Radiation cooling nanofiber composite membrane, preparation method and application thereof

Abstract

The application discloses a kind of radiation cooling nanofiber composite membranes and preparation method and application thereof, the PVDF-HFP / SiO2 nanofiber composite membrane is prepared by the airflow spinning and electrostatic spraying technology of synchronous implementation in the present application, SiO2 is uniformly anchored in the surface of PVDF-HFP fiber membrane, and it is not easy to fall off.PVDF-HFP nanofiber contains C-F and other characteristic vibration groups, not only can provide strong infrared emission capacity, but also can give fiber strong hydrophobicity and show good self-cleaning capacity, help its long-term stable application in complex environment outdoors;SiO2 can effectively scatter sunlight, because SiO2 microspheres particle size distribution is widespread, in 0.25 μm-2.5 μm wavelength range, SiO2 will scatter sunlight, combined with the structure of nanofiber itself, the solar reflectivity of fiber membrane is improved, and the radiation cooling performance of composite membrane can be significantly improved.

Description

Technical Field

[0001] This invention relates to the field of radiation cooling materials technology, specifically to a radiation cooling nanofiber composite membrane, its preparation method, and its application. Background Technology

[0002] Against the backdrop of global climate change and the continuous rise in cooling energy consumption, passive radiation cooling technology has attracted much attention due to its unique working mechanism. This technology utilizes the atmospheric transparent window (8-13μm) to directly radiate heat from the Earth's surface to the outer space at a temperature of about 3K, achieving a cooling effect without additional energy input. It demonstrates significant potential for energy saving and emission reduction, opening up new avenues for the development of sustainable cooling technology.

[0003] Currently, the core challenges facing passive radiative cooling materials in practical applications are as follows: First, existing preparation processes struggle to achieve stable anchoring and efficient exposure of functional particles on the fiber surface while simultaneously forming the fiber. Traditional processes often embed functional particles inside the fiber or merely loosely attach them to the surface, resulting in suppressed particle scattering function or insufficient durability. Second, existing radiative cooling fiber membranes struggle to maintain high solar reflectivity and high-to-medium infrared emissivity while constructing an effective thermal barrier to suppress environmental parasitic heat gain. In hot environments, convective and conductive heat transfer significantly reduce the actual cooling effect, making it impossible to guarantee the stability of radiative cooling for long-term outdoor applications. Summary of the Invention

[0004] This invention provides a radiative cooling nanofiber composite membrane, its preparation method, and its application, in order to solve the technical problem that existing passive radiative cooling materials are unable to maintain high solar reflectivity and high-infrared emissivity while constructing an effective thermal barrier to suppress environmental parasitic heat gain. In hot environments, the actual cooling effect is significantly reduced due to convective and conductive heat transfer, which makes it impossible to guarantee the stability of radiative cooling for long-term outdoor applications.

[0005] The present invention provides a radiation cooling nanofiber composite membrane, comprising PVDF-HFP nanofibers and inorganic oxide microspheres. The inorganic oxide microspheres are anchored on the surface of the PVDF-HFP nanofibers and partially embedded in the shallow layer of the PVDF-HFP nanofibers to form discretely distributed scattering sites on the surface of the PVDF-HFP nanofibers.

[0006] In some embodiments of the present invention, the mass fraction of inorganic oxide microspheres on PVDF-HFP nanofibers is 3wt%-15wt%.

[0007] In some embodiments of the present invention, the inorganic oxide microspheres include at least one of SiO2 microspheres, TiO2 microspheres, and alumina microspheres.

[0008] The present invention also provides a method for preparing the aforementioned radiation-cooling nanofiber composite membrane, comprising the following steps: PVDF-HFP spinning solution is shaped into PVDF-HFP nanofibers by airflow stretching and deposited on the surface of a rotating drum receiver to form a PVDF-HFP nanofiber network. Simultaneously, inorganic oxide microsphere dispersion is formed into charged microdroplets by electrostatic atomization and then directionally deposited onto the forming PVDF-HFP nanofibers under the action of an electric field. This allows the inorganic oxide microspheres to collide with and partially embed into the shallow surface layer of the PVDF-HFP nanofibers during the quasi-solid stage of the PVDF-HFP nanofibers' transformation from solution jet to solid fiber, forming a physical interlocking and anchoring.

[0009] In some embodiments of the present invention, PVDF-HFP polyvinylidene fluoride-hexafluoropropylene is added to a mixed solution of acetone and DMF and mixed evenly to obtain a PVDF-HFP spinning solution.

