Engineering Papers⌕ Search

Engineering topics

Nuth, J. A.

Publications and source records attributed to Nuth, J. A..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Microgravity particle research on the Space Station - The gas-grain simulation facility

In the gravitational field on earth, the large settling rate of micron-sized particles and the effects of gravity-induced convection prohibit many interesting studies of phenomena such as coagulation, collisions, and mutual interactions of droplets, dust grains and other particles. Examples of exobiology experiments involving these phenomena are the simulation of organic aerosol formation in Titan's atmosphere, studies of the role of comets in prebiotic chemical evolution, and simulations of carbon grain interactions in various astrophysical environments. The Gas-Grain Simulation Facility (GGSF) is a proposed earth-orbital laboratory that will allow present ground-based experimental programs which study processes involving small particles and weak interactions to be extended to a new domain. Physics issues that scientists wishing to propose GGSF experiments must consider are reviewed in this paper. Specifically, coagulation, motion in gases and vacua, and wall deposition of particles in a microgravity environment are discussed.

Fogleman, G.↗
NASA NTRS1988

Experimental studies of circumstellar, interstellar and interplanetary refractory grain analogs

Experimental techniques for studying the processes which occur during the life cycle of a refractory grain are reviewed. Consideration is given to the construction of a differentially-pumped beam-mass spectrometer which is expected to determine the relative stability of small clusters in the Fe-Mg-Si-O and Ca-Ti-Al-O systems as a function of temperature and pressure. Also, a smoke generator has been constructed to produce gram quantities of amorphous grains for use in thermal annealing and hydrous alteration studies of smokes containing Fe-Mg-Al-Ti-Ca-Si-O and Cl. Topics of the studies include the properties of grains produced in a flow condensation apparatus, the sorption efficiency of Si2O3 smokes for AR gas, the processing of ices containing refractory precursors such as SiH4 or Fe(CO)5 plus water, and the residue from 1 MeV bombardment and warming of an amorphous Fe-SiO smoke.

Nuth, J. A.↗
NASA NTRS1986

Experimental studies of the vapor phase nucleation of refractory compounds. III - The condensation of silver

Measurements are reported of the critical supersaturation necessary to initiate the vapor phase nucleation of silver over the ambient temperature range 500-950 K at total pressures of 250 and 500 torr of hydrogen. The results are compared with the predictions of both classical nucleation theory and the theory of a physical spinodal developed by Rasmussen (1982), neither of which adequately describes the present observations. The nucleation of silver appears to be dependent on the total pressure of the system in a manner which suggests that three-body reactions are necessary in order to form the initial silver cluster (e.g., dimers) from the monoatomic vapor.

Nuth, J. A.↗
NASA NTRS1986

Nucleation experiments in a microgravity environment

A simple experimental apparatus is described in which a wide variety of vapor phase nucleation studies of refractory materials could be performed aboard NASA's KC-135 Research Aircraft. The chief advantage of a microgravity environment for these studies is the expected absence of thermally driven convective motions in the gas. The absence of convection leads to much more accurate knowledge of both the temperature distribution in the system and the time evolution of the refractory vapor concentration as a function of distance from the crucible.The evolution of the apparatus will be described as more experience is gained with the microgravity environment. Such experiments will be used to prepare for similar ones carried out aboard either the shuttle or Space Station where considerably longer duration experiments are possible.

Nuth, J. A.↗
NASA NTRS1986

Analytical electron microscopy of Mg-SiO smokes - A comparison with infrared and XRD studies

Analytical electron microscopy conducted for Mg-SiO smokes (experimentally obtained from samples previously characterized by IR spectroscopy) indicates that the microcrystallinity content of unannealed smokes increases with increased annealing for up to 30 hr. The growth of forsterite microcrystallites in the initially nonstoichiometric smokes may give rise to the contemporaneous growth of the SiO polymorph tridymite and MgO; after 4 hr of annealing, these react to form enstatite. It is suggested that XRD analysis and IR spectroscopy should be conducted in conjunction with detailed analytical electron microscopy for the detection of emerging crystallinity in vapor-phase condensates.

