Graphite / silicon carbide composite phase change energy storage unit and preparation method thereof

Abstract

The application discloses a graphite / silicon carbide composite phase change energy storage unit and a preparation method thereof. The graphite / silicon carbide composite phase change energy storage unit is prepared by the following basic processes: (1) 3D printing of mixed powder; (2) selective laser sintering of a phase change energy storage unit blank; (3) multiple carbonization and vacuum pressure impregnation densification pretreatment; (4) molten silicon infiltration sealing strengthening treatment; (5) loading of high-temperature molten salt; and (6) packaging. The graphite / silicon carbide composite phase change energy storage unit provided by the application has the advantages of simple preparation process, low cost, easy industrial production, high-temperature resistance, corrosion resistance, good mechanical properties, good packaging performance, high loading rate and the like. The existence of the three-dimensional porous graphite skeleton improves the energy conversion efficiency and heat transfer uniformity, and multiple phase change energy storage units are assembled together to form an energy storage device, which is expected to be applied to peak load shifting, waste heat recycling and reuse and periodic new energy rapid storage and release.

Description

Technical Field

[0001] This invention relates to a graphite / silicon carbide composite phase change energy storage unit and its preparation method, belonging to the technical field of inorganic non-material composite forming and high-efficiency phase change energy storage device manufacturing. Background Technology

[0002] Energy storage technology has garnered widespread attention due to its ability to coordinate energy supply and demand, peak shaving and valley filling, and the effective allocation and utilization of renewable energy. Among these technologies, phase change energy storage (PCS) technology primarily relies on the heat absorption and release characteristics of PCS materials during the phase change process. Compared to other energy storage methods, it boasts advantages such as small volume change during the phase change process, high energy density, and near-isothermal temperature during the phase change process, making it a hot topic. High-temperature molten salts, as heat storage and release PCS materials, are highly favored due to their large latent heat of phase change, low price, low viscosity, and excellent thermal stability. However, the low thermal conductivity, strong corrosiveness, difficulty in encapsulation, and susceptibility to leakage of high-temperature molten salts limit the industrial application of PCS energy storage devices. The key to designing and manufacturing high-temperature molten salt PCS thermal storage units lies in the selection of encapsulation materials and how to achieve rapid and uniform heat storage / release while improving heat exchange efficiency.

[0003] Common high-temperature encapsulation materials include metals and ceramics. Metals such as gold and platinum have good corrosion resistance at high temperatures, but their high price limits their widespread application. Stainless steel is relatively inexpensive and has some corrosion resistance, but its poor thermal conductivity limits its improvement on the thermal conductivity of high-temperature molten salts. Ceramic materials have stable chemical properties, good high-temperature and corrosion resistance, and excellent mechanical and thermal conductivity, making them ideal encapsulation materials for phase change energy storage. However, ceramic materials have poor thermal shock resistance and are brittle, making them prone to breakage and leakage. Designing and manufacturing an efficient three-dimensional heat-conducting network is an important means to improve the heat exchange efficiency of phase change energy storage units. An efficient three-dimensional heat-conducting structure can uniformly and quickly transfer heat to all parts of the high-temperature molten salt, greatly accelerating the overall melting rate and effectively improving energy exchange efficiency. However, at present, most phase change energy storage devices mainly enhance heat exchange by adding simple structures such as metal fins and heat pipes, with unsatisfactory heat exchange effects. Furthermore, traditional processes such as extrusion, turning, and welding are insufficient to construct complex and efficient three-dimensional heat-conducting networks.

[0004] Currently, phase change energy storage unit encapsulation technologies mainly fall into two categories: microencapsulation and macroencapsulation. Microencapsulation primarily refers to the microencapsulation of high-temperature phase change thermal storage materials with a maximum size of less than 1 mm. It offers advantages such as high latent heat of phase change and wide applicability. However, the microcapsule preparation process is generally complex and costly, and the shell thickness is difficult to control precisely, hindering industrialization. Macroencapsulation mainly targets high-temperature phase change thermal storage materials with a size greater than 1 mm, including large capsule preparation and tubular encapsulation. Macroencapsulation of high-temperature phase change materials offers advantages such as better thermal cycling performance and less brittle shells. However, it also suffers from disadvantages such as complex preparation processes and low latent heat storage. Furthermore, the phase change thermal storage material is prone to bonding with metals, leading to rapid degradation of thermal storage performance and poor thermal cycling performance.

[0005] In the research of phase change energy storage heat transfer enhancement technology, existing technologies mainly focus on two aspects: the phase change material itself and the heat storage system. These include improving the thermal conductivity of the phase change material, expanding the contact area between the phase change material and the heat source, and increasing the temperature difference between the heat source and the phase change material. Currently, improving the thermal conductivity of phase change materials is mainly achieved by adding highly thermally conductive fillers (such as metal / non-metal nanoparticles, metal foam, expanded graphite, etc.). This process is simple to operate, but because the thermally conductive fillers do not form a continuous thermally conductive network, a large amount of thermal resistance exists, thus the improvement in thermal conductivity is very limited. Expanding the heat transfer area can directly improve heat exchange performance, and this is currently mainly achieved by adding metal fins or by unitizing the phase change material through encapsulation. Furthermore, the temperature decreases after heat exchange between the heat source and the phase change material, leading to problems such as slower heat transfer efficiency in the latter half of the heat transfer tube, uneven melting of the phase change material, and low heat storage / release efficiency. Currently, multi-stage phase change materials are mainly used to improve heat transfer uniformity, which undoubtedly increases the difficulty of phase change material development.

