Optical Solar Reflector Research Articles

Electrostatic charge mitigation by graphene-based thin films for optical solar reflectors in spacecraft

In the field of spacecraft engineering, in particular for geostationary and geosynchronous applications, a significant need exists for a thermal control film to be electrically conductive, to effectively manage the dissipation of accumulated charges. In this study, transparent-electrically conductive reduced graphene oxide is strategically applied to heated rigid (quartz glass) and flexible (Fluorinated Ethylene propylene) optical solar reflector substrates. This innovation harnesses the unique properties of graphene to seamlessly integrate electrical conductivity into space-appropriate substrates with minimum deviation in their optical transparency and thermal properties. The tape peel-off test, conducted in accordance with space materials standards (ASTM D903) using 3 M Scotch tape, reveals the better adhesion of the developed coating. A noteworthy aspect of the present work involves visualizing charge dissipation using Field Emission Scanning Electron Microscope (FESEM) electron bombardment through obtained images and decreased surface potential measured by Kelvin Probe Force microscopy (KPFM). Furthermore, a basic OSR model is fabricated by depositing a reflective back with Aluminium and graphene thin film protecting from the top to investigate the alteration in reflection by varying incident angles. This research contributes to the advancement of spacecraft materials and coatings aimed at improving spacecraft durability and functionality.

Study of the photo-stability of ceramic thermal control coating based on aluminium oxide

The study examined the photo-stability of the thermal control coating (TCC), a class of “Optical Solar Reflectors”, used in spacecrafts. This TCC is achieved by applying a layer of dielectric ceramic paste (DCP) onto the substrate using a printer (PCB printing). The DCP was prepared from filler (ground Al2O3 ceramic) and solvent with thickener (terpineol with ethylcellulose). The coating obtained by high-temperature firing (T = 850 оC, 2 h) has high reflectivity in the solar range of the spectrum (0.2–2.5 μm) with the solar absorptance (αs) of DCP equal to 0.143. It has high adhesion and photo-stability that exceeds photo-stability parameters of ZnO, TiO2, BaSO4, and CaSiO3 pigments. The developed coating can be recommended not only as a TCC for spacecrafts, but also as a protective or dielectric layer in manufacturing of printed circuit boards in various fields of technology, where increased resistance to solar radiation or its ultraviolet region is required.

Robust radiative cooling via surface phonon coupling-enhanced emissivity from SiO2 micropillar arrays

Silicon dioxide (SiO2) is a prominent candidate for radiative cooling applications due to its low absorption in solar wavelengths (0.25–2.5 µm) and exceptional stability. However, its bulk phonon-polariton band results in a strong reflection peak in the atmospheric transparency window (8–13 μm), making it difficult to meet the requirements for sub-ambient passive radiative cooling. Herein, we demonstrate that SiO2 micropillar arrays can effectively suppress infrared reflection at 8–13 μm and enhance the infrared emissivity by optimizing the micropillar array structure. We created a pattern with a height, spacing, and diameter of approximately 1.45 μm, 0.15 μm, and 0.35 μm, respectively, on top of a bulk SiO2 substrate using reactive ion etching. The resulting surface phonon coupling of the micropillar array led to an increase in the thermal emissivity from 0.79 to 0.94. Outdoor tests show that the SiO2 cooler with an optimized micropillar array can generate an average temperature drop of 5.5 °C throughout the daytime underneath an irradiance of 843.1 W m−2 at noon. Furthermore, the micropillar arrays endow the SiO2 cooler with remarkable hydrophobic properties, attributed to the formation of F/C compounds introduced during the etching process. Finally, we also replicated the micropillar pattern onto the surface of industrial optical solar reflectors (OSRs), demonstrating similar emissivity and hydrophobicity enhancements. Our findings revealed an effective strategy for modifying the thermal management features of durable SiO2 layers, which can be harnessed to cool OSRs and other similar sky-facing devices.

