The future will bring transparent and highly resistant materials that have been investigated as part of an EU project.
Will they soon be translucent or transparent?
“Research has shown that the main prerequisite for translucence is a general harmonization of the optical properties of the composite components,” explains Professor Dagmar Hülsenberg, Head of the Institute for Materials Technology of the Technical University of Ilmenau. However, producing such composites is very tricky because one has to optimize the seemingly contradictory requirements for the desired maximum reinforcement and for the best possible transparency. This method is facilitated when intermediate layers made from, for example, titanium oxide or boron nitride are applied to the fibers by means of a chemical vapor deposition (CVD) process. The improved sturdiness and fracture toughness are tied to the efficacy of various reinforcement mechanisms. This is where the intermediate layers of about 30 nanometers (30 billionths of a meter) play a crucial role.
The Fraunhofer Institute for Ceramic Technologies and Sintered Materials (IKTS) in Dresden is also involved in developing transparent and extremely resistant materials. The researchers first focused on the development of halogen bulbs that have to withstand extremely hot plasma. Neither quartz glass nor the ceramic materials commonly used up to now can easily cope with the high internal gas pressure and even tend to burst. The IKTS found the solution with aluminum oxide, also known as alumina or corundum. Pure corundum, like its colored forms sapphire and ruby, has a melting point above 2,000° C. The problem is that it cannot easily be melted and subsequently cast or blown. The melt forms crystals when it solidifies, thus leading to undesirable mechanical properties. Existing production processes work with temperatures around 200 K below the melting point at which the powder grains sinter; in other words, they merely blend together on their surface. The particles grow and, as a result, the porosity diffuses light. This is the reason why these advanced ceramics are usually cloudy, like translucent glass.
The Fraunhofer Institute found one alternative as part of the “Starelight” project sponsored by the EU. “The most important factor in obtaining transparency is that the grain size of the raw materials should be well below one third of a micron. In the finished product the particles are not much more than half a micron, which is why they scarcely diffuse the light,” stresses Dr. Andreas Krell, Project Manager of the IKTS. This is possible at a sintering temperature that is 600 K below conventional temperatures. “We optimized the processes so that the final ceramic material is virtually poreless despite the low temperature, thus leading to transparence and good strength,” says Krell. This interesting approach with nanopowders may be able to be used with other systems, such as fiber-reinforced glass.