Carbon nanotube / graphene composite heat conduction film and preparation method thereof

Abstract

The invention provides a carbon nanotube / graphene composite heat conduction film and a preparation method thereof, and relates to the technical field of graphene, and the preparation method comprises the following steps: (a) sanding a carbon nanotube suspension to obtain a carbon nanotube stripping dispersion slurry; (b) diluting the slurry, mixing the diluted slurry with crystalline flake graphite, and performing spray drying to obtain mixed powder; (c) carrying out intercalation oxidation reaction on the mixed powder to obtain a composite material; (d) preparing the composite material into slurry, performing homogeneous stripping treatment to obtain homogenized slurry, and sequentially performing film forming treatment, graphitization treatment and calendering treatment on the homogenized slurry to obtain a calendered composite graphitized film; and (e) carrying out film stacking and graphitization treatment on the composite graphitized film and the graphene carbonized film to obtain the carbon nanotube / graphene composite heat-conducting film. The heat-conducting property and the bending resistance of the graphene heat-conducting film can be improved, and the urgent demand of the market on the graphene heat-conducting film with the high heat-conducting property can be met.

Description

Carbon nanotube / graphene composite thermal conductive film and its preparation method Technical Field

[0001] This invention relates to the technical field of graphene, and in particular to a carbon nanotube / graphene composite thermal conductive film and its preparation method. Background Technology

[0002] Graphene thermal conductive films are carbon-based thermal management materials with excellent thermal conductivity. Their perfect long-range ordered six-membered ring structure boasts a theoretical thermal conductivity as high as 5300 W / mK. However, with the thermal conductivity of graphene thermal conductive films surpassing the 1500 W / mK threshold, the development of high-performance graphene thermal conductive films with scalable production capabilities has gradually stalled. Although cutting-edge research on graphene thermal conductive films shows that their thermal conductivity has already exceeded 2000 W / mK, this is often accompanied by extremely demanding preparation conditions and high production costs. For example, graphene thermal conductive films prepared from highly dispersed, ultra-large-sized, low-defect graphene oxide sheets in cutting-edge research are difficult to mass-produce. Like many cutting-edge scientific research technologies, they cannot be truly converted to production due to outdated hardware conditions and remain shelved. The vast majority of cutting-edge research on high-performance graphene thermal conductive films cannot be mass-produced; current production equipment and processes are incompatible with many cutting-edge research processes and technologies. With the urgent market demand for high thermal conductivity materials, there is a pressing need for a lightweight thermal management material with high thermal conductivity and its preparation technology. Moreover, the process technology must be compatible with existing production equipment and process conditions, and must be capable of large-scale production.

[0003] Carbon nanotubes, like graphene, possess the exact same six-membered carbon ring structure. The theoretical thermal conductivity of single-walled carbon nanotubes at room temperature is as high as 6600 W / mK, even higher than that of graphene. This is mainly attributed to their perfectly ordered long-range covalent junction structure (i.e., SP). 2 The hybrid six-membered ring structure), one-dimensional quantum confinement effect (i.e., the one-dimensional nanostructure of carbon nanotubes endows phonons with unidirectional one-dimensional transmission characteristics), and huge mean free path of phonon transmission. The first point is an intrinsic feature shared by carbon nanotubes and graphene, while the second and third points are unique to the one-dimensional structure of carbon nanotubes.

[0004] Currently, graphene films prepared by conventional large-scale processes all undergo a homogeneous exfoliation and dispersion process of graphene oxide, and then rely on the oxygen-containing groups carried by the graphene oxide fragments to self-assemble into films. This inevitably introduces a large number of boundary defects, causing phonons to be continuously captured and scattered by "phonon traps" during transmission. The mean free path of phonon transmission becomes shorter. This is the fundamental reason why the development of high thermal conductivity graphene thermal conductive films has entered a stagnation phase after the thermal conductivity of the mass-produced graphene thermal conductive films has exceeded the 1500W / mk threshold. The reason is the result of macroscopic competition between phonon scattering caused by defects and the release of the intrinsic thermal conductivity of graphene.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] One of the objectives of this invention is to provide a method for preparing a carbon nanotube / graphene composite thermal conductive film, which can improve the thermal conductivity and bending resistance of the graphene thermal conductive film, thereby meeting the urgent market demand for high thermal conductivity graphene thermal conductive films.

[0007] The second objective of this invention is to provide a carbon nanotube / graphene composite thermally conductive film with high thermal conductivity and a thermal diffusivity of 900 mmHg. 2 / s-1000mm 2 / s.

