A method for purifying carbon nanotubes

Abstract

This invention provides a method for purifying carbon nanotubes, belonging to the field of nanomaterial purification technology. The invention involves mixing the carbon nanotubes to be purified with a thermally conductive enhancing material to obtain a mixture; the thermal conductivity of the thermally conductive enhancing material is higher than that of the carbon nanotubes to be purified; the mixture is then heat-treated to purify the carbon nanotubes. This invention, by mixing the carbon nanotubes to be purified with the thermally conductive enhancing material, yields a mixture with high thermal conductivity. During heat treatment, it accelerates the heat transfer and homogenization process from the surface to the core of the carbon nanotubes, thereby improving the production capacity per unit time and the uniformity of the purified carbon nanotube product. It achieves high impurity removal efficiency and good results. Compared to traditional direct heating methods, it significantly saves energy consumption per unit product, lowers the heat treatment temperature, and reduces the problem of increased product resistivity due to overheating.

Description

Technical Field

[0001] This invention relates to the field of nanomaterial purification technology, and in particular to a method for purifying carbon nanotubes. Background Technology

[0002] Carbon nanotubes are tubular one-dimensional nanomaterials, typically prepared by catalytic cracking and deposition of carbon-containing compounds, especially hydrocarbons, using transition metal catalysts. During this process, transition metal catalysts can remain in the resulting coarse carbon nanotube powder. In applications requiring high purity, such as power batteries, it is necessary to control the content of residual transition metal impurities in the coarse carbon nanotube powder; therefore, further purification of the coarse carbon nanotube powder is required.

[0003] Currently, the mainstream purification methods fall into two main categories: chemical purification and high-temperature physical purification. Chemical purification methods, such as acid washing, have unique advantages in removing transition metal impurities. For example, the reaction process is simple, generally limited to changes at the surface and interface, and is less likely to form overall structural defects. It can control the residual transition metal impurities in coarse carbon nanotube powder to within 500 ppm. However, if further reduction of impurities in carbon nanotubes is required, chemical purification is difficult to achieve. Therefore, high-temperature physical purification is generally used for further purification. High-temperature physical purification has two processing methods: batch furnace and continuous furnace. In batch furnace processing, the temperature is typically raised to 2100–2300℃ and held under vacuum for 8–12 hours. However, the current high-temperature physical purification method has low throughput per unit time and requires significant investment in high-temperature equipment. Furthermore, the uneven processing temperature results in inconsistent impurity content in the purified product; typically, the surface impurity content is low, while the core impurity content is high. The conductivity of overheated parts of the product decreases significantly, leading to inconsistent product quality across batches. Summary of the Invention

[0004] The purpose of this invention is to provide a method for purifying carbon nanotubes. By adding thermally conductive enhancing materials, a mixture with high thermal conductivity can be obtained using the method provided by this invention, which is beneficial for improving the production capacity per unit time and the uniformity of the purified carbon nanotube product.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] This invention provides a method for purifying carbon nanotubes, comprising the following steps:

[0007] The carbon nanotubes to be purified are mixed with a thermally conductive enhancement material to obtain a mixture; the thermal conductivity of the thermally conductive enhancement material is higher than that of the carbon nanotubes to be purified.

[0008] The mixture is subjected to heat treatment to purify the carbon nanotubes to be purified.

[0009] Preferably, the thermally conductive enhancement material includes one or more of carbon materials, oxide ceramic materials, carbide ceramic materials, and nitride ceramic materials.

[0010] Preferably, the carbon material includes one or more of carbon nanotubes, graphene, carbon fibers, diamond, and graphite; the oxide ceramic material includes alumina ceramic material; the carbide ceramic material includes silicon carbide ceramic material; and the nitride ceramic material includes one or more of aluminum nitride ceramic material, silicon nitride ceramic material, and boron nitride ceramic material.

[0011] Preferably, the particle size of the thermally conductive reinforcing material is 1μm to 10mm, and the thermal conductivity is 10 to 5000W / m·K.

[0012] Preferably, the carbon nanotubes to be purified have a length of 10–200 μm and an outer diameter of 1–100 nm; the aggregate size of the carbon nanotubes to be purified is 10–2000 μm.

