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119 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS2007

Development of a High Efficiency UVR/IRR Coverglass for Triple Junction Solar Cells

Cover glasses have been a necessary and integral part of space solar arrays since their inception. The main function of the cover glass is to protect the underlying solar cell from the harsh radiation environment of space. They are formed either from fused silica or specially formulated ceria doped glass types that are resistant to radiation damage, for example Pilkington's CMX, CMG, CMO. Solar cells have steadily increased in performance over the past years, from Silicon cells through textured Silicon cells to GaAs cells and the multijunction cells of today. The optimum coverglass solution for each of these cells has been different. The glass itself has also evolved. In some cases it has had its expansion coefficient matched to the cell substrate material, and in addition, added value has been derived from the application of thin film optical coatings to the coverglass. In the majority of cases this has taken the form of a single layer of MgF2 which acts as an antireflection coating. There are also conductive coatings to address electrostatic discharge issues (ESD) and Ultra Violet Reflective (UVR) and Infrared Reflective (IRR) coatings designed for thermal enhancement. Each type of coating can be applied singly or in combination. This paper describes a new type of UVR/IRR (or blue red reflector BRR) specifically designed for triple junction solar cells. For space applications, where radiation is the principal mechanism for removing heat from the satellite, it is the emittance and solar absorptance that primarily determine the temperature of the array. It is therefore essential that any coatings designed to have an effect on the temperature by reducing the solar absorption have a minimal effect on the overall emittance.

Russell, John↗
NASA NTRS2016

Optical Evaluation of Digital Micromirror Devices (DMDs) with UV-Grade Fused Silica, Sapphire, and Magnesium Fluoride Windows and Longterm Reflectance of Bare Devices

Digital micromirror devices (DMDs) are commercial micro-electromechanical systems, consisting of millions of mirrors which can be individually addressed and tilted into one of two states (+/-12deg). These devices were developed to create binary patterns in video projectors, in the visible range. Commercially available DMDs are hermetically sealed and extremely reliable. Recently, DMDs have been identified as an alternative to microshutter arrays for space-based multi-object spectrometers (MOS). Specifically, the MOS at the heart of the proposed Galactic Evolution Spectroscopic Explorer (GESE) uses the DMD as a reprogrammable slit mask. Unfortunately, the protective borosilicate windows limit the use of DMDs in the UV and IR regimes, where the glass has insufficient throughput. In this work, we present our efforts to replace standard DMD windows with custom windows made from UV-grade fused silica, low-absorption optical sapphire (LAOS) and magnesium fluoride (MgF2). We present transmission measurements of the antireflection coated windows and the reflectance of bare (window removed) DMDs. Furthermore, we investigated the long-term stability of the DMD reflectance and experiments for coating DMD active area with a layer of pure aluminum (Al) to boost reflectance performance in the UV spectral range (200−400 nm).

reflectance↗
NASA NTRS2017

Enhanced Aluminum Reflecting and Solar-Blind Filter Coatings for the Far-Ultraviolet

The advancement of far-ultraviolet (FUV) coatings is essential to meet the specified throughput requirements of the Large UV/Optical/IR (LUVOIR) Surveyor Observatory which will cover wavelengths down to the 100 nm range. The biggest constraint in the optical thin film coating design is attenuation in the Lyman-Alpha Ultraviolet range of 100-130 nm in which conventionally deposited thin film materials used in this spectral region (e.g. aluminum [Al] protected with Magnesium fluoride [MgF2]) often have high absorption and scatter properties degrading the throughput in an optical system. We investigate the use of optimally deposited aluminum and aluminum tri-fluoride (AlF3) materials for reflecting and solar blind band-pass filter coatings for use in the FUV. Optical characterization of the deposited designs has been performed using UV spectrometry. The optical thin film design and optimal deposition conditions to produce superior reflectance and transmittance using Al and AlF3 are presented.

optical thin films↗
NASA NTRS2018

E-Beam Generated Plasma Etching for Developing High-Reflectance Mirrors for Far-Ultraviolet Astronomical Instrument Applications

Astronomical space telescopes to study astrophysical phenomena from the far-ultraviolet (FUV) to the near infrared (NIR) will require mirror coatings with high reflectance over this entire spectral region. While coatings for the optical and NIR part of the spectrum are fairly well developed with proven performance, the FUV presents significant challenges. The U.S. Naval Research Laboratory (NRL) has developed a processing system based on an electron beam-generated plasma that provides for controlled fluorination and/or etching of surfaces with near monolayer precision and minimal changes to surface morphology. In this paper, we report recent results of samples treated in the NRL Large Area Plasma Processing System (LAPPS) where restoration of the high intrinsic reflectance in the FUV spectral range have been observed of aluminum (Al) mirrors protected with a magnesium di-fluoride (MgF2) overcoat. This paper will also extend these studies to other Al mirrors protected with aluminum tri-fluoride (AlF3) in order to realize the high intrinsic reflectance Al down to FUV wavelengths (100−200 nm), while still maintaining the high reflectance in the optical and NIR spectral regions. Laboratory test data and optical diagnostic techniques used to verify surface scattering and durability of selected coatings will be presented. Finally, we will discuss the scalability of the LAPPS etching process in order to realize these high-reflectivity coatings on mirror segments as large as those proposed for the Large Ultraviolet, Optical, and Infrared (LUVOIR) astronomical telescope system (1+meter class).

