ENHANCING DIELECTRIC PERMITTIVITY FOR ENERGY STORAGE DEVICES

Abstract

The demand for energy storage ceramic materials with high dielectric constant, low loss, good thermal stability, and high energy storage density are increasing daily with the rapid advance ment of power electronics towards miniaturization. In the last few decades, lead-based perov skite ceramic material occupied a large area in this field. Still, due to its toxicity and harming the environment, the interest in researching other lead-free materials has increased that can fulfil all the criteria of energy storage material. Barium titanate (BaTiO3) (BT) based material is greatly preferred among lead-free perovskite materials. BT possesses high dielectric permit tivity, strong ferroelectric property, room temperature coercive field, strong piezoelectric prop erty, and high remanent polarization. By choosing suitable dopants that can be substituted at its A and B site, the property of BT can be controlled, and its application area can be broadened. Among the BT-based lead-free material, BaZr0.1Ti0.9O3(BZT) is highly demanded due to its exceptional characteristics. The present thesis is divided into four chapters followed by a bibliography. A summary of the work presented in each of the chapters is as under : Chapter 1 contains the introduction and general background part. An overview of perovskite material has been given in this section followed by a brief description of different types of ceramic dielectric materials about ferroelectric, relaxor ferroelectric, and antiferroelectric and their corresponding dielectric properties. This part also includes a literature review on BZT and NN-based dielectric materials. Chapter 2 deals with the description of the synthesis of Ba(1-x)SrxZr0.1Ti0.9O3 + 2wt % MgO (x=0,0.02,0.04) via the solid-state reaction method. In the first section, a brief introduction to the Solid-state synthesis procedure has been discussed. MgO is used as a sintering aid for liquid phase sintering that helps form dense structures. We studied the structural, morphological, and dielectric properties of these composites. Powder X-ray diffraction pattern confirms the for mation of a tetragonal structure as a main phase of Ba(1-x)SrxZr0.1Ti0.9O3 for x= 0 and the phase of the material changes to cubic phase with the increase of Sr concentration. FESEM micro graph of these compositions reveals that the sample is adequately sintered and dense. In die lectric measurement, the r -T and tanδ -T is plotted for different composition at a different iii frequency. The result shows the variation of dielectric property with the variation of tempera ture, which is characteristic of the temperature-stable dielectric materials, observed in all com posites. For all the compositions it has been found that there is a decrease in dielectric constant with the frequency which is due to a decrease in polarisations of the atoms or molecules with the increase in frequency. The flatness of the dielectric curve confirms that this studied material is highly preferable for X8R, X8T type class II dielectric materials. The broadening of the dielectric peak is observed for all the compositions because of the diffusive phase transition. The variation of capacitance or ϵr in terms of TCC is -33% to +22% for a wide range of tem peratures from -55 °C to +150 °C. In Chapter 3 deals with the synthesisation of ceramic composite of BSZT-NN. NaNbO3(NN) is a majorly used antiferroelectric material with low dielectric constant, high breakdown strength, and high energy storage density. Due to these properties, NN(NaNbO3) has been added with BSZT(Ba0.94Sr0.96Zr0.1Ti0.9O3) to get (1-x)BSZT- xNN (x = 0, 0.02, 0.04, 0.06 and 0.08) to achieve overall high performance of the ceramic composite. XRD analysis of pure NN shows the formation of the monoclinic phase of the sample along with high porosity as revealed by FESEM images. Upon study of structural analysis, the composite revealed that there is the formation of a tetragonal phase of BSZT-NN with no additional peak or presence of a secondary phase. W-H plot reveals that there has been a formation of almost equal crys tallite size along with the decrease in the strain values. FESEM micrographs show the for mation of dense microstructure with distinguished grain boundaries in the sample. In the die lectric study, with the addition of NN in BSZT there has been a decrement in the maximum dielectric constant observed. All the compositions have been showing diffuse phase transition behaviour around curie temperature. On addition of monoclinic-phased NaNbO3 does not con tribute to the increase in the breakdown strength of the material. The highest breakdown field (Eb = 75kV/cm) is achieved for pure NaNbO3. The highest obtained recoverable energy density, Wrec = 0.17 J/cm3 has been observed for x = 0.08.