[0013] Preferably, the mass of the thermally conductive enhancement material is 1-90% of the mass of the carbon nanotubes to be purified.

[0014] Preferably, the heat treatment includes:

[0015] The mixture is placed in a crucible and compacted or vibrated to obtain a green body; the crucible is provided with an exhaust vent.

[0016] The crucible containing the blank is placed in a high-temperature furnace for heat treatment.

[0017] Preferably, the density of the green body is 0.05–0.5 g / cm³. 3 Its thermal conductivity is above 1 W / m·K.

[0018] Preferably, the heat treatment is carried out in a high-temperature furnace with a pressure of less than 100 Pa or in a protective atmosphere; the temperature of the heat treatment is 1500-2500℃, the holding time is 2-12 h, and the heating rate to the heat treatment temperature is 3-30℃ / min.

[0019] Preferably, the heat treatment further includes separation, wherein the separation method includes screening based on material size or sorting based on material specific gravity.

[0020] This invention provides a method for purifying carbon nanotubes, comprising the following steps: mixing the carbon nanotubes to be purified with a thermally conductive reinforcing material to obtain a mixture; wherein the thermal conductivity of the thermally conductive reinforcing material is higher than that of the carbon nanotubes to be purified; and subjecting the mixture to heat treatment to achieve purification of the carbon nanotubes. This invention, by mixing the carbon nanotubes to be purified with a thermally conductive reinforcing material, can obtain a mixture with high thermal conductivity. During heat treatment, it can accelerate the heat transfer and homogenization process from the surface to the core of the carbon nanotubes to be purified, thereby improving the unit time production capacity and the uniformity of the purified carbon nanotube product. It achieves high impurity removal efficiency and good effect. Compared with traditional direct high-temperature physical purification methods and chemically assisted high-temperature physical purification methods, it significantly saves unit product energy consumption, lowers the heat treatment temperature, and reduces the problem of increased product resistivity due to overheating, while achieving the same purity and impurity uniformity. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the crucible used in purifying carbon nanotubes according to the present invention. Detailed Implementation

[0022] This invention provides a method for purifying carbon nanotubes, comprising the following steps:

[0023] The carbon nanotubes to be purified are mixed with a thermally conductive enhancement material to obtain a mixture; the thermal conductivity of the thermally conductive enhancement material is higher than that of the carbon nanotubes to be purified.

[0024] The mixture is subjected to heat treatment to purify the carbon nanotubes to be purified.

[0025] This invention mixes carbon nanotubes to be purified with a thermally conductive reinforcing material to obtain a mixture; the thermal conductivity of the thermally conductive reinforcing material is higher than that of the carbon nanotubes to be purified. By using a thermally conductive reinforcing material with a higher thermal conductivity than the carbon nanotubes to be purified, this invention can accelerate the heat transfer and homogenization process from the surface to the core of the carbon nanotubes during heat treatment, thereby improving the production capacity per unit time and the uniformity of the purified carbon nanotube product. It also achieves high impurity removal efficiency and good results. Compared with traditional direct heating methods, it significantly saves energy consumption per unit product, lowers the heat treatment temperature, and reduces the problem of increased product resistivity due to overheating.

[0026] This invention does not specifically limit the source of the carbon nanotubes to be purified; any carbon nanotubes requiring purification that are well known to those skilled in the art can be used. In this invention, the carbon nanotubes to be purified are preferably coarse carbon nanotube powder (i.e., unrefined carbon nanotube powder) prepared by catalytic cracking and deposition of carbon-containing compounds using a transition metal catalyst, or carbon nanotubes that have undergone chemical purification (e.g., acid washing), wherein the carbon nanotubes to be purified contain residual transition metal catalyst. In this invention, the main impurity in the carbon nanotubes to be purified includes iron, and the iron content is preferably 3300–5000 ppm. In this invention, the length of the carbon nanotubes to be purified is preferably 10–200 μm, more preferably 10–50 μm; the outer diameter is preferably 1–100 nm, more preferably 7–20 nm; the agglomeration size of the carbon nanotubes to be purified is preferably 10–2000 μm, more preferably 100–1000 μm. In this invention, the coarse carbon nanotube powder prepared by catalytic cracking and deposition of carbon-containing compounds using transition metal catalysts usually exists in agglomerate form. The agglomeration size of the carbon nanotubes to be purified in this invention specifically refers to the size of the agglomerates formed by the coarse carbon nanotube powder. In this invention, the resistivity of the purified carbon nanotubes is preferably 100–115 ohm·cm; the conductivity of the carbon nanotubes is specifically characterized by electrode resistivity, and the test method will be described in detail later.