[0010] In some embodiments of the present invention, the mass ratio of PVDF-HFP to the volume ratio of the mixed solution is (5-10) g: (40-60) ml, and the volume ratio of acetone to DMF is 3:7.

[0011] In some embodiments of the present invention, the process parameters for airflow stretching spinning include: the inner diameter of the spinning needle is 0.1 mm-0.2 mm, the feed rate of the spinning solution is 5 mL / h-10 mL / h, the receiving distance is 15 cm-20 cm, and the air pressure is 20 kPa-30 kPa.

[0012] In some embodiments of the present invention, inorganic oxide microspheres are added to a DMF solution, and the solution is magnetically stirred and ultrasonically dispersed to obtain an inorganic oxide microsphere dispersion.

[0013] In some embodiments of the present invention, the process parameters for electrostatic spraying include: voltage 15kV-25kV, drum speed 375r / min, and liquid supply rate 0.5mL / h.

[0014] The present invention also provides the application of the aforementioned radiation-cooling nanofiber composite membrane in the preparation of radiation-cooling materials.

[0015] Compared with the prior art, the advantages and positive effects of the present invention are: This invention employs a simultaneous composite film-forming process combining airflow spinning and electrostatic spraying. The process of stretching a polymer solution with high-speed airflow to form nanofibers is synchronized with the process of electrostatically atomizing a functional microsphere dispersion and directional deposition, all within a unified spatiotemporal constraint. During the transition from solution jet to solid fiber, charged functional microspheres are impacted and partially embedded in the shallow surface of the fiber, forming a heterogeneous interface structure where the microsphere surface is exposed to the outside of the fiber and physically interlocked with the fiber matrix. This heterogeneous interface structure allows the infrared emitting functional groups provided by the fiber matrix and the solar scattering function provided by the microspheres to synergize spatially. Simultaneously, the low surface energy of fluorine in the fluoropolymer fiber, combined with the micro-nano rough structure of the fiber surface, imparts hydrophobicity to suppress environmental heat convection. The low thermal conductivity resulting from the porous structure of the fiber suppresses environmental heat conduction. This, in turn, synergizes with the high solar reflectance and high-infrared emission optical functions to achieve passive radiative cooling that integrates photothermal and hydrophobic properties.

[0016] This invention utilizes simultaneous air-jet spinning and electrostatic spraying techniques to prepare PVDF-HFP / SiO2 nanofiber composite membranes, which promotes the uniform anchoring of SiO2 on the surface of the PVDF-HFP fiber membrane, preventing it from easily detaching. Compared to conventional electrospinning processes for nanofiber preparation, air-jet spinning is simpler, more efficient, and enables large-scale nanofiber production.

[0017] The synergistic effect of PVDF-HFP and SiO2 on the radiative cooling mechanism of PVDF-HFP / SiO2 nanofiber composite membranes: On the one hand, PVDF-HFP nanofibers contain characteristic vibrational groups such as CF, which not only provide strong infrared emission capabilities but also endow the fibers with strong hydrophobicity and good self-cleaning ability, contributing to their long-term stable application in complex outdoor environments; on the other hand, SiO2 can effectively scatter sunlight because SiO2 microspheres have a wide particle size distribution. In the wavelength range of 0.25μm-2.5μm, SiO2 scatters sunlight. Combined with the structure of the nanofibers themselves, the two work together to improve the solar reflectivity of the fiber membrane, ultimately significantly enhancing the radiative cooling performance of the PVDF-HFP / SiO2 fiber composite membrane. Attached Figure Description

[0018] Figure 1 A schematic diagram of the apparatus for preparing air-jet spun PVDF-HFP / SiO2 nanofiber composite membrane according to the present invention; Figure 2 This is a SEM image of the PVDF-HFP / SiO2 nanofiber composite membrane of Example 1 of the present invention; Figure 3 The contact angles of the PVDF-HFP / SiO2 nanofiber composite membranes in Examples 1-4 of this invention; Figure 4 shows the temperature reduction range of the PVDF-HFP / SiO2 nanofiber composite membranes in Examples 1-4 of this invention. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. Those skilled in the art should understand that the specific description below is illustrative rather than restrictive and should not be construed as limiting the scope of protection of this invention.

[0020] This invention provides a radiation-cooling nanofiber composite membrane comprising a PVDF-HFP nanofiber network and inorganic oxide microspheres. The inorganic oxide microspheres are anchored to the surface of the PVDF-HFP nanofibers and partially embedded within the shallow layer of the PVDF-HFP nanofibers, thereby forming discretely distributed scattering sites on the surface of the PVDF-HFP nanofibers.

