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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Preceramic Polymers for Use as Fiber Coatings

Polymeric precursors to Si-C-O, SI-B-N and Si-C were evaluated for use as ceramic interfaces in ceramic matrix composites. Use of the preceramic polymers allows for easy dip coating of fibers from dilute solutions of a polymer, which are then pyrolyzed to obtain the ceramic. SCS-0 fibers (Textron Specialty Materials, Lowell, MA) were coated with polymers from three systems: polysilsesquioxanes, polyborosilazanes and polycarbosilanes. The polysilsesquioxane systems were shown to produce either silicon oxycarbide or silicon oxynitride, depending on the pyrolysis conditions, and demonstrated some promise in an RBSN (reaction-bonded silicon nitride) matrix model system. Polyborosilazanes were shown, in studies of bulk polymers, to give rise to oxidation resistant Si-B-N ceramics which remain amorphous to temperatures of 1600 C, and should therefore provide a low modulus interface. Polycarbosilanes produce amorphous carbon-rich Si-C materials which have demonstrated oxidation resistance.

Heimann, P. J.↗
NASA NTRS1997

New Approaches to Waterproofing of Space Shuttle Insulating Materials

Future reusable space vehicles will be in service much more frequently than current space shuttles. Therefore, rapid reconditioning of spacecraft will be required. Currently, the waterproofing of space shuttles after each re-entry takes 72 hours and requires substantial labor. In addition, the currently used waterproofing reagent, DiMethylEthoxySilane (DMES), is considered toxic, and ethanol fumes are released during its hydrolytic activation. Consequently, a long time period, which is not acceptable for future operations, is needed to ensure that 0 the excess volatile compounds are removed before further maintenance of the space vehicle can be performed. The objective of this project was to assist NASA Ames in finding improved waterproofing systems by identifying suitable waterproofing agents that can be applied by vapor phase deposition and will be less toxic, bond more rapidly to the insulation material surface, and potentially have higher thermal stability than the DMES system. Several approaches to achieve faster waterproofing with less toxicity were assessed using the following alternatives: Reactive volatile compounds that are rapidly deposited by chemical bonding at the surface and leave no toxic volatiles. Reactive reagents that are the least toxic. Nonvolatile reagents that are very reactive and bond strongly to the insulating material surface. Three specific types of potential reagents were chosen for evaluation in this project: 1. Volatile reagents with Si-Cl functional groups for vapor deposition 2. Volatile reagents with Si-H functional groups for vapor deposition 3. Nonvolatile oligomeric or polymeric reactive siloxanes that are assumed to have higher thermal stability and/or strong bonding to the insulating material. The chemistry involved in the project was targeted at the generation of intermediates having reactive Si-OH bonds for the formation of either volatile species or polymeric species that bond rapidly to the surface and also cure rapidly. We focused on two chemical reactions@-hydrolysis of Si-Cl bonds and catalytic dehydrocoupling of Si-H bonds.

Blum, Yigal D.↗
NASA NTRS1998

Buried Porous Silicon-Germanium Layers in Monocrystalline Silicon Lattices

Monocrystalline semiconductor lattices with a buried porous semiconductor layer having different chemical composition is discussed and monocrystalline semiconductor superlattices with a buried porous semiconductor layers having different chemical composition than that of its monocrystalline semiconductor superlattice are discussed. Lattices of alternating layers of monocrystalline silicon and porous silicon-germanium have been produced. These single crystal lattices have been fabricated by epitaxial growth of Si and Si-Ge layers followed by patterning into mesa structures. The mesa structures are strain etched resulting in porosification of the Si-Ge layers with a minor amount of porosification of the monocrystalline Si layers. Thicker Si-Ge layers produced in a similar manner emitted visible light at room temperature.

