Making metal wires – no heating required

Engineers from North Carolina State University, have developed a simple route to fabricating metallic wires at room temperature

By their very nature, today’s electronic devices based on silicon and germanium are hard, stiff and unwieldy, but the rapid development of organic and polymer based circuits are beginning to change that. Producing flexible, conductive wires for the smallest-scale devices remains a challenge though, but it is one that a new paper published in Extreme Mechanics Letters [DOI: 10.1016/j.eml.2016.03.010] has taken on.

liquid metal

A team of engineers from North Carolina State University looked to eutectic gallium indium (EGaIn) – a metal with a melting point of ∼15.5 °C – to produce thin wires at room temperature. Conventional electrical wires are fabricated by using large forces to repeatedly pull and elongate a metal rod that had been produced at high temperatures. The approach taken by Prof. Michael Dickey and his team is rather different.

First, they extrude a small-volume strip of EGaIn onto a polymer substrate using a syringe. Then, the polymer is stretched by hand, under typical strain rates of ∼1 cm/s. This also stretches the metal, producing an encapsulated wire with a diameter that decreases with strain. Key to the success of the wire-making process is the surface oxide that naturally forms on EGaIn. It plays two roles – one, it adheres to the polymer, allowing both materials to stretch at the same rate. And two, it adds mechanical stability to the metal, which allows it to achieve the high aspect ratio geometries usually disallowed by surface tension mechanics.

The researchers demonstrated that by chemically modifying certain polymers (e.g. polydimethylsiloxane and polyvinyl alcohol) during post-processing, it was possible to form both elastomeric and stiff wires. Exposed wires that retain their shape were produced following the same stretching routine, followed by removal of the polymer casing with formic acid, once the wire is in its desired position.

This simple stretching approach doesn’t require any specialist equipment, and can be carried out at room temperature. And yet, it can produce stretchable liquid metal wires with diameters approaching 10 um – similar to those achieved by standard line patterning techniques. It might even find use in making ‘wires on demand’, for repairing electrical connections.

This story appeared here: http://www.materialstoday.com/metals-alloys/news/making-metal-wires-no-heating-required/

Original research paper (£): Y. Lin, C. Ladd, S. Wang, A. Martin, J. Genzer, S.A. Khan, M.D. Dickey, “Drawing liquid metal wires at room temperature”, Extreme Mechanics Letters 7 (2016) 55–63. DOI: 10.1016/j.eml.2016.03.010