[0006] In summary, current methods for preparing high-temperature molten salt phase change energy storage units still suffer from many shortcomings, such as slow heat storage / release rates, poor heat exchange performance, uneven heat transfer, and easy leakage, making it difficult to meet the requirements of industrial applications. Therefore, it is urgent to invent new high-temperature molten salt phase change energy storage units and their preparation methods to achieve the synergistic effect of the above-mentioned multiple objectives. Summary of the Invention

[0007] To address the problems of poor thermal conductivity, high phase change temperature, strong corrosivity, and difficult encapsulation of high-temperature molten salts, this invention proposes a graphite / silicon carbide composite phase change energy storage unit and its preparation method. The main inventive ideas are as follows: High-temperature resistant and corrosion-resistant graphite / silicon carbide is selected as the high-temperature molten salt encapsulation material; considering the poor processability and difficulty in device fabrication of graphite and silicon carbide ceramic powders, based on the development of graphite 3D printing materials, selective laser sintering technology is used to shape them, obtaining a graphite / silicon carbide composite phase change energy storage unit; to improve the heat exchange capacity of the phase change energy storage unit, a corrugated surface is "constructed" on its exterior to increase the heat exchange area, and a three-dimensional high thermal conductivity skeleton is designed and manufactured inside to achieve rapid heat release, improve heat exchange efficiency, and ensure heat transfer uniformity; it is proposed to densify it by repeatedly impregnating it with liquid phenolic resin under vacuum pressure, and eliminate internal pores through high-temperature melt silicon infiltration treatment (silicon reacts in situ with carbon at high temperature to generate silicon carbide), thereby improving the mechanical properties, thermal conductivity, and sealing performance of the graphite ceramic composite energy storage unit; the internal pores of the three-dimensional high thermal conductivity skeleton are used to "fill" the high-temperature molten salt, and its storage capacity can be adjusted by changing the structure of the three-dimensional high thermal conductivity skeleton to ensure energy storage density.

[0008] The graphite / silicon carbide composite phase change energy storage unit consists of a housing and a cover. A highly thermally conductive porous framework is designed and manufactured inside the housing, directly connected to the housing to achieve rapid heat storage and output, improving heat storage efficiency. Low thermal conductivity high-temperature molten salt is filled within it. Multiple graphite ceramic-based phase change energy storage units are assembled together to form a phase change energy storage system, with a heat pipe network arranged between them for heat exchange. The fabrication process includes 3D printing of mixed powder preparation, selective laser sintering of the phase change energy storage single-element blank, multiple vacuum pressure impregnation, carbonization, molten silicon infiltration, filling with high-temperature molten salt, and encapsulation.

[0009] The present invention has the following advantages: (1) The phase change energy storage unit consists of a box, a box cover, a porous graphite skeleton, and a phase change material. The porous graphite skeleton is integrated with the box and forms a continuous three-dimensional heat conduction path inside the phase change energy storage unit, eliminating the interfacial contact thermal resistance and improving the heat conduction effect. The outer wall of the box is designed with a corrugated surface to increase the heat exchange area and improve the heat exchange efficiency.

[0010] (2) The phase change material is filled in the pores of the porous graphite skeleton. By optimizing the structural parameters of the porous graphite skeleton, the amount of phase change material stored can be controlled. At the same time, the area and number of heat exchange interfaces can be changed. Heat can be quickly conducted to the interior of the phase change energy storage unit through the porous graphite skeleton, realizing the rapid storage and release of thermal energy and ensuring the uniformity of temperature field distribution.

[0011] (3) Silicon carbide ceramics are controllably composited into natural flake graphite to ensure that the phase change energy storage unit has high temperature resistance, corrosion resistance, and good thermal conductivity and sealing performance.

[0012] (4) Compared with the traditional phase change energy storage unit forming method, selective laser sintering technology and its post-processing technology can be used to quickly prepare any complex phase change energy storage unit, providing a new technology for the preparation of complex phase change energy storage units, solving the technical problem of easy leakage of high temperature molten salt, and achieving the synergistic effect of multiple technical goals such as high storage capacity and efficient heat exchange by constructing a three-dimensional heat conduction network skeleton inside the phase change thermal energy storage unit.