Fathauer, Robert W.↗
NASA NTRS1998

Microstructural, Chemical and Mechanical Characterization of Polymer-Derived Hi-Nicalon Fibers with Surface Coatings

Room temperature tensile strengths of as-received Hi-Nicalon fibers and those having BN/SiC, p-BN/SiC, and p-B(Si)N/SiC surface coatings, deposited by chemical vapor deposition, were measured using an average fiber diameter of 13.5 microns. The Weibull statistical parameters were determined for each fiber. The average tensile strength of uncoated Hi-Nicalon on was 3.19 +/- 0.73 GPa with a Weibull modulus of 5.41. Strength of fibers coated with BN/SiC did not change. However, coat with p-BN/SiC and p-B(Si)N/SiC surface layers showed strength loss of approx. 10 and 35 percent, respectively, compared with as-received fibers. The elemental compositions of the fibers and the coatings were analyzed using scanning Auger microprobe and energy dispersive x-ray spectroscopy. The BN coating was contaminated with a large concentration of carbon and some oxygen. In contrast, p-BN, p-B(Si)N, and SiC coatings did not show any contamination. Microstructural analyses of the fibers and the coatings were done by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction. Hi-Nicalon fiber consists of the P-SIC nanocrystals ranging in size from 1 to 30 nm embedded in an amorphous matrix. TEM analysis of the BN coating revealed four distinct layers with turbostatic structure. The p-BN layer was turbostratic and showed considerable preferred orientation. The p-B(Si)N was glassy and the silicon and boron were uniformly distributed. The silicon carbide coating was polycrystalline with a columnar structure along the growth direction. The p-B(Si)N/SiC coatings were more uniform, less defective and of better quality than the BN/SiC or the p-BN/SiC coatings.

Bansal, Narottam P.↗
NASA NTRS2001

Spectroscopic and Geochemical Analyses of Ferrihydrite from Hydrothermal Springs in Iceland and Applications to Mars

Ferrihydrite samples were collected from a thermal spring and a cold stream in the Landmannalaugar region of Iceland. Chemical and spectroscopic analyses have been performed on the air-dried and fine-grained fractions of these samples. The ferrihydrite from the cold stream is a pure sample, containing small amounts of Ca, P and Si, which do not form minerals detectable with X-ray diffraction (XRD) or reflectance and transmittance spectroscopy. The ferrihydrite from the thermal pool is a less pure sample, containing larger amounts of amorphous Si and P. The XRD and spectral features for this sample are also consistent with a less crystalline structure. Some of the Si is incorporated in the structure of the ferrihydrite. The Ca, P and possibly some of the Si may be biogenic. The spectral character of these Icelandic ferrihydrites is compared with those of synthetic ferrihydrites and other iron oxide/oxyhydroxide minerals. Ferrihydrite is characterized by a broad Fe3+ excitation band near 0.92 microns (approx. 10900/cm) and a strong Fe-O absorption feature near 475/cm (approx. 21 microns) in transmittance spectra. Multiple bands due to H2O and OH are also present for ferrihydrite. Natural ferrihydrites frequently exhibit a band near 950-1050/cm (approx. 10 microns) that is typically not observed for synthetic ferrihydrites and may be due to some Si in the structure. An additional pair of spectral bands near 1400 and 1500/cm (approx. 7 microns) are characteristic of pure ferrihydrites from natural and synthetic sources. Hydrothermal springs may have been present at one time on Mars in association with volcanic activity. Ferrihydrite formation in such an environment may have contributed to the ferric oxide-rich surface material on Mars.