Rietmeijer, F. J. M.↗
NASA NTRS1986

Hydrous alteration of amorphous silicate smokes - First results

Results of the initial studies of the hydrous alteration of amorphous Mg-SiO smokes indicate that, although these materials readily adsorb water, the silicate structure is much more stable at 360 K than expected. Drastic changes in the relative absorption strengths of the 10- and 20-micron 'silicate' features that appear quite rapidly at 750 K were observed; these observations might have important implications for the interpretation of cometary dust spectra. Observations of the development of 3.4-3.5 micron features possibly due to hydrocarbons in the spectra of processed Mg-SiO smokes have raised the exciting possibility that these amorphous condensates could act as Fischer-Tropsch type catalysts to produce hydrocarbons in the primitive solar nebula.

Nuth, J. A.↗
NASA NTRS1986

Ultraviolet spectroscopy of meteoric debris of comets

It is proposed to carry out slitless spectroscopy at ultraviolet wavelengths from orbit of meteoric debris associated with comets. The Eta Aquarid and Orionid/Halley and the Perseid/1962 862 Swift-Tuttle showers would be principal targets. Low light level, ultraviolet video technique will be used during night side of the orbit in a wide field, earthward viewing mode. Data will be stored in compact video cassette recorders. The experiment may be configured as a GAS package or in the HITCHHIKER mode. The latter would allow flexible pointing capability beyond that offered by shuttle orientation of the GAS package, and doubling of the data record. The 1100 to 3200 A spectral region should show emissions of atomic, ionic, and molecular species of interest on cometary and solar system studies.

Wdowiak, T. J.↗
NASA NTRS1985

Meteoritic evidence that graphite is rare in the interstellar medium

Carbonaceous material from several distinct astrophysical environments that has been identified in primitive meteorites is discussed. Most of this material is not well crystallized graphite, but ranges from kerogenlike macromolecular organic matter to poorly graphitized carbon. Of the small fraction of the carbonaceous material which is graphite, most may have resulted from the graphitization of macromolecular precursors in the solar nebula. Since graphite is more stable than those precursors, preexisting interstellar graphite should have survived in meteorites to at least the same extent as did other forms of carbon. It is concluded from this dearth of graphite that graphitic carbon was not a major component of the interstellar dust at the time of the formation of the solar system.

Nuth, J. A.↗
NASA NTRS1985

Effects of multiquantum transitions on molecular populations in grain-forming circumstellar environments

Simplification of astrophysical calculations can be achieved by invoking the condition of local thermodynamic equilibrium; however, recent investigations suggest that this assumption may not be valid for certain astrophysical regions. To examine the effects of multiquantum translation to vibration transitions in expanding circumstellar envelopes, vibrational populations of the lowest 20 levels of CO have been calculated as a function of pressure and radiation density for H atom-CO collisions. Significant departure from local thermodynamic equilibrium is indicated, which implies lower dissociation rates for molecular components and a subsequent enhancement in the rate of grain formation by many orders of magnitude. Stabilization of intermediate species before they can dissociate may facilitate the formation of refractory grain cores in very hot, dilute outflows. As the present calculations indicate, laboratory measurements of state-to-state translation to vibration rates are needed for a more complete understanding of circumstellar chemistry.

Nuth, J. A.↗
NASA NTRS1985

Laboratory infrared spectra of predicted condensates in carbon-rich stars

Laboratory spectra and mass absorption coefficients of MgS, CaS, FeS, SiS2, FeS2, Fe3C, and a commercial iron carbide are presented over the wavelength range 125-15 microns. These spectra confirm that MgS is the most likely source of the unidentified 30-micron emission in carbon-rich sources and that FeS, Fe3C, and 'iron carbide' cannot be responsible for this feature although they could contribute to the continuum in this region. CaS and FeS2 may contribute to the 30-micron feature; however, both higher resolution and higher precision astronomical observations are needed before their presence can be established. SiS2 has a peak near 22 microns and therefore cannot be a significant component of the dust in such regions.

Nuth, J. A.↗
NASA NTRS1985

Does nucleation theory apply to the formation of refractory circumstellar grains?

It is pointed out that kinetic factors inherent in cosmic systems will prevent the attainment of an approximate equilibrium precondensation cluster distribution, and that therefore such systems will violate one of the most fundamental assumptions of nucleation theory. More importantly, it is shown that of the 11 refractory metal/metal oxide systems which have been studied experimentally none are consistent with the predictions of either classical nucleation theory or the Lothe-Pound modification. Since no valid method is known by which these deficiencies can be circumvented, it is concluded that nucleation theory cannot be made to work in expanding circumstellar shells. However, a stochastic, kinetic treatment which makes use of experimental and theoretical data for specific refractory systems does appear to be feasible.

