Results Of Molecular Dynamics Modeling Research Articles

Enhanced corrosion inhibition performance of 5-amino-3‑bromo-1-methylindazole on copper in sulfuric acid environments

The corrosion inhibitor 5-amino-3‑bromo-1-methylindazole (ABMI) with multiple substituents can effectively protect the copper substrate. According to the electrochemical experiment results, at 298 K, 3 mM ABMI may suppress copper corrosion by 92.1 %. Furthermore, going from 298 K to 308 K raises the temperature at which 3 mM ABMI may continue to suppress corrosion with an efficiency exceeding 90 %. The weight loss test, SEM, AFM, and XRD data all reveal that the presence of ABMI efficiently prevents the corrosion media from dissolving the copper substrate. The ΔGads0 value of ABMI is −32.97 kJ/mol, demonstrating that it works on the metal/solution interface through physical adsorption and chemisorption. The results of molecular dynamics modeling and quantum chemical computation show that ABMI adsorbs to the metal superficies in a nearly parallel way, minimizing the exposure area among the metal superficies with the medium with corrosive substances and providing the best possible protection for copper.

DETECTION OF STRUCTURAL CHANGES IN A BCC CRYSTAL-LE DYNAMICS UNDER LASER INFLUENCE BY THE METHOD OF MOLECULAR

The presented work presents the results of molecular dynamics modeling of changes in the surface layer of the computational cell under a short-term high-energy impact. Interest in this topic is because the processes occurring in the surface layer, which is in a liquid state, will subsequently have an impact during its crystallization, and, as a result, will affect various physical and geometric characteristics of the surface of the material as a whole. The model constructed and described in the work, in which the temperature of the computational cell is distributed in accordance with the solution of the linear problem of heat conduction, made it possible to reveal the discontinuity of the surface layer, which consists in the localization of excessfree volume in the form of a group of spherical pores. The sizes of these imperfections, as well as the duration of their existence, have differences when modeling different energy densities of laser radiation. Further research made it possible to reveal the conditions under which the pores remain stable throughout the entire simulation time, as well as to reveal the relationship between the crystallographic orientation of the “solid-liquid” interface and the sizes of the formed pores.

Study of the Effect of Laser Ablation on the Structure of a BCC Crystal Using Molecular Dynamics Simulation

An external high-energy impact on the surface of a solid, for example, by ultrashort ultra-high-power laser pulses, can lead to phase transitions, as a result of which the surface structure undergoes cardinal changes. This paper presents the results of molecular dynamics modeling of changes in the surface layer of the computational cell under a short-term high-energy impact. The model constructed and described in the paper, in which the temperature of the computational cell is distributed in accordance with the solution of the linear problem of heat conduction, made it possible to reveal a violation of the continuity of the surface layer, which consists in the localization of excess free volume in the form of a group of spherical pores. The sizes of these imperfections, as well as the duration of their existence, have differences when modeling different energy densities of laser radiation. Further research made it possible to reveal the conditions under which the pores remain stable throughout the entire simulation time, as well as to reveal the relationship between the crystallographic orientation of the "solid — liquid" interface and the sizes of the formed pores.

Relation between defects and crystalline thermal conduction

Modeling of the heat transfer in ideal crystal lattice with defects is performed for measuring the heat conductivity coefficient. A non-steady process in closed system is studied. The method is based on comparison of the results of molecular dynamics simulation and solution of the heat equation. Two-dimensional and three-dimensional structures with dense packing of particles are considered. Defects are modeled by removing or changing the mass of randomly selected lattice atoms. Based on the results of molecular-dynamics modeling, an empirical dependence of the thermal diffusivity on the density of defects is elaborated. It also turns out in a good agreement with experimental data.

