Publications

Establishing the atomic structure and composition of interfaces in thermoelectric materials is important to understanding how these defects mediate thermal and electronic transport. Here, we discuss our experimental observations and theoretical calculations of the Bi{sub 2}Te{sub 3} (0001) basal twin in nanocrystalline Bi{sub 2}Te{sub 3}. This interface is important both because it is common in tetradymite-structured thermoelectric compounds and because it serves as a useful model system for more complex interfaces. Macroscopically, the (0001) twin corresponds to a 180 rotation of the crystal about the [0001] axis, which reverses the stacking of the basal planes. The basal planes of Bi{sub 2}Te{sub 3} are arranged in 5-plane groupings of alternating Bi and Te layers. Microscopically, one envisions three possible interface terminations: at the Te layer in the middle of the 5-layer packet, at a Bi layer, or at the Te-double layer at the junction of the 5-layer packet. Using aberration-corrected HAADF-STEM imaging, we have established that the twin boundary terminates at the Te-double layer. This result is consistent with ab initio calculations, which predict that the lowest energy for the three candidate structures is for this termination.

Interfaces are a critical determinant of the full range of materials properties, especially at the nanoscale. Computational and experimental methods developed a comprehensive understanding of nanograin evolution based on a fundamental understanding of internal interfaces in nanocrystalline nickel. It has recently been shown that nanocrystals with a bi-modal grain-size distribution possess a unique combination of high-strength, ductility and wear-resistance. We performed a combined experimental and theoretical investigation of the structure and motion of internal interfaces in nanograined metal and the resulting grain evolution. The properties of grain boundaries are computed for an unprecedented range of boundaries. The presence of roughening transitions in grain boundaries is explored and related to dramatic changes in boundary mobility. Experimental observations show that abnormal grain growth in nanograined materials is unlike conventional scale material in both the level of defects and the formation of unfavored phases. Molecular dynamics simulations address the origins of some of these phenomena.

Thermoelectric materials have many applications in the conversion of thermal energy to electrical power and in solid-state cooling. One route to improving thermoelectric energy conversion efficiency in bulk material is to embed nanoscale inclusions. This report summarize key results from a recently completed LDRD project exploring the science underpinning the formation and stability of nanostructures in bulk thermoelectric and the quantitative relationships between such structures and thermoelectric properties.

We analyze a thin (~1 nm) hexagonal-close-packed (HCP) intergranular layer at a 29°110 tilt grain boundary in gold. Our analysis, which is based on HRTEM observations and atomistic calculations, shows that this boundary consists of a dense array of 60°1/2 110 crystal lattice dislocations that are distributed one to every two {111} planes. These dislocations dissociate into paired Shockley partial dislocations, creating a stacking fault on every other plane and thereby producing the abab, or HCP, stacking sequence. This distribution of dislocations is consistent both with the measured intergranular misorientation and with the calculated rigid-body translation along the tilt axis. By establishing the interfacial dislocation arrangement, we also show how the HCP layer at the 29°boundary observed here is geometrically related to that found previously at the 80.6°∑ = 43 110 boundary. This result helps to link dislocation-based descriptions for boundary structures between the high- and low-angle misorientation regimes. © 2009 Springer Science+Business Media, LLC.

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Twinning is ubiquitous in electroplated metals. Here, we identify and discuss unique aspects of twinning found in electrodeposited Ni-Mn alloys. Previous reports concluded that the twin boundaries effectively refine the grain size, which enhances mechanical strength. Quantitative measurements from transmission electron microscopy (TEM) images show that the relative boundary length in the as-plated microstructure primarily comprises twin interfaces. Detailed TEM characterization reveals a range of length scales associated with twinning beginning with colonies ({approx}1000 nm) down to the width of individual twins, which is typically <50 nm. We also consider the connection between the crystallographic texture of the electrodeposit and the orientation of the twin planes with respect to the plating direction. The Ni-Mn alloy deposits in this work possess a 110-fiber texture. While twinning can occur on {l_brace}111{r_brace} planes either perpendicular or oblique to the plating direction in {l_brace}110{r_brace}-oriented grains, plan-view TEM images show that twins form primarily on those planes parallel to the plating direction. Therefore, grains enclosed by twins and multiply twinned particles are produced. Another important consequence of a high twin density is the formation of large numbers of twin-related junctions. We measure an area density of twin junctions that is comparable to the density of dislocations in a heavily cold-worked metal.