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.     

High temperature oxidation behavior of porous MoAlB ceramic from 800 °C to 1100 °C in air

MoAlB possesses the characteristics of both metals and ceramic materials, which has attracted extensive attention due to its excellent high-temperature oxidation resistance. For this reason, porous MoAlB is considered applicable to the practice of filtration under harsh environment. In this study, the high-temperature oxidation behavior of porous MoAlB ceramics is systematically studied at the temperatures ranging from 800 to 1100 °C. According to the results, the porous MoAlB exhibits good oxidation resistance at a maximum temperature of 1000 °C. The oxidation kinetics of porous MoAlB can be divided into three stages, and the estimated activation energies of the three stages are 253.83 kJ·mol⁻¹, 367.48 kJ·mol⁻¹ and 317.84 kJ·mol⁻¹, respectively. In the stable stage at 1000 °C, the quadratic mass gain per unit area shows linearity over time, and the oxidation rate of porous MoAlB reaches 37.31 mg²·cm⁻⁴·h⁻¹. As revealed by the analysis of the composition and microstructure of oxide layers, the main components of the oxide layer include MoO₃, MoO₂, Al₂O₃, B₂O₃. With the extension of oxidation time, the content of Al₂O₃ in the oxide films increases. The average pore size, permeability and open pore porosity of porous MoAlB show a trend of first decreasing and then tending to be stable. In addition, a discussion is conducted on the high-temperature oxidation mechanism of porous MoAlB.     

Ablation behavior and mechanisms of 3D-Cf/Ta0.8Hf0.2C-SiC composite at temperatures up to 2500 °C

In this work, ablation behavior and mechanisms of 3D-C_f/Ta_0.8Hf_0.2C-SiC composite were studied via air plasma test at temperatures up to 2500 °C. At temperatures below 2000 °C, a continuous oxides layer composed of o-Ta₂O_5(H) – t-Ta₂O₅ – Hf₆Ta₂O_17(H) – SiO₂ is formed on the ablation surface, which turns to o-Ta₂O_5(H) – SiO₂ above 2200 °C. During ablation, o-Ta₂O_5(H) precipitates from the glassy SiO₂ and grows up following Oswald ripening. Although the volatilization of SiO₂ aggravates with ablation temperature increase and ablation time extension, o-Ta₂O_5(H) – SiO₂ melt can still serve as effective self-healing similar to SiO₂ glass. Accordingly, the multiphase oxides layer formed on the ablation surface provides a stable protective effect for the internal composite under all the tested ablation conditions. As a consequence, the 3D-C_f/Ta_0.8Hf_0.2C-SiC composite presents outstanding ablation-resistant performance even at 2500 °C for 300 s, with a linear recession rate of ～ 5.7 µm/s and a mass recession rate of 2.91 mg/s.     

Solid-state pressureless sintering of silicon carbide below 2000 °C

To date, solid-state pressureless sintering of silicon carbide powder requires sintering aids and high sintering temperature (>2100 °C) in order to achieve high sintered density (>95% T.D.). Two-step sintering (TSS) method can allow to set sintering temperature lower than that conventionally required. So, pressureless two-step sintering process was successfully applied for solid-state sintering (boron carbide and carbon as sintering additives) of commercial SiC powder at 1980 °C. Microstructure and mechanical properties of TSS-SiC were evaluated and compared to those obtained with the conventional sintering (SSiC) process performed at 2130 °C. TSS-SiC showed finer microstructure and higher flexural strength than SSiC with very similar density (98.4% T.D. for TSS-SiC and 98.6% T.D. for SSiC).     

