Structural, Vibrational, Dielectric, Ferroelectric and Electrical Properties of (1-x) Na0.5Bi0.5TiO3–x BaTiO3 Solid Solutions and The Effect of Ion Irradiations on Functional Properties of Ferroelectric Ceramic-Polymer Composites

Abstract

Dielectric oxides are an important class of materials, which are widely used in modern electronic and optoelectronic device applications. Ferroelectrics are the non-linear dielectrics, which possess piezoelectric, pyroelectric, and ferroelectric properties. Ferroelectric ceramic oxides are extensively utilized in various devices such as piezoelectric sensors, actuators, IR detectors, capacitors, energy storage, energy harvesting, and memory devices due to their outstanding physical properties. Among the different structural families, ferroelectric oxides belonging to the perovskite structure are widely used due to the possibility of tuning the physical properties as per the requirement of device applications. In order to further enhance the electromechanical, dielectric and ferroelectric properties, the fabrication of solid solutions with different types of perovskites are one of the suitable approaches. Around the morphotropic phase boundary (MPB) compositions of the ferroelectric solid solutions, anomalous enhancement of dielectric permittivity, polarization, electromechanical and piezoelectric properties are observed. In view of the processing and environmental issues pertaining to leadbased ferroelectric materials, investigations on lead-free ferroelectrics are carried out intensively in recent years. The 1st part of this work is mainly focused on the synthesis and characterization of high quality lead free ferroelectric ceramic oxides having general formula: (1-x) Na0.5Bi0.5TiO3–x BaTiO3 (NBT-BT) solid solutions (x = 0.00, 0.02, 0.04, 0.05, 0.06, 0.07, 0.08 and 0.10). Among the available lead free ferroelectric ceramics, the A-site distorted perovskite (Na0.5Bi0.5)TiO3 (NBT) system has drawn immense attention due to their excellent dielectric, and ferroelectric properties. However, it has some limitations such as (i) high coercive field, (ii) high conductivity, (iii) high dielectric loss and (iv) high leakage current, which is against the use of this system in various device applications. In order to overcome these limitations, fabrication of solid solutions of NBT with BaTiO3 (BT) system has been studied. The NBT-BT ceramics are prepared by sol–gel auto combustion method followed by the sintering using microwave sintering technique. Structural, vibrational, dielectric, ferroelectric, and electrical properties of NBT-BT solid-solution are investigated using a wide variety of experimental techniques. The formation of single phase material with perovskite structure is confirmed from the X ray diffraction (XRD) patterns. A compositional driven structural phase transition from R3c (x = 0.0 to 0.05) to P4mm (x = 0.08 to 0.10) through an intermediate co-existence of R3c + P4mm (x = 0.06 and 0.07) is observed from X-ray Rietveld refinement and Raman spectroscopic studies. Existence of MPB composition has been observed in (1-x) Na0.5Bi0.5TiO3–x BaTiO3 solid solutions at x = 0.06. The same observation is also clearly seen in Raman spectroscopic studies. The scanning electron micrographs confirmed the presence of grains and grain boundaries with dense microstructure. It has been found that the grain size decreases with increasing of BaTiO3 (BT) concentration. The ferroelectric property has been studied by measuring P-E hysteresis loop after electrical poling and observed enhanced and welldeveloped ferroelectric loops after poling the ceramic samples. The highest polarization (2Pr O is observed for x = 0.06 sample. This enhancement of the ferroelectric properties could be resulted due to the presence of the MPB, i.e. the presence of both the rhombohedral and tetragonal phases. The temperature variation of dielectric properties shows two types of phase transitions such as (i) Relaxor ferroelectric to ferroelectric (TFR) and (ii) ferroelectric to paraelectric (FE-PE) phase transition (Tm or TC), for all compositions. It has been observed that the value of Tm is decreased with the increasing x, whereas there is a decrease in value of TFR with increase in composition up to x = 0.06 and thereafter it increases again. On the other hand, the value of dielectric permittivity at Tm (εrmax) increases with an increase in the composition up to x = 0.06 but with further increasing x, it decreases. The observed