Porous calcium hexaluminate-fused salt phase change composite material and preparation method thereof

Abstract

The invention relates to a porous calcium hexaluminate-fused salt phase change composite material and a preparation method thereof. According to the technical scheme, the preparation method comprises the following steps: mixing activated aluminum oxide and aragonite to obtain a mixture I; adding acrylamide, N-hydroxymethyl acrylamide and N, N-methylene bisacrylamide into the mixture I, and uniformly mixing to obtain a mixture II; adding deionized water into the mixture II, and stirring to obtain ceramic slurry; adding a nonionic foaming agent into the ceramic slurry, and stirring to obtain foamed slurry; and adding tert-butyl hydroperoxide into the foamed slurry, stirring, pouring into a mold, drying, demolding, heating an obtained porous calcium hexaluminate blank, and preserving heat to obtain the porous calcium hexaluminate. The porous calcium hexaluminate is buried in a fused salt-containing corundum crucible, and heating, heat preservation and cooling are performed to prepare the porous calcium hexaluminate-fused salt phase change composite material. The preparation method is simple in process, short in production period and low in cost, and the prepared product is high in compression strength, high in fused salt content, high in energy storage density and good in thermal cycling stability.

Description

Technical Field

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

[0002] With the development of the economy, the demand for energy in human society is constantly expanding, and the problems of energy shortage and uneven distribution are becoming increasingly prominent. In order to achieve the rational distribution and utilization of energy, the development of heat storage technology is an essential link. The heat energy storage technology stores heat energy when there is an excess of heat energy and releases heat energy when there is an energy shortage, realizing the distribution of energy at different times and spaces, thereby improving the energy utilization efficiency.

[0003] Molten salt phase change materials are excellent choices for high - temperature heat storage applications due to their high energy density, wide phase change temperature range, good physical / chemical stability, and low price. However, problems such as the low thermal conductivity of molten salts, corrosion to containers, and easy leakage after phase change limit their applications. Using porous ceramics with excellent high - temperature performance to load molten salts can well solve the above problems. The framework of porous ceramics is resistant to molten salt erosion and strong enough, and is not affected by micro - encapsulated salts. However, the currently developed porous ceramics have a low molten salt loading rate and poor thermal cycle stability, making it difficult to meet the usage requirements of heat storage systems in fields such as solar thermal power generation and industrial waste heat recovery. Therefore, the preparation of heat storage materials for high - temperature heat storage fields such as solar thermal power generation and industrial waste heat recovery has attracted the attention of those skilled in the art.

[0004] For the patented technology of "A Phase - Change Molten Salt Heat Storage Material with Enhanced Thermal Conductivity of Carbon Nanotube - Grafted Silicon Carbide Ceramics and Its Preparation Method" (CN202311073775.7), this technology uses porous silicon carbide ceramics as the matrix to adsorb nickel acetate tetrahydrate. After drying to remove the adsorbed water, it is heated and kept warm in a nitrogen / n - hexane atmosphere to obtain a carbon nanotube - grafted silicon carbide ceramic sample, and then the phase - change molten salt heat storage material is prepared by vacuum impregnation to encapsulate the molten salt. This method has a high production cost, complicated process, harsh process conditions, and the molten salt penetration rate is only 43.77 - 54.49 wt%, which is not suitable for large - scale application production.

[0005] The patented technology of "A ceramic-based molten salt composite phase change heat storage material and its preparation method" (202211273446.2) uses secondary aluminum ash, steel slag, and silicon-rich aluminum additive as raw materials to prepare porous ceramics, and obtains the ceramic-based molten salt composite phase change heat storage material by impregnating high-temperature molten salt NaCl-KCl (mass ratio 44:56) under normal pressure. Although the raw material cost of this preparation method is low and the process is simple, the phase change heat storage value is low, only 74.6 - 79.6 J / g. The theoretical phase change latent heat value of the NaCl-KCl (mass ratio 44:56) molten salt is 204 J / g. Through calculation, its molten salt content is only 36.6 - 39.0 wt%, the molten salt permeability is extremely low, and the heat storage performance does not meet the requirements of practical applications.

[0006] The patented technology of "A high-temperature composite phase change heat storage material and its preparation method" (CN201210018441.5) prepares this material by co-firing inorganic salts, ceramic matrix, and high thermal conductivity materials. Although the preparation process of this method is simple and has good thermal conductivity, it is prone to leakage and has poor thermal cycle stability.

[0007] Literature I (Wang H, Ran X, Zhong Y, et al. Ternary chloride salt–porous ceramic composite as a high-temperature phase change material.) reports that a eutectic salt-silicon carbide porous ceramic high-temperature phase change material is prepared using silicon nitride porous ceramic, sodium chloride, potassium chloride, and magnesium chloride. This method conducts molten salt impregnation under vacuum high-temperature conditions and requires argon gas to be filled for pressurization. The process is complex and the equipment is expensive. Moreover, the molten salt impregnation content is only 38.67 - 39.67 wt%. After 300 thermal cycles, the molten salt content decreased by 9.3%. The low molten salt content, low energy storage density, and poor thermal cycle stability do not meet the actual application conditions.

