Method for efficiently and losslessly purifying macroscopic body of carbon nano tube

Abstract

The invention relates to the technical field of nanometer carbon material purification, in particular to a method for efficiently and losslessly purifying a carbon nanotube macroscopic body. According to the method, rapid heating (the heating rate is 100-3000 DEG C / s)-short-time (1-300 s) high-temperature (700-2500 DEG C) heat treatment-rapid cooling is carried out on a carbon nanotube macroscopic body in a vacuum / inert atmosphere in modes of laser irradiation, Joule heat radiation, large-current pulse and the like, so that residual impurities such as amorphous carbon and a metal catalyst in the carbon nanotube are removed while the crystallinity of the carbon nanotube is improved; and a high-quality and high-purity carbon nanotube macroscopic body is obtained. The purification method has the characteristics of no damage, controllability and high efficiency, and intrinsic structures such as orientation, length and diameter of the carbon nanotube in a macroscopic body are not influenced. The purified carbon nanotube macroscopic body can avoid the influence of metal impurities, and the physical and chemical properties of the carbon nanotube macroscopic body are expected to be effectively improved.

Description

Technical Field

[0001] The invention relates to the technical field of nano carbon material purification, in particular to a method for efficiently and non-destructively purifying carbon nano tube macroscopic bodies. Background Art

[0002] Carbon nanotubes are composed of carbon atoms in the form of sp 2 A one-dimensional hollow tubular structure with a high aspect ratio formed by hybridization. Macroscopic bodies such as films, fibers, arrays, and aerogels constructed by carbon nanotubes have high thermal conductivity, high electrical conductivity, large specific surface area, high chemical stability, and excellent mechanical properties. They show broad application prospects in many fields such as energy storage and conversion, lightweight and high-strength composite materials, and flexible electronics. The growth of carbon nanotubes requires metal nanoparticle catalysts and carbon sources. During their growth, the metal nanoparticles will be coated and inactivated by the carbon layer. Therefore, the prepared carbon nanotube macroscopic body inevitably contains amorphous carbon and metal catalyst particles (such as iron, cobalt, nickel, etc.), which makes the purity and performance of the carbon nanotube macroscopic body unable to match the high-precision application requirements.

[0003] At present, researchers have developed a variety of carbon nanotube purification methods, which can be summarized into three categories, namely gas phase oxidation, acid treatment and high temperature heat treatment. The gas phase oxidation method is to use oxidizing gases such as oxygen to treat at high temperature, so that impurities such as amorphous carbon in carbon nanotubes react with oxidizing gases to generate gases such as carbon dioxide and remove them, thereby achieving the purpose of purifying carbon nanotubes (Nawal Berrada.et.al.J.Phys.Chem.C,2019,123,14725.). This method can keep the structure of the carbon nanotube macrobody intact, but it cannot remove the metal catalyst nanoparticles therein. Acid treatment purification usually uses strong acid or mixed acid to react chemically with impurities in carbon nanotubes to improve their purity (AbdulazizS.R.Bati.et.al.Nanoscale,2018,10,22087.). However, acid treatment may cause certain damage to the wall structure of carbon nanotubes and introduce functional groups (hydroxyl, carboxyl, etc.) on their surface, thereby affecting their intrinsic properties. In particular, it is difficult for carbon nanotube macroscopic bodies treated in liquid phase to maintain their original structure.

[0004] In addition, high-temperature heat treatment can also achieve purification by evaporating non-carbon impurities and increasing the degree of graphitization of carbon nanotubes. Liu et al. used a high-temperature graphitization furnace to heat treat carbon nanotubes at 1400-2800 ° C, which removed the iron catalyst to a certain extent and improved the crystallinity of carbon nanotubes (Yang Liu. et al. Carbon, 2024, 222, 119004.). Lee et al. healed structural defects and enhanced the interaction between adjacent carbon nanotubes by high-temperature graphitization of carbon nanotube fibers, significantly improving their mechanical and electrical properties (Dongju Lee. et al. Sci Adv., 2022, 8, eabn0939.), but long-term high-temperature heating changes the intrinsic morphology and structure of the carbon nanotube macrostructure. At the same time, high-temperature heating using a graphitization furnace not only has a slow heating rate (<20°C / min) and extremely high energy consumption, but also the volatilization of the heating elements in the graphite furnace at high temperatures inevitably causes secondary pollution (Masaki Okada. et al. Carbon, 2017, 116, 737.). In summary, there is still a lack of a method for low-cost, efficient, and controllable purification of carbon nanotube macrostructures. Summary of the invention

