This thesis aims to investigate structural, optical and photoelectrochemical behavior of CdS nanowires and their heterojunctions with CdTe and polypyrrole nanowires. In the first part, CdS nanowires have been synthesized via electrochemical template-based route. It has been observed that synthesis conditions, such as bias voltage and deposition time, affect the morphology, optical and photoelectrochemical characteristics of CdS nanowires. Depending on the deposition time, length of the CdS nanowires changed from 100-200 nm to 3-4 m. Also the diameter of the nanowires increased with increasing the deposition time. Structure of the CdS nanowires has been confirmed by X-ray diffraction spectrometry and EDX analysis. Phototelectrochemical performances of the CdS nanowires have been changed dramatically with bias voltage and deposition time.In the second part of this thesis, CdTe nanostructures have been deposited on CdS nanowires. Change in optical and photoelectrochemical behavior of CdS nanowires after CdTe deposition has been investigated. Organic semiconductors and their composites with inorganic materials have been gaining attention due to tunable optical, electrical and magnetic properties. Also, ease of fabrication techniques, and therefore, low cost made these materials attractive for lots of applications including photovoltaic devices and flexible electronics. In the last part of this thesis, heterojunctions of CdS and Polypyrrole (Ppy) nanowires have been synthesized. Like CdS/CdTe heteronanostructures, first the CdS nanowires have been electrochemically deposited in anodized alumina template and then Ppy has been successfully deposited on CdS nanowires. In order to investigate the effects of polypyrrole synthesis conditions on CdS/Ppy heteronanostructures, CdS nanowire synthesis conditions have been kept constant. It has been observed that morphology and photoelectrochemical behavior of the Ppy nanowires has been affected from Ppy synthesis conditions. The photoelectrochemical performance changes of CdS/Ppy heteronanostructures have been also investigated in this part.

One of the aim of this study is to prepare conductive polymer nanocomposites of polypropylene to obtain better mechanical and electrical properties. Composite materials based on conductive fillers dispersed within insulating thermoplastic matrices have wide range of application. For this purpose, conductive polymer nanocomposites of polypropylene with nano dimentional conductive fillers like carbon black, carbon nanotube and fullerene were prepared. Their mechanical, electrical and thermal properties were investigated. Polypropylene (PP)/carbon black (CB) composites at different compositions were prepared via melt blending of PP with CB. The effect of CB content on mechanical and electrical properties was studied. Test samples were prepared by injection molding and compression molding techniques. Also, the effect of processing type on mechanical and electrical properties was investigated. Composites become semiconductive with the addition of 2 wt% CB. Polypropylene (PP) / Carbon Nanotube (CNT) and Polypropylene / Fullerene composites were prepared by melt mixing. CNT&rsquo / s and fullerenes were surface functionalized with HNO3 : H2SO4 before composite preparation. The CNT and fullerene content in the composites were varied as 0.5, 1.0, 2.0 and 3.0 % by weight. For the composites which contain surface modified CNT and fullerene four different compatibilizers were used. These were selected as TritonX-100, Poly(ethylene-block-polyethylene glycol), Maleic anhydride grafted Polypropylene and Cetramium Bromide. The effect of surface functionalization and different compatibilizer on mechanical, thermal and electrical properties were investigated. Best value of these properties were observed for the composites which were prepared with maleic anhydride grafted polypropylene and cetramium bromide. Another aim of this study is to built and characterize transistors which have polyethylene as dielectric layers. While doing this, polyethylene layer was deposited on gate electrode using vacuum evaporation system. Fullerene , Pentacene ve Indigo were used as semiconductor layer. Transistors work with low voltage and high on/off ratio were built with Aluminum oxide - PE and PE dielectrics.

