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Commercial carbon fibers have been coated by a brush-like layer of ZnO nanowires using a seeded chemical bath deposition. Both single carbon fibers or large tows can be functionalized in such a way. Exploiting the electrical conductivity of carbon fibers and the functional properties of ZnO nanostructures, we demonstrated that the intersection at the crossing of two functionalized carbon fibers can act not only as a piezoelectric strain sensor, but also as a chemoresistive gas sensor. The small volume of added nanostructures prevents delamination issues. The “crossing” configuration, which is typical of carbon fiber fabrics used for composites, paves the way to transform a typical texture with simple mechanical and structural properties into a “smart” material with noninvasive integrated arrays of micron-scale sensors.

A piezoelectric investigation of a nano-engineered transducer consisting of zinc oxide nanorods grown in situ on carbon fibers has been performed by means of dynamic hysteresis and capacitance measurements. The device has been stimulated using both static and dynamic stress: the occurrence of characteristic current vs voltage polarization lobes of a ferroelectric material (stressed piezoelectric) and the corresponding saturation polarization of 290 mC cm-2 (at 2.4 V mm-1 electric field) have been recorded under static stress application. Under dynamic stress conditions a 400% capacitance increase has been measured with respect to the unstressed device. It is noteworthy that these results have been achieved using the carbon fiber itself as conductive element, without the need for external wiring, providing a true integration of the piezoelectric transducer into carbon fiber based materials. The presented nano- engineered device acts as stress sensor and can actively react to the applied stress to hinder the deformation, enabling the design of smart carbon fiber based composites.

The physical and operating principle of a stress sensor, based on two crossing carbon fibers functionalized with ZnO nanorod-shaped nanostructures, was recently demonstrated. The functionalization process has been here extended to tows made of one thousand fibers, like those commonly used in industrial processing, to prove the idea that the same working principle can be exploited in the creation of smart sensing carbon fiber composites. A stress-sensing device made of two functionalized tows, fixed with epoxy resin and crossing like in a typical carbon fiber texture, was successfully tested. Piezoelectric properties of single nanorods, as well as those of the test device, were measured and discussed.