Dissertations / Theses on the topic 'Octahedral structure'

Thesis advisor: Chia-kuang Frank Tsung<br>Pt-based alloy and core-shell nanoparticles have been intensively studied to regulate its size and shape. It has known that these nanoparticles show enhanced catalytic activity in various important fields such as heterogeneous catalysis, and electrochemical energy storage including fuel cells and metal-air batteries. Here, we report a facile hydrothermal synthesis of sub-10 nm PdPt alloy and sub-20 nm Au@PdPt core-shell structures. By using a mild reducing agent in aqueous solution, metal precursors are co-reduced. Specific gases are introduced during the synthesis to optimize the reaction conditions. The PdPt alloy and Au@PdPt core-shell nanostructures were characterized and confirmed by TEM, HRTEM, EDS, ICP-OES and XRD. The resulting PdPt and Au@PdPt particles are monodispersed single crystalline and octahedral shape enclosed by (111) facets. The electrocatalytic activity for the oxidation of formic acid was tested. It was found that the catalytic activity toward the formic acid oxidation of Au@PdPt core-shell particles were much higher than those of PdPt alloy particles. In addition, Pt-rich compositions were the most active in both PdPt alloy and Au@PdPt core-shell nanoparticles. Further studies on thinner alloy-shell core-shell nanoparticles reveal that there is a volcano-curve relationship between the lattice strain strength related to alloy-shell thickness and the catalytic performance. It is proposed that there are three key parameters that can determine the catalytic activity: the alloy composition, the presence of the gold core, and the thickness of alloy-shell<br>Thesis (MS) — Boston College, 2012<br>Submitted to: Boston College. Graduate School of Arts and Sciences<br>Discipline: Chemistry

There has been significant interest in BiFeO₃ over the past decade. This interest has focused on the magnetic and electrical properties, which in the long term may prove useful in device applications. This thesis focuses on the synthesis, electrical characterisation, and structural origin of the electrical properties of rare earth doped bismuth ferrite. Two systems have been studied: BiFeO₃ doped with lanthanum and neodymium (Bi₁₋ₓREₓFeO₃ RE= La, Nd). Specific examples have been highlighted focusing on a detailed structural analysis of a lanthanum doped bismuth ferrite, Bi₀.₅La₀.₅FeO₃, and a neodymium analogue, Bi₀.₇Nd₀.₃FeO₃. Both adopt an orthorhombic GdFeO₃-type structure (space group: Pnma) with G-type antiferromagnetism. Structural variations were investigated by Rietveld refinement of temperature dependent powder neutron diffraction using a combination of both conventional “bond angle/bond length” and symmetry-mode analysis. The latter was particularly useful as it allowed the effects of A-site displacements and octahedral tilts/distortions to be considered separately. This in-depth structural analysis was complemented with ac-immittance spectroscopy using the multi-formulism approach of combined impedance and modulus data to correlate structural changes with the bulk electrical properties. This approach was essential due to the complex nature of the electrical response with contributions from different electroactive regions. The structural variations occur due to a changing balance between magnetic properties and other bonding contributions in the respective systems. This results in changes in the magnitude of the octahedral tilts, and A-site displacements giving rise to phenomena such as negative thermal expansion and invariant lattice parameters i.e., the invar effect. More specifically, analysis of Bi₀.₅La₀.₅FeO₃ highlights a structural link between changes in the relative dielectric permittivity and changes in the FeO₆ octahedral tilt magnitudes, accompanied by a structural distortion of the octahedra with corresponding A-site displacement along the c-axis; this behaviour is unusual due to an increasing in-phase tilt mode with increasing temperature. The anomalous orthorhombic distortion is driven by magnetostriction at the onset of antiferromagnetic ordering resulting in an Invar effect along the magnetic c-axis and anisotropic displacement of the A-site Bi³⁺ and La³⁺ along the a-axis. This contrasts with the neodymium analogue Bi₀.₇Nd₀.₃FeO₃ in which a combination of increasing A-site displacements in the ac-plane and decrease in both in-phase and anti-phase tilts combine with superexchange giving rise to negative thermal expansion at low temperature. The A-site displacements correlate with the orthorhombic strain. By carefully changing the synthesis conditions, a significant change in bulk conductivity was observed for a number for Bi₁₋ₓLaₓFeO₃ compositions. A series of Bi₀.₆La0.₄FeO₃ samples are discussed, where changes in the second step of the synthesis result in significantly different bulk conductivities. This behaviour is also observed in other compositions e.g. Bi₀.₇₅La₀.₂₅FeO₃. Changes in the electrical behaviour as a function of temperature are discussed in terms of phase composition and concentration gradients of defects. Activation energies associated with the conduction process(es) in Bi₁₋ₓLaₓFeO₃ samples, regardless of composition, fall within one of two broad regimes, circa. 0.5 eV or 1.0 eV, associated with polaron hopping or migration of charge via oxygen vacancies, respectively. The use of symmetry-mode analysis, in combination with conventional crystallographic analysis and electrical analysis using multi-formulism approach, presents a new paradigm for investigation of structure-property relationships in rare earth doped BiFeO₃.

