Nanostructures from directionally solidified NiAl–W eutectic alloys

A directionally solidified eutectic NiAl–W alloy was employed as a source for NiAl nanopore arrays, W-nanowire arrays and W-nanowires. The NiAl–W eutectic alloy containing 1.5 at.% W. A growth rate of 30 mm h−1 was used at a temperature gradient of 40 K cm−1 in a Bridgman-type directional solidification furnace. A combined stability diagram was derived from the Pourbaix diagrams of the three elements involved (Ni, Al, W). It allowed predicting proper conditions for the selective dissolution of either of the phases. Etching in a mixture of HCl:H2O2 released parallel aligned W-nanowires with a wire diameter of ∼200 nm. The aspect ratio observed in these measurements was over 400. Selective electrodissolution of the W minor phase was possible in a neutral buffer at 0.5 V versus SHE. This procedure yielded regular nanopore arrays with pore diameters of ∼200 nm. The pore density obtained was 8 × 1010 m−2. The relative coverage of the wires or the corresponding pores in a cross section was 0.3%. For the material employed in this study a 12 μm2 area corresponds to a single nanowire electrode. The potential of this material and the processing developed is discussed in terms of the production of nanofilters, nanoelectrode sensors, nanowire arrays and quantitative amounts of single crystalline tungsten nanowires.