WO2017178841A1

WO2017178841A1 - Heater elements, heat exchangers and heater element arrays

Info

Publication number: WO2017178841A1
Application number: PCT/GB2017/051058
Authority: WO
Inventors: Lukasz KURZEPA; Catharina PAUKNER; Krzysztof Kazimierz Koziol; Nicola MAZZUCATO; Michael John SCHMID
Original assignee: Fgv Cambridge Nanosystems Limited
Priority date: 2016-04-15
Filing date: 2017-04-13
Publication date: 2017-10-19
Also published as: GB201706030D0; DK3443810T3; GB2551250B; EP3443810B1; GB2551250A; PL3443810T3; EP3443810A1

Abstract

A heater element comprising: a substrate; a first electrode and a second electrode disposed on the substrate; and a carbon nanomaterial-containing layer disposed on the substrate, the carbon nanomaterial-containing layer being disposed between and electrically connected to the first and second electrodes. Also provided are a heat exchanger, a heater element array, integrated devices, and manufacturing methods.

Description

HEATER ELEMENTS, HEAT EXCHANGERS

AND HEATER ELEMENT ARRAYS

Field of the Invention

The present invention relates to electrically-powered heater elements and arrays thereof, and also to heat exchangers.

Background to the Invention

Conventional electrically-powered heater elements are in widespread use in a range of applications. Such heater elements use Joule heating (also known as ohmic heating or resistive heating) to generate heat by passing an electric current through an electrically conductive element.

However, in many cases, conventional heater elements are not well suited for use on (or in) flexible substrates. This can make it difficult to apply heating to curved or irregularly-shaped surfaces. Typically, conventional heater elements are also not well suited for use in applications in which rapid heating and cooling rates are required. Likewise, conventional heat exchangers, which transfer heat from a heater element or other heat source to a fluid (e.g. a gas such as air), are not well suited for use on (or in) flexible substrates, or in applications in which rapid heating and cooling rates are required. There is therefore a desire for heater elements (and likewise heat exchangers) which are better suited for use on (or in) flexible substrates and which enable rapid heating and cooling rates to be achieved.

Summary of the Invention

According to a first aspect of the present invention there is provided a heater element as defined in Claim 1 of the appended claims. Thus, there is provided a heater element comprising: a substrate; a first electrode and a second electrode disposed on the substrate; and a carbon nanomaterial-containing layer disposed on the substrate, the carbon nanomaterial-containing layer being disposed between and electrically connected to the first and second electrodes. Advantageously, the substrate may be flexible, and consequently the resulting heater element may also be flexible. Additionally, the heater element is able to achieve rapid heating and cooling rates.

Optional features are defined in the dependent claims. Thus, the carbon nanomaterial-containing layer may for example comprise graphene, or carbon nanotubes, or a mixture of graphene and carbon nanotubes.

In one embodiment the carbon nanomaterial-containing layer may include a first sub-layer comprising a first carbon nanomaterial and a second sub-layer comprising a second carbon nanomaterial. For example, the first sub-layer may comprise graphene and the second sub-layer may comprise carbon nanotubes. This enables the two types of carbon nanomaterials to be deposited separately, in a manner optimised for each of the carbon nanomaterials in question - for example using separate dispersions tailored to suit the specific respective carbon nanomaterial therein. In such a manner, agglomeration of the carbon nanomaterials can be avoided, and thereby better control and better heat- generating behaviour of the resulting heating element can be achieved.

In certain embodiments the substrate may comprise a flexible polymer film. For example, the substrate may comprise a polyimide film, such as Kapton (RTM).

In other embodiments the substrate may comprise a breathable fabric through which air can flow. More particularly, the substrate may comprise a woven fabric (e.g. a woven glass fibre fabric) or a non-woven fabric (e.g. a felt). By virtue of the substrate being a breathable fabric through which air can flow, this enables heat to be transferred efficiently from the heater element to such a flow of air. Preferably the carbon nanomaterial-containing layer at least partly overlaps each of the first and second electrodes, thereby ensuring good electrical conduction between the electrodes and the carbon nanomaterial-containing layer. A cover layer may be disposed over the carbon nanomaterial-containing layer. The cover layer may comprise a polymer film. For example, the cover layer may comprise a polyimide film, such as Kapton (RTM).

In certain embodiments the carbon nanomaterial-containing layer may incorporate one or more holes to enable the cover layer to be bonded to the substrate at one or more points across the carbon nanomaterial-containing layer, thereby enhancing the structural integrity of the heater element.

In certain embodiments the heater element may have a thickness of the order of 0.1 mm, thereby rendering it flexible.

In certain embodiments the carbon nanomaterial-containing layer may be formed so as to have one or more regions that are devoid of carbon nanomaterials, in order to cause localised concentration of the current flowing between the first and second electrodes in use, and thereby enable a variation in the heat distribution across the heater element to be achieved.

In embodiments in which the substrate comprises a breathable fabric through which air can flow, preferably the cover layer comprises an electrically-insulating breathable fabric through which air can flow. Accordingly, this enables air to flow through both the substrate and the cover layer. Similarly, the heater element may further comprise a layer of electrically-insulating breathable fabric on the opposite side of the substrate from the cover layer. A stack of heater elements may be formed in which each heater element is constructed using breathable fabric. The stack may be arranged such that air can flow through the entire thickness of the stack. With such a stack, greater overall heating ability of the airflow may be achieved.

According to a second aspect of the invention there is provided a device comprising a heater element or stack of heater elements in accordance with the first aspect of the invention, coupled to a heat exchanger. This enhances the transfer of heat from the heater element to a fluid (e.g. a gas such air) passing through the heat exchanger. In certain embodiments the heat exchanger comprises first and second interlayers and a sealing layer, wherein: the sealing layer is attached to the first interlayer; the first interlayer is attached to the second interlayer; the heater element is coupled to the second interlayer; the first and/or second interlayer is provided with an air inlet; the first and/or second interlayer is provided with an air outlet; and the first interlayer incorporates a first arrangement of apertures and the second interlayer incorporates a second arrangement of apertures, the first and second arrangements of apertures being configured such as to form a network of channels within the first and second interlayers, extending across the heat exchanger from the air inlet to the air outlet.

For example, the first arrangement of apertures may comprise a plurality of parallel slots oriented in a first direction, and the second arrangement of apertures comprises a plurality of parallel slots oriented in a second direction, the first and second arrangements of apertures being configured such that slots of the first arrangement cross slots of the second arrangement to thereby form the network of channels.

The first and second directions may be at substantially 90° to one another. The first arrangement of apertures may comprise an alternating arrangement of relatively long slots and relatively short slots, wherein, between successive parallel relatively long slots, a plurality of relatively short slots are collinear with one another.

The second arrangement of apertures may comprise a staggered arrangement of relatively short parallel slots.

One or both of the interlayers may incorporate a first channel into which the air inlet feeds. Similarly, one or both of the interlayers may incorporate a second channel which feeds to the air outlet.

The first and/or second channels may be oriented in the first direction. In certain embodiments the sealing layer may comprise a polymer film. For example, the sealing layer may comprise a polyimide film, such as Kapton (RTM).

Further, the first and second interlayers may comprise a polymer film. For example, the first and second interlayers may comprise a polyimide film, such as Kapton (RTM). Merely by way of example, each of the first and second interlayers may have a thickness of the order of 0.5 mm.

Alternatively, the heat exchanger may comprise one or more layers of a mesh material, such as a fibreglass mesh, to provide flexibility and resilience.

For example, the heat exchanger may comprise a first layer of the mesh material and a second layer of the mesh material, the second layer being oriented at an angle (e.g. 45°) relative to the first layer. A plurality of said first and second layers of the mesh material may be provided, in an alternating manner. The apertures within the mesh layers, when the layers are attached together, form a labyrinthine network of channels for the air to flow through, for heat exchange purposes. The heat exchanger may be contained within a bag or sleeve arrangement.

According to a third aspect of the invention there is provided an article comprising a heater element or device in accordance with the first or second aspects of the invention, the heater element being connected to a power supply.

According to a fourth aspect of the invention there is provided an assembly comprising a plurality of heater elements, stacks of heater elements, or devices in accordance with the first or second aspects of the invention, wherein the heater elements are connected in parallel to a power supply.

According to a fifth aspect of the invention there is provided an assembly comprising a plurality of heater elements, stacks of heater elements, or devices in accordance with the first or second aspects of the invention, wherein the heater elements are connected in series to a power supply.

According to a sixth aspect of the invention there is provided an assembly comprising a plurality of heater elements, stacks of heater elements, or devices in accordance with the first or second aspects of the invention, wherein the heater elements are connected to a power control unit, the power control unit being configured to supply power to selected individual heater elements or selected subsets of the heater elements.

In certain embodiments, two or more neighbouring heater elements may share a common electrode.

Advantageously, the power control unit may be configured to vary over time which of the heater elements is/are activated. For example, the power control unit may be configured to cause the activated heater element(s) to change over time in such a manner that the heated region of the assembly moves across the assembly in a cyclic wave-like manner. This has beneficial therapeutic applications, e.g. to provide a person with a heat massage in which waves of heat traverse their body. Many other applications are also possible, in no way limited to therapeutic purposes.

Thus, according to a seventh aspect of the invention there is provided an article comprising an assembly in accordance with the sixth aspect of the invention, selected from a group comprising: a massage mat or pad; a vehicle seat; an interior panel of a vehicle; a spacecraft such as a satellite; a cooking hob; an item of clothing or other wearables; fabrics, blinds and shutters. According to an eighth aspect of the invention there is provided a method of making a heater element, the method comprising: arranging a first electrode and a second electrode on a substrate; and depositing a carbon nanomaterial-containing dispersion on the substrate so as to form a carbon nanomaterial-containing layer between and electrically connected to the first and second electrodes. The sequence of these operations may be reversed (i.e. the carbon nanomaterial- containing dispersion may be deposited on the substrate first, and then the electrodes attached).

