Preparation method of low-temperature phase change microcapsule with high thermal conductivity and high durability

Abstract

The application discloses a kind of high heat-conducting high endurance low-temperature phase change microcapsule preparation methods, comprising: step 1: using C7, C9 alkane preparation cryogenic phase change material emulsion;Step 2: using melamine resin prepolymer containing polyethylene glycol and hydrophobic silicon carbide powder containing nano silicon carbide preparation modified wall material prepolymer;Step 3: using cryogenic phase change material emulsion and modified wall material prepolymer preparation phase change microcapsule;Step 4: using silver mirror reaction to phase change microcapsule surface silver plating, obtain surface silver plating microcapsule.The low-temperature phase change microcapsule prepared, using C7, C9 low-temperature mixed alkane as microcapsule core material;Using melamine resin as microcapsule wall material, and wall material contains polyethylene glycol modified material, nano silicon carbide modified material to improve the toughness of microcapsule, thermal conductivity;Microcapsule surface silver plating to resist volatility, eliminate residual formaldehyde in wall material.The application solves the problem of low thermal conductivity and poor durability of cryogenic phase change material microcapsule.

Description

Technical Field

[0001] This invention belongs to the field of cryogenic phase change material microcapsule preparation technology, specifically relating to a method for preparing low-temperature phase change microcapsules with high thermal conductivity and high durability. Background Technology

[0002] The storage and transportation of high-grade cold energy is one of the important means to improve the performance of cryogenic systems such as LNG cold power generation systems, liquid air energy storage systems, and large-scale air separation systems. Recovering and utilizing the high-quality cold energy (approximately -100°C) generated during the liquefaction and regasification processes of natural gas or air in cryogenic systems is of great significance for improving system efficiency, saving energy, reducing carbon emissions, and enhancing economic viability.

[0003] Phase change material (PCM) energy storage is an effective energy storage method. PCMs can store and release energy through their own phase changes, achieving excellent thermal energy storage performance. Organic alkane PCMs have advantages such as high latent heat of phase change, controllable phase change point, continuous and stable phase change process, strong environmental adaptability, and non-toxicity and odorlessness, making them highly valuable for applications. However, they also have problems such as easy leakage, overcooling, and low thermal conductivity. Microencapsulation technology can encapsulate PCMs within a wall material shell to create MEPCMs, thereby overcoming the leakage and volume expansion problems of alkanes. Microencapsulation not only reduces the interaction between the PCM and the external environment, improving the storage and release efficiency of latent heat of phase change, but also enhances the stability and durability of the energy conversion process.

[0004] In recent years, there have been numerous literature reports and patent publications concerning microcapsules and their preparation processes, but examples of the preparation of cryogenic microcapsule phase change materials are rare. Moreover, cryogenic microcapsules suffer from problems such as volatility, leakage, and poor thermal conductivity, requiring modification to improve their low-temperature performance.

[0005] For example, patent CN 1621483 A discloses a microcapsule-type phase change refrigerant for air conditioning and its preparation method. The phase change material used in this method is tetradecane, pentadecane, hexadecane, or a mixture of two or three of them, and the phase change temperature of the obtained product is 4-12℃. Patent CN 103642462 A discloses a low-temperature microcapsule phase change material and its preparation method, with the core material being n-tetradecane or dodecanol / decyl alcohol, and the phase change temperature being around 0℃. Although these two patents also achieve a low-temperature range, they are still far from achieving cryogenics of -100℃ and cannot meet the storage requirements of high-grade cold energy.

[0006] Regarding the thermal conductivity enhancement of microcapsules, patent CN 115746794 A discloses a thermally modified microcapsule based on boron nitride lignin hybrid Pickering emulsion. Its drawback is that the modified material is doped into the core material, occupying the space of the core material and weakening the energy storage performance to a certain extent.

[0007] Regarding the durability enhancement of microcapsules, patent CN 105670568 A discloses a method for preparing carbon nanotube-reinforced phase change material microcapsules. However, the core material used in this patent is butyl stearate, which is not a cryogenic phase change material and therefore does not have the problem of easy volatilization at room temperature. Therefore, there is currently no technology specifically for enhancing the durability of cryogenic phase change material microcapsules.

