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At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Structure and superconducting properties of ((Ln(1-x)Ln*(x) 1/2 (Ba(1-y)Sr(y) 1/3 Ce 1/6) 8Cu6O(z)

A variety of new oxide superconductors were prepared. The crystallographic structures of the oxides were all tetragonal and of the (Ln(+), Ce)4(Ln(+),Ba)4Cu6Oz (Ln(+) = Nd, Sm or Eu) type which had been previously discovered by Akimitsu et al. As the Sr content, y, increased when Ln = Ln(excited state) = Nd, the oxygen content, z, monotonically increased and the superconducting transition temperature, T(sub c), varied exhibiting a maximum. When z was controlled directly by means of high oxygen pressure sintering techniques, T(sub c) was changed accordingly. T(sub c's) of samples with different combinations of Ln and Ln(excited state) and different values of x and y were found to depend on the magnitude of the bond valence sum for a Cu atom located in the bottom plane of the Cu-O5 pyramid. Transport and magnetization measurements were carried out to investigate the magnetic field dependence of superconducting properties and to determine the phenomenological parameters. The Hall coefficients were positive below room temperature and varied yielding a maximum with respect to temperature.

Yamauchi, H.↗
NASA NTRS1991

Structure and superconducting properties of ((Ln(1-x)Ln*(x) 1/2 (Ba(1-y)Sr(y) 1/3 Ce 1/6)8Cu6O(z)

A variety of new oxide superconductors were prepared. The crystallographic structures of the oxides were all tetragonal and of the (Ln(+),Ce)4(Ln(+),Ba)4Cu6Oz (Ln(+) = Nd, Sm or Eu) type which had been previously discovered by Akimitsu et al. As the Sr content, y, increased when Ln = Ln(excited state) = Nd, the oxygen content, z, monotonically increased and the superconducting transition temperature, T sub c, varied exhibiting a maximum. When z was controlled directly by means of high oxygen pressure sintering techniques, T sub c was changed accordingly. T sub c's of samples with different combinations of Ln and Ln(excited state) and different values of x and y were found to depend on the magnitude of the bond valence sum for a Cu atom located in the bottom plane of the Cu-O5 pyramid. Transport and magnetization measurements were carried out to investigate the magnetic field dependence of superconducting properties and to determine the phenomenological parameters. The Hall coefficients were positive below room temperature and varied yielding a maximum with respect to temperature.

Yamauchi, H.↗
NASA NTRS1983

Structure and other properties of Jupiter's distant magnetotail

Analyses of data from Voyager 2 experiments provide evidence for, and characteristics of, a Jovian magnetotail extending at least to 9,000 Jovian radii from the planet. During approximately (25 day) periodic sightings of the tail, the magnetic field tended to point radially towards or away from Jupiter, indicating preservation to large distances of the bipolar, lobe like structure observed near the planet. This periodicity, along with various properties of the solar wind at this time, indicates that the tail is apparently influenced by recurrent solar wind features. Anomalous magnetic fields, not aligned with the nominal tail axis, also exist within the tail, especially in the low density, central (core) region, indicating some complexity of internal structure.

Lepping, R. P.↗
NASA NTRS1983

Structure and other properties of Jupiter's distant magnetotail

Analyses of data from Voyager 2 experiments provide evidence for, and characteristics of, a Jovian magnetotail extending at least to 9,000 Jovian radii from the planet. During approximately (25 day) periodic sightings of the tail, the magnetic field tended to point radially towards or away from Jupiter, indicating preservation to large distances of the bipolar, lobe like structure observed near the planet. This periodicity, along with various properties of the solar wind at this time, indicates that the tail is apparently influenced by recurrent solar wind features. Anomalous magnetic fields, not aligned with the nominal tail axis, also exist within the tail, especially in the low density, central (core) region, indicating some complexity of internal structure. Previously announced in STAR as N83-29153

Lepping, R. P.↗
NASA NTRS1985

Effects of wear on structure-sensitive magnetic properties of ceramic ferrite in contact with magnetic tape

Wear experiments and electron microscopy and diffraction studies were conducted to examine the wear and deformed layers in single-crystal Mn-Zn (ceramic) ferrite magnetic head material in contact with magnetic tape and the effects of that contact on magnetic properties. The crystalline state of the single-crystal magnetic head was changed drastically during the sliding process. A nearly amorphous structure was produced on its wear surface. Deformation in the surficial layer of the magnetic head was a critical factor in readback signal loss above 2.5 dB. The signal output level was reduced as applied normal load was increased. Considerable plastic flow occurred on the magnetic tape surface with sliding, and the signal loss due to the tape wear was approximately 1 dB.

