IMPROVEMENTS RELATING TO THERMOELECTRIC MATERIALS

Abstract

A thermoelectric material comprising carbon nanotubes and lignin. The carbon nanotubes are present as fibres and the lignin is present in pores and/or voids in the carbon nanotube fibres. The lignin may act as a dopant to increase the thermoelectric efficiency of the carbon nanotubes, multi-walled carbon nanotubes in particular. A method of forming a thermoelectric material involving impregnating fibres of carbon nanotubes with lignin, is also provided. A thermoelectric element, a fabric and a thermoelectric device comprising the thermoelectric material are also provided. The thermoelectric material may be particularly useful for the production of wearable thermoelectric devices.

Claims

1. A thermoelectric material comprising carbon nanotubes and lignin.

2. The thermoelectric material according to claim 1, wherein the carbon nanotubes are multi-walled carbon nanotubes.

3. The thermoelectric material according to claim 1, wherein the carbon nanotubes are in the form of fibres having a specific surface area between 50 and 2000 cm.sup.3/g.

4. The thermoelectric material according to claim 1, wherein the lignin is an organosolve lignin.

5. The thermoelectric material according to claim 1, comprising at least 2 wt % lignin.

6. The thermoelectric material according to claim 1, comprising at least 10 wt % lignin.

7. The thermoelectric material according to claim 1, wherein the thermoelectric material is a yarn comprising fibres of the carbon nanotubes.

8. A thermoelectric element comprising a thermoelectric material according to claim 1.

9. A fabric comprising a thermoelectric material according to claim 1.

10. A thermoelectric device comprising a thermoelectric material according to claim 1.

11. The thermoelectric device according to claim 10, wherein the thermoelectric device is adapted to be arranged in use on a user's body and to generate electricity from heat from said body of said user.

12. A method of forming a thermoelectric material comprising carbon nanotubes and lignin, the method comprising the steps of: a) providing fibres of carbon nanotubes; b) impregnating the fibres of carbon nanotubes with lignin.

13. The method according to claim 12, wherein step a) involves producing the fibres of carbon nanotubes by chemical vapour deposition.

14. The method according to claim 12, wherein step a) involves producing the fibres of carbon nanotubes by direct spinning from a gas phase.

15. The method according to claim 12, wherein step b) involves treating the fibres of carbon nanotubes with a solution comprising the lignin and a solvent.

16. A thermoelectric device comprising a thermoelectric element according to claim 8.

17. A thermoelectric device comprising a fabric according to claim 9.

Description

EXAMPLES

Sample Preparation—Fabric

[0064] In a non-limiting example, a non-woven unidirectional fabric was produced by winding fibres of carbon nanotubes onto a bobbin and densifying the material with a solvent. In an example particularly relevant for industrialisation, such a fabric was produced from carbon nanotube fibres that were directly collected from the gas-phase during their synthesis by chemical vapour deposition, as described in EP2615193B1, EP2631330B1 and EP2631331A1. Briefly, this method of preparing the carbon nanotube fibres involves the production of a carbon nanotube agglomerate, comprising the steps of: passing a flow of one or more gaseous reactants into a reactor; reacting the one or more gaseous reactants within a reaction zone of the reactor to form an aerogel; agglomerating the aerogel into an agglomerate; and applying a force to the agglomerate to displace it continuously away from the reaction zone.

[0065] The porous nature of fibres of carbon nanotubes formed from said methods (and therefore fabrics formed from said fibers) allows the infiltration of lignin into the fabric. Infiltration with lignin can be carried out in-line as the fibre is produced and wound as a fabric, or in a subsequent step.

[0066] To impregnate the carbon nanotube fibre fabric with lignin, the carbon nanotube fibre fabrics were immersed in a 5 wt % solution of lignin (organosolv hardwood provided by Thecnaro, Germany) in THF for 5 minutes. The carbon nanotube fibre fabric was then dried at 70° C. for 1 hour.

Sample Preparation—Yarns

[0067] In order to form the yarns described herein, a plurality of carbon nanotube fibres prepared as described above were collected on a spool and rolled off the spool sideways to provide a “yarn”.

[0068] Carbon nanotube fibre yarns were impregnated with lignin using the same methods as described above. Table 1 below shows the different concentrations of lignin/THF solutions which were used and the wt % of lignin in the CNT/lignin yarns produced, measured by calculating the difference between the weight of the samples before and after impregnation with lignin and drying.

TABLE-US-00001 TABLE 1 Yarns Lignin concentration Amount of Yarn in THF Solution lignin in yarns sample no. wt. % wt. % 1 1.00% 13% 2 2.50% 23% 3 5.00% 34% 4 10.00% 53% 5 20.00% 56%

SEM Images

[0069] FIG. 1 shows scanning electron microscope (SEM) images of the fabric (a) before and (b) after lignin infusion. These SEM images show the morphology of the fabric before and after lignin infusion. A comparison of the SEM images indicates how lignin has filled the gaps/voids between the carbon nanotube fibres in the fabric.

