Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-05-05T18:57:13Z No. of bitstreams: 1 Dissertacao-AndreCavaignac.pdf: 10851210 bytes, checksum: da7f6d04592241587693042b0ee75bd7 (MD5) / Made available in DSpace on 2017-05-05T18:57:13Z (GMT). No. of bitstreams: 1 Dissertacao-AndreCavaignac.pdf: 10851210 bytes, checksum: da7f6d04592241587693042b0ee75bd7 (MD5) Previous issue date: 2015-06-12 / Structural, magnetic and magnetocaloric properties of the RMn2Si2 compounds (R = Tm, Dy and Tb) were studied. X ray diffraction analysis and structure Rietveld refinement shows that the samples crystallize with the desired tetragonal I4/mmm structural phase. Magnetization measurements shows second order magnetic phase transition from paramagnetic (PM) to ferromagnetic (FM) state for TmMn2Si2 compound around T = 5.2 K. On the other hand, DyMn2Si2 and TbMn2Si2 compounds present multiple phase transitions below 100 K. DyMn2Si2 present four magnetic phase transitions while TbMn2Si2 present two magnetic phase transition. Both compounds present first order FM phase transitions originated from ordering of rare earth sub-lattice. In addition, DSC measurements indicated that DyMn2Si2 and TbMn2Si2 compounds present high temperature phase transition at T~ 425 K and 444 K associated to the AFM coupling in the Mn sub-lattice. Magnetic hysteresis loop was used to characterize exchange bias effect in the DyMn2Si2 observed for magnetic fields higher than 35 kOe in which was associated to interactions between AFM and FM magnetic domain present in this compound. The largest magnetocaloric effect (MCE) was observed for TmMn2Si2 compound, when compared with other studied compound. The maximum values of entropy variation change −∆𝑆𝑀 𝑚á𝑥 and the Relative Cooling Power (RCP) found for TmMn2Si2 were, respectively, 18.5 J/kg.K and 247.5 J/kg for a magnetic field change (H) of 50 kOe. Significant values of −∆𝑆𝑀 𝑚á𝑥 (~8.2 J/kg.K and ~9.7 J/kg.K@50 kOe, respectively) and RCP (124.6 J/kg and 233 J/kg@50 kOe, respectively) as well as successive magnetic phase transitions were observed for DyMn2Si2 and TbMn2Si2 compounds. Also these two compound exhibits a table like EMC presenting a wide working window for practical applications. The results obtained for compounds motivate the preparation of a composite sample with the following concentrations 10% de TmMn2Si2, 15% de HoCoSi, 35% de DyMn2Si2 e 40% de TbMn2Si2, aiming further increase in the temperature range of maximum EMC. The maximum entropy change variation obtained for the composite sample was ~4.6 J/kg.K over a temperature range of ~80𝐾. Our results show that the compounds RMn2Si2 present important characteristics for application in magnetic refrigeration for cryogenic temperatures. Besides, it is possible to get a larger working region, when these compounds are associated forming a composite material. / Neste trabalho foi realizado um estudo das propriedades estruturais, magnéticas e magnetocalóricas dos compostos da série RMn2Si2 (R = Tm, Dy e Tb). A análise estrutural por difração de raios X e refinamento dos difratogramas pelo método de Rietveld mostraram que as amostras cristalizam na fase tetragonal grupo espacial I4/mmm. Com a medida de magnetização foi possível observar a transição de fase magnética de segunda ordem do estado paramagnético (PM) para ferromagnético (FM) para o composto TmMn2Si2 em torno de 5,2 K. Enquanto que para os compostos DyMn2Si2 e TbMn2Si2 observou-se a presença de transições múltiplas abaixo de 100 K. O DyMn2Si2 apresentou quatro transições de fase magnética enquanto que TbMn2Si2 apresentou duas. Em ambos os compostos, as transições ferromagnéticas atribuídas ao ordenamento da sub-rede da terra rara são de primeira ordem. Medidas DSC