Apr 6 2004
Infrared cameras create images by detecting the heat given off by an object, including the body of a soldier hidden in the dark of night. Now, researchers have developed a technique for imaging how fast heat can move through an object.
Thermal conductivity – the rate at which heat flows through a material – is a fundamental property, but can be difficult to measure and evaluate. Scientists at the University of Illinois at Urbana-Champaign have imaged this property at micron-scale resolution.
“Our imaging technique tells us how the thermal conductivity varies by position in the material,” said David Cahill, a professor of materials science and engineering and a Willett Faculty Scholar at Illinois. “The image clearly reveals the type of structure through which the heat must flow.”
Cahill and his colleagues create an image by measuring and mapping the thermal conductivity at thousands of locations across the sample. The procedure, based upon a technique called time-domain thermoreflectance, requires a single measurement at each location. The procedure is fast and accurate.
“Thermal conductivity depends on much more than just the material composition, so the property is difficult both to predict and to measure,” Cahill said. “The image we generate helps us present and manipulate the data for further analysis.”
To demonstrate the power of their technique, the researchers imaged the thermal conductivity of a cross section of a sample containing niobium, silicon and titanium. Niobium-based alloys have been proposed for next-generation, high-temperature turbine blades, to replace the nickel-based materials currently used.
“Viewing the image, we were surprised to find significant variations in the thermal conductivity of individual crystal grains,” Cahill said. “The grains were randomly oriented in the sample, and how effectively each grain conducted heat depended on its orientation.”
Next, the researchers plan to image the cross section of an actual turbine blade. “The image should reveal how the thermal properties of the metal, bond coat and thermal barrier coating vary by position as we go through the structure,” Cahill said. “This information may help improve the thermal performance and operating efficiency of future turbines.”
Collaborators included research scientist Jeffrey White and postdoctoral research associate Scott Huxtable at Illinois and scientist Ji-Cheng Zhao at General Electric Global Research. The National Science Foundation and the U.S. Department of Energy funded the work.