29 research outputs found
High-precision molecular dynamics simulation of UO2-PuO2: pair potentials comparison in UO2
Our series of articles is devoted to high-precision molecular dynamics
simulation of mixed actinide-oxide (MOX) fuel in the approximation of rigid
ions and pair interactions (RIPI) using high-performance graphics processors
(GPU). In this first article 10 most recent and widely used interatomic sets of
pair potentials (SPP) are assessed by reproduction of solid phase properties of
uranium dioxide (UO2) - temperature dependences of the lattice constant, bulk
modulus, enthalpy and heat capacity. Measurements were performed with 1K
accuracy in a wide temperature range from 300K up to melting points. The best
results are demonstrated by two recent SPPs MOX-07 and Yakub-09, which both had
been fitted to the recommended thermal expansion in the range of temperatures
300-3100K. They reproduce the experimental data better than the widely used
SPPs Basak-03 and Morelon-03 at temperatures above 2500K.Comment: 11 pages, 9 figures, 4 table
High-precision molecular dynamics simulation of UO2-PuO2: Anion self-diffusion in UO2
Our series of articles is devoted to high-precision molecular dynamics
simulation of mixed actinide-oxide (MOX) fuel in the approximation of rigid
ions and pair interactions (RIPI) using high-performance graphics processors
(GPU). In this article we study self-diffusion mechanisms of oxygen anions in
uranium dioxide (UO2) with the ten recent and widely used sets of interatomic
pair potentials (SPP) under periodic (PBC) and isolated (IBC) boundary
conditions. Wide range of measured diffusion coefficients (from 10^-3 cm^2/s at
melting point down to 10^-12 cm^2/s at 1400 K) made possible a direct
comparison (without extrapolation) of the simulation results with the
experimental data, which have been known only at low temperatures (T < 1500 K).
A highly detailed (with the temperature step of 1 K) calculation of the
diffusion coefficient allowed us to plot temperature dependences of the
diffusion activation energy and its derivative, both of which show a wide
(~1000 K) superionic transition region confirming the broad lambda-peaks of
heat capacity obtained by us earlier. It is shown that regardless of SPP the
anion self-diffusion in model crystals without surface or artificially embedded
defects goes on via exchange mechanism, rather than interstitial or vacancy
mechanisms suggested by the previous works. The activation energy of exchange
diffusion turned out to coincide with the anti-Frenkel defect formation energy
calculated by the lattice statics.Comment: 18 pages, 11 figures, 5 table
Molecular dynamics simulation of UO2 nanocrystals surface
In this article we investigated surface of nanocrystals (NC) of uranium
dioxide (UO2) using molecular dynamics (MD) under isolated (non-periodic)
boundary conditions with the approximation of pair potentials and rigid ions.
It is shown that a cubic shape of the model NCs is metastable and the stable
equilibrium is reached in the process of structural relaxation to the
octahedral shape over a time of 1000 ns (200 million MD steps), which increases
with the size of NC. We measured the size dependences of the lattice parameter
and the surface energy density for NC of cubic and octahedral shape with volume
up to 1000 nm3 (50000 particles) at temperatures of 2200K and 2300K. For the
surfaces {100} and {111} we obtained the energy density
{\sigma}100=1.602\pm0.016 J/m^2, {\sigma}111=1.137\pm0.032 J/m^2 and surface
tension constant {\gamma}111=0.875\pm0.008 J/m^2. The resulting ratio of
{\sigma}100/{\sigma}111=1.408\pm0.042 within the error coincides with the
experimental value of 1.42\pm0.05 measured for microscopic cavities in
monocrystals of UO2.Comment: 11 pages, 8 figures, 5 table
Investigation of cation self-diffusion mechanisms in UO2+-x using molecular dynamics
This article is devoted to investigation of cation self-diffusion mechanisms,
taking place in UO2, UO2+x, and UO2-x crystals simulated under periodic (PBC)
and isolated (IBC) boundary conditions using the method of molecular dynamics
in the approximation of rigid ions and pair interactions. It is shown that
under PBC the cations diffuse via an exchange mechanism (with the formation of
Frenkel defects) with activation energy of 15-22 eV, while under IBC there is
competition between the exchange and vacancy (via Schottky defects) diffusion
mechanisms, which give the effective activation energy of 11-13 eV near the
melting temperature of the simulated UO2.00 nanocrystals. Vacancy diffusion
with lower activation energy of 6-7 eV was dominant in the non-stoichiometric
crystals UO2.10, UO2.15 and UO1.85. Observations showed that a cation vacancy
is accompanied by different number of anion vacancies depending on the
deviation from stoichiometry: no vacancies in UO2.15, single vacancy in UO2.00
and four vacancies in UO1.85. The corresponding law of mass action formulas
derived within the Lidiard-Matzke model allowed explaining the obtained
activation energies and predicting a change in the activation energy within the
temperature range of the superionic phase transition. The diffusion of cations
on the surface of nanocrystals had activation energy of 3.1-3.6 eV.Comment: 22 pages, 6 tables, 15 figure
High-precision molecular dynamics simulation of UO2-PuO2: superionic transition in uranium dioxide
Our series of articles is devoted to high-precision molecular dynamics
simulation of mixed actinide-oxide (MOX) fuel in the rigid ions approximation
using high-performance graphics processors (GPU). In this article we assess the
10 most relevant interatomic sets of pair potential (SPP) by reproduction of
the Bredig superionic phase transition (anion sublattice premelting) in uranium
dioxide. The measurements carried out in a wide temperature range from 300K up
to melting point with 1K accuracy allowed reliable detection of this phase
transition with each SPP. The {\lambda}-peaks obtained are smoother and wider
than it was assumed previously. In addition, for the first time a pressure
dependence of the {\lambda}-peak characteristics was measured, in a range from
-5 GPa to 5 GPa its amplitudes had parabolic plot and temperatures had linear
(that is similar to the Clausius-Clapeyron equation for melting temperature).Comment: 7 pages, 6 figures, 1 tabl
Real-space renormalization group study of the anisotropic antiferromagnetic Heisenberg model of spin S=1 on a honeycomb lattice
The real-space RG approach is applied to study critical temperatures of
system consisting of interacting spin chains of spin S=1 with an inner
antiferromagnetic exchange which form a honeycomb crystal lattice. Using
anisotropic Heisenberg model we calculate critical temperature as a function of
anisotropic parameter and the ratio of interchain and intrachain interactions.
