Researchers have introduced a new passive radiative cooling (PRC) material designed to boost energy efficiency in the construction industry, offering a promising tool in the fight against global warming.

Application in a building envelope, with the white cooling ceramic applied on the roof. Image: City University of Hong Kong.

Researchers at the City University of Hong Kong (CityU) have created a novel material called cooling ceramic.

As per the official release, the newly designed material has showcased “high-performance optical properties for energy-free and refrigerant-free cooling generation”.

"Our cooling ceramic achieves advanced optical properties and has robust applicability. The colour, weather resistance, mechanical robustness and ability to depress the Leidenfrost effect – a phenomenon that prevents heat transfer and makes liquid cooling on the hot surface ineffective – are key features ensuring the durable and versatile nature of the cooling ceramic,” says Edwin Tso Chi-yan, associate professor at the university, and one of the corresponding authors of the paper. 

The uniqueness of this material

The study highlights that this material’s fundamental strength and uniqueness come from its 'hierarchically porous structure'.

This structure is simple to construct using ordinary inorganic elements such as alumina. The procedure consists of two basic steps: phase inversion and sintering. Notably, this technique does not require complex equipment or expensive ingredients, making large-scale production of cooling ceramics more practical and achievable.

Moreover, the cooling ceramic reduces sunlight absorption due to the high bandgap of alumina.

"The cooling ceramic is made of alumina, which provides the desired UV resistance degradation, which is a concern typical of most polymer-based PRC designs. It also exhibits outstanding fire resistance by withstanding temperatures exceeding 1,000°C, which surpasses the capabilities of most polymer-based or metal-based PRC materials," says Professor Tso. 

The researchers also added two additional properties to the material to improve its overall efficiency. The first attribute was acquired from the extraordinarily bright insects known as Cyphochilus beetles, while the second was based on Mie scattering, a phenomenon in which particles scatter light. 

The bio-inspired high-solar-reflectivity ceramic mimics the bio-whiteness of the Cyphochilus beetle. Image: City University of Hong Kong.

The cooling ceramic was able to scatter virtually all sunlight wavelengths by replicating the natural brightness of these beetles and optimising the porous structure based on this scattering principle. 

This resulted in an exceptional solar reflectivity of 99.6%, one of the highest values ever measured. Interestingly, the improved optical qualities outperform the capabilities of currently available ceramic materials in the market.

Furthermore, the cooling ceramic performs well under various weather conditions, and showcases chemical stability as well as substantial mechanical strength, making it well-suited for extensive outdoor usage.

"Our experiment found that applying the cooling ceramic on a house roof can achieve more than 20% electricity for space cooling, which confirms the great potential of cooling ceramic in reducing people's reliance on traditional active cooling strategies and provides a sustainable solution for avoiding electricity grid overload, greenhouse gas emissions and urban heat islands," says Prof Tso.

Its low cost and durability make it a great candidate for commercialisation in a variety of applications, mainly building construction, in the near future. 

The technology represents a significant advancement in the pursuit of sustainable and environmentally friendly construction methods.

As global temperatures rise, so does the need for cooling systems, which contributes to climate change. Sustainable materials provide a solution to this rising need without worsening the issue.

The new material has been reported in the journal Science.