Thesis (M.Sc.)--City University of Hong Kong, 2005. / At head of title: City University of Hong Kong, Department of Physics and Materials Science, Master of Science in materials engineering & nanotechnology dissertation. On t.p. the "x" of "CdxZn₁-xS" are subscript. Title from title screen (viewed on Sept. 4, 2006) Includes bibliographical references.

The metallurgy and superconductivity of the Nb-Ti-Ta ternary alloy system were studied. The Nb-Ti, and Ta-Ti binary samples, and Nb-Ti-Ta ternary samples were precipitation heat treated under different schedules. After the precipitating heat treatment, the samples were characterized by X-Ray Diffraction (XRD) techniques. Equilibrium binary and ternary phase diagrams based on the different alloy compositions and heat treatment temperatures were developed. The Ta-Ti binary phase diagram is very close to the ASM standard phase diagram. The ��-phase boundary of Nb-Ti binary phase diagram developed here is at a higher temperature relative to the ASM standard one. A working ternary equilibrium phase diagram for the Nb-Ti-Ta system has been developed that is based on the experimental measurements and quantitative thermodynamic calculations. Measurements of superconducting critical temperature, Tc, show a good agreement with previous measurements of Tc in the ternary alloys. / Graduation date: 2002

Abstract not available

Abstract not available

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

Metal membranes play a vital role in hydrogen purification. Defect-free membranes can exhibit effectively infinite selectivity for hydrogen. Membranes must meet multiple objectives, including providing high fluxes, resistance to poisoning, long operational standards, and be cost effective. Alloys offer an alternate route in improving upon membranes based on pure metal such as Pd. Development of new membranes is hampered by the large effort and time required not only to experimentally develop these membranes but also to properly test these materials. We show how first principle calculations combined with coarse-grained modeling can accurately predict H2 fluxes through binary and ternary alloy membranes as a function of alloy composition, temperature and hydrogen pressures. Our methods require no experimental input apart from the knowledge of the bulk crystal structure. Our approach is demonstrated for pure Pd, Pd-rich binary alloys, PdCu binary alloys, and PdCu-based ternary alloys. PdCu alloys have experimentally shown to have potential for resistance to sulfur poisoning. First, we used plane wave Density Functional Theory to study the binding and local motion of hydrogen for the alloys of interest. This data was used in combination with a Cluster Expansion Method along with the Leave-One-Out analysis to generate comprehensive models to predict hydrogen behavior in the interstitial binding sites within the bulk of the alloys of interest. These models not only were required to correctly fit our calculated data, but they were also required to properly predict behaviors for local conditions for which we had not collected information. These models were then used to predict hydrogen solubility and diffusivity at elevated temperatures. Although we are capable of combining first principle theory calculations with coarse grain modeling, we have explored a pre-screening method in order to determine which a particular material are worth performing additional calculations. Our heuristic lattice model is a simplified model involving as few factors as possible. It is by no means intended to predict the exact macroscopic H properties in the bulk of fcc materials, but it is intended as a guide in determining which materials merit additional characterization.

Thesis (M.S.)--University of Texas at El Paso, 2008. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

