ELECTRICALLY CONDUCTIVE MATERIALS COMPRISING GRAPHENE

Abstract

The present invention relates to electrically conductive materials. The present invention also relates to processes for the preparation of these materials and to electronic circuits, electronic devices and textile garments that comprise them.

Claims

1. An electrically conductive material comprising: a porous substrate material; a hydrophobic surface coating covering at least a portion of a surface of the porous substrate material; and an electrically conductive track or film disposed on the hydrophobic surface coating; wherein: (i) the hydrophobic coating forms a hydrophobic surface on the porous substrate material having an equilibrium contact angle of water against air, at 25 C., of greater than or equal to 60 and less than or equal to 175; and (ii) the electrically conductive track or film comprises graphene and/or reduced graphene oxide.

2. An electrically conductive material according to claim 1, wherein the porous substrate material is selected from a textile or cellulosic material (e.g. paper).

3. An electrically conductive material according to claim 1, wherein the porous substrate material is a textile (e.g. cotton).

4. An electrically conductive material according to claim 1, wherein the hydrophobic coating covering at least a portion of a surface of the porous substrate material is a hydrophobic material selected from the group consisting of styrene, (meth)acrylate, acrylate, ester, olefin, vinyl ester, vinyl pyrrolidone and vinylpyridine based polymers.

5. An electrically conductive material according to claim 1, wherein the hydrophobic coating comprises particles formed from a hydrophobic polymeric material.

6. An electrically conductive material according to claim 5, wherein the hydrophobic coating comprises particles formed from co-polymers comprising styrene, divinylbenzene and hydroxyl methacrylate.

7. An electrically conductive material according to claim 1, wherein the hydrophobic coating forms a hydrophobic surface on the porous substrate material having an equilibrium contact angle of water against air, at 25 C., of greater than or equal to 90 and less than or equal to 165.

8. An electrically conductive material according to claim 1, wherein the hydrophobic coating forms a hydrophobic surface on the porous substrate material having an equilibrium contact angle of water against air, at 25 C., of greater than or equal to 90 and less than or equal to 145.

9. An electrically conductive material according to claim 1, wherein the hydrophobic coating forms a hydrophobic surface on the porous substrate material having an equilibrium contact angle of water against air, at 25 C., of greater than or equal to 90 and less than or equal to 135.

10. An electrically conductive material according to claim 1, wherein the hydrophobic coating forms a hydrophobic surface on the porous substrate material having an equilibrium contact angle of water against air, at 25 C., of greater than or equal to 100 and less than or equal to 125.

11. An electrically conductive material according to claim 1, wherein the electrically conductive track or film comprises: (i) graphene; (ii) reduced graphene oxide; or (iii) graphene or reduced graphene oxide in combination with one or more additional conductive agents.

12. An electrically conductive material according to claim 11, wherein the additional conductive agent is selected from a silver precursor, silver nanoparticles, carbon nanotubes, or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT/PSS).

13. A process for forming an electrically conductive material according to claim 1, wherein the process comprises: (i) providing a porous substrate material; (ii) digitally printing a hydrophobic surface coating ink formulation onto at least a portion of a surface of the porous substrate material to form a hydrophobic surface on the substrate having an equilibrium contact angle of water against air, at 25 C., of greater than or equal to 60 and less than or equal to 175; (iii) digitally printing or digitally applying an electrically conductive ink formulation comprising graphene and/or graphene oxide onto the hydrophobic surface of the substrate to form a film or track and thereafter reducing any graphene oxide present to form a track or film comprising reduced graphene oxide; and (iv) optionally, and if necessary, heating the printed electronically conductive formulation to between 50 C. and 300 C. so as to cure the electronically conductive formulation.

14. A process according to claim 13, wherein the digital printing in steps (ii) and (iii) is inkjet printing.

15. A process according to claim 13, wherein the porous substrate material is selected from a textile or cellulosic material (e.g. paper).

16. A process according to claim 13, wherein the porous substrate material is a textile (e.g. cotton).

17. A process according to claim 13, wherein the hydrophobic coating forms a hydrophobic surface on the substrate material having an equilibrium contact angle of water against air, at 25 C., of greater than or equal to 80 and less than or equal to 120.

18. A process according to claim 13, wherein the hydrophobic coating ink formulation comprises particles of a hydrophobic polymeric material in an aqueous vehicle.

19. A process according to claim 18, wherein the concentration of particles of hydrophobic polymeric material in the aqueous vehicle is within the range of 0.5-10 wt-%.

20. A process according to claim 13, wherein the hydrophobic coating ink formulation has a viscosity within the range of 2 to 300 cPs at 25 C.