The carbon nanomaterial-containing dispersion may for example comprise graphene, or carbon nanotubes, or a mixture of graphene and carbon nanotubes. For example, the dispersion may comprise small graphene nanoplatelets less than 2 μιτΊ in size. Alternatively, or in addition, the dispersion may comprise graphene nanoplatelets greater than 2 μνη in size. In certain embodiments the depositing may comprise depositing a first dispersion comprising a first carbon nanomaterial (e.g. graphene) and a second dispersion comprising a second carbon nanomaterial (e.g. carbon nanotubes). If desired, the depositing of the first dispersion may be performed substantially simultaneously with the depositing of the second dispersion (e.g. by using two spray nozzles simultaneously). However, in other instances the depositing of the first dispersion may be performed non-simultaneously with the depositing of the second dispersion. For example, the first and second dispersions may be performed a plurality of times, in an alternating manner, to build up a layered structure within the carbon nanomaterial-containing layer.

The or each carbon nanomaterial-containing dispersion may be deposited by spraying, or by printing, or by painting, for example.

The method may further comprise attaching a cover layer over the carbon nanomaterial-containing layer. This attaching may performed at an elevated temperature (e.g. of the order of 170°C) and under applied pressure (e.g. of the order of 250 PSI). The elevated temperature may be provided by hot pressing. Alternatively, the elevated temperature may be created by passing an electric current through the carbon nanomaterial-containing layer, thus causing the heater element to heat up and effectively self-cure. This can be very beneficial if the geometry of the heater element is not suited to hot pressing.

Alternatively the attaching may be performed using UV-curing epoxy, for example.

In embodiments in which the substrate and cover layer both comprise a breathable fabric through which air can flow, the attaching may be performed by sewing using a non-conductive thread. The sewing may also pass through the first and second electrodes, to secure (or further secure) the electrodes in place.

The method may further comprise attaching electrical supply wires to the first and second electrodes, for example by soldering or using a clip-on connector.

According to a ninth aspect of the invention there is provided a heat exchanger substantially as described above in relation to the device of the second aspect.

According to a tenth aspect of the invention there is provided a method of making such a heat exchanger. According to an eleventh aspect of the invention there is provided a method of making a heater element with an integrated heat exchanger, comprising the method of the eighth aspect in combination with the method of the tenth aspect, with the heater element being coupled to the heat exchanger.

According to a twelfth aspect of the invention there is provided a method of controlling an array of heater elements so as to cause the activated heater element(s) to change over time in such a manner that the heated region of the array moves across the array in a cyclic wave-like manner.

Finally, according to a thirteenth aspect of the invention there is provided conductive ink for deposition on a substrate to form a heater element, the conductive ink comprising a dispersion which contains one or more carbon nanomaterials.

For example, the dispersion may contain graphene, or carbon nanotubes, or a mixture of graphene and carbon nanotubes. The dispersion may contain small graphene nanoplateiets less than 2 μηι in size. Alternatively, the dispersion may contain graphene nanoplateiets greater than 2 μπι in size, !n a further alternative the dispersion may comprise a mixture of small graphene nanoplateiets less than 2 μιη in size, and graphene nanoplateiets greater than 2 μηι in size.

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 a is a schematic plan view of a carbon nanomaterial-based heater element (with a cover layer having been removed to expose a carbon nanomaterial-containing layer disposed between and electrically connected to first and second electrodes);

Figures 1 b and 1 c are schematic cross-sectional views of the carbon nanomaterial-based heater element of Figure 1 a, taken along lines A-AA and B-

BB respectively; Figure 1 d is a schematic cross-sectional view of the carbon nanomaterial-based heater element of Figure 1 a, across its entire breadth (i.e. effectively along line A- BB);

Figure 2a is a schematic plan view of a heat exchanger suitable for use with the heater element of Figures 1 a-d, the heat exchanger comprising first and second interlayers;

Figure 2b is a schematic cross-sectional view of the heat exchanger of Figure 2a;

Figure 3a illustrates an exemplary geometry of a first interlayer of the heat exchanger, and the flow of air (denoted by the arrows) in the heat exchanger in the "x" direction, through the slots in the first interlayer;

Figure 3b illustrates an exemplary geometry of a second interlayer of the heat exchanger, and the flow of air (denoted by the arrows) in the heat exchanger in the "y" direction, through the slots in the second interlayer;

Figure 4 illustrates, by way of example only, possible dimensions (in millimetres) of the slots in the second interlayer;

Figure 5 is a schematic cross-sectional view of the carbon nanomaterial-based heater element of Figures 1 a-d (in particular Figure 1 d) bonded to the heat exchanger of Figures 2a-b;

Figure 6 illustrates an alternative heat exchanger construction, with (a) showing a first piece of mesh in a first orientation that serves as the first interlayer; (b) showing a second piece of the same mesh in a second orientation (at 45° to the first orientation) that serves as the second interlayer; and (c) showing the first and second pieces of mesh attached on top of one another and trimmed to shape, for use in the heat exchanger;

Figure 7 is a schematic cross-sectional view of an alternative heat exchanger configuration, bonded to the carbon nanomaterial-based heater element of

Figures 1 a-d (in particular Figure 1 d);

Figure 8 shows, in plan view, further details of the arrangement depicted in Figure

7;

Figure 9 is a schematic circuit diagram showing a carbon nanomaterial-based heater element connected to a power supply; Figure 10 is a schematic circuit diagram showing a plurality of carbon nanomaterial-based heater elements connected in parallel to a power supply; Figure 1 1 is a schematic circuit diagram showing a plurality of carbon nanomaterial-based heater elements connected in series to a power supply;

Figure 12 is a schematic circuit diagram showing an array of individually- addressable carbon nanomaterial-based heater elements connected to a power control unit;

Figure 13 shows the heater array of Figure 12 incorporated into a substrate (for example in the form of, or incorporated in, a flexible massage mat/pad, or a vehicle seat, or an interior panel of a vehicle);

Figures 14a, 14b and 14c show examples of heating sequences to which the individual heater elements of the arrays of Figures 12 and 13 may be subjected, to generate what we term a "heat wave" effect;

Figure 15 illustrates a variant of the carbon nanomaterial-based heater element of Figures 1 a-d, wherein, in this variant, the insulating substrate surrounds (i.e. extends outwards beyond) the electrodes;

Figure 16 illustrates the use of a clip-on connector to provide electrical power to the electrodes of a carbon nanomaterial-based heater element (with Figure 16a showing an enlarged view of the clip-on connector);

Figure 17a is a schematic plan view of another variant of the carbon nanomaterial- based heater element of Figure 1 a, in this case employing a breathable fabric as the substrate for the carbon nanomaterial-containing layer (with a cover layer having been removed to expose the carbon nanomaterial-containing layer and the first and second electrodes);

Figure 17b is a schematic cross-sectional view of the carbon nanomaterial-based heater element of Figure 17a, taken along line A-B, showing an underlying electrically-insulating fabric layer, the breathable fabric substrate on which the carbon nanomaterial-containing layer is deposited, and an electrically-insulating fabric cover layer;

Figures 17c and 17d show, in each case, a schematic cross-sectional view of a stack of a plurality (two in these examples, but more than two are also possible) of carbon nanomaterial-based heater elements according to Figures 17a and 17b (with, in Figure 17d, a single layer of electrically-insulating fabric being used between the carbon nanomaterial-coated fabric substrates);

Figure 18 is a schematic plan view of another variant of the carbon nanomaterial- based heater element of Figure 1 a, in this case incorporating holes in the carbon nanomaterial-containing layer, through which holes the substrate may be bonded to the cover layer to enhance the structural integrity of the heater element;

Figure 19 is a schematic plan view of another variant of the carbon nanomaterial- based heater element of Figure 1 a, in this case with regions of the carbon nanomaterial-containing layer being devoid of carbon nanomaterials, in order to concentrate the electrical current flowing between the electrodes in certain places and thereby achieve a variation in the heat distribution across the heater element; and

Figure 20 illustrates the use of a common electrode shared by two adjacent carbon nanomaterial-based heater elements.

In the figures, like elements are indicated by like reference numerals throughout. 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.

Overview

The present work provides carbon nanomaterial-based heater elements which generate heat by Joule heating when an electric current is passed through a carbon nanomaterial layer (also referred to herein as a "carbon nanomaterial- containing layer"). The carbon nanomaterial layer comprises carbon nanomaterials such as graphene or carbon nanotubes, or a combination thereof. It should be noted that, in the present work, and in accordance with commonly- accepted usage, the term "carbon nanomaterial" does not encompass graphite. It should also be noted that the term "carbon nanomaterial layer" (or "carbon nanomaterial-containing layer") as used herein does not preclude the presence of other species within the layer, such as a binder, for example. The present work also provides a heat exchanger which may be integrated with an abovementioned carbon nanomaterial-based heater element, and an array of such heater elements (optionally with integrated heat exchangers).

Carbon nanomaterial-based heater element

Figure 1 a shows a schematic plan view of a carbon nanomaterial-based heater element 10, and Figures 1 b and 1 c show schematic cross-sectional views of the heater element 10 of Figure 1 a, taken along lines A-AA and B-BB respectively.

The heater element 10 comprises a substrate layer 12 on which a first electrode 14 and a second electrode 16 are provided. In a presently-preferred embodiment the substrate 12 comprises a flexible polymer film (e.g. a polyimide film, such as Kapton (RTM)), and the first and second electrodes 14, 16 are made of copper, although other suitable materials may alternatively be used for these components. A carbon nanomaterial-containing layer 18 is formed on the substrate 12, such as to extend between the first and second electrodes 14, 16 (as illustrated in Figure 1 a) and to at least partly overlap the first and second electrodes 14, 16 (as illustrated in Figures 1 b and 1 c). In such a manner, the carbon nanomaterial- containing layer 18 is electrically connected to the first and second electrodes 14, 16.

Figure 1 d is a schematic cross-sectional view across the entire breadth of the heater element 10, i.e. effectively along line A-BB of Figure 1 a. It should be noted that, in Figure 1 d, the carbon nanomaterial-containing layer 18 is not shown as overlapping the first and second electrodes 14, 16. However, this is merely for the sake of clarity of the diagram, and in practice the carbon nanomaterial-containing layer 18 will, preferably, at least partly overlap the first and second electrodes 14, 16, as illustrated in the above-described Figures 1 b and 1 c.