Bishop, Janice↗
NASA NTRS2004

Durability of Environmental Barrier Coatings in a Water Vapor/Oxygen Environment

Silicon carbide (Sic) and silicon nitride (Si3N4) show potential for application in the hot sections of advanced jet engines. The oxidation behavior of these materials has been studied in great detail. In a pure oxygen environment, a silica (SiO2) layer forms on the surface and provides protection from further oxidation. Initial oxidation is rapid, but slows as silica layer grows; this is known as parabolic oxidation. When exposed to model fuel-lean combustion applications (standard in jet engines), wherein the partial pressure of water vapor is approximately 0.5 atm., these materials exhibit different characteristics. In such an environment, the primary oxidant to form silica is water vapor. At the same time, water vapor reacts with the surface oxide to form gaseous silicon hydroxide (Si(OH)4). The simultaneous formation of both silica and Si(OH)4 -the latter which is lost to the atmosphere- the material continues to recede. Recession rates for uncoated Sic and Si3N4 are unacceptably high, for use in jet engines, - on the order of 1mm/4000h. External coatings have been developed that protect Si-based materials from water vapor attack. One such coating consists of a Ba(0.75)Sr(0.25)Al2Si2O8 (BSAS) topcoat, a mullite/BSAS intermediate layer and a Si bond coat. The key function of the topcoat is to protect the Si-base material from water vapor; therefore it must be fairly stable in water vapor (recession rate of about 1mm/40,000h) and remain crack free. Although BSAS is much more resistant to water vapor attack than pure silica, it exhibits a linear weight loss in 50% H2O - 50% O2 at 1500 C. The objective of my research is to determine the oxidation behavior of a number of alternate hot-pressed monolithic top coat candidates. Potential coatings were exposed at 1500 C to a 50% H2O - 50% O2 gas mixture flowing at 4.4 cm/s . These included rare- earth silicates, barium-strontium aluminosilicates. When weight changes were measured with a continuously recording microbalance, linear weight loss was observed. BSAS materials have a fairly high volatility at this temperature, but rare-earth mono-silicate compounds were significantly more stable.

Holchin, John E.↗
NASA NTRS1994

Long-Wavelength Stacked SiGe/Si Heterojunction Internal Photoemission Infrared Detectors Using Multiple SiGe/Si Layers

Utilizing low temperature silicon molecular beam epitaxy (MBE) growth, long-wavelength stacked SiGe/Si heterojunction internal photoemission (HIP) infrared detectors with multiple SiGe/Si layers have been fabricated and demonstrated. Using an elemental boron source, high doping concentrations (approximately equal to 4 x 10(sup 20) cm(sup -3)) has been achieved and high crystalline quality multiple Si(sub 0.7)Ge(sub 0.3)/Si layers have been obtained. The detector structure consists of several periods of degenerately boron doped (approximately equal to 4 x 10(sup 20) cm(sup -3)) thin (less than or equal to 50 u Si(sub 0.7)Ge(sub 0.3) layers and undoped thick (approximately equal to 300u Si layers. The multiple p(sup +) - Si(sub 0.7)Ge(sub 0.3)/undoped-Si layers show strong infrared absorption in the long-wavelength regime mainly through free carrier absorption. The stacked Si(sub 0.7)Ge(sub 0.3)/Si HIP detectors with p = 4 x 10(sup 20) cm(sup -3) exhibit strong photoresponse at wavelengths ranging from 2 to 20 (micro)m with quantum efficiencies of about 4% and 1.5% at 10 and 15 (micro)m wavelengths, respectively. The detectors show near ideal thermionic-emission limited dark current characteristics.

beam epitaxy↗
NASA NTRS2007

Measurement of Damage Profiles from Solar Wind Implantation

NASA's Genesis Mission launched from Cape Canaveral in August of 2001 with the goal of collecting solar wind in ultra-pure materials. The samples were returned to Earth more than three years later for subsequent analysis. Although the solar wind is comprised primarily of protons, it also contains ionized species representing the entire periodic table. The Genesis mission took advantage of the natural momentum of these ionized species to implant themselves in specialized collectors including single crystal Si and SiC. The collectors trapped the solar wind species of interest and sustained significant damage to the surface crystal structure as a result of the ion bombardment. In this work, spectroscopic ellipsometry has been used to evaluate the extent of this damage in Si and SiC samples. These results and models are compared for artificially implanted samples and pristine non-flight material. In addition, the flown samples had accumulated a thin film of molecular contamination as a result of outgassing in flight, and we demonstrate that this layer can be differentiated from the material damage. In addition to collecting bulk solar wind samples (continuous exposure), the Genesis mission actually returned silicon exposed to four different solar wind regimes: bulk, high speed, low speed, and coronal mass ejections. Each of these solar wind regimes varies in energy, but may vary in composition as well. While determining the composition is a primary goal of the mission, we are also interested in the variation in depth and extent of the damage layer as a function of solar wind regime. Here, we examine flight Si from the bulk solar wind regime and compare the results to both pristine and artificially implanted Si. Finally, there were four samples which were mounted in an electrostatic "concentrator" designed to reject a large fraction (>85%) of incoming protons while enhancing the concentration of ions mass 4-28 amu by a factor of at least 20. Two of these samples were single crystal 6H silicon carbide. (The others were polycrystalline CVD diamond and amorphous carbon that were not examined in the work.) The ion damaged SiC samples from the concentrator were studied in comparison to the flight Si from the bulk array to understand differences in the extent of the damage.

