Thermally conductive heat storage material based on cnf / nr dual crosslinked network and preparation method and application thereof

Abstract

The application discloses a preparation method of a heat-conducting and heat-storing material based on a CNF / NR double crosslinking network, and comprises the following steps: 1) dispersing a heat-conducting filler into a CNF water suspension, and performing homogenization treatment to obtain a first mixed solution; 2) uniformly mixing the first mixed solution with phase change capsules to obtain a second mixed solution; 3) dropping natural rubber latex into the second mixed solution, and uniformly stirring to obtain a third mixed solution; and 4) performing freezing, drying and pressing on the third mixed solution to obtain the heat-conducting and heat-storing material. In the application, the MPCM is fixed on the surface of the CNF, and the microencapsulation is added to the continuous network structure of the NR and the CNF, and then the BNNS and the MWCNT are combined as binary synergistic heat-conducting fillers and are self-assembled and dispersed on the continuous network structure to form an efficient heat-conducting path, so that the heat-conducting coefficient of the polymer-based flexible heat-conducting and heat-storing material is effectively improved. The heat-conducting and heat-storing material can be applied to effectively manage the heat of a battery, and the application scenarios of the battery are greatly expanded.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of phase change heat storage materials, and particularly relates to a heat-conducting heat storage material based on a CNF / NR double crosslinked network and a preparation method and application thereof. BACKGROUND

[0002] In recent years, lithium ion batteries have become the main choice for electric vehicle energy storage systems due to their high energy density, low self-discharge rate, long cycle life and environmental friendliness.

[0003] In the existing reported PCM-based BTMS, the most widely used PCM is a solid-liquid organic PCM, especially polyethylene glycol (PEG) and paraffin (or n-alkane); among them, paraffin is concerned due to its high latent heat, non-toxicity and non-corrosiveness, excellent cycle stability and low cost, and is widely used in BTMS. However, the inherent low thermal conductivity of PCM, the unresolvable thermal expansion, the leakage problem in the molten state and the certain degree of supercooling phenomenon inevitably hinder its wide practical application.

[0004] In combination with the existing research, it is considered that encapsulating the phase change material into microcapsules is an effective method. The phase change microcapsule (MPCM) is a spherical filler, and only a small area of the fillers is overlapped, and it is difficult to form a heat-conducting network connected with each other; therefore, the functionality of the phase change microcapsule usually needs to be assisted by a carrier medium. The phase change microcapsule can be dispersed in a polymer crosslinked network to form a blended flexible composite phase change material with required properties and functions. However, the inherent low thermal conductivity of the organic MPCM and the polymer makes the prepared polymer-based flexible CPCM face the problem of low overall thermal conductivity, and due to the high specific surface area of the MPCM, it is easy to form serious aggregates in the polymer matrix. The nano-thermal conductive filler is easy to agglomerate and deposit in the polymer matrix, and cannot establish effective interface contact; therefore, how to effectively control the distribution structure of the thermal conductive filler to improve the thermal conductivity while realizing the good mechanical properties of the flexible phase change material is the key to obtain the high-performance flexible phase change material. SUMMARY

[0005] The present application aims to at least solve one of the technical problems in the related art. To this end, the main purpose of the present application is to provide a heat-conducting heat storage material based on a CNF / NR double crosslinked network and a preparation method thereof, which aims to solve the problems of poor dispersion effect, liquid phase leakage, high rigidity and low thermal conductivity of the existing flexible phase change material in the preparation process.

[0006] The present application also provides the application of the heat-conducting heat storage material based on the CNF / NR double crosslinked network in photothermal materials and wearable materials

[0007] The purpose of the present application is realized by the following technical solutions:

[0008] A preparation method of a heat-conducting and heat-storing material based on a CNF / NR double crosslinked network, the process flow thereof is as shown in Figure 1 The preparation method comprises the following steps:

[0009] 1) A certain amount of heat-conducting filler is dispersed into a CNF water suspension for homogenization treatment to obtain a first mixed solution;

[0010] 2) The first mixed solution prepared in step 1) is mixed uniformly with phase change capsules to obtain a second mixed solution;

[0011] 3) Natural rubber latex is dropped into the second mixed solution prepared in step 2) and stirred uniformly to obtain a third mixed solution;

[0012] 4) The third mixed solution prepared in step 3) is subjected to freezing, drying and pressing to obtain the heat-conducting and heat-storing material.

[0013] In some specific embodiments, the heat-conducting material comprises BNNS and MWCNT, and the mass ratio of the BNNS and the MWCNT is (3-8):(2-5).

[0014] Further, the mass ratio of the BNNS and the MWCNT is 7:3.

[0015] In some specific embodiments, the process parameters of the homogenization treatment in step 1) are homogenization at 4000-8000 rpm for 5-15 min at room temperature.

[0016] In some specific embodiments, the freezing condition in step 4) is freezing at -25 to -15℃ for 6-18 h; the drying condition is freeze-drying at a pressure less than 20 Pa and a temperature of -60 to -50℃ for 40-50 h; and the pressing condition is pressing at room temperature and a pressure of 1-3 MPa for 3-8 min.

[0017] In some specific embodiments, the mass percentage of the heat-conducting filler in the heat-conducting and heat-storing material is 4-12%.

