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At least 865 records · Page 48

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NASA NTRS1978

Conductivity of materials made of aluminum nitride and silicon nitride mixtures

To establish the possible mechanism for conductivity in aluminum nitride a study was made of the electric conductivity of pure AlN and its mixtures with silicon nitride at different temperatures and partial pressures of nitrogen in the gas phase. The thermoelectromotive force was also measured. The experiments used polycrystalline samples of cylindrical shape 18 mm in diameter made of powders by hot pressing in graphite press molds at a temperature of 1973-2273 K and pressure 1,470,000 n/sqm. The items obtained by this method had porosity not over 5%. After pressing, the samples were machined to remove carbon from the surface, and were annealed in a stream of dry ammonia for 10 h at a temperature of 1273-1373 K. Electric conductivity was measured according to the bridge scheme on an alternating current of frequency 10 kHz. In order to guarantee close contact of the platinum electrodes with the surface of the samples, a thin layer of platinum was sprayed on them. Experiments were conducted in the temperature interval 1273-1573 K with a half hour delay at each assigned temperature with heating and cooling.

Gorbatov, A. G.↗

NASA NTRS1977

Atomic chlorine and the chlorine monoxide radical in the stratosphere - Three in situ observations

Three simultaneous observations of atomic chlorine (Cl) and the clorine monoxide radical (ClO) are reported which encompass the altitude interval between 25 and 45 kilometers. Together, Cl and ClO form a gas-phase catalytic cycle potentially capable of depleting stratospheric ozone. Observed Cl and ClO densities, although variable, imply that chlorine compounds constitute an important part of the stratospheric ozone budget. The results are compared with recent models of stratospheric photochemistry which have been used as a basis for predicting ozone depletion resulting from fluorocarbon release.

Anderson, J. G.↗

NASA NTRS1977

Results of ionization measurements in the middle atmosphere

Positive-ion electrical-conductivity measurements, which are now performed with 4-cm diameter meteorological rocket payloads, provide a simple means of monitoring the NO concentration in the mesosphere, as well as the ionization variability. A wavelike dynamical structure is observed at low latitudes. Parachute-borne Gerdien mobility analyzers show, in addition, ion mobility spectra which spread from much greater to much less than that expected for simple gas-phase ions in the mesosphere and also indicate an ion clustering chain in the stratosphere. Large particulate ions apparently form in the daytime and grow very large at night. An instrument combining the mobility analyzer with a flashing source of ionizing radiation could be used as an NO(x) spectrometer.

Croskey, C. L.↗

NASA NTRS1977

Burner rig alkali salt corrosion of several high temperature alloys

The hot corrosion of five alloys was studied in cyclic tests in a Mach 0.3 burner rig into whose combustion chamber various aqueous salt solutions were injected. Three nickel-base alloys (IN-792, IN-738, and IN-100), a cobalt-base alloy (MM-509), and an iron-base alloy (304 stainless steel) were studied at temperatures of 700, 800, 900, and 1000 C with various salt concentrations and compositions. The relative resistance of the alloys to hot corrosion attack was found to vary with temperature and with both the concentration and composition of the injected salt solution. Results indicate that the corrosion of these alloys is a function of both the presence of salt condensed as a liquid on the surface and of the composition of the gas phases present.

Deadmore, D.↗

NASA NTRS1977

Ultraviolet absorption spectrum of hydrogen peroxide vapor

The ultraviolet absorption cross sections of hydrogen peroxide vapor have been determined over the wavelength range 210 to 350 nm at 296 K. At the longer wavelengths, the gas phase absorptivities are significantly larger than the corresponding values in condensed phase. The atmospheric H2O2 photodissociation rate for overhead sun at the earth's surface is estimated to be about 1.3 x 10 to the -5th/sec.

Molina, L. T.↗

NASA NTRS1977

Interaction of NaCl/g/ and HCl/g/ with condensed Na2SO4

Na2SO4(l)-NaCl(g) interactions were studied at a total pressure of one atmosphere of air or oxygen for various temperatures of Na2SO4(l) and for various partial pressures of NaCl(g) and H2O(g). Mass spectrometric sampling techniques were used to identify and monitor gas phase species. Continuous recording thermomicrogravimetric measurements were conducted to determine condensed phase weight change rates. Experimental measurements were supplemented with thermodynamic calculations. Numerous experiments were performed at sample temperatures of 900 and 1000 C with 300 ppm NaCl(g). In these experiments, the reproducibility of the Na2SO4 vaporization weight loss rate and initial weight gain upon addition of NaCl(g) were found to be satisfactory. It was found that the addition of NaCl(g) to air flowing over Na2SO4(l) at 900 and 1000 C enhances the rate of weight loss of the Na2SO4(l). This enhancement increases when H2O(g) is also added to the air flow.

