Graphene powder, method for preparing the same by using carbon dioxide as carbon source and application thereof

Abstract

The application provides a graphene powder and a method for preparing the graphene powder by using carbon dioxide as a carbon source and application of the graphene powder, and the method comprises the following steps: under high vacuum conditions, a stable glow plasma is spontaneously generated in a microwave plasma device by using an inert gas as background gas under the action of microwaves; the flow of the inert gas is increased, the output power of the microwaves is increased, and the pressure of the system is increased to a micro-negative pressure state, so that the glow plasma is transformed, and the system is in a state where the glow plasma and the electric arc coexist; carbon dioxide and a deoxidizing agent are introduced into the microwave plasma device, under the action of the glow plasma and the electric arc, carbon dioxide molecules are cracked by high-energy particles, and carbon species after cracking are nucleated and grown to obtain the graphene powder. The application synchronously provides energy required for carbon source cracking and graphene nucleation and growth by microwave plasma discharge, and does not need additional heat sources, templates and other conditions, and the prepared graphene has high purity and high quality.

Description

Technical Field

[0001] This invention relates to a graphene powder and a method for preparing it using carbon dioxide as a carbon source, as well as its application, belonging to the field of graphene material technology. Background Technology

[0002] Graphene, with its superior properties in sound, light, electricity, and heat, has attracted widespread attention from researchers and has broad application prospects. Currently, the mainstream methods for graphene preparation are divided into two main categories: top-down and bottom-up methods. The top-down method mainly involves exfoliating graphite raw materials to prepare graphene powder, a process involving solution handling that can lead to the retention of more contaminants in the graphene powder. The bottom-up method generally requires higher energy to achieve carbon source pyrolysis to obtain high-quality graphene, and its growth conditions are harsh, resulting in higher production costs. Both contaminant retention and high production costs can affect the application of graphene.

[0003] Currently, the raw materials used to prepare high-quality graphene are mainly high-purity small-molecule substances such as hydrocarbons (methane, ethylene, acetylene, etc.) and alcohols (methanol, ethanol, etc.), which are costly. In the field of fossil energy technology, CO2 emissions remain high, and carbon capture and storage (CFS) technology is an effective way to reduce CO2 emissions. After capturing CO2 through CFS, the captured CO2 can be converted into high-value-added chemicals and fuels. Therefore, CFS technology can realize the resource utilization of CO2, which is an important carbon emission reduction technology and has significant implications for alleviating the energy crisis.

[0004] The standard Gibbs free energy (ΔG) of CO2 is -394.38 kJ / mol. Its inherent thermodynamic stability and kinetic inertness result in low efficiency when directly synthesizing high-value-added chemicals from CO2, with harsh reaction conditions and low product yields. However, the application of plasma technology can effectively reduce the reaction conditions of CO2. For example, microwave plasma can simultaneously provide energy for carbon source cracking and nucleation growth. Therefore, it is necessary to conduct in-depth research in this area. Breakthroughs in this direction are of great significance for reducing CO2 emissions and realizing the resource utilization of CO2.

[0005] Therefore, providing a novel graphene powder and a method for preparing it using carbon dioxide as a carbon source, as well as its application, has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To address the aforementioned shortcomings and deficiencies, one objective of this invention is to provide a method for preparing graphene powder using carbon dioxide as a carbon source. The method provided by this invention uses carbon dioxide, a major greenhouse gas, as the carbon source and employs microwave plasma technology to achieve carbon source decomposition, nucleation, and growth processes within a reaction region, in a reactor (such as...). Figure 1The graphene powder can be collected at the end of the quartz tube, eliminating the need for post-processing and realizing the transformation of carbon dioxide into high-quality, high-value-added graphene powder carbon materials.

[0007] Another object of the present invention is to provide a graphene powder, which is prepared by the above-described method of preparing graphene powder using carbon dioxide as a carbon source.

[0008] Another object of the present invention is the application of the above-described graphene powder as a reinforcing material.

[0009] To achieve the above objectives, the present invention provides a method for preparing graphene powder using carbon dioxide as a carbon source, wherein the method includes the following steps:

[0010] Step (1): Under high vacuum conditions, a stable glow plasma is spontaneously generated in a microwave plasma device with an inert gas as the background gas under the action of microwaves.

[0011] Step (2): Increase the flow rate of the inert gas, increase the output power of the microwave, and increase the system pressure to a slightly negative pressure state so that the glow plasma will change and the system will be in a state where glow plasma and electric arc coexist.

[0012] Step (3): Carbon dioxide and deoxidizer are introduced into the microwave plasma device. Under the action of glow plasma and electric arc, carbon dioxide molecules are bombarded by high-energy particles, namely electrons and ions formed in the process of generating stable glow plasma in step (1), and the carbon species after decomposition nucleate and grow to obtain graphene powder.

