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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Probing hot-carrier transport and elastic scattering using ballistic-electron-emission microscopy

Ballistic-electron-emission microscopy (BEEM) has been used to characterize electron transport and scattering in metal/semiconductor structures. A SiO2 layer at the Au/Si interface was patterned to form transmitting and nontransmitting regions. By analyzing the BEEM current profiles at the boundaries of these regions, information on the spatial distribution of electrons after transport through the Au layer can be derived. A detailed comparison is made between the results presented here and models which involve modification of the electron distribution by scattering.

Milliken, A. M.↗
NASA NTRS1993

Characterization of Si3N4/SiO2 optical channel waveguides by photon scanning tunneling microscopy

Photon scanning tunneling microscopy (PSTM) is used to characterize Si3N4/Si02 optical channel waveguides being used for integrated optical-micromechanical sensors. PSTM utilizes an optical fiber tapered to a fine point which is piezoelectrically positioned to measure the decay of the evanescent field intensity associated with the waveguide propagating mode. Evanescent field decays are recorded for both ridge channel waveguides and planar waveguide regions. Values for the local effective refractive index are calculated from the data for both polarizations and compared to model calculations.

Wang, Yan↗
NASA NTRS1993

Scanning Tunneling Microscopy analysis of space-exposed polymer films

The characterization of the surface of selected space-exposed polymer films by Scanning Tunneling Microscopy (STM) is reported. Principles of STM, an emerging new technique for materials analysis, are reviewed. The analysis of several films which received up to 5.8 years of low Earth orbital (LEO) exposure onboard the NASA Long Duration Exposure Facility (LDEF) is discussed. Specimens included FEP Teflon thermal blanket material, Kapton film, and several experimental polymer films. Ultraviolet and atomic oxygen-induced crazing and erosion are described. The intent of this paper is to demonstrate how STM is enhancing the understanding of LEO space environmental effects on polymer films.

Kalil, Carol R.↗
NASA NTRS1992

Scanning electron acoustic microscopy of residual stresses in ceramics - Theory and experiment

The paper presents a three-dimensional mathematical model of signal generation and contrast in brittle materials and uses the model to simulate the effect of residual stress fields on the scanning electron acoustic microscopy (SEAM)-generated electron acoustic signal. According to the model, a positive (tensile) strain produces an increase in the output signal, whereas a negative (compressive) strain produces a decrease in the ouput signal. Dark field contrast conditions occur at a chopping frequency at which V2 - V1 is greater than 0 (where V2 = V is the SEAM output in a region of residual stresses, and V1 is the output in a stress-free region of the sample). Under ideal conditions (maximum contrast) V1 approaches zero. It was found that tensile strains of the order 0.2-0.3 percent, possible in brittle materials, would produce a variation of the acoustic output signal of the order 10 nV (about 1 percent), well within the image contrast and signal processing capability of the SEAM electronics.

Cantrell, John H.↗
NASA NTRS1994

Examining Thermally Sprayed Coats By Fluorescence Microscopy

True flaws distinquished from those induced by preparation of specimens. Fluorescence microscopy reveals debonding, porosity, cracks, and other flaws in specimens of thermally sprayed coating materials. Specimen illuminated, and dye it contains fluoresces, emitting light at different wavelength. Filters emphasize contrast between excitation light and emission light. Specimen viewed directly or photographed on color film.

