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NASA NTRS1972

New space processing experiments for the Skylab missions.

The M512 Materials Processing Facility, which is the main item of equipment for the Skylab space manufacturing experiments is described. It is basically an electron-beam welding apparatus, consisting of a battery power supply, a 20-kV electron beam source, and a spherical 40-cm-diam vacuum chamber. It will be used to perform the M551 Metals Melting Experiment, the M552 Exothermic Brazing Experiment, the M553 Sphere Forming Experiment, the M554 Composite Casting Experiment, the M555 Gallium Arsenide Crystal Growth Experiment, the M561 Whisker-Reinforced Composites Experiment, the M562 Indium Antimonide Crystal Growth, Experiment, the M563 Mixed III-V Crystal Growth Experiment, the M564 Alkali Halide Eutectics Experiment, the M565 Silver Grids Melted in Space Experiment, and the M566 Copper-Aluminum Eutectic Experiment. Three apparatus systems designed to broaden the technical scope of the experiments and to gain experience with new experimental techniques are described. These are an electrophoretic separator, an electromagnetic levitation system, and a versatile electric furnace with an electronic control system.

Bredt, J. H.↗

NASA NTRS1973

Correlation of materials properties with the atomic density concept

Progress is described in developing, through mathematical expressions, values for the parameters which apparently determine the values of atomic density, melting point, and principal elastic constant for the alkali halides. This necessarily required the review and refinement of these data for the monohalides in general and for LiH. A more sensitive and dimensionless expression of the mass factor appears to have promise and a still more sensitive one is suggested for future study. A periodicity factor based on inert gas core configuration has been suggested to replace the atomic number previously used.

Bradstreet, S. W.↗

NASA NTRS1975

Space processing of chalcogenide glasses

Chalcogenide glasses are discussed as good infrared transmitters, possessing the strength, corrosion resistance, and scale-up potential necessary for large 10.6-micron windows. The disadvantage of earth-produced chalcogenide glasses is shown to be an infrared absorption coefficient which is unacceptably high relative to alkali halides. This coefficient is traced to optical nonhomogeneities resulting from environmental and container contamination. Space processing is considered as a means of improving the infrared transmission quality of chalcogenides and of eliminating the following problems: optical inhomogeneities caused by thermal currents and density fluctuation in the l-g earth environment; contamination from the earth-melting crucible by oxygen and other elements deleterious to infrared transmission; and, heterogeneous nucleation at the earth-melting crucible-glass interface.

Larsen, D. C.↗

NASA NTRS1975

Correlation of materials properties with the atomic density concept

Based on the hypothesis that the number of atoms per unit volume, accurately calculable for any substance of known real density and chemical composition, various characterizing parameters (energy levels of electrons interacting among atoms of the same or different kinds, atomic mass, bond intensity) were chosen for study. A multiple exponential equation was derived to express the relationship. Various properties were examined, and correlated with the various parameters. Some of the properties considered were: (1) heat of atomization, (2) boiling point, (3) melting point, (4) shear elastic modulus of cubic crystals, (5) thermal conductivity, and (6) refractive index for transparent substances. The solid elements and alkali halides were the materials studied. It is concluded that the number of different properties can quantitively be described by a common group of parameters for the solid elements, and a wide variety of compounds.

Source record↗

NASA NTRS1976

Dielectric breakdown induced by picosecond laser pulses

The damage thresholds of transparent optical materials were investigated. Single picosecond pulses at 1.06 microns, 0.53 microns and 0.35 microns were obtained from a mode locked Nd-YAG oscillator-amplifier-frequency multiplier system. The pulses were Gaussian in space and time and permitted the determination of breakdown thresholds with a reproducibility of 15%. It was shown that the breakdown thresholds are characteristic of the bulk material, which included nine alkali halides, five different laser host materials, KDP, quartz, sapphire and calcium fluoride. The extension of the damage data to the ultraviolet is significant, because some indication was obtained that two- and three-photon absorption processes begin to play a role in determining the threshold. Throughout the visible region of the spectrum the threshold is still an increasing function of frequency, indicating that avalanche ionization is the dominant factor in determining the breakdown threshold. This was confirmed by a detailed study of the damage morphology with a high resolution microscope just above the threshold. The influence of self focusing is discussed, and evidence for beam distortion below the power threshold for complete self focusing is presented, confirming the theory of Marburger.

