Study on thermal shock resistance of Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings

In order to reveal the effect of Sc₂O₃ and Y₂O₃ co-doping system on the thermal shock resistance of ZrO₂ thermal barrier coatings, Y₂O₃ stabilized ZrO₂ thermal barrier coatings (YSZ TBCs) and Sc₂O₃–Y₂O₃ co-stabilized ZrO₂ thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology. The surface and cross-section micromorphologies of YSZ ceramic coating and ScYSZ ceramic coatings were compared, and their phase composition before and after heat treatment at 1200 °C was analyzed. Whereupon, the thermal shock experiment of the two TBCs at 1100 °C was carried out. The results show that the micromorphologies of YSZ ceramic coating and ScYSZ ceramic coating were not much different, but the porosity of the latter was slightly higher. Before heat treatment, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was a single T′ phase. After heat treatment, the phase composition of YSZ ceramic coating was a mixture of M phase, T phase, and C phase, while that of ScYSZ ceramic coating was still a single T′ phase, indicating ScYSZ ceramic coating had better T′ phase stability, which could be attributed to the co-doping system of Sc₂O₃ and Y₂O₃ facilitated the formation of defect clusters. In the thermal shock experiment, the thermal shock life of YSZ TBCs was 310 times, while that of ScYSZ TBCs was 370 times, indicating the latter had better thermal shock resistance. The difference in thermal shock resistance could be attributed to the different sintering resistance of ceramic coatings and the different growth rates of thermally grown oxide in the two TBCs. Furthermore, the thermal shock failure modes of YSZ TBCs and ScYSZ TBCs were different, the former was delamination, while the latter was delamination and shallow spallation.     

Asymmetric alumina-based ultrathin composite ceramic membranes with interfacial modification of black talc nanosheets

In this work, ultrathin planar alumina-based ceramic membranes with asymmetric structure and thickness less than 0.85 mm were successfully prepared by one-step molding phase transformation/sintering method using low-cost black talc (BT) nanosheets for the first time. The microstructure, pore structure, mechanical strength and permeability of novel ceramic membranes were systematically investigated with different BT amount and sintering temperatures. The doping of BT nanosheets effectively modulated the interfacial bonding area and strength between the grains, achieving significant increase in flexural strength through the evolution of the dense layer structure. The asymmetric structural features formed by the phase transformation/sintering process in combination with polymer substrate significantly reduced the thickness of effective separation layer, thus weakening the loss of flux caused by the densification of the film layer due to the interfacial modification process. Moreover, the organic carbon layers between BT layers were oxidized during the sintering process, forming fine pores and increasing the porosity, which showed to be unique characteristic different from other clay mineral materials. The prepared composite membrane had the pure water flux up to 16335 L m⁻² h⁻¹/bar at 1350 °C sintering, which achieved stable permeation of ∼5200 L m⁻² h⁻¹/bar and high retention over 90% for O/W emulsions.     

Oxygen barrier resistance of HfB2-MoSi2-TaB2 coatings in a wide temperature region

HfB₂-MoSi₂-based ultra-high temperature ceramic (UHTC) coatings have shown remarkable antioxidant effects owing to the formation of silicate glass layers with low oxygen permeability in high-temperature environments, which shows great potential in the antioxidation of carbon structural materials. To further enhance the oxidation resistance of the HfB₂-MoSi₂-based coating in a wide temperature region, the influence of volume ratio between HfB₂ and TaB₂ on the antioxidant capacity of the HfB₂-MoSi₂-TaB₂ coatings was investigated. The addition of 15 vol% TaB₂ in the 60HfB₂-40MoSi₂ coating delays the initial oxidation temperature of the 60HfB₂-40MoSi₂ sample from 300 °C to 500 °C, which decreased the oxidation loss by 75.85% during dynamic oxidation. In oxidation process at 900 °C and 1700 °C, the weight gains of the 45HfB₂-40MoSi₂–15TaB₂ coating reduced by 78.56% and 63.14%, respectively. Due to the coexistence of 45 vol%HfB₂ and 15 vol%TaB₂, the suitable Ta⁵⁺ promoted the homogenization and dispersion of Hf/Ta-oxides, which forms the coral-like Hf/Ta oxides skeleton in the glass layer, thus preventing the oxygen penetration and decreasing the inert factor of the HfB₂-MoSi₂ coating at 1700 °C by 51.19%. However, excessive TaB₂ weakened the self-healing ability of the Ta-Hf-Si-O glass layer and inhibited the oxygen barrier effect of the HfB₂-MoSi₂-TaB₂ coating.     

