High-thermal-conductivity shaped composite phase change material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of heat storage systems, and particularly relates to a high-thermal-conductivity shaped composite phase change material as well as a preparation method and application thereof. The graphite foam with a three-dimensional porous structure is prepared by mixing graphite powder, carbon nitride and ammonium bicarbonate and adopting a pressing and drying method. A series of graphite foam matrixes with different adsorption effects are prepared by regulating and controlling the addition amount of ammonium bicarbonate, and a porous matrix with high heat conductivity and a good packaging effect is successfully prepared. The method is helpful for the phase change material to show excellent thermal conductivity and leakage resistance, and has huge application potential in the fields of heat dissipation and heat storage. Meanwhile, by means of the preparation method of the high-thermal-conductivity shaped composite phase change material, the thermal conductivity of the composite phase change material is effectively improved, the composite phase change material shows excellent heat storage / heat release rate and low leakage performance, and it is indicated that the preparation method has good application prospects in the aspect of solar heating systems.

Description

Technical Field

[0001] This invention belongs to the field of thermal storage system technology, and more specifically relates to a high thermal conductivity shaped composite phase change material, its preparation method and application. Background Technology

[0002] Thermal energy storage technology has received widespread attention in recent years as an important means to address the imbalance between energy supply and demand and improve the efficiency of renewable energy utilization. Among various thermal energy storage methods, latent heat storage exhibits unique advantages due to its high energy density and near-constant temperature heat storage and release characteristics. Its core lies in the absorption or release of a large amount of latent heat by phase change materials during the phase transition process. Organic phase change materials have become a research hotspot due to their good chemical stability, high phase change enthalpy, and small supercooling. Among them, 1-octadecyl alcohol, as a typical fatty alcohol phase change material, has a high phase change enthalpy of about 240~260 J / g and a phase change temperature close to the human comfort zone (55~60℃), showing broad application prospects in building energy conservation, thermal management of electronic equipment, and solar thermal energy storage.

[0003] However, while single phase change materials (such as 1-octadecyl alcohol) possess excellent thermal storage properties, their inherent low thermal conductivity and liquid leakage problems severely limit their practical applications. Current research mainly focuses on improving thermal conductivity by introducing thermally conductive fillers. However, in traditional methods, the fillers are randomly distributed within the matrix. With the cyclic use of the phase change material, the fillers are prone to sedimentation, leading to the destruction of the thermal conductivity network and a decline in the performance of the composite phase change material. Therefore, how to solve the problems of poor thermal performance and easy liquid leakage during melting in composite phase change materials has become a pressing challenge for those skilled in the art. Summary of the Invention

[0004] The purpose of this invention is to provide a high thermal conductivity shaped composite phase change material, its preparation method, and its application, so as to solve the problems existing in the prior art.

[0005] To achieve the above objectives, the present invention provides the following solution: One of the technical solutions of this invention is to provide a method for preparing graphite foam, comprising the following steps: Carbon nitride was obtained by calcining melamine and urea as reactants. Phenolic resin is added to ethanol and mixed evenly to obtain a phenolic resin-ethanol solution; The carbon nitride, graphite powder, and ammonium bicarbonate are added to the phenolic resin-ethanol solution, stirred, sonicated, and dried. The solution is then ground, sieved, pressed into shape, and subjected to segmented heating treatment to obtain the graphite foam.

[0006] Furthermore, the mass ratio of melamine to urea is 1:9.

[0007] Furthermore, the calcination treatment is carried out at a temperature of 550°C, a heating rate of 5°C / min, and a holding time of 3 hours.

[0008] Furthermore, the mass ratio of the phenolic resin to ethanol is 1:105.

[0009] Furthermore, the mixing speed is 500 rpm, the temperature is 40°C, and the time is 30 min.

[0010] Furthermore, the mass ratio of the graphite powder to carbon nitride is 9:1.

[0011] Furthermore, the mass ratio of the sum of the graphite powder and carbon nitride to the mass of ammonium bicarbonate is 1:3-5.

[0012] Furthermore, the mass ratio of the sum of the masses of carbon nitride, graphite powder, and ammonium bicarbonate to the mass ratio of the phenolic resin-ethanol solution is 12-19:15-16.

