Improved light transmission in nanocrystalline aluminum nitride (AlN)—Enabling a lightweight, thermal shock resistant transparent ceramic

Abstract

Optically transparent ceramics were traditionally only viable using optically isotropic materials and limited birefringent materials with small Δn such as alumina (Δn≈0.008). Thus heavily birefringent materials such as AlN (Δn≈0.045), polycrystalline ceramics are usually not considered for optical applications due to their high scattering loss, despite its exceptionally appealing thermal conductivity, mechanical stiffness/toughness and relatively low density—attributes that are ideal for light weight optical windows for harsh environments. To further improve the transparency over coarse grain AlN ceramics that have been reported, subwavelength grain size is desired. Here we present significant improvements in visible, NIR and mid-IR transparency in AlN ceramics by reducing the average grain size to 230 nm. A newly developed, non-carbon thermal reduction for producing clean powders at low temperatures is key to the success. The nanocrystalline AlN ceramics reach full density at a densification temperature as low as 1500 °C and showed an in-line transparency of over 55% at 2.5 μm and over 70% in the mid-IR which is the highest transparency in AlN ceramics reported. The successful demonstration of transparent nanocrystalline AlN ceramics makes it the ideal choice for optical windows in quickly fluctuating temperature environments where weight is a consideration such as air and space vehicles.