[0018] In some specific embodiments, the natural rubber latex is NR natural rubber emulsion, the phase change capsules are phase change paraffin microcapsules, and the mass fraction of the CNF water suspension is 1-3%.

[0019] Further, the mass fraction of the CNF water suspension is 1.3%.

[0020] In some specific embodiments, the mass ratio of the phase change capsules to the CNF is (5-15):1.

[0021] Further, the mass ratio of the phase change capsule to CNF is 10:1.

[0022] In some specific embodiments, the mass ratio of the phase change capsule to NR is (4-8):(2-5).

[0023] Further, the mass ratio of the phase change capsule to NR is 7:3.

[0024] A heat-conducting heat storage material prepared by the foregoing preparation method.

[0025] An application of the foregoing heat-conducting heat storage material in photothermal materials and wearable materials.

[0026] 1) The present application uses cellulose nanofiber (CNF) as a dispersant to effectively disperse phase change paraffin microcapsules (MPCM) in a natural rubber (NR) matrix. The CNF network directly penetrates the NR matrix, promoting the dispersion of MPCM while making the double network structure more compact, and the heat-conducting heat storage material has excellent anti-leakage performance.

[0027] 2) The heat-conducting heat storage material of the present application is physically mixed throughout the preparation process, is uniformly dispersed, and CPCM is prepared by mixing and pressing at room temperature; the process is simple and efficient, without harsh experimental conditions such as high temperature and high pressure, and is suitable for large-scale production and application.

[0028] (3) The CNF / BNNS / MWCNT network formed by the self-assembly strategy plays a role in constructing a connected heat-conducting network. When the addition content of BNNS / MWCNT is 12wt%, a perfect connected heat-conducting network is constructed, heat can be quickly transferred, and the thermal conductivity reaches 1.023 W / m·K, which is 499% of the thermal conductivity of NC@M, while maintaining a relatively high latent heat value of 110.04 J / g.

[0029] (4) Based on the triple synergistic encapsulation effect of the core-shell structure of paraffin phase change microcapsules, the cross-linked network of unsaturated bonds of NR, and the cross-linked network of hydrogen bonds of CNF, the obtained flexible NCBM@M heat-conducting heat storage material exhibits excellent complete anti-leakage performance, and also exhibits excellent cycle stability and thermal stability, showing great potential in practical applications.

[0030] (5) The continuous NR flexible cross-linked network in the present application endows the heat-conducting heat storage material with excellent room temperature flexibility and stretchability. NCBM@M can be bent, stretched and twisted at room temperature, and has a wide flexibility temperature range. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to more clearly illustrate the specific embodiments of the present application, the drawings required to be used in the specific embodiments or prior art description will be briefly introduced as follows.

[0032] Figure 1 Preparation flow chart of the heat-conducting heat storage material provided in the present application;

[0033] Figure 2 FT-IR graph (a) and XED test graph (b) of MPCM and NC@M, NCBM4@M, NCBM8@M and NCBM12@M in the present application;

[0034] Figure 3 SEM graph of MPCM and NC@M, NCBM4@M, NCBM8@M and NCBM12@M in the present application;

[0035] Figure 4 Thermal conductivity graph of NC@M, NCBM4@M, NCBM8@M and NCBM12@M in Examples 1-4 in the present application;

[0036] Figure 5 Heating process DSC curve graph (a) and heating process DSC curve graph (b) of MPCM and NC@M, NCBM4@M, NCBM8@M and NCBM12@M in the present application;

[0037] Figure 6 Flexibility test graph of NC@M, NCBM4@M, NCBM8@M and NCBM12@M in Examples 1-4 in the present application; wherein graph (a) is a digital image of NCBM12@M; (b-c) are digital images of the bending and twisting processes of NCBM12@M; (d) is a stress-strain curve of the sample; (e) is the tensile strength and elongation at break of the sample; (f) is a digital image of NCBM12@M pulling a 500g weight;

[0038] Figure 7 Digital images of NCBM4@M, NCBM8@M and NCBM12@M in Examples 2-4 in the present application under 80℃ and 500g pressure and after continuous heating at 80℃ for 120h;

[0039] Figure 8 Phase transition temperature and phase transition enthalpy of NCBM12@M in Example 4 in the present application before and after 100 heating and cooling cycles;

[0040] Figure 9 TGA curve graph (a) and DTG curve graph (b) of MPCM and NC@M, NCBM12@M in the present application;

[0041] Figure 10The thermal management test diagram of NC@M, NCBM3.6@M and NCBM12@M in the present application;

[0042] Figure 11 The temperature distribution diagram of the surface of NC@M and NCBM12@M materials in the present application over time. DETAILED DESCRIPTION

[0043] The present application will be further described in conjunction with the accompanying drawings and examples, the following examples are only descriptive, not limiting, and cannot limit the protection scope of the present application.

[0044] When a certain amount, concentration or other value or parameter is expressed in a range, a preferred range, or a preferred upper and lower limit of a value, it is understood that any range of values, whether or not specifically recited, is specifically disclosed by the disclosure of any one of the upper or preferred values of the range combined with any one of the lower or preferred values of the range. Unless otherwise indicated, the numerical ranges listed herein are inclusive of the endpoints of the range, and all integers and fractions within the range.

[0045] Unless otherwise specified, all percentages, parts, ratios, etc. herein are by weight.

