GB2576014A

GB2576014A - Flexible Thermoelectric Generator

Info

Publication number: GB2576014A
Application number: GB1812546.8A
Authority: GB
Inventors: Fletcher Thomas
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 2018-08-01
Filing date: 2018-08-01
Publication date: 2020-02-05
Also published as: GB201812546D0

Abstract

A flexible substrate for a flexible thermoelectric generator 6 comprising: a metal foil layer 2; an electrode 5; and, a dielectric layer 3, disposed between the foil and electrode, comprising electrically-insulating particles embedded in an electrically-insulating polymer, the particles having a higher thermal conductivity than the polymer. The polymer is preferably epoxy resin, whilst the particles are preferably ceramic or a large band-gap semiconductor. When forming the flexible substrate, the dielectric layer may be treated such that it bonds to the metal foil, and may be provided by spin coating or by diluting the layer first. A thermoelectric generator 6 is formed by sandwiching a thermoelectric element 7 between the electrodes 51 52 of two flexible substrates.

Description

Fig. 6

FLEXIBLE THERMOELECTRIC GENERATOR

BACKGROUND

Embodiments of the present disclosure relate to flexible thermoelectric generators, and more particularly, but not by way of limitation to a flexible thermoelectric generator 5 including a flexible substrate comprising a dielectric layer with high thermal conductivity.

A thermoelectric generator (TEG) can be used to generate electrical power or as a heating/cooling device. The TEG includes at least one thermoelectric element 10 sandwiched between a pair of electrical contacts (or “electrodes”) provided on substrates.

Typically, the TEG may include a plurality of n-type thermoelectric elements and p-type thermoelectric elements. The thermoelectric elements are interposed between a pair of substrates. An array of electrodes is provided on each substrate. The electrodes are 15 arranged to connect the thermoelectric elements in series, with the thermoelectric elements being spaced along a first direction. The thermoelectric elements are arranged in a sequence of alternating n-type and p-type thermoelectric elements.

A temperature difference may be applied across the TEG in a second direction 20 perpendicular to the first direction. The second direction intersects a contactthermoelectric element boundary. In response to the temperature difference, a voltage is generated by the thermoelectric elements. This voltage can be used to drive a current through the TEG. Alternatively, a current may be driven through the TEG to produce a temperature difference across the TEG which can be used to cool or heat a thermal load.

By improving the thermal performance of the TEG, the thermal efficiency of the TEG can be increased. This increase in the thermal efficiency results in a higher power output of the TEG for a given temperature difference across the TEG.

To improve the thermal performance of the TEG, the thermal conductivity of the substrates may be increased in relation to the thermal conductivity of thermoelectric elements, so that the temperature difference can be transferred effectively through the substrates to the thermoelectric elements.

-1In a rigid TEG, substrates are usually made from thermally-conductive ceramics. To improve the thermal performance of the rigid device, the thickness of the thermoelectric elements maybe increased relative to the thickness of the substrates. This can decrease the thermal conductance of the thermoelectric elements.

In a flexible TEG, a rigid substrate structure may not be viable. In a flexible TEG, the thickness of the substrates and thermoelectric elements is limited to hundreds of microns to help ensure that the TEG is flexible. Furthermore, ceramic substrates typically used in rigid TEGs are difficult to manufacture and generally have poor 10 mechanical properties for flexing.

As a result, substrates used in flexible TEGs tend to be made of plastic films. These plastic films have a desired thickness in a range of tens to hundreds of microns. However, plastic films tend to have very high thermal resistance and therefore cannot provide the 15 desired thermal performance.

United States Patent No. 7,999,172 (the “T72 patent”) describes a flexible thermoelectric device and a manufacturing method thereof. In the T72 patent, the flexible thermoelectric device comprises flexible substrates, offering a flexible property and tensile property to the thermoelectric device.

SUMMARY

A summary of aspects of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of 25 aspects and/or a combination of aspects that may not be set forth.

According to a first aspect of some embodiments of the present disclosure there is provided a flexible TEG including a first flexible substrate and a second flexible substrate. Both the first and the second flexible substrates each comprises a metal foil layer and an electrode with a dielectric layer/electrically-insulating layer disposed between the metal 30 foil layer and the electrode. In embodiments of the present disclosure, the dielectric layer comprises electrically-insulating particles embedded in an electrically-insulating polymer, where the electrically-insulating particles have a high thermal conductivity, which is higher than the thermal conductivity of the electrically-insulating polymer.

-2In some embodiments, the electrically-insulating polymer may comprise an epoxy. The epoxy may comprise an epoxy resin, a polymer with an epoxide functional group, a polyepoxide and/or the like. For forming the dielectric layer on the metal foil, the epoxy may comprise an epoxy resin, a diluent and an epoxy curative.

