Composite phase-change material containing oriented hierarchical pore heat-conducting network and preparation method of composite phase-change material

Abstract

The invention discloses a composite phase change material containing a directional hierarchical pore heat conduction network and a preparation method thereof.The method comprises the steps that an aqueous solution of polyamic acid salt, an aqueous solution of graphene oxide and modified expandable graphite flakes are mixed and then subjected to directional freezing and freeze drying, and a porous network framework is prepared; sequentially carrying out imidization, thermal expansion and graphitization treatment on the porous network skeleton in an inert atmosphere to prepare a graphite-based directional hierarchical pore network skeleton; and under vacuum, dipping the graphite-based directional hierarchical pore network skeleton in the phase change material to prepare the composite phase change material containing the directional hierarchical pore heat conduction network. By modifying expandable graphite flakes to optimize dispersibility and combining ice crystal template induced assembly and a controllable expansion process, an axial ordered heat conduction network and a porous synergistic confinement structure are constructed, the heat conduction and heat storage efficiency of the material is effectively improved, capillary adsorption of multi-scale apertures is utilized, and the heat storage efficiency of the material is improved. And the leakage problem of the solid-liquid phase change material is effectively solved.

Description

Technical Field

[0001] The invention belongs to the technical field of functional composite materials and relates to a composite phase change material containing a directional multi-level pore heat-conducting network and a preparation method thereof. Background Art

[0002] As the power density of high-frequency electronic devices such as 5G communications and artificial intelligence chips exceeds 100W / cm 2 At this level, traditional active cooling solutions such as air cooling and liquid cooling can no longer meet the temperature control requirements under transient thermal shock. Phase change thermal management materials, with their latent heat storage capacity of up to 200-300 J / g, have become an ideal choice for solving the transient overheating problem of electronic devices. However, existing organic phase change material systems have two major shortcomings: (1) low intrinsic thermal conductivity (<0.5W / (m·K)), resulting in a hysteresis in thermal response and significant temperature fluctuations of the chip under high heat flux density; (2) the risk of volume expansion and liquid leakage during the solid-liquid phase change process, which causes the material's energy storage efficiency to decrease after multiple thermal cycles, reducing its effectiveness in thermal management.

[0003] To simultaneously improve thermal conductivity and reduce the risk of phase change material leakage, researchers typically introduce highly thermally conductive fillers into organic phase change materials to create a three-dimensional network structure. For example, Chinese patent application CN110205100B discloses a graphene oxide / expanded graphite aerogel phase change composite material and its preparation method. This application uses graphene oxide / expanded graphite aerogel to construct a three-dimensional structure. After compounding with paraffin wax, the thermal conductivity of the composite phase change material reaches 0.69 W / (m·K). Furthermore, when heated to 63°C, the pure paraffin wax completely melts, while the surface of the composite phase change material only exhibits slight precipitation. Chinese patent application CN117659955A discloses a method for preparing a graphene aerogel phase change composite material. This application prepares a graphene aerogel with a directional thermally conductive skeleton and compounds it with paraffin wax. The thermal conductivity of the composite material is increased to 1.02 W / (m·K), and its stable three-dimensional network structure enables the composite phase change material to maintain a stable shape. However, incomplete reduction of the graphene oxide skeleton can cause strong interfacial phonon scattering, which in turn limits the improvement of the overall thermal conductivity of the composite phase-change material. To overcome this problem, Chinese patent publication number CN110804420B discloses a phase-change composite material based on a highly thermally conductive anisotropic graphene skeleton and its preparation method. This application introduces polyimide into the three-dimensional graphene network and performs a graphitization treatment, effectively reducing the contact thermal resistance, interfacial thermal resistance, and defect-induced phonon scattering of the graphene thermal conductive network. The result is a composite phase-change material with a thermal conductivity of 4.16 W / (m·K).

[0004] Although the oriented graphene skeleton significantly improves the thermal conductivity of the composite phase change material, its open pore structure (porosity >90%) makes it difficult to effectively inhibit the flow and leakage of the molten phase change material. This leakage problem is particularly prominent under high-temperature thermal cycling conditions, leading to packaging failure and making it difficult to meet the actual needs of chip packaging. Therefore, there is an urgent need to develop phase change thermal conductive composite materials with excellent shape stability and thermal conductivity to meet the urgent demand for efficient thermal management materials in the modern chip industry. Summary of the Invention

[0005] In response to the problems existing in the prior art, the present invention provides a composite phase change material containing a directional multi-level pore thermal conductive network and a preparation method thereof, thereby solving the technical problem in the prior art that the open high-porosity structure of the directional skeleton improves the thermal conductivity of the composite phase change material, but cannot effectively suppress the flow leakage of the molten phase change material under high-temperature thermal cycles, resulting in insufficient shape stability and packaging failure.

[0006] The present invention is achieved through the following technical solutions:

[0007] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0008] S1: performing polarity modification treatment on expandable graphite flakes to obtain modified expandable graphite flakes;

[0009] S2: uniformly mixing an aqueous solution of polyamic acid salt, an aqueous solution of graphene oxide and the modified expandable graphite flakes, and then directionally freezing and freeze-drying the mixture to obtain an oriented porous network skeleton;

[0010] S3: Under inert gas conditions, sequentially performing imidization treatment, thermal expansion treatment, and graphitization treatment on the oriented porous network skeleton to obtain a graphite-based oriented multi-level porous network skeleton;

[0011] S4: placing the graphite-based oriented multi-level pore network skeleton in a phase change material under vacuum, and performing an impregnation treatment to obtain the composite phase change material containing the oriented multi-level pore thermal conductive network.

[0012] Preferably, in step S1, the polarity modification treatment of the expandable graphite flakes includes chemical oxidation, silane coupling agent treatment or polymer coating.

[0013] Preferably, the chemical oxidation method is to immerse the expandable graphite flakes in a mixture of concentrated sulfuric acid and concentrated nitric acid, add potassium permanganate, and react to obtain the modified expandable graphite flakes;

[0014] The silane coupling agent treatment method comprises adding a silane coupling agent to an aqueous solution of ethanol to prepare a modified treatment solution, adjusting the pH value of the system to 4-5, stirring to hydrolyze the silane coupling agent, and then adding expandable graphite flakes. After the reaction, modified expandable graphite flakes are obtained.

