Nanotechnology researchers at the Georgia Institute of Technology have developed a means of generating electricity from pairs of textile fibres covered with zinc oxide nanowires.
The researchers say that combining the current from many such fibre pairs woven into a shirt or jacket could allow the wearer’s body movement to power a range of portable electronic devices.
‘If we can combine many fibres in double or triple layers in clothing, we could provide a flexible, foldable and wearable power source that, for example, would allow people to generate their own electrical current while walking,’ said Zhong Lin Wang, a Regents professor in the School of Materials Science and Engineering at the Georgia Institute of Technology.
The microfibre-nanowire hybrid system builds on a nanowire nanogenerator that Wang’s research team announced in April 2007. That system generates current from arrays of vertically-aligned zinc oxide (ZnO) nanowires that flex beneath an electrode containing conductive platinum tips. The nanowire nanogenerator was designed to harness energy from environmental sources such as ultrasonic waves, mechanical vibrations or blood flow.
The nanogenerators developed by Wang’s research group take advantage of the unique coupled piezoelectric and semiconducting properties of zinc oxide nanostructures, which produce small electrical charges when they are flexed. The original nanogenerators – which are two by three millimetres square – can produce up to 800 nanoamperes and 20 millivolts.
The microfibre generators rely on the same principles, but are made from soft materials and designed to capture energy from low-frequency mechanical energy. They consist of DuPont Kevlar fibres on which zinc oxide nanowires have been grown radially and embedded in a polymer at their roots, creating what appear to be microscopic baby-bottle brushes with billions of bristles. One of the fibres in each pair is also coated with gold to serve as the electrode and to deflect the nanowire tips.
Zhong Lin Wang holds a prototype microfibre nanogenerator composed of two fibres that rub together to produce a small electrical current
‘The two fibres scrub together just like two bottle brushes with their bristles touching, and the piezoelectric-semiconductor process converts the mechanical motion into electrical energy,’ Wang explained. ‘Many of these devices could be put together to produce higher power output.’
Wang and collaborators Xudong Wang and Yong Qin have made more than 200 of the fibre nanogenerators. Each is tested on an apparatus that uses a spring and wheel to move one fibre against the other. The fibres are rubbed together for up to 30 minutes to test their durability and power production.
So far, the researchers have measured current of about four nanoamperes and output voltage of about four millivolts from a nanogenerator that included two fibres that were each one centimetre long. With a much improved design, Wang estimates that a square metre of fabric made from the special fibres could theoretically generate as much as 80 milliwatts of power.
Schematic shows how pairs of fibres coated with zinc oxide nanowires and alternately with gold would rub together to produce a small electrical current
Fabrication of the microfibre nanogenerator begins with coating a 100 nanometre seed layer of zinc oxide onto the Kevlar using magnetron sputtering. The fibres are then immersed in a reactant solution for approximately 12 hours, which causes nanowires to grow from the seed layer at a temperature of 80oC. The growth produces uniform coverage of the fibres, with typical lengths of about 3.5 microns and several hundred nanometres between each fibre.
To help maintain the nanowires’ connection to the Kevlar, the researchers apply two layers of tetraethoxysilane (TEOS) to the fibre. ‘First we coat the fibre with the polymer, then with a zinc oxide layer,’ Wang explained. ‘Then we grow the nanowires and re-infiltrate the fibre with the polymer. This helps to avoid scrubbing off the nanowires when the fibres rub together.’
Finally, the researchers apply a 300 nanometre layer of gold to some of the nanowire-covered Kevlar. The two different fibres are then paired up and entangled to ensure that a gold-coated fibre contacts a fibre covered only with zinc oxide nanowires. The gold fibres serve as a Schottky barrier with the zinc oxide, substituting for the platinum-tipped electrode used in the original nanogenerator.
To ensure that the current they measured was produced by the piezoelectric-semiconductor effect and not just static electricity, the researchers conducted several tests. They tried rubbing gold fibres together, and zinc oxide fibres together, neither of which produced current. They also reversed the polarity of the connections, which changed the output current and voltage.
By allowing nanowire growth to take place at temperatures as low as 80oC, the new fabrication technique would allow the nanostructures to be grown on virtually any shape or substrate.
As a next step, the researchers want to combine multiple fibre pairs to increase the current and voltage levels. They also plan to improve conductance of their fibres.
However, one significant challenge lies head for the power shirt, namely washing it. Zinc oxide is sensitive to moisture, so in real shirts or jackets, the nanowires would have to be protected from the effects of the washing machine, Wang noted.