FABRICATION AND CHARACTERIZATION OF FLEXIBLE PIEZOELECTRIC GENERATOR COMPOSED OF SODIUM POTASSIUM NIOBATE (KNN) BASED CERAMICS AND POLYMERS FOR ENERGY HARVESTING APPLICATIONS

Abstract

In the recent era, the urgent need for sustainable and renewable energy solutions in day- to-day life has gradually increased due to the global energy crisis, driven by increasing demand and the depletion of conventional fossil fuel reserves. In this context, piezoelectric energy harvesting technology has gained significant attention due to its ability to convert abundantly available mechanical energy into useful electrical energy. The present thesis work reports the development of composite-type flexible piezoelectric generators (PEGs) which can potentially provide power to the self- powered and wearable devices by harvesting the abundantly available mechanical energy into useful electrical energy. For the construction of the PEG device, pure and Lithium (Li), Tantalum (Ta), and Antimony (Sb) modified Potassium Sodium Niobate (KNaNbO3) (KNN) have been used as filler materials in the poly (vinylidene fluoride) (PVDF) matrix. Pure KNN has been synthesized by two methods: solid state reaction method and hydrothermal method. On the other hand, Li, Ta, and Sb modified KNN was synthesized by solid state method followed by high energy ball milling. The phase and structure identification of the prepared filler materials and flexible composite films were done by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). The dielectric and ferroelectric properties of the fabricated flexible composite films were also studied. The filler content in the PVDF matrix has been systematically varied to analyze its effect on the piezoelectric output performance of the constructed PEG device. The obtained results indicated the dependency of the β phase of PVDF, dielectric, piezoelectric and ferroelectric properties of composite films on the filler content in the PVDF matrix. The generated output voltage and current of the fabricated PEG devices were also recorded. The generated instantaneous power by the constructed PEG devices has also been measured by connecting the devices with different resistors showing gradual rise in voltage accompanied by the steadily decreasing current as a function of load resistance. Furthermore, to increase the content of the β phase of PVDF and enhance the energy harvesting performance of PVDF based PEGs, some conducting fillers: ZnO nanorods viii synthesized by Co-precipitation method and Multiwall Carbon Nanotube (MWCNT) purchased from Sigma Aldrich were dispersed in the PVDF matrix along with the ceramic particles. The addition of some conducting fillers in the polymer matrix can enhance the nucleation and improve the formation of β phase of PVDF and also helps in transfer of the generated charges. It is the combined effect of the filler materials and polymer in the PVDF-based composites which enhance the piezoelectric performance of the devices for wider applications. To further evaluate the performance of the constructed PEG devices in harvesting mechanical energy from human body motions such as finger tapping, wrist movement, elbow bending etc. the device was attached to the human body and generated voltage was recorded. The obtained result indicated that the device is very sensitive to the applied force and can harvest energy from the human body movements. The electrical energy generated by the fabricated PEG devices by harvesting external mechanical vibration have been used to light up light emitting diodes (LEDs) and powering small electronic devices. The present thesis work demonstrated that high-performance flexible PEG device with simple structure and high sensitivity can be developed by using modified KNN ceramics along with some conducting fillers in the PVDF matrix that can be used in self-powered and wearable devices. Chapter 1 of the thesis begins by systematically addressing the energy crisis and the growing need for energy harvesting, highlighting the existing shortfalls in meeting these demands. It then reviews various energy harvesting techniques, emphasizing particularly on piezoelectric energy harvesting as a promising method, which converts mechanical energy into electrical energy by using piezoelectric generators (PEGs). Owing to their versatility and effectiveness, PEGs have emerged as a focal point of research, since they can harvest energy from human body motions and mechanical vibrations by using piezoelectric effect while also serving as self-powered devices in a wide range of applications. Furthermore, various types of piezoelectric materials, including inorganic, organic, and composite-based materials have been discussed which can be employed for the construction of PEGs. A review of existing literature on piezoelectric generators is conducted to identify research gaps and to establish the primary objectives of the present thesis work. ix Chapter 2 elaborates the experimental methods utilized for the synthesis of filler particles in the PVDF matrix and the fabrication of composite films. For the