Densification of the entropy stabilized oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O

The first entropy-stabilized oxide, (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O, was reported in 2015. Initial studies synthesized this material using solid state processing and were limited to densities < 80%. Here, we report a straightforward solid state route to sinter samples to densities up to 98% of the theoretical by identifying the role of oxygen and promoting the resulting mechanisms in densification. Previous works have studied effects of cation stoichiometry on the entropy-driven reaction to form a single phase, but few have explored the associated effects of anion stoichiometry and/or redox chemistry on both phase stability and densification. We demonstrate here that tuning heating rate and pO₂ during heating of initially-homogeneous calcined powders can enhance densifying diffusion processes and enable reliable sintering of dense Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O samples.     

High-entropy ferroelastic rare-earth tantalite ceramic: (Y0.2Ce0.2Sm0.2Gd0.2Dy0.2)TaO4

In this study, high-entropy rare-earth tantalate ceramics (Y_0.2Ce_0.2Sm_0.2Gd_0.2Dy_0.2)TaO₄ ((5RE_0.2)TaO₄) have been successfully fabricated. The possibility of formation of (5RE_0.2)TaO₄ was verified via first-principles calculations. In addition, the phase structure, ferroelastic toughening mechanism, thermophysical, and mechanical properties were systematically investigated. The (5RE_0.2)TaO₄ ceramics have lower phonon thermal conductivity (1.2–2.6 W·m^–1·K^–1) in the entire temperature range than that of RETaO₄ and YSZ. (5RE_0.2)TaO₄ has a higher fracture toughness and lower brittleness index than YSZ. The thermal expansion coefficients of (5RE_0.2)TaO₄ are as high as 10.3 × 10^-6 K^–1 at 1200°C and Young's modulus is 66–189 GPa, and thus, (5RE_0.2)TaO₄ possesses great potential for application in thermal barrier coatings (TBCs).     

Grain coalescence in (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C during spark plasma sintering

High-entropy (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C ceramics (HEC) are fabricated via spark plasma sintering using different die configurations, including the conductive and insulating dies. Compared to the conductive die, the grain sizes of samples sintered in the insulating die are significantly larger, which is attributed to the higher local temperature as a result of the higher current density in the sample. Furthermore, the microstructure evolution and grain growth mechanism of HEC are investigated for the first time. We find that at moderate temperatures (∼1600°C), the grain growth of HEC can occur by a grain coalescence mechanism, forming numerous irregular grains in the porous sample. Three factors are crucial to induce grain coalescence, including the formation of partial melting layers on particle surfaces, nanograin rearrangement via rotation and sliding, and the formation of low-angle grain boundaries. During the final sintering stage, the irregular grains will change into polyhedral shapes by grain boundary migration. These findings are of assistance to better understand and control the microstructure evolution of HEC and other ultrahigh-temperature carbide ceramics.     

High-temperature oxidation behavior of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramics in air

High-entropy metal carbides have recently been arousing considerable interest. Nevertheless, their high-temperature oxidation behavior is rarely studied. Herein the high-temperature oxidation behavior of (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy metal carbide (HEC-1) was investigated at 1573-1773 K in air for 120 minutes. The results showed that HEC-1 had good oxidation resistance and its oxidation obeyed a parabolic law at 1573-1673 K, while HEC-1 was completely oxidized after isothermal oxidation at 1773 K for 60 minutes and thereby its oxidation followed a parabolic-linear law at 1773 K. An interesting triple-layered structure was observed within the formed oxide layer at 1673 K, which was attributed to the inward diffusion of O₂ and the outward diffusion of Ti element and CO or CO₂ gaseous products.     