[0008] In summary, existing technologies have neither achieved the preparation of cryogenic phase change material microcapsules, nor have they provided effective means to improve the thermal conductivity and durability of cryogenic phase change material microcapsules. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a method for preparing low-temperature phase change microcapsules with high thermal conductivity and high durability, which addresses the shortcomings of the prior art. The method involves cryogenic high thermal conductivity and high durability microcapsule phase change energy storage materials with a phase change temperature of around -100℃ and their preparation process, which can meet the storage requirements of low-temperature high-grade cold energy (-100℃) and improve its thermal conductivity and durability.

[0010] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows:

[0011] A method for preparing low-temperature phase change microcapsules with high thermal conductivity and high durability includes:

[0012] Step 1: Prepare cryogenic phase change material emulsions using C7 and C9 alkanes;

[0013] Step 2: Prepare modified wall material prepolymer using melamine resin prepolymer containing polyethylene glycol and hydrophobic silicon carbide powder containing nano-silicon carbide;

[0014] Step 3: Prepare phase change microcapsules using cryogenic phase change material emulsion and modified wall material prepolymer;

[0015] Step 4: The phase change microcapsules are surface-plated with silver using a silver mirror reaction to obtain silver-plated microcapsules.

[0016] To optimize the above technical solution, the specific measures also include:

[0017] Step 1 above specifically involves mixing C7 and C9 alkanes in a specific ratio at a set phase change temperature, then adding emulsifier and distilled water and mixing them evenly. The mixture is then stirred and emulsified using a high-speed mixer to form an O / W emulsion, which is a cryogenic phase change material emulsion.

[0018] The mixing ratio of C7 and C9 alkanes in step 1 above is 9:1 to 8:2, and the phase transition temperature is below -100℃;

[0019] The emulsifier is a mixture of nonionic and anionic emulsifiers;

[0020] The nonionic emulsifiers include Span80, Span60, Tween80, Tween60, polyvinyl alcohol solution, gelatin solution, or xanthan gum solution; the anionic emulsifiers include sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), or styrene-maleic anhydride (SMA).

[0021] The emulsifier is 3% to 8% of the mass of the microcapsule core material, and the mass ratio of nonionic emulsifier to anionic emulsifier is 3:1 to 2:1.

[0022] The emulsification temperature, rotation speed, and time were 30–40℃, 1500–7000 r / min, and 10–15 min, respectively.

[0023] Step 2 above specifically includes:

[0024] Step (1): Mix melamine, polyethylene glycol and distilled water and heat until polyethylene glycol is completely dissolved. Then add formaldehyde solution, mix and stir. Adjust the pH of the solution to alkaline and continue stirring until transparent to obtain melamine resin prepolymer.

[0025] Step (2): Disperse the silane coupling agent in anhydrous ethanol, add nano silicon carbide powder, heat and stir in a water bath, and obtain hydrophobic silicon carbide powder after filtration and drying.

[0026] Step (3): Add hydrophobic silicon carbide powder to melamine resin prepolymer to obtain modified wall material prepolymer.

[0027] In step (1) above, the mass ratio of melamine to formaldehyde solution is 1.5:1 to 2:1, and the mass of polyethylene glycol is 0.7% to 1.1% of the mass of melamine.

[0028] The stirring time for melamine and formaldehyde solutions was controlled at 30 min, with the stirring temperature and speed being 65–75 °C and 550–700 rpm / min, respectively.

[0029] The pH of the solution was adjusted to 8–10 using alkaline reagents such as triethanolamine solution, 10 wt% sodium hydroxide solution, or 10 wt% sodium carbonate solution.

[0030] In step (2) above, the silane coupling agent is methacryloxysilane, and the models include KH-570, KH-571, KH-572, KH-573, and KH-574.

[0031] The mass ratio of silane coupling agent to anhydrous ethanol is 1:10 to 1:20;

[0032] The mass of silicon carbide powder added to the anhydrous ethanol and silane coupling agent solution is 1% to 2% of the solution mass. The stirring time in the anhydrous ethanol and silane coupling agent solution is not less than 1 hour. After filtration, it cannot be washed with water. It is dried by vacuum heating at a temperature of 80-90℃ for 30 minutes.

[0033] Step 3 above specifically involves: adding the modified wall material prepolymer prepared in step 2 dropwise to the emulsion prepared in step 1, while simultaneously mechanically stirring and heating in a water bath; after the prepolymer is completely added, adjusting the pH of the solution to acidic, and continuing to stir until the solution turns white; finally, filtering and washing to obtain phase change microcapsules.

[0034] In step 3 above, acidic reagents such as glacial acetic acid, citric acid, or dilute hydrochloric acid are used to adjust the pH of the solution to 3-5.

