Engineered Microstructures for Enhanced IR Aperture Performance

The Navy has demonstrated within its laboratories that a composite ceramic exhibiting IR transparency can be produced with distinct phases, microstructural feature sizes below 100 nm in all phases, and porosity content below 0.1% on component parts as large as 0.4 inch in lateral dimension. Phase identity, crystal structure, phase chemistry, microstructure feature size, and porosity content were established via X-ray diffraction (XRD) and scanning electron microscopy (SEM) cross-sections with energy dispersive spectroscopy (EDS) analysis. Retention of sub-100 nm feature sizes across all phases is critical to achieving the desired mechanical and thermal properties of the composite; porosity above 0.1% similarly degrades mechanical performance. All three specifications -- sub-100 nm microstructural features in all phases, porosity below 0.1%, and Short-Wave to Mid-Wave Infrared (SWIR/MWIR) transparency of at least 80% -- must be achieved at scale for the material to be suitable for transition. Producing IR-transparent composite ceramics meeting these specifications at sample sizes relevant to aperture applications is a significant technical challenge. Conventional ceramic processing routes that rely on mechanical mixing or co-precipitation of constituent phases typically result in microstructural coarsening during densification that precludes achieving sub-100 nm feature sizes in all phases. Innovative powder synthesis and consolidation approaches capable of satisfying all microstructural, porosity, and optical transparency requirements at sample sizes exceeding 2" x 4" x 0.5" are sought. Furthermore, the proposed processes should be industrially scalable in both capacity and final component size, with a path towards producing components in the 3" x 9" range or larger. In addition to these requirements, the composite ceramic must retain its microstructure and IR transparency after exposure to temperatures up to 1200 degrees C for 10 minutes. Phase transformation or grain growth during high-temperature service would degrade both mechanical and optical performance. The composite must also exhibit a low ratio of thermal expansion coefficient (alpha) to thermal conductivity (k). The threshold requirement for alpha/k is 1.0 micrometers/W and the target is 0.5 micrometers/W at 1000C.