[0013] As a specific embodiment of the method described above in this invention, the inert gas includes one or a combination of argon, nitrogen, and helium.

[0014] As a specific embodiment of the method described above in this invention, in step (1), the high vacuum condition includes a pressure of -0.05 to -0.1 MPa.

[0015] In a specific embodiment of the method described above in this invention, in step (1), the flow rate of the inert gas is 200-500 sccm, and the output power of the microwave is 300-600W.

[0016] In step (1) of the method described above in this invention, under high vacuum conditions, the working gas (i.e., the inert gas) is accelerated by a small number of free electrons in the microwave high-frequency electric field to have kinetic energy and to force them to collide with other unionized inert gas molecules, thereby generating enough secondary electrons. When the number of generated electrons is greater than or equal to the number of lost electrons, inert gas breakdown will occur and trigger discharge, at which time a uniform and stable glow plasma can be generated.

[0017] In a specific embodiment of the method described above in this invention, in step (2), the flow rate of the inert gas is 1000-5000 sccm, and the output power of the microwave is 1000-2000W.

[0018] As a specific embodiment of the method described above in this invention, in step (2), the pressure corresponding to the micro-negative pressure state is greater than or equal to -0.02 MPa and less than 0 MPa, that is, the range of pressure (P) is -0.02 ≤ P < 0.

[0019] In one specific embodiment of the method described above, the deoxidizer includes hydrogen and / or water, etc. In the method of this invention, the deoxidizer is used to combine oxygen species generated from CO2 cracking to form atomic C species, thereby forming solid carbon.

[0020] In one specific embodiment of the method described above in this invention, in step (3), the flow rate of carbon dioxide is 5-200 sccm. When the deoxidizer is hydrogen, its flow rate is 40-60% of the carbon dioxide flow rate. When the deoxidizer is water, the water flow rate is controlled by the carbon dioxide flow rate, but it cannot be quantified by monitoring equipment. As long as the carbon dioxide flow rate is 5-200 sccm, the purpose of this invention can be achieved.

[0021] The carbon dioxide used in this invention comes from a wide range of sources, including but not limited to CO2 produced by processes such as fossil fuel combustion and chemical product manufacturing.

[0022] In one specific embodiment of the method described above in this invention, the reaction time in step (3) is more than 1 hour.

[0023] In steps (2) and (3) of the method described above, a slightly negative pressure system is conducive to the removal of reaction products from the reaction system. From the perspective of reaction equilibrium, this can promote the reaction. Step (1) first achieves self-initiation of plasma under high vacuum conditions, i.e., negative pressure conditions. Then, adjusting the system pressure to a slightly negative pressure state helps the plasma maintain an excited state for a long time, preventing chaos or even extinction over time. The system feedback microwave power can remain zero for a long time, meaning that all microwave energy is applied to the working gas, avoiding energy waste. Furthermore, the slightly negative pressure state... Under these conditions, glow plasma and electric arc coexist in a dynamic equilibrium, which can effectively expand the arc range, i.e., expand the nucleation range of carbon source pyrolysis, thus facilitating the occurrence of carbon source pyrolysis nucleation process. In addition, glow plasma and local electric arc (mainly the electric arc part) have ultra-high temperatures, reaching several thousand K, and the effective range is small, presenting a large temperature gradient with the edge of the reactor in the microwave plasma device. The temperature of several thousand K is sufficient to provide energy for carbon source pyrolysis, and with the change of temperature gradient, the pyrolysis components gradually undergo nucleation and growth processes under the above-mentioned ultra-high temperature conditions to obtain graphene powder. Furthermore, the micro-negative pressure state is conducive to maintaining the stability of the plasma state for a long time, which can reach more than 1 hour. Therefore, the growth time in step (3) is greatly increased, thereby reducing the difficulty of operation.

[0024] On the other hand, the present invention also provides a graphene powder, wherein the graphene powder is prepared by the method described above for preparing graphene powder using carbon dioxide as a carbon source.

[0025] As a specific embodiment of the graphene powder described above in this invention, Raman results show that the graphene powder has I D / I G <1, I 2D / I G The concentration is 0.3-0.4%, and the impurity content is ~0%.

[0026] In another aspect, the present invention also provides the application of the above-described graphene powder as a reinforcing material.

