High-entropy Ceramics Research Articles

Nanosecond-laser cutting of (TiZrHfNbTa)C high-entropy ceramics

(TiZrHfNbTa)C high-entropy ceramics have wide range of application prospects due to their excellent performance, but their hardness and brittleness make them difficult to process. In this study, novel method based on nanosecond laser was used to cut (TiZrHfNbTa)C high-entropy ceramics. Results show that laser machining is efficient strategy for high-precision machining of (TiZrHfNbTa)C high-entropy ceramics. Slit width obtained via nanosecond-laser cutting is only 26.4 % of that obtained via mechanical cutting, and average roughness and maximum height difference of cut surface are 2.078 and 20.497 μm, respectively, which are smaller than those obtained through mechanical cutting (4.296 and 29.706 μm, respectively). Furthermore, removal mechanism under the action of nanosecond laser was investigated, and it was found that cut surface of (TiZrHfNbTa)C high-entropy ceramics formed thin oxide recast layer together with carbide underneath. This study provides important results for precision machining of (TiZrHfNbTa)C high-entropy ceramics.

Prediction of grain boundary strength and mechanical responses of high-entropy carbides reinforced by hybrid graphene and SiC nanowire

The excellent performance of high entropy ceramics (HECs) has attracted extreme attention in the most advanced functional applications and engineering structures, yet the poor toughness limits the application of HECs in many fields. As a consequence, advanced technology and new doping phases are urgently demanded to enhance the comprehensive mechanical properties of HEC composites. The microstructure model of 0.2 wt% graphene (G) and SiC nanowires (SiCnw) doped in (HfNbTaTiZr)C is established in Abaqus with Python language. The effects of HEC-HEC interface strength and HEC-G interface strength on crack propagation and mechanical properties of HEC composites were investigated. The improvement of mechanical properties of (HfNbTaTiZr)C with different content of SiCnw was predicted respectively. The composites obtain the optimal combination of mechanical properties at the content of 1.5 wt% SiCnw. The accuracy of the simulation is verified by the experimental data of mechanical properties changing trend and toughening mechanism, providing theoretical support for toughening HEC composites.

Dual sinter-aids to fabricate high-entropy transition metal carbide ceramic and their properties

Due to excellent performances and easy adjustability, more and more researchers pay attention to high-entropy ceramics, which can be applied in many fields. In this work, a high-entropy transition metal carbide ceramic, (Hf0.2Ta0.2Zr0.2Nb0.2W0.2)C, was fabricated by pressureless sintering method. To obtain highly dense ceramic, dual sintering aids (Cr3C2 and C), are employed in the sintering process. Owing to suitable microstructure (grain size and defect density), the ceramics exhibit excellent electric conductivity(0.7040 MS/m) and flexural strength (378 ± 11.4 MPa). Meanwhile, the relationship between electromagnetic shielding effectiveness and different aid content in the X-band is also investigated. The P3 ceramic with a thickness of 1 mm can achieve an EMI shielding efficiency of 92 dB in the X-band as a result of the stronger interface polarization and relaxation effects. High-entropy ceramic obtained in this work would give a different way to fabricate shielding material and can promote the use of high-entropy ceramics as effective EMI shielding materials.

High-temperature dimensional stability and radiation behavior of high-entropy rare earth tungstate ceramics

Materials with high-temperature dimensional stability and resistance to radiation damage are receiving increasing attention in advanced nuclear energy systems. In this research, (Sm0.2Eu0.2Gd0.2Dy0.2A0.2)2W3O12-type (A = Y, Tm,Ho) high-entropy ceramics is presented. The coefficients of thermal expansion of (Sm0.2Eu0.2Gd0.2Dy0.2Y0.2)2W3O12 (HE-RE2W3O12) is 7.21 × 10−6 K−1 at 300–1100K, which is the lowest among the three synthesized high-entropy ceramics and indicates good dimensional stability at high temperatures. After irradiation with Kr+ fluence of 4.58 × 1016 ions/cm2 at 400 °C, both HE-RE2W3O12 and Gd2W3O12 maintained crystalline, but the lattice volume expansion rate of HE-RE2W3O12 (0.71 %) is much lower than that of Gd2W3O12 (1.49 %). Additionally, in the post-irradiated HE-RE2W3O12, the Raman spectra vibration peak remained unchanged and no radiation-induced segregation was observed. Nanoindentation tests displayed that the degradation of hardness and elastic modulus in HE-RE2W3O12 is rarely degrade. Overall, these results provide new insights for research on advanced nuclear materials in the future.

