Material of the month – Wire

Materials World magazine
,
1 Dec 2014

What do lightbulbs have in common with pianos? Suspension bridges and toasters? Futuristic computers with ancient jewellery? All of these, from the everyday to the quantum, rely on wire technology.

The first example of metal being formed into wire comes from ancient Egypt almost 5,000 years ago. Gold and silver wires for jewellery were made by hammering or twisting strips of metal into a narrow, hollow tube and then rolling this between two flat surfaces to form a thin, solid wire. The method involved drawing a thin ribbon of metal through other metals or stone dyes by hand, and repeating passes to form tubes, which eventually close to produce solid wire. The Vikings laid the foundations for the modern draw-plate set-up, and since then there have been further developments in hard materials over the past 150 years to fine-tune wire drawing into the process used today.

The current method of wire drawing involves pulling ductile metal through a small hole in a die at room temperature. This cold works the metal when it plastically deforms so it can fit through the narrow aperture. Dislocations or line defects in ductile materials can easily slip and climb through the crystal lattice. Cold working by wire drawing makes metals stronger due to the generation, entanglement and pinning of dislocations. In this way, wires with a tensile strength of up to 2,600MPa can be produced. This strength is used in high-carbon piano strings to withstand repeated tightening and slackening during tuning, and constant hammering when played.

Wire drawing differs from extrusion because the wire is pulled, not pushed, through the die by a rotating block at the end of the process, onto which the wire is wound. Usually a series of dies is required to sequentially reduce the wire diameter. These intermediate annealing steps are necessary to soften the material by removing excessive dislocations and produce the required ductility for subsequent draws. The final product can be annealed again to maximise ductility and electrical conductivity.

Dies must be made from materials that are significantly harder than the drawn metal. These include tool steel, tungsten carbide or single–crystal diamond for the very finest wires, lubricated with oil or a copper coating. Although a circular cross-section is most common, hexagonal, square, rectangular or any other shape is possible by changing the outline of the die.

There are relatively few metals and alloys that are both ductile and strong enough at room temperature to withstand cold drawn into wire. Platinum, silver, iron, aluminium, gold, copper, and alloys such as brass and bronze all have suitable properties. For enhanced thermal protection, copper wires can be plated in tin, nickel or silver. Wires used for electrical applications are usually coated in insulating polymers such as polyethylene or PVC.


Electrical wires

Perhaps the most familiar form of wire in the home is hook-up wire, used to carry electric current. Often, these wires comprise a number of smaller strands, which are bundled to provide more flexibility and increased conductivity than an equivalent solid wire of the same diameter. The smallest number of strands used in wire bundles is seven – one in the centre with six surrounding it. The more strands in the bundle, the stronger and more flexible it becomes, so it is common for the strongest and most flexible wires to contain up to 100 individual strands. Bundles can even be wound into super-bundles, which are combined to make the final cable. Helical winding of bundles means the wires twist to relieve stress when the cable is flexed. Such cables made from galvanised steel are used to carry extreme loads in suspension bridges.

Conducting metal wires can be coiled, often around a metallic core, to form a solenoid. Passing electric current through a solenoid produces a magnetic field, and vice versa if a magnetic field interacts with a solenoid. This is used in electric motors to produce mechanical motion from electricity and the reverse in generators. Transformers make use of electromagnetic induction to step-up or step-down voltage in the electric grid through the interaction of two coils with a multiple number of turns.


Applications

How many inventors did it take to change the lightbulb? Joseph Swan and Thomas Edison’s work was built on contributions from more than 20 other individuals. Their success came after creating an effective filament material, a superior vacuum achieved by a Sprengel pump and high resistance, which made the product economically viable.

The use of tungsten as a filament material was patented by Alexander Friedrich Just and Franjo Hanaman in 1904, and bulbs were filled with inert argon gas rather than a vacuum, which improved luminosity. However, in the early iterations of tungsten lamps, the filaments suffered from severe drooping. The microstructure was such that the bamboo-like grains had little resistance to creep deformation under the high operating temperatures (2,500oC) and, consequently, the filaments reduced in cross-section, which led to local heating and premature filament failure. However, William Coolidge of the General Electric Company developed a method to produce ductile tungsten filaments from powdered tungsten with an overlapping grain structure, which was commercialised in 1911. The doping of tungsten with potassium results in the formation of gaseous potassium bubbles due to potassium’s relatively low boiling point (759oC). These bubbles prevent creep by pinning grain boundaries to provide significantly longer lamp lifetime.

These filaments produce light due to incandescence, whereby electromagnetic radiation is emitted through high resistance to electron conduction. Tungsten’s high temperatures allow incandescent temperatures to increase without melting. Around 10% of this radiation falls in the visible spectrum, therefore the filament produces light. However, the majority falls in the infra-red part of the spectrum, which is given out as heat and severely diminishes the bulb’s efficiency. This energy waste has resulted in the phasing out of incandescent lightbulbs worldwide, to be replaced with energy-efficient LEDs and halogen alternatives. However, heat released by incandescence can be used to our advantage – to toast our bread in the morning, for instance, using heating elements made from high-resistance nichrome wire.

Wires are also finding application in the extraordinary realm of nanotechnology. These nanowires are one-dimensional materials, made from metallic (Ni, Pt, Au), semiconducting (Si, InP, GaN) or insulating (SiO2, TiO2) materials. They can be synthesised by top-down approaches such as lithography or electrophoresis – or, more commonly, bottom-up processes, by combining ad-atoms using electrochemical deposition or vapour deposition.

At such minute scales, quantum mechanical effects take hold. Electrons are laterally quantum confined, so their energy levels become quantised, making them behave differently from those occupying the continuum of energy levels in bulk materials. One consequence of this is that there is a restraint on the number of electrons that can travel through the wire and so the wire exhibits discrete conductance.

Nanowires can be incorporated into quantum devices, which exploit these electronic properties. Semiconducting nanowires in particular doping configurations, can form logic gates used in quantum computing. Sensors made from superconducting niobium nitride nanowires are able to detect single photons of visible and infra-red light, and silicon-based nanowires can sense proteins and chemicals due to a change in charge density upon contact, causing a surface difference and a measurable change in conduction.

Wire technology continues to brighten our lives, with influences including electronics, music, computing, engineering and energy distribution. Although consumers are increasingly demanding wireless technologies, the applications of wire are so widespread that its longevity and utility is unquestionably guaranteed for good.