A phosphogypsum-based composite low-temperature thermal storage material based on lattice defects and its preparation method

Abstract

This invention proposes a phosphogypsum-based composite low-temperature thermal storage material based on lattice defects and its preparation method. The raw materials include hydrateable calcium sulfate, a zinc-containing compound, and a thermally conductive framework material. The hydrateable calcium sulfate material includes one or more of anhydrous phosphogypsum and hemihydrate phosphogypsum. The thermally conductive framework material includes any one of expanded graphite, graphene, carbon nanotubes, boron nitride, or silicon carbide. The material prepared by this invention possesses low phase transition temperature, high thermal density, high thermal conductivity, and excellent cycle stability, overcoming the key technical bottlenecks of traditional hydrated salt and phosphogypsum-based thermal storage materials.

Description

Technical Field

[0001] This invention relates to the fields of phase change energy storage materials and industrial solid waste resource utilization, and particularly to a phosphogypsum-based composite low-temperature thermal energy storage material based on lattice defects and its preparation method. Background Technology

[0002] With increasing energy demand and stricter environmental protection requirements, the development of efficient and environmentally friendly energy storage technologies is crucial. Phase change thermal energy storage technology has attracted much attention due to its high thermal density and near-isothermal heat storage and release processes. Among them, hydrated salt phase change thermal energy storage materials, such as magnesium sulfate heptahydrate and calcium chloride hexahydrate, are considered highly promising low-temperature thermal energy storage media due to their high thermal density and relatively low cost. However, existing hydrated salt materials generally suffer from problems such as high supercooling, severe phase separation, and poor cycle stability, which limit their large-scale application.

[0003] On the other hand, phosphogypsum (PG), a massive industrial solid waste generated during wet-process phosphoric acid production, not only occupies a large amount of land when stockpiled, but also poses environmental risks due to its content of impurities such as phosphorus and fluorine. The main component of phosphogypsum is calcium sulfate dihydrate (CSD), with a theoretical thermal storage density as high as approximately 530 J / g, making it a highly promising thermal storage material. However, directly applying phosphogypsum to thermal storage faces three major technical bottlenecks:

[0004] 1. High phase change temperature: The dehydration phase change (endothermic energy storage) temperature window of pure calcium sulfate dihydrate is relatively high (approximately 124°C at the beginning and approximately 145°C at the peak). It is difficult to utilize the widely available low-temperature industrial waste heat (usually below 100°C) or solar energy, requiring additional high-grade energy to drive the process, resulting in poor economic efficiency.

[0005] 2. Poor thermal conductivity: Calcium sulfate dihydrate is a poor conductor of heat with extremely low thermal conductivity (approximately 0.1-0.2 W / (m·K)), resulting in slow heat storage and release rates, which cannot meet the power density requirements of practical applications.

[0006] 3. Poor cycle stability: During multiple dehydration-hydration heat storage and release cycles, calcium sulfate dihydrate crystals are prone to pulverization and agglomeration, leading to material structure disintegration, a sharp decline in heat and mass transfer performance, and a short service life.

[0007] Therefore, effectively reducing the phase transition temperature of phosphogypsum-based materials while improving their thermal conductivity and cycle stability is key to achieving a leapfrog upgrade from industrial solid waste to high-performance energy storage materials, and is also a technical problem that urgently needs to be solved in this field. Specifically, the technical problems that urgently need to be solved include: (1) How to reduce the thermal storage operating temperature of phosphogypsum-based materials from above 120°C to a low-temperature range below 100°C through low-cost technical means, so that they can efficiently utilize industrial waste heat or solar energy. (2) How to significantly improve the macroscopic thermal conductivity of phosphogypsum-based materials and solve the problem of slow thermal storage and release rates. (3) How to improve the structural stability of the material during multiple thermal storage and release cycles, suppress pulverization and performance degradation, and extend its service life. Summary of the Invention

[0008] In view of this, the present invention proposes a phosphogypsum-based composite low-temperature thermal storage material based on lattice defects and its preparation method. The present invention introduces zinc ions to modify the "lattice defects" to reduce the phase transition temperature, and constructs a three-dimensional network with a highly thermally conductive framework material to enhance thermal conductivity and structural stability. This aims to solve the technical bottlenecks of high phase transition temperature, low thermal conductivity, and poor cycle stability in existing phosphogypsum-based thermal storage materials.

