US researchers have developed a material that is more translucent than glass and at the same time has excellent insulating properties. It could significantly reduce global energy consumption in buildings. Potential applications range from windows to greenhouses to space exploration.
Scientists from the USA presented a material that could significantly reduce the energy requirements of buildings. It is more transparent than glass and insulates as well as a solid wall. When applied to window panes, it can greatly minimize heat exchange through windows, thereby reducing the energy consumption of homes. Heating and cooling buildings accounts for around 40 percent of global energy consumption, as the US team led by Amit Bhardwaj from the University of Colorado in Boulder reports in the journal “Science”.
Although windows only make up an average of eight percent of the building envelope, they are responsible for around 50 percent of heat loss, the study authors explain. Double and triple glazed windows have already reduced heat exchange. Cellulose-based airgel films also show good insulation values - although the transparency decreases if they are more than a few millimeters thick. This is because the pore size is difficult to control when producing aerogels and varies in the range from nanometers (millionths of a millimeter) to micrometers (thousandths of a millimeter).
Bhardwaj and his team therefore looked for a process that would enable pore sizes in the range of 2 to 50 nanometers. Such pores are smaller than the wavelengths of visible light, resulting in minimal light reflection or absorption. At the same time, the small pore size ensures that molecules can hardly transfer heat. The pore walls are made of silicone, which also has low thermal conductivity.
Mochi is light and extremely transparent
In order to precisely control the pore size, the researchers use surfactants, similar to the detergent active substances in detergents. In a solution, the surfactants form self-organizing, cylindrical aggregates that structure the silicone tubes. Surfactants and solvents are then replaced by air in the drying process. The researchers call the finished material Mochi (Mesoporous Optically Clear Heat Insulator).
This material is very light and achieves an optical transparency of over 99 percent. For comparison: glass achieves less than 92 percent. Thermal conductivity is only 10 milliwatts per degree of temperature difference per meter, while still air – such as that found in down jackets or between panes of double-glazed windows – reaches around 27 milliwatts.
From greenhouses to space exploration
Due to the drying process, mochi sheets are currently limited to an area of one square meter. However, the excellent optical properties also allow the production of thicker and larger mochi elements. According to the researchers, Mochi could expand the uses of insulating glass in future building envelopes, such as windows, skylights and daylighting systems. “Other applications include greenhouse covers, protective and thermal insulation clothing as well as space research – wherever optical transparency and thermal insulation are required at the same time,” write the study authors.
In a commentary in Science, Longnan Li and Wei Li of the Chinese Academy of Sciences in Changchun confirm: “The concept presented by Bhardwaj et al. marks a transition from traditional aerogels with disordered pore structures to metamaterials with ordered nanostructures that simultaneously control thermal and optical properties on the macroscale.” From an energetic and economic perspective, the potential of the silicone metamaterial is enormous. In this way, optically transparent surfaces and even textiles could be transformed into energy-efficient surfaces and energy requirements could be significantly reduced.