[0008] There is a literature II (Liu J, Zhang Y, Su Z, et al. Novel low-cost anorthite porous ceramic-based binary chlorate high-temperature thermal energy storage material: preparation and characterization.) reporting that porous anorthite was prepared using blast furnace slag, bentonite and fly ash as raw materials, and a barium feldspar / NaCl-KCl phase change composite material was prepared by the melt infiltration method. Although this method has low raw material cost and simple process, the compressive strength of its porous anorthite is only 2.8 MPa, the molten salt impregnation rate is 39 wt%, and the molten salt content decreases by 4% after 100 thermal cycles, making it difficult to ensure the structural stability at high temperature and the demand for thermal energy storage after multiple thermal cycles. Summary of the Invention

[0009] The present invention aims to overcome the defects of the prior art, and the purpose is to provide a preparation method of a porous hexaaluminate-molten salt phase change composite material with simple process, short production cycle and low cost. The porous hexaaluminate-molten salt phase change composite material prepared by this method has high compressive strength, high molten salt content, high energy storage density and good thermal cycle stability.

[0010] To achieve the above purpose, the specific steps of the technical solution adopted by the present invention are as follows:

[0011] Step 1: Mix 80-90 wt% of activated alumina and 10-20 wt% of aragonite to obtain mixture I.

[0012] According to the mass ratio of mixture I: acrylamide: N-hydroxymethylacrylamide: N,N-methylenebisacrylamide of 100: 3-5: 0.2-2: 0.2-1, add acrylamide, N-hydroxymethylacrylamide and N,N-methylenebisacrylamide to mixture I and mix evenly to obtain mixture II.

[0013] Step 2: According to the mass ratio of mixture II: deionized water of 100: 25-45, add deionized water to mixture II and stir for 5-10 minutes to obtain a ceramic slurry.

[0014] According to the mass ratio of the ceramic slurry: non-ionic foaming agent of 100: 0.2-1, add a non-ionic foaming agent to the ceramic slurry and stir for 3-5 minutes to obtain a foamed slurry.

[0015] Add tert - butyl hydroperoxide to the foaming slurry according to the mass ratio of foaming slurry∶tert - butyl hydroperoxide of 10000∶0.1 - 1, stir for 2 - 4 minutes, pour into a mold, and dry at 110 - 160 °C for 12 - 24 hours, then demold to obtain a porous hexaaluminate blank.

[0016] Step 3: Under an air atmosphere and normal pressure conditions, heat the porous hexaaluminate blank to 1400 - 1600 °C and hold for 3 - 5 hours to obtain porous hexaaluminate.

[0017] Step 4: According to the mass ratio of porous hexaaluminate∶molten salt of 1∶10 - 15, bury the porous hexaaluminate in a corundum crucible containing the molten salt, then heat the corundum crucible to 20 - 40 °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 porous hexaaluminate to prepare a porous hexaaluminate - molten salt phase change composite material.

[0018] The purity of the activated alumina is greater than 99.8% and the particle size is less than 2 μm.

[0019] The purity of the aragonite is greater than 99.8%, the shape is needle - shaped or firecracker - shaped, and the particle size is less than 10 μm.

[0020] The purities of the acrylamide, N - hydroxymethylacrylamide, and N,N - methylenebisacrylamide are all greater than 99%, and the particle sizes are all less than 20 μm.

[0021] The rotation speed of the stirring in Step 2 is the same, and the rotation speed of the stirring is 500 - 800 revolutions per minute.

[0022] The non - ionic foaming agent is one of alcohols, ether alcohols, ethers, and esters.

[0023] The purity of the tert - butyl hydroperoxide is greater than 99%.

[0024] The purity of the molten salt is greater than 98% and the particle size is less than 100 μm.

[0025] The molten salt is one of sodium chloride, potassium chloride, sodium sulfate, potassium sulfate, sodium carbonate, and potassium carbonate. 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; the melting point of pure potassium sulfate is 1069 °C; the melting point of sodium carbonate is 851 °C; the melting point of pure potassium carbonate is 891 °C.

[0026] Due to the adoption of the above - mentioned technical solution, the present invention has the following positive effects compared with the prior art:

[0027] 1. The present invention uses sintered alumina, aragonite and molten salt (one of sodium chloride, potassium chloride, sodium sulfate, potassium sulfate, potassium carbonate and sodium carbonate) as main raw materials to prepare a porous calcium hexaaluminate-molten salt phase change composite material. The raw materials used are non-toxic, not easy to cause harm to the human body, low cost and environmentally friendly.

[0028] The present invention is different from the cold pressing and sintering method. The porous calcium hexaaluminate prepared by the foaming method has high compressive strength, excellent corrosion resistance and high-temperature stability after high-temperature sintering. Therefore, the product has high compressive strength, excellent corrosion resistance and good high-temperature stability, and can better ensure safety during thermal cycling.

[0029] The present invention is different from the vacuum impregnation method or the vacuum pressure impregnation method. The present invention adopts a molten salt spontaneous infiltration process and uses capillary force to make the molten salt spontaneously infiltrate into the porous calcium hexaaluminate, which not only simplifies the production process but also saves energy and resources.

