Method for removing silicon and purifying carbon nanotubes and carbon nanotubes

Abstract

The present invention discloses a method for removing silicon from purified carbon nanotubes and carbon nanotubes, and belongs to the field of carbon nanotube purification. The method uses vermiculite as a substrate, generates carbon nanotubes on the surface of the substrate by a CVD method, and then pre-oxidizes the carbon nanotubes to obtain oxidized carbon nanotubes; the oxidized carbon nanotubes are mixed with an alkaline solution and an alkali-resistant water-soluble dispersant for alkali washing; the alkali-washed carbon nanotubes are sequentially washed with water, centrifuged, and dried to obtain dried carbon nanotubes; the dried carbon nanotubes are subjected to a first purification treatment with a chlorine-containing gas under a protective atmosphere, and then to a second purification treatment with a fluorine-containing gas to obtain purified carbon nanotubes. The method utilizes three methods, gas phase oxidation, alkali washing, and atmosphere high-temperature purification, to purify the carbon nanotubes, significantly improving the purity of the carbon nanotubes while not destroying the wall structure of the carbon nanotubes and enhancing their electrical conductivity.

Description

Technical Field

[0001] The present invention relates to a method for removing silicon and purifying carbon nanotubes and carbon nanotubes, and in particular to a purification method for carbon nanotubes grown by CVD on a Si / SiO2 substrate, which is subjected to pre-oxidation, alkali washing, water washing and atmosphere purification, and belongs to the field of carbon nanotube purification. Background Art

[0002] Vermiculite has a typical layered and microporous structure, making it an ideal template for preparing carbon nanotubes and providing an ideal substrate for the orderly arrangement and directional growth of carbon nanotubes. During the CVD process, the carbon source can undergo catalytic cracking between the layers of the vermiculite, thereby generating carbon nanotubes. The vermiculite substrate has good thermal and chemical stability, is not easily reactive with common catalysts and carbon sources, and can maintain structural stability in high-temperature and corrosive environments. This helps maintain the morphology and structure of the carbon nanotubes during the CVD preparation process, thereby improving their application performance and ensuring the stability and controllability of the preparation process. At the same time, vermiculite is a widely distributed mineral with abundant resources and easy access. Therefore, the preparation of carbon nanotubes using vermiculite as a raw material is low-cost and easy to achieve large-scale production.

[0003] However, the main component of vermiculite is magnesium-containing hydroaluminosilicate, and its SiO2 content accounts for approximately 37-42%. Therefore, in addition to the common impurities of amorphous carbon and metal catalyst particles, the carbon nanotubes prepared by CVD on vermiculite also contain a large amount of other silicon-containing impurities such as Si / SiO2. The presence of silicon-containing impurities will directly affect the performance and application of carbon nanotubes. Silicon-containing impurities in carbon nanotubes are extremely difficult to remove. They will cause changes in the carrier mobility in carbon nanotubes, leading to increased charge scattering or reduced charge transfer efficiency, which increases the resistance of carbon nanotubes. Conventional oxidation and high-temperature purification methods cannot remove them; acid washing methods require the use of highly toxic hydrofluoric acid to remove silicon, which further increases the cost and safety risks of the purification process.

[0004] A Chinese patent (publication number CN 117735535 A) discloses a method for purifying carbon nanotubes. Carbon nanotubes obtained by CVD are placed in a reactor and oxidized by the introduction of an oxidizing gas to produce primary oxidized carbon nanotubes. The primary oxidized carbon nanotubes are then immersed in an acidic oxidizing solution to undergo an oxidation reaction, producing secondary oxidized carbon nanotubes. A protective atmosphere and a halogen atmosphere are then introduced into the secondary oxidized carbon nanotubes for halogenation and volatilization, yielding purified carbon nanotubes. However, while this method effectively removes most metal impurities from the carbon nanotubes, it cannot purify carbon nanotubes with high silicon content.

