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At least 109 records · Page 6

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NASA NTRS1989

Isotopic study of oxygen diffusion in oxide coatings

Diffusion of oxygen in thin films of silicon dioxide was studied using oxygen isotopically enriched in oxygen of atomic mass 18 (O-18). This subject is of interest because thin films of dielectrics such as SiO2 are proposed for use as a protective coatings for solar mirrors in low Earth orbit, which is a strongly oxidizing environment. Films of this material were prepared with a direct current magnetron using reactive sputtering techniques. To produce (O-18)- enriched SiO2, a standard 3.5-in.-diameter silicon wafer was reactively sputtered using (O-18)-enriched (95 percent) oxygen as the plasma feed gas. The films were characterized using Rutherford backscattering and Secondary Ion Mass Spectrometer (SIMS) to establish stoichiometry and purity. Subsequently, the films were exposed to an air-derived oxygen plasma in a standard laboratory plasma reactor for durations of up to 10 hr. The concentration ratio of O-16 as a function of depth was determined using SIMS profiling and compared to a baseline, nonplasma exposed sample. A value for the diffusivity of oxygen near the surface of these films was obtained and found to be about 10(-15)sq cm/sec.

Gulino, Daniel A.↗

NASA NTRS1989

Reusable high-temperature heat pipes and heat pipe panels

A reusable, durable heat pipe which is capable of operating at temperatures up to about 3000 F in an oxidizing environment and at temperatures above 3000 F in an inert or vacuum environment is produced by embedding a refractory metal pipe within a carbon-carbon composite structure. A reusable, durable heat pipe panel is made from an array of refractory-metal pipes spaced from each other. The reusable, durable, heat-pipe is employed to fabricate a hypersonic vehicle leading edge and nose cap.

Camarda, Charles J.↗

NASA NTRS1990

Anode materials for electrochemical waste destruction

Electrochemical Oxidation (ECO) offers promise as a low-temperature, atmospheric pressure method for safe destruction of hazardous organic chemical wastes in water. Anode materials tend to suffer corrosion in the intensely oxidizing environment of the ECO cell. There is a need for cheaper, more resistant materials. In this experiment, a system is described for testing anode materials, with examples of several common anodes such as stainless steel, graphite, and platinized titanium. The ECO system is simple and safe to operate and the experiment can easily be expanded in scope to study the effects of different solutions, temperatures, and organic materials.

Molton, Peter M.↗

NASA NTRS1989

Iridium-coated rhenium thrusters by CVD

Operation of spacecraft thrusters at increased temperature reduces propellant requirements. Inasmuch as propellant comprises the bulk of a satellite's mass, even a small percentage reduction makes possible a significant enhancement of the mission in terms of increased payload. Because of its excellent high temperature strength, rhenium is often the structural material of choice. It can be fabricated into free-standing shapes by chemical vapor deposition (CVD) onto an expendable mandrel. What rhenium lacks is oxidation resistance, but this can be provided by a coating of iridium, also by CVD. This paper describes the process used by Ultramet to fabricate 22-N (5-lbf) and, more recently, 445-N (100-lbf) Ir/Re thrusters; characterizes the CVD-deposited materials; and summarizes the materials effects of firing these thrusters. Optimal propellant mixture ratios can be employed because the materials withstand an oxidizing environment up to the melting temperature of iridium, 2400 C (4350 F).

Harding, J. T.↗

NASA NTRS1990

Volatile compounds in shergottite and nakhlite meteorites

Shergottites ALHA77005, EETA79001, and Shergotty, and the nakhlite Nakhla are all found to contain oxidized sulfur (sulfate) of preterrestrial origin, In EETA79001, which is composed of three different lithologies, the degree of sulfur oxidation increased from A to C. Significant bulk carbonate was confirmed in Nakhla and trace carbonate was confirmed for EETA79001, all of which seems to be preterrestrial in origin. Chlorine in shergottites and Nakhla is found to covary with oxidized sulfur, whereas carbonate and sulfate are related inversely. Sulfate and carbonate in shergottites and Nakhla were probably formed by aqueous geochemistry in a highly oxidizing environment sometime near the late-stage histories of the rocks that are now represented as meteorites.

