Fused salt heat storage material based on lithium nitrate and polyvinylidene fluoride hybridized zeolite imidazate framework structure and preparation method of fused salt heat storage material

Abstract

The invention discloses a fused salt heat storage material based on a lithium nitrate and polyvinylidene fluoride hybridized zeolite imidazate framework structure. The fused salt heat storage material comprises the following components in parts by mass: 20-35 parts of sodium nitrate, 25-40 parts of potassium nitrate, 10-20 parts of lithium nitrate, 10-25 parts of calcium nitrate and 0.5-2 parts of a polyvinylidene fluoride hybridized zeolite imidazate framework structure material, the polyvinylidene fluoride hybridized zeolite imidazate skeleton structure material is prepared by the following steps: step 1, adding a zinc nitrate solution into a 2-methylimidazole solution under a stirring condition, standing, carrying out a crystallization reaction, washing and drying to obtain a zeolite imidazate skeleton structure material; and step 2, mixing the zeolite imidazate skeleton structure material, polyvinylidene fluoride and dimethylformamide, and drying to obtain the polyvinylidene fluoride hybridized zeolite imidazate skeleton structure material. The fused salt heat storage material is high in specific heat capacity and low in viscosity.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of molten salt heat storage materials, in particular to a molten salt heat storage material based on lithium nitrate and polyvinylidene fluoride hybrid zeolite imidazolate framework structure and a preparation method thereof. BACKGROUND

[0002] The molten salt heat storage material using molten salt substances for heat energy storage realizes efficient storage and release of heat energy through a cycle process of heat absorption, heat storage and heat release. In the field of energy, especially in the application of medium and high temperature heat storage, the material plays a crucial role. Although the molten salt heat storage materials on the market have good thermal stability and low cost, they mainly depend on alkali metal / alkaline earth metal salts, and have problems of high melting point and insufficient specific heat capacity.

[0003] The specific heat capacity of the current commercial molten salt heat storage material is not high, and even after multi-component compounding, the specific heat capacity is only 1.8 J / (g oC), which still cannot meet the requirement of the specific heat capacity of heat transfer fluid of solar thermal power generation of more than 2.25 J / (g oC), resulting in a large volume and low energy efficiency of the heat storage system.

[0004] In order to solve the above problems, the prior art compensates for the above problems by increasing the amount of alkali metal / alkaline earth metal salt or multi-component compounding, but the excessive addition of lithium ions and calcium ions in the prior art will increase the viscosity of the molten salt by 20%-30%, significantly reduce the flowability, increase the pumping energy consumption, and the single inorganic ion modification is difficult to synergistically optimize the melting point, specific heat capacity, thermal conductivity and other parameters, and the comprehensive performance of the material is limited. How to provide a molten salt heat storage material with low melting point and high specific heat capacity has become a technical problem to be solved in the field. SUMMARY

[0005] The purpose of the present application is to solve the technical defects of the prior art, such as low specific heat capacity of molten salt heat storage material, large volume of heat storage system and low energy efficiency, and to provide a molten salt heat storage material based on lithium nitrate and polyvinylidene fluoride hybrid zeolite imidazolate framework structure and a preparation method thereof.

[0006] The technical scheme adopted to achieve the purpose of the present application is as follows: A molten salt heat storage material based on lithium nitrate and polyvinylidene fluoride hybrid zeolite imidazolate framework structure, comprising the following components in mass fraction: 20-35 parts of sodium nitrate, 25-40 parts of potassium nitrate, 10-20 parts of lithium nitrate, 10-25 parts of calcium nitrate and 0.5-2 parts of polyvinylidene fluoride hybrid zeolite imidazolate framework structure material; The polyvinylidene fluoride hybrid zeolite imidazolate framework structure material is prepared by the following steps: Step 1, after stirring, zinc nitrate solution is added to 2-methyl imidazole solution, and then crystallization reaction is carried out by standing, washing and drying, to obtain a zeolite imidazolate framework material; Step 2, after mixing the zeolite imidazolate framework material, polyvinylidene fluoride and dimethylformamide, drying is carried out to obtain a polyvinylidene fluoride hybrid zeolite imidazolate framework material.

[0007] In the above technical solution, in step 1, the 2-methyl imidazole solution is a 2-methyl imidazole methanol solution, and the zinc nitrate solution is a methanol solution of zinc nitrate hexahydrate; the concentration of 2-methyl imidazole in the 2-methyl imidazole methanol solution is 0.5-1 mol / L; the concentration of zinc nitrate hexahydrate in the methanol solution of zinc nitrate hexahydrate is 0.05-0.1 mol / L; the volume ratio of the methanol solution of zinc nitrate hexahydrate to the 2-methyl imidazole methanol solution is 1: (3-5).

