9,297 research outputs found
Strain-induced kinetics of intergrain defects as the mechanism of slow dynamics in the nonlinear resonant response of humid sandstone bars
A closed-form description is proposed to explain nonlinear and slow dynamics
effects exhibited by sandstone bars in longitudinal resonance experiments.
Along with the fast subsystem of longitudinal nonlinear displacements we
examine the strain-dependent slow subsystem of broken intergrain and
interlamina cohesive bonds. We show that even the simplest but
phenomenologically correct modelling of their mutual feedback elucidates the
main experimental findings typical for forced longitudinal oscillations of
sandstone bars, namely, (i) hysteretic behavior of a resonance curve on both
its up- and down-slopes, (ii) linear softening of resonant frequency with
increase of driving level, and (iii) gradual recovery (increase) of resonant
frequency at low dynamical strains after the sample was conditioned by high
strains. In order to reproduce the highly nonlinear elastic features of
sandstone grained structure a realistic non-perturbative form of strain
potential energy was adopted. In our theory slow dynamics associated with the
experimentally observed memory of peak strain history is attributed to
strain-induced kinetic changes in concentration of ruptured inter-grain and
inter-lamina cohesive bonds causing a net hysteretic effect on the elastic
Young's modulus. Finally, we explain how enhancement of hysteretic phenomena
originates from an increase in equilibrium concentration of ruptured cohesive
bonds that are due to water saturation.Comment: 5 pages, 3 figure
Insights into the mechanism of energy transfer with poly(heptazine imide)s in deoximation reaction
Following our previous studies on potassium poly(heptazine imide) (K-PHI) – catalyzed photooxidative [3+2] aldoxime-to-nitrile addition to form 1,2,4-oxadiazoles, we discovered that electron-rich oximes yield the parent aldehydes instead of target products. In this work, the mechanism of this singlet oxygen-mediated deoximation process was established using a series of control reactions and spectroscopic measurements such as steady-state and time-resolved fluorescence quenching experiments. Additionally, singlet-triplet energy gap value was obtained for K-PHI in suspension, and the reaction scope was broadened to include ketoximes
A general scaling relation for the critical current density in Nb3Sn
We review the scaling relations for the critical current density (Jc) in
Nb3Sn wires and include recent findings on the variation of the upper critical
field (Hc2) with temperature (T) and A15 composition. We highlight deficiencies
in the Summers/Ekin relations, which are not able to account for the correct
Jc(T) dependence. Available Jc(H) results indicate that the magnetic field
dependence for all wires can be described with Kramer's flux shear model, if
non-linearities in Kramer plots are attributed to A15 inhomogeneities. The
strain (eps) dependence is introduced through a temperature and strain
dependent Hc2*(T,eps) and Ginzburg- Landau parameter kappa1(T,eps) and a strain
dependent critical temperature Tc(eps). This is more consistent than the usual
Ekin unification, which uses two separate and different dependencies on Hc2*(T)
and Hc2*(eps). Using a correct temperature dependence and accounting for the
A15 inhomogeneities leads to a remarkable simple relation for Jc(H,T,eps).
Finally, a new relation for s(eps) is proposed, based on the first, second and
third strain invariants.Comment: Accepted Topical Review for Superconductor, Science and Technolog
Double quantum dot with tunable coupling in an enhancement-mode silicon metal-oxide semiconductor device with lateral geometry
We present transport measurements of a tunable silicon
metal-oxide-semiconductor double quantum dot device with lateral geometry.
Experimentally extracted gate-to-dot capacitances show that the device is
largely symmetric under the gate voltages applied. Intriguingly, these gate
voltages themselves are not symmetric. Comparison with numerical simulations
indicates that the applied gate voltages serve to offset an intrinsic asymmetry
in the physical device. We also show a transition from a large single dot to
two well isolated coupled dots, where the central gate of the device is used to
controllably tune the interdot coupling.Comment: 4 pages, 3 figures, to be published in Applied Physics Letter
Detector for imaging of explosions: present status and future prospects with higher energy X-rays
The detector for imaging of explosions (DIMEX) is in operation at the
synchrotron radiation (SR) beam-line at VEPP-3 electron ring at Budker INP
since 2002. DIMEX is based on one-coordinate gas ionization chamber filled with
Xe-CO2(3:1) mixture at 7atm, and active Frisch-grid made of Gas Electron
Multiplier (GEM). The detector has spatial resolution of ~0.2mm and dynamic
range of ~100 that allows to realize the precision of signal measurement at a
percent level. The frame rate can be tuned up to 8 MHz (125 ns per image) and
up to 32 images can be stored in one shot. At present DIMEX is used with the
X-ray beam from 2T wiggler that has ~20 keV average energy. Future possibility
to install similar detector at the SR beam-line at VEPP-4 electron ring is
discussed.Comment: 14 pages, 15 figures. Submitted to JINS
Forward Flux Sampling-type schemes for simulating rare events: Efficiency analysis
We analyse the efficiency of several simulation methods which we have
recently proposed for calculating rate constants for rare events in stochastic
dynamical systems, in or out of equilibrium. We derive analytical expressions
for the computational cost of using these methods, and for the statistical
error in the final estimate of the rate constant, for a given computational
cost. These expressions can be used to determine which method to use for a
given problem, to optimize the choice of parameters, and to evaluate the
significance of the results obtained. We apply the expressions to the
two-dimensional non-equilibrium rare event problem proposed by Maier and Stein.
For this problem, our analysis gives accurate quantitative predictions for the
computational efficiency of the three methods.Comment: 19 pages, 13 figure
Multisite PCET with photocharged carbon nitride in dark
A combination of photochemistry and proton coupled electron transfer (PCET) is a primary strategy employed by biochemical systems and synthetic chemistry to enable uphill reactions under mild conditions. Degenerate nanometer-sized n-type semiconductor nanoparticles (SCNPs) with the Fermi level above the bottom of the conduction band are strongly reducing and act more like metals than semiconductors. Application of the degenerate SCNPs is limited to few examples. Herein, we load microporous potassium poly(heptazine imide) (K-PHI) nanoparticles with electrons (e–) and charge balancing protons (H+) in an illumination phase using sacrificial agents. e–/H+ in the K-PHI nanoparticles are weakly bound and therefore could be used in a range of PCET reactions in dark, such as generation of aryl radicals from aryl halides, ketyl radicals from ketones, and 6e–/6H+ reduction of nitrobenzene to aniline. The integration of several features that until now were intrinsic for plants and natural photosynthesis into a transition metal free nanomaterial composed of abundant elements (C, N, and K) offers a powerful tool for synthetic organic chemistry
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