Composite phase change material with three-layer core-shell structure and preparation method thereof

Abstract

The present application relates to a kind of composite phase change material of three-layer core-shell structure and its preparation method, belong to thermal insulation energy storage material technical field.To solve the problem that the double-layer core-shell structure of silica aerogel composite phase change material is easy to leak and difficult to disperse uniformly in concrete matrix, the present application provides a kind of composite phase change material of three-layer core-shell structure, with porous fly ash hollow microsphere as outer shell layer, with silica aerogel as intermediate shell layer, with n-octadecane phase change material as inner core.The present application increases the heat transfer area of phase change material, thereby improving the thermal conductivity and heat storage and release rate.The critical stress volume fraction of the pore size on porous fly ash hollow microsphere is greater than the critical stress volume fraction of n-octadecane, which can prevent phase change material from leaking and maintain stable thermal insulation effect.The present application increases the bulk density of composite phase change material, so that it can be uniformly dispersed in concrete, and by increasing the mixing amount, the thermal insulation effect can be better played, and the thermal conductivity of concrete is reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of thermal insulation and energy storage materials, and particularly relates to a composite phase change material with a three-layer core-shell structure and a preparation method thereof. BACKGROUND

[0002] Silica aerogel is a new type of material with a three-dimensional network structure, has the characteristics of high porosity, large specific surface area and low thermal conductivity, is a kind of high-temperature-resistant aerogel that is most studied in the field of thermal insulation at present, and has gradually become a mainstream thermal insulation material. Researchers have found that organic or inorganic phase change materials can be filled into the aerogel, which can enhance the degree of heat absorption and release without changing the thermal insulation performance.

[0003] Liquid paraffin as a phase change material can absorb or release a large amount of heat when the temperature changes, improve the energy utilization efficiency, and has significant application in building energy saving. Adding phase change materials to concrete can significantly improve the thermal insulation performance of concrete, prolong the service life and reduce energy consumption. The invention patent with the publication number CN115124976B “Composite phase change gel thermal insulation and energy storage material and preparation method thereof” fills paraffin phase change material into the nanopores of aerogel to obtain phase change aerogel. This kind of phase change aerogel with double-layer core-shell structure often cannot avoid the loss of phase change material, and the phase change material will leak in the process of continuous phase change, resulting in the decline of the thermal insulation and heat storage capacity of the composite material. Moreover, the composite material has a low bulk density and a large difference in density with the concrete matrix, which is not easy to disperse uniformly in the concrete, not only affecting the mechanical properties and durability of the whole concrete, but also reducing the heat transfer efficiency of the phase change material in the concrete. SUMMARY

[0004] To solve the problems of easy leakage of the silica aerogel composite phase change material with double-layer core-shell structure and difficulty in uniform dispersion in the concrete matrix, the application provides a composite phase change material with a three-layer core-shell structure and a preparation method thereof.

[0005] The technical scheme of the application:

[0006] A preparation method of a composite phase change material with a three-layer core-shell structure, comprising the following steps:

[0007] Step one, preparing porous fly ash hollow microspheres:

[0008] The ice crystals are used as electrolyte and dissolved in water to obtain an electrolyte solution. A graphite rod is used as two electrodes, and the fly ash hollow microspheres are added. The electrolysis is carried out under the conditions of an electrolysis temperature of 970-1050℃ and an electrolysis voltage of 0.7-1.40V, to obtain porous fly ash hollow microspheres with a pore size of 50-150nm on the surface. The obtained porous fly ash hollow microspheres are washed to neutral and fully dried;

[0009] Step two, preparation of phase change silica hydrosol:

[0010] Mix water glass with deionized water in the volume ratio of 1:3-6, and mechanically stir to obtain a water glass solution. The obtained water glass solution is passed through a strong acid cation exchange resin at a certain flow rate to obtain a silica hydrosol with pH of 3-5. The obtained silica hydrosol is mixed with n-octadecane at room temperature, and stirred uniformly to obtain a phase change silica hydrosol.

[0011] Step three, preparation of phase change silica hydrogel-porous fly ash hollow microsphere:

[0012] In a vacuum environment, the phase change silica hydrosol obtained in step two is mixed with the porous fly ash hollow microsphere obtained in step one. After vacuum treatment for a certain period of time, the solid material floating on the upper layer of the obtained vacuum mixed system is collected and treated in a water bath at a certain temperature to obtain a phase change silica hydrogel-porous fly ash hollow microsphere.