[0015] The polymer coating method comprises immersing expandable graphite flakes in a Tris buffer solution of dopamine hydrochloride, oscillating the solution to form a polydopamine coating layer on the surface of the expandable graphite flakes, filtering and drying the solution to obtain the modified expandable graphite flakes.

[0016] Preferably, in step S2, the mass volume concentration of graphene oxide in the aqueous solution of graphene oxide is 1 to 20 mg / mL.

[0017] Preferably, in step S2, the mass ratio of polyamic acid salt to graphene oxide is 100:(5-20).

[0018] Preferably, in step S2, the mass ratio of the polyamic acid salt to the modified expandable graphite flakes is 100:(100-300).

[0019] Preferably, in step S3, the temperature of the imidization treatment is 300°C; the temperature of the thermal expansion treatment is 600-900°C; and the temperature of the graphitization treatment is 2000-2800°C.

[0020] Preferably, in step S4, the phase change material is one of erythritol, lauric acid, paraffin, stearic acid, pentaerythritol, mannitol and palmitic acid.

[0021] A composite phase change material containing a directional multi-level pore heat-conducting network is prepared by the above method; the composite phase change material containing a directional multi-level pore heat-conducting network has a thermal conductivity of 6.2 to 9.5 W / (m·K), an encapsulation rate of 88.3% to 99.2%, and a leakage rate of less than 1%.

[0022] The above-mentioned application of the composite phase change material containing a directional multi-level hole thermal conductive network in electronic devices.

[0023] Compared with the prior art, the present invention has the following beneficial technical effects:

[0024] The present invention discloses a method for preparing a composite phase change material containing a directional multi-level pore heat conductive network:

[0025] First, the method first modifies the surface polarity of expandable graphite flakes by grafting polar functional groups on the surface of the graphite flakes to regulate their dispersibility in the graphene oxide and polyamic acid salt composite system, so that they can still maintain good dispersion stability at a solid content of 40wt%, laying the foundation for the construction of a three-dimensional thermal conductive network;

[0026] Secondly, the added polyamic acid salt is a precursor of polyimide, which is dehydrated and cyclized to form polyimide in the subsequent high-temperature imidization treatment, giving the skeleton high thermal stability and chemical inertness. The viscosity of its aqueous solution bonds the graphene oxide and modified graphite flakes into an integral structure during the freeze-drying stage, preventing the components from being discrete and ensuring the integrity of the directional porous skeleton; the two-dimensional layer structure of graphene oxide is interspersed between the graphite flakes, and is combined with the polyamic acid salt and modified graphite through hydrogen bonds and π-π interactions to enhance the compressive strength of the skeleton. In addition, its surface oxygen-containing functional groups (-COOH, -OH) improve hydrophilicity, promote the uniform dispersion of graphene oxide in the aqueous phase, avoid flake agglomeration, and ensure the uniformity of the porous network. In addition, the graphene oxide sheets are oriented along the direction of ice crystal growth during freeze-drying to form a continuous heat conduction path; the graphene oxide sheets and modified graphite flakes work together to construct a double-layer heat conduction system of "macro flake channels + micro graphene oxide networks", effectively improving the thermal conductivity of the composite material.

[0027] Third, during the directional freezing process, polyamic acid salt molecular chains co-crystallize with water molecules to form an ice template. The direction of ice crystal growth determines the directional arrangement of pores. The anisotropy of ice crystal growth along the crystal direction drives graphene oxide and graphite flakes to form axially ordered continuous heat conduction channels.

[0028] Fourthly, a stepwise heat treatment under an inert atmosphere achieves a dynamic balance between the imidization shrinkage of the polyamic acid salt and the thermal expansion of the graphite flakes, constructing an in-situ interconnected porous network. The carbon skeleton is then reconstructed at high temperatures to achieve a directional graphitized skeleton. Finally, when combined with a phase-change material, the material achieves a 92% encapsulation efficiency at low loadings and a thermal conductivity of 9.5 W / (m·K), resolving the difficult balance between high thermal conductivity and high enthalpy encountered in conventional composite phase-change materials.

[0029] In summary, the present invention effectively improves the heat storage efficiency of the material, and the multi-level structure regulation strategy provides a thermal management solution with rapid thermal response, precise temperature control and long-term stability for high-frequency thermal shock scenarios such as 5G chips and power batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0031] Figure 1 This is a microscopic morphology of the expandable graphite flakes before expansion in the present invention;

[0032] Figure 2 This is a microscopic morphology of the expandable graphite flakes after expansion treatment in the present invention;

[0033] Figure 3 The stability test results of unmodified expandable graphite flakes and modified expandable graphite flakes, wherein (a) is a mixed solution of unmodified expandable graphite flakes and polyamic acid salt, (b) is a mixed solution of modified expandable graphite flakes and polyamic acid salt in Example 1 of the present invention, and (a1) and (b1) are macroscopic photographs of the solutions in (a) and (b) after standing for 15 days, respectively;

[0034] Figure 4 These are cross-sectional morphologies of the oriented multi-level porous network skeleton obtained in Example 1 of the present invention at different magnifications, wherein the scale bar in (a) is 300 μm and the scale bar in (b) is 50 μm;

[0035] Figure 5 Schematic diagram of heat transfer of the directional multi-level porous network skeleton prepared by the present invention;

[0036] Figure 6 The adsorption-desorption isotherm curve (a) and pore size distribution diagram (b) of the oriented multi-level porous network skeleton prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0037] To facilitate understanding of the features and effects of the present invention by those skilled in the art, the following provides a general description and definition of the terms and expressions used in the specification and claims. Unless otherwise indicated, all technical and scientific terms used herein have the ordinary meanings as understood by those skilled in the art regarding the present invention. In the event of conflict, the definitions in this specification shall prevail.

[0038] The theories or mechanisms described and disclosed herein, whether correct or incorrect, should not limit the scope of the present invention in any way, that is, the present invention can be implemented without being limited by any specific theory or mechanism.

