Ablation behavior of (Hf–Ta–Zr–Nb–Ti)C high-entropy carbide and (Hf–Ta–Zr–Nb–Ti)C–xSiC composites

The ablation behavior of (Hf–Ta–Zr–Nb–Ti)C high-entropy carbide (HEC-0) was investigated using a plasma flame in air for different times (60, 90, and 120 s) at about 2100°C. The effect of SiC content on the ablation resistance of HEC–xSiC composites (x = 10 and 20 vol%) was also studied. The linear ablation rate of HEC-0 decreases with increasing ablation time, showing the positive role of the oxide layer with a complex composition. The linear ablation rate of HEC–10 vol% SiC (0.3 µm s⁻¹) is only a 10th of that of HEC-0, showing a significant improvement in ablation resistance, probably due to the formation of a protective oxide layer containing melted SiO₂ and refractory Hf–Zr–Si–O oxides.     

Oxidation behavior of high-entropy carbide (Hf0.2Ta0.2Zr0.2Ti0.2Nb0.2)C at 1400–1600 °C

Oxidation behavior of high-entropy carbide (Hf_0.2Ta_0.2Zr_0.2Ti_0.2Nb_0.2)C (HTZTNC) was investigated over temperature range of 1400–1600 °C. Results showed improved oxidation resistance of high-entropy carbide compared with individual carbide ceramics. In oxide layer, Ta₂O₅ and Nb₂O₅ were found to be dominant phases at 1400 °C, whereas ZrTiO₄ and HfTiO₄ were main phases obtained at 1500 and 1600 °C. Moreover, these complex dense oxide layer structures on the surface of HTZTNC at high temperature led to excellent oxidation resistance. The observation of Ti-depleted layer at 1500 and 1600 °C after 20 min of oxidation indicated that oxidation mechanism involved outward diffusion of titanium oxide, which was further confirmed by reoxidation experiments. In sum, these findings are promising for future development of high-entropy ultrahigh temperature ceramics with good oxidation resistance.     

(Ti0.2V0.2Cr0.2Nb0.2Ta0.2)2AlC–(Ti0.2V0.2Cr0.2Nb0.2Ta0.2)C high-entropy ceramics with low thermal conductivity

In recent years, the microstructure and physicochemical properties of high-entropy ceramics have received much interest by the combination of multiple principal elements. Herein, (Ti_0.2V_0.2Cr_0.2Nb_0.2Ta_0.2)₂AlC–(Ti_0.2V_0.2Cr_0.2Nb_0.2Ta_0.2)C high-entropy ceramics (M₂AlC-MC HECs) were prepared by the spark plasma sintering (SPS) technique, attributing to the structural and chemical diversity of MAX phases. The microstructure of M₂AlC-MC HECs was characterized from micron to atomic scales, and the phase composition of M₂AlC-MC HECs was analyzed by a combination of Maud and Rietveld analysis. The results indicate the successful solid solution of Ti, V, Cr, Nb, and Ta atoms in the M-site of the 211-MAX configuration, and all the samples show a classic layered structure. The weight percentage of (Ti_0.2V_0.2Cr_0.2Nb_0.2Ta_0.2)₂AlC in the M₂AlC-MC HECs was more than 90%. Furthermore, the thermoelectric properties of M₂AlC-MC HECs were investigated for the first time in this study, and the electrical conductivity and thermal conductivity of HECs are 3278 S cm⁻¹ and 2.78 W m⁻¹ K⁻¹at 298 K, respectively.     

(Ti,Mo,W,Ta,V,Nb)(C,N)多元陶瓷相的价电子结构

运用固体与分子经验电子理论(EET理论)计算了(Ti,Mo,W,Ta,V,Nb)(C,N)多元陶瓷相的价电子结构.结果表明,价电子结构参数(nA)随碳化物添加量的增加而增加.不同碳化物对价电子结构参数的影响不同,其中VC的影响最为显著.价电子结构参数(nA)可以用来评价金属陶瓷的力学性能,提出了相关的判据关系式.     

Pressureless sintering of dual-phase,high-entropy boride–carbide ceramics

Dual-phase, high-entropy boride–carbide ceramics were densified by pressureless sintering. Relative densities up to approximately 96% were obtained for ceramics containing about 30 vol% high-entropy boride and 70 vol% high-entropy carbide. Isostatic pressing at 200 MPa resulted in higher relative densities of both the green bodies and final ceramics compared to uniaxial pressing. The highest relative density of 96.3% was achieved for a ceramic that was isostatically pressed at 200 MPa and sintered at 2300°C for 2 h. Grain sizes of the resulting ceramics were approximately 2 µm. This is the first report of pressureless sintering of dual-phase, high-entropy boride–carbide ceramics.     