Dielectric and Magnetic properties of (1 − x)BiFeO3–xBa0.8Sr0.2TiO3 ceramics

The polycrystalline samples of (1 ? x)BiFeO₃–xBa_0.8Sr_0.2TiO₃ (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4 and x = 1) were prepared by the conventional solid state reaction method. The effect of substitution in BiFeO₃ by Ba_0.8Sr_0.2TiO₃ on the structural, dielectric and magnetic properties was investigated. X-ray diffraction study showed that these compounds crystallized at room temperature in the rhombohedral distorted perovskite structure for x ≤ 0.3 and in cubic one for x = 0.4. As Ba_0.8Sr_0.2TiO₃ content increases, the dielectric permittivity increases. This work suggests also that the Ba_0.8Sr_0.2TiO₃ substitution can enhance the magnetic response at room temperature. A remanent magnetization M_r and a coercive magnetic field H_C of about 0.971 emu/g and 2.616 kOe, respectively were obtained in specimen with composition x = 0.1 at room temperature.     

A new type of Sr(Zr0.2Hf0.2Ce0.2Yb0.2Me0.2)O3−x (Me = Y,Gd) high-entropy ceramics used in thermal barrier coatings

Seeking for new ceramics with excellent thermophysical properties as thermal barrier coatings candidate materials has become a hot research field. In this study, Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x high-entropy ceramic powders were successfully synthesized by the method of solid-state reaction, and the ceramics with single phase were prepared by pressureless sintering at 1600°C. The phase composition, microstructure, element distribution, high-temperature thermal stability, and thermophysical properties of the ceramics were studied. The results showed that Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x ceramics were composed of SrZrO₃ phase and the second phase of AB₂O₄ spinel (i.e., SrY₂O₄ and SrGd₂O₄). The content of the second phase was gradually increased after heat treatment at 1400°C, which significantly improved the thermophysical and mechanical properties of the ceramics. The microhardness and fracture toughness of the ceramics were improved compared with that of SrZrO₃. The thermal conductivities of Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x (Me = Y, Gd) ceramics were 1.30 and 1.28 W m⁻¹ K⁻¹ at 1000°C, which were about 35% and 40% lower than that of SrZrO₃ (1.96 W m⁻¹ K⁻¹) and yttria-stabilized zirconia (2.12 W m⁻¹ K⁻¹), respectively. The thermal expansion coefficients of Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x (Me = Y, Gd) ceramics were 12.8 × 10⁻⁶ and 14.1 × 10⁻⁶ K⁻¹ at 1300°C, respectively, which was more closer to the superalloys compared with SrZrO₃ ceramic (11.0 × 10⁻⁶ K⁻¹).     

Oxidation behavior of ZrB2-SiC-ZrC at 1700 °C

The oxidation behaviors of four compositions of ZrB₂-SiC-ZrC and one composition of ZrB₂-SiC were studied at 1700 °C in air and under low oxygen partial pressure. Volatility diagrams for ZrB₂-SiC-ZrC and ZrB₂-SiC were used to thermodynamically elucidate the oxidation mechanisms. SiO₂ and ZrO₂ layers formed on the surfaces of ZrB₂-SiC-ZrC and ZrB₂-SiC oxidized at 1700 °C. A SiC-depleted layer only formed on the surface of the ZrB₂-SiC oxidized under low oxygen partial pressure. The oxide layer thickened with increasing ZrC volume content during oxidation in air and under low oxygen partial pressure. The ZrB₂-SiC-ZrC oxide surface exploded in air when the ZrC volume content was more than 50%. Under low oxygen partial pressure, the oxide surfaces of all the ZrB₂-SiC-ZrC specimens bubbled.     

(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.     

Heteroepitaxial ZnO/sapphire (0 0 0 1) structure prepared by sol–gel process

ZnO thin films were grown on sapphire (0 0 0 1) substrates by sol–gel process and their structural and optical properties were characterized in detail. High-quality texture was obtained by using precursor solution of zinc acetate and ethanolamine in 2-methoxyethanol, pyrolyzed at 300 °C, then heated at 500 °C, and finally annealed at 750 °C. Highly c-axis oriented ZnO films were confirmed by X-ray θ–2θ scan. A relatively high transmittance in the visible spectra range and clear absorption edge of the film were observed. Epitaxial relationship between ZnO and sapphire and photoluminescence of the film were examined by using a X-ray pole-figure analysis and He–Cd laser. Near-band-edge emission with a deep-level emission was observed.     

