Thesis (Ph.D.)--Ohio University, November, 2005. / Title from PDF t.p. Includes bibliographical references (p. 89-93)

The main objective of this thesis aims at developing a new generation of software products, in order to obtain a fully automatic simulator predicting the entire Czochralski process while handling correctly the switches between the different growth stages. First of all, new efficient, robust and high-quality automatic mesh generation algorithms with enough flexibility for any complex geometry were implemented, including a 1D mesh generator by global grade-adaptive method, a 2D initial triangulation algorithm by improved sweep line technique and an automatic 2D shape-quality unstructured mesh generator by modified incremental Delaunay refinement technique. Secondly, a Finite Element Navier-Stokes solver based on unstructured meshes was developed and validated. Enhanced turbulence models based on the classical mixing-length or k-l model, together with a generic transformation method to avoid negative k when solving the turbulent kinetic energy equation by the Newton-Raphson iterative method were introduced and implemented. Moreover, laminar and turbulent mathematical models governing the gas convection, thermal distribution and oxygen concentration were developed, and Finite Element numerical methods to solve these governing equations on unstructured meshes were implemented, while appropriate numerical approaches to capture the wall shear stress exerted by the gas flow and experienced by the silicon melt were investigated. Finally, a series of numerical experiments devoted to investigate the industrial Czochralski crystal growth process under various growth conditions are presented based on all the developments implemented. Comparisons of the simulation results with literature and available experimental observations are also presented, and conclusions are drawn based on these simulation results and observations.

The aim of the present investigation is to increase theunderstanding of the role played by the solvent in inhibitingor enhancing crystal growth. Drowning-out crystallizationexperiments has been performed by the controlled addition ofwater or ethanol water mixtures to a saturated solution ofbenzoic acid in ethanol-water mixtures. Crystal habitcontrolling factors have been identified.Seededcrystallization experiments have been carried out to evaluatethe effect of solvent composition on crystal habit at constantsupersaturation. The solubility of benzoic acid inethanol-water mixtures at the working temperatures has beendetermined. Electro-zone sensing determinations and microscopicmeasurements are used to characterize the final crystallineproduct. It has been found that the shape of the benzoic acidcrystals grown from ethanol-water solutions ranges from needlesto platelets. Platy particles possess a predominant basal plane(001), bound by (010) and (100) faces, while needles aredeveloped along the b-axis. Long needle-shaped particles havebeen produced at low initial bulk concentration and highethanol concentration in the feed. Small platelets are obtainedat high initial bulk concentrations and high waterconcentration in the feed. The effect of solvent composition on the growth rate hasbeen evaluated at constant supersaturation. Seed crystals arecharacterized by image analysis measurement both before andafter each experiment. Length and width dimensions have beenmeasured on the particle silhouette. The growth rate, thesolid-liquid interfacial energy and the surface entropy factorfor the (010) faces (length dimension) and (100) faces (widthdimension) have been estimated. The interfacial energy andsurface entropy factor decreases in the direction of increasingethanol concentration due to increasing solubility. The results suggest that at low ethanol concentration(xEtOH&lt;60%) growth proceeds by screw dislocation mechanism,and adsorption of ethanol molecules may reduce the growth rate.As the ethanol concentration increases above a critical value(xEtOH ≥60%), the growth mechanism shifts to surfacenucleation and the growth rate increases with increasingethanol concentration. It has been suggested that the observedeffect of the solvent composition on crystal habit is theresult of two conflicting effects here referred as the kineticand interfacial energy effects. High interactions of the pairethanol-benzoic acid seem to be responsible of the growthretardation (kinetic effect) exerted by the solvent. On theother hand, increased ethanol concentration leads to reduceinterfacial energy and increasing surface nucleation whichmight contribute to enhance growth kinetics. <b>Keywords:</b>drowning-out crystallisation, solventcomposition, benzoic acid, solubility, crystal growth,interfacial energy, surface entropy factor, growth mechanism,crystal shape distribution.

This dissertation has been focused on the synthesis and characterization of novel functionalized silica mesoporous crystals by using cationic surfactant and co–structure directing agents (CSDA), the central concept of the synthesis method is to build proper organic/inorganic interactions by introducing CSDA into the synthesis system. By using cationic surfactant as template and anionic CSDA, carboxylic group functionalized mesoporous silicas were successfully synthesized. Well ordered 2D p6mm, cubic Fm-3m, mixture of CCP (Fm-3m) and HCP (P63/mmc), and cubic Fd-3m with uniform carboxylic group distribution have been obtained. Besides, we have investigated the Fm-3m/Fd-3m type intergrowth and new type defects observed in the Fd-3m structure using transmission electron microscopy (TEM) and proposed a “polyhedron packing” model. New amphoteric, inorganic amino acid with highly ordered mesopores were synthesized. Uniform distribution of acid and base organic groups on the mesopore surfaces were formed by interactions between the counter charged surfactant head groups and ionic parts of CSDAs. It has been demonstrated that organic (–NH2 and –COOH) pairs incorporated in the mesopore walls behave as natural amino acids, collectively exhibiting an isoelectric point of ~6.0. Moreover, we have demonstrated that the inorganic amino acid is an efficient catalyst for the reaction between aldehydes and carbon nucleophiles.

