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Silicone Contamination Titles |
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Titles:
Snyder, A., Banks, B., Miller, S., Stueber, T., and Sechkar, E., “Modeling of Transmittance Degradation Caused by Optical Surface Contamination by Atomic Oxygen Reaction With Adsorbed Silicones,” prepared for the 45th Annual International Symposium on Optical Science and Technology Meeting, San Diego, California, June 2001. A numerical procedure is presented to calculate transmittance degradation caused by contaminant films on spacecraft surfaces produced though the interaction of orbital atomic oxygen (AO) with volatile silicones and hydrocarbons from spacecraft component. In the model, contaminant accretion is dependent on the adsorption of species, depletion reactions due to gas-surface collisions, desorption, and surface reactions between AO and silicone producing SiOx (where x is near 2). A detailed description of the procedure used to calculate the constituents of the contaminant layer is presented, including the equations that govern the evolution of fractional coverage by specie type. As an illustrative example of film growth, calculation results using a prototype code that calculates the evolution of surface coverage by specie type is presented and discussed. An example of the transmittance degradation caused by surface interaction of AO with deposited contaminant is presented for the case of exponentially decaying contaminant flux. These examples are performed using hypothetical values for the process parameters.
The continued presence and use of silicones on spacecraft in low Earth orbit (LEO) has been found to cause the deposition of contaminant films on surfaces which are also exposed to atomic oxygen. The composition and optical properties of the resulting SiOx-based (where x is near 2) contaminant films may be dependent upon the relative rates of arrival of atomic oxygen, silicone contaminant, and hydrocarbons. This paper presents results of in-space silicone contamination tests, ground laboratory simulation tests, and analytical modeling to identify controlling processes that affect contaminant characteristics.
Banks, B. A., de Groh, K. K., Baney-Barton, E.,Sechkar, E. A., Hunt, P. K., Willoughby, A., Bemer, M., Hope, S., Koo, J., Kaminski, C., and Youngstrom, E., "A Space Experiment to Measure the Atomic Oxygen Erosion of Polymers and Demonstrate a Technique to Identify Sources of Silicone Contamination", prepared for the 34th Intersociety Energy Conversion Engineering Conference (IECEC) sponsored by the Society of Automotive Engineers, Inc., Vancouver, British Columbia, Aug. 1-5, 1999. A low Earth orbital space experiment entitled, "Polymers Erosion and Contamination Experiment" (PEACE) has been designed as a Get-Away Special (GAS Can) experiment to be accommodated as a Shuttle in-bay environmental exposure experiment. The first objective is to measure the atomic oxygen erosion yields of ~40 different polymeric materials by mass loss and erosion measurements using atomic force microscopy. The second objective is to evaluate the capability of identifying sources of silicone contamination through the use of a pin-hole contamination camera, which utilizes environmental atomic oxygen to produce a contaminant source image on an optical substrate.
In January 1998 during the STS-89 mission, an eight section Russian solar array panel was retrieved after more than ten years exposure to the orbital space environment on the Russian space station Mir. Two darkened handrail samples from the Russian solar array have been evaluated for contamination; a section of a white paint covered rigid handrail and a section of woven fabric over-wrapped around a flexible handhold. The handrail samples were evaluated using optical microscopy (OM), field emission scanning electron microscopy (FESEM) and energy dispersive spectroscopy (EDS). Optical properties were also obtained. Microscopy has shown the discolored areas to have thick layers of contaminant that has crazed and spalled off in regions. Energy dispersive spectroscopy revealed that the brown contaminant is composed of oxidized silicon. No silicon was present on the unexposed fabric over-wrap, and very small amounts were present in the white paint. Therefore, the contaminant layer on both samples is attributed to silicone contamination from other spacecraft materials that have been oxidized by atomic oxygen while in orbit. A significant source of the silicone contamination appears to be from the solar array itself.
The exposure of silicones to atomic oxygen in low Earth orbit causes oxidation of the surface, resulting in conversion of silicone to silica. This chemical conversion increases the elastic modulus to the surface and initiates the development of a tensile strain. Ultimately, with sufficient exposure, tensile strain leads to cracking of the surface enabling the underlying unexposed silicone to be converted to silica resulting in additional depth and extent of cracking. The use of silicone coatings for the protection of materials from atomic oxygen attack is limited because of the eventual exposure of underlying unprotected polymeric material due to deep tensile stress cracking of the oxidized silicone. The use of moderate to high volatility silicones in low Earth orbit has resulted in a silicone contamination arrival at surfaces which are simultaneously being bombarded with atomic oxygen, thus leading to conversion of the silicone contaminant to silica. As a result of these processes, a gradual accumulation of contamination occurs leading to deposits, which at times have been up to several microns thick (as in the case of a Mir solar array after 10 years in space). The contamination species typically consist of silicon, oxygen, and carbon, which in the synergistic environment of atomic oxygen and UV radiation leads to increased solar absorptance and reduced solar transmittance. A comparison of the results of atomic oxygen interaction with silicones and silicone contamination will be presented based on the LDEF, EOIM-III, Offeq-3 spacecraft and Mir solar array in-space results. The design of a contamination pin-hole camera space experiment, which uses atomic oxygen to produce an image of the sources of silicone contamination, will also be presented.