Microstructure, mechanical, and thermal properties of (MoTaTiVW)CX high entropy ceramics with different carbon stoichiometries

In present work, high entropy (MoTaTiVW)CX ceramics with different carbon stoichiometries (X = 3–6) were produced using spark plasma sintering. The microstructure, mechanical properties, and thermal properties of (MoTaTiVW)CX with different carbon stoichiometries were studied. At X = 3, the carbon deficiency resulted in a multiphase structure of the sample, (MoTaTiVW)C3 with a Mo-rich carbide phase. At X = 3.5, the single-phase rock salt structure was formed despite carbon vacancies of up to 30 %. The relative density of (MoTaTiVW)CX gradually decreases as the carbon stoichiometry increases from X = 3 to X = 6. The concentration of carbon vacancies affects the sintering activation energy, which can enhance mass transfer and promote the densification process. The Vickers hardness of (MoTaTiVW)CX shows an overall decreasing trend as the carbon stoichiometry increases from X = 3.5 to X = 6, peaking at 20.4 ± 0.4 GPa at X = 3.5. The fracture toughness increases and then decreases, peaking at 4.4 ± 0.2 MPa m1/2 at X = 4. The thermal conductivity of (MoTaTiVW)CX gradually increases, then decreases and finally increases with the increase of carbon stoichiometry. At room temperature, the maximum value of thermal conductivity is 6.35 ± 0.29 W/m·K of (MoTaTiVW)C5 and the minimum value is 5.32 ± 0.35 W/m·K of (MoTaTiVW)C5.5. At 1000 °C, the thermal conductivity of (MoTaTiVW)CX ranges from 15.94 ± 0.37 W/m·K to 19.56 ± 0.45 W/m·K. As the concentration of carbon vacancies decreases, the decrease in point defect concentration will reduce the degree of lattice distortion, thereby reducing phonon scattering and affecting thermal conductivity. The study suggests that carbon stoichiometry can be used to adjust the microstructure, mechanical properties, and thermal properties of high entropy carbide ceramics.

Effect of A-site rare-earth ions on structure and magnetic properties of novel (Ln0.2Gd0.2La0.2Nd0.2Sm0.2)MnO3 (Ln = Eu, Ho, Yb) high-entropy perovskite ceramics

Single-phase high-entropy perovskite ceramics (Ln0.2Gd0.2La0.2Nd0.2Sm0.2)MnO3 (Ln-GLNSMO HEPCs) have been synthesized using solid-state reactions. The effect of A-site rare-earth ions (Ln = Eu, Ho, Yb) on the structure and magnetic properties was systematically investigated. The results show that all samples are orthorhombic phases (with a space group of Pbnm) and exhibit a strong crystallization trend sintered at 1300 °C for 16 h. All the HEPCs are composed of irregular spherical particles with smooth surfaces and show obvious hysteresis effects at T = 5 K. The changing Mn–O bond distance, Mn–O–Mn bond angle and the increase of Mn4+ ion concentration enhance the double exchange effect of Mn3+-O2--Mn4+, leading to different magnetic interactions. The residual magnetization Mr (coercivity Hc) of the samples is significantly enhanced, which is increasing from 0.13 emu/g (16.34 Oe) to 3.17 emu/g (97.07 Oe) due to the increase of configuration entropy, lattice distortion and the double exchange effect (DE). In addition, there is a strong relationship between the magnetic properties and the average radius of the A-site ions, in which Ho-GLNSMO HEPCs have a high Ms magnetism. This work is a valuable reference for the magnetic regulation of high-entropy ceramics based on rare-earth and entropy-stabilized perovskite types.

