No description available.

Mixed valence manganites with the perovskite structure R1-xAxMnO3 (where R = La, Nd, Pr and A = Ba, Ca, Sr, Pb) have been a popular subject of contemporary research because of their interesting physical properties such as competing magnetic orders, metal-insulator transitions and colossal magnetoresistance. A complex interplay between structure, electronic and magnetic properties results in rich phase diagrams involving various metallic, insulating and magnetic phases. A review of the literature related to rare-earth managnites clearly reveals that the systems with Pb as a divalent dopant are relatively less explored. This may be due to the volatile nature of lead based compounds which are used as precursors for preparing these systems. This has motivated us to take up research on Pb doped rare earth manganites. This thesis is divided into eight chapters. The first introductory chapter gives a brief review of the work on manganites which have already been reported in the literature following which the motivation for carying out the present investigation is given. The second chapter deals with technical details of various instruments used in the present reasearch work. The third chapter is related to growth of single crystals, their preliminary characterization, magnetization and resistivity studies. Single crystals of Pr1-xPbxMnO3 are grown by flux technique for different compositions. Crystals are characterized by energy dispersive x-ray analysis (EDAX) and inductively coupled plasma atomic emission spectroscopy (ICPAES) for compositional analysis. Magnetization and resistivity studies are carried out on Pr1-xPbxMnO3 for three compositions viz. x = 0.2, 0.23 and 0.3. The magnetization vs. temperature plots show that all the three compositions undergo a transition from paramagnetic to ferromagnetic state. The magnetization in the low temperature ferromagnetic region obeys Bloch`s law. The susceptibility in the paramagnetic region is fitted to Curie Weiss law. Deviation of susceptibilty from Curie Weiss law, a feature observed in all the three crystals has been attributed to formation of ferromagnetic clusters at ~ 250 K. The cluster formation has its implications on all other properties in the temperature range from TC to 250 K where TC is the magnetic transition temperature. Resistivity measurements are carried out on the same three compositions. The x = 0.2 and 0.23 compositions undergo a transition from paramagnetic insulating to ferromagnetic insulating phases. The x = 0.3 composition shows a metal – insulator transition at nearly 35 K above TC. Chapter 4 describes the critical behaviour of Pr1-xPbxMnO3 for two compositions, viz. x = 0.23 and 0.3. For critical studies, magnetization vs. field measurements are carried out in the temperature range TC ± 10 K. Using modified Arrott plots and Kouvel-Fisher method the critical exponents and precise value of TC are obtained. The x = 0.23 composition shows results which indicate a conventional second order phase transition shown by a 3D Heisenberg ferromagnet. It also obeys the universal scaling behaviour. However, the x = 0.3 composition shows deviation from this behaviour. Hence to probe further into the nature of magnetic transition of this compound the effective critical exponents are calculated as a function of reduced temperature ε (=(T-TC)/TC). Based on the behaviour of effective exponents the nature of the transition in the x = 0.3 composition is described in detail. The unconventional ordering is attributed to presence of possible magnetic frustration in the system. In chapter 5 the resistivity and magnetoresistance behaviour of the x = 0.23 and 0.3 crystals are discussed. Initially the nature of plots of temperature and field variation of resistivity are described for both the cases. Detailed measurements are carried out at the magnetic transition region. The analysis is carried out in terms of critical scattering behaviour at the transition region. The zero field resistivity is analyzed in terms of theory of Fisher and Langer, while the magnetoresistance is fitted to scaling theory at the critical region developed by Balberg and Helman. It is seen that the x = 0.23 crystal shows a critical behaviour in resistivity for zero field as well as in magnetoresistance close to TC. However, the behaviour of the x = 0.3 composition is more complex. A simpler critical scattering theory alone cannot explain its large negative magnetoresistance. Chapter 6 contains the EPR studies on the x = 0.23 and 0.3 compositions. Analysis is carried out in the paramagnetic region. The EPR signals are fitted to a modified Dysonian equation. The