As we continue to carry around items that insist on requiring electricity to work, portable—even wearable—energy-generating systems are looking very attractive. A group of researchers has recently looked into the use of piezoelectric materials, which generate an electric field or potential when placed under mechanical stress. By placing these materials on a rubbery or flexible surface, they created a material that can generate the highest rate of energy conversion reported for similar systems. While these are still far from the market, the metrics of the flexible piezoelectrics so far are very promising.
Piezoelectric materials are not new. Certain crystals and ceramics generate electricity when mechanically stressed, but coupling these brittle items with flexible substrates is not easy— often the materials need a significant stress to produce an electric field, but that stress could break them into pieces, rendering them ineffective. There are a couple of polymer piezoelectrics that can flex, but their voltage coefficient is an order of magnitude smaller than the crystal materials.
To make a system that generated a significant amount of power, the researchers focused on embedding the brittle piezoelectric materials into flexible substrates. To make the two substances work together, they generated very thin crystalline piezoelectric films on a crystal substrate. They then etched the film into strips that were only micrometers long and nanometers thick, stuck a piece of flexible plastic on top, and ripped it off. The ribbons stuck to the plastic and came off the crystal intact thanks to van der Waals forces, resulting in a piece of "piezo-rubber" ready to generate power.
A typical problem with using piezo materials in such small amounts is that the crystals will often have a high number of internal defects, reducing performance. This team found they had to tune the growth of the crystals to make sure their structure was preserved when they were ripped away by the plastic, and put them through several microscopy and spectroscopy tests the ensure the integrity of their structure.
To compare the energy conversion abilities, the researchers tested both the film form and the ribbon-on-plastic form. For the film, they had a vertical deflection of 57 picometers per volt of potential applied (using the piezo materials usually requires the opposite relationship—bending to generate a voltage—but the order of events don't matter for testing).
To generate another measure of efficiency, the crystals were also "poled," or placed in a potential to magnetically align them, at which point they'll respond more strongly. Poling will happen naturally over time, and measuring it helps researchers see how the crystals will perform once they settle into a poled configuration. Once poled, the films' constant was 113 picometers per volt.
When they tested the piezo rubber, they found the crystals' performance was barely diminished. The ribbons deflected 54.2 picometers per volt, and 101 picometers per volt once poled. Researchers noted that the performance of the piezo-rubber was much better than polymer piezo materials, which deflect around 25 picometers per volt.
The fact that the coefficients and power-generating ability was preserved at all is encouraging. The piezo-rubber has the potential (pun intended) to be integrated into a number of everyday products to harvest power: tires, spandex, even shoe soles, which have been a potential target of piezo power-harvesting for some time now.
While this test was a good first step toward the integration of piezo materials into something that could potentially be used in clothing or a durable, portable system, scientists have yet to put the piezo-rubber through enough tests necessary to determine how usable it really is. They'll need to know what the physical limits of the material is, for example, or how to prevent your cell phone from being blown into bits when the piezo-rubber it's connected to gets accidentally folded in half in your pocket. The researchers acknowledge there is still a lot to be done, including longevity testing, before power-generating sneakers reach the market.
ACS Nano Letters, 2010. DOI: 10.1021/nl903377u
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