Effect of multi-component rare-earth doping on maintaining structure stability of RE2Zr2O7 (RE = La,Sm, Gd,Y, Yb) coatings under thermal cycling

Inspired by the high entropy effects of high-entropy components, a novel high-entropy rare-earth zirconate (La_1/5Gd_1/5Y_1/5Sm_1/5Yb_1/5)₂Zr₂O₇ (HEC-LZ) was designed and successfully synthesized in this work. In addition, two binary rare-earth doped zirconates (RE-LZ), (La_1/3Sm_1/3Yb_1/3)₂Zr₂O₇ (LSYZ) and (La_1/3Gd_1/3Y_1/3)₂Zr₂O₇ (LGYZ), were proposed using the same rare-earth elements for comparison. The thermal barrier coatings with LZ-based ceramic top layer were prepared by spray granulation, solid-phase synthesis and atmospheric plasma spraying techniques. The as-synthesized LZ-based ceramics are all dominated by the pyrochlore phase. Under 1000 °C, the thermal cycling performances of the three coatings were studied. The microstructure evolution and crack expansion during the failure process were investigated in detail. The strengthening mechanism and the cause of coating spallation are proposed in combination with mechanical properties and thermal matching analysis. The results showed that compared with the undoped LZ coating, the thermal shock life of LGYZ coating, LSYZ coating and HEC-LZ coating is improved by nearly 46%, 27% and 57%, respectively. Due to the characteristics of high randomness, HEC-LZ ceramic has a large lattice distortion than RE-LZ ceramics, resulting in a higher coefficient of thermal expansion and fracture toughness, which contributes to maintaining the structure stability of coatings under thermal stress.     

Magnetic hierarchically porous biomass carbon for realizing broadband and strong absorption of electromagnetic wave

Hierarchically porous carbon materials provide favorable conditions for electromagnetic wave loss enhancement due to the superimposed positive influence of multilevel pores. However, high production costs and complex preparation limit their large-scale production. Biomass carbon with a natural hierarchically porous structure offers an alternative; however, the impedance mismatch and single-loss mechanism prevent biomass carbon from being an ideal absorbent for broadband and strong absorption. In this study, a series of magnetic hierarchically porous biomass carbons were prepared using a facile adsorption-inert calcination method. The natural hierarchical porous structure of the loofah sponge provides numerous adsorption sites for ferric ions, which are transformed in situ into Fe₃O₄ during calcination to regulate the conductivity. The impedance matching and electromagnetic loss properties of the biomass carbon/Fe₃O₄ composites were adjusted by varying the amount of ferric nitrate. Optimal performance occurs when ferric nitrate weighs 0.8 g, and the calcination temperature is 600 °C. Under these conditions, the effective absorption bandwidth reaches 5.28 GHz (11.84–17.12 GHz, 2.5 mm), and the minimum reflection loss (RL_min) is as low as −52.54 dB (4.5 mm), which is achieved by superior impedance matching and strong conduction loss together with polarization loss due heterogeneous interfaces and carbon defects. Our work provides a new perspective and a simple method for the large-scale production of high-efficiency biomass-based electromagnetic wave absorbents.     

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.     

