Method for preparing ultra-pure carbon material through microwave plasma CVD (Chemical Vapor Deposition) thermochemistry

Abstract

The invention discloses a method for preparing an ultra-pure carbon material through microwave plasma CVD thermochemistry, and belongs to the technical field of nanometer carbon material synthesis. The method comprises the following steps: firstly, introducing hydrogen and trace oxygen to etch a substrate; then introducing mixed gas of high-purity methane gas and hydrogen, exciting plasma by adopting a microwave source, and cracking and depositing a carbon source under a thermochemical auxiliary condition; and finally, carrying out annealing treatment to obtain the ultra-pure carbon material. The form of the ultra-pure carbon material can be regulated and controlled to be of a diamond, graphene-like or amorphous carbon structure, the impurity content is lower than one million, and the carbon purity reaches the N6 level or above. The method disclosed by the invention has the advantages of mild process, high energy efficiency and strong impurity removal capability, and the prepared ultrapure carbon material is remarkably improved in electrical, thermal and electromagnetic shielding properties.

Description

Technical Field

[0001] This invention belongs to the field of nano-carbon material synthesis technology, specifically relating to a method for preparing ultrapure carbon materials using microwave plasma chemical vapor deposition (MPCVD) thermochemical method. Background Technology

[0002] With the rapid development of next-generation information technology, the energy industry, and high-end manufacturing, the demand for basic functional materials is increasing daily, especially for carbon materials with excellent electrical, thermal, and mechanical properties. Carbon materials, such as diamond, graphene, diamond-like carbon films, and amorphous carbon, have been widely used in semiconductors, quantum information, aerospace, and energy storage. However, these high-end applications place extremely stringent requirements on material purity and structural integrity, especially in quantum devices and power electronics, where even impurities at the ppm level can significantly affect material performance.

[0003] Currently, common methods for preparing carbon materials mainly include high-temperature high-pressure (HPHT), hot-wire chemical vapor deposition (HFCVD), conventional chemical vapor deposition (CVD), and arc discharge. Although these methods can synthesize carbon materials of a certain quality, they still have significant shortcomings: First, the purity of the product is limited, as impurities such as nitrogen, oxygen, and metal ions are difficult to completely remove, and the carbon purity is usually difficult to exceed 99.9%; Second, some methods, such as the HPHT process, require extreme temperature and pressure conditions, resulting in high energy consumption and complex equipment, making them difficult to scale up; Third, conventional CVD deposition rates are relatively low, leading to insufficient efficiency in preparing thick films or bulk materials; Fourth, some processes easily introduce additional impurities during the reaction, resulting in a decrease in electrical and optical properties.

[0004] Microwave plasma chemical vapor deposition (MPCVD), a rapidly developing carbon material synthesis technology in recent years, boasts advantages such as high plasma density, low electron temperature, high energy utilization, and controllable atmosphere, enabling the effective decomposition and deposition of carbon precursors under relatively mild conditions. However, existing MPCVD technologies primarily focus on the growth of single-crystal diamond or graphene, and still face the following prominent challenges in the preparation of ultrapure carbon materials: a narrow process window, making it difficult to achieve high deposition rates while maintaining high purity; insufficient impurity removal mechanisms, making it difficult to control residual elements such as nitrogen and oxygen in the products to below one part per million; and limited morphology control, making it difficult to achieve high-purity preparation of various forms such as diamond, graphene-like materials, and amorphous carbon through process parameter adjustments.

[0005] The above background information is provided only to aid in understanding the concept and technical solution of this invention. It does not necessarily belong to the prior art of this patent application. In the absence of clear evidence that the above information was still disclosed before the filing date of this patent application, the above background information should not be used to evaluate the novelty and inventiveness of this application. Summary of the Invention

[0006] The purpose of this invention is to address the problems of high impurity content, low deposition efficiency, harsh process conditions, and limited morphology control in existing carbon material preparation methods by providing a microwave plasma CVD thermochemical method for preparing ultrapure carbon materials. This method can achieve efficient decomposition and purification of carbon precursors under low energy consumption conditions, obtaining ultrapure carbon materials with a carbon purity of N6 or higher and an impurity content of less than one part per million. Furthermore, different morphologies of ultrapure carbon materials can be achieved through process parameter control, resulting in significant improvements in electrical, thermal, and electromagnetic shielding properties.

