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MAX phases are a member of ternary carbide and nitride, with a layered crystal structure and a mixed nature of chemical bonds (covalent-ionic-metallic) that promote MAX phases embracing both ceramic and metal characteristics. As a result, MAX phase ceramics emerge with remarkable properties unique from other traditional ceramics. In this review, we focus on alternate processing approaches for MAX phases that are cost-effective and energy-saving. The MAX phase purity, formation of other unwanted phases, microstructure, and properties are influenced by many parameters during processing. Therefore, we highlight the effect of numerous factors, which alternately diminish the efficiency and performance of materials. Here, the impact of several parameters, such as starting materials, stoichiometric composition, temperature, pressure, particle size, porosity, microstructure, mechanical alloying, mechanical activation, ion irradiation, and doping, are summarized to reveal their influence on the synthesis and properties of MAX phases. The potential applications of MAX phases are considered for their development on a commercial scale toward the industry.

• A novel high entropy pyrochlore ceramic (La 0.3 Gd 0.3 Ca 0.4) 2 (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2) 2 O 7 with significant atomic radius and mass fluctuation is proposed for the first time. • Enhanced fracture toughness and amorphous-like low thermal conductivity was reported for the first time. • Close thermal expansion coefficient to Al 2 O 3 was demonstrated for the first time. • The influences of disorders in the atomic weight, ionic size and electronegativity of substitutional cations, and intrinsic oxygen vacancies on thermal conductivity reduction are investigated deeply. Low thermal conductivity and excellent mechanical strength are essential to pyrochlore A 2 B 2 O 7 ceramic for environmental/thermal barrier coating applications. To collaboratively tailor the mechanical and thermal properties of A 2 B 2 O 7 ceramic, a novel high entropy pyrochlore ceramic (La 0.3 Gd 0.3 Ca 0.4) 2 (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2) 2 O 7 with significant atomic radius and mass fluctuation is proposed by simultaneously introducing various elements with different valence states at A and B cation sites. The as-synthesized (La 0.3 Gd 0.3 Ca 0.4) 2 (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2) 2 O 7 exhibits enhanced fracture toughness (1.68 MPa m1/2), amorphous-like low thermal conductivity (1.45 W m–1 K–1 at 900 °C) and matched thermal expansion coefficient (9.0 × 10–6 K–1 at 1200 °C) with Al 2 O 3 /Al 2 O 3 CMCs. The extensive misfits in atomic weight, ionic radius among the substitutional cations in combination with the intrinsic oxygen vacancies in the anion sublattice play significant roles in the thermal conductivity reduction of (La 0.3 Gd 0.3 Ca 0.4) 2 (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2) 2 O 7 ceramic. The combination of outstanding mechanical and thermal properties indicates that this type of material has a good application prospect for environmental/thermal barrier coatings. [Display omitted] [ABSTRACT FROM AUTHOR]

• A simple yet efficient strategy for long term storage of MXene suspension by introducing glycerol, a typical polyhydric alcohol, was developed. • The effectiveness of the strategy is evidenced by structural compositional and morphological investigations. • Glycerol protects the defective sites of MXene flakes through restricting water and/or oxygen molecules from reactive sites. • The strategy based on polyhydric alcohols has the potential to be extended to other MXenes, solving the most urgent challenge in the field of MXene engineering. MXenes have promises in myriad applications by virtue of two-dimensional nature and adjustable functional groups. To achieve the applications, MXenes are always first prepared in the form of aqueous suspension. However, fast degradation caused by the attack of dissolved oxygen and water molecules is the main obstacle to the application of MXenes. It has come to light that the degradation preferentially takes place at defective sites and edges where defects enrich. To tackle this problem and increase the stability, herein, using Ti 3 C 2 T x MXene as a model material, we report a simple yet efficient strategy for long term storage of MXene suspension by introducing glycerol, a typical polyhydric alcohol. The effectiveness of the strategy is evidenced by structural compositional and morphological investigations. Glycerol protects the defective sites of MXene flakes through restricting water and/or oxygen molecules from reactive sites. This is supported by ab initio molecular dynamics simulations that form hydrogen bonds between MXene and glycerol molecules just over defective sites. Following this mechanism, other polyhydric alcohols, such as ethylene glycol and propylene glycol, are also effective in stabilizing Ti 3 C 2 T x MXene suspension. The strategy based on polyhydric alcohols has the potential to be extended to other MXenes, solving the most urgent challenge in the field of MXene engineering. [Display omitted] [ABSTRACT FROM AUTHOR]

• The high entropy rare earth hafnate as a promising nuclear engineering material was designed and prepared, and the ion irradiation properties with 400 keV Kr+ at 400 ℃ were reported for the first time. • Excellent radiation-induced amorphization resistance of high entropy ceramics after irradiation at 120 dpa was reported for the first time. • Compared with single components, the high entropy ceramics exhibit better radiation resistance and high temperature stability. Nuclear engineering materials are required to possess outstanding extreme environmental tolerance and irradiation resistance. A promising novel pyrochlore-type of (Sm 0.2 Eu 0.2 Gd 0.2 Dy 0.2 Er 0.2) 2 Hf 2 O 7 high-entropy ceramic (HE-RE 2 Hf 2 O 7) for control rod was prepared by solid-state reaction method. The ion irradiation of HE-RE 2 Hf 2 O 7 with 400 keV Kr+ at 400 °C was investigated using a 400 kV ion implanter and compared with single-component pyrochlore Gd 2 Hf 2 O 7 to evaluate the irradiation resistance. For HE-RE 2 Hf 2 O 7 , the phase transition from pyrochlore to defective fluorite is revealed after irradiation at 60 dpa. After irradiation at 120 dpa, it maintained crystalline, which is comparable to Gd 2 Hf 2 O 7 but superior to the titanate pyrochlores previously studied. Moreover, the lattice expansion of HE-RE 2 Hf 2 O 7 (0.22%) is much lower than that of Gd 2 Hf 2 O 7 (0.62%), indicating excellent irradiation damage resistance. Nanoindentation tests displayed an irradiation-induced increase in hardness and a decrease in elastic modulus by about 2.6%. Irradiation-induced segregation of elements is observed on the surface of irradiated samples. In addition, HE-RE 2 Hf 2 O 7 demonstrates a more sluggish grain growth rate than Gd 2 Hf 2 O 7 at 1200 °C, suggesting better high-temperature stability. The linear thermal expansion coefficient of HE-RE 2 Hf 2 O 7 is 10.7 × 10-6 K-1 at 298–1273 K. In general, it provides a new strategy for the design of the next advanced nuclear engineering materials. [Display omitted] [ABSTRACT FROM AUTHOR]

• The medium and high-entropy transition mental disilicides present significantly reduced electrical conductivities. • The medium and high-entropy transition mental disilicides are promoting infrared emissivity materials for thermal protection applications. • The mechanism for the reduced thermal conductivity and high infrared emissivity are disclosed. Transition metal disilicides are widely used as heating elements and infrared emission coatings. However, the limited intrinsic infrared emissivity and high thermal conductivity are the main limitations to their applications as infrared emission coatings in the thermal protection system. To cope with these problems, four medium and high-entropy transition metal disilicides, i.e., (V 0.25 Ta 0.25 Mo 0.25 W 0.25)Si 2 (ME-1), (Nb 0.25 Ta 0.25 Mo 0.25 W 0.25)Si 2 (ME-2), (V 0.2 Nb 0.2 Ta 0.2 Mo 0.2 W 0.2)Si 2 (HE-1), and (Cr 0.2 Nb 0.2 Ta 0.2 Mo 0.2 W 0.2)Si 2 (HE-2), were designed and synthesized by spark plasma sintering method using transition metal binary disilicides as precursors. The introduction of multi-elements into transition metal disilicides not only improved the infrared emissivity but also reduced the electrical and thermal conductivity. Among them, (Cr 0.2 Nb 0.2 Ta 0.2 Mo 0.2 W 0.2)Si 2 (HE-2) had the lowest electrical conductivity of 3789 S cm–1, which is over one order of magnitude lower than that of MoSi 2 (50000 S cm–1), and total infrared emissivity of 0.42 at room temperature, which is nearly double of that of TaSi 2. Benefiting from low electrical conductivity and phonon scattering due to lattice distortion, the medium and high-entropy transition metal disilicides also demonstrated a significant decline in thermal conductivity compared to their binary counterparts. Of all samples, HE-2 exhibited the lowest thermal conductivity of 6.4 W m − 1 K − 1 . The high-entropy transition metal disilicides also present excellent oxidation resistance at high temperatures. The improved infrared emissivity, reduced thermal conductivity, excellent oxidation resistance, and lower densities of these medium and high-entropy transition metal disilicides portend that they are promising as infrared emission coating materials for applications in thermal protection systems. [ABSTRACT FROM AUTHOR]

In this article, authors mainly introduce new digital technology in facial bone contouring surgery. In our experience, these new technologies are crucial in ensuring the satisfaction of surgical accuracy. Our previous studies have shown surgeons can use precise pre-operative design to reduce operative time, reduce bleeding during surgery. Additionally, augmented reality can enhance the perspective perception of surgeons combining virtuality and reality. What’s more, robot-assisted surgical technology also has a strong application prospect in facial contouring surgery. In the future, the combination of soft tissue contouring surgery will make the facial bone contouring surgery safer and more effective.

