Nanometer aerogel material with excellent radiation cooling and millimeter wave transparent performance and preparation method thereof

Abstract

The application provides a nano-aerogel material with excellent radiation refrigeration and millimeter wave wave transmission performance and a preparation method thereof, the nano-aerogel material is formed by a high-crosslinking-density silicon-containing polymer matrix and a trisilanol ethyl polyhedral oligomeric silsesquioxane through a crosslinking reaction and supercritical fluid drying, the nano-aerogel material has a bimodal nanopore structure composed of small pores and large pores, the porosity is 92% to 94%, and the nano-aerogel material has the radiation refrigeration and millimeter wave wave transmission performance. The nano-aerogel material has a wave transmission rate of at least 99.5% in the Ku band, and even can reach 99.7%, and the passive cooling performance of the nano-aerogel material can reach a delta T of 9.26 DEG C, and the nano-aerogel material has excellent millimeter wave wave transmission performance and radiation refrigeration performance.

Description

Technical Field

[0001] The present invention belongs to the field of polymer nanocomposite radiation cooling and wave-transmitting materials, and relates to a nano aerogel material having both excellent radiation cooling and millimeter wave transmission properties and a preparation method thereof. Background Art

[0002] Maintaining a suitable indoor temperature and efficient millimeter wave communication efficiency is of great significance to industrial manufacturing, agricultural production and human indoor activities. The current indoor temperature regulation method is mainly adjusted by consuming electricity, such as using air conditioning. Studies have shown that human buildings consume about 30% of the total energy consumption each year and produce 10% of greenhouse gases. Compared with traditional cooling methods, radiative cooling materials can significantly improve the indoor temperature environment without consuming additional energy (such as without consuming additional electricity). In addition, radiative cooling materials with millimeter wave transparency are helpful in the design and construction of new energy-saving and intelligent communication buildings.

[0003] Porous polymer materials have many advantages such as light weight, flexibility and corrosion resistance, and are currently widely used in many fields. Among them, nanoaerogel materials have unique structures such as nanopore size, high specific surface area, a large number of Si-O-Si chemical bonds and high porosity, which help to simultaneously reduce the thermal conductivity of the material and improve the millimeter wave transmission ability of the material. However, the preparation of nanoaerogels with both radiative cooling performance and high-frequency wave transmission performance still faces the following problems: (1) Since the pore size in the nanoaerogel (30-40nm) is smaller than the wavelength of visible light (390-780nm), visible light can penetrate the nanoaerogel material, thereby significantly reducing its radiative cooling performance; (2) The large number of Si-OH functional groups contained in the nanoaerogel will significantly reduce the hydrophobicity of the material. After absorbing water, the dielectric loss of the nanoaerogel material will increase significantly, thereby reducing its millimeter wave transmission performance; (3) Traditional inorganic nanoaerogel materials are brittle, which will significantly reduce the mechanical performance of the material. The present invention is proposed to address these problems, hoping to effectively improve the radiation cooling performance and millimeter wave transmission performance of nano-aerogel materials at the same time. Summary of the Invention

[0004] In view of the deficiency of existing technology that it is difficult to prepare materials with both excellent radiative cooling performance and high millimeter wave transmission performance, the present invention provides a nano aerogel material with excellent radiative cooling and millimeter wave transmission performance and a preparation method thereof, so as to simultaneously improve the material's radiative cooling performance and millimeter wave transmission performance.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is as follows:

[0006] A nano-aerogel material with excellent radiative cooling and millimeter wave transmission properties. The nano-aerogel material is formed by cross-linking reaction and supercritical fluid drying of a silicon-containing polymer matrix polyvinyltrimethylsilane and trisilanolethyl polyhedral oligomeric silsesquioxane with a high cross-linking density. The nano-aerogel material has a bimodal nanoporous structure consisting of small pores and large pores, with a porosity of 92% to 94%. The nano-aerogel material has both radiative cooling and millimeter wave transmission properties.

[0007] Preferably, in the technical solution of the nano-aerogel material having both excellent radiative cooling and millimeter wave transmission properties, the nano-aerogel material is formed by a cross-linking reaction and supercritical fluid drying of a silicon-containing polymer matrix with a high cross-linking density and trisilanolethyl polyhedral oligomeric silsesquioxane in a ratio of 15wt% to 25wt% of the total mass of the silicon-containing polymer matrix with a high cross-linking density and the trisilanolethyl polyhedral oligomeric silsesquioxane.

[0008] In the above technical solution of the nano-aerogel material having both excellent radiative cooling and millimeter wave transmission properties, the pore diameter of the small pores in the nano-aerogel material is 30-40 nm, and the pore diameter of the large pores is 400-600 nm.

[0009] In the above-mentioned technical solution of nano-aerogel material with excellent radiative cooling and millimeter wave transmission performance, the cross-linking density of the high cross-linking density silicon-containing polymer matrix should meet the requirements of constructing a nanoporous structure. A feasible high cross-linking density silicon-containing polymer matrix can be polyvinyltrimethylsilane.

[0010] In the above-mentioned technical solution of the nanoaerogel material having both excellent radiative cooling and millimeter wave transmission properties, the nanoaerogel material has a wave transmittance in the Ku band of at least 99.5%. Preferably, when the nanoaerogel material is formed by cross-linking reaction and supercritical fluid drying of polyvinyltrimethylsilane and trisilanolethyl polyhedral oligomeric silsesquioxane in a ratio of trisilanolethyl polyhedral oligomeric silsesquioxane to 20wt% of the sum of the mass of polyvinyltrimethylsilane and trisilanolethyl polyhedral oligomeric silsesquioxane, the wave transmittance of the nanoaerogel material in the Ku band can reach 99.7%.

[0011] In the technical solution of the nano-aerogel material with both excellent radiative cooling and millimeter wave transmission performance, the specific surface area of ​​the nano-aerogel material is 700-750m 2 / g.

