High energy density phase change composite materials, their preparation methods and applications

Abstract

This invention relates to high-energy-density phase change composite materials, their preparation methods, and applications, belonging to the field of phase change composite material technology. To address the problems of low energy density, insufficient mechanical properties, and poor processability of current phase change materials, this invention provides a high-energy-density phase change composite material, specifically a lightweight porous cellulose phase change composite aerogel. Internally, it consists of a three-dimensional network framework formed by the cross-linking of vinyl-functionalized cellulose nanocrystals and cellulose nanofibers. The phase change material is uniformly loaded within the pores of this three-dimensional network, with a loading amount of 65.53~91.02 wt%. The phase change temperature range of the composite material is 20~80℃. This invention's phase change composite material, relying on a hierarchical confinement structure, achieves efficient encapsulation and high loading of the phase change material, with a phase change enthalpy as high as 133.2 J / g. It also possesses excellent mechanical and thermal cycling stability and 3D printing performance, making it suitable for scenarios with wide temperature fluctuations and applicable to the field of intelligent thermal management.

Description

Technical Field

[0001] This invention belongs to the field of phase change composite material technology, and particularly relates to high energy density phase change composite materials, their preparation methods and applications. Background Technology

[0002] With the continuous rise in global energy demand, energy conservation, emission reduction, and green low-carbon development have become core directions in the energy sector, making efficient and energy-saving intelligent thermal management technology a research hotspot in materials science. Currently, building indoor thermal control heavily relies on heating, ventilation, and air conditioning systems. These active temperature control devices are energy-intensive, have low energy utilization rates, and generate large carbon emissions, making them unsuitable for long-term green and energy-saving development needs. Phase change materials (PCMs) can absorb and release a large amount of latent heat through solid-liquid phase change within a constant temperature range, achieving passive thermal management without external energy sources. This provides a green and innovative path for building energy conservation and temperature control optimization, becoming a core research direction for new energy-saving thermal management materials.

[0003] To address the issue of liquid phase leakage in traditional phase change materials, current mainstream technologies often employ microencapsulation or porous matrix physical adsorption strategies. However, these methods require the introduction of a large number of inactive supporting components without heat storage function, which significantly dilutes the proportion of effective phase change function in the system. This results in a significant decrease in the overall energy storage density and heat storage performance of the material, while also increasing the cost of raw materials and preparation. Consequently, these methods fail to meet the two core requirements of shape stability and high heat storage capacity.

[0004] In addition, existing encapsulated phase change materials still have multiple application limitations. Most systems only possess a single phase change temperature, making it difficult to adapt to scenarios with wide fluctuations in ambient temperature, thus limiting the applicable range of temperature control. Simultaneously, the rigid framework structure used in encapsulation significantly reduces the mechanical flexibility of the composite phase change material, failing to meet the application requirements of dynamic contact interfaces, flexible electronic devices, and complex curved surface structures. More critically, traditional phase change composite materials generally lack customizable molding capabilities, making it difficult to fabricate structurally stable and complex intelligent thermal management devices through digital processing methods, further limiting their large-scale application in high-end fields such as building integration and flexible temperature control equipment. Summary of the Invention

[0005] To address the current challenges in the application of phase change materials in building thermal regulation and intelligent thermal management, such as the difficulty in balancing energy storage density and shape stability, insufficient mechanical properties, and poor formability, this invention provides high-energy-density phase change composite materials, their preparation methods, and applications.

[0006] The technical solution of the present invention:

[0007] A high-energy-density phase change composite material is a lightweight porous cellulose phase change composite aerogel. The interior is formed by cross-linking vinyl functionalized cellulose nanocrystals and cellulose nanofibers to form a three-dimensional network skeleton. The phase change material is uniformly loaded in the three-dimensional network pores, and the phase change material loading is 65.53~91.02wt%. The phase change temperature range of the phase change composite material is 20~80℃.

[0008] A method for preparing a high energy density phase change composite material, comprising the following steps:

[0009] Step 1: Preparation of vinyl-functionalized cellulose nanocrystal powder:

[0010] Cellulose nanocrystals were dispersed in pyridine solvent and mixed and stirred to prepare a cellulose nanocrystal / pyridine suspension. The modifier 10-undecenoyl chloride was added to the obtained cellulose nanocrystal / pyridine suspension and stirred at 55~60℃ for 50~60 min. Anhydrous ethanol was added to the mixture obtained from the reaction, and the precipitate was collected by centrifugation. The precipitate was purified, dried and ground to obtain vinyl-functionalized cellulose nanocrystal powder.

[0011] Step 2: Constructing a composite three-dimensional network system:

[0012] The vinyl-functionalized cellulose nanocrystal powder obtained in step one was dispersed in deionized water and ultrasonically treated to obtain homogeneous suspension I. After thorough mixing of suspension I and cellulose nanofiber suspension, a thiol crosslinking agent was added and ultrasonically mixed at 45~50℃ to obtain a composite three-dimensional network system.

