Gotowa bibliografia na temat „BaTiO3 (BT)”

AbstractAiming to improve the inferior properties of polyvinyl chloride polymer, (BaTiO3)x(PVC)100-x composite samples were prepared and investigated. The structural changes of the composite (BaTiO3)x(PVC)100-x were studied as a function of BT content using FTIR, XRD and DSC measurements. Attention is paid to the tetragonality changes during composition changes.It was found that the hindrance to the PVC crystallization becomes less and less serious with the increase of BT ratio in the composite. This behavior could be attributed to the interaction between Ba+2 ions and Chlorine in the polymer. The FTIR spectra indicate a clear interaction between PVC and BaTiO3 particles as is concluded from XRD results.The Scherrer formula was used to estimate the grain size for the included BT in the (BT)x (PVC)100x composite samples. The composite samples show abnormally small tetragonality for its BT content less than unity (?/a < 1). It seems that in (BaTiO3)x(PVC)100-x composite samples, the stress that stabilized the tetragonal phase of the core regions of BT decreased, leading to lower tetragonality (c/a ratio). It seems that Tg of the composite samples increases with the increase of its BT content. Also DSC results reveal the increase of crystallization with the increase of BT content in the composite.

Flexible PVC/BT (Polyvinyl chloride/Barium Titanate) composite thick films with (0–30%) volume fractions of BaTiO3 were fabricated via the solution casting method. The effects of BaTiO3 filler on the phase, microstructure and dielectric properties of composite films were investigated. The XRD results revealed that BT particles are embedded in the PVC matrix with no chemical reaction taking place between the two phases. It was observed that the glass transition temperature of PVC had increased with the addition of BT. The frequency dispersion in the dielectric constant versus temperature curves indicated the relaxor nature of the composites. The dielectric constant (εr) measured at 40 °C, increased from 7.6 for pure PVC to 16.1 for 30% of BaTiO3 content in PVC polymer matrix. It is suggested that BaTiO3 ceramic powder enhanced the dielectric properties of PVC and may be used as a flexible dielectric material.

AbstractThe reaction mechanism of BaCO3+CaCO3+TiO2 by solid state methods has been studied in this work using thermal analysis (DSC-TG) from 500 to 1500 °C and in situ X-ray diffraction (XRD) from room temperature to 800 °C. In the mixed powders, the CaO is firstly formed followed by presence of an intermediate Ba2TiO4 phase and finally the formation of CaTiO3, BaTiO3 and/or (Ba,Ca)TiO3, where the presence of CaO or CaTiO3 (CT) has slowed down the formation of BaTiO3 (BT). Raman microscopy of a BT-CT diffusion couple has shown that Ca2+ firstly diffuses into the BT grain boundaries and then into the BT core.

In the research, the properties of potassium sodium niobate – barium titanate [(1-x)K0.02Na0.98NbO3–(x)BaTiO3: (1-x)KNN–(x)BT] ceramics prepared by molten salt method with various molecular weight of BT or x are 0 and 0.05 were investigated. The calcined powders of pure K0.02Na0.98NbO3 and (0.95) K0.02Na0.98NbO3-(0.05) BaTiO3 were pressed and sintered at 1250 – 1325 °C and 1225 – 1300 °C for 2h, respectively. It was found that, the samples showed phase structure changing from monoclinic to orthorhombic with small amount BaTiO3 addition. The densification of K0.02Na0.98NbO3 ceramics and dielectric properties were improved with the addition of BaTiO3. The (0.95)K0.02Na0.98NbO3–(0.05)BaTiO3 ceramics sintered at 1250 °C showed maximum density and dielectric constant (∼8035), which was even comparable with that of K0.02Na0.98NbO3 ceramics sintered at 1225 – 1300 °C.

Barium titanate (BaTiO3: BT) is widely used in various shapes depending on specific applications. Recently, 1-D ferroelectric ceramics are eagerly studied to innovate on new applications. Electrospinning is a versatile method to prepare 1-D nanomaterials. Here, we have prepared BaTiO3/poly (vinyl alcohol) (BT/PVA) composite fibers via the electrospinning method. Suspensions of BT/PVA aq. were prepared with weight ratio of BT:PVA:H2O = x:0.1:0.9-x (x = 0.2 and 0.3). Dried electrospun BT/PVA fibers were sintered at 900-1000°C for 2 h in air. Morphological change of the 1-D BT fibers before and after sintering was investigated by using scanning electron microscopy (SEM) observation. Distribution of the fiber diameter before and after sintering was characterized; average diameters of the 1-D BT fibers before and after sintering were 720 nm and 640 nm, respectively.

