High-temperature electrochemical synthesis of carbides, silicides and borides of VI-group metals in ionic melts

Possible systems and conditions have been determined for the electrosynthesis of carbides, silicides and borides of chromium, molybdenum and tungsten on the basis of thermodynamic analysis of chemical and electrochemical reactions involved in high temperature electrochemical synthesis. Electrosynthesis of powdered molybdenum and tungsten carbides has been carried out under thermodynamic conditions from halide–oxide, under excess CO₂ pressure, and tungstate–molybdate–carbonate melts. Electrosynthesis of powdered silicides and borides of chromium, molybdenum and tungsten has been realized under kinetic conditions. Oxide and halide–oxide electrolytes have been developed for the electrodeposition of molybdenum and tungsten carbide coat ings.     

Densification,mechanical, and tribological properties of ZrB2-ZrCx composites produced by reactive hot pressing

ZrB₂-ZrC_x composites were produced using Zr:B₄C powder mixtures in the molar ratios of 3:1, 3.5:1, 4:1, and 5:1 by reactive hot pressing (RHP) at 4-7 MPa, 1200°C for 60 minutes. X-ray diffraction analyses confirmed the formation of nonstoichiometric zirconium carbide (ZrC_x) with different lattice parameters and enhanced carbide formation by increasing the Zr mole fraction. An increase in applied pressure from 4 to 7 MPa was responsible for the improved relative density (RD) of 4Zr:B₄C composition from 86% to 99%. Microstructural studies on Zr-rich composites showed a reduction in unreacted B₄C particles and enriched elongated ZrB₂ platelets. Reaction and densification mechanism in 4Zr:B₄C composition were studied as a function of temperature increased from 600 to 1200°C at an applied constant pressure of 7 MPa. After 1000°C, <40 vol.% of unreacted Zr was observed during the densification process. Concurrently, low energies of carbon diffusion and carbon vacancy formation were found to enhance nonstoichiometric ZrC_x formation, which was found to be responsible for the completion of the reaction. The plastic deformation of unreacted Zr was responsible for the densification of the ZrB₂-ZrC_x composite. The results clearly showed that the applied pressure is five times lower than the reported values. Moreover, a temperature of 1200°C was sufficient to produce dense ZrB₂-ZrC_x composites. The improved microhardness, flexural strength, fracture toughness, and specific wear rate were 8.2-15 GPa, 265-590 MPa, 2.82-6.33 MPa.m^1/2, and 1.43-0.376 × 10⁻² mm²/N, respectively.     

Dissolving and stabilizing soft WB2 and MoB2 phases into high-entropy borides via boron-metals reactive sintering to attain higher hardness

Five single-phase WB₂- and MoB₂-containing high-entropy borides (HEBs) have been made via reactive spark plasma sintering of elemental boron and metals. A large reactive driving force enables the full dissolution of 10−20 mol. % WB₂ to form dense, single-phase HEBs, including (Ti_0.2Zr_0.2Hf_0.2Mo_0.2W_0.2)B₂, (Ti_0.2Ta_0.2Cr_0.2Mo_0.2W_0.2)B₂, (Zr_0.2Hf_0.2Nb_0.2Ta_0.2W_0.2)B₂, and (Zr_0.225Hf_0.225Ta_0.225Mo_0.225W_0.1)B₂; the successful fabrication of such single-phase WB₂-containing HEBs has not been reported before. In the processing science, this result serves perhaps the best example demonstrating that the phase formation in high-entropy ceramics can strongly depend on the kinetic route. A scientifically interesting finding is that HEBs containing softer WB₂ and/or MoB₂ components are significantly harder than (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂ (with harder binary boride components). This exemplifies that high-entropy ceramics can achieve unexpected properties.     

In situ reaction synthesis of high-entropy carbide/diboride composite with high mechanical properties

A novel technique to simultaneously lower the synthesis temperature of high-entropy carbides and maintain their high mechanical properties was proposed. Certain amount of carbon vacancies was first introduced to significantly lower the temperature down to 2000°C for uniform elemental distributions in high-entropy carbides. Those carbon vacancies were then fully eliminated through the reaction between high-entropy carbides and certain amount of boron carbide. Concomitantly, the high-entropy boride phase was formed. The elimination of carbon vacancies and the formation of high-entropy boride phase significantly improved the mechanical properties of the high-entropy carbides. A high mechanical strength over 500 MPa can be obtained by phase optimization.     