Street, Kenneth W., Jr.↗
NASA NTRS1994

High Resolution Transmission Electron Microscopy (HRTEM) of nanophase ferric oxides

Iron oxide minerals are the prime candidates for Fe(III) signatures in remotely sensed Martian surface spectra. Magnetic, Mossbauer, and reflectance spectroscopy have been carried out in the laboratory in order to understand the mineralogical nature of Martian analog ferric oxide minerals of submicron or nanometer size range. Out of the iron oxide minerals studied, nanometer sized ferric oxides are promising candidates for possible Martian spectral analogs. 'Nanophase ferric oxide (np-Ox)' is a generic term for ferric oxide/oxihydroxide particles having nanoscale (less than 10 nm) particle dimensions. Ferrihydrite, superparamagnetic particles of hematite, maghemite and goethite, and nanometer sized particles of inherently paramagnetic lepidocrocite are all examples of nanophase ferric oxides. np-Ox particles in general do not give X-ray diffraction (XRD) patterns with well defined peaks and would often be classified as X-ray amorphous. Therefore, different np-Oxs preparations should be characterized using a more sensitive technique e.g., high resolution transmission electron microscopy (HRTEM). The purpose of this study is to report the particle size, morphology and crystalline order, of five np-Ox samples by HRTEM imaging and electron diffraction (ED).

Golden, D. C.↗
NASA NTRS1994

Investigation of nucleation and growth processes of diamond films by atomic force microscopy

The nucleation and growth of plasma-enhanced chemical-vapor deposited polycrystalline diamond films were studied using atomic force microscopy (AFM). AFM images were obtained for (1) nucleated diamond films produced from depositions that were terminated during the initial stages of growth, (2) the silicon substrate-diamond film interface side of diamond films (1-4 micrometers thick) removed from the original surface of the substrate, and (3) the cross-sectional fracture surface of the film, including the Si/diamond interface. Pronounced tip effects were observed for early-stage diamond nucleation attributed to tip convolution in the AFM images. AFM images of the film's cross section and interface, however, were not highly affected by tip convolution, and the images indicate that the surface of the silicon substrate is initially covered by a small grained polycrystalline-like film and the formation of this precursor film is followed by nucleation of the diamond film on top of this layer. X-ray photoelectron spectroscopy spectra indicate that some silicon carbide is present in the precursor layer.

George, M. A.↗
NASA NTRS1992

Scanning electron acoustic microscopy of residual stresses in ceramics: Theory and experiment

Several reviews have highlighted a number of applications of scanning electron acoustic microscopy (SEAM) to metals and semiconductors which show that SEAM can provide new information on surface and near-surface features of such materials, but there have been few studies attempting to determine the capabilities of SEAM for characterizing ceramic materials. We have recently observed image contrast in SEAM from residual stress fields induced in brittle materials by Vickers indentations that is strongly dependent on the electron beam chopping frequency. We have also recently developed a three-dimensional mathematical model of signal generation and contrast in SEAM, appropriate to the brittle materials studied, that we use as a starting point in this paper for modeling the effect of residual stress fields on the generated electron acoustic signal. The influence of the electron beam chopping frequency is also considered under restrictive assumptions.

Cantrell, John H.↗
NASA NTRS1995

Investigation of nucleation and growth processes of diamond films by atomic force microscopy

The nucleation and growth of plasma enhanced chemical vapor deposited (PECVD) polycrystalline diamond films were studied using atomic force microscopy (AFM). AFM images were obtained for: (1) nucleated diamond films produced from depositions that were terminated during the initial stages of growth, (2) the silicon substrate-diamond film interface side of diamond films (1-4 micrometers thick) removed from the original surface of the substrate, and (3) cross-sectional fracture surface of the film, including the Si/diamond interface. Pronounced tip effects were observed for early-stage diamond nucleation attributed to tip convolution in the AFM images. AFM images of the films cross-section and interface however were not affected by tip convolution, and the images indicate that the surface of the silicon substrate is initially covered by small grained polycrystalline-like film and the formation of this precursor film is followed by nucleation of the diamond film on top of this layer. X-ray photoelectron spectroscoy (XPS) spectra indicates that some silicon carbide is present in the precursor layer.