In certain embodiments the carbon nanomaterial-containing layer 18 comprises graphene. However, in other embodiments the carbon nanomaterial-containing layer 18 may comprise carbon nanotubes, or a mixture of graphene and carbon nanotubes. The carbon nanomaterial-containing layer 18 may also comprise other species, such as a binder, for example. As discussed below, the carbon nanomaterial-containing layer 18 is preferably formed by depositing a carbon nanomaterial-containing dispersion (which we refer to as an "ink") on the substrate 12. A range of compositions for the ink are discussed below. One end 14a of the first electrode 14, and one end 16a of the second electrode 16, function as terminals for connection to a power supply. In both cases, terminal ends 14a and 16a are distal from the carbon nanomaterial-containing layer 18.

As shown in Figures 1 b, 1 c and 1 d, a cover layer 20 is provided over the carbon nanomaterial-containing layer 18. In the presently-preferred embodiment the cover layer 20 comprises a flexible polymer film (e.g. a polyimide film, such as Kapton (RTM)), although other suitable materials may alternatively be used.

Merely by way of example, in one embodiment we have made the heater element 10 has dimensions of 300 mm x 280 mm and the overall thickness (i.e. of the flexible substrate 12, the carbon nanomaterial layer 18 with electrodes 14, 16, and the flexible cover layer 20) is approximately 0.1 mm. The overall heater element is therefore inherently very flexible. As further examples, other heater elements 10 we have made have dimensions of 80 mm x 80 mm, and 30 mm x 70 mm.

More generally, with our presently-preferred embodiments, by using a flexible substrate 12 and a flexible cover layer 20, the heater element 10 is flexible. However, if desired, a rigid substrate may be used instead, to form a rigid heater element.

Although, in the illustrated examples, the heater element 10 is substantially rectangular in shape, in alternative embodiments it may be other shapes, such as circles, squares, or more complex geometries.

In use, the heater element 10 generates heat based on the Joule heating principle. Electric power is delivered to the carbon nanomaterial (e.g. graphene) layer 18 by means of the electrodes 14, 16 (via terminals 14a and 16a). The carbon nanomaterial layer 18 heats up when the electric current is passed through it, thereby generating heat.

More particularly, when an electric current is passed through the carbon nanomaterial layer 18, a temperature rise is obtained due to Joule heating, as the carbon nanomaterial layer 18 has an electrical resistance due to the contact between the individual carbon nanomaterial particles (e.g. graphene platelets or carbon nanotubes) within the layer 18. The electrical resistance and thus the heat generating capability of the heater element 10 is a function of the density of the carbon nanomaterial particles in the carbon nanomaterial layer.

Finally, it should be noted that our heater elements are, in general, not transparent; this is primarily due to the high concentration of carbon nanomaterials used in the carbon nanomaterial layer 18.

Heat exchanger

Figures 2a and 2b are schematic plan and cross-sectional views of a heat exchanger 30 suitable for use with the above-described heater element 10. The heat exchanger 30 is a multilayer device which may be bonded directly underneath the above-described heater element 10 (as described below in relation to Figure 5). The heat exchanger 30 comprises a first interlayer 34, a second interlayer 36, and a sealing layer 32. The sealing layer 32 is bonded to the first interlayer 34, and the first interlayer 34 is bonded to the second interlayer 36 around the outer edges (regions 42 in Figure 2a, as described further below).

In a presently-preferred embodiment the sealing layer 32 comprises a thin polymer film (e.g. a polyimide film, such as Kapton (RTM)), and the first and second interlayers 34, 36 are each made of a thicker polymer film (e.g. a polyimide film, such as Kapton (RTM)), although other suitable materials may alternatively be used for these components.

Merely by way of example, in one embodiment each of the first and second interlayers 34, 36 has a thickness of 0.5 mm, and the thinner sealing layer 32 has a thickness of 0.1 mm.

In the illustrated embodiment, a first channel 39 is formed within the second interlayer 36. One end of the first channel 39 forms an air inlet 38. A second channel 40 is also formed within the second interlayer 36, at the opposite end of the heat exchanger from the first channel 39. One end of the second channel 40 forms an air outlet 41.

In use, the heat exchanger 30 (more particularly the second interlayer 36) is bonded to the heater element 10, or is otherwise coupled in a manner which allows heat transfer between the heater element 10 and the heat exchanger 30. A stream of cold air is received via the inlet 38 and the first channel 39 and is blown through the two interlayers 34, 36 to the second channel 40 and thence the outlet 41 . Air can also flow between layer 32 and interlayer 34, as well as between interlayer layer 36 and the heater element 10, as a consequence of interlayers 34 and 36 being bonded to layer 12 and heater element 10 only at the outer edges of the interlayers 34, 36 (regions 42 in Figure 2a), as described further below. As a consequence of the heat exchange function of the heat exchanger 30, heat is taken away from the heater element 10, such that the air that leaves the outlet 41 is warmer than the air that enters the inlet 38.

In more detail, the first and second interlayers 34, 36 include a specially designed pattern of apertures (holes and slots), arranged to create a multiplicity of small channels via which the air passes from the first channel 39 to the second channel 40. The sealing layer 32 does not incorporate any such holes or slots, thereby restricting the multiplicity of small channels, and the airflow, to the first and second interlayers 34, 36.

As shown in Figures 3a and 3b (both of which are effectively rotated through 90° relative to Figure 2a), each interlayer 34, 36 has a different slots pattern. The slot pattern of the first interlayer 34 comprises an alternating arrangement of parallel relatively long slots 35a and parallel relatively short slots 35b, aligned with the "x" direction. Between successive parallel relatively long slots 35a, a plurality of relatively short slots 35b are collinear with one another. The slot pattern of the second interlayer 36 comprises a staggered arrangement of parallel relatively short slots 37, aligned with the "y" direction (the "y" direction being at 90° to the "x" direction). As illustrated in Figure 2a, the "y" direction corresponds to the direction across the heat exchanger from the first channel 39 to the second channel 40, whereas the "x" direction corresponds to the orientation of the first and second channels 39, 40, at each edge of the heat exchanger.

The slot patterns of the first and second interlayers are designed in such a way that the slots 35a, 35b of the first interlayer 34 cross with the slots 37 of the second interlayer 36, thereby creating a network of channels. More particularly, the slots 35a, 35b in the first interlayer 34 distribute air in the "x" direction, whereas air flows through the slots 37 in the second interlayer 36 in the "y" direction (the airflow being denoted by the arrows in Figures 3a and 3b). This causes the air to be spread uniformly across the whole surface of the heat exchanger and to efficiently extract heat from all the heat-generating area of the heater element 10. The heat exchange is performed in such a way that cold air flowing through the slots in the second interlayer 36 directly comes into contact with the substrate 12 of the heater element 10 and takes heat away from the heater element 10 via the air outlet 41 .

Figure 4 illustrates, by way of example only, possible dimensions (in millimetres) of the slots 37 in the second interlayer 36. In this example, the slots are small enough (2 mm wide and 6 mm long) such that the sealing layer 32 does not bend into the slot (which would potentially cause blockage of the channel) when a load is applied perpendicular to the surface and the device is bent.

Alternative heat exchanger configuration

Although in the above-described embodiment the heat exchanger 30 is made from polymer layers (e.g. polyimide layers) 32, 34, 36 bonded together, with crisscrossing slots being provided in interlayers 34 and 36, in an alternative embodiment multiple layers of orthogonal-grid mesh (e.g. made of fibreglass to provide flexibility and resilience, and using e.g. five layers of mesh) may be used as layers 34, 36 of the heat exchanger. Each layer of mesh may be, for example, 0.4 mm thick, and the size of each of the apertures in the grid may be of the order of a couple of millimetres. In one embodiment, and with reference to Figure 6, a first piece 70 of mesh in a first orientation may be used as the first interlayer (34); and a second piece 72 of the same mesh in a second orientation (at 45° to the first orientation) may be used as the second interlayer (36). The first and second pieces of mesh 70, 72 are attached on top of one another (74) and trimmed to shape to form the active core of the heat exchanger. It should be noted that the illustrations in Figure 6 represent just a small part of what would typically be a larger heat exchanger construction. The apertures within the mesh layers, when the layers are attached together, form a labyrinthine network of channels for the air to flow through, for heat exchange purposes. Carbon nanomaterial-based heater element with integrated heat exchanger

Figure 5 is a schematic cross-sectional view of the above-described carbon nanomaterial-based heater element 10 bonded to the above-described (polymer layer based) heat exchanger 30, so as to create an integrated multilayer device 44.

More particularly, in the illustrated embodiment, the heater element 10 is placed on top of the heat exchanger 30, with substrate 12 of the heater element 10 being bonded (or otherwise coupled) to the second interlayer 36 of the heat exchanger 30. As will be apparent to those skilled in the art, in an alternative embodiment the cover layer 20 of the heater element 10 may be bonded (or otherwise coupled) to the second interlayer 36 of the heat exchanger 30.

The resulting integrated device 44 is capable of generating heat (via the heater element 10) as well as extracting heat (via the heat exchanger 30) from any surfaces or components placed on top of the heater element 10. Cold air can be delivered via flexible tubing attached to the air inlet 38 of the heat exchanger 30 (via an appropriately-shaped connector to connect the tubing with the flat and thin air inlet 38 of the heat exchanger 30). Any suitable source of cold air may be used.

Many techniques are possible for providing cold air to the heat exchanger 30, as those skilled in the art will appreciate. For example, one way of doing this is by using a small fan and/or an air pump, and a Peltier cell with an air-sealed radiator. The fan and/or air pump can be used to pump air through the radiator placed on the Peltier cell. The cooled-down air at the outlet of the Peltier cell can then be pumped via flexible tubing into the heat exchanger 30 via the inlet 38.

It will be appreciated that the heat exchanger 30, with cold air flowing through it, may be used as a cooler in respect of hot surfaces or components placed on top of the heater element 10, without power being supplied to the heater element 10. Figure 7 illustrates an alternative heat exchanger configuration 80, shown bonded to the above-described carbon nanomaterial-based heater element 10. In this alternative heat exchanger 80, one or more interlayers 82 are contained within a bag-like heat exchanger body 84.