[0008] To achieve the above-mentioned objectives of this invention, the following technical solution is adopted: Firstly, a method for preparing a carbon nanotube / graphene composite thermal conductive film, comprising the following steps: (a) milling a carbon nanotube suspension to obtain a carbon nanotube exfoliated dispersion slurry; (b) diluting the carbon nanotube exfoliated dispersion slurry and mixing it with flake graphite, then spray-drying to obtain a carbon nanotube / flake graphite mixed powder; (c) subjecting the carbon nanotube / flake graphite mixed powder to an intercalation oxidation reaction to obtain an oxidized... (d) The carbon nanotube / graphene oxide composite material is prepared into a slurry and then subjected to homogenization and exfoliation treatment to obtain a homogenized slurry. The homogenized slurry is then subjected to film formation treatment, graphitization treatment and calendering treatment in sequence to obtain a calendered carbon nanotube / graphene composite graphitized film; (e) The calendered carbon nanotube / graphene composite graphitized film and a graphene carbonization film are stacked together and graphitized to obtain the carbon nanotube / graphene composite thermal conductive film.

[0009] Furthermore, the carbon nanotubes include carbon nanotubes after acid washing and impurity removal; preferably, the acid solution used for acid washing and impurity removal includes hydrochloric acid.

[0010] Furthermore, the solid content of the carbon nanotube suspension is 0.5%-4%; preferably, the sand milling speed is 1000r / min-2500r / min.

[0011] Furthermore, the carbon nanotube exfoliated dispersion slurry is diluted to a solid content ≤1%; preferably, the mass ratio of the flake graphite to the carbon nanotubes is 10:0.1-5; preferably, the spray drying temperature is 150℃-200℃.

[0012] Furthermore, the intercalation oxidation includes first intercalating with concentrated sulfuric acid, and then oxidizing with potassium permanganate.

[0013] Furthermore, the carbon nanotube / graphene oxide composite material is prepared into a slurry by mixing the carbon nanotube / graphene oxide composite material, water, and ammonia water to prepare the slurry; preferably, the solid content of the prepared slurry is 4%-6%, and the pH is 6.2-6.9.

[0014] Furthermore, the homogenization stripping process includes first homogenizing at a pressure of 1000 bar to 1300 bar, and then homogenizing at a pressure of 600 bar to 1000 bar.

[0015] Furthermore, the film-forming process sequentially includes the steps of slurry degassing, coating, drying, pretreatment, and carbonization; preferably, the calendering density of the calendering process is ≥1.5 g / cm³. 3 .

[0016] Furthermore, the lamination method includes intermittent lamination in an ABAB lamination pattern.

[0017] Secondly, a carbon nanotube / graphene composite thermal conductive film is prepared by any one of the preparation methods described above; the thermal diffusivity of the carbon nanotube / graphene composite thermal conductive film is 900 mmHg. 2 / s-1000mm 2 / s.

[0018] Compared with the prior art, the present invention has at least the following beneficial effects: The preparation method of the carbon nanotube / graphene composite thermal conductive film provided by the present invention introduces carbon nanotubes into the graphene thermal conductive film. Utilizing the unique one-dimensional quantum confinement effect and the huge mean free path of phonon transport of carbon nanotubes, through covalent bridging, phonons can cross a large number of boundary defect traps on the graphene surface during transport. Phonon transport can be achieved through cross-regional transport via the fast thermal conductive channels constructed by carbon nanotubes. This achieves the technical effect of rapidly improving the thermal conductivity and bending resistance of the graphene thermal conductive film while being compatible with existing production equipment and processes, thus meeting the urgent market demand for high thermal conductivity graphene thermal conductive films.

[0019] The carbon nanotube / graphene composite thermally conductive film provided by this invention has high thermal conductivity and a thermal diffusivity of 900 mmHg. 2 / s-1000mm 2 / s. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0021] Figure 1 shows the thermal conductivity test results of the thermal conductive film provided in Embodiment 1 of the present invention; Figure 2 shows the thermal conductivity test results of the thermal conductive film provided in Embodiment 2 of the present invention; Figure 3 shows the thermal conductivity test results of the thermal conductive film provided in Embodiment 3 of the present invention; Figure 4 shows the thermal conductivity test results of the thermal conductive film provided in Embodiment 4 of the present invention; Figure 5 shows the thermal conductivity test results of the thermal conductive film provided in Embodiment 5 of the present invention; Figure 6 shows the thermal conductivity test results of the thermal conductive film provided in Comparative Example 1; Figure 7 shows the thermal conductivity test results of the thermal conductive film provided in Comparative Example 2; Figure 8 shows an internal SEM image of the film material provided in Comparative Example 2; Figure 9 shows the thermal conductivity test results of the thermal conductive film provided in Comparative Example 3; Figure 10 shows an internal SEM image of the film material provided in Comparative Example 3. Detailed Implementation