[0012] In the technical solution of the nano-aerogel material with both excellent radiative cooling and millimeter wave transmission performance, the density of the nano-aerogel material is 0.1-0.2 g / cm 3 .

[0013] In the above technical solution of the nano-aerogel material with excellent radiation refrigeration and millimeter wave transmission performance, the water contact angle of the nano-aerogel material is 125°-145°.

[0014] In the above technical solution of the nano-aerogel material with excellent wave absorption performance and infrared stealth performance, the thermal conductivity of the nano-aerogel material is not more than 26 mW·m -1 K -1 , and is usually 22-25.5 mW·m -1 K -1 .

[0015] In the above technical solution of the nano-aerogel material with excellent wave absorption performance and infrared stealth performance, the nano-aerogel material has excellent radiation refrigeration performance. Under the test adjustment in the present application, the passive cooling performance (ΔT) of the nano-aerogel material in the present application reaches at least 7°C, and is usually 7-10°C. For example, when the nano-aerogel material is formed by cross-linking reaction and supercritical fluid drying of polyvinyltrimethylsilane and trisilanol ethyl polyhedral oligomeric silsesquioxane at a proportion of 20 wt% of trisilanol ethyl polyhedral oligomeric silsesquioxane based on the total mass of polyvinyltrimethylsilane and trisilanol ethyl polyhedral oligomeric silsesquioxane, the passive cooling performance (ΔT) of the nano-aerogel material can reach 9.26°C. Under the same test conditions, when no trisilanol ethyl polyhedral oligomeric silsesquioxane is added, the passive cooling performance (ΔT) of the prepared polyvinyltrimethylsilane nano-aerogel material is only 2.66°C.

[0016] The present application also provides a preparation method of the above nano-aerogel material with excellent radiation refrigeration and millimeter wave transmission performance, comprising the following steps:

[0017] (1) dissolving a high-crosslinking-density silicon-containing polymer matrix in an anhydrous solvent to obtain a silicon-containing polymer matrix solution; dissolving trisilanol ethyl polyhedral oligomeric silsesquioxane in a solvent to obtain a trisilanol ethyl polyhedral oligomeric silsesquioxane solution; mixing the silicon-containing polymer matrix solution and the trisilanol ethyl polyhedral oligomeric silsesquioxane solution to obtain a mixed solution, then adding ammonia water to induce spinodal decomposition, and after mixing uniformly after adding the ammonia water, obtaining a gel precursor solution; transferring the gel precursor solution to a mold, placing in an oven at 35-45°C until the gel precursor solution is converted into a gel state to obtain a wet gel; and immersing the obtained wet gel in a solvent for aging;

[0018] In this step, the amount of the silicon-containing polymer matrix alkane solution and the trisilanolethyl polyhedral oligomeric silsesquioxane solution added is controlled so that the content of the trisilanolethyl polyhedral oligomeric silsesquioxane in the mixed solution is 15wt% to 25wt%, and the amount of ammonia water added is controlled so that the molar ratio of water to Si element in the mixed solution is (8 to 12):1;

[0019] (2) The sample aged in step (1) is subjected to supercritical carbon dioxide drying to obtain a nano-aerogel material having both excellent radiative cooling and millimeter wave transmission properties.

[0020] In the technical solution of the above-mentioned method for preparing a nano-aerogel material having both excellent radiative cooling and millimeter wave transmittance, the solvent in step (1) is preferably anhydrous ethanol, and the main purpose of immersing the obtained wet gel in the solvent for aging in step (1) is to further increase the crosslinking density of the silicon-containing polymer matrix (e.g., polyvinyltrimethylsilane).

[0021] In the technical solution of the preparation method of the nano-aerogel material with excellent radiative cooling and millimeter wave transmission properties, the concentration of the silicon-containing polymer matrix solution is 0.1-0.2 g / mL, and the concentration of the trisilanolethyl polyhedral oligomeric silsesquioxane solution is 3-45 mg / mL.

[0022] In the technical solution of the method for preparing the nano-aerogel material having both excellent radiative cooling and millimeter wave transmission properties, the aging time in step (1) is 20 to 30 hours.

[0023] In the technical solution of the above-mentioned method for preparing a nano-aerogel material having both excellent radiative cooling and millimeter wave transmission properties, when the silicon-containing polymer matrix is ​​polyvinyltrimethylsilane, a feasible preparation method of polyvinyltrimethylsilane is as follows:

[0024] The thermal initiator is dissolved in the vinyltrimethoxysilane monomer, and the mass ratio of the thermal initiator to the vinyltrimethoxysilane monomer is controlled to be 1:(5-10). The reaction is stirred at 150-160° C. in a nitrogen atmosphere for 3-5 hours, and the unreacted thermal initiator and vinyltrimethoxysilane monomer are removed to obtain the product.

[0025] The preparation process of the nano-aerogel material with excellent radiative cooling and millimeter wave transmission performance of the present invention is as follows:

[0026] The present invention first adopts free radical polymerization to prepare a silicon-containing polymer matrix (such as polyvinyltrimethylsilane, PVTMS) with high inorganic crosslinking points. The high crosslinking density of the silicon-containing polymer matrix is ​​conducive to the preparation of aerogels with nanopores and high specific surface area, and the organic-inorganic hybrid molecular structure is conducive to improving the mechanical properties of nano aerogel materials. Then, POSS containing -Si-OH groups is introduced into the silicon-containing polymer matrix. POSS can react with the silicon-containing polymer matrix under ammonia catalysis conditions. Therefore, the mobility and crosslinking density of the molecular chain of the obtained material (such as POSS / PVTMS) will be reduced to some extent. During the spinodal decomposition process, large pores (~500nm) can be formed in the material. Finally, the nano aerogel material is obtained by scCO2 drying, retaining the nanoporous structure and high specific surface area of ​​the material. At the same time, due to the introduction of POSS with ethyl functional groups, the nano aerogel material prepared by the present invention has hydrophobicity, and its water contact angle reaches 137°. In addition, the nano-aerogel material prepared by the present invention has a nano-scale bimodal pore structure, and the rich heterogeneous interfaces and high porosity are beneficial to improving its passive cooling performance and EMW transmission performance.