[0005] Therefore, developing an efficient and safe purification method is of great significance for the purification of carbon nanotubes containing silicon impurities. Summary of the Invention

[0006] In view of the problems existing in the prior art of removing silicon-containing impurities from carbon nanotubes, such as poor removal effect, low purity of the obtained carbon nanotubes and high energy consumption, the purpose of the present invention is to provide a method for removing silicon and purifying carbon nanotubes. This method utilizes three methods, namely gas phase oxidation-alkaline washing-atmosphere high-temperature purification, to purify carbon nanotubes, thereby significantly improving the purity of carbon nanotubes and enhancing their conductivity without destroying the carbon nanotube wall structure.

[0007] In order to achieve the above object, the present invention provides a method for removing silicon and purifying carbon nanotubes, which comprises the following steps:

[0008] (1) Using vermiculite as a substrate, generating carbon nanotubes on the surface of the substrate by a CVD method, and then pre-oxidizing the carbon nanotubes to obtain oxidized carbon nanotubes;

[0009] (2) mixing the oxidized carbon nanotubes with an alkaline solution and an alkali-resistant water-soluble dispersant for alkaline washing; the alkali-resistant water-soluble dispersant is selected from at least one of polyvinyl pyrrolidone, polyvinyl alcohol, hydroxyethylene diphosphonic acid, an alkali-resistant cellulose compound, and a compound containing a polyoxyethylene ether functional group;

[0010] (3) washing the alkali-washed carbon nanotubes with water, centrifuging them, and drying them in sequence to obtain dry carbon nanotubes;

[0011] (4) The dried carbon nanotubes are subjected to a first purification treatment with a chlorine-containing gas under a protective atmosphere, and then subjected to a second purification treatment with a fluorine-containing gas to obtain purified carbon nanotubes.

[0012] Using vermiculite as a substrate, carbon nanotubes are grown on the surface via a CVD method. These carbon nanotubes typically have a purity of 88-92%, containing 6,000-22,000 ppm of metallic impurities, a small amount of amorphous carbon, and some silicon-containing impurities. The key technology of this invention lies in first subjecting the carbon nanotubes to a vapor-phase oxidation treatment, exposing them to air to initially oxidize the amorphous carbon impurities adhering to their surfaces. Subsequently, an alkaline solution and an alkali-resistant water-soluble dispersant are added to the carbon nanotubes. After ultrasonic stirring for a period of time, the carbon nanotubes are heated and stirred in a water bath (i.e., alkaline washing). After the reaction is complete, the carbon nanotubes are washed with hot pure water and centrifuged until the pH of the washing solution reaches <8, followed by drying. During the alkaline washing process, a certain amount of alkaline solution and any other chemicals that may be produced will adsorb on the surface of the carbon nanotubes. Hot water, with its high solubility, effectively dissolves and removes these impurities, ensuring the purity of the carbon nanotubes and achieving preliminary silicon removal.

[0013] Finally, under a protective atmosphere, the carbon nanotubes are first purified with chlorine-containing gas and then purified with fluorine-containing gas in a second step, so that the metal impurities and silicon-containing impurities in the carbon nanotubes react fully to generate metal chlorides and silicon-containing fluorides with lower boiling points, which are finally volatilized and removed at high temperature to achieve the purpose of purification.

[0014] The inventors discovered that in an alkaline environment, the surface of carbon nanotubes will take on a negative charge. This change is due to the adsorption and reaction of hydroxide ions on the surface of carbon nanotubes. Hydroxyl ions not only adhere tightly to the surface of carbon nanotubes, but also interact with functional groups or defect sites on the surface, thereby increasing the negative charge and strengthening the electrostatic repulsion between carbon nanotubes. This electrostatic repulsion makes it more difficult for carbon nanotubes to approach each other and agglomerate, thereby presenting a better dispersion state in the solution. More importantly, this improvement in dispersion also helps to fully expose silicon-containing impurities. In an alkaline solution, well-dispersed carbon nanotubes make it easier for impurities to react with the alkali in the solution, thereby improving its impurity removal effect. The alkaline environment enhances its dispersibility by affecting the charge state of the carbon nanotube surface, and promotes the exposure of silicon-containing impurities and the impurity removal process of the alkaline solution. It provides new ideas and methods for the purification and application of carbon nanotubes.

