High-thermal-conductivity and high-strength aluminum oxide-molten salt phase change composite material and preparation method thereof

Abstract

The invention relates to a high-thermal-conductivity and high-strength aluminum oxide-molten salt phase change composite material and a preparation method thereof. According to the technical scheme, the preparation method comprises the following steps: mixing sintered aluminum oxide, activated aluminum oxide and silica powder to obtain a mixture; adding polyacrylamide, sodium tripolyphosphate and deionized water into the mixture, and carrying out ball milling to obtain ceramic slurry; adding a nonionic foaming agent into the ceramic slurry, and stirring to obtain foamed slurry; adding aluminum citrate into the foamed slurry, stirring, drying in a mold, and demolding to obtain an aluminum oxide porous skeleton blank; then heating to 1450-1600 DEG C, and preserving heat to obtain an aluminum oxide porous framework; and burying the alumina porous skeleton in a fused salt-containing corundum crucible, heating the corundum crucible to 10-50 DEG C higher than the melting point of the fused salt, preserving heat, and naturally cooling to obtain the high-thermal-conductivity and high-strength alumina-fused salt phase change composite material. The preparation method is simple in process, low in cost and high in safety, and the prepared product is high in compression strength, good in heat-conducting property, large in energy storage density and excellent in stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of phase change composite materials. In particular, it relates to a high thermal conductivity and high strength alumina-molten salt phase change composite material and a preparation method thereof. Background Art

[0002] With the development of the economy, the contradiction between energy demand and the shortage of traditional fossil fuels has become increasingly prominent. Thermal energy storage technologies applicable to fields such as solar thermal power generation, industrial waste heat recovery, and power grid peak shaving have attracted people's attention. In existing technical research, phase change materials have advantages such as high heat storage density, high output temperature and energy, and low volume change rate. However, their low thermal conductivity and corrosiveness at high temperatures make it difficult to meet the usage requirements of thermal energy storage systems in fields such as solar thermal power generation and industrial waste heat recovery. Therefore, the preparation of a high thermal conductivity and high strength alumina-molten salt phase change composite material is of great significance for the development of thermal energy storage materials applied to fields such as solar thermal power generation and industrial waste heat recovery. The technology for preparing phase change energy storage composite materials has attracted the attention of those skilled in the art.

[0003] The patented technology of "A Composite Phase Change Energy Storage Material and a Preparation Method Thereof" (CN201410220101.X) selects a mixture of palmitic acid and stearic acid as the phase change material and expanded graphite as the support material to improve the thermal conductivity of the composite material, obtaining a palmitic acid-stearic acid-expanded graphite composite phase change material with a thermal conductivity of 2-2.6 W / m·K. However, palmitic acid and stearic acid are toxic liquids and are prone to causing harm to the human body during use. Among them, stearic acid is flammable and belongs to dangerous drugs. At the same time, its high temperature stability is poor, and the strength of the expanded graphite skeleton is low at high temperatures, and the composite material is prone to fragmentation, resulting in the leakage of the phase change material.

[0004] The patented technology of "A Phase Change Molten Salt Heat Storage Material with Enhanced Thermal Conductivity Grafted with Carbon Nanotubes on Silicon Carbide Ceramics and a Preparation Method Thereof" (CN202311073775.7) adsorbs nickel acetate tetrahydrate on a porous silicon carbide ceramic matrix, dries to remove the adsorbed water, and then heats and holds it in a nitrogen / n-hexane atmosphere to obtain a carbon nanotube grafted silicon carbide ceramic sample, and then encapsulates the molten salt by vacuum impregnation to prepare a phase change molten salt heat storage material. The raw materials and equipment used in this method are relatively expensive, the production cost is high, and the process is cumbersome and the process conditions are harsh, which limits its practical application.

[0005] The literature (Yuping Wu, TaoWang. Preparationandcharacterizationofhydratedsalts / silica compositeasshape-stabilizedphasechangematerialviasol–gelprocess.) reported that hydrated salt / silica composites were prepared by a sol-gel method using sodium silicate, ammonium bicarbonate, sodium sulfate decahydrate, and disodium hydrogen phosphate dodecahydrate as raw materials. This method requires several washings, wasting water resources. Moreover, its process requires gelation for 5 hours, water bath for 48 hours, ultrasonic treatment for 3 hours, a long time of vigorous stirring, and drying for 48 hours, with a long preparation cycle, small output, and difficulty in large-scale production.