[0013] Step 3: Preparation of cellulose phase change composite hydrogel:

[0014] Under conditions of 55~65℃, phase change material and photoinitiator are added to the composite three-dimensional network system obtained in step two, and mechanically stirred until the system is homogeneous and stable to obtain cellulose phase change composite ink. It is then molded or 3D printed and irradiated with ultraviolet light to obtain cellulose phase change composite hydrogel.

[0015] Step 4: Preparation of high energy density phase change composite materials:

[0016] The cellulose phase change composite hydrogel obtained in step three is pre-frozen and freeze-dried to obtain a lightweight and porous cellulose phase change composite aerogel, which is a high energy density phase change composite material.

[0017] Furthermore, in step one, the concentration of cellulose nanocrystals in the cellulose nanocrystal / pyridine suspension is 25~35 mg / mL, and the molar ratio of 10-undecenoyl chloride to the anhydrous glucose unit in the cellulose nanocrystals is 1.5:1~2.5:1; the diameter of the obtained vinyl-functionalized cellulose nanocrystal powder is 3~20 nm, and the length is 50~300 nm.

[0018] Furthermore, the concentration of vinyl-functionalized cellulose nanocrystal powder in suspension I obtained in step two is 30~35 mg / mL, the concentration of the cellulose nanofiber suspension is 2.9~3.05 wt%, and the length of the cellulose nanofiber is 0.5~1.0 μm; suspension I and cellulose nanofiber suspension are mixed at a solid content mass ratio of 3:10; the thiol crosslinking agent is at least one of 1-octadecanethiol, 1-hexadecanethiol, 1-tetradecanethiol or 1-dodecanethiol; the amount of thiol crosslinking agent added is controlled according to the molar ratio of vinyl-functionalized cellulose nanocrystal to thiol crosslinking agent of 1:0.5~1:3.2.

[0019] Furthermore, in step two, the suspension I and the cellulose nanofiber suspension are thoroughly mixed by reciprocating extrusion blending, with 10 to 16 reciprocations. Through mechanical shearing and reciprocating extrusion, the components are thoroughly mixed and the fibers are physically entangled.

[0020] Furthermore, in step two, the ultrasonic cell disruptor used for ultrasonic treatment of the suspension I has a power of 650W, an ultrasonic power of 20%~45%, and a treatment time of 12~20h; the ultrasonic power of the ultrasonic mixing of the composite three-dimensional network system is 750W, the frequency is 38~42kHz, and the treatment time is 20~40min.

[0021] Furthermore, the phase change material mentioned in step three is one or more of polyethylene glycol 1000, polyethylene glycol 2000, polyethylene glycol 4000, polyethylene glycol 6000, polyethylene glycol 8000, paraffin wax, and rice bran wax; the mass of the phase change material accounts for 65.53~91.02% of the total mass of the cellulose phase change composite system, and the photoinitiator is Irgacure2959 or lithium phenyl(2,4,6-trimethylbenzoyl)phosphate, the mass of the photoinitiator accounting for 0.8~1.3% of the total mass of the cellulose phase change composite system.

[0022] Furthermore, the 3D printing described in step three adopts a room temperature direct-write 3D printing process with the following specific parameters: printing needle inner diameter 0.9~1.1mm, printing advance rate 3~4mm / s, printing platform temperature 40~45℃, and printing air pressure 6.08~6.47 MPa; the ultraviolet irradiation parameters are: ultraviolet wavelength 254~365nm, ultraviolet light power 600~800W, irradiation distance 5~15cm, and irradiation time 20~30min.

[0023] Furthermore, the pre-freezing in step four is pre-freezing at a temperature of -30 to -20°C for 14 to 24 hours, and the freeze-drying process is freeze-drying at a vacuum degree of 5 to 15 Pa and a temperature of -40 to -30°C for 30 to 50 hours.

[0024] An application of a high energy density phase change composite material provided by the present invention in the field of intelligent thermal management.

[0025] The beneficial effects of this invention are:

[0026] The cellulose-based phase change composite material provided by this invention imparts click chemical reactivity to cellulose nanocrystals through vinyl functionalization modification, constructs a covalent cross-linked network through thiol-ene click reaction, and forms a physically entangled network with cellulose nanofibers. These two elements synergistically form a hierarchical confinement structure. Through the synergistic effect of covalent confinement, physical entanglement, and multiple hydrogen bonds, efficient and stable encapsulation of the phase change material is achieved, overcoming the problems of easy leakage and low encapsulation efficiency in traditional phase change materials. The hierarchical confinement structure not only increases the loading capacity of the phase change material but also constrains the molecular chain arrangement, improving its crystallinity and crystallization efficiency, maximizing the heat storage potential of the phase change material, achieving a phase change enthalpy of up to 133.2 J / g.

[0027] The phase change composite material of this invention has excellent mechanical properties, thermal cycling stability and 3D printing adaptability, which solves the problems of weak mechanical properties, difficult molding and poor environmental adaptability of traditional phase change materials. In particular, it can adapt to complex application scenarios with wide temperature fluctuations, which greatly improves its practicality.

