Showing papers by "Carl V. Thompson published in 2015"

Rate-Dependent Nucleation and Growth of NaO2 in Na-O2 Batteries.

Abstract: Understanding the oxygen reduction reaction kinetics in the presence of Na ions and the formation mechanism of discharge product(s) is key to enhancing Na–O2 battery performance. Here we show NaO2 as the only discharge product from Na–O2 cells with carbon nanotubes in 1,2-dimethoxyethane from X-ray diffraction and Raman spectroscopy. Sodium peroxide dihydrate was not detected in the discharged electrode with up to 6000 ppm of H2O added to the electrolyte, but it was detected with ambient air exposure. In addition, we show that the sizes and distributions of NaO2 can be highly dependent on the discharge rate, and we discuss the formation mechanisms responsible for this rate dependence. Micron-sized (∼500 nm) and nanometer-scale (∼50 nm) cubes were found on the top and bottom of a carbon nanotube (CNT) carpet electrode and along CNT sidewalls at 10 mA/g, while only micron-scale cubes (∼2 μm) were found on the top and bottom of the CNT carpet at 1000 mA/g, respectively.

Metal assisted anodic etching of silicon

Abstract: Metal assisted anodic etching (MAAE) of Si in HF, without H2O2, is demonstrated. Si wafers were coated with Au films, and the Au films were patterned with an array of holes. A Pt mesh was used as the cathode while the anodic contact was made through either the patterned Au film or the back side of the Si wafer. Experiments were carried out on P-type, N-type, P+-type and N+-type Si wafers and a wide range of nanostructure morphologies were observed, including solid Si nanowires, porous Si nanowires, a porous Si layer without Si nanowires, and porous Si nanowires on a thick porous Si layer. Formation of wires was the result of selective etching at the Au–Si interface. It was found that when the anodic contact was made through P-type or P+-type Si, regular anodic etching due to electronic hole injection leads to formation of porous silicon simultaneously with metal assisted anodic etching. When the anodic contact was made through N-type or N+-type Si, generation of electronic holes through processes such as impact ionization and tunnelling-assisted surface generation were required for etching. In addition, it was found that metal assisted anodic etching of Si with the anodic contact made through the patterned Au film essentially reproduces the phenomenology of metal assisted chemical etching (MACE), in which holes are generated through metal assisted reduction of H2O2 rather than current flow. These results clarify the linked roles of electrical and chemical processes that occur during electrochemical etching of Si.

Nanostructured Thin Film Silicon Anodes for Li-Ion Microbatteries.

Abstract: Thin film microbatteries require electrode materials with high areal specific capacities and good cyclability. Use of vapor-deposited silicon thin films as anodes in Li-ion microbatteries offers the advantage of high capacity as well as compatibility with other processes used for microsystem fabrication. Unfortunately, monolithic silicon films greater than 200 nm in thickness pulverize during lithiation and delithiation. We have used metal-assisted-chemical-etching of sputter-deposited amorphous silicon films to make nanoporous silicon layers and arrays of silicon nanopillars as a means of achieving anodes with high areal capacity and good cyclability. We have compared the performance of these nanostructured layers with the performance of monolithic silicon films in Li half-cells. A reduced first cycle coulombic efficiency was observed in all cases and was attributed to the irreversible formation of Li2O due to the presence of oxygen in the sputter-deposited silicon films. This was controlled through modifications of the sputtering conditions. As expected, monolithic films thicker than 200 nm showed poor cycling performance due to pulverization of the film. Nanoporous silicon showed good initial cycling performance but the performance degraded due to porosity collapse and delamination. Arrays of silicon nanopillars made from 750 nm silicon films exhibited good cycling, rate performance and an areal capacity (0.20 mA h cm(-2)) 1.6 times higher than what could be obtained with monolithic Si films with similar cyclability.

Crystallographic analysis of the solid-state dewetting of polycrystalline gold film using automated indexing in a transmission electron microscope

Abstract: We analyzed the effect of crystallographic anisotropy on the morphological evolution of a 12-nm-thick gold film during solid-state dewetting at high temperatures using automated indexing tool in a transmission electron microscopy. Dewetting initiated at grain-boundary triple junctions adjacent to large grains resulting from abnormal grain growth driven by (111) texture development. Voids at the junctions developed shapes with faceted edges bounded by low-index crystal planes. The kinetic mobility of the edges varied with the crystal orientation normal to the edges, with a predominance of specific edges with the slowest retraction rates as the annealing time was increased.

Origin of physical degradation in AlGaN/GaN on Si high electron mobility transistors under reverse bias stressing

Abstract: We have investigated the role of threading dislocations in pit formation during stressing of AlGaN/GaN on Si high electron mobility transistors under high reverse bias. Upon stressing, the drain current saturation (I D-saturation ) decreases over time. The amount of I D-saturation degradation correlates well with pit formation at the gate-edge, where the electric field is the highest. Using a transmission electron microscope weak-beam technique, it is found that pits tend to nucleate at threading dislocations that have a screw component, even when these dislocations are at locations away from the gate-edge. An explanation based on an electrochemical oxidation model is proposed.

Stress engineering using low oxygen background pressures during Volmer–Weber growth of polycrystalline nickel films

Abstract: A robust strategy for controlling the level of residual stress in polycrystalline films remains elusive, owing to the complex coevolution of the surface, microstructure, and intrinsic stress during Volmer–Weber film growth. Recent improvements in the understanding of stress evolution mechanisms have led to the possibility of engineering the intrinsic stress through the control of thin film growth conditions. Here, the authors demonstrate stress engineering during deposition of polycrystalline Ni films through control of the oxygen partial pressure. The physical mechanisms of stress management during codeposition of nickel and oxygen are investigated using in situ stress measurements and ex situ structural and chemical characterizations. The intrinsic stress in Ni films is affected by grain growth during deposition (which causes a tensile stress) and by Ni adatom trapping at grain boundaries and oxygen incorporation in the Ni lattice (which cause a compressive stress). The authors show direct evidence that a small amount of oxygen suppresses grain growth during deposition. They suggest that the presence of chemisorbed oxygen limits surface diffusion of Ni adatoms, thereby limiting adatom trapping at grain boundaries. The presence of oxygen therefore affects the mechanisms for development of both tensile and compressive stresses, providing a direct method for engineering the residual stress in as-deposited Ni films. Finally, the authors demonstrate a process for evaporative deposition of “zero” stress Ni films by introducing a very low level of background impurities, with the resultant films containing only 1.2 at. % oxygen.

Characterization of the Young?s modulus, residual stress and fracture strength of Cu?Sn?In thin films using combinatorial deposition and micro-cantilevers

Abstract: Microcantilevers coupled with combinatorial deposition were used to characterize the Young?s modulus, residual stress and fracture strength of Cu?Sn?In thin films over a broad range of compositions. Measurement inaccuracies due to cantilever non-idealities were corrected using finite element simulations and deflection measurements at multiple locations. ??phase with a composition Cu53Sn25In22 was discovered to have the highest fracture strength and therefore has potential in thin film solder bonding applications. This study provides a database for the mechanical properties of a wide range of Cu?Sn?In alloys. Moreover, the techniques developed in this study provide a highly efficient approach to finding an intermetallic compound composition with the most desired mechanical properties.

Rate-Dependent Nucleation and Growth of NaO[subscript 2] in Na-O[subscript 2] Batteries

Abstract: Seventh Framework Programme (European Commission) (Marie Curie International Outgoing Fellowship, 2007-2013))