Evaluating the performance of materials on the exterior of spacecraft id of continuing interest, particularly in anticipation of those applications that will require a long duration in low Earth orbit. The Passive Optical Sample Assembly (POSA) experiment flown on the exterior of Mir as a risk mitigation experiment for the International Space Station was designed to better understand the interaction of materials with the low Earth orbit environment and to better understand the potential contamination threats that may be present in the vicinity of spacecraft. Deterioration in the optical performance of candidate space power materials due to the low Earth orbit environment, the contamination environment, or both, must be evaluated in order to propose measures to mitigate such deterioration. The thirty-two samples of space power materials studied here include solar array blanket materials such as polyimide Kapton H and SiOx coated polyimide Kapton H, front surface aluminized sapphire, solar dynamic concentrator materials such as silver on spin coated polyimide and aluminum on spin coated polyimide, CV1144 silicone, and the thermal control paint Z-93-P. The physical and optical properties that were evaluated prior to and after the POSA flight include mass, total, diffuse, and specular reflectance, solar absorptance, and infrared emittance. Additional post flight evaluation included scanning electron microscopy to observe surface features caused by the low Earth orbit environment and the contamination environment, and variable angle spectroscopic ellipsometry to identify contaminant type and thickness. This paper summarizes the results of pre- and post-flight measurements, identifies the mechanisms responsible for optical properties deterioration, and suggests improvements for the durability of materials in future missions.
The probability of atomic oxygen reacting with polymeric materials is orders of magnitude lower at thermal energies (<0.1 eV) than at orbital impact energies (4.5 eV). As a result, absolute atomic oxygen fluxes at thermal energies must be orders of magnitude higher than orbital energy fluxes, to produce the same effective fluxes (or same oxidation rates) for polymers. These differences can cause highly pessimistic durability predictions for protected polymers, and polymers which develop protective metal oxide surfaces as a result of oxidation if one does not make suitable calibrations. A comparison was conducted of undercut cavities below defect sites in protected polyimide Kapton samples flown on the Long Duration Exposure Facility (LDEF) with similar samples exposed in thermal energy oxygen plasma. The results of this comparison were used to quantify predicted material loss in space based on material loss in ground laboratory thermal energy plasma testing. A microindent hardness comparison of surface oxidation of a silicone flown on the Environmental Oxygen Interaction with Materials III (EOIM-III) experiment with samples exposed in thermal energy plasmas was similarly used to calibrate the rate of oxidation of silicone in space relative to samples in thermal energy plasmas exposed to polyimide Kapton effective fluences.
Space solar power systems for use in low Earth orbit (LEO) environment experience a variety of harsh environmental conditions. Materials used for solar power generation in LEO need to be durable to environmental threats such as atomic oxygen, ultraviolet (UV) radiation, thermal cycling, and micrometeoroid and debris impact. Another threat to LEO solar power performance is due to contamination from other spacecraft components. This paper gives an overview of these LEO environmental issues as they relate to space solar power system materials. Issues addressed include atomic oxygen erosion of organic materials, atomic oxygen undercutting of protective coatings, UV darkening of ceramics, UV embrittlement of Teflon, effects of thermal cycling on organic composites, and contamination due to silicone and organic materials. Specific examples of samples from the Long Duration Exposure Facility (LDEF) and materials returned from the first servicing mission of the Hubble Space Telescope (HST) are presented. Issues concerning ground laboratory facilities which simulate the LEO environment are discussed along with ground-to-space correlation issues.
For spacecraft in low Earth orbit (LEO), contamination can occur from thruster fuel, sputter contamination products, and from products of silicone degradation. This paper describes laboratory testing in which solar cell materials and thermal control surfaces were exposed to simulated spacecraft environmental effects including contamination, atomic oxygen, ultraviolet radiation and thermal cycling. The objective of these experiments was to determine how the interaction of the natural LEO environmental effects with contaminated spacecraft surfaces impacts the performance of these materials. Optical properties of samples were measured and solar cell performance data was obtained. In general, exposure to contamination by thruster fuel resulted in degradation of solar absorptance for fused silica and various thermal control surfaces and degradation of solar cell performance. Fused silica samples which were subsequently exposed to an atomic oxygen/vacuum ultraviolet radiation environment showed reversal of this degradation. These results imply that solar cells and thermal control surfaces which are susceptible to thruster fuel contamination and which also receive atomic oxygen exposure may not undergo significant performance degradation. Materials which were exposed to only vacuum ultraviolet radiation subsequent to contamination showed, slight additional degradation in solar absorptance.
de Groh, K. K. and McCollum, T. A., "Low Earth Orbit Durability of Protected Silicone for Refractive Photovoltaic Concentrator Arrays", Journal of Spacecraft and Rockets, Vol. 32, Num. 1, p.103-109. Photovoltaic power systems with novel refractive silicone
solar
concentrators are being developed for use in low Earth orbit (LEO).
Because
of the vulnerability of silicones to atomic oxygen and ultraviolet
radiation,
these lenses are coated with a multi-layer metal oxide protective
coating.
The objective of this work was to evaluate the effects of atomic oxygen
and
thermal exposures on multi-layer coated silicone. Samples were exposed
to
high-fluence ground-laboratory and low-fluence in-space atomic oxygen.
Ground
testing resulted in decreases in both total and specular transmittance,
while
in-space exposure resulted in only small decreases in specular
transmittance.
A contamination film, attributed to exposed silicone at coating crack
sites,
was found to cause transmittance decreases during ground testing.
Propagation
of coating cracks was found to be the result of sample heating during
exposure.
The potential for silicone exposure, with the resulting degradation of
optical
properties from silicone contamination, indicates that his multi-layer
coated
silicone is not durable for LEO space applications where thermal
exposures
will cause coating crack development and propagation. |
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