[0005] The purpose of the present invention is to provide a method for purifying carbon nanotube macrostructures in an efficient and non-destructive manner, thereby overcoming the problems of the prior art in terms of structural destruction of carbon nanotube macrostructures, long purification time, and high energy consumption.

[0006] The technical solution of the present invention is:

[0007] A method for purifying a carbon nanotube macrobody efficiently and non-destructively comprises placing the carbon nanotube macrobody on a heating table in a sealed cavity, rapidly heating the carbon nanotube macrobody in a vacuum or inert atmosphere (heating rate 100-3000°C / s) by ultrafast heating - subjecting the carbon nanotube macrobody to a short-time (1-300s) high-temperature (700-2500°C) heat treatment - rapid cooling, and utilizing the thermal radiation generated by the high temperature of the heating table to purify the carbon nanotube macrobody at high temperature, remove residual metal catalysts therein, and improve the purity and crystallinity of the carbon nanotube macrobody without destroying its structure.

[0008] The method for highly efficient and non-destructive purification of carbon nanotube macro-bodies is applicable to types of carbon nanotube macro-bodies including but not limited to films, fibers, arrays or aerogels.

[0009] The method for efficiently and non-destructively purifying carbon nanotube macroscopic bodies adopts a cyclic heating method to treat carbon nanotube macroscopic bodies. During the treatment process, the heating temperature is 700-2500°C, the cavity pressure is 0.1-600kPa, the treatment time is 1-300s, the heating rate is 100-3000°C / s, and the heating times are 1-30 times.

[0010] In the method for efficiently and non-destructively purifying carbon nanotube macro-bodies, the pressure in the heating chamber is a vacuum condition maintained by a mechanical pump or a normal pressure or high pressure condition under the protection of an inert atmosphere, and the inert atmosphere is argon or nitrogen with a volume purity of >99.99%.

[0011] The highly efficient and non-destructive method for purifying carbon nanotube macroscopic bodies realizes the regulation of different heating rates and heating temperatures by controlling the ultrafast heating method, heating power, and the material and thickness of the heating platform.

[0012] The method for efficiently and non-destructively purifying carbon nanotube macroscopic bodies comprises an ultrafast heating method including one of direct current / pulse power supply heating, laser irradiation, flash heating, plasma discharge and Joule thermal radiation.

[0013] The method for efficiently and non-destructively purifying carbon nanotube macrostructures is characterized in that the material of the heating platform is one or more of high-purity graphite (purity > 99.99wt%), high-purity molybdenum (purity > 99.99wt%), high-purity tungsten (purity > 99.99wt%) and high-purity hexagonal boron nitride (purity > 99.99wt%).

[0014] The method for efficiently and non-destructively purifying carbon nanotube macroscopic bodies, the purity and crystallinity of the purified carbon nanotube macroscopic bodies are related to the rapid heating heating rate, the maximum heating temperature, the number of rapid heating times, the size of the carbon nanotube macroscopic bodies and the matching degree of the heating stage.

[0015] The highly efficient and non-destructive method for purifying carbon nanotube macroscopic bodies, for a self-supporting ultra-thin single-walled carbon nanotube film with a transmittance of >90% at 550nm, after purification by Joule thermal radiation, the network structure remains unchanged, and the carbon nanotubes have high crystallinity. G / I D >150, and no metal nanoparticle impurities remain.

