Metal Oxide Nanopowders for the Production of Transparent Windows

In the Phase I STTR work, YAG (yttrium aluminum garnet) was identified as the superior choice for use as a UV-VIS window in advanced propulsion applications, based on measured properties and literature data.  YAG provides a better combination of retained high temperature strength, creep resistance, optical isotropy and resistance to neutron irradiation.  The Phase II STTR program is focused on the continued development of transparent YAG hollow cylinders through both slip casting and extrusion of YAG tubes. Slip cast prototypes were produced by UES under the Phase I program and the method was successful; however, extruding tubes should allow for a higher production rate and the University of Michigan will develop this process.  Additions of fugitive elements will also be examined for the production of internal channels.  Polishing of the cylinders will be studied in collaboration with a commercial source.  Information on the response of YAG at high temperature, including microstructural stability, mechanical properties, optical transmission, and radiation resistance will be determined in this work.  Further, the lifetime behavior of YAG cylinders under expected use conditions will be predicted using these results. Transparent YAG cylinders will be delivered to the Air Force for further evaluation in actual use environments.    BENEFIT: At the end of the Phase II work, it is expected that dense, transparent, complex-shaped ceramic YAG windows can be produced that meet all of the necessary optical and mechanical requirements for advanced propulsion applications: high transmittance, high refractive index homogeneity, low bulk optical absorption, high strength and high fracture toughness.   Examples of potential military applications are sensor or laser windows for extreme aero-thermal environments, window components in advanced propulsion systems utilizing high intensity radiative energy, transparent armor such as vehicle vision blocks, windshields, blast shields and IR transparent windows for radomes.  Commercial applications include high intensity industrial lighting, commercial laser gain products, and optical ceramics, such as high temperature envelopes for metal gas lamps. An environmental impact can also be expected, if solar concentrators can be utilized with these UV transparent window materials.