Retarded thermal oxidation of Si nanowires is investigated. The oxidation behavior strongly depends on the wire curvature. The effect starts to evolve for a curvature larger than 0.05nm, i.e., an original nanowire radius of 35nm. For longer oxidation time and lower oxidation temperature, the effect of retarded oxidation gets stronger. The average values of the oxidation rate for small wires are reduced nearly by a factor of 2 compared to bulk ⟨100⟩ silicon. The authors suggest that the increased stress is responsible for the mechanism of retarded oxidation which cannot be decreased by the viscous flow of the oxide.

Thermal oxidation of MBE-grown silicon nanowires with a gold droplet on their tips was investigated. Two kinds of oxidation behavior were observed: (i) enhanced axial oxidation, if there was a direct contact between the gold droplet and the nanowire, and (ii) conventional oxidation for nanowires when there was no direct contact between the gold and silicon. For a dry atmosphere, such enhanced oxidation takes place at temperatures down to 500 • C. Under a wet atmosphere, remarkable oxidation was observed even for temperatures down to 250 • C. 2. Experiment Undoped single-crystalline 1 1 1 Si NWs were grown in a MBE chamber at a temperature of 525 • C and at a pressure

This paper deals with different structuring methods for high temperature resistant nickel alloys. The ideal structured surface for a possible application on the blades of aeroengines combines high oxidation resistance with low drag in a hot gas flow. The effect of drag reduction due to riblet structured surfaces was originally inspired by shark scales, which have a drag reducing riblet structure. The necessary riblet sizes for effective drag reduction depend on the temperature, pressure and velocity of the flowing medium (gas or liquid). These riblet sizes were calculated for the different sections in an aeroengine. The riblets were successfully produced on a NiCoCrAlY coating by picosecond laser treatment. This method is suitable for larger structures within the range of some tens of micrometers. Furthermore, experiments were performed by depositing different materials through polymer and metal masks via electrodeposition and physical vapor deposition. All fabricated structures were oxidized at 900-1000 • C for up to 100 h to simulate the temperature conditions in an aeroengine. The resulting shape of the riblets was characterized using scanning electron microscopy. The most accurate structures were obtained by using photolithography with a subsequent electrodeposition of nickel. This method is suited for single digit micrometer structures. The reduction of the wall shear stress was measured in an oil channel. The riblet structures prior to oxidation showed a reduction of the wall shear stress of up to 4.9% compared to a normal smooth surface. This proves that the fabricated riblet design can be used as a drag reducing surface.

We observe pore formation with diameters in the 10 nm range in silicon when it is covered with gold particles. This pore etching occurs when the sample is put in 5 wt % hydrofluoric acid (HF) solution for a few minutes. The pores form along the 100 direction, which is also the preferred direction of macro- and mesopores electrochemically etched into silicon. No etching occurs if the dissolved oxygen is removed from the aqueous HF solution or the gold is removed from the silicon surface. This leads to the assumption that the dissolved oxygen acts as an oxidant as in the case of stain etching with gold as cathodic material. A tentative model is suggested to explain why all of the observed nanopores have roughly the same diameter of about 10 nm. These pores can occur for inhomogeneously gold-covered planar silicon surfaces but also in MBE (molecular beam epitaxy) grown silicon nanowires since these nanowires are covered unintentionally with gold nanoclusters at their cylindrical surface.

The shark skin effect is the mechanism of wall friction reduction of a fluid due to a riblet structured surface. A new application for riblet surfaces may be jet engine blades. Riblet structured coatings on the blades would act as oxidation protection and additionally reduce the skin drag on the surface. In this study structuring surface areas of high temperature nickel‐based alloys is investigated. These alloys are used for compressor and turbine blades near the combustion chamber. Experiments are performed by depositing titanium on a nickel base alloy through particular metal grids using magnetron sputtering. For single‐digit micrometer structures, photolithography with subsequent electrodeposition of nickel as well as sputter deposition and thermal evaporation of several materials are investigated. Successfully fabricated structures are oxidized at 900–1 000 °C for up to 100 h and the resulting shape is characterized using scanning electron microscopy. The most accurate structures are obtained using photolithography and subsequent electrodeposition. The reduction of the wall shear stress was measured in an oil channel. The riblet structures prior to oxidation show a reduction of the wall shear stress of up to 4.9%.

Silicon nanowires are frequently grown involving a liquid gold droplet at their tips. Here we show that under certain circumstances the thermal oxidation of a silicon nanowire is drastically enhanced by the presence of this gold droplet. Such a gold-enhanced oxidation was observed in a temperature range from 1000 °C down to 250 °C. As a consequence, instead of the slow radial oxidation expected and desired for thinning the nanowires, a fast axial oxidation may occur catalyzed by the gold tip. This leads to a shrinking of the length of the Si nanowire and its replacement by a longer nanowire consisting of silicon dioxide. During this gold-enhanced oxidation process the gold droplet migrates from the tip to the base of the nanowire. Our experiments demonstrate that gold droplets lead to an enhanced dissolution of silicon during oxidation in the case that these remain in intimate contact with the Si nanowires.

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