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The present work was aimed at the investigation of effect of High Energy Ball Milling (HEBM) time on the sintering kinetics, structure, and properties of heavy tungsten alloy (HTA) W-7%Ni-3%Fe. The HTA samples were obtained from the nanopowders (20-80 nm) by conventional liquid phase sintering (LPS) in hydrogen and by Spark Plasma Sintering (SPS) in vacuum. The HTA density was shown to depend on the HEBM time non-monotonously that originates from the formation of non-equilibrium solid solutions in the W-Ni-Fe systems during HEBM. The SPS kinetics of the HTA nanopowders was shown to have a two-stage character, the intensity of which depends on the Coble diffusion creep rate and on the intensity of diffusion of the tungsten atoms in the crystal lattice of the -phase. The kinetics of sintering of the initial submicron powders have a single-stage character originating from the intensity of the grain boundary diffusion in the -phase. The dependencies of the hardness and of the yield strength on the grain sizes were found to obey the Hall-Petch relation. The hardness, strength, and dynamic strength in the compression tests of the fine-grained tungsten alloys obtained by SPS and LPS were studied.

The special aspects of spark plasma sintering (SPS) of plasma-chemical nanopowders of tungsten carbide with higher oxygen content were investigated. It was demonstrated that shrinkage rate of the WC nanopowders during SPS is limited by the rate of grain-boundary diffusion with the abnormally low values of activation energy. Decrease in the activation energy of SPS is caused by the influence of oxygen on the diffusion permeability of boundaries of the tungsten carbide grains at the stage of intense densification, as well as by abnormal grain growth. Kinetics of SPS of the WC-W2C-WO3-W nanopowder compositions at the stage of severe densification is controlled by the rate of sticking together the oxide particles, with their simultaneous transformation into W2C particles, and then by the plastic flow of W2C particles in the presence of the tungsten particles at the stage of high-temperature sintering. Ceramics with a high density ratio (98–99%) and ultra-fine-grained structure (the mean grain size less than 0.3 μm) with higher hardness HV = 30.5 GPa and fracture toughness of ~6.5 MPa m1/2 were obtained using the SPS method.

Features of high-speed sintering of WC–Co nanopowders with various contents of excess carbon (colloidal graphite) were studied. Powders were obtained in a process that included plasma-chemical and low-temperature syntheses and a chemical-metallurgical method of applying ultrathin cobalt layers by precipitation from a solution of salts. Powder materials were consolidated by high-speed spark plasma sintering. It was found that an increase in the concentration of free carbon (colloidal graphite) has the greatest effect on the shrinkage and the sintering rate at the stage of intense shrinkage of WC–Co nanopowders. It was revealed that the process of nanopowder compaction at the intense shrinkage stage is determined by the intensity of the plastic flow and the intensity of diffusion along the boundaries of cobalt grains. It was shown that the mechanism of plastic deformation of the cobalt-based γ phase corresponds to the Coble diffusion creep. It was found that an increase in carbon content lowers the activation energy at the stage of intense shrinkage and does not significantly affect the activation energy at stage III of sintering, where the shrinkage intensity is observed to decrease. It was shown that a decrease in the sintering activation energy is due to a decrease in the tungsten concentration in the γ phase.

The shrinkage of the WC nanopowders with increased oxygen content during Spark Plasma Sintering (SPS) was shown to be limited by the grain boundary diffusion with abnormally low activation energy. It was found that the reduction of the SPS activation energy may originate from the effect of oxygen on the diffusion permeability of the grain boundaries in tungsten carbide at the intensive compaction stage as well as from an abnormal grain growth at the high-temperature sintering stage. The kinetics of SPS of the WC-W2C–WO3–W nanopowders at the intense compaction stage is controlled by the sintering rate of the oxide particles with simultaneous transformation of these ones into the W2C particles and then, at the high-temperature sintering stage – by the plastic flow of the W2C particles in the presence of tungsten ones. The ceramic specimens with high density (98–99%) and ultrafine grained structure (mean grain size

Improvement of the physical and mechanical properties of hard alloys based on WC – Co widely used in manufacturing of structural and tool products nowadays results from the use of novel technologies providing formation of a homogeneous high-density structures. Slight deviations of the carbon content from the equilibrium state lead to the formation of brittle η-phases (in particular, Co3W3C) and, accordingly, to deterioration of the mechanical properties of the product. We present the results of studying the homogeneity of the phase composition of the samples of hard alloys WC + 10% Co, obtained using advanced technologies of plasma-chemical synthesis and spark plasma sintering (SPS). The layer-by-layer X-ray phase analysis revealed the heterogeneity of the phase composition in depth: the brittle η-phase (Co3W3C) appears at a depth of ≥100 μm and reaches a constant value of 18 ± 1 wt.% at >200 μm, which indirectly confirms the hypothesis of carbon diffusion from graphite punches contacting with the surface of sintered samples and makes it possible to expand the range of parameters affecting the process of spark plasma sintering.

