A facile route for the synthesis of high-entropy transition carbides/borides at low temperatures

As the main candidates in the field of ultra-high temperature ceramics, high entropy carbides/borides (HECs/HEBs) have good oxidation resistance properties, high hardness, as well as excellent thermal and electrical conductivities, which are the focused points of research nowadays. In the current study, (Hf,Ta,Zr,Nb,Mo,Ti)C powders were successfully synthesized by a three-step process, including the mixing process of raw oxides and carbon black with spaying Fe(NO₃)₃ solution, carbothermal reduction and subsequent calcium posttreatment. For the preparation of (Hf,Ta,Zr,Nb,Mo,Ti)B₂ powders, during the calcium posttreatment process, equal stoichiometric ratio of B₄C was added for the purpose of boriding reaction. The relevant X-ray diffraction and SEM characterizations indicate the successful preparations of face-centered cubic HECs and hexagonal HEBs. However, slight Mo local segregation was found in the prepared (Hf,Ta,Zr,Nb,Mo,Ti)B₂ powders. The iron generated from Fe(NO₃)₃ promotes the solid solution process between monocarbides during the carbothermal reduction process via the dissolution-diffusion-precipitation mechanism. In the calcium posttreatment process, the liquid calcium ensures the boriding reaction take place at a low temperature. In addition, the residual carbon could be combined with calcium to generate CaC₂ which is easy to be removed by acid leaching, and meanwhile, the added Fe could also be finally eliminated to produce pure HEC/HEB powders. The current method does not require the long-time high energy ball milling of raw materials, but only simple and mild mixing is enough. Therefore, such a facile route has a great potential application prospect for industrially preparing high entropy phase powders in a large scale.     

Phase stability,mechanical and thermodynamic properties of (Hf,Zr, Ta,M)B2 (M= Nb,Ti, Cr,W) quaternary high-entropy diboride ceramics via first-principles calculations

As the high-entropy design concept applied to the diboride ceramic system, high-entropy diboride ceramics with a wide range of composition control, is expected to become a new high-performance material for extreme high-temperature environments. Herein, the effects of four transition metal elements (Nb, Ti, Cr, W) on the phase stability and properties of (Hf, Zr, Ta)B₂-based high-entropy diboride ceramics are systematically investigated via the first-principles calculations. All components were identified as thermodynamically, mechanically and dynamically stable from enthalpy of formation, elastic and phonon spectrum calculations. Among these, compared with the (Hf, Zr, Ta)B₂ ceramics, the addition of Nb and Ti on the metal sublattice is beneficial to improve the mechanical properties of ceramics, including Young's modulus, hardness and fracture toughness, while the introduction of Cr and W weakens the strength of covalently and ionic bonds inside the material, reducing its mechanical properties. The predicted thermophysical properties show that the high-entropy diboride ceramics containing Nb and Ti have better high-temperature comprehensive performance, including higher Debye temperature, thermal conductivity and lower thermal expansion characteristics, which is conducive to the application in extreme high-temperature environments. This research will provide important guidance for the design and development of new high-performance high-entropy diboride ceramics.     

Compositional disorder and sintering of entropy stabilized (Hf,Nb,Ta,Ti,Zr)B2 solid solution powders

Entropy-stabilized (Hf,Nb,Ta,Ti,Zr)B₂ solid solution powders produced by a carbo/boro-thermal reduction followed by solid solution formation were first analysed by synchrotron radiation x-ray diffraction, and their long range periodicity (i.e. lattice parameters) as well as the micro-strain intended as lattice disorder were quantitatively determined. A model to describe the micro-strain was proposed. The as-synthesized (Hf,Nb,Ta,Ti,Zr)B₂ solid solution powders were then hot-pressed at 2200 K and 50 MPa until near full densification was achieved. The hot-pressed material had a residual micro-porosity of 1.3 vol.% and consisted of a (Hf,Nb,Ta,Ti,Zr)B₂ ceramic matrix, 0.3-1 μm grain size range, and of a residual 10 vol.% B₄C particulate component, grain size in the range 0.2-2 μm. B₄C was a side product of the former synthesis and, after hot-pressing, remained trapped along the grain boundaries of the primary (Hf,Nb,Ta,Ti,Zr)B₂ solid solution ceramic matrix. Micro-hardness HV_0.2 = 22.7 ± 1.9 GPa for 1.96 N applied force was measured.     