[0007] Technical Solution: To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical method involves first introducing hydrogen and trace amounts of oxygen to etch the substrate to remove the surface oxide layer and adsorbed impurities; then introducing a mixture of high-purity methane gas and hydrogen gas, using a microwave source to excite plasma, and promoting the decomposition and deposition of carbon source under thermochemical-assisted conditions; finally, obtaining ultrapure carbon materials through annealing treatment.

[0009] The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical treatment involves a substrate etching time of 5-30 min, a hydrogen flow rate of 100-500 mL / min, an oxygen flow rate of 1-10 mL / min, and a power of 4-8 kW; preferably, the bulk etching time is 10 min, the hydrogen flow rate is 200 mL / min, the oxygen flow rate is 10 mL / min, and the power is 6 kW.

[0010] The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical treatment involves pretreatment of the substrate before etching. Specifically, the substrate is polished on one side, ensuring that the polished surface faces upwards, and the surface thickness is no more than 5 mm and the surface roughness is less than 0.8 micrometers. Then, the substrate is ultrasonically cleaned in an organic solvent for 5-15 min and rinsed with deionized water. Preferably, the substrate surface thickness is 5 mm, and it is ultrasonically cleaned in acetone, anhydrous ethanol and deionized water for 15 min each.

[0011] The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical deposition involves the following steps: when the volume concentration of methane is 0.1%-0.9% and the deposition temperature is 900-1000 °C, a diamond film is formed; when the volume concentration of methane is 1%-3% and the deposition temperature is 1000-1100 °C, graphene material is formed; when the volume concentration of methane is 3%-5% and the deposition temperature is 750-900 °C with a deposition time of 10 h, a carbon nanotube composite material is formed. Preferably, the volume concentration of methane is controlled at 3.5% and the deposition temperature is 750 °C for depositing the carbon nanotube composite material.

[0012] The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical deposition includes the addition of argon gas during the deposition process. The total gas flow rate during deposition is 510-2600 mL / min, the methane flow rate is 10-100 mL / min, the hydrogen flow rate is 500-2000 mL / min, and the argon flow rate is 0-500 mL / min. The purity of all three gases is not less than 99.9999% to ensure that no exogenous impurities are introduced during the deposition process. Methane is used as the carbon source gas, hydrogen is used to activate the surface and remove amorphous carbon, and argon is used as a dilution and plasma stabilizing gas. Preferably, the flow rates of the three gases are 20 mL / min for methane, 500 mL / min for hydrogen, and 50 mL / min for argon.

[0013] The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical methods includes annealing treatment performed under a hydrogen atmosphere or vacuum conditions. When using a hydrogen atmosphere, the hydrogen flow rate is controlled at 200-800 mL / min, the annealing temperature is 900-1100 ℃, and the time is 1-5 h to further remove residual impurities and amorphous phases from the carbon surface. When using vacuum conditions, the vacuum degree is 10... -3 -10 -5 Torr, with an annealing temperature of 950-1050 ℃ and an annealing time of 2-4 h, to reduce the content of impurities such as oxygen and nitrogen and improve lattice integrity; preferably, annealing is carried out at 900 ℃ for 90 min in a hydrogen atmosphere with a flow rate of 400 mL / min.

[0014] The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical methods includes the following preparation steps:

[0015] (a) Substrate pretreatment: The substrate is subjected to organic solvent, deionized water ultrasonic cleaning and hydrogen plasma treatment in sequence to remove surface impurities and defects, and hydrogen and trace oxygen are introduced to perform etching pretreatment on the substrate.

[0016] (b) Atmosphere configuration: A high-purity mixed gas consisting of methane, hydrogen and argon is introduced into the reaction chamber; the reaction chamber is made of quartz or high-purity stainless steel, and its inner wall is treated with high vacuum coating or inert isolation coating to reduce the risk of metal ions being released from the chamber wall under high-energy plasma bombardment, thereby avoiding impurity migration and ensuring the ultra-high purity of the deposited carbon layer.

