Quasi-self-similar structures with multifunctional properties
Dental enamel, sea shells, bones — living nature has produced many different kinds of very hard biological materials. These bio-materials have remarkable characteristics: Compared with pure minerals such as marble, they are less brittle, i.e. they do not break or splinter easily. This comes from their special internal structure: If one studies tooth enamel under a microscope, one first finds a complex of long ceramic crystals that are divided up by thin, soft 'bands' containing more protein. When the magnification is increased, it becomes clear that each ceramic crystal consists of a bundle containing many ceramic fibres, and that the whole bundle is enveloped in a thin layer of protein. Under even greater magnification it is possible to see the individual fibres of the bundle in detail. They have a diameter of a few nanometres and are also each individually wrapped in an extremely thin polymer layer.
The advantage of this hierarchical 'self-similar' structure: Although dental enamel is the hardest material in the human body, it only seldom breaks and is not likely to develop cracks. This is because dental enamel is not only hard, but relatively pliable. When subjected to a mechanical load it can deform quite a lot. Usually, one expects metals to behave in such a way. The reason for this is as follows: The soft polymer layers and envelopes that are present in the dental enamel in very varying orders of magnitude have the effect of a mechanical buffer.
The specialists working in Area A use this fact as the starting point to develop methods for manufacturing a new class of materials. How can this be done? The most basic units are tiny amounts of material of a defined size, such as ceramic spheres measuring some nanometres or micrometres in diameter. These particles, themselves very hard and brittle, are provided with a soft covering layer. Such a layer generally consists of organic polymers, i.e. plastic. The layer functions like an adhesive, causing several of the particles to clump together to form a larger agglomeration. This agglomeration is itself coated with a new and thicker polymer layer — and can then join up with other agglomerations to form an even larger entity. The result of such a hierarchical structuring process is a ceramic material containing its own 'shock absorber'. The mineral nanoparticles provide for hardness and stability, while the polymers, which are to account for five to ten percent of the material, increase the material's resistance to fracturing and make it somewhat more flexible. Ideally, the material would become as hard and resistant to scratching as a mineral, but would be less prone to breakage or splintering.
An interesting application would be mobile phone or laptop casings that would be significantly better able to resist scratching than today's metal or plastic casings, but still almost as light as aluminium. And there are further potential advantages: In principle, they could be designed in such a way that radio waves could pass through them (in contrast to metal casings), which would improve mobile phone reception.
Ingenious optical effects are possible as well: The material could be made to shimmer like mother-of-pearl (nacre), but without the need to apply paint. If the nanoparticles were to be made of so-called piezoelectric ceramics, the material could even be used as a sensor. Piezoelectric crystals have a special property: When put under pressure, they generate a very small electrical voltage. This would make it possible to integrate pressure sensors into the casing itself, for instance for switching a device on or off.
And it would appear possible to use the opposite effect as well: By applying an electrical voltage to the material, it deforms slightly, making it possible to use it as an actuator (i.e. as a switch for triggering a certain action). It is also likely that such materials can be used to reduce unwanted vibrations or for active sound insulation.
Another plus-point: In contrast to the production processes for currently available high-performance ceramics, it is not necessary to use temperatures in excess of 1000 °C. Temperatures of no more than 200 °C are sufficient. This makes the manufacturing process more energy-efficient, cheaper and climate-friendly.
In the long-term, it is possible that the plastic polymers could be replaced by biomolecules. This would make the material biodegradable. In other words: It could, in principle, be thrown onto the compost heap when its service life has come to an end.
The scientists have already managed to develop some preparatory modelling systems. For instance, they have succeeded in coating ceramic nanospheres with defined quantities of polymer molecules. Now Area A of SFB 986 is giving priority to the following questions:What are the most suitable combinations of nanoparticles and polymers?
- Having found suitable candidates, what are the best proportions by weight or volume between the nanoparticles and the polymers?
- What options are available for fine-tuning material properties by varying the size and shape of the nanoparticles as well as the agglomerations made up of them?
- How can we obtain a better understanding of the details of the processes involved through the development of physical theories and computer models?
To produce the nanoparticles, i.e. the 'building blocks' of the new material, the researchers prepare a chemical solution with the appropriate ingredients, e.g. calcium and carbon dioxide. Under certain conditions, these two chemicals combine to form calcium carbonate nanoparticles. Then a polymer is added to the solution. The polymer molecules attach themselves to the particles and form an envelope around them.
The scientific challenge here is to ensure that the polymers adhere closely to the particle surfaces, and at the same time combine with each other to form a dense network. Only then can the coated particles join up to form larger agglomerations in the required manner. In preparation for the next step, the agglomerations must first be rinsed and cleaned. In order to provide them with a new, larger covering, another kind of polymer is added to the solution
In principle, this step can be repeated several times, so that ever larger agglomerations develop. At each individual stage of this hierarchical process, the covering can be configured to have particular properties. This means that it is possible to tailor the material properties as required, e.g. with regard to its plasticity.
Finally, the material is dried and, if necessary, compressed: The spherical agglomerations become flat 'pancakes'. This trick has the effect of increasing the transmission of forces within the polymers — the material becomes more stable.
The Centrum für Angewandte Nanotechnologie (Centre for Applied Nanotechnology, CAN GmbH) is already working to automate the nanoparticle production so that larger quantities can be manufactured. This is an essential prerequisite for using them on an industrial scale.
Another production method being investigated by the SFB 986 that appears to be suitable for mass production is the so-called 'fluid bed granulation' process. Here, a fine powder is caused to swirl above a bed of air while being coated with a polymer. The polymer makes the powder particles accumulate to form larger ones. A number of SFB 986's scientists are involved in finding out more about the various theoretical systems that have been developed to describe this process.
Some of these describe the nanoparticles at the quantum level, i.e. the level of individual atoms: What are the exact chemical bonds between the polymer molecules and the particles? Which polymers are suitable for which nanoparticles — and which are not? Other theoreticians are working at the next level of magnitude, on the scale of micrometres. Their task is to find out more about the quality of the bonds between the agglomerations. Yet other experts are developing computer models of the process technology. They are using highly complex routines to investigate the behaviour of the rotating layer of air during the fluid bed granulation process.
The work of the theoreticians and that of the experimental scientists complement each other: The theories provide starting points for the experiments and make it clear what sort of polymers are most likely to produce the desired results. Equally, the experimental results produce important information for the theoreticians: They provide feedback as to the reliability of the theories and where there may be room for improvement.