intensity, linewidth, and asymmetry parameter are obtained as a function of temperature from fitting. The parameters are obtained till 210 K for both compositions. The intensity is fitted to a Curie Weiss law. The linewidth shows a “bottleneck” mechanism and is proportional to conductivity. Hence it is fitted to activated behaviour. In addition, a secondary signal develops at low fields from 240 K and is present till 200 K in both the compositions. This is explained by means of phase separation. In chapter 7 the specific heat of the x = 0.23 and 0.3 compositions are discussed. The measurements are carried out from 2 to 300 K in zero field and also in the presence of 3 Tesla magnetic field. The analysis is carried out in two separate sections. The first section deals with the low temperature analysis from 2 to 80 K where apart from the usual lattice, electron and magnetic terms, presence of Schottky anomaly is also discussed. The Schottky peak occurs at a relatively higher temperature of around 40 K. Due to presence of higher order lattice terms the Schottky effect is not easily discernible. It is extracted only from fitting. In the second section, the specific heat associated with ferromagnetic – paramagnetic transition is extracted. The lattice term in the entire temperature range from 10 to 300 K except at the transition region is fitted to Einstein function. The magnetic specific heat is obtained by subtracting the Einstein specific heat from the total specific heat. The change in entropy due to magnetic transition is also calculated for both compositions. In chapter 8 the general conclusions derived from the work presented in this thesis are summarized along with the scope for future work in this system.

Semiconductor nanostructures have attracted great interest owing to their unique physical properties and potential applications in nanoscale functional devices. The enhancement of the physical properties of semiconductor nanostructures and their performance in devices requires a deeper understanding of their fundamental microstructural properties. Thus this thesis is focused on the experimental and theoretical studies of the microstructural properties of two important semiconductor nanostructures: axial heterostructured silicon nanowires with varying doping and indium nitride colloidal nanoparticles. In this thesis, axial heterostructured silicon nanowires with varying doping were synthesized on an oxide-removed Si{111} substrate using a vapour-liquid-solid approach. Their fundamental microstructural properties, including the crystalline structure, wire growth direction and morphologies, were studied using various characterization techniques. It is found that a very small fraction of the silicon nanowires crystallize in a hexagonal (wurtzite) phase, which is thermodynamically unstable in bulk silicon under ambient conditions, while a large majority of the synthesized silicon nanowires exhibit the expected diamond cubic crystalline structure. About 75% of the diamond cubic silicon nanowires synthesized grow in a single <111> direction, while the rest contain growth-related kinks, where the nanowire switches to another direction during the growth. The ~109° silicon nanowire kinks are the most commonly observed, and the growth direction before and after such ~109° kink are both <111>. The sidewalls of silicon nanowires do not change abruptly at the ~109° kink, but exhibit an elbow-shaped structure. It is also found that the nanowire sidewalls exhibit periodic nanofaceting, which is strongly doping-dependent. The nanofaceting is found to occur during the enhanced sidewall growth that arises when the diborane dopant gas is introduced. A thermodynamic model predicting the dependence of nanofacet period on the wire diameter is developed. Another semiconductor nanostructure studied in this thesis is indium nitride colloidal nanoparticles, which were grown using a solution-phase chemical method. The formation of such indium nitride colloidal nanoparticles is confirmed by studying their compositions, crystalline structures and shape using various electron microscopy techniques. The size of the indium nitride colloidal nanoparticles was controlled by varying the time of solution-phase reactions. The most probable size of the colloidal nanoparticles increases and the size distribution broadens with the increase of reaction time. The crystalline structures of the indium nitride colloidal nanoparticles are found to be particle size dependent. The observed dependence of the band gap blueshift of the indium nitride colloidal nanoparticles on the reaction time (hence the particle size) is explained by the quantum-size effect.

