Made available in DSpace on 2014-10-09T12:29:37Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:59:41Z (GMT). No. of bitstreams: 1 01257.pdf: 1883679 bytes, checksum: 16dcaa8a2de2f071f89d7e49a9912605 (MD5) / Dissertacao (Mestrado) / IEA/D / Instituto de Fisica, Universidade de Sao Paulo - IF/USP

Made available in DSpace on 2014-10-09T12:35:01Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:02Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:35:01Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:02Z (GMT). No. of bitstreams: 0 / O presente estudo está dividido em duas partes. A primeira parte está relacionada à preparação de pós de Cr3C2-25(Ni20Cr) nanoestruturados através do processo de moagem de alta energia, bem como à caracterização dos pós moídos e no estado como recebido. A análise dos dados obtidos nesta etapa do trabalho foi feita utilizando-se uma abordagem essencialmente teórica. A segunda parte deste estudo refere-se à produção e caracterização de revestimentos preparados com os pós de Cr3C2-25(Ni20Cr) nanoestruturados e como recebido. O comportamento destes revestimentos sob erosão-oxidação em alta temperatura foi comparado com base em uma abordagem de caráter mais tecnológico. O tamanho médio de cristalito do pó de Cr3C2-25(Ni20Cr) decresceu rapidamente de 145 nm para 50 nm nos estágios iniciais de moagem e, posteriormente, com o aumento do tempo de moagem, decresceu mais lentamente até atingir um estado estacionário para um tamanho de cristalito em torno de 10 nm. Este estado estacionário corresponde ao início do processo de recuperação dinâmica. A máxima deformação da rede cristalina (δ = 1,17%) foi observada para pós moídos por 16 horas, caracterizando um tamanho crítico de cristalito da ordem de 28 nm. Por outro lado, o parâmetro de rede atingiu um mínimo para pós moídos por 16 horas. Após atingir o tamanho crítico de cristalito, a densidade de discordâncias praticamente não mais varia (estado estacionário) e toda deformação plástica posteriormente introduzida no material é acomodada através de eventos que ocorrem nos contornos de grão, particularmente por meio do processo designado deslizamento de contorno de grão (grain boundary sliding). A energia de deformação armazenada na rede cristalina dos pós de Cr3C2-25(Ni20Cr) moídos com diferentes tempos de moagem foi determinada por meio de medidas da variação de entalpia. Estes resultados indicaram que a máxima variação de entalpia (ΔH = 722 mcal) também ocorreu para pós moídos por 16 horas. Analogamente, a máxima variação do calor específico (ΔCp = 0,278 cal/gK) ocorreu para pós moídos por 16 horas. As seguintes propriedades mecânicas dos revestimentos de Cr3C2-25(Ni20Cr), preparados utilizando-se o processo HVOF de aspersão térmica, foram determinadas: microdureza Vickers, módulo de Young e tenacidade à fratura. As propriedades dos revestimentos preparados com os pós nanoestruturados e como recebido foram comparadas. A dureza e o módulo de Young dos revestimentos preparados com os pós nanoestruturados foram aproximadamente 26% maiores que aqueles preparados com os pós como recebido. A tenacidade à fratura dos revestimentos nanoestruturados foi aproximadamente 36% maior do que o verificado para os revestimentos produzidos com pós no estado como recebido. A resistência à erosão-oxidação do revestimento produzido com o pó nanoestruturado foi em torno de 52% maior do que a do revestimento preparado com o pó no estado como recebido, a 800ºC. Ambos os revestimentos mostraram um aumento da taxa de erosão-oxidação para temperaturas acima de 450ºC. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:29:37Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:59:41Z (GMT). No. of bitstreams: 1 01257.pdf: 1883679 bytes, checksum: 16dcaa8a2de2f071f89d7e49a9912605 (MD5) / Dissertacao (Mestrado) / IEA/D / Instituto de Fisica, Universidade de Sao Paulo - IF/USP

No description available.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Francisco Javier Carrion. / M.S.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Khaled Awni Touqan. / Ph.D.

