423 research outputs found
A model for time-dependent grain boundary diffusion of ions and electrons through a film or scale, with an application to alumina
A model for ionic and electronic grain boundary transport through thin films,
scales or membranes with columnar grain structure is introduced. The grain
structure is idealized as a lattice of identical hexagonal cells - a honeycomb
pattern. Reactions with the environment constitute the boundary conditions and
drive the transport between the surfaces. Time-dependent simulations solving
the Poisson equation self-consistently with the Nernst-Planck flux equations
for the mobile species are performed. In the resulting Poisson-Nernst-Planck
system of equations, the electrostatic potential is obtained from the Poisson
equation in its integral form by summation. The model is used to interpret
alumina membrane oxygen permeation experiments, in which different oxygen gas
pressures are applied at opposite membrane surfaces and the resulting flux of
oxygen molecules through the membrane is measured. Simulation results involving
four mobile species, charged aluminum and oxygen vacancies, electrons, and
holes, provide a complete description of the measurements and insight into the
microscopic processes underpinning the oxygen permeation of the membrane. Most
notably, the hypothesized transition between p-type and n-type ionic
conductivity of the alumina grain boundaries as a function of the applied
oxygen gas pressure is observed in the simulations. The range of validity of a
simple analytic model for the oxygen permeation rate, similar to the Wagner
theory of metal oxidation, is quantified by comparison to the numeric
simulations. The three-dimensional model we develop here is readily adaptable
to problems such as transport in a solid state electrode, or corrosion scale
growth
Surface structure and water adsorption on Fe<sub>3</sub>O<sub>4</sub>(111): Spin-density functional theory and on-site Coulomb interactions
The surface structure of magnetite Fe3O4(111) in contact with oxygen and water is investigated using spin density functional theory plus on-site Coulomb interactions. The present results unravels apparent contradictions in the experimental data regarding the equilibrium stoichiometry of the bare surface termination. Both for 298 K and 1200 K, the equilibrium structure is terminated by 1/4 monolayer (ML) of iron (Fe) on top of a full oxygen layer, consistent with an earlier low-energy electron diffraction analysis. Nontheless, the calculated negative slope of the surface energies vs oxygen partial pressure shows that a 1/2 ML Fe termination would become stable under oxygen poor conditions at high temperatures, in agreement to interpretation of scanning tunneling microscopy experiments. Initial water adsorption is dissociative and saturates when all Fe sites are occupied by OH groups while the H atoms bind to surface oxygen. Further water bridges the OH and H groups resulting in a quite unique type of H-bonded molecular water with its oxygen forming a hydronium ion like structure OH3+-OH. This water structure is different from the water dimeric structures found as yet on oxide and metal surfaces for partially dissociated (H2O-OH-H) overlayers
Magnetic tight-binding and the iron-chromium enthalpy anomaly
We describe a self consistent magnetic tight-binding theory based in an
expansion of the Hohenberg-Kohn density functional to second order, about a non
spin polarised reference density. We show how a first order expansion about a
density having a trial input magnetic moment leads to the Stoner--Slater rigid
band model. We employ a simple set of tight-binding parameters that accurately
describes electronic structure and energetics, and show these to be
transferable between first row transition metals and their alloys. We make a
number of calculations of the electronic structure of dilute Cr impurities in
Fe which we compare with results using the local spin density approximation.
