Glass, whose transparency can be varied by applying an electric current, is available from various manufacturers. Two teams of researchers from the U.S. universities of Delaware and Pittsburgh are now exploring new and probably much cheaper routes.
The group of researchers from Pittsburgh is pinning its hopes on microstructured glass. Using a reactive ion etching procedure, they expose structures that resemble blades of grass. Tiny polymer particles on the surface serve as a stencil. “Nanograss glass” is what they call the result of this process. The length of the blades in this grass mat made of glass can be controlled during the etching process. Measurements carried out using a spectrophotometer in the UV, visible (VIS) and near infrared (NIR) light range show that the structure with 2.5 micrometer long and a few hundred nanometers thin blades lets through 93.5 percent of the light falling on it with 97.5 percent clouding.
The structure only scatters light this much if it is dry. If, however, it is flooded with water, acetone or methyl benzene – whose refractive index is similar to that of glass – the cloudy glass becomes clear. Group leader Paul W. Leu believes that this glass could in future be used for smart windows which can be made opaque within seconds if necessary. He added that “Nanograss glass” is also an attractive proposition for increasing the efficiency of photovoltaic cells and LEDs. Not only the considerable scattering of light but also the self-cleaning properties of the mat structure offer advantages.
A team from the University of Delaware is also trying to influence the transparency of glass with the help of liquids. In this case, however, the researchers headed by Keith Goosen and Daniel Wolfe are working with an additional light-reflecting layer of plastic. Over the course of the project, they optimized the design of this layer using 3D printing. One advantage is that they do not need to produce a new mold for every change as is the case with injection molding. The result is a honeycomb-like structure which, when dry, reflects light in exactly the direction from which it came. If, however, methyl salicylate (whose refractive index is close to that of the plastic used) is pumped into the honeycombs, the structure becomes clear and transparent. Depending on the switching status, the transmission of the smart glass from Delaware can be anywhere between eight and 85 percent.
The researchers expect their switchable glass to cost just a tenth of what customers previously paid for electrically switchable glass. One reason for this is that the structure developed on a 3D printer can be injection molded for commercial use. Possible applications include windows and roofs of buildings which reflect sunlight in summer and let it through in winter, or car windows which prevent the vehicle heating up in summer.
The Fraunhofer Institute for Mechanics of Materials IWM in Freiburg is carrying out research in an entirely different area. Here too, they are looking at smart sheet glass, though in this case forming it. The researchers are pinning their hopes on a combination of an oven, a laser and gravity. In the oven, they heat sheet glass to a temperature where it is just about to become viscous. They then use a scanner system to guide the beam of a high-performance CO2 laser onto the heated glass. In the areas where the beam strikes the glass, it becomes soft and starts to bend under the weight of the rest of the sheet.
The IWM researchers have optimized the new bending procedure to the point where they can program the laser process exactly in advance. Using the desired geometric data, they calculate where the laser should heat the pre-heated glass and for how long, how often and with what power. The fully automated, computer-controlled laser bending procedure makes it possible to produce individual glass objects in small numbers and even individual items economically. According to information from Tobias Rist, the IWM group leader responsible, the entire procedure takes around 30 minutes. For maximum productivity, the bent glass cools outside the oven while the procedure for bending the next sheet of glass can begin.