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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Deformation and fracture of single-crystal and sintered polycrystalline silicon carbide produced by cavitation

An investigation was conducted to examine the deformation and fracture behavior of single-crystal and sintered polycrystalline SiC surfaces exposed to cavitation. Cavitation erosion experiments were conducted in distilled water at 25 C by using a magnetostrictive oscillator in close proximity (1 mm) to the surface of SiC. The horn frequency was 20 kHz, and the double amplitude of the vibrating disk was 50 microns. The results of the investigation indicate that the SiC (0001) surface could be deformed in a plastic manner during cavitation. Dislocation etch pits were formed when the surface was chemically etched. The number of defects, including dislocations in the SiC (0001) surface, increased with increasing exposure time to cavitation. The presence of intrinsic defects such as voids in the surficial layers of the sintered polycrystalline SiC determined the zones at which fractured grains and fracture pits (pores) were generated. Single-crystal SiC had superior erosion resistance to that of sintered polycrystalline SiC.

Miyoshi, Kazuhisa↗
NASA NTRS1987

Dome craters on Ganymede

Voyager observations reveal impact craters on Ganymede that are characterized by the presence of broad, high albedo, topographic domes situated within a central pit. Fifty-seven craters with central domes were identified in images covering approx. 50% of the surface. Owing to limitations in resolution, and viewing and illumination angles, the features identified are most likely a subset of dome craters. The sample appears to be sufficiently large to infer statistically meaningful trends. Dome craters appear to fall into two distinct populations on plots of the ratio of dome diameter to crater rim diameter, large-dome craters and small-dome craters. The two classes are morphologically distinct from one another. In general, large dome craters show little relief and their constituent landforms appear subdued with respect to fresh craters. The physical attributes of small-dome craters are more sharply defined, a characteristic they share with young impact craters of comparable size observed elsewhere in the solar system. Both types of dome craters exhibit central pits in which the dome is located. As it is difficult to produce domes by impact and/or erosional processes, an endogenic origin for the domes is reasonably inferred. Several hypotheses for their origin are proposed. These hypotheses are briefly reviewed.

Moore, J. M.↗
NASA NTRS1986

Mechanism of strength degradation for hot corrosion of alpha-SiC

Sintered alpha SiC was corroded by thin films of Na2SO4 and Na2CO3 molten salts at 1000 percent. This hot corrosion attack reduced room temperature strengths by as much as 50 percent. Strength degradation was proportional to the degree and uniformity of corrosion pitting attack as controlled by the chemistry of the molten salt. Extensive fractography identified corrosion pits as the most prevalent source of failure. A fracture mechanics treatment of the strength/pit depth relationship produced an average K sub IC equal to 2.6 MPa sub m 1/2, which is cnsistent ith published values.

Smialek, James L.↗
NASA NTRS1988

Air-Operated Sump Pump

Pump removes liquid seepage from small, restricted area and against large pressure head. Developed for moving small amounts of water and oil from sump pit 85 ft (25.91 m) deep. Fits in space only 6 1/2 in. (16.5 cm) in diameter and 18 in. (45.7 cm) long. In discharge part of pumping cycle, air forces liquid out of pump chamber through pipe. During filling part of pumping cycle, water enters pump chamber from sump pit. Float in chamber next to pump chamber controls pressurization through timer and solenoid valve.

Nolt, Gary D.↗
NASA NTRS1988

Evaluation of candidate alloys for the construction of metal flex hoses in the STS launch environment

Various vacuum jacketed cryogenic supply lines at the Shuttle launch site use convoluted flexible expansion joints. The atmosphere at the launch site has a very high salt content, and during a launch, fuel combustion products include hydrochloric acid. This extremely corrosive environment has caused pitting corrosion failure in the flex hoses, which were made of 304L stainless steel. A search was done to find a more corrosion resistant replacement material. This study focused on 19 metal alloys. Tests which were performed include electrochemical corrosion testing, accelerated corrosion testing in a salt fog chamber, long term exposure at the beach corrosion testing site, and pitting corrosion tests in ferric chloride solution. Based on the results of these tests, the most corrosion resistant alloys were found to be (in order) Hastelloy C-22, Inconel 625, Hastelloy C-276, Hastelloy C-4, and Inco Alloy G-3. Of these top five alloys, the Hastelloy C-22 stands out as being the best of those tested for this application.

