Highly dispersive graphene materials

Abstract

The present invention provides an efficient and effective method for producing graphene material by a high-shear mechanical process for exfoliating a dispersion of natural graphite in a solvent, followed by supercritical exfoliation and drying. This method exfoliates all graphite flakes, resulting in mostly few-layer graphene flakes. This method is more efficient than conventional mechanical exfoliation techniques and completely avoids the need for multiple sampling / centrifugation cycles. The graphene flakes are generally uniform in both size (area) and thickness and do not exhibit agglomeration or aggregation. After drying, this graphene material is highly dispersible in appropriate solvents, and the prepared graphene dispersion is stable for at least three months without showing any signs of settling or separation.

Description

[Technical Field]

[0001] The present invention relates to a method for producing graphene material by a high shear mechanical process for exfoliating a dispersion of natural graphite in a solvent, followed by supercritical exfoliation and drying. The present invention also relates to graphene material flakes having a generally uniform size and thickness, with at least 90% of the flakes having a thickness of less than 25 nm. [Background technology]

[0002] By its strict scientific definition, graphene is an allotrope of carbon consisting of a single layer of atoms arranged in a two-dimensional honeycomb lattice. Graphene is a carbon sp 2 Graphene is a hybrid form of carbon, i.e., the graphitic form of carbon. A variety of remarkable properties have been attributed to graphene, including high electrical and thermal conductivity, strength at least 100 times greater than that of steel, uniform absorption of light across the visible and near-infrared portions of the spectrum, and potential suitability for use in spin transport. Graphene possesses very high electron mobility, providing a high level of electronic conduction due to the occurrence of free pi (π) electrons for each carbon atom. However, true single-layer graphene (SLG) is currently available only in small quantities, primarily for laboratory use, and large-scale industrial applications of true graphene are unlikely unless current issues of high cost and low throughput are adequately addressed.

[0003] Indeed, the scientific and industrial communities use the term "graphene" for a variety of carbon-based materials that are not actually SLGs. For example, the term "few-layer graphene" or FLG is used to refer to carbon-based materials consisting of flakes with 2–5 or 2–10 atomic layers, depending on the source. The term "graphene nanoplatelets" or GNPs is used to refer to graphitic carbon in the form of flakes up to 25 or 50 atomic layers thick, again depending on the source. Carbon in the form of flakes with more than 25 or 50 layers is often referred to as "graphite," although some refer to such materials as "micrographite." Functionalized graphene materials, such as graphene oxide (GO) and reduced graphene oxide (rGO), are often referred to simply as "graphene" in the literature.

[0004] Atomic force microscopy (AFM) is a common technique used to observe the morphology, particularly thickness, of individual graphene flakes, i.e., SLGs, FLGs, and GNPs. Imaging and thickness measurement of graphene flakes is relatively easy with AFM. Statistical AFM data on flake thickness, while difficult to collect, can reveal the quality of a particular batch of graphene in terms of the overall distribution of flake thickness (and area) within the batch. However, reference sources have shown various values ​​for the thickness of SLG flakes, ranging from approximately 0.4 nm to 1.7 nm. While the interlayer spacing in graphite is known to be approximately 0.34 nm, in practice, this number is difficult to reproduce when measuring the thickness of graphene flakes via AFM, and measured thicknesses are often larger than the theoretical thickness. For example, some published papers use 0.5 nm as a conversion factor for estimating the number of atomic layers from AFM thickness measurements, while others use 1.0 nm.

[0005] For this reason, AFM thickness measurements in nm cannot be directly converted to thickness in terms of atomic layers. However, AFM remains a useful and practical method for comparing the relative quality of different graphene materials. It is generally accepted that thinner graphene flakes (SLG, FLG, GNP) are preferable to thicker flakes because they tend to have more "graphene-like" and less "graphite-like" features.

[0006] Although FLG and GNP may not exhibit the same unique properties as SLG, they still exhibit exceptional and useful electrical and thermal conductivity, mechanical properties, high chemical stability, and non-toxicity, which play important roles in a variety of applications such as composites, displays, sensors, medical devices, and energy storage. Currently, graphene dispersions are being widely investigated for use in printed electronics and surface coatings, prepared by printing, vacuum plating, and spray coating.

