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NASA NTRS1997

A Chemical and Structural Study of the AlN-Si Interface

The growth of low defect density heteroepitaxial AlN has great implications for optoelectronic and high power devices since the AlN can be used either as device material or as a buffer layer for the overgrowth of other group-III nitrides. In this work, the results of transmission electron microscopy (TEM) involving both high resolution imaging and electron energy loss spectroscopy (EELS) of AlN/Si layers is reported and the relationship between Si-Al-N interactions and the misorientation of AlN nuclei is elucidated.

epitaxial optoelectronic silicon gallium nitride↗

NASA NTRS1981

Bidirectional reflectance spectroscopy. I - Theory

An approximate analytic solution is derived for the radiative transfer equation describing particulate surface light scattering, taking into account multiple scattering and mutual shadowing. Analytical expressions for the following quantities are found: bidirectional reflectance, radiance coefficient and factor, the normal, Bond, hemispherical, and physical albedos, integral phase function and phase integral, and limb-darkening profile. Scattering functions for mixtures can be calculated, as well as corrections for comparisons of experimental transmission or reflection spectra with observational planetary spectra. The theory should be useful for the interpretation of reflectance spectroscopy of laboratory surfaces and the photometry of solar system objects.

Hapke, B.↗

NASA NTRS2013

Nanoscale Mineralogy and Composition of Experimental Regolith Agglutinates Produced under Asteroidal Impact Conditions

On the Moon, the energetics of smaller impactors and the physical/chemical characteristics of the granular regolith target combine to form a key product of lunar space weathering: chemically reduced shock melts containing optically-active nanophase Fe metal grains (npFe0) [1]. In addition to forming the optically dark glassy matrix phase in lunar agglutinitic soil particles [1], these shock melts are becoming increasingly recognized for their contribution to optically active patina coatings on a wide range of exposed rock and grain surfaces in the lunar regolith [2]. In applying the lessons of lunar space weathering to asteroids, the potential similarities and differences in regolith-hosted shock melts on the Moon compared to those on asteroids has become a topic of increasing interest [3,4]. In a series of impact experiments performed at velocities applicable to the asteroid belt [5], Horz et al. [6] and See and Horz [7] have previously shown that repeated impacts into a gabbroic regolith analog target can produce melt-welded grain aggregates morphologically very similar to lunar agglutinates [6,7]. Although these agglutinate-like particles were extensively analyzed by electron microprobe and scanning electron microscopy (SEM) as part of the original study [7], a microstructural and compositional comparison of these aggregates to lunar soil agglutinates at sub-micron scales has yet to be made. To close this gap, we characterized a representative set of these aggregates using a JEOL 7600 field-emission scanning electron microscope (FE-SEM), and JEOL 2500SE field-emission scanning transmission electron microscope (FE-STEM) both optimized for energy dispersive X-ray spectroscopy (EDX) compositional spectrum imaging at respective analytical spatial resolutions of 0.5 to 1 micron, and 2 to 4 nm.

Christoffersen, Roy↗

NASA NTRS2015

Empirical InfraRed Line Lists for Five SO2 Isotopologues: 32/33/34/36S16O2 and 32S18O2

Using the latest published, empirically refined potential energy surface (PES) Ames-1 and purely ab initio CCSD(T)/aug-cc-pVQZ dipole moment surface (DMS), we have computed Infrared line lists for five symmetric isotopologues of sulfur dioxide: 32S16O2 (626), 33S16O2 (636), 34S16O2 (646), 36S16O2 (666), and 32S18O2 (828). The line lists are based on J=0-80 rovibrational variational calculations with E’ ≤ 8000 cm-1. The 34S16O2 and 33S16O2 line lists are compared to the experiment-based models in the HIgh-resolution TRANsmission molecular absorption database (HITRAN2012, http://www.cfa.harvard.edu/hitran/) and the Cologne Database for Molecular Spectroscopy, CDMS (http://www.astro.uni-koeln.de/cdms/). The accuracy for computed 646 band origins is similar to what has been reported for the main isotopologue, i.e. 0.01 – 0.03 cm-1 for bands up to 5500 cm-1. For rovibrational transitions, the 646 line position and intensity deviation patterns are much simpler and more self-consistent than those of the main isotopologue 626. The discrepancies are mainly found for higher Ka/J transitions. 626 and 646 exhibit comparable line position and intensity agreement for lower Ka/J transitions. The line position deviations for the 636 purely rotational band are parallel to those of 626 and 646, while its line intensity deviations do not show branching patterns as we found in the 626 and 646 cases. Predictions for the other minor isotopologues are expected to exhibit similar accuracy. These line lists are accurate enough to provide alternatives for missing bands of 626 and the minor isotopologues. It may significantly facilitate the laboratory spectroscopic measurement and analysis, as well as to identify these isotopologues in various astrophysical environments.

