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Calorimetric measurement of source and broadband spectral absorptances of spacecraft thermal control coatings during exposure to UV in vacuum environment
The fluorescence properties of UV sensitive organic phosphors and the radiometric properties of phosphor coated silicon detectors in the VUV, UV, and visible wavelengths are described. With evaporated films of coronene and liumogen, effective quantum efficiencies of up to 20% have been achieved on silicon photodiodes in the vacuum UV. With thin films of methylmethacrylate (acrylic), which are doped with organic laser dyes and deposited from solution, detector quantum efficiencies of the order of 15% for wavelengths of 120-165 nm and of 40% for wavelengths above 190 nm have been obtained. The phosphor coatings also act as antireflection coatings and thereby enhance the response of coated devices throughout the visible and near IR.
The NASA Exoplanet program and the Cosmic Origins program are exploring technical options to combine the visible to NIR performance requirements of a space coronagraph with the general astrophysics requirements of a space telescope covering the deep UV spectrum. Are there compatible options in terms of mirror coatings and telescope architecture to satisfy both goals? In this paper, we address some of the main concerns, particularly relating to polarization in the visible and throughput in the UV. Telescope architectures employing different coating options compatible with current technology are considered in this trade study.
The U.S. Department of Energy Wind Energy Technologies Office (DOE WETO), and the National Renewable Energy Laboratory convened a workshop entitled the State of the Science and Technology for Minimizing Impacts to Bats from Wind Energy on 13–14 November 2019. The objectives of the workshop were to (1) Identify the current impact minimization measures that are, or can be, used to reduce bat fatalities at wind energy facilities, (2) Assess the current effectiveness of those minimization measures, (3) Identify and assess the research and development (R&D) opportunities needed to optimize and improve the effectiveness of current minimization and deterrent technologies and inform the development of future solutions; and (4) Identify potential emerging or novel methods for informing impact minimization measures at or around wind energy facilities. Specifically, the workshop focused on deterrent and curtailment strategies. For deterrents, the discussion centered on the existing technology (i.e., ultrasonic deterrents, dim-UV light, and texture coating), integration with wind turbines (either retrofitting or out of the box installation), effectiveness, validation studies, and cost (e.g., technology, installation, validation, and maintenance). For curtailment, the conversation was divided into blanket curtailment (i.e., based on time and wind speed) and smart curtailment (i.e., blanket curtailment plus additional variables such as temperature or bat activity). The workshop included plenary presentations, panels, and breakout sessions to share data and stakeholder perspectives, and engage participants. Although there are several priority topics related to bat and wind energy, this workshop focused the discussion on the current technologies and strategies that are, or can be, used to reduce bat fatalities at wind energy facilities, status of research and development (R&D) of minimization measures, opportunities to optimize costs and improve effectiveness, and potential emerging or novel approaches to explore. This workshop took a holistic approach and discussed all aspects associated with advancing deterrent technologies and curtailment strategies, including the technological, biological, economic, and regulatory barriers faced by the wind energy and wildlife community.
Thermal control coating spectral sensitivity to UV induced degradation, noting rapidly increasing damage as incident radiation wavelength decreases
Calorimetric measurement of source and broadband spectral absorptances of spacecraft thermal control coatings during exposure to UV in vacuum environment
A series of space simulation tests on FEP Teflon and other polymeric thermal control coatings was conducted using UV, electrons, protons and Lyman-alpha wavelength vacuum UV radiation. Interest was centered on simulating synchronous altitude radiation environment conditions. Physical changes in materials included loss of specularity and introduction of a high density of photographable tracking defects on a microscopic scale. An example shows the discharge of trapped electrons when dielectric strength is exceeded. This leaves a portion of the Teflon volume no longer clear, but diffusely transmitting and reflecting. Thermophysical changes included degradation of solar absorptance coefficients as much as five-fold. The situation that prevails for earth orbit contrasted with polymer performance in the solar wind. It is suggested that conductive overcoatings be developed for polymers and other flexible materials, comparable or superior to those available for fused silica SSMs.