Barium titanate (BaTiO3: BT) is widely used in various shapes depending on specific applications. Recently, 1-D ferroelectric ceramics are eagerly studied to innovate on new applications. Electrospinning is a versatile method to prepare 1-D nanomaterials. Here, we have prepared BaTiO3/poly (vinyl alcohol) (BT/PVA) composite fibers via the electrospinning method. Suspensions of BT/PVA aq. were prepared with weight ratio of BT:PVA:H2O = x:0.1:0.9-x (x = 0.2 and 0.3). Dried electrospun BT/PVA fibers were sintered at 900-1000°C for 2 h in air. Morphological change of the 1-D BT fibers before and after sintering was investigated by using scanning electron microscopy (SEM) observation. Distribution of the fiber diameter before and after sintering was characterized; average diameters of the 1-D BT fibers before and after sintering were 720 nm and 640 nm, respectively.

Highly dispersed barium titanate (BaTiO3, BT) nanoparticles were prepared by the new 2-step thermal decomposition method of barium titanyl oxalate of 30 nm in size. The nanoparticles were heated at 450 °C for 5 hours in air to yield intermediate product: Ba2Ti2O5CO3. Highly dispersed BaTiO3 nanoparticles were prepared by rotationally stirring it at the reduced pressure of 0.2 Pa at various temperatures between 800 °C and 900 °C. The particle size and morphology of the BaTiO3 nanoparticles were investigated by X-ray diffraction and scanning electron microscopy. These measurements showed that the BT nanoparticles were highly dispersed and well-crystallized.

Nucleation and particle growth conditions of barium titanate (BaTiO3, BT) were investigated for preparation of the BT/strontium titanate (SrTiO3, ST) multilayered nanoparticles. The conditions with and without BT nucleation were clarified. Epitaxial growth of the BT layer on the ST substrate particles was studied using both conditions. The formation of the BT layer on the ST substrate particles was confirmed using the condition with BT nucleation.

In order to investigate the influence of (Bi0.5M0.5)TiO3 (M=Li, Na, K, Rb) addition on the Curie temperature (Tc) and positive temperature coefficient (PTC) effect of BaTiO3-based ceramics, BaTiO3-(Bi0.5M0.5)TiO3 (M=Li, Na, K, Rb) solid solutions were prepared by a conventional solid sintering reaction using high-purity reagents. It was found that the Tc of the samples would vary with (Bi0.5M0.5)TiO3 of different alkali ions, in which BT-BKT ceramics had the highest value (about 148 °C). Moreover, the incorporation of alkali ions would influence the PTC effect of the sample, which can be defined by the resistivity jump with the ratio of maximum to minimum resistivity (ρmax/ρmin). Under the present conditions, the ρmax/ρmin of BT-BRT and BT-BLT ceramics were almost equal and higher than those of BT-BNT and BT-BKT ceramics.

We report the temperature and frequency dependence impedance spectroscopy of (1 - x) ( Bi 0.5 Na 0.5) TiO 3-x BaTiO 3 (abbreviated as BNT–BT) ceramics with 0 ≤ x ≤ 0.07 prepared by conventional solid-state route. X-ray diffraction analysis indicated that a solid solution is formed when BaTiO3 diffuses into the (Bi0.5Na0.5)TiO3 lattice and a morphotropic phase boundary between rhombohedral and tetragonal locates at x = 0.07. The microstructure indicated that the grain size reduces and the shape changes from rectangular to quasi-spherical with increase in BaTiO3 content. Complex Impedance Spectroscopy analysis suggested the presence of temperature-dependent relaxation process in the materials. The modulus mechanism indicated the non-Debye type of conductivity relaxation in the materials, which is supported by impedance data. The activation energies have been calculated from impedance, electric modulus studies and dc conductivity which suggests that the conductions are ionic in nature. The activation energy increases with increase of BT content up to x = 0.05 and decreases at x = 0.07 which also indicates the presence of morphotropic phase boundary at x = 0.07.

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.

