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Phase field modeling of ferroelectrics with point defects

Kowalewsky, Olga (2005) Theory of complex lattice quasicontinuum and its application to ferroelectrics

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Courses of Study | IIT Gandhinagar

Introduction to traditional and advanced materials, their properties and applications, Origin of these properties, Electrical materials: metals, semiconductors, dielectrics, ferroelectrics, multiferroics, piezoelectrics, Optical materials: reflectors, absorbers, transparent materials, Optoelectronic materials, Magnetic materials: diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, ferrimagnetic, soft and hard magnetic materials, spintronics, superconductors, Biocompatible materials, Nanomaterials, Nuclear materials, Case studies: Advanced energy materials, healthcare materials, electronic materials, etc.

DATE: Thursday, October 18, 2012 12:00H LOCATION: C1002, Building C1, Campus Nord UPC

(2015) Synthesis and Characterization of Intergrowth Bismuth Layered Structure Ferroelectrics in the System SrO-Bi2O3-TiO2. PhD thesis.

Electrical Engineering < University of Dayton

Thesis on ferroelectrics How To Write A Science Paper Essay Writing Quotes Top 10 Essay Writing Services

Intergrowth bismuth layered structure ferroelectrics (BLSFs) in the system SrO-Bi2O3-TiO2 have been investigated with an objective to improve the dielectric and ferroelectric properties of them. There are three BLSFs in this system namely Bi4Ti3O12 (BIT), SrBi4Ti4O15 (SBTi4) and Sr2Bi4Ti5O18 (SBTi5) and one stable intergrowth BLSF compound between BIT-SBTi4. All these BLSFs compounds have been synthesized through a modified oxalate route. Their phase formation behavior during synthesis has been investigated in details using TG/DSC and XRD analysis. The crystal structures of all the compounds were refined through Rietveld
analysis to find accurate lattice parameters. Densification characteristics, microstructure
development, dielectric and ferroelectric properties of all compounds were analysed and
correlated with each other. The major findings were:
i. BIT-SBTi4 intergrowth compound showed an enhanced 2Pr than their individual constituent BIT and SBTi4. ii. Only a short range intergrowth structure between SBTi4-SBTi5 could be possible, which showed a broaden permittivity temperature characteristics.
iii. The remnant polarization of BIT-SBTi4intergrowth has been enhanced with Nb doping and doping helps in reducing oxygen vacancy in the ceramics.
iv. The ferroelectric properties of Nb modified BIT-SBTi4 intergrowth can be further enhanced by the La substitution for Bi in the structure, however with a disadvantage of decreased Tc.
v. Keeping the Tc unchanged, the ferroelectric property of the intergrowth can be improved by CuO addition, which helps in better densification of the ceramics.

Complex lattice Quasicontinuum theory is developed and applied to the description of ferroelectric phenomena. Quasicontinuum theory is a multiscale theory that provides a unified description of materials by combining atomistic and continuum approaches. It provides a seamless transition between atomistics and continuum, but the description of the material is derived directly from the underlying atomic structure, using the computationally expensive atomistics only where needed, at the location of phenomena of atomistic origin.

Complex Lattice Quasicontinuum theory can be applied to complex lattice crystals consisting of many kinds of atoms. One highlight of it is treatment of each component lattice as separately and independently as possible. The component Quasicontinua are coupled through the microscopic forces within nodal clusters, making the complex atomistics of the heterogeneous lattice the basis of the description.

Ferroelectrics are especially suited to the application of Quasicontinuum theory. The nature of defects in ferroelectric materials is atomistic, but their influence over the material is long ranged due to induced elastic fields. Many different ferroelectric phenomena involving the perovskite ferroelectrics Barium Titanate and Lead Titanate are investigated and simulated. For Barium Titanate: the 180 degree domain wall structure and quasistatic crack under load. For Lead Titanate: the 180 degree domain wall structure and a domain wall step.

The results for the domain walls show that the domain wall thickness is atomistically small, of the order of few lattice constants, which is in agreement with recent ab initio molecular dynamics simulations, but we also observe long range effects resulting from the presence of the wall. During crack loading in the sample of Barium Titanate we observe polarization changes around the crack tip which are consistent with experimental observations of an increase of fracture toughness. The quasicontinuum study of a domain wall step gives an atomistical view into the equilibrium structure of the step.

Quasicontinuum is able to model these phenomena with atomistic precision around the defects and non-homogeneities, and also capture the influence of long-ranging effects in the samples. These studies could also give valuable modeling input for larger scale continuum approaches.

Optical & Electronic Materials

Parida, Geetanjali (2015) Synthesis and Characterization of Intergrowth Bismuth Layered Structure Ferroelectrics in the System SrO-Bi2O3-TiO2. PhD thesis.

First, we introduce the concept of hybrid improper ferroelectricity and discuss some intriguing domain features in the non-magnetic HIF, Sr-doped Ca3Ti2O7.

(2005) Theory of complex lattice quasicontinuum and its application to ferroelectrics. Dissertation (Ph.D.), California Institute of Technology.

Theoretical investigations of complex oxides: Understanding bulk ferroelectrics and oxide -metal interfaces. Valentino R Cooper, University of Pennsylvania
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    by: Amir Abdollahi Hosnijeh

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This thesis is an experimental and theoretical investigation of the photorefractive properties of barium titanate and tungsten bronze ferroelectrics, and applications thereof. In the first part, the physics of the photorefractive effect is presented. The band transport equations are solved for three cases that describe the photorefractive mechanism in a crystal with one photorefractive species and either one or two types of charge carriers, or in a crystal with two photorefractive species, but only one type of charge carrier.

In the second part, the coupled wave theory of two-wave mixing in photorefractive crystals is reviewed. Effects of energy coupling between the two interacting beams are discussed along with experimental methods for determining the two-beam coupling coefficient and the photorefractive response time.

The photorefractive crystals barium titanate, strontium barium niobate, and barium strontium potassium sodium niobate are described in the third part. A summary of their optical, physical and electrical properties is presented for use in subsequent sections of the thesis.

In the fourth part, the photorefractive properties of these crystals are presented. Data from two-beam coupling experiments are used to obtain the two-beam coupling coefficients and the photorefractive response times of the crystals under a variety of experimental conditions. The band transport models are then applied to these results.

Figures of merit are defined in the fifth part that are then used in a comparison of several photorefractive materials, including the barium titanate, the strontium barium niobate, and the barium strontium potassium sodium niobate crystals. Both ferroelectric and non-ferroelectric materials are considered.

In the last part, applications using barium titanate and the tungsten bronze ferroelectrics are described. By using the self-focusing properties of barium titanate and strontium barium niobate, the compensation of nonlinear optical distortions with phase conjugation is demonstrated. A passive phase conjugator that relies on no external mirrors and uses strontium barium niobate as the nonlinear medium is also described. Finally, a thresholding passive phase conjugate mirror is then presented along with several devices that can perform mathematical operations on images such as subtraction and differentiation.

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