Σκοπός της παρούσας Πτυχιακής Εργασίας είναι ανάπτυξη μιας διαδικασίας παρασκευής νανοσύνθετων εποξικής ρητίνης/ πολλαπλών νανοσωληνίσκων άνθρακα (MWCNT) σε ένα εύρος περιεκτικοτήτων από 0.1 έως και 1 % κατά βάρος (κ.β) MWCNT. Τα εμποτισμένων δοκίμια έναντι αυτών της καθαρής ρητίνης παρουσίασαν ενισχυμένες μηχανικές ιδιότητες όπως αντοχή σε εφελκυσμό και αυξημένο μέτρο ελαστικότητας. Η αύξηση αυτή μπορεί να αποδοθεί στο υψηλό λόγο μήκους /διαμέτρου καθώς και στην μεγάλη ελεύθερη επιφάνεια των νανοσωληνίσκων (CNTs). Επίσης από τα πειράματα δυναμικής ανάλυσης παρατηρήθηκε αύξηση της θερμοκρασίας υαλώδους μετάβασης με την αύξηση της περιεκτικότητας των CNTs. Στα πλαίσια της ίδιας εργασίας μελετήθηκαν οι ηλεκτρικές ιδιότητες καθώς και οι αισθητήριες ιδιότητες των MWCNT και εξερευνήθηκε η χρήση τους ως νανοαισθητήρες για την παρακολούθηση βλάβης στην εμποτισμένη εποξική ρητίνη. Για τον σκοπό αυτό πραγματοποιήθηκαν πειράματα φόρτισης-αποφόρτισης μονοαξονικού εφελκυσμού με ταυτόχρονη παρακολούθηση της ηλεκτρικής αντίστασης του δοκιμίου. Από την παραπάνω διαδικασία παρατηρήθηκε πως όσο μεγαλύτερη είναι η περιεκτικότητα CNTs στην ρητίνη, τόσο μεγαλύτερη είναι η ευαισθησία της ηλεκτρικής αντίστασης στις αλλαγές του εφαρμοζόμενου φορτίου. Σημειώνεται επίσης πως η εμποτισμένη σε CNTs εποξική ρητίνη παρουσιάζει ηλεκτρική αγωγιμότητα παρουσιάζοντας σε περιεκτικότητα 1% κ.β συμπεριφορά αγωγού, γεγονός που οφείλεται στην αγώγιμη φύση των CNTs. Από τις μετρήσεις ηλεκτρικής αγωγιμότητας παρατηρήθηκε πως το «κατώφλι» αγωγιμότητας επιτυγχάνεται σε περιεκτικότητα 0.3% κ.β MWCNT ενώ, επιβεβαιώνεται η ισχύς της θεωρίας «διήθησης» : σ~ (V-Vc)t δίνοντας τιμή «κρίσιμου» εκθέτη t ίση με 2.05. / The goal of the present study is the development of a manufacturing process of epoxy resin compounds with several multi-wall carbon nanotube (MWCNT) contents per weight. Enhanced mechanical properties of the doped specimens epoxy against the neat epoxy testpieces e.g. tensile strength and modulus of elasticity was achieved and attributed to the high surface area and high aspect ratio of the nanotubes. Moreover the dynamic properties of the nano-doped epoxy polymers were investigated and the relation of glass transition temperature with increasing CNT content was found to be inverse. Another goal of the present work was to use the electrical/sensing properties of MWCNTs as a nano-sensor for the damage detection within the doped matrix material. Therefore loading-unloading tensile tests were performed, along with on-line conductivity monitoring for the nano-doped epoxy polymers. It was noted that all the nano-doped samples were more sensitive to load changes and thus resistance changes. The higher the CNT content per weight was, the higher the sensitivity in load changes. The conductive nature of CNTs has produced conductive epoxy polymers, which exhibit “percolation threshold” at the content of 0.3% wt. MWCNT and enhanced sensing properties. The measurements of electrical conductivity confirm the validity of “percolation” theory: σ~ (V-Vc)t with the critical exponent t equal to 2.05.

In this study, the High Density Polyethylene (HDPE) and Low Density Polyethylene (LDPE) blends prepared by Plasticorder Brabender were strengthened by adding Carbon Black (CB). Blends were prepared at 190 &deg / C. Amounts of LDPE were changed to 30, 40, 50 and 60 percent by the volume and the percent amounts of CB were changed to 5, 10,15, 20 and 30 according to the total volume. Thermal and morphological properties were investigated by using Differential Scanning Calorimeter (DSC), Scanning Electron Microscope (SEM). Mechanical properties were investigated by tensile test and hardness measurements. Melt flow properties were studied by Melt Flow Index (MFI) measurements. Electrical conductivities were measured by four probe and two probe techniques. Temperature dependence of electrical conductivity was also studied. In general, it is observed that stress at break and MFI values decrease by the addition of CB / however, modulus and hardness increase. DSC results indicated that the crystallization of the polymer blend was decreased by the addition of CB. SEM results showed that the components were mixed homogenously. Increasing CB content increased electrical conductivity. Furthermore, by increasing the temperature, positive temperature coefficient behavior was observed which increases when CB content decreased.