[0017] (c) Vapor deposition: A 2.45 GHz, 4-8 kW microwave source is used to excite plasma and provide a thermal field to promote the complete decomposition of carbon precursors and the removal of impurities. Carbon atoms are deposited and grow in an orderly manner on the substrate surface under a pressure of 10-200 Torr. The microwave power control range needs to be matched with parameters such as cavity pressure, gas flow rate and stage height to maintain the stability of the plasma fireball. Too low microwave power may lead to incomplete decomposition of carbon source gas, while too high microwave power may cause local overheating of the substrate surface, both of which will cause fluctuations in deposition rate and uneven carbon film thickness.

[0018] (d) Annealing treatment: Ultrapure carbon material is obtained by annealing under hydrogen or vacuum conditions.

[0019] The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical methods, wherein the substrate is any one of silicon wafers, Ni wafers, or Fe wafers.

[0020] The ultrapure carbon material prepared by the above method has a defect density of less than 1×10⁻⁶. 5 cm -2 Structural defects such as dislocations and vacancies are significantly reduced; the carbon purity is above N6 level, and the content of non-carbon elements (such as O, N, and metal ions) is less than one part per million.

[0021] The ultrapure carbon material has an electrical conductivity of up to 10. 3 -10 5 The S / m thermal conductivity reaches 1180.3 W / m·K, and the SE can reach 72.4 dB in the 9-13 GHz frequency band; both of which are significantly better than carbon materials prepared by conventional CVD.

[0022] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0023] (1) Extremely high purity: The purity of the obtained carbon material can reach N6 level or above, and the impurity content is less than one part per million, which effectively overcomes the problem of incomplete impurity removal in traditional CVD process;

[0024] (2) Low defect density: The material defect density is less than 1×10 5 cm -2 This significantly reduces structural defects such as dislocations and vacancies, ensuring the lattice integrity and stability of the material;

[0025] (3) Superior performance: The prepared ultrapure carbon material has an electrical conductivity of 10. 3 -10 5 With a thermal conductivity of 1180.3 W / m, its electrical, thermal, and mechanical properties are significantly superior to those of carbon materials prepared by conventional CVD.

[0026] (4) Mild process conditions: The synergistic effect of microwave plasma and thermochemistry is used to achieve low energy consumption and high efficiency deposition, avoiding the limitations of high temperature and high pressure conditions on equipment and energy consumption;

[0027] (5) Broad application prospects: The prepared ultrapure carbon materials can be widely used in heat dissipation materials and electromagnetic shielding, meeting the application needs of next-generation semiconductors, quantum information and aerospace devices. Attached Figure Description

[0028] Figure 1 This is a process flow diagram of the present invention;

[0029] Figure 2 Raman diagrams of the carbon materials prepared in Example 1 and Comparative Example 1;

[0030] Figure 3 SEM images of the carbon materials prepared in Example 1 and Comparative Examples 1-2;

[0031] Figure 4 A comparison chart of the impurity content of the carbon materials prepared in Example 1 and Comparative Examples 1-2;

[0032] Figure 5 The graph shows a comparison of the electromagnetic shielding effectiveness (EMI SE) of the carbon materials prepared in Example 1 and Comparative Examples 1-2. Detailed Implementation

[0033] The present invention will be further illustrated below with reference to specific embodiments. These embodiments are implemented based on the technical solutions of the present invention, and it should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0034] Example 1

[0035] A method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical methods includes the following steps:

[0036] (a) High-purity Ni wafers were selected to ensure a substrate surface thickness of 5 mm and a surface roughness of less than 0.8 μm. The wafers were ultrasonically cleaned for 15 min each with acetone, anhydrous ethanol, and deionized water, and then dried. The wafers were then bombarded with plasma for 10 min in a hydrogen-oxygen mixed atmosphere to remove surface impurities and activate the surface (hydrogen flow rate was controlled at 200 mL / min, oxygen flow rate at 10 mL / min, and microwave power was set to 6 kW).

[0037] (b) Secondary vacuum in the cavity, preparation of reaction gas: deposition is carried out in a mixed atmosphere of methane, hydrogen and argon, with the flow rates of the three gases being 20 mL / min for methane, 500 mL / min for hydrogen and 50 mL / min for argon, and the purity of all the gases used is ≥99.9999%; the volume concentration of methane in the cavity is controlled at 3.5%, and the deposition temperature is 750 ℃, for the deposition of carbon nanotube composite materials;

[0038] (c) A 2.45 GHz microwave source was used, the power was set to 8 kW, the reaction chamber was made of high-purity quartz material, and the inner wall was treated with high vacuum coating; during the deposition reaction, the reaction pressure was maintained at 40 Torr, the substrate temperature was controlled at 750 ℃, and the reaction duration was 10 hours to obtain the initially deposited carbon material.

