A functional group functionalized carbon nanotube composite heat conducting framework and a preparation method and application thereof

Abstract

This invention discloses a functionalized carbon nanotube composite thermally conductive framework, its preparation method, and its application, relating to the field of phase change thermal energy storage engineering applications. The preparation method involves: immersing carbon felt in a carbon nanotube dispersion, removing and drying it, and then subjecting it to high-temperature heat treatment under inert gas protection to form interfacial cross-linking between the carbon nanotubes and the carbon felt fiber surface; in-situ grafting of fusible salt functional groups onto the surfaces of the carbon felt and carbon nanotubes to obtain a thermally conductive framework with a multi-level micro / nano structure; and filling the pores of the thermally conductive framework with a phase change thermal energy storage molten salt using a vacuum impregnation method to obtain a composite phase change thermal energy storage material. This invention effectively solves the problems of high microscopic contact thermal resistance and poor carbon / salt interface compatibility in traditional carbon-based frameworks through the synergistic effect of physically constructing a strongly cross-linked thermally conductive network and chemically grafting fusible salt functional groups in situ. The resulting composite phase change thermal energy storage material possesses high thermal conductivity, high molten salt loading rate, and excellent long-term thermal cycling stability.

Description

Technical Field

[0001] This invention relates to the field of phase change thermal energy storage engineering applications, specifically to a functionalized carbon nanotube composite thermal conductive framework, its preparation method, and its applications. More specifically, it relates to a method for preparing a carbon felt thermal conductive framework by in-situ growth of carbon nanotubes to enhance thermal conductivity and the introduction of molten salt-loving functional groups to improve interfacial compatibility, as well as a composite phase change thermal energy storage material based on this framework. Background Technology

[0002] Phase change thermal energy storage technology utilizes the latent heat of phase change materials for energy storage, which is a key means to improve energy utilization efficiency. In the field of medium and high temperature thermal energy storage, inorganic molten salts, represented by binary and ternary nitrates, are widely used due to their large latent heat and low cost. However, their extremely low thermal conductivity severely restricts the heat charging and discharging rate of the thermal energy storage system.

[0003] To address thermal conductivity bottlenecks, porous carbon materials such as carbon felt are often introduced as thermally conductive reinforcing frameworks. However, existing technologies suffer from two significant drawbacks in practical applications: First, the inter-fiber contact thermal resistance of ordinary carbon felt is relatively high. Although attempts have been made to physically dopant carbon nanotubes to construct microscopic thermally conductive channels, the physical bonding force is weak, and the carbon nanotubes are prone to agglomeration or detachment, making it difficult to maintain a long-term stable heat transfer network. Second, and more critically, is the interfacial compatibility issue. The difference between the nonpolarity of the carbon felt surface and the ionic polarity of the inorganic molten salt leads to severe interfacial non-wetting. Experiments show that pure carbon felt or frameworks with carbon nanotubes loaded only physically are almost impossible to wet with molten salt under normal pressure, resulting in extremely low molten salt loading in the composite material. Furthermore, a large number of air gaps exist at the interface, generating significant interfacial thermal resistance, which severely affects the heat storage density and heat transfer efficiency of the composite material.

[0004] Therefore, there is an urgent need to develop a modification technique that can simultaneously achieve the robust construction of a microscopic thermally conductive network and a significant improvement in interfacial compatibility, in order to prepare composite phase change thermal storage materials with both high thermal conductivity and excellent thermal storage density. Summary of the Invention

[0005] This invention constructs a carbon nanotube network with interfacial crosslinking on the surface of carbon felt fiber in situ through high-temperature solid-state reaction, and introduces molten salt functional groups to prepare a composite phase change thermal storage material with high thermal conductivity, high loading rate and cycle stability with multi-level micro-nano structure. The aim is to provide a method for preparing a thermally conductive framework of functionalized carbon nanotube composite and a composite phase change thermal storage material based on this framework.

