Composite phase change material and preparation method therefor

Abstract

The present invention provides a composite phase change material and a preparation method therefor. The composite phase change material is a mixture of a phase change material, expanded graphite, and fine graphite; the phase change material is a material which has an enthalpy change exceeding 100 J / g during melting and solidification; the expanded graphite is low-density graphite of a porous structure; and the fine graphite fills pores of the expanded graphite, so that thermal conductivity between the pores of the expanded graphite is improved. In the formula of the present invention, the phase change material can solve the problem of energy storage, the expanded graphite builds a thermally conductive framework, the fine graphite serves as a mixing agent that mixes and introduces the phase change material into the expanded graphite and achieves uniform mixing between the phase change material and the expanded graphite without phase separation; and the phase change material, the expanded graphite, and the fine graphite are mixed to realize the most crucial point of the present invention.

Description

Composite phase change material and preparation method thereof

Technical field

[0001] The present invention relates to the technical field of phase change materials, and in particular to a composite phase change material and a preparation method thereof. [Background Technology]

[0002] With the global shortage of energy, phase change materials have been widely used in many fields, especially in thermal management and energy storage. The so-called phase change material is a type of material with heat conduction function that can use its own phase change to absorb or release a large amount of heat from the surrounding environment, thereby realizing thermal management function.

[0003] Phase change materials have three very critical parameters: 1. Phase change temperature: also known as the melting point. The higher the purity of the material, the higher the phase change temperature and the more stable the phase change temperature, especially the stability of the melting point of the material between batches; 2. Enthalpy: the energy change generated when the phase change material undergoes a phase change, such as the energy change during the transformation of the three forms: solid-liquid, gas-liquid, and solid-gas; 3. Thermal conductivity: the better the thermal conductivity of the material, the higher the efficiency of heat storage and release.

[0004] There are a series of problems in the preparation of phase change materials in the existing technology. For example, carbon materials such as graphene and carbon nanotubes have great potential and advantages as thermal conductive fillers due to their ultra-high thermal conductivity and low density. However, these materials are expensive and not suitable for industrial mass production needs.

[0005] However, using low-cost individual fine graphite for modification requires a very high proportion of graphite, which results in a serious reduction in the enthalpy value of the composite phase change material per unit weight, which in fact leads to a significant reduction in the energy storage efficiency of the phase change material.

[0006] In addition to the above problems, composite phase change materials with added graphite will lead to severe phase separation if they are in a molten state for a long time. In the molten state, the existing phase change materials have obvious upper and lower stratification, that is, the density of graphite is greater than that of the phase change material. Once it absorbs heat and melts into a liquid state, the graphite sinks to the bottom due to its high density, while the phase change material floats on the surface, resulting in uneven enthalpy values ​​and uneven thermal conductivity coefficients in space. At the same time, severe phase separation will gradually make it lose the meaning of a composite phase change material.

[0007] [Summary of the invention]

[0008] In view of the existing problems, the present invention proposes a composite phase change material that takes into account both thermal conductivity and enthalpy value, improves thermal conductivity and takes into account enthalpy value requirements, and does not phase separate during use, and can better ensure the stability of the phase change point.

[0009] The present invention uses expanded graphite as a skeleton and optimizes the mixing ratio of fine graphite and phase change material, the preparation process and the parameter setting to ensure that the performance of the composite phase change material reaches the best state.

[0010] Specifically, the present invention provides a composite phase change material, which includes a phase change material, expanded graphite and fine graphite; the phase change material is selected from an organic material or an inorganic material that has an enthalpy change of more than 100 J / g during melting and solidification; the pores in the skeleton formed by the expanded graphite are filled with a mixture of the fine graphite and the phase change material, and the mixture and the graphite layer of the expanded graphite generate convective heat exchange, and the heat is conducted along the pores in the skeleton.

[0011] The expanded graphite is a porous, low-density graphite used to build a thermally conductive skeleton; the mixture formed by mixing the fine graphite and the phase change material infiltrates the expanded graphite until the expanded graphite fully absorbs the mixture, thereby improving the thermal conductivity between the pores of the expanded graphite.

[0012] The phase change material is sorbitol, methyl octadecanoate, pentacosane or octadecanoic acid.

[0013] The mixing ratio of the fine graphite and the phase change material is 3% to 40%.

[0014] The mixing ratio of the fine graphite and the phase change material is 10% to 25%.

[0015] The fine graphite is equal to or greater than 5000 mesh, that is, less than or equal to 2.6 μm.

[0016] The expansion ratio of the expanded graphite is 300-360 times, and less than 40g of the expanded graphite is mixed in 1000g of the mixture of the fine graphite and the phase change material.