Failure criteria play a vital role in the numerical analysis of reinforced concrete structures. The current failure criteria can be classified into two types, namely the empirical and theoretical failure criteria. Empirical failure criteria normally lack reasonable theoretical backgrounds, while theoretical ones either involve too many parameters or ignore the effects of intermediate principal stress on the concrete strength. Based on the octahedral shear stress model and the concrete tensile strength under the state of triaxial and uniaxial stress, a new failure criterion, that is, the simplified unified strength theory (UST), is developed by simplifiing the five-parameter UST for the analysis of reinforced concrete structures. According to the simplified UST failure criterion, the concrete strength is influenced by the maximum and intermediate principal shear stresses together with the corresponding normal stresses. Moreover, the effect of hydrostatic pressure on the concrete strength is also taken into account. The failure criterion involves three concrete strengths, namely the uniaxial tensile and compressive strengths and the equal biaxial compressive strength. In the numerical analysis, a degenerated shell element with the layered approach is adopted for the simulation of concrete structures. In the layered approach, concrete is divided into several layers over the thickness of the elements and reinforcing steel is smeared into the corresponding number of layers of equivalent thickness. In each concrete layer, three-dimensional stresses are calculated at the integration points. For the material modelling, concrete is treated as isotropic material until cracking occurs. Cracked concrete is treated as an orthotropic material incorporating tension stiffening and the reduction of cracked shear stiffness. Meanwhile, the smeared craclc model is employed. The bending reinforcements and the stirrups are simulated using a trilinear material model. To verify the correctness of the simplified UST failure criterion, comparisons are made with concrete triaxial empirical results as well as with the Kupfer and the Ottosen failure criteria. Finally, the proposed failure criterion is used for the flexural analysis of simply supported reinforced concrete beams. Also conducted are the punching shear analyses of single- and multi-column-slab connections and of half-scale flat plate models. In view of its accuracy and capabilities, the simplified UST failure criterion may be used to analyse beam- and slab-type reinforced concrete structures.

Failure criteria play a vital role in the numerical analysis of reinforced concrete structures. The current failure criteria can be classified into two types, namely the empirical and theoretical failure criteria. Empirical failure criteria normally lack reasonable theoretical backgrounds, while theoretical ones either involve too many parameters or ignore the effects of intermediate principal stress on the concrete strength. Based on the octahedral shear stress model and the concrete tensile strength under the state of triaxial and uniaxial stress, a new failure criterion, that is, the simplified unified strength theory (UST), is developed by simplifiing the five-parameter UST for the analysis of reinforced concrete structures. According to the simplified UST failure criterion, the concrete strength is influenced by the maximum and intermediate principal shear stresses together with the corresponding normal stresses. Moreover, the effect of hydrostatic pressure on the concrete strength is also taken into account. The failure criterion involves three concrete strengths, namely the uniaxial tensile and compressive strengths and the equal biaxial compressive strength. In the numerical analysis, a degenerated shell element with the layered approach is adopted for the simulation of concrete structures. In the layered approach, concrete is divided into several layers over the thickness of the elements and reinforcing steel is smeared into the corresponding number of layers of equivalent thickness. In each concrete layer, three-dimensional stresses are calculated at the integration points. For the material modelling, concrete is treated as isotropic material until cracking occurs. Cracked concrete is treated as an orthotropic material incorporating tension stiffening and the reduction of cracked shear stiffness. Meanwhile, the smeared craclc model is employed. The bending reinforcements and the stirrups are simulated using a trilinear material model. To verify the correctness of the simplified UST failure criterion, comparisons are made with concrete triaxial empirical results as well as with the Kupfer and the Ottosen failure criteria. Finally, the proposed failure criterion is used for the flexural analysis of simply supported reinforced concrete beams. Also conducted are the punching shear analyses of single- and multi-column-slab connections and of half-scale flat plate models. In view of its accuracy and capabilities, the simplified UST failure criterion may be used to analyse beam- and slab-type reinforced concrete structures.<br>Thesis (Masters)<br>Master of Philosophy (MPhil)<br>School of Engineering<br>Full Text

