ELECTROSPUN CONDUCTIVE CARBON FIBERS

Abstract

A conductive carbonaceous fiber is provided, comprising a carbonaceous material obtained from carbonizing an electrospun fiber wherein said fiber comprises at least one conductive metal precursor. The electrospun fibers can be formed into fibrous mats during spinning, stabilization and carbonization that are conductive materials which can be used to make stretchable conductors for flexible electronic devices. The invention relates also to the process for making the fibers, corresponding elastomeric fibrous mesh/polymer composites as well as use of these composites for making stretchable electrical conductors. The obtainable elastomeric composite films (with a thickness in the range of 0.8 to 1.5 mm) exhibit good electrical conductivity and excellent electromechanical stability under mechanical deformations (e.g. elongating, twisting and bending). The scalable fabrication process and low-cost precursors make the elastic electrospun carbon fibers/polymer composite conductors promising materials for applications in flexible electronic devices, displays, sensors, wearable conducting clothes, implantable medical devices, etc.

Claims

1.-25. (canceled)

26. A stretchable, conductive fibrous mesh/elastomer composite material comprising a mesh of conductive carbonaceous fibers comprising a carbonaceous material obtained from carbonizing an electrospun fiber wherein said electrospun fiber is derived from lignin and comprises at least one metal precursor, and wherein said mesh is integrated into an elastomer matrix.

27. The composite material according to claim 26, wherein the metal precursor is converted to a conductive metal particle.

28. The composite material according to claim 26, wherein the metal precursor is selected from the group consisting of a copper and nickel salt.

29. The composite material according to claim 26, wherein the lignin is selected from the group consisting of organosolv lignin, softwood kraft lignin, hardwood kraft lignin and lignosulfonate.

30. The composite material according to claim 26, wherein the precursor partly or fully converts to a corresponding conductive metal nanoparticle via pre-oxidation and reduction during a stabilization and carbonization process.

31. A process for making a stretchable, conductive fibrous mesh/elastomer composite material comprising a mesh of conductive carbonaceous fibers comprising a carbonaceous material obtained from carbonizing an electrospun fiber wherein said electrospun fiber is derived from lignin and comprises at least one metal precurson, and wherein said mesh is integrated into an elastomer matrix, comprising the operations of a) dispersing the conductive metal precursor into a spinning composition; b) electrospinning of the obtained composition; c) carbonizing the obtained fiber to fully or partially convert the metal precursor to conductive metal nanoparticles; d) putting a layer of a fibrous mesh of the obtained fiber on top of a layer of elastomer wherein said elastomer layer is optionally supported by a substrate; e) casting a layer of fully or partially uncured elastomer on top of the fibrous mesh optionally supported by degassing in a vacuum; and f) curing the top elastomer layer.

32. The process according to claim 31 wherein the spinning composition comprises the lignin in admixture with at least one other polymer in a polar solvent.

33. The process according to claim 31 wherein the carbonization step comprises a stabilization as pre-operation.

34. The process according to claim 33 wherein the stabilization pre-operation comprises heat treating the electrospun fibers in an inert atmosphere optionally supported by annealing steps.

35. The composite material according to claim 26 wherein the elastomer is selected from one or more polysiloxanes, polyurethanes, rubbers or a combination thereof.

36. The composite material according to claim 35 wherein the mesh is formed to a layer which is integrated in an elastomer layer.

37. The composite material according to claim 36 wherein the thickness of the integrated composite layer is about 0.1 to 10 mm.

38. The process according to claim 31, wherein the elastomer is pre-stretched in operation d) when putting the fibrous mesh on top of the elastomer.

39. A stretchable electrical conductor comprising a stretchable, conductive fibrous mesh/elastomer composite material comprising a mesh of conductive carbonaceous fibers comprising a carbonaceous material obtained from carbonizing an electrospun fiber wherein said electrospun fiber is derived from lignin and comprises at least one metal precurson, and wherein said mesh is integrated into an elastomer matrix.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0085] The accompanying drawings illustrate a disclosed embodiment and serve to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition or the limitation of the invention.