[0017] When the mixing ratio of the fine graphite to the phase change material is 10%, the enthalpy value is 216 J / g and the thermal conductivity is 2.07 W / (m·K);

[0018] When the mixing ratio of the fine graphite to the phase change material is 15%, the enthalpy value is 204 J / g and the thermal conductivity is 4.10 W / (m·K);

[0019] When the mixing ratio of the fine graphite to the phase change material is 20%, the enthalpy value is 192 J / g and the thermal conductivity is 6.87 W / (m·K);

[0020] When the mixing ratio of the fine graphite to the phase change material is 25%, the enthalpy value is 180 J / g and the thermal conductivity is 10.37 W / (m·K).

[0021] The fine graphite and the phase change material are melted and mixed in a proportion to obtain a liquid mixed material; the liquid mixed material is filled into the pores of the expanded graphite until the expanded graphite fully absorbs the liquid mixture; and the mixture is stirred at a temperature greater than the melting point of the phase change material to a temperature lower than the flash point of the phase change material to obtain the composite phase change material.

[0022] The pore size distribution of expanded graphite prepared by the high-temperature electric furnace method is 30 to 8000 nm. That is to say, the pore size of general expanded graphite is less than 8 μm. The diameter of the fine graphite of the present invention is less than 8 μm. However, since the internal voids of expanded graphite are not very uniform, in order to fill most of the voids with fine graphite, the fine graphite particle size selected in the present invention needs to be less than 3 μm. The diameter of 5000 mesh is 2.6 μm.

[0023] The composite phase change material involved in the present invention has a "framework"-like structure composed of expanded graphite, and the pores therein are filled with a mixture of fine graphite and phase change material. The heat conduction process is conducted along the "framework" path, with high thermal conductivity and high enthalpy value, making it suitable for use as an energy storage device. At the same time, because the expanded graphite solves the problem of thermal conductivity skeleton, while the fine graphite and phase change material solve the problems of efficient energy storage and phase separation, it can provide a relatively perfect composite phase change material suitable for industrial application in energy storage devices. It has many advantages such as low industrial cost, stable repeated phase change process, overcoming the phase separation problem, high enthalpy value, and high thermal conductivity efficiency.

[0024] Expanded graphite has a unique internal structure. First, it's obtained by chemically or physically treating natural flake graphite. During the expansion process, intercalation compounds are inserted into the graphite, increasing the distance between the graphite layers and creating an expanded state.

[0025] The skeleton structure of expanded graphite is porous and has a high specific surface area. These pores are formed by the spaces between graphite layers and the cracks formed during the expansion process. These pores not only provide more surface area but also facilitate rapid heat transfer.

[0026] Internally, expanded graphite's skeleton is composed of numerous graphite crystallites. These crystallites are interconnected by weak van der Waals forces, forming a sponge-like structure. This structure gives expanded graphite excellent elasticity and toughness, making it adaptable to a variety of complex applications.

[0027] In addition, the skeleton of expanded graphite has good thermal and chemical stability, which makes it remain stable under high temperature and strong oxidants, thus ensuring the durability of its thermal conductivity.

[0028] In summary, the internal structure of expanded graphite is porous, has a high specific surface area, is composed of graphite microcrystals, and has good thermal and chemical stability. These characteristics make expanded graphite have broad application prospects in fields such as thermal conductivity, adsorption, and catalysis.

[0029] The skeleton of expanded graphite provides a unique mechanism for its thermal conductivity. First, the basic graphite structure of expanded graphite is composed of continuous graphite layers, each composed of carbon atoms arranged in a hexagonal lattice to form a flat grid. This structure inherently provides excellent electrical and thermal conductivity.

[0030] Secondly, when expanded graphite is exposed to high temperatures, it forms a structure with multiple pores and a high specific surface area. These pores not only provide more transmission channels for heat, but also increase the specific surface area of ​​graphite, thereby improving its thermal conductivity.

[0031] In the thermal conductivity path, heat is primarily transferred through the expanded graphite through the high thermal conductivity of the graphite itself, resulting in convection heat transfer between the liquid mixture and the graphite layers within the expanded graphite pores. This convection heat transfer allows heat to flow more efficiently through the expanded graphite skeleton, achieving efficient heat conduction. Furthermore, the fine graphite enhances the thermal conductivity of the phase change material within the expanded graphite pores, thus achieving efficient heat conduction.

[0032] In addition, the skeleton structure of expanded graphite has a certain elasticity and can absorb external impact forces, which also helps to maintain the stability of the heat conduction path and prevent the degradation of thermal conductivity due to external factors.

