Advanced alumina ceramics with pseudo‐isotropic optical properties for infrared windows

We successfully fabricated polycrystalline alumina ceramics with a hexagonal structure that exhibit excellent light transmittance in the near‐ to mid‐infrared spectrum. Although composed of hexagonal crystals, these ceramics exhibit minimal light leakage due to birefringence, with transmittance values below 0.2% when measured under cross nicols beyond the near‐infrared wavelength bands. The extinction ratio exceeds 30 dB, comparable to that of birefringence‐free materials such as cubic Y 2 O 3 ceramics and c‐axis single‐crystal sapphire. Using the near‐infrared (0.9–1.7 µm) and mid‐infrared (3–5 µm) cameras, the images obtained for the fabricated alumina windows ( t = 2–6 mm) exhibited similar levels of brightness and clarity to those observed through the sapphire single crystal windows. These advanced alumina ceramics exhibit exceptional mechanical properties, including a Vickers hardness (H v1 ) of 2205, a bending strength of 945 MPa, and a fracture toughness (K 1C ) of 3.4 MPa.m 0.5 . This transparent alumina ceramic exhibits pseudo‐optical isotropy in the infrared region, providing window characteristics comparable to those of cubic polycrystalline ceramics and c‐axis sapphire single crystals.