i Spray-on clear coatings – a step forward for cheaper smart windows

Spray-on clear coatings – a step forward for cheaper smart windows

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RMIT researchers have developed a method for making clear coatings that can block heat, conduct electricity and may radically cut the cost of energy-saving windows and heat-repelling glass.

The spray-on coatings are ultra-thin, cost-effective and rival the performance of current industry standards for transparent electrodes. A transparent electrode is a highly conductive clear coating, combining the best properties of glass and metals in a single component, that allows visible light through.

The coatings – key components of technologies including smart windows, touchscreen displays, LED lighting and solar panels – are currently made through time-consuming processes that rely on expensive raw materials. RMIT researchers say their new spray-on method is fast, scalable and based on cheaper materials that are readily available.

The method could simplify the fabrication of smart windows, which can be both energy-saving and dimmable, as well as low-emissivity glass, where a conventional glass panel is coated with a special layer to minimise ultraviolet and infrared light. 

Lead investigator Dr Enrico Della Gaspera says the pioneering approach could be used to substantially bring down the cost of energy-saving windows and potentially make them a standard part of new builds and retrofits.

“Smart windows and low-emissivity glass can help regulate temperatures inside a building, delivering major environmental benefits and financial savings, but they remain expensive and challenging to manufacture,” says Della Gaspera, a senior lecturer and Australian Research Council DECRA (Discovery Early Career Researcher Award) Fellow at RMIT. “We’re keen to collaborate with industry to further develop this innovative type of coating. The ultimate aim is to make smart windows much more widely accessible, cutting energy costs and reducing the carbon footprint of new and retrofitted buildings.”

The new method can also be precisely optimised to produce coatings tailored to the transparency and conductivity requirements of the many different applications of transparent electrodes. The standard approach for manufacturing transparent electrodes is based on indium, a rare and expensive element, and vacuum deposition methods, which are bulky, slow and costly. This makes transparent electrodes a major cost in the production of any optoelectronic device. 

In the new study, published in the journal Advanced Materials Interfaces, researchers in RMIT’s School of Science made transparent electrodes using tin oxide, a cheaper material, ‘spiked’ with a special combination of chemicals to enhance conductivity and transparency.
The ultra-thin transparent coatings, which are more than 100 times thinner than a human hair, only allow visible light through, while blocking both harmful UV light and heat in the form of infrared radiation.

The scientists used a process called ‘ultrasonic spray pyrolysis’ to fabricate smooth, uniform coatings of high optical and electrical quality.

A precursor solution is nebulised, using commercially available technology to create a fine spray mist that forms ultra-small and uniformly-sized droplets. This solution is sprayed on a heated support layer, such as glass. 

When the solution hits the hot layer a chemical reaction is triggered, decomposing the precursor into a solid residue that is deposited as an ultra-thin coating. All the by-products of the reaction are eliminated as vapours, leaving a pure coating with the desired composition.

The global market size for smart glass and smart windows is expected to reach AU$6.9 billion by 2022, while the global low-emissivity glass market is set to reach an estimated AU$39.4 billion by 2024.

New York’s Empire State Building reported energy savings of US$2.4 million and cut carbon emissions by 4000 metric tonnes after installing smart glass windows.

Eureka Tower in Melbourne features a dramatic use of smart glass in its Edge tourist attraction, a glass cube that projects three metres out of the building and suspends visitors 300 metres over the city. The glass is opaque as the cube moves out over the edge of the building and becomes clear once fully extended.

Author Jaewon Kim, a PhD researcher in Applied Chemistry at RMIT, says the next steps in the research will be developing precursors that will decompose at lower temperatures, allowing the coatings to be deposited on plastics and used in flexible electronics, as well as producing larger prototypes by scaling up the deposition. “The spray coater we use can be automatically controlled and programmed, so fabricating bigger proof-of-concept panels will be relatively simple,” says Kim.

The research was supported through funding from the Australian Research Council, with key imaging and analysis conducted at RMIT’s Microscopy and Microanalysis Facility. The work was enabled in part by use of the Central Analytical Research Facility at the Queensland University of Technology Institute for Future Environments.

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