WO2018183518A1 - Method and apparatus for producing non-oxidized graphene plates - Google Patents

Abstract

The present invention is directed at the apparatus and method for producing chemically pure non-oxidized graphene nano-sheets with a thickness of 3 nm or less from natural or synthetic graphite crystal by applying sonic and electromagnetic waves to a solution comprising a mix of water and graphite particles.

Description

METHOD AND APPARATUS FOR PRODUCING NON-OXIDIZED GRAPHENE

PLATES

This application claims the benefit of U.S. Provisional Application No. 62/477,726, filed

03/28/2017

FIELD OF THE INVENTION

[001] Graphene is an ultra-thin, practically two-dimensional carbon sheet, a basic structural unit of graphite. It can be produced by slicing or peeling off graphite layers from graphite crystals by vaporization, chemical or mechanical means. The present invention is in the field of slicing or peeling off graphite layers mechanically, and is directed at production of two- dimensional graphene platelets, also known as graphene sheets or graphene flakes. This invention describes an apparatus and method for producing high-quality, defect-free non- oxidized graphene sheets from either natural or synthetic graphite crystals. The apparatus of the present invention is represented by a portable reactor that can be easily transported and maintained anywhere in order to minimize both transportation and graphene production costs. The apparatus is also highly efficient and capable of producing graphene platelets having the number of graphene layers from one to five. BACKGROUND OF THE INVENTION

[002] Since graphene was discovered in 2004, there have been introduced many methods of graphene production. Those methods generally fall into three main categories: synthetic methods, i.e. vapor deposition or chemical vapor deposition (CVD), mechanical methods, i.e. liquid exfoliation and application of sheer force, and chemical synthesis, i.e. oxidation/reduction process of graphite using chemicals and electrochemical processes. Known graphene production methods, even though capable of producing amounts of graphene sufficient for use in research and development applications, are very expensive and largely inefficient to fulfil present industrial demands. Therefore, there is a need for a cost-effective method of production pf industrial volumes of graphene.

[003] By way of example, a scalable process for producing exfoliated graphene is described in the U.S. Patent Application No. : 2014/773011. Another graphite intercalation and exfoliation process was described in the U.S. Patent No. : 7,105, 108. Yet other methods for producing nano- scaled graphene platelets with a high length-to-width aspect ratio were presented in the U.S. Patent No. : 7,790,285, U.S. Patent Applications Nos. : 2011/787442 and 2016/0009561.

[004] One of the problems the foregoing prior art does not solve is that graphene sheets produced by those methods may be chemically and mechanically impure, and neither they are free from physical defects. This, in turn, may cause the resulting graphene sheets to have less than optimal thermo- or electricity conductive qualities, as well as other undesirable physical and chemical properties, such as toxicity, chemical instability, etc. Also important is that some of the chemicals used in the production of graphene by the foregoing methods are not biocompatible or eco-friendly, and, as the result, their use may cause negative environmental consequences.

[005] This present invention offers a different approach for production of high purity graphene sheets from natural or synthetic graphite crystals. This approach is based on use of a combination of electromagnetic and concentrated sonic fields under low temperatures and in the absence of any chemical inhibitors. As a result, graphene sheets produced by this method are free from physical defects, chemically pure, non-oxidized and have a high length-to-width aspect ratio. More importantly, since no chemical inhibitors are used, no unwanted byproducts are released into the environment. The process is clean and environmentally friendly, while capable of producing pure graphene of the superior quality. [006] Moreover, no known prior art describes a chemical-free application of electromagnetic and ultrasonic (sonic cavitation) energies in production of graphene sheets or platelets.. The invented method and apparatus are highly energy efficient, yet capable of producing high quantities of pure graphene and practically any location.

[007] Based on the current and anticipated demands for high quality graphene, there is a clear need for a simple, energy-efficient, environmentally-friendly and inexpensive apparatus and method for producing defect-free, chemically and pure non-oxidized graphene sheets having high length-to-width aspect ratios. This present invention describes a cost-effective solution that addresses that need.

