Nanofibrous materials for passive thermal control on earth and in space

Abstract

Nanofibrous materials and their fabrication by electrospinning are disclosed for passive temperature control on earth and in outer space. The materials combine high solar reflectivity with high thermal emittance in the infrared region, including in the long wavelength atmospheric window region between 8 μm and 13 μm. The materials include nanofibrous PTFE / PEO and silica materials. The physical properties of the materials are suitable for extraterrestrial as well as terrestrial applications.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Patent Application Nos. 63 / 633,284, filed Apr. 12, 2024; 63 / 664,769, filed Jun. 27, 2024; and 63 / 664,771, filed Jun. 27, 2024; the disclosures of which are hereby incorporated by reference in their entireties.GOVERNMENT LICENSE RIGHTS

[0002] This invention was made with government support under grant number 80NSSC21K0072, awarded by the National Aeronautics and Space Administration. The government has certain rights in the invention.TECHNICAL FIELD

[0003] The present disclosure relates to materials, devices, uses and methods of passive control of heat influx and efflux, on earth, and in space.BACKGROUND

[0004] Passive heat management is useful in space, especially for extended missions involving protection from sunlight. Thermal coatings with desirable optical properties can drastically reduce the power consumed by active cooling systems, thereby reserving more resources for other systems onboard. Specifically, materials with wavelength-dependent reflectance and emittance are desirable for managing incident sunlight and self-cooling by thermal emission. On earth, passive thermal control offers a method to regulate system temperature without significant energy input. Active temperature regulation methods in buildings, such as air conditioners, entail high energy consumption and cost. Additionally, these systems often utilize refrigerants that emit environmentally harmful volatile compounds. Incorporating passive systems presents an opportunity to reduce reliance on active thermal controls.SUMMARY

[0005] According to some embodiments, a passive temperature-regulating nanofibrous material is disclosed, the material comprising nanofibers having an average diameter of between about 200 nm and about 1500 nm, wherein the material has a thickness of greater than about 400 μm; an average solar absorptance (<as>) of less than about 0.1, the average solar absorptance being determined over the range from 0.3 μm to 2.5 μm; an average thermal emittance (<εIR>) of greater than about 0.75 at 300 K, the average thermal emittance being determined over the range from 2.5 μm to 15 μm; and a figure of merit greater than about 20, the figure of merit being the ratio <εIR> / <as>. The material can have a porosity of greater than about 0.5.

[0006] According to some embodiments, the figure of merit of the material is greater than about 500. According to some embodiments, <as> of the nanofibrous material can be less than about 0.05, and can be less than about 0.005. The nanofibers of the material can be silica nanofibers.

[0007] According to some embodiments, the nanofibers can comprise a water soluble polymer, and nanobeads of a fluoropolymer, the nanobeads of the fluoropolymer being attached to surfaces of the water soluble polymer. For some such embodiments, the figure of merit is greater than about 800. For some such embodiments, the fluoropolymer is selected from the group consisting of polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy alkane (PFA), ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-hexafluoropropylene (PDVF-HFP), ethylene chlorotrifluoroethylene (ECTFE), polychlorotrifluoroethylene (PCTFE), and combinations thereof. According to some embodiments, the nanofibers comprise polyethylene oxide (PEO) nanofibers and polytetrafluoroethylene (PTFE) nanobeads, wherein the PTFE nanobeads are disposed on surfaces of the PEO nanofibers.

[0008] According to some embodiments, the weight ratio of PTFE to PEO is between about 75:25 and about 95:5. For some such embodiments, the PTFE nanobeads have an average diameter of between about 150 nm and about 300 nm. For some such embodiments, the material has a figure of merit greater than about 800.

[0009] According to some embodiments, the passive temperature-regulating material is obtained by a process of electrospinning.

[0010] According to some embodiments, a method is disclosed of obtaining a passive temperature-regulating nanofibrous material, the method comprising:

[0011] (1) dissolving a water soluble polymer in water to form a water soluble polymer solution;

[0012] (2) adding fluoropolymer nanobeads to the water soluble polymer solution to form a mixture of fluoropolymer nanobeads and water soluble polymer in water, the weight ratio of fluoropolymer to water soluble polymer being between about 75:25 and about 95:5, the fluoropolymer nanobeads having an average diameter of between about 150 nm and about 300 nm; and

[0013] (3) electrospinning the mixture of fluoropolymer nanobeads and water soluble polymer in water to obtain the passive temperature-regulating material.

[0014] According to some embodiments of this method, the water soluble polymer is PEO and the fluoropolymer nanobeads are PTFE nanobeads. According to some such embodiments, the ratio of PTFE to PEO is about 90:10.

[0015] According to some embodiments, a method is disclosed of obtaining a passive temperature-regulating nanofibrous material, the method comprising:

[0016] (1) mixing together a tetraalkyl orthosilicate, an alcohol, water, and a strong acid to form a mixture;

[0017] (2) stirring the mixture; and

[0018] (3) electrospinning the mixture to obtain the passive temperature-regulating nanofibrous material.

[0019] For some such methods, the tetraalkyl orthosilicate is tetraethyl orthosilicate, the alcohol is ethanol, and the strong acid is hydrochloric acid.BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 compares the extraterrestrial figure of merit <εIR> / <as> of electrospun PTFE / PEO and silica-based materials of the current application with materials commonly used for outer space applications.

[0021] FIG. 2 compares the terrestrial figure of merit <εATW> / <as> of electrospun PTFE / PEO materials of the current application with other materials that have been considered for passive cooling applications on earth.

[0022] FIG. 3A shows a scanning electron microscope (SEM) image for a 90:10 wt. % PTFE / PEO material, using standard fabrication conditions as described below.

[0023] FIG. 3B shows the fiber diameter size distribution for the sample of FIG. 3A.

[0024] FIG. 3C shows a scanning electron microscope (SEM) image for an 80:20 wt. % PTFE / PEO material, using standard fabrication conditions.

[0025] FIG. 3D shows the fiber diameter size distribution for the sample of FIG. 3C.

[0026] FIG. 4A shows a schematic of a procedure for preparing electrospun silica.

[0027] FIG. 4B shows an SEM image of silica nanofibers obtained using the procedure of FIG. 4A, under standard fabrication conditions.

[0028] FIG. 4C shows the fiber diameter size distribution for the sample of FIG. 4B.

[0029] FIG. 5 shows the experimental setup used for electrospinning.

[0030] FIG. 6 shows a sample of PTFE nanobeads prepared by electrospraying under standard fabrication conditions.

[0031] FIG. 7 provides experimental stress-strain curves for PTFE / PEO materials with different compositions.

[0032] FIG. 8 provides experimental determinations of ultimate tensile strength, yield strength, and toughness as a function of PTFE / PEO wt. %.

[0033] FIG. 9 provides experimental determinations of tensile modulus and elongation at break as a function of PTFE / PEO wt. %.

[0034] FIG. 10 compares the spectral normal-hemispherical reflectance in the 0.3-2.5 μm wavelength range for a PTFE / PEO 90:10 sample to the reflectance from a 80:20 sample.

[0035] FIG. 11 is an SEM image of randomly arranged fibers for a PTFE / PEO sample obtained with a rotating collector speed of 500 rpm.

[0036] FIG. 12 is an SEM image of oriented fibers for a PTFE / PEO sample obtained with a rotating collector speed of 1400 rpm.

[0037] FIG. 13 illustrates the dependence of the spectral normal-hemispherical reflectance in the 0.3-2.5 μm wavelength range on thickness of PTFE / PEO material.

[0038] FIG. 14 displays the solar reflectance of a 1.2 mm thick 90:10 PTFE / PEO sample and a 1.6 mm thick silica sample across the 0.3-2.5 μm solar wavelength range, with extraterrestrial solar irradiance at air mass 0 (AM 0) overlaid for reference.

[0039] FIG. 15 compares the IR emittance at 300 K of 90:10 PTFE / PEO, silica, double-aluminized Mylar, and aluminum foil.

[0040] FIG. 16 plots the thermal stability of silica nanofiber material and PTFE / PEO nanofiber material as characterized by thermogravimetric analysis.

[0041] FIG. 17 compares the spectral emittance (at 300 K) of PTFE / PEO 90:10 samples with thicknesses of 1.2 mm and 3.36 mm across the mid-infrared (MIR) and long-wave infrared (LWIR) spectral range (2.5-15 μm).DETAILED DESCRIPTIONMaterials for Extraterrestrial Applications.

[0042] The existence of extreme temperatures is among the most significant challenges that make space exploration difficult, with sunlight being a primary radiation source heating objects in space. While some sunlight is reflected, energy is also dissipated into the extremely cold void of space by thermal radiation emitted from the spacecraft. Unlike terrestrial conditions, convective heat transfer by ambient air is absent in outer space, making temperature control even more challenging. Without an atmosphere to absorb a portion of the solar energy, space vehicles also experience the full impact of the sun, leading to undesirably high temperatures. Hence, temperature control is desirable for space applications ranging from satellites and space stations to storage of cryogens, including propellants such as liquid hydrogen for interplanetary exploration.

[0043] In order to maintain temperatures, space missions employ various active and passive cooling technologies to reject heat. Active technologies include thermoelectric coolers, cryocoolers, pumped fluid loops, and active thermal architectures. Using active cooling alone is impractical due to large power demands and complex control systems prone to failure in the harsh space environment, making missions cost-prohibitive. Practical solutions require passive cooling to reduce power consumption. Current passive technologies used for space applications include paints, coatings, thermally conductive tapes, straps and louvers, sun shields and deployable radiators, heat pipes, phase change materials, and multilayer insulation (MLI).

[0044] Thermal coatings and MLI are typically the shiny outermost coatings of space vehicles and can include several layers of thin polymeric reflectors made of polyethylene terephthalate or polyimide films coated with vapor-deposited metal like aluminum, silver, and gold on one or both sides. For better thermal insulation, the reflector films are often separated by polymeric spacers. Table I provides optical properties of typical materials used for space applications. For such typical materials, solar absorptance <as> (averaged over wavelengths of 0.3 μm to 2.5 μm) and infrared (IR) emittance <εIR> (averaged over wavelengths of 2.5 μm to 15 μm) are around <as>=0.12-0.28 and <εIR>=0.02-0.05, respectively, which helps reflect sunlight, but the low IR emittance prevents radiative heat dissipation to space. Consequently, typical MLI and thermal coatings still absorb a sizeable portion of solar radiation while not emitting much energy to space. There is a need for advanced materials with ultra-low solar absorptance and strong IR emittance, yielding a large ratio of <εIR> / <as>.

[0045] A related measure of the solar reflective properties of materials for thermal coatings is the solar reflectance, <ρs> is defined as <ρs>=1-<as>. With this definition, ultra-low solar absorptance corresponds to solar reflectance approaching the limiting value of 1.

[0046] Some embodiments of the present disclosure present a material for passive heat management and spacecraft temperature control by employing nano-engineered, porous, spectrally selective materials with ultra-high solar reflectance and strong mid-infrared emittance. The materials are manufactured using an electrospinning process—a versatile, scalable, and economical fabrication technique to create materials with nano and microscale features. According to the present disclosure, electrospinning is used to create nanofibrous materials made of silica and of polytetrafluoroethylene (PTFE) nanobeads coating nanofibers of polyethylene oxide (PEO) (herein referenced as PTFE / PEO nanofibers). The spectral properties of these electrospun materials are also included in Table I, for comparison to the conventional passive thermal control materials used for space applications. A plot of the ratio of <εIR> / <as> for these materials is provided in FIG. 1.

[0047] The nanofibers of the electrospun materials allow strong scattering of solar radiation, yielding a high solar reflectance. In contrast to the other materials in FIG. 1, these electrospun materials also provide a high degree of thermal emittance. As we discuss further below, the mechanical (tensile) properties and the ultraviolet and atomic oxygen durability of these electrospun materials are suitable for long-duration space missions in the low Earth orbit. Because of these favorable spectral and materials properties, these electrospun materials present a paradigm for passive thermal control of spacecraft and long-duration cryogenic fluid storage for space missions.TABLE 1Comparison of the spectral properties of passive thermal control materials used forspace applications. Electrospun PTFE / PEO and silica materials of the current application are also included in the table. All values are determined at 300 K.SolarSolarThermal Materialreflectance (ρs)absorptance (αs)emittance (εIR)εIRαsKapton film withKapton-Al0.540.460.861.87Aluminum backingTeflon with goldTeflon-Au0.780.220.813.68backingMylar film withMylar-Al-10.810.190.774.05Aluminum backing 1Mylar film withMylar-Al-20.830.170.764.47Aluminum backing 2GSFC White paintGSFC-NS43C0.80.200.924.6NS43CSumitomo BakeliteS. Bakelite0.830.170.824.82GSFC White paintGSFC-MS740.830.170.925.41MS74Teflon withTeflon-Al-10.870.130.816.23Aluminumbacking 1Teflon withTeflon-Al-20.870.130.876.69Aluminumbacking 2Zinc oxide pigmentZ93SC550.860.140.946.71in potassium silicatebinderZinc ortho-titanateZnTiOx-0.870.130.927.08with PotassiumKSiOxsilicateAluminum compositeAl composite-0.910.090.728coating overAl foilAluminum foilQioptiq StandardQ-S-CMX0.90.100.868.6Optical SolarReflector CMXTeflon with silverTeflon-Ag0.910.090.889.78backingAZ TechnologyAZW / LA-II0.910.090.9110.11AZW / LA-IIQioptiq Plain OSRQ-CMO0.9150.0850.8710.24CMOHelios secondHelios-SSM-0.930.070.7911.29surface mirror / silverAgbackingOptical SolarOSR0.940.060.813.33ReflectorBarium sulphate withBaSO4-PVA0.940.060.8814.67polyvinyl alcoholElectrospun SilicaSilica-0.90.9640.0360.88524.580.9 mmElectrospun SilicaSilica-1.60.9670.0330.89527.121.6 mmElectrospun SilicaSilica-3.30.970.030.88429.473.3 mmElectrospunPTFE:PEO-1.20.9940.0040.78195PTFE / PEO 1.2 mmElectrospunPTFE:PEO-0.9990.0010.81810PTFE / PEO 3.36 mm3.36Materials for Terrestrial Applications.

