Elastic and thermodynamic properties of potentially superhard carbon boride materials

Boron icosahedral structures are the basic building structures of many important hard borides and especially В₄С. The structural and thermodynamic properties of В₄С have been examined applying molecular dynamics simulation with the use of both ab initio and bond order Tersoff potentials. Various physical quantities of В₄С including the elastic constants, thermal expansion coefficients, and specific heat have been examined.Ікосаедрічні структури бору є основними будівельними конструкціями багатьох важливих твердих боридів і особливо це стосується В₄С. Досліджено структурні та термодинамічні властивості В₄С при застосуванні моделювання динаміки молекул з використанням як ab initio розрахунків, так і потенціалів Терсоффа. Розглянуті різні фізичні характериcтики В₄С, включаючи пружні константи, коефіцієнти теплового розширення і теплоємність.Икосаэдрические структуры бора являются основными строительными конструкциями многих важных твердых боридов и особенно это касается В₄С. Исследованы структурные и термодинамические свойства В₄С с применением моделирования динамики молекул, используя как ab initio расчеты, так и потенциалы Терсоффа. Рассмотрены различные физические характериcтики В₄С, включая постоянные упругости, коэффициенты теплового расширения и теплоемкость

Order parameter model for unstable multilane traffic flow

We discuss a phenomenological approach to the description of unstable vehicle motion on multilane highways that explains in a simple way the observed sequence of the phase transitions "free flow -> synchronized motion -> jam" as well as the hysteresis in the transition "free flow synchronized motion". We introduce a new variable called order parameter that accounts for possible correlations in the vehicle motion at different lanes. So, it is principally due to the "many-body" effects in the car interaction, which enables us to regard it as an additional independent state variable of traffic flow. Basing on the latest experimental data (cond-mat/9905216) we assume that these correlations are due to a small group of "fast" drivers. Taking into account the general properties of the driver behavior we write the governing equation for the order parameter. In this context we analyze the instability of homogeneous traffic flow manifesting itself in both of the mentioned above phase transitions where, in addition, the transition "synchronized motion -> jam" also exhibits a similar hysteresis. Besides, the jam is characterized by the vehicle flows at different lanes being independent of one another. We specify a certain simplified model in order to study the general features of the car cluster self-formation under the phase transition "free flow synchronized motion". In particular, we show that the main local parameters of the developed cluster are determined by the state characteristics of vehicle motion only.Comment: REVTeX 3.1, 10 pages with 10 PostScript figure

Mechanical properties of vanadium carbide and a ternary vanadium tungsten carbide

N.R.F (South.Africa)-Sida (Sweden); STINT; STSAb initio total energy calculations are performed on non-stoichiometric vanadium carbide with supercells representing vacancy concentrations of VC(0.875) and VC(0.75). The VC(0.875) supercell retains a cubic symmetry whilst in the case of the VC(0.75) supercell C vacancies located in close proximity have the lowest energy configuration and the cubic lattice slightly distorts to a monoclinic symmetry. Using a stress strain calculational procedure, the elastic constants of both the cubic and the monoclinic systems are deduced. In all cases C vacancies decrease the elastic moduli. A similar analysis is then applied to consider when W is incorporated into VC. In this case it is found that the elastic moduli increase with W content suggesting that a V-W-C alloy could have significant potential as a novel hard material. (C) 2010 Elsevier Ltd. All rights reserved

Thermodynamic model of hardness: Particular case of boron-rich solids

A number of successful theoretical models of hardness have been developed recently. A thermodynamic model of hardness, which supposes the intrinsic character of correlation between hardness and thermodynamic properties of solids, allows one to predict hardness of known or even hypothetical solids from the data on Gibbs energy of atomization of the elements, which implicitly determine the energy density per chemical bonding. The only structural data needed is the coordination number of the atoms in a lattice. Using this approach, the hardness of known and hypothetical polymorphs of pure boron and a number of boron-rich solids has been calculated. The thermodynamic interpretation of the bonding energy allows one to predict the hardness as a function of thermodynamic parameters. In particular, the excellent agreement between experimental and calculated values has been observed not only for the room- temperature values of the Vickers hardness of stoichiometric compounds, but also for its temperature and concentration dependencies

MAGNETIC FLUCTUATIONS, EXCITATIONS, AND INDUCED MOMENTS IN A SYSTEM HAVING TWO SINGLET LEVELS

L'influence des excitations et des fluctuations sur les moments induits dans un système à deux niveaux singulets est étudiée en utilisant la technique du diagramme de Vaks, Larkin et Pikin. On applique ces résultats au calcul de l'aimantation du sous-réseau du composé UN. On montre que la prise en compte des excitations magnétiques rend la courbe d'aimantation moins rigide que celle obtenue dans l'approximation du champ moyen, ce qui conduit à un résultat meilleur en accord avec l'expérience.The influence of magnetic excitations and fluctuations on the induced moments in a system having two singlet levels is studied using the diagram technique of Vaks, Larkin and Pikin. Application of these results are made to a calculation of the sublattice magnetization of uranium mononitride. It is shown that the inclusion of magnetic excitations renders the magnetization curve less rigid than that obtained from mean field theory, leading to a better agreement with experiment