In some embodiments, the electrically-insulating particles may comprise an electricallyinsulating ceramic with a high thermal conductivity. For example aluminium nitride is a wide-band gap semiconductor with a high thermal conductivity of about 177 W m^K¹for individual crystals of the aluminium nitride. In some embodiments, the electricallyinsulating particles may have thermal conductivities of greater than 50 W m flk¹,100 W 10 nUK'¹ or 150 W nUK¹. In some embodiments, the electrically-insulating particles may comprise aluminium nitride, boron nitride, sapphire, diamond, hexagonal boron nitride, aluminium oxide, silicon dioxide, zinc oxide, silicon nitride, silicon carbide (SiC), graphene oxide, tungsten carbide and/or the like.

In some embodiments, the metal foil layer is patterned. In some embodiments, the flexible substrate may be made thin enough to provide that the substrate can flex. In some embodiments, the flexible substrate may be resistant to solvents that it may be exposed to during thermoelectric generator fabrication and usage.

In some embodiments, the dielectric layer may have a thickness of between 5 and 15 pm.

In some embodiments, the electrically-insulating layer may have a thermal conductivity 20 of between about 1 and 10 W m^K¹.

In some embodiments, the metal foil layer may have a thickness of between 30 and too pm.

In some embodiments, the metal foil layer may have a thermal conductivity of between about 35 and 500 W m flC¹.

In some embodiments, the metal foil layer may comprise aluminium, gold, copper and/or the like.

In some embodiments of the present disclosure, the TEG may comprise a thermoelectric element disposed between the first electrode and the second electrode.

-3In some embodiments, a first base and a second base may have a thermal conductivity of at least 30 W irHK’¹, wherein the first and second bases are each formed of the metal foil layer and the dielectric layer.

In some embodiments of the present disclosure, there is provided a method of forming a 5 flexible TEG. The method comprises disposing a dielectric layer on a metal foil layer to form a first base and disposing the dielectric layer on the metal foil layer to form a second base, where the electrically-insulating layer comprises electrically-insulating particles embedded in an electrically-insulating polymer.

In embodiments of the present disclosure, the electrically-insulating particles have a 10 higher thermal conductivity than the electrically-insulating polymer.

In some embodiments, the dielectric layer may comprise an epoxy resin, a diluent, an epoxy curative and a plurality of electrically-insulating particles, wherein the electricallyinsulating particles comprise: a wide-band gap semiconductor with a high thermal conductivity; a ceramic with a high thermal conductivity; one or more high-thermal 15 conductivity, electrically-insulating materials selected from the group of aluminium nitride, boron nitride, sapphire, diamond, hexagonal boron nitride, aluminium oxide, silicon dioxide, zinc oxide, silicon nitride, silicon carbide (SiC), graphene oxide, and tungsten carbide; and/or the like. In some embodiments, the dielectric layer may comprise an epoxy resin, a diluent, an epoxy curative and a plurality of electrically20 insulating particles may be coupled with the first and the second substrates by curing.

In some embodiments, the method may further comprise providing the dielectric layer on the metal foil layer by spin coating the electrically-insulating layer onto a surface of the metal foil layer.

In some embodiments, the method may comprise treating the dielectric layer such that 25 the dielectric layer bonds to the metal foil layer, and bringing the first base into contact with a first electrode, the dielectric layer being arranged between the metal foil layer and the first electrode.

In some embodiments, the method may comprise cleaning the surface of the metal foil layer by applying water and isopropyl alcohol prior to metal foil layer and then disposing 30 the dielectric layer on the metal foil layer and applying propylene glycol methyl ether acetate to the surface of the metal foil layer.

-4In some embodiments, the method may comprise diluting the electrically-insulating layer to form a diluted electrically-insulating layer, and providing the diluted electricallyinsulating layer on the metal foil layer.

In some embodiments, the method may comprise drying the first base, and treating the 5 electrically-insulating layer by heating the first base at 180⁰ for 30 minutes.

In some embodiments, the method may comprise heating the first and the second bases in an oven or on a hotplate.

In some embodiments, the method may comprise providing the first electrode on the first base by sputtering or printing.

In some embodiments, the method may comprise patterning the first electrode by deposition and photolithography, or printing.

In some embodiments, the method may comprise forming a second electrode on the second base and providing a thermoelectric element between the first electrode and the second electrodes.

In some embodiments, the method may comprise providing the thermoelectric element on the first electrode by dispense printing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is described in conjunction with the appended figures. It is emphasized that, in accordance with the standard practice in the industry, various 20 features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

In the appended figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the 25 similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.

