WO2020260881A1

WO2020260881A1 - Method of forming a thermoelectric device

Info

Publication number: WO2020260881A1
Application number: PCT/GB2020/051540
Authority: WO
Inventors: Thomas Fletcher
Original assignee: Sumitomo Chemical Co., Ltd; Cambridge Display Technology Limited
Priority date: 2019-06-25
Filing date: 2020-06-25
Publication date: 2020-12-30
Also published as: GB2585045A; GB201909147D0

Abstract

A method of forming a thermoelectric device having n-type and p-type thermoelectric legs disposed in apertures of an electrically insulating bank structure comprising hollow, electrically insulating particles and a binder in which formation of the electrically insulating bank structure comprises deposition of a composition comprising the hollow, electrically insulating particles and the binder or a precursor of the binder. The composition may be free of any solvent. The composition may be deposited by a printing technique.

Description

METHOD OF FORMING A THERMOELECTRIC DEVICE

BACKGROUND

Embodiments of the present disclosure relate to a flexible thermoelectric device, for example a flexible thermoelectric generator (TEG). A thermoelectric device can be used to generate electrical power or as a heating/cooling device. A thermoelectric device includes at least one thermoelectric couple. A thermoelectric couple includes an n-type thermoelectric leg electrically coupled to a p-type thermoelectric leg. The thermoelectric device may comprise a plurality of electrically connected thermoelectric couples, forming a plurality of alternating n-type thermoelectric legs and p-type thermoelectric legs electrically connected across each leg in series.

In use, a temperature difference may be applied across the thermoelectric device in a second direction that is different to and/or orthogonal to the first direction. The temperature difference is applied across a contact-thermoelectric 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 thermoelectric device. Alternatively, a current may be driven through the thermoelectric device to produce a temperature difference across the device which can be used to cool or heat a thermal load.

Thermal and electrical resistance between the contacts and the thermoelectric element affects the power efficiency of the thermoelectric device. US 7,999,172 describes a flexible thermoelectric device and a manufacturing method thereof.

US 2016/0163948 discloses a bonding of a thermoelectric material leg to a header or an electrical connector.

US20170317261 discloses a flexible thermoelectric generator. 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 aspects and/or a combination of aspects that may not be set forth. Embodiments of the present disclosure provide a method of forming a thermoelectric device having a plurality of electrically connected thermoelectric couples, each thermoelectric couple having an n-type thermoelectric leg electrically connected to a p- type thermoelectric leg. Each n-type thermoelectric leg and each p-type thermoelectric leg may be disposed in an aperture of an electrically insulating bank structure containing hollow, electrically insulating particles and a binder. Formation of the electrically insulating bank structure may include deposition of a composition containing the hollow, electrically insulating particles and the binder or a precursor of the binder.

Optionally, the hollow, electrically insulating particles are glass or plastic microspheres. Optionally, the composition is printed in a pattern defining the apertures.

Optionally, the composition contains a crosslinkable precursor of the binder and the composition is subjected to a crosslinking treatment following the deposition of the composition.

In some embodiments, the composition does not contain a solvent. In some embodiments, the composition contains one or more solvent materials which are evaporated following the deposition of the composition.

Optionally, the composition is screen-printed.

Optionally, formation of the p-type and n-type thermoelectric legs comprises printing the components of the thermoelectric legs into the apertures of the electrically insulating bank structure.

Optionally, the thermoelectric device is flexible.

Optionally, the thermoelectric device is disposed on a substrate.

Optionally, the thermoelectric device is disposed between a first and second substrate.

Optionally, formation of the thermoelectric device includes: formation of a first part of the electrically insulating bank structure comprising deposition of the composition over the first substrate; formation of a second part of the electrically insulating bank structure comprising deposition of the composition over the second substrate; printing the components of the thermoelectric legs into the apertures of each of the first part and second part of the electrically insulating bank structure; and bringing the first and second parts of the electrically insulating bank structure into direct contact. Optionally, the, or each, substrate has a metal foil layer and an insulating layer.

In some embodiments there is provided a thermoelectric device obtainable by a method as described herein.

