Onset of chaotic dynamics in a ball mill: attractors merging and crisis induced intermittency

In mechanical treatment carried out by ball milling, powder particles are subjected to repeated high-energy mechanical loads which induce heavy plastic deformations together with fracturing and cold-welding events. Owing to the continuous defect accumulation and interface renewal, both structural and chemical transformations occur. The nature and the rate of such transformations have been shown to depend on variables, such as impact velocity and collision frequency that depend, in turn, on the whole dynamics of the system. The characterization of the ball dynamics under different impact conditions is then to be considered a necessary step in order to gain a satisfactory control of the experimental set up. In this paper we investigate the motion of a ball in a milling device. Since the ball motion is governed by impulsive forces acting during each collision, no analytical expression for the complete ball trajectory can be obtained. In addition, mechanical systems exhibiting impacts are strongly nonlinear due to sudden changes of velocities at the instant of impact. Many different types of periodic and chaotic impact motions exist indeed even for simple systems with external periodic excitation forces. We present results of the analysis on the ball trajectory, obtained from a suitable numerical model, under growing degree of impact elasticity. A route to high dimensional chaos is obtained. Crisis and attractors merging are also found

Unconventional magnetism of non-uniform distribution of Co in TiO2 nanoparticles

High-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) analysis, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), magnetic methods, and density-functional theory (DFT) calculations were applied for the investigations of Co-doped anatase TiO2 nanoparticles (∼20 nm). It was found that high-spin Co2+ ions prefer to occupy the interstitial positions in the TiO2 lattice which are the most energetically favourable in compare to the substitutional those. A quantum mechanical model which operates mainly on two types of Co2+ – Co2+ dimers with different negative exchange interactions and the non-interacting paramagnetic Co2+ ions provides a satisfactorily description of magnetic properties for the TiO2:Co system. © 2020 Elsevier B.V.Russian Foundation for Basic Research. Ministry of Science and Higher Education of the Russian Federatio

Magnetic properties and structure of TiO2-Mn (0.73 %) nanopowders: The effects of electron irradiation and vacuum annealing

Nanopowder TiO2-0.73 % Mn was synthesized by the sol-gel method. Thermal treatment of the samples was carried out in vacuum at a temperature of 500°C. Magnetic properties were studied in the temperature range from 2 to 850 K. The effects of electron irradiation and vacuum annealing on the EPR spectra and magnetic properties of TiO2-Mn powder are discussed. It was established that a part of manganese ions in the anatase crystal lattice interacts antiferromagnetically, which causes a decrease in magnetization as compared to the result of the calculation for non-interacting ions. Vacuum annealing leads to the formation of oxygen vacancies and, at the same time, to a noticeable increase in the ferromagnetic contribution to magnetization, especially, after preliminary electron irradiation. We assume that the ferromagnetic contribution to the magnetization appears either due to incomplete compensation of antiferromagnetically directed moments of manganese ions, or due to positive exchange interactions of Mn ions via defects in the TiO2 lattice. It is shown that the temperature of magnetic disordering in samples with a spontaneous magnetic moment exceeds 600°C. © 2019, Institute for Metals Superplasticity Problems of Russian Academy of Sciences. All rights reserved

NMR study of magnetic nanoparticles Ni@C

The 61Ni, 13C NMR spectra of carbon encapsulated nickel nanoparticles have been obtained. It has been shown that the cores of the particles consist of metallic nickel with face-centered cubic structure, nickel carbide Ni3C and carbon-nickel solid solution. The carbon shell of nanoparticles is a highly defective structure and close to an amorphous glassy-like carbon. © Published under licence by IOP Publishing Ltd.Ural Branch, Russian Academy of Sciences, UB RAS: 18-10-2-37.The study was performed within the state assignments of the Mikheev Institute of Metal Physics of the Ural Branch of the Russian Academy of Sciences: state program «Function» No АААА-А19-119012990095-0; state program «Magnit» No АААА-А18-118020290129-5 and state program «Alloys». The research also was supported by the project of the complex program of Ural Branch of Russian Academy of Sciences № 18-10-2-37

Two different types of ferromagnetic state in TiO2-Co nanopowders

TiO2-Co nanopowders were synthesized by the sol-gel method and gas condensation method. The structure of powders and its change during various heat treatments in air and hydrogen was investigated. The samples obtained by the gas condensation method contain ferromagnetic regions in the as-prepared state. This ferromagnetism does not disappear when annealed in air at 773 K. The magnetization curves of these powders are measured in the temperature range from 2 up to 773 K and the temperature dependence of the ferromagnetic contribution is analyzed. © Published under licence by IOP Publishing Ltd

Carbon States in Carbon-Encapsulated Nickel Nanoparticles Studied by Means of X-Ray Absorption, Emission, and Photoelectron Spectroscopies

Electronic structure of nickel nanoparticles encapsulated in carbon was characterized by photoelectron, X-ray absorption, and X-ray emission spectroscopies. Experimental spectra are compared with the density of states calculated in the frame of the density functional theory. The carbon shell of Ni nanoparticles has been found to be multilayer graphene with significant (about 6%) amount of Stone--Wales defects. Results of the experiments evidence protection of the metallic nanoparticles from the environmental degradation by providing a barrier against oxidation at least for two years. Exposure in air for 2 years leads to oxidation only of the carbon shell of Ni@C nanoparticles with coverage of functional groups.Comment: 16 pages, 6 figures, accepted in J. Phys. Chem.