[0006] To achieve the above objectives, in a first aspect, the present invention provides a method for preparing a thermally conductive framework of functionalized carbon nanotube composites, comprising the following steps: S1. Carbon nanotubes are uniformly dispersed in a mixed solvent composed of deionized water and anhydrous ethanol to obtain a carbon nanotube dispersion. Preferably, the concentration of carbon nanotubes is controlled at 0.2-0.7 mg / mL. S2. The carbon felt is immersed in the carbon nanotube dispersion, taken out and dried, and then placed under inert gas protection for high-temperature heat treatment at a temperature of 700-900 ℃, so that the carbon nanotubes and the surface of the carbon felt fiber form an interfacial cross-linking effect to obtain a carbon felt precursor loaded with carbon nanotubes. S3. Dissolve aromatic amines with sulfonic acid, nitro, aldehyde or carboxyl groups substituted on the benzene ring in hydrochloric acid to form an aromatic amine-hydrochloric acid solution, and then add NaNO2 solution to form an aromatic amine solution. The molar ratio of aromatic amine to HCl is 1:(4.8-5.2), and the molar ratio of NaNO2 to HCl is 1:(0.8-1.2). S4. The carbon felt precursor is added to the aromatic amine solution at 0-5 °C for reaction, preferably for 0.5-1.5 h; then the temperature is raised to room temperature and the reaction continues, preferably for 1-2 h; then the reacted carbon felt is taken out, washed, and dried (preferably vacuum dried at 50-60 °C for 10-12 h) to obtain a thermally conductive framework of functionalized carbon nanotube composite.

[0007] As a further preferred embodiment of the present invention, the aromatic amine is at least one of p-aminobenzenesulfonic acid (sulfonic acid group), p-nitroaniline (nitro group), 4-aminobenzaldehyde (aldehyde group), and p-aminobenzoic acid (carboxyl group).

[0008] As a further preferred technical solution of the present invention, the carbon felt in step S2 includes a pretreatment step before impregnation, specifically: the carbon felt is placed in anhydrous ethanol for ultrasonic cleaning, then cleaned with deionized water, and dried to obtain the pretreated carbon felt.

[0009] As a further preferred embodiment of the present invention, the mixed solvent is composed of deionized water and anhydrous ethanol in a volume ratio of 1-2:1.

[0010] As a further preferred embodiment of the present invention, the heating rate of the high-temperature heat treatment is 5-10 ℃ / min, and the holding time is 1-2 hours; and / or, the inert gas is argon.

[0011] As a further preferred embodiment of the present invention, the concentration of aromatic amine in the aromatic amine solution is 0.2 mmol / mL, and 1 L of the aromatic amine solution can effectively modify 10-20 g of functionalized carbon nanotube composite thermally conductive framework.

[0012] According to a second aspect of the present invention, the present invention also provides a functionalized carbon nanotube composite thermally conductive framework, which is prepared by the preparation method of the first aspect described above. The thermally conductive framework has a multi-level micro / nano structure, including a micron-scale carbon fiber framework, a carbon nanotube network with strong interfacial cross-linking with the carbon fiber surface, and functional groups grafted onto the carbon fiber surface; the functional groups improve the surface wettability of the thermally conductive framework.

[0013] According to a third aspect of the present invention, the present invention also provides a composite phase change thermal storage material, comprising a thermally conductive framework of functionalized carbon nanotubes composite as described in the second aspect above, and a phase change thermal storage molten salt filling the pores of the thermally conductive framework of the functionalized carbon nanotubes composite. Preferably, the phase change thermal storage molten salt is filled by physical impregnation or vacuum adsorption.

[0014] As a further preferred embodiment of the present invention, the phase change thermal storage molten salt is an inorganic molten salt, comprising at least one of nitrates, chlorides, or carbonates. More preferably, the phase change thermal storage molten salt is a Hitec salt (53 wt% KNO3 + 40 wt% NaNO2 + 7 wt% NaNO3).