Fabrication of high-entropy carbide (TiZrHfTaNb)С by high-energy ball milling

In this study, we report on the process to fabricate powder of high-entropy carbide (TiTaNbZrHf)С. We have obtained a high-entropy carbide phase by two-step high-energy ball milling (HEBM) of metallic precursors and carbon. At the first stage of HEBM, a powder of high-entropy alloy TiTaNbZrHf was prepared. 15 min of HEBM were found to be adequate to converse the initial metallic precursors into a bcc solid solution. High-entropy carbide was fabricated at the second stage of HEBM from TiTaNbZrHf high-entropy alloy and carbon. The formation of the fcc carbide phase under HEBM occurs in two stages within 15–30 min. The first one is reaction diffusion of carbon into a metal matrix forming an intermediate bcc solid solution of TiTaNbZrHf + C followed by its second stage where a bcc structure transforms into an fcc one. The fabricated powder of high-entropy carbide (TiTaNbZrHf)С demonstrates thermal stability up to 1200 °C during 6 h.     

Strength of single-phase high-entropy carbide ceramics up to 2300°C

The mechanical properties of single-phase (Hf,Zr,Ti,Ta,Nb)C high-entropy carbide (HEC) ceramics were investigated. Ceramics with relative density >99% and an average grain size of 0.9 ± 0.3 µm were produced by a two-step process that involved carbothermal reduction at 1600°C and hot pressing at 1900°C. At room temperature, Vickers hardness was 25.0 ± 1.0 GPa at a load of 4.9 N, Young's modulus was 450 GPa, chevron notch fracture toughness was 3.5 ± 0.3 MPa·m^1/2, and four-point flexural strength was 421 ± 27 MPa. With increasing temperature, flexural strength stayed above ~400 MPa up to 1800°C, then decreased nearly linearly to 318 ± 21 MPa at 2000°C and to 93 ± 10 MPa at 2300°C. No significant changes in relative density or average grain size were noted after testing at elevated temperatures. The degradation of flexural strength above 1800°C was attributed to a decrease in dislocation density that was accompanied by an increase in dislocation motion. These are the first reported flexural strengths of HEC ceramics at elevated temperatures.     

Ablation behavior of high-entropy carbides ceramics (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C upon exposition to an oxyacetylene torch at 2000 °C

The ablation performance of a high-entropy ceramic carbide, (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C, was performed by oxyacetylene ablation flame, simulating the extreme service environment at 2000 ºC. Phase and microstructure characterization at multi-length scales was carried out. During ablation, a compositionally and microstructurally complex oxidation layer formed on the ablation surface, which consisted of a combination of (Zr_xHf_1?x)₆(Nb_yTa_1?y)₂O₁₇, Ti(Nb_xTa_1?x)₂O₇, and Ti_x(Zr_aHf_bNb_cTa_1?a-b-c)_1?xO₂. Based on the microstructure information, the ablation mechanisms were proposed considering the oxidation thermodynamics and kinetics. Comparable rates of O inward diffusion and Ti outward diffusion are suggested, and a particular innermost dense layer composed of isolated (Zr_xHf_1?x)₆(Nb_yTa_1?y)₂O₁₇ grains embedded in a continuous Ti(Nb_xTa_1?x)₂O₇ matrix is considered to be beneficial for a better ablation resistance.     

Multifunctionality of Active Centers in (Amm)oxidation Catalysts: From Bi–Mo–O x to Mo–V–Nb–(Te, Sb)–O x

