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NASA NTRS1987

Solidification of undercooled Ni-Sn eutectic alloy under microgravity conditions in the Space Shuttle

The Space Shuttle Columbia carried an Alloy Undercooling Experiment on its STS 61-C mission in January, 1986. The experiment was performed in an electromagnetic levitator. A sample of Ni-32.5 wt pct Sn eutectic was melted and solidified under microgravity conditions in the Space Shuttle. The specimen achieved only a fairly small undercooling, probably less than 30 K. The specimen was examined by optical and scanning electron microscopy. The surface and cross-sectional microstructures were primarily composed of normal lamellar eutectic, but showed several interesting features, including an apparent surface nucleation site, curved dendrites with nonorthogonal secondary arms, dendrite fragments with extremely fine arm spacing, submicron precipitates, and faceted crystals. The results of the space experiment are presented and compared with ground-based results obtained with the same alloy.

Piccone, T. J.↗

NASA NTRS1987

Two stage dual gate MESFET monolithic gain control amplifier for Ka-band

A monolithic two stage gain control amplifier has been developed using submicron gate length dual gate MESFETs fabricated on ion implanted material. The amplifier has a gain of 12 dB at 30 GHz with a gain control range of over 30 dB. This ion implanted monolithic IC is readily integrable with other phased array receiver functions such as low noise amplifiers and phase shifters.

Sokolov, V.↗

NASA NTRS1988

A new instability of Saturn's ring

Perturbations in the Saturn ring's mass density are noted to be prone to instabilities through the sporadic elevation of submicron-size dust particles above the rings, which furnishes an effective angular momentum exchange between the rings and Saturn. The dust thus elevated from the ring settles back onto it at a different radial distance. The range of wavelength instability is determinable in light of the dust charge, the average radial displacement of the dust, and the fluctuation of these quantities. It is suggested that at least some of the B-ring's ringlets may arise from the instability.

Goertz, C. K.↗

NASA NTRS1988

Analysis of chondritic interplanetary dust thin-sections

Chondritic interplanetary dust particles (IDPs) are heterogeneous aggregates of predominantly submicron mineral grains and carbonaceous material, whose bulk compositions agree within a factor of two with type CI/CM carbonaceous chondrites. The mineralogy and petrography of 25 such particles were studied by analytical electron microscopic examination of ultramicrotomed thin sections (500-1000 A thick). Four classes of chondritic IDPs were recognized, referred to as pyroxene, olivine, smectite, and serpentine, and their relative abundances were 9:4:10:2, respectively. Quantitative thin-film analyses indicate that pyroxene particles most closely resemble material emitted from comet Halley. Smectite particles may have formed from pyroxene particles by aqueous alteration of glass and enstatite crystals. Serpentine particles are the only class that are similar to the matrices of carbonaceous chondrites, but these are the least abundant chondritic IDPs. Collectively, chondritic particles are a mineralogically diverse group of extraterrestrial materials.

Bradley, John P.↗

NASA NTRS1988

Ice in the Antarctic polar stratosphere

On six occasions during the 1987 Airborne Antarctic Ozone Experiment, the Polar Stratospheric Cloud (PSC) ice crystals were replicated over the Palmer Peninsula at approximately 70 deg South. The sampling altitude was approximately 60 to 65 thousand feet, the temperature range was -83.5 to -72C and the atmosphere was subsaturated in all cases. The collected crystals were predominantly complete and hollow prismatic columns with maximum dimensions up to 217 microns. Evidence of scavenging of submicron particles was detected on several crystals. While the replicated crystal sizes were larger than anticipated, their relatively low concentration results in a total surface area less than one tenth that of the sampled aerosol particles. The presence of large crystals suggest that PSC ice crystals can play a very important role in stratospheric dehydration processes.