[0039] Herein, all features such as values, amounts, amounts, and concentrations defined in numerical ranges or percentage ranges are for brevity and convenience only. Accordingly, the description of numerical ranges or percentage ranges should be considered to include and specifically disclose all possible subranges and individual values ​​within the range (including integers and fractions).

[0040] In this document, unless otherwise specified, “include,” “including,” “contains,” “has” or similar terms cover the meanings of “consisting of” and “mainly consisting of,” for example, “A includes a” covers the meanings of “A includes a and other” and “A only includes a.”

[0041] In this document, for the sake of brevity, not all possible combinations of the various technical features in each embodiment or example are described. Therefore, as long as there are no contradictions in the combination of these technical features, the various technical features in each embodiment or example can be combined in any way, and all possible combinations should be considered to be within the scope of this specification.

[0042] The present invention provides a method for preparing a composite phase change material containing a directional multi-level pore thermal conductive network. The specific technical solution is as follows:

[0043] S1, preparation of modified expandable graphite flakes: first, the expandable graphite flakes are crushed and screened, and the expandable graphite flakes modified with polar functional groups are prepared by surface modification technology, that is, the surface of the expandable graphite flakes is polar modified;

[0044] The expandable graphite flakes were purchased from Qingdao Tengshengda Carbon Machinery Co., Ltd. The expansion multiple of the expandable graphite flakes is 10 to 500 times.

[0045] The size of the expandable graphite flakes after crushing and screening is 500 to 28000 meshes.

[0046] The polar modified expandable graphite flakes can be prepared by chemical oxidation, silane coupling agent treatment or polymer coating.

[0047] The chemical oxidation method is as follows: the expandable graphite flakes are immersed in a mixture of 98% concentrated sulfuric acid and 68% concentrated nitric acid, potassium permanganate is added, and the reaction is carried out at 35°C for 3 hours. Subsequently, the reaction is terminated with 30% hydrogen peroxide to obtain polar modified expandable graphite flakes. The polar functional groups on the surface of the modified expandable graphite flakes are mainly hydroxyl groups and carboxyl groups.

[0048] The mass ratio of the mixed solution of concentrated sulfuric acid and concentrated nitric acid to potassium permanganate is 1:5; the volume ratio of concentrated sulfuric acid to concentrated nitric acid in the mixed solution of concentrated sulfuric acid and concentrated nitric acid is 3:1;

[0049] The silane coupling agent treatment method is specifically as follows: a silane coupling agent is added to an aqueous solution of ethanol to prepare a modified treatment solution, the pH value of the system is adjusted to 4-5, and the silane coupling agent is stirred to hydrolyze the silane coupling agent, and then expandable graphite flakes are added. After curing, a chemical bonding interface is formed to obtain a modified expandable graphite flake, and the polar functional groups on the surface of the expandable graphite flakes are amino and epoxy groups;

[0050] Wherein, the mass ratio of the silane coupling agent to the aqueous solution of ethanol is 5:100;

[0051] The mass ratio of ethanol to water in ethanol aqueous solution is 9:1;

[0052] The mass percentage concentration of the silane coupling agent in the modified treatment liquid is 3wt% to 5wt%;

[0053] The silane coupling agent is KH-550 (γ-aminopropyltriethoxysilane);

[0054] The hydrolysis time is 3 to 24 hours, preferably 12 hours;

[0055] The curing temperature is 50°C to 100°C, preferably 80°C.

[0056] The polymer coating method is specifically as follows: the expandable graphite flakes are immersed in a Tris buffer solution of dopamine hydrochloride and shaken for 12 hours to form a polydopamine coating layer with a thickness of 50 to 200 nm on the surface of the expandable graphite flakes, and the modified expandable graphite flakes are obtained after filtration and drying. The polar functional groups on the surface of the expandable graphite flakes are amino groups;

[0057] Wherein, the concentration of dopamine hydrochloride is 1 to 6 mg / mL, preferably 3 mg / mL;

[0058] The pH of Tris buffer is 8.5;

[0059] S2, preparation of polyamic acid salt: synthesizing polyamic acid salt by polycondensation method;

[0060] Specifically, pyromellitic dianhydride is added to diaminodiphenyl ether and dimethylacetamide, and after the reaction, the obtained precipitate is filtered, washed, and vacuum dried to obtain polyamic acid, and the polyamic acid reacts with triethylamine to generate water-soluble polyamic acid salt.

[0061] More specifically, a feasible reaction process is as follows: first, in a three-necked flask, 10 mmol of diaminodiphenyl ether is completely dissolved in 25 mL of dimethylacetamide with mechanical stirring. 10 mmol of pyromellitic dianhydride is gradually added, and the reactor is placed in an ice-water bath and stirred for 12 hours. Then, 3 mL of triethylamine is added and stirred for an additional 3 hours. Afterwards, 100 mL of acetone is added dropwise until a water-soluble polyamic acid salt precipitates as a white powder. The white powder is then filtered and washed three times with acetone.

[0062] S3, construction of an oriented porous network skeleton: modified expandable graphite flakes, graphene oxide, water-soluble polyamic acid salt and water are mixed, directionally frozen, and then freeze-dried to obtain an oriented porous network skeleton;

[0063] Specifically, an aqueous solution of polyamic acid salt, an aqueous solution of graphene oxide, and modified expandable graphite flakes are mixed, subjected to ultrasonic treatment for 30 minutes, stirred for 12 hours, and then subjected to directional freezing treatment. During the directional freezing treatment, liquid nitrogen is used to cool a copper block. The mixed solution is then placed in a polytetrafluoroethylene container and placed on the cooled copper block for directional freezing. The directional freezing is then followed by freeze drying until the water is completely volatilized to obtain a directional porous network skeleton.

[0064] The volume concentration of the polyamic acid salt in the aqueous solution of the polyamic acid salt is 40 mg / mL;

[0065] The volume concentration of graphene oxide in the aqueous solution of graphene oxide is 1 to 20 mg / mL;

[0066] The mass ratio of polyamic acid salt to graphene oxide is 100:(5-20);

[0067] The mass ratio of polyamic acid salt to modified expandable graphite flakes is 100:(100-300), preferably 100:200.