Ferroelectric ceramics are very promising materials for a variety of piezoelectric applications such as high permittivity dielectrics, piezoelectric sensors, piezoelectric/electrostrictive transducers, actuators, electro-optic devices, etc. Among the commercially viable ferroelectric ceramics, the lead-zircon ate-titivate Pb(Zr1-xTix)O3 (PZT) based ceramics have dominated the market due to their superior piezoelectric and dielectric property along with other advantages like high electromechanical coupling, ease of processing and low cost. However, the toxicity of lead based materials, and its volatility at processing temperatures is a serious health and environmental concern. Several legislations against lead-based products have been passed all over the world in order to encourage identification of alternative lead-free materials for these applications. As a consequence, there has been a remarkable surge in efforts by researchers on identifying lead-free alternatives for piezoelectric applications. A larger emphasis has been placed on perovskite based ceramics since in addition to possessing excellent properties, their relatively simple structure facilitates understanding structure-property relationships which are important for developing novel high performance materials. Na1/2Bi1/2TiO3 (NBT) and its solid solutions are one of the leading classes of perovskite ceramics, which show promising ferroelectric, piezoelectric and dielectric property thereby having the potential to replace PZT based ferroelectrics. The parent compound NBT is ferroelectric with large ferroelectric polarization (~40 C/cm2), promising piezoelectric properties with 0.08% maximum strain and longitudinal piezoelectric coefficient (d33) ~ 80 pC/N. Though NBT was discovered nearly six decades ago, a clear understanding of its structure remained elusive for a long time since different characterization techniques yielded contradicting reports on its structure and nature of phase transformation. However, rapid advances in characterization techniques in recent years have led to uncovering of new results, thereby shedding light on the true structure of NBT. X-ray and neutron diffraction studies in the past have shown that NBT exhibits rhombohedral (R3c) structure at room temperature, which undergoes a gradual transformation into tetragonal (P4bm) structure at ~230oC. However, recent characterization of both single crystal and powder of NBT using high resolution x-ray diffraction showed that the room temperature structure is not purely rhombohedral and the structure can be better modeled with a monoclinic (Cc) structure. In contrast to x-ray and neutron diffraction, electron diffraction studies have shown evidence for the presence of planar disorders, corresponding to in-phase octahedral tilts in the sample which cannot be accounted for by either R3c or Cc structure. In addition, EXAFS, x-ray and neutron total scattering studies, diffuse scattering studies, etc. have shown that the structural parameters obtained from bulk diffraction techniques are significantly different from the local structure of the material. Similar ambiguities have been observed even in NBT based solid solutions like BaTiO3, K1/2Bi1/2TiO3, etc. which show enhanced properties at the morphotropic phase boundary (MPB). A major breakthrough in understanding the structural complexity involved in NBT based solid solutions was achieved when it was found that the structure of the MPB compositions were sensitive to electric field. This led to solving the mystery behind the appearance of cubic-like phase at some of the MPB compositions where the application of electric-field resulted in the transformation of the structure into a co-existence of rhombohedral and tetragonal phases. Observation of an electric-field-induced structural transition at the MPB compositions was expected, because the MPB signifies instability in the system and even a minor external force can significantly influence the system. However, we have found that the structure of even pure NBT is significantly influenced by electric field and the framework of this thesis is based on this particularly important result. The intrinsic tendency of the electric field to affect the structure of NBT is a major factor which needs to be considered when studying similar phase transitions in the MPB compositions of NBT-substituted systems also. This was not taken into account by other research groups, and they assumed that the instability associated with the MPB was the only major factor involved in the electric-field induced transitions. A simple but highly effective strategy of grinding the electrically poled pellet into fine powder and then collecting x-ray diffraction patterns, facilitated elimination of preferred orientation in the sample. Thus, structural analysis by Rietveld refinement was possible even on the poled sample, which has not been carried out by any other groups on any ferroelectric ceramics so far. The initial part of the thesis deals with addressing the structural complexity of pure NBT, wherein the effect of electric field on the bulk structure as well as the local structure was studied in great detail. Later on, similar concepts are used to investigate BaTiO3 and K1/2Bi1/2TiO3 substituted NBT also. The first chapter of the thesis provides a brief introduction to the field of ferroelectrics, perovskite structure and their phase transition. An exposure to concepts like relaxor ferroelectrics, morphotrophic phase boundary, octahedral tilting, etc. has been provided. Then, a detailed overview of the existing literature related to the structure of NBT and its phase transition studies with temperature has been discussed. The details of the experimental procedures, characterization techniques used, and some theory behind these techniques have been provided in chapter 2. The third chapter deals with the room temperature structural characterization of pure NBT using techniques like x-ray diffraction, neutron diffraction, electron diffraction and EXAFS analysis. The results of these structural characterizations are also complemented with first-principles calculation study of the ground state structure of NBT, dielectric and ferroelectric characterization, and ageing studies. While x-ray and neutron diffraction clearly established electric-field induced structural transition from a monoclinic (Cc) to rhombohedral (R3c) structural transition, first principles calculation showed that the monoclinic phase is not stable and hence cannot be the ground state structure of NBT. Also, the possibility of the monoclinic features appearing in the x-ray diffraction solely due to micro structural effects by nano-sized domains was discussed. Comparison of electron diffraction of poled and unpoled samples of NBT showed that the in-phase tilted regions were greatly suppressed in the poled samples. Even HRTEM images showed that the unpoled samples had a very high concentration of strain heterogeneity in them, which was absent in the poled samples. This gave a direct evidence of correlation between observation of monoclinic phase and the presence of in-phase tilted regions in the unpoled samples. It was proposed that the strain caused by these in-phase tilted disorders caused distortion in the original rhombohedral lattice thereby making the structure appear monoclinic. Application of electric field causes the in-phase octahedral tilt disorders to vanish, thereby even the monoclinic features observed in the XRD also disappear. The fourth chapter discusses the consequences of poling on the high temperature phase transition behavior of NBT. We have used temperature dependent x-ray and neutron diffraction, Raman spectroscopy and EXAFS analysis whose results were correlated with the anomalies observed in temperature dependent dielectric and polarization studies. We found that the poled sample shows a sharp anomaly at the depolarization temperature (Td) in all the characterization techniques used, in contrast to a diffuse or negligible effect seen in the unpoled sample. The depolarization temperature was found to be associated with introduction of structural disorder in the sample in the form of in-phase octahedral tilts. This also gave rise to a normal to relaxor ferroelectric transition at Td, and this relaxor behavior persisted even after cooling the sample to room temperature. An intermediate cubiclike phase was observed from x-ray diffraction at around 300C wherein the rhombohedral phase vanishes and the tetragonal phase begins to appear. Even single crystal study of NBT in the past showed sudden disappearance of the domains at 300C, even though they were visible at both above and below this temperature. This effect was called isotropization, and was postulated to arise due to extremely small domains which made the system isotropic. However, our neutron diffraction pattern showed that in-phase tilted superlattice reflections persisted at this temperature which meant that the structure was not truly cubic at this temperature. Further, a neutron diffraction study at higher temperatures showed that the in-phase tilted superlattice reflections persisted even above the cubic phase transition temperature, in corroboration with similar reports from high temperature electron diffraction. Chapter five deals with the BaTiO3 substituted NBT system, which has gained tremendous interest in the last decade as a viable piezoelectric ceramic for commercial applications. Though a large number of groups have already carried out exhaustive studies on this system, most of the work concentrated mainly on the MPB compositions which showed enhanced piezoelectric properties. In this chapter, we highlight some important findings in the pre-MPB and post-MPB compositions. Using room-temperature and high temperature x-ray diffraction, we show that there exists a subtle compositional phase boundary at x = 0.03, which disappears upon poling the sample. While the monoclinic phase in pure NBT becomes cubiclike at this composition, the depolarization temperature (Td) also slightly increases up to this composition and then decreases upon further Ba substitution. Similar studies were also carried out with compositions containing slightly excess bismuth in them (0.1 mol %), whose purpose was to negate the effects of Bi-vaporization during sintering. It was found that while the compositional phase boundary remained essentially unchanged, there was a decrease in Td for all the compositions. It was also realized that the addition of excess bismuth improved the overall piezoelectric property of the system. Studies on higher compositions of Ba in the post-MPB regions further revealed two additional compositional phase boundaries. A criticality in the coercive field and spontaneous tetragonal strain was observed at x = 0.2, which was found to be associated with crossover from a long-period modulated tetragonal phase (for x < 0.2) to a no modulated tetragonal phase (for x > 0.2). Near the BT rich end (x ~ 0.7), the system exhibits a crossover from normal to a diffuse/relaxor ferroelectric transition with increasing Na1/2Bi1/2 substitution. The onset of relaxor state by Na1/2Bi1/2 substitution on the Ba-site, was shown to increase the spontaneous tetragonal strain in the system. This was because of the enhancement in the covalent character of the A-O bond by virtue of the Bi+3 6s2 lone pair effect, and it also led to a sudden increase in the tetragonal-to-cubic transition temperature. This was in contrast to other chemical modifications of BT reported in the past (like Zr, Sn, Sr, etc.) where the relaxor state is accompanied by a weakening of the ferroelectric distortion and a decrease in the critical temperature. Finally, chapter six covers the effect of electric field induced phase transition in K1/2Bi1/2TiO3 substituted NBT. Visual observation showed that while the compositions (x < 0.2) behaved similar to pure NBT, wherein the structure became purely rhombohedral upon poling, the effect of electric field was negligible in the post-MPB compositions (x > 0.2). In addition, the peaks in the annealed samples were considerably overlapping which made identifying the structural transitions at the MPB compositions difficult using Rietveld analysis. However, comparison of the FWHM of the {200}pc peaks of compositions x < 0.2 showed that the FWHM began to increase suddenly for x > 0.15 indicating emergence of the tetragonal phase. Also, all the compositions showed an increase in the {200}pc peak FWHM by 0.03 after poling. The depolarization temperature showed only slight variation in the pre-MPB compositions, but showed a minimum at the MPB compositions. Temperature dependent dielectric studies further showed that for the post-MPB compositions, the system remains relaxor even after poling.

