Carbothermic production of hexagonal BN by using boric acid and ground colemanite mined from Bigadi&ccedil / Region in Turkey was investigated by subjecting pellets prepared from B2O3, activated carbon and colemanite mixtures to nitrogen gas at 1500&deg / C. Similar to CaCO3 addition, colemanite addition to the B2O3-C mixtures resulted in higher amounts of h-BN in the final products. As a result of the experiments conducted with colemanite and CaCO3 additions providing the same quantity of CaO to the initial mixtures, similar amounts of hexagonal BN in the reaction products were observed. As a result of the experiments conducted with different compositions of colemanite- B2O3- C mixtures, 5 wt % colemanite addition was determined to be the optimum composition giving the highest amount of hexagonal BN in the reaction products. Increasing duration of the experiments increased the amount and particle size of h-BN formed in the products. Optimum amount of colemanite addition resulted in higher amounts and coarser particles of h-BN in the products than the optimum amounts of CaCO3 addition.

Thermal behavior and molecular packing of a series of £\-helical poly(L-glutamates), with n-alkyl side chain of various lengths (m(number of carbons in the alkyl group) = 1, 2, 6, 12,18), were studied by means of differential scanning calorimetry, polarizing light microscopy and X-ray diffraction. For polymers of m = 1 and 2, There is a pseudohexagonal structure below ca. 130 oC and above this temperature the stable phase is the hexagonal columnar phase. There exists a layered structure in the polymer of m = 6, as well as a solvent induced hexagonal columnar structure which formed during solution casting process. In the polymer of m = 12, a layered structure was formed in the temperature range between 20 to 255 oC. However, for longer side chain, m = 18, tendency of crystallization of alkyl long side chain forced the backbone to pack into layer structure. There are two distinct melting temperature at ca. 60 oC, and the enthalpy are ca. 53 and 19 J/g, which corresponding to the melting of hexagonal and monoclinic side chain crystallines. The polymers with longer side chain (m = 6, 12 and 18) tend to be lyotropic liquid crystalline phase within lamellar inter-rod distance of 1.25 nm in solution state, and the structure will remain after drying. However, the inter-rod distance will collapse at the temperature above ca. 200 ¢J and will not recover after cooling.

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ABSTRACT Structural evolution and its effect on optical absorption/emission behavior of derivative of PPVs upon isothermal heat treatment at elevated temperatures were studied by means of a combination of polarized light microscopy, x-ray diffraction, transmission electron microscopy, ultraviolet-visible spectroscopy, and photoluminescence spectroscopy. The main physical picture drawn from results of this study over a series of PPVs with flexible side-chains may be summarized as the following: (1) They are generally liquid-crystalline in nature, typically biaxially nematic in optical texture but morphologically characterized as of lamellar or hexagonal columnar structure. This is consistent with the nematogenic nature one would expect from the rigid backbone as well as the smectogenic nature one would expect from the aliphatic side-chains. (2) The aggregates formed in solutions and the supramolecular assemblies formed in the bulk state are structurally similar (in terms of the similar level of conjugation), and hence possibly of the same thermodynamic origin. This surfactant-like self-ordering behavior is consistent with the tendency towards segregation between the aromatic, rigid backbone and the aliphatic, flexible side-chains. (3) The collapse of these conjugated polymers with flexible side-chains into aggregates appears to be a general phenomenon upon slow to moderate solvent removal and not limited to the present case of poor solvency power. This is consistent with the strong tendency toward phase separation in rigid rod solutions delineated by Flory some 30 years ago. (4) All the above observations may be explained in terms of lyotropic or thermotropic self-assembly of hairy-rod chains into coiled helical conformation with ellipsoidal cross section for the conjugated backbone as shown schematically in Figure 4-42 and 4-43. The flexible side-chains generally tend to fill the space within the ellipsoidal cylindrical structure. As the side-chain length is increased, the increased Van der Waals attraction among side-chains results in more extended period of helical twist or more straighten backbone conformation, rendering preference of lamellar structure over hexagonal helical structure. (5) As a consequence, supramolecular aggregation is basically enhanced by increased side-chain length or backbone rigidity. This in turn results in more extended conjugation length or more fully developed

Zirconium and its alloys have been extensively used in both heavy and light water nuclear reactors. Like other Hexagonal Close-Packed (HCP) materials, e.g. magnesium, zirconium alloys develop different textures during manufacturing process which result in highly anisotropic materials with different responses under different loading conditions. Slip and twinning are two major deformation mechanisms during plastic deformation of zirconium. This dissertation uses various experimental techniques and a crystal plasticity scheme in the finite element framework to study deformation mechanisms in HCP materials with an emphasis on twinning in Zircaloy-2. The current study is presented as a manuscript format dissertation comprised of four manuscript chapters. After a literature review in Chapter 2, Chapter 3 reports steps in developing a crystal plasticity finite element user material subroutine for modeling deformation in Zircaloy-2 at room temperature. It is shown in Chapter 3 that the developed rate dependent equations are capable of capturing evolution of key features, e.g., texture, lattice strains, and twin volume fractions, during deformation by twinning and slip. Chapter 4 reports various assumptions and approaches in modeling twinning where results are compared against neutron diffraction measurements from the literature. It is shown in Chapter 4 that the predominant twin reorientation scheme can explain texture development more precisely than the other schemes discussed. Chapter 5 and 6 are two connected chapters where in the first one the formation of twins is studied statistically and in the second one, local inception and propagation of twins is studied. Numerical results of these two chapters are compared with 2D electron backscattered diffraction measurements, both carried out by the author and from the literature. Results from these two connected chapters emphasize the important role of grain boundary geometry and stress concentration sites on twin nucleation and growth. The four manuscript chapters are followed by summarizing conclusions and suggestions for future work in Chapter 7. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2012-04-23 11:50:33.751

