Ferroelectricity Driven Mechanical and Thermal Energy Harvesters of Polymer and 2D-van der Waals Materials

Abstract

The increasing global energy demand makes renewable energy resources a primary focus of interest. The self-powered electronic devices based on mechanical and thermal stimuli could minimize the energy demand and provide alternate solutions to use radially available mechanical vibrations and dissipated heat from our surroundings. In this scenario, ferroelectric materials are one of the ideal choices due to their electrical, thermal (pyroelectric) and mechanical (piezoelectric) stimuli-responsive properties. Traditionally, ferroelectricity is reported mainly in bulk insulating materials that limit their optical functionalities and application in thin film-based electronics. The discovery of 2D van der Waals opens up a new domain in low dimensional devices due to their synergistic electronic, optical and mechanical properties of materials. In this quest, it is expected that reducing the dimensionality of ferroelectric material could enhance the performance and thus the range of applications. This thesis investigates the dimensionality effect on mechanical and thermal stimuli driven ferroelectric active polymers and 2D van der Waals materials so as envisioned as self-powered devices. The limiting performance of bulk-3D flexible ferroelectric polymer-based devices as compared to their oxide counterpart for mechanical and thermal energy harvesting is overcome by introducing the concept of ferroelectret-based energy harvesters. In this work, a 3D printing process is used to fabricate a porous ferroelectret structure followed by high-voltage corona discharge. The charged ferroelectret exhibits ferroelectric-type hysteresis and a 40 times higher piezoelectric coefficient as compared to the film counterpart. The temperate-dependent analysis suggests a reverse polarity of the pyroelectric coefficient with a monotonically increasing trend till the melting temperature, whereas the film counterpart shows maximum pyroelectric coefficient at the Curie transition temperature (105 oC) as expected in commonly known ferroelectric material. As pyroelectricity saturates at the Curie transition temperature, the P(VDF-TrFE-CFE) terpolymer is chosen to achieve the optimum pyroelectricity at room temperature so as to target diverse biomedical applications. The molecular orientation controlled ferroelectric phase of terpolymer exhibits a maximum pyroelectric coefficient of 0.45 μC/cm2K at room temperature (30 oC), which is higher than P(VDF-TrFE) copolymer. The pyroelectric and piezoelectric functionalities of 2D van der Waals materials are aimed to investigate in different classes of 2D-ferroelectric materials such as mono-elemental (Bi), monochalcogenides (SnSe), metal dichalcogenides (MoTe2), halide chalcogenides (CrTeI) and tri-chalcogenides (TiSe2S). The free chemical bond in the out-of-plane direction in reduced v dimensions in 2D structures shows the greatest dimensionality effect with enhanced piezo- and pyro-electricity that ensures the possibility of building effective mechanical and thermal energy management systems. The effect of 1D confinement on the mechanical and thermal properties of ferroelectric materials is investigated in electron-spun nanofibers. The nanoconfinement of water-soluble ferroelectric molecular complex ([Cu(L-phe)(bpy)(H2O)]PF6⋅H2O) is carried out in aqueous processable polymer for harvesting waste mechanical and thermal energy. It is found that the 1D-confinement promotes the enhanced density of states around the Fermi-edge for boosted thermoelectric performance and compressive strain in 1D-fiber for efficient piezoelectric response. It is aimed to have higher piezoelectric performance in 1D polymer nanofiber. For instance, the electro-spun bias polarity control in P(VDF-CTFE) 1D-nanofibers provides enhanced molecular orientation perpendicular to the fiber axis that gives rise to an enhanced piezoelectric coefficient. It ensures the improved mechano-electrical sensitivity in both longitudinal (voltage (VsL) = 0.3 V/kPa, current (IsL) = 0.07 μA/kPa) as well as transverse (voltage (VsT) = 1.0 V/kPa, current (IsT) = 0.8 μA/kPa) directions. To counter the ambiguity of high transverse response as compared to longitudinal in electrospinning fiber-based devices, an approach is proposed to isolate the ferroelectret, triboelectric and piezoelectric signals in a fiber-based hybrid device with their independent charge generation mechanisms.