[0027] Compared with the prior art, the beneficial technical effects achieved by the present invention include:

[0028] 1) The method for preparing graphene powder using carbon dioxide as a carbon source provided by this invention obtains graphene powder with low impurity content, high degree of graphitization and good quality after treating carbon dioxide with microwave plasma discharge. During the preparation process, the energy required for carbon dioxide cracking and graphene nucleation and growth is provided simultaneously by microwave plasma discharge, without the need for additional heat sources, templates and other conditions, thus avoiding the use of additional energy and promoting energy conservation and consumption reduction. Moreover, the prepared graphene has high purity and high quality, and has great market competitiveness in the fields of reinforcing materials and biomedicine.

[0029] 2) The method for preparing graphene powder using carbon dioxide as a carbon source provided by this invention uses CO2 as the raw material. CO2 is widely available and is a major source of gases that cause the greenhouse effect. This method is expected to realize the conversion of CO2 into high-value-added carbon materials (i.e., graphene powder), which is of great significance for CO2 emission reduction and resource utilization.

[0030] 3) The method for preparing graphene powder using carbon dioxide as a carbon source provided by this invention is simple to operate, easy to achieve batch preparation, and suitable for industrial production. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a schematic diagram of the structure of the microwave plasma device used in an embodiment of the present invention.

[0033] Figure 2 This is a schematic diagram of the microwave plasma device used in Comparative Example 1.

[0034] Figure 3 The image shows the Raman spectrum of the graphene powder provided in Example 1 of this invention.

[0035] Figure 4 Thermogravimetric curve of graphene powder provided in Example 2 of the present invention.

[0036] Figure 5 This is a scanning electron microscope image of the graphene powder provided in Example 3 of the present invention.

[0037] Figure 6 The image shows a scanning electron microscope image (top view of the substrate surface) of the product obtained in Comparative Example 2.

[0038] Figure 7This is a scanning electron microscope image (front view, also known as a cross-sectional view of the substrate surface) of the product obtained in Comparative Example 2.

[0039] Figure 8 This is a transmission electron microscope image of graphene powder provided in Embodiment 3 of the present invention.

[0040] Explanation of main icon numbers:

[0041] 1a-Carbon dioxide storage tank, 1b-Inert gas storage tank, 1c-Deoxidizer storage tank, 2-Short-circuit piston, 3-Microwave irradiation area, 4-Quartz tube, 5-Three-pin regulator, 6-Circulator, 7-Microwave generator (magnetic head), 8-High voltage power supply, 9-Graphene powder collection tank, 10-Vacuum pump, 11-Exhaust gas pipeline. Detailed Implementation

[0042] It should be noted that the term "comprising" and any variations thereof in the specification, claims, and accompanying drawings of this invention are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or devices.

[0043] The "range" disclosed in this invention is given in the form of a lower limit and an upper limit. It can be one or more lower limits and one or more upper limits, respectively. A given range is defined by selecting a lower limit and an upper limit. The selected lower and upper limits define the boundaries of the particular range. All ranges defined in this way are composable, meaning that any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for specific parameters, it is also expected that ranges of 60-110 and 80-120 are also expected. Furthermore, if the listed minimum range values ​​are 1 and 2, and the listed maximum range values ​​are 3, 4, and 5, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5.

[0044] In this invention, unless otherwise specified, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this invention, and "0-5" is simply a shortened representation of these numerical combinations.

[0045] In this invention, unless otherwise specified, all embodiments and preferred embodiments mentioned in this invention can be combined with each other to form new technical solutions.

[0046] In this invention, unless otherwise specified, all technical features and preferred features mentioned in this invention can be combined with each other to form new technical solutions.

[0047] In this invention, unless otherwise specified, all steps mentioned herein may be performed sequentially or randomly, but are preferably performed sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0048] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. The embodiments described below are some, but not all, embodiments of this invention, and are only used to illustrate the invention, and should not be considered as limiting the scope of the invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0049] Example 1

[0050] This embodiment provides a method for preparing graphene powder using carbon dioxide as a carbon source, which employs a structural schematic diagram as shown below. Figure 1 The microwave plasma device shown is implemented as follows, wherein the microwave plasma device includes:

[0051] The system includes a carbon dioxide storage tank 1a, an inert gas storage tank 1b, a deoxidizer storage tank 1c, a short-circuit piston 2, a microwave irradiation area 3, a quartz tube 4, a three-pin regulator 5, a circulator 6, a microwave generator (magnetic head) 7, a high-voltage power supply 8, a graphene powder collection tank 9, a vacuum pump 10, and an exhaust gas pipeline 11. The carbon dioxide storage tank 1a, inert gas storage tank 1b, and deoxidizer storage tank 1c are connected to the inlet of the quartz tube 4 via pipelines. The short-circuit piston 2 and the three-pin regulator 5 are located on both sides of the microwave irradiation area 3. The quartz tube 4 is located within the microwave irradiation area 3 and is connected to the microwave generator (magnetic head) 7 via the three-pin regulator 5 and the circulator 6. The microwave generator (magnetic head) 7 is powered by the high-voltage power supply 8. The outlet of the quartz tube 4 is connected to the first opening of the graphene powder collection tank 9 via a pipeline. The second opening of the graphene powder collection tank 9 is connected to the exhaust gas pipeline 11 via the vacuum pump 10.