[0039] (d) Annealing was performed at 900 °C for 90 min under a hydrogen atmosphere (400 mL / min). The temperature stability was controlled within ±5 °C to effectively eliminate lattice defects and reduce impurities, followed by programmed cooling.

[0040] (e) The surface morphology and carbon skeleton structure of ultrapure carbon were analyzed by SEM and Raman spectroscopy.

[0041] (f) The obtained ultrapure carbon material was prepared into a carbon nanotube composite material, and its electrical conductivity, thermal conductivity and electromagnetic shielding performance were tested.

[0042] Comparative Example 1

[0043] Steps (a)-(c) of Example 1 were repeated, but the annealing treatment in step (d) was omitted, and the material properties were tested directly. The results showed that the defect density of the obtained carbon material was approximately 10. 7 cm -2 The Raman D / G peak ratio was 0.87, and the electromagnetic shielding effectiveness was only 52.7 dB, indicating that the annealing process is crucial for improving the purity and performance of carbon materials.

[0044] Figure 2 Raman diagrams of the carbon materials prepared in Example 1 and Comparative Example 1. Figure 2 It can be seen that the D / G peak ratio of the material obtained by the present invention is as low as 0.46, while the D / G peak ratio of Comparative Example 1 (unannealed) is as high as 0.87, indicating that the annealing process significantly reduces the defect density of carbon materials.

[0045] Comparative Example 2

[0046] Deposition is performed using a traditional hot-wire CVD process at the same gas ratio and temperature. The process includes the following steps:

[0047] (a) High-purity Ni wafers were selected as the substrate, and the surface was ultrasonically cleaned and dried in sequence with acetone, anhydrous ethanol and deionized water.

[0048] (b) Install multiple tungsten wires as heating wires in the vacuum chamber, with a distance of about 10 mm between the wires and the substrate.

[0049] (c) A mixture of methane, hydrogen and argon is introduced, with gas flow rates of 20 mL / min for methane, 500 mL / min for hydrogen and 50 mL / min for argon, and the system pressure is maintained at 40 Torr.

[0050] (d) Carbon material was deposited at a substrate temperature of about 750 °C by heating a tungsten filament to about 2000 °C to crack methane, and the deposition time was 10 hours.

[0051] (e) After the reaction is complete, the sample is naturally cooled under an argon atmosphere to obtain a carbon material sample.

[0052] Figure 3 SEM images of the carbon materials prepared in Example 1 and Comparative Examples 1-2. Figure 3 As can be seen, the ultrapure carbon film prepared in Example 1 has a smooth and dense surface, uniformly distributed grains, and no obvious cracks or impurity agglomerations, indicating that the method of the present invention can effectively suppress the non-uniform deposition problem that occurs in the traditional hot-wire CVD process. The carbon layer of the sample in Comparative Example 1 is damaged, indicating that the defect density of the carbon material is significantly reduced during the annealing process. The carbon layer of the sample in Comparative Example 2 has a high content of amorphous carbon and some carbon agglomeration structures, indicating that the method proposed in this invention can effectively improve the regularity and uniformity of the carbon material.

[0053] Figure 4 This is a comparison chart of the impurity content of the carbon materials prepared in Example 1 and Comparative Examples 1-2. Figure 4 It can be seen that the carbon sample obtained by this invention has a purity of N6 or higher, and the content of non-carbon elements (such as O, N, and metal ions) is less than one part per million, while Comparative Example 1 has 96 ug / g and Comparative Example 2 has 4277 ug / g. This shows that the use of microwave plasma CVD and optimized annealing process can significantly improve the purity of carbon sample of material.

[0054] Figure 5 This is a comparison chart of the electromagnetic shielding effectiveness (EMI SE) of the carbon materials prepared in Example 1 and Comparative Examples 1-2. Figure 5It can be seen that the carbon material obtained by this invention has an electromagnetic shielding effectiveness of up to 72.4 dB in the 8-12 GHz frequency band, and exhibits good electromagnetic shielding performance across the entire 2-18 GHz frequency band. Furthermore, it maintains a performance better than 60 dB even at high frequencies (10-18 GHz), indicating its suitability for electromagnetic shielding in high-frequency electronic devices and communication equipment. In contrast, Comparative Example 1 shows only 52.7 dB, and Comparative Example 2 shows 38.8 dB, demonstrating that the use of microwave plasma CVD and optimized annealing processes can significantly improve the electromagnetic shielding performance of the material.