[0068] S4, high temperature treatment: under inert gas conditions, sequentially performing imidization treatment, thermal expansion treatment and graphitization treatment on the oriented porous network skeleton to obtain a graphite-based oriented multi-level porous network skeleton;

[0069] Wherein, the inert gas may be argon;

[0070] The temperature of the imidization treatment is 300° C. until the imidization is completely achieved;

[0071] The temperature of the thermal expansion treatment is 600-900°C until the thermal expansion is fully achieved;

[0072] The temperature of the graphitization treatment is 1000-2800°C until graphitization is completely achieved;

[0073] S5, vacuum-assisted impregnation: placing the graphite-based oriented multi-level pore network skeleton in a phase change material under vacuum, and performing an impregnation treatment to obtain the composite phase change material containing the oriented multi-level pore thermal conductive network;

[0074] Wherein, the phase change material is one of erythritol, lauric acid, paraffin, stearic acid, pentaerythritol, mannitol and palmitic acid.

[0075] In addition, the dipping temperature is 20° C. higher than the phase change temperature of the phase change material to ensure that the phase change material is completely in a molten state.

[0076] The invention also discloses that the composite phase change material containing directional multi-level hole heat conduction network prepared by the above method has a thermal conductivity of 6.2-9.5W / (m·K), an encapsulation rate of 88%-99.2%, and a leakage rate of less than 1%.

[0077] The present invention can significantly improve the dispersion stability of expandable graphite flakes in a composite system by grafting polar functional groups on the surface of expandable graphite flakes, so that expandable graphite flakes still maintain good dispersibility when high solid content. The surface-grafted polar functional groups (such as hydroxyl-OH, amino-NH2, carboxyl-COOH) form hydrogen bonds, electrostatic interactions or covalent bonds with polar groups in a polymer matrix (such as polyamic acid salt), thereby enhancing the interfacial bonding force between the filler and the matrix. This strong interaction enables the matrix molecular chains to "tow" the filler in a specific direction (along the directional temperature gradient direction) during stirring, thus achieving directional assembly of the thermally conductive filler in the polymer matrix, and laying the foundation for subsequent construction of an efficient three-dimensional heat-conducting network.

[0078] Furthermore, the present invention forms nanoscale gaps between the layers of expandable graphite flakes through a controlled expansion method, and at the same time, combines directional freezing technology to construct millimeter-scale axial macroporous channels, giving the material excellent thermal conductivity and stable packaging characteristics. First, the nanoscale gaps establish an efficient thermal conduction path through the π-π stacking effect of graphene nanosheets inside the expandable graphite. Secondly, the millimeter-scale pores enable the directional transmission of phonons at the graphitized interface, thereby constructing a three-dimensional interpenetrating thermal conductive network. Ultimately, the thermal conductivity of the composite phase change material is as high as 9.5W / (m·K), which significantly improves the thermal conductivity of the material and overcomes the problem of low thermal conductivity of traditional phase change materials. In addition, the multi-level pores work synergistically through the capillary effect to effectively limit the migration of the phase change material. After 1000 thermal shock tests (temperature range from 20°C to 140°C), the composite material still maintains 98% of the phase change enthalpy value and zero leakage characteristics. This greatly reduces the risk of leakage of the phase change material during the solid-liquid phase transition process and ensures the long-term stability of the material. At the same time, the material also has a high encapsulation rate of 92% of the phase change material, ensuring the high enthalpy value of the composite phase change material.

[0079] In summary, the present invention discloses a composite phase change material containing a directional multi-level pore thermal conductive network and a preparation method thereof. The material is composed of surface-modified graphite flakes, graphene oxide, polyamic acid salt and phase change material. Its core preparation process is divided into four stages: (1) interface modification: by grafting polar functional groups on the surface of graphite flakes, its dispersibility in the composite system of graphene oxide and polyamic acid salt is regulated, so that it can still maintain good dispersion stability at a solid content of 40wt%, laying the foundation for the construction of a three-dimensional thermal conductive network; (2) ice crystal template induced assembly: using a directional freezing technology with a gradient cooling of -30℃, the anisotropy of ice crystal growth along the crystal direction is utilized to drive graphene oxide and graphite flakes to form an axially ordered continuous thermal conductive channel; (3) thermal expansion regulation: step-by-step heat treatment is carried out under an inert atmosphere, and a dynamic balance is achieved through the imidization shrinkage of polyamic acid salt and the thermal expansion of graphite flakes, and a through-type porous network is constructed in situ; (4) multi-level pore structure graphitization: the carbon skeleton is reconstructed at high temperature to obtain a directional graphitized skeleton. After compounding with phase change materials, a 92% encapsulation rate of phase change materials is achieved at a low filling amount, and the thermal conductivity is as high as 9.5W / (m·K), which solves the contradiction between high thermal conductivity and high enthalpy values ​​in traditional composite phase change materials. After 1000 times of thermal shock from 20°C to 140°C, 98% of the phase change enthalpy value and low leakage characteristics are still maintained. In addition, the multi-level structure control strategy of the present invention provides a thermal management solution with rapid thermal response, precise temperature control and long-term stability for high-frequency thermal shock scenarios such as 5G chips and power batteries.

[0080] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0081] The following examples were prepared using conventional instruments and equipment in the art. Experimental methods in the following examples, where specific conditions are not specified, were generally performed under conventional conditions or according to the conditions recommended by the manufacturer. The various raw materials used in the following examples, unless otherwise specified, were conventional commercially available products, with specifications conventional in the art. In the present specification and the following examples, unless otherwise specified, "%" indicates percentage by weight, "part" indicates parts by weight, and "ratio" indicates weight ratio.

[0082] Example 1

[0083] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0084] S1: 0.8 g of expandable graphite flakes were crushed and screened to a particle size of 1000 mesh, then immersed in a 3 mg / mL dopamine hydrochloride solution (Tris buffer pH = 8.5), stirred for 12 h to form a polydopamine coating, and filtered and dried to obtain modified expandable graphite flakes (expansion multiple 200 times);

[0085] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 4 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried for 48 h to prepare a directional porous framework;

[0086] S3: The oriented porous skeleton is first imidized at 300°C for 180 minutes in argon, and then subjected to expansion heat treatment at 800°C for 180 minutes and graphitization at 2800°C for 180 minutes to obtain a graphite-based oriented multi-level porous network skeleton, and finally vacuum impregnated with erythritol to obtain a composite phase change material.

