High-Performance Flexible Piezoelectric Nanogenerator Based on Electrospun PVDF-BaTiO3 Nanofibers for Self-Powered Vibration Sensing Applications

In the present era of intelligent electronics and Internet of Things (IoT), the demand for flexible and wearable devices is very high. Here, we have developed a high-output flexible piezoelectric nanogenerator (PENG) based on electrospun poly(vinylidene fluoride) (PVDF)-barium titanate (BaTiO₃) (ES PVDF-BT) composite nanofibers with an enhanced electroactive phase. On addition of 10 wt % BaTiO₃ nanoparticles, the electroactive β-phase of the PVDF is found to be escalated to ∼91% as a result of the synergistic interfacial interaction between the tetragonal BaTiO₃ nanoparticles and the ferroelectric host polymer matrix on electrospinning. The fabricated PENG device delivered an open-circuit voltage of ∼50 V and short-circuit current density of ∼0.312 mA m^-2. Also, the PVDF-BT nanofiber-based PENG device showed an output power density of ∼4.07 mW m^-2, which is 10 times higher than that of a pristine PVDF nanofiber-based PENG device. Furthermore, the developed PENG has been newly demonstrated for self-powered real-time vibration sensing applications such as for mapping of mechanical vibrations from faulty CPU fans, hard disk drives, and electric sewing machines.

Compressibility and crystalline structures of PVDF membranes under elevated gravity acceleration by two-axis spin coating technology

Polymers play an important role in designing and manufacturing lithium-ion batteries and sensors. This study investigates the compressibility and crystalline structure of polyvinylidene fluoride (PVDF) membranes spin-coated under different gravity conditions. Particle Flow Code (PFC) numerically models the compressibility of membranes. The models show a 10% reduction in the thickness of the membrane when the gravity is artificially elevated to 500g. The time of solvents' release from the free surface of membranes based on Stokes' law is simulated by MATLAB. The results show that the solvents' release time decreases significantly when the artificial gravity increases. Novel experiments are conducted to validate the results of numerical modeling and MATLAB simulations. Four PVDF membranes are spin-coated by a novel two-axis spin coater under 1, 100, 300, and 500g. The SEM images experimentally report a 21% reduction in the thickness of the membrane spin-coated under 500g. The weights of membranes are measured to verify the results of MATLAB simulations. The results show that 99% of the whole solvents are evaporated while increasing the gravity to noticeable values. XRD, FTIR, and SEM characterize the polymeric crystalline structure of membranes. The crystalline structure of spin-coated PVDF membranes varies under various gravity conditions. The XRD measurements report phase transitions, while the gravity is artificially elevated during membranes' fabrication. FTIR spectroscopy revealed that the observed phase transition is from γ toward β while increasing the gravity acceleration.

Nonbonding interaction analyses on PVDF/[BMIM][BF4] complex system in gas and solution phase

The present study provides a detailed quantum chemical description of the physicochemical interactions between poly-vinylidene fluoride (PVDF) and 1-butyl-3-methyl-imidazolium tetrafluoro borate ([BMIM][BF₄]) ionic liquid (IL). Geometry optimization and frequency calculations are carried out for four monomer units of α- and β-PVDF, [BMIM][BF₄], and PVDF/[BMIM][BF₄] using dispersion corrected density functional theory. The effects of solvation on the systems under study are demonstrated for three polar aprotic solvents, namely tetra-hydrofuran (THF), acetone, and n,n-dimethyl formamide (DMF) using the integral equation formalism polarizable continuum model (IEFPCM). Calculated negative solvation free energy values suggest solution phase stability of the systems under study. Binding and interaction energies for β-PVDF/IL are found higher in magnitude than those for α-PVDF/IL. The nonbonding interaction phenomenon of β-PVDF/[BMIM][BF₄] is elucidated on the basis of natural bond orbital (NBO), Bader's quantum theory of atoms in molecules (QTAIM), delocalization indices, Hirshfeld surface, and reduced density gradient (RDG) analyses. Both anions and cations of ionic liquids are found to show weak van der Waals interaction with PVDF molecule but the anion ([BF₄]^-)/PVDF interaction is found to be stronger than cation ([BMIM]⁺)/PVDF interaction. Inter-unit C-H⋯F type hydrogen bonds are found to show improper (causing blue shifts in vibrational frequencies) nature. Frontier molecular orbital analysis is carried out, and different chemical parameters like electronegativity, chemical potential, chemical hardness and softness, and electrophilicity index are calculated using Koopmans' theorem. Thermochemical calculations are also performed, and the variation in different standard thermodynamic parameters with temperature is formulated. Graphical abstract (a) Hirshfeld surface mapped onto electron density and (b) NCI isosurfaces showing inter-unit interactions of β-PVDF/[BMIM][BF₄].

Ice-templated poly(vinylidene fluoride) ferroelectrets

Ferroelectrets are piezoelectrically-active polymer foams that can convert externally applied loads into electric charge for sensor or energy harvesting applications. Existing processing routes used to create pores of the desired geometry and degree of alignment appropriate for ferroelectrets are based on complex mechanical stretching and chemical dissolution steps. In this work, we present the first demonstration of the use of freeze casting as a cost effective and environmentally friendly approach to produce polymeric ferroelectrets. The pore morphology, phase analysis, relative permittivity and direct piezoelectric charge coefficient (d33) of porous poly(vinylidene fluoride) (PVDF) based ferroelectrets with porosity volume fractions ranging from 24% to 78% were analysed. The long-range alignment of pore channels produced during directional freezing is shown to be beneficial in forming a highly polarised structure and high d33 ∼ 264 pC N-1 after breakdown of air within the pore channels during corona poling. This new approach opens a way to create tailored pore structures and voids in ferroelectret materials for transducer applications related to sensors and vibration energy harvesting.

A critical analysis of the α, β and γ phases in poly(vinylidene fluoride) using FTIR

A universal but simple procedure for identifying the α, β and γ phases in PVDF using FTIR is proposed and validated. An integrated quantification methodology for individual β and γ phase in mixed systems is also proposed.

Enhanced ferroelectric properties of electrospun poly(vinylidene fluoride) nanofibers by adjusting processing parameters

This contribution investigates the ferroelectric properties and polymorphism evolution of poly(vinylidene fluoride) (PVDF) mats prepared through electrospinning solutions of PVDF in pure N,N-dimethylformamide (DMF) and DMF/acetone mixtures with different weight ratios.