The bag-like body 84 of the alternative heat exchanger 80 is illustrated in more detail in Figure 8, in plan view. The heat exchanger body 84 is formed from a thermal plastic sleeving (for example polytubing). After the one or more interlayers 82 and a gas inlet 86 have been inserted into the sleeving 84, the sleeving 84 is welded as per welding lines 88, for example using polymer welding tongs. The carbon nanomaterial-based heater element 10 is then attached to the heat exchanger 80 by means of an adhesion compound such as glue or epoxy.

Preparation of carbon nanomaterial-containing inks

(for forming the carbon nanomaterial layer 18)

This section details the preparation of carbon nanomaterial-containing inks, comprising combinations of graphene and carbon nanotubes with binders (polymers) as set out in the table below, that have been successfully tested for application by spraying or brushing to form the carbon nanomaterial layer 18 of a heater element 10 as described above. Each binder requires dispersion of the carbon nanomaterial in a suitable solvent. The binder (polymer) itself was dissolved in a solvent where indicated, or used as-is, as in the case of green latex and epoxy resin.

By way of example, the graphene material used to prepare inks in the present work, in the form of graphene nanoplatelet (GNP) powder, was produced by FGV Cambridge Nanosystems Limited and is commercially available under the brand name GamGraph (RTM). The powder showed a 99% carbon purity and contained graphene with an average diameter of 450 nm (from 150 nm to 750 nm) and a thickness of up to 5 nm. However, highly effective inks can also be made using larger graphene flakes, of the order of 100 pm in size, or any size up to that. Although the processing used to prepare the inks will likely result in a reduction in size of such large graphene flakes, the original size of the graphene flakes is no hindrance to the production of effective inks.

One graphene based ink we have devised, suitable for use in the present work, contains small-size graphene nanopiateiets less than 2 μηι in size. Another graphene based ink we have devised contains larger graphene nanopiateiets greater than 2 m in size. A further graphene based ink we have devised contains a mixture of the small-size graphene nanopiateiets less than 2 μνη in size and the larger graphene nanopiateiets greater than 2 in size.

With regard to the thickness of the graphene flakes, we have found that the fewer the layers (or the thinner the flakes), the better the performance, but significantly thicker graphene material can also be used, for example up to 50 nm in thickness.

The carbon nanotubes used in the present work were multiwali carbon nanotubes (MWCNTs) according to the following parameters:

Outer diameter: 50-120 nm

Inner diameter: 5-10 nm

Length before processing: 1 -3 mm

Length after processing: 10-20 μιη

Active surface: 60 m²/g

Resistivity (along CNT axis): 10^"4 Qm

Bulk density as grown: 0.18 g/cnr*

Density under maximum compression (10 tons): 2.1 g/cm³

Purity >95%

With regard to the abovementioned "length after processing" of 10-20 μηι, it should be noted that, if one were to start with CNTs of that length then, after processing had taken place, one would not achieve the same performance from the ink. For good electrical conductivity properties a highly crystalline CNT material is preferred, with long CNTs to provide wide reach, to bind well the deposited layer (e.g. to prevent cracking and to achieve percolation).

We have identified various binders for successful heater ink formulation. These include water-based systems (such as latex, polyvinyl alcohol and others), alcohol based systems (such as isopropanol, ethanol, and others for use with polyamides), and a series of other organic solvents (such as /V-Methyl-2- pyrrolidone (NMP), dimethylformamide (DMF), tetrahydrofuran (THF), and others for use with polyimides) - as well as from solvent free processes such as resins or molten polymers.

We have also found that combinations of different carbon nanomaterials (e.g. MWCNTs and graphene nanopowders) lead to successful heater ink formulation. Carbon nanotubes from multi-wall CNT arrays and graphene from graphene nanopowders can be used individually in certain inks, or combined in a range of combinations to form other inks. Thus, one ink may comprise only carbon nanotubes as the carbon nanomaterial. One ink may comprise only graphene as the carbon nanomaterial. Another ink may comprise a mixture of carbon nanotubes and graphene as the carbon nanomaterials. For instance, one ink we have devised, suitable for use in the present work, contains a mixture of small-size graphene nanoplatelets (less than 2 μιη in size) and carbon nanotubes.

Whilst a single ink comprising a mixture of carbon nanomaterials (e.g. graphene and carbon nanotubes) has benefits in terms of ease of application (enabling continuous application from a single source), in some cases it can be advantageous to form the carbon nanomateriai-containing layer 18 by applying separate inks which differ in respect of their nanomaterial content. For example, a graphene-containing ink may be sprayed, followed by a carbon nanotube containing ink, or vice versa, and repeated as many times as necessary, in an alternating manner. The use of separate inks in this manner enables each ink to be tailored to suit the specific carbon nanomaterial therein (e.g. in respect of the solvent or binder used), so as to avoid agglomeration of the carbon nanomaterial and thereby achieve better control and better heat-generating behaviour of the resulting heating element.

While the carbon nanomaterial concentration in the final heating layer varies between 0 and 40 wt.% (the remainder being the binder or a combination of the binders), their concentration in the ink would typically vary between 0.05 and 5%. Usually, higher relative concentrations of CNTs would require higher concentration of the filler in the ink. Water based inks and solvent free resin systems would have filler concentrations at the higher end of the spectrum.

The following table presents a summary of ink bases (solvents) and possible binders, together with nanocarbon concentration in the final coating/heater layer.

Summary of ink bases (solvents) and possible binders, together with nanocarbon concentration in the final coating/heater layer

To summarise some of the trends we have observed:

the lower the carbon content, the smoother the layer 18 that is produced the layer 18 is smoother with graphene than with carbon nanotubes the higher the carbon content, the higher the conductivity of the layer 18 carbon nanotubes yield higher conductivities than graphene due to their larger size

the maximum concentration is determined by how much carbon the binder can contain before the carbon starts to rub off the final layer Solvent based inks are prepared by dissolving the binder in a suitable solvent at the highest concentration possible. The carbon filler in a combination suitable for the application is dispersed in a solvent that is both compatible with the solvent used for the binder and wets the nanocarbon well. The concentration of the nanocarbon component is dependent on the intended application of the final heater as low voltage applications have to display very high conductivity and will require a higher loading fraction. Higher voltage applications (e.g. mains power) may require more binder (to prevent overheating) or one may want to have a higher binder fraction for aesthetic reasons (a shiny surface, etc.).

The above table shows that a great number of different binders can be used to formulate inks that can lead to heater layers. As mentioned, the solvent would be determined by the chosen binder but may also play an important role in deciding on a system.

Manufacturing processes for carbon nanomaterial-based heater element 10, heat exchanger 30, and integrated multilayer device 44 Pressing and curing technique

This is a technique for bonding two or more layers together, which we use in a number of the method steps below. Each layer of Kapton (RTM) (our presently- preferred polymer film for the substrate 12 and cover layer 20 of the heater element 10, and for the sealing layer 32 of the heat exchanger 30) has adhesive on one side. We have found that, to bond all the layers together, they should preferably be treated in a certain way. More particularly, the layers should preferably be heated up to around 170°C and a pressure of around 250 PSI applied for 30-60 minutes. The pressure and temperature treatment melts and cures the adhesive, resulting in good bonding between the Kapton (RTM) layers. This "pressing and curing" technique is referred to a number of times in the method steps which follow below. In the "pressing and curing" technique, the heating may be effected by putting the layers into a hot press or similar apparatus. Alternatively (and particularly if the geometry of the heater element is not suited to hot pressing) an electric current may be passed through the carbon nanomaterial layer, to cause the heater element to heat up and effectively self-cure.

Other techniques for bonding two or more layers together may be used instead. For example, layers may be bonded using UV-curing epoxy, which does not require the application of pressure and temperature and so may again be advantageous if the geometry of the heater element is not suited to hot pressing.

A: Manufacture of carbon nanomaterial-based heater element 10

Step 1 - Preparation of copper electrodes (copper edging) 14, 16 on the substrate 12

Firstly, copper foil is bonded onto the substrate 12 (which in our presently- preferred embodiment is made of Kapton (RTM)). A chemical etching process may then be used to remove excess copper and to leave the desired pattern of copper electrodes 14, 16.

Step 2 - Deposition of carbon nanomaterial-containing ink onto the substrate 12 Multiple layers of carbon nanomaterial-containing ink may then be sprayed onto the substrate 12 onto which the copper electrodes 14, 16 have been formed. The ink is deposited such that it overlaps with at least part of each of the electrodes 14, 16, thereby ensuring good electrical connection between the resulting carbon nanomaterial layer 18 and the electrodes 14, 16. A single spray operation produces a carbon nanomaterial layer that is around 3 pm in thickness. By spraying a number of successive layers the thickness of the resulting carbon nanomaterial layer 18 can be increased accordingly, enabling the resistance of the resulting carbon nanomaterial layer 18 to be controlled to suit voltage requirements (e.g. 12V DC, 230V AC, or others). Spraying is also advantageous in that it enables the carbon nanomaterial layer 18 to be deposited on complex (e.g. three dimensional) substrate geometries.

Other deposition techniques for the carbon nanomaterial-containing ink may be used instead of spraying. One such alternative is printing of the conductive ink. With the printing method, the carbon nanomaterial layer 18 will be made out of multiple lines of conductive ink. Printing provides higher precision in comparison to the spraying method. This advantageously allows us to vary the thickness of the resulting carbon nanomaterial layer 18 in various places. As the thickness of the carbon nanomaterial layer 18 varies, the current density which passes though the layer 18 during heating will also vary. Thus, intentionally varying the thickness of the carbon nanomaterial layer 18 can be used to advantage, to compensate for geometrical effects which can otherwise cause parts of the heater element to be hotter or cooler than others, or to deliberately provide temperature variation at different places on the layer 18.