Impact of reducing conditions on the stabilization of Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O high-entropy oxide

The entropy stabilization effect has expanded the compositional diversity of inorganic compounds. Nevertheless, the correlation between processing conditions, properties, and microstructure remains poorly understood. So far, little attention has been dedicated to identifying processing strategies to facilitate (i.e., reduce processing temperature) the synthesis of multi-elements oxides. In this work, we have evaluated the impact of spark plasma sintering (SPS) on stabilizing Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O high entropy oxide (HEO). Compared to conventional sintering in air, the reducing conditions achieved in SPS allowed a ≈ 200 °C decrease in the temperature required to stabilize the HEO phase. The interpretation of such effect was guided by DSC/TGA, XRD, and XPS analyses. The results suggest a strong correlation between processing atmosphere (i.e., reducing, inert or oxidizing), interfacial resistance and volumetric capacity of the high entropy anode materials.     

Microstructures and oxidation mechanisms of (Zr0.2Hf0.2Ta0.2Nb0.2Ti0.2)B2 high-entropy ceramic

A nano dual-phase powder with great sinterability was synthesized by molten-salt assisted borothermal reductions at 1100 °C using B, ZrO₂, HfO₂, Ta₂O₅, Nb₂O₅ and TiO₂ powders as raw materials. Single-phase (Z_r0.2Hf_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ high-entropy ceramic was prepared by spark plasma sintering using the as-synthesized nano dual-phase powder. Oxidation behavior of the (Zr_0.2Hf_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ ceramic was investigated over the range of 30–1400 °C in air and the result indicated that the rapid oxidation of ceramic began at 1300 °C. The phenomenon could be ascribed to the rapid volatilization of B₂O₃ from oxide scale. A layered structure was formed at the cross section of (Zr_0.2Hf_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ ceramic after oxidation. The relationship between partial pressures of gaseous metal oxides and oxygen partial pressures was calculated, which inferred that the formation of layered structure could be ascribed to the active oxidation of (Zr_0.2Hf_0.2Ta_0.2Nb_0.2Ti_0.2)B₂, the generation of gaseous metal oxides, their outward diffusion and further oxidation.     

Diffusion bonding of (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2)C high-entropy ceramic with metallic Ni foil

To prepare large-sized and complex-shaped components, the feasibility of direct diffusion bonding of (Hf_0.2Zr_0.2Ti_0.2Ta_0.2Nb_0.2)C high-entropy ceramic (HEC) and its diffusion bonding with a metallic Ni foil was investigated, and the interfacial microstructure and mechanical properties of HEC/HEC and HEC/Ni/HEC joints were analyzed. For the direct diffusion bonding, reliable joints with a shear strength of 146 MPa could be achieved when the bonding temperature reached 1500 °C under a pressure of 30 MPa. By introducing a metallic Ni foil as the interlayer, the HEC was successfully bonded at the diffusion temperatures from 1150 °C to 1250 °C under 10 MPa through the formation of Ti₂Ni compound phase. Meanwhile, the HEC(Ni) phase formed by the diffusion of Ni into HEC and Ni(s, s) bulks precipitated in the bonding transition zone. The maximum joint shear strength of 151 MPa was obtained by optimizing the Ni-foil thickness, bonding temperature, and holding time.     

Novel (Sm0.2Lu0.2Yb0.2Y0.2Dy0.2)3TaO7 high-entropy ceramic for thermal barrier coatings

A novel (Sm_0.2Lu_0.2Dy_0.2Yb_0.2Y_0.2)₃TaO₇ (SLT-5RE_0.2) oxide with a single-fluorite structure was synthesized via an optimized sol-gel and sintering method, and its crystal structure, mechanical and thermophysical properties were investigated. The results indicate that the calcined nanoscale powder is of high crystallinity, and bulk sample is of a uniform elemental distribution. Compared to YSZ (6–8 wt.% Y₂O₃ partially stabilized by ZrO₂), SLT-5RE_0.2 exhibits lower Young's modulus, less mean acoustic velocity, and higher Vickers microhardness. Owing to the strengthened anharmonic vibration and phonon scattering, SLT-5RE_0.2 exhibits low thermal conductivity (1.107 W K^?1·m^?1, 900 °C). The high thermal expansion coefficient (11.3 × 10^?6 K^?1, 1200 °C) of SLT-5RE_0.2 ceramic can be ascribed to the reduced lattice energy and ionic spacing as well as the cocktail effect of high-entropy ceramics. The excellent mechanical and thermophysical properties, and excellent phase steadiness during the whole testing temperature cope, indicate that SLT-5RE_0.2 high-entropy ceramic can be a candidate material for thermal barrier coatings.     

(Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramics with low thermal conductivity

A novel high‐entropy carbide ceramic, (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C, with a single‐phase rock salt structure, was synthesized by spark plasma sintering. X‐ray diffraction confirmed the formation of a single‐phase rock salt structure at 26‐1140°C in Argon atmosphere, in which the 5 metal elements may share a cation position while the C element occupies the anion position. (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C exhibits a much lower thermal diffusivity and conductivity than the binary carbides HfC, ZrC, TaC, and TiC, which may result from the significant phonon scattering at its distorted anion sublattice. (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C inherits the high elastic modulus and hardness of the binary carbide ceramics.     

First-principles study,fabrication, and characterization of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramic

The formation possibility of (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy ceramic (HHC-1) was first analyzed by the first-principles calculations, and then, it was successfully fabricated by hot-pressing sintering technique at 2073 K under a pressure of 30 MPa. The first-principles calculation results showed that the mixing enthalpy and mixing entropy of HHC-1 were −0.869 ± 0.290 kJ/mol and 0.805R, respectively. The experimental results showed that the as-prepared HHC-1 not only had an interesting single rock-salt crystal structure of metal carbides but also possessed high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, it exhibited extremely high nanohardness of 40.6 ± 0.6 GPa and elastic modulus in the range from 514 ± 10 to 522 ± 10 GPa and relatively high electrical resistivity of 91 ± 1.3 μΩ·cm, which could be due to the presence of solid solution effects.     

Novel high entropy (Y0.2Sm0.2Gd0.2Er0.2Ho0.2)3NbO7 nanofibers with ultralow thermal conductivity

Thermal insulation materials can provide thermal protection in extreme environments. Ceramic fibers have played an important role in the thermal protection field at high temperatures due to the advantages of low density, high strength, low thermal conductivity, and excellent thermal stability. In this work, high entropy (Y_0.2Sm_0.2Gd_0.2Er_0.2Ho_0.2)₃NbO₇ (5RE₃NbO₇) nanofibers were fabricated by electrospinning and subsequent calcination. Defective fluorite-structured 5RE₃NbO₇ nanofibers were obtained when heated at 900°C. The research indicates that 5RE₃NbO₇ nanofiber based porous ceramics present an ultralow thermal conductivity (0.0992 W/m·K, porosity of 78.18%), good thermal stability, and high spectral reflectance, which establish the foundation for applications in thermal insulation.     

Phase evolution and thermal stability of novel high-entropy (Mo0.2Nb0.2Ta0.2V0.2W0.2)Si2 ceramics

Owing to the high melting points and high-temperature stability, transition-metal disilicides are potential components for aerospace, automotive, and industrial engineering applications. However, unwanted oxidation known as PEST oxidation severely limits their application owing to the formation of volatile transition metal oxides, especially in the temperature range of 500–1000 °C. To overcome this problem, a new class of high-entropy disilicides, (Mo_0.2Nb_0.2Ta_0.2V_0.2W_0.2)Si₂, was selected by first-principles calculations and then successfully fabricated using a hot-pressing sintering technique. Furthermore, the phase evolution, thermal expansion behavior, thermal conductivity, and oxidation behavior were systematically investigated. Compared with MoSi₂, (Mo_0.2Nb_0.2Ta_0.2V_0.2W_0.2)Si₂ possessed a lower thermal conductivity (10.9–14.7 W·m^?1·K^?1) at 25–1000 °C, higher thermal expansion coefficients (8.6 ± 1.3^–6 K^–1) at 50–1200 °C, and especially an excellent thermal stability at 500–1000 °C owing to slow diffusion and selective oxidation. This work provides a strong foundation for the synthesis and application of high-entropy disilicides.     

