Surfaces exposed to sunlight absorb solar heat and radiate it in the form of infrared radiation. When the heat emitted is greater than the heat absorbed, the surface cools. This is called passive cooling. To achieve this, you need materials that strongly reflect sunlight while emitting long-wave infrared light that can pass through the atmosphere without being reflected back.

Ceramic materials made from silica and alumina can meet these requirements and are also wear-resistant, thermostable and water-resistant, making them attractive for use in the built environment. Two separate groups of researchers have now published two similar microporous materials that are capable of passive cooling in Science. This could eventually provide a more energy-efficient alternative to conventional, energy-guzzling air conditioning.

How a material refracts and reflects light depends on its refractive index and extinction coefficient. A material suitable for passive cooling has a high refractive index, a low extinction coefficient in the visible spectrum and a high extinction coefficient in the infrared part of the solar spectrum. Unfortunately, materials with the desired extinction coefficients often have a refractive index that is (too) low. A solution to this problem is to modify the material at the nanoscale to scatter the incident light. This can be done, for example, with silica and alumina particles - these scatter light in the wavelength range close to their own diameter, increasing the reflectivity.

The research groups of Xinpeng Zhao (University of Maryland) and Kaixin Lin (City University of Hong Kong) have optimised the microstructures of their silica and alumina to reflect sunlight almost perfectly. The two cooling materials have different structures, but the underlying design principles are similar. Zhao’s team made a glassy ceramic coating of microporous SiO₂ containing Al₂O₃ nanoparticles. They mixed aluminium oxide particles averaging 0.5 µm in size through the silica so that they scatter incoming light. The nanoparticles also ensure that the microporous structure does not clog up during the manufacturing process. According to the authors, their coating can provide up to 4°C of cooling.

Instead, Lin’s group made a structure out of Al₂O₃ and managed to make light scatter in the alumina pores. He was inspired by the shield of Cyphochilus, the whitest beetle on earth. He reports a cooling capacity of 130 watts per square metre at midday. They carried out tests on two model huts, one with a roof made of their white ceramic cooling material and the other with white commercial roof tiles. The difference between the indoor temperatures reached a maximum of 2.5°C over a period of four days. They also compared air conditioning consumption. When they set the indoor temperature to 25, 23 and 20°C, the energy savings in the hut with their ceramic tiles were 26.8, 22.6 and 19.6% respectively. A simulation of the energy consumption of a typical four-storey apartment building, with the material on both the roof and the walls, showed that in tropical areas it can save more than 10% energy per year.

Zhao et al. (2023) Science https://doi.org/10.1126/science.adi2224

Lin et al. (2023) Science https://doi.org/10.1126/science.adi4725