[0033] In summary, the skeleton of expanded graphite provides an efficient heat transfer path for its thermal conductivity. The high thermal conductivity of graphite itself allows for convective heat transfer between the liquid mixture and the graphite layers within the pores. Furthermore, the fine graphite within the pores of expanded graphite acts as a heat transfer medium, enabling rapid heat transfer. This makes expanded graphite promising for broad applications in thermal management.

[0034] Micro-graphite is a high-quality carbon material with extremely fine particles, many unique properties and a wide range of applications.

[0035] First, fine graphite has excellent electrical conductivity, which makes it important in electronics and the manufacture of conductive materials. The π electrons in its layered structure move freely between layers, allowing electrons to flow freely in the material, thus achieving efficient conductivity.

[0036] Secondly, fine graphite also has good thermal conductivity. Due to the presence of van der Waals forces between graphite molecular layers, the bonding between layers is relatively weak, which is conducive to the rapid conduction of heat in the planar direction, thereby improving the heat dissipation performance of the material.

[0037] Micro-graphite also possesses lubricity and chemical stability. Its layered structure results in a low coefficient of friction between layers, thus providing lubrication. Furthermore, graphite remains relatively stable under most chemical conditions, enabling it to perform well in a variety of environments.

[0038] The particle size of the fine graphite selected in the present invention is greater than or equal to 5000 meshes, which fills the pores of the expanded graphite and improves the thermal conductivity between the pores of the expanded graphite.

[0039] In the present invention, expanded graphite is used as a skeleton, and inorganic or organic phase change materials and fine graphite are mixed and filled into the gaps of the expanded graphite skeleton. This combination will exhibit unique performance and broad application prospects.

[0040] First, as a skeleton material, the porous structure of expanded graphite provides a stable support and dispersion system for the organic phase change material and fine graphite. This structure not only helps to enhance the overall stability of the composite material, but also promotes uniform distribution and rapid transfer of heat within the material.

[0041] In particular, organic phase-change materials (PCMs) are materials that undergo phase changes in response to temperature changes, absorbing or releasing large amounts of heat. Their combination with finely divided graphite may further enhance the thermal performance of PCMs. Finely divided graphite, with its excellent electrical and thermal conductivity, can enhance the thermal conductivity of PCMs, leading to improved performance in thermal energy storage and regulation.

[0042] Furthermore, the porous structure of expanded graphite enhances the composite's adsorption properties. This makes the composite material potentially valuable for applications in environmental remediation, energy storage, and temperature control. For example, it can be used to prepare high-efficiency thermal energy storage materials, enabling efficient energy storage and on-demand release. It can also serve as an adsorbent to remove harmful substances from water or air.

Brief Description of the Drawings

[0043] FIG1 is a schematic diagram of an electron microscope of a double graphite phase change material according to the present application;

[0044] FIG2 is a schematic diagram of an electron microscope image of a conventional phase change material;

[0045] FIG3 is a line graph showing changes in enthalpy and thermal conductivity obtained by mixing fine graphite and octadecanoic acid in different proportions according to the present application.

[0046] Figure 4 shows the enthalpy values ​​and thermal conductivity coefficients of micro-graphite and phase change materials tested at different proportions.

[0047] FIG5 is a comparison photo of the existing phase change material and the phase change material of the present invention.

[0048] FIG. 6 is a top view showing comparative photographs of the conventional phase change material shown in FIG. 5 and the phase change material of the present invention. [Specific implementation method]

[0049] The present invention will be described in detail below with reference to the accompanying drawings and embodiments, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention and are not to be construed as limiting the present invention.

[0050] The present invention relates to a composite phase-change material comprising a mixture of a phase-change material, expanded graphite, and fine graphite. The key to the present invention is the construction of a system in which the phase-change material addresses energy storage, the expanded graphite forms a thermally conductive framework, and the fine graphite and phase-change material are mixed and introduced into the expanded graphite, whereby the mixture is uniformly mixed in the expanded graphite to prevent phase separation. This three-component mixture achieves the key point of the present invention. Furthermore, as shown in the graph of FIG3 , test experiments conducted according to the present invention clearly demonstrate that the ratio of the phase-change material to the fine graphite can be infinitely adjusted. This means that as the proportion of fine graphite increases, the thermal conductivity improves while the enthalpy decreases, allowing the system to be maintained within a balanced range according to specific requirements.

[0051] Phase change materials are organic or inorganic materials that have an enthalpy change of more than 100 J / g during melting and solidification. The pores in the skeleton composed of expanded graphite are filled with a mixture of fine graphite and phase change material. The mixture and the graphite layer of the expanded graphite generate convection heat transfer, and the heat is conducted along the pores in the skeleton.

[0052] The ratio we limit does not mean that the effect of this solution is only effective within this range, but rather that it has achieved a reasonable range with greater application value from an industrial perspective.