-5Figure i is a schematic side view of a flexible substrate for a flexible TEG, in accordance with some embodiments of the present disclosure.

Figure 2 is a process flow diagram of a method of manufacturing a flexible substrate for a flexible TEG, in accordance with some embodiments of the present disclosure.

Figure 3 is a schematic side view of a flexible thermoelectric generator comprising a flexible substrate, in accordance with some embodiments of the present disclosure.

Figure 4 illustrates a measurement system for verifying thermal performance of a flexible substrate for a flexible TEG, in accordance with some embodiments of the present disclosure.

Figure 5 illustrates a measurement system for verifying thermal efficiency of a flexible TEG, in accordance with some embodiments of the present disclosure.

Figure 6 shows graphs of voltage against temperature difference for a flexible TEG, in accordance with some embodiments of the present disclosure, and a comparative example of a TEG.

DETAILED DESCRIPTION

In some embodiments of the present disclosure there is provided a flexible TEG including a first flexible substrate and a second flexible substrate. Both the first and the second flexible substrates each comprises a metal foil layer and an electrode with a dielectric layer/electrically-insulating layer disposed between the metal foil layer and the 20 electrode. In embodiments of the present disclosure, the dielectric layer comprises electrically-insulating particles embedded in an electrically-insulating polymer, where the electrically-insulating particles have a high thermal conductivity, which is higher than the thermal conductivity of the electrically-insulating polymer.

Flexible TEGs have previously been manufactured using flexible ceramics - which have 25 high thermal conductivity (of the order of around 2 to 3 watts per meter-Kelvin (W m^Kfl), but are generally hard and brittle so problematic for incorporating in the TEG - or flexible plastic materials. The flexible plastic materials generally have a low thermal conductance, of the order of around less than 1W nUK¹. As such, for TEGs comprising flexible plastics, the thickness of the flexible plastic needs to be balanced to have a 30 thickness that provides for complete electrical isolation (thicker plastic layer) and a low

-6thermal resistance (thinner plastic layer). In practice, this is a difficult balance to achieve and ultimately limits the thermal conductivity of the substrate.

In embodiments ofthe present disclosure, a flexible substrate is provided that comprises a metal foil layer, an electrode and a dielectric layer that is disposed between the metal 5 foil layer and the electrode. The dielectric layer comprises an epoxy containing particles that have a higher thermal conductivity than the epoxy, and thereby form a material with a higher thermal conductivity than a dielectric layer comprising only epoxy. The epoxy makes the dielectric layer flexible and provides a mechanism for coupling the dielectric layer with the metal foil layer. The particles comprise an electrically insulating material, 10 such as a ceramic, high band-gap semiconductor and/or the like. In some embodiments, the particles may comprise greater than 50% or greater than 75% by volume of the dielectric layer to provide that the dielectric layer is flexible, as a result of the presence of the epoxy, has a high thermal conductivity and is electrically insulating. The material may be deposited on the metal foil layer to a thickness that provides complete electrical 15 insulation and has high thermal conductance.

In some embodiments, the epoxy may comprise an epoxy resin, a polymer with an epoxide functional group, a polyepoxide and/or the like. In some embodiments, to form the dielectric layer on the metal foil, the epoxy may comprise an epoxy resin, a diluent and an epoxy curative. In some embodiments of the present disclosure, a method of 20 manufacturing a flexible substrate comprising the flexible dielectric layer is provided in which the flexible dielectric layer may be spin coated, printed and/or the like onto the metal foil layer to a thickness that provides electrical isolation. For example, in some embodiments, the dielectric layer may have a thickness of between 5 and 15 pm.

In some embodiments, the electrically-insulating particles may comprise an electrically25 insulating ceramic with a high thermal conductivity. For example aluminium nitride is a wide-band gap semiconductor with a high thermal conductivity of about 177 W m^K¹for individual crystals of the aluminium nitride. In some embodiments, the electricallyinsulating particles may have thermal conductivities of greater than 50 W nr’K'¹, too W nUK'¹ or 150 W nr’K'¹. In some embodiments, the electrically-insulating particles may 30 comprise aluminium nitride, boron nitride, sapphire, diamond, hexagonal boron nitride, aluminium oxide, silicon dioxide, zinc oxide, silicon nitride, silicon carbide (SiC), graphene oxide, tungsten carbide and/or the like.

-7Figure i is a schematic side view of a flexible substrate for a flexible TEG, in accordance with some embodiments of the present disclosure.