[0048] Considerable research has been devoted to exploring passive radiative cooling here on Earth. For terrestrial applications, efficient passive radiative cooling requires low solar absorptance <as> (i.e. high solar reflectance) in the solar spectrum (0.3 μm to 2.5 μm) combined with high thermal emittance <εATW> in the long wavelength infrared atmospheric transmission window (8-13 μm). Emittance in the atmospheric transmission window (ATW) permits the outward emission of thermal radiation from Earth to outer space. By combining low solar absorptance <as> with high thermal emittance <εATW> in the atmospheric transmission window, terrestrial self-cooling can be achieved without the need for substantial active energy consumption. Materials having a high value of the ratio <εATW> / <as> will function more effectively as passive radiative cooling materials for terrestrial applications. A comparison of this ratio at 300 K for various state-of-the-art terrestrial passive radiative cooling materials is shown in FIG. 2. Notably, for terrestrial applications, electrospun PTFE / PEO materials of the present disclosure show orders of magnitude greater values of <εATW> / <as> than other state-of-the-art terrestrial passive radiative cooling materials. Consequently, these electrospun PTFE / PEO materials provide exceptional performance characteristics (as monitored by <εIR> / <as> and by <εATW> / <as>) for passive thermal control applications in both extraterrestrial and terrestrial environments.Fabrication of Electrospun Nanofibrous Materials

[0049] The passive temperature-regulating nanofibrous materials of the present application can be prepared by electrospinning. As shown in the scanning electron micrograph (SEM) of FIGS. 3A and 3C, in an embodiment suitable for both terrestrial and extraterrestrial applications, the electrospun material can include nanofibers 310 of a water soluble polymer, onto which nanobeads 320 of a fluoropolymer are disposed. In the embodiments of FIGS. 3A and 3B, the water soluble polymer is PEO, and the fluoropolymer is polytetrafluoroethylene (PTFE). FIGS. 3B and 3D show the fiber diameter distributions for the SEMs of FIGS. 3A and 3C, respectively. For FIGS. 3A and 3B, the ratio of PTFE to PEO is 90:10. For FIGS. 3C and 3D, the ratio of PTFE to PEO is 80:20. The preparation of the materials of FIG. 3A-3D is discussed further below.

[0050] Solutions for electrospinning are prepared by dissolving the water soluble polymer in water to form an aqueous solution of the water soluble polymer, adding fluoropolymer nanobeads to the aqueous solution to form a mixture of fluoropolymer nanobeads and water soluble polymer in water. In some embodiments, the weight ratio of fluoropolymer to water soluble polymer is between about 75:25 and about 95:5. In some such embodiments, the weight ratio is between about 85:15 and 95:5. In some such embodiments, the weight ratio is about 90:10, or about 85:15, or about 85:20. The fluoropolymer nanobeads can have an average diameter of between about 150 nm and about 300 nm. According to some embodiments, the water soluble polymer is PEO. For some embodiments, the water soluble polymer can be polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyvinylpyrrolidone (PVP) or carboxymethylcellulose (CMC). The fluoropolymer can be selected from the group consisting of polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy alkane (PFA), ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-hexafluoropropylene (PDVF-HFP), ethylene chlorotrifluoroethylene (ECTFE), polychlorotrifluoroethylene (PCTFE), and combinations thereof. According to some embodiments, the fluoropolymer is PTFE. According to some embodiments, the average fiber diameter is between about 200 nm and about 1500 nm. According to the embodiments of FIG. 3A-3D, the water soluble polymer is PEO and the fluoropolymer is PTFE.

[0051] In other embodiments, according to the reaction pathway of FIG. 4A and the SEM of FIG. 4B, electrospun materials composed of silica fibers 410 provide materials suitable for extraterrestrial applications. Compared to the nanofibrous PTFE / PEO materials of FIGS. 3A-3D, the silica nanofibers of FIG. 4B are thinner, having an average diameter 261±86, with a distribution of diameters shown in FIG. 4C. For these silica nanofibrous materials, solutions for electrospinning can be prepared by mixing together and stirring a tetraalkyl orthosilicate, an alcohol, water, and a strong acid to form a mixture, followed by electrospinning the material to obtain a passive temperature-regulating nanofibrous material composed of silica fibers 410. In the example embodiment of FIGS. 4A to 4C, the tetralkyl orthosilicate is tetraethyl orthosilicate, the alcohol is ethanol, and the strong acid is hydrochloric acid.

[0052] As illustrated in FIG. 5, an electrospinning apparatus 510 of the type used in the experiments described below includes a syringe pump 520 with a syringe needle 530 connected to a high voltage power supply 540 and a grounded rotating collector 550, configured to rotate with a D.C. motor 560. During the electrospinning process, a polymeric solution 570 for electrospinning is extruded from the syringe needle at a constant rate. As the liquid droplet becomes charged, electrostatic repulsion overcomes surface tension of the droplet, causing a stream of polymeric solution 570 to travel from the syringe toward the rotating grounded collector. As the stream of polymeric solution 570 moves towards the collector, the liquid evaporates, causing a layer of polymeric nanofibers to deposit on the surface of the grounded rotating collector 550.Characterization of Electrospun Nanofibrous Materials

[0053] The morphology of the nanofibrous materials of the present disclosure were characterized by scanning electron microscopy (SEM), recorded using a Carl Zeiss Supra 55 Field Emission Scanning Electron Microscope (FESEM) at a working distance of 3 mm, an accelerating voltage of 1.5 kV, and a current of 5 pA. Before imaging each sample, a 1 cm square piece was cut and placed in a Denton Platinum Sputter System, and a 2 nm-thick layer of platinum was deposited on the sample surface. Image J software was used for data processing and analysis of the SEM images.

[0054] The spectral normal-hemispherical reflectance and transmittance of the samples were characterized using a UV-Visible-NIR spectrometer (Perkin-Elmer Lambda 950) equipped with a 6-inch integrating sphere using a NIST traceable Labsphere Spectralon® diffuse reflectance standard. The average solar reflectance <ρs> was calculated using Equation 1:<ρs>=∫λ=0.3 μ⁢mλ=2.5 μ⁢mρλ.Iλ⁢d⁢λ∫λ=0.3 μ⁢mλ=2.5 μ⁢mIλ⁢d⁢λ(1)where pλ represents the spectral normal-hemispherical reflectance obtained from the spectrometer and Iλ is the solar spectral intensity using the extraterrestrial spectrum (ASTM E490 Air Mass 0). The average solar absorptance <as> is calculated as <as>=1-<ρs>.To characterize materials for extraterrestrial applications, the spectral optical properties in the infrared wavelength range (2.5-15 μm) were characterized using a Fourier-Transform Infrared spectrometer (ThermoFisher Scientific Nicolet™ iS20) interfaced with a 3-inch gold-coated integrating sphere (Pike Technologies Mid-IR IntegratIR™) and a mercury cadmium telluride detector. The average emittance <εIR> of the materials is calculated using the following equation:εIR=∫λ=2.5 μ⁢mλ=1.5 μ⁢mελ.IB⁢B(λ,T)⁢d⁢λ∫λ=2.5 μ⁢mλ=1.5 μ⁢mIB⁢B(λ,T)⁢d⁢λ(2)where ελ represents the spectral normal-hemispherical emittance obtained from the FTIR spectrometer and IBB(λ, T) is the spectral blackbody emissive radiance at 300 K:IB⁢B(λ,T)=2⁢h⁢c2λ5(eh⁢cλ⁢KB⁢T-1)(3)where h, c, λ, KB, T represent Planck's constant, speed of light in vacuum, wavelength, Boltzmann's constant, and absolute temperature of the blackbody, respectively.For terrestrial applications, the relevant thermal emittance is the thermal emittance <εATW> in the long wavelength infrared atmospheric transmission window between 8-13 μm, calculated as:εA⁢T⁢W=∫λ=8⁢ μ⁢mλ=13⁢ μ⁢mελ.IB⁢B(λ,T)⁢d⁢λ∫λ=8⁢ μ⁢mλ=13⁢ μ⁢mIB⁢B(λ,T)⁢d⁢λ(4)where IBB is the spectral blackbody emissive radiance according to equation (1).The thermal degradation behavior of the nanofibrous materials was studied using thermogravimetric analysis (TGA-Q50) by heating 7.5 mg of each sample from 25° C. to 800° C. at a temperature ramp rate of 10° C. / min.Durability tests were conducted to evaluate the resilience of electrospun nanofibrous materials in environments simulating the harsh conditions of outer space. The electrospun materials were subjected to ultraviolet (UV) radiation, atomic oxygen (AO) and extreme temperature swing cycles, followed by characterization, as described in more detail below.EXAMPLESPreparation of Precursor Electrospinning SolutionsTo fabricate the PTFE / PEO materials, PTFE (particle size 234±59 nm) was purchased from Sigma-Aldrich as a 60 wt % dispersion in water, and PEO powder (molecular weight ˜5 million) was purchased from Beantown Chemical.PTFE / PEO solution of the desired mass ratio (Table 2) was prepared by first dissolving PEO powder in deionized water and stirring at 60° C. and 600 rpm for 4 h using a magnetic stirrer hot plate (Thermo Scientific Cimarec) to form a 4 wt % aqueous solution. Then, PTFE dispersion was added to the PEO solution and stirred at room temperature and 600 rpm for 6 h to obtain the final PTFE / PEO composite solution. The PTFE / PEO solution was then drawn into a syringe and used for electrospinning to fabricate the electrospun nanofibrous PTFE / PEO materials.TABLE 2Composition of PTFE / PEO electrospinning solutionsPTFE (60 wt. % in water) (g)PEO (g)water (g)PTFE:PEO (g / g)2.50.1674.0090:102.50.2656.3585:152.50.3759.0080:20The electrospinning solution for silica nanofibers was prepared by mixing tetramethyl orthosilicate, ethanol, deionized water, and a 37% aqueous solution of hydrochloric acid. This mixture was stirred at 40° C. and 200 rpm for six hours to ensure thorough mixing. After preparation, the solution was transferred to a syringe for use in the electrospinning process.Fabrication ProcessGeneral ProcedureThe electrospinning process used 22-gauge needles (0.508 mm internal diameter), syringes, and an aluminum foil substrate (99% purity) purchased from McMaster-Carr. Deionized water used in the fabrication was obtained from the RPI's Center for Materials, Devices, and Integrated Systems (CMDIS).

[0063] As shown in FIG. 5, the electrospinning setup 510 included a variable high-voltage power supply 540 (Gamma High Voltage Research), a grounded rotating drum nanofiber collector 550, a syringe pump 520 interfaced with the 22-gauge syringe needle 530. The electrospinning process was performed in a temperature and humidity controlled chamber with temperature and humidity sensors (B&K Precision 725 datalogging humidity and temperature meter, not shown).

[0064] The process was performed within a temperature- and humidity-controlled chamber equipped with a temperature and pressure sensor (BK Precision 725 Datalogging Humidity and Temperature Meter). The electrospinning solution-loaded syringe was mounted on the pump, and the high-voltage power supply 540 was connected to the tip of the syringe needle 530. The grounded rotating drum collector 550 was overlaid with an aluminum foil substrate (not shown) on which the electrospun nanofibers were deposited. The solution was made to flow through the syringe needle 530 by operating the syringe pump 520 while an electric potential was applied between the syringe needle 530 and the rotating collector 550. As the applied potential was increased, the polymeric solution 570 at the tip of the needle changed from a hemispherical shape to the desired Taylor cone. As the voltage was increased beyond a threshold voltage, a the polymeric solution 570 was ejected as a jet from the tip of the Taylor cone. The jet comprising the polymeric solution 570 dried out while undergoing an in-flight whipping motion as it approached the collector. The nanoscale fibers accumulated on the collecting substrate to form a physically visible layer of bright white color. The electrospinning process was stopped when the desired thickness of the electrospun material was obtained. The overall thickness was measured using a vernier caliper at different sample locations, and an average thickness was recorded. The electrospun nanofibrous materials with the aluminum foil substrate were removed from the collector and allowed to dry for about 12 h before material characterization.Parametric Study

[0065] A baseline study of the effect of fabrication parameters on physical and optical properties of the electrospun materials was performed to define standard parameters. For the PTFE / PEO system, the first parameter tested was the relative solution concentrations of PTFE and PEO. As a baseline, a dispersion of PTFE was deposited on the collector using the following fabrication parameters: a needle to collector distance of 12 cm, a potential of 10 kV between the syringe needle 530 and the grounded rotating collector 550, a solution flow rate of 0.5 ml / h, ambient temperature 21±3° C., and relative humidity of 20±4%. An SEM of the deposited material is shown in FIG. 6. The baseline experiment shows that shows that although the fabrication process deposits PTFE on the collector substrate, it produces nano and microscale beads with 234±59 nm particles and 5±1.5 μm clumps. These conditions do not produce nanofibers. The PTFE beads lack sufficient adhesion to the substrate and could be easily separated from the substrate due to the low surface energy of PTFE.

[0066] The formation of beads by electro-spraying and fibers via electrospinning depends on several factors, including the dispersion's molecular weight, conductivity, surface tension, and viscosity. Although PTFE provides good UV stability and chemical resistance, it exhibits relatively high electrical resistance and dielectric strength. These properties, along with the high surface tension and low viscosity of the PTFE dispersion, favor electrospraying of beads. With the addition of PEO, the PTFE:PEO composite solution yields nanofibers via electrospinning. With the addition of PEO, while the PTFE clumps are no longer produced, the individual PTFE beads (234±59 nm) append to the PEO nanofibers, as shown in FIGS. 3A and 3C.

