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Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 248-253). Available also in a digital version from Dissertation Abstracts.

Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-06-02T21:28:34Z No. of bitstreams: 1 EduardoDiniz.pdf: 974608 bytes, checksum: 134920c92050dd5dee6af94ec24287a3 (MD5) / Made available in DSpace on 2017-06-02T21:28:34Z (GMT). No. of bitstreams: 1 EduardoDiniz.pdf: 974608 bytes, checksum: 134920c92050dd5dee6af94ec24287a3 (MD5) Previous issue date: 2006-02-22 / Fundação de Amparo à Pesquisa e ao Desenvolvimento Científico e Tecnológico do Maranhão (FAPEMA) / In this work static atomistic simulations were performed to investigate two systems: rare-earth trifluorides, REF3, with tysonite structure (RE = La, Ce, Pr and Nd) and rare-earth mixed metallic oxides, RE(TiTa)O6, with RE = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er and Yb. In the rare-earth trifluorides was investigated the pressure induced structural phase transition identified by X-ray powder difraction in RE = La and Ce. The results shown that the crystalline structure is possibly tetragonal, in concordance with quantum mechanical calculations. For this, an interionic potential set that describes better the structural data of each compound and the physical observable was obtained. Besides, it was observed that critical pressure values were closed. The study involving the rare-earth mixed metallic oxides was focused in determining their bulk and structural properties. Thus, firstly a set of reliable interionic potentials was looked for to reproduce the crystalline structure and the physical observable, trough the investigation of three methodologies for obtaining interionic potentials, assuming the methodology that reproduces more reliable the dielectric constants. Furthermore, others bulk properties as lattice energy formation, elastic constants and bulk modulii were calculated. All then shown a rare-earth ionic radii dependence that revealed the morphotropic phase boundary between aeschynite and euxenite crystalline structures presented by these crystals. / Neste trabalho foram empregadas simulações atomistícas estáticas para se investigar dois sistemas: trifluoretos de terra-rara, REF3, com estrutura tisonita (RE = La, Ce, Pr e Nd) e óxidos metálicos mistos de terras-raras, RE(TiTa)O6, com RE = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er e Yb. Nos trifuoretos de terra-rara foi investigada a transição de fase estrutural que ocorre para altas pressões identificada por difração de raios-X em pó para RE = La e Ce. Os resultados mostraram que a estruturas cristalinas à altas pressões são possivelmente tetragonais, concordando com os resultados obtidos por cálculos quânticos. Para tanto, obtivemos um conjunto de potenciais interiônicos que proporcionassem uma descrição confiável da estrutura cristalina de cada composto, bem como o cálculo de observáveis físicos que produzissem valores próximos aos observados experimentalmente. O estudo envolvendo os óxidos metálicos mistos de terras-raras, dedicou-se em determinar suas propriedades de bulk e estruturais. Para isso, primeiramente obtivemos um conjunto de potenciais interiônicos confiáveis para se reproduzir a estrutura cristalina e os observáveis físicos, através da investigação de três metodologias para a obtenção de potenciais interiônicos, optando-se por aquela que melhor reproduziu as constantes dielétricas. Além da constante dielétrica, outros observáveis como a energia de formação da rede, constantes elásticas e módulo de bulk, foram calculados. Todos apresentaram uma dependência com o raio iônico do íon terra-rara que evidenciou o contorno de fase morfotrópico entre as estruturas aeschinita e euxenita apresentada por estes compostos.

Abstract The purpose of this task is to synthesize a dielectric substrate material through a sintering process, which can be used for non-invasive physiological sensor development. Low loss, high dielectric constant ceramic material is used. Sintering process is employed to ensure stable structure and homogeneous dielectric properties of the substrate. Samples were prepared with TiO2 and in combination with CuO and Al2O3. All samples were measured and validated in 500 MHz to 20 GHz frequency range. Characterization measurements were performed with a Vector Network Analyzer, FieldFox N9918A, and connected to Keysight, open ended coaxial probe and performance probe. Reflection based measurement method was used due to its simplicity, speed and requirement of wideband data. The dielectric measurement results of developed samples show non-frequency dispersive behaviour, high dielectric constant and data was also selected at 2.45 GHz in aligned to the industrial, scientific and medical band. The resulting measurements show the highest dielectric constant of 16.6 at 2.45 GHz with a very low loss behaviour. / Målet är att syntetisera ett material, genom sintringsprocess, som kan användas som ett dielektriskt substrat för utveckling av sensorer. Det dielektriska materialet har keramisk materialstruktur och innehar högt dielektricitetskonstant med låga dielektriska materialförluster. Denna uppgift kräver att dielektriska materialet ska vara stabilt och inneha isotropiska egenskaper efter att genomgått sintringsprocess. Proverna förberedes med TiO2 och även i kombination med TiO2 tillsammans med CuO och Al2O3. Proverna mäts i frekvensområdet 500 MHz till 20 GHz. Mätningarna utförs med Vector Network Analyzer, FieldFox N9918A från Keysight. Resultat som visas och jämförs i arbetet är tagna vid 2.45 GHz eftersom det används och är standardiserat för medicinskt frekvensband. Högsta uppmätta dielektricitetskonstant har värdet av 16,6. Resultaten visar även låga förluster i dielektrikumet.