Etching with atomic precision by using low electron temperature plasma

There has been a steady increase in sub-nm precision requirement for many critical plasma etching processes in the semiconductor industry. In addition to high selectivity and low controllable etch rate, an important requirement of atomic precision etch processes is no (or minimal) damage to the remaining material surface. It has traditionally not been possible to avoid damage in conventional radio-frequency (RF) plasma processing systems, even during layer-by-layer or ‘atomic layer’ etch. To meet these increasingly stringent requirements, it is necessary to have an accurate control over ion energy and ion/radical composition during plasma processing. In this work, a new plasma etch system designed to facilitate atomic precision plasma processing is presented. An electron sheet beam parallel to the substrate surface is used to produce a plasma in this system. This plasma has a significantly lower electron temperature Te ~ 0.3 eV and ion energy Ei < 3 eV (without applied bias) compared to inductively and capacitively coupled RF plasmas. Electron beam plasmas also have a higher ion–to–radical ratio compared to RF plasmas, so this plasma etch system employs an independent radical source for accurate control over relative ion and radical concentrations. A low frequency RF bias capability that allows control of ion energy in the 2–50 eV range is another important component of this plasma etch system. The results of etching of a variety of materials and structures in this low-electron temperature plasma system are presented in this study: (1) layer-by-layer etching of p-Si at Ei ~ 25–50 eV using electrical and gas cycling is demonstrated; (2) continuous etching of epi-grown µ-Si in Cl2-based plasmas is performed, showing that surface damage can be minimized by keeping Ei < 10 eV. Also presented are the results of molecular dynamics modeling of atomic precision etching at low Ei.

Radiation stability of nanomaterials

Attention is called to three possible results of nanomaterial irradiation: interfaces working as sinks for radiation defects, nanostructure amorphization, and the enhancement of recrystallization. In this context, the radiation effect on the structure and physicomechanical properties of metal-, alloy-, and compound-based nanomaterials is analyzed. Microscopic approaches and results of molecular dynamics modeling are described. Little-studied questions are pointed out.

Three-dimensional atomistic modeling of microcrack propagation in iron

This paper presents the results of molecular dynamics modeling of quasi-brittle microcrack propagation on the (0 0 1) plane of bcc iron. The nucleation and propagation of such cracks is accompanied by the emission of slip bands from the crack tips. The 3D nature of the model, in conjunction with a Voronoi decomposition post-processing technique, allows us to clarify the atomistic structure of slip bands and to follow the detailed kinetics of slip band expansion and interaction with other microstructural defects. The effects of simulation temperature and alternative defects (in particular, vacancies and micro-voids) on the crack propagation kinetics are discussed.

STUDY ON NANOMETRIC CUTTING MECHANISM AND BURR FORMATION USING MOLECULAR DYNAMICS SIMULATION

Since no physical approach can be employed to study the mechanism in micro cutting, the molecular dynamics simulation is becoming more and more important. In this study, the results of molecular dynamics modeling and analysis on the nanometric machining on silicon surface are presented. According to the simulation, some phenomena in the nanometric cutting process are found. First, surface elastic rebound happens on the cut surface after cutter moving away. The value of the surface elastic rebound is calculated in the simulation. Second, the atoms near the corner of work piece swirl up following the cutter moving direction at the initial stage of removing atoms from the work piece. Third, the simulation results show that no matter how small material removal is, the burr is always formed at the edge of work piece.

The influence of a nanocrystal size on the results of molecular-dynamics modeling

In connection with the rapid progress in nanotechnology, a need has presently arisen in obtaining nanostructures with desirable properties by processes using various thermomechanical actions exerted on starting structures. Previously, we performed a molecular-dynamics study of the mechanical failure of a three-dimensional defect-free copper nanocrystal under uniaxial tension. Based on the mesoanalysis, such mechanical characteristics of the nanocrystal were obtained as the stress vs strain relation and the Young modulus. Local criteria of dynamical fracture have been established. In the present work, data concerning the effect of nanocrystal size on atomistically modeled characteristics of copper nanocrystals are reported. A certain critical size of six crystal cells has been found in a copper nanocrystal. The nanostructure sizes being over this value, the calculated characteristics do not depend on nanocrystal size.