[0052] The method includes the following specific steps:

[0053] Step 1): Check the airtightness of the device;

[0054] Step 2): Turn on the vacuum pump 10. When the system pressure drops to the lowest gauge pressure (generally, a pointer vacuum gauge with a range of -0.1 to 0.15 MPa is used, and the lowest gauge pressure is -0.1 MPa), turn off the vacuum pump 10 and introduce nitrogen gas for gas replacement. Repeat this several times to remove the residual air in the device.

[0055] Step 3): Keep the vacuum pump on. At this time, the system pressure drops to the gauge pressure of -0.1MPa. Introduce 200sccm of nitrogen gas, turn on the high-voltage power supply 8 and set the output power to 300W. After the microwave output stabilizes, the nitrogen gas is broken down to produce glow plasma. The plasma is uniformly light purple. It can be observed that the glow plasma coverage area has exceeded the microwave irradiation range.

[0056] Step 4): After the glow plasma stabilizes, increase the nitrogen flow rate to 1000 sccm and adjust the output power of the high-voltage power supply 8 to 1200W. At this time, the system pressure gradually increases to a slightly negative pressure state (the system pressure rises to approximately -0.01 MPa as indicated by the gauge pressure). The glow plasma state changes, gradually changing from a uniform glow state to a state of mixed glow and arc, that is, arc discharge is generated in a large-scale glow. It can be observed that the plasma coverage area shrinks to slightly larger than the microwave irradiation range. In addition, the stability of the plasma can also be determined by the microwave feedback power. When the microwave feedback power is zero, it indicates that the microwave energy is used to generate plasma and there is no energy waste.

[0057] Step 5): After the plasma stabilizes, 20 sccm of carbon dioxide and 10 sccm of hydrogen are introduced. After the gas flow stabilizes, the plasma color changes to a bright green. Since the end of the system is closer to the vacuum pump at this time, the vacuum level is high and no black powder is seen falling. As the reaction time increases, a large amount of black powder can be seen adhering to the tube wall at the end of the quartz tube. Throughout the reaction process, the high voltage power supply shows that the microwave feedback power remains at zero.

[0058] Step 6): After the reaction lasted approximately 60 minutes, the carbon dioxide flow meter was turned off. Once the plasma color returned to a pale purple, the hydrogen flow meter was turned off. Observation revealed that only a small amount of carbon was deposited in the microwave irradiated area, with an area of ​​approximately 1-2 cm². 2 Adjust the output power of the high voltage power supply 8 to 800W, introduce 20sccm of oxygen to etch the carbon deposits on the tube wall. The carbon deposits on the tube wall can be eliminated in about 5 minutes. Stop the oxygen supply. During the entire etching process, the microwave feedback power displayed by the high voltage power supply remains at zero.

[0059] Step 7): Repeat steps 5) and 6) multiple times to obtain a large amount of graphene powder.

[0060] Example 2

[0061] This embodiment provides a method for preparing graphene powder using carbon dioxide as a carbon source, which employs a structural schematic diagram as shown below. Figure 1 The microwave plasma device shown is implemented as follows, wherein the microwave plasma device includes:

[0062] The system includes a carbon dioxide storage tank 1a, an inert gas storage tank 1b, a deoxidizer storage tank 1c, a short-circuit piston 2, a microwave irradiation area 3, a quartz tube 4, a three-pin regulator 5, a circulator 6, a microwave generator (magnetic head) 7, a high-voltage power supply 8, a graphene powder collection tank 9, a vacuum pump 10, and an exhaust gas pipeline 11. The carbon dioxide storage tank 1a, inert gas storage tank 1b, and deoxidizer storage tank 1c are each connected to the inlet of the quartz tube 4 via pipelines. A gas mixing system is also installed on the pipeline between the carbon dioxide storage tank 1a and the inlet of the quartz tube 4. A bottle (not shown in the figure), a short-circuit piston 2 and a three-pin adjuster 5 are arranged on both sides of the microwave irradiation area 3. A quartz tube 4 is arranged in the microwave irradiation area 3 and is connected to a microwave generator (magnetic head) 7 through the three-pin adjuster 5 and the circulator 6. The microwave generator (magnetic head) 7 is powered by a high-voltage power supply 8. The outlet of the quartz tube 4 is connected to the first opening of the graphene powder collection tank 9 through a pipeline. The second opening of the graphene powder collection tank 9 is connected to the exhaust gas pipeline 11 through a pipeline via a vacuum pump 10.