[0006] The patented technology of "A preparation method of a phase change material with energy storage and composite network structure" (CN202311090610.0) mixes a pore-forming agent, a matrix binder, and a heat-conducting filler evenly and then performs heat treatment to form a porous carrier, which is then compounded with a two-dimensional material and freeze-dried to form a composite network structure, and finally adsorbs a phase change material to obtain a product. This method requires a crusher, a mixer, a vacuum oven, a flat vulcanizer, a water bath, and a freeze dryer, with a complex process route. Moreover, the thermal conductivity of the obtained product is only 0.6800 - 0.7521 W / m·K, with a slow heat storage and release rate and a low energy storage density, making it difficult to meet higher heat storage requirements. Summary of the Invention

[0007] The present invention aims to overcome the defects of the prior art. The purpose is to provide a preparation method of a high-thermal-conductivity and high-strength alumina-molten salt phase change composite material with a simple production process, low production cost, and high safety. The products prepared by this method have high compressive strength, large energy storage density, good thermal conductivity, and excellent high-temperature stability.

[0008] To achieve the above purpose, the specific steps of the technical solution adopted by the present invention are as follows:

[0009] Step 1: Mix 70 - 85 wt% of sintered alumina, 10 - 25 wt% of activated alumina, and 1 - 6 wt% of silica powder to obtain a mixture.

[0010] According to the mass ratio of polyacrylamide: sodium tripolyphosphate: deionized water: the mixture being 1.5 - 3: 0.1 - 0.3: 16 - 19: 100, add polyacrylamide, sodium tripolyphosphate, and deionized water to the mixture in sequence, and ball mill to obtain a ceramic slurry.

[0011] Step 2: According to the mass ratio of a non-ionic foaming agent: the ceramic slurry being 0.5 - 2: 100, add a non-ionic foaming agent to the ceramic slurry and stir for 3 - 5 minutes to obtain a foamed slurry.

[0012] Step 3: Add aluminum citrate to the foaming slurry at a mass ratio of aluminum citrate to the foaming slurry of 0.8 - 1.6:100, stir for 1 - 3 minutes, pour it into a mold, dry it at 50 - 80°C for 24 - 36 hours, demold to obtain a green body of an alumina porous framework; then, under an air atmosphere and normal pressure conditions, heat the green body of the alumina porous framework to 1450 - 1600°C and hold for 3 - 5 hours to obtain an alumina porous framework.

[0013] Step 4: Immerse the alumina porous framework in a corundum crucible containing the molten salt at a mass ratio of the alumina porous framework to the molten salt of 1:10 - 15, heat the corundum crucible to 10 - 50°C above the melting point of the molten salt, hold for 3 - 5 hours, cool naturally, and remove the excess molten salt on the surface of the alumina porous framework to prepare a high thermal conductivity and high strength alumina - molten salt phase change composite material.

[0014] The purity of the sintered alumina is greater than 99.6%, and the particle size is less than 45μm.

[0015] The purity of the activated alumina is greater than 99.8%, and the particle size is less than 5μm.

[0016] The purity of the silica fume is greater than 99.99%, the shape is spherical, and the particle size is less than 50nm.

[0017] Add polyacrylamide, sodium tripolyphosphate and deionized water in sequence, where: the purities of polyacrylamide and sodium tripolyphosphate are both greater than 99%, and the particle sizes are both less than 20μm.

[0018] The ball - milling time is 0.5 - 2 hours, and the ball - milling speed is 200 - 400 revolutions per minute.

[0019] The non - ionic foaming agent is one of alcohols, ether alcohols, ethers and esters.

[0020] The purity of the aluminum citrate is greater than 99%, and the particle size is less than 20μm.

[0021] The purity of the molten salt is greater than 98%, and the particle size is less than 100μm; the molten salt is one of sodium chloride, potassium chloride, sodium sulfate and potassium sulfate; among them, the melting point of sodium chloride is 801°C, the melting point of potassium chloride is 770°C, the melting point of sodium sulfate is 884°C, and the melting point of potassium sulfate is 1069°C.

[0022] Due to the adoption of the above - mentioned technical solution, the present invention has the following positive effects compared with the prior art:

[0023] 1. The present invention uses sintered alumina, activated alumina, ultra-pure and ultra-fine spherical silica powder, and a molten salt (one of sodium chloride, potassium chloride, sodium sulfate, and potassium sulfate) as the main raw materials to prepare a high thermal conductivity and high strength alumina-molten salt phase change composite material. The raw materials used are non-toxic and not likely to cause harm to the human body, so the cost is low and the safety is high.

[0024] Different from the cold pressing sintering method, the alumina porous skeleton prepared by the foaming method in the present invention has high thermal conductivity, high strength, excellent erosion resistance, and high temperature stability. The prepared product has faster heat transfer, high safety, and excellent use stability during the thermal cycling process; different from the vacuum impregnation method, the molten salt spontaneous infiltration process is adopted, and the molten salt spontaneously infiltrates into the alumina porous skeleton under atmospheric pressure by capillary action, which not only simplifies the production process but also saves energy.