[0021] The anchoring of inorganic oxide microspheres onto the surface of PVDF-HFP nanofibers is achieved through a specific fabrication process. Specifically, the inorganic oxide microspheres are deposited onto the PVDF-HFP nanofibers in the quasi-solid stage, during the transition from a solution jet to a solid fiber state, via electrostatic atomization in the form of charged droplets. The quasi-solid stage refers to the intermediate phase in the formation process of the PVDF-HFP nanofibers, transitioning from a solution jet state to a fully solidified state. During this stage, the fiber surface is not yet fully hardened and still possesses a certain degree of viscoelasticity and deformability, enabling it to accept and physically intercalate external particles. Once the fiber is fully solidified, the intercalated particles are firmly locked within the shallow surface layer of the fiber, forming a physical intercalation and anchoring.

[0022] This invention involves the directional deposition of inorganic oxide microspheres as electrostatically atomized charged droplets in the quasi-solid stage of PVDF-HFP nanofibers. Utilizing the quasi-solid-state characteristics of the incompletely solidified fibers, the charged droplets are driven by an electric field to collide with and partially embed into the shallow surface of the fibers, forming a physical interlocking anchor. This anchoring method exposes the microsphere surface to the fiber exterior, maximizing its solar scattering function. Simultaneously, the physical interlocking structure prevents the microspheres from detaching, ensuring structural stability for long-term outdoor applications. The carbon-fluorine bonds in the PVDF-HFP nanofibers provide infrared emission capabilities, synergizing with the scattering function of the microspheres anchored to the fiber surface at the heterogeneous interface. Combined with the porous structure of the fiber network, this further enhances light scattering, achieving a passive radiative cooling effect that combines high solar reflectivity, high-to-medium infrared emissivity, and excellent structural stability.

[0023] PVDF-HFP nanofibers contain characteristic CF vibrational groups. The stretching and deformation vibration frequencies of the CF chemical bonds lie within the atmospheric window range of 8-13 micrometers, providing high infrared emissivity in this atmospheric transparency window and efficiently dissipating heat into outer space through thermal radiation. PVDF-HFP nanofibers absorb and radiate infrared electromagnetic waves in this band through intrinsic chemical bond vibrations, providing highly efficient selective infrared emission capabilities without the need for externally added emitting fillers. Simultaneously, the abundant CF groups enriched on the fiber surface and bulk endow the fiber with extremely low surface energy, resulting in a macroscopically hydrophobic and self-cleaning fiber network. This reduces the risk of environmental moisture and pollutant adhesion, contributing to the long-term maintenance of stable radiative cooling performance in humid and dusty outdoor environments.

[0024] PVDF-HFP nanofibers have diameters ranging from 100 nm to 500 nm. Controlling the diameter to the submicron scale optimizes the match between the fiber's flexibility and its optical scattering function. Within this range, the fibers possess sufficient bending flexibility, facilitating the construction of a high-porosity three-dimensional porous elastic network and increasing the number of effective sites participating in infrared radiation per unit volume, resulting in enhanced bulk emission. The 100 nm-500 nm range is on the same order of magnitude as or slightly smaller than the wavelengths of visible to near-infrared light in the solar spectrum. Based on Mie scattering theory, fibers with diameters of 100 nm-500 nm can serve as efficient scattering centers, strongly scattering sunlight. If the fiber diameter is much smaller than 100 nm, the Mie scattering efficiency decreases significantly, weakening its contribution to solar reflectivity. If the fiber diameter exceeds 900 nm, fiber rigidity increases, network flexibility decreases, and the specific surface area per unit mass of fiber decreases, making it unfavorable for constructing a uniform nanoscale porous network.

[0025] Inorganic oxide microspheres include at least one of SiO2 microspheres, TiO2 microspheres, and alumina microspheres.

[0026] In some embodiments of the present invention, the inorganic oxide microspheres can be SiO2 microspheres. The silicon-oxygen-silicon bond phonon polarization resonance frequency of amorphous SiO2 is located within the atmospheric window band of 8-13 micrometers. This phonon polarization resonance characteristic allows silicon dioxide to absorb and re-radiate infrared electromagnetic waves in this band, which complements the CF bond vibration absorption of PVDF-HFP nanofibers and synergistically enhances the infrared emission capability of the entire fiber composite membrane in the atmospheric window band.