[0053] The phase change material is a material having an enthalpy change of more than 100 J / g during melting and solidification;

[0054] The expanded graphite is a porous, low-density graphite;

[0055] The particle size of the fine graphite is greater than or equal to 5000 mesh, that is, less than or equal to 2.6 μm, and fills the pores of the expanded graphite to improve the thermal conductivity between the pores of the expanded graphite.

[0056] When the fine graphite and the phase change material are mixed, the mass fraction of the fine graphite is 3% to 40%, and the phase change material makes up the balance, and the total is 100%.

[0057] Furthermore, a better solution is that the mass fraction of the fine graphite is 10% to 25%.

[0058] The attached table shows that when the proportion of fine graphite is 10%, the enthalpy of the composite phase change material is 216 J / g and the thermal conductivity is 2.07 W / (m·K). The enthalpy decreases by 10%, while the thermal conductivity is about 10 times that of the original phase change material. When the proportion of fine graphite is 25%, the enthalpy of the composite phase change material is 180 J / g and the thermal conductivity is 10.37 W / (m·K). The enthalpy decreases by 25%, while the thermal conductivity increases by almost 50 times.

[0059] The phase change material is a material that can undergo thermal changes during the melting and solidification process, including but not limited to alkanes, organic acids, esters and alcohols.

[0060] The phase change material is sorbitol, methyl octadecanoate, pentacosane or octadecanoic acid.

[0061] When the phase change material is octadecanoic acid, its thermal conductivity is 0.21 W / (m·K). The phase change material selected here is octadecanoic acid, which is a synthetic product. Therefore, its composition is very clear and single, its melting point is very stable, and there is basically no difference between different batches. It has excellent stability and is very suitable for use in industrial standardized production lines.

[0062] When the phase change material is an alkane, it can be considered that the phase change material is pentacosane, which has a thermal conductivity of 0.20-0.22 W / (m·K). The phase change material selected here is pentacosane, which is a commonly used material in the industry and has stable chemical properties.

[0063] When the phase change material is an ester, methyl octadecanoate can be considered, and its thermal conductivity is 0.1-0.2 W / (m·K). The phase change material selected here is methyl octadecanoate, which is a synthetic product with low price. Due to artificial synthesis, batch purity control and stability are good.

[0064] When the phase change material is an alcohol, sorbitol can be considered, which has a thermal conductivity of 0.13-0.17 W / (m·K). The phase change material we selected here is sorbitol, which is a synthetic product, so its composition is very clear and single, its melting point is very stable, there is basically no difference between different batches, and its stability is very good, which is very suitable for use in industrial standardized production lines.

[0065] The expanded graphite has different expansion ratios, and the pores of the expanded graphite are filled with the phase-change material containing fine graphite. The expansion ratio is related to the size of the pores between the graphite, which in turn determines the amount of the mixture of fine graphite and phase-change material that can be filled and adsorbed. The expansion ratio of expanded graphite ranges from 300 to 360 times. In a specific experiment, a maximum of 40g of expanded graphite was added to 1000g of the mixture of fine graphite and phase-change material.

[0066] Therefore, the amount and proportion of expanded graphite cannot be numerically limited, nor is it necessary to do so. The key is to use the expanded graphite until it can absorb the maximum amount of the mixture and no more.

[0067] Specifically, the composite phase change material includes: a mixture of phase change material, expanded graphite and fine graphite;

[0068] The phase change material is a material that can undergo thermal changes during the melting and solidification process, including but not limited to alkanes, organic acids, esters and alcohols.

[0069] The phase change material includes but is not limited to sorbitol, methyl octadecanoate, pentacosane or octadecanoic acid;

[0070] The phase change material is a material having an enthalpy change of more than 100 J / g during melting and solidification;

[0071] The expanded graphite is a porous, low-density graphite;

[0072] The fine graphite fills the pores of the expanded graphite and improves the thermal conductivity between the pores of the expanded graphite.

[0073] The mass fraction of the fine graphite is 3% to 40%.

[0074] Furthermore, a better solution is that the mass fraction of the fine graphite is 10% to 25%.

[0075] The phase change material is octadecanoic acid, and its thermal conductivity is 0.21 W / (m·K).

[0076] The expanded graphite has different expansion multiples, and the pores of the expanded graphite are filled with a phase change material containing fine graphite.

[0077] A method for preparing a composite phase change material, characterized in that:

[0078] S1: preparing the phase change material, expanded graphite and fine graphite according to a predetermined ratio;

[0079] S2: Mixing fine graphite and phase change material in a ratio of 3% to 40% by mass of fine graphite and the remaining amount of phase change material, melting and mixing to obtain a liquid mixed material;

[0080] S3: adding the liquid mixed material in S2 to the expanded graphite and mixing until the expanded graphite fully absorbs the liquid mixture;

[0081] S4: Stirring for 4-6 hours at a temperature greater than the melting point of the phase change material until the temperature is lower than the flash point of the phase change material;

[0082] S5: Obtain the composite phase change material and keep it for future use.

