Mechanical Forces That Change Chemistry Brian W. Sheldon , Brown University, DMR 0805172

1. Mechanical Forces That Change ChemistryBrian W. Sheldon, Brown University, DMR 0805172 Outcome: Research at Brown University demonstrates that technologically important chemical processes in certain materials are strongly influenced by mechanical forces that are produced by an effect that has not been previously documented. Impact: Controlling these forces can improve the performance of key materials that are used in a wide range of applications, including fuel cells, batteries, and medical devices. Explanation: Surface atoms in solids are bonded differently than those that are far away from the surface. In a broad class of oxide materials these surface atoms influence the subsurface atoms, in a region sometimes referred to as a “space-charge” layer. Recent research at Brown University demonstrates that the inherent structure of this space charge region produces extremely large mechanical forces on the surrounding atoms. The resulting stresses induced by these forces can be much larger those produced by the space charge layer (~3 to 30 atom layers) surface atoms bulk crystal large forces due to changes in atomic scale “defects” surface atoms alone. These stresses alter the motion of atoms in the space charge layer, and the chemical processes that occur between neighboring small crystals. Professor Brian W. Sheldon of Brown University’s School of Engineering led the team which discovered this effect. bulk “defects” and properties Schematic showing the subsurface “space charge” layer. Small changes in the composition of this layer can produce extremely large internal stresses.

2. Multiple Grain-Boundary Mechanisms Dominate Compositional Stresses in Nanocrystalline CeriaBrian W. Sheldon, Brown University, DMR 0805172 Compositional changes in a material generally produce volume changes, which can in turn induce large stresses in ceramic thin films and coatings where atomic mobilities are limited. In a well-characterized material like ceria (CeO2-X), the volume changes during oxidation / reduction are a well-defined thermodynamic property (i.e., the partial molar of oxygen). However, in nanocrystalline films, our work shows that grain boundary contributions to these stresses are dominant. Furthermore, recent work demonstrates that there is more than one competing grain boundary mechanism. Oxidation (pure O2) / Reduction (PO2 = 4(10)-30 atm) Stresses Tensile at higher temperature due to grain boundary (not bulk) phase transformation MOCVD CeO2-X avg grain size = 92 nm Competing mechanisms at intermediate temperatures Compressive: consistent with bulk behavior, but much larger magnitude with nanograins

3. Stress Evolution and Degradation During Field Induced Crystallization of Ta CapacitorsBrian W. Sheldon, Brown University, DMR 0805172 Industrial Collaborators: Joachim Hossick-Schott and Mark Viste, Medtronic High-voltage Ta capacitors (i.e., Ta/anodic Ta oxide systems) are employed in high reliability military, aerospace and medical systems. Prior work suggests that significant internal stresses occur in the oxide layer, and our work provides the first direct measurements of these stresses. A new mechanism that can limit crystallization has also been proposed. National Science Foundation Model Experiments with Anodically Grown Ta Oxide on Ta Thin Films Crystals induced by the field protrude above amorphous oxide (see AFM image below) (1) known to be a critical failure mechanism (2) competing stress generation mechanisms were detected: tensile stresses can lead to cracking compressive stresses can lead to delamination / loss of contact near crystal 10 mm