By Susan Dieterle
A unique, compact furnace combined with high-energy X-rays is giving researchers at Ames Laboratory the unprecedented ability to directly record the chemical and structural changes of complex materials at high temperatures under real processing conditions.
This information is crucial to understanding and controlling the composition and microstructure of new materials. It previously took months or years to collect such data through the laborious process of heating, quenching and then analyzing numerous samples. But the Ames Lab researchers can now gather the data in just a few days while getting a more detailed picture of what happens to a material's crystal structure as it heats and cools.
The new system is ideal for complex materials such as structural ceramics, superconducting wires and nanostructured materials. The in-sights gained through the Ames Lab system may speed the development of new materials for use in fields such as aerospace engineering, electrical distribution systems and microelectronics.
"We're seeing details of the phase transitions that I don't think anybody has ever described before," says scientist Matt Kramer, who helped design the furnace built by Ames Lab's Engineering Services Group.
The furnace uses an analytical technique known as X-ray diffraction in which an X-ray beam is focused on a small sample of material. The beam is diffracted by the crystal structure of each material, producing a unique pattern of concentric circles, called "Debye rings." By capturing images of the changes in the ring pattern as the material is heated and cooled, scientists gain a better fundamental understanding of what happens to the material's crystal structure at various temperatures.
In 1997, Kramer began working with senior materials scientist Bill McCallum, senior physicist Alan Goldman, assistant metallurgist Kevin Dennis and then-Ames Lab graduate student Larry Margulies on a design for a compact, portable furnace that would enable them to rotate samples during an experiment. They also wanted to be able to subject the samples to a variety of environments, such as inert or oxidizing atmospheres, encountered in processing conditions.
What emerged was a scaled-down version of the standard laboratory tube furnace, measuring about 18 inches in length and 6 inches in diameter and capable of heating samples to 1,500 C (2,732 F). The furnace has an indirect, magnetic coupling system that connects to a motor shaft, which rotates the sample.
Samples are held at the end of a long tube and aligned with a 3-millimeter opening in the side of the furnace. The X-ray beam enters through the opening and the diffracted rays emerge through a slot in the furnace.
Kramer says the new system is an improvement over current high-temperature X-ray diffraction systems, in which samples rest on a flat plate. This doesn't allow the sample to be rotated and sometimes causes the liquid and solid phases of the material to draw apart. Also, the flat-plate systems don't always heat the sample uniformly, producing large temperature variations in the material that make it difficult to correlate the temperatures with changes in the crystal structure.
"The excellent control we have with our furnace means that we can select an exact temperature setting for our measurement and know that the whole sample is that temperature," Kramer says. "And with the confined geometry, we can melt things and know that the liquid and solid aren't separating."
In addition to the scientists who developed the furnace, the device has been used by Ames Lab scientists Doug Finnemore and Dan Sordelet as well as researchers at DOE's Brookhaven National Laboratory.
The experiments are conducted at off-site facilities where high-energy X-rays of between 35 and 100 kilovolts are available. Most of the experiments take place at the Advanced Photon Source at the Department of Energy's Argonne National Laboratory near Chicago and the Cornell High Energy Synchrotron Source in New York.
At Argonne's APS facility, the furnace has now found a home on a sector reserved for the Midwest Universities Collaborative Access Team, which is operated by Ames Lab and Iowa State University (see MUCAT story).
Before an experiment begins, the furnace must be aligned with the X-ray beam -- a painstaking process because the beam itself is about 1 millimeter wide and 0.5 millimeter high. "That's the hard part," Kramer says. "The first time we did this, it took us three days to align the furnace to the beam and then another hour or two to align the sample to the beam itself. But we've gotten the process down well enough now that it only takes us about a half a day to line it up and minutes to put the sample in."
The researchers have also found that "shuttering" the beam, rather than using a continuous ray, enables them to take better sequential images from the diffracted rays. Kramer says the reactions are monitored with a time resolution of less than two seconds, fast enough to make a virtual movie of images that capture the material's structural transformation during temperature-driven processing.
Kramer adds that colleagues at Argonne and the European Synchrotron Research Facilities have asked about having Ames Lab build similar furnaces for them. He notes that the Ames Lab group is also willing to let other researchers use the furnace.
For more information:
Matt Kramer, (515) 294-0276
mjkramer@ameslab.gov
Research funded by:
DOE Office of Basic Energy Sciences
Last revision: 9/15/00 sd
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