ALKAN GÜRSEL SELMIYE (TR)
BAKHTIARI ROKHSAREH (TR)
GHOBADI SAJJAD (TR)
ÖZDEN YENIGÜN ELIF (TR)
TEMIZ VEDAT (TR)
KAYA ISMAIL (TR)
EP2837716A1 | 2015-02-18 | |||
CN105603582A | 2016-05-25 |
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REKHA NARAYAN ET AL: "Graphene Oxide Liquid Crystals: Discovery, Evolution and Applications", ADVANCED MATERIALS, vol. 28, no. 16, 1 April 2016 (2016-04-01), DE, pages 3045 - 3068, XP055353875, ISSN: 0935-9648, DOI: 10.1002/adma.201505122
WEIHUA CAI ET AL: "A spinneret as the key component for surface-porous graphene fibers in high energy density micro-supercapacitors", JOURNAL OF MATERIALS CHEMISTRY A: MATERIALS FOR ENERGY AND SUSTAINABILITY, vol. 3, no. 9, 22 January 2015 (2015-01-22), GB, pages 5060 - 5066, XP055353388, ISSN: 2050-7488, DOI: 10.1039/C5TA00365B
STANKOVICH ET AL., CARBON, vol. 45, no. 7, 30 June 2007 (2007-06-30), pages 1558 - 65
Claims: 1. Method for preparation of graphene-based fibers, comprising the following steps a) releasing of an aqueous graphene oxide (GO) suspension with a GO concentration of at least 20 g/L, through a nozzle, into an ionic solution, thus formation a longitudinal fiber; and then b) taking the fiber out of the ionic solution and then contacting the fiber with a solvent having a higher polarity than that of water. 2. Method according to the claim 1, wherein the nozzle is positioned such that the flow direction inside the nozzle is parallel to that of the gravity vector; and the nozzle opening is dipped into the ionic solution, such that the suspension is directly surrounded with the ionic solution upon being released from the nozzle. 3. Method according to any of the claims 1 or 2, wherein the suspension released from the nozzle has a volumetric flow rate within the range between 50 and 800 microliters per minute; the nozzle is longitudinal and has a length-to-diameter (L/D) ratio within the range between 30 and 225; and the GO suspension is provided in a receptacle connected with the nozzle, and the receptacle mainly being in form of a cylinder having a length to diameter (L/D) ratio within the range between 1 and 10. 4. Method according to any of the claims 1 to 3, wherein the ionic solution of step (a) is an aqueous calcium chloride (CaCI2) solution and the concentration of calcium chloride is within the range between 3 wt.% and 5 wt.%. 5. Method according to any of the claims 1 to 4, wherein the solvent of step (b) comprises methanol. 6. Method according to the claim 5, wherein the solvent of step (b) is methanol. 7. Method according to any of the claims 1 to 6, comprising a further step (c) for at least partly de-oxidation of the graphene oxide to an extent where an electrically conductive fiber is obtained. 8. Method according to the claim 7, wherein the step (c) comprises contacting the fiber with an aqueous solution of sodium borohydride (NaBH4). 9. Method according to the claim 8, wherein the step (c) is performed by dipping a coil of fiber into the sodium borohydride solution. 10. An aqueous graphene oxide suspension with a graphene oxide concentration of at least 20 g/L. 11. Mixture according to the claim 10, wherein the solvent is deionized water; and the graphene oxide concentration is at least about 25 g/L, and preferably at least about 30 g/L. 12. Mixture according to any of the claims 9 or 10, consisting of water and graphene oxide. 13. System for obtainment of graphene-based longitudinal fibers from an aqueous graphene oxide (GO) suspension with a GO concentration of at least 20 g/L, comprising a longitudinal nozzle having a length-to-diameter (L/D) ratio within the range between 30 and 225, a receptacle for receiving the GO suspension, to be connected with the nozzle, the receptacle being mainly in form of a cylinder having a length-to-diameter (L/D) ratio within the range between 1 and 10, and the system is provided with a pushing plug movable inside said receptacle adapted for pushing the GO suspension out of the nozzle in a volumetric flow rate within the range between 50 and 800 microliters per minute. 14. System according to the claim 13, wherein the pushing plug is adapted to push the suspension at a stabilized linear velocity, such that in use, when a certain volumetric flow rate is selected, the flow rate remains in ± 5% tolerance limits around said volumetric flow rate. 15. System according to any of the claims 13 or 14, comprising a threaded shaft rotatable around its longitudinal axis in accordance with being rotated an electric motor, and a longitudinal guide for guiding the threaded shaft to move linearly along a direction parallel to said longitudinal axis; said threaded shaft being connectable to the pushing plug such that said linear movement of the threaded shaft is plug is transferred to the pushing plug; and the motor speed is adjustable for adjusting the volumetric flow rate. |
The present invention relates to a stable graphene oxide suspension, graphene- based fibers and a method for production of such fibers.