[0009] The technical solution of this invention is implemented as follows:

[0010] In a first aspect, the present invention provides a phosphogypsum-based composite low-temperature thermal storage material based on lattice defects, the raw materials of which include hydrateable calcium sulfate material, zinc-containing compound and thermally conductive framework material;

[0011] The hydrateable calcium sulfate material includes one or more of anhydrous phosphogypsum and hemihydrate phosphogypsum.

[0012] The thermally conductive framework material includes any one of expanded graphite, graphene, carbon nanotubes, boron nitride, or silicon carbide.

[0013] Based on the above technical solutions, the main component of the anhydrous phosphogypsum is anhydrous calcium sulfate, and the main component of the hemihydrate phosphogypsum is hemihydrate calcium sulfate.

[0014] Based on the above technical solutions, the thermally conductive skeleton material is preferably expanded graphite.

[0015] Based on the above technical solutions, the zinc-containing compound further includes one or more of the following: zinc nitrate, zinc chloride, zinc sulfate, zinc acetate, and their hydrates.

[0016] Based on the above technical solutions, the zinc nitrate hydrate further includes zinc nitrate hexahydrate, the zinc chloride hydrate includes zinc chloride tetrahydrate, the zinc sulfate hydrate includes zinc sulfate heptahydrate, and the zinc acetate hydrate includes zinc acetate dihydrate.

[0017] Based on the above technical solution, the raw materials further include 50% to 80% hydrateable calcium sulfate material, 15% to 40% zinc-containing compound, and 1% to 15% thermally conductive skeleton material.

[0018] In a second aspect, the present invention provides a method for preparing a phosphogypsum-based composite low-temperature thermal storage material based on lattice defects, comprising the following steps: S1, after drying the hydrateable calcium sulfate material, grinding and mixing it with a zinc-containing compound and a thermally conductive framework material, pressing it into shape to obtain a blank;

[0019] S2. The green blank is subjected to in-situ deliquescent hydration to obtain a sample;

[0020] S3. The sample is dried to obtain the phosphogypsum-based composite low-temperature thermal storage material based on lattice defects.

[0021] In the presence of a zinc ion source, hydrateable calcium sulfate is subjected to a hydration reaction, thereby introducing lattice defects caused by zinc ions replacing calcium ions into the generated dihydrate calcium sulfate crystal through a "forced bulk doping" mechanism; and then, during or after the hydration process, it is combined with a thermally conductive framework material.

[0022] This invention is based on Zn 2+ Synergistic effect of ion-induced "lattice defects" and three-dimensional network of thermally conductive framework material.

[0023] Zn 2+ Inducing defects and lowering the phase transition temperature: During in-situ hydration, Zn 2+ Due to its unique electronic structure and ionic radius, anhydrous phosphogypsum can introduce numerous nanoscale pores, dislocations, and other defects into CSD crystals through surface adsorption and lattice substitution. 2+ In the presence of ions, it transforms into defective calcium sulfate dihydrate. These defects disrupt the lattice's binding of the water of crystallization, introducing a huge lattice strain energy, thereby significantly reducing the energy barrier of the dehydration (heat storage) process.