[0087] The thermal conductivity of the composite phase change material is 9.5 W / (m·K), and the encapsulation rate is 92.0%.

[0088] The packaging rate and thermal conductivity in the present invention are both initial test results, and the leakage rate is the leakage rate after 1000 thermal cycles.

[0089] Example 2

[0090] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0091] S1: 0.6 g of expandable graphite flakes were crushed and screened to a particle size of 1000 mesh, then immersed in a 3 mg / mL dopamine hydrochloride solution (Tris buffer pH = 8.5), stirred for 12 h to form a polydopamine coating, and filtered and dried to obtain modified expandable graphite flakes (expansion multiple 200 times);

[0092] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 4 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried for 48 h to prepare a porous framework;

[0093] S3: imidization was carried out at 300°C for 180 min in argon, followed by expansion heat treatment at 750°C for 180 min and graphitization at 2000°C for 180 min, and finally erythritol was impregnated under vacuum to obtain a composite phase change material.

[0094] The thermal conductivity of the composite phase change material is 8.5 W / (m·K), and the encapsulation rate is 90.1%.

[0095] Example 3

[0096] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0097] S1: 1.2 g of expandable graphite flakes were crushed and screened to a particle size of 1000 mesh, then immersed in a 3 mg / mL dopamine hydrochloride solution (Tris buffer pH = 8.5), stirred for 12 h to form a polydopamine coating, and filtered and dried to obtain modified expandable graphite flakes (expansion multiple 200 times);

[0098] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 4 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried for 48 h to prepare a porous framework;

[0099] S3: imidization was performed at 300°C for 180 min in argon, followed by expansion heat treatment at 900°C for 180 min and graphitization at 2800°C for 180 min, and finally impregnation with lauric acid under vacuum to obtain a composite phase change material.

[0100] The thermal conductivity of the composite phase change material is 8.9 W / (m·K), and the encapsulation rate is 90.1%.

[0101] Example 4

[0102] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0103] S1: 0.8 g of expandable graphite flakes were crushed and screened to a particle size of 1000 mesh, then immersed in a 6 mg / mL dopamine hydrochloride solution (Tris buffer pH = 8.5), stirred for 12 h to form a polydopamine coating, and filtered and dried to obtain modified expandable graphite flakes (expansion multiple 200 times);

[0104] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 4 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried for 48 h to prepare a porous framework;

[0105] S3: imidization was performed at 300°C for 180 min in argon, followed by expansion heat treatment at 850°C for 180 min and graphitization at 2800°C for 180 min, and finally paraffin impregnation was performed under vacuum to obtain a composite phase change material.

[0106] The thermal conductivity of the composite phase change material is 9.0 W / (m·K), and the encapsulation rate is 90.2%.

[0107] Example 5

[0108] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0109] S1: 0.8 g of expandable graphite flakes were crushed and screened to a particle size of 1000 mesh, then immersed in a 1 mg / mL dopamine hydrochloride solution (Tris buffer pH = 8.5), stirred for 12 h to form a polydopamine coating, and filtered and dried to obtain modified expandable graphite flakes (expansion multiple 200 times);

[0110] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 4 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried for 48 h to prepare a porous framework;

[0111] S3: imidization was performed at 300°C for 180 min in argon, followed by expansion heat treatment at 700°C for 180 min and graphitization at 2800°C for 180 min, and finally paraffin immersion (100°C in vacuum for 12 h) to obtain a composite phase change material.

[0112] The thermal conductivity of the composite phase change material is 9.2 W / (m·K), and the encapsulation rate is 90.5%.

[0113] Example 6

[0114] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0115] S1: 0.6 g of expandable graphite flakes were crushed and screened to a particle size of 1000 mesh, then immersed in a 3 mg / mL dopamine hydrochloride solution (Tris buffer pH = 8.5), stirred for 12 h to form a polydopamine coating, and filtered and dried to obtain modified expandable graphite flakes (expansion multiple 60 times);

[0116] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 4 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried for 48 h to prepare a porous framework;

[0117] S3: imidization was performed at 300°C for 180 min in argon, followed by expansion heat treatment at 800°C for 180 min and graphitization at 2200°C for 180 min, and finally impregnation with stearic acid to obtain a composite phase change material.

[0118] The thermal conductivity of the composite phase change material is 8.8 W / (m·K) and the encapsulation rate is 90.5%.

[0119] Example 7

[0120] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0121] S1: 0.8 g of expandable graphite flakes were crushed and screened to a particle size of 1000 mesh, mixed with KH-550 silane coupling agent, reacted at 80°C for 12 h, washed and dried to obtain modified graphite flakes (expansion multiple 200 times);

[0122] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 4 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried for 48 h to prepare a porous framework;

[0123] S3: imidization was performed at 300°C for 180 min in argon, followed by expansion heat treatment at 800°C for 180 min and graphitization at 2800°C for 180 min, and finally pentaerythritol was impregnated under vacuum to obtain a composite phase change material.

[0124] The thermal conductivity of the composite phase change material is 8.6 W / (m·K), and the encapsulation rate is 91.6%.

[0125] Example 8

[0126] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0127] S1: 0.8 g of expandable graphite flakes were crushed and screened to a particle size of 1000 mesh, mixed with KH-550 silane coupling agent, reacted at 80°C for 12 h, washed and dried to obtain modified graphite flakes (expansion multiple 60 times);

[0128] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 4 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried for 48 h to prepare a porous framework;

[0129] S3: imidization was performed at 300°C for 180 min in argon, followed by expansion heat treatment at 800°C for 180 min and graphitization at 2800°C for 180 min, and finally mannitol was impregnated under vacuum to obtain a composite phase change material.

[0130] The thermal conductivity of the composite phase change material is 7.9 W / (m·K), and the packaging rate is 90.8%.

