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NASA NTRS1992

Oxidation kinetics of CVD silicon carbide and silicon nitride

The long-term oxidation behavior of pure, monolithic CVD SiC and Si3N4 is studied, and the isothermal oxidation kinetics of these two materials are obtained for the case of 100 hrs at 1200-1500 C in flowing oxygen. Estimates are made of lifetimes at the various temperatures investigated. Parabolic rate constants for SiC are within an order of magnitude of shorter exposure time values reported in the literature. The resulting silica scales are in the form of cristobalite, with cracks visible after exposure. The oxidation protection afforded by silica for these materials is adequate for long service times under isothermal conditions in 1-atm dry oxygen.

Fox, Dennis S.↗

NASA NTRS2003

Oxidation of Ultra-High Temperature Ceramics in Water Vapor

Ultra high temperature ceramics (UHTCs) including HfB2 + SiC (20% by volume), ZrB2 + SiC (20% by volume) and ZrB2 + SiC (14% by volume) + C (30% by volume) have historically been evaluated as reusable thermal protection systems for hypersonic vehicles. This study investigates UHTCs for use as potential combustion and aeropropulsion engine materials. These materials were oxidized in water vapor (90%) using a cyclic vertical furnace at 1 atm. The total exposure time was 10 hours at temperatures of 1200, 1300, and 1400 C. CVD SiC was also evaluated as a baseline comparison. Weight change measurements, X-ray diffraction analyses, surface and cross-sectional SEM and EDS were performed. These results will be compared with tests ran in static air at temperatures of 1327, 1627, and 1927 C. Oxidation comparisons will also be made to the study by Tripp. A small number of high pressure burner rig (HPBR) results at 1100 and 1300 C will also be discussed. Specific weight changes at all three temperatures along with the SIC results are shown. SiC weight change is negligible at such short duration times. HB2 + SiC (HS) performed the best out of all the tested UHTCS for all exposure temperatures. ZrB2 + Sic (ZS) results indicate a slightly lower oxidation rate than that of ZrBl + SiC + C (ZCS) at 1200 and 1400 C, but a clear distinction can not be made based on the limited number of tested samples. Scanning electron micrographs of the cross-sections of all the UHTCs were evaluated. A representative area for HS is presented at 1400 C for 26 hours which was the composition with the least amount of oxidation. A continuous SiO2 scale is present in the outer most edge of the surface. An image of ZCS is presented at 1400 C for 10 hours, which shows the most degradation of all the compositions studied. Here, the oxide surface is a mixture of ZrSiO4, ZrO2 and SO2.

Nguyen, QuynhGiao N.↗

NASA NTRS2005

Going to the Bottom of the Explosion that Formed Cas A

The purpose of this INTEGRAL AO 1 proposal was to observe the gamma-ray lines at 1157 keV, as well as 68 keV and 78 keV of the (44)Ti decay chain from the youngest known galactic supernova remnant Cas A. The radioactive decay of (44)Ti in Cas A was first detected by COMPTEL in the 1157 keV line and later confirmed in the lower energy lines using BeppoSAX data. Despite the second confirming measurement, the flux value still has a considerable error, due to a low signal over background ratio and the poor definition of the continuum background in the range of 70 - 100 keV above the line energies. In addition to improving this flux measurement, INTEGRAL has the unique capability, in principle, to measure the line width of the emission. This type of measurement can further constrain supernova explosion models. Unfortunately, the INTEGRAL instrumental background, especially in the SPI spectrometer, turned out to be significantly higher than estimated at the time of proposal submission before launch. This, and to a smaller extent the reduced effective exposure time of about 1.5 Ms compared to requested 2.5 Ms, has precluded us from reaching the sensitivity necessary for this measurement. Studies of the background variability with the goal of improving background modeling are ongoing, and additional 2.5 Ms observing time has just been approved for the next observing cycle.

Oberlack, Uwe↗

NASA NTRS1966

Log-normal model for microbial survival in heat sterilization

An analytical model is developed for the survival times of organisms in heat sterilization in which the probability of inactivation as a function of exposure time is log-normally distributed. Experimental data is examined relative to this model and it is concluded that the model is valid except during the initial period of heating when an additional interaction between the organisms and their surrounding medium appears to be present. At long heating times, the log-normal model appears more accurate for extrapolating to low survival probabilities than the usual logarithmic survivor curves and is generally more conservative.