[0025] 2. The present invention uses ultra-pure and ultra-fine spherical silica powder as an additive. The added ultra-pure and ultra-fine spherical silica powder has extremely high activity. After sintering, the alumina grains are smooth and the interfacial energy is small, which is conducive to the spontaneous infiltration of the molten salt. The ultra-pure and ultra-fine spherical silica powder can be evenly distributed at the interface of alumina particles, promote the sintering connection between alumina particles, reduce the interfacial thermal resistance between particles, and at the same time form mullite to fill the gaps between alumina particles, reducing the heat transfer path of particle-pore-particle. The alumina skeleton matrix is also denser, comprehensively improving the mechanical strength and heat transfer performance of the high thermal conductivity and high strength alumina-molten salt phase change composite material.

[0026] 3. The present invention uses polyacrylamide and aluminum citrate as the gel system. Among them, the polynuclear hydroxy-bridged complex ions of aluminum form polar bonds and coordination bonds with the carboxyl groups of polyacrylamide for cross-linking. When drying at a higher temperature (50 - 80 °C), the release rate of aluminum ions from aluminum citrate increases, and the reverse reaction rate of the cross-linking of complex ions and polymers also increases, reducing the gel stability, which is conducive to the expansion and movement of the foam, so that the foams contact and connect with each other to form a large number of window pores. Therefore, the obtained alumina porous skeleton has a large number of "bubble-window" structures and high through-hole properties, accelerating the infiltration of the molten salt during the spontaneous infiltration process of the molten salt. At the same time, the small-diameter window pore structure prevents the outward leakage of the molten salt during high-temperature use, comprehensively improving the molten salt permeability, energy storage density, and high temperature stability of the high thermal conductivity and high strength alumina-molten salt phase change composite material. [[ID=,11]]

[0027] The high thermal conductivity and high strength alumina-molten salt phase change composite material prepared by the present invention is detected: the main crystal phases are alumina and molten salt, and the chemical compatibility is good; the proportion of infiltrated molten salt is 38.37 - 51.09 wt%, and the bulk density is 1.92 - 2.17 g / cm 3, the normal temperature compressive strength is 82.8 - 100.1 MPa, the normal temperature thermal conductivity is 5.23 - 6.40 W / m·K, the phase change temperature is 802.4 - 1068.2 °C, and the energy storage density is 994 - 1264 J / g; fully meeting the application requirements in the field of high-temperature heat storage.

[0028] Therefore, the process of the present invention is simple, the production cost is low, and the safety is high. The prepared high thermal conductivity and high strength alumina - molten salt phase change composite material has high compressive strength, large energy storage density, good thermal conductivity, and excellent high-temperature stability. Its performance is significantly superior to the phase change materials in the existing phase change heat storage systems (such as heat energy storage in solar thermal power plants, industrial waste heat recovery, regenerative combustion technology, etc.) in the field of high-temperature heat storage, and has good application prospects. Description of the Drawings

[0029] Figure 1 It is a SEM image of the fracture morphology of a high thermal conductivity and high strength alumina - molten salt phase change composite material prepared by the present invention;

[0030] Figure 2 is Figure 1 The SEM image of the fracture morphology of the alumina porous skeleton of the high thermal conductivity and high strength alumina - molten salt phase change composite material shown. Specific Embodiments

[0031] The following further describes the present invention in combination with specific embodiments, which is not a limitation to the protection scope of the present invention.

[0032] A high thermal conductivity and high strength alumina - molten salt phase change composite material and its preparation method. The preparation method of this specific embodiment is as follows:

[0033] Step 1: Mix 70 - 85 wt% of sintered alumina, 10 - 25 wt% of activated alumina, and 1 - 6 wt% of silica powder to obtain a mixture.

[0034] According to the mass ratio of polyacrylamide : sodium tripolyphosphate : deionized water : the mixture being 1.5 - 3 : 0.1 - 0.3 : 16 - 19 : 100, add polyacrylamide, sodium tripolyphosphate, and deionized water to the mixture in sequence, and ball mill to obtain a ceramic slurry.

[0035] Step 2: According to the mass ratio of non - ionic foaming agent : the ceramic slurry being 0.5 - 2 : 100, add a non - ionic foaming agent to the ceramic slurry and stir for 3 - 5 minutes to obtain a foamed slurry.