The behaviour of nitrogen in silicon is investigated using the dislocation unlocking technique. Specimens containing well-ordered arrays of dislocations are isothermally annealed for a controlled duration, during which nitrogen segregates to and pins the dislocations. The stress required to unlock the dislocations is then measured by three-point bending at elevated temperature. By analysing the dependence of this unlocking stress on anneal duration and temperature, information about nitrogen's transport and interaction with dislocations can be deduced. Experiments are performed at anneal temperatures of 500 - 1050C using float-zone silicon with [N] = 2x10^15 cm^-3. The results are analysed to give an expression for nitrogen's effective diffusivity of D = 173,000 exp(-3.24eV/kT)cm^2 s^-1 in the 500 - 750C range, showing for the first time that nitrogen transport at low temperatures behaves in the same way as measured at higher temperatures by other groups using secondary ion mass spectrometry. If analysed in terms of monomer-dimer dissociative transport, the results give a nitrogen monomer diffusivity of D_1 = 28 exp(-(1.1 to 1.4 eV)/kT) cm^2 s^-1, which is similar to that found by another analysis in the literature. The measurements also show that nitrogen's dislocation locking strength measured at 550C is dependent on anneal temperature, peaking at 600 - 700C and falling towards zero above 1000C. The dislocation unlocking technique itself is also investigated and characterized. It is found that the measured unlocking stress is dependent on the three-point bend duration, falling with increasing duration. Analysis of these results in terms of the theory of release of dislocations from pinning points indicates that nitrogen dislocation locking is likely to be by an atomic species. This effect also has implications for the results of previous nitrogen dislocation unlocking experiments, and the technique has been modified so that a standardised set of conditions is used for every test. Other measurements show that nitrogen's dislocation locking effect is lessened by the presence of transition metal contamination, and that dislocation velocity in silicon may be affected by the nitrogen present in the material. A modified dislocation unlocking technique is developed to measure dislocation locking from near-surface ion-implanted impurities. Results from heavily N-implanted silicon show that nitrogen implantation can provide additional dislocation locking strength to that already given by the oxygen in the material. The scale of the dislocation locking effect in these experiments may provide evidence that nitrogen's effective diffusivity is reduced at high concentrations, indicating that nitrogen transport may be by a dissociative mechanism.

Thermally induced phase transformation in NiTi shape memory alloys (SMA) shows strong size and shape, collectively termed length scale effects, at the nano to micrometer scales, and that has important implications for the design and use of devices and structures at such scales. This paper, based on a recently developed multiscale model that utilizes molecular dynamics (MD) simulations at small scales and MD-verified phase field (PhF) simulations at larger scales, reports results on specific length scale effects, i.e. length scale effects in martensite phase fraction evolution, transformation temperatures (martensite and austenite start and finish) and in the thermally cyclic transformation between austenitic and martensitic phase. The multiscale study identifies saturation points for length scale effects and studies, for the first time, the length scale effect on the kinetics (i.e. developed internal strains) in the B19 phase during phase transformation. The major part of the work addresses small scale single crystals in specific orientations. However, the multiscale method is used in a unique and novel way to indirectly study length scale and grain size effects on evolution kinetics in polycrystalline NiTi, and to compare the simulation results to experiments. The interplay of the grain size and the length scale effect on the thermally induced martensite phase fraction (MPF) evolution is also shown in this present study. Finally, the multiscale coupling results are employed to improve phenomenological material models for NiTi SMA.