Photodeposition of thin polymer (PDAMNA) films from diacetylene monomer (DAMNA) solutions onto gold coated silica substrates with UV light was studied using the technique of in-situ real time spectroscopic ellipsometry. The dielectric function of PDAMNA was determined; and the thickness of the growing PDAMNA films over a 4 hour time interval was determined using a linear regression fit of the experimental data to a homogeneous four phase optical model. It was found that stirring the solution dramatically lowers the rate of film deposition, suggesting that convection can affect the attachment kinetics of the film to the substrate. Lastly, the morphology of the PDAMNA films was examined using non-contact mode atomic force microscopy.
Evaporated ZnS films used as a component in reflectance enhancing or decreasing multilayer coatings for mirrors, transparent optical materials, and vacuum UV reflecting optics, are investigated with reference to the reflectance loss under UV irradiation and the formation of ZnO as a result of the decomposition of ZnS in the presence of oxygen. Reflectance measurements over a broad wavelength range reveal that the UV induced reflectance losses are generally restricted to wavelengths shorter than 4000 A and are most severe in the vacuum UV region. After 134 hr of UV exposure initially polycrystalline film of 150-200 A thick is completely converted to amorphous ZnO. The results demonstrate that caution should be exercised in employing ZnS as the outer layer in an optical coating system designed for use in the UV and vacuum UV regions.
A simple technique to assess the reactivity of photocatalytic coatings sprayed onto transmissive glass surfaces was developed. This new method uses ultraviolet (UV) gallium nitride (GaN) light-emitting diodes (LEDs) to drive a photocatalytic reaction (the photocatalytic breakdown of a UV-resistant dye applied to a surface coated with the semiconductor titanium dioxide); and then a combination of a stabilized white light LED and a spectrometer to track the dye degradation as a function of time. Simple, standardized evaluation techniques that assess photocatalytic materials over a variety of environmental conditions, including illumination level, are not generally available and are greatly needed prior to in situ application of photocatalytic technologies. To date, much research pertaining to this aspect of photocatalysis has been limited and has focused primarily on laboratory experiments using mercury lamps. Mercury lamp illumination levels are difficult to control over large ranges and are temporally modulated by line power, limiting their use in helping to understand and predict how photocatalytic materials will behave in natural environmental settings and conditions. The methodology described here, using steady-state LEDs and time series spectroradiometric techniques, is a novel approach to explore the effect of UV light on the photocatalytic degradation of a UV resistant dye (crystal violet). GaN UV LED arrays, centered around 365 nm with an adjustable DC power supply, are used to create a small, spatially uniform light field where the steady state light level can be varied over three to four orders of magnitude. For this study, a set of glass microscope slides was custom coated with a thinly sprayed layer of photocatalytic titanium dioxide. Crystal violet was then applied to these titanium-dioxide coated slides and to uncoated control slides. The slides were then illuminated at various light levels from the dye side of the slide by the UV LED array. To monitor dye degradation on the slides over time, a temperature-stabilized white light LED was used to illuminate the opposite side of the slides. As the dye degraded, the amount of light from the white light LED transmitted through the slide was monitored with a spectrometer and subsequently analyzed to determine and compare the rate of dye degradation for photocatalytically coated versus uncoated slide surfaces. The long-term stability of the spectrometer/white light LED combination, which required only a single reference spectra to be taken for a time series sequence of several hours, enabled accurate measurements of transmitted light over time. Time series transmission curves were generated and results demonstrated that over time the transmission increased much more rapidly on the coated slides than on the control slides. This experimental configuration and methodology for photocatalytic activity measurement minimizes many external variable effects and allows low light level studies to be performed. This study also compares the advantages of this novel LED light source design to traditional mercury lamp systems and non-LED lamp approaches that have conventionally been used. The methodology and experimental design research summarized in this abstract is partly funded by the Department of Homeland Security, Science and Technology Directorate, and by the NASA Stennis Space Center Innovative Partnerships Program.
