Composite flexible films prepared by hot pressing for low-energy harvesting and storage

Abstract

The important task of scientific community nowadays is finding the way to use enormous amount of mechanical energy released everywhere around us as a renewable and safe energy source. This research was focused on the preparation of flexible composite films, by combining a highly flexible polyvinylidene fluoride (PVDF) polymer matrix with lead-free piezoelectric perovskites, 0.94[(Bi0.5Na0.5)TiO3]-0.06BaTiO3 (NBT-BT), in different ratios, using the hot pressing method. A crucial point of this investigation is to show that this material is quite versatile and possesses functional properties which are sensitive to both microscopic and chemical modifications. Detailed investigation of processed flexible films led to the main conclusion that electrical properties of these composites can be affected by different factors. Firstly, the hotpressing method itself induces the formation of electroactive β-phase of PVDF polymer, the NBT-BT as filler with negatively charged surface enables a predominant formation of desirable piezoelectric PVDF phase as well and additionally, there is an influence of concentration and type (pre-preparation method) of the added filler. Dielectric permittivity values of composites were up to 110 and highly depend on the filler amount. A very useful zone around room temperature as a plateau with relatively constant dielectric permittivity and losses was noticed in each film`s dielectric spectra. Anelastic measurements have shown a complete agreement with dielectric properties in which the temperature dependence of the Young’s modulus and the losses are dominated by those of the polymer. Regarding the resulting dielectric and ferroeletric properties of the flexible composites, the potential of these materials for the energy storage application was investigated. Energy density efficiencies obtained for investigated materials have shown a decreasing trend with increasing amount of filler with values of 66-74 %. Assembled energy harvesting units were made by proper wiring and covering the flexible film with Kapton protection layer. The obtained output voltage while applying the impact force was from 1 V to 7 V, depending on the type of the flexible film. The main conclusion derived from this study is that composite flexible films made of lead-free NBT-BT filler and PVDF, have high potential to be used for environmentally safe lowenergy storage and energy harvesting devices.