Method for high-precision, mass measurement of carbon nanotube size

Abstract

This invention relates to a high-precision, high-volume measurement method for carbon nanotube dimensions, belonging to the field of carbon materials technology. The method includes the following steps: mixing carbon nanotubes with graphene powder, and expanding the mixture 5-10 times with supercritical carbon dioxide; the mass of the carbon nanotubes is 1-4 times the mass of the graphene; the expansion method involves treating the mixed carbon nanotube and graphene powder with supercritical carbon dioxide at a pressure of 8-20 MPa, a temperature of 35-45℃, and a treatment time of 20-60 min, followed by a rapid pressure reduction to atmospheric pressure. Samples are then prepared and subjected to scanning electron microscopy (SEM) testing, and the size data of the carbon nanotubes are acquired from the images. This invention significantly improves the problems of carbon nanotube aggregation and entanglement, facilitates the identification of complete single carbon nanotubes from SEM images, and makes the batch acquisition of carbon nanotube size data easy, meeting the requirements for high-precision, high-volume statistical analysis of carbon nanotube dimensions.

Description

Technical Field

[0001] This invention relates to a high-precision, high-volume measurement method for carbon nanotube dimensions, belonging to the field of carbon materials technology. Background Technology

[0002] The size of carbon nanotubes is a key structural parameter determining their electrical, mechanical, and composite reinforcement properties. Accurate characterization of carbon nanotube size is crucial for both fundamental research and engineering applications. Size measurement methods primarily include solution light scattering and electron microscopy. However, light scattering, based on Mie scattering theory, suffers from insufficient accuracy in measuring carbon nanotube sizes, making it difficult to meet the accuracy requirements of specific applications. Electron microscopy, particularly atomic force microscopy and transmission electron microscopy, suffers from high cost, low efficiency, sparse carbon nanotube distribution within the field of view, and a limited number of samples that can be measured in a single run, hindering the large-scale statistical analysis of carbon nanotube sizes.

[0003] Compared to other detection methods, scanning electron microscopy (SEM) measurement of carbon nanotube dimensions offers advantages such as low cost, high testing efficiency, high measurement accuracy, and convenient sample preparation. Theoretically, it can meet the needs of large-scale, high-precision statistical analysis of carbon nanotube dimensions. Currently, the diameter of carbon nanotubes can be directly measured using SEM or TEM. However, measuring the length of carbon nanotubes is more difficult because they are often entangled and aggregated, making it difficult to clearly distinguish the outline and end boundaries of a single carbon nanotube in the electron microscope image. This makes it impossible to perform effective size measurement of a single carbon nanotube, greatly limiting the accuracy and applicability of the electron microscopy image measurement method.

[0004] The reasons for the entanglement and aggregation of carbon nanotubes are as follows: In preparation methods such as chemical vapor deposition, when carbon nanotubes grow from catalyst particles, the space constraints and random orientation naturally lead to the formation of entangled bundle structures; the hexagonal grid structure of sp² hybridized carbon atoms in the tube walls, with a uniform electron cloud distribution, results in strong attraction between the tubes; carbon nanotubes have a huge specific surface area, which increases the contact opportunities and interaction area between tubes, further promoting aggregation; the extremely high aspect ratio of carbon nanotubes increases the possibility of entanglement. The severe entanglement and aggregation of carbon nanotubes are related to their inherent properties. Dispersing carbon nanotubes using conventional methods during electron microscopy sample preparation is insufficient to overcome the entanglement and aggregation problem. Scanning electron microscopy imaging cannot clearly distinguish the outline and end boundaries of individual carbon nanotubes, failing to meet the high-precision, large-scale statistical requirements for carbon nanotube size detection. Summary of the Invention Technical issues

[0005] Conventional methods for preparing carbon nanotube samples for scanning electron microscopy (SEM) often result in carbon nanotubes becoming entangled and difficult to distinguish from their outlines and boundaries, limiting the accuracy of carbon nanotube size detection during SEM testing. Therefore, there is an urgent need for a new SEM method for carbon nanotube size measurement that meets the requirements for high precision and large-scale statistical analysis. Technical solution

[0006] To address the aforementioned technical problems, this invention utilizes the adsorption effect of graphene on carbon nanotubes and the dispersing effect of rapid pressure relief and expansion of supercritical carbon dioxide. Before scanning electron microscopy (SEM) testing, carbon nanotubes are mixed and expanded with graphene, significantly improving the aggregation and entanglement problems in carbon nanotube sample preparation for SEM. This results in carbon nanotubes with appropriate density, clear edge contours, clear end boundaries, and distinct individual nanotubes in the SEM field of view, meeting the requirements for high-precision testing and large-scale statistical analysis of carbon nanotube dimensions.

