Phase stability,mechanical, and thermal properties of high-entropy transition and rare-earth metal diborides from first-principles calculations

The narrow composition design space of high-entropy transition metal diborides (HE TMB₂) limits their further development. In this study we designed six quaternary and quinary high-entropy transition metal and rare-earth diborides (HE TMREB₂) and investigated their phase stability using the energy distribution of the local mixing enthalpy of all possible configurations. The results show that both quaternary and quinary HE TMREB₂ have higher enthalpic driving forces, which facilitates the formation of single-phase AlB₂-type structures between TMB₂ and REB₂. Calculations of elastic constants show that the TMB₂ component has the greatest effect on the c₄₄ elastic constant and shear modulus G, while REB₂ significantly influences the bulk modulus B. Furthermore, LuB₂ and TmB₂ substantially affect the elastic modulus anisotropy of HE TMB₂. Rare-earth atoms in HE TMREB₂ can enhance the nonharmonic interactions between phonons, which results in a significant hindrance in the thermal transport of low-frequency phonons as well as an increase in the volume thermal expansion coefficients. Thus, the incorporation of REB₂ into HE TMB₂ has a significant impact on the phase stability and properties.     

Low-temperature sintering of single-phase,high-entropy carbide ceramics

Dense (Hf, Zr, Ti, Ta, Nb)C high-entropy ceramics were produced by hot pressing (HP) of carbide powders synthesized by carbothermal reduction (CTR). The relative density increased from 95% to 99.3% as the HP temperature increased from 1750°C to 1900°C. Nominally phase pure ceramics with the rock salt structure had grain sizes ranging from 0.6 µm to 1.2 µm. The mixed carbide powders were synthesized by high-energy ball milling (HEBM) followed by CTR at 1600°C, which resulted in an average particle size of ~100 nm and an oxygen content of 0.8 wt%. Low sintering temperature, high relative densities, and fine grain sizes were achieved through the use of synthesized powders. These are the first reported results for low-temperature densification and fine microstructure of high-entropy carbide ceramics.     

Low-temperature reactive sintering of carbon vacant high-entropy carbide ceramics with in-situ formed silicon carbide

It is thought that the sintering of high-entropy (HE) ceramics is generally more difficult when compared to that of the corresponding single-component ceramics. In this paper, we report a novel approach to densify the HE carbide ceramics at relatively low temperatures with a small amount of silicon. Reactive spark plasma sintering (SPS) was used to densify the ceramics using powders of HE carbide and silicon as starting materials. Dense ceramics can be obtained at 1600 -1700°C. X-ray diffraction analysis reveals that only non-stoichiometric HE carbide phase with carbon vacancy and SiC phase exist in the obtained ceramics. The in-situ formed SiC phase inherits the morphology of the starting silicon powder owing to the slower diffusion of silicon atoms compared to that of the carbon atoms in HE carbide phase. The mechanical properties of the prepared ceramics were preliminarily studied.     

Influence of grain boundary and grain size on the mechanical properties of polycrystalline ceramics: Grain-scale simulations

The Orowan-Petch relation is a famous model to describe the strength of polycrystalline ceramics covering a wide range of grain sizes. However, it becomes difficult to explain the strength trend when the grain size decreases to the sub-microscale or nanoscale. This is because some microstructural parameters (such as grain size, grain boundary fracture energy, and grain boundary defects) vary with different processing technologies, and their coupling effects on mechanical properties are still unclear. In this study, a finite element method (FEM) was applied to investigate the dependence of mechanical properties, such as strength and damage resistance, on the abovementioned microstructural parameters on example of alumina. The numerical results show that the grain boundary energy is weakly coupled with the grain size and grain boundary defects. The grain size and grain boundary are intercoupling, which affects mechanical properties. The mechanical properties could be improved by increasing the grain boundary fracture energy and decreasing the grain size and the grain boundary defect density.     