[0013] Step four, preparation of composite phase change material with three-layer core-shell structure:

[0014] The phase change silica hydrogel-porous fly ash hollow microsphere obtained in step three is soaked in anhydrous ethanol, treated in a water bath at a certain temperature, and the solid material is collected by filtration to obtain a phase change silica alcogel-porous fly ash hollow microsphere.

[0015] The obtained phase change silica alcogel-porous fly ash hollow microsphere is soaked in a trimethylchlorosilane / n-hexane mixed solution, treated in a water bath at a certain temperature, and the solid material is collected by filtration to obtain a phase change silica silylated gel-porous fly ash hollow microsphere.

[0016] The obtained phase change silica silylated gel-porous fly ash hollow microsphere is soaked in a n-hexane solution, the solid material is collected by filtration, and after standing for a certain period of time, it is dried under a step-by-step heating environment to obtain a three-layer core-shell structure of phase change silica aerogel-porous fly ash hollow microsphere composite phase change material.

[0017] Further, step one also includes the treatment of screening fly ash hollow microspheres before electrolysis: pour fly ash hollow microspheres into water, select fly ash hollow microspheres with complete hollow structure floating on the water surface, and dry thoroughly for standby.

[0018] Further, the particle size of the fly ash hollow microspheres in step one is 50-200 μm; the mass concentration of cryolite in the electrolyte is 20-30%; the electrolysis time is 60-120 min, and the sufficient drying condition is drying at 80-110°C for not less than 12 h.

[0019] Further, the modulus of the water glass in step two is 3.3M; the mechanical stirring is 300-360r / min for 30-40min, and the flow rate of the water glass solution through the strong acidic cation exchange resin is 200-300mL / h.

[0020] Further, the mass ratio of the silica hydrosol to n-octadecane in step two is 0.5-0.8:0.15-0.24.

[0021] Further, the solid-liquid ratio of the phase change silica hydrosol to the porous fly ash hollow microsphere in step three is 20-40g:200mL, the vacuum degree of the vacuum environment is -0.03--0.08MPa, the vacuum treatment time is 30min, the temperature of the water bath treatment is 40℃, and the water bath treatment time is 6h.

[0022] Further, the solid-liquid ratio of the phase change silica hydrogel-porous fly ash hollow microsphere to anhydrous ethanol in step four is 40-60g:400mL, the solid-liquid ratio of the phase change silica hydrogel-porous fly ash hollow microsphere to the trimethylchlorosilane / n-hexane mixed solution is 40-60g:200mL, the molar ratio of trimethylchlorosilane to n-hexane in the trimethylchlorosilane / n-hexane mixed solution is 1:2-1:3; and the solid-liquid ratio of the phase change silica silylated gel-porous fly ash hollow microsphere to the n-hexane solution is 40-60g:200mL.

[0023] Further, the water bath treatment in step four is constant temperature water bath at 40℃ for 24h, the standing is for 24h, and the drying treatment by gradual temperature increase is drying at 60℃, 80℃, 120℃ and 160℃ respectively for 2h.

[0024] The three-layer core-shell structure composite phase change material prepared by the preparation method of the three-layer core-shell structure composite phase change material of the present application has an outer shell layer of porous fly ash hollow microspheres, an intermediate shell layer of silica aerogel, and a core layer of n-octadecane.

[0025] Further, the phase change temperature of the composite phase change material is 27-30℃.

[0026] The present application has the following advantages:

[0027] The application provides a three-layer core-shell structure composite phase change material, which takes porous fly ash hollow microspheres with a pore diameter of 50-150 nm etched on the surface as an outer shell layer, takes silica aerogel as an intermediate shell layer, and takes n-octadecane phase change material as an inner core.

[0028] Compared with the porous fly ash hollow microspheres and the silica aerogel, the three-layer core-shell structure composite phase change material has an increased volume density, can be uniformly dispersed in a concrete base material, can better play a heat preservation effect by increasing the mixing amount, and can reduce the thermal conductivity of the concrete.