Design of VO2-based spacecraft smart radiator with low solar absorptance

Thermochromic film with the ability to self-adaptively switch emittance is important for spacecraft missions experiencing rapidly increasing demand. Current research on thermochromic films primarily focuses on optimizing emittance tunability, while the solar absorptance of these films remains high, limiting their practical application in spacecraft thermal control. In this work, a VO2-based spacecraft smart radiator with low solar absorptance and high emittance tunability is proposed. The smart radiator is a multilayer structured film composed of a switchable resonator (i.e., a superposed Fabry-Perot cavity) and a solar reflector (i.e., a Distributed Bragg Reflector). The performance of the smart radiator is optimized by combining the Rigorous Coupled Wave Analysis method and Genetic Algorithm, achieving a normal solar absorptance of 0.121 and emittance tunability of 0.825. The solar absorptance and emittance tunability of the smart radiator remain excellent even at large incident angles. The underlying mechanisms involved in the smart radiator are attributed to the combination of superposed Fabry-Perot resonance and multiple solar reflectance. In addition, we provide a numerical example using a geosynchronous orbit satellite to demonstrate that the regulation range of the net heat dissipation flux density can increase from 146.5–463.9 W/m2 by using Optical Solar Reflector to 0–494.6 W/m2 when using the proposed smart radiator within the permissible temperature range (263.15–318.15 K) of spacecraft. This VO2-based spacecraft smart radiator with outstanding properties which outperform the existing thermochromic films reveals the promising potential of thermochromic films for spacecraft intelligent thermal control applications.

On the possibility of obtaining thermal control coatings for spacecraft by printing

There has been considered a method to produce “Optical Solar Reflector” (OSR) class thermal control coatings (TCC) for spacecraft by applying a paste of required composition on a substrate by a printed circuit boards (PCB) printer. 2 types of pastes were applied: dielectric ceramic paste (DCP), consisting of a filler – ground polycor (Al2O3) – and a solvent with a thickener (terpineol with ethyl cellulose); conductor paste of PP-17 grade consisting of silver particles and an organic terpineol-based binding medium. After heating the pastes at 150 °C and subsequent annealing at 850 °C for 2 h, layers of aluminum oxide or silver are deposited on the surface of the substrate, which have a high reflectance in the solar spectral range (0.2–2.5 μm). The solar absorptance (αs) is 0.143 for the coating based on the DCP paste and 0.23 for the coating based on the PP-17 paste, and these values follow the requirements of the OSR class TCC. The coating based on the DCP paste features a high radiation resistance (αs = 0.173 after irradiation by electrons with energy of 30 keV and fluence of 3·1016cm−2).

Flexible thin film optical solar reflectors with Ta2O5-based multimaterial coatings for space radiative cooling

Optical Solar Reflectors (OSRs) combine low solar radiation absorption (α) and high broadband infrared emissivity (ε) and are applied to the external surface of spacecraft for its thermal management. Bulk glass OSR tiles are the incumbent, but ultra-lightweight and thin-film flexible OSR coatings are raising considerable interest for both space and terrestrial radiative cooling applications. In this work, a genetic algorithm combined with a transfer matrix method is used for the design and optimization of multimaterial thin-film OSRs for broadband radiative cooling. The algorithm simultaneously optimizes the spectral performance of the OSR at two parts of the wavelength spectrum, solar (0.3–2.5 μm) and thermal infrared (2.5–30 μm). The designed optimized OSR structure consists of 18 alternating layers of three materials, SiN, SiO2, and Ta2O5, on top of an Al mirror backreflector, with a total thickness of only 2.088 μm. The optimized multilayer stack contributes distributed Bragg reflections that reduce the residual solar absorption below that of an uncoated Al mirror. The optimized OSR is demonstrated experimentally on a 150 mm (6 in.) Si wafer and on a flexible polyimide substrate using a production level reactive sputtering tool. The fabricated thin film OSR shows good thermal-optical property with α = 0.11 and ε = 0.75 and achieves a net cooling power of 150.1 W/m2 under conditions of one sun total solar irradiance in space. The ultrathin coating fabricated using hard inorganic materials facilitates its integration onto flexible foils and enables large-scale manufacture of low-cost OSRs for broadband radiative cooling applications.