Oxidation behavior characteristics of an aluminized Ni-based single crystal superalloy CM186LC between 900 °C and 1100 °C in air

Oxidation behavior of an aluminized Ni-based single crystal superalloy CM186LC was performed between 900 °C and 1100 °C in air. The oxidation kinetics approximately followed a parabolic oxidation law at 900 °C and 1100 °C. The mass gains were significantly increased owing to the formation of θ-Al₂O₃ during initial oxidation stage. After 100 h oxidation, the mass gain rates were then decreased due to the transformation from θ-Al₂O₃ to α-Al₂O₃. The microstructures after 500 h oxidation at all temperatures generally consisted of scale, coating layer, interdiffusion zone (IDZ), substrate diffusion zone (SDZ) accompanied with the topologically close-packed (TCP) and substrate.     

Liquid–Liquid Equilibria of Ternary Systemscis-1,2-Dimethylcyclohexane + Toluene + Sulfolane

Liquid–Liquid Equilibrium (LLE) data for three Ternary Systems comprising cis-1,2-dimethylcyclohexane + toluene + sulfolane were measured at 298.15, 313.15 and 328.15 K under atmospheric pressure. The phase diagrams for the ternary systems were presented. The reliability of the experiment data was tested using the Othmer–Tobias correlation. The LLE data were then correlated with the universal functional activity coefficient for liquid–liquid systems (UNIF-LL) and non-random two liquid using dataset 2 (NRTL/2) activity coefficient models to obtain the binary interaction parameters as programmed by the Aspen Plus simulation. The results showed that the experimental data were satisfactorily represented by both the UNIF-LL and the NRTL/2 models as revealed from the very small values of the root mean square error and the absolute deviation in composition.     

Synthesis of hexagonal boron nitride fibers within two hour annealing at 500 °C and two hour growth duration at 1000 °C

Several advantageous features like low density, large surface area, high porosity and tight pore size make the nanofiber suitable for a wide range of applications from medical to consumer products and industrial to high-tech applications. Present study concerns the synthesis of hexagonal boron nitride fibers (Ø=70–350 nm) from a mixture of Boron, Magnesium oxide and Iron oxide powders via a simple CVD technique. A relatively long annealing and growth duration of two hours at 500 °C and 1000 °C, respectively were utilized in this synthesis. The synthesized samples seem to have the BNFs of irregular curved human hair-like morphologies in lower resolution and solid cylinder-like structures in high resolution transmission electron microscopy. The presence of Boron and Nitrogen in the synthesized BNF's sample were confirmed via the B 1s peak at 190.7 eV and N 1s peak at 398.3 eV in the XPS survey whereas a major peak at 1370 (cm⁻¹) in the Raman spectrum corresponds to the vibration of E_2g mode in h-BN. The sharp peaks in the XRD pattern verify the h-BN phase and highly crystalline nature of the synthesized BNFs.     

Demixing pressures of hydroxy-terminated poly(dimethylsiloxane)–carbon dioxide binary mixtures at 313.2 K, 323.2 K and 333.2 K

The phase behavior of PDMS(OH)–CO₂ binary mixtures was investigated. Two different molecular weight PDMS(OH) were utilized and the demixing pressures were determined at three temperatures for a wide composition range. Both of these polymers were found to form miscible mixtures with CO₂ at all compositions at pressures lower than 31 MPa in the temperature range 313.2–333.2 K. Depending on the composition of the binary mixtures, two types of phase separation was observed during depressurization; the bubble point and the cloud point. In addition, at specific weight fractions a color change was also observed which was attributed to the mixture critical point. The demixing pressures were observed to increase with temperature and decrease with increasing polymer weight fraction. In addition, higher demixing pressures were obtained for the higher molecular weight polymer mixtures. The bubble point data were modeled by using Sanchez–Lacombe equation of state (SLEoS) and the binary interaction parameters were regressed at the studied temperatures. It was observed that the binary interaction parameters decreased with increasing temperature.     

