Method for purifying carbon nanotubes

Abstract

The invention belongs to the technical field of carbon nanotube purification, and provides a carbon nanotube purification method. The method comprises the following steps: placing unpurified carbon nanotube raw powder in a fixed bed Joule thermal reactor; introducing weak oxidizing gas, heating to enable the temperature of the reaction zone to reach 100-400 DEG C, generating first Joule heat at the carbon nanotube raw powder, and removing an impurity carbon layer coating the surfaces of the metal catalyst particles; heating to enable the temperature of the reaction zone to reach 200-700 DEG C, generating second Joule heat at the carbon nanotube raw powder, and removing impurity carbon on the surface of the carbon nanotube; and mixing the treated carbon nanotube raw powder with acid, and removing the metal catalyst particles. According to the present invention, the weak oxidation gas is adopted as the reaction gas, the electric field is introduced under the external low temperature condition to perform the stepped purification, and the Joule heat is selectively generated on the different types of the impurity carbon and the metal catalyst micro-region so as to achieve the effect of selectively removing the multiple types of the impurity carbon under the mild condition.

Description

Technical Field

[0001] The present disclosure relates to the technical field of carbon nanotube purification, and in particular to a method for purifying carbon nanotubes. Background Art

[0002] As an advanced functional material, carbon nanotubes have always been a hot research topic in the nano field due to their excellent electrical and mechanical properties and broad application prospects. However, whether carbon nanotubes are prepared by arc discharge or chemical vapor deposition, the products contain more or less impurities such as amorphous carbon, graphite-like fragments, catalyst particles, etc. For example, carbon nanotubes prepared by chemical vapor deposition often produce some impurity carbon, such as graphite carbon and amorphous carbon. On the one hand, these impurity carbons will wrap the metal catalysts, making it difficult for the metal catalysts to contact the acid solution and be removed during the subsequent impurity removal process, affecting the purity of the carbon nanotubes; on the other hand, the presence of these impurity carbons will affect the overall mechanical and electrical properties of the carbon nanotubes. Therefore, the purification of carbon nanotubes is an important step to improve the purity and quality of the product.

[0003] At present, the carbon nanotube purification technologies that have been developed and widely used include: centrifugal separation, electrophoresis purification, acid solution / gas phase oxidation, chemical modification, filtration and chromatography, microwave heating, etc. Among the above purification technologies, the gas phase oxidation method (oxygen, air, CO2, ozone, etc.) and the acid solution dissolution method (hydrochloric acid, sulfuric acid, nitric acid, etc.) are generally used to treat large quantities of carbon nanotubes, but this method has major disadvantages. First, it may introduce oxidizing functional groups and cause more serious damage to the structure of carbon nanotubes; secondly, the process often requires a higher temperature, resulting in high energy consumption. For example, when using CO2 oxidation, the temperature for removing impurity carbon is as high as 1000°C; finally, it is difficult to control the carbon nanotubes from being oxidized when oxidizing impurity carbon, which often causes a part of the carbon nanotubes to be sacrificed. Another way to remove impurity carbon is to pass current through the carbon nanotube fiber bundle. The current instantly generates high temperature, causing a large amount of thermal expansion of the impurity carbon, thereby peeling off from the carbon nanotubes, and generating electric sparks to be burned by air / oxygen. This method is still quite intense, the spark temperature can reach thousands of degrees Celsius, which will cause a certain degree of carbon nanotube loss and pose certain safety risks. It can be seen that since the thermal stability of carbon nanotubes and impurity carbon is similar, overly intense oxidation conditions will destroy a certain amount of carbon nanotubes while removing impurity carbon, which not only reduces the yield of carbon nanotubes, but also loses the mechanical and electrical properties of carbon nanotubes.

[0004] Therefore, it is necessary to develop mild and highly selective technologies to remove impurity carbon to minimize damage and loss to carbon nanotubes. Summary of the invention

[0005] The present disclosure provides a method for purifying carbon nanotubes to at least solve the above technical problems existing in the prior art.