Preparation of a (Ca,Sr,Ba)ZrO3 Crucible by Slip Casting for the Vacuum Induction Melting of NiTi Alloy.

Vacuum induction melting is a more energy-efficient process for the preparation of a titanium alloy with good homogeneity and low cost. But the crucial problem for this technology is in developing a crucible refractory with high stability. In the present work, a novel (Ca,Sr,Ba)ZrO3 crucible was prepared by slip casting and its performance in melting NiTi alloy was studied. The results showed that a single solid solution was formed with a homogeneous distribution of metal elements after sintering at 1500 °C. It was found that the total content of oxygen and nitrogen remaining in the TiNi alloy after melting in the (Ca,Sr,Ba)ZrO3 crucible was 0.0173 wt.%, which fulfills the ASTM standard on biomedical TiNi alloys. The good resistance of the (Ca,Sr,Ba)ZrO3 crucible to molten NiTi has a relationship with the sluggish diffusion effect of high-entropy ceramics. This study provides insights into the process of designing highly suitable crucible material for melting a NiTi alloy.

Secondary phase evolution of high-entropy ceramics under heavy-ion irradiation in high-temperature coupling-induced environment

The formation of new phases in high-entropy ceramics can be induced by both heavy-ion irradiation and annealing, meaning that these transitions can simultaneously occur under high-temperature irradiation. Consequently, the radiation resistance of high-entropy ceramics can involve complex mechanisms upon this coupled environment. In this study, the irradiation resistance of the high-entropy (ZrHfTiSn)O2 ceramic was investigated under heavy-ion irradiation at different temperatures. The results show that after irradiation, the secondary triclinic phase exists in all the ion-injected ranges. The triclinic phase can withstand irradiation, leading to dislocations that trigger a local increase in entropy and result in layered amorphization. Amorphization is accompanied by volume expansion that compresses adjacent regions, causing angular differences in the triclinic phase unit cell, and recrystallization of the amorphous region at 500 °C can release this compression. This study provides experimental data and theoretical support for further improving the radiation resistance of related materials.

Dielectric property optimization of polymer nanocomposites using BaTiO3 based high-entropy ceramic filler with Dirac-cone effect

BaTiO3 (BTO) based nanoceramics play an important role in electronic devices. Here, we synthesized three BTO-based high-entropy nanoceramics and calculated their band gaps via density functional theory (DFT). Firstly, cubic high-entropy ceramic powders, namely (La0.2Ca0.2Sr0.2Ba0.2Li0.2)TiO3, Ba(Ti0.2Sn0.2Zr0.2Hf0.2Nb0.2)O3 and Ba(Ti0.2Sn0.2Hf0.2Zr0.2Ta0.2)O3, were synthesized. Then, composition and morphology of powders was determined. Ultimately, their band gaps were calculated. We clarified the influence of metal co-doping on band gaps. Dirac-cone band-structure was found in Ba(Ti0.2Sn0.2Zr0.2Hf0.2Nb0.2)O3. Compared with BTO (band gap 1.821 eV), BTO-based high-entropy ceramics can have the reduced band gaps (1.549 eV; 0.001 eV; 0 eV) using various co-doping. By high-entropy strategy, conductive ability of BTO-based ceramic is regulated. Experimental results of polymer/ceramic composite dielectrics and theoretical results of ceramics are consistent. Dirac-cone ceramic filler is promising for preparing polymer-based composites for energy storage. This research enables mass preparation of high-entropy ceramic/polymer dielectric composites.

How does cold sintering affect the microstructural evolution of high-entropy pyrochlore during pressureless sintering?