Catalytic centers in selective (allylic) oxidation and ammoxidation catalysts are multimetallic and multifunctional. In the historically important bismuth molybdates, used for propylene (amm)oxidation, they are composed of (Bi³⁺)(Mo⁶⁺)₂ complexes in which the Bi³⁺ site is associated with the -H abstraction and the (Mo⁶⁺)₂ site with the propylene chemisorption and O or NH insertion. An updated reaction mechanism is presented. In the Mo–V–Nb–Te–O _x systems, three crystalline phases (orthorhombic Mo_7.5V_1.5NbTeO₂₉, pseudohexagonal Mo₆Te₂VO₂₀, and monoclinic TeMo₅O₁₆) were identified, with the orthorhombic phase being the most important one for propane (amm)oxidation. Its active centers contain all necessary key catalytic elements (2V⁵⁺/Mo⁶⁺, 1V⁴⁺/Mo⁵⁺, 2Mo⁶⁺/Mo⁵⁺, 2Te⁴⁺) for this reaction wherein a V⁵⁺ surface site (V⁵⁺ = O ⁴⁺V–O) is associated with paraffin activation, a Te⁴⁺ site with -H abstraction once the olefin has formed, and a (Mo⁶⁺)₂ site with the NH insertion. Four Nb⁵⁺ centers, each surrounded by five molybdenum octahedra, stabilize and structurally isolate the catalytically active centers from each other (site isolation), thereby leading to high selectivity of the desired acrylonitrile product. A detailed reaction mechanism of propane ammoxidation to acrylonitrile is proposed. Combinatorial methodology identified the nominal composition Mo_0.6V_0.187Te_0.14Nb_0.085O _x for maximum acrylonitrile yield from propane, 61.8% (86% conversion, 72% selectivity at 420 °C). We propose that this system, composed of 60% Mo_7.5V_1.5NbTeO₂₉, 40% Mo₆Te₂VO₂₀, and trace TeMo₅O₁₆, functions with a combination of compositional pinning of the optimum orthorhombic Mo_7.5V_1.5±x Nb_1±y Te_1±z O_29± phase and symbiotic mop-up of olefin intermediates through phase cooperation. Under mild reaction conditions, a single optimum orthorhombic composition might suffice as the catalyst; under demanding conditions this symbiosis is additionally required. Improvements in catalyst performance could be attained by further optimization of the elemental distributions at the active catalytic center of Mo_7.5V_1.5NbTeO₂₉, by promoter/modifier substitutions, and incorporation of compatible cocatalytic phases (preferably epitaxially matched). High-throughput methods will greatly accelerate the rational catalyst design processes.     

Creep behavior of a zirconium diboride–silicon carbide composite

Flexural creep studies of ZrB₂–20 vol% SiC ultra-high temperature ceramic were conducted over the range of 1400–1820 °C in an argon shielded testing apparatus. A two decade increase in creep rate, between 1500 and 1600 °C, suggests a clear transition between two distinct creep mechanisms. Low temperature deformation (1400–1500 °C) is dominated by ZrB₂ grain or ZrB₂–SiC interphase boundary and ZrB₂ lattice diffusion having an activation energy of 364 ± 93 kJ/mol and a stress exponent of unity. At high temperatures (>1600 °C) the rate-controlling processes include ZrB₂–ZrB₂ and/or ZrB₂–SiC boundary sliding with an activation energy of 639 ± 1 kJ/mol and stress exponents of 1.7 < n < 2.2. In addition, cavitation is found in all specimens above 1600 °C where strain-rate contributions agree with a stress exponent of n = 2.2. Microstructure observations show cavitation may partially accommodate grain boundary sliding, but of most significance, we find evidence of approximately 5% contribution to the accumulated creep strain.     

Thermophysical properties of the most refractory carbide Ta0.8Hf0.2C under high temperatures (2000–5000 K)

We studied the carbide Ta_0.81Hf_0.19C_0.94 in the form of a thin deposited layer (1 μm) under rapid heating (5 μs) by electric current pulse. We obtain the properties of this carbide: melting heat; specific heat, and enthalpy in the solid phase from 2000 K up to T_melt = 4300 K, and for the first time in the liquid state up to 5000 K. We measured the temperature by high-speed radiation pyrometer calibrated against the tungsten temperature lamp. Here, we confirm the earlier obtained experimental data on the melting temperature of this compound obtained by black body model and thus, we confirm that this compound has the maximum melting temperature as against the other refractory carbides (TaC and HfC). Under fast heating, the steep increase of the specific heat of the carbide Ta_0.81Hf_0.19C_0.94 for 400 K before melting was observed.     

A vibrational study of the activation sequence of C–H and C–C bonds of isobutene and 1-butene on Mo(110) and (4 × 4)-C/Mo(110) surfaces

The thermal decomposition pathways of isobutene and 1-butene on both Mo(110) and 4 × 4-C/Mo(110) surfaces have been studied using high-resolution electron energy loss spectroscopy (HREELS) in order to highlight the substantially different activities of these two surfaces towards the cleavage of C–H and C–C bonds. On clean Mo(110), the CH₂ group of isobutene decomposes upon heating to 150 K, producing either a /-bonded isobutenylidene [(CH₃)₂CCH] species or a 1,1-di-/-bonded isobutenyl [(CH₃)₂CC] species. Upon further heating, extensive C–H bond scission occurs to form hydrocarbon fragments which do not contain CH₃ or CH₂ groups, but appear to have largely intact carbon skeletons. By contrast, isobutene is molecularly adsorbed on the carbide-modified surface at 150 K. Further heating produces isobutylidyne [(CH₃)₂HCC] by 300 K, which subsequently decomposes via C–C bond scission to generate surface methyl groups. The different activation sequence of the C–H and C–C bonds of isobutene on clean and carbide-modified Mo(110) surfaces is also qualitatively confirmed by comparative studies of 1-butene on the two surfaces.     