Nos últimos 30 anos o crescimento paralelo das Ciências de Superfície tradicionais, em ambiente de ultra-alto vácuo (UHV), com a Eletroquímica levou ao nascimento de um novo campo interdisciplinar: Física de Superfície e Eletroquímica. Técnicas de ambas as áreas dão informações complementares e assim, quando realizadas em conjunto podem fornecer muitas respostas em nível atômico, estrutural e eletrônico quando o eletrodo está em contato com a solução eletrolítica. A intenção primordial dessa Dissertação foi o estudo fundamental das fases ricas em oxigênio presentes na superfície de Ru(0001) através de caracterizações eletroquímicas e morfológicas utilizando um sistema que permitiu o acoplamento de uma célula eletroquímica miniatura de fluxo a câmaras de UHV. Inicialmente exibi-se a modificação e a construção de equipamentos necessários para a preparação do sistema binário Au-Pt(111) e do óxido monocristalino Ru2O(110). Imagens de STM em escala morfológica mostraram o crescimento anisotrópico do filme de RuO2(110) sobre um substrato monocristalino de Ru(0001). Resultados obtidos através da técnica de Voltametria Cíclica na eletrooxidação de CO em RuO2(110) corroboraram cálculos teóricos sobre a estrutura da superfície quando esta em ambiente úmido. Superfícies modelos baseadas em ouro, crescido epitaxialmente sobre um substrato de Pt(111), foram preparadas no sistema de UHV. Dados eletroquímicos foram correlacionados às composições superficiais destas, mostrando o efeito do substrato prevalecendo sobre o efeito eletrônico. / In the last 30 years the parallel growth of the traditional Surface Science, under UHV environment, and Electrochemistry gave rise to a new interdisciplinary field: Surface Science and Electrochemistry. Techniques from both sciences give complementary information. Thus, in tandem, they are able to elucidate many atomic, structural and electronic phenomena, of an electrode in contact with a solution. The main goal of this Dissertation was the fundamental study of the Oxygen-rich Ru(0001) surface through electrochemical and morphologic characterizations using a coupled system which allowed the attachment of a miniature flow cell to UVH-chambers. Initially it is shown the construction and modifications of required equipments for the preparation of the binary system Au-Pt(111) and single crystal RuO2(110) oxide. Attainable morphological STM images demonstrated the anisotropic growth of the RuO2(110) over a Ru(0001) substrate. Results of the electrooxidation of CO on RuO2(110), obtained by means of Cyclic Voltammetry, corroborated theoretical calculations concerning the oxide superficial structure in a humid environment. Model surfaces based on Au, epitaxialy grown on a Pt(111) substrate, were prepared under UHV conditions. Electrochemical data and superficial composition were correlated, confirming that the substrate effect overcomes electronic strain effects.

Utilizando a técnica de ataque químico e microdureza realizamos um estudo sistemático dos defeitos estruturais e da microplasticidade em monocristais pertencentes ao sistema (BSCC) e em monocristais de LiNbO3. Desenvolvemos uma solução apropriada com HNO3 e H2O nas proporções de 1ml:30ml durante 20 segundos para o ataque químico dos compostos do sistema BSCC e revelamos padrões de defeitos estruturais como deslocações, terráceos (terraceds) e agrupamentos (bunchs). Além disso, fizemos uma tentativa de identificar a composição real destes compostos usando as técnicas de microscopia ótica, difração de raio-x, Laue, fluorescência de raio-x, medidas de resistividade, microdureza e ataque químico. O comportamento plástico/frágil dos cristais de Niobato de Lítio foi estudado nos planos (00.1), (01.4) e (01.0). Analisamos também a distribuição das deslocações em torno das deformações plásticas produzidas por indentações de uma ponta Vickers nas superfícies de pólos de domínios positivo e negativo sobre plano (00.1). Alguns desses resultados, como no caso da revelação de defeitos estruturais e microdureza dos compostos do sistema BSCC, podem ser considerados inéditos. Os cristais de niobato de lítio crescidos em nossos laboratórios mostraram um comportamento frágil isotrópico não convencional para esse composto, quando submetidos a tratamentos