Top PDF Atomistic simulations of barium titanate

Atomistic simulations of barium titanate

Atomistic simulations of barium titanate

vn Contents Acknowledgements iv Abstract V 1 1 2 Introduction 1.1 Ferroelectrics and BaTiO3 1.2 Origin of FerroeleetrilQ 1.3 Charge Transfer and Electronic Polarization 1.4 Displacive, O[r]

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Point Defects in Strontium and Barium Titanate from First Principles: Properties and Thermodynamics.

Point Defects in Strontium and Barium Titanate from First Principles: Properties and Thermodynamics.

The work presented in this thesis would not have been possible without help from numerous collaborators and friends, the support of friends and family, and generous funding and computing time provided by the Air Force Office of Scientific Research. I would like to thank Professor Douglas Irving for all of his advice and guidance during my graduate career; not only have I grown as a scientist during my time here, I have also grown as a person, and I feel that I have learned many life lessons that will serve me well the rest of my career. I would like to thank my colleagues Preston Bowes, Joshua Harris, Yifeng Wu, Kelsey Mirrielees, Dan Long, Nikki Creange, Brian Behrhorst, Ben Gaddy, and Changning Niu for always being willing to discuss research issues and making themselves available to bounce ideas off of. I would like to acknowledge contributions by Preston Bowes, Joshua Harris, Brian Behrhorst, and Ben Gaddy to software that I now use almost daily. I would also like to thank people who performed experiments complementing my simulations: Dan Long, Nikki Creange, Ali Moballegh, and Biya Cai. I would like to especially thank Professors Ramo ´ n Collazo and Elizabeth Dickey for always providing guidance and support during this endeavour. Preston Bowes deserves a special additional acknowledgement; in an attempt to split the workload during this project, I ran some of the underlying DFT calculations, and Preston ran others. This project is very much a joint effort between me and Preston.
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Electromechanical Response of Polycrystalline Barium Titanate Resolved at the Grain Scale

Electromechanical Response of Polycrystalline Barium Titanate Resolved at the Grain Scale

Such correlated mesoscale phenomena have often proven difficult to study, and benefit greatly from a combination of modeling and imaging techniques. 17,22 – 24 Powder diffrac- tion studies give insight into the average mechanics, while grain-scale information revealed by imaging techniques is generally limited to two-dimensional surface measurements. Simulations and modeling offer an opportunity for in-depth investigation of length scales that are inaccessible experimen- tally. Phase field models have in particular contributed to the understanding of domain structures under an electrical field. 25 – 27 However, models require experimental input for
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Liquid-Phase Processing of Barium Titanate Thin Films.

Liquid-Phase Processing of Barium Titanate Thin Films.

Thermodynamics and phenomenological models describe many experimental results well, but are dependent on significant input from experimental work. First principle based simulations of ferroelectric materials proved difficult, in part because of the generally accepted view of polarization in materials. Polarization as a bulk material property has long been utilized to explain observed properties such as permittivity, piezoelectricity, pyroelectricity, and ferroelectricity. However, theories of polarization using classical treatments have ambiguities stemming from how we terminate our crystal and the introduction of surface charges. By taking a quantum approach to material polarization it is evident that the bulk material polarization arises from the adiabatic change of state of the material and is analogous to the differences in polarization measured in experiments.
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First principles study of intrinsic point defects in hexagonal barium titanate

First principles study of intrinsic point defects in hexagonal barium titanate

Density functional theory (DFT) calculations have been used to study the nature of intrinsic defects in the hexagonal polymorph of barium titanate. Defect formation energies are derived for multiple charge states and due consideration is given to finite-size effects (elastic and electrostatic) and the band gap error in defective cells. Correct treatment of the chemical potential of atomic oxygen means that it is possible to circumvent the usual errors associated with the inaccuracy of DFT calculations on the oxygen dimer. Results confirm that both mono- and di-vacancies exist in their nominal charge states over the majority of the band gap. Oxygen vacancies are found to dominate the system in metal-rich conditions with face sharing oxygen vacancies being preferred over corner sharing oxygen vacancies. In oxygen-rich conditions, the dominant vacancy found depends on the Fermi level. Binding energies also show the preference for metal-oxygen di-vacancy formation. Calculated equilibrium concentrations of vacancies in the system are presented for numerous temperatures. Comparisons are drawn with the cubic polymorph as well as with previous potential-based simulations and experimental results.
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Radio Frequency Applications of Barium Strontium Titanate Thin Film Tunable Capacitors