[0063] The method includes the following specific steps:

[0064] Step 1): Check the airtightness of the device;

[0065] Step 2): Turn on vacuum pump 10. When the system pressure drops to the lowest gauge pressure (-0.1MPa), turn off vacuum pump 10 and introduce helium gas for gas replacement. Repeat this several times to remove residual air from the device.

[0066] Step 3): Keep the vacuum pump on (-0.1MPa), introduce 300sccm of helium, turn on the high voltage power supply 8 and set the output power to 500W. After the microwave output stabilizes, the helium will be broken down to produce glow plasma. The plasma is uniformly light red and can be observed to cover an area that exceeds the microwave irradiation range.

[0067] Step 4): After the glow plasma stabilizes, increase the helium flow rate to 3000 sccm and adjust the output power of the high-voltage power supply 8 to 1500W. At this time, the system pressure gradually increases to a slightly negative pressure state (the system pressure rises to approximately -0.01 MPa as indicated by the gauge pressure). The glow plasma state changes, gradually changing from a uniform glow state to a state of mixed glow and arc, that is, arc discharge is generated in a large-scale glow. It can be observed that the plasma coverage area shrinks to slightly larger than the microwave irradiation range. In addition, the stability of the plasma can also be determined by the microwave feedback power. When the microwave feedback power is zero, it means that the microwave energy is used to generate plasma and there is no energy waste.

[0068] Step 5): Liquid water, which serves as a deoxidizer, is poured into the gas mixing bottle beforehand. After the plasma stabilizes, 60 sccm of carbon dioxide is introduced (water enters the reaction system along with the carbon dioxide gas flow, i.e., water molecules are carried into the reaction system by the carbon dioxide gas flow). After the gas flow stabilizes, the plasma color changes to a bright green. Since the end of the system is closer to the vacuum pump at this time, the vacuum level is high, and no black powder is seen falling. As the reaction time increases, a large amount of black powder can be seen adhering to the tube wall at the end of the quartz tube. Throughout the reaction process, the microwave feedback power of the high voltage power supply remains at zero.

[0069] Step 6): After approximately 90 minutes of reaction, the carbon dioxide flow meter was turned off. Once the plasma color returned to a pale red, observation revealed only a small amount of carbon buildup in the microwave irradiated area, with an area of ​​approximately 2 cm². 2 Adjust the output power of the high voltage power supply 8 to 800W, introduce 30sccm of oxygen to etch the carbon deposits on the tube wall. The carbon deposits on the tube wall can be eliminated in about 5 minutes. Stop the oxygen supply. During the entire etching process, the microwave feedback power of the high voltage power supply remains at zero.

[0070] Step 7): Repeat steps 5) and 6) multiple times to obtain a large amount of graphene powder.

[0071] Example 3

[0072] This embodiment provides a method for preparing graphene powder using carbon dioxide as a carbon source, which employs a structural schematic diagram as shown below. Figure 1 The microwave plasma device shown is implemented as follows, wherein the microwave plasma device includes:

[0073] The system includes a carbon dioxide storage tank 1a, an inert gas storage tank 1b, a deoxidizer storage tank 1c, a short-circuit piston 2, a microwave irradiation area 3, a quartz tube 4, a three-pin regulator 5, a circulator 6, a microwave generator (magnetic head) 7, a high-voltage power supply 8, a graphene powder collection tank 9, a vacuum pump 10, and an exhaust gas pipeline 11. The carbon dioxide storage tank 1a, inert gas storage tank 1b, and deoxidizer storage tank 1c are connected to the inlet of the quartz tube 4 via pipelines. The short-circuit piston 2 and the three-pin regulator 5 are located on both sides of the microwave irradiation area 3. The quartz tube 4 is located within the microwave irradiation area 3 and is connected to the microwave generator (magnetic head) 7 via the three-pin regulator 5 and the circulator 6. The microwave generator (magnetic head) 7 is powered by the high-voltage power supply 8. The outlet of the quartz tube 4 is connected to the first opening of the graphene powder collection tank 9 via a pipeline. The second opening of the graphene powder collection tank 9 is connected to the exhaust gas pipeline 11 via the vacuum pump 10.

[0074] The method includes the following specific steps:

[0075] Step 1): Check the airtightness of the device;

[0076] Step 2): Turn on vacuum pump 10. When the system pressure drops to the lowest gauge pressure (-0.1MPa), turn off vacuum pump 10 and introduce argon gas for gas replacement. Repeat this several times to remove residual air in the device.

