Avinash M. Dongare

Atomic scale studies of spall behavior in nanocrystalline Cu

The micromechanisms related to ductile failure during dynamic loading of nanocrystalline Cu are investigated in a series of large-scale molecular dynamics simulations. Void nucleation, growth, and coalescence is studied for a nanocrystalline Cu system with an average grain size of 6 nm under conditions of impact of a shock piston with velocities of 250, 500, 750, and 1000 m/s and compared to that observed in single crystal copper. Higher impact velocities result in higher strain rates and higher values of spall strengths for the metal as well as nucleation of larger number of voids in smaller times. For the same impact velocity, the spall strength of the nanocrystalline metal, however, is lower than that for single crystal copper. The results obtained for void nucleation and growth in nanocrystalline Cu for various impact velocities and for single crystal copper [001] suggests two distinct stages of evolution of voids. The first stage (I) corresponds to the fast nucleation of voids followed by the second ...

Effects of Uniaxial and Biaxial Strain on Few-Layered Terrace Structures of MoS2 Grown by Vapor Transport

One of the most fascinating properties of molybdenum disulfide (MoS2) is its ability to be subjected to large amounts of strain without experiencing degradation. The potential of MoS2 mono- and few-layers in electronics, optoelectronics, and flexible devices requires the fundamental understanding of their properties as a function of strain. While previous reports have studied mechanically exfoliated flakes, tensile strain experiments on chemical vapor deposition (CVD)-grown few-layered MoS2 have not been examined hitherto, although CVD is a state of the art synthesis technique with clear potential for scale-up processes. In this report, we used CVD-grown terrace MoS2 layers to study how the number and size of the layers affected the physical properties under uniaxial and biaxial tensile strain. Interestingly, we observed significant shifts in both the Raman in-plane mode (as high as -5.2 cm(-1)) and photoluminescence (PL) energy (as high as -88 meV) for the few-layered MoS2 under ∼1.5% applied uniaxial tensile strain when compared to monolayers and few-layers of MoS2 studied previously. We also observed slippage between the layers which resulted in a hysteresis of the Raman and PL spectra during further applications of strain. Through DFT calculations, we contended that this random layer slippage was due to defects present in CVD-grown materials. This work demonstrates that CVD-grown few-layered MoS2 is a realistic, exciting material for tuning its properties under tensile strain.

Synthesis and characterization of copper-stabilized zirconia as an anode material for SOFC

Solid oxide fuel cells (SOFC) are now being seriously considered for alternate energy source because of the environmental hazards associated with the use of fossil fuels and their limited availability. As a part of the development of these fuel cells, we have synthesized a series of CuO–ZrO2 samples with varying concentration of CuO. In this paper, we present the study of copper-stabilized zirconia as an anode material in SOFC. The role of copper oxide in stabilizing zirconia and the possible reasons for the increase in catalytic activity of the relevant composition have been discussed using the experimental data.

Theoretical study on strain induced variations in electronic properties of 2H-MoS2 bilayer sheets

The strain dependence of the electronic properties of bilayer sheets of 2H-MoS2 is studied using ab initio simulations based on density functional theory. An indirect band gap for bilayer MoS2 is observed for all variations of strain along the basal plane. Several transitions for the indirect band gap are observed for various strains for the bilayer structure. The variation of the band gap and the carrier effective masses for the holes and the electrons for the bilayer MoS2 structure under conditions of uniaxial strain, biaxial strain, as well as uniaxial stress is investigated.

Dislocation evolution and peak spall strengths in single crystal and nanocrystalline Cu

The dynamic evolution and interaction of defects under the conditions of shock loading in single crystal and nanocrystalline Cu are investigated using a series of large-scale molecular dynamics simulations for an impact velocity of 1 km/s. Four stages of defect evolution are identified during shock simulations that result in deformation and failure. These stages correspond to: the initial shock compression (I); the propagation of the compression wave (II); the propagation and interaction of the reflected tensile wave (III); and the nucleation, growth, and coalescence of voids (IV). The effect of the microstructure on the evolution of defect densities during these four stages is characterized and quantified for single crystal Cu as well as nanocrystalline Cu with an average grain size of 6 nm, 10 nm, 13 nm, 16 nm, 20 nm, and 30 nm. The evolution of twin densities during the shock propagation is observed to vary with the grain size of the system and affects the spall strength of the metal. The grain sizes o...

