Field of the invention

The present disclosure relates to but not exclusively to methods and apparatus to produce high power, small footprint and physically robust semiconductor heat sink and solving other problematic applications where a diamond or synthetic diamond coating on ceramics including boron nitride ceramics can provide the solution.

Background

It has been demonstrated that diamond can be doped to produce p type and n type semiconductors indeed Professor Mutusko Hatano, along with the National Institute of Advanced Industrial Science and Technology in Japan, has made a lateral p-n junction in diamond which was then created into a J-FET transistor prototype. It is expected that in the next few years diamond semiconductors will start to appear on the market in the form of diodes, transistors and integrated circuits. Diamond semiconductor devices will have significant impacts on high-power devices primarily due to diamond's high electrical resistivity and high heat dissipation capabilities. The very high electrical resistivity would allow for very high-voltage-rated devices which would be beneficial in applications including power distribution and high voltage handling (such as electric trains, electric cars etc.). The heat conductivity would allow for smaller power devices as the diamond material would help to dissipate more heat (which would keep the device cooler for larger current draw as compared to current silicon devices). There is a problem yet to be solved that of getting the heat transferred out of the device to keep it cool at very high voltages and at low cost.

Aspects of the disclosure may solve this problem.

Table 1 Showing diamond properties against other materials.

Summary of the invention

Aspects of the invention are as set out in the independent claims and optional features are set out in the dependent claims. Aspects of the invention may be provided in conjunction with each other and features of one aspect may be applied to other aspects.

The desire of the electronics components industry is to have power devices which will work reliably at voltages up to 10 kV ac that means that if diamond or synthetic diamond is used as an insulator the insulation needs to have a voltage breakdown in excess of 10 kV ac e.g. 15 kV ac.

The thermal conductivity of boron nitride is very high almost as good as copper or silver and therefore makes a very good heat sink. It is also a very good electrical insulator; parallel pressed boron nitrate (with boric oxide binder) has an ac dielectric strength of 95 kV/mm. There are also problems using boron nitride which have to be addressed namely that the material is porous and therefore loses its high dielectric strength as it absorbs moisture and it is a relatively soft material which is badly affected by abrasion.

The invention is an extension to the work carried out by Ultra Biotecs Ltd. (published patent application W02018/142138 A1) and uses a process whereby a thin diamond coating is produced on the whole surface of a thin boron nitride base material e.g. a plate. One face of the diamond coated boron nitride plate is available to deposit the diamond semiconductor material to manufacture the electronic component or circuit and the other face is bonded to a high thermal conductivity plate allowing the combination to become an integrated heat sink thereby efficiently removing the heat from the diamond semiconductor. Preferably the high thermal conductivity plate is made of metal such as copper.

Aspects of the disclosure seek to solve the heat conduction problem outlined in the "Background of the invention section". The inventors have surprisingly found that a coating that is a minimum of 2 pm thick effectively resolves the porosity and the abrasion problems associated with boron nitride.

In an aspect there is provided a high power heat sink comprising a thin boron nitride plate coated with diamond or synthetic diamond and having a metallic plate or other suitable material plate whose thermal conductivity is at least 30 W/M.K bonded to one surface of the diamond coated born nitride plate.

When a doped diamond or doped synthetic diamond semiconductor is grown directly onto the other face of the diamond coated boron nitride plate it has intimate contact with the high power heat sink and is able to conduct heat away from the semiconductor circuit extremely efficiently. The implications for the electronics industry are profound, much smaller package size for the same or higher power ratings, fast manufacturing speeds, high power with higher frequencies, much higher currents (1000 x) than Silicon semiconductors etc.

In another aspect there is provided a capacitor such as a supercapacitor. Supercapacitors require very large surface area and high dielectric strength and the ability to be manufactured into relatively small components. This can be achieved with diamond coated boron nitride formed (for example, impregnated) into a matrix of boron nitride agglomerates.

From the diamond information table 1 , the dc breakdown voltage is 1 ,000,000 volts / mm therefore the ac breakdown voltage will be approximately 330,000 volts / mm or 330 volts / pm. If the boron nitride is coated with diamond or synthetic diamond the combination would need to be at least 0.158 mm thick to withstand 15 kV ac. To get a diamond coating of sufficient quality for electronic components the minimum thickness of diamond coating would be 5 pm. In another aspect there is described how a boron nitride plate coated with diamond that can be used as the dielectric for an electrostatic capacitor.