SURVIVAL↗

NASA NTRS1975

Mechanical behavior of the directionally solidified gamma/gamma prime - delta eutectic alloy

The eutectic alloy Ni-20.0%Cb-2.5%Al-6.0%Cr was tested in short-term creep and long-term exposure to service conditions to assess its suitability for high temperature turbine blade applications. Long-time exposure showed the lamellar microstructure of the alloy to be exceptionally stable. Other properties tested were notch sensitivity, isothermal and thermomechanical fatigue strength, shear strength, and transverse ductility. It was shown that this alloy is superior to the best currently available directionally solidified superalloys over the temperature/stress conditions encountered in turbine airfoils.

Barkalow, R. H.↗

NASA NTRS1977

Optical and electrical properties of ion beam textured Kapton and Teflon

An electron bombardment argon ion source was used to ion etch polyimide (Kapton) and fluorinated ethylene, FEP (Teflon). Samples of polyimide and FEP were exposed to (0.5-1.0) keV Ar ions at ion current densities of (1.0-1/8) mA/sq cm for various exposure times. Changes in the optical and electrical properties of the samples were used to characterize the exposure. Spectral reflectance and transmittance measurements were made between 0.33 and 2.16 micron m using an integrating sphere after each exposure. From these measurements, values of solar absorptance were obtained. Total emittance measurements were also recorded for some samples. Surface resistivity was used to determine changes in the electrical conductivity of the etched samples. A scanning electron microscope recorded surface structure after exposure. Spectral optical data, resistivity measurements, calculated absorptance and emittance measurements are presented along with photomicrographs of the surface structure for the various exposures to Ar ions.

Mirtich, M. J.↗

NASA NTRS1979

Solar power satellite system definition study. Volume 4: Silicon solar cell annealing test, phase 1

Laser annealing tests were conducted on ten 50 micron cells. Two were control cells that were not irradiated. These showed no loss in output due to exposure to the laser. Two cells were broken in handling. Six cells were successfully tested. All cells tested without breakage showed some recovery. One cell was subjected to two cycles and showed recovery on both cycles. Cells that were moderately degraded appeared to recover more completely than those more severly degraded. Exposure times ranged from two to ten seconds at 500 degrees centigrade. There was some indication that longer exposure was beneficial.

Walker, F.↗

NASA NTRS2004

Central Stars of Planetary Nebulae in the LMC

In FUSE cycle 2's program B001 we studied Central Stars of Planetary Nebulae (CSPN) in the Large Magellanic Could. All FUSE observations have been successfully completed and have been reduced, analyzed and published. The analysis and the results are summarized below. The FUSE data were reduced using the latest available version of the FUSE calibration pipeline (CALFUSE v2.2.2). The flux of these LMC post-AGB objects is at the threshold of FUSE's sensitivity, and thus special care in the background subtraction was needed during the reduction. Because of their faintness, the targets required many orbit-long exposures, each of which typically had low (target) count-rates. Each calibrated extracted sequence was checked for unacceptable count-rate variations (a sign of detector drift), misplaced extraction windows, and other anomalies. All the good calibrated exposures were combined using FUSE pipeline routines. The default FUSE pipeline attempts to model the background measured off-target and subtracts it from the target spectrum. We found that, for these faint objects, the background appeared to be over-estimated by this method, particularly at shorter wavelengths (i.e., < 1000 A). We therefore tried two other reductions. In the first method, subtraction of the measured background is turned off and and the background is taken to be the model scattered-light scaled by the exposure time. In the second one, the first few steps of the pipeline were run on the individual exposures (correcting for effects unique to each exposure such as Doppler shift, grating motions, etc). Then the photon lists from the individual exposures were combined, and the remaining steps of the pipeline run on the combined file. Thus, more total counts for both the target and background allowed for a better extraction.