[0028] The phase change composite material prepared by this invention exhibits a compressive stress of up to 35 MPa under 65% strain conditions, demonstrating ultra-high compressive strength and excellent load-bearing capacity, as well as superior structural rigidity and mechanical stability. Even after undergoing multiple thermal cycles at a high temperature of 60°C, or coping with wide fluctuations in ambient temperature, the phase change composite material of this invention can still maintain its complete structural morphology, with no significant leakage of phase change material and almost no decrease in heat storage capacity, resulting in significantly improved shape stability and thermal durability.

[0029] Benefiting from the unique shear-thinning properties of cellulose substrates, the cellulose phase change composite ink prepared in this invention exhibits excellent 3D printing adaptability. Combined with UV curing and freeze-drying technologies, it can precisely fabricate complex geometric configurations, ultimately yielding lightweight porous aerogel materials. This invention, by controlling the ratio of crosslinking agent to phase change material, enables precise customization of material properties, further expanding the practical application scenarios of phase change composite materials in the field of intelligent thermal management. Attached Figure Description

[0030] Figure 1 The images show actual photographs of components made by 3D printing using the cellulose phase change composite ink prepared in step three of Example 6. Figure 1 shows a spider web pattern, Figure 2 shows a grid pattern, Figure 3 shows a star pattern, and Figure 4 shows a pyramid pattern.

[0031] Figure 2 Figure 1 shows the high-temperature load-bearing capacity and lightweight properties of the high-energy-density phase change composite material prepared in Example 8. Figure 2 shows the high-temperature load-bearing capacity and Figure 3 shows the lightweight properties.

[0032] Figure 3 Figure 1 shows the DSC melting curves of the phase change composite materials prepared by the methods of Examples 1-8 and Comparative Example 1. Figure (a) shows the DSC melting curves of the phase change composite materials with different amounts of thiol crosslinking agent, and Figure (b) shows the DSC melting curves of the phase change composite materials with different PEG loadings.

[0033] Figure 4 The image shows the DSC curves of the phase change composite materials prepared by the methods of Examples 1-8 and Comparative Example 1 after 25 thermal cycles. Detailed Implementation

[0034] The technical solution of the present invention will be further described below with reference to embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention. In the following embodiments, the process equipment or apparatus not specifically specified are all conventional equipment or apparatus in the art. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available; unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0035] Example 1

[0036] This embodiment provides a method for preparing a high-energy-density phase change composite material, the steps of which are as follows:

[0037] Step 1: Preparation of vinyl-functionalized cellulose nanocrystal powder:

[0038] Cellulose nanocrystals (CNC) were dispersed in pure pyridine solvent at room temperature and mixed to obtain a CNC / pyridine suspension with a CNC concentration of 30 mg / mL. 10-Undecenoyl chloride was added to the obtained CNC / pyridine suspension and the mixture was stirred at 60 °C for 60 min, wherein the molar ratio of 10-undecenoyl chloride to the anhydrous glucose unit in the CNC was 2:1. Anhydrous ethanol equivalent to half its volume was added to the resulting mixture to completely precipitate the modified product. The precipitated solid product was collected by centrifugation and further purified by repeated washing and centrifugation with anhydrous ethanol to remove residual unreacted substances and reagents. The purified solid product was then freeze-dried and ground to obtain vinyl-functionalized cellulose nanocrystal powder with a diameter of 3-20 nm and a length of 50-300 nm.

[0039] Step 2: Constructing a composite three-dimensional network system:

[0040] The vinyl-functionalized cellulose nanocrystal powder obtained in step one was dispersed in deionized water to prepare a suspension with a concentration of 30 mg / mL. The suspension was ultrasonically treated for 12 h in an ultrasonic cell disruptor (JY92-IIN) with a power of 650 W and an ultrasonic power of 40% to obtain homogeneous suspension I.

[0041] A cellulose nanofiber suspension with a concentration of 3.05 wt% and a length of 0.5–1.0 μm was prepared. The suspensions were extruded and mixed 10 times using a syringe at a solid content ratio of 3:10. Mechanical shearing and extrusion were used to achieve thorough mixing and physical entanglement of the components and fibers. Subsequently, 1-octadecyl mercaptan was added to the extruded mixture, with the amount added controlled according to a molar ratio of 1:0.5 between vinyl functionalized cellulose nanocrystals and 1-octadecyl mercaptan. The mixture was ultrasonically mixed for 30 min at 45°C, with an ultrasonic cleaner power of 750 W and a frequency of 38 kHz to construct a composite three-dimensional network system.