[0006] According to a first aspect of the present disclosure, a method for purifying carbon nanotubes is provided, comprising the following steps:

[0007] S1: placing unpurified carbon nanotube raw powder in a fixed bed Joule heat reactor; introducing a weak oxidizing gas into the fixed bed Joule heat reactor, heating the reaction zone to 100° C. to 400° C., generating a first Joule heat at the unpurified carbon nanotube raw powder to remove the impurity carbon layer coated on the surface of the metal catalyst particles in the carbon-encapsulated metal structure of the unpurified carbon nanotube raw powder;

[0008] S2: heating the reaction zone to a temperature of 200° C. to 700° C., generating a second Joule heat at the carbon nanotube raw powder after being treated in step S1, so as to remove impurity carbon attached to the surface of the carbon nanotubes in the carbon nanotube raw powder;

[0009] S3: mixing the carbon nanotube raw powder treated in step S2 with an acid to remove the metal catalyst particles exposed in the carbon-enclosed metal structure.

[0010] Specifically, the carbon nanotube raw powder prepared by chemical vapor deposition and other methods is a complex mixture of carbon nanotubes, impurity carbon (graphite carbon, amorphous carbon, etc.), and metal catalysts (i.e., the metal catalyst particles mentioned above). The resistivity of the carbon nanotubes is much lower than the resistivity of the impurity carbon and the metal catalyst. Therefore, when a certain power current is passed through the carbon nanotube raw powder to generate Joule heat, the Joule heat is mainly concentrated on the impurity carbon and the metal catalyst and heats both, so that both generate a temperature much higher than that of the carbon nanotubes. At this time, combined with weak oxidizing gas at a relatively low temperature (100°C to 400°C. Since it is easy to ablate carbon nanotubes during the purification process under strong oxidizing atmospheres such as oxygen and temperatures above 400°C, the combination of weak oxidizing atmosphere and low temperature is selected to ensure that the carbon nanotubes are not damaged during the purification process) step-by-step purification can selectively oxidize different types of impurity carbon to ensure that the carbon nanotubes are not damaged, solving the high energy consumption problem and incomplete removal of impurity carbon in the traditional weak oxidizing atmosphere treatment of carbon nanotubes. Then, an auxiliary acid treatment is performed. Since the outer carbon shell of the carbon-metal structure has been removed, the exposed metal catalyst particles can be effectively dissolved by the acid solution, thereby improving the purity of the carbon nanotubes.

[0011] In one possible implementation, the unpurified carbon nanotube raw powder is synthesized by at least one of chemical vapor deposition, arc discharge, and laser evaporation.

[0012] Specifically, the method for synthesizing unpurified carbon nanotube raw powder is not limited to the above method, and any method may be used as long as it can synthesize carbon nanotubes.

[0013] In one embodiment, the weak oxidizing gas is selected from at least one of carbon dioxide, water vapor, carbon monoxide, and nitric oxide.

[0014] In one embodiment, the weak oxidizing gas accounts for 0.1% to 50% of the total volume of the gas introduced into the fixed bed Joule heat reactor, and the remaining gas is an inert protective gas.

[0015] In one embodiment, the inert protective gas is selected from at least one of argon, helium and nitrogen.

[0016] In one embodiment, the metal particles are selected from at least one of chromium, manganese, iron, cobalt, nickel, rhodium, palladium and molybdenum.

[0017] In one embodiment, step S1 generates the first Joule heat by applying a current having a first input power to the unpurified carbon nanotube raw powder.

[0018] Specifically, the first input power is 0.1W to 20W.

[0019] Specifically, the time for applying the current with the first input power is 0.1 min to 150 min.

[0020] In one embodiment, conductive filter elements are provided at both ends of the fixed bed Joule heat reactor to fix the carbon nanotube raw powder; the positive and negative electrodes of the power supply are respectively in contact with the conductive filter elements at both ends of the carbon nanotube raw powder, thereby achieving electrical connection with the carbon nanotube raw powder.

[0021] In one possible implementation manner, after generating the first Joule heat, step S1 further includes the following step: using a temperature measuring element to measure the actual temperature of the carbon nanotube raw powder after the first Joule heat treatment.

[0022] Specifically, the actual temperature of the carbon nanotube raw powder is measured to be 200°C to 1000°C.

[0023] Specifically, the purpose of testing the actual temperature reached by the raw carbon nanotube powder at a macroscopic level is to compare it with the temperature of gas phase purification without using Joule heat, thereby demonstrating the advantage of this method in lowering the reaction temperature.

[0024] Specifically, after the treatment in step S1, the impurity carbon layer coated on the surface of the metal catalyst particles in the unpurified carbon nanotube raw powder is removed, and the metal catalyst particles coated with the impurity carbon are exposed and more easily removed by acid dissolution.

[0025] In one embodiment, step S2 generates the second Joule heat by applying a current having a second input power to the carbon nanotube raw powder.