Radio Frequency Applications of Barium Strontium Titanate Thin Film Tunable Capacitors

It can be seen that the dielectric tunability decreases with increasing the RF signal amplitude. It is suggested that the tunability drop is proportional to the rms value of the RF signal amplitude. The small signal tunability curve of this BST capacitor (Figure 4.14) was fit to a 14 th order polynomial, which was then used in the nonlinear capacitor model of HP-ADS. Through a Large Signal S-Parameter simulation in HP-ADS, predicted results for the tunability at large RF signal amplitudes were obtained (Figure 4.15). The MATLAB program for curvefitting and the simulation setup can be seen in Appendix C and Appendix D, respectively. The results obtained from this simulation are in good agreement with the measured results. Almost the same tunability drops from the measurements and the simulations are observed. The small discrepancy in peak dielectric constants is due to imperfect curvefitting of the BST’s tunability to the polynomial.
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Atomistic molecular dynamics simulations of shock compressed quartz

Atomistic molecular dynamics simulations of shock compressed quartz

We have performed atomistic non-equilibrium molecular dynamics simulations of shock wave compression on quartz. We chose the widely used BKS interatomic potential. How- ever, in order to avoid complications with the unphysical be- haviour at high pressures in the pair-interaction part of the BKS potential, we created a simple (2, 6) polynomial expres- sion with an analytically determined fit to the BKS potential at the point of inflection and its derivatives. Our systems were periodic in three-dimensions but had a vacuum gap between the end of the sample atoms and the start of the flyer-plate atoms as a means of preventing unwanted interactions due to periodic boundary conditions. We used the Ewald summation correction for 3D systems as proposed by Yeh and Berkowitz as a means of allowing a geometry optimisation to be suc- cessfully performed, in order to eliminate the dipole moment caused by cleaving a slab from bulk quartz. The optimised structure was found to be that of the high-temperature poly- morph β -quartz and not α-quartz. We found a phase change from β → α quartz at 6 GPa by performing static compres- sion calculations. Our analysis of the radial distribution func- tions showed that the shock compression of quartz formed an amorphous phase.
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Analysis of boundary conditions for crystal defect atomistic simulations

Analysis of boundary conditions for crystal defect atomistic simulations

These results are generally useful for the analysis of crystalline defects, how- ever, our own primary motivation was to provide a foundation for the analysis of atomistic multi-scale simulation methods, which in this context can be thought of as different means to produce boundary conditions for an atomistic defect core simulation. To demonstrate the applicability of our framework we analyzed sim- ple variants of some of the most commonly employed schemes: Dirichlet boundary conditions, periodic boundary conditions, far-field approximation via linearised lat- tice elasticity and via nonlinear continuum elasticity (Cauchy–Born, atomistic-to- continuum coupling). In parallel works [12,23,24,36] this framework has already been exploited resulting in new and improved formulations of atomistic/continuum and quantum/atomistic coupling schemes.
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The defect chemistry of UO2±x from atomistic simulations

The defect chemistry of UO2±x from atomistic simulations

Atomic scale simulations are well suited to investigate the behaviour of point defects and their influ- ence on material properties. The description of interatomic forces in a system can be represented using either density function theory (DFT) or empirical potentials. The former has the advantage of accurately describing complex interactions, including charge transfer, from first principles, while the computational efficiency of the latter enables the dynamical simulation of systems inaccessible to electronic structure methods. Both approaches have been widely applied to defects in UO 2 ± x [27–42]. DFT has been reported
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Atomistic Simulations of Formation of Elementary Zr I Systems

Atomistic Simulations of Formation of Elementary Zr I Systems

We report results of simulations on the formation of simple zirconium iodide molecules. Previous work by Wimmer et al. [1] explored the relationship between iodine and a zirconium surface. We investigate the re- action schemes through atomistic simulations to better understand the nature of Zr-I interactions through iso- lated molecules. The computed energy values of varying Zr-I systems suggests a strong binding mechanism between zirconium and iodine, and offer predictions of likely reaction products. The computed results pre- dict condensation of volatile ZrI 4 with ZrI 2 to form Zr 2 I 6 .
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Structure Evolution and Dielectric Behavior of Polystyrene Capped Barium Titanate Nanoparticles