[0013] Furthermore, the stirring speed is 800 rpm and the stirring time is 1 hour.

[0014] Furthermore, the temperature of the ultrasound is 40°C, and the duration is 40 minutes.

[0015] Furthermore, the drying process involves drying in a forced-air drying oven for 6 hours to remove ethanol.

[0016] Furthermore, the mesh size of the grinding and sieving process is 80 mesh.

[0017] Furthermore, the pressing pressure is 50 MPa, and the holding time is 5 min.

[0018] Furthermore, the segmented heating process involves holding the temperature at 80°C for 2 hours, followed by heating to 120°C and holding for 2 hours.

[0019] The second technical solution of the present invention is to provide a graphite foam prepared by the above preparation method.

[0020] In existing technologies, conventional graphite foam (such as Comparative Example 3) is generally prepared by mixing graphite powder and ammonium bicarbonate, pressing into shape, and then heat-treating. Compared to conventional graphite foam, the graphite foam prepared in this invention incorporates carbon nitride prepared from melamine and urea, which improves the leak-proof performance of the prepared graphite foam. Simultaneously, phenolic resin acts as a binder, stabilizing the graphite foam skeleton, ultimately resulting in a shape-stable composite phase change material.

[0021] The third technical solution of the present invention provides an application of the above-mentioned graphite foam as a phase change material carrier in a solar thermal storage system.

[0022] Fourth technical solution of the present invention: Provides a high thermal conductivity shaped composite phase change material, comprising, by mass percentage: The above-mentioned graphite foam contains 22-32% and the balance is 1-octadecyl alcohol.

[0023] Fifth technical solution of the present invention: A method for preparing the above-mentioned high thermal conductivity shaped composite phase change material, comprising the following steps: Graphite foam is immersed in molten 1-octadecyl alcohol, and after vacuum adsorption and cooling, the high thermal conductivity shaped composite phase change material is obtained.

[0024] Furthermore, the vacuum adsorption temperature is 80°C and the time is 1 hour.

[0025] The sixth technical solution of this invention provides an application of the above-mentioned high thermal conductivity shaped composite phase change material in a solar thermal storage system.

[0026] The present invention discloses the following technical effects: The graphite foam provided by this invention utilizes the thermal decomposition properties of ammonium bicarbonate to mix with graphite powder and carbon nitride to construct a three-dimensional porous matrix-graphite foam, achieving directional distribution of thermally conductive fillers and forming a three-dimensional continuous thermally conductive network, which significantly improves the thermal conductivity of subsequent composite phase change materials.

[0027] The unique porous structure of the graphite foam provided by this invention can effectively fix the phase change material and prevent leakage of the phase change material in the molten state. In addition, the introduced carbon nitride further enhances the structural stability of the composite phase change material and synergistically reduces the leakage risk of the phase change material.

[0028] The high thermal conductivity shaped composite phase change material provided by this invention is prepared by pre-fabricating a thermally conductive framework (graphite foam) with a porous structure, and then introducing the phase change material. This porous thermally conductive framework not only improves the overall thermal conductivity, but its pore structure also prevents leakage of the phase change material, solving the problems of poor thermal performance and easy liquid-phase leakage during melting in composite phase change materials.

[0029] The high thermal conductivity shaped composite phase change material provided by this invention has a thermal conductivity of up to 3.12 W / (m·K), which is 940.0% higher than that of 1-octadecyl alcohol. The heat storage / release rate is 321.7% and 223.0% higher than that of 1-octadecyl alcohol, respectively. After 300 thermal cycles, the phase change temperature and latent heat of the prepared shaped composite material hardly change, exhibiting good thermal cycling stability. Attached Figure Description

[0030] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 The thermal conductivity of 1-octadecyl alcohol, Examples 1-3 and Comparative Examples 1-2 and 4 is given, wherein (a) is the thermal conductivity of 1-octadecyl alcohol and Examples 1-3, and (b) is the thermal conductivity of Examples 1 and Comparative Examples 1-2 and 4.

[0031] Figure 2 The results of high-temperature leakage tests on the composite phase change materials of Examples 1-3 are shown. (a) is the sample before high-temperature treatment, and (b) is the filter paper after removing the sample after high-temperature treatment.