Bianchi, Luciana↗

NASA NTRS2024

X-Ray Polarization Measurement of the Gold Standard of Radio-Quiet Active Galactic Nuclei: NGC 1068

Context.NGC 1068 is the most observed radio-quiet active galactic nucleus (AGN) in polarimetry, yet its high-energy polarization has never been probed before due to a lack of dedicated polarimeters.Aims.Using the first X-ray polarimeter sensitive enough to measure the polarization of AGNs, we want to probe the orientation and geometric arrangement of (sub)parsec-scale matter around the X-ray source. Methods. We used the Imaging X-ray Polarimetry Explorer (IXPE) satellite to measure, for the first time, the 2–8 keV polarization of NGC 1068.We pointed IXPE at the target for a net exposure time of 1.15 Ms, in addition to using two Chandra snapshots of∼10 ks each in order to account for the potential impact of several ultraluminous X-ray sources (ULXs) within IXPE’s field of view. Results. We measured a 2–8 keV polarization degree of 12.4%±3.6% and an electric vector polarization angle of 101◦±8◦at a 68% confidence level. If we exclude the spectral region containing bright Fe K lines and other soft X-ray lines where depolarization occurs, the polarization fraction rises to 21.3%±6.7% in the 3.5–6.0 keV band, with a similar polarization angle. The observed polarization angle is found to be perpendicular to the parsec-scale radio jet. Using a combined Chandra and IXPE analysis plus multiwavelength constraints, we estimated that the circum nuclear“torus” may sustain a half-opening angle of 50–55◦(from the vertical axis of the system). Conclusions. Thanks to IXPE, we have measured the X-ray polarization of NGC 1068 and found comparable results, both in terms of the polarization angle orientation with respect to the radio jet and the torus half-opening angle, to the X-ray polarimetric measurement achieved for the other archetypal Compton-thick AGN: the Circinus galaxy. Probing the geometric arrangement of parsec-scale matter in extragalactic objects is now feasible thanks to X-ray polarimetry

active galaxies↗

NASA NTRS2000

Far Ultraviolet Imaging from the Image Spacecraft: Wideband FUV Imaging - 2

The Far Ultraviolet Wideband Imaging Camera (WIC) complements the magnetospheric images taken by the IMAGE satellite instruments with simultaneous global maps of the terrestrial aurora. Thus, a primary requirement of WIC is to image the total intensity of the aurora in wavelength regions most representative of the aurora] source and least contaminated by dayglow, have sufficient field of view to cover the entire polar region from spacecraft apogee and have resolution that is Sufficient to resolve auroras on a scale of 1 to 2 latitude degrees, The instrument is sensitive in the spectral region from 140- 190 nm. The WIC is mounted on the rotating, IMAGE spacecraft viewing radially outward and has a field of view of 17 deg in the direction parallel to the spacecraft spin axis. Its field of view is 30 deg in the direction perpendicular to the spin axis, although only a 17 deg x 17 deg image of the Earth is recorded. The optics was an all-reflective, inverted Cassegrain Burch camera using concentric optics with a small convex primary and a large concave secondary mirror. The mirrors were coated by a special multi-layer coating, which has low reflectivity in the visible and near UV region, The detector consists of a MCP-Intensified CCD. The MCP is curved to accommodate the focal surface of the concentric optics. Tile phosphor of the image intensifier is deposited on a concave fiberoptic window, which is then Coupled to the CCD with a fiberoptic taper. The camera head operates in a fast frame transfer mode with the CCD being read approximately 30 full frames (512 by 256 pixel) per second with an exposure time of 0.033 s. The image motion (file to the satellite spin is minimal during such a short exposure. Each image is electronically distortion corrected using the look up table scheme. An offset is added to each memory address that is proportional to the image shift due to satellite rotation, and the charge signal is digitally summed in memory. On orbit, approximately 300 frames will be added to produce one WIC image in memory. The advantage of the electronic motion compensation and distortion correction is that it is extremely flexible, permitting several kinds of corrections including motions parallel and perpendicular to the predicted axis of rotation. File instrument was calibrated by applying ultraviolet light through a vacuum monochromator and measuring the absolute responsivity of the instrument. To obtain the data for the distortion look up table the camera was turned through various angles and the input angles corresponding to a pixel matrix were recorded. It was found that the spectral response peaked at 150 nm and fell off in either direction. The equivalent aperture of the camera, including mirror reflectivities and effective photocathode quantum efficiency, is about 0.04 sq cm. Thus, a 100 Rayleigh LBH aurora is expected to produce 23 equivalent counts per pixel per 10 s exposure at the peak of instrument response.

