Method for purifying multi-walled carbon nanotubes by spark plasma sintering technology

Abstract

The invention discloses a method for purifying a multi-walled carbon nanotube by a spark plasma sintering technology. The method comprises the following steps: pre-treating a multi-walled carbon nanotube crude product prepared by a CVD (Chemical Vapor Deposition) method; taking a proper amount of treated multi-walled carbon nanotubes and putting the treated multi-walled carbon nanotubes into a graphite crucible; and in vacuum, putting the crucible into a discharge plasma sintering furnace, starting pressureless sintering, taking out a sample and immediately performing vacuum sealing after a set temperature is reached and heat preservation is performed for a corresponding time, thereby obtaining the purified multi-walled carbon nanotube. According to the purification method disclosed by the invention, the multi-walled carbon nanotubes are purified by adopting a spark plasma sintering technology, so that defects, free-state carbon particles, metal catalyst particles and other impurities generated in the preparation process of the multi-walled carbon nanotubes can be removed only at a relatively low temperature in a relatively short time, the multi-walled carbon nanotubes can be better graphitized, and the purpose of purification is achieved. The purification method is also applicable to graphene and other carbon materials.

Description

Technical Field

[0001] The invention relates to a method for purifying carbon nanotubes, graphene and carbon materials by using spark plasma sintering technology, belonging to the technical field of purification. Background Art

[0002] Carbon nanotubes have good mechanical, electrical and physicochemical properties, and are widely used in composite materials, electrochemical devices, field effect transistors, and even artificial muscles. At present, the main methods for preparing carbon nanotubes are arc discharge, hydrocarbon catalytic pyrolysis, laser evaporation, electrolytic thermal synthesis, chemical vapor deposition, etc. These preparation methods will inevitably produce impurities, such as metal catalysts Fe, Co and Ni and other carbon impurities. These impurities not only reduce the purity of carbon nanotubes, but also affect their physical and chemical properties such as magnetic permeability, thermal conductivity, and antioxidant properties, which have an extremely adverse effect on the performance of carbon nanotubes in composite materials.

[0003] At present, the purification methods of carbon nanotubes can be divided into physical purification and chemical purification. Among them, the advantage of physical purification is that it will not destroy the structure of carbon nanotubes, and the advantage of chemical purification is that it generally does not require special experimental equipment, the reaction conditions are easy to control, and the operation is simple and easy. However, these purification methods all have some shortcomings, such as it is not easy to remove impurities such as metal catalyst particles, which can easily cause irreversible damage to the structure of carbon nanotubes. In addition, traditional high-temperature purification technology usually requires a high temperature of more than 3000°C and a processing time of several hours, and the energy efficiency is extremely low. Spark plasma sintering has many advantages such as short sintering time, controllable structure and grain size. During the sintering process, the plasma temperature generated by the local discharge at the tip of the particle can be as high as 4000-10999°C. The present invention utilizes the local high temperature characteristics of spark plasma sintering to purify multi-walled carbon nanotubes, and only needs to remove defects, free carbon and metal catalyst particles in multi-walled carbon nanotubes at a lower temperature and a shorter time, so that multi-walled carbon nanotubes can be better graphitized to achieve the purpose of purification. Summary of the invention

[0004] In order to overcome the problems in the background technology, the present invention proposes a method for purifying multi-walled carbon nanotubes using spark plasma sintering technology. By using spark plasma sintering technology to heat treat multi-walled carbon nanotubes at high temperature, the graphitization process is essentially a dynamic balance process of structural repair and destruction. When the temperature is appropriate, the defect repair rate is greater than the rate of structural damage caused by high temperature, and the graphitization effect is improved at this time; when the temperature is too high, the damage to the structure caused by high temperature is greater than the repair rate, and the graphitization effect is reduced at this time. In the traditional graphitization process, this temperature is usually above 3000°C, and it takes several hours of insulation time to achieve the purpose. The present invention uses spark plasma sintering to heat treat multi-walled carbon nanotubes. The results show that only a lower temperature (around 1600°C) and a shorter time (10-15min) are required to achieve a good graphitization effect. This is because the heating characteristics of spark plasma sintering are different from traditional thermal radiation heating methods. Spark plasma sintering uses pulsed plasma discharge heating, which makes it easier to generate plasma discharge sparks at defects and conductive particles (free carbon, metal catalyst particles) to reach local high temperatures to repair defects and vaporize impurity particles. Therefore, good purification effects can be achieved at lower heat treatment temperatures.

