Introducing: Ceramic- and glass-based passive radiative cooling materials resistance to harsh environments

Two studies highlight new glass- and ceramic-based passive radiative cooling materials. Unlike passive radiative cooling approaches that rely on polymers, these hard materials are more durable and versatile, making them more attractive for a wide range of outdoor passive cooling applications, including those that could help reduce the need for air conditioning. The energy demand for cooling continues to rise, particularly in regions rapidly warming due to climate change. To make matters worse, the growing carbon footprint of cooling systems further contributes to global warming, exacerbating the need for cooling solutions. Passive radiative cooling (PRC) materials, which are designed to reflect solar radiation and emit long-wavelength infrared (LWIR) thermal radiation through the atmosphere’s infrared window and back into outer space, are promising technologies that could mitigate both rising temperatures and cooling costs. However, developing effective PRC materials that are both environmentally robust and practical to manufacture has proven challenging. In this pair of studies, Xinpeng Zhao and colleagues and Kaixin Lin and colleagues, respectively describe microporous materials – a glass-based ceramic coating and a ceramic composite, respectively – that exhibit passive daytime radiative cooling and resistance to harsh environments.

The cooling glass-based ceramic coating developed by Zhao et al. uses a microporous glass silicon dioxide framework embedded with aluminum oxide (Al₂O₃) nanoparticles. The dual-particle approach produces a material that has both high solar reflectance and selective LWIR emission. What’s more, the addition of Al₂O₃ prevents densification of the microporous structure, which is crucial to its functionality, during manufacturing. According to the authors, the microporous glass coating enables a temperature drop of ~3.5 to 4 degrees Celsius, even under high-humidity conditions in both the daytime and nighttime respectively. It also maintains its high solar reflectance when exposed to harsh environments.

Inspired by the carapace of the whitest known insect on earth, Lin et al. developed a cooling ceramic composite composed of a hierarchically structured microporous Al₂O₃ framework that can achieve highly efficient light scattering, high thermal emission, and a near-perfect solar reflection of 99.6%. According to Lin et al., the ceramic demonstrated continuous sub-ambient cooling with a power of over 130 watts per square meter outdoors and at noon. “Although some structures with dynamic radiative cooling capabilities have been proposed and experimentally demonstrated recently, attaining large-scale applications remains a substantial challenge,” write Donliang Zhao and Huajie Tang in a related Perspective. “Nevertheless, the findings of Zhao et al. and Lin et al. advance cooling approaches that could, if commercially applied to buildings, drive down the electrical demand of air conditioners and benefit the environment.”