Aluminum-Nitride Infrared Window Materials for High Heat Load Applications

Feasibility of fabricating IR window materials from aluminum nitride (AlN) will be demonstrated. Aluminum nitride windows are sought as replacements for sapphire windows on high speed seeker missiles because of their higher thermal shock resistance. Our approach is to consolidate high-purity ultrafine powders of AlN to obtain a nanocrystalline window. Small grain size is expected to minimize IR scattering, and preliminary data suggest this is the case. Theoretical estimates show that a grain size of 200 nm should yield high thermal conductivity and IR transparency. A novel aspect of this work is a flame synthesis process for producing ultrafine AlN powders encapsulated in a removable coating material to ensure that they retain their purity until consolidation. This sodium/halide flame and encapsulation technology is easily scaleable and the powders produced are unagglomerated and low in oxygen. We will also employ a novel consolidation process that sequentially removes the encapsulation material (sodium chloride) prior to vacuum hot pressing so that AlN powders are never directly exposed to atmosphere. For this program, ultrafine AlN powders will be synthesized and consolidated into 1 mm thick disks. Both powder and compacts will be characterized for IR transmission, elemental composition and crystal phases to determine the feasibility of using such materials for infrared transparent windows.