New optical ceramics: High-entropy sesquioxide X2O3 multi-wavelength emission phosphor transparent ceramics

Highly transparent X₂O₃ sesquioxide ceramics were obtained from a solid solution of five different oxides (Lu₂O₃, Y₂O₃, Yb₂O₃, Gd₂O₃, and Dy₂O₃), mixed in an equal molar ratio according to the principle of high-entropy. The fabricated (Lu, Y, Yb, Gd, Dy)₂O₃ ceramics achieved 99.97 % of the relative density and exhibited a high degree of optical transparency with the in-line transmittance of almost 80 % in the visible wavelength range. Emissions of Gd³⁺ (⁶P_J → ⁸S_7/2 at 312 nm), Dy³⁺ (⁴F_9/2 → ⁶H_15/2 at 492 nm and ⁴F_9/2 → ⁶H_13/2 at 572 nm), and Yb³⁺ (²F_5/2 → ²F_7/2 at 1031 nm) suggested a potential application of the high-entropy ceramics as multi-wavelength emission phosphor transparent ceramics. High-entropy ceramics also exhibited lower specific heat and thermal conductivity compared to single-element sesquioxide ceramics. This work demonstrated that highly transparent oxide ceramics, with complex chemical compositions and good optical properties, could be obtained using the high-entropy principle.     

High-entropy transparent (Y0.2La0.2Gd0.2Yb0.2Dy0.2)2Zr2O7 ceramics as novel phosphor materials with multi-wavelength excitation and emission properties

High-entropy ceramics have been extensively studied because of their novel intrinsic properties and have significant potential for application in various fields. In this study, a novel high-entropy transparent ceramic phosphor (Y_0.2La_0.2Gd_0.2Yb_0.2Dy_0.2)₂Zr₂O₇ was successfully prepared via a solid-state reaction and vacuum sintering. X-ray diffraction and scanning electron microscopy analyses were performed to analyze the phases and microstructures of the as-prepared powders and sintered ceramics. The highest in-line transmittance of the developed ceramic was 74 % in both visible and infrared regions. To reveal its luminescent properties as a potential WLED material, the photoluminescence of ceramic samples was analyzed using multi-excitation and emission spectra. Strong emissions originating from Dy³⁺ and Gd³⁺ were observed, and the emission color was effectively regulated under multi-wavelength excitation. Combining excellent optical transmittance with unique photoluminescence performance, the (Y_0.2La_0.2Gd_0.2Yb_0.2Dy_0.2)₂Zr₂O₇ high-entropy transparent ceramics can find potential applications as a novel WLED material with multi-wavelength excitation and tunable emission.     

Synthesis and thermal stability of novel high-entropy metal boron carbonitride ceramic powders

High-entropy metal boron carbonitride ceramic powders including (Ta_0.2Nb_0.2Zr_0.2Hf_0.2W_0.2)BCN, (Ta_0.2Nb_0.2Zr_0.2Hf_0.2Ti_0.2)BCN, and (Ta_0.2Nb_0.2Zr_0.2Ti_0.2W_0.2)BCN, were successfully synthesized via mechanical alloying at room temperature. Results show that for the first step of 10 h milling, the amorphous BCN phases are observed. After 24 h of second step milling, the as-synthesized high-entropy ceramics exhibit a single face-centered cubic solid solution structure with high compositional uniformity from nano-scale to micron-scale. When heated to 1500 °C for 30min in flowing Ar, the as-prepared high-entropy ceramic powders still show relatively high thermal stability; however, some metals oxides like HfO₂ and ZrO₂ are detected due to the pre-existing oxides on sample surfaces. After heat treatment, some amorphous phases are still retained. This work suggests a new processing route on the synthesis of high-entropy metal boron carbonitride ceramics.     

Effect of carbon content on the microstructure and mechanical properties of high-entropy (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)Cx ceramics

High-entropy (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x ceramics, with different carbon contents (x=0.55?1), were prepared by spark plasma sintering using powders synthesized via a carbothermal reduction approach. Single-phase, high-entropy (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x ceramics could be obtained when using a carbon content of x=0.70?0.85. Combined ZrO₂ and Mo-rich carbide phases, or residual graphite, existed in the ceramics due to either a carbon deficiency or excess at x=0.55 and 1, respectively. With the carbon content increased from x=0.70 to x=0.85, the grain size decreased from 4.36 ± 1.55 μm to 2.00 ± 0.91 μm, while the hardness and toughness increased from 23.72 ± 0.26 GPa and 1.69 ± 0.21 MPa·m^1/2 to 25.45 ± 0.59 GPa and 2.37 ± 0.17 MPa·m^1/2, respectively. This study showed that the microstructure and mechanical properties of high-entropy carbide ceramics could be adjusted by the carbon content. High carbon content is conducive to improving hardness and toughness, as well as reducing grain size.     

