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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.

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.

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.

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.

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.