21. A process according to claim 13, wherein the hydrophobic coating ink formulation has a viscosity within the range of 2 to 30 cPs at 25 C.

22. A process according to claim 13, wherein the hydrophobic coating ink formulation has surface tension within the range of 10 to 72 mN/m.

23. A process according to claim 13, wherein the electrically conductive ink formulation comprises a plurality of flakes of pristine graphene and/or graphene oxide and, optionally, particles of additional conductive agents in an aqueous vehicle.

24. A process according to claim 23, wherein the concentration of flakes of pristine graphene and/or graphene oxide in the aqueous vehicle is within the range of 0.01 to 10 mg/ml.

25. A process according to claim 13, wherein the electrically conductive ink formulation has a viscosity within the range of 2 to 30 cPs at 25 C.

26. A process according to claim 13, wherein the electrically conductive ink formulation has a surface tension within the range of 10 to 72 mN/m.

27. An electronic circuit comprising an electronically conductive material according to claim 1.

28. An electronic device comprising an electronic circuit according to claim 27.

29. A textile garment comprising an electronically conductive material according to claim 1.

Description

EXAMPLES

[0110] Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings, in which:

[0111] FIG. 1 shows the contact angle of distilled water versus time at 25 C. on inkjet printed cotton fabrics (BD022) with polystyrene nanoparticles: (a) before wash and (b) after wash, wherein: (.square-solid.) control fabric; (.circle-solid.) NP1 without MF pre-treatment; (.box-tangle-solidup.) NP1 with MF pre-treatment; and (.Math.) NP5 with MF pre-treatment.

[0112] FIG. 2 shows the contact angle of distilled water versus time at 25 C. on inkjet printed polyester fabrics (MK14) with nanoparticles before wash.

[0113] FIG. 3 shows the particle size distribution of cross-linked polystyrene nanoparticles (NP1). The Z-Average particle size of the polystyrene nanoparticle was found to be 63.12 nm (PDI=0.055)

[0114] FIG. 4 shows the Raman spectra of a BS8 pristine graphene dispersion drop casted onto a Si+SiO.sub.2wafer.

[0115] FIG. 5 shows the SEM images for 6 layers of inkjet printed composite ink (ink C) onto 12 layers of the polystyrene hydrophobic coating (NP1) printed on MF pre-treated 100% cotton fabric, at both a) 500 optical zoom and b) 10000 optical zoom.

[0116] FIG. 6 shows SEM images for 6 layers of inkjet printed composite ink (ink C) onto 12 layers of the polystyrene hydrophobic coating (NP1) printed on untreated 100% cotton fabric, at both a) 500 optical zoom and b) 10000 optical zoom.

[0117] FIG. 7 shows a LED light connected to an electrically conductive textile material of the present invention and a suitable power supply.

MATERIALS

[0118] Styrene (St), divinylbenzene (DVB), hydroxyethyl methacrylate (HEMA), sodium dodecyl sulphate (SDS), ammonium persulfate (APS), glycerol, melamine formaldehyde (MF), para toluene sulfonic acid (PTSA), silver nanoparticle inks (30-35 wt. %) and Triton X-100 were purchased from Sigma-Aldrich, UK and used as received. Protective Chemical FC-3548 and Aerosil R202 fumed silica were supplied by 3M and Evonik Industries, respectively. BD022 (100% Cotton), MK14 (100% Polyester) and KG308 (35% Cotton, 65% Polyester) fabrics were provided by Royal TenCate, Netherlands.

[0119] Highly concentrated water-based graphene dispersion (BS8, 8 wt.-%) was supplied by BGT Materials Limited, UK. Silver nanoparticle inks (SA-Ag, 30-35 wt.-%), Triethylene glycol monomethyl ether (TEGMME), Polyvinylpyrrolidone (PVP) of 10 K molecular weight and Triton X-100 were purchased from Sigma-Aldrich. Nano 60 PEL paper was purchased from Printed Electronics Limited, UK. 100% Cotton fabrics (BD022) were supplied by Royal TenCate, Netherlands.