[0131] Example 9

[0132] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0133] S1: 0.8 g of expandable graphite flakes were crushed and screened to a particle size of 1000 mesh, mixed with KH-550 silane coupling agent, reacted at 80°C for 12 h, washed and dried to obtain modified graphite flakes (expansion multiple 200 times);

[0134] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 2 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried for 48 h to prepare a porous framework;

[0135] S3: imidization was performed at 300°C for 180 min in argon, followed by expansion heat treatment at 800°C for 180 min and graphitization at 2800°C for 180 min, and finally palmitic acid was impregnated under vacuum to obtain a composite phase change material.

[0136] The thermal conductivity of the composite phase change material is 7.1 W / (m·K), and the encapsulation rate is 89.9%.

[0137] Example 10

[0138] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0139] S1: 0.8 g of expandable graphite flakes were crushed and screened to a particle size of 1000 mesh, mixed with KH-550 silane coupling agent, reacted at 80°C for 12 h, washed and dried to obtain modified graphite flakes (expansion multiple 200 times);

[0140] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 8 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried for 48 h to prepare a porous framework;

[0141] S3: imidization was performed at 300°C for 180 min in argon, followed by expansion heat treatment at 800°C for 180 min and graphitization at 2800°C for 180 min, and finally paraffin impregnation was performed under vacuum to obtain a composite phase change material.

[0142] The thermal conductivity of the composite phase change material is 8.3 W / (m·K), and the encapsulation rate is 88.4%.

[0143] Example 11

[0144] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0145] S1: 0.6 g of expandable graphite flakes were crushed and screened to a particle size of 500 mesh, mixed with a mixed solution of concentrated sulfuric acid and concentrated nitric acid, and then potassium permanganate was added. The mixture was oxidized at 35°C for 3 h, and then washed and dried to obtain modified graphite flakes (expansion multiple 200 times);

[0146] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 4 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried for 48 h to prepare a porous framework;

[0147] S3: Imidization was performed at 300°C for 180 min in argon, followed by expansion heat treatment at 800°C for 180 min and graphitization at 2500°C for 180 min, and finally paraffin impregnation was performed under vacuum (100°C for 12 h) to obtain a composite phase change material.

[0148] The thermal conductivity of the composite phase change material is 7.2 W / (m·K), and the encapsulation rate is 90.5%.

[0149] Example 12

[0150] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0151] S1: 0.8 g of expandable graphite flakes were crushed and screened to a particle size of 1000 mesh, mixed with a mixed solution of concentrated sulfuric acid and concentrated nitric acid, and then potassium permanganate was added. The mixture was oxidized at 35°C for 3 h, and then washed and dried to obtain modified graphite flakes (expansion multiple 200 times);

[0152] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 4 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried for 48 h to prepare a porous framework;

[0153] S3: imidization was performed at 300°C for 180 min in argon, followed by expansion heat treatment at 800°C for 180 min and graphitization at 2800°C for 180 min, and finally paraffin impregnation was performed under vacuum to obtain a composite phase change material.

[0154] The thermal conductivity of the composite phase change material is 7.3 W / (m·K), and the encapsulation rate is 89.6%.

[0155] Example 13

[0156] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0157] S1: 1.5 g of expandable graphite flakes were crushed and screened to a particle size of 1800 mesh, mixed with a mixed solution of concentrated sulfuric acid and concentrated nitric acid, and then potassium permanganate was added. The mixture was oxidized at 35°C for 3 h, and then washed and dried to obtain modified graphite flakes (expansion multiple 200 times);

[0158] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 4 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried for 48 h to prepare a porous framework;

[0159] S3: imidization was performed at 300°C for 180 min in argon, followed by expansion heat treatment at 800°C for 180 min and graphitization at 2800°C for 180 min, and finally paraffin impregnation was performed under vacuum to obtain a composite phase change material.

[0160] The thermal conductivity of the composite phase change material is 6.9 W / (m·K), and the encapsulation rate is 88.3%.

[0161] Example 14

[0162] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0163] S1: 0.8 g of expandable graphite flakes were crushed and screened to a particle size of 2800 mesh, mixed with a mixed solution of concentrated sulfuric acid and concentrated nitric acid, and then potassium permanganate was added. The mixture was oxidized at 35°C for 3 h, and then washed and dried to obtain modified graphite flakes (expansion multiple 200 times);

[0164] S2: 10 mL of a 4 wt% water-soluble polyamic acid salt solution and 4 mL of a 10 mg / mL graphene oxide solution were mixed with the modified expandable graphite flakes, ultrasonicated for 30 min and stirred for 12 h, and then directionally frozen in liquid nitrogen and freeze-dried to prepare a porous framework;

[0165] S3: imidization was performed at 300°C for 180 min in argon, followed by expansion heat treatment at 800°C for 180 min and graphitization at 2800°C for 180 min, and finally paraffin immersion (100°C in vacuum for 12 h) to obtain a composite phase change material.

[0166] The thermal conductivity of the composite phase change material is 6.2 W / (m·K), and the packaging rate is 88.5%.

[0167] Example 15

[0168] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0169] S1: crushing and screening the expandable graphite flakes to obtain expandable graphite flakes with a size of 800 mesh, and immersing the screened expandable graphite flakes in a mixture of 98% concentrated sulfuric acid and 68% concentrated nitric acid, adding potassium permanganate, and reacting at 35° C. for 3 hours to obtain modified expandable graphite flakes;

[0170] S2: After uniformly mixing an aqueous solution of a water-soluble polyamic acid salt, an aqueous solution of graphene oxide and the modified expandable graphite flakes, a graphite-based porous network skeleton is obtained after directionally freezing and freeze-drying; wherein the mass volume concentration of graphene oxide in the aqueous solution of graphene oxide is 1 mg / mL; the mass ratio of water-soluble polyamic acid salt to graphene oxide is 100:5; the mass ratio of the water-soluble polyamic acid salt to the modified expandable graphite flakes is 100:100.