LUVOIR↗
NASA NTRS2019

ZERODUR Substrates for Application of High-Temperature Protected-Aluminum Far Ultraviolet Coatings

Recent development in coating deposition processes for aluminum mirrors that are protected with a metal-fluoride overcoat (such as LiF, MgF2, or LiF) have improved reflectance performance particularly in the far-ultraviolet (FUV) part of the optical spectrum. The active research in this area is motivated by the fact that these gains in reflectance are expected to significantly increase the throughput of any future FUV-sensitive NASA missions into the Lyman Ultraviolet. These reflectance improvements are attributed, in part, by performing the metal-fluoride overcoat depositions with the substrates at an elevated temperature as high as 220-250 degrees Celsius. ZERODUR® is a widely used material as a mirror substrate because, among other things, it exhibits a low coefficient of thermal expansion (CTE) over a wide range of temperatures. Moreover, ZERODUR® has recently been proposed for several future NASA concept missions where this improved FUV mirror coating may be used. Given the elevated temperature at which these improved FUV coatings are produced, it is imperative to make sure that heating of the substrate will not significantly impact the final figure of the coated mirror. In this paper, we will study and report the effects of heating ZERODUR® up to the highest temperature mentioned above (250 degrees Celsius) during a simulated coating process. These studies are relevant since it has been reported the CTE will change slightly if ZERODUR® is cooled down from application temperatures between 130 degrees Celsius and 320 degrees Celsius with rates that differ from the initial production annealing rate of 3 degrees Celsius per hour.

Engineering↗
NASA NTRS2017

Mirror coatings for large aperture UV optical infrared telescope optics

Large space telescope concepts such as LUVOIR and HabEx aiming for observations from far UV to near IR require advanced coating technologies to enable efficient gathering of light with important spectral signatures including those in far UV region down to 90nm. Typical Aluminum mirrors protected with MgF2 fall short of the requirements below 120nm. New and improved coatings are sought to protect aluminum from oxidizing readily in normal environment causing severe absorption and reduction of reflectance in the deep UV. Choice of materials and the process of applying coatings present challenges. Here we present the progress achieved to date with experimental investigations of coatings at JPL and at GSFC and discuss the path forward to achieve high reflectance in the spectral region from 90 to 300nm without degrading performance in the visible and NIR regions taking into account durability concerns when the mirrors are exposed to normal laboratory environment as well as high humidity conditions. Reflectivity uniformity required on these mirrors is also discussed.

Quijada, Manuel↗
NASA NTRSDate not supplied

Advanced Al Mirrors Protected with LiF Overcoat to Realize Stable Mirror Coatings for Astronomical Telescopes

Pure Aluminum (Al) exhibits intrinsic high reflectance over the proposed Large UV/Optical/IR (LUVOIR) Surveyor observatory target spectral range (90-2500 nm). However, Al coatings have to be protected from oxygen exposure in order to prevent the formation of the naturally occurring Al2O3 oxide layer, which limit performance for use only above 160 nm. Aluminum protected with fluorides such as LiF or MgF2 have been the most commonly used solutions. But below 102 nm and down to 90 nm, no transparent material is available to protect Al and coating mirror reflectance stays below 40%. But even above 102 nm, the reflectance of protected Al is limited by the residual absorption of the fluoride overcoats. Hence, this paper will report on recent advances in producing enhanced Al-based mirror coatings with the highest possible far-ultraviolet (FUV) reflectance, while exhibiting a more stable LiF protection layer. The process starts with a bare optically smooth glass substrate that is coated with Al in an ultra-high vacuum chamber by using the physical vapor deposition (PVD) process, which has been shown to provide the best aluminum thin-films when compared to other approaches. The next step is done by in-situ exposure of the freshly made Al film to a reactive XeF2 gas that will grant a thin AlF3 overcoat (2-3 nm) to the Al film that prevents further oxidation. This fluorinated Al film is then coated with a final layer of a LiF metal-fluoride overcoat. The coating process is finalized with a second exposure to the XeF2 precursor gas. The preparation of these mirror coatings will be studied and analyzed as a function of the XeF2 exposure time and deposition rate of the LiF layer during the PVD process. This paper will present and discuss characterization of a number of Al+XeLiF witness coupons produced with this reactive PVD (rPVD) process. These studies include characterization of specular reflectance in the 90-2500 nm spectral range, micro-roughness, long-term stability, as well as polarization characteristics in the visible and near-infrared spectral regions. These studies have been performed in order to demonstrate the improved reflectance performance, longer durability, and less hygroscopic nature of protected Al mirrors produced with the rPVD process (through exposure to XeF2) and in comparison with the standard PVD process.