[0022] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] According to a first aspect of the present invention, a method for preparing a carbon nanotube / graphene composite thermal conductive film is provided, comprising the following steps: (a) milling a carbon nanotube suspension to obtain a carbon nanotube exfoliated dispersion slurry; (b) diluting the carbon nanotube exfoliated dispersion slurry and mixing it with flake graphite, then spray drying it to obtain a carbon nanotube / flake graphite mixed powder; (c) subjecting the carbon nanotube / flake graphite mixed powder to an intercalation oxidation reaction to obtain an oxidized carbon nanotube / oxidized graphene composite material; (d) preparing the oxidized carbon nanotube / oxidized graphene composite material into a slurry and then subjecting it to a homogenized exfoliation treatment to obtain a homogenized slurry, and then subjecting the homogenized slurry to a film-forming treatment, a graphitization treatment, and a calendering treatment to obtain a calendered carbon nanotube / graphene composite graphitized film; (e) stacking the calendered carbon nanotube / graphene composite graphitized film with a graphene carbonization film, and then subjecting it to a graphitization treatment to obtain a carbon nanotube / graphene composite thermal conductive film.

[0024] The present invention introduces carbon nanotubes into a graphene thermal conductive film. Utilizing the unique one-dimensional quantum confinement effect and the large mean free path of phonon transport of carbon nanotubes, phonon transport crosses numerous boundary defect traps on the graphene surface through covalent bridging. Phonon transport can be achieved across regions through the rapid thermal conductive channels constructed by carbon nanotubes. This achieves the technical effect of rapidly improving the thermal conductivity and bending resistance of the graphene thermal conductive film while being compatible with existing production equipment and processes, thus meeting the urgent market demand for high thermal conductivity graphene thermal conductive films.

[0025] In a preferred embodiment, the carbon nanotubes may be selected from carbon nanotubes that have been acid-washed and purified.

[0026] In this invention, the acid solution used for pickling and impurity removal includes, but is not limited to, hydrochloric acid.

[0027] Acid washing to remove impurities from carbon nanotubes is necessary, primarily due to their growth mechanism. The catalyst-catalyzed growth mechanism of carbon nanotubes inevitably results in the presence of numerous metal and metal oxide catalyst impurities in the virgin carbon nanotube powder. In acidic graphene oxide slurry, carbon nanotubes ionize, releasing a large number of positively charged metal ions. These metal ions preferentially bind to the negatively charged carboxyl groups hydrolyzed on the graphene oxide sheets, deactivating the carboxyl groups and reducing the number of effective carboxyl groups. This leads to poorer self-assembly of the graphene oxide sheets, introducing numerous boundary defects and hindering the improvement of the thermal conductivity of the graphene thermal conductive film. Acid washing also increases the hydrophilicity of the carbon nanotube powder, facilitating the grafting of oxygen-containing groups during the subsequent mixing and oxidation stage with flake graphite. Carbon nanotubes carrying oxygen-containing groups can then participate in the self-assembly reaction of the graphene oxide sheets through covalent bonding, enabling the establishment of rapid thermal conduction channels for phonon transport.

[0028] Carbon nanotubes can be purchased from Qingdao Chaorui Nanomaterials Technology Co., Ltd., specifically the CR8253 model. This model initially has a high impurity content, with Fe content >3000ppm and Co content >1000ppm. The main impurities are Fe-based and Co-based metal catalyst particles. The CR8253 carbon nanotube powder is immersed in a 20% hydrochloric acid solution, ensuring complete immersion. The solution is slowly stirred for ≥2 hours, and solid-liquid separation is achieved through filtration, completing the first hydrochloric acid washing treatment. After two hydrochloric acid washing treatments, the powder is washed with a large amount of deionized water until the pH value is neutral, completing the acid washing and impurity removal process to obtain CR8253 carbon nanotubes with low impurity content.

[0029] CR8253 carbon nanotubes with low impurity content can be prepared into carbon nanotube suspensions using deionized water.

[0030] In a preferred embodiment, the solid content of the carbon nanotube suspension can be 0.5%-4%, with typical but non-limiting solid contents such as 0.5%, 1%, 2%, 3%, and 4%.

