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 1000oC in an oxidizing environment, transmission in the midwave infrared (MWIR) range, and high emissivity at wavelengths shorter than 3 microns as heat must be dissipated by radiation. The development of radomes faces similar challenges with materials maintaining high broadband emissivity and high RF transparency at temperatures over 1250oC. Nanoionix and subcontractor Johns Hopkins University Applied Physics Laboratory (JHU/APL) propose to solve this problem by engineering novel multicomponent metal oxide ceramic materials that can be used as a thin film coating on their exterior surface. These are exceptionally thermodynamically stable and robust materials, even at temperatures >1700oC. Oxides represent the best opportunity to withstand the highly oxidizing environments experienced by hypersonic missiles. During Phase I the Nanoionix team developed two classes of materials: (1) materials with spectrally controlled emissivity (high visible and near infrared emissivity (>0.8) and high MWIR transparency (>98% transparent at 4 microns)) and (2) materials with high broadband emissivity (>0.9 from 0.5 – 16 micron) with ~100% RF transparency and coated them on relevant substrates. Both classes of materials are stable to at least 1500oC in air for one hour, maintain their emissivity and transparency at elevated temperatures, are ~100% transparent to RF from 10 – 40 GHz, and cool the underlying substrate by at least 400oC. Coatings are resilient to thermal shock with no peeling or scaling. Materials’ performance is not sensitive to small changes in stoichiometry providing a broad process window for materials scale-up. This type of emissivity control and thermal stability are critical to addressing the thermal issues associated with hypersonic missile windows and radomes in flight. During the Phase II effort the Nanoionix team will further optimize and characterize these materials, coat them onto IR-transparent windows and RF-transparent radome substrates, and test their optical, electronic, mechanical, and thermal performance at elevated temperatures under simulated flight operating (shock/plasma) conditions. Optimization will be guided by experimental results, artificial intelligence-driven modeling/ab initio predictive capabilities, and optical modeling at JHU/APL.  Phase II deliverables include a data package with all the material properties that a system designer would need in the future to implement multicomponent metal oxide coatings on both infrared windows and radomes and multiple samples (including notional full-scale prototypes of both IR windows and radomes) provided to the Navy and missile/window/radome suppliers for independent evaluation.