[0013] The implementation process of this invention is as follows: (1) Structural design of phase change energy storage unit The phase change energy storage unit consists of a box, a box cover, a porous graphite skeleton connected to the inner wall of the box, and high-temperature molten salt. The box cover and the box are connected as one unit by a high-temperature adhesive. The porous graphite skeleton is evenly distributed inside the box, and the high-temperature molten salt fills the pores. The phase change treatment loading is not less than 60%. The outer surface of the box and the box cover is designed with grooves to obtain a corrugated surface, which helps to increase the heat exchange area.

[0014] The enclosure is a cube or cuboid with one open end. Both the lid and the enclosure have stepped surfaces to ensure they fit together. The enclosure wall thickness is 10–30 mm. The outer surfaces of the enclosure and lid have semi-circular grooves with a radius of 1–3 mm. The porous graphite skeleton is an air-ball structure, a Kelvin structure, or a diamond-like structure, with a minimum solid feature size controlled at 3–5 mm. (See attached image) Figure 1 .

[0015] (2) Preparation of 3D printing mixed powder To improve the thermal conductivity and mechanical properties of graphite ceramic-based phase change energy storage units, an appropriate amount of high-purity silicon powder is added to the material formulation. At high temperature, it undergoes an in-situ sintering reaction with glassy carbon (the residue after carbonization of thermosetting phenolic resin) to generate a silicon carbide phase, which, together with the glassy carbon, connects with the natural flake graphite powder.

[0016] The preparation of 3D printing hybrid powder includes processes such as coating pretreatment, drying, crushing, sieving, and powder mixing. The graphite 3D printing hybrid powder composition is as follows: 50-70% by weight of natural flake graphite powder (carbon content greater than 99.5%, particle size 100-250 mesh), 20-40% by weight of spherical graphite powder (carbon content greater than 99%, particle size 600-3000 mesh), and 10-20% by weight of high-purity silicon powder (purity greater than 99%, particle size 200-300 mesh). Natural flake graphite powder, spherical graphite powder, and high-purity silicon powder are added to a ball mill and mixed evenly according to a certain mass ratio. Then, liquid phenolic resin (concentration 10wt%) is added at a powder mass ratio of 5:1, and the mixture is further milled for 30-60 minutes. After drying, it is crushed and sieved to obtain a -150 mesh hybrid powder.

[0017] (3) Selective laser sintering for forming phase change energy storage single-element billets Selective laser sintering (SLS) technology was used to rapidly print and prepare prototypes of phase change energy storage units, and a secondary curing process was completed to obtain blanks for the phase change energy storage unit housing and cover. The SLS process parameters were optimized to improve the bending strength and forming accuracy of the blanks after selective laser sintering of mixed powders.

[0018] The selective laser sintering (SLS) process parameters are as follows: layer thickness 0.1–0.3 mm, scanning spacing 0.1–0.2 mm, scanning speed 500–2000 mm / s, and laser power 15–25 W. The secondary curing process parameters are: heating to 160℃ and holding for 10–15 minutes.

[0019] Multiple carbonization and vacuum pressure impregnation densification processes are employed. The aim is to adjust the internal pore state of the graphite ceramic-based phase change energy storage unit preform by optimizing the mixed powder formulation (including adjusting the proportions of natural flake graphite, spherical graphite, silicon powder, and phenolic resin), and to optimize the carbonization and vacuum pressure impregnation processes (including selecting appropriate impregnating agents) to achieve vacuum pressure impregnation densification.

[0020] During carbonization, the phase change energy storage single-element preform is placed in a vacuum carbonization furnace. First, the vacuum level is evacuated to below 100 Pa, then 99% pure nitrogen is introduced. The temperature is then increased to 600℃ at a rate of 60–150℃ / h and held for 0.5 h. Finally, the temperature is increased to 800℃ at a rate of 180–240℃ / h and held for 0.5 h. The preform is then cooled to room temperature in the furnace to obtain the phase change energy storage unit preform. For vacuum pressure impregnation, the vacuum level is first evacuated to below 200 Pa. Then, the phase change energy storage unit preform is immersed in 30–40 wt% liquid phenolic resin and held under pressure of 0.1–0.5 MPa for 1–3 min. It is then removed, its surface is cleaned with ethanol, and it is dried at 60–70℃ and cured (curing temperature 160℃, time 0.5–1.0 h). The above carbonization and vacuum pressure impregnation processes are repeated 2–3 times.

[0021] (5) Fused silicon infiltration surface sealing and strengthening treatment The phase change energy storage unit's housing, cover, and porous graphite skeleton undergo molten silicon infiltration treatment. At high temperatures, molten silicon reacts with the graphite matrix to generate silicon carbide, which strengthens the surface seal, resulting in an open porosity of less than 0.5% and a pore size of less than 5 micrometers, thus preventing leakage of the high-temperature molten salt phase change thermal energy storage material.

[0022] The phase change energy storage unit was placed in a graphite crucible and embedded in a mixture of high-purity silicon and boron nitride powder (mass ratio of 10:1). The mixture was then placed in a vacuum carbonization furnace, evacuated to 100 Pa, and then heated to 1450 °C at a rate of 180–360 °C / h. The temperature was held for 0.5–1 h, and the unit was removed after cooling in the furnace to obtain a phase change energy storage unit that had undergone sealing and strengthening treatment. (6) Filling with high-temperature molten salt The selected high-temperature molten salt is melted and filled into the pores of the internal framework of the phase change energy storage unit. The high-temperature molten salt used is a ternary chloride salt of NaCl-KCl-CaCl2.