Os nitretos do grupo III (BN, AIN,Gan e InN) e suas ligas ternárias Al-GaN e InGaN proporcionam, recentemente, um extraordinário avanço na fabricação de dispositivos opto-eletrônicos operando na região do espectro correspondente ao verde-azul-UV e na produção de dispositivos eletrônicos de alta frequência, alta temperatura e alta potência. Estes materiais semicondutores de gap largo atraíram enorme atenção dos pesquisadores nos últimos anos. O objetivo desta tese é o estudo das propriedades vibracionais dos nitretos do grupo III referente tanto ao cristal perfeito, quanto ao cristal com defeito. Utilizamos como base a Teoria Clássica do Crital Harmônico e o Método das Funções de Green. Com a Teoria Clássica do Cristal Harmônico, juntamente com o Método do Valence Force Filed e o Método da Soma de Ewald, que permitem gerar a matriz dinâmica do sistema, determinamos o comportamento vibracional dos nitretos binários e das ligas ternárias. A utilização destes métodos permitiu a obtenção do espectro de fônons dos nitretos binários, e o estudo do comportamento dos modos ópticos em para as ligas ternárias. A partir da Função de Green do cristal perfeito e da Função de Green do cristal com defeito, obtivemos as frequências e os modos vibracionais localizados e ressonantes introduzidos pela impureza de C e As em GaN. A partir das densidades de estados do cristal perfeito e do cristal com defeito, calculamos a entropia de formação da vacância de N em GaN. Os resultados obtidos foram usados na interpretação de dados experimentais disponíveis na literatura, relativos às propriedades vibracionais dos nitretos na estrutura wurtzita, e na predição e análise de dados experimentais obtidos pelo grupo do Laboratório de Novos Materiais Semicondutores do Instituto de Física da USP para os nitretos zincblende. / The group-III nitrides (BN, AIN, GaN and InN) and their ternary alloys AlGaN and InGaN generated recently an extraordirlary progress in the production of optoelectronic devices operating in the green-blue-UV region of the spectrum, and in the production of electronic devices of high frequency, high temperature and high power. These wide gap semiconductor materials attracted enormous attention in the last years. The objective of this Thesis was to study the vibrational properties of the bulk III nitrides, without and with defects. To accomplish this study we used the Classic Theory of the Harmonic Crystal and the Method of the Green\'s Functions. With the Classic Theory of the Harmonic Crystal, together with the Valence Force Field Method and the Method of the Ewald\'s Sum, that allow to generate the dynamic matrix of the system, we determined the vibrational behavior of the binary nitrides and of the ternary alloys. The use of these methods allowed us to obtain the phonon spectra of the binary nitrides and to study the behavior of the optical modes at of the ternary alloys. Starting from the Green\'s Function of the perfect crystal and the Green\'s Function of the crystal with defect, we obtained the frequencies and the localized and resonant vibrational modes introduced by the C and As impurities in GaN. Starting from the densities of states of the perfect crystal and of the crystal with defect, we calculated the formation entropy of the N vacancy in GaN. The obtained results were used in the interpretation of experimental data related to the vibrational properties of the wurtzite nitrides available in the literature, and in the prediction and analysis of experimental results obtained for zincblende nitrides by the group of the New Semiconductors Materials Laboratory of t11c Physics Institute at USP.

Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / In this work the synthesis of MgCdS ternary semiconductor nanocrystal alloys has been carried out by aqueous route through a bottom-up approach, using conventional hydrothermal heating as well as in situ onto graphene matrices. In the synthesis of MgCdS nanocrystals, the effect of each reaction parameter on the spectroscopic properties was studied aiming to understand the possibilities to control the optical properties. Emission spectra of MgCdS samples obtained in the experiments designed to optimize reaction parameters exhibited a single emission band reflecting nanocrystal growth, with quantum yields as high as 85%. Based on the presence of two bands in absorption spectra as well as atomic absorption spectrometry (AAS) data it was possible to propose that nanocrystals are composed of Cd and Mg. Concerning the structural architecture, it has been proposed that nanocrystals show a core-shell structure with a diffuse interface. Data from AAS also showed that the final composition of nanocrystals is generally different from the initial reaction Cd:Mg proportion, as the metal precursors have distinct reactivities. Morphological analyses by transmission electron microscopy (TEM) of nanocrystals evidenced the predominance of spherical shapes and sizes below 4 nm. Studies of the formation of nanocrystal alloys with Mg1-xCdxS and Cd1-xMgxS composition, by ion exchange from the binary components MgS and CdS helped the discussion of spectroscopic behavior of the ternary system MgCdS. It was possible to confirm that the introduction of a second cation (Cd2+ or Mg2+) into each binary structure (MgS or CdS) is consistent with the observation of two absorption bands and only one emission band. The addition of graphene derivatives during the synthesis of MgCdS nanocrystals was carried out aiming to improve the properties of the materials, as well as providing a physical support to the nanocrystals, favoring future applications. The presence of graphene induced shifts in the emission bands to larger wavelengths concomitant with intensity reduction, which can be taken as evidence of interactions between the materials. The morphologies of composites were characterized by typical graphene sheets decorated with spherical nanocrystals. / Neste trabalho foram realizadas as sínteses de nanocristais (NCs) semicondutores ternários de MgCdS via síntese aquosa através da metodologia bottom-up, assistida por tratamento térmico hidrotermal convencional, além da síntese in situ de nanocompósitos de MgCdS em matrizes de grafeno. A síntese do nanocristal de MgCdS foi avaliada através do efeito da variação de cada parâmetro de síntese sobre as propriedades espectroscópicas do material, a fim de se compreender as possibilidades de controle das propriedades ópticas. Os espectros de emissão dos NCs de MgCdS, referente ao estudo de otimização dos parâmetros de síntese, apresentaram uma única banda de emissão intensa que reflete o crescimento do nanocristal, com rendimentos quânticos de fotoluminescência elevados, chegando a 85%. Com base na presença de duas bandas de absorção no espectro de UV-visível, bem como de dados de espectrofotometria de absorção atômica (AAS), pode-se inferir que os nanocristais são compostos pelos metais de Cd e Mg, propondo-se a hipótese de uma arquitetura caroço-casca com interface difusa. Os dados obtidos através de AAS mostraram também que, como os precursores tem reatividades distintas, a composição dos materiais formados tende a diferir da proporção Cd2+:Mg2+ utilizada na reação. As análises morfológicas realizadas por microscopia eletrônica de transmissão (TEM) permitiram verificar o contorno esférico e uniforme das nanoesferas e estimar o tamanho dos nanocristais, sendo abaixo de 4 nm. Estudos de formação de ligas do tipo Mg1-xCdxS e Cd1-xMgxS, por troca iônica a partir dos componentes binários MgS e CdS permitiram compreender melhor os dados espectroscópicos dos nanocristais formados introduzindo ambos precursores simultaneamente. Confirmou-se que a introdução do segundo cátion (Cd2+ ou Mg2+) em cada estrutura binária (MgS ou CdS) de fato causa a formação de duas bandas de absorção e somente uma de emissão. A implementação de derivados de grafeno na síntese do MgCdS foi realizada a fim de aprimorar as propriedades gerais do material, bem como de propiciar um suporte físico aos nanocristais de MgCdS, favorecendo aplicações. A presença do grafeno na síntese do nanocristal proporcionou deslocamento da banda de emissão para maiores comprimentos de onda com redução da intensidade luminescente, evidenciando interações entre os materiais. As morfologias dos compósitos apresentam folhas de grafeno decoradas com nanocristais esféricos. / São Cristóvão, SE