[0032] Figure 3 The results of high-temperature leakage tests on the composite phase change materials of Comparative Examples 1-4 are shown. (a) is the sample before high-temperature treatment, and (b) is the filter paper after removing the sample after high-temperature treatment.

[0033] Figure 4 The images show SEM images of the graphite foam and the high thermal conductivity shaped composite phase change material prepared in Examples 1-3, where (a) is the graphite foam of Example 3, (b) is the graphite foam of Example 2, (c) is the graphite foam of Example 1, (d) is the high thermal conductivity shaped composite phase change material of Example 3, (e) is the high thermal conductivity shaped composite phase change material of Example 2, and (f) is the high thermal conductivity shaped composite phase change material of Example 1.

[0034] Figure 5 The N2 adsorption-desorption isotherms and pore size distribution curves of carbon nitride and graphite foam in Examples 1-3 are shown, where (a) is the N2 adsorption-desorption isotherm of carbon nitride, (b) is the pore size distribution curve of carbon nitride, (c) is the N2 adsorption-desorption isotherm of graphite foam, and (d) is the pore size distribution curve of graphite foam.

[0035] Figure 6 The XRD diffraction patterns are of 1-octadecyl alcohol, graphite powder, carbon nitride, and the composite phase change materials of Examples 1-3.

[0036] Figure 7 The heat storage / release temperature curves of 1-octadecyl alcohol and the composite phase change materials of Examples 1-3 are shown, wherein (a) is the heat storage temperature curve of 1-octadecyl alcohol, (b) is the heat release temperature curve of 1-octadecyl alcohol, (c) is the heat storage temperature curve of the composite phase change material, and (d) is the heat release temperature curve of the composite phase change material.

[0037] Figure 8The DSC curves are for 1-octadecyl alcohol and the composite phase change materials of Examples 1-3, where (a) is the first DSC curve of 1-octadecyl alcohol and the composite phase change materials of Examples 1-3, and (b) is the DSC curve before and after 300 cycles of CPCM5. Detailed Implementation

[0038] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0039] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0040] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0041] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0042] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0043] Unless otherwise specified, all raw materials and reagents involved in the specific embodiments of this invention are commercially available products.

[0044] Unless otherwise specified, room temperature and ambient temperature in the specific embodiments of this invention refer to 20-30℃.

[0045] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0046] In some specific embodiments, the present invention provides a highly thermally conductive shaped composite phase change material, comprising, by mass percentage: Graphite foam 22-32% and the balance 1-octadecyl alcohol.

[0047] In the technical solution of this invention, graphite foam is first prepared by mixing graphite powder, carbon nitride and ammonium bicarbonate, and then immersed in molten 1-octadecyl alcohol to adsorb 1-octadecyl alcohol. The proportion of graphite foam before and after adsorption of 1-octadecyl alcohol is calculated by the mass difference (22%, 28% and 32%, respectively).

[0048] Taking Example 1 as an example, the 22% graphite foam content means that the maximum amount of 1-octadecyl alcohol that the graphite foam can adsorb is 78% (100%-22%=78%). If we want to increase the graphite foam content to 28%, that is, reduce the 1-octadecyl alcohol content to 72%, it cannot be artificially controlled. This is related to the porosity characteristics of the material itself. More pores can absorb more 1-octadecyl alcohol. Therefore, by changing the preparation parameters of the graphite foam, the purpose of indirectly adjusting the graphite foam content in the high thermal conductivity shaped composite phase change material is achieved.