[0023] (7) Packaging The housing and cover are connected together using a high-temperature adhesive to obtain a phase change energy storage unit filled with high-temperature molten salt. The high-temperature adhesive is characterized by being prepared by mixing liquid thermosetting phenolic resin (55.6%), silica (3%), and B4C (41.4%).

[0024] The graphite / silicon carbide composite phase change energy storage unit described in this invention consists of a housing, a porous graphite skeleton, a housing cover, and high-temperature molten salt. The porous graphite skeleton is uniformly distributed inside the housing and fixedly connected to the inner wall of the housing, providing a rapid channel for heat exchange with external sources. The high-temperature molten salt fills the pores of the porous graphite skeleton. The housing cover and housing are connected as a single unit by a high-temperature adhesive. The outer surfaces of the housing and housing cover are designed with a corrugated surface to increase the heat exchange area and ensure heat exchange efficiency. The graphite / silicon carbide composite phase change energy storage unit provided by this invention has a simple manufacturing process, low cost, and is easy to industrialize. The prepared graphite / silicon carbide composite phase change energy storage unit has advantages such as high temperature resistance, corrosion resistance, good mechanical properties, good encapsulation, and high loading rate. The presence of the three-dimensional porous graphite skeleton improves energy conversion efficiency and heat transfer uniformity. Multiple phase change energy storage units are assembled together to form an energy storage device, which is expected to be applied in peak shaving and valley filling, waste heat recovery and reuse, and rapid storage and release of periodic new energy sources. Attached Figure Description

[0025] Figure 1 a. Schematic diagram of a three-dimensional porous graphite skeleton with Kelvin structure; b. Schematic diagram of a three-dimensional porous graphite skeleton with air sphere structure (1 is an air sphere, 2 is a graphite ceramic skeleton); c. Schematic diagram of a three-dimensional porous graphite skeleton with diamond-like carbon structure. Detailed Implementation

[0026] Example 1 (1) Structural design of phase change energy storage unit The phase change energy storage unit is a cube with one end open. Both the cover and the body are designed with a 3mm stepped surface. The body wall thickness is 15mm. The outer surface of the body and the cover is designed with a semi-circular groove with a radius of 1mm. The porous graphite skeleton is an air ball structure with its minimum solid feature size controlled at 3mm. The high-temperature melting loading rate is not less than 70%.

[0027] (2) Preparation of graphite 3D printing mixed powder The composition of the graphite 3D printing mixed powder is as follows: natural flake graphite powder (carbon content greater than 99.5%, particle size 100 mesh) accounts for 63% by mass, spherical graphite powder (carbon content greater than 99%, particle size 600 mesh) accounts for 18% by mass, and high-purity silicon powder (purity greater than 99%, particle size 200 mesh) accounts for 19% by mass. The natural flake graphite powder, spherical graphite powder, and high-purity silicon powder are added to a ball mill and mixed evenly. Then, liquid phenolic resin (concentration 10wt%) is added at a powder mass ratio of 5:1, and the mixture is further milled for 45 minutes. After drying, the powder is crushed and sieved to obtain a -150 mesh mixed powder.

[0028] (3) 3D printing of phase change energy storage single-element billet Selective laser sintering (SLS) technology was used to 3D print a phase change energy storage unit (PCS) preform from a graphite / phenolic resin mixed powder. The SLS process parameters were: laser power 25W, layer thickness 0.15mm, infill spacing 0.15mm, infill speed 1500mm / s, contour scanning infill method, and infill power 8W. The PCS preform was then placed in an oven for secondary curing. The secondary curing process parameters were: heating to 160℃ and holding for 10 minutes.

[0029] (4) Multiple carbonization and vacuum pressure impregnation densification treatment During carbonization, the phase change energy storage single-element preform is placed in a vacuum carbonization furnace. First, the vacuum level is evacuated to below 100 Pa, then 99% pure nitrogen is introduced. The temperature is then increased to 600℃ at 150℃ / h and held for 0.5h. Finally, the temperature is increased to 800℃ at 180℃ / h and held for 0.5h. The preform is then cooled to room temperature in the furnace to obtain the phase change energy storage unit preform. For vacuum pressure impregnation, the vacuum level is first evacuated to below 200 Pa. Then, the phase change energy storage unit preform is immersed in 35wt% liquid phenolic resin and held at 0.2MPa for 1.5min. It is then removed, its surface is cleaned with ethanol, and dried / cured at 60℃ (curing temperature 160℃, time 0.5~1.0h). The above carbonization and vacuum pressure impregnation processes are repeated three times.