Miyoshi, K.↗
NASA NTRS1952

Correlation of physical properties with molecular structure for some dicyclic hydrocarbons having high thermal-energy release per unit volume -- 2-alkylbiphenyl and the two isomeric 2-alkylbicyclohexyl series

Three homologous series of related dicyclic hydrocarbons are presented for comparison on the basis of their physical properties, which include net heat of combustion, density, melting point, boiling point, and kinematic viscosity. The three series investigated include the 2-n-alkylbiphenyl, 2-n-alkylbicyclohexyl (high boiling), and 2-n-alkylbiphenyls (low boiling) series through c sub 16, in addition to three branched-chain (isopropyl, sec-butyl, and isobutyl) 2-alkylbiphenyls and their corresponding 2-alkylbicyclohexyls. The physical properties of the low-boiling and high-boiling isomers of 2-sec-butylbicyclohexyl and 2-isobutylbicyclohexyl are reported herein for the first time.

Goodman, Irving A↗
NASA NTRS1990

Structural and optical properties for typical solid mirror shapes

A method is developed to determine the weight, center of gravity, areal properties, and mass inertial properties for typical mirrors. A number of support conditions were considered to examine optical surface deflections, surface quality, and fundamental natural frequency for single- and double-arch mirror shapes. Structural performance estimates were made with the NASTRAN program, and optical performances were evaluated with the FRINGE program, using an SXA 40-in mirror. To show the behavior of element types from the NASTRAN program, finite element validity and sensitivity studies were performed in optical model applications. Material parameters, contoured back shapes, and support locations are shown to have significant effects on structural and optical performances. Optimal support locations and support points are given. Fundamental natural frequencies for some shapes are found with the closed-form solution. The plate models results may not be acceptable for determining real mirror optical performances.

Cho, Myung K.↗
NASA NTRS1979

Effects of isothermal and cyclic exposures on interface structure and mechanical properties of FPalpha-Al2O3/aluminum composites

The microstructural and interface stability of FPalpha-Al203/Al-Li composites are investigated as a function of isothermal exposure at 500 C or thermal cycling between 140 and 500 C with hold time at Tmax. Interfacial morphology, growth kinetics, crystal structure, and composition of interfacial reaction products are characterized. Strength is monitored in the transverse orientation, and fracture mechanics is analyzed in terms of interface reaction products. The interfacial reaction product in FP/Al is Li2O.5Al2O3. Significant fiber-matrix reaction occurs during fabrication. The number of thermal cycles rather than total time at Tmax is the determining factor in strength degradation, thermal cycling giving rise to voids at the fiber-matrix interface. Extensive interface failures occur at composite fracture stresses below about 128 MPa; above this stress level failure is attributed to ductile matrix fracture.

Kim, W. M.↗
NASA NTRS1980

Structure and optical properties of evaporated films of the Cr- and V-group metals

Thin films of Cr, Mo, and W rapidly evaporated in high vacuum (5 x 10 to the -7th torr) onto room-temperature substrates show anomalously low reflectance (compared to bulk samples). From electron and X-ray diffraction and electron microscopy, the normal bcc crystal structure is found, but with very fine grains. Columnar grains about 100 A in diameter were separated by a less dense grain-boundary network about 10-A wide. The measured optical conductivity agrees with an inhomogeneous-medium model that assumes the normal crystalline conductivity for the grain interiors, with model parameters that correlate to the observed columnar grain size. In contrast, V and Nb films rapidly evaporated onto room-temperature substrates have the reflectance of bulk crystalline material. On liquid-nitrogen temperature substrates, however, V and Nb have normal bcc crystal structure but with small flat-plate grains, and the same model, with appropriate parameters, accounts for the optical conductivity. The difference between these two groups apparently depends on residual gases segregated at the grain boundaries in the Cr-group films.

Nestell, J. E., Jr.↗
NASA NTRS1996

Towards an understanding of the structural and functional properties of MscL, a mechanosensitive channel in bacteria

Whether it be to sense a touch, arterial pressure, or an osmotic gradient across a cell membrane, essentially all living organisms require the capability of detecting mechanical force. Electrophysiological evidence has suggested that mechanosensitive ion channels play a major role in many systems where mechanical force is detected. But, despite their biological importance, determination of the most basic structural and functional features of mechanosensitive channels has only recently become possible. A gene called mscL, which was isolated from Escherichia coli, was the first gene shown to encode a mechanosensitive channel activity. This channel directly responds to tension in the membrane; no other proteins are required. MscL appears to be a homohexamer of a 136 amino acid polypeptide that is highly alpha helical, contains two transmembrane domains, and has both the amino and carboxyl termini in the cytoplasm. The study of the MscL protein remains, to date, one of the most viable options for understanding the structural and functional characteristics of a mechanosensitive channel.