[0030] 2. The present invention uses aragonite as one of its raw materials. Unlike the common cubic calcite-type calcium carbonate, the aragonite is needle-shaped or firecracker-shaped, with extremely high specific surface area and reactivity. The calcium hexaaluminate flake grains generated after sintering are large, and the interlaced growth forms an interlocking structure, which enhances the mechanical strength of the porous calcium hexaaluminate. At the same time, aragonite decomposes at high temperatures to form calcium oxide, which then reacts with aluminum oxide to form calcium hexaaluminate. This process leaves needle-shaped or firecracker-shaped in-situ pores on the calcium hexaaluminate skeleton. At the same time, the development and interlaced growth of the calcium hexaaluminate flakes form interstitial pores. These in-situ pores and interstitial pores have small pore diameters and strong capillary forces, which can generate adsorption force on molten salt under high temperature conditions, thereby improving the thermal cycling stability of the porous calcium hexaaluminate-molten salt phase change composite material.

[0031] 3. The present invention uses acrylamide, N-hydroxymethyl acrylamide, and N,N-methylenebisacrylamide as the gel system. Unlike existing gel systems, the gel system used in the present invention needs to be kept warm at high temperature (110-160°C) for 8-18 hours to completely gel, which can give full play to the bubble drainage and bubble merging effects of the foaming slurry. Since a plateau interface is formed between adjacent bubbles, under the action of surface tension, bubble drainage and bubble merging will cause the bubble wall at the location with large curvature of the plateau interface to become thinner and thinner until window holes are formed to connect the bubbles, thereby forming a large number of through holes. Therefore, the porous calcium hexaaluminate in the present invention has a large number of "bubble-window" structures and high permeability, which accelerates the penetration of molten salt during the spontaneous infiltration of molten salt. At the same time, the window hole structure with a small diameter prevents the molten salt from leaking outward during the thermal cycle, significantly improving the molten salt content and thermal cycle stability of the porous calcium hexaaluminate-molten salt phase change composite material.

[0032] 4. The present invention is different from other porous ceramics used for phase change composite materials. The present invention combines the foaming gel casting method and the in-situ pore formation technology, so that the porous calcium hexaaluminate of the present invention has a hierarchical porous structure. During the impregnation process, the capillary force generated at the window holes with small diameters is greater than the atmospheric pressure, resulting in a suction effect, realizing the rapid penetration of the molten salt and further infiltrating into the interior of the porous calcium hexaaluminate. During the thermal cycling process, the large pores are filled with the molten salt, the internal pressure is less than the atmospheric pressure, and the capillary force of the in-situ pores on the inner wall of the large pores will generate a suction force on the molten salt, thus stabilizing the molten salt and preventing the molten salt from leaking from the window holes. High molten salt content, high energy storage density, and excellent thermal cycling stability of the porous calcium hexaaluminate-molten salt phase change composite material are achieved.

[0033] Therefore, the production process of the present invention is simple, the production cycle is short, and the cost is low. The prepared porous calcium hexaaluminate-molten salt phase change composite material has high compressive strength, high molten salt content, high energy storage density, and good thermal cycling stability. Its performance is significantly superior to the phase change composite 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. Brief Description of the Drawings

[0034] Figure 1 SEM micrograph of the fracture surface of a porous calcium hexaaluminate-molten salt phase change composite material prepared according to the present invention.

[0035] Figure 2 is Figure 1 SEM micrograph of the fracture surface of the shown porous calcium hexaaluminate-molten salt phase change composite material after 500 thermal cycles

[0036] Figure 3 is Figure 1 SEM micrograph of the aragonite raw material used in the shown porous calcium hexaaluminate-molten salt phase change composite material;

[0037] Figure 4 is Figure 1 SEM micrograph of the fracture surface of the porous calcium hexaaluminate used in the shown porous calcium hexaaluminate-molten salt phase change composite material;

[0038] Figure 5 is Figure 4 Mercury intrusion pore size distribution diagram of the shown porous calcium hexaaluminate. Detailed Embodiments

[0039] The following further describes the present invention in conjunction with the detailed embodiments, which is not a limitation to the protection scope of the present invention.

[0040] A porous calcium hexaaluminate-molten salt phase change composite material and its preparation method. The preparation method of this detailed embodiment is as follows:

[0041] Step 1: Mix 80 - 90 wt% of activated alumina and 10 - 20 wt% of aragonite to obtain mixture I.

[0042] According to the mass ratio of mixture I∶acrylamide∶N - hydroxymethylacrylamide∶N,N - methylenebisacrylamide being 100∶3 - 5∶0.2 - 2∶0.2 - 1, add acrylamide, N - hydroxymethylacrylamide, and N,N - methylenebisacrylamide to mixture I and mix evenly to obtain mixture II.

[0043] Step 2: According to the mass ratio of mixture II∶deionized water being 100∶25 - 45, add deionized water to mixture II and stir for 5 - 10 minutes to obtain a ceramic slurry.

[0044] According to the mass ratio of the ceramic slurry∶non - ionic foaming agent being 100∶0.2 - 1, add the non - ionic foaming agent to the ceramic slurry and stir for 3 - 5 minutes to obtain a foamed slurry.

[0045] According to the mass ratio of the foamed slurry∶tert - butyl hydroperoxide being 10000∶0.1 - 1, add tert - butyl hydroperoxide to the foamed slurry, stir for 2 - 4 minutes, pour it into a mold, and dry it at 110 - 160 °C for 12 - 24 hours, then demold to obtain a porous calcium hexaaluminate green body.

[0046] Step 3: Under an air atmosphere and normal pressure conditions, heat the porous calcium hexaaluminate green body to 1400 - 1600 °C and hold for 3 - 5 hours to obtain porous calcium hexaaluminate.