[0046] The materials, methods and examples herein are exemplary and should not be construed as limiting unless specifically stated.

[0047] In the following examples, the heat-conducting material used is a mixture of multi-walled carbon nanotubes (MWCNT) and boron nitride nanosheets (BNNS); the phase change capsule used is a phase change paraffin microcapsule; and the natural rubber latex used is NR natural rubber latex. The main raw materials used and technical indicators are shown in Table 1:

[0048] Table 1 Experimental materials and specifications

[0049]

[0050] In the following examples, the materials used are characterized by physical properties and performance tests:

[0051] 1) The surface morphology and microstructure of the materials were analyzed and tested by scanning electron microscopy (SEM). The samples were surface gold-plated before testing to improve the electrical conductivity of the materials and make the imaging clearer. The SEM images were taken by Quattro S (Thermo Fisher) at an acceleration voltage of 5-20 kV.

[0052] 2) Fourier Transform Infrared (FT-IR) was used to analyze the chemical structure of the material; FT-IR spectra were obtained by Nicolet iS50 infrared spectrometer (Thermo Fisher Scientific) at room temperature, the measurement range was 4000-400 cm -1 , and the resolution was 4 cm -1 .

[0053] 3) X-ray Diffraction (XRD) was used to characterize and analyze the crystal structure of the material; XRD spectra were obtained by Empyrean (PANalytical, Netherlands), with a voltage of 40 kV, a current of 40 mA, a scanning range of 5°-80°, and a scanning speed of 5° / min.

[0054] 4) The thermal conductivity of the PCM sample was directly measured by a Hot Disk TPS2500S based on the transient plane source method; the test temperature was 25°C, the test was repeated 3 times, and the average value was taken as the final result.

[0055] 5) Differential Scanning Calorimeter (DSC) was used to study the phase change characteristics of the PCM sample, and the thermal performance parameters such as phase change temperature and phase change latent heat were determined; under the purging environment of nitrogen (50 ml / min), the heating / cooling rate was 5°C / min, and the scanning range was 20-60°C.

[0056] 6) The cycle stability of the sample was judged by thermal cycle test to ensure that its phase change characteristics did not change during long-term operation; specifically, by characterizing the DSC curves of the sample before and after the thermal cycle test, the stability was ensured.

[0057] 7) Dynamic Thermomechanical Analysis (DMA) was used to test the flexibility of the sample.

[0058] 8) Thermo-Gravimetric Analysis (TGA) was used to study the thermal stability of the material; the sample mass was 3-5 mg, the test was carried out under nitrogen (50 ml / min) atmosphere, the test temperature range was 30°C to 600°C, and the heating rate was 10°C / min.

[0059] 9) In order to test the shape stability of the sample, the PCM series sample is placed in a constant temperature drying oven for continuous heating at 80℃ for a period of time, then taken out and weighed again by an electronic balance to calculate the mass leakage rate of the sample; the sample leakage rate after heating n times is calculated according to formula (2.5):

[0060]

[0061] Wherein L n and M n respectively represent the sample leakage rate and the remaining mass of the n th test; M0 is the initial mass of the sample.

[0062] Example 1

[0063] The embodiment provides a preparation method of a heat-conducting heat storage material based on a CNF / NR double crosslinked network, and specifically comprises the following steps:

[0064] 1) A CNF water suspension with a mass concentration of 1.3% is uniformly mixed with phase change paraffin microcapsules (MPCM), wherein the mass ratio of the phase change paraffin microcapsules to the CNF is 10:1, to obtain a mixed solution A;

[0065] 2) NR natural rubber latex is dropped into the mixed solution A prepared in step 1), wherein the mass ratio of the phase change paraffin microcapsules (MPCM) in the mixed solution A to the NR natural rubber latex is 7:3, and the mixed solution is uniformly stirred to obtain a third mixed solution;

[0066] 3) The third mixed solution prepared in step 2) is frozen at-20℃ for 12h, and then immediately transferred to a freeze dryer for freeze drying for 48h (-55℃, <20Pa), and the obtained solid material is pressed at room temperature and 2MPa for 5min to eliminate internal air, thereby obtaining a heat-conducting heat storage material (referred to as NC@M).

[0067] Example 2

[0068] The embodiment provides a preparation method of a heat-conducting heat storage material based on a CNF / NR double crosslinked network, and specifically comprises the following steps:

[0069] 1) A heat-conducting filler (the proportion of the heat-conducting filler in the heat-conducting heat storage material is 4%) with a mass ratio of 7:3 of BNNS and MWCNT is dispersed into a CNF water suspension with a mass concentration of 1.3%, and homogenization treatment is performed to obtain a first mixed solution;

[0070] 2) The first mixed solution prepared in step 1) is uniformly mixed with phase change paraffin microcapsules (MPCM), wherein the mass ratio of the phase change paraffin microcapsules (MPCM) to the CNF in the first mixed solution is 10:1, to obtain a second mixed solution;

[0071] 3) Drop NR natural rubber latex into the second mixed solution prepared in step 2), wherein the mass ratio of phase change paraffin microcapsules (MPCM) to NR natural rubber latex in the second mixed solution is 7:3, and stir uniformly to obtain a third mixed solution;

[0072] 4) Freeze the third mixed solution prepared in step 3) at -20°C for 12h, and then immediately transfer it to a freeze dryer for freeze-drying for 48h (-55°C, <20Pa), and press the obtained solid material at room temperature and 2MPa for 5min to eliminate internal air, thereby obtaining a heat-conducting and heat-storing material (referred to as NCBM4@M for short).

