Method for removing silicon and purifying carbon nano tube and carbon nano tube

Abstract

The invention discloses a method for removing silicon and purifying a carbon nano tube and the carbon nano tube, and belongs to the field of carbon nano tube purification. The method comprises the following steps: taking vermiculite as a substrate, generating a carbon nanotube on the surface of the substrate through a CVD (Chemical Vapor Deposition) method, and then carrying out pre-oxidation treatment on the carbon nanotube to obtain an oxidized carbon nanotube; uniformly mixing the oxidized carbon nanotubes with an alkali solution and an alkali-resistant water-soluble dispersant, and carrying out alkali washing; sequentially carrying out water washing, centrifugal treatment and drying treatment on the carbon nano tube subjected to alkali washing to obtain a dried carbon nano tube; and carrying out first purification treatment on the dried carbon nano tube and chlorine-containing gas in a protective atmosphere, and then carrying out second purification treatment on the dried carbon nano tube and fluorine-containing gas to obtain the purified carbon nano tube. According to the method, the carbon nanotubes are purified by combining the gas-phase oxidation method, the alkali washing method and the atmosphere high-temperature purification method, the purity of the carbon nanotubes is greatly improved while the tube wall structures of the carbon nanotubes are not damaged, and the conductivity of the carbon nanotubes is improved.

Description

Technical Field

[0001] The 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 Si / SiO2 as a 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 structure and microporous structure, which makes it an ideal template for the preparation of carbon nanotubes and provides an ideal substrate for the orderly arrangement and directional growth of carbon nanotubes. During the CVD process, the carbon source can be catalytically cracked between the layers of vermiculite to generate carbon nanotubes. The vermiculite substrate has good thermal and chemical stability, is not easy to react with common catalysts and carbon sources, and can maintain structural stability in high temperature and corrosive environments, which helps to maintain the morphology and structure of carbon nanotubes during the CVD preparation process, thereby improving its application performance and ensuring the stability and controllability of the preparation process. At the same time, vermiculite is also a widely distributed mineral with abundant resources and easy to obtain. Therefore, the preparation of carbon nanotubes using vermiculite as raw material has a low cost and is easy to achieve large-scale production.

[0003] However, the main component of vermiculite is magnesium-containing hydroaluminosilicate, and its SiO2 content accounts for about 37~42%. Therefore, in addition to the common impurities of amorphous carbon and metal catalyst particles, the carbon nanotubes prepared by CVD on the vermiculite substrate 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, which will change the carrier mobility in carbon nanotubes, leading to increased charge scattering or reduced charge transfer efficiency, and increase the resistance of carbon nanotubes. Conventional oxidation and high-temperature purification methods cannot remove them; acid washing method for silicon removal requires the use of highly toxic hydrofluoric acid, 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, wherein carbon nanotubes obtained by CVD are placed in a reactor, and oxidizing gas is introduced for oxidation treatment to obtain primary oxidized carbon nanotubes; the primary oxidized carbon nanotubes are soaked in an oxidant acid solution to undergo oxidation reaction to obtain secondary oxidized carbon nanotubes; protective atmosphere and halogen atmosphere are introduced into the secondary oxidized carbon nanotubes for halogenation and volatilization to obtain purified carbon nanotubes. However, this method can effectively remove most of the metal impurities in carbon nanotubes, but cannot purify carbon nanotubes with a 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 of poor removal effect, low purity of the obtained carbon nanotubes and high energy consumption in the methods of the prior art for removing silicon-containing impurities in carbon nanotubes, the purpose of the present invention is to provide a method for removing silicon and purifying carbon nanotubes. The method utilizes three methods, namely gas phase oxidation-alkaline washing-atmosphere high temperature purification, to purify the carbon nanotubes, thereby greatly improving the purity of the carbon nanotubes and enhancing their conductivity without destroying the wall structure of the carbon nanotubes.

[0007] In order to achieve the above object, the present invention provides a method for removing silicon and purifying carbon nanotubes, the method comprising 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 at least one selected from the group consisting 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 and drying them in sequence to obtain dry carbon nanotubes; (4) The dried carbon nanotubes are first 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.

