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.     

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.     

Low-temperature molten salt synthesis of high-entropy carbide nanopowders

Synthesis of the powders is critical for achieving the extensive applications of high-entropy carbides (HECs). Previously reported studies focus mainly on the high-temperature (>2000 K) synthesis of HEC micro/submicropowder, while the low-temperature synthesis of HEC nanopowders is rarely studied. Herein we reported the low-temperature synthesis of HEC nanopowders, namely (Ta_0.25Nb_0.25Ti_0.25V_0.25)C (HEC-1), via molten salt synthesis for the first time. The synthesis possibility of HEC-1 nanopowders was first theoretically demonstrated by analyzing lattice size difference and chemical reaction thermodynamics based on the first-principle calculations, and then the angular HEC-1 nanopowders were successfully synthesized via molten salt synthesis at 1573 K. The as-synthesized nanopowders possessed the single-crystal rock-salt structure of metal carbides and high compositional uniformity from nanoscale to microscale. In addition, their formation mechanism was well interpreted by a classical molten salt-assisted growth.     

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.     

Low-temperature synthesis of high-entropy (Hf0.2Ti0.2Mo0.2Ta0.2Nb0.2)B2 powders combined with theoretical forecast of its elastic and thermal properties

A theoretical calculation combined with experiment was used to study high-entropy (Hf_0.2Ti_0.2Mo_0.2Ta_0.2Nb_0.2)B₂ (HEB-HfTiMoTaNb). The theoretical calculation suggested HEB-HfTiMoTaNb could be stable over a wide temperature range. Then, a novel solvothermal/molten salt-assisted borothermal reduction method was proposed to efficiently pre-disperse transitional metal atoms in a precursor and synthesize (Hf_0.2Ti_0.2Mo_0.2Ta_0.2Nb_0.2)B₂ nanoscale powders at 1573 K for 6 h, which is nearly 300 K lower than previous reports. The characterization results indicated that the as-synthesized nanoscale HEB-HfTiMoTaNb powder was hexagonal single-phase with homogeneous elements distribution and uniform size, and the oxygen content of the particles is 0.97 wt%. Simultaneously, the mechanical properties, anisotropic nature, and thermal properties of HEB-HfTiMoTaNb were investigated by density functional theory (DFT) calculations. The Cannikin's law was adopted to explain the improvement of comprehensive mechanical properties. In addition, a significant reduction of thermal conductivity was observed for HEB-HfTiMoTaNb and it only was 1/15 of the value of HfB₂. This work suggests a reliable technique for synthesis of nanosized HEB powders and discovery of high-entropy materials under the guidance of first-principle theory.     

高熵(Zr,Hf,Nb,Ta)C微米长方体的制备、吸波性能和抗氧化性研究

针对高熵碳化物制备困难,本文采用ZrC、HfC、NbC和TaC粉为原料,Ni粉为熔剂,通过低温无压烧结工艺成功制备出三种不同成分的高熵(Zr, Hf, Nb, Ta)C粉体。结果表明,三种粉体均为微米长方体,且暴露(100)晶面。(Zr_1/4Hf_1/4Nb_1/4Ta_1/4)C微米长方体因具有高介电常数而展现出优异的吸波性能,在厚度为3.5 mm、频率为6.16 GHz时,最低反射损耗值可达-48.86 dB。高熵(Zr, Hf, Nb, Ta)C微米长方体在800～1 200℃下展示出优异的抗氧化性,且氧化产物均由正交相(Nb_xTa_1-x)₂O₅固溶体、单斜相(Zr_xHf_1-x)O₂固溶体和正交相HfO₂所组成,与氧化温度和过渡金属的物质的量比无关。Zr、Hf、Nb和Ta的协同作用导致其氧化机制与单组元碳化物截然不同,Hf的存...     

Synthesis and characterization of (Zr1/3Nb1/3Ti1/3)C metal carbide solid-solution ceramic

(Zr_1/3Nb_1/3Ti_1/3)C metal carbide solid-solution ceramic has been successfully fabricated by hot pressing sintering at 2473 K using ZrC, NbC, and TiC powders as raw materials. The results show that the as-prepared solid-solution ceramic possesses a single rock-salt crystal structure of metal carbides and simultaneously exhibits high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, it shows relatively high hardness of 38.8 ± 4.4 Gpa and elastic modulus of 481.8 ± 31.0 Gpa and relatively low thermal conductivity of 17.1 ± 0.3 W/(m·K) and thermal diffusivity of 6.1 ± 0.1 mm²/s, which may attribute primarily to the presence of solid solution effects.     