[0027] In this invention, the thermally conductive reinforcing material preferably comprises one or more of carbon materials, oxide ceramic materials, carbide ceramic materials, and nitride ceramic materials; the carbon material preferably comprises one or more of carbon nanotubes, graphene, carbon fibers, diamond, and graphite; the oxide ceramic material preferably comprises alumina ceramic materials; the carbide ceramic material preferably comprises silicon carbide ceramic materials; and the nitride ceramic material preferably comprises one or more of aluminum nitride ceramic materials, silicon nitride ceramic materials, and boron nitride ceramic materials. In this invention, the particle size of the thermally conductive reinforcing material is preferably 1 μm to 10 mm, more preferably 25 μm to 5 mm, and even more preferably 100 μm to 1 mm; the thermal conductivity is preferably 10 to 5000 W / m·K, more preferably 30 to 1000 W / m·K, even more preferably 100 to 500 W / m·K, and even more preferably 200 to 400 W / m·K. In this invention, the mass of the thermally conductive enhancement material is preferably 1-90% of the mass of the carbon nanotubes to be purified, more preferably 10-50%, further preferably 15-40%, and even more preferably 20-30%.

[0028] In this invention, the heat treatment preferably includes:

[0029] The mixture is placed in a crucible and compacted or vibrated to obtain a green body; the crucible is provided with an exhaust vent.

[0030] The crucible containing the blank is placed in a high-temperature furnace for heat treatment.

[0031] This invention places the mixture in a crucible and compacts or vibrates it to obtain a green body; the crucible is provided with an exhaust vent. In this invention, the crucible includes a crucible body and a crucible lid. The outer surface of the feeding port of the crucible body is provided with external threads, and the inner surface of the crucible lid is provided with internal threads matching the external threads. The feeding port and the crucible lid are specifically connected through the external and internal threads. This invention preferably uses a crucible with the above structure, which facilitates vacuuming before subsequent heat treatment and allows for horizontal placement during subsequent heat treatment. In this invention, the exhaust vent can be specifically located at the internal thread of the crucible lid, or at the bottom or side wall of the crucible body; the diameter of the exhaust vent is preferably ≤0.5mm. In this invention, during the heat treatment process, substances produced by the decomposition of impurities (such as transition metal catalysts) in the carbon nanotubes to be purified at high temperatures will overflow through the exhaust vent. Simultaneously, the diameter of the exhaust vent should not be too large to prevent the mixture from spraying out. In this invention, the crucible is preferably a graphite crucible. Preferably, the mixture is added to the crucible body through the feeding port, compacted or vibrated to form a blank, filling the crucible body, and then the crucible lid is placed on top. Figure 1 As shown. In this invention, the density of the green body is preferably 0.05 to 0.50 g / cm³. 3 More preferably, it is 0.18–0.30 g / cm³. 3 More preferably, it is 0.20–0.25 g / cm³. 3 The thermal conductivity is preferably 1 W / m·K or higher, more preferably 10 to 200 W / m·K, even more preferably 20 to 100 W / m·K, and even more preferably 40 to 70 W / m·K.

