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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Photochemistry of acetylene at 1849 A

It is pointed out that the photochemistry of acetylene in the 1500-2000 A region at low pressure, particularly the quantum yield of C2H production and the reactions of C2H with H2 and hydrocarbons, is important in understanding the photochemistry of Jovian and Titan atmospheres. The present investigation was initiated to obtain information on these issues. The 1849 A light source used in the study was a sealed quartz tube containing a drop of Hg and 2 torr of Ar excited by a microwave generator. The lamp was attached to a Pyrex reaction cell. The amount of diacetylene produced was measured on the basis of absorption near 1650 A. Reaction mixtures of acetylene with N2, He, C2H6, or C3H8 were prepared. Attention is given to the absorption cross section, the photolysis of acetylene, and the photolysis of acetylene in the presence of other gases. It is found that the photochemical process of acetylene at 1849 A may represent that of acetylene in the Titan atmosphere.

Okabe, H.↗
NASA NTRS1982

Thermal infrared pushbroom imagery acquisition and processing

A 9-element focal plane detector array and signal processing electronics was developed and delivered in December 1977. It was integrated into a thermal infrared imaging system using LSI microprocessor image processing and CRT display. After three years of laboratory operation, the focal plane has demonstrated high reliability and performance. On the basis of the 9-channel breadboard, the 90-element Aircraft Pushbroom IR/CCD Focal Plane Development Program was funded in October 1977. A follow-on program was awarded in July 1979, for the construction of a field test instrument and image processing facility. The objective of this project was to demonstrate thermal infrared pushbroom hard-copy imagery. It is pointed out that the successful development of the 9-element and 90-element thermal infrared hybrid imaging systems using photoconductive (Hg,Cd)Te has verified the operational concept of 8 to 14 micrometer pushbroom scanners.

Brown, T. J.↗
NASA NTRS1983

A theoretical study on the mechanism of electronic to vibrational energy transfer in Hg/3P/ + CO

The mechanism of electronic-to-vibrational (E-V) energy transfer in Hg(3P) + CO collisions has been studied theoretically. The configuration interaction (CI) method was employed to calculate potential energy surfaces of the collision system. A simplified theoretical model, based on the reaction coordinate concept and the calculated potential energy characteristics, was used to discuss the mechanism of the singlet-triplet transition and the energy disposal in the collision. The results obtained were that: (a) the quenching process processed via a collision complex mechanism; and that (b) the triplet-singlet transition occurs near the collinear geometry. A model classical trajectory calculation gives a product CO vibrational distribution in good agreement with the experimental result.

Kato, S.↗
NASA NTRS1983

Interstellar C2 molecules in a Taurus dark cloud

Five relatively strong interstellar absorption lines of C2 in the (2-0) Phillips band near 8670 A have been detected toward the heavily reddened, late B star HD 29647, which lies behind a substantial part of one of the darkest regions of the nearest molecular cloud complex, the dark clouds of Taurus. Stringent upper limits on the C2 absorption toward two lightly reddened stars, Nu Cyg and Omicron And, are also reported. The observations yield a column density of about 9 x 10 to the 13th/sq cm, comparable to those of widely distributed molecules like CH and H2CO. Theoretical treatment of the data yields a kinetic temperature of 14 K and a mean density of less than about 1000/cu cm. A narrow, anomalously strong, stellar Mn II line yields a projected rotational velocity of less than about 7 km/s and is explained by previous identifications of the star as Hg-Mn peculiar.

Hobbs, L. M.↗
NASA NTRS1982

Atomic fluorescence study of high temperature aerodynamic levitation

Ultraviolet laser induced atomic fluorescence has been used to characterize supersonic jet aerodynamic levitation experiments. The levitated specimen was a 0.4 cm sapphire sphere that was separately heated at temperatures up to 2327 K by an infrared laser. The supersonic jet expansion and thermal gradients in the specimen wake were studied by measuring spatial variations in the concentration of atomic Hg added to the levitating argon gas stream. Further applications of atomic fluorescence in containerless experiments, such as ideal gas fluorescence thermometry and containerless process control are discussed.

