|
|
|
![[GRC Hanger link to Glenn Research Center Home Page]](../images/hangarsm.jpg)
![[NASA Logo link to NASA national site]](../plateimg/top_left_globe.gif)
|
 |
Chemical Interactions Studies Titles |
|
|
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.
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.
Illingsworth, M. L., Banks, B. A., Smith, J. W., Jayne, D., Garlick, R. G., Rutledge, S. K., and de Groh, K. K., "Plasma and Beam Facility Atomic Oxygen Erosion of a Transition Metal Complex", Plasma Chemistry and Plasma Processing, Vol. 16, No. 1, 1996.
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 Ĺ and b=3.889 Ĺ) but a 0.06 Ĺ c axis expansion to 11.75 Ĺ 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.
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.
Small bars of ceramic Ba2YCu3O(7-x) were fabricated and subjected to environments similar to those that might be encountered during some NASA missions. These conditions include ambient conditions, high humidity, vacuum, and high fluences of electrons and protons. The normal state resistivity or critical current density (J sub c) were monitored during these tests to assess the stability of the material. When normal state resistivity is used as a criterion, the ambient stability of these samples was relatively good, exhibiting only a 2 percent degradation over a 3 month period. The humidity stability was shown to be very poor, and to be a steep function of temperature. Samples stored at 50 C for 40 min increased in normal state resistivity by four orders of magnitude. Kinetic analysis indicates that the degradation reaction is second order with water vapor concentration. It is suspected that humidity degradation also accounts for the ambient instability. The samples were stable to vacuum over a period of at least 3 months. Degradation of J sub c in a 1 MeV electron fluence of 9.7 x 10 to the 14th e(-)/sq cm was determined to be no more than about 2 percent. Degradation of J sub c in a 8.7 x 10 to the 14th p(+)/sq cm of 42 MeV protons was found to be grain size dependent. Samples with smaller grain size and initial J sub c of about 240 A/sq cm showed no degradation, while that with larger grain size and an initial J sub c of about 30 A/sq cm degraded to 37 percent of its original value.
|
![[horizontal rule]](file:///W:/epbranch/images/rule.gif){kind=small}
|
![[horizontal rule]](../images/rule.gif){kind=small}