[0036] Step 3: Add aluminum citrate to the foaming slurry at a mass ratio of aluminum citrate to the foaming slurry of 0.8 - 1.6:100, stir for 1 - 3 minutes, pour it into a mold, dry at 50 - 80°C for 24 - 36 hours, demold to obtain a green body of an alumina porous skeleton; then, under air atmosphere and normal pressure conditions, heat the green body of the alumina porous skeleton to 1450 - 1600°C and hold for 3 - 5 hours to obtain an alumina porous skeleton.

[0037] Step 4: Immerse the alumina porous skeleton in a corundum crucible containing the molten salt at a mass ratio of the alumina porous skeleton to the molten salt of 1:10 - 15, heat the corundum crucible to 10 - 50°C above the melting point of the molten salt, hold for 3 - 5 hours, cool naturally, and remove the excess molten salt on the surface of the alumina porous skeleton to prepare a high thermal conductivity and high strength alumina - molten salt phase change composite material.

[0038] The time for ball milling is 0.5 - 2 hours, and the rotational speed of ball milling is 200 - 400 revolutions per minute.

[0039] The non - ionic foaming agent is one of alcohols, ether alcohols, ethers, and esters.

[0040] The molten salt is one of sodium chloride, potassium chloride, sodium sulfate, and potassium sulfate.

[0041] In this specific embodiment:

[0042] The purity of the sintered alumina is greater than 99.6%, and the particle size is less than 45μm.

[0043] The purity of the activated alumina is greater than 99.8%, and the particle size is less than 5μm.

[0044] The purity of the silica powder is greater than 99.99%, the shape is spherical, and the particle size is less than 50nm.

[0045] Polyacrylamide, sodium tripolyphosphate, and deionized water are added in sequence, where: the purities of polyacrylamide and sodium tripolyphosphate are both greater than 99%, and the particle sizes are both less than 20μm.

[0046] The purity of the aluminum citrate is greater than 99%, and the particle size is less than 20μm.

[0047] The purity of the molten salt is greater than 98%, and the particle size is less than 100μm; among them, the melting point of sodium chloride is 801°C, the melting point of potassium chloride is 770°C, the melting point of sodium sulfate is 884°C, and the melting point of potassium sulfate is 1069°C.

[0048] Details are not elaborated in the examples.

[0049] Example 1

[0050] A high thermal conductivity and high strength alumina - molten salt phase change composite material and its preparation method. The steps of the preparation method in this embodiment are as follows:

[0051] Step 1: Mix 80 wt% of sintered alumina, 19 wt% of activated alumina, and 1 wt% of silica powder to obtain a mixed material.

[0052] According to the mass ratio of polyacrylamide∶sodium tripolyphosphate∶deionized water∶the mixed material being 2∶0.2∶17∶100, add polyacrylamide, sodium tripolyphosphate, and deionized water to the mixed material in sequence, and perform ball milling to obtain a ceramic slurry.

[0053] Step 2: According to the mass ratio of non - ionic foaming agent∶the ceramic slurry being 2∶100, add a non - ionic foaming agent to the ceramic slurry, and stir for 3 minutes to obtain a foamed slurry.

[0054] Step 3: According to the mass ratio of aluminum citrate∶the foamed slurry being 1∶100, add aluminum citrate to the foamed slurry, stir for 3 minutes, pour it into a mold, dry it at 80 °C for 30 hours, demold to obtain an alumina porous skeleton green body; then, under air atmosphere and normal pressure conditions, heat the alumina porous skeleton green body to 1600 °C and hold for 3.5 hours to obtain an alumina porous skeleton.

[0055] Step 4: According to the mass ratio of the alumina porous skeleton∶molten salt being 1∶15, bury the alumina porous skeleton in a corundum crucible containing the molten salt, heat the corundum crucible to 20 °C higher than the melting point of the molten salt, hold for 5 hours, cool naturally, and remove the excess molten salt on the surface of the alumina porous skeleton to prepare a high thermal conductivity and high strength alumina - molten salt phase change composite material.

[0056] The time of the ball milling is 1 hour, and the rotation speed of the ball milling is 300 revolutions per minute.

[0057] The non - ionic foaming agent is an alcohol.

[0058] The molten salt is sodium sulfate.

[0059] The high thermal conductivity and high strength alumina - molten salt phase change composite material prepared in Example 1 is as shown in the attached figure; Figure 1 It is the SEM image of the fracture morphology of the high thermal conductivity and high strength alumina - molten salt phase change composite material prepared in Example 1; Figure 2 It is Figure 1 The SEM image of the fracture morphology of the alumina porous skeleton of the high thermal conductivity and high strength alumina - molten salt phase change composite material shown. It can be seen from Figure 1 that after the sodium sulfate molten salt penetrates into the alumina porous skeleton, the recrystallization is full, and the sodium sulfate molten salt combines well with the alumina porous skeleton, as shown in Figure 1The spherical molten salt at position 1 in China, Figure 1 The hemispherical molten salt and shell-shaped molten salt disconnected at position 2 in China, and Figure 1 The connecting part of the molten salt balls at position 3 in China, that is, the connection part of the "bubble-window" structure, indicating that the prepared high thermal conductivity and high strength alumina-molten salt phase change composite material has a high molten salt content, mechanical properties and stability. From Figure 2 It can be seen that: the pore structure of the alumina porous skeleton is mostly the above-mentioned "bubble-window" structure.