Inspectability and POD investigation for optical solar reflector bonded satellite panels

Optical Solar Reflectors (OSR) are fully light-reflective materials, which are used in satellite panels that bonded with metallic sheets with a layer of silicon type adhesive. During the bonding process, entrapping of air bubbles in the bondline is the main problem, and the detection of air bubbles is crucial for the thermal integrity of the panel. The inspectability investigation established in this work is a complete methodology that consists of the determination of the applicable NDT method, parameter optimization, reference specimen design, and most importantly, execution of the POD analysis for the detection reliability to implement the method into post-manufacturing NDI operations of OSR panels. A comprehensive comparison of conventional and advanced inspection methods were performed in this study, including sonic, ultrasonic, radiographic and thermographic techniques. After trials on generic scale panels, high sensitivity surfaces of OSR and large density differences of bonded elements lead to the selection of the lockin thermography technique. Next, an appropriate reference specimen was designed for the POD approach with thermally representative artificial defects. The results show that the lock-in thermography technique can detect air bubbles at the adhesive layer for sizes larger than 20 mm2 with a probability of detection of 90% at a confidence level of 95%.

Metamaterial-based smart and flexible Optical Solar Reflectors

In the frame of projects funded by the European Commission and the ESA, we developed a new type of Optical Solar Reflector (OSR) that combines the flexibility and easy handling of Second Surface Mirrors with the temperature-variable emittance necessary to ensure both effective radiative cooling in the hot phase and reduced heat losses in the cold phase. The new smart OSR consists of a metamaterial coating deposited on Kapton film. The coating is made of two functional blocks, namely a variable emitter topped by a dielectric solar reflector. The variable emitter is a Metamaterial Perfect Absorber designed for strong and broadband plasmonic resonance absorption in the thermal IR. It consists of a metal back-reflector, a dielectric spacer, and an array of doped VO2 thermochromic micro-antennas that are switched-off when the temperature drops below the metal-to-insulator transition point. The solar reflector is a wideband dielectric filter made of materials that are transparent across the entire spectrum from the VIS to the thermal IR. All the layers of the two blocks are deposited by standard vacuum techniques, while the array is patterned by Nanoimprint Lithography, a technique that is often performed at the wafer level but allows for up-scaling via roll-to-roll or roll-to-plate production setups. The paper reports on the characterization and testing of samples of size up to 100 mm x 100 mm, at the Beginning of Life and after thermal, humidity, irradiation and handling tests.

Characterisation of lunar and Martian dust simulants and impacts on thermo-optical properties of space materials

Characterisation analyses were carried out on several available lunar and Martian simulants in order to provide standardised comparative data to guide testing. Thermal conductivity measurements were performed on regolith powder under ambient conditions Reflectance spectra were obtained for several simulants including separated size-fractions in order to calculate solar absorptance (αS ) values. The change in thermo-optical properties of optical solar reflectors (OSRs) due to lunar simulant EAC-1A <100 μm fraction dust deposition was investigated, showing a linear increase in αS and infrared emittance (εIR ) values. Facilities are in development for future work involving dusty testing under simulated lunar environments.

Annealing effects on Al/polyimide adhesion in flexible optical solar reflectors

Flexible optical solar reflectors are made of single and multi-layered metal thin films on polymer substrates and will encounter around 6000 thermal cycles exceeding +/-100°C during one year of operation in low earth orbit. The candidate thin film system of Inconel/silver (Ag)/Teflon (FEP) recently demonstrated early damage formation (cracks and voids) after only a few thermal cycles, most likely due to the poor interfacial properties between Ag and FEP. An alternative material system that could be used is colourless polyimide, Tormed, instead of FEP. Additionally, aluminium (Al) has demonstrated very good interfacial properties with polyimide even after thermal cycling and has suitable optical properties. The adhesion of the Al/Tormed and Ag/Tormed interfaces were evaluated with tensile induced delamination. Generally, Al/Tormed has a much higher interface adhesion energy compared to Ag/Tormed, and there is no significant degradation after bake-out in vacuum (200°C, 10-6 mbar) for 10 and 24 hrs. Thus, the Al/Tormed system could be a more robust coating system for future flexible solar reflectors.