[0073] Example 3

[0074] The embodiment provides a preparation method of a heat-conducting and heat-storing material based on a CNF / NR double crosslinking network, and specifically comprises the following steps:

[0075] 1) Disperse heat-conducting fillers (BNNS and MWCNT, the proportion of the heat-conducting fillers in the heat-conducting and heat-storing material is 8%) with a mass ratio of 7:3 into a CNF water suspension with a mass concentration of 1.3%, and perform homogenization treatment to obtain a first mixed solution;

[0076] 2) Mix the first mixed solution prepared in step 1) with phase change paraffin microcapsules (MPCM) uniformly, wherein the mass ratio of the phase change paraffin microcapsules (MPCM) to CNF in the first mixed solution is 10:1, to obtain a second mixed solution;

[0077] 3) Drop NR natural rubber latex into the second mixed solution prepared in step 2), wherein the mass ratio of phase change paraffin microcapsules (MPCM) to NR natural rubber latex in the second mixed solution is 7:3, and stir uniformly to obtain a third mixed solution;

[0078] 4) Freeze the third mixed solution prepared in step 3) at -20°C for 12h, and then immediately transfer it to a freeze dryer for freeze-drying for 48h (-55°C, <20Pa), and press the obtained solid material at room temperature and 2MPa for 5min to eliminate internal air, thereby obtaining a heat-conducting and heat-storing material (referred to as NCBM8@M for short).

[0079] Example 4

[0080] The embodiment provides a preparation method of a heat-conducting and heat-storing material based on a CNF / NR double crosslinking network, and specifically comprises the following steps:

[0081] 1) The heat-conducting filler of BNNS and MWCNT with a mass ratio of 7:3 (the proportion of the heat-conducting filler in the heat-conducting heat storage material is 12%) is dispersed into a CNF water suspension with a mass concentration of 1.3%, and homogenization treatment is performed to obtain a first mixed solution;

[0082] 2) The first mixed solution prepared in step 1) is mixed with phase change paraffin microcapsules (MPCM) uniformly, wherein the mass ratio of the phase change paraffin microcapsules (MPCM) to the CNF in the first mixed solution is 10:1 to obtain a second mixed solution;

[0083] 3) NR natural rubber latex is dropped into the second mixed solution prepared in step 2), wherein the mass ratio of the phase change paraffin microcapsules (MPCM) to the NR natural rubber latex in the second mixed solution is 7:3, and stirring is performed uniformly to obtain a third mixed solution;

[0084] 4) The third mixed solution prepared in step 3) is frozen at -20°C for 12h, and then immediately transferred to a freeze dryer for freeze drying for 48h (-55°C, <20Pa). The obtained solid material is pressed at room temperature and 2MPa for 5min to eliminate internal air, and a heat-conducting heat storage material (referred to as NCBM12@M) is obtained.

[0085] Performance test:

[0086] The heat-conducting heat storage materials prepared in Examples 1-4 are subjected to performance tests, specifically:

[0087] (1) Chemical compatibility and crystallinity

[0088] In this test example, Examples 1 and 2 are taken as examples, and FT-IR characterization is performed on MPCM, NC@M (Example 1) and NCBM@M (Example 2) respectively. The results are shown in Figure 2 (a), from the spectrum of MPCM, it can be seen that the absorption peaks at 2916cm -1 and 2848cm -1 are attributed to the stretching vibration of C-H bond in -CH3 and -CH2 group, which is a typical characteristic of alkane. The asymmetric stretching vibration and rocking vibration of -CH2 are respectively at 1471cm -1 and 717cm -1 band. It is worth noting that compared with pure MPCM, the characteristic peaks of cellulose material appear in NC@M and NCBM@M, the absorption peak at 3335cm -1 can be attributed to the stretching vibration of O-H, and the absorption peak at 891cm -1 corresponds to the cellulose glycosidic bond; after adding NR, NC@M and NCBM@M have an absorption peak at 842cm -1The new absorption peak generated at 1310 cm⁻¹ corresponds to the in-plane rocking vibration of -CH₂; NCBM@M at 1310 cm⁻¹ -1 and 769cm -1 The two characteristic peaks appearing at 1660 cm⁻¹ correspond to the bending vibration of BNB and the tensile vibration of BN, respectively; NCBM@M at 1660 cm⁻¹ -1 The new absorption peak appearing corresponds to the C=C stretching vibration. Furthermore, the main absorption peaks of MPCM and NC@M were observed in the spectrum of NCBM@M, indicating that MPCM successfully combined with other components through physical interactions without generating new functional groups. The physical crosslinking between MPCM, NR, CNF, BNNS, and MWCNT without chemical reactions demonstrates the good chemical compatibility of NCBM@M.

