Strong Orientation Dependence Research Articles

Effect of lamellar plates on creep resistance in near gamma TiAl alloys

Abstract Effect of interlamellar spacings on creep rate in Ti-48 at% Al alloy with fully transformed lamellar structure (FL) has been investigated at 1123 K/98 MPa. In addition, the correlation between creep rate and lamellar orientation to stress axis was elucidated by conducting creep tests at 1148 K/68.6 MPa for three single crystal (SC) specimens of Ti-48 at% Al with different lamellar orientations to stress axis. The difference in creep rate among FL specimens having different interlamellar spacings could not be defined in the early stage of transient creep. The onset of accelerating creep was slightly retarded by decreasing the inter-lamellar spacing. While the creep rate of the SC specimen whose lamellar orientation to stress axis 30–63 ° gradually decreases with increasing strain, and is larger than that of the FL specimen, the creep rate of the SC specimen with lamellar orientation nearly parallel to stress axis drastically decreases within small strain, and is 1 50 smaller than that of the FL specimen. This strong lamellar orientation dependence of creep rate is interpreted by the correlation between dislocation slip system and the lamellar plate.

Dependence of electrical properties on the crystallographic orientation of CBr 4-doped GaAs epilayers grown on GaAs substrates by atmospheric pressure metalorganic chemical vapor deposition

Carbon incorporation into GaAs epilayers has been performed by atmospheric pressure metalorganic chemical vapor deposition using CBr4. The electrical properties of CBr 4-doped GaAs epilayers grown on the GaAs substrates with various surface crystallographic orientations between (100) and (111)A were investigated. The electrical properties of the epilayers showed a strong crystallographic orientation dependence. On increasing the surface offset angle, the hole concentration of CBr4-doped GaAs epilayers rapidly decreased with a hump at (311)A. The lower hole concentration at the high offset angle can be explained by its higher desorption rate than that of the (100) surface. This hole concentration dependence on the offset angle was not changed in spite of the growth temperature and the V/III ratio variation given in this work. The above behaviors indicate that the surface kinetics plays an important role in the C incorporation into the non-planar GaAs epilayers.

Origin of cartilage laminae in MRI.

To understand the origin of the laminated appearance of cartilage in MRI (the magic angle effect), microscopic MRI (mu MRI) experiments were performed at 14-microns pixel resolution on normal canine articular cartilage from the shoulder joints. Two-dimensional images of the spin-spin relaxation time (T2) of the cartilage-bone plug at two angles (0 degree and 55 degrees) were calculated quantitatively. A distinct T2 anisotropy was observed as a function of the cartilage tissue depth. The surface and the deep regions exhibit strong orientational dependence of T2, whereas the upper-middle region exhibits little orientational dependence of T2. These three mu MRI regions correspond approximately to the three histologic zones in cartilage tissue. The results from the bulk T2 measurements agreed with these mu MRI results. Our studies show that the laminated appearance of cartilage in MRI is caused by T2 anisotropy of the tissue. We further suggest that the molecular origin of the T2 anisotropy is the nuclear dipolar interaction. The structure of the cartilage tissue indicates that the collagen meshwork defines this T2 anisotropy. The results show that the T2 anisotropy provides an indirect but sensitive indicator for the orientation of macromolecular structures in cartilage. The clinical implications of this anisotropy are discussed.

Dependence of carbon incorporation on crystallographic orientation of GaAs and AlGaAs grown by metalorganic chemical vapor deposition using CBr4

The electrical properties of CBr4-doped GaAs and Al0.3 Ga0.7As epilayers grown on GaAs substrates with various surface crystallographic orientations from (100) toward (111)A were investigated. Carbon incorporation into GaAs and AlGaAs epilayers was performed by atmospheric pressure metalorganic chemical vapor deposition using CBr4. The electrical properties of the epilayers showed a strong crystallographic orientation dependence. With an increase of the surface offset angle, the hole concentration of CBr4-doped GaAs and AlGaAs epilayers rapidly decreased showing a hump at (311)A. The trend of the hole concentration dependence on the offset angle was not changed with growth temperatures in the range of 550–650 °C. Carbon incorporation is much higher in AlGaAs than in GaAs.

Inner-shell ionization cross sections and aperture size in electron energy-loss spectroscopy

Electron-energy-loss spectra have been recorded as a function of incident beam orientation for $K$-shell ionization of C in diamond. The results show a strong orientation dependence. We show that a common approximation that is often used in calculating the (total) cross section for energy-dispersive x-ray analysis cannot be made for electron-energy-loss spectroscopy when the acceptance angle of the detector is sufficiently small.

