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ion Mechanical Forces That Change Chemistry

Brian W. Sheldon, Brown University, DMR 0805172Outcome: 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

bulk crystal

space charge layer(~3 to 30 atom layers)

bulk “defects” and properties

large forces due to changes in atomic scale “defects”

surface atomssurface 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.

Schematic showing the subsurface “space charge” layer. Small changes in the composition of this layer

can produce extremely large internal stresses.

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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.

Multiple Grain-Boundary Mechanisms Dominate Compositional Stresses in Nanocrystalline Ceria

Brian W. Sheldon, Brown University, DMR 0805172

Oxidation (pure O2) / Reduction (PO2 = 4(10)-30 atm) Stresses

Compressive: consistent with bulk behavior, but much larger magnitude with nanograins

Tensile at higher temperature due to grain boundary (not

bulk) phase transformation

Competing mechanisms at intermediate temperatures

MOCVD CeO2-X avg grain size = 92 nm

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ion 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

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

Model Experiments with Anodically Grown Ta Oxide on Ta Thin Films

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.

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10 mm