[0008] In the above technical solution, in step 1, the stirring speed is 2000-3000 r / min, the standing temperature is 25-35℃, and the time is 12-24 h.

[0009] In the above technical solution, in step 1, the drying temperature is 60-80℃, and the time is 12-18 h.

[0010] In the above technical solution, in step 2, the mass ratio of the zeolite imidazolate framework material to polyvinylidene fluoride is 1: (5-10), and the amount ratio of the zeolite imidazolate framework material to dimethylformamide is 1 g: (5-15) mL.

[0011] In the above technical solution, in step 2, the mixing is carried out under stirring, the stirring speed is 500-1000 r / min, the time is 6-10 h, and the temperature is 60-80℃; the drying temperature is 25-35℃, and the time is 10-20 h.

[0012] Another aspect of the present application also includes a preparation method of a molten salt heat storage material based on lithium nitrate and polyvinylidene fluoride hybrid zeolite imidazolate framework, comprising the following steps: Step S1, after mixing sodium nitrate, potassium nitrate, lithium nitrate and calcium nitrate, dissolving in deionized water to form an inorganic molten salt solution; Step S2, after mixing the inorganic molten salt solution and the polyvinylidene fluoride hybrid zeolite imidazolate framework material, drying is carried out to obtain a mixed powder; Step S3, the mixed powder is heated to melt, and after cooling, a molten salt heat storage material is obtained.

[0013] The preparation method provided by the application realizes the molecular-level uniform mixing of various nitrate salts by using water as a solvent, avoids the component segregation caused by traditional mechanical mixing, and meanwhile, Ca²⁺ dissociated from calcium nitrate in the aqueous solution can form a pre-coordination structure with lithium nitrate; the inorganic molten salt solution, the antioxidant and the polyvinylidene hybrid zeolite imidazolate framework material are uniformly distributed in the molten salt matrix by the solution dispersion method, so that the chemical stability and dispersion uniformity of the mixed powder are improved.

[0014] In the step S1, the sodium nitrate, the potassium nitrate, the lithium nitrate and the calcium nitrate are mixed and then dissolved in the deionized water under stirring, the stirring temperature is 60-80 DEG C, the stirring time is 1-2 h, and the stirring speed is 100-200 r / min.

[0015] In the step S2, the drying is vacuum drying, the vacuum drying temperature is 80-100 DEG C, and the vacuum drying time is 12-24 h.

[0016] In the step S3, the melting is carried out in an inert atmosphere, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere, the heating rate for melting is 5-10 DEG C / min, after melting, the temperature is kept for 0.5-1 h, and the cooling rate is 1-5 DEG C / min.

[0017] Compared with the prior art, the application has the following beneficial effects: 1. The lithium nitrate destroys the lattice order arrangement due to the small ionic radius, cooperates with the calcium nitrate to regulate the eutectic system, significantly reduces the melting point of the molten salt, and the sodium nitrate and the potassium nitrate form a basic framework to ensure the stability of the system at medium and high temperatures; the polyvinylidene hybrid zeolite imidazolate framework material enhances the adsorption and coordination of the molten salt ions through the high specific surface area and porous structure of the zeolite imidazolate framework, and the polyvinylidene improves the interfacial compatibility of the molten salt through the flexible network of the polymer chain, thereby avoiding the agglomeration problem of the traditional nanofiller, and forming a ternary synergistic system of "low melting point-high heat storage-strong stability".

[0018] 2. The porous structure of the polyvinylidene hybrid zeolite imidazolate framework material in the molten salt heat storage material provides additional heat storage sites for the molten salt, improves the latent heat and specific heat capacity through the physical adsorption and chemical coordination mechanisms, and at the same time, the inorganic framework of the zeolite imidazolate framework material constructs a heat conduction network, the molecular chain of the polyvinylidene hybrid inhibits the ion migration resistance of the molten salt at high temperature, so that the thermal conductivity and fluidity are simultaneously improved; the calcium nitrate not only further reduces the eutectic point, but also enhances the interface bonding between the molten salt and the hybrid material through the strong coordination ability of calcium ions, inhibits high-temperature decomposition, and prolongs the service life. DETAILED DESCRIPTION

[0019] The application will be further described in connection with the specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the application and should not be used to limit the application.