[0089] This test case also included XRD tests on MPCM, NC@M (Example 1), and NCBM@M (Example 2), with results as follows: Figure 2 As shown in (b), the three characteristic diffraction peaks of MPCM are located at 19.4°, 23.4°, and 24.9°, respectively, representing the (110), (200), and (210) crystal planes of the phase change paraffin microcapsules (MPCM). All diffraction peaks of MPCM and NC@M were observed in the diffraction pattern of the thermally conductive and heat-storing material NCBM@M. The positions of the diffraction peaks did not shift significantly, only the intensity decreased slightly. Among them, the new diffraction peaks located at 26.7°, 41.7°, 43.6°, 49.8°, 55.1°, and 75.9° in the diffraction pattern of NCBM@M also correspond to the (002), (100), (101), (102), (004), and (110) crystal planes of BNNS, respectively. In summary, this indicates that only physical mixing occurred during the entire synthesis process of the thermally conductive and heat-storing material, without any chemical reaction between the melting and mixing of any individual components.

[0090] (2) Microstructure and morphology

[0091] This application presents microstructure and morphology tests on MPCM and the thermally conductive and heat-storing materials in Examples 1-4, and their SEM images are shown below. Figure 3 As shown, where Figure 3 (a) is a SEM image of MPCM, showing microclusters formed by the mutual attraction of many MPCMs. This is because MPCMs are small in size, have a large specific surface area, and high surface energy, making them prone to self-assembly into aggregates. If these MPCM aggregates are directly and randomly dispersed in the NR matrix, island-like structures similar to polymer blends can easily appear, which introduces more interfacial defects into the NR matrix. Figure 3(b) shows that the addition of CNF apparently improves the compatibility between MPCM and NR. CNF wraps around the NR / MPCM composite in the form of a belt, while achieving uniform dispersion of MPCM in the NR matrix. The crosslinking network derived from the unsaturated bonds of NR and the crosslinking network derived from the hydrogen bonding of CNF interweave with each other, and the three-dimensional continuous CNF network directly penetrates the NR matrix, making the encapsulation network more compact. The double flexible network formed can well impart flexibility to the heat-conducting thermal storage material. That is, the double encapsulation network formed by NR and CNF will also impart excellent anti-leakage performance to the heat-conducting thermal storage material. As shown in Figure 3 As shown in (c-e), with the introduction of BNNS / MWCNT binary heat-conducting fillers, the CNF skeleton becomes obviously thicker, indicating that the BNNS / MWCNT is successfully attached to the CNF skeleton, which is conducive to building a highly interconnected and uniform heat-conducting network; with the increase of the content of BNNS / MWCNT on CNF, the CNF / BNNS / MWCNT network also becomes more compact.

[0092] (3) Thermal conductivity

[0093] This test example tests the thermal conductivity of the heat-conducting thermal storage material prepared in Examples 1-4 at 25°C, and the results are shown in Figure 4 As can be seen from the figure, the thermal conductivity of NCBM@M increases continuously with the increase of the content of BNNS / MWCNT binary heat-conducting fillers. From the SEM results, the CNF crosslinking network directly penetrates the entire NR matrix, which is conducive to building a highly interconnected heat-conducting path in the NR matrix. Therefore, when the content of BNNS / MWCNT increases from 0wt% to 12wt%, the BNNS / MWCNT fills and contacts each other on the CNF crosslinking network, forming and improving the heat-conducting network. The thermal conductivity of NCBM4@M is 0.462 W / m·K, and the thermal conductivity of NCBM8@M is 0.727 W / m·K, which is 225% and 355% of the thermal conductivity of NC@M, respectively. The thermal conductivity of NCBM12@M reaches a maximum value of 1.023 W / m·K, which is 499% of the thermal conductivity of NC@M. This is mainly due to the fact that BNNS / MWCNT builds and improves a better heat-conducting network at an addition amount of 12wt%, so that heat can be quickly transferred through the channel of the network, thereby greatly improving the thermal conductivity, that is, the heat-conducting thermal storage material of the present application has excellent heat-conducting performance.

[0094] (4) Phase change behavior and heat storage performance

[0095] This test example determines the phase change characteristics of MPCM and NCBM@M heat-conducting thermal storage materials by DSC to evaluate the heat storage capacity of the materials. The results are shown in Figure 5 show the DSC curves of the series of samples, in which Figure 5(a) the phase change characteristics of MPCM and NCBM@M heat-conducting thermal storage materials during the heating process, Figure 5 (b) the phase change characteristics of MPCM and NCBM@M heat-conducting thermal storage materials during the cooling process. Table 2 lists the melting temperature (T m ), melting enthalpy (ΔH m ), crystallization temperature (T f ), crystallization enthalpy (ΔH f ) and supercooling degree (ΔT) of MPCM and NCBM@M.

[0096] Table 2 Phase change temperature and phase change enthalpy and supercooling degree of MPCM and series NCBM@M heat-conducting thermal storage materials

[0097]

[0098] In combination Figure 5 with the data in Table 2, it can be seen that MPCM has a higher melting enthalpy (188.64 J / g) and crystallization enthalpy (184.82 J / g), and the melting temperature and crystallization temperature are 44.25℃ and 32.84℃, respectively. In the crystallization process, the freezing point of the prepared NCBM@M heat-conducting thermal storage material is higher than that of MPCM. This is because the NCBM@M heat-conducting thermal storage material with higher thermal conductivity has a faster heat transfer rate, which can absorb or release more heat in the same duration of time, thereby accelerating the phase change. Therefore, its crystallization occurs earlier than pure MPCM, and the freezing point temperature increases. Among them, the melting temperature and crystallization temperature of NCBM12@M heat-conducting thermal storage material are 44.32℃ and 38.14℃, respectively, and the supercooling degree of NCBM12@M heat-conducting thermal storage material is reduced from 11.41℃ to 6.18℃ compared with pure MPCM, which is beneficial to the utilization of phase change materials.