Synthesis,mechanical, and thermophysical properties of high-entropy (Zr,Ti,Nb,Ta,Hf)C0.8 ceramic

Two high-entropy carbides, including stoichiometric (Zr,Ti,Nb,Ta,Hf)C and nonstoichiometric (Zr,Ti,Nb,Ta,Hf)C_0.8, were prepared from monocarbides and ZrH₂. Their sinterability, microstructures, mechanical properties, thermophysical properties, and oxidation behaviors were systematically compared. With the introduction of carbon vacancy, the sintering temperature was lowered up to 300°C, Vickers hardness was almost unaffected, whereas the strength decreased significantly generally due to the decrease of covalent bonds. The thermal conductivity shows a 50% decrease for nonstoichiometry high-entropy carbide, which is a major consequence of the lower electrical conductivity. The oxidation resistance in high temperature water vapor was not sensitive to carbon stoichiometry.     

Pressureless sintering of high-entropy boride ceramics

High-entropy boride ceramics were densified by pressureless sintering. Green densities of the ceramics varied by composition with the highest green density of 53.6 % for (Hf, Nb, Ta, Ti, Zr)B₂. After pressureless sintering, relative densities up to ∼100 % were obtained for (Cr, Hf, Ta, Ti, Zr)B₂ and (Hf, Ta, Ti, V, Zr)B₂. Two compositions, (Hf, Ta, Ti, W, Zr)B₂ and (Hf, Mo, Ti, W, Zr)B₂ contained secondary phases and did not reach full density. All compositions had average grain sizes less than 10 µm and less than 2 vol % of residual B₄C. This is the first report of conventional pressureless sintering of high-entropy boride ceramics powder compacts without evidence of liquid phase formation.     

Synthesis of rare earth containing single-phase multicomponent metal carbides via liquid polymer precursor route

Novel high-entropy carbide ceramics (HEC) containing rare earth metals, namely (Ti, Zr, Hf, Ta, La, Y)C, (Ti, Zr, Hf, Ta, Nb, La, Y)C, and (Ti, Zr, Hf, Ta, Nb, Mo, W, La)C were prepared with single-phase structure by polymer precursor method. Controlled co-hydrolysis and polycondensation of equiatomic metal-containing monomers were conducted successively, followed by blending allyl-functional novolac resin as carbon source, and the polymer precursors were obtained as clear viscous liquid solutions. The single-phase formation possibility was theoretically analyzed from the aspects of size-effect parameter δ of the designed compositions. All as-obtained ceramics possessed single face-centered-cubic structure of metal carbides and high-compositional uniformity from nanoscale to microscale. The (Ti, Zr, Hf, Ta, Nb, Mo, W, La)C ceramic powder pyrolyzed at 1800°C exhibited low-oxygen impurity content of 1.2 wt%. Thus, multicomponent high-entropy carbide nanoceramics with over five metal elements containing even rare earth element were firstly synthesized and characterized.     

Hot-corrosion of refractory high-entropy ceramic matrix composites synthesized by alloy melt-infiltration

Carbon fiber-containing refractory high-entropy ceramic matrix composites (C/RHECs) were fabricated through a reaction with carbon powders, transition metal carbides, and Zr–Ti alloys as a novel heat resistant material used for components of hypersonic vehicles cruising at Mach 7–10. With the infiltration of alloys at 1750 °C into a composite preform containing carbon and carbide powders for 15 min, a high-entropy matrix was successfully formed in situ. Arc-jet tests were conducted in the temperature range of 1800–1900 °C. Results showed the formation of an oxidized region composed of complex oxides, such as (Zr, Hf)O₂, (Nb, Ta)₂(Zr, Hf)₆O₁₇, (Zr, Hf)TiO₄, and Ti(Nb, Ta)₂O₇, with an average thickness of ~600 μm, under which an unoxidized region remained. The porous oxidized region resulted from the evolution of CO_(g) during oxidation, while a dense oxide region formed as the outermost region. This indicates that the dense oxide region acted as a barrier to oxygen diffusion for the unoxidized region during oxidation.     