[0042] Step 3: Preparation of cellulose phase change composite hydrogel:

[0043] At 60°C, polyethylene glycol (PEG) 1000, a phase change material, and Irgacure 2959, a photoinitiator were added to the obtained composite three-dimensional network system. The system was mechanically stirred for 55-60 min until it was homogeneous and stable, thus obtaining a cellulose phase change composite ink. The mass of PEG 1000 accounted for 69.68 wt% of the total mass of the cellulose phase change composite system, and the mass of Irgacure 2959 accounted for 1% of the total mass of the cellulose phase change composite system.

[0044] After the obtained cellulose phase change composite ink is cast into a mold, it is immediately irradiated with ultraviolet light under the following conditions: ultraviolet wavelength 365nm, ultraviolet light power 800W, irradiation distance 10cm, and irradiation time 25min, so as to fully activate the photoinitiator and drive the thioene click crosslinking reaction to complete, thereby obtaining a fully cured and elastic cellulose phase change composite hydrogel.

[0045] Step 4: Preparation of high energy density phase change composite materials:

[0046] The cellulose phase change composite hydrogel obtained in step three was pre-frozen at -30°C for 16 hours and then freeze-dried at -40°C under a vacuum of 10 Pa for 40 hours to obtain a lightweight and porous cellulose phase change composite aerogel, which is a high-energy-density phase change composite material, denoted as DCNC-O. 0.5 -PEG1, with a phase transition temperature of 38~48℃.

[0047] Example 2

[0048] The only difference between this embodiment and Example 1 is that in step two of this embodiment, the amount of 1-octadecyl mercaptan added is controlled according to the molar ratio of vinyl functionalized cellulose nanocrystals to 1-octadecyl mercaptan of 1:1. The high energy density phase change composite material prepared is denoted as DCNC-O1-PEG1, and the phase change temperature is 38~48℃.

[0049] Example 3

[0050] The only difference between this embodiment and Example 1 is that in step two of this embodiment, the amount of 1-octadecyl mercaptan added is controlled according to the molar ratio of vinyl functionalized cellulose nanocrystals to 1-octadecyl mercaptan of 1:2. The high energy density phase change composite material prepared is denoted as DCNC-O2-PEG1, and the phase change temperature is 38~48℃.

[0051] Example 4

[0052] The only difference between this embodiment and Example 1 is that in step two of this embodiment, the amount of 1-octadecyl mercaptan added is controlled according to the molar ratio of vinyl functionalized cellulose nanocrystals to 1-octadecyl mercaptan of 1:3. The high energy density phase change composite material prepared is denoted as DCNC-O3-PEG1, and the phase change temperature is 38~48℃.

[0053] Example 5

[0054] The only difference between this embodiment and Example 1 is that in step two of this embodiment, the amount of 1-octadecyl mercaptan added is controlled according to the molar ratio of vinyl functionalized cellulose nanocrystals to 1-octadecyl mercaptan of 1:1, and in step three, the mass of phase change material PEG1000 accounts for 81.19 wt% of the total mass of the cellulose phase change composite system. The high energy density phase change composite material prepared is denoted as DCNC-O1-PEG2, and the phase change temperature is 38~48℃.

[0055] Example 6

[0056] The only difference between this embodiment and Example 1 is that in step two of this embodiment, the amount of 1-octadecyl mercaptan added is controlled according to the molar ratio of vinyl functionalized cellulose nanocrystals to 1-octadecyl mercaptan of 1:1, and in step three, the mass of phase change material PEG1000 accounts for 86.37 wt% of the total mass of the cellulose phase change composite system. The high energy density phase change composite material prepared is denoted as DCNC-O1-PEG3, and the phase change temperature is 38~48℃.

[0057] Example 7

[0058] The only difference between this embodiment and Example 1 is that in step two of this embodiment, the amount of 1-octadecyl mercaptan added is controlled according to the molar ratio of vinyl functionalized cellulose nanocrystals to 1-octadecyl mercaptan of 1:1, and in step three, the mass of phase change material PEG1000 accounts for 89.22 wt% of the total mass of the cellulose phase change composite system. The high energy density phase change composite material prepared is denoted as DCNC-O1-PEG4, and the phase change temperature is 38~48℃.

[0059] Example 8

[0060] The only difference between this embodiment and Example 1 is that in step two of this embodiment, the amount of 1-octadecyl mercaptan added is controlled according to the molar ratio of vinyl functionalized cellulose nanocrystals to 1-octadecyl mercaptan of 1:1, and in step three, the mass of phase change material PEG1000 accounts for 91.02 wt% of the total mass of the cellulose phase change composite system. The high energy density phase change composite material prepared is denoted as DCNC-O1-PEG5, and the phase change temperature is 38~48℃.

