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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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Quantitative void characterization in structural ceramics by use of scanning laser acoustic microscopy

The ability of scanning laser acoustic microscopy (SLAM) to characterize artificially seeded voids in sintered silicon nitride structural ceramic specimens was investigated. Using trigonometric relationships and Airy's diffraction theory, predictions of internal void depth and size were obtained from acoustic diffraction patterns produced by the voids. Agreement was observed between actual and predicted void depths. However, predicted void diameters were generally much greater than actual diameters. Precise diameter predictions are difficult to obtain due to measurement uncertainty and the limitations of 100 MHz SLAM applied to typical ceramic specimens.

Roth, D. J.↗
NASA NTRS1988

Application of Hoffman modulation contrast microscopy coupled with three-wavelength two-beam interferometry to the in situ direct observation of the growth process of a crystal in microgravity

Direct visualization of three dimensional transfer process of both heat and mass around a growing crystal and mono-molecular growth layers on the surface is possible in situ by means of high resolution Hoffman modulation contrast microscopy coupled with three wavelength two beam Mach-Zehnder interferometry. This in situ observation is very suitable for the verification of the growth mechanism of a crystal in a solution or a melt in microgravity.

Tsukamoto, Katsuo↗
NASA NTRS1988

Flaw characterization in structural ceramics using scanning laser acoustic microscopy

The ability of scanning laser acoustic microscopy (SLAM) to characterize artifically seeded voids in sintered silicon nitride structural ceramic specimens was investigated. The voids ranged from 20 to 430 microns in diameter and were embedded up to 2 mm beneath the surface of the specimens. Probability of detection was determined as a function of void depth and size. Trigonometric relationships and Airy's diffraction theory were used to obtain predictions of void depth and size from acoustic diffraction patterns produced by the voids. Agreement was observed between actual and predicted void depths. However, predicted void diameters were generally much greater than actual diameters. Precise diameter predictions are difficult to obtain because of measurement uncertainty and the limitations of the 100 MHz SLAM applied to typical ceramic specimens.

Roth, Don J.↗
NASA NTRS1986

Nondestructive Evaluation (NDE) of structural ceramics by photoacoustic microscopy

Photoacoustic microscopy (PAM) was utilized to detect surface and subsurface defects in structural ceramic materials. A computerized PAM data acquisition, color imaging and analysis system was developed and used. Subsurface simulated cylindrical holes can be detected to about 1 mm below the interrograting surface. Simulated tight surface cracks of 96 microns length and 48 microns depth can be detected in these materials under optimum conditions.

Khandelwal, P. K.↗
NASA NTRS1988

Direct control and characterization of a Schottky barrier by scanning tunneling microscopy

Scanning tunneling microscopy (STM) methods are used to directly control the barrier height of a metal tunnel tip-semiconductor tunnel junction. Barrier behavior is measured by tunnel current-voltage spectroscopy and compared to theory. A unique surface preparation method is used to prepare a low surface state density Si surface. Control of band bending with this method enables STM investigation of semiconductor subsurface properties.

Bell, L. D.↗
NASA NTRS1988

Transmission electron microscopy study of the formation of epitaxial CoSi2/Si (111) by a room-temperature codeposition technique

Co and Si have been codeposited on Si (111) substrates near room temperature in a stoichiometric 1:2 ratio in a molecular beam epitaxy system. Annealing of these deposits yields high-quality single-crystal CoSi2 layers. Transmission electron microscopy has been used to examine as-deposited layers and layers annealed at 300, 500, and 600 C. Single-crystal epitaxial grains of CoSi2 embedded in a matrix of amorphous Co/Si are observed in as-deposited samples, while the layer is predominantly single-crystal, inhomogeneously strained CoSi2 at 300 C. At 600 C, a homogeneously strained single-crystal layer with a high density of pinholes is observed. In contrast to other solid phase epitaxy techniques used to grow CoSi2 on Si (111), no intermediate silicide phases are observed prior to the formation of CoSi2.