NON-REACTIVE ORIENTATIONS OF MOLECULES AT SURFACES

The role of the orientation of a molecule in its interaction with a surface is examined for the specific case of NO interaction with Pt(111). For this system molecular chemisorption occurs, mediated by a strong chemisorption well. Experimental results concerning sticking, angular distributions of scattered molecules, steric effects in scattering, and rotational excitation will be presented. Classical trajectory calculations using a model potential energy hypersurface can reproduce most experimental findings. Analysis of the trajectories shows that there is a strong orientation dependence of rotational excitation and sticking. The O-end of the molecule turns out to be non-reactive. The N-end of the molecule is very reactive. Its behaviour can almost be described using statistical methods.

Anisotropic hole scattering in hexagonal GaN

We calculate the hole scattering rate owing to the interaction with polar optical phonons and ionized impurities in bulk wurtzite GaN. The valence band states of this hexagonal material are obtained from a matrix Hamiltonian. The calculated scattering rate shows strong orientational dependence with respect to the crystal c-axis. Analysis shows that both anisotropy in the dispersion relations and the Bloch overlap factors are important for a proper description of hole scattering.

A comparison of ortho, meta, and para substituent effects in tetraphenylporphyrins: insights into the nature of the porphyrin-phenyl electronic interaction

Electron-withdrawing substituents on the phenyl rings of tetraphenylporphyrins exert very different electronic effects on the porphyrin core, depending on their ortho, meta, or para location. Specifically, the electronic effects of electron-withdrawing ortho substituents are remarkably small compared with those of their meta or para counterparts. This is ascribed to a large dipolar component in the porphyrin-phenyl electronic interaction and the strong orientational dependence of this polar effect.

Interaction of process variables with the fracture behavior of amorphous polystyrene during attrition

AbstractThis study investigates the fracture of oriented general‐purpose polystyrene. Specimen microstructure was controlled using an extrusion‐sheeting technique where draw ratio, polymer melt temperature, cooling roll temperature, and nip roll pressure changed according to a designed experiment. Sharp‐notched tension results display a 4X difference in longitudinal vs. transverse specimen values. Shrinkage measurements show contraction in the machine direction amplifies with increasing draw ratio. Craze formation is visible only in those specimens tested in the longitudinal direction. Fracture mechanisms have strong orientation dependence and the extrusion‐sheeting process produces an ideal system to study the effect on attrition. Comminution studies illustrate draw ratio has an effect on the final median particle size and width distribution of the powder. Increasing the draw ratio reduces the volume median particle size and minimizes the formation of fine particles. A monotonic fracture mechanism was initially considered but rejected in favor of a low cycle fatigue model based on estimates of fracture strength.

Effect of anisotropy on magneto-optical properties of uniaxial crystals: Application to CrO2.

The effect of the magnetization direction m on the dielectric tensor of uniaxial crystals is described by a simple dependence of the gyration vector g(\ensuremath{\omega}) on m. It is shown that the vectors g(\ensuremath{\omega}) and m as well as the orbital magnetic moment 〈L^〉 and m are generally aligned noncollinearly in contrast to an isotropic case. Formulas describing the polar Kerr effect are derived for crystals with their principal axis c\ensuremath{\perp}m and for polycrystals having c randomly oriented in the sample plane. Using these analytical results and performing ab initio calculations, we correctly reproduce anisotropy in optical spectra of ${\mathrm{CrO}}_{2}$ and the main features in magneto-optical spectra of polycrystalline films of ${\mathrm{CrO}}_{2}$. The maximal optical anisotropy and orientation dependence of g(\ensuremath{\omega}) of 100% are found in the energy interval \ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\le} 2.1 eV coinciding with the direct half-metallic ferromagnetic gap of ${\mathrm{CrO}}_{2}$. The noncollinearity effects in this interval are also very large. The obtained results correlate well with strong orientation dependence of 〈L^〉 found in our calculations. \textcopyright{} 1996 The American Physical Society.

Charged steps on III-V compound semiconductor surfaces.

The electrical charge of steps on (110) surfaces of InP, GaP, and GaAs is probed by scanning tunneling microscopy. It is demonstrated that step edges, with and without kinks, are charged and have localized defect states in the band gap. The charge of indium-terminated step edges on p-type doped InP(110) after annealing is found to be independent of the step orientation, while a strong orientation dependence is observed for phosphorus-terminated steps. This is explained by a partial compensation of the charge due to rebonding after phosphorus desorption. The magnitude of charge is estimated to be in the range of +1e to +3e per lattice spacing. \textcopyright{} 1996 The American Physical Society.

