Real-Time X-Ray Studies of Materials Processes

Karl Ludwig

Our research investigates how materials evolve on atomic and nano- length scales as they change from one form to another. In particular, we use real-time x-ray techniques to examine structural evolution during phase transitions, thin film growth and surface processing. Many of the experiments use the high brightness of synchrotron x-ray sources – the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory on Long Island, the Advanced Photon Source (APS) at Argonne National Laboratory outside of Chicago, and the Linac Coherent Light Source (LCLS) at SLAC.  We are actively involved in new beamlines planned for the NSLS-II project now under construction at Brookhaven and Ludwig is Spokesperson for the Integrated In-Situ and Resonant (ISR) beamline project there.  Where possible, our research makes contact with fundamental theory and simulation.

In the last few years, our detailed interest has been in three directions – understanding surface and thin film processes, investigating nanoscale dynamics in metallic alloys using coherent x-ray scattering and studying the relationship between atomic structure and function in solid oxide fuel cell cathodes. Many of our in-situ studies utilize a unique ultra-high vacuum growth and surface modification facility that we have helped develop on the insertion-device beamline X21 at the NSLS. We have been using it to examine surface morphology evolution during ion bombardment (which can cause the spontaneous growth of surface nanostructures) and issues related to the growth of wide-bandgap group III-V semiconductor films using plasma-assisted molecular beam epitaxy (in collaboration with Prof. Moustakas in Electrical Engineering).

Coherent x-ray scattering provides the ability to probe nanoscale dynamics in metallic alloys and other materials systems. Partially coherent x-ray beams are created using small (10 micron) slits in conjunction with a high-brilliance 3rd generation synchrotron source, such as the APS. The disorder in the alloys produces speckle patterns in the scattered x-ray intensity. The evolution of the speckle pattern can then be related to the underlying dynamics of structural changes (e.g. ordering, phase separation or stacking fault rearrangement) in the alloy.  The LCLS is the world's first hard x-ray laser and offers unique new opportunies for coherent scattering on femtosecond time scales that we are now exploring.

Solid oxide fuel cells offer the potential for highly efficient energy conversion, but improvements in cathode function are needed before their potential can be fully realized. In collaboration with Profs. Pal, Basu and Gopalan in Engineering and Prof. Smith in Physics, we are examining in-situ the near-surface atomic structure of cathode materials in order to better understand the relationship between function and structure.