Characterization of early transition metal carbides and nitrides by NEXAFS

Powder materials of a series of early transition metal (groups 4–6B) carbides and nitrides, including TiC, VC, NbC, Mo₂C, WC, TiN, VN and Mo₂N, have been characterized by nearedge X-ray absorption fine structure (NEXAFS). A comparison of the carbon and nitrogen K-edge features reveals systematic trends in the electronic properties of these materials. These results are compared to an earlier NEXAFS characterization of thin VC films produced on a single crystal V(110) surface. In addition, the NEXAFS data are also compared to existing band-structure calculations for carbides and nitrides of early transition metals.     

Effects of (Ti,Ta,Nb,W)(C,N) on the microstructure,mechanical properties and corrosion behaviors of WC-Co cemented carbides

Homogeneous solid-solution (Ti, Ta, Nb,W)(C,N) powders were synthesized through carbothermal reduction-nitridation method. The effects of (Ti, Ta, Nb,W)(C,N) powders on the microstructure, mechanical properties and corrosion resistance of WC-10Co cemented carbides were investigated using XRD, SEM, electrochemical test and mechanical properties tests. The results showed that cemented carbides with pre-alloyed powder addition had a similar microstructure appearance: weak core/rim structure consisting of solid-solution phase embedded in the WC-Co system. The black core and gray rim, both of which contained similar elements, were identified as (Ti, Ta, Nb,W)(C,N), but the latter contained higher amount of heavy elements.With the addition of (Ti, Ta, Nb,W)(C,N) powders, the density, transverse rupture strength and fracture toughness of samples decreased monotonously. However, the hardness rose sharply at first, reached a peak at 15 wt% solid-solution addition, then slightly decreased, and finally increased again. Results also revealed that increasing (Ti, Ta, Nb,W)(C,N) made the open circuit potential (OCP) in 1 M sulphuric acid solution more negative than that of WC-Co, and all specimens exhibited pseudo-passivation phenomenon in the test solution. In addition, increasing pre-alloyed powders led to decreasing corrosion current density, which implies that (Ti, Ta, Nb,W)(C,N) could remarkably improve the corrosion resistance of WC-Co cemented carbides.     

One-step synthesis of coral-like high-entropy metal carbide powders

The synthesis of high-entropy metal carbide powders is critical for implementing their extensive applications. However, the one-step synthesis of high-entropy metal carbide powders is rarely studied. Herein, the synthesis possibility of high-entropy metal carbide powders, namely (Zr_0.25Ta_0.25Nb_0.25Ti_0.25)C (ZTNTC), via one-step carbothermal reduction was first investigated theoretically by analyzing chemical thermodynamics and lattice size difference based on the first-principle calculations, and then the ZTNTC powders with particle size of 0.5-2 μm were successfully synthesized experimentally. The as-synthesized powders not only had a single rock-salt crystal structure of metal carbides, but also possessed high-compositional uniformity from nanoscale to microscale. More interestingly, they exhibited the distinguished coral-like morphology with the hexagonal step surface, whose growth was governed by a classical screw dislocation growth mechanism.     

Partial oxidation of methane to synthesis gas over some titanates based perovskite oxides

Ca_1–x - _x Sr _x TiO₃-based mixed oxide catalysts containing chromium, iron, cobalt or nickel were prepared and used in the oxidation of methane. The catalyst containing cobalt or nickel showed high activity for the synthesis gas production from methane. In the case of nickel containing catalyst, nickel oxide originally separated from the perovskite structure was easily reduced to nickel metal, which showed synthesis gas production activity. In the case of the cobalt containing catalyst, pretreatment with methane was required for high activity. Reduced metallic cobalt was formed from the perovskite structure, which revealed relatively high selectivity for the oxidative coupling of methane, and afforded synthesis gas production. Both the catalysts also catalyzed carbon dioxide reforming of methane and especially both high activity and selectivity were observed over the nickel containing catalyst.     

Effects of Nb/TiC/TaC/VC and Co addition on the microstructure and properties of WC-based cemented carbides

A series of WC-based cemented carbides with Nb/TiC/TaC/VC and Co was prepared through spark plasma sintering (SPS) at a low sintering temperature of 1300°C, and their microstructures and mechanical properties were investigated. The nonstoichiometric multicomponent carbide Nb/TiC/TaC/VC with a rock-salt structure ($Fm\overline{3}m$) has a high atomic solution capacity. In the sintering process, partial WC and Co may dissolve in Nb/TiC/TaC/VC. With a high concentration of carbon vacancies, Nb/TiC/TaC/VC plays a beneficial role as a mass transfer intermediary. Good mass transfer facilitates the formation of a more accommodating and stable bonding between WC, Nb/TiC/TaC/VC, and Co, thereby preserving the hardness of the sintered bulks and preventing the initiation and propagation of cracks. When 6 wt.% Nb/TiC/TaC/VC and 4 wt.% Co are added to WC, the sintered bulk with fine grains exhibits superior hardness (23.27 ± .63 GPa) and toughness (10.45 ± .56 MPa·m^1/2).     