An In-Orbit Stereo Navigation Camera Self-Calibration Method for Planetary Rovers with Multiple Constraints

In order to complete the high-precision calibration of the planetary rover navigation camera using limited initial data in-orbit, we proposed a joint adjustment model with additional multiple constraints. Specifically, a base model was first established based on the bundle adjustment model, second-order radial and tangential distortion parameters. Then, combining the constraints of collinearity, coplanarity, known distance and relative pose invariance, a joint adjustment model was constructed to realize the in orbit self-calibration of the navigation camera. Given the problem of directionality in line extraction of the solar panel due to large differences in the gradient amplitude, an adaptive brightness-weighted line extraction method was proposed. Lastly, the Levenberg-Marquardt algorithm for nonlinear least squares was used to obtain the optimal results. To verify the proposed method, field experiments and in-orbit experiments were carried out. The results suggested that the proposed method was more accurate than the self-calibration bundle adjustment method, CAHVOR method (a camera model used in machine vision for three-dimensional measurements), and vanishing points method. The average error for the flag of China and the optical solar reflector was only 1 mm and 0.7 mm, respectively. In addition, the proposed method has been implemented in China’s deep space exploration missions.

Simple Double-Layer Coating for Efficient Daytime and Nighttime Radiative Cooling

The passive radiative cooling approach refers to the physical process that pumps heat into outer space via the atmospheric window (8–13 μm) without energy input. The ability to continuously adjust the emissivity of thermal emitters in the sky window while maintaining high reflectivity in the solar spectrum remains a challenge. In order to achieve this task, a novel design referred to as double-layer nanoparticle-based coating is proposed. Our proposed emitter is appropriate for both high solar reflection and strong mid-infrared emissivity. The bottom and top layers are Al2O3 embedded with Ni nanoparticles and a super-hydrophilic TiO2-SiO2 layer. The bottom layer is designed to achieve high emissivity in “the atmospheric transparency window”. The top layer is designed to block solar illumination and to favor an enhanced cleanability of the coated design. Our double-layer coating as an optical solar reflector has excellent solar irradiation (0.96) and is strongly emissive (0.97) across the “full sky window” at room temperature. Furthermore, a detailed numerical energy study has been performed, evaluating the temperature reduction and the radiative cooling performance under different conditions. The proposed simple coating can be used as an efficient radiative cooler on a large scale for energy conservation and thermoelectric devices.

Preparation of porous Mo-doped VO2 films via atomic layer deposition and post annealing

Mo-doped VO2 films were fabricated on fused silica substrates by the atomic layer deposition of VO2/MoO3 nanolaminates and subsequent post-annealing. The effect of the number of VO2/MoO3 interfaces as well as annealing conditions on the film morphology, structure, and thermochromic properties were systematically studied. Among them, increasing the annealing temperature has the most significant effect on the formation of the porous structure in Mo-doped VO2 films. The porous Mo-doped films, with their phase transition temperature of near room temperature, have a large potential on the application of smart windows and smart optical solar reflectors.

Property changes in materials due to atomic oxygen in the low Earth orbit

We expect satellites at altitude below 300 km, very low Earth orbit (VLEO), making observations of the Earth at optical wavelength with increasingly higher resolution. The density of atomic oxygen (AO) at VLEO is significantly higher than that at LEO; severe degradation of spacecraft materials (polymers) due to the high-flux AO is a serious concern. To clarify VLEO environmental effects on spacecraft materials, we designed the Material Degradation Monitor (MDM) and MDM2 missions. The MDM is a material exposure experiment onboard the Super Low-Altitude Test Satellite (SLATS). It aims to understand reactions and degradation of polymeric materials depending on AO fluence in VLEO. In the MDM, samples of spacecraft material were exposed at altitude of 160–560 km; their degradation behaviors were observed optically by a CCD camera for 1.8 years. The MDM2 is a material exposure experiment onboard the International Space Station (ISS) and aims to correctly understand surface reactions and degradation of the same samples used in the MDM at a given AO fluence. In the MDM2, the samples were exposed at altitude of 400 km for 1 year and then returned to Earth for analysis. Based on the results from both missions, we will help in the molecular design of more-durable materials, and establish design standards for future VLEO satellites. This study aims to quantitatively understand the surface reactions and degradation of the 11 types of thermal control materials exposed on the ISS in the MDM2. Five types of multilayer insulation (MLI) films (three types of Si-containing AO protective materials (a silsesquioxane-(SQ-) containing coated polyimide film, two types of polysiloxane-block polyimide (BSF-30) films), an ITO-coated polyimide film, and a Beta Cloth), and flexible optical solar reflectors (flexible OSRs) were found to have a high durability against erosion by AO. This was determined by measuring their loss of mass and thermo-optical properties. The Ag/Inconel layer’s discoloration and peeling were observed for three types of FEP/Ag films as determined by the Ag layer’s oxidation by AO. Also, X-ray photoelectron spectroscopy (XPS) showed that reactions of the Si-containing materials, the SQ-coated polyimide film and the BSF-30 film, form a layer of silica that protects against AO. Even though the concentration of Si in the SQ-coating is the same or greater than in the BSF-30 film, the amount of the SQ-coating that reacted was larger than that of the BSF-30 film under the same AO fluence. Moreover, the effective ability of the UV-shielding coating, composed of ITO and CeO2 coated onto one of the BSF-30 films, was demonstrated by UV–Vis spectrometry. Its sufficient AO protection was confirmed by mass measurements, XPS analyses, and FE-SEM observations.