indicaram que DyMn2Si2 e TbMn2Si2 apresentam uma transição de fase em T~ 425 K e 444 K, respectivamente, ambas relacionadas ao acoplamento antiferromagnético na sub-rede do Mn. Com as medidas do loop de histerese magnética foi possível caracterizar o efeito de Exchange Bias (EB) para o DyMn2Si2, em campos magnéticos superiores a 35 kOe, o qual foi atribuído a interações entre os domínios AFM e FM presentes no material. A caracterização das propriedades magnetocalóricas do composto TmMn2Si2 mostrou uma variação de entropia magnética (-∆SM) mais intensa quando comparada aos outros compostos deste estudo. Os valores máximos obtidos para a variação isotérmica da entropia (−∆𝑆𝑀 𝑚á𝑥 ) e para o poder de resfriamento relativo (RCP) foram, respectivamente 18,5 J/kg.K e 247,5 J/kg, para ∆𝐻 = 50 kOe. Já os compostos DyMn2Si2 e TbMn2Si2 mostraram valores significativos de −∆𝑆𝑀 𝑚á𝑥 (~8,2 J/kg.K e ~9,7 J/kg.K@50 kOe, respectivamente) e RCP (124,6 J/kg e 233,6 J/kg@50 kOe, respectivamente) além de duas transições de fase magnética sucessivas, o que resultou em um EMC com comportamento do tipo table-like, caracterizado por uma ampla janela de trabalho com máximo EMC. Estes resultados motivaram a preparação de um compósito com as seguintes concentrações 10% de TmMn2Si2, 15% de HoCoSi, 35% de DyMn2Si2 e 40% de TbMn2Si2, visando ampliar ainda mais o intervalo de máximo EMC. A variação máxima de entropia obtida para o compósito foi da ordem de 4,6 J/kg.K num intervalo de temperatura de ~80𝐾. Os resultados obtidos sugerem que os compostos desta série apresentam características importantes para aplicação na refrigeração magnética em temperaturas criogênicas. Além disso, quando estes estão associados na forma de um compósito é possível obter uma grande ampliação na região de trabalho.

Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-05-08T20:28:10Z No. of bitstreams: 1 DelcicleideReis.pdf: 3837410 bytes, checksum: d063417815b0c49287d6fdb8d46998bd (MD5) / Made available in DSpace on 2017-05-08T20:28:10Z (GMT). No. of bitstreams: 1 DelcicleideReis.pdf: 3837410 bytes, checksum: d063417815b0c49287d6fdb8d46998bd (MD5) Previous issue date: 2016-07-29 / Fundação de Amparo à Pesquisa e ao Desenvolvimento Científico e Tecnológico do Maranhão (FAPEMA) / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPQ) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Structural, magnetic and magnetocaloric properties of the RMn2Si2 compounds (R = Tm, Dy and Tb) were studied. X ray diffraction analysis and structure Rietveld refinement shows that the samples crystallize with the desired tetragonal I4/mmm structural phase. Magnetization measurements shows second order magnetic phase transition from paramagnetic (PM) to ferromagnetic (FM) state for TmMn2Si2 compound around T = 5.2 K. On the other hand, DyMn2Si2 and TbMn2Si2 compounds present multiple phase transitions below 100 K. DyMn2Si2 present four magnetic phase transitions while TbMn2Si2 present two magnetic phase transition. Both compounds present first order FM phase transitions originated from ordering of rare earth sub-lattice. In addition, DSC measurements indicated that DyMn2Si2 and TbMn2Si2 compounds present high temperature phase transition at T~ 425 K and 444 K associated to the AFM coupling in the Mn sub-lattice. Magnetic hysteresis loop was used to characterize exchange bias effect in the DyMn2Si2 observed for magnetic fields higher than 35 kOe in which was associated to interactions between AFM and FM magnetic domain present in this compound. The largest magnetocaloric effect (MCE) was observed for TmMn2Si2 compound, when compared with other studied compound. The maximum values of entropy variation change −∆𝑆𝑀𝑚á𝑥 and the Relative Cooling Power (RCP) found for