Electroactive properties and piezo-tribo hybrid energy harvesting performances of PVDF-AlFeO composites: role of crystal symmetry and agglomeration of fillers.

Inorganic filler-loaded PVDF-based composites have been very widely used for electrical and energy harvesting applications in recent times. In this regard, the effects of different parameters of fillers like size, shape, chemical states, distribution, functional properties, and many others on the output performance of PVDF have been widely studied. However, the effect of another important parameter, namely the crystal symmetry of the filler, in tuning the energy harvesting performance of PVDF has been rarely explored. Therefore, to explore this fact, here we develop PVDF-based composite films by using two types of AlFeO fillers, one with rhombohedral 3̄ symmetry (AFRH) and another with an orthorhombic 2 structure. Ferrite-based oxides have been chosen here as fillers due to their good dielectric compatibility with PVDF. On the other hand, AlFeO has been chosen due to the simplicity of synthesizing it with both centrosymmetric and non-centrosymmetric crystal structures and the scarcity of reports exploring the energy-harvesting performance of AlFeO-based polymer composites. A significant difference in particle agglomeration has also been observed here between the mentioned two types of AlFeO fillers which was mainly due to their specific synthesis conditions. The electroactive properties of PVDF have been observed to be mostly dependent on filler agglomeration. However, the crystal symmetry has shown a strong effect on the piezoelectric energy harvesting performances. As a result of these facts, the piezo-tribo hybrid energy harvesting performance, which depends on both the dielectric permittivity and piezoelectric activity, has been observed to be better for the AFRH5-based hybrid device (AFRH5H) (with ∼72 V open circuit voltage and ∼45 μW cm power density) compared to that of the AFOR5-based hybrid device (AFOR5H). The real-life applications of all the energy harvesting devices have also been demonstrated here.