[0015] According to a fourth aspect of the present invention, the present invention also provides the application of the composite phase change thermal storage material of the third aspect above in solar thermal power generation thermal storage systems, industrial waste heat recovery systems or high-temperature thermal management systems.

[0016] Compared with the prior art, the present invention can achieve the following beneficial effects: 1. This invention utilizes high-temperature heat treatment to induce a strong interfacial cross-linking of carbon nanotubes on the surface of carbon felt fibers, constructing a multi-level micro / nano interpenetrating thermal network of "micron-scale carbon fibers-nano-scale carbon nanotubes," effectively reducing contact thermal resistance during phonon transport. Test results show that, compared with unmodified "carbon felt-molten salt composite material," the thermal conductivity of the composite phase change thermal storage material prepared by this invention is increased by 3 times, significantly improving the thermal response rate of the thermal storage system.

[0017] 2. This invention successfully grafts molten salt-loving functional groups onto the carbon skeleton surface of a molten salt, significantly reducing the wetting angle of the inorganic molten salt on the carbon surface and solving the problem of interfacial non-wetting. This allows the molten salt to spontaneously and fully fill the microporous structure of the carbon felt, achieving a molten salt loading rate of over 85 wt% in the composite material, effectively ensuring the heat storage density.

[0018] 3. Benefiting from the interfacial cross-linking between carbon nanotubes and the matrix, as well as the chemical adsorption of molten salt by functional groups, the composite material of this invention exhibits excellent structural stability during long-term thermal cycling. After 305 cycles of thermal cycling, the thermal conductivity decay rate is less than 10%, and the molten salt loading rate remains above 85% of the initial value, effectively solving the problems of carbon nanotube shedding and molten salt leakage that easily occur in traditional physical adsorption materials during thermal cycling. Attached Figure Description

[0019] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0020] Figure 1 The diagram shows the mass changes of the thermally conductive framework before grafting sulfonic acid groups, aldehyde groups, nitro groups, and carboxyl groups into the composite phase change materials in Examples 1, 2, 3, and 4 of this invention, as well as the mass changes of the composite phase change materials after grafting.

[0021] Figure 2 The contact angle tests are for Embodiments 1, 2, 3, and 4 of the present invention, as well as Comparative Examples 1 and 2.

[0022] Figure 3 These are scanning electron microscope (SEM) images of the surfaces of the composite phase change materials in Examples 1, 2, 3, and 4, and Comparative Examples 1 and 2 of the present invention.

[0023] Figure 4 This is a comparison diagram of the thermal conductivity of the composite phase change materials in Examples 1, 2, 3, and 4 of the present invention, as well as Comparative Examples 1 and 2.

[0024] Figure 5 This is a comparison chart of the mass retention rate of the composite phase change materials in Examples 1, 2, 3, and 4 of the present invention, as well as Comparative Examples 1 and 2, after 305 thermal cycles.

[0025] Figure 6 This is a comparison chart of the phase change enthalpy retention rates of the composite phase change materials in Examples 1, 2, 3, and 4 of the present invention, as well as Comparative Examples 1 and 2, after 305 thermal cycles.

[0026] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0027] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0028] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this invention pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0029] Example 1 This embodiment provides a method for preparing a thermally conductive framework and a composite phase change thermal storage material composed of sulfonic acid functionalized carbon nanotubes, specifically including the following steps: Step 1: Carbon felt pretreatment Take a commercially available porous carbon felt of suitable size, immerse it completely in anhydrous ethanol, and use an ultrasonic cleaner to ultrasonically wash it to remove surface oil and impurities; then take out the carbon felt, rinse it repeatedly with deionized water, and finally place the carbon felt in a forced-air drying oven to dry it, and obtain pretreated clean carbon felt.

[0030] Step 2, Preparation of carbon nanotube dispersion Measure 14 mL of deionized water and 7 mL of anhydrous ethanol, and mix them thoroughly to obtain a mixed solvent with a volume ratio of 2:1. Weigh 10 mg of multi-walled carbon nanotubes and add them to the mixed solvent. Sonicate the mixture for 10 minutes until there are no visible agglomerated particles in the dispersion, resulting in a carbon nanotube dispersion with a concentration of approximately 0.47 mg / mL.

