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 Y₂O₃ 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.