[0032] After obtaining the blank, the present invention places the crucible containing the blank in a high-temperature furnace for heat treatment. In the present invention, the heat treatment is preferably carried out in a furnace chamber with a pressure of less than 100 Pa or in a protective atmosphere. The present invention does not have a special limitation on the type of protective gas providing the protective atmosphere, and any protective gas well known to those skilled in the art can be used, such as nitrogen or argon. The temperature of the heat treatment is preferably 1500-2500℃, more preferably 1600-2000℃, and even more preferably 1700-1800℃. The holding time is preferably 2-12h, more preferably 2-8h, and even more preferably 2-5h. The heating rate to the heat treatment temperature is preferably 3-30℃ / min, more preferably 5-10℃ / min. In this invention, after placing the crucible containing the blank in a high-temperature furnace, it is preferable to first perform a vacuum treatment to replace the air in the crucible and the furnace cavity, and then maintain the pressure in the furnace cavity less than 100 Pa or supplement protective gas, and heat the furnace from room temperature (25°C) to 1500-2500°C at a heating rate of 3-30°C / min, and hold the temperature for 2-12 hours for heat treatment.

[0033] In this invention, the purified material obtained after heat treatment is a mixture of purified carbon nanotubes and thermally conductive reinforcing materials. Preferably, the heat treatment further includes cooling to obtain the mixture of purified carbon nanotubes and thermally conductive reinforcing materials, which can then be used directly or further separated as needed. Specifically, when the high-temperature furnace is a batch furnace, the present invention preferably performs furnace cooling after the heat treatment; when the high-temperature furnace is a continuous furnace, the present invention preferably cools the obtained material after discharge following the heat treatment. Specifically, when the thermally conductive reinforcing material is only a carbon material, it is preferable to use the purified material directly or further separate the purified material to obtain purified carbon nanotubes as needed; when the thermally conductive reinforcing material contains non-carbon materials (i.e., at least one of oxide ceramic materials, carbide ceramic materials, and nitride ceramic materials), it is preferable to further separate the purified material to remove the non-carbon materials. In this invention, the separation method preferably includes sieving based on material size or sorting based on material specific gravity. Specifically, when the thermally conductive reinforcing material is a carbon material, sieving is preferably performed based on the size of the purified carbon nanotubes and the thermally conductive reinforcing material. When the thermally conductive reinforcing material is a non-carbon material, sieving can be performed based on the size of the purified carbon nanotubes and the thermally conductive reinforcing material, or sorting can be performed based on the specific gravity of the purified carbon nanotubes and the thermally conductive reinforcing material. When the thermally conductive reinforcing material contains both carbon and non-carbon materials, sieving can be performed based on the size of the purified carbon nanotubes and the thermally conductive reinforcing material, and the above-mentioned methods can be used to separate the carbon and non-carbon materials stepwise. This invention does not impose any special limitations on this.

[0034] After purification using the method provided by this invention, the resistivity of the purified carbon nanotubes is preferably 118–185 ohm·cm, and the average iron content is preferably 28–265 ppm. The difference in iron content between the surface and core of the purified carbon nanotubes is 13–410 ppm, all of which are improvements over carbon nanotubes obtained by directly treating the carbon nanotubes to be purified at high temperatures in a vacuum furnace or atmosphere-protected furnace. In the embodiments of this invention, after purification using the method provided by this invention, the resistivity of the purified carbon nanotubes is 130–185 ohm·cm, and the average iron content is 58–265 ppm. The range of the difference in iron content between the surface and core of the purified carbon nanotubes is related to the average iron content. Specifically, the lower limit of the difference in iron content between the surface and core of the purified carbon nanotubes is 51–76% of the average iron content, and the upper limit of the difference in iron content between the surface and core of the purified carbon nanotubes is 1.1–1.6 times the average iron content.

[0035] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0036] In this invention, the resistivity of carbon nanotubes is measured according to the following steps:

[0037] The carbon nanotubes (CNTs) to be tested were unwashed carbon nanotubes, with iron as the main impurity, and a content of 3300 ppm; the length of the CNTs was 10-50 μm, the outer diameter was 7-20 nm, and the aggregate size was 100-1000 μm;

[0038] A mixture of 4 wt% CNTs, 1 wt% polyvinylpyrrolidone (PVP), and 95 wt% N-methylpyrrolidone (NMP) was placed in a 2L container. 0.5mm zirconium balls were ground at a ball-to-material ratio of 60:1 at 5000 rpm for 2 hours. The zirconium balls were then separated by sieving to obtain a carbon nanotube slurry. A mixture of 1 wt% CNTs, 0.25 wt% PVP, 1 wt% polyvinylidene fluoride (PVDF), and 97.75 wt% lithium iron phosphate (LFP) was stirred at 800 rpm for 2 hours in a 100mL beaker to obtain an electrode slurry. A 200μm thick wet film of the electrode slurry was coated onto a flat PET film and dried in an oven at 110℃. The dried material was cut into φ25mm discs, and the resistivity was directly read using a four-probe measurement. The average value of three points was taken as the final value.