[0026] Specifically, the second input power is 0.1W to 20W.

[0027] Specifically, the time for applying the current with the second input power is 0.1 min to 150 min.

[0028] In one possible implementation manner, after generating the second Joule heat, step S2 further includes the following step: using a temperature measuring element to measure the actual temperature of the carbon nanotube raw powder after the second Joule heat treatment.

[0029] Specifically, the actual temperature of the carbon nanotube raw powder is measured to be 200°C to 1000°C.

[0030] Specifically, after the processing in step S2, the impurity carbon in the carbon nanotube raw powder that is more stably attached to the surface of the carbon nanotubes is removed, thereby further improving the purity of the carbon nanotubes.

[0031] In one embodiment, the acid in step S3 is selected from at least one of hydrochloric acid, nitric acid and sulfuric acid.

[0032] In one embodiment, in step S3, the carbon nanotube raw powder is immersed in the acid, and heated with stirring at 50°C to 100°C for 6h to 48h; then solid-liquid separation is performed, and the obtained solid sample is dried to remove the metal catalyst particles exposed in the carbon-metal structure.

[0033] In one embodiment, the solid-liquid separation method is selected from any one of centrifugation and filtration.

[0034] In one embodiment, after step S3, the following step is further included: detecting the purity of the carbon nanotubes after the treatment in step S3, if the purity reaches the required standard, the purification is completed; if the purity does not reach the required standard, repeating steps S1 and S2 until the purity reaches the required standard.

[0035] Specifically, the method for detecting the purity of the carbon nanotubes includes detecting by means of at least one instrument selected from the group consisting of a scanning electron microscope, a thermogravimetric analyzer, and an inductively coupled plasma analyzer.

[0036] According to an implementable embodiment of the present disclosure, at least the following beneficial effects are achieved:

[0037] The present disclosure introduces an electric field after compacting the raw carbon nanotube powder sample, which is applicable to different types of carbon nanotubes prepared by different methods and has a wider range of application value. The present disclosure provides a weak oxidizing gas as a reaction gas, introduces an electric field under the condition of a relatively low external temperature for step-by-step purification, selectively generates Joule heat on different types of impure carbon and metal catalyst micro-regions, and the voltage and power applied by the electric field are relatively mild, ultimately achieving the effect of selectively removing various types of impurity carbon under suitable mild conditions, and the purification efficiency is relatively high.

[0038] It should be understood that the content described in this section is not intended to identify the key or important features of the embodiments of the present disclosure, nor is it intended to limit the scope of the present disclosure. Other features of the present disclosure will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The above and other objects, features and advantages of the exemplary embodiments of the present disclosure will become readily understood by reading the detailed description below with reference to the accompanying drawings. In the accompanying drawings, several embodiments of the present disclosure are shown in an exemplary and non-limiting manner, in which:

[0040] In the drawings, the same or corresponding reference numerals represent the same or corresponding parts.

[0041] Figure 1 A scanning electron microscope photograph of the purified single-walled carbon nanotubes in Example 1 of the present disclosure is shown;

[0042] Figure 2 A scanning electron microscope photograph of the purified single-walled carbon nanotubes in Comparative Example 1 of the present disclosure is shown. DETAILED DESCRIPTION

[0043] In order to make the purpose, features, and advantages of the present disclosure more obvious and easy to understand, the technical solutions in the embodiments of the present disclosure will be clearly and completely described below in conjunction with the drawings in the embodiments of the present disclosure. Obviously, the described embodiments are only part of the embodiments of the present disclosure, not all of the embodiments. Based on the embodiments in the present disclosure, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present disclosure.

[0044] Example 1

[0045] In this embodiment, nitrogen (90 vol%) is used as carrier gas, 10 vol% CO2 gas is introduced, and the carbon nanotube raw powder is purified under electric heating. The specific process is as follows:

[0046] (1) 10g of single-walled carbon nanotube powder synthesized by chemical vapor deposition method using a metal iron-based catalyst is placed in the center of a fixed-bed Joule heat reactor, and the positive and negative electrodes of the power supply are contacted respectively through the conductive filter elements at both ends of the reactor, and a K-type thermocouple is used to measure the temperature at the contact point between the electrode and the conductive filter element (i.e., the actual temperature of the carbon nanotube powder after Joule heat treatment). CO2 gas is continuously introduced into the fixed-bed Joule heat reactor, and the furnace temperature of the fixed-bed Joule heat reactor is raised to 300°C. A current with a first input power of 15W is applied to the single-walled carbon nanotube powder to generate the first Joule heat, and the current is maintained for 15 minutes. At this time, the temperature of the single-walled carbon nanotube powder is measured to be as high as 541°C;

