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Metal carbides are widely used in various applications as catalysts, components of composite materials, cutting tools, and abrasive materials where the particle/grain size has a great impact on target properties. Here, we report a facile way to synthesize nano- and ultra-fine carbides of IVb-Vb groups, including high-entropy carbides, which are of interest nowadays due to their superior properties, by the combination of calcium-hydride reduction of metal oxides and the direct reaction of the reduced metals with carbon in one production step. By tailoring the synthesis parameters, it is possible to obtain the carbide powders of a size in the range of 50–1500 nm with high crystallinity. The average particle size is found to be dependent on the homologous synthesis temperature and follows a power law. We have also shown the possibility of obtaining ultrafine-grained bulk carbides (d = 0.24–0.9 μm) via spark-plasma sintering at a temperature of 1500–1650 °C with a relative density >95 % without using additives. The HEC (Ti,Zr,Nb,Hf,Ta)C has a higher activation energy of grain growth (collective recrystallization) compared to binary TaC x (x = 0.8 and 1), which confirms the effect of lattice distortion on diffusion coefficients. The suggested fabrication route can be widely used for nano and ultrafine carbide powder preparation on the industrial scale. [ABSTRACT FROM AUTHOR]

The present work reports the direct production of a high-entropy (HE) intermetallic CoNi0.3Fe0.3Cr0.15Al material with a B2 structure from mechanically activated elemental powder mixtures. Fast and efficient combustion synthesis (CS), spark plasma sintering (SPS), and reactive SPS (RSPS) methods were used to synthesize the HE powders and bulks. The formation of the main B2 phase along with some amounts of secondary BCC and FCC phases are reported, and L12 intermetallic (CS scheme) and BCC based on Cr (CS + SPS and RSPS schemes at 1000 °C) were observed in all samples. The interaction between the components during heating to 1600 °C of the mechanically activated mixtures and CS powders has been studied. It has been shown that the formation of the CoNi0.3Fe0.3Cr0.15Al phase occurs at 1370 °C through the formation of intermediate intermetallic phases (Al9Me2, AlCo, AlNi3) and their solid solutions, which coincidences well with thermodynamic calculations and solubility diagrams. Compression tests at room and elevated temperatures showed that the alloy obtained by the RSPS method has enhanced mechanical properties (σp = 2.79 GPa, σ0.2 = 1.82 GPa, ε = 11.5% at 400 °C) that surpass many known alloys in this system. High mechanical properties at elevated temperatures are provided by the B2 ordered phase due to the presence of impurity atoms and defects in the lattice. [ABSTRACT FROM AUTHOR]

The three primary steps in the production of tungsten carbide WC and titanium carbide TiC powders are the preparation of the green mixture, carbidization by furnace annealing, and ball milling of the annealed products. This work performed a comprehensive parametric investigation of these three steps. The impact of several factors was examined including the carbon precursor, the mass and diameter of the milling bodies (balls), the milling time and speed, the temperature and length of the annealing process, the height of the powder in the furnace boats, and the rate at which the furnace boats move. Regression models for every stage of the process were verified by 10-fold validation and used to optimize the synthesis sequence, resulting in high-quality WC and TiC with a grain size below 2 microns and a content of free carbon below 0.1%. Additionally, solid solution (W,Ti)C was fabricated by mechanochemical synthesis from the elemental mixtures; however, further modification of this technique is necessary because of the observed relatively high concentration of residual free carbon (0.2–0.8%) and contamination by Fe. [ABSTRACT FROM AUTHOR]

The nanocomposite Cu–Cr powder was consolidated by flash spark plasma sintering (FSPS), which involves applying an extremely rapid change in the electrical power passing through the bulk of the sample. It was demonstrated that an essentially fully dense material could be obtained in 15 s. Such short-term treatment typically preserves the nanostructured features of the material. However, investigation revealed a nonuniformity in the microstructure of the alloys obtained under such extreme conditions. To better understand the observed effects, the FSPS process was simulated. It was observed that a rapid change in the applied electrical power resulted in nonuniform distributions of current density and temperature along the body of the consolidated material. Specifically, the current density was higher on the periphery of the sample, and the temperature was higher in the middle. These findings explain the observed structural transformation during FSPS and suggest an optimization strategy to avoid microstructural nonuniformity. [ABSTRACT FROM AUTHOR]

BaCe 0.7 Y 0.2 In 0.1 O 3–δ (BCYI) compositions were prepared by a modified Pechini method, following this the ceramic samples were consolidated using conventional sintering (CS) and spark plasma sintering (SPS) at 1250–1500 °C for 3–10 min. The structural and microstructural characteristics of the samples were determined using XRD, SEM and TEM. The total, bulk and grain boundary ionic conductivities were evaluated using the AC impedance method in dry air, wet air and dry Ar. It was shown that application of SPS in case of nanocrystalline BCYI allows to reduce the sintering time, and in case of microcrystalline BCYI application of SPS after CS allows to improve hardness and total conductivity through reduction of grain boundary resistance. [ABSTRACT FROM AUTHOR]