Another alternative deposition technique for the carbon nanomaterial-containing ink is to paint the ink onto the substrate 12, e.g. using a brush. Step 3 - Introduction of cover layer 20 onto the carbon nanomaterial layer 18

A protective cover layer 20 of polymer film (which in our presently-preferred embodiment is made of Kapton (RTM)) is then put in place to cover the carbon nanomaterial layer 18 and the electrodes 14, 16. Step 4 - Bonding of cover layer 20 to carbon nanomaterial layer 18

This is performed using the "pressing and curing" technique described above.

Step 5 - Cutting to shape

The resulting laminate is then cut to shape and pattern, for example using a CNC (Computer Numerical Control) router. With this method the external shape of the heater element 10, as well as any holes or cut-outs within the heater element, can be made. Other cutting techniques, such as laser cutting, may be used instead, as those skilled in the art will appreciate.

Step 6 - Connecting electrical supply wires

Copper electrical supply wires are then soldered (e.g. manually, or using an automated soldering process) to the electrodes 14, 16, at ends 14a and 16a. Other attachment techniques for connecting the electrical supply wires to the electrodes 14, 16 may be used instead, as those skilled in the art will appreciate. An example of an alternative way of attaching the electrical supply wires to the ends 14a and 16a of the electrodes 14, 16, using a clip-on connector, is discussed below with reference to Figures 16 and 16a.

B: Manufacture of heat exchanger 30 Step 1 - Cutting the first interlayer 34 and second interlayer 36 to shape, with the pattern of internal slots (e.g. using a CNC router).

Step 2 - Bonding the first and second interlayers 34, 36 together, using the "pressing and curing" technique described above.

Step 3 - Bonding the first interlayer 34 to the sealing layer 32, using the "pressing and curing" technique described above.

C: Integration of carbon nanomaterial-based heater element 10 and heat exchanger 30

Bonding of the heat exchanger 30 to the carbon nanomaterial-based heater element 10 to form the integrated device 44 may be performed manually using a suitable adhesive (for example a sealant). The adhesive is applied to the periphery of the upper surface of the heat exchanger 30, only in the regions outside the slotted area. Such bonding regions are denoted by reference numeral 42 in Figure 2a. Heater arrays and practical applications

For some practical applications, a single carbon nanomaterial-based heater element 10 may be used, either with or without an accompanying heat exchanger 30. Such applications would typically be those where localised heating and/or cooling is desired, e.g. to pre-heat or cool an electronic component or a circuit board in an electronic device (e.g. a mobile phone, portable computer or tablet device). To illustrate this, Figure 9 is a schematic circuit diagram showing a single carbon nanomaterial-based heater element 10 connected to a power supply 46.

However, for other practical applications, it is advantageous to employ a plurality of carbon nanomaterial-based heater elements 10, again either with or without accompanying heat exchangers. For instance, using a plurality of heater elements 10 enables the area being heated to be increased, and/or enables a complex (potentially three-dimensional) geometry to be heated.

Figure 10 is a schematic circuit diagram showing a plurality of carbon nanomaterial-based heater elements 10 connected in parallel to a power supply 46. In this example three heater elements 10 are connected in parallel, by connecting the first electrode 14 of each the heater elements 10 together, and by connecting the second electrode 16 of each the heater elements 10 together. Naturally, as those skilled in the art will appreciate, other numbers of heater elements (e.g. two, or more than three) may be connected in such a manner, and/or in other geometrical configurations.

To illustrate an alternative wiring arrangement, Figure 1 1 is a schematic circuit diagram showing a plurality of carbon nanomaterial-based heater elements 10 connected in series to a power supply 46. In this example two heater elements 10 are connected in series, but as those skilled in the art will appreciate, other numbers of heater elements (i.e. more than two) may be connected in such a manner, and/or in other geometrical configurations, The carbon nanomaterial-based heater elements 10 of the present work can reach high temperatures. In general, the maximum operational temperature (i.e. the maximum temperature that can reliably be maintained) is determined by the substrate which supports the carbon nanomaterial layer. For example, using a polyimide substrate we are able to reach an operational temperature of the order of 200°C, and possibly higher. With a different choice of substrate we are able to achieve an operational temperature of the order of 450°C-500°C.

Without being bound by theory, we attribute the high temperatures achievable by our heater elements to be due to one or more of the following:

- The low resistance of the carbon nanomaterial layer (which we have measured as being of the order of 7-9 Ω/D). This is due to the ink composition used, and also, in practice, is as a result of one or both of the following:

- The large quantity of ink deposited when forming the carbon nanomaterial layer. Typically a total of approximately 1 ml of ink per square centimetre is deposited, this being done by means of a number of successive layers (e.g. approximately 10 layers).

- The application of pressure when joining the various layers together to form the heater element. We also use a relatively large spray nozzle, greater than 1 mm in diameter, which enables a relatively thick layer to be produced.

Further, the carbon nanomaterial-based heater elements 10 of the present work achieve rapid heating and cooling rates. For example, a heater element 10 of the present work can heat up at a rate as fast as 4°C/second, and can cool down at a rate of around 2-3°C/second. Whilst these rapid heating and cooling rates can be applied to single heater elements and series- or parallel-connected heater elements, they can be used to particular advantage in an array of individually powered (i.e. selectively heatable) carbon nanomaterial-based heater elements 10. By using a suitable power controller and appropriate circuitry to selectively supply power to individual heater elements, it is possible to vary with time (e.g. in a matter of seconds) which part of the array is heated.

As an example, Figure 12 is a schematic circuit diagram of such an array 50, comprising nine individually-addressable carbon nanomaterial-based heater elements 10 (denoted A1 , A2, A3, B1 , B2, B3, C1 , C2 and C3) that are connected to a power control unit 52. It will of course be appreciated that, in other embodiments, the array may comprise different numbers of heater elements, e.g. any number between two and eight, or more than nine, and/or may be arranged in different geometrical configurations. The power control unit 52 is operable to supply power to any of the heater elements in the array, individually or in certain combinations, by supplying electrical current via ports A, B and C (individually or in any combination) and via ports 1 , 2 and 3 (individually or in any combination) of the power control unit 52. By way of example, to activate heater element C2 by itself, power is supplied via port C and port 2 of the power control unit 52. To activate heater elements A1 , A2 and A3 in combination, power is supplied via port A and ports 1 , 2 and 3 of the power control unit 52. To activate heater elements A3 and C3 in combination, power is supplied via port 3 and ports A and C of the power control unit 52. The power control unit 52 may be configured such that, when power is supplied to more than one heater element, the overall power supplied to the elements is increased proportionately, so that the activated heater elements are heated to the same temperature, irrespective of the number of heater elements that are activated.

It will be appreciated that the addressing circuitry of Figure 12 does not permit the simultaneous activation of heater elements which differ in both row letter and column number, without also activating other heater elements. (For example, heater elements A2 and B1 cannot be simultaneously activated without also activating elements A1 and B2.) However, as those skilled in the art will appreciate, alternative addressing circuitry may be used which enables any combination of heater elements in the array to be activated. This may be done, for example, by connecting one of the electrodes (e.g. electrode 14) of each heater element to a common return path and, for each of the heater elements, providing a separate, dedicated, element-specific control path from the power control unit to the other electrode (e.g. electrode 16) of the heater element.

An array 50 of carbon nanomaterial-based heater elements 10, as described above, may be employed in a variety of practical applications. Figure 13 shows the heater array 50 of Figure 12 incorporated into (or onto) a substrate 60, with the control unit 52 to one side. In the interest of clarity, the interconnections between each of the heater elements and the control unit 52 are not illustrated. In some applications the substrate 60 may be flexible, whereas in other applications the substrate 60 may be rigid. Furthermore, in some applications the heater elements 10 may individually controllable, whereas in other applications the heater elements 10 may be connected in parallel so as to operate as one.

Examples of potential applications for arrays of the carbon nanomaterial-based heater elements 10 are as follows:

• A heated massage mat or pad (typically using a flexible substrate).

Preferably the heater elements are individually controllable, to permit the heated region of the mat or pad to change over time, thereby enabling "waves" of heat to move across the mat or pad, e.g. from one side to another (our so-called "heat wave" effect). This is discussed in greater detail below.

• A heated vehicle seat (typically with a flexible substrate). The heater elements may be individually controllable, as per the massage mat/pad example above, or may be connected in parallel so as to operate as one. Optionally the heater elements 10 may be provided with accompanying heat exchangers 30 to provide a cooling function, e.g. for hot climates, to supplement the vehicle's air conditioning. The term "vehicle" as used herein should be interpreted broadly, to encompass road vehicles such as cars, coaches and lorries, air vehicles such as aeroplanes, and waterborne vehicles such as ships and boats.

A heated interior panel (e.g. a roof panel or a door panel) of a vehicle (using either a flexible substrate or a rigid substrate). The heater elements would typically be connected in parallel so as to operate as one, but alternatively may be configured so as to be individually controllable if so desired. Optionally the heater elements 10 may be provided with accompanying heat exchangers 30 to provide a cooling function, e.g. for hot climates, to supplement the vehicle's air conditioning.

Heating (or cooling) components of spacecraft such as satellites (typically using a rigid substrate, but potentially using a flexible substrate if employed on a flexible part of the spacecraft, such as on an unfurlable reflector of a satellite). The heater elements would typically be connected in parallel so as to operate as one, but alternatively may be configured so as to be individually controllable if so desired. Optionally the heater elements 10 may be provided with accompanying heat exchangers 30 to provide a cooling function.

Cooking hobs (typically using a rigid substrate).

Clothing and other wearables, fabrics (e.g. blankets and curtains), blinds, shutters etc. (typically using a flexible substrate). Depending on the specific application, the heater elements may be connected in parallel so as to operate as one, or may be configured so as to be individually controllable. Optionally the heater elements 10 may be provided with accompanying heat exchangers 30 to provide a cooling function, e.g. in hot weather.

Many other applications for the present work are foreseeable, as those skilled in the art will appreciate.

In general, and in relation to all of the above examples, the use of carbon nanomaterial-based heater elements confers the benefits of flexibility (i.e. bendability), light weight and compactness / space-saving.

