The passive radiative cooling (PRC) material, developed at the City University of Hong Kong (CityU), could be applied to buildings to reduce demands on energy-intensive cooling processes.
“By reducing the thermal load of buildings and providing stable cooling performance, even in diverse weather conditions in all climates, cooling ceramic enhances energy efficiency and can combat global warming,” the researchers said.
PRC is considered one of the most promising green cooling technologies for curbing soaring demand for space cooling and reducing environmental pollution, said Professor Edwin Tso Chi-yan, one of the corresponding authors of a paper on the work.
Current PRCs use nanophotonic structures that are limited by high cost and poor compatibility with applications, however, while polymeric photonic alternatives lack weather resistance and effective solar reflection.
“Our cooling ceramic achieves advanced optical properties and has robust applicability,” said Professor Tso. “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.”
The new material has a ‘hierarchically porous’ structure as a bulk ceramic material, which is easily fabricated using accessible inorganic materials such as aluminium oxide (also known as alumina), through a simple two-step process involving phase inversion and sintering. No delicate equipment or costly materials are required, the researchers claimed, making scalable manufacturing feasible.
By mimicking the bio-whiteness of the Cyphochilus beetle and optimising the porous structure, the cooling ceramic efficiently scatters almost all the wavelength of sunlight, reportedly resulting in near-ideal solar reflectivity of 99.6% and a high mid-infrared thermal emission of 96.5%. These optical properties surpass those of current state-of-the-art materials, the team said.
“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,” said Professor Tso.
The researchers said that beyond its exceptional optical performance, the cooling ceramic exhibits excellent weather resistance, chemical stability and mechanical strength.
The work was published in Science.
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