[0024] Synergistic reinforcement of thermally conductive framework material network: The three-dimensional network constructed by the thermally conductive framework material plays multiple roles: 1. The thermally conductive framework material plays a role in efficient heat conduction; 2. The flexible network of the thermally conductive framework material, as a mechanical framework, effectively suppresses the volume change and pulverization of CSD crystals during cycling, ensuring macroscopic shape stability and playing the role of structural framework; 3. The micron-sized pores in the thermally conductive framework material network also provide channels for the release and entry of water vapor, accelerating reaction kinetics and playing the role of mass transfer channels.

[0025] Based on the above technical solution, the drying process further involves completely converting the hydrated calcium sulfate material into type III anhydrous calcium sulfate.

[0026] Based on the above technical solution, the drying temperature is further specified as 180°C to 220°C, and the drying time is further specified as 3 to 4 hours.

[0027] Based on the above technical solutions, the pressing pressure is further specified as 10~50 MPa.

[0028] Based on the above technical solutions, the relative humidity of the in-situ deliquescence hydration is above 80%, the temperature is between 15℃ and 40℃, and the time is between 8h and 24h.

[0029] Based on the above technical solution, further, the drying in step S3 includes placing the sample in an environment with a relative humidity of 30% and a temperature of 25°C for 24 hours.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) The invention realizes the transformation of phosphogypsum from waste to treasure and its high-value utilization: The invention transforms phosphogypsum, an industrial solid waste with almost zero cost and a burden on the environment, into a high-performance low-temperature heat storage material by combining it with low-cost additives, which is both economical and environmentally friendly.

[0032] (2) Successfully expanded the thermal storage working temperature range to the low temperature field: Through "corrosion defect engineering", this invention successfully controlled the peak dehydration temperature of the material from above 140°C to below 90°C, enabling it to efficiently match and utilize widely existing industrial waste heat, data center waste heat, and low temperature solar energy sources, greatly improving energy utilization efficiency.

[0033] (3) Excellent comprehensive performance, solving industry pain points: The material prepared by this invention has low phase change temperature (<90℃), high heat storage density (>560 J / g), high thermal conductivity (~1.94 W / (m·K)) and excellent cycle stability. It overcomes the key technical bottlenecks of traditional hydrated salt and phosphogypsum-based heat storage materials and has broad application prospects in building heating, agricultural product drying, industrial process constant temperature and other fields. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 The following are DSC test results analysis graphs of the materials prepared in Example 1 and Comparative Examples 1-3;

[0036] Figure 2 The images shown are electron microscope (SEM) images and EDS scan analysis images of the material prepared in Example 1. In the images, a is a high-magnification SEM image, b is a low-magnification SEM image, c is the EDS analysis area, d is the elemental distribution map of Zn, e is the elemental distribution map of Ca and f is the elemental distribution map. Detailed Implementation

[0037] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0038] In the following specific implementation, anhydrous phosphogypsum and hemihydrate phosphogypsum were purchased from Hubei Yihua Group; expanded graphite was purchased from Nanjing Greenfa Carbon Materials Co., Ltd.

[0039] Example 1

[0040] This embodiment provides a phosphogypsum-based composite low-temperature thermal storage material based on lattice defects and its preparation method.

[0041] The raw materials include 45g of anhydrous phosphogypsum, 20g of zinc nitrate hexahydrate, and 5g of expanded graphite.

[0042] Its preparation method includes the following steps:

[0043] S1. Bake industrial anhydrous phosphogypsum at 180°C for 4 hours to ensure complete conversion into type III anhydrous calcium sulfate.

[0044] S2. Grind and mix the above-mentioned type III anhydrous calcium sulfate, zinc nitrate hexahydrate and expanded graphite evenly in an agate mortar.

[0045] S3. Place the mixed powder in a stainless steel mold and press it under 30 MPa pressure for 5 minutes to form a cylindrical blank with a diameter of 15 mm and a thickness of 3 mm.

[0046] S4. Place the green embryo in a constant temperature and humidity chamber with a relative humidity of 90% and a temperature of 25℃, and let it stand for 16 hours to carry out in-situ deliquescence and hydration.

