397 research outputs found
Hybrid expansions for local structural relaxations
A model is constructed in which pair potentials are combined with the cluster
expansion method in order to better describe the energetics of structurally
relaxed substitutional alloys. The effect of structural relaxations away from
the ideal crystal positions, and the effect of ordering is described by
interatomic-distance dependent pair potentials, while more subtle
configurational aspects associated with correlations of three- and more sites
are described purely within the cluster expansion formalism. Implementation of
such a hybrid expansion in the context of the cluster variation method or Monte
Carlo method gives improved ability to model phase stability in alloys from
first-principles.Comment: 8 pages, 1 figur
First-principles equation of state and phase stability for the Ni-Al system under high pressures
The equation of state (EOS) of alloys at high pressures is generalized with
the cluster expansion method. It is shown that this provides a more accurate
description. The low temperature EOSs of Ni-Al alloys on FCC and BCC lattices
are obtained with density functional calculations, and the results are in good
agreement with experiments. The merits of the generalized EOS model are
confirmed by comparison with the mixing model. In addition, the FCC phase
diagram of the Ni-Al system is calculated by cluster variation method (CVM)
with both spin-polarized and non-spin-polarized effective cluster interactions
(ECI). The influence of magnetic energy on the phase stability is analyzed. A
long-standing discrepancy between ab initio formation enthalpies and
experimental data is addressed by defining a better reference state. This aids
both evaluation of an ab initio phase diagram and understanding the
thermodynamic behaviors of alloys and compounds. For the first time the
high-pressure behavior of order-disorder transition is investigated by ab
initio calculations. It is found that order-disorder temperatures follow the
Simon melting equation. This may be instructive for experimental and
theoretical research on the effect of an order-disorder transition on shock
Hugoniots.Comment: 27 pages, 12 figure
Charge redistribution at Pd surfaces: ab initio grounds for tight-binding interatomic potentials
A simplified tight-binding description of the electronic structure is often
necessary for complex studies of surfaces of transition metal compounds. This
requires a self-consistent parametrization of the charge redistribution, which
is not obvious for late transition series elements (such as Pd, Cu, Au), for
which not only d but also s-p electrons have to be taken into account. We show
here, with the help of an ab initio FP-LMTO approach, that for these elements
the electronic charge is unchanged from bulk to the surface, not only per site
but also per orbital. This implies different level shifts for each orbital in
order to achieve this orbital neutrality rule. Our results invalidate any
neutrality rule which would allow charge redistribution between orbitals to
ensure a common rigid shift for all of them. Moreover, in the case of Pd, the
power law which governs the variation of band energy with respect to
coordination number, is found to differ significantly from the usual
tight-binding square root.Comment: 6 pages, 2 figures, Latex; Phys.Rev. B 56 (1997
A Nonzero Gap Two-Dimensional Carbon Allotrope from Porous Graphene
Graphene is considered one of the most promising materials for future
electronic. However, in its pristine form graphene is a gapless material, which
imposes limitations to its use in some electronic applications. In order to
solve this problem many approaches have been tried, such as, physical and
chemical functionalizations. These processes compromise some of the desirable
graphene properties. In this work, based on ab initio quantum molecular
dynamics, we showed that a two-dimensional carbon allotrope, named biphenylene
carbon (BPC) can be obtained from selective dehydrogenation of porous graphene.
BPC presents a nonzero bandgap and well-delocalized frontier orbitals.
Synthetic routes to BPC are also addressed.Comment: Published on J. Phys. Chem. C, 2012, 116 (23), pp 12810-1281
Compositional Analysis of Lignocellulosic Feedstocks. 2. Method Uncertainties
The most common procedures for characterizing the chemical components
of lignocellulosic feedstocks use a two-stage sulfuric acid hydrolysis
to fractionate biomass for gravimetric and instrumental analyses.
