•High-entropy boride-carbide two-phase ultrahigh temperature ceramics are made.•This work further extends the emerging field of high-entropy ceramics.•A novel reactive spark plasma sintering route ...produces > ∼99 % dense specimens.•A thermodynamic relation governing the equilibrium phase compositions is discovered.•The hardness is higher than the weighted average of the two high-entropy phases.
A series of dual-phase high-entropy ultra-high temperature ceramics (DPHE-UHTCs) are fabricated starting from N binary borides and (5-N) binary carbides powders. > ∼99 % relative densities have been achieved with virtually no native oxides. These DPHE-UHTCs consist of a hexagonal high-entropy boride (HEB) phase and a cubic high-entropy carbide (HEC) phase. A thermodynamic relation that governs the compositions of the HEB and HEC phases in equilibrium is discovered and a thermodynamic model is proposed. These DPHE-UHTCs exhibit tunable grain size, Vickers microhardness, Young’s and shear moduli, and thermal conductivity. The DPHE-UHTCs have higher hardness than the weighted linear average of the two single-phase HEB and HEC, which are already harder than the rule-of-mixture averages of individual binary borides and carbides. This study extends the state of the art by introducing dual-phase high-entropy ceramics (DPHECs), which provide a new platform to tailor various properties via changing the phase fraction and microstructure.
A novel high‐entropy carbide ceramic, (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C, with a single‐phase rock salt structure, was synthesized by spark plasma sintering. X‐ray diffraction confirmed the formation of a ...single‐phase rock salt structure at 26‐1140°C in Argon atmosphere, in which the 5 metal elements may share a cation position while the C element occupies the anion position. (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C exhibits a much lower thermal diffusivity and conductivity than the binary carbides HfC, ZrC, TaC, and TiC, which may result from the significant phonon scattering at its distorted anion sublattice. (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C inherits the high elastic modulus and hardness of the binary carbide ceramics.
The formation possibility of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramic (HHC‐1) was first analyzed by the first‐principles calculations, and then, it was successfully fabricated by ...hot‐pressing sintering technique at 2073 K under a pressure of 30 MPa. The first‐principles calculation results showed that the mixing enthalpy and mixing entropy of HHC‐1 were −0.869 ± 0.290 kJ/mol and 0.805R, respectively. The experimental results showed that the as‐prepared HHC‐1 not only had an interesting single rock‐salt crystal structure of metal carbides but also possessed high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, it exhibited extremely high nanohardness of 40.6 ± 0.6 GPa and elastic modulus in the range from 514 ± 10 to 522 ± 10 GPa and relatively high electrical resistivity of 91 ± 1.3 μΩ·cm, which could be due to the presence of solid solution effects.
High‐entropy metal carbides have recently been arousing considerable interest. Nevertheless, their high‐temperature oxidation behavior is rarely studied. Herein the high‐temperature oxidation ...behavior of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy metal carbide (HEC‐1) was investigated at 1573‐1773 K in air for 120 minutes. The results showed that HEC‐1 had good oxidation resistance and its oxidation obeyed a parabolic law at 1573‐1673 K, while HEC‐1 was completely oxidized after isothermal oxidation at 1773 K for 60 minutes and thereby its oxidation followed a parabolic‐linear law at 1773 K. An interesting triple‐layered structure was observed within the formed oxide layer at 1673 K, which was attributed to the inward diffusion of O2 and the outward diffusion of Ti element and CO or CO2 gaseous products.
High‐entropy single‐phase rare earth titanates (RE0.2Gd0.2Ho0.2Er0.2Yb0.2)2Ti2O7 (RE = Sm, Y, Lu) were designed and synthesized successfully, in which their lattice distortion was quantitatively ...described by mass disorder and size disorder. It is worth mentioning that (Y0.2Gd0.2Ho0.2Er0.2Yb0.2)2Ti2O7 could obtain the low thermal conductivity (1.51 W·m−1·K−1, 1500°C), high thermal expansion coefficient (average, 11.69×10−6 K−1, RT ∼1500°C) and excellent high‐temperature stability. In addition, the relationship between the microstructure and thermal transport behaviors has been studied at the atomic scale. Due to the disorder of A‐site ions, severe lattice distortion occurred in specific crystal planes, and the large mass difference between Y3+ and other RE3+ further causes mass fluctuation and results in lower thermal conductivity. Compared with YSZ, the high‐entropy rare earth titanate (Y0.2Gd0.2Ho0.2Er0.2Yb0.2)2Ti2O7 has lower thermal conductivity, higher thermal expansion coefficient, and excellent high‐temperature stability, which has great potential for application in the thermal protection field.