[0047] Step 4: According to the mass ratio of porous calcium hexaaluminate∶molten salt being 1∶10 - 15, bury the porous calcium hexaaluminate in a corundum crucible containing the molten salt, then heat the corundum crucible to 20 - 40 °C higher than the melting point of the molten salt, hold for 3 - 5 hours, and cool naturally to remove the excess molten salt on the surface of the porous calcium hexaaluminate to prepare a porous calcium hexaaluminate - molten salt phase change composite material.

[0048] The stirring speed in Step 2 is the same, and the stirring speed is 500 - 800 revolutions per minute.

[0049] The non - ionic foaming agent is one of alcohols, ether alcohols, ethers, and esters.

[0050] The molten salt is one of sodium chloride, potassium chloride, sodium sulfate, potassium sulfate, sodium carbonate, and potassium carbonate.

[0051] In this specific embodiment:

[0052] The purity of the activated alumina is greater than 99.8%, and the particle size is less than 2 μm.

[0053] The purity of the said wollastonite is greater than 99.8%, its shape is needle-like or firecracker-like, and its particle size is less than 10 μm.

[0054] The purities of the said acrylamide, N-methylolacrylamide and N,N-methylenebisacrylamide are all greater than 99%, and their particle sizes are all less than 20 μm.

[0055] The purity of the said tert-butyl hydroperoxide is greater than 99%.

[0056] The purity of the said molten salt is greater than 98%, and its particle size is less than 100 μm.

[0057] The melting point of the said sodium chloride is 801 °C; the melting point of the said potassium chloride is 770 °C; the melting point of the said sodium sulfate is 884 °C; the melting point of the said potassium sulfate is 1069 °C; the melting point of the said sodium carbonate is 851 °C; the melting point of the said potassium carbonate is 891 °C.

[0058] Details are not repeated in the examples.

[0059] Example 1

[0060] A porous calcium hexaaluminate-molten salt phase change composite material and a preparation method thereof. The preparation method in this example is as follows:

[0061] Step 1: Mix 86 wt% of activated alumina and 14 wt% of wollastonite to obtain mixture I.

[0062] According to the mass ratio of mixture I∶acrylamide∶N-methylolacrylamide∶N,N-methylenebisacrylamide being 100∶5∶2∶1, add acrylamide, N-methylolacrylamide and N,N-methylenebisacrylamide to mixture I and mix evenly to obtain mixture II.

[0063] Step 2: According to the mass ratio of mixture II∶deionized water being 100∶36, add deionized water to mixture II and stir for 10 minutes to obtain a ceramic slurry.

[0064] According to the mass ratio of the ceramic slurry∶non-ionic foaming agent being 100∶0.4, add the non-ionic foaming agent to the ceramic slurry and stir for 5 minutes to obtain a foamed slurry.

[0065] According to the mass ratio of the foamed slurry∶tert-butyl hydroperoxide being 10000∶0.1, add tert-butyl hydroperoxide to the foamed slurry, stir for 4 minutes, pour it into a mold, dry it at 160 °C for 24 hours, and demold to obtain a porous calcium hexaaluminate green body.

[0066] Step 3: Under an air atmosphere and normal pressure conditions, heat the said porous calcium hexaaluminate green body to 1600 °C and hold for 4 hours to obtain porous calcium hexaaluminate.

[0067] Step 4: Bury the porous hexaaluminate in a corundum crucible containing the molten salt at a mass ratio of porous hexaaluminate to molten salt of 1:12. Then heat the corundum crucible to 40°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 porous hexaaluminate to obtain a porous hexaaluminate-molten salt phase change composite material.

[0068] The stirring speed in Step 2 is the same, and the stirring speed is 800 revolutions per minute.

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

[0070] The molten salt is potassium chloride.

[0071] The porous hexaaluminate prepared in this example was detected: the apparent porosity was 81.09%; the cold compressive strength was 24.46 MPa; the molten salt permeability was 61.46 wt%.

[0072] The porous hexaaluminate-molten salt phase change composite material prepared in this example was detected: the cold compressive strength was 56.84 MPa; the phase change temperature was 851.6°C; the phase change heat storage capacity was 216.95 J / g; the molten salt retention rate after 500 thermal cycles was 97.75%.

[0073] Example 2

[0074] A porous hexaaluminate-molten salt phase change composite material and its preparation method. The preparation method in this example is as follows:

[0075] Step 1: Mix 90 wt% of activated alumina and 10 wt% of aragonite to obtain mixture I.

[0076] According to the mass ratio of mixture I: acrylamide: N-methylolacrylamide: N,N'-methylenebisacrylamide of 100:3.5: 0.8:0.4, add acrylamide, N-methylolacrylamide and N,N'-methylenebisacrylamide to mixture I and mix evenly to obtain mixture II.

[0077] Step 2: According to the mass ratio of mixture II: deionized water of 100:45, add deionized water to mixture II and stir for 8 minutes to obtain a ceramic slurry.

[0078] According to the mass ratio of ceramic slurry: non-ionic foaming agent of 100:1, add the non-ionic foaming agent to the ceramic slurry and stir for 3.5 minutes to obtain a foamed slurry.

[0079] According to the mass ratio of foamed slurry: tert-butyl hydroperoxide of 10000:0.8, add tert-butyl hydroperoxide to the foamed slurry, stir for 2.5 minutes, pour it into a mold, and dry it at 145°C for 16 hours, then demold to obtain a porous hexaaluminate green body.