[0007] The first objective of this invention is to provide a pretreatment method for carbon nanotube scanning electron microscopy testing, comprising the steps of: mixing carbon nanotubes with graphene powder and expanding them with supercritical carbon dioxide 5 to 10 times; the mass of the carbon nanotubes being 1 to 4 times the mass of the graphene; the expansion method being to treat the mixed powder of carbon nanotubes and graphene with supercritical carbon dioxide at a pressure of 8 to 20 MPa, a temperature of 35 to 45°C, and a treatment time of 20 to 60 min, and then rapidly reducing the pressure to atmospheric pressure; the process of one pressurization and heating followed by rapid pressure release is referred to as one expansion.

[0008] As one embodiment of the present invention, the pressure drop to atmospheric pressure refers to the reduction of the pressure in the supercritical carbon dioxide reactor to 0.1 ± 0.02 MPa within 30 seconds.

[0009] In one embodiment of the present invention, the carbon nanotube scanning electron microscope test is used to observe the morphology and size information of the carbon nanotubes; the size information includes length, diameter, and degree of curvature.

[0010] A scanning electron microscope (SEM) is an electron beam device that uses scanning and focusing an electron beam to collect signals emitted from the sample surface in order to characterize the microstructure of a material surface through imaging.

[0011] In one embodiment of the present invention, the number of coaxial tube wall layers of carbon nanotubes is 1 to 20.

[0012] In one embodiment of the present invention, the carbon nanotubes are directionally grown carbon nanotubes or freely oriented carbon nanotubes.

[0013] Directionally grown carbon nanotubes are arrayed tubes. These carbon nanotubes are usually grown in one direction on a substrate coated with a catalyst, and the growth of carbon nanotubes is relatively uniform.

[0014] Free-oriented carbon nanotubes are entangled tubes. These carbon nanotubes are usually grown non-directionally on catalyst particles, and the grown carbon nanotubes are entangled together.

[0015] In one embodiment of the present invention, the graphene is preferably 0.01~10 nm thick and 3~8 μm in diameter.

[0016] In one embodiment of the present invention, the stirring speed for supercritical carbon dioxide treatment is 500~2000 rpm.

[0017] In one embodiment of the present invention, carbon nanotubes and graphene powder are premixed before carbon dioxide treatment; the premixing is selected from one or more of ball milling and sand milling.

[0018] In one embodiment of the invention, gentle grinding conditions are used during premixing to avoid damaging the carbon nanotubes. Proper grinding facilitates the uniform adsorption of carbon nanotubes by graphene.

[0019] In one embodiment of the present invention, the ball milling method involves adding zirconium beads to a mixed powder of carbon nanotubes and graphene. The zirconium beads have a diameter of 0.3 to 0.6 mm, the volume of the carbon nanotube powder is 2 to 4 times the volume of the zirconium beads, the rotation speed is 200 to 600 rpm, and the milling time is 1 to 4 hours.

[0020] In one embodiment of the present invention, the grinding method is as follows: zirconium beads are added to the mixed powder of carbon nanotubes and graphene. The diameter of the zirconium beads is 0.6-1 mm, the volume of the carbon nanotube powder is 2-4 times the volume of the zirconium beads, the rotation speed is 500-2000 rpm, and the grinding time is 2-6 h.

[0021] The second objective of this invention is to provide a scanning electron microscope (SEM) testing method for carbon nanotubes, comprising the steps of: pretreating the carbon nanotubes to be tested using the above-mentioned pretreatment method, preparing the sample, and performing SEM testing.

[0022] In one embodiment of the present invention, the sample preparation is carried out in accordance with the sample preparation requirements of a scanning electron microscope.

[0023] In one embodiment of the present invention, the carbon nanotube scanning electron microscope test is used to observe the morphology and size information of the carbon nanotubes; the size information includes length, diameter, and degree of curvature.

[0024] The third objective of this invention is to provide a method for measuring the size of carbon nanotubes, comprising the steps of: processing the carbon nanotubes to be measured using the above-mentioned pretreatment method, capturing scanning electron microscope images, and acquiring the size data of the carbon nanotubes from the images.