The Gateway Passive Thermal Control System (PTCS) Group has identified charged particle and UV degradation of Gateway materials as a knowledge gap for the Near Rectilinear Halo Orbit (NRHO) and transit environment. Candidate materials and coatings were subjected to expected mission UV and charged particle environments at NASA Goddard Space Flight Center (GSFC). In situ change in reflectivity was observed at specified intervals during testing such that change in solar absorptivity could be plotted as a function of time of solar exposure. Gateway will be an outpost in cislunar space designed to survive a 15-year mission to support human moon landings and future deep space exploration. The Gateway Passive Thermal Control System (PTCS) Group has identified charged particle and UV degradation of potential Gateway materials as a knowledge gap for the Near Rectilinear Halo Orbit (NRHO) and transit environment, especially the slow spiral transit planned for the first two modules. Solar absorptivity degradation is important to quantify because the end-of-life absorptivity impacts thermal performance of the system. To address this, ground testing is being completed at NASA Goddard Space Flight Center in which potential Gateway materials, including radiators, multi-layer insulation (MLI), and structural materials are subjected to the expected transit plus 15-year on-orbit charged particle fluence (9.86x1015 protons/cm2 at 2.5 keV and 3.1x1016 electrons/cm2 at 10 keV) and up to 5000 Equivalent Solar Hours (ESH). Reflectivity of a single sample of each material was measured in atmosphere and in vacuum inside the chamber after exposure to incremental levels of ESH and charged particle fluence. Increase in absorptivity of most samples were seen throughout the test including large differences in absorptivity of materials of the same category. Future testing will validate these results and include additional materials of interest.
Dew accelerates the soiling rates and increases the dust adhesion. To use dew for self-cleaning, a fluorinated ethylene propylene coating was applied to suppress the reactions between dust and glass as well as to facilitate the dew to condense as mobile droplets. An array of rectangular hydrophilic channels in the coating increases the condensation rates and droplet slide-off diameters. Furthermore, the durability of the coating was evaluated by artificial UV weathering. Four different types of soilants were used in the artificial soiling tests to assess the effect of soilant type and surface properties on the soiling rates and self-cleaning efficacy under the simulated dew conditions. Soil deposition and self-cleaning mechanisms are being reported.
To realize the many potential applications of ZnO films, it is vital to produce films with high optical quality that exhibit strong UV luminescence. By combining annealing at an optimal temperature followed by the deposition of a coating, one can achieve a significant enhancement of photoluminescence (PL). The effectiveness of the coating over time is a crucial point to be considered. Three types of coating materials were investigated: MgO, SiO 2 , and Al 2 O 3 . Due to its strong bond energy, MgO was found to be the most effective coating material for passivation of the surfaces of the ZnO films; SiO 2 was the second best. The UV-PL intensity of MgO coated ZnO was found to increase by a factor of 52 relative to an uncoated film. The effectiveness of the coatings exhibited a linear correlation with their bond energies and is discussed in terms of competing mechanisms to surface passivation such as the adsorption of OH-groups; these can act as surface traps and diminish the UV-PL intensity. Annealing at 900 °C prior to the deposition of the coating was found to be an important step in realizing the optimal performance of the coating due to the reduction of Zn interstitials accompanied by improved crystallinity. Exposure to the environment, up to 294 days, results in the degradation of the UV-PL of the MgO coated film; this effect was not observed for the film coated with SiO 2 . This effect is discussed in terms of the strong reactivity of MgO with environmental contaminants from the OH-groups.
