form a stack, wherein the thermal conductivity properties of the stack are essentially those of a 5 monolithic object formed of the constituent material and having the same dimensions and internal crystallographic planar orientations as in the stack.

67. A method of producing a structure comprising a plurality of layers of a 0 constituent material, each layer of the constituent material having an in- plane thermal conductivity significantly greater than the thermal conductivity in the through-thickness direction, the method comprising: stacking and joining the said plurality of layers to form a stack, wherein the layers are sufficiently compressed in order to provide interlayer 5 forces comparable with the forces present in a single crystal or monolithic object formed of the same constituent material, such that the thermal conductivity properties of the stack are essentially those of a single crystal or monolithic object formed of the same constituent material and having the same dimensions and internal crystallographic0 planar orientations as the stack.

68. A method of producing a material substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings. 5

69. A structure substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings.

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70. Apparatus substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings.

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