Spark plasma sintering (SPS) of plasma-chemical nanopowders tungsten carbide with a high oxygen content are studied. The results show that the nanopowder WC shrinkages during SPS is limited by grain boundary diffusion with abnormally low activation energies. It is established that the decrease in the activation energy can be caused by the influence of oxygen on the diffusion permeability of the tungsten carbide grain boundaries at the stage of intense compaction, as well as by abnormal grain growth at the stage of high-temperature sintering. The SPS kinetics of WC-W2C-WO3-W nanopowder compositions at the stage of intense compaction is controlled by the rate of oxide particle sintering with their simultaneous transformation into W2C particles, and then, by the plastic flow of W2C particles in the presence of W particles at the stage of high-temperature sintering. Using the SPS method, we obtained ceramics with a high density (98-99%), ultrafine-grained structure (the average grain size is less than 0.3 mm), having an increased hardness HV = 30.5 GPa with a Palmquist crack resistance of ~6.5 MPa·m1/2.

We investigate the constructive role of an external noise signal, in the form of a low-rate Poisson sequence of pulses supplied to all inputs of a spiking neural network, consisting in maintaining for a long time or even recovering a memory trace (engram) of the image without its direct renewal (or rewriting). In particular, this unique dynamic property is demonstrated in a single-layer spiking neural network consisting of simple integrate-and-fire neurons and memristive synaptic weights. This is carried out by preserving and even fine-tuning the conductance values of memristors in terms of dynamic plasticity, specifically spike-timing-dependent plasticity-type, driven by overlapping pre- and postsynaptic voltage spikes. It has been shown that the weights can be to a certain extent unreliable, due to such characteristics as the limited retention time of resistive state or the variation of switching voltages. Such a noise-assisted persistence of memory, on one hand, could be a prototypical mechanism in a biological nervous system and, on the other hand, brings one step closer to the possibility of building reliable spiking neural networks composed of unreliable analog elements.

The results of investigations of spark plasma sintering (SPS) kinetics of tungsten carbide nanopowders with different Co content (0.3, 0.6, and 1.0 wt%) and graphite (0.3 and 0.5 wt%) are described. The α-WC nanopowders were obtained by direct current (DC) arc plasma synthesis followed by annealing in hydrogen. Ultrathin Co layers were deposited onto the nanoparticles by a chemical-metallurgical method from a salt solution. The excess carbon was introduced by mixing the WC-Co powders with graphite that resulted in the increase of the oxygen concentration. The carbonization of the tungsten carbide hard alloy specimens was demonstrated to take place in SPS. It resulted in different phase compositions, along with hardness within surface and central layers of sintered ceramic specimens. The effects of the initial particle size, of the Co concentration, and of the graphite one on the stages of the SPS of the ultra-low Co hard alloys were studied. The specimens with uniform fine-grained structure, increased density, and improved mechanical characteristics were obtained. The hardness Hv for WC-0.6%Co-0.3%C hard alloy with averaged grain size 1.0–1.5 μm was 20.2–20.5 GPa at the minimum fracture toughness coefficient KIC = 9.2–10.4 MPa m1/2.

Ceramics obtained by spark plasma sintering of tungsten carbide powder were studied by X-ray diffraction layer-by-layer analysis. The surface of the ceramic samples was mechanical grinded and polished in several stages. The distribution of crystal phases by the depth of ceramics was investigated.

The features of spark plasma sintering (SPS) of plasma-chemical nanopowders WC-(0.3, 0.6, 1) wt.% Co were studied. The SPS process of ultralow-cobalt hard alloys can be sequentially represented as a change of the following mechanisms: rearrangement of particles at lower temperatures (Stage I) → sintering of WC-Co particles due to Coble diffusion creep of cobalt, the intensity of which is determined by the grain boundary diffusion rate (Stage II ) → sintering due to diffusion creep, the rate of which is limited by the bulk diffusion in cobalt (Stage III-1) → sintering of tungsten carbide particles along the intergranular boundaries of WC / WC under conditions of intensive grain growth (Stage III-2). Samples with a high density (96.4-98.4%) and high mechanical properties were obtained (for the WC-0.3% Co hard alloy: Hv ∼ 20.5 GPa, KIC = 7.1 MPa · m1/2).