Combustion synthesis of high-performance high-entropy dielectric ceramics for energy storage applications

High-entropy dielectric ceramics have demonstrated a promising prospect for applications in energy storage recently. However, most high-entropy dielectrics synthesized by conventional solid-state reaction (SSR) method demonstrated unsatisfactory performance for energy storage. Therefore, it is meaningful to develop a feasible way to fabricate high-performance high-entropy dielectric ceramics. Herein, high-entropy (Sr_0.6Bi_0.2Na_0.2)(Ti_1-xZr_x/2Al_x/4Nb_x/4)O₃ ceramics are prepared by a solution combustion synthesis (SCS) method. The SCS fabricated ceramics (x = 0.25) demonstrate a high recoverable energy density of ∼4.46 J/cm³ at a high critical electric field of 520 kV/cm, a high energy efficiency ∼88.52%, a large power density of ∼176.65 MW/cm³ (at 400 kV/cm), an ultrafast discharge time of ∼48 ns, and a high Vikers hardness of ∼7.09 GPa. The key energy storage parameters are much better than those of the samples prepared by the SSR method owing to the absence of unexpected impurity phases, and the refined grain size at the submicrometer scale in our SCS fabricated high-entropy ceramics. The study provides a facile way to fabricate high-performance high-entropy dielectric ceramics for energy storage, indicating that the SCS routine is notably advantageous for preparing high-entropy dielectric energy ceramics.     

Preparation of multicomponent A2Zr2O7 transparent ceramics by vacuum sintering

Recently, high-entropy oxide ceramics have become a hot topic in the field of high entropy materials. In this paper, multicomponent pyrochlore A₂Zr₂O₇ transparent ceramics were prepared via vacuum sintering using combustion synthesized nanopowders. The phase analysis results indicate that the powders exhibit defective fluorite structure and the ceramics are in pyrochlore structure. The structural order degree of ceramics varies with the increase of incorporated components. It is found that the grain size of A₂Zr₂O₇ ceramics is related with the component of A-site. The main fracture mode of final ceramics exhibit typical transgranular fracture. The multicomponent A₂Zr₂O₇ ceramics exhibit excellent optical transmittance, and the highest in-line transmittance reaches to 80% for #A2ZO ceramic at 1880 nm.     

High-entropy yttrium pyrochlore ceramics with glass-like thermal conductivity for thermal barrier coating application

A₂B₂O₇-type oxides with low thermal conductivities are potential candidates for next-generation thermal barrier coatings. The formation of high-entropy ceramics is considered as a newly effective way to further lower their thermal conductivities. High-entropy Y₂(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)₂O₇ (5HEO) and Y₂(Ti_0.25Zr _0.25Hf_0.25Ta_0.25)₂O₇ (4HEO) ceramics were prepared by in situ solid reaction sintering, considering the important roles of B-site cations on thermal conductivities of the A₂B₂O₇-type oxides. Reaction process, phase structures, microstructures, and thermal conductivities of the as-sintered ceramics were investigated. Lattice distortion effects on their thermal conductivities were also discussed by using the proposed criterion based on the supercell volume difference of the individual compounds. Near fully-dense 5HEO and 4HEO ceramics were obtained after being sintered at 1600°C. The former one had a dual-phase structure containing high-entropy Y₂(Ti_0.227Zr_0.227Hf_0.227Nb_0.136Ta_0.182)₂O_7.318 pyrochlore oxide (5HEO-P) and Y(Nb, Ta)O₄ solid solution, while the latter one was a single-phase pyrochlore oxide (4HEO-P) with homogeneous element distribution. The formed 5HEO-P oxide has larger lattice distortion than 4HEO-P oxide due to the larger total amounts of Nb and Ta cations at B sites in the 5HEO-P oxide. It results in lower thermal conductivity of 5HEO ceramics (keeping at 1.8 W·m^–1·K^–1) than those of 4HEO ceramics (ranging from 1.8 to 2.5 W·m^–1·K^–1) at temperatures from 25°C to 1400°C. Their glass-like thermal conductivities were determined by the selection of B site cations and high-entropy effects. These results provide some useful information for the material design of novel thermal barrier coating materials.     

