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Ultraviolet and Vacuum Ultraviolet Radiation Titles |
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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.
This paper describes testing to investigate the effects of vacuum ultraviolet (VUV) radiation on Teflon® fluorinated ethylene propylene (FEP) film, examining differences in mechanical properties degradation for samples of 50.8 m thickness exposed to VUV of various lower cut-off wavelengths. Samples were illuminated in a high vacuum facility by deuterium lamps, which provide radiation in the 115-400 nm wavelength range, but with the highest intensity being below 200 nm. Windows of fused silica, crystalline quartz, and magnesium fluoride provided lower cut-off wavelengths of 155, 140, and 115 nm, respectively. Lamp intensity was measured in the 115-200 nm wavelength range throughout the sample exposures. The determined intensities were used to estimate intensity and incident energy of various wavelength ranges, between 115 and 400 nm. Samples were analyzed for tensile strength and elongation at failure. The effects of radiation exposures of different wavelength ranges were compared and discussed in terms of the expected depth to which radiation of various wavelengths is deposited into FEP.
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.Vacuum ultraviolet ellipsometric optical measurements were made on DC93-500 silicone to determine the depth of absorption of vacuum ultraviolet light as a function of wavelength. These data indicate the depth within which VUV radiation can cause material degradation. Laboratory VUV exposures were used to examine effects of various VUV exposure wavelength ranges and various VUV exposure intensities to determine whether there exist wavelength or intensity dependencies of degradation. In one set of experiments, transmittance degradation of DC93-500 was examined as a function of exposure to narrow wavelength bands (~ 20 nm bandwidth) of VUV in the 140 to 200 nm wavelength range. In another set of experiments, broad spectrum VUV exposures (greater than 115 nm) were used to examine effects of VUV intensity on rates of optical and mechanical properties degradation. Correlations between observed degradation and the measured depth of VUV penetration will be discussed.
Dever, J. A., and de Groh, K. K., “Vacuum Ultraviolet Radiation and Atomic Oxygen Durability Evaluation of HST Bi-Stem Boom Thermal Shield Materials,” NASA TM-2002-211364, February 2002. Bellows-type thermal shields were proposed for use on the Hubble Space Telescope (HST) solar array bi-stem booms to reduce the thermal gradient-induced jitter during orbital thermal cycling. Candidate thermal shield materials included aluminized FEP Teflon with and without protective coatings for durability to atomic oxygen (AO) and combined AO and ultraviolet (UV) radiation. NASA Lewis (now Glenn) Research Center performed vacuum ultraviolet (VUV) radiation and AO durability testing of candidate materials as part of an overall program coordinated by NASA Goddard Space Flight Center (GSFC) to evaluate the on-orbit durability of these thermal shield materials. Coating adhesion problems were observed for samples having AO- and the combined AO/UV-protective coatings which were attributed to exposure to rapid thermal cycling used to simulate thermal cycling on orbit. Such adhesion problems led to production of coating flakes from the material which could pose a significant risk to HST optics if the coated materials were used for the bi-stem boom thermal shields. No serious degradation was observed for the uncoated aluminized Teflon as evaluated by optical microscopy, although atomic force microscopy (AFM) revealed that an embrittled surface layer would build up on the uncoated Teflon surface due to ultraviolet radiation exposure. This embrittled layer was not completely removed by AO erosion. Despite the formation of this embrittled layer, no cracks or particle flakes were produced for the uncoated material upon exposure to VUV and AO. Uncoated aluminized FEP Teflon was determined to be the most appropriate thermal shield material and was used on the replacement solar arrays installed during the December 1993 First HST Servicing Mission.
Vacuum ultraviolet (VUV) radiation of wavelengths between 115 and 200 nm produced by the sun in the space environment can cause degradation to polymer films producing changes in optical, mechanical, and chemical properties. These effects are particularly important for thin polymer films being considered for ultra-lightweight space structures, because, for most polymers, VUV radiation is absorbed in a thin surface layer. NASA Glenn Research Center has developed facilities and methods for long-term ground testing of polymer films to evaluate space environmental VUV radiation effects. VUV exposure can also be used as part of sequential simulated space environmental exposures to determine combined damaging effects. This paper will describe the effects of VUV on polymer films and the necessity for ground testing. Testing practices used at Glenn Research Center for VUV exposure testing will be described including characterization of the VUV radiation source used, calibration procedures traceable to the National Institute of Standards and Technology (NIST), and testing techniques for VUV exposure of polymer surfaces.
Spacecraft in low Earth orbit (LEO) are subject to degradation
in thermal and optical performance of components and materials through
interaction with atomic oxygen and vacuum ultraviolet radiation which
are predominant in LEO. Due to the importance of LEO durability and
performance to manufacturers and users, an international test program
for assessing the durability
of spacecraft materials and components was initiated. Initial tests
consisted of exposure of samples representing a variety of thermal
control paints and multiplayer insulation materials that have been used
in space. Materials donated from various international sources were
tested alongside a material whose performance is well known such as
Teflon FEP or Kapton H for multiplayer insulation, or Z-93-P for white
thermal control paints. The optical, thermal or mass loss data
generated during the test was then provided to the participating
material supplier. Data was not published unless the participant
donating the material consented to the publication. This paper presents
a description of the types of test and facilities that have been used
for the test
program as well as some examples of data that have been generated. The
test program is intended to give spacecraft builders and users a better
understanding of degradation processes and effects to enable improved
prediction of spacecraft performance. This paper describes the vacuum ultraviolet (VUV) radiation durability screening testing of thin (12.7 to 25.4 µm) polyimide films proposed for use on the Next Generation Space Telescope (NGST) sunshield. Materials included in this screening test were Kapton®E, Kapton®HN, Upilex®S, CP1, CP1 with vapor deposited aluminum (VDA) on its back surface, and CP2 with a VDA coating on its back surface. Samples were exposed to approximately 1000 equivalent sun hours (ESH) of VUV radiation and examined for changes in solar absorptance, thermal emittance, ultimate tensile strength, and elongation-to-failure. Changes in solar absorptance were observed for some materials, and additionally, significant changes in spectral reflectance were observed in the ultraviolet to visible wavelength region for all polyimide materials tested. Changes in ultimate tensile strength and elongation at failure were within the experimental uncertainty for all samples. Longer exposures are needed to verify the observed trends and to develop performance predictions for these materials on the NGST sunshield.
NASA Glenn Research Center at Lewis Field designed and
developed a steady state vacuum ultraviolet automated (SSVUVa) facility
with in
situ VUV intensity calibration capability. The automated feature
enables
a constant accelerated VUV radiation exposure over long periods of
testing without breaking vacuum. This test facility is designed to
simultaneously accommodate four isolated radiation exposure tests
within the SSVUVa vacuum chamber. Computer-control of the facility for
long term continuous operation also provides control and recording of
thermocouple temperatures, periodic recording of VUV lamp intensity,
and monitoring of vacuum facility status. This paper discusses the
design and capabilities of the SSVUVa facility. |
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