A group of Georgia Institute of Technology researchers have fabricated super-small nanoscale power generators able to generate enough power to run a small LCD. The nanoscale generators that harvest mechanical energy from the environment using an array of tiny nanowires.
Merely compressing a nanogenerator between two fingers puts out enough energy to run the small display. Demonstration devices can generate as much as three volts and 300 nanoamps. Researchers say these nanogenerators will never produce large amounts of electricity but could power nanoscale and microscale devices – and even to recharge pacemakers or iPods.
The nanogenerators rely on the piezoelectric effect seen in crystalline materials such as zinc oxide, in which an electric charge potential arises when structures made from the material are flexed or compressed. Devices built so far combine the charges from millions of nanoscale zinc oxide wires.
The researchers reported recent improvements in the nanogenerators, including a simpler fabrication technique, in the journal Nano Letters.
The earliest zinc oxide nanogenerators used arrays of nanowires grown on a rigid substrate and topped with a metal electrode. Later versions embedded both ends of the nanowires in polymer and produced power by simple flexing. Regardless of the configuration, the devices required careful growth of the nanowire arrays and painstaking assembly. But researchers Zhong Lin Wang, Youfan Hu, Yan Zhang, Chen Xu, Guang Zhu and Zetang Li came up with much simpler fabrication techniques. First, they grew arrays of a new type of nanowire that has a conical shape. These wires were cut from their growth substrate and placed into an alcohol solution.
The solution containing the nanowires was then dripped onto a thin metal electrode and a sheet of flexible polymer film. After the alcohol dried, they added another layer. Multiple nanowire/polymer layers were built up into a kind of composite, using a process that Wang believes could be scaled up to industrial production.
When flexed, the nanowire sandwiches – which are about 2 × 1.5 cm – generated enough power to drive a commercial display borrowed from a pocket calculator.
Wang says the nanogenerators are now close to producing enough current for a self-powered system that, for example, might monitor the environment for a toxic gas, then broadcast a warning. The system would include capacitors able to store up the small charges until there was enough power to send out a burst of data.
While even the current nanogenerator output remains below the level required for such devices as iPods or cardiac pacemakers, Wang expects to hit those levels within three to five years. The current nanogenerator, he notes, is nearly 100 times more powerful than what his group had developed just a year ago.
Writing in a separate paper published in October in the journal Nature Communications, group members Sheng Xu, Benjamin J. Hansen and Wang reported on a new technique for fabricating piezoelectric nanowires from lead zirconate titanate – also known as PZT. The material is already used industrially, but is difficult to grow because it requires temperatures of 650 °C.
In the paper, Wang’s team reported the first chemical epitaxial growth of vertically-aligned single-crystal nanowire arrays of PZT on a variety of conductive and non-conductive substrates. They used a process known as hydrothermal decomposition, which took place at just 230 °C.
And in another paper published in Nano Letters, Wang and group members Guang Zhu, Rusen Yang and Sihong Wang reported a way to boost nanogenerator output. Their approach, called “scalable sweeping printing,” includes a two-step process of (1) transferring vertically-aligned zinc oxide nanowires to a polymer receiving substrate to form horizontal arrays and (2) applying parallel strip electrodes to connect all of the nanowires together.
Using a single layer of this structure, the researchers produced an open-circuit voltage of 2.03 V and a peak output power density of approximately 11 mW/cm3.