[0039] The results showed that the resistivity of the carbon nanotube under test was 100 ohm·cm.

[0040] The resistivity of carbon nanotubes in this invention was measured using the method described above, and will not be repeated hereafter.

[0041] Comparative Example 1

[0042] In this comparative example, the carbon nanotubes to be purified are unwashed carbon nanotubes with a resistivity of 100 ohm·cm and iron as the main impurity at a content of 3300 ppm. The length of the carbon nanotubes to be purified is 10–50 μm, the outer diameter is 7–20 nm, and the cluster size is 100–1000 μm.

[0043] The method for purifying the carbon nanotubes to be purified includes the following steps:

[0044] The carbon nanotubes to be purified are added to a graphite crucible and compacted by vibration to form a preform, ensuring the preform fills the graphite crucible. The crucible is then covered with a lid, the inner thread of which has a 0.5 mm diameter vent hole. The density of the preform is 0.15 g / cm³. 3 Its thermal conductivity is 20 W / m·K;

[0045] The graphite crucible containing the blank was placed in a high-temperature furnace. The furnace was evacuated and filled with nitrogen three times to replace the air in the graphite crucible and the furnace cavity. The pressure in the furnace cavity was kept less than 100 Pa. The temperature was increased from room temperature (25°C) to 1800°C at a rate of 5°C / min. The temperature was held for 8 hours for purification. After that, the furnace was cooled to obtain purified carbon nanotubes.

[0046] The resistivity of the purified carbon nanotubes in this comparative example is 285 ohm·cm, and the average iron content is 350 ppm; the difference in iron content between the surface and the core of the purified carbon nanotubes is 200–800 ppm.

[0047] Comparative Example 2

[0048] In this comparative example, the carbon nanotubes to be purified are unwashed carbon nanotubes with a resistivity of 100 ohm·cm and iron as the main impurity at a content of 3300 ppm. The length of the carbon nanotubes to be purified is 10–50 μm, the outer diameter is 7–20 nm, and the cluster size is 100–1000 μm.

[0049] The method for purifying the carbon nanotubes to be purified includes the following steps:

[0050] The carbon nanotubes to be purified are added to a graphite crucible and compacted by vibration to form a preform, ensuring the preform fills the graphite crucible. The crucible is then covered with a lid, the inner thread of which has a 0.5 mm diameter vent hole. The density of the preform is 0.15 g / cm³. 3Its thermal conductivity is 20 W / m·K;

[0051] The graphite crucible containing the blank was placed in a high-temperature furnace. The furnace was evacuated and filled with argon three times to replace the air in the graphite crucible and the furnace cavity. Then, argon was added to maintain the pressure in the furnace cavity at 200 Pa. The temperature was increased from room temperature (25°C) to 2800°C at a rate of 5°C / min and held for 1 hour for purification. After that, the furnace was cooled in the cooling section to obtain purified carbon nanotubes.

[0052] The resistivity of the purified carbon nanotubes in this comparative example is 252 ohm·cm, and the average iron content is 302 ppm; the difference in iron content between the surface and the core of the purified carbon nanotubes is 205–430 ppm.

[0053] Example 1

[0054] In this embodiment, the carbon nanotubes to be purified are unwashed carbon nanotubes with a resistivity of 100 ohm·cm and iron as the main impurity at a content of 3300 ppm. The carbon nanotubes to be purified have a length of 10–50 μm, an outer diameter of 7–20 nm, and a cluster size of 100–1000 μm.