[0027] Compared with the existing technology, the technical solution provided by the present invention can produce the following beneficial technical effects:

[0028] 1. The present invention provides a nano-aerogel material with both excellent radiative cooling and millimeter-wave transmission properties. The nano-aerogel material is formed by cross-linking a high-crosslink density silicon-containing polymer matrix and trisilanolethyl polyhedral oligomeric silsesquioxane and drying with a supercritical fluid. The nano-aerogel material has a bimodal nanoporous structure consisting of small pores (30-40 nm) and large pores (400-600 nm), with a porosity of 92%-94%. The large pores in the nano-aerogel material enhance the reflectivity and scattering of visible light (400-780 nm), while the small pores effectively block heat conduction and convection through the Knudsen effect, thereby improving passive cooling performance. The nano-aerogel material's high specific surface area facilitates infrared reflection, thereby improving passive cooling performance. Furthermore, the nano-aerogel material's high porosity facilitates thermal insulation, thereby improving passive cooling performance. The high porosity also facilitates millimeter-wave transmission.

[0029] 2. The present invention has experimentally confirmed that the nano-aerogel material provided by the present invention has excellent radiative cooling performance, with a passive cooling performance ΔT of up to 9.26°C. The nano-aerogel material also has excellent millimeter wave transmission performance, with a wave transmittance of up to 99.7% in the Ku band (12.4-18GHz). At the same time, the nano-aerogel material provided by the present invention is also hydrophobic, with a water contact angle of up to 137°, and also has low dielectric properties, with an average D k=1.14. In addition, the passive cooling performance of the nano-aerogel material provided by the present invention is less affected by surface contamination. When the surface of the nano-aerogel material is contaminated, the passive cooling performance is only slightly reduced.

[0030] 3. The present invention also provides a method for preparing the aforementioned nanoaerogel material with both excellent radiative cooling and millimeter wave transmission properties. A silicon-containing polymer matrix (e.g., PVTMS) with a high number of inorganic crosslinking points is first prepared by free radical polymerization. POSS is then introduced by spinodal decomposition to prepare a wet gel. Finally, the nanoaerogel material is prepared by scCO2 drying. Both nanopores and nanopores are simultaneously introduced into the nanoaerogel material, forming a nanoscale bimodal pore structure. This invention provides new ideas and guidance for the nanoscale structural design of porous materials for passive cooling and EMW transmission applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Figure 1 is a schematic diagram of the preparation process of the POSS / PVTMS nanoaerogel material of the present invention, wherein (a) is a schematic diagram of the PVTMS free radical polymerization, (b) is a schematic diagram of the sol-gel synthesis of wet gel and the bimodal pore structure formed therefrom, and (c) is a schematic diagram of scCO2 drying and the test results of density and water contact angle.

[0032] Figure 2 Figures (a) to (e) are digital micrographs of PVTMS nanoaerogel material and POSS / PVTMS nanoaerogel material covered on the light source, respectively. Figure 2 Figures (a') to (e') and (a") to (e") are the SEM images of PVTMS nanoaerogel material and POSS / PVTMS nanoaerogel material at different magnifications, respectively. Figure 2 Figure (f) is the nitrogen adsorption-desorption isotherms of PVTMS nanoaerogel material and POSS / PVTMS-20wt%, POSS / PVTMS-30wt%. Figure 2 Figure (g) shows the pore size distribution curves of PVTMS nano-aerogel material and POSS / PVTMS-20wt%, POSS / PVTMS-30wt%. Figure 2 Figures (h) to (k) are the total XPS spectra of the prepared PVTMS nanoaerogel material and POSS / PVTMS-20wt%, element content test results, Si 2P spectrum and C1s XPS spectrum.

[0033] Figure 3 Figure (a) shows the porosity of PVTMS nanoaerogel material and POSS / PVTMS nanoaerogel material. Figure 3Figure (b) shows the thermal conductivity of PVTMS nano-aerogel material and POSS / PVTMS nano-aerogel material. Figure 3 Figure (c) is the TGA test results of PVTMS nano-aerogel material and POSS / PVTMS-20wt%. Figure 3 Figure (d) is the infrared stealth performance test result of POSS / PVTMS-20wt%, Figure 3 Figure (e) is the thermal resistance performance test results of POSS / PVTMS-20wt%, where the first row of figures are the test results of the experimental group, and the second row of figures are the test results of the control group. Figure 3 Figure (f) is the test results of solar reflectivity and infrared emissivity of POSS / PVTMS-20wt%.

[0034] Figure 4 Figure (a) is a schematic diagram of using a homemade passive cooling test chamber to test the passive cooling performance of nano aerogel materials.

[0035] Figure 4 Figures (b) and (d) show how the ambient temperature (Ambient) and the temperature below the aerogel cooler (Aerogel cooler) change with time when PVTMS nano-aerogel material and POSS / PVTMS-20wt% are used as aerogel coolers, respectively. Figure 4 Figure (c) shows the passive cooling performance (ΔT) of POSS / PVTMS nanoaerogels with different POSS contents.

[0036] Figure 5 Figure (a) is a schematic diagram of the chemical bond and radiation emission mechanism of the nano-aerogel cold material and a photo of the test process.

[0037] Figure 5 Figure (b) shows the passive cooling performance of POSS / PVTMS-20wt% with different thicknesses. Figure 5 Figure (c) is a summary of the average values ​​of passive cooling performance (ΔT) of POSS / PVTMS-20wt% with different thicknesses. Figure 5 Figure (d) shows the passive cooling performance test results of contaminated and uncontaminated POSS / PVTMS-20wt%. Figure 5 Figure (e) shows the average ΔT of contaminated and uncontaminated POSS / PVTMS-20wt%.

