Hierarchical porous ceramics: Design strategies, structure-property trade-offs, and multifunctional applications in aerospace, biomedical, and energy systems

Hierarchically porous ceramics integrate micro- (<2 nm), meso- (2–50 nm), and macropores (>50 nm) to overcome inherent property trade-offs, such as mechanical strength versus permeability, thermal insulation versus structural reliability, and bioactivity versus load-bearing capacity. This review critically evaluates the design strategies, fabrication methods, and multifunctional applications of these materials in aerospace, biomedical, and energy systems. The principal results show that freeze casting enables anisotropic, aligned pore structures for directional transport; template-assisted synthesis offers precise hierarchical control; additive manufacturing provides deterministic design freedom for graded porosity; and sol-gel processing yields high-purity nano-to-mesoporous networks. In solid oxide fuel cells, hierarchical anodes increase the triple-phase boundary density by 40–60% while maintaining mechanical strength. In thermal barrier coatings, graded porosity reduces the thermal conductivity by approximately 50% and extends the cyclic lifespan beyond 1000 cycles at 1100°C. In bioactive scaffolds, hierarchical hydroxyapatite achieves porosities ≥ 75% with a compressive strength of 5–8 MPa, accelerating bone formation by 4–6 weeks. The major conclusions are as follows: (i) decoupling transport and mechanical functions across pore scales is key to resolving trade-offs; (ii) scalable, low-energy processing remains a critical barrier; (iii) integrating artificial intelligence and machine learning with multi-scale modeling enables predictive design; and (iv) future systems should incorporate adaptive, self-healing, and stimuli-responsive porosity. This review provides a structured framework for engineering next-generation porous ceramics tailored for high-performance aerospace, biomedical, and energy applications.