térmicos / We used etching and microindentation techniques to study the structural defects, microplasticity and microhardness in Bi-Sr-Ca-Cu-O and LiNbO3 single crystals. To revel structural defects in Bi-Sr-Ca-Cu-O crystals was developed a suitable solution of HNO3 in the ratio 1:30. The results obtained showed that there are patterns of structural defects as dislocations, terraceds and bunches in these compounds. We also did an attempt to identify the actual compositions of these compounds using optical microscopy, x-ray diffraction, Laue pattern, X-ray fluorescence, resistivity, microhardness and etching techniques. In LiNbO3 single crystals we analyzed the dislocations distributions around the plastic deformations produced by indentation experiments. The behavior plastic/fragile in LiNbO3 crystals were studied on three different perpendicular directions: to the Z-axis, to the 128 and to the (01.0). Some of these results, as the structural defects revelation and the microhardness in Bi-Sr-Ca-Cu-O compounds as well, were obtained at the first time. Also, unusual fragile behavior isotropic of LiNbO3 grown in our laboratory was found

Realizamos crescimento de monocristal de Hexafluoreto de Fósforo e Potássio:KPF6 pelo método de crescimento em solução aquosa e o estudo das propriedades elétricas. Para isso foram feitas medidas, de solubilidade, tempo de incubação, espectroscopia dispersiva no infra-vermelho, difração de raios-X, condutividade elétrica e constante dielétrica no intervalo de temperatura de hélio líquido a 300&#176C. Foram detectadas três transições de fases estruturais em APROXIMADAMENTE 0&#176C, -14,4&#176C e 193&#176C e impurezas de CO-23, responsável pelo aumento da condutividade elétrica na região extrínsica. Na defermação do grupo PF6 durante a transição de fase estrutural em -14,4&#176C há surgimento de um dipolo elétrico com valor de 12,2x10-18ues.cm. E observamos o congelamento do grupo PF6 por volta de -41&#C / It was made the growth of a Potassium Hexafluophosphate: KPF6 monocrystal method of growth in aqueous solution and the study of its electrical properties. With this purpose, it was use of solubility, incubation time, dispersive espectroscopy on the infrared region, X-ray diffraction, electrical conductivity and dieletrical Constant in the range of temperature between liquid helium and 573K. It was detected three strutural phase transitions in 273K, 258,6K and 80K and impurities like CO-23. In the deformation of the PF6 group, during the structural phase transition in 258,6K, electrical dipole appears with a value 12,2.10-18ues.cm. It was observed the freezing of PF6 group near 232K

O presente trabalho mostra um estudo sobre a presença de pares de íons de CN- - Cu+ e OCN- -Cu+, em cristais de KCl, usando técnicas de medidas de corrente de despolarização termoestimulada (I.T.C.) e Absorção óptica. A detecção destes pares foi feita através de um estudo sistemática de correlação entre as diferentes amostras aqui usadas: KCl + 1% KCN; KCl + 1% CuCl; KCl + 1% KOCN; KCl + 1% KOCN + 1% CuCl e KCl + 1% KCN + 1% CuCl. Deve ser ressaltado que a última amostra apresenta um tipo de impureza não intencional OCN- , cuja associação com o íon Cu+ também é observada. Na absorção ótica observamos bandas à temperatura ambiente em 210, 228, 242, 252, 260 e 290 nm para o cristal de KCl + 1% CuCl + 1% KCN, sendo que para o cristal de KCl + 1% KOCN + 1% CuCl não foi observado o último pico. As diferentes bandas são atribuídas às possíveis interações entre Cu+ e CN-, bem como Cu+ e OCN-. Quanto à técnica de ITC, as curvas obtidas por esta, para os três últimos cirstais citados acima, foram melhor ajustadas pela somatória de duas curvas singulares de ITC, cujas temperaturas de pico estão nas posições de 53 e 55 K; 54,3 e 56,6 K; 53,3 e 56,1 K, respectivamente. Em KCl + 1% KOCN a curva de mais baixa temperatura (curva 1) de ITC é atribuída à molécula (16O12C14N-) e a curva de mais alta temperatura (curva 2) é atribuída à molécula isotrópica (18O12C14N-). Para as amostras de KCl + 1% KOCN + 1% CuCl a curva 1 foi atribuída ao OCN- isolado e a curva 2 ao Cu+ perturbado pela presença do OCN-. Quanto ao cristal de KCl + 1% KCN + 1% CuCl, as duas técnicas propostas não foram suficientes para identificar a presença de