First-principles prediction,fabrication and characterization of (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)B2 high-entropy borides

The microstructure and mechanical properties of (Hf_0.2Nb_0.2Ta_0.2Ti_0.2Zr_0.2)B₂ high-entropy boride (HEB) were first predicted by first-principles calculations combined with virtual crystal approximation (VCA). The results verified the suitability of VCA scheme in HEB studying. Besides, single-phase (Hf_0.2Nb_0.2Ta_0.2Ti_0.2Zr_0.2)B₂ ceramics were successfully fabricated using boro/carbothermal reduction (BCTR) method and subsequent spark plasma sintering (SPS); furthermore, the effects of different amounts of B₄C on microstructure and mechanical properties were evaluated. Due to the addition of B₄C and C, all samples formed single-phase solid solutions after SPS. When the excess amount of B₄C increased to 5 wt%, the sample with fine grains exhibited superior comprehensive properties with the hardness of 18.1 ± 1.0 GPa, flexural strength of 376 ± 25 MPa, and fracture toughness of 4.70 ± 0.27 MPa m^1/2. Nonetheless, 10 wt% excess of B₄C coarsened the grains and decreased the strength of the ceramic. Moreover, the nanohardness (34.5–36.9 GPa) and Young's modulus (519–571 GPa) values with different B₄C contents just showed a slight difference and were within ranges commonly observed in high-entropy diboride ceramics.     

A novel high-entropy monoboride (Mo0.2Ta0.2Ni0.2Cr0.2W0.2)B with superhardness and low thermal conductivity

A new high-entropy monoboride (Mo_0.2Ta_0.2Ni_0.2Cr_0.2W_0.2)B ceramic with a WB-type orthogonal structure was designed and synthesised by in-situ reactive hot pressing at 2000 °C and 30 MPa for 1.5 h under an argon atmosphere. The microstructure of the sintered samples was comprehensively characterised, and the formation of a high-entropy monoboride (Mo_0.2Ta_0.2Ni_0.2Cr_0.2W_0.2)B ceramic was confirmed. Owing to the high density of the dislocations and strengthening metal-boron bonds, the high-entropy (Mo_0.2Ta_0.2Ni_0.2Cr_0.2W_0.2)B ceramic exhibited a hardness of 48.51 ± 4.07 GPa, which enabled it to be classed as a new superhard material. In addition, the thermal conductivity (2.05 ± 0.10 W/(m·K) at 400 °C) and electric conductivity (132.30 S/cm) were determined.     

Formation and properties of Ca2+ substituted (Ce0.2Zr0.2Ti0.2Sn0.2Hf0.2)O2 high-entropy ceramics

Five equimolar multicomponent oxides were synthesized by replacing one of five cations in (Ce_0.2Zr_0.2Ti_0.2Sn_0.2Hf_0.2)O₂ with Ca²⁺. The results reveal that except for the one in which Ce⁴⁺ replaced by Ca²⁺, the other four components can form single-phase high-entropy fluorite oxides (HEFOs) at different temperatures, which indicates that Ce⁴⁺ is very important for the formation of single-phase HEFOs. The sintering behavior, lattice parameter and properties containing density, porosity, flexural strength and thermal conductivity of the four single-phase HEFOs were investigated. With the change of substituted ions, grain size, relative density, flexural strength and thermal conductivity of the materials vary greatly, which are correlated to the size disorder and mass disorder of these materials. The results of this paper provide a reference for the composition designing and performance tailoring of equimolar HEFOs.     