[0047] (2) continuously introducing CO2 gas, continuing to raise the furnace temperature of the fixed bed Joule heat reactor to 550°C, applying a current with a second input power of 15 W to the single-walled carbon nanotube raw powder treated in step (1) to generate a second Joule heat, and maintaining the current for 20 minutes. At this time, it is measured that the temperature of the single-walled carbon nanotube raw powder is as high as 733°C;

[0048] (3) The raw single-walled carbon nanotube powder treated in step (2) was immersed in 20 wt% sulfuric acid, stirred and heated at 60°C for 24 h, filtered, and the sample was washed with ultrapure water until neutral, and dried to obtain single-walled carbon nanotubes with a purity of 99.0%.

[0049] The purified samples were observed under a scanning electron microscope. Figure 1 shown. Figure 1 It showed that the impure carbon and iron-based metal catalyst particles on the single-walled carbon nanotubes were removed and the sample was very clean.

[0050] Example 2

[0051] In this embodiment, nitrogen (90 vol%) is used as carrier gas, 10 vol% CO2 gas is introduced, and the carbon nanotube raw powder is purified under electric heating. The specific process is as follows:

[0052] (1) 10g of single-walled carbon nanotube powder synthesized by arc discharge method is loaded into the center of a fixed-bed Joule heat reactor, and the conductive filters at both ends of the reactor are respectively contacted with the positive and negative electrodes of the power supply, and a K-type thermocouple is used to measure the temperature at the contact point between the electrode and the conductive filter (i.e., the actual temperature of the carbon nanotube powder after Joule heat treatment). CO2 gas is continuously introduced into the fixed-bed Joule heat reactor, and the furnace temperature of the fixed-bed Joule heat reactor is raised to 300°C. A current with a first input power of 15W is applied to the single-walled carbon nanotube powder to generate the first Joule heat, and the current is maintained for 15 minutes. At this time, the temperature of the single-walled carbon nanotube powder is measured to be 522°C at most;

[0053] (2) continuously introducing CO2 gas, continuing to raise the furnace temperature of the fixed bed Joule heat reactor to 550°C, applying a second current with a second input power of 15 W to the single-walled carbon nanotube raw powder treated in step (1) to generate a second Joule heat, and maintaining the current for 20 minutes, at which time the temperature of the single-walled carbon nanotube raw powder is measured to be a maximum of 696°C;

[0054] (3) The raw single-walled carbon nanotube powder treated in step (2) was immersed in 20 wt% sulfuric acid, stirred and heated at 60°C for 24 h, filtered, and the sample was washed with ultrapure water until neutral, and dried to obtain single-walled carbon nanotubes with a purity of 99.2%.

[0055] Example 3

[0056] In this embodiment, nitrogen (90 vol%) is used as carrier gas, 10 vol% CO2 gas is introduced, and the carbon nanotube raw powder is purified under electric heating. The specific process is as follows:

[0057] (1) 10g of single-walled carbon nanotube powder synthesized by laser evaporation is loaded into the center of a fixed-bed Joule heat reactor, and the conductive filters at both ends of the reactor are respectively in contact with the positive and negative electrodes of the power supply, and a K-type thermocouple is used to measure the temperature at the contact point between the electrode and the conductive filter (i.e., the actual temperature of the carbon nanotube powder after Joule heat treatment). CO2 gas is continuously introduced into the fixed-bed Joule heat reactor, and the furnace temperature of the fixed-bed Joule heat reactor is raised to 300°C. A current with a first input power of 15W is applied to the single-walled carbon nanotube powder to generate the first Joule heat, and the current is maintained for 15 minutes. At this time, the temperature of the single-walled carbon nanotube powder is measured to be as high as 566°C;

[0058] (2) continuously introducing CO2 gas, continuing to raise the furnace temperature of the fixed bed Joule heat reactor to 550°C, applying a current with a second input power of 15 W to the single-walled carbon nanotube raw powder treated in step (1) to generate a second Joule heat, and maintaining the current for 20 minutes. At this time, it is measured that the temperature of the single-walled carbon nanotube raw powder is as high as 765°C;

[0059] (3) The raw single-walled carbon nanotube powder treated in step (2) was immersed in 20 wt% sulfuric acid, stirred and heated at 60°C for 24 h, filtered, and the sample was washed with ultrapure water until neutral, and dried to obtain single-walled carbon nanotubes with a purity of 99.1%.