[0055] In this embodiment, pre-purified carbon nanotubes are used as the thermal conductivity enhancement material. The pre-purified carbon nanotubes have a thermal conductivity of 200 W / m·K and a cluster size of 1–1000 μm. The preparation method of the pre-purified carbon nanotubes is as follows:

[0056] Carbon nanotubes with a metallic impurity content of 500 ppm were added to a graphite crucible and compacted to form a green body, which filled the graphite crucible. The crucible was then covered with a lid, the inner thread of which had a 0.5 mm diameter vent hole. The density of the green body was 0.15 g / cm³. 3 Its thermal conductivity is 200 W / m·K;

[0057] The graphite crucible containing the blank was placed in a high-temperature furnace. The furnace was evacuated and filled with nitrogen three times to replace the air in the graphite crucible and the furnace cavity. The pressure in the furnace cavity was kept less than 100 Pa. The temperature was increased from room temperature (25°C) to 2500°C at a rate of 3°C / min. The temperature was held for 8 hours for purification. After that, the furnace was cooled to obtain pre-purified carbon nanotubes.

[0058] The pre-purified carbon nanotubes have a resistivity of 200 ohm·cm and an average metal element content of 5 ppm; the difference in iron content between the surface and the core of the pre-purified carbon nanotubes is 0.1 to 1 ppm.

[0059] The method for purifying the carbon nanotubes to be purified includes the following steps:

[0060] The carbon nanotubes to be purified are mixed with a thermally conductive reinforcing material (i.e., pre-purified carbon nanotubes) at a mass ratio of 1:1 to obtain a mixture. This mixture is then added to a graphite crucible and compacted by vibration to form a green body, ensuring the green body fills the crucible. The crucible is then covered with a lid, the inner thread of which has a 0.5 mm diameter vent hole. The density of the green body is 0.18 g / cm³. 3 Its thermal conductivity is 110 W / m·K;

[0061] The graphite crucible containing the blank was placed in a high-temperature furnace. The furnace was evacuated and filled with nitrogen three times to replace the air in the graphite crucible and the furnace cavity. The pressure in the furnace cavity was kept less than 100 Pa. The temperature was increased from room temperature to 1800°C at a rate of 5°C / min and held for 2 hours for purification. After discharge, the material was cooled to obtain the purified material. The purified material is a mixture of purified carbon nanotubes and thermally conductive reinforcing materials, which can be used for subsequent applications without further separation.

[0062] In this embodiment, the resistivity of the purified material is 130 ohm·cm, and the average iron content is 58 ppm; the difference in iron content between the surface and the core of the purified material is 30-85 ppm.

[0063] Example 2

[0064] In this embodiment, the carbon nanotubes to be purified are unwashed carbon nanotubes with a resistivity of 110 ohm·cm and iron as the main impurity at a content of 5000 ppm. The carbon nanotubes to be purified have a length of 10–50 μm, an outer diameter of 7–20 nm, and a cluster size of 100–1000 μm.

[0065] The method for purifying the carbon nanotubes to be purified includes the following steps:

[0066] Artificial graphite is used as a thermal conductivity enhancement material, wherein the thermal conductivity of the artificial graphite is 100 W / m·K and the D50 is 25 μm;

[0067] The carbon nanotubes to be purified are mixed with a thermally conductive reinforcing material at a mass ratio of 1:0.3 to obtain a mixture. This mixture is then added to a graphite crucible and compacted by vibration to form a blank, ensuring the blank fills the crucible completely. The crucible is then covered with a lid, the inner thread of which has a 0.5 mm diameter vent hole. The density of the blank is 0.2 g / cm³. 3 Its thermal conductivity is 40 W / m·K;

[0068] The graphite crucible containing the blank was placed in a high-temperature furnace. The furnace was evacuated and filled with nitrogen three times to replace the air in the graphite crucible and the furnace cavity. The pressure in the furnace cavity was kept less than 100 Pa. The temperature was increased from room temperature to 2000°C at a rate of 5°C / min and held for 2 hours for purification. After discharge, the material was cooled to obtain the purified material. The purified material was a mixture of purified carbon nanotubes and thermally conductive reinforcing material. After sieving, purified carbon nanotubes were obtained.