[0020] Example 1 The present embodiment provides a molten salt heat storage material based on lithium nitrate and polyvinylidene fluoride hybrid zeolitic imidazolate framework, which comprises the following components in mass fraction: 20 parts of sodium nitrate, 25 parts of potassium nitrate, 10 parts of lithium nitrate, 10 parts of calcium nitrate and 0.5 parts of polyvinylidene fluoride hybrid zeolitic imidazolate framework material.

[0021] The preparation method of the polyvinylidene fluoride hybrid zeolitic imidazolate framework material comprises the following steps: Step 1: 100g of zinc nitrate hexahydrate is dissolved in 3.36L of methanol to obtain solution A, and 413g of 2-methylimidazole is dissolved in 0.52L of methanol to obtain solution B. Under the condition of 25℃, solution B is poured into solution A under the condition of stirring at 2000r / min. After standing for 12h under the condition of 25℃, a white precipitate is generated. After filtration and washing with methanol, the washed precipitate is dried at 60℃ for 12h to obtain a zeolitic imidazolate framework material.

[0022] Step 2: 1g of the zeolitic imidazolate framework material is added to 50mL of dimethylformamide and ultrasonically dispersed for 30min to form a dispersion liquid. Then, 5g of polyvinylidene fluoride is added, and the mixture is stirred at 60℃ and 500r / min for 6h. After that, the obtained solution is dried at 25℃ for 10h to obtain a polyvinylidene fluoride hybrid zeolitic imidazolate framework material.

[0023] The preparation method of the molten salt heat storage material comprises the following steps: S1: After mixing sodium nitrate, potassium nitrate, lithium nitrate and calcium nitrate, they are dissolved in deionized water under the condition of stirring at 100r / min and 60℃ for 1h to form an inorganic molten salt solution; S2: After mixing the inorganic molten salt solution and the polyvinylidene fluoride hybrid zeolitic imidazolate framework material, vacuum drying is performed at 80℃ for 12h to obtain a mixed powder; S3: The mixed powder is heated to 150℃ at a heating rate of 5℃ / min, and then kept at 150℃ for 0.5h. After that, the temperature is cooled to room temperature at a cooling rate of 1℃ / min to obtain a molten salt heat storage material.

[0024] Example 2 The present embodiment provides a molten salt heat storage material based on lithium nitrate and polyvinylidene fluoride hybrid zeolitic imidazolate framework: The molten salt heat storage material comprises components in the following mass fractions: 35 parts of sodium nitrate, 40 parts of potassium nitrate, 20 parts of lithium nitrate, 25 parts of calcium nitrate and 2 parts of polyvinylidene fluoride hybrid zeolitic imidazolate framework material.

[0025] The preparation method of the polyvinylidene fluoride hybrid zeolitic imidazolate framework material comprises the following steps: Step 1: 100 g of zinc nitrate hexahydrate is dissolved in 6.72 L of methanol to obtain solution A, and 2758 g of 2-methylimidazole is dissolved in 3.49 L of methanol to obtain solution B. Under the condition of 35 DEG C, solution B is poured into solution A under the condition of stirring at 3000 r / min. After standing for 24 h under the condition of 35 DEG C, a white precipitate is generated. After filtration and washing with methanol, the washed precipitate is dried at 80 DEG C for 18 h to obtain a zeolitic imidazolate framework material.

[0026] Step 2: 1 g of the zeolitic imidazolate framework material is added to 50 mL of dimethylformamide and ultrasonically dispersed for 60 min to form a dispersion liquid. Then, 10 g of polyvinylidene fluoride is added, and the obtained solution is stirred at 80 DEG C and 1000 r / min for 10 h. After that, the obtained solution is dried at 35 DEG C for 20 h to obtain a polyvinylidene fluoride hybrid zeolitic imidazolate framework material.

[0027] The preparation method of the molten salt heat storage material comprises the following steps: S1: sodium nitrate, potassium nitrate, lithium nitrate and calcium nitrate are mixed and dissolved in deionized water under the condition of stirring at 200 r / min and 80 DEG C for 2 h to form an inorganic molten salt solution; S2: the inorganic molten salt solution and the polyvinylidene fluoride hybrid zeolitic imidazolate framework material are mixed and vacuum dried at 100 DEG C for 24 h to obtain a mixed powder; S3: the mixed powder is heated to 200 DEG C at a heating rate of 10 DEG C / min, and then kept at 200 DEG C for 1 h. After that, the mixed powder is cooled to room temperature at a cooling rate of 5 DEG C / min to obtain a molten salt heat storage material.

