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.     

Fine-grained dual-phase high-entropy ceramics derived from boro/carbothermal reduction

In the current work, fine-grained dual-phase, high-entropy ceramics (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂-(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C with different phase ratios were prepared from powders synthesized via a boro/carbothermal reduction approach, by adjusting the content of B₄C and C in the precursor powders. Phase compositions, densification, microstructure, and mechanical properties were investigated and correlated. Due to the combination of pinning effect and the boro/carbothermal reduction approach, the average grain size (～0.5?1.5 μm) of the dual-phase high-entropy ceramics was roughly one order of magnitude smaller than previously reported literature. The dual-phase high-entropy ceramics had residual porosity ranging from 0.3 to 3.2 % upon sintering by SPS and the material with about 18 vol% boride phase exhibited the highest Vickers hardness (24.2±0.3 GPa) and fracture toughness (3.19±0.24 MPam^1/2).     

Synthesis of high-entropy diboride nanopowders via molten salt-mediated magnesiothermic reduction

Powder synthesis is critical for implementing the wide applications of high-entropy diborides (HEBs). However, the low-temperature synthesis of HEB powders was rarely reported. Herein, the low-temperature synthesis of the single-phase HEB nanopowders via molten salt-mediated magnesiothermic reduction (MMR) method was reported for the first time. The results showed that the as-synthesized nanopowders consisted of the single-phase HEBs and their average particle sizes are in the range of 28-56 nm. Meanwhile, they possessed the good compositional homogeneity and the low-content oxygen impurity in the range of 4.13-6.12 at%. In addition, their formation mechanism could be well interpreted by a classical MMR growth process.     

One-step synthesis of coral-like high-entropy metal carbide powders

The synthesis of high-entropy metal carbide powders is critical for implementing their extensive applications. However, the one-step synthesis of high-entropy metal carbide powders is rarely studied. Herein, the synthesis possibility of high-entropy metal carbide powders, namely (Zr_0.25Ta_0.25Nb_0.25Ti_0.25)C (ZTNTC), via one-step carbothermal reduction was first investigated theoretically by analyzing chemical thermodynamics and lattice size difference based on the first-principle calculations, and then the ZTNTC powders with particle size of 0.5-2 μm were successfully synthesized experimentally. The as-synthesized powders not only had a single rock-salt crystal structure of metal carbides, but also possessed high-compositional uniformity from nanoscale to microscale. More interestingly, they exhibited the distinguished coral-like morphology with the hexagonal step surface, whose growth was governed by a classical screw dislocation growth mechanism.     

Synthesis of rod-like ZrB2 crystals by boro/carbothermal reduction

Rod-like ZrB₂ crystals were synthesized at 1600 °C in Ar atmosphere by boro/carbothermal reduction using ZrOCl₂⋅8H₂O, B₄C and carbon powders as raw materials. The optimum molar ratio of raw materials required to form pure ZrB₂ grains was found to be 2: 1.2: 3. With increase in temperature and subsequent heat preservation stage, ZrB₂ powders grew into a rod-like morphology along the c axis. The rod-like ZrB₂ grains obtained at 1600 °C have diameters of 0.5–3 μm and high aspect ratios of >8. Effects of molar ratio of raw materials, heating temperature and holding time on the phase composition and final morphology were investigated. Growth mechanism of rod-like ZrB₂ grains was also analyzed.     

Microstructure and mechanical properties of WB2–B4C composites fabricated by boro/carbothermal reduction and SPS

The phase composition, microstructure, and mechanical properties of the WB₂–B₄C composites fabricated by a combination of boro/carbothermal reduction and spark plasma sintering (SPS) method with WO₃, B₄C, and graphite as raw materials were investigated in this study. The experimental results showed that the relative density of the as-sintered WB₂–B₄C composites was ∼93.1% and ∼99.5%, respectively, after being SPS sintered at 1600°C under the applied load of 30 MPa for 10 min. Scanning electron microscope analysis showed that a network structure with WB₂ grains surrounded by B₄C grains was observed after sintering. Analyses of high-resolution TEM showed semi-coherent interface and lattice distortion transition region between WB₂ and B₄C grains. The Vickers hardness of WB₂–B₄C composite increased to 22.3 ± 0.9 GPa at 9.8 N owing to the fully dense, solid solution of C, and three-dimensional network structure. Moreover, the fracture toughness and flexural strength of WB₂–B₄C composite reach 6.04 ± 0.81 MPa m^1/2 and 750 ± 80 MPa, respectively, which could be attributed to the semi-coherent interface between WB₂ and B₄C grains.     