Stearns, C. A.↗

NASA NTRS1977

Rate constants for reactions of ClO/x/ of atmospheric interest

Chemical kinetics measurements on 82 gas phase reactions of chlorine containing species are reviewed. Recommended rate constants are given. The principal species of interest are Cl, Cl2, ClO, Cl2O, ClOO, OClO, CINO, HCl and halo derivatives of methane and ethane. Absorption spectra are given for 21 species. In addition the chemical kinetics methods used to obtain these data are discussed with regard to their applicability and reliability.

Watson, R. T.↗

NASA NTRS1977

Photodecomposition of chloromethanes adsorbed on silica surfaces

Irradiation of CCl4, CFCl3, and CF2Cl2 in the presence of C2H6 in vessels containing silica sand or fused quartz tubing results in the formation of chlorine-containing products. The formation of these compounds occurs at wavelengths extending up to approximately 400 nm, that is, at wavelengths well beyond the absorption threshold of the chloromethanes in the gas phase. It is suggested that CCl4 adsorbed on silica surfaces photodissociates to yield CCl3 and CCl2 species. The poor material balance obtained in these experiments indicates that several of the chlorine-containing fragments are strongly adsorbed on the surface. At a CCl4 pressure of 13 Pa (0.1 torr), photolysis with 366 nm light in the presence of sand results in the decomposition of one molecule for every 10,000 photons striking the surface. Under otherwise identical conditions, the photon-induced breadkdown of CFCl3 and CF2Cl2 is respectively only 10% or 3% as efficient.

Ausloos, P.↗

NASA NTRS1978

Carbon vaporization into a nonequilibrium, stagnation-point boundary layer

The heat transfer to the stagnation point of an ablating carbonaceous heat shield, where both the gas-phase boundary layer and the heterogeneous surface reactions are not in chemical equilibrium, is examined. Specifically, the nonequilibrium changes in the mass fraction profiles of carbon species calculated for frozen flow are studied. A set of equations describing the steady-state, nonequilibrium laminar boundary layer in the axisymmetric stagnation region, over an ablating graphite surface, is solved, with allowance for the effects of finite rate of carbon vaporization.

Suzuki, T.↗

NASA NTRS1975

Volatiles on the surface of Apollo 15 green glass and trace-element distributions among Apollo 15 soils

Zn, Ge, Cd, In, and Au have been detected in surficial deposits on Apollo 15 green-glass spherules, and it is suggested that these deposits are condensates from the magmatic gas phase which was responsible for the pneumatic expulsion of the green glass from the lunar interior. Thermodynamic data indicate that chlorides and fluorides were the dominant forms of the volatile metals. The Ar-40x content of a nongreen-glass soil fraction is greater than that found in green-glass. Mare and low-K Fra Mauro basalts seem to be the most prominent components of Apollo 15 soil. The correlation of Zn with Ar-40x and with Pb-204 is studied, and the distribution of quartz-normative and olivine-normative basalts is considered.

Chou, C.-L.↗

NASA NTRS1978

The ratio of carbon monoxide to molecular hydrogen in interstellar dark clouds

LTE (C-13)O column densities are compared with the corresponding values of beam-convolved visual extinction at more than 100 locations within 38 different interstellar dark clouds. A roughly linear correlation is found to exist between these two quantities for visual extinctions in the range from about 1.5 to 5 mag. It is argued that this correlation can be extended up to about 10 mag and that the standard gas-to-extinction ratio can be expected to remain valid in the sources studied. The correlation of LTE (C-13)O column density with visual extinction is used to obtain an equation for the H2 column density associated with a given (C-13)O column density. It is shown that if the clouds studied are assumed to be chemically homogeneous, the equation obtained implies that at least 12% of all gas-phase carbon is in the form of CO. Comparison of the observational data with various theories proposed for molecule formation in dark clouds indicates that Langer's (1977) ion-molecule scheme accounts well for the observations when the fractionation channel of Watson et al. (1976) is included.

Dickman, R. L.↗

NASA NTRS1978

Process feasibility study in support of silicon material task 1

Process system properties are analyzed for materials involved in the alternate processes under consideration for solar cell grade silicon. The following property data are reported for trichlorosilane: critical constants, vapor pressure, heat of vaporization, gas heat capacity, liquid heat capacity, density, surface tension, viscosity, thermal conductivity, heat of formation, and Gibb's free energy of formation. Work continued on the measurement of gas viscosity values of silicon source materials. Gas phase viscosity values for silicon tetrafluoride between 40 C and 200 C were experimentally determined. Major efforts were expended on completion of the preliminary economic analysis of the silane process. Cost, sensitivity and profitability analysis results are presented based on a preliminary process design of a plant to produce 1,000 metric tons/year of silicon by the revised process.

Li, K. Y.↗

NASA NTRS1978

Decomposition of nitrous oxide and chloromethanes absorbed on particulate matter

The effect of pressure on the heterogeneous thermal and pyrolytic decomposition of nitrous oxides adsorbed on sand was studied. Results indicate that N20 adsorbed on certain sand surfaces can be decomposed by photons which nitrous oxide cannot absorb in the gas phase. There is also a thermal heterogeneous decomposition of nitrous oxide which also produces nitrogen. The photolysis of CC14, CFC13, CF2C12 adsorbed on fused quartz and on different types of sand was also investigated. There was no thermal heterogeneous reaction with any of these chloromethanes. Apparently the larger bond energy of approximately 74 kcal for the C-C1 bond compared to approximately 40 kcal for the N-O bond in N2O makes the thermal reaction inoperative for the chloromethanes.