Mende, S. B.↗

NASA NTRS2007

Improved C/SiC Ceramic Composites Made Using PIP

Improved carbon-fiber-reinforced SiC ceramic-matrix composite (C/SiC CMC) materials, suitable for fabrication of thick-section structural components, are producible by use of a combination of raw materials and processing conditions different from such combinations used in the prior art. In comparison with prior C/SiC CMC materials, these materials have more nearly uniform density, less porosity, and greater strength. The majority of raw-material/processing-condition combinations used in the prior art involve the use of chemical vapor infiltration (CVI) for densifying the matrix. In contrast, in synthesizing a material of the present type, one uses a combination of infiltration with, and pyrolysis of, a preceramic polymer [polymer infiltration followed by pyrolysis (PIP)]. PIP processing is performed in repeated, tailored cycles of infiltration followed by pyrolysis. Densification by PIP processing takes less time and costs less than does densification by CVI. When one of these improved materials was tested by exposure to a high-temperature, inert-gas environment that caused prior C/SiC CMCs to lose strength, this material did not lose strength. (Information on the temperature and exposure time was not available at the time of writing this article.) A material of the present improved type consists, more specifically, of (1) carbon fibers coated with an engineered fiber/matrix interface material and (2) a ceramic matrix, containing SiC, derived from a pre-ceramic polymer with ceramic powder additions. The enhancements of properties of these materials relative to those of prior C/SiC CMC materials are attributable largely to engineering of the fiber/ matrix interfacial material and the densification process. The synthesis of a material of this type includes processing at an elevated temperature to a low level of open porosity. The approach followed in this processing allows one to fabricate not only simple plates but also more complexly shaped parts. The carbon fiber reinforcement in a material of this type can be in any of several alternative forms, including tow, fabric, or complex preforms containing fibers oriented in multiple directions.

Easler, Timothy↗

NASA NTRS1981

Combined single-pulse holography and time-resolved laser schlieren for flow visualization

A pulsed ruby laser and continuous-wave argon ion laser were used in a combined setup at the Langley Expansion Tube for single pulse holography and time resolved laser schlieren with a common optical axis. The systems can be operated simultaneously for a single run. For a single frame, the pulsed holographic setup offers the options of shadowgraph, Schlieren, and interferometry from the reconstructed hologram as well as the advantage of post-run sensitivity adjustments. For flow establishment studies the time resolved laser Schlieren provides visualization of the flow field every 12.5 microns for up to 80 frames with an exposure time per frame of 5.4 microns.

Burner, A. W.↗

NASA NTRS2000

Physiological effects of light on the human circadian pacemaker

The physiology of the human circadian pacemaker and its influence and on the daily organization of sleep, endocrine and behavioral processes is an emerging interest in science and medicine. Understanding the development, organization and fundamental properties underlying the circadian timing system may provide insight for the application of circadian principles to the practice of clinical medicine, both diagnostically (interpretation of certain clinical tests are dependent on time of day) and therapeutically (certain pharmacological responses vary with the time of day). The light-dark cycle is the most powerful external influence acting upon the human circadian pacemaker. It has been shown that timed exposure to light can both synchronize and reset the phase of the circadian pacemaker in a predictable manner. The emergence of detectable circadian rhythmicity in the neonatal period is under investigation (as described elsewhere in this issue). Therefore, the pattern of light exposure provided in the neonatal intensive care setting has implications. One recent study identified differences in both amount of sleep time and weight gain in infants maintained in a neonatal intensive care environment that controlled the light-dark cycle. Unfortunately, neither circadian phase nor the time of day has been considered in most clinical investigations. Further studies with knowledge of principles characterizing the human circadian timing system, which governs a wide array of physiological processes, are required to integrate these findings with the practice of clinical medicine.

Review, Tutorial↗

NASA NTRSDate not supplied

Scroll Pump Dust Tolerance Test for Martian Atmospheric Acquisition

The ability to generate oxygen on the Martian surface will be essential to establishing a human presence on Mars. Flow generating devices such as pumps, compressors, and blowers will be crucial components in Martian atmospheric acquisition and processing systems that need to work reliably during the mission duration. A concern with the reliability of the system is its ability to continue to perform nominally when Martian dust, if it bypasses the inlet filter, enters into the system. A series of tests were conducted to simulate the ingestion of Martian dust on a small scroll pump, similar to the one used on the MOXIE payload. The inlet of the pump was connected to a large volume closed-loop pipe system, known as the Mars Atmospheric Flow Loop, containing pure CO2 gas at a Martian pressure of 7 Torr. A length of stainless steel tubing was extended from the inlet port of scroll pump, which was mounted outside the flow loop, to the inside volume of the flow loop using a feed-through compression fitting. A steady low-speed flow was generated inside the flow loop to continuously disperse and transport the dust toward the inlet tubing. JSC-Mars 1 Martian simulant was used to challenge the pump. Pump performance parameters such as flow rate, pump speed, pressure and temperature were monitored during these tests. Samples of the dust entering the pump inlet were taken prior to the exposure tests, using an inline filter element to determine the rate of dust ingestion into the internal components of the pump. After two tests with exposure times of one hour, the data indicated that small changes in pump performance took place under high rates of dust exposure.