[0015] Furthermore, an alkali-resistant water-soluble dispersant is added during the alkali washing process. The alkali-resistant water-soluble dispersant can effectively reduce the surface tension of the carbon nanotubes, further reduce the agglomeration between the carbon nanotubes, make them uniformly dispersed in the alkali solution, and improve production efficiency.

[0016] In a preferred embodiment, the pre-oxidation treatment is performed at a temperature of 400-700°C for 30-100 minutes. Lower reaction temperatures result in incomplete reaction, resulting in a higher concentration of amorphous carbon, which in turn affects the purity of the carbon nanotubes and their performance and applications. Higher temperatures lead to oxidation of the carbon nanotubes, resulting in higher carbon loss.

[0017] In a preferred embodiment, the mass ratio of the oxidized carbon nanotubes to the alkali-resistant water-soluble dispersant and the alkaline solution is 100:0.5-2:30-50; the alkaline solution is measured on a dry basis. Too little alkali-resistant water-soluble dispersant may not effectively disperse the carbon nanotubes, while too much may introduce other impurities. Excessive alkali concentrations may cause excessive corrosion of the carbon nanotubes, while too low a concentration may result in poor silicon removal.

[0018] In a preferred embodiment, the alkali-resistant water-soluble dispersant is selected from polyvinyl pyrrolidone and / or polyvinyl alcohol.

[0019] In a preferred embodiment, the concentration of the alkaline solution is 0.5-4 mol / L.

[0020] In a preferred embodiment, the alkaline solution is a sodium hydroxide solution and / or a potassium hydroxide solution.

[0021] In a preferred embodiment, the method further comprises mixing the oxidized carbon nanotubes under ultrasonic conditions, wherein the mixing condition is: the ultrasonic time is 5 to 10 minutes.

[0022] In a preferred embodiment, the alkali wash is performed in a water bath heated at a temperature of 70-90°C, a stirring rate of 100-500 rpm, and a duration of 8-12 hours. These preferred conditions facilitate the removal of residual alkali, improve cleaning efficiency, reduce water molecule adsorption, and facilitate subsequent purification.

[0023] In a preferred embodiment, the alkali washing is carried out in a Teflon reactor, and the stirring device for the alkali washing is a stirring paddle and a stirring shaft, both surfaces of which are coated with Teflon.

[0024] In a preferred embodiment, the water washing conditions are: the water temperature is 70-100° C., and the water washing conditions are controlled so that the pH value of the washing liquid obtained after the water washing is less than 8.

[0025] In a preferred embodiment, the drying treatment conditions are: temperature of 120° C. to 150° C., and time of 2 to 15 hours.

[0026] In a preferred embodiment, the chlorine-containing gas is selected from at least one of hydrogen chloride and chlorine; the fluorine-containing gas is selected from at least one of Freon and fluorine. The chlorine-containing gas can remove metal impurities from the carbon nanotubes, and the fluorine-containing gas can remove silicon-containing impurities from the carbon nanotubes, generating metal chlorides and silicon-containing fluorides with lower boiling points, respectively.

[0027] In a preferred embodiment, the protective atmosphere is selected from at least one of argon, nitrogen and helium.

[0028] In a preferred embodiment, the conditions for the first purification treatment are: temperature of 800-1400° C., time of 1-8 hours; and flow ratio of the chlorine-containing gas to the protective atmosphere of 1:1-10.

[0029] In a more preferred embodiment, the flow ratio of the chlorine-containing gas to the protective atmosphere is 1:1 to 3. The inventors have found that when the flow ratio is 1:1 to 3, the purity of the obtained carbon nanotubes is higher.

[0030] In a preferred embodiment, the second purification treatment is performed at a temperature of 1500-1800° C. for 2-4 hours, and at a flow rate ratio of 1:1-10 between the fluorine-containing gas and the protective atmosphere.

