By Will Soutter
A team at Lawrence Berkeley National Lab have developed a new type of smart window coating which can selectively filter out heat and visible light using a nanostuctured composite of indium tin oxide (ITO) and niobium oxide.
Some smart glass technology, such as electrochromic glass, is already available on the market. This can typically be used to switch the glass between clear and frosted states using an electrical sugnal, or to reduce the amount of light passing through the glass without obscuring the view.
This new development, described by the Berkeley Lab team in a Nature paper, takes smart glass one step further - controlling heat transmission (in the form of near-infrared radiation) as well as visible light.
This is achieved using a composite of two separate radiation-absorbing materials with different properties. The bulk glass material is made from niobium oxide, which darkens when an electrical current is applied to it.
Embedded into this electrochromic glass are nanocrystals of ITO (indium tin oxide), a common transparent conducting ceramic used in touch screens and solar panels. When these nanocrystals are triggered with a voltage spike, they jump into a state which can filter out around 35% of the incoming NIR radiation.
This nanocomposite coating provides a three-stage filter for external light and heat - in it's normal state, the glass will act as normal. With a small voltage, the ITO crystals will absorb some of the heat, without reducing the amount of light passing through. Increasing the voltage will darken the niobium oxide glass, filtering out the light as well.
This could lead to huge efficiency benefits in buildings, decreasing the need for both air conditioning and artificial lighting. Anna Llourdes, a member of the Berkeley team, commented:
“We’re very excited about the combination of unique optical function with the low-cost and environmentally friendly processing technique. That’s what turns this ‘universal smart window’ concept into a promising competitive technology.”
Delia Milliron, Guillermo Garcia, and Anna Llordés created a new electrochromic material that can dynamically control transmission of both visible light and near-infrared light, enabling a new generation of smart windows. (Team member Jaume Gazquez not shown.) Image credit: Roy Kaltschmidt/Berkeley Lab
The researchers also discovered an additional effect, which enhances the performance of the coating even further. The ITO nanocrystals actually alter the structure of the niobium oxide glass slightly, opening up space in the matrix and allowing charge to flow more readily - making the electrical triggers more effective and strengthening the electrochromic effect.
This discovery could have much wider-reaching applications than just in smart glass, as it demonstrates the power of nanocomposites to access properties which aren't available with conventional materials. Delia Milliron, corresponding author on the Nature paper, commented:
“From a materials-design perspective, we’ve shown that you can combine very dissimilar materials to create new properties that are not accessible in a homogeneous single phase material, either amorphous or crystalline, by taking nanocrystals and putting them in glass.
"The most exciting part has been taking this project all the way from synthesizing a new material, to understanding it in great detail, and finally to realizing a completely new functionality that can have a big impact on technology."