McNamara, K. M.↗
NASA NTRS1997

Method of producing buried porous silicon-geramanium layers in monocrystalline silicon lattices

Lattices of alternating layers of monocrystalline silicon and porous silicon-germanium have been produced. These single crystal lattices have been fabricated by epitaxial growth of Si and Si--Ge layers followed by patterning into mesa structures. The mesa structures are stain etched resulting in porosification of the Si--Ge layers with a minor amount of porosification of the monocrystalline Si layers. Thicker Si--Ge layers produced in a similar manner emitted visible light at room temperature.

Fathauer, Robert W.↗
NASA NTRS2007

Fabrication of Buried Nanochannels From Nanowire Patterns

A method of fabricating channels having widths of tens of nanometers in silicon substrates and burying the channels under overlying layers of dielectric materials has been demonstrated. With further refinement, the method might be useful for fabricating nanochannels for manipulation and analysis of large biomolecules at single-molecule resolution. Unlike in prior methods, burying the channels does not involve bonding of flat wafers to the silicon substrates to cover exposed channels in the substrates. Instead, the formation and burying of the channels are accomplished in a more sophisticated process that is less vulnerable to defects in the substrates and less likely to result in clogging of, or leakage from, the channels. In this method, the first step is to establish the channel pattern by forming an array of sacrificial metal nanowires on an SiO2-on-Si substrate. In particular, the wire pattern is made by use of focused-ion-beam (FIB) lithography and a subsequent metallization/lift-off process. The pattern of metal nanowires is then transferred onto the SiO2 layer by reactive-ion etching, which yields sacrificial SiO2 nanowires covered by metal. After removal of the metal covering the SiO2 nanowires, what remains are SiO2 nanowires on an Si substrate. Plasma-enhanced chemical vapor deposition (PECVD) is used to form a layer of a dielectric material over the Si substrate and over the SiO2 wires on the surface of the substrate. FIB milling is then performed to form trenches at both ends of each SiO2 wire. The trenches serve as openings for the entry of chemicals that etch SiO2 much faster than they etch Si. Provided that the nanowires are not so long that the diffusion of the etching chemicals is blocked, the sacrificial SiO2 nanowires become etched out from between the dielectric material and the Si substrate, leaving buried channels. At the time of reporting the information for this article, channels 3 m long, 20 nm deep, and 80 nm wide (see figure) had been fabricated by this method.

Choi, Daniel↗
NASA NTRS2010

The Biological Effectiveness of Silicon Ions is Significantly Higher than Iron Ions for the Induction of Chromosome Damage in Human Lymphocytes

Chromosome aberrations were measured in human peripheral blood lymphocytes after in vitro exposure to Si-28-ions with energies ranging from 90 to 600 MeV/u, or Fe-56-ions with energies ranging from 200 to 5,000 MeV/u. The LET of the various Fe beams in this study ranged from 145 to 440 keV/micron and the LET Si ions ranged from 48 to 158 keV/micron. Doses delivered were in the 10 to 200 cGy range. Dose response curves for chromosome exchanges in cells at first division after exposure, measured using fluorescence in situ hybridization (FISH) with whole chromosome probes, were fitted with linear or linear-quadratic functions. The relative biological effectiveness (RBE) was estimated from the initial slope of the dose response curve for chromosome damage with respect to gamma-rays. The estimates of RBE(sub max) values for total chromosome exchanges ranged from 4.4+/-0.4 to 31.5+/-2.6 for Fe ions, and 11.8+/-1.0 to 42.2+/-3.3 for Si ions. The highest RBE(sub max) value for Fe ions was obtained with the 600 Mev/u beam and 170 MeV/u beam produced the highest RBE(sub max) value for Si ions. For both ions the RBE(sub max) values increased with LET, reaching a maximum at about 180 keV/micron for Fe and about 100 keV/micron for Si, and decreased with further increase in LET.