[0083] Furthermore, the expanded graphite can be prepared by oneself or purchased.

[0084] The preparation process is as follows: first, graphite is selected, an oxidant and an intercalant are added to the graphite, expandable graphite is obtained through reaction, and finally the expandable graphite is placed in a high temperature environment above 300°C for heating to obtain expanded graphite, which is then cooled for standby use.

[0085] Furthermore, if a better mixing effect is required, a vacuum adsorption step can be added before step S3, wherein the vacuum adsorption step is as follows: the expanded graphite is placed in a vacuum reactor, the reactor is started to extract the air in the pores of the expanded graphite, and while maintaining the vacuum in the reactor, the liquid mixed material in step S3 is added to the vacuum reactor and infiltrated on the expanded graphite. The mixing process described in step S4 is completed under vacuum conditions, and the expanded graphite can no longer absorb more liquid mixture.

[0086] Furthermore, the material after cooling in step S5 is uniformly crushed by a crusher.

[0087] Example 1:

[0088] Materials: 40g graphite, 750g (75% by weight) of pentacosane with a thermal conductivity of 0.20-0.22W / (m·K) as a phase change material, and 250g (25% by weight) of 5000 mesh fine graphite.

[0089] Prepare expanded graphite by adding an oxidant and an intercalating agent to 40 g of graphite to obtain expandable graphite, and finally heat the expandable graphite at a high temperature of 300° C. to obtain expanded graphite, and cool it for later use;

[0090] The expanded graphite is placed in a vacuum reactor with a vacuum degree of -75kPa to extract the air from the pores of the expanded graphite.

[0091] 250 g (25% by mass) of 5000 mesh fine graphite and 750 g (75% by mass) of pentacosane with a thermal conductivity of 0.20-0.22 W / (mK) were melted and mixed at 100° C. to obtain a liquid mixed material.

[0092] While maintaining the vacuum in the reactor, the liquid mixed material is added to the vacuum reactor and infiltrated onto the expanded graphite, allowing the liquid mixed material to fill the pores of the expanded graphite until the expanded graphite can no longer absorb the liquid mixture. The mixture is stirred at 100° C. to 110° C. for 4 hours to keep the temperature below the flash point of the phase change material, 127° C., to obtain the composite phase change material having a thermal conductivity of 9.8 W / (m·K).

[0093] Example 2:

[0094] Materials: 40g graphite, 600g (60% by mass) of methyl octadecanoate with a thermal conductivity of 0.1-0.2W / (m·K) as a phase change material, and 400g (40% by mass) of 5000 mesh fine graphite;

[0095] Prepare expanded graphite by adding an oxidant and an intercalating agent to 40 g of graphite to obtain expandable graphite, and finally heat the expandable graphite at a high temperature of 300° C. to obtain expanded graphite, and cool it for later use;

[0096] The expanded graphite is placed in a vacuum reactor with a vacuum degree of -75kPa to extract the air from the pores of the expanded graphite.

[0097] 400 g (40% by mass) of 5000 mesh fine graphite and 600 g (60% by mass) of methyl octadecanoate with a thermal conductivity of 0.1-0.2 W / (m·K) are mixed and melted at 100° C. to obtain a liquid mixed material.

[0098] The composite phase change material is obtained by stirring at 100° C. to 110° C. for 5 hours to lower the temperature to 169° C., which is a flash point of the phase change material, and has a thermal conductivity of 26.2 W / (m·K).

[0099] Example 3:

[0100] Sorbitol has unique phase transition properties. At room temperature and pressure, sorbitol is solid. However, as the temperature rises, it undergoes a phase transition from solid to liquid. This phase transition temperature, often called the melting point, is a key parameter in sorbitol's physical properties. During this phase transition, sorbitol absorbs heat and gradually transforms from solid to liquid. This phase transition is reversible; that is, when the liquid sorbitol cools, it transforms back into a solid.

[0101] Materials: 40g graphite, 80g (80% by weight) of sorbitol with a thermal conductivity of 0.13-0.17W / (m·K) as a phase change material, and 20g (20% by weight) of 5000 mesh fine graphite;

[0102] Prepare expanded graphite by adding an oxidant and an intercalating agent to 40 g of graphite to obtain expandable graphite, and finally heat the expandable graphite at a high temperature of 300° C. to obtain expanded graphite, and cool it for later use;

[0103] Expanded graphite was placed in a vacuum reactor at -75 kPa to remove air from the pores of the expanded graphite. 20 g (20% by weight) of 5000-mesh fine graphite and 80 g (80% by weight) of methyl octadecanoate with a thermal conductivity of 0.1-0.2 W / (m·K) were mixed and melted at 100°C to 110°C to obtain a liquid mixed material.