[0060] The high thermal conductivity and high strength alumina-molten salt phase change composite material prepared in Example 1 was tested: the main crystal phases are alumina and sodium sulfate, and the chemical compatibility is good; the molten salt permeability is 51.09 wt%, the bulk density is 2.17 g / cm 3 , the room temperature compressive strength is 100.1 MPa, the room temperature thermal conductivity is 6.40 W / m·K, the phase change temperature is 888.5 °C, and the energy storage density is 1129 J / g.

[0061] Example 2

[0062] A high thermal conductivity and high strength alumina-molten salt phase change composite material and its preparation method. The steps of the preparation method described in this example are:

[0063] Step 1: Mix 70 wt% of sintered alumina, 124 wt% of activated alumina and 6 wt% of silica powder to obtain a mixture.

[0064] According to the mass ratio of polyacrylamide: sodium tripolyphosphate: deionized water: the mixture is 1.5:0.1:16:100, add polyacrylamide, sodium tripolyphosphate and deionized water to the mixture in sequence, and ball mill to obtain a ceramic slurry.

[0065] Step 2: According to the mass ratio of non-ionic foaming agent: the ceramic slurry is 0.5:100, add a non-ionic foaming agent to the ceramic slurry, stir for 3.5 minutes to obtain a foamed slurry.

[0066] Step 3: According to the mass ratio of aluminum citrate: the foamed slurry is 0.8:100, add aluminum citrate to the foamed slurry, stir for 2 minutes, pour it into a mold, dry it at 50 °C for 24 hours, demold to obtain an alumina porous skeleton blank; then under air atmosphere and normal pressure conditions, heat the alumina porous skeleton blank to 1450 °C and keep it warm for 3 hours to obtain an alumina porous skeleton.

[0067] Step 4: Bury the alumina porous framework in a corundum crucible containing the molten salt at a mass ratio of the alumina porous framework to the molten salt of 1:10. Heat the corundum crucible to 10 °C above the melting point of the molten salt, keep it warm for 3 hours, and then cool it naturally. Remove the excess molten salt on the surface of the alumina porous framework to obtain a high thermal conductivity and high strength alumina-molten salt phase change composite material.

[0068] The time of the ball milling is 0.5 hour, and the rotation speed of the ball milling is 200 revolutions per minute.

[0069] The non-ionic foaming agent is ether alcohol.

[0070] The molten salt is sodium chloride.

[0071] The high thermal conductivity and high strength alumina-molten salt phase change composite material prepared in Example 2 is detected: the main crystal phases are alumina and sodium chloride, and the chemical compatibility is good; the molten salt permeability is 38.37 wt%, the bulk density is 1.92 g / cm 3 , the normal temperature compressive strength is 82.8 MPa, the normal temperature thermal conductivity is 5.23 W / m·K, the phase change temperature is 802.4 °C, and the energy storage density is 1103 J / g.

[0072] Example 3

[0073] A high thermal conductivity and high strength alumina-molten salt phase change composite material and a preparation method thereof. The steps of the preparation method in this example are as follows:

[0074] Step 1: Mix 73 wt% of sintered alumina, 25 wt% of activated alumina, and 2 wt% of silica powder to obtain a mixture.

[0075] Add polyacrylamide, sodium tripolyphosphate, and deionized water to the mixture in sequence according to the mass ratio of polyacrylamide:sodium tripolyphosphate:deionized water:the mixture of 2.5:0.3:18:100, and perform ball milling to obtain a ceramic slurry.

[0076] Step 2: Add a non-ionic foaming agent to the ceramic slurry according to the mass ratio of the non-ionic foaming agent to the ceramic slurry of 1.2:100, and stir for 5 minutes to obtain a foamed slurry.

[0077] Step 3: Add aluminum citrate to the foamed slurry according to the mass ratio of aluminum citrate to the foamed slurry of 1.5:100, stir for 1 minute, pour it into a mold, dry it at 60 °C for 28 hours, and demold to obtain an alumina porous framework green body; then, under the conditions of an air atmosphere and normal pressure, heat the alumina porous framework green body to 1550 °C and keep it warm for 4 hours to obtain an alumina porous framework.

