3,195 research outputs found
Chemical Bonding Analysis on Amphoteric Hydrogen - Alkaline Earth Ammine Borohydrides
Usually the ions in solid are in the positive oxidation states or in the
negative oxidation state depending upon the chemical environment. It is highly
unusual for an ion having both positive as well as negative oxidation state in
a particular compound. Structural analysis suggest that the alkaline earth
ammine borohydrides (AABH) with the chemical formula M (BH4)2(NH3)2 (M = Mg,
Ca, or Sr) where hydrogen is present in +1 and -1 oxidation states. In order to
understand the oxidation states of hydrogen and also the character of chemical
bond present in AABH we have made charge density, electron localization
function, Born effective charge, Bader effective charge, and density of states
analyses using result from the density functional calculations. Our detailed
analyses show that hydrogen is in amphoteric behavior with hydrogen closer to
boron is in negative oxidation state and that closer to nitrogen is in the
positive oxidation state. Due to the presence of finite covalent bonding
between the consitutents in AABH the oxidation state of hydrogen is
non-interger value. The confirmation of the presence of amphtoric behavior of
hydrogen in AABH has implication in hydrogen storage applications
Ti4+ Substituted Magnesium Hydride as Promising Material for Hydrogen Storage and Photovoltaic Applications
In order to overcome the disadvantages of MgH2 towards its applications in
on-board hydrogen storage, first principle calculations have been performed for
Ti (2+, 3+, and 4+) substituted MgH2. Our calculated enthalpy of formation and
H site energy implies that Ti substitution in Mg site reduces the stability of
MgH2 which improve the hydrogen storage properties and Ti prefers to be in +4
oxidation state in MgH2. The bonding analyses through partial density of
states, electron localization function and Bader charge of these systems
confirm the existence of iono-covalent bonding. Electronic structure obtained
from hybrid functional calculations show that intermediate bands (IB) are
formed in Ti4+ substituted MgH2 which could improve the solar cell efficiencies
due to multiple photon absorption from valence band to conduction band via IBs
and converts low energy photons in the solar spectrum also into electricity.
Further, our calculated carrier effective masses and optical absorption spectra
show that Ti4+ substituted MgH2 is suitable for higher efficiency photovoltaic
applications. Our results suggest that Ti4+ substituted MgH2 can be considered
as a promising material for hydrogen storage as well as photovoltaic
applications
Neutrino Mass Constraints on R violation and HERA anomaly
R parity violating trilinear couplings of the minimal
supersymmetric standard model (MSSM) are constrained from the limit on the
electron neutrino mass. Strong limits on these couplings follow from the
earlier neglected contribution due to sneutrino vacuum expectation values. The
limits on most of the couplings derived here are stronger than
the existing ones for a wide range in parameters of MSSM. These limits strongly
constrain the interpretation of recent HERA results in scattering in
terms of production of squarks through violating couplings. In particular,
the interpretation in terms of production off strange quark as
suggested recently is not viable for wide ranges in MSSM parameters.Comment: 15 pages with 5 figures, minor typos corrected and a new reference
adde
Ab-initio investigation of the covalent bond energies in the metallic covalent superconductor MgB2 and in AlB2
The contributions of the covalent bond energies of various atom pairs to the
cohesive energy of MgB2 and AlB2 are analysed with a variant of our recently
developed energy-partitioning scheme for the density-functional total energy.
The covalent bond energies are strongest for the intralayer B-B pairs. In
contrast to the general belief, there is also a considerable covalent bonding
between the layers, mediated by the metal atom. The bond energies between the
various atom pairs are analysed in terms of orbital- and energy-resolved
contributions.Comment: 6 pages, 1 figure, 2 tables, submitted to PR
On the Dynamics of Controlled Magnetohydrodynamic Systems
In this paper we study the long time behavior of solutions for an optimal control problem associated with the viscous incompressible electrically conducting fluid modeled by the magnetohydrodynamic (MHD) equations in a bounded two dimensional domain through the adjustment of distributed controls. We first construct a quasi-optimal solution for the MHD systems which possesses exponential decay in time. We then derive some preliminary estimates for the long-time behavior of all admissible solutions of the MHD systems. Next we prove the existence of a solution for the optimal control problem for both finite and infinite time intervals. Finally, we establish the long-time decay properties of the solutions for the optimal control problem
Prediction of material damage in orthotropic metals for virtual structural testing
Models based on the Continuum Damage Mechanics principle are increasingly used for predicting the initiation and growth of damage in materials. The growing reliance on 3-D finite element (FE) virtual structural testing demands implementation and validation of robust material models that can predict the material behaviour accurately. The use of these models within numerical analyses requires suitable material data. EU aerospace companies along with Cranfield University and other similar research institutions have created the MUSCA (non-linear MUltiSCale Analysis of large aero structures) project to develop virtual structural testing prediction. The MUSCA project focuses on static failure testing of large aircraft components. It aims to reduce laboratory tests using advanced numerical analysis to predict failure in order to save overall cost and development time. This thesis aims to improve the current capability of finite element codes in predicting orthotropic material behaviour, primarily damage. The Chow and Wang damage model has been implemented within ABAQUS as a VUMAT subroutine. This thesis presents the development of a numerical damage prediction model and an experimental study to develop a damage material characterisation process that can easily be performed using standard tensile test specimen and equipment already available in the aerospace industry. The proposed method makes use of Digital Image Correlation (DIC), a non-contact optical strain field measurement technique.
Experiments were conducted at Cranfield University material testing facility on aerospace aluminium alloy material AA-2024-T3 and AA-7010-T7651. After thorough literature survey a complete new method was formulated to implement Chow and Wang damage model in Abaqus Explicit numerical code. The damage model was successfully implemented for isotropic and orthotropic behaviour using single element model, multi-element coupon test model and a simple airframe structure. The simulation results were then verified with the similar experimental results by repeating the experimental procedure using simulation for each material type and found matching results. The model is then compared with experimentally determined orthotropic material parameter for AA2024 and AA7010 for validation and found agreeable results for practical use. The material characterisation of damage parameters from standard tensile specimen using DIC technique was also demonstrated and the procedures were established. In this research the combination of experimental work and numerical analysis with clear and simpler calibration strategy for damage model is demonstrated. This is the important contribution of this research work and the streamlined procedures are vital for the industry to utilise the new damage prediction tools. The damage model implementation and test procedures developed through this research provide information and processes involved in fundamentally predicting the ductile damage in metals and metal alloys. The numerical damage model developed using the well-defined verification and validation procedures explained in this research work with new streamlined damage material characterisation using recent contact less DIC technique has wider implication in the material model development for ductile metals in general. The thesis ultimately delivered a fully verified, validated robust damage model numerical simulation code with a new DIC damage characterisation procedure for practical application. The model is now used by the aerospace industry for predicting damage of large aircraft structures
Penalization of Dirichlet Optimal Control Problem for Magneto-Hydrodynamics
We study the approximation of unsteady magnetohydrodynamic equations with Dirichlet control by equations with penalized Neumann control. We prove the existence of optimal solutions to the penalized control problem. We prove the convergence of solutions of penalized control problem to the corresponding solutions of the Dirichlet control problem, as the penalty parameter goes to zero. First order order necessary and second order sufficient optimality condition are developed. Numerical results are provided
- …