Sulfur Dioxide↗

NASA NTRS1999

Surface Diagnostics in Tribology Technology and Advanced Coatings Development

This paper discusses the methodologies used for surface property measurement of thin films and coatings, lubricants, and materials in the field of tribology. Surface diagnostic techniques include scanning electron microscopy, transmission electron microscopy, atomic force microscopy, stylus profilometry, x-ray diffraction, electron diffraction, Raman spectroscopy, Rutherford backscattering, elastic recoil spectroscopy, and tribology examination. Each diagnostic technique provides specific measurement results in its own unique way. In due course it should be possible to coordinate the different pieces of information provided by these diagnostic techniques into a coherent self-consistent description of the surface properties. Examples are given on the nature and character of thin diamond films.

Miyoshi, Kazuhisa↗

NASA NTRS1999

Measurements and Diagnostics of Diamond Films and Coatings

The commercial potential of chemical-vapor-deposited (CVD) diamond films has been established and a number of applications have been identified through university, industry, and government research studies. This paper discusses the methodologies used for property measurement and diagnostic of CVD diamond films and coatings. Measurement and diagnostic techniques studied include scanning electron microscopy, transmission electron microscopy, atomic force microscopy, stylus profilometry, x-ray diffraction, electron diffraction, Raman spectroscopy, Rutherford backscattering, elastic recoil spectroscopy, and friction examination. Each measurement and diagnostic technique provides unique information. A combination of techniques can provide the technical information required to understand the quality and properties of CVD diamond films, which are important to their application in specific component systems and environments. In this study the combination of measurement and diagnostic techniques was successfully applied to correlate deposition parameters and resultant diamond film composition, crystallinity, grain size, surface roughness, and coefficient of friction.

Miyoshi, Kazuhisa↗

NASA NTRS2018

Segregation and Phase Transformations Along Superlattice Intrinsic Stacking Faults in Ni-Based Superalloys

In this study, local chemical and structural changes along superlattice intrinsic stacking faults combine to represent an atomic-scale phase transformation. In order to elicit stacking fault shear, creep tests of two different single crystal Ni-base superalloys, ME501 and CMSX-4, were performed near 750 degrees Centigrade using stresses of 552 megapascals and 750 megapascals, respectively. Through high resolution Scanning Transmission Electron Microscopy (STEM) and state-of-the-art energy dispersive x-ray spectroscopy, ordered compositional changes were measured along SISFs (Superlattice Intrinsic Stacking Faults) in both alloys. For both instances, the elemental segregation and local crystal structure present along the SISFs are consistent with a nanoscale to D019 (ordering of a hexagonal close-packed crystal) phase transformation. Another notable observation is prominent Cr- and Co-rich Cottrell atmospheres and new evidence of more complex reordering processes responsible for the formation of these faults. These findings are further supported using density functional theory calculations and High Angle Annular Dark Field (HAADF) STEM image simulations.

STEM-EDS↗

NASA NTRS1973

Investigation of photoelectron spectroscopy

The problem of obtaining true and meaningful branching ratios from the photoelectron spectra is investigated. The problem consists of understanding the transmission of an electron energy analyzer for electrons with different energies, understanding the effects of using partially polarized radiation from different vacuum monochromators, and in understanding the effects of the angular distribution of photoelectrons ejected from different orbitals. An analysis of the degree of polarization of monochromatic radiation and of the problem of varying angular distributions led to the construction of a cylindrical mirror electron energy analyzer set at the special angle of 54 deg 44 min so that no discrimination would occur for electrons of different angular distributions. With the analyzer properly calibrated for transmission of electrons of different energies, data were taken at several wavelengths and for several atmospheric gases.

Samson, J. A. R.↗

NASA NTRS1992

Numerical study of low-current steady arcs

The development of a high-efficiency CW YLF laser doped with Er,Tm,Ho: and featuring a strongly focusing resonator that collects a high density of pump power on the active crystal is described. The emission is investigated at 2.06 microns and a tuning range both at liquid-nitrogen (77 K) and at dry-ice (210 K) temperature. The noise characteristics and the long-term power stability of the laser is studied with an eye to employing this source for high-resolution spectroscopy in the 2-micron wavelength region. The detection of several absorption lines of NH3 at low pressure is described. The output power of the laser as a function of the power impinging on the crystal for different transmission of the output mirror is illustrated. The best result obtained is 1.46 W output for 3.2 W of argon pump. The minimum threshold achieved is 3.5 mW with a 1-percent transmission mirror. It is concluded that it is possible to develop a highly efficient Ho:YLF laser featuring low noise and sufficient tunability for high-resolution spectroscopy in the 2-micron region.