Characterisation

[0120] The Raman spectra were obtained from a low power (<1 mVV) HeNe laser (1.96 eV, 633 nm) in Renishaw 2000 spectrometer. The viscosity of formulated inks was measured using a Brookfield DV-II+PRO programmable digital viscometer at 25 C. temperature and surface tension was measured by using a torsion balance (model OS) for surface and interfacial tension measurement. Thermogravimetric Analysis (TGA) was conducted to investigate the thermal stability of formulated inks using a TGA Q500 (TA Instruments, USA). A Philips XL 30 Field Emission Gun Scanning Electron Microscope (SEM) was used to analyse the surface topography. Printed samples were gold-palladium (AuPd) coated for 90 seconds and assessed under FEG SEM with the following operating parameters: 6.0 KV, spot size 2.0, 10 mm WD and magnification: 500 to 40000. A Jandel four-point probe system (Jandel Engineering Ltd, Leighton, UK) was employed to measure the sheet resistance of the conductive patterns. The sheet resistance was calculated from the average of six measurements and multiplied by a correction factor of 4.5324.

[0121] The particle size of the nanoparticle dispersion was measured using Dynamic Light Scattering (DLS) techniques (Nano Z-Series, Malvern Instruments).

[0122] The hydrophobicity was assessed by measuring the contact angle (CA) of a distilled water droplet on the treated substrate, and the change of WCA with time was also measured using a Kruss Dynamic Shape Analyser DSA 100. The WCA readings were taken at every 5 min and the respective graphs were plotted.

Hydrophobic Surface Coating

[0123] Synthesis of Polystyrene Based Nanoparticles

[0124] Hydroxyl functionalised cross-linked styrene/divinylbenzene nanoparticles were synthesised using conventional emulsion polymerisation containing either 1 wt.-% (NP1) or 5 wt.-% HEMA (NP5) on total monomer. 250 ml of deionised water and 20 ml of a 3.38 mmol, solution of SDS were added to 500 ml flange flask fitted with a condenser, nitrogen flow, a 5 blade impeller mechanical stirrer and a thermometer; stirred for 15 min at 600 rpm under nitrogen flow. St (21 g, 216 mmol), DVB (2.1 g, 16.1 mmol) and HEMA were then added and stirred at 600 rpm whilst being degassed for 1 hour and heated to 80 C. APS (1 g, 11.6 mmol), dissolved in 10 ml of deionised water and degassed for 30 min in a vial, added to the reaction flask. The reaction was run for 24 hr; stopped and run another 2 hr for cooling. The resultant suspension was passed through 50 m nylon gauze to remove any coagulant; and nanoparticles were used without any further treatment.

Surface Pre-Treatment and Inkjet Printing

[0125] A 5:1 mixture of 50% w/v MF in methanol and 1% w/v PTSA in methanol was deposited onto textiles using a Kruss DSA100 (NE43 needle, 0.7 mm) and dried at 130 C. for 30 min. Candidate inkjet inks were formulated using glycerol or 2-butanol and Triton X-100 to increase the viscosity and reduce the surface tension of the dispersions, respectively. Inks were filtered through a 0.45 m filter to remove any impurities and large particles that could block the nozzles.

[0126] A Dimatix DMP-2800 inkjet printer (Fujifilm Dimatix Inc., Santa Clara, USA) was used in this study, equipped with a disposable piezo inkjet cartridge. This printer can create and define patterns over an area of about 200300 mm and handle substrates up to 25 mm thick, being adjustable in the Z direction. The nozzle plate consists of a single row of 16 nozzles of 21.5 m diameter spaced 254 m with typical drop diameter of 27 m and 10 pl drop size. Print head height was adjusted to 0.75 mm; formulated inks were jetted reliably and reproducibly at 24 V and ambient temperature. It was important however to use the primed-head within 48 hours to avoid non recoverable nozzle dry out. In order to compare the hydrophobicity achieved using both the conventional padding method and the digital inkjet printing method, the fabrics supplied were also padded into an acidic solution containing 40 g/L Protective Chemical FC-3548; dried at 100 C. for 5 min and thermally fixed by curing at 180 C. for 1 min.

[0127] The inkjet printing of nanoparticles onto a range of textile materials such as cotton, polyester and their blends significantly improved water repellent properties, achieving a higher WCA up to 160 as illustrated in FIGS. 1a and 1b.

[0128] During contact angle measurement, the water droplets falling onto untreated cotton fabrics were absorbed almost immediately after hitting the surface, FIG. 1a, as the cotton fibres provide higher polarity, hydrogen-bonding and wettability in their natural form.

[0129] The inkjet printing of a few layers of polystyrene nanoparticles onto cotton fabric introduced surface hydrophobicity and imparted measureable WCA onto printed pattern.

[0130] The WCAs for NP1 printed on 100% cotton fabrics were found to be 131.2 and 132.9 for the fabrics without and with MF pre-treatment, respectively (FIG. 1a).

[0131] The WCAs for NP1 printed on polyester fabric, without any MF treatment, imparted a relatively high WCA of 143.3 (FIG. 2).