[0077] Step 3): Keep vacuum pump 10 on (-0.1MPa), introduce 500sccm of argon gas, turn on high voltage power supply 8 and set the output power to 600W. After the microwave output stabilizes, the argon gas will be broken down to produce glow plasma. The plasma is uniformly light purple and it can be observed that the coverage of the glow plasma has exceeded the microwave irradiation range.

[0078] Step 4): After the glow plasma stabilizes, increase the argon flow rate to 5000 sccm and adjust the output power of the high-voltage power supply 8 to 1800W. At this time, the system pressure gradually increases to a slightly negative pressure state (the system pressure rises to approximately -0.01 MPa as indicated by the gauge pressure). The glow plasma state changes, gradually changing from a uniform glow state to a state of mixed glow and arc, that is, arc discharge is generated in a large glow area, and it can be observed that the plasma coverage area shrinks to slightly larger than the microwave irradiation range. In addition, the stability of the plasma can also be determined by the microwave feedback power. When the microwave feedback power is zero, it means that the microwave energy is used to generate plasma and there is no energy waste.

[0079] Step 5): After the plasma stabilizes, 100 sccm of carbon dioxide and 40 sccm of hydrogen are introduced. After the gas flow stabilizes, the plasma color changes to a bright green. Since the end of the system is closer to the vacuum pump at this time, the vacuum level is high, and no black powder is seen falling. As the reaction time increases, a large amount of black powder can be seen adhering to the tube wall at the end of the quartz tube. Throughout the reaction process, the high voltage power supply shows that the microwave feedback power remains at zero.

[0080] Step 6): After the reaction lasted approximately 120 minutes, the carbon dioxide flow meter was turned off. Once the plasma color returned to a pale purple, the hydrogen flow meter was turned off. Observation revealed that only a small amount of carbon was deposited in the microwave irradiated area, with an area of ​​approximately 2-3 cm². 2 Adjust the output power of the high voltage power supply 8 to 800W, introduce 50sccm of oxygen to etch the carbon deposits on the tube wall. The carbon deposits on the tube wall can be eliminated in about 7 minutes. Stop the oxygen supply. During the entire etching process, the microwave feedback power of the high voltage power supply remains at zero.

[0081] Step 7): Repeat steps 5) and 6) multiple times to obtain a large amount of graphene powder.

[0082] Comparative Example 1

[0083] This comparative example provides a method for preparing graphene powder using carbon dioxide as a carbon source, which employs a structural schematic diagram as shown below. Figure 2The microwave plasma device shown includes a carbon dioxide storage tank 1a, an inert gas storage tank 1b, a deoxidizer storage tank 1c, a short-circuit piston 2, a microwave irradiation area 3, a quartz tube 4, a three-pin regulator 5, a circulator 6, a microwave generator (magnetic head) 7, a high-voltage power supply 8, and a graphene powder collection tank 9. The carbon dioxide storage tank 1a, inert gas storage tank 1b, and deoxidizer storage tank 1c are connected to the inlet of the quartz tube 4 via pipelines. The short-circuit piston 2 and the three-pin regulator 5 are located on both sides of the microwave irradiation area 3. The quartz tube 4 is located within the microwave irradiation area 3 and is connected to the microwave generator (magnetic head) 7 via the three-pin regulator 5 and the circulator 6. The microwave generator (magnetic head) 7 is powered by the high-voltage power supply 8. The outlet of the quartz tube 4 is connected to the first opening of the graphene powder collection tank 9 via a pipeline.

[0084] The method includes the following specific steps:

[0085] Step 1): Check the airtightness of the device;

[0086] Step 2): Purge the reaction system with 1200 sccm of argon gas to purge any residual air from the system;

[0087] Step 3): Keep 1200 sccm of argon gas flowing in, turn on the high voltage power supply 8 and set the output power to 700W. After the microwave output is stable, place the Faraday coil in the microwave irradiation area to initiate plasma, which will then generate plasma arc discharge. The plasma coverage area almost overlaps with the microwave irradiation area and the color is light purple.

[0088] Step 4): After the arc plasma stabilizes, 20 sccm of carbon dioxide and 10 sccm of hydrogen are introduced into the system. After the synthesis gas flow stabilizes, the color of the arc plasma changes from light purple to green. Only a very small amount of powder can be seen falling at the end of the quartz tube. As the reaction time increases, the microwave feedback power begins to fluctuate. After 20 minutes of reaction, the microwave feedback power gradually increases from the initial 0W to 15W. It is necessary to adjust the three-pin regulator 5 to control the microwave feedback power. As the time continues to extend, it is no longer possible to simply adjust the three-pin regulator 5 to control the microwave feedback power. The microwave feedback power continues to deviate from the normal state (0W), and the plasma state becomes unstable. Black substances remain on the quartz tube wall in the microwave irradiation area, and local red heat is observed.