Graceful behavior during CMAS corrosion of a high-entropy rare-earth zirconate for thermal barrier coating material

The corrosion resistance to calcium-magnesium-alumino-silicates (CMAS) is critically important for the thermal barrier coatings (TBCs). High-entropy zirconate (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ (HEZ) ceramics with low thermal conductivity, high coefficient of thermal expansion and good durability to thermal shock is expected to be a good candidate for the next-generation TBCs. In this work, the CMAS corrosion of HEZ at 1300°C was firstly investigated and compared with the well-studied La₂Zr₂O₇ (LZ). It is found that the HEZ ceramics showed a graceful behavior to CMAS corrosion, obviously much better than the LZ ceramics. The HEZ suffered from CMAS corrosion only through dissolution and re-precipitation, while additional grain boundary corrosion existed in the LZ system. The precipitated high-entropy apatite showed fine-grained structure, resulting in a reaction layer without cracks. This study reveals that HEZ is a promising candidate for TBCs with extreme resistance to CMAS corrosion.     

Fabrication and characterization of polymer-derived high-entropy carbide ceramic powders

Polymer-derived ceramic (PDC) route has been widely used to fabricate various ceramics or ceramic-matrix composites in recent years. However, the synthesis of high-entropy ceramics via PDC route has rarely been reported until now. Herein, we successfully synthesized a class of high-entropy carbides, namely (Hf_0.25Nb_0.25Zr_0.25Ti_0.25)C (HEC-1), via PDC route. The polymer-derived HEC-1 ceramics consisted of numerous superfine particles with the average particle size ~800 nm. Meanwhile, they possessed a rock-salt structure of metal carbides and high-compositional uniformity from nanoscale to microscale. In addition, the as-obtained HEC-1 ceramics had a low oxygen impurity content of 0.51% and a low free carbon impurity content of 2.56%. This work will open up a new research field on the fabrication of high-entropy ceramics or high-entropy ceramic-matrix composites via PDC route.     

Achieving high energy storage properties in perovskite oxide via high-entropy design

In recent years, “high-entropy” materials have attracted great attention in various fields due to their unique design concepts and crystal structures. The definition of high-entropy is also more diverse, gradually expanding from a single phase with an equal molar ratio to a multi-phase with a non-equimolar ratio. This study selected (Na_0.2Bi_0.2Ba_0.2Sr_0.2Ca_0.2)TiO₃ high entropy ceramics with excellent relaxation behavior. The A-site elements are divided into (x = Na, Bi, Ba) and ((1-3x)/2 = Sr, Ca) according to their inherent properties. A novel ABO₃ structural energy storage ceramics (NaBaBi)_x(SrCa)_(1-3x)/2TiO₃ (x = 0.19, 0.195, 0.2, 0.205 and 0.21) was successfully fabricated using the high entropy design concept. The ferroelectric and dielectric properties of non-equimolar ratio high-entropy ceramics were studied in detail. It was found that the dielectric constant of ∼4920 and the recoverable energy storage density of 3.86 J/cm³ (at 335 kV/cm) can be achieved simultaneously at x = 0.205. The results indicate that the design concept of high-entropy materials with a non-equal molar ratio is an effective means to achieve distinguished energy storage performance in lead-free ceramics.     

A review on structure–property relationships in dielectric ceramics using high-entropy compositional strategies

High-entropy strategy is a design that uses multiple elements to increase configurational entropy. As an emerging research field, high-entropy was originally applied to metals, but it has shown great potential in inorganic nonmetallic materials. Since the first discovery of colossal dielectric permittivity in high-entropy ceramics, it indicates that the high-entropy strategy is feasible in dielectric ceramics. The multi-ion solid solution will bring a broader space for the material structure and property tailoring. With the research of domestic and foreign scholars in high-entropy dielectrics, we believe that entropic engineering can effectively regulate dielectric properties. The review paper explores the potential of the high-entropy strategy in dielectric ceramics. It provides a detailed overview of the structural properties and the potential benefits of utilizing high-entropy materials in dielectric ceramics. The review also covers the current research advancements in the field and provides insights into future directions for further development of high-performance dielectric ceramics.     

New class of high-entropy defect fluorite oxides RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE = Y,Ho, Er,or Yb) as promising thermal barrier coatings

High-entropy ceramics exhibit great application potential as thermal barrier coating (TBC) materials. Herein, a series of novel high-entropy ceramics with RE₂(Ce_0.2Zr_0.2Hf_0.2Sn_0.2Ti_0.2)₂O₇ (RE₂HE₂O₇, RE = Y, Ho, Er, or Yb) compositions were fabricated via a solid-state reaction. X-ray diffraction (XRD) and energy dispersive spectrometry (EDS) mapping analyses confirmed that RE₂HE₂O₇ formed a single defect fluorite structure with uniform elemental distribution. The thermophysical properties of the RE₂HE₂O₇ ceramics were investigated systematically. The results show that RE₂HE₂O₇ ceramics have excellent high-temperature phase stability, high thermal expansion coefficients (10.3–11.7 × 10^?6 K^-1, 1200 ℃), and low thermal conductivities (1.10-1.37 W m^-1 K^-1, 25 ℃). In addition, RE₂HE₂O₇ ceramics have a high Vickers hardness (13.7–15.0 GPa) and relatively low fracture toughness (1.14-1.27 MPa m^0.5). The outstanding properties of the RE₂HE₂O₇ ceramics indicate that they could be candidates for the next generation of TBC materials.     