[0031] In a more preferred embodiment, the flow ratio of the fluorine-containing gas to the protective atmosphere is 1:1 to 3. The inventors have found that when the flow ratio is 1:1 to 3, the purity of the obtained carbon nanotubes is higher.

[0032] The main purpose of the purification treatment of the present invention is that the gases generated by the high-temperature decomposition of chlorine-containing gas and fluorine-containing gas can react with the metal impurities and silicon-containing impurities in the carbon nanotubes respectively to generate metal chlorides and silicon-containing fluorides with lower boiling points, and these substances can be vaporized and volatilized at high temperatures, thereby achieving the purpose of purification.

[0033] The present invention provides a method for removing silicon and purifying carbon nanotubes, which specifically comprises the following steps:

[0034] S1: Carbon nanotubes are prepared by chemical vapor deposition. A vermiculite-based catalyst is placed in a reactor, and an inert protective gas and a reducing gas are continuously introduced. The reactor is heated until the temperature reaches 500-900°C, and finally a carbon source gas is introduced to react and obtain carbon nanotubes.

[0035] S2: placing the carbon nanotubes in a tube furnace, keeping both ends of the tube furnace connected to air, heating the furnace to 400-700°C in an air atmosphere for oxidation treatment for 30-100 minutes to obtain oxidized carbon nanotubes;

[0036] S3: mixing the oxidized carbon nanotubes with an alkaline solution and an alkali-resistant water-soluble dispersant to perform alkaline washing; sequentially washing the carbon nanotubes with water, centrifuging, and drying to obtain dry carbon nanotubes; the alkaline washing allows the silicon-containing impurities to react with the alkaline solution;

[0037] S4: placing the dried carbon nanotubes in a tubular furnace, introducing a protective gas, first performing a first purification treatment with a chlorine-containing gas at a temperature of 800-1400°C for 1-8 hours, and then performing a second purification treatment with a fluorine-containing gas at a temperature of 1500-1800°C for 2-4 hours to obtain purified carbon nanotubes.

[0038] In a preferred embodiment, the catalyst used in the CVD method for preparing carbon nanotubes contains one or more metals such as iron, silicon, aluminum, molybdenum, magnesium, chromium, nickel, manganese, sodium, and calcium.

[0039] In a preferred embodiment, the reducing gas is hydrogen.

[0040] In a preferred embodiment, the carbon source gas is one of methane, ethylene, acetylene, and propylene, and the reaction time after the carbon source is introduced is 60 to 100 minutes.

[0041] The present invention also provides carbon nanotubes prepared by the above-mentioned method for removing silicon and purifying carbon nanotubes. The carbon nanotubes have high purity and the tube wall structure of the carbon nanotubes remains intact.

[0042] Compared with the prior art, the present invention has at least the following advantages:

[0043] (1) The purification method of the present invention combines gas phase oxidation-alkaline washing-atmosphere high temperature purification to purify carbon nanotubes, which greatly improves the removal rate of metal impurities and silicon-containing impurities without destroying the wall structure of the carbon nanotubes, effectively improving the purity of the carbon nanotubes, thereby effectively improving the electrical properties and thermal conductivity of the carbon nanotubes.

[0044] (2) The purification method of the present invention solves the problem in the prior art that silicon-containing impurities are difficult to remove or the removal cost is high, and economically and effectively reduces the impurity content in carbon nanotubes, thereby improving the purity and quality of carbon nanotubes.

[0045] (3) The present invention first uses gas oxidation to initially purify and remove amorphous carbon, then further removes silicon-containing impurities in the carbon nanotubes by soaking them in a dispersant-alkaline solution. Finally, under certain temperature conditions, chlorine-containing and fluorine-containing gases react with metal impurities and silicon-containing impurities to form metal chlorides and silicon-containing fluorides, which are then volatilized and carried away. Therefore, the present invention achieves deep purification of carbon nanotubes through the synergistic effects of gas oxidation, alkaline washing purification, and high-temperature gas purification.