[0005] In order to achieve the above object, the present invention is implemented by the following technical solutions:

[0006] The method for purifying multi-walled carbon nanotubes by spark plasma sintering technology comprises the following steps:

[0007] (1) pretreating a crude multi-walled carbon nanotube prepared by a CVD method;

[0008] (2) taking an appropriate amount of multi-walled carbon nanotubes treated in step (1) and placing them in a graphite crucible;

[0009] (3) placing the multi-walled carbon nanotubes and graphite crucible in step (2) into a plasma sintering furnace and starting pressureless sintering until the set temperature is reached and the temperature is kept for a corresponding time, and after the sintering is completed, purified multi-walled carbon nanotubes are obtained.

[0010] (4) After sintering is completed, the sample from step (3) is taken out and immediately vacuum-sealed to prevent the sample from absorbing impurities such as air.

[0011] Preferably, in step (1), the inner diameter of the multi-walled carbon nanotube is 5-10 nm, the outer diameter is 20-30 nm, and the length is 0.5-2 μm.

[0012] Preferably, the amount of multi-walled carbon nanotubes used in step (1) is 0.1 g.

[0013] Preferably, in step (1), the pretreatment is to place the multi-walled carbon nanotubes in a drying oven and dry them at a temperature of 40 to 90° C. for 12 hours to remove moisture and gas molecules adsorbed in the carbon nanotubes.

[0014] Preferably, in step (3), the sintering temperature ranges from 1400 to 2000° C., and the heat treatment time is 5 to 15 minutes.

[0015] Preferably, in step (3), the sintering temperature is 1500-1800° C. and the holding time is 5-10 min.

[0016] Preferably, in step (3), the sintering temperature is 1600-1700° C. and the holding time is 5-10 min.

[0017] Preferably, in step (3), the sintering temperature is 1600° C. and the holding time is 10 min.

[0018] Preferably, in step (3), the plasma sintering furnace needs to be evacuated to below 6 Pa.

[0019] Preferably, in step (3), the heating rate of the multi-walled carbon nanotubes is 100°C / min, the temperature is measured by infrared radiation, and after reaching the set sintering temperature, the temperature is kept for a corresponding time and then naturally cooled to room temperature.

[0020] Beneficial effects of the present invention:

[0021] The present invention adopts the spark plasma sintering technology, utilizes the pulse plasma discharge heating, and is more likely to generate plasma discharge sparks at defects and conductive particles (free carbon, metal catalyst particles) to locally reach high temperature, and can obtain multi-walled carbon nanotubes with good purification effect at a lower sintering temperature and in a shorter processing time. This method is simple to operate and has obvious effects. In addition, the method of the present invention is easy to operate and control, the sintering temperature is low, the processing time is short, the practicability is strong, the energy efficiency ratio is high, and it is convenient for large-scale promotion. This purification method is also applicable to graphene and other carbon materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is the XRD spectrum of the carbon nanotubes after spark plasma sintering in Examples 1-6 of the present invention at different temperatures and different heat treatment times in a vacuum environment.

[0023] Figure 2 These are high-resolution field emission TEM images of the multi-walled carbon nanotubes of Example 1 of the present invention before and after vacuum heat treatment. (a) and (b) are original multi-walled carbon nanotubes; (c) and (d) are multi-walled carbon nanotubes of Example 1 after vacuum heat treatment at 1600°C for 10 min.

[0024] Figure 3 These are high-resolution field emission TEM images of the multi-walled carbon nanotubes of Example 2 of the present invention before and after vacuum heat treatment, (a) is the original multi-walled carbon nanotubes; (b) is the multi-walled carbon nanotubes of Example 2 after vacuum heat treatment at 1400°C for 5 minutes.

[0025] Figure 4 This is the thermogravimetric analysis result of the multi-walled carbon nanotubes of Example 2 of the present invention under air conditions. The left side is the original multi-walled carbon nanotubes; the right side is the multi-walled carbon nanotubes of Example 2 after vacuum heat treatment at 1400°C for 5 minutes. DETAILED DESCRIPTION

[0026] The present invention will be further described below in conjunction with specific embodiments.