The knowledge of the packing behaviour of small organic compounds in crystal lattices is of great importance for industries dealing with solid state materials. The properties of materials depend on how the molecules arrange themselves in a crystalline environment. Crystal structure prediction provides a theoretical approach through the application of computational strategies to seek possible crystal packing arrangements (or polymorphs) a compound may adopt. Based on the chemical diagrams, this thesis investigates polymorphism of several small organic compounds. Plausible crystal packings of those compounds are generated, and their lattice energies are minimised using molecular mechanics and/or quantum mechanics methods. Most of the work presented here is conducted using two software packages commercially available in this field, Polymorph Predictor of Materials Studio 4.0 and GRACE 1.0. In general, the computational techniques implemented in GRACE are very good at reproducing the geometries of the crystal structures corresponding to the experimental observations of the compounds, in addition to describing their solid state energetics correctly. Complementing the CSP results obtained using GRACE with isostructurality offers a route by which new potential polymorphs of the targeted compounds might be crystallised using the existing experimental data. Based on all calculations in this thesis, four new potential polymorphs for four different compounds, which have not yet been determined experimentally, are predicted to exist and may be obtained under the right crystallisation conditions. One polymorph is expected to crystallise under pressure. The remaining three polymorphs might be obtained by using a seeding technique or the utilisation of suitable tailor made additives. / University of Bradford

Twenty-five muscovites have been systematically studied chemically, optically, and structurally. Multiple linear regressions were performed on these data to determine the correlations of the compositional components with both the optical properties and the unit cell parameters. The results of this study indicate refractive indices increase to the extent Fe and Ti constitute the octahedral cations, and to the extent OH rather than F coordinates to these cations. The optic angle 2VX decreases nearly linearly with the Ti content. The b cell edge follows a near-linear trend with ∑(Mg + Fe(total)), similar to that found by Guidotti (1984). / Master of Science / incomplete_metadata

Vibrations of nuclei in crystals govern various properties such as thermal expansion, phase transitions, and elasticity, and the quasiharmonic approximation (QHA) is the simplest nontrivial approximation which includes the effects of vibrational anharmonicity into temperature dependent observables. Nonetheless, the QHA is often implemented with additional approximations due to the complexity of computing phonons under arbitrary strains, and the generalized QHA, which employs constant stress boundary conditions, has not been completely developed. Here we formulate the generalized QHA, providing a practical algorithm for computing the strain and other observables as a function of temperature and true stress. We circumvent the complexity of computing phonons under arbitrary strains by employing irreducible second order displacement derivatives of the Born-Oppenheimer potential and their strain dependence, which are efficiently and precisely computed using the lone irreducible derivative approach. We formulate two complementary strain parametrizations: a discretized strain grid interpolation and a Taylor series expansion in symmetrized strain. We illustrate the quasiharmonic approximation by evaluating the temperature and pressure dependence of select elastic constants and the thermal expansion in thoria (ThO₂) using density functional theory with three exchange-correlation functionals. The convergence of the two complementary strain parametrizations is evaluated for the computed thermal expansion. The temperature dependent lattice parameter and thermal expansion computed within the QHA is compared with experimental measurements. The QHA results are compared to measurements of the elastic constant tensor using time domain Brillouin scattering and inelastic neutron scattering. We then demonstrate the generalized quasiharmonic approximation in a non-cubic material, ferroelectric lead titanate, computing the temperature and stress dependence of the full elastic constant tensor. The irreducible derivative approach is employed for computing strain dependent phonons using finite difference, explicitly including dipole-quadrupole contributions. We use density functional theory, computing all independent elastic constants and piezoelectric strain coefficients at finite temperature and stress. There is good agreement between the quasiharmonic approximation and the experimentally measured lattice parameters close to 0 K. The quasiharmonic approximation overestimates the measured temperature dependence of the lattice parameters and elastic constant tensor, demonstrating that a higher level of strain dependent anharmonic vibrational theory is needed. The next material we study is zirconium nitride, employing the quasiharmonic approximation with the irreducible derivative approach to compute the phonons and thermal expansion. Density functional theory is used with two exchange-correlation functionals. We investigate the difference between the measured and computed optical phonon branches, showing that volume effects, two-phonon scattering, and nitrogen vacancies do not explain the discrepancy between the measurement and computation. The temperature dependent lattice parameter is computed within the QHA, where the thermal expansion is overestimated as compared with existing experimental measurements.