[0080] Step 3: Under an air atmosphere and normal pressure conditions, heat the porous hexaaluminate green body to 1550 °C and hold for 3 hours to obtain porous hexaaluminate.

[0081] Step 4: According to the mass ratio of porous hexaaluminate to molten salt of 1:15, bury the porous hexaaluminate in a corundum crucible containing the molten salt, then heat the corundum crucible to 20 °C above the melting point of the molten salt, hold for 5 hours, cool naturally, and remove the excess molten salt on the surface of the porous hexaaluminate to prepare a porous hexaaluminate-molten salt phase change composite material.

[0082] The stirring speed in Step 2 is the same, and the stirring speed is 800 revolutions per minute.

[0083] The non-ionic foaming agent is ether alcohol.

[0084] The molten salt is sodium chloride.

[0085] The porous hexaaluminate-molten salt phase change composite material prepared in this example is shown in the attached figure: Figure 1 is the SEM image of the fracture morphology of the porous hexaaluminate-molten salt phase change composite material prepared in this example; Figure 2 is Figure 1 the SEM image of the fracture morphology of the porous hexaaluminate-molten salt phase change composite material shown after 500 thermal cycles; Figure 3 is Figure 1 the SEM image of the aragonite raw material used in the porous hexaaluminate-molten salt phase change composite material shown; Figure 4 is Figure 1 the SEM image of the fracture morphology of the porous hexaaluminate used in the porous hexaaluminate-molten salt phase change composite material shown; Figure 5 is Figure 4 the mercury intrusion pore size distribution diagram of the porous hexaaluminate shown.

[0086] From Figure 1 and Figure 2 comparison, it can be seen that after 500 thermal cycles, the sodium chloride molten salt in the porous hexaaluminate-molten salt phase change composite material also maintains a good cubic crystal structure, without structural damage or erosion, and its microstructure remains consistent with that before thermal cycling, indicating that the prepared porous hexaaluminate has excellent high-temperature structural stability and mechanical strength, and the prepared porous hexaaluminate-molten salt phase change composite material has excellent thermal cycling stability. From Figure 3 it can be seen that the aragonite used in this example is needle-shaped or firecracker-shaped, and the in-situ pores generated can generate an adsorption force on the molten molten salt under high-temperature conditions, improving the thermal cycling stability of the porous hexaaluminate-molten salt phase change composite material.

[0087] From Figure 4(1) It can be seen that the porous calcium hexaaluminate has a porous structure characterized by spherical pores; from Figure 4 (2) It can be seen that the porous calcium hexaaluminate has a hierarchical porous structure, which can be divided into four types of pores: macropores formed by the foaming gel casting method, window pores formed by the drainage and coalescence of bubbles (see Figure 4 (3)(4) and (5)), in-situ pores generated by the decomposition of aragonite (see Figure 4 (6)), and interstitial pores formed by the stacking of calcium hexaaluminate flaky grains (see Figure 4 (6)); from Figure 5 It can be seen that the pore size distribution of the prepared porous calcium hexaaluminate by mercury intrusion porosimetry, in which the macropores that make up most of the volume improve the molten salt storage capacity, and the smaller pores (less than 30 μm) have high capillary forces and can quickly absorb the molten salt during the molten salt infiltration process, while ensuring the stability of the molten salt during the thermal cycling process.

[0088] The porous calcium hexaaluminate prepared in this example was detected: the apparent porosity was 85.02%; the cold crushing strength was 14.09 MPa; the molten salt permeability was 66.07 wt%.

[0089] The porous calcium hexaaluminate - molten salt phase change composite material prepared in this example was detected: the cold crushing strength was 49.02 MPa; the phase change temperature was 802.1 °C; the phase change heat storage capacity was 318.46 J / g; the molten salt retention rate after 500 thermal cycles was 97.20%.

[0090] Example 3

[0091] A porous calcium hexaaluminate - molten salt phase change composite material and its preparation method. The preparation method described in this example is as follows:

[0092] Step 1: Mix 83 wt% of activated alumina and 17 wt% of aragonite to obtain mixture I.

[0093] According to the mass ratio of mixture I∶acrylamide∶N - hydroxymethylacrylamide∶N,N - methylenebisacrylamide being 100∶4∶1.1∶0.6, add acrylamide, N - hydroxymethylacrylamide, and N,N - methylenebisacrylamide to mixture I and mix evenly to obtain mixture II.

[0094] Step 2: According to the mass ratio of mixture II∶deionized water being 100∶28, add deionized water to mixture II and stir for 7 minutes to obtain a ceramic slurry.

[0095] According to the mass ratio of the ceramic slurry∶non - ionic foaming agent being 100∶0.8, add the non - ionic foaming agent to the ceramic slurry and stir for 4 minutes to obtain a foamed slurry.

[0096] Add tert-butyl hydroperoxide to the foaming slurry at a mass ratio of foaming slurry:tert-butyl hydroperoxide of 10,000:0.5, stir for 3 minutes, pour it into a mold, dry it at 130 °C for 20 hours, and demold to obtain a porous calcium hexaaluminate green body.

[0097] Step 3: Under an air atmosphere and normal pressure conditions, heat the porous calcium hexaaluminate green body to 1500 °C and hold for 3.5 hours to obtain porous calcium hexaaluminate.