[0078] Step 4: Bury the alumina porous framework in a corundum crucible containing the molten salt at a mass ratio of the alumina porous framework to the molten salt of 1:11. Heat the corundum crucible to 40°C above the melting point of the molten salt, keep it warm for 4 hours, cool it naturally, and remove the excess molten salt on the surface of the alumina porous framework to obtain a high thermal conductivity and high strength alumina-molten salt phase change composite material.

[0079] The time of the ball milling is 1.8 hours, and the rotation speed of the ball milling is 350 revolutions per minute.

[0080] The non-ionic foaming agent is an ether.

[0081] The molten salt is potassium chloride.

[0082] The high thermal conductivity and high strength alumina-molten salt phase change composite material prepared in Example 3 is detected: the main crystal phases are alumina and potassium chloride, and the chemical compatibility is good; the molten salt permeability is 39.63 wt%, the bulk density is 1.96 g / cm 3 , the normal temperature compressive strength is 86.6 MPa, the normal temperature thermal conductivity is 5.43 W / m·K, the phase change temperature is 771.5°C, and the energy storage density is 1264 J / g.

[0083] Example 4

[0084] A high thermal conductivity and high strength alumina-molten salt phase change composite material and a preparation method thereof. The steps of the preparation method in this example are:

[0085] Step 1: Mix 85 wt% of sintered alumina, 10 wt% of activated alumina, and 5 wt% of silica powder to obtain a mixture.

[0086] Add polyacrylamide, sodium tripolyphosphate, and deionized water to the mixture in sequence according to the mass ratio of polyacrylamide:sodium tripolyphosphate:deionized water:the mixture of 3:0.15:19:100, and perform ball milling to obtain a ceramic slurry.

[0087] Step 2: Add a non-ionic foaming agent to the ceramic slurry according to the mass ratio of the non-ionic foaming agent to the ceramic slurry of 1.5:100, and stir for 4 minutes to obtain a foamed slurry.

[0088] Step 3: Add aluminum citrate to the foamed slurry according to the mass ratio of aluminum citrate to the foamed slurry of 1.6:100, stir for 2.5 minutes, pour it into a mold, dry it at 65°C for 36 hours, demold it to obtain an alumina porous framework green body; then, in an air atmosphere and under normal pressure conditions, heat the alumina porous framework green body to 1580°C and keep it warm for 5 hours to obtain an alumina porous framework.

[0089] Step 4: Bury the alumina porous framework in a corundum crucible containing the molten salt according to the mass ratio of the alumina porous framework to the molten salt of 1:13. Heat the corundum crucible to 50°C above the melting point of the molten salt, keep it warm for 3.5 hours, cool it naturally, and remove the excess molten salt on the surface of the alumina porous framework to obtain a high thermal conductivity and high strength alumina-molten salt phase change composite material.

[0090] The time of the ball milling is 2 hours, and the rotation speed of the ball milling is 400 revolutions per minute.

[0091] The non-ionic foaming agent is an ester.

[0092] The molten salt is potassium sulfate.

[0093] The high thermal conductivity and high strength alumina-molten salt phase change composite material prepared in Example 4 was detected: the main crystal phases are alumina and potassium sulfate, and the chemical compatibility is good; the molten salt permeability is 43.25 wt%, the bulk density is 2.03 g / cm 3 , the normal temperature compressive strength is 92.3 MPa, the normal temperature thermal conductivity is 5.76 W / m·K, the phase change temperature is 1068.2°C, and the energy storage density is 944 J / g.

[0094] Comparative Example 1

[0095] An alumina-sodium sulfate phase change composite material and a preparation method thereof. Comparative Example 1 is the same as Example 1 except for the following:

[0096] The silica powder used in Comparative Example 1 has a purity greater than 95% and a particle size less than 10 μm;

[0097] The silica powder used in Example 1 has a purity greater than 99.99%, a spherical shape, and a particle size less than 50 nm.

[0098] The alumina-sodium sulfate phase change composite material prepared in Comparative Example 1 was detected: the main crystal phases are alumina and sodium sulfate, and the chemical compatibility is good; the molten salt permeability is 36.21 wt%, the bulk density is 1.86 g / cm 3 , the normal temperature compressive strength is 76.2 MPa, the normal temperature thermal conductivity is 5.06 W / m·K, the phase change temperature is 888.5°C, and the energy storage density is 953 J / g.

[0099] Comparative Example 2

[0100] A low molten salt content alumina-sodium sulfate phase change composite material and a preparation method thereof. Comparative Example 2 is the same as Example 1 except for the following:

[0101] Step 1: Mix 80 wt% of sintered alumina, 19 wt% of activated alumina, and 1 wt% of silica powder to obtain a mixture.