The rigid band model provides a powerful means for interpreting complex
magnetic configurations in alloys; using this approach we are able to advance a
simple and readily understood explanation for the observed anomaly in the
enthalpy of mixing.Comment: Submitted to Phys Rev
Diffusion of oxygen in Mg-doped α-Al2O3: the corundum conundrum explained
It has been a puzzle for over two decades that the enhancement of oxygen diffusion in α-Al_{2}O_{3} ,with respect to the amount of Mg doping, is several orders of magnitude less than expected. The standard model, which envisages that transport is mediated by oxygen vacancies induced to compensate the charge of Mg 2+ ions substituting Al 3+ ions, has not been able to explain this anomaly. Here, we report a detailed study of populations of point defects and defect clusters in Mg-doped α-Al_{2}O_{3}. By taking into account calculated defect formation energies from the literature, the condition of charge neutrality, and the environmental parameters (chemical potentials) under which the anomalous trend in oxygen diffusivities were previously observed, we are able to arrive at an explanation. A non-linear relationship between Mg concentration in the system and key native point defects, which serve as mediators of self-diffusion in α-Al_{2}O_{3_ , is predicted: the concentrations of such defects increase much more slowly in the supersaturation regime than in the pre-saturation regime, matching the anomalous result previously observed in α-Al_{2}O_{3} . We identify the reason for this as buffering by positively charged Mg interstitials and Mg–oxygen vacancy clusters, which compensate the negative charges of Mg substitutional defects (Mg^{1−}Al ). This study answers part of the long-standing question about self-diffusion in alumina, referred to by Heuer and Lagerlöf in 1999 as the Corundum Conundrum
Electronic structure and total energy of interstitial hydrogen in iron: Tight binding models
An application of the tight binding approximation is presented for the
description of electronic structure and interatomic force in magnetic iron,
both pure and containing hydrogen impurities. We assess the simple canonical
d-band description in comparison to a non orthogonal model including s and d
bands. The transferability of our models is tested against known properties
including the segregation energies of hydrogen to vacancies and to surfaces of
iron. In many cases agreement is remarkably good, opening up the way to quantum
mechanical atomistic simulation of the effects of hydrogen on mechanical
properties
Prediction and Observation of the bcc Structure in Pure Copper at a \u3cem\u3e\u3cstrong\u3eΣ\u3c/strong\u3e\u3c/em\u3e3 Grain Boundary
We have used molecular dynamics and simulated annealing to study an asymmetrical Σ3 tilt grain boundary with ⟨211⟩ rotation axis in Cu. The boundary plane was inclined at 84° with respect to the {}(111) plane. A simple central force N-body interatomic potential was used. The most stable configuration shows a broad band of predominantly bcc structure in the boundary region. Samples of the bicrystal with the same misorientation and inclination of the boundary plane were observed in a 1250 kV transmission electron microscope, confirming the predicted structure with atomic resolution
The stabilizing role of itinerant ferromagnetism in inter-granular cohesion in iron
We present a simple, general energy functional for ferromagnetic materials
based upon a local spin density extension to the Stoner theory of itinerant
ferromagnetism. The functional reproduces well available ab initio results and
experimental interfacial energies for grain boundaries in iron. The model shows
that inter-granular cohesion along symmetric tilt boundaries in iron is
dependent upon strong magnetic structure at the interface, illuminates the
mechanisms underlying this structure, and provides a simple explanation for
relaxation of the atomic structure at these boundaries.Comment: In review at Phys. Rev. Lett. Submitted 23 September 1997; revised 16
March 199
Structural and chemical embrittlement of grain boundaries by impurities: a general theory and first principles calculations for copper
First principles calculations of the Sigma 5 (310)[001] symmetric tilt grain
boundary in Cu with Bi, Na, and Ag substitutional impurities provide evidence
that in the phenomenon of Bi embrittlement of Cu grain boundaries electronic
effects do not play a major role; on the contrary, the embrittlement is mostly
a structural or "size" effect. Na is predicted to be nearly as good an
embrittler as Bi, whereas Ag does not embrittle the boundary in agreement with
experiment. While we reject the prevailing view that "electronic" effects
(i.e., charge transfer) are responsible for embrittlement, we do not exclude
the role of chemistry. However numerical results show a striking equivalence
between the alkali metal Na and the semi metal Bi, small differences being
accounted for by their contrasting "size" and "softness" (defined here). In
order to separate structural and chemical effects unambiguously if not
uniquely, we model the embrittlement process by taking the system of grain
boundary and free surfaces through a sequence of precisely defined gedanken
processes; each of these representing a putative mechanism. We thereby identify
three mechanisms of embrittlement by substitutional impurities, two of which
survive in the case of embrittlement or cohesion enhancement by interstitials.
Two of the three are purely structural and the third contains both structural
and chemical elements that by their very nature cannot be further unravelled.
We are able to take the systems we study through each of these stages by
explicit computer simulations and assess the contribution of each to the nett
reduction in intergranular cohesion. The conclusion we reach is that
embrittlement by both Bi and Na is almost exclusively structural in origin;
that is, the embrittlement is a size effect.Comment: 13 pages, 5 figures; Accepted in Phys. Rev.
A simple environment-dependent overlap potential and Cauchy violation in solid argon
We develop an analytic and environment-dependent interatomic potential for
the overlap repulsion in solid argon, based on an approximate treatment of the
non-orthogonal Tight-Binding theory for the closed-shell systems. The present
model can well reproduce the observed elastic properties of solid argon
including Cauchy violation at high pressures, yet very simple. A useful and
novel analysis is given to show how the elastic properties are related to the
environment-dependence incorporated into a generic pairwise potential. The
present study has a close link to the broad field of computational materials
science, in which the inclusion of environment dependence in short-ranged
repulsive part of a potential model is sometimes crucial in predicting the
elastic properties correctly.Comment: 10 pages, 3 figure
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