β‑Seed‑Induced Selective Exaggerated Growth Enables Designed Bimodal Microstructures in Low‑Cost Si ₃ N ₄ Ceramics

ABSTRACT Combustion synthesis (CS) presents a promising, cost‐effective alternative to expensive commercial powders for fabricating high‐performance silicon nitride (Si ₃ N ₄ ) ceramics, which are urgent candidates for next‐generation power electronic substrates; however, unlocking the full potential of CS‐derived ceramics remains hindered by the challenge of precise microstructural tailoring. In this work, the influence of the initial α/β phase ratio in CS powders on the densification kinetics and microstructural evolution of Si ₃ N ₄ ceramics was systematically investigated. The dispersed β‐particles acted as pre‐existing nuclei during liquid‐phase sintering, which significantly accelerated the densification process and fostered the preferential growth of elongated β‐grains. This “seeding effect” successfully engineered a pronounced bimodal microstructure characterized by an interlocking network of high‐aspect‐ratio crystals. The optimized microstructure provides a synergistic enhancement of properties, yielding a thermal conductivity of 120 W·m ⁻¹ ·K ⁻¹ , a fracture toughness of 9.17 MPa·m ^1/2 , and a flexural strength of 661 MPa. This study not only elucidates the mechanism of phase‐regulated microstructural evolution but also demonstrates a viable, cost‐effective route for manufacturing structural‒functional integrated Si ₃ N ₄ ceramics.