Particulate Polymer Foam Composites: For Piezoelectric Sensing Applications

摘要

In recent years the number of applications relying on piezoelectric sensors has increased rapidly. New applications lead to new requirements concerning material properties and functionalities. In many of the (future) applications, such as artificial robotic skin, structural health monitoring and flexible keyboards, the necessity for flexibility and conformability of the sensor material is evident. The sensor material should be able to nicely follow the surface of complex shapes and sensing efficiency should be enhanced further. Therefore, the main goal of this project was to reduce the dielectric constant and improve flexibility and conformability of traditional di-phase flexible piezocomposites consisting of PZT particles in a dense polymeric matrix, by adding a third (gaseous) phase to the system. The presence of the gaseous component in the polymer matrix lowers its dielectric constant and increases the piezoelectric voltage constant, g33, which is the scalar measure for the touch sensitivity of the system. The focus of the optimisation in this project is on the polymer matrix phase and the (micro)structure of the particulate polymer foam composite. Lead Zirconium Titanate (PZT) with composition Pb(Zr0.52 Ti0.4)O3 was selected as the piezoceramic filler phase. Elastomeric Polyurethane (PU) was found to be the most optimal polymer matrix system, showing excellent flexibility, conformability, viscosity behaviour upon curing and processability. Another advantage of the selected PU system is the large operating temperature range, which was found to be between 0 ?C and 160 ?C. Furthermore, component A (diisocyanates) of the PU system is able to react with water, resulting in the release of CO2. Thereby the gaseous volume fraction of the foam could be fine-tuned and increased significantly. For open-mould production processes (0-3 random composites), adding 0.4 vol% demineralised water (microliter-range) to the system resulted in gaseous volume fractions of >55%, whereas closed-mould production processes (1-3 quasi-structured composites) required only 0.2 vol% demineralised water to obtain similar porosity. This indicates that approximately 50% of the total amount of CO2 formed, escapes the system in an open-mould approach. The reduction in dielectric constant was found to be proportional to the increase in gaseous volume fraction. The PZT-PU foam composites showed remarkably high g33 values up to maximum values of 95 mV.m/N for unstructured 40%PZT-PU foam and 170 mV.m/N for quasi-structured 10%PZT-PU foam, primarily resulted from the significant reduction of the dielectric constant. Note that the volume fraction of piezoceramic loading has to be corrected for the increase in total volume due to the addition of the gaseous phase. The best performing composite (1-3 10%PZT-PU) has a corrected volumetric composition of: 4.3%PZT - 39.1%PU - 56.6%air. The great performance of the new concept opens up routes for further improvement of porous piezoelectric composites and we are now one step closer to the realisation of the material that meets the (future) requirements for flexible piezoelectric sensors.