886 research outputs found
Segregation, precipitation, and \alpha-\alpha' phase separation in Fe-Cr alloys: a multi-scale modelling approach
Segregation, precipitation, and phase separation in Fe-Cr systems is
investigated. Monte Carlo simulations using semiempirical interatomic
potential, first-principles total energy calculations, and experimental
spectroscopy are used. In order to obtain a general picture of the relation of
the atomic interactions and properties of Fe-Cr alloys in bulk, surface, and
interface regions several complementary methods has to be used. Using Exact
Muffin-Tin Orbitals method the effective chemical potential as a function of Cr
content (0-15 at.% Cr) is calculated for a surface, second atomic layer and
bulk. At ~10 at.% Cr in the alloy the reversal of the driving force of a Cr
atom to occupy either bulk or surface sites is obtained. The Cr containing
surfaces are expected when the Cr content exceeds ~10 at.%. The second atomic
layer forms about 0.3 eV barrier for the migration of Cr atoms between bulk and
surface atomic layer. To get information on Fe-Cr in larger scales we use
semiempirical methods. Using combined Monte Carlo molecular dynamics
simulations, based on semiempirical potential, the precipitation of Cr into
isolated pockets in bulk Fe-Cr and the upper limit of the solubility of Cr into
Fe layers in Fe/Cr layer system is studied. The theoretical predictions are
tested using spectroscopic measurements. Hard X-ray photoelectron spectroscopy
and Auger electron spectroscopy investigations were carried out to explore Cr
segregation and precipitation in Fe/Cr double layer and Fe_0.95Cr_0.05 and
Fe_0.85Cr_0.15 alloys. Initial oxidation of Fe-Cr was investigated
experimentally at 10^-8 Torr pressure of the spectrometers showing intense
Cr_2O_3 signal. Cr segregation and the formation of Cr rich precipitates were
traced by analysing the experimental spectral intensities with respect to
annealing time, Cr content, and kinetic energy of the exited electron.Comment: 16 pages, 14 figures, 52 reference
Soliton-like Spin State in the A-like Phase of 3He in Anisotropic Aerogel
We have found a new stable spin state in the A-like phase of superfluid 3He
confined to intrinsically anisotropic aerogel. The state can be formed by
radiofrequency excitation applied while cooling through the superfluid
transition temperature and its NMR properties are different from the standard
A-like phase obtained in the limit of very small excitation. It is possible
that this new state is formed by textural domain walls pinned by aerogel.Comment: 9 pages, 3 figures. Submitted to J. of Low Tem. Phys. (QFS2007
Proceedings
Half-Metallic Ferrimagnetism in Mn_2VAl
We show that Mn_2VAl is a compound for which the generalized gradient
approximation (GGA) to the exchange-correlation functional in density
functional theory makes a qualitative change in predicted behavior compared to
the usual local density approximation (LDA). Application of GGA leads to
prediction of Mn_2VAl being a half-metallic ferrimagnet, with the minority
channel being the conducting one. The electronic and magnetic structure is
analyzed and contrasted with the isostructural enhanced semimetal Fe_2VAl.Comment: 5 pages, Latex, 6 postscript figures. Description and figures of the
(minority) Fermi surfaces have been adde
Frost quakes in wetlands in northern Finland during extreme winter weather conditions and related hazard to urban infrastructure
The paper reports the first results of an experiment in northern Finland during winter 2022â2023 that aimed at studying seismic events caused by seasonal freezing in the so-called critical zone (CZ) of the Earth. These events have attracted public attention recently, as multiple reports about them from local inhabitants in Arctic and sub-Arctic areas appeared on social networks. To make an instrumental study of such events, to reveal the relationship between their occurrence and winter weather conditions, and to evaluate the possible hazard, we installed two high-resolution seismic arrays with co-located soil temperature stations at two sites in Finland, one of them being in the city of Oulu in the sub-Arctic area (65.04°âN, 25.61°âE) and the other one above the polar circle in the municipality of SodankylĂ€ (67.36°âN, 26.63°âE). The equipment recorded continuous seismic and soil temperature data during November 2022âApril 2023. Based on reports from the inhabitants of Talvikangas (Oulu) about the ground shaking and unusual noises on 6 January 2023 and their observations of new fractures on the roads there, we selected the time interval for the identification of frost quakes that originated during that day from continuous seismic records in Talvikangas and in SodankylĂ€. During the selected time interval, the extremely rapid air temperature drops of about â1.4â°Câhâ1 in Talvikangas and â0.88â°Câhâ1 in SodankylĂ€ were observed. We identified and located two types of seismic events â namely, frost quakes with frequencies of about 10â20âHz, with waveforms like those of tectonic events, and irregularly shaped frost tremors with frequencies of about 120â180âHz. The sources of frost quakes in Talvikangas are mainly located on irrigated wetland, while in SodankylĂ€, about 50â% of registered frost quakes were caused by ice fracturing on the Kitinen River. However, several relatively strong events, with their origin in the wetlands, were also recorded. A significant number of sources of frost tremors are confined to wetland areas cut by irrigation channels and to roads cleaned from snow during winter both in Talvikangas and in SodankylĂ€. We calculated ground accelerations and ground velocities for the strongest events from both groups and compared them to equivalent properties of other seismic signals, like distant local earthquakes in the area, mining production blasts, and cargo train vibration. Our study shows that high-frequency frost tremors corresponding to surface fracture opening in the uppermost frozen surface layer of a thickness of up to 5âcm can directly damage infrastructure objects like roads and basements of buildings. Surface waves, produced by frost quakes and propagating inside the shallow soil layer, have large enough ground accelerations at epicentral distances of hundreds of metres. Therefore, frost quakes should be considered a phenomenon that can potentially damage infrastructures, and they have to be taken into account in seismic hazard assessments.