High-intensity high-temperature testing of materials for the Bepi Colombo MPO and MTM solar arrays

The Bepi Colombo spacecraft was successfully launched on 20th October 2018 and is now on its way to Mercury. The materials and coatings on the Mercury Planetary Orbiter (MPO) and Mercury Transfer Module (MTM) solar arrays will need to withstand a prolonged extreme environment of high-intensity UV radiation at high-temperature (HIHT). The solar arrays must be gradually off-pointed at close sun distances in order to avoid over-heating, up to an angle of over 70° at 0.3 AU for the MPO. This means that some of the materials and coatings, as well as the solar cells, will always be operating reasonably close to their temperature limits. Extensive on-ground testing was performed to qualify the solar arrays for the BepiColombo mission environment and during this testing some materials and coatings were pushed beyond acceptable performance limits. In this paper we give an overview of selected test programmes related to the solar array materials, and we present results of experimental investigations which provide some scientific insight into the likely causes of the materials degradation observed, and the lessons learnt in the cases where over-testing had occurred. This included testing on bare and bonded optical solar reflectors, solar cell cover glass and adhesive, and the solar array edge shields.

Thin nanoporous anodic alumina film on aluminium for passive radiative cooling

We demonstrate a simple, low-cost, and passive radiative cooler based on a monolithic design consisting of thin nanoporous anodic alumina (NAA) films grown on aluminium sheets. The NAA/Al structure maintains a high broadband reflectivity close to 98 $$\%$$ within the solar spectrum (0.4–2.2 $$\mu $$ m) and simultaneously exhibits a high average emissivity of 88 $$\%$$ within the atmospheric infrared (IR) transmission window of 8–13 $$\mu $$ m with the peak IR emission approaching 99 $$\%$$ at a wavelength of 10 $$\mu $$ m. Optical modelling of the system using optical parameters of the materials confirms that the high solar reflectance arises due to the transparent nature of NAA and high reflectivity of bottom Al, while the large thermal IR emissivity arises from the interference effects of the NAA film and the high absorption of IR light due to phonon resonances in alumina at wavelength larger than 10 $$\mu $$ m. Further, we estimate the average cooling power of NAA/Al to be about 136 W $$\hbox {m}^{-2}$$ at ambient temperature even after including the contribution to heat input from external non-radiative processes. This robust and lightweight NAA/Al can be projected as an excellent alternative to optical solar reflectors used in spacecraft for thermal heat management and rooftop cooling green technologies.

Effect of Al2O3 nanoparticle modification on increase in reflectivity of BaSO4 pigment

The effect of modification with Al2O3 nanoparticles (1–10 wt%) on the reflection spectra of BaSO4 powders in the range of 0.2–2.2 μm and integral absorption coefficient of solar radiation (as) was studied. With a lower reflectivity of Al2O3 nanoparticles in comparison with BaSO4 powder, its increase in the entire range of the solar spectrum in the modified powder was obtained. The modification efficiency, determined by the ratio of the absorption coefficient as of the unmodified and modified powders, is 1.77 times at the optimum concentration of nanoparticles equals 5 wt%. The determining factor of such increase was experimentally established – that is the change in the size and concentration of grains and granules of BaSO4 powder during modification. The obtained results were compared with the results of BaSO4 powder modification by SiO2 nanoparticles.The achieved effect is a significant result for spacecraft, in which BaSO4 powders can be used as a pigment for thermal control coatings of the “optical solar reflectors” class.