The ternary or quaternary layered compounds called MAB phases are frequently mentioned recently together with the well-known MAX phases. However, MAB phases are generally referred to layered transition metal borides, while MAX phases are layered transition metal carbides and nitrides with different types of crystal structure although they share the common nano-laminated structure characteristics. In order to prove that MAB phases can share the same type of crystal structure with MAX phases and extend the composition window of MAX phases from carbides and nitrides to borides, two new MAB phase compounds Zr2SeB and Hf2SeB with the Cr2AlC-type MAX phase (211 phase) crystal structure were discovered by a combination of first-principles calculations and experimental verification in this work. First-principles calculations predicted the stability and lattice parameters of the two new MAB phase compounds Zr2SeB and Hf2SeB. Then they were successfully synthesized by using a thermal explosion method in a spark plasma sintering (SPS) furnace. The crystal structures of Zr2SeB and Hf2SeB were determined by a combination of the X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The lattice parameters of Zr2SeB and Hf2SeB are a= 3.64398 Å, c= 12.63223 Å and a= 3.52280 Å, c= 12.47804 Å, respectively. And the atomic positions are M at 4f (1/3, 2/3, 0.60288 [Zr] or 0.59889 [Hf]), Se at 2c (1/3, 2/3, 1/4), and B at 2a (0, 0, 0). And the atomic stacking sequences follow those of the Cr2AlC-type MAX phases. This work opens up the composition window for the MAB phases and MAX phases and will trigger the interests of material scientists and physicists to explore new compounds and properties in this new family of materials.

High-entropy rare-earth aluminate (Y0.2Yb0.2Lu0.2Eu0.2Er0.2)3Al5O12(HE-RE3Al5O12) has been considered as a promising thermal protection coating (TPC) material based on its low thermal conductivity and close thermal expansion coefficient to that of Al2O3. However, such a coating has not been experimentally prepared, and its thermal protection performance has not been evaluated. To prove the feasibility of utilizing HE-RE3Al5O12as a TPC, HE-RE3Al5O12coating was deposited on a nickelbased superalloy for the first time using the atmospheric plasma spraying technique. The stability, surface, and cross-sectional morphologies, as well as the fracture surface of the HE-RE3Al5O12coating were investigated, and the thermal shock resistance was evaluated using the oxyacetylene flame test. The results show that the HE-RE3Al5O12coating can remain intact after 50 cycles at 1200 °C for 200 s, while the edge peeling phenomenon occurs after 10 cycles at 1400 °C for 200 s. This study clearly demonstrates that HE-RE3Al5O12coating is effective for protecting the nickel-based superalloy, and the atmospheric plasma spraying is a suitable method for preparing this kind of coatings.

• Two medium-entropy (ME) oxide ceramics, ME (Y,Ti) 0.1 (Zr,Hf,Ce) 0.9 O 2 and ME (Ta,Ti) 0.1 (Zr,Hf,Ce) 0.9 O 2 , were designed and prepared successfully. • Good high-temperature thermal radiation shielding performance and CMAS resistance of the two ME oxides were demonstrated for the first time. • The relationship between high-temperature thermal radiation shielding performance, i.e., infrared absorbance at the waveband of 1 to 5 μm and band gap in ME (Me,Ti) 0.1 (Zr,Hf,Ce) 0.9 O 2 (Me= Y and Ta) was investigated systematically. • The mechanism for the CMAS corrosion resistance was proposed for ME (Me,Ti) 0.1 (Zr,Hf,Ce) 0.9 O 2 (Me= Y and Ta). With continuous enhancement of gas-turbine inlet temperature and rapid increase of radiant heat transfer, thermal barrier coating (TBC) materials with a combination of low thermal conductivity and good high-temperature thermal radiation shielding performance play vital roles in ensuring the durability of metallic blades. However, yttria-stabilized zirconia (YSZ), as the state-of-the-art TBC and current industry standard, is unable to meet such demands since it is almost translucent to high-temperature thermal radiation. Besides, poor corrosion resistance of YSZ to molten calcia-magnesia-alumina-silicates (CMAS) also impedes its application in sand, dust, or volcanic ash laden environments. In order to improve the high-temperature thermal radiation shielding performance and CMAS resistance of YSZ and further reduce its thermal conductivity, two medium-entropy (ME) oxide ceramics, ME (Y, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 and ME (Ta, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 , were designed and prepared by pressureless sintering of binary powder compacts in this work. ME (Y, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 presents cubic structure but a trace amount of secondary phase, while ME (Ta, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 displays a combination of tetragonal phase (81.6 wt.%) and cubic phase (18.4 wt.%). Both ME (Y, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 and ME (Ta, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 possess better high-temperature thermal radiation shielding performance than YSZ. Especially, the high-temperature thermal radiation shielding performance of ME (Ta, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 is superior to that of ME (Y, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 due to its narrower band gap and correspondingly higher infrared absorbance (above 0.7) at the waveband of 1 to 5 μm. The two ME oxides also display significantly lower thermal conductivity than YSZ and close thermal expansion coefficients (TECs) to YSZ and Ni-based superalloys. In addition, the two ME oxides possess excellent CMAS resistance. After attack by molten CMAS at 1250 °C for 4 h, merely ∼2 μm thick penetration layer has been formed and the structure below the penetration layer is still intact. These results demonstrate that ME (Me , Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 (Me = Y and Ta), especially ME (Ta, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 , are promising thermal barrier materials for high-temperature thermal radiation shielding and CMAS blocking. [ABSTRACT FROM AUTHOR]

• Grain boundary segregation in medium/high entropy carbides is studied for the first time. • Spontaneous segregation is common in medium/high entropy carbides. • Segregation is dominated by atomic size effect. • Grain boundaries are usually strengthened due to segregation. • Grain boundary segregation is a universal mechanism to develop high performance UHTCs. Ultra-high temperature ceramics (UHTCs) exhibit a unique combination of excellent properties, including ultra-high melting point, excellent chemical stability, and good oxidation resistance, which make them promising candidates for aerospace and nuclear applications. However, the degradation of high-temperature strength is one of the main limitations for their ultra-high temperature applications. Thus, searching for mechanisms that can help to develop high-performance UHTCs with good high-temperature mechanical properties is urgently needed. To achieve this goal, grain boundary segregation of a series of carbides, including conventional, medium entropy, and high entropy transition metal carbides, i.e., Zr 0.95 W 0.05 C, TiZrHfC 3 , ZrHfNbTaC 4 , TiZrHfNbTaC 5 , were studied by atomistic simulations with a fitted Deep Potential (DP), and the effects of segregation on grain boundary strength were emphasized. For all the studied carbides, grain boundary segregations are realized, which are dominated by the atomic size effect. In addition, tensile simulations indicate that grain boundaries (GBs) will usually be strengthened due to segregation. Our simulation results reveal that grain boundary segregation may be a universal mechanism in enhancing the high-temperature strength of both conventional UHTCs and medium/high entropy UHTCs, since GBs play a key role in controlling the fracture of UHTCs at elevated temperatures. [Display omitted] [ABSTRACT FROM AUTHOR]

• High-entropy transition metal carbides with regulatable carbon stoichiometry (HE TMC x) achieve the promoting of formation ability from carbon stoichiometry deviations. • Bulk (Zr 0.25 Hf 0.25 Ta 0.25 Nb 0.25)C x (SPS-ZHTNC x) with carbon stoichiometry deviations show promoted sintering. • Bulk (Zr 0.25 Hf 0.25 Ta 0.25 Nb 0.25)C x (SPS-ZHTNC x) present tunable mechanical properties. Tremendous efforts have been dedicated to promote the formation ability of high-entropy transition metal carbides. However, the majority of methods for the synthesis of high-entropy transition metal carbides still face the challenges of high temperature, low efficiency, additional longtime post-treatment and uncontrollable properties. To cope with these challenges, high-entropy transition metal carbides with regulatable carbon stoichiometry (HE TMC x) were designed and synthesized, achieving improved ability for single phase solid solutions formation, promoting of sintering and controllable mechanical properties. Two typical composition series, i.e., easily synthesized (Zr 0.25 Hf 0.25 Ta 0.25 Nb 0.25)C (ZHTNC) and difficultly synthesized (Zr 0.25 Hf 0.25 Ta 0.25 Ti 0.25)C (ZHTTC) are selected to demonstrate the promoting formation ability of single phase solid solutions from carbon stoichiometry deviations. Single phase high-entropy ZHTTC, which has been proven difficult in forming a single phase solid solution, can be prepared with the decrease of C/TM ratio under 2000 °C; while the high-entropy ZHTNC, which has been proven easy in forming a single phase solid solution, can be synthesized at lower temperatures with the decrease of C/TM ratio. The synergistic effect of entropy stabilization and reduced chemical bond strength gaining from carbon stoichiometry deviations is responsible for the formation of single phase solid solutions and the promoted sintering of HE TMC x. For example, the relative density of bulk (Zr 0.25 Hf 0.25 Ta 0.25 Nb 0.25)C x (SPS-ZHTNC x) increases from 90.98% to 94.25% with decreasing the C/TM atomic ratio from 0.9 to 0.74. More importantly, the room temperature flexural strength, fracture toughness and brittleness index of SPS-ZHTNC x can be tuned in the range of 384 MPa–419 MPa, 4.41 MPa⋅m1/2–4.73 MPa⋅m1/2 and 3.679 μm−1/2–4.083 μm−1/2, respectively. Thus, the HE TMC x prepared by adjusting the ratio of carbon to refractory transition metal oxides have great potential for achieving low temperature synthesis, promoted sintering and tunable properties. [ABSTRACT FROM AUTHOR]