NASA Discipline Cell Biology↗
NASA NTRS1975

Surface structure and electronic properties of materials

A surface potential model is developed to explain dopant effects on chemical vapor deposition. Auger analysis of the interaction between allotropic forms of carbon and silicon films has shown Si-C formation for all forms by glassy carbon. LEED intensity measurements have been used to determine the mean square displacement of surface atoms of silicon single crystals, and electron loss spectroscopy has shown the effect of structure and impurities on surface states located within the band gap. A thin film of Al has been used to enhance film crystallinity at low temperature.

Siekhaus, W. J.↗
NASA NTRS2006

Flame Structure and Scalar Properties in Microgravity Laminar Fires

Recent results from microgravity combustion experiments conducted in the Zero Gravity Facility (ZGF) 5.18 second drop tower are reported. Emission mid-infrared spectroscopy measurements have been completed to quantitatively determine the flame temperature, water and carbon dioxide vapor concentrations, radiative emissive power, and soot concentrations in a microgravity laminar ethylene/air flame. The ethylene/air laminar flame conditions are similar to previously reported experiments including the Flight Project, Laminar Soot Processes (LSP). Soot concentrations and gas temperatures are in reasonable agreement with similar results available in the literature. However, soot concentrations and flame structure dramatically change in long duration microgravity laminar diffusion flames as demonstrated in this paper.

Feikema, D. A.↗
NASA NTRS1997

AB Initio Study of the Structure and Spectroscopic Properties of Halogenated Thioperoxy Radicals

Thioperoxy (XSO or XOS) radicals exist in a variety of chemical environments, and they have as a consequence drawn some interest. HSO, an important species in the chemistry of the troposphere, has been examined both experimentally. The halogenated (X = F, Cl or Br) peroxy species and isovalent thioperoxy species have been studied less, but they too are potentially interesting because oxidized sulfur species and halogen sources are present in the atmosphere. Learning the fate of XSO and XOS radicals is important to understanding the atmospheric oxidation chemistry of sulfur compounds. Of these, FSO and ClSO are particularly interesting because they have been directly detected spectroscopically. Recent studies in our laboratory on the photochemistry of thionyl halides (X2SO; where X = F or Cl) have suggested new ways to generate XSO species. The laser-induced photodissociation of thionyl fluoride, F2SO, at 193 nm and thionyl chloride, ClSO, at 248 nm is characterized by a radical mechanism, X2SO -> XSO + X. The structure of FSO has been characterized experimentally by Endo et cd. employing microwave spectroscopy. Using the unrestricted Hartree-Fock (UHF) self-consistent field (SCF) method, Sakai and Morokuma computed the electronic structure of the ground (sup 2)A" and the first excited (sup 2)A' states of FSO. Electron correlation was not taken into account in their study. In a laser photodissociation experiment, Huber et al. identified ClSO mass spectromctrically. ClSO has also been detected in low temperature matrices by EPR and in the gas phase by far IR laser magnetic resonance. Although the structure of FSO is known in detail, the only study, experimental or theoretical, of CISO has been an ab initio HFSCF study by Hinchliffe. Electron correlation corrections were also excluded from this study. In order to better understand the isomerization and dissociation dynamics of the radical species, we have performed ab initio correlated studies of the potential energy surfaces (PES) of ClSO and its isomer ClOS at the QCISD(T)/6-31 G* level of theory. For FSO and FOS, more extensive QCISD/6-31 1G(2df) calculations have been possible, and the results are summarized here.

Munoz, Luis A.↗
NASA NTRS1989

A 3D visualization system for molecular structures

The properties of molecules derive in part from their structures. Because of the importance of understanding molecular structures various methodologies, ranging from first principles to empirical technique, were developed for computing the structure of molecules. For large molecules such as polymer model compounds, the structural information is difficult to comprehend by examining tabulated data. Therefore, a molecular graphics display system, called MOLDS, was developed to help interpret the data. MOLDS is a menu-driven program developed to run on the LADC SNS computer systems. This program can read a data file generated by the modeling programs or data can be entered using the keyboard. MOLDS has the following capabilities: draws the 3-D representation of a molecule using stick, ball and ball, or space filled model from Cartesian coordinates, draws different perspective views of the molecule; rotates the molecule on the X, Y, Z axis or about some arbitrary line in space, zooms in on a small area of the molecule in order to obtain a better view of a specific region; and makes hard copy representation of molecules on a graphic printer. In addition, MOLDS can be easily updated and readily adapted to run on most computer systems.

Green, Terry J.↗