Triboelectric Generators

July 25, 2012

One physical phenomenon, especially annoying to small children, is the electrical spark that jumps from their finger to switchplates in low humidity, usually in the winter. Winter is also the time when they are likely to have socks, slippers, or shoes on their feet, and that's important to the process. It's the rubbing of the dissimilar materials of their foot covering and the room carpet that generates an electrical charge in their body by the triboelectric effect.

I wrote about the triboelectric effect in two previous articles (Triboelectricity, July 17, 2007, and William Duddell's Singing Sparks, May 15, 2012. The triboelectric effect was known in ancient Greece, where Thales of Miletus (c. 600 BC) made the first written record, and the triboelectricity of amber was mentioned by Theophrastus in his book, "On Stones" (see figure).

Portion of "On Stones," a discourse on minerals (c. 300 BC) by Theophrastus, mentioning the attractive property of amber.[1]

The Greek's noted that rabbit fur and amber make a good triboelectric couple; in fact, the name of the negative charge carrier, the electron, comes from the Greek word for amber, ηλεκτρον. The word triboelectric comes from the Greek verb for rubbing, tribo, (τριβω), but actual rubbing is not required. What's required is that the materials contact each other, and then separate. The physical mechanism is that chemical bonds form between the materials, and separating the materials will also separate a charge. Rubbing helps by increasing the contact area.

It took quite a few centuries for triboelectricity to go from a novelty to an accessible source of continuous electricity. James Wimshurst (1832-1903), a nineteenth century British engineer designed and built his Wimshurst machine (see figure). The Wimshurst machine rubbed two disks together to generate a static electrical charge that was collected by metal patches and directed to electrodes.

The Winshurst machine. Left image, a scan by Andy Dingley from the 1903, Electrical Installations (Volumes V) by Rankin Kennedy, Caxton Press, London. Right image, a modern replica. Images from Wikimedia Commons.

Although Michael Faraday's electrical generator and electromagnetic dynamos replaced generators such as the Wimshurst machine as voltage sources for most applications, they weren't very good for high voltage generation.

Princeton University physics professor, Robert Van de Graaff(1901-1967), developed the eponymous Van de Graaff generator for such high voltage applications. This triboelectric generator is very simple, as the figure shows. It's an improved topology of the Wimshurst machine, and it was an inexpensive means of generating millions of volts for early nuclear physics experiments.

Diagram of a Van De Graaf generator The triboelectric version of this device has a pulley acting as one member of a triboelectric couple with an insulating belt. Charge is transferred through a small gap between the belt and the brushes. (Via Wikimedia Commons, modified).

Scientists from the Georgia Institute of Technology (Atlanta, GA) have developed a new triboelectric generator using transparent polymers that's suitable for some environmental energy-harvesting applications.[2-3] The secret ingredient, of course, is nanotechnology. Zhong Lin Wang, professor of Materials Science and Engineering at the Georgia Tech, explains that "this generator can convert random mechanical energy from our environment into electric energy."[2]

The generator is made from a sheet of polyester, a electron donor, that rubs against a sheet of polydimethylsiloxane, an electron acceptor. The polymer surfaces rub together, and produce a charge when separated.[2] Continually rubbing and separating the sheets generates a small alternating current.[2] The current production is enhanced by micropatterning the surfaces (see photograph). Pyramidal shapes seemed to be optimum, with as much as 18 volts at about 0.13 microamps per square centimeter being generated.[2-3]

Micrograph of pyramid patterns created in a polymer sheet for the Georgia Tech triboelectric generator. (Georgia Tech image by Zhong Lin Wang).[2]

Any good physical effect can easily be turned into a sensor, and this triboelectric device can be used as a pressure sensor. The sensitivity is quite good, since it can detect an 8 milligram water droplet (3.6 Pa contact pressure); or, a 20 milligram falling feather (0.4 Pa in contact pressure). The lower limit of pressure detection is 13 millipascals.[2-3] These devices have been tested for more than 100,000 operating cycles.[2]

This research was funded by the National Science Foundation, the Department of Energy and the U.S. Air Force.[2] Details were reported in the June issue of the journal, Nano Letters.[3]

References:

1. Earle Radcliffe Caley and John F.C. Richards, "Theophrastus on Stones: Introduction, Greek Text, English Translation, and Commentary," Ohio State University (Columbus, Ohio, 1956). Greek text, p. 23; English translation, p. 51, as follows:
   

   "... And since amber is also a stone—for the kind that is dug up is found in Liguria—the power of attraction would belong to this too. The stone that attracts iron is the most remarkable and conspicuous example. This also is rare and occurs in few places. This stone too should be listed as having a similar power."

2. John Toon, "Plastic Power: Triboelectric Generator Produces Electricity by Harnessing Frictional Forces Between Transparent Polymer Surfaces," Georgia Tech Press Release, July 9, 2012.
3. Feng-Ru Fan, Long Lin, Guang Zhu, Wenzhuo Wu, Rui Zhang, and Zhong Lin Wang, "Transparent Triboelectric Nanogenerators and Self-Powered Pressure Sensors Based on Micropatterned Plastic Films," Nano Letters, vol. 12, no. 6 (June 13, 2012), pp. 3109-3114.