[0104] The composite phase change material is obtained by stirring at 100° C. to 110° C. for 5 hours to lower the temperature to 182° C., which is a flash point of the phase change material, and has a thermal conductivity of 7.2 W / (m·K).

[0105] Example 4:

[0106] Prepare 40g of graphite phase change material: 900g (90% by mass) of octadecanoic acid with a thermal conductivity of 0.21W / (m·K) and 100g (10% by mass) of 5000 mesh fine graphite.

[0107] Prepare expanded graphite by adding an oxidant and an intercalating agent to 40 g of graphite to obtain expandable graphite, and finally heat the expandable graphite at a high temperature of 300° C. to obtain expanded graphite, and cool it for later use;

[0108] The expanded graphite is placed in a vacuum reactor with a vacuum degree of -75kPa to extract the air from the pores of the expanded graphite.

[0109] 100 g (10% by mass) of 5000 mesh fine graphite and 900 g (90% by mass) of 0.21 W / (m·K) octadecanoic acid were melted and mixed at 100° C. to 110° C. to obtain a liquid mixed material.

[0110] While maintaining the vacuum in the reactor, the liquid mixed material is added into the vacuum reactor and soaked on the expanded graphite, allowing the liquid mixed material to fill the pores of the expanded graphite until the expanded graphite can no longer absorb more liquid mixture.

[0111] The composite phase change material is obtained by stirring at 100° C. to 110° C. for 5 hours to lower the temperature to 212° C., which is the flash point of the phase change material, and has a thermal conductivity of 2.07 W / (m·K).

[0112] Example 5:

[0113] Prepare 40g of graphite phase change material: 800g (80% by mass) of octadecanoic acid with a thermal conductivity of 0.21W / (m·K) and 200g (20% by mass) of 5000 mesh fine graphite.

[0114] Prepare expanded graphite by adding an oxidant and an intercalating agent to 40 g of graphite to obtain expandable graphite, and finally heat the expandable graphite at a high temperature of 300° C. to obtain expanded graphite, and cool it for later use;

[0115] The expanded graphite is placed in a vacuum reactor with a vacuum degree of -75kPa to extract the air from the pores of the expanded graphite.

[0116] 200 g (20% by mass) of 5000 mesh fine graphite and 800 g (80% by mass) of 0.21 W / (m·K) octadecanoic acid were melted and mixed at 100° C. to 110° C. to obtain a liquid mixed material.

[0117] While maintaining the vacuum in the reactor, the liquid mixed material is added into the vacuum reactor and soaked on the expanded graphite, allowing the liquid mixed material to fill the pores of the expanded graphite until the expanded graphite can no longer absorb more liquid mixture.

[0118] The composite phase change material is obtained by stirring at 100° C. to 110° C. for 5 hours to lower the temperature to 212° C., which is a flash point of the phase change material, and has a thermal conductivity of 6.87 W / (m·K).

[0119] The enthalpy and thermal conductivity test data of different proportions of fine graphite and octadecanoic acid phase change materials are shown in Figure 4, where:

[0120] When the ratio of fine graphite to phase change material is 10%, the enthalpy value is 216 J / g and the thermal conductivity is 2.07 W / (m·K);

[0121] When the ratio of fine graphite to phase change material is 15%, the enthalpy value is 204 J / g and the thermal conductivity is 4.10 W / (m·K);

[0122] When the ratio of fine graphite to phase change material is 20%, the enthalpy value is 192 J / g and the thermal conductivity is 6.87 W / (m·K);

[0123] When the ratio of fine graphite to phase change material is 25%, the enthalpy value is 180 J / g and the thermal conductivity is 10.37 W / (m·K);

[0124] When the ratio of fine graphite to phase change material is 3%, the enthalpy value is 232.8 J / g and the thermal conductivity is 0.46 W / (m·K);

[0125] When the ratio of fine graphite to phase change material is 30%, the enthalpy value is 168 J / g and the thermal conductivity is 14.60 W / (m·K);

[0126] When the ratio of fine graphite to phase change material is 40%, the enthalpy value is 144J / g and the thermal conductivity is 25.27W / (m·K).