[0061] Example 9

[0062] This embodiment provides a method for preparing a high-energy-density phase change composite material, the steps of which are as follows:

[0063] Step 1: Preparation of vinyl-functionalized cellulose nanocrystal powder:

[0064] Cellulose nanocrystals (CNC) were dispersed in pure pyridine solvent at room temperature and mixed to obtain a CNC / pyridine suspension with a CNC concentration of 35 mg / mL. 10-Undecenoyl chloride was added to the obtained CNC / pyridine suspension and the mixture was stirred at 60 °C for 50 min, wherein the molar ratio of 10-undecenoyl chloride to the anhydrous glucose unit in the CNC was 2.5:1. Anhydrous ethanol equivalent to half its volume was added to the resulting mixture to completely precipitate the modified product. The precipitated solid product was collected by centrifugation and further purified by repeated washing and centrifugation with anhydrous ethanol to remove residual unreacted substances and reagents. The purified solid product was then freeze-dried and ground to obtain vinyl-functionalized cellulose nanocrystal powder with a diameter of 3-20 nm and a length of 50-300 nm.

[0065] Step 2: Constructing a composite three-dimensional network system:

[0066] The vinyl-functionalized cellulose nanocrystal powder obtained in step one was dispersed in deionized water to prepare a suspension with a concentration of 30 mg / mL. The suspension was ultrasonically treated for 12 h in an ultrasonic cell disruptor (JY92-IIN) with a power of 650 W and an ultrasonic power of 40% to obtain homogeneous suspension I.

[0067] A cellulose nanofiber suspension with a concentration of 3.05 wt% was prepared, wherein the length of the cellulose nanofibers was 0.5~1.0 μm. The suspensions were then extruded and mixed 10 times in a syringe at a solid content ratio of 3:10, using mechanical shearing and extrusion to achieve thorough mixing and physical entanglement of the components. Subsequently, 1-octadecyl mercaptan was added to the extruded mixture, with the amount of 1-octadecyl mercaptan added based on a molar ratio of 1:1 between vinyl functionalized cellulose nanocrystals and mercaptan crosslinking agent. The mixture was ultrasonically mixed for 30 min at 45~50℃, with an ultrasonic cleaner power of 750W and a frequency of 38kHz to construct a composite three-dimensional network system.

[0068] Step 3: Preparation of cellulose phase change composite hydrogel:

[0069] Under conditions of 55~65℃, a phase change material and a photoinitiator Irgacure2959 were added to the obtained composite three-dimensional network system. In this embodiment, the phase change material was an equal volume mixture of PEG1000 and PEG2000. The mixture was mechanically stirred for 60 min until the system was homogeneous and stable, thus obtaining a cellulose phase change composite ink. The mass of the phase change material accounted for 91.02 wt% of the total mass of the cellulose phase change composite system, and the mass of the photoinitiator Irgacure2959 accounted for 1.2% of the total mass of the cellulose phase change composite system.

[0070] After the obtained cellulose phase change composite ink is cast into a mold, it is irradiated with ultraviolet light under the following conditions: ultraviolet wavelength 365nm, ultraviolet light power 800W, irradiation distance 10cm, and irradiation time 25min, so as to fully activate the photoinitiator and drive the thioene click crosslinking reaction to complete, thereby obtaining a fully cured and elastic cellulose phase change composite hydrogel.

[0071] Step 4: Preparation of high energy density phase change composite materials:

[0072] The cellulose phase change composite hydrogel obtained in step 3 was pre-frozen at -30℃ for 24 hours and freeze-dried at a vacuum of 15 Pa and a temperature of -30℃ for 50 hours to obtain a lightweight and porous cellulose phase change composite aerogel, which is a high energy density phase change composite material with a phase change temperature of 38~48℃.

[0073] Example 10

[0074] The only difference between this embodiment and embodiment 9 is that in this embodiment, step 3 involves adding an equal volume mixture of PEG4000 and paraffin with a carbon chain length of C24 to the phase change material, resulting in a high energy density phase change composite material with a phase change temperature of 48~53℃.

[0075] Example 11

[0076] The only difference between this embodiment and embodiment 9 is that in this embodiment, step 3 involves adding an equal volume mixture of PEG6000 and rice bran wax to the phase change material, resulting in a high energy density phase change composite material with a phase change temperature of 30~70℃.

[0077] Comparative Example 1

[0078] This comparative example provides a composite phase change material prepared solely through physical entanglement without the addition of a thiol crosslinking agent, thus preventing the thiol-olefin click crosslinking reaction. The specific preparation method is as follows:

[0079] Step 1: Preparation of vinyl-functionalized cellulose nanocrystal powder:

[0080] Cellulose nanocrystals (CNC) were dispersed in pure pyridine solvent at room temperature and mixed to obtain a CNC / pyridine suspension with a CNC concentration of 30 mg / mL. 10-Undecenoyl chloride was added to the obtained CNC / pyridine suspension and the mixture was stirred at 60 °C for 60 min, wherein the molar ratio of 10-undecenoyl chloride to the anhydrous glucose unit in the CNC was 2:1. Anhydrous ethanol equivalent to half its volume was added to the resulting mixture to completely precipitate the modified product. The precipitated solid product was collected by centrifugation and further purified by repeated washing and centrifugation with anhydrous ethanol to remove residual unreacted substances and reagents. The purified solid product was then freeze-dried and ground to obtain vinyl-functionalized cellulose nanocrystal powder with a diameter of 3-20 nm and a length of 50-300 nm.