[0035] After the solution pH is adjusted, continue the reaction for 1-2 hours at a temperature of 60-80℃ and a stirring speed of 400-600 rpm.

[0036] Step 4 above specifically refers to:

[0037] First, mix the microcapsules obtained in step 3 with distilled water and sonicate until the microcapsules are completely dispersed to obtain a microcapsule dispersion.

[0038] The microcapsule dispersion was then activated in a stannous chloride solution, followed by the addition of silver ammonia solution and glucose, and the silver mirror reaction was initiated by heating in a water bath.

[0039] Finally, the filtration, washing, and drying processes are repeated to obtain silver-plated microcapsules.

[0040] In step 4 above, the concentration of the stannous chloride solution is 0.5 wt% to 1 wt%.

[0041] The reducing agent used in the silver mirror reaction is residual formaldehyde in the wall material, and then 10 wt% glucose solution or 37 wt% formaldehyde solution is added until the reaction is complete;

[0042] The temperature for the silver mirror reaction was controlled at 70–90℃, and the reaction time was 5–10 min.

[0043] In the washing process of step 4, anhydrous ethanol is first used to wash away the residual alkanes and acidic substances on the surface, and then distilled water is used to wash away the residual silver ammonia solution on the surface; during the drying process, the drying temperature is controlled at 40-50℃ and the drying time is 2-3 hours.

[0044] The high thermal conductivity and high durability low-temperature phase change microcapsules obtained by the method described above are made using n-heptane (C7H). 16 (hereinafter referred to as C7), n-nonane C9H 20 The microcapsule core material is a low-temperature mixed alkane (phase change temperature range of around -100℃) of (hereinafter referred to as C9); melamine resin is used as the microcapsule wall material, and the wall material contains polyethylene glycol modified material and nano silicon carbide modified material to improve the toughness and thermal conductivity of the microcapsule; the surface of the microcapsule is silver-plated to resist volatility and eliminate residual formaldehyde in the wall material.

[0045] The present invention has the following beneficial effects:

[0046] 1. This invention uses a mixture of C7 and C9 alkanes as the phase change material, with a phase change temperature below -100℃, meeting the requirements for cryogenic storage; the microcapsule melting enthalpy is 70.1 J / g, and the encapsulation rate is 42.9%, exhibiting a relatively ideal encapsulation effect;

[0047] 2. This invention uses highly thermally conductive nano-silicon carbide powder to modify the wall material, thereby improving the thermal conductivity of the microcapsules;

[0048] 3. This invention uses a compound emulsifier that combines anionic and nonionic emulsifiers. The anionic emulsifier ensures that the wall material polymerizes on the surface of the core material, while the nonionic emulsifier stabilizes the droplets and inhibits volatilization.

[0049] 4. This invention uses a silver mirror reaction to coat the surface of the wall material with another layer of metallic silver. Since silver has excellent thermal conductivity and density, it can improve the thermal conductivity of the capsule while improving its sealing performance, inhibiting the volatilization of the core material, and improving the durability of the capsule.

[0050] 5. In the preparation of the wall material prepolymer, polyethylene glycol is added. Polyethylene glycol condenses with melamine to form a compound with a more flexible bis-triazine ring structure. This replaces the methylene group that connects two adjacent triazine rings in the melamine resin molecule with polyethylene glycol, which increases the distance between adjacent triazine rings and improves the elasticity and toughness of the wall material.

[0051] 6. Formaldehyde residue is unavoidable in the conventional preparation of melamine resin capsules. Formaldehyde has an irritating odor and is harmful to the human body. The silver mirror reaction silver plating process used in this invention can oxidize the residual formaldehyde into harmless water and carbon dioxide, making the capsules more environmentally friendly. Attached Figure Description

[0052] Figure 1 is a microscopic morphology diagram of the microcapsules of the present invention;

[0053] Figure 2 These are macroscopic morphology images of the microcapsules of this invention before and after silver plating;

[0054] Figure 3 This is a diagram of the microcapsule coating structure of the present invention;

[0055] Figure 4 This is a DSC curve of the microcapsules of the present invention. Detailed Implementation

[0056] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0057] Although the steps in this invention are arranged by reference numerals, this is not intended to limit the order of the steps. Unless the order of the steps is explicitly stated or the execution of a step requires other steps as a basis, the relative order of the steps can be adjusted. It is understood that the term "and / or" as used herein refers to and covers any and all possible combinations of one or more of the associated listed items.