[0038] Figure 6 Figures (a) and (a') show the dielectric constant and dielectric loss test results of PVTMS nano-aerogel material and POSS / PVTMS-20wt% in the low frequency band. Figure 6Figures (b) and (b') are the dielectric constant and dielectric loss test results of PVTMS nanoaerogel materials and POSS / PVTMS-20wt% in the high frequency band (Ku band), Figure (c) of 6 is the EMW transmission schematic diagram of the nanoaerogel sample, and Figure (c') of 6 is the EMW transmission performance test results of PVTMS nanoaerogel materials and POSS / PVTMS-20wt% in the Ku band. DETAILED DESCRIPTION

[0039] The following examples further illustrate the nano-aerogel material with excellent radiative cooling and millimeter wave transmission performance and its preparation method according to the present invention. The following examples are only some embodiments of the present invention, not all embodiments. Based on the present invention and the examples, other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0040] In the following examples and comparative examples, the thermal initiators di-tert-butyl peroxide (DTBP, 98%) and vinyltrimethoxysilane (VTMS, 98%) were purchased from Sigma Aldrich. The solvent, anhydrous ethanol (100%), and the alkaline catalyst, aqueous ammonia (28%-30% (NH3), ACS grade), were purchased from GreenField Global and VWR, respectively. All chemicals were used as received. Trisilanolethyl polyhedral oligomeric silsesquioxane (POSS) was purchased from Hybrid Plastics, USA. scCO2 drying was performed using carbon dioxide (≥99% purity, Linde Gas).

[0041] Example 1

[0042] In this embodiment, a POSS / PVTMS nano-aerogel material with excellent radiative cooling and millimeter wave transmission performance is prepared. The preparation process is shown in the figure below. Figure 1 As shown, the steps are as follows:

[0043] (1) Preparation of PVTMS by free radical polymerization

[0044] The thermal initiator di-tert-butyl peroxide (DTBP) was dissolved in the monomer vinyltrimethoxysilane (VTMS), and the mass ratio of DTBP to VTMS was controlled to be 1:10. The resulting solution was poured into a four-necked flask equipped with a condenser and a stirrer, and reacted at 150°C and a stirring speed of 200 rpm in a nitrogen atmosphere for 3 hours. After that, the four-necked flask was placed in a vacuum oven at 150°C to remove unreacted monomers and thermal initiator to obtain polyvinyltrimethylsilane (PVTMS).

[0045] (2) Constructing nanoporous structures using cross-linking chemical reactions

[0046] PVTMS was dissolved in anhydrous ethanol to obtain a 0.2 g / mL PVTMS solution. The PVTMS solution was stirred at 40°C for 0.5 h. POSS was then added to anhydrous ethanol and stirred at 40°C for 0.5 h to obtain a POSS solution. The PVTMS solution and POSS solution were mixed and stirred for 0.5 h to obtain a mixed solution. A basic ammonia catalyst was then added at a molar ratio of water to Si in the mixed solution (water / Si) of 9:1 to induce spinodal decomposition (spinodal phase separation). After adding ammonia, the mixture was stirred for 1 min. The resulting gel precursor solution was then transferred to a mold. The mold containing the gel precursor solution was placed in an oven at 40°C until the gel precursor solution transformed into a gel state, which took approximately 10 to 12 h. The resulting wet gel (POSS / PVTMS wet gel) was then aged in anhydrous ethanol for 24 h.

[0047] In this step, multiple experiments were conducted. In each experiment, when mixing the PVTMS solution and the POSS solution, the addition amounts of the two were controlled so that the amount of POSS in the mixed solution was 5wt%, 10wt%, 20wt%, or 30wt% of the PVTMS solution.

[0048] (3) Supercritical carbon dioxide (scCO2) drying

[0049] The sample obtained by aging in step (2) was solvent exchanged with liquid CO2 under a pressure of 10.34 MPa (1500 psi) and a temperature of 25°C, and finally the CO2 was released under a pressure of 10.34 MPa (1500 psi) and a temperature of 45°C to complete the drying process to obtain a circular sheet of POSS / PVTMS nanoaerogel material (POSS / PVTMS aerogel).

[0050] The POSS / PVTMS nanoaerogel materials prepared in each group of experiments in this embodiment are respectively recorded as POSS / PVTMS-5wt% (abbreviated as POSS-5wt%), POSS / PVTMS-10wt% (abbreviated as POSS-10wt%), POSS / PVTMS-20wt% (abbreviated as POSS-20wt%), and POSS / PVTMS-30wt% (abbreviated as POSS-30wt%) according to the amount of POSS in the mixed solution described in step (2).

[0051] Comparative Example 1

[0052] In this comparative example, a PVTMS nano-aerogel material (PVTMS aerogel) was prepared. The preparation process of this comparative example was basically the same as that of Example 1, except that no POSS solution was added in step (2), and the prepared PVTMS aerogel was recorded as POSS-0wt%.

[0053] Example 2

[0054] In this example, the POSS / PVTMS nanoaerogel material prepared in Example 1 was characterized.

[0055] The microstructure of the PVTMS nanoaerogel material (PVTMSaerogel) prepared in Comparative Example 1 and the POSS / PVTMS nanoaerogel material prepared in Example 1 was observed using a scanning electron microscope (SEM). The pore size distribution and surface area of ​​the PVTMS nanoaerogel material prepared in Comparative Example 1 and the POSS / PVTMS-20wt% and POSS / PVTMS-30wt% prepared in Example 1 were determined using the BET (Brunner-Emmett-Teller) test method of Autosorb IQ (Quanta chrome Instruments). The elemental content of the PVTMS nanoaerogel material prepared in Comparative Example 1 and the POSS / PVTMS-20wt% prepared in Example 1 was tested using an X-ray photoelectron spectrometer (XPS). The hydrophobicity (water contact angle) of the POSS / PVTMS nanoaerogel material prepared in Example 1 was tested using a water contact angle (WCA) measuring device, and each measurement was repeated at least 3 times.