[0007] However, widespread application of graphene materials is hindered by the fact that commercially available materials are generally of low quality. These commercially available graphene materials contain a large fraction of graphite. They also have a wide distribution of flake sizes (i.e., areas), typically varying from less than 1 μm to hundreds of microns. Some commercially available graphene materials also exhibit a D band (sp 3 hybridized carbon) and G band (sp 2 They exhibit high levels of defects in their carbon bonding structure, as determined by the ratio of the intensities of the carbon nanotubes (representing hybridized carbons) to the carbon nanotubes (representing hybridized carbons). Such low-quality graphene materials do not exhibit the expected levels of electrical / thermal conductivity or mechanical reinforcement when used in composites. They are also difficult to disperse and apply as coatings or films due to the large fraction of graphite flakes.

[0008] The separate layers of graphene in graphite are held together by van der Waals forces, which can be overcome during the exfoliation of graphite to graphene. Exfoliation in this context refers to the process of overcoming the van der Waals forces holding the individual atomic carbon layers together, thereby separating thicker graphite flakes into SLGs, FLGs, or GNPs. This can be achieved, for example, by subjecting graphite to sufficiently high mechanical shear forces.

[0009] Researchers have developed many different methods for graphene production, which can generally be categorized as bottom-up and top-down techniques. However, both routes have several drawbacks. Bottom-up techniques, such as micromechanical cleavage, chemical vapor deposition, and growth on different substrates, are extremely difficult to use for effective mass production, in contrast to top-down techniques. Among the main top-down methods, graphite can be exfoliated by mechanical and chemical (redox) routes. Chemical exfoliation processes produce defective graphene materials, such as graphene oxide and reduced graphene oxide, which has low conductivity. Mechanical exfoliation is simpler, more scalable, and does not involve harsh chemicals. In the past few years, supercritical fluids have been explored for the exfoliation of graphite in carbon dioxide, ethanol, and dimethylformamide. However, these methods do not achieve complete exfoliation, and the unexfoliated material, i.e., residual graphite flakes, must be removed by multiple sampling / centrifugation cycles, making the process expensive and impractical for commercial-scale production.

[0010] Currently known techniques for preparing graphene materials often result in a heterogeneous distribution of flakes with different sizes (lateral dimensions ranging from tens of nanometers to tens of microns) and thicknesses (number of individual carbon layers ranging from one to hundreds or more). Because structural parameters strongly influence mechanical, electrical, and other types of properties, highly polydispersed materials can lead to unpredictable behavior, which is a significant bottleneck for many applications.

[0011] There is a need to develop new, simpler methods for synthesizing graphene that use inexpensive starting graphite materials and do not involve harsh chemicals or high temperatures. [Brief explanation of the drawings]

[0012] [Figure 1] Block diagram of the process for producing graphene from graphite via mechanical and supercritical exfoliation. Tc: critical temperature. Pc: critical pressure. [Figure 2] 1 is a SEM / STEM image of a graphene material of the present invention. [Figure 3A] Comparative SEM / STEM images of commercial products. [Figure 3B] Comparative SEM / STEM images of commercial products. [Figure 4] 2D and 3D AFM images of (A) the graphene material of the present invention; (B) the commercial product "B." [Figure 5] FIG. 1 shows the statistical distribution of flake thickness for the graphene material of the present invention (top), and for commercial graphene products A (bottom left) and B (bottom right). [Figure 6] FIG. 1 shows the percentage of few-layer graphene (FLG), graphene nanoplatelets (GNP), and graphite in the graphene material of the present invention (top), and in commercial graphene products A (bottom left) and B (bottom right). [Figure 7] FIG. 1 shows TGA, Raman, and BET surface area data for the graphene material of the present invention (top), and commercial graphene products A (middle) and B (bottom). [Figure 8] FIG. 1 shows dispersions in IPA of graphene material of the present invention (left and center) and comparative commercial product B (right). DETAILED DESCRIPTION OF THE INVENTION

[0013] [Definition] The terms "graphene" or "graphene material" as used herein are interchangeable and both are defined as a mixture of graphitic carbon flakes having a thickness of 25 nm or less as measured by AFM. The terms graphene or graphene material refer to a material consisting of flakes of SLG, FLG, GNP, or any mixture of the three in any ratio. Carbon flakes with a thickness greater than 25 nm are defined herein as "graphite." A "supercritical fluid" is any substance at a temperature and pressure above its critical point where no distinct liquid and gas phases exist, but below the pressure required to compress it into a solid. "Supercritical conditions" are characterized by the critical temperature, critical pressure, and concentration of the initial reactants in the solvent that produce the supercritical fluid. For example, the critical temperature and pressure of water are approximately 374°C and 3200 psi, respectively. The critical temperature and pressure of IPA are approximately 235.3°C and 690 psi.