[0099] The sample is prepared by simple physical blending without chemical reaction, so the theoretical latent heat value ΔH of the sample should follow the following formula:

[0100] ΔH = ΔH MPCM × α (4.1)

[0101] Where, ΔH MPCMThe latent heat value of MPCM, and a represents the mass fraction of MPCM in the thermal conductive and heat storage material. The results show that the latent heat value of NCBM@M thermal conductive and heat storage material is not much different from the theoretical calculation result, indicating that MPCM is uniformly distributed in NCBM@M. Due to the incorporation of non-phase change components NR, CNF, BNNS and MWCNT, the latent heat value of the prepared NCBM@M thermal conductive and heat storage material is significantly lower than that of MPCM. The melting enthalpy and crystallization enthalpy of NCBM12@M thermal conductive and heat storage material are 110.04 J / g and 107.56 J / g, respectively, which are 41.67% and 41.80% lower than those of MPCM, respectively. Among them, the heat storage density of NCBM12@M thermal conductive and heat storage material reaches 110.04 J / g, which meets the requirements of battery thermal management.

[0102] (5) Flexible properties

[0103] The flexibility and stretchability of the thermal conductive and heat storage materials prepared in Examples 1-4 were tested, and the results are shown in Figure 6 , wherein the actual photos of Figure 6 (a) show that the thickness of NCBM@M thermal conductive and heat storage material is 1.2 mm, and the surface is smooth and flat. As shown in Figure 6 (b) and 6(c), the prepared NCBM@M thermal conductive and heat storage material can be easily bent and twisted at a large angle, showing excellent room temperature flexibility. The tensile strength of NCBM@M thermal conductive and heat storage material depends largely on the loading amount of NR.

[0104] The mechanical properties of NCBM@M samples (13x5x1.2mm 3 ) were tested by a tensile model. The results show that NCBM12@M samples still maintain good mechanical properties. According to the typical stress-strain curve of NCBM@M thermal conductive and heat storage material, the tensile strength and breaking elongation were obtained, as shown in 6(d) and 6(e). NCBM12@M thermal conductive and heat storage material can withstand a stress of 2.55 MPa, and the corresponding tensile rate can reach 11.12%, indicating that it has excellent flexibility. The enhancement of the mechanical properties of the thermal conductive and heat storage material is due to the uniform dispersion and fewer defects of CNF / BNNS / MWCNT in the continuous NR network. As shown in Figure 6 (f), 0.2 g of NCBM12@M with a cross-sectional size of 5 mm x 1.5 mm can lift a weight of 500 g, which intuitively reflects its high strength and good flexibility. This means that NCBM12@M thermal conductive and heat storage material with excellent flexibility has considerable application potential in real life.

[0105] (6) Stability

[0106] (61) Shape stability

[0107] The heat storage materials prepared in Examples 1-4 were placed on a constant temperature heating table and heated to 80°C to test the shape stability and leakage prevention performance of the NCBM@M heat storage materials, and the results are shown in Figure 7 As can be seen from the figure, when heated above the phase change temperature, under the pressure of 500 g of weight, NCBM4@M, NCBM8@M and NCBM12@M all did not observe obvious shape change and liquid leakage. In order to further quantify the leakage of PCM, the NCBM@M series samples were placed in a constant temperature drying oven at 80°C for continuous heating for 120h. It can be seen that the surface of the glass dish still remains dry after 120h, and the heat storage material has almost no mass loss. Obviously, based on the triple encapsulation effect of the core-shell structure of phase change paraffin microcapsules, the three-dimensional cross-linked network of NR and the three-dimensional cross-linked network of CNF, the obtained flexible NCBM@M heat storage materials all exhibit excellent complete leakage prevention performance, and show great potential in practical application.

[0108] (62) Cycle stability and thermal stability

[0109] The cycle stability and thermal stability of the heat storage materials prepared in Examples 1-4 were tested, and the results are shown in Figure 8 Figure 8 The DSC curves of NCBM12@M heat storage material before and after 100 times of heating / cooling cycles are shown. The results show that compared with the first heating / cooling cycle of NCBM12@M heat storage material, the phase change temperature and enthalpy of NCBM12@M heat storage material after 100 cycles have almost no change, maintaining a high enthalpy (115.87 J / g) and a loss of less than 1%, showing excellent cycle stability. The chemical structure of NCBM12@M heat storage material remains stable after 100 times of heating / cooling cycles.