The results of studies of the kinetics of high-speed spark plasma sintering (SPS) of plasma-chemical nanosized and industrial micron tungsten carbide powders are described. During SPS the carburization of the surface layer of tungsten carbide samples takes place, which leads to differences in the phase composition and hardness of the surface and central regions of sintered ceramics. The process of SPS of tungsten carbide can be sequentially represented as a change of the following mechanisms: rearrangement of particles at lower temperatures (Stage I) → sintering of particles due to grain boundary diffusion (Stage II) → sintering due to diffusion in the crystal lattice (Stage III- 1) → sintering under conditions of intensive grain growth with an abnormally low diffusion activation energy (Stage III-2).

The features of spark plasma sintering (SPS) of plasma-chemical nanopowders with an increased oxygen content are investigated. It is shown that the process of shrinkage of nanopowders during SPS is limited by the rate of grain-boundary diffusion with anomalously low values of the activation energy. It has been established that a decrease in the activation energy of SPS may be due to the effect of oxygen on the diffusion permeability of grain boundaries of tungsten carbide at the stage of intense compaction, as well as anomalous grain growth at the stage of high-temperature sintering. The SPS kinetics of WC-W2C-WO3-W nanopowder compositions at the stage of intensive compaction is controlled by the rate of sintering of oxide particles with their simultaneous transformation into W2C particles, and then, at the stage of high-temperature sintering, by the process of plastic flow of W2C particles in the presence of tungsten particles. Ceramic samples with high density (98-99%), ultrafine-grained structure (average grain size less than 0.3 μm), having increased hardness were obtained by the SPS method. Hv = 30.5 GPa at Palmquist crack resistance ∼ 6.5 MPa · m1/2.

The features of the compaction of plasma-chemical WC - (0.3; 0.6; 1) %Co nanopowders with the addition of 0.3 and 0.5% graphite during high-speed spark plasma sintering have been investigated. It was shown that an increased concentration of oxygen adsorbed on the surface of plasma-chemical nanoparticles during mixing with graphite, as well as the effect of graphite itself, which leads to a decrease in the activation energy of sintering due to a decrease in the intensity of the formation of particles of the η-phase in the “oxidized” WC-Co nanopowders, are the main structure features of ultralow-cobalt hard alloys. Samples of finegrained ultralow-cobalt hard alloys with increased hardness and fracture toughness were obtained (for a WC-0.6% Co-0.3% C hard alloy with an average grain size of ∼ 1-1.5 pm, the hardness is Hv = 20.2-20.5 GPa with a minimum crack resistance coefficient K ic = 9.2-10.4 MPa-m1/2).

The effect of carbon on the kinetics of spark plasma sintering (SPS) of submicron powder compositions WC-10wt.%Co is investigated. Free carbon in the form of graphite was added into submicron powders by mixing. The activation energy of solid-phase sintering under the conditions of isothermal hold and continuous heating rate sintering was determined. It was shown that an increase in the carbon content leads to a decrease in the volume fraction of particles of the η-phase and a shift of the shrinkage curve to the region of lower heating temperatures. It was established that an increase in the graphite content does not significantly affect the activation energy of sintering of submicron powders in the region of “average” heating temperatures, the value of which is close to the activation energy of grain boundary diffusion in cobalt. It has been shown that an increase in graphite content leads to a significant decrease in the activation energy of compaction of WC-Co powders in the region of “high” sintering temperatures due to a decrease in the concentration of tungsten atoms in the γ phase based on cobalt. The kinetics of sintering of fine-grained WC-Co hard alloys is limited by the intensity of the cobalt diffusion creep by Coble.

Spin injection light-emitting diodes based on heterostructures with InGaAs/GaAs quantum wells and a ferromagnetic metal (Co) contact layer are fabricated and studied. The effect of the penetration of cobalt in GaAs is experimentally detected. It is demonstrated that the use of tunnel-thin (1–3 nm) Al2O3 layers as diffusion barriers leads to a decrease in the concentration of cobalt atoms in the semiconductor surface layers. In diodes with the Co/Al2O3/GaAs contact, circularly polarized radiation is detected, which is caused by the injection of spin-polarized holes from the ferromagnetic contact into the semiconductor.