Nordine, P. C.↗
NASA NTRS1983

Effect of variable thermal conductivity on isotherms in Bridgman growth

A change in thermal conductivity associated with melting or solidification can have a profound influence on the isotherms near the solidification interface if the material is being directionally solidified in an ampoule whose walls carry a substantial portion of the heat. This analysis was prompted by a recent discovery that the thermal conductivity of Hg(1-x)CD(x)Te increased dramatically as the material is heated above the solidus curve. An illustrative example is shown in which the sample is approximated as an infinite cylinder with constant but diffferent thermal properties in the solid and melt. The boundary conditions are fixed on the surface by a conductive ampoule in a two-zone Bridgman furnace with an adiabatic region separating the two zones. The effect of the adiabatic zone in this case is to intensify the curvature of the interface rather than to lessen it.

Naumann, R. J.↗
NASA NTRS1984

Test bed ion engine development

A test bed ion (TBI) engine was developed to serve as a tool in exploring the limits of electrostatic ion thruster performance. A description of three key ion engine components, the decoupled extraction and amplified current (DE-AC) accelerator system, field enhanced refractory metal (FERM) hollow cathode and divergent line cusp (DLC) discharge chamber, whose designs and operating philosophies differ markedly from conventional thruster technology is given. Significant program achievements were: (1) high current density DE-AC accelerator system operation at low electric field stress with indicated feasibility of a 60 mA/sq cm argon ion beam; (2) reliable FERM cathode start up times of 1 to 2 secs. and demonstrated 35 ampere emission levels; (3) DLC discharge chamber plasma potentials negative of anode potential; and (4) identification of an efficient high plasma density engine operating mode. Using the performance projections of this program and reasonable estimates of other parameter values, a 1.0 Newton thrust ion engine is identified as a realizable technology goal. Calculations show that such an engine, comparable in beam area to a J series 30 cm thruster, could, operating on Xe or Hg, have thruster efficiencies as high as 0.76 and 0.78 respectively, with a 100 eV/ion discharge loss.

Aston, G.↗
NASA NTRS1984

Investigation of the influence of the zeta-potential on the filtration rate in the presence of collectors

The value of the zeta-potential does not have an explicit effect, which is expressed by a simple math correlation, on filtration rate when a solution of the tested collector is filtered through a cake prepared under standard conditions from the examined particulate material. The zeta-potential measurements and filtration tests were carried out on silica and galena with solutions contg. a cationic container ANP and Et xanthane, resp. at PH = 6.5, varying concentration of the agent (0-2500 g/ton), and under a vacuum of 100 to 600 mm Hg.

Provirnina, E. V.↗
NASA NTRS1983

Vibration-rotation line strengths of the gas-phase OH radical

Absorption line strengths have been measured at 295 K for the first time for four vibration-rotation transitions of the OH radical, using a tunable, infrared diode-laser in conjunction with a molecular modulation spectrometer. A periodically varying OH concentration was created by photolysis, using a sinusoidally modulated 2537-A Hg lamp, of a flowing mixture of O3 and H2O contained within a quartz-walled White cell. The modulated absorption owing to the OH radical was subsequently observed by phase-sensitive detection. The absolute modulation amplitude of the OH number density was obtained by numerical simulation of the complete time-dependent photochemical system. The strongest transition measured was at 3407.607 kaysers, with a strength of 3.3 + or - 1.5 x 10 to the -20th per(cm molecule) sq cm.

Podolske, J.↗
NASA NTRS1985

An underground cosmic ray muon telescope for observation of cosmic ray anisotropy

A telescope housed in a tunnel laboratory has an overburden of 573 hg cm(-2) and is located under the center of a saddle-shaped landscape. It is composed of triple layers of proportional counters, each layer of area approx. 4m x 2m and their separation 0.5m. Events are selected by triple coincidence and software track identification. The telescope is in operation for over a year and the overall count rate is 1280 hr(-1). The structure and operation of the system is reported.

Lee, Y. W.↗
NASA NTRS1984

Advanced solar electric propulsion (ASEP) for planetary missions

This paper defines an advanced SEP concept based upon demonstrated ion thruster technology improvements and projected improvements in solar array technology. The ASEP concept utilizes an 'integrated' rather than a 'stage' approach and assumes 30 cm Hg ion thrusters capable of operation near 7 kWe each. The mission performance using the ASEP system is presented for three missions: (1) Saturn orbiter, (2) comet rendezvous and (3) comet sample return. The ASEP system provides significant mission performance improvements. The simpler, fewer thruster ASEP system design may provide cost as well as performance benefits for the missions examined when compared to ballistic or SEP stage mission options.