• Single-phase formation abilities of 120 two-component metal (transition metal and rare earth metal) diborides (TCBs) are studied by first-principles. • The thermodynamic stability of single-phase high-entropy transition and rare-earth metal diborides (HE TMREB 2 s) were quantified using the energy distribution of the local mixing enthalpies of all possible configurations. • HE TMREB 2 s were predicted to be enthalpy-stabilized materials, i.e., mixing enthalpy plays a critical role in single-phase stability. • The experimental results confirmed that enthalpy dominates the thermodynamic domain and drives the stability of REB 2 s in HE TMREB 2 s. Transition metal diborides (TMB 2 s) are the materials of choice in extreme environments due to their excellent thermal and chemical stabilities. However, the degradation of oxidation resistance of TMB 2 s at elevated temperature still hinders their applications. To cope with this challenge, it is effective to incorporate rare earth elements to form high-entropy transition and rare-earth metal diborides (HE TMREB 2 s). To obtain thermodynamically stable single-phase structures for HE TMREB 2 s, a "16 × 16 mixed enthalpy matrix" is constructed using first-principles calculations to predict the single-phase formation ability of 120 two-component diborides (TCBs). Through the use of the "16 × 16 mixed enthalpy matrix" of TCBs, specific combinations of TMB 2 s and REB 2 s that are most likely to form single-phase HE TMREB 2 s are confirmed. Subsequently, based on the energy distribution of the local mixing enthalpies of all possible configurations, the enthalpy and entropy descriptors of HE TMREB 2 s (RE = Sc, Lu, Tm, Er, Ho and Dy) are investigated. It is found that the mixing enthalpy plays a critical role in the stability of the single-phase HE TMREB 2 s, i.e., HE TMREB 2 s are enthalpy-stabilized materials. The experimental results further confirm that enthalpy dominates the thermodynamic domain and drives the stability of REB 2 s in HE TMREB 2 s. This study validates that enthalpy-stabilized HE TMREB 2 s can further expand the compositional space of ultrahigh temperature ceramics (UHTCs) and is expected to further improve the oxidation resistance and high temperature properties of UHTCs. [Display omitted] [ABSTRACT FROM AUTHOR]

Willemite Zn2SiO4crystallizes in such a way that Zn and Si are tetrahedrally coordinated with O in an ionic-covalent manner to form ZnO4and SiO4tetrahedra as the building units. The tetrahedra are corner-sharing, of which one SiO4 tetrahedron connects eight ZnO4tetrahedra, and one ZnO4tetrahedron links four ZnO4tetrahedra and four SiO4tetrahedra. The unique crystallographic configuration gives rise to parallel tunnels with a diameter of 5.7 Å along the c-axis direction. The tunnel structure of Zn2SiO4definitely correlates with its interesting elastic and thermal properties. On the one hand, the elastic modulus, coefficient of thermal expansion (CTE), and thermal conductivity are low. Zn2SiO4has low Vickers hardness of 6.6 GPa at 10 N and low thermal conductivity of 2.34 W/(m·K) at 1073 K. On the other hand, the elastic modulus and CTE along the c-axis are significantly larger than those along the a- and b-axes, showing obvious elastic and thermal expansion anisotropy. Specifically, the Young’s modulus along the zdirection (Ez= 179 GPa) is almost twice those in the xand ydirections (Ex= Ey= 93 GPa). The high thermal expansion anisotropy is ascribed to the empty tunnels along the c-axis, which are capable of more accommodating the thermal expansion along the a- and b-axes. The striking properties of Zn2SiO4in elastic modulus, hardness, CTE, and thermal conductivity make it much useful in various fields of ceramics, such as low thermal expansion, thermal insulation, and machining tools.

Ferrites are the most widely used microwave absorbing materials to deal with the threat of electromagnetic (EM) pollution. However, the lack of sufficient dielectric loss capacity is the main challenge that limits their applications. To cope with this challenge, three high-entropy (HE) spinel-type ferrite ceramics including (Mg0.2Mn0.2Fe0.2Co0.2Ni0.2)Fe2O4, (Mg0.2Fe0.2Co0.2Ni0.2Cu0.2)Fe2O4, and (Mg0.2Fe0.2Co0.2Ni0.2Zn0.2)Fe2O4were designed and successfully prepared through solid state synthesis. The results show that all three HE MFe2O4samples exhibit synergetic dielectric loss and magnetic loss. The good magnetic loss ability is due to the presence of magnetic components; while the enhanced dielectric properties are attributed to nano-domain, hopping mechanism of resonance effect and HE effect. Among three HE spinels, (Mg0.2Mn0.2Fe0.2Co0.2Ni0.2)Fe2O4shows the best EM wave absorption performance, e.g., its minimum reflection loss (RLmin) reaches −35.10 dB at 6.78 GHz with a thickness of 3.5 mm, and the optimized effective absorption bandwidth (EAB) is 7.48 GHz from 8.48 to 15.96 GHz at the thickness of 2.4 mm. Due to the easy preparation and strong EM dissipation ability, HE MFe2O4are promising as a new type of EM absorption materials.

Electronic devices pervade everyday life, which has triggered severe electromagnetic (EM) wave pollution. To face this challenge, developing EM wave absorbers with ultra-broadband absorption capacity is critically required. Currently, nano-composite construction has been widely utilized to realize impedance match and broadband absorption. However, complex experimental procedures, limited thermal stability, and interior oxidation resistance are still unneglectable issues. Therefore, it is appealing to realize ultra-broadband EM wave absorption in single-phase materials with good stability. Aiming at this target, two high-entropy transition metal carbides (HE TMCs) including (Zr,Hf,Nb,Ta)C (HE TMC-2) and (Cr,Zr,Hf,Nb,Ta)C (HE TMC-3) are designed and synthesized, of which the microwave absorption performance is investigated in comparison with previously reported (Ti,Zr,Hf,Nb,Ta)C (HE TMC-1). Due to the synergistic effects of dielectric and magnetic losses, HE TMC-2 and HE TMC-3 exhibit better impedance match and wider effective absorption bandwidth (EAB). In specific, the exclusion of Ti element in HE TMC-2 endows it optimal minimum reflection loss (RLmin) and EAB of −41.7 dB (2.11 mm, 10.52 GHz) and 3.5 GHz (at 3.0 mm), respectively. Remarkably, the incorporation of Cr element in HE TMC-3 significantly improves the impedance match, thus realizing EAB of 10.5, 9.2, and 13.9 GHz at 2, 3, and 4 mm, respectively. The significance of this study lays on realizing ultra-broadband capacity in HE TMC-3 (Cr, Zr, Hf, Nb, Ta), demonstrating the effectiveness of high-entropy component design in tailoring the impedance match.