[0067] The effect of PEO concentration. The PTFE / PEO mixture combines the complementary properties of both polymers to create a PTFE / PEO composite structure. FIGS. 3A and 3C show the different materials produced from PTFE / PEO dispersions of 90:10 and 80:20 composite solutions using the following electrospinning parameters: a needle-to-collector distance of 12 cm, a potential of 10 kV between the needle and the collector, a solution flow rate of 0.5 mL / h, a rotating collector speed of 500 rpm, ambient temperature of 21±3° C., and relative humidity of 20±4%. These materials comprise smooth and fine PEO nanofibers 310 with PTFE beads 320 attached to the PEO nanofiber surface.

[0068] The 90:10 PTFE / PEO solution produces fibers with an average diameter of 1050 nm (FIG. 3A), which is significantly larger than the average fiber diameter of 516 nm produced using the 80:20 solution (FIG. 3C). An intermediate average fiber diameter of 616 nm is produced using an 85:15 PTFE / PEO solution (data not shown). The 90:10 solution fibers contain PTFE nanobeads of diameter 234±59 nm. The PTFE nanobeads embed themselves on the PEO fibers during electrospinning. Thus, comparing the baseline experiment containing only PTFE (FIG. 6) with these various PTFE / PEO solutions, it is clear that PEO facilitates the formation of fibers attached to PTFE nanobeads, unlike the baseline (FIG. 6), where PTFE is clumped together. Decreasing the amount of PTFE in the solution reduces the fiber diameter since fewer PTFE beads are attached to the much thinner PEO fibers (˜300 nm diameter).

[0069] The effect of the fiber morphology due to a variation in PTFE and PEO concentrations affects the mechanical performance, as determined from tensile strength measurements. Three samples of different PTFE / PEO ratios, specifically 90:10, 85:15, and 80:20 samples, were characterized in tension using the Instron 3345 Universal Testing Machine, following the ASTM D882-18 testing procedure. This procedure defines the standard testing method for characterizing the tensile properties of thin polymeric materials. The samples of similar dimensions, namely length×width×thickness of 45 mm×17 mm×0.35 mm, were cut into dog-bone shapes with mid-region dimensions of 8.5 mm width and 35 mm length, in line with the testing procedure to prevent rupture of the samples at the instrument grips and to ensure that deformation took place at the neck (mid-region). Each end of a sample was glued between cardboard sheets, and the glued cardboard sheets were placed between the instrument grips. This gluing to cardboard sheets was done to prevent the puncture and tearing of the fibrous coatings by the knurled teeth of the instrument grips. A slow crosshead speed of 1 mm / min was maintained across all three samples to allow uniform deformation and reduce inaccuracies in results from the tensile loads being applied too quickly. The measured tensile properties are given in Table 3, and plotted in FIGS. 7 through 9. FIG. 7 shows stress vs. strain plots for the 90:10 PTFE / PEO sample 710, the 85:15 PTFE / PEO sample 720, and the 80:20 PTFE / PEO sample 730. FIG. 8 shows the ultimate tensile strength 810 (solid line), the yield strength 820 (dashed line), and the toughness 830 (dotted line) plotted against the wt. % of PTFE / PEO. FIG. 9 shows the tensile modulus 910 (solid line) and the elongation at break 920 (dashed line).TABLE 3Tensile properties of electrospun nanofibrous coatings.UltimateTensileYieldTensileElongationPTFE / PEOStrengthStrengthmodulusToughnessatratio(MPa)(MPa)(MPa)(MJ / m3)break (%)90:100.1500.132.430.02943085:150.4270.292.350.16896080:200.4310.373.50.080627

[0070] The tests reveal that the concentration of PEO was increased from 10 wt. % to 20 wt. % (consistent with a decrease in PTFE content), the yield strength of the coating increases. This result implies that coatings with a higher PEO content allow more elastic deformation before the onset of plastic deformation. The ultimate tensile strength also increases with the increase in PEO content contained in the coatings. This increase in coating strength is traceable to the smaller individual fiber diameters obtained with the increase in PEO content. This decrease in fiber diameter causes a reduction in the cross-sectional area of the fiber, allowing more stress to be borne by the individual fiber. This result is in agreement with the equation:σ=FA(5)where σ represents the stress (in MPa), F is the force or load (in N) borne by the fiber, and A (m2) represents the cross-sectional area of the cylindrical fiber.Because all three coatings were of similar thickness, width, and cross-sectional area when they were prepared for testing, the increase in strength while increasing PEO concentration (decrease in fiber diameter) indicates that the diameters of the individual fibers play a significant role in determining the material strength achievable.

[0072] Increasing the PEO content from 10 wt. % (PTFE:PEO 90:10) to 15 wt. % (PTFE:PEO 85:15) also led to a decrease in the tensile modulus of elasticity of the coating (reduction in coating stiffness), with a significant increase in deformation as seen in FIG. 9.

[0073] The tensile (Young's) modulus E is defined by E=σε where σ represents the stress (in MPa), and ε represents the strain. The PTFE / PEO 85:15 sample was more ductile, allowing the most plastic deformation, and it had the highest toughness of all three materials (FIG. 8). The toughness (unit energy per unit volume) was obtained by integrating the area under the stress-strain plot, and that the PTFE / PEO 85:15 coating had the largest area means that it absorbed the most energy before fracturing at 60% elongation. A further increase in the PEO content by 5 wt. % (PTFE / PEO 80:20) led to a 55% decrease in the elongation-at-break, a 50% increase in stiffness, and a 52% decrease in the toughness of the coating. The PTFE / PEO 85:15 coating had the best tensile properties due to its superior strength, ductility, toughness, and resistance to plastic deformation before breaking. However, all three ratios of PTFE to PEO provided materials with suitable mechanical properties to function efficiently in outer space applications.

[0074] FIG. 10 compares the spectral normal-hemispherical reflectance in the 0.3-2.5 μm wavelength range for the PTFE / PEO 90:10 sample 1010 to the reflectance from the 80:20 sample 1020. For the sake of clarity, the curve for the 85:15 sample is not shown, since it falls squarely between the 90:10 and the 80:20 curves. The (terrestrial) solar spectral intensity 1030 is shown for reference.

[0075] Both samples, each with a thickness of 0.3 mm, demonstrate exceptional reflectivity to incident solar radiation, with the average spectral reflectance (<ρs>) nearing 100%. However, the 90:10 sample 1010 exhibited a slightly higher average spectral reflectance of 96.7% compared to the 80:20 sample 1020 (96%). This marginal difference can be attributed to the larger fibers and broader distribution of fiber diameters in the 90:10 sample 1010, which likely contributed to enhanced reflectance by scattering a wider range of incident radiation spectra compared to the 80:20 sample 1020.

[0076] The effect of rotating collector speed. FIGS. 11 and 12 compare the effects of varying the rotational speed of the grounded collector using 90:10 PTFE:PEO solution and maintaining the same fabrication parameters while changing the rotating drum collector speed. Four different speeds of 500, 1000, 1400, and 2100 rpm were used in this study. FIG. 11 shows an SEM image for a speed of 500 rpm, whereas FIG. 12 shows an SEM image for a speed of 1400 rpm. As seen in FIG. 11, a rotating collector speed of 500 rpm resulted in randomly oriented fibers. With speeds exceeding 1000 rpm, the fiber orientation changes, with most fibers aligning in the direction of rotation, as can be seen in FIG. 12, which shows an SEM image for a speed of 1400 rpm.

[0077] Higher rotational speeds also decrease the diameters of the nanofibers, consistent with the fibers experiencing increased stretching and drying, leading to further thinning compared to the baseline case, which involves a lower speed. By doubling the speed of the drum from 500 to 1000 rpm, there is a sharp decrease in the average fiber diameter size by more than half, from 1035±163 nm to 463±107 nm. The SEM images of FIGS. 11 and 12 clearly show this reduction in fiber sizes. Subsequent increases in the speeds led to a further decrease in the fiber diameter, with an average diameter of 374±88 at 1400 rpm (FIG. 12). Hence, changing the speed of the rotating drum is one verifiable way of altering both the fiber geometry and orientation simultaneously. At four times the baseline speed (2100 rpm), the sample has an average diameter of 341 nm, a 3-times reduction from the 1035 nm average diameter of the baseline sample at 500 rpm. The reduction in fiber diameter at higher speeds correlates with a corresponding reduction in reflectance. A comparison of two samples having the same thickness of 350 μm shows an increase in the average solar reflectance from 95.2% to 96.2% on increasing the collector speed from 500 to 2100 rpm. Without being bound by theory, this increase in reflectance for the 2100 rpm sample compared to the 500 rpm sample can most reasonably be attributed to the thinner fibers of the 2100 rpm samples, which are better at scattering light even at shorter wavelengths. In addition, the 2100 rpm fibers show a uniaxial alignment due to the higher collecting speeds, which allows closer packing, affecting light propagation through the sample. A wider range of diameters would perform well in reflecting longer wavelength light, allowing broadband reflectance in the NIR region. The 500 rpm sample could be marginally more transmissive due to the large pore sizes between the larger fiber strands.

[0078] The effect of material thickness. FIG. 13 shows the effect of material thickness on the average solar reflectance <ρs> of a 90:10 ratio sample of PTFE / PEO, prepared with a rotating collector speed of 500 rpm. For relatively thick layers (420±12 to 3360±20 μm) the effect of the substrate is insignificant since the electrospun layer is nearly opaque to the incident light (300-2500 nm). Thicker samples have little to no transmittance, and the incident light is scattered by the fibers, causing diffuse reflection. While thinner samples also exhibit relatively high reflectance, they tend to be marginally transparent. Consequently, some light is transmitted through the fibers to reach the substrate. In this case, the substrate or background material can affect the average solar reflectance. This study uses a thin aluminum foil as a substrate since it is relatively inexpensive compared to alternatives like silver coatings used in space applications. Besides offering superior reflectivity, thicker samples would be helpful in space applications requiring insulation against thermal conduction. The thicker materials with low thermal conductivities of the polymers can enhance the thermal resistance to insulate objects from excessive heating.

[0079] FIG. 13 demonstrates that the average solar reflectance <ρs> of the 90:10 PTFE / PEO material, while high for all thicknesses examined, increases as the material thickness increases from 220 to 3360 μm, ranging from 94.7±0.38% for a 220 μm sample to 99.75±0.40% at 1560±20 μm.

[0080] Beyond this thickness, the reflectance saturates at 99.9%. The average thermal emittance at 300 K of relatively thick samples having zero transmittance also increases with thickness. Specifically, for sample thicknesses of 1200, 1560, and 3360 μm, the average thermal emittance at 300 K is 78.32, 81.14, and 81.42%, respectively.Standard Parameters

[0081] Based on the above parametric studies, similar electrospinning parameters (500-rpm rotating collector speed, 0.5 ml / hr. flow rate, and 12 cm needle-to-collector distance) were used to fabricate both silica and PTFE / PEO materials. However, 10 kV needle-collector DC potentials were used to fabricate the PTFE / PEO materials and 18 kV potentials were used for the silica materials. The electrospinning chamber was maintained at a temperature of 20±5° C. and a relative humidity of 20±4%. Unless otherwise indicated, PTFE / PEO materials were 90:10 wt. % PTFE:PEO.Characterization of Materials for Extraterrestrial Applications

[0082] Space materials must meet rigorous requirements, including chemical and thermal stability, along with resistance to ultraviolet radiation and atomic oxygen. These criteria guided the selection of PTFE, known for its chemical stability, and silica, for its high-temperature resistance and excellent atomic oxygen resilience.Spectral Properties

[0083] Computational simulations predict a material's optical response by modeling its interaction with light by solving Maxwell's equations. The simulations indicate that the reflectances of these materials, although affected by many geometric features, generally improve with increased material thickness, as is observed experimentally (FIG. 13). These simulations and experimental results informed our choice to fabricate thicker samples for reflectance, transmittance, and absorptance characterization. FIG. 14 displays the solar reflectance of a 1.2 mm thick 90:10 PTFE / PEO sample 1410 and a 1.6 mm thick silica sample 1420 across the 0.3-2.5 μm solar wavelength range, with extraterrestrial solar irradiance 1450 at air mass 0 (AM 0) overlaid for reference. The spectral solar reflectance of conventional materials-double-aluminized Mylar 1430 and aluminum foil 1440 are included in this plot for comparison. FIG. 15 shows the IR emittance at 300 K of 90:10 PTFE / PEO 1510, of silica 1520, of double-aluminized Mylar 1530, and of aluminum foil 1540. Both nanofibrous materials exhibit strong selectivity, with high solar reflectance and strong infrared emittance, which is useful for space applications demanding low solar absorption and passive self-cooling. Electrospun silica shows more spectral selectivity compared to PTFE / PEO due to its smaller average fiber diameter, which enhances scattering at the shorter wavelengths. Conversely, PTFE / PEO, with a broader fiber distribution, effectively scatters across UV, visible, and infrared ranges, yielding a higher overall solar reflectance.