In some embodiments there is provided a composition containing a mixture of a crosslinkable material and hollow, electrically insulating particles. Optionally, the hollow, electrically insulating particles are glass or plastic microspheres.

The composition may or may not contain one or more solvent materials.

In some embodiments there is provided a thermoelectric device comprising a plurality of electrically connected thermoelectric couples, each thermoelectric couple comprising an n-type thermoelectric leg electrically connected to a p-type thermoelectric leg, wherein each n-type thermoelectric leg and each p-type thermoelectric leg is disposed in an aperture of an electrically insulating bank structure comprising hollow, electrically insulating particles and a binder.

Elements of the thermoelectric device, including the substrate, n-type thermoelectric leg, p-type thermoelectric leg, electrical contacts, substrate and electrically insulating bank structure may be as described anywhere herein.

In some embodiments there is provided sensor apparatus comprising a temperature sensor and a thermoelectric device as described herein wherein the thermoelectric device is a thermoelectric generator configured to supply electrical power the temperature sensor.

BRIEF DESCRIPTION OF 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 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 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.

Figure 1 provides a schematic side view of a thermoelectric device according to some embodiments;

Figure 2 provides a schematic side view of a process of forming a thermoelectric device according to some embodiments in which thermoelectric materials are deposited into apertures of a single insulating structure;

Figure 3 provides a schematic side view of a process of forming a thermoelectric device according to some embodiments in which thermoelectric materials are deposited into apertures of insulating structures on two separate substrates, the insulating structures being brought together after deposition of the thermoelectric materials; and

Figure 4 is a schematic illustration of a thermoelectric system according to some embodiments.

DETAILED DESCRIPTION Fig. l illustrates a thermoelectric device too according to some embodiments of the present disclosure. The thermoelectric device contains a plurality of electrically connected thermoelectric couples, each thermoelectric couple having an n-type thermoelectric leg 107h electrically connected to a p-type thermoelectric leg I07p. For simplicity, Fig. 1 illustrates a thermoelectric device having two thermoelectric couples connected in series, however it will be appreciated that any number of thermoelectric couples may be connected in series and / or in parallel. Preferably, some or all of the plurality of thermoelectric couples are connected in series.

The thermoelectric device includes electrically insulating bank structure 105 having apertures extending between a first surface 105A and a second surface 105B thereof n- type thermoelectric legs 107h and a p-type thermoelectric legs 107P are disposed in separate apertures of the electrically insulating bank structure. Each thermoelectric leg extends between a first end at first surface 105A of the electrically insulating bank structure and a second end at an opposing second surface 105B. The thermoelectric device comprises a plurality of first contacts 103 and a plurality of second contacts 109 electrically connecting the thermoelectric legs. Each thermoelectric leg is disposed between a first contact and a second contact. Each first contact may be in direct contact with the first end of at least one of a p-type and an n-type thermoelectric leg. Each second contact may be in direct contact with the second end of at least one of a p-type and an n-type thermoelectric leg. A first or second contact at an end of a chain of electrically connected thermoelectric couples maybe connected to only one of an n-type and p-type thermoelectric leg.

The length of thermoelectric legs 107P and 107h, i.e. the distance between the first and second ends thereof, maybe in the range of about too microns - 1 mm, optionally 200-550 microns. The width of the thermoelectric legs i07p and 107h maybe in the range of about 0.5-4 mm.

The thermoelectric device maybe supported on a substrate 101. The first contacts 103 may be disposed between the electrically insulating bank structure 105 and the substrate 101. The substrate may be a flexible substrate.

The electrically insulating layer comprises a matrix having hollow particles distributed therein. The matrix may consist of a single material or may be a mixture of two or more materials. It will be understood that the material or materials of the matrix and the hollow particles are electrically insulating. In the embodiment of Fig. 1, the electrically insulating bank structure is a single layer having apertures formed therethrough. In other embodiments, the electrically insulating bank structure has two or more layers having apertures formed therethrough, at least one of the layers comprising hollow particles.

The present inventors have found that the presence of hollow particles in the electrically insulating bank structure of a thermoelectric device as described herein may increase the temperature gradient across the device when in use as compared to a thermoelectric device in which no hollow particles are present in the electrically insulating bank structure. This may allow for a greater voltage to be generated when the device is used as a thermoelectric generator. The layer of the electrically insulating bank structure containing the hollow particles may further contain a binder.