Gooding, James L.↗

NASA NTRS1992

CVD of silicon carbide on structural fibers: Microstructure and composition

Structural fibers are currently being considered as reinforcements for intermetallic and ceramic materials. Some of these fibers, however, are easily degraded in a high temperature oxidative environment. Therefore, coatings are needed to protect the fibers from environmental attack. Silicon carbide (SiC) was chemically vapor deposited (CVD) on Textron's SCS6 fibers. Fiber temperatures ranging from 1350 to 1500 C were studied. Silane (SiH4) and propane (C2H8) were used for the source gases and different concentrations of these source gases were studied. Deposition rates were determined for each group of fibers at different temperatures. Less variation in deposition rates were observed for the dilute source gas experiments than the concentrated source gas experiments. A careful analysis was performed on the stoichiometry of the CVD SiC coating using electron microprobe. Microstructures for the different conditions were compared. At 1350 C, the microstructures were similar; however, at higher temperatures, the microstructure for the more concentrated source gas group were porous and columnar in comparison to the cross sections taken from the same area for the dilute source gas group.

Veitch, Lisa C.↗

NASA NTRS1992

Wear mechanisms found in angular contact ball bearings of the SSME's LOX turbopump

Extensive experimental investigations were carried out on used flight bearings of the Phase 2 high-pressure oxygen turbopump (HPOTP) of the Space Shuttle Main Engine (SSME) in order to determine the wear mechanisms, dominant wear modes, and their extent and causes. The report shows methodology, surface analysis techniques used, result, and discussion. The mode largely responsible for heavy bearing wear in LOX was identified as adhesive/shear peeling of the upper layers of bearing balls and rings. The mode relies on the mechanisms of scale formation, breakdown, and removal, all of which are greatly enhanced by the heavy oxidation environment of the HPOTP. Major causes of the high wear in bearings appear to be lubrication and cooling, both inadequate for the imposed conditions of operation. Numerous illustrations and evidence are given.

Chase, T. J.↗

NASA NTRS1992

Damage mechanisms in three-dimensional woven reinforced ceramic matrix composites under tensile and flexural loading at room and elevated temperatures

Three-dimensional Nicalon SiC/SiC matrix composites made through a chemical vapor infiltration process were studied under tensile and flexural loading at 23, 1200, and 1550 C in air. In situ damage accumulation monitoring NDE techniques were utilized to identify failure mechanisms in these materials. The effectiveness and durability of a chemical vapor deposition SiC surface coating were also evaluated in severe oxidizing environment. Results show that the failure response was very similar for the 23 and 1200 C specimens, while at 1550 C there were significant changes in both the composite mechanical behavior and the matrix microstructure. Extensive fiber pull-out was observed only in the 1550 C specimen. It is also found that the SiC surface coating can protect the composite up to the critical matrix cracking strength.

Chulya, Abhisak↗

NASA NTRS1992

Failure mechanisms of 3-D woven SiC/SiC composites under tensile and flexural loading at room and elevated temperatures

Nicalon silicon carbide 3D yarn with silicon carbide matrix composites made through a chemical vapor infiltration (CVI) process were investigated under tensile and flexural loading at 23, 1200 and 1550 C in air. The effectiveness of a chemical vapor deposition (CVD) SiC surface coating was also evaluated in severe oxidizing environment. Acoustic emission sensors and in situ optical microscopy were used at room temperature to monitor the failure mechanisms. It is shown that the level of tensile stress at which nonlinear behavior begins is not drastically reduced at 1200 and 1550 C when composites were protected by a SiC surface coating. Extensive fiber pull-out was observed only in the 1550 C specimen. Similar behaviors were also found in flexural specimens.