[0056] Figure 2 Figures (a) to (e) are digital micrographs of the PVTMS nanoaerogel material prepared in Comparative Example 1 and the POSS / PVTMS nanoaerogel material prepared in Example 1 covered on a light source. It can be seen from this group of figures that with the increase of POSS content, the light transmittance of the nanoaerogel material gradually decreases. Figure 2 Figures (a') to (e') and (a") to (e") are SEM images of the PVTMS nanoaerogel material prepared in Comparative Example 1 and the POSS / PVTMS nanoaerogel material prepared in Example 1 at different magnifications, respectively. It can be seen from these two groups of figures that the POSS / PVTMS nanoaerogel material prepared in Example 1, in addition to containing abundant small pores, also gradually produces pores with larger pore size (pore size ~ 500nm). The large pore size (pore size ~ 500nm) is beneficial to enhancing the reflectivity and scattering of visible light (400~780nm), thereby reducing the visible light transmittance.

[0057] Figure 2Figure (f) shows the nitrogen adsorption-desorption isotherms of the PVTMS nanoaerogel material prepared in Comparative Example 1 (Pristine PVTMS in this figure) and POSS / PVTMS-20wt% and POSS / PVTMS-30wt% prepared in Example 1. As can be seen from the figure, all samples show type IV isotherms, which are characteristic of mesoporous materials. The hysteresis loop is generated by capillary condensation in mesopores with a diameter greater than 4nm. Nitrogen adsorption-desorption analysis is also used to measure the specific surface area and pore size of the samples. Figure 2 As shown in the pore size distribution curve of Figure (g), POSS / PVTMS-20wt% has a high specific surface area (728m 2 / g) and nanopore size (30-40nm). The Pristine PVTMS in this figure represents the PVTMS nanoaerogel material prepared in Comparative Example 1. Combined with the SEM image, it can be seen that POSS / PVTMS-20wt% not only has a small pore structure with a pore size in the range of 30-40nm, but also has a macropore structure with a pore size of 500nm, showing a bimodal nanopore structure. For POSS / PVTMS-30wt%, the excessively high POSS content significantly reduces the crosslinking density of the PVTMS molecular chain, thereby deteriorating the nanopore structure of the sample.

[0058] Figure 2 Figure (h) is the total XPS spectra of the PVTMS nanoaerogel material prepared in Comparative Example 1 (Pristine PVTMS in the figure) and the POSS / PVTMS-20wt% prepared in Example 1. Figure 2 Figure (i) shows the element content test results of the PVTMS nanoaerogel material prepared in Comparative Example 1 (Pristine PVTMS in the figure) and the POSS / PVTMS-20wt% prepared in Example 1. As can be seen from the figure, after the introduction of POSS, the C element content decreases, and the O and Si element contents increase, which is mainly due to the introduction of POSS increasing the content of Si-O-Si bonds. Figure 2 Figures (j) to (k) are the Si 2P spectra and C1s XPS spectra of the PVTMS nanoaerogel material prepared in Comparative Example 1 (PVTMS in the figure) and the POSS / PVTMS-20wt% prepared in Example 1 (POSS / PVTMS in the figure). As can be seen from the figure, POSS / PVTMS-20wt% has CC and CO chemical bonds. With the introduction of POSS nanofillers, the CO chemical bond content increases. Si-O, CC, and CO chemical bonds are beneficial to enhancing the passive cooling performance of POSS / PVTMS nanoaerogel materials through thermal radiation.

[0059] The hydrophobicity (water contact angle) of the POSS / PVTMS nano-aerogel material prepared in Example 1 was tested using a water contact angle (WCA) measuring device, and each measurement was repeated at least 3 times. Figure 1 As shown in Figure (c), the water contact angle of POSS / PVTMS-20wt% is 137°±7.9°. This is mainly due to the ethyl functional groups in the introduced POSS, which increases the hydrophobicity of the POSS / PVTMS nanoaerogel material.

[0060] The density of the POSS / PVTMS nano-aerogel material prepared in Example 1 was further tested. The results showed that the density of the POSS / PVTMS nano-aerogel material was between 0.1 and 0.2 g / cm 3 Between, such as Figure 1 As shown in Figure (c), the density of POSS / PVTMS nanoaerogel material is placed on fresh flowers, which can be supported by the fresh flowers without causing deformation of the petals.

[0061] Example 3

[0062] In this example, the porosity, thermal conductivity, thermal stability, infrared stealth performance, thermal resistance, solar reflectivity and infrared emissivity of the POSS / PVTMS nanoaerogel material prepared in Example 1 were characterized.

[0063] The thermal conductivity of the PVTMS nanoaerogel material (PVTMS aerogel) prepared in Comparative Example 1 and the POSS / PVTMS nanoaerogel material prepared in Example 1 was measured using a hot plate TPS2500S thermal constant analyzer. The thermal stability of the PVTMS nanoaerogel material prepared in Comparative Example 1 and the POSS / PVTMS-20wt% prepared in Example 1 was determined by thermogravimetric analysis (TGA). The test was carried out in a nitrogen environment, with a test temperature range of 50 to 800°C and a heating rate of 10°C / min. An experimental group was formed by placing POSS / PVTMS-20wt% on a hot table and then placing a fresh flower on the top surface of the POSS / PVTMS-20wt%. Infrared thermal images of the POSS / PVTMS-20wt% at different surface temperatures (heated on a 100°C hot table for 0, 14, and 26 minutes) were taken using a Fluke-Ti32S infrared imager. A control group was formed by directly placing a fresh flower on the hot table. The top surface temperature of the POSS / PVTMS-20wt% was also measured when the POSS / PVTMS-20wt% was heated on a 100°C hot table for 0, 14, and 26 minutes. The solar reflectance of the POSS / PVTMS-20wt% prepared in Example 1 was measured using a UV-vis-NIR spectrophotometer in the 0.2-2.5 μm range. The emissivity spectrum of the POSS / PVTMS-20wt% prepared in Example 1 in the infrared region (3-18 μm) was measured using an FTIR spectrometer.