[0049] Example 1 The preparation steps of highly thermally conductive shaped composite phase change materials include: S1. Melamine and urea are mixed at a mass ratio of 1:9, placed in a covered alumina crucible, wrapped with tin foil, heated to 550°C at 5°C / min in a muffle furnace and held for 3 hours. After natural cooling, the mixture is ground to obtain a light yellow powder of carbon nitride (CN). S2. Add phenolic resin to anhydrous ethanol and stir at 500 rpm for 30 minutes until homogeneous to obtain a phenolic resin-ethanol solution; the mass ratio of phenolic resin to anhydrous ethanol is 1:105. S3. Add the carbon nitride, graphite powder (GP) and ammonium bicarbonate obtained in step S1 to the phenolic resin-ethanol solution obtained in step S2, place it in a magnetic stirrer, stir at 800 rpm for 1 hour, then ultrasonically vibrate in an ultrasonic cleaner at 40°C for 40 minutes, and then dry in a forced-air drying oven for 6 hours to remove ethanol. Grind through an 80-mesh sieve to obtain a mixed powder. The mass ratio of graphite powder to carbon nitride is 9:1; the mass ratio of the sum of graphite powder and carbon nitride to ammonium bicarbonate is 1:5; and the mass ratio of the sum of carbon nitride, graphite powder, and ammonium bicarbonate to the phenolic resin-ethanol solution is 18.75:15.93. S4. Place the composite powder into a circular mold, press it at 50 MPa for 5 minutes to form a shape, then heat it to 80℃ and hold it for 2 hours, then heat it to 120℃ and hold it for 2 hours to obtain graphite foam (GP@CN5). S5. Prepare the following raw material components according to the mass percentage: The graphite foam obtained in step S4 is 22%, with the balance being 1-octadecyl alcohol (OD), totaling 100%. S6. Heat 1-octadecyl alcohol to a molten state, add graphite foam, place in a vacuum oven at 80°C and keep warm for 1 hour, then cool naturally to obtain a high thermal conductivity shaped composite phase change material (OD / GP@CN5), denoted as CPCM5.

[0050] Example 2 The preparation steps of highly thermally conductive shaped composite phase change materials include: S1. Melamine and urea are mixed at a mass ratio of 1:9, placed in a covered alumina crucible, wrapped with tin foil, heated to 550°C at 5°C / min in a muffle furnace and held for 3 hours. After natural cooling, the mixture is ground to obtain a light yellow powder of carbon nitride (CN). S2. Add phenolic resin to anhydrous ethanol and stir at 500 rpm for 30 min until homogeneous to obtain a phenolic resin-ethanol solution; the mass ratio of phenolic resin to anhydrous ethanol is 1:105. S3. Add the carbon nitride, graphite powder (GP) and ammonium bicarbonate obtained in step S1 to the phenolic resin-ethanol solution obtained in step S2, place it in a magnetic stirrer, stir at 800 rpm for 1 hour, then ultrasonically vibrate in an ultrasonic cleaner at 40°C for 40 minutes, and then dry in a forced-air drying oven for 6 hours to remove ethanol. Grind through an 80-mesh sieve to obtain a mixed powder. The mass ratio of graphite powder to carbon nitride is 9:1; the mass ratio of the sum of the masses of graphite powder and carbon nitride to the mass ratio of ammonium bicarbonate is 1:4; and the mass ratio of the sum of the masses of carbon nitride, graphite powder, and ammonium bicarbonate to the mass ratio of phenolic resin-ethanol solution is 15.60:15.93. S4. Place the composite powder into a circular mold, press it at 50 MPa for 5 minutes to form a shape, then heat it to 80℃ and hold it for 2 hours, then heat it to 120℃ and hold it for 2 hours to obtain graphite foam (GP@CN4). S5. Prepare the following raw material components according to the mass percentage: The graphite foam obtained in step S4 is 28%, with the balance being 1-octadecyl alcohol (OD), totaling 100%. S6. Heat 1-octadecyl alcohol to a molten state, add graphite foam, place in a vacuum oven at 80°C and keep warm for 1 hour, then cool naturally to obtain a high thermal conductivity shaped composite phase change material (OD / GP@CN4), denoted as CPCM4.