[0069] In this embodiment, the resistivity of the purified carbon nanotubes is 132 ohm·cm, and the average iron content is 105 ppm; the difference in iron content between the surface and the core of the purified material is 80–140 ppm.

[0070] Example 3

[0071] In this embodiment, the carbon nanotubes to be purified are unwashed carbon nanotubes with a resistivity of 100 ohm·cm and iron as the main impurity at a content of 3300 ppm. The carbon nanotubes to be purified have a length of 10–50 μm, an outer diameter of 7–20 nm, and a cluster size of 100–1000 μm.

[0072] The method for purifying the carbon nanotubes to be purified includes the following steps:

[0073] Alumina ceramic microspheres (containing 99 wt% Al2O3) were used as thermal conductivity enhancement materials. The thermal conductivity of the alumina ceramic microspheres was 30 W / m·K, and the diameter was 0.5 mm.

[0074] The carbon nanotubes to be purified are mixed with a thermally conductive reinforcing material at a mass ratio of 1:0.4 to obtain a mixture. This mixture is then added to a graphite crucible and compacted by vibration to form a blank, ensuring the blank fills the crucible. The crucible is then covered with a lid, the inner thread of which has a 0.5 mm diameter vent hole. The density of the blank is 0.25 g / cm³. 3 Its thermal conductivity is 20 W / m·K;

[0075] The graphite crucible containing the blank was placed in a high-temperature furnace. The furnace was evacuated and filled with nitrogen three times to replace the air in the graphite crucible and the furnace cavity. The pressure in the furnace cavity was kept less than 100 Pa. The temperature was increased from room temperature to 1600°C at a rate of 5°C / min and held for 2 hours for purification. After discharge, the material was cooled to obtain the purified material. The purified material was a mixture of purified carbon nanotubes and thermally conductive reinforcing material. After sieving, purified carbon nanotubes were obtained.

[0076] In this embodiment, the resistivity of the purified carbon nanotubes is 185 ohm·cm, and the average iron content is 265 ppm; the difference in iron content between the surface and the core of the purified material is 200–410 ppm.

[0077] Example 4

[0078] In this embodiment, the carbon nanotubes to be purified are unwashed carbon nanotubes with a resistivity of 115 ohm·cm and iron as the main impurity at a content of 3500 ppm. The carbon nanotubes to be purified have a length of 10–50 μm, an outer diameter of 7–20 nm, and a cluster size of 100–1000 μm.

[0079] The method for purifying the carbon nanotubes to be purified includes the following steps:

[0080] Silicon nitride ceramic microspheres (containing 99 wt% Si3N4) were used as thermal conductivity enhancement materials. The thermal conductivity of the silicon nitride ceramic microspheres was 400 W / m·K and the diameter was 0.5 mm.

[0081] The carbon nanotubes to be purified were mixed with a thermally conductive reinforcing material at a mass ratio of 1:0.15 to obtain a mixture. This mixture was then added to a graphite crucible and compacted by vibration to form a blank, ensuring the blank filled the crucible. The crucible was then covered with a lid, the inner thread of which had a 0.5 mm diameter vent hole. The density of the blank was 0.2 g / cm³. 3 Its thermal conductivity is 70 W / m·K;

[0082] The graphite crucible containing the blank was placed in a high-temperature furnace. The furnace was evacuated and filled with nitrogen three times to replace the air in the graphite crucible and the furnace cavity. The pressure in the furnace cavity was kept less than 100 Pa. The temperature was increased from room temperature to 1700°C at a rate of 5°C / min and held for 2 hours for purification. After discharge, the material was cooled to obtain the purified material. The purified material was a mixture of purified carbon nanotubes and thermally conductive reinforcing materials. After sieving, purified carbon nanotubes were obtained.

[0083] In this embodiment, the resistivity of the purified carbon nanotubes is 140 ohm·cm, and the average iron content is 85 ppm; the difference in iron content between the surface and the core of the purified material is 62–98 ppm.