New data on the structure of the diluted magnetic semiconductor Si:Mn with the Curie point to 500 K synthesized by the laser method have been presented. High-resolution electron microscopy and diffraction in the directions 〈110〉 and 〈100〉 of epitaxial layers of the diluted magnetic semiconductor Si:15% Mn with the elimination of the contribution from the GaAs substrate and interface have been applied. It has been established that the diluted magnetic semiconductor Si:Mn is a previously unknown compound with the variable composition Si3 − xMnx (0 < x < 1), single-phase diamond-like structure, high crystal perfection, and self-organized formation of the superlattice structure with the period of the triple distance between the nearest (110) atomic layers and the interval between the (110) layers doped with Mn atoms and oriented along the growth direction of the Si:Mn film. The layers Si:15% Mn (or Si2.5Mn0.5) consist of 15- to 50-nm blocks with the mutually perpendicular orientations of the superlattice modulations. Manganese atoms in the (110) layers doped with this impurity are located in the form of single-atom stripes, which alternate with the silicon single-atom stripes in agreement with the X-ray spectral analysis and ferromagnetic resonance data. The ferromagnetism of Si3 − xMnx is associated with a specific feature of the impurity band.

A comprehensive analysis of double tunnel-coupled InGaAs/GaAs quantum well heterostructures is carried out. The real composition profiles of the structures are obtained by high-resolution transmission electron microscopy and energy-dispersive spectrometry. The resultant profiles are compared with the profile obtained by computer simulation. By solving the Schrodinger equation in combination with the Poisson equation, the energy states for quantum-confined heterostructures with initially specified and real composition profiles are calculated. The influence of a number of factors, such as the well width, barrier thickness, and the background doping level on the properties of the heterostructure is thoroughly analyzed. In this manner, the optical characteristics and their dependence on the growth technology and geometric parameters of the structures are studied. Such an approach makes it possible to refine the real geometric parameters of wells and barriers and to correct the parameters of the structure and growth technology in order to improve the optical characteristics.

The results of complex analysis of the parameters of the GaAs/AlGaAs semiconductor superlattice with a complex unit cell using mathematical modeling and experimental methods of transmission electron microscopy, energy-dispersive analysis, and spectroscopy of photoluminescence and photocurrent are presented. Suppression of the intraband static negative differential conductivity under strong interaction conditions between minibands and the inharmonic electron-dispersion law is shown.

Heterostructures containing single GaAsSb/GaAs quantum wells and bilayer GaAsSb/InGaAs quantum wells are produced by metal-organic vapor-phase epitaxy at atmospheric pressure. The growth temperature of the quantum-confined layers is 500–570°C. The structural quality of the samples and the quality of heterointerfaces of the quantum wells are studied by the high-resolution transmission electron microscopy of cross sections. The emission properties of the heterostructures are studied by photoluminescence measurements. The structures are subjected to thermal annealing under conditions chosen in accordance with the temperature and time of growth of the upper cladding p-InGaP layer during the formation of GaAs/InGaP laser structures with an active region containing quantum-confined GaAsSb layers. It is found that such heat treatment can have a profound effect on the emission properties of the active region, only if a bilayer GaAsSb/InGaAs quantum well is formed.

The possibility of the epitaxial growth of ferromagnetic manganese gallide (Mn3Ga5) layers on a the GaAs(100) surface was demonstrated. The ferromagnetic properties of epitaxial Mn3Ga5 at room temperature were evaluated from measurements of the anomalous Hall effect. A diode structure based on the Mn3Ga5/GaAs contact was formed, and the low-temperature electroluminescence of this diode was measured. The possibility of electroluminescence and the high crystal quality of the structures under study showed promises of their application in light-emitting diodes with spin injection.

Using high-resolution transmission electron microscopy and photoelectric spectroscopy methods, the effect of Mn δ layer embedding and GaAs coating layer growth techniques in structures with In(Ga)As/GaAs quantum dots and wells on their structural and optoelectronic characteristics is studied. It is shown that the low-temperature GaAs coating layer in a structure with a Mn δ layer is structurally inhomogeneous and can cause a decrease in the quantum-dot photosensitivity.

The crystalline structure and composition of a GaMnSb film is studied. The film is produced on a GaAs (100) substrate by deposition from laser plasma in a hydrogen flux at a temperature of 400°C. The formation of GaMn inclusions in the GaSb:Mn matrix is detected. The stoichiometry ratio of the inclusions corresponds to that of the Ga162.5Mn101.5 compound.

The photoluminescence spectra of erbium centers in SiO2 films with ion-synthesized silicon nanoclusters under nonresonant excitation were investigated. Erbium was introduced into thermal SiO2 films by ion implantation. The dependences of photoluminescence intensity on the dose, the order of ion implantation of Si and Er, the annealing temperature, and additional Ar+ and P+ ion irradiation regimes, i.e., factors determining the influence of radiation damage and doping on sensitization of erbium luminescence by silicon nanoclusters, were determined. It was found that the sensitization effect and its amplification due to doping with phosphorus are most pronounced under the conditions where nanoclusters are amorphous. The quenching of photoluminescence due to radiation damage in this case manifests itself to a lesser extent than for crystalline nanoclusters. The role of various factors in the observed regularities was discussed in the framework of the existing concepts of the mechanisms of light emission and energy exchange in the system of silicon nanoclusters and erbium centers.