Background of the Invention
Among carbon-based materials, graphene has superior mechanical, thermal and electrical properties in the nanoscale. Moving such properties of the nanoscale graphene to the macro-scale still remains as a challenge due to several limitations of related manufacturing processes. Accordingly, graphene in form of fiber can be regarded as advantageous, especially considering the ease of carbon processing fibers. Graphene-based and graphene oxide based fibers have potential application in a wide variety of technical fields including energy, biotechnology, electrical and composite applications.
Conventional carbon fiber fabrication systems have several drawbacks including high process costs due to high temperature operation conditions and emission of environmentally harmful substances and by-products.
Objects of the Invention
Primary object of the present invention is to overcome the abovementioned shortcomings of the prior art.
Another object of the present invention is provision of a method for obtaining insulating GO-based fibers as well as electrically conductive graphene fibers.
A further object of the present invention is provision of GO-based fibers having high flexibility, mechanical strength, and fiber density.
A further object of the present invention is provision of a stable graphene oxide suspension in form of liquid crystals for use in production of such GO-based fibers. Summary of the Invention
The present invention proposes a method for preparation of graphene-based fibers, comprising releasing of an aqueous graphene oxide (GO) suspension with a GO concentration of at least 20 g/L, through a nozzle, into an ionic solution, thus formation a longitudinal fiber, and then taking the fiber out of the ionic solution and then contacting the fiber with a solvent having a higher polarity than that of water. The present invention further proposes a stable aqueous graphene oxide suspension allowing formation of fibers therefrom.
Detailed Description of the Invention
The present invention proposes graphene-based high flexibility and high strength fibers, and method for obtainment thereof.
The proposed method for preparation of graphene-based fibers comprises the following steps of:
a) releasing of an aqueous graphene oxide (GO) suspension with a GO concentration of at least 20 g/L, through a nozzle, into an ionic solution, thus formation a longitudinal fiber; and then
b) taking the fiber out of the ionic solution and then contacting the fiber with a solvent having a higher polarity than that of water.
Preferably, the nozzle is positioned such that the flow direction inside the nozzle is parallel to that of the gravity vector; and the nozzle opening is dipped into the ionic solution (which can be considered as a coagulation bath), such that the suspension is directly surrounded with the ionic solution upon being released from the nozzle.
It is observed that the alignment of GO flakes inside the GO suspension released through the nozzle is more favorable in case where the suspension released from the nozzle has a volumetric flow rate within the range between 50 microliters per minute and 800 microliters per minute, even more favorable with a volumetric flow rate within the range between 450 and 500 microliters per minute. In that case, it is further preferable that the nozzle is longitudinal and has a length-to-diameter (L/D) ratio within the range between 30 and 225; the GO suspension is provided in a receptacle connected with the nozzle, and the receptacle being mainly in form of a cylinder having a length to diameter (L/D) ratio within the range between 1 and 10, more preferably between 3 and 5. It is observed that the alignment of GO flakes inside the GO suspension is better preserved, a highly aligned GO stream is obtained when released through the nozzle, and the final product has a high strength along with a good elasticity.
It is observed that a very high flexibility along with enhanced fiber density values can be achieved in the fibers, when the ionic solution of step (a) is a calcium chloride (CaCI 2 ) solution and the concentration of calcium chloride is within the range between 3 wt.% and 5 wt.%. In said solution, an ethanol/water mixture was used as solvent. The ethanol to water volumetric ratio was selected to be 3: 1.
In case where the solvent of step (b) (i.e. washing solvent) comprises methanol, facilitated, rapid and efficient removal of ionic material from the surfaces of the fibers, can be achieved.
At the end of the above steps, a GO fiber with reduced or eliminated electric conductivity can be obtained. For rendering the fiber electrically conductive, the GO is to be de-oxidized, such that charge-carrying capability of the material is restored by formation of delocalized electron pairs. To this end, the method may comprise further step (c) for at least partly de-oxidation of the graphene oxide to an extent where an electrically conductive fiber is obtained.
In case where the step (c) comprises contacting the fiber with an aqueous solution of sodium borohydride (NaBH 4 ), a safe version of the method is achieved by eliminating employment of toxic acids in the process.
