|
|
|
![[GRC Hanger link to Glenn Research Center Home Page]](../images/hangarsm.jpg)
![[NASA Logo link to NASA national site]](../plateimg/top_left_globe.gif)
|
 |
Concentrators Titles |
|
|
Titles:
Dever, J. A., Banks, B. A., Yan., L., “Effects
of Vacuum Ultraviolet Radiation on Dow Corning (DC) 93-500 Silicone,"
Journal of Spacecraft and Rockets, Vol. 43, No. 2,
March-April
2006, pp. 386-392. Vacuum ultraviolet radiation is among the space environment
elements
that can be hazardous to DC93-500 silicone film, which has been
proposed
for use on spacecraft exterior surfaces. Investigations have been
conducted
to examine vacuum ultraviolet effects on DC93-500 film. Laboratory
exposure
tests were used to determine the effectiveness of various wavelength
ranges
in causing optical and mechanical degradation and to determine
intensity-dependence
of optical and mechanical properties degradation. Results
indicated that
wavelengths between 185 nm and 200 nm were significantly more effective
in
causing degradation than wavelengths between 140 nm and 185 nm.
These findings
were consistent with results of vacuum ultraviolet ellipsometric
optical
measurements which provided data on depth of penetration in DC93-500 as
a
function of wavelength. Wavelengths between 185 and 200 nm
penetrate to
depths between 1 m and 3 m in DC93-500, depths where bulk degradation
is
likely, whereas the penetration of shorter wavelengths is much more
shallow
and more likely to result only in surface degradation. Results of
exposures
of DC93-500 film samples to vacuum ultraviolet of intensities between
1.5
and 5.5 times the sun’s intensity indicated no intensity-dependence of
optical
and mechanical property degradation.
Joyce Dever, Bruce Banks, and Li Yan, “Vacuum
Ultraviolet Radiation Effects on Dow Corning (DC) 93-500 Silicone Film,”
presented at the “7th International Conference on Protection of
Materials and Structures in the Space Environment (ICPMSE),” held in
Toronto, Canada, May 2004, in Kleiman, Jacob I. (Ed.), Protection of
Materials and Structures from the Space Environment: ICPMSE-7,
Springer Publishing, 2006. A space-qualified silicone polymer (Dow Corning DC93-500) has
been used as a spacecraft solar cell adhesive and has been more
recently proposed for use in a Fresnel lens solar concentrator for
space power applications. Potential future applications of
DC93-500 for exterior spacecraft surfaces require an understanding of
its overall space environment durability. Vacuum ultraviolet
(VUV) radiation is among the space environment elements that can
be hazardous to the properties of DC93-500. This paper describes
investigations into the effects of VUV radiation on DC93-500 silicone
film.
Jaworske, D. A., and Skowronski, T. J., “Optical Efficiency of a Refractive Secondary Concentrator,” prepared for the 35th IECEC, Las Vegas, NV, AIAA-2000-2994, July 2000. Refractive secondary concentrators are being considered as an optical component in solar thermal power and propulsion systems. To evaluate the impact of a refractive secondary concentrator on the overall efficiency of the optical system, the optical efficiency of an acrylic refractive secondary concentrator prototype was evaluated on an optics bench. Light from a laser was allowed to enter the refractive secondary concentrator from a variety of positions, and an integrating sphere was used to collect the exiting light. Energy output of the refractive secondary concentrator was compared to energy input. Several mathematical models were used to describe potential flux distributions impinging on the front surface of the refractive secondary concentrator. For uniform illumination, the optical throughput was found to be 0.874. Different flux profiles resulted in optical throughputs in the range of 0.857 to 0.889. An antireflective coating added to the refractive secondary concentrator should increase optical throughput further.
Jaworske, D. A., de Groh, K. K., Skowronski, T. J., McCollum, T., Pippin, G., and Bungay, C., "Evaluation of Space Power Materials Flown on the Passive Optical Sample Assembly", NASA/TM 1999-209061. Evaluating the performance of materials on the exterior of spacecraft is 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.
Jaworske, D. A., Skowronski, T. J., and Miles, B. J., "Optical Evaluation of an As-Manufactured Compound Secondary Concentrator", prepared for the Space Technology and Applications Forum, 1999. Secondary concentrators are needed in solar thermal propulsion to further concentrate the energy collected by large lightweight primary concentrators. Although the physics of secondary concentrators has been worked on in detail and the manufacturing has been successfully completed for a ground demonstration, there is a need to quantify the specific performance of as-manufactured concentrators. This paper summarizes the properties of a secondary concentrator manufactured for the Integrated Solar Upper Stage engine ground demonstration in 1997 and presents data obtained from the optic that describe the performance of the as-manufactured component.
