Mesut Kirca

Ligament and joint sizes govern softening in nanoporous aluminum

The present computational study demonstrates that softening of an open cell nanoporous aluminum structure subjected to tensile loading can be significantly reduced when the size of ligaments and the joints that connect them in the structure is designed to be sufficiently small. It is found using molecular dynamics simulations that the softening becomes slightly slower with increasing porosity for the structures with porosity less than or equal to 72%, and stress localization is observed during softening. In contrast, for structures with more than 75% porosity, softening is much slower, and stress delocalization occurs during softening. It is argued that at relatively high porosity, softening is governed by both the ligament size and the joint size because their compliance becomes high enough to allow the overloading stress due to ligament rupture to be redistributed more effectively throughout the structure.

Mechanical properties of SWNT X-Junctions through molecular dynamics simulation

The mechanical behavior of seven different carbon nanotube (CNT) X-junctions with a varying number of bonds was investigated through molecular dynamics simulations. The X-junctions are composed of two (6,0) single-walled carbon nanotubes (SWNTs) created via vibration-assisted heat welding. The junctions, containing anywhere between one and seven bonds, are subject to uniaxial tensile, shear and torsional strain, and then the stiffness values are determined for each case. When subjected to tensile and shear strain, both the arrangement and orientation of bonds are found to affect the stiffness of junctions more substantially than the number of bonds, bond length or bond order. Surprisingly, anisotropic shear behavior is observed in the X-junctions, which can be attributed to the junctions bond orientation. Also, the stiffness of X-junctions tested under an applied torque (torsion) differs from the stiffness under tensile and shear strain, however, in that it is more substantially affected by the number of bonds present in the junction than by any other property.

Surface structure and properties of functionalized nanodiamonds: a first-principles study

The goal of this work is to gain fundamental understanding of the surface and internal structure of functionalized detonation nanodiamonds (NDs) using quantum mechanics based density functional theory (DFT) calculations. The unique structure of ND assists in the binding of different functional groups to its surface which in turn facilitates binding with drug molecules. The ability to comprehensively model the surface properties, as well as drug-ND interactions during functionalization, is a challenge and is the problem of our interest. First, the structure of NDs of technologically relevant size (∼5 nm) was optimized using classical mechanics based molecular mechanics simulations. Quantum mechanics based density functional theory (DFT) was then employed to analyse the properties of smaller relevant parts of the optimized cluster further to address the effect of functionalization on the stability of the cluster and reactivity at its surface. It is found that functionalization is preferred over reconstruction at the (100) surface and promotes graphitization in the (111) surface for NDs functionalized with the carbonyl oxygen (C = O) group. It is also seen that the edges of ND are the preferred sites for functionalization with the carboxyl group (-COOH) vis-à-vis the corners of ND.

Atomistic simulation of Voronoi-based coated nanoporous metals

In this study, a new method developed for the generation of periodic atomistic models of coated and uncoated nanoporous metals (NPMs) is presented by examining the thermodynamic stability of coated nanoporous structures. The proposed method is mainly based on the Voronoi tessellation technique, which provides the ability to control cross-sectional dimension and slenderness of ligaments as well as the thickness of coating. By the utilization of the method, molecular dynamic (MD) simulations of randomly structured NPMs with coating can be performed efficiently in order to investigate their physical characteristics. In this context, for the purpose of demonstrating the functionality of the method, sample atomistic models of Au/Pt NPMs are generated and the effects of coating and porosity on the thermodynamic stability are investigated by using MD simulations. In addition to that, uniaxial tensile loading simulations are performed via MD technique to validate the nanoporous models by comparing the effective Youngs modulus values with the results from literature. Based on the results, while it is demonstrated that coating the nanoporous structures slightly decreases the structural stability causing atomistic configurational changes, it is also shown that the stability of the atomistic models is higher at lower porosities. Furthermore, adaptive common neighbour analysis is also performed to identify the stabilized atomistic structure after the coating process, which provides direct foresights for the mechanical behaviour of coated nanoporous structures.

Effects of ultrathin coating on the tensile behavior of nanoporous gold

In this study, the mechanical properties of nanoporous gold (np-Au) coated with different ultrathin metallic materials (i.e., platinum and silver) are studied through molecular dynamics simulations. A new atomistic modelling technique, which is based on the Voronoi tessellation method providing periodic atomistic specimens, is used for the geometric representation of np-Au structure. Three different coating thickness values are used to examine the role of thickness on the coating performance under tensile loading at a constant strain rate. Several parameters, including Youngs modulus, yield, and ultimate strengths, are utilized to compare the mechanical characteristics of coated and uncoated np-Au specimens. Moreover, adaptive common neighbor analyses are performed on the specimens for the purpose of understanding the deformation mechanisms of coated and uncoated nanoporous specimens comprehensively by monitoring the microstructural evolution of the crystal structure of the specimens within the deformati...

Structure and Surface Properties of Nanodiamonds: A First-Principles Multiscale Approach

The goal of this work is to gain fundamental understanding of the surface structure of functionalized detonation nanodiamonds (NDs) using quantum mechanics (QM) based multiscale modeling and simulation. The study entails a multiscale approach to bridge the length scale between the real sizes of ND being fabricated (∼4 nm) and the size allowed by employing first-principles based modeling (< 1 nm). At first, the structure of NDs of technologically relevant size (∼4 nm) was optimized using classical mechanics based molecular mechanics simulations. QM based density functional theory (DFT) was then employed to simulate the structure and analyze the properties of relevant parts of the optimized cluster. This work is extended to NDs functionalized with carboxylic acid (-COOH) and carbonyl oxygen (=O), which help to guide further experiments on functionalization of NDs and their use as carriers of drug molecules to the desired site.Copyright