Phase transition and mechanical properties of tungsten nanomaterials from molecular dynamic simulation

Molecular dynamic simulation is used to systematically find out the effects of the size and shape of nanoparticles on phase transition and mechanical properties of W nanomaterials. It is revealed that the body-centered cubic (BCC) to face-centered cubic (FCC) phase transition could only happen in cubic nanoparticles of W, instead of the shapes of sphere, octahedron, and rhombic dodecahedron, and that the critical number to trigger the phase transition is 5374 atoms. Simulation also shows that the FCC nanocrystalline W should be prevented due to its much lower tensile strength than its BCC counterpart and that the octahedral and rhombic dodecahedral nanoparticles of W, rather than the cubic nanoparticles, should be preferred in terms of phase transition and mechanical properties. The derived results are discussed extensively through comparing with available observations in the literature to provide a deep understanding of W nanomaterials.     

Molecular dynamic simulation of the size- and shape-dependent lattice parameter of small Platinum nanoparticles

The molecular dynamics simulation method has been used to study the size- and shape-dependent lattice parameter of unsupported small Pt nanoparticles, where the shapes concerned are sphere, cube, and cuboctahedron. It is shown that the lattice parameters decrease with decreasing the particle size in specific shape. The lattice variations of cubic shapes are higher than those of cuboctahedral shapes, and those of cuboctahedral shapes are higher than spherical ones. Furthermore, the shape effect on lattice parameter increases with decreasing the particle size. By linear fitting the simulated results, it is found that the particle shape can contribute to 7% of the total lattice parameter variation for cubic shape and to 5% for cuboctahedral shape. The present simulation results are qualitatively consistent with experimental values and the predictions by Continuous-Medium (CM) model.     

Large-scale synthesis and magnetic properties of cubic CoO nanoparticles

We present the synthesis, microstructural and magnetic characterization of cubic CoO nanoparticles with well-controlled size and shape. The as-synthesized CoO nanoparticles are stable because of the organic coating that occurred in situ. The Néel temperature is 225 and 280 K for the 42 and 74 nm CoO particles, respectively. The CoO nanoparticles exhibit anomalous magnetic properties, such as large moments, coercivities and loop shifts. These results provide evidence for the formation of spin compensated random system in CoO. The structurally distorted and magnetically disordered surface layer ferromagnetic phase played an important role in the magnetic behavior of CoO nanoparticles. The smaller is the particle size, the stronger is the contribution of the ferromagnetic phase and the more is the surface layer helpful to enhance the observed coercivity and the exchange bias.     

Structural transition of sheared-liquid metal in quenching state

In this Letter, the effects of shear rate on structural properties of liquid Al in quenching process were investigated via molecular dynamics (MD) simulations based on the EAM potential. Analyses in internal energy and pair correlation functions (PCF) reveal an increasing structural transition temperature as the shear rate is enhanced in the liquid. Results of pair analysis indicate that for liquid Al under normal condition, face center cubic (FCC) structure is clearly detected upon cooling; while in sheared liquid, structural transition from FCC to body center cubic (BCC) at temperature of 800 K is manifested, leading to the dominance of BCC structural order at low temperatures.     

Molecular dynamics investigation of the structural stability of body-centered cubic zirconium nanofilms

The influence of the film thickness and temperature on the phase stability of body-centered cubic (BCC) zirconium in infinite films with different crystallographic orientations has been investigated using the molecular dynamics method with a many-body interatomic interaction potential obtained within the embedded atom model. The calculations have been performed for BCC zirconium films with thicknesses ranging from 2 to 13 nm and with low Miller indices (001), (110), and (111). It has been shown that the BCC(001) zirconium nanofilms with thicknesses up to 6.1 nm, which are formed in the temperature range from 500 to 1300 K, undergo a reorientational phase transition through an intermediate metastable face-centered cubic (FCC) phase with the subsequent transformation into the hexagonal close-packed (HCP) structure (BCC(001)-FCC-BCC??(110)-HCP). When the temperature of initialization of the films is 500 K and below, the BCC-FCC transformation is observed and the FCC phase remains stable. The (110) films are characterized by a strong dependence of the temperature of the BCC-HCP phase transition on the film thickness up to values of 5.8 nm. In the (111) films, the amorphization of the initial BCC phase with the subsequent formation of the BCC phase with a twin structure is observed.     