[0064] Figure 3 Figure (a) shows the porosity of the PVTMS nano-aerogel material prepared in comparative example 1 and the POSS / PVTMS nano-aerogel material prepared in example 1. Figure 3 Figure (b) shows the thermal conductivity of the PVTMS nano-aerogel material prepared in comparative example 1 and the POSS / PVTMS nano-aerogel material prepared in example 1. Figure 3 As shown in Figures (a) and (b), with the increase of POSS content, the porosity of the nano-aerogel material increases from 90.9% to 94.6%, and the thermal conductivity increases from 21.9mW·m -1 K -1 Increased to 35mW·m -1 K -1 The increase in the porosity of the nano-aerogel material can be attributed to the decrease in the shrinkage of the material during the scCO2 drying process, and the increase in the thermal conductivity of the nano-aerogel material can be attributed to the large pore structure generated in the POSS / PVTMS aerogel material. At the same time, we noticed that the thermal conductivity of POSS / PVTMS-20wt% was only 25.5mW·m -1 K -1, lower than the thermal conductivity of air 26 mW·m -1 K -1 . The low thermal conductivity can be attributed to the high porosity (large amount of air) and the nanoporous structure of POSS / PVTMS-20wt% (Knudsen effect).

[0065] Figure 3 Figure (c) of the drawings is the TGA test results of the PVTMS nanogel material prepared in Comparative Example 1 and the POSS / PVTMS-20wt% prepared in Example 1. From the figure, it can be seen that the thermal decomposition temperature of the POSS / PVTMS-20wt% is greatly improved, which is mainly due to the increase of Si-O-Si content, which increases the thermal resistance performance.

[0066] Figure 3 Figure (d) of the drawings is the infrared stealth performance test results of the POSS / PVTMS-20wt%. Figure 3 Figure (e) of the drawings is the thermal resistance performance test results of the POSS / PVTMS-20wt%, wherein the first row of figures is the test results of the experimental group, and the second row of figures is the test results of the control group. From the figure, it can be seen that the thermal resistance of the experimental group is higher than that of the control group, which is mainly due to the low thermal conductivity of the POSS / PVTMS-20wt%. Figure 2 From Figures (d) and (e) of the drawings, it can be seen that due to the low thermal conductivity of the POSS / PVTMS-20wt%, it is used to effectively block heat transfer, thereby inhibiting the water evaporation process of fresh flowers on the surface of the POSS / PVTMS nanogel material.

[0067] Figure 3 Figure (f) of the drawings is the solar reflectance and infrared emissivity test results of the POSS / PVTMS-20wt%. From the figure, it can be seen that the POSS / PVTMS-20wt% has high solar reflectance and infrared emissivity, and the high solar reflectance is mainly due to the large pores generated in the nanogel material, and the high infrared emissivity is mainly due to the chemical bonds in the nanogel material, including Si-O, C-C and C-O.

[0068] Example 4

[0069] In this example, the passive cooling performance of the POSS / PVTMS nanogel material prepared in Example 1 is characterized.

[0070] As shown in Figure (a) of the drawings, the passive cooling performance of the POSS / PVTMS nanogel material prepared in Example 1 is tested. The test is carried out in a dark room, and the temperature of the room is controlled at 25°C. The test is carried out for 24 hours, and the temperature of the fresh flower is measured every 2 hours. The results are shown in Figure (b) of the drawings. Figure 4The homemade passive cooling test box shown in Figure (a) tests the passive cooling performance of the PVTMS nano aerogel material (PVTMS aerogel) prepared in Comparative Example 1 and the POSS / PVTMS nano aerogel material prepared in Example 1. A quartz halogen light source illuminator with an adjustable power between 0 and 200W is used as a light source to illuminate the homemade passive cooling test box from above the homemade passive cooling test box, and a thermocouple is used to simultaneously record the temperature above and below the nano aerogel material within 180 minutes. The homemade passive cooling test box is prepared by a rectangular parallelepiped polymer foam material, a cavity is dug on the upper surface of the foam material, a piece of nano aerogel material is horizontally installed in the cavity as an aerogel cooler (Aerogel cooler), the opening of the cavity is sealed with a PE film, and thermocouples are respectively installed in the space above and below the aerogel cooler. The temperatures of the space above and below the aerogel cooler are recorded as the ambient temperature and the temperature below the aerogel cooler, respectively. The polymer foam material is placed on an insulating board, and the entire foam material is wrapped with tin foil.

[0071] Figure 4 Figures (b) and (d) show how the ambient temperature (Ambient) and the temperature below the aerogel cooler (Aerogelcooler) change with time when the PVTMS nano aerogel material prepared in Comparative Example 1 and the POSS / PVTMS-20wt% prepared in Example 1 are used as aerogel coolers. Figure 4 Figure (c) shows the passive cooling performance (ΔT) of POSS / PVTMS nanoaerogels with different POSS contents. Figure 4 It can be seen that for the PVTMS nano-aerogel material prepared in Comparative Example 1, although it exhibits the best thermal insulation performance (thermal conductivity from 21.9mW·m -1 K -1 ), however, most solar light will be able to pass through its nanoporous structure (pore size 30-40nm), so its passive cooling performance only reaches 2.66°C; with the increase of POSS content, the passive cooling gradually increases, which may be due to the gradual enlargement of the pores in the POSS / PVTMS nanoaerogel material, resulting in enhanced light reflectivity. At the same time, the nanoporous structure around the large pores can effectively block thermal conduction through the Knudsen effect. Compared with POSS / PVTMS-20wt%, the passive cooling performance of POSS / PVTMS-30wt% decreases, which may be due to the deterioration of the nanostructure of the nanoaerogel material caused by the introduction of too much POSS in the PVTMS system. The thermal conductivity of POSS / PVTMS-30wt% is significantly higher than that of POSS / PVTMS-20wt%. POSS / PVTMS-20wt% has the best passive cooling performance, with a ΔT of 9.26°C.