[0102] According to the mass ratio of sodium tripolyphosphate∶deionized water∶the mixture being 0.2∶17∶100, sodium tripolyphosphate and deionized water are successively added to the mixture, and ball milling is carried out to obtain a ceramic slurry.

[0103] Step 3: Pour the foaming slurry into a mold, dry it at 80°C for 30 hours, and demold to obtain an alumina porous skeleton green body; then, under an air atmosphere and normal pressure conditions, heat the alumina porous skeleton green body to 1600°C and hold for 3.5 hours to obtain an alumina porous skeleton.

[0104] The low-melting-salt-content alumina-sodium sulfate phase change composite material prepared in Comparative Example 2 was detected: the main crystal phases are alumina and sodium sulfate, and the chemical compatibility is good; the molten salt permeability is 28.63 wt%, the bulk density is 1.78 g / cm 3 , the normal temperature compressive strength is 66.4 MPa, the normal temperature thermal conductivity is 4.83 W / m·K, the phase change temperature is 888.5°C, and the energy storage density is 861 J / g.

[0105] The following positive effects exist when this specific embodiment is compared with the prior art:

[0106] 1. This specific embodiment uses sintered alumina, activated alumina, ultra-pure and ultra-fine spherical silica powder, and a molten salt (one of sodium chloride, potassium chloride, sodium sulfate, and potassium sulfate) as the main raw materials to prepare a high thermal conductivity and high strength alumina-molten salt phase change composite material. The raw materials used are non-toxic and not likely to cause harm to the human body, so the cost is low and the safety is high.

[0107] This specific embodiment is different from the cold pressing sintering method. The alumina porous skeleton prepared by the foaming method has high thermal conductivity, high strength, excellent erosion resistance, and high temperature stability. The products prepared have faster heat transfer, high safety, and excellent use stability during the thermal cycling use process; this specific embodiment is different from the vacuum impregnation method. The molten salt spontaneous infiltration process is adopted, and the molten salt spontaneously infiltrates into the alumina porous skeleton under normal pressure by capillary action, which not only simplifies the production process but also saves energy.

[0108] 2. This specific embodiment uses ultra-pure and ultra-fine spherical silica powder as an additive. The added ultra-pure and ultra-fine spherical silica powder has extremely high activity. After sintering, the alumina grains are smooth and the interfacial energy is small, which is conducive to the spontaneous infiltration of the molten salt. The ultra-pure and ultra-fine spherical silica powder can be evenly distributed at the interface of the alumina particles, promote the sintering connection between the alumina particles, reduce the interfacial thermal resistance between the particles, and at the same time form mullite to fill the gaps between the alumina particles, reducing the heat transfer path of particle-pore-particle. The alumina skeleton matrix is also more dense, comprehensively improving the mechanical strength and heat transfer performance of the high thermal conductivity and high strength alumina-molten salt phase change composite material.

[0109] 3. This specific embodiment uses polyacrylamide and aluminum citrate as the gel system. Among them, the polynuclear hydroxy-bridged complex ions of aluminum form polar bonds and coordination bonds with the carboxyl groups of polyacrylamide for crosslinking. When drying at a relatively high temperature (50 - 80 °C), the release rate of aluminum ions from aluminum citrate increases, and the reverse reaction rate of the crosslinking between the complex ions and the polymer also increases, resulting in a decrease in gel stability, which is conducive to the expansion and movement of the foam, so that the foams contact and connect with each other to form a large number of window pores. Therefore, the obtained alumina porous framework has a large number of "bubble-window" structures and high through-hole properties, which accelerates the penetration of molten salt during the spontaneous infiltration process of molten salt. At the same time, the small-diameter window pore structure prevents the outward leakage of molten salt during high-temperature use, comprehensively improving the molten salt permeability, energy storage density, and high-temperature stability of the high thermal conductivity and high-strength alumina-molten salt phase change composite material.

[0110] The high thermal conductivity and high-strength alumina-molten salt phase change composite material prepared by this specific embodiment is detected: the main crystal phases are alumina and molten salt, and the chemical compatibility is good; the proportion of infiltrated molten salt is 38.37 - 51.09 wt%, and the bulk density is 1.92 - 2.17 g / cm 3 , the compressive strength at room temperature is 82.8 - 100.1 MPa, the thermal conductivity at room temperature is 5.23 - 6.40 W / m·K, the phase change temperature is 802.4 - 1068.2 °C, and the energy storage density is 994 - 1264 J / g; it fully meets the application requirements in the field of high-temperature heat storage.