Synthesis of single-phase high-entropy diboride powders and their spheroidization process

Spherical (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ powders with a uniform particle size distribution are successfully prepared using a novel industrial approach, which combines spray-drying process and thermal plasma sintering technology together. In this, single-phase (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ powders are first synthesized via a borothermal reduction process using a mixture of individual metallic oxides and boron powders as starting materials. The influence of boron powder content on the structure of prepared powders is researched. Then, (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ granules are prepared after wet-grinding and spray-drying process, which exhibit a spherical shape and homogeneous element distribution. RF induction thermal plasma is finally used to sinter the granulated particle, and the apparent density of sintered spherical powders is increased to 2.57 g/cm³ from 1.43 g/cm³. Such powders are in potential demand for additive manufacturing techniques, and the successful synthesis of spherical (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ powders may guide the way toward the preparation of many other spherical high-entropy diboride powders.     

Preparation of tungsten diboride by a combination of boro/carbothermal reduction process and spark plasma sintering

Submicron tungsten diboride (WB₂) powder was successfully synthesized with the ratio of WO₃:B₄C:C = 2:1.1:5. The effects of carbon sources (carbon black or graphite) and heat treatment temperatures (1100, 1200, 1300 ℃) on the phase composition and microstructure of the as-synthesized WB₂ powder samples were studied. The results showed that ultrafine WB₂ powders with oxygen content of 1.65 wt% and 2.04 wt% were obtained by carbon black and graphite at 1300 ℃, respectively. The relatively density of the as-sintered WB₂ samples achieve ~ 91 % and ~ 87 % without any kind of sintering additive after spark plasma sintering at 1500 °C under 30 MPa for 10 min. The formation mechanism of the WB₂ powders synthesized by boro/carbothermal reduction was proposed and verified by thermodynamic calculation according to the phases present in the powder synthesized at different temperatures.     

Microstructure and mechanical properties of high-entropy borides derived from boro/carbothermal reduction

Starting from metal oxides, B₄C and graphite, a suite of high-entropy boride ceramics, formulated (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)B₂, (Hf_0.2Zr_0.2Mo_0.2Nb_0.2Ti_0.2)B₂ and (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ derived from boro/carbothermal reduction at 1600 °C were fabricated by spark plasma sintering at 2000 °C. It was found that the synthetic high-entropy boride crystalized in hexagonal structure and the yield of the targeting phase was calculated to be over 93.0 wt% in the sintered ceramics. Benefitting from the nearly full densification (96.3% ˜ 98.5% in relative density) and the refined microstructure, the products exhibited the relatively high Vickers hardness. The indentation fracture toughness was determined to be comparable with the single transition metal-diboride ceramics. It should be noted that the formation of high-entropy boride ceramics were featured with the relatively high hardness at no expense of the fracture toughness.     

High-pressure sintering of ultrafine-grained high-entropy diboride ceramics

Herein the ultrafine-grained (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ high-entropy diboride ceramics were successfully fabricated by high-pressure sintering technology for the first time. The results showed that the grain size, relative density, and Vickers hardness of the as-fabricated samples all increased gradually with increasing sintering temperatures from 1373 K to 1973 K. The relative density and mean grain size of the as-sintered samples at 1973 K were 97.2% and 684 nm, respectively, and simultaneously they exhibited excellent comprehensive mechanical properties, combining a Vickers hardness of 26.2 GPa and a fracture toughness of 5.3 MPa·m^1/2, which were primary attributed to the fine grain strengthening mechanism and microcrack deflection toughening mechanism.     

Fabrication and characterization of polymer-derived high-entropy carbide ceramic powders

Polymer-derived ceramic (PDC) route has been widely used to fabricate various ceramics or ceramic-matrix composites in recent years. However, the synthesis of high-entropy ceramics via PDC route has rarely been reported until now. Herein, we successfully synthesized a class of high-entropy carbides, namely (Hf_0.25Nb_0.25Zr_0.25Ti_0.25)C (HEC-1), via PDC route. The polymer-derived HEC-1 ceramics consisted of numerous superfine particles with the average particle size ~800 nm. Meanwhile, they possessed a rock-salt structure of metal carbides and high-compositional uniformity from nanoscale to microscale. In addition, the as-obtained HEC-1 ceramics had a low oxygen impurity content of 0.51% and a low free carbon impurity content of 2.56%. This work will open up a new research field on the fabrication of high-entropy ceramics or high-entropy ceramic-matrix composites via PDC route.     