• Three dense bulk high-entropy REMgAl 11 O 19 and LaMAl 11 O 19 ceramics are designed and successfully prepared via solid state reaction in a single step. • The factors affecting the phase stability of LaMgAl 11 O 19 are firstly investigated via the electronic structure. • Enhanced thermal stability and infrared emissivity properties are demonstrated for the first time. • Mechanism for excellent thermal stability and infrared emissivity properties is revealed for the first time. LaMgAl 11 O 19 (LMA), characterized by high melting point, low density and thermal conductivity as well as good infrared emissivity, is regarded as a potential candidate for the thermal protection of hypersonic vehicles. Nevertheless, the unsatisfied phase stability at high temperature results in declining of the emissivity below 6 μm, which limits the extensive applications of LaMgAl 11 O 19. In order to overcome this obstacle, three dense bulk high-entropy ceramics, (La 0.2 Nd 0.2 Gd 0.2 Sm 0.2 Pr 0.2)MgAl 11 O 19 (HE LMA-1), (La 0.2 Nd 0.2 Gd 0.2 Sm 0.2 Dy 0.2)Mg Al 11 O 19 (HE LMA-2) and La(Mg 0.2 Fe 0.2 Co 0.2 Ni 0.2 Zn 0.2)Al 11 O 19 (HE LMA-3), were designed and successfully prepared through solid state reaction at 1700 °C for 4 h in one step. XRD analyses show that the phase compositions of HE LMA-1, HE LMA-2 and HE LMA-3 are single-phase solid solutions with the relative density of 95.61%, 95.49% and 94.31%, respectively. Heat treatment experiments demonstrate that the phase composition of HE LMA-1 remains a single phase after high-temperature heating, while second phase appears in other two samples. The stability of HE LMA-1 is attributed to small average size difference δ (∼4%) of constitute elements. Intriguingly, the average emissivity of HE LMA-1 in the range of 3–6 μm reaches 0.9, which is significantly higher than that of LMA and other two HE LMA samples. The emissivity of all samples remains above 0.95 from 6 to 10 μm. In the far infrared region (10–14 μm), although the emissivity of these specimens decreases slightly, it still exceeds 0.85. The UV–Vis absorption spectra indicate that the formation of many discrete impurity energy levels with small intervals in HE LMA-1 promotes the f electrons to transit between adjacent impurity energy levels and conduction band, which enhances the infrared emission of HE LMA-1 below 6 μm. In a word, with improved phase stability and thermal emissivity in infrared range, high-entropy REMgAl 11 O 19 , especially (La 0.2 Nd 0.2 Gd 0.2 Sm 0.2 Pr 0.2)MgAl 11 O 19 (HE LMA-1), is a promising candidate in thermal protection coatings of hypersonic vehicles. [Display omitted] [ABSTRACT FROM AUTHOR]

• Segregation of solid solutes in ZrC grain boundaries are studied by first-principles. • The segregation is dominated by atomic size effect. • Segregation energy can be predicted by machine learning method. • ZrC grain boundaries can be strengthened by segregation of smaller atoms. • Strengthening grain boundaries can improve high temperature strengths of ZrC. ZrC is a promising candidate for the application in ultra-high temperature regime due to its unique combination of excellent properties, such as high melting point, good chemical inertness and high temperature stability. The rapid decrease of strength at high temperatures, however, is one of the obstacles that impedes its practical services. Strengthening of grain boundaries by solute segregation is believed to be an effective way to improve its high temperature performance. Therefore, the segregation tendency of ten solid solute atoms, including Sc, Ti, V, Cr, Y, Nb, Mo, Hf, Ta, W, in ZrC grain boundaries, and the strengthening/weakening effects on grain boundaries due to segregation are investigated by first-principles calculations. The segregation tendency is found dominated by the size effect, which is confirmed by both a qualitative analysis and a quantitative approach based on support vector regression. It means that big atoms tend to segregate to grain boundary sites with local expansions, while small atoms tend to segregate to grain boundary sites with local compressions. Simulations on stress-strain responses indicate that segregation of small atoms (Ti, V, Cr, Nb, Ta, Mo, W) can usually improve grain boundary strengths by inducing compression strains to grain boundaries, even though there is also an exception. In contrast, segregation of Sc and Y will soften grain boundaries. The results reveal that strengthening of grain boundaries by solute segregation is a valuable avenue to enhance high temperature mechanical properties of ZrC, providing guidelines for further design of ZrC based materials. [Display omitted] [ABSTRACT FROM AUTHOR]

Interfacial reaction between Cu and Ti2SnC during the processing of Cu–Ti2SnC composite was investigated by using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. It is shown that the reaction is closely related to the reaction temperature, reaction time, and relative ratio of Cu and Ti2SnC. The reacting products are Cu(Sn) solid solution and TiCx, and the transporting process during interfacial reaction is described. The interfacial reaction products are further confirmed by measuring the hardness across the interface using nanoindentation test. It is shown that the reaction is controlled by the de-intercalation of Sn from Ti2SnC to form Cu(Sn) solid solution and TiCx, and crystallographic relations of [01̅1] TiCx// [010] Ti2SnC and (11̅1̅) TiCx// (001) Ti2SnC were observed at the interface. The effect of interfacial reaction on the mechanical and electrical properties of the composite is also discussed.

Various sample thicknesses and punch diameters were used in punch-shear tests for investigating the size effects in evaluating the shear strength of machinable ceramics. The ratio of the sample thickness to the punch diameter was defined as the relative thickness that strongly affects the measured shear strength in a certain range. It was found that there was a threshold value in the relative thicknesses (~ 0.1 for Ti3AlC2and ~ 0.13 for Ti3SiC2) above which the measured shear strength increased linearly with increasing thickness. The threshold provided a thickness-requirement for valid punch-shear tests, and stable shear strength was obtained under this requirement using 3 mm and 1.8 mm punch diameters, respectively. Experiments using various loading rates suggested that the shear strength measured by the punch-shear test was almost immune from the loading rates. The mechanism of the size effects was analysed through optical and SEM observations.

Ti2SnC dispersion-strengthened (DS) copper matrix composites were prepared by the hot-pressing method. The change of microstructure, mechanical properties, and electrical resistivity of the composites as a function of Ti2SnC volume fraction were studied. The results demonstrated that the grain size of Cu decreased pronouncedly by incorporating Ti2SnC, and the strengthening effect was significant. Improvements in yield strength of up to four times that of pure copper were found in Cu-1 vol.% Ti2SnC, however, the conductivity of the composite was still 85.6% of pure copper. The high strength and low electrical resistivity indicated that Ti2SnC is a promising reinforcement for copper.

Various sample thicknesses and punch diameters were used in punch-shear tests for investigating the size effects in evaluating the shear strength of machinable ceramics. The ratio of the sample thickness to the punch diameter was defined as the relative thickness that strongly affects the measured shear strength in a certain range. It was found that there was a threshold value in the relative thicknesses (~ 0.1 for Ti3AlC2and ~ 0.13 for Ti3SiC2) above which the measured shear strength increased linearly with increasing thickness. The threshold provided a thickness-requirement for valid punch-shear tests, and stable shear strength was obtained under this requirement using 3 mm and 1.8 mm punch diameters, respectively. Experiments using various loading rates suggested that the shear strength measured by the punch-shear test was almost immune from the loading rates. The mechanism of the size effects was analysed through optical and SEM observations.

Single-phase bulk Cr2AlC, a non-Ti-containing carbide with superior expected properties, was successfully fabricated by in-situ hot pressing/solid-liquid reaction process. The reaction process was investigated using differential scanning calorimetry. The products were characterized using X-ray diffraction and scanning electron microscopy. Four distinguished stages of the reactions from Cr, Al, and C elemental powders were discussed. The obtained X-ray diffraction data are in good agreement with calculated ones and those from JCPDS card No. 29-0017. A new set of X-ray diffraction data comprising reflections, 2θ, and intensities of Cr2AlC is presented. Lattice parameters obtained by Rietveld XRD refinement of Cr2AlC are a= 2.858 Å and c= 12.818 Å, respectively. The measured Vickers hardness of the as-synthesized Cr2AlC is 5.5 ± 0.4 GPa, which is twice that of Ti2AlC. Cr2AlC displays excellent oxidation resistance. The parabolic rate constant of Cr2AlC was decreased by 3 –4 orders of magnitude compared with those of Ti3SiC2at 1200 °C.

The high-temperature mechanical properties of both Ti3AlC2and Ti3AlC2/10 vol.%Al2O3composite, including compressive strength, bending strength, and fracture behavior, were investigated. The strengthening effect caused by Al2O3strongly depended on the mode of failure: significant enhancement in strength was only observed under the brittle mode of failure; whereas under the ductile mode of failure the strengthening effect was impaired due to Ti3AlC2matrix softening and stress relaxation.

The high-temperature mechanical properties of both Ti3AlC2and Ti3AlC2/10 vol.%Al2O3composite, including compressive strength, bending strength, and fracture behavior, were investigated. The strengthening effect caused by Al2O3strongly depended on the mode of failure: significant enhancement in strength was only observed under the brittle mode of failure; whereas under the ductile mode of failure the strengthening effect was impaired due to Ti3AlC2matrix softening and stress relaxation.

The high-temperature mechanical properties of both Ti3AlC2and Ti3AlC2/10vol.%Al2O3composite, including compressive strength, bending strength, and fracture behavior, were investigated. The strengthening effect caused by Al2O3strongly depended on the mode of failure: significant enhancement in strength was only observed under the brittle mode of failure; whereas under the ductile mode of failure the strengthening effect was impaired due to Ti3AlC2matrix softening and stress relaxation.