D'Anterroches, Cecile↗
NASA NTRS1987

Multielement mapping of alpha-SiC by scanning Auger microscopy

Fine second-phase particles, numerous in sintered alpha-SiC, were analyzed by scanning Auger microscopy and conventional techniques. The Auger analysis utilized computer-controlled data acquisition, multielement correlation diagrams, and a high spatial resolution of 100 nm. This procedure enabled construction of false color maps and the detection of fine compositional details within these particles. Carbon, silicon oxide, and boron-rich particles (qualitatively as BN or B4C) predominated. The BN particles, sometimes having a carbon core, are believed to result from reaction between B4C additives and nitrogen sintering atmospheres.

Browning, Ray↗
NASA NTRS1987

Diffusion length measurement in bulk and epitaxially grown III-V semiconductors using charge collection microscopy

Diffusion lengths and surface recombination velocities were measured in GaAs diodes and InP finished solar cells. The basic technique used was charge collection microscopy, also known as electron beam induced current (EBIC). The normalized currents and distances from the pn junction were read directly from the calibrated curves obtained while using the line-scan mode in an SEM. These values were then equated to integral and infinite series expressions resulting from the solution of the diffusion equation with both extended-generation and point-generation functions. This expands previous work by examining both thin and thick samples. The surface recombination velocity was either treated as an unknown in a system of two equations or measured directly using low e(-) beam accelerating voltages. These techniques give accurate results by accounting for the effects of surface recombination and the finite size of the generation volume.

Leon, R. P.↗
NASA NTRS1988

Scanning tunneling microscopy characterization of the geometric and electronic structure of hydrogen-terminated silicon surfaces

Scanning tunneling microscopy (STM) methods are used to characterize hydrogen-terminated Si surfaces prepared by a novel method. The surface preparation method is used to expose the Si-SiO2 interface. STM images directly reveal the topographic structure of the Si-SiO2 interface. The dependence of interface topography on oxide preparation conditions observed by STM is compared to the results of conventional surface characterization methods. Also, the electronic structure of the hydrogen-terminated surface is studied by STM spectroscopy. The near-ideal electronic structure of this surface enables direct tunnel spectroscopy measurements of Schottky barrier phenomena. In addition, this method enables probing of semiconductor subsurface properties by STM.

Kaiser, W. J.↗
NASA NTRS1988

Imaging graphite in air by scanning tunneling microscopy - Role of the tip

Atomically resolved images of highly oriented pyrolytic graphite (HOPG) in air at point contact have been obtained. Direct contact between tip and sample or contact through a contamination layer provides a conduction mechanism in addition to the exponential tunneling mechanism responsible for scanning tunneling microscopy (STM) imaging. Current-voltage (I-V) spectra were obtained while scanning in the current imaging mode with the feedback circuit interrupted in order to study the graphite imaging mechanism. Multiple tunneling tips are probably responsible for images without the expected hexagonal or trigonal symmetry. The observations indicate that the use of HOPG for testing and calibration of STM instrumentation may be misleading.

Colton, R. J.↗
NASA NTRS1987

A microscopy study of composite delamination

Double cantilever beam specimens of carbon fiber-epoxy composites were tested for interlaminar fracture energy. While still under load, the specimens were potted in a clear epoxy polymer. The potted specimens were cut longitudinally into sections 0.6 mm (0.025 in.) or less in thickness and examined using reflection and transmission light microscopy. Massive fiber bridging was observed especially at the crack tip.

Bascom, W. D.↗
NASA NTRS1989

Analytical electron microscopy of biogenic and inorganic carbonates

In the terrestrial sedimentary environment, the mineralogically predominant carbonates are calcite-type minerals (rhombohedral carbonates) and aragonite-type minerals (orthorhombic carbonates). Most common minerals precipitating either inorganically or biogenically are high magnesium calcite and aragonite. High magnesium calcite (with magnesium carbonate substituting for more than 7 mole percent of the calcium carbonate) is stable only at temperatures greater than 700 C or thereabouts, and aragonite is stable only at pressures exceeding several kilobars of confining pressure. Therefore, these carbonates are expected to undergo chemical stabilization in the diagenetic environment to ultimately form stable calcite and dolomite. Because of the strong organic control of carbonate deposition in organisms during biomineralization, the microchemistry and microstructure of invertebrate skeletal material is much different than that present in inorganic carbonate cements. The style of preservation of microstructural features in skeletal material is therefore often quite distinctive when compared to that of inorganic carbonate even though wholesale recrystallization of the sediment has taken place. Microstructural and microchemical comparisons are made between high magnesium calcite echinoderm skeletal material and modern inorganic high magnesium calcite inorganic cements, using analytical electron microscopy and related techniques. Similar comparisons are made between analogous materials which have undergone stabilization in the diagenetic environment. Similar analysis schemes may prove useful in distinguishing between biogenic and inorganic carbonates in returned Martian carbonate samples.