[0029] The three-layer core-shell structure composite phase change material has simple and easy-to-control preparation process, low cost and easy-to-obtain raw materials, and is conducive to realizing large-scale production of the composite phase change material. The three-layer core-shell structure composite phase change material prepared by the application is suitable for the fields of building energy saving and energy storage, and has a wide application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 A microscopic photo of the porous fly ash hollow microspheres prepared in Example 1;

[0031] Figure 2 A microscopic photo of the three-layer core-shell structure composite phase change material prepared in Example 1. DETAILED DESCRIPTION

[0032] The technical solutions of the application are further described below in combination with examples, but are not limited thereto. Any modification or equivalent replacement to the technical solutions of the application without departing from the spirit and scope of the technical solutions of the application should be included in the protection scope of the application. The process equipment or device not specifically mentioned in the following examples is the conventional equipment or device in the field, and the raw materials used in the examples of the application can be commercially available if not specifically mentioned. The technical means used in the examples of the application is the conventional means known to those skilled in the art if not specifically mentioned.

[0033] Example 1

[0034] The example provides a three-layer core-shell structure composite phase change material and a preparation method thereof.

[0035] The required materials are prepared according to the following weight parts in the example.

[0036] Fly ash hollow microsphere 1.8 parts, particle size 100-120 μm,

[0037] n-octadecane 0.18 parts,

[0038] water glass 0.7 parts, modulus 3.3M,

[0039] strong acid cation exchange resin 3 parts,

[0040] absolute ethanol 3 parts,

[0041] n-hexane 3 parts,

[0042] trimethylchlorosilane 0.7 parts,

[0043] deionized water 0.7 parts.

[0044] The properties of the preparation materials used in this example are described as follows:

[0045] (1) Fly ash hollow microsphere, particle size 100-120 μm, smooth surface, silver-white spherical, with fine pinholes on the shell, wall thickness 3-10 μm, specific surface area 0.35-0.40 m 2 / g, bulk density 250-450 kg / m 3 , apparent specific gravity 0.6-0.75 g / cm 3 , 1 / 3 of fly ash, specific resistance 108-1011 Ω·m, refractoriness 1600-1700℃, load softening point 1200℃, thermal conductivity at room temperature 0.081-0115 W / (m·k). The content of SiO2, Al2O3, CaO, MgO, Na2O, K2O, Fe2O3, TiO2, SO3 in fly ash hollow microsphere is 56.60%, 34.60%, 0.92%, 0.47%, 0.56%, 1.44%, 3.37%, 1.30%, 0.40% respectively, 80% of the phase is glass phase, and the crystalline phase contains mullite and quartz and other mineral components.

[0046] (2) Phase change material n-octadecane, melting point about 25.2-29.9℃, latent heat of phase change 243.5 KJ / kg, density 785 kg / m 3 , a very stable substance.

[0047] (3) Particle size of strong acid cation exchange resin 0.3-1.2 mm, exchange capacity ≥1.8 Na. Its role is to remove sodium ions in water glass (formula 1), to prevent the prepared aerogel from containing sodium chloride crystals affecting the thermal conductivity.

[0048] Na2SiO3+ H ++ H2O → Si(HO)4 (1)

[0049] (4) n-Hexane, molecular weight 86.18 g / mol, density 0.655 g / cm 3 (20℃), surface tension about 18.4 mN / m. Its functions: ① as a solvent of surface hydrophobic reagent, combined with silane reagent TMCS (trimethylchlorosilane), to make silica aerogel surface hydrophobic, prevent aerogel from moisture absorption and aging; ② replace water in aerogel pores, low surface tension of n-hexane greatly reduces capillary pressure, making the drying process more gentle.

[0050] (5) Trimethylchlorosilane (TMCS), molecular weight 108.64 g / mol, density 0.857 g / cm 3 (25℃). Its function: the gel contains a large number of hydrophilic silanol groups (Si-OH), TMCS reacts with silanol groups to form trimethylsiloxy groups (-Si(CH3)3) and hydrogen chloride, which changes the surface of the gel from hydrophilic to hydrophobic (Formula 2), making the aerogel-porous hollow microsphere moisture resistant.

[0051] (CH3)3SiCl + Si-OH → Si-O-Si(CH3)3 + HC (2)

[0052] (6) Cryolite (Na3AlF6): white fine crystalline, melting point 1009℃, its functions: ① can reduce the loss in the melting point reduction process of oxides; ② enhance the conductivity of the electrolyte to improve the efficiency of electrolysis.

[0053] (7) Anhydrous ethanol: purity 99.5%. Its function: anhydrous ethanol can promote the chemical reaction of silica hydrogel during the aging process, making the siloxane bond (Si-O-Si) further cross-linked, enhancing the network structure and mechanical stability of the gel.