[0027] In other preferred embodiments, the inorganic oxide microspheres may be partially or entirely composed of alumina microspheres. The alumina microspheres' characteristic vibrational absorption and emission capabilities in the mid-infrared region, particularly the aluminum-oxygen bond characteristics, can replace or supplement the mid-infrared emission enhancement function of SiO2 microspheres. Simultaneously, alumina microspheres possess high mechanical strength and chemical stability, providing additional mechanical reinforcement and improved weather resistance to the fiber composite membrane, thus meeting the requirements of applications demanding higher structural strength and durability.

[0028] This invention also provides a method for preparing a radiation-cooling nanofiber composite membrane, comprising the following steps: PVDF-HFP spinning solution is shaped into PVDF-HFP nanofibers by airflow stretching and deposited on the surface of a rotating drum receiver to form a PVDF-HFP nanofiber network. Simultaneously, inorganic oxide microsphere dispersion is formed into charged microdroplets by electrostatic atomization and then directionally deposited onto the forming PVDF-HFP nanofibers under the action of an electric field. This allows the inorganic oxide microspheres to collide with and partially embed into the shallow surface layer of the PVDF-HFP nanofibers during the quasi-solid stage of the PVDF-HFP nanofibers' transformation from solution jet to solid fiber, forming a physical interlocking and anchoring.

[0029] Figure 1 This is a schematic diagram of the apparatus for preparing air-spun PVDF-HFP / SiO2 nanofiber composite membranes according to the present invention, which can realize the batch preparation of fiber composite membranes.

[0030] In the step of air-jet stretching to form PVDF-HFP nanofibers, the PVDF-HFP spinning solution is loaded into a syringe and connected to an air-jet spinning device. The spinning solution is extruded through the spinneret of the air-jet spinning device, while a high-speed airflow is ejected from around the spinneret to stretch and refine the polymer solution jet. During the stretching process, the solvent evaporates simultaneously, and the fibers solidify and form. The fibers are deposited on the surface of a rotating drum receiver, forming a randomly arranged PVDF-HFP nanofiber network. In the concurrent electrostatic atomization step, an inorganic oxide microsphere dispersion is loaded into another syringe and connected to an electrostatic spraying device. Under the action of a high-voltage electrostatic field, the dispersion ejected from the nozzle is atomized into charged microdroplets. The droplets migrate towards the receiver under the drive of the electric field. By setting the process parameters of air-jet spinning and electrostatic spraying, the fibers and microdroplets converge synchronously on the rotating drum receiver surface. At the convergence, the fibers are in a quasi-solid state, and the charged microdroplets collide with the fiber surface and partially embed into the shallow layer of the fiber, forming a physical intercalation and anchoring. Once the fiber is fully cured, the inorganic oxide microspheres are firmly anchored to the fiber surface.

[0031] In the above preparation method, the simultaneous process of airflow stretching to form fibers and electrostatic atomization directional deposition of microspheres integrates fiber forming and microsphere anchoring into a single process both spatially and temporally. Airflow stretching uses high-pressure airflow instead of the high-voltage electrostatic field required for traditional electrospinning as the driving force for fiber stretching, reducing equipment complexity and safety risks while improving spinning efficiency. Simultaneous electrostatic atomization utilizes an independent electrostatic field to atomize and directionally transport the microsphere dispersion, allowing the microspheres to collide with and partially embed into the shallow surface layer of the fiber during the quasi-solid stage before the fiber is fully solidified, forming a physical interlocking anchor. This method overcomes the bottleneck in traditional stepwise methods where the fiber is fully solidified, resulting in weak adhesion as particles can only physically adhere to the surface. It achieves integrated and simultaneous completion of fiber forming and stable particle anchoring, resulting in a highly efficient, continuous, and controllable process suitable for large-scale production.

[0032] The PVDF-HFP nanofiber network is formed by stretching the PVDF-HFP spinning solution jet with airflow. Electrostatic atomization and airflow stretching occur simultaneously, both converging towards the same rotating drum receiving surface under unified spatiotemporal constraints. Airflow stretching uses high-pressure airflow instead of the high-voltage electrostatic field required for traditional electrospinning as the driving force for fiber stretching, reducing equipment complexity and safety risks while improving spinning efficiency. Simultaneous electrostatic atomization utilizes an independent electrostatic field to atomize and directionally transport the microsphere dispersion, precisely controlling the deposition timing of the microspheres in the quasi-solid stage of the fiber.

[0033] The process parameters for airflow stretching spinning include: the inner diameter of the spinning needle is 0.1 mm - 0.2 mm, the feed rate of the spinning solution is 5 mL / h - 10 mL / h, the receiving distance is 15 cm - 20 cm, and the air pressure is 20 kPa - 30 kPa.