"Heat wave" effect

As mentioned above, the present carbon nanomaterial-based heater elements 10 provide rapid heating and cooling rates. By virtue of these rapid heating and cooling rates, by producing an array of individually controlled (i.e. selectively heatable) carbon nanomaterial-based heater elements 10 it is possible to vary with time (e.g. in a matter of seconds) which part of the array (e.g. a specific heater element or a subset of the heater elements) is activated. Under the control of the power control unit 52 (in which a microprocessor would be suitably programmed with instruction code), the heated region of the device (e.g. massage mat or pad, or heated car seat) can be made to change over time, thereby causing "waves" of heat to move across the device, e.g. from one side to another, or up and down. This is considered beneficial for therapeutic purposes. A single device may be programmed with a variety of different "heat wave" sequences. Figures 14a, 14b and 14c show examples of heating sequences to which the individual heater elements of the arrays of Figures 12 and 13 may be subjected, to generate such "heat wave" effects. Each 3x3 grid is an array 50 of carbon nanomaterial-based heater elements 10, as discussed above. The stars denote, at each point in time, which elements of the array receive electrical power and generate heat. In each of Figures 14a, 14b and 14c one complete cycle of the heating sequence is depicted. In practice, the cycle in question would be repeated. In Figure 14a the heating sequence is such that rows of heater elements are activated in turn, from top to bottom and then back again. Such a sequence may again be effected using the circuitry of Figure 12, under the control of the power control unit 52. For one complete cycle of the heating sequence, the heater elements that are activated (in combination) at each step of the sequence, and the corresponding ports of the power control unit via which power would be supplied, are as follows: Heater elements Ports of the power control unit via which power is supplied

A1 , B1 , C1 A, B, C, 1

A2, B2, C2 A, B, C, 2

A3, B3, C3 A, B, C, 3

A2, B2, C2 A, B, C, 2

A1 , B1 , C1 A, B, C, 1

In Figure 14b the heating sequence is such that columns of heater elements are activated in turn, from left to right and then back again. Such a sequence may be effected using the circuitry of Figure 12, under the control of the power control unit 52. For one complete cycle of the heating sequence, the heater elements that are activated (in combination) at each step of the sequence, and the corresponding ports of the power control unit via which power would be supplied, are as follows:

Heater elements Ports of the power control unit via which power is supplied A1 , A2, A3 A, 1 , 2, 3

B1 , B2, B3 B, 1 , 2, 3

C1 , C2, C3 C, 1 , 2, 3

B1 , B2, B3 B, 1 , 2, 3

A1 , A2, A3 A, 1 , 2, 3

In Figure 14c the heating sequence is such that diagonal lines of heater elements are activated in turn, from the top left corner of the array to the bottom right corner and then back again. Due to the activation of diagonal lines of heater elements, the circuitry of Figure 12 is not suitable to effect this heating sequence, and it would be necessary to use alternative addressing circuitry which enables any combination of heater elements in the array to be activated. As discussed above, this may be done, for example, by connecting one of the electrodes (e.g. electrode 14) of each heater element to a common return path and, for each of the heater elements, providing a separate, dedicated, element-specific control path from the power control unit to the other electrode (e.g. electrode 16) of the heater element. For one complete cycle of the heating sequence, the heater elements that are activated (in combination) at each step of the sequence are as follows:

Heater elements

A1

A2, B1

A3, B2, C1

B3, C2

C3

B3, C2

A3, B2, C1

A2, B1

A1

Following the above examples, those skilled in the art will be able to envisage other heater element activation sequences to create other "heat wave" sequences.

Summary and further implementational details

The present work provides highly accurate heat delivery devices which can be precisely controlled for heat output, accurate temperature settings and electrical energy input. By virtue of the heating properties of carbon nanomaterials (e.g. graphene platelets or carbon nanotubes), it elevates the provision of heat energy to a level of accuracy and control not achievable with traditional techniques.

More particularly, the present work provides a highly controllable heater element in the form of a flexible sheet, the heater element comprising a thin, flexible carbon nanomaterial layer (e.g. comprising graphene and/or carbon nanotubes) disposed on a flexible substrate. Alternatively, if flexibility is not required, the substrate, and thus the heater element as a whole, may be rigid. Electrodes (e.g. made of copper) are provided at opposing edges of the carbon nanomaterial layer, electrically connected to the carbon nanomaterial layer.

The application of a voltage across the electrodes (and thus across the carbon nanomaterial layer) will produce a temperature rise due to Joule heating, as the carbon nanomaterial layer has an electrical resistance due to the contact between the individual carbon nanomaterial particles (e.g. graphene platelets or carbon nanotubes). Without being bound by theory, it is understood that each platelet to platelet (or nanotube to nanotube) resistance will vary slightly, but over the whole sheet these average out to a repeatable, measurable value, provided the concentration of carbon nanomaterial particles across the sheet is consistent. The electrical resistance and thus the heat generating capability of the heater element is a function of the density of the carbon nanomaterial particles in the carbon nanomaterial layer.

Such sheets can be used either singly or in modular form.

Such sheets can replace traditional heating wires and elements and can be applied where other heating systems cannot be employed (e.g. on curved and/or flexible surfaces). If desired, carbon nanomaterial layers (with appropriate electrodes, to thereby function as heater elements) can be formed on both the top side and the underside of a single substrate. If a fluid (e.g. air) is passed across and/or through the sheet the heating element can act as a space heater. To enable the passage of fluid through the sheet, holes may be provided in the sheet. Alternatively, a heat exchanger arrangement as described above may be employed.

It will be understood that many different physical arrangements of the present principles are possible, and the implementations described above are by way of example only.

Modifications and alternatives

Detailed embodiments and some possible alternatives have been described above. As those skilled in the art will appreciate, a number of modifications and further alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein.

For example, in the embodiment of the carbon nanomaterial-based heater element 10 illustrated in Figures 1 a-d, the first and second electrodes 14, 16 are shown as being situated along outer edges of the device. However, as illustrated in Figure 15, the substrate 12 (made of an electrically-insulating material, e.g. Kapton) may surround (i.e. extend outwards beyond) the electrodes 14, 16. That is to say, the electrodes 14, 16 are positioned inwards of the outer edges of the substrate 12. The substrate 12 surrounding the electrodes 14, 16 facilitates the sealing of the cover layer 20 to the substrate 12 and also provides electrical insulation around the outside of the electrodes 14, 16, thus enhancing the electrical safety of the device.

In the above-described manufacturing method of the carbon nanomaterial-based heater element 10, the electrical supply wires are soldered to the ends 14a and 16a of the electrodes 14, 16. However, in an alternative technique, as illustrated in Figure 16, the electrical supply wires may be attached to the ends 14a and 16a of the electrodes 14, 16 using a clip-on connector 1 1 . The clip-on connector incorporates a clip mechanism which, when closed, secures the connector above and below (and around the outside of) the ends 14a and 16a of the electrodes 14, 16. As illustrated in greater detail in the enlarged view of the connector 1 1 shown in Figure 16a, internally the connector 1 1 includes two electrically-conductive contact regions 13a, 13b, that are respectively pressed against the surfaces of the electrode ends 14a, 16a when the clip-on connector is closed, thereby effecting electrical connection between the contact regions 13a, 13b and the electrodes 14, 16. The contact regions 13a, 13b extend into a port region 15 of the connector 1 1 . The port region 15 incorporates a set of electrical terminals (not shown) that are electrically connected to the contact regions 13a, 13b and adapted to receive the electrical supply wires. Thus, when inserted, the electrical supply wires become electrically connected to the electrodes 14, 16. In the embodiment of the carbon nanomaterial-based heater element 10 illustrated in Figures 1 a-d, the carbon nanomaterial-containing layer is disposed on a substrate 12 that comprises a flexible polymer film (e.g. Kapton). However, with reference initially to Figure 17a, in an alternative embodiment of a carbon nanomaterial-based heater element 10' a breathable fabric may be used as a substrate 18' onto which the electrically-conductive carbon nanomaterial- containing dispersion or ink is applied. The breathable fabric substrate 18' may for example be woven, or alternatively may be non-woven. For example, the breathable fabric substrate may be a woven glass fibre fabric, or may be made of a textile material. The breathable fabric substrate 18' advantageously allows air to flow through it, which is advantageous in respect of transferring heat from the heater element 10' to such a flow of air. The carbon nanomaterial-containing dispersion (e.g. containing graphene, carbon nanotubes, or a mixture thereof) may be sprayed onto the breathable fabric substrate 18', although other application techniques are also possible to coat the fabric substrate 18' (such as dipping the fabric substrate 18' into the dispersion, or painting the dispersion onto the fabric substrate 18'). The electrodes 16, 18 are arranged along opposing sides of the coated fabric substrate 18'. As shown in Figures 17a and 17b, an underlying layer 12' of electrically-insulating breathable fabric may be used to support the coated fabric substrate 18' and the electrodes and to provide electrical insulation (as, in practice, it is likely that both sides of the coated fabric substrate 18' will become electrically conductive once the carbon nanomaterial-containing dispersion has been applied, due to the open structure of the fabric). A cover layer 20', also of electrically-insulating breathable fabric, may also be applied over the top of the coated fabric substrate 18'. The underlying fabric layer 12', the coated fabric substrate 18' and the cover layer 20' (and potentially the electrodes 14, 16 too) may all be joined together by sewing using a non-conductive thread, although other joining techniques are also possible, such as gluing (or welding, if the fabric substrate 18' and layers 12' and 20' are polymer-based). As shown in Figure 17c, a stack of a plurality (two in this example, but more than two are also possible) of carbon nanomaterial-based heater elements 10' may be formed. The individual heater elements 10' in the stack may be joined together by sewing or other appropriate techniques (e.g. as mentioned above). Airflow is possible through the entire thickness of the stack. With such a stack, greater overall heating ability of the airflow may be achieved. As shown in Figure 17d, a variant of the stack principle may include only a single layer 12" of electrically- insulating fabric between the (or each) neighbouring pair of carbon nanomaterial- coated fabric substrates 18'. Figure 18 is a schematic plan view of another variant of the carbon nanomaterial- based heater element 10 of Figure 1 a, but in this case incorporating one or more holes 17 in the carbon nanomaterial-containing layer 18. By virtue of the hole(s) 17 the substrate 12 may be bonded to the cover layer 20 (refer back e.g. to Figure 1 d) at one or more points across the carbon nanomaterial-containing layer 18 (i.e. not just around the sides of it), thereby enhancing the structural integrity of the heater element 10. The provision of holes in the carbon nanomaterial-containing layer 18 also causes the electrical current flowing between the electrodes 14, 16 to be concentrated into certain areas, and this principle can be used to achieve a variation in the heat distribution across the heater element (as greater heating is obtained in regions where greater current flows). This principle is illustrated further in Figure 19, in which regions 19 of the carbon nanomaterial-containing layer 18 are devoid of carbon nanomaterials. This causes the electrical current flowing between the electrodes 14, 16 to concentrate in region 21 , thereby achieving a variation in the heat distribution across the heater element 10 (that is to say, a greater degree of heating in region 21 , and a lesser degree of heating away from region 21 ). Thus, by changing the arrangement of the carbon nanomaterial-containing layer 18, the heat distribution across the heater element 10 can also be changed.