Fig. 2 illustrates a process of forming a thermoelectric device according to some embodiments. A plurality of first electrical contacts 103 are provided on a substrate 101. The plurality of first electrical contacts 103 may be formed by any suitable deposition and / or patterning technique, e.g. by photolithography.

An electrically insulating bank structure 105 comprising or consisting of a layer containing hollow particles is formed over the first electrical contacts. The electrically insulating bank structure 105 has a plurality of apertures extending through the electrically insulating bank structure to expose at least part of the surface of a plurality of first electrical contacts 103. The exposed surface of a first contact 103 forms the base of a well defined by the aperture.

Material for forming the n-type thermoelectric legs 107h may be deposited into wells defined by some of the apertures, and material for forming the p-type thermoelectric legs i07p is deposited into wells defined by others of the apertures. The material for forming the n-type thermoelectric legs and the material for forming the p-type thermoelectric legs maybe deposited in any order or sequence and maybe deposited simultaneously.

In some embodiments, as illustrated in Figs. 1 and 2, the first end of each thermoelectric leg is in direct contact with a first electrical contact 103. In some embodiments (not shown), one or more intermediate conductive layers may be disposed between a thermoelectric leg and its associated first electrical contact.

Second contacts 109 extending between the second ends of the n-type and p-type thermoelectric legs are formed to electrically connect the thermoelectric legs. In some embodiments, the second end of each thermoelectric leg is in direct contact with a second electrical contact 109. In some embodiments (not shown), one or more intermediate conductive layers may be disposed between a thermoelectric leg and its associated second electrical contact.

Fig. 3 illustrates a process of forming a thermoelectric device according to some further embodiments wherein a first part of the electrically insulating bank structure 105’ is formed over first contacts 103 on a first substrate loiA and a second part of the electrically insulating bank structure 105” is formed over second contacts 109 on a second substrate 101B.

First parts of the n-type and p-type thermoelectric legs, 107’p and 107’n respectively, and second parts of the n-type and p-type thermoelectric legs, i07”p and i07”n respectively, are deposited into wells. Wells of the first part are defined by the first part of the electrically insulating bank structure 105’ and the first contacts 103. Wells of the second part are defined by the second part of the electrically insulating bank structure 105” and the second contacts 109. The first and second parts of the electrically insulating bank structures and the thermoelectric legs are then brought into contact. The first and second parts of the thermoelectric legs maybe heated when brought into contact, for example by applying heat to an outer surface of one or both of the first and second substrates. Optionally, the first and second thermoelectric legs comprise a polymer binder. Heat treatment may be at a temperature above the glass transition temperature of the polymer. If the first and second thermoelectric legs comprise a thermosetting polymer, for example an epoxy resin, then curing may result in binding between polymer in the first and second parts of the thermoelectric legs. In some embodiments, heating is at a temperature of about 150° to 250°. In some embodiments, heating is for a time period of about 1 to 3 hours.

The first and second substrates may be pressed together using any suitable apparatus, for example by use of a jig.

An adhesive may be applied between the first and second substrates and extending around a perimeter of the device.

A partial thermoelectric leg selected from one or more of 107’p, 107’n, I07”p and I07”n may have an end which protrudes beyond the second surface 105B of the electrically insulating bank structure, optionally by up to 50 microns, e.g. 10-50 microns. A protruding end may be formed by overfilling of a well-defined by the electrically insulating bank structure, i.e. by depositing a volume of ink into the well which is greater than a volume of the well. A high contact angle of the ink at the second surface 105B may prevent spreading of the ink across surface 105B.

In some embodiments, first substrate 101A and second substrate 101B are both flexible. Electrically insulating bank structure In some embodiments, the electrically insulating bank structure comprises or consists of a layer containing hollow particles. The hollow particles are electrically insulating, e.g. glass or plastic. The hollow particles may be hollow spheres.

Optionally, 50 % of the hollow particles have a diameter of 1-1000 pm, optionally 1-100 pm. The size of the particles may be selected according to the thickness of the electrically insulating layer containing the particles. Optionally, the hollow particles have an effective top size of less than 500 microns. By “effective top size of less than 500 microns” is meant that no more than 3 weight % of the hollow particles pass through a sieve having a mesh size of 500 microns.

