1,851 research outputs found
A parallel multistate framework for atomistic non-equilibrium reaction dynamics of solutes in strongly interacting organic solvents
We describe a parallel linear-scaling computational framework developed to
implement arbitrarily large multi-state empirical valence bond (MS-EVB)
calculations within CHARMM. Forces are obtained using the Hellman-Feynmann
relationship, giving continuous gradients, and excellent energy conservation.
Utilizing multi-dimensional Gaussian coupling elements fit to CCSD(T)-F12
electronic structure theory, we built a 64-state MS-EVB model designed to study
the F + CD3CN -> DF + CD2CN reaction in CD3CN solvent. This approach allows us
to build a reactive potential energy surface (PES) whose balanced accuracy and
efficiency considerably surpass what we could achieve otherwise. We use our PES
to run MD simulations, and examine a range of transient observables which
follow in the wake of reaction, including transient spectra of the DF
vibrational band, time dependent profiles of vibrationally excited DF in CD3CN
solvent, and relaxation rates for energy flow from DF into the solvent, all of
which agree well with experimental observations. Immediately following
deuterium abstraction, the nascent DF is in a non-equilibrium regime in two
different respects: (1) it is highly excited, with ~23 kcal mol-1 localized in
the stretch; and (2) not yet Hydrogen bonded to the CD3CN solvent, its
microsolvation environment is intermediate between the non-interacting
gas-phase limit and the solution-phase equilibrium limit. Vibrational
relaxation of the nascent DF results in a spectral blue shift, while relaxation
of its microsolvation environment results in a red shift. These two competing
effects result in a post-reaction relaxation profile distinct from that
observed when DF vibration excitation occurs within an equilibrium
microsolvation environment. The parallel software framework presented in this
paper should be more broadly applicable to a range of complex reactive systems.Comment: 58 pages and 29 Figure
Electrical transport properties of bulk MgB2 materials synthesized by the electrolysis on fused mixtures of MgCl2, NaCl, KCl and MgB2O4
Electrolysis was carried out on fused mixtures of MgCl2, NaCl, KCl and MgB2O4
under an Ar flow at 600C. Electrical resistivity measurements for the grown
deposits show an onset of superconducting transition at 37 K in the absence of
applied magnetic field. The resistivity decreases down to zero below 32 K. From
an applied-field dependence of resistivity, an upper critical field and a
coherence length were calculated to be 9.7 T and 5.9 nm at 0 K, respectively
Transport critical current of Solenoidal MgB2/Cu Coils Fabricated Using a Wind-Reaction In-situ Technique
In this letter, we report the results of transport Jc of solenoid coils upto
100 turns fabricated with Cu-sheathed MgB2 wires using a wind-reaction in-situ
technique. Despite the low density of single core and some reaction between Mg
and Cu-sheath, our results demonstrate the decrease in transport Jc with
increasing length of MgB2 wires is insignificant. Solenoid coils with diameter
as small as 10 mm can be readily fabricated using a wind-reaction in-situ
technique. The Jc of coils is essentially the same as in the form of straight
wires. A Jc of 133,000 A/cm2 and 125,000 A/cm2 at 4 K and self field has been
achieved for a small coil wound using Cu-sheathed tape and Cu-sheathed wire
respectively. These results indicate that the MgB2 wires have a great potential
for lage scale applicationsComment: 6 pages, 4 figures, 1 tabl
Improved Current Densities in MgB2 By Liquid-Assisted Sintering
Polycrystalline MgB2 samples with GaN additions were prepared by reaction of
Mg, B, and GaN powders. The presence of Ga leads to a low melting eutectic
phase which allowed liquid phase sintering and produces plate-like grains. For
low-level GaN additions (5% at. % or less), the critical transition
temperature, Tc, remained unchanged and in 1T magnetic field, the critical
current density, Jc was enhanced by a factor of 2 and 10, for temperatures of
\~5K and 20K, respectively. The values obtained are approaching those of hot
isostatically pressed samples.Comment: 12 pages, 1 table, 4 figures, accepted in Applied Physics Letter
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