There are few places with conditions more demanding than the interior of a gas turbine. The components inside jet engines and electricity generating turbines have to withstand extremely high temperatures, fast flows of gases, corrosive atmosphere and high angular velocities, with all the stresses this implies. One of the major research areas for material scientists since the 1950s has been to identify materials that can operate under these conditions. The Purdue team has developed a ceramic coating which, they claim, could make a significant contribution towards higher performance turbines.
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Ceramic coatings are standard inside the hottest part of turbines; for example, they coat the turbine blades that bear the brunt of the high temperatures, protecting the nickel alloy that forms the blade structure and allowing it to operate above the melting point of the metal. But although these coatings are vital for the structural stability of the blade, they do not actually control how the blade radiates heat, which compromises the performance and efficiency of the turbine. The Purdue team, led by electrical and computing engineer Zubin Jacob, describes in Nano Letters how they developed and built a coating comprised of ceramic nanotubes which act as thermal antennae, controlling both the direction and the electromagnetic spectrum of heat radiation.
Jacob has been researching the heat radiating properties of ceramics for some time, focusing on meta-materials. In the Nano Letters paper, he reports a new class of materials known as polaritronic ceramics, based on boron nitride. This controls radiation through oscillations of light and matter, called polaritrons, inside the ceramic material. Jacob, along with professors Luna Lu and Tongcang Li, has formed the boron nitride into nanotubes of diameter around 5 nm which, they state, couple the polaritrons to outgoing heat radiation.
Changing the way the nanotubes are deposited on the surface controls how they emit radiation, the researchers added. According to Jacob, they could accelerate the rate of radiation, perform enhanced cooling of the underlying material or even send information in specific directions or wavelengths. "By controlling radiation at these high temperatures, we can increase the lifetime of the coating. The performance of the engine would also increase because it could be kept hotter with more isolation for longer periods of time," he said.