Finally, in the carbon nanomaterial-based heater elements described thus far (e.g. as shown in Figures 10, 1 1 and 12), each carbon nanomaterial-based heater element 10 has its own dedicated pair of electrodes 14, 16. However, under certain circumstances, a pair of neighbouring heater elements (or indeed more than two heater elements that are near to one another) may share a common electrode. Such an arrangement is illustrated in the schematic circuit diagram of Figure 20, in which a common electrode 23 is shared by two adjacent carbon nanomaterial-based heater elements 10a, 10b. Each heater element has its own dedicated power supply, with power supply 46a providing power to heater element 10a, and power supply 46b providing power to heater element 10b. It will naturally be appreciated that the polarity of either power supply may be reversed if so desired.

Claims

1 . A heater element comprising:

a substrate;

a first electrode and a second electrode disposed on the substrate; and a carbon nanomaterial-containing layer disposed on the substrate, the carbon nanomaterial-containing layer being disposed between and electrically connected to the first and second electrodes.

2. A heater element as claimed in claim 1 , wherein the carbon nanomaterial- containing layer comprises graphene.

3. A heater element as claimed in claim 1 , wherein the carbon nanomaterial- containing layer comprises carbon nanotubes.

4. A heater element as claimed in claim 1 , wherein the carbon nanomaterial- containing layer comprises a mixture of graphene and carbon nanotubes.

5. A heater element as claimed in claim 1 , wherein the carbon nanomaterial- containing layer includes a first sub-layer comprising a first carbon nanomaterial and a second sub-layer comprising a second carbon nanomaterial.

6. A heater element as claimed in claim 5, wherein the first sub-layer comprises graphene and the second sub-layer comprises carbon nanotubes.

7. A heater element as claimed in any preceding claim, wherein the substrate comprises a flexible polymer film.

8. A heater element as claimed in claim 7, wherein the substrate comprises a polyimide film, for example Kapton.

9. A heater element as claimed in any of claims 1 to 6, wherein the substrate comprises a breathable fabric through which air can flow.

10. A heater element as claimed in claim 9, wherein the substrate comprises a woven fabric.

1 1 . A heater element as claimed in claim 10, wherein the substrate comprises a woven glass fibre fabric.

12. A heater element as claimed in claim 9, wherein the substrate comprises a non-woven fabric.

13. A heater element as claimed in any preceding claim, wherein the carbon nanomaterial-containing layer at least partly overlaps each of the first and second electrodes.

14. A heater element as claimed in any preceding claim, further comprising a cover layer disposed over the carbon nanomaterial-containing layer.

15. A heater element as claimed in claim 14, wherein the cover layer comprises a polymer film.

16. A heater element as claimed in claim 15, wherein the cover layer comprises a polyimide film, for example Kapton.

17. A heater element as claimed in claim 14, wherein the carbon nanomaterial- containing layer incorporates one or more holes to enable the cover layer to be bonded to the substrate at one or more points across the carbon nanomaterial- containing layer.

18. A heater element as claimed in any preceding claim, having a thickness of the order of 0.1 mm.

19. A heater element as claimed in any preceding claim, wherein the carbon nanomaterial-containing layer is formed so as to have one or more regions that are devoid of carbon nanomaterials, in order to cause localised concentration of the current flowing between the first and second electrodes in use, and thereby achieve a variation in the heat distribution across the heater element.

20. A heater element as claimed in claim 14 when dependent on any of claims 9 to 12, wherein the cover layer comprises an electrically-insulating breathable fabric through which air can flow.

21 . A heater element as claimed in claim 20, further comprising a layer of electrically-insulating breathable fabric on the opposite side of the substrate from the cover layer.

22. A stack of heater elements as claimed in any of claims 9 to 12, 20 or 21 , arranged such that air can flow through the entire thickness of the stack.

23. A device comprising a heater element or stack of heater elements as claimed in any preceding claim, coupled to a heat exchanger.

24. A device as claimed in claim 23, wherein the heat exchanger comprises first and second interlayers and a sealing layer, wherein:

the sealing layer is attached to the first interlayer;

the first interlayer is attached to the second interlayer;

the heater element is coupled to the second interlayer;

the first and/or second interlayer is provided with an air inlet;

the first and/or second interlayer is provided with an air outlet; and the first interlayer incorporates a first arrangement of apertures and the second interlayer incorporates a second arrangement of apertures, the first and second arrangements of apertures being configured such as to form a network of channels within the first and second interlayers, extending across the heat exchanger from the air inlet to the air outlet.

25. A device as claimed in claim 24, wherein the first arrangement of apertures comprises a plurality of parallel slots oriented in a first direction, and the second arrangement of apertures comprises a plurality of parallel slots oriented in a second direction, the first and second arrangements of apertures being configured such that slots of the first arrangement cross slots of the second arrangement to thereby form the network of channels.

26. A device as claimed in claim 25, wherein the first and second directions are at substantially 90° to one another.

27. A device as claimed in claim 25 or claim 26, wherein the first arrangement of apertures comprises an alternating arrangement of relatively long slots and relatively short slots, wherein, between successive parallel relatively long slots, a plurality of relatively short slots are collinear with one another.

28. A device as claimed in claim 27, wherein the second arrangement of apertures comprises a staggered arrangement of relatively short parallel slots.

29. A device as claimed in any of claims 24 to 28, wherein one or both of the interlayers incorporates a first channel into which the air inlet feeds.

30. A device as claimed in claim 29, wherein one or both of the interlayers incorporates a second channel which feeds to the air outlet.

31 . A device as claimed in claim 29 or claim 30 when dependent on claim 25, wherein the first and/or second channels are oriented in the first direction.

32. A device as claimed in any of claims 24 to 31 , wherein the sealing layer comprises a polymer film.

33. A device as claimed in claim 32, wherein the sealing layer comprises a polyimide film, for example Kapton.

34. A device as claimed in any of claims 24 to 33, wherein the first and second interlayers comprise a polymer film.

35. A device as claimed in claim 34, wherein the first and second interlayers comprise a polyimide film, for example Kapton.

36. A device as claimed in any of claims 24 to 35, wherein each of the first and second interlayers has a thickness of the order of 0.5 mm.

37. A device as claimed in claim 23, wherein the heat exchanger comprises one or more layers of a mesh material.

38. A device as claimed in claim 37, wherein the heat exchanger comprises a first layer of the mesh material and a second layer of the mesh material, the second layer being oriented at an angle relative to the first layer.

39. A device as claimed in claim 30, wherein said angle is approximately 45°.

40. A device as claimed in claim 38 or claim 39, wherein the heat exchanger comprises a plurality of said first and second layers of the mesh material, in an alternating manner.

41 . A device as claimed in any of claims 23 to 40, wherein the heat exchanger is contained within a bag or sleeve arrangement.

42. An article comprising a heater element, a stack of heater elements, or device as claimed in any preceding claim, the heater element(s) being connected to a power supply.

43. An assembly comprising a plurality of heater elements, stacks of heater elements, or devices as claimed in any of claims 1 to 41 , wherein the heater elements are connected in parallel to a power supply.

44. An assembly comprising a plurality of heater elements, stacks of heater elements, or devices as claimed in any of claims 1 to 41 , wherein the heater elements are connected in series to a power supply.

45. An assembly comprising a plurality of heater elements, stacks of heater elements, or devices as claimed in any of claims 1 to 41 , wherein the heater elements are connected to a power control unit, the power control unit being configured to supply power to selected individual heater elements or selected subsets of the heater elements.

46. An assembly as claimed in claim 45, wherein the power control unit is configured to vary over time which of the heater elements is/are activated.

47. An assembly as claimed in claim 46, wherein the power control unit is configured to cause the activated heater element(s) to change over time in such a manner that the heated region of the assembly moves across the assembly in a cyclic wave-like manner.

48. An assembly as claimed in any of claims 43 to 47, wherein two or more neighbouring heater elements share a common electrode.

49. An article comprising an assembly as claimed in any of claims 43 to 48 selected from a group comprising:

a massage mat or pad;

a vehicle seat;

an interior panel of a vehicle;

a spacecraft such as a satellite; a cooking hob;

an item of clothing or other wearables;

fabrics, blinds and shutters.

50. A method of making a heater element, the method comprising:

arranging a first electrode and a second electrode on a substrate; and depositing a carbon nanomaterial-containing dispersion on the substrate so as to form a carbon nanomaterial-containing layer between and electrically connected to the first and second electrodes.

51 . A method as claimed in claim 50, wherein the carbon nanomaterial- containing dispersion comprises graphene.

52. A method as claimed in claim 50, wherein the carbon nanomaterial- containing dispersion comprises carbon nanotubes.

53. A method as claimed in claim 50, wherein the carbon nanomaterial- containing dispersion comprises a mixture of graphene and carbon nanotubes.

54. A method as claimed in claim 51 or claim 53, wherein the dispersion comprises small graphene nanoplatelets less than 2 μηι in size.

55. A method as claimed in claim 51 or claim 53, wherein the dispersion comprises graphene nanoplatelets greater than 2 μηη in size.