[0016] In the method for efficiently and non-destructively purifying carbon nanotube macro-bodies, the rapid cooling method is natural cooling to room temperature.

[0017] The design concept of the present invention is:

[0018] The present invention provides a method for efficiently and non-destructively purifying a carbon nanotube macro-body. The carbon nanotube macro-body is treated by a cyclic heating method of rapid heating (heating rate 100-3000°C / s)-short time (1-300s) high temperature (700-2500°C) heat treatment-rapid cooling, and the residual impurities such as amorphous carbon and metal catalysts in the carbon nanotubes are removed while the crystallinity of the carbon nanotubes is improved, so as to obtain a high-quality and high-purity carbon nanotube macro-body. The method can evaporate and remove the impurities such as residual metal catalysts and amorphous carbon in the macro-body in a short time and improve the crystallinity of the carbon nanotubes, and the method does not affect the structure and morphology of the macro-body, and is particularly suitable for macro-bodies with structures sensitive to purification conditions such as carbon nanotube films with nanometer-level thickness. In addition, by designing and selecting suitable heating carriers and heating methods, high-precision control of heating temperature, heating rate, and heating time is achieved, so as to realize rapid and efficient purification of carbon nanotube macro-bodies.

[0019] The advantages and beneficial effects of the present invention are:

[0020] 1. The present invention can efficiently purify carbon nanotube macro-bodies. By rapidly heating up in a short period of time, volatile impurities and metal catalysts in the macro-bodies can be quickly evaporated without the need for long-term high-temperature treatment, which greatly shortens the purification time and reduces energy consumption.

[0021] 2. The present invention is a method for non-destructively purifying carbon nanotube macro-bodies, which ensures the purification effect while not destroying the intrinsic structure of the carbon nanotube macro-bodies and improving their thermal conductivity, electrical conductivity and mechanical properties.

[0022] 3. The method proposed in the present invention is universal and can be applied to the purification of various carbon nanotube macroscopic bodies, especially thin films and ultrafine fibers with easily destroyed intrinsic structures, and is easy to realize industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 .Optical photograph of single-walled carbon nanotube film before purification.

[0024] Figure 2 . Transmission electron micrograph of single-walled carbon nanotube film before purification.

[0025] Figure 3 .Optical photograph of purified single-walled carbon nanotube film.

[0026] Figure 4 . Transmission electron micrograph of purified single-walled carbon nanotube film.

[0027] Figure 5 . Transmission electron micrograph of tube bundles in purified single-walled carbon nanotube film.

[0028] Figure 6.Raman spectra of single-walled carbon nanotube films before and after purification under heating conditions of 1500℃. The horizontal axis Raman shift represents the Raman shift (cm -1 ), the vertical axis Intensity represents the relative intensity (au).

[0029] Figure 7 .Optical photograph of the sample and its carrier after treatment in a graphitization furnace at 2300℃. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention is described in detail below through embodiments and drawings, but this is not intended to limit the scope of protection of the present application.

[0031] Example 1

[0032] In this embodiment, the process of purifying the ultra-thin single-walled carbon nanotube film by Joule thermal radiation is as follows:

[0033] (1) Collection and transfer of carbon nanotube films.

[0034] The single-walled carbon nanotube film collected on the cellulose filter was transferred to a boron nitride ring with a pore diameter of 8 mm to form a self-supporting single-walled carbon nanotube film ( Figure 1 ), the film thickness is about 100nm, the transmittance of 550nm visible light is about 80%, it is mainly composed of irregularly overlapped small tube bundles of single-walled carbon nanotubes, the diameter of the tube bundles is 5 to 50nm, and the residual nanoparticle catalyst can be clearly seen by transmission electron microscopy observation ( Figure 2 ).

[0035] (2) Purification of single-walled carbon nanotube films by thermal radiation heating.