Mechanical and optical degradation of flexible optical solar reflectors during simulated low earth orbit thermal cycling

Multilayer thin film systems on flexible polymer substrates are used as flexible optical solar reflectors or thermal insulation of satellites and spacecraft. During one year of operation, a satellite in low earth orbit typically encounters 6000 thermal cycles of ±100 °C. Due to the different coefficients of thermal expansion between the individual layers and the substrate it is important to investigate the thermo-mechanical stability of the multilayers as a function of the cyclic heat load. Scanning electron microscopy and focused ion beam cross-sectioning revealed that Inconel-Ag bilayers on fluorinated ethylene propylene (FEP) substrate severely degrade during thermal cycling of ±150 °C in a gaseous N2 atmosphere. After only 100 cycles through thickness cracks and subsurface voids in the Ag layer form as a result of equi-biaxial thermal stresses caused by the large difference in thermal expansion between film and substrate. Transmission Kikuchi Diffraction (TKD) before and after thermal cycling also revealed grain growth and twin widening in the Ag layer. Cracking and void formation are detrimental to application relevant material properties including corrosion protection (Inconel) and reflectivity (Ag). Reflectance measurements revealed that the amount of reflected energy as well as the reflection mode (specular vs. diffuse) significantly change during the first 100 cycles. Saturation of reflection characteristics was observed after 25 cycles, which correlates to a turning point in the evolution of Ag voids. Results of this study indicate that special focus should be directed towards thermal stress control (Δα) and tailoring of the metal-polymer interface to improve resistance of versatile metal-polymer systems against thermal cycling.

Embedded Metal Oxide Plasmonics Using Local Plasma Oxidation of AZO for Planar Metasurfaces.

New methods for achieving high-quality conducting oxide metasurfaces are of great importance for a range of emerging applications from infrared thermal control coatings to epsilon-near-zero nonlinear optics. This work demonstrates the viability of plasma patterning as a technique to selectively and locally modulate the carrier density in planar Al-doped ZnO (AZO) metasurfaces without any associated topographical surface profile. This technique stands in strong contrast to conventional physical patterning which results in nonplanar textured surfaces. The approach can open up a new route to form novel photonic devices with planar metasurfaces, for example, antireflective coatings and multi-layer devices. To demonstrate the performance of the carrier-modulated AZO metasurfaces, two types of devices are realized using the demonstrated plasma patterning. A metasurface optical solar reflector is shown to produce infrared emissivity equivalent to a conventional etched design. Second, a multiband metasurface is achieved by integrating a Au visible-range metasurface on top of the planar AZO infrared metasurface. Independent control of spectral bands without significant cross-talk between infrared and visible functionalities is achieved. Local carrier tuning of conducting oxide films offers a conceptually new approach for oxide-based photonics and nanoelectronics and opens up new routes for integrated planar metasurfaces in optical technology.

Thermal regulation of satellites using adaptive polymeric materials

This paper focuses on the thermal performances of an adaptive radiator made of 16 electroemissive devices (EEDs) acting as electro-active materials for thermal control of satellites. EEDs are similar to an electrochemical cell where one electrode serves as active layer in the infrared and is made of a conducting polymer, the poly(3,4-ethylenedioxythiophene) (PEDOT), interpenetrated into a polyethylene oxide/nitrile butadiene rubber (PEO/NBR) solid gel electrolyte as supporting matrix. First, EEDs were placed in moderate vacuum (10-3 bars) to estimate their optical cyclability c.a. 4000 cycles in neutral and oxidized state. Then, an electro-emissive radiator (EER) assembled with 16 EEDs (total area of 80cm2) was tested under space vacuum conditions. “Hot” and “cold” operating cases of the satellite were simulated and ability of the radiator to control radiative exchanges between the satellite and its environment was estimated by comparison with the currently used optical solar reflector (OSR) radiators. In low emissivity state, the EER can slow down the rejection of heat in a cold environment which is an interesting feature in terms of energy saving. Also when the EER is not directly exposed to the sun, it is able to reject the heat in the same way as the OSR radiator in a hot phase. By comparing the electrical consumption with a system operating with OSR, an energy saving in the order of 25% was demonstrated. Finally the EER exhibits bistability during many hours, i.e. memory effect once electrically switched into a given emissivity state, allowing to further reduce the electrical consumption.