In another aspect there is disclosed a heatsink for an electronic assembly or component. The heatsink comprises a boron nitride base material, a non-porous electrically resistive coating layer, enclosing the base material, a thermally conductive element coupled to one side of the coated base material, and an electronic assembly or component coupled to another (such as an opposite) side of the coated base material, such that the coated base material is between the thermally conductive element and the electronic component or assembly. The thermally conductive element may be coupled to the coated base material with ceramic adhesive, or it may be soldered or sputtered, or coupled via a Sol Gel process.

The boron nitride base material may comprise a porous matrix of boron nitride agglomerates formed into a plate-like structure. A plate-like structure may be one that has at least one, and preferably two (such as height and width), dimensions that are an order of magnitude greater than a third dimension (such as depth) - for example so that the structure is relatively long and wide and flat. It may be rectangular, square, circular, or may take any other regular or irregular geometric form. Preferably the structure will have a relatively uniform thickness - for example to within a selected degree of tolerance such as +/- 0.1 pm (the thickness in this context being the distance between the electrical component on one side and the thermally conductive element on the other).

The particle sizes of the agglomerates may range from 50 to 500 pm in diameter. In some examples a range of different diameter agglomerates may be selected, for example to achieve a desired density. In other examples the diameters of the agglomerates may be selected based on the desired dielectric properties of the base material. For example, a smaller diameter may be selected so as to provide a higher surface area of the agglomerates which will increase the dielectric strength of the base material. The boron nitride may comprise a-BN or hexagonal boron nitride (also known as h-BN) or“white graphite”, and in some examples may consist solely of a-BN or hexagonal boron nitride. A boric oxide binder may be used to bind the agglomerates together into a porous matrix. It will be understood, however, that in other examples the boron nitride base material may take other forms. For example, the boron nitride base material may comprise, and in some examples consist of parallel-pressed boron nitride that has been pressed under heat and formed into a plate-like structure, optionally with a boric oxide binder.

It will be understood that the non-porous coating layer may be waterproof. The coating layer may comprise diamond, and in some examples the diamond may be doped, for example with boron. It will be understood that in examples where the diamond is doped, only certain selected regions of the coating may be doped with boron, for example where an electrical component is configured to be coupled to the coated base material.

The coating layer may provide a layer at least 2 pm thick enclosing the base material. The coating layer may be applied via chemical vapour deposition, CVD, by using a seeding solution, and/or by using a Sol Gel process.

The boron nitride base material may be 0.5 mm thick or less between the side coupled to the thermally conductive element and the side coupled to the electronic assembly or component.

The thermally conductive element may comprise a metal such as copper. The electronic component may comprise diamond, optionally diamond doped with boron.

In another aspect there is provided a method of providing a heat sink for an electronic component. The method comprises applying a non-porous electrically resistive coating to a boron nitride base material, and attaching the electronic component to the coated base material. It will be understood that the thermally conductive element may be coupled to the coated base material with ceramic adhesive, or it may be soldered or sputtered, or coupled via a Sol Gel process.

The method may further comprise attaching a thermally conductive element to the coated base material on an opposite side of the coated base material to the electronic component. Applying a coating to the boron nitride base material may comprise applying a diamond coating to the boron nitride base material. Applying a coating to the boron nitride base material may comprise: seeding the boron nitride base material with a seeding solution, and forming a coating on the seeded boron nitride base material by a process of chemical vapour deposition. The seeding solution may have a positive zeta potential. For example, the seeding solution may be a hydrogen terminated diamond seed solution. In some examples the electronic component comprises diamond, optionally diamond doped with boron.

In some examples the method further comprises moulding a collection of boron nitride agglomerates into a matrix having a plate-like shape before applying the coating. This may comprise using a boric oxide binder. Moulding the collection of boron nitride agglomerates may comprise placing the collection of boron nitride agglomerates in a mould, and sintering the collection of boron nitride agglomerates in the mould at a temperatures of at least 1800°C for at least 10 minutes. The agglomerates may be dried in a vacuum, such as at least -2 bar or -200000 pascal, for at least 30 minutes and at a temperature of at least 150°C.