Rebbert, R. E.↗

NASA NTRS1979

Analysis of ultraviolet spectrophotometric data from Copernicus

Ultraviolet spectral data from the OAO 3 satellite are being used to study interstellar absorption lines and stellar and circumstellar lines in hot stars. The interstellar data are beneficial in analyzing the depletions of heavy elements from the gas phase and in elucidating how these depletions depend on physical conditions. Abundances in separate velocity components were determined from line profiles. Observations were carried out for interstellar abundances, both atomic and molecular, towards a number of stars. The better quality data are being analyzed for profile information and the lesser data are being used in curve-of-growth analyses. Molecular observations were carried out as well, N2 was sought; interstellar C2 was detected and its rotational excitation utilized to establish limits in interstellar cloud temperatures. An extensive search for H2O resulted in a tentative identification which will produce new information on chemical reaction rates. Interstellar depletions and grain properties in the rho Ophiuchi cloud, stellar wind variability, and circumstellar lines are also under study.

Snow, T. P., Jr.↗

NASA NTRS1978

Experimental Studies of the Formation/Deposition of Sodium Sulfate in/from Combustion Gases

Processes related to the hot corrosion of gas turbine components were examined in two separate investigations. Monochromatic laser light was used to probe condensation onset and condensate film growth (via interference of reflected light) on electrically heated ribbons immersed in seeded, flat flame combustion product gases. Boron trichloride is used as the seed gas in these preliminary experiments conducted to obtain precise measurements of the dew point/deposition rates. Because of the importance of gaseous Na(g) as a precursor to NaSO4 formation, the kinetics and mechanisms of the heterogeneous reaction H(g) + NaCl(s) yields Na(g) + HCl(g) was studied using atomic absorption spectroscopy combined with microwave discharge-vacuum flow reactor techniques at moderate temperatures. Preliminary results indicate the H-atom attack of solid NaCl vaporization is negligible; hence the corresponding gas phase (homogeneous) reaction no role in the observed Na(g) production.

Rosner, D. E.↗

NASA NTRS1979

The chemistry of sodium chloride involvement in processes related to hot corrosion

Thermodynamic and mass transport calculations, and laboratory experiments elucidating the behavior of sodium chloride in combustion environments, in the deposition process, and in reactions with certain oxides on the surfaces of superalloys are summarized. It was found that some of the ingested salt is separated out of the air stream by the compressor. However, sodium chloride does pass from the compressor to the combustor where numerous chemical reactions take place. Here some of the salt is vaporized to yield gaseous sodium chloride molecules. Hydrogen and oxygen atoms present in the combustion products react with some sodium chloride to yield other gaseous species such as sodium, and a fraction of the salt remains as particulates. Both the gas phase and condensed sodium chloride can lead to sodium sulfate formation by various routes, all of which involve reaction with sulfur oxides and oxygen. In addition to contributing to the formation of sodium sulfate, the sodium chloride can contribute to corrosion directly.

Stearns, C. A.↗

NASA NTRS1978

The formation of molecules in interstellar clouds from singly and multiply ionized atoms

The suggestion is considered that multiply ionized atoms produced by K- and L-shell X-ray ionization and cosmic-ray ionization can undergo ion-molecule reactions and also initiate molecule production. The role of X-rays in molecule production in general is discussed, and the contribution to molecule production of the C(+) radiative association with hydrogen is examined. Such gas-phase reactions of singly and multiply ionized atoms are used to calculate molecular abundances of carbon-, nitrogen-, and oxygen-bearing species. The column densities of the molecules are evaluated on the basis of a modified version of previously developed isobaric cloud models. It is found that reactions of multiply ionized carbon with H2 can contribute a significant fraction of the observed CH in diffuse interstellar clouds in the presence of diffuse X-ray structures or discrete X-ray sources and that substantial amounts of CH(+) can be produced under certain conditions.

Langer, W. D.↗

NASA NTRS1978

Laser-enhanced dynamics in molecular rate processes

The present discussion deals with some theoretical aspects associated with the description of molecular rate processes in the presence of intense laser radiation, where the radiation actually interacts with the molecular dynamics. Whereas for weak and even moderately intense radiation, the absorption and stimulated emission of photons by a molecular system can be described by perturbative methods, for intense radiation, perturbation theory is usually not adequate. Limiting the analysis to the gas phase, an attempt is made to describe nonperturbative approaches applicable to the description of such processes (in the presence of intense laser radiation) as electronic energy transfer in molecular (in particular atom-atom) collisions; collision-induced ionization and emission; and unimolecular dissociation.

George, T. F.↗