An angular-dependent embedded atom method (A-EAM) interatomic potential to model thermodynamic and mechanical behavior of Al/Si composite materials

A new interatomic potential is developed for the Al/Si system in the formulation of the recently developed angular-dependent embedded atom method (A-EAM). The A-EAM is formulated by combining the embedded atom method potential for Al with the Stillinger–Weber potential for Si. The parameters of the Al/Si cross-interactions are fitted to reproduce the structural energetics of Al/Si bulk alloys determined based on the results of density functional theory calculations and the experimentally observed mixing behavior of the AlSi liquid alloy at high temperatures. The ability to investigate the thermodynamic properties of the Al/Si system is demonstrated by computing the binary phase diagram of the Al–Si system as predicted by the A-EAM potential and comparing with that obtained using experiments. The ability to study the mechanical behavior of the Al/Si composite systems is demonstrated by investigating the micromechanisms related to dynamic failure of the Al/Si nanocomposites using MD simulations.

Edge effects on band gap energy in bilayer 2H-MoS2 under uniaxial strain

The potential of ultrathin MoS2 nanostructures for applications in electronic and optoelectronic devices requires a fundamental understanding in their electronic structure as a function of strain. Previous experimental and theoretical studies assume that an identical strain and/or stress state is always maintained in the top and bottom layers of a bilayer MoS2 film. In this study, a bilayer MoS2 supercell is constructed differently from the prototypical unit cell in order to investigate the layer-dependent electronic band gap energy in a bilayer MoS2 film under uniaxial mechanical deformations. The supercell contains an MoS2 bottom layer and a relatively narrower top layer (nanoribbon with free edges) as a simplified model to simulate the as-grown bilayer MoS2 flakes with free edges observed experimentally. Our results show that the two layers have different band gap energies under a tensile uniaxial strain, although they remain mutually interacting by van der Waals interactions. The deviation in their ba...

Shock wave propagation and spall failure in single crystal Mg at atomic scales

Large scale molecular dynamics (MD) simulations are carried out to investigate the wave propagation and failure behavior of single crystal Mg under shock loading conditions. The embedded atom method interatomic potential, used to model the Mg systems, is first validated by comparing the predicted Hugoniot behavior with that observed using experiments. The first simulations are carried out to investigate the effect of loading orientation on the wave propagation and failure behavior by shock loading the system along the [0001] direction (c-axis) and the [101¯0] direction using a piston velocity of 1500 m/s. The spall strength (peak tensile pressure prior to failure) is predicted to be higher for loading along the [101¯0] direction than that predicted for loading along the [0001] direction. To investigate the effect of shock pressure on the failure behavior and spall strength of the metal, the MD simulations are carried out using piston velocities of 500 m/s, 1000 m/s, 1500 m/s, and 2000 m/s for loading alon...

Quasi-coarse-grained dynamics: modelling of metallic materials at mesoscales

A computationally efficient modelling method called quasi-coarse-grained dynamics (QCGD) is developed to expand the capabilities of molecular dynamics (MD) simulations to model behaviour of metallic materials at the mesoscales. This mesoscale method is based on solving the equations of motion for a chosen set of representative atoms from an atomistic microstructure and using scaling relationships for the atomic-scale interatomic potentials in MD simulations to define the interactions between representative atoms. The scaling relationships retain the atomic-scale degrees of freedom and therefore energetics of the representative atoms as would be predicted in MD simulations. The total energetics of the system is retained by scaling the energetics and the atomic-scale degrees of freedom of these representative atoms to account for the missing atoms in the microstructure. This scaling of the energetics renders improved time steps for the QCGD simulations. The success of the QCGD method is demonstrated by the prediction of the structural energetics, high-temperature thermodynamics, deformation behaviour of interfaces, phase transformation behaviour, plastic deformation behaviour, heat generation during plastic deformation, as well as the wave propagation behaviour, as would be predicted using MD simulations for a reduced number of representative atoms. The reduced number of atoms and the improved time steps enables the modelling of metallic materials at the mesoscale in extreme environments.

Photoemission study of ternary to penternary Fe-based metallic glasses: Chemical analysis of surface and bulk

Bulk metallic glasses consisting of Fe, Mo, Cr, C, B, and Er have been investigated by x-ray photoelectron spectroscopy, aimed to elucidate the local atomic structure of the amorphous phase. In order to examine the electronic properties of this class of material, photon energy dependent measurements in combination with argon-ion irradiation were employed to identify and separate surface and bulk contributions to the spectra. The core levels suggest the presence of a carbon-rich surface layer with oxidized boron and metals, and metal carbides and borides in the bulk. Exposure to molecular oxygen and annealing experiments probe the chemical reactivity of the material. Formation of boron oxides at comparably low temperatures (300°C) might have consequences for the stability of the amorphous phase. We observe variations in binding energy of the Fe 3p core level with respect to the alloy composition, which indicate changes in the chemical state of iron.