Applying a coating to the boron nitride base material may comprise applying a coating to the exterior of the matrix having a plate-like shape. Alternatively, applying a coating to the boron nitride base material may comprise impregnating the matrix having a plate-like shape with a diamond solution containing diamond particulates suspended in a carrier liquid. In such examples the impregnated matrix may be subsequently dried, for example at a temperature of at least 220°C, to drive off the carrier volatiles. It will be understood that the thickness of the diamond coating may be selected such that it leaves a porous diamond-coated matrix structure. Preferably this is at least 2 pm. The thickness may be controlled, for example, by repeating the above impregnating process and/or by controlling the amount of time that the matrix of agglomerates is impregnated with the diamond solution. For example, the method may comprise repeating the application of the coating process a plurality of times to obtain a diamond coating thickness of at least 2 m. In some examples impregnating the matrix with a diamond solution comprises subjecting the matrix to a vacuum to encourage the diamond solution to penetrate the matrix. In some examples the impregnated matrix may be finally dried for at least 30 minutes at 500 to 550 °C.

In another aspect there is provided a capacitor. The capacitor comprises a boron nitride base material, a non-porous electrically resistive coating layer, enclosing the base material, a first electrode coupled to one side of the coated base material, and a second electrode coupled to an opposite side of the coated base material.

The boron nitride base material may comprise a porous matrix of boron nitride agglomerates formed into a plate-like structure. A plate-like structure may be one that has at least one, and preferably two (such as height and width), dimensions that are an order of magnitude greater than a third dimension (such as depth) - for example so that the structure is relatively long and wide and flat. It may be rectangular, square, circular, or may take any other regular or irregular geometric form. Preferably the structure will have a relatively uniform thickness - for example to within a selected degree of tolerance such as +/- 0.1 pm (the thickness in this context being the distance between the first electrode on one side and the second electrode on the other.

The matrix may be filled with an electrolyte solution. The electrolyte solution may be based on ethylene glycol and boric acid. Additionally or alternatively the electrolyte solution may be an anhydrous electrolyte based on organic solvents, such as dimethylformamide (DMF), dimethylacetamide (DMA), or g-butyrolactone (GBL). The particle sizes of the agglomerates may range from 50 to 500 pm. In some examples a range of different diameter agglomerates may be selected, for example to achieve a desired density. In other examples the diameters of the agglomerates may be selected based on the desired dielectric properties of the base material. For example, a smaller diameter may be selected so as to provide a higher surface area of the agglomerates which will increase the dielectric strength of the base material. The boron nitride may comprise a-BN or hexagonal boron nitride (also known as h-BN) or“white graphite”, and in some examples may consist solely of a-BN or hexagonal boron nitride. A boric oxide binder may be used to bind the agglomerates together into a porous matrix. It will be understood, however, that in other examples the boron nitride base material may take other forms. For example, the boron nitride base material may comprise, and in some examples consist of parallel-pressed boron nitride that has been pressed under heat and formed into a plate-like structure, optionally with a boric oxide binder.

It will be understood that the non-porous coating layer may be waterproof. The coating layer may comprise diamond, and in some examples the diamond may be doped, for example with boron. It will be understood that in examples where the diamond is doped, only certain selected regions of the coating may be doped with boron, for example where an electrical component is configured to be coupled to the coated base material.

The coating layer may provide a layer at least 2 pm thick enclosing the base material. The coating layer may be applied via chemical vapour deposition, CVD, by using a seeding solution, and/or by using a Sol Gel process.

The boron nitride base material may be 0.5 mm thick or less between the side coupled to the thermally conductive element and the side coupled to the electronic assembly or component.

In another aspect there is provided a method of manufacturing a capacitor. The method comprises applying a non-porous electrically resistive coating to a boron nitride base material, attaching a first electrode to one side of the coated base material, and attaching a second electrode to an opposite side of the coated base material.

Applying a coating to the boron nitride base material may comprise applying a diamond coating to the boron nitride base material. Applying a coating to the boron nitride base material may comprise: seeding the boron nitride base material with a seeding solution, and forming a coating on the seeded boron nitride base material by a process of chemical vapour deposition. The seeding solution may have a positive zeta potential. For example, the seeding solution may be a hydrogen terminated diamond seed solution. In some examples the electronic component comprises diamond, optionally diamond doped with boron.