[0008] Vermiculite is used as a substrate, and carbon nanotubes are generated on the surface of the substrate by CVD method. The general purity is 88-92%, which contains 6000-22000ppm of metal impurities, a small amount of amorphous carbon, and some silicon-containing impurities. The key technology of the present invention is that the carbon nanotubes are first subjected to gas phase oxidation treatment, the carbon nanotubes are exposed to the air, and the amorphous carbon impurities adhering to the surface of the carbon nanotubes are first oxidized; then an alkaline solution and an alkaline-resistant water-soluble dispersant are added to the carbon nanotubes, and after ultrasonic stirring for a period of time, water bath heating and stirring (i.e., alkaline washing) are carried out. After the reaction is completed, the carbon nanotubes are washed and centrifuged with hot pure water, and the pH value of the washing liquid is <8, and then drying is carried out. During the alkaline washing process, a certain amount of alkaline solution and other chemical substances that may be produced will be adsorbed on the surface of the carbon nanotubes. Hot water has a high solubility, which can effectively dissolve and take away these residues, ensure the purity of the carbon nanotubes, and preliminarily remove silicon.

[0009] Finally, under a protective atmosphere, the carbon nanotubes are first purified with a chlorine-containing gas and then purified with a fluorine-containing gas, 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.

[0010] The inventors found that in an alkaline environment, the surface of the carbon nanotubes will be negatively charged, and this change is due to the adsorption and reaction of hydroxide ions on the surface of the carbon nanotubes. The hydroxide ions not only fit tightly on the surface of the carbon nanotubes, but also interact with the functional groups or defect sites on the surface, thereby increasing the negative charge and enhancing the electrostatic repulsion between the carbon nanotubes. This electrostatic repulsion makes it more difficult for the carbon nanotubes to approach each other and agglomerate, thereby presenting a better dispersion state in the solution. More importantly, this improvement in dispersibility 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.

[0011] 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 evenly dispersed in the alkali solution, and improve production efficiency.

[0012] In the preferred embodiment, the pre-oxidation treatment conditions are: temperature of 400-700°C and time of 30-100 min. When the reaction temperature is low, the reaction is incomplete, and more amorphous carbon affects the purity of carbon nanotubes, further affecting their performance and application. At higher temperatures, carbon nanotubes are oxidized, resulting in a higher loss rate.

[0013] In a preferred embodiment, 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 by its dry basis mass. Too little alkali-resistant water-soluble dispersant cannot effectively disperse the carbon nanotubes, while too much alkali may introduce other impurities; too much alkali may cause excessive corrosion of the carbon nanotubes, while too low a concentration may result in poor silicon removal effect.

[0014] In a preferred embodiment, the alkali-resistant water-soluble dispersant is selected from polyvinyl pyrrolidone and / or polyvinyl alcohol.

[0015] In a preferred embodiment, the concentration of the alkaline solution is 0.5-4 mol / L.

[0016] In a preferred embodiment, the alkaline solution is a sodium hydroxide solution and / or a potassium hydroxide solution.

[0017] 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.

[0018] In the preferred embodiment, 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 100-500r / min, and time of 8-12h. Under this preferred condition, it is beneficial to remove residual alkali solution, improve cleaning efficiency, reduce water molecule adsorption, and is more conducive to subsequent purification treatment.

[0019] 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.

[0020] In a preferred embodiment, the water washing condition is as follows: the water temperature is 70-100° C., and the water washing condition is controlled so that the pH value of the washing liquid obtained after the water washing is less than 8.

[0021] In a preferred embodiment, the drying treatment is carried out at a temperature of 120° C. to 150° C. for 2 to 15 hours.

[0022] 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 in the carbon nanotubes, and the fluorine-containing gas can remove silicon-containing impurities in the carbon nanotubes, respectively generating metal chlorides and silicon-containing fluorides with lower boiling points.

[0023] In a preferred embodiment, the protective atmosphere is selected from at least one of argon, nitrogen and helium.

[0024] In a preferred embodiment, 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.

[0025] In a more preferred embodiment, the flow ratio of the chlorine-containing gas to the protective atmosphere is 1:1 to 3. The inventors found that under the condition of a flow ratio of 1:1 to 3, the purity of the obtained carbon nanotubes is higher.

[0026] In a preferred embodiment, the conditions of the second purification treatment are: temperature of 1500-1800°C, time of 2-4 hours, and flow ratio of the fluorine-containing gas to the protective atmosphere of 1:1-10.

[0027] 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.

[0028] The main purpose of the purification treatment of the present invention is that the gases generated by the high-temperature decomposition of chlorine-containing gases and fluorine-containing gases can react with the metal impurities and silicon-containing impurities in the carbon nanotubes 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.

