Equivalent Sun Hours Research Articles

UiO-66 uniformly assembled foldable polyimide films with high transmittance and excellent ultraviolet resistance

As a protective layer for the flexible solar cell, a flexible protective material should be of high transmittance, excellent ultraviolet (UV) resistance and mechanical property. Herein, a foldable transparent polyimide (PI) composite film assembled with the zirconium-based metal-organic frameworks (UiO-66-X, X = H, NH2, NO2) is prepared by the direct dispersion or in-situ growth methods. The results show that the transmittances of UiO-66-X@PI films (U@PI, NU@PI and OU@PI) prepared with the in-situ growth are higher than those of UiO-66-X/PI films prepared with the direct dispersion, and the former's mechanical properties are also better than those of the latter. This may be because that the UiO-66-X in the former are embedded into the PI matrix to construct multipoint bonding network. The U@PI exhibits stronger UV-resistance than the Pristine PI, NU@PI and OU@PI after 5000 equivalent sun hours (ESHs) UV irradiation. The attenuations of average transmittance and tensile strength for U@PI are 2.19 % and 18.68 %, respectively, which are only one half and one-third of those for the Pristine PI (4.59 % and 56.58 %), respectively. And the average transmittance of U@PI has been over that of the Pristine PI after 5000 ESHs UV irradiation. This may be because that the porous UiO-66 increases the UV transmission path and the electron-hole recombination center, thereby inhibits the destructive effect of UV.

Finely tuning polyimide film structure via green plasticizer's disentangling chain to improve the trade-off between transmittance and mechanical property

To achieve the prevention of toxic waste while improving the intended functionality and durability of the film, polyimide (PI) based on TFMB-6FDA was tuned via the green plasticizer acetyl tributyl citrate (ATBC) to prepare a flexible film with high transmittance, high strength and ultraviolet (UV)-resistance. All the properties of ATBC tuned film (PI/T1, PI/T2, and PI/T3), average transmittance (AT), tensile strength (σb), elongation at break (ɛb), and elastic modulus (E) increase first and then decrease with the increase of ATBC amount. The three formers reach their own maximum values with 0.5 mole ratio of ATBC, 91.20 %, 244.0 MPa, and 12.2 %, respectively, higher than 90.50 %, 184.1 MPa, and 5.34 % of PI. The water contact angle of PI/T2 obtains the maximum value of 94.72°, and its water contact angle is 14.2 % higher than that of the pure PI (82.93°). The UV-resistance of PI/T2 is far better than that of PI after 2000 equivalent sun hours (ESHs) UV irradiation, and the AT attenuation of PI/T2 is just about one third of PI, so ATBC can retard the attenuation of PI transmittance and mechanical property.

Evaluation of the abilities of atomic oxygen/ultraviolet resistance for the filled silicone rubber coating: Structures, properties, and aging mechanism

A series of experimental techniques were used to study the erosion-damage effect of a silica-filled silicone rubber coating exposed to simulated space environment (atomic oxygen and ultraviolet irradiation). The elastomer is known to be chemically and thermally stable, but there are insufficient data on its durability in the space environment. In the present work, atomic oxygen doses up to 3.8 × 1021 atoms·cm−2 and ultraviolet irradiation doses up to 360 equivalent sun-hours (ESH) were applied from Space Environment Simulation Measurement System, respectively. Since the aging and failure processes of the filled silicone rubber relate to different aspects of chemical structure and physics form, therefore, this study explored the use of chemical-mechanics methods to comprehensively evaluate the erosion-damage effect of materials. Morphology and chemical changes due to radiation were investigated by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), and nanoindentation test probed surface mechanical properties changed with exposure dose. Results demonstrated that, with increased exposure time and irradiation-induced free radicals，the aggregation state and molecular chain structure of the silica-filled silicone rubber have changed, which caused an increase in surface hardness and modulus. Taken together, the results indicated that oxidative cross-linking was the dominant degradation mechanism under the irradiation dose in this paper.

