High Boride Research Articles

Oxidation behavior of (Hf0.2Zr0.2Ta0.2Ti0.2Me0.2)B2 (Me=Nb,Cr) high-entropy ceramics at 1200 °C in air

The oxidation behavior of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2 and (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2 high entropy boride ceramics at 1200 °C was investigated to provide insights into their microstructural evolution and oxidation mechanism. The results showed that the oxide layer thickness of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2 was about 112 μm, much thicker than the counterpart of (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2 (∼62 μm). The differences in oxidation response and oxidation resistance between these two material systems were mainly pertained to the generation of different structures of oxide layer. For the (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2 sample, the oxide layer was composed of four-layer structure (porous-dense-porous-dense). In contrast, the oxide layers of (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2 sample exhibited a distinctly laminated structure.

Dense nine elemental high entropy diboride ceramics with unique high temperature mechanical and physical properties

Due to their huge composition space and superior performance characteristics, high entropy borides have garnered great interest in many fields, particularly where their high-temperature performances at high temperatures are critical. Herein, we first discovered that dense (Ti1/9Zr1/9Hf1/9Nb1/9Ta1/9V1/9Cr1/9Mo1/9W1/9)B2 (HEB9) ceramics prepared at 1850 °C showed an exceptionally low temperature coefficient of electrical resistivity (4.28 × 10−4 K−1), which is ∼38% of that of (Ti1/5Zr1/5Hf1/5Nb1/5Ta1/5)B2 (HEB5) and nearly an order of magnitude lower than those of previously reported ZrB2 and ZrB2-30 vol% SiC ceramics. Their mechanical, thermophysical properties at elevated temperatures were also investigated systematically. Although the thermal conductivity of HEB9 increased with elevated temperatures, it remained at a very low level (∼29 W/(m·K)) at 1273 K, nearly half that of HEB5. Interestingly, it is found that the thermal conduction of HEB9 and HEB5 was mainly contributed by electrons, suggeting their thermal conductivity could be roughly estimated from corresponding electrical conductivity values. Moreover, HEB9 exhibited a geometrically necessary dislocation density over 30 times greater than HEB5, likely contributing to its higher Vickers hardness and unique physical properties. Notably, the flexural strength of HEB9 at 1600 °C was even improved to 650.0 ± 86.3 MPa, compared to the value (559.7 ± 36.7 MPa) at room temperature without degradation, which was comparable to that of HEB5, although thermodynamic calculations indicated a lower melting point of HEB9 and grain boundary softening was occurred in HEB9 at higher temperatures. The excellent mechanical, electrical and thermophysical properties of HEB9 at elevated temperatures make it a competitive candidate for various high-temperature applications.

Toward a low‐temperature sintering of carbon fiber toughened high entropy boride composite with Co addition

AbstractDense Cf/(ZrHfNbTaCr)B2–SiC (CBS‐Co) composite is successfully prepared at a low temperature of 1500°C by adding 5 vol.% Co. Liquid Co accelerates the particle rearrangement and significantly enhances the relative density of CBS‐Co (95.2%). In situ formed Co2B phase by diffusion reaction bonds tightly with high entropy diboride (HEB) and SiC grains in CBS‐Co. Due to the promotion of relative density and interfacial bonding strength, CBS‐Co exhibits a high flexural strength (283 ± 23 MPa). Structural damages of carbon fibers are effectively prevented by compact fiber coating. The well‐preserved fibers play dominant roles to increase the fracture toughness of CBS‐Co (4.77 ± .5 MPa·m1/2). Moreover, SiO2‐rich oxide layer can effectively heal flaws and inhibit oxygen diffusion, achieving relatively high flexural strength after oxidation at 1300°C of CBS‐Co composite. This work provides a stepping stone for developing high‐performance carbon fiber toughened HEB composites based on low temperature liquid‐phase sintering technology.