[0047] S5. Transfer the hydrated sample to a dry environment with a relative humidity of 30% and a temperature of 25°C, and slowly dry it for 24 hours to obtain the final composite low-temperature thermal storage material.

[0048] Example 2

[0049] This embodiment provides a phosphogypsum-based composite low-temperature thermal storage material based on lattice defects and its preparation method.

[0050] The raw materials include 35g of hemihydrate phosphogypsum, 28g of zinc sulfate heptahydrate, and 7g of graphene.

[0051] Its preparation method includes the following steps:

[0052] S1. Bake industrial hemihydrate phosphogypsum at 200℃ for 3.5 hours to ensure that it is completely converted into type III anhydrous calcium sulfate.

[0053] S2. Grind and mix the above-mentioned type III anhydrous calcium sulfate, zinc sulfate heptahydrate and graphene evenly in an agate mortar.

[0054] S3. Place the mixed powder in a stainless steel mold and press it under 10 MPa pressure for 10 minutes to form a cylindrical blank with a diameter of 15 mm and a thickness of 3 mm.

[0055] S4. Place the green embryo in a constant temperature and humidity chamber with a relative humidity of 80% and a temperature of 15℃, and let it stand for 24 hours to carry out in-situ deliquescence and hydration.

[0056] S5. Transfer the hydrated sample to a dry environment with a relative humidity of 30% and a temperature of 25°C, and slowly dry it for 24 hours to obtain the final composite low-temperature thermal storage material.

[0057] Example 3

[0058] This embodiment provides a phosphogypsum-based composite low-temperature thermal storage material based on lattice defects and its preparation method.

[0059] The raw materials include 28g anhydrous phosphogypsum, 28g hemihydrate phosphogypsum, 10.5g zinc chloride tetrahydrate, and 3.5g carbon nanotubes.

[0060] Its preparation method includes the following steps:

[0061] S1. Bake industrial anhydrous phosphogypsum and hemihydrate phosphogypsum at 220°C for 3 hours to ensure complete conversion into type III anhydrous calcium sulfate.

[0062] S2. Grind and mix the above-mentioned type III anhydrous calcium sulfate, zinc chloride tetrahydrate and carbon nanotubes evenly in an agate mortar.

[0063] S3. Place the mixed powder in a stainless steel mold and press it under 50 MPa pressure for 5 minutes to form a cylindrical blank with a diameter of 15 mm and a thickness of 3 mm.

[0064] S4. Place the green embryo in a constant temperature and humidity chamber with a relative humidity of 100% and a temperature of 15℃, and let it stand for 16 hours to carry out in-situ deliquescence and hydration.

[0065] S5. Transfer the hydrated sample to a dry environment with a relative humidity of 30% and a temperature of 25°C, and slowly dry it for 24 hours to obtain the final composite low-temperature thermal storage material.

[0066] Example 4

[0067] This embodiment provides a phosphogypsum-based composite low-temperature thermal storage material based on lattice defects and its preparation method. The difference from Embodiment 1 is that the raw materials in this embodiment include 56g of anhydrous phosphogypsum, 10.5g of zinc acetate dihydrate, and 3.5g of boron nitride.

[0068] Example 5

[0069] This embodiment provides a phosphogypsum-based composite low-temperature thermal storage material based on lattice defects and its preparation method. The difference from Embodiment 1 is that the raw materials in this embodiment include 44.8g of anhydrous phosphogypsum, 24.5g of zinc chloride, and 0.7g of silicon carbide.

[0070] Example 6

[0071] This embodiment provides a phosphogypsum-based composite low-temperature thermal storage material based on lattice defects and its preparation method. The difference from Embodiment 1 is that the raw materials include 35g of anhydrous phosphogypsum, 14g of zinc nitrate hexahydrate, 14g of zinc chloride, and 7g of expanded graphite.

[0072] Comparative Example 1

[0073] The difference between this comparative example and Example 1 is that this comparative example does not contain expanded graphite.