[0025] As one embodiment of the present invention, the method for collecting carbon nanotube size data is to calculate the actual size of the carbon nanotube based on the size of the carbon nanotube pattern in the image and the scale or magnification of the scanning electron microscope image.

[0026] As one embodiment of the present invention, actual size data of 20 to 500 carbon nanotubes are collected; these data are used to calculate the mean, median, mode, maximum, minimum, range, variance, standard deviation, and coefficient of variation, or to analyze distribution patterns.

[0027] Beneficial effects: The pretreatment method for carbon nanotube scanning electron microscopy testing described in this invention utilizes the adsorption effect of graphene on carbon nanotubes and the dispersion effect of rapid decompression and expansion of supercritical carbon dioxide to significantly improve the aggregation and entanglement problems of carbon nanotubes. This results in a suitable density of carbon nanotubes in the field of view of the scanning electron microscope, with clear edge contours and distinct individual nanotubes. This facilitates the identification of complete single carbon nanotubes from the scanning electron microscope image, making it easier to collect carbon nanotube size data in batches and meeting the requirements for high-precision and large-scale statistical analysis of carbon nanotube size. Attached Figure Description

[0028] Figure 1 The image is an SEM image taken in Example 1.

[0029] Figure 2 The image shown is a SEM image taken for Comparative Example 1.

[0030] Figure 3 This is a SEM image taken for comparison example 4. Detailed Implementation Example 1

[0031] A method for measuring the length of multi-walled carbon nanotubes, comprising the following steps.

[0032] (1) Carbon nanotube premix Commercially available multi-walled carbon nanotube powder and high-energy ball milling zirconium beads were placed in a high-energy ball mill for grinding. The diameter of the high-energy ball milling zirconium beads was 0.6 mm, the volume of the added multi-walled carbon nanotube powder was 3 times the volume of the zirconium beads, the grinding time was 2 hours, and the high-energy ball mill speed was 500 rpm.

[0033] (2) Carbon nanotube expansion and dispersion The multi-walled carbon nanotube powder and graphene powder processed in step (1) were mixed, with the mass of carbon nanotubes being twice the mass of graphene. The mixed powder was then expanded eight times. The expansion method was as follows: the mixed powder was placed in a supercritical carbon dioxide reactor, the reactor pressure was set to 12 MPa, the reactor temperature was set to 37°C, the rotor stirring speed was set to 1000 rpm, and the mixture was stirred for 30 min. Then, the pressure inside the reactor was rapidly released, causing the pressure to drop sharply to atmospheric pressure to expand the powder inside the reactor.

[0034] (3) Measurement of carbon nanotube dimensions The product from step (2) is evenly sprinkled onto the conductive adhesive. Unadhesive powder is blown away with a syringe, and SEM testing is performed, with images captured. Line segments or curves are drawn on the SEM image using image processing software, ensuring the length of the segments or curves matches that of the multi-walled carbon nanotubes (MWC). The length data of the segments or curves are collected, and the actual length of the MWC is calculated based on the scale of the SEM image. Actual length data from 20 to 500 MWC are collected to create a length distribution map of the MWC or to calculate the average length.

[0035] The SEM image captured in Example 1 is as follows: Figure 1 As shown, multi-walled carbon nanotubes are clearly distributed on the graphene surface. The carbon nanotubes are not entangled or aggregated, and each one is distinct, making it easy to collect dimensional data of the multi-walled carbon nanotubes manually. Example 2

[0036] A method for measuring the length of carbon nanotubes, comprising the following steps.

[0037] (1) Carbon nanotube dispersion Commercially available single-walled carbon nanotube powder and graphene powder were mixed, with the mass of carbon nanotubes being twice the mass of graphene. The mixed powder was then expanded 10 times. The expansion method involved placing the mixed powder in a supercritical carbon dioxide reactor, setting the internal pressure to 8 MPa, the reactor temperature to 45°C, and the rotor stirring speed to 1500 rpm for 20 minutes. Then, the pressure inside the reactor was rapidly released, causing the pressure to drop sharply to atmospheric pressure to expand the powder inside the reactor.