Processing and characterization of Ultra High Temperature High‐Entropy (Ti0.2Zr0.2Hf0.2Mo0.2W0.2)B2-based Ceramics: Effect of W granulometry, graphite, and SiC addition

A highly dense and single phase (Ti0.2Zr0.2Hf0.2Mo0.2W0.2)B2 ceramic product is obtained in this work at 1950°C (20 min, 20 MPa) by Spark Plasma Sintering (SPS) from powders prepared by Self-propagating High-temperature Synthesis (SHS). The formation of the (W,Mo)B2 secondary phase is avoided using fine W precursors and adding 1 wt% graphite to the SHS powders before SPS. Kinetic limitations responsible for hindering the synthesis of the high entropy boride are correspondingly eliminated. The resulting 98.5 % dense sample exhibits a homogeneous microstructure, with Vickers hardness of 26.8 GPa. The introduction of 20 vol% SiC produces an increase of the KIC values from 2.32 to 5.11 MPa m1/2. Very relevant is that the volatilization of Mo- and W-oxides occurring during sample oxidation at high temperature, which leads to its rapid degradation with the formation of a very porous oxide scale, can be strongly inhibited by the silicate phases generated in the composite ceramic.

Tribological behavior of medium entropy boride (Ta,Zr,Ti)B2 densified at a relatively low temperature of 1600 °C

Medium and High entropy borides with high hardness and corrosion resistance are promising wear resistant materials. High temperature (>2000 °C) is always necessary for the densification of medium and high entropy borides. Using a mixture of 10 vol% metallic cobalt and 20 vol% SiC as additives, medium entropy (Ti,Zr,Ta)B2 ceramics were prepared at a relatively low temperature of 1600 °C by SPS. The reduced sintering temperature is attributed to the synergistic effect of Co and SiC that wet the grain boundary and promote the interdiffusion. The prepared (Ti,Zr,Ta)B2 obtains the microhardness of 22.54 ± 0.92 GPa, comparable with other medium entropy boride ceramics prepared at 2000 °C. The tribo-tests results of (Ti,Zr,Ta)B2 sliding against steel suggests that the tribological performance are load dependent. As the load increases, the friction coefficient first rises and then falls at a load of 10 N, which is associated with the chemical differences of the tribofilm. The tribofilm is primarily composed of O and Fe, along with Zr, Ti and Ta oxides. The iron oxide in the tribofilm will deteriorate the tribological performance. Abrasive wear is the main wear mechanism at lower load of 1 N, while adhesive wear is the predominant wear mechanism at 10 N.

Enhanced fracture toughness of high entropy boride by adding SiC

The effect of SiC content (10 vol%, 20 vol% and 30 vol%) and sintering temperature (1800 °C, 1900 °C and 2000 °C) on the microstructure and mechanical properties of high entropy boride-silicon carbide (HEB-SiC) composites were systematically investigated in this work. The densification was gradually increased with the increase of SiC content and sintering temperature. More specially, the densification of HEB-30 vol% SiC composite obtained at 2000 °C was as high as 99.1 %. As for the mechanical properties, the dense HEB-30S-2000 composite possessed the highest fracture toughness (5.24 MPa m1/2) and Vickers hardness (28.5 GPa). The above significantly enhanced fracture toughness was closely pertained to the fracture modes adjusting. In details, after the addition of SiC, the branching and bridging of crack, the rupture of large SiC grain, and the deflection of crack existing near the HBE matrix and SiC particles jointly promote the improvement of fracture toughness. Besides, the Vickers hardness increased linearly with the relative density increasing. A simplified equation was established, which could effectually reflect the linear connection between the Vicker hardness and relative density (SiC content) of HEB-SiC composites. It will certainly play an important guiding role in the further development of HEB-SiC composites.

Synthesis and properties of (Hf,Mo,Ti,W,Zr)B2–(Hf,Mo,Ti,W,Zr)C dual phase ceramics