High energy storage density and efficiency in nanostructured (Bi0.2Na0.2K0.2La0.2Sr0.2)TiO3 high-entropy ceramics

High-entropy ceramics (HECs) (Bi_0.2Na_0.2K_0.2La_0.2Sr_0.2)TiO₃ (BNKLST) with single-phase perovskite structure have been successfully prepared by a modified citrate acid method. In comparison to (Bi_0.5Na_0.5)TiO₃ (BNT) ceramics prepared by the same synthesis route, the BNKLST HECs exhibit dense nanostructures with grain sizes as small as 45 nm, which are suggested to be responsible for the significantly improved electric breakdown fields and reduced leakage currents in the ceramics, and they have much enhanced elastic modulus owing to the entropy-stabilized perovskite structure. The electrical and dielectric characterizations reveal that BNKLST has high electrical resistances and dielectric constants at elevated temperatures, and, in particular, a recoverable energy storage density of 0.959 J/cm³ can be achieved under an applied electric field of 180 kV/cm. Moreover, the energy storage efficiency in BNKLST can be maintained to be larger than 90% at 40–200°C. These excellent properties suggest that entropy-stabilized BNT-based ceramics are promising dielectrics for electrical energy storage applications.     

High-entropy La(Fe0.2Co0.2Ni0.2Cr0.2Mn0.2)O3 ceramic exhibiting high emissivity and low thermal conductivity

A novel, high-entropy, perovskite-structured, solid solution La(Fe_0.2Co_0.2Ni_0.2Cr_0.2Mn_0.2)O₃ ceramic was successfully synthesized via high-temperature solid-state reaction. The crystal structure, microstructure, infrared emissivity, and thermophysical properties were investigated. The experimental results indicated that La(Fe_0.2Co_0.2Ni_0.2Cr_0.2Mn_0.2)O₃ exhibited an infrared emissivity as high as .92 in the near-infrared region of .76–2.50 μm. The thermal conductivity was 1.38–1.72 W m⁻¹ K⁻¹ in the temperature range of 25–1200°C.     

Microstructure and dielectric properties of high entropy Ba(Zr0.2Ti0.2Sn0.2Hf0.2Me0.2)O3 perovskite oxides

A series of high entropy Ba(Zr_0.2Ti_0.2Sn_0.2Hf_0.2Me_0.2)O₃ (Me=Y³⁺,Nb⁵⁺,Ta⁵⁺,V⁵⁺,Mo⁶⁺,W⁶⁺) perovskite oxides were synthesized by using a solid state reaction method. Three multiple-cation solid solutions formed pure phase compounds, and only two compounds were sintered into ceramics. Microstructure analysis showed the influence of configurational entropy on phase stability and grain growth. Dielectric measurements showed that the high entropy ceramics possessed decent temperature stability of permittivity from 25 °C to 200 °C, low dielectric loss (<0.002) from 20 Hz to 2 MHz, high resistance and moderate breakdown strength (290 kV/cm, 370 kV/cm). Evidence strongly confirmed that controlling configurational entropy could be a feasible perspective to set up highly tunable perovskite structures and explore novel species of dielectric materials.     

Characterization of novel high-entropy (La0.2Nd0.2Sm0.2Dy0.2Yb0.2)2Zr2O7 electrospun ceramic nanofibers

We present a simple way to fabricate high-entropy (La_0.2Nd_0.2Sm_0.2Dy_0.2Yb_0.2)₂Zr₂O₇ (HE-RE₂Zr₂O₇) ceramic nanofibers using the electrospinning and annealing processing in this work. The microstructure of nanofibers was characterized by thermal gravity-differential scanning calorimetry (TG-DSC), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. Meanwhile, the diameter and grain growth of HE-RE₂Zr₂O₇ nanofibers under 1000 °C was analyzed. Results indicate that HE-RE₂Zr₂O₇ nanofibers can be prepared at temperatures above 400 °C and the crystallite size can be controlled by annealing temperature. Both diameter and the grain growth of HE-RE₂Zr₂O₇ nanofibers are lower than that of La₂Zr₂O₇ nanofibers, attributed to the sluggish diffusion effect. The advantages of HE-RE₂Zr₂O₇ nanofibers can further enlarge the application of nanofibers in the aspect of high-temperature thermal insulation materials.