[0029] The present invention provides a method for removing silicon and purifying carbon nanotubes, which specifically comprises the following steps: 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 temperature in the reactor is heated to 500-900°C, and finally a carbon source gas is introduced to react and obtain carbon nanotubes.

[0030] S2: placing the carbon nanotubes in a tubular furnace, keeping both ends of the tubular furnace connected to the air, heating the furnace to 400-700° C. in an air atmosphere for oxidation treatment for 30-100 min to obtain oxidized carbon nanotubes; S3: mixing the oxidized carbon nanotubes with an alkaline solution and an alkaline-resistant water-soluble dispersant for alkaline washing; washing the carbon nanotubes after the alkaline washing with water, centrifuging and drying them in sequence to obtain dry carbon nanotubes; the alkaline washing allows the silicon-containing impurities to react with the alkaline solution; S4: placing the dried carbon nanotubes in a tubular furnace, introducing a protective gas, firstly subjecting the dried carbon nanotubes to a first purification treatment with a chlorine-containing gas at a temperature of 800-1400° C. for 1-8 hours, and then subjecting the dried carbon nanotubes to 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.

[0031] 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, calcium, etc.

[0032] In a preferred embodiment, the reducing gas is hydrogen.

[0033] 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.

[0034] 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.

[0035] Compared with the prior art, the present invention has at least the following advantages: (1) The purification method of the present invention combines gas phase oxidation-alkali 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 improves the purity of the carbon nanotubes, and thus effectively improves the electrical properties and thermal conductivity of the carbon nanotubes.

[0036] (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.

[0037] (3) The present invention first uses gas oxidation for preliminary purification to remove amorphous carbon, then further removes silicon-containing impurities in carbon nanotubes by soaking in a dispersant and alkaline solution, and finally, under certain temperature conditions, chlorine-containing gas and fluorine-containing gas react with metal impurities and silicon-containing impurities to generate metal chlorides and silicon-containing fluorides, which are then volatilized and taken away. Therefore, the present invention achieves deep purification of carbon nanotubes under the synergistic effect of gas oxidation, alkaline washing purification and gas high-temperature purification.

[0038] (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

[0039] The endpoints and any values ​​of the ranges disclosed in this article 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 endpoint values ​​of each range, the endpoint values ​​of each range and the 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 as specifically disclosed in this article.

[0040] The present invention is further described below in conjunction with specific embodiments, but the protection scope 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 creative work still belong to the protection scope of the present invention.

[0041] 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.

[0042] In the following examples, in step (2), the amount of multi-walled carbon nanotubes used is 100 g.

[0043] Example 1

[0044] (1) A Fe-Si catalyst system with vermiculite as a carrier was prepared by chemical vapor deposition, i.e., the catalyst was spread flat in a CVD tube furnace, nitrogen as a protective gas and hydrogen as a reducing gas were introduced, the reaction was carried out at 650°C for 5 min, and then propylene as a carbon source gas was introduced and the reaction was carried out for 1 h, thereby obtaining multi-walled carbon nanotubes with a purity of 88-92% (i.e., an ash content of 8-12%), 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. (2) placing the prepared multi-walled carbon nanotubes in a tubular furnace, keeping both ends of the tubular furnace connected to the outside air, and heating the reactor to 550°C for 30 min; (3) The oxidized carbon nanotubes were immersed in a sodium hydroxide aqueous solution (concentration of 1 mol / L), polyvinyl pyrrolidone was added, and the mixture was stirred thoroughly and ultrasonicated for 5 min. Then, the mixture was placed in a water bath and stirred at 200 r / min and 85 °C for 12 h. After the reaction, the mixture was washed with 85-100 °C hot pure water and centrifuged until the pH was <8. Then, the mixture was dried at 150 °C for 12 h.

[0045] 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 by its dry basis mass; (4) The dried carbon nanotubes were placed in a tube furnace, heated to 1200°C under the protection of argon, and then reacted for 1.5 hours at a ratio of hydrogen chloride flow rate: argon flow rate = 1:4. The hydrogen chloride flow rate was 300 mL / min. The temperature was then raised to 1500°C, and the hydrogen chloride was replaced with freon. The reaction was carried out at a ratio of freon flow rate: argon flow rate = 1:4 for 2 hours. The freon flow rate was 300 mL / min. Finally, the mixture was cooled to room temperature under the protection of argon.