In a quest to design novel deployable structures, flexible polyhedra provide interesting insights. This work follows the discovery of flexible polyhedra and aims to make flexible polyhedra more useful. The dissertation describes how flexible polyhedra can be made. The flexible polyhedra first considered in this dissertation have a rotational degree of freedom. The range of this rotational movement is measured and maximised in this work by numerical maximisation. All polyhedra are established computationally: an iterative solution method is used to find vertex coordinates; several clash detecting methods are described to define whether each rotational position of a flexible polyhedron is physically possible; then a range of motion is defined between occurrences of clashes at the two ends; finally, an optimisation tool is used to maximise the range of motion. By using these tools, the range of motion of two types of simplest flexible polyhedra are maximised. The first type is a series of flexible polyhedra generalised from the Steffen flexible polyhedron. The range of motion of this type is improved to double that of Steffen’s original, from 27° to 59°. Another type of flexible polyhedron is expanded from a model provided by Tachi. Based on the understanding of Steffen’s flexible polyhedron, optimisation parameters are carefully given. This new type has achieved a wider range of motion, so now the range of motion of flexible polyhedron is tripled to 80°. After enlarging the range of motion of the degree of freedom in the 1-dof systems, the dissertation found multiple degrees of freedom in one polyhedron. The multiple mechanisms can be even repetitive, so that an n-dof polyhedron is found. A polyhedron of two degrees of freedom is first presented. Then, a unit cell for any number of mechanisms is found. As a repetitive structure, a 3-dof polyhedron is presented. Finally, this work presents the possibility of configuring a flexible polyhedral torus and a closed polyhedral surface that is able to flex without the need to stop.

碩士<br>國立中山大學<br>電機工程學系研究所<br>106<br>In this study, we investigate different nanostructures of cuprous oxide (Cu2O) on ITO/glass substrate with silver catalyst for hydrogen peroxide (H2O2) sensors. We use sol-gel method to grow Cu2O nanoparticles. Different shapes of Cu2O morphology including cubic, truncated octahedron and octahedron are discussed for their effects to H2O2 sensing ability. The octahedral Cu2O nanoparticles are respectively combined with silver catalyst by photoelectrochemical, hydrothermal, settling, and ultrasonic assistance methods. Improvements of metal catalyst to H2O2 sensors are also analyzed. According to the results, the octahedral Cu2O nanoparticles have the best sensitivity of 28.0 μAmM-1cm-2. Octahedron has more reaction sites than other shapes. Besides, the higher chemical properties and conductivity of (111) plane make the sensor has higher sensitivity. However, although the truncated octahedron has a larger (111) plane than the cubic, the dense structure let it has the lowest sensitivity. Combination of silver catalyst by photoelectrochemical method has the highest sensitivity of 41.9 μAmM-1cm-2. Due to the octahedral nanostructure and metal catalyst, the sensitivity of the H2O2 sensor has improved 49%.

High performance perovskite (ABO3) based piezoceramics are used as actuators, pressure sensors, and transducers in wide-ranging applications spanning sectors like health, space, defence and automobiles. For over four decades, Pb(ZrxTi1-x)O3, commonly known as PZT, has been the preferred choice for such applications because this system exhibits high electromechanical response due to interferroelectric instability at its morphotropic phase boundary (MPB), which separates ferroelectric tetragonal (space group P4mm) phase from a ferroelectric rhombohedral (space group R3m) phase. Increased environmental concerns and governmental directives in the past two decades have sought to replace toxic Pb-based piezoelectrics with Pb-free alternatives for the commercial use. In this context the lead-free Na0.5Bi0.5TiO3 (NBT)-based piezoelectrics made a remarkable impact as it demonstrated seemingly MPB characteristics. The parent compound NBT exhibits local in-phase octahedral tilt embedded in ferroelectric (R3c) matrix. Population of in-phase tilt increases towards the MPB of NBT-based systems (especially Na0.5Bi0.5TiO3 -K0.5Bi0.5TiO3 (NBT-KBT) and Na0.5Bi0.5TiO3 -BaTiO3 (NBT-BT)), where both KBT and BT are tetragonal (P4mm) ferroelectrics and makes the systems complicated in contrast to conventional MPB of PZT. In this thesis work, we have demonstrated enormous impact of in-phase octahedral tilt on structure and properties of NBT-based piezoceramics. We have shown first time weakening of ferroelectricity at the MPB of NBT-KBT and NBT-BT due to increasing intervention of in-phase tilt. We also have addressed some unsolved issues related to depoling process of NBT-based system. So far it was believed in perovskite based ferroelectrics that normal ferroelectric to relaxor transition occurs in gradual manner du to increase in local random fields. First time we have shown an abrupt ferroelectric to relaxor crossover within tetragonal regime of NBT-KBT system caused by onset of in-phase tilt disorder. Simultaneously large piezoelectric response (~208pC/N), large electrostrain (~0.6%) and high depolarization temperature (~180 oC) have been observed at single tetragonal composition of NBT-KBT system without any doping among NBT-based systems due to enhanced reversible domain switching, restoring force for reversibility is provided by retained in-phase tilt as disorder.