[0060] Example 4

[0061] In this embodiment, argon (85 vol%) is used as carrier gas, 15 vol% CO2 gas is introduced, and the carbon nanotube raw powder is purified under electric heating. The specific process is as follows:

[0062] (1) 10g of single-walled carbon nanotube powder synthesized by chemical vapor deposition using a metal nickel-based catalyst is placed in the center of a fixed-bed Joule heat reactor, and the conductive filters at both ends of the reactor are respectively contacted with the positive and negative electrodes of the power supply, and a K-type thermocouple is used to measure the temperature at the contact point between the electrode and the conductive filter (i.e., the actual temperature of the carbon nanotube powder after Joule heat treatment). CO2 gas is continuously introduced into the fixed-bed Joule heat reactor, and the furnace temperature of the fixed-bed Joule heat reactor is raised to 350°C. A current with a first input power of 20W is applied to the single-walled carbon nanotube powder to generate the first Joule heat, and the current is maintained for 20 minutes. At this time, the temperature of the single-walled carbon nanotube powder is measured to be as high as 595°C;

[0063] (2) continuously introducing CO2 gas, continuing to raise the furnace temperature of the fixed bed Joule heat reactor to 600°C, applying a current with a second input power of 20 W to the single-walled carbon nanotube raw powder treated in step (1) to generate a second Joule heat, and maintaining the current for 20 minutes, at which time the temperature of the single-walled carbon nanotube raw powder is measured to be a maximum of 815°C;

[0064] (3) The raw single-walled carbon nanotube powder treated in step (2) was immersed in 20 wt% sulfuric acid, stirred and heated at 60°C for 24 h, filtered, and the sample was washed with ultrapure water until neutral, and dried to obtain single-walled carbon nanotubes with a purity of 99.5%.

[0065] Example 5

[0066] In this embodiment, argon (95 vol%) is used as carrier gas, 5 vol% water vapor is introduced, and the carbon nanotube raw powder is purified under electric heating. The specific process is as follows:

[0067] (1) 10g of single-walled carbon nanotube powder synthesized by chemical vapor deposition method using metal cobalt-based catalyst is loaded into the center of a fixed-bed Joule heat reactor, and the conductive filters at both ends of the reactor are respectively contacted with the positive and negative electrodes of the power supply, and a K-type thermocouple is used to measure the temperature at the contact point between the electrode and the conductive filter (i.e., the actual temperature of the carbon nanotube powder after Joule heat treatment). Water vapor is continuously introduced into the fixed-bed Joule heat reactor, and the furnace temperature of the fixed-bed Joule heat reactor is raised to 250°C. A current with a first input power of 10W is applied to the single-walled carbon nanotube powder to generate the first Joule heat, and the current is maintained for 20 minutes. At this time, the temperature of the single-walled carbon nanotube powder is measured to be as high as 435°C;

[0068] (2) continuously introducing water vapor, and continuously raising the furnace temperature of the fixed bed Joule heat reactor to 500° C., applying a current having a second input power of 10 W to the single-walled carbon nanotube raw powder treated in step (1) to generate a second Joule heat, and maintaining the current for 20 minutes, and measuring that the temperature of the single-walled carbon nanotube raw powder at this time is the highest at 645° C.;

[0069] (3) The raw single-walled carbon nanotube powder treated in step (2) was immersed in 20 wt% sulfuric acid, stirred and heated at 80°C for 48 hours, filtered, and the sample was washed with ultrapure water until neutral, and dried to obtain single-walled carbon nanotubes with a purity of 98.4%.

[0070] Example 6

[0071] In this embodiment, argon (85 vol%) is used as carrier gas, 15 vol% water vapor is introduced, and the carbon nanotube raw powder is purified under electric heating. The specific process is as follows:

[0072] (1) 10g of single-walled carbon nanotube powder synthesized by chemical vapor deposition method using a metal iron-based catalyst is placed in the center of a fixed-bed Joule heat reactor, and the positive and negative electrodes of the power supply are contacted respectively through the conductive filter elements at both ends of the reactor, and a K-type thermocouple is used to measure the temperature at the contact point between the electrode and the conductive filter element (i.e., the actual temperature of the carbon nanotube powder after Joule heat treatment). Water vapor is continuously introduced into the fixed-bed Joule heat reactor, and the furnace temperature of the fixed-bed Joule heat reactor is raised to 350°C. A current with a first input power of 20W is applied to the single-walled carbon nanotube powder to generate the first Joule heat, and the current is maintained for 20 minutes. At this time, the temperature of the single-walled carbon nanotube powder is measured to be as high as 564°C;

