An Atomistic Study of the Tensile Deformation of Carbon Nanotube-Polymethylmethacrylate Composites.

There has been growing interest in polymer/carbon nanotube (CNT) composites due to an exceptional enhancement in mechanical, structural, thermal, and electronic properties resulting from a small percentage of CNTs. However, the performance of these composites is influenced by the type of polymer used. PMMA is a polymer of particular interest among many other polymers because of its biomaterial applications due to its biocompatibility, non-toxicity, and non-biodegradability. In this research, we utilized a reactive force field to conduct molecular dynamics simulations to investigate changes in the mechanical properties of single-walled carbon nanotube (SWCNT)-reinforced Poly (methyl methacrylate) (PMMA) matrix composites. To explore the potential of SWCNT-reinforced PMMA composites in these applications, we conducted simulations with varying CNT diameters (0.542-1.08 nm), CNT volume fractions (8.1-16.5%), and temperatures (100 K-700 K). We also analyzed the dependence of Young's modulus and interaction energy with different CNT diameters, along with changes in fracture toughness with varying temperatures. Our findings suggest that incorporating a small amount of SWCNT into the PMMA polymer matrix could significantly enhance the mechanical properties of the resulting composite. It is also found that the double-walled carbon nanotube has roughly twice the tensile strength of SWCNT, while maintaining the same simulation cell dimensions.

Tuning the mechanical properties of functionally graded nickel and aluminium alloy at the nanoscale.

In this article, Molecular Dynamics (MD) simulation is used to investigate the tensile mechanical properties of functional graded Ni-Al (Ni3Al) alloy with Ni coating. The grading profile, temperature, crystallographic direction, and concentration of vacancy defects have been varied and corresponding changes in the tensile properties are reported. In general, it has been revealed that functional grading may reduce the ultimate tensile strength (UTS) of this homogeneous alloy but increase Young's modulus (YM). Furthermore, MD simulations suggest that elliptically graded Ni-Al alloy has the highest UTS at low temperature while at high temperature, the largest UTS is recorded for the parabolic grading. Besides, at any temperature, the parabolically graded Ni-Al alloy shows the largest YM, followed by linear grading and elliptical grading. Moreover, it is also observed that the [111] crystallographic direction for this alloy demonstrates the highest UTS and YM. At extremely low temperatures, lattice mismatch is also observed to exert a significant impact on the failure characteristics of functional graded Ni-Al alloys. This investigation also suggests that the vacancy defects introduced via removing either Al or Ni atoms degrades the UTS and YM of FGM alloys remarkably. Besides, it is also found that the UTS and YM of Ni-Al FGM alloys are very sensitive to Ni vacancies compared to Al vacancies. Parabolic grading demonstrates more resilience against vacancy defects, followed by linear and elliptical grading. This paper provides a comprehensive understanding of the mechanical properties of Ni-Al FGM alloys at the atomic level as a potential substitute for homogeneous alloys.

Investigation on the mechanical properties and fracture phenomenon of silicon doped graphene by molecular dynamics simulation.

Silicon doping is an effective way to modulate the bandgap of graphene that might open the door for graphene to the semiconductor industries. However, the mechanical properties of silicon doped graphene (SiG) also plays an important role to realize its full potential application in the electronics industry. Electronic and optical properties of silicon doped graphene are well studied, but, our understanding of mechanical and fracture properties of the doped structure is still in its infancy. In this study, molecular dynamics (MD) simulations are conducted to investigate the tensile properties of SiG by varying the concentration of silicon. It is found that as the concentration of silicon increases, both fracture stress and strain of graphene reduces substantially. Our MD results also suggest that only 5% of silicon doping can reduce the Young's modulus of graphene by ∼15.5% along the armchair direction and ∼13.5% along the zigzag direction. Tensile properties of silicon doped graphene have been compared with boron and nitrogen doped graphene. The effect of temperature, defects and crack length on the stress-strain behavior of SiG has also been investigated. Temperature studies reveal that SiG is less sensitive to temperature compared to free stranding graphene, additionally, increasing temperature causes deterioration of both fracture stress and strain of SiG. Both defects and cracks reduce the fracture stress and fracture strain of SiG remarkably, but the sensitivity to defects and cracks for SiG is larger compared to graphene. Fracture toughness of pre-cracked SiG has been investigated and results from MD simulations are compared with Griffith's theory. It has been found that for nano-cracks, SiG with larger crack length deviates more from Griffith's criterion and the degree of deviation is larger compared to graphene. Fracture phenomenon of pre-cracked SiG and the effect of strain rate on the tensile properties of SiG have been reported as well. These results will aid the design of SiG based semiconducting nanodevices.