[0086] FIG. 1 shows the fabrication process of electrospun carbon fibrous mats from lignin.

[0087] FIG. 2a shows an SEM image of a lignin-derived copper/carbon nanofiber.

[0088] FIG. 2b shows an SEM image of a lignin-derived copper/carbon nanofiber.

[0089] FIG. 2c shows a cross-sectional TEM image of the copper/carbon nanofibers.

[0090] FIG. 3a shows an XRD spectrum of the lignin-derived electrospun copper/carbon nanofibers.

[0091] FIG. 3b shows a Raman spectra of the lignin-derived electrospun carbon nanofibers with and without copper.

[0092] FIG. 4 shows the fabrication process of electrospun carbon fibrous mat/PDMS composites.

[0093] FIG. 5a shows a photograph of the PDMS composite film embedded with electrospun carbon fibrous mat.

[0094] FIG. 5b shows a cross-sectional SEM image of the electrospun carbon fiber/PDMS composite before stretch-release test.

[0095] FIG. 5c shows cross-sectional SEM image of the electrospun carbon fiber/PDMS composite after stretch-release test.

[0096] FIG. 6 shows typical stress-strain curves of PDMS and electrospun copper/carbon fiber/PDMS composites with 0.7 wt % carbon fiber loading.

[0097] FIG. 7 shows the sheet resistance vs. strain of electrospun carbon fiber/PDMS composite films with or without copper.

[0098] FIG. 8a shows the sample shape induced by bending and twisting.

[0099] FIG. 8b shows the sample shape induced by bending and twisting.

[0100] FIG. 8c shows the dependence of normalized resistance (R/R.sub.0) on the number of bending (with a bending radius of 4 mm) and twisting cycles.

[0101] FIG. 9a shows the sample shape induced by stretching and releasing.

[0102] FIG. 9b shows the sample shape induced by stretching and releasing.

[0103] FIG. 9c shows the dependence of normalized resistance (R/R.sub.0) on the number of stretching and releasing cycles with different maximum applied strain (20%, 40% and 60%).

DETAILED DESCRIPTION OF DRAWINGS

[0104] Referring to FIG. 1, it is shown that the disclosed lignin-derived hybrid carbon nanofibers are fabricated via electrospinning the lignin/DMF solutions incorporated with metal precursors (such as for instance copper acetate, copper chloride, nickle nitrate, etc.) followed by thermal stabilization and subsequent carbonization.

[0105] Referring to FIGS. 2a, b and c, the FIGS. 2a and b present the typical SEM images of the electrospun copper/carbon nanofibers derived from lignin and copper acetate with diameters around 500 nm. The surface of the fibers is not smooth because of the loading of metals. The cross-sectional TEM image (FIG. 2c) reveals the uniform dispersion of copper in the resultant carbon fibers.

[0106] Referring to FIG. 3, FIG. 3a gives evidence for the uniform dispersion of copper in the resultant carbon fibers by their X-ray diffraction (XRD) spectrum which shows very pure copper diffraction peaks at 43.3, 50.4 and 74.1°. The uniformly embedded metal nanoparticles (such as copper, nickle, etc.) not only improve the electrical conductivity of the fibers, but also act as catalyst to generate higher degree of graphitization of the lignin-derived carbon. The Raman spectra of the lignin-derived electrospun carbon fibers with and without copper are shown in FIG. 3b. The peak intensity ratio of D band (1350 cm.sup.−1, disordered carbon) to G-band (1580 cm.sup.−1, sp.sup.2 carbon) decreased from 1.79 to 1.34 after incorporation of 40 wt % copper, indicating that larger proportions of graphitic carbon is formed under the catalysis of copper. The electrical conductivity of the lignin-derived pure electrospun carbon fibrous mats is 287 S/m, which could be enhanced to 1024 S/m after loading with 40 wt % copper.