Size, shape and structure dependent cohesive energy and phase stability of metallic nanocrystals

A model to account for the size, shape and structure dependent cohesive energy of metallic nanocrystals is developed in this contribution. It is predicted that the cohesive energy of nanocrystals decreases with decreasing the crystal size in specific shape, and decreases with increasing the shape factor in specific size. Furthermore, the model can be applied to predict the size and shape dependent phase stability of nanocrystal. To take Cr nanocrystal as an example, we found that there exists FCC structure for Cr crystal (the bulk structure is BCC) when the crystal size is small enough, and critical size of phase transition ranges from 249 to 824 atoms due to crystal shape variation, which is consistent with the corresponding experimental results.     

Effect of gamma irradiation on synthesis and characterization of bio-nanocomposite SF/Ag nanoparticles

This study describes the synthesis of silver nanoparticles (AgNPs) using aqueous silk fibroin (SF) solution obtained from Bombyx mori silk under gamma radiation environment. The obtained AgNPs were characterized using UV–visible (UV–Vis) spectroscopy, X-ray diffraction (XRD) measurements, dynamic light scattering experiment (DLS) and transmission electron microscope (TEM) images. The UV–Vis absorption spectra of the samples confirmed the formation of AgNPs by showing surface plasmon resonance (SPR) band in the range of (= 428–435?nm. The XRD study revealed metal silver with the face-centered cubic (FCC) crystal structure. DLS measurements showed the dose-dependent average size of the AgNPs. TEM images showed formed AgNPs are nearly spherical in shape with smooth edges. From this study, it was found that the increasing radiation dose increases the rate of reduction and decreases the particle size. The size of the AgNPs can be tuned by controlling the radiation dose.     

Investigation of the activation-parameters of low-temperature slip in cubic metals

The macroscopic critical resolved shear stress (CRSS)τ of 9 body-centred cubic (BCC) and 5 face-centred cubic (FCC) metals has been found to vary with temperatureT in the range 0 to 300 K as given by: lnτ=A − BT, whereA andB are positive constants. Theτ−T data have been analysed within the framework of a kink-pair nucleation (KPN) model of plastic flow in crystals. The microscopic parameters of the unit activation process of yielding, e.g. the initial length of the glide dislocation segment, the critical height of the kink-pair nucleated in it, the activation volume associated with the CRSS, and the binding energy per interatomic spacing along the glide dislocation in the slip plane etc., have been evaluated. A consistent picture of the dislocation kinetics involved in the yielding of BCC and FCC metals emerges, which is adequately described by the KNP model of plastic flow in crystals.     

Stability of cubic zirconia and of stoichiometric zirconia nanoparticles

Using the electron density functional method, it is shown that the oxygen sublattice of cubic zirconia is unstable with respect to random displacements of oxygen atoms, which results in general instability of bulk cubic zirconia at low temperatures. A comparison of the equilibrium atomic structures and total energies of stoichiometric ZrO₂ nanoparticles about 1 nm in size shows that particles with cubic symmetry are more stable than those with rhombic (close-to-tetragonal) symmetry. The electronic structure of nanoparticles exhibits an energy gap at the Fermi level; however, this gap (depending on the symmetry and size of the particle) can be much narrower than the energy gap of the bulk material.     

Thermodynamics and molecular dynamics investigation of a possible new critical size for surface and inner cohesive energy of Al nanoparticles

In this study, the authors first review the previously developed, thermodynamics-based theory for size dependency of the cohesion energy of free-standing spherically shaped Al nanoparticles. Then, this model is extrapolated to the cubic and truncated octahedron Al nanoparticle shapes. A series of computations for Al nanoparticles with these two new shapes are presented for particles in the range of 1–100 nm. The thermodynamics computational results reveal that there is a second critical size around 1.62 and 1 nm for cubes and truncated octahedrons, respectively. Below this critical size, particles behave as if they consisted only of surface-energy-state atoms. A molecular dynamics simulation is used to verify this second critical size for Al nanoparticles in the range of 1–5 nm. MD simulation for cube and truncated octahedron shapes shows the second critical point to be around 1.63 and 1.14 nm, respectively. According to the modeling and simulation results, this second critical size seems to be a material property characteristic rather than a shape-dependent feature.     

