image: Figure | The directional radiative cooling thermal protective window based on double-sided nanophotonic-based films. (a), Schematic Diagram of the Structure. (b), Ideal optical properties.
Credit: Qiang Li et al.
The research team integrated a visible transparent broadband directional thermal emitter and Low-E film with commercial PC windows to develop a directional radiative cooling thermal protective window based on double-sided nanophotonic-based films, as schematically illustrated in Figure 1. The backside of this thermal protective window features a Low-E film with high visible transparency (transmittance> 0.8), primarily composed of PET film coated with indium tin oxide (ITO). Due to ITO's high infrared conductivity, this Low-E film exhibits low emissivity (corresponding to high infrared reflectivity) across the 3-14 μm infrared spectrum. Consequently, the backside effectively reflects substantial thermal radiation from heat sources, preventing the window from absorbing heat and increasing temperature.
The front side of the thermal protective window features a visible transparent (transmittance> 0.8) broadband directional emitter. To address the issue of previous directional emitters being non-transparent to visible light, the research team replaced metal substrates with ITO films that combine visible transparency and high infrared reflectivity, developing a visible transparent broadband directional thermal emitter based on a Al2O3/ZnS/Al2O3/Low-E multilayer film structure. Compared to conventional broadband directional emitters using multiple epsilon-near-zero (ENZ) dielectric materials, this design achieves broadband directional thermal emission with a single ENZ material, significantly reducing costs and simplifying fabrication processes. Moreover, this breakthrough overcomes the limitation of narrow-band emission caused by a single ENZ material, providing greater material control flexibility for future research on bandwidth tuning in directional thermal emitters.
The fabricated directional radiative cooling thermal protective window has excellent optical, thermal, and mechanical properties, including high visible transparency, and high-temperature, scratch, and impact resistance, which is essential for special protection work, such as firefighting and steel-smelting. Ultimately, we believe that it has the potential for application in various fields, including space heating and cooling, and thermal management.
Journal
Light: Advanced Manufacturing
Article Title
Photonic control of thermal radiation for protective windows