[0014] The present invention provides an efficient and economical method for graphene synthesis that uses inexpensive starting graphite materials and does not involve harsh chemicals or high temperatures. The method produces high-quality graphene material containing bulk quantities of mostly individual particles or flakes (i.e., FLGs) less than 10 nm thick, with a high degree of uniformity and homogeneity among individual flakes in terms of the number (thickness) and size (planar area) of carbon layers in each flake. Such materials are useful as thin-film coatings for electrical, electronic, or sensor devices, as corrosion-resistant or flame-retardant coatings, or as additives in polymer, metal, or ceramic matrix composites for improved electrical, thermal, or mechanical properties. The method is scalable to industrial levels and cost-effective.

[0015] The present invention provides a method for preparing graphene material, comprising the steps of: (a) dispersing graphite powder in a solvent to form a dispersion; (b) dispersing the dispersion to 10 6 ~10 9 s -1 (c) shearing the sheared dispersion of step (b) at a shear rate of 1000 rpm to exfoliate the graphite into graphene material; (c) heating the sheared dispersion of step (b) to reach and maintain supercritical conditions for a period of time; and (d) drying the graphene material by removing the solvent from the dispersion while maintaining the dispersion above the critical temperature of the solvent, thereby preserving the exfoliated state of the graphene material. The method combines and optimizes several individual steps, including exfoliation by mechanical and supercritical fluid techniques. The method provides graphene material with little or no structural damage and a high degree of uniformity in both flake size and thickness.

[0016] The starting material is preferably high-purity graphite powder to ensure that the properties of the final graphene material product are not compromised by contamination. The graphite is preferably at least 99% pure, more preferably at least 99.5% or 99.9% pure. The graphite powder may be natural graphite obtained, for example, from mines, or synthetic graphite. Natural graphite is preferred primarily for cost reasons. Natural graphite is commercially available in high quality and purity and is produced in large quantities. Examples of such material include natural crystalline graphite from RS Mines (Sri Lanka) or Graphit Kropfmuhl GmbH (Germany). The starting material is preferably pretreated with an intercalation compound and does not contain expandable graphite, which adds cost and impurities to the process.

[0017] The method of the present invention for producing graphene from graphite is summarized in Figure 1. In step 102, graphite powder is first combined with a suitable solvent and dispersed to form a mixture in a suitable container. A suitable solvent has critical temperatures and pressures that are within the capabilities of existing laboratory or industrial equipment. A suitable solvent is chemically compatible with the equipment. For example, the equipment used in the process may have O-rings, seals, fittings, etc. that come into contact with the dispersion, so the materials comprising these parts must be resistant to the selected solvent. Suitable solvents include organic solvents such as isopropanol (IPA), ethanol, butanol, dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), toluene, ortho-dichlorobenzene, and N,N-dimethylacetamide. IPA, ethanol, DMF, and NMP are preferred solvents. Suitable solvents also include aqueous solvents such as water. However, ionic or nonionic surfactants (e.g., sodium deoxycholate) can be added to keep the graphite suspended in water and prevent it from settling. The listed organic solvents have much lower critical pressures, about 40-63 bar, or about 580-914 psi, compared to the critical pressure of water, which is 220 bar or about 3200 psi.

[0018] The mixture is then mixed, for example, by sonication, mechanical stirring, homogenizer mixing, and / or rotor-stator mixing, to form a dispersion. To effectively pulverize the graphite powder particles and uniformly disperse them in the solvent, the sonication time is typically 1 to 3 hours, and the shear mixing time is 30 minutes to 1 hour. The graphite concentration in the dispersion can be 1 to 100 g / L, preferably 3 to 48 g / L, and more preferably 6 to 24 g / L. The preferred concentration range is selected to provide an appropriate balance between process efficiency and effective exfoliation of graphite to graphene. Higher concentrations increase productivity but somewhat reduce exfoliation effectiveness and product quality. Lower concentrations result in better-quality graphene material but at the expense of throughput.

[0019] At 104, the dispersion is subjected to a high shear rate to exfoliate the graphite into graphene material. 6 ~10 9 s -1 This refers to a shear rate of 1000 psi. Any method capable of achieving such a shear rate is within the scope of the present invention. In one embodiment, high shear rates are achieved by forcing the dispersion through an orifice under high pressure. Commercially available equipment is available for processing dispersions in this manner. Forcing the dispersion through an orifice under high pressure is preferred over other mechanical exfoliation methods because high shear rates can be applied throughout the dispersion. Process parameters such as orifice size, applied pressure, and graphite concentration in the dispersion can be optimized. For example, a smaller orifice and / or higher system pressure both result in further exfoliation, thinner flakes, and larger specific surface areas due to the cumulative effect on the graphite particles or increased shear forces applied to them. A higher initial concentration of graphite in the dispersion allows a larger amount of graphene material to be prepared in a given time period.

