Browsing by Subject "Crystal growth"

Now showing 1 - 7 of 7
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    Characterizing The Mechanism Of Nanocrystalline Anatase To Rutile Phase Transformation
    (2014-10) Sabyrov, Kairat
    Phase transformations are important processes by which materials evolve in both natural and synthetic materials. Understanding the nature, mechanisms, and kinetics of phase transformations, as well as the micro structural changes that accompany them, require comprehensive characterization so as to gain a deeper understanding of atomic scale mechanisms and better control materials properties. A combination of experimental and theoretical techniques has led to improved understanding of how phase transformations are initiated at interfaces and then propagate by growth of the more stable phase at the expense of meta-stable phase. In this work, nanocrystalline TiO2 is used as a model system to systematically explore the atomic level mechanism of particle mediated phase transformation. First, a number of important research studies are highlighted to improve our understanding of this aggregation based phase transformation. Second, the dependence of anatase to rutile phase transformation and crystal growth kinetics on crystallite size and aggregation state of the particles in aqueous suspension is explored by varying reaction conditions. Rates of anatase growth and its transformation to rutile increase with decreasing initial grain size under hydrothermal conditions. Overall, rates are slower at the higher pHs employed. Furthermore, densely aggregated particles show higher transformation and growth rates, compared to loosely aggregated ones. Third, macroscopic modeling was used to characterize the kinetics of anatase to rutile phase transformation. Kinetic data under acidic, hydrothermal conditions are consistent with a two-step phase transformation mechanism: interface-nucleation followed by dissolution-precipitation. Fourth, a kinetic model that enables quantitative assessment of the contribution to the rate of phase transformation by dissolution-precipitation and by interface-nucleation has been developed. Generally speaking, interface-nucleation plays a critical role during the early stages of the transformation, regardless of pH, whereas dissolution-precipitation dominates the later stages of the transformation. Finally, anatase to rutile phase transformation kinetics was exploited to produce nanoporous rutile nanocrystals by controlling the solubility of TiO2 nanocrystals. All in all, the observations and results obtained in this work might enable new insights into the mechanism of particle-mediated phase transformation and better control over the mechanism to produce materials with desired properties.
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    Computational Modeling For The Vertical Bridgman Growth Of Babrcl:Eu Crystal
    (2020-03) Zhang, Chang
    In recent years, many new scintillator crystals for X-ray or gamma-ray detection have been discovered. They have great potential to be used in security devices or medical imaging devices. However, there are a couple challenges need to be overcome before these scintillator crystals can be commercialized. Firstly, the internal physical processes during the growth of these crystals are hard to be observed, making it difficult to control and optimize the processes. Secondly, cracking is a main issue that hinders the growth of high quality, large size scintillator crystals. Slow cooling, a conventional way to reduce thermo-elastic stress, fails to completely prevent cracking in scintillator crystals. In this thesis, we, together with our experimental collaborators, will demonstrate that computational modeling and advanced experimental tools can help researchers overcome these challenges and manufacture high quality, large size scintillator crystals. BaBrCl:Eu crystal and vertical Bridgman method are chosen as the candidate material and candidate crystal growth method in this thesis. Tremsin and coworkers developed a neutron imaging system to observe the vertical Bridgman growth process of scintillator crystals. Their measurements provided a direct observation of segregation and interface shape within a vertical gradient freeze system (VGF) that is large enough to exhibit the complex interplay of heat transfer, fluid flow, segregation, and phase change characteristic of an industrially relevant melt-growth process. We have applied continuum models to simulate a VGF growth process of BaBrCl:Eu crystal conducted in the neutron imaging system. Our models provide a rigorous framework in which to understand the mechanisms that are responsible for the complicated evolution of interface shape and dopant distribution in the growth experiment. We explain how a transition in the solid/liquid interface shape from concave to convex is driven by changes in radial heat transfer caused by furnace design. We also provide a mechanistic explanation of how dynamic growth conditions and changes of the flow structure in the melt result in complicated segregation patterns in this system. Onken and coworkers used neutron diffraction to measure the crystal structure evolution of BaBrCl:Eu at different temperature levels. Their results showed that the chemical stress induced by the lattice mismatch between Eu dopant and BaBrCl is responsible for the cracking of BaBrCl:Eu crystal during cooling process. We developed finite element models to analyze the chemical stress in BaBrCl:Eu crystal under different growth conditions based on the study of Onken and coworkers. To our knowledge, these are the first computations for chemical stress in bulk crystal growth process. Our results showed that the melt/crystal interface shape and the associated melt flows have a strong influence on the radial segregation outcome of Eu, which determines the chemical stress profile in the crystal. Counterintuitively, growing this crystal at slow growth rates can lead to high stress levels and tensile stress states near the cylindrical surface that promote cracking. However, a slightly faster growth rate can produce Eu radial concentration gradients that provide a protective, compressive force layer that would suppress cracking. Our results show that the chemical stress could be tailored by designing appropriate interface shapes and melt flows.