Manipulating a crystalline material's configurational entropy through the introduction of unique atomic species can produce novel materials with desirable mechanical and electrical properties. From a ...thermal transport perspective, large differences between elemental properties such as mass and interatomic force can reduce the rate at which phonons carry heat and thus reduce the thermal conductivity. Recent advances in materials synthesis are enabling the fabrication of entropy‐stabilized ceramics, opening the door for understanding the implications of extreme disorder on thermal transport. Measuring the structural, mechanical, and thermal properties of single‐crystal entropy‐stabilized oxides, it is shown that local ionic charge disorder can effectively reduce thermal conductivity without compromising mechanical stiffness. These materials demonstrate similar thermal conductivities to their amorphous counterparts, in agreement with the theoretical minimum limit, resulting in this class of material possessing the highest ratio of elastic modulus to thermal conductivity of any isotropic crystal.
The thermal, mechanical, and structural properties of a new class of high‐entropy ceramics, entropy‐stabilized oxides, are reported, to reveal that local ionic charge disorder can effectively reduce thermal conductivity without compromising mechanical stiffness. This work demonstrates the benefits of entropy stabilization to achieve unique combinations of thermophysical properties.
(Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramics (HEC) with a submicron grain size of 400 to 600 nm were fabricated by spark plasma sintering using a two‐step sintering process. Both X‐ray and ...neutron diffractions confirmed the formation of single‐phase with rock salt structure in the as‐fabricated (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C samples. The effect of submicron grain size on the thermal stability and mechanical properties of HEC was investigated. The grain growth kinetics in the fine‐grained HEC was small at 1300 and 1600°C, suggesting high thermal stability that was possibly related to the compositional complexity and sluggish diffusion in HEC. Compared to the coarse‐grain HEC with a grain size of 16.5 µm, the bending strength and fracture toughness of fine‐grained HEC were 25% and 20% higher respectively. The improvement of mechanical properties in fine‐grained HEC may be attributed to micromechanistic mechanisms such as crack deflection.
High‐entropy oxides (HEOs) are a new class of materials promising for a wide range of applications. Designing HEOs needs to consider both geometric compatibility and electrical balance. However, ...there is currently no available method to systematically consider these two factors when selecting constituent materials for making HEOs. Here we propose a two‐step strategy, where a HEO system is first partitioned into multiple subsystems based on the valence combinations of constituted cations; the geometric compatibility is then considered in selecting suitable constituted cations. We demonstrate this strategy by using A(5B0.2)O3 perovskite as a model system. We show that the system can be partitioned into 12 subsystems. A single‐phase cubic perovskite has been synthesized in ten of the subsystems. We anticipate that this strategy is applicable to other oxide systems.
A nano dual-phase powder with great sinterability was synthesized by molten-salt assisted borothermal reductions at 1100 °C using B, ZrO2, HfO2, Ta2O5, Nb2O5 and TiO2 powders as raw materials. ...Single-phase (Zr0.2Hf0.2Ta0.2Nb0.2Ti0.2)B2 high-entropy ceramic was prepared by spark plasma sintering using the as-synthesized nano dual-phase powder. Oxidation behavior of the (Zr0.2Hf0.2Ta0.2Nb0.2Ti0.2)B2 ceramic was investigated over the range of 30–1400 °C in air and the result indicated that the rapid oxidation of ceramic began at 1300 °C. The phenomenon could be ascribed to the rapid volatilization of B2O3 from oxide scale. A layered structure was formed at the cross section of (Zr0.2Hf0.2Ta0.2Nb0.2Ti0.2)B2 ceramic after oxidation. The relationship between partial pressures of gaseous metal oxides and oxygen partial pressures was calculated, which inferred that the formation of layered structure could be ascribed to the active oxidation of (Zr0.2Hf0.2Ta0.2Nb0.2Ti0.2)B2, the generation of gaseous metal oxides, their outward diffusion and further oxidation.
Herein the ultrafine‐grained (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)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·m1/2, which were primary attributed to the fine grain strengthening mechanism and microcrack deflection toughening mechanism.
PDF