The challenge of producing dense fine-grained high-entropy ceramics is hereby addressed by a combined cold and conventional pressureless sintering method for the first time. It provides nearly fully dense high-entropy pyrochlores with better microstructural homogeneity, and only half the grain size of conventional pressurelessly sintered samples with identical density. Both the finer and more homogeneous microstructures benefit better mechanical properties in hardness and reduced Young’s modulus. The kinetics and activation energies of both densification and grain growth were quantified and discussed to understand how cold sintering affects the microstructural evolution during the following pressureless sintering. More importantly, cathodoluminescence analysis suggested that cold sintering could cause more defects on the surface of the original particles, which could promote the grain boundary diffusion and have drag effects on grain boundary motion. The lessons learned here offer scientific understanding and technological guidance towards pressureless sintering of dense bulk high-entropy ceramics.

Microstructure development of plasma sprayed dual-phase high entropy ceramic coating derived from spray dried and induction plasma spheroidized powder

Dual phase high entropy ceramics (DPHECs) exhibit excellent high temperature mechanical properties and oxidation resistance. Nevertheless, they cannot be applied in the structural components on large scale due to their intrinsic brittleness. Hence, DPHEC coating is a necessary development direction for industrial applications. In this study, we first fabricated micron-order DPHEC powder with high quality by precursor synthesis, spray drying (SD), and induction plasma spheroidization (IPS). The DPHEC coating was prepared by supersonic atmospheric plasma spraying (SAPS). The microstructure, phase compositions, and mechanical properties of the DPHEC powder and coating are investigated in detail. Results show that the SD&IPS powder and SAPS coating are all composed of HEB and HEC phases, showing slight composition segregation and typical global eutectic microstructure. The nanohardness and Young's modulus are 22.17 ± 2.76 GPa and 309.79 ± 35.71 GPa, respectively. The fracture toughness was calculated to be 2.23 ± 0.17 MPa m1/2 from the energy analysis method. The mechanical properties are enhanced by the solid solution and fine grain strengthening effects and are weakened by the intrinsic defects of plasma sprayed coating. This work provides a new method and a potential material for thermal spraying ultra-high temperature ceramic coatings.

Remarkable energy-storage density together with efficiency of above 92% in high-entropy ferroelectric ceramics

Recently, the use of “entropy engineering” to form high-entropy ceramic dielectric materials is considered to be an effective means to break through the traditional doping which modified local structures. However, the low energy storage efficiency (η) of most high-entropy ceramics cannot match their excellent energy storage density (Wrec). This work is the first to combine scheelite structure (SmTaO4) with high-entropy perovskite structure ((NaBiBaSrCa)0.2TiO3). On the one hand, the superposition of multiple structures makes the inherent lattice distortion and sluggish diffusion of high-entropy materials more obvious, thus playing a role in refining grains, reducing leakage current density, and obtaining a high breakdown electric field (Eb). On the other hand, the introduction of donor elements inhibits the generation and migration of oxygen vacancies, reducing losses to improve η and Eb. Ultimately, the goal of obtaining a remarkable Wrec (5.6 J/cm3) and η (92.2%) at 445 kV/cm was successfully achieved. This work offers a practical approach to attain superior overall performances in high-entropy ceramics.

Pressureless two-step sintering of ultrafine-grained high-entropy zirconate ceramics with excellent mechanical properties

The challenge of preparing dense ultrafine-grained high-entropy ceramics through conventional pressureless sintering is hereby addressed by a simple two-step sintering method. Ultrafine-grained (Ce0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 high-entropy zirconate with 99.2 % theoretical density and 135 nm grain size has been fabricated by pressureless two-step sintering for the first time. Compared to the conventional sintering, two-step sintering provides finer grain sizes and better microstructural uniformity, which yield excellent comprehensive mechanical properties with high hardness of 16.0 GPa, large elastic modulus of 262 GPa and high fracture toughness of 2.8 MPa·m1/2. It is believe that pressureless two-step sintering can be utilized to prepare other high-entropy ceramics with fine grain size and high quality.