Ontiveros, Cordelia↗
NASA NTRS1988

Hot corrosion of ceramic engine materials

A number of commercially available SiC and Si3N4 materials were exposed to 1000 C in a high velocity, pressurized burner rig as a simulation of a turbine engine environment. Sodium impurities added to the burner flame resulted in molten Na2SO4 deposition, attack of the SiC and Si4N4 and formation of substantial Na2O-x(SiO2) corrosion product. Room temperature strength of the materials decreased. This was a result of the formation of corrosion pits in SiC, and grain boundary dissolution and pitting in Si3N4. Corrosion regimes for such Si-based ceramics have been predicted using thermodynamics and verified in rig tests of SiO2 coupons. Protective mullite coatings are being investigated as a solution to the corrosion problem for SiC and Si3N4. Limited corrosion occurred to cordierite (Mg2Al4Si5O18) but some cracking of the substrate occurred.

Fox, Dennis S.↗
NASA NTRS1989

Deformation and fracture of single-crystal and sintered polycrystalline silicon carbide produced by cavitation

An investigation was conducted to examine the deformation and fracture behavior of single-crystal and sintered polycrystalline SiC surfaces exposed to cavitation. Cavitation erosion experiments were conducted in distilled water at 25 C by using a magnetostrictive oscillator in close proximity (1 mm) to the surface of SiC. The horn frequency was 20 kHz, and the double amplitude of the vibrating disk was 50 microns. The results of the investigation indicate that the SiC (0001) surface could be deformed in a plastic manner during cavitation. Dislocation etch pits were formed when the surface was chemically etched. The number of defects, including dislocations in SiC (0001) surface, increased with increasing exposure time to cavitation. The presence of intrinsic defects such as voids in the surficial layers of the sintered polycrystalline SiC determined the zones at which fractured grains and fracture pits (pores) were generated. Single-crystal SiC had superior erosion resistance to that of sintered polycrystalline SiC.

Miyoshi, Kazuhisa↗
NASA NTRS1989

MEVTV Workshop on Tectonic Features on Mars

The state of knowledge of tectonic features on Mars was determined and kinematic and mechanical models were assessed for their origin. Three sessions were held: wrinkle ridges and compressional structure; strike-slip faults; and extensional structures. Each session began with an overview of the features under discussion. In the case of wrinkle ridges and extensional structures, the overview was followed by keynote addresses by specialists working on similar structures on the Earth. The first session of the workshop focused on the controversy over the relative importance of folding, faulting, and intrusive volcanism in the origin of wrinkle ridges. The session ended with discussions of the origin of compressional flank structures associated with Martian volcanoes and the relationship between the volcanic complexes and the inferred regional stress field. The second day of the workshop began with the presentation and discussion of evidence for strike-slip faults on Mars at various scales. In the last session, the discussion of extensional structures ranged from the origin of grabens, tension cracks, and pit-crater chains to the origin of Valles Marineris canyons. Shear and tensile modes of brittle failure in the formation of extensional features and the role of these failure modes in the formation of pit-crater chains and the canyons of Valles Marineris were debated. The relationship of extensional features to other surface processes, such as carbonate dissolution (karst) were also discussed.

Watters, Thomas R.↗
NASA NTRS1989

Transient-mode liquid phase epitaxial growth of GaAs on GaAs-coated Si substrates prepared by migration-enhanced molecular beam epitaxy

Planar oxide-maskless growth of GaAs was demonstrated by transient-mode liquid phase epitaxy (TMLPE) on GaAs-coated Si substrates that were prepared by migration-enhanced molecular beam epitaxy (MEMBE). In TMLPE, the cool substrate was brought into contact with hot melts for a short time. A GaAs layer as thick as 30 microns was grown in 10 sec. The etch pits observed in TMLPE-grown layers became longer in one direction and decreased in density with increasing the TMLPE epilayer thickness. The density of etch pits in a 20 micron-thick layer was approximately 5 x 10 the 6th/sq cm. Strong bandgap emission elliptically polarized with a major axis perpendicular to the surface was observed at about 910 nm, while deep-level emission from the TMLPE/MEMBE GaAs interface was detected at 980 nm. The photoluminescence intensity divided by the carrier concentration of the TMLPE-grown layer was about 270 times larger than that of the MEMBE-grown layer used as a substrate.