Boro/carbothermal reduction synthesis of uranium tetraboride and its oxidation behavior in dry air

Uranium tetraboride (UB₄) was successfully synthesized by boro/carbothermal reduction of UO₂ with B₄C and carbon as combined reduction agents under flowing argon. The effects of processing temperature and mole ratios of starting materials on phase evolution were studied. XRD results demonstrated that UB₄ was obtained with 3.75 mol% B₄C in excess based on the UO₂/B₄C/C molar ratios of 1:1:1 at 1500°C. SEM observation revealed that submicrometer-sized quasi-spherical UB₄ particles cemented together to be an aggregate. The laser particle size analysis showed that the particle size was in the range of 1-10 μm. The oxidation behavior of UB₄ was also investigated by TG and XRD. The oxidation of UB₄ started at about 500°C and it showed better oxidation resistance than other basic uranium nuclear fuels (UO₂, UC, UN and U₃Si₂). The oxidation chemical process of UB₄ was presented as a three-step process: (a) the formation of U₃O₈ and B₂O₃ oxidation products; (b) the formation of UB₂O₆ intermediate product by the interaction of U₃O₈ and B₂O₃; (c) the decomposition of UB₂O₆ to get U₃O₈.     

Synthesis of nano-titanium diboride powders by carbothermal reduction

The nano-sized titanium diboride particles were synthesized by carbothermal reduction process. In this study, carbothermal reduction process was used by controlling reaction rate and duration time. TiO₂, B₂O₃ and carbon resin were used as starting materials with a molar composition; TiO₂:B₂O₃:C = 1:2:5. The mixture was placed in a graphite crucible and pushed into a heating zone maintained at 1500 °C and Ar was flown for a period of 20 min. After reaction, the crucible was pulled out from the heating zone to cooling zone of the furnace for the rapid cooling. The average particle size of the agglomerated product was found to be ∼500 nm, which was composed with small primary particles of <100 nm in size. After milling, the large agglomerate was reduced to primary particles.     

In situ synthesis of Al2O3–SiC powders via molten-salt-assisted aluminum/carbothermal reduction method

Al₂O₃–SiC composite powder (ASCP) was successfully synthesized using a novel molten-salt-assisted aluminum/carbothermal reduction (MS-ACTR) method with silica fume, aluminum powder, and carbon black as raw materials; NaCl–KCl was used as the molten salt medium. The effects of the synthesis temperature and salt-reactant ratio on the phase composition and microstructure were investigated. The results showed that the Al₂O₃–SiC content increased with an increase in molten salt temperature, and the salt–reactant ratio in the range of 1.5:1–2.5:1 had an impact on the fabrication of ASCP. The optimum condition for synthesizing ASCP from NaCl–KCl molten salt consisted of maintaining the temperature at 1573 K for 4 h. The chemical reaction thermodynamics and growth mechanism indicate that the molten salt plays an important role in the formation of SiC whiskers by following the vapor-solid growth mode in the MS-ACTR treatment. This study demonstrates that the addition of molten salt as a reaction medium is a promising approach for synthesizing high-melting-point composite powders at low temperatures.     

Phase stability,mechanical, and thermal properties of high-entropy transition and rare-earth metal diborides from first-principles calculations

The narrow composition design space of high-entropy transition metal diborides (HE TMB₂) limits their further development. In this study we designed six quaternary and quinary high-entropy transition metal and rare-earth diborides (HE TMREB₂) and investigated their phase stability using the energy distribution of the local mixing enthalpy of all possible configurations. The results show that both quaternary and quinary HE TMREB₂ have higher enthalpic driving forces, which facilitates the formation of single-phase AlB₂-type structures between TMB₂ and REB₂. Calculations of elastic constants show that the TMB₂ component has the greatest effect on the c₄₄ elastic constant and shear modulus G, while REB₂ significantly influences the bulk modulus B. Furthermore, LuB₂ and TmB₂ substantially affect the elastic modulus anisotropy of HE TMB₂. Rare-earth atoms in HE TMREB₂ can enhance the nonharmonic interactions between phonons, which results in a significant hindrance in the thermal transport of low-frequency phonons as well as an increase in the volume thermal expansion coefficients. Thus, the incorporation of REB₂ into HE TMB₂ has a significant impact on the phase stability and properties.     

