When thinking about ceramics, the first thing that comes to mind is crockery or sharp knives. However, much more can be done with this hard and brittle material beyond the already impressive list of sensors, capacitors, etc., is the opinion of Professor Jürgen Rödel from the research group for non-metallic inorganic materials at TU Darmstadt. He is searching for new applications for the materials comprising many tiny crystals (polycrystals).
Rödel’s approach seems paradoxical initially. The materials researcher aims to improve ceramics by disrupting their atomic structure. “Our aimis to do this in a controlled way, however”, he says. His team is focusing on a type of crystal defect, the creation of which appears trivial for metals, but for hard ceramics has hitherto appeared virtually unthinkable.
Reinhart Koselleck project
The “Research of Dislocations in Ceramics” is supported by DFG as a Reinhart Koselleck project with 1.25 million euros for five years. The programme targets areas for particularly innovative and in a positive sense risky research and thus attracts prestigious scientists and researchers. Professor Jürgen Rödel was the first person at TU Darmstadt to gain such DFG support.
“For this purpose, we use methods that neither chemists nor physicists use”, Rödel explains. These include the mechanical deformation of ceramics under controlled pressure and temperature. Moreover, as the Darmstadt team masters the methods for characterising materials, they see themselves as ideal experts for the task. Some types of crystal defects have been well researched. One of these is the absence of an atom in an otherwise regularly formed crystal lattice.
Such a “point defect” resembles an empty seat in an otherwise full cinema. Point defects increase the conductivity of semiconductors in electronics. Two-dimensional defects have also been well researched. These are interfaces that separate two grains in a polycrystal. The intermediate case of a one-dimensional defect has, however, remained untouched territory as chemistry alone was not enough, Rödel says. In the case of such a dislocations, the defect takes the form of a straight line through the crystal. Seen in the light of the “cinema” metaphor, this would be an empty row of seats.
Dislocations in ceramics are electrically charged, which makes them technically interesting. They serve as channels for the electrical charge, thus increasing the conductivity. As they simultaneously retard the propagation of heat, they are well-suited as thermoelectric elements. These materials convert waste heat into electricity. Rödel mentions a further useful effect increasing the efficiency of fuel cells: “At the end of the dislocations, namely on the surface of the crystal, oxygen can be incorporated or removed”. Furthermore, dislocations remain stable up to 500 degrees Celsius whereas point defects already move at around 100 degrees.
The prerequisite for technical usage is to incorporate dislocations into a ceramic in a planned way. This has hardly been achieved hitherto. Rödel’s team aims to change that. “We are attempting to create as high a density of dislocations in ceramics as possible”, Rödel explains. One of the challenges is to find the optimum temperature, electrical potential and other parameters for the mechanical deformation. “The result is still open”, Rödel confesses. But we are making progress: The first partners in his network are already working on dislocation-based photovoltaic systems in England and on high-resolution electron microscopy in Japan.