A comparison our results with those obtained from RGRS calculations for the
same model of spin-1/2 on a square lattice is given
High-Precision Molecular Dynamics Simulation of UO2-PuO 2: Pair Potentials Comparison in UO2
Our series of articles is devoted to high-precision molecular dynamics simulation of mixed actinide-oxide (MOX) fuel in the approximation of rigid ions and pair interactions (RIPI) using high-performance graphics processors (GPU). In this first article ten most recent and widely used interatomic sets of pair potentials (SPP) are assessed by reproduction of solid phase properties of uranium dioxide (UO2) - temperature dependences of the lattice constant, bulk modulus, enthalpy and heat capacity. Measurements were performed with 1 K accuracy in a wide temperature range from 300 K up to melting points. The best results are demonstrated by two recent SPPs MOX-07 and Yakub-09, which both had been fitted to the recommended thermal expansion in the range of temperatures 300-3100 K. They reproduce the experimental data better than the widely used SPPs Basak-03 and Morelon-03 at temperatures above 2500 K. © 2011 Elsevier B.V. All rights reserved
Real-Space Renormalization Group Study of the Anisotropic Antiferromagnetic Heisenberg Model of Spin S = 1 on a Honeycomb Lattice
The real-space renormalization group approach is applied to study critical temperatures of a system consisting of interacting spin chains of spin S = 1, with an inner antiferromagnetic exchange, which form a honeycomb crystal lattice. Using the anisotropic Heisenberg model we calculate the critical temperature as a function of the anisotropic parameter and the ratio of the interchain and intrachain interactions. A comparison our results with those obtained from RSRG calculations for the same model of spin-1/2 on a square lattice is given. © 2005 IOP Publishing Ltd
Quantum Dissipation Theory of Slow Magnetic Relaxation Mediated by Domain-Wall Motion in the One-Dimensional Chain Compound [Mn (hfac)2 BN OH]
Based on a quantum dissipation theory of open systems, we present a theoretical study of slow dynamics of magnetization for the ordered state of the molecule-based magnetic complex [Mn (hfac)2 BN OH] composed from antiferromagnetically coupled ferrimagnetic (5 2,1) spin chains. Experimental investigations of the magnetization process in pulsed fields have shown that this compound exhibits a metamagnetic AF-FI transition at a critical field in the order of the interchain coupling. A strong frequency dependence for the ac susceptibility has been revealed in the vicinity of the AF-FI transition and was associated with an AF-FI interface kink motion. We model these processes by a field-driven domain-wall motion along the field-unfavorable chains correlated with a dissipation effect due to a magnetic system-bath coupling. The calculated longitudinal magnetization has a two-step relaxation after the field is switched off and are found in good agreement with the experiment. The relaxation time determined from the imaginary part of the model ac susceptibility agrees qualitatively with that found from the remanent magnetization data. © 2006 The American Physical Society.The authors would like to thank E. Z. Kuchinskii for discussions. This work was partly supported by Grant No. NREC-005 of US CRDF Civilian Research and Development Foundation. Two of the authors V.E.S. and A.S.B. thank the Foundation Dynasty Moscow for support
Quantum Dissipation Theory of Slow Magnetic Relaxation Mediated by Domain-Wall Motion in the One-Dimensional Chain Compound [Mn (hfac)2 BN OH]
Based on a quantum dissipation theory of open systems, we present a theoretical study of slow dynamics of magnetization for the ordered state of the molecule-based magnetic complex [Mn (hfac)2 BN OH] composed from antiferromagnetically coupled ferrimagnetic (5 2,1) spin chains. Experimental investigations of the magnetization process in pulsed fields have shown that this compound exhibits a metamagnetic AF-FI transition at a critical field in the order of the interchain coupling. A strong frequency dependence for the ac susceptibility has been revealed in the vicinity of the AF-FI transition and was associated with an AF-FI interface kink motion. We model these processes by a field-driven domain-wall motion along the field-unfavorable chains correlated with a dissipation effect due to a magnetic system-bath coupling. The calculated longitudinal magnetization has a two-step relaxation after the field is switched off and are found in good agreement with the experiment. The relaxation time determined from the imaginary part of the model ac susceptibility agrees qualitatively with that found from the remanent magnetization data. © 2006 The American Physical Society.The authors would like to thank E. Z. Kuchinskii for discussions. This work was partly supported by Grant No. NREC-005 of US CRDF Civilian Research and Development Foundation. Two of the authors V.E.S. and A.S.B. thank the Foundation Dynasty Moscow for support