Introduction
Piezoelectric transducers such as piezo-ceramic lead zirconate titanate (PZT) and poly(vinylidene fluoride) (PVDF) piezo-polymers have been widely adopted for sensing [
1], actuation [
2], control [
3], and energy harvesting [
4]. Unique to piezoelectric materials is their inherent ability to simultaneously serve as sensors and actuators. For instance, one can take advantage of the direct piezoelectric effect for dynamic strain sensing and energy harvesting where an electrical potential drop develops in response to an applied strain. The converse is also true, and the piezoelectric transducer can be used as an actuator as it strains due to an applied electric field (i.e., due to the indirect piezoelectric effect) [
3,
5]. More importantly, piezoelectric materials are advantageous in that they do not rely on external power sources (e.g., batteries or alternating current (AC) power) for continued operations, unlike strain gages [
6], fiber optics [
7], wireless sensor nodes [
8], micro-electromechanical systems (MEMS) devices [
9], and other types of sensing systems.
Unfortunately, PZT and PVDF suffer from fundamental limitations intrinsic to their material. Although piezo-ceramic PZT transducers possess high piezoelectricity and
d
33 piezoelectric constants approximately 200–400 pC N
−1 [
10], they are extremely brittle, have high loss factors, and are characterized by highly hysteretic behavior [
11]. On the other hand, piezoelectric polymers such as PVDF and PVDF-copolymers are flexible, conformable, and can be fabricated to different sizes and thicknesses [
12]. However, they possess considerably lower piezoelectric constants as compared to PZTs (~10 pC N
−1) and require intricate mechanical stretching to enhance their bulk film piezoelectricity [
5,
13]. Furthermore, both PVDF films and PZTs require high-voltage poling so as to enhance their piezoelectricity. Thus, in order to use piezoelectric transducers for sensing applications in complex laboratory and field environments, it is desirable for them to simultaneously possess high piezoelectricity and excellent mechanical properties.
On the other hand, the nanotechnology domain offers a diverse suite of new materials and composite fabrication methodologies for high-performance piezoelectrics [
14‐
16]. Among the plethora of nanomaterials, zinc oxide (ZnO) nanostructures (e.g., nanowires, nanosprings, and nanoparticles, among others) can be readily synthesized and exhibit inherent piezoelectricity [
17,
18]. For example, Wang and Song [
17] has synthesized aligned ZnO nanowire arrays and has characterized their electrical energy generation via deformation induced by an atomic force microscope tip. Scrymgeour et al
. [
19] has employed a solution technique for growing ZnO nanorods on silver films, and piezoelectric force microscopy studies estimate that the
d
33 constants of individual nanorods are approximately 4.41 ± 1.73 pm V
−1. On the other hand, Lin et al
. [
20] has fabricated a PZT/ZnO nanowhisker nanocomposite; when compared to monolithic PZT, although the piezoelectric constant of the nanocomposite decreased slightly, the nanocomposite’s flexural strength and fracture toughness increased by 40 and 30%, respectively. In general, the aforementioned studies have confirmed the piezoelectric performance of ZnO nanostructures, and it is possible to fabricate piezoelectric ZnO-based structures without the need for high-voltage poling. On the other hand, ZnO nanomaterials have also found diverse applications in polymer solar cells [
21] or other nanocomposite systems [
22].
The objective of this research is to characterize the mechanical and piezoelectric performance of ZnO nanoparticle-based thin films for dynamic strain monitoring. First, ZnO nanoparticles are dispersed in polyelectrolyte solutions and evaporated in Petri dish molds for film fabrication. A total of seven unique sets of nanocomposites are fabricated, where each set is characterized by a unique nanoparticle weight fraction; the various films are used as is (i.e., without further post-thin film fabrication treatment or high-voltage poling) for the remainder of this study. Second, the as-fabricated films are imaged using scanning electron microscopy (SEM) to evaluate their micro- and nano-scale physical morphology. Then, monotonic uniaxial tensile testing is employed for investigating thin film mechanical properties. Finally, to evaluate the nanocomposite’s piezoelectric performance, the films are also affixed onto cantilevered beam specimens, and their generated voltages are sampled during free vibration response of the beam. The normalized generated voltages are compared to surface strain measurements and PVDF generated voltages, and the relationship between ZnO weight fraction and dynamic strain sensitivity is discussed.
Conclusions
In summary, the objective of this study is to investigate the mechanical and piezoelectric performance of (ZnO–PSS/PVA)wf nanocomposites for dynamic strain sensing. A total of seven unique sets of thin films with different ZnO nanoparticle weight fractions have been fabricated via an evaporation technique. First, specimens from each sample set are subjected to monotonic uniaxial tensile testing to investigate their stress–strain response. The stress–strain diagrams suggest that the films are characterized by an initial linear region, and some films of low ZnO weight fractions can exhibit slight yielding followed by fracture failure. It has also been shown that even though the bulk nanocomposite stiffness (or modulus of elasticity) increases with increasing ZnO weight fractions, there is a tradeoff since the material becomes more brittle (i.e., decreased ultimate failure strain). On the other hand, when compared to PVDF, ZnO nanocomposites are characterized by lower modulus of elasticity and ultimate strength regardless of ZnO weight fraction.
The second major portion of this study has focused on characterizing the piezoelectric performance of the proposed ZnO-based nanocomposites. It has been confirmed through free vibration cantilevered beam tests that the (ZnO–PSS/PVA)wf films are in fact piezoelectric and generate voltage in response to dynamic strain of the substrate. More importantly, one can begin to tune the piezoelectricity of the film by controlling the film’s ZnO weight fraction. The experimental results suggest that increasing ZnO weight fraction leads to a near-linear increase in their dynamic strain sensitivity and piezoelectricity. The piezoelectric and dynamic strain sensing performance of the ZnO-based films are also compared to that of commercial PVDF thin films. It has been found that 50 and 60% films exhibit comparable dynamic strain sensitivity and piezoelectricity while not requiring high-voltage poling and mechanical stretching as do PVDF-based films.
While this preliminary study has only focused on exploring the relationship between ZnO weight fraction and bulk film mechanical performance and piezoelectricity, other factors may also be used and explored for improving their performance. For instance, it is hypothesized that high-voltage poling applied to (ZnO–PSS/PVA)
wf thin films can further improve their piezoelectricity, and further research is currently underway to explore this idea. On the other hand, future testing will also investigate the effects of film thickness. Other studies related to carbon nanotube-based thin films have shown that increasing nanocomposite thickness can decrease strain sensitivity while improving mechanical performance [
30]. It is possible that the mechanical properties of ZnO-based film will improve with increasing film thickness since the nanocomposites will become less sensitive to thin film inhomogeneity due to nanoparticle agglomerations and stress concentrations.
Acknowledgements
The authors would like to express their gratitude to the College of Engineering, University of California, Davis for the support of this research. The authors would also like to thank Dr. Frank Yaghmaie, Ms. Andrea Gusman, Ms. Yingjun Zhao, and the Northern California Nanotechnology Center (NC2) for assistance with obtaining the SEM images.