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Modeling of Transmittance Degradation
Caused by
Optical Surface Contamination by Atomic Oxygen Reaction With Adsorbed
Silicones
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.
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Hall Effect Thruster Plume Contamination
and Erosion Study
The objective of the Hall effect thruster plume contamination and
erosion study was to evaluate the impact of a xenon ion plume on
various samples placed in the vicinity of a Hall effect thruster for a
continuous 100 hour exposure. NASA Glenn Research Center was
responsible for the pre- and post-test evaluation of three sample types
placed around the thruster: solar cell cover glass, RTV silicone,
and Kapton®. Mass and profilometry were used to identify the degree of
deposition and/or erosion on the solar cell cover glass,
RTV silicone, and Kapton® samples. Transmittance, reflectance, solar
absorptance,
and room temperature emittance were used to identify the degree of
performance
degradation of the solar cell cover glass samples alone. Auger
spectroscopy
was used to identify the chemical constituents found on the surface of
the
exposed solar cell cover glass samples. Chemical analysis indicated
some
boron nitride contamination on the samples, from boron nitride
insulators
used in the body of the thruster. However, erosion outweighed
contamination. All samples exhibited some degree of erosion, with ±90°
positions. For the solar cell cover glass samples, erosion progressed
through the antireflective coating and into the microsheet glass
itself. Erosion occurred in the solar cell cover glass, TRV silicone,
and Kapton® at different rates. All optical properties changed with the
degree of erosion. The transmittance of some samples decreased while
the reflectance of some samples increased and others decreased. All
results are consistent with an energetic plume of xenon ions serving as
a source for erosion.
Reaction of YBa2Cu3O7-d
with Gold, Silver, Bismuth, and Lead: Substitution Chemistry and
Composite Fabrication
The reaction of YBa2Cu3O7-d with Au,
Ag, Bi, and Pb ions or metal is described. Three types of materials
were produced: a well-defined series of homogeneous superconductors was
obtained for Au ion substitution with little effect on Tc;
attempted Ag and Bi ion substitution resulted in multi-phase samples
with slightly enhanced Tc; finally, attempts to produce
superconducting metal/superconducting ceramics composites with Pb and
Bi powders resulted in multi-phase samples with drastically diminished
superconducting properties. For Au-substituted superconductors, YBa2(Cu1-xAux)3O7-d
, a substitution series (x= 0-0.1) has been synthesized.
For x=0.1 there was no change in the a and b lattice parameters
(a=3.826 Å and b=3.889 Å) but a 0.06 Å c axis expansion to 11.75 Å was
observed. The valence of Cu and Au in YBa2Au3Cu2.7O7-d
was investigated using x-ray absorption near-edge structure
(XANES).
X-ray studies indicate that Au goes into the Cu(1) site and Cu K edge
XANES
shows that this has little effect on the oxidation state of the
remaining
copper. A small effect on Tc is observed (Tc=89
for
x=0.10). Ag and Bi addition results in a rise in Tc and a
decrease
in D Tc at low levels (x=0.10 Ag, Tc=94K and D Tc=0.5K;
x= 0.02 Bi, Tc=94K and D Tc=1K) relative to
typical values for YBa2Cu3O7-d (Tc=91K,
D Tc=2K). Attempts at fabrication of Pb- and Pb1-xBix-superconductor
composites are described. Cold pressing followed by low temperature
(200° C) sintering resulted in a composite which excluded flux below
90K but did not show zero electrical resistance until the metal (alloy)
superconducting transition. X-ray diffraction showed the presence of
perovskite and metal. Processing at moderate (450° C) or high (950° C)
temperatures resulted in oxygen-depleted perovskite and/or metal
oxides. These materials displayed greatly degraded superconducting
properties. Processing at 800° C resulted in high Tc only
for composites containing 90% weight fraction ceramic.
Reaction of metal with YBa2Cu3O7-d formed
superconducting lead/bismuth-based oxides and other binary oxides.
Chemical Interactions Between Candidate
Materials and Simulated Lunar Basalt at Temperatures up to 1273 K
The reactivity of various metal substrates with simulated lunar basalt
was studied by mass balance. Candidate materials were submerged in
simulated lunar basalt powder and heated to temperatures in excess of
1273 K. After exposure, energy dispersive x-ray analysis was used to
provide supporting evidence of the elemental composition of the metal
substrates. Niobium, niobium-1%zirconium, and UDIMET 720 gained weight
as a result of the high temperature exposure to the basalt and energy
dispersive x-ray analysis revealed
the presence of iron in these substrates, suggesting that iron diffused
from
the basalt to the metal substrate. In addition, the basalt lost weight
as
a result of the high temperature exposure, suggesting the evolution of
a
volatile species. Other substrates, such as molybdenum and carbon
showed no change in weight as a result of the high temperature exposure
to the basalt.
The results of the high temperature testing using the simulated lunar
basalt
will be discussed. |
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