[0054] The preparation method of the three-layer core-shell structure composite phase change material of the embodiment includes the following steps:

[0055] A preparation method of a three-layer core-shell structure composite phase change material includes the following steps:

[0056] Step one, preparation of porous fly ash hollow microspheres:

[0057] Fly ash hollow microspheres with a particle size of 100-120 μm were poured into water, and fly ash hollow microspheres with complete hollow structure floating on the water surface were selected, and dried at 105℃.

[0058] The cryolite is used as electrolyte, which is dissolved in water to obtain electrolyte with 25% mass concentration of cryolite, graphite rods are used as two electrodes, and the screened fly ash hollow microspheres are added, and then electrolysis is carried out under the conditions of electrolysis temperature of 1010℃ and electrolysis voltage of 1.08-1.26V for 90min. During the electrolysis process, the oxides such as silicon dioxide and aluminum oxide in the fly ash hollow microspheres are reduced to form elements to form pores on the surface, and the porous fly ash hollow microspheres with 100-120nm pore diameter on the surface are obtained. The obtained porous fly ash hollow microspheres are washed to neutral, and then dried at 105℃ for more than 12h for standby.

[0059] Figure 1 The porous fly ash hollow microspheres prepared in the embodiment can be seen from the figure, and the fly ash hollow microspheres are etched with nanoscale pore diameter on the surface.

[0060] Step two, preparation of phase change silica hydrosol:

[0061] The modulus of 3.3M water glass is mixed with deionized water in a volume ratio of 1:4 to obtain a water glass solution, and the obtained water glass solution is dropped into a strong acid cation exchange resin, and the dropping speed of the liquid drops is 250mL / h, and a silica hydrosol with pH of 3 is obtained. In an acidic environment, an aerogel with larger specific surface area and smaller pore diameter can be prepared compared with an alkaline environment.

[0062] The obtained silica hydrosol is mixed with n-octadecane at room temperature at a mass ratio of 0.7:0.18, and stirred uniformly to obtain a phase change silica hydrosol.

[0063] Step three, preparation of phase change silica hydrogel-porous fly ash hollow microspheres:

[0064] The phase change silica hydrosol obtained in step two and the porous fly ash hollow microspheres prepared in step one are poured into a vacuum cylinder at a solid-liquid ratio of 20g:200mL, and after ensuring that the phase change silica hydrosol is submerged in the porous fly ash hollow microspheres, the vacuum cylinder is closed, the vacuum pump is started, and the vacuum treatment is carried out under the condition of vacuum degree of-0.08MPa for 30min. The solid material floating on the upper layer of the obtained vacuum mixed system is collected and placed in a 40℃ constant temperature water bath for 6h to obtain phase change silica hydrogel-porous fly ash hollow microspheres.

[0065] Step four, preparation of composite phase change material with three-layer core-shell structure:

[0066] The phase change silica hydrogel-porous fly ash hollow microspheres obtained in step three are soaked in anhydrous ethanol at a solid-liquid ratio of 40g:400mL, and then placed in a 40℃ constant temperature water bath for 24h. The solid material is collected by filtration to obtain phase change silica alcogel-porous fly ash hollow microspheres.

[0067] The obtained phase change silica sol-gel-porous fly ash hollow microsphere is immersed in a trimethylchlorosilane / n-hexane mixed solution with a solid-liquid ratio of 40 g: 200 mL, the molar ratio of trimethylchlorosilane to n-hexane in the trimethylchlorosilane / n-hexane mixed solution is 1:2, and the temperature is kept at 40°C in a constant temperature water bath for 24 hours. The solid material is collected by filtration to obtain phase change silica silylated gel-porous fly ash hollow microsphere.

[0068] The obtained phase change silica silylated gel-porous fly ash hollow microsphere is immersed in a n-hexane solution with a solid-liquid ratio of 40 g: 400 mL, the solid material is collected by filtration, and is left to stand for 24 hours. The three-layer core-shell structure phase change silica aerogel-porous fly ash hollow microsphere composite phase change material is obtained by drying at 60°C, 80°C, 120°C and 160°C respectively for 2 hours.

[0069] Figure 2 The micrograph of the three-layer core-shell structure composite phase change material prepared in this example shows that the outer shell layer of the composite material is porous fly ash hollow microsphere, the middle shell layer is silica aerogel, and the core layer is n-octadecane.

[0070] The specific role and phase change principle of the n-octadecane adsorbed to the silica aerogel in this example are as follows:

[0071] ① In the heat release process: when the temperature is below 27°C, the liquid solidifies into a solid and releases heat. The aerogel is a three-dimensional continuous porous solid material with low density, high porosity, high specific surface area and low thermal conductivity. By adsorbing between the pore walls of the aerogel, the heat transfer area is increased, the interfacial thermal conductivity is improved, and the heat storage and release rate of the phase change material is accelerated.