The aim of this work is to make electrochemical metal nanowire growth a competitive method, being up to par with more standardized procedures, like e.g. lithography. This includes on the one hand the production of nanowires as reliable and reproducible parts, potentially suited for nanoelectronic circuit design. Therefore, this work presents a systematic investigation of the causes of nanowire branching, the necessary conditions to achieve straight growth and the parameters affecting the diameter of the wires. The growth of ultrathin (down to 15 nm), straight and unbranched platinum nanowires assembly is demonstrated. On the other hand, it is the objective to go beyond purely electronic applications. An examination of the crystallography of the wires reveals nanoclusters inside the wire with a common crystallographic orientation. The versatility of the wires is illustrated by implementing them into an impedimetric sensor capable of the detection of single nanoscaled objects, such as bacteria. / Die Zielstellung der vorliegenden Arbeit ist es, die elektrochemische Herstellung von metallischen Nanodrähten zu einer wettbewerbsfähigen Methode zu machen, die sich mit standardisierten Prozessen, wie z. B. der Lithographie messen kann. Dies beinhält auf der einen Seite die Produktion der Nanodrähte als zuverlässige und reproduzierbare Bauteile, die im nanoelektrischen Schaltungsdesign Verwendung finden können. Daher befasst sich diese Arbeit mit einer systematischen Untersuchung der Ursachen für die Verzweigung von Nanodrähten, den notwendigen Bedingungen um gerades Wachstum zu erlangen und mit den Parametern, die Einfluss auf den Durchmesser der Drähte haben. Der Wuchs von sehr dünnen (bis zu 15 nm), geraden und unverzweigten Nanodrähten aus Platin wird gezeigt. Auf der anderen Seite ist es erklärtes Ziel, über rein elektronische Anwendungen hinaus zu gehen. Eine Untersuchung der Kristallographie der Nanodrähte zeigt, dass die Drähte aus Nanopartikeln bestehen, die eine gemeinsame kristallographische Orientierung aufweisen. Die Vielseitigkeit der Drähte wird anhand einer Sensoranwendung gezeigt, mit der es möglich ist, einzelne nanoskalige Objekte (wie z. B. Bakterien) zu detektieren.

Control of bulk melt crystal growth techniques is desirable for producing semiconductors with the highest purity and ternary alloys with tunable electrical properties. Because these molten materials are electrically conducting, external magnetic fields are often employed to regulate the flow in the melt. However, complicated by the coupled flow, thermal, electromagnetic and chemical physics, such magnetic control is typically empirical or even an educated guess. Two magnetic flow control mechanisms: flow damping by steady magnetic fields, and flow stirring by alternating magnetic fields, are investigated numerically. Magnetic damping during optically-heated float-zone crystal growth is modeled using a spectral collocation method. The Marangoni convection at the free melt-gas interface is suppressed by applying a steady magnetic field, measured by the Hartmann number Ha. Using normal mode linear stability analyses, suppression of detrimental flow instabilities is quantitatively determined in a range applicable to experiments (up to Ha = 300 for Pr = 0.02, and up to Ha = 500 for Pr = 0.001). The hydrodynamic flow instability for small Prandtl number P r float-zone is confirmed by energy analyses. Rotating magnetic field stirring during confined crystal growth in an ampoule is also modeled. Decoupled from the flow field at small magnetic Reynolds number, the electromagnetic field is solved in a finite element solver. At low AC frequencies, the force is only in the azimuthal direction but penetrates deep into the melt. In contrast, the magnetic shielding effect is observed at high alternating current (AC) frequencies, where the external magnetic field penetrates only by a skin depth into the electrically conducting media within the short AC cycle. As a result, the electromagnetic body force is primarily confined to the ampoule surface. At these high AC frequencies the magnetic flux lines are drastically distorted within the melt. The body force is fully three-dimensional and is much stronger than at low AC frequencies, but is confined to near the ampoule surface due to the magnetic shielding effect. These models promote fundamental understanding of flow dynamics regulated by electromagnetic body forces. They provide quantitative guidance for crystal growth to minimize trial and error experimentation that is slow and expensive.

