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We have studied the hydrolytic stability of Y2.5Nd0.5Al5O12-based garnet ceramics produced by spark plasma sintering. The ceramics have been tested under hydrothermal conditions in an autoclave and, for comparison, in static mode at room temperature. The mechanisms of yttrium and neodymium leaching from the ceramics have been identified.

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.

A glycine-nitrate self-propagating high-temperature synthesis (SHS) was developed to produce composite MgO-Gd2O3 nanopowders. The X-ray powder diffraction (XRD) analysis confirmed the SHS-product consists of cubic MgO and Gd2O3 phases with nanometer crystallite size and retains this structure after annealing at temperatures up to 1200 °C. Near full dense high IR-transparent composite ceramics were fabricated by spark plasma sintering (SPS) at 1140 °C and 60 MPa. The in-line transmittance of 1 mm thick MgO-Gd2O3 ceramics exceeded 70% in the range of 4–5 mm and reached a maximum of 77% at a wavelength of 5.3 mm. The measured microhardness HV0.5 of the MgO-Gd2O3 ceramics is 9.5±0.4 GPa, while the fracture toughness (KIC) amounted to 2.0±0.5 MPa·m1/2. These characteristics demonstrate that obtained composite MgO-Gd2O3 ceramic is a promising material for protective infra-red (IR) windows.

High-density (95.3–95.8%) CeO2–xα-SiC (x = 10, 20 vol %) composite ceramics have been produced for the first time by spark plasma sintering (SPS) as promising mineral-like materials for inert fuel matrices. The SPS of CeO2–SiC ceramics has been shown to be a three-stage process. The addition of α-SiC particles shifts characteristic temperatures of CeO2 sintering to higher values. SPS leads to an increase in the hardness, fracture toughness, and thermal conductivity of cerium dioxide-based ceramics.

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.

The results of X-ray diffraction analysis of qualitative and quantitative phase composition of powder and solid polycrysral samples of the system a-WC/W 2 C are presented. Powder (nanopowder) samples were obtained by plasmochemical synthesis from tungsten oxide and hydrocarbon. High-density fine-grained structure of ceramics was formed by electropulse plasma sintering (spark plasma sintering) of initial industrial a-WC powders (AlfaAesar). Experimental data were obtained on a Shimadzu XRD-7000 X-ray diffractometer (CuKa, λ = 1.54 A). The optimal modes of X-ray shooting of the ceramic samples based on tungsten carbide are presented. In the absence of reference standards for initial crystalline phases (WC and W2C), quantitative phase analysis was carried out using the reference intensity ratio (RIR) or corundum number (instead of the method of calibration curve). The required quantitative ratios were calculated using the structural parameters of corundum a-Al 2 O 3 and phases a-WC, W 2 C. The results of determining the repeatability of the intensity values were used to select the optimal exposure time and width of the receiving slit of the detector of diffracted X-rays. Study of the effect of the quality of sample preparation on the sensitivity of qualitative X-ray phase analysis showed the feasibility of grinding of the surface of solid ceramic samples and their pre-polishing with a diamond paste (grain size not less than 5/3 pm). The accuracy of quantitative phase analysis of powder and sintered ceramic samples was evaluated. The layer-by-layer phase analysis of the ceramic preform revealed a preferential crystallite orientation (texture) in the surface layer and the absence of the impurity phase W 2 C. The sensitivity of X-ray phase analysis to the content of a-W 2 C phase in nanopowders of tungsten monocarbide has been estimated.

The results of investigation of thermoelectric materials fabricated by spark plasma sintering and based on Si1 –xGex solid solutions doped with Sb to a concentration of 0–5 at % are presented. It was found that, at Sb concentration below 1 at %, efficient doping of the solid solution was carried out during the sintering process, which allowed us to form a thermoelectric material with a relatively high thermoelectric figure of merit. An increase in the concentration of antimony in the range of 1–5 at % led to a change in the mechanism of doping, which resulted in an increase in the resistance of materials and the segregation of Sb into large clusters. For such materials, a significant decrease in the Seebeck coefficient and thermoelectric figure of merit was noted. The highest obtained thermoelectric figure of merit (ZT) with Sb doping was 0.32 at 350°C, which is comparable with known analogues for the GexSi1 –x solid solution.