Ferroelectric ceramics are very promising materials for a variety of piezoelectric applications such as high permittivity dielectrics, piezoelectric sensors, piezoelectric/electrostrictive transducers, actuators, electro-optic devices, etc. Among the commercially viable ferroelectric ceramics, the lead-zircon ate-titivate Pb(Zr1-xTix)O3 (PZT) based ceramics have dominated the market due to their superior piezoelectric and dielectric property along with other advantages like high electromechanical coupling, ease of processing and low cost. However, the toxicity of lead based materials, and its volatility at processing temperatures is a serious health and environmental concern. Several legislations against lead-based products have been passed all over the world in order to encourage identification of alternative lead-free materials for these applications. As a consequence, there has been a remarkable surge in efforts by researchers on identifying lead-free alternatives for piezoelectric applications. A larger emphasis has been placed on perovskite based ceramics since in addition to possessing excellent properties, their relatively simple structure facilitates understanding structure-property relationships which are important for developing novel high performance materials. Na1/2Bi1/2TiO3 (NBT) and its solid solutions are one of the leading classes of perovskite ceramics, which show promising ferroelectric, piezoelectric and dielectric property thereby having the potential to replace PZT based ferroelectrics. The parent compound NBT is ferroelectric with large ferroelectric polarization (~40 C/cm2), promising piezoelectric properties with 0.08% maximum strain and longitudinal piezoelectric coefficient (d33) ~ 80 pC/N. Though NBT was discovered nearly six decades ago, a clear understanding of its structure remained elusive for a long time since different characterization techniques yielded contradicting reports on its structure and nature of phase transformation. However, rapid advances in characterization techniques in recent years have led to uncovering of new results, thereby shedding light on the true structure of NBT. X-ray and neutron diffraction studies in the past have shown that NBT exhibits rhombohedral (R3c) structure at room temperature, which undergoes a gradual transformation into tetragonal (P4bm) structure at ~230oC. However, recent characterization of both single crystal and powder of NBT using high resolution x-ray diffraction showed that the room temperature structure is not purely rhombohedral and the structure can be better modeled with a monoclinic (Cc) structure. In contrast to x-ray and neutron diffraction, electron diffraction studies have shown evidence for the presence of planar disorders, corresponding to in-phase octahedral tilts in the sample which cannot be accounted for by either R3c or Cc structure. In addition, EXAFS, x-ray and neutron total scattering studies, diffuse scattering studies, etc. have shown that the structural parameters obtained from bulk diffraction techniques are significantly different from the local structure of the material. Similar ambiguities have been observed even in NBT based solid solutions like BaTiO3, K1/2Bi1/2TiO3, etc. which show enhanced properties at the morphotropic phase boundary (MPB). A major breakthrough in understanding the structural complexity involved in NBT based solid solutions was achieved when it was found that the structure of the MPB compositions were sensitive to electric field. This led to solving the mystery behind the appearance of cubic-like phase at some of the MPB compositions where the application of electric-field resulted in the transformation of the structure into a co-existence of rhombohedral and tetragonal phases. Observation of an electric-field-induced structural transition at the MPB compositions was expected, because the MPB signifies instability in the system and even a minor external force can significantly influence the system. However, we have found that the structure of even pure NBT is significantly influenced by electric field and the framework of this thesis is based on this particularly important result. The intrinsic tendency of the electric field to affect the structure of NBT is a major factor which needs to be considered when studying similar phase transitions in the MPB compositions of NBT-substituted systems also. This was not taken into account by other research groups, and they assumed that the instability associated with the MPB was the only major factor involved in the electric-field induced transitions. A simple but highly effective strategy of grinding the electrically poled pellet into fine powder and then collecting x-ray diffraction patterns, facilitated elimination of preferred orientation in the sample. Thus, structural analysis by Rietveld refinement was possible even on the poled sample, which has not been carried out by any other groups on any ferroelectric ceramics so far. The initial part of the thesis deals with addressing the structural complexity of pure NBT, wherein the effect of electric field on the bulk structure as well as the local structure was studied in great detail. Later on, similar concepts are used to investigate BaTiO3 and K1/2Bi1/2TiO3 substituted NBT also. The first chapter of the thesis provides a brief introduction to the field of ferroelectrics, perovskite structure and their phase transition. An exposure to concepts like relaxor ferroelectrics, morphotrophic phase boundary, octahedral tilting, etc. has been provided. Then, a detailed overview of the existing literature related to the structure of NBT and its phase transition studies with temperature has been discussed. The details of the experimental procedures, characterization techniques used, and some theory behind these techniques have been provided in chapter 2. The third chapter deals with the room temperature structural characterization of pure NBT using techniques like x-ray diffraction, neutron diffraction, electron diffraction and EXAFS analysis. The results of these structural characterizations are also complemented with first-principles calculation study of the ground state structure of NBT, dielectric and ferroelectric characterization, and ageing studies. While x-ray and neutron diffraction clearly established electric-field induced structural transition from a monoclinic (Cc) to rhombohedral (R3c) structural transition, first principles calculation showed that the monoclinic phase is not stable and hence cannot be the ground state structure of NBT. Also, the possibility of the monoclinic features appearing in the x-ray diffraction solely due to micro structural effects by nano-sized domains was discussed. Comparison of electron diffraction of poled and unpoled samples of NBT showed that the in-phase tilted regions were greatly suppressed in the poled samples. Even HRTEM images showed that the unpoled samples had a very high concentration of strain heterogeneity in them, which was absent in the poled samples. This gave a direct evidence of correlation between observation of monoclinic phase and the presence of in-phase tilted regions in the unpoled samples. It was proposed that the strain caused by these in-phase tilted disorders caused distortion in the original rhombohedral lattice thereby making the structure appear monoclinic. Application of electric field causes the in-phase octahedral tilt disorders to vanish, thereby even the monoclinic features observed in the XRD also disappear. The fourth chapter discusses the consequences of poling on the high temperature phase transition behavior of NBT. We have used temperature dependent x-ray and neutron diffraction, Raman spectroscopy and EXAFS analysis whose results were correlated with the anomalies observed in temperature dependent dielectric and polarization studies. We found that the poled sample shows a sharp anomaly at the depolarization temperature (Td) in all the characterization techniques used, in contrast to a diffuse or negligible effect seen in the unpoled sample. The depolarization temperature was found to be associated with introduction of structural disorder in the sample in the form of in-phase octahedral tilts. This also gave rise to a normal to relaxor ferroelectric transition at Td, and this relaxor behavior persisted even after cooling the sample to room temperature. An intermediate cubiclike phase was observed from x-ray diffraction at around 300C wherein the rhombohedral phase vanishes and the tetragonal phase begins to appear. Even single crystal study of NBT in the past showed sudden disappearance of the domains at 300C, even though they were visible at both above and below this temperature. This effect was called isotropization, and was postulated to arise due to extremely small domains which made the system isotropic. However, our neutron diffraction pattern showed that in-phase tilted superlattice reflections persisted at this temperature which meant that the structure was not truly cubic at this temperature. Further, a neutron diffraction study at higher temperatures showed that the in-phase tilted superlattice reflections persisted even above the cubic phase transition temperature, in corroboration with similar reports from high temperature electron diffraction. Chapter five deals with the BaTiO3 substituted NBT system, which has gained tremendous interest in the last decade as a viable piezoelectric ceramic for commercial applications. Though a large number of groups have already carried out exhaustive studies on this system, most of the work concentrated mainly on the MPB compositions which showed enhanced piezoelectric properties. In this chapter, we highlight some important findings in the pre-MPB and post-MPB compositions. Using room-temperature and high temperature x-ray diffraction, we show that there exists a subtle compositional phase boundary at x = 0.03, which disappears upon poling the sample. While the monoclinic phase in pure NBT becomes cubiclike at this composition, the depolarization temperature (Td) also slightly increases up to this composition and then decreases upon further Ba substitution. Similar studies were also carried out with compositions containing slightly excess bismuth in them (0.1 mol %), whose purpose was to negate the effects of Bi-vaporization during sintering. It was found that while the compositional phase boundary remained essentially unchanged, there was a decrease in Td for all the compositions. It was also realized that the addition of excess bismuth improved the overall piezoelectric property of the system. Studies on higher compositions of Ba in the post-MPB regions further revealed two additional compositional phase boundaries. A criticality in the coercive field and spontaneous tetragonal strain was observed at x = 0.2, which was found to be associated with crossover from a long-period modulated tetragonal phase (for x < 0.2) to a no modulated tetragonal phase (for x > 0.2). Near the BT rich end (x ~ 0.7), the system exhibits a crossover from normal to a diffuse/relaxor ferroelectric transition with increasing Na1/2Bi1/2 substitution. The onset of relaxor state by Na1/2Bi1/2 substitution on the Ba-site, was shown to increase the spontaneous tetragonal strain in the system. This was because of the enhancement in the covalent character of the A-O bond by virtue of the Bi+3 6s2 lone pair effect, and it also led to a sudden increase in the tetragonal-to-cubic transition temperature. This was in contrast to other chemical modifications of BT reported in the past (like Zr, Sn, Sr, etc.) where the relaxor state is accompanied by a weakening of the ferroelectric distortion and a decrease in the critical temperature. Finally, chapter six covers the effect of electric field induced phase transition in K1/2Bi1/2TiO3 substituted NBT. Visual observation showed that while the compositions (x < 0.2) behaved similar to pure NBT, wherein the structure became purely rhombohedral upon poling, the effect of electric field was negligible in the post-MPB compositions (x > 0.2). In addition, the peaks in the annealed samples were considerably overlapping which made identifying the structural transitions at the MPB compositions difficult using Rietveld analysis. However, comparison of the FWHM of the {200}pc peaks of compositions x < 0.2 showed that the FWHM began to increase suddenly for x > 0.15 indicating emergence of the tetragonal phase. Also, all the compositions showed an increase in the {200}pc peak FWHM by 0.03 after poling. The depolarization temperature showed only slight variation in the pre-MPB compositions, but showed a minimum at the MPB compositions. Temperature dependent dielectric studies further showed that for the post-MPB compositions, the system remains relaxor even after poling.