[0072] Figure 5 Figure (a) is a schematic diagram of the chemical bond and radiation emission mechanism of POSS / PVTMS nanoaerogel materials. In POSS / PVTMS-20wt%, the CC bond (900cm -1 ), Si-OC bond (840cm -1 ) and Si-O-Si bonds (1105 cm -1 ) exhibits strong stretching vibrations in the mid-infrared region. When POSS / PVTMS-20wt% is used as an aerogel cooler, upon receiving external radiation energy (hν) from the external environment, the aerogel cooler accelerates the molecular motion of the nano-aerogel material, causing the internal atoms to undergo strong stretching vibrations. Simultaneously, electrons in the high-energy band transition to the low-energy band, releasing photons. Finally, the aerogel cooler can emit heat in the form of electromagnetic waves in the mid-infrared region.

[0073] Figure 5 Figure (b) shows the passive cooling performance of POSS / PVTMS-20wt% with different thicknesses, and the average values ​​are summarized in Figure 5 It is worth noting that due to the combined advantages of mid-infrared radiation, light reflectivity and low thermal conductivity, the passive cooling performance of POSS / PVTMS-20wt% does not change significantly with the change of sample thickness, showing stable passive cooling performance.

[0074] In this example, the passive cooling performance of dirty and clean POSS / PVTMS-20wt% was also tested. Figure 5 Figure (d) shows the passive cooling performance test results of contaminated and uncontaminated POSS / PVTMS-20wt%. Figure 5 Figure (e) shows the average ΔT of contaminated and uncontaminated POSS / PVTMS-20wt%. Figure 5 It can be seen from Figures (d) and (e) that although the upper surface of POSS / PVTMS-20wt% is contaminated, compared with the uncontaminated case, the temperature above POSS / PVTMS-20wt% (ambient temperature) increases from 33.7°C to 46.7°C due to the enhanced light absorption of the black pollution on the upper surface. However, the temperature below POSS / PVTMS-20wt% (the temperature below the aerogel cooler) only decreases from 9.26°C to 7.86°C, and the passive cooling performance decreases only slightly. This is mainly because the rich nanopore structure reduces the chemical bonds of thermal conduction and mid-infrared radiation.

[0075] Example 5

[0076] In this example, the EMW transmission performance of the POSS / PVTMS nano-aerogel material prepared in Example 1 was tested.

[0077] A broadband dielectric spectrometer (Keysight, E4990A) and a PNA-X network analyzer (Keysight N5232B) were used to measure the low frequency band (1×10 3 ~1×10 7 The dielectric constant and dielectric loss of the PVTMS nanoaerogel material (PVTMS aerogel) prepared in Example 1 and the POSS / PVTMS nanoaerogel material prepared in Example 1 were characterized in the Ku band (12.4-18 GHz). The EMW transmittance of the nanoaerogel material in the Ku band was tested using the Keysight N5232B waveguide method, and the transmittance was calculated according to formula (1):

[0078]

[0079] In formula (1), S 21s Represents the wave transmission factor under the conditions of nano-aerogel material sample loading, S 21g It represents the wave transmission factor under no-load sample conditions.

[0080] Figure 6 Figures (a) and (a') are the test results of the dielectric constant (Figure (a)) and dielectric loss (Figure (a')) of the PVTMS nanoaerogel material prepared in Comparative Example 1 and the POSS / PVTMS-20wt% prepared in Example 1 in the low frequency band. Figure 6 Figures (b) and (b') show the dielectric constant (Figure (b)) and dielectric loss (Figure (b')) test results of the PVTMS nanoaerogel material prepared in Comparative Example 1 and the POSS / PVTMS-20wt% prepared in Example 1 at high frequency band (Ku band). As can be seen from the figure, after the introduction of POSS, the dielectric constant and dielectric loss of the nanoaerogel material are reduced. The main reasons for the decline in dielectric properties include: (1) Due to the introduction of POSS, the porosity of the nanoaerogel material is slightly increased from 90.9% to 93%; (2) The -Si-OH group of PVTMS reacts with the -Si-OH group, the number of polar functional groups is reduced, and the dielectric constant and dielectric loss are reduced.

[0081] For the design of new generation communication technology, the passive cooling material should also be able to have high frequency EMW transmission characteristics, such as Figure 6 The EMW transmission performance of the PVTMS nano-aerogel material (PVTMS aerogel) prepared in Comparative Example 1 and the POSS / PVTMS-20wt% prepared in Example 1 in the Ku band is shown in FIG. Figure 6As shown in Figure (c'). Compared with the PVTMS nanoaerogel material prepared in Comparative Example 1, the POSS / PVTMS nanoaerogel material of the present invention has a lower dielectric constant and dielectric loss due to the introduction of POSS, and the EMW transmittance of the nanoaerogel material is effectively improved. The average EMW transmittance of POSS / PVTMS-20wt% in the Ku band reaches 99.7%.

[0082] Example 6

[0083] In this embodiment, the steps for preparing a POSS / PVTMS nano-aerogel material having both excellent radiative cooling and millimeter wave transmission performance are as follows:

[0084] (1) Preparation of PVTMS by free radical polymerization

[0085] The thermal initiator di-tert-butyl peroxide (DTBP) was dissolved in the monomer vinyltrimethoxysilane (VTMS), and the mass ratio of DTBP to VTMS was controlled to be 1:10. The resulting solution was poured into a four-necked flask equipped with a condenser and a stirrer, and reacted at 150°C and a stirring speed of 200 rpm in a nitrogen atmosphere for 3 hours. After that, the four-necked flask was placed in a vacuum oven at 150°C to remove unreacted monomers and thermal initiator to obtain polyvinyltrimethylsilane (PVTMS).