[0073] (2) continuously introducing water vapor, and continuously raising the furnace temperature of the fixed bed Joule heat reactor to 600° C., applying a current having a second input power of 20 W to the single-walled carbon nanotube raw powder treated in step (1) to generate a second Joule heat, and maintaining the current for 20 minutes, and measuring that the temperature of the single-walled carbon nanotube raw powder at this time is a maximum of 781° C.;

[0074] (3) The raw single-walled carbon nanotube powder treated in step (2) was immersed in 20 wt% sulfuric acid, stirred and heated at 80°C for 24 h, filtered, and the sample was washed with ultrapure water until neutral, and dried to obtain single-walled carbon nanotubes with a purity of 99.4%.

[0075] Example 7

[0076] In this embodiment, argon (95 vol%) is used as carrier gas, 5 vol% CO gas is introduced, and the carbon nanotube raw powder is purified under electric heating. The specific process is as follows:

[0077] (1) 10g of single-walled carbon nanotube powder synthesized by chemical vapor deposition method using a metal iron-based catalyst is loaded into the center of a fixed-bed Joule heat reactor, and the conductive filters at both ends of the reactor are respectively contacted with the positive and negative electrodes of the power supply, and a K-type thermocouple is used to measure the temperature at the contact point between the electrode and the conductive filter (i.e., the actual temperature of the carbon nanotube powder after Joule heat treatment). CO gas is continuously introduced into the fixed-bed Joule heat reactor, and the furnace temperature of the fixed-bed Joule heat reactor is raised to 200°C. A current with a first input power of 15W is applied to the single-walled carbon nanotube powder to generate the first Joule heat, and the current is maintained for 15 minutes. At this time, the temperature of the single-walled carbon nanotube powder is measured to be as high as 460°C;

[0078] (2) continuously introducing CO gas, continuing to raise the furnace temperature of the fixed bed Joule heat reactor to 450° C., applying a second input power of 15 W to the single-walled carbon nanotube raw powder treated in step (1) to generate a second Joule heat, and maintaining the current for 20 min. At this time, it is measured that the temperature of the single-walled carbon nanotube raw powder is at most 660° C.;

[0079] (3) The raw single-walled carbon nanotube powder treated in step (2) was immersed in 20 wt% sulfuric acid, stirred and heated at 80°C for 48 hours, filtered, and the sample was washed with ultrapure water until neutral, and dried to obtain single-walled carbon nanotubes with a purity of 99.1%.

[0080] Example 8

[0081] In this embodiment, nitrogen (80 vol%) is used as carrier gas, 20 vol% CO gas is introduced, and the carbon nanotube raw powder is purified under electric heating. The specific process is as follows:

[0082] (1) 10g of multi-walled carbon nanotube powder synthesized by chemical vapor deposition method using a metal iron-based catalyst is loaded into the center of a fixed-bed Joule heat reactor, and the conductive filters at both ends of the reactor are respectively contacted with the positive and negative electrodes of the power supply, and a K-type thermocouple is used to measure the temperature at the contact point between the electrode and the conductive filter (i.e., the actual temperature of the carbon nanotube powder after Joule heat treatment). CO gas is continuously introduced into the fixed-bed Joule heat reactor, and the furnace temperature of the fixed-bed Joule heat reactor is raised to 300°C. A current with a first input power of 20W is applied to the multi-walled carbon nanotube powder to generate the first Joule heat, and the current is maintained for 15 minutes. At this time, the maximum temperature of the multi-walled carbon nanotube powder is measured to be 586°C;

[0083] (2) continuously introducing CO gas, continuing to raise the furnace temperature of the fixed bed Joule heat reactor to 550° C., applying a current with a second input power of 20 W to the multi-walled carbon nanotube raw powder treated in step (1) to generate a second Joule heat, and maintaining the current for 20 min. At this time, it is measured that the temperature of the multi-walled carbon nanotube raw powder is as high as 785° C.;

[0084] (3) The multi-walled carbon nanotube powder treated in step (2) was immersed in 20 wt% sulfuric acid, stirred and heated at 80°C for 24 h, filtered, and the sample was washed with ultrapure water until neutral, and dried to obtain multi-walled carbon nanotubes with a purity of 99.2%.