[0081] Step 2: Preparation of cellulose mixture:

[0082] The vinyl-functionalized cellulose nanocrystal powder obtained in step one was dispersed in deionized water to prepare a suspension with a concentration of 30 mg / mL. The suspension was ultrasonically treated for 12 h in an ultrasonic cell disruptor (JY92-IIN) with a power of 650 W and an ultrasonic power of 40% to obtain homogeneous suspension I.

[0083] A cellulose nanofiber suspension with a concentration of 3.05 wt% was prepared, wherein the length of the cellulose nanofibers was 0.5~1.0 μm. The suspensions were then loaded into a syringe and repeatedly extruded and mixed 10 times at a solid content ratio of 3:10. Mechanical shearing and reciprocating extrusion were used to achieve thorough mixing and physical entanglement of the components and fibers. The mixture was then ultrasonically mixed for 30 min at 45℃, with an ultrasonic cleaner power of 750W and a frequency of 38kHz to obtain a cellulose mixture.

[0084] Step 3: Preparation of composite hydrogel:

[0085] At 60°C, polyethylene glycol (PEG) 1000, a phase change material, and Irgacure 2959, a photoinitiator were added to the obtained cellulose mixture. The mixture was mechanically stirred for 55-60 min until the system was homogeneous and stable, thus obtaining a cellulose composite ink. The mass of PEG 1000 accounted for 70.57 wt% of the total mass of the cellulose composite system, and the mass of Irgacure 2959 accounted for 1% of the total mass of the cellulose composite system.

[0086] After the obtained cellulose composite ink was cast into a mold, it was immediately irradiated with ultraviolet light under the following conditions: ultraviolet wavelength 365nm, ultraviolet light power 800W, irradiation distance 10cm, and irradiation time 25min, to obtain cellulose composite hydrogel. Since the system does not contain thiol crosslinking agent, it cannot undergo thiol-olefin click covalent crosslinking and only forms a physical entanglement structure, thus obtaining cellulose composite hydrogel.

[0087] Step 4: Preparation of phase change composite materials:

[0088] The cellulose composite hydrogel obtained in step 3 was pre-frozen at -30℃ for 16 hours and freeze-dried at a vacuum of 10 Pa and a temperature of -40℃ for 40 hours to obtain the cellulose composite aerogel, which is the phase change composite material, denoted as DCNC-O0-PEG1.

[0089] Experimental Example 1: Testing the 3D printing adaptability of high energy density phase change composite materials.

[0090] The cellulose phase change composite ink prepared in step three of Example 6 was used in a room temperature direct-write 3D printing process. Specific parameters were: printing needle inner diameter 1.0 mm, printing advance rate 3 mm / s, printing platform temperature 40°C, printing air pressure 6.37 MPa; ultraviolet wavelength 365 nm, ultraviolet light power 800 W, irradiation distance 10 cm, and freeze-drying after 20 min to obtain the desired results. Figure 1 The four configurations shown are spider web, grid, star, and pyramid. Figure 1 As can be seen, all four complex configurations can be formed completely, with continuous and uniform filaments, clear structural outlines, and no obvious filament breaks / collapses / deformations. This indicates that the printing precision is high, the forming stability is good, and it can accurately reproduce the complex geometric shape of the design.

[0091] Experimental Example 2: Testing the mechanical properties of high energy density phase change composite materials.

[0092] Using the cellulose phase change composite ink prepared in step three of Examples 1-8 and the composite ink prepared in step three of Comparative Example 1 as raw materials, they were filled into cylindrical molds with a diameter of 5 mm and a height of 3 mm. After irradiation at a UV wavelength of 365 nm, UV light power of 800 W, and a distance of 10 cm for 20 min, the samples were freeze-dried to obtain various cellulose phase change composite aerogel samples. The compressive strength of these samples was tested using a universal testing machine equipped with a 1 kN mechanical sensor. The displacement rate was set to 3 mm / min, and the compressive strain was set to 60-70%. Young's modulus and ultimate compressive strength were calculated based on the stress-strain curves, and the results are shown in Table 1.

[0093] Table 1

[0094]

[0095] Table 1 shows that this invention can precisely control the mechanical properties of cellulose-based phase change composites by adjusting the molar ratio of vinyl-functionalized cellulose nanocrystals to thiol crosslinking agents and the loading of polyethylene glycol. With a fixed PEG loading of 1g, the Young's modulus and ultimate compressive strength of the material initially increased and then decreased with increasing thiol crosslinking agent content, reaching their optimal values ​​at a 1:1 molar ratio of vinyl-functionalized cellulose nanocrystals to thiol crosslinking agents, with a Young's modulus of 5.91 MPa and an ultimate compressive strength of 2.00 MPa. This indicates that the covalent crosslinking network and the physical entanglement network work most effectively at this crosslinking ratio. With a fixed molar ratio of 1:1, the Young's modulus initially increased and then decreased with increasing PEG loading from 1g to 5g, reaching a peak of 15.94 MPa at a PEG loading of 3g. Meanwhile, the ultimate compressive strength continued to increase significantly, from 2.00 MPa to 34.78 MPa, demonstrating that the hierarchical confinement structure can still effectively confine the phase change material under high loading, achieving a significant enhancement in mechanical properties.