[0031] The carbon nanotube suspension can be sand-milled at least twice to complete the exfoliation and dispersion of carbon nanotubes, and obtain a carbon nanotube exfoliated dispersion slurry.

[0032] The sand milling process for carbon nanotubes primarily involves separating them from their clustered and physically entangled state, resulting in a high quantity, less entanglement, less agglomeration, and highly dispersed carbon nanotubes. This helps improve the uniform dispersion of carbon nanotubes in graphene thermal conductive films and reduces agglomeration and entanglement. The high quantity and uniform dispersion of carbon nanotubes can maximize the construction of numerous cross-sectional phonon transport thermal conductive channels in the graphene thermal conductive film, reducing phonon scattering encountered by interfacial defects during phonon transport in the graphene plane. Along with the reduction of carbon nanotube agglomeration and entanglement in the graphene thermal conductive film, the covalent bonding between carbon nanotubes through grafted oxygen-containing groups is also reduced, greatly increasing the effective mean free path of phonon transport using carbon nanotubes as fast thermal conductive channels, and maximizing the release of the intrinsic thermal conductivity of carbon nanotubes.

[0033] In a preferred embodiment, the sand milling speed can be 1000 r / min to 2500 r / min, with typical but non-limiting speeds such as 1000 r / min, 1500 r / min, 2000 r / min, and 2500 r / min, which is more conducive to improving the exfoliation and dispersion effect of carbon nanotubes.

[0034] In a preferred embodiment, the carbon nanotube exfoliated dispersion slurry is diluted to a solid content of ≤1% to obtain a low-solids carbon nanotube dispersion.

[0035] Flake graphite powder was added to a low-solids-content carbon nanotube dispersion and stirred until uniformly dispersed to obtain a carbon nanotube / flake graphite dispersion. While stirring, a peristaltic pump was used to pump the carbon nanotube / flake graphite dispersion into a spray dryer, where it was spray-dried at 150℃-200℃ to achieve solid-liquid separation, obtaining a carbon nanotube / flake graphite mixed powder. The spray-dried carbon nanotube / flake graphite mixed powder was then transferred to an oven and baked at 200℃-250℃ for 5h-24h to obtain a dry carbon nanotube / flake graphite powder.

[0036] In a preferred embodiment, the intercalation oxidation includes, but is not limited to, first intercalation with concentrated sulfuric acid and then oxidation with potassium permanganate.

[0037] Conventional auxiliary material compounding often results in the physical deposition of auxiliary materials within the film, hindering the self-assembly of graphene oxide sheets. However, the carbon nanotube hybrid oxidation scheme of this invention not only solves the problems of physical deposition of auxiliary materials between graphene sheets and hindering the self-assembly of graphene oxide sheets, but also enables carbon nanotubes carrying oxygen-containing groups to assist in the covalent bonding of graphene oxide, thereby enhancing the self-assembly effect of the graphene film.

[0038] Carbon nanotube / flake graphite dry powder was used as a precursor for intercalation oxidation reaction. First, concentrated sulfuric acid with a concentration of 95% or higher was used for intercalation, followed by oxidation with potassium permanganate. After washing and filtration, carbon nanotube / graphene oxide composite cake was obtained.

[0039] It should be noted that carbon nanotubes and flake graphite can be oxidized together. Like graphite, carbon nanotubes can be grafted with oxygen-containing groups through the oxidation process. Both can use the same oxidation process and equipment, which is highly feasible and fully compatible with existing production equipment and processes.

[0040] In a preferred embodiment, carbon nanotube / graphene oxide composite material, deionized water, and ammonia are prepared into a composite slurry with a solid content of 4%-6%. The ammonia serves two purposes: first, to adjust the pH of the slurry to a near-neutral range of 6.2-6.9; and second, to allow the hydrolyzed ammonium ions to combine with the residual sulfate ions in the slurry to form ammonium sulfate salts, which are then volatilized and removed during the subsequent high-temperature treatment stage.

[0041] In a preferred embodiment, the homogenization stripping process includes first homogenizing at a pressure of 1000-1300 bar, and then homogenizing at a pressure of 600-1000 bar.

[0042] The first high-pressure homogenization is to exfoliate the graphene oxide sheets. The optimal exfoliation state is when the graphene oxide sheets are exfoliated into a single layer or a few layers, while the carbon oxide nanotubes still maintain their structural integrity. The second high-pressure homogenization is to reduce the viscosity of the composite slurry.

