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Posted: Feb 07, 2013

Gold squeezed into micro-Velcro

(Nanowerk News) Velcro consists of one surface with loops, and another with hooks that latch onto the loops, joining opposing surfaces strongly. A miniaturised version of Velcro could be used in micro- and nanotechnology, but to form the surfaces, microwires are needed with properties that provide strength and durability.
Several different approaches have been used to construct 'micro-Velcro', but the most promising are those that use self-assembling or self-organising techniques, where microwires are 'squeezed' from a composite material by compression. Researchers at Ruhr University Bochum, Germany, have used this technique to produce gold microwires that have suitable properties for micro-Velcro ("Synthesis of Au microwires by selective oxidation of Au–W thin-film composition spreads").
a variety of gold microwire morphologies depending on the overall gold content
SEM micrographs showing a variety of Au microwire morphologies depending on the overall Au content: a) Short and straight wires appear at 42 at.% Au. b) With increasing Au content to 44 at.% the wires become longer and start bending. c) They achieve a maximum length and a hook-like shape at 48 at.% Au. d) False-color picture generated from elemental composition maps for 46 at.% Au (Au: yellow, W: blue).
The scientists created thin films of composite materials containing gold and tungsten metal. These were then heated to very high temperatures, causing the tungsten to react with oxygen and form tungsten oxide. This increased the volume of the tungsten and caused compression within the composite, 'squeezing' the softer gold out as 'whiskers'.
Different ratios of gold to tungsten were tried in the original composite, and these were heated to different temperatures and for different times to find the optimal conditions. The best result produced gold microwires approximately 35 micrometres long - similar to the width of a human hair - and 2 micrometres in diameter.
The resulting gold microwires have larger diameters than indium metal microwires that had previously been made using a similar technique, making them more suitable for micro Velcro. The results demonstrate that this new approach is a feasible one for producing the microwires that could be used to make micro-Velcro.
Source: National Institute for Materials Science
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