[0020] In one embodiment, the dispersion is passed through an orifice having a size of 200 μm or less, preferably 100 μm or less.

[0021] In one embodiment, the dispersion is passed through the orifice under an applied pressure of about 20,000 to about 40,000 psi, preferably about 25,000 to 35,000 psi.

[0022] In one embodiment, the dispersion is passed through an orifice having a size of 100 μm or less under an applied pressure of about 25,000 to 35,000 psi.

[0023] It may be advantageous to pass the dispersion through the orifice under pressure multiple times. Such repeated high shear treatments can have a cumulative effect on the graphite particles, providing improved exfoliation, thinner flakes, and greater specific surface area. In one embodiment, the dispersion is passed through the orifice 10 to 200 times, preferably 50 to 150 times.

[0024] Graphene is formed in the dispersion after the high shear process of 104. However, subsequent steps 106 and 108 are important to maintain the exfoliated state of the graphene material as it is processed into a dry powder.

[0025] At 106, the sheared dispersion is processed by supercritical exfoliation by placing an appropriate amount of the dispersion in a pressure vessel or autoclave so that supercritical conditions for the solvent can be achieved without exceeding the temperature or pressure limits of the vessel. Supercritical conditions are characterized by a critical temperature, critical pressure, and the concentration of the initial reactants in the solvent that create a supercritical medium. When a substance is subjected to pressure and temperature above its critical point, it undergoes a physical change and is called a "supercritical fluid."

[0026] The vessel is heated above the solvent's critical temperature (e.g., 235.3°C for IPA) so that, with the appropriate amount of solvent present, the pressure within the vessel exceeds the solvent's critical pressure (e.g., 47.6 bar for IPA). Typically, the sheared dispersion is heated from room temperature to above the solvent's critical temperature within 12 to 48 hours. The vessel is held at these conditions for a minimum of approximately 1 hour to ensure complete transition of the solvent from an equilibrium liquid and vapor state to a homogeneous fluid (supercritical fluid). After this step, the conversion of graphite to graphene material is complete.

[0027] At 108, the graphene material is dried by removing the solvent from the dispersion while maintaining the dispersion above the solvent's critical temperature to maintain the exfoliated state of the graphene material. The supercritical fluid is slowly removed from the vessel by opening a valve that allows control of the outward flow rate while maintaining a constant vessel temperature. As the fluid is released, the pressure inside the vessel is reduced and the fluid is converted to vapor. The rate of pressure reduction inside the vessel is approximately 0.5 to 30 psi / min, preferably 1 to 20 psi / min or 1 to 10 psi / min, and more preferably 2 to 8 psi / min. The released vapor condenses back into the liquid solvent and is recovered for reuse. All vapor is removed, the vessel is fully depressurized, and then cooled to room temperature. The final product, a graphene powder containing mostly SLG and FLG with a smaller proportion of GNPs, is removed from the vessel.

[0028] Drying of graphene materials in a pressure vessel under supercritical conditions of the dispersing solvent provides further exfoliation and higher specific surface area compared to other drying techniques, while enabling scale-up of the process.

[0029] The supercritical processing and drying steps 106 and 108 at least prevent the graphene from restacking into thicker flakes. Supercritical processing can actually further exfoliate the graphene flakes in the dispersion into thinner flakes. Other drying methods (e.g., air drying, oven drying, vacuum drying, freeze drying) can cause the exfoliated flakes to restack, resulting in thicker flakes, lower specific surface area, and reduced properties.

[0030] When preferred solvents are used, the process does not require high pressures (greater than about 100 bar or about 1450 psi) which would be a significant obstacle to large-scale production, since pressure vessels capable of withstanding higher pressures tend to be very expensive, especially on an industrial scale.

[0031] In this process, once the dispersion is prepared, it is moved from one step to the next, preferably without any changes, modifications, or interruptions, until the final step, where dry graphene powder is obtained as the final product. The process is efficient and does not require multiple post-processing steps of washing, filtering, and drying. The process also does not require a centrifugation step after exfoliation to separate unexfoliated material from the graphene. The process is fully scalable for commercial use, most easily by increasing the size of the mechanical exfoliation equipment and / or pressure vessel.

[0032] In this process, over 90% of the solvent is easily recoverable and reusable. There are minimal process-related losses. The process does not require the use of multiple solvents or any solvent exchange procedures, as is required, for example, when drying under supercritical carbon dioxide (CO2).