Influence of Mo contents on microstructures and mechanical properties of (Ti,W,Mo)(CN)–Ni cermets

The introduction of N₂ gas during a sintering and carbothermal reduction process causes the separation of the WC phase in (Ti,W)(CN)–Ni cermet. Furthermore, the addition of secondary transition-metal carbides such as Mo₂C, VC, and TaC not only promotes phase separation but also controls grain growth by the differences in their thermodynamic stabilities. Increased N₂ flow during sintering increases the precipitation and coalescence of WC particles. The addition of Mo₂C of 0.05?mol fraction suppresses the precipitation and coalescence of WC. However, increases in both secondary carbides by >?0.05?mol fraction and the N₂ flow by >?4 kPa (≒ 30?Torr), respectively, induces significant grain growth by coalescence. Consequently, the pore levels, hardness, and fracture toughness of the specimens are substantially affected by changes in the precipitation and grain growth behaviors of the WC particles. The fracture toughness of a (Ti_0.7-xW_0.3Mo_x)(C_0.7N_0.3)–20Ni cermet, sintered under 1.33 kPa (≒ 10?Torr) N₂, is significantly enhanced from 9 to 14?MPa?m^0.5 by crack bridging and deflection.     

Microstructure of friction surface developed on carbon fibre reinforced carbon–silicon carbide (Cf/C–SiC)

We have used TEM to study the microstructure of friction surface of carbon fibre/carbon–silicon carbide composites brake discs after multi braking stop by using organic pads. A friction surface layer was developed consistently on the top of Si regions of the composites, but inconsistently on that of SiC and C. Inside the layer, amorphous silicon/silicon oxides appeared extensively with various non-metallic and metallic crystallites dispersed inside with sizes ranging from a few nanometers to several microns. A coherent interface between the friction layer and the composite surface was established under the braking conditions, whilst its sustainability varied notably in SiC and C regions. Microcracking near the friction surface appeared in SiC and C_f/C regions largely due to the extensive ductile deformation of SiC and weak interfaces between C and C_f. Material joining mechanisms were discussed to enlighten the friction transfer layer development on the surface of the composite discs.     

Mechanical behavior of zirconium diboride–silicon carbide ceramics at elevated temperature in air

The mechanical properties of zirconium diboride–silicon carbide (ZrB₂–SiC) ceramics were characterized from room temperature up to 1600 °C in air. ZrB₂ containing nominally 30 vol% SiC was hot pressed to full density at 1950 °C using B₄C as a sintering aid. After hot pressing, the composition was determined to be 68.5 vol% ZrB₂, 29.5 vol% SiC, and 2.0 vol% B₄C using image analysis. The average ZrB₂ grain size was 1.9 μm. The average SiC particles size was 1.2 μm, but the SiC particles formed larger clusters. The room temperature flexural strength was 680 MPa and strength increased to 750 MPa at 800 °C. Strength decreased to ～360 MPa at 1500 °C and 1600 °C. The elastic modulus at room temperature was 510 GPa. Modulus decreased nearly linearly with temperature to 210 GPa at 1500 °C, with a more rapid decrease to 110 GPa at 1600 °C. The fracture toughness was 3.6 MPa·m^½ at room temperature, increased to 4.8 MPa·m^½ at 800 °C, and then decreased linearly to 3.3 MPa·m^½ at 1600 °C. The strength was controlled by the SiC cluster size up to 1000 °C, and oxidation damage above 1200 °C.     

Electrochemical noise analysis on the crevice corrosion behavior of Ni–Cr–Mo–V high strength steel using recurrence plots

This article makes a study of electrochemical noise analysis on the crevice corrosion behavior of Ni–Cr–Mo–V high strength steel using recurrence plots. The crevice corrosion behavior of Ni–Cr–Mo–V high strength steel was investigated by the electrochemical noise (EN) technique and SEM observation. The experimental results reveal that the crevice corrosion could be distinguished by three stages including induction stage, transformation stage, and stable stage. While increasing the growth probability of metastable corrosion, the presence of crevice decreases the initiation rate of metastable corrosion. In the case of crevice, the metastable corrosion is easy to develop into stable one.     