[0046] (4) The purification method of the present invention has the advantages of simple operation and low cost, and is suitable for large-scale production. It provides new technical support and important application prospects for the purification and application of carbon nanotubes. DETAILED DESCRIPTION

[0047] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0048] The present invention is further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments. Obviously, the embodiments described below are only a part of the embodiments, and all other embodiments obtained by professionals in this field without making creative efforts are still within the scope of protection of the present invention.

[0049] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0050] In the following examples, in step (2), the amount of multi-walled carbon nanotubes used is 100 g.

[0051] Example 1

[0052] (1) A Fe-Si catalyst system with vermiculite as the carrier was prepared by chemical vapor deposition. The catalyst was spread flat in a CVD tube furnace, nitrogen as the protective gas and hydrogen as the reducing gas were introduced, and the reaction was carried out at 650°C for 5 minutes. Then, propylene as the carbon source gas was introduced and the reaction was carried out for 1 hour. Multi-walled carbon nanotubes with a purity of 88-92% (i.e., an ash content of 8-12%) and an Fe content of 9054 ppm, an Al content of 10273 ppm, a Mg content of 3077 ppm, and a Si content of 1118 ppm were obtained.

[0053] (2) Place the prepared multi-walled carbon nanotubes in a tubular furnace, keep both ends of the tubular furnace connected to the outside air, and heat the reactor to 550°C for 30 minutes;

[0054] (3) Immerse the oxidized carbon nanotubes in a sodium hydroxide aqueous solution (concentration of 1 mol / L), add polyvinylpyrrolidone, stir thoroughly and ultrasonicate for 5 minutes, then place in a water bath heating pot, stir and react at 200 r / min and 85°C for 12 hours. After the reaction is completed, wash with 85-100°C hot pure water and centrifuge until the pH is less than 8, and then dry at 150°C for 12 hours.

[0055] The mass ratio of the oxidized carbon nanotubes to the polyvinyl pyrrolidone and the alkaline solution is 100:0.6:40; wherein the alkaline solution is measured on a dry basis;

[0056] (4) The dried carbon nanotubes were placed in a tube furnace and heated to 1200°C under the protection of argon gas. The mixture was then aerated at a ratio of 1:4 for 1.5 hours, with a flow rate of 300 mL / min of hydrogen chloride gas. The mixture was then heated to 1500°C, and the hydrogen chloride gas was replaced with freon gas. The mixture was aerated at a ratio of 1:4 for 2 hours, with a flow rate of 300 mL / min of freon gas. The mixture was then cooled to room temperature under the protection of argon gas.

[0057] The final carbon nanotube solid sample was subjected to ash content and ICP impurity full scan testing. After calcination at 950°C for 5 hours, the ash content was found to be 0.23%. The Fe content in the carbon nanotubes was found to be 6 ppm, the Al content was 15 ppm, the Mg content was 10 ppm, and the Si content was 82 ppm.

[0058] Example 2

[0059] (1) A Fe-Si catalyst system with vermiculite as the carrier was prepared by chemical vapor deposition. The catalyst was spread flat in a CVD tube furnace, nitrogen as the protective gas and hydrogen as the reducing gas were introduced, and the reaction was carried out at 650°C for 5 minutes. Then, propylene as the carbon source gas was introduced and the reaction was carried out for 1 hour. Multi-walled carbon nanotubes with a purity of 88-92% (i.e., an ash content of 8-12%) and an Fe content of 9054 ppm, an Al content of 10273 ppm, a Mg content of 3077 ppm, and a Si content of 1118 ppm were obtained.

