Modelling, simulation, synthesis and structural characterization of Ni-Fe based nanoalloys

Irmak, Ece Arslan
There is a growing interest in the simulation and production of nanoalloys because the unique chemical and physical properties of nanoalloys can be tuned, and completely new structural motifs can be created by varying the type and composition of constituent elements, the atomic ordering, size, and shape of the nanoparticles. As an important magnetic material, Fe-Ni based nanoalloys have promising applications in the chemical industry, aerospace and stealth industry, magnetic biomedical applications and computer hardware industry. The purpose of this study is to analyze the structural properties of the magnetic nanoalloys at atomistic level and to establish a bridge between theoretical and experimental studies, in order to interpret many of experimental results and to predict the physical and chemical properties of the nanoalloys. In the theoretical part, structural evolutions of Fe-Ni based nanoalloys have been studied by using molecular dynamics (MD) method in Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). In this regard, structural evolution of the bimetallic FeNi3 crystalline and amorphous nanoalloys has been investigated by means of MD simulation combined with Embedded Atom Model (EAM) with taking into account the effect of temperature (300-1700 K), particle size (2 nm-6 nm) and shape (spherical and cubic) on radial distribution functions, inter-atomic distances, coordination numbers, core-to-surface concentration profiles, surface energies and Voronoi analysis. From the molecular dynamics simulations, it has been clearly observed that the structural evolution, melting point and atomic arrangements of the nanoparticles exhibited strongly size and shape dependent behavior. As the particle size of the simulated nanoparticles increased, the particles became more heat-resistant and mostly preserved their stable crystalline structure, shape and mixing pattern at high temperatures. Also, it has been observed that the 6 nm nanoparticles owned the FCC lattice structure at room temperature which is consistent with the L12-type ordered structure of the synthesized via mechanical alloying FeNi3 nanoparticles with soft magnetic properties. In the experimental part of the study, FeNi3 bimetallic nanoalloys were synthesized via mechanical alloying in a planetary high energy ball milling. The experimental studies were carried out in three parts. Firstly, mechanical alloying in high energy dry planetary ball milling with 250 and 400 rpm was applied to obtain FeNi3 nanoparticles. Afterward, two-step mechanical alloying was performed in which dry milling was followed by surfactant-assisted ball milling to investigate the surfactant (oleic acid and oleylamine) and solvent (heptane) effect on the structure, size, and properties of the FeNi3 nanoalloys. The structural and magnetic properties of the alloyed nanoparticles have been analyzed using XRD, SEM, EDS, and VSM techniques. In terms of the particle size, it was found that the amount of nano-sized particles raised with increasing milling time and milling speed, and consequently the magnetic properties of the particles varied. However, no significant effect of surfactants on the particle size was observed. The smallest, L12-type ordered FeNi3 nanopowders with 5.82 nm crystallite size, -0.46% strain value, and 3.54263 Å lattice parameter, showing soft magnetic properties, were synthesized by mechanical alloying with 400rpm under dry atmosphere after 80 h milling time.