Nanofibrous Materials for Passive Thermal Control on Earth and in Space

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.

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.

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

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.

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 (<a>) 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 (<ε>) 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 <ε>/<a>. The material can have a porosity of greater than about 0.5.

According to some embodiments, the figure of merit of the material is greater than about 500. According to some embodiments, <a> 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.

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.

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.

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

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

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.

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

For some such methods, the tetraalkyl orthosilicate is tetraethyl orthosilicate, the alcohol is ethanol, and the strong acid is hydrochloric acid.

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.

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).

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 <a> (averaged over wavelengths of 0.3 μm to 2.5 μm) and infrared (IR) emittance <ε> (averaged over wavelengths of 2.5 μm to 15 μm) are around <a>=0.12-0.28 and <ε>=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 <ε>/<a>.

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

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 <ε>/<a> for these materials is provided in.

The nanofibers of the electrospun materials allow strong scattering of solar radiation, yielding a high solar reflectance. In contrast to the other materials in, 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.

Considerable research has been devoted to exploring passive radiative cooling here on Earth. For terrestrial applications, efficient passive radiative cooling requires low solar absorptance <a> (i.e. high solar reflectance) in the solar spectrum (0.3 μm to 2.5 μm) combined with high thermal emittance <ε> 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 <a> with high thermal emittance <ε> 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 <ε>/<a> 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. Notably, for terrestrial applications, electrospun PTFE/PEO materials of the present disclosure show orders of magnitude greater values of <ε>/<a> than other state-of-the-art terrestrial passive radiative cooling materials. Consequently, these electrospun PTFE/PEO materials provide exceptional performance characteristics (as monitored by <ε>/<a> and by <ε>/<a>) for passive thermal control applications in both extraterrestrial and terrestrial environments.

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

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, the water soluble polymer is PEO and the fluoropolymer is PTFE.

In other embodiments, according to the reaction pathway ofand the SEM of, electrospun materials composed of silica fibersprovide materials suitable for extraterrestrial applications. Compared to the nanofibrous PTFE/PEO materials of, the silica nanofibers ofare thinner, having an average diameter 261±86, with a distribution of diameters shown in. 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. In the example embodiment of, the tetralkyl orthosilicate is tetraethyl orthosilicate, the alcohol is ethanol, and the strong acid is hydrochloric acid.

As illustrated in, an electrospinning apparatusof the type used in the experiments described below includes a syringe pumpwith a syringe needleconnected to a high voltage power supplyand a grounded rotating collector, configured to rotate with a D.C. motor. During the electrospinning process, a polymeric solutionfor 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 solutionto travel from the syringe toward the rotating grounded collector. As the stream of polymeric solutionmoves towards the collector, the liquid evaporates, causing a layer of polymeric nanofibers to deposit on the surface of the grounded rotating collector.

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.

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 <ρ> was calculated using Equation 1:

where prepresents the spectral normal-hemispherical reflectance obtained from the spectrometer and Iis the solar spectral intensity using the extraterrestrial spectrum (ASTM E490 Air Mass 0). The average solar absorptance <a> is calculated as <a>=1-<ρ>.

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 <ε> of the materials is calculated using the following equation:

where εrepresents the spectral normal-hemispherical emittance obtained from the FTIR spectrometer and I(λ, T) is the spectral blackbody emissive radiance at 300 K:

where h, c, λ, K, 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 <ε> in the long wavelength infrared atmospheric transmission window between 8-13 μm, calculated as:

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.

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.

The 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.

The 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).

As shown in, the electrospinning setupincluded a variable high-voltage power supply(Gamma High Voltage Research), a grounded rotating drum nanofiber collector, a syringe pumpinterfaced with the 22-gauge syringe needle. 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).

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 supplywas connected to the tip of the syringe needle. The grounded rotating drum collectorwas overlaid with an aluminum foil substrate (not shown) on which the electrospun nanofibers were deposited. The solution was made to flow through the syringe needleby operating the syringe pumpwhile an electric potential was applied between the syringe needleand the rotating collector. As the applied potential was increased, the polymeric solutionat 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 solutionwas ejected as a jet from the tip of the Taylor cone. The jet comprising the polymeric solutiondried 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.

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 needleand the grounded rotating collector, 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. 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.

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.

The effect of PEO concentration. The PTFE/PEO mixture combines the complementary properties of both polymers to create a PTFE/PEO composite structure.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 nanofiberswith PTFE beadsattached to the PEO nanofiber surface.

The 90:10 PTFE/PEO solution produces fibers with an average diameter of 1050 nm (), which is significantly larger than the average fiber diameter of 516 nm produced using the 80:20 solution (). 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 () with these various PTFE/PEO solutions, it is clear that PEO facilitates the formation of fibers attached to PTFE nanobeads, unlike the baseline (), 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).