[0084] The average solar reflectance for 1.2 mm thick PTFE / PEO and 1.6 mm silica samples is 99.85% and 96.56%, respectively, with thermal emittance values of 78.3% and 89.5%. These results indicate excellent passive radiative heat dissipation due to low solar absorptance and high emissivity. The bright, diffusely white appearance of these materials underscores their strong solar reflectivity. Additional tests with various thicknesses showed that a 3.36 mm thick PTFE / PEO sample can achieve close to ˜99.9% reflectance, marking one of the highest solar reflectances for passive thermal control materials.Morphology

[0085] The morphologies of the electrospun PTFE / PEO and silica materials, shown in FIG. 3A-3D and FIGS. 4A-D, respectively, reveal porous, randomly oriented nanoscale fibers, which promote light scattering, resulting in strong reflectance. Additionally, the porous structure contributes to the material's lightweight properties, a critical requirement for space applications. Different precursor materials yield distinct fiber textures—the silica nanofibers have a smooth surface, while PTFE / PEO fibers, with PTFE beads of 234±59 nm attached to PEO fibers, show a rough texture. SEM images reveal that PTFE / PEO fibers tested here are about four times thicker than silica fibers. This size dissimilarity means that the smaller silica fibers are more effective in scattering shorter wavelengths, while the PTFE / PEO's broader range of diameters scatter light across a wider wavelength range encompassing the UV, visible, and infrared. Both materials, however, demonstrate outstanding reflective properties and thermal stability at high temperatures.Thermal Stability

[0086] The thermal stability of silica nanofiber material and PTFE / PEO nanofiber material was characterized by thermogravimetric analysis, as plotted in FIG. 16. The PTFE / PEO nanofiber mass 1610 experienced a two-stage mass-loss decomposition process involving decomposition of PEO followed by the decomposition of PTFE, leaving behind a residual weight of less than 1% at temperatures above 650° C. By contrast, the silica nanofiber mass 1620 exhibited a gradual mass loss as the temperature increased, due to the cleavage of chemical bonds between the silica and other substituent groups attached to the silica. The plateauing of the silica nanofiber mass at a stable residual weight of ˜75% at temperatures above 650° C. indicates good thermal stability for high temperature applications.Durability in Space

[0087] The extraterrestrial environment presents harsh conditions that could adversely impact the performance of spacecraft materials. Therefore, it is essential to evaluate these materials through Earth-based experiments before in-flight space tests. Ground testing provides valuable performance data, allowing for further design and manufacturing optimization before undertaking the costly and logistically complex in-space evaluations. The electrospun materials were subjected to ultraviolet (UV) radiation, atomic oxygen (AO) and extreme temperature swing cycles, followed by characterization, as described subsequently

[0088] Atomic oxygen. Atomic oxygen (AO) in low Earth orbit (LEO) results from the photodissociation of molecular oxygen by high-energy ultraviolet radiation. Due to its high reactivity and impact energy, AO is known to oxidize and erode the external surfaces of spacecraft materials. The resilience of external surface coatings to AO is crucial for determining their long-term durability and the retention of optical properties, making it essential to assess the impact of AO on these materials.

[0089] SEM images indicate that the fibrous, porous structures of both materials remained largely intact following AO exposure (data not shown). The silica fibers retained their smooth morphologies post-exposure. PTFE:PEO nanofibers which features PTFE nanobeads embedded on PEO fibers, also preserved its rough morphology, although some small, random strands appear to have been oxidized during AO exposure. Overall, the preservation of the porous structure suggests that the materials' strong light-scattering properties, crucial for high solar reflectance, remain unaffected.

[0090] Both materials display strong spectral reflectance after AO exposure. Particularly, the spectral line profile of silica after exposure is identical to its pre-exposure profile, indicating minimal effect of AO on the material. The PTFE / PEO spectral profile post-exposure exhibits a broadband behavior, evidenced by the absence of the troughs in the near-infrared spectrum. By physical observation, while the baseline Kapton material (see supplementary note 3 for full description), shows discoloration, both electrospun materials retain their bright white appearance indicating strong diffuse reflectivity. Despite the slight reduction in average solar reflectance observed for both materials-0.66% for silica and 3.3% for PTFE / PEO, they exhibited strong resistance to AO.

[0091] UV exposure. The impact of ultraviolet (UV) radiation on the materials was studied to assess their UV resistance, quantify any potential effects, and predict their in-space performance. For this study, 1.2 mm thick PTFE:PEO and 1.6 mm thick silica samples were exposed to UV radiation with an intensity of 12 W / m2 for six days (144 hours), resulting in a total dosage of 6.22 MJ / m2. Post-exposure characterization was conducted using SEM and optical spectroscopy to understand the effects of UV radiation on the materials. No observable difference could be discerned in the SEM images of either PTFE / PEO or silica nanofibers following UV exposure, with the porous structures and fiber morphologies remaining intact, and high solar reflectance being maintained. As summarized in Table 4, PTFE / PEO exhibited no significant change in average solar reflectance over the 0.3 μm to 2.5 μm range, likely due to the strength of PTFE's carbon-fluorine bonds, which impart high UV resistance to the composite. In contrast, silica showed a slight decrease of 1.3% in its average solar reflectance. Despite these minor changes, both materials retained high solar reflectance post-UV exposure, demonstrating their strong resistance to UV radiation.TABLE 4The average solar reflectance <ρs> of the electrospunmaterials before and after exposure to UV radiation.Sample<ρs> before UV exposure<ρs> after UV exposurePTFE / PEO99.8599.45Silica96.5695.2

[0092] Thermal cycling. The thermal environment of space exposes materials to rapid changes in temperature when the spacecraft moves from the planetary shadow (very low temperature) to the sun's view (high temperature). These thermal swings could induce changes in the material's structure and properties, depending on the number of thermal cycles the material is exposed to. To estimate the impact of thermal swings on the nanofibrous samples, we conducted rapid thermal shock tests on both materials, subjecting them to sudden temperature changes (between −196° C. and 75-125° C.). Scanning electron micrographs indicate that the structures and morphologies of both materials are preserved after the thermal tests. The samples remain diffusely white and bright after the tests, indicating the retainment of their strong reflective properties after the cycles. This conclusion is supported by the minimal changes in spectral and average solar reflectance (0.03% and 0.40% for PTFE / PEO and silica nanofibers, respectively) post-exposure.Materials for Terrestrial Applications

[0093] In addition to their favorable characteristics for extraterrestrial applications, electrospun PTFE / PEO materials provide exceptional performance characteristics for passive thermal control applications in terrestrial environments (as monitored by <εATW>, <as>, <ρs>, and <εATW> / <as>).

[0094] For characterizing such materials for terrestrial applications, the spectral optical properties in the long wavelength atmospheric window (ATW) of 8 μm-13 μm were determined using a Fourier-transform infrared spectrometer (ThermoFisher Scientific Nicolet™ iS20) equipped with a three inch gold-coated integrating sphere (Pike Technologies Mid-IR IntegratIR™) and a mercury cadmium telluride (MCT) detector. The average emittance of the materials was calculated according to equation (4).

[0095] FIG. 17 shows the spectral emittance (at 300 K) of PTFE / PEO 90:10 samples with thicknesses of 1.2 mm 1710 and 3.36 mm 1720 across the mid-infrared (MIR) and long-wave infrared (LWIR) spectral range (2.5-15 μm). Of particular significance is the emittance within the 8-13 μm range, between the dashed lines, which corresponds to the atmospheric transmission window. Within this wavelength range, materials can effectively couple with outer space at a temperature of 3 K, emitting thermal radiation and facilitating cooling. The 1.2 mm sample 1710 exhibited an average emittance <εATW> of 84.42% within the atmospheric transmission window, while the 3.36 mm sample 1720 averaged an <εATW> of 86.69%. As discussed above, and as is shown in FIG. 13, the average solar reflectance increases with thickness, surpassing 99% beyond a thickness of 1 mm, and saturating at 99.9% for thicknesses exceeding 3 mm.

[0096] The cooling capacity of the PTFE / PEO material can be further assessed by radiative cooling power analysis. When exposed to the sky during daytime at a temperature T, a material of surface area A, would receive solar irradiance as well as atmospheric radiation (equivalent to the ambient temperature, Tamb). The material would also experience parasitic heat fluxes due to thermal conduction and atmospheric convection, and the net radiative cooling power of the material (Pnet) is given by:Pn⁢e⁢t(T)=Po⁢u⁢t(T)-Pa⁢t⁢m(Ta⁢m⁢b)-Ps⁢o⁢l-Pp⁢a⁢r(6)where the power radiated outward Pout(T) by the material is:Po⁢u⁢t(T)=A⁢∫d⁢Ω⁢ε⁡(λ)⁢∫0∞d⁢λ⁢ε⁡(λ)⁢Eb⁢λ(λ,T)(7)where∫d⁢Ω=∫02⁢π∫0π / 2dθ sinθdφ is the angular integral over a hemisphere and Ebλ(λ, T) is the spectral blackbody emissive radiance at temperature T.The atmospheric thermal radiation absorbed by the material is given by:Pa⁢t⁢m(Ta⁢m⁢b)=A⁢∫d⁢ΩcosΘ⁢∫0∞d⁢λ⁢ε⁡(λ)⁢εa⁢t⁢m(λ)⁢Eb⁢λ(λ,Ta⁢m⁢b)(8)where εatm(λ) is directional emissivity of the atmosphere given by the equation:εatm(λ)=1-τatm(λ)1 / cos⁢ Θ(9)and τatm(λ)1 / cosθ is the atmospheric transmittance in the normal direction.The absorbed solar power is given by:Ps⁢o⁢l=A⁢∫0∞d⁢λ⁢ε⁡(λ)⁢Is⁢o⁢l⁢a⁢r(λ)(10)Using Kirchhoff's radiation law and assuming a diffuse surface, ε(λ)=a(λ) where a(λ) is the spectral solar absorptance.The parasitic heat flux is estimated as:Pp⁢a⁢r=A⁢hc(Ta⁢m⁢b-T)(11)where hc is the non-radiative, combined heat transfer coefficient to account for both conduction through support structures and convection to the ambient air.Here, the net cooling power Pnet is calculated assuming the material is held at T=Tamb=300K. Consequently, Ebλ(λ,T) is evaluated at 300K and the parasitic heat flux is zero.Radiative cooling power analysis was conducted of PTFE / PEO 90:10 samples with thicknesses of 1.2 mm and 3.36 mm, considering their optical properties. The analysis yielded the cooling power densities shown in Table 5. Both materials exhibited exceptional performance, with the 3.36 mm sample achieving a net cooling power exceeding 106 W / m2. Such a substantial cooling capacity would lead to a sizeable reduction in the need for active cooling in buildings and cars.TABLE 5Radiative cooling performance of PTFE / PEO 90:10 samples.MaterialthicknessPout (W / m2)Patm(W / m2)Psolar(W / m2)Pnet(W / m2) 1.2 mm202.9998.64.4699.933.36 mm211.03103.650.89106.49Since passive radiative cooling performance is lowered when the materials have high solar absorptance <as>, it is necessary to develop materials with large solar reflectance <ρs>=1-<as>. Likewise, a strong ATW emittance is desirable in the atmospheric transmission window <εATW> to enable self-cooling. Considering both aspects, the figure of merit, <εATW> / <as> should be large for high-performance radiative cooling materials.Referring back, FIG. 2 compares the figure of merit, <εATW> / <as> for various state-of-the-art passive radiative cooling materials having low <as> and high <εATW>. Specifically, since this study considers a surface held at 300 K, a comparison of <εATW> / <as> for different passive thermal control materials with the electrospun materials developed in this study shows that the electrospun materials outperform the state-of-the-art by orders of magnitude, indicating that they would perform remarkably well as passive radiative cooling materials, on earth, as well as in outer space.What has been described and illustrated herein is an example along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Many variations are possible within the spirit and scope of the subject matter, which is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated.

Examples

examples

Preparation of Precursor Electrospinning Solutions

To fabricate the PTFE / PEO materials, PTFE (particle size 234±59 nm) was purchased from Sigma-Aldrich as a 60 wt % dispersion in water, and PEO powder (molecular weight ˜5 million) was purchased from Beantown Chemical.

PTFE / PEO solution of the desired mass ratio (Table 2) was prepared by first dissolving PEO powder in deionized water and stirring at 60° C. and 600 rpm for 4 h using a magnetic stirrer hot plate (Thermo Scientific Cimarec) to form a 4 wt % aqueous solution. Then, PTFE dispersion was added to the PEO solution and stirred at room temperature and 600 rpm for 6 h to obtain the final PTFE / PEO composite solution. The PTFE / PEO solution was then drawn into a syringe and used for electrospinning to fabricate the electrospun nanofibrous PTFE / PEO materials.

TABLE 2Composition of PTFE / PEO electrospinning solutionsPTFE (60 wt. % in water) (g)PEO (g)water (g)PTFE:PEO (g / g)2.50.1674.0090:102.50.2656.3585:152.50.3759.0080:20

The electrosp...

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Patent Application Nos. 63 / 633,284, filed Apr. 12, 2024; 63 / 664,769, filed Jun. 27, 2024; and 63 / 664,771, filed Jun. 27, 2024; the disclosures of which are hereby incorporated by reference in their entireties.GOVERNMENT LICENSE RIGHTS

[0002] This invention was made with government support under grant number 80NSSC21K0072, awarded by the National Aeronautics and Space Administration. The government has certain rights in the invention.TECHNICAL FIELD

[0003] The present disclosure relates to materials, devices, uses and methods of passive control of heat influx and efflux, on earth, and in space.BACKGROUND

[0004] Passive heat management is useful in space, especially for extended missions involving protection from sunlight. Thermal coatings with desirable optical properties can drastically reduce the power consumed by active cooling systems, thereby reserving more resources for other systems onboard. Specifically, materials with wavelength-dependent reflectance and emittance are desirable for managing incident sunlight and self-cooling by thermal emission. On earth, passive thermal control offers a method to regulate system temperature without significant energy input. Active temperature regulation methods in buildings, such as air conditioners, entail high energy consumption and cost. Additionally, these systems often utilize refrigerants that emit environmentally harmful volatile compounds. Incorporating passive systems presents an opportunity to reduce reliance on active thermal controls.SUMMARY

[0005] According to some embodiments, a passive temperature-regulating nanofibrous material is disclosed, the material comprising nanofibers having an average diameter of between about 200 nm and about 1500 nm, wherein the material has a thickness of greater than about 400 μm; an average solar absorptance (<as>) of less than about 0.1, the average solar absorptance being determined over the range from 0.3 μm to 2.5 μm; an average thermal emittance (<εIR>) of greater than about 0.75 at 300 K, the average thermal emittance being determined over the range from 2.5 μm to 15 μm; and a figure of merit greater than about 20, the figure of merit being the ratio <εIR> / <as>. The material can have a porosity of greater than about 0.5.