In Fig. 1, a flexible substrate i includes a metal foil layer 2, an electrically-insulating layer in contact with the metal foil layer 2, and a first electrode 5, 51 in contact with the electrically-insulating layer 3, such that the electrically-insulating layer 3 is sandwiched between the metal foil layer 2 and the first electrode 51. The metal foil layer 2 and the electrically-insulating layer 3 form a first flexible base 4,41 onto which the first electrode 5i may be disposed.

In some embodiments, the metal foil layer 2 may comprise an aluminium, gold or copper foil layer. In some embodiments, the metal foil layer 2 is unpatterned. In some embodiments, the metal foil layer may have a thermal conductivity of between about 35 and 500 W nUK¹. In some embodiments, the metal foil layer 2 may have a thickness, ti, of between about 30 and too pm, where thickness is taken to be in the direction in which the materials are layered or stacked in the flexible substrate 1 (in Fig. 1, thickness is shown to along the z-axis).

In embodiments of the present disclosure, using the metal foil layer 2 increases the thermal conductivity of the first flexible base 41 compared to comparative bases, for example, a base consisting of plastic film or the like. In embodiments of the present disclosure, the electrically-insulating layer 3 comprises electrically-insulating particles and an electrically-insulating polymer.

In embodiments of the present disclosure, the electrically-insulating particles have a higher thermal conductivity than the electrically-insulating polymer. In some embodiments, the electrically-insulating layer 3 has a thermal conductivity of between about 1 and 10 W nUK¹. In some embodiments, the electrically-insulating layer 3 may 25 have a thickness, t₂, of between about 5 and 15 pm.

In some embodiments of the present disclosure, the electrically-insulating layer 3 may be formed of thermally-conductive epoxy. An example of a suitable thermallyconductive epoxy is product 122-07SP available from Creative Materials Inc., Ayer, USA.

In embodiments of the present disclosure, the first electrode 51 may include a single conductive layer or a stack of multiple conductive layers. The or each conductive layer may consist of a single conductive material or two or more materials, for example, in the

-8form of an alloy. Examples of conductive materials include metals, such as aluminium, copper, silver or gold. In embodiments of the present disclosure, the metal maybe suspended in a binder (“printable metal”). Other examples of conductive materials include conductive metal oxides and conductive carbon allotropes, such as graphite. In 5 some embodiments, the first electrode 51 may have a thickness, t₃, of between about i and 20 pm.

In some embodiments of the present disclosure, the flexible substrate i has a bend radius of about 70 mm or less, or 20 mm or less. In some embodiments, the bend radius may be at least 5 mm or at least 10 mm.

The thickness, t₂, of the electrically-insulating layer 3 may be chosen so that the metal foil layer 2 is electrically-isolated from the first electrode 51. To improve the electrical isolation of the metal foil layer 2, the thickness, t₂, of the electrically-insulating layer 3 may be increased. Increasing the thickness, t₂, of the electrically-insulating layer 3 can also help to decrease the likelihood of pinholes in the layer 3, which can increase the 15 manufacturing yield of the flexible substrate 1.

The thermal resistance of the electrically-insulating layer 3 increases with increasing thickness. However, the thermal conduction properties of the electrically-insulating layer 3 helps to counter e.g. offset or even balance, the increase in thermal resistance caused by the increase in thickness of the electrically-insulating layer 3.

The thickness, ti, of the metal foil layer 2 may be chosen to help provide mechanical support to the flexible substrate 1. The thickness, t₂, of the electrically-insulating layer 3 cannot be increased to help provide mechanical support to the flexible substrate 1 without increasing the thermal resistance of the electrically-insulating layer 3. Thus, in some embodiments, the metal foil layer 2 has thickness, ti, greater than the thickness, 25 t₂, of the electrically-insulating layer 3.

Fabrication of flexible electronic device

Figure 2 is a flow-type diagram of a method of manufacturing the flexible substrate 1, illustrated in Fig. 1, in accordance with some embodiments of the present disclosure.

In some embodiments of the present disclosure, the method involves providing the 30 electrically-insulating layer 3 on the metal foil layer 2. In some embodiments, to aid

-9fabrication, the metal foil layer 2 may be laminated to a carrier substrate (not shown) to form a workpiece prior to providing the electrically-insulating layer 3 on the metal foil layer 2 (step Si).

In some embodiments, the carrier substrate (not shown) may take the form of a square glass plate having a thickness 1.1 mm and dimensions 355 x 355 mm. In some embodiments, the carrier substrate (not shown) maybe laminated with a double-sided adhesive gel film. An example of a suitable double-sided adhesive gel film is Gel-Pak® DGL Film X4.