Dense, dual phase (Hf,Mo,Ti,W,Zr)B2–(Hf,Mo,Ti,W,Zr)C ceramics were synthesized with varying contents of Mo and W. The final (Hf0.317Mo0.025Ti0.317W0.025Zr0.317)C–(Hf0.317Mo0.025Ti0.317W0.025Zr0.317)B2 was a nominally pure, dual phase ceramic, while compositions with higher amounts of Mo and W contained multiple phases. The final microstructures had submicron grains due to pinning effects. Vickers hardness values were up to 48.6 ± 2.2 GPa for an applied load of 0.49 N for the ceramic with optimized composition and densification. The solubility limits for Mo and W into the (Hf,Mo,Ti,W,Zr)B2–(Hf,Mo,Ti,W,Zr)C ceramics were mitigated by decreasing their contents to 2.5 at% each, which produced a nominally pure dual phase ceramic that could be densified by spark plasma sintering or hot pressing. A synergistic hardening effect was observed for optimized ceramic with 5 at% of Mo and W, whereby it had a higher hardness than individual high entropy carbide and high entropy boride phases containing the same transition metals.

High entropy borides as efficient catalysts for electrochemical reduction of nitrate to ammonia

Electrochemical reduction of nitrate (NO3-RR) to ammonia has attracted great attention as a strategy for simultaneously achieving pollutant removal and economic product acquisition. To achieve high Faradaic efficiency of ammonia (FE) and ammonia selectivity, it is necessary to develop highly active catalysts with strong hydrogenation ability. In this work, we report high-entropy-borides (HEB) with tunable cation composition as high-performance catalysts for NO3-RR. Taking (TiVCrMo)B2 as the model material systems, we systematically investigate the effect of doping (Co, Fe, and Ni) on the activity of NO3-RR. Electrochemical test results show that the HEB doped with Co element exhibits the optimal performance, with a FE reaching 97.9 %. Combining several advanced spectroscopic techniques, we find that Co dopant can effectively facilitate the hydrogenation process of NO3-RR. This work demonstrates high-entropy ceramics as an excellent platform for rational design of highly active catalysts for energy and environmental applications.

A super‐hard high entropy boride containing Hf, Mo, Ti, V, and W

AbstractSuper‐hard (Hf,Mo,Ti,V,W)B2 was synthesized by boro‐carbothermal reduction and densified by spark plasma sintering. This composition was produced for the first time as a single‐phase ceramic in the present research. The optimized ceramic had a single hexagonal AlB2‐type crystalline phase with a grain size of 3.8 µm and homogeneous distribution of the constituent metals. The Vickers hardness exhibited the indentation size effect, increasing from 27 GPa at a load of 9.8 N to as high as 66 GPa at a load of 0.49 N. This is the highest hardness reported to date for high entropy boride ceramics.

Processing and optical behavior of dense (Hf,Zr)B2 solid solutions for solar energy receivers

While individual borides and, more recently, quinary High Entropy Transition Metal Borides have been investigated, the fabrication and characterization of bulk binary to quaternary solid solutions are barely explored. In this work, dense (Hf0.5Zr0.5)B2 is produced by Spark Plasma Sintering (SPS) from powders prepared by Self-propagating High-temperature (SHS). SHS produced a multiphase product, whose secondary phases are fully converted into the desired diborides during the subsequent SPS step. Optical properties of (Hf0.5Zr0.5)B2 are evaluated for the first time with focus on the possible use as novel high-temperature solar thermal absorber, by hemispherical reflectance measurements and calculation of solar absorptance, temperature-dependent spectral selectivity and absorber opto-thermal efficiency at various solar concentration ratios. To optimize the material, a chemically etched surface texture was realized to modify the optical properties. The etched sample showed a higher solar absorptance (0.71) and a lower spectral selectivity than the unetched one, with consequent higher opto-thermal efficiency at all temperatures for solar concentration ratios 1000 ÷ 3000, while at lower concentration ratios and temperatures >1400 ÷ 1600 K, the unetched sample shows the highest efficiency. These results show the promising properties of binary diborides for solar thermal applications.