Jones, R. M.↗
NASA NTRS1983

Short wavelength infrared hybrid focal plane arrays

The employment of area focal plane arrays (FPA) has made it possible to obtain second generation infrared imaging systems with high resolution and sensitivity. The Short Wavelength Infrared (SWIR) region (1-2.5 microns) is of importance for imaging objects at high temperature and under conditions of reflected sunlight. The present investigation is concerned with electrooptical characterization results for 32 x 32 SWIR detector arrays and FPAs which are suitable for use in a prototype imaging spectrometer. The employed detector material is Hg(1-x)Cd(x)Te grown by liquid phase epitaxy on a CdTe transparent substrate. Attention is given to details of processing, the design of the detector array, the multiplexer, the fabrication of the hybrid FPA, and aspects of performance.

Vural, K.↗
NASA NTRS1985

Observation of adsorbate-induced surface states by elastic electron tunneling spectroscopy

Electronic structure induced by adsorbates at the interface of Al/AlOx/Au tunnel junctions has been observed by elastic electron tunneling spectroscopy. Strong structures appearing in the tunneling spectra above approximately 1 eV after exposure to I, Hg, Bi, and organohalides, have been interpreted in terms of adsorbate-induced surface states. The spectroscopic capabilities of elastic electron tunneling spectroscopy may be useful in the area of chemical detection. In the broader sense, the observation of adsorbate-induced unoccupied electronic states below the vacuum energy, makes elastic electron tunneling spectroscopy a potentially useful technique for the study of surfaces.

Leduc, H. G.↗
NASA NTRS1984

Combined distillation and normal freezing to purify elements of groups II and VI

A practical system and its application to the purification of Te and Cd is described. Single crystals are grown directly in vitreous silica ampoules subsequently used for sealed Bridgman growth of (Hg-Cd)Te. The system also prepares the ampoules by heating in high vacuum. Purification of the elements is by the combined effect of distillation and normal freezing. Transport and segregation are discussed.

Holland, L. R.↗
NASA NTRS1984

Effect of growth parameters on compositional variations in directionally solidified HgCdTe alloys

A series of Hg(1-x)Cd(x)Te alloy crystals was grown by directional solidification with compositions ranging from x = 0.2 to x = 0.4. The measured axial compositional profiles were interpreted in terms of an exact numerical solution of the appropriate diffusion equation that takes into account both the variations of the segregation coefficient and solidification rate with composition. The solutions for all growth rates agree generally well with the experimental data, and confirm earlier observations that employed approximate analytical solutions. The effective mass diffusion coefficients showed no significant composition or melt temperature dependence. Slightly higher diffusion coefficients were obtained, however, for the highest growth rates.

Szofran, F. R.↗
NASA NTRS1984

A method for interface shape control during Bridgman type crystal growth of HgCdTe alloys

A method is described for interface shape control of Hg(1-x)Cd(x)Te alloys during Bridgman-type crystal growth. The authors have used this method for several years to obtain the flat or convex interface shapes required for simultaneously reducing radial compositional variations and crystal defects. The method turns to advantage the thick-walled, fused-silica ampules required for growing these alloys. A proper combination of furnace geometry (two isothermal zones separated by a thin barrier), upper- and lower-zone temperatures, and growth rate is required. Examples of results are reported.

Szofran, F. R.↗
NASA NTRS1985

Processing of Complex Compound Semiconductors in Space

The purpose of this first year ground based research effort was to develop growth and characterization facilities for bulk single crystals of compound semiconductor alloys. As a system of 2-6 alloy crystal growth, C sub x Mn sub (1-x)Te was chosen since it has a relatively narrow solidus-liquidus separation serving as a suitable first model for zone melting and solution growth. Also, it allows, in principle, perfect lattice matching to Cd sub x Hg sub(1-x)Te that needs to be studied as a model for the investigation of heteroepitaxial growth from Te-solutions. A vertical zone melting furnace was built and used in the synthesis of Cd sub x Mn sub(1-x)Te from the elements and crystal growth in carbon coated fused silica crucibles of Bridgman growth geometry. Photoluminescence spectra of the crystals reveal band to band recombination at energies that agree with the reported literature data. In addition, deep luminescence is present at energies corresponding to proportional shifts from the 1.4 eV deep emission observed for bulk single crystals of CdTe.

Bachmann, K.↗