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ABSTRACTHigh-entropy carbide (Ti0.2xZr0.2xHf0.2x + (1-x)Nb0.2xTa0.2x)Cx(HEC-(1-x)Hf, x = 0.6-1) ceramics with 0-40 at.% nominal carbon vacancy were prepared by adding metal Hf into (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C (HEC) as a case study. The samples prepared by spark plasma sintering at 2000oC exhibit relative densities higher than 99%, single-phase rock salt structure, and homogeneous metal elements distribution. Their lattice parameters increase gradually with the added Hf content. The lattice parameter of HEC-0.4Hf is 0.86% higher than HEC's. The average grain size of the samples decreased initially, followed by an increase as Hf content increased, which is controlled by the comprehensive effect of Hf addition and carbon vacancy. The thermal conductivities of the samples decrease with the increase of Hf addition. HEC-0.4Hf has a very low thermal conductivity of 5.6 W/m·K at room temperature, which decreases 67.8% compared to HEC. The increased phonon scattering by the carbon vacancies can be the main reason for the reduced thermal conductivity.

High-entropy carbide (Ti0.2xZr0.2xHf0.2x+ (1-x)Nb0.2xTa0.2x)Cx(HEC-(1-x)Hf, x = 0.6-1) ceramics with 0-40 at.% nominal carbon vacancy were prepared by adding metal Hf into (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C (HEC) as a case study. The samples prepared by spark plasma sintering at 2000oC exhibit relative densities higher than 99%, single-phase rock salt structure, and homogeneous metal elements distribution. Their lattice parameters increase gradually with the added Hf content. The lattice parameter of HEC-0.4Hf is 0.86% higher than HEC's. The average grain size of the samples decreased initially, followed by an increase as Hf content increased, which is controlled by the comprehensive effect of Hf addition and carbon vacancy. The thermal conductivities of the samples decrease with the increase of Hf addition. HEC-0.4Hf has a very low thermal conductivity of 5.6 W/m·K at room temperature, which decreases 67.8% compared to HEC. The increased phonon scattering by the carbon vacancies can be the main reason for the reduced thermal conductivity.

• After irradiation with He ions, only 'black-dot' type defects were observed while no helium-induced cavities, cracks or extended dislocations were detected in ZrC 1- x. • The lattice parameters would recover after annealing, and the degree of recovery varied with different stoichiometric and annealing temperatures. • Helium-induced cavities and extended dislocations were generated in ZrC 1- x during annealing. • With increasing x value in ZrC 1- x , the size of cavities increased while the length and density of extended dislocations decreased. The dynamic study of radiation-induced defects with annealing is critical for the material design for next-generation nuclear energy systems. The native vacancy could affect the development of defects, which lacks study. In the present work, the as-hot pressed ZrC 1- x (x =0, 0.15, 0.3) ceramics which comprised crystallites of a few microns in size with different amounts of carbon vacancies were irradiated by 540 keV He2+ ions at room temperature with a fluence of 1 × 1017/cm2. The radiation-induced lattice expansion was found to be a common phenomenon in a sequence of ZrC 0.85 ≥ZrC 1.0 >ZrC 0.7. Both X-ray and electron diffractions confirmed maintenance of structural integrity without amorphization after irradiation. Inside the irradiated region, only "black-dot" type defects, i.e., clusters of point defects were observed while no helium-induced cavities, cracks, or extended dislocations were detected. The as-irradiated ZrC 1- x were then annealed at different high temperatures. Upon annealing at 800 °C, very tiny helium-induced cavities were found to be generated and the crystal lattice recovered to a great extent, especially for the sub-stoichiometric samples. While annealed at 1500 °C, all the samples almost fully recovered the crystal lattices close to those of as-hot pressed ones. Meanwhile, large cavities and extended dislocations were generated. With increasing amount of native carbon vacancies, the size of cavities increased while the length and density of extended dislocations decreased. Inverse changes of lattice parameters during irradiation and annealing processes have been interpreted by the kinetics of defects. Finally, the correlation between native vacancies and damage behavior is discussed. [ABSTRACT FROM AUTHOR]