Nakamura, Shuji↗
NASA NTRS1990

Burner rig hot corrosion of silicon carbide and silicon nitride

A number of commercially available SiC and Si3N4 materials were exposed to 1000 C for 40 h in a high-velocity, pressurized burner rig as a simulation of an aircraft turbine environment. Na impurities (2 ppm) added to the burner flame resulted in molten Na2SO4 deposition, attack of the SiC and Si3N4, and formation of substantial Na2O+x(SiO2) corrosion product. Room-temperature strength of the materials decreased as a result of the formation of corrosion pits in SiC and grain-boundary dissolution and pitting in Si3N4.

Fox, Dennis S.↗
NASA NTRS1988

Evaluation of candidate alloys for the construction of metal flex hoses in the STS launch environment

Various vacuum jacketed cryogenic supply lines at the Shuttle launch site use convoluted flexible expansion joints. The atmosphere at the launch site has a very high salt content, and during a launch fuel combustion products include hydrochloric acid. This extremely corrosive environment has caused pitting corrosion failure in the flex hoses, which were made out of 304L stainless steel. A search was done to find a more corrosion resistant replacement material. Nineteen metal alloys were tested. Tests which were performed include electrochemical corrosion testing, accelerated corrosion testing in a salt fog chamber, long term exposure at the beach corrosion testing site, and pitting corrosion tests in ferric chloride solution. Based on the results, the most corrosion resistant alloys were found to be, in order, Hastelloy C-22, Inconel 625, Hastelloy C-276, Hastelloy C-4, and Inco Alloy G-3. Of these top five alloys, the Hastelloy C-22 stands out as being the best of the alloys tested.

Macdowell, Louis G., III↗
NASA NTRS1991

The effect of zinc additions on the environmental stability of Alloy 8090 (Al-Li-Cu-Mg-Zr)

Stress corrosion cracking (SCC) remains a problem in both Al-Li and conventional Al heat treatable alloys. It has recently been found that relatively small additions (less than or approximately 1 wt-percent) of Zn can dramatically improve the SCC performance of alloy 8090 (Al-Li-Cu-Mg-Zr). Constant load time to failure experiments using cylindrical tensile samples loaded between 30 and 85 percent of TYS indicate improvements of orders of magnitude over the baseline 8090 for the Zn-containing alloys under certain aging conditions. However, the toughnesses of the alloys were noticeably degraded due to the formation of second phase particles which primarily reside on grain and subgrain boundaries. EDS revealed that these intermetallic particles were Cu and Zn rich. The particles were present in the T3 condition and were not found to be the result of quench rate, though their size and distribution were. At 5 hours at 160 C, the alloys displayed the greatest susceptibility to SCC but by 20 hours at 160 C the alloys demonstrated markedly improved TTF lifetimes. Aging past this time did not provide separable TTF results, however, the alloy toughnesses continued to worsen. Initial examination of the alloys microstructures at 5 and 20 hours indicated some changes most notably the S' and delta' distributions. A possible model by which this may occur will be explored. Polarization experiments indicated a change in the trend of E(sub BR) and passive current density at peak aging as compared to the baseline 8090. Initial pitting experiments indicated that the primary pitting mechanism in chloride environments is one occurring at constituent (Al-Fe-Cu) particles and that the Cu and Zn rich boundary precipitates posses a breakaway potential similar to that of the matrix acting neither anodic or cathodic in the first set of aerated 3.5 w/o NaCl experiments. Future work will focus on the identification of the second phase particles, evaluation of K(sub 1SCC) and plateau da/dt via both DCB and slow strain rate techniques. A lower Zn content variant will be examined in the near future in hopes of optimizing toughness, density and SCC performance.