[0030] (5) Fused silicon infiltration surface sealing and strengthening treatment The phase change energy storage unit was placed in a graphite crucible and embedded with a mixture of high-purity silicon and boron nitride powder (mass ratio of 10:1). The mixture was then placed in a vacuum carbonization furnace, evacuated to 100 Pa, and then heated to 1450 °C at a rate of 200 °C / h. The temperature was held for 0.5 h, and the unit was removed after cooling in the furnace to obtain a phase change energy storage unit that had undergone sealing and strengthening treatment. (6) Filling with high-temperature molten salt The selected high-temperature molten salt is melted and filled into the pores of the internal framework of the phase change energy storage unit. The high-temperature molten salt is a ternary chloride salt of NaCl-KCl-CaCl2 with a melting point of 300℃~700℃; (7) Packaging The housing and cover are connected together using a high-temperature adhesive to obtain a phase change energy storage unit filled with high-temperature molten salt. The high-temperature adhesive is characterized by being prepared by mixing liquid thermosetting phenolic resin (55.6%), silica (3%), and B4C (41.4%).

[0031] The comprehensive performance test results are as follows: the density of the high-temperature molten salt phase change energy storage unit is 2.15 g / cm³. 3 The thermal conductivity is 60.25 W / (m·K), the compressive strength of the phase change energy storage unit is 45 MPa, its porosity is less than 0.5%, and the pore size is less than 5 micrometers, effectively preventing leakage of the high-temperature molten salt phase change energy storage material; the high-temperature molten salt loading rate is greater than 70%, its melting point is 354.9℃, the latent heat of phase change is 219.9 kJ / kg, and the heat storage and release efficiency is improved by 28.4%. The prepared graphite-silicon carbide composite high-temperature molten salt phase change energy storage unit has high thermal conductivity, high molten salt loading rate, and excellent heat storage and release efficiency. The adopted process technology is simple and easy to implement, can be mass-produced, has low production cost, and has broad industrial application prospects.

[0032] Example 2 (1) Structural design of phase change energy storage unit The phase change energy storage unit is a cube with one open end. Both the cover and the body are designed with a 4.5mm stepped surface. The body wall thickness is 20mm. The outer surface of the body and the cover is designed with a semi-circular groove with a radius of 1.25mm. The porous graphite skeleton has a Kelvin structure and its minimum solid feature size is controlled at 4mm. The high-temperature melting loading rate is not less than 75%.

[0033] (2) Preparation of graphite 3D printing mixed powder The composition of the graphite 3D printing mixed powder is as follows: natural flake graphite powder (carbon content greater than 99.5%, particle size 250 mesh) accounts for 65% by mass, spherical graphite powder (carbon content greater than 99%, particle size 600 mesh) accounts for 18% by mass, and high-purity silicon powder (purity greater than 99%, particle size 200 mesh) accounts for 17% by mass. The natural flake graphite powder, spherical graphite powder, and high-purity silicon powder are added to a ball mill and mixed evenly. Then, liquid phenolic resin (concentration 10wt%) is added at a powder mass ratio of 5:1, and the mixture is further milled for 35 minutes. After drying, the powder is crushed and sieved to obtain a -150 mesh mixed powder.

[0034] (3) 3D printing of phase change energy storage single-element billet Selective laser sintering (SLS) technology was used to 3D print a phase change energy storage unit (PCS) preform from a graphite / phenolic resin mixed powder. The SLS process parameters were: laser power 18W, layer thickness 0.15mm, infill spacing 0.12mm, infill speed 1500mm / s, contour scanning infill method, and infill power 10W. The PCS preform was then placed in an oven for secondary curing. The secondary curing process parameters were: heating to 160℃ and holding for 10 minutes.

[0035] (4) Multiple carbonization and vacuum pressure impregnation densification treatment During carbonization, the phase change energy storage single-element preform is placed in a vacuum carbonization furnace. First, the vacuum level is evacuated to below 100 Pa, then 99% pure nitrogen is introduced. The temperature is then increased to 600℃ at 135℃ / h and held for 0.5h. Finally, the temperature is increased to 800℃ at 200℃ / h and held for 0.5h. The furnace is then cooled to room temperature to obtain the phase change energy storage unit preform. For vacuum pressure impregnation, the vacuum level is first evacuated to below 200 Pa. Then, the phase change energy storage unit preform is immersed in 35wt% liquid phenolic resin and held at 0.1MPa for 1.5min. It is then removed, its surface is cleaned with ethanol, and dried / cured at 60℃ (curing temperature 160℃, time 0.5~1.0h). The above carbonization and vacuum pressure impregnation processes are repeated three times.