[0043] Homogenization was performed at 1000-1300 bar (first high-pressure homogenization). The viscosity of the homogenized slurry was significantly higher than that before homogenization. This is mainly attributed to the large number of graphene oxide sheets being peeled off, increasing the basic number of graphene oxide sheets. Most of the graphene oxide sheets carry oxygen-containing groups, and the high viscosity is a result of the mutual exclusion of oxygen-containing groups in the high-quantity graphene oxide sheets and the short free path of the graphene oxide sheets. Homogenization was performed at 600-1000 bar (second high-pressure homogenization). The homogenization pressure was relatively low. This was mainly achieved by reducing the homogenization pressure and expanding the gap in the homogenization working chamber. The cavity effect was used to regulate the viscosity of the slurry, thereby reducing the viscosity of the homogenized slurry and improving the liquid crystal arrangement effect of graphene oxide in the slurry.

[0044] In a preferred embodiment, the film-forming process sequentially includes the steps of slurry degassing, coating, drying, pretreatment, and carbonization.

[0045] After homogenization, the slurry is subjected to a series of film-forming steps, including slurry degassing, coating, drying, pretreatment, and carbonization, to obtain a carbon nanotube / graphene composite carbonized film.

[0046] The carbon nanotube / graphene composite carbonized film was subjected to a first graphitization treatment at 2800℃ for 0.5h-3h to obtain a first-graphitized carbon nanotube / graphene composite graphitized film. The first-graphitized carbon nanotube / graphene composite graphitized film was then calendered using a plate press to a density ≥1.5g / cm³. 3 .

[0047] The first graphitization process is carried out at a relatively low temperature. At the same time, a calendered film is used to lock the carbon in the carbon nanotubes to prevent them from burning out at high temperatures and losing their thermal conductivity.

[0048] The calendered carbon nanotube / graphene composite graphitized film is stacked alternately with a conventional graphene carbonized film in an ABAB stacking manner and then transferred to a graphitization furnace again. The furnace is sealed with graphite paper or graphite discs of the same diameter as the crucible in the graphitization furnace and subjected to a second graphitization treatment at 3150℃ for 2-4 hours to obtain the target carbon nanotube / graphene composite thermally conductive film.

[0049] Since the graphene carbonized film still retains a small number of oxygen-containing functional groups, at the temperature of the second graphitization treatment, the remaining small number of oxygen-containing functional groups will fall off, and at the same time, active carbon-containing free radicals will be released. The carbon-containing free radicals can be partially deposited at the defect points of the target film material through recombination reaction during the second graphitization process to form new CC bonds, which will help the film material after the first graphitization to be further repaired and improve the thermal conductivity.

[0050] According to a second aspect of the present invention, a carbon nanotube / graphene composite thermal conductive film is provided, which is prepared by any of the preparation methods described above.

[0051] The carbon nanotube / graphene composite thermally conductive film of this invention has high thermal conductivity and a thermal diffusivity of 900 mmHg. 2 / s-1000mm 2 / s.

[0052] The present invention will be further illustrated by the following examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or purchased directly from the market.