The effect of Nb or Ta substitution into the M1 phase of the MoV(Nb,Ta)TeO selective oxidation catalyst

We use aberration corrected high-angle annular dark field (HAADF) imaging to systematically study, atomic column by atomic column, the effects of substituting Nb or Ta into the M1 phase of the MoV(Nb,Ta)TeO propane (amm)oxidation catalyst. The HAADF results indicate that the x,y coordinates of the metal sites within the M1 framework are unaffected by the substitution of either Nb or Ta for Mo. The HAADF analysis of the Ta-substituted catalyst demonstrated that the Ta preferentially substitutes into the pentagonal bipyramidal site, and by analogy, we anticipate that Nb substitutes similarly. Compositional analysis of the entire framework suggests that Ta/Nb behaves as a director of V among the octahedra that link the pentagonal rings, and the variable V occupancy may be correlated with variations in catalytic activities and selectivities. Finally, HAADF imaging provided evidence of coexistence of Ta-rich and Ta-poor domains. Similar phase segregation behavior may be present in Nb-substituted specimens, but would be very difficult to detect.     

Ultrafast high-temperature synthesis and densification of high-entropy carbides

Recently, high-entropy carbides have attracted great attention due to their remarkable component complexity and excellent properties. However, the high melting points and low self-diffusion coefficients of carbides lead to the difficulties in forming solid solution and sintering densification. In this work, six dense multicomponent carbides (containing 5–8 cations) were prepared by a novel ultrafast high-temperature sintering (UHS) technique within a full period of 6 min, and three of them formed a single-phase high-entropy solid solution. The solid solubility of the UHSed multicomponent carbides was highly sensitive to the compositional variation. The presence of Cr₃C₂ liquid had significant contributions to the formation of solid solution and the densification of multicomponent carbides. All UHSed multicomponent carbides exhibited high hardness, which, unexpectedly, did not simply increase with increasing number of the components. The highest nanohardness with a value of 36.6 ± 1.5 GPa was achieved in the (Ti_1/5Cr_1/5Nb_1/5Ta_1/5V_1/5)C_x high-entropy carbide. This work is expected to expedite the development of high-entropy carbides and broaden the application of UHS in the synthesis and densification of advanced ceramics.     

Compositional disorder and sintering of entropy stabilized (Hf,Nb,Ta,Ti,Zr)B2 solid solution powders

Entropy-stabilized (Hf,Nb,Ta,Ti,Zr)B₂ solid solution powders produced by a carbo/boro-thermal reduction followed by solid solution formation were first analysed by synchrotron radiation x-ray diffraction, and their long range periodicity (i.e. lattice parameters) as well as the micro-strain intended as lattice disorder were quantitatively determined. A model to describe the micro-strain was proposed. The as-synthesized (Hf,Nb,Ta,Ti,Zr)B₂ solid solution powders were then hot-pressed at 2200 K and 50 MPa until near full densification was achieved. The hot-pressed material had a residual micro-porosity of 1.3 vol.% and consisted of a (Hf,Nb,Ta,Ti,Zr)B₂ ceramic matrix, 0.3-1 μm grain size range, and of a residual 10 vol.% B₄C particulate component, grain size in the range 0.2-2 μm. B₄C was a side product of the former synthesis and, after hot-pressing, remained trapped along the grain boundaries of the primary (Hf,Nb,Ta,Ti,Zr)B₂ solid solution ceramic matrix. Micro-hardness HV_0.2 = 22.7 ± 1.9 GPa for 1.96 N applied force was measured.     

Ultra-high temperature oxidation resistance of a novel (Mo,Hf, W,Ti)Si2 ceramic coating with Nb interlayer on Ta substrate

In order to solve the challenge of recyclability of tantalum substrates in high temperature oxidation environments, a novel MoSi₂-WSi₂-HfSi₂-TiSi₂ composite ceramic coating containing an Nb interlayer was prepared on the surface of tantalum substrate by a three-step method. The mix ceramic silicide coating exhibited superior performance and effective protection for 10.2 h at 1800 °C, possibly due to the formation of an outer SiO₂-HfO₂-HfSiO₄ composite oxide film with low oxygen permeability, moderate viscosity and thermal expansion coefficient, as well as good self-healing ability. Furthermore, the coating successfully passed 537 thermal cycles from room temperature to 1800 °C. The presence of Nb interlayer significantly mitigated the thermal mismatch between the ceramic coating and the tantalum substrate, and the bidirectional diffusion of Nb element during the high temperature oxidation and thermal shock process further reduced the tendency of the coating to crack.     