[0085] Comparative Example 1

[0086] In this comparative example, nitrogen (90 vol%) is used as carrier gas, 10 vol% CO2 gas is introduced, and the carbon nanotube raw powder is purified under heating conditions. The difference from Example 1 is that this comparative example does not use an electric heating method. The specific process is as follows:

[0087] (1) 10 g of single-walled carbon nanotube powder synthesized by chemical vapor deposition using a metallic iron-based catalyst was loaded into a fixed bed reactor. CO2 gas was continuously introduced into the fixed bed reactor, and the furnace temperature of the fixed bed reactor was raised to 750°C and maintained for 30 minutes;

[0088] (2) The raw single-walled carbon nanotube powder treated in step (1) was immersed in 20 wt% sulfuric acid, stirred and heated at 60°C for 24 h, filtered, and the sample was washed with ultrapure water until neutral, and dried to obtain single-walled carbon nanotubes with a purity of 90.2%.

[0089] The purified samples were observed under a scanning electron microscope. Figure 2 shown. Figure 2 It shows that some impurity carbon and metal catalyst particles on the single-walled carbon nanotubes have not been removed.

[0090] Comparative Example 2

[0091] In this comparative example, nitrogen (90 vol%) is used as carrier gas, 10 vol% CO2 gas is introduced, and the carbon nanotube raw powder is purified under heating conditions. The difference from Example 1 is that this comparative example does not use an electric heating method. The specific process is as follows:

[0092] (1) 10 g of single-walled carbon nanotube powder synthesized by chemical vapor deposition using a metallic iron-based catalyst was loaded into a fixed bed reactor. CO2 gas was continuously introduced into the fixed bed reactor, and the furnace temperature of the fixed bed reactor was raised to 1000°C and maintained for 30 minutes;

[0093] (2) The raw single-walled carbon nanotube powder treated in step (1) was immersed in 20wt% sulfuric acid, stirred and heated at 60°C for 24h, filtered, and washed with ultrapure water until the sample was neutral, and dried to obtain single-walled carbon nanotubes with a purity of 99.2%. However, nearly 50% of the single-walled carbon nanotubes were lost.

[0094] Comparative Example 3

[0095] This comparative example is implemented according to the method of the embodiment in the patent "A method for purifying carbon nanotube fibers" (application number: 201610462342.4). If the carbon nanotubes are electrified and purified according to Example 1, the specific process is as follows:

[0096] Cut a uniform carbon nanotube fiber with a length of 2.5 cm and a diameter of 54 μm, stick its two ends on the conductive glue, coat it with silver glue, and make the fiber suspended. Use a multimeter to measure the resistance of the fiber, which is 32Ω. According to the resistance, choose a suitable voltage value of 40V to power on. The power-on time is about 0.5 to 1 second to prevent the carbon nanotube fiber from burning due to excessive power-on time or excessive voltage and current. Remove the carbon nanotube fiber and soak it in acid for 6 hours. After washing it with deionized water, soak it in hydrogen peroxide for 6 hours. Soak it in deionized water for 24 hours, and finally dry it in a forced air drying oven at 80°C for 4 hours.

[0097] The above method is suitable for purifying macroscopic carbon nanotube fibers, but has strict requirements on the growth method of carbon nanotubes (such as floating catalyst chemical vapor deposition method), and cannot be used to purify carbon nanotubes grown in large quantities by other growth methods, and it is difficult to effectively promote it for industrial applications. In addition, the data recorded therein show that the voltage loaded at both ends of the carbon nanotube fiber may be as high as 135V (Example 4). Although the electric spark generated by air can remove the impurity carbon, there is also a risk of burning the carbon nanotube fiber. This purification method is relatively intense and causes greater damage to the carbon nanotubes.

[0098] In contrast, the present disclosure introduces an electric field after compacting the raw carbon nanotube powder, which is applicable to different types of carbon nanotubes prepared by different methods and has a wider range of application value. The present disclosure provides a weak oxidizing gas as a reaction gas, introduces an electric field under a relatively low temperature condition for step-by-step purification, selectively generates Joule heat on different types of impure carbon and metal catalyst micro-regions, and the voltage and power applied by the electric field are relatively mild, ultimately achieving the effect of selectively removing various types of impure carbon under suitable mild conditions.

[0099] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps recorded in this disclosure can be executed in parallel, sequentially or in different orders, as long as the desired results of the technical solution of this disclosure can be achieved, and this document is not limited here.

