260 research outputs found
Response of armour steel plates to localised air blast load : a dimensional analysis
We report on the results of dimensional analyses on the dynamic plastic response of square armour steel plates due to detonation of proximal cylindrical charges and ensued air blast loading. By assuming a generic function for the blast load, which is multiplicative comprising its spatial and temporal parts, a set of 14 dimensionless parameters, representative of the load and plate deformation, were identified and recast in the form of dimensionless functions of stand-off to charge diameter ratio. Parametric studies were performed using commercial code ABAQUS’s module of Finite Element hydrocode using MMALE and MMAE techniques, and combined with regression analyses to quantify the dimensional parameters and the expressions for dimensionless functions. A few numerical studies with various FE mesh types were also performed to validate the transient deflections against the small-scale experiments. For pulse loading due to proximal charges of small orders of stand-off/charge diameter ratio, the magnitude of the transverse deflection increased abruptly with incremental decrease in stand-off, in contradistinction to the plate deformations at higher stand-offs where variations in displacement are smooth. This confirmed the existence of a stand-off at which a transition in behaviour takes place. For stand-off values less than charge diameter, a dimensionless energy absorbing effectiveness factor was considered to investigate the prediction of rupture in the plate corresponding to different charge masses. This factor is measured as a baseline parameter to predict, using solely numerical means, the blast loads which ensue rupture on full-scale prototypes
Phonons from neutron powder diffraction
The spherically averaged structure function \soq obtained from pulsed
neutron powder diffraction contains both elastic and inelastic scattering via
an integral over energy. The Fourier transformation of \soq to real space, as
is done in the pair density function (PDF) analysis, regularizes the data, i.e.
it accentuates the diffuse scattering. We present a technique which enables the
extraction of off-center phonon information from powder diffraction experiments
by comparing the experimental PDF with theoretical calculations based on
standard interatomic potentials and the crystal symmetry. This procedure
(dynamics from powder diffraction(DPD)) has been successfully implemented for
two systems, a simple metal, fcc Ni, and an ionic crystal, CaF. Although
computationally intensive, this data analysis allows for a phonon based
modeling of the PDF, and additionally provides off-center phonon information
from powder neutron diffraction
Storage Device Sizing for a Hybrid Railway Traction System by Means of Bicausal Bond Graphs
In this paper, the application of bicausal bond graphs for system design in electrical engineering is emphasized. In particular, it is shown how this approach is very useful for model inversion and parameter dimensioning. To illustrate these issues, a hybrid railway traction device is considered as a case study. The synthesis of a storage device (a supercapacitor) included in this system is then discussed
Nano-magnetic droplets and implications to orbital ordering in La1-xSrxCoO3
Inelastic cold neutron scattering on LaCoO3 provided evidence for a distinct
low energy excitation at 0.6 meV coincident with the thermally induced magnetic
transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic
(AFM) correlations that are dynamic follow the activation to the excited state,
identified as the intermediate S=1 spin triplet. This is indicative of
dynamical orbital ordering favoring the observed magnetic interactions. With
hole doping as in La1-xSrxCoO3, the FM correlations between Co spins become
static and isotropically distributed due to the formation of FM droplets. The
correlation length and condensation temperature of these droplets increase
rapidly with metallicity due to the double exchange mechanism.Comment: To appear in Phys. Rev. Let
Resistivity of Mixed-Phase Manganites
The resistivity of manganites is studied using a
random-resistor-network, based on phase-separation between metallic and
insulating domains. When percolation occurs, both as chemical composition and
temperature vary, results in good agreement with experiments are obtained.