[0027] Example 1

[0028] A method for purifying multi-walled carbon nanotubes using spark plasma sintering technology comprises the following steps:

[0029] (1) pretreating the prepared crude multi-walled carbon nanotubes by placing the multi-walled carbon nanotubes in a drying oven at a drying temperature of 60° C. for 12 h;

[0030] (2) taking 0.1 g of the multi-walled carbon nanotubes treated in step (1) and placing them in a graphite crucible;

[0031] (3) placing the multi-walled carbon nanotubes and graphite crucible of step (2) into a plasma sintering furnace with a vacuum of 6 Pa and starting pressureless sintering at a heating rate of 100°C / min. After heating to 1600°C, the temperature is kept at that temperature for 10 minutes. After the temperature is kept at that temperature, the temperature is naturally cooled to room temperature. After the sintering is completed, the purified multi-walled carbon nanotubes are obtained.

[0032] (4) After sintering is completed, the sample from step (3) is taken out and immediately vacuum-sealed to prevent the sample from absorbing impurities such as air.

[0033] Example 1 XRD pattern and high-resolution field emission TEM of vacuum heat treatment at 1600°C for 10 min are as follows Figure 1 and Figure 2 shown.

[0034] Example 2

[0035] A method for purifying multi-walled carbon nanotubes using spark plasma sintering technology comprises the following steps:

[0036] (1) pretreating the prepared crude multi-walled carbon nanotubes by placing the multi-walled carbon nanotubes in a drying oven at a drying temperature of 60° C. for 12 h;

[0037] (2) taking 0.1 g of the multi-walled carbon nanotubes treated in step (1) and placing them in a graphite crucible;

[0038] (3) placing the multi-walled carbon nanotubes and graphite crucible of step (2) into a plasma sintering furnace with a vacuum of 6 Pa and starting pressureless sintering at a heating rate of 100°C / min. After heating to 1400°C, the temperature is kept at that temperature for 5 minutes. After the temperature is kept at that temperature, the temperature is naturally cooled to room temperature. After the sintering is completed, the purified multi-walled carbon nanotubes are obtained.

[0039] (4) After sintering is completed, the sample from step (3) is taken out and immediately vacuum-sealed to prevent the sample from absorbing impurities such as air.

[0040] Example 2 The XRD pattern, high-resolution field emission TEM pattern and thermogravimetric analysis of the samples after vacuum heat treatment at 1400°C for 5 min are as follows: Figure 1 , Figure 3 and Figure 4 shown.

[0041] Example 3

[0042] A method for purifying multi-walled carbon nanotubes using spark plasma sintering technology comprises the following steps:

[0043] (1) pretreating the prepared crude multi-walled carbon nanotubes by placing the multi-walled carbon nanotubes in a drying oven at a drying temperature of 60° C. for 12 h;

[0044] (2) taking 0.1 g of the multi-walled carbon nanotubes treated in step (1) and placing them in a graphite crucible;

[0045] (3) placing the multi-walled carbon nanotubes and graphite crucible of step (2) into a plasma sintering furnace with a vacuum of 6 Pa and starting pressureless sintering at a heating rate of 100°C / min. After heating to 1400°C, the temperature is kept at that temperature for 10 minutes. After the temperature is kept at that temperature, the temperature is naturally cooled to room temperature. After the sintering is completed, the purified multi-walled carbon nanotubes are obtained.

[0046] (4) After sintering is completed, the sample from step (3) is taken out and immediately vacuum-sealed to prevent the sample from absorbing impurities such as air.

[0047] The XRD pattern of Example 3 after vacuum heat treatment at 1400°C for 20 min is as follows: Figure 1 shown.

[0048] Example 4

[0049] A method for purifying multi-walled carbon nanotubes using spark plasma sintering technology comprises the following steps:

[0050] (1) pretreating the prepared crude multi-walled carbon nanotubes by placing the multi-walled carbon nanotubes in a drying oven at a drying temperature of 60° C. for 12 h;

[0051] (2) taking 0.1 g of the multi-walled carbon nanotubes treated in step (1) and placing them in a graphite crucible;

[0052] (3) placing the multi-walled carbon nanotubes and graphite crucible of step (2) into a plasma sintering furnace with a vacuum of 6 Pa and starting pressureless sintering at a heating rate of 100°C / min. After heating to 1800°C, the temperature is kept at that temperature for 5 minutes. After the temperature is kept at that temperature, the temperature is naturally cooled to room temperature. After the sintering is completed, the purified multi-walled carbon nanotubes are obtained.

[0053] (4) After sintering is completed, the sample from step (3) is taken out and immediately vacuum-sealed to prevent the sample from absorbing impurities such as air.

[0054] The XRD pattern of Example 4 after vacuum heat treatment at 1800°C for 5 min is shown in Figure 1 shown.