Kilmer, Raymond J.↗
NASA NTRS1992

Automatic Detection Of Faults In Turbomachinery Bearings

System of analog and digital signal-processing equipment, computers, and programs detects faults in ball bearings in turbomachines. System operates in real time, extracting information from vibrations sensed by accelerometers, strain gauges, and/or acoustical sensors. Vibrations caused when pits in balls contact races, and pits in races contact balls.

Higbie, Nathan B.↗
NASA NTRS1993

Fast image decompression for telebrowsing of images

Progressive image transmission (PIT) is often used to reduce the transmission time of an image telebrowsing system. A side effect of the PIT is the increase of computational complexity at the viewer's site. This effect is more serious in transform domain techniques than in other techniques. Recent attempts to reduce the side effect are futile as they create another side effect, namely, the discontinuous and unpleasant image build-up. Based on a practical assumption that image blocks to be inverse transformed are generally sparse, this paper presents a method to minimize both side effects simultaneously.

Miaou, Shaou-Gang↗
NASA NTRS1992

The canyon system on Mars

Individual Martian equatorial troughs are described, and their stratigraphy, geomorphology and structure are discussed. Possible origins and the overall sequence of events are addressed. Wall rock, interior layered deposits, irregular floor deposits, fractured floor material, and surficial deposits are examined. Chasma walls, wall stability, pits and pit chains, tributary canyons, and the transition from troughs to channels are also discussed.

Lucchitta, B. K.↗
NASA NTRS1993

Electromagnetic propulsion for spacecraft

Three electromagnetic propulsion technologies, solid propellant pulsed plasma thrusters (PPT), magnetoplasmadynamic (MPD) thrusters, and pulsed inductive thrusters (PIT) have been developed for application to auxiliary and primary spacecraft propulsion. Both the PPT and MPD thrusters have been flown in space, though only PPTs have been used on operational satellites. The performance of operational PPTs is quite poor, providing only about 8 percent efficiency at about 1000 sec specific impulse. Laboratory PPTs yielding 34 percent efficiency at 5170 sec specific impulse have been demonstrated. Laboratory MPD thrusters have been demonstrated with up to 70 percent efficiency and 7000 sec specific impulse. Recent PIT performance measurements using ammonia and hydrazine propellants are extremely encouraging, reaching 50 percent efficiency for specific impulses between 4000 and 8000 sec.

Myers, Roger M.↗
NASA NTRS1993

Atomic probe microscopy of 3C SiC films grown on 6H SiC substrates

The surface of 3C SiC films grown on 6H SiC substrates has been studied by atomic probe microscopy in air. Atomic-scale images of the 3C SiC surface have been obtained by STM which confirm the 111 line type orientation of the cubic 3C layer grown on the 0001 plane type surface of the hexagonal 6H substrate. The nearest-neighbor atomic spacing for the 3C layer has been measured to be 3.29 +/- 0.2 A, which is within 7 percent of the bulk value. Shallow terraces in the 3C layer have been observed by STM to separate regions of very smooth growth in the vicinity of the 3C nucleation point from considerably rougher 3C surface regions. These terraces are oriented at right angles to the growth direction. Atomic force microscopy has been used to study etch pits present on the 6H substrate due to high temperature HCl cleaning prior to CVD growth of the 3C layer. The etch pits have hexagonal symmetry and vary in depth from 50 nm to 1 micron.

Steckl, A. J.↗
NASA NTRS1952

Investigations of Air-cooled Turbine Rotors for Turbojet Engines II : Mechanical Design, Stress Analysis, and Burst Test of Modified J33 Split-disk Rotor / Richard H. Kemp and Merland L. Moseson

A full-scale J33 air-cooled split turbine rotor was designed and spin-pit tested to destruction. Stress analysis and spin-pit results indicated that the rotor in a J33 turbojet engine, however, showed that the rear disk of the rotor operated at temperatures substantially higher than the forward disk. An extension of the stress analysis to include the temperature difference between the two disks indicated that engine modifications are required to permit operation of the two disks at more nearly the same temperature level.

TURBINES - WHEELS, DISK - STRENGTH↗