optics↗
NASA NTRS2023

Cryolite Overcoated Aluminum Reflectors for Far-Ultraviolet Spectroscopy

Aluminum (Al) mirrors are conventionally protected with metal-fluoride coatings (e.g., MgF2, LiF, or AlF3) immediately after deposition to prevent oxidation and preserve its far-ultraviolet (FUV) spectral efficiency. However, the resulting FUV reflectance of the aluminum reflector is limited by the metal-fluoride overcoat film index of refraction, morphology, stoichiometry, and its absorption cut-off in the lower end of the FUV spectra. Cryolite (sodium hexafluoroaluminate, Na3AlF6) emerges as a potential candidate to preserve the aluminum FUV reflectance due to its relatively lower index of refraction in the visible to ultraviolet; therefore, allowing for the thin-film design of highly spectral efficient reflectors over a wide spectral range. We investigate the use of cryolite in aluminum reflector FUV coating design. The deposited aluminum reflector overcoated with cryolite will be examined in terms of spectral efficiency and environmental durability. The deposited cryolite overcoat will be evaluated in terms of optical constants and structural properties. Preliminary results have shown that the use of cryolite as an overcoat to protect aluminum would yield unprecedented results as an optimal Hydrogen Lyman-alpha (HLyα) spectral line reflector, with experimental reflectance values >96%.

optical coatings↗
NASA NTRSDate not supplied

Metrological and Spectral Characterization of the Aspera Flight Optics

Aspera is a NASA's Astrophysics Pioneers mission, led by the University of Arizona. The aim of this mission is to study the presence of hot gasses (T=10^5-10^6 K) in the circumgalactic medium (CGM) and how the flow of these gasses affects galactic formation. One key enabling technology in the Aspera optical system is the use of more efficient UV-reflective optical coatings particularly at around 103.2 nm where the O VI emission line is located . In order to meet radiometric effective area requirements, the Goddard Space Flight Center (GSFC) code 551 Optics Branch applied Al (aluminum) + eLiF (enhanced lithium fluoride) on these optics, where the Al+LiF is annealed at elevated temperature (~250˚C) after the deposition and thus providing an improved version of the Al+LiF used in FUSE . Furthermore, these optics are encapsulated with a thin film of atomic layer deposited MgF2 at the Jet Propulsion Laboratory for enhanced durability . The efficiency and durability of these coatings depends on the quality of the optical surface in terms of cleanliness and roughness. Roughness increases the scatter off the coating and reduces the specular reflectance, and surface contaminants can accommodate moisture and other contaminants, increase scattering, and are weak points in the coating that may affect adhesion and subsequently the longevity of these coatings. Extensive optical metrology is necessary to minimize and quantify the impact of surface roughness and contaminants on optical surfaces. In this proceeding we report the inspection procedure implemented for Aspera optics at GSFC, from initial receiving of the optics to post-coating. Inspection includes Atomic Force Microscopy (AFM), Scanning White Light Interferometry (SWILI), Dark Field Microscopy (DFM), Phase Shifting Interferometry (PSI), and Vacuum ultraviolet (VUV) spectroscopy. Combinations of these techniques are used to inspect flight optics at each of the following steps: i) as-received, pre-cleaning inspection, ii) pre-coating, post-cleaning inspection, and iii) post-coating inspection. The evolution of roughness and other surface inspections such as contaminants are compared between each step to quantify the effect each step has on the flight optics. The final far ultraviolet (FUV) spectral performance of witness samples coated with the flight optics are presented.

Far Ultraviolet (FUV)↗
NASA NTRSDate not supplied

Environmental and Polarization Characterizations of E-Beam Plasma-Based AlF 3 -Passivated Aluminum Mirrors for Astronomical Telescopes

Astronomical space telescopes to study astrophysical phenomena from the far ultraviolet (FUV) to the near infrared (NIR) will require mirror coatings with high reflectance over this entire spectral region. While coatings for the optical and NIR part of the spectrum are fairly well developed with proven performance, the FUV range has presented significant challenges, particularly below 120nm. Recent developments in electron-beam (e-Beam) generated plasma treatment in a SF 6 environments has enabled the effective passivation of aluminum (Al) coatings for applications in the FUV, by native oxide removal and the formation of a AlF 3 passivation layer which could be tuned to any desired AlF 3 thickness. These results have been produced through a collaboration between the Goddard Space Flight Center (GSFC) and the Naval Research Laboratory (NRL). The passivation experiments have been carried out using the Large Area Plasma Processing System (LAPPS) at NRL using bare aluminum samples and provided by the coating group at GSFC. This novel procedure has demonstrated improved Al mirrors with state-of-the-art FUV reflectivity (e.g. R=91% at 121.6nm). In this paper, we will be reporting on environmental testing, micro-roughness, as well as polarization studies of these E-beam treated samples. These characterizations are being done in order to advance the Technology Readiness Level (TRL) for these Al+AlF 3 mirror coatings produced at LAPPS. The ultimate goal is to demonstrate the promise of using this coating technology to deliver reflectance performance plus stability and uniformity over a large area for a future IR/O/UV space telescope observatory.

FUV reflectance↗