[0051] Example 3 The preparation steps of highly thermally conductive shaped composite phase change materials include: S1. Melamine and urea are mixed at a mass ratio of 1:9, placed in a covered alumina crucible, wrapped with tin foil, heated to 550°C at 5°C / min in a muffle furnace and held for 3 hours. After natural cooling, the mixture is ground to obtain a light yellow powder of carbon nitride (CN). S2. Add phenolic resin to anhydrous ethanol and stir at 500 rpm for 30 min until homogeneous to obtain a phenolic resin-ethanol solution; the mass ratio of phenolic resin to anhydrous ethanol is 1:105. S3. Add the carbon nitride, graphite powder (GP) and ammonium bicarbonate obtained in step S1 to the phenolic resin-ethanol solution obtained in step S2, place it in a magnetic stirrer, stir at 800 rpm for 1 hour, then ultrasonically vibrate in an ultrasonic cleaner at 40°C for 40 minutes, and then dry in a forced-air drying oven for 6 hours to remove ethanol. Grind through an 80-mesh sieve to obtain a mixed powder. The mass ratio of graphite powder to carbon nitride is 9:1; the mass ratio of the sum of graphite powder and carbon nitride to ammonium bicarbonate is 1:3; and the mass ratio of the sum of carbon nitride, graphite powder, and ammonium bicarbonate to the phenolic resin-ethanol solution is 12.45:15.93. S4. Place the composite powder into a circular mold, press it at 50 MPa for 5 minutes to form a shape, then heat it to 80℃ and hold it for 2 hours, then heat it to 120℃ and hold it for 2 hours to obtain graphite foam (GP@CN3). S5. Prepare the following raw material components according to the mass percentage: The graphite foam obtained in step S4 is 32%, with the balance being 1-octadecyl alcohol (OD), totaling 100%. S6. Heat 1-octadecyl alcohol to a molten state, add graphite foam, place in a vacuum oven at 80°C and keep warm for 1 hour, then cool naturally to obtain a high thermal conductivity shaped composite phase change material (OD / GP@CN3), denoted as CPCM3.

[0052] Comparative Example 1 Compared with Example 1, the only difference is that the graphite foam was replaced with an equal mass of expanded graphite (from commercially available expanded graphite with an expansion ratio of 200 mL / g, 80 mesh, purity ≥99.5%, purchased from Qingdao Tengshengda Auto Machinery Co., Ltd.).

[0053] S1. Place an appropriate amount of expandable graphite powder in an 80℃ vacuum drying oven and dry for 12 hours to remove moisture. Then place it in a quartz beaker and expand it in a muffle furnace at 900℃ for 1 minute to obtain EG.

[0054] S2. Place 1-octadecyl alcohol in a beaker containing EG, stir thoroughly with a glass rod, and then place in an 80℃ water bath for 2 hours, stirring for 30 minutes. After cooling, place the composite material in an 80℃ vacuum drying oven and heat for 3 hours. After natural cooling under vacuum, press into blocks to prepare a shaped composite phase change material. The material contains 22% expanded graphite and the balance is 1-octadecyl alcohol.

[0055] Comparative Example 2 The preparation steps of composite phase change materials include: S1. Melamine and urea are mixed at a mass ratio of 1:9, placed in a covered alumina crucible, wrapped with tin foil, heated to 550°C at 5°C / min in a muffle furnace and held for 3 hours. After natural cooling, the mixture is ground to obtain a light yellow powder of carbon nitride. S2. Grind the carbon nitride, graphite powder, ammonium bicarbonate, and phenolic resin obtained in step S1 through an 80-mesh sieve (simple physical blending) to obtain a mixed powder; wherein, the mass ratio of graphite powder to carbon nitride is 9:1; the mass ratio of the sum of the masses of graphite powder and carbon nitride to the mass ratio of ammonium bicarbonate is 1:5; and the mass ratio of the sum of the masses of carbon nitride, graphite powder, and ammonium bicarbonate to the mass ratio of phenolic resin is 125:1. S3. Place the composite powder into a circular mold, press it at 50MPa for 5 minutes to form a shape, then heat it to 80℃ and hold it for 2 hours, then heat it to 120℃ and hold it for 2 hours to obtain graphite foam. S4. Prepare the following raw material components according to the mass percentage: The graphite foam obtained in step S3 is 22%, with the balance being 1-octadecyl alcohol, totaling 100%. S5. Heat 1-octadecyl alcohol to a molten state, add graphite foam, place in a vacuum oven at 80°C for 1 hour, and allow to cool naturally to obtain a composite phase change material.

[0056] Comparative Example 3 The preparation steps of composite phase change materials include: S1. Grind graphite powder and ammonium bicarbonate through an 80-mesh sieve to obtain a mixed powder; wherein the mass ratio of graphite powder to ammonium bicarbonate is 1:5. S2. Place the composite powder into a circular mold, press it at 50MPa for 5 minutes to form a shape, then heat it to 80℃ and hold it for 2 hours, then heat it to 120℃ and hold it for 2 hours to obtain graphite foam. S4. Prepare the following raw material components according to the mass percentage: The graphite foam obtained in step S3 comprises 35%, with the remainder being 1-octadecyl alcohol, totaling 100%. S5. Heat 1-octadecyl alcohol to a molten state, add graphite foam, place in a vacuum oven at 80°C for 1 hour, and allow to cool naturally to obtain a composite phase change material.

