This makes the new material resistant to high temperatures, even though it is amorphous. Amorphous materials are assumed to be thermally unstable, which is why Jansen had his work cut out for him convincing the materials scientists. The reason, again, is the energy. If one imagines it as a mountainous landscape, the crystals correspond to the rocks that have rolled down into the deepest energy valleys. However, on their fall into the energy valley, rocks, which correspond to amorphous structures, get stuck in an energy hollow on the slope. If increasing temperatures now shake the energy landscape like strong earthquakes, these rocks tend to jump out of their perilous metastable position again: they roll further into the energy valley, and the amorphous structure reorders itself to form a crystal. The material thus radically changes its properties, which would destroy a machine. In the Stuttgart concept, however, the local energy hollows are so deep that the rocks stay inside, and the ceramic material retains its amorphous network.
This project has been aimed at industrial application right from the start. “This is why the synthesis of the precursor molecule was designed to be environmentally and economically sustainable,” says Jansen. All constituents are low-cost, readily available chemicals. The only waste product produced is hydrochloric acid, which can be reused as a chemical.
The manufacture of the ceramic required an unconventional method. The possible solid starting compounds decompose before melting, and after cooling, the desired amorphous network would not be obtained. The network must therefore be constructed step by step from basic molecular components. In the center of these molecules is a nitrogen atom that bonds one silicon and one boron atom. At the edge are groups that act like the components of a superglue. In the second step, the “polycondensation,” they allow the basic molecular components to combine in a flash. “This must work like instant glue,” says Jansen. The polymer network thus formed already largely corresponds to the amorphous structure of the ceramic, but residues of glue still remain between the boron, nitrogen and silicon atoms. In the final step, the chemists heat the polymer in order to drive out these residues. Starting at 600 degrees Celsius, the organic substances escape from the network as pyrolysis gas. The pyrolysis is therefore the only step in the synthesis sequence in which material is lost.
Jansen proudly shows one of the results of 20 years of research: the pitch black fiber was produced from the Stuttgart ceramic by the Fraunhofer Institute for Silicate Research (ISC) in Würzburg, a longstanding cooperation partner. It is surprising that this fluffy material is a ceramic – and that it withstands temperatures above 1,500 degrees Celsius without significant loss of mechanical strength. The new ceramic can be processed in many different ways. It can be ground to a powder, for example, that can be sintered at high temperatures to produce components.
“It is also possible to manufacture coatings or infiltrations,” says Jansen, “and to draw these fibers here.” They are the most developed. At the ISC it is possible to witness how they are manufactured: the Würzburg-based researchers built a pilot plant for this, an intermediate step between laboratory and industrial production. This is where they draw the initially colorless ‘green’ fibers out of the polymer before heating them in an oven. Today, the plant already manufactures 50 kilograms of polymer per run. “We have improved the synthesis again and again, changed the constituents and thus significantly improved the yield and the purity of the ceramic,” says Dieter Sporn, who headed the project at the ISC for many years.
One variation of the fiber whose network also contains carbon in addition to silicon, boron and nitrogen has proven to be particularly heat resistant. This SiBNC fiber withstands temperatures of up to 1,500 degrees Celsius in air without chemically decomposing. “No mass loss occurs in helium, even at temperatures of up to 2,000 degrees,” explains Jansen. The new fiber is thus significantly superior to expensive ceramic fibers made of silicon carbide (SiC), which
are already commercially available. Although it reacts just like these in air with oxygen at 1,500 degrees Celsius, it forms a double layer that protects it from the aggressive oxygen. “The commercial SiC fiber, in contrast, simply corrodes through,” says Jansen.