Ultrafast high-temperature synthesis and densification of high-entropy carbides

Recently, high-entropy carbides have attracted great attention due to their remarkable component complexity and excellent properties. However, the high melting points and low self-diffusion coefficients of carbides lead to the difficulties in forming solid solution and sintering densification. In this work, six dense multicomponent carbides (containing 5–8 cations) were prepared by a novel ultrafast high-temperature sintering (UHS) technique within a full period of 6 min, and three of them formed a single-phase high-entropy solid solution. The solid solubility of the UHSed multicomponent carbides was highly sensitive to the compositional variation. The presence of Cr₃C₂ liquid had significant contributions to the formation of solid solution and the densification of multicomponent carbides. All UHSed multicomponent carbides exhibited high hardness, which, unexpectedly, did not simply increase with increasing number of the components. The highest nanohardness with a value of 36.6 ± 1.5 GPa was achieved in the (Ti_1/5Cr_1/5Nb_1/5Ta_1/5V_1/5)C_x high-entropy carbide. This work is expected to expedite the development of high-entropy carbides and broaden the application of UHS in the synthesis and densification of advanced ceramics.     

Synthesis of high-purity high-entropy metal diboride powders by boro/carbothermal reduction

Synthesis of high-purity high-entropy metal diboride powders is critical to implementing their extensive applications. However, the related studies are rarely reported. Herein we first theoretically studied the synthesis possibility of high-purity high-entropy diboride powders, namely (Hf_0.25Ta_0.25Nb_0.25Ti_0.25)B₂ (HTNTB), via boro/carbothermal reduction by analyzing the thermodynamics of the possible chemical reactions and then successfully synthesized the high-purity and superfine HTNTB powders via boro/carbothermal reduction for the first time. The as-prepared powders exhibited low-oxygen impurity content of 0.49 wt% and small average particle size of 260 nm. Meanwhile, they possessed good single-crystal hexagonal structure of metal diborides and high-compositional uniformity from nanoscale to microscale. This work will open up a new research field on the synthesis of high-purity high-entropy metal diboride powders.     

Selection principle of the synthetic route for fabrication of HfB2 and HfB2-SiC ceramics

Densification, microstructure, and mechanical properties of spark plasma sintered HfB₂ and HfB₂-SiC ceramics using HfB₂ powders from borothermal reduction and boro/carbothermal reduction were investigated and compared. It was found that HfB₂ceramics obtained by boro/carbothermal reduction exhibited a significantly higher sinterability compared to that by borothermal reduction. Inversely, HfB₂-SiC ceramics obtained by borothermal reduction exhibited a refined microstructure and better mechanical properties (Vickers hardness: 23.60 ± 2.43 GPa; fracture toughness: 5.89 ± 0.30 MPa.m^1/2) than that by boro/carbothermal reduction. These results indicated that optimal fabrication of HfB₂-based ceramics could be achieved by the selection of synthetic route of HfB₂ powders.     

A novel approach to the rapid synthesis of high-entropy carbide nanoparticles

A novel strategy for the rapid synthesis of high-entropy carbide particles is proposed that involves the transformation of multicomponent intermetallic intermediates to multicomponent carbides (high-entropy carbide precursors). (Ti_0.25V_0.25Nb_0.25Ta_0.25)C nanoparticles with a uniform solute distribution were successfully synthesized in an Al matrix by heating Al-Ti-V-Nb-Ta-C powder mixtures at 1500°C for 10 minutes. The multicomponent aluminide intermediates led to the rapid formation of multicomponent carbides during heating to 1100°C, which transformed into a high-entropy solid solution during heating to 1500°C. We developed a new rapid approach for the synthesis of high-entropy ceramic particles.     