Novel non-equimolar SrLa(Al0.25Zn0.125Mg0.125Ga0.25Ti0.25)O4 high-entropy ceramics with excellent mechanical and microwave dielectric properties

Novel non-equimolar high-entropy SrLa(Al_0.25Zn_0.125Mg_0.125Ti_0.25Ga_0.25)O₄ (SLAZMTG) ceramics with a layered perovskite structure have been prepared via the standard solid-state reaction method. The high-entropy composition belongs to the tetrahedral structure with a space group of I4/mmm, which is confirmed by the XRD and TEM analyses. Excellent microwave dielectric properties with a suitable dielectric constant (ε_r = 22.5), high quality factor (Qf = 83,003 GHz), and near-zero τ_f value of −1.7 ppm/°C are obtained in SLAZMTG ceramics sintered at 1400 °C. Meanwhile, a significant enhancement in compressive strength was achieved due to the improvement of configuration entropy, 912 MPa for SLAZMTG compared to 578 MPa in the pure SrLaAlO₄ composition. Additionally, the high-entropy engineering in the present work suggests great potential in achieving low thermal conductivities. SLAZMTG ceramics exhibit low thermal conductivities ranging from 2.86 W/m•K at 323 K to 1.99 W/m•K at 673 K, much lower than those of SrLaAlO₄ and other perovskite ceramics.     

Microstructure and ferroelectric properties of high-entropy perovskite oxides with A-site disorder

High-entropy oxides with complex compositions can be designed as new ferroelectric materials with interesting physical consequences. Here, a series of high-entropy perovskite ceramics (Bi_0.2Na_0.2Ba_0.2Sr_0.2Ca_0.2TiO₃, Bi_0.2Li_0.2Ba_0.2Sr_0.2Pb_0.2TiO₃, Bi_0.2Na_0.2Ba_0.2Sr_0.2Pb_0.2TiO₃, Bi_0.2K_0.2Ba_0.2Sr_0.2Pb_0.2TiO₃, and Bi_0.2Ag_0.2Ba_0.2Sr_0.2Pb_0.2TiO₃) was proposed, which selected various elements to diminish the formational enthalpy and thus to achieve a single-phase structure. Detailed crystal structure and microstructure characterizations indicated that the high-entropy perovskites exhibited a single tetragonal phase with excellent chemical homogeneity. The equiatomic ratios of the A-site cations in perovskites could be used to maximize the entropy stabilization effect and effectively disordered the symmetry of the crystal structure. A robust ferroelectric polarization reaching 20 μC/cm² under 50 kV/cm was achieved in Bi_0.2Na_0.2Ba_0.2Sr_0.2Pb_0.2TiO₃ high-entropy ferroelectrics. This work provides an effortless approach to discover new high-entropy ferroelectrics in materials with unexplored compositional complexity and gives additional opportunities to design and tailor the functional properties in entropy-stabilized ferroelectrics.     

Gradient microstructure development and grain growth inhibition in high-entropy carbide ceramics prepared by reactive spark plasma sintering

High-entropy carbide ceramics (Ti_0.2Hf_0.2Nb_0.2Ta_0.2W_0.2)C is prepared from five transition metal oxides and graphite by reactive spark plasma sintering. X-ray diffraction indicates the synthesized ceramics with the single-phase face-centered cubic structure. The elemental distribution maps by energy dispersive spectroscopy demonstrate homogeneous distribution of the five metal elements in both central and circumferential regions of the sample. SEM and corresponding back scattered electron observations show the residual graphite particles locating at the grain boundaries of high-entropy carbide ceramics. Moreover, the content of the residual graphite decreases and the grain size of the high-entropy carbide phase increases from central to circumferential region of the sample. Thermodynamic calculation results indicate that gradient gas pressure inside the sample affects the carbothermal reduction reactions during sintering and consequently results in the existence of residual graphite with gradient distribution feature. This study points out an effective way to inhibit the grain growth of high-entropy carbide phase during sintering process by the incorporation of graphite as the second phase particles acting as grain growth inhibitor.     