[0058] The present invention provides a method for preparing cryogenic phase change microcapsules with high thermal conductivity and high durability, comprising the following four typical steps:

[0059] Step 1: Preparation of cryogenic phase change material emulsion: Cryogenic phase change material emulsion was prepared using C7 and C9 alkanes;

[0060] At a set phase change temperature, C7 and C9 alkanes are mixed in proportion, then emulsifier and distilled water are added and mixed evenly. The mixture is then stirred and emulsified using a high-speed mixer to form an O / W emulsion, i.e., a cryogenic phase change material emulsion.

[0061] Step 2: Preparation of modified wall material: Modified wall material prepolymer is prepared using melamine resin prepolymer containing polyethylene glycol and hydrophobic silicon carbide powder containing nano-silicon carbide.

[0062] Step 1: Mix melamine, polyethylene glycol and distilled water and heat until polyethylene glycol is completely dissolved. Then add formaldehyde solution, mix and stir. Adjust the pH of the solution to alkaline and continue stirring until transparent to obtain melamine resin prepolymer.

[0063] Step 2: Disperse the silane coupling agent in anhydrous ethanol, add nano silicon carbide powder, heat and stir in a water bath, and obtain hydrophobic silicon carbide powder after filtration and drying.

[0064] Step 3: Add hydrophobic silicon carbide powder to the previously prepared melamine resin prepolymer to obtain the modified wall material prepolymer.

[0065] Step 3: Microcapsule preparation: Phase change microcapsules were prepared using cryogenic phase change material emulsion and modified wall material prepolymer;

[0066] Add the modified wall material prepolymer prepared in step 2 dropwise to the emulsion prepared in step 1, while mechanically stirring and heating in a water bath; after the prepolymer is completely added, adjust the pH of the solution to acidic, and continue stirring until the solution turns white;

[0067] Finally, the mixture is filtered and washed to obtain phase change microcapsules.

[0068] Step 4: Surface silver plating: The phase change microcapsules are surface silvered using a silver mirror reaction to obtain silver-plated microcapsules.

[0069] First, mix the microcapsules obtained in step 3 with distilled water and sonicate until the microcapsules are completely dispersed to obtain a microcapsule dispersion.

[0070] The microcapsule dispersion was then activated in a stannous chloride solution, followed by the addition of silver ammonia solution and glucose, and the silver mirror reaction was initiated by heating in a water bath.

[0071] Finally, the filtration, washing, and drying processes are repeated to obtain silver-plated microcapsules.

[0072] In specific implementation, in step 1, the mixing ratio of the C7 and C9 alkanes is 9:1 to 8:2, and the phase transition temperature is below -100℃.

[0073] The emulsifier is a mixture of nonionic and anionic emulsifiers;

[0074] The nonionic emulsifiers include Span80, Span60, Tween80, Tween60, polyvinyl alcohol solution, gelatin solution, or xanthan gum solution; the anionic emulsifiers include sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), or styrene-maleic anhydride (SMA).

[0075] The emulsifier is 3% to 8% of the core material mass, and the mass ratio of nonionic emulsifier to anionic emulsifier is 3:1 to 2:1.

[0076] Emulsification was performed using a high-speed homogenizer at a temperature of 30–40°C, a rotation speed of 1500–7000 r / min, and a time of 10–15 min.

[0077] In specific implementation, in step 2, the mass ratio of melamine and formaldehyde in step (1) is 1.5:1 to 2:1, and the mass of polyethylene glycol is 0.7% to 1.1% of the mass of melamine.

[0078] The stirring time for melamine and formaldehyde solutions was controlled at 30 min, with the stirring temperature and speed being 65–75 °C and 550–700 rpm / min, respectively.

[0079] The alkaline reagents used to adjust the pH of the prepolymer are triethanolamine solution, 10 wt% sodium hydroxide solution, or 10 wt% sodium carbonate solution, and the pH of the prepolymer is adjusted to 8-10.

[0080] In step (2), the silane coupling agent is methacryloxysilane, including models KH-570, KH-571, KH-572, KH-573, and KH-574;

[0081] The mass ratio of silane coupling agent to anhydrous ethanol is 1:10 to 1:20.

[0082] The silicon carbide powder has a particle size of nanometers. The mass of silicon carbide powder added to the anhydrous ethanol and silane coupling agent solution is 1% to 2% of the solution mass. The stirring time in the anhydrous ethanol and silane coupling agent solution is not less than 1 hour. After filtration, it cannot be washed with water. It is dried by vacuum heating at a temperature of 80-90℃ for 30 minutes.