An introduction to titanium minerals is provided along with a brief history of high tension roll (HTR) separation and its applications. Current HTR theory is critically reviewed and deficiencies in previous modelling work are identified. These problems are recognised as the likely reason why past models required empirical manipulation to reproduce observed separation behaviour. An updated theory of charge decay in a particle bed is developed for HTR applications. Particle bed interactions and their effects on electrostatic forces and discharge rates are examined, leading to a reviewed analysis of force balance. The theoretical contributions are incorporated into a new HTR model which takes a mechanistic approach to describing machine operation, as opposed to previous empirical-based work in the area. A simplified electrostatic field model is also developed. Atmospheric effects on separation behaviour are ignored in the modelling. Experimental separations are used to validate new model performance, where comparisons are specifically designed to test the theoretical developments on charge decay and force balance in a particle bed. The new model is also compared to the Dance (1992) HTR model, embedded in JKSimSand, the most recent HTR model with an empirical basis. An error analysis shows the new model performs as well as, or better, than JKSimSand in most respects. The fact that this is achieved with a mechanistic approach validates the new developments in particle bed charge decay and force theory, and sets the foundation for mechanistic HTR simulation.

Resumo: Neste trabalho foram estudadas as propriedades elétricas e dielétricas de um compósito à base de poliuretano derivado de óleo de mamona (PU) e negro de fumo (NF). As amostras de PU/NF foram preparadas com quantidades fixas de prépolímero e poliol (seguindo uma fração 10/7 em massa) e com diferentes quantidades de NF. Medidas de espectroscopia de impedância permitiram analisar as propriedades elétricas do compósito em campo elétrico alternado em diferentes temperaturas, enquanto que, o método de quatro pontas foi utilizado para calcular a condutividade dc das diferentes amostras. Neste trabalho, também, foi desenvolvido um modelo estatístico bidimensional de redes de resistores e capacitores que gera a estrutura morfológica e calcula a condutividade alternada do compósito PU/NF a partir de uma técnica de matriz de transferência proposto por Derrida . As curvas da permissividade real e imaginária em função da frequência para amostras de PU/NF com diferentes proporções de NF demonstraram o mesmo comportamento qualitativo, ou seja, possuindo maiores valores para baixas frequências e altas temperaturas e decrescendo com aumento da frequência. As medidas de permissividade dielétrica imaginária apresentaram picos de relaxação devido ao processo de polarização interfacial entre o negro de fumo e a matriz polimérica. No entanto, esses picos são mais pronunciados quando se utiliza o módulo elétrico, que minimiza os efeitos de condução dc. O modelo estatístico reproduziu bem os dados experimentais obtidos das medidas de espectroscopia de impedância. A partir dos ajustes teórico-experimentais pode-se observar que a condutividade elétrica para a amostra 99/01 ocorre predominantemente por condução eletrônica nas regiões isolantes, obedecendo ao modelo modificado de Abrahams e Miller. Entretanto, para as amostras 95/05 e 90/10 o modelo de Abrahams e... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: In this work we investigated the electrical properties of the polyurethane derived from castor oil (PUR) and carbon black (CB) composite. Samples of PUR/CB were prepared by fixed quantities of the pre-polymer and polyol (following a ratio of 10/7 by weight) and mixing different amount of the CB. Measurement of impedance spectroscopy was used to analyze the transport properties of the composite as function of applied electric field at different temperatures. The method of four probes was used to measurement the dc conductivity of the PUR/CB composites. A statistical model two-dimensional was developed based on networks of resistors and capacitors that generates the morphological structure and calculates the alternating conductivity of the PUR/CB composite using a transfer matrix technique proposed by Derrida et al. The