[0028] Example 3 The present embodiment provides a molten salt heat storage material based on lithium nitrate and polyvinylidene fluoride hybrid zeolitic imidazolate framework: The molten salt heat storage material comprises components in the following mass fractions: 27 parts of sodium nitrate, 32 parts of potassium nitrate, 15 parts of lithium nitrate, 18 parts of calcium nitrate and 1 part of polyvinylidene fluoride hybrid zeolitic imidazolate framework material.

[0029] The preparation method of the polyvinylidene fluoride hybrid zeolitic imidazolate framework material comprises the following steps: Step 1: Dissolve 100g of zinc nitrate hexahydrate in 4.52L of methanol to obtain solution A, and dissolve 1580g of 2-methylimidazole in 2.49L of methanol to obtain solution B. Under the condition of 30℃ and stirring at 2500r / min, pour solution B into solution A. Under the condition of 30℃ and standing for 18h, a white precipitate is formed. After filtration, the precipitate is washed with methanol. The washed precipitate is dried at 70℃ for 15h to obtain zeolite imidazole ester framework structure material.

[0030] Step 2: Add 1g of zeolite imidazole ester framework material to 50mL of dimethylformamide, ultrasonically disperse for 45min to form a dispersion, then add 7g of polyvinylidene fluoride, and stir for 8h at 70℃ and 750r / min. Then dry the resulting solution at 30℃ for 15h to obtain polyvinylidene fluoride hybrid zeolite imidazole ester framework material.

[0031] The preparation method of the molten salt thermal storage material includes the following steps: S1. Sodium nitrate, potassium nitrate, lithium nitrate and calcium nitrate are mixed and stirred at 150 r / min and 70℃ for 1.5 h to dissolve in deionized water to form an inorganic molten salt solution. S2. The inorganic molten salt solution and the polyvinylidene fluoride hybrid zeolite imidazole ester framework structure material are mixed and then vacuum dried at 90°C for 18 hours to obtain a mixed powder. S3. The mixed powder is heated to 175°C at a heating rate of 7°C / min, held at that temperature for 0.75h, and then cooled to room temperature at a cooling rate of 3°C to obtain molten salt thermal storage material.

[0032] Comparative Example 1 This comparative example provides a molten salt thermal storage material and its preparation method, which specifically includes the following steps: The molten salt thermal storage material comprises the following components in parts by weight: 27 parts sodium nitrate, 32 parts potassium nitrate, and 15 parts lithium nitrate.

[0033] The preparation method of the molten salt thermal storage material includes the following steps: S1. Sodium nitrate, potassium nitrate and lithium nitrate are mixed and stirred at 150 r / min and 70℃ for 1.5 h to dissolve in deionized water to form an inorganic molten salt solution. S2. The inorganic molten salt solution is vacuum dried at 90°C for 18 hours to obtain a mixed powder; S3. The mixed powder is heated to 175°C at a heating rate of 7°C / min, held at that temperature for 0.75h, and then cooled to room temperature at a cooling rate of 3°C to obtain molten salt thermal storage material.

[0034] Comparative Example 2 This comparative example provides a molten salt thermal storage material and its preparation method, which specifically includes the following steps: The molten salt thermal storage material comprises the following components in parts by weight: 27 parts sodium nitrate, 32 parts potassium nitrate, 15 parts lithium nitrate, and 18 parts calcium nitrate.

[0035] The preparation method of the molten salt thermal storage material includes the following steps: S1. Sodium nitrate, potassium nitrate, lithium nitrate and calcium nitrate are mixed and stirred at 150 r / min and 70℃ for 1.5 h to dissolve in deionized water to form an inorganic molten salt solution. S2. The inorganic molten salt solution is vacuum dried at 90°C for 18 hours to obtain a mixed powder; S3. The mixed powder is heated to 175°C at a heating rate of 7°C / min, held at that temperature for 0.75h, and then cooled to room temperature at a cooling rate of 3°C to obtain molten salt thermal storage material.

[0036] The performance of the molten salt thermal storage materials in Examples 1-3 and Comparative Examples 1-2 of this invention was tested. The test parameters included melting point, decomposition point, latent heat of phase change, specific heat capacity, thermal conductivity, and viscosity. The test results are shown in Table 1. The melting point, decomposition point, and latent heat of phase change were determined by differential scanning calorimetry and thermogravimetric analysis. The specific heat capacity was measured using a simultaneous thermal analyzer, the thermal conductivity was measured using a thermal conductivity meter, and the viscosity was measured using a rheometer.

