Scratch-resistant and lightweight casings for smart phones and laptops; featherweight aircraft wings that are highly stable as well; turbine coatings that can withstand extreme heat; photovoltaic systems that can generate electricity efficiently from 'waste' heat. All of these areas of use have one thing in common: The materials available to us today fulfil these requirements only to a certain extent, or not at all. Therefore the researchers at the SFB 986 Collaborative Research Centre have set themselves the task of creating a new 'genus' of materials — so-called 'Tailor-Made Multi-Scale Materials Systems'. The SFB 986 was inaugurated by the German Research Foundation in 2012. It involves around 70 scientists from the Hamburg University of Technology, the University of Hamburg and the Helmholtz Centre for Materials and Coastal Research in Geesthacht.
What sets the new materials apart from conventional ones is the fact that their structure is fundamentally different. For instance, a straightforward lump of iron consists almost entirely of iron atoms that are combined to form small crystals. These 'crystallites', about a micrometre in size, form the 'building blocks' of larger, massive workpieces, e.g. rails, struts or ships' hulls. The properties of the basic metal can be improved by adding other elements such as chrome or carbon — this turns iron into steel. By varying the proportions of these additives, it is possible to make steels that are harder, more heat resistant and/or more supple than the original iron.
In comparison, the materials that are being developed by the experts of SFB 986 in about 20 interdisciplinary sub-projects have a significantly more complex structure. In most cases, the basis is formed by a tiny nanostructure — here, the 'building blocks' are measured in billionths of a metre. These 'nanobricks', only visible through special microscopes, are then put together to form larger units. Then, these units are used to build up even larger conglomerates. These are what constitute the visible material. Other materials under investigation are based on extremely thin layers of minute hollow spheres, or they consist of complex nano-networks made of carbon or porous metal. In many cases, the inner structure of such materials ranges over several orders of magnitude. Therefore the experts speak of 'multi-scale' systems that are sometimes also characterized by a hierarchical configuration.
Substances of different natures are often used together so as to exploit their properties in such a way that they complement each other. In one of the projects, the basic elements are small, hard ceramic spheres. These spheres are shrouded by microscopically thin, soft plastic layers. If it should prove possible to combine both substances in an appropriate hierarchical structure, the result would be a new kind of material with some astonishing properties: It would be as hard and scratch-resistant as ceramics, but at the same time as plastic and impact-resistant as a metal.
Another class of materials — the nanoporous metals — can display fascinating characteristics when moulded in a certain way and combined with polymers. For instance, they can become electrically functional. One of the visions: A material that can be used for structural elements in aviation engineering, and at the same time receive and conduct electrical signals — a load-bearing material and sensor in one and the same component.
With other materials the aim is to design them so that they can reflect heat very efficiently. This is a requirement for heat shields in power station turbines, for instance. Or perhaps the exact opposite is required, e.g. in applications where heat radiation needs to be conducted in a very specific way. Such a property will be very useful in the coming generation of photovoltaic plants that will be capable of converting heat energy directly into electricity.
Some of the SFB 986 projects get their inspiration directly from nature: Substances like mother-of-pearl (or nacre) or dental enamel also have a hierarchical structure. The smallest basic units of dental enamel consist of nanofibres made of the mineral hydroxyapatite and 'glued' together by special proteins. In the same way as a rope is made, several such nanofibres make up a bundle that itself forms a microfibre. These microfibres are visible to the naked eye. They form a basket-like network that we see as dental enamel. This structure is what gives the enamel its characteristic properties: hard, but not brittle.
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