Applicability of Spark Plasma Sintering has been studied for production of high-density (98.6–99.5%) composite ceramics Y2.5Nd0.5Al5O12 (YAG:Nd) – x MgO (x = 5, 10, 20 vol %). Powder YAG:Nd-MgO compositions have been obtained by precipitation of the MgO phase from Mg(NO3)2 water solution of magnesium nitrate onto the surface of garnet nanoparticles YAG:Nd. It has been demonstrated that the intensity of sintering fine-grained YAG:Nd-MgO composites is governed by volume diffusion at low temperatures and by grain boundary diffusion at high temperatures. Faster sintering at low temperatures is caused by formation of MgAl2O4 spinel particles and internal stress fields that are manifested as broadening and shifting of garnet peaks in the sintered composites.

Fine-grained SrWO4 and NaNd(WO4)2 ceramics with the scheelite structure having high relative densities (99 and 95.8%), which can appear candidate matrices for radioactive waste (RAW) management, are prepared by spark plasma sintering (SPS). The phase identity of the ceramics is determined by X-ray powder diffraction; their microstructure is studied by X-ray photoelectron spectroscopy. The tungstates under study are sintered at rather low temperatures (580–665°C). The intensity of compaction of the tungstates at the early sintering stage is determined by the degree of powder agglomeration. The activation energy of fine-grained scheelite ceramics at high temperatures corresponds with the activation energy of grain-boundary oxygen diffusion.

A bulk nanostructured material based on oxidized silicon nanopowder was fabricated using a spark plasma sintering technique. Structural investigations revealed that this material has the composition of ∼14 nm core Si granules inside an SiO2 shell. Photoluminescence measurements have shown that the emission spectra lie in the energy range of 0.6–1.1 eV, which is not typical of the emissions of the Si/SiO2 nanostructures reported in numerous papers. This result can be explained by the formation of energy states in the bandgap and the participation of these states in both electronic transport and photoluminescence emission. Annealing of the sample leads to a decrease in defect density, which in turn leads to quenching of the 0.6–1.1 eV photoluminescence. In this case ∼1.13 eV inter-band transitions in the Si core start to play a dominant role in radiative recombination. Thus, the possibility of controlling the photoluminescence emission over a broad wavelength range was demonstrated.

This paper is a study of sintering mechanisms, structure, and mechanical properties of ultrafine-grained 95W-Ni-Fe tungsten heavy alloys. Powder particle sizes were controlled by mechanical activation (MA) of original coarse-grained components and by addition of ultrafine particles. W-Ni-Fe alloys were obtained by sintering in hydrogen and Spark Plasma Sintering (SPS) in a vacuum. The dependence of ultrafine-grained (UFG) alloy density on sintering temperatures has been found to be non-monotonic with a maximum corresponding to the optimal sintering temperature. It has been demonstrated that the sintering activation energy of UFG alloys is significantly lower than that of coarse-grained alloys. It has been shown that the optimal SPS temperature for mechanically activated nanopowders goes down by 350–400 °C in comparison with the optimal sintering temperature in hydrogen for coarse-grained 95W-Ni-Fe powder composition. The reason for a lower optimal sintering temperature lies in a decreased activation energy of grain-boundary diffusion and formation of a non-equilibrium solid solution of nickel and iron in the surface layer of tungsten α-W particles during high-energy MA. High-energy MA and SPS were used to obtain samples of UFG tungsten alloys with high mechanical properties: macro-elastic limit – up to 2250 MPa, yield stress – up to 2500 MPa.