Goodman, J. K.↗

NASA NTRS1988

Observations of condensation nuclei in the 1987 airborne Antarctic ozone experiment

The condensation nucleus counter (CNC) flown of the NASA ER-2 in the Airborne Antarctic Ozone Experiment provides a measurement of the number mixing ratio of particles which can be grown by exposure to supersaturated n-butyl alcohol vapor to diameters of a few microns. Such particles are referred to as condensation nuclei (CN). The ER-2 CNC was calibrated with aerosols of known size and concentration and was found to provide an accurate measure of the number concentration of particles larger than about 0.02 micron. Since the number distribution of stratospheric aerosols is usually dominated by particles less than a few tenths of micron in diameter, the upper cutoff of the ER-2 CNC has not been determined experimentally. However, theory suggests that the sampling and counting efficiency should remain near one for particles as large as 1 micron in diameter. Thus, the CN mixing ratio is usually a good measure of the mixing ratio of submicron particles.

Wilson, J. C.↗

NASA NTRS1988

The search for refractory interplanetary dust particles from preindustrial aged Antarctic ice

In a study of refractory interplanetary dust particles, preindustrial-aged Antarctic ice samples have been collected, melted, and filtered to separate the particle load. Particles containing a significant amount of aluminum, titanium, and/or calcium were singled out for detailed SEM and STEM characterization. The majority of these particles are shown to be volcanic tephra from nearby volcanic centers. Six spherical aggregates were encountered that consist of submicron-sized grains of rutile within polycrystalline cristobalite. These particles are probably of terrestrial volcanic origin, but have not been previously reported from any environment. One aggregate particle containing fassaite and hibonite is described as a probable interplanetary dust particle. The constituent grain sizes of this particle vary from 0.1 to 0.3 microns, making it significantly more fine-grained than meteoritic calcium-aluminum-rich inclusions. This particle is mineralogically and morphologically similar to recently reported refractory interplanetary dust particles collected from the stratosphere, and dissimilar to the products of modern spacecraft debris.

Zolensky, Michael E.↗

NASA NTRS1988

Resolution of the circumstellar disk of Beta Pictoris at 10 and 20 microns

Observations at 10 and 20 microns with about 5 arcsec resolution are presented of the central, approximately 13 arcsec diameter region of the A5 dwarf Beta Pictoris. These observations resolve the inner disk and provide important constraints for models of the star and a basis for removing some of their ambiguities. The evidence indicates that submicron-sized grains emit much of the excess 10 and 20 micron radiation from the star. The characteristics of the dust are discussed, and the possibility that comets form the dust reservoir is considered.

Telesco, C. M.↗

NASA NTRS1984

Effect of particulate thermophoresis in reducing the fouling rate advantages of effusion-cooling

To predict small-particle diffusional mass transfer (deposition), including particle thermophoresis, transpiration cooling, and variable properties, the coupled ordinary differential equations governing self-similar laminar boundary layers are solved numerically. Under typical combustion turbine conditions, although diffusional deposition rates can be dramatically reduced by transpiration cooling (e.g., by some 5-decades for mainstream submicron particles corresponding to a Schmidt number of about 100 and a wall transpiration-cooled to Tw/Te = 0.8), actual deposition rate reductions will be smaller than previously expected (by about 1 decade for particles with Sc of about 100), owing to thermophoretic particle drift caused by the colder wall. Such microdroplets, small enough to behave like heavy molecules in combustion systems, are often important because they can cause adherence of the much larger ash particles which inertially impact on the same surface.

Gokoglu, S. A.↗

NASA NTRS1985

Mass transfer across combustion gas thermal boundary layers - Power production and materials processing implications

The effects of Soret diffusion (for vapors) and thermophoresis (for particles) are illustrated using recent optical experiments and boundary layer computations. Mass transfer rate augmentations of up to a factor of 1000 were observed and predicted for submicron-particle capture by cooled solid surfaces, while mass transfer suppressions of more than 10 to the -10th-fold were predicted for 'overheated' surfaces. It is noted that the results obtained are of interest in connection with such technological applications as fly-ash capture in power generation equipment and glass droplet deposition in optical-waveguide manufacture.

