The formation mechanism of eutectic microstructures in NiAl-Cr composites.

NiAl-based eutectic alloys, consisting of an ordered bcc matrix (B2) and disordered bcc fibers (A2), have been a subject of intensive efforts aimed at tailoring the properties of many of the currently used nickel-based superalloys. A thermodynamic phase field model was developed on a thermodynamic foundation and fully integrated with a thermo-kinetic database of the Ni-Al-Cr ternary system to elucidate the resulting peculiar eutectic microstructure. Invoking a variation of the liquid/solid interfacial thickness with temperature, we simulated the characteristic sunflower-like eutectic microstructures in the NiAl-Cr composites, consistent with experimental observations. The mechanism that governs the formation of the peculiar eutectic morphology was envisioned from the modeled evolutions associated with six sequential steps. Our calculations show that the conditional spinodal decomposition occurring in sequence could further trim and revise the microstructure of the eutectics by generating fine-domain structures, thereby providing an additional method to explore the novel NiAl-based eutectic composites with tunable properties at elevated temperatures.

Reactivity of gold nanobelts with unique {110} facets.

Gold nanobelts were synthesized by directional solidification of the Fe-Au eutectoid followed by selective phase dissolution. Cleaning from organic molecules was performed in alkaline solution by PbO(2) deposition/dissolution to avoid surface reconstruction. The electrochemical behaviour of the Au nanobelts was determined by structure-sensitive electrochemical reactions, and the findings confirm the results obtained by selected area electron diffraction (SAED). The underpotential deposition (UPD) of lead under alkaline conditions and cyclic voltammograms (CVs) in sulphuric acid revealed an unusual large amount of (110) domains (>65%). Finally, after cleaning the Au nanobelts showed a higher and stable electrocatalytic behaviour toward methanol oxidation in alkaline media. The possible mechanism and the potential applications of the Au nanobelts are discussed.

Single crystalline molybdenum nanowires, nanowire arrays and nanopore arrays in nickel-aluminium.

This work describes a novel fabrication method of single crystalline Mo nanowires and nanowire arrays. The method utilizes directional solidification (ds) of a NiAl-Mo eutectic alloy and its subsequent electrochemical processing. In the first step, a self-organized array of Mo nanowires embedded in a NiAl matrix is obtained. By combining the Pourbaix diagrams of the three elements involved, a strategy for selective removal of either of the two phases is derived. An oxidizing acidic solution of pH 0.2 dissolved the matrix and released an array of long and uniform Mo wires. Even a complete extraction of the wires is possible through entire dissolution of the matrix. On the other hand, electrodissolution of the Mo with a simultaneous passivation of the NiAl matrix at the pH 6 and the potential of 200 mV SHE yielded nanopore arrays with rectangular pores. This method has several advantages. First of all, it is one of the few top-down methods that allow the production of large amounts of nanostructures. In addition, both the wires and the matrix are single crystalline which makes them favorable for various applications. Further, the obtained nanostructures exhibit extremely high aspect ratios (> 1000), unreachable by most other techniques. This technique has the potential for the production of nanowire arrays either for employment in sensors or in field emission.

Arrays of iso-oriented gold nanobelts.

For the first time single crystalline gold nanobelt arrays with identical crystallographic orientation were obtained. A combined method consisting of directional solid-state transformation of a Fe-Au eutectoid and a well controlled electrochemical treatment enables production of arrays of nanobelts with a desired length. They have an average thickness of 25 nm and width of 200-250 nm, respectively. The obtained gold nanobelt arrays were characterized by electron backscattered diffraction (EBSD), X-ray diffraction, and XPS. The underlying mechanisms and the potential of this method for the production of nanosensors are discussed.

Model systems with extreme aspect ratio, tunable geometry, and surface functionality for a quantitative investigation of the Lotus effect.

Superhydrophobicity and superhydrophilicity of surfaces are key properties for fabrication of self-cleaning surfaces (Lotus effect). It is well known that the mechanism behind this is based on the surface roughness and surface functionalization. To obtain an understanding of the details of the underlying mechanism, a metal system based on a eutectic is suggested. In this study, a wide range tunability of its needlelike narrow size distributed nanostructure is demonstrated. The length of the needles as well as their density can be varied independently. In addition, an important parameter for the wettability, the roughness, is related directly to the growth parameters, which lead to excellent controllable and reproducible eutectic structures. Simply by varying etching time very high aspect ratios can be achieved, allowing studying the interaction of the very long needles with liquids. Moreover, the surface functionality can be tuned by RF-magnetron sputtering of PTFE onto the metal needles. As those layers can be very thin, our system allows, in principle, studying the transition from a metal to a polymer surface using submonolayers. Furthermore, the first contact angle measurements on the nanostructured and functionalized eutectic structures are presented and discussed.

Effect of the growth conditions on the spatial features of Re nanowires produced by directional solidification.

The effects of the solidification parameters, such as growth rate and temperature gradient, on the distance and diameter of Re nanowires have been examined. Both the spacing and diameter increase with decreasing growth rate and temperature gradient, respectively. The ratio of fiber spacing to diameter is 9.1. In addition, it was demonstrated that the temperature gradient influences interface undercooling in the same way as the growth rate and may be used as an additional parameter to control fiber spacing and diameter.