Kim, S. C.↗

NASA NTRS1987

Origin and evolution of osmoregulatory mechanisms in blue-green algae (cyanobacteria) as a function of metabolic and structural complexity: Reflections of Precambrian paleobiology

Major accomplishments underlying the basic understanding of cyanobacterial resistance to salt tolerance and osmotic stress were made. The methodology proposed included: the tracing of the pathways of formation of osmoregulatory solutes by traditional methods involving C-14 labelled substrates; gas chromatography; amino acid analysis; X-ray analysis using scanning transmission electron microscopy; and most importantly, C-13 labelled substrates, followed by Nuclear Magnetic Resonance (NMR) spectroscopy. It was found that the cyanobacteria employ a diversity of organic, osmoregulatory solutes. Osmoregulatory solutes were found to serve four functions: adjustment of water activity, noninhibition of enzymes; lowering of K sub m of enzymes to allow functioning at normal levels when the intracellular salt accumulates, and extending the pH optimum of enzymes as intracellular pH rises due to proton-potassium ion pump action during osmoregulation. Differences in osmoregulatory solutes may, but are not always, be attributed to differences in nutritional capabilities. The mechanism of osmoregulation and concomitant salt tolerance in halophilic cyanobacteria was elucidated. The activities of betaine and S-Adenosylhomocysteine hydrolase are discussed.

Yopp, John H.↗

NASA NTRS1988

Tunable diode laser spectral diagnostic studies of a pressure modulator radiometer

The successful application of high-resolution tunable diode laser spectroscopy to the diagnostic measurement of a pressure modulator radiometer (PMR) is reported, and recorded line transmission profiles for a single N2O line at 1272/cm are presented. PMR measurements obtained for mean cell pressures of about 10, 20, and 30 Torr were found to be consistent with theoretical predictions. The validity of single-pressure and two-pressure approximation representations of the mean and modulated PMR transmission spectra has been verified.

May, Randy D.↗

NASA NTRS1993

Enhancement of photoluminescence intensity of GaAs with cubic GaS chemical vapor deposited using a structurally designed single-source precursor

A two order-of-magnitude enhancement of photoluminescence intensity relative to untreated GaAs has been observed for GaAs surfaces coated with chemical vapor-deposited GaS. The increase in photoluminescence intensity can be viewed as an effective reduction in surface recombination velocity and/or band bending. The gallium cluster /(t-Bu)GaS/4 was used as a single-source precursor for the deposition of GaS thin films. The cubane core of the structurally characterized precursor is retained in the deposited film producing a cubic phase. Furthermore, a near-epitaxial growth is observed for the GaS passivating layer. Films were characterized by transmission electron microscopy, X-ray powder diffraction, and X-ray photoelectron and Rutherford backscattering spectroscopies.

Macinnes, Andrew N.↗

NASA NTRS1993

Photoluminescence intensity enhancement of GaAs by vapor-deposited GaS - A rational approach to surface passivation

A two order-of-magnitude enhancement of photoluminescence intensity relative to untreated GaAs has been observed for GaAs surfaces coated with chemical vapor-deposited GaS. The increase in photoluminescence intensity can be viewed as an effective reduction in surface recombination velocity and/or band bending. The gallium cluster (/t-Bu/GaS)4 was used as a single-source precursor for the deposition of GaS thin films. The cubane core of the structurally-characterized precursor is retained in the deposited film producing a cubic phase. Furthermore, a near-epitaxial growth is observed for the GaS passivating layer. Films were characterized by transmission electron microscopy, X-ray powder diffraction, and X-ray photoelectron and Rutherford backscattering spectroscopies.

Jenkins, Phillip P.↗

NASA NTRS2012

Overview of C/C-SiC Composite Development for the Orion Launch Abort System

Past and present efforts by the authors to further understanding of the ceramic matrix composite (CMC) material used in the valve components of the Orion Launch Abort System (LAS) Attitude Control Motor (ACM) will be presented. The LAS is designed to quickly lift the Orion Crew Exploration Vehicle (CEV) away from its launch vehicle in emergency abort scenarios. The ACM is a solid rocket motor which utilizes eight throttleable nozzles to maintain proper orientation of the CEV during abort operations. Launch abort systems have not been available for use by NASA on manned launches since the last Apollo ]Saturn launch in 1975. The CMC material, carbon-carbon/silicon-carbide (C/C-SiC), is manufactured by Fiber Materials, Inc. and consists of a rigid 4-directional carbon-fiber tow weave reinforced with a mixed carbon plus SiC matrix. Several valve and full system (8-valve) static motor tests have been conducted by the motor vendor. The culmination of these tests was the successful flight test of the Orion LAS Pad Abort One (PA ]1) vehicle on May 6, 2010. Due to the fast pace of the LAS development program, NASA Marshall Space Flight Center assisted the LAS community by performing a series of material and component evaluations using fired hardware from valve and full ]system development motor tests, and from the PA-1 flight ACM motor. Information will be presented on the structure of the C/C-SiC material, as well as the efficacy of various non ]destructive evaluation (NDE) techniques, including but not limited to: radiography, computed tomography, nanofocus computed tomography, and X-ray transmission microscopy. Examinations of the microstructure of the material via scanning electron microscopy and energy dispersive spectroscopy will also be discussed. The findings resulting from the subject effort are assisting the LAS Project in risk assessments and in possible modifications to the final ACM operational design.