Hollow particles for use in the electrically insulation layer having different size

distributions, e.g. different mean average diameters, may be mixed together.

In the case of a non-spherical particle,“diameter” used herein is meant the longest straight line distance through the particle between two points on the particle’s surface.

In some embodiments, the electrically insulating bank structure contains 10-90 % by volume, optionally 30 -80 % by volume, hollow particles. The remaining volume of the electrically insulating bank structure may comprise or consist of a binder material. The binder material may be, without limitation, a polymer. The polymer may be a crosslinked polymer. The polymer may be, e.g. a positive or negative photoresist. The polymer may be a silicone.

The electrically insulating bank structure may be formed by any known method. In some embodiments, an electrically insulating layer containing the hollow particles is formed by depositing a mixture containing the glass particles dispersed in a liquid comprising or consisting of a precursor of the binder. In some embodiments, the mixture contains a binder precursor dissolved in one or more solvent materials. In some embodiments, the binder precursor is a liquid at 20°C and the mixture is free from any solvent materials. The absence of any solvent materials to be evaporated from the mixture following deposition may allow for formation of a layer containing the hollow particles which undergoes little or no change in its dimensions and / or size or shape of any apertures formed by printing of the mixture.

Optionally, the mixture contains 0-20 mass % of solvent material. In some embodiments, a uniform electrically insulating layer is formed by spin-coating a mixture containing the hollow particles and a binder precursor, followed by curing of the binder precursor and formation of apertures through the electrically insulating layer and any other layers of the electrically insulating bank structure. Apertures may be formed by photopatterning and etching. In some embodiments, the electrically insulating layer is printed in a pattern including the apertures, e.g. by screen printing a mixture containing a precursor of the binder and the hollow particles. The binder precursor may be a material which is crosslinked following deposition to form the binder. A curing agent may be mixed with the binder precursor prior to printing. The curing agent may be activated by heating and / or irradiation of the deposited mixture, e.g. UV irradiation. The desired thickness of the layer containing the binder and the hollow particles may be achieved by selecting the viscosity of the mixture to be deposited and / or by additive deposition of multiple layers of the mixture.

Optionally, an electrically insulating layer comprising hollow particles as described herein has a thermal conductivity measured by a steady state heat flux method of no more than 0.2 WmA¹, optionally no more than o.i Wnr’K ¹.

Thermoelectric leg materials

The p-type semiconductor and the n-type semiconductor of, respectively, the p-type thermoelectric legs and the n-type thermoelectric legs may be selected from known thermoelectric materials as disclosed in, for example, J. Mater. Chem. C, 2015, 3, 10362 and Chem. Soc. Rev., 2016, 45, 6147-6164, the contents of which are incorporated herein by reference.

In some embodiments, each n-type thermoelectric leg may comprise or consist of an alloy of bismuth, and tellurium or selenium. In some embodiments, the semiconducting material of each n-type thermoelectric leg may comprise or consist of an alloy of bismuth (Bi), and tellurium (Te) or selenium (Se), for example, Bi₂Te₃, and Bi₂Se₃, and/ or optionally an n-type dopant. Examples of n-type dopants include selenium (Se), bismuth (Bi), sulfur (S), iodine (I) and/or the like. In some embodiments, the concentration of the n-type dopant may be between 1 and 10 weight %.

In some embodiments, the p-type thermoelectric element may comprise or consist of an alloy of bismuth, tellurium and antimony. In some embodiments, the semiconducting material of each p-type thermoelectric leg may comprise or consist of an alloy of bismuth (Bi), tellurium (Te), and antimony (Sb), for example Bh ₅Sbo_.5Te₃, and optionally a p-type dopant. In some embodiments, the second semiconducting particles may comprise or consist of an alloy of lead (Pb) and tellurium (Te), an alloy of tin (Sn) and selenium (Se), or an alloy of silicon (Si) and germanium (Ge), and optionally a p-type dopant. Examples of p-type dopants include tellurium (Te), selenium (Se), sulfur (S), arsenic (As), antimony (Sb), phosphorus (P), bismuth (Bi) and the halogens. The concentration of the p-type dopant may be between 1 and 10 weight %. In some embodiments, at least one of the n-type semiconductor and the p-type semiconductor is in particulate form. The semiconductor particles may be dispersed in an organic or inorganic binder, optionally a polymeric binder. In some embodiments, the binder may comprise or consist of a thermoplastic polymer. In some embodiments, the binder may comprise or consist of a thermosetting polymer. In some embodiments, the binder may comprise or consist of a curable epoxy resin.