[0006] According to some embodiments, the figure of merit of the material is greater than about 500. According to some embodiments, <as> of the nanofibrous material can be less than about 0.05, and can be less than about 0.005. The nanofibers of the material can be silica nanofibers.

[0007] According to some embodiments, the nanofibers can comprise a water soluble polymer, and nanobeads of a fluoropolymer, the nanobeads of the fluoropolymer being attached to surfaces of the water soluble polymer. For some such embodiments, the figure of merit is greater than about 800. For some such embodiments, the fluoropolymer is selected from the group consisting of polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy alkane (PFA), ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-hexafluoropropylene (PDVF-HFP), ethylene chlorotrifluoroethylene (ECTFE), polychlorotrifluoroethylene (PCTFE), and combinations thereof. According to some embodiments, the nanofibers comprise polyethylene oxide (PEO) nanofibers and polytetrafluoroethylene (PTFE) nanobeads, wherein the PTFE nanobeads are disposed on surfaces of the PEO nanofibers.

[0008] According to some embodiments, the weight ratio of PTFE to PEO is between about 75:25 and about 95:5. For some such embodiments, the PTFE nanobeads have an average diameter of between about 150 nm and about 300 nm. For some such embodiments, the material has a figure of merit greater than about 800.

[0009] According to some embodiments, the passive temperature-regulating material is obtained by a process of electrospinning.

[0010] According to some embodiments, a method is disclosed of obtaining a passive temperature-regulating nanofibrous material, the method comprising:

[0011] (1) dissolving a water soluble polymer in water to form a water soluble polymer solution;

[0012] (2) adding fluoropolymer nanobeads to the water soluble polymer solution to form a mixture of fluoropolymer nanobeads and water soluble polymer in water, the weight ratio of fluoropolymer to water soluble polymer being between about 75:25 and about 95:5, the fluoropolymer nanobeads having an average diameter of between about 150 nm and about 300 nm; and

[0013] (3) electrospinning the mixture of fluoropolymer nanobeads and water soluble polymer in water to obtain the passive temperature-regulating material.

[0014] According to some embodiments of this method, the water soluble polymer is PEO and the fluoropolymer nanobeads are PTFE nanobeads. According to some such embodiments, the ratio of PTFE to PEO is about 90:10.

[0015] According to some embodiments, a method is disclosed of obtaining a passive temperature-regulating nanofibrous material, the method comprising:

[0016] (1) mixing together a tetraalkyl orthosilicate, an alcohol, water, and a strong acid to form a mixture;

[0017] (2) stirring the mixture; and

[0018] (3) electrospinning the mixture to obtain the passive temperature-regulating nanofibrous material.

[0019] For some such methods, the tetraalkyl orthosilicate is tetraethyl orthosilicate, the alcohol is ethanol, and the strong acid is hydrochloric acid.BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 compares the extraterrestrial figure of merit <εIR> / <as> of electrospun PTFE / PEO and silica-based materials of the current application with materials commonly used for outer space applications.

[0021] FIG. 2 compares the terrestrial figure of merit <εATW> / <as> of electrospun PTFE / PEO materials of the current application with other materials that have been considered for passive cooling applications on earth.

[0022] FIG. 3A shows a scanning electron microscope (SEM) image for a 90:10 wt. % PTFE / PEO material, using standard fabrication conditions as described below.

[0023] FIG. 3B shows the fiber diameter size distribution for the sample of FIG. 3A.

[0024] FIG. 3C shows a scanning electron microscope (SEM) image for an 80:20 wt. % PTFE / PEO material, using standard fabrication conditions.

[0025] FIG. 3D shows the fiber diameter size distribution for the sample of FIG. 3C.

[0026] FIG. 4A shows a schematic of a procedure for preparing electrospun silica.

[0027] FIG. 4B shows an SEM image of silica nanofibers obtained using the procedure of FIG. 4A, under standard fabrication conditions.

[0028] FIG. 4C shows the fiber diameter size distribution for the sample of FIG. 4B.

[0029] FIG. 5 shows the experimental setup used for electrospinning.

[0030] FIG. 6 shows a sample of PTFE nanobeads prepared by electrospraying under standard fabrication conditions.

[0031] FIG. 7 provides experimental stress-strain curves for PTFE / PEO materials with different compositions.

[0032] FIG. 8 provides experimental determinations of ultimate tensile strength, yield strength, and toughness as a function of PTFE / PEO wt. %.

[0033] FIG. 9 provides experimental determinations of tensile modulus and elongation at break as a function of PTFE / PEO wt. %.

[0034] FIG. 10 compares the spectral normal-hemispherical reflectance in the 0.3-2.5 μm wavelength range for a PTFE / PEO 90:10 sample to the reflectance from a 80:20 sample.

[0035] FIG. 11 is an SEM image of randomly arranged fibers for a PTFE / PEO sample obtained with a rotating collector speed of 500 rpm.

[0036] FIG. 12 is an SEM image of oriented fibers for a PTFE / PEO sample obtained with a rotating collector speed of 1400 rpm.

[0037] FIG. 13 illustrates the dependence of the spectral normal-hemispherical reflectance in the 0.3-2.5 μm wavelength range on thickness of PTFE / PEO material.

[0038] FIG. 14 displays the solar reflectance of a 1.2 mm thick 90:10 PTFE / PEO sample and a 1.6 mm thick silica sample across the 0.3-2.5 μm solar wavelength range, with extraterrestrial solar irradiance at air mass 0 (AM 0) overlaid for reference.

[0039] FIG. 15 compares the IR emittance at 300 K of 90:10 PTFE / PEO, silica, double-aluminized Mylar, and aluminum foil.

[0040] FIG. 16 plots the thermal stability of silica nanofiber material and PTFE / PEO nanofiber material as characterized by thermogravimetric analysis.

[0041] FIG. 17 compares the spectral emittance (at 300 K) of PTFE / PEO 90:10 samples with thicknesses of 1.2 mm and 3.36 mm across the mid-infrared (MIR) and long-wave infrared (LWIR) spectral range (2.5-15 μm).DETAILED DESCRIPTIONMaterials for Extraterrestrial Applications.

[0042] The existence of extreme temperatures is among the most significant challenges that make space exploration difficult, with sunlight being a primary radiation source heating objects in space. While some sunlight is reflected, energy is also dissipated into the extremely cold void of space by thermal radiation emitted from the spacecraft. Unlike terrestrial conditions, convective heat transfer by ambient air is absent in outer space, making temperature control even more challenging. Without an atmosphere to absorb a portion of the solar energy, space vehicles also experience the full impact of the sun, leading to undesirably high temperatures. Hence, temperature control is desirable for space applications ranging from satellites and space stations to storage of cryogens, including propellants such as liquid hydrogen for interplanetary exploration.

[0043] In order to maintain temperatures, space missions employ various active and passive cooling technologies to reject heat. Active technologies include thermoelectric coolers, cryocoolers, pumped fluid loops, and active thermal architectures. Using active cooling alone is impractical due to large power demands and complex control systems prone to failure in the harsh space environment, making missions cost-prohibitive. Practical solutions require passive cooling to reduce power consumption. Current passive technologies used for space applications include paints, coatings, thermally conductive tapes, straps and louvers, sun shields and deployable radiators, heat pipes, phase change materials, and multilayer insulation (MLI).

[0044] Thermal coatings and MLI are typically the shiny outermost coatings of space vehicles and can include several layers of thin polymeric reflectors made of polyethylene terephthalate or polyimide films coated with vapor-deposited metal like aluminum, silver, and gold on one or both sides. For better thermal insulation, the reflector films are often separated by polymeric spacers. Table I provides optical properties of typical materials used for space applications. For such typical materials, solar absorptance <as> (averaged over wavelengths of 0.3 μm to 2.5 μm) and infrared (IR) emittance <εIR> (averaged over wavelengths of 2.5 μm to 15 μm) are around <as>=0.12-0.28 and <εIR>=0.02-0.05, respectively, which helps reflect sunlight, but the low IR emittance prevents radiative heat dissipation to space. Consequently, typical MLI and thermal coatings still absorb a sizeable portion of solar radiation while not emitting much energy to space. There is a need for advanced materials with ultra-low solar absorptance and strong IR emittance, yielding a large ratio of <εIR> / <as>.

[0045] A related measure of the solar reflective properties of materials for thermal coatings is the solar reflectance, <ρs> is defined as <ρs>=1-<as>. With this definition, ultra-low solar absorptance corresponds to solar reflectance approaching the limiting value of 1.

[0046] Some embodiments of the present disclosure present a material for passive heat management and spacecraft temperature control by employing nano-engineered, porous, spectrally selective materials with ultra-high solar reflectance and strong mid-infrared emittance. The materials are manufactured using an electrospinning process—a versatile, scalable, and economical fabrication technique to create materials with nano and microscale features. According to the present disclosure, electrospinning is used to create nanofibrous materials made of silica and of polytetrafluoroethylene (PTFE) nanobeads coating nanofibers of polyethylene oxide (PEO) (herein referenced as PTFE / PEO nanofibers). The spectral properties of these electrospun materials are also included in Table I, for comparison to the conventional passive thermal control materials used for space applications. A plot of the ratio of <εIR> / <as> for these materials is provided in FIG. 1.

[0047] The nanofibers of the electrospun materials allow strong scattering of solar radiation, yielding a high solar reflectance. In contrast to the other materials in FIG. 1, these electrospun materials also provide a high degree of thermal emittance. As we discuss further below, the mechanical (tensile) properties and the ultraviolet and atomic oxygen durability of these electrospun materials are suitable for long-duration space missions in the low Earth orbit. Because of these favorable spectral and materials properties, these electrospun materials present a paradigm for passive thermal control of spacecraft and long-duration cryogenic fluid storage for space missions.TABLE 1Comparison of the spectral properties of passive thermal control materials used forspace applications. Electrospun PTFE / PEO and silica materials of the current application are also included in the table. All values are determined at 300 K.SolarSolarThermal Materialreflectance (ρs)absorptance (αs)emittance (εIR)εIRαsKapton film withKapton-Al0.540.460.861.87Aluminum backingTeflon with goldTeflon-Au0.780.220.813.68backingMylar film withMylar-Al-10.810.190.774.05Aluminum backing 1Mylar film withMylar-Al-20.830.170.764.47Aluminum backing 2GSFC White paintGSFC-NS43C0.80.200.924.6NS43CSumitomo BakeliteS. Bakelite0.830.170.824.82GSFC White paintGSFC-MS740.830.170.925.41MS74Teflon withTeflon-Al-10.870.130.816.23Aluminumbacking 1Teflon withTeflon-Al-20.870.130.876.69Aluminumbacking 2Zinc oxide pigmentZ93SC550.860.140.946.71in potassium silicatebinderZinc ortho-titanateZnTiOx-0.870.130.927.08with PotassiumKSiOxsilicateAluminum compositeAl composite-0.910.090.728coating overAl foilAluminum foilQioptiq StandardQ-S-CMX0.90.100.868.6Optical SolarReflector CMXTeflon with silverTeflon-Ag0.910.090.889.78backingAZ TechnologyAZW / LA-II0.910.090.9110.11AZW / LA-IIQioptiq Plain OSRQ-CMO0.9150.0850.8710.24CMOHelios secondHelios-SSM-0.930.070.7911.29surface mirror / silverAgbackingOptical SolarOSR0.940.060.813.33ReflectorBarium sulphate withBaSO4-PVA0.940.060.8814.67polyvinyl alcoholElectrospun SilicaSilica-0.90.9640.0360.88524.580.9 mmElectrospun SilicaSilica-1.60.9670.0330.89527.121.6 mmElectrospun SilicaSilica-3.30.970.030.88429.473.3 mmElectrospunPTFE:PEO-1.20.9940.0040.78195PTFE / PEO 1.2 mmElectrospunPTFE:PEO-0.9990.0010.81810PTFE / PEO 3.36 mm3.36Materials for Terrestrial Applications.

[0048] Considerable research has been devoted to exploring passive radiative cooling here on Earth. For terrestrial applications, efficient passive radiative cooling requires low solar absorptance <as> (i.e. high solar reflectance) in the solar spectrum (0.3 μm to 2.5 μm) combined with high thermal emittance <εATW> in the long wavelength infrared atmospheric transmission window (8-13 μm). Emittance in the atmospheric transmission window (ATW) permits the outward emission of thermal radiation from Earth to outer space. By combining low solar absorptance <as> with high thermal emittance <εATW> in the atmospheric transmission window, terrestrial self-cooling can be achieved without the need for substantial active energy consumption. Materials having a high value of the ratio <εATW> / <as> will function more effectively as passive radiative cooling materials for terrestrial applications. A comparison of this ratio at 300 K for various state-of-the-art terrestrial passive radiative cooling materials is shown in FIG. 2. Notably, for terrestrial applications, electrospun PTFE / PEO materials of the present disclosure show orders of magnitude greater values of <εATW> / <as> than other state-of-the-art terrestrial passive radiative cooling materials. Consequently, these electrospun PTFE / PEO materials provide exceptional performance characteristics (as monitored by <εIR> / <as> and by <εATW> / <as>) for passive thermal control applications in both extraterrestrial and terrestrial environments.Fabrication of Electrospun Nanofibrous Materials

[0049] The passive temperature-regulating nanofibrous materials of the present application can be prepared by electrospinning. As shown in the scanning electron micrograph (SEM) of FIGS. 3A and 3C, in an embodiment suitable for both terrestrial and extraterrestrial applications, the electrospun material can include nanofibers 310 of a water soluble polymer, onto which nanobeads 320 of a fluoropolymer are disposed. In the embodiments of FIGS. 3A and 3B, the water soluble polymer is PEO, and the fluoropolymer is polytetrafluoroethylene (PTFE). FIGS. 3B and 3D show the fiber diameter distributions for the SEMs of FIGS. 3A and 3C, respectively. For FIGS. 3A and 3B, the ratio of PTFE to PEO is 90:10. For FIGS. 3C and 3D, the ratio of PTFE to PEO is 80:20. The preparation of the materials of FIG. 3A-3D is discussed further below.