Rosner, D. E.↗

NASA NTRS1988

A source map for dust jets observed in the coma of Comet P/Halley

In this paper, the large-scale enhancements of about 50 s duration in the flux of submicron dust particles from Comet P/Halley recorded by the Dust Counter and Mass Analyzer carried aboard the two Vega spaceprobes are traced back to their source departure points on the nucleus. Evidence is presented that such enhancements which correspond to regions of enhanced particle density in the coma with spatial widths of about 4000 km, are superimposed upon a background flux distribution which is well described by a simple fountain model. Thus, dust production on the sunlit hemisphere of the nucleus is characterized by discrete jet sources superimposed upon relatively low-level isotropic emission. The jet sources at the nucleus responsible for the major enhancements in recorded particle impact rates are narrow and discrete, with each jet accelerating about 10 to the -13th g particles to a narrow range of terminal velocities which decrease with increasing particle mass.

Rabinowitz, D. L.↗

NASA NTRS1989

Titanium carbide and titania phases on Antarctic ice particles of probable extraterrrestrial origin

Two unique titania-rich particles, found within ancient Antarctic ice have been discovered and characterized, and are believed to be of extraterrestrial origin. Both particles contain abundant submicron-sized crystals of Magneli phases (Ti(n)O(2n-1). In addition, one particle contains a core of TiC. Whereas the Magneli phases would have been stable in the early solar nebula, and so probably formed there, the TiC is more likely to have condensed in the cool, dusty, carbon-rich outer shell of a red giant star. It is suggested that both particles are interplanetary dust particles whose Magneli phases carry a record of the PO2-T conditions of the early solar nebula. It is further suggested that the TiC grain in particle 705 is remnant interstellar dust.

Zolensky, M. E.↗

NASA NTRS1989

Collisions of macroscopic fluffy aggregates in the primordial solar nebula and the formation of planetesimals

The characteristics of submicron-size grains lead to the conclusion that, with relative velocities under tens of meters per second deduced for the early stages of accretion in the solar nebula, bodies of meter dimension, were low-density, compressible aggregates. The collision mechanism appropriate to such objects is explored here. The results of representative collisions are summarized which show that the penetration is only a fraction of the radius of the impacting body. Hence, all but near-grazing collisions lead to merger of the two bodies. Aggregate growth occurs without substantially increasing the mean density, but small region with densities about 60 percent of the bulk density will occur in a collision.

Donn, B.↗

NASA NTRS1989

NbN/MgO/NbN SIS tunnel junctions for submm wave mixers

The authors report on the fabrication and testing of all-refractory NbN/MgO/NbN SIS (superconductor-insulator-superconductor) tunnel junctions for use as high-frequency mixers. Progress in the development of techniques for the fabrication of submicron-area tunnel junctions is described. Junction structures which have been investigated include mesa, crossline, and edge geometries. Using reactive sputtering techniques, NbN tunnel junctions with critical currents in excess of 104 A/sq cm have been fabricated with Vm values as high as 65 mV and areas down to 0.1 sq micron. Specific capacitance measurements on NbN/MgO/NbN mesa-type tunnel junctions give values in the range 60-90 fF/sq micron. These SIS tunnel junctions have been integrated with antennas and coupling structures for mixer tests in a waveguide receiver at 207 GHz. Preliminary mixer results are reported.

Stern, J. A.↗

NASA NTRS1989

The effects of ion gun beam voltage on the electrical characteristics of NbCN/PbBi edge junctions

The authors have succeeded in fabricating high-quality submicron NbCN edge junctions using a technique which is commonly used to make Nb edge junctions. A modified commercial ion gun was used to cut an edge in SiO2/NbCN films partially covered with photoresist. An insulating barrier was then formed on the exposed edge by reactive ion beam oxidation, and a counterelectrode of PbBi was deposited. The electrical quality of the resulting junctions was found to be strongly influenced by the ion beam acceleration voltages used to cut the edge and to oxidize it. For low ion beam voltages, the junction quality parameter was as high as Vm = 55 mV (measured at 3 mV), but higher ion beam voltages yielded strikingly poorer quality junctions. In light of the small coherence length of NbN, the dependence of the electrical characteristics on ion beam voltage is presumably due to mechanical damage of the NbCN surface. In contrast, for similar ion beam voltages, no such dependence was found for Nb edge junctions.

