2, Department of Engineering, La Trobe University, Bundoora, Victoria, Australia
3, Note Issue Department, Reserve Bank of Australia, Craigieburn, Victoria, Australia
The global need for sustainable and green power generation methods has resulted in renewable energy harvesting technologies such as photovoltaics. This method has low energy conversion efficiencies <15% and only operates at maximum efficiency in direct sunlight. For the purposes of always-on, self-powered sensors and portable/wearable electronics, novel reliable energy scavenging techniques are required. Piezoelectric generators convert mechanical energy from external sources to electricity, with energy conversion efficiencies >35%.1 Furthermore, recent advances in piezoelectric fluoropolymers suggest prospects for flexible polymer-based piezoelectric generators with high visible-wavelength transparency.
Despite the promising nature of piezoelectric polymers, they are difficult to process into the polar (all-trans) β phase, with high energy methods commonly utilized as an additional step to orient dipoles after deposition as thin films.2 The β phase of fluoropolymers is desired as it exhibits the highest electromechanical coupling properties. Shear stresses have previously been found to reorient fluoropolymers into the polar β phase.3 Our computational fluid dynamics models show high shear stresses at the exit of pressure-based 3D printing nozzles, showing the potential for 3D printing techniques to induce the β phase. Increases in aspect ratio of the polymers are linked to an improved electrical output, further enhancing the potential of 3D printing as a deposition method for polymer piezoelectric generators.4 Recent literature investigating effects of ionic additives to fluoropolymers suggests increased re-orientation from non-polar to polar phase due to ion-dipole interactions, with further enhancement upon doping with low (<1 wt%) concentrations of graphitic carbon nanomaterials (such as carbon nanotubes).5-6
This study presents the prospects of 3D printing of fluoropolymers for their use as piezoelectric generators, removing the requirement of post-processing to align dipoles. The scope for the use of 3D printed piezoelectric polymer microstructures as a method of enhancing voltage output up to 4.5x over that of thin films is discussed. Diffusion kinetics of solvent evaporation-assisted 3D printing is investigated and linked to nucleation into β phase and hence additional increases in voltage of 2x at optimised conditions. Further self-orientation of 3D printed fluoropolymers at room temperature is shown through the utilization of polymer-ionic liquid additive matrices, previously only used in high temperature processing such as melt mixing and extrusion.
1. Martins, P., et al., Prog. Polym. Sci. 2014, 39 (4), 683-706.
2. Ramadan, K. S., et al., Smart Mater. Struct. 2014, 23 (3), 033001-033026.
3. Yang, J. H., et al., Org. Electron. 2016, 2867-72.
4. Chen, X. L., et al., Small 2017, 13 (23), 1604245.
5. Zhu, Y., et al., Mater. Chem. Phys. 2014, 144 (1), 194-198.
6. Xing, C., et al., J. Phys. Chem. B 2012, 116 (28), 8312-8320.