In some embodiments, the metal-foil layer 2 may be pre-treated and the electrically10 insulating layer 3 may be diluted (step S4) before the electrically-insulating layer 3 is deposited onto the metal foil layer 2 (step S5). Step S4 maybe performed before or after step S2 and step S3. The electrically-insulating layer 3 may be deposited onto the metal foil layer 2 by spin coating, printing, curing and/or the like.

In some embodiments, the surface of the metal foil layer 2 maybe cleaned with water 15 and isopropyl alcohol (IPA) (step S2) and wetted with propylene glycol methyl ether acetate (PGMEA) for uniform coating (step S3). The electrically-insulating layer 3 (EIL) may be diluted (step S4) with a thinner, such as PGMEA. For example, the electricallyinsulating layer 3 maybe diluted with PGMEA to a ratio of about 4:1 (EIL: PGMEA).

In some embodiments, the electrically-insulating layer 3 may be spin coated onto the 20 metal foil layer 2 (step S5), by a 3-phase spin coating process, to form the first flexible base 4i.

In some embodiments of the present disclosure, a volume of approximately 20 ml of the electrically-insulating layer 3 is dispensed in the centre of the metal foil layer 2 and spin coated in 3 phases as set out in Table 1 below:

Table 1

Phase	Speed/rad s^_1 (rpm)	Acceleration/rad s^-2 (rpm s^_1)	Time/s
1	31 (300)	10 (too)	10
2	83 (800)	31 (300)	15
3	10 (too)	10 (too)	120

-10Diluting the electrically-insulating layer 3 and performing a 3-phase spin coating process can help to ensure that the electrically-insulating layer 3 provided on the metal foil layer 2 has a desired thickness to provide electrical isolation and thermal conductivity. For example, diluting and performing the 3-phase spin coating process can help the 5 electrically-insulating layer 3 to be deposited on the metal foil layer 2 to a thickness of pm.

After the electrically-insulating layer 3 is provided on the metal foil layer 2, the first flexible base 4.1 maybe dried (step S6). In some embodiments, the first flexible base 4.1 may be dried on a hot plate at 6o° for 5 minutes.

In some embodiments, the electrically-insulating layer 3 is treated (“crosslinked” or “cured”) (step S7) so that the electrically-insulating layer 3 bonds to the metal foil layer 2. In some embodiments, the electrically-insulating layer 3 maybe cured by heating the first flexible base 41 at 180⁰ for 30 minutes. In some embodiments, the first flexible base 4i may be heated in an oven or heated on a hotplate.

The method further includes providing the first electrode 51 on the electrically-insulating layer 3 (step S8). In some embodiments, the first electrode 51 maybe provided on the surface of the electrically-insulating layer 3 by sputtering, printing and/or the like. In some embodiments, the first electrode 51 maybe patterned (step S9) by deposition and photolithography, printing and/or the like.

Thermoelectric generator

In Fig. 3, a flexible thermoelectric generator 6 includes a thermoelectric element 7, first and second flexible bases 4, 41,42, and first and second electrodes 5, 51, 52. The second flexible base 42 is similar to the first flexible base 41 previously described. The second electrode 52 is similar to the electrode 51 previously described.

The thermoelectric element 7 is disposed between the first and second electrodes 51, 52. The first electrode 51 is provided directly on the first flexible base 41 such that the first electrode 51 is provided directly on the electrically-insulating layer 3. The second

-11electrode 52 is provided on the second flexible base 42 such that the second electrode 52 is provided directly on the electrically electrically-insulating layer 3.

In some embodiments, the thermoelectric element 7 is formed of an epoxy-based ink. In some embodiments, the epoxy-based ink may comprise or consist of an alloy of 5 tellurium (Te), and bismuth (Bi) or antimony (Sb). Merely by way of example, in some embodiments, the epoxy-based ink may comprise or consist of Bi₂Te₃, Sb₂Te₃ and/or the like. In some embodiments, the thermoelectric element 7 may have a thickness, t₄, of between about 20 and 400 pm.

In some embodiments of the present disclosure, the thermoelectric element 7 may be deposited on the first electrode 51 by dispense printing or the like. The second electrode 52, which is provided on the second flexible base 42, is brought into contact with the thermoelectric element 7. In some embodiments, a carrier substrate (not shown) is removed prior to the second electrode 52 being provided on the thermoelectric element7.

A thermoelectric generator may include one of a selection of different substrate configurations depending on the requirements of that generator. Thermoelectric generators of different substrate configurations may exhibit different thermal performances.

The thermal performance of a thermoelectric generator can be quantified by a ratio of a temperature difference across the thermoelectric element (also referred to as “active material”) to a temperature difference applied across the generator:

_ ^material ^applied

Wherein: 5T_materia| indicates the temperature difference across the thermoelectric element (“active material”) 7, and 5T_app|_ied indicates the temperature difference across 25 the thermoelectric generator 6.