Photocatalytic self-cleaning coatings with a high surface area are important for a wide range of applications, including optical coatings, solar panels, mirrors, etc. Here, we designed a highly porous TiO 2 coating with photoinduced self-cleaning characteristics and very high hydrophilicity. This was achieved using the swelling-assisted sequential infiltration synthesis (SIS) of a block copolymer (BCP) template, which was followed by polymer removal via oxidative thermal annealing. The quartz crystal microbalance (QCM) was employed to optimize the infiltration process by estimating the mass of material infiltrated into the polymer template as a function of the number of SIS cycles. This adopted swelling-assisted SIS approach resulted in a smooth uniform TiO 2 film with an interconnected network of pores. The synthesized film exhibited good crystallinity in the anatase phase. The resulting nanoporous TiO 2 coatings were tested for their functional characteristics. Exposure to UV irradiation for 1 h induced an improvement in the hydrophilicity of coatings with wetting angle reducing to unmeasurable values upon contact with water droplets. Furthermore, their self-cleaning characteristics were tested by measuring the photocatalytic degradation of methylene blue (MB). The synthesized porous TiO 2 nanostructures displayed promising photocatalytic activity, demonstrating the degradation of approximately 92% of MB after 180 min under ultraviolet (UV) light irradiation. Thus, the level of performance was comparable to the photoactivity of commercial anatase TiO 2 nanoparticles of the same quantity. Our results highlight a new robust approach for designing hydrophilic self-cleaning coatings with controlled porosity and composition.
Here, we used grazing-incidence small-angle neutron scattering (GISANS) to probe the depth-dependent orientation of self-assembled morphologies of block copolymers. A hydrophobic–hydrophobic block copolymer, poly(solketal methacrylate)-block-polystyrene (PSM-b-PS), was synthesized and spin-coated into films tens of nanometers in thickness where the polymers adopted a disordered state. PSM-b-PS was then hydrolyzed by exposing the polymer thin film to trifluoroacetic acid vapor or embedding a photoacid generator in the film during spin coating, followed by exposure to UV light. The hydrolysis converted the block copolymer into a hydrophilic–hydrophobic block copolymer, poly(glycerol monomethacrylate)-block-polystyrene (PGM-b-PS), significantly increasing the segmental interaction parameter (χ) and causing microphase separation of the copolymers. GISANS was used to study the depth-dependent structure of the films and revealed that the orientation of the microphase-separated structure differed at the polymer–air and polymer–substrate interfaces. In the regions where the lamellae were parallel to the interfaces, the orientation propagated into the film from the interfaces. However, in the areas where the lamellae were perpendicular to the interfaces, the orientation did not persist with increasing distance from the interfaces.
The ability to engineer material at its nanoscale has shown great potentials in overcoming significant obstacles in numerous fields. Because of its abundance, biodegradability, renewability, and eco-friendliness nature, lignin has become an ideal precursor for producing nanoscale materials from both research and industrial communities. Nanolignin, also known as colloidal lignin particle (CLP) or lignin nanoparticle (LNP), has received significant interest in recent years and its production and application in numerous fields represent an important aspect of lignin valorization. LNPs could be produced via several techniques including sonication, solvent exchange, and precipitation, which could be divided into wet and dry methods. Applications of LNPs in UV protection, cosmetics, food package, coatings, and drug delivery have been reported in the literature. Despite being extensively studied, LNPs as well as their derived materials with controlled shape and architecture are still largely underexploited. This book chapter mainly covers recent advances in the preparation of LNPs and their use in the novel and advanced applications in different areas.
Silicate treated zinc oxide thermal control coatings, discussing effective stabilization against UV induced degradation in IR region of solar spectrum
A technique for obtaining detailed quantitative temperature distributions on test models in hypersonic wind tunnels is presented. This technique is based on the ratio of blue to green (450, 520 nm) emission from an UV (365 nm) excited phosphor coating. Separately filtered images are recorded from a three-tube color camera, utilizing off-the-shelf front-end video optics to discriminate wavelengths. Two demonstration studies in a 31-inch Mach 10 tunnel are discussed. One study presents the windward surface temperature-time history for a transatmospheric vehicle, and the other illustrates nosetip heating on a spherically blunted slender cone.