Every year in Australia, more than 5000 children are rescued after being left unattended in a car. Not all of those children are saved before they succumb to death by heatstroke, with at least ten Australian children dying in the past five years after being left inside a hot car. The number of animal deaths has not been recorded but according to the RSPCA, is lightly likely to significantly exceed the number of human deaths. A dog can overheat in six minutes inside a car during an Australian summer.

Researchers from the Centre’s RMIT team are working to eliminate these tragedies by developing a meta-surface coating that can be sprayed on to existing car windows and windshields at room temperature. This coating, as reported on in ACS Applied Nano Materials, is made from vanadium dioxide (VO2), a thermochromic material with optical properties that change as it changes in temperature. The VO2 coating transmits both visible light and the infrared waves responsible for much of the sun’s heat. Once the coating reaches a certain temperature, its properties change. It still transmits visible light but once it switches from a semiconductor to a metal, it reflects infrared, preventing most of the heat from passing through the window into the car.

Previous attempts to solve the problem of overheating cars using light filters have had moderate success. There are some electrochromic glasses on the market that can switch between being transparent and opaque by applying voltage. This requires an electric charge, and it blocks all light, not just infrared. The Centre’s VO2 coating is solar powered so needs no connection to an electricity source and it allows for the user to see through the glass while the heat is being reflected.

Earlier versions of thermochromic coatings were applied to substances using a process called annealing, which heats the substance to a very high temperature in a vacuum environment before slowly letting it cool, an expensive process that might work for new car windows and windshields but doesn’t solve the problem of overheating in the almost 15 million passenger cars currently on the road in Australia.

In addition, these earlier coatings corroded over time when exposed to oxygen, making them impractical for everyday use. This new VO2 coating includes other polymers that give the coating added stability against oxidization and greater uniformity, which improved performance.

Lead researcher Sumaiya Kabir says “This is a huge step in the development of a glass coating that reflects heat while still transmitting visible light. The threshold for the transition from infrared-transmitting semiconducting insulator to infrared-reflective metal is currently 68 degrees and the next steps in the research is to lower that temperature threshold by combining it with other materials.”

Chief Investigator Madhu Bhaskaran says, “This metasurface coating has a wide range of applications beyond smart windows. It could be used for smart wearables and reconfigurable electronics also. The strength of it is in its compatibility with different scalable manufacturing processes such as dip coating, drop casting, and screen printing, offering great feasibility for further scaling up.”

For more information about this research, please email connect@tmos.org.au

Solution-Processed VO2 Nanoparticle/Polymer Composite Films for Thermochromic Applications

Sumaiya Kabir, Dan Yang, Aminuddin Bin Ahmad Kayani, Huihui Zhang, Shruti Nirantar, Sharath Sriram, Sumeet Walia, and Madhu Bhaskaran

ACS Applied Nano Materials, 20^th July 2022

Thin films composed of vanadium dioxide (VO2), a well-known thermochromic material with reversible insulator-to-metal-transition near room temperature, are intriguing for intelligent and energy-efficient heat-blocking applications. However, the conventional vacuum-based deposition methods often involve a high-temperature annealing process, and oxidation of VO2 under air exposure further limits their practical applications. In this work, we demonstrate a room-temperature solution process to prepare VO2-based thermochromic thin films using a smart ink composed of crystalline VO2 nanoparticles. To enhance their chemical stability against oxidation and assist in the uniform deposition of the VO2 thin films, polymers were used as both capping agents and for surface modification of the VO2 nanocrystals. Specifically, the concentration of VO2 nanocrystals, the type of polymers, and the molar ratio between VO2 and polymers are systematically tailored, and their effects on the thermochromic performance are also explored. It is revealed that the inclusion of optimum polymers enhanced the thermochromic performance with an almost 4-fold increase in IR switching with a visible luminous transmittance of 86% and a solar modulation of 17.61%. In addition, the inks are compatible with an array of scalable manufacturing processes. We demonstrate uniform films on different substrates, both rigid and flexible, by dip coating, drop casting, and screen printing, offering great feasibility for further scaling up.