In some examples the method further comprises moulding a collection of boron nitride agglomerates into a matrix having a plate-like shape before applying the coating. This may comprise using a boric oxide binder. Moulding the collection of boron nitride agglomerates may comprise placing the collection of boron nitride agglomerates in a mould, and sintering the collection of boron nitride agglomerates in the mould at a temperatures of at least 1800°C for at least 10 minutes. The agglomerates may be dried in a vacuum, such as at least -2 bar or -200000 pascal, for at least 30 minutes and at a temperature of at least 150°C.

Applying a coating to the boron nitride base material may comprise applying a coating to the exterior of the matrix having a plate-like shape. Alternatively, applying a coating to the boron nitride base material may comprise impregnating the matrix having a plate-like shape with a diamond solution containing diamond particulates suspended in a carrier liquid. In such examples the impregnated matrix may be subsequently dried, for example at a temperature of at least 220°C, to drive off the carrier volatiles. It will be understood that the thickness of the diamond coating may be selected such that it leaves a porous diamond-coated matrix structure. Preferably this is at least 2 pm. The thickness may be controlled, for example, by repeating the above impregnating process and/or by controlling the amount of time that the matrix of agglomerates is impregnated with the diamond solution. For example, the method may comprise repeating the application of the coating process a plurality of times to obtain a diamond coating thickness of at least 2 pm. In some examples impregnating the matrix with a diamond solution comprises subjecting the matrix to a vacuum to encourage the diamond solution to penetrate the matrix. In some examples the impregnated matrix may be finally dried for at least 30 minutes at 500 to 550 °C. Drawings

Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 shows scanning electron microscope pictures of the diamond coating on the boron nitride plate;

Fig. 2 shows a scanning electron microscope picture of spherical agglomerates of boron nitride;

Fig. 3 shows a sectioned view of an example integrated high power heat sink;

Fig. 4 shows a sectioned view of an example porous diamond coated boron nitride matrix;

Fig. 5 shows a sectioned view of an example electrostatic capacitor;

Fig. 6 shows an example method of providing a heat sink for an electronic component; and

Fig. 7 shows an example method of manufacturing a capacitor.

Specific description

As described above, in an aspect of the disclosure there is provided a high power heat sink comprising a thin boron nitride plate coated with diamond or synthetic diamond and having a metallic element such as a plate or other suitable material plate whose thermal conductivity is at least 30 W/M.K coupled (e.g. bonded) to one surface of the diamond coated boron nitride plate. This assembly is operable to produce an overall thermal conductivity of better than 30 W/M.K and an ac dielectric strength of at least 95 kV/mm.

When a doped diamond or doped synthetic diamond semiconductor is grown directly onto the other face of the diamond coated boron nitride base material, such as a plate, it has intimate contact with the high power heat sink and is able to conduct heat away from the semiconductor circuit extremely efficiently. The implications for the electronics industry are profound, much smaller package size for the same or higher power ratings, fast manufacturing speeds, high power with higher frequencies, much higher currents (1000 x) than Silicon semiconductors etc.

Work carried out on the coating process has shown that the coating must be inclusion and pin hole free to meet the needs of the electronic industry and therefor great care need to be taken in the surface preparation of the boron nitride. It needs to be clean and as smooth as is practically possible and the boron nitride material must be moisture free. The boron nitride was shaped into 10 X 10 X 0.5mm plates then cleaned in an ultrasonic cleaner using a none-water based solvent and then vacuum dried under heat.

The boron nitride sample was then placed into a microwave chemical vapour deposition reactor to ascertain whether the diamond film could be coated directly onto the boron nitride. The inventors surprisingly found that the diamond coating could not be directly grown on the boron nitride and therefore the boron nitride required diamond seeding to allow growth to proceed. The zeta potential of the boron nitride was measured to select the diamond seeds for the growth. The zeta potential of the boron nitride was found to be negative and so an H- treated (for example, hydrogen-terminated) diamond seed solution with positive zeta potential was selected. Diamond was grown on the seeded boron nitride using a microwave chemical vapour deposition system. The diamond on the boron nitride was imaged with scanning electron microscopy. Fig. 1 shows SEM images of diamond grown on the boron nitride. Panels A and B show the over view and zoomed image of the growth. Panel C shows the substrate exposed to growth plasma without any seeding. It is clear from the images that spontaneous growth of diamond on the boron nitride is not possible, even though the surface of the boron nitride was quite rough.