Thermoelectric leg formation

A p-type or n-type thermoelectric leg as described herein may be formed using an ink containing the material or materials of the thermoelectric leg, or precursors thereof, dissolved or dispersed in a solvent. The ink may be deposited into a well followed by evaporation of the solvent.

Suitable techniques for depositing an ink are coating or printing methods including, without limitation, roll-coating, spray coating, doctor blade coating, slit coating, ink jet printing, screen printing, dispense printing, gravure printing, stencil printing and flexographic printing.

In dispense printing, each thermoelectric leg is formed by depositing a continuous flow of ink from a nozzle positioned above the first electrode. It will be understood that no ink is dispensed in regions between each thermoelectric leg.

The solvent of an ink as described herein may include one or more solvents. In some embodiments, the ink comprises two or more solvents. Exemplary solvents include, without limitation, benzene substituted with one or more C - ₀ alkyl or alkoxy groups, e.g. anisole; ketones, e.g. methyl isobutyl ketone; (MIBK) and carboxylic acid esters e.g. propylene glycol methyl ether acetate (PGMEA).

If present, a binder or a precursor thereof may be dissolved in the ink. Electrical contacts

Each electrical contact described herein comprises one or more conductive layers. Examples of conductive materials for the conductive layer or layers of the electrical contacts include metals, such as copper or gold; metal alloys; conductive metal oxides; and conductive polymers. Optionally, each contact described herein has a thickness between 1 and 5 pm. Each electrical contact described herein may be formed by any suitable technique known to the skilled person, for example sputtering, thermal evaporation or printing, e.g. printing of a metallic paste. Substrate

In some embodiments the or each substrate comprises a flexible ceramic, plastic film, metal foil or the like. The or each substrate may independently consist of a single layer or may comprise a plurality of layers, for example a metal foil coated on one surface with an electrically-insulating substrate layer, e.g. a polymer layer. The electrically-insulating substrate layer may be thinner than the metal foil. In some embodiments, the electrical contacts may be disposed directly on the electrically insulating substrate layer. In some embodiments, the or each substrate is independently a flexible substrate having a thickness between 30 and 60 pm. Applications

Referring to Fig. 4, sensor apparatus 19 comprising a thermoelectric device as described herein is shown. The apparatus 19 may be used to measure a surface temperature of a remote component (not shown) and transmit temperature data to a remote receiver (not shown). When a given temperature difference dT is applied across the layers of the thermoelectric generator, a voltage is induced across the thermoelectric legs. This voltage drives a current through the contacts and thermoelectric legs.

The system 19 comprises the thermoelectric device 13, a temperature sensor 20, a signal processor unit 21, a controller unit 22, and a transmitter unit 23. The thermoelectric device 13 is provided on a surface of the remote component. The component may be a pipe or a moving mechanical element, for example an actuator. The flexible thermoelectric device 13 may be flexible to maintain surface contact with the remote component to reduce thermal resistance at the contact surface.

The thermoelectric device 13 converts the waste heat produced by the component into useful energy to power the temperature sensor. The signal processor 21 receives at least one signal from the temperature sensor 20. The signal indicates the temperature of the component.

In response to receiving control signals from the controller 22, the signal processor 21 processes the signal. The transmitter 23 receives the processed signal and transmits the processed signal to the external receiver (not shown).