The preparation of high-entropy (HE) ceramics with designed composition is essential for verifying the formability models and evaluating the properties of the ceramics. However, inevitable oxygen contamination in non-oxide ceramics will result in the formation of metal oxide impurity phases remaining in the specimen or even escaping from the specimen during the sintering process, making the elemental compositions of the HE phase deviated from the designed ones. In this work, the preparation and thermodynamic analysis during the processing of equiatomic 9-cation HE carbide (HEC9) ceramics of the IVB, VB, and VIB groups were studied focusing on the removing of the inevitable oxygen impurity existed in the starting carbide powders and the oxygen contamination during the powder mixing processing. The results demonstrate that densification by spark plasma sintering (SPS) by directly using the mixed powders of the corresponding single-component carbides will inhibit the oxygen-removing carbothermal reduction reactions, and most of the oxide impurities will remain in the sample as (Zr,Hf)O2phase. Pretreatment of the mixed powders at high temperatures in vacuum will remove most part of the oxygen impurity but result in a remarkable escape of gaseous Cr owing to the oxygen-removing reaction between Cr3C2and various oxide impurities. It is found that graphite addition enhances the oxygen-removing effect and simultaneously prevents the escape of gaseous Cr. On the other hand, although WC, VC, and Mo2C can also act as oxygen-removing agents, there is no metal-containing gaseous substance formation in the temperature range of this study. By using the heat-treated powders with added graphite, equiatomic HEC9 ceramics were successfully prepared by SPS.

• Three new ternary borides Nb 2 PB 2 , Nb 2 SB 2 and Nb 2 PB are identified, and they are dynamically and energetically stable. • The overlap between acoustic-optical phonon states driven by the A layer as well as the gaps can be regarded d as the vibrational signatures. • The structural and compositional variation lead to different Fermi surfaces, enlightening the anomalous behavior in the heat conductance. • An ultra-high thermal conductor Nb 2 PB 2 with a κ t o t of 65 W/mK is identified. • An ultra-low thermal conductor Nb 2 SC is found with a κ t o t lowering to 5 W/mK. The emergence of MAX borides as well as MAB phases attracted great attention because of the renewable developments of ternary ceramics and offering great opportunities in potential applications. However, the number of borides remains limited, and further fundamental descriptions and detailed investigations on various properties are still lacking. In this report, we employ an integrated computational scheme that combines density functional theory with the evolutional algorithm to search for the favorable structures of P - and S -glued ternary borides terminated by Nb metal. We discover that the structures of 212-type, as e.g. Nb 2 PB 2 and Nb 2 SB 2 , belong to the P 6 ¯ m2 space group, while those of 211-type, as e.g. Nb 2 PB and Nb 2 SB, prefer to crystallize in the P6 3 /mmc space group, and the corresponding carbides Nb 2 PC and Nb 2 SC are also considered for the sake of completeness and comparative analsys. The predicted Nb 2 PB 2 , Nb 2 PB, Nb 2 SB, Nb 2 PC and Nb 2 SC are energetically stable, as revealed by the negative formation energies and by the proposed reaction paths with respect to the most competing phases, as well as dynamically stable, as suggested by the non-imaginary phonon spectra. The thermal conductivities of the six materials show unusual behaviors, particularly for the acoustic and optical contributions, and are accompanied by a strong anisotropy. Most importantly, Nb 2 PB 2 is found to be an excellent thermal conductor with a total thermal conductivity of ~65 W/(m K), while Nb 2 SC is found to be an ultra-low thermal conductor, with a total thermal conductivity of ~5 W/(m K). These values are clearly outside the currently reported range of thermal conductivities, which makes Nb 2 PB 2 and Nb 2 SC extremely interesting for fundamental research as well as prospective applications with the aid of artificial tunings on the almost independent MB block and the A layer. The discovery of these novel materials is expected to contribute substantially to the rapid development of ternary ceramics and to accelerate attempts in the applicability of MAX phases for heat conduction. [Display omitted] [ABSTRACT FROM AUTHOR]

