Growth of ZnO nanorods on carbon fibers for in-situ stress measurements

Carbon fiber based composites (CFC), owing to the exceptional ratio between mechanical properties and weight, represent a fundamental technology for several applications, ranging from aerospace to automotive, from civil infrastructure to biomedical engineering. Due to the intense mechanical stress often involved, strain and deformation on CFC made structures have to be constantly monitored. Furthermore univocal mechanical models doesn't exist, since carbon fiber (CF) patches can be textured with CF oriented in different directions depending on necessity. Nowadays strain sensing on CFC is carried out with optical fiber and piezoelectric ceramics. However this technologies present three main drawbacks, that are large size (compared to CF size), weight addition to the structure and use of precious metals wires. Recently the use of different materials functionalized with piezoelectric zinc oxide (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 in-situ growth is made by a low-temperature and low-cost two-step process: functionalization of CF with ZnO seed-layer by Successive Ionic Layer Adsorption and Reaction (SILAR) technique and 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 such structure using ferroelectric virtual ground, PUND and DHM techniques. The emergence of a voltage-invariant capacitance variation upon stress application is ascribed as the fingerprint of piezoelectricity.