[0034] If the air pressure used for airflow stretching is too low, the stretching and refining effect of the airflow on the PVDF-HFP spinning solution jet will be insufficient, resulting in fibers with larger diameters, which is detrimental to light scattering efficiency. If the air pressure is too high, it may lead to instability in the PVDF-HFP spinning solution jet or fiber breakage, affecting film uniformity. In one embodiment of this application, the air pressure used for airflow stretching is 20kPa-30kPa, which can apply sufficient shear and airflow stretching forces to the PVDF-HFP spinning solution jet, refining it to a diameter of several hundred nanometers. The inner diameter of the spinneret is 0.1mm-0.2mm, and the feed rate of the spinning solution is 5mL / h-10mL / h, which can ensure continuous and stable fiber formation; the receiving distance is 15cm-20cm, which allows the fibers sufficient distance to complete solvent evaporation and curing before reaching the receiving face.

[0035] In this invention, PVDF-HFP polyvinylidene fluoride-hexafluoropropylene is added to a mixed solution of acetone and DMF and mixed evenly to obtain PVDF-HFP spinning solution.

[0036] The mass ratio of PVDF-HFP to the volume ratio of the mixed solution is (5-10) g: (40-60) ml, and the volume ratio of acetone to DMF is 3:7.

[0037] PVDF-HFP nanofibers have a fiber surface layer and an internal porous structure. The fiber surface layer is induced by the preferential evaporation of highly volatile solvents from the PVDF-HFP spinning solution jet, while the internal porous structure is induced by delayed phase separation of low-volatile solvents. Together, the fiber surface layer and the internal porous structure construct micro-nano rough textures on the surface of the PVDF-HFP nanofibers.

[0038] During the spinning process, highly volatile solvents rapidly escape from the jet, causing a rapid increase in the concentration of PVDF-HFP on the jet surface layer, which then preferentially solidifies to form a dense fiber surface layer. Simultaneously, low-volatile solvents gradually diffuse into the fiber interior and slowly evaporate, initiating delayed liquid-phase separation and thermally induced phase separation within the fiber, forming an internal porous structure. This fiber surface layer and internal porous structure together construct a micro / nano-rough texture on the fiber surface, resulting in an uneven surface with micro-protrusions or wrinkles. The construction of this micro / nano-rough texture does not introduce additional pore-forming agents or templates; it is achieved solely through the differences in the solvent's own evaporation characteristics. This micro / nano-rough texture generates more scattering interfaces with incident sunlight, increasing the solar reflectivity of the fiber membrane. Furthermore, this rough structure synergizes with the low surface energy of fluorine in PVDF-HFP, allowing air to be trapped in the rough grooves, increasing the apparent water contact angle, enhancing the hydrophobicity of the fiber membrane surface, and reducing the solid-phase thermal conductivity of the fiber due to air filling.

[0039] The highly volatile solvent is acetone, and the low-volatile solvent is N,N-dimethylformamide (DMF), with a volume ratio of acetone to DMF of 3:7. During air-jet spinning, acetone, with its high saturated vapor pressure, rapidly escapes from the jet, causing the polymer concentration on the jet surface to rise rapidly and preferentially solidify to form a skin layer. The low-volatility DMF gradually diffuses into the fiber interior and slowly volatilizes, initiating delayed liquid-phase separation and thermally induced phase separation within the fiber, resulting in a dense surface and porous internal fiber microstructure. If the proportion of acetone is too high, the skin layer forms too quickly and is too thick, potentially inhibiting the normal volatilization and phase separation process of the internal DMF, leading to insufficient development of internal pores and hindering the low thermal conductivity and hydrophobic reinforcement effects. If the proportion of acetone is too low, the skin layer formation is insufficient, resulting in inadequate fiber surface roughness, which is detrimental to increasing the water contact angle and enhancing light scattering. By using an acetone to DMF volume ratio of 3:7, the requirements for the skin formation rate and the degree of internal pore development can be balanced, and the synergistic optimization of hydrophobic enhancement, reflection enhancement and thermal conductivity reduction can be achieved during the preparation process.

[0040] The anchoring loading of inorganic oxide microspheres in PVDF-HFP nanofibers can be controlled by the following method: during the electrostatic spraying process of directionally depositing inorganic oxide microspheres onto PVDF-HFP nanofibers, the supply rate of the dispersion containing inorganic oxide microspheres is kept constant, and the loading of inorganic oxide microspheres deposited on PVDF-HFP nanofibers per unit time is adjusted by using the concentration of inorganic oxide microspheres in the dispersion as a single variable.