From these tests it is clear from the results that it is possible to grow good quality diamond coatings on boron nitride ceramics and that a similar strategy can be applied for the growth of diamond on other types of ceramics. Saint - Gobain Ceramic Materials produce boron nitride agglomerates which are engineered particles in the size range 50 pm to 500 pm (as shown, for example, in the SEM image of Fig. 2).

The boron nitride agglomerates are placed in a mould to retain the agglomerates in the desired shape (thin plate for example) and then are pressure-less or low pressure sintered to form a porous template structure. Preferably the sintering temperature is 1800°C. The templates are coated with diamond or synthetic diamond using a multi-dip Sol Gel process or a multi-dip nano-diamond particulate suspended in a suitable carrier liquid. After each dip the template is heated to remove the volatiles from the film leaving the diamond coating intact, when the diamond coating is of the required thickness the template has a final high temperature treatment to finalise the coating process causing the diamond coating to coalesce providing a porous diamond film that can be a component of a supercapacitor. Tests have shown that the diamond film coalesces at between 500°C and 550°C and forms a continuous homorganic diamond film. In the first embodiment referring to Fig. 3 a thin plate of boron nitride is formed into the desired shape and is prepared to receive a diamond coating on its exterior. Preferably the boron nitride plate 1 is less than 0.5 mm thick.

Preparation

1) The surface of the boron nitride plate 1 is ground to a smooth finish. Preferably the finish is as good as or better than 5 pm finish.

2) The smooth boron nitride plate 1 is washed and ultrasonically cleaned in an ultrasonic bath for at least 10 minutes in non-aqueous solvent. Preferably the boron nitride 1 plate is cleaned for 20 minutes.

3) The smooth boron nitride plate 1 is vacuum dried under heat for at least 30 minutes. Preferably the vacuum is at least -2 bars and the heat is at least 150° C.

The clean, smooth boron nitride plate 1 can now be coated with diamond or synthetic diamond. Coating

4) A chemical vapour deposition system is prepared to receive the prepared boron nitride plate 1.

5) The zeta potential of the boron nitride was found to be negative and so an H-treated diamond seeding solution with positive zeta potential is selected.

6) The whole of the boron nitride surface is coated with a diamond seeding liquid preferably using a dip process or a spraying system.

7) The whole of the prepared and seeded boron nitride plate is coated with diamond or synthetic diamond 2 preferably using a CVD process. Preferably the coating thickness is at least 2 pm.

The diamond 2 coated boron nitride plate 1 is ready to receive the doped diamond semiconductor material 3.

8) The doped diamond semiconductor material 3 is applied to one side of the diamond coated boron nitride plate 1 using standard semiconductor processing equipment.

Attaching the metallic heat conductor

9) The metallic heat conductor 4 is attached to the opposite side of the coated boron nitride plate 1 to the applied semiconductor side using commercially available high temperature, high thermal conductivity ceramic adhesive or another process such as high temperature soldering process or a sputtering process or a Sol Gel process or any other suitable process known to those skilled in the art. In the second embodiment referring to Figs. 2 and 4, a porous diamond film is shown suitable for use in a supercapacitor or in a conventional electrostatic capacitor.

Saint - Gobain and several other manufacturers of ceramic materials produce boron nitride agglomerates which are engineered particles in the size range 1 pm to 10 pm and 50 pm to 500 pm (as shown in the SEM image of Fig. 2) which can be made into a boron nitride matrix using the following process:

Preparation

1) The boron nitride agglomerates 5 are washed and ultrasonically cleaned in an ultrasonic bath for at least 10 minutes in non-aqueous solvent. Preferably the boron nitride agglomerates 5 are cleaned for 20 minutes.

2) The boron nitride agglomerates 5 are vacuum dried under heat for at least 30 minutes. Preferably the vacuum is at least minus 2 bars and the heat is at least 150°C.