[0089] Step 5): If the reaction continues, after 28 minutes, the plasma state will fluctuate violently and will be unable to maintain the plasma initiation state, resulting in plasma extinction.

[0090] Step 6): Based on step 5), after the reaction has proceeded for 25 minutes, turn off the carbon dioxide flow meter and the hydrogen flow meter. After the plasma color returns to light purple, introduce 20 sccm of oxygen into the system. The carbon deposits on the tube wall can be eliminated in about 5 minutes. Then stop the oxygen supply. During the etching process, the microwave feedback power gradually returns to 0W.

[0091] Step 7): Repeat steps 4) and 6) for about 5 hours. The amount of graphene powder obtained at the tail end is small, and there is obvious local damage to the quartz tube wall (caused by local high temperature due to carbon accumulation on the quartz tube wall, and carbon reacts with the SiO2 material of the quartz tube). White particles are found in the powder collected at the tail end.

[0092] Comparative Example 2

[0093] This comparative example provides a method for preparing graphene using carbon dioxide as a carbon source, which includes the following specific steps:

[0094] Step 1): Check the airtightness of the device;

[0095] Step 2): Place the nickel foil substrate in the center of the microwave irradiation area, turn on the vacuum pump, and when the system pressure drops to the lowest gauge pressure (-0.1MPa), turn off the vacuum pump and introduce argon gas for gas replacement. Repeat this several times to remove the residual air in the device.

[0096] Step 3): Keep the vacuum pump on (-0.1MPa), introduce 200sccm of argon gas, turn on the high voltage power supply and set the output power to 300W. After the microwave output stabilizes, the argon gas will be broken down to produce glow plasma. The plasma is uniformly light purple and it can be observed that the glow plasma coverage area has exceeded the microwave irradiation range.

[0097] Step 4): After the glow plasma stabilizes, introduce 20 sccm of carbon dioxide and 10 sccm of hydrogen into the system. After the gas flow stabilizes, start timing. As time goes on, the nickel foil substrate gradually shows a red-hot state.

[0098] Step 5): After the reaction has lasted for about 60 minutes, the carbon dioxide flow meter and hydrogen flow meter are turned off. By observing that a large amount of carbon deposits are attached to the quartz tube wall near the nickel foil substrate, the nickel foil substrate can hardly be observed. The vacuum pump is turned off. When the system pressure returns to normal pressure, the argon gas is turned off. After the system temperature drops to room temperature, the nickel foil substrate is taken out and it is found that it is covered with black material, while no black powder is collected at the tail end.

[0099] Comparing the embodiments of the present invention with Comparative Example 2, it can be seen that the method provided by the present invention uses microwave plasma as the main generation means. The plasma is sufficient to provide the energy required for carbon source cracking, nucleation, and growth. No external heat source, temperature control, or substrate is required to obtain graphene powder. In contrast, Comparative Example 2 uses low-pressure plasma-enhanced chemical vapor deposition with CO2 as the carbon source to prepare graphene. It uses a nickel foil substrate, a commonly used graphene growth substrate. The carbon source (CO2 in this case) forms a buffer layer under the high temperature of the plasma. Under the action of the plasma electric field, the carbon source continues to grow graphene on the buffer layer. The resulting product is not graphene powder, but rather graphene sheets that tend to be vertically aligned, such as... Figure 6 and Figure 7 As shown.

[0100] Test Example 1

[0101] This test example uses Raman spectroscopy to characterize the graphene powder provided in Example 1. The obtained Raman spectrum is shown below. Figure 3 As shown. From Figure 3 As can be seen from this, it mainly exhibits three distinct peaks, namely the one located at 1340 cm⁻¹. -1 The D peak is located at 1570 cm. -1 The G peak and located at 2680cm -1 Based on the above results, it can be concluded that the product obtained in Example 1 of this invention is indeed graphene.

[0102] Test Example 2

[0103] This test example involves thermogravimetric analysis (at an oxygen atmosphere) of the graphene powder provided in Example 2. The heating range is 100-900℃, and the resulting thermogravimetric curves are shown below. Figure 4 As shown. From Figure 4 As can be seen, after the temperature reaches 700℃, the graphene powder provided in Example 2 of this invention is completely burned off with almost no residue, indicating that the graphene powder does not contain impurities that cannot be decomposed in an oxygen environment.