Characterization and analysis of high-entropy boride ceramics sintered at low temperature

Multicomponent transition metal boride composite–sintered bodies were prepared by spark plasma sintering, and the composite sintered bodies prepared at different sintering temperatures (1500–1900°C) were characterized. The experimental results showed that several other compounds diffused into the TiB_x phase at lower sintering temperatures under the combined effect of temperature and pressure due to the nonstoichiometric ratio of TiB_1.5 vacancies. When the temperature reached 1900°C, only the hexagonal phase remained. With the continuous increase of sintering temperature, the Vickers hardness and fracture toughness of the sintered bodies had a trend of increasing first and then decreasing, due to the continuous reduction of the porosity of the cross section of the sintered bodies and the growth of the grain size. The Vickers hardness and fracture toughness of sintered body obtained at 1800°C are the best, which are 24.4 ± 1.8 GPa and 5.9 ± 0.2 MPa m^1/2. At 1900°C, the sintered body was a single-phase hexagonal high-entropy diboride. Its Vickers hardness and fracture toughness were 21.9 ± 1.5 GPa and 5.4 ± 0.2 MPa m^1/2, respectively; it showed a clear downward trend.     

Bipolar high-power impulse magnetron sputtering synthesis of high-entropy carbides

In this study, we report high-entropy carbides synthesis with reactive bipolar high-power impulse magnetron sputtering (HiPIMS). Uncontrolled microstructure and stoichiometry development with reactive gas flow rate are major limitations of conventional direct current (DC) and radio frequency (RF) magnetron sputtering of multicomponent carbides. With HiPIMS these chemically disordered crystals structurally and compositionally transform from a carbon-deficient metallic (C/M < 1), to a stoichiometric ceramic zone (C/M ∼ 1), and to a nanocomposite embodiment (C/M > 1), as a function of the carbon content during HiPIMS deposition. X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, and nanoindentation hardness measurements are combined to demonstrate the three regions of synthesis domain. HiPIMS provides access to metallic, ceramic, and composite carbides with great control over the microstructure and stoichiometry, which is elusive in case of conventional DC and RF magnetron sputtering. Notably, the stoichiometric ceramic zone maintains a constant carbon to metal ratio (C/M ∼ 1) over an extended amount of methane flow before transitioning to a nanocomposite microstructure (C/M > 1). The transition zone breadth depends on materials affinity for carbon that correlates with valence electron concentration (VEC). As such, synthesis conditions for new high-entropy carbides can be understood and predicted based on VEC.     

Low-temperature molten salt synthesis of high-entropy carbide nanopowders

Synthesis of the powders is critical for achieving the extensive applications of high-entropy carbides (HECs). Previously reported studies focus mainly on the high-temperature (>2000 K) synthesis of HEC micro/submicropowder, while the low-temperature synthesis of HEC nanopowders is rarely studied. Herein we reported the low-temperature synthesis of HEC nanopowders, namely (Ta_0.25Nb_0.25Ti_0.25V_0.25)C (HEC-1), via molten salt synthesis for the first time. The synthesis possibility of HEC-1 nanopowders was first theoretically demonstrated by analyzing lattice size difference and chemical reaction thermodynamics based on the first-principle calculations, and then the angular HEC-1 nanopowders were successfully synthesized via molten salt synthesis at 1573 K. The as-synthesized nanopowders possessed the single-crystal rock-salt structure of metal carbides and high compositional uniformity from nanoscale to microscale. In addition, their formation mechanism was well interpreted by a classical molten salt-assisted growth.     

The role of microstructure on high-temperature oxidation behavior of hafnium carbide

Microstructure development of the products formed upon oxidation of hafnium carbide (HfC_x, x = 0.65, 0.81, or 0.94) at 1300°C and 0.8 mbar oxygen pressure was investigated using Raman spectroscopy, X-ray diffraction, electron microscopy, and electron energy-loss spectroscopy. For all specimens a multilayered oxide scale was observed featuring an outermost porous hafnia layer and an interlayer adjacent to the parent carbide containing hafnia interspersed with carbon. The outermost hafnia features coarse pores presumably formed during initial stages of oxidation to allow rapidly evolving gaseous products to escape from the oxidation front. As the oxidation scale thickens, diffusional resistance results in slower oxidation rates and smaller quantities of gaseous products that are removed via networks of increasingly fine pores until the local oxygen partial pressure is sufficiently low to selectively oxidize the parent carbide. Electron microscopy studies suggest that the oxidation sequence at this stage begins with the transformation of parent carbide to an amorphous material having empirical formula HfO₂C_x that subsequently phase separates into hafnia and carbon domains. Hafnia polymorphs in the phase-separated region vary from cubic to monoclinic as grains coarsen from ca. 2–20 nm, respectively. Immediately adjacent to the phase-separated region is carbon-free mesoporous hafnia whose pore morphology is inherited from that of prior carbon domains. The average pore size and pore volume fraction observed in mesoporous hafnia are consistent with predictions from kinetic models that ascribe gaseous diffusion through a pore network as the rate determining step in oxidation behavior of hafnium carbide. These observations imply that high-temperature oxidation behavior of hafnium carbide under the employed test conditions is linked to microstructure development via phase separation and coarsening behaviors of an initially formed amorphous HfO₂C_x product.     