3) The boron nitride agglomerates 5 are placed in a mould to retain the agglomerates in the desired shape (for example a thin walled cylinder or a thin plate) and sintered at 1800°C with a 10 minute soak so that the agglomerates 5 fuse together to form a porous boron nitride ridged matrix or template. Coating

4) A diamond coating 6 is grown on the surface of the template using a multi-dip diamond Sol Gel process or a multi-dip nano-diamond particulate suspended in a suitable carrier liquid process which is encouraged to permeate through the template to coat the template inside and out. When using a multi-dip nano-diamond particulate suspended in a suitable carrier liquid of low viscosity a straight forward dip process is adequate to get good results. When using a higher viscosity Sol Gel solution the solution may need vacuum or pressure to force the liquid into the boron nitride template. When the template is saturated with the solution the whole template is heated to at least 220°C to drive off the carrier volatiles, leaving the diamond coating on the surface of the boron nitride agglomerates effectively producing a porous diamond coated boron nitride matrix or template.

5) The film can be thickened by repeating the coating process until the required thickness is produced (minimum 2 pm).

6) The coating is finished by heating the diamond coating to at least 500°C - 550°C for at least 30 minutes.

The resultant porous diamond coated boron nitride matrix is ready to be further processed into the final electronic component.

By selecting the size (such as the diameter) of the agglomerates the capacitance range can be adjusted, the more agglomerates per mm ³ the more the capacitance range increases because the surface area of the agglomerates increases.

An alternative way to form a porous diamond film is by forming the diamond film on the agglomerates first using the process outlined in embodiment 1. Putting the diamond coated agglomerates into a mould of the desired shape and then sintering it at 500°C 550°C to produce the template.

The diamond coating can be made conductive by doping it with chemical elements such as boron as the diamond coating is formed leading to further applications in the electronic components industry. In the third embodiment referring to Fig. 5, an electrostatic can be formed using a thin plate of boron nitride. The thin plate of boron nitride is formed into the desired shape and is prepared to receive a diamond coating on its exterior. Preferably the boron nitride plate 3 is less than 0.5 mm thick. The diamond coated boron nitride constitutes the dielectric of the capacitor.

Preparation

1) The boron nitride plate 3 is washed and ultrasonically cleaned in an ultrasonic bath for at least 10 minutes in non-aqueous solvent. Preferably the boron nitride plate 3 is cleaned for 20 minutes.

2) The boron nitride plate 3 is vacuum dried under heat for at least 30 minutes. Preferably the vacuum is at least minus 2 bars and the heat is at least 150° C.

The clean, boron nitride plate 3 can now be coated with diamond or synthetic diamond 4.

Coating

3) A chemical vapour deposition system is prepared to receive the prepared boron nitride plate 3.

4) The zeta potential of the boron nitride was found to be negative and so an H-treated diamond seeding solution with positive zeta potential is selected.

5) The whole of the boron nitride plate 3 surfaces are coated with a diamond seeding liquid preferably using a dip process or a spraying system.

6) The whole of the prepared and seeded boron nitride plate is coated with diamond or synthetic diamond 4 preferably using a CVD process or a multi-dip diamond Sol Gel process or a multi-dip nano-diamond particulate suspended in a suitable carrier liquid process. Preferably the coating thickness is at least 2 pm.

7) The diamond coated boron nitride plate requires electrodes 6 attached to each side of the diamond coated plate to form a capacitor. The electrodes can be formed by a high temperature soldering process or a sputtering process or a Sol Gel process or any other suitable process known to those skilled in the art.

If a very high capacitance capacitor is required then the porous diamond coated boron nitride processes used in the second embodiment to produce a very high surface area dielectric material can be used by attaching electrodes to both sides of the dielectric as described in embodiment 3.

Fig. 6 shows an example method of providing a heat sink for an electronic component. The method comprises moulding 701 a collection of boron nitride agglomerates into a matrix having a plate-like shape before applying the coating. This may comprise using a boric oxide binder. Moulding 701 the collection of boron nitride agglomerates may comprise placing the collection of boron nitride agglomerates in a mould, and sintering the collection of boron nitride agglomerates in the mould at a temperatures of at least 1800°C for at least 10 minutes. The agglomerates may be dried in a vacuum, such as at least -2 bar or -200000 pascal, for at least 30 minutes and at a temperature of at least 150°C.