Electrical Conductivity Measurements on Fabrics and Yarns

[0070] The electrical conductivity of the fabric prepared as described above was obtained using the van der Pauw method. This method can be utilized to determine the conductivity of thin films wherein the distance between contacts is much larger than the sample thickness. Four contacts to the fabric were used in this method to eliminate the effect of the contact resistance. The van der Pauw equation used in this method is given below:

e.sup.−π.Math.d.Math.R.sup.1.sup.σ+.sup.−π.Math.d.Math.R.sup.2.sup.σ=1

wherein σ is the conductivity, d is the sample thickness, and R.sub.1 and R.sub.2 are the resistance values. To calculate R.sub.1 and R.sub.2, the following equations were used:

R.sub.1=VBD/IAC and R.sub.2=VAB/ICD

wherein V and I are the voltage and intensity across the sample respectively, and A, B, C and D are the four contacts with the fabric.

[0071] The electrical conductivity of the yarns prepared as described above (prior to formation into fabric) was determined by using a four point probe method as is shown in FIG. 5. Using this method, the resistance was obtained and the conductivity was calculated using the following equation:

[00002]$\sigma =\frac{\ln 2}{\pi \mathrm{wR}}$

where σ is the conductivity of the yarn, w is the yarn diameter, and R is the resistance measured in the yarn.

[0072] The Seebeck coefficient (S) is determined as the ratio between the electrical potential, ΔV, and the temperature difference, ΔT:

[00003]$S=\frac{\Delta V}{\Delta T}$

[0073] In order to create a temperature difference, the sample was placed between two Peltier modules powered by a power supply. The temperature was measured by k-type thermocouples while the voltage was recorded using a Keithley 2000 multimeter.

[0074] Table 2 shows the thermoelectric properties of the fabric before (“Comp.” denotes comparative sample) and after lignin infusion (samples 2 to 5). Table 3 shows the same data for yarn samples 1-5 and the lignin-free “comparative” sample. The thermoelectric efficacy can be quantified by the Power factor (PF=σ.Math.S.sup.2). These values are useful for comparing samples with similar thermal conductivity. The results in Table 1 indicate that after lignin infusion the power factor increases significantly, mainly due to the increase in the Seebeck coefficient.

TABLE-US-00002 TABLE 2 Thermoelectric parameters of the fabric samples. Conductivity Power Factor Fabric Lignin Conductivity using R1 and Power Factor (μV/mK.sup.2) sample conc. in R1 R2 using R1 R.sub.2 Seebeck (μV/mK.sup.2) using no. THF (Ω) (Ω) (S/cm) (S/cm) (μV/K) using R.sub.1 R.sub.1 and R.sub.2 Comp. 0.0% 0.60 7.00 241.00 55.745 48.343 56.3228 13.03 2 2.5% 0.50 1.80 350.00 173.967 96.339 324.8421 161.46 3 5.0% 1.10 2.10 286.00 199.505 94.396 254.8433 177.77 4 10.0% 0.50 3.45 490.00 155.844 69.964 239.8531 76.29 5 20.0% 0.14 6.50 358.00 30.812 76.49 209.4558 18.03

TABLE-US-00003 TABLE 3 Thermoelectric properties of yarn samples. Lignin concentration in Power Yam sample THF Solution Resistance Conductivity Seebeck Factor no. wt. % Ω S/cm μV/K μW/K.sup.2m Comparative    0% 1.10 60.173 56.901  19.48 1  1.0% 0.84 157.595 83.64  110.25 2  2.5% 0.70 135.082 98.933 132.21 3  5.0% 0.50 120.346 94.621 107.75 4 10.0% 0.59 59.046 77.742  35.69 5 20.0% 0.61 31.002 72.428  16.26

[0075] FIGS. 2 and 3 show voltage generated as a function of the temperature difference of the “pristine” and “impregnated” fabrics and yarns respectively. With the addition of lignin, both of the parameters.Math.electrical conductivity and Seebeck coefficient.Math.increase. Therefore the thermoelectric efficiency given by the PF is higher compared to the comparative sample which comprises carbon nanotubes with no lignin impregnation. These results show a significant increase of approximately an order of magnitude in the PF with the lignin impregnated samples 2-5. This means that thermoelectric materials and thermoelectric devices comprising such lignin impregnated carbon nanotube fibres may generate electricity more efficiently that known thermoelectric materials and devices, particularly compared to known multi-walled carbon nanotube thermoelectric materials and devices.