SUMMARY OF THE INVENTION

[008] The present invention introduces apparatus and method for producing high-purity, defect-free non-oxidized graphene platelets from natural and synthetic graphite crystals. The apparatus uses a combination of certain frequencies of electromagnetic and sonic energies applied to a reactor containing a mixture of water and graphite fakes.

[009] The method and apparatus are based on the mixture of graphite and water passing through the inlet pipes and into the reactor. Forces produced by the electromagnetic emitter and sonicator are applied to the reactor at certain frequencies; thus, generating sufficient energy to peel off graphene sheets from the flakes in the presence of water. Use of the water is two-fold: first, it is used as medium to peel off graphene sheets, and second, it acts as a stabilizer for peeled off graphene sheets.

[0010] Graphene sheets produced by the method and apparatus described in this invention can be as thin as 3 nm or less and chemically pure, since no other reagent but water is used in their production. Moreover, since neither physical force nor chemicals are used in the present invention, the resulting graphene sheets are not curved, have a plane surface and, therefore, have superb thermo- and electrical conductivity and other advanced physical properties.

[0011] The method described herewith comprises: a) preparing a mixture of natural or synthetic graphite flakes in the water; b) centrifuging or otherwise mechanically treating the mixture so that sizes of graphite particles remain substantially similar with impurities washed away; c) circulating the graphite water mixture in the cavitation reactor; d) applying sonic energy to the mixture ("sonication"); e) applying electromagnetic energy to the mixture; and f) separating resulting graphene nano-sheets from the water by filtration.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The features of this state of art invention will be more readily understood from following detailed description of various aspects of the invention taken in conjunction with the accompanying drawings and figures.

[0013] FIG. l depicts a schematic diagram of the apparatus for producing graphene sheets or platelets from a mixture of water and graphite flakes.

[0014] FIG 2 shows dry high purity graphene platelets produced by the described apparatus.

[0015] FIG.3A and FIG.3B show SEM (Scanning Electron Microscope) images of graphene platelets produced by the apparatus.

[0016] FIG.4 depicts a TEM (Transmission Electron Microscope) image of graphene platelets produced by the apparatus.

[0017] FIG.5 depicts measured Raman spectra of high purity graphene flakes produced by the apparatus.

[0018] FIG.6. depicts FTIR (Fourier Transform Infra-Red) spectra of graphene platelets produced by the apparatus. [0019] FIG.7 depicts XRD (X-ray diffraction) pattern of graphene platelets, which represents a standard graphitic structure.

[0020] FIG.8 shows sheet size distribution of produced graphene platelets. [0021] TABLE 1. Elemental analysis of the produced graphene

Element % present Detection Limit

Carbon (C) 99.58 ^~ 0.05513

Oxygen (O) 0_39 0.30583

Chloride (CI) | 0.025 | 0.00451

DETAILED DESCRIPTION OF THE INVENTION

[0022] The following is a detailed description of the invention provided to aid those skilled in the art in practicing in the field of the present invention. Those of ordinary skill in the art may make modifications and variations in the embodiments described herein without departing from the spirit or scope of the present invention. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for describing particular embodiments only and is not intended to be limiting of the invention. All publications, patent applications, patents, figures and other references mentioned herein are expressly incorporated by reference in their entirety.

[0023] The present invention is directed at the apparatus and method for eco- friendly production of high-purity, defect-free non-oxidized graphene sheets from either natural or synthetic graphite crystals and without any chemical reagents.

[0024] The main component of the proposed method and apparatus is sonication and cavitation of graphite's particles in a water-based mixture, which causes intercalation of graphite layers into graphene sheets through increased graphite layers' spacing and reduction of interlayer adhesion.

[0025] Applying sonic energy to the reactor containing a water-based mixture of graphite crystals results in cavitation, causing the graphite crystals to split into graphite flakes. Furthermore, continuous circulation of the mixture in the reactor and application of electromagnetic energy increases graphite-water friction, which, in turn, enhances intercalation of graphite flakes into graphene sheets or platelets.

[0026] Referring to FIG. l, the graphite/water mixture is delivered into the reactor 104 through input pipes 102. The mixture in the reactor 104 is circulating continuously. The purpose of this circulation is two-fold. First, the circulation ensures even application of the sonic and electromagnetic energies to the mixture. Second, it prevents re-aggregation of resulted individual graphene sheets back into graphite flakes.

