Is this anything semi-related? ...mechanical vibration causing certain kinds of ceramics to absorb such vibration and in turn create tiny electrical fields...possible that other materials create a bit of heat?
Turning vibrations into energy, nanowire style
A new device based on piezoelectric nanowires aims to turn mechanical …
by Adam Stevenson - Nov 11, 2008 3:33pm GMT
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Considering the problems inherent to fossil fuel energy production and the increasing saturation of mobile electronic devices, one would think that we could waste less of the energy that we produce and carry with us. However, it turns out that we lose a huge fraction of our energy as excess heat and vibration. While some of the lost energy is impossible to recover (in this journal, we follow the laws of thermodynamics!), most waste heat and vibration can be captured and converted back into useable energy.
Piezoelectricity has been seen as the key to harnessing waste vibrations in mechanical devices. Piezoelectricity is a material property that is a characteristic of many ceramics with noncentrosymmetric crystal structures. When these materials are strained, they develop an electric field. Straining the material causes positive and negative ions in each unit cell to displace in different directions. This produces a small electrical field but, when it's summed over moles of unit cells, it can be quite substantial. To convert vibrations into energy, piezoelectric materials must be packaged into a device so that the vibrations cause strain that can be extracted through a connection to an appropriate electrical circuit.
As with most engineering problems today, nanotubes and nanowires are being tested for their piezoelectric potential. Nanowire mechanical energy harvesting devices place piezoelectric ZnO nanowires vertically on a flat substrate, and contact them with a zig-zag shaped electrode. As the device vibrates, the free-moving, zig-zag electrode flexes the nanowires, generating an electrical potential. While these devices can produce substantial fields and do not require control over the placement of the nanowires, the design leads to mechanical breakdown of the nanowires and short service lifetimes. Sealing difficulties also make this design prone to environmental attack.
Top (A): SEM image of ZnO nanowires
Middle (B): Schematic of a flexural based measurement system that is similar to "zig-zag" designs
Bottom (C): Plot of electrical response versus position showing the fields generated by discrete ZnO nanorods (credit for all three images: Georgia Institute of Technology through nsf.gov)
In this week's Nature Nanotechnology, researchers from Georgia Institute of Technology have modified the zig-zag design to increase its durability while maintaining efficiency. Their method was brilliantly simple—just lay the nanowires down to produce a laterally-packaged piezoelectric generator. In this system, ZnO nanowires were placed on a flexible polyimide film with both ends attached to a circuit. The researchers found that, when the film was flexed, a single nanowire could generate a 50 mV field with 6.8% efficiency. Clearly, the efficiency needs to be improved, but the device solves all of the major problems inherent to the previous design: the nanowires are not exposed to breaking stresses and the laterally packaged device is easily sealed.
The key to the design (and the real insight of the paper) is that at least one end of the nanowire must be electrically connected by a Shottkey contact rather than a simple ohmic contact. Ohmic contacts are characterized by a change in resistance at the junction, but Shottkey contacts create an electrical potential barrier that limits electron conduction. Non-stoichiometric defects at the wire ends perturb the band structure, resulting in a potential barrier in ZnO nanowires.
In laterally packaged piezoelectric systems, ohmic contacts allow electrical conduction that immediately negates the piezoelectric field, thus rendering the system useless. However, by incorporating Shottkey barriers on at least one end of the ZnO wire, electron conduction is severely limited because the piezoelectric field generated cannot overcome the Shottkey potential barrier. This causes electrons to pile up at the Shottkey barrier when the device is flexed; a discharge occurs when the strain is relieved. The result is an alternating field and current that can be easily harnessed for storage or used to drive an electrical device.
The device described in this study is a clear incremental step over the current zig-zag nanowire designs, but it is by no means a revolution—the efficiency is far too low and the fabrication has substantial scaling issues. However, with future research, both of these problems should be overcome and the substantial engineering advantages of this system could lay the groundwork for future functional devices.
Nature Nanotechnology DOI:10.1038/nnano.2008.314