[0110] In addition, the thermal stability of the heat storage materials prepared in Examples 1-4 was evaluated by thermogravimetric analysis (TGA), and the results are shown in Figure 9 Figure 9 ​​(a) shows the TGA curves of MPCM, NC@M and NCBM12@M. It can be seen from the figure that the components of the materials remain stable within 0-180℃; the thermal stability of MPCM is low, and it begins to perform thermal decomposition behavior at 180℃, and the main mass loss occurs between 180 and 450℃, accompanied by three thermal decomposition stages; the mass loss of NC@M and NCBM12@M thermal conductive heat storage materials occurs between 190 and 450℃, accompanied by two-step thermal decomposition. It is shown that the thermal decomposition rate increases with the increase of temperature. It is well known that BNNS and MWCNT have extremely stable chemical properties and will not perform obvious thermal decomposition at a temperature as high as 800℃. The results show that the charring residue of NCBM12@M is about 11.8%. It is basically consistent with the loading ratio of MWCNT and BNNS in the corresponding NCBM12@M composite material. In order to further reveal the thermal decomposition behavior of the phase change material, the derivative thermogravimetric (DTG) curve of the material is obtained based on the TGA data, as shown in Figure 9 (b). For the thermal conductive heat storage material, the first step on the TGA curve is the thermal decomposition process of MCPW. Compared with pure MPCM, the prepared NC@M and NCBM12@M thermal conductive heat storage materials are all completed at a higher temperature. After 350℃, NR and CNF experience rapid thermal decomposition due to the gradual decomposition of low molecular weight compounds and volatilization of the molecular chain and oligomers. In summary, the weight loss rate of NCBM12@M is slower, and the weight loss is completed at a higher temperature, which can prove that the prepared NCBM12@M has better thermal stability than pure MPCM, which is very beneficial to its safe use in thermal management systems.

[0111] 7) Thermal management and resistivity test

[0112] This test example takes Example 4 as an example, and the prepared thermal conductive heat storage material uses NCBM12@M module for temperature control. The module uses a polyimide (PI) heating belt as a heat source inserted into NCBM12@M. Figure 10 (a) shows the experimental device and the schematic diagram of the heat storage / release process, including the arrangement of the thermocouple.

[0113] Figure 10 (b) shows the change of the surface temperature of NCBM12@M module with time. The two test points show consistent trends, indicating that the temperature is uniform throughout the heat storage and heat release cycle. In the discharge phase, the first phase change leads to the release of latent heat from point C to point D. Subsequently, after the release of part of the sensible heat (from point D to point E), the second stage of latent heat release (from point E to point F) begins, keeping the surface temperature between 30 and 31℃ for 6 minutes. F point represents the end of the entire phase change. The results prove the two-stage temperature control capability and high thermal efficiency potential of NCBM12@M. Figure 10(c) The effect of module thickness on the two-stage constant surface temperature was further analyzed. The results provide guidance for designing module specifications that meet practical requirements.

[0114] In addition, this test example also tested the resistivity (ρ) of thermally conductive and heat-storing materials NC@M, NCM3.6@M (NCBM12@M sample without BNNS, specifically prepared using the same method as in Example 4, except that BNNS was not added) and NCBM12@M. The results are shown below. Figure 10 (d) The resistivity of the NC@M sample, the NCM3.6@M sample (NCBM12@M sample without BNNS), and the NCBM12@M sample are approximately 2.47 × 10¹⁰ Ω⁻ m, 2.64 × 10⁸ Ω⁻ m, and 7.77 × 10⁹ Ω⁻ m, respectively. These findings indicate that NR and MPCM have good insulation properties. The resistivity significantly decreased after the addition of MWCNTs, mainly due to the enhanced conductivity of MWCNTs. BNNS has a very high resistivity, and its addition significantly enhances the insulation properties of CPCM. This suggests that BNNS and MWCNTs...

[0115] It can be effectively dispersed in the CPCM to form an interconnected network, thereby significantly suppressing the conductivity of the CPCM.

[0116] The application of the thermally conductive and heat-storing material in this application as a wearable material specifically involves: by reducing the peak surface operating temperature or achieving excellent temperature control performance, we further apply the NCBM12@M module to the human arm (…). Figure 10 (f) As an example of hyperthermia, its effectiveness in personal thermal management was evaluated. Due to the greater flexibility and bioavailability of the NR / CNF substrate, we were able to fabricate large-format NCBM12@M modules that could be bent to fit the curvature of the human arm without leakage of NCBM12@M. For ethical reasons, we used a 3.5 mm thick temperature control module to allow subjects to test the behavior of multiple heat-cooling cycles. Figure 10 The heat charge-release curves in (e) show that the optimal human body temperature range of 36-40°C can be maintained for more than 10 minutes after the first charge. Furthermore, after the second and third short charges, the dwell time related to thermal comfort temperature is almost identical to that of the first charge. Infrared thermal imaging also shows that the F-FSPCMs module can evenly release heat to the arm after charging, giving it excellent thermotherapy effects and demonstrating better thermal cycle stability and longer service life in practical applications.

[0117] 8) Photothermal conversion performance test

[0118] The light-heat conversion performance of the heat-conducting heat storage materials prepared in Examples 1-4 was tested, specifically as follows:

[0119] The light-heat conversion performance test system in this test example was composed of a xenon lamp, an infrared camera and a temperature collector (as shown in Figure 11 (a)). The PCM sample with a diameter of 2 cm and a height of 0.4 mm received a constant irradiance of 100 mW / cm2 through a solar simulator; the temperature fluctuation at the bottom of the PCM sample was recorded using a thermocouple and a temperature collector; and the temperature distribution on the surface of the PCM sample over time was recorded using an infrared camera.