The temperature differences are shown schematically in Figure 3 (below).

Theoretical temperature differences may be calculated using reference materials and thermal conductivity data. From this, a theoretical X may be calculated for thermoelectric generators having different substrate configurations.

-12For the values in Table 2 below, the thermoelectric element 7 is considered to have a thickness of 50 pm and thermal conductivity of 0.45W nUK¹.

Table 2

Substrate configuration	Thermal Conductivity W nr¹ K¹	Thickness pm	X	Comments
Polyethylene naphthalate (PEN)	0.2	125	0.08	Poor thermal performance
Thermosilicone on polyimide	1.05	150	Ο.27	Poor thermal performance
Flexible ceramic	2.7	40	0.79	Hard and brittle, requires specialised handling
Spin on glass (1) on aluminium foil (2)	1) 1 2) 237	1) 6 2) 40	0.90	Requires high temperature processing
SU-8 (1) on aluminium foil (2)	1) 0.38 2) 237	1) 6 2) 40	0.78	Low temperature processing, solvent resistant
Thermallyconductive epoxy (1) on aluminium foil (2)	1) 5 2) 237	1) 10 2) 40	0.96	Low temperature processing, solvent resistant, increased durability, improved performance compared to other substrate configurations, for example flexible ceramic and SU-8 on aluminium foil

The PEN is product Teonex® Q65HA available from DuPont Teijin Films. The 5 thermosilicone on polyimide comprises thermo-silicone interface material KU-KC15 available from Aavid-Kunze. The flexible ceramic is product E-Strate® available from ENrG Inc. The glass is soda-lime glass. The SU-8 is product SU-8 3025 available from MicroChem Corp. The thermal conductivity of glass is typical, for example as recited in “Transport phenomena data companion”, VSSD, 3^rd Edition, 2006.

The thermal conductivity of aluminium foil is taken from Handbook of Chemistry and Physics, CRC Press, 67th Edition, D-185.

The thermal conductivity of PEN has been measured against a glass reference. The thermal conductivity of SU-8 photoresist (“SU-8”) has also been measured against a glass reference. The glass used for reference is soda-lime glass with thickness of 0.7 mm. 15 The thermally-conductive epoxy referred to in Table 2 is Creative Materials Inc. product 122-07SP.

-13Table 2 shows that the substrate including thermally-conductive epoxy on aluminium foil gives the largest theoretical X compared to the comparative examples of substrates in Table 2. In some embodiments of the present disclosure, the flexible substrate 1 may comprise an epoxy with particles having a higher thermal conductivity than the 5 epoxy as the electrically-insulating layer 3 provided on an aluminium metal foil layer.

Verification of Increased Performance

The thermal performance of the flexible substrate 1, as illustrated in Fig, 1, maybe tested and compared to other flexible substrates. The flexible substrates used for comparison (“comparative example devices”) include a substrate comprising a PEN base material 10 and a substrate comprising SU-8 on a foil base.

Referring to Figure 4, a measurement system for verifying the thermal performance of the flexible substrate 1 is shown. The performance of each comparative example device is measured using a similar measurement system.

Before measuring the performance of the flexible substrate 1, the first electrode 51 is 15 patterned in a serpentine pattern to form a high resistance structure. This makes electrode 51 sensitive to temperature changes. In other words, the electrode 51 can function as a thermistor.

The flexible substrate 1 is cut to 50x50 mm pieces. To test the performance of the flexible substrate 1, a stack 8 is assembled.

The stack 8 includes two identical flexible substrates 1 and a separation element 9. The flexible substrates 1 face each other. The separation element 9 is provided between the identical flexible substrates 1 such that the separation element 9 is in direct contact with the electrode 51 of each flexible substrate 1. The separation element 9 is formed of PEN and has a thickness, t₅, of 50 pm.

The stack 8 is provided between first and second aluminium blocks 10, lOi, io₂. Thermally conductive paste is provided between each aluminium block 10 and metal foil layer 2 of each flexible substrate 1. The aluminium blocks 10 hold the stack 8 in place. The aluminium blocks 10 are temperature controlled.

-14The thermal performance of the flexible substrate i is indicated by the ratio X. Equation i is used to calculate the ratio X. The ratio X for the flexible substrate i is shown in Table 3·

The first aluminium block lOi is set at temperature Ti. The second aluminium block io₂5 is set at temperature T₂, wherein T_x A T₂. The difference between these temperatures gives the temperature difference, 5T_appiied, across the stack 8.