George, M. A.↗
NASA NTRS1996

Scanning Tunneling Microscopy Studies of Diamond Films and Optoelectronic Materials

We present a summary of the research, citations of publications resulting from the research and abstracts of such publications. We have made no inventions in the performance of the work in this project. The main goals of the project were to set up a Chemical Vapor Deposition (CVD) diamond growth system attached to an UltraHigh Vacuum (UHV) atomic resolution Scanning Tunneling Microscopy (STM) system and carry out experiments aimed at studying the properties and growth of diamond films using atomic resolution UHV STM. We successfully achieved these goals. We observed, for the first time, the atomic structure of the surface of CVD grown epitaxial diamond (100) films using UHV STM. We studied the effects of atomic hydrogen on the CVD diamond growth process. We studied the electronic properties of the diamond (100) (2x1) surface, and the effect of alkali metal adsorbates such as Cs on the work function of this surface using UHV STM spectroscopy techniques. We also studied, using STM, new electronic materials such as carbon nanotubes and gold nanostructures. This work resulted in four publications in refereed scientific journals and five publications in refereed conference proceedings.

Perez, Jose M.↗
NASA NTRS1996

Advancement of X-Ray Microscopy Technology and its Application to Metal Solidification Studies

The technique of x-ray projection microscopy is being used to view, in real time, the structures and dynamics of the solid-liquid interface during solidification. By employing a hard x-ray source with sub-micron dimensions, resolutions of 2 micrometers can be obtained with magnifications of over 800 X. Specimen growth conditions need to be optimized and the best imaging technologies applied to maintain x-ray image resolution, contrast and sensitivity. It turns out that no single imaging technology offers the best solution and traditional methods like radiographic film cannot be used due to specimen motion (solidification). In addition, a special furnace design is required to permit controlled growth conditions and still offer maximum resolution and image contrast.

Kaukler, William F.↗
NASA NTRS1994

Ballistic-Electron-Emission-Microscopy of Strained Si(sub 1-x)Ge(sub x) Layers

Ballistic-electron-emission microscopy (BEEM) has been used to investigate the effects of strain on Si(sub 1-x)Ge(sub x) alloys. Lifting of the degeneracy of the conduction-band minimum of Si(sub 1-x)Ge(sub x), due to lattice deformation has been directly measured by application of BEEM spectroscopy to Ag/Si structures. Experimental values for this conduction-band splitting agree well with calculations. In addition, an unexpected heterogeneity in the strain of the Si(sub 1-x)Ge(sub x) layer is introduced by deposition of Au. This effect, not observed with Ag, is attributed to species interdiffusion and has important implications for metal-semiconductor devices based oil pseudomorphic Si(sub 1-x)Ge(sub x)/Si material systems.

Bell, L. D.↗
NASA NTRS1996

Advancement of X-Ray Microscopy Technology and Its Application to Metal Solidification Studies

The technique of x-ray projection microscopy is being used to view, in real time, the structures and dynamics of the solid-liquid interface during solidification. By employing a hard x-ray source with sub-micron dimensions, resolutions of 2 microns can be obtained with magnifications of over 800 X. Specimen growth conditions need to be optimized and the best imaging technologies applied to maintain x-ray image resolution, contrast and sensitivity. It turns out that no single imaging technology offers the best solution and traditional methods like radiographic film cannot be used due to specimen motion (solidification). In addition, a special furnace design is required to permit controlled growth conditions and still offer maximum resolution and image contrast.

Kaukler, William F.↗
NASA NTRS1996

Evidence of Momentum Conservation at a Nonepitaxial Metal/Semiconductor Interface Using Ballistic Electron Emission Microscopy

Ballistic-Electron-Emission Microscopy (BEEM) spectroscopy has been performed on Au/Si(111) structures as a function of Au thickness and temperature. At 77 K a direct signature of parallel momentum conservation at the Au/Si interface is observed in the BEEM spectra. The variation in spectral shape with both Au thickness and temperature places restrictions on allowable values of inelastic and elastic mean-free paths in the metal, and also requires the presence of multiple electron passes within the Au layer. An independent indication of multiple reflections is directly observed in the attenuation of BEEM current with Au thickness.

