SYNTHESIS AND CHARACTERIZATION OF BaTiO3-BASED MAGNETOELECTRIC COMPOSITES FOR ENERGY STORAGE APPLICATIONS

Abstract

The upsurge in energy usage in the field of electronics and communication has increased dependency on storage of energy from renewable as well as nonrenewable sources. Consequently, the major need of the world is to prepare efficient materials capable of storing and providing environment friendly sustainable and clean energy. Different research groups have put strenuous efforts for the preparation of advanced electronic materials as cheaper and alternative for energy storage elements in which batteries have up surged but the main focus is on the fabrication of dielectric materials with remarkable charging and discharging capacities and thermal stability. In context of energy storage devices, ceramics are found to exhibit outstanding electrical properties, prominent stability, and high rigidity under severe environmental conditions. The electrical energy storage application requires that these materials exhibit strong and large spontaneous polarization (Ps) as well as low remnant polarization. The dielectrics and ferroelectrics exhibit strong energy storage property and are subsequently desired in modern electrics and pulsed capacitors for power electronic system. Dielectric materials display excellent power density and strong discharge capability. In this context, oxide-based systems have inspired to show good results in these applications. In recent years, various types of lead-free ceramic materials based on titanate are extensively investigated because of their important applications in energy storage devices. The probing of multifunctional materials with energy storage property is essential for technological advancements. Among these viii materials, there exist unique magnetoelectric (ME) materials, which display controlling attribute of manipulation of ferroelectric ordering by applied magnetic field or ferromagnetic ordering by applied electric field. The first artificial ME material was an eutectic composite of BaTiO3 and CoFe2O4, which was formed by mixing the ferroelectric and ferromagnetic constituents. The implemented magnetic field and the voltage generated do not vary linearly as in the case of single-phased compounds due to the complexity of ME coupling amongst these phases. The research on ME coupling of composite materials has been thoroughly investigated. An enormous effort has been focused on materials with large ME effect in the field of physics and material science for building new types of multistate memory devices. There are two classes of ME materials: Single phase magnetoelectrics and two-phase magnetoelectrics. Single phase ME materials show the coupling in a single phase material where the coupling arises out of two or more ferroic orders. Further, two phase magnetoelectrics or composite ME exhibit large magnitudes of the ME voltage and are therefore preferred over single phase magnetoelectrics. Usually, a ME composite consists of ferroelectric and ferromagnetic for piezoelectricity and magnetostriction to exhibit multiferroism. The composite ME materials exhibit tensorial product property as a consequence of mutually connected electric and magnetic phases resulting in indirect mechanical strain transfer at the interface of two phases and enhanced ME coupling. Motivated by the above-mentioned facts, different magnetic material based - BaTiO3 composites have been synthesized by solid state reaction ix route by varying the composition to explore the magnetoelectric properties comprehensively. Dielectric, ferroelectric and energy storage properties have also been discussed in detail. Based on the extensive characterization and measured physical properties, the outcome of the research work has been organized into eight chapters and the chapter wise summary of the same is as follows: Chapter 1 begins with a brief introduction, origin of problem, motivation for the research work and an overview of the current work. This chapter includes origin of magnetoelectricity and multiferroics. Subsequently, the types of magnetoelectric materials and importance of the composite materials have been discussed in detail. The following section describes about the structure of perovskite and spinel materials. The electrical and magnetic properties of materials have been discussed briefly. A short description on importance of BaTiO3 and ferrimagnetic (CoFe2O4 and Co0.5Ni0.5Fe2O4), ferromagnetic (Bi0.85La0.15FeO3) and