[0041] During electrostatic atomization, the supply rate of the inorganic oxide microsphere dispersion is kept constant, and the concentration of inorganic oxide microspheres in the dispersion is used as a single variable to adjust the number of inorganic oxide microspheres deposited on PVDF-HFP nanofibers per unit time. For example, with a fixed supply rate of 0.5 mL / h, composite films with different loadings can be prepared by adding different masses of inorganic oxide microspheres to 50 mL of N,N-dimethylformamide to change the dispersion concentration. This control method transforms the complex control process that typically requires simultaneous adjustment of multiple coupled process parameters into a single-variable precise control problem. The operator only needs to change the microsphere concentration in the dispersion to linearly adjust the effective deposition amount per unit time, simplifying the process parameter optimization process. It can quickly establish the correspondence between concentration and product performance, which is beneficial for process-oriented customization of product performance and industrial quality control for different application scenarios.

[0042] The voltage used for electrostatic atomization is 15kV-25kV. Within this voltage range, the microsphere dispersion can be fully atomized into uniformly charged microdroplets, and a sufficient electric field force can be obtained to drive their directional migration to the fiber surface. If the voltage is lower than 15kV, the atomization effect is insufficient and the electric field driving force is weak, resulting in a decrease in microsphere deposition efficiency. If the voltage is higher than 25kV, unstable phenomena such as corona discharge may occur, affecting deposition uniformity and process safety.

[0043] The PVDF-HFP nanofiber network is a three-dimensional porous structure formed by the random stacking of nanofibers. The three-dimensional porous structure contains network gaps between fibers and micro-nano pores inside the fibers, which makes the overall thermal conductivity of the PVDF-HFP nanofiber network lower than that of a dense polymer film made of the same material.

[0044] When randomly arranged nanofibers are deposited on the receiver surface, they randomly overlap and stack, forming numerous interfiber voids. Simultaneously, micro- and nano-pores within the fibers are formed by solvent-delayed phase separation. Air fills the network voids between fibers and the micro- and nano-pores within the fibers. Since the thermal conductivity of air is much lower than that of the polymer solid phase, the overall solid-phase thermal conductivity of the composite membrane is significantly reduced. When the composite membrane covers the surface of an object being cooled, the low thermal conductivity of the fiber membrane suppresses heat conduction from the high ambient temperature to the surface of the object. Simultaneously, the low surface energy of fluorine in PVDF-HFP, combined with the hydrophobicity imparted by the rough structure of the fiber surface, suppresses environmental heat convection. The suppression of heat conduction and heat convection together constitute a dual thermal barrier, significantly reducing parasitic heat gain from the environment and ensuring effective radiative cooling under hot conditions.

[0045] This fiber composite membrane exhibits solar reflectivity in the 0.3–2.5 μm band of the solar spectrum and mid-infrared emissivity in the 8–13 μm band of the atmospheric transparency window, with its surface water contact angle indicating hydrophobicity. The solar reflectivity arises from the synergistic effect of surface scattering by the inorganic oxide microspheres and scattering by the porous structure of the PVDF-HFP nanofiber network; the mid-infrared emissivity arises from the synergistic effect of carbon-fluorine bond vibrations in the PVDF-HFP nanofibers and chemical bond vibrations in the inorganic oxide microspheres; and the hydrophobicity arises from the synergistic effect of the low surface energy of fluorine in the PVDF-HFP nanofibers and the micro / nano rough structure of the PVDF-HFP nanofiber surface.

[0046] The three synergistic effects described above are not simply the sum of the independent effects of each component, but rather an organic integration of physicochemical synergistic effects. Inorganic oxide microspheres, anchored to the fiber surface and fully exposed, work in conjunction with the porous scattering of the fiber network to form a multi-interface, multi-scale solar scattering system. Both carbon-fluorine bond vibrations and silicon-oxygen-silicon bond vibrations are active in the infrared band, and their synergy at the heterogeneous interface enhances infrared emissivity. The low surface energy of fluorine provides a hydrophobic basis, and the micro-nano rough structure of the fiber surface further amplifies the hydrophobic effect. These three synergistic effects together construct an integrated photothermal-hydrophobic passive radiative cooling system, enabling the product to simultaneously achieve efficient solar reflection, efficient thermal radiation heat dissipation, and suppression of environmental parasitic heat gain in hot outdoor environments, ensuring a long-term stable passive cooling effect.