Effects of refractory metal additives on diboride-based ultra-high temperature ceramics: A review

Diboride-based ultra-high temperature ceramics (UHTCs) are a special class of ceramics with excellent comprehensive properties, which have extensive potential applications in extreme environments. However, their practical applications are limited, mainly due to the poor fracture toughness and thermal shock resistance. Refractory metals have high melting points, good ductility, and high toughness, which have huge potential to improve the properties of diboride-based ceramics. As a special class of additives, they have been adopted to promote densification, improve microstructure, and properties. However, diboride-based ceramics containing refractory metals have not received adequate attention due to relatively weak practical effects on property improvement. The present review highlights the progress and existing problems of transition metal diborides with refractory metal additives, including W, Ta, Mo, Nb, Hf, V, Cr, and Zr, focusing mainly on the microstructure change and property improvements, followed by challenges and possible future development strategies.     

Synthesis of the ternary metal carbide solid-solution ceramics by polymer-derived-ceramic route

The polymer-derived-ceramic (PDC) route has been widely used to fabricate the transition-metal carbides (TMCs). Previously reported works focused mainly on the synthesis of the single or binary TMCs, while the synthesis of the ternary or more component TMCs was rarely reported. Herein, a class of the ternary TMCs, namely (Nb_1/3Zr_1/3Ta_1/3)C solid-solution ceramics, was successfully synthesized via PDC route for the first time. The as-synthesized ceramics exhibited the particle-like morphology with an average particle size of ~250 nm and showed a single rock-salt crystal structure of metal carbides. At the same time, they had high compositional uniformity from nanoscale to microscale. In addition, they possessed low-oxygen impurity content of 0.79 wt% and moderate-carbon impurity content of 8.98 wt%. Such work provides a novel route to fabricate the ternary or more component TMCs.     

Thermal and electrical properties of a high entropy carbide (Ta,Hf, Nb,Zr) at elevated temperatures

The thermal and electrical properties were measured for a high entropy carbide ceramic, consisting of (Hf, Ta, Zr, Nb)C. The ceramic was produced by spark plasma sintering a mixture of the monocarbides and had a relative density of more than 97.6%. The resulting ceramic was chemically homogeneous as a single-phase solid solution formed from the constituent carbides. The thermal diffusivity (0.045–0.087 cm²/s) and heat capacity (0.23–0.44 J/g•K) were measured from room temperature up to 2000°C. The thermal conductivity increased from 10.7 W/m•K at room temperature to 39.9 W/m•K at 2000°C. The phonon and electron contributions to the thermal conductivity were investigated, which showed that the increase in thermal conductivity was predominantly due to the electron contribution, while the phonon contribution was independent of temperature. The electrical resistivity increased from 80.9 μΩ•cm at room temperature to 114.1 μΩ•cm at 800°C.     

Synthesis of transition metal carbides and high-entropy carbide TiZrNbHfTaC5 in self-shielding DC arc discharge plasma

The paper shows the feasibility of synthesizing micro- and nano-sized particles of binary metal carbides (Me–C) and high-entropy carbide (HEC) TiZrNbHfTaC₅ by vacuum-free electric arc method. The method is based on the effect of self-shielding of the reaction volume from atmospheric oxygen by carbon monoxide CO, which is generated during arcing in air. The synthesis results in a solid solution with a NaCl-type carbide with a cubic lattice, which simultaneously contains atoms of titanium, zirconium, niobium, hafnium, tantalum, and carbon. The lattice parameter of the HEC TiZrNbHfTaC₅ phase is ～4.532 Å that is in line with the known data on this compound. The synthesis product contains micro-sized particle agglomerates of transition metal carbides. The synthesis products also contain nano-sized particles with a shell-core structure, in which the core can consist of metal carbide (TiC, ZrC, NbC, HfC, TaC) or HEC TiZrNbHfTaC₅, and the shell is a graphite phase.     