maximum value of dielectric permittivity at Tm and a minimum value of TFR for x = 0.06 may be due to the existence of MPB. Complex impedance, complex electrical modulus formalism, and frequency dependent ac conductivity analysis have also been carried out to study the relaxation and conduction mechanism. The presence of grain- and grain boundary contribution to impedance spectra in NBT–BT ceramics are analyzed using complex impedance plot (Nyquist plot) in association with complex modulus plot. The experimental data of these materials are fitted using suitable equivalent circuit to explain the electrical response of the materials. The frequency dependent of ac conductivity of these materials fits well with the double power law. The demand for miniaturized, flexible and light weight devices, leads to the development of flexible dielectric materials. There are two types of dielectric materials namely ceramics and polymers are widely used for storing the capacitive energy in capacitor. In view of this, ferroelectric polymer ceramic composites are one of the important R & D activities in the field of materials science. The polymer matrix in the polymer composites has the functionalities such as flexibility, easy processing, low cost and exhibit high breakdown strength. However, polymers are the materials having low dielectric permittivity. On the other hand, ferroelectric ceramic oxides have high dielectric permittivity but low breakdown strength. Therefore, the fabrication of ceramic-polymer composites can be a suitable solution for the problems associated with the ceramics and polymers, when considered separately for the energy storage. Solution-casting technique is used to prepare the free standing and flexible ferroelectric ceramic- polymer composite having general formula PVDF (Polyvinylidene fluoride) + ϕ wt.% of 0.94(Na0.5Bi0.5TiO3)-0.06BaTiO3 (BNBT) (ϕ = 0, 5, 10, 15, 20, 25, 30, 35, 40 and 50) with 0-3 connectivity. This MPB composition BNBT has been chosen as filler as it possesses high dielectric permittivity and maximum polarization in the entire BNBT series. The semicrystalline nature and formation of composite due to the addition of BNBT filler to PVDF is confirmed from XRD analysis. The surface morphology of the prepared samples is studied using Field Emission Scanning Electron Microscope (FE-SEM), which shows the presence of spherulite and homogeneous distribution of ceramic filler particles in PVDF confirming the semicrystalline nature of the samples. In the polymeric chain of PVDF, systematic packing of parallel dipoles of fluorine atoms on one side yields higher electronegativity and hydrogen atoms on the other side (less electronegativity as compared to fluorine) with carbon as a backbone results in polar β-phase. FTIR and XRD results suggest that the fraction of the electro active β-phase increases with increase in filler concentrations and peaked for 35 wt.% of the ceramic filler. The increase in the fraction of β-phase has been explained based on ion (negatively charged surface ion of the ferroelectric ceramic filler)-dipole (-CH2 dipole of the polymer matrix) interactions, as evidenced from FTIR spectra. It has been observed that dielectric permittivity keep on increasing with addition of ceramic filler up to 35 wt.%. However, above 35 wt.% a decrease in the dielectric permittivity value has been observed for all the frequencies. Swift Heavy Ion (SHI) irradiation is one of the most effective, powerful and emerging techniques for tailoring the physico-chemical properties of the material suitable for a particular application. The effect of Swift Heavy Li3+ ion beam (50 MeV) irradiation with different fluence (ranging from 1×1011 to 3.3×1013 ions/cm2) on the structural, morphological, vibrational, dielectric and ferroelectric properties of PVDF and PVDF + 35 wt.% BNBT (PVDF-BNBT) composite are studied. XRD patterns show an increase of β-phase and degree of crystallinity upon irradiation for the respective films. The scanning electron microscopic study showed a systematic increase in the spherulites size with irradiation. Dielectric permittivity and ferroelectric polarization of PVDF and PVDF-BNBT composite is increased with increase of fluence and the highest value is observed for the highest fluence. So the interaction of Li3+ ions with polymer composite leading to the enhancement β-phase, which plays the decisive role for the enhancement of the functional properties such as dielectric and ferroelectric properties