Tawfiqur Rakib

Graphene and its elemental analogue: A molecular dynamics view of fracture phenomenon

Abstract Graphene and some graphene like two dimensional materials; hexagonal boron nitride (hBN) and silicene have unique mechanical properties which severely limit the suitability of conventional theories used for common brittle and ductile materials to predict the fracture response of these materials. This study revealed the fracture response of graphene, hBN and silicene nanosheets under different tiny crack lengths by molecular dynamics (MD) simulations using LAMMPS. The useful strength of these two dimensional materials are determined by their fracture toughness. Our study shows a comparative analysis of mechanical properties among the elemental analogues of graphene and suggested that hBN can be a good substitute for graphene in terms of mechanical properties. We have also found that the pre-cracked sheets fail in brittle manner and their failure is governed by the strength of the atomic bonds at the crack tip. The MD prediction of fracture toughness shows significant difference with the fracture toughness determined by Griffths theory of brittle failure which restricts the applicability of Griffiths criterion for these materials in case of nano-cracks. Moreover, the strengths measured in armchair and zigzag directions of nanosheets of these materials implied that the bonds in armchair direction have the stronger capability to resist crack propagation compared to zigzag direction.

Atomistic Representation of Anomalies in the Failure Behaviour of Nanocrystalline Silicene

Silicene, a 2D analogue of graphene, has spurred a tremendous research interest in the scientific community for its unique properties essential for next-generation electronic devices. In this work, for the first time, we present a molecular dynamics (MD) investigation to determine the fracture strength and toughness of nanocrystalline silicene (nc-silicene) sheet of varying grain sizes and pre-existing cracks at room temperature. Our results suggest a transition from an inverse pseudo Hall-Petch to a pseudo Hall-Petch behaviour in nc-silicene at a critical grain size of 17.32 nm. This phenomenon is also prevalent in nanocrystalline graphene. However, nc-silicene with pre-existing cracks exhibits anomalous crack propagation and fracture toughness behaviour. We observed two distinct types of failure mechanisms (crack sensitive and insensitive failure) and devised mechano-physical conditions under which they occur. The most striking outcome is: despite the presence of a pre-existing crack, the crack sensitivity of nc-silicene is found to be dependent on the grain size and their orientations. The calculated Fracture toughness from both Griffith’s theory and MD simulations indicate that the former over-predicts the fracture toughness of nc-silicene. Finally, this study is the first direct comparison of atomistic simulations to the continuum theories to predict the anomalous behaviour in deformation and failure mechanisms of nc-silicene.

MHD mixed convection analysis in an open channel by obstructed Poiseuille flow of non-Newtonian power law fluid

This paper demonstrates magneto-hydrodynamic (MHD) mixed convection flow through a channel with a rectangular obstacle at the entrance region using non-Newtonian power law fluid. The obstacle is kept at uniformly high temperature whereas the inlet and top wall of the channel are maintained at a temperature lower than obstacle temperature. Poiseuille flow is implemented as the inlet velocity boundary condition. Grid independency test and code validation are performed to justify the computational accuracy before solving the present problem. Galerkin weighted residual method has been appointed to solve the continuity, momentum and energy equations. The problem has been solved for wide range of pertinent parameters like Richardson number (Ri = 0.1 - 10) at a constant Reynolds number (Re = 100), Hartmann number (Ha = 0 - 100), power index (n = 0.6 - 1.6). The flow and thermal field have been thoroughly discussed through streamline and isothermal lines respectively. The heat transfer performance of the given st...

Temperature and size effect on the mechanical properties of indium phosphide nanowire: An atomistic study