Experimental determination of the 1300 °C and 1400 °C isotherms for CaO–SiO2–TiO2-10 wt% Al2O3 system in air

Phase equilibrium relations for the CaO–SiO₂–TiO₂-10 wt%Al₂O₃ system were studied at 1300 °C and 1400 °C by a high temperature equilibration and quenching method, with the purpose of extending the equilibrium phase information for the TiO₂-containing slag systems. Equilibrium phase assemblies and phase compositions were analyzed by Scanning Electron Microscopy (SEM) equipped with an X-ray Energy Dispersive Spectroscopy (EDS). The equilibrium solid phases of perovskite CaO·TiO₂, wollastonite CaO·SiO₂, rutile TiO₂, silica SiO₂ and sphene CaO·SiO₂·TiO₂ were confirmed to coexist with a liquid phase. Based on the experimental results, the 1300 °C and 1400 °C isotherms were constructed in the CaO–SiO₂–TiO₂-10 wt% Al₂O₃ quasi-ternary phase diagram. Significant discrepancies were found between the present results and the simulated results by FactSage and MTDATA databases, as well as the results from previous literature.     

Heterogeneous oxidation of 2-octanol on 5 wt%Pt–1 wt%Bi/Carbon catalyst

The low activity of 5%Pt–1%Bi/Carbon for the oxidation of 2-octanol at atmospheric pressure and 343 K was investigated. Using solvents such as heptane and p-xylene, it was shown that the reaction rates decrease dramatically shortly after the start of the reaction due to poisoning by product adsorption. Hence this work investigates the effect of using different solvent mixtures on the oxidation reaction of 2-octanol with 5%Pt–1%Bi/Carbon. Mixtures with different volumetric ratios of heptane and dioxane were investigated to find the best composition capable of effectively removing the adsorbed amphiphilic ketone. It is apparent that the reaction rate is correlated with the adsorption coefficient of ketone on the catalyst, such that the maximum reaction rate occurs at the lowest ketone adsorption coefficient, corresponding to a concentration of 16–18%v/v dioxane. A model based on a Langmuir–Hinshelwood mechanism has been successfully fitted to the experimental rates.     

Oxidation behaviors of ZrB2–SiC binary composites above 2000 °C

Rapid oxidation testing for monolithic ZrB₂ and ZrB₂–SiC binary composites with different SiC contents (0–30 vol%) was performed using an electric heating system above 2000 °C. The system used in this study achieved the high heating rate of 250 °C/s. The experimental results showed that the morphologies of the oxidized specimens depended strongly on the SiC content. The formation mechanism of SiC-depleted layers beneath the surface scale above 2000 °C differed completely from that below 2000 °C. Although the holding time was below 10 s, SiC-depleted layers were formed because the oxygen partial pressure of the air atmosphere was not enough to form SiO₂ by the oxidation of SiC. It was determined that ZrB₂–20 vol% SiC showed the best oxidation resistance above 2000 °C at high heating rates.     

Ablation behavior and mechanism of C/C–ZrC–SiC composites under an oxyacetylene torch at 3000 °C

C/C–ZrC–SiC composites with continuous ZrC–SiC ceramic matrix were prepared by a multistep technique of precursor infiltration and pyrolysis process. Ablation properties of the composites were tested under an oxyacetylene flame at 3000 °C for 120 s. The results show that the linear ablation rate of the composites was about an order lower than that of pure C/C and C/C–SiC composites as comparisons, and the mass of the C/C–ZrC–SiC composites increased after ablation. Three concentric ring regions with different coatings appeared on the surface of the ablated C/C–ZrC–SiC composites: (i) brim ablation region covered by a coating with layered structure including SiO₂ outer layer and ZrO₂–SiO₂ inner layer; (ii) transition ablation region, and (iii) center ablation region with molten ZrO₂ coating. Presence of these coatings which acted as an effective oxygen and heat barrier is the reason for the great ablation resistance of the composites.