Amitava Moitra

Modified embedded atom method potential for Al, Si, Mg, Cu, and Fe alloys

A set of modified embedded atom method (MEAM) potentials for the interactions between Al, Si, Mg, Cu, and Fe was developed from a combination of each element’s MEAM potential in order to study metal alloying. Previously published MEAM parameters of single elements have been improved for better agreement to the generalized stacking fault energy (GSFE) curves when compared with ab initio generated GSFE curves. The MEAM parameters for element pairs were constructed based on the structural and elastic properties of element pairs in the NaCl reference structure garnered from ab initio calculations, with adjustment to reproduce the ab initio heat of formation of the most stable binary compounds. The new MEAM potentials were validated by comparing the formation energies of defects, equilibrium volumes, elastic moduli, and heat of formation for several binary compounds with ab initio simulations and experiments. Single elements in their ground state crystal structure were subjected to heating to test the potentials at elevated temperatures. An Al potential was modified to avoid formation of an unphysical solid structure at high temperatures. The thermal expansion coefficient of a compound with the composition of AA 6061 alloy was evaluated and compared with experimental values. MEAM potential tests performed in this work, utilizing the universal atomistic simulation environment (ASE), are distributed to facilitate reproducibility of the results.

Melting tungsten nanoparticles : a molecular dynamics study

We report a molecular dynamics simulation of melting of tungsten (W) nanoparticles. The modified embedded atom method interatomic potentials are used to describe the interaction between tungsten atoms. The melting temperature of unsupported tungsten nanoparticles of different sizes are found to decrease as the size of the particles decreases. The melting temperature obtained in this study is approximately a decreasing function of inverse radius, in good agreement with the predictions of thermodynamic models. We also observed that the melting of a W nanoparticle is preceded by the premelting of its outer skin at a temperature lower than its melting temperature.

Semi-Empirical Potential Methods for Atomistic Simulations of Metals and Their Construction Procedures

General theory of semi-empirical potential methods including embedded-atom method and modified-embedded-atom method (MEAM) is reviewed. The procedures to construct these potentials are also reviewed. A multi-objective optimization (MOO) procedure has been developed to construct MEAM potentials with minimal manual fitting. This procedure has been applied successfully to develop a new MEAM potential for magnesium. The MOO procedure is designed to optimally reproduce multiple target values that consist of important material properties obtained from experiments and first-principle calculations based on density-functional theory. The optimized target quantities include elastic constants, cohesive energies, surface energies, vacancy-formation energies, and the forces on atoms in a variety of structures. The accuracy of the present potential is assessed by computing several material properties of Mg including their thermal properties. We found that the new MEAM potential shows a significant improvement over previously published potentials, especially for the atomic forces and melting temperature calculations.

Conversion of Nano-Sized Spherical Magnetite to Spherical Barium Ferrite Nanoparticles for High Density Particulate Recording Media

We report 24-30 nm spherical barium ferrite (S-BaFe) particles having extremely narrow size distribution without any superparamagnetic phase. We have converted spherical magnetite (S-Mag) nanoparticles to S-BaFe nanoparticles using a unique adsorption-diffusion process. The synthesized S-BaFe nanoparticles were characterized by X-ray diffractometer, Mossbauer spectrometer, transmission electron microscope (TEM), and vibrating magnetometer (VSM) for magnetic and physical properties. Saturation magnetization and coercivity of the S-BaFe nanoparticles were found to be 41.4 emu/g and 4075 Oe, respectively. The thermal stability of Ku V/kB T ap 107 was estimated for the S-BaFe nanoparticles from time-dependent remanent coercivity measurement.

Investigation on Sintering Mechanism of Nanoscale Tungsten Powder Based on Atomistic Simulation

Atomistic simulations focusing on sintering of crystalline tungsten powders at the submicroscopic level are performed to shed light on the processing on the nanoscale powders. The neck growth and shrinkage were calculated during these sintering simulations, so it is possible to extend these results to the global physical property evolution via sintering. The densification and grain growth during sintering were calculated with variations in temperature, pressure, particle configuration, additives, and crystalline misalignment between particles. These findings lay a foundation for a virtual approach to setting the processing cycles and materials design applicable to nanoscale powders.

Laser raman optical sensor for monitoring gas mixtures using photomultiplier tube detector

A Photomultiplier Tube (PMT) based optical fiber Raman sensor was developed for online monitoring of nitrogen/oxygen concentration ratios in gaseous mixtures. The sensor employed a frequency doubled 532 nm continuous wave (CW) Nd:YAG laser and a modified In-Photonics fiber optic state-of-art miniaturized Raman Probe. The gaseous mixture was enclosed in a high pressure cell and subjected to varying degrees of pressure. Raman signal of gaseous nitrogen and oxygen were first analyzed with a miniature spectrometer. The detection system was then replacing by a Labview interfaced PMT module for fast data acquisition and real time monitoring of relative Raman signals of nitrogen and oxygen. Instrumentation features and sensor performances with different detection systems (i.e. spectrometer and PMT) is presented in the paper.