MAB phases are layered ternary compounds with alternative stacking of transition metal boride layers and group A element layers. Until now, most of the investigated MAB phases are concentrated on compounds with Al as the A element layers. In this work, the family of M5SiB2 (M = IVB—VIB transition metals) compounds with silicon as interlayers were investigated by density functional theory (DFT) methods as potential MAB phases for high-temperature applications. Starting from the known Mo5SiB2, the electronic structure, bonding characteristics, and mechanical behaviors were systematically investigated and discussed. Although the composition of M5SiB2does not follow the general formula of experimentally reported (MB)2zAx(MB2)y(z= 1, 2; x= 1, 2; y= 0, 1, 2), their layered structure and anisotropic bonding characteristics are similar to other known MAB phases, which justifies their classification as new members of this material class. As a result of the higher bulk modulus and lower shear modulus, Mo5SiB2has a Pugh’s ratio of 0.53, which is much lower than the common MAB phases. It was found that the stability and mechanical properties of M5SiB2compounds depend on their valence electron concentrations (VECs), and an optimum VEC exists as the criteria for stability. The hypothesized Zr and Hf containing compounds, i.e., Zr5SiB2and Hf5SiB2, which are more interesting in terms of high-temperature oxidation/ablation resistance, were found to be unfortunately unstable. To cope with this problem, a new stable solid solution (Zr0.6Mo0.4)5SiB2was designed based on VEC tuning to demonstrate a promising approach for developing new MAB phases with desirable compositions.

A novel solid – liquid reaction synthesis and simultaneous densification process was developed to fabricate Ti2AlC material using the stoichiometric mixture of Ti, Al and graphite powders as starting materials. This method is a one-step process for the preparation of dense poycrystalline Ti2AlC. Moreover, it provides the advantages of low synthesis temperature, short reaction time and simultaneous synthesis and densification. The microstructure evolution during the high temperature processing was described and the properties of polycrystalline Ti2AlC prepared by this method at 1400 °C were investigated. The results showed that Ti2AlC exhibited metallic electrical conductivity and high mechanical properties. The electrical conductivity of Ti2AlC at room temperature was 4.42⨯106Ω– 1⨯ m– 1and increased with decreasing temperature. The Vickers hardness of Ti2AlC was 2.8 GPa. No indentation cracks were observed and the microdamage is confined in the immediate vicinity of the indent indicating that Ti2AlC is damage tolerant. The flexural strength and the fracture toughness were 275 MPa and 6.5 MPa ⨯ m1/2, respectively.

Chemical reactions and stability of Ti3SiC2in Cu during processing of Cu/Ti3SiC2composites in the temperature range of 900 – 1070 °C were investigated using X-ray diffraction and scanning electron microscopy. The results indicated that Cu reacted with Ti3SiC2above 900 °C, and the reaction products were closely related to the reaction temperature and the relative ratio of Cu and Ti3SiC2. At low Ti3SiC2content or temperatures below 1000 °C, Cu(Si) solid solution and TiCxwere formed, whereas at high reaction temperature and high Ti3SiC2content, Cu–Si intermetallic compounds like Cu5Si, Cu15Si4and (Cu, Si) η'as well as TiCxwere observed. The transporting process for the reaction was investigated and described. It was found that de-intercalation of Si from Ti3SiC2dominated the reaction, which dissolved in copper to form Cu(Si) solid solution or Cu – Si intermetallic compounds such as Cu5Si, Cu15Si4and (Cu, Si) η'. TiCxwas always present as the decomposition product of Ti3SiC2.

The advance in communication technology has triggered worldwide concern on electromagnetic wave pollution. To cope with this challenge, exploring high-performance electromagnetic (EM) wave absorbing materials with dielectric and magnetic losses coupling is urgently required. Of the EM wave absorbers, transition metal diborides (TMB2) possess excellent dielectric loss capability. However, akin to other single dielectric materials, poor impedance match leads to inferior performance. High-entropy engineering is expected to be effective in tailoring the balance between dielectric and magnetic losses through compositional design. Herein, three HE TMB2powders with nominal equimolar TM including HE TMB2-1 (TM = Zr, Hf, Nb, Ta), HE TMB2-2 (TM = Ti, Zr, Hf, Nb, Ta), and HE TMB2-3 (TM = Cr, Zr, Hf, Nb, Ta) have been designed and prepared by one-step boro/carbothermal reduction. As a result of synergistic effects of strong attenuation capability and impedance match, HE TMB2-1 shows much improved performance with the optimal minimum reflection loss (RLmin) of −59.6 dB (8.48 GHz, 2.68 mm) and effective absorption bandwidth (EAB) of 7.6 GHz (2.3 mm). Most impressively, incorporating Cr in HE TMB2-3 greatly improves the impedance match over 1–18 GHz, thus achieving the RLminof −56.2 dB (8.48 GHz, 2.63 mm) and the EAB of 11.0 GHz (2.2 mm), which is superior to most other EM wave absorbing materials. This work reveals that constructing high-entropy compounds, especially by incorporating magnetic elements, is effectual in tailoring the impedance match for highly conductive compounds, i.e., tuning electrical conductivity and boosting magnetic loss to realize highly efficient and broadband EM wave absorption with dielectric and magnetic coupling in single-phase materials.

[Display omitted] • The intrinsic room and high temperature reflectance spectra of TMSi 2 (TM = Ta, Mo) have been calculated theoretically. • Two simplified models are proposed to simulate the optical property of two typical application scenarios of TMSi 2. • Non-negligible significance of oxides to the emittance of silicides is clarified. • Useful guidelines for improving the emission performance of silicides and searching for potential high emissivity agents are provided. TM Si 2 (TM = Ta, Mo) are extensively used as thermal emissivity agents in high emission coatings due to their well-known "high" emissivity in infrared range. However, there is a paucity of the high temperature (HT) emissivity property of these two silicides. Moreover, room temperature (RT) spectrometer measurements have demonstrated that the emittance in infrared range of two silicides was considerably low. Therefore, providing critical HT data and satisfactory elucidation on the emission incompatibility of TM Si 2 is eagerly needed. In this contribution, combining first principles calculations and Drude model, the reflectance spectra of TM Si 2 were predicted at both RT and HT. Consistent with spectrometer measurements, the intrinsic emittance of silicides was relatively low in the entire investigated temperatures. To explain the incompatible emission behavior, two simplified models including the majority of high emissivity coating, SiO 2 , were proposed. Intriguingly, with SiO 2 considered in simulations, no matter covered on the surface or blended in the composites, the emittance of the TM Si 2 enhanced significantly. Our theoretical results demonstrate the non-negligible significance of oxides on the high temperature performance of silicides and provide the guidelines for improving the emission performance of silicides and searching for potential high emissivity agents. [ABSTRACT FROM AUTHOR]

[Display omitted] • The strategy on high throughput first principles calculations is introduced. • The application of high throughput first principles calculations on prediction of new ceramics is summarized. • The perspective of high throughput first principles calculations on high performance ceramics is discussed. Recent technical progress in the industry has led to an urgent requirement on new materials with enhanced multi-properties. To meet this multi-property requirement, the materials consisting of three and more elements have attracted increasing attention. However, facing to the nearly unknown huge multi-component materials system, the traditional trial and error method cannot provide sufficient data efficiently. Therefore, an efficient material innovation strategy is significant. The first-principles calculation based on the density functional theory is a powerful tool for both the accurate prediction of material properties and the identification of its underlying thermodynamics and dynamics. At the same time, the advances of computational methods and computer calculation abilities that are orders of magnitude faster than before make the high throughput first-principles calculations popular. At present, the simulation-assisted material design has become a main branch in the material research field and a great many successes have been made. In this article, the advances of the high throughput first-principles calculations are reviewed to show the achievements of the first-principles calculations and guide the future directions of its applications in ceramics. [ABSTRACT FROM AUTHOR]