For a given temperature difference hT_appiied, a resistance of each electrode 5i is recorded.

As previously mentioned, the resistance of each electrode 5i indicates the temperature of that electrode 5i. The temperature difference between the electrodes 5i gives the io temperature difference, hTmatermi, across the separation element 9. The ratio X can be measured for each comparative example device using a similar method to the method for measuring the ratio X of the flexible substrate 1.

The ratio X for each comparative example device is shown in Table 3.

Table 3

Substrate	X measured
125 pm PEN	0.2
SU-8 on foil	0.6
TCE on foil	0.76

The flexible substrate 1 exhibits improved thermal performance, compared to the comparative example devices. For example, the ratio X improves by approximately a factor 1.27 between the “SU-8 on foil” device and the flexible substrate 1, including the thermally-conductive epoxy.

Thermoelectric generator comparison

Referring to Figure 5, measurement of the thermal efficiency of the flexible thermoelectric generator 6 will now be described.

The thermal efficiency of the flexible thermoelectric generator 6 may be measured and compared to comparative examples of a flexible thermoelectric generator, such as a thermoelectric generator including an SU-8 on foil base.

-15The thermal efficiency of the flexible thermoelectric generator 6 is measured by placing the flexible thermoelectric generator 6 between the aluminium blocks to previously described. The flexible thermoelectric generator 6 is arranged between the blocks io so that the blocks io are in thermal contact with the metal foil layer 2. A series of different 5 temperature differences bT_appiied are applied across the flexible thermoelectric generator

6, similar to how a temperature difference may be applied to the stack 8 (Fig. 4) as hereinbefore described.

A thermoelectric voltage (“output voltage”) across the thermoelectric element 7 is directly proportional to the temperature difference, fiTmateriai, across the thermoelectric 10 element 7. Thus, measuring the thermoelectric voltage provides an indication of the thermal efficiency of the flexible thermoelectric generator 6. The thermal efficiency of a comparative example of a thermoelectric generator may be measured in a similar way.

Referring to Fig. 6, graphs of thermoelectric voltage (“Vtherm”) against temperature difference (“δΤ_;ψρΐί«ι”) are shown. A first graph corresponds to the flexible thermoelectric 15 generator 6 (Figure 3) (“Foil+TCE thermoelectric generator”). A second graph corresponds to the comparative example of a thermoelectric generator including an SU8 on foil base (“Foil+SU-8 thermoelectric generator”).

The Foil+TCE thermoelectric generator 6 produces a significantly higher output voltage than the Foil+SU-8 thermoelectric generator for a given temperature difference 5T_appiied· 20 This improvement significantly enhances the power output of the thermoelectric generator, as power (P) has quadratic proportionality to output voltage (U) for a given resistance (R):

where, power is measured in Watts (W), voltage is measured in Volts (V), and resistance 25 is measured in Ohms (Ω).

As shown, the flexible thermoelectric generator 6 (Figure 3), including the flexible substrate 1, provides improved thermal efficiency compared to a comparative example of a flexible thermoelectric generator including, for example, an SU-8 on foil base.

The description above provides preferred exemplary embodiment(s) only, and is not 30 intended to limit the scope, applicability or configuration of the invention. Rather, the

-16ensuing description of the preferred exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing a preferred exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements, including combinations of features from 5 different embodiments, without departing from the scope of the invention.

Unless the context clearly requires otherwise, throughout the description and the claims, the words comprise, comprising, and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of including, but not limited to. As used herein, the terms connected, coupled, or any 10 variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, electromagnetic, or a combination thereof. Additionally, the words herein, above, below, and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the 15 context permits, words in the Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word or, in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.

The teachings of the technology provided herein can be applied to other systems, not necessarily the system described below. The elements and acts of the various examples described below can be combined to provide further implementations of the technology. Some alternative implementations of the technology may include not only additional elements to those implementations noted below, but also may include fewer elements.

These and other changes can be made to the technology in light of the following detailed description. While the description describes certain examples of the technology, and describes the best mode contemplated, no matter how detailed the description appears, the technology can be practiced in many ways. Details of the system may vary considerably in its specific implementation, while still being encompassed by the 30 technology disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the technology should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the technology with which that terminology is associated. In

-17general, the terms used in the following claims should not be construed to limit the technology to the specific examples disclosed in the specification, unless the Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the technology encompasses not only the disclosed examples, but also all equivalent ways of 5 practicing or implementing the technology under the claims.

To reduce the number of claims, certain aspects of the technology are presented below in certain claim forms, but the applicant contemplates the various aspects of the technology in any number of claim forms. For example, while some aspect of the technology may be recited as a computer-readable medium claim, other aspects may 10 likewise be embodied as a computer-readable medium claim, or in other forms, such as being embodied in a means-plus-function claim.

