Will nanogenerators replace batteries?
Researchers at the Georgia Institute of Technology have built prototypes of a nanogenerator providing continuous electrical power by "harvesting mechanical energy from such environmental sources as ultrasonic waves, mechanical vibration or blood flow." According to the scientists, the prototype could produce as much as 4 watts per cubic centimeter. This should be largely enough to power a broad range of nanoscale devices used for defense, environmental and biomedical applications, including biosensors implanted in the body or nanoscale robots. But will these nanogenerators be able to power bigger devices, such as our cell phones? The researchers think it's possible.
These nanogenerators are based on arrays of vertically-aligned zinc oxide nanowires and could one day power nanoscale devices without batteries or other external power sources. They have been developed by Zhong Lin Wang, professor in the School of Materials Science and Engineering at the Georgia Institute of Technology, and the members of his Nanoscience research group
Before going further, below is an image showing how these nanogenerators are driven by ultrasonic waves (Credit: Zhong Lin Wang's lab). And here is the full caption: "(A) Schematic diagram showing the design and structure of the nanogenerator. Aligned ZnO NWs grown on a solid/polymer substrate are covered by a zigzag electrode. The substrate and the electrode are directly connected to an external load. (B) Aligned ZnONWs grown on a GaN substrate. The gold catalyst particles used for the growth had been mostly vaporized; thus, the final NWs were purely ZnO with flat top ends. (C) Zigzag trenched electrode fabricated by the standard etching technique after being coated with 200 nm of Pt. The surface features are due to nonuniform etching. (D) Cross-sectional SEM image of the nanogenerator, which is composed of aligned NWs and the zigzag electrode. (Inset) A typical NW that is forced by the electrode to bend."
Here are some more details about these nanogenerators.
The nanogenerators take advantage of the unique coupled piezoelectric and semiconducting properties of zinc oxide nanostructures, which produce small electrical charges when they are flexed. Fabrication begins with growing an array of vertically-aligned nanowires approximately a half-micron apart on gallium arsenide, sapphire or a flexible polymer substrate. A layer of zinc oxide is grown on top of substrate to collect the current. The researchers also fabricate silicon “zig-zag” electrodes, which contain thousands of nanometer-scale tips made conductive by a platinum coating.
The electrode is then lowered on top of the nanowire array, leaving just enough space so that a significant number of the nanowires are free to flex within the gaps created by the tips. Moved by mechanical energy such as waves or vibration, the nanowires periodically contact the tips, transferring their electrical charges. By capturing the tiny amounts of current produced by hundreds of nanowires kept in motion, the generators produce a direct current output in the nano-Ampere range.
For more information, this research work has been published by Science under the name "Direct-Current Nanogenerator Driven by Ultrasonic Waves" (Volume 316, Number 5821, Pages 102-105, April 6, 2007). Here are links to the abstract and to the full paper (PDF format, 5 pages, 479 KB). The above illustration comes from this article.
You might also want to read a previous Georgia Tech news release about these tiny but powerful devices, "Nanogenerators convert mechanical energy to electrical energy for self-powered nanoscale devices" (April 13, 2006).
But will these nanogenerators be able to power bigger devices, such as our cell phones? "If you had a device like this in your shoes when you walked, you would be able to generate your own small current to power small electronics," Wang noted. "Anything that makes the nanowires move within the generator can be used for generating power." So let's wait.
Sources: Georgia Institute of Technology Research News, April 5, 2007; and various websites
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