Lichtenberger, A. W.↗

NASA NTRS1989

Tin in a chondritic interplanetary dust particle

Submicron platey Sn-rich grains are present in chondritic porous interplanetary dust particle (IDP) W7029 A and it is the second occurrence of a tin mineral in a stratospheric micrometeorite. Selected Area Electron Diffraction data for the Sn-rich grains match with Sn2O3 and Sn3O4. The oxide(s) may have formed in the solar nebula when tin metal catalytically supported reduction of CO or during flash heating on atmospheric entry of the IDP. The presence of tin is consistent with enrichments for other volatile trace elements in chondritic IDPs and may signal an emerging trend toward nonchondritic volatile element abundances in chondritic IDPs. The observation confirms small-scale mineralogical heterogeneity in fine-grained chondritic porous interplanetary dust.

Rietmeijer, Frans J. M.↗

NASA NTRS1987

Zirconia toughened SiC whisker reinforced alumina composites small business innovation research

The objective of this phase 1 project was to develop a ceramic composite with superior fracture toughness and high strength, based on combining two toughness inducing materials: zirconia for transformation toughening and SiC whiskers for reinforcement, in a controlled microstructure alumina matrix. The controlled matrix microstructure is obtained by controlling the nucleation frequency of the alumina gel with seeds (submicron alpha-alumina). The results demonstrate the technical feasibility of producing superior binary composites (Al2O3-ZrO2) and tertiary composites (Al2O3-ZrO2-SiC). Thirty-two composites were prepared, consolidated, and fracture toughness tested. Statistical analysis of the results showed that: (1) the SiC type is the key statistically significant factor for increased toughness; (2) sol-gel processing with a-alumina seed had a statistically significant effect on increasing toughness of the binary and tertiary composites compared to the corresponding mixed powder processing; and (3) ZrO2 content within the range investigated had a minor effect. Binary composites with an average critical fracture toughness of 6.6MPam sup 1/2, were obtained. Tertiary composites with critical fracture toughness in the range of 9.3 to 10.1 MPam sup 1/2 were obtained. Results indicate that these composites are superior to zirconia toughened alumina and SiC whisker reinforced alumina ceramic composites produced by conventional techniques with similar composition from published data.

Loutfy, R. O.↗

NASA NTRS1989

Precipitation strengthened high strength, high conductivity Cu-Cr-Nb alloys produced by chill block melt spinning

A series of Cu-based alloys containing 2 to 10 a/o Cr and 1 to 5 a/o Nb were produced by chill block melt spinning (CBMS). The melt spun ribbons were consolidated and hot rolled to sheet to produce a supersaturated Cu-Cr-Nb solid solution from which the high melting point intermetallic compound Cr2Nb could be precipitated to strengthen the Cu matrix. The results show that the materials possess electrical conductivities in excess of 90 percent that of pure Cu at 200 C and above. The strengths of the Cu-Cr-Nb alloys were much greater than Cu, Cu-0.6 Cr, NARloy-A, and NARloy-Z in the as-melt spun condition. The strengths of the consolidated materials were less than Cu-Cr and Cu-Cr-Zr below 500 C and 600 C respectively, but were significantly better above these temperatures. The strengths of the consolidated materials were greater than NARloy-Z, at all temperatures. The GLIDCOP possessed similar strength levels up to 750 C when the strength of the Cu-Cr-Nb alloys begins to degrade. The long term stability of the Cu-Cr-Nb alloys was measured by the microhardness of aged samples and the growth of precipitates. The microhardness measurements indicate that the alloys overage rapidly, but do not suffer much loss in strength between 10 and 100 hours which confirms the results of the electrical resistivity measurements taken during the aging of the alloys at 500 C. The loss in strength from peak strength levels is significant, but the strength remains exceptionally good. Transmission electron microscopy (TEM) of the as-melt spun samples revealed that Cr2Nb precipitates formed in the liquid Cu during the chill block melt spinning, indicating a very strong driving force for the formation of the precipitates. The TEM of the aged and consolidated materials indicates that the precipitates coarsen considerably, but remain in the submicron range.

Ellis, David L.↗