The method then comprises applying 703 a non-porous electrically resistive coating to a boron nitride base material. Applying 703 a coating to the boron nitride base material may comprise applying a diamond coating to the boron nitride base material. Applying 703 a coating to the boron nitride base material may comprise: seeding the boron nitride base material with a seeding solution, and forming a coating on the seeded boron nitride base material by a process of chemical vapour deposition. The seeding solution may have a positive zeta potential. For example, the seeding solution may be a hydrogen terminated diamond seed solution.

Applying 703 a coating to the boron nitride base material may comprise applying a coating to the exterior of the matrix having a plate-like shape. Alternatively, applying 703 a coating to the boron nitride base material may comprise impregnating the matrix having a plate-like shape with a diamond solution containing diamond particulates suspended in a carrier liquid. In such examples the impregnated matrix may be subsequently dried, for example at a temperature of at least 220°C, to drive off the carrier volatiles. It will be understood that the thickness of the diamond coating may be selected such that it leaves a porous diamond-coated matrix structure. Preferably this is at least 2 pm. The thickness may be controlled, for example, by repeating the above impregnating process and/or by controlling the amount of time that the matrix of agglomerates is impregnated with the diamond solution. For example, the method may comprise repeating the application of the coating process a plurality of times to obtain a diamond coating thickness of at least 2 pm. In some examples impregnating the matrix with a diamond solution comprises subjecting the matrix to a vacuum to encourage the diamond solution to penetrate the matrix. In some examples the impregnated matrix may be finally dried for at least 30 minutes at 500 to 550 °C.

The method then comprises attaching 705 the electronic component to the coated base material. It will be understood that the thermally conductive element may be coupled to the coated base material with ceramic adhesive, or it may be soldered or sputtered, or coupled via a Sol Gel process.

The method further comprises attaching 707 a thermally conductive element to the coated base material on an opposite side of the coated base material to the electronic component.

Fig. 7 shows an example method of manufacturing a capacitor. The method comprises moulding 703 a collection of boron nitride agglomerates into a matrix having a plate-like shape before applying the coating. This may comprise using a boric oxide binder. Moulding 703 the collection of boron nitride agglomerates may comprise placing the collection of boron nitride agglomerates in a mould, and sintering the collection of boron nitride agglomerates in the mould at a temperatures of at least 1800°C for at least 10 minutes. The agglomerates may be dried in a vacuum, such as at least -2 bar or - 200000 pascal, for at least 30 minutes and at a temperature of at least 150°C. The method may then comprise applying 703 a non-porous electrically resistive coating to a boron nitride base material. Applying 703 a coating to the boron nitride base material may comprise applying a diamond coating to the boron nitride base material. Applying 703 a coating to the boron nitride base material may comprise: seeding the boron nitride base material with a seeding solution, and forming a coating on the seeded boron nitride base material by a process of chemical vapour deposition. The seeding solution may have a positive zeta potential. For example, the seeding solution may be a hydrogen terminated diamond seed solution. Applying 703 a coating to the boron nitride base material may comprise applying a coating to the exterior of the matrix having a plate-like shape. Alternatively, applying a coating to the boron nitride base material may comprise impregnating the matrix having a plate-like shape with a diamond solution containing diamond particulates suspended in a carrier liquid. In such examples the impregnated matrix may be subsequently dried, for example at a temperature of at least 220°C, to drive off the carrier volatiles. It will be understood that the thickness of the diamond coating may be selected such that it leaves a porous diamond-coated matrix structure. Preferably this is at least 2 pm. The thickness may be controlled, for example, by repeating the above impregnating process and/or by controlling the amount of time that the matrix of agglomerates is impregnated with the diamond solution. For example, the method may comprise repeating the application of the coating process a plurality of times to obtain a diamond coating thickness of at least 2 pm. In some examples impregnating the matrix with a diamond solution comprises subjecting the matrix to a vacuum to encourage the diamond solution to penetrate the matrix. In some examples the impregnated matrix may be finally dried for at least 30 minutes at 500 to 550 °C.

The method may then comprise attaching 705 a first electrode to one side of the coated base material, and attaching 707 a second electrode to an opposite side of the coated base material. The first and/or second electrodes may span an entire respective face of each side of the coated base material, or may only cover a region of a respective face of the base material.

The above embodiments are to be understood as illustrative examples. Further embodiments are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

In the context of the present disclosure other examples and variations of the apparatus and methods described herein will be apparent to a person of skill in the art.