[0086] (2) Constructing nanoporous structures using cross-linking chemical reactions

[0087] PVTMS was dissolved in anhydrous ethanol to obtain a 0.2 g / mL PVTMS solution. The PVTMS solution was stirred at 40°C for 0.5 h. POSS was then added to anhydrous ethanol and stirred at 40°C for 0.5 h to obtain a POSS solution. The PVTMS solution and POSS solution were mixed and stirred for 0.5 h to obtain a mixed solution. A basic ammonia catalyst was then added at a molar ratio of water to Si in the mixed solution (water / Si) of 9:1 to induce spinodal decomposition (spinodal phase separation). After adding ammonia, the mixture was stirred for 1 min. The resulting gel precursor solution was then transferred to a mold. The mold containing the gel precursor solution was placed in an oven at 40°C until the gel precursor solution transformed into a gel state, which took approximately 10 to 12 h. The resulting wet gel was then aged in anhydrous ethanol for 24 h.

[0088] In this step, multiple groups of experiments were conducted. In each group of experiments, when mixing the PVTMS solution and the POSS solution, the addition amounts of the two were controlled so that the amount of POSS in the mixed solution was 15 wt % or 25 wt % of the PVTMS solution.

[0089] (3) Supercritical carbon dioxide (scCO2) drying

[0090] The sample obtained from step (2) was subjected to solvent exchange with liquid CO2 at a pressure of 10.34 MPa (1500 psi) and finally dried by releasing CO2 at 45 °C to obtain a round disk-shaped POSS / PVTMS nanogel material.

[0091] The POSS / PVTMS nanogel materials prepared in each group of experiments in this example are denoted as POSS / PVTMS-15wt%, POSS / PVTMS-25wt% according to the amount of POSS in the mixed solution described in step (2) being 15wt% and 25wt% of PVTMS, respectively.

Claims

1. A nano-aerogel material with both excellent radiative cooling and millimeter wave transmission performance, characterized in that: The nano-aerogel material is formed by a cross-linking reaction and supercritical fluid drying of a silicon-containing polymer matrix with a high cross-linking density and trisilanolethyl polyhedral oligomeric silsesquioxane, with the trisilanolethyl polyhedral oligomeric silsesquioxane accounting for 15 wt% to 25 wt% of the total mass of the silicon-containing polymer matrix with a high cross-linking density and the trisilanolethyl polyhedral oligomeric silsesquioxane. The nano-aerogel material has a bimodal nanoporous structure consisting of small pores and large pores, a porosity of 92% to 94%, and a pore diameter of 30 to 40 nm for the small pores and 400 to 600 nm for the large pores. The silicon-containing polymer matrix with a high cross-linking density is polyvinyltrimethoxysilane. The nano-aerogel material has both radiative cooling and millimeter wave transmission properties. The nano aerogel is prepared by the following method: A silicon-containing polymer matrix with a high cross-linking density is dissolved in an anhydrous solvent to obtain a silicon-containing polymer matrix solution; trisilanolethyl polyhedral oligomeric silsesquioxane is dissolved in a solvent to obtain a trisilanolethyl polyhedral oligomeric silsesquioxane solution; the silicon-containing polymer matrix solution and the trisilanolethyl polyhedral oligomeric silsesquioxane solution are mixed, and ammonia water is added to induce spinodal decomposition, and after adding ammonia water and mixing evenly, a gel precursor solution is obtained, and the gel precursor solution is allowed to stand until it is converted into a gel state to obtain a wet gel; the wet gel is immersed in a solvent for aging; and the aged sample is subjected to supercritical carbon dioxide drying to obtain the obtained product.

2. The nano-aerogel material having both excellent radiative cooling and millimeter wave transmission performance according to claim 1, characterized in that: The nano aerogel material is u The wave transmittance of the band is at least 99.5%.

3. The method for preparing the nano-aerogel material having both excellent radiative cooling and millimeter wave transmission properties according to claim 1 or 2, characterized in that: The following steps are involved: (1) A silicon-containing polymer matrix with a high cross-linking density is dissolved in an anhydrous solvent to obtain a silicon-containing polymer matrix solution; trisilanolethyl polyhedral oligomeric silsesquioxane is dissolved in a solvent to obtain a trisilanolethyl polyhedral oligomeric silsesquioxane solution; the silicon-containing polymer matrix solution and the trisilanolethyl polyhedral oligomeric silsesquioxane solution are fully mixed to obtain a mixed solution, and then ammonia water is added to induce spinodal decomposition. After adding ammonia water and mixing evenly, a gel precursor solution is obtained, and the gel precursor solution is transferred to a mold and heated at 35-45°C. o C oven until the gel precursor solution is transformed into a gel state to obtain a wet gel; the obtained wet gel is immersed in a solvent for aging; In this step, the amount of the silicon-containing polymer matrix alkane solution and the trisilanolethyl polyhedral oligomeric silsesquioxane solution added is controlled so that the content of the trisilanolethyl polyhedral oligomeric silsesquioxane in the mixed solution is 15 wt% to 25 wt%, and the amount of ammonia water added is controlled so that the molar ratio of water to Si element in the mixed solution is (8-12):1; (2) The sample aged in step (1) is dried with supercritical carbon dioxide to obtain a nano-aerogel material having both excellent radiative cooling and millimeter wave transmission properties.

4. The method for preparing the nano-aerogel material having both excellent radiative cooling and millimeter wave transmission performance according to claim 3, characterized in that: The concentration of the silicon-containing polymer matrix solution is 0.1-0.2 g / mL, and the concentration of the trisilanolethyl polyhedral oligomeric silsesquioxane solution is 3-45 mg / mL.

5. The method for preparing the nano-aerogel material having both excellent radiative cooling and millimeter wave transmission performance according to claim 3, characterized in that: The aging time in step (1) is 20 to 30 h.

6. The method for preparing the nano-aerogel material having both excellent radiative cooling and millimeter wave transmission properties according to any one of claims 3 to 5, characterized in that: Polyvinyltrimethoxysilane was prepared as follows: The thermal initiator was dissolved in the vinyl trimethoxysilane monomer, and the mass ratio of the thermal initiator to the vinyl trimethoxysilane monomer was controlled to be 1: (5-10). o C is stirred for reaction for 3-5 hours, and the unreacted thermal initiator and vinyltrimethoxysilane monomer are removed to obtain the product.

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