[0053] Example 1: A method for preparing a carbon nanotube / graphene composite thermal conductive film, comprising the following steps: S1 (acid washing and impurity removal of carbon nanotubes): CR8253 carbon nanotubes from Qingdao Chaorui Nanomaterials Technology Co., Ltd. are purchased. The metal content of this type of carbon nanotube powder is 3730.58 ppm, with iron and cobalt being the main impurity metal elements. The CR8253 carbon nanotube powder is immersed in a 20% hydrochloric acid solution, ensuring complete immersion (solid content 4.5%). The solution is slowly stirred for 5 hours, and solid-liquid separation is achieved using vacuum filtration, thus completing one hydrochloric acid washing treatment. After two hydrochloric acid washing treatments, a large amount of deionized water is used... Washing until the pH value is neutral completes the acid washing and impurity removal process, yielding CR8253 carbon nanotubes with low impurity content; S2 (carbon nanotube exfoliation treatment): The CR8253 carbon nanotubes purified by acid washing in step S1 are dispersed in deionized water to prepare a suspension with a solid content of 2%. This suspension is then milled twice using a sand mill at a speed of 2000 r / min to complete the exfoliation and dispersion of the carbon nanotubes, obtaining a carbon nanotube exfoliated dispersion slurry; S3 (spray drying treatment): The solid content of the carbon nanotube slurry obtained in step S2 is diluted to 0.8% using deionized water to obtain a low-solids carbon nanotube dispersion; At a mass ratio of flake graphite powder to carbon nanotubes of 10:1, the... 500-mesh flake graphite powder was added to a low-solids carbon nanotube dispersion and stirred at 800 rpm for 30 minutes using a stirring paddle to thoroughly disperse the carbon nanotubes and flake graphite, obtaining a carbon nanotube / flake graphite dispersion. While stirring at 500 rpm, a peristaltic pump was used to pump the carbon nanotube / flake graphite dispersion into a spray dryer, where it was spray-dried at 180°C to achieve solid-liquid separation, obtaining a uniformly mixed carbon nanotube / flake graphite powder. S4 (Intercalation Oxidation Reaction): Using the carbon nanotube / flake graphite powder obtained in step S3 as a precursor, the Hummers process was performed according to conventional oxygenation... The graphene preparation process involves an intercalation oxidation reaction. First, concentrated sulfuric acid with a concentration of 95% or higher is used for intercalation, followed by oxidation with potassium permanganate (flake graphite becomes graphene oxide, and carbon nanotubes are oxidized by grafting oxygen-containing groups). After washing twice with 5% dilute sulfuric acid and pressure filtration, a carbon nanotube / graphene oxide composite cake is obtained. S5 (slurry preparation): The carbon nanotube / graphene oxide composite cake, deionized water, and ammonia are mixed to prepare a composite slurry with a solid content of 5%. The ammonia adjusts the pH of the slurry to 6.8, and the hydrolyzed ammonium ions combine with the residual sulfate ions in the slurry to form ammonium sulfate, which is volatilized and removed in the subsequent high-temperature treatment stage.

[0054] S6 (Homogenization and Stripping): The composite slurry prepared in step S5 is homogenized twice using a high-pressure homogenizer. The pressure for the first homogenization is 1100 bar, and the pressure for the second homogenization is 800 bar. Water cooling is used to cool the slurry throughout the homogenization process. The water cooling heat exchanger is connected to the homogenizer outlet to ensure the homogenized slurry temperature is maintained at 19°C. S7 (Film Formation): The homogenized slurry obtained in step S6 is degassed. The degassed slurry is then coated onto a breathable PET substrate using a flatbed coater to a thickness of 5000 μm. The coated film is transferred to a forced-air drying oven at 45°C to remove moisture, obtaining a carbon oxide nanotube / graphene oxide composite dry film. The obtained carbon oxide nanotube / graphene oxide composite dry film is then laminated using graphite paper as a separator. The film was then transferred to a forced-air drying oven and heated to 240℃ at a rate of 0.5℃ / min under a 50kg load, and held at 240℃ for 1 hour to complete the pretreatment of the composite dry film, obtaining a pretreated film. The pretreated film was then transferred onto a graphite fixture, placed in a carbonization furnace, and heated to 1300℃ at a rate of 2℃ / min, and held at 1300℃ for 2 hours to complete the carbonization process, obtaining a carbonized film. The carbonized film, along with the graphite fixture, was transferred to a graphitization furnace and sealed with five circular graphite papers of the same size as the inner diameter of the graphite crucible. Under a high-purity argon atmosphere, the temperature was raised to 2800℃ at a rate of 2℃ / min and held at 2800℃ for 1 hour to complete the first graphitization process. Finally, the graphitized film was calendered using a plate press to a density of 1.9 g / cm³. 3 The calendered graphitized film and a conventional 50-micron pure graphene carbonized film were stacked alternately on a graphite fixture using an ABAB stacking method. The stacked film material, along with the graphite fixture, was transferred to a graphitization furnace. Five circular graphite papers of the same size as the inner diameter of the graphite crucible were used for sealing. Under the protection of a high-purity argon atmosphere, the temperature was raised to 3150℃ at a heating rate of 2℃ / min and held at 3150℃ for 3 hours to complete the second graphitization treatment, thereby obtaining the target carbon nanotube / graphene composite thermally conductive film.

[0055] After calendering the obtained target carbon nanotube / graphene composite thermally conductive film using a flatbed die-cutting machine, the sample film was tested using an LFA467 tester. The test results showed that its thermal diffusivity was 958.203 mm. 2 / s, see Figure 1.

[0056] Example 2 The only difference between this example and Example 1 is that step S1 was not performed, that is, no impurity removal treatment was performed on the CR8253 carbon nanotubes; the remaining steps are the same as in Example 1, and the target carbon nanotube / graphene composite thermal conductive film is obtained.