[0027] The mixture, while passing through the reactor 104, is subjected to sonic pressure from a plurality of sonic emitters, 106; said sonic pressure causes graphite crystals to break into graphite flakes.

[0028] Water is a natural dielectric, yet it does display certain magnetic qualities when it is air- saturated, which, in this application, is a result of the cavitation in the reactor. Applying electromagnetic forces through a plurality of electromagnetic emitters, 108 to the currents of the water- graphite mixture results in higher levels of graphite-water friction, which, in turn, enhances intercalation of graphite flakes into graphene sheets.

[0029] In one exemplary embodiment, the mixture of water and graphite is passed through sonic pressure of 3-5 kW operating at a frequency of 18-22 kHz with an amplitude of displacement of the ends of the waveguides of at least 100 microns. In that embodiment, two sources of acoustic pressure, such as Helmholtz resonators, for example, are placed directly across of one another and remit acoustic waves in a frequency equal to 3/4 of the frequency of ultrasound.

[0030] In another exemplary embodiment, the mixture is prepared from three liters of distilled water and 30 grams of natural flake graphite with a particle size of 350 mesh. The resulting solution is passed through the reactor at a speed of 2 m/sec. For the first 10 minutes, electromagnetic waves with a frequency of 40 Hz and the reactor's pressure of 12,000 kilopascals are applied to the solution. After that, and for the next 20 minutes, the mixture is subjected to sonic pressure with the frequency of 20 kHz and power of 200 w/cm2. This causes the pressure inside the reactor to increase to 18,000 kilopascals, which, in turn, triggers graphite flakes in the mixture to intercalate into graphene sheets. Once the entire mixture is processed, resulting graphene sheets can be extracted from the resulting aqueous paste by either filtering or dehydration.

[0031] The present invention provides for production of high purity graphene sheets that have high electrical and thermal conductivity. For example, if the source graphite crystals have conductivity of 14,000 s/m, the resulting graphene sheet's conductivity is at the 70,000 s/m level. Moreover, if the surface area of the source graphite crystals is about 3.5 m2/g, the resulting graphene sheet may have the surface area of 150-250 m2/ g.

[0032] Referring now to FIG.2, dry high-purity graphene sheets actually produced by the present invention are shown. It can be appreciated that the resulting product is gray-black in color, which is indicative of high-quality graphene.

[0033] Referring now to FIG.3A and FIG.3B showing SEM (Scanning Electron Microscope) images of graphene sheets produced by the present invention. It can be seen that flakes are mostly straight, defect-free and have uniformed dimensions. Some scrambled graphene sheets also can be noticed, which suggests that it could be due to bilayer graphene sheets. Notably, the maximum number of layers should not exceed five. Also, there are no impurities can be seen.

[0034] Referring now to FIG.4, TEM (Transmission Electron Microscope) image of graphene sheets is shown. It can be noticed that the sheets are semitransparent, which is indicative of multi-layer graphene with the number of layers not exceeding five. It can also be noticed that the sheets are agglomerated. Worth to mention that this is a bulk material, and the sheets can be further separated by applying high power ultra-sonication. The image is an additional testament to the quality of graphene produced by the described method and apparatus.

[0035] Referring now to FIG.5, measured Raman spectra of high purity graphene sheets produced by apparatus is shown. Raman spectra collected by measuring bulk material of graphene sheets. Raman analysis is very important and used to identify graphene structure, number of layers, level of crystallinity and the extent of defects present in the sheets. G peak is universal for any types of carbon based materials, sometimes it could be wider or narrower depending on the material. For high quality graphitic structure, this G band should be narrower and higher in intensity, as it is seen in the spectra. The most characteristic peaks for different types of graphene materials are D and 2D. D peak represents extent or level of defects present in the graphene structure. The lower the D peak intensity the better the material. For the graphene of the ideal purity this peak should not be visible at all. From the graph, it can be seen that D peak intensity is very low compare to other characteristic peaks, which means that the produced graphene is substantially defect-free. On the other hand, 2D peak, which identifies lack of defects, is significantly higher and prominent (for a defective material, this peak is much lower and wider). From the spectra, it can be said that the produced graphene has crystalline and ordered structure. A slight 2D shoulder can be seen in the spectra as well, which correspond to the presence of the multilayer graphene structure. Still, the number of layers does not exceed five.