The distribution of elastic strains in a system consisting of a quantum-dot layer and a buried GaAs x P1 − x layer is studied using geometric phase analysis. A hypothesis is offered concerning the possibility of controlling the process of the formation of InAs quantum dots in a GaAs matrix using a local isovalent phosphorus impurity.

An integrated approach to the analysis of tunnel-coupled InGaAs/GaAs quantum well heterostructures is suggested. In the approach, both experimental and theoretical investigation methods are used. Transmission electron microscopy combined with energy-dispersive X-ray spectrometry is used to determine the spatial distribution of the InGaAs alloy’s composition. The photoluminescence and photoconductivity spectra of the structures are recorded experimentally. In order to interpret the results in more detail, computer simulation of the epitaxial growth is performed. By simultaneously solving the Schrodinger equation and the Poisson equation, the energy states are calculated for the quantum-confined hetertostructure with initial and real composition profiles. The results of calculations are correlated with the data obtained for interband optical transitions from the analysis of the photoluminescence and photoconductivity spectra. Good agreement between the experimental and theoretical results is gained. The approach suggested in the study provides a means for refining the real geometrical features of the structure, for correlating the spectral results with the real composition profile of the structure, and for correcting the structural and growth parameters to improve the optical characteristics of the structure.

A comprehensive study of the crystal structure and distribution of impurities in GaAs nanoheterosystems containing InGaAs quantum wells and delta ‹Mn›-doped layers is performed. High-resolution transmission electronmicroscopy, energy dispersive spectroscopy, and X-ray diffraction are used in particular. It is shown that our technique allows growing structures with embedded delta layers of magnetic impurities epitaxially.

The Hall effect in InMnAs layers with MnAs inclusions of 20--50 nm in size is studied both theoretically and experimentally. We find that the anomalous Hall effect can be explained by the Lorentz force caused by the magnetic field of ferromagnetic inclusions and by an inhomogeneous distribution of the current density in the layer. The hysteretic dependence of the average magnetization of ferromagnetic inclusions on an external magnetic field results in a hysteretic dependence of ${R}_{\mathrm{H}}({H}_{\mathrm{ext}})$. Thus, we show the possibility of a hysteretic ${R}_{\mathrm{H}}({H}_{\mathrm{ext}})$ dependence (i.e., observation of the anomalous Hall effect) in thin conductive layers with ferromagnetic inclusions in the absence of carriers' spin polarization.

Current interest in nanomaterials is focused mostly on artificial materials fabricated for various applications. However, naturally occurring nanocrystal arrays, like those recently found in the meteorite that fell near Chelyabinsk in Russia on 15 February, 2013, can provide some new insights into the nature of nanomaterials, including the conditions for their natural occurrence. Here we report the results of our spectroscopic investigation of a fragment of the Chelyabinsk meteorite. The atomic structure of a fragment of the Chelyabinsk meteorite was studied using high-resolution transmission electron microscopy. Elemental and phase analysis of the object that was carried out by energy dispersive X-ray spectroscopy and electron diffraction revealed the presence of crystalline phases of different chemical compounds specific to meteorites of the LL group. In addition to single-crystal inclusions, extensive areas with ferropericlase nanocrystals having characteristic sizes from 3 to 15 nm were found in the structure of the meteorite. The study of the meteorite employing combination scattering of light (Raman) and photoluminescence spectroscopy methods has revealed quantum effects of ferropericlase nanoparticles and related photoluminescence with a maximum in the range of 675–800 nm.

Bipolar resistive switching phenomenon in the Au/Zr/ZrO2-Y2O3/TiN/Ti memristive devices deposited by magnetron sputtering has been studied. The structure of devices and electrical measurements data for the temperature range from 77 to 490 K are analyzed. The stable switching is demonstrated at room temperature, but the decrease in the resistive switching performance at elevated temperatures is observed.

Elastic strains have been measured quantitatively and clearly demonstrated, resulting in vertical stacking of Ge(Si) self-assembled islands in a silicon matrix using the geometric phase analysis method. The strains are calculated on the basis of the data on the elemental composition and crystal-lattice distortions. An empirical approach on the basis of the experiment has allowed the determination of geometric phase method in accuracy.