STRUCTURAL DIELECTRIC AND ELECTRICAL PROPERTIES OF LEAD FREE CERAMICS

Abstract

Ferroelectric materials have importance in the electronic industry because of their wide applications like in sensors, transducers, ferroelectric memories, high dielectric constant capacitors and ferroelectric tunnel junctions etc. Lead (Pb)-based materials are dominating ferroelectric materials due to their excellent properties and flexibility in terms of chemical modifications. But due to increasing concern on the toxicity of lead in Pb-based materials, there is an urgent need to develop the effective lead-free alternatives. The most promising lead-free alternatives are (K,Na)NbO3, (Bi0.5Na0.5)TiO3 and BaTiO3. It is difficult to select one of them for complete substitution of Pb–based materials due to their inferior properties. (K,Na)NbO3 is a promising lead-free material due to its high Curie temperature and ferroelectric properties. The properties of a ceramic can be tailored by using dopants, making solid solution and incorporating additives. Moreover the composites of these lead-free ferroelectric materials with a ferromagnetic phase may exhibit the coupling between two order parameters: polarization and magnetization. This coupling between polarization and magnetization is known as magnetoelectric effect which is an interesting phenomenon due to the control of the magnetic (spin) state via electric field and/or the control of electric polarization via magnetic field. These materials have potentials application in multifunctional devices such as high sensitivity magnetic field sensors, spintronic devices, solid-state transformers and actuators etc. In the present thesis, lead-free (K,Na)NbO3 ceramics were synthesized at different phase boundaries. Effect of different dopants on the (K,Na)NbO3 system were studied. Further the composites of (Bi0.5Na0.5)TiO3 and (1-x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 lead-free ferroelectric phase and ferromagnetic cobalt ferrite (CoFe2O4) phase have also been synthesized and characterized. Magnetodielectric effect was studied in these composites to confirm the magnetoelectric coupling which may provide the indication for their applicability in devices. Lead-free (K,Na)NbO3 system doped with Mo6+, making solid solution with (Ba0.9Sr0.1)(Zr0.1Ti0.9)O3 and using WO3 additive were synthesized and discussed in Chapter 3. In lead-free (K0.5Na0.5)MoxNb1-xO3 ceramic Mo6+ ion has limited solubility up to 6 mol %. These compounds exhibited the negative temperature coefficient of resistance (NTCR) behaviour. Mo6+ doped (K0.5Na0.5)NbO3 ceramics show two emission bands in photoluminescence spectrum. Abstract ii The solid solution of (1-x)(K0.5Na0.5)NbO3-x(Ba0.9Sr0.1)(Zr0.1Ti0.9)O3 exhibit the presence of morphotropic phase boundary (MPB) i.e. the coexistence of orthorhombic and tetragonal phase in the composition region 0.02 ≤ x ≤ 0.04. Improved dielectric and ferroelectric properties are found for x=0.4 composition in this solid solution. In (K0.17Na0.83)NbO3, the incorporation of WO3 leads to the improvement in electrical properties at room temperature. WO3 addition promotes densification in the system and the optimum values of remnant polarization (Pr) ~24 μm/cm2, maximum dielectric constant r-max~8535 and optical band gap (Eg) ~3.50 eV have been found for 5 wt.% WO3 in (K0.17Na0.83)NbO3 . The magnetoelectric composites (1-x)(Bi0.5Na0.5)TiO3/xCoFe2O4 and xPoly(vinylidene-fluoride)-(1-x)[0.8(Bi0.5Na0.5TiO3)-0.2CoFe2O4] have been synthesized and results of the study are summarized in Chapter 4. The remnant magnetization and magnetocapacitance were found to enhance with increasing concentration of CoFe2O4 in (1-x)(Bi0.5Na0.5)TiO3/xCoFe2O4 composites whereas dielectric constant reduces upto 30 % CoFe2O4 content and further increases. Polymer composite films of xPoly(vinylidenefluoride)-( 1-x)[0.8(Bi0.5Na0.5TiO3)-0.2CoFe2O4] have been synthesized by hot polymer press and showed higher value of magnetocapacitance and reduced value of dielectric loss. The magnetoelectric interaction coefficient γ was calculated from 2  ~ M relation in these prepared composites. 0.55Ba(Zr0.2Ti0.8)O3-0.45(Ba0.7Ca0.3)TiO3 ceramic and xCoFe2O4-(1- x)[0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3] composites have been prepared and discussed in Chapter 5. The sol-gel synthesized 0.55Ba(Zr0.2Ti0.8)O3-0.45(Ba0.7Ca0.3)TiO3 ceramic showed high dielectric constant ~6985 at low dielectric loss ~0.013 at 1 kHz frequency at room temperature. The photoluminescence spectrum of this ceramic showed two emission bands. The magnetoelectric composites xCoFe2O4-(1-x)[0.5Ba(Zr0.2Ti0.8)O3- 0.5(Ba0.7Ca0.3)TiO3] were synthesized and found that these composites show soft ferromagnetic properties with increase in magnetization. The magnetodielectric effect in these composites increases with increase of CoFe2O4 content. These composites revealed relaxor behaviour and decrease in optical band gap with increasing CoFe2O4 content. Finally the conclusion of the present thesis is given in Chapter 6. The structural, electrical, magnetic and optical properties of aforementioned lead-free ceramic open the possibility of these lead–free compounds to be used in memory, energy storage and optical devices.