Oxyacetylene ablation of (Hf0.2Ti0.2Zr0.2Ta0.2Nb0.2)C at 1350 − 2050 °C

Ablation resistance of a multi-component carbide (Hf_0.2Ti_0.2Zr_0.2Ta_0.2Nb_0.2)C (HTZTNC) was investigated using an oxyacetylene flame apparatus. When the surface temperature of the HTZTNC was below 1800 °C, (Nb, Ta)₂O₅, (Hf, Zr)TiO₄, and (Hf, Zr)O₂ were found to be the main oxidation products, while at higher temperature, formation of (Hf, Zr, Ti, Ta, Nb)O_x was favored and its content gradually increased with the increase in ablation temperature. Based on the ablation results and thermodynamic simulation analysis, a possible ablation mechanism of HTZTNC was proposed. Active oxidation of TiC and outward diffusion of TiO were demonstrated to occur during the ablation process, which constitute the critical steps for the ablation of HTZTNC. These results can contribute to the design of ablation resistant ultra-high-temperature ceramics.     

Synthesis of all equiatomic five-transition metals High Entropy Carbides of the IVB (Ti,Zr, Hf) and VB (V,Nb, Ta) groups by a low temperature route

The six possible equiatomic five-transition metal High Entropy Carbides (HECs) of the IVB (Ti, Zr, Hf) and VB (V, Nb, Ta) groups of the periodic table, i.e., TiZrHfVNbC₅, TiZrHfVTaC₅, TiZrHfNbTaC₅, TiZrVNbTaC₅, TiHfVNbTaC₅ and ZrHfVNbTaC₅, were successfully obtained via a powder metallurgy route at room temperature, specifically, by one-step diffusion mechanosynthesis starting from the elemental constituents (using graphite as the carbon source). Three of those HECs, TiZrHfVTaC₅, TiZrVNbTaC₅ and ZrHfVNbTaC₅, were developed for the first time. Their development was possible without any subsequent thermal treatment, in contrast to the usual way (reactive sintering at 1800–2200 °C), and in a powder form, make them potential advanced raw ceramics for hard, refractory and oxidation resistance coatings or matrix phase composites.     

Boro/carbothermal reduction co-synthesis of dual-phase high-entropy boride-carbide ceramics

Dense, dual-phase (Cr,Hf,Nb,Ta,Ti,Zr)B₂-(Cr,Hf,Nb,Ta,Ti,Zr)C ceramics were synthesized by boro/carbothermal reduction of oxides and densified by spark plasma sintering. The high-entropy carbide content was about 14.5 wt%. Grain growth was suppressed by the pinning effect of the two-phase ceramic, which resulted in average grain sizes of 2.7 ± 1.3 µm for the high-entropy boride phase and 1.6 ± 0.7 µm for the high-entropy carbide phase. Vickers hardness values increased from 25.2 ± 1.1 GPa for an indentation load of 9.81 N to 38.9 ± 2.5 GPa for an indentation load of 0.49 N due to the indentation size effect. Boro/carbothermal reduction is a facile process for the synthesis and densification of dual-phase high entropy boride-carbide ceramics with both different combinations of transition metals and different proportions of boride and carbide phases.     

Effects of Ti,Y, and Hf additions on the thermal properties of ZrB2

Reactive hot pressing was utilized to synthesize and densify four ZrB₂ ceramics with impurity contents low enough to avoid obscuring the effects of dopants on thermal properties. Nominally pure ZrB₂ had a thermal conductivity of 141 ± 3 W/m K at 25 °C. Additions of 3 at% of Ti, Y, or Hf decreased the thermal conductivity by 20 %, 30 %, and 40 %, respectively. The thermal conductivity of (Zr,Hf)B₂ was similar to ZrB₂ synthesized from commercial powders containing the natural abundance of Hf as an impurity. This is the first study to demonstrate that Ti and Y additions decrease the thermal conductivity of ZrB₂ ceramics and report intrinsic values for thermal conductivity and electrical resistivity of ZrB₂ containing transition metal additions. Previous studies were unable to detect these effects because the natural abundance of Hf present masked the effects of these additions.     