[0104] Test Example 3

[0105] In this test example, the graphene powder provided in Example 3 and the product obtained in Comparative Example 2 were characterized by scanning electron microscopy to obtain their surface morphology. The obtained scanning electron micrographs are shown below. Figure 5 , Figure 6 and Figure 7 As shown. Figure 5The image shown is a scanning electron microscope image of the graphene powder provided in Embodiment 3 of the present invention. The testing method is to directly coat the graphene powder onto the conductive adhesive and then perform SEM testing. Since there is no substrate present during the growth of the graphene powder, the graphene sheets tend to form wrinkled and clustered graphene spherical particles. Therefore, the SEM image mostly shows this wrinkled particle shape.

[0106] Comparative Example 2 used a substrate in the preparation of graphene. Generally speaking, it is easier to obtain graphene sheets perpendicular to the substrate when growing graphene on a substrate under plasma irradiation. Figure 6 This is a top view obtained from SEM testing of graphene on a substrate. It is a flower-like shape when viewed from a height perpendicular to the substrate surface. This is a representative top view of vertically arranged graphene. Figure 7 This is a representative SEM front view (also known as a substrate surface cross-sectional view) of the graphene obtained in Comparative Example 2, from... Figure 7 As can be seen, this graphene mainly manifests as vertically arranged graphene sheets perpendicular to the substrate surface, with the area above the white line being the graphene sheets and the area below the white line being the substrate. This type of graphene is not free-living; it requires attachment to a substrate to exhibit this state. Without a substrate, it simply consists of self-supporting graphene sheets. In summary, from... Figures 6-7 As can be seen, the product obtained in Comparative Example 2 is a graphene sheet that tends to be vertically aligned.

[0107] Test Example 4

[0108] This test example uses transmission electron microscopy (TEM) to characterize the graphene powder provided in Example 3 to obtain its sheet morphology. The obtained TEM images are shown below. Figure 8 As shown. When characterizing graphene powder using transmission electron microscopy, it is often necessary to ultrasonically disperse the graphene powder in anhydrous ethanol. Under the action of ultrasound, the graphene powder sheets open up, forming individual sheets, thus allowing for... Figure 8 The transmission electron microscope image shown indicates that the graphene powder provided in Example 3 of this invention is in the form of thin sheets.

[0109] The above description is merely a specific embodiment of the present invention and should not be construed as limiting the scope of the invention. Therefore, any substitution of equivalent components or equivalent changes and modifications made within the scope of protection of this patent should still fall within the scope of this patent. Furthermore, the technical features, technical features and technical inventions, and technical inventions in this invention can be freely combined and used.

Claims

1. A method for preparing graphene powder using carbon dioxide as a carbon source, characterized in that, The method includes the following steps: Step (1): Under high vacuum conditions, a stable glow plasma is spontaneously generated in a microwave plasma device with an inert gas as the background gas under the action of microwaves; in step (1), the flow rate of the inert gas is 200-500 sccm, the output power of the microwave is 300-600 W, and the high vacuum conditions include: pressure of -0.05 to -0.1 MPa; Step (2): Increase the flow rate of the inert gas, increase the output power of the microwave, and increase the system pressure to a slightly negative pressure state so that the glow plasma will transform and the system will be in a state where glow plasma and electric arc coexist; In step (2), the flow rate of the inert gas is 1000-5000 sccm, the output power of the microwave is 1000-2000 W, and the pressure corresponding to the slightly negative pressure state is greater than or equal to -0.02 MPa and less than 0 MPa; Step (3): Under the condition of maintaining the system pressure at a slightly negative pressure state, carbon dioxide and deoxidizer are introduced into the microwave plasma device. Under the action of glow plasma and electric arc, carbon dioxide molecules are bombarded by high-energy particles and decomposed. The decomposed carbon species nucleate and grow to obtain graphene powder.

2. The method according to claim 1, characterized in that, The inert gas includes one or a combination of argon, nitrogen, and helium.

3. The method according to claim 1, characterized in that, The deoxidizer includes hydrogen.

4. The method according to claim 1 or 3, characterized in that, In step (3), the flow rate of carbon dioxide is 5-200 sccm, and the deoxidizer is hydrogen, the flow rate of which is 40-60% of the flow rate of carbon dioxide.

5. The method according to claim 1 or 2, characterized in that, In step (3), the reaction time is more than 1 hour.

6. A graphene powder, characterized in that, The graphene powder is prepared by the method for preparing graphene powder using carbon dioxide as a carbon source as described in any one of claims 1-5, wherein Raman results show that the graphene powder has I D / I G <1, I 2D / I G It is 0.3-0.

4.

7. The application of the graphene powder according to claim 6 as a reinforcing material.

Images