Tech Briefs
Shedding new light on advanced materials

A unique compact furnace combined with high energy X-rays is giving researchers at Ames Laboratory, operated for the Department of Energy (DOE) by Iowa State University, the ability to record directly 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. 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 insights gained through the Ames Lab system may speed the development of new materials for use in electrical distribution systems and microelectronics.

"We're seeing details of the phase transitions in the materials that I don't think anybody has ever described before," said scientist Matt Kramer, who helped design the new furnace. The furnace uses 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.

The new system is a scaled-down version of the standard laboratory tube furnace, measuring about 46-cm (18-in) tall, with a 15-cm (6-in) diameter, and capable of heating samples to 1500°C (2700°F). The top has an indirect, magnetic coupling system that connects to a motor shaft, which can rotate the sample holder up to 1000 times per minute. One end of the sample holder—a long pipette that can be capped—is placed in the coupling system while the end containing the sample is aligned with a 3-mm (0.12-in) 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 said the new system is an improvement over current high-temperature X-ray diffraction systems, in which samples rest on a flat plate. This does not 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 do not 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 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 said. "And with the confined geometry, we can melt things and know that the liquid and solid aren't separating."

Before an experiment begins, the furnace must be aligned with the X-ray beam—a painstaking process because the beam itself is about 1-mm (0.04-in) wide and 0.5-mm (0.02-in) high. "That's the hard part," Kramer said. "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 got 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 said the reactions are monitored with a time resolution of less than 2 s, fast enough to make a virtual movie of images that capture the material's structural transformation during temperature-driven processing.

Jean L. Broge