The step (c) can be performed by dipping a coil of fiber into the sodium borohydride solution. This can be applied upon preparing the coil from fiber upon being subjected to the step (b).
The present invention further proposes a high stability liquid mixture (aqueous graphene oxide suspension) for use as raw material in manufacturing said fibers. The aqueous graphene oxide suspension has a graphene oxide concentration of at least 20 g/L. This suspension has very appreciable suspension stability, and it can be directly used in the above described step (a); forms well aligned and stabile GO fiber without interruption. Since stabilizing agents (such as surface active materials or dispersion agents) would surround solids (graphene) in mixtures, addition of stabilizing agents would negatively affect the alignment of GO flakes by suppressing Van der Waals attraction forces between the flakes. Therefore the mixture preferably does not comprise any stabilizing agents, in order to obtain highly aligned fibers from well aligned graphene flakes distributed in the suspension. Since the mixture includes GO flakes instead of graphene flakes, the flakes align with neighboring flakes and also maintain a distance from each other preventing fully collapsing onto one another. That provides stability to the suspension without necessitating additional stabilizing agents, by keeping it from sedimentation due to increased density and gravity.
In a preferred embodiment, the solvent is deionized water; and also the graphene oxide concentration is at least about 25 g/L. This value corresponds to a highly suitable abundance of GO flakes in the solvent, for encountering other GO flakes to align with. Preferably, the GO concentration is at least 25 g/L, and more preferably at least about 30 g/L. The mixture may consist of water and graphene oxide, for making sure that the mixture does not include any additional material risking the alignment of GO flakes.
The present invention further proposes a system for obtainment of graphene-based longitudinal fibers from an aqueous graphene oxide (GO) suspension with a GO concentration of at least 20 g/L, comprising:
- a longitudinal nozzle having a length-to-diameter (L/D) ratio within the range between 30 and 225, and more preferably within the range between 40 and 50,
- a receptacle for receiving the GO suspension, to be connected with the nozzle, the receptacle being mainly in form of a cylinder having a length-to-diameter (L/D) ratio within the range between 1 and 10, more preferably between 3 and 5.
Especially in case where the suspension is in a state of a liquid crystal (by GO flakes being aligned with each other); for a favorable alignment of GO flakes when forming the fiber, the system is provided with a pushing plug movable inside said receptacle, adapted for pushing the GO suspension out of the nozzle in a volumetric flow rate within the range between 50 and 800 microliters per minute, more preferably within the range between 450 and 500 microliters per minute. It is observed that these values are very suitable to maintain the stability of the suspension in the receptacle during formation of fibers, thus ensures undisturbed diameter and enhanced length values at graphene-based fibers production.
In a preferred embodiment of the system according to the present invention, the pushing plug is adapted to push the suspension at a stabilized linear velocity, such that in use, when a certain volumetric flow rate is selected, the flow rate remains in ± 5% tolerance limits around said volumetric flow rate.
Preferably, the system comprises a threaded shaft rotatable around its longitudinal axis in accordance with being rotated an electric motor, and a longitudinal guide for guiding the threaded shaft to move linearly along a direction parallel to said longitudinal axis; said threaded shaft being connectable to the pushing plug such that said linear movement of the threaded shaft is plug is transferred to the pushing plug; and the motor speed is adjustable for adjusting the volumetric flow rate.
Several exemplary lab-scale experiments according to the method of the present invention are described below in detail. By giving said examples, it is solely intended to provide a better understanding of the present invention, and the examples as such are not intended to limit the scope of the appended claims.
EXAMPLES
Preparation ofGraohene Oxide (GO) Flakes
Two dimensional GO has been synthesized from graphite flakes (CAS number: 7782- 42-5 and particle size: 100 mesh) by chemical oxidation in accordance with the modified Hummers Method (Stankovich et al., Carbon. 2007 Jun 30; 45(7): 1558- 65). As the modification, phosphoric acid / sulphuric acid mixture (a liquid mixture) was used instead of nitric acid, for the sake of process safety. Natural graphite flakes were first mechanically mixed with solid potassium permanganate (a solid mixture is obtained), and said liquid mixture was then slowly added onto the solid mixture. This step took place in an ice bath to suppress sudden temperature raises. This reaction mixture was kept under reflux for 24 hours, and then ice and hydrogen peroxide were subsequently added for termination of the oxidation reaction. Then, the products were centrifuged and washed for several times in a washing bath. An ethanol/water mixture with a pH value of about 3.5 was used as washing bath. Obtained GO was separated by filtering (here, using ceramic filters), and dried in a stove for 48 hours.
