Basic materials science at Sandia/California consists of a highly integrated combination of theory and experimental work. Our overarching goal is to discover and elucidate the fundamental mechanisms that govern the behavior and properties of materials — particularly the mechanisms that control the evolution and stability of interfaces in materials and that dictate how interfaces interact with their surrounding environments.
Our work is concerned with solids and encompasses both free surfaces and internal interfaces such as grain boundaries. Throughout our research, we seek to determine how elementary, atomic-scale structures and processes are related to the longer-range interactions that ultimately control interfacial behavior and properties. Thus, for instance, we concentrate significant effort on determining how collective groupings of atoms — such as dislocations and steps — bridge between atomic and macroscopic scales. In studying the response of interfaces to their surroundings, we seek to understand the interactions of interfaces with both the external environment and the interior bulk of the solid.
Nanoscale crystals move down atomic “staircases” to grow taller, thereby reducing contact area with the substrate.
We are quantifying the fundamental atomic processes governing the dynamics of surface structure and morphology. We use state-of-the-art microscopy to measure the time evolution of surface structure on nanometer-length scales. We then compose precise equations of motion to describe the observed time dependence and relate these equations of motion to atomic processes. This approach has afforded new — and often surprising — insights into the mechanisms that underlie surface alloying, metal oxidation, thin-film dewetting, two-dimensional self-assembly, surface motion, thermal surface smoothing, and misfit dislocation.
We seek to establish the fundamental principles that underpin the structure and behavior of interfaces in metals. We hope to explain how the incompatibilities and discontinuities that arise at an interface are accommodated and then explore the implications of these relaxations for the behavior and properties of the interface. To accomplish this, we first make detailed, experimental observations on carefully controlled materials systems using high-resolution transmission electron microscopy, scanning tunneling microscopy, and atom probe tomography. We then integrate these observations with theory and computation that encompasses continuum elasticity, interfacial crystallography, atomistic simulations, and first-principles simulations. By using this process, we have developed a comprehensive, dislocation-based understanding of interfacial structure. This understanding enables us to explain the mechanisms of strain accommodation at interfaces and the formation of transitional interfacial layers. This knowledge has also led to the discoveries of new types of structural-phase transitions.
Lanthanum halide crystals could revolutionize gamma-ray scintillation spectroscopy if the brittle crystals can be grown to sufficient size. Sandia is identifying optimal processing conditions for this crystal growth.
For detecting gamma rays, lanthanum (La) halides offer sharper resolution and faster response than conventional sodium iodide scintillators. However, La-halide crystals are highly brittle and difficult to grow to the needed size. We are evaluating the thermo-mechanical properties of halide melts and crystals to eliminate sources of stress-induced cracking.
To detect fission-spectrum neutrons in low-energy applications — such as passive imaging of shielded, highly enriched uranium — we are investigating direct electronic detection using organic semiconductors. Semiconducting materials would eliminate the need for optics and vacuum tubes and could enable high-spatial resolution imaging, particle tracking, recoil collimation, and gamma rejection.
Emerging research areas in materials science include thermoelectric materials and diamond-based neutron detectors.
For more information, contact Sarah Allendorf at (925) 294-3379.