Zinc oxide nanoparticle-polymeric thin films for dynamic strain sensing

Piezoelectric transducers are becoming increasingly popular for dynamic strain monitoring due to their small form factors and their ability to generate an electrical voltage drop in response to strain. Although numerous types of piezoelectric thin films have been adopted for strain sensing, it has been shown that piezo-ceramics are expensive, brittle, and can fail during operation, while piezo-polymers possess lower piezoelectricity and mechanical stiffness. Thus, the objective of this study is to develop a piezoelectric thin film characterized by high piezoelectricity (i.e., high dynamic strain sensitivities) and favorable mechanical properties (i.e., being conformable to structural surfaces yet stiff). First, zinc oxide (ZnO) nanoparticles are dispersed in polyelectrolyte solutions, and the excess solvent is evaporated for thin film fabrication. The amount of ZnO nanoparticles embedded within the films is varied to yield seven unique sample sets with ZnO weight fractions ranging from 0 to 60%. Upon film fabrication, specimens are mounted in a load frame for monotonic uniaxial testing to explore the films’ stress–strain performance and to subsequently determine their mechanical properties (namely, modulus of elasticity, ultimate strength, and ultimate failure strain). Finally, film specimens are also mounted onto cantilevered beams undergoing free vibration due to an applied initial displacement. The generated voltages in response to induced strains in the beams are recorded, and the piezoelectric performance and dynamic strain sensitivities for the different weight fraction films are calculated and compared. Commercial PVDF thin films are also employed in this study for performance comparison.