In confined bulk crystal growth techniques such as the traveling heater method, base materials in an ampoule are melted and resolidified as a single crystal. During this process, flow control is desired so that the resulting alloy semiconductors are uniform in composition and have minimal defects. Such control allows for tuned lattice parameters and bandgap energy, properties necessary to produce custom materials for specific electro-optical applications. For ternary alloys, bulk crystal growth methods suffer from slow diffusion rates between elements, severely limiting growth rates and reducing uniformity. Exposing the electrically conducting melt to an external alternating magnetic field can accelerate the mixing. A rotating magnetic field (RMF) can be used to stir the melt in the azimuthal direction, which reduces temperature variations and controls the shape at the solidification front. A traveling magnetic field (TMF) imposes large body forces in the radial and axial directions, which helps reduce the settling of denser components and return them to the growth front. In either case, mixing is desired, but turbulence is not. At large magnetic Taylor numbers the flow becomes unstable to first laminar and then turbulent transitions. It is imperative that crystal growers know when these transitions will occur and how the flow physics is affected. Here, the melt driven by electromagnetic forces is analyzed through the use of 3D numerical simulations of the flow field up to and beyond the point of laminar instability. The analysis aims to emulate laboratory conditions for generating electromagnetic forces for both types of alternating magnetic fields and highlights the differences between laboratory forces and the analytical approximations that are often assumed. Comparisons are made between the resulting forces, flow fields, and points of instability as the frequency of the alternating field varies. Critical Taylor numbers and the resulting unstable flow fields are compared to the results from linear stability theory.

Single crystals of cerium doped lutetium yttrium orthosilicate scintillator (Ce:(Lu(1-X)YX)2SiO5; Ce:LYSO) were grown through Czochralski method to investigate the affection of different growth conditions on the crystal growth. The relationship between thermal field and crystal growth was investigated. Different growth parameters and insulating system were adopted and compared to decrease the temperature gradient. It is found that the new insulating bricks can decrease effectively the temperature gradient and solve the crack problem of grown crystal. The relationship between materials ratio and the impurities in crystals. It is found that the optimized material ratio is Lu2O3:Y2O3:CeO2:SiO2=1.8563:0.1397:0.008:1.99. Under this ratio, we can get the crystals with best macroscopic optical quality. The scintillation properties of grown Ce:LYSO single crystal were measured. Through comparing the data with those in published literatures, it is found that the grown Ce:LYSO crystals present outstanding optical transmission, reaching the theoretical value about 84% in the visible light region. The transmission, UV-excitation and emission spectra are almost identical with the reported ones. The luminescence efficiency (LE) of random chosen 43 Ce:LYSO¡@samples were measured on the £^-ray multi-channel energy spectrum. It is found that the Ce:LYSO samples present stable luminescence efficiency with minimum channel number 3636.37, maximum channel number 4293.78, maximum deviation of 9.91% and standard deviation of 152.24 (3.77%). Annealing treatments were carried out in the air atmosphere on the small pieces of Ce:LYSO single crystals to eliminate the oxygen vacancies in the sample. It is found that the optimized annealing condition to improve the luminescence efficiency of Ce:LYSO is 1400 ¢J, 80 h under air atmosphere.

¡@¡@This paper mainly focuses on the discussion about how to grow a large-sized highly-doped laser crystal- Neodymium yttrium aluminum garnet( {NdxY1-x}3Al5O3; Nd:YAG). The higher concentration of Nd:YAG crystal can allow the better efficiency of the laser. Since the ion size doped with Nd3+ ( R= 1.12A ) is larger than the ion size of Y3+ (R= 1.01A ), in the lattice of YAG, it¡¦s not easy to mix the smaller Y3+ site with the larger diameter of the Nd3 + ion. Therefore, the higher concentration of the laser crystal we want to grow, the more difficult work it is. ¡@¡@This experiment works under the use of the Czochralski technique to grow the laser crystal: Nd:YAG, and explore the impacts of different crystal pulling growth conditions on the growth of Nd:YAG crystal. Through adjusting the parameters of crystal growth, the crystal growth environments and the thermal fields, I discuss how to solve the problems of the scattering, cracking and spiral growing during the crystal growth process in order to improve the quality of the crystals. ¡@¡@Finally, comparing the results of a variety of spectral analysis (X-ray diffraction, UV / Vis Spectroscopy, Raman Spectroscopy, PL Spectroscopy) on the slice samples of Nd:YAG crystal which grew by our laboratory with the literature results, we can find that the X-ray diffraction pattern, the absorption spectroscopy, the Raman spectroscopy and the Photoluminescence spectroscopy of the Nd:YAG crystals grew in this experiment are consistent with the literature.

The major innovation of this work is an establishment of a novel chiral separation process using preferential crystallization coupled with a membrane barrier. This hybrid process was proved to be promising from a significant increase in product yield and purity compared to existing chiral separation processes. This work sets up a process design platform to extend the use of this hybrid process to a separation of other mixtures. This novel process especially is a promising alternative for chiral separation of pharmaceutical compounds which include more than fifty percent of approved drugs world-wide. A better performance chiral separation technique contributes to cut the operating cost and to reduce the price of chiral drugs.