[0083] In specific implementation, in step 3, the acidic reagent used to adjust the pH value of the solution is glacial acetic acid, citric acid or dilute hydrochloric acid, and the pH value of the solution is adjusted to 3 to 5.

[0084] After the solution pH is adjusted, continue the reaction for 1-2 hours at a temperature of 60-80℃ and a stirring speed of 400-600 rpm.

[0085] In specific implementation, in step 4, the silver ammonia solution is prepared from 0.1–0.2 mol / L silver nitrate solution and 5–20 wt% dilute ammonia water. The specific method is as follows:

[0086] Dilute ammonia solution is added dropwise to silver nitrate solution. After a yellowish-brown precipitate appears, the addition continues until the precipitate is completely dissolved, thus obtaining the desired silver ammonia solution.

[0087] The concentration of the stannous chloride solution used to activate the microcapsules is 0.5 wt% to 1 wt%.

[0088] The reducing agent used in the silver mirror reaction is the residual formaldehyde in the wall material. Depending on the specific reaction situation, an appropriate amount of 10 wt% glucose solution or 37 wt% formaldehyde solution is added until the reaction is complete.

[0089] The temperature for the silver mirror reaction was controlled at 70–90℃, and the reaction time was 5–10 min.

[0090] During the washing process of silver-plated microcapsules, anhydrous ethanol is first used to wash away the residual alkanes and acidic substances on the surface, and then distilled water is used to wash away the residual silver ammonia solution on the surface.

[0091] During the drying process of the silver-plated microcapsules, the drying temperature is controlled at 40-50℃ and the drying time is 2-3 hours.

[0092] The high thermal conductivity and high durability low-temperature phase change microcapsules obtained by the above method are made using n-heptane (C7H). 16 (hereinafter referred to as C7), n-nonane C9H 20 The microcapsule core material is a low-temperature mixed alkane (phase transition temperature around -100℃) of C9; melamine resin is used as the microcapsule wall material. Adding polyethylene glycol as a modifier to the wall material improves the toughness of the microcapsules, adding nano-silicon carbide as a modifier improves the thermal conductivity of the microcapsules, and silver plating on the surface of the microcapsules improves their anti-volatility and eliminates residual formaldehyde in the wall material. Example 1

[0093] A method for preparing cryogenic phase change microcapsules with high thermal conductivity and high durability according to Embodiment 1 of the present invention includes the following steps:

[0094] Step 1: Preparation of cryogenic phase change material emulsion: Mix 1 part C9 and 9 parts C7 alkanes, then add 0.8 parts SMA and 100 parts distilled water. Stir using a high-speed mixer at 1500 rpm to form an O / W emulsion.

[0095] Step 2: Preparation of Modified Wall Material: Mix 3 parts melamine, 0.1 parts polyethylene glycol, and 100 parts distilled water, heat until the polyethylene glycol is completely dissolved, then add 6 parts formaldehyde solution and mix. Heat and stir in a 50°C water bath at 500 rpm until the solution is transparent. Then adjust the pH of the mixture to 8 using sodium hydroxide solution, and continue stirring until the solution is transparent to obtain the melamine resin prepolymer. Disperse 3 parts silane coupling agent in 30 parts anhydrous ethanol, add 0.1 parts nano-silicon carbide powder and stir for 1 hour. Filter and dry for 30 minutes to obtain hydrophobic silicon carbide powder. Add the hydrophobic silicon carbide powder to the prepolymer to obtain the modified wall material prepolymer.

[0096] Step 3: Microcapsule preparation: The modified wall material prepolymer prepared in Step 2 was added dropwise to the O / W emulsion prepared in Step 1, while mechanical stirring and water bath heating were performed at 500 rpm and 60℃. 10 wt% glacial acetic acid was added to adjust the pH to 4, and stirring continued until the solution turned white. Finally, the solution was filtered and washed to obtain phase change microcapsules.