[0127] Based on the data obtained from the test results, a line graph of the enthalpy and thermal conductivity changes of the composite phase change material at different ratios can be generated. Please refer to Figure 3, which shows the enthalpy and thermal conductivity changes obtained by mixing octadecanoic acid and fine graphite at different ratios. It can be seen that in actual production, in order to ensure the energy storage effect of the phase change material, the higher the enthalpy value (greater than 180 J / g) and the higher the thermal conductivity (greater than 2 W / (m·K)), the lower the cost and the best effect are selected.

[0128] Experimental model calculations show that the actual enthalpy value is the enthalpy value of the fine graphite multiplied by the addition ratio during mixing. The enthalpy value decreases as the fine graphite and phase change material are mixed in different proportions. The thermal conductivity increases nonlinearly, increasing according to the addition ratio when the fine graphite and phase change material are mixed. In other words, when fine graphite and phase change material are mixed in different proportions, the enthalpy value decreases and the thermal conductivity increases. The optimal mixing ratio, which satisfies the requirements of an enthalpy value greater than 180 J / g and a thermal conductivity greater than 2 W / (m·K), is 10% to 25%.

[0129] For example, if the original enthalpy of octadecanoic acid is 240 J / g and 50% of fine graphite is added, the enthalpy of the composite material is 240*50%=120 J / g, and the thermal conductivity will also be high, but it must be between 0.21-151 W / (m·K), while the thermal conductivity is approximately =146.89*50%^2+3.9049*50%+0.21=38.88 W / (m·K).

[0130] FIG1 and FIG2 are electron microscope micrographs showing the microstructures of the composite phase change material according to the present application and the original phase change material.

[0131] Figure 1 clearly shows a skeleton made of expanded graphite, with the pores filled with a mixture of fine graphite and phase change material. Heat conduction occurs along the internal path of the skeleton, resulting in extremely high thermal conductivity. The lower portion of Figure 1 shows expanded graphite, with a thermal conductivity of 151 W / (m·K).

[0132] The top portion of Figure 1 shows a mixture of fine graphite and phase change material. While the thermal conductivity is relatively low, the distance between the fine graphite and the framework is minimal, resulting in minimal thermal resistance. The framework maintains this relatively large distance, ensuring overall thermal conductivity.

[0133] Figure 2 shows the microstructure of the original phase change material, in which the phase change material and graphite are evenly mixed. The whole is in a chaotic and intertwined state, and the conduction path is disordered, resulting in poor thermal conductivity.

[0134] FIG5 and FIG6 are comparative photographs of the phase change material in the prior art and the phase change material of the present invention. The left side is the prior art, which shows phase separation and stratification; the right side is the present invention, which does not show phase separation.

[0135] The composite phase change material involved in the present invention has high thermal conductivity and high enthalpy value, and is suitable for use as an energy storage device. At the same time, since the expanded graphite solves the problem of the thermal conductive skeleton, and the fine graphite and phase change material solve the problems of efficient energy storage and phase separation, it can provide a relatively perfect composite phase change material suitable for industrial application in energy storage devices. It has many advantages such as low industrial cost, stable repeated phase change process, overcoming the phase separation problem, high enthalpy value, and high thermal conductivity efficiency.

[0136] As described above, it is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention is disclosed as above with a preferred embodiment, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modifications to equivalent embodiments of equivalent changes by using the technical content disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modifications, equivalent changes and modifications made to the above embodiments according to the technology of the present invention are all within the scope of the technical solution of the present invention without departing from the content of the technical solution of the present invention.

Claims

1. A composite phase change material, characterized in that: The composite phase change material comprises a phase change material, expanded graphite and fine graphite; the phase change material is selected from an organic material or an inorganic material having an enthalpy change of more than 100 J / g during melting and solidification; the pores in the skeleton formed by the expanded graphite are filled with a mixture of the fine graphite and the phase change material, and the mixture and the graphite layer of the expanded graphite generate convective heat exchange, and the heat is conducted along the pores in the skeleton.

2. The composite phase change material according to claim 1, characterized in that: The expanded graphite is a porous, low-density graphite used to construct a heat-conducting skeleton; the mixture formed by mixing the fine graphite and the phase change material infiltrates the expanded graphite until the expanded graphite fully absorbs the mixture.

3. The composite phase change material according to claim 1 or 2, characterized in that: The phase change material is sorbitol, methyl octadecanoate, pentacosane or octadecanoic acid.

4. The composite phase change material according to claim 1 or 2, characterized in that: The mixing ratio of the fine graphite and the phase change material is 3% to 40%.

5. The composite phase change material according to claim 4, characterized in that: The mixing ratio of the fine graphite to the phase change material is 10% to 25%.

6. The composite phase change material according to claim 1 or 2, characterized in that: The expansion multiple of the expanded graphite is 300-360 times.

7. The composite phase change material according to claim 1 or 2, characterized in that: Less than 40 g of the expanded graphite is mixed into 1000 g of the mixture of the fine graphite and the phase change material.