TmMn2Si2 were, respectively, 18.5 J/kg.K and 247.5 J/kg for a magnetic field change (H) of 50 kOe. Significant values of −∆𝑆𝑀𝑚á𝑥(~8.2 J/kg.K and ~9.7 J/kg.K@50 kOe, respectively) and RCP (124.6 J/kg and 233 J/kg@50 kOe, respectively) as well as successive magnetic phase transitions were observed for DyMn2Si2 and TbMn2Si2 compounds. Also these two compound exhibits a table like EMC presenting a wide working window for practical applications. The results obtained for compounds motivate the preparation of a composite sample with the following concentrations 10% de TmMn2Si2, 15% de HoCoSi, 35% de DyMn2Si2 e 40% of TbMn2Si2, aiming further increase in the temperature range of maximum EMC. The maximum entropy change variation obtained for the composite sample was ~4.6 J/kg.K over a temperature range of ~80𝐾. Our results show that the compounds RMn2Si2 present important characteristics for application in magnetic refrigeration for cryogenic temperatures. Besides, it is possible to get a larger working region, when these compounds are associated forming a composite material. / Neste trabalho foi realizado um estudo das propriedades estruturais, magnéticas e magnetocalóricas dos compostos da série RMn2Si2 (R = Tm, Dy e Tb). A análise estrutural por difração de raios X e refinamento dos difratogramas pelo método de Rietveld mostraram que as amostras cristalizam na fase tetragonal grupo espacial I4/mmm. Com a medida de magnetização foi possível observar a transição de fase magnética de segunda ordem do estado paramagnético (PM) para ferromagnético (FM) para o composto TmMn2Si2 em torno de 5,2 K. Enquanto que para os compostos DyMn2Si2 e TbMn2Si2 observou-se a presença de transições múltiplas abaixo de 100 K. O DyMn2Si2 apresentou quatro transições de fase magnética enquanto que TbMn2Si2 apresentou duas. Em ambos os compostos, as transições ferromagnéticas atribuídas ao ordenamento da sub-rede da terra rara são de primeira ordem. Medidas DSC indicaram que DyMn2Si2 e TbMn2Si2 apresentam uma transição de fase em T~425 K e 444 K, respectivamente, ambas relacionadas ao acoplamento antiferromagnético na sub-rede do Mn. Com as medidas do loop de histerese magnética foi possível caracterizar o efeito de Exchange Bias (EB) para o DyMn2Si2, em campos magnéticos superiores a 35 kOe, o qual foi atribuído a interações entre os domínios AFM e FM presentes no material. A caracterização das propriedades magnetocalóricas do composto TmMn2Si2 mostrou uma variação de entropia magnética (-∆SM) mais intensa quando comparada aos outros compostos deste estudo. Os valores máximos obtidos para a variação isotérmica da entropia (−∆𝑆𝑀𝑚á𝑥) e para o poder de resfriamento relativo (RCP) foram, respectivamente 18,5 J/kg.K e 247,5 J/kg, para ∆𝐻 = 50 kOe. Já os compostos DyMn2Si2 e TbMn2Si2 mostraram valores significativos de −∆𝑆𝑀𝑚á𝑥 (~8,2 J/kg.K e ~9,7 J/kg.K@50 kOe, respectivamente) e RCP (124,6 J/kg e 233,6 J/kg@50 kOe, respectivamente) além de duas transições de fase magnética sucessivas, o que resultou em um EMC com comportamento do tipo table-like, caracterizado por uma ampla janela de trabalho com máximo EMC. Estes resultados motivaram a preparação de um compósito com as seguintes concentrações 10% de TmMn2Si2, 15% de HoCoSi, 35% de DyMn2Si2 e 40% de TbMn2Si2, visando ampliar ainda mais o intervalo de máximo EMC. A variação máxima de entropia obtida para o compósito foi da ordem de 4,6 J/kg.K num intervalo de temperatura de ~80𝐾. Os resultados obtidos sugerem que os compostos desta série apresentam características importantes para aplicação na refrigeração magnética em temperaturas criogênicas. Além disso, quando estes estão associados na forma de um compósito é possível obter uma grande ampliação na região de trabalho.