[0100] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of the features. In the description of the present disclosure, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0101] The above is only a specific embodiment of the present disclosure, but the protection scope of the present disclosure is not limited thereto. Any person skilled in the art who is familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present disclosure, which should be included in the protection scope of the present disclosure. Therefore, the protection scope of the present disclosure should be based on the protection scope of the claims.

Claims

1. A method for purifying carbon nanotubes, characterized in that: The following steps are involved: S1: placing unpurified carbon nanotube raw powder in a fixed bed Joule heat reactor; introducing a weak oxidizing gas into the fixed bed Joule heat reactor, heating the reaction zone to 100° C. to 400° C., generating a first Joule heat at the unpurified carbon nanotube raw powder to remove the impurity carbon layer coated on the surface of the metal catalyst particles in the carbon-encapsulated metal structure of the unpurified carbon nanotube raw powder; S2: heating the reaction zone to a temperature of 200° C. to 700° C., generating a second Joule heat at the carbon nanotube raw powder after being treated in step S1, so as to remove impurity carbon attached to the surface of the carbon nanotubes in the carbon nanotube raw powder; S3: mixing the carbon nanotube raw powder treated in step S2 with an acid to remove the metal catalyst particles exposed in the carbon-enclosed metal structure.

2. The purification method according to claim 1, characterized in that The unpurified carbon nanotube raw powder is synthesized by at least one of chemical vapor deposition, arc discharge and laser evaporation; Preferably, the weak oxidizing gas is selected from at least one of carbon dioxide, water vapor, carbon monoxide, and nitric oxide; Preferably, the weak oxidizing gas accounts for 0.1% to 50% of the total volume of the gas introduced into the fixed bed Joule heat reactor, and the remaining gas is an inert protective gas; Preferably, the inert protective gas is selected from at least one of argon, helium and nitrogen; Preferably, the metal particles are selected from at least one of chromium, manganese, iron, cobalt, nickel, rhodium, palladium and molybdenum.

3. The purification method according to claim 1, characterized in that In step S1, the first Joule heat is generated by applying a current having a first input power to the unpurified carbon nanotube raw powder; the first input power is 0.1W to 20W, and the time for applying the current having the first input power is 0.1min to 150min.

4. The purification method according to claim 1, characterized in that Conductive filter elements are arranged at both ends of the fixed bed Joule heat reactor to fix the carbon nanotube raw powder; the positive and negative electrodes of the power supply are respectively in contact with the conductive filter elements at both ends of the carbon nanotube raw powder, thereby achieving electrical connection with the carbon nanotube raw powder.

5. The purification method according to claim 1, characterized in that After the first Joule heat is generated, step S1 further includes the following steps: using a temperature measuring element to measure the actual temperature of the carbon nanotube raw powder after the first Joule heat treatment; the measured actual temperature of the carbon nanotube raw powder is 200°C to 1000°C.

6. The purification method according to claim 1, characterized in that Step S2 generates the second Joule heat by applying a current with a second input power to the carbon nanotube raw powder; the second input power is 0.1W to 20W, and the time for applying the current with the second input power is 0.1min to 150min.

7. The purification method according to claim 1, characterized in that After the second Joule heat is generated in step S2, the step further includes the following steps: using a temperature measuring element to measure the actual temperature of the carbon nanotube raw powder after the second Joule heat treatment; the measured actual temperature of the carbon nanotube raw powder is 200°C to 1000°C.

8. The purification method according to claim 1, characterized in that The acid in step S3 is selected from at least one of hydrochloric acid, nitric acid and sulfuric acid.

9. The purification method according to claim 1, characterized in that Step S3: immersing the carbon nanotube raw powder in the acid, stirring and heating at 50° C. to 100° C. for 6 h to 48 h; then performing solid-liquid separation, and drying the obtained solid sample to remove the metal catalyst particles exposed in the carbon-encapsulated metal structure.

10. The purification method according to claim 1, characterized in that: After step S3, the method further includes the following steps: detecting the purity of the carbon nanotubes after the treatment in step S3, if the purity reaches the required standard, the purification is completed; if the purity does not reach the required standard, repeating steps S1 and S2 until the purity reaches the required standard; Preferably, the method for detecting the purity of the carbon nanotubes comprises detecting by means of at least one of a scanning electron microscope, a thermogravimetric analyzer, and an inductively coupled plasma analyzer.

Images