[0076] FIG. 4 shows a schematic representation of a thermoelectric device (100) according to the fourth aspect of the present invention. The thermoelectric device (100) comprises a fabric (110) according to the third aspect of the present invention and a nickel wire (120) wound through the fabric (110). The fabric (110) comprises carbon nanotube fibres and lignin, and was prepared as described above. The thermoelectric device (100) is a wearable thermoelectric device for body heat recovery. The temperature difference between the user's skin (20) and ambient air (30) (outside of the user's skin (20) and the thermoelectric device (100)) produces a temperature difference between the skin side (130) and the air side (140) of the thermoelectric device (100). This temperature difference causes a voltage to be established across the fabric (110) which drives an electrical current through the nickel wire (120). The current can then be used to power a functionality of the device, for example a health sensor or monitor, or a communications device. Therefore the thermoelectric device (100) can generate useful electricity from the user's body heat which would otherwise be lost to the atmosphere. The thermoelectric device (100) may be able to do this more effectively than if the lignin was not present in the fabric.

[0077] FIG. 5 shows the test set-up (40) used to determine the electrical conductivity of the yarn samples (150). Four electrical contacts (11) are made in the sample yarn (150) with silver paint. The electric current (A) is applied between end contacts (12) and the voltage (V) is measured between the internal contacts (13).

[0078] FIG. 6 shows a schematic of a thermoelectric device (200) comprising twenty pieces of carbon nanotube fibre yarn impregnated with lignin (150). The device was prepared by first preparing twenty 5 mm long yarns impregnated with lignin (from a 2.5 wt % lignin solution in THF), which may be considered “thermoelectric elements”, and grouping these into four sets of five carbon nanotube fibre yarns (250). Each of these sets were bound together with cooper wire (220), with the top of one yarn being tied to the bottom of the next. A schematic of such a group of carbon nanotube fibre yarns is shown in FIG. 6(a). The copper wire was secured to the yarns with silver paint to ensure good contact. Once these four sets were made, they were arranged so that there would be two groups of ten yarns in series with each other, and these two groups would be parallel to each other, as shown in FIG. 6(b). These were then secured to a copper plate (260) coated in a thermally non-conductive coating using a non-conducting silicon glue. When worn by a user, a temperature difference is created between the top and the bottom part of the device (you can create this temperature gradient heating the bottom part using a heat source for example the human body). FIG. 7 demonstrates the power output of the thermoelectric device (200) in use, showing the evolution of power as a function of the electrical current generated at temperature gradients of 30° K and 20° K. The device (200) showed a maximum power value of around 3.5 μW. This is a surprisingly high power output for such a thermoelectric device having twenty thermoelectric elements. Known devices having a similar number of thermoelectric elements have been reported to produce power outputs in the nW range, therefore orders of magnitude lower than the power output which may be provided by the thermoelectric device of the present invention.

[0079] In summary, the present invention provides a thermoelectric material comprising carbon nanotubes and lignin. The carbon nanotubes are present as fibres and the lignin is present in pores and/or voids in the carbon nanotube fibres. The lignin may act as a dopant to increase the thermoelectric efficiency of the carbon nanotubes, multi-walled carbon nanotubes in particular. A method of forming a thermoelectric material involving impregnating fibres of carbon nanotubes with lignin, is also provided. A thermoelectric element, a fabric and a thermoelectric device comprising the thermoelectric material are also provided. The thermoelectric material may be particularly useful for the production of wearable thermoelectric devices.

[0080] Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

[0081] Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of other components. The term “consisting essentially of” or “consists essentially of” means including the components specified but excluding other components except for materials present as impurities, unavoidable materials present as a result of processes used to provide the components, and components added for a purpose other than achieving the technical effect of the invention. Typically, when referring to compositions, a composition consisting essentially of a set of components will comprise less than 5% by weight, typically less than 3% by weight, more typically less than 1% by weight of non-specified components.

[0082] The term “consisting of” or “consists of” means including the components specified but excluding addition of other components.

[0083] Whenever appropriate, depending upon the context, the use of the term “comprises” or “comprising” may also be taken to encompass or include the meaning “consists essentially of” or “consisting essentially of”, and may also be taken to include the meaning “consists of” or “consisting of”.

[0084] For the avoidance of doubt, wherein amounts of components in a composition are described in wt %, this means the weight percentage of the specified component in relation to the whole composition referred to. For example, “suitably the thermoelectric material comprises at least 2 wt % lignin” means that 2 wt % of the thermoelectric material is provided by lignin.

[0085] The optional features set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional features for each aspect or exemplary embodiment of the invention as set out herein are also to be read as applicable to any other aspect or exemplary embodiments of the invention, where appropriate. In other words, the skilled person reading this specification should consider the optional features for each exemplary embodiment of the invention as interchangeable and combinable between different exemplary embodiments.

[0086] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0087] All of the features disclosed in this specification (including any accompanying claims, and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

[0088] Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0089] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.