[0096] Figure 2 Figure 1 shows the high-temperature load-bearing capacity and lightweight properties of the high-energy-density phase change composite material prepared in Example 8. Figure 2 shows the load-bearing performance of the cellulose-based phase change composite material of the present invention at a high temperature of 60°C: the sample can stably bear a 500g weight, with no obvious structural deformation and no leakage of phase change material, which intuitively proves that it achieves efficient encapsulation of phase change material and excellent high-temperature mechanical stability by relying on the hierarchical confinement structure.

[0097] Figure ii illustrates the lightweight properties of the high-energy-density phase change composite material of this invention: the material density is only 0.45 g / cm³. 3 It can be placed stably on the blade surface without bending it, demonstrating the characteristics of a lightweight and porous aerogel structure, achieving a synergy of high strength and lightweight, and further broadening its application scenarios in the field of intelligent thermal management.

[0098] Experimental Example 3: Testing the heat storage capacity of high energy density phase change composite materials.

[0099] Using the cellulose phase change composite ink prepared in step three of Examples 1-8 and the composite ink prepared in step three of Comparative Example 1 as raw materials, the inks were filled into cylindrical molds with a diameter of 5 mm and a height of 3 mm. The samples were irradiated with ultraviolet light at a wavelength of 365 nm, an ultraviolet light power of 800 W, and a distance of 10 cm for 20 min, and then freeze-dried to obtain various cellulose phase change composite aerogel samples. After eliminating the thermal history, all samples were heated and cooled at a heating rate of 5 °C / min within the range of -10 °C to 60 °C. The thermal stability of the samples was tested by DSC for 25 cycles, with parameters remaining consistent with previous tests.

[0100] The thermal storage capacity of all prepared aerogel samples was tested using a TA Q2000 instrument via differential scanning calorimetry (DSC). The tests were conducted under a nitrogen atmosphere. After eliminating the thermal history, all samples were heated from 25°C to 60°C at a heating rate of 5°C / min, held at that temperature for 1 min, and then cooled to 25°C at the same rate. Furthermore, the thermal stability of the samples was tested using 25 cycles of DSC, with parameters consistent with previous tests. The enthalpy of melting and enthalpy of crystallization were calculated from the DSC curves, and the stability was observed. The results are as follows: Figure 3 As shown in Table 2.

[0101] Table 2

[0102]

[0103] Figure 3 As shown in Table 2, when the PEG loading is fixed at 1g, the melting enthalpy first increases and then decreases as the molar ratio of vinyl functionalized cellulose nanocrystals to thiol crosslinking agent increases from 1:0 to 1:3, reaching a peak of 77.78 J / g when the molar ratio of vinyl functionalized cellulose nanocrystals to thiol crosslinking agent is 1:1. This indicates that moderate crosslinking can optimize the hierarchical confined structure, improve the regularity of PEG crystallization and thermal storage efficiency, while excessive crosslinking will restrict PEG crystallization and lead to a decrease in energy storage performance.

[0104] When the molar ratio of vinyl-functionalized cellulose nanocrystals to thiol crosslinking agent is 1:1, the melting enthalpy increases significantly as the PEG loading increases from 1g to 5g. The melting enthalpy of DCNC-O1-PEG5 reaches 133.2J / g and the crystallization enthalpy reaches 139.1J / g, proving that the hierarchical confinement network can still efficiently encapsulate PEG under high loading, achieving high energy density energy storage while maintaining good crystallization behavior.

[0105] Figure 4 The results show that the DSC curves of 25 thermal cycles highly overlap, which intuitively proves that the composite material of the present invention has almost no decay in heat storage performance after multiple thermal cycles, stable phase change behavior, and excellent thermal cycling stability and shape stability, and can adapt to complex scenarios with wide fluctuations in ambient temperature.

Claims

1. A high-energy-density phase change composite material, characterized in that, The phase change composite material is a lightweight porous cellulose phase change composite aerogel. The interior is formed by cross-linking of vinyl functionalized cellulose nanocrystals and cellulose nanofibers to form a three-dimensional network skeleton. The phase change material is uniformly loaded in the three-dimensional network pores, and the phase change material loading is 65.53~91.02wt%. The phase change temperature range of the phase change composite material is 20~80℃.