[0098] Step 4: At a mass ratio of porous calcium hexaaluminate:molten salt of 1:14, bury the porous calcium hexaaluminate in a corundum crucible containing the molten salt, then heat the corundum crucible to 30 °C above the melting point of the molten salt, hold for 4 hours, cool naturally, and remove the excess molten salt on the surface of the porous calcium hexaaluminate to prepare a porous calcium hexaaluminate-molten salt phase change composite material.

[0099] The rotation speed of the stirring in Step 2 is the same, and the rotation speed of the stirring is 600 revolutions per minute.

[0100] The non-ionic foaming agent is an ether.

[0101] The molten salt is sodium sulfate.

[0102] The porous calcium hexaaluminate prepared in this example was detected: the apparent porosity was 84.02%; the normal temperature compressive strength was 15.90 MPa; the molten salt permeability was 65.47 wt%.

[0103] The porous calcium hexaaluminate-molten salt phase change composite material prepared in this example was detected: the normal temperature compressive strength was 53.59 MPa; the phase change temperature was 885.6 °C; the phase change heat storage capacity was 108.03 J / g; the molten salt retention rate after 500 thermal cycles was 96.36%.

[0104] Example 4

[0105] A porous calcium hexaaluminate-molten salt phase change composite material and a preparation method thereof. The preparation method in this example is as follows:

[0106] Step 1: Mix 80 wt% of activated alumina and 20 wt% of aragonite to obtain mixture I.

[0107] Add acrylamide, N-hydroxymethylacrylamide, and N,N-methylenebisacrylamide to mixture I at a mass ratio of mixture I:acrylamide:N-hydroxymethylacrylamide:N,N-methylenebisacrylamide of 100:3:0.2:0.2, and mix evenly to obtain mixture II.

[0108] Step 2: Add deionized water to mixture II at a mass ratio of mixture II:deionized water of 100:25, stir for 5 minutes to obtain a ceramic slurry.

[0109] Add a non-ionic foaming agent to the ceramic slurry at a mass ratio of ceramic slurry: non-ionic foaming agent of 100:0.2, and stir for 3 minutes to obtain a foamed slurry.

[0110] Add tert-butyl hydroperoxide to the foamed slurry at a mass ratio of foamed slurry: tert-butyl hydroperoxide of 10,000:1, stir for 2 minutes, pour it into a mold, dry it at 110 °C for 12 hours, demold it, and obtain a porous calcium hexaaluminate green body.

[0111] Step 3: Under an air atmosphere and normal pressure conditions, heat the porous calcium hexaaluminate green body to 1400 °C and hold for 5 hours to obtain porous calcium hexaaluminate.

[0112] Step 4: At a mass ratio of porous calcium hexaaluminate: molten salt of 1:10, bury the porous calcium hexaaluminate in a corundum crucible containing the molten salt, then heat the corundum crucible to 35 °C above the melting point of the molten salt, hold for 3 hours, cool naturally, and remove the excess molten salt on the surface of the porous calcium hexaaluminate to prepare a porous calcium hexaaluminate-molten salt phase change composite material.

[0113] The stirring speed in Step 2 is the same, and the stirring speed is 500 revolutions per minute.

[0114] The non-ionic foaming agent is an ester.

[0115] The molten salt is sodium carbonate.

[0116] The porous calcium hexaaluminate prepared in this example was detected: the apparent porosity was 79.05%; the normal temperature compressive strength was 19.86 MPa; the molten salt permeability was 59.98 wt%.

[0117] The porous calcium hexaaluminate-molten salt phase change composite material prepared in this example was detected: the normal temperature compressive strength was 50.25 MPa; the phase change temperature was 770.3 °C; the phase change heat storage value was 98.97 J / g; the molten salt retention rate after 500 thermal cycles was 98.05%.

[0118] Example 5

[0119] A porous calcium hexaaluminate-molten salt phase change composite material and its preparation method. Except for the molten salt, Example 5 is the same as Example 1:

[0120] The molten salt is potassium sulfate.

[0121] The porous calcium hexaaluminate prepared in this example was detected: the apparent porosity was 81.09%; the normal temperature compressive strength was 24.46 MPa; the molten salt permeability was 61.23 wt%.

[0122] The prepared porous calcium hexaaluminate-salt phase change composite material in this example was tested: the compressive strength at room temperature was 56.66 MPa; the phase change temperature was 1069.2 °C; the phase change heat storage capacity was 129.81 J / g; the molten salt retention rate after 500 thermal cycles was 96.66%.

[0123] Example 6

[0124] A porous calcium hexaaluminate-salt phase change composite material and its preparation method. In Example 6, except for the molten salt, it is the same as Example 1:

[0125] The molten salt is potassium sulfate.

[0126] The prepared porous calcium hexaaluminate in this example was tested: the apparent porosity was 81.09%; the compressive strength at room temperature was 24.46 MPa; the molten salt permeability was 62.76 wt%.

[0127] The prepared porous calcium hexaaluminate-salt phase change composite material in this example was tested: the compressive strength at room temperature was 57.63 MPa; the phase change temperature was 891.1 °C; the phase change heat storage capacity was 116.41 J / g; the molten salt retention rate after 500 thermal cycles was 98.43%.

