Metal Glass in Thin Film Structure Improves IR Thermal System

An advanced design for IR thermal technology, developed by researchers at National Taiwan University (NTU), takes advantage of the tunable optical properties of metallic glass to combine IR camouflage and IR thermal management functions in a single system. The system provides dual functionality for IR camouflage and IR thermal management within the same wavelength region of the atmospheric window.

A conventional, low-emissivity design approach to thermal camouflage is only useful for concealing targets at temperatures greater than 350 °C. It is not effective for applications that aim to reduce the ability to detect targets in the near-room to medium-high temperatures below the 350 °C range.

Also, given the typically high thermal emissivity of environments on a day-to-day basis, the thermal emissivity of a target’s background environment should be a consideration in the design of an IR thermal camouflage and management system.

To achieve versatile thermal camouflage while maintaining effective thermal management, the researchers combined metallic glass with the Berreman mode of epsilon-near-zero (ENZ) thin films (SiO₂, Al₂O₃, and TiO₂), and stacked the ENZ thin films on a metal-based bottom layer. The adjustable emissivity of the metallic glass enabled the system to accommodate diverse IR thermal camouflage scenarios.

ENZ thin films stack on a metal-based bottom layer within a dual-functional system for thermal IR camouflage and thermal management within the atmospheric window. Courtesy of NTU.

In the LWIR regions of 8 μm to 14 μm, the researchers found that the small viewing angle of the thermal management system exhibited the optical properties of metallic glass. When the viewing angle increased above 45°, the system demonstrated high thermal emissivity in transverse-magnetic polarization. The system was thus able to provide an effective thermal management function without compromising the performance of the thermal camouflage function. Increases to the viewing angle were driven by the multiple Berreman modes of the ENZ thin films.

The team found that the cooling power of ENZ thin films on metallic glass surpassed that of the conventional, low-emissivity design strategy for thermal camouflage by a factor of 1.79. Moreover, the thermal images from the NTU system indicated over 97% similarity in thermal radiation between the target and background environments.

By introducing metallic glass into IR thermal camouflage technology and exploiting the adjustable emissivity properties of the glass, the NTU team led by professor Hsuen-Li Chen created a dual-function system for IR camouflage and thermal management within an identical wavelength region of the atmospheric window.

This innovative, multilayer thin-film design approach for thermal management could contribute to the development of new approaches to reducing the detectability of a target using thermal imaging devices.

The research was published in Materials Horizons (www.doi.org/10.1039/D4MH00711E).