Electrically Conductive Formulations

Synthesis and Characterisation

[0132] In order to find the optimum percolation threshold for diluted SA-Ag ink, a series of composite inks were formulated by blending BS8, TEGMME and 1% PVP (in TEGMME) with SA-Ag inks. The formulated composite inks were deposited onto PEL paper using a triple reservoir cube film applicator (TQC, Netherland) and cured at 150 C. for 1 hr to form 90 m thick conductive films.

TABLE-US-00001 TABLE 1 The composition of electrically conductive composite inks % Materials (as supplied) 1% PVP BS8 SA-Ag (TEGMME) Ink A 40 60 0 Ink B 35 60 5 Ink C 30 60 10 Ink D 25 60 15 Ink E 20 60 20

[0133] The formulated inks were printed using a Dimatix DMP-2800 inkjet printer (Fujifilm Dimatix Inc., Santa Clara, USA) which can create and define patterns over an area of 200300 mm and handle substrates up to 25 mm thick, being adjustable in the Z direction. This printer is equipped with a disposable piezo inkjet cartridge and the nozzle plate consists of a single row of 16 nozzles of 21.5 m diameter spaced 254 m with typical drop diameter 27 m and 10 pl drop size. Print head height was adjusted to 0.75 mm and the formulated inks were jetted at 37 C. temperature, using frequent cleaning cycles during the printing. A few layers (1-5 layers) of composite inks were printed to produce a conductive pattern of 1 cm.sup.2 area and thermally-cured at 150 C. for 1 hr in an oven to sinter the conductive inks.

[0134] In order to demonstrate the potential electronic textiles applications of graphene-based composite inks, a hydrophobic coating was inkjet printed onto 100% cotton plain twill fabrics (B022) by depositing 12 layers of nanoparticles (NP1) as detailed above. Subsequently, six layers (6 L) of graphene inks (formulated from BS8 dispersion) or composite inks C were inkjet printed onto hydrophobic areas of cotton fabrics.

[0135] The viscosity and surface tension of the BS8 pristine graphene dispersion was found to be 1.32 cP and 71 mN/m, respectively.

[0136] The BS8 pristine graphene dispersion was supplied by BGT Materials Limited and was found to have an average flake size of approximately 1 m.

[0137] The Raman spectra of BS8 shows a very well-defined 2D band at 2686.36 cm.sup.1, a G band at 1579.98 cm.sup.1 and a D band at 1334.6 cm.sup.1 (FIG. 4). The G-peak indicates a graphite carbon structure, whereas the D peak, only observed at the sample edge, indicates defects typically attributed to the structural edge effects such as epoxides covalently bonded to the basal plane of graphene [10, 11]. It is possible to identify the number of graphene layers from the shape of the 2D peak [12], which is very sharp for monolayer graphene. It can therefore be implied that BS8 graphene dispersion contains multi-layer graphene sheets.

Electronic Textile Application

[0138] Table 3 shows the sheet resistances of conductive patterns printed on untreated and 12 layer NP1 inkjet printed hydrophobic textiles using graphene ink and Composite ink C. The sheet resistance of NP1 printed textiles with BS8 ink was found to be 161.55 /sq. and that for untreated cotton was 2238.45 /sq; which were significantly reduced to 2.11 /sq. and 30.89 /sq. for composite Ink C, Table 3. FIGS. 5 and 6 show the SEM images of inkjet printed conductive textiles with composite Ink C. The inkjet printing of hydrophobic NP1 onto cotton fabrics provided inter-fibre bonding, FIGS. 5a and 5b, which helped to produce a continuous film of Ag NPs and imparted very good inter-connections between graphene sheets. Therefore, the sheet resistances of the conductive patterns onto NP1 printed cotton were found to be much lower than that of untreated textiles.

TABLE-US-00002 TABLE 3 Sheet resistances of inkjet printed conductive patterns with composite Ink C and graphene inks onto 100% cotton fabrics Sheet Resistance Inkjet Inks and Substrates (/sq.) 6L BS8 inks printed onto NP1 (12L) printed 161.55 100% Cotton 6L BS8 inks printed onto untreated 100% Cotton 2238.45 6L Composite Ink C printed onto NP1 (12L) 2.11 printed 100% Cotton 6L Composite Ink C printed onto untreated 30.89 100% Cotton

[0139] In order to demonstrate a potential application, an LED light was illuminated by connecting it with a power supply and conductive textiles as shown in FIG. 7.

[0140] While specific embodiments of the invention have been described herein for the purpose of reference and illustration, various modifications will be apparent to a person skilled in the art without departing from the scope of the invention as defined by the appended claims.

REFERENCES

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