Inherent mechanical properties of bilayer germanene coupled by covalent bonding.

Germanene, a two-dimensional buckled hexagonal structure of germanium atoms, has attractive mechanical, optical, thermal and electronic features. Recently it has been reported that covalent bonding between two monolayer germanene sheets leads to the integration of intrinsic magnetism and band gap opening that makes it attractive to future nanoelectronics. In order to use the captivating features of this structure, its mechanical characterization needs to be studied. In this study, molecular dynamics simulations have been performed using optimized Tersoff potential to analyze the effect of chirality, temperature and strain rate on the uniaxial tensile properties of this structure. This study suggests that bonded bilayer germanene shows higher mechanical strength compared to monolayer germanene. Uniaxial loading in the armchair direction shows higher fracture strength and strain compared to the zigzag direction which is contrary to the monolayer germanene. It also reports that with increasing temperature, both the fracture strength and strain of the structure decrease. It has been found that at a higher strain rate, the material exhibits higher fracture strength and strain. Mechanical properties and fracture mechanisms of defected structures have also been reported below the curie temperature. Moreover, the interlayer shear characteristics of the bilayer structure have been looked into. These results will provide significant insight to the investigation of this structure as a potential nano-electronics substitute.

Atomistic investigations on the mechanical properties and fracture mechanisms of indium phosphide nanowires.

The mechanical properties of indium phosphide (InP) nanowires are an emerging issue due to the promising applications of these nanowires in nanoelectromechanical and microelectromechanical devices. In this study, molecular dynamics simulations of zincblende (ZB) and wurtzite (WZ) crystal structured InP nanowires (NWs) are presented under uniaxial tension at varying sizes and temperatures. It is observed that the tensile strengths of both types of NWs show inverse relationships with temperature, but are independent of the size of the nanowires. Moreover, applied load causes brittle fracture by nucleating cleavage on ZB and WZ NWs. When the tensile load is applied along the [001] direction, the direction of the cleavage planes of ZB NWs changes with temperature. It is found that the {111} planes are the cleavage planes at lower temperatures; on the other hand, the {110} cleavage planes are activated at elevated temperatures. In the case of WZ NWs, fracture of the material is observed to occur by cleaving along the (0001) plane irrespective of temperature when the tensile load is applied along the [0001] direction. Furthermore, the WZ NWs of InP show considerably higher strength than their ZB counterparts. Finally, the impact of strain rate on the failure behavior of InP NWs is also studied, and higher fracture strengths and strains at higher strain rates are found. With increasing strain rate, the number of cleavages also increases in the NWs. This paper also provides in-depth understanding of the failure behavior of InP NWs, which will aid the design of efficient InP NWs-based devices.

Tuning the mechanical properties of silicene nanosheet by auxiliary cracks: a molecular dynamics study.

Silicene has become a topic of interest nowadays due to its potential application in various electro-mechanical nanodevices. In our previous work on silicene, fracture stresses of single crystal and polycrystalline silicene have been investigated. Existence of defects in the form of cracks reduces the fracture strength of silicene nanosheets to a great extent. In this study, an engineering way has been proposed for improving the fracture stress of silicene nanosheets with a pre-existing crack by incorporating auxiliary cracks symmetrically in a direction perpendicular to the main crack. We call this mechanism the "Failure shielding mechanism". An extensive molecular dynamics simulation based analysis has been performed to capture the atomic level auxiliary crack-main crack interactions. It is found that the main crack tip stress distribution is significantly changed with the presence of auxiliary cracks for loading along both armchair and zigzag directions. The effects of temperature and the crack propagation speed of silicene have also been studied. Interestingly, in the case of loading along the zigzag direction, SW defect formation is observed at the tip of main crack. This leads to a reduction of the tip stress resulting in a more prominent failure shielding in case of zigzag loading than in armchair loading. Moreover, the position and length of the cracks as well as the loading directions have significant impacts on the tip stress distribution. Finally, this study opens the possibilities of strain engineering for silicene by proposing an engineering way to tailor the fracture strength of silicene.

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.