Smith, W. L.↗

NASA NTRS1978

Growth of fungi in NaCl-MgSO4 brines

Previous investigations have shown that common fungi of the Penicillium-Aspergillus group can be grown in a variety of brines or on moist salt crystals. This simulates salt flats as well as sizeable waterbodies stabilized as saturated brines such as Don Juan Pond (Antarctica), the Great Salt Lake of Utah, and the Dead Sea in the Jordan Valley. In general, salt media rich in KCl are favored over other alkali halides; the media become more selective as the salt concentration rises and nutrient requirements become more complex. In the present paper, it is shown that media which resemble the Dead Sea salt mix can, in fact, support the growth of selected fungal strains, even in the absence of reduced organic nutrients other than glucose. Such media may serve as models for localized microhabitats on Mars.

Siegel, S. M.↗

NASA NTRS1979

Status of equation of state of solids

The results of equation of state (pressure-volume-temperature) measurements for the rare gas solids, alkali metals and alkali halides are reviewed. The precision and errors associated with the techniques most commonly used to obtain measurements of volume as a function of pressure, including piston volume displacement, ultrasonic, shock wave, and Brillouin scattering techniques, length change measurement, and combined length change and ultrasonic transit techniques, are presented. The various equations of state which are normally used to represent these measurements are discussed.

Ruoff, A. L.↗

NASA NTRS1979

The structure of small, vapor-deposited particles. II - Experimental study of particles with hexagonal profile

'Multiply-twinned' gold particles with hexagonal bright field TEM profile were determined to be icosahedra composed of 20 identical and twin-related tetrahedral building units that do not have an fcc structure. The crystal structure of these slightly deformed tetrahedra is rhombohedral. Experimental evidence supporting this particle model was obtained by selected-zone dark field and weak beam dark field electron microscopy. In conjunction with the results of part I, it has been concluded that multiply-twinned gold particles of pentagonal or hexagonal profile that are found during the early stages of the vapor deposition growth process on alkali halide surfaces do not have an fcc crystal structure, which is in obvious contrast to the structure of bulk gold.

Yacaman, M. J.↗

NASA NTRS1983

On the method of positron lifetime measurement

A fast-slow coincidence system was constructed for the measurement of positron lifetimes in material. The time resolution of this system was 270 ps for the (60)Co gamma rays. Positron lifetime spectra for 14 kinds of alkali halides were measured with this system. Two lifetime components and their intensities were derived from analyses of the lifetime spectra.

Nishiyama, F.↗

NASA NTRS1981

Electron bombardment charge-coupled device /CCD/ detectors for the vacuum ultraviolet

Development work is underway at Princeton and NRL on electronic-readout imaging detectors for the VUV which are based on electron-bombardment CCD arrays and opaque alkali-halide photocathodes. These detectors provide single-photoelectron detection capability and wide dynamic range combined with almost the maximum possible quantum efficiency (in excess of 50% in the wavelength range below 1300 A). The Princeton work is based on an oblique-focusing electron optical configuration, while the NRL program is based on a Schmidt optical system with opaque photocathode. Results of preliminary tests at Princeton and overall system concepts for the Princeton and NRL cameras are discussed, and applications to possible space astronomy studies are examined.

Lowrance, J. L.↗

NASA NTRS1982

Integration and compaction of optical systems in the mid-infrared

Problems in the design of optical systems for use with mid-IR (MIR; 10.6-micron) lasers are reviewed. The cryogenically operated lead-chalcogenide tunable diode lasers currently used as sources emit complex far-field patterns and require sophisticated beam collection, shaping, and guide optics. Since conventional optics using large mirrors and Ge lenses are often awkward to use and sensitive to vibration, and since optical fibers for the MIR have not been perfected, current research concentrates on hollow flexible rectangular metallic pipes. These pipes are shown to provide transmission of 95 percent or better over a 1-m length, with beam shaping and polarization control. The low transmission losses and wideband IR transmission of single-crystal alkali halides suggest the feasibility of their use in MIR optical fibers and waveguides.

Yu, C.↗

NASA NTRS1985

Conducting a thermal conductivity survey

A physically transparent approximate theory of phonon decay rates is presented starting from a pair potential model of the interatomic forces in an insulator or semiconductor. The theory applies in the classical regime and relates the 3-phonon decay rate to the third derivative of the pair potential. Phonon dispersion relations do not need to be calculated, as sum rules relate all the needed quantities directly to the pair potential. The Brillouin zone averaged phonon lifetime turns out to involve a dimensionless measure of the anharmonicity multiplied by an effective density of states for 3-phonon decay. Results are given for rare gas and alkali halide crystals. For rare gases, the results are in good agreement with more elaborate perturbation calculations. Comparison to experimental data on phonon linewidths and thermal conductivity are made.