The uncertainty (i.e., dispersion of values from repeated measurement)
in the primary data is of general interest to those with technical
or financial interests in biomass conversion technology. The composition
of a homogenized corn stover feedstock (154 replicate samples in 13
batches, by 7 analysts in 2 laboratories) was measured along with
a National Institute of Standards and Technology (NIST) reference
sugar cane bagasse, as a control, using this laboratory's suite of
laboratory analytical procedures (LAPs). The uncertainty was evaluated
by the statistical analysis of these data and is reported as the standard
deviation of each component measurement. Censored and uncensored versions
of these data sets are reported, as evidence was found for intermittent
instrumental and equipment problems. The censored data are believed
to represent the âbest caseâ results of these analyses,
whereas the uncensored data show how small method changes can strongly
affect the uncertainties of these empirical methods. Relative standard
deviations (RSD) of 1â3% are reported for glucan, xylan, lignin,
extractives, and total component closure with the other minor components
showing 4â10% RSD. The standard deviations seen with the corn
stover and NIST bagasse materials were similar, which suggests that
the uncertainties reported here are due more to the analytical method
used than to the specific feedstock type being analyzed
Biorefining of wheat straw:accounting for the distribution of mineral elements in pretreated biomass by an extended pretreatmentâseverity equation
BACKGROUND: Mineral elements present in lignocellulosic biomass feedstocks may accumulate in biorefinery process streams and cause technological problems, or alternatively can be reaped for value addition. A better understanding of the distribution of minerals in biomass in response to pretreatment factors is therefore important in relation to development of new biorefinery processes. The objective of the present study was to examine the levels of mineral elements in pretreated wheat straw in response to systematic variations in the hydrothermal pretreatment parameters (pH, temperature, and treatment time), and to assess whether it is possible to model mineral levels in the pretreated fiber fraction. RESULTS: Principal component analysis of the wheat straw biomass constituents, including mineral elements, showed that the recovered levels of wheat straw constituents after different hydrothermal pretreatments could be divided into two groups: 1) Phosphorus, magnesium, potassium, manganese, zinc, and calcium correlated with xylose and arabinose (that is, hemicellulose), and levels of these constituents present in the fiber fraction after pretreatment varied depending on the pretreatment-severity; and 2) Silicon, iron, copper, aluminum correlated with lignin and cellulose levels, but the levels of these constituents showed no severity-dependent trends. For the first group, an expanded pretreatment-severity equation, containing a specific factor for each constituent, accounting for variability due to pretreatment pH, was developed. Using this equation, the mineral levels could be predicted with R(2)â>â0.75; for some with R(2) up to 0.96. CONCLUSION: Pretreatment conditions, especially pH, significantly influenced the levels of phosphorus, magnesium, potassium, manganese, zinc, and calcium in the resulting fiber fractions. A new expanded pretreatment-severity equation is proposed to model and predict mineral composition in pretreated wheat straw biomass
Nucleation of Al3Zr and Al3Sc in aluminum alloys: from kinetic Monte Carlo simulations to classical theory
Zr and Sc precipitate in aluminum alloys to form the compounds Al3Zr and
Al3Sc which for low supersaturations of the solid solution have the L12
structure. The aim of the present study is to model at an atomic scale this
kinetics of precipitation and to build a mesoscopic model based on classical
nucleation theory so as to extend the field of supersaturations and annealing
times that can be simulated. We use some ab-initio calculations and
experimental data to fit an Ising model describing thermodynamics of the Al-Zr
and Al-Sc systems. Kinetic behavior is described by means of an atom-vacancy
exchange mechanism. This allows us to simulate with a kinetic Monte Carlo
algorithm kinetics of precipitation of Al3Zr and Al3Sc. These kinetics are then
used to test the classical nucleation theory. In this purpose, we deduce from
our atomic model an isotropic interface free energy which is consistent with
the one deduced from experimental kinetics and a nucleation free energy. We
test di erent mean-field approximations (Bragg-Williams approximation as well
as Cluster Variation Method) for these parameters. The classical nucleation
theory is coherent with the kinetic Monte Carlo simulations only when CVM is
used: it manages to reproduce the cluster size distribution in the metastable
solid solution and its evolution as well as the steady-state nucleation rate.
We also find that the capillary approximation used in the classical nucleation
theory works surprisingly well when compared to a direct calculation of the
free energy of formation for small L12 clusters.Comment: submitted to Physical Review B (2004
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