Overview of the Project Area C

General Information

Materials systems for photonics at high temperatures

Whether in a power station's combustion chamber or a jet aircraft turbine — many important technological processes take place at temperatures on the hot side of 1000 degrees. This means that materials are required that can not only fulfil their specific function, but are also extremely heat resistant. The researchers from Area C are studying the fundamentals of a new generation of heat-resistant materials.

These new materials, as well as being able to withstand extreme temperatures, are to be capable of fulfilling some interesting new functions: They are to be able to manipulate the heat radiation present at these high temperatures in ingenious ways. In electronics, information processing is carried out through the manipulation of electrons. In analogy to this, experts speak of photonics where light manipulation is involved. Photonics is regarded as one of the most promising key technologies of the coming decades.

Specifically, the SFB 986 experts are developing so-called multi-scale materials, i.e. materials that have different structures at different size scales. Such systems can, for instance, reflect heat especially well, perhaps as a highly effective heat shield in a power station turbine. Or the opposite property may be exploited: They may be used to conduct heat very well and deflect it in a particular direction (e.g. onto a thermovoltaic cell). Such a thermovoltaic cell can convert heat radiation directly into electric current — something that engineers have been dreaming of for a long time

The following principle applies both to thermal power stations as well as to aircraft engines: The higher the temperature in the turbine, the greater the efficiency rating of the system concerned. Power station turbine blades are generally made of nickel alloys at present. To prevent them from melting, they have to be water-cooled and also coated with a special ceramic 'heat shield' (known as thermal barrier coating) that is usually made of zirconium oxide.

This is the same material that the SFB 986 researchers have selected as their starting point. However, they are pursuing a new strategy: Instead of using zirconium oxide crystallites as the 'building blocks' of the material they are developing (as in conventional engineering) they employ tiny, similar spheres as elementary units. These spheres have diameters that range from 100 nanometres to a few micrometres. They may consist entirely of zirconium oxide, have a metal core, be coated with other materials — or they may even be hollow. Just as a greengrocer piles oranges into tiers, these spherelets can be arranged to form artificial crystals that display a more or less periodic structure.

This kind of structure has a very marked effect on infra-red radiation, which is – at these high temperatures – the dominant form of heat energy. The structure reflects infra-red radiation like a mirror.

The researchers are pursuing the following strategy: By selecting the size and alignment of the spheres carefully, they can exactly determine the wavelength that is to be reflected. If several such structures can be combined in a given material, a broad range of wavelengths can be reflected. The result would be a highly effective heat shield. Using such a shield in a turbine would mean that considerably higher gas-flow temperatures could be achieved. Power stations would be more efficient, and aircraft engines would use less fuel. Alternatively: If the combustion temperature were to be maintained as before, the thermal stress on the turbine blades could be reduced significantly, resulting in longer service lives.

On the other hand, the new materials could serve to conduct heat radiation very efficiently. This property is required in thermo-photovoltaics. This is the technology in which 'solar cells' generate electricity not directly from sunlight, but from other sources of heat such as waste heat from power stations and industrial plants. The problem to be overcome: The cell must not be exposed directly to the heat, because it would then operate inefficiently or even be destroyed. The solution is to place a special device between the heat source and the cell — a tandem consisting of an absorber and an emitter. This pair has the purpose of converting the heat effectively to a narrow infra-red frequency band for the photovoltaic cell.

Best results are obtained with an extremely small displacement between the absorber and the emitter — less than 100 nanometres. This space must be kept under vacuum conditions to insulate the heat.

These requirements represent a huge technological challenge. To find a way round it, the experts are experimenting with a new class of materials, so-called hyperbolic optical materials. These materials display certain physical characteristics that make it possible to increase the distance between the absorber and the emitter to several micrometers. This reduces the technological challenge considerably. Suitable materials could be combined in a 'sandwich' technique, e.g. nanometre thick layers of gold and silicon. Such a system would be able to transmit a large amount of infra-red radiation, but with a suitable design practically no other types of heat energy would be transmitted. This would mean that the photovoltaic cell would be 'fed' with exactly the right kind of photons. It would be able to produce electricity with a high degree of efficiency, but without overheating.

The research questions

  • How can thermal barrier coatings be produced without cracks developing? How can they be stabilized and within which temperature limits can they operate?
  • How should the layers be arranged so that they can reflect heat radiation in the same way that a bathroom mirror reflects light?
  • What materials systems are best suited to achieve the most effective transfer of heat radiation in thermo-photovoltaic plants?
  • How can heat conduction be suppressed in such systems?

How stable are they across given temperature ranges?

Methods and instruments

So-called inverted zirconium oxide structures represent promising candidates for innovative heat protection coatings. Here, the spaces between stacked spherelets are first filled in, and then the spherelets are removed. Such structures are very light, and their internal structure consists of thin connecting struts. This means that they only conduct small amounts of heat — which is what is required in this application. The hollow spheres are produces by means of an ingenious process: The starting point is provided by a mixture of tiny plastic spheres that are dispersed within a fluid. A substrate is then dipped into this mixture. When the fluid evaporates, a regular, microscopically thin layer of plastic spherelets forms on the substrate.

In the next step, the researchers coat the spherelets with a zirconium oxide compound that is only one molecule thick. This process can be repeated several times, until there are approximately ten identical layers of zirconium oxide. This product is then heated. The plastic evaporates, leaving a highly ordered layer consisting of thin-walled zirconium oxide shells — this is the 'inverted' structure.

Because of its three-dimensional, periodic arrangement, this layer is able to reflect a certain spectral range of heat radiation efficiently. The same process can be used with either smaller or larger spherelets, so a range of inverted structures consisting of zirconium oxide layers can be manufactured. Each such layer reflects a specific spectral range. By stacking different layers to form a hierarchical meta-structure, a broad range of the spectrum can be covered.

For use in thermo-photovoltaic applications, layer systems are required consisting of extremely thin layers of gold and silicon, for example. Today, special vacuum vapour deposition techniques are available that allow such layer systems to be produced very precisely. For instance, the silicon layers are just 40 nanometres thick, whilst the layers of gold that separate them are a mere five nanometres thick. As an alternative to this, it is also possible to use arrangements of tiny metallic rods. The rods are only a few dozen nanometres thick, and they are embedded in membranes. These systems also display the hyperbolic optical properties that are required.