[0033] The process produces highly uniform, undamaged graphene flakes that are free of impurities, have a narrow combustion temperature range, and a high specific surface area.

[0034] The graphene material obtained by this process (i) uses a graphite powder starting material in the micron range, (ii) applies a high shear process by passing the dispersion through an orifice at high pressure to exfoliate the graphite into graphene, and (iii) further exfoliates and dries the material under supercritical conditions of the dispersion solvent, thereby reducing the flake thickness (number of carbon layers) while maintaining the flake size obtained in the final dried powder material, resulting in a very uniform flake size (area) and flake thickness (number of carbon layers).

[0035] In one embodiment, the present invention relates to a graphene material having less than 20% or less than 10% unexfoliated graphite with flakes greater than 25 nm in thickness as determined by statistical analysis by atomic force microscopy (AFM).

[0036] In one embodiment, the present invention relates to a graphene material having more than 80%, more than 85%, or more than 90% of the flakes having a thickness of less than 25 nm.

[0037] In one embodiment, the present invention relates to a graphene material having more than 60%, or more than 65%, or more than 70% of the flakes less than 10 nm thick as determined by statistical analysis by AFM.

[0038] In one embodiment, the present invention relates to a graphene material having more than 80%, more than 85%, or more than 90% of the flakes less than 25 nm thick, and more than 60%, or more than 65%, or more than 70% of the flakes less than 10 nm thick, as determined by statistical analysis by AFM.

[0039] In one embodiment, the present invention relates to graphene materials having an average flake thickness of about 8-12 nm, or about 10 nm.

[0040] The graphene material of the present invention can be dispersed in organic solvents such as IPA without additional ingredients such as dispersants or surfactants, and the dispersion is stable for at least 3 months and shows no settling, separation, or aggregation, whereas dispersions of commercial products show complete separation and settling of the solid material within about 1 day.

[0041] The stable dispersions of the graphene materials of the present invention are useful for applying thin films or coatings, for example, by spray coating, dip coating, inkjet printing, screen printing, stencil printing, etc. Such films may be useful in applications such as corrosion-resistant coatings, thermal barrier coatings, electromagnetic interference (EMI) shielding, or as conductive or active electrode layers in energy storage devices. The stable dispersions of graphene are also useful for adding graphene as a filler to polymers in extrusion compounding processes to create composites with improved mechanical, electrical, and / or thermal properties. Such composites may be useful, for example, as structural components, EMI shielding layers, thermal barriers or insulators, or as flame retardants.

[0042] The following examples further illustrate the present invention. These examples are intended to be merely illustrative of the invention and should not be construed as limiting. [Example]

[0043] Example 1. Mechanical and Supercritical Exfoliation of Graphite Dispersions in IPA A graphite dispersion with a concentration of 6 g / L was prepared by combining 3.6 grams of natural crystalline graphite powder (Graphit Kropfmuhl / AMG Graphite type SGA 20, average particle size 20 μm) in 0.6 L of IPA. The mixture was treated by ultrasonic bath treatment for 3 hours, followed by rotor-stator mixing for 1 hour to form a dispersion. The entire volume of the dispersion was then subjected to a high shear process (>10 s) by passing it through a 100 μm orifice under a pressure of 30,000 psi. 6 s -1The exfoliated material dispersed in IPA was transferred to a glass beaker, which was then placed in a 1-liter stainless steel pressure vessel. The vessel was sealed and heated to 240°C within 48 hours, after which the IPA was removed, thereby drying the graphene material.

[0044] The detailed procedure for drying in the pressure vessel was as follows. (a) The graphene / IPA dispersion was weighed into a glass beaker, which was then placed in a container. For a 6 g / L dispersion, 183 g of dispersion was used, which corresponds to approximately 181.6 g of IPA and approximately 1.4 g of graphene material. (b) The vessel was heated from room temperature to 240°C within 48 hours. (c) During heating, pressure and temperature were monitored and recorded to ensure that conditions followed the liquid-vapor equilibrium curve of IPA, exceeding critical conditions after reaching 240°C. The critical temperature and pressure for IPA are approximately 235.3°C and approximately 47.6 bar (690 psi). In this example, the maximum pressure reached at 240°C was 740 psi. Thus, supercritical conditions were achieved. (d) The vessel was held at 240°C for approximately 1 hour and then depressurized by slowly releasing supercritical IPA fluid over a period of approximately 3 hours using a low-flow stainless steel metering valve. The depressurization rate was approximately 4 psi / min. The released IPA fluid was condensed and recovered for recycling or reuse. (e) After atmospheric pressure was reached, nitrogen was passed through the vessel to ensure all IPA vapors were purged. The vessel was allowed to cool naturally to room temperature, and then the dried graphene material was removed for characterization.