Multicomponent (Hf0.25Zr0.25Ti0.25Cr0.25)B2 ceramic modified SiC–Si composite coatings: In-situ synthesis and high-temperature oxidation behavior

High-entropy ceramics, a novel type of multicomponent materials with broad application prospects, have stirred up world-wide interests for over a decade. In the current work, in-situ high-entropy (Hf_0.25Zr_0.25Ti_0.25Cr_0.25)B₂ ceramic modified SiC–Si (HETMB₂-SiC-Si) coating was deposited on carbon/carbon (C/C) composites via gaseous reactive infiltration of Si assisted slurry painting (GRSI-SP) method, to improve the oxidation protective ability of C/C composites at 1973 K. The formation and oxidation mechanisms of the coating was explored by first-principles simulation, experiment and thermodynamic analyses. The coating prepared at 2373 K shows dense mosaic structure filled with HETMB₂-rich Si-based multiphase. This coating adheres well with the C/C substrate, which is ascribed to the formed zigzagged SiC–Si transition layer. This coating protected C/Cs from oxidation for more than 205 h at 1973 K. The enhanced oxidation protective ability is mostly ascribed to the subsequently generated compact and stable Hf-Zr-Ti-Cr-Si-O composite oxidation scale. This research will start up novel research ares of developing high-entropy materials modified coatings with improved protective ability under extreme environments.     

Porous (Ce0.2Zr0.2Ti0.2Sn0.2Ca0.2)O2-δ high-entropy ceramics with both high strength and low thermal conductivity

Single-phase (Ce_0.2Zr_0.2Ti_0.2Sn_0.2Ca_0.2)O_2-δ porous high-entropy ceramics have been in-situ fabricated by foam-gelcasting-freeze drying method at different temperatures. The microstructure, phase composition, and properties of the obtained ceramics were investigated. The results indicate that compared with other porous ceramics reported in the literatures, this type of ceramics exhibits excellent performance. The sample prepared at 1350 °C shows high porosity (88.6 %), low thermal conductivity (0.023 W m^-1 K^-1), and high compressive strength (1.48 MPa). The current study suggests that porous (Ce_0.2Zr_0.2Ti_0.2Sn_0.2Ca_0.2)O_2-δ high entropy ceramics are promising candidates for thermal insulation applications.     

Grain growth inhibition by sluggish diffusion and Zener pinning in high-entropy diboride ceramics

This work reported the grain growth kinetics of high-entropy diboride (HEB) and HEB-SiC ceramics containing 10, 20, and 30 vol% SiC during heat treatment at 1800°C. The coarsening of HEB phase occurred in the four kinds of ceramics during heat treatment, especially in HEB ceramics. The kinetic analysis showed that the grain growth of HEB phase in HEB and HEB-SiC ceramics is controlled by interface-controlled kinetics and grain-boundary pinning, respectively. The growth rate constant of HEB grains is lower than ZrB₂, which is related to the low grain-boundary energy and the sluggish diffusion effect in dynamics of high-entropy materials. The growth rate of matrix phase in HEB-SiC ceramics is similar to that in ZrB₂–SiC ceramics, indicating that the pinning effect of the SiC second-phase played the dominant role in inhibiting the grain growth of the high-entropy matrix phase and disguised the sluggish diffusion effect. This study reveals that the grain growth inhibition through sluggish diffusion effect in a high-entropy ceramic system may be magnified by the possible existence of segregated second-phase particles located at the grain boundaries.     

Dissolving and stabilizing soft WB2 and MoB2 phases into high-entropy borides via boron-metals reactive sintering to attain higher hardness

Five single-phase WB₂- and MoB₂-containing high-entropy borides (HEBs) have been made via reactive spark plasma sintering of elemental boron and metals. A large reactive driving force enables the full dissolution of 10−20 mol. % WB₂ to form dense, single-phase HEBs, including (Ti_0.2Zr_0.2Hf_0.2Mo_0.2W_0.2)B₂, (Ti_0.2Ta_0.2Cr_0.2Mo_0.2W_0.2)B₂, (Zr_0.2Hf_0.2Nb_0.2Ta_0.2W_0.2)B₂, and (Zr_0.225Hf_0.225Ta_0.225Mo_0.225W_0.1)B₂; the successful fabrication of such single-phase WB₂-containing HEBs has not been reported before. In the processing science, this result serves perhaps the best example demonstrating that the phase formation in high-entropy ceramics can strongly depend on the kinetic route. A scientifically interesting finding is that HEBs containing softer WB₂ and/or MoB₂ components are significantly harder than (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂ (with harder binary boride components). This exemplifies that high-entropy ceramics can achieve unexpected properties.