[Display omitted] • Five high-entropy rare earth hexaborides/tetraboridescomposites were designed and successfully synthesized. • Superior electromagnetic (EM) wave absorbing properties were demonstrated for the first time. • Mechanisms for the excellent electromagnetic wave absorbing properties HE REB 6 /HE REB 4 composite were investigated for the first time. • This work opens a new widow to enhance EM wave absorbing properties of the high-entropy rare-earth hexaborides (HE REB 6) by promoting dielectric loss without sacrificing magnetic loss. The increasing electromagnetic hazards including electromagnetic interference and electromagnetic pollution, which were stemmed from massive usage of electromagnetic technology, have triggered widespread concerns. To cope with this challenge, electromagnetic wave absorbing materials with high performance are greatly needed. Composite construction has been widely applied in electromagnetic (EM) wave absorbing materials to achieve high permittivity, high permeability and impedance matching. However, high-temperature stability, oxidation and corrosion resistance are still unignorable issues. Herein, high entropy hexaborides/tetraborides (HE REB 6 /HE REB 4) composites with synergistic dielectric and magnetic losses were designed and successfully synthesized through a one-step boron carbide reduction method. The five as-prepared (Y 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2)B 6 /(Y 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2)B 4 , (Y 0.2 Nd 0.2 Sm 0.2 Er 0.2 Yb 0.2)B 6 /(Y 0.2 Nd 0.2 Sm 0.2 Er 0.2 Yb 0.2)B 4 , (Y 0.2 Nd 0.2 Eu 0.2 Er 0.2 Yb 0.2)B 6 /(Y 0.2 Nd 0.2 Eu 0.2 Er 0.2 Yb 0.2)B 4 , (Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2 Yb 0.2)B 6 /(Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2 Yb 0.2)B 4 and (Y 0.2 Sm 0.2 Eu 0.2 Er 0.2 Yb 0.2)B 6 /(Y 0.2 Sm 0.2 Eu 0.2 Er 0.2 Yb 0.2)B 4 contain two phases of HE REB 6 and HE REB 4. Among them (Y 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2)B 6 /(Y 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2)B 4 (HE REB 6 /HE REB 4 -1) and (Y 0.2 Nd 0.2 Sm 0.2 Er 0.2 Yb 0.2)B 6 /(Y 0.2 Nd 0.2 Sm 0.2 Er 0.2 Yb 0.2)B 4 (HE REB 6 /HE REB 4 -2) exhibit excellent EM wave absorption properties. The optimal minimum reflection loss (RL min) and effective absorption bandwidth (E AB) of HE REB 6 /HE REB 4 -1 and HE REB 6 /HE REB 4 -2 are –53.3 dB (at 1.7 mm), 4.2 GHz (at 1.5 mm) and –43.5 dB (1.3 mm), 4.2 GHz (1.5 mm), respectively. The combination of conducting HE REB 4 with magnetism into HE REB 6 as a second phase enhances dielectric and magnetic losses, which lead to enhanced EM wave absorption performance. Considering superior high-temperature stability, oxidation and corrosion resistance of HE REB 6 and HE REB 4 , HE REB 6 /HE REB 4 composite ceramics are promising as a new type of high-performance EM wave absorbing materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel high-entropy (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Mo 0.2)B was designed and prepared in both powder and bulk form for the first time. • Although MnB, FeB, CoB, CrB and MoB have different crystal structures, HE (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Mo 0.2)B exhibits MnB type structure. • The lattice parameters, microstructure, electrical conductivity and Vickers hardness of HE (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Mo 0.2)B were determined. • HE (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Mo 0.2)B shows good EMI shielding performance, which is promising as a novel EMI shielding material. A novel equimolar high-entropy (HE) transition metal monoboride, (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Mo 0.2)B, was designed and prepared in powder and bulk form by high temperature elemental reaction method and spark plasma sintering (SPS) method, respectively. XRD analysis shows that HE (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Mo 0.2)B possesses orthorhombic structure with Pnma space group. Through Rietveld refinement, the lattice parameters of HE (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Mo 0.2)B are a = 5.6675, b = 2.9714, c = 4.2209 and the theoretical density is 6.95 g/cm3. The Vickers hardness and electrical conductivity of HE (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Mo 0.2)B bulk with relative density of 90 % is 12.3 ± 0.5 GPa and 0.49 ± 0.04 × 106 S/m, respectively. Due to high electrical conductivity, HE (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Mo 0.2)B bulk with 3.0 mm thickness displays superior EMI shielding performance in 18.0–26.5 GHz (K-band), and the average values of SE T , SE R , and SE A are 23.3 dB, 13.9 dB, and 9.4 dB, respectively. The EMI shielding mechanism of HE (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Mo 0.2)B mainly results from reflection. [ABSTRACT FROM AUTHOR]

High-entropy ceramics (HECs) are solid solutions of inorganic compounds with one or more Wyckoff sites shared by equal or near-equal atomic ratios of multi-principal elements. Although in the infant stage, the emerging of this new family of materials has brought new opportunities for material design and property tailoring. Distinct from metals, the diversity in crystal structure and electronic structure of ceramics provides huge space for properties tuning through band structure engineering and phonon engineering. Aside from strengthening, hardening, and low thermal conductivity that have already been found in high-entropy alloys, new properties like colossal dielectric constant, super ionic conductivity, severe anisotropic thermal expansion coefficient, strong electromagnetic wave absorption, etc., have been discovered in HECs. As a response to the rapid development in this nascent field, this article gives a comprehensive review on the structure features, theoretical methods for stability and property prediction, processing routes, novel properties, and prospective applications of HECs. The challenges on processing, characterization, and property predictions are also emphasized. Finally, future directions for new material exploration, novel processing, fundamental understanding, in-depth characterization, and database assessments are given.

Y2O3is regarded as one of the potential environmental barrier coating (EBC) materials for Al2O3f/Al2O3ceramic matrix composites owing to its high melting point and close thermal expansion coefficient to Al2O3. However, the relatively high thermal conductivity and unsatisfactory calcium-magnesium-aluminosilicate (CMAS) resistance are the main obstacles for the practical application of Y2O3. In order to reduce the thermal conductivity and increase the CMAS resistance, four cubic bixbyite structured high-entropy oxides RE2O3, including (Eu0.2Er0.2Lu0.2Y0.2Yb0.2)2O3, (Sm0.2Er0.2Lu0.2Y0.2Yb0.2)2O3, (Sm0.2Eu0.2Er0.2Y0.2Yb0.2)2O3, and (Sm0.2Eu0.2Lu0.2Y0.2Yb0.2)2O3were designed and synthesized, among which (Eu0.2Er0.2Lu0.2Y0.2Yb0.2)2O3and (Sm0.2Er0.2Lu0.2Y0.2Yb0.2)2O3bulks were prepared by spark plasma sintering (SPS) to investigate their mechanical and thermal properties as well as CMAS resistance. The mechanical properties of (Eu0.2Er0.2Lu0.2Y0.2Yb0.2)2O3and (Sm0.2Er0.2Lu0.2Y0.2Yb0.2)2O3are close to those of Y2O3but become more brittle than Y2O3. The thermal conductivities of (Eu0.2Er0.2Lu0.2Y0.2Yb0.2)2O3and (Sm0.2Er0.2Lu0.2Y0.2Yb0.2)2O3(5.1 and 4.6 W·m−1·K−1) are only 23.8% and 21.5% respectively of that of Y2O3(21.4 W·m−1·K−1), while their thermal expansion coefficients are close to those of Y2O3and Al2O3. Most importantly, HE RE2O3ceramics exhibit good CMAS resistance. After being attacked by CMAS at 1350 °C for 4 h, the HE RE2O3ceramics maintain their original morphologies without forming pores or cracks, making them promising as EBC materials for Al2O3f/Al2O3composites.

[Display omitted] • The electromagnetic wave absorption properties of TM Cs (TM =Ti, Zr, Hf, Nb and Ta) and high entropy (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C were investigated for the first time. • Superior EMW absorbing properties were reported for the first time. • Mechanisms for the superior EMW absorbing properties, were reported for the first time. • Most importantly, this work opens a new window to design single phase high performance EMW absorbing materials by dielectric/magnetic loss coupling in carbides, nitrides and possibly borides. Electromagnetic wave (EMW) absorbing materials play a vital role in modern communication and information processing technologies to inhibit information leakage and prevent possible damages to environment and human bodies. Currently, most of EMW absorbing materials are either composites of two or more phases or in the form of nanosheets, nanowires or nanofibers in order to enhance the EMW absorption performance through dielectric loss, magnetic loss and dielectric/magnetic loss coupling. However, the combination of complex shapes/multi phases and nanosizes may compound the difficulties of materials processing, composition and interfaces control as well as performance maintenance during service. Thus, searching for single phase materials with good stability and superior EMW absorbing properties is appealing. To achieve this goal, the EMW absorbing properties of transition metal carbides TM Cs (TM =Ti, Zr, Hf, Nb and Ta) and high entropy (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C which belong to ultrahigh temperature ceramics, were investigated in this work. Due to the good electrical conductivity and splitting of d orbitals into lower energy t 2g level and higher energy e g level in TM C 6 octahedral arrangement, TM Cs (TM =Ti, Zr, Hf, Nb and Ta) exhibit good EMW absorbing properties. Especially, HfC and TaC exhibit superior EMW absorbing properties. The minimum reflection loss (RL min) value of HfC is −55.8 dB at 6.0 GHz with the thickness of 3.8 mm and the effective absorption bandwidth (E AB) is 6.0 GHz from 12.0 to 18.0 GHz at thickness of 1.9 mm; the RL min value of TaC reaches −41.1 dB at 16.2 GHz with a thickness of 2.0 mm and the E AB is 6.1 GHz with a thickness of 2.2 mm. Intriguingly, the electromagnetic parameters, i.e., complex permittivity and permeability are tunable by forming single phase solid solution or high entropy (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C. The RL min value of high entropy (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C is −38.5 dB at 9.5 GHz with the thickness of 1.9 mm, and the E AB is 2.3 GHz (from 11.3 to 13.6 GHz) at thickness of 1.5 mm. The significance of this work is that it opens a new window to design single phase high performance EMW absorbing materials by dielectric/magnetic loss coupling through tuning the conductivity and crystal field splitting energy of d orbitals of transition metals in carbides, nitrides and possibly borides. [ABSTRACT FROM AUTHOR]