We have studied the feasibility of preparing high-density (98.6–99.5%) Y2.5Nd0.5Al5O12 (YAG)–xMgO (x = 5, 10, 20 vol %) composite ceramics by spark plasma sintering. YAG–MgO powder materials have been prepared via MgO precipitation from an aqueous solution of magnesium nitrate, Mg(NO3)2, on the surface of garnet particles. The sintering rate of the YAG–MgO composites has been shown to be controlled by volume diffusion at low temperatures and by grain-boundary diffusion at elevated temperatures.

Thermoelectric materials on the basis of SiGe produced by spark plasma sintering are studied. The degree of doping of the initial materials, the degree of mixing of Si and Ge, and the properties of the grain structure of the formed samples are varied. It is established that the use of initial materials doped with donor or acceptor impurities provides a means for controlling the charge-carrier concentration in the sintered samples. It is shown that the charge-carrier concentration has the most profound effect on the energy characteristics (power factor) of thermoelectrics. For structures with the highest charge-carrier concentration, the thermoelectric coefficients can be controlled by varying the degree of mixing of Ge and Si and the degree of uniformity of the distribution of impurities.

The mechanisms for the sintering of ultrafine-grained 95%W–3.5%Ni–1.5%Fe heavy tungsten alloy powders have been investigated. It has been established that a decrease in the activation energy of grain boundary diffusion and the formation of a nonequilibrium solid solution of nickel and iron in the surface layer of tungsten particles upon high-energy ball milling are responsible for the decrease in optimal sintering temperature.

The paper dwells on the research conducted into high-rate consolidation of pure tungsten carbide (WC) nanopowders using the Spark Plasma Sintering technology. Studies included the effect that the original size of WC nanoparticles and their preparation modes have on density, structure parameters, and mechanical properties of tungsten carbide. Samples of high-density nanostructured tungsten carbide characterized by high hardness (up to 31–34 GPa) and improved fracture toughness (4.3–5.2 MPa m 1/2 ) were obtained. It has been found that materials that show abnormal grain growth during sintering have lower values of sintering activation energy as compared to materials the structure of which is more stable during high-rate heating. A qualitative model is proposed that explains this effect through the dependence of the grain boundary diffusion coefficient on the grain boundary migration rate.

Using wet chemistry techniques, we have obtained ultrafine powders of complex oxides Y2.5Nd0.5Al5O12 with garnet structure and coated with layers of metals (Mo, W, Ni) by means of precipitation from salt solutions. Spark plasma sintering (SPS) was used to obtain fine-grain ceramic composites Y2.5Nd0.5Al5O12 – Met (Met = Mo, W, Ni) with 92–99% density. We have determined sintering activation energies and demonstrated that the dependence of shrinkage of powders on the heating temperature has a two-stage character: the stage of compaction of nanoparticles due to their regrouping and plastic flow under the action of applied stress and the direct sintering stage.

NaxSr1 – 2xNdxMoO4 (x = 0–0.5, Δx = 0.1) solid solutions with the scheelite structure have been synthesized for the first time and their crystallographic parameters have been determined as functions of composition and temperature. Their thermal expansion coefficients have been determined in the temperature range 25–1000°C: αa = (12.9–14.9) × 10–6 °C–1 and αc = (24.9–25.9) × 10–6 °C–1. Using spark plasma sintering at tmax ≈ 872–985°C and τ ≈ 650–750 s, we have prepared ceramics with relative densities in the range 97–99%., This work was supported by the Russian Science Foundation, project no. 16-13-10464: Promising Mineral-Like Ceramic Materials with Improved and Controlled Functional Characteristics: Development, Synthesis, and Characterization.

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.

Investigations of plastic deformation of fine-grained ceramics based on alumina obtained by the method of spark plasma sintering in the temperature range of 1150-1320 ° C at an applied stress of 40-100 MPa are presented. Based on the experimental data, the parameters of rheological equations were established. It is shown that the mechanism of high-temperature deformation of alumina is superplasticity. In the process of plastic deformation, the grains of alumina do not stretch, which confirms the mechanism of superplasticity - grain boundary sliding. The activation energy for superplastic deformation was Q = 17.8 kTm at d = 6 μm and Q = 24.6 kTm at d = 1 μ m, which is very close to the activation energy of grain boundary diffusion in aluminum oxide Qb = 20 kTm. For a coarse-grained material, an assumption was made about a possible mechanism for lowering the activation energy due to the generation of dislocations by the grain boundary.