[0055] Example 5

[0056] A method for purifying multi-walled carbon nanotubes using spark plasma sintering technology comprises the following steps:

[0057] (1) pretreating the prepared crude multi-walled carbon nanotubes by placing the multi-walled carbon nanotubes in a drying oven at a drying temperature of 60° C. for 12 h;

[0058] (2) taking 0.1 g of the multi-walled carbon nanotubes treated in step (1) and placing them in a graphite crucible;

[0059] (3) placing the multi-walled carbon nanotubes and graphite crucible of step (2) into a plasma sintering furnace with a vacuum of 6 Pa and starting pressureless sintering at a heating rate of 100° C. / min. After heating to 1800° C., the temperature is kept at that temperature for 15 min. After the temperature is kept at that temperature, the temperature is naturally cooled to room temperature. After the sintering is completed, the purified multi-walled carbon nanotubes are obtained.

[0060] (4) After sintering is completed, the sample from step (3) is taken out and immediately vacuum-sealed to prevent the sample from absorbing impurities such as air.

[0061] The XRD pattern of Example 5 after vacuum heat treatment at 1800°C for 15 min is as follows: Figure 1 shown.

[0062] Example 6

[0063] A method for purifying multi-walled carbon nanotubes using spark plasma sintering technology comprises the following steps:

[0064] (1) pretreating the prepared crude multi-walled carbon nanotubes by placing the multi-walled carbon nanotubes in a drying oven at a drying temperature of 60° C. for 12 h;

[0065] (2) taking 0.1 g of the multi-walled carbon nanotubes treated in step (1) and placing them in a graphite crucible;

[0066] (3) placing the multi-walled carbon nanotubes and graphite crucible of step (2) into a plasma sintering furnace with a vacuum of 6 Pa and starting pressureless sintering at a heating rate of 100°C / min. After heating to 2000°C, the temperature is kept at that temperature for 5 minutes. After the temperature is kept at that temperature, the temperature is naturally cooled to room temperature. After the sintering is completed, the purified multi-walled carbon nanotubes are obtained.

[0067] (4) After sintering is completed, the sample from step (3) is taken out and immediately vacuum-sealed to prevent the sample from absorbing impurities such as air.

[0068] The XRD pattern of Example 6 after vacuum heat treatment at 2000°C for 5 min is as follows: Figure 1 shown.

[0069] Example 7

[0070] A method for purifying multi-walled carbon nanotubes using spark plasma sintering technology comprises the following steps:

[0071] (1) pretreating the prepared crude multi-walled carbon nanotubes by placing the multi-walled carbon nanotubes in a drying oven at a drying temperature of 40° C. for 12 h;

[0072] (2) taking 0.1 g of the multi-walled carbon nanotubes treated in step (1) and placing them in a graphite crucible;

[0073] (3) placing the multi-walled carbon nanotubes and graphite crucible of step (2) into a plasma sintering furnace with a vacuum of 6 Pa and starting pressureless sintering at a heating rate of 100°C / min. After heating to 1500°C, the temperature is kept at that temperature for 10 minutes. After the temperature is kept at that temperature, the temperature is naturally cooled to room temperature. After the sintering is completed, the purified multi-walled carbon nanotubes are obtained.

[0074] (4) After sintering is completed, the sample from step (3) is taken out and immediately vacuum-sealed to prevent the sample from absorbing impurities such as air.

[0075] Example 8

[0076] A method for purifying multi-walled carbon nanotubes using spark plasma sintering technology comprises the following steps:

[0077] (1) pretreating the prepared crude multi-walled carbon nanotubes by placing the multi-walled carbon nanotubes in a drying oven at a drying temperature of 90° C. for 12 h;

[0078] (2) taking 0.1 g of the multi-walled carbon nanotubes treated in step (1) and placing them in a graphite crucible;

[0079] (3) placing the multi-walled carbon nanotubes and graphite crucible of step (2) into a plasma sintering furnace with a vacuum of 6 Pa and starting pressureless sintering at a heating rate of 100°C / min. After heating to 1700°C, the temperature is kept at that temperature for 10 minutes. After the temperature is kept at that temperature, the temperature is naturally cooled to room temperature. After the sintering is completed, the purified multi-walled carbon nanotubes are obtained.

[0080] (4) After sintering is completed, the sample from step (3) is taken out and immediately vacuum-sealed to prevent the sample from absorbing impurities such as air.