Pressureless densification and properties of high-entropy boride ceramics with B4C additions

High entropy boride ceramics have great potential as structural materials serving in extreme environments. However, their applications are limited by the difficulty of sintering. In the present study, dense (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 ceramics with B4C additions were prepared through pressureless sintering at as low as 1900 °C. Calculations based on the CALPHAD approach predict that (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 starts to melt at about 3315 °C whilst B4C additions reduce the temperature and broaden the temperature region where solid and liquid coexist. Results showed that the introduction of B4C could trigger the densification of (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 at a lower temperature and promote their densification significantly. The relative density of samples with 5 wt% of B4C additions sintered at 1900 and 2000 °C was 97.7 % and 99.7 %, respectively. While the sintering temperature was further increased to 2100 °C, the liquid phase was reactively formed, leading to the rapid grain coarsening in samples with B4C additions. Strengthened by well-dispersed B4C grains, the sample with 5 wt% B4C sintered at 2000 °C exhibited excellent mechanical properties with the Vickers hardness, flexural strength, and fracture toughness of 21.07 ± 2.09 GPa, 547 ± 45 MPa, and 5.24 ± 0.14 MPa m1/2, which are comparable or even higher than counterparts sintered under pressure.

Design and characterization of LaB6/NbNiTaTi refractory high-entropy alloy coatings: Effect of in-situ TaB phase on strengthening mechanism

The excellent strength and hardness obtained from the customized phase compositions provide vast spaces for surface protection application of refractory high entropy alloy (RHEA) coatings in titanium alloys. However, the introduction of high-strength brittle phase in RHEA coatings commonly increases the cracking tendency and affects their service life. In this work, we propose a design strategy to enhance the mechanical properties of RHEA coatings by introducing an extremely stable high strength boride combined with a ductile phase. Experimentally, LaB6/NbNiTaTi RHEA coatings mainly consist of BCC phase, in-situ TaB phase and NiTi/IM eutectic phase. TaB phase with high-density (110) nanotwins inside presents needle-like morphology with the long side axis parallel to the [001] direction. Excessive needle-like TaB phase is fully fused and fractured due to its preferential growth along a specific direction. The fracture surface analysis shows that BCC phase bears the main deformation, NiTi phase can relieve stress concentration at the boundary while bearing a small amount of deformation. By contrast, the low-density dislocation distribution and the low energy extended dislocation structure in TaB phase exhibit the phase stability. In addition, the amorphous islands are formed near the high-deformation regions when the defect concentration reaches the critical value.

High Entropy Borides Synthesized by the Thermal Reduction of Metal Oxides in a Microwave Plasma.

Metal oxide thermal reduction, enabled by microwave-induced plasma, was used to synthesize high entropy borides (HEBs). This approach capitalized on the ability of a microwave (MW) plasma source to efficiently transfer thermal energy to drive chemical reactions in an argon-rich plasma. A predominantly single-phase hexagonal AlB2-type structural characteristic of HEBs was obtained by boro/carbothermal reduction as well as by borothermal reduction. We compare the microstructural, mechanical, and oxidation resistance properties using the two different thermal reduction approaches (i.e., with and without carbon as a reducing agent). The plasma-annealed HEB (Hf0.2, Zr0.2, Ti0.2, Ta0.2, Mo0.2)B2 made via boro/carbothermal reduction resulted in a higher measured hardness (38 ± 4 GPa) compared to the same HEB made via borothermal reduction (28 ± 3 GPa). These hardness values were consistent with the theoretical value of ~33 GPa obtained by first-principles simulations using special quasi-random structures. Sample cross-sections were evaluated to examine the effects of the plasma on structural, compositional, and mechanical homogeneity throughout the HEB thickness. MW-plasma-produced HEBs synthesized with carbon exhibit a reduced porosity, higher density, and higher average hardness when compared to HEBs made without carbon.

A uniform rule between hardness and structure hierarchy of ceramics in both direct and inverse Hall–Petch regimes

AbstractThe MicroStructure Hierarchy Descriptor (SHD) has built a bridge in linking the structure hierarchy and properties of ceramic materials. Here, a new variable , which stands for the average of scales present in a ceramic microstructure, based on the SHD, is constructed and calibrated. It is found that the is not only related to the size scales, but also the morphological features in microstructures, due to an inheritance from the SHD. The is positively linear correlated with the average grain size d, but the relationship changes to be nonlinear when the morphology of grains is considered. In the Hall–Petch regime, the Vickers hardness HV of both, ZrO2–SiC composite ceramics and a class of high entropy metallic borides, is found to increase with the decrease of index. For the case study of ZrO2 ceramic, a uniform rule is found to be valid in both the direct and inverse Hall–Petch regimes that the varies in an opposite trend as the HV varies with d.