Chulya, Abhisak↗

NASA NTRS1992

Development of an optical flameout detection system for High Temperature Wind Tunnels

This paper outlines the design and development of a flameout detection system for future use in the 8-Foot High Temperature Wind Tunnel (8' HTT) at the NASA Langley Research Center. The proposed system is required to function reliably in an oxidizing environment with pressures up to 28 MPa (4,000 psi), survive temperatures reaching 1700 C (3100 F), and respond with a shut-down signal upon flameout in less than 100 milliseconds. The 7-Inch High Temperature Pilot Tunnel (7" HTT), a one-twelfth scale version of the 8' HTT, was used to develop a prototype system and to obtain data on the spectral content of the flame during combustion. The design and development of the spectral data acquisition system for the 7" HTT is presented here.

Borg, Stephen E.↗

NASA NTRS1992

CVD of silicon carbide on structural fibers - Microstructure and composition

Veitch, Lisa C.↗

NASA NTRS1993

The x ray microprobe determination of chromium oxidation state in olivine from lunar basalt and kimberlitic diamonds

The synchrotron x-ray microprobe is being used to obtain oxidation state information on planetary materials with high spatial resolution. Initial results on chromium in olivine from various sources including laboratory experiments, lunar basalt, and kimberlitic diamonds are reported. The lunar olivine was dominated by Cr(2+) whereas the diamond inclusions had Cr(2+/Cr(3+) ratios up to about 0.3. The simpliest interpretation is that the terrestrial olivine crystallized in a more oxidizing environment than the lunar olivine.

Sutton, S. R.↗

NASA NTRS1995

Films Composed Of Diamond And Diamondlike Carbon

Proposed films composed of diamond and diamondlike carbon useful as wear-resistant and self-lubricating protective and tribological coats at extreme temperatures and in corrosive and oxidizing environments. Films have wide variety of industrial applications.

Shing, Yuh-Han↗

NASA NTRS1995

Polyimides That Contain Cyclobutene-3,4-Dione Moieties

Linear aromatic polyimides containing cyclobutene-3,4,-dione moieties synthesized in experiments. Polymers exhibit high thermal and thermo-oxidative stabilities, making them useful in structural components exposed to hot and/or oxidative environments, as in jet engines and on outer surfaces of supersonic aircraft. Electrons in cyclobutene-3,4-dione moieties readily excited to states of higher energy upon exposure to ultraviolet light, giving rise to electronic properties with potential utility in photovoltaic, photoconductive, and/or photoemissive devices.

St. Clair, Terry L.↗

NASA NTRS1994

Theoretical Basis for Estimated Test Times and Conditions for Drop Tower and Space-Based Droplet Burning Experiments With Methanol and N-Heptane

In order to develop an extensive envelope of test conditions for NASA's space-based Droplet Combustion Experiment (DCE) as well those droplet experiments which can be performed using a drop tower, the transient vaporization and combustion of methanol and n-heptane droplets were simulated using a recently developed fully time-dependent, spherically symmetric droplet combustion model. The transient vaporization of methanol and n-heptane was modeled to characterize the instantaneous gas phase composition surrounding the droplet prior to the introduction of an ignition source. The results for methanol/air showed that the entire gas phase surrounding a 2 mm methanol droplet deployed in zero-g .quickly falls outside the lean flammability limit. The gas phase surrounding an identically-sized n-heptane droplet, on the other hand, remains flammable. The combustion of methanol was then modeled considering a detailed gas phase chemical kinetic mechanism (168 steps, 26 species) and the effect of the dissolution of flame-generated water into the liquid droplet. These results were used to determine the critical ignition diameter required to achieve quasi-steady droplet combustion in a given oxidizing environment. For droplet diameters greater than the critical ignition diameter, the model predicted a finite diameter at which the flame would extinguish. These extinction diameters were found to vary significantly with initial droplet diameter. This phenomenon appears to be unique to the transient heat transfer, mass transfer and chemical kinetics of the system and thus has not been reported elsewhere to date. The extinction diameter was also shown to vary significantly with the liquid phase Lewis number since the amount of water present in the droplet at extinction is largely governed by the rate at which water is transported into the droplet via mass diffusion. Finally, the numerical results for n-heptane combustion were obtained using both 2 step and 96 step semi-emperical chemical kinetic mechanisms. Neither mechanism exhibited the variation of extinction diameter with initial diameter.