[0081] Depend on Figure 1 It can be seen that the intensity of the diffraction peak of the multi-walled carbon nanotubes heat-treated at 1600℃ for 10 minutes is the strongest, the wall of the multi-walled carbon nanotubes is smooth, the defects are greatly reduced, the crystallization quality and graphitization effect of the carbon nanotubes are better. The intensity of the diffraction peak of the multi-walled carbon nanotubes heat-treated at 1800℃ and 2000℃ is weakened.

[0082] Depend on Figure 2 It can be seen that the wall of the multi-walled carbon nanotubes that have been heat treated at 1600℃ in vacuum for 10 minutes becomes smooth, the defects are greatly reduced, and the surface defects of the carbon nanotubes are repaired by graphitization; while there are a large number of defects on the wall of the multi-walled carbon nanotubes that have not been heat treated, and the wall is uneven.

[0083] Depend on Figure 3 It can be seen that the metal catalyst particles in the multi-walled carbon nanotubes after vacuum heat treatment at 1400°C for 5 minutes are greatly reduced, and the purpose of purifying the multi-walled carbon nanotubes can be achieved.

[0084] Depend on Figure 4 It can be seen that the weight of multi-walled carbon nanotubes after vacuum heat treatment at 1400℃ for 5min begins to decrease significantly at about 553℃, and reaches a stable mass of 0.17wt.% at 780℃. The weight of the original multi-walled carbon nanotubes decreases significantly at about 446℃, because the free carbon and defects in the sample begin to oxidize, and decrease sharply with the increase of temperature, reaching a stable mass of 2.28wt.% at 667℃. The remaining part is mainly the oxide residue of metal catalyst particles. The comparison results show that after heat treatment at 1400℃ for 5min, the carbon nanotubes are well graphitized, and their stable temperature in air increases by about 107K. The final residual weight of the sample is 0.17wt.%, indicating that the spark plasma sintering technology can effectively vaporize the metal catalyst impurities and free carbon in the sample, achieving a good effect of purifying carbon nanotubes.

[0085] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A method for purifying multi-walled carbon nanotubes using spark plasma sintering technology, characterized in that: The steps include: (1) pretreating a crude multi-walled carbon nanotube prepared by a CVD method; (2) taking an appropriate amount of multi-walled carbon nanotubes treated in step (1) and placing them in a graphite crucible; (3) placing the multi-walled carbon nanotubes and graphite crucible of step (2) into a plasma sintering furnace in a vacuum and starting pressureless sintering to a set temperature and keeping the temperature for a corresponding time; (4) After sintering is completed, the sample from step (3) is taken out and immediately vacuum-sealed to prevent the sample from absorbing impurities such as air, thereby obtaining purified multi-walled carbon nanotubes.

2. The method for purifying multi-walled carbon nanotubes by spark plasma sintering technology according to claim 1, characterized in that: In the step (1), the inner diameter of the multi-walled carbon nanotube is 5-10 nm, the outer diameter is 20-30 nm, and the length is 0.5-2 μm.

3. The method for purifying multi-walled carbon nanotubes by spark plasma sintering technology according to claim 2, characterized in that: In the step (1), the pretreatment is to place the multi-walled carbon nanotubes in a drying oven and dry them at a temperature of 40 to 90° C. for 12 hours.

4. The method for purifying multi-walled carbon nanotubes by spark plasma sintering technology according to claim 1, characterized in that: In the step (3), the sintering temperature ranges from 1400 to 2000° C., and the heat treatment time is 5 to 15 minutes.

5. The method for purifying multi-walled carbon nanotubes by spark plasma sintering technology according to claim 1, characterized in that: In the step (3), the plasma sintering furnace needs to be evacuated to below 6 Pa.

6. The method for purifying multi-walled carbon nanotubes by spark plasma sintering technology according to claim 1, characterized in that: In the step (3), the temperature of the multi-walled carbon nanotubes is raised at a rate of 100° C. / min, and the temperature is quickly reached to a set sintering temperature, kept at a corresponding time, and then cooled to room temperature in the furnace.

7. The method for purifying multi-walled carbon nanotubes by spark plasma sintering technology according to claim 1 or 4, characterized in that: In the step (3), the sintering temperature is 1500-1800° C. and the holding time is 5-10 minutes.

8. The method for purifying multi-walled carbon nanotubes by spark plasma sintering technology according to claim 7, characterized in that: In the step (3), the sintering temperature is 1600-1700° C. and the holding time is 5-10 minutes.

Images