[0031] Step 3: Prepare carbon felt precursor by loading carbon nanotubes. The dried carbon felt from step one was completely immersed in the carbon nanotube dispersion prepared in step two, and kept still until the carbon felt was completely wetted and reached adsorption saturation. The saturated carbon felt was removed and placed in a drying oven to dry until the solvent completely evaporated. The dried carbon felt was placed in a tube furnace and protected with argon gas. Under an argon atmosphere, it was heated to 800℃ at a heating rate of 5℃ / min and held at that temperature for 1.5 hours, allowing the carbon nanotubes and the carbon felt fiber surface to form an interfacial crosslinking reaction through thermal reaction. After the reaction, it was naturally cooled to room temperature to obtain a carbon nanotube-loaded carbon felt precursor, which was then cut into multiple cubes with a side length of 1 cm and a thickness of 5 mm. The mass of the multiple cubic carbon felt precursors was weighed and recorded, and the average value (81.2 mg) was taken.

[0032] Step 4: Prepare an aromatic amine solution grafted with sulfonic acid groups. 0.866 g (5 mmol) of p-aminobenzenesulfonic acid was dissolved in 25 mL of 1.0 mol / L hydrochloric acid at 3 °C, and stirred until completely dissolved to form a p-aminobenzenesulfonic acid solution. 1.725 g of NaNO₂ was dissolved in 25 mL of deionized water, and stirred until completely dissolved to form a 1.0 mol / L NaNO₂ aqueous solution, which was then cooled to 0 °C. The pre-cooled NaNO₂ aqueous solution was slowly added dropwise to the p-aminobenzenesulfonic acid solution at 3 °C, with continuous stirring for 20 minutes to obtain a sulfonic acid-grafted acidic aromatic amine solution.

[0033] Step 5: Introduce sulfonic acid groups using a low-temperature liquid phase method. Five cubes of carbon nanotube-loaded carbon felt precursors, with an average mass of 81.2 mg, prepared in step four, were added to a solution of p-sulfonic acid-grafted acidic aromatic amines at 3°C ​​and stirred for 1 hour. The temperature was then increased to 25°C at a rate of 2°C / min, and the reaction continued for 2 hours. The reacted carbon felts were removed and rinsed with deionized water and anhydrous ethanol until neutral. They were then vacuum-dried at 60°C for 10 hours and allowed to cool naturally to room temperature in air to obtain the thermally conductive framework of sulfonic acid-functionalized carbon nanotube composites.

[0034] Step 6: Loading phase change thermal storage molten salt Hitec ternary salt was prepared according to a mass fraction of 53% KNO3, 40% NaNO2, and 7% NaNO3. The cubic thermally conductive framework grafted with sulfonic acid groups from step five was placed in an 80°C vacuum oven for 1 hour to remove adsorbed water vapor. The mixed nitrate powder was heated to 180°C in the oven and held for 1 hour to fully melt and mix, forming a Hitec molten salt liquid. The cubic thermally conductive framework was then immersed in the Hitec molten salt liquid and thoroughly impregnated for 1 hour in a vacuum oven at 180°C with a relative vacuum controlled between -0.090 MPa and -0.098 MPa, ensuring that the Hitec molten salt liquid fully fills the internal pores of the carbon fiber felt. The sample was removed and placed on filter paper, allowed to cool naturally to room temperature, and then reheated to 180°C, with the filter paper being replaced continuously until no Hitec molten salt precipitates on the filter paper, thus removing excess Hitec molten salt from the sample surface, ultimately obtaining a multi-level micro / nano-structured composite phase change thermal storage material.

[0035] Example 2 This embodiment provides a method for preparing a thermally conductive framework and a composite phase change thermal storage material composed of aldehyde-functionalized carbon nanotubes. All specific steps are basically the same as in Example 1, except that in step four, 0.866 g (5 mmol) of p-aminobenzenesulfonic acid is replaced with 0.606 g (5 mmol) of 4-aminobenzaldehyde.

