Ni-silicide growth kinetics in Si and Si/SiO2 core/shell nanowires.

A systematic study of the kinetics of axial Ni silicidation of as-grown and oxidized Si nanowires (SiNWs) with different crystallographic orientations and core diameters ranging from ∼ 10 to 100 nm is presented. For temperatures between 300 and 440 °C the length of the total axial silicide intrusion varies with the square root of time, which provides clear evidence that the rate limiting step is diffusion of Ni through the growing silicide phase(s). A retardation of Ni-silicide formation for oxidized SiNWs is found, indicative of a stress induced lowering of the diffusion coefficients. Extrapolated growth constants indicate that the Ni flux through the silicided NW is dominated by surface diffusion, which is consistent with an inverse square root dependence of the silicide length on the NW diameter as observed for (111) orientated SiNWs. In situ TEM silicidation experiments show that NiSi(2) is the first forming phase for as-grown and oxidized SiNWs. The silicide-SiNW interface is thereby atomically abrupt and typically planar. Ni-rich silicide phases subsequently nucleate close to the Ni reservoir, which for as-grown SiNWs can lead to a complete channel break-off for prolonged silicidation due to significant volume expansion and morphological changes.

Comparison of relative flexoelectric coefficients measured by different techniques

While it is well known that it is possible to determine the effective flexoelectric coefficient of nematic liquid crystals using hybrid cells [1], this technique can be difficult due to the necessity of using a D.C. field. We have used a second method[2], requiring an A.C. field, to determine this parameter and here we compare the two techniques. The A.C. method employs the linear flexoelectrically induced linear electro-optic switching mechanism observed in chiral nematics. In order to use this second technique a chiral nematic phase is induced in an achiral nematic by the addition of a small amount of chiral additive (∼3% concentration w/w) to give helix pitch lengths of typically 0.5-1.0 μm. We note that the two methods can be used interchangeably, since they produce similar results, and we conclude with a discussion of their relative merits.

Forcing of self-excited round jet diffusion flames

In this experimental and numerical study, two types of round jet are examined under acoustic forcing. The first is a non-reacting low density jet (density ratio 0.14). The second is a buoyant jet diffusion flame at a Reynolds number of 1100 (density ratio of unburnt fluids 0.5). Both jets have regions of strong absolute instability at their base and this causes them to exhibit strong self-excited bulging oscillations at welldefined natural frequencies. This study particularly focuses on the heat release of the jet diffusion flame, which oscillates at the same natural frequency as the bulging mode, due to the absolutely unstable shear layer just outside the flame. The jets are forced at several amplitudes around their natural frequencies. In the non-reacting jet, the frequency of the bulging oscillation locks into the forcing frequency relatively easily. In the jet diffusion flame, however, very large forcing amplitudes are required to make the heat release lock into the forcing frequency. Even at these high forcing amplitudes, the natural mode takes over again from the forced mode in the downstream region of the flow, where the perturbation is beginning to saturate non-linearly and where the heat release is high. This raises the possibility that, in a flame with large regions of absolute instability, the strong natural mode could saturate before the forced mode, weakening the coupling between heat release and incident pressure perturbations, hence weakening the feedback loop that causes combustion instability. © 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.