High-pressure sintering of ultrafine-grained high-entropy diboride ceramics

Herein the ultrafine-grained (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ high-entropy diboride ceramics were successfully fabricated by high-pressure sintering technology for the first time. The results showed that the grain size, relative density, and Vickers hardness of the as-fabricated samples all increased gradually with increasing sintering temperatures from 1373 K to 1973 K. The relative density and mean grain size of the as-sintered samples at 1973 K were 97.2% and 684 nm, respectively, and simultaneously they exhibited excellent comprehensive mechanical properties, combining a Vickers hardness of 26.2 GPa and a fracture toughness of 5.3 MPa·m^1/2, which were primary attributed to the fine grain strengthening mechanism and microcrack deflection toughening mechanism.     

The new complex high-entropy metal boron carbonitride: Microstructure and mechanical properties

In this contribution, the ternary BCN anion systems of high-entropy ceramics (HEC) are consolidated by hot-pressing sintering and the impacts of sintering temperature and the content of amorphous BCN addition on microstructural evolution and mechanical performance were evaluated. Results confirmed that high-entropy, oxide, and BN(C) phases were precipitated for (Ta_0.2Nb_0.2Zr_0.2Hf_0.2Ti_0.2)(B, C, N) ceramics after sintering at 1900°C. With the decrease of BCN addition, a new phase of MiB₂ (Mi representing the metal atoms) occurred. The Vickers hardness, bending strength, elastic modulus, and fracture toughness of the optimized bulk HECs were investigated, obtained at 24.5 ± 2.3 GPa, 522.0 ± 2.6 MPa, 478.9 ± 11.1 GPa, and 5.36 ± 0.56 MPa m^1/2, respectively.     

Microstructural evolution and mechanical properties of (Mg,Co,Ni,Cu,Zn)O high-entropy ceramics

The reaction sequence and mechanical properties were studied for (Mg,Co,Ni,Cu,Zn)O high-entropy ceramics that were synthesized using field-assisted sintering technology. The evolution from binary oxide starting powders to a single-phase rock salt structure exhibited a distinct incorporation sequence. For the rock salt oxides, MgO and CoO had the lowest vacancy formation energies and were the first to be incorporated into the high-entropy ceramic followed by NiO, which had a higher vacancy formation energy. Both CuO and ZnO had different crystal structures, and were incorporated into the single phase structure after the rock salt oxides due to the additional energy barrier associated with the transformations from their original structures to the rock salt structure. Distinctive morphological features including Cu-rich regions and lattice distortion were observed in the high entropy ceramic. In addition, a trade-off between densification and grain growth resulted in a maximum in strength (323 MPa) and elastic modulus (108 GPa) after densification at 900°C. This study has revealed new information that can be used to design other high-entropy ceramics including selection criteria for constituent compounds based on crystal structure and defect formation energy as well as the effects of grain size and porosity on control strength and elastic modulus.     

Simultaneous hardening and toughening of a high-entropy (NbTaZrW)C ceramic carbide using SiC particle

Previously, we have found that (NbTaZrW)C exhibits a good combination of nanohardness and toughness. In this report, we explore the possibility to further increase the overall properties of this high-entropy carbide ceramic (HECC) through introducing SiC particle (SiC_P). To this end, a series of (NbTaZrW)C–xSiC ceramic composites (x = 0/5/15/30/50 vol.%) were fabricated using spark plasma sintering (SPS), their microstructure and mechanical properties were characterized. Our results reveal a grain refinement effects of SiC_P, an agglomeration of SiC_P with (1 0 0) plane preferentially perpendicular to the SPS-pressing direction and the formation of a transition region with various stoichiometric ratio of (NbTaZrW)_xC_1−x in the (NbTaZrW)C–SiC_P vicinity. The elastic modulus, microhardness, and flexural strength of the HECCs show tight positive relations with the SiC_P content and the beneficial effect of SiC_P to the fracture toughness of (NbTaZrW)C becomes evident once the content of SiC_P reaches 30 vol.%. Altogether, (NbTaZrW)C–50%SiC, which has a microhardness of 22 GPa, a flexural strength of 455 MPa, and an indentation fracture toughness of 6.54 MPa m^1/2, presents the optimal combination of mechanical properties among the investigated composites. Mechanistically, the strengthening effect of SiC_P introduction arises from the intrinsic high hardness of SiC_P and the SiC_P-induced grain refinement and the toughening effect is mainly associated with crack bridging mechanism.     