[0057] Comparative Example 4 The preparation steps of composite phase change materials include: S1. Grind graphite powder, ammonium bicarbonate and phenolic resin through an 80-mesh sieve to obtain a mixed powder; wherein the mass ratio of graphite powder to ammonium bicarbonate is 1:5; the mass ratio of the sum of the masses of graphite powder and ammonium bicarbonate to the mass ratio of phenolic resin-ethanol solution is 125:1. S3. Place the composite powder into a circular mold, press it at 50MPa for 5 minutes to form a shape, then heat it to 80℃ and hold it for 2 hours, then heat it to 120℃ and hold it for 2 hours to obtain graphite foam. S4. Prepare the following raw material components according to the mass percentage: The graphite foam obtained in step S3 comprises 31%, with the remainder being 1-octadecyl alcohol, totaling 100%. S5. Heat 1-octadecyl alcohol to a molten state, add graphite foam, place in a vacuum oven at 80°C for 1 hour, and allow to cool naturally to obtain a composite phase change material.

[0058] Test case Figure 1 The figures show the thermal conductivity of the high thermal conductivity shaped composite phase change materials prepared from 1-octadecyl alcohol, Examples 1-3, and Comparative Examples 1-2 and 4. (a) shows the thermal conductivity of 1-octadecyl alcohol and Examples 1-3, and (b) shows the thermal conductivity of Comparative Examples 1-2 and 4. As can be seen from the figures, the addition of graphite foam effectively improves the heat transfer performance of the composite phase change materials. With the increase in the proportion of graphite powder, the thermal conductivity of the material also increases. When the mass fraction of graphite foam (CPCM5) is 22%, the thermal conductivity of the composite material increases to 3.12 W / (m·K), which is 940.0% higher than that of 1-octadecyl alcohol. This is attributed to the three-dimensional thermally conductive network of graphite foam providing a fast channel for phonons with low heat dissipation, replacing the originally disordered and inefficient heat transfer path. In Comparative Example 1, due to the high thermal conductivity of expanded graphite... The addition of expanded graphite significantly improved the thermal conductivity of the composite phase change material. However, excessive addition caused agglomeration of the material, resulting in uneven distribution of the thermally conductive material and preventing the thermally conductive filler from fully utilizing its function. In Comparative Example 2, the dry powder mixing failed to achieve uniform dispersion of the material, leading to ineffective bonding between the materials and a slight decrease in thermal conductivity compared to Example 1. The thermal conductivity of Comparative Example 3 could not be investigated due to its lack of a molded structure. In Comparative Example 4, the increased mass percentage of graphite powder resulted in a higher thermal conductivity compared to Example 1.

[0059] High-temperature leakage test: The sample was placed on filter paper and heated to 70°C in a drying oven. After holding at that temperature for 30 minutes, the oil stain area on the filter paper was used for evaluation (if oil penetration was observed, leakage occurred; if no obvious oil penetration was observed, leakage was minimal or nonexistent).

[0060] Figure 2 The figures show the high-temperature leakage test results of the composite phase change materials in Examples 1-3. (a) shows the sample before high-temperature treatment, and (b) shows the filter paper after removing the sample following high-temperature treatment. As can be seen from the figures, no obvious leakage was observed in the composite phase change materials of Examples 1-3, indicating that the material has good encapsulation effect and no leakage occurred.

[0061] Figure 3 The figures show the high-temperature leakage test results of the composite phase change materials in Comparative Examples 1-4. (a) shows the sample before high-temperature treatment, and (b) shows the filter paper after removing the sample following high-temperature treatment. As can be seen from the figures, the composite phase change material in Comparative Example 1 did not exhibit leakage. The composite phase change material in Comparative Example 2 showed a small amount of leakage due to uneven mixing. The composite phase change material in Comparative Example 3 collapsed because the graphite foam skeleton was not tightly connected due to the lack of resin. The composite phase change material in Comparative Example 4, lacking carbon nitride, could not completely contain octadecyl alcohol within its porous structure, resulting in some octadecyl alcohol leakage.