George, Kerry↗
NASA NTRS2011

Enhanced Low Dose Rate Sensitivity at Ultra-Low Dose Rates

We have presented results of ultra-low dose rate irradiations (< or = 10 mrad(Si)/s) for a variety of radiation hardened and commercial linear bipolar devices. We observed low dose rate enhancement factors exceeding 1.5 in several parts. The worst case of dose rate enhancement resulted in functional failures, which occurred after 10 and 60 krad(Si), for devices irradiated at 0.5 and 10 mrad(Si)/s, respectively. Devices fabricated with radiation hardened processes and designs also displayed dose rate enhancement at below 10 mrad(Si)/s. Furthermore, the data indicated that these devices have not reached the damage saturation point. Therefore the degradation will likely continue to increase with increasing total dose, and the low dose rate enhancement will further magnify. The cases presented here, in addition to previous examples, illustrate the significance and pervasiveness of low dose rate enhancement at dose rates lower than 10 mrad(Si). These results present further challenges for radiation hardness assurance of bipolar linear circuits, and raise the question of whether the current standard test dose rate is conservative enough to bound degradations due to ELDRS.

Chen, Dakai↗
NASA NTRS2016

Making Mercury's Core with Light Elements

Recent results obtained from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft showed the surface of Mercury has low FeO abundances (less than 2 wt%) and high S abundances (approximately 4 wt%), suggesting the oxygen fugacity of Mercury's surface materials is somewhere between 3 to 7 log10 units below the IW buffer. The highly reducing nature of Mercury has resulted in a relatively thin mantle and a large core that has the potential to exhibit an exotic composition in comparison to the other terrestrial planets. This exotic composition may extend to include light elements (e.g., Si, C, S). Furthermore, has argued for a possible primary floatation crust on Mercury composed of graphite, which may require a core that is C-saturated. In order to investigate mercurian core compositions, we conducted piston cylinder experiments at 1 GPa, from 1300 C to 1700 C, using a range of starting compositions consisting of various Si-Fe metal mixtures (Si5Fe95, Si10Fe90, Si22Fe78, and Si35Fe65). All metals were loaded into graphite capsules used to ensure C-saturation during the duration of each experimental run. Our experiments show that Fe-Si metallic alloys exclude carbon relative to more Fe-rich metal. This exclusion of carbon commences within the range of 5 to 10 wt% Si. These results indicate that if Mercury has a Si-rich core (having more than approximately 5 wt% silicon), it would have saturated in carbon at low C abundances allowing for the possible formation of a graphite floatation crust as suggested by. These results have important implications for the thermal and magmatic evolution of Mercury.

Vander Kaaden, Kathleen E.↗
NASA NTRS2018

Patterns in Mobility and Modification of Middle and High-Latitude Southern Hemisphere Dunes on Mars

Change detection analyses of aeolian bedforms (dunes and ripples), using multitemporal imagesacquired by the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (HiRISE), canreveal migration of bedforms on Mars. Here we investigated bedform mobility (evidence of wind-drivenmigration or activity), from analysis of HiRISE temporal image pairs, and dune field modification (i.e., apparentpresence/lack of changes or degradation due to nonaeolian processes) through use of a dune stability indexor SI (1–6; higher numbers indicating increasing evidence of stability/modification). Combining mobility dataand SI for 70 dune fields south of 40°S latitude, we observed a clear trend of decreasing bedform mobility withincreasing SI and latitude. Both dunes and ripples were more commonly active at lower latitudes, althoughsome high-latitude ripples are migrating. Most dune fields with lower SIs (3) were found to be active whilethose with higher SIs were primarily found to be inactive. A shift in prevalence of active to apparently inactivebedforms and to dune fields with SI 2 occurs at ~60°S latitude, coincident with the edge of highconcentrations of H2O-equivalent hydrogen observed by the Mars Odyssey Neutron Spectrometer. This result isconsistent with previous studies suggesting that stabilizing agents, such as ground ice, likely stabilize bedformsand limit sediment availability. Observations of active dune fields with morphologies indicative of stability(i.e., migrating ripples in SI = 3 dune fields) may have implications for episodic phases of reworking or dunebuilding, and possibly geologically recent activation or stabilization corresponding to shifts in climate