The densification kinetics for sintering of titanium spherical particles under two different experimental schemes, i.e., current-assisted and current-insulated were investigated. It was shown that measurable densification rate differences between the two schemes are recognized only for the preheating stage. For current-assisted experiments, consolidation starts at lower temperatures than for current-insulated samples. Also at high heating rates, the change of sample porosity all through the preheating stage is higher for current-assisted conditions, while at relatively low heating rates ( i.e., less than 100 K/min) they are comparable. All through the isothermal sintering stage, at a temperature of 1073 K (800 °C), the shrinkage rates are comparable for both experimental schemes within the measurement accuracy. The explanation of the observed effects within the framework of conventional sintering theory is also provided. [ABSTRACT FROM AUTHOR]

We report on thermoelectric properties of n-type nanostructured bulk SiGe with the addition of FeSi prepared via two sintering routes: the conventional spark plasma sintering method and a direct current pressing technique. The thermal conductivity, the electrical conductivity, and the Seebeck coefficient have been determined over the temperature range from 25°C to 900°C in a helium atmosphere. The highest ZT value for the multiphase nanostructured composite was reached at ∼0.6 at 900°C. Embedding of 10 at.% FeSi phase had a positive impact on thermal properties but dramatically affected the power factor, which eventually resulted in a drop of the thermoelectric efficiency. It was also shown that the orthorhombic β-FeSi2 phase transforms to a tetragonal α-FeSi2 phase during high temperature sintering. [ABSTRACT FROM AUTHOR]

In this work silicon carbide (SiC) sub-micron powders were synthesized using four different combustion routes, which include the following reactive systems: Si/C/Teflon (T); Si/C/N 2 (N); Si/C mechanically activated composite particles (MA) and reduction SiO 2 /Mg/C reaction (R). The specific surface area of the as-synthesized SiC T , SiC N , SiC MA and SiC R powders are: 4 ± 0.5; 17 ± 2; 20 ± 2 and 145 ± 5 m 2 /g respectively. These powders were subjected to spark plasma sintering (SPS) without any additives at 2073 K for 10 min. The densification kinetics for each powder were measured and discussed. The obtained ceramics possess a relative density in the range 80–95% and Vickers micro-hardness ( H v ) from 7 to 20 GPa. To illustrate the capability of the method, essentially pore-free ceramics were prepared from SiC MA powder by 1 h SPS without additives at 2273 K, with H v = 24 GPa and fracture toughness K 1C = 3–5 MPa m 1/2 . [ABSTRACT FROM AUTHOR]

High-entropy carbides (HECs) are paid great attention owing to superior properties, and various fabrication methods have been used to date to produce high-quality material. Here, a novel approach, in the case of HECs, is used to prepare powder and bulk (Ti,Zr,Hf,Nb,Ta)C: the calcium-hydride reduction (CHR) of oxides, followed by pressureless sintering (PS) and spark plasma sintering (SPS). The material obtained is characterized via TEM, SEM, and XRD. It has been shown that the CHR provides the formation of the nano-sized powder with a multiphase structure consisting of binary carbides. Subsequent PS and SPS lead to the formation of a single-phase structure; however, porosity differs significantly. As a bulk state, (Ti,Zr,Hf,Nb,Ta)C exhibits typical high hardness (20.4 GPa) and good fracture toughness (4.2 MPa∙m1/2). The results have shown that calcium-hydride reduction process, with proper development, can provide a new cost-effective technology for the synthesis of nano and submicron powders of high-entropy carbides. [ABSTRACT FROM AUTHOR]

Spark plasma sintering (SPS) is widely used for the consolidation of different materials. Copper-based pseudo alloys have found a variety of applications including as electrodes in vacuum interrupters of high-voltage electric circuits. How does the kinetics of SPS consolidation for such alloys depend on the heating rate? Do SPS kinetics depend on the microstructure of the media to be sintered? These questions were addressed by the investigation of SPS kinetics in the heating rate range of 0.1 to 50 K/s. The latter conditions were achieved through flash spark plasma sintering (FSPS). We also compared the sintering kinetics for the conventional copper–chromium mixture and for the mechanically induced copper/chromium nanostructured particles. It was shown that, under FSPS conditions, the observed maximum consolidation rates were 20–30 times higher than that for conventional SPS with a heating rate of 100 K/min. Under the investigated conditions, the sintering rate for mechanically induced composite Cu/Cr particles was 2–4 times higher compared to the conventional Cu + Cr mixtures. The apparent sintering activation energy for the Cu/Cr powder was twice less than that for Cu–Cr mixture. It was concluded that the FSPS of nanostructured powders is an efficient approach for the fabrication of pseudo-alloys. [ABSTRACT FROM AUTHOR]