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    Data for Crystal-Chemical Origins of the Ultrahigh Conductivity of Metallic Delafossites
    (2023-11-09) Zhang, Yi; Tutt, Fred; Evans, Guy N; Sharma, Prachi; Haugstad, Greg; Kaiser, Ben; Ramberger, Justin; Bayliff, Samuel; Tao, Yu; Manno, Mike; Garcia-Barriocanal, Javier; Chaturvedi, Vipul; Fernandes, Rafael M; Birol, Turan; Seyfried Jr, William E; Leighton, Chris; leighton@umn.edu; Leighton, Chris; Leighton Electronic and Magnetic Materials Lab
    Despite their highly anisotropic complex-oxidic nature, certain delafossite compounds (e.g., PdCoO2, PtCoO2) are the most conductive oxides known, for reasons that remain poorly understood. Their room-temperature conductivity can exceed that of Au, while their low-temperature electronic mean-free-paths reach an astonishing 20 um. It is widely accepted that these materials must be ultrapure to achieve this, although the methods for their growth (which produce only small crystals) are not typically capable of such. Here, we first report a new approach to PdCoO2 crystal growth, using chemical vapor transport methods to achieve order-of-magnitude gains in size, the highest structural qualities yet reported, and record residual resistivity ratios (>440). Nevertheless, the first detailed mass spectrometry measurements on these materials reveal that they are not ultrapure, typically harboring 100s-of-parts-per-million impurity levels. Through quantitative crystal-chemical analyses, we resolve this apparent dichotomy, showing that the vast majority of impurities are forced to reside in the Co-O octahedral layers, leaving the conductive Pd sheets highly pure (~1 ppm impurity concentrations). These purities are shown to be in quantitative agreement with measured residual resistivities. We thus conclude that a previously unconsidered “sublattice purification” mechanism is essential to the ultrahigh low-temperature conductivity and mean-free-path of metallic delafossites. This dataset contains all digital data in the published paper of the same name.
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    Early stages of zeolite growth.
    (2010-08) Kumar, Sandeep
    Zeolites are crystalline nonporous aluminosilicates with important applications in separation, purification, and adsorption of liquid and gaseous molecules. However, an ability to tailor the zeolite microstructure, such as particle size/shape and pore-size, to make it benign for specific application requires control over nucleation and particle growth processes. But, the nucleation and crystallization mechanisms of zeolites are not fully understood. In this context, the synthesis of an all-silica zeolite with MFI-type framework has been studied extensively as a model system. Throughout chapters 2, 4 and 5, MFI growth process has been investigated by small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM). Of fundamental importance is the role of nanoparticles (~5 nm), which are present in the precursor sol, in MFI nucleation and crystallization. Formation of amorphous aggregates and their internal restructuring are concluded as essential steps in MFI nucleation. Early stage zeolite particles have disordered and less crystalline regions within, which indicates the role of structurally distributed population of nanoparticles in growth. Faceting occurs after the depletion of nanoparticles. The chapter 6 presents growth studies in silica sols prepared by using a dimer of tertaprpylammonium (TPA) and reports that MFI nucleation and crystallization are delayed with a more pronounced delay in crystal growth.
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    Modeling and control of cadmium zinc telluride grown via an electro-dynamic gradient freeze furnace.
    (2007-12) Lun, Lisa San
    In this thesis, numerical models are used to study the effect of novel processing methods to grow bulk, single crystal cadmium zinc telluride (CZT) in a vertical Bridgman (VB) furnace. Additionally, we investigate new mathematical algorithms for improved solving capability of equations that describe such crystal growth systems. A two-dimensional crystal growth model for the simulation of bulk crystal growth in a VB system is presented. This model consists of conservation equations for coupled continuum level transport of heat, mass, and momentum. Thermodynamic relations associated with phase change are also included. The Galerkin finite element method is used to discretize the spatial portion of the governing equations. The resulting sets of nonlinear algebraic equations are solved using Newton's method. Novel processing methods that are not practical to attempt in experiments are investigated using numerical modeling. A two-dimensional, planar, crystal growth model is used to explore the effect of ampoule tilting on zinc segregation in a CZT crystal. Tilting is shown to improve lateral segregation. We also analyze the use of closed-loop control to improve the macroscopic melt-crystal interface shape during growth by changing the furnace temperature gradient. Targeted closed-loop control on the temperature gradient adjacent to the solid only gave the best results and unexpectedly produced a favorable convex shape. A multi-scale crystal growth model is developed by coupling pre-existing codes, one which specializes in modeling the complex crystal growth process and the other which specializes in modeling the heat transfer effects in a furnace. Previously, a coupling algorithm based on the Gauss-Seidel method was used but it converged unreliably [136, 196]. Here, we use an Approximate Block Newton approach where we approximate Newton's method used to solve the two separate models as if they were one monolithic model. A Schur complement formulation is employed to solve the free-boundary problem associated with melt crystal growth systems. With this form, the difficult interface location part of the problem is mapped away from the equations governing transport. We assess the behavior of this method using two-dimensional simulations, but the goal is to improve solvability of three-dimensional problems.