Refractive secondary concentrators are bring considered for solar thermal applications because of their ability to achieve maximum efficiency through the use of total internal reflection for the concentration and distribution of solar energy. A prototype refractive secondary concentrator was built based on ray tracing analysis to demonstrate this collection and distribution concept. The design included a conical secondary concentrator and a faceted extractor. The objective of this effort was to functionally evaluate the performance of the refractive secondary concentrator/extractor prototype and to compare the results with the modeling. Most of the light was found to exit the refractive secondary concentrator through the extractor. In addition, the degree of attenuation encountered by the light as it passed through the refractive secondary concentrator was of interest. Quantifying optical output and validating the modeling will provide further understanding of the efficiency of the prototype and will provide insight for additional design and materials selection activities.
A bench top facility was used to evaluate the reflectivity of several candidate coating-substrate combinations in vacuum at elevated temperatures. Silver was selected as the reflective coating of choice, while copper, nickel, electroless nickel on copper, and 304 stainless steel were selected as substrates. Pure silver, with no coating at all, was also evaluated. An optically flat silver-coated sapphire substrate was used as a standard. All metal substrates were either metallurgically polished or diamond turned to mirror finish prior to silver deposition. Silicon dioxide was used as a protective coating in most cases. Reflectivity measurements were made at room temperature in the visible range with a spectrophotometer, and at elevated temperatures up to 800°C with a helium-neon laser at 632 nm. Results from the high temperature reflectivity measurements will be presented.
Two light-weight graphite epoxy composite mirrors, each having
a silver reflective layer and a silicon dioxide protection layer, were
exposed to two levels of atomic oxygen fluence in a ground-based plasma
asher facility for the purpose of evaluating their atomic oxygen
durability. Total reflectivity and specular reflectivity were monitored
during the course of atomic oxygen exposure. Optical microscope
photographs were also taken during the course of exposure to document
the process of atomic oxygen undercutting at pin window defect sites.
Although there was evidence of atomic oxygen undercutting at pin window
defects sites, functional performance of the mirrors remained fair over
the course of atomic oxygen exposure.
Second surface silver microsheet glass concentrators are being developed for potential use in future solar dynamic space power systems. Traditional concentrators are aluminum honeycomb sandwich composites with either aluminum or graphite epoxy face sheets, where a reflective aluminum layer is deposited onto an organic leveling layer on the face sheet. To protect the underlying layers, a SiO2 layer is applied on top of the aluminum reflective layer. These concentrators may be vulnerable to atomic oxygen degradation due to possible atomic oxygen attack of the organic layers at defect sites in the protective and reflective coatings. A second surface microsheet glass concentrator would be inherently more atomic oxygen durable than these first surface concentrators. In addition, a second surface microsheet glass concentrator design provides a smooth optical surface and allows for silver to be used as a reflective layer, which would improve the reflectivity of the concentrator and the performance of the system. A potential threat to the performance of second surface microsheet glass concentrators is atomic oxygen attack of the underlying silver at seams and edges or at micrometeoroid and debris (MMD) impact sites. Second surface silver microsheet glass concentrator samples were fabricated and tested for atomic oxygen durability. The samples were initially exposed to an atomic oxygen environment in a plasma asher. Samples were evaluated for potential degradation at fabrication seams, simulated MMD impact sites, and edges. Optical microscopy was used to evaluate atomic oxygen degradation. Reflectance was obtained for an impacted sample prior to and after atomic oxygen exposure. After an initial atomic oxygen exposure to an effective fluence of » 1 x 1021 atoms/cm², oxidation of that silver at defect sites and edges was observed. Exposure to an additional » 1 x 1021 atoms/cm² caused no observed increase in oxidation. Oxidation at an impact site caused negligible changes in reflectance. In all cases oxidation was found to be confined to the vicinity of the seams, impact sites, edges, or defect sites. Asher to in-space atomic oxygen correlation issues will be addressed.
Pinholes or other defect sites in a protective oxide coating provide pathways for atomic oxygen in low Earth orbit to reach underlying material. One concept for enhancing the lifetime of materials in low Earth orbit is to apply a leveling coating to the material prior to the material prior to applying any reflective and protective coatings. Using a surface-tension-leveling coating concept, a low-viscosity epoxy was applied to the surface of several composite coupons. A protective layer of 1000 Å of SiO2 was deposited on top of the leveling coating, and the coupons were exposed to an atomic oxygen environment in a plasma asher. Pinhole populations per unit area were estimated by counting the number of undercut sites observed by scanning electron microscopy. Defect density values of 180,000 defects/cm² were reduced to about 1000 defects/cm² as a result of applied leveling coating. These improvements occur at a mass penalty of about 2.5 mg/cm².
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. |
|
|
![[horizontal rule]](file:///W:/epbranch/images/rule.gif){kind=small}
![[horizontal rule]](../images/rule.gif){kind=small}