[0171] S3: Under inert gas conditions, sequentially subjecting the graphite-based porous network skeleton to imidization treatment, thermal expansion treatment, and graphitization treatment, wherein the imidization treatment temperature is 300° C. and the time is 60 min; the thermal expansion treatment temperature is 600° C. and the time is 240 min; and the graphitization treatment temperature is 2800° C. and the time is 240 min, to obtain a graphite-based hierarchical porous skeleton;

[0172] S4: placing the graphite-based multi-level porous skeleton in paraffin under vacuum, and performing an immersion treatment to obtain the composite phase change material containing a directional multi-level porous thermal conductive network.

[0173] The thermal conductivity of the composite phase change material containing the oriented multi-level pore heat-conducting network prepared in this embodiment is 7.5 W / (m·K), the encapsulation rate is 92%, and the leakage rate is 0.4%.

[0174] Example 16

[0175] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0176] S1: crushing and screening the expandable graphite flakes to obtain expandable graphite flakes with a size of 15000 mesh, and adding the screened expandable graphite flakes to the hydrolyzed silane coupling agent to obtain modified expandable graphite flakes;

[0177] S2: After uniformly mixing an aqueous solution of a water-soluble polyamic acid salt, an aqueous solution of graphene oxide and the modified expandable graphite flakes, the mixture is directionally frozen and freeze-dried for 72 hours to obtain a graphite-based porous network skeleton; wherein the mass volume concentration of graphene oxide in the aqueous solution of graphene oxide is 15 mg / mL; the mass ratio of water-soluble polyamic acid salt to graphene oxide is 100:15; the mass ratio of the water-soluble polyamic acid salt to the modified expandable graphite flakes is 100:200.

[0178] S3: Under inert gas conditions, sequentially subjecting the graphite-based porous network skeleton to imidization treatment, thermal expansion treatment, and graphitization treatment, wherein the imidization treatment temperature is 300° C. and the time is 200 min; the thermal expansion treatment temperature is 700° C. and the time is 200 min; and the graphitization treatment temperature is 2500° C. and the time is 200 min, to obtain a graphene-graphite polyimide skeleton;

[0179] S4: placing the graphene-graphite polyimide skeleton in paraffin under vacuum, and performing an immersion treatment to obtain the composite phase change material containing a directional multi-level pore thermal conductive network.

[0180] The thermal conductivity of the composite phase change material containing the oriented multi-level pore heat-conducting network prepared in this embodiment is 8.3 W / (m·K), the packaging rate is 91.2%, and the leakage rate is 0.6%.

[0181] Example 17

[0182] A method for preparing a composite phase change material containing a directional multi-level pore heat conductive network, comprising the following steps:

[0183] S1: crushing and screening the expandable graphite flakes to obtain expandable graphite flakes with a size of 28000 mesh, and adding the screened expandable graphite flakes to a Tris buffer solution of dopamine hydrochloride, shaking for 12 hours to form a polydopamine coating layer on the surface of the expandable graphite flakes, filtering and drying, to obtain modified expandable graphite flakes;

[0184] S2: After uniformly mixing an aqueous solution of a water-soluble polyamic acid salt, an aqueous solution of graphene oxide and the modified expandable graphite flakes, the mixture is directionally frozen and freeze-dried for 72 hours to obtain a graphite-based porous network skeleton; wherein the mass volume concentration of graphene oxide in the aqueous solution of graphene oxide is 20 mg / mL; the mass ratio of water-soluble polyamic acid salt to graphene oxide is 100:20; the mass ratio of the water-soluble polyamic acid salt to the modified expandable graphite flakes is 100:300.

[0185] S3: Under inert gas conditions, sequentially subjecting the graphite-based porous network skeleton to imidization treatment, thermal expansion treatment, and graphitization treatment, wherein the imidization treatment temperature is 300° C. and the time is 360 min; the thermal expansion treatment temperature is 900° C. and the time is 120 min; and the graphitization treatment temperature is 2800° C. and the time is 60 min, to obtain a graphene-graphite polyimide skeleton;

[0186] S4: placing the graphene-graphite polyimide skeleton in paraffin under vacuum, and performing an immersion treatment to obtain the composite phase change material containing a directional multi-level pore thermal conductive network.

[0187] The thermal conductivity of the composite phase change material containing the oriented multi-level pore heat-conducting network prepared in this embodiment is 8.8 W / (m·K), the packaging rate is 89.2%, and the leakage rate is 0.4%.

[0188] Table 1 shows the phase change parameters of the composite phase change materials prepared in Examples 1 to 17 of the present invention. As can be seen from Table 1, the thermal conductivity of the composite phase change materials prepared using the scheme of the present invention is 6.2 to 9.5 W / (m·K), the encapsulation rate is 88.3% to 99.2%, and the leakage rate is less than 1%, which shows good encapsulation effect.

[0189] Table 1 Phase change parameters of the composite phase change materials prepared in Examples 1 to 17 of the present invention

[0190]

[0191] Figure 1 This is a microscopic morphology of the expandable graphite flakes in the present invention before expansion. As can be seen from the figure, it presents a scaly morphology, a relatively flat surface, an irregular edge, and a flake width of 10 to 50 μm.

[0192] Figure 2 This is a microscopic morphology of the expandable graphite flakes after expansion treatment in the present invention. As can be seen from the figure, the interior is composed of graphene nanolayers with a wrinkled morphology. These layers are loosely connected to each other, forming obvious gaps and voids.

[0193] Figure 3 These are the stability test results of unmodified expandable graphite flakes and modified expandable graphite flakes, wherein (a) is a mixed solution of unmodified expandable graphite flakes and polyamic acid salt, (b) is a mixed solution of modified expandable graphite flakes and polyamic acid salt in Example 1 of the present invention, (a1) and (b1) are macroscopic photographs of the solutions in (a) and (b) after standing for 15 days, respectively. It can be seen from the figure that the expandable graphite flakes have good dispersion stability after surface modification and less sedimentation, which also indicates that the modified groups on the surface of polydopamine are combined with the carboxylate groups of the polyamic acid salt, significantly improving the interfacial bonding force.

