Controlling anisotropic shrinkage in freeze-cast porous Al2O3 through two-step sintering

Anisotropic shrinkage during sintering remains a challenge in architectured porous ceramics, yet its mechanistic origin and controllability are not fully understood. Here, the evolution of anisotropic shrinkage in freeze-cast Al 2 O 3 is quantitatively analyzed using high-resolution in-situ dilatometry combined with microstructural characterization. A clear directional transition is observed during heating: shrinkage is initially dominated by the perpendicular direction at low temperatures, but reverses to the parallel direction above ∼1396 °C, resulting in a final anisotropy of ∼1.44%. This behavior is consistent with the competition between grain-boundary and lattice diffusion, indicating that anisotropic shrinkage depends on the dominant densification mechanism rather than on structural anisotropy alone. By linking curvature-driven capillarity ( γ / r ) to direction-dependent mass transport pathways, a mechanistic framework is proposed to explain the emergence of shrinkage anisotropy. Based on this framework, two-step sintering (TSS) is employed to promote grain-boundary diffusion while suppressing lattice diffusion, leading to a near-complete elimination of anisotropic shrinkage (∼0.2%) without sacrificing densification. These findings suggest that shrinkage anisotropy can be effectively regulated through diffusion-pathway control and provide useful guidance for designing sintering strategies to achieve dimensional stability in porous materials.