Piezoelectric materials generate an electric charge when their shape is changed.

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Were he to have used the conventional generators found in regular windmills, his minimills would only achieve one percent efficiency, as opposed to the 18 percent they can reach using the piezoelectric materials.

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The devices developed by the Georgia Tech researchers rely on a different physical phenomenon - tiny polarisation changes when so-called "piezoelectric" materials such as zinc oxide are moved or placed under strain.

BBC: Sensors

The technique only works in materials that have both piezoelectric and semiconducting properties.

BBC: Sensors