antiferromagnetic (NdMnO3) materials have also been discussed in this chapter. Finally, the objectives of the thesis based on the review of the literature have been incorporated. Chapter 2 describes the synthesis procedure and characterization techniques used in the current thesis. The solid state reaction method has been used to synthesize desired perovskite BaTiO3, magnetic constituents (CoFe2O4, Co0.5Ni0.5Fe2O4, Bi0.85La0.15FeO3 and NdMnO3) and their composites. The stoichiometry in these composites has been varied to enhance the obtained magnetoelectric coupling and energy storage x properties. This chapter elaborates the utility of many sophisticated experimental techniques such as x-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, dielectric LCR meter and Impedance analyzer, vibrating sample magnetometer (VSM) and P-E ferroelectric hysteresis loop tracer in order to study the various properties such as structural, morphological, dielectric, magnetic, ferroelectric, magnetoelectric and energy storage properties, respectively. Chapter 3 presents the comprehensive study on the composites of BaTiO3-CoFe2O4 (BT-CFO) for energy storage and magnetoelectric applications. This chapter focuses on the basic ferroelectric BaTiO3 (BT) and the effect of ferrite composition on magnetoelectric and energy storage properties of BT. The structural and morphological optimization of BT-CFO was systematically studied to obtain bi-phasic ferroelectric-ferrite system. The dielectric studies revealed Maxwell-Wagner polarization and thermal activated non-Debye type relaxation process in BT-CFO composites with 0.95BT - 0.05CFO composite exhibiting low dielectric loss ≈ 0.3 in frequency range of 100 Hz - 1 MHz and promised for industrial application. The maximum value of magneto-dielectric coupling achieved was 1.2 % at 7 kOe for 0.95BT - 0.05CFO composite. The impedance and conduction studies revealed high resistive nature in the composites and dominant polaron tunneling conduction mechanism. The ferroelectric P–E loop measurement confirmed the ferroelectric nature in BT-CFO composites. The maximum energy storage density and efficiency achieved for 0.95BT - 0.05CFO composite were 8.33 mJ/cm3 and 59.7 % respectively. The xi magnetoelectric coupling coefficient (α) was estimated by studying the effect of magnetic field on ferroelectric hysteresis loop measurements. The value of α was the highest for 0.95BT - 0.05CFO composite and was 13.33 mV/cm/Oe. The enhanced dielectric, ferroelectric, magnetoelectric characteristics suggest the scope of BT-CFO composites in energy storage applications. (The results of this chapter have been published in Journal of Mater Sci: Mater Electron 29 (2018) 18352–18357 (IF: 2.22) and Materials Chemistry and Physics 234 (2019) 110–121 (IF: 3.408)). Chapter 4 describes the multiferroic and magnetoelectric properties of CoFe2O4-BaTiO3 (CFO-BT) for energy storage and magnetoelectric applications. This chapter focuses on basic ferromagnetic CFO and the effect of ferroelectric BT concentration on magnetoelectric and energy storage properties of CFO. The composites of CFO-BT exhibited interplay of magnetism, ferroelectricity and display strong magnetoelectric behavior arising out of charge disordering. The structural analysis from the combination of XRD, Raman, and FT-IR measurements of CFO-BT composites established the co-existence of cubic and tetragonal phases. The dielectric measurements confirmed non-Debye type Maxwell-Wagner polarization and temperature-dependent relaxation in CFO-BT composites with 0.7CFO - 0.3BT composite showing an unexpected low dielectric loss ≈ 0.5 above 1 kHz and exhibited potential for device applications. The magnetic measurements revealed an enormous increase in the coercivity of 0.7CFO - 0.3BT composite, which was identified in terms of movement of ferromagnetic domains arising due to inclusion of trapping centers of BT in CFO. The impedance spectroscopy and conductivity measurements xii confirmed high impedance behavior and crossover from barrier hopping to polaron conduction in CFO-BT composites. The addition of BT in CFO initiated the structural modification and resulted in conductivity cross-over with improved conductivity. The ferroelectric properties displayed a low leakage charge density of 0.0031 mC/cm2 and prevalent asymmetry arising due to spatial disordering of charge distribution. The maximum energy storage density and efficiency achieved for 0.7CFO - 0.3BT composite were 