Nano-structured multi-phase materials systems
To date, engineers have distinguished between structural materials and functional materials. The first category includes steel, concrete or aluminium, which are used for the load-bearing structures in houses, bridges and vehicles. Functional materials, on the other hand, provide the basis for active components: Chips made of silicon do the calculations in our PCs, piezo-ceramics force diesel fuel through injection jets and ink through the printhead of an inkjet printer.
The experts of Area B are working on the basic properties of materials that can do both: The new materials are to provide for a solid, stable structure and at the same time be capable of performing useful functions, for instance as sensors or actuators. An example for this is provided by aircraft wings made of carbon fibre reinforced plastic (CFRP). They are very light and also extremely strong. Their disadvantage: As CFRP does not conduct electricity, it is liable to be damaged when struck by lightning. Therefore CFRP wings have to be coated with metal at present — an expensive procedure. Also, the metal coating makes it more difficult to perform routine inspections, e.g. for hair cracks in the wing.
These disadvantages could be avoided with a carbon-based material that is stable and at the same time can conduct electricity. It would be able to conduct lightning strikes so that they do not cause any damage, without having to be given a metal coating. In principle, such a material could even function as a sensor: The material could send electrical signals while the wing oscillates. The signals could be used to detect micro-cracks as they develop. In other words: What is basically a structural material becomes a sensor as well.
To turn such applications into reality, the researchers are developing new kinds of composite materials. In contrast to currently available CFRP materials, the basic elements would not be carbon fibres of micrometre dimensions, but significantly thinner ones in the nanometre range. These range from carbon nanotubes, for instance, to complex nanofibre meshes or aerographite. Aerographite, the lightest material in the world, was first manufactured by researchers from the Hamburg University of Technology some time ago. The idea: By filling the spaces between these 'nano-bricks' with a suitable polymer, one would have a new kind of carbon fibre composite material. This could be lighter and more versatile than conventional CFRP materials and perhaps even easier to manufacture.
Sponge-like metallic structures also appear very promising. These are metals that have innumerable nanometre-sized pores that make them significantly lighter than a compact piece of metal of the same size. In principle, such materials can also serve as sensors or actuators: If such a metal mesh is subjected to an electrical voltage, it will either expand or contract — the basis for an actuator. If one exerts pressure on the material, the electrical voltage will change — it becomes a sensor.
Also, the researchers intend to fill the tiny pores with polymers in a certain manner, producing yet another class of composite materials. One of the ideas arising from this: Some of these polymers contain channels of nanometre size that could conduct water. The researchers hope to be able to use this to convey electrical charges to the surface of the metal. This would present another means of providing such metallic foams with switching and control functionality. For instance, they could be used as simple expansion sensors such as are needed in many technological applications.
It could also prove worthwhile to integrate a range of size scales and hierarchical levels into the materials: In most cases, it is advantageous for material functionality if it has a large number of nanopores, meaning that its internal surface area is very large. However, in order to exploit this functionality effectively, i.e. switch it on and off reliably, additional channels on the micrometre scale are needed. Such channels would greatly increase switching process transmission speeds. By way of illustration, one can imagine how business is conducted in a city: At the end of the retail chain, the basic transactions are conducted in individual shops. But these shops are supplied by means of relatively few, but large traffic channels: motorways, railways and rivers.
- The mechanical properties of structural materials improve as the size of their basic elements decreases. What may be the reason for this? What basic physical principles play a role here?
- To date, it has proved possible to manufacture nanoporous gold and platinum. How can one achieve this with other metals, especially titanium and aluminium?
- How can one devise ways of introducing pores at different levels of magnitude into a metal?
- How can the mesh density of carbon nanofibres be increased?
Nanoporous metal foams are produced by means of electrochemical corrosion. In this technique, a solid lump of metal, e.g. an alloy of gold and silver, is immersed in an ultra-pure acid. Then an electric current is conducted through the metal. This is sufficient to corrode the silver out of the alloy. The remaining gold 'rearranges' itself and forms a porous piece of metal with nanostructures of varying complexity. In order to introduce larger channels into the foam to improve signal transmission, the first corrosion process can be followed by a second one with different process parameters.
If these tiny pores are to be filled with polymers, the porous metal is immersed in liquid synthetic resin under vacuum conditions. It is then reaerated, and the pressure of the air forces the resin into the pores of the metal. Finally, the resin is allowed to harden, and this makes the porous metal more stable.
In order to produce new carbon-based composite materials, a lump of plastic is subjected to a pyrolytic process — i.e. to the influence of heat — that degrades it. Plastic consists mainly of hydrocarbons. At high temperatures the hydrogen component 'evaporates' out of the material. The remaining carbon reverts to a porous, networked substance. Depending on the processing conditions, these nano-meshes and nano-frameworks can have a range of densities. These structures can then be mechanically stabilized by filling the interstices with polymers.
Materials based on nanotubes can be 'cooked' by vaporizing carbon over a silicon surface in a special furnace. The result is an extremely thin layer consisting of tubes that can be readily detached from the silicon. Then several such layers are laid over each other. They are made more dense by compression by means of a roller. The nanotubes are also soaked in liquid synthetic resin. This makes the material more stable after the resin has hardened.
In order to find out what properties a material produced in the laboratory displays, the researchers employ special measurement techniques. Amongst other things, they bombard it with the extremely brilliant X-ray radiation source supplied by the PETRA III accelerator at the DESY research centre in Hamburg. This makes it possible to determine how the atoms and pores of the material are distributed.
A number of theoreticians are closely involved in these activities: They investigate how well a polymer bonds with a carbon nanotube, describe the behaviour of internal interface surfaces and calculate the mechanical properties of macroscopic material samples. The results obtained in this way enable them to predict what kind of laboratory experiments are likely to be of particular interest.