[0045] For quality and purity assessment of the final graphene product, various microscopic and spectroscopic techniques were used, including thermogravimetric analysis (TGA), Raman spectroscopy, scanning electron microscopy in secondary and transmission modes (SEM / STEM), atomic force microscopy (AFM), and nitrogen physisorption analysis (BET surface area). Results were compared side-by-side with comparable commercially available graphene materials (Product A and Product B).

[0046] [Scanning Electron and Scanning Transmission Electron Microscopy (SEM / STEM)] SEM / STEM images of the graphene material product were recorded at various voltages and magnifications to obtain topographical and morphological information of the graphene material. Based on the microscopic images, the present method successfully exfoliated all graphite flakes into few-layer graphene (FLG) flakes, most of which were 10 nm thick or less and approximately 0.5-1.0 μm in area size. Based on the STEM images, the graphene flakes were generally translucent to the electron beam, uniform, and showed no agglomerations or aggregations (Figure 2). In contrast, the commercially available graphene materials, Product A and Product B, showed significant presence of opaque flakes with widely varying area sizes (Figure 3).

[0047] The purpose of Figures 2 and 3 is to compare the appearance of graphene flakes produced by this process, shown in Figure 2, with that of commercially available material, shown in Figure 3. Two types of micrographs are shown. The top set of two images are SEM images taken in "secondary electron" mode, a standard SEM mode. This provides information primarily about the surface features and topography of the material. The bottom set of two images are in transmission mode (STEM). In these images, we are looking "through" the material. In the STEM images, the gray background in the shape of a white circle is the carbon film, i.e., the sample holder, onto which the graphene flakes are deposited for observation.

[0048] Figure 2 (top left) shows that the flakes of the present invention are highly uniform in size, i.e., area (approximately 0.5-1.0 μm per side). The flakes of the commercially available materials (Products A and B), shown in Figure 3, are larger and have a much broader size distribution. The size uniformity of the flakes of the present invention provides an advantage in that the properties and behavior of the material are more consistent and predictable.

[0049] Figure 2 (bottom right) shows that the graphene flakes of the present invention are translucent to an electron beam, i.e., they are extremely thin, with most flakes consisting of 10 or fewer atomic carbon layers. In contrast, commercially available graphene materials have a large proportion of much thicker flakes that appear completely opaque (dark), as seen in the images below Products A and B in Figure 3. These flakes are more accurately described as graphite rather than graphene. Materials of the present invention, having primarily thin flakes (more than 90% less than 25 nm thick), are advantageous for use, for example, as conductive coatings or as conductive fillers in polymer matrix composites to enhance their thermal, electrical, or mechanical properties, such as their electrical conductivity, thermal conductivity, or tensile strength.

[0050] [Atomic Force Microscopy (AFM)] Graphene produced by the method described in this example, as well as comparable commercially available materials, Product A and Product B, were each dispersed in dimethylformamide (DMF) by sonication for 1 hour and deposited dropwise onto freshly cleaved mica substrates. AFM was performed under ambient conditions in tapping mode. Imaging was achieved simultaneously in the regimes of tip-sample interaction, recording height, Z-sensor, amplitude, and phase retrace. For each material, a statistically significant number of individual flakes were measured for thickness to generate a distribution of flake fraction versus thickness in the material. Typical 2D and 3D images of the graphene material of the present invention and the commercially available material are shown in Figure 4.

[0051] The 2D image on the left of Figure 4 shows where graphene flakes are located on the mica substrate. The graphene flakes appear as white, irregular shapes, while the exposed mica is dark gray. The image in the top left of Figure 4 (A) is of the graphene material of the present invention; the flakes appear as white spots, all very small, less than 1 μm on a side. The image in the bottom left of the commercial material (B) shows larger flakes up to 4–5 μm in size, as well as smaller flakes.

[0052] The 3D image on the right of Figure 4 shows the approximate height of the flakes that appear in the 2D image. The positions of the protruding "spikes" in the 3D image correspond to the positions of the flakes in the 2D image.

[0053] Based on AFM analysis, the graphene material prepared by the method of the present invention exhibits individual flake thicknesses in the range of 1-120 nm, whereas Product A and Product B were found to have flakes in the range of 3-900 nm and 1-1000 nm, respectively.