[0074] Comparative Example 1 was prone to differentiation after several cycles, exhibiting poor structural stability, demonstrating the importance of the EG framework for cycle stability.

[0075] Comparative Example 2

[0076] The difference between this comparative example and Example 1 is that this comparative example contains only anhydrous phosphogypsum, resulting in pure calcium sulfate dihydrate (CSD).

[0077] Comparative Example 3

[0078] The difference between this comparative example and Example 1 is that this comparative example does not contain zinc nitrate hexahydrate.

[0079] Performance testing

[0080] 1. DSC tests (heating rate of 5 K / min) were performed on the samples prepared in Examples 1-6 and Comparative Examples 1-3, respectively. The dehydration onset temperature, peak temperature, and phase transition enthalpy are shown in Table 1 below. Figure 1 As shown.

[0081] Table 1. DSC test results of the examples and comparative examples.

[0082]

[0083] Depend on Figure 1 As can be seen, unlike Comparative Examples 2 and 3, which exhibit endothermic peaks at approximately 140°C, the DSC curve of Example 1 shows a sharp single endothermic peak at a low temperature below 100°C, with a peak temperature of 87.4°C. This indicates that the defect engineering strategy of this invention successfully lowered the material's operating temperature range from the high-temperature region to the low-temperature region. The sharp and concentrated peak shape indicates a rapid and consistent phase change process, which is beneficial for efficient heat storage and release.

[0084] As shown in Table 1, comparing Example 1 and Comparative Example 1, although the dehydration initiation temperature of Comparative Example 1 was also lower, its phase transition enthalpy was only 326.1 J / g, which is much lower than that of Example 1, indicating that the presence of the EG network can make the hydration reaction more complete. Furthermore, the material prepared in Comparative Example 1 is prone to pulverization after several cycles, exhibiting poor structural stability.

[0085] A comparison of Example 1 and Comparative Example 2 shows that the dehydration initiation temperature of pure CSD is as high as 120°C or more, and the peak dehydration temperature is as high as 141°C or more. The phase transition temperature is excessively high, indicating that the present invention, by introducing Zn... 2+ By inducing lattice defects, the peak temperature was successfully reduced by more than 50°C, solving the bottleneck that the material could not utilize low-temperature heat sources.

[0086] A comparison of Example 1 and Comparative Example 3 shows that the dehydration initiation temperature of the material prepared in Comparative Example 3 is as high as 120°C or higher, and the peak dehydration temperature is still above 141°C. Its dehydration temperature is basically consistent with that of pure CSD, indicating that expanded graphite itself cannot change the phase transition temperature. 2+ The introduction of [a specific substance] and the induction of lattice defects through "bonding chemical mismatch" are the fundamental reasons and key factors for achieving a phase transition temperature that drops from the high-temperature region to the low-temperature region.

[0087] 2. The thermal conductivity of the samples prepared in Examples 1-6 and Comparative Examples 2-3 was tested, and the results are shown in Table 2. Comparative Example 1 did not have its thermal conductivity tested because the material itself absorbs moisture due to poor structural stability.

[0088] Table 2 Thermal conductivity test results of the examples and comparative examples

[0089]

[0090] As shown in Table 2, the thermal conductivity of Example 1 is 1.94 W / m. -1 K -1 The thermal conductivity of the composite material is significantly higher than that of pure CSD in Comparative Example 2, indicating that the present invention greatly improves the inherent defect of poor thermal conductivity in the phosphogypsum matrix. The thermal conductivity of Example 1 and Comparative Example 3 are similar, indicating that the significant improvement in the thermal conductivity of the composite material of the present invention is mainly attributed to the three-dimensional thermally conductive network constructed by expanded graphite (EG), while the thermal conductivity is improved by Zn... 2+ Defect engineering achieved through ion doping successfully reduced the phase transition temperature without negatively impacting the material's high thermal conductivity, thus achieving a synergistic effect between the two major advantages of low-temperature phase transition and efficient heat transfer.