Surface mode absorption and infrared optical properties of gallium phosphide nanoparticles

The surface optical (SO) mode of ellipsoidal gallium phosphide (GaP) nanoparticles is investigated by infrared transmission spectroscopy. The surface mode theory of diatomic cubic particles is generalized and applied to GaP nanoparticles in a systematic treatment. The Fröhlich mode of GaP nanoparticles has been observed in our experiment. As far as surface mode frequency is concerned, the result of the experiment agrees with that of the theoretical calculation. The characteristics of the SO mode peak, including frequency shift, broadening and line shape, are analyzed. The frequency shift is attributed to the surrounding medium effect, surface oxidation and the aggregation effect as well as intrinsic point defects; the broadening is mainly due to the non-spherical particle shape, aggregation and quantum confinement effect; and the line shape is related to the particle shape and the damping function.     

Simulation of the thermally induced austenitic phase transition in NiTi nanoparticles

The reverse martensitic (“austenitic”) transformation upon heating of equiatomic nickel-titanium nanoparticles with diameters between 4 and 17 nm is analyzed by means of molecular-dynamics simulations with a semi-empirical model potential. After constructing an appropriate order parameter to distinguish locally between the monoclinic B19′ at low and the cubic B2 structure at high temperatures, the process of the phase transition is visualized. This shows a heterogeneous nucleation of austenite at the surface of the particles, which propagates to the interior by plane sliding, explaining a difference in austenite start and end temperatures. Their absolute values and dependence on particle diameter are obtained and related to calculations of the surface induced size dependence of the difference in free energy between austenite and martensite.     

Relative contribution of various factors to the formation of the energy spectrum of fast, medium-energy, charged particles and ions transmitted through a thin target with fluctuations of the target thickness

The characteristics of the energy spectra of kiloelectron-volt protons transmitted through a free-standing foil are investigated theoretically and experimentally as functions of the angle of incidence of the beam on the target. Analytical expressions for the average characteristics of the transmitted-particle energy spectrum are determined for the case of small-angle scattering. The combined influence of various factors affecting the formation of the energy spectra is taken into account: systematic stopping of particles in the medium, fluctuations of the particle energy losses in inelastic collisions, bending of the particle trajectories due to multiple elastic scattering, and fluctuations of the target thickness. It is shown that the contributions of these factors to the width of the transmitted-particle energy spectrum depend differently on the angle of incidence of the beam on the target surface. On the basis of this differentiation it is inferred from the experimental dependence of the width of the energy spectra of kiloelectron-volt protons transmitted through a free-standing foil on the angle of incidence of the beam that fluctuations of the particle energy losses in inelastic collisions are the predominant factor in the formation of the proton energy spectra. Zh. Tekh. Fiz. 67, 81–93 (May 1997)     

Formation of porous GaSb compound nanoparticles by electronic-excitation-induced vacancy clustering

Porous semiconductor compound nanoparticles have been prepared by a new technique utilizing electronic excitation. The porous structures are formed in GaSb particles, when vacancies are efficiently introduced by electronic excitation and the particle size is large enough to confine the vacancy clusters. The capture cross section of the surface layer in particles for the vacancies is smaller than that for the interstitials. Under the condition of supersaturation of vacancies in the particle core, porous structures are produced through the vacancy clusters to a void formation.     

