Developing near-non-ablative ultra-high-temperature ceramic matrix composites (UHTCMCs) capable of enduring long-term severe aerodynamic heating remains a critical challenge for hypersonic vehicle thermal protection systems. A dense C f /HfB 2 -SiC composite with a low open porosity of 1.87% was successfully fabricated through pressure slurry impregnation combined with liquid polycarbosilane infiltration. The composite exhibited exceptional ablation and oxidation resistance under oxy-acetylene torch exposure at heat fluxes of 2–4 MW/m 2 and prolonged oxy-acetylene torch exposure at 2527 ± 37 °C for 1000 s, maintaining remarkably low mass and linear ablation rates of 0.78 mg/s and 0.115 μm/s, respectively. Microstructural characterization coupled with thermodynamic and kinetic analyses revealed that this superior performance originated from the formation of a hierarchical, self-adaptive oxide layer regulated jointly by local oxygen partial pressure ( P O 2 ) and temperature gradients. A highly sintered HfO 2 -rich outer surface acted as the principal refractory barrier, while residual Si-containing oxide species retained in the subsurface may have contributed to local pore filling and oxide-layer densification. Conversely, the inner oxygen-depleted subsurface region underwent active SiC oxidation, yielding a porous intermediate buffer layer that effectively accommodates thermal stresses and regulates mass transport. The high apparent activation energy of 475 kJ/mol indicated a formidable kinetic barrier against aggressive oxidation. This work demonstrates that the long-term structural integrity of the composite at temperatures above 2500 °C does not solely rely on a single continuous SiO 2 glassy layer but rather arises from the synergistic interplay among the refractory HfO 2 skeleton, residual Si-containing sealing phases, and the carbon-fiber reinforcement retained in the underlying unoxidized region. These findings establish a clear correlation between microstructure and properties, offering key insights for the design of low-ablation-rate UHTCMCs for extreme aerospace applications.
Closed-pore structures are pivotal for achieving high specific capacity in hard carbon (HC) anodes for sodium-ion batteries. Yet, their precise formation remains poorly controlled due to limited molecular-level understanding. Here, renewable cellulose as a model precursor, and a molecular pre-cleavage strategy is introduced to synthesize HC with expanded interlayer spacing and abundant tunable closed pores. Experimental characterizations combined with kinetic simulations reveal that closed-pore formation is governed by site-selective anchored oxidation at defined molecular positions, while the pre-cleaved molecular architecture suppresses graphitization, yielding numerous closed pores enclosed by short-range graphitic domains that enhance plateau capacity. The optimized HC anode delivers a high reversible capacity of 373.7 mAh g−1 at 20 mA g−1 and retains up to 97.7% capacity over 800 cycles at 2 A g−1. Furthermore, a full-cell with Na4Fe3(PO4)2P2O7 exhibits 93.2% capacity retention after 900 cycles at 2C. This work provides molecular-level design principles for engineering high-performance HC anodes, offering a pathway toward advanced SIBs with superior energy storage capabilities.