EXAMPLES Example l - Electrically Insulating Laver l

Hollow glass microbubbles grade S22 obtained from 3M (60 vol %), were mixed with negative epoxy resist SU-8 3025 obtained from Microchem 3025. Cyclopentanone was used to dilute the mixture to modify the viscosity to enable better mixing. The mixture was spin coated on to PEN substrates and cured at 95°C for 15 minutes, exposed to broadband UV from a handheld UV wand for 2 minutes and then baked for a further 5 minutes at 95°C, followed by a hard bake at 150°C for 1 hour, resulting in a film about too microns thick. Samples were cut (48x37mm) from the substrates for thermal conductivity measurements.

The thermal conductivity was measured using a steady state heat flux method. Layers of PEN coated with the SU-8/glass bubbles were assembled between two temperature controlled clamps. PEN substrates of known thermal resistance with resistive temperature sensors were incorporated between the temperature controlled clamps. A series of temperature gradients were applied between the clamps to provide a steady state heat flux. The heat flux was determined by measuring the temperature drop across the known thermal resistance. The unknown thermal resistance of the PEN+SU-8/glass bubbles layers was then determined from the temperature drop across the layers and the already determined heat flux. The thermal resistance of the PEN substrate was subtracted to give the thermal resistance of the SU-8/glass bubble layers only. Table 1 shows the measured thermal conductivities. Table 1

^Literature value

Example 2 - Electrically Insulating Laver 2 S22 glass bubbles (40 vol %) were mixed with the base resin of flowable silicone elastomer Sylgard 184 obtained from Dow Corning (60 vol%). This mixture may be stored for long periods at room temperature provided it is re-mixed before use. Before use, the curing agent supplied with the base resin was added to the mixture at a concentration of about 10 wt% and the mixture was immediately screen-printed onto a substrate. Curing was then accelerated by heating the substrate to ~120°C for 10 minutes. In this way, an insulating layer having apertures formed therein was formed from a 100%“solids content” mixture, i.e. a mixture free from any solvents.

Example 3 - Thermoelectric Device 1 First and second substrates of a metal foil with a low thermal resistance insulating layer were provided. Electrical contacts were formed on the insulating layer of the first and second substrates in a complementary fashion so as to provide interconnected thermoelectric couples upon combination of the first and second substrates.

The mixture as described in Example 2 was screen printed onto each substrate to form an electrically insulating layer with apertures extending through each layer to expose the underlying contacts. Screen printing was performed using a DEK Horizon 031X machine in a cleanroom with settings as shown in Table 2.

The layer thickness was 150 microns. The apertures had an area of 2 mm x 2 mm with a imm spacing between apertures. Table 2

A mechanically alloyed powder of bismuth telluride (n-type) or bismuth antinomy telluride (p-type), was formulated with a heat curing epoxy resin to form n-type and p-type inks in a semiconductor particle : epoxy resin weight ratio of 82 : 18. A blend of solvents of MIBK : Anisole (7:3 w/w) made up 18 weight % of the inks. The inks were deposited by dispense printing into the wells of the first and second substrates, with the n-type and p-type inks being deposited in alternate wells. Solvent was removed by heating to yield a substantially dry film.

The first and second substrates were aligned and brought into intimate contact, by applying an even sustained pressure across the device with a clamping jig. The assembled device sealed within the jig was heated in an oven between i8o°C and 250°C (i.e. in a curing temperature range of the epoxy).

For the purpose of comparison, devices were made as described above except that an insulating layer not containing hollow particles were made using either photopatterned SU- 8 photoresist or a laminated laser-cut polyimide film. The thermovoltage generated by each device was measured under a passive cooling regime.

The thermovoltage is directly proportional to the temperature gradient across the thermoelectric materials and thus can be used as a measurement of performance.

Initially the thermovoltage response was calibrated against known temperature gradients. The device was clamped between temperature controlled aluminium blocks and various setpoints were applied, generating temperature gradients up to 20K and the resulting voltage measured. Plotting the voltage against temperature gradient shows a linear relationship, where the slope is the calibration factor.

In the second part of the measurement a finned aluminium heatsink was placed on the top clamp and an elevated temperature was applied to the bottom clamp, thus generating a temperature gradient to the ambient room temperature, which was dropped across the thermoelectric module and heatsink. The thermovoltage was measured and using the calibration factor measured earlier, the temperature gradient across the module was derived. It should be noted that by deriving the temperature gradient in this fashion the insulative properties of the bank can be assessed independently of the performance of the thermoelectric materials.