[0072] ② In the heat absorption process: when the temperature is higher than 27°C, the solid melts into a liquid and absorbs heat. The phase change material is adsorbed into the mesopores. By this composite method of n-octadecane and silica aerogel, the super-insulation performance of the silica aerogel material is exerted. The cells are divided into countless nanoscale pores. Since the nanoscale pore size is smaller than the average free path of the gas, the gas in the cells adheres to the inner wall of the nanoscale pores, loses fluidity, and thus reduces the heat transfer generated by the gas convection.

[0073] The overall thermal conductivity formula of the three-layer core-shell structure phase change silica aerogel-porous fly ash hollow microsphere composite phase change material is as follows:

[0074] Because the heat conduction of the porous material is composed of heat convection, gas heat conduction, solid heat conduction and heat radiation, the overall thermal conductivity of the phase change silica aerogel-porous fly ash hollow microsphere composite phase change material can be represented as:

[0075] λtot = λconv + λgas + λsolid + λrad (3)

[0076] where λconv is the heat conduction due to the heat convection caused by the temperature gradient of the gas in the pore structure. The contribution of the heat convection to the heat conduction can be neglected when the diameter of the selected porous fly ash cenospheres is less than 1 mm.

[0077] where λgas is the heat conduction due to the molecular collision in the structure. This part of the heat conduction is calculated by the following formula:

[0078] λgas = λga / (1 + 2βKn) (4)

[0079] Kn = Lmean / Φ (5)

[0080] λga is the heat conduction of the free gas, β is the coefficient of energy exchange between the gas molecules and the pore wall, generally 1.5-2.0, Kn is the Knudsen number, Lmean is the average free path of the gas molecules, and Φ is the pore size of the porous material.

[0081] As can be seen from equations (4) and (5), the heat conduction of the gas mainly depends on the pore size in the porous structure and the average free path of the gas molecules. When the phase change material occupies the volume of the silica aerogel, the collision of the gas molecules is further restricted, thereby reducing the heat conduction.

[0082] λsolid is the heat conduction due to the lattice vibration in the skeleton of the porous structure. The calculation formula of this part of the heat conduction is as follows:

[0083] λsolid = 1 / (3pCvVaΛph) (6)

[0084] where p is the solid density, Cv is the specific heat capacity of the crystal under the Debye model, Va is the average speed of phonons, and Λph is the average free path of phonons. The diameter of the primary particles of the silica aerogel is about 2-5 nm, and the average free path of phonons is about 0.58 nm, both of which are in the same order of magnitude. Therefore, the solid heat conduction on the silica skeleton will be significantly reduced due to the microscale effect. In addition, the silica aerogel skeleton is a three-dimensional porous structure, and after absorbing the phase change material, the solid heat conduction path is further complicated, thereby significantly reducing the solid heat conduction.

[0085] λrad is the heat conduction due to the radiation heat conduction. The calculation formula of this part of the heat conduction is as follows:

[0086] λrad = 16n 2 σB / (3ρK) em T 3 (7)

[0087] where n is the refractive index, k is the Stefan-Boltzmann constant, and p is the material density, k is the average extinction coefficient, and T is the temperature. It can be seen from the formula that the thermal radiation is related to the temperature, material density and extinction coefficient. The thermal conductivity increases with the increase of temperature. In addition, the high porosity of the silica aerogel adsorbs the phase change material, which has a strong scattering effect on thermal radiation, thereby effectively reducing the heat transfer by radiation. 2 ·K4), p is the material density, K em is the average extinction coefficient, and T is the temperature. It can be seen from the formula that the thermal radiation is related to the temperature, material density and extinction coefficient. The thermal conductivity increases with the increase of temperature. In addition, the high porosity of the silica aerogel adsorbs the phase change material, which has a strong scattering effect on thermal radiation, thereby effectively reducing the heat transfer by radiation.

[0088] Therefore, the silica aerogel can effectively weaken the three heat transfer factors of gas heat conduction, solid heat conduction and radiation heat conduction by adsorbing n-octadecane.

[0089] The porous fly ash hollow microsphere as the shell has the following effects:

[0090] ①It has a hard shell that can protect the porous structure of the silica aerogel from being damaged;

[0091] ②The porous fly ash hollow microsphere as the shell can prevent the leakage of the phase change material.