[0037] Table 1 Performance test results of molten salt thermal storage materials Observing Table 1, the molten salt thermal storage materials shown in Examples 1-3 of the present invention have significantly lower melting points compared to the comparative examples. At the same time, their thermal stability, latent heat of phase change, specific heat capacity, and thermal conductivity have all been significantly improved. In addition, their viscosity has also been improved, meeting the requirements of solar thermal power generation for the specific heat capacity of the heat transfer fluid.

[0038] The above description is only a preferred embodiment of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A molten salt thermal storage material based on a lithium nitrate and polyvinylidene fluoride hybrid zeolite imidazole ester framework structure, characterized in that, The components include the following parts by weight: 20 to 35 parts sodium nitrate, 25 to 40 parts potassium nitrate, 10 to 20 parts lithium nitrate, 10 to 25 parts calcium nitrate, and 0.5 to 2 parts polyvinylidene fluoride hybrid zeolite imidazole ester framework structural material. The polyvinylidene fluoride hybrid zeolite imidazole ester framework structure material is prepared by the following steps: Step 1: Under stirring conditions, zinc nitrate solution was added to 2-methylimidazole solution, and the mixture was allowed to stand for crystallization reaction. After washing and drying, zeolite imidazole ester framework structure material was obtained. Step 2: After mixing the zeolite imidazole ester framework structural material, polyvinylidene fluoride and dimethylformamide, the mixture is dried to obtain polyvinylidene fluoride hybrid zeolite imidazole ester framework structural material.

2. The molten salt thermal storage material as described in claim 1, characterized in that, In step 1, the 2-methylimidazole solution is a methanol solution of 2-methylimidazole, and the zinc nitrate solution is a methanol solution of zinc nitrate hexahydrate; the concentration of 2-methylimidazole in the methanol solution of 2-methylimidazole is 0.5~1 mol / L; the concentration of zinc nitrate hexahydrate in the methanol solution of zinc nitrate hexahydrate is 0.05~0.1 mol / L; and the volume ratio of the methanol solution of zinc nitrate hexahydrate to the methanol solution of 2-methylimidazole is 1:(3~5).

3. The molten salt thermal storage material as described in claim 1, characterized in that, In step 1, the stirring speed is 2000~3000 r / min, the settling temperature is 25~35℃, and the time is 12~24h.

4. The molten salt thermal storage material as described in claim 1, characterized in that, In step 1, the drying temperature is 60~80℃ and the time is 12~18h.

5. The molten salt thermal storage material as described in claim 1, characterized in that, In step 2, the mass ratio of the zeolite imidazole ester framework structural material to polyvinylidene fluoride is 1:(5~10), and the dosage ratio of the zeolite imidazole ester framework structural material to dimethylformamide is 1g:(5~15)mL.

6. The molten salt thermal storage material as described in claim 1, characterized in that, In step 2, the mixing is carried out under stirring conditions, the stirring speed is 500~1000 r / min, the time is 6~10 h, and the temperature is 60~80℃; the drying temperature is 25~35℃, and the time is 10~20 h.

7. The method for preparing molten salt thermal storage material based on a lithium nitrate and polyvinylidene fluoride hybrid zeolite imidazole ester framework structure as described in claim 1, characterized in that, Includes the following steps: Step S1: Sodium nitrate, potassium nitrate, lithium nitrate and calcium nitrate are mixed and dissolved in deionized water to form an inorganic molten salt solution; Step S2: After mixing the inorganic molten salt solution and the polyvinylidene fluoride hybrid zeolite imidazole ester framework structural material, the mixture is dried to obtain a mixed powder. Step S3: Heat the mixed powder until it melts, and after cooling, obtain molten salt thermal storage material.

8. The preparation method according to claim 7, characterized in that, In step S1, sodium nitrate, potassium nitrate, lithium nitrate and calcium nitrate are mixed and then dissolved in deionized water under stirring conditions. The stirring temperature is 60~80℃, the stirring time is 1~2h, and the stirring speed is 100~200r / min.

9. The preparation method according to claim 7, characterized in that, In step S2, the drying is vacuum drying, and the temperature of the vacuum drying is 80~100℃, and the time is 12~24h.

10. The preparation method according to claim 7, characterized in that, In step S3, the melting is carried out in an inert atmosphere, which is a nitrogen atmosphere or an argon atmosphere. The heating rate to melt is 5~10℃ / min. After melting, the process includes heat preservation for 0.5~1h and cooling rate of 1~5℃ / min.

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