Data analytics approach to predict the hardness of copper matrix composites

Copper matrix composite materials have exhibited a high potential in applications where excellent conductivity and mechanical properties are required. In this study, the machine learning models have been applied to predict the hardness of the copper matrix composite materials (CuMCs) produced via the powder metallurgy technique. Six different machine learning regression models were employed. The observed CuMCs were reinforced with two different volume fractions (2 vol.% and 7vol.%) of ZrB2 particles. Based on experimental work, we extracted the independent variables (features) like the milling time (MT, Hours), dislocation density (DD, m-2), average particle size (PS, μm), density (ρ, g/cm3), and yield stress (σ, MPa) while the Vickers hardness (MPa) was used as the dependent variable. Feature selection was performed by calculation the Pearson correlation coefficient (PCC) between the independent and dependent variables. The predictive accuracy higher than 80% was achieved for Cu-7vol.% ZrB2 and lower for the Cu-2vol.% ZrB2

Ab initio calculations of structural and electronic properties of CdTe clusters

We present results of a study of small stoichiometric $Cd_{n}Te_{n}$ ($1{\leq}n{\leq}6$) clusters and few medium sized non-stoichiometric $Cd_{m}Te_{n}$ [($m,n= 13, 16, 19$); ($m{\neq}n$)] clusters using the Density Functional formalism and projector augmented wave method within the generalized gradient approximation. Structural properties {\it viz.} geometry, bond length, symmetry and electronic properties like HOMO-LUMO gap, binding energy, ionization potential and nature of bonding {\it etc.} have been analyzed. Medium sized non-stoichiometric clusters were considered as fragments of the bulk with T{$_{d}$} symmetry. It was observed that upon relaxation, the symmetry changes for the Cd rich clusters whereas the Te rich clusters retain their symmetry. The Cd rich clusters develop a HOMO-LUMO gap due to relaxation whereas there is no change in the HOMO-LUMO gap of the Te rich clusters. Thus, the symmetry of a cluster seems to be an important factor in determining the HOMO-LUMO gap.Comment: 8 pages 16 figure

First principle study of free and surface terminated CdTe nanoparticles

Density functional calculations of structural and electronic properties of stoichiometric and nonstoichiometric CdTe clusters, containing up to few tens of atoms, are carried out using projector augmented wave method. Molecular dynamics has been performed for Cd12Te12 and Cd15Te15 to predict the structure corresponding to global energy minimum. Cage type structures and bulk fragments, both in zinc blende and wurtzite structures, are used as starting geometries and conjugate gradient method is used to locate the local energy minima for other clusters. The aim of these calculations is to get the energetically favorable probable structures, to be compared with the experimentally known structures. Clusters are relaxed both in vacuum and in the presence of surface passivating ligands and the resulting structural rearrangement is analyzed. As expected, passivation increases the stability of an individual cluster, as indicated by specific properties like binding energy, vertical detachment energy, electron affinity etc. Passivation also locks the symmetry for three-dimensional structures but the small CdnTen (1 ≤ n ≤ 6) clusters, which are planar, attain higher symmetry structures on passivation. We observe `self-healing' mechanism viz., opening of optical gap on relaxation without the aid of passivating ligand, in CdTe clusters as observed in CdSe clusters [A. Puzder et al., Phys. Rev. Lett. 92, 217401 (2004)]. However, we note that 'self-healing' is a stoichiometry dependent phenomenon. Te atoms are found to achieve a total coordination of 4 on passivation, a fact useful in chemical synthesis of nanoclusters

Defect studies in small CdTe clusters

We study Cd vacancy formation in prototype stoichiometric and non-stoichiometric CdTe clusters with and without passivation. For certain clusters like Cd13Te16, vacancy leads to severe distortion of the geometry due to propagation of defect. Annealing of the vacancy out of the cluster is observed in all unpassivated clusters. Passivated clusters retain their initial geometry and vacancy induced structural distortions are not seen in these clusters since the defect gets localized. Vacancy also induces intragap states. However, it was observed that the passivation of the dangling bonds created by the vacancy removes the intragap states. In an attempt to have CdTe clusters with extrinsic carriers, we substituted a Cd atom by its adjacent atoms Pd/Ag/In/Sn in these CdTe clusters. Substitutional doping of Cd by metal atoms increases the stability of unpassivated clusters. For certain clusters, metal atom doping leads to a half-metallic character. Pd/Ag-doped clusters are p-type semiconductors whereas In-doped clusters are n-type semiconductors. Sn doping in these clusters does not result in n-type semiconductors

Effect of Si on the oxidation reaction of α-Ti(0 0 0 1) surface: ab initio molecular dynamics study

We present our ab initio molecular dynamics (MD) study of the effect of Si on the oxidation of α-Ti(0 0 0 1) surfaces. We varied the Si concentration in the first layer of the surface from 0 to 25 at.% and the oxygen coverage (θ) on the surface was varied up to 1 monolayer (ML). The MD was performed at 300, 600 and 973 K. For θ = 0.5 ML, oxygen penetration into the slab was not observed after 16 ps of MD at 973 K while for θ > 0.5 ML, oxygen penetration into the Ti slab was observed even at 300 K. From Bader charge analysis, we confirmed the formation of the oxide layer on the surface of the Ti slab. At higher temperatures, the Si atoms diffused from the first layer to the interior of the slab, while the Ti atoms moved from second layer to the first layer. The pair correlation function shows the formation of a disordered Ti-O network during the initial stage of oxidation. Si was found to have a strong influence on the penetration of oxygen in the Ti slab at high temperatures

A Molecular Dynamics Study of the Role of Adatoms in SAMs of Methylthiolate on Au(111): A New Force Field Parameterized from Ab Initio Calculations

Starting from ab initio calculations and using a force matching procedure, we have developed a new force field for molecular dynamics simulations of self-assembled monolayers of methylthiolate (MT) on Au(111) surfaces. This new force field is able to reproduce several observed features of SAMs of MT on Au(111) surface, such as the formation of gold vacancy islands and the (√3 × √3)<i>R</i>30 lattice. We have studied the dynamics of Au adatoms and monatomic vacancies on the Au(111) surface for the SAM of MT at room temperature. It is observed that monatomic vacancies coarsen to form large vacancy islands while the adatoms group to form clusters. Both results are in agreement with experiments. At elevated temperatures, Au adatoms that are lifted from the surface leave an atomic vacancy on it. The liquid-like diffusion of gold adatoms on the SAM surface occurs by hopping between pairs of methylthiolate to which the adatom is temporarily bound. Our findings indicate that structural models of the <i>c</i>(4 × 2) unit cell including adatoms and vacancies at room temperature need to be revisited