Evidence for crossover effects in the spin dynamics of the two-dimensional antiferromagnet Sr2CuO2Cl2 from 35Cl nuclear magnetic resonance

35Cl nuclear magnetic resonance (NMR) and relaxation measurements have been performed in stacked Sr2CuO2Cl2 single crystals with Néel temperature TN=257 K for two crystal orientations with respect to the applied field: H∥c and H⊥c. All the parameters of 35Cl NMR are found to exhibit a strong orientation dependence varying in temperature. In particular, the unconventional anisotropy and strong temperature dependence of the 35Cl nuclear spin–spin relaxation rate, T−12, are presented and discussed in terms of a crossover of the Cu2+ spin dynamics from Heisenberg to XY-like correlation for T→T+N. The onset of anisotropic broadening and shift of the 35Cl NMR line was observed well above TN, which is in agreement with the previous results.

Molecular dynamics simulations of the bis-intercalated complexes of ditercalinium and Flexi-Di with the hexanucleotide d(GCGCGC)2: theoretical analysis of the interaction and rationale for the sequence binding specificity.

The X-ray crystal structures of the complexes of ditercalinium and Flexi-Di with d(CGCG)2 have been studied by computational chemistry methods in an attempt to rationalize their distinct structural features. In addition, the complexes of these two bisintercalating drugs with d(GCGCGC)2 have been modeled and subjected to 0.5 ns of molecular dynamics simulations in explicit solvent with the aim of evaluating the relative importance of hydrogen bonding and stacking interactions in the sequence binding specificity of these compounds. According to our calculations, the electrostatic term is attractive for the stacking interactions between the pyridocarbazole chromophores of these drugs and the base pairs that make up the sandwiched GpC step. On the contrary, this energy term is repulsive for the base pairs that make up the boundaries of the bisintercalation site. This differential electrostatic binding energy component, which is shown to have a strong orientational dependence, could lie at the origin of the observed binding preferences of these drugs. In addition, both the Lennard-Jones and the electrostatic energy terms contribute to stabilizing the underwound central GpC step. The attractive electrostatic interactions between the linkers and the major groove are in concert with the stacking specificities for the sandwiched GpC step, which is thus very effectively stapled by the drugs. The hydrogen-bonding potential of the linkers, however, appears to be reduced in an aqueous medium due to competing interactions with water. Binding of either ditercalinium or Flexi-Di to d(GCGCGC)2 appears to favor the A-type conformation that this DNA molecule most likely adopts in the free state. The possible relevance of these findings to the process of bis-intercalation and to the pharmacological action of these compounds is discussed.

Microstructure of Ambient and Supercritical Water. Direct Comparison between Simulation and Neutron Scattering Experiments

Molecular dynamics simulations of SPC, SPC/E, TIP4P, and ST2 water models are performed at ambient and two supercritical conditions to make a direct comparison with recent microstructural data obtained by neutron diffraction with isotopic substitution (NDIS) experiments. The models generally fail to accurately predict details of the NDIS results at supercritical conditions, even though they are somewhat successful at ambient conditions. The failure is not as pronounced as that expected by Postorino et al. (Lett. Nature 1993, 366, 668−670) because of an unusual density dependence in the structure predicted by two of the models. We also evaluate a model for supercritical water denoted SPCG, a modification of the SPC and SPC/E models, in which the dipole moment is reduced to the bare dipole moment of water. For this model, the predicted structure at supercritical conditions is in much better agreement with experiment. A geometric definition of hydrogen bonding is used to gain insight into the angular dependence of the H···O pair distribution function gOH(r,ω). The simulation results for the five models indicate a strong orientational dependence for the gOH(r,ω) along the H-bonding orientations, with an approximately constant relative strength from ambient to supercritical conditions, suggesting that the angle-averaged radial distribution function, gOH(r), and its volume integral over the first solvation shell, nOH(r), may not in themselves be good measures of the strength of the H-bonding.

ＳｉＣのウィスカあるいは粒子で強化した６０６１アルミニウム合金のねじり疲労

Torsional fatigue tests of SiC whisker or SiC particulate reinforced 6061-T6 aluminum matrix composite (SiCw/Al or SiCp/Al), fabricated by a high pressure infiltration method, were performed. From a comparison with the properties of the monolithic aluminum alloy, it became clear that the torsional fatigue strength of 6061Al alloy was increased by reinforcing either with SiC whiskers or SiC particulates. Unlike strong orientation dependency of the tensile strength or fatigue strength in rotating bending, no orientation dependency was observed in the torsional fatigue strength of the SiCw/Al with highly aligned whiskers. In the SiCp/Al, cracks both parallel and transverse to the axial direction of test piece were observed. On the other hand, cracks in the SiCw/Al were always formed parallel to the direction of aligned whiskers. That is, cracks parallel to the axis were observed in the L oriented specimen whose axis was parallel to the direction of aligned whiskers and cracks transverse to the axis were observed in the T oriented specimen whose axis was transverse to the aligned whiskers. Importance of shear deformation in initiating fatigue cracks was clarified.