The GO suspension and alignment of GO flakes therein
GO (in form of flakes) was mixed with water (here: deionized water), and this suspension was energized along with visually observing the extent of alignment between GO. Here, the energizing was performed by subjecting the suspension to ultrasound waves. As a way thereof, an ultrasonic probe was used for a first time period (e.g. approximately 1 hour), and the extent of alignment between GO flakes was visually followed using polarized optical microscope. Then, the suspension was further energized by subjecting it to ultrasonic bath, for a second time period (e.g. approximately 1 hour).
For the exemplary case, it is realized that specific amounts of energy (in kiloJoules per milliliter, kJ/mL, of suspension; variable with GO concentration in grams per liter, g/L) was sufficient for achieving an optically observable extent of GO alignment. The below Table 1 shows the correlation between said specific amounts of energy with the GO concentration in the suspension, which can be considered as a liquid crystal upon said alignment:
Table 1: Amounts of energy (via ultrasonic waves) applied onto unit volume (1 mL) of the GO suspension for obtaining a liquid crystal (LC)
Starting from the above table, the energy amount required for GO alignment (i.e. obtainment of liquid crystal) was thus formulated as follows:
E (kJ/mL) = 98.4 ■ (GO concentration, in g/L) + 1121
Considering the above empirical equation, a still advantageous extent of alignment can be considered achievable within a tolerance (of ±10%) around the respective correlation. Hence, a minimum amount of energy (E min ) to achieve said advantageous extent of alignment is considered calculable using the below equation: E min (kJ/mL) = 98.4 (± 9.84) ■ (GO concentration, in g/L) + 1121 (± 112.1)
Wet Spinning of GO fibers
The aqueous suspension of GO was fed into an ionic solution which serves as a coagulation bath. The suspension can be considered as a liquid crystal by being a liquid mixture containing aligned GO flakes. A special pump was designed for better control of flow regime in feeding of GO suspension into the ionic solution, compared to that obtainable when a syringe pump is employed. The pump was suitable to provide a controlled displacement to a pushing plug movable in the container of the GO suspension, adapted for pushing the GO suspension out of the nozzle in a volumetric flow rate within the range between 50 and 400 microliters per minute. This provides a high uniformity of the thickness (or diameter) along the final product (i.e. GO based fiber). Thus, a constant linear velocity within ± 5% tolerance limits around a selected linear velocity corresponding to a flow rate calculable using geometry of the container receiving the GO suspension, was obtained. In this case, the pump was voltage-controlled, which means the speed of the pump was arrangeable as a function of the electric potential difference applied onto the pump motor; but other means may also be adapted to precise arrangement of the speed of the pump.
In this experimental case, the ionic solution was still in terms of fluids mechanics, i.e. no turbulence occurred in the coagulation bath, for provision of a more uniform alignment between GO flakes. The nozzle exit was positioned inside the ionic solution for avoiding any contact between air and GO suspension before the stream exiting from the nozzle is surrounded with the ionic solution. Furthermore, the angle of the nozzle was arranged such that the GO suspension exits the nozzle in the same direction with the gravity, which is also for provision of an even more uniform alignment between GO flakes. These considerations enable a more incessant production of GO based longitudinal fiber.
The longitudinal fiber was then directed using a rotating guide (which was in this case a cylinder made of TEFLON; preferably rotating around a horizontal axis, in accordance with the linear velocity of the fiber), into a further bath containing a solvent having a higher polarity than that of water. This bath dissolves and removes the ionic content on the fiber remaining from the coagulation bath, and therefore renders the fiber more flexible (less brittle, less fragile) as a final product.
Evaluation of alternative liquid phases for use in the GO suspension
Dimethyl formamide (DMF), n-methyl propanol (NMP) and deionized water (DI- water) were tried as alternative liquid phases of GO suspensions; and dispersions with 2.5, 5, 7.5, 10, 20, 25, 30 and 40 g/L (grams per liter) of GO concentrations were prepared with each solvent. Each suspension were subjected for ultrasonic bath four times for 40 hours, and re-homogenized in an ultrasonic-probe sonicator for one hour prior to being fed (injected) into the coagulation bath. Upon a 24 hours observation period, water (Dl-water) was considered to have provided a better stability to the GO suspension. Furthermore, water is preferred also because it was the most environment-friendly and low-cost fluid amongst its alternatives. GO concentrations lower than 20 g/L were considered insufficient for alignment of GO flakes. The reason thereof can be that the distance between neighboring flakes being too long for achieving alignment in accordance with Van der Waals forces therebetween. Higher GO concentrations of at least about 25 g/L, especially of at least about 30 g/L resulted in an even more favorable alignment between GO flakes, and thus an even more favorable orientation of the material forming the fiber. It is considered that the alignment is rendered more favorable with increasing GO concentrations, as soon as the suspension remains stable and pumpable. Yet, investigation of SEM photographs upon mechanical tensile tests (according to ASTM D3379-75) on fibers obtained from different suspensions of various GO concentrations showed that a slightly more regular alignment was achieved using GO concentrations of 30 g/L, in comparison with that at 40 g/L. Elongation (%), specific strain (in MPa/g.cm 3 ) and specific modulus (in MPa/g.cm 3 ) values were derived from the data obtained from said tensile tests.
