Fabrication and characterisation of integrated ZnO nanostructures-polymer piezoelectric devices
Jeronimo Martinez, Karina
Flexible piezoelectric pressure sensors have been developed and characterised using different ZnO nanostructures and different fabrication methods. Through the development process, this work explores the integration of ZnO nanostructures with different polymers including Polydimethylsiloxane (PDMS) and Thermoplastic Polyurethane (TPU) in films and fibre mats offering excellent properties like flexibility, durability, compatibility, and conformability enabling a wide variety of applications. One of the approaches consisted of fabricating randomly dispersed PDMS-ZnO piezocomposites using ZnO nanoparticles and in-house synthesised ZnO nanoflowers while the other approach consisted of vertically grown ZnO nanowires arrays from flexible substrates. The design of the devices was similar to a nanogenerator based on a sandwich-like structure where the ZnO nanostructures were embedded into a dielectric polymer matrix between a top and a bottom electrode. The characterisation of the materials involved was carried out using several techniques in order to understand and overcome limitations related to the combination of inorganic and organic flexible materials into the conventional microfabrication processes. ZnO nanowires (NWs), nanosheets (NSs), and ZnO nanoflowers (NFs) were synthesized using a hydrothermal method. The size, shape and geometry of the ZnO nanomaterials were demonstrated to influence the piezoelectric response when embedded into a polymer matrix. When vertically grown, the selection of the substrate combined with the reaction conditions determines the alignment and aspect ratio of the NWs. The sensors made of piezocomposites at 10% w/w produced a voltage of 1.86V for NPs and 4.2V for NFs with a very good sensitivity of 21.6mV/kPa and 59.3mV/kPa respectively. On the other hand, the TPU-based sensors with vertically grown ZnO NWs generated an output voltage of 596mV with a sensitivity of 0.38mV/kPa. To explore the capability of the TPU-based sensor to be potentially used for structural health monitoring (SHM) on aircraft, preliminary tests were carried out by embedding the TPU-based sensor into a composite material. The NWs integrity was found to be intact after the embedding process as well as the Low-velocity impact tests using loads similar to in-service conditions. A finite element model (FEM) simulation was performed using CoventorWare10 in order to calculate the piezopotential of the ZnO devices based on the experimental data obtained from the materials involved. The effect of the substrate type, the length of NWs and polymer dielectric matrix on the output generated voltage was simulated. Finally, electrospinning was introduced as an alternative way to produce aligned fibre mats for flexible substrates. The effect of the process parameters in the final morphology of the fibre mats was discussed as well as their mechanical and thermal properties. The growth of ZnO nanowires was successfully conducted on the fibre mats giving an interesting brush-like microstructure in which the NWs were grown radially from the fibres. The outcomes of this research can be valuable in the design and fabrication of piezoelectric nano/microsensors and nanogenerators based on ZnO nanostructures in areas such as smart skin sensors, implantable medical devices, human vital sign monitoring and structural health monitoring (SHM).