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The passivation of polycrystalline nickel surfaces against hydrogen uptake by oxygen is investigated experimentally with low energy ion scattering (LEIS), direct recoil spectroscopy (DRS), and thermal desorption spectroscopy (TDS). These techniques are highly sensitive to surface hydrogen, allowing the change in hydrogen adsorption in response to varying amounts of oxygen exposure to be measured. The chemical composition of a nickel surface during a mixed oxygen and hydrogen exposure was characterized with LEIS and DRS, while the uptake and activation energies of hydrogen on a nickel surface with preadsorbed oxygen were quantified with TDS. By and large, these measurements of how the oxygen and hydrogen surface coverage varied in response to oxygen exposure were found to be consistent with predictions of a simple site-blocking model. This finding suggests that, despite the complexities that arise due to polycrystallinity, the oxygen-induced passivation of a polycrystalline nickel surface against hydrogen uptake can be approximated by a simple site-blocking model.

In this study, we report on the thermal conductivity of amorphous carbon generated in diamond via nitrogen ion implantation (N 3 + at 16.5 MeV). Transmission electron microscopy techniques demonstrate amorphous band formation about the longitudinal projected range, localized approximately 7 μm beneath the sample surface. While high-frequency time-domain thermoreflectance measurements provide insight into the thermal properties of the near-surface preceding the longitudinal projected range depth, a complimentary technique, steady-state thermoreflectance, is used to probe the thermal conductivity at depths which could not otherwise be resolved. Through measurements with a series of focusing objective lenses for the laser spot size, we find the thermal conductivity of the amorphous region to be approximately 1.4 W m − 1 K − 1, which is comparable to that measured for amorphous carbon films fabricated through other techniques.

Uniform metal contacts are critical for advanced thermoelectric devices. The uniformity of the contact resistance for gold, tungsten, and SrRuO3 electrodes on polycrystalline ternary Bi2Te3-based alloys for different types of surface cleaning procedures was characterized. The presence of a nanometer-thick native oxide layer on the Bi2Te3 surface leads to large and non-uniform contact resistance. Surface treatments included solvent cleans and chemical and dry etching prior to metallization of the Bi2Te3. Only etching the surface led to a significant improvement in contact resistance uniformity. None of the tested contacts reacted with the underlying Bi2Te3 substrate. Etching resulted in the removal of the native oxide on the Bi2Te3 surface, which was characterized using X-ray photoelectron spectroscopy (XPS). The average thickness, chemistry, and dry etch rate of the native oxide was further characterized using XPS. The non-uniformity in contact resistance suggests that the native oxide grows non-uniformly on polycrystalline bismuth telluride surfaces.

We investigate experimentally and theoretically the long-term chemical and morphological stability of Au contacts on Bi2Te3-based materials. Electron microscopy and energy dispersive spectroscopy experiments show that thermal annealing severely degrades the integrity of micron-thick Au films on n-type Bi2Te3, eventually leading to their complete dissolution. To explain this result, we have used density functional theory to calculate defect formation energies and diffusion barriers of Au within Bi2Te3. We identify an interstitial binding site consistent with previous reports of (rapid) anisotropic diffusion of Au in Bi2Te3. We find, however, that substitutional Au has lower formation energies. We suggest that these substitutional defects may be active in our experiments and account for the relatively long time scale of the contact degradation.

In some cases, nanoscale microstructures improve thermoelectric efficiency, but this phenomenon has rarely been studied systematically for precipitates in bulk materials. We quantified the influence of nanostructuring on the thermoelectric figure of merit (zT) by embedding Sb2Te3 inclusions, from nanometer to micron sizes, in an Sb-rich AgSbTe2 matrix through solid-state precipitation. Nucleation/growth and coarsening regimes of precipitate formation had a clear effect on transport properties, which could be understood using the effective medium theory of a two-phase composite. The majority of precipitates nucleated heterogeneously at grain boundaries and at planar defects found in the matrix phase, forming a complex interconnected network. This heterogeneous nucleation causes the precipitate/matrix system to follow effective medium theory even at small precipitate sizes, thus lowering the figure of merit. Therefore, heterogeneous nucleation is a major obstacle to efficiency improvement using nanoscale pr...

The authors present a method to fabricate confined, oriented, single crystals of ternary alloys within an inert ceramic matrix. Pulsed-laser deposition of a polycrystalline CuNiFe film fills lithographically defined surface cavities in a sapphire single crystal. Solid-state diffusion bonding to a second sapphire crystal internalizes the metal-filled cavities. Electron microscopy verifies that subsequent heat treatment converts the thin, fully constrained films into single crystals of specific orientation by nucleation-controlled liquid-phase epitaxy during cooling from above the alloy melting temperature. The resulting films provide an ideal medium for fundamental studies of a wide range of volumetrically constrained phase transformations.