[0047] The radiative cooling nanofiber composite membrane of this invention can be used to prepare radiative cooling materials and can be applied in the field of radiative cooling. For example, the radiative cooling nanofiber composite membrane of this invention can be used in scenarios where cooling is required without external energy input, such as building roofs and exterior walls, outdoor equipment shells, vehicle surfaces, and cold chain transportation packaging. In hot climates, covering the surface of the object being cooled with this composite membrane can achieve a passive cooling effect by reflecting sunlight and radiating heat into outer space. Example 1

[0048] The preparation method of the radiation-cooling nanofiber composite membrane in this embodiment includes the following steps: (1) Preparation of PVDF-HFP spinning solution Weigh 7 g of PVDF-HFP and add it to 50 mL of a mixed solution of acetone and DMF (the volume ratio of acetone to DMF is 3:7). Stir in an oil bath at 50 °C for 4 h.

[0049] (2) Preparation of SiO2 microsphere dispersion Add 0.21 g of SiO2 microspheres (3wt% by mass) to 50 mL of solution, stir with a magnetic stirrer for 12 h, and then sonicate with an ultrasonic instrument for 30 min to make the SiO2 microspheres more uniformly dispersed in the solution system.

[0050] (3) Preparation of PVDF-HFP / SiO2 nanofiber composite membrane Simultaneously, air-jet spinning was used to prepare PVDF-HFP fibers, and electrostatic atomization was used to directionally deposit SiO2 microspheres.

[0051] The process parameters for preparing PVDF-HFP fibers by air-jet spinning are as follows: the inner diameter of the spinning needle is 0.16 mm, the feed rate of the spinning solution is 8 mL / h, the receiving distance is 18 cm, and the air pressure is 25 kPa; the process parameters for electrostatic spraying are as follows: voltage 20 kV, roller speed 375 r / min, receiving distance 18 cm, and flow rate 0.5 mL / h.

[0052] The PVDF-HFP / SiO2 nanofiber composite membrane prepared in this embodiment is named PVDF-HFP / SiO2-3. Example 2

[0053] The preparation process of the radiation cooling nanofiber composite membrane in this embodiment is basically the same as that in Example 1. The difference is that the mass of SiO2 microspheres in step (2) is 0.42g and the mass fraction of SiO2 microspheres is 6wt%.

[0054] The PVDF-HFP / SiO2 nanofiber composite membrane prepared in this embodiment is named PVDF-HFP / SiO2-6. Example 3

[0055] The preparation process of the radiation cooling nanofiber composite membrane in this embodiment is basically the same as that in Example 1. The difference is that the mass of SiO2 microspheres in step (2) is 0.63g and the mass fraction of SiO2 microspheres is 9wt%.

[0056] The PVDF-HFP / SiO2 nanofiber composite membrane prepared in this embodiment is named PVDF-HFP / SiO2-9. Example 4

[0057] The preparation process of the radiation cooling nanofiber composite membrane in this embodiment is basically the same as that in Example 1. The difference is that the mass of SiO2 microspheres in step (2) is 0.84 g and the mass fraction of SiO2 microspheres is 12 wt%.

[0058] The PVDF-HFP / SiO2 nanofiber composite membrane prepared in this embodiment is named PVDF-HFP / SiO2-12.

[0059] Figure 2 The image shown is a SEM image of the PVDF-HFP / SiO2 nanofiber composite membrane of Example 1 of this invention. Figure 2 As shown, in the nanofiber composite membrane prepared by air-jet spinning and electrostatic spraying technologies, SiO2 is relatively uniformly anchored on the surface of PVDF-HFP nanofibers without exhibiting agglomeration, which will help to give full play to the respective roles of the two technologies.

[0060] Figure 3 The contact angles of the PVDF-HFP / SiO2 nanofiber composite membranes in Examples 1-4 of this invention are as follows: Figure 3 As shown, the PVDF-HFP / SiO2 fiber composite membrane exhibits hydrophobicity. With the increase of SiO2 content in the PVDF-HFP / SiO2 fiber composite membrane, the contact angle of the composite membrane shows a decreasing trend, namely 118.4° (PVDF-HFP / SiO2-3), 116.9° (PVDF-HFP / SiO2-6), 113.7° (PVDF-HFP / SiO2-9), and 108.3° (PVDF-HFP / SiO2-12).