High entropy diborides are new categories of ultra-high temperature ceramics, which are believed promising candidates for applications in hypersonic vehicles. However, knowledge on high temperature thermal and mechanical properties of high entropy diborides is still lacking unit now. In this work, variations of thermal and elastic properties of high entropy (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)B 2 with respect to temperature were predicted by molecular dynamics simulations. Firstly, a deep learning potential for Ti-Zr-Hf-Nb-Ta-B diboride system was fitted with its prediction error in energy and force respectively being 9.2 meV/atom and 208 meV/Å, in comparison with first-principles calculations. Then, temperature dependent lattice constants, anisotropic thermal expansions, anisotropic phonon thermal conductivities, and elastic properties of high entropy (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)B 2 from 0 °C to 2400 °C were evaluated, where the predicted room temperature values agree well with experimental measurements. In addition, intrinsic lattice distortions of (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)B 2 were analyzed by displacements of atoms from their ideal positions, which are in an order of 10-3 Å and one order of magnitude smaller than those in (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C. It indicates that lattice distortions in (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)B 2 is not so severe as expected. With the new paradigm of machine learning potential, deep insight into high entropy materials can be achieved in the future, since the chemical and structural complexly in high entropy materials can be well handled by machine learning potential. [ABSTRACT FROM AUTHOR]

A novel entropy-stabilized (ES) (Ca,Sr,Ba)ZrO 3 ceramic has been designed and synthesized by pressureless sintering of CaZrO 3 , SrZrO 3 and BaZrO 3 powders mixtures at 1450 °C, 1500 °C and 1550 °C for 3 h. X-ray diffraction, scanning electron microscopy and transmission electron microscopy analyses collectively indicate that a single solid solution is formed with a homogeneous distribution of metal elements after sintering at 1550 °C. The relative density and hardness of the ES (Ca,Sr,Ba)ZrO 3 ceramic sintered at 1550 °C are 97.79 % and 10.84 ± 0.33 GPa, respectively. This ES (Ca,Sr,Ba)ZrO 3 exhibits lower thermal conductivity from 373 K to 1073 K than their constituting zirconates, CaZrO 3 , SrZrO 3 and BaZrO 3. Most importantly, the ES (Ca,Sr,Ba)ZrO 3 ceramic possesses good corrosion resistance to TiNi alloy melt and no distinct reaction layer exists between TiNi alloy and ES (Ca,Sr,Ba)ZrO 3 ceramic in the contact region. The combination of these properties indicates that ES (Ca,Sr,Ba)ZrO 3 ceramic is promising for use as a novel crucible material for the melting of titanium alloys. [ABSTRACT FROM AUTHOR]

Considering the emergence of severe electromagnetic interference problems, it is vital to develop electromagnetic (EM) wave absorbing materials with high dielectric, magnetic loss and optimized impedance matching. However, realizing the synergistic dielectric and magnetic losses in a single phase material is still a challenge. Herein, high entropy (HE) rare earth hexaborides (REB6) powders with coupling of dielectric and magnetic losses were designed and successfully synthesized through a facial one-step boron carbide reduction method, and the effects of high entropy borates intermedia phases on the EM wave absorption properties were investigated. Five HE REB6ceramics including (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6, (Ce0.2Eu0.2Sm0.2Er0.2Yb0.2)B6, (Ce0.2Y0.2Eu0.2Er0.2Yb0.2)B6, (Ce0.2Y0.2Sm0.2Eu0.2Yb0.2)B6, and (Nd0.2Y0.2Sm0.2Eu0.2Yb0.2)B6possess CsCl-type cubic crystal structure, and their theoretical densities range from 4.84 to 5.25 g/cm3. (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6powders with the average particle size of 1.86 µm were found to possess the best EM wave absorption properties among these hexaborides. The RLminvalue of (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6reaches -33.4 dB at 11.5 GHz at thickness of 2 mm; meanwhile, the optimized effective absorption bandwidth (EAB) is 3.9 GHz from 13.6 to 17.5 GHz with a thickness of 1.5 mm. The introduction of HE REBO3(RE = Ce, Y, Sm, Eu, Er, Yb) as intermediate phase will give rise to the mismatching impedance, which will further lead to the reduction of reflection loss. Intriguingly, the HEREB6/HEREBO3still possess wide effective absorption bandwidth of 4.1 GHz with the relative low thickness of 1.7 mm. Considering the better stability, low density, and good EM wave absorption properties, HE REB6ceramics are promising as a new type of EM wave absorbing materials.

Searching for new materials with extra low thermal conductivities is significant in numerous fields like thermal barrier coatings and thermoelectric devices. Traditional multiple-component design has successfully reduced the thermal conductivity, but it also dramatically increases the complexity of manufactural technologies and the risk of material failures. In this work, a specific category known as ABO4scheelites that with both simple crystal structure and the structural signature of the low lattice thermal conductivity is explored. High-throughput calculations are employed to screen for the materials with the targeted performance by multi-dimensional mechanical/thermal property criteria and a database of 46 stable scheelites is constructed. Seven scheelites with both ultra-low thermal conductivities (<1.2 W/(m∙K)) and quasi-ductility are predicted to be novel thermal insulation materials. Low thermal conductivities prefer the scheelites with large valence disparity between “A” and “B” cations and/or small ionic radius ratio. The adopted strategy starting from the structural fingerprint and the data-driven material selection is expected to be a reference of future structural and functional ceramics design.

The critical requirements for the environmental barrier coating (EBC) materials of silicon-based ceramic matrix composites (CMCs) include good tolerance to harsh environments, thermal expansion matches with the interlayer mullite, good high-temperature phase stability, and low thermal conductivity. Cuspidine-structured rare-earth aluminates RE4Al2O9have been considered as candidates of EBCs for their superior mechanical and thermal properties, but the phase transition at high temperatures is a notable drawback of these materials. To suppress the phase transition and improve the phase stability, a novel cuspidine-structured rare-earth aluminate solid solution (Nd0.2Sm0.2Eu0.2Y0.2Yb0.2)4Al2O9was designed and successfully synthesized inspired by entropy stabilization effect of high-entropy ceramics (HECs). The as-synthesized HE (Nd0.2Sm0.2Eu0.2Y0.2Yb0.2)4Al2O9exhibits a close thermal expansion coefficient (6.96×10-6K-1at 300–1473 K) to that of mullite, good phase stability from 300 to 1473 K, and low thermal conductivity (1.50 W·m–1·K–1at room temperature). In addition, strong anisotropic thermal expansion has been observed compared to Y4Al2O9and Yb4Al2O9. The mechanism for low thermal conductivity is attributed to the lattice distortion and mass difference of the constituent atoms, and the anisotropic thermal expansion is due to the anisotropic chemical bonding enhanced by the large size rare-earth cations.

Guided by the theoretical prediction, a new MAX phase V2SnC was synthesized experimentally for the first time by reaction of V, Sn, and C mixtures at 1000 °C. The chemical composition and crystal structure of this new compound were identified by the cross-check combination of first-principles calculations, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and high resolution scanning transmission electron microscopy (HR-STEM). The stacking sequence of V2C and Sn layers results in a crystal structure of space group P63/mmc. The a- and c-lattice parameters, which were determined by the Rietveld analysis of powder XRD pattern, are 0.2981(0) nm and 1.3470(6) nm, respectively. The atomic positions are V at 4f (1/3, 2/3, 0.0776(5)), Sn at 2d (2/3, 1/3, 1/4), and C at 2a (0, 0, 0). A new set of XRD data of V2SnC was also obtained. Theoretical calculations suggest that this new compound is stable with negative formation energy and formation enthalpy, satisfied Born-Huang criteria of mechanical stability, and positive phonon branches over the Brillouin zone. It also has low shear deformation resistance c44(second-order elastic constant, cij) and shear modulus (G), positive Cauchy pressure, and low Pugh’s ratio (G/B= 0.500 < 0.571), which is regarded as a quasi-ductile MAX phase. The mechanism underpinning the quasi-ductility is associated with the presence of a metallic bond.

