Methods for the fabrication of IR windows from nanoparticulates

Windows for infrared sensors on missiles and aircraft must be of high optical quality and durable enough to protect delicate sensors from harsh operating conditions. If the grain size is smaller than the wavelength of the IR signal, then optical materials with excellent mechanical properties and IR transmission will be likely. This paper describes work being done for IR window applications on Yttrium oxide, which represents one potential candidate material for infrared windows because of its high infrared transparency, and a nanocomposite of Alumina:Zirconia having a grain size under 50 nm. Nanoparticles of yttria precursors and alumina-zirconia are synthesized using a sonochemical technique. These are passivated and dispersed as a colloid. Various colloidal processing approaches are utilized to make solid preforms, which are further processed to remove liquid, organics and to convert precursors to crystalline materials. Concentrated colloids can be used to directly fabricate a solid preform using more than one consolidation technique. The preform becomes the starting point for further processing and densification to form an IR transparent material. Pressure-assisted sintering is being done using sinter forging done on solid preforms that were partially sintered. Near 100% of the theoretical density was attained at modest temperatures with this method. Samples containing zirconia need to be further oxygenated between 1200°C-1275°C for 12 to 24 hours. While this work is still on-going, one sample showed 50% IR transmission. IR transmission was also found to increase with longer oxygenation times.