Allen, P. B.↗

NASA NTRS1984

Electron impact spectroscopy of high temperature species

The application of the methods of electron impact spectroscopy to the study of high temperature species is discussed. Electron scattering by metal vapors (Ba, Bi, Cu, Mn, Tl, Zn), alkali halide molecules, and KOH are described as demonstrative examples.

Trajmar, S.↗

NASA NTRS1986

Analysis of the physical atomic forces between noble gas atoms, alkali ions and halogen ions

The physical forces between atoms and molecules are important in a number of processes of practical importance, including line broadening in radiative processes, gas and crystal properties, adhesion, and thin films. The components of the physical forces between noble gas atoms, alkali ions, and halogen ions are analyzed and a data base for the dispersion forces is developed from the literature based on evaluations with the harmonic oscillator dispersion model for higher order coefficients. The Zener model of the repulsive core is used in the context of the recent asymptotic wave functions of Handler and Smith; and an effective ionization potential within the Handler and Smith wave functions is defined to analyze the two body potential data of Waldman and Gordon, the alkali-halide molecular data, and the noble gas crystal and salt crystal data. A satisfactory global fit to this molecular and crystal data is then reproduced by the model to within several percent. Surface potentials are evaluated for noble gas atoms on noble gas and salt crystal surfaces with surface tension neglected. Within this context, the noble gas surface potentials on noble gas and salt crystals are considered to be accurate to within several percent.

Wilson, J. W.↗

NASA NTRS1986

Finite size corrections to Madelung number

It is customary in the study of ionic crystals to assume that the ions are point charges at their respective lattice sites; the corresponding electrostatic energy of one such ion is reducible to Madelung's form, where the Madelung number has a value of 1.7467. This paper considers the modifications in the electrostatic energy when the atomic finite size is treated in more detail. The results are tabulated as a direct correction to Madelung's number for alkali halide cubic crystals.

Wilson, J. W.↗

NASA NTRS1987

The role of electronic mechanisms in surface erosion and glow phenomena

Experimental studies of desorption induced by electronic transitions (DIET) are described. Such studies are producing an increasingly complete picture of the dynamical pathways through which incident electronic energy is absorbed and rechanneled to produce macroscopic erosion and glow. These mechanistic studies can determine rate constants for erosion and glow processes in model materials and provide valuable guidance in materials selection and development. Extensive experiments with electron, photon, and heavy particle irradiation of alkali halides and other simple model materials have produced evidence showing that: (1) surface erosion, consisting primarily in the ejection or desorption of ground-state neutral atoms, occurs with large efficiencies for all irradiated species; (2) surface glow, resulting from the radiative decay of desorbed atoms, likewise occurs for all irradiating species; (3) the typical mechanism for ground-state neutral desorption is exciton formation, followed by relaxation to a permanent, mobile electronic defect which is the precursor to bond-breaking in the surface or near-surface bulk of the material; and (4) the mechanisms for excited atom formation may include curve crossing in atomic collisions, interactions with surface defect or impurity states, or defect diffusion.

Haglund, Richard F., Jr.↗

NASA NTRS1987

The use of molten salts as physical models for the study of solidification in metals and semiconductors

It is presently noted that molten salts possess attributes rendering them attractive as physical models of cast metals in solidification studies. Molten alkali halides have an approximately correct Prandtl number for this modeling of metallic melts, and are transparent to visible light. Attention is given to solidification in the LiCl-KCl system, in order to determine whether such phenomena as solute rejection can be observed and characterized through the application of laser schlieren imaging.

Koziol, Jurek K.↗

NASA NTRS1988

Atomic forces between noble gas atoms, alkali ions, and halogen ions for surface interactions

The components of the physical forces between noble gas atoms, alkali ions, and halogen ions are analyzed and a data base developed from analysis of the two-body potential data, the alkali-halide molecular data, and the noble gas crystal and salt crystal data. A satisfactory global fit to this molecular and crystal data is then reproduced by the model to within several percent. Surface potentials are evaluated for noble gas atoms on noble gas surfaces and salt crystal surfaces with surface tension neglected. Within this context, the noble gas surface potentials on noble gas and salt crystals are considered to be accurate to within several percent.

Wilson, J. W.↗