56. A method as claimed in claim 51 or claim 53, wherein the dispersion comprises a mixture of small graphene nanoplatelets less than 2 μηπ in size, and graphene nanoplatelets greater than 2 μιη in size.

57. A method as claimed in claim 50, wherein the depositing comprises depositing a first dispersion comprising a first carbon nanomaterial and a second dispersion comprising a second carbon nanomaterial.

58. A method as claimed in claim 57, wherein the first dispersion comprises graphene and the second dispersion comprises carbon nanotubes.

59. A method as claimed in claim 57 or claim 58, wherein the depositing of the first dispersion is performed substantially simultaneously with the depositing of the second dispersion.

60. A method as claimed in claim 57 or claim 58, wherein the depositing of the first dispersion is performed non-simultaneously with the depositing of the second dispersion.

61 . A method as claimed in claim 60, wherein the depositing of the first and second dispersions are performed a plurality of times, in an alternating manner.

62. A method as claimed in any of claims 50 to 61 , wherein the or each carbon nanomaterial-containing dispersion is deposited by spraying.

63. A method as claimed in any of claims 50 to 61 , wherein the or each carbon nanomaterial-containing dispersion is deposited by printing.

64. A method as claimed in any of claims 50 to 61 , wherein the or each carbon nanomaterial-containing dispersion is deposited by painting.

65. A method as claimed in any of claims 50 to 64, wherein the or each carbon nanomaterial-containing dispersion is deposited such that it at least partly overlaps each of the first and second electrodes.

66. A method as claimed in any of claims 50 to 65, wherein the substrate comprises a flexible polymer film.

67. A method as claimed in claim 66, wherein the substrate comprises a polyimide film, for example Kapton.

68. A method as claimed in any of claims 50 to 64, wherein the substrate comprises a breathable fabric through which air can flow.

69. A method as claimed in any of claims 50 to 68, further comprising attaching a cover layer over the carbon nanomaterial-containing layer.

70. A method as claimed in claim 69, wherein the attaching is performed at an elevated temperature and under applied pressure.

71 . A method as claimed in claim 70, wherein the elevated temperature is of the order of 170°C.

72. A method as claimed in claim 70 or claim 71 , wherein the applied pressure is of the order of 250 PSI.

73. A method as claimed in any of claims 70 to 72, wherein the elevated temperature is provided by hot pressing.

74. A method as claimed in any of claims 70 to 72, wherein the elevated temperature is created by passing an electric current through the carbon nanomaterial-containing layer.

75. A method as claimed in claim 69, wherein the attaching is performed using UV-curing epoxy.

76. A method as claimed in any of claims 69 to 75, wherein the cover layer comprises a polyimide film, for example Kapton.

77. A method as claimed in any of claims 50 to 76, wherein the heater element has a thickness of the order of 0.1 mm.

78. A method as claimed in any of claims 50 to 77, further comprising cutting the heater element to shape.

79. A method as claimed in claim 78, wherein the cutting is performed using a CNC router.

80. A method as claimed in claim 69 when dependent on claim 68, wherein the cover layer comprises an electrically-insulating breathable fabric through which air can flow.

81 . A method as claimed in claim 80, wherein the attaching is performed by sewing using a non-conductive thread.

82. A method as claimed in claim 81 , wherein the sewing passes through the first and second electrodes.

83. A method as claimed in any of claims 50 to 82, further comprising attaching electrical supply wires to the first and second electrodes by soldering.

84. A method as claimed in any of claims 50 to 82, further comprising attaching electrical supply wires to the first and second electrodes using a clip-on connector.

85. A heat exchanger comprising first and second interlayers and a sealing layer, wherein:

the sealing layer is attached to the first interlayer;

the first interlayer is attached to the second interlayer;

the second interlayer is for coupling to a heat source;

the first and/or second interlayer is provided with an air inlet;

86. A heat exchanger as claimed in claim 85, wherein the first arrangement of apertures comprises a plurality of parallel slots oriented in a first direction, and the second arrangement of apertures comprises a plurality of parallel slots oriented in a second direction, the first and second arrangements of apertures being configured such that slots of the first arrangement cross slots of the second arrangement to thereby form the network of channels.

87. A heat exchanger as claimed in claim 86, wherein the first and second directions are at substantially 90° to one another.

88. A heat exchanger as claimed in claim 86 or claim 87, wherein the first arrangement of apertures comprises an alternating arrangement of relatively long slots and relatively short slots, wherein, between successive parallel relatively long slots, a plurality of relatively short slots are collinear with one another.

89. A heat exchanger as claimed in claim 88, wherein the second arrangement of apertures comprises a staggered arrangement of relatively short parallel slots.

90. A heat exchanger as claimed in any of claims 85 to 89, wherein one or both of the interlayers incorporates a first channel into which the air inlet feeds.

91 . A heat exchanger as claimed in claim 90, wherein one or both of the interlayers incorporates a second channel which feeds to the air outlet.

92. A heat exchanger as claimed in claim 90 or claim 91 when dependent on claim 86, wherein the first and/or second channels are oriented in the first direction.

93. A heat exchanger as claimed in any of claims 85 to 92, wherein the sealing layer comprises a polymer film.

94. A heat exchanger as claimed in claim 93, wherein the sealing layer comprises a polyimide film, for example Kapton.

95. A heat exchanger as claimed in any of claims 85 to 94, wherein the first and second interlayers comprise a polymer film.

96. A heat exchanger as claimed in claim 95, wherein the first and second interlayers comprise a polyimide film, for example Kapton.

97. A heat exchanger as claimed in any of claims 85 to 96, wherein each of the first and second interlayers has a thickness of the order of 0.5 mm.

98. A heat exchanger as claimed in claim 85, wherein the first and second interlayers are layers of a mesh material.

99. A heat exchanger as claimed in claim 98, wherein the mesh material of the first interlayer is oriented at an angle relative to the mesh material of the second interlayer.

100. A heat exchanger as claimed in claim 99, wherein said angle is approximately 45°.

101 . A method of making a heat exchanger, the method comprising:

forming a first interlayer and a second interlayer such that first interlayer incorporates a first arrangement of apertures and the second interlayer incorporates a second arrangement of apertures, the first and second arrangements of apertures being configured such as to form a network of channels within the first and second interlayers, extending across the heat exchanger, when the first and second interlayers are placed on top of one another;

attaching the first and second interlayers together;

attaching a sealing layer to the first interlayer;

providing the first and/or second interlayer with an air inlet; and

providing the first and/or second interlayer with an air outlet.

102. A method as claimed in claim 101 , wherein the first arrangement of apertures comprises a plurality of parallel slots oriented in a first direction, and the second arrangement of apertures comprises a plurality of parallel slots oriented in a second direction, the first and second arrangements of apertures being configured such that, when the first and second interlayers are placed on top of one another, slots of the first arrangement cross slots of the second arrangement to thereby form the network of channels.

103. A method as claimed in claim 102 wherein, when the first and second interlayers are attached together, the first and second directions are at substantially 90° to one another.

104. A method as claimed in claim 102 or claim 103, wherein the first arrangement of apertures comprises an alternating arrangement of relatively long slots and relatively short slots, wherein, between successive parallel relatively long slots, a plurality of relatively short slots are collinear with one another.

105. A method as claimed in claim 104, wherein the second arrangement of apertures comprises a staggered arrangement of relatively short parallel slots

106. A method as claimed in any of claims 101 to 105, wherein one or both of the interlayers incorporates a first channel into which the air inlet feeds.

107. A method as claimed in claim 106, wherein one or both of the interlayers incorporates a second channel which feeds to the air outlet.

108. A method as claimed in claim 106 or claim 107 when dependent on claim 102, wherein the first and second channels are oriented in the first direction.

109. A method as claimed in any of claims 101 to 108, wherein the forming is performed using a CNC router.

1 10. A method as claimed in any of claims 101 to 109, wherein the attaching is performed at an elevated temperature and under applied pressure.

1 1 1 . A method as claimed in claim 1 10, wherein the elevated temperature is of the order of 170°C.

1 12. A method as claimed in claim 1 10 or claim 1 1 1 , wherein the applied pressure is of the order of 250 PSI.

1 13. A method as claimed in any of claims 1 10 to 1 12, wherein the elevated temperature is provided by hot pressing.

1 14. A method as claimed in any of claims 101 to 109, wherein the attaching is performed using UV-curing epoxy.

1 15. A method of making a heater element with an integrated heat exchanger, comprising the method of any of claims 50 to 84 in combination with the method of any of claims 101 to 1 14, with the heater element being coupled to the heat exchanger.

1 16. A method of controlling an array of heater elements so as to cause the activated heater element(s) to change over time in such a manner that the heated region of the array moves across the array in a cyclic wave-like manner.

1 17. Conductive ink for deposition on a substrate to form a heater element, the conductive ink comprising a dispersion which contains one or more carbon nanomaterials.

1 18. Conductive ink as claimed in claim 1 17, wherein the dispersion contains graphene.

1 19. Conductive ink as claimed in claim 1 17, wherein the dispersion contains carbon nanotubes.

120. Conductive ink as claimed in claim 1 17, wherein the dispersion contains a mixture of graphene and carbon nanotubes.

121 . Conductive ink as claimed in claim 1 18 or claim 120, wherein the dispersion contains small graphene nanoplatelets less than 2 μηι in size.

122. Conductive ink as claimed in claim 1 18 or claim 120, wherein the dispersion contains graphene nanoplatelets greater than 2 μιτι in size.

123. Conductive ink as claimed in claim 1 18 or claim 120, wherein the dispersion contains a mixture of small graphene nanoplatelets less than 2 μπΊ in size, and graphene nanoplatelets greater than 2 m in size.

124. A heater element substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings.

125. A heat exchanger substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings.

126. An integrated device comprising a heater element and a heat exchanger substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings.

127. An assembly comprising a plurality of heater elements or devices substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings.

128. A method of making a heater element and/or a heat exchanger substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings.

129. Conductive ink substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings, for deposition on a substrate to form a heater element.