[0036] A graphite plate with a thickness of 1 mm is fixed between the graphite electrodes on both sides as a heating platform, and the single-walled carbon nanotube film sample to be purified is placed on the graphite plate. The cavity pressure is pumped to <100 Pa by a mechanical pump, and then high-purity argon gas (volume purity 99.999%) is introduced to normal pressure. The graphite plate is heated by a DC power supply, the applied voltage is 300 V, the total heating time is 10 s, the heating rate is about 200 ° C / s, and the sample is heated by thermal radiation at high temperature of the graphite plate. The highest temperature of the sample is 1500 ° C. After naturally cooling to room temperature, a purified single-walled carbon nanotube film is obtained.

[0037] (3) Characterization of purified single-walled carbon nanotube films.

[0038] From the optical photos, it can be seen that after purification, the film still maintains a self-supporting structure without obvious morphological changes ( Figure 3The purified single-walled carbon nanotube film was dispersed on a copper mesh microgrid after ultrasonic treatment and observed under a transmission electron microscope. No metal nanoparticles were observed on the carbon nanotube network ( Figure 4 ). High-magnification transmission electron microscopy images further confirmed that the catalyst had been removed, leaving only the outer carbon coating structure ( Figure 5 a), and the wall structure of carbon nanotubes is clear and highly crystalline ( Figure 5 b). Raman spectroscopy characterization shows that the intensity ratio of the Raman G peak to the D peak of the film (I G / I D ) from 141 to 171 ( Figure 6 ), proving that the purification process improves the crystallinity of carbon nanotubes.

[0039] Example 2

[0040] In this embodiment, the process of purifying carbon nanotube fibers in vacuum by high-frequency pulse heating is as follows:

[0041] (1) Preparation of carbon nanotube fibers.

[0042] Single-walled carbon nanotubes are prepared by floating catalyst chemical vapor deposition, dispersed and then obtained through wet spinning process to obtain carbon nanotube fibers, whose internal impurities are mainly amorphous carbon and residual solvents in the liquid phase treatment process.

[0043] (2) High-frequency pulse heating to purify carbon nanotube fibers.

[0044] A flexible graphite paper with a thickness of 40 μm is fixed between the graphite electrodes on both sides as a heating platform, and the carbon nanotube fiber sample to be purified is placed on the flexible graphite paper. The cavity pressure is pumped to <100Pa by a mechanical pump, and then high-purity argon gas (volume purity 99.999%) is introduced to normal pressure. After repeating this process 3 times to remove the air in the cavity, the cavity is kept in a low pressure state (~100Pa). The flexible graphite paper is heated by an AC pulse power supply with an output voltage of 50V, an output frequency of 10Hz, a duty cycle of 50%, and 30 pulses. The process uses the thermal radiation generated by the flexible graphite paper under the high temperature of pulse heating to heat the sample, and the highest temperature of the sample is 1800℃. After heating, it is naturally cooled to room temperature to obtain purified carbon nanotube fibers.

[0045] (3) Characterization of purified single-walled carbon nanotube fibers.

[0046] In this embodiment, the diameter and orientation of the purified carbon nanotube fibers remain unchanged. Raman spectroscopy and transmission electron microscopy observation show that the crystallinity is improved and the internal amorphous carbon and residual solvent impurities are removed.

[0047] Example 3

[0048] In this embodiment, the process of purifying carbon nanotube foam by flash heating is as follows:

[0049] (1) Preparation of carbon nanotube foam.

[0050] The carbon nanotube foam is directly prepared by a floating catalyst chemical vapor deposition method, wherein the impurities are mainly iron catalyst and a small amount of amorphous carbon.

[0051] (2) Flash heating to purify carbon nanotube foam.

[0052] Place a block of carbon nanotube foam with a size of 5×5×10mm in a heating chamber, clamp and press its two ends with graphite electrodes. Adjust the degree of compression until the resistance at both ends of the graphite electrode is about 1Ω. Use a mechanical pump to evacuate the chamber pressure to <100Pa, and then introduce high-purity argon gas (volume purity 99.999%) to normal pressure. Use a capacitor with a capacity of 90mF for flash discharge: set the capacitor voltage to 110V, the discharge time to 1s, the number of flash evaporations to 1, and the maximum temperature to 2500℃. After the flash heating is completed, wait for the carbon nanotube foam to cool naturally to room temperature and then take it out to complete the purification.