[0036] Example 3 This embodiment provides a method for preparing a thermally conductive framework and a composite phase change thermal storage material composed of nitro functionalized carbon nanotubes. All specific steps are basically the same as in Example 1, except that in step four, 0.866 g (5 mmol) of p-aminobenzenesulfonic acid is replaced with 0.691 g (5 mmol) of p-nitroaniline.

[0037] Example 4 This embodiment provides a method for preparing a thermally conductive framework and a composite phase change thermal storage material composed of carboxyl functional group-functionalized carbon nanotubes. All specific steps are basically the same as in Example 1, except that in step four, 0.866 g (5 mmol) of p-aminobenzenesulfonic acid is replaced with 0.686 g (5 mmol) of p-aminobenzoic acid.

[0038] Comparative Example 1 As a control experiment for Example 1, all specific steps were basically the same as in Example 1, except that in step four, 0.866 g (5 mmol) of p-aminobenzenesulfonic acid was replaced with 0.466 g (5 mmol) of p-aniline.

[0039] Comparative Example 2 A carbon felt loaded with only carbon nanotubes was composited with Hitec salt. The carbon felt precursor loaded with carbon nanotubes was prepared according to steps one to three in Example 1, cut into the same size, and without functional group grafting, was directly placed in molten Hitec salt at 180 degrees Celsius for vacuum impregnation. After cooling and removing excess surface molten salt, a composite phase change material containing only carbon nanotubes was obtained.

[0040] Performance testing and comparative analysis: The average mass of the cubic carbon felt precursor was 81.2 mg. The average mass of the composite phase change material sample prepared in Example 1 was 625.3 mg, with a molten salt loading rate of 87.01 wt%; the average mass of the composite phase change material sample prepared in Example 2 was 618.7 mg, with a molten salt loading rate of 86.88 wt%; the average mass of the composite phase change material sample prepared in Example 3 was 611.5 mg, with a molten salt loading rate of 86.72 wt%; and the average mass of the composite phase change material sample prepared in Example 4 was 602.2 mg, with a molten salt loading rate of 86.52 wt%. Figure 1 The molten salt loading rates of the composite phase change materials prepared in Examples 1-4 were all higher than 85 wt%.

[0041] The contact angle tests of Examples 1-4 and Comparative Examples 1-2 show that ( Figure 2 The grafting of molten salt functional groups solved the problem of non-wetting of Hitec salt on carbon fiber surfaces. SEM images show that ( Figure 3In Examples 1-4, the functional groups modified with strong adsorption and the carbon nanotube network were uniformly coated with Hitec molten salt, resulting in good interfacial contact; while the carbon fibers in Comparative Examples 1-2 were obviously exposed and had extremely poor macroscopic wettability.

[0042] The thermal conductivity of the sample was tested using the transient plane heat source method. Figure 4 The results showed that the thermal conductivity of the composite phase change thermal storage material using pure carbon felt to support carbon nanotubes (Comparative Example 2) was approximately 1.3 W / (m·K). The thermal conductivity of the composite phase change thermal storage material obtained in Example 1 reached 4.1 W / (m·K), while that in Examples 2 and 3 reached 3.8 W / (m·K), and that in Example 4 reached 3.6 W / (m·K), nearly three times that of Comparative Example 2. This is attributed to the strong adsorption energy functional groups significantly improving the interfacial contact state and effectively reducing the interfacial thermal resistance. The thermal conductivity of Comparative Example 1 also improved, reaching 1.9 W / (m·K), far lower than the improvement observed in Examples 1-4.