Bell, L. D.↗
NASA NTRS1998

Real-Time X-Ray Transmission Microscopy for Fundamental Studies Solidification

High resolution real-time X-ray Transmission Microscopy, XTM, has been applied to obtain information fundamental to solidification of optically opaque metallic systems. We have previously reported the measurement of solute profile in the liquid, phase growth, and detailed solid-liquid interfacial morphology of aluminum based alloys with exposure times less than 2 seconds. Recent advances in XTM furnace design have provided an increase in real-time magnification (during solidification) for the XTM from 4OX to 16OX. The increased magnification has enabled for the first time the XTM imaging of real-time growth of fibers and particles with diameters of 5 micrometers. We have applied this system to study of the kinetics of formation and morphological evolution of secondary fibers and particles in Al-Bi monotectic alloys to observe a previously unreported velocity dependent thermo-capillary depletion mechanism for Bi rich liquid which can penetrate many fiber diameters into the solid-liquid interface. In this talk we will discuss application of the XTM to the study the fundamentals of monotectic and eutectic solidification, the enhancement of XTM data with precise solid liquid interfacial temperature and thermal gradient measurement techniques, and the application of this technology to the study of the fundamentals of solidification in microgravity,

Curreri, Peter A.↗
NASA NTRS1998

Chemical Silver Coating of Fiber Tips in Near-Field Scanning Optical Microscopy

We report what is believed to be the first experimental demonstration of silver coating by a wet chemical process on tapered fiber tips used in near-field scanning optical microscopy. The process is at room temperature and pressure and takes only a few minutes to complete. Many tips can be simultaneously coated.

Vikram, Chandra S.↗
NASA NTRS1998

Determining the Molecular Packing Arrangements on Protein Crystal Faces by Atomic Force Microscopy

Periodic Bond Chain (PBC) analysis of the packing of tetragonal lysozyme crystals have revealed that there are two possible molecular packing arrangements for the crystal faces. The analysis also predicted that only one of these, involving the formation of helices about the 4(sub 3) axes, would prevail during crystal growth. In this study high resolution atomic force microscopy (AFM) was employed to verify these predictions for the (110) crystal face. A computer program was developed which constructs the expected AFM image for a given tip shape for each possible molecular packing arrangement. By comparing the actual AFM image with the predicted images the correct packing arrangement was determined. The prediction of an arrangement involving 4(sub 3) helices was confirmed in this manner,"while the alternate arrangement was not observed. The investigation also showed the protein molecules were packed slightly closer about the 4(sub 3) axes than in the crystallographic arrangement of the crystal interior. This study demonstrates a new approach for determining the molecular packing arrangements on protein crystal faces. It also shows the power of combining a theoretical PBC analysis with experimental high resolution AFM techniques in probing protein crystal growth processes at the molecular level.

Li, Huayu↗
NASA NTRS1999

Determining the Molecular Growth Mechanisms of Protein Crystal Faces by Atomic Force Microscopy

A high resolution atomic force microscopy (AFM) study had shown that the molecular packing on the tetragonal lysozyme (110) face corresponded to only one of two possible packing arrangements, suggesting that growth layers on this face were of bimolecular height. Theoretical analyses of the packing also indicated that growth of this face should proceed by the addition of growth units of at least tetramer size corresponding to the 43 helices in the crystal. In this study an AFM linescan technique was devised to measure the dimensions of individual growth units on protein crystal faces as they were being incorporated into the lattice. Images of individual growth events on the (110) face of tetragonal lysozyme crystals were observed, shown by jump discontinuities in the growth step in the linescan images as shown in the figure. The growth unit dimension in the scanned direction was obtained from these images. A large number of scans in two directions on the (110) face were performed and the distribution of lysozyme growth unit sizes were obtained. A variety of unit sizes corresponding to 43 helices, were shown to participate in the growth process, with the 43 tetramer being the minimum observed size. This technique represents a new application for AFM allowing time resolved studies of molecular process to be carried out.

Nadarajah, Arunan↗