[0054] To generate statistical data, we measured the height of each flake individually. For the material of the present invention, Product A, and Product B, we performed individual measurements of 1026 flakes, 785 flakes, and 1017 flakes, respectively. These measurements provide a clear distinction between the qualities of the materials (Figure 5).

[0055] Figure 5 shows the distribution of flake thickness for each material in terms of the percentage of all flakes measured. For clarity, the x-axis (thickness) in these plots is limited to 500 nm for all materials, and the insets provide further clarity for the graphene materials of the present invention. The solid line in each chart indicates cumulative % data, and the arrows point to the second y-axis (cumulative %) on the right side of each chart. The difference in thickness distribution between the graphene materials of the present invention and commercial products is readily apparent.

[0056] Further data analysis reveals that, as shown in Figure 6, 91% of the flakes in the graphene material of the present invention are in the 1-25 nm range, 70% of the flakes are 1-10 nm thick, and 21% of the flakes are 11-25 nm thick. In contrast, only 48-49% of the flakes in Product A and Product B are in the 1-25 nm range. The commercially available material contains a mixture of few-layer graphene (FLG), graphene nanoplatelets (GNP), and unexfoliated graphite flakes, with the latter representing over 50% of the total flakes. Based on measurements of 1,026 individual flakes, the average flake thickness of the graphene material prepared by the method of the present invention was 10 nm. Based on measurements of 785 and 1,017 individual flakes, the average flake thicknesses of Products A and B were 43 nm and 21 nm, respectively.

[0057] [Thermogravimetric analysis (TGA)] TGA measurements were performed using a Pyris 1 analyzer (Perkin Elmer, Waltham, MA), and weight was recorded as a function of increasing temperature. Measurements were performed with samples in a purified air atmosphere with a gas flow rate of 20 mL / min. The samples were heated from 25°C to 120°C at 20°C / min, held at 120°C for 60 minutes, then heated from 120°C to 900°C at 5°C / min, and held at 900°C for 2 hours.

[0058] The properties of different graphene materials can be qualitatively compared by examining their respective differential weight loss curves, as shown in Figure 7. The material prepared by the current method exhibits a narrow combustion range, with a peak occurring at a relatively low temperature of 693 °C. Combustion is already complete when temperatures reach approximately 750 °C. In contrast, commercial product A exhibits a broad combustion temperature range that continues to 900 °C. The predicted peak combustion temperature was 942 °C. Commercial product B exhibits a lower peak combustion temperature compared to product A, but still exhibits a much broader combustion range compared to the material of the present invention, with combustion continuing to nearly 900 °C. In the TGA profile, the differential peak position corresponds to the average combustion temperature. The differential peak width is an indicator of the combustion temperature distribution. The higher combustion temperature and wider combustion temperature distribution of the graphene material correspond to a more inhomogeneous material with a larger overall number of carbon layers and flakes with a wider distribution of carbon layers, respectively.

[0059] Based on the higher combustion temperature and wider combustion temperature distribution, commercially available graphene materials contain a mixture of exfoliated flakes (SLG, FLG, and GNP) and unexfoliated graphite, as combustion is observed at temperatures up to at least 900 °C. In contrast, the graphene material of the present invention exhibits a lower combustion temperature and a very narrow combustion temperature distribution, indicating that the graphene flakes are very uniform. Furthermore, it was confirmed that all carbon was consumed before reaching 750 °C, and the graphite was almost completely exfoliated into SLG, FLG, and GNP.

[0060] [Raman spectroscopy] Raman spectroscopy is extremely sensitive to the geometric structure and bonding within a molecule. Differences in bonding between various allotropes of carbon cause different peak shifts or bands in their Raman spectra. Graphene materials characteristically exhibit bands designated as the G band, D band, and 2D band. The G band is the sp band, which is the normal structure of graphite and graphene. 2 The D band corresponds to the sp 3It corresponds to hybridized carbon bonds and indicates imperfections or defects in the structure. The 2D band is the result of two phonon lattice vibration processes and does not indicate defects, but is present in all graphene materials. The intensity ratio of the D band to the G band (I D / I G ) is a useful indicator of the concentration of defects in a material.

[0061] Raman data for the graphene material of the present invention and the commercially available graphene material were collected using a 532 nm green laser. As expected, the Raman spectra of all graphene materials contained the G band, D band, and 2D band. Among the three materials, the highest I of 0.3 (most defects) was observed. D / I G The ratio was calculated for commercial product A, but product B had an I of 0.18. D / I G The material prepared by the method of the present invention exhibited a Raman I ratio of 0.18 (Figure 7). D / I G The ratios shown were comparable to or better than commercially available materials.