Donn, B.↗
NASA NTRS1984

Remarks on a vibronic origin for the diffuse band spectrum

It has been demonstrated that the diffuse band spectrum could not originate as a series of vibronic transitions from atomic impurities in interstellar MgO grains as has recently been proposed since laboratory spectra of such materials show no evidence for such complex structure. Furthermore, recent observational evidence indicates that the extinction feature at 160 nm in the standard interstellar extinction curve upon which the identification of interstellar MgO was founded is actually the result of a calibration error in the original observational data.

Nuth, J. A.↗
NASA NTRS1984

Thermal metamorphism of Si2O3 - (A circumstellar dust analog)

The thermal behavior of Si2O3, the metastable condensate from SiO vapor,has been studied experimentally. Si2O3 and its successor, amorphous quartz, have previously been shown to have infrared spectral features similar to some that appear in the spectra of grains in oxygen-rich circumstellar regions. Thermal annealing experiments show that only one Si2O3 decay process operates over the range between 750-1000 K. This process is a unimolecular disproportionation. The rate of this transformation can be expressed as k/hr = 10 to the 9th exp (-40 kcal/mole/RT). By using this rate constant, it is found that a significant fraction of freshly nucleated circumstellar grains can survive passage through a typical circumstellar shell virtually unaltered in structure. It is emphasized that this is only the first in a series of laboratory experiments intended to study the metamorphism of newly condensed circumstellar material ejected into the interstellar medium. Grains similar to these might have been incorporated into the primitive solar nebula, provided that they could also survive passage through the general interstellar medium.

Nuth, J. A.↗
NASA NTRS1983

A note on the composition and structure of interstellar grains

The plausibility of relatively pure diatomic oxide crystals forming in the expanding atmospheres of cool stars is commented on, and the likelihood of SiO grains condensing is discussed on the basis of laboratory nucleation experiments. Results of annealing experiments are used to consider the further evolution of silicon oxide and silicate grains. The evidence shows that SiO solids form only when SiO condenses directly onto surfaces, and that the probability of pure crystalline SiO grains forming is rather small.

Nuth, J. A.↗
NASA NTRS1983

On a vibronic origin for the diffuse band spectrum

Duley (1982) has proposed that many of the diffuse interstellar bands in the wavelength interval 542-677 nm arise from vibronic transitions of Cr (3+) ions in MgO grains. No explanation has been offered for the fact that as many as 85 of the possible 108 transitions of this system have not been observed in the interstellar medium. Moreover, the relative intensities of the diffuse bands which are observed appear to be inconsistent with their assignment. It is therefore concluded that this model is not consistent with the observations.

Nuth, J. A.↗
NASA NTRS1983

Laboratory studies of the condensation and properties of amorphous silicate smokes

The results of experiments intended for defining the vapor phase condensation of refractory materials are reported, and the usefulness of applying laboratory data to astrophysical conditions is assessed. The testing apparatus used produced Mg-SiO smokes by boiling powders in an evaporator. Critical temperatures for the onset of nucleation were measured for SiO-H2 and Mg-SiO-H2 systems. Spectroscopic examination of the grains produced were determined to be similar to the spectra obtained from both carbonaceous chondrites and several stratospheric micrometeorites. The experiments demonstrated that direct nucleation of refractory silicates from the vapor phase yielded amorphous, metastable particles, resembling more circumstellar grains than a chondritic matrix. It is suggested that the formation of SiO(x) may control nucleation in oxygen-rich stars.

Nuth, J. A.↗
NASA NTRS1982

The vacuum ultraviolet spectra of HCN, C2N2, and CH3CN

Quantitative vacuum ultraviolet absorption spectra for HCN, C2N2, and CH3CN have been obtained over the wavelength range 60-160 nm. Where comparison is possible, the measurements of the absorption coefficients for HCN and C2N2 are consistent with previous studies. Because of the superior resolution of this work (0.05 nm), vibrational assignments in the valence and Rydberg transitions of HCN have been extended while higher members of the Rydberg series in CH3CN have been identified.

Nuth, J. A.↗