With climbing worldwide oil prices and global warming, energy crisis poses a threat to living standards, economy, and even global environment. Nanodielectrics become one of the new materials activated to play a unique role in sustainable and clean energy production, energy transportation, energy storage, and end usage. Based on our recent research on frequency-dependent dielectric properties of BaTiO3 (BT) single domain, BT/parylene nanodielectric composites have been examined in wide-ranging frequency at room temperature by several theoretical models. The projected models combined with Debye type of dissipation and soft mode theory to obtain more precise frequency dependent dielectric spectrum. Among the others, Wiener mixture rule, Lichtnecker model, Rayleigh model, Yamada rule, Maxwell-Wagner model, modified Kerner model were used to find frequency dependent nanocomposites dielectric spectrum. The predicted results are compared with our experimental results and explored the frequency dependent dielectric behavior of nanodielectric composites. The dielectric constant decreases while dielectric loss increases with increasing frequency due to at very high frequency, only electronic polarization can occur. This investigation provides the fundamental knowledge on dielectric properties of nanocomposites with a wide frequency range instead of trial-and-error strategy of experiments for the future development of energy storage devices.

When coherent optical beams cross in a photorefractive crystal such as BaTiO3, SBN, there is an automatic 90° phase shift between beams A and B at the place of maximum refractive index of the photorefractive grating, because there is a 90° phase shift between the grating and the interference pattern of two beams A, B (A, B also indicate the amplitude of beams A, B). Because of the energy conservation law there is a 90° phase difference between A t and A d , Bt and B d (beams A and B will be divided into transmitted beam A t , B t and diffractive beam A d , B d , respectively).