[0038] (2) Measurement of carbon nanotube dimensions The product from step (1) was evenly sprinkled onto the conductive adhesive. Unadhered powder was blown away with a syringe, and SEM testing was performed, with images captured. Line segments or curves were plotted on the SEM image using image processing software, ensuring the length of the segments or curves matched that of the single-walled carbon nanotubes (SUVs). The length data of the segments or curves were collected, and the actual length of the SUVs was calculated based on the SEM image scale. Actual length data from 20 to 500 SUVs were collected to create a length distribution map of SUVs or to calculate the average length.

[0039] In the SEM image taken in Example 2, single-walled carbon nanotubes are clearly distributed on the graphene surface. The carbon nanotubes are not entangled or aggregated, and each one is distinct, making it easy to collect the size data of the single-walled carbon nanotubes manually. Example 3

[0040] A method for measuring the length of carbon nanotubes, comprising the following steps.

[0041] (1) Carbon nanotube dispersion Commercially available multi-walled carbon nanotube powder and graphene powder were mixed, with the mass of carbon nanotubes being four times the mass of graphene. The mixed powder was then expanded six times. The expansion method involved placing the mixed powder in a supercritical carbon dioxide reactor, setting the internal pressure to 18 MPa, the reactor temperature to 35°C, and the rotor stirring speed to 600 rpm for 50 minutes. Then, the pressure inside the reactor was rapidly released, causing the pressure to drop sharply to atmospheric pressure to expand the powder inside the reactor.

[0042] (2) Measurement of carbon nanotube dimensions The product from step (1) is evenly sprinkled onto the conductive adhesive. Unadhesive powder is blown away with a syringe, and SEM testing is performed, with images captured. Line segments or curves are drawn on the SEM image using image processing software, ensuring the length of the segments or curves matches that of the multi-walled carbon nanotubes (MWC). The length data of the segments or curves are collected, and the actual length of the MWC is calculated based on the scale of the SEM image. Actual length data from 20 to 500 MWC nanotubes are collected to create a length distribution map of the MWC, or to calculate the average length.

[0043] In the SEM image taken in Example 3, multi-walled carbon nanotubes are clearly distributed on the graphene surface. The carbon nanotubes are not entangled or aggregated, and each one is distinct, making it easy to collect the size data of the multi-walled carbon nanotubes manually. Comparative Example 1

[0044] A method for measuring multi-walled carbon nanotubes differs from Example 1 only in that graphene powder is not added in step (2), i.e., multi-walled carbon nanotube powder is directly expanded.

[0045] SEM images taken for Comparative Example 1 are as follows: Figure 2 As shown, multi-walled carbon nanotubes are severely entangled and aggregated, making it difficult to distinguish complete carbon nanotubes with the naked eye, and making it impossible to manually collect the size data of multi-walled carbon nanotubes from SEM images.

[0046] Compared to Comparative Example 1, the SEM images taken in Example 1 made it easier to acquire size data of multi-walled carbon nanotubes. This may be because both carbon nanotubes and graphene are composed of sp... 2 Composed of hybrid carbon atoms, it has a complete large π-π conjugated electron system, and strong π-π stacking interaction can be generated between the two. At the same time, carbon nanotubes and graphene have large specific surface areas and significant intermolecular van der Waals forces. Therefore, unentangled carbon nanotubes can be easily and stably adsorbed on the graphene surface, making it easy to observe a single complete carbon nanotube. Comparative Example 2

[0047] A method for measuring multi-walled carbon nanotubes differs from Example 1 only in that the mass of the carbon nanotubes is 7 times the mass of the graphene.

[0048] In the SEM image of Comparative Example 2, the multi-walled carbon nanotubes are severely entangled and aggregated, making it difficult to distinguish complete carbon nanotubes with the naked eye, and making it impossible to manually collect the size data of multi-walled carbon nanotubes from the SEM image. Comparative Example 3

[0049] A method for measuring multi-walled carbon nanotubes differs from Example 1 only in that the mass of the carbon nanotubes is 0.2 times the mass of the graphene.

[0050] In the SEM image of Comparative Example 3, the distribution of multi-walled carbon nanotubes is too sparse, making it difficult to manually collect sufficient multi-walled carbon nanotube size data from the SEM image. Comparative Example 4

[0051] Commercially available carbon nanotube powder was evenly sprinkled onto conductive adhesive, and any unadhered powder was blown away with a bulb syringe. SEM testing was then performed and images were captured.

[0052] SEM images taken in Comparative Example 4 are as follows: Figure 3 As shown, multi-walled carbon nanotubes are severely entangled and aggregated, making it difficult to distinguish complete carbon nanotubes with the naked eye, and making it impossible to manually collect the size data of multi-walled carbon nanotubes from SEM images.