[0036] (5) Fused silicon infiltration surface sealing and strengthening treatment The phase change energy storage unit was placed in a graphite crucible and embedded in a mixture of high-purity silicon and boron nitride powder (mass ratio of 10:1). The mixture was then placed in a vacuum carbonization furnace, evacuated to 100 Pa, and then heated to 1450 °C at a rate of 240 °C / h. The temperature was held for 0.5 h, and the unit was removed after cooling in the furnace to obtain a phase change energy storage unit that had undergone sealing and strengthening treatment. (6) Filling with high-temperature molten salt The selected high-temperature molten salt is melted and filled into the pores of the internal framework of the phase change energy storage unit. The high-temperature molten salt is a ternary chloride salt of NaCl-KCl-CaCl2 with a melting point of 300℃~700℃; (7) Packaging The housing and cover are connected together using a high-temperature adhesive to obtain a phase change energy storage unit filled with high-temperature molten salt. The high-temperature adhesive is characterized by being prepared by mixing liquid thermosetting phenolic resin (55.6%), silica (3%), and B4C (41.4%).

[0037] The comprehensive performance test results are as follows: the density of the high-temperature molten salt phase change energy storage unit is 2.20 g / cm3, the thermal conductivity is 75.18 W / (m·K), the compressive strength of the phase change energy storage unit is 50 MPa, its porosity is less than 0.5%, and the pore size is less than 5 micrometers, effectively preventing leakage of the high-temperature molten salt phase change thermal storage material; the high-temperature molten salt loading rate is greater than 75%, its melting point is 354.9℃, the latent heat of phase change is 219.9 kJ / kg, and the heat storage and release efficiency is improved by 30.1%. The prepared graphite and silicon carbide composite high-temperature molten salt phase change energy storage unit has high thermal conductivity, high-temperature molten salt loading rate, and excellent heat storage and release efficiency. The adopted process technology is simple and easy to implement, can be mass-produced, has low production cost, and has broad industrial application prospects.

[0038] Example 3 (1) Structural design of phase change energy storage unit The phase change energy storage unit is a cuboid with one open end. Both the cover and the body are designed with a 3.5mm stepped surface. The body wall thickness is 21mm. The outer surface of the body and the cover is designed with a semi-circular groove with a radius of 1.25mm. The porous graphite skeleton is a diamond-like structure with a minimum solid feature size controlled at 2.5mm. The high-temperature melting loading rate is not less than 80%.

[0039] (2) Preparation of graphite 3D printing mixed powder The composition of the graphite 3D printing mixed powder is as follows: natural flake graphite powder (carbon content greater than 99.5%, particle size 150 mesh) accounts for 65% by mass, spherical graphite powder (carbon content greater than 99%, particle size 600 mesh) accounts for 16% by mass, and high-purity silicon powder (purity greater than 99%, particle size 200 mesh) accounts for 18% by mass. The natural flake graphite powder, spherical graphite powder, and high-purity silicon powder are added to a ball mill and mixed evenly. Then, liquid phenolic resin (concentration 10wt%) is added at a powder mass ratio of 5:1, and the mixture is further milled for 35 minutes. After drying, the powder is crushed and sieved to obtain a -150 mesh mixed powder.

[0040] (3) 3D printing of phase change energy storage single-element billet Selective laser sintering (SLS) technology was used to 3D print a phase change energy storage unit (PCS) preform from a graphite / phenolic resin mixture. The SLS process parameters were: laser power 19 W, layer thickness 0.15 mm, infill spacing 0.1 mm, infill speed 1600 mm / s, contour scanning infill method, and infill power 10 W. The PCS preform was then placed in an oven for secondary curing. The secondary curing process parameters were: heating to 160℃ and holding for 10 minutes.

[0041] (4) Multiple carbonization and vacuum pressure impregnation densification treatment During carbonization, the phase change energy storage single-element preform is placed in a vacuum carbonization furnace. First, the vacuum level is evacuated to below 100 Pa, then 99% pure nitrogen is introduced. The temperature is then increased to 600℃ at 150℃ / h and held for 0.5h. Finally, the temperature is increased to 800℃ at 160℃ / h and held for 0.5h. The furnace is then cooled to room temperature to obtain the phase change energy storage unit preform. For vacuum pressure impregnation, the vacuum level is first evacuated to below 200 Pa. Then, the phase change energy storage unit preform is immersed in 30wt% liquid phenolic resin and held at 0.2MPa for 1.5min. It is then removed, its surface is cleaned with ethanol, and dried / cured at 60℃ (curing temperature 160℃, time 1.0h). The above carbonization and vacuum pressure impregnation processes are repeated twice.

[0042] (5) Fused silicon infiltration surface sealing and strengthening treatment The phase change energy storage unit was placed in a graphite crucible and embedded in a mixture of high-purity silicon and boron nitride powder (mass ratio of 10:1). The mixture was then placed in a vacuum carbonization furnace, evacuated to 100 Pa, and then heated to 1450 °C at a rate of 240 °C / h. The temperature was held for 0.5 h, and the unit was removed after cooling in the furnace to obtain a phase change energy storage unit that had undergone sealing and strengthening treatment. (6) Filling with high-temperature molten salt The selected high-temperature molten salt is melted and filled into the pores of the internal framework of the phase change energy storage unit. The high-temperature molten salt is a ternary chloride salt of NaCl-KCl-CaCl2 with a melting point of 300℃~700℃; (7) Packaging The housing and cover are connected together using a high-temperature adhesive to obtain a phase change energy storage unit filled with high-temperature molten salt. The high-temperature adhesive is characterized by being prepared by mixing liquid thermosetting phenolic resin (55.6%), silica (3%), and B4C (41.4%).