The role of Cr addition on the processing and mechanical properties of high entropy carbides

Group VI transition metals do not form room temperature stable carbides with a rock salt structure, however, they can be incorporated into a rock salt high entropy carbide lattice. Novel 5-metal high entropy carbides (Cr, Zr, Nb, Hf, Ta)C (HEC5-Cr) were produced using spark plasma sintering and compared with 4-metal carbide (Zr_0.25Nb_0.25Hf_0.25Ta_0.25)C (HEC4) and 8-metal carbide containing Cr (HEC8-Cr). The HEC5-Cr ceramics had higher density and smaller grain size (~14 µm) compared with HEC4 (~28 µm). The solubility limit of Cr on the metal site increased from ~2.5 at% for HEC5-Cr to ~6.0 at% for HEC8-Cr, implying that the high entropy effect increased the solubility of Cr. A significant Cr enrichment was observed at the grain boundaries of HEC5-Cr, and it showed a ~14% increase in nanohardness and a similar indentation modulus compared with HEC4. The nanohardness of HEC5-Cr was up to 41.2 GPa due to increased solid solution strengthening.     

Pressureless sintering and properties of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramics: The effect of pyrolytic carbon

A novel (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy ceramic was successfully prepared by pressureless sintering at 2200 °C. With increasing content of resin-derived-carbon, the density, and mechanical and thermal properties increased up to a maximum content of 2～4 wt% resin addition, after which further addition was detrimental. All specimens showed high strength (≥347±36 MPa), with the highest value achieving 450±64 MPa, and fracture toughness significantly higher (>20 %) than those of the corresponding monocarbides and Ta_0.5Hf_0.5C, (Ta_1/3Zr_1/3Nb_1/3)C. The thermal conductivity was approximately equivalent to the lowest value of the corresponding mono-carbides, which was assumed to be due to the lattice distortion effect.     

Unveiling enhanced oxidation resistance and mechanical integrity of multicomponent ultra-high temperature carbides

The development of a new class of multicomponent ultra-high temperature ceramics (MC-UHTCs), often referred to as high-entropy UHTCs, has gained increased interest due to the possibility of improved thermomechanical and oxidation properties. In this study, a systematic approach by gradual addition in the UHTC components ranging from a binary to a dense quaternary (Ta,Nb,Hf,Ti)C is synthesized using spark plasma sintering (SPS). The solid solutioning was the critical factor in homogenizing the composition in the multicomponent system. The segregation of NbC and HfC was seen in binary and ternary UHTC systems, while a single-phase homogeneity was observed in the quaternary UHTC improving its hardness up to 34.8 GPa. The presence of closely spaced slip lines in the MC-UHTCs enhances resistance to indentation damage up to 72% at an applied load of 200 N. The formation of complex mixed oxide phase of Hf₆Ta₂O₁₇ ensued in the lower to negligible oxidation even up to 3 min of plasma exposure with temperature exceeding 2800°C. In sum, though the entropy remains medium (0.96R) for the selected system, the quaternary UHTC system undoubtedly has significantly better thermomechanical performance when compared to established baseline UHTCs. This raises the debate on the justification for calling a multicomponent system a “high entropy” to be seen in a new light. The developed MC-UHTCs elicits the paradigm of this new class of UHTCs expanding their potential in thermal protection systems for hypersonic applications.     

过渡金属改性Ce-Zr固溶体催化氧化丙烯和CO

丙烯和CO是大气中主要的有害气体,需有效去除。选取Cu,Co,Ni,Fe为活性组分,采用共沉淀法制备过渡金属M(M=Cu,Co,Ni,Fe)与Ce+Zr+M原子比为1∶40的复合氧化物催化剂(M-CZO)催化燃烧两种有害气体,并采用XRD,BET,TPR对催化剂进行了表征。结果表明,过渡金属改性Ce-Zr固溶体对两种气体催化活性相对CZO大大提高。其中催化氧化CO时,活性顺序为Cu>Co>Ni>Fe,Cu-CZO活性最高,T50和T90分别为95℃和185℃。H2-TPR结果表明,表面氧越活泼,催化氧化CO的活性越高。C3H6催化氧化活性顺序为Co>Cu>Fe>Ni,随着温度升高,CO相对生成量增加,Cu-CZO催化氧化CO活性远高于其他三种催化剂,使C3H6→CO→CO2连续反应快速进行,从而使C3H6总转化率提高,导致Co-CZO的T90比Cu-CZO略高。整体看来,Cu-CZO催化燃烧C3H6效果最好,T50和T90分别为345℃和376℃,CO的浓度低于0.05 mg/L,二次污染最小。