High Emissivity Hypersonic Window Coatings

The development of IR transparent windows for hypersonic flight must solve three critical problems simultaneously: the ability to withstand temperatures above 1000C in an oxidizing environment, transmission in the MWIR range, and high emissivity at wavelengths shorter than 3 microns as heat must be dissipated by radiation. Nanoionix and subcontractor Johns Hopkins Applied Physics Laboratory (APL) propose to solve this problem by engineering novel multicomponent metal oxide ceramic materials that can be added to existing IR windows or used as a thin film coating on their exterior surface. These are exceptionally thermodynamically stable and robust materials, even at temperatures >1700C. Oxides represent the best opportunity to withstand the highly oxidizing environments experienced by hypersonic missiles. The spectral properties of most oxides are not ideal as they do not emit in the spectral bands of highest Planck function energy (0.7 – 3-micron) at temperature >1000C. The key is to engineer compositions and stoichiometry (e.g., modulating the bandgap and presence/absence of oxygen and cation vacancies) which provide the desired spectrally selective emission attributes of increased emissivity in the 0.7 – 3-micron region and high transmission/low emissivity at longer wavelengths (3 – 5-micron for MWIR windows). This type of spectral control is critical to addressing the thermal issues associated with hypersonic missile windows in flight. Building on data from the literature and its experience in high-temperature IR materials, the Nanoionix team has already synthesized and demonstrated a series of robust multicomponent metal oxide materials and demonstrated an emissivity of 0.9 below 3 microns, without changing the emissivity between 3 and 5 microns. While the materials developed to date are not perfect, they provide a clear pathway to reaching the Navy’s goals. Applicability to other seeker window functions will also be considered. Today’s LWIR windows are not thermally stable at extreme temperatures and oxide coatings with enhanced emission would allow them to run cooler. Similarly, oxide coatings would provide an enhanced cooling function for RF radomes and can be engineered to have low electrical conductivity to not block the RF signal. During the Phase I effort the Nanoionix team will further optimize and characterize these materials, coat them onto IR-transparent windows, and test at elevated temperatures. Optimization will be guided both by experimental results and the extensive knowledge-base and artificial intelligence-driven modeling/ab initio predictive capabilities of APL. A Phase II program will further optimize material performance and downselect the most promising candidates for high temperature testing under plasma conditions. Implementation approaches will be considered with oxides applied as coatings or integrated into window materials. Samples will be provided to the Navy for independent evaluation.