[0062] Figure 4 The images show SEM images of the graphite foam and the high thermal conductivity shaped composite phase change material prepared in Examples 1-3. (a) shows the graphite foam of Example 3, (b) shows the graphite foam of Example 2, (c) shows the graphite foam of Example 1, (d) shows the high thermal conductivity shaped composite phase change material of Example 3, (e) shows the high thermal conductivity shaped composite phase change material of Example 2, and (f) shows the high thermal conductivity shaped composite phase change material of Example 1. As can be seen from the images, comparing (a), (b), and (c), it can be observed that with the increase of ammonium bicarbonate content, the overall structure of the graphite foam becomes more porous. This allows for the redistribution and aggregation of a fixed amount of graphite powder between the pore walls, promoting the directional distribution and tight connection of graphite powder within the graphite foam framework, forming a more continuous and efficient thermally conductive network. Comparing (d), (e) and (f), it can be found that as the pore density increases, 1-octadecyl alcohol is able to achieve more complete and uniform filling due to the provision of richer and more continuous transport channels and accommodation space, and finally forms an exceptionally smooth and dense coating layer on the surface of the composite phase change material.

[0063] Figure 5The figures show the N2 adsorption-desorption isotherms and pore size distribution curves for carbon nitride and graphite foam in Examples 1-3. (a) shows the N2 adsorption-desorption isotherm for carbon nitride, (b) shows the pore size distribution curve for carbon nitride, (c) shows the N2 adsorption-desorption isotherm for graphite foam, and (d) shows the pore size distribution curve for graphite foam. As can be seen from the figures, (a) and (b) demonstrate that carbon nitride has an extremely high specific surface area. (c) and (d) show that when high-specific-surface-area carbon nitride is introduced into a low-specific-surface-area graphite matrix, the specific surface area of ​​the resulting composite material stabilizes in the range of 24.05-32.32 m² / g. Therefore, graphite foam can be effectively used as a supporting material for phase change materials.

[0064] Figure 6 The figures show the XRD diffraction patterns of 1-octadecyl alcohol, graphite powder, carbon nitride, and the composite phase change materials of Examples 1-3. As can be seen from the figures, no new diffraction peaks appeared in the composite phase change materials, indicating no chemical reaction was involved, and the materials were physically mixed during the preparation of the composite materials.

[0065] Figure 7 The figures show the heat storage / release temperature curves of 1-octadecyl alcohol and the composite phase change materials of Examples 1-3, where (a) is the heat storage temperature curve of 1-octadecyl alcohol, (b) is the heat release temperature curve of 1-octadecyl alcohol, (c) is the heat storage temperature curve of the composite phase change material, and (d) is the heat release temperature curve of the composite phase change material. As can be seen from the figures, the heat storage / release rate of the composite phase change material (CPCM5) is increased by 321.7% and 223.0% compared to 1-octadecyl alcohol, respectively. Typically, 1-octadecyl alcohol has a low thermal conductivity, therefore, there is no effective heat transfer channel during the heat storage / release process, resulting in a low heat storage / release rate. After adding the graphite foam skeleton, heat is rapidly transferred along the fixed heat conduction channels, greatly improving the heat storage / release rate of the material.

[0066] Figure 8The figures show the DSC curves of 1-octadecyl alcohol and the composite phase change materials of Examples 1-3, where (a) is the initial DSC curve of 1-octadecyl alcohol and the composite phase change materials of Examples 1-3, and (b) is the DSC curve of CPCM5 before and after 300 cycles. As can be seen from the figures, the decrease in latent heat of melting and solidification is inevitable with the addition of graphite foam. This is because graphite foam does not participate in the phase change process, leading to a reduction in the latent heat of the composite phase change material. However, as the proportion of graphite foam decreases, the latent heat of phase change continuously increases, and the reduction in latent heat is alleviated to some extent. Therefore, the composite phase change material still maintains a high heat storage capacity. After 300 cycles, the latent heat of melting and solidification of CPCM5 decreased by 0.44% and 0.60%, respectively, and the phase change temperature changed by +1.23℃ and -0.02℃. This indicates that the composite phase change material still has a high latent heat and a suitable phase change temperature after multiple cycles, exhibiting excellent thermal cycling stability.