[0050] Solutions for electrospinning are prepared by dissolving the water soluble polymer in water to form an aqueous solution of the water soluble polymer, adding fluoropolymer nanobeads to the aqueous solution to form a mixture of fluoropolymer nanobeads and water soluble polymer in water. In some embodiments, the weight ratio of fluoropolymer to water soluble polymer is between about 75:25 and about 95:5. In some such embodiments, the weight ratio is between about 85:15 and 95:5. In some such embodiments, the weight ratio is about 90:10, or about 85:15, or about 85:20. The fluoropolymer nanobeads can have an average diameter of between about 150 nm and about 300 nm. According to some embodiments, the water soluble polymer is PEO. For some embodiments, the water soluble polymer can be polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyvinylpyrrolidone (PVP) or carboxymethylcellulose (CMC). The fluoropolymer can be selected from the group consisting of polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy alkane (PFA), ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-hexafluoropropylene (PDVF-HFP), ethylene chlorotrifluoroethylene (ECTFE), polychlorotrifluoroethylene (PCTFE), and combinations thereof. According to some embodiments, the fluoropolymer is PTFE. According to some embodiments, the average fiber diameter is between about 200 nm and about 1500 nm. According to the embodiments of FIG. 3A-3D, the water soluble polymer is PEO and the fluoropolymer is PTFE.

[0051] In other embodiments, according to the reaction pathway of FIG. 4A and the SEM of FIG. 4B, electrospun materials composed of silica fibers 410 provide materials suitable for extraterrestrial applications. Compared to the nanofibrous PTFE / PEO materials of FIGS. 3A-3D, the silica nanofibers of FIG. 4B are thinner, having an average diameter 261±86, with a distribution of diameters shown in FIG. 4C. For these silica nanofibrous materials, solutions for electrospinning can be prepared by mixing together and stirring a tetraalkyl orthosilicate, an alcohol, water, and a strong acid to form a mixture, followed by electrospinning the material to obtain a passive temperature-regulating nanofibrous material composed of silica fibers 410. In the example embodiment of FIGS. 4A to 4C, the tetralkyl orthosilicate is tetraethyl orthosilicate, the alcohol is ethanol, and the strong acid is hydrochloric acid.

[0052] As illustrated in FIG. 5, an electrospinning apparatus 510 of the type used in the experiments described below includes a syringe pump 520 with a syringe needle 530 connected to a high voltage power supply 540 and a grounded rotating collector 550, configured to rotate with a D.C. motor 560. During the electrospinning process, a polymeric solution 570 for electrospinning is extruded from the syringe needle at a constant rate. As the liquid droplet becomes charged, electrostatic repulsion overcomes surface tension of the droplet, causing a stream of polymeric solution 570 to travel from the syringe toward the rotating grounded collector. As the stream of polymeric solution 570 moves towards the collector, the liquid evaporates, causing a layer of polymeric nanofibers to deposit on the surface of the grounded rotating collector 550.Characterization of Electrospun Nanofibrous Materials

[0053] The morphology of the nanofibrous materials of the present disclosure were characterized by scanning electron microscopy (SEM), recorded using a Carl Zeiss Supra 55 Field Emission Scanning Electron Microscope (FESEM) at a working distance of 3 mm, an accelerating voltage of 1.5 kV, and a current of 5 pA. Before imaging each sample, a 1 cm square piece was cut and placed in a Denton Platinum Sputter System, and a 2 nm-thick layer of platinum was deposited on the sample surface. Image J software was used for data processing and analysis of the SEM images.

[0054] The spectral normal-hemispherical reflectance and transmittance of the samples were characterized using a UV-Visible-NIR spectrometer (Perkin-Elmer Lambda 950) equipped with a 6-inch integrating sphere using a NIST traceable Labsphere Spectralon® diffuse reflectance standard. The average solar reflectance <ρs> was calculated using Equation 1:<ρs>=∫λ=0.3 μ⁢mλ=2.5 μ⁢mρλ.Iλ⁢d⁢λ∫λ=0.3 μ⁢mλ=2.5 μ⁢mIλ⁢d⁢λ(1)where pλ represents the spectral normal-hemispherical reflectance obtained from the spectrometer and Iλ is the solar spectral intensity using the extraterrestrial spectrum (ASTM E490 Air Mass 0). The average solar absorptance <as> is calculated as <as>=1-<ρs>.To characterize materials for extraterrestrial applications, the spectral optical properties in the infrared wavelength range (2.5-15 μm) were characterized using a Fourier-Transform Infrared spectrometer (ThermoFisher Scientific Nicolet™ iS20) interfaced with a 3-inch gold-coated integrating sphere (Pike Technologies Mid-IR IntegratIR™) and a mercury cadmium telluride detector. The average emittance <εIR> of the materials is calculated using the following equation:εIR=∫λ=2.5 μ⁢mλ=1.5 μ⁢mελ.IB⁢B(λ,T)⁢d⁢λ∫λ=2.5 μ⁢mλ=1.5 μ⁢mIB⁢B(λ,T)⁢d⁢λ(2)where ελ represents the spectral normal-hemispherical emittance obtained from the FTIR spectrometer and IBB(λ, T) is the spectral blackbody emissive radiance at 300 K:IB⁢B(λ,T)=2⁢h⁢c2λ5(eh⁢cλ⁢KB⁢T-1)(3)where h, c, λ, KB, T represent Planck's constant, speed of light in vacuum, wavelength, Boltzmann's constant, and absolute temperature of the blackbody, respectively.For terrestrial applications, the relevant thermal emittance is the thermal emittance <εATW> in the long wavelength infrared atmospheric transmission window between 8-13 μm, calculated as:εA⁢T⁢W=∫λ=8⁢ μ⁢mλ=13⁢ μ⁢mελ.IB⁢B(λ,T)⁢d⁢λ∫λ=8⁢ μ⁢mλ=13⁢ μ⁢mIB⁢B(λ,T)⁢d⁢λ(4)where IBB is the spectral blackbody emissive radiance according to equation (1).The thermal degradation behavior of the nanofibrous materials was studied using thermogravimetric analysis (TGA-Q50) by heating 7.5 mg of each sample from 25° C. to 800° C. at a temperature ramp rate of 10° C. / min.Durability tests were conducted to evaluate the resilience of electrospun nanofibrous materials in environments simulating the harsh conditions of outer space. The electrospun materials were subjected to ultraviolet (UV) radiation, atomic oxygen (AO) and extreme temperature swing cycles, followed by characterization, as described in more detail below.EXAMPLESPreparation of Precursor Electrospinning SolutionsTo fabricate the PTFE / PEO materials, PTFE (particle size 234±59 nm) was purchased from Sigma-Aldrich as a 60 wt % dispersion in water, and PEO powder (molecular weight ˜5 million) was purchased from Beantown Chemical.PTFE / PEO solution of the desired mass ratio (Table 2) was prepared by first dissolving PEO powder in deionized water and stirring at 60° C. and 600 rpm for 4 h using a magnetic stirrer hot plate (Thermo Scientific Cimarec) to form a 4 wt % aqueous solution. Then, PTFE dispersion was added to the PEO solution and stirred at room temperature and 600 rpm for 6 h to obtain the final PTFE / PEO composite solution. The PTFE / PEO solution was then drawn into a syringe and used for electrospinning to fabricate the electrospun nanofibrous PTFE / PEO materials.TABLE 2Composition of PTFE / PEO electrospinning solutionsPTFE (60 wt. % in water) (g)PEO (g)water (g)PTFE:PEO (g / g)2.50.1674.0090:102.50.2656.3585:152.50.3759.0080:20The electrospinning solution for silica nanofibers was prepared by mixing tetramethyl orthosilicate, ethanol, deionized water, and a 37% aqueous solution of hydrochloric acid. This mixture was stirred at 40° C. and 200 rpm for six hours to ensure thorough mixing. After preparation, the solution was transferred to a syringe for use in the electrospinning process.Fabrication ProcessGeneral ProcedureThe electrospinning process used 22-gauge needles (0.508 mm internal diameter), syringes, and an aluminum foil substrate (99% purity) purchased from McMaster-Carr. Deionized water used in the fabrication was obtained from the RPI's Center for Materials, Devices, and Integrated Systems (CMDIS).

[0063] As shown in FIG. 5, the electrospinning setup 510 included a variable high-voltage power supply 540 (Gamma High Voltage Research), a grounded rotating drum nanofiber collector 550, a syringe pump 520 interfaced with the 22-gauge syringe needle 530. The electrospinning process was performed in a temperature and humidity controlled chamber with temperature and humidity sensors (B&K Precision 725 datalogging humidity and temperature meter, not shown).

[0064] The process was performed within a temperature- and humidity-controlled chamber equipped with a temperature and pressure sensor (BK Precision 725 Datalogging Humidity and Temperature Meter). The electrospinning solution-loaded syringe was mounted on the pump, and the high-voltage power supply 540 was connected to the tip of the syringe needle 530. The grounded rotating drum collector 550 was overlaid with an aluminum foil substrate (not shown) on which the electrospun nanofibers were deposited. The solution was made to flow through the syringe needle 530 by operating the syringe pump 520 while an electric potential was applied between the syringe needle 530 and the rotating collector 550. As the applied potential was increased, the polymeric solution 570 at the tip of the needle changed from a hemispherical shape to the desired Taylor cone. As the voltage was increased beyond a threshold voltage, a the polymeric solution 570 was ejected as a jet from the tip of the Taylor cone. The jet comprising the polymeric solution 570 dried out while undergoing an in-flight whipping motion as it approached the collector. The nanoscale fibers accumulated on the collecting substrate to form a physically visible layer of bright white color. The electrospinning process was stopped when the desired thickness of the electrospun material was obtained. The overall thickness was measured using a vernier caliper at different sample locations, and an average thickness was recorded. The electrospun nanofibrous materials with the aluminum foil substrate were removed from the collector and allowed to dry for about 12 h before material characterization.Parametric Study

[0065] A baseline study of the effect of fabrication parameters on physical and optical properties of the electrospun materials was performed to define standard parameters. For the PTFE / PEO system, the first parameter tested was the relative solution concentrations of PTFE and PEO. As a baseline, a dispersion of PTFE was deposited on the collector using the following fabrication parameters: a needle to collector distance of 12 cm, a potential of 10 kV between the syringe needle 530 and the grounded rotating collector 550, a solution flow rate of 0.5 ml / h, ambient temperature 21±3° C., and relative humidity of 20±4%. An SEM of the deposited material is shown in FIG. 6. The baseline experiment shows that shows that although the fabrication process deposits PTFE on the collector substrate, it produces nano and microscale beads with 234±59 nm particles and 5±1.5 μm clumps. These conditions do not produce nanofibers. The PTFE beads lack sufficient adhesion to the substrate and could be easily separated from the substrate due to the low surface energy of PTFE.

[0066] The formation of beads by electro-spraying and fibers via electrospinning depends on several factors, including the dispersion's molecular weight, conductivity, surface tension, and viscosity. Although PTFE provides good UV stability and chemical resistance, it exhibits relatively high electrical resistance and dielectric strength. These properties, along with the high surface tension and low viscosity of the PTFE dispersion, favor electrospraying of beads. With the addition of PEO, the PTFE:PEO composite solution yields nanofibers via electrospinning. With the addition of PEO, while the PTFE clumps are no longer produced, the individual PTFE beads (234±59 nm) append to the PEO nanofibers, as shown in FIGS. 3A and 3C.

[0067] The effect of PEO concentration. The PTFE / PEO mixture combines the complementary properties of both polymers to create a PTFE / PEO composite structure. FIGS. 3A and 3C show the different materials produced from PTFE / PEO dispersions of 90:10 and 80:20 composite solutions using the following electrospinning parameters: a needle-to-collector distance of 12 cm, a potential of 10 kV between the needle and the collector, a solution flow rate of 0.5 mL / h, a rotating collector speed of 500 rpm, ambient temperature of 21±3° C., and relative humidity of 20±4%. These materials comprise smooth and fine PEO nanofibers 310 with PTFE beads 320 attached to the PEO nanofiber surface.

[0068] The 90:10 PTFE / PEO solution produces fibers with an average diameter of 1050 nm (FIG. 3A), which is significantly larger than the average fiber diameter of 516 nm produced using the 80:20 solution (FIG. 3C). An intermediate average fiber diameter of 616 nm is produced using an 85:15 PTFE / PEO solution (data not shown). The 90:10 solution fibers contain PTFE nanobeads of diameter 234±59 nm. The PTFE nanobeads embed themselves on the PEO fibers during electrospinning. Thus, comparing the baseline experiment containing only PTFE (FIG. 6) with these various PTFE / PEO solutions, it is clear that PEO facilitates the formation of fibers attached to PTFE nanobeads, unlike the baseline (FIG. 6), where PTFE is clumped together. Decreasing the amount of PTFE in the solution reduces the fiber diameter since fewer PTFE beads are attached to the much thinner PEO fibers (˜300 nm diameter).