Tailoring Ni3ZnC0.7/graphite heterostructure in N-rich laminated porous carbon nanosheets for highly efficient microwave absorption

Bimetallic transition carbide nanoparticles (Ni₃ZnC_0.7) encapsulated with graphitic shells were successfully embedded into N-rich laminated porous carbon nanosheets (NC–ZnNi_1.5) by one-step pyrolysis of bimetallic organic frameworks. It was found that C atoms penetrated octahedral interstitial spaces of the Ni lattice to form Ni₃ZnC_0.7. The charge states and distribution of metal atoms were influenced by the interstitial C atoms, which promoted polarization relaxation and facilitated dielectric loss. Simultaneously, the volatilization of Zn influenced recrystallization and rearrangement of the crystalline domains, facilitated graphitization and established a 3D conductive network to optimize the conductive loss. Besides, multi-level heterogeneous interface and nitrogen doping also further optimized the impedance matching. Benefiting from these advantages, the minimum reflection loss of NC-ZnNi_1.5 was −69.1 dB at 12.7 GHz, and the effective absorption bandwidth was 6.5 GHz. The mechanism of bimetallic carbide's dielectric loss was explained in detail, providing a new pathway for the development of multi-component carbon-based microwave absorbers in the future.     

Simultaneous enhancement of mechanical and ablation properties of C/C composites modified by (Hf-Ta-Zr)C solid solution ceramics

In order to improve the mechanical and ablative resistance of C/C composites, (Hf-Ta-Zr)C single-phase solid solution ceramics were introduced into C/C composites by polymer infiltration and pyrolysis (PIP) to fabricate (Hf-Ta-Zr)C modified C/C composites (HTZ). Their mechanical property and ablation resistance were studied. The results showed that HTZ achieved simultaneous enhancement of mechanical property and ablative resistance. Their flexural strength and modulus could reach 219.34 MPa and 24.82 GPa, respectively. In addition, the mass and linear ablation rate of HTZ were 0.379 mg/s and 0.667 µm/s, respectively after the 90 s oxyacetylene ablation. A dense Hf-Ta-Zr-O multiphase oxide layer was formed on the surface of the HTZ during ablation process, which protected the interior modified C/C composites from ablation. Our work expands a rational design of modified C/C composites and broaden the application of solid solution ceramic in the field of ultra-high temperature ablation resistance for carbon or ceramic-based composites.     

Effect of the incorporation of SiC nanowire with double protective layers on SiC coating for C/C composites

To maintain the thermal stability of SiC nanowires during SiC coating fabrication process, carbon and SiC double protective layers were covered on the surface of nanowires. And SiC nanowires with double protective layers toughened SiC coating were prepared by pack cementation. The results showed that after introducing the SiC nanowires with double protective layers, the fracture toughness of the SiC coating was increased by 88.4 %. The coating protected C/C for 175 h with a mass loss of 3.67 %, and after 51 thermal shock cycles, the mass losses of the oxidized coating were 3.96 %. The double protective layers are beneficial to improve the thermal stability of nanowires, leading to good fracture toughness and thermal shock resistance of SiC coating. SiC nanowires consume the energy of crack propagation by fracture, pullout and bridging, leading to an increase in fracture toughness.     

Rapid and low-cost carbon/carbon composites by using graphite slurry impregnated prepregs

A rapid and low-cost carbon/carbon (C/C) composites preparation method is proposed: graphite prepreg-coated carbon fiber fabric (CFF) is formed by hot pressing, followed by hot isostatic pressing and high temperature graphitization, to prepare C/C composite with low porosity and high crystallinity. In this method, the carbon fiber (CF) mass fraction can be precisely regulated in the range of 40–95% by the impregnation process conditions of CFF in graphite prepreg. The graphite particles in the preform were graphitized and bonded with CFF by high temperature graphitization. Finally, a ZrO₂ anti-ablative layer was applied using sol-gel method. The results show that when the CF mass fraction is 50%, the C/C composite with a crystallinity of 92.21 and a porosity of 3.47% can be obtained, with mass ablation rate of 0.23 mg/s and density of 1.62 g/m³. The method can prepare C/C composites with uniform density and high ablation resistance.