Ultra-high temperature ceramics (UHTCs) are generally referred to the carbides, nitrides, and borides of the transition metals, with the Group IVB compounds (Zr & Hf) and TaC as the main focus. The UHTCs are endowed with ultra-high melting points, excellent mechanical properties, and ablation resistance at elevated temperatures. These unique combinations of properties make them promising materials for extremely environmental structural applications in rocket and hypersonic vehicles, particularly nozzles, leading edges, and engine components, etc. In addition to bulk UHTCs, UHTC coatings and fiber reinforced UHTC composites are extensively developed and applied to avoid the intrinsic brittleness and poor thermal shock resistance of bulk ceramics. Recently, highentropy UHTCs are developed rapidly and attract a lot of attention as an emerging direction for ultra-high temperature materials. This review presents the state of the art of processing approaches, microstructure design and properties of UHTCs from bulk materials to composites and coatings, as well as the future directions.

[Display omitted] • Ion irradiation of ZrB 2 leads to boron vacancies evidenced experimentally and quantified by DFT calculation. • Ion irradiation of ZrB 2 leads to lattice line defects as long dislocations and Frank dislocations. • ZrB 2 keeps structural integrity with mild damage at lattice level after ion irradiation. Ultra-high temperature ceramics have been considered as good candidates for plasma facing materials due to their combination of high melting point, high strength and hardness, high thermal conductivity as well as good chemical inertness. In this study, zirconium diboride has been chosen to investigate its irradiation damage behavior. Irradiated by 4 MeV Au2+ with a total fluence of 2.5 × 1016 cm−2, zirconium diboride ceramic shows substantial resilience to irradiation-induced damage with its structural integrity well maintained but mild damage at lattice level. Grazing incident X-ray diffraction evidences no change of the hexagonal structure in the irradiated region but its lattice parameter a increased and c decreased, giving a volume shrinkage of ∼0.46%. Density functional theory calculation shows that such lattice shrinkage corresponds to a non-stoichiometric compound as ZrB 1.97. Electron energy-loss spectroscopy in a transmission electron microscope revealed an increase of valence electrons in zirconium, suggesting boron vacancies were indeed developed by the irradiation. Along the irradiation depth, long dislocations were observed inside top layer with a depth of ∼750 nm where the implanted Au ions reached the peak concentration. Underneath the top layer, a high density of Frank dislocations is formed by the cascade collision down to a depth of 1150 nm. All the features show the potential of ZrB 2 to be used as structural material in nuclear system. [ABSTRACT FROM AUTHOR]

High-entropy ceramics attract more and more attention in recent years. However, mechanical properties especially strength and fracture toughness for high-entropy ceramics and their composites have not been comprehensively reported. In this work, high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta 0.2)B2(HEB) monolithic and its composite containing 20 vol% SiC (HEB–20SiC) are prepared by hot pressing. The addition of SiC not only accelerates the densification process but also refines the microstructure of HEB, resulting in improved mechanical properties. The obtained dense HEB and HEB–20SiC ceramics hot pressed at 1800 ℃ exhibit four-point flexural strength of 339±17 MPa and 447±45 MPa, and fracture toughness of 3.81±0.40 MPa·m1/2and 4.85±0.33 MPa·m1/2measured by single-edge notched beam (SENB) technique. Crack deflection and branching by SiC particles is considered to be the main toughening mechanisms for the HEB–20SiC composite. The hardness Hv0.2of the sintered HEB and HEB–20SiC ceramics is 23.7±0.7 GPa and 24.8±1.2 GPa, respectively. With the increase of indentation load, the hardness of the sintered ceramics decreases rapidly until the load reaches about 49 N, due to the indentation size effect. Based on the current experimental investigation it can be seen that the room temperature bending strength and fracture toughness of the high-entropy diboride ceramics are within ranges commonly observed in structure ceramics.