Reduced graphene oxide supported Fe2B as robust catalysts for oxygen reduction reaction

Transition metal borides have great potential to be low-cost, high-performance catalysts for novel energies despite the synthesis is rather difficult. In this paper, the reduced graphene oxide (rGO) supported iron boride (Fe2B/rGO) based catalysts are synthesized by a facile reduction method. The successful synthesis of Fe2B is confirmed by X-ray diffraction, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray photo-electron spectroscopy (XPS) and other tests. HRTEM tests showed that the constructed Fe2B was embedded in the rGO, where B played the role of coordination atoms which could regulate the electronic structure of the catalysts and improve the catalytic performance towards oxygen reduction reaction (ORR). The electrochemistry tests showed that the peak current intensity of the Fe2B/rGO catalyzed ORR could be reached up to 7.6 mA/cm2, which surpassed that of the Pt/C (20 wt%) catalyst. The current intensity can be kept at 82.47% after continuous running 20,000 s, which is higher than the Pt/C catalyst (79.4%). The onset potential reaches up to 0.95 V, which is only 0.06 V lower than that of Pt/C (20 wt%) catalyst. Both RDE and RRDE tests confirmed that the Fe2B/rGO catalyzed ORR major happed through 4-electron pathway. The redistributed electron between iron and boron atoms promoted the happening of ORR on Fe2B/rGO catalysts. The results of this work provide a novel way to develop high performance transition metal boride based catalysts for ORR.

Application of transition metal high entropy boride in electrocatalytic hydrogen evolution reaction

For the growth of hydrogen energy, it is essential to create electrocatalysts that are affordable, effective, and stable. The transition metal high-entropy boride electrocatalyst is synthesized with the molten salt-assisted boron thermal reduction method. It discusses the influence of element composition, temperature, and the ratio of salt to material on electrocatalytic hydrogen evolution. The results show that WMoVNbMnB synthesized at the ratio of salt to material of 15:1 and sintering temperature of 1000 °C has nano-flake structure. The overpotential at a current density of 10 mA cm−2 at 1 M KOH is as low as 115 mV, with the Tafel slope of 92 mV·dec−1. According to theoretical study, the transition metals (V, Mo, and Mn) are a considerable contributor to the electron around the Fermi level. Under the synergistic effect of multiple components, the V 3d orbit possesses excellent polarization, which thus improves the migration ability of the carrier. In the process of water decomposition, WMoVNbMnB electrocatalyst only needs to cross the 0.24 eV energy barrier to successfully dissociation, with the excellent properties of a glass carbon catalyst. It offers a workable reference for creating water electrolysis technology because of its highly effective catalytic activity.

Influence of Cr content on sintering, textured structure, and properties of (Hf, Zr, Ta, Cr, Ti)B2 high-entropy boride ceramics

In order to provide insights into the influence of Cr content on the microstructure and corresponding properties of (Hf, Zr, Ta, Cr, Ti)B2, the effects of 10 at.% to 30 at.% Cr contents on texture development, phase composition, microstructure, and mechanical properties of (Hf, Zr, Ta, Cr, Ti)B2, which were prepared via a combination of boro/carbothermal reduction and SPS, were investigated in this study. The results showed that (Hf, Zr, Ta, Cr, Ti)B2 added with 15 at.% Cr content samples formed a textured microstructure due to the formation of CrB in the (Hf, Zr, Ta, Cr, Ti)B2 high-entropy boride powder. With the increase of Cr content up to 30 at.%, the density, grain size, and fracture toughness of (Hf, Zr, Ta, Cr, Ti)B2 high entropy boride ceramics increased, then for higher content, they decreased, and tended to remain constant. The hardness of (Hf, Zr, Ta, Cr, Ti)B2 decreased with the increase of Cr content. The (Hf, Zr, Ta, Cr, Ti)B2 with 15 at.% Cr content possessed the best comprehensive mechanical performance (Hv0.2: TS:26.9 ± 0.8 GPa, SS: 26.5 ± 0.6 GPa; KIc: TS:4.21 ± 0.46 MPa·m1/2, SS: 3.16 ± 0.18 MPa·m1/2) due to a combination of high relative density and unique textured structure.