[0091] 3. The material prepared in Example 1 was observed by electron microscopy and analyzed by EDS surface scanning. The results are as follows: Figure 2 As shown.

[0092] Depend on Figure 2 It can be seen that in Zn 2+ In the presence of the sample in Example 1, CSD crystals exhibit irregular plate-like or columnar morphology. Their surface quality is extremely rough, especially under high magnification (…). Figure 2 a) Numerous disordered etchpits, depressions, and irregular growth steps are clearly visible. The presence of these macroscopic defects indicates that the crystallization process has been severely disrupted, exhibiting a unique "corrosive" growth pattern, which is a direct manifestation of "corrosive defect engineering" in macroscopic morphology. EDS surface scan analysis ( Figure 2Further analysis (d, e, f, g) reveals that the Zn signal exhibits a highly uniform and diffuse distribution throughout the crystal cluster, largely overlapping with the distribution of Ca and S elements, and no independent zinc-rich phase particles were observed. This uniform distribution at the microscale strongly supports the existence of Zn. 2+ The ions have entered the crystal structure of CSD through "forced bulk doping" rather than simply adsorbed on the surface, providing direct evidence for the defect formation mechanism of this invention.

[0093] In summary, this invention, guided by the theory of "bonding chemical mismatch," utilizes a "corrosion defect engineering" strategy with Zn... 2+ The synergistic effect of ion-induced lattice defects and expanded graphite (EG) three-dimensional network results in a material with low phase transition temperature, high heat storage density, high thermal conductivity and excellent cycle stability, which overcomes the key technical bottlenecks of traditional hydrated salt and phosphogypsum-based heat storage materials.

[0094] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A lattice-defect-based ardealite-based composite low-temperature heat storage material, characterized in that, The raw materials include a hydratable calcium sulfate material, a zinc-containing compound, and a heat-conducting skeleton material; The hydratable calcium sulfate material includes one or more of anhydrous phosphogypsum and hemihydrate phosphogypsum. The heat-conducting skeleton material includes any one of expanded graphite, graphene, carbon nanotube, boron nitride, or silicon carbide. The zinc-containing compound includes one or more of soluble zinc salts of zinc nitrate, zinc chloride, zinc sulfate, and zinc acetate, and hydrates thereof. The raw materials include the hydratable calcium sulfate material with a mass fraction of 50% to 80%, the zinc-containing compound with a mass fraction of 15% to 40%, and the heat-conducting skeleton material with a mass fraction of 1% to 15%. The preparation method of the phosphogypsum-based composite low-temperature heat storage material based on lattice defects includes the following steps: S1. After drying the hydratable calcium sulfate material, grinding the zinc-containing compound and the heat-conducting skeleton material, mixing them uniformly, and pressing them to form a green body; S2. The green body is left to be in situ deliquescent hydration to obtain a sample; S3. The sample is dried to obtain the phosphogypsum-based composite low-temperature heat storage material based on lattice defects.

2. The lattice defect-based ardealite-based composite low-temperature thermal storage material of claim 1, wherein, The drying is to completely convert the hydratable calcium sulfate material into type III anhydrous calcium sulfate.

3. The lattice defect based ardealite based composite low temperature thermal storage material according to claim 1, wherein, The pressure for the pressing is 10 to 50 Mpa.

4. The lattice defect based ardealite based composite low temperature thermal storage material according to claim 1, wherein, The relative humidity for the in situ deliquescent hydration is above 80%, the temperature is 15 to 40℃, and the time is 8 to 24 hours.

5. The lattice defect-based ardealite-based composite low-temperature thermal storage material of claim 1, wherein, The drying in step S3 includes placing the sample in an environment with a relative humidity of 30% and a temperature of 25℃ for 24 hours.

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