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.     

Medium-entropy (Ti,Zr, Hf)2SC MAX phase

Medium- and high-entropy alloys or ceramics for tuning the physicochemical properties of materials by the combination of multiple principal elements have received much interest. Herein, a medium-entropy (Ti, Zr, Hf)₂SC phase was synthesized attributing to the structural and chemical diversity of MAX phases. The crystal structure of (Ti, Zr, Hf)₂SC was determined by the Rietveld refinement of XRD, SEM, and atom-resolved TEM along with EDS elemental analysis. Phase evolution of X-ray diffraction patterns and TG/DSC curves were employed to reveal the synthesis mechanism of (Ti, Zr, Hf)₂SC from 2TiC–Zr–ZrC-2HfH₂-3.2FeS reactant system. The Vicker's hardness and the electrical resistivity of (Ti, Zr, Hf)₂SC were found higher than those of Ti₂SC, but the thermal conductivity of (Ti, Zr, Hf)₂SC was lower.     

Preparation of (Ti,Zr,Hf,Nb,Ta)C high-entropy carbide ceramics through carbosilicothermic reduction of oxides

Fully dense high-entropy carbide (HEC) ceramic has been prepared from a mixture of the group IV and V transition metal oxides by a two-step technique, which involved the vacuum carbosilicothermic reduction (VCSTR) synthesis of a composite powder containing 75 vol.% HEC, 20 vol.% (Nb_1-xMe_x)Si₂ (where Me = Ti, Zr, Hf, Ta), and 5 vol.% SiC followed by hot pressing of the as-synthesized product. It was found that the reaction between (Nb_1-xMe_x)Si₂ and HEC took place during hot pressing, thereby allowing effective sintering to occur. The mechanical properties of the obtained nearly single-phase HEC ceramic were comparable to or even slightly better than those of HEC ceramics prepared by other methods. The use of VCSTR synthesis as a key step in the preparation of fully dense HEC ceramic was concluded to be effective both in lowering the sintering temperature and in improving the mechanical properties.     

Room-temperature mechanical properties of a high-entropy diboride

The mechanical properties of a (Hf,Mo,Nb,Ta,W,Zr)B₂ high-entropy ceramic were measured at room temperature. A two-step synthesis process was utilized to produce the (Hf,Mo,Nb,Ta,W,Zr)B₂ ceramics. The process consisted of a boro/carbothermal reduction reaction followed by solid solution formation and densification through spark plasma sintering. Nominally, phase pure (Hf,Mo,Nb,Ta,W,Zr)B₂ was sintered to near full density (8.98 g/cm³) at 2000°C. The mean grain size was 6 ± 2 µm with a maximum grain size of 17 µm. Flexural strength was 528 ± 53 MPa, Young's modulus was 520 ± 12 GPa, fracture toughness was 3.9 ± 1.2 MPa·m^1/2, and hardness (HV_0.2) was 33.1 ± 1.1 GPa. A Griffith-type analysis determined the strength limiting flaw to be the largest grains in the microstructure. This is one of the first reports of a variety of mechanical properties of a six-component high-entropy diboride.     

Room-Temperature Mechanosynthesis of Carbides by Grinding of Elemental Powders

The direct room-temperature synthesis (mechanochemical synthesis or mechanosynthesis—MS) of most metal carbides by milling metal—carbon powders (size some tens of micrometers) mixtures is described. The particle size of the carbides obtained is of the order of 20 nm. Stable, metastable, mixed, and new carbides can be formed. Moreover, metal—carbon alloys can be obtained, such as in the Fe—C system. The synthesized compounds have been characterized by X-ray diffraction and Mössbauer spectroscopy for iron-containing systems. Carbides of the following elements were obtained: Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Hf, Ta, W, Re, Al and Si.     