Submitted by Maria Aparecida (cidazen@gmail.com) on 2017-05-17T13:28:23Z No. of bitstreams: 1 Geanso Miranda.pdf: 3684473 bytes, checksum: 4d358200b337e74396e2c65531625ec1 (MD5) / Made available in DSpace on 2017-05-17T13:28:23Z (GMT). No. of bitstreams: 1 Geanso Miranda.pdf: 3684473 bytes, checksum: 4d358200b337e74396e2c65531625ec1 (MD5) Previous issue date: 2015-05-28 / FAPEMA / In this work, L-Histidine hydrobromide monohydrate (C6H12N3O3Br) (LHHBr) and L-Histidine hydrochloride monohydrate (C6H12N3O3Cl) (LHHCl) crystals are studied by Thermogravimetric Analysis (TGA), Differential Thermal Analysis (DTA), Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD) and Raman Spectroscopy as a function of the temperature. The thermal analysis experiments (TGA, DTA and DSC) were performed between 30 and 500 °C, and XRD experiments were performed between 30 and 190 °C due to melting point. It has been observed that LHHBr is stable up to 100 °C and undergoes a structural phase transition between 110 and 130 °C, whereas LHHCl is stable up to 130 °C and undergoes a structural phase transition between 135 to 150 °C. The Le Bail method was used to determine the samples structure after the phase transition (orthorhombic-monoclinic) as a function of the temperature. Moreover, it was possible to determine the lattice parameters of the new phase. For LHHBr, where obtained lattice parameters: a= 12,159(1) Å, b= 16,755(2) Å, c= 19,278(9) Å, β= 108,49(6)°. For LHHCl the unit cell dimensions were obtained: a= 10,775(6) Å, b= 15,340(9) Å, c= 21,699(6) Å, β= 102,20(3)°. Through the XRD measurements as function of temperature was also possible to determine the expansion coefficient of orthorhombic phase. The expansion coefficients obtained for the LHHBr along the crystallographic axes were: α[100]= 6,41(5) x 10-6 °C-1, α[010]= 28,7(6) x 10-6 °C-1 and α[001]= 52,2(9) x 10-6 °C-1. For LHHCl the expansion coefficient obtained for the orthorhombic phase in the crystallographic axes were: α[100]= 4,92(3) x 10-6 °C-1, α[010]= 32,08(2) x 10-6 °C-1 and α[001]= 39,60(6) x 10-6 °C-1. The thermal expansion results have shown an anisotropic expansion. The analysis shown that the transitions are due to loss of structural water and the difference in thermal stability can be attributed to factors such as: high number of hydrogen bonds in the LHHCl crystal, more stable bonds due the higher electronegativity and smaller atomic radius compared chloride for bromide atom inside the unit cell. / Neste trabalho, cristais de L-Histidina hidrobromídrica monohidratada (C6H12N3O3Br) (LHHBr) e L-Histidina hidroclorídrica monohidratada (C6H12N3O3Cl) (LHHCl) foram estudados por Termogravimetria (TGA), Análise Térmica Diferencial (DTA), Calorimetria Exploratória Diferencial (DSC), Difração de raios-X (DRX) e Espectroscopia Raman variando-se a temperatura como parâmetro termodinâmico. Os experimentos de análise térmica (TGA, DTA e DSC) foram realizados entre 30 e 500 °C, enquanto que os experimentos de DRX foram realizados entre 30 e 190 °C devido ao ponto de fusão das amostras. Dos experimentos realizados, observou-se que a LHHBr é estável até 100 °C e sofre uma transição estrutural entre 110 e 130 °C, enquanto que a LHHCl é estável até 130 °C e sofre uma transição estrutural entre 135 e 150 °C. Através do método Le Bail foi possível determinar a estrutura das amostras após a transição de fase (ortorrômbica-monoclínica) em função da temperatura. Os parâmetros de rede da nova fase foram determinados sendo para a LHHBr a= 12,159(1) Å, b= 16,755(2) Å, c= 19,278(9) Å, β= 108,49(6)° e para LHHCl a= 10,775(6) Å, b= 15,340(9) Å, c= 21,699(6) Å, β= 102,20(3)°. Através das medidas de DRX em função da temperatura também foi possível determinar o coeficiente de dilatação para a fase ortorrômbica. O coeficiente de dilatação obtido para a LHHBr nos eixos cristalográficos foram: α[100]= 6,41(5) x 10-6 °C-1, α[010]= 28,7(6) x 10-6 °C-1 e α[001]= 52,2(9) x 10-6 °C-1. Para a LHHCl o coeficiente de dilatação obtido para a fase ortorrômbica nos eixos cristalográficos foram: α[100]= 4,92(3) x 10-6 °C-1, α[010]= 32,08(2) x 10-6 °C-1 e α[001]= 39,60(6) x 10-6 °C-1. Os resultados de expansão térmica obtidos evidenciam uma dilatação anisotrópica. As análises mostraram que as transições são atribuídas à perda de água estrutural e a diferença na estabilidade térmica deve-se aos seguintes fatores: maior número de ligações de hidrogênio na amostra de LHHCl, ligações mais estáveis devido à maior eletronegatividade e menor raio atômico do átomo de cloro em comparação ao átomo de bromo no interior da célula unitária.