[0194] Figure 4 The cross-sectional morphology of the directional multilevel pore network skeleton obtained in Example 1 of the present invention at different magnifications, wherein the scale of (a) is 300 μm and the scale of (b) is 50 μm. It can be seen from the figure that a multilevel pore structure skeleton with gradient expansion characteristics is successfully constructed by combining the ice crystal template induced assembly with a stepped heat treatment process. The skeleton cross section presents a vertically arranged highly ordered parallel layered porous feature, and the modified expandable graphite flakes increase in volume after high-temperature expansion, forming a hierarchical porous nanoscale secondary structure on the pore wall surface. This stable multilevel porous channel structure is suitable as a carrier for phase change materials.

[0195] Figure 5 This is a schematic diagram of the heat transfer of the directional multi-level porous network skeleton produced by the present invention. As can be seen from the figure, its cross-section forms continuous axially directional heat conduction channels. Due to the action of the graphitized interface, heat is efficiently and directionally transferred along the axial direction. Simultaneously, the interconnected graphene nanosheets generated in situ between the graphite flakes further construct a three-dimensional through-hole heat conduction network, significantly improving the heat transfer rate of the composite phase-change material. Furthermore, the millimeter-scale axial macroporous channels formed by the directional freezing technique and the nanometer-scale interlayer gaps generated by the heat treatment synergistically regulate the migration of the phase-change material through the capillary effect, allowing the composite phase-change material to maintain a low leakage rate during multiple thermal cycles.

[0196] Figure 6 The adsorption-desorption isotherm curve (a) and pore size distribution diagram of the oriented multi-level porous network skeleton prepared in Example 1 of the present invention are shown in FIG. Figure 6 As shown in (a), the adsorption-desorption isotherm of the oriented multi-level porous network skeleton prepared in this embodiment shows a typical IV type isotherm characteristic, reflecting the presence of layered aggregates or slit-like mesoporous / macroporous structures inside the material; and the desorption branch has an obvious hysteresis effect, which indicates that the adsorbate in the mesopores has undergone strong capillary condensation, further confirming the hierarchical pore feature of the coexistence of mesopores and macropores in the material. This hierarchical pore feature plays a key role in suppressing leakage. In addition, Figure 6 As shown in (b), the main peak is concentrated in the 2-5 nm range, indicating that mesopores are the dominant pore structure of the material. Furthermore, the pore size distribution of the material ranges from 2 to 120 nm. This result not only confirms the presence of a hierarchical pore structure within the oriented hierarchical porous framework but also demonstrates its excellent phase change material loading capacity. The abundant mesopores and macropores provide ample loading space for the phase change material and impart excellent leakage resistance to the material.

[0197] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a composite phase change material containing a directional multi-level pore thermal conductive network, characterized in that: The following steps are involved: S1: performing polarity modification treatment on expandable graphite flakes to obtain modified expandable graphite flakes; S2: uniformly mixing an aqueous solution of polyamic acid salt, an aqueous solution of graphene oxide and the modified expandable graphite flakes, and then directionally freezing and freeze-drying the mixture to obtain an oriented porous network skeleton; S3: Under inert gas conditions, sequentially performing imidization treatment, thermal expansion treatment, and graphitization treatment on the oriented porous network skeleton to obtain a graphite-based oriented multi-level porous network skeleton; S4: placing the graphite-based oriented multi-level pore network skeleton in a phase change material under vacuum, and performing an impregnation treatment to obtain the composite phase change material containing the oriented multi-level pore thermal conductive network.

2. The method for preparing a composite phase change material containing a directional multi-level pore thermal conductive network according to claim 1, characterized in that: In step S1, the expandable graphite flakes are subjected to polarity modification treatment, including chemical oxidation, silane coupling agent treatment or polymer coating.

3. The method for preparing a composite phase change material containing a directional multi-level pore thermal conductive network according to claim 2, characterized in that: The chemical oxidation method comprises immersing expandable graphite flakes in a mixture of concentrated sulfuric acid and concentrated nitric acid, adding potassium permanganate, and reacting to obtain the modified expandable graphite flakes. The silane coupling agent treatment method comprises adding a silane coupling agent to an aqueous solution of ethanol to prepare a modified treatment solution, adjusting the pH value of the system to 4-5, stirring to hydrolyze the silane coupling agent, and then adding expandable graphite flakes. After the reaction, modified expandable graphite flakes are obtained. The polymer coating method comprises immersing expandable graphite flakes in a Tris buffer solution of dopamine hydrochloride, oscillating the solution to form a polydopamine coating layer on the surface of the expandable graphite flakes, filtering and drying the solution to obtain the modified expandable graphite flakes.

4. The method for preparing a composite phase change material containing a directional multi-level pore thermal conductive network according to claim 1, characterized in that: In step S2, the concentration of graphene oxide in the aqueous solution of graphene oxide is 1 to 20 mg / mL.

5. The method for preparing a composite phase change material containing a directional multi-level pore thermal conductive network according to claim 1, characterized in that: In step S2, the mass ratio of polyamic acid salt to graphene oxide is 100:(5-20).

6. The method for preparing a composite phase change material containing a directional multi-level pore thermal conductive network according to claim 1, characterized in that: In step S2, the mass ratio of the polyamic acid salt to the modified expandable graphite flakes is 100:(100-300).

7. The method for preparing a composite phase change material containing a directional multi-level pore thermal conductive network according to claim 1, characterized in that: In step S3, the temperature of the imidization treatment is 300°C; the temperature of the thermal expansion treatment is 600-900°C; and the temperature of the graphitization treatment is 2000-2800°C.

8. The method for preparing a composite phase change material containing a directional multi-level pore thermal conductive network according to claim 1, characterized in that: In step S4, the phase change material is one of erythritol, lauric acid, paraffin, stearic acid, pentaerythritol, mannitol and palmitic acid.

9. A composite phase change material containing a directional multi-level pore thermal conductive network, characterized in that: The composite phase change material containing a directional multi-level pore heat-conducting network has a thermal conductivity of 6.2 to 9.5 W / (m·K), an encapsulation rate of 88.3% to 99.2%, and a leakage rate of less than 1%.

10. Use of the composite phase change material containing a directional multi-level pore heat conductive network as claimed in claim 9 in electronic devices.

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