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Title: Effect of functionalization of (Ba,Sr)TiO3 on dielectric and energy storage behavior of PVDF-(Ba,Sr)TiO3 nanocomposites
Authors: Jaidka, Sachin
Supervisor: Singh, Dwijendra P.
Keywords: Polymer ceramic nanocomposites;High energy density capacitors;Surface functionalization;Saifraces/interfaces
Issue Date: 30-May-2024
Abstract: High-energy density capacitors are essential components in modern electronics due to their application in pulsed power systems such as military, aerospace and hybrid electric vehicles. The high energy density capacitors store large amounts of electrostatic energy per unit volume, which is readily available for delivering to a load in a very short span of time. There are certain ceramic materials which are found to exhibit extremely high energy density in the form of thin films, but they cannot be processed over larger areas in the capacitors constituting energy banks. The dielectric polymer-ceramic nanocomposites have the ease of processability in the form of flexible thin films, which could serve the purpose of high energy density as well as miniaturization of modern electronic devices. These electronic devices, e.g., avionics in aerospace cover 70% of its volume by capacitors. Despite of having high mechanical and breakdown strength, the dielectric polymers have a drawback of low dielectric constant. On the other hand, the ceramics have high dielectric constant but relatively very low breakdown strength. Therefore, dielectric polymer ceramic nanocomposites having a moderate dielectric constant, high breakdown strength and low tangent loss could serve the purpose of high energy density capacitor materials. A large number of polymers are used as a matrix, such as polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), etc., with particular interest surrounding PVDF due to its favorable functional attributes, high breakdown strength, flexibility, and ease of processing. On the other hand, some examples of ceramics fillers are PbZrTiO3 (PZT), PbTiO3 (PTO), BaTiO3 (BTO), BaSrTiO3 (BST), HfO2, etc., which are highly appealing for their applications in various fields as they have high dielectric constant. Among these ceramic fillers, barium strontium titanate (BST) exhibits an exceptionally high dielectric constant (e.g., 2500-3000) at room temperature and maintains a nearly constant dielectric constant over a wide temperature range (-173°C to 120°C) and frequency range (100 Hz to 1 MHz). Extensive literature has documented thorough examinations of the energy storage characteristics of polymer-ceramic nanocomposites, emphasizing on the pivotal role of breakdown strength and dielectric performance. The investigations carried out have adopted several approaches based on the engineered processing, such as the use of nanowires, nanotubes etc., functionalization, poling, incorporation of wide band gap nanofillers and engineered multilayer structure in the dielectric polymer ceramic nanocomposites. Moreover, the use of polymer-ceramic nanocomposite thick films in harsh conditions (room temperature to 100 °C) is very important to operating it for strategic purposes. Therefore, understanding the cause that deteriorates its energy storage performance at high temperatures requires deeper understanding since the understanding will lead towards the solution. The existing literature survey necessitates and demands a thorough investigation of polyvinylidene fluoride (PVDF)-(Ba,Sr)TiO3 (BST) nanocomposites towards improvement in dielectric properties, breakdown strength, discharge energy density and energy efficiency. Consequently, the present thesis is divided into seven chapters consisting of aforesaid physical parameters, which are as follows: Chapter 1 provides a concise synopsis of polyvinylidene fluoride (PVDF) polymers, ceramics, and polymer-based ceramic nanocomposites. PVDF polymers exhibit a high breakdown strength but a low dielectric constant, whereas ceramics have a high dielectric constant but a low breakdown strength. The combination of these materials presents an opportunity to create a nanocomposite with a moderate dielectric constant and high breakdown strength. Such nanocomposite materials, leveraging polymers and ceramics, are recognized as promising materials for high-energy density capacitors. The chapter emphasizes the potential dielectric ceramic fillers to be included in the polymeric matrix, including barium strontium titanate ((Ba,Sr)TiO3), BST), as they stand out for their impressive dielectric constant (~3000) across a broad frequency range (100Hz - 1MHz) and temperature range (-173 °C-120 °C). The chapter concludes with the methods and approaches adapted in the existing literature leading towards the motivation and objectives of the thesis entitled "Effect of functionalization of (Ba,Sr)TiO3 on dielectric and energy storage behavior of PVDF-(Ba,Sr)TiO3 nanocomposites". Chapter 2 is focused on the synthesis procedures and experimental methodologies utilized for the analysis of synthesized materials, namely Ba0.8Sr0.2TiO3 (BST) nanopowder and polyvinylidene fluoride-Ba0.8Sr0.2TiO3 (PVDF-BST) nanocomposite thick films. The BST nanopowder is synthesized employing the hydrothermal method, whereas the fabrication of PVDF-BST nanocomposite thick films is accomplished through the tape-casting technique. Chapter 3 is related to studies of the effect of an applied