Ytterbium aluminum garnet (Yb 3 Al 5 O 12) is considered as a promising thermal barrier material. However, the main limitations of Yb 3 Al 5 O 12 for thermal barrier applications are relative low thermal expansion coefficient and high thermal conductivity. In order to overcome these obstacles, herein, a new high entropy (Y 0.2 Yb 0.2 Lu 0.2 Eu 0.2 Er 0.2) 3 Al 5 O 12 ceramic was designed, and then powders and bulk were prepared through solid-state reaction method and spark plasma sintering (SPS), respectively. The thermal expansion coefficient of HE (Y 0.2 Yb 0.2 Lu 0.2 Eu 0.2 Er 0.2) 3 Al 5 O 12 is (8.54 ± 0.29) × 10−6 K-1 at 673 K–1273 K, which is about 9% higher than that of Yb 3 Al 5 O 12. The thermal conductivity of HE (Y 0.2 Yb 0.2 Lu 0.2 Eu 0.2 Er 0.2) 3 Al 5 O 12 ceramic is 3.81 W·m-1 K-1 at 300 K, which is about 18 % lower than that of Yb 3 Al 5 O 12. Moreover, there is no reaction between HE (Y 0.2 Yb 0.2 Lu 0.2 Eu 0.2 Er 0.2) 3 Al 5 O 12 and thermally grown (TG) Al 2 O 3 even at 1600 °C. After annealing at 1590 °C for 18 h, the average grain size of HE (Y 0.2 Yb 0.2 Lu 0.2 Eu 0.2 Er 0.2) 3 Al 5 O 12 increases only from 1.56 μm to 2.27 μm. Close thermal expansion coefficient to TG Al 2 O 3 , low thermal conductivity, good phase stability, excellent chemical compatibility with TG Al 2 O 3 and slow grain growth rate make HE (Y 0.2 Yb 0.2 Lu 0.2 Eu 0.2 Er 0.2) 3 Al 5 O 12 promising for thermal barrier applications. [ABSTRACT FROM AUTHOR]

Developing electromagnetic (EM) wave absorbing materials with low reflection coefficient and optimal operating frequency band is urgently needed on account of the increasingly serious EM pollution. However, the applications of common EM absorbing materials are encumbered by poor high-temperature stability, poor oxidation resistance, narrow absorption bandwidth or high density. Herein, the strong EM absorption capability and wide efficient absorption bandwidth of high entropy ceramics are reported for the first time, which are designed by a combination of the novel high entropy (HE) rare earth silicide carbides/rare earth oxides (RE 3 Si 2 C 2 /RE 2 O 3). Three HE powders, i.e., HERSC-1 (HE (Tm 0.2 Y 0.2 Dy 0.2 Gd 0.2 Tb 0.2) 3 Si 2 C 2), HERSC-2 HE (Tm 0.2 Y 0.2 Pr 0.2 Gd 0.2 Dy 0.2) 3 Si 2 C 2 /HE (Tm 0.2 Y 0.2 Pr 0.2 Gd 0.2 Dy 0.2) 2 O 3) and HERSC-3 (HE (Tm 0.2 Y 0.2 Pr 0.2 Gd 0.2 Tb 0.2) 3 Si 2 C 2 /HE (Tm 0.2 Y 0.2 Pr 0.2 Gd 0.2 Tb 0.2) 2 O 3), are synthesized. Although HERSC-1 exhibits a limited absorption effect (the minimum reflection loss (RL min) is -11.6 dB at 3.4 mm) and a relatively narrow effective absorption bandwidth (EAB) of 1.7 GHz, the optimal absorption RL min value and EAB of HERSC-2 and HERSC-3 are -40.7 dB (at 2.9 mm), 3.4 GHz and-50.9 dB (at 2.0 mm), 4.5 GHz, respectively, demonstrating strong microwave absorption capability and wide absorption bandwidth. Considering the better stability, low density and strong EM absorption effect, HE ceramics are promising as a new type of EM absorbing materials. [ABSTRACT FROM AUTHOR]

Yttrium aluminum perovskite (YAlO 3) is a promising candidate material for environmental barrier coatings (EBCs) to protect Al 2 O 3f /Al 2 O 3 ceramic matrix composites (CMCs) from the corrosion of high-temperature water vapor in combustion environments. Nevertheless, the relatively high thermal conductivity is a notable drawback of YAlO 3 for environmental barrier coating application. Herein, in order to make RE AlO 3 more thermal insulating, a novel high-entropy rare-earth aluminate ceramic (Y 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2)AlO 3 was designed and synthesized. The as-prepared (Y 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2)AlO 3 ceramic possesses close thermal expansion coefficient (9.02 × 10―6 /oC measured from room temperature to 1200 °C) to that of Al 2 O 3. The thermal conductivity of (Y 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2) AlO 3 at room temperature is 4.1 W·m−1 K−1, which is almost one third of the value of YAlO 3. Furthermore, to effectively prevent the penetration of water vapor from possible pores/cracks of coating layer, which are often observed in T/EBCs, a tri-layer EBC system RE AlO 3 / RE 3 Al 5 O 12 /(Al 2 O 3f /Al 2 O 3 CMCs) is designed. Close thermal expansion coefficient to Al 2 O 3 and low thermal conductivity of (Y 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2)AlO 3 , as well as the formation of dense garnet layer at (Y 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2)AlO 3 /Al 2 O 3 interface, indicate that this new type of high-entropy ceramic is suitable as a candidate environmental barrier coating material for Al 2 O 3f /Al 2 O 3 CMCs. [ABSTRACT FROM AUTHOR]

Rare-earth tantalates and niobates (RE3TaO7and RE3NbO7) have been considered as promising candidate thermal barrier coating (TBC) materials in next generation gas-turbine engines due to their ultra-low thermal conductivity and better thermal stability than yttria-stabilized zirconia (YSZ). However, the low Vickers hardness and toughness are the main shortcomings of RE3TaO7and RE3NbO7that limit their applications as TBC materials. To increase the hardness, high entropy (Y1/3Yb1/3Er1/3)3TaO7, (Y1/3Yb1/3Er1/3)3NbO7, and (Sm1/6Eu1/6Y1/6Yb1/6Lu1/6Er1/6)3(Nb1/2Ta1/2)O7are designed and synthesized in this study. These high entropy ceramics exhibit high Vickers hardness (10.9–12.0 GPa), close thermal expansion coefficients to that of single-principal-component RE3TaO7and RE3NbO7(7.9×10−6-10.8×10−6C−1at room temperature), good phase stability, and good chemical compatibility with thermally grown Al2O3, which make them promising for applications as candidate TBC materials.

High entropy materials (HEMs, e.g. high entropy alloys, high entropy ceramics) have gained increasing interests due to the possibility that they can provide challenge properties unattainable by traditional materials. Though a large number of HEMs have emerged, there is still in lack of theoretical predictions and simulations on HEMs, which is probably caused by the chemical complexity of HEMs. In this work, we demonstrate that the machine learning potentials developed in recent years can overcome the complexity of HEMs, and serve as powerful theoretical tools to simulate HEMs. A deep learning potential (DLP) for high entropy (Zr 0.2 Hf 0.2 Ti 0.2 Nb 0.2 Ta 0.2)C is fitted with the prediction error in energy and force being 9.4 meV/atom and 217 meV/Å, respectively. The reliability and generality of the DLP are affirmed, since it can accurately predict lattice parameters and elastic constants of mono-phase carbides TMC (TM = Ti, Zr, Hf, Nb and Ta). Lattice constants (increase from 4.5707 Å to 4.6727 Å), thermal expansion coefficients (increase from 7.85×10-6 K-1 to 10.58×10-6 K-1), phonon thermal conductivities (decrease from 2.02 W·m-1·K-1 to 0.95 W·m-1·K-1), and elastic properties of high entropy (Zr 0.2 Hf 0.2 Ti 0.2 Nb 0.2 Ta 0.2)C in temperature ranging from 0 °C to 2400 °C are predicted by molecular dynamics simulations. The predicted room temperature properties agree well with experimental measurements, indicating the high accuracy of the DLP. With introducing of machine learning potentials, many problems that are intractable by traditional methods can be handled now. It is hopeful that deep insight into HEMs can be obtained in the future by such powerful methods. [ABSTRACT FROM AUTHOR]

Synergy effect of high stiffness and good damage tolerance is always the focus of the development of novel structural materials. Herein, a new strategy on the future damage tolerant material design is proposed to merge the strong covalent bonds into the easy shear deformed A 3 B metallic box. This goal is realized by studying 126 A 3 BX phases and establishing a database on their mechanical properties through high-throughput first principles calculations. The combination strategies of A 3 B metallic box and XA 3 octahedra show intensive influences on the expected mechanical properties. The family includes 49 quasi-ductile compounds. Among them, four compounds (Ti 3 AlN, Mn 3 CuN, Ti 3 TlN and Ni 3 SnN) exhibit excellent damage tolerance and the other six compounds (Mn 3 NiN, Mn 3 GaC, Mn 3 GaN, Mn 3 SnC, Cr 3 SnN, Co 3 AlC) show both damage tolerance and high stiffness. Their competitive high temperature properties are demonstrated through the detailed investigation on the typical cases of Co 3 AlC and Ti 3 TlN. This study leads a novel direction for the design of the future quasi-ductile and high stiffness ceramics. [ABSTRACT FROM AUTHOR]