Abstract A promising material for optical applications is transparent polycrystalline alumina (Al 2 O 3 ) ceramics, owing to their excellent mechanical properties and thermal and chemical stability. However, the light transmittance of Al 2 O 3 polycrystalline ceramics is mainly limited by grain scattering (birefringence) and pore scattering, resulting in a lower transmittance than single‐crystal sapphire. An effective approach to minimize grain scattering and pore scattering is to refine grains and reduce porosity and pore sizes. To achieve near‐theoretical transparency in Al 2 O 3 ceramics, in our present work, the grains in Al 2 O 3 polycrystalline ceramics are refined to 13 nm, from our homemade nanoparticles and by high‐pressure/low‐temperature sintering. Typically, the synthesized Al 2 O 3 nanocrystalline ceramic exhibits 82% in‐line transmittance at 0.8 mm thickness and a wavelength of 640 nm, which is not only superior to the reported doped, undoped, and textured Al 2 O 3 ceramics (78%) but even close to that of single‐crystal sapphire (86%). The reasons behind this are analyzed and discussed. Our results confirm that refinement of nanograins is an effective approach to enhance the transparency of anisotropic polycrystalline ceramics.
Kang et al. (2026) studied this question.