[0060] (2) Place the prepared multi-walled carbon nanotubes in a tubular furnace, keep both ends of the tubular furnace connected to the outside air, and heat the reactor to 550°C for 30 minutes;

[0061] (3) Immerse the oxidized carbon nanotubes in a sodium hydroxide aqueous solution (concentration of 2 mol / L), add polyvinylpyrrolidone, stir thoroughly and ultrasonicate for 5 minutes, then place in a water bath heating pot, stir and react at 200 r / min and 85 °C for 12 hours. After the reaction is completed, wash with 85-100 °C hot pure water and centrifuge until the pH is less than 8, and then dry at 150 °C for 12 hours.

[0062] The mass ratio of the oxidized carbon nanotubes to the polyvinyl pyrrolidone and the alkaline solution is 100:1.2:50; wherein the alkaline solution is measured on a dry basis;

[0063] (4) The dried carbon nanotubes were placed in a tube furnace and heated to 1200°C under the protection of argon gas. The mixture was then aerated at a ratio of 1:4 for 1.5 hours, with a flow rate of 300 mL / min of hydrogen chloride gas. The mixture was then heated to 1500°C, and the hydrogen chloride gas was replaced with freon gas. The mixture was aerated at a ratio of 1:4 for 2 hours, with a flow rate of 300 mL / min of freon gas. The mixture was then cooled to room temperature under the protection of argon gas.

[0064] The resulting solid carbon nanotube sample was tested for ash content and ICP impurity full scan. After calcination at 950°C for 5 hours, the ash content was 0.12%. The Fe content, Al content, Mg content, and Si content in the carbon nanotubes were 11 ppm, 8 ppm, 3 ppm, and 55 ppm, respectively. Example 2 differed from Example 1 only in that the amounts of sodium hydroxide and polyvinylpyrrolidone were increased during the alkaline washing step, resulting in higher purity and lower silicon content in the obtained carbon nanotubes. The results indicate that increasing the alkali concentration within a certain range during the alkaline washing step improves impurity removal.

[0065] Example 3

[0066] (1) A Fe-Si catalyst system with vermiculite as the carrier was prepared by chemical vapor deposition. The catalyst was spread flat in a CVD tube furnace, nitrogen as the protective gas and hydrogen as the reducing gas were introduced, and the reaction was carried out at 650°C for 5 minutes. Then, propylene as the carbon source gas was introduced and the reaction was carried out for 1 hour. Multi-walled carbon nanotubes with a purity of 88-92% (i.e., an ash content of 8-12%) and an Fe content of 9054 ppm, an Al content of 10273 ppm, a Mg content of 3077 ppm, and a Si content of 1118 ppm were obtained.

[0067] (2) Place the prepared multi-walled carbon nanotubes in a tubular furnace, keep both ends of the tubular furnace connected to the outside air, and heat the reactor to 550°C for 30 minutes;

[0068] (3) Immerse the oxidized carbon nanotubes in a sodium hydroxide aqueous solution (concentration of 1 mol / L), add polyvinylpyrrolidone, stir thoroughly and ultrasonicate for 5 minutes, then place in a water bath heating pot, stir and react at 200 r / min and 85°C for 12 hours. After the reaction is completed, wash with 85-100°C hot pure water and centrifuge until the pH is less than 8, and then dry at 150°C for 12 hours.

[0069] The mass ratio of the oxidized carbon nanotubes to the polyvinyl pyrrolidone and the alkaline solution is 100:0.6:40; wherein the alkaline solution is measured on a dry basis;

[0070] (4) The dried carbon nanotubes were placed in a tube furnace and heated to 1200°C under the protection of argon gas. The mixture was then aerated at a ratio of 1:4 for 1.5 hours. The flow rate of 300 mL / min of HCl was used. The mixture was then heated to 1500°C and replaced with Freon gas. The mixture was aerated at a ratio of 1:2 for 2 hours. The flow rate of Freon gas was 300 mL / min. The mixture was finally cooled to room temperature under the protection of argon gas.

[0071] The resulting solid carbon nanotube sample was subjected to ash content and ICP impurity full scan testing. After calcination at 950°C for 5 hours, the ash content was 0.18%. The Fe content in the carbon nanotubes was 5 ppm, the Al content was 4 ppm, the Mg content was 6 ppm, and the Si content was 14 ppm.