Ultra-high temperature ceramics (UHTCs) are considered as a family of nonmetallic and inorganic materials that have melting point over 3000 °C. Chemically, nearly all UHTCs are borides, carbides, and nitrides of early transition metals (e.g., Zr, Hf, Nb, Ta). Within the last two decades, except for the great achievements in the densification, microstructure tailoring, and mechanical property improvements of UHTCs, many methods have been established for the preparation of porous UHTCs, aiming to develop high-temperature resistant, sintering resistant, and lightweight materials that will withstand temperatures as high as 2000 °C for long periods of time. Amongst the synthesis methods for porous UHTCs, sol-gel methods enable the preparation of porous UHTCs with pore sizes from 1 to 500 urn and porosity within the range of 60%-95% at relatively low temperature. In this article, we review the currently available sol-gel methods for the preparation of porous UHTCs. Templating, foaming, and solvent evaporation methods are described and compared in terms of processing-microstructure relations. The properties and high temperature resistance of sol-gel derived porous UHTCs are discussed. Finally, directions to future investigations on the processing and applications of porous UHTCs are proposed.

High-entropy pyrochlore-type structures based on rare-earth zirconates are successfully produced by conventional solid-state reaction method. Six rare-earth oxides (La2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, and Y2O3) and ZrO2are used as the raw powders. Five out of the six rare-earth oxides with equimolar ratio and ZrO2are mixed and sintered at different temperatures for investigating the reaction process. The results demonstrate that the high-entropy pyrochlores (5RE1/5)2Zr2O7have been formed after heated at 1000°C. The (5RE1/5)2Zr2O7are highly sintering resistant and possess excellent thermal stability. The thermal conductivities of the (5RE1/5)2Zr2O7high-entropy ceramics are below 1 W·m–1·K–1in the temperature range of 300–1200°C. The (5RE1/5)2Zr2O7can be potential thermal barrier coating materials.

A high-entropy silicide (HES), (Ti0.2Zr0.2Nb0.2Mo0.2W0.2)Si2with close-packed hexagonal structure is successfully manufactured through reactive spark plasma sintering at 1300 °C for 15 min. The elements in this HES are uniformly distributed in the specimen based on the energy dispersive spectrometer analysis except a small amount of zirconium that is combined with oxygen as impurity particles. The Young’s modulus, Poisson’s ratio, and Vickers hardness of the obtained (Ti0.2Zr0.2Nb0.2Mo0.2W0.2)Si2are also measured.

High electrical conductivity and super high hardness are two sought-after material properties, but both are contradictory because the effective suppression of dislocation movement generally increases the scattering of conducting electrons. Here we synthesized a high-entropy dodecaboride composite (HEDC) with a large number of atomic-scale interlocking layers. It shows a Vickers hardness of 51.2 ± 3.6 GPa under an applied load of 0.49 N and an electrical resistivity of 44.5 μΩ·cm at room temperature. Such HEDC achieves superhardness by inheriting the high intrinsic hardness of its constituent phases and restricting the dislocation motion to further enhance the extrinsic hardness through forming numerous atom-scale interlocks between different slip systems. Moreover, the HEDC maintains the excellent electrical conductivity of the constituent borides, and the competition between two correlating structures produces the special kind of coherent boundary that minimizes the scattering of conducting electrons and does not largely deteriorate the electrical conductivity.

An efficient strategy that can guide the synthesis of materials with superior mechanical properties is important for advanced material/device design. Here, we report a feasible way to enhance hardness in transition-metal monocarbides (TMCs) by optimally filling the bonding orbitals of valence electrons. We demonstrate that the intrinsic hardness of the NaCl- and WC-type TMCs maximizes at valence electron concentrations of about 9 and 10.25 electrons per cell, respectively; any deviation from such optimal values will reduce the hardness. Using the spark plasma sintering technique, a number of W1–xRexC (x= 0–0.5) have been successfully synthesized, and powder X-ray diffractions show that they adopt the hexagonal WC-type structure. Subsequent nanoindentation and Vickers hardness measurements corroborate that the newly developed W1–xRexC samples (x= 0.1–0.3) are much harder than their parent phase (i.e., WC), marking them as the hardest TMCs for practical applications. Furthermore, the hardness enhancement can be well rationalized by the balanced occupancy of bonding and antibonding states. Our findings not only elucidate the unique hardening mechanism in a large class of TMCs but also offer a guide for the design of other hard and superhard compounds such as borides and nitrides.