[0043] The comprehensive performance test results are as follows: the density of the high-temperature molten salt phase change energy storage unit is 2.04 g / cm³. 3The thermal conductivity is 65.31 W / (m·K), the compressive strength of the phase change energy storage unit is 57 MPa, its porosity is less than 0.5%, and the pore size is less than 5 micrometers, effectively preventing leakage of the high-temperature molten salt phase change energy storage material; the high-temperature molten salt loading rate is greater than 80%, its melting point is 354.9℃, the latent heat of phase change is 219.9 kJ / kg, and the heat storage and release efficiency is improved by 36.5%. The prepared graphite-silicon carbide composite high-temperature molten salt phase change energy storage unit has high thermal conductivity, high-temperature molten salt loading rate, and excellent heat storage and release efficiency. The adopted process technology is simple and easy to implement, can be mass-produced, has low production cost, and has broad industrial application prospects.

[0044] Comparative Example 1 (1) Structural design of phase change energy storage unit Completely identical to Example 2 (the box is a cube with one end open, the lid and the step surface of the box are 4.5mm, the wall thickness is 20mm, the radius of the semi-circular groove on the outer surface is 1.25mm, the porous graphite skeleton is a Kelvin structure, the minimum solid feature size is 4mm, and the design loading rate is not less than 75%).

[0045] (2) Preparation of graphite 3D printing mixed powder ~ (5) Fused silicon infiltration surface sealing and strengthening treatment All process parameters and material formulations are completely consistent with those in Example 2, without any adjustments.

[0046] (6) Filling with contrast molten salt The high-temperature molten salt is replaced with KNO3-NaNO3 binary nitrate (mass ratio 50:50, industrial grade purity ≥99.0%). This molten salt is commonly used in traditional solar thermal power generation, with a melting point of 207℃ and a maximum operating temperature of ≤565℃. It is filled into the pores of the porous graphite skeleton using the same melting and filling process.

[0047] (7) Packaging Consistent with Example 2, the same high-temperature adhesive (55.6% liquid thermosetting phenolic resin, 3% silica, and 41.4% B4C) was used for encapsulation.

[0048] Overall performance test results The density of the high-temperature molten salt phase change energy storage unit is 2.08 g / cm³. 3The thermal conductivity is 52.36 W / (m·K), the compressive strength is 48 MPa, and the porosity is ≤0.5% (consistent with the example, and the sealing performance meets the standard); the high-temperature molten salt loading rate is 75%; the latent heat of phase change is 186.5 kJ / kg (lower than 219.9 kJ / kg in Example 2); the heat storage and release efficiency is improved by 22.3% (significantly lower than 30.1% in Example 2); after 500 thermal cycles (300℃-550℃), slight corrosion marks appeared inside the box, and a small amount of decomposition products (NO) appeared in the molten salt. x (Volatile residue), and its thermal stability score was lower than that of Example 2.

[0049] Comparative Example 2 (1) Structural design of phase change energy storage unit Completely consistent with Example 2.

[0050] (2) Preparation of graphite 3D printing mixed powder ~ (5) Fused silicon infiltration surface sealing and strengthening treatment All process parameters and material formulations are completely consistent with those in Example 2, without any adjustments.

[0051] (6) Filling with contrast molten salt The high-temperature molten salt is replaced with a NaCl-KCl binary chloride salt (mass ratio 50:50, industrial grade purity ≥99.0%). This molten salt is a common binary chloride salt system with a melting point of 640℃ and a maximum operating temperature of ≤800℃. It is filled into the pores of the porous graphite skeleton using the same melt filling process.

[0052] (7) Packaging Consistent with Example 2.

[0053] Overall performance test results The density of the high-temperature molten salt phase change energy storage unit is 2.15 g / cm³. 3 The thermal conductivity is 68.72 W / (m·K) (lower than 75.18 W / (m·K) in Example 2). The compressive strength is 49 MPa; the high-temperature molten salt loading rate is 75%; the latent heat of phase change is 201.3 kJ / kg (lower than 219.9 kJ / kg in Example 2); the heat storage and release efficiency is improved by 26.7% (lower than 30.1% in Example 2); the liquid phase temperature range is 640℃-800℃ (narrower than 550℃-900℃ for the ternary chloride in Example 2); after 800 thermal cycles (650℃-800℃), the fluidity of the molten salt decreases slightly, and the high melting point makes it impossible to achieve phase change heat storage in the low-temperature range (300℃-600℃), thus limiting the applicable scenarios.