[0107] Referring to FIG. 4, the schematic fabrication process of the PDMS composites incorporated with electrospun carbon fibrous mats is shown. First, the electrospun hybrid carbon fibrous mats were laid on the surface of a pre-stretched PDMS film with a length of L+ΔL. Then a thin layer of uncured PDMS was cast on top of the carbon fiber, followed by vacuum infiltration at room temperature for 0.5 h and thermal curing at 70° C. in the air for 1 h.

[0108] Referring to FIG. 5, it is shown that the composite film was finally peeled off and exhibited a curved shape (as shown in FIG. 5a) because there is stain on one side of the film. It can be mentioned that the interaction between the carbon fibers and PDMS was so strong that no obvious defects were observed in the cross-sectional SEM image of the composite (FIG. 5b). Incorporation of PDMS matrix does not damage the scaffold of the carbon fibrous mats, because the electrical conductivity of the carbon fibers showed nearly no change after infiltration with PDMS. The thickness of the resulting film was in the range of 0.8 to 1.5 mm with a weight ratio of carbon fiber less than 1 wt %. The composite films exhibit good flexibility, and can be bent, stretched and twisted without breaking.

[0109] Referring to FIG. 6, it is shown that incorporation of the electrospun carbon fibers obviously improves the mechanical properties of the PDMS matrix, as revealed in the stress-strain curves shown in FIG. 6. The ultimate strength of the carbon fiber/PDMS composites with a pre-stretched length of 30% is increased by a factor of 1.25 compared with the pure PDMS films.

[0110] Referring to FIG. 7, it is shown that the electrical sheet resistance of the composite films as a function of applied tensile strain from 0 to 60%. The initial sheet resistance of the composite with copper/carbon fibers (40 wt % loading of copper) was as low as 0.16 kΩ/□ which increased slowly with the increase of tensile stain. When the film was stretched to 60% longer and released, the sheet resistance still kept at a low level around 0.85 kΩ/□ (increased by 4 times). The value is much lower than most reported CNT/PDMS composites. The increase in sheet resistance is mainly due to less local interconnections or decreased contact area between adjacent carbon fibers after stretching to a certain degree. As a comparison, PDMS composite films with pure electrospun carbon fibers were also fabricated. However, they exhibited much higher sheet resistance from 0.73 to 4.92 kΩ/□ with 0 to 60% applied tensile strain. In order to investigate the reversibility of this conductor, repetitive mechanical deformation cycles were applied.

[0111] Referring to FIG. 8, the dependence of normalized resistance (R/R.sub.0) on the number of bending (with a bending radius of 4 mm) and twisting cycles is shown. It was found that the electrical conductivity of the composite sheets revealed little sensitivity to repeated bending and the change of resistance under 100 cycles of twisting deformations is less than 7%.

[0112] Referring to FIG. 9, the variations of the normalized resistance of the composites as a function of tensile strain up to 60% in the first twenty stretch-release cycles are presented. It can be seen that that the resistance change of the composite sheets quickly stabilizes after the first 5 elongation-contraction cycles. After such initial conditioning, the resistance remains almost constant for various tensile strains (20%, 40% and 60%). FIG. 5c additionally shows the cross-sectional image of the composite after repeated stretch-release test. The interconnection of the carbon fibers within the polymer matrix exhibits an accordion phenomenon. The fibers break when stretched and connected with one another again upon release, leading to good stability of their electrical conductivity. Due to the good electromechanical properties, inexpensive precursors and easy fabrication, the lignin-derived electrospun hybrid carbon fiber/PDMS composites can be produced in large scale and applied in flexible, stretchable and foldable electronics

EXAMPLES

[0113] Non-limiting examples of the invention and a comparative example will be further described in greater detail, which should not be construed as in any way limiting the scope of the invention.