Low-temperature sintered (Ti,Zr, Nb,Ta, Mo)C-based composites toughened with damage-free SiCw

The high sintering temperature would have a great tendency to damage the morphology and thus properties of the silicon carbide whisker (SiC_w) in high entropy carbide-silicon carbide whisker (HEC-SiC_w) composites, which, in turn, would impact the effectiveness of the operative toughening mechanisms. The objective of this study was to achieve full contributions to the toughening effects of SiC_w by preparing (Ti, Zr, Nb, Ta, Mo)C-SiC_w composites at low temperature (1600 ℃) using cobalt as additives. Results showed that the fracture toughness of the (Ti, Zr, Nb, Ta, Mo)C bulk reinforced with 20 vol% SiC_w and 5 vol% Co was 7.2 MPa?m^1/2, which was much higher than that of the (Ti, Zr, Nb, Ta, Mo)C bulk only sintered with 5 vol% Co (3.4 MPa?m^1/2). Meanwhile, it was also higher than that of the reported HEC-20 vol% SiC_w composite sintered at 2000 ℃ (4.3 MPa?m^1/2). For the fracture toughness of HEC-SiC_w composites, it was significantly increased by the introduction of damage-free SiC_w.     

Two-step synthesis process for high-entropy diboride powders

High-entropy diboride powders were produced by a two-step synthesis process consisting of boro/carbothermal reduction followed by solid solution formation. Nominally phase-pure (Hf,Zr,Ti,Ta,Nb)B₂ in a single-phase hexagonal structure had an average particle size of just over 400 nm and contained 0.3 wt% carbon and 0.3 wt% oxygen. The fine particle size was due to the use of high-energy ball milling prior to boro/carbothermal reduction, which led to a relatively low synthesis temperature of 1650°C. Oxygen and carbon contents were minimized by completion of the boro/carbothermal reduction reactions under vacuum. This is the first report of synthesis of a nominally phase pure high-entropy diboride powder from oxides using a two-step process.     

The effect of transition metal carbides MeC (Me = Ti,Zr, Nb,Ta, and W) on mechanical properties of B4C ceramics fabricated via pressureless sintering

In this study, boron carbide-metallic boride (B₄C-MeB_x, Me = Ti, Zr, Nb, Ta, or W) multiphase ceramics were fabricated via in situ pressureless sintering at 2250 °C for 1 h. The effects of transition metal carbides, namely, TiC, ZrC, NbC, TaC, and WC, on the phase composition, microstructure, and mechanical properties of the ceramics were investigated. The results showed that MeC could facilitate the sintering densification of B₄C by distributing second-phase particles uniformly throughout the B₄C. Additionally, the main phases observed were B₄C and (Me, W)B_x (Me = Ti, Zr, Nb, or Ta) due to the doping of a small amount of WC during the ball milling process. As a result, the mechanical properties of B₄C-MeB_x showed significant improvements when compared with those of single-phase B₄C ceramics. B₄C–NbB₂ ceramics were found to exhibit the best mechanical properties, with an elastic modulus of 393.0 GPa, a hardness of 28.7 GPa, a flexural strength of 368.0 MPa, and a fracture toughness of 6.94 MPa m^1/2.     

Bestimmung von Spurenverunreinigungen in Tantalpentoxid mit ICP-Atomemissionsspektrometrie (ICP-AES) und ICP-Massenspektrometrie (ICP-MS) nach multielementfähigen Spuren-Matrix-Trennungen

Determination of Trace Impurities in Tantalum Pentoxide by ICP-AES and ICP-MS after Trace-Matrix-Separation A trace–matrix separation procedure based on anion exchange is presented. 500 mg Ta₂O₅ are dissolved in hydrofluoric acid in a Teflon bomb. Ta is retained on the resin column in the form of different fluoro complexes. Most analytes pass the column as cations. Others, like Ti, Zr, Hf, V, Nb also form fluoro complexes. Based on a thorough investigation of the complex equilibria two selective elution procedures using 1M HCl or 0.5M HNO₃ + 0.5M HF are developed. Both methods require a minimum of equipment and reagents, thus drastically reducing the risk of contaminations. The separation procedures are suitable for on-line coupling with both ICP spectrometric techniques. More than 30 analytes can be determined including Ti, Zr, Hf, V, Nb, Mo, and W. Taking an aliquot of 100 mg Ta₂O₅ detection limits are less than 1 μg for ICP-AES or 10 ng for ICP-MS for most elements. For example, 1.3 ng of Nb can be detected in 100 mg Ta₂O₅ by ICP-MS after separation from the matrix.