[0057] Compared to Example 1, the test results of the thermally conductive film in this example show that its thermal diffusivity is 840.433 mm. 2 / s, see Figure 2, a decrease of 12.29% compared to Example 1. This is mainly attributed to the fact that the CR8253 carbon nanotubes were not purified. In the acidic liquid phase, the metal impurities of the carbon nanotubes ionized into metal cations. The metal cations preferentially combined with the hydrolyzed carboxyl groups carried by the graphene oxide, which reduced the effective number of carboxyl groups carried by the graphene oxide. This was not conducive to the self-assembly reaction, and there were many boundary defects, which led to the decrease in the thermal conductivity of the carbon nanotube / graphene composite thermal conductive film.

[0058] Example 3 The only difference between this example and Example 1 is that in step S1, CR5111 carbon nanotubes are used instead of CR8253 carbon nanotubes; the remaining steps are the same as in Example 1, and the target carbon nanotube / graphene composite thermal conductive film is obtained.

[0059] Compared to Example 1, the test results of the thermally conductive film in this example show that its thermal diffusivity is 987.838 mm. 2 / s, see Figure 3, which is 3.09% higher than that of Example 1. This is mainly attributed to the fact that the CR5111 carbon nanotube is longer than the CR8253 carbon nanotube. The longer the average length of the carbon nanotube, the more graphene sheet interfaces it crosses, that is, the more interface defects it crosses. The effect of phonon traversal is more obvious, and the scattering probability of phonons during transmission is lower. Therefore, the thermal conductivity of the film material is better.

[0060] Example 4 The only difference between this example and Example 1 is that in step S6, the composite slurry is homogenized five times using a high-pressure homogenizer; the pressure of the first homogenization is 1100 bar, and the pressure of the second, third, fourth and fifth homogenizations is 800 bar; the remaining steps are the same as in Example 1, and the target carbon nanotube / graphene composite thermal conductive film is obtained.

[0061] Compared to Example 1, the test results of the thermally conductive film in this example show that its thermal diffusivity is 928.279 mm. 2 / s, see Figure 4, a decrease of 3.12% compared to Example 1. This is mainly attributed to the fact that multiple homogenization processes caused some carbon nanotubes in the slurry to break down. The broken carbon nanotube interfaces do not carry oxygen-containing groups and do not participate in the self-assembly reaction of the subsequent coating film. Even the two ends of the broken carbon nanotubes in the middle section do not carry oxygen-containing groups. They exist in the form of physical deposition between graphene sheets, do not undergo covalent bonding, do not participate in phonon transport, and the broken carbon nanotubes greatly shorten the average phonon transport free path, resulting in a decrease in thermal conductivity.

[0062] Example 5: The only difference between this example and Example 3 is that in step S6, the composite slurry is homogenized five times using a high-pressure homogenizer; the pressure of the first homogenization is 1100 bar, and the pressure of the second, third, fourth and fifth homogenizations is 800 bar; the remaining steps are the same as in Example 3, and the target carbon nanotube / graphene composite thermal conductive film is obtained.

[0063] Compared to Example 3, the test results of the thermally conductive film in this example show that its thermal diffusivity is 902.591 mm. 2 / s, see Figure 5, which is 8.63% lower than that of Example 3. The longer the carbon nanotube, the more fragments it breaks into during the homogenization stage, and the greater the decrease in the thermal conductivity of the film.

[0064] Comparative Example 1 differs from Example 1 only in that steps S1, S2, and S3 were not performed; that is, 500-mesh flake graphite was directly used for intercalation oxidation reaction, and carbon nanotubes were not introduced into the graphene thermal conductive film. The remaining steps are the same as in Example 1, and a pure graphene thermal conductive film is obtained.

[0065] Compared to Example 1, the test results of this comparative thermal conductive film show that its thermal diffusivity is 805.536 mm. 2 / s, see Figure 6, which is 16.25% lower than that of Example 1.

[0066] It can be seen that the thermal conductivity of the carbon nanotube / graphene composite thermal conductive film with added carbon nanotubes is significantly higher than that of the pure graphene thermal conductive film. This is mainly attributed to the combined increase of the ultra-high theoretical thermal conductivity of carbon nanotubes, the one-dimensional confinement effect, and the huge mean free path of phonon transmission.

[0067] Comparative Example 2 differs from Example 1 only in that step S2 was not performed, i.e., the carbon nanotubes were not stripped and dispersed; the remaining steps were the same as in Example 1, resulting in a carbon nanotube / graphene composite thermal conductive film.