[0084] Example 5

[0085] Pre-purified carbon nanotubes were used as thermal conductivity enhancement materials, wherein the thermal conductivity of the pre-purified carbon nanotubes was 200 W / m·K and the aggregate size was 1–1000 μm.

[0086] The carbon nanotubes to be purified are mixed with a thermally conductive reinforcing material at a mass ratio of 1:1 to obtain a mixture. This mixture is then added to a graphite crucible and compacted by vibration to form a blank, ensuring the blank fills the crucible completely. The crucible is then covered with a lid, the inner thread of which has a 0.5 mm diameter vent hole. The density of the blank is 0.18 g / cm³. 3 Its thermal conductivity is 110 W / m·K;

[0087] The graphite crucible containing the blank was placed in a high-temperature furnace. The furnace was evacuated and filled with argon gas three times to replace the air in the graphite crucible and the furnace cavity. Then, argon gas was added to maintain the pressure in the furnace cavity at about 200 Pa. The temperature was increased from room temperature (25°C) to 2800°C at a rate of 5°C / min and held for 1 hour for purification. After that, the furnace was cooled in the cooling section to obtain purified carbon nanotubes.

[0088] In this embodiment, the resistivity of the purified carbon nanotubes is 118 ohm·cm, and the average iron content is 28 ppm; the difference in iron content between the surface and the core of the purified carbon nanotubes is 13–40 ppm.

[0089] As can be seen from the above embodiments and comparative examples, the method provided by the present invention for purifying carbon nanotubes, by adding thermally conductive enhancing materials, can produce a mixture with high thermal conductivity, which is beneficial to improving the unit time production capacity and the uniformity of the purified carbon nanotube products.

[0090] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for purifying carbon nanotubes, comprising the following steps: The carbon nanotubes to be purified are mixed with a thermally conductive enhancement material to obtain a mixture; the thermal conductivity of the thermally conductive enhancement material is higher than that of the carbon nanotubes to be purified; the iron content in the carbon nanotubes to be purified is 3300~5000ppm; The mixture is subjected to heat treatment, or heat treatment followed by separation, to purify the carbon nanotubes to be purified. The heat treatment is carried out in a high-temperature furnace at a pressure of less than 100 Pa or in a protective atmosphere. The heat treatment temperature is 1500~2500℃, and the holding time is 2~12h. The heating rate to the heat treatment temperature is 3~30℃ / min. The separation method includes sieving based on material size or sorting based on material specific gravity. The average iron content in the purified carbon nanotubes is 28~265ppm, and the difference in iron content between the surface and the core of the purified carbon nanotubes is 13~410ppm.

2. The method according to claim 1, characterized in that, The thermally conductive enhancement material includes one or more of carbon materials, oxide ceramic materials, carbide ceramic materials, and nitride ceramic materials.

3. The method according to claim 2, characterized in that, The carbon material includes one or more of carbon nanotubes, graphene, carbon fibers, diamond, and graphite; the oxide ceramic material includes alumina ceramic material; the carbide ceramic material includes silicon carbide ceramic material; and the nitride ceramic material includes one or more of aluminum nitride ceramic material, silicon nitride ceramic material, and boron nitride ceramic material.

4. The method according to any one of claims 1 to 3, characterized in that, The particle size of the thermally conductive reinforcing material is 1μm~10mm, and the thermal conductivity is 10~5000W / m·K.

5. The method according to claim 1, characterized in that, The carbon nanotubes to be purified have a length of 10~200μm and an outer diameter of 1~100nm; the aggregate size of the carbon nanotubes to be purified is 10~2000μm.

6. The method according to claim 1, characterized in that, The mass of the thermally conductive enhancement material is 1-90% of the mass of the carbon nanotubes to be purified.

7. The method according to claim 1, characterized in that, The heat treatment includes: The mixture is placed in a crucible and compacted or vibrated to obtain a green body; the crucible is provided with an exhaust vent. The crucible containing the blank is placed in a high-temperature furnace for heat treatment.

8. The method according to claim 7, characterized in that, The density of the blank is 0.05~0.5 g / cm³. 3 Its thermal conductivity is above 1 W / m·K.

Images