Synthesis,mechanical, and thermophysical properties of high-entropy (Zr,Ti,Nb,Ta,Hf)C0.8 ceramic

Two high-entropy carbides, including stoichiometric (Zr,Ti,Nb,Ta,Hf)C and nonstoichiometric (Zr,Ti,Nb,Ta,Hf)C_0.8, were prepared from monocarbides and ZrH₂. Their sinterability, microstructures, mechanical properties, thermophysical properties, and oxidation behaviors were systematically compared. With the introduction of carbon vacancy, the sintering temperature was lowered up to 300°C, Vickers hardness was almost unaffected, whereas the strength decreased significantly generally due to the decrease of covalent bonds. The thermal conductivity shows a 50% decrease for nonstoichiometry high-entropy carbide, which is a major consequence of the lower electrical conductivity. The oxidation resistance in high temperature water vapor was not sensitive to carbon stoichiometry.     

High thermal conductivity of spark plasma sintered silicon carbide ceramics with yttria and scandia

A fully dense SiC ceramic with a room‐temperature thermal conductivity of 262 W·(m·K)^?1 was obtained via spark plasma sintering β‐SiC powder containing 0.79 vol% Y₂O₃‐Sc₂O₃. High‐resolution transmission electron microscopy revealed two different SiC‐SiC boundaries, that is, amorphous and clean boundaries, in addition to a fully crystallized junction phase. A high thermal conductivity was attributed to a low lattice oxygen content and the presence of clean SiC‐SiC boundaries.     

Thermophysical and electrical properties of rare-earth-cerate high-entropy ceramics

A new series of rare-earth-cerate high-entropy ceramics with compositions of (La_0.2Nd_0.2Sm_0.2Gd_0.2Dy_0.2)₂Ce₂O₇ (HEC1), (La_0.2Nd_0.2Sm_0.2Gd_0.2Yb_0.2)₂Ce₂O₇ (HEC2), (La_0.2Nd_0.2Sm_0.2Yb_0.2Dy_0.2)₂Ce₂O₇ (HEC3), (La_0.2Nd_0.2Yb_0.2Gd_0.2Dy_0.2)₂Ce₂O₇ (HEC4), (La_0.2Yb_0.2Sm_0.2Gd_0.2Dy_0.2)₂Ce₂O₇ (HEC5) as well as a single component of Nd₂Ce₂O₇ are fabricated via sintering the corresponding sol–gel-derived powders at 1600°C for 10 h. HEC1–5 samples exhibit a single-cerate phase with fluorite structure and high configurational entropy. Compared with Nd₂Ce₂O₇, HEC1–5 samples have a lower grain growth rate owing to the sluggish diffusion effect. The chemical compositional uniformity of HEC1–5 as well as Nd₂Ce₂O₇ does not apparently change after annealing at 1500°C for different time intervals (1, 6, 12, and 18 h). Compared with 8YSZ, HEC1–5 samples display the decreased thermal conductivity and increased thermal expansion coefficient. The lattice size disorder parameter of HEC1–5 is negatively related to the thermal conductivity in 26–450°C. Furthermore, HEC1–5 and Nd₂Ce₂O₇ exhibit lower oxygen-ion conductivity, meaning an increased resistance to oxygen diffusion.     

Effects of sintering aids on microstructure,mechanical, and optical properties of AlON ceramics synthesized by SPS

Aluminum oxynitride (AlON) ceramics doped with different sintering aids were synthesized by spark plasma sintering process. The microstructures, mechanical, and optical properties of the ceramics were investigated. The results indicate that the optimal amount of sintering aids is 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO. The addition of La³⁺ and Mg²⁺ decreases the rate of grain boundary migration in ceramics, promotes pore elimination, and inhibits grain growth. The addition of Y³⁺ facilitates liquid-phase sintering of AlON ceramics. Moreover, the addition of Mg²⁺ effectively promotes twin formation in the ceramics, which hinders crack propagation and dislocation motion when the ceramics are loaded. Hence, the AlON ceramic doped with 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO exhibits a relative density of 99.95%, an average grain size of 9.42 μm, and a twin boundary content of 10.3%, which contributes to its excellent mechanical and optical properties.     