[0043] The composite material was placed in a hydrothermal reactor, sealed, and then placed in an oven. It was heated from room temperature to 220°C at a heating rate of 10°C / min and held for 1 hour, followed by natural cooling to room temperature and holding for 1 hour. Sample mass was weighed before each test and every 10 test cycles, repeating this process until 305 cycles were completed. After 305 thermal cycles, the mass retention rate of the four examples exceeded 75%, fully meeting the current industrial performance requirements for composite phase change thermal storage materials. In contrast, the mass retention rates of Comparative Example 1 and Comparative Example 2 were less than 70% and 55%, respectively. Figure 5 The phase transition enthalpy was tested using differential scanning calorimetry (DSC). The test results showed that after 305 thermal cycles, the latent heat retention rate of the composite material in Example 1 was as high as 84.7%, in Example 2 it was 80.9%, in Example 3 it was 81.5%, and in Example 4 it was 78.6%. Comparative Example 1 showed a significant improvement over Comparative Example 2, but it was still lower than the implementation conditions claimed in this patent. Figure 6 ).

[0044] While specific embodiments of the present invention have been described above, those skilled in the art should understand that these are merely illustrative examples, and various changes or modifications can be made to these embodiments without departing from the principles and essence of the present invention. The scope of protection of the present invention is defined only by the appended claims.

Claims

1. A method for preparing a thermally conductive framework of functionalized carbon nanotube composites, characterized in that, Includes the following steps: S1. Carbon nanotubes are uniformly dispersed in a mixed solvent consisting of deionized water and anhydrous ethanol to obtain a carbon nanotube dispersion. S2. The carbon felt is immersed in the carbon nanotube dispersion, taken out and dried, and then placed under inert gas protection for high-temperature heat treatment at a temperature of 700-900 ℃, so that the carbon nanotubes and the surface of the carbon felt fiber form an interfacial cross-linking effect to obtain a carbon felt precursor loaded with carbon nanotubes. S3. Dissolve aromatic amines with sulfonic acid, nitro, aldehyde or carboxyl groups substituted on the benzene ring in hydrochloric acid to form an aromatic amine-hydrochloric acid solution, and then add NaNO2 solution to form an aromatic amine solution. The molar ratio of aromatic amine to HCl is 1:(4.8-5.2), and the molar ratio of NaNO2 to HCl is 1:(0.8-1.2). S4. The carbon felt precursor is added to the aromatic amine solution at 0-5 °C to carry out the reaction, and then the temperature is raised to room temperature to continue the reaction; the reacted carbon felt is then taken out, washed, and dried to obtain a thermally conductive framework of functionalized carbon nanotube composite.

2. The preparation method according to claim 1, characterized in that, Step S2, before impregnation, also includes a pretreatment step for the carbon felt, specifically: The carbon felt was ultrasonically cleaned in anhydrous ethanol, then rinsed with deionized water, and dried to obtain the pretreated carbon felt.

3. The preparation method according to claim 1, characterized in that, The mixed solvent consists of deionized water and anhydrous ethanol in a volume ratio of 1-2:

1.

4. The preparation method according to claim 1, characterized in that, The heating rate of the high-temperature heat treatment is 5-10℃ / min, and the holding time is 1-2 hours; and / or, the inert gas is argon.

5. The preparation method according to claim 1, characterized in that, The concentration of aromatic amine in the aromatic amine solution is 0.2 mmol / mL, and 1 L of the aromatic amine solution can effectively modify 10-20 g of functionalized carbon nanotube composite thermally conductive framework.

6. A thermally conductive framework composed of functionalized carbon nanotubes, characterized in that, It is prepared by the preparation method according to any one of claims 1-5.

7. A composite phase change thermal storage material, characterized in that, It includes a thermally conductive framework of functionalized carbon nanotube composites as described in claim 6, and a phase change thermal storage molten salt filling the pores of the thermally conductive framework of the functionalized carbon nanotube composites.

8. The composite phase change thermal storage material according to claim 7, characterized in that, The phase change thermal storage molten salt is an inorganic molten salt, which includes at least one of nitrates, chlorides, or carbonates.

9. The application of the composite phase change thermal storage material as described in claim 7 or 8 in solar thermal power generation thermal storage systems, industrial waste heat recovery systems, or high-temperature thermal management systems.

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