[0046] The final carbon nanotube solid sample was subjected to ash content and ICP impurity full scan test. After calcination at 950°C for 5 hours, the ash content was detected to be 0.23%. The Fe content in the carbon nanotubes was measured to be 6ppm, the Al content was 15ppm, the Mg content was 10ppm, and the Si content was 82ppm.

[0047] Example 2

[0048] (1) A Fe-Si catalyst system with vermiculite as a carrier was prepared by chemical vapor deposition, i.e., the catalyst was spread flat in a CVD tube furnace, nitrogen as a protective gas and hydrogen as a reducing gas were introduced, the reaction was carried out at 650°C for 5 min, and then propylene as a carbon source gas was introduced and the reaction was carried out for 1 h, thereby obtaining multi-walled carbon nanotubes with a purity of 88-92% (i.e., an ash content of 8-12%), 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. (2) placing the prepared multi-walled carbon nanotubes in a tubular furnace, keeping both ends of the tubular furnace connected to the outside air, and heating the reactor to 550°C for 30 min; (3) The oxidized carbon nanotubes were immersed in a sodium hydroxide aqueous solution (concentration of 2 mol / L), polyvinyl pyrrolidone was added, and the mixture was stirred thoroughly and ultrasonicated for 5 min. Then, the mixture was placed in a water bath and stirred at 200 r / min and 85 °C for 12 h. After the reaction, the mixture was washed with 85-100 °C hot pure water and centrifuged until the pH was <8. Then, the mixture was dried at 150 °C for 12 h.

[0049] 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 by its dry basis mass; (4) The dried carbon nanotubes were placed in a tube furnace, heated to 1200°C under the protection of argon, and then reacted for 1.5 hours at a ratio of hydrogen chloride flow rate: argon flow rate = 1:4. The hydrogen chloride flow rate was 300 mL / min. The temperature was then raised to 1500°C, and the hydrogen chloride was replaced with freon. The reaction was carried out at a ratio of freon flow rate: argon flow rate = 1:4 for 2 hours. The freon flow rate was 300 mL / min. Finally, the mixture was cooled to room temperature under the protection of argon.

[0050] The solid sample of carbon nanotubes finally obtained was subjected to ash content and ICP impurity full scan test. After being calcined at 950°C for 5h, the ash content was detected to be 0.12%, and the Fe content in the carbon nanotubes was measured to be 11ppm, the Al content was 8ppm, the Mg content was 3ppm, and the Si content was 55ppm. The only difference between Example 2 and Example 1 is that in the alkali washing stage, the amount of sodium hydroxide and polyvinylpyrrolidone was increased, and the purity of the obtained carbon nanotubes was higher and the silicon content was lower. The results show that increasing the concentration of alkali within a certain range in the alkali washing stage has a better impurity removal effect.

[0051] Example 3

[0052] (1) A Fe-Si catalyst system with vermiculite as a carrier was prepared by chemical vapor deposition, i.e., the catalyst was spread flat in a CVD tube furnace, nitrogen as a protective gas and hydrogen as a reducing gas were introduced, the reaction was carried out at 650°C for 5 min, and then propylene as a carbon source gas was introduced and the reaction was carried out for 1 h, thereby obtaining multi-walled carbon nanotubes with a purity of 88-92% (i.e., an ash content of 8-12%), 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. (2) placing the prepared multi-walled carbon nanotubes in a tubular furnace, keeping both ends of the tubular furnace connected to the outside air, and heating the reactor to 550°C for 30 min; (3) The oxidized carbon nanotubes were immersed in a sodium hydroxide aqueous solution (concentration of 1 mol / L), polyvinyl pyrrolidone was added, and the mixture was stirred thoroughly and ultrasonicated for 5 min. Then, the mixture was placed in a water bath and stirred at 200 r / min and 85 °C for 12 h. After the reaction, the mixture was washed with 85-100 °C hot pure water and centrifuged until the pH was <8. Then, the mixture was dried at 150 °C for 12 h.

[0053] 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 by its dry basis mass; (4) The dried carbon nanotubes were placed in a tube furnace, heated to 1200°C under the protection of argon, and then reacted for 1.5 hours at a ratio of hydrogen chloride flow rate: argon flow rate = 1:4. The hydrogen chloride flow rate was 300 mL / min. The temperature was then raised to 1500°C, the hydrogen chloride was replaced with freon, and the reaction was carried out at a ratio of freon flow rate: argon flow rate = 1:2 for 2 hours. The freon flow rate was 300 mL / min. Finally, the mixture was cooled to room temperature under the protection of argon.