In the description above, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of implementations of the disclosed technology. It will be apparent, however, to one skilled in the art that 15 embodiments of the disclosed technology may be practiced without some of these specific details.

Claims

WHAT IS CLAIMED IS:

1. A flexible substrate configured for a thermal electric generator, comprising:

a metal foil layer;

an electrode; and a dielectric layer disposed between the metal foil layer and the electrode, wherein the dielectric layer comprises electrically-insulating particles embedded in an electrically-insulating polymer and the electrically-insulating particles have a higher thermal conductivity than the electrically-insulating polymer.
2. The flexible substrate according to claim i, wherein the electricallyinsulating polymer comprises a thermally-conductive epoxy.
3. The flexible substrate according to claim 1 or claim 2, wherein the thermally-conductive epoxy comprises an epoxy resin.
4. The flexible substrate according to any of the preceding claims, wherein the electrically-insulating particles comprise an electrically-insulating ceramic with a high thermal conductivity and/or a large band-gap semiconductor.
5. The flexible substrate according to any of the preceding claims, wherein the electrically-insulating particles comprise at least one of aluminium nitride, boron nitride, sapphire, diamond, hexagonal boron nitride, aluminium oxide, silicon dioxide, zinc oxide, silicon nitride, silicon carbide (SiC), graphene oxide and tungsten carbide.
6. The flexible substrate according to any of the preceding claims, wherein the electrode comprises an unpatterned electrode.
7. The flexible substrate according to any of the preceding claims, wherein the dielectric layer comprises a thickness of between about 5 and 15 pm.
8. The flexible substrate according to any of the preceding claims, wherein the metal foil layer comprises a thickness of between about 30 and too pm.

-199- The flexible substrate according to any of the preceding claims, wherein the dielectric layer comprises a thermal conductivity of between about i and to W nUKA
10. The flexible substrate according to any of the preceding claims, wherein the metal foil layer has thermal conductivity of between about 35 and 500 W nUKA
11. The flexible substrate according to any of the preceding claims, wherein the metal foil layer comprises aluminium, gold or copper.
12. The flexible substrate according to any of the preceding claims, wherein a first base has a thermal conductivity of at least 30 W nUK¹, wherein the first base is formed of the metal foil layer and the dielectric layer.
13. A thermoelectric generator comprising a flexible substrate according to any of the preceding claims.
14. A method for forming a flexible substrate configured for a thermoelectric generator, the method comprising:

providing an electrically-insulating layer on a metal foil layer to form a first base, wherein the electrically-insulating layer comprises electrically-insulating particles embedded in an electrically-insulating polymer, the electrically-insulating particles having a higher thermal conductivity than the electrically-insulating polymer; and treating the electrically-insulating layer such that the electrically-insulating layer bonds to the metal foil layer; and bringing the first base into contact with a first electrode, the electricallyinsulating layer being arranged between the metal foil layer and the first electrode.
15. The method according to claim 14, further comprising:

cleaning the surface of the metal foil layer by applying water and isopropyl alcohol prior to providing the electrically-insulating layer on the metal foil layer; and

-20applying propylene glycol methyl ether acetate to the surface of the metal foil layer.
16. The method according to claim 14 or 15, further comprising:

providing the electrically-insulating layer on the metal foil layer by spin coating the electrically-insulating layer onto a surface of the metal foil layer.
17. The method according to any one of claims 14 to 16, further comprising:

diluting the electrically-insulating layer to form a diluted electricallyinsulating layer; and providing the diluted electrically-insulating layer on the metal foil layer.
18. The method according to any one of claims 14 to 17, further comprising:

drying the first base; and treating the electrically-insulating layer by heating the first base at 180⁰ for 30 minutes.
19. The method of claim 18, wherein the first base is heated in an oven or on a hotplate.
20. The method according to any one of claims 14 to 19, further comprising:

providing the first electrode on the first base by sputtering or printing.
21. The method according to any one of claims 14 to 20, further comprising:

patterning the first electrode by deposition and photolithography, or printing.

19. The method according to any one of claims 11 to 18, further comprising:

-21providing a thermoelectric element on the first electrode;

forming a second base, the second base comprising the electricallyinsulating layer on the metal foil layer;

bringing the second base into contact with a second electrode, the electrically-insulating layer being arranged between the metal foil layer and the second electrode; and bringing the second electrode into contact with the thermoelectric element such that the thermoelectric element is provided between the first and second bases.

20. The method according to claim 19, further comprising:

providing the thermoelectric element on the first electrode by dispense printing.