8. The composite phase change material according to claim 1 or 2, characterized in that: When the mixing ratio of the fine graphite to the phase change material is 10%, the enthalpy value is 216 J / g and the thermal conductivity is 2.07 W / (m·K); when the mixing ratio of the fine graphite to the phase change material is 15%, the enthalpy value is 204 J / g and the thermal conductivity is 4.10 W / (m·K); when the mixing ratio of the fine graphite to the phase change material is 20%, the enthalpy value is 192 J / g and the thermal conductivity is 6.87 W / (m·K); or when the mixing ratio of the fine graphite to the phase change material is 25%, the enthalpy value is 180 J / g and the thermal conductivity is 10.37 W / (m·K).

9. The method for preparing the composite phase change material according to claims 1-8, characterized in that: The fine graphite and the phase change material are melted and mixed in a proportion to obtain a liquid mixed material; the liquid mixed material is filled into the pores of the expanded graphite until the expanded graphite fully absorbs the liquid mixture; and the mixture is stirred under a temperature greater than the melting point of the phase change material to make the temperature lower than the flash point of the phase change material to obtain the composite phase change material.

10. The preparation method according to claim 9, characterized in that: 25% by weight of 5000 mesh fine graphite and 75% by weight of pentacosane with a thermal conductivity of 0.20-0.22 W / (m·K) are melted and mixed at 100° C. to obtain a liquid mixed material. While maintaining the vacuum state of the reactor, the liquid mixed material is added into a vacuum reactor and infiltrated on the expanded graphite to allow the liquid mixed material to fill the pores of the expanded graphite. The mixture is stirred at 100° C. to 110° C. to make the temperature lower than the flash point of the phase change material, 127° C., to obtain a composite phase change material with a thermal conductivity of 9.8 W / (m·K).

11. The preparation method according to claim 9, characterized in that: 40% by weight of 5000 mesh fine graphite and 60% by weight of methyl octadecanoate with a thermal conductivity of 0.1-0.2 W / (m·K) are melted and mixed at 100° C. to 110° C. to obtain a liquid mixed material. While maintaining the vacuum state of the reactor, the liquid mixed material is added into a vacuum reactor and infiltrated on the expanded graphite to allow the liquid mixed material to fill the pores of the expanded graphite. The composite phase change material with a thermal conductivity of 26.2 W / (m·K) is obtained by stirring at 100° C. to 110° C. to make the temperature lower than the flash point of the phase change material, 169° C., to obtain a composite phase change material with a thermal conductivity of 26.2 W / (m·K).

12. The preparation method according to claim 9, characterized in that: 20% by weight of 5000 mesh fine graphite and 80% by weight of methyl octadecanoate with a thermal conductivity of 0.1-0.2 W / (m·K) are melted and mixed at 100°C to 110°C to obtain a liquid mixed material. While maintaining the vacuum state of the reactor, the liquid mixed material is added into a vacuum reactor and infiltrated on the expanded graphite to allow the liquid mixed material to fill the pores of the expanded graphite. The composite phase change material with a thermal conductivity of 7.2 W / (m·K) is obtained by stirring at 100°C to 110°C to make the temperature lower than the flash point of the phase change material, 182°C.

13. The preparation method according to claim 9, characterized in that: 10% by weight of 5000 mesh fine graphite and 90% by weight of 0.21 W / (m·K) octadecanoic acid are melted and mixed at 100°C to 110°C to obtain a liquid mixed material, which is stirred at 100°C to 110°C to make the temperature lower than the flash point of the phase change material 212°C to obtain a composite phase change material with a thermal conductivity of 2.07 W / (m·K).

14. The preparation method according to claim 9, characterized in that: 20% by weight of 5000 mesh fine graphite and 80% by weight of 0.21 W / (m·K) octadecanoic acid are melted and mixed at 100° C. to 110° C. to obtain a liquid mixed material; while maintaining the vacuum state of the reactor, the liquid mixed material is added into a vacuum reactor, infiltrated on the expanded graphite, and the liquid mixed material is allowed to fill the pores of the expanded graphite; the composite phase change material with a thermal conductivity of 6.87 W / (m·K) is obtained by stirring at 100° C. to 110° C. to make the temperature lower than the flash point of the phase change material 212° C.

15. The preparation method according to claim 9, characterized in that: An oxidant and an intercalant are added to graphite to obtain expandable graphite, the expandable graphite is placed at a high temperature of 300° C. for heating to obtain expanded graphite, and the expanded graphite is cooled for standby use; the expanded graphite is placed in a vacuum reactor with a vacuum degree of -75 kPa, and air in the pores of the expanded graphite is extracted.

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