[0062] [Physical adsorption analysis] The specific surface area of ​​the graphene materials was determined by standard nitrogen physisorption analysis according to the model of Brunauer, Emmett, and Teller or the well-known BET method.

[0063] The BET specific surface area of ​​the graphene product of the present invention, processed by mechanical and supercritical exfoliation of graphite in IPA, is approximately 62 m 2 / g. In contrast, commercial products A and B were measured to be 19 and 35 m / g, respectively. 2 It was determined that the specific surface area was 1000 Å / g (FIG. 7).

[0064] It should be noted that BET surface areas determined using conventional nitrogen adsorption / desorption methods do not accurately represent the true specific surface area of ​​most types of graphene materials. This is because individual flakes tend to stack on top of each other when in dry powder form, effectively blocking much of the surface area available for adsorption of nitrogen molecules.

[0065] The theoretical specific surface area of ​​SLG is 2630m 2 / g, and the BET surface area of ​​the material of the present invention is 62 m 2 / g. Based on these numbers, the average number of atomic layers is about 42 (i.e., 2630 / 62). Via AFM, the material of the present invention exhibits an average flake thickness of about 10 nm, which, when converted using a graphite interlayer spacing of 0.34 nm, equates to about 30 atomic layers.

[0066] Products A and B are 19 and 35 m 2 They exhibited BET surface areas of 138 and 75 atomic layers based on the theoretical surface area of ​​SLG, while their average AFM flake thicknesses of 42 and 21 nm were equivalent to approximately 123 and 62 atomic layers, respectively, using a graphite interlayer spacing of 0.34 nm.

[0067] Clearly, whether compared by BET surface area or AFM thickness measurements, the material of the present invention consists of substantially thinner flakes with, on average, significantly fewer atomic layers than either of the commercial graphene products A and B. Furthermore, the material of the present invention has a much narrower distribution of flake thicknesses.

[0068] Example 2 [Preparation of graphene dispersion in IPA] Dispersions of commercial products A and B and the dry powder graphene material of the present invention were prepared in IPA at concentrations of 0.5 g / L and 3 g / L by 3-hour ultrasonic bath treatment. No dispersants or surfactants were added to the graphene material and IPA mixture. Dispersions of the graphene product of the present invention processed by mechanical and supercritical exfoliation were stable for at least 3 months and showed no visible settling, separation, or aggregation that precipitated from the solvent or settled to the bottom of the mixture when mechanical agitation was not applied (Figure 8, left and center). In contrast, dispersions prepared using commercial products A (not shown) or B (Figure 8, right) were unstable and showed complete separation and settling of the solid material within approximately one day.

[0069] From the foregoing, it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the scope of the invention.

Claims

1. 1. A method for preparing a graphene material, comprising: (a) dispersing graphite powder in a solvent to form a dispersion; (b) Mix the dispersion with 10 ６ ~10 ９ s －１ shearing the graphite at a high shear rate of 0.1 to 1.5 to exfoliate the graphite into graphene material; (c) heating the sheared dispersion of step (b) to reach and maintain supercritical conditions of the solvent for a period of time; (d) drying the graphene material by removing the solvent from the dispersion while maintaining the dispersion above the critical temperature of the solvent to maintain the graphene material in an exfoliated state; A method comprising:

2. 10. The method of claim 1, wherein the dispersing in step (a) comprises homogenizer mixing, mechanical stirring, sonication, rotor-stator mixing, or any combination thereof.

3. 10. The method of claim 1, wherein the shearing step (b) comprises forcing the dispersion through an orifice under pressure.

4. 2. The method of claim 1, wherein the concentration of graphite powder in the solvent is 3 to 48 g / L.

5. 10. The method of claim 1, wherein the concentration of graphite powder in the solvent is 6 to 24 g / L.

6. The method of claim 1 wherein the solvent is isopropyl alcohol.

7. The method of claim 3 , wherein the orifice size is less than 200 μm.

8. The method of claim 3, wherein the orifice size is 100 μm or less.

9. 4. The method of claim 3, wherein the pressure is 20,000 psi or greater.

10. The method of claim 3, wherein the dispersion is passed through the orifice 50 to 150 times.

11. 10. The method of claim 1, wherein steps (c) and (d) are carried out with the dispersion inside a pressurized vessel or autoclave.

12. 12. The method of claim 11, wherein in step (c), the dispersion is heated from room temperature to above the critical temperature of the solvent within 12 to 48 hours.

13. 12. The method of claim 11, wherein in step (d), the solvent is removed from the dispersion at a pressure drop rate of 1 to 10 psi / min.

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