At first sight, a transition temperature of 60 °C might seem to be too high for outdoor applications, but this value can be brought down by several degrees celsius by doping with molybdenum or tungsten. And, of course, air temperatures in countries like Australia can quite frequently go above 40 °C, with the temperature of a sunlit painted metal surface under such conditions easily exceeding 60 °C.
The differences between low and high-temperature states are accentuated in the near-infrared part of the spectrum, exactly where you would want them to be in a smart paint. But are these changes sufficient to make a difference to the heat load on a painted structure?
In a recent paper published in Nanotechnology, the team explored this question by preparing a quantity of vanadium dioxide pigment by a new, simplified wet chemical process and then carefully measuring and modelling the scattering properties of coatings made with the pigment.
The group found the answer to be both yes and no. Yes, because changing the temperature of the coating does modulate the amount of infrared light absorbed, and indeed, the changes closely follow the theoretical predictions of the authors' models. But no, because the changes are sufficient to only make a several percent or so change to the heat load on a structure.
Once again, vanadium dioxide, with its wonderful phase change at about 60 °C, is left hanging as a solution looking for a problem.