[0120] The temperature-time curves of the light-heat conversion experiment are shown in Figure 11 (b) and 11(c). As can be seen from the figures, under the simulated lighting conditions, the temperature of the PCM sample (NC@M and NCBM12@M) was rising. After 25 minutes of irradiation, the temperature of NC@M only reached 37.3℃, which was much lower than its melting point (about 39℃). This indicates that its light-heat conversion performance is relatively poor. In contrast, the temperature at the bottom of NCBM12@M rapidly increased with the extension of the irradiation time. In addition, the phase change process was clearly visible when the temperature was between 38-40℃. The surface temperature distribution of NCBM12@M and NC@M showed a similar pattern. In addition, it was also observed that the surface temperature of NCBM12@M was higher than the temperature measured by the thermocouple. The reason for this difference is that the thermocouple is placed at the bottom of the NCBM12@M sample; when the surface of the NCBM12@M sample is irradiated, the temperature will rise; however, the heat transfer to the bottom has not yet reached equilibrium. In addition, the heat transfer of the NCBM12@M sample to the surrounding environment is also one of the reasons for this difference. It was calculated that the light-heat conversion efficiency of the NCBM12@M sample was 89.26%. The conversion and storage efficiency of solar energy to heat energy h can be determined by calculating the ratio of the heat energy stored in the material to the incident light radiation energy received during the phase change process, as shown in the following formula:

[0121]

[0122] where m represents the mass of the PW composite material, H m represents the melting enthalpy of the PCM, P indicates the irradiance of the solar simulator, S represents the irradiated surface area of the PCM, and t1 and t2 represent the phase change time of the PCM under solar driving before and after the phase change, respectively.

[0123] In summary, the addition of MWCNTs and BNNS significantly improves the light-heat conversion and heat storage performance of the CPCM. This enhancement can be attributed to the MWCNTs acting as effective light absorbers and molecular heaters, which can effectively convert solar energy into heat energy and promote the heat storage process of the CPCM.

[0124] In summary, the heat-conducting and heat-storing material provided by the present application effectively avoids the agglomeration of MPCM by fixing the MPCM on the surface of CNF; the triple support of microencapsulation plus NR and CNF continuous network is expected to give the heat-conducting and heat-storing material significant anti-leakage performance. NR serves as a flexible matrix, and the NR crosslinked skeleton after curing gives the MPCM room temperature flexibility, so that the CPCM based on MPCM can be bent or twisted without breaking. In addition, BNNS and MWCNT as a binary synergistic heat-conducting filler self-assemble and disperse on the CNF crosslinked network to form an efficient heat-conducting path, providing technical support for the thermal conductivity of the synergistically enhanced material; at the same time, the CNF / BNNS / MWCNT heat-conducting path based on the CNF crosslinked network is introduced into the polymer matrix, effectively improving the thermal conductivity of the polymer-based flexible heat-conducting and heat-storing material. The resulting heat-conducting and heat-storing material that combines flexibility, anti-leakage and heat-conducting enhancement can be applied to effectively manage the heat of the battery, greatly expanding its application scenarios.

[0125] The above examples are only used to illustrate the technical solutions of the present application, but not to limit it; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application, and they should be covered in the scope of the claims and the specification of the present application.

Claims

1. A method for preparing a thermally conductive and heat-storing material based on a CNF / NR dual crosslinked network, characterized in that, Includes the following steps: 1) A certain amount of thermally conductive filler is dispersed into CNF aqueous suspension and homogenized to obtain the first mixture; 2) Mix the first mixture obtained in step 1) with the phase change capsules until homogeneous to obtain the second mixture; 3) Add natural rubber latex to the second mixture prepared in step 2), stir well, and obtain the third mixture; 4) Freeze, dry and press the third mixture obtained in step 3) to obtain a thermally conductive and heat-storing material; The thermally conductive filler is 4-12% by mass in the thermally conductive and heat-storing material, and the thermally conductive material includes BNNS and MWCNT, with a mass ratio of BNNS to MWCNT of (3-8):(2-5); the mass ratio of phase change capsule to CNF is (5-15):1; and the mass ratio of phase change capsule to NR is (4-8):(2-5).

2. The method for preparing the thermally conductive and heat-storing material based on a CNF / NR dual crosslinked network according to claim 1, characterized in that, The homogenization process parameters in step 1) are: homogenization at 4000-8000 rpm for 5-15 min at room temperature.

3. The method for preparing the thermally conductive and heat-storing material based on a CNF / NR dual crosslinked network according to claim 1, characterized in that, The freezing conditions described in step 4) are freezing at -25 ~ -15℃ for 6-18 hours; the drying conditions are freeze-drying at a pressure of less than 20Pa and a temperature of -60 ~ -50℃ for 40-50 hours; and the pressing conditions are pressing at room temperature and 1-3MPa for 3-8 minutes.

4. The method for preparing the thermally conductive and heat-storing material based on a CNF / NR dual crosslinked network according to claim 3, characterized in that, The natural rubber latex is NR natural rubber latex, the phase change capsule is phase change paraffin microcapsule, and the CNF aqueous suspension has a mass fraction of 1-3%.

5. A thermally conductive and thermally storing material prepared by the preparation method according to any one of claims 1-4.

6. The application of the thermally conductive and thermally storing material according to claim 5 in photothermal materials and wearable materials.

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