[0111] Therefore, this specific embodiment has simple process, low production cost, and high safety. The prepared high thermal conductivity and high-strength alumina-molten salt phase change composite material has high compressive strength, large energy storage density, good thermal conductivity, and excellent high-temperature stability. Its performance is significantly superior to the phase change materials of the phase change heat storage system (such as thermal energy storage in solar thermal power plants, industrial waste heat recovery, regenerative combustion technology, etc.) in the existing high-temperature heat storage field, and has good application prospects.

Claims

1. A preparation method of a high thermal conductivity and high strength alumina-molten salt phase change composite material, characterized in that, The steps of the preparation method are as follows: Step 1: Mix 70-85 wt% of sintered alumina, 10-25 wt% of activated alumina, and 1-6 wt% of silica powder to obtain a mixed material. According to the mass ratio of polyacrylamide:sodium tripolyphosphate:deionized water:the mixed material being 1.5-3:0.1-0.3:16-19:100, sequentially add polyacrylamide, sodium tripolyphosphate, and deionized water to the mixed material, and ball mill to obtain a ceramic slurry. Step 2: According to the mass ratio of non-ionic foaming agent:the ceramic slurry being 0.5-2:100, add a non-ionic foaming agent to the ceramic slurry and stir for 3-5 minutes to obtain a foamed slurry. Step 3: According to the mass ratio of aluminum citrate:the foamed slurry being 0.8-1.6:100, add aluminum citrate to the foamed slurry, stir for 1-3 minutes, pour it into a mold, and dry it at 50-80 °C for 24-36 hours, then demold to obtain an alumina porous framework green body; then, under air atmosphere and normal pressure conditions, heat the alumina porous framework green body to 1450-1600 °C and keep it warm for 3-5 hours to obtain an alumina porous framework. Step 4: According to the mass ratio of the alumina porous framework:molten salt being 1:10-15, bury the alumina porous framework in a corundum crucible containing the molten salt, heat the corundum crucible to 10-50 °C higher than the melting point of the molten salt, keep it warm for 3-5 hours, and naturally cool, and remove the excess molten salt on the surface of the alumina porous framework to prepare a high thermal conductivity and high strength alumina-molten salt phase change composite material.

2. The preparation method of the high thermal conductivity and high strength alumina-molten salt phase change composite material according to claim 1, wherein The purity of the sintered alumina is greater than 99.6%, and the particle size is less than 45 μm.

3. The preparation method of the high thermal conductivity and high strength alumina-molten salt phase change composite material according to claim 1, characterized in that, The purity of the activated alumina is greater than 99.8%, and the particle size is less than 5 μm.

4. The preparation method of the high thermal conductivity and high strength alumina-molten salt phase change composite material according to claim 1, wherein, The purity of the silica powder is greater than 99.99%, the shape is spherical, and the particle size is less than 50 nm.

5. The method for preparing the high thermal conductivity and high strength alumina-molten salt phase change composite material according to claim 1, characterized in that: When sequentially adding polyacrylamide, sodium tripolyphosphate, and deionized water, among them: the purities of polyacrylamide and sodium tripolyphosphate are both greater than 99%, and the particle sizes are both less than 20 μm.

6. The method for preparing the high thermal conductivity and high strength alumina-molten salt phase change composite material according to claim 1, characterized in that: The ball milling time is 0.5-2 hours, and the ball milling speed is 200-400 revolutions per minute.

7. The method for preparing the high thermal conductivity and high strength alumina-molten salt phase change composite material according to claim 1, characterized in that: The non-ionic foaming agent is one of alcohols, ether alcohols, ethers, and esters.

8. The preparation method of the high thermal conductivity and high strength alumina-molten salt phase change composite material according to claim 1, characterized in that, The purity of the aluminum citrate is greater than 99%, and the particle size is less than 20 μm.

9. The preparation method of the high thermal conductivity and high strength alumina-molten salt phase change composite material according to claim 1, characterized in that, The purity of the molten salt is greater than 98%, and the particle size is less than 100 μm; the molten salt is one of sodium chloride, potassium chloride, sodium sulfate, and potassium sulfate; among them, the melting point of sodium chloride is 801 °C, the melting point of potassium chloride is 770 °C, the melting point of sodium sulfate is 884 °C, and the melting point of potassium sulfate is 1069 °C.

10. A high thermal conductivity and high strength alumina-molten salt phase change composite material, characterized in that The high thermal conductivity and high strength alumina-molten salt phase change composite material is the high thermal conductivity and high strength alumina-molten salt phase change composite material prepared according to the preparation method of the high thermal conductivity and high strength alumina-molten salt phase change composite material described in any one of claims 1-9.

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