3.009 mJ/cm3 and 27.3 % respectively. The highest value of α obtained was 22 mV/cm/Oe at a field of 5000 Gauss for 0.9CFO - 0.1BT composite. These results were useful for exploring energy storage devices based on magnetoelectric CFO-BT composites. (The results of this chapter have been published in Journal of Alloys and Compounds 779 (2019) 918-925 (IF: 4.65) and Journal of Electronic Materials 49 (2020) 472–484 (IF: 1.774)). Chapter 5 deals with magnetoelectric bulk composites of Co0.5Ni0.5Fe2O4-BaTiO3 (CNFO-BT). The structural studies of CNFO-BT composites confirmed lattice distortion and enlarged strain owing to increasing BT in CNFO. The dielectric and impedance measurement exhibited conventional Maxwell-Wagner polarization and confirmed the existence of grain dominated non-Debye relaxations phenomena in CNFOBT composites. The magnetic hysteresis curves revealed strong ferromagnetic behavior in all composites. The maximum energy storage density and efficiency achieved for 0.8CNFO - 0.2BT composite were 4.25 mJ/cm3 and 31.6 % respectively. The highest value of magnetoelectric coupling obtained was 5 mV/cm/Oe at a field of 4000 Oersted for 0.8CNFO - 0.2BT composite. These results revealed lattice distortion, interfacial xiii charge polarization and restricted ferromagnetic domain wall rotation arising from substitution of BT in CNFO and indicate that CNFO-BT composites have potential for energy storage applications. (The results of this chapter have been communicated to Journal of Electroceramics (IF: 2.58)). Chapter 6 focused on the comprehensive study of Bi0.85La0.15FeO3- BaTiO3 (BLFO-BT) for magnetoelectric and energy storage applications. The structural analysis revealed phase purity in BLFO and a structural transformation from rhombohedral to cubic phase with increasing content of BT confirming the co-existence of composite phase with lattice compression. The dielectric measurements displayed peak broadening in temperature–permittivity plot and confirm relaxor behavior in BLFO-BT composite ceramics. The magnetic measurements confirmed the existence of weak ferromagnetism in BLFO-BT composites and novel superparamagnetism in 0.9BLFO - 0.1BT composite ceramic. The ferroelectric hysteresis P-E loop measurements produced unsaturated ovalshaped loops with high leakage and displayed a lossy dielectric nature. The 0.9BLFO - 0.1BT composite displayed an improved recoverable energy storage density of 16 mJ/cm3 with an improved efficiency of 60 %. The highest value of magnetoelectric coupling obtained was 16 mV/cm/Oe at a field of 3000 Oersted for 0.9BLFO - 0.1BT composite. The superparamagnetic behavior and magnetic field-dependent energy storage capacity of BLFO-BT composite ceramics made them potential candidate for magnetoelectric devices. (The results of this chapter have been published in Journal of Mater Sci: Mater Electron 31 (2020) 12226–12237 (IF: 2.22)). xiv Chapter 7 is focused on the multiferroic magnetoelectric composites of NdMnO3-BaTiO3 (NMO - BT). The structural investigations revealed the evolution and co-existence of orthorhombic structure of NMO and tetragonal structure of BT in NMO-BT composites and confirm lattice stabilization in terms of symmetry. The dielectric measurements revealed step-like decrease in frequency dependent dielectric constant which confirmed improved conduction nature in NMO-BT composites. The addition of BT phase in NMO improved the remnant magnetization and saturated ferroelectric polarization owing from lattice stability establishing multiferroism in NMO-BT system. The impedance and conductivity measurements confirmed non-Debye type thermally activated conduction behavior and hopping assisted mechanism dominating in NMO-BT composites. The 0.8NMO - 0.2BT composite displayed an enhanced energy storage density of 1.544 mJ/cm3 with an improved efficiency of 50.4 %. The highest value of magnetoelectric coupling obtained was 22 mV/cm/Oe at a field of 5000 Oersted for 0.8NMO-0.2BT composite. The enhancement in energy storage efficiency of 0.8NMO-0.2BT composite and improved magnetoelectric coupling validates its potential for energy storage devices. (The results of this chapter have been communicated to Journal of Alloys and Compounds (IF: 4.65)). Chapter 8 includes summary of the research work described in the previous chapters for optimization of efficient lead free ferroelectric BaTiO3 based magnetoelectric composites for energy storage applications and outlines the future scope of this work.