Similar conclusions are reached using quantum calculations and microscopic
considerations. Above the Curie temperature, it is argued that ferromagnetic
clusters should exist in Mn-oxides. Small magnetic fields induce large
changes and a bad-metal state with (disconnected) insulating
domains.Comment: 4 pages, 4 eps figure
Measurement of the local Jahn-Teller distortion in LaMnO_3.006
The atomic pair distribution function (PDF) of stoichiometric LaMnO_3 has
been measured. This has been fit with a structural model to extract the local
Jahn-Teller distortion for an ideal Mn(3+)O_6 octahedron. These results are
compared to Rietveld refinements of the same data which give the average
structure. Since the local structure is being measured in the PDF there is no
assumption of long-range orbital order and the real, local, Jahn-Teller
distortion is measured directly. We find good agreement both with published
crystallographic results and our own Rietveld refinements suggesting that in an
accurately stoichiometric material there is long range orbital order as
expected. The local Jahn-Teller distortion has 2 short, 2 medium and 2 long
bonds.Comment: 5 pages, 3 postscript figures, minor change
Evidence for charge localization in the ferromagnetic phase of La_(1-x)Ca_(x)MnO_3 from High real-space-resolution x-ray diffraction
High real-space-resolution atomic pair distribution functions of
La_(1-x)Ca_(x)MnO_3 (x=0.12, 0.25 and 0.33) have been measured using
high-energy x-ray powder diffraction to study the size and shape of the MnO_6
octahedron as a function of temperature and doping. In the paramagnetic
insulating phase we find evidence for three distinct bond-lengths (~ 1.88, 1.95
and 2.15A) which we ascribe to Mn^{4+}-O, Mn^{3+}-O short and Mn^{3+}-O long
bonds respectively. In the ferromagnetic metallic (FM) phase, for x=0.33 and
T=20K, we find a single Mn-O bond-length; however, as the metal-insulator
transition is approached either by increasing T or decreasing x, intensity
progressively appears around r=2.15 and in the region 1.8 - 1.9A suggesting the
appearance of Mn^{3+}-O long bonds and short Mn^{4+}-O bonds. This is strong
evidence that charge localized and delocalized phases coexist close to the
metal-insulator transition in the FM phase.Comment: 8 pages, 8 postscript figures, submitted to Phys. Rev.
Nanoscale anisotropic structural correlations in the paramagnetic and ferromagnetic phases of Nd0.5Sr0.5 MnO3
We report x-ray scattering studies of short-range structural correlations and
diffuse scattering in Nd0.5Sr0.5MnO3. On cooling, this material undergoes a
series of transitions, first from a paramagnetic insulating (PI) to a
ferromagnetic metallic (FM) phase, and then to a charge-ordered (CO) insulating
state. Highly anisotropic structural correlations were found in both the PI and
FM states. The correlations increase with decreasing temperature, reaching a
maximum at the CO transition temperature. Below this temperature, they abruptly
collapsed. Single-polaron diffuse scattering was also observed in both the PI
and FM states suggesting that substantial local lattice distortions are present
in these phases. We argue that our measurements indicate that nanoscale regions
exhibiting layered orbital order exist in the paramagnetic and ferromagnetic
phases of Nd0.5Sr0.5MnO3.Comment: 5 pages, 4 embedded figure
Magnetic-field-induced collapse of charge-ordered nanoclusters and the Colossal Magnetoresistance effect in Nd(0.3)Sr(0.3)MnO(3)
We report synchrotron x-ray scattering studies of charge/orbitally ordered
(COO) nanoclusters in NdSrMnO. We find that the COO
nanoclusters are strongly suppressed in an applied magnetic field, and that
their decreasing concentration follows the field-induced decrease of the sample
electrical resistivity. The COO nanoclusters, however, do not completely
disappear in the conducting state, suggesting that this state is inhomogeneous
and contains an admixture of an insulating phase. Similar results were also
obtained for the zero-field insulator-metal transition that occurs as
temperature is reduced. These observations suggest that these correlated
lattice distortions play a key role in the Colossal Magnetoresistance effect in
this prototypical manganite.Comment: 5 pages, 3 embedded eps figures; to appear in PRB Rapid
Commumication
Charge melting and polaron collapse in
X-ray and neutron scattering measurements directly demonstrate the existence
of polarons in the paramagnetic phase of optimally-doped colossal
magnetoresistive oxides. The polarons exhibit short-range correlations that
grow with decreasing temperature, but disappear abruptly at the ferromagnetic
transition because of the sudden charge delocalization. The "melting" of the
charge ordering as we cool through occurs with the collapse of the
quasi-static polaron scattering, and provides important new insights into the
relation of polarons to colossal magnetoresistance.Comment: 4 pages (RevTex), 3 postscript-formatted figures (Figs. 1 and 2 are
color figures
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