Improved ablation resistance of C–C composites using zirconium diboride and boron carbide

Zirconium diboride and boron carbide particles were used to improve the ablation resistance of carbon–carbon (C–C) composites at high temperature (1500 °C). Our approach combines using a precursor to ZrB₂ and processing them with B₄C particles as filler material within the C–C composite. An oxyacetylene torch test facility was used to determine ablation rates for carbon black, B₄C, and ZrB₂–B₄C filled C–C composites from 800 to 1500 °C. Ablation rates decreased by 30% when C–C composites were filled with a combination of ZrB₂–B₄C particles over carbon black and B₄C filled C–C composites. We also investigated using a sol–gel precursor method as an alternative processing route to incorporate ZrB₂ particles within C–C composites. We successfully converted ZrB₂ particles within C–C composites at relatively low temperatures (1200 °C). Our ablation results suggest that a combination of ZrB₂–B₄C particles is effective in inhibiting the oxidation of C–C composites at temperatures greater than 1500 °C.     

Oxidation of isobutane over Mo–V–Sb mixed oxide catalyst

The catalytic performances of Mo–V–Sb mixed oxide catalysts have been studied in the selective oxidation of isobutane into methacrolein. V–Sb mixed oxide showed the activity for oxidative dehydrogenation of isobutane to isobutene. The selectivity to methacrolein increased by the addition of molybdenum species to the V–Sb mixed oxide catalyst. In a series of Mo–V–Sb oxide catalysts, Mo₁V₁Sb₁₀O_x exhibited the highest selectivity to methacrolein at 440°C. The structure analyses by XRD, laser Raman spectroscopy and XPS showed the coexistence of highly dispersed molybdenum suboxide, VSbO₄ and -Sb₂O₄ phases in the Mo₁V₁Sb₁₀O_x. The high catalytic activity of Mo₁V₁Sb₁₀O_x can be explained by the bifunctional mechanism of highly dispersed molybdenum suboxide and VSbO₄ phases. It is likely that the oxidative dehydrogenation of isobutane proceeds on the VSbO₄ phase followed by the oxidation of isobutene into methacrolein on the molybdenum suboxide phase.     

Substituted Mo–V(Ti)–Te(Ce)-oxide M2 Catalysts for Propene Ammoxidation

One of the most effective propane to acrylonitrile ammoxidation catalyst is comprised of the two phases M1 (orthorhombic) Mo_7.5V_1.5NbTeO₂₉ and M2 (pseudo-hexagonal) Mo₄V₂Te₂O₂₀. Under reaction conditions, the two phases work in symbiosis with each other where M1 is the paraffin activating component and M2 is the olefin activating component. Since the catalytic improvement of either phase should result in an enhancement of the overall acrylonitrile yield, controlled substitution of certain elements in either or both phases might result in the desired improvement. Our current study concentrates on the partial substitutions of V with Ti and Te with Ce in the M2 phase. Ti substitution results in a considerable propene activity improvement, whereas the selectivity to acrylonitrile is unaffected. Substitution with Ce, on the contrary, substantially improves the selectivity to acrylonitrile. Also, a minor improvement of the activity is notable. The acrylonitrile selectivity improvement is a result of better NH₃ utilization and comes at the expense of reduced acrolein make. XRD reveals that all of the substituted compositions retain the M2 structure and essentially are monophasic. XANES recordings show for the bulk that the Mo is 6+, the V is 4+, or 4+ and 5+ when Ce is present, the Ti is 4+, the Ce is 3+, and the Te 4+ with some 6+ also present. According to the ESR data, in the M2 with Ce (7Te/3Ce) only 21% of the V is 4+, the remainder being 5+, which tentatively can be explained by the existence of some cation vacancies in the hexagonal channels. HRTEM imaging reveals little if any differences between the materials, all have the typical pseudo-hexagonal habit of the M2 phase and expose a 1–2 nm thick surface layer without any apparent long-range ordering. XPS data show that all catalysts, including the base, are highly enriched at the surface with Te at the expense of other metals. The 7Te/3Ce composition exhibits also substantial Ce surface enrichment. Moreover, the valences of the cations at the surface differ from the bulk in that for all fresh catalysts V is 5+ and Te is 6+ on the surface. Characterization by XPS of catalysts used in propene ammoxidation, reveals reduction of Te and, except when Ce is present, also Mo. Therefore, it might be inferred that the surfaces of the catalysts studied here are comprised essentially of one or a few monolayers of TeMoO or TeCeMoO on an interacting M2 crystalline base.