Clay Culture: Ceramic Coatings Laurel Sheppard

Appears in the April 2024 issue of Ceramics Monthly.

As global temperatures rise, how we cool ourselves is a growing concern. In an attempt to move away from emission-causing air conditioners, some researchers are looking to ceramic radiative coatings as one answer.

More people are at risk from heat-related diseases and death than ever before as global temperatures continue to rise. Though air conditioners may appear to be a solution, use of this technology leads to the emission of hydrofluorocarbons and greenhouse gases, which drive climate change. While energy-efficient air conditioners help reduce emissions from these devices, other cooling methods that do not cause any emissions are needed. 

Radiative coatings can provide passive cooling without the use of mechanical refrigeration equipment. These coatings are designed to reflect solar radiation and emit thermal radiation to the cold outer space, thereby achieving electricity-free spontaneous cooling. Researchers have made numerous advancements in radiative coatings in recent years, thanks largely to innovations in micro/ nanofabrication. The study described in the section below demonstrates one such recent development. 

Inspired by Beetles 

Researchers at several Hong Kong universities designed a new ceramic radiative coating that exhibits a near-perfect solar reflectivity of 99.6%. The coating’s impressive properties are due to its nanostructure, which was inspired by the Cyphochilus beetle. 

The Cyphochilus beetle, native to Southeast Asia, is considered the whitest insect on Earth. Its coloring is due to the arrangement of tiny tear-shaped scales that cover the beetle’s whole exoskeleton. 

These scales, only 6 μm thick, form a highly connected and dense network of chitin, i.e., a long-chain polymer that gives strength to the exoskeletons of crustaceans, insects, and the cell walls of fungi. Chitin scatters light extremely efficiently, resulting in the ultrawhite appearance. 

Previous studies have drawn inspiration from the Cyphochilus beetle to create sustainable and biocompatible ultrawhite coatings. But the new study took this inspiration a step further by creating a coating that is both aesthetic and functional. 

The Hong Kong researchers fabricated the ceramic coating through a process that can be easily scaled for mass production. First, they cast a solution of polyethersulfone (PES), N-methyl-2-pyrrolidone (NMP), and alpha-alumina onto a flat substrate and immersed it in ethanol, which caused the NMP to dissolve. They then sintered the material to remove the PES and bond the alumina particles in a porous pattern that resembles the Cyphochilus beetle scales. 

In addition to a record-high solar reflectivity of 99.6%, the final alumina coating exhibited an infrared thermal emission of 96.5% and withstood temperatures of more than 1000°C (1832°F). When applied to a house roof, the coating reduced the amount of electricity used for space cooling by 20%. 

Additional Properties 

Other characteristics of the ceramic radiative coating include: 

• Ultralow thickness. The coating requires a thickness of only 150 μm to achieve a reflectance above 95%. Conventional high-performance roof cooling coatings typically require a thickness above 1 mm. 
• High mechanical strength. The coating demonstrates a high mechanical strength of more than 100 MPa (building envelopes require a minimum of 35 MPa). 
• Low reflectivity. The coating has low reflectivity within the atmospheric window transmittance range at any thickness, making it suitable for coating concrete and similar substrates. 
• Subambient cooling. The coating achieves subambient cooling above 4°C (39°F) even around midday (between 11am and 2pm), resulting in lower temperatures compared to white commercial tiles. 
• Either water-loving or water-repelling. The coating can be converted from superhydrophilic (attracted to water) to hydrophobic (repels water) by impregnation with organosilicon compounds. This change to the coating causes only a small drop in solar reflectance. 
• Resistant to environmental stimuli. The coating resists pollutants when treated with fluorosilane, maintaining a solar reflectance above 97%. The coating also exhibits resistance to ultraviolet radiation and fire. 
• Recyclable. The coating is recyclable and can be turned into a new material with well-preserved optical properties. 
• Color options. The coating can be colored using a dual-layer design while mostly retaining its reflective properties. For example, yellow, red, and green coatings exhibited reflectivity in the near-infrared region of 95%, 96%, and 87%, respectively. 
  

“[This study] 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 Chi Yan Tso, associate professor of energy and environment at the City University of Hong Kong, in a university press release. 

The paper, published in Science, is “Hierarchically structured passive radiative cooling ceramic with high solar reflectivity” (DOI: 10.1126/science.adi4725). 

*www.cityu.edu.hk/research/stories/2023/11/10/new-cooling-ceramic-can-enhance-energy-efficiency-construction-sector-and-help-combat-global-warming-cityu-research

the author Laurel M. Sheppard is an award-winning writer and editor who has worked on numerous trade and association publications, including The American Ceramic Society Bulletin, Advanced Materials & Processes, Materials Engineering, Society of Women Engineers, and Photonics Spectra. She also currently writes energy content for Questline and articles for the Ohio Genealogical Society.