Size control synthesis of starch capped-gold nanoparticles

Metallic gold nanoparticles have been synthesized by the reduction of chloroaurate anions [AuCl₄]⁻ solution with hydrazine in the aqueous starch and ethylene glycol solution at room temperature and at atmospheric pressure. The characterization of synthesized gold nanoparticles by UV–vis spectroscopy, high resolution transmission electron microscopy (HRTEM), electron diffraction analysis, X-ray diffraction (XRD), and X-rays photoelectron spectroscopy (XPS) indicate that average size of pure gold nanoparticles is 3.5 nm, they are spherical in shape and are pure metallic gold. The concentration effects of [AuCl₄]⁻ anions, starch, ethylene glycol, and hydrazine, on particle size, were investigated, and the stabilization mechanism of Au nanoparticles by starch polymer molecules was also studied by FT-IR and thermogravimetric analysis (TGA). FT-IR and TGA analysis shows that hydroxyl groups of starch are responsible of capping and stabilizing gold nanoparticles. The UV–vis spectrum of these samples shows that there is blue shift in surface plasmon resonance peak with decrease in particle size due to the quantum confinement effect, a supporting evidence of formation of gold nanoparticles and this shift remains stable even after 3 months.     

Structural and magnetic properties of Fe–Ni mecanosynthesized alloys

Fe_100???x Ni _x samples with x?=?22.5, 30.0 and 40.0 at.% Ni were prepared by mechanical alloying (MA) with milling times of 10, 24, 48 and 72 h, a ball mass to powder mass (BM/PM) ratio of 20:1 and rotation velocity of 280 rev/min. Then the samples were sintered at 1,000°C and characterized by X-ray diffraction (XRD) and transmission Mössbauer spectrometry (TMS). From the refinement of the X ray patterns we found in this composition range two crystalline phases, one body centered cubic (BCC), one face centered cubic (FCC) and some samples show FeO and Fe₃O₄ phases. The obtained grain size of the samples shows their nanostructured character. Mössbauer spectra were fitted using a model with two hyperfine magnetic field distributions (HMFDs), and a narrow singlet. One hyperfine field distribution corresponds to the ferromagnetic BCC grains, the other to the ferromagnetic FCC grains (Taenite), and the narrow singlet to the paramagnetic FCC grains (antitaenite). Some samples shows a paramagnetic doublet which corresponds to FeO and two sextets corresponding to the ferrimagnetic Fe₃O₄ phase. In this fit model we used a texture correction in order to take into account the interaction between the particles with flake shape and the Mössbauer $\upgamma$ -rays.     

An investigation into the mechanical properties of silicon nanoparticles using molecular dynamics simulations with parallel computing

This study investigates the mechanical properties of cubic silicon nanoparticles with side lengths ranging from 2.7 to 16.3 nm using molecular dynamics (MD) simulation with parallel computing technique. The results reveal that the surface energy of the particles increases significantly as the particle size decreases. Furthermore, having passed the point of maximum compressive load, the phase transformation region of the particles gradually transfers from the core to the surface. The small volume of the current nanoparticles suppresses the nucleation of dislocations, and as a result, the maximum strength and Young’s modulus values of all but the smallest of the current nanoparticles are greater than the corresponding values in bulk silicon. Finally, it is found that the silicon nanoparticles with a side length of 10.86 nm exhibit the greatest maximum strength (24 GPa). In nanoparticles with shorter side lengths, the maximum strength decreases significantly as the volume of the nanoparticle is reduced.     

Size-dependent nanoparticle reaction enthalpy: Oxidation of aluminum nanoparticles

Here we present a model describing the particle size dependence of the oxidation enthalpy of aluminum nanoparticles. The model includes the size dependence of the cohesive energy of the reactant particles, the size dependence of the product lattice energy, extent of product agglomeration, and surface capping effects. The strongest effects on aluminum nanoparticle energy release occur for particle diameters below 10 nm, with enhanced energy release for agglomerated oxide products and decreased energy release for nanoscale oxide products. An unusual effect is observed with all nanoparticle reaction enthalpies converging to the bulk value when agglomeration of the products approaches the transition between nanoparticle→nanoparticle and nanoparticle→bulk energetics. Optimal energy output for Al NP oxidation should occur for sub-10-nm particles reacting with significant agglomeration.