Fabrication of Zinc Oxide Nanostructure/ Poly(vinylidene fluoride) Hybrid Thin Films and Their Application for Pressure Sensors and Acoustic Actuators

Abstract

As a representative piezoelectric material, poly(vinylidene difluoride) (PVDF) and its copolymers have been widely used owing to their light weight, non-toxicity and ease of processing. In general, the piezoelectricity refers to the ability of a material to generate a piezoelectric potential in response to an external force, and thus the enhanced dielectric property like permittivity is a key factor for improving the sensitivity of PVDF-based electronic devices. For this reason, inorganic fillers with high dielectric constants such as zinc oxide (ZnO) have been dispersed into the PVDF matrix. Since the piezoelectric behavior occurred in crystals that have anisotropic unit cells or in macroscopically separated regions with different charges, the coefficient value for piezoelectric strain constant of PVDF increased via addition of the inorganic fillers, which resulted in the enhanced PVDF based device ability. This dissertation describes the three different ZnO/PVDF thin films were prepared for tactile sensor, wireless health care system and acoustic actuator by piezoelectricity and pyroelectricity enhanced hybrid film. First, an 80-μm-thick film (which is around 15% of the thickness of the human epidermis), which is a highly sensitive hybrid bifunctional gauge sensor, and was fabricated from PVDF and rod-like ZnO nanostructures with graphene electrodes. Using this film, we were able to simultaneously measure pressure and temperature in real time. The pressure was monitored from the change in the electrical resistance via the piezoresistance of the material, and the temperature was inferred based on the recovery time of the signal. Our thin film system enabled us to detect changes in pressure as small as 10 Pa that is pressure detection limit was 103-fold lower than the minimum level required for artificial skin, and to detect temperatures in the range 20–120 °C. Second, highly sensitive, wearable and wireless heart rate monitoring system was successfully fabricated based on the PVDF/ZnO nanoneedle hybrid film. The nanoneedle structure of ZnO with large aspect ratio and hexagonal vertical grown pyramid form could lower the elastic modulus of the hybrid film compared to the rod-like ZnO. Due to its high permittivity, low polarization response time and outstanding durability, the hybrid film can be applied for a real-time pressure sensor to monitor the heart rate. Notably, the lowest detectable pressure of the hybrid film was as small as 4 Pa. Furthermore, the reduced graphene oxide (rGO) electrode-based Bluetooth antenna attained high peak gain (2.70 dBi) and radiation efficiency (78.38%), which was applicable to be used as an omnidirectional antenna to transmit wireless signal to the smart phone. Interestingly, the received wireless heart beat signal within a distance of 8 m was more sensitively measured on the radial artery than carotid artery without distortion and time delay, and it had a similar oscillation in comparison with the wire pressure sensor. This approach offers a valuable and promising tool for producing the commercial and continuous wireless pressure sensor for use in routine biomedical research and critical healthcare. Lastly, a bass frequency response enhanced flexible PVDF based thin film acoustic actuator was successfully fabricated. High concentration of urchin-like ZnO was embedded in PVDF matrix, enhancing the β phase content and the dielectric property of the composite thin film. ZnO acted as a nucleation agent for the crystallization of PVDF. A chemical vapor deposition (CVD) grown graphene was used as electrodes, enabling high electron mobility for the distortion free acoustic signals. The frequency response of the fabricated acoustic actuator was studied as a function of the film thickness and filler content. The optimized film had the thickness of 80 μm with 30 wt% filler content, and showed 72% and 42 % frequency response enhancement in bass and midrange compared to the commercial PVDF, respectively. In addition, the total harmonic distortion decreased 82 % and 74 % in the bass and midrange regions, respectively. Furthermore, the composite film showed a promising potential for microphone applications. Most of all, it is demonstrated that acoustic actuator performance is strongly influenced by degree of PVDF crystalline.