Advancing ultra‐high‐temperature materials requires superior oxidation resistance for hypersonic flight, atmospheric reentry, and propulsion systems, yet their service temperatures remain limited to below 3000°C. Herein, we put forward a component modulation strategy for the fabrication of SiC‐enhanced high‐entropy ceramics, with SHC‐3 demonstrating the lowest mass ablation rate (1.58 mg·s −1 ) and linear ablation rate (0.67 µm·s −1 ) while exhibiting outstanding ablation resistance under various extreme high‐temperature conditions. The incorporation of SiC particles significantly enhances the densification of SHC‐3, increasing its hardness to 33.1 GPa, while the observed crack deflection induced by the SiC particles is considered a key mechanism for strengthening and toughening the material. The superior ablation resistance of SHC‐3 is attributed to the synergistic effects of SiC‐assisted densification and crack deflection, the formation of a SiO 2 ‐rich protective phase under ultra‐high‐temperature ablation conditions, and the lattice‐distortion‐related diffusion retardation of the high‐entropy carbide matrix, which collectively suppress oxygen penetration and slow material degradation. This study demonstrates significant progress in the application of high‐entropy ceramics at extreme temperatures up to 3000°C and lays the foundation for designing advanced materials capable of operating at even higher temperatures.
Polymer‐derived ceramic (PDC) coatings on carbon fibers (CFs) protect against high‐temperature oxidation; however, thermomechanical stress and cracking during precursor pyrolysis can expose fiber surfaces, creating pathways for oxygen ingress. In this study, a boron‐ and hafnium‐modified polysilazane precursor is used to prepare PDC coatings on CF single plies via a modified polymer infiltration and pyrolysis (PIP) process. Rather than studying high‐temperature stability limits of SiHfBCN‐coated CF composites, we track macroscale mass changes and localized crack evolution. Up to three PIP cycles for fabrication and an oxidation temperature of 550°C were used as a model system to investigate defects in PDC coating, oxygen ingress, and CF degradation. The relationship among coating layers, crack width, crack spacing, ceramic mass loading, and fiber mass retention is investigated using electron microscopy. After oxidation at 550°C for over an hour, SiHfBCN‐coated samples exhibited the lowest average crack width of 5.6 ± 1.5 µm and the highest crack spacing of 38.5 ± 10.9 µm compared to SiBCN and SiCN coatings. At 36% ceramic mass loading, SiHfBCN retained 25.7% of the fiber mass, compared with 12% for SiCN, 9.2% for SiBCN, and 2.9% for uncoated CFs. These findings provide mechanistic insights into coating defects, microcrack evolution, and CF degradation in PDC‐CF composites.
ABSTRACT Yttria‐stabilized zirconia ceramic (3YSZ) was densified to only 60% relative density (RD) by cold‐sintering process (500 MPa pressure, 180°C temperature) but yielded majority of the pores in nano‐scale range. In contrast, 3YSZ conventionally sintered at low temperature of 1200°C yields a high RD of ∼97% with large macro pores ∼2 µm size. To explore further, cold sintering was done as a pretreatment followed by conventional low‐temperature sintering at 1200°C. This method yielded exceptional properties with a majority of tetragonal phases and a complete densification with a remarkably minimal pore size of ∼0.01 µm having Vickers hardness of 12.6 ± 0.3 GPa and flexural strength of 450 ± 40 MPa. The ceramic specimens obtained by combined cold and conventional sintering (CS) were thoroughly characterized using mercury porosimetry, x‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Raman, confocal Raman, impedance spectroscopy, and AFM. The evolution of pore size during the combined cold and CS stages closely mirrors the behavior seen in the well‐known two‐step sintering process. The combined cold and CS process provides an energy‐efficient two‐step sintering route for densification of micron‐sized 3YSZ ceramics, which are important in ceramic industry from a sustainability point of view.
ABSTRACT Graphene aerogels are limited by poor high‐temperature stability and insufficient mechanical strength in harsh thermal environments. Herein, SiCN‐Al 2 O 3 dual ceramics were incorporated into a nitrogen‐doped reduced graphene oxide (N‐rGO) framework through hydrothermal self‐assembly, supercritical drying, and polymer‐derived ceramization to construct composite aerogels with nanoscale interpenetrating networks. At 20 wt% ceramic precursor, interconnected ceramic nanoparticles uniformly penetrated the N‐rGO sheets, forming a continuous and homogeneous architecture. The flexible graphene layers bridged rigid ceramic domains to dissipate stress and suppress crack propagation, while the ceramic network anchored the graphene sheets, inhibiting restacking and oxidation‐induced ablation. After pyrolysis at 1200°C, the optimized aerogel achieved a low thermal conductivity of 0.034 W m −1 K −1 , a yield strength of 0.92 MPa, and a two‐stage fracture behavior, demonstrating improved damage tolerance. This interpenetrating ceramic/graphene network enables lightweight aerogels with improved thermal insulation, oxidation resistance, and mechanical robustness.
ABSTRACT Developing robust ceramic thermoelectric materials that maintain stable performance at high temperatures remains a critical challenge. This article reports the first systematic investigation of the structural, morphological, and thermoelectric properties of alkaline earth metal–doped spinel ferrites, ZnFe 1.975 M 0.025 O 4 (M = Mg, Ca, Sr, and Ba), synthesized via the sol–gel method. Among the compositions studied, Mg proved the most effective dopant, with ZnFe 1.975 Mg 0.025 O 4 exhibiting the highest electrical conductivity of 525 S/m and a maximum power factor of 20 µW/mK 2 at 850 K. An enhanced figure of merit ( z T) of 3.52 × 10 − 3 was achieved at 750 K. This improvement is attributed to the substitution of lower valent dopants at higher valent cation sites, which generates oxygen vacancies and substitutional point defects. The key novelty of this work lies not only in providing a comprehensive comparison of alkaline earth metal dopants in spinel zinc ferrite under identical synthesis conditions, but also in showing that ionic radii–driven site occupancy, rather than aliovalent charge compensation alone, governs oxygen vacancy formation and carrier generation across the dopant series in ZnFe 2 O 4 . These findings offer valuable insights into dopant chemistry, defect engineering design rules, and charge transport, supporting the optimization of high‐performance spinel zinc ferrite oxide ceramics.