Glass formation in amorphous SiO2 as a percolation phase transition in a system of network defects

Thermodynamic parameters of defects (presumably, defective SiO molecules) in the network of amorphous SiO2 are obtained by analyzing the viscosity of the melt with the use of the Doremus model. The best agreement between the experimental data on viscosity and the calculations is achieved when the enthalpy and entropy of the defect formation in the amorphous SiO2 network are Hd=220 kJ/mol and Sd=16.13R, respectively. The analysis of the network defect concentration shows that, above the glass-transition temperature (Tg), the defects form dynamic percolation clusters. This result agrees well with the results of molecular dynamics modeling, which means that the glass transition in amorphous SiO2 can be considered as a percolation phase transition. Below Tg, the geometry of the distribution of network defects is Euclidean and has a dimension d=3. Above the glass-transition temperature, the geometry of the network defect distribution is non-Euclidean and has a fractal dimension of df=2.5. The temperature Tg can be calculated from the condition that percolation arises in the defect system. This approach leads to a simple analytic formula for the glass-transition temperature: Tg=Hd/((Sd+1.735R). The calculated value of the glass-transition temperature (1482 K) agrees well with that obtained from the recent measurements of Tg for amorphous SiO2 (1475 K).

The application of fractal and quantum geometry to brittle fracture

This paper examines three apparently disparate views of fracture in brittle materials with the purpose of showing the interrelationship of the fracture process at different length scales. Quantitative fractographic analysis of brittle fracture surfaces shows that there are characteristic markings on the surfaces that are self-similar and scale invariant, implying that fractal analysis is a reasonable approach to analyzing these surfaces. The fractal dimension is directly proportional to the fracture energy, γ, during fracture for many brittle materials, i.e., γ= 1 2 Ea 0D ∗ where E is the elastic modulus, a 0 a structural parameter and D * is the fractal dimensional increment. Analysis of previous results of molecular dynamics modeling shows that the fractal nature of the fracture surface is consistent with the predicted surface produced during simulated fracture. The fractal dimensional increment, D *, of the simulated fracture surface in a silica glass and silicon single crystal over a single order of magnitude matched well with those values measured on fracture surfaces of beams fractured in flexure for the same materials at length scales 1000–100 000 times larger. Finally, a ring contraction mechanism, modeled using semi-empirical quantum mechanical molecular orbital methods, is shown to be a likely step in the fracture of silica tetrahedra along the crack front. The geometry of the structure formed at the atomic scale from these ring contractions can be related to a 0 and leads to the basis for propagation into a fractal structure. Thus, we have identified a reasonable atomic level foundation for the structural parameter, a 0. Based on a comparison of atomic and molecular modeling with macroscopically measured values of D * and a 0, we suggest that the fracture process is a percolation of a series of ring contractions along the crack front, which result in the observed fracture surfaces for several brittle materials.

Structural Characterization of Ti-Silicalite-1: A Synchrotron Radiation X-Ray Powder Diffraction Study

We present high-resolution X-ray powder diffraction (XRPD) data collected at the BM16 beamline at ESRF on a set of dehydrated Ti-silicalite-1 (TS-1) samples having Ti content (molar ratio)x=[Ti]/([Ti]+[Si]) in the range 0–0.022. The cell volume values resulting from Rietveld refinements of the powder diffraction data exhibit very good linear correlation (r=0.99994) with Ti content:V(x)=2093.0x+V(0) [V(0)=5335.8 Å3]. This work represents a continuation of the important characterization work of the EniRicerche group [J. Catal.137, 497 (1992)] performed on TS-1 previously treated with ammonium acetate, calcined, and measured in atmosphere (i.e., under partially hydrous conditions). Our results indicate that when XRPD measurements are performed under carefully controlled vacuum conditions a highly linear correlation betweenVandxis obtained without any need for sample pretreatment. Finally, the very high quality of our experimental data allows us to comment on the presence of some preferential framework T sites for Ti substitution. Such sites were recently proposed by two different groups on the basis of the results of molecular dynamics modeling.