Our research is the first instrumental study of seismic events originating from wetland areas. These events occur as a result of the interaction between the uppermost layer of the solid Earth's CZ and atmosphere processes that deserves further study.</p
Applications of the generalized gradient approximation to ferroelectric perovskites
The Perdew-Burke-Ernzerhof generalized gradient approximation to the density
functional theory is tested with respect to sensitivity to the choice of the
value of the parameter , which is associated to the degree of
localization of the exchange-correlation hole. A study of structural and
dynamical properties of four selected ferroelectric perovskites is presented.
The originally proposed value of =0.804 %(best suited for atoms and
molecules) works well for some solids, whereas for the ABO perovskites it
must be decreased in order to predict equilibrium lattice parameters in good
agreement with experiments. The effects on the structural instabilities and
zone center phonon modes are examined. The need of varying from one
system to another reflects the fact that the localization of the
exchange-correlation hole is system dependent, and the sensitivity of the
structural properties to its actual value illustrates the necessity of finding
a universal function for .Comment: 15 pages, 2 figures, PRB in pres
Alloy Steel: Properties and Use First-Principles Quantum Mechanical Approach to Stainless Steel Alloys
Accurate description of materials requires the most advanced atomic-scale techniques from both experimental and theoretical areas. In spite of the vast number of available techniques, however, the experimental study of the atomic-scale properties and phenomena even in simple solids is rather difficult. In steels the challenges become more complex due to the interplay between the structural, chemical and magnetic effects. On the other hand, advanced computational methods based on density functional theory ensure a proper platform for studying the fundamental properties of steel materials from first-principles. In 1980âs the first-principles description of the thermodynamic properties of elemental iron was still on the borderline of atomistic simulations. Today the numerous application- oriented activities at the industrial and academic sectors are paired by a rapidly increasing scientific interest. This is reflected by the number of publications on ab initio steel research, which has increased from null to about one thousand within the last two decades. Our research group has a well established position in developing and applying computational codes for steel related applications. Using our ab initio tools, we have presented an insight to the electronic and magnetic structure, and micromechanical properties of austenite and ferrite stainless steel alloys. In the present contribution, we review the most important developments within the ab initio quantum mechanics aided steel design with special emphasis on the role of magnetism on the fundamental properties of alloy steels
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Stability of an Embedded Mesh Method for Coupling Lagrangian and ALE Finite Element Models
The evolution of cooperation and altruism--a general framework and a classification of models.
One of the enduring puzzles in biology and the social sciences is the origin and persistence of intraspecific cooperation and altruism in humans and other species. Hundreds of theoretical models have been proposed and there is much confusion about the relationship between these models. To clarify the situation, we developed a synthetic conceptual framework that delineates the conditions necessary for the evolution of altruism and cooperation. We show that at least one of the four following conditions needs to be fulfilled: direct benefits to the focal individual performing a cooperative act; direct or indirect information allowing a better than random guess about whether a given individual will behave cooperatively in repeated reciprocal interactions; preferential interactions between related individuals; and genetic correlation between genes coding for altruism and phenotypic traits that can be identified. When one or more of these conditions are met, altruism or cooperation can evolve if the cost-to-benefit ratio of altruistic and cooperative acts is greater than a threshold value. The cost-to-benefit ratio can be altered by coercion, punishment and policing which therefore act as mechanisms facilitating the evolution of altruism and cooperation. All the models proposed so far are explicitly or implicitly built on these general principles, allowing us to classify them into four general categories
Mathematical modelling long-term effects of replacing Prevnar7 with Prevnar13 on invasive pneumococcal diseases in England and Wales
England and Wales recently replaced the 7-valent pneumococcal conjugate vaccine (PCV7) with its 13-valent equivalent (PCV13), partly based on projections from mathematical models of the long-term impact of such a switch compared to ceasing pneumococcal conjugate vaccination altogether. A compartmental deterministic model was used to estimate parameters governing transmission of infection and competition between different groups of pneumococcal serotypes prior to the introduction of PCV13. The best-fitting parameters were used in an individual based model to describe pneumococcal transmission dynamics and effects of various options for the vaccination programme change in England and Wales. A number of scenarios were conducted using (i) different assumptions about the number of invasive pneumococcal disease cases adjusted for the increasing trend in disease incidence prior to PCV7 introduction in England and Wales, and (ii) a range of values representing serotype replacement induced by vaccination of the additional six serotypes in PCV13. Most of the scenarios considered suggest that ceasing pneumococcal conjugate vaccine use would cause an increase in invasive pneumococcal disease incidence, while replacing PCV7 with PCV13 would cause an overall decrease. However, the size of this reduction largely depends on the level of competition induced by the additional serotypes in PCV13. The model estimates that over 20 years of PCV13 vaccination, around 5000â62000 IPD cases could be prevented compared to stopping pneumococcal conjugate vaccination altogether. Despite inevitable uncertainty around serotype replacement effects following introduction of PCV13, the model suggests a reduction in overall invasive pneumococcal disease incidence in all cases. Our results provide useful evidence on the benefits of PCV13 to countries replacing or considering replacing PCV7 with PCV13, as well as data that can be used to evaluate the cost-effectiveness of such a switch
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