[0097] Step 4: Surface silver plating: Mix the microcapsules obtained in Step 3 with a 0.5 wt% stannous chloride solution, sonicate until the microcapsules are completely dispersed, and filter to obtain surface-activated microcapsules. Take 10 parts of a 0.1 mol / L silver nitrate solution, add 5% dilute ammonia solution dropwise until the precipitate is completely dissolved, then add 5 parts of formaldehyde aqueous solution and mix with the surface-activated microcapsules. Heat in a 60℃ water bath for 10 min. Finally, filter, wash with anhydrous ethanol and distilled water, and dry in a 40℃ drying oven for 2 h to obtain phase change microcapsules. Example 2

[0098] A method for preparing cryogenic phase change microcapsules with high thermal conductivity and high durability according to Embodiment 2 of the present invention includes the following steps:

[0099] Step 1: Preparation of cryogenic phase change material emulsion: Mix 1.2 parts C9-8.8 parts C7 alkanes, then add 0.8 parts SMA and 100 parts distilled water. Stir using a high-speed mixer at 1500 rpm to form an O / W emulsion.

[0100] Step 2: Preparation of Modified Wall Material: Mix 3 parts melamine, 0.1 parts polyethylene glycol, and 100 parts distilled water, heat until the polyethylene glycol is completely dissolved, then add 6 parts formaldehyde solution and heat and stir at 500 rpm in a 50°C water bath. Adjust the pH of the mixture to 8.5 using sodium hydroxide solution, and continue stirring until the solution is clear to obtain the melamine resin prepolymer. Disperse 3 parts silane coupling agent in 30 parts anhydrous ethanol, add 0.1 parts nano-silicon carbide powder and stir for 1 hour, filter and dry for 30 minutes to obtain hydrophobic silicon carbide powder. Add the hydrophobic silicon carbide powder to the prepolymer to obtain the modified wall material prepolymer.

[0101] Step 3: Microcapsule preparation: The modified wall material prepolymer prepared in Step 2 was added dropwise to the O / W emulsion prepared in Step 1, while mechanical stirring and water bath heating were performed at 500 rpm and 60℃. 10 wt% glacial acetic acid was added to adjust the pH to 4, and stirring continued until the solution turned white. Finally, the solution was filtered and washed to obtain phase change microcapsules. Example 3

[0102] A method for preparing cryogenic phase change microcapsules with high thermal conductivity and high durability according to Embodiment 3 of the present invention includes the following steps:

[0103] Step 1: Preparation of cryogenic phase change material emulsion: Mix 1.3 parts C9 – 8.7 parts C7 alkanes, then add 0.8 parts SMA and 100 parts distilled water. Stir using a high-speed mixer at 3000 rpm to form an O / W emulsion.

[0104] Step 2: Preparation of modified wall material: Mix 3 parts melamine, 0.1 parts polyethylene glycol, and distilled water, and heat until the polyethylene glycol is completely dissolved. Then add 6 parts formaldehyde solution and mix under a 50°C water bath with stirring at 600 rpm until transparent. Then adjust the pH of the mixture to 8.5 and stir stably to obtain melamine resin prepolymer.

[0105] Step 3: Microcapsule Preparation: The prepolymer was added dropwise to the O / W emulsion prepared in Step 1 while mechanically stirring. 10 wt% glacial acetic acid was added to adjust the pH to 4. After the melamine resin prepolymer was completely added, stirring continued for a period of time until the solution turned white. Finally, the solution was filtered and washed to obtain phase change microcapsules.

[0106] Step 4: Surface Silver Plating: Mix the microcapsules obtained in Step 3 with a 0.5 wt% stannous chloride solution, sonicate until the microcapsules are completely dispersed, and filter to obtain surface-activated microcapsules. Take 10 parts of a 0.1 mol / L silver nitrate solution, add 5% dilute ammonia solution dropwise until the precipitate is completely dissolved, then add 5 parts of formaldehyde aqueous solution and mix with the surface-activated microcapsules. Heat in a 60℃ water bath for 10 min. Finally, filter, wash with anhydrous ethanol and distilled water, and dry in a 40℃ drying oven for 2 h to obtain phase change microcapsules. Example 4

[0107] A method for preparing cryogenic phase change microcapsules with high thermal conductivity and high durability according to Embodiment 4 of the present invention includes the following steps:

[0108] Step 1: Preparation of cryogenic phase change material emulsion: Mix 1.8 parts C9 – 8.2 parts C7 alkanes, then add 0.8 parts SMA and 100 parts distilled water and mix thoroughly. Stir using a high-speed mixer at 3000 rpm to form an O / W emulsion.