1,3,5-tricarboxamida-ciclo-hexano são compostos capazes de se autoagregarem formando colunas supramoleculares as quais se mantêm unidas não só devido às interações das cadeias laterais mas também devido às ligações de hidrogênio de cada um dos três grupos amida por monômero. Cada monômero possui momento de dipolo elétrico associado aos grupos amida. Quando as amidas dos vários monômeros dentro da mesma coluna estão apontadas para a mesma direção, os momentos de dipolo individuais de todas as amidas se somam formando elevado dipolo ao longo do eixo da coluna, chamado de macrodipolo, o qual influencia as interações intercolunares. Neste trabalho foram investigadas quatro conformações as quais diferem entre si em relação à orientação dos grupos carbonila: a conformação Up-Up contém grupos carbonilas paralelos dentro das colunas e colunas paralela, a conformação Up-Down possui grupos carbonilas paralelos dentro das colunas e colunas antiparalelas, a conformação Intra-Up-Up contém grupos carbonilas antiparalelos dentro das colunas e colunas paralelas e a conformação Intra-Up-Down possui grupos carbonilas antiparalelos dentro das colunas e colunas antiparalelas. Foi usado Dinâmica Molecular Clássica para investigar o efeito das interações macrodipolo-macrodipolo das quatro diferentes conformações sobre a estabilidade térmica de três diferentes compostos derivados de 1,3,5-tricarboxamida-ciclo-hexano. Foi verificado que as conformações com colunas antiparalelas tendem a ser ligeiramente mais estáveis do que as conformações com orientação paralela. O efeito da orientação dos grupos carbonila dentro das colunas sob a estabilidade do material está relacionado a vários fatores, tais como cargas atômicas parciais, arranjo colunar ou natureza das cadeias laterais, e os resultados não são tão diretos como quando se compara as orientações entre colunas. Outro tópico investigado foi o comportamento do material durante a transição da fase colunar para a fase desordenada. As colunas podem se desmontar em três diferentes formas: elas podem completamente se desintegrar rapidamente, podem primeiro se desintegrar lentamente e então perder a ordem colunar ou primeiro perdem a ordem colunar e então se desmontam em um processo demorado. Tais comportamentos estão associados com as interações dentro e entre colunas. / 1,3,5-cyclohexanetricarboxamides are self-assembling compounds able to form supramolecular columnar aggregates which are stacked together not only due to the interaction between the side chains, but also due to the hydrogen bonds of each of three amide groups per monomer. Each monomer possesses an electrical dipole moment related to the amide groups. When the amide groups of various monomers within the same column are pointed in the same direction, the individual dipole moments of all amides add up, resulting in a high dipole along the column axis, called macrodipole, which plays an important role in intercolumnar interaction. In this work, four types of conformations, differing from each other in the orientation of the carbonyl groups, were investigated: Up-Up conformation contains parallel carbonyl groups inside the columns as well as parallel columns, Up-Down conformation has parallel carbonyl groups inside the columns and antiparallel columns, Intra-Up-Up conformation contains antiparallel carbonyl groups inside the columns and parallel columns, and the Intra-Up-Down conformation has antiparallel carbonyl groups inside the columns as well as antiparallel columns. Classical Molecular Dynamics was used to investigate the effect of macrodipole-macrodipole