Under harsh environments, atoms in a material often migrate and change properties such as strength or electrical conductivity. Before now, scientists had to remove an interesting region from the surrounding crowd of materials to study it. In addition, they had to destroy the material to create smaller pieces and precisely analyze the defects. Scientists found a better way. Their new technique uses penetrating 3-D x-ray diffraction. With this approach, they tracked changes to an individual particle within a heated thin film. They tracked the particle's shape and internal defect evolution.
This technique monitors materials while they are exposed to heat, pressure, chemicals, and other real-world conditions. The technique tracks how the particle boundaries change. Also, it can image mismatches between shifting internal rows of atoms. By tracking behavior in 3-D under actual use conditions, scientists can improve materials for use in functional environments such as chemical processing and solar energy generation. In addition, this data could aid scientists in creating more corrosion-resistant materials.
We rely on particulate materials every day in demanding environments, such as structural metals for vehicles and complex, layered thin films for electronics. Often their unique properties come from the way microscopic particles are formed together into a macroscopic structure. The shape and boundaries of particles in films determine mechanical and electrical properties. They also affect the durability of devices during use in demanding environments. Because atoms move when heated or exposed to changing chemical bonds, the microscopic structure of a material evolves during processing and use. Materials design for real-life applications requires accurate measurement of particle shape, transition boundaries, and deviations in atomic positions that cause defects. The most helpful measurements must be done in place, in actual working configurations, and under harsh operating conditions.
Advantageously, an x-ray beam can penetrate a sample and its operating environment. The way the x-rays scatter outwards is sensitive to the atomic positions inside. This research developed and demonstrated a new sensitive 3-D x-ray imaging technique. Researchers exposed a thin gold film made of connected small particles to high temperatures. The x-ray images tracked changes in individual particles and particle boundaries. A key feature of this new technique is that it can resolve the shape of each particle from within a collection of neighboring particles that are fused together into a polycrystalline thin film. It can also resolve atomic-scale misalignments caused by local deformation, and oddly shaped boundaries that affect the crystalline ordering. When the film was heated, the gold particles grew preferentially in locations where atom stacking was mismatched and deformation existed.
This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (synchrotron x-ray grain Bragg coherent diffractive imaging technique and analysis) and the National Science Foundation (sample synthesis and electron microscopy). The research used resources of the Advanced Photon Source, a DOE Office of Science user facility.
A. Yau, W. Cha, M.W. Kanan, G.B. Stephenson, and A. Ulvestad, "Bragg coherent diffractive imaging of single-grain defect dynamics in polycrystalline films." Science 356, 739-742 (2017). [DOI: 10.1126/science.aam6168]
Argonne National Laboratory news release: Special X-ray technique allows scientists to see 3-D deformations