[0069] The effect of the fiber morphology due to a variation in PTFE and PEO concentrations affects the mechanical performance, as determined from tensile strength measurements. Three samples of different PTFE / PEO ratios, specifically 90:10, 85:15, and 80:20 samples, were characterized in tension using the Instron 3345 Universal Testing Machine, following the ASTM D882-18 testing procedure. This procedure defines the standard testing method for characterizing the tensile properties of thin polymeric materials. The samples of similar dimensions, namely length×width×thickness of 45 mm×17 mm×0.35 mm, were cut into dog-bone shapes with mid-region dimensions of 8.5 mm width and 35 mm length, in line with the testing procedure to prevent rupture of the samples at the instrument grips and to ensure that deformation took place at the neck (mid-region). Each end of a sample was glued between cardboard sheets, and the glued cardboard sheets were placed between the instrument grips. This gluing to cardboard sheets was done to prevent the puncture and tearing of the fibrous coatings by the knurled teeth of the instrument grips. A slow crosshead speed of 1 mm / min was maintained across all three samples to allow uniform deformation and reduce inaccuracies in results from the tensile loads being applied too quickly. The measured tensile properties are given in Table 3, and plotted in FIGS. 7 through 9. FIG. 7 shows stress vs. strain plots for the 90:10 PTFE / PEO sample 710, the 85:15 PTFE / PEO sample 720, and the 80:20 PTFE / PEO sample 730. FIG. 8 shows the ultimate tensile strength 810 (solid line), the yield strength 820 (dashed line), and the toughness 830 (dotted line) plotted against the wt. % of PTFE / PEO. FIG. 9 shows the tensile modulus 910 (solid line) and the elongation at break 920 (dashed line).TABLE 3Tensile properties of electrospun nanofibrous coatings.UltimateTensileYieldTensileElongationPTFE / PEOStrengthStrengthmodulusToughnessatratio(MPa)(MPa)(MPa)(MJ / m3)break (%)90:100.1500.132.430.02943085:150.4270.292.350.16896080:200.4310.373.50.080627

[0070] The tests reveal that the concentration of PEO was increased from 10 wt. % to 20 wt. % (consistent with a decrease in PTFE content), the yield strength of the coating increases. This result implies that coatings with a higher PEO content allow more elastic deformation before the onset of plastic deformation. The ultimate tensile strength also increases with the increase in PEO content contained in the coatings. This increase in coating strength is traceable to the smaller individual fiber diameters obtained with the increase in PEO content. This decrease in fiber diameter causes a reduction in the cross-sectional area of the fiber, allowing more stress to be borne by the individual fiber. This result is in agreement with the equation:σ=FA(5)where σ represents the stress (in MPa), F is the force or load (in N) borne by the fiber, and A (m2) represents the cross-sectional area of the cylindrical fiber.Because all three coatings were of similar thickness, width, and cross-sectional area when they were prepared for testing, the increase in strength while increasing PEO concentration (decrease in fiber diameter) indicates that the diameters of the individual fibers play a significant role in determining the material strength achievable.

[0072] Increasing the PEO content from 10 wt. % (PTFE:PEO 90:10) to 15 wt. % (PTFE:PEO 85:15) also led to a decrease in the tensile modulus of elasticity of the coating (reduction in coating stiffness), with a significant increase in deformation as seen in FIG. 9.

[0073] The tensile (Young's) modulus E is defined by E=σε where σ represents the stress (in MPa), and ε represents the strain. The PTFE / PEO 85:15 sample was more ductile, allowing the most plastic deformation, and it had the highest toughness of all three materials (FIG. 8). The toughness (unit energy per unit volume) was obtained by integrating the area under the stress-strain plot, and that the PTFE / PEO 85:15 coating had the largest area means that it absorbed the most energy before fracturing at 60% elongation. A further increase in the PEO content by 5 wt. % (PTFE / PEO 80:20) led to a 55% decrease in the elongation-at-break, a 50% increase in stiffness, and a 52% decrease in the toughness of the coating. The PTFE / PEO 85:15 coating had the best tensile properties due to its superior strength, ductility, toughness, and resistance to plastic deformation before breaking. However, all three ratios of PTFE to PEO provided materials with suitable mechanical properties to function efficiently in outer space applications.

[0074] FIG. 10 compares the spectral normal-hemispherical reflectance in the 0.3-2.5 μm wavelength range for the PTFE / PEO 90:10 sample 1010 to the reflectance from the 80:20 sample 1020. For the sake of clarity, the curve for the 85:15 sample is not shown, since it falls squarely between the 90:10 and the 80:20 curves. The (terrestrial) solar spectral intensity 1030 is shown for reference.

[0075] Both samples, each with a thickness of 0.3 mm, demonstrate exceptional reflectivity to incident solar radiation, with the average spectral reflectance (<ρs>) nearing 100%. However, the 90:10 sample 1010 exhibited a slightly higher average spectral reflectance of 96.7% compared to the 80:20 sample 1020 (96%). This marginal difference can be attributed to the larger fibers and broader distribution of fiber diameters in the 90:10 sample 1010, which likely contributed to enhanced reflectance by scattering a wider range of incident radiation spectra compared to the 80:20 sample 1020.

[0076] The effect of rotating collector speed. FIGS. 11 and 12 compare the effects of varying the rotational speed of the grounded collector using 90:10 PTFE:PEO solution and maintaining the same fabrication parameters while changing the rotating drum collector speed. Four different speeds of 500, 1000, 1400, and 2100 rpm were used in this study. FIG. 11 shows an SEM image for a speed of 500 rpm, whereas FIG. 12 shows an SEM image for a speed of 1400 rpm. As seen in FIG. 11, a rotating collector speed of 500 rpm resulted in randomly oriented fibers. With speeds exceeding 1000 rpm, the fiber orientation changes, with most fibers aligning in the direction of rotation, as can be seen in FIG. 12, which shows an SEM image for a speed of 1400 rpm.

[0077] Higher rotational speeds also decrease the diameters of the nanofibers, consistent with the fibers experiencing increased stretching and drying, leading to further thinning compared to the baseline case, which involves a lower speed. By doubling the speed of the drum from 500 to 1000 rpm, there is a sharp decrease in the average fiber diameter size by more than half, from 1035±163 nm to 463±107 nm. The SEM images of FIGS. 11 and 12 clearly show this reduction in fiber sizes. Subsequent increases in the speeds led to a further decrease in the fiber diameter, with an average diameter of 374±88 at 1400 rpm (FIG. 12). Hence, changing the speed of the rotating drum is one verifiable way of altering both the fiber geometry and orientation simultaneously. At four times the baseline speed (2100 rpm), the sample has an average diameter of 341 nm, a 3-times reduction from the 1035 nm average diameter of the baseline sample at 500 rpm. The reduction in fiber diameter at higher speeds correlates with a corresponding reduction in reflectance. A comparison of two samples having the same thickness of 350 μm shows an increase in the average solar reflectance from 95.2% to 96.2% on increasing the collector speed from 500 to 2100 rpm. Without being bound by theory, this increase in reflectance for the 2100 rpm sample compared to the 500 rpm sample can most reasonably be attributed to the thinner fibers of the 2100 rpm samples, which are better at scattering light even at shorter wavelengths. In addition, the 2100 rpm fibers show a uniaxial alignment due to the higher collecting speeds, which allows closer packing, affecting light propagation through the sample. A wider range of diameters would perform well in reflecting longer wavelength light, allowing broadband reflectance in the NIR region. The 500 rpm sample could be marginally more transmissive due to the large pore sizes between the larger fiber strands.

[0078] The effect of material thickness. FIG. 13 shows the effect of material thickness on the average solar reflectance <ρs> of a 90:10 ratio sample of PTFE / PEO, prepared with a rotating collector speed of 500 rpm. For relatively thick layers (420±12 to 3360±20 μm) the effect of the substrate is insignificant since the electrospun layer is nearly opaque to the incident light (300-2500 nm). Thicker samples have little to no transmittance, and the incident light is scattered by the fibers, causing diffuse reflection. While thinner samples also exhibit relatively high reflectance, they tend to be marginally transparent. Consequently, some light is transmitted through the fibers to reach the substrate. In this case, the substrate or background material can affect the average solar reflectance. This study uses a thin aluminum foil as a substrate since it is relatively inexpensive compared to alternatives like silver coatings used in space applications. Besides offering superior reflectivity, thicker samples would be helpful in space applications requiring insulation against thermal conduction. The thicker materials with low thermal conductivities of the polymers can enhance the thermal resistance to insulate objects from excessive heating.

[0079] FIG. 13 demonstrates that the average solar reflectance <ρs> of the 90:10 PTFE / PEO material, while high for all thicknesses examined, increases as the material thickness increases from 220 to 3360 μm, ranging from 94.7±0.38% for a 220 μm sample to 99.75±0.40% at 1560±20 μm.

[0080] Beyond this thickness, the reflectance saturates at 99.9%. The average thermal emittance at 300 K of relatively thick samples having zero transmittance also increases with thickness. Specifically, for sample thicknesses of 1200, 1560, and 3360 μm, the average thermal emittance at 300 K is 78.32, 81.14, and 81.42%, respectively.Standard Parameters

[0081] Based on the above parametric studies, similar electrospinning parameters (500-rpm rotating collector speed, 0.5 ml / hr. flow rate, and 12 cm needle-to-collector distance) were used to fabricate both silica and PTFE / PEO materials. However, 10 kV needle-collector DC potentials were used to fabricate the PTFE / PEO materials and 18 kV potentials were used for the silica materials. The electrospinning chamber was maintained at a temperature of 20±5° C. and a relative humidity of 20±4%. Unless otherwise indicated, PTFE / PEO materials were 90:10 wt. % PTFE:PEO.Characterization of Materials for Extraterrestrial Applications

[0082] Space materials must meet rigorous requirements, including chemical and thermal stability, along with resistance to ultraviolet radiation and atomic oxygen. These criteria guided the selection of PTFE, known for its chemical stability, and silica, for its high-temperature resistance and excellent atomic oxygen resilience.Spectral Properties

[0083] Computational simulations predict a material's optical response by modeling its interaction with light by solving Maxwell's equations. The simulations indicate that the reflectances of these materials, although affected by many geometric features, generally improve with increased material thickness, as is observed experimentally (FIG. 13). These simulations and experimental results informed our choice to fabricate thicker samples for reflectance, transmittance, and absorptance characterization. FIG. 14 displays the solar reflectance of a 1.2 mm thick 90:10 PTFE / PEO sample 1410 and a 1.6 mm thick silica sample 1420 across the 0.3-2.5 μm solar wavelength range, with extraterrestrial solar irradiance 1450 at air mass 0 (AM 0) overlaid for reference. The spectral solar reflectance of conventional materials-double-aluminized Mylar 1430 and aluminum foil 1440 are included in this plot for comparison. FIG. 15 shows the IR emittance at 300 K of 90:10 PTFE / PEO 1510, of silica 1520, of double-aluminized Mylar 1530, and of aluminum foil 1540. Both nanofibrous materials exhibit strong selectivity, with high solar reflectance and strong infrared emittance, which is useful for space applications demanding low solar absorption and passive self-cooling. Electrospun silica shows more spectral selectivity compared to PTFE / PEO due to its smaller average fiber diameter, which enhances scattering at the shorter wavelengths. Conversely, PTFE / PEO, with a broader fiber distribution, effectively scatters across UV, visible, and infrared ranges, yielding a higher overall solar reflectance.

[0084] The average solar reflectance for 1.2 mm thick PTFE / PEO and 1.6 mm silica samples is 99.85% and 96.56%, respectively, with thermal emittance values of 78.3% and 89.5%. These results indicate excellent passive radiative heat dissipation due to low solar absorptance and high emissivity. The bright, diffusely white appearance of these materials underscores their strong solar reflectivity. Additional tests with various thicknesses showed that a 3.36 mm thick PTFE / PEO sample can achieve close to ˜99.9% reflectance, marking one of the highest solar reflectances for passive thermal control materials.Morphology

[0085] The morphologies of the electrospun PTFE / PEO and silica materials, shown in FIG. 3A-3D and FIGS. 4A-D, respectively, reveal porous, randomly oriented nanoscale fibers, which promote light scattering, resulting in strong reflectance. Additionally, the porous structure contributes to the material's lightweight properties, a critical requirement for space applications. Different precursor materials yield distinct fiber textures—the silica nanofibers have a smooth surface, while PTFE / PEO fibers, with PTFE beads of 234±59 nm attached to PEO fibers, show a rough texture. SEM images reveal that PTFE / PEO fibers tested here are about four times thicker than silica fibers. This size dissimilarity means that the smaller silica fibers are more effective in scattering shorter wavelengths, while the PTFE / PEO's broader range of diameters scatter light across a wider wavelength range encompassing the UV, visible, and infrared. Both materials, however, demonstrate outstanding reflective properties and thermal stability at high temperatures.Thermal Stability

[0086] The thermal stability of silica nanofiber material and PTFE / PEO nanofiber material was characterized by thermogravimetric analysis, as plotted in FIG. 16. The PTFE / PEO nanofiber mass 1610 experienced a two-stage mass-loss decomposition process involving decomposition of PEO followed by the decomposition of PTFE, leaving behind a residual weight of less than 1% at temperatures above 650° C. By contrast, the silica nanofiber mass 1620 exhibited a gradual mass loss as the temperature increased, due to the cleavage of chemical bonds between the silica and other substituent groups attached to the silica. The plateauing of the silica nanofiber mass at a stable residual weight of ˜75% at temperatures above 650° C. indicates good thermal stability for high temperature applications.Durability in Space

[0087] The extraterrestrial environment presents harsh conditions that could adversely impact the performance of spacecraft materials. Therefore, it is essential to evaluate these materials through Earth-based experiments before in-flight space tests. Ground testing provides valuable performance data, allowing for further design and manufacturing optimization before undertaking the costly and logistically complex in-space evaluations. The electrospun materials were subjected to ultraviolet (UV) radiation, atomic oxygen (AO) and extreme temperature swing cycles, followed by characterization, as described subsequently

[0088] Atomic oxygen. Atomic oxygen (AO) in low Earth orbit (LEO) results from the photodissociation of molecular oxygen by high-energy ultraviolet radiation. Due to its high reactivity and impact energy, AO is known to oxidize and erode the external surfaces of spacecraft materials. The resilience of external surface coatings to AO is crucial for determining their long-term durability and the retention of optical properties, making it essential to assess the impact of AO on these materials.

[0089] SEM images indicate that the fibrous, porous structures of both materials remained largely intact following AO exposure (data not shown). The silica fibers retained their smooth morphologies post-exposure. PTFE:PEO nanofibers which features PTFE nanobeads embedded on PEO fibers, also preserved its rough morphology, although some small, random strands appear to have been oxidized during AO exposure. Overall, the preservation of the porous structure suggests that the materials' strong light-scattering properties, crucial for high solar reflectance, remain unaffected.

