Carbon fibers functionalized with piezoelectric ZnO nanorod for mechanical stress sensing

Carbon fiber composites (CFC) represent a fundamental technology for mechanical applications. Due to the intense mechanical stress involved, deformation on CFC structures have to be constantly monitored. Nowadays strain sensing on CFC is carried out through optical fiber and piezoelectric ceramics. However these technologies present three main drawbacks: large size (compared to CF), weight addition and use of precious metal wires. Recently the use of piezoelectric ZnO nanostructures started to get a foothold for sensing and energy harvesting. Piezoelectric effect, owing to its dual peculiarity relating deformation with electric properties, lends itself to both sensing and actuating applications. Therefore functionalization of CF with ZnO piezoelectric nanostructures allow to realize a fully integrated piezoelectric sensor/actuator within CFC structure, thanks also to the fact that conductive CF themselves act as electrical wires. The ZnO growth is made by a low-temperature and low-cost two-step process: 1.growth of ZnO seed-layer by SILAR technique; 2.growth of ZnO nanorods by chemical bath deposition. Measuring the piezoelectric effect in ZnO nanostructures is still a debatable topic, since the typical I-V characterization is not generally accepted. In this work piezoelectric investigation is carried out for the first time on this type of structure using DHM technique and capacitance measurements. When a stress is applied, a strong capacitance increase occurs. Under specific frequency conditions, this is ascribed as the fingerprint of piezoelectricity. During this work, in collaboration with the CFC manufacturer Bercella s.r.l. in Varano de' Melegari (Parma), an international patent was registered and published.