[0092] The porous fly ash hollow microsphere is selected as the shell: because n-octadecane is dispersed in the porous fly ash hollow microsphere during the heat absorption process, the microsphere has many small pores, which form a more small and interconnected random network. When these networks are not appropriate, it is difficult for n-octadecane to pass through. There is a critical value between the carbon particles from the state of not being able to pass through to the state of being able to pass through. When the planar stress volume sphere formed by the particles gradually increases to the percolation threshold, more and more particles are associated, and the superposition of the shear stress of the particles exceeds the shear yield stress, and then a percolation channel appears.

[0093] At this time:

[0094] S = Sc = dc + tc (8)

[0095] ∮s = ∮sc = (Sc / dc)3∮rc (9)

[0096] From the above:

[0097] ∮sc = (dc+tc / dc)3∮rc (10)

[0098] Where S is the stress volume sphere diameter, Sc is the critical planar stress volume sphere diameter, dc is the critical phase change material particle size, ∮sc is the critical planar stress volume sphere fraction, ∮rc is the critical material volume fraction, and tc is the distance between particles.

[0099] It can be concluded that the stress volume fraction of n-octadecane d / S < 0.8, the stress volume fraction of the critical state is basically unchanged, the stress volume fraction of the critical state is 0.206, the percolation threshold remains unchanged, each particle remains independent in shape and cannot be associated, when the selected pore size is between 50-150 nm, the critical stress volume fraction is 0.213-0.345 according to the above formula, so the critical stress volume fraction of the porous fly ash hollow microsphere is greater than that of n-octadecane, that is, the shear yield stress of the porous fly ash hollow microsphere is greater than the shear stress between n-octadecane particles, and the selected particle size of the porous fly ash hollow microsphere can effectively prevent the leakage of n-octadecane.

[0100] Example 2

[0101] The present embodiment provides a three-layer core-shell structure composite phase change material and a preparation method thereof.

[0102] The difference between the present embodiment and Example 1 is that the particle size of the fly ash hollow microsphere used in the present embodiment is 120-160 μm.

[0103] Example 3

[0104] The present embodiment provides a three-layer core-shell structure composite phase change material and a preparation method thereof.

[0105] The difference between the present embodiment and Example 1 is that the particle size of the fly ash hollow microsphere used in the present embodiment is 160-200 μm.

[0106] Example 4

[0107] The present embodiment provides a three-layer core-shell structure composite phase change material and a preparation method thereof.

[0108] The difference between the present embodiment and Example 1 is that in step one, the porous fly ash hollow microsphere is prepared as follows:

[0109] The fly ash hollow microsphere with a particle size of 100-120 μm is poured into water, and the fly ash hollow microsphere with an intact hollow structure floating on the water surface is selected, and is dried at 105°C.

[0110] The cryolite is used as an electrolyte, is dissolved in water and stirred to obtain an electrolyte with a mass concentration of 25% of cryolite, a graphite rod is used as two electrodes, the screened fly ash hollow microsphere is added, and the fly ash hollow microsphere is electrolyzed at an electrolysis temperature of 1010°C and an electrolysis voltage of 0.7-0.85V for 90min, during the electrolysis process, the oxides such as silicon dioxide and aluminum oxide in the fly ash hollow microsphere are reduced to form an element to form a hole on the surface, and the porous fly ash hollow microsphere with a pore size of 50-80 nm on the surface is obtained, the obtained porous fly ash hollow microsphere is washed to neutral, and is dried at 105°C for 12h or more for standby.

[0111] Example 5

[0112] The present example provides a composite phase change material with a three-layer core-shell structure and a preparation method thereof.

[0113] The difference between the present example and Example 1 is that, in the present example, the porous fly ash hollow microspheres are prepared in Step 1 as follows:

[0114] Fly ash hollow microspheres with a particle size of 100-120 μm are poured into water, and fly ash hollow microspheres with an intact hollow structure floating on the water surface are selected and dried at 105°C.

[0115] An electrolyte of 25% cryolite is prepared by dissolving cryolite in water and stirring until uniform. Graphite rods are used as two electrodes, and the screened fly ash hollow microspheres are added. The fly ash hollow microspheres are electrolyzed at an electrolysis temperature of 1010°C and an electrolysis voltage of 0.85-1.08 V for 90 min. During the electrolysis process, the oxides such as silicon dioxide and aluminum oxide in the fly ash hollow microspheres are reduced to form elements, thereby forming pores on the surface. The porous fly ash hollow microspheres with a pore size of 80-100 nm on the surface are obtained. The obtained porous fly ash hollow microspheres are washed to neutral and dried at 105°C for 12 h or more for standby use.