pares Cu+ - CN-, necessitando, desta maneira, da utilização de outra técnica experimental, como por exemplo a da luminescência. A correlação dos resultados experimentais foi feita através dos parâmetros determinados do momento de dipolo (p), energia de ativação (Ea&#8727) e a temperatura de pico (Tm&#8727) / The present work gives a study on CN- - Cu+ and OCN- -Cu+ coupled pairs doped KCl single crystal, by using Ionic Thermal Current (I.T.C.) and Optical absorption techniques. These pairs were detected through a systematic correlation study between different samples: KCl + 1% KCN; KCl + 1% CuCl; KCl + 1% KOCN; KCl + 1% KOCN + 1% CuCl and KCl + 1% KCN + 1% CuCl, were the last sample shows unporposely doping OCN-, compled with Cu+. The observed optical absorption bands, at room temperature, were in 210, 228, 242, 252, 260 and 290 nm for KCl + 1% CuCl + 1% KCN crystal. Only the last absorption band was not seen in KCl + 1% KOCN + 1% CuCl samples. Diferent bands are associated with possible interactions between Cu+ and CN-, as also Cu+ with OCN-. The ITC curves for the last three crystals were better fitted with two single ITC´s curves, whose temperature peaks are at 53 and 55 K; 54,3 and 56,6 K; 53,3 and 56,1 K, respectively. The lowest temperature ITC band (curve 1) in KCl + 1% KOCN is attributed to the (16O12C14N-) molecule and the highest temperature ITC band (curve 2) is due to the (18O12C14N-) isotropic molecule. For KCl + 1% KOCN + 1% CuCl samples curve 1 was associated with isolated OCN- and curve 2 with Cu+ perturbed with OCN-. With KCl + 1% KCN + 1% CuCl, the two techniques were not good enough to identify Cu+ - CN-, compling, it should be necessary to apply another type of techniques, like luminescence. The correlations of the experimental results were done through the determination electric dipole moment (p), activation energy (Ea&#8727) and temperature peak (Tm&#8727) parameters

No presente trabalho foi estudado catalisadores dispersos em carbono contendo Pt, PtRh e PtRhRu para eletroxidação de etanol. Os catalisadores foram preparados em ultra-som e caracterizados por EDX, DRX e MET e sua atividade catalítica estudada através das técnicas de voltametria cíclica e DEMS. Os resultados experimentais indicaram que a adição de ródio diminui o potencial de início de oxidação de etanol, embora também diminua a corrente faradáica. Os estudos de DEMS mostraram que com a presença de ródio há um aumento na produção de CO2. Devido a isso, estudos fundamentais em monocristais de Pt (111), Pt (110) e Pt (100) modificados com depósitos de Rh foram realizados. Os depósitos foram feitos pelo método eletroquímico e diferentes graus de recobrimentos foram estudados para a eletroxidação de etanol. A atividade catalítica foi avaliada por voltametria cíclica, cronoamperometria e FTIR in situ. Os resultados mostraram que quanto maior a quantidade de ródio na superfície menor é a corrente obtida na oxidação de etanol. Dentre os eletrodos monocristalinos estudados a Pt (100) apresentou melhor atividade para a oxidação etanol. Os estudos com monocristais corroboraram com os resultados obtidos para os catalisadores dispersos em carbono. / In this work, carbon dispersed catalysts containing Pt, PtRh and PtRhRu were studied for electroxidation of ethanol. The catalysts were prepared using ultrasound and characterized by EDX, XRD and TEM and its catalytic activity studied using the techniques of cyclic voltammetry and DEMS. Experimental results indicated that the addition of rhodium decreases the potential onset of ethanol oxidation, but also decrease the faradaic current. DEMS studies showed that the presence of rhodium promotes an increase in CO2 production. Because of this, fundamental studies on single crystal of Pt (111), Pt (110) and Pt (100) modified with Rh deposits were done. The deposits were made by electrochemical method and different degrees of coatings were studied for ethanol electroxidation The catalytic activity was evaluated by cyclic voltammetry, chronoamperometry and in situ FTIR. The results showed that the greater the amount of rhodium on the surface the lower the current obtained in the oxidation of ethanol. Among the electrodes studied monocrystalline Pt (100) showed the best activity for ethanol oxidation.