[0054] Comparative analysis conclusions Compared with binary nitrates (Comparative Example 1): the ternary chloride NaCl-KCl-CaCl2 exhibits a wider liquidus temperature range (500-900℃ vs 207-565℃) and higher thermal conductivity (75.18 vs 52.36 W / (m²)). It has higher latent heat of phase change (219.9 vs 186.5 kJ / kg) and higher heat storage and release efficiency (30.1% vs 22.3%), and exhibits no significant corrosion at high temperatures, with superior thermal stability, thus solving the problems of high-temperature decomposition of nitrates and limited operating temperature.

[0055] Compared with binary chloride (Comparative Example 2): ternary chloride, by introducing CaCl2, lowers the melting point (550℃ vs 640℃), broadens the liquid phase temperature range (550-900℃ vs 640-800℃), and improves the heat storage and release efficiency (30.1% vs 26.7%), while maintaining the advantages of low corrosion and high thermal stability of chloride, making it more suitable for high-temperature heat storage scenarios with a wide temperature range.

[0056] In summary, the NaCl-KCl-CaCl2 ternary chloride salt outperforms traditional binary nitrates and binary chlorides in terms of thermal conductivity, heat storage density, operating temperature range, thermal stability, and compatibility. It is the optimal high-temperature molten salt system suitable for the graphite / silicon carbide composite phase change energy storage unit of this invention.

Claims

1. A graphite / silicon carbide composite phase change energy storage unit, characterized in that, Includes the enclosure, porous graphite skeleton, enclosure lid, and high-temperature molten salt; The porous graphite skeleton is disposed inside the box and is fixedly connected to the inner wall of the box. The high-temperature molten salt fills the pores of the porous graphite skeleton; The lid and body of the box are sealed together by a high-temperature adhesive. The outer surfaces of the box body and the box cover have a corrugated surface structure.

2. The graphite / silicon carbide composite phase change energy storage unit according to claim 1, characterized in that, The porous graphite skeleton has a porosity of not less than 60%, and its structure is one of Kelvin structure, air ball structure or diamond-like structure.

3. The graphite / silicon carbide composite phase change energy storage unit according to claim 1, characterized in that, The high-temperature molten salt is a ternary chloride salt of NaCl-KCl-CaCl2 with a melting point of 300℃~700℃.

4. A method for preparing a graphite / silicon carbide composite phase change energy storage unit according to any one of claims 1-3, characterized in that, Includes the following steps: (1) Preparation of 3D printing hybrid powder; (2) Selective laser sintering is used to obtain a single-element billet for phase change energy storage; (3) The green blank is subjected to multiple carbonization and vacuum pressure impregnation densification treatments; (4) Perform fused silica sealing and strengthening treatment; (5) Fill the treated energy storage unit with high-temperature molten salt; (6) Encapsulate the box and cover.

5. The preparation method according to claim 4, characterized in that, The preparation of the 3D printing hybrid powder in step (1) includes coating pretreatment, drying, crushing, sieving and mixing processes; The mixed powder comprises natural flake graphite powder, spherical graphite powder, and high-purity silicon powder; The natural flake graphite powder has a carbon content greater than 99.5%, a particle size of 100-250 mesh, and a mass percentage of 50-70%. The spherical graphite powder has a carbon content greater than 99%, a particle size of 600-3000 mesh, and a mass percentage of 20-40%. The high-purity silicon powder has a purity greater than 99%, a particle size of 200-300 mesh, and a mass percentage of 10-20%.

6. The preparation method according to claim 4, characterized in that, The process parameters for selective laser sintering in step (2) are as follows: Laser power 15-25W, layer thickness 0.1-0.3mm, scanning spacing 0.1-0.2mm, scanning speed 500-2000mm / s.

7. The preparation method according to claim 4, characterized in that, The carbonization process described in step (3) is carried out in a vacuum carbonization furnace, including: Evacuate to below 100 Pa, introduce nitrogen, raise the temperature to 600℃ at 60-150℃ / h and hold for 0.5-1h, then raise the temperature to 800℃ at 180-240℃ / h and hold for 0.5-1h, and cool with the furnace. The vacuum pressure impregnation process includes: Vacuum the material to below 200 Pa, immerse the preform in 30-40 wt% liquid phenolic resin, hold it under pressure of 0.1-0.5 MPa for 1-3 minutes, remove it and dry it to cure. The carbonization and impregnation steps are repeated 2 to 3 times.

8. The preparation method according to claim 4, characterized in that, The fused silicon infiltration process described in step (4) includes: The energy storage unit is embedded in a mixture of high-purity silicon and boron nitride powder. The mixture is then evacuated to 100 Pa in a vacuum carbonization furnace, heated to 1450 °C at a rate of 180–360 °C / h, and held at that temperature for 0.5–1 h before being cooled in the furnace. The mass ratio of the high-purity silicon to boron nitride powder is 10–20:

1.

9. The preparation method according to claim 4, characterized in that, The high-temperature molten salt mentioned in step (5) is a ternary chloride salt of NaCl-KCl-CaCl2.

10. The preparation method according to claim 4, characterized in that, The high-temperature adhesive described in step (6) consists of the following components: 50-60 wt% liquid thermosetting phenolic resin, 1-5 wt% silica, and 35-45 wt% B4C.

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