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    The study of oriented aggregation: a nonclassical nanocrystal growth mechanism
    (2013-02) Burrows, Nathan Dennis
    Oriented aggregation is a nonclassical crystal growth mechanism resulting in new secondary nanoparticles composed of crystallographically aligned primary crystallites. These secondary crystals often have unique and symmetry-defying morphologies, can be twinned, and can contain stacking faults and other significant defects. A wide range of important materials, such as titanium dioxide, iron oxides, selenides and sulfides, and metal oxyhydroxides, are known to grow by oriented aggregation under certain conditions. Evidence for oriented aggregation also has been observed in natural materials. However questions remain about what conditions are the most importing in facilitating purposeful control over nanoparticle size, size distribution, and morphology. Kinetic models for oriented aggregation point to important variables such as ionic strength, pH, temperature, and choice of dispersing solvent as being the key or keys to gaining control of this natural phenomenon and moving it towards a tool to be used in designing novel nanomaterials. The main technique used in this research is transmission electron microscopy with temporal resolution to characterize the population of growing nanocrystals. Cryogenic transmission electron microscopy is employed to observe the various stages of crystal growth. With extensive image analysis, it is possible to determine the kinetics of growth and the effects of systematically changing these key growth conditions. Additional complimentary techniques are employed, such as dynamic light scattering as well as various methods of characterization, such as powder X-ray diffraction. As our fundamental understanding of oriented aggregation improves, novel and complex functional materials are expected to emerge.
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    Tailoring the Microstructure of 2D Molecular Sieve Materials for Thin Film Applications
    (2018-05) Shete, Meera
    Zeolites and metal organic frameworks (MOFs) are microporous materials, with pores of molecular dimensions, that are of interest in a variety of applications including catalysis, adsorption, ion-exchange, separation membranes etc. With a global need of developing clean energy resources and reducing the carbon footprint of existing processes, they are being increasingly sought after as catalysts for the conversion of biomass to chemicals and fuels, in separation membranes to replace the existing energy intensive industrial separations with clean energy-efficient processes and for capture and storage of carbon dioxide. Their performance in these applications depends mainly on their pore size but also on our ability to tune their microstructure (crystal morphology and size, orientation, phase purity, defect densities etc.) as desired for an optimum performance. Recent advances in synthesis of molecular sieve materials have resulted in the development of advanced morphologies such as hierarchical materials, core-shell catalysts, two-dimensional nanosheets and thin films. However, a lot of the reports in the literature focus on conventional crystals and studies focusing on nanoscale crystal growth control are still in their infancy. This dissertation focuses on developing synthetic methods that will enable us to tailor the microstructure of 2D molecular sieve materials at a nanoscale approaching single-unit-cell dimensions with a goal of optimizing their performance in thin film applications. A novel coating technique was applied to isolate 2D MFI zeolite nanosheets and form monolayer coatings on versatile supports such as Si wafers. Using this as a prototype, growth conditions were developed that lead to unprecedented control of zeolite MFI growth at a scale approaching single-unit-cell dimensions. It was demonstrated that these growth conditions are robust enough and can be used to grow zeolite MFI crystals of varied sizes and morphology. It also enabled us to precisely control the microstructure of MFI thin films leading to the development of a material that had one of the lowest reported dielectric constant. Furthermore, the nanoscale growth control also allowed us to tailor the design of hierarchical catalysts by controllably thickening the zeolite domains in them and open opportunities to design multifunctional catalysts. A scalable and direct synthesis of Cu(BDC) MOF nanosheets was developed. Hybrid nanocomposites incorporating the MOF nanosheets in polymer matrices were fabricated which demonstrated significantly improved performance for CO2/CH4 separation.