2. A method for preparing the high energy density phase change composite material as described in claim 1, characterized in that, The steps are as follows: Step 1: Preparation of vinyl-functionalized cellulose nanocrystal powder: Cellulose nanocrystals were dispersed in pyridine solvent and mixed and stirred to prepare a cellulose nanocrystal / pyridine suspension. The modifier 10-undecenoyl chloride was added to the obtained cellulose nanocrystal / pyridine suspension and stirred at 55~60℃ for 50~60 min. Anhydrous ethanol was added to the mixture obtained from the reaction, and the precipitate was collected by centrifugation. The precipitate was purified, dried and ground to obtain vinyl-functionalized cellulose nanocrystal powder. Step 2: Constructing a composite three-dimensional network system: The vinyl-functionalized cellulose nanocrystal powder obtained in step one was dispersed in deionized water and ultrasonically treated to obtain homogeneous suspension I. After thorough mixing of suspension I and cellulose nanofiber suspension, a thiol crosslinking agent was added and ultrasonically mixed at 45~50℃ to obtain a composite three-dimensional network system. Step 3: Preparation of cellulose phase change composite hydrogel: Under conditions of 55~65℃, phase change material and photoinitiator are added to the composite three-dimensional network system obtained in step two, and mechanically stirred until the system is homogeneous and stable to obtain cellulose phase change composite ink. It is then molded or 3D printed and irradiated with ultraviolet light to obtain cellulose phase change composite hydrogel. Step 4: Preparation of high energy density phase change composite materials: The cellulose phase change composite hydrogel obtained in step three is pre-frozen and freeze-dried to obtain a lightweight and porous cellulose phase change composite aerogel, which is a high energy density phase change composite material.

3. The method for preparing the high energy density phase change composite material according to claim 2, characterized in that, In step one, the concentration of cellulose nanocrystals in the cellulose nanocrystal / pyridine suspension is 25-35 mg / mL, and the molar ratio of 10-undecenoyl chloride to the anhydrous glucose unit in the cellulose nanocrystals is 1.5:1-2.5:1; the diameter of the obtained vinyl-functionalized cellulose nanocrystal powder is 3-20 nm, and the length is 50-300 nm.

4. The method for preparing the high energy density phase change composite material according to claim 2 or 3, characterized in that, The concentration of vinyl-functionalized cellulose nanocrystal powder in suspension I obtained in step two is 30~35 mg / mL, and the concentration of the cellulose nanofiber suspension is 2.9~3.05 wt%, wherein the length of the cellulose nanofiber is 0.5~1.0 μm; suspension I and cellulose nanofiber suspension are mixed at a solid content mass ratio of 3:10; the thiol crosslinking agent is at least one of 1-octadecanethiol, 1-hexadecanethiol, 1-tetradecanethiol or 1-dodecanethiol; the amount of thiol crosslinking agent added is controlled according to the molar ratio of vinyl-functionalized cellulose nanocrystal to thiol crosslinking agent of 1:0.5~1:3.

2.

5. The method for preparing the high energy density phase change composite material according to claim 4, characterized in that, In step two, the suspension I and the cellulose nanofiber suspension are thoroughly mixed by reciprocating extrusion blending, with 10 to 16 reciprocations. Through mechanical shearing and reciprocating extrusion, the components are thoroughly mixed and the fibers are physically entangled.

6. The method for preparing the high energy density phase change composite material according to claim 5, characterized in that, In step two, the ultrasonic cell disruptor used for ultrasonic treatment of the suspension I has a power of 650W, an ultrasonic power of 20%~45%, and a treatment time of 12~20h; the ultrasonic power of the ultrasonic mixing of the composite three-dimensional network system is 750W, the frequency is 38~42kHz, and the treatment time is 20~40min.

7. The method for preparing the high energy density phase change composite material according to claim 6, characterized in that, The phase change material mentioned in step three is one or more of polyethylene glycol 1000, polyethylene glycol 2000, polyethylene glycol 4000, polyethylene glycol 6000, polyethylene glycol 8000, paraffin wax, and rice bran wax; the mass of the phase change material accounts for 65.53~91.02% of the total mass of the cellulose phase change composite system, and the photoinitiator is Irgacure2959 or lithium phenyl(2,4,6-trimethylbenzoyl)phosphate, the mass of the photoinitiator accounting for 0.8~1.3% of the total mass of the cellulose phase change composite system.

8. The method for preparing the high energy density phase change composite material according to claim 7, characterized in that, Step 3 describes a 3D printing process using room temperature direct-write 3D printing. Specific parameters are: printing nozzle inner diameter 0.9~1.1mm, printing advance rate 3~4mm / s, and printing platform temperature. The printing air pressure is 6.08~6.47 MPa; the ultraviolet irradiation parameters are: ultraviolet wavelength 254~365nm, ultraviolet light power 600~800W, irradiation distance 5~15cm, and irradiation time 20~30min.

9. The method for preparing the high energy density phase change composite material according to claim 8, characterized in that, The pre-freezing in step four is to pre-freeze at a temperature of -30 to -20°C for 14 to 24 hours, and the freeze-drying process is to freeze-dry at a vacuum degree of 5 to 15 Pa and a temperature of -40 to -30°C for 30 to 50 hours.

10. An application of the high energy density phase change composite material as described in claim 1 in the field of intelligent thermal management.

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