Structure Evolution and Dielectric Behavior of Polystyrene Capped Barium Titanate Nanoparticles

The ferroelectric phase of BaTiO3 generally has a higher dielectric constant than the cubic paraelectric phase.33 Another contributing factor is the high volume fraction and uniform disp[r]

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Dielectric and Ferroelectric Properties of Ho2O3 Doped Barium Strontium Titanate Ceramicsq

Dielectric and Ferroelectric Properties of Ho2O3 Doped Barium Strontium Titanate Ceramicsq

[2] A. Kaur, A. Singh, L. Singh, S. K. Mishra, P. D. Babu, K. Asokan, S. Kumar, C. L. Chen, K. S. Yang, D. H. Wei, Structural, magnetic and electronic properties of iron doped barium strontium titanate, RSC Adv., 6 (2016) 112363–112369, doi: 10.1039/C6RA21458D.

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The Room-Temperature Sodium-Free Organosol Precipitation of Barium Titanate Nanocrystals

The Room-Temperature Sodium-Free Organosol Precipitation of Barium Titanate Nanocrystals

Where RCOOH denotes oleic acid. Thus, a bright- orange transparent solution of barium oleate was formed, which was cooled down to room temperature. In a separate flask, 3.05mL, 10mmol, titanium isopropoxide was dissolved in toluene, 40 mL, and stirred magnetically under nitrogen flow to ensure that a homogenous solution has been formed. The prepared titanium isopropoxide/toluene solution was added to the Ba-oleate solution and mixed magnetically for two hours for complete reaction and obtaining a homogenous BTO precursor solution (2).
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Design of Frequency Agile Circuits Using Barium Strontium Titanate

Varactor

Design of Frequency Agile Circuits Using Barium Strontium Titanate Varactor

Multi-standard and multi-band radio are the next milestone in the ever developing radio industry. Frequency agile circuits are an enabling technology which allows us to achieve this goal. It is a low cost and efficient technology that provides the nec- essary flexibility required to implement multi-standard and multi-band radios. One important circuit component which comes easily to one’s mind for frequency tunabil- ity is a varactor; with so many constraints of high tuning range, fast tuning speed and low loss on varactor, new technologies have come up to full fill these require- ments. Among various varactor technologies, Barium Strontium Titanate (BST) has shown promise in the implementation of various tunable circuits like filters, voltage controlled oscillators (VCO), and tunable matching networks with good performance.
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Superelasticity of Carbon Nanocoils from Atomistic Quantum Simulations

Superelasticity of Carbon Nanocoils from Atomistic Quantum Simulations

With their unique three-dimensional (3D) helical struc- tures, the CNCs are expected to exhibit spring-like behavior in their mechanical properties. In an experiment by Chen et al. [24], multi-walled CNCs with outer tubular diameter of *126 nm have been elastically elongated to a maximum strain of *42%. A spring constant of 0.12 N/m in the low strain region was obtained. According to the structural parameters of nanocoil given by Chen et al. [24] (tubular diameter of 120 nm, coil radius of 420 nm, and pitch of 2,000 nm), Fonseca et al. [25] computed the CNC’s Young’s modulus within the framework of the Kirchhoff rod model and obtained a value of 6.88 GPa. Using finite element analysis at the continuum level, Sanada et al. also predicted a similar result (about 4.5 GPa) for carbon nanocoil with tubular radius of 240 nm, coil radius of 325 nm, and coil pitch of 1,080 nm [26]. How- ever, the experimentally measured Young’s modulus val- ues are much higher than these theoretical predictions. Volodin et al. [27] reported a Young’s modulus *0.7 TPa for CNCs with coil diameter [ 170 nm from AFM mea- surement. Using a manipulator-equipped SEM, Pan et al. determined the Young’s modulus of CNCs to be up to 0.1 TPa for coil diameter ranging from 144 to 830 nm [28]. The large discrepancy between experiment and theory has been attributed to the usage of mechanical parameters of bulk materials in the continuum mechanics simulations [25].
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Atomistic Simulations of Material Properties under Extreme Conditions