Evaluation of atomic oxygen effects on nano-coated carbon-carbon structures for re-entry applications

Since the beginning of Space Age, the re-entry phase represents a crucial task for both manned and unmanned aerospace missions. Along with high temperatures and plasma created by the spacecraft/atmosphere dramatic interaction, several issues must be considered in order to use new technologies and materials for re-entry systems. In particular, the effects of space environment due to ultraviolet (UV) and atomic oxygen (AO) have to be taken into account, in order to design and realize aerospace structures useful also for long duration missions. Such damaging factors, in fact, affect the spacecraft outer surfaces by degrading the thermo-mechanical properties of the materials designed to withstand re-entry conditions, thus frustrating their specific effectiveness. Carbon/Carbon (C/C) composites are widely used for high temperatures applications, including Thermal Protection Systems (TPS) for re-entry vehicles. In this work, a treatment of artificial aging was performed over in-house produced C/C material by means of AO ground facility at the Aerospace Systems Laboratory (LSA) of Sapienza University of Rome, evaluating erosion rate and surface oxides creation due to UV + AO irradiation. The experimental test was carried out setting the AO generator at a fluence of 8.7 × 1020 atoms/cm2, for an exposure time of 3000 equivalent sun hours. The material involved was manufactured by a customized vapor infiltration process; moreover, a protective coating was applied by using an aluminum oxide varnish charged with silicon di-oxide nano-spheres. The effect of coating is established by comparing with the results obtained for the uncoated material; an insight about the coating degradation due to the exposure is then assessed by morphological and chemical analyses.

Environmental Testing of Thermal Control Materials at Elevated Temperature and Intense Ultraviolet Radiation

This paper presents two test campaigns in the frame of the ongoing program for the European Space Agency’s mission toMercury. Improvements on the experimental setup are presented that enable ground-based simulation of such a harsh space environment. The thermal endurance of white ceramic paints is investigated at 350 C. The solar absorptance of all paints increased significantly. Two types of ceramic woven fabrics were irradiated by 17 solar constants of ultraviolet light at an elevated temperature. The available data from the solar absorptance as a function of an ultraviolet dose up to 52,000 equivalent sun hours was extrapolated to the mission’s end-of-life dose of 100,000 equivalent sun hours. The custom-baked Astroquartz 2 was the best performing material with an estimated end-of-life solar absorptance of 0.35.

Time dependent degradation of photovoltaic modules by ultraviolet light

For high efficiency silicone encapsulated solar modules for concentrator photovoltaic systems or space applications, the question of performance degradation due to long term exposure to ultraviolet (UV) light is addressed. The transparent silicones DC93-500 and Elastosil S690 and S695 were subjected to a time and wavelength resolved UV test with a total accumulated dose of 10 000 equivalent sun hours. Based on a model for the kinetics of photodegradation, it is shown that the measured decrease in transmittance translates into a universal logarithmic time dependence for the module short circuit current. The UV related performance impact over 15years under the AM0 spectrum in space and the terrestrial AM1.5 spectrum with 500× concentration is quantified.

Effects of Vacuum Ultraviolet Radiation on Thin Polyimide Films

This paper describes the vacuum ultraviolet (VUV) radiation durability screening testing of thin (12.7–25.4 μm) polyimide films proposed for use on the Next Generation Space Telescope (NGST) sunshield. Materials included in this screening test were Kapton®E, Kapton®HN, Upilex®S, CP1, CP1 with vapour deposited aluminium (VDA) on its back surface, and CP2 with a VDA coating on its back surface. Samples were exposed to approximately 1000 equivalent sun hours (ESH) of VUV radiation and examined for changes in solar absorptance, thermal emittance, ultimate tensile strength, and elongation at failure. Changes in the solar absorptance were observed for some materials, and, additionally, significant changes in spectral reflectance were observed in the ultraviolet to visible wavelength region for all of the polyimide materials tested. Changes in the ultimate tensile strength and elongation at failure were within the experimental uncertainty for all samples. Longer exposures are needed to verify the observed trends and to develop performance predictions for these materials on the NGST sunshield.

Environmental Exposure Conditions for Teflon® Fluorinated Ethylene Propylene on the Hubble Space Telescope

The outer layer of Teflon® fluorinated ethylene propylene (FEP) multi-layer insulation (MLI) on the Hubble Space Telescope (HST) was observed to be significantly cracked at the time of the Second HST Servicing Mission (SM2), 6.8 years after HST was launched into low Earth orbit (LEO). Comparatively minor embrittlement and cracking were also observed in the FEP materials retrieved from solar-facing surfaces on the HST at the time of the First Servicing Mission (3.6 years exposure). After SM2, a failure review board was convened to address the problem of degradation of MLI on the HST. In order for this board to determine possible degradation mechanisms, it was necessary to consider all environmental constituents to which the FEP MLI surfaces were exposed. Based on measurements and various models, the environmental exposure conditions for the FEP surfaces on the HST were estimated, including: the number and temperature ranges of thermal cycles; equivalent sun hours; fluence and absorbed radiation dose of x-rays, trapped protons and electrons and plasma electrons and protons; and atomic oxygen (AO) fluence. This paper presents the environmental exposure conditions for FEP on the HST, briefly describing the possible roles of the environmental factors in the observed FEP embrittlement and providing references to the published works which describe in detail testing and analysis related to FEP degradation on the HST.