[0036] Referring now to FIG.6, FTIR (Fourier Transform Infra-Red) spectra of bulk graphene sheets produced by the apparatus is shown. FTIR analysis was carried out in order to identify unwanted elements and impurities chemically or physically attached to the graphene structure. From the spectra, it is clear that no foreign entity or impurity attached to the graphene

surface/structure either physically or chemically. Prominent CO2 absorption peak can be seen in the spectra, which corresponds to the atmospheric absorption. These spectra are indicative of the high-quality graphene.

[0037] Referring now to FIG.7, XRD (X-ray diffraction) pattern of bulk graphene sheets is shown, representing a standard graphitic structure. A high intensity sharp peak corresponds to hexagonal structure that can be seen in the 2Θ range 23-27. From the spectra, it is clear that that the produced high purity graphene sheets are multilayered and highly crystalline.

[0038] Referring now to FIG.8 showing sheets size distribution of produced graphene flakes. The average sheets dimension is 21 microns and median 13 microns. The produced graphene sheets' dimensions were compared with the raw graphitic material, which found them to be substantially the same. This suggests that the proposed method and apparatus d not break or alter graphitic sheet or structure.

[0039] Referring now to TABLE 1. Elemental analysis is the most quantitative and effective way to identify quantity and identity chemical elements presented in the material. Elemental analysis of bulk material was carried out by quantitative XRF (X ray fluorescence spectroscopy) technique. Using this analysis technique, it was determined that the produced graphene was 99.58%. The oxygen content was detected at 0.3%, which is too high and, therefore, was most likely erroneous. Among other foreign elements only chloride was detected, which originated in the water.

[0040] The description of the present embodiment has been presented for purposes of illustration, but is not intended to be exhaustive or to limit the invention to the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art.

[0041] Those of ordinary skill in the art will appreciate that the elements depicted herein may vary depending on the implementation. The depicted examples are not meant to imply architectural limitations with respect to the present invention.

[0042] Herein above, or in the following claims, the term "comprises" is synonymous with "includes." The use of terminology such as "X comprises A, B and C" is not intended to imply that A, B and C are necessarily the only components or most important components of X.

[0043] Unless clearly and explicitly stated, the claims that follow are not intended to imply any particular sequence of actions. The inclusion of labels, such as a), b), c) or 1), 2), 3) etc., for portions of the claims does not, by itself, imply any particular sequence, but rather is merely to facilitate reference to the portions.

[0044] To reiterate, the embodiments were chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention. Various other embodiments having various modifications may be suited to a particular use contemplated, but may be within the scope of the present invention

Claims

CLAIMS What is claimed is:

1. A method of producing graphene sheets comprising:

preparing a substantially homogeneous mixture of plurality of graphite crystals and water;

centrifuging said mixture so that sizes of said plurality of graphite crystals remain substantially similar;

placing said mixture into a reactor;

causing said mixture to be circulated in said reactor;

applying sonic energy to said reactor and to said mixture circulating in said reactor;

applying electromagnetic energy to said reactor and to said mixture circulating in said reactor;

extracting said mixture from said reactor; and

separating resulting graphene sheets from said mixture.

2. The method as in Claim 1, wherein said graphite crystals are made of natural

graphite.

3. The method as in Claim 1, wherein said graphite crystals are made of natural

graphite.

4. The method as in Claim 1, wherein said water is substantially distilled.

5. The method as in Claim 1, wherein said reactor is substantially spherical.

6. The method as in Claim 1, wherein said separating of said resulting graphene sheets is by filtering.

7. The method as in Claim 1, wherein said separating of said resulting graphene sheets is by centrifuging.

8. An apparatus for production of graphene sheets comprising:

a reactor;

a plurality of electromagnetic emitters;

a plurality of sonic emitters.

9. The apparatus as in Claim 8, wherein said reactor is substantially spherical.