[0053] (3) Characterization of purified carbon nanotube foam.

[0054] In this embodiment, the pore structure of the purified carbon nanotube foam remains unchanged. Raman spectroscopy and transmission electron microscopy show that its crystallinity is improved, and the internal amorphous carbon and residual metal nanoparticle impurities are removed.

[0055] Example 4

[0056] In this embodiment, the process of purifying the vertical array of carbon nanotubes by laser irradiation heating is as follows:

[0057] (1) Preparation and transfer of vertical arrays of carbon nanotubes.

[0058] The vertical array of carbon nanotubes was prepared by plasma enhanced chemical vapor deposition, in which the impurities were mainly metal catalysts and a small amount of amorphous carbon. The vertical array grown on the silicon wafer was directly peeled off to obtain the sample to be purified, which was 1×1 cm in size and about 50 μm in thickness.

[0059] (2) Laser heating and purification of carbon nanotube vertical arrays.

[0060] A tungsten foil with a thickness of 100 μm is fixed in a vacuum chamber as a high-temperature resistant substrate, and the vertical array of carbon nanotubes to be purified is placed in the center of the tungsten foil. A mechanical pump is used to evacuate the cavity pressure to <100 Pa, and then high-purity argon gas (volume purity 99.999%) is introduced to normal pressure. A ruby ​​laser with a wavelength of 694.3 nm is used as a heat source, and the output power is set to 50 W, the pulse frequency is 5 Hz, and the number of pulses is 10 times to heat the vertical array of carbon nanotubes. In this embodiment, the heating rate of the sample is about 1500 ° C / s, and the maximum temperature is about 2000 ° C. After stopping the laser irradiation, the vertical array of carbon nanotubes is naturally cooled to room temperature and then taken out to complete the purification.

[0061] (3) Characterization of vertical arrays of purified carbon nanotubes.

[0062] In this embodiment, the density and alignment of the vertical array of carbon nanotubes remain unchanged after purification. Raman spectroscopy and transmission electron microscopy observation show that the crystallinity is improved and the internal amorphous carbon and residual metal nanoparticle impurities are successfully removed.

[0063] Comparative Example: Purification of single-walled carbon nanotube film by heating in a graphitization furnace

[0064] (1) Collection and transfer of carbon nanotube films.

[0065] The single-walled carbon nanotube film was prepared by floating catalyst chemical vapor deposition, in which the impurities were mainly iron catalyst and a small amount of amorphous carbon. The film collected on the cellulose filter was pressed and transferred to a boron nitride ring with a hole diameter of 12 mm to form a self-supporting single-walled carbon nanotube film with a thickness of about 100 nm and a transmittance of about 80% for 550 nm visible light.

[0066] (2) Slowly heating and purifying the single-walled carbon nanotube film.

[0067] The boron nitride ring loaded with carbon nanotube film is placed in a boron nitride crucible and placed in a graphitization furnace. The pressure in the cavity is pumped down to below 100Pa by a vacuum pump, and then high-purity argon gas is introduced to normal pressure. The cavity is heated from room temperature to 2300℃ (the heating rate is 20℃ / min below 1600℃ and 10℃ / min between 1600℃ and 2300℃) under an argon protective atmosphere. After reaching the set temperature, the temperature is kept for 30 minutes and then cooled down with the furnace to complete the purification.

[0068] (3) Characterization of purification results.

[0069] After opening the furnace, the carbon nanotube film on the boron nitride ring has been damaged, and the surfaces of the boron nitride ring and the boron nitride crucible are seriously contaminated with carbon ( Figure 7). Therefore, the graphite furnace heating method is not suitable for the purification of single-walled carbon nanotube films with nanometer thickness due to the slow oxidation and carbon volatilization caused by long-term high-temperature heating.