A general micromechanics-based model for precipitate strengthening and fracture toughness in polycrystal high entropy alloys

High-entropy alloys (HEAs) usually exhibit exceptional mechanical properties attributed to one of important core effects for serious lattice strain to impede dislocation motion compared to the traditional alloys. However, their roles on the quantitative measurement for precipitate strengthening and fracture toughness are lack using the existing physical model. Here, we propose a mechanistic modelling to study effect of heterogeneous strain caused by lattice distortion on the precipitate strengthening and fracture toughness in the HEAs, and then verify this role using atomic simulation. The results indicate that the lattice distortion and precipitate synergistically impede the grain boundary migration, increasing the strength. In the dilute alloy with a low lattice distortion, the grain boundary migration process is less sensitive to the precipitate size. The stress field generated by the lattice distortion relieves the stress concentration at the crack tip under external force. This in turn alleviates the accumulation of dislocations and reduces the probability of crack extension. Furthermore, the heterogeneous strain caused by lattice distortion counteracts some of the applied stress and raises the critical stress for crack extension, which enhances the plasticity and the critical stress intensity factor. The developed unified model would be applicable to high entropy ceramics in similar scenario.

High entropy ceramics for applications in extreme environments

Compositionally complex materials have demonstrated extraordinary promise for structural robustness in extreme environments. Of these, the most commonly thought of are high entropy alloys, where chemical complexity grants uncommon combinations of hardness, ductility, and thermal resilience. In contrast to these metal–metal bonded systems, the addition of ionic and covalent bonding has led to the discovery of high entropy ceramics (HECs). These materials also possess outstanding structural, thermal, and chemical robustness but with a far greater variety of functional properties which enable access to continuously controllable magnetic, electronic, and optical phenomena. In this experimentally focused perspective, we outline the potential for HECs in functional applications under extreme environments, where intrinsic stability may provide a new path toward inherently hardened device design. Current works on high entropy carbides, actinide bearing ceramics, and high entropy oxides are reviewed in the areas of radiation, high temperature, and corrosion tolerance where the role of local disorder is shown to create pathways toward self-healing and structural robustness. In this context, new strategies for creating future electronic, magnetic, and optical devices to be operated in harsh environments are outlined.

Enhanced energy storage performance of (Bi0·4Ba0·2K0·2Na0.2)TiO3–NaNbO3 high-entropy ceramics under moderate electric fields

High-entropy perovskite oxides (HEPOs) have attracted considerable attention as dielectric ceramics. However, the energy-storage performance of these HEPOs still requires improvement. In this study, a series of (1-x)(Bi0.4Ba0.2K0.2Na0.2)TiO3-xNaNbO3 high-entropy ceramics were designed and prepared using a high-entropy strategy. The introduction of antiferroelectric NaNbO3 into the (Bi0.4Ba0.2K0.2Na0.2)TiO3 medium-entropy matrix increased the configurational entropy and cationic disorder, resulting in improved energy storage performance. The ceramic with an optimal composition of 0.775(Bi0.4Ba0.2K0.2Na0.2)TiO3-0.225NaNbO3 achieved a high recoverable energy density (Wrec) of 2.523 J/cm3 and a high efficiency (η) of 83.7% at an electric field of 220 kV/cm. Furthermore, the material demonstrated good temperature and frequency stability, as well as good anti-fatigue properties. This paper presents a dielectric ceramic suitable for energy storage under low to moderate electric fields.

Dielectric properties and excellent energy storage density under low electric fields for high entropy relaxor ferroelectric (Li0.2Ca0.2Sr0.2Ba0.2La0.2)TiO3 ceramic

High entropy relaxor ferroelectrics, are a representative type of dielectric with exceptional properties and play an indispensable role in the next-generation pulsed power capacitor market. In this paper, a high-entropy relaxor ferroelectric ceramic (Li0.2Ca0.2Sr0.2Ba0.2La0.2)TiO3 successfully designed and synthesized using the traditional solid-state reaction method. The microstructure, dielectric properties, and energy storage characteristics of the ceramics were systematically investigated. The ceramic exhibited a dense microstructure with a single-phase perovskite structure and small grain size. The unique microstructure and high insulation not only wides the band gap, but also minimizes residual polarization and resulting in higher energy density (Eb) 1.51 J/cm3 and efficiency (η) of 89.6% at a low field strength of 260 kV/cm. Furthermore, the observed anomalous behavior in dielectric relaxation was explained within the context of high entropy ceramics. The material also exhibited excellent temperature/frequency stability and fast charging-discharging speed (∼35 ns). These results demonstrate the effectiveness of high entropy strategy to develop high-performance energy storage capacitors.