Os nitretos do grupo III (BN, AIN,Gan e InN) e suas ligas ternárias Al-GaN e InGaN proporcionam, recentemente, um extraordinário avanço na fabricação de dispositivos opto-eletrônicos operando na região do espectro correspondente ao verde-azul-UV e na produção de dispositivos eletrônicos de alta frequência, alta temperatura e alta potência. Estes materiais semicondutores de gap largo atraíram enorme atenção dos pesquisadores nos últimos anos. O objetivo desta tese é o estudo das propriedades vibracionais dos nitretos do grupo III referente tanto ao cristal perfeito, quanto ao cristal com defeito. Utilizamos como base a Teoria Clássica do Crital Harmônico e o Método das Funções de Green. Com a Teoria Clássica do Cristal Harmônico, juntamente com o Método do Valence Force Filed e o Método da Soma de Ewald, que permitem gerar a matriz dinâmica do sistema, determinamos o comportamento vibracional dos nitretos binários e das ligas ternárias. A utilização destes métodos permitiu a obtenção do espectro de fônons dos nitretos binários, e o estudo do comportamento dos modos ópticos em para as ligas ternárias. A partir da Função de Green do cristal perfeito e da Função de Green do cristal com defeito, obtivemos as frequências e os modos vibracionais localizados e ressonantes introduzidos pela impureza de C e As em GaN. A partir das densidades de estados do cristal perfeito e do cristal com defeito, calculamos a entropia de formação da vacância de N em GaN. Os resultados obtidos foram usados na interpretação de dados experimentais disponíveis na literatura, relativos às propriedades vibracionais dos nitretos na estrutura wurtzita, e na predição e análise de dados experimentais obtidos pelo grupo do Laboratório de Novos Materiais Semicondutores do Instituto de Física da USP para os nitretos zincblende. / The group-III nitrides (BN, AIN, GaN and InN) and their ternary alloys AlGaN and InGaN generated recently an extraordirlary progress in the production of optoelectronic devices operating in the green-blue-UV region of the spectrum, and in the production of electronic devices of high frequency, high temperature and high power. These wide gap semiconductor materials attracted enormous attention in the last years. The objective of this Thesis was to study the vibrational properties of the bulk III nitrides, without and with defects. To accomplish this study we used the Classic Theory of the Harmonic Crystal and the Method of the Green\'s Functions. With the Classic Theory of the Harmonic Crystal, together with the Valence Force Field Method and the Method of the Ewald\'s Sum, that allow to generate the dynamic matrix of the system, we determined the vibrational behavior of the binary nitrides and of the ternary alloys. The use of these methods allowed us to obtain the phonon spectra of the binary nitrides and to study the behavior of the optical modes at of the ternary alloys. Starting from the Green\'s Function of the perfect crystal and the Green\'s Function of the crystal with defect, we obtained the frequencies and the localized and resonant vibrational modes introduced by the C and As impurities in GaN. Starting from the densities of states of the perfect crystal and of the crystal with defect, we calculated the formation entropy of the N vacancy in GaN. The obtained results were used in the interpretation of experimental data related to the vibrational properties of the wurtzite nitrides available in the literature, and in the prediction and analysis of experimental results obtained for zincblende nitrides by the group of the New Semiconductors Materials Laboratory of t11c Physics Institute at USP.