synthesis of the filler materials, solid state method, hydrothermal method and co-precipitation method were used and are discussed in detail in the chapter. Further, for the fabrication of the flexible composite films, drop casting method was employed. In addition, the working principle of various experimental techniques and equipment that were used for the characterization of the prepared samples have also been provided in this chapter to get a comprehensive understanding. To study the phase formation and morphology of the prepared powders and flexible composite films, X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Field Emission Scanning Electron Microscopy (FESEM) and Fourier Transform Infrared Spectroscopy (FTIR) were used. Furthermore, dielectric, ferroelectric and piezoelectric properties of the fabricated composite films were also measured using Impedance analyzer, Polarisation vs. Electric field (P-E) loop tracer and Strain measurement system (Butterfly loop), respectively. With the aid of a vibrator shaker and finger tapping, force was applied on the surface of the constructed PEG devices and generated open circuit voltage and short circuit current were measured by using digital storage oscilloscope and an electrometer, respectively. Furthermore, load resistances were also attached to the PEG devices to measure the output power of the manufactured PEG device. Chapter 3 is divided into two sections. The first section focuses on the fabrication and characterization of piezoelectric generator (PEG) based on flexible composite films composed of Potassium Sodium Niobate (KNN) and Poly (vinylidene fluoride) (PVDF), with varying concentrations of KNN ceramic particles (0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, and 20 wt.%) in the polymer matrix. In the first section, KNN was synthesized using the solid-state reaction method and used for the construction of PEG. The second section explores the effect of KNN particle size synthesized by hydrothermal method as filler in the PVDF matrix on the piezoelectric output performance of the constructed piezoelectric generator device. The obtained result demonstrates a considerable enhancement in the piezoelectric output performance of a device constructed with KNN synthesized by hydrothermal method over the PEG device constructed using KNN synthesized by solid-state method. x Chapter 4 presents the development of piezoelectric energy harvesters based on PVDF/KNN/ZnO nanocomposite films. Initially, KNN nanoparticles were synthesized using the hydrothermal method, as described in Chapter 3. Zinc oxide (ZnO) nanorods were prepared through co-precipitation method and subsequently integrated into the PVDF polymer matrix along with KNN nanoparticles to enhance the overall piezoelectric performance of the fabricated piezoelectric nanogenerator (PENG). ZnO is well-known for its semiconducting properties and remarkable piezoelectric characteristics, making it an ideal candidate for boosting the energy harvesting efficiency of the constructed PENG device. The synergistic effect of KNN and ZnO within the PVDF matrix is expected to result in superior piezoelectric output, contributing to the development of efficient energy harvesting PENG devices. Chapter 5 presents the fabrication of a flexible piezoelectric generator (PEG) based on a composite film of (Li, Ta, Sb)-modified (K, Na)NbO₃ (KNNLTS) and poly(vinylidene fluoride) (PVDF). KNNLTS particles were synthesized using the solid- state method and subsequently incorporated into the PVDF polymer to develop PVDF/KNNLTS-based piezoelectric energy harvesters. The integration of KNNLTS into the PVDF matrix significantly enhanced the composite's ferroelectric, piezoelectric, and dielectric properties, leading to improved energy harvesting performance. Chapter 6 focuses on the development of PVDF/KNNLTS/MWCNT-based piezoelectric energy harvester devices. KNNLTS nanoparticles were synthesized using a high-energy ball milling process, while multi-walled carbon nanotubes (MWCNTs) were procured from Sigma-Aldrich. Both KNNLTS nanoparticles and MWCNTs were incorporated into the PVDF polymer matrix to enhance overall piezoelectric performance of the PENG device. MWCNTs, known for their excellent electrical conductivity, also promote the formation of the electroactive β-phase in PVDF by acting as an effective nucleating agent, thereby enhancing the piezoelectric response of the composite. The combined incorporation of KNNLTS ceramic nanoparticles and MWCNTs into the PVDF matrix significantly improves the overall piezoelectric performance of the piezoelectric nanogenerator (PENG) device, making it a promising candidate for energy harvesting applications. xi Chapter 7 provides a conclusion of all findings of the present thesis work. The findings of this research lay the groundwork for further advancements in the field of flexible energy harvesters. Additionally, it outlines the future scope, potential social impact, and the challenges associated with the research. References also form part along with bibliography at the end of each chapter.