electric field on the dielectric and structural properties of polyvinylidene fluoride-Ba0.8Sr0.2TiO3 (PVDF-BST) nanocomposite thick films synthesized using a tape-casting technique. These films were exposed to an electric field for varying durations. The study reveals that the electric field induces changes in the crystalline phases of PVDF, specifically enhancing the β phase. The dielectric behavior of the nanocomposite films is significantly improved under the electric field, with the film exposed for 60 minutes showing the highest dielectric constant (~25) and low tangent loss (~0.02) at 1 kHz. The enhancement is attributed to increased dipolar density resulting from modifications in structural and interfacial behavior, as well as molecular motion of the dipoles in the PVDF chain. The findings suggest that electric field-induced modifications could serve as an effective strategy for developing flexible nanocomposite films with low ceramic filler loading for various electronic applications. Chapter 4 is associated with one of the important requirements for utilizing polyvinylidene fluoride-Ba0.8Sr0.2TiO3 (PVDF-BST) nanocomposites for high energy density capacitors operating in harsh conditions, i.e., in high-temperature electronics and electrical power systems (RT-100oC). This requirement necessitates the investigation of the temperature-dependent dielectric behavior, AC conductivity, and impedance of flexible PVDF-Ba0.8Sr0.2TiO3 nanocomposite thick films. Tape-casting is used to synthesize flexible PVDF-Ba0.8Sr0.2TiO3 nanocomposite thick films with varying BST concentrations (0.75%, 1.5%, 2.25%, and 3% by volume). An increase in the dielectric constant and a decrease in the tangent loss is found with an increase in the loading of BST nanoparticles in the nanocomposite thick films. The highest dielectric constant (~25) and the lowest tangent loss (~0.03) are observed for 3 vol% BST-loaded PVDF-BST nanocomposite at 1 kHz; the dielectric constant and the tangent loss increase to ~93 and ~1.64, respectively, at 150 °C. The dielectric constant and the tangent loss of all the PVDF-BST nanocomposites are thermally stable up to 70 °C and then increase with further increases in temperature. A phenomenological model is proposed to explain the experimentally observed behavior, which might be attributed to the thermally induced translational motion in the polymeric chains of PVDF, the motion of ions, and the migration of space charge in the interfacial layer of the PVDF matrix and BST nanoparticles. Chapter 5 focuses on the processing of polyvinylidene fluoride-Ba0.8Sr0.2TiO3 (PVDF-BST) nanocomposite films to achieve high discharge energy density, higher breakdown strength, and high efficiency. The articulated synthesis process of fabrication of PVDF-BST trilayered nanocomposites has been adopted with varying BST nanoparticle concentrations (0.75%, 1.50%, 2.25%, and 3.00% by volume). By using the tape casting technique, the upper and lower layers of the nanocomposites are cast in the same direction, while the middle layer is cast in the opposite direction. The 3.00 vol% BST-loaded nanocomposite demonstrates notable dielectric properties, including high dielectric permittivity (~25), low tangent loss (~0.03), and moderately high breakdown strength (~282 MV/m). Additionally, it exhibits a high discharge energy density (~7.8 J/cc at 1400 kV/cm) and efficiency (~93%). A mechanism is proposed that involves the interfacial dipoles and the distribution of the local electric field, contributing to the enhanced energy storage behavior. The results suggest the potential application of these trilayered nanocomposites in high-energy density capacitors for pulsed power systems. Chapter 6 is dedicated to exploring the role of surface functionalization on dielectric and energy storage behavior. For this purpose, the surface of BST nanoparticles is functionalized with -OH and 〖PO〗_4^(3-) using H2O2 and H3PO4 surface modifiers. The surface modification of BST nanoparticles is confirmed through Fourier transform infrared (FTIR) spectra and energy dispersive spectroscopy (EDS) mapping. The resulting polyvinylidene fluoride-Ba0.8Sr0.2TiO3 (PVDF-BST) nanocomposites, particularly H3PO4 modified, exhibit improved dielectric constants, with the highest value of ~30 at 1 kHz. The addition of BST nanoparticles enhances energy storage capabilities, as indicated by increased discharge energy density and efficiency. The H3PO4-modified nanocomposite film demonstrates superior performance with higher saturation polarization, lower remanent polarization, increased discharge energy density, and efficiency. The breakdown strength of nanocomposites, however, decreases compared to pure PVDF. It is reported that surface modification using H3PO4 leads to an increase in the mechanical strength of the nanocomposites. This enhancement in the mechanical strength further leads to an increment in the dielectric constant and electrical breakdown strength of the H3PO4-modified PVDF-BST nanocomposite as compared to H2O2-modified and unfunctionalized PVDF-BST nanocomposite thick films. Chapter 7 presents the summary of the findings and conclusions derived from the conducted investigations. Additionally, potential directions for future research are proposed to expand on the current insights and contribute to the ongoing scientific investigations in this field.
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