[0054] The obtained carbon nanotube solid sample was subjected to ash content and ICP impurity full scan test. After being calcined at 950°C for 5 hours, the ash content was 0.18%, and the Fe content, Al content, Mg content, and Si content in the carbon nanotube were 5ppm, 4ppm, 6ppm, and 14ppm, respectively.

[0055] 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.

[0056] Example 4

[0057] Step (1) and step (2) are the same as in Example 1; Step (3): immerse the oxidized carbon nanotubes in a sodium hydroxide aqueous solution (concentration of 5 mol / L), add polyvinyl pyrrolidone, stir thoroughly and then ultrasonicate for 5 minutes, then put into a water bath heating pot, stir and react for 12 hours at 200 r / min and 85°C. After the reaction is completed, wash with 85~100°C hot pure water and centrifuge until the pH is less than 8, then dry at 150°C for 12 hours.

[0058] 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 by its dry basis mass; Step (4) is the same as in Example 1.

[0059] The final carbon nanotube solid sample was subjected to ash content and ICP impurity full scan test. After being calcined at 950°C for 5 hours, the ash content was 0.52%, and the Fe content in the carbon nanotubes was 18ppm, the Al content was 22ppm, the Mg content was 31ppm, and the Si content was 118ppm.

[0060] 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, indicating that in the process of alkali washing of carbon nanotubes, controlling the concentration of the alkali solution can improve the silicon removal efficiency.

[0061] Comparative Example 1 Step (1) and step (2) are the same as in Example 1; 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 ultrasonicate for 5 minutes, then put into a water bath heating pot, stir and react for 12 hours at 200r / min and 85°C, wash and centrifuge with 85-100°C hot pure water after the reaction, and then dry at 150°C for 12 hours. The mass ratio of carbon nanotubes to polyvinyl pyrrolidone is 100:0.6.

[0062] Step (4) is the same as in Example 1 to obtain purified carbon nanotubes.

[0063] The final carbon nanotube solid sample was subjected to ash content and ICP impurity full scan test. After being calcined at 950°C for 5 hours, the ash content was 0.86%, and the Fe content in the carbon nanotubes was 36ppm, the Al content was 12ppm, the Mg content was 23ppm, and the Si content was 437ppm.

[0064] The only difference between Comparative Example 1 and Example 1 is that: in the alkaline washing stage, no alkali is added, and the sodium hydroxide aqueous solution is replaced by pure water of equal mass. 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 alkaline washing. In the preparation process of carbon nanotubes, the use of alkali in the alkaline washing stage is necessary.

[0065] Comparative Example 2 Step (1), step (2), and step (3) are the same as in Example 1; Step (4): placing the dried carbon nanotubes in a tubular furnace, heating them to 1200°C under the protection of argon, and then aerating them for 1.5 hours at a ratio of hydrogen chloride flow rate: argon flow rate = 1:4, wherein the hydrogen chloride flow rate is 300 mL / min, and then cooling them to room temperature under the protection of argon.

[0066] The final carbon nanotube solid sample was subjected to ash content and ICP impurity full scan test. After being calcined at 950°C for 5 hours, the ash content was 1.88%, and the Fe content in the carbon nanotubes was 42ppm, the Al content was 11ppm, the Mg content was 12ppm, and the Si content was 862ppm.

[0067] 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.

[0068] 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 can be subjected to a variety of simple modifications, and each technical feature can be combined in any other suitable manner. These simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for removing silicon and purifying carbon nanotubes, characterized in that: 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 at least one selected from the group consisting 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 and drying them in sequence to obtain dry carbon nanotubes; (4) The dried carbon nanotubes are first 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, characterized in that: The pre-oxidation treatment conditions are: temperature of 400-700° C. and time of 30-100 min.

3. A 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 by its dry basis mass; And / or, the concentration of the alkaline solution is 0.5~4 mol / L.

4. A 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. A method for removing silicon and purifying carbon nanotubes according to claim 1 or 2, characterized in that: The alkaline solution is a sodium hydroxide solution and / or a 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. A method for removing silicon and purifying carbon nanotubes according to claim 1 or 2, characterized in that: 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.

7. A 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. A 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. A 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 purifying carbon nanotubes by removing silicon according to any one of claims 1 to 9.