[0128] Comparative Example 1

[0129] A calcium hexaaluminate-salt phase change composite material and its preparation method. In Comparative Example 1, except for the following, the rest is the same as Example 1:

[0130] Step 1: Mix 86 wt% of activated alumina and 114 wt% of calcite-type calcium carbonate to obtain mixture I.

[0131] The purity of the calcite-type calcium carbonate is greater than 99.8%, the shape is cubic, and the particle size is less than 10 μm.

[0132] The prepared porous calcium hexaaluminate in this example was tested: the apparent porosity was 80.96%; the compressive strength at room temperature was 15.92 MPa; the molten salt permeability was 52.45 wt%.

[0133] The prepared calcium hexaaluminate-salt phase change composite material in this example was tested: the compressive strength at room temperature was 47.40 MPa; the phase change temperature was 851.6 °C; the phase change heat storage capacity was 185.14 J / g; the molten salt retention rate after 500 thermal cycles was 90.07%.

[0134] Comparative Example 2

[0135] A directly foamed calcium hexaaluminate-salt phase change composite material and its preparation method. In Comparative Example 2, except for the following, the rest is the same as Example 1:

[0136] Step 1: Mix 86 wt% activated alumina and 14 wt% aragonite to obtain mixture I;

[0137] Step 2: Add deionized water to mixture II according to the mass ratio of mixture I: deionized water being 100:36, and stir for 10 minutes to obtain a ceramic slurry;

[0138] Add a non-ionic foaming agent to the ceramic slurry according to the mass ratio of ceramic slurry: non-ionic foaming agent being 100:0.4, and stir for 5 minutes to obtain a foamed slurry;

[0139] Stir the foamed slurry for 4 minutes, pour it into a mold, dry it at 160 °C for 24 hours, and demold to obtain a porous calcium hexaaluminate green body;

[0140] The porous calcium hexaaluminate prepared in this example was tested: the apparent porosity was 81.16%; the cold compressive strength was 28.08 MPa; the molten salt permeability was 48.07 wt%.

[0141] The directly foamed calcium hexaaluminate - molten salt phase change composite material prepared in this example was tested: the cold compressive strength was 53.72 MPa; the phase change temperature was 851.6 °C; the phase change heat storage capacity was 169.69 J / g; the molten salt retention rate after 500 thermal cycles was 82.96%.

[0142] This specific embodiment has the following positive effects compared with the prior art:

[0143] 1. This specific embodiment uses sintered alumina, aragonite and molten salt (one of sodium chloride, potassium chloride, sodium sulfate, potassium sulfate, potassium carbonate and sodium carbonate) as the main raw materials to prepare a porous calcium hexaaluminate - molten salt phase change composite material. The raw materials used are non-toxic, not likely to cause harm to the human body, low in cost, and environmentally friendly.

[0144] This specific embodiment is different from the cold pressing sintering method. The porous calcium hexaaluminate prepared by the foaming method used has high compressive strength, excellent corrosion resistance and high-temperature stability after high-temperature sintering. Therefore, the prepared product has high compressive strength, excellent corrosion resistance and good high-temperature stability, and can better ensure safety during the thermal cycling use process.

[0145] This specific embodiment is different from the vacuum impregnation method or the vacuum pressure impregnation method. This specific embodiment adopts the molten salt spontaneous infiltration process, and uses capillary force to make the molten salt spontaneously infiltrate into the porous calcium hexaaluminate, which not only simplifies the production process, but also saves energy and resources.

[0146] 2. This specific embodiment uses aragonite as one of the raw materials. Unlike the common cubic calcite-type calcium carbonate, the aragonite is needle-shaped or firecracker-shaped, with extremely high specific surface area and reactivity. The calcium hexaaluminate flake grains generated after sintering are large, and the interlaced growth forms an interlocking structure, which enhances the mechanical strength of the porous calcium hexaaluminate. At the same time, aragonite decomposes at high temperatures to form calcium oxide, which then reacts with aluminum oxide to form calcium hexaaluminate. This process leaves needle-shaped or firecracker-shaped in-situ pores on the calcium hexaaluminate skeleton. At the same time, the development and interlaced growth of the calcium hexaaluminate flakes form interstitial pores. These in-situ pores and interstitial pores have small pore sizes and strong capillary forces, which can generate adsorption force on molten salt under high temperature conditions, thereby improving the thermal cycling stability of the porous calcium hexaaluminate-molten salt phase change composite material.

[0147] 3. This specific embodiment adopts acrylamide, N-hydroxymethyl acrylamide, and N,N-methylene bis acrylamide as the gel system. Unlike the existing gel system, the gel system adopted in this specific embodiment needs to be kept warm for 8-18 hours under high temperature (110-160°C) to completely gel, which can give full play to the bubble drainage and bubble merging effects of the foaming slurry. Since a plateau interface is formed between adjacent bubbles, under the action of surface tension, bubble drainage and bubble merging will cause the bubble wall at the location with large curvature of the plateau interface to become thinner and thinner until window holes are formed to connect the bubbles, thereby forming a large number of through holes. Therefore, the porous calcium hexaaluminate in this specific embodiment has a large number of "bubble-window" structures and high permeability, which accelerates the penetration of molten salt during the spontaneous infiltration of molten salt. At the same time, the window hole structure with a small diameter prevents the molten salt from leaking outward during the thermal cycle, significantly improving the molten salt content and thermal cycle stability of the porous calcium hexaaluminate-molten salt phase change composite material.