Review of phase stability in the group IVB and VB transition‐metal carbides

This review summarizes the phase stability in the group IVB (Ti‐C; Zr‐C; Hf‐C) and group VB (V‐C; Nb‐C; Ta‐C) transition‐metal carbides. The order parameter functional (OPF) method and density functional theory (DFT) method have been used to predict phase equilibria in these systems. Extensive experimental investigations have attempted to determine both phase stability as a function of composition as well the crystal structures present using X‐ray diffraction, neutron diffraction, electron backscatter detection, and selected area electron diffraction. These investigations have demonstrated that the structures that form are based on the close‐packing of the metal atoms and the arrangement of the carbon atoms in the octahedral interstices. In general, the rocksalt B1 phase is stable for all of the transition‐metal carbides, with their substoichiometry tolerance increasing with temperature; vanadium carbide is the exception due to its negative vacancy formation energy. Vacancy‐ordered M₆C₅ phases have been predicted and experimentally confirmed in both groups of carbides; however, kinetic limitations often inhibit the formation of vacancy‐ordered phases, which has contributed to controversy in phase identification. The vacancy‐ordered M₄C₃ phase has been predicted for select carbides and has only been observed in zirconium carbide. In contrast, the stacking fault phase ζ‐M₄C_3?x has been readily reported in the group VB carbides (but not in the group IVB carbides). The vacancy‐ordered M₃C₂ has been predicted by DFT for the group IVB carbides but not in the group VB carbides, whereas OPF predicts its stability in both carbides. Vacancy‐ordered M₃C₂ phases have been experimentally observed in the Ti‐C and Hf‐C systems. Finally, the M₂C phase has been predicted in both group carbides, except for hafnium carbide, with an order‐disorder transition with temperature. These factors result in phase diagrams that are similar among all the carbides, but each phase diagram is unique due to subtle differences in bonding that result in slight differences in thermodynamically stable phases.     

Measurement of the melting temperature of ZrB2 as determined by laser heating and spectrometric analysis

The melting temperatures of two different ZrB₂ ceramics were studied using laser induced melting. ZrB₂ having a low Hf content, produced by reaction hot pressing, had a melting temperature of 3546 K and a commercial grade ZrB₂ had a melting temperature of 3553 K. Uncertainty of the temperature measurements was 1% of the absolute temperature, or ~35 K for both materials based upon 2-sigma and a 95% confidence interval. While these values were consistent with the previously reported ZrB₂ melting temperature of 3518 K, this study was able to measure T_m with less uncertainty than previous studies (±45 K). Furthermore, this study assessed the effect of Hf content on melting temperature, finding that melting temperature did not change significantly for hafnium contents of 1.75 to 0.01 at%. This study also measured a normal spectral emissivity of 0.34 for ZrB₂ at 3000 K. The emissivity decreased to 0.28 at the melting temperature, then, stabilized at 0.30 in a liquid phase.     

Effects of SiC nanowires and whiskers on high-entropy carbide-based composites sintered at low and high temperatures

This study aimed to investigate the toughening effects of SiC nanowires (SiC_nw) and SiC whiskers (SiC_w) on high-entropy carbide based composites prepared at different temperatures (1600°C and 2000°C). At low temperature (1600°C), SiC_nw and SiC_w maintain their original morphology and properties, and exhibit the good toughening effects. The SiC_nw with larger aspect ratio and more curly wires exhibit a much stronger toughening effect on the (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 composites reinforced with 15 vol.% SiC_nw, which shows the highest value of fracture toughness about 6.7 MPa∙m^1/2. However, at high sintering temperature (2000°C), SiC_nw and SiC_w are prone to thermal-induced damages, which significantly reduces their mechanical properties, and thus, toughening effects on (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 composites. The addition of SiC_w, which have better thermal stability at 2000°C, results in the (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8–15 vol.% SiC_w composite exhibiting relatively better fracture toughness, about 3.7 MPa∙m^1/2. Based on the results of the current study, the critical influence of SiC_nw and SiC_w on the toughening of (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 composites is highly dependent on their high-temperature thermal stability.