Making windows more flexible

A smart window material that is also flexible could revolutionize architecture and vehicle design allowing control of heat and light to improve efficiency as well as potentially opening new solutions to as yet unrecognized problems.

Delia Milliron of the University of Texas at Austin, USA, and colleagues have devised a low-temperature acid-catalyzed condensation of polyniobate clusters process for making depositing a smart coating on to a plastic substrate. The same approach also allows "nanocrystal-in-glass" composites, i.e. tin-doped indium oxide (ITO) nanocrystals embedded in NbOx glass to be prepared. The method gives researchers an alternative to attempting to make transparent composites with glass itself. The team has demonstrated their flexible electrochromic device, which responds to a 4 volt input to lighten or darken the material and affect the degree to which it transmits near-infrared radiation. The work was carried out in collaboration with scientists at the European Synchrotron Radiation Facility and CNRS in France, and Ikerbasque in Spain. [Milliron et al., Nature Mater. (2016) DOI: 10.1038/nmat4734]

The nanostructured material, niobium oxide, in common with those made using sputtering techniques or by solution coating with high temperature annealing at high temperature is amorphous but has a unique local arrangement of linear chains of atoms. These chains allow ions to flow in and out of the material. "There's relatively little insight into amorphous materials and how their properties are impacted by local structure," Milliron explains. "But, we were able to characterize with enough specificity what the local arrangement of the atoms is, so that it sheds light on the differences in properties in a rational way."

UT's Graeme Henkelman adds that the determination of the atomic structure for amorphous materials is far more difficult than for crystalline materials and so the team had to use a combination of X-ray scattering and spectroscopic characterization to obtain an atomic structure that was consistent with both experiment and computer simulations. "Such collaborative efforts that combine complementary techniques are, in my view, the key to the rational design of new materials," he suggests.

The same insights that have emerged from this work might also be exploited in the design of other amorphous materials for a wide range of engineering applications such as the development of supercapacitors for storing electrical energy from sustainable but intermittent or periodic generation sources such as wind and solar power.

The team's next challenge will be to optimize the flexible material so that their low-temperature process makes substances that exceed the performance of conventional electrochromic materials. "We want to see if we can marry the best performance with this new low-temperature processing strategy," Milliron says.

"The next step is to apply the newly developed low temperature process to materials which have enhanced optical performance," Milliron told Materials Today. "Specifically, we want to be able to block more infrared and tint the windows to a darker hue than is possible using the materials that we used in our proof-of-concept study just published. We have demonstrated enhanced optical coloration previously but it required multiple high temperature processing steps, so we need to figure out how to use the low temperature process on these other, improved materials.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".