[0061] Figure 4 shows the temperature drop of the PVDF-HFP / SiO2 nanofiber composite membranes of Examples 1-4 of the present invention. The composite membranes of Examples 1-4 were placed outdoors under sunlight, with an average solar intensity of 634 W / m². 2 The average ambient humidity was 20%, and the sunlight exposure time was from 11:00 AM to 2:00 PM. After the irradiation, the front temperature of the composite membrane and the back temperature of the aerogel were measured. The front temperature was the ambient temperature, and the difference between the front and back temperatures was... Figure 3 The temperature reduction indicator clearly shows that the back surface of the composite film is cooler than the ambient temperature, indicating that the surface composite film possesses excellent heat insulation and cooling properties. For example... Figure 4As shown, when the SiO2 addition is 3 wt%, the temperature below the PVDF-HFP / SiO2 fiber composite membrane is 2.63 ℃ lower than the ambient temperature. When the SiO2 addition is 6 wt% and 9 wt%, the temperature drop of the PVDF-HFP / SiO2 fiber composite membrane decreases to 2.89 ℃ and 3.39 ℃, respectively. When the SiO2 addition is 12 wt.%, the average temperature drop of the PVDF-HFP / SiO2 fiber composite membrane is 2.96 ℃. Therefore, comparatively, the PVDF-HFP / SiO2 fiber composite membrane exhibits the largest temperature drop when the SiO2 addition is 9 wt%.

[0062] The front of the composite membrane faces the sunlight, while the back of the composite membrane faces away from the sunlight.

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A radiation-cooling nanofiber composite membrane, characterized in that, It includes PVDF-HFP nanofibers and inorganic oxide microspheres. The inorganic oxide microspheres are anchored to the surface of the PVDF-HFP nanofibers and partially embedded in the shallow layer of the PVDF-HFP nanofibers to form discretely distributed scattering sites on the surface of the PVDF-HFP nanofibers.

2. The radiation-cooling nanofiber composite membrane according to claim 1, characterized in that, The mass fraction of inorganic oxide microspheres on PVDF-HFP nanofibers is 3wt%-15wt%.

3. The radiation-cooling nanofiber composite membrane according to claim 1, characterized in that, Inorganic oxide microspheres include at least one of SiO2 microspheres, TiO2 microspheres, and alumina microspheres.

4. A method for preparing a radiation-cooling nanofiber composite membrane as described in any one of claims 1-3, characterized in that, Includes the following steps: PVDF-HFP spinning solution is shaped into PVDF-HFP nanofibers by airflow stretching and deposited on the surface of a rotating drum receiver to form a PVDF-HFP nanofiber network. Simultaneously, inorganic oxide microsphere dispersion is formed into charged microdroplets by electrostatic atomization and then directionally deposited onto the forming PVDF-HFP nanofibers under the action of an electric field. This allows the inorganic oxide microspheres to collide with and partially embed into the shallow surface layer of the PVDF-HFP nanofibers during the quasi-solid stage of the PVDF-HFP nanofibers' transformation from solution jet to solid fiber, forming a physical interlocking and anchoring.

5. The method for preparing the radiation-cooling nanofiber composite membrane according to claim 4, characterized in that, PVDF-HFP polyvinylidene fluoride-hexafluoropropylene was added to a mixed solution of acetone and DMF and mixed evenly to obtain PVDF-HFP spinning solution.

6. The method for preparing the radiation-cooling nanofiber composite membrane according to claim 5, characterized in that, The mass ratio of PVDF-HFP to the volume ratio of the mixed solution is (5-10) g: (40-60) ml, and the volume ratio of acetone to DMF is 3:

7.

7. The method for preparing the radiation-cooling nanofiber composite membrane according to claim 4, characterized in that, The process parameters for airflow stretching spinning include: the inner diameter of the spinning needle is 0.1mm-0.2mm, the feed rate of the spinning solution is 5mL / h-10mL / h, the receiving distance is 15cm-20cm, and the air pressure is 20kPa-30kPa.

8. The method for preparing the radiation-cooling nanofiber composite membrane according to claim 4, characterized in that, Inorganic oxide microspheres were added to the DMF solution, and the mixture was magnetically stirred and ultrasonically dispersed to obtain an inorganic oxide microsphere dispersion.

9. The method for preparing the radiation-cooling nanofiber composite membrane according to claim 4, characterized in that, The process parameters for electrostatic spraying include: voltage 15kV-25kV, drum speed 375r / min, and liquid supply rate 0.5mL / h.

10. The application of the radiation-cooling nanofiber composite membrane according to any one of claims 1-3 in the preparation of radiation-cooling materials.

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