Allen, Lee R.↗

NASA NTRS2015

Problems at the Leading Edge of Space Weathering as Revealed by TEM Combined with Surface Science Techniques

Both transmission electron micros-copy (TEM) and surface analysis techniques such as X-ray photoelectron spectroscopy (XPS) were instrumen-tal in making the first characterizations of material generated by space weathering in lunar samples [1,2]. Without them, the nature of nanophase metallic Fe (npFe0) correlated with the surface of lunar regolith grains would have taken much longer to become rec-ognized and understood. Our groups at JSC and UVa have been using both techniques in a cross-correlated way to investigate how the solar wind contributes to space weathering [e.g., 3]. These efforts have identified a number of ongoing problems and knowledge gaps. Key insights made by UVa group leader Raul Barag-iola during this work are gratefully remembered.

Christoffersen, R.↗

NASA NTRS1992

Cathodoluminescence and transmission electron microscopy study of dark line defects in thick In(0.2)Ga(0.8)As/GaAs multiple quantum wells

TEM and cathodoluminescence (CL) imaging and spectroscopy have been performed on In(0.2)Ga(0.8)As/GaAs MQW structures. Cross-sectional and plan-view TEM demonstrates that misfit dislocations (MDs) are confined to the MQW-to-GaAs interfacial regions. The observed large variation in the exciton luminescence intensity is interpreted as due to the presence of nonradiative recombination centers spread homogeneously in the MQW region away from interface MDs. These nonradiative recombination centers compete with exciton and midgap radiative centers at wavelengths of 950 nm and 1000-1600 nm, respectively, resulting in spatiallty correlated dark line defects for all CL imaging wavelengths.

Rich, D. H.↗

NASA NTRS1994

Simultaneous soft and hard X-ray spectroscopy of AM Herculis with EXOSAT: Discovery of photospheric absorption features

We present 0.1-10 keV spectroscopic observations of AM Herculis obtained with the Transmission Grating Spectrometers and Medium Energy experiments on EXOSAT, taken when the object was in its 'reversed X-ray mode.' The observation covers over six binary orbits without interruption, enabling us to analyze the phase and intensity dependence of both the hard and the soft spectrum simultaneously. We resolve the optically thick soft X-ray spectrum, and find definite evidence for time- and phase-dependent photospheric absorption structure arising in the white dwarf atmosphere. We present a simple empirical analysis of the combined soft and hard X-ray spectra, to examine whether the effect of a better determination of the column density of neutral absorbing material, afforded by our data, would solve the problem of the large relative soft X-ray overluminosity previously observed in AM Her. We find that a single absorbing column fits the entire spectrum, and that the column densities implied are indeed substantially lower than previously estimated. However, during half the binary orbit we still determine a strong lower limit to the soft-to-hard luminosity ratio of L(sub soft)/L(sub hard) is greater than or approximately equal to 10, in conflict with the simple radiative shock models for the accretion region. We argue that this indicates the need to reexamine the luminosity problem using explicit models for the emission spectrum based on a full solution of the atmospheric radiative transfer problem.

Paerels, Frits↗

NASA NTRS2004

Defect Clustering and Nano-phase Structure Characterization of Multicomponent Rare Earth-Oxide-Doped Zirconia-Yttria Thermal Barrier Coatings

Advanced thermal barrier coatings (TBCs) have been developed by incorporating multicomponent rare earth oxide dopants into zirconia-based thermal barrier coatings to promote the creation of the thermodynamically stable, immobile oxide defect clusters and/or nanophases within the coating systems. In this paper, the defect clusters, induced by Nd, Gd, and Yb rare earth dopants in the zirconia-yttria thermal barrier coatings, were characterized by high-resolution transmission electron microscopy (TEM). The TEM lattice imaging, selected area diffraction (SAD), and electron energy-loss spectroscopy (EELS) analyses demonstrated that the extensive nanoscale rare earth dopant segregation exists in the plasma-sprayed and electron-physical-vapor-deposited (EB PVD) thermal barrier coatings. The nanoscale concentration heterogeneity and the resulting large lattice distortion promoted the formation of parallel and rotational defective lattice clusters in the coating systems. The presence of the 5-to 100-nm-sized defect clusters and nanophases is believed to be responsible for the significant reduction of thermal conductivity, improved sintering resistance, and long-term high temperature stability of the advanced thermal barrier coating systems.

Zhu, Dongming↗