[0109] Step 2: Preparation of Modified Wall Material: Mix 3 parts melamine and 6 parts formaldehyde solution and heat and stir at 700 rpm in a 60°C water bath until transparent. Then, adjust the pH of the mixture to 8.5 and stir steadily to obtain a melamine resin prepolymer. Disperse 3 parts silane coupling agent in 30 parts anhydrous ethanol, add 0.1 parts nano-silicon carbide powder and stir evenly. Filter and dry to obtain hydrophobic silicon carbide powder. Add the hydrophobic silicon carbide powder to the prepolymer to obtain the modified wall material prepolymer.

[0110] Step 3: Microcapsule Preparation: The prepolymer was added dropwise to the O / W emulsion prepared in Step 1 while mechanically stirring. 10 wt% glacial acetic acid was added to adjust the pH to 5. After the melamine resin prepolymer was completely added, stirring continued for a period of time until the solution turned white. Finally, the solution was filtered and washed to obtain phase change microcapsules.

[0111] Step 4: Surface silver plating: Mix the microcapsules obtained in Step 3 with a 0.5 wt% stannous chloride solution, sonicate until the microcapsules are completely dispersed, and filter to obtain surface-activated microcapsules. Take 10 parts of a 0.1 mol / L silver nitrate solution, add 5% dilute ammonia solution dropwise until the precipitate is completely dissolved, then add 5 parts of formaldehyde aqueous solution and mix with the surface-activated microcapsules. Heat in a 60℃ water bath for 10 min. Finally, filter, wash with anhydrous ethanol and distilled water, and dry in a 40℃ drying oven for 2 h to obtain phase change microcapsules.

[0112] The microscopic morphology images of the microcapsules of this invention, the macroscopic morphology images of the microcapsules before and after silver plating, the structural diagram of the microcapsules, and the DSC curves of the microcapsules are shown below. Figure 1-4 As shown, the performance of the microcapsules in each embodiment is illustrated in Table 1. According to... Figure 1-4 As shown in Table 1, the phase change temperature of this invention is below -100℃, which meets the requirements of cryogenic storage. The enthalpy of melting is 70.1 J / g, which has a relatively ideal coating effect, improves the thermal conductivity of the microcapsules, can suppress the volatilization of the core material, and improves the durability of the capsules.

[0113] Table 1. Performance of microcapsules in each embodiment

[0114] Example Preparation characteristics Freezing point Coverage Thermal conductivity W / m·K Evaporation time h 1 Polyethylene glycol and silicon carbide modified, with silver plating on the surface. -113.4℃ 43.9% 0.192 72 2 Polyethylene glycol and silicon carbide modified, surface not silver-plated -118.6℃ 45.8% 0.078 53 3 Polyethylene glycol modified with silver plating -114.1℃ 44.8% 0.127 74 4 Silicon carbide modification, surface silver plating -111.3℃ 44.1% 0.184 68

[0115] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0116] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing low-temperature phase change microcapsules with high thermal conductivity and high durability, characterized in that, include: Step 1: Prepare cryogenic phase change material emulsions using C7 and C9 alkanes; At a set phase change temperature, C7 and C9 alkanes are mixed in proportion, then emulsifier and distilled water are added and mixed evenly. The mixture is then stirred and emulsified using a high-speed mixer to form an O / W emulsion, i.e., a cryogenic phase change material emulsion. The emulsifier is a blend of nonionic and anionic emulsifiers; the nonionic emulsifiers include Span80, Span60, Tween80, Tween60, polyvinyl alcohol solution, gelatin solution, or xanthan gum solution; the anionic emulsifiers include sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), or styrene-maleic anhydride (SMA). Step 2: Prepare modified wall material prepolymer using melamine resin prepolymer containing polyethylene glycol and hydrophobic silicon carbide powder containing nano-silicon carbide: Step (1): Mix melamine, polyethylene glycol and distilled water and heat until polyethylene glycol is completely dissolved. Then add formaldehyde solution, mix and stir. Adjust the pH of the solution to alkaline and continue stirring until transparent to obtain melamine resin prepolymer. Step (2): Disperse the silane coupling agent in anhydrous ethanol, add nano silicon carbide powder, heat and stir in a water bath, and obtain hydrophobic silicon carbide powder after filtration and drying. Step (3): Add hydrophobic silicon carbide powder to melamine resin prepolymer to obtain modified wall material prepolymer; Step 3: Prepare phase change microcapsules using cryogenic phase change material emulsion and modified wall material prepolymer; Step 4: The phase change microcapsules are surface-plated with silver using a silver mirror reaction to obtain silver-plated microcapsules.