[0055] Table 1 Comparison of Defect Density, Impurity Content, and Electromagnetic Shielding Effect of Various Materials

[0056]

[0057] In summary, the microwave plasma CVD preparation method of this invention can obtain ultrapure carbon materials with low defect density and extremely low impurity content. Compared with conventional CVD methods, this material exhibits significant improvements in electrical, thermal, and electromagnetic shielding properties, making it particularly suitable for high-end applications such as conductive materials, heat dissipation materials, and electromagnetic shielding.

[0058] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical methods, characterized in that, First, hydrogen and trace amounts of oxygen are introduced to etch the substrate; then, a mixture of high-purity methane and hydrogen is introduced, and plasma is excited by a microwave source to deposit carbon source under thermochemical-assisted conditions; finally, ultrapure carbon material is obtained through annealing.

2. The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical methods according to claim 1, characterized in that, The substrate is subjected to hydrogen plasma etching for 5-30 min, with a hydrogen flow rate of 100-500 mL / min, an oxygen flow rate of 1-10 mL / min, and a power of 4-8 kW.

3. The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical methods according to claim 1, characterized in that, Before etching, the substrate is pretreated by polishing one side, ensuring that the polished side faces upwards, and that the surface thickness is no more than 5 mm and the surface roughness is less than 0.8 micrometers. Then, the substrate is ultrasonically cleaned in organic solvent and deionized water in sequence, and rinsed with deionized water.

4. The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical methods according to claim 1, characterized in that, When the volume concentration of methane is 0.1%-0.9% and the deposition temperature is 900-1000 ℃, diamond films are formed; when the volume concentration of methane is 1%-3% and the deposition temperature is 1000-1100 ℃, graphene materials are formed; when the volume concentration of methane is 3%-5% and the deposition temperature is 750-900 ℃, carbon nanotube composite materials are formed.

5. The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical methods according to claim 1, characterized in that, Argon gas is added during the deposition process. The total gas flow rate during deposition is 510-2600 mL / min, the methane flow rate is 10-100 mL / min, the hydrogen flow rate is 500-2000 mL / min, and the argon flow rate is 0-500 mL / min. The purity of all three gases is not less than 99.9999%.

6. The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical method according to claim 1, characterized in that, The annealing treatment is carried out in a hydrogen atmosphere or under vacuum conditions. When using a hydrogen atmosphere, the hydrogen flow rate is 200-800 mL / min, the annealing temperature is 900-1100 ℃, and the time is 1-5 h. When using a vacuum atmosphere, the vacuum degree is 10. -3 -10 -5 Torr, annealing temperature is 950-1050 ℃, annealing time is 2-4 h.

7. The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical methods according to claim 1, characterized in that, The preparation steps include the following: (a) Substrate pretreatment: The substrate is pretreated to remove surface impurities and defects, and hydrogen and trace amounts of oxygen are introduced to perform etching pretreatment on the substrate; (b) Atmosphere configuration: A high-purity mixed gas consisting of methane, hydrogen and argon is introduced into the reaction chamber; (c) Vapor deposition: A 2.45 GHz, 4-8 kW microwave source is used to excite plasma to provide a thermal field, and carbon atoms are deposited and grow in an orderly manner on the substrate surface under a pressure of 10-200 Torr. (d) Annealing treatment: Ultra-pure carbon material is obtained after annealing treatment.

8. The method for preparing ultrapure carbon materials by microwave plasma CVD thermochemical method according to claim 1, characterized in that, The substrate is any one of silicon wafer, Ni wafer, or Fe wafer.

9. The ultrapure carbon material prepared by the method according to any one of claims 1-8, characterized in that, Defect density less than 1×10 5 cm -2 The carbon element purity is N6 or higher, and the non-carbon element content is less than one part per million.

10. The ultrapure carbon material according to claim 9, characterized in that, The electrical conductivity of the material can reach 10. 3 -10 5 The thermal conductivity reaches 1180.3 W / m·K, and the SE reaches 72.4 dB in the 9-13 GHz band.

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