Alternative materials tested for being contained in the coaaulation bath and in the further bath:
Alternative baths were prepared for evaluating their effect on the fiber production process, and each bath combination (each run# shown in the below Table 2) were evaluated in terms of suitability for the method according to the present invention. Run numbers (Run #) 1 to 7 were found to be unsuitable, since only loose and low strength fibers were obtained using aqueous CTAB solution. Table 2: Preparations as the ionic bath (coagulation bath) and the further bath (washing bath)
Aqueous KOH and CuS0 4 solutions were also tried as coagulation bath media (as ionic solutions), yet none of them were found to be enabling in fiber formation. Use of NaOH solutions in runs 8 to 11 resulted in fragile fibers upon drying the final product. Yet, even at run 10 where NaOH was used in the ionic bath (coagulation bath), obtainment of a fiber having a length of 270 cm was achieved with the laboratory-scale system, and the experiment was terminated by the consent of the investigators. Runs 13, 15, 16, 20, 22 and 23 gave even higher strength and flexibility to the fiber; and Runs 13 and 20 resulted even better than said runs in the same terms.
According to the mechanical tensile tests mentioned above; for run 24, specific strain and strain of the fiber from the suspension with GO concentration at 30 g/L were higher than that at 40 g/L; but the % elongation of the latter was higher. For run 18, each of the specific strain, strain and % elongation of the latter was higher at the fiber from the suspension with GO concentration of 30 g/L were higher than that at 40 g/L.
Best % elongation values were achieved with 30 g/L of GO in suspension, coagulated in 5 wt% CaCI 2 in water (at a value of 10±2 %); along with a specific strain of 7±0.5 MPa/g.cm 3 , and a specific modulus value of 6±2.9 MPa/g.cm 3 . Second best % elongation values were achieved with 30 g/L of GO in suspension, coagulated in 3 wt% CaCI 2 in water (at a value of 8±0.6 %); along with a specific strain of 3±1.2 MPa/g.cm 3 , and a specific modulus value of 6±2 MPa/g.cm 3 . Third best % elongation values were achieved with 40 g/L of GO in suspension, coagulated in 5 wt% CaCI 2 in water (at a value of 6±0.5 %); along with a specific strain of 10±1 MPa/g.cm 3 , and a specific modulus value of 4±0.9 MPa/g.cm 3 . Thus, the method according to the present invention provided high flexibility and high mechanical strength to the fibers.
Reduction of GO fibers
For obtainment of electrically conductive fibers, the GO fibers were further subjected to a chemical reduction process. To that end, the GO fibers obtained with the method according to the present invention were subjected to reduction each one of the below mentioned reductants.
Reductants such as hydroiodic acid (HI, e.g. at a concentration of about 55 wt% in an aqueous solution, used for a reaction temperature of about 100°C under reflux), ascorbic acid (AA, e.g. at a concentration of about 2 mM in an aqueous solution, used for a reaction temperature of about 25°C) and sodium borohydride (NaBH , e.g. at a concentration of about 150 mM in an aqueous solution, used for a reaction temperature of about 25°C) were considered as suitable for being environmentally friendly and affordable especially for larger scale applications, when compared to ionic liquids suitable rather for laboratory scale applications, including l-ethyl-3- methylimidazolium acetate and l-ethyl-3- methylimidazolium bromide. According to observations made using Raman spectroscopy, use of NaBH 4 resulted in a higher increase in conductive double bonds, yet both HI and AA also visibly increased the conductivity of fibers (measured with CR-Cascade Microtech CP4 conductivity measurement system).
Thus the following objects are achieved by the present invention:
- overcoming the shortcomings of the prior art,
provision of:
- a method for obtainment electrically conductive or insulating GO- based fibers,
- GO-based fibers having high flexibility, mechanical strength, and fiber density.