[0070] The implementation results show that the purification method of the present invention is non-destructive, controllable and efficient, and does not affect the intrinsic structure of the carbon nanotubes in the macroscopic body, such as orientation, length and diameter. The purified carbon nanotube macroscopic body can avoid the influence of metal impurities, and is expected to effectively improve the physical and chemical properties of the carbon nanotube macroscopic body. The design concept and implementation scheme of the present invention are described in detail above, but some modifications and improvements can still be made on the basis of the present invention. These modifications or improvements made without departing from the spirit of the present invention are all within the scope of protection required by the present invention.

Claims

1. A method for efficiently and non-destructively purifying a macroscopic carbon nanotube, characterized in that: The carbon nanotube macrobody is placed on a heating table in a sealed cavity, and the carbon nanotube macrobody is rapidly heated (heating rate 100-3000°C / s) in a vacuum or inert atmosphere by ultrafast heating - short-time (1-300s) high-temperature (700-2500°C) heat treatment - rapid cooling, and the carbon nanotube macrobody is purified by high-temperature thermal radiation generated by the high temperature of the heating table to remove residual metal catalysts therein, thereby improving its purity and crystallinity without destroying the structure of the carbon nanotube macrobody.

2. A method for efficiently and non-destructively purifying carbon nanotube macrostructures according to claim 1, characterized in that: The method is applicable to types of carbon nanotube macroscopic bodies including but not limited to films, fibers, arrays or aerogels.

3. A method for efficiently and non-destructively purifying carbon nanotube macrostructures according to claim 1, characterized in that: The carbon nanotube macrobody is treated by a cyclic heating method. During the treatment process, the heating temperature is 700-2500°C, the chamber pressure is 0.1-600kPa, the treatment time is 1-300s, the heating rate is 100-3000°C / s, and the heating times are 1-30 times.

4. A method for efficiently and non-destructively purifying carbon nanotube macroscopic bodies according to claim 1 or 3, characterized in that: The pressure in the heating chamber is a vacuum condition maintained by a mechanical pump or a normal pressure or high pressure condition under the protection of an inert atmosphere, and the inert atmosphere is argon or nitrogen with a volume purity of >99.99%.

5. A method for efficiently and non-destructively purifying carbon nanotube macrostructures according to claim 1, characterized in that: The regulation of different heating rates and heating temperatures is achieved by controlling the ultrafast heating method, heating power, material and thickness of the heating table.

6. A method for efficiently and non-destructively purifying carbon nanotube macro-bodies according to claim 1 or 5, characterized in that: The ultrafast heating method includes one of direct current / pulse power supply heating, laser irradiation, flash heating, plasma discharge and Joule thermal radiation.

7. A method for efficiently and non-destructively purifying carbon nanotube macro-bodies according to claim 1 or 5, characterized in that: The material of the heating stage is one or more of high-purity graphite (purity>99.99wt%), high-purity molybdenum (purity>99.99wt%), high-purity tungsten (purity>99.99wt%) and high-purity hexagonal boron nitride (purity>99.99wt%).

8. A method for efficiently and non-destructively purifying carbon nanotube macro-bodies according to claim 1, 3 or 5, characterized in that: The purity and crystallinity of the purified carbon nanotube macrobody are related to the rapid heating rate, the highest heating temperature, the number of rapid heating times, the size of the carbon nanotube macrobody and the matching degree of the heating stage.

9. A method for highly efficient and non-destructive purification of carbon nanotube macro-bodies according to claim 1, characterized in that: For the self-supporting ultra-thin single-walled carbon nanotube film with a transmittance of >90% at 550nm, the network structure remains unchanged after purification by Joule thermal radiation, and the carbon nanotubes have high crystallinity. G / I D >150, and no metal nanoparticle impurities remain.

10. A highly efficient and non-destructive method for purifying carbon nanotube macroscopic bodies according to claim 1, characterized in that: The rapid cooling method is to cool naturally to room temperature.

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