[0072] The only difference between Example 3 and Example 1 is that: in the purification stage, the Freon gas flow rate is increased by two times, and the obtained carbon nanotubes have higher purity and lower silicon content. The results show that increasing the amount of Freon within a certain range in the atmosphere purification stage can achieve better impurity removal effect.

[0073] Example 4

[0074] Step (1) and step (2) are the same as in Example 1;

[0075] Step (3): Immerse the oxidized carbon nanotubes in a sodium hydroxide aqueous solution (concentration of 5 mol / L), add polyvinylpyrrolidone, stir thoroughly and ultrasonicate for 5 minutes, then place in a water bath heating pot, stir and react at 200 r / min and 85°C for 12 hours, after which the reaction is completed, wash with 85-100°C hot pure water and centrifuge until the pH is less than 8, and then dry at 150°C for 12 hours.

[0076] The mass ratio of the oxidized carbon nanotubes to the polyvinyl pyrrolidone and the alkaline solution is 100:0.6:60; wherein the alkaline solution is measured on a dry basis;

[0077] Step (4) is the same as in Example 1.

[0078] Ash content and ICP impurity full-scan testing were performed on the resulting solid carbon nanotube sample. After calcination at 950°C for 5 hours, the ash content was 0.52%. The Fe content, Al content, Mg content, and Si content in the carbon nanotubes were 18 ppm, 22 ppm, 31 ppm, and 118 ppm, respectively.

[0079] The only difference between Example 4 and Example 1 is that the amount of sodium hydroxide used is increased to a greater extent, but the Si content in the test results does not decrease significantly, but increases slightly. This shows that in the process of alkaline washing of carbon nanotubes, controlling the concentration of the alkaline solution can improve the silicon removal efficiency.

[0080] Comparative Example 1

[0081] Step (1) and step (2) are the same as in Example 1;

[0082] Step (3): Immerse the oxidized carbon nanotubes in water (the volume of water is the same as that of the sodium hydroxide aqueous solution), add polyvinyl pyrrolidone, stir thoroughly, and then ultrasonicate for 5 minutes. Then place the carbon nanotubes in a water bath and stir at 200 r / min and 85°C for 12 hours. After the reaction, wash with 85-100°C hot pure water and centrifuge, then dry at 150°C for 12 hours. The mass ratio of carbon nanotubes to polyvinyl pyrrolidone is 100:0.6.

[0083] Step (4) is the same as in Example 1 to obtain purified carbon nanotubes.

[0084] The resulting solid carbon nanotube sample was subjected to ash content and ICP impurity full-scan testing. After calcination at 950°C for 5 hours, the ash content was 0.86%. The carbon nanotubes contained 36 ppm of Fe, 12 ppm of Al, 23 ppm of Mg, and 437 ppm of Si.

[0085] The only difference between Comparative Example 1 and Example 1 is that no alkali was added in the alkali washing stage, and the sodium hydroxide aqueous solution was replaced with an equal mass of pure water. However, the purity of the obtained carbon nanotubes decreased and the silicon content increased. The results show that the impurities cannot be effectively removed without alkali washing. In the preparation process of carbon nanotubes, the use of alkali in the alkali washing stage is necessary.

[0086] Comparative Example 2

[0087] Step (1), step (2), and step (3) are the same as in Example 1;

[0088] Step (4): Place the dried carbon nanotubes in a tube furnace, heat them to 1200°C under the protection of argon, and then aerate them at a ratio of hydrogen chloride flow rate: argon flow rate = 1:4 for 1.5 hours, wherein the hydrogen chloride flow rate is 300 mL / min, and then cool them to room temperature under the protection of argon.

[0089] The resulting solid carbon nanotube sample was subjected to ash content and ICP impurity full-scan testing. After calcination at 950°C for 5 hours, the ash content was 1.88%. The carbon nanotubes contained 42 ppm of Fe, 11 ppm of Al, 12 ppm of Mg, and 862 ppm of Si.