[0116] Example 6

[0117] The present example provides a composite phase change material with a three-layer core-shell structure and a preparation method thereof.

[0118] The difference between the present example and Example 1 is that, in the present example, the porous fly ash hollow microspheres are prepared in Step 1 as follows:

[0119] Fly ash hollow microspheres with a particle size of 100-120 μm are poured into water, and fly ash hollow microspheres with an intact hollow structure floating on the water surface are selected and dried at 105°C.

[0120] An electrolyte of 25% cryolite is prepared by dissolving cryolite in water and stirring until uniform. Graphite rods are used as two electrodes, and the screened fly ash hollow microspheres are added. The fly ash hollow microspheres are electrolyzed at an electrolysis temperature of 1010°C and an electrolysis voltage of 1.26-1.40 V for 90 min. During the electrolysis process, the oxides such as silicon dioxide and aluminum oxide in the fly ash hollow microspheres are reduced to form elements, thereby forming pores on the surface. The porous fly ash hollow microspheres with a pore size of 120-150 nm on the surface are obtained. The obtained porous fly ash hollow microspheres are washed to neutral and dried at 105°C for 12 h or more for standby use.

[0121] Comparative Example 1

[0122] The comparative example provides a three-layer core-shell structure composite phase change material and a preparation method thereof.

[0123] The difference between the comparative example and example 1 is only that, in step one of the comparative example, the porous fly ash hollow microsphere is prepared as follows:

[0124] The fly ash hollow microspheres with a particle size of 50-100 μm are poured into water, and the fly ash hollow microspheres with an integral hollow structure floating on the water surface are selected and dried at 105 ℃.

[0125] The cryolite is used as an electrolyte, and the cryolite is dissolved in water to obtain an electrolyte with a mass concentration of 25% of the cryolite. A graphite rod is used as two electrodes, and the screened fly ash hollow microspheres are added. The fly ash hollow microspheres are electrolyzed at an electrolysis temperature of 1010 ℃ and an electrolysis voltage of 1.08-1.26 V for 90 min. During the electrolysis process, the oxides such as silicon dioxide and aluminum oxide in the fly ash hollow microspheres are reduced to form an element to form a hole on the surface, and the porous fly ash hollow microspheres with a hole diameter of 100-120 nm on the surface are obtained. The obtained porous fly ash hollow microspheres are washed to be neutral, and dried at 105 ℃ for more than 12 h for standby.

[0126] The properties of the three-layer core-shell structure composite phase change materials prepared in examples 1-6 and the comparative example 1 are detected by using conventional detection methods in the art, and the results are shown in Table 1.

[0127] Table 1

[0128]

[0129] As shown by the comparison of the data in Table 1, the three-layer core-shell structure composite phase change material provided by the application has a lower thermal conductivity, and can better achieve a stable heat preservation effect. At the same time, the phase change loss of the three-layer core-shell structure composite phase change material is only 0.06-0.21% after 150 phase change cycles, which indicates that the critical stress volume fraction of the hole diameter on the outer shell layer of the porous fly ash hollow microspheres is greater than the critical stress volume fraction of the n-octadecane, and can prevent the leakage of the phase change material and effectively reduce the loss of the phase change material.