2. The method for preparing high thermal conductivity and high durability low-temperature phase change microcapsules according to claim 1, characterized in that, The mixing ratio of C7 and C9 alkanes in step 1 is 9:1 to 8:2, and the phase transition temperature is below -100℃; The emulsifier is 3% to 8% of the mass of the microcapsule core material, and the mass ratio of nonionic emulsifier to anionic emulsifier is 3:1 to 2:

1. The emulsification temperature, rotation speed, and time were 30–40℃, 1500–7000 r / min, and 10–15 min, respectively.

3. The method for preparing high thermal conductivity and high durability low-temperature phase change microcapsules according to claim 1, characterized in that, In step (1), the mass ratio of melamine to formaldehyde solution is 1.5:1 to 2:1, and the mass of polyethylene glycol is 0.7% to 1.1% of the mass of melamine. The stirring time for melamine and formaldehyde solutions was controlled at 30 min, with the stirring temperature and speed being 65–75 °C and 550–700 r / min, respectively. The pH of the solution was adjusted to 8–10 using alkaline reagents such as triethanolamine solution, 10 wt% sodium hydroxide solution, or 10 wt% sodium carbonate solution. In step (2), the silane coupling agent is methacryloxysilane, including models KH-570, KH-571, KH-572, KH-573, and KH-574. The mass ratio of silane coupling agent to anhydrous ethanol is 1:10 to 1:20; The mass of silicon carbide powder added to the anhydrous ethanol and silane coupling agent solution is 1% to 2% of the solution mass. The stirring time in the anhydrous ethanol and silane coupling agent solution is not less than 1 hour. After filtration, it cannot be washed with water. It is dried by vacuum heating at a temperature of 80 to 90°C for 30 minutes.

4. The method for preparing high thermal conductivity and high durability low-temperature phase change microcapsules according to claim 1, characterized in that, Step 3 specifically involves: adding the modified wall material prepolymer prepared in step 2 dropwise to the emulsion prepared in step 1, while simultaneously mechanically stirring and heating in a water bath; after the prepolymer is completely added, adjusting the pH of the solution to acidic, and continuing to stir until the solution turns white; finally, filtering and washing to obtain phase change microcapsules.

5. The method for preparing high thermal conductivity and high durability low-temperature phase change microcapsules according to claim 4, characterized in that, In step 3, acidic reagents such as glacial acetic acid, citric acid, or dilute hydrochloric acid are used to adjust the pH of the solution to 3-5. After the solution pH is adjusted, continue the reaction for 1-2 hours at a temperature of 60-80℃ and a stirring speed of 400-600 rpm.

6. The method for preparing high thermal conductivity and high durability low-temperature phase change microcapsules according to claim 1, characterized in that, Step 4 specifically involves: First, mix the microcapsules obtained in step 3 with distilled water and sonicate until the microcapsules are completely dispersed to obtain a microcapsule dispersion. The microcapsule dispersion was then activated in a stannous chloride solution, followed by the addition of silver ammonia solution and glucose, and the silver mirror reaction was initiated by heating in a water bath. Finally, the filtration, washing, and drying processes are repeated to obtain silver-plated microcapsules.

7. The method for preparing high thermal conductivity and high durability low-temperature phase change microcapsules according to claim 6, characterized in that, In step 4, the concentration of the stannous chloride solution is 0.5 wt% to 1 wt%. The reducing agent used in the silver mirror reaction is residual formaldehyde in the wall material, and then 10 wt% glucose solution or 37 wt% formaldehyde solution is added until the reaction is complete; The temperature for the silver mirror reaction was controlled at 70–90℃, and the reaction time was 5–10 min. In the washing process of step 4, anhydrous ethanol is first used to wash away the residual alkanes and acidic substances on the surface, and then distilled water is used to wash away the residual silver ammonia solution on the surface; during the drying process, the drying temperature is controlled at 40-50℃ and the drying time is 2-3 hours.

8. The high thermal conductivity and high durability low-temperature phase change microcapsules obtained by the method according to any one of claims 1-7, characterized in that, The low-temperature phase change microcapsules use low-temperature mixed alkanes of C7 and C9 as the core material; melamine resin is used as the wall material, and the wall material contains polyethylene glycol modified material and nano-silicon carbide modified material to improve the toughness and thermal conductivity of the microcapsules; the surface of the microcapsules is silver-plated to resist volatility and eliminate residual formaldehyde in the wall material.

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