Application of rare-earth high entropy boride in electrocatalytic hydrogen evolution reaction

The preparation of simple, and low-cost and efficient catalysts is essential and challenging for achieving large-scale water electrolysis for hydrogen production. Three new electrocatalysts with high-entropy borides were synthesized by the short-process molten salt-assisted boron thermal reduction method. This paper has discussed the influence of synthesis conditions and rare-earth species on the catalyst performance. At 1 M KOH solution, WMoVNbCeB has the best electrochemical hydrogen evolution activity when the salt-to-material ratio and sintering temperature are 15:1 and 1000 °C, respectively The overpotential of the current density of 10 mA∙cm−2 is as low as 117 mV, the Tafel slope is 111 mV∙dec−1, and it has a large active area. The calculation results show that the combination of the electron orbit of the outer layer of each element promotes the hydrogen evolution reaction. The synergy between the f-orbit of the rare-earth metal and the d-orbit of other transition metals accelerates the charge transfer, and promotes the dissociation of the adsorbent, and then reduces the reaction energy barrier of the hydrogen precipitation process. It is confirmed that the rare-earth high entropy boride has a positive electrochemical activity. This provides a new way and new materials for industrial hydrogen production and has broad guidance and development prospects.

Novel (Hf0.2Zr0.2Ta0.2V0.2Nb0.2)B2 high entropy diborides with superb hardness sintered by SPS under a mild condition

The composition and synthesis approach of high entropy ceramics have significant influences on their microstructures and mechanical properties. A new (Hf0.2Zr0.2Ta0.2V0.2Nb0.2)B2 high entropy diboride ceramic with excellent mechanical properties was successfully prepared by spark plasma sintering (SPS) at a relatively “low temperature” (1800 °C) in this work. The effects of two mixing strategies, grinding and ball milling, on the solid solubility of the calcined powders were studied. Furthermore, the effects of these two mixing strategies on the phase and morphology of the calcined powders and the mechanical properties of the SPS sintered ceramics were analyzed. Experimental results indicated that, compared with a single solid solution phase within the high entropy boride powders obtained by ball milling, incomplete solid-solution presented in the high entropy boride powders obtained using grinding treatment. It promoted the solid-state diffusion and in-situ reaction of the diborides in the ceramics during the sintering process, thereby improving the densification and the hardness of the high entropy diboride ceramics. By grinding treatment, (Hf0.2Zr0.2Ta0.2V0.2Nb0.2)B2 high entropy ceramic with a relative density of 94% and a hardness of up to 25.34 ± 1.5 GPa at an indentation load of 9.8 N was obtained at 1800 °C. This work not only expanded the family of high-entropy diboride ceramics but also proved the advantage of grinding treatment in preparation of high entropy diboride ceramics.

Properties of high entropy borides synthesized via microwave-induced plasma

Microwave-induced plasma was used to anneal precursor powders containing five metal oxides with carbon and boron carbide as reducing agents, resulting in high entropy boride ceramics. Measurements of hardness, phase structure, and oxidation resistance were investigated. Plasma annealing for 45 min in the range of 1500–2000 °C led to the formation of predominantly single-phase (Hf, Zr, Ti, Ta, Mo)B2 or (Hf, Zr, Nb, Ta, Mo)B2 hexagonal structures characteristic of high entropy borides. Oxidation resistance for these borides was improved by as much as a factor of ten when compared to conventional commercial diborides. Vickers and nanoindentation hardness measurements show the indentation size effect and were found to be as much as 50% higher than that reported for the same high entropy boride configuration made by other methods, with average values reaching up to 38 GPa (for the highest Vickers load of 200 gf). Density functional theory calculations with a partial occupation method showed that (Hf, Zr, Ti, Ta, Mo)B2 has a higher hardness but a lower entropy forming ability compared to (Hf, Zr, Nb, Ta, Mo)B2, which agrees with the experiments. Overall, these results indicate the strong potential of using microwave-induced plasma as a novel approach for synthesizing high entropy borides.