interactions of the four different conformations on the thermal stability of three different 1,3,5-cyclohexanetricarboxamide derivatives. The research shows that antiparallel column conformations tend to be slightly more stable than parallel ones. The effect of the orientation of carbonyl groups within columns is related to a variety of factors, such as partial atomic charges, columnar arrangement or nature of the side chain, and results are not as straightforward as when looking at inter-column orientation. A second issue investigated was the behavior of the material during the phase transition. Columns can break apart in three different ways: they can fully disintegrate quickly, disassemble more slowly losing columnar order as they go, or first lose their columnar order and then break into smaller pieces in a slow process. Such behaviors are related to interactions inside and between columns.

Apresentamos o estudo de três modelos estocásticos governadas por equações mestras que descrevem a propagação de epidemias em uma comunidade de indivíduos; / We present a study of three stochastic models, governed by master equations, that decribe the epidemic spreding in a community of individuals;

Orientador: Makoto Yoshida / Resumo: Apresenta-se um estudo das transições de fase de um material ferromagnético representado pelo modelo Blume-Capel. A investigação é realizada através da teoria de campo médio implementada através da aproximação de Bethe-Peierls. Como tarefa preliminar é proposta uma revisão detalhada da aproximação de Weiss para investigação dos fenômenos críticos de sistemas magnéticos. Nesta etapa, tanto o modelo de Ising quanto o modelo Blume-Capel são considerados. Em seguida, uma revisão do modelo de Ising através da aproximação de Bethe-Peierls, tida como mais precisa, também é realizada e de posse da experiência adquirida, o modelo Blume-Capel é detalhadamente investigado. / Abstract: The study of the phase transition of Blume-Capel ferromagnet is carried out by means of Bethe-Peierls approximation. A detailed review of 2D Ising model and the Weiss/Bethe-Peierls mean field theory is presented as the preliminar task. This is followed by a review of Blume-Capel model and finally by the investigations of its critical phenomena in the Bethe-Peierls approximation. / Mestre

<p>The present thesis deals with the application of <i>ab-initio</i> electronic structure calculations based on density functional theory for material design.</p><p>The correlation between magnetic properties and electronic structures has been investigated in detail for diluted magnetic semiconductors (DMS), which have promising application for spintronics devices. The point defects, acting as electron donor or electron acceptor, have been studied for their key role in mediating the long ranged ferromagnetic interaction between transition metal (TM) ions. The electron holes induced by electron acceptor are completely spin polarized in semiconductor and exhibit a very significant efficiency to the ferromagnetic coupling between TM ions. While the electron donor shows a negative effect to the ferromagnetism in the system. The common trend of magnetic interaction and electron charge transfer between TM ions and electron acceptors or electron donators have been outlined. The Coulomb correlation <i>U</i> of <i>d</i> electrons, which could change the energy levels of TM <i>d</i> band respective to the host semiconductor band, also shows a significant influence on the magnetic behavior in DMS. </p><p>The crystallography phase transition under high pressure has also been studied for the iron doped with light element, carbon. Our calculated results show that interstitial carbon defect has little effect on the iron's bcc to hcp phase transition under high pressure. The other carbon iron phases, like Fe₃C, has also been studied in a wide pressure range.</p><p>We also present a first-principles description on the temperature dependence of elastic constant for solids. The total temperature effects are approximated as a sum of two separated parts, the thermal expansion contribution, which gives the normal linearly decreasing effect on the elastic constant with increasing temperature, and the electronic band contribution, which could lead anomalous behavior for thermal elastic constants.</p>