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.

Parameters of powder WC + 10% Co sintering modes that exclude the possibility of η-phase (Co3W3C) formation were determined. The phase composition was controlled by introducing colloidal graphite into the original powder.

Ca0.75 + 0.5xZr1.5Fe0.5(PO4)3 –x(SiO4)x (x = 0–0.5) solid solutions have been synthesized by a sol–gel process and characterized by X-ray diffraction, IR spectroscopy, and differential scanning calorimetry. As expected, the synthesized phosphatosilicates crystallize in a NaZr2(PO4)3-type structure (trigonal symmetry, sp. gr. R $$\bar {3}$$ c). The thermal expansion of the solid solutions has been studied by high-temperature X-ray diffraction in the temperature range from 25 to 800°C. Their thermal expansion parameters have been calculated and analyzed as functions of composition. High-density ceramics based on the Ca0.875Zr1.5Fe0.5(PO4)2.75(SiO4)0.25 phosphatosilicate have been produced by spark plasma sintering and their structure and properties have been studied in detail.

The article presents the results of the investigation of the initial sintering stage of alumina. The authors determined the activation energy of shrinkage at the initial stage of sintering. The study shows that the additives do not change the kinetics of shrinkage at the initial stage of SPS.

The article shows the use of an experimental-calculation method for the refinement of thermoelectric constants of graphite materials and alumina ceramics by experimentally studying the heating of graphite equipment and refinement thermoelectric constants of materials using the finite element method (ANSYS WORKBENCH). To simulate the thermal fields of graphite equipment based of the data on electric current, voltage and heat exchange conditions, a coupled boundary electric and non-stationary temperature problem was realized. The application of the developed approach made it possible to clarify the thermoelectric constant of materials, which helped to reduce the difference between the experimental and simulated temperatures from 20 to 5%.

The article presents the results of the investigation of the initial sintering stage of alumina. The authors determined the activation energy of shrinkage at the initial. The study shows that the effect of additives is observed only at the final stage of sintering.

This is the first ever paper to address the prospects of using Spark Plasma Sintering (SPS) for high-rate diffusion welding of a high-strength ultrafine-grained (UFG) α-titanium alloy Ti–5A–2V. The effect of growing diffusion welding intensity in UFG titanium alloys is also described. Welds of a UFG α-titanium alloy Ti–5Al–2V obtained with SPS are characterized by high density, strength, and corrosion resistance. The rate of weld sealing in UFG alloys is shown to be in nonmonotonic (maximum) dependence on the heating rate.

The results of the qualitative and quantitative X-ray powder diffraction analysis of the W–C system samples are presented. Powder samples were nanopowders of α-WC/W2C obtained by plasma chemical synthesis. High-density fine-grained structure of ceramics was formed using the method of electro pulse plasma sintering (spark plasma sintering) from the initial powder of the α-WC. The optimal experimental conditions for the X-ray diffraction proceeding of the tungsten carbide based ceramic samples were chosen according to the preliminary experiments. Reference intensity ratio method for quantitative phase analysis was used in this study.

We have obtained powders of garnet-type complex oxide Y 2.5 Nd 0.5 Al 5 O 12 – x vol.% SiC (x = 0, 10, 20) using wet chemistry techniques. The ceramics based on the studying compounds were sintered using Spark Plasma Sintering (SPS) (Т sintering = 1320-1350 °С, Р = 80 MPa, t sintering = 7 min, t shrinkage = 2 min) with relative density ~98%. Microstructure of the obtained composites and influence of the concentration of silicon carbide on their mechanical (microhardness, fracture toughness) properties were investigated.