Marchese, Anthony J.↗

NASA NTRS1994

A Coupled Soil-Atmosphere Model of H2O2 on Mars

The Viking Gas Chromatograph Mass Spectrometer failed to detect organic compounds on Mars, and both the Viking Labeled Release and the Viking Gas Exchange experiments indicated a reactive soil surface. These results have led to the widespread belief that there are oxidants in the martian soil. Since H2O2 is produced by photochemical processes in the atmosphere of Mars, and has been shown in the laboratory to reproduce closely the Viking LR results, it is a likely candidate for a martian soil oxidant. Here, we report on the results of a coupled soil/atmosphere transport model for H202 on Mars. Upon diffusing into the soil, its concentration is determined by the extent to which it is adsorbed and by the rate at which it is catalytically destroyed. An analytical model for calculating the distribution of H202 in the martian atmosphere and soil is developed. The concentration of H202 in the soil is shown to go to zero at a finite depth, a consequence of the nonlinear soil diffusion equation. The model is parameterized in terms of an unknown quantity, the lifetime of H202 against heterogeneous catalytic destruction in the soil. Calculated concentrations are compared with a H202 concentration of 30 nmoles/cu cm, inferred from the Viking Labeled Release experiment. A significant result of this model is that for a wide range of H202 lifetimes (up to 105 years), the extinction depth was found to be less than 3 m. The maximum possible concentration in the top 4 cm is calculated to be approx. 240 nmoles/cu cm, achieved with lifetimes of greater than 1000 years. Concentrations higher than 30 nmoles/cu cm require lifetimes of greater than 4.3 terrestrial years. For a wide range of H202 lifetimes, it was found that the atmospheric concentration is only weakly coupled with soil loss processes. Losses to the soil become significant only when lifetimes are less than a few hours. If there are depths below which H202 is not transported, it is plausible that organic compounds, protected from an oxidizing environment, may still exist. They would have been deposited by meteors, or be the organic remains of past life.

Bullock, Mark A.↗

NASA NTRS1997

The Effects of Sooting and Radiation on Droplet Combustion

The burning of liquid hydrocarbon fuels accounts for a significant portion of global energy production. With predicted future increases in demand and limited reserves of hydrocarbon fuel, it is important to maximize the efficiency of all processes that involve conversion of fuel. With the exception of unwanted fires, most applications involve introduction of liquid fuels into an oxidizing environment in the form of sprays which are comprised of groups of individual droplets. Therefore, tremendous benefits can result from a better understanding of spray combustion processes. Yet, theoretical developments and experimental measurements of spray combustion remains a daunting task due to the complex coupling of a turbulent, two-phase flow with phase change and chemical reactions. However, it is recognized that individual droplet behavior (including ignition, evaporation and combustion) is a necessary component for laying the foundation for a better understanding of spray processes. Droplet combustion is also an ideal problem for gaining a better understanding of non-premixed flames. Under the idealized situation producing spherically-symmetric flames (produced under conditions of reduced natural and forced convection), it represents the simplest geometry in which to formulate and solve the governing equations of mass, species and heat transfer for a chemically reacting two phase flow with phase change. The importance of this topic has promoted extensive theoretical investigations for more than 40 years.

Lee, Kyeong-Ook↗

NASA NTRS1996

Simulation of Fatigue Behavior of High Temperature Metal Matrix Composites

A generalized relatively new approach is described for the computational simulation of fatigue behavior of high temperature metal matrix composites (HT-MMCs). This theory is embedded in a specialty-purpose computer code. The effectiveness of the computer code to predict the fatigue behavior of HT-MMCs is demonstrated by applying it to a silicon-fiber/titanium-matrix HT-MMC. Comparative results are shown for mechanical fatigue, thermal fatigue, thermomechanical (in-phase and out-of-phase) fatigue, as well as the effects of oxidizing environments on fatigue life. These results show that the new approach reproduces available experimental data remarkably well.

Tong, Mike T.↗