Example 1

Preparation of Electrospun Copper/Carbon Fibrous Mats from Lignin

[0114] 436 mg Alcell lignin and 48 mg polyethylene oxide (Mw 600K) were dispersed into 2 mL N,N-dimethylformamide (DMF) under magnetic stirring and the suspension was heated at 60° C. for 0.5 hours. Then 545 mg copper acetate monohydrate was added to the mixture and stirring was continued at 60° C. for 1 hour. After cooling down to room temperature naturally under continuous stirring, the solution was placed in a 1 mL plastic syringe fitted with a flap tip 22 G needle and was electrospun using a horizontal electrospinning setup with air humidity lower than 40%. Electrospinning of the above suspensions was carried out using a conventional single-spinneret electrospinning setup (model: nanon-01A of MECC Co., Ltd., Japan). Typically, electrospinning was performed at 7.5 to 8.5 kV with a feeding rate of 1.5 mL/h and the needle tip-to-plate substrate distance was 10 cm. The nanofibers were collected on aluminium foil and dried at 70° C. under vacuum overnight. The dried nanofibers were thermostabilized in a tube furnace under atmospheric environment. The temperature was ramped from 25 to 200° C. at 1° C. min.sup.−1 and kept at 200° C. for 2 hours. The stabilized fibers were then heated from 200 to 900° C. at 10° C. min.sup.−1 under a flow of argon (150 cm.sup.3 STP/min) and carbonized at 900° C. for 3 hours.

Example 2

Preparation of PDMS Substrate

[0115] The PDMS substrate was prepared by mixing a silicone-elastomer base and curing agent (Sylgard 184, Dow Corning) at a ratio of 10:1 by weight. The mixture was first degassed under stirring in vacuum for 1 hour and then poured onto a glass substrate, followed by curing at 70° C. in the air for 1 h. The thickness of the resulting film was in the range of 0.4-0.6 mm.

Example 3

Preparation of Electrospun Carbon Fiber/PDMS Composites

[0116] The electrospun carbon fibrous mats were laid on a PDMS substrate pre-stretched with a strain of 30%. Then a thin layer of uncured PDMS was cast on top of the carbon fiber, followed by degassing in a vacuum oven at room temperature for 0.5 h and thermal curing at 70° C. in the air for 1 h. The thickness of the resulting film was in the range of 0.8 to 1.5 mm.

Example 4

Characterization

[0117] The electrical conductivity of the electrospun carbon fiber/PDMS composites was measured by a two-probe digital multimeter at room temperature. Thin copper wires were embedded and connected to the carbon fibrous mats with electronically conductive silver paint (RS 186-3593, RS Components Ltd, UK) before infiltration with PDMS pre-polymer. The mechanical strength of the carbon fiber/PDMS composite films were measured with an Instron 5569 universal testing machine equipped with 500 N loading cells. The stain ramp rate was maintained at 10 mm per minute for all the tests.

[0118] Morphology of the carbon nanofibers were observed under JEOL JSM 6700 field-emission scanning electron microscope at an accelerating voltage of 5 kV. All samples were coated with a thin gold layer before SEM imaging. To observe the cross-sectional morphology, the copper/carbon fibrous mats were embedded into epoxy and cut into 50 nm slices using a microtome (Leica) before attaching onto copper grids. High resolution TEM images were obtained with a JEOL 2100 transmission electron microscope. Wide-angle X-ray diffraction (XRD) measurements were performed using a Bruker D8 Discover GADDS X-ray diffraction meter with Cu Ka radiation and Raman spectra were recorded on Jobin Yvon T64000 triple spectrograph micro-Raman system. The metal content in the resultant hybrid carbon fibers was measured by inductively coupled plasma mass spectrometry (ICP-MS) analysis.

INDUSTRIAL APPLICABILITY

[0119] The fibers and composites described in this disclosure may be useful as materials in stretchable electric conductors. The good conductivity that is retained upon twisting and bending makes them very useful for devices that employ or could employ such conductors. There can be mentioned as examples for such devices: flexible displays, skin sensors on moving body parts, stretchable circuits, wearable electronic on functional clothes and pressure gauges etc. Lignin can be used as the base material of the fibers which is inexpensive and abundant.

[0120] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.