AES and SEM/EDS Analysis of Silvered Teflon Laminates of Optical Solar Reflector

The Optical Solar Reflector (OSR) of Earth Observing System-PM (EOS-PM) is made of a flexible function layer of silvered Teflon/Inconel/Kapton laminate bonded to a honeycomb panel. After having been thermal cycled in vacuum, the laminate/panel was irradiated in air with UV at 2.2 equivalent suns intensity for 500 hours. Severe darkening was observed in areas exposed to UV but little or no darkening occurred in adjacent but shaded areas. Drastic increase of absorptance was observed-due to the darkening.In order to investigate the cause of the degradation, test samples were irradiated at different configurations for 1000 equivalent sun hours (ESH). These samples were freestanding laminates and irradiated in vacuum and air, respectively. No darkening was observed on the sample that was irradiated in vacuum. Severe darkening was observed on the sample that was irradiated in the air. A laminate sample was irradiated in dry nitrogen for 1000 ESR and found no darkening either.

Calorimetric measurements of thermal control surfaces at geosynchronous orbit

Calorimeter data for up to 18,000 equivalent sun-hours are presented for a.number of satellite thermal control surfaces. This has been an ongoing study by Aerojet ElectroSystems. Solar absorptance values as a function of time were collected at geosynchronou s orbit on samples of zinc orthotitanate paint, silver-alumina-silica, silvered Teflon, aluminized Teflon, silica cloth, and clean second-surface mirrors. Over 6000 equivalent sunhours of calorimeter data were also collected on precontaminated second-surface mirror samples (also on-orbit). A useful collection of solar absorptance degradation equations is the result. These equations are of major significance in predicting satellite useful life. Nomenclature t - time T = temperature am - maximum solar absorptance a0 = initial solar absorptance as = solar absorptance £ = emissivity r = time constant

Effects of ultraviolet and electron radiations on graphite-reinforced polysulfone and epoxy resins

Degradation mechanisms have been investigated for graphite/polysulfone and graphite/epoxy laminates exposed to ultraviolet and high-energy electron radiations in vacuum up to 960 equivalent sun hours and 109 rads, respectively. Based on GC and combined GC/MS analysis of volatile by-products evolved during irradiation, several free radical mechanisms of composite degradation have been identified. All the composite materials evaluated have shown high electron radiation stability and relatively low ultraviolet stability as indicated by low G values and high quantum yields for gas formation. Mechanical property measurements of irradiated samples did not reveal significant changes, with the possible exception of UV exposed polysulfone laminates. Hydrogen and methane have been identified as the main byproducts of irradiation, along with unexpectedly high levels of CO and CO2. Initial G values for methane relative to hydrogen formation are higher in the presence of isopropylidene linkages, which occur in bisphenol-A based resins.

Fundamental studies of the stability of thermal-control coatings.

Particle size and thermal history have been observed to affect the stability of thermal control coating pigments to space environment radiation. This paper describes research designed to evaluate these effects on pigment materials, with a view toward revealing useful fundamental information. Tests have been completed on the first of a series of materials. Bulk specimens of carefully controlled purity were prepared employing three particle size ranges with three degrees of heat treatment each. Specimens were exposed in vacuum to 300 equivalent sun hours of ultraviolet radiation from a mercury-arc lamp. Degradation was evaluated in terms of the decrease in reflectance in the wavelength range 0.25 to 2.0 fji because of the exposure. Results show that increasing the particle size or the degree of heat treatment consistently reduces the degradation. For instance, increasing the particle size from a mean of about 3 IJL to a mean of about 18 M reduces the degradation in reflectance by as much as 30%, whereas heat treating the particles can reduce the degradation by a comparable magnitude. Results are interpreted to confirm the theory that structural and surface defects are major contributors to degradation in small particles.