A composite material based on polyethylene terephthalate (PET, about 1% by mass) and Al85Y8Ni5Co2 metallic glass was obtained by mechanical alloying and consequent spark plasma sintering. The spark plasma sintering was performed at a temperature near to the super cooled liquid region of the metallic glass. Mechanical properties and the structural characterization of the composite material were obtained. It was conceived that composite samples (Al85Y8Ni5Co2/PET) have a better thermal conductivity in comparison with pure PET samples. The formation of the crystalline phases causes degradation of physical properties. It was calculated that the activation energy for crystallization of the Al85Ni5Y8Co2 metallic glass is higher than that of the other types of metallic glasses (Mg67.5Ca5Zn27.5 and Cu54Pd28P18) used for composite preparation previously. This denotes a good thermal stability of the chosen metallic glass. [ABSTRACT FROM AUTHOR]

By a combination of mechanical activation, combustion synthesis, and spark plasma sintering methods, bulk materials in the Hf-Zr-C-N system with various Hf/Zr atomic ratios were produced. The mechanical activation process in a low-energy mode (350 rpm) for 90 min resulted in the formation of layered Hf/Zr/C composite particles 10–30 μm in size, depending on the Hf/Zr ratio. The diffraction spectra of the as-synthesized powders were described by several isomorphic (Hf,Zr)(C,N)ss cubic phases with different nitrogen content. Spark plasma sintering contributed to the homogenization of the chemical composition and allowed production of the dense (∼98%) single-phase hafnium-zirconium carbonitrides with an FCC structure. The bulk ceramics exhibited superior hardness (∼17 GPa) and fracture toughness (∼5 MPa m1/2), relative to (Hf,Zr)C carbides prepared by similar methods. [ABSTRACT FROM AUTHOR]

Reactive spark plasma sintering (RSPS) is an effective method for the consolidation of refractory compounds, including intermetallics. RSPS involves the SPS consolidation process and chemical reactions. SPS parameters such as the heating rate, maximum temperature, and dwell time influence the kinetics of the chemical reaction. To control the sintering process, it is critical to understand the relationship between the SPS parameters and different reaction modes. For a highly reactive Ni–Al system, we directly compare the consolidation kinetics under various SPS conditions. Comparing the RSPS and SPS schemes, we unequivocally conclude that the reactive mode permits an overall higher densification for the considered system. Two RSPS strategies are identified: i) RSPS under self-sustained reaction conditions occurring under a high heating rate and large volume-to-surface ratio, and ii) RSPS under relatively slow reaction rate conditions. The question of which strategy is best is addressed based on an analysis of the microstructures and mechanical properties of the fabricated materials. • Direct comparison of SPS, RSPS and RFSPS consolidate mode for intermetallic system. • Unequivocally showed that the reactive modes permit a higher densification. • The optimum consolidation strategies are suggested. • Material with exceptionally high mechanical properties fabricated in a minute. [ABSTRACT FROM AUTHOR]

The effect of pulsed direct current (PDC) on solid-state diffusion in the Ni–Al binary system was investigated. Two experimental schemes were employed: in the presence and absence of an electric field. The diffusion couples were heat-treated for 1.5, 3, and 5 h at 803, 833, and 863 K. Under the investigated conditions, only two intermetallic phases (NiAl 3 and Ni 2 Al 3) formed at the boundary of the metals. It was shown that the PDC passing through the diffusion couple significantly enhanced the growth rates of both phases. The apparent reaction–diffusion coefficients were D NiA l 3 = 4.0 × 10 − 9 exp (− 7.6 × 10 4 RT) and D N i 2 A l 3 = 9.7 × 10 − 9 exp (− 8.4 × 10 4 RT) in the field-assisted scheme, whereas their corresponding values in the field-insulated scheme were D NiA l 3 = 6.1 × 10 − 5 exp (− 1.55 × 10 5 RT) and D N i 2 A l 3 = 3.7 × 10 − 4 exp (− 1.71 × 10 5 RT). The results directly imply that the effective activation energy of diffusion decreases by approximately two times when the PDC passes through the media. [Display omitted] [ABSTRACT FROM AUTHOR]

Heusler alloys have attracted much attention due to their low toxicity, mechanical and thermal stability, relative abundance, and potential for thermoelectric power generation. Herein, we demonstrate that TiFe 2 Sn full-Heusler alloy can be obtained by a mechanochemical synthesis of metal precursors followed by spark plasma sintering. It is shown that thus prepared samples exhibit the ordered structure (Heusler phase, MnCu 2 Al-type). The influence of processing parameters on microstructure, phase composition, and thermoelectric properties of TiFe 2 Sn is studied. The thermoelectric figure of merit of the sample fabricated under optimal mechanochemical processing parameters is comparable to that of TiFe 2 Sn synthesized using the conventional route. Thus, our results indicate that the mechanochemical synthesis is a facile, scalable, and cost-effective route for the preparation of TiFe 2 Sn Heusler alloys. • TiFe 2 Sn prepared by mechanochemical synthesis followed by spark plasma sintering. • Mechanochemical synthesis performed at room temperature. • Figure of merit similar to that of TiFe 2 Sn synthesized by a conventional path. [ABSTRACT FROM AUTHOR]