Ti₃C₂T_x MXene shows great potential in the application as microwave absorbers due to its high attenuation ability. However, excessively high permittivity and self-stacking are the main obstacles that constrain its wide range of applications. To tackle these problems, herein, the microspheres of SiO₂@Ti₃C₂T_x@CoNi with the hydrangea-like core–shell structure were designed and prepared by a combinatorial electrostatic assembly and hydrothermal reaction method. These microspheres are constructed by an outside layer of CoNi nanosheets and intermediate Ti₃C₂T_x MXene nanosheets wrapping on the core of modified SiO₂, engendering both homogenous and heterogeneous interfaces. Such trilayer SiO₂@Ti₃C₂T_x@CoNi microspheres are "magnetic microsize supercapacitors" that can not only induce dielectric loss and magnetic loss but also provide multilayer interfaces to enhance the interfacial polarization. The optimized impedance matching and core–shell structure could boost the reflection loss (RL) by electromagnetic synergy. The synthesized SiO₂@Ti₃C₂T_x@CoNi microspheres demonstrate outstanding microwave absorption (MA) performance benefited from these advantages. The obtained RL value was −63.95 dB at an ultra-thin thickness of 1.2 mm, corresponding to an effective absorption bandwidth (EAB) of 4.56 GHz. This work demonstrates that the trilayer core–shell structure designing strategy is highly efficient for tuning the MA performance of MXene-based microspheres.

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.

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 M₅SiB₂ does not follow the general formula of experimentally reported (MB)_2zA_x(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, Mo₅SiB₂ has 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 M₅SiB₂ compounds 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., Zr₅SiB₂ and Hf₅SiB_2, 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 (Zr_0.6Mo_0.4)₅SiB₂ was designed based on VEC tuning to demonstrate a promising approach for developing new MAB phases with desirable compositions.

Willemite Zn₂SiO₄ crystallizes in such a way that Zn and Si are tetrahedrally coordinated with O in an ionic-covalent manner to form ZnO₄ and SiO₄ tetrahedra as the building units. The tetrahedra are corner-sharing, of which one SiO₄ tetrahedron connects eight ZnO₄ tetrahedra, and one ZnO₄ tetrahedron links four ZnO₄ tetrahedra and four SiO₄ tetrahedra. 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 Zn₂SiO₄ definitely 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. Zn₂SiO₄ has 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 z direction (E_z = 179 GPa) is almost twice those in the x and y directions (E_x = E_y = 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 Zn₂SiO₄ in 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 (Mg_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)Fe₂O₄, (Mg_0.2Fe_0.2Co_0.2Ni_0.2Cu_0.2)Fe₂O₄, and (Mg_0.2Fe_0.2Co_0.2Ni_0.2Zn_0.2)Fe₂O₄ were designed and successfully prepared through solid state synthesis. The results show that all three HE MFe₂O₄ samples 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, (Mg_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)Fe₂O₄ shows the best EM wave absorption performance, e.g., its minimum reflection loss (RL_min) 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 MFe₂O₄ are 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 (RL_min) 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.

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 TMB₂ powders 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 (RL_min) 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 RL_min of -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.

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.

Y₂O₃ is regarded as one of the potential environmental barrier coating (EBC) materials for Al₂O_3f/Al₂O₃ ceramic 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 (Eu_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃, (Sm_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃, (Sm_0.2Eu_0.2Er_0.2Y_0.2Yb_0.2)₂O₃, and (Sm_0.2Eu_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ were designed and synthesized, among which (Eu_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ and (Sm_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ bulks were prepared by spark plasma sintering (SPS) to investigate their mechanical and thermal properties as well as CMAS resistance. The mechanical properties of (Eu_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ and (Sm_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ are close to those of Y₂O₃ but become more brittle than Y₂O₃. The thermal conductivities of (Eu_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ and (Sm_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)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 Y₂O₃ and Al₂O₃. Most importantly, HE RE₂O₃ ceramics exhibit good CMAS resistance. After being attacked by CMAS at 1350 ℃ for 4 h, the HE RE₂O₃ ceramics maintain their original morphologies without forming pores or cracks, making them promising as EBC materials for Al₂O_3f/Al₂O₃ composites.

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 REB₆ ceramics including (Ce_0.2Y_0.2Sm_0.2Er_0.2Yb_0.2)B₆, (Ce_0.2Eu_0.2Sm_0.2Er_0.2Yb_0.2)B₆, (Ce_0.2Y_0.2Eu_0.2Er_0.2Yb_0.2)B₆, (Ce_0.2Y_0.2Sm_0.2 Eu_0.2Yb_0.2)B₆, and (Nd_0.2Y_0.2Sm_0.2Eu_0.2 Yb_0.2)B₆ possess CsCl-type cubic crystal structure, and their theoretical densities range from 4.84 to 5.25 g/cm³. (Ce_0.2Y_0.2Sm_0.2Er_0.2 Yb_0.2)B₆ powders with the average particle size of 1.86 μm were found to possess the best EM wave absorption properties among these hexaborides. The RL_min value of (Ce_0.2Y_0.2Sm_0.2Er_0.2Yb_0.2)B₆ reaches -33.4 dB at 11.5 GHz at thickness of 2 mm; meanwhile, the optimized effective absorption bandwidth (E_AB) 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/HEREBO₃ still 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 REB₆ ceramics are promising as a new type of EM wave absorbing materials.