Martian Atmospheric↗

NASA NTRSDate not supplied

The Microbiology of Microgreens Grown in Controlled Environment Chambers under ISS Conditions

Microgreens have been identified as a new type of pick-and-eat salad crop that can be utilized in space crop production systems. The majority of traditionally grown leafy green crops can be grown as microgreens, in addition to crops such as legumes, sunflower, buckwheat, most herbs, and corn, presenting hundreds of microgreen crop options. Notably, microgreens are nutrient dense, high in beneficial compounds like antioxidants, Vitamins C and K, and exhibit a variety of desirable flavors and textures.The short growth cycles (7-14 days), low water requirements and volume optimization potential make them a viable option for sustainable production of nutritious and flavorful crops in space.The crop production team at Kennedy Space Center is investigating the food safety aspects of microgreens grown under spaceflight relevant conditions for crew consumption. Microbiological analysis and screening for potential foodborne pathogens was performed on over 20 varieties of microgreens that have demonstrated positive horticultural attributes.Additionally, a comparison of microgreens grown hydroponically under ISS environmental conditions and similar varieties from local markets was completed to collect baseline data on the microbial load on microgreens. In aneffort to improve microgreen quality, strategies to reduce the microbial load were tested, including bulk seed sanitization, harvest age, exposure to high blue light, and post-harvest chemical disinfection. The efficacy of a citric acid-based produce wash currently used for ISS grown produce and 1% H2O2were investigated at different exposure times for reduction in bacterial and fungal counts on a variety of microgreens. Limited log reduction was achieved depending on exposure time.Our testing also demonstrated that seed sanitization impacted microbial load on microgreens and systems.

Mary E. Hummerick↗

NASA NTRS1975

Report on computation of repetitive hyperbaric-hypobaric decompression tables

The tables were constructed specifically for NASA's simulated weightlessness training program; they provide for 8 depth ranges covering depths from 7 to 47 FSW, with exposure times of 15 to 360 minutes. These tables were based up on an 8 compartment model using tissue half-time values of 5 to 360 minutes and Workmanline M-values for control of the decompression obligation resulting from hyperbaric exposures. Supersaturation ratios of 1.55:1 to 2:1 were used for control of ascents to altitude following such repetitive dives. Adequacy of the method and the resultant tables were determined in light of past experience with decompression involving hyperbaric-hypobaric interfaces in human exposures. Using these criteria, the method showed conformity with empirically determined values. In areas where a discrepancy existed, the tables would err in the direction of safety.

Edel, P. O.↗

NASA NTRS1978

The IRAS radiation environment

Orbital flux integration for three selected mission altitudes and geographic instantaneous flux-mapping for nominal flight-path altitude were used to determine the external charged particle radiation predicted for the Infrared Astronomy Satellite. A current field model was used for magnetic field definitions for three nominal circular trajectories and for the geographic mapping positions. Innovative analysis features introduced include (1) positional fluxes as a function of time and energy for the most severe pass through the South Atlantic Anomaly; (2) total positional doses as a function of time and shield thickness; (3) comparison mapping fluxes for ratios of positional intensities to orbit integrated averages; and (4) statistical exposure-time history of a trajectory as a function of energy indicating, in percent of total mission duration, the time intervals over which the instantaneous fluxes would exceed the orbit integrated averages. Results are presented in tables and graphs.

Stassinopoulos, E. G.↗

NASA NTRS1980

Research plan for establishing the effects of time varying noise exposures on community annoyance and acceptability

The design of a community noise survey to determine the effects of time varying noise exposures in residential communities is presented. Complex physical and human variables involved in the health and welfare effects of environmental noise and the number-level tradeoffs and time of day penalties are among the factors considered. Emphasis is placed on community reactions where noise exposures are equal in day or evening but differ in the night time, and the effects of ambient noise on more intense aircraft noise exposures. Thirteen different times of day and types of operation situations with exposed populations up to 8-10 miles from the airport are identified. A detailed personal interview questionnaire as well as specific instructions to interviewers are included.

Borsky, P. N.↗