Table 3 shows the measured temperature gradient dT under the passive cooling regime with a hot block temperature of 70°C for devices with different electrically insulating layers. A significant improvement in temperature gradient across the device is achieved when using glass bubbles containing bank.

Table 3

The parameter dT is dependent upon device thickness. As such the thermal conductivity of the material was estimated according to the following procedure:

The electrically insulating layers were prepared on thermally conductive flexible substrates described above. Two substrates were assembled facing each other, clamped between temperature controlled aluminium blocks, replicating the TEG device configuration. An additional PEN substrate of known thermal resistance was added in series between the bottom clamp and the bottom substrate.

Temperature gradients were applied between the top and bottom clamps and the resulting temperature gradients across the PEN (dTPEN) and the bank materials (dTBank) measured by reference to the clamp temperatures and the temperature of the substrates (measured by change in electrical resistance of electrodes). Heat flux (Q) was determined from the temperature gradient across the known thermal resistance of the PEN (RPEN), and the thermal resistance of the bank material (RBank) determined from the heat flux.

Thermal conductivity was estimated by assuming all thermal conduction is through the bank material (the air in the open apertures does not conduct any heat) and using the nominal thickness of the bank material. It is assumed that the thermal resistance of the thermally conductive substrates and aluminium clamps is negligible clT_PEN 72 71 d7 P_:EN

R dr B.ank.

Q R Bank

d7 Biank T3 - 72 PEN Q

The description above provides preferred exemplary embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the invention. Rather, the ensuing 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 maybe made in the function and arrangement of elements, including combinations of features from 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 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 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 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 general, 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 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 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 embodiments of the disclosed technology may be practiced without some of these specific details.

Claims

CLAIMS:

1. A method of forming a thermoelectric device comprising a plurality of electrically connected thermoelectric couples, each thermoelectric couple comprising an n-type thermoelectric leg electrically connected to a p-type thermoelectric leg, wherein each n-type thermoelectric leg and each p-type thermoelectric leg is disposed in an aperture of an electrically insulating bank structure comprising hollow, electrically insulating particles and a binder and wherein formation of the electrically insulating bank structure comprises deposition of a composition comprising the hollow, electrically insulating particles and the binder or a precursor of the binder, wherein the composition does not comprise a solvent and wherein formation of the p-type and n-type

thermoelectric legs comprises printing the components of the thermoelectric legs into the apertures of the electrically insulating bank structure.

2. The method according to claim l wherein the hollow, electrically insulating particles are glass or plastic microspheres. 3. The method according to claim 1 or 2 wherein the composition is printed in a pattern defining the apertures.

4. The method according to any one of the preceding claims wherein the composition comprises a crosslinkable precursor of the binder and wherein the composition is subjected to a crosslinking treatment following the deposition of the composition.

5. The method according to any one of the preceding claims wherein the composition is screen-printed.

6. The method according to any one of the preceding claims wherein the thermoelectric device is flexible. 7. The method according to any one of the preceding claims wherein the thermoelectric device is disposed on a substrate.

8. The method according to any one of claims 1-7 wherein the thermoelectric device is disposed between a first and second substrate.

9. The method according to claim 8 comprising: formation of a first part of the electrically insulating bank structure comprising deposition of the composition over the first substrate; formation of a second part of the electrically insulating bank structure comprising deposition of the composition over the second substrate; printing the components of the thermoelectric legs into the apertures of each of the first part and second part of the electrically insulating bank structure; and bringing the first and second parts of the electrically insulating bank structure into direct contact. to. The method according to any one of claims 7 to 9 wherein the, or each, substrate comprises a metal foil layer and an insulating layer.

11. A thermoelectric device obtainable by a method according to any one of the preceding claims.

12. A composition comprising a mixture of a crosslinkable material and hollow, electrically insulating particles wherein the composition does not comprise a solvent material.

13. A composition according to claim 15 wherein the hollow, electrically insulating particles are glass or plastic microspheres.

14. Sensor apparatus comprising a temperature sensor and a thermoelectric device according claim 11 wherein the thermoelectric device is a thermoelectric generator configured to supply electrical power the temperature sensor.