碩士<br>國立清華大學<br>化學系<br>99<br>We have prepared Pd nanocrystals with systematic shape evolution from octahedral to cubic structures through a one-pot synthesis approach. Octahedral, cuboctahedral, and cubic structures can be synthesized by using cetyltrimethylammonium chloride (CTAC) surfactant as capping agent, H2PdCl4, ascorbic acid as reducing agent and a very small amount of KBr. Fine tuning in the amount of KBr added to the growth solution enabled the fine control of nanocrystal morphology. Structural characterization confirmed that octahedra are bounded by {111} facets, whereas the cubes are bounded by {100} facets. The final shapes of Pd nanocrystals resulted from the different growth rates in the <100> direction to that of <111> direction. We have successfully used a systematic method to synthesize Pd nanocrystals with entirely {111} and {100} facets and found the relationships among these shapes. These nanocrystals should allow the examination of their various properties as a function of particles shapes and surfaces. According to our experiments regarding the study of growth mechanism of the Pd nanocrystals, we found that bromide ions play a critical role to influence the reduction kinetics of Pd precursors and relative rates of Pd addition on the surfaces of nanoparticles. With the increase of KBr added to the growth solution, the whole reaction rate was increased. Furthermore, the accelerated growth rate in the <111> direction suppressed that of <100> direction to increase the proportion of {100} facets of Pd nanocrystals. In the future, we will apply these nanoparticles with specific facets as catalysts for the facet-dependent organic reactions.

碩士<br>國立清華大學<br>化學系<br>100<br>Semiconductor nanoparticles are often synthesized in organic solvents with the use of high reaction temperatures. If nanoparticles can be synthesized in aqueous phase, it should reduce the energy cost and the process is more environmentally friendly. According to the literature, PbS nanocrystals are usually synthesized in organic solvents. When they are prepared in aqueous solutions, the reaction mixtures can frequently be heated for 12–24 hours. In this study, we have developed a seeding growth method to synthesize PbS nanocrystals in aqueous solution. The method involves addition of a small volume of preheated lead acetate and thioacetamide (TAA) mixture to an aqueous growth solution of lead acetate, thioacetamide, cetyltrimethylammonium bromide, and nitric acid. By varying the amount of thioacetamide added to the growth solution, PbS nanocrystals with different morphologies were generated in 2 h at 90 ºC. The PbS nanocrystals have sizes of 30–60 nm. Transmission electron microscopy (TEM), powder X-ray diffraction (PXRD) patterns, and scanning electron microscopy (SEM) have been employed to characterize the nanocrystals. Nanocube sizes can also be tuned within a range. UV–vis absorption spectra of PbS cubes, cuboctahedra, and octahedra all show decreasing but continuous absorption from 300 nm to beyond 1000 nm. By monitoring the speed of darkening of solution color, particle growth rate was found to be fastest for nanocubes, followed by truncated cubes, cuboctahedra, and octahedra. The production of different particle morphologies of PbS nanocrystals is linked to their reaction rates. Lowering the concentration of TAA in the reaction mixture can retard the reaction rate, and this favors the formation of octahedra. These monodisperse nanocrystals can readily form self-assembled structures. Truncated cubes and octahedra forming monolayer and multilayer packing arrangements have been studied. PXRD was used to confirm these assembled structures. Intensities of certain peaks in the PXRD patterns are enhanced due to preferred orientations of the nanocrystal packing structures.

博士<br>國立臺灣大學<br>化學研究所<br>104<br>There are two chapters in this dissertation. In Chapter 1, a modified elastic theory for $ ext{sp}^2$ pure carbon molecules has been proposed. The theory includes high order term in curvature, the bond stretching term and the tight-binding correction. The validity of this model has been examined by various graphitic-like molecules with different topology. The model can be applied on DFT (Density Functional Theory) optimized geometry or AIREBO (Adaptive Intermolecular Reactive Empirical Bond-Order) optimized geometry. Finally the most stable fullerene was found by constructing the candidates via the new model first and then verifying by VASP. In Chapter 2, a construction scheme of octahedral fullerenes has been built. After investigating the topological constraint, a fundamental polygon compatible with the octahedral symmetry was found. The fundamental polygon can be specified by four integers called index. However, the octahedral fullerene does not specified by a unique index and there is redundancy in the indexes which we called index symmetries. Besides symmetries corresponded to the geometrical symmetries of the graphene, there are symmetries originated from different dissection ways of the octahedral fullerenes. Finally all the possible orbits are clarified and an algorithm to eliminate these redundancy has been suggested.