Atomistic Simulations of Material Properties under Extreme Conditions

For the analysis of numerous reactions during the ReaxFF simulation, we need a systematic criterion to identify molecular fragments. To enable the automatic and systematic analysis of chemical reactions from ReaxFF simulation trajectories, we developed a molecular fragment analysis program, BondFrag. One general criterion determining the atomic connectivity is based on the comparison of inter-atomic distances with the van der Waals (vdW) radii. However, it is inadequate to apply the vdW radii defined under ambient conditions into the highly compressed and detonative conditions. Instead, we used the bond-order values defined in ReaxFF, ranging 0 to 1 for the systems considered here, to provide a quantitative criterion for defining the presence of chemical bonds. We optimized the bond order cutoff values from simulations of several energetic materials systems. These cutoff values are tabulated at Table 3.1 for various atom pairs. Table 3.1 Bond order cutoff values for different atom pairs. BondFrag program uses these values as a default parameter set (can be adjusted by the user) to determine molecular fragments.
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Investigation on the Physical Properties of Denture Base Resin Filled with Nano-Barium Titanate

Investigation on the Physical Properties of Denture Base Resin Filled with Nano-Barium Titanate

Background: Poly(methyl methacrylate) (PMMA) is the material of choice for denture base construction because of its many good qualities. However, PMMA suffers from polymerization shrinkage and low strength, and its slow water absorption causes serious warpage and dimensional change in the material. Objective: This study aimed to evaluate the effect of nano-barium titanate (NBT) inclusion on the water absorption, polymerization shrinkage, and surface hardness of PMMA denture base. Results: The PMMA composites showed lower water absorption and polymerization shrinkage than pure PMMA. The density of the PMMA composites was increased by 5%, and a marked improvement in surface hardness was observed on the filled samples. Conclusion: NBT inclusion considerably reduced the water absorption and polymerization shrinkage, as well as improved the surface hardness and density, of PMMA. Such enhancements could promote the longevity of the composites.
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Structural and electrical properties of Barium Titanate (BaTiO3) and Neodymium doped BaTiO3 (Ba0.995Nd0.005TiO3)

Structural and electrical properties of Barium Titanate (BaTiO3) and Neodymium doped BaTiO3 (Ba0.995Nd0.005TiO3)

X-ray diffraction analysis reveals the changes in the lattice parameter and unit cell volume of the pure perovskite tetragonal structure with space group (P4mm).. Electrical analysis is[r]

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Elastic dipoles of point defects from atomistic simulations

Elastic dipoles of point defects from atomistic simulations

Point-defects in crystalline solids, such as vacancies, self-interstitial atoms, solute atoms or their small clus- ters, play a crucial role in controlling materials prop- erties and their kinetic evolutions, particularly through their interaction with other defects, like dislocations, sur- faces, interfaces, grain boundaries and also other point- defects. While ab initio calculations give an accurate description of such interactions at short range, this mod- eling approach is not tractable to characterize the long- range part because of the inherent size limitation of these simulations. For neutral defects, the long range inter- action is elastic and elasticity theory appears therefore as a natural modeling approach. A point-defect can be described within elasticity theory through an equivalent distribution of point forces. 1{3 Of particular importance
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NemoViz: a visual interactive system for atomistic simulations design

NemoViz: a visual interactive system for atomistic simulations design

Three simulation design process models exist in the lit- erature. The first documented definition of a simulation workflow model was proposed by Kruger (1970) (Krüger 1975). Kruger presented that the simulation workflow begins with the problem definition and proceeds to the stages of data collection and model building, valida- tion, data analysis and interpretation, and documentation. Allen et al. (1990) proposed a scientific simulation work- flow designed for fluid simulations (Allen and Tildesley 2017). This model highlighted the importance of con- ducting experiments in the real world and compared the results of the experiments with the simulation results using theoretical models. Most recently, Romanowska (2015) includes coding, testing, and result replication as part of the workflow (Romanowska 2015). This model classified the workflow steps into three main phases: the conceptual, technical, and dissemination phases shown in Fig. 1.
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