Technology update
Ceramic fiber diet for gas turbines?
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The good news is that ceramics demonstrate great heat resistance; the bad news is that they are brittle and vulnerable to mechanical stress. Specialists at Bayer's Central Research Division in cooperation with researchers at the University of Bonn in Germany have developed a ceramic that is resistant to heat and corrosion, and can be stretched into fibers and used in gas turbine engines.
The new material, which was discovered nine years ago, is being refined, improved, and tested for possible use in gas turbines in a large-scale project, reveals Bayer. At present, turbine manufacturers and operators alike have to make do with a compromise and use relatively heat-sensitive alloys such as nickel and chrome (superalloys). Since extremely high temperatures are generated during the operation of a turbine, their metal parts have to be permanently cooled. This ensures that even at temperatures up to 1000 °C, the metal does not soften. But the cooling process is expensive and energy consuming.
However, ceramics can endure temperatures up to twice that figure. Their Achilles' heel is that they are brittle, so uses are confined to applications where mechanical stresses are limited. Bayer and Bonn University are now developing ceramics that are strengthened using fibers. The route they have taken and are still taking is highly complex. They have managed to create a pre-ceramic substance that can be stretched into fibers. This is rare in ceramic production, as fibers cannot be spun from commonly used pre-ceramic powders. Bayer's process engineers are now working with researchers at Germany's Fraunhofer Institute for Silicate Research in Würzburg to develop the fibers. These fibers can be "fired" into inorganic ceramics through pyrolysis in nitrogen at 1200 °C. They are resistant to creep, oxidation, and heat—three of the crucial criteria that must be achieved for turbine materials. A pilot plant to produce the fibers was established in 1991. Since 1996 Bayer has been involved in a joint project with manufacturers of turbines and ceramic components together with universities and institutes, to develop the fibers for production turbine applications.
Because turbine combustors and blades cannot be constructed of fibers alone, the project partners also developed a ceramic matrix that surrounds the threads to form fiber-reinforced ceramic tiles. Additionally, a fabric made from several layers of the fiber can be embedded in a polymer (e.g., a phenolic resin) and the resulting component can then be finished in the desired form. Carbonization follows, with the polymer pyrolisis in a vacuum at 900 °C to leave only pure carbon. "A ceramic fiber-reinforced carbon component is produced, onto which liquid silicon is poured at 1600 °C. The silicon reacts with the carbon to form a silicon-carbide ceramic.
"The resulting tile is unique," states Bayer. "Until now, no fiber has been able to withstand rigors of this nature. Should the tile be subjected to heavy mechanical loads, the fibers absorb some of the energy and distribute it in different directions. The force of an impact striking a fiber-reinforced ceramic is absorbed in this manner, which means that considerably greater force would be required to cause disastrous fracture behavior."
Bayer believes there are several applications for fiber-reinforced tiles within a gas turbine engine. The tiles could be used to line combustors or for turbine blades. At present, researchers involved in the current project are trying to produce a diffuser ring from fiber-reinforced ceramics. Since this component connects the high and low pressure turbines, it is exposed to extreme temperature and pressure fluctuations.
What all this means in terms of hard cash for airline operators is a fuel savings of up to 20% because fuel burn efficiency would be improved in a higher temperature environment made possible by this development. Emissions could also be improved; nitrous oxide emissions could be reduced by as much as 60%, believes Bayer.
The company admits that a rigorous and lengthy test schedule lies ahead before ceramic tiles could be used in turbines. However, it believes that for the first time there is now a real prospect of ceramic technology finding a role in aircraft engine design, bringing with it the benefits in terms of operating efficiency and environmental gains, that have been one of the more elusive elements of modern aerospace engineering.
Stuart Birch
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