[0053] Compared to Comparative Example 4, the SEM images of Comparative Example 1 show that the carbon nanotubes are less tightly entangled and more loosely packed, indicating an improvement in the entanglement and aggregation of carbon nanotubes. However, it is still impossible to clearly distinguish the outline of a single carbon nanotube and its two ends. Comparative Example 5

[0054] Commercially available single-walled carbon nanotube powder was mixed with NMP solvent at a concentration of 0.1 wt%. An appropriate amount of dispersant was added and stirred to form a dispersion. One to two drops of the carbon nanotube dispersion were dropped onto the conductive adhesive, the solvent was dried, and SEM testing was performed and images were captured.

[0055] In Comparative Example 5, carbon nanotubes were dispersed with a dispersant during sample preparation. The carbon nanotubes were agglomerated and entangled in the SEM images, and the outline and two-end boundaries of a single carbon nanotube could not be clearly distinguished. Therefore, it was impossible to collect high-precision dimensional data of carbon nanotubes in large quantities from the SEM images.

[0056] The embodiments provided above are not intended to limit the scope of the invention, nor are the described steps intended to limit the order of execution. Any obvious modifications made to the invention by those skilled in the art based on existing common knowledge also fall within the scope of protection defined by the claims.

Claims

1. A pretreatment method for carbon nanotube scanning electron microscopy testing, characterized in that, The steps include: mixing carbon nanotubes and graphene powder, and expanding them 5 to 10 times with supercritical carbon dioxide; the mass of carbon nanotubes is 1 to 4 times the mass of graphene; the expansion method is to treat the mixed powder of carbon nanotubes and graphene with supercritical carbon dioxide at a pressure of 8 to 20 MPa, a temperature of 35 to 45°C, and a treatment time of 20 to 60 min, and then rapidly reducing the pressure to atmospheric pressure; the process of pressurizing and heating and then rapidly releasing the pressure is recorded as one expansion.

2. The pretreatment method according to claim 1, characterized in that, The carbon nanotube scanning electron microscope test is used to observe the morphology and size information of carbon nanotubes.

3. The pretreatment method according to claim 1, characterized in that, The number of coaxial tube wall layers of carbon nanotubes ranges from 1 to 20; the carbon nanotubes are either directionally grown carbon nanotubes or freely oriented carbon nanotubes.

4. The pretreatment method according to claim 1, characterized in that, The graphene has a thickness of 0.01~10 nm and a sheet diameter of 3~8 μm.

5. The pretreatment method according to claim 1, characterized in that, Before carbon dioxide treatment, carbon nanotubes and graphene powder are premixed; the premixing is selected from one or more of ball milling and sand milling.

6. The pretreatment method according to claim 5, characterized in that, The ball milling method involves adding zirconium beads with a diameter of 0.3–0.6 mm to the mixed powder of carbon nanotubes and graphene. The volume of the carbon nanotube powder is 2–4 times the volume of the zirconium beads. The rotation speed is 200–600 rpm, and the milling time is 1–4 h. The sand milling method involves adding zirconium beads with a diameter of 0.6–1 mm to the mixed powder of carbon nanotubes and graphene. The volume of the carbon nanotube powder is 2–4 times the volume of the zirconium beads. The rotation speed is 500–2000 rpm, and the milling time is 2–6 h.

7. A scanning electron microscope method for testing carbon nanotubes, characterized in that, The steps include: treating the carbon nanotubes to be tested using any of the pretreatment methods described in claims 1 to 6, preparing the sample, and performing scanning electron microscopy testing.

8. A method for measuring the size of carbon nanotubes, characterized in that, The steps include: taking scanning electron microscope images of the carbon nanotubes to be tested using the scanning electron microscope testing method described in claim 7, and collecting the size data of the carbon nanotubes from the images.

9. The measurement method according to claim 8, characterized in that, The method for collecting carbon nanotube size data is to calculate the actual size of the carbon nanotube based on the size of a single carbon nanotube pattern in the image and the scale or magnification of the scanning electron microscope image.

10. The measurement method according to claim 8 or 9, characterized in that, Collect actual size data of 20-500 carbon nanotubes; calculate the mean, median, mode, maximum, minimum, range, variance, standard deviation, and coefficient of variation, or analyze the distribution pattern.

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