[0068] Compared to Example 1, the test results of this comparative thermal conductive film show that its thermal diffusivity is 800.907 mm. 2 / s, see Figure 7, which is 16.42% lower than that of Example 1. As can be seen from the internal SEM image of the film material (Figure 8), carbon nanotubes exist in the graphene layers in a tangled and aggregated manner. It is difficult to build a large number of fast heat conduction channels with carbon nanotubes as bridges for cross-phonon transmission. The one-dimensional confinement effect of carbon nanotubes is difficult to return, and the thermal conductivity of the graphene thermal conductive film cannot be significantly improved.

[0069] The only difference between Comparative Example 3 and Example 1 is that no calendering process was performed in step S7; the remaining steps are the same as in Example 1, resulting in a carbon nanotube / graphene composite thermal conductive film.

[0070] Compared to Example 1, the test results of this comparative example thermal conductive film show that its thermal diffusivity is 888.928 mm. 2 / s, see Figure 9, the attenuation was 7.23% compared to Example 1. This is mainly attributed to the lack of calendering treatment, which resulted in carbon nanotubes being ablated, broken, and unwound at a high temperature of 3150℃. The internal SEM image of the film is shown in Figure 10. The burned carbon nanotubes lost their thermal bridging function. The carbon-containing free radicals that sublimated at high temperature from the carbon nanotubes escaped along the micro-nano pore channels in the film because they were not calendered and locked in carbon. They did not diffuse between graphene sheets and could not be captured by the active carbon atoms in the graphene defect region for defect repair.

[0071] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a carbon nanotube / graphene composite thermally conductive film, characterized in that, Includes the following steps: (a) The carbon nanotube suspension is milled to obtain a carbon nanotube exfoliated dispersion slurry; (b) The carbon nanotube exfoliated dispersion slurry is diluted and mixed with flake graphite, then spray-dried to obtain a carbon nanotube / flake graphite mixed powder; (c) The carbon nanotube / flake graphite mixed powder is subjected to an intercalation oxidation reaction to obtain an oxidized carbon nanotube / graphene oxide composite material; (d) The oxidized carbon nanotube / graphene oxide composite material is prepared into a slurry and then subjected to homogenization exfoliation treatment to obtain a homogenized slurry. The homogenized slurry is then subjected to film formation treatment, graphitization treatment, and calendering treatment in sequence to obtain a calendered carbon nanotube / graphene composite graphitized film; (e) The calendered carbon nanotube / graphene composite graphitized film is stacked with a graphene carbonization film and graphitized to obtain the carbon nanotube / graphene composite thermal conductive film.

2. The preparation method according to claim 1, characterized in that, The carbon nanotubes include carbon nanotubes after acid washing and impurity removal; preferably, the acid solution used for acid washing and impurity removal includes hydrochloric acid.

3. The preparation method according to claim 1, characterized in that, The solid content of the carbon nanotube suspension is 0.5%-4%; preferably, the sand milling speed is 1000r / min-2500r / min.

4. The preparation method according to claim 1, characterized in that, The carbon nanotube exfoliated dispersion slurry is diluted to a solid content ≤1%; preferably, the mass ratio of the flake graphite to the carbon nanotubes is 10:0.1-5; preferably, the spray drying temperature is 150℃-200℃.

5. The preparation method according to claim 1, characterized in that, The intercalation oxidation involves first intercalating with concentrated sulfuric acid, and then oxidizing with potassium permanganate.

6. The preparation method according to any one of claims 1-5, characterized in that, The preparation of the carbon nanotube / graphene oxide composite material into a slurry involves mixing the carbon nanotube / graphene oxide composite material, water, and ammonia to form a slurry; preferably, the solid content of the prepared slurry is 4%-6%, and the pH is 6.2-6.

9.

7. The preparation method according to claim 6, characterized in that, The homogenization stripping process includes first homogenizing at a pressure of 1000-1300 bar, and then homogenizing at a pressure of 600-1000 bar.

8. The preparation method according to claim 6, characterized in that, The film-forming process sequentially includes the steps of slurry degassing, coating, drying, pretreatment, and carbonization; preferably, the calendering density of the calendering process is ≥1.5 g / cm³. 3 .

9. The preparation method according to claim 6, characterized in that, The lamination method includes intermittent lamination using an ABAB lamination pattern.

10. A carbon nanotube / graphene composite thermal conductive film, characterized in that, The carbon nanotube / graphene composite thermally conductive film is prepared by the preparation method according to any one of claims 1-9; the thermal diffusivity of the film is 900 mmHg. 2 / s-1000mm 2 / s.

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