Enhanced thermal shock resistance of gradient high-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Ce2O7/YSZ thermal barrier coatings

High-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Ce2O7 (LECO) has been verified as a potential candidate for thermal barrier coatings (TBCs). In this work, two different structured LECO coatings were constructed and the thermal shock behavior was systematically investigated. In the classical double-layered LECO coating (D-LECO), the cracks initially originated from the edge and completely exfoliated from substrate after failure. To strengthen the thermal stress resistance of the coating, a gradient multilayered LECO/YSZ coating (M-LECO) was designed with five ceramic sub-layers. The thermal shock life of M-LECO was greatly enhanced by approximately 4 times. Unlike the typical TBCs, the delamination first appeared at an annular region on the surface, located away from the center and edge. Besides, the spalling only occurred in the topmost LECO layer with a thickness loss of less than 5%. After failure, the underneath coating could still provide efficient thermal protection to substrate. The better thermal shock resistance of M-LECO was mainly attributed to the fine gradient structure of the coating and the intrinsic property of high-entropy LECO. The microhardness and fracture toughness in M-LECO were strengthened layer by layer along the thickness direction, alleviating the thermal stress accumulation in the coating and greatly improving the cracking resistance of the interface. Moreover, the high-entropy effect and sluggish diffusion effect of LECO improved its sintering resistance, thereby enhancing the thermal shock resistance of M-LECO. Such design not only fully utilizes the unique thermo-physical properties of high-entropy ceramics, but also effectively solves the problem of weak adhesion between the LECO and the substrate during thermal shock.

Turning of high entropy alloy into high entropy carbides by modifying carbon content during reactive sintering

High entropy alloys containing equimolar multi-element composition enhance physical properties over traditional alloys. The concept was applied also for high entropy ceramics, but the solid-state reaction is necessary. In this work, high entropy alloy VNbMoTaW reacted with carbon to form high entropy carbide. Direct reaction in Spark Plasma Sintering (SPS) conditions leads to high entropy or entropy aided carbides depending on the amount of present carbon. The entropy aided carbide formed at low carbon concentrations below 0.8 of equimolar content contained all metallic elements, but two separated phases were identified: Nb, Ta or W, Mo rich. The microstructure of the entropy aided carbides is lamellar and refined compared to high entropy carbide (V0.2Nb0.2Mo0.2Ta0.2W0.2)C0.8. The presence of entropy aided carbide decreased the Vickers hardness from 23 GPa to 19 GPa, however, the unique microstructure might enhance other physical properties.

Regulating nitrogen fraction, phasic composition and mechanical properties in (Ti0.2V0.2Nb0.2Ta0.2W0.2)(CxNy) high-entropy ceramic via carbon stoichiometry

Regulation of nitrogen content and phasic composition can be directly controlled by adjusting raw carbon allocation based on the certain cationic component in high-entropy carbide and carbonitride. By adopting this method, a series of high-entropy ceramics with the nominal composition of (Ti0.2V0.2Nb0.2Ta0.2W0.2)(CxNy) were synthesized. Within, chemical composition, phase evolution, phasic formation mechanism, mechanical performance and the strengthening mechanism were systematically probed. The matrix has undergone the evolution from double phases to single phase and then to the complex phases with the decrease of carbon and the increase of nitrogen. The monophasic carbide exhibits the best hardness (24.59 ± 1.20 GPa), and the single-phase carbonitride with 8:1 carbon-nitrogen ratio shows the best flexure strength (282.4 ± 16.2 MPa). Precipitation of (W,M)2C also contributes to the crack deflection and the fine-grain strengthening and enhances fracture toughness (for the best of 5.44 ± 0.24 MPa∙m1/2).