[0148] 4. This specific embodiment is different from other porous ceramics used for phase change composite materials. This specific embodiment combines the foaming gel injection molding method and the in-situ pore forming technology, so that the porous calcium hexaaluminate of this specific embodiment has a hierarchical porous structure. During the impregnation process, the capillary force generated at the window holes with small diameters is greater than the atmospheric pressure, resulting in an imbibition effect, which enables the rapid penetration of the molten salt and further infiltration into the porous calcium hexaaluminate. During the thermal cycle, the inside of the large pores is filled with molten salt, the internal pressure is less than the atmospheric pressure, and the capillary force of the in-situ pores on the inner wall of the large pores will generate suction on the molten salt, thereby stabilizing the molten salt and making it difficult for the molten salt to leak from the window holes. The porous calcium hexaaluminate-molten salt phase change composite material has a high molten salt content, high energy storage density, and excellent thermal cycle stability.

[0149] Therefore, the production process of this specific implementation method is simple, the production cycle is short, and the production cost is low. The prepared porous calcium hexaaluminate-salt phase change composite material has high compressive strength, high salt content, high energy storage density, and good thermal cycle stability. Its performance is significantly superior to the phase change composite materials in the existing phase change heat storage systems in the field of high-temperature heat storage (such as heat energy storage in solar thermal power plants, industrial waste heat recovery, regenerative combustion technology, etc.), and has good application prospects.

Claims

1. A preparation method of a porous calcium hexaaluminate-salt phase change composite material, characterized in that The steps of the preparation method are as follows: Step 1: Mix 80-90 wt% of activated alumina and 10-20 wt% of aragonite to obtain mixture I; According to the mass ratio of mixture I: acrylamide: N-methylolacrylamide: N,N-methylenebisacrylamide being 100: 3-5: 0.2-2: 0.2-1, add acrylamide, N-methylolacrylamide and N,N-methylenebisacrylamide to mixture I and mix evenly to obtain mixture II; Step 2: According to the mass ratio of mixture II: deionized water being 100: 25-45, add deionized water to mixture II and stir for 5-10 minutes to obtain a ceramic slurry; According to the mass ratio of the ceramic slurry: non-ionic foaming agent being 100: 0.2-1, add the non-ionic foaming agent to the ceramic slurry and stir for 3-5 minutes to obtain a foamed slurry; According to the mass ratio of the foamed slurry: tert-butyl hydroperoxide being 10000: 0.1-1, add tert-butyl hydroperoxide to the foamed slurry and stir for 2-4 minutes, pour it into a mold, and dry it at 110-160 °C for 12-24 hours, then demold to obtain a porous calcium hexaaluminate blank; Step 3: Under air atmosphere and normal pressure conditions, heat the porous calcium hexaaluminate blank to 1400-1600 °C and hold for 3-5 hours to obtain porous calcium hexaaluminate; Step 4: According to the mass ratio of the porous calcium hexaaluminate: molten salt being 1: 10-15, bury the porous calcium hexaaluminate in a corundum crucible containing the molten salt, then heat the corundum crucible to 20-40 °C higher than the melting point of the molten salt, hold for 3-5 hours, and cool naturally to remove the excess molten salt on the surface of the porous calcium hexaaluminate to prepare a porous calcium hexaaluminate-molten salt phase change composite material.

2. The preparation method of the porous calcium hexaaluminate-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 2 μm.

3. The preparation method of the porous calcium hexaaluminate-salt phase change composite material according to claim 1, characterized in that The purity of the aragonite is greater than 99.8%, the shape is needle-like or firecracker-like, and the particle size is less than 10 μm.

4. The preparation method of the porous calcium hexaaluminate-salt phase change composite material according to claim 1, characterized in that The purities of the acrylamide, N-methylolacrylamide and N,N-methylenebisacrylamide are all greater than 99%, and the particle sizes are all less than 20 μm.

5. The preparation method of the porous calcium hexaaluminate-salt phase change composite material according to claim 1, wherein The stirring speed in Step 2 is the same, and the stirring speed is 500-800 revolutions per minute.

6. The preparation method of the porous calcium hexaaluminate-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.

7. The preparation method of the porous calcium hexaaluminate-salt phase change composite material according to claim 1, wherein The purity of the tert-butyl hydroperoxide is greater than 99%.

8. The preparation method of the porous calcium hexaaluminate-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.

9. The preparation method of the porous calcium hexaaluminate-salt phase change composite material according to claim 1 or claim 8, characterized in that, The molten salt is one of sodium chloride, potassium chloride, sodium sulfate, potassium sulfate, sodium carbonate and potassium carbonate. Among them: the melting point of the sodium chloride is 801 °C; the melting point of the potassium chloride is 770 °C; the melting point of the sodium sulfate is 884 °C; the melting point of the pure potassium sulfate is 1069 °C; the melting point of the sodium carbonate is 851 °C; the melting point of the pure potassium carbonate is 891 °C.

10. A porous calcium hexaaluminate-molten salt phase change composite material, characterized in that The porous calcium hexaaluminate-molten salt phase change composite material is the porous calcium hexaaluminate-molten salt phase change composite material prepared according to the preparation method of the porous calcium hexaaluminate-molten salt phase change composite material described in any one of claims 1-9.

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