In situ reaction synthesis of high-entropy carbide/diboride composite with high mechanical properties

A novel technique to simultaneously lower the synthesis temperature of high-entropy carbides and maintain their high mechanical properties was proposed. Certain amount of carbon vacancies was first introduced to significantly lower the temperature down to 2000°C for uniform elemental distributions in high-entropy carbides. Those carbon vacancies were then fully eliminated through the reaction between high-entropy carbides and certain amount of boron carbide. Concomitantly, the high-entropy boride phase was formed. The elimination of carbon vacancies and the formation of high-entropy boride phase significantly improved the mechanical properties of the high-entropy carbides. A high mechanical strength over 500 MPa can be obtained by phase optimization.     

Spark plasma sintering of boron nitride micron tubes reinforced boron carbide ceramics with excellent mechanical property

In this paper, the novel boron nitride micron tubes (BNMTs) were used to reinforce commercial boron carbide (B₄C) ceramics prepared via spark plasma sintering technology. The effects of the sintering parameters, sintering temperature, the holding time, and the BNMTs content on the microstructure and mechanical properties of B₄C/BNMTs composite ceramics were studied. The results indicated that adding a proper amount of BNMTs could inhibit the grain growth of B₄C and improve the fracture toughness of the B₄C/BNMTs composite ceramics. The prepared composite ceramic sample with 5 wt% BNMTs at 1850°C, 8 min and 30 MPa displayed the best mechanical properties. The relative density, hardness, fracture toughness, and bending strength of the samples were 99.7% ± .1%, 35.62 ± .43 GPa, 6.23 ± .2 MPa m^1/2, and 517 ± 7.8 MPa, respectively. Therein, the corresponding value of hardness, fracture toughness, and bending strength was increased by 10.3%, 43.59%, and 61.5%, respectively, than that of the B₄C/BNMTs composite ceramic without BNMTs. It was proved that the high interface binding energy and bridging effect between boron carbide and BNMTs were the toughening principle of BNMTs.     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

Effect of carbon nanoparticle reinforcement on mechanical and thermal properties of silicon carbide ceramics

This research presents an analysis of the influence of graphene reinforcement on the thermal and mechanical properties of silicon carbide ceramics, at 2.5% (wt%) graphene content. The SiC composites, containing various carbon nanofillers (graphene oxide and graphene nanoparticles), were sintered by the classical two stage spark plasma sintering method. Two current modes were used, the continuous mode and the pulsed current mode. The results from photothermal radiometry and investigations of the mechanical properties showed that graphene additives significantly improve the thermal properties and toughness of material, sintered from a SiC powder. An 45% growth in the toughness was observed, which increased from 1.21 to 1.75?MPa/m^1/2. The thermal diffusivity value also increased from 0.60 to 0.71?cm²/s and giving an improvement in thermal properties of 18%. The friction coefficient reached 7% giving an increase in value from 0.62 to 0.66. Microscopic investigations supported the photothermal radiometry (PTR) results. Whilst, thermal imaging revealed homogeneity of the local thermal properties of the products fabricated from the starting SiC powder.     

Low-temperature synthesis of six-principal-component high-entropy transition-metal carbide aerogel thermal insulator

High-entropy transition-metal (IVB–VIB) carbide (HETMC) ceramics consisting of multiple principal components generally correspond to higher configuration entropy, and exhibit better overall performance. However, they also present certain synthesis challenges, for example, in the synthesis of a three-dimensional six-principal-component HETMC aerogel. In the present work, as an example a novel (Ti_0.167Cr_0.167V_0.167Mo_0.167Nb_0.167Ta_0.167)C aerogel was prepared at a relatively low temperature of 1773 K by an in-situ carbothermal reduction/partial sintering technique following the successful preparation of (Ti_0.2V_0.2Mo_0.2Nb_0.2Ta_0.2)C and (Ti_0.2Cr_0.2Mo_0.2Nb_0.2Ta_0.2)C five-principal-component HETMC aerogels. The synthesized 6-HETMC aerogel exhibited a homogeneous microstructure with grain phases and pores of 100–300 nm and 0.2–10 μm, respectively, a density of 0.45 g cm⁻³, a high porosity and compressive strength of 94.5% and 0.8 MPa, respectively, a low thermal conductivity of 0.128 W (m K)⁻¹ at 298 K, and a good high-temperature stability at least up to 1673 K in Ar. This research provided a novel strategy for future development of HETMC ceramic aerogels for high-temperature applications.     