Banks, Maria E.↗
NASA NTRS1972

Exploration of alloy surface and slurry modification to improve oxidation life of fused silicide coated niobium alloys

Edge and surface modifications of niobium alloys were investigated prior to coating with Si-20Cr-20Fe and slurry composition modification for performance in a 1370 C ambient pressure slow cycle test. The best coating obtained was Si-20Cr-20Mn with an average life of 63 cycles, compared to 40 for Si-20Cr-20Fe on FS-85 (100 percent improvement in weight parity life). Edge beading extended the lives of Si-20Cr-20Fe-coated Cb-752 and FS-85 to 57 and 41 cycles respectively (50 and 20 percent improvements in weight parity life respectively). W, Al2O3 and ZrO2(CaO) surface modifications altered coating crack frequency and microstructure and increased life somewhat.

Levine, S. R.↗
NASA NTRS1972

Rocket spectroscopy of zeta Orionis.

Analysis of a spectrum of zeta Ori extending from 922 to 1453 A with approximately 0.8 A resolution recorded at rocket altitudes. All lines used in existing models of stellar atmospheres appear in the recorded spectrum with the exception of those masked by telluric N2 or strong P Cygni-type profiles and by an O V line at 1371.29 A. Fifteen multiplets of subordinate lines have been reliably identified, indicating an approximate range of excitation from 0 to 50 eV. Transitions in C III (1176 A), N III (991 A), N V (1239, 1243 A), O VI (1032, 1038 A), Si IV (1394, 1403 A), S IV (1063, 1074 A), and S VI (933, 944 A) have been observed as P Cygni-type profiles presumably arising in a circumstellar envelope. The degree of ionization, transitions present, and mean radial velocities are all consistent with viewing the envelope as a hot (about 100,000 K), rarefied plasma in which collisional ionization is important. Interstellar lines in C I (1277, 1280 A), C II (1036, 1334 A), N I (1134-1135 A), N I (1200-1201 A), N II (1084-1086 A), O I (1302, 1305 A), Si II (1190, 1193 A), Si II (1260 A), and Si II (1304 A) have been definitely identified. Other transitions in Ar II, S I, C I, and Fe II are tentatively identified. The equivalent width of the L alpha line is found to be 10.4 plus or minus 1.6 A, corresponding to a columnar density of 2.0 plus or minus 0.7 x 10 to the 20th per cu cm.

Smith, A. M.↗
NASA NTRS1972

Exploration of alloy surface and slurry modification to improve oxidation life of fused silicide coated niobium alloys.

Edge and surface modification of niobium alloys prior to coating with Si-20Cr-20Fe and slurry composition modification were investigated to improve performance in a 1370 C, ambient pressure, slow-cycle test. The best coating obtained was Si-20Cr-20Mn with an average life of 63 cycles compared to 40 for Si-20Cr-20Fe on FS-85 (100 percent improvement in weight parity life). Edge beading extended the lives of Si-20Cr-20Fe coated Cb-752 and FS-85 to 57 and 41 cycles, respectively (50 and 20 percent improvements in weight parity life, respectively).

Levine, S. R.↗
NASA NTRS1971

Effects of positive ion implantation into antireflection coating of silicon solar cells

The state of technological development of Si solar cells for highest obtained efficiency and radiation resistance is summarized. The various theoretical analyses of Si solar cells are reviewed. It is shown that factors controlling blue response are carrier diffusion length, surface recombination, impurity concentration profile in surface region, high level of surface impurity concentration (degeneracy), reflection coefficient of oxide, and absorption coefficient of Si. The theory of ion implantation of charge into the oxide antireflection coating is developed and side effects are discussed. The experimental investigations were directed at determining whether the blue response of Si solar cells could be improved by phosphorus ion charges introduced into the oxide antireflection coating.

Middleton, A. E.↗