The mechanical properties of Indium Phosphide (InP) nanowire is an emerging issue due to its application as optoelectronic material. In this paper, atomistic simulations are conducted to find thermo-mechanical properties of Indium Phosphide (InP) nanowire under uniaxial tension. Vashishta potential is employed to define the atomic interactions between the atoms. The effect of variation of temperatures (100K-500K) on the tensile response of the InP nanowires is investigated in this study. Also, size effect is investigated for the temperature of 300 K by varying the cross sectional area of the nanowire. Results suggest that increment of temperature results in the failure of InP nanowire at a lower value of stress (from 8.60 GPa at 100K to 6.50 GPa at 500K) along with the decrement of Young’s modulus. Results also suggest that size has little effect on the tensile properties of this nanowire. Finally, failure mechanisms of indium phosphide nanowire are also investigated from the atomic images obtained from the simulation results.The mechanical properties of Indium Phosphide (InP) nanowire is an emerging issue due to its application as optoelectronic material. In this paper, atomistic simulations are conducted to find thermo-mechanical properties of Indium Phosphide (InP) nanowire under uniaxial tension. Vashishta potential is employed to define the atomic interactions between the atoms. The effect of variation of temperatures (100K-500K) on the tensile response of the InP nanowires is investigated in this study. Also, size effect is investigated for the temperature of 300 K by varying the cross sectional area of the nanowire. Results suggest that increment of temperature results in the failure of InP nanowire at a lower value of stress (from 8.60 GPa at 100K to 6.50 GPa at 500K) along with the decrement of Young’s modulus. Results also suggest that size has little effect on the tensile properties of this nanowire. Finally, failure mechanisms of indium phosphide nanowire are also investigated from the atomic images obtained from t...

Shear based analysis of nickel nano-plate by molecular dynamics simulations

The determination of shear based properties of Nickel (Ni) has a great importance since it is more likely to fail by shear than tension due to its ductile nature. It also features a wide variety of applications in structure, thin film, tubes, and plates due to its unique thermal and electrical properties. Molecular Dynamics Simulations were performed on Ni nano-plate subjected to shear loading to study the effect of voids in the structure using embedded atom method (EAM) potential. The shear stress-strain behavior was observed for Ni nano-plate with voids of 1.0 nm, 1.5 nm, and 2.0 nm radius. Snapshots taken at different strains show the formation of slip planes, crack propagation, and dislocation activity. Simulation results show that the modulus of rupture decreases with the increase of void radius due to more dislocation activity for larger void. Lastly, the effect of different void size on the shear modulus of rigidity is also incorporated.The determination of shear based properties of Nickel (Ni) has a great importance since it is more likely to fail by shear than tension due to its ductile nature. It also features a wide variety of applications in structure, thin film, tubes, and plates due to its unique thermal and electrical properties. Molecular Dynamics Simulations were performed on Ni nano-plate subjected to shear loading to study the effect of voids in the structure using embedded atom method (EAM) potential. The shear stress-strain behavior was observed for Ni nano-plate with voids of 1.0 nm, 1.5 nm, and 2.0 nm radius. Snapshots taken at different strains show the formation of slip planes, crack propagation, and dislocation activity. Simulation results show that the modulus of rupture decreases with the increase of void radius due to more dislocation activity for larger void. Lastly, the effect of different void size on the shear modulus of rigidity is also incorporated.

Influence of defects on thermal properties of stanene

Stanene is a two-dimensional, graphene-like honeycomb structure material, has been synthesized in a recent experimental study. Theoretically, it is expected to have a super conductive property near room temperature due to its spin orbital coupling effect. It is a potential material for the next generation nano-electronics application. Therefore, studying its thermal property is of particular interest. In this paper, we investigated the effect of different types of defects on the thermal conductivity of stanene nanosheets. Molecular Dynamics simulations are performed to calculate the thermal conductivity as a function of various types of defects. MEAM potential is used to describe the inter-atomic forces. It has been found that the presence of defects reduces the thermal conductivity significantly. Finally, vibrational density of states (DOS) are calculated to elucidate the underlying mechanisms of the reduction of thermal conductivity.

MHD mixed convection analysis of non-Newtonian power law fluid in an open channel with round cavity

In this study, magneto-hydrodynamic (MHD) mixed convection flow through a channel with a round cavity at bottom wall using non-Newtonian power law fluid is analysed numerically. The cavity is kept at uniformly high temperature whereas rest of the bottom wall is insulated and top wall of the channel is maintained at a temperature lower than cavity temperature. Grid independency test and code validation are performed to justify the computational accuracy before solving the present problem. Galerkin weighted residual method is appointed to solve the continuity, momentum and energy equations. The problem is solved for wide range of pertinent parameters like Rayleigh number (Ra= 103 – 105), Hartmann number (Ha= 0 - 60) and power law index (n= 0.5 – 1.5) at constant Richardson number Ri= 1.0. The flow and thermal field have been thoroughly discussed through streamline and isothermal lines respectively. The heat transfer performance of the given study is illustrated by average Nusselt number plots. Result of thi...