Stress Induced Martensitic Transformation in Aged Titanium Nickel Single Crystals

The orientation dependence of mechanical behavior of Ti-51at.%Ni single crystals as a function of structural state and test temperature has been investigated. It has been shown that mechanical properties (ductility, strain localization, martensite structure and superelasticity effect) of solution treated single crystals princi pally are different from those in polycrystals of close composition. Strong orientation dependence of ductility in B2-phase and in the temperature interval of stress induced martensitic transformation has been found. It is detected that strong influence of precipitates on kinetic of martensitic transformation development and on structure of martensite and thus on superelasticity behavior occurs.

Yielding and plastic flow behavior of B2-type Fe-39.5 mol.% A1 single crystals in compression

Yielding and plastic flow behavior of B2-type Fe-39.5 mol.% Al were investigated by deforming single crystals in the temperature range from room temperature to 1073 K. Yield stress exhibits a distinct positive temperature dependence followed by a peak for all the orientations examined. The temperatures of the anomalous peak are located between 823 and 873 K for all the orientations except the near-[111] orientation. Only for the near-[111] orientation the peak temperature is located between 773 and 823 K. The slip transition from 〈111〉 direction at intermediate temperatures to 〈100〉 at high temperatures occurs at the peak temperatures. The yield stress at 773 K exhibits a strong orientation dependence and has a good correlation with respect to non-glide stress component. Specimens having compression axes of χ ⪖ 0° exhibit serrations in stress-strain curves below the peak temperatures, whereas the serrations are not observed in those of χ < 0°. In addition, a yield drop is observed around the peak temperatures for all the orientations. Below the peak temperatures, even as low as at room temperature, the yield stress hardly depends on the applied strain rate. This indicates that the motion of 〈111〉-type superdislocations has very small strain-rate sensitivity in the temperature range. On the other hand, there is a strong strain-rate dependence at the peak temperature and above, indicating that the motion of 〈100〉-type dislocations is strongly rate sensitive. The positive temperature dependence of yield stress in B2 FeAl is discussed on the basis of the present results.

Ionization of oriented Rydberg states by subpicosecond half-cycle electromagnetic pulses.

The ionization of Rydberg states by subpicosecond, unipolar electromagnetic-field pulses shows a strong orientation dependence. A small static electric field permits us to study Rydberg-Stark states with oriented permanent electric dipole moments. The probability for ionizing a Stark state depends on its energy and the orientation of its electric dipole moment with respect to the direction of the pulsed electric field. This orientation dependence is most prominent for pulse durations comparable to the classical orbit period. Classical simulations reproduce many of the observed features.

Orientation dependence of X-ray reflection from BN hex in the energy region of B and N K-edges

The reflection spectra fine structure from two surfaces of the hexagonal BN hex cut parallely and perpendicularly to the c-axis of the crystal in the energy region 40–850 eV, i.e. including the B K and N K absorption edges, have been obtained for different angles of incidence using s-polarized synchrotron radiation. The B K and N K absorption spectra fine structure for both crystal orientations were calculated from the reflection spectra at 86° using the Kramers-Kronig dispersion relations and were combined in a common scale by means of XPS-data. A strong orientation dependence of both reflection and absorption spectra was found. The comparison of the B K and N K absorption spectra leads to the following conclusion: the N K-spectra demonstrate features similar to the B K-spectra, that means both B and N 2p states participate in the formation of N2 pπ ∗, B2 pπ ∗, N2 pσ ∗ and B2 pσ ∗ resonance states in BN hex.

Growth and Characterization of Fept Compound thin Films

ABSTRACTFePt alloy films were deposited at 50 and 49°C onto amorphous SiO2 and single crystal [001] MgO and [0001] Al2O3 using DC magnetron cosputtering, resulting in polycrystalline and [001] and [111] epitaxial films, respectively. High temperature deposition resulted in ordered films with the tetragonal Ll structure and out-of-plane magnetic easy axes while low temperature deposition yielded chemically disordered fcc alloys with in-plane easy axes. Significant modification of the magneto-optic Kerr spectrum is observed for ordered relative to disordered alloys for all orientations. The Kerr rotation has a strong orientation dependence for the ordered, but not disordered films.