[0067] In summary, the method for preparing the high thermal conductivity shaped composite phase change material of this invention effectively improves the thermal conductivity of the composite phase change material, exhibiting excellent heat storage / release rates and low leakage performance, indicating that this preparation method has good application prospects in solar heating systems. Graphite foam with a three-dimensional porous structure was prepared by mixing graphite powder, carbon nitride, and ammonium bicarbonate and using a pressing and drying method. By controlling the amount of ammonium bicarbonate added, a series of novel graphite foam matrices with different adsorption effects were prepared, successfully creating a porous matrix that combines high thermal conductivity and good encapsulation effect. This method helps the phase change material exhibit excellent thermal conductivity and leakage resistance, and also has great application potential in the fields of heat dissipation and heat storage. When the mass ratio of graphite powder and carbon nitride to ammonium bicarbonate is 1:5, the proportion of graphite foam prepared is 22%. The thermal conductivity of the composite phase change material is 3.12 W / (m·K), which is 940.0% higher than that of 1-octadecyl alcohol. The heat storage / release rate is 321.7% and 223.0% higher than that of 1-octadecyl alcohol, respectively. After 300 thermal cycling experiments, the phase change temperature and latent heat of phase change of the prepared composite material hardly changed, showing good thermal cycling stability.

[0068] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0069] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing graphite foam, characterized in that the steps include... include: Carbon nitride was obtained by calcining melamine and urea as reactants. Phenolic resin is added to ethanol and mixed evenly to obtain a phenolic resin-ethanol solution; The carbon nitride, graphite powder, and ammonium bicarbonate are added to the phenolic resin-ethanol solution, stirred, sonicated, and dried. The solution is then ground, sieved, pressed into shape, and subjected to segmented heating treatment to obtain the graphite foam.

2. The preparation method according to claim 1, characterized in that, The mass ratio of melamine to urea is 1:9; And / or, the calcination treatment is performed at a temperature of 550°C, a heating rate of 5°C / min, and a holding time of 3h; And / or, the mass ratio of the phenolic resin to ethanol is 1:105; And / or, the mixing speed is 500 rpm, the temperature is 40°C, and the time is 30 min.

3. The preparation method according to claim 1, characterized in that, The mass ratio of graphite powder to carbon nitride is 9:1; And / or, the mass ratio of the sum of the mass of the graphite powder and carbon nitride to the mass of ammonium bicarbonate is 1:3-5; And / or, the mass ratio of the sum of the masses of carbon nitride, graphite powder and ammonium bicarbonate to the mass ratio of the phenolic resin-ethanol solution is 12-19:15-16.

4. The preparation method according to claim 1, characterized in that, The stirring speed is 800 rpm, and the time is 1 hour; And / or, the temperature of the ultrasound is 40°C and the duration is 40 min; And / or, the drying is to remove ethanol by drying in a forced-air drying oven for 6 hours; And / or, the mesh size of the grinding and sieving is 80 mesh; And / or, the pressing pressure is 50 MPa and the holding time is 5 min; And / or, the segmented heating process involves holding the temperature at 80°C for 2 hours, then heating to 120°C and holding for 2 hours.

5. A graphite foam prepared by the preparation method according to any one of claims 1-4.

6. The application of the graphite foam of claim 5 as a phase change material carrier in a solar thermal storage system.

7. A highly thermally conductive, shape-stabilized composite phase change material, characterized in that, By mass percentage, the components include: The graphite foam of claim 5 comprises 22-32% and the balance being 1-octadecyl alcohol.

8. A method for preparing the high thermal conductivity shaped composite phase change material according to claim 7, comprising the following steps: Graphite foam is immersed in molten 1-octadecyl alcohol, and after vacuum adsorption and cooling, the high thermal conductivity shaped composite phase change material is obtained.

9. The preparation method according to claim 8, characterized in that, The vacuum adsorption was performed at a temperature of 80°C for 1 hour.

10. The application of the high thermal conductivity shaped composite phase change material as described in claim 7 in a solar thermal storage system.

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