[0090] Both materials display strong spectral reflectance after AO exposure. Particularly, the spectral line profile of silica after exposure is identical to its pre-exposure profile, indicating minimal effect of AO on the material. The PTFE / PEO spectral profile post-exposure exhibits a broadband behavior, evidenced by the absence of the troughs in the near-infrared spectrum. By physical observation, while the baseline Kapton material (see supplementary note 3 for full description), shows discoloration, both electrospun materials retain their bright white appearance indicating strong diffuse reflectivity. Despite the slight reduction in average solar reflectance observed for both materials-0.66% for silica and 3.3% for PTFE / PEO, they exhibited strong resistance to AO.

[0091] UV exposure. The impact of ultraviolet (UV) radiation on the materials was studied to assess their UV resistance, quantify any potential effects, and predict their in-space performance. For this study, 1.2 mm thick PTFE:PEO and 1.6 mm thick silica samples were exposed to UV radiation with an intensity of 12 W / m2 for six days (144 hours), resulting in a total dosage of 6.22 MJ / m2. Post-exposure characterization was conducted using SEM and optical spectroscopy to understand the effects of UV radiation on the materials. No observable difference could be discerned in the SEM images of either PTFE / PEO or silica nanofibers following UV exposure, with the porous structures and fiber morphologies remaining intact, and high solar reflectance being maintained. As summarized in Table 4, PTFE / PEO exhibited no significant change in average solar reflectance over the 0.3 μm to 2.5 μm range, likely due to the strength of PTFE's carbon-fluorine bonds, which impart high UV resistance to the composite. In contrast, silica showed a slight decrease of 1.3% in its average solar reflectance. Despite these minor changes, both materials retained high solar reflectance post-UV exposure, demonstrating their strong resistance to UV radiation.TABLE 4The average solar reflectance <ρs> of the electrospunmaterials before and after exposure to UV radiation.Sample<ρs> before UV exposure<ρs> after UV exposurePTFE / PEO99.8599.45Silica96.5695.2

[0092] Thermal cycling. The thermal environment of space exposes materials to rapid changes in temperature when the spacecraft moves from the planetary shadow (very low temperature) to the sun's view (high temperature). These thermal swings could induce changes in the material's structure and properties, depending on the number of thermal cycles the material is exposed to. To estimate the impact of thermal swings on the nanofibrous samples, we conducted rapid thermal shock tests on both materials, subjecting them to sudden temperature changes (between −196° C. and 75-125° C.). Scanning electron micrographs indicate that the structures and morphologies of both materials are preserved after the thermal tests. The samples remain diffusely white and bright after the tests, indicating the retainment of their strong reflective properties after the cycles. This conclusion is supported by the minimal changes in spectral and average solar reflectance (0.03% and 0.40% for PTFE / PEO and silica nanofibers, respectively) post-exposure.Materials for Terrestrial Applications

[0093] In addition to their favorable characteristics for extraterrestrial applications, electrospun PTFE / PEO materials provide exceptional performance characteristics for passive thermal control applications in terrestrial environments (as monitored by <εATW>, <as>, <ρs>, and <εATW> / <as>).

[0094] For characterizing such materials for terrestrial applications, the spectral optical properties in the long wavelength atmospheric window (ATW) of 8 μm-13 μm were determined using a Fourier-transform infrared spectrometer (ThermoFisher Scientific Nicolet™ iS20) equipped with a three inch gold-coated integrating sphere (Pike Technologies Mid-IR IntegratIR™) and a mercury cadmium telluride (MCT) detector. The average emittance of the materials was calculated according to equation (4).

[0095] FIG. 17 shows the spectral emittance (at 300 K) of PTFE / PEO 90:10 samples with thicknesses of 1.2 mm 1710 and 3.36 mm 1720 across the mid-infrared (MIR) and long-wave infrared (LWIR) spectral range (2.5-15 μm). Of particular significance is the emittance within the 8-13 μm range, between the dashed lines, which corresponds to the atmospheric transmission window. Within this wavelength range, materials can effectively couple with outer space at a temperature of 3 K, emitting thermal radiation and facilitating cooling. The 1.2 mm sample 1710 exhibited an average emittance <εATW> of 84.42% within the atmospheric transmission window, while the 3.36 mm sample 1720 averaged an <εATW> of 86.69%. As discussed above, and as is shown in FIG. 13, the average solar reflectance increases with thickness, surpassing 99% beyond a thickness of 1 mm, and saturating at 99.9% for thicknesses exceeding 3 mm.

[0096] The cooling capacity of the PTFE / PEO material can be further assessed by radiative cooling power analysis. When exposed to the sky during daytime at a temperature T, a material of surface area A, would receive solar irradiance as well as atmospheric radiation (equivalent to the ambient temperature, Tamb). The material would also experience parasitic heat fluxes due to thermal conduction and atmospheric convection, and the net radiative cooling power of the material (Pnet) is given by:Pn⁢e⁢t(T)=Po⁢u⁢t(T)-Pa⁢t⁢m(Ta⁢m⁢b)-Ps⁢o⁢l-Pp⁢a⁢r(6)where the power radiated outward Pout(T) by the material is:Po⁢u⁢t(T)=A⁢∫d⁢Ω⁢ε⁡(λ)⁢∫0∞d⁢λ⁢ε⁡(λ)⁢Eb⁢λ(λ,T)(7)where∫d⁢Ω=∫02⁢π∫0π / 2dθ sinθdφ is the angular integral over a hemisphere and Ebλ(λ, T) is the spectral blackbody emissive radiance at temperature T.The atmospheric thermal radiation absorbed by the material is given by:Pa⁢t⁢m(Ta⁢m⁢b)=A⁢∫d⁢ΩcosΘ⁢∫0∞d⁢λ⁢ε⁡(λ)⁢εa⁢t⁢m(λ)⁢Eb⁢λ(λ,Ta⁢m⁢b)(8)where εatm(λ) is directional emissivity of the atmosphere given by the equation:εatm(λ)=1-τatm(λ)1 / cos⁢ Θ(9)and τatm(λ)1 / cosθ is the atmospheric transmittance in the normal direction.The absorbed solar power is given by:Ps⁢o⁢l=A⁢∫0∞d⁢λ⁢ε⁡(λ)⁢Is⁢o⁢l⁢a⁢r(λ)(10)Using Kirchhoff's radiation law and assuming a diffuse surface, ε(λ)=a(λ) where a(λ) is the spectral solar absorptance.The parasitic heat flux is estimated as:Pp⁢a⁢r=A⁢hc(Ta⁢m⁢b-T)(11)where hc is the non-radiative, combined heat transfer coefficient to account for both conduction through support structures and convection to the ambient air.Here, the net cooling power Pnet is calculated assuming the material is held at T=Tamb=300K. Consequently, Ebλ(λ,T) is evaluated at 300K and the parasitic heat flux is zero.Radiative cooling power analysis was conducted of PTFE / PEO 90:10 samples with thicknesses of 1.2 mm and 3.36 mm, considering their optical properties. The analysis yielded the cooling power densities shown in Table 5. Both materials exhibited exceptional performance, with the 3.36 mm sample achieving a net cooling power exceeding 106 W / m2. Such a substantial cooling capacity would lead to a sizeable reduction in the need for active cooling in buildings and cars.TABLE 5Radiative cooling performance of PTFE / PEO 90:10 samples.MaterialthicknessPout (W / m2)Patm(W / m2)Psolar(W / m2)Pnet(W / m2) 1.2 mm202.9998.64.4699.933.36 mm211.03103.650.89106.49Since passive radiative cooling performance is lowered when the materials have high solar absorptance <as>, it is necessary to develop materials with large solar reflectance <ρs>=1-<as>. Likewise, a strong ATW emittance is desirable in the atmospheric transmission window <εATW> to enable self-cooling. Considering both aspects, the figure of merit, <εATW> / <as> should be large for high-performance radiative cooling materials.Referring back, FIG. 2 compares the figure of merit, <εATW> / <as> for various state-of-the-art passive radiative cooling materials having low <as> and high <εATW>. Specifically, since this study considers a surface held at 300 K, a comparison of <εATW> / <as> for different passive thermal control materials with the electrospun materials developed in this study shows that the electrospun materials outperform the state-of-the-art by orders of magnitude, indicating that they would perform remarkably well as passive radiative cooling materials, on earth, as well as in outer space.What has been described and illustrated herein is an example along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Many variations are possible within the spirit and scope of the subject matter, which is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated.

Examples

examples

Preparation of Precursor Electrospinning Solutions

To fabricate the PTFE / PEO materials, PTFE (particle size 234±59 nm) was purchased from Sigma-Aldrich as a 60 wt % dispersion in water, and PEO powder (molecular weight ˜5 million) was purchased from Beantown Chemical.

PTFE / PEO solution of the desired mass ratio (Table 2) was prepared by first dissolving PEO powder in deionized water and stirring at 60° C. and 600 rpm for 4 h using a magnetic stirrer hot plate (Thermo Scientific Cimarec) to form a 4 wt % aqueous solution. Then, PTFE dispersion was added to the PEO solution and stirred at room temperature and 600 rpm for 6 h to obtain the final PTFE / PEO composite solution. The PTFE / PEO solution was then drawn into a syringe and used for electrospinning to fabricate the electrospun nanofibrous PTFE / PEO materials.

TABLE 2Composition of PTFE / PEO electrospinning solutionsPTFE (60 wt. % in water) (g)PEO (g)water (g)PTFE:PEO (g / g)2.50.1674.0090:102.50.2656.3585:152.50.3759.0080:20

The electrosp...

Claims

1. A temperature-regulating material comprising:nanofibers having an average diameter of between about 200 nm and about 1500 nm, wherein the material has:a thickness of greater than about 400 μm;an average solar absorptance (<as>) of less than about 0.1, the average solar absorptance being determined over the range from 0.3 μm to 2.5 μm;an average thermal emittance (<εIR>) of greater than about 0.75 at 300 K, the average thermal emittance being determined over the range from 2.5 μm to 15 μm; anda figure of merit greater than about 20, the figure of merit being the ratio <εIR> / <as>.

2. The temperature-regulating material of claim 1, wherein the figure of merit of the material is greater than about 500.

3. The temperature-regulating material of claim 1, wherein the <as> of the material is less than about 0.05.

4. The temperature-regulating material of claim 1, wherein the <as> of the material is less than about 0.005.

5. The temperature-regulating material of claim 1, wherein the nanofibers are silica-based nanofibers.

6. The temperature-regulating material of claim 1, wherein the nanofibers comprise a water soluble polymer, and nanobeads of a fluoropolymer, the nanobeads of the fluoropolymer being attached to surfaces of the water soluble polymer.

7. The temperature-regulating material of claim 6, wherein the water soluble polymer is selected from the group consisting of polyethylene oxide, polyvinyl alcohol, polyacrylic acid, polyvinylpyrrolidone, and carboxymethylcellulose.

8. The temperature-regulating material of claim 6, wherein the fluoropolymer is selected from the group consisting of polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy alkane (PFA), ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-hexafluoropropylene (PDVF-HFP), ethylene chlorotrifluoroethylene (ECTFE), polychlorotrifluoroethylene (PCTFE), and combinations thereof.

9. The temperature-regulating material of claim 1, wherein the nanofibers comprise polyethylene oxide (PEO) nanofibers, the material further comprising polytetrafluoroethylene (PTFE) nanobeads,wherein the PTFE nanobeads are disposed on surfaces of the PEO nanofibers,wherein the weight ratio of PTFE to PEO is between about 75:25 and about 95:5.

10. The temperature-regulating material of claim 6, wherein the material has a figure of merit greater than about 800.

11. The temperature-regulating material of claim 9, wherein the PTFE nanobeads have an average diameter of between about 150 nm and about 300 nm.

12. The temperature-regulating material of claim 9, wherein the material has a figure of merit greater than about 800.

13. The temperature-regulating material of claim 1, wherein the material has porosity of greater than about 0.5.

14. The temperature-regulating material of claim 1, wherein the temperature-regulating material is obtained by a process of electrospinning.

15. The temperature-regulating material of claim 5, wherein the temperature-regulating material is obtained by a process of electrospinning.

16. A method of obtaining a temperature-regulating material, the method comprising:dissolving a water soluble polymer in water to form a water soluble polymer solution;adding fluoropolymer nanobeads to the water soluble polymer solution to form a mixture of fluoropolymer nanobeads and water soluble polymer in water, the weight ratio of fluoropolymer to water soluble polymer being between about 75:25 and about 95:5, the fluoropolymer nanobeads having an average diameter of between about 150 nm and about 300 nm; andelectrospinning the mixture of fluoropolymer nanobeads and water soluble polymer in water to obtain the temperature-regulating material of claim 6.

17. A method of obtaining a temperature-regulating material, the method comprising:dissolving PEO in water to form a PEO solution;adding PTFE nanobeads dispersed in water to the PEO solution to form a mixture of PTFE nanobeads and PEO in water, the weight ratio of PTFE to PEO being between about 75:25 and about 95:5, the PTFE nanobeads having an average diameter of between about 150 nm and about 300 nm; andelectrospinning the mixture of PTFE nanobeads and PEO in water to obtain the temperature-regulating material of claim 9.

18. The method of claim 17, wherein the ratio of PTFE to PEO is about 90:10.

19. A method of obtaining a temperature-regulating material, the method comprising:mixing together a tetraalkyl orthosilicate, an alcohol, water, and a strong acid to form a mixture;stirring the mixture; andelectrospinning the mixture to obtain the temperature-regulating material of claim 5.

20. The method of claim 19, wherein the tetraalkyl orthosilicate is tetraethyl orthosilicate, the alcohol is ethanol, and the strong acid is hydrochloric acid.

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