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.     

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.     

Solute distributions in tantalum-containing zirconium diboride ceramics

Solute segregation was examined in zirconium diboride and zirconium-tantalum diboride solid solution ceramics that were produced by reactive hot pressing. Microstructural analysis demonstrated that the ZrB₂ and (Zr,Ta)B₂ ceramics reached nearly full relative density and were nominally phase-pure. X-ray diffraction was consistent with full incorporation of Ta into solid solution within the ZrB₂ structure, and energy-dispersive spectroscopy demonstrated that tantalum was well distributed throughout the bulk of the Ta-doped specimens. The weak characteristic X-rays for B led to inaccurate results for total atom concentrations in boride ceramics by energy-dispersive spectroscopy. Atom probe tomography was used to analyze the amount and spatial distribution of Ta species. No obvious Ta segregation was observed in grains or grain boundaries. However, nitrogen strongly segregated to a grain boundary. This study demonstrated that atom probe tomography is an accurate method for characterizing the amount and spatial distribution of metallic and nonmetallic species in ZrB₂ ceramics.     

Synthesis and thermal stability of novel high-entropy metal boron carbonitride ceramic powders

High-entropy metal boron carbonitride ceramic powders including (Ta_0.2Nb_0.2Zr_0.2Hf_0.2W_0.2)BCN, (Ta_0.2Nb_0.2Zr_0.2Hf_0.2Ti_0.2)BCN, and (Ta_0.2Nb_0.2Zr_0.2Ti_0.2W_0.2)BCN, were successfully synthesized via mechanical alloying at room temperature. Results show that for the first step of 10 h milling, the amorphous BCN phases are observed. After 24 h of second step milling, the as-synthesized high-entropy ceramics exhibit a single face-centered cubic solid solution structure with high compositional uniformity from nano-scale to micron-scale. When heated to 1500 °C for 30min in flowing Ar, the as-prepared high-entropy ceramic powders still show relatively high thermal stability; however, some metals oxides like HfO₂ and ZrO₂ are detected due to the pre-existing oxides on sample surfaces. After heat treatment, some amorphous phases are still retained. This work suggests a new processing route on the synthesis of high-entropy metal boron carbonitride ceramics.     

Synthesis and characterization of the ternary metal diboride solid-solution nanopowders

The synthesis of the multi-component transition-metal diboride (MeB₂) solid-solution powders has been recently attracting considerable attentions. However, the synthesis of the ternary or more component MeB₂ solid-solution powders has rarely been reported until now. To fabricate the ternary MeB₂ solid-solution powders, herein we utilized two kinds of the ternary MeB₂ solid solutions as prototypes, namely (Hf_1/3Zr_1/3Ti_1/3)B₂ (HZTB) and (Ta_1/3Nb_1/3Ti_1/3)B₂ (TNTB). The formation possibility of HZTB and TNTB was first analyzed by the first-principles calculations and then we attempted of fabricated them by a simple molten salt synthesis technique. The first-principles calculations results showed that the mixing Gibbs free energy at room temperature and lattice size difference at 0 K of HZTB and TNTB were (1.666 kJ/mol and 3.146%) and (−3.030 kJ/mol and 1.254%), respectively. This suggested that TNTB solid solution was more prone to being fabricated than HZTB solid solution. The experimental results showed the high purity TNTB solid-solution nanopowders were successfully synthesized by the molten salt synthesis technique at 1373 K with 30% excessive B as precursors while the HZTB solid solution was not able to be synthesized by the molten salt synthesis technique. The as-synthesized TNTB solid-solution nanopowders exhibited the distinguished nanorod morphology with the diameters of 20-40 nm and lengths of 100-200 nm. Meanwhile, they possessed the good single-crystal hexagonal structure and high compositional uniformity from nanoscale to microscale. In addition, their formation mechanism associated to the possible chemical reactions was well interpreted by the thermodynamics analysis.