Grain size regulation in high‐thermal‐conductivity Si ₃ N ₄ ceramics: Role of additive size and powder characteristics

Abstract This study systematically investigates the influence of sintering aid particle size and powder characteristics on the phase composition, microstructure, and properties of Si ₃ N ₄ ceramics. Results demonstrate that sintering aid size critically governs liquid‐phase homogeneity, α → β phase transformation, and grain growth. Smaller aids facilitate more uniform liquid distribution, enhancing densification while suppressing abnormal grain growth. In contrast, larger additives induce localized Y‐rich regions or Mg‐rich regions, accelerating β‐Si ₃ N ₄ nucleation and grain coarsening. Powder properties, especially oxygen content and particle morphology, further dictate liquid‐phase formation and phase transformation. High‐oxygen powders increase weight loss via the formation of volatile SiO (g) and restrict grain growth via steric hindrance, whereas rod‐like β‐phase seeds in the raw material trigger abnormal growth, forming a bimodal microstructure. Using raw powders with low oxygen content and rod‐shaped grains, along with nanoscale MgO (20 nm) and micrometer‐sized Y ₂ O ₃ (2.5 µm) as sintering aids, the SN‐C6 sample achieved a bimodal microstructure with elongated β‐Si ₃ N ₄ grains, yielding superior performance with a thermal conductivity of 125.5 W·m ^‒1 ·K ^‒1 , flexural strength of 685 ± 31 MPa, and fracture toughness of 10.1 ± 0.4 MPa·m ^1/2 after sintering at 1910°C for 12 h, followed by a 1600°C heat treatment for 8 h.