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This work developed a stylized three dimensional benchmark problem based on Argonne National Laboratory's conceptual Advanced Burner Test Reactor design. This reactor is a sodium cooled fast reactor designed to burn recycled fuel to generate power while transmuting long term waste. The specification includes heterogeneity at both the assembly and core levels while the geometry and material compositions are both fully described. After developing the benchmark, 15 group cross sections were developed so that it could be used for transport code method verification. Using the aforementioned benchmark and 15 group cross sections, the Coarse-Mesh Transport Method (COMET) code was compared to Monte Carlo code MCNP5 (MCNP). Results were generated for three separate core cases: control rods out, near critical, and control rods in. The cross section groups developed do not compare favorably to the continuous energy model; however, the primary goal of these cross sections is to provide a common set of approachable cross sections that are widely usable for numerical methods development benchmarking. Eigenvalue comparison results for MCNP vs. COMET are strong, with two of the models within one standard deviation and the third model within one and a third standard deviation. The fission density results are highly accurate with a pin fission density average of less than 0.5% for each model.

Aluminum hexagonal honeycombs loaded in their tubular direction have extremely good mechanical properties, including high stiffness to weight and energy absorption capacities. The corresponding load-displacement curve exhibits a long plateau accompanied by small fluctuations. These fluctuations are due to the propagation of a folding front through the studied sample, which is due to the creation of folds. This plateau load makes honeycombs the perfect candidates for use as energy-dissipative devices such as bumpers. Previous studies have largely focused on the study of the plateau load with less attention given to the length of the folds. However, it will be seen that this parameter is crucial for both the complete understanding of the mechanics of the folding and the derivation of the plateau load. We present first an introduction to the thematic of honeycomb. Then, the first model focuses precisely on the fold length. Two hypotheses are considered, a correlation between elastic buckling and folding first and a local propagation of the existing fold secondly. The second hypothesis is found to be correct, and the results are good for thin foils. For thick foils, a geometric limitation occurs, which makes the results less precise. Then, we are able to use the previous kinematics for the folding and derive a new set of formulas for the plateau load. The results are compared with experimental results and past formulas, and are found to be good, especially for thin foils, where our results for the fold length are more precise.

In this paper, the coarse mesh transport method is extended to hexagonal geometry. This stochastic-deterministic hybrid transport method calculates the eigenvalue and explicit pin fission density profile of hexagonal reactor cores. It models the exact detail within complex heterogeneous cores without homogenizing regions or materials, and neither block-level nor core-level asymmetry poses any limitations to the method. It solves eigenvalue problems by first splitting the core into a set of coarse meshes, and then using Monte Carlo methods to create a library of response expansion coefficients, found by expanding the angular current in phase-space distribution using a set of polynomials orthogonal on the angular half-space defined by mesh boundaries. The coarse meshes are coupled by the angular current at their interfaces. A deterministic sweeping procedure is then used to iteratively construct the solution. The method is evaluated using benchmark problems based on a gas-cooled, graphite-moderated high temperature reactor. The method quickly solves problems to any level of detail desired by the user. In this paper, it is used to explicitly calculate the fission density of individual fuel pins and determine the reactivity worth of individual control rods. In every case, results for the core multiplication factor and pin fission density distribution are found within several minutes. Results are highly accurate when compared to direct Monte Carlo reference solutions; errors in the eigenvalue calculations are on the order of 0.02%, and errors in the pin fission density average less than 0.1%.

Title: Local structure of hexagonal ferrites studied by NMR Author: Karel Kouřil Department/Institute: Department of Low Temperature Physics Supervisor of the doctoral thesis: prof. RNDr. Helena Štěpánková, CSc. Abstract: Hexagonal ferrites of M, W, X and Y structure types were studied by means of NMR, electronic structure calculations and magnetoelectric experi- ments. Presented results deal with cation distribution, localization of ferrous ions, interpretation of NMR spectra of studied materials and effects of size reduction. In oriented layers M type strontium ferrite and submicron particles of M type barium ferrite pronounced effects of reduced size were observed on 57 Fe NMR spectra. Performance of magnetoelectric barium-strontium Y type hexaferrites with divalent zinc cations improved upon thermal treatment of samples while distribution of zinc was not significantly altered. In Sc sub- stituted BaM, Sc content was found to be uniform throughout TSSG grown crystal. In LaSrM systems electron localization in 2a sublattice was observed, co-substitution with La+Zn, La+Co and La+Cu was found to lead to partial charge compensation. In strontium ferrites of type W and X localization of ferrous ions in octahedral sites in SS block pair was observed. Keywords: NMR, hexagonal ferrites, structure,...