1,194 research outputs found
Plastic flow around rigid spherical inclusions
The extent of plastic flow in a spherical solid (assumed to be homogeneous and elastically and plastically isotropic), surrounding a concentric rigid sphere was calculated as a function of applied external pressure. The applied pressure necessary to cause plastic deformation throughout the solid was obtained
Stress anisotropy and concentration effects in high pressure measurements
Sodium chloride is used as an internal pressure standard in high pressure research. Possible corrections are discussed which are needed in the calibration of this standard due to the independent effects of stress anisotropy and stress concentration in pressure vessels. The first is due to the lack of a truly hydrostatic state of stress in solid state pressure vessels. The second is due to the difference in the compressibilities between the pressure transmitting substances (sodium chloride) and a stiffer test specimen. These two corrections are then combined and a total correction, as a function of measured pressure, is discussed for two systems presently in use. The predicted value of the combined effect is about 5-10% of the pressure at 30 GPa
Evolution equations for the perturbations of slowly rotating relativistic stars
We present a new derivation of the equations governing the oscillations of
slowly rotating relativistic stars. Previous investigations have been mostly
carried out in the Regge-Wheeler gauge. However, in this gauge the process of
linearizing the Einstein field equations leads to perturbation equations which
as such cannot be used to perform numerical time evolutions. It is only through
the tedious process of combining and rearranging the perturbation variables in
a clever way that the system can be cast into a set of hyperbolic first order
equations, which is then well suited for the numerical integration. The
equations remain quite lengthy, and we therefore rederive the perturbation
equations in a different gauge, which has been first proposed by Battiston et
al. (1970). Using the ADM formalism, one is immediately lead to a first order
hyperbolic evolution system, which is remarkably simple and can be numerically
integrated without many further manipulations. Moreover, the symmetry between
the polar and the axial equations becomes directly apparent.Comment: 13 pages, no figures, MSRAS typesetting, cleaning of the
inadvertently disfigured equation
Pressure-induced insulator-to-metal transition in low-dimensional TiOCl
We studied the transmittance and reflectance of the low-dimensional
Mott-Hubbard insulator TiOCl in the infrared and visible frequency range as a
function of pressure. The strong suppression of the transmittance and the
abrupt increase of the near-infrared reflectance above 12 GPa suggest a
pressure-induced insulator-to-metal transition. The pressure-dependent
frequency shifts of the orbital excitations, as well as the pressure
dependences of the charge gap and the spectral weight of the optical
conductivity above the phase transition are presented.Comment: 4 pages, 6 figure
Studies on silane to 70 GPa
Raman and X-ray diffraction studies were made on silane in the diamond anvil cell using three different gaskets, stainless steel, tungsten and rhenium. The structure existing between 10 and 27 GPa is well characterized by the monoclinic space group P21c (#14). While the Gibbs free energy of formation of silane is positive at one atmosphere, it is calculated from the equation of state of silane and its reactants that this becomes negative near 4 GPa and remains negative until 13 GPa and then becomes positive again. At about 27 GPa, where quasi-quantum mechanical calculations suggest there should be a transformation from 4-fold to 6-fold (or even higher), the sample turns black. The Raman modes seize to exist beyond 30 GPa after showing softening above 25 GPa. At higher pressures it turns silvery. The gaskets play a different role as will be discussed. The sample brought back from 70 GPa contains amorphous Si (with attached hydrogen) as well as crystalline silicon. The lowest free energy system at high pressure is the decomposed reactants as observed
Effect of pressure on the polarized infrared optical response of quasi-one-dimensional LaTiO
The pressure-induced changes in the optical properties of the
quasi-one-dimensional conductor LaTiO were studied by
polarization-dependent mid-infrared micro-spectroscopy at room temperature. For
the polarization of the incident radiation parallel to the conducting
direction, the optical conductivity spectrum shows a pronounced mid-infrared
absorption band, exhibiting a shift to lower frequencies and an increase in
oscillator strength with increasing pressure. On the basis of its pressure
dependence, interpretations of the band in terms of electronic transitions and
polaronic excitations are discussed. Discontinuous changes in the optical
response near 15 GPa are in agreement with a recently reported pressure-induced
structural phase transition and indicate the onset of a dimensional crossover
in this highly anisotropic system.Comment: 7 pages, 7 figure
The effects of the station teaching model of co-teaching on students with learning disabilities
The purpose of this study was: (a) to examine the effects of using the station teaching model of co-teaching to improve academic performance of students with learning disabilities, (b) to examine the effects of using the station teaching model of co-teaching to improve attention and engagement of students with learning disabilities, and (c) to evaluate student satisfaction of the station teaching intervention. Eight elementary students, five second graders and three fourth graders participated in the study. All eight students, three males and five females, were eligible for special education services as classified with varying learning disabilities. A single subject ABAB design was used. During the baseline phases, students were instructed in writing using a traditional workshop model where instruction was provided whole-group and then students were provided independent writing time. During the intervention, instruction in writing was provided in small-groups using stations among two teachers in the classroom. Students\u27 academic performance and attention/engagement was assessed throughout all baseline and intervention phases. Results indicate that students\u27 academic performance and attention/engagement increased when the intervention was provided. The student satisfaction survey suggests that students overall enjoyed using the station teaching model. Further research is suggested to investigate the effects of station teaching for students with learning disabilities
Pressure-induced metallization and structural phase transition of the Mott-Hubbard insulator TiOBr
We investigated the pressure-dependent optical response of the
low-dimensional Mott-Hubbard insulator TiOBr by transmittance and reflectance
measurements in the infrared and visible frequency range. A suppression of the
transmittance above a critical pressure and a concomitant increase of the
reflectance are observed, suggesting a pressure-induced metallization of TiOBr.
The metallic phase of TiOBr at high pressure is confirmed by the presence of
additional excitations extending down to the far-infrared range. The
pressure-induced metallization coincides with a structural phase transition,
according to the results of x-ray powder diffraction experiments under
pressure.Comment: 4 pages, 3 figure
Nanoscale Weibull Statistics
In this paper a modification of the classical Weibull Statistics is developed
for nanoscale applications. It is called Nanoscale Weibull Statistics. A
comparison between Nanoscale and classical Weibull Statistics applied to
experimental results on fracture strength of carbon nanotubes clearly shows the
effectiveness of the proposed modification. A Weibull's modulus around 3 is,
for the first time, deduced for nanotubes. The approach can treat (also) a
small number of structural defects, as required for nearly defect free
structures (e.g., nanotubes) as well as a quantized crack propagation (e.g., as
a consequence of the discrete nature of matter), allowing to remove the
paradoxes caused by the presence of stress-intensifications
General Relativistic Rossby-Haurwitz waves of a slowly and differentially rotating fluid shell
We show that, at first order in the angular velocity, the general
relativistic description of Rossby-Haurwitz waves (the analogues of r-waves on
a thin shell) can be obtained from the corresponding Newtonian one after a
coordinate transformation. As an application, we show that the results recently
obtained by Rezzolla and Yoshida (2001) in the analysis of Newtonian
Rossby-Haurwitz waves of a slowly and differentially rotating, fluid shell
apply also in General Relativity, at first order in the angular velocity.Comment: 4 pages. Comment to Class. Quantum Grav. 18(2001)L8
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