Oxide materials exhibit a wide variety of structures and properties. In particular, transition metal oxides tend to be highly stable while exhibiting a wide range of properties that can be used for numerous applications. This work focuses on investigating how the structures��� of 4d and 5d transition metal oxides influences their properties. Specifically oxides of Ru, Rh and Ir were investigated. A complete solid solution was found between isostructural Pb���Mn���O������ and Pb���Rh���O������. Pb���Rh���O������ shows a Verwey-type transition at 185 K. This transition remains with a 3 % substitution of Mn for Rh but disappears with a 4 % substitution of Mn for Rh. The structure was found to expand in the direction perpendicular to the layers of the structure, which is the c-axis, despite a contracting unit cell. Bi for Pb substitution in Pb���Mn���O������ was found to be limited as compared to in Pb���Rh���O������. Alkali metal substitution on the A-site of the orthorhombic perovskite SrRuO��� showed only low substitution levels were possible. Nonetheless, the substituted phases showed decreased ferromagnetic Curie temperatures, increased electrical resisitivity and relatively unchanged Seebeck coefficients. Thermoelectric studies of Sr[subscript 2-x]La[subscript x]CoRuO��� perovskite phases showed Sr���.���La���.���CoRuO��� with the best thermoelectric performance. This system showed possible correlations between cation ordering on the B-site and the charge carrier transport. A similar thermoelectric study of (RhV)[subscript 1+x]Ti[subscript 1-2x]O��� phases crystallizing in a disordered trirutile structure was done. Electron carriers were found to be dominant and dependent on Ti content. The electron carriers appear to become diminished at higher temperatures. Sr���IrO��� crystallizes in a K���NiF���-type structure. Effects of Ti, Fe and Co substitution for Ir were investigated. A complete Sr���Ir[subscript 1-x]Ti[subscript x]O��� solid solution was synthesized and characterized while limited solubility was found for Fe and Co substitutions. All substitutions showed a decrease in the c-cell parameter coupled with a decrease in octahedral tilting. All substitutions also showed a decrease in magnetic susceptibility and an increase in the paramagnetic effective moment was observed for Co and Fe doped samples. An incomplete solid solution was formed for Sr���Ti[subscript 1-x]Rh[subscript x]O��� phases; however effects of increased octahedral tilting with higher Rh content were observed. / Graduation date: 2013

<p>In this thesis we report on the metal-mediated-thermally induced changes of the structural and optical properties of hydrogenated amorphous silicon deposited by hot-wire CVD, where aluminium and nickel were used to induce crystallization. The metal-coated amorphous silicon was subjected to a thermal annealing regime of between 150 and 520&deg / C. The structural measurements, obtained by Raman spectroscopy, show partial crystallization occurring at 350 &deg / C. At the higher annealing temperatures of 450&deg / C and 520&deg / C complete crystallization occurs. Reflection and transmission measurements in the UV-visible range were then used to extract the optical properties. By adopting the effective medium approximation a single optical model could be constructed that could successfully model material that was in different structural phases, irrespective of metal contamination. Changes in the absorption of the material in various stages of transition were confirmed with a directly measured absorption technique, and the modelled absorption closely followed the same trends This study forms part of the larger overall solar cell research project, of which the primary aim is to eventually develop a silicon solar panel that optimises the characteristics for best performance.</p>

No description available.