### Nickel-Copper Nanowires

June 5, 2012

The well-known formula for the electrical resistance R of a wire contains a lot of useful information; viz.,
R = ρL/A

where ρ is the material resistivity, measured in ohm-meters, L is the length of the wire, and A is its cross-sectional area. A low resistance wire, which is what's needed in most applications, will be made from a low resistivity (high conductivity) material, will have a large cross-sectional area, and will be as short as possible.

Sometimes, your object is a high resistance, as provided by these resistors on a circuit board from a Univac 9200 Computer (c. 1966). Also shown are diodes (regular and zener), transistors (round objects), and some antique capacitors (rectangular objects). (Photo by the author.)

The wiring that concerns most people is the wiring in the walls of their house. In the US, these copper wires are usually 12 or 14 AWG (American Wire Gauge). What's confusing to most people, but not to machinists, is that lower gauge numbers are for thicker wires. Some properties of these wires are listed in the table, below.

 Copper Wire Properties
 AWG dia. (mm) dia. (inch) mΩ/m mΩ/foot 12 2.053 0.0808 5.211 1.588 14 1.628 0.0641 8.286 2.525

The ohms/foot value is significant, since it specifies what the voltage drop and resistance heating of the wire will be. For fifteen amperes, which is about the current draw of a hair dryer, a fifty foot run of 14 AWG wire would have a voltage drop of nearly two volts because of its 0.126 ohm resistance. This will generate 28 watts of heat inside the walls of your house. Fortunately, this heat is distributed along the length of the wire, but it's a significant energy loss (~2% for the hair dryer).

At one time, aluminum wiring was used for some residential wiring in the US, since it was less expensive than copper and almost as conductive. Aluminum, however, was not quite up to the task for a variety of reasons, the most important of which is mechanical creep at wire joins and joins of wire to switches and receptacles. An originally tight connection can become loose over time, causing a high contact resistance.

Although copper and aluminum are the best solution for most wiring applications, including conductors on integrated circuits, they aren't the only high conductivity metals, as the following table shows.

 Electrical Conductors (Data at 20°C from Wikipedia)
 Material ρ (10-8Ω-m) Material ρ (10-8Ω-m) Silver 1.59 Iron 10 Copper 1.68 Platinum 10.6 Gold 2.44 Tin 10.9 Aluminium 2.82 1010 Carbon steel 14.3 Calcium 3.36 Lead 22 Tungsten 5.60 Titanium 42 Zinc 5.90 Stainless Steel † 69 Nickel 6.99 Mercury 98 Lithium 9.28 Nichrome 110
† 18% Cr, 8% Ni Austenitic

One thing that the table shows is that most pure elements have lower resistivity than alloys, a fact that's easily understood. The high resistivity of alloys works to an advantage in some applications, such as transformer cores, where you want a high resistance to prevent core losses from eddy currents. Silicon steel (47.2 x 10−8 ohm-meter) is used in transformer core applications. Amorphous metals have about three times the resistivity of silicon steel (100-150 x 10−8 ohm-meter).

Early computers were called "Big Iron" for a reason. There were a lot of transformers, like this, with silicon steel laminated cores. Switched-mode power supplies have made everything lighter.

(Photo by Arnold G. Reinhold, via Wikimedia Commons.)

When we consider nanowires, we're immediately faced with the reality of a very low cross-sectional area, perhaps mitigated by a small length. The cross-sectional area scales as the square of the dimension, so it's a major problem. Nonetheless, nanowires have been researched as a way to produce the conductive, yet transparent, electrodes required for displays.

As I wrote in a previous article (Transparent and Conductive, June 10, 2011), carbon nanotubes, the wonder material of the decades before graphene, mixed in a polymer, have been investigated for such an application by scientists at the Eindhoven University of Technology (Eindhoven, The Netherlands).[1-2] Silver nanowires, overcoated with a conductive polymer, likewise form a good quality transparent electrode.[3-4]

I reviewed the silver nanowire work by scientists at the University of California, Los Angeles (UCLA) in another article (Silver Nanowire Transparent Electrodes December 19, 2011). The important feature of the UCLA nanowires is that a conductive polymer overcoating of PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)) provides a conductive path between individual nanowires to enhance electrical conductivity.[3-4]

Copper is as conductive as silver, but it's a thousand times more abundant and less expensive; so, copper nanowires are a potential material for this application. Copper nanowires, however, have an orange color, and they oxidize when exposed to air. Recent research at Duke University has addressed both these problems by putting a nickel coating over the copper nanowires.[5-6]

Benjamin Wiley, an assistant professor of chemistry at Duke, and his students examined the sheet resistance of uncoated copper nanowire films, and they found that it doubles after three months at room temperature. Silver nanowire films perform better, doubling resistance after three years,[6] but the sheet resistance of copper nanowires, coated with 20 mol-% nickel, will double only after hundreds of years. The Ni-coated wires have a gray color that's less noticeable in displays.[5]

Image of a nickel-coated copper nanowire.

(Image supplied by Benjamin Wiley, used with permission)

At this point, the conductivity of the nickel-coated copper nanowire films is less than that of indium-tin oxide (ITO) of the same transparency. However, this nanowire material is promising on many counts, and Wiley has started NanoForge Corp., a Durham, North Carolina, startup company, to develop this material.[5]

### References:

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