[0090] The only difference between Comparative Example 2 and Example 1 is that no fluorine-containing gas is introduced for purification. However, the purity of the obtained carbon nanotubes decreases and the silicon content increases. The results show that the impurities cannot be effectively removed without introducing fluorine-containing gas.

[0091] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention may be subjected to a variety of simple modifications, and the various technical features may be combined in any other appropriate manner. These simple modifications and combinations shall also be regarded as disclosed by the present invention and fall within the scope of protection of the present invention.

Claims

1. A method for removing silicon and purifying carbon nanotubes, characterized by: The method comprises the following steps: (1) Using vermiculite as a substrate, generating carbon nanotubes on the surface of the substrate by a CVD method, and then pre-oxidizing the carbon nanotubes to obtain oxidized carbon nanotubes; (2) mixing the oxidized carbon nanotubes with an alkaline solution and an alkali-resistant water-soluble dispersant for alkaline washing; the alkali-resistant water-soluble dispersant is selected from at least one of polyvinyl pyrrolidone, polyvinyl alcohol, hydroxyethylene diphosphonic acid, an alkali-resistant cellulose compound, and a compound containing a polyoxyethylene ether functional group; (3) washing the alkali-washed carbon nanotubes with water, centrifuging them, and drying them in sequence to obtain dry carbon nanotubes; the water temperature for the washing is 70-100°C; (4) The dried carbon nanotubes are subjected to a first purification treatment with a chlorine-containing gas under a protective atmosphere, and then subjected to a second purification treatment with a fluorine-containing gas to obtain purified carbon nanotubes.

2. The method for removing silicon and purifying carbon nanotubes according to claim 1, wherein: The pre-oxidation treatment conditions are: temperature of 400-700° C. and time of 30-100 min.

3. The method for removing silicon and purifying carbon nanotubes according to claim 1 or 2, characterized in that: The mass ratio of the oxidized carbon nanotubes to the alkali-resistant water-soluble dispersant and the alkali solution is 100:0.5-2:30-50; wherein the alkali solution is measured on a dry basis; And / or, the concentration of the alkaline solution is 0.5-4 mol / L.

4. The method for removing silicon and purifying carbon nanotubes according to claim 1 or 2, characterized in that: The alkali-resistant water-soluble dispersant is selected from polyvinyl pyrrolidone and / or polyvinyl alcohol.

5. The method for removing silicon and purifying carbon nanotubes according to claim 1 or 2, characterized in that: The alkaline solution is sodium hydroxide solution and / or potassium hydroxide solution; And / or, the alkali washing is carried out under the condition of water bath heating, and the conditions of the alkali washing are: temperature of 70-90° C., stirring rate of 100-500 r / min, and time of 8-12 h.

6. The method for removing silicon and purifying carbon nanotubes according to claim 1 or 2, characterized in that: The water washing conditions are controlled so that the pH value of the washing liquid obtained after the water washing is less than 8.

7. The method for removing silicon and purifying carbon nanotubes according to claim 1 or 2, characterized in that: The drying conditions are as follows: temperature of 120° C. to 150° C. and time of 2 to 15 hours.

8. The method for removing silicon and purifying carbon nanotubes according to claim 1 or 2, characterized in that: The chlorine-containing gas is selected from at least one of hydrogen chloride and chlorine; the fluorine-containing gas is selected from at least one of Freon and fluorine.

9. The method for removing silicon and purifying carbon nanotubes according to claim 1 or 2, characterized in that: The conditions of the first purification treatment are: temperature of 800-1400° C., time of 1-8 hours; flow ratio of the chlorine-containing gas to the protective atmosphere of 1:1-10; And / or, the conditions of the second purification treatment are: temperature of 1500-1800° C., time of 2-4 hours; flow ratio of the fluorine-containing gas to the protective atmosphere of 1:1-10.

10. Carbon nanotubes prepared by the method for removing silicon and purifying carbon nanotubes according to any one of claims 1 to 9.