Claims

1. A method for preparing a three-layer core-shell composite phase change material, characterized in that, Includes the following steps: Step 1: Preparation of porous fly ash hollow microspheres: Cryolite was dissolved in water and stirred evenly to obtain an electrolyte. Graphite rods were used as two electrodes, and hollow fly ash microspheres with a particle size of 100-120 μm were added. Electrolysis was carried out at an electrolysis temperature of 970-1050 ℃ and an electrolysis voltage of 0.7-1.40 V to obtain porous hollow fly ash microspheres with a surface pore size of 50-150 nm. The obtained porous hollow fly ash microspheres were washed until neutral and thoroughly dried. Step 2: Preparation of phase change silica hydrosol: Water glass and deionized water are mixed at a volume ratio of 1:3~6 and mechanically stirred to obtain a water glass solution. The obtained water glass solution is passed through a strong acid cation exchange resin at a certain flow rate to obtain a silica hydrosol with a pH of 3~5. The obtained silica hydrosol is mixed with n-octadecane at room temperature and stirred evenly to obtain a phase change silica hydrosol. Step 3: Preparation of phase change silica hydrogel-porous fly ash hollow microspheres: Under vacuum, the phase change silica hydrosol obtained in step two is mixed with the porous fly ash hollow microspheres obtained in step one. After vacuum treatment for a certain period of time, the solid material floating on the upper layer of the obtained vacuum mixing system is collected and placed in a water bath at a certain temperature to obtain phase change silica hydrogel-porous fly ash hollow microspheres. Step 4: Preparation of composite phase change materials with a three-layer core-shell structure: The phase change silica hydrogel-porous fly ash hollow microspheres obtained in step 3 were immersed in anhydrous ethanol, treated with a water bath at a certain temperature, and the solid material was collected by filtration to obtain phase change silica alcohol gel-porous fly ash hollow microspheres. The obtained phase change silica alcohol gel-porous fly ash hollow microspheres were immersed in a trimethylchlorosilane / n-hexane mixed solution, treated with a water bath at a certain temperature, and the solid material was collected by filtration to obtain phase change silica silyl alkylation gel-porous fly ash hollow microspheres. The obtained phase change silica silylated gel-porous fly ash hollow microspheres were immersed in a hexane solution, the solid material was collected by filtration, and after standing for a certain period of time, it was dried under a stepwise heating environment. The stepwise heating drying process was carried out at 60 ℃, 80 ℃, 120 ℃ and 160 ℃ for 2 h each to obtain a three-layer core-shell structured phase change silica aerogel-porous fly ash hollow microsphere composite phase change material.

2. The method for preparing a three-layer core-shell composite phase change material according to claim 1, characterized in that, Step one also includes the pre-electrolysis screening of fly ash hollow microspheres: pour the fly ash hollow microspheres into water, select the fly ash hollow microspheres with complete hollow structures that float on the water surface, and dry them thoroughly for later use.

3. The method for preparing a three-layer core-shell composite phase change material according to claim 1 or 2, characterized in that, The mass concentration of cryolite in the electrolyte in step one is 20-30%; the electrolysis time is 60-120 min; and the sufficient drying conditions are drying at 80-110 ℃ for no less than 12 h.

4. The method for preparing a three-layer core-shell composite phase change material according to claim 3, characterized in that, The modulus of the water glass in step two is 3.3 M; the mechanical stirring is carried out at 300~360 r / min for 30~40 min; and the flow rate of the water glass solution through the strong acid cation exchange resin is 200~300 mL / h.

5. The method for preparing a three-layer core-shell composite phase change material according to claim 4, characterized in that, The mass ratio of silica hydrosol to n-octadecane in step two is 0.5~0.8:0.15~0.

24.

6. The method for preparing a three-layer core-shell composite phase change material according to claim 5, characterized in that, In step three, the solid-liquid ratio of the phase change silica hydrosol to the porous fly ash hollow microspheres is 20~40 g:200 mL, the vacuum degree of the vacuum environment is -0.03~-0.08 MPa, the vacuum treatment time is 30 min, the water bath treatment temperature is 40 ℃, and the water bath treatment time is 6 h.

7. A method for preparing a three-layer core-shell composite phase change material according to claim 6, characterized in that, In step four, the solid-liquid ratio of the phase change silica hydrogel-porous fly ash hollow microspheres to anhydrous ethanol is 40-60 g: 400 mL; the solid-liquid ratio of the phase change silica alcohol gel-porous fly ash hollow microspheres to the trimethylchlorosilane / n-hexane mixed solution is 40-60 g: 200 mL; the molar ratio of trimethylchlorosilane to n-hexane in the trimethylchlorosilane / n-hexane mixed solution is 1:2-1:3; and the solid-liquid ratio of the phase change silica silylated gel-porous fly ash hollow microspheres to the n-hexane solution is 40-60 g: 200 mL.

8. A method for preparing a three-layer core-shell composite phase change material according to claim 7, characterized in that, The water bath treatment in step four is a constant temperature water bath at 40 ℃ for 24 h, and the standing time is 24 h.

9. A composite phase change material with a three-layer core-shell structure prepared by the preparation method of a composite phase change material with a three-layer core-shell structure as described in any one of claims 1-8, characterized in that, The outer shell is made of porous fly ash hollow microspheres, the middle shell is made of silica aerogel, and the core layer is made of n-octadecane.

10. The three-layer core-shell composite phase change material according to claim 9, characterized in that, The phase transition temperature of the composite phase change material is 27~30 ℃.

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