Blake, David F.↗
NASA NTRS1988

Analytical electron microscopy of a hydrated interplanetary dust particle

Properties of a hydrated interplanetary dust particle (IDP), Ames-Dec86-11, were investigated using TEM and analytical electron microscopy. The particle was found to have mineralogy and chondritic composition indicating an absence of direct kinship with known carbonaceous chondrites. The available data on the Ames-Dec86-11 suggest that at least one aqueous alteration event took place in this hydrated IDP, during which fine-grained material, possibly glass, was transformed to smectite. This event appears to be unique to hydrated IDPs.

Blake, David F.↗
NASA NTRS1988

Observation of interface band structure by ballistic-electron-emission microscopy

The paper reports an advanced ballistic electron spectroscopy technique that was used to directly measure semiconductor band structure properties at a subsurface interface. Two interface systems having contrasting band structures were investigated by this method: Au-Si and Au-GaAs. It is concluded that the proposed method, based on scanning tunneling microscopy, enables the spatially resolved carrier-transport spectroscopy of interfaces.

Bell, L. D.↗
NASA NTRS1988

Process dependent morphology of the Si/SiO2 interface measured with scanning tunneling microscopy

A new experimental technique to determine Si/SiO2 interface morphology is described. Thermal oxides of silicon are chemically removed, and the resulting surface topography is measured with scanning tunneling microscopy. Interfaces prepared by oxidation of Si (100) and (111) surfaces, followed by postoxidation anneal (POA) at different temperatures, have been characterized. Correlations between interface structure, chemistry, and electrical characteristics are described.

Hecht, Michael H.↗
NASA NTRS1989

Electron microscopy of a Gd-Ba-Cu-O superconductor

An electron microscopy study has been carried out to characterize the microstructure of a sintered Gd-Ba-Cu-O superconductor alloy. The GdBa2Cu3O(7-x) phase in the oxygen annealed sample is orthorhombic, while in the vacuum annealed sample it is tetragonal. It is shown that the details of the fine structure in the 001-line zone axis convergent beam patterns can be used to distinguish between the orthorhombic form and the tetragonal form. In addition to this matrix phase, an amorphous phase is frequently observed at the triple grain junctions. Gd-rich inclusions have been observed inside the matrix phase.

Ramesh, R.↗
NASA NTRS1989

Ballistic-electron-emission microscopy investigation of Schottky barrier interface formation

Ballistic-electron-emission microscopy (BEEM) has been used to investigate the origin of defects at the Au/GaAs(100) Schottky barrier interface. In addition, molecular beam epitaxy (MBE) and in situ fabrication methods have been employed to control Schottky barrier interface properties. BEEM characterization combined with MBE methods has enabled the development of a near-ideal Schottky barrier interface with drastically reduced defect density.

Hecht, M. H.↗
NASA NTRS1989

In situ transmission electron microscopy study on the epitaxial growth of CoSi2 on Si(111) at temperatures below 150 C

This paper reports an in situ transmission electron microscopy study on the epitaxial growth of CoSi2 on Si(111) from a 10-nm-thick amorphous mixture of Co and Si in the ratio 1:2, which was formed by codeposition of Co and Si near room temperature. Nuclei of CoSi2 are observed in the as-deposited film. These nuclei are epitaxial and extend through the whole film thickness. Upon annealing, these columnar epitaxial CoSi2 grains grow laterally at temperatures as low as 50 C. The kinetics of this lateral epitaxial growth was studied at temperatures between 50 and 150 C. The activation energy of the growth process is 0.8 + or - 0.1 eV.

Nieh, C. W.↗