Optimized piezoelectric properties and temperature stability in PSN-PMN-PT by adjusting the phase structure and grain size

For relaxor ferroelectric materials, improving the piezoelectric properties and temperature stability simultaneously is still a great challenge up to now. In this work, the structure, electric properties, and thermal stability of xPSN-(1 − x)PMN-0.4PT (x = 0.15, 0.29, 0.43, and 0.5) ceramics were studied systematically by experiment and phase field simulation. A high Curie temperature T_c of 255℃ and good longitudinal electricmechanical coupling factor k₃₃ of 0.75 and piezoelectric constant d₃₃ of 650 pC/N are achieved in x = 0.43 ceramics with monoclinic C and tetragonal phases coexistence at room temperature. At 30℃, this composition ceramics sintered at 1260℃ shows the remnant polarization P_r and coercive field E_c are about 36.8 µC/cm² and 8.2 kV/cm respectively. Moreover, as the temperature increases to 150℃, these values remain as high as 22.6 µC/cm² and 5.7 kV/cm. In the temperature range of 30–230℃, the variation of k₃₃ and d₃₃ is about 24% and 25%. These high piezoelectric performance and superior temperature stability are related to the more complex domain structures caused by phase coexistence and larger grains with more stable domain structure due to internal stress. The former is beneficial in improving the piezoelectric properties, and the latter dominates the enhanced temperature stability.     

Processing of dense high-entropy boride ceramics

Dense (Hf_0.2,Zr_0.2,Ti_0.2,Ta_0.2,Nb_0.2)B₂ high-entropy ceramics with high phase purity were produced by two-step spark plasma sintering of precursor powders synthesized by boro/carbothermal reduction of oxides. The reacted powders had low oxygen (0.404 wt%) and carbon (0.034 wt%) contents and a sub-micron average particle size (∼0.3 μm). Powders were synthesized by optimizing the excess B₄C content of the reaction mixture and densified by a two-step spark plasma sintering process. The relative density increased from 98.9% to 99.9% as the final sintering temperature increased from 2000 °C to 2200 °C. The resulting ceramics were nominally single-phase (Hf,Zr,Ti,Ta,Nb)B₂ with oxygen contents as low as 0.004 wt% and carbon as low as 0.018 wt%. The average grain size increased from 2.3 ± 1.2 μm after densification at 2000 °C to 4.7 ± 1.8 μm after densification at 2100 °C, while significant grain growth occurred during sintering at 2200 °C. The high relative densities, low oxygen and carbon contents, and fine grain sizes achieved in the present study were attributed to the use of synthesized precursor powders with high purity and fine particle size, and the two-step synthesis-densification process. These are the first reported results for dense high-entropy boride ceramics with high purity and fine grain size.     

Spark plasma sintering of (5RE.2)Ta3O9 high-entropy ceramics and their thermal properties

Six rare-earth tantalate high-entropy ceramics of (5RE_.2)Ta₃O₉ (RE represents any five elements selected from La, Ce, Nd, Sm, Eu, Gd) were designed and prepared by spark plasma sintering process at 1400°C in this study. The (5RE_.2)Ta₃O₉ ceramics only consist of a single-phase solid solution with perovskite structure. Their relative densities are all above 90%, and the average grain size is in the range of 1.47–2.92 μm. The thermal conductivity of (5RE_.2)Ta₃O₉ ceramics is in 2.24–1.90 W m⁻¹ K⁻¹ (25°C–500°C), which is much lower than that of yttria-stabilized zirconia. In six samples, (La_.2Nd_.2Sm_.2Gd_.2Eu_.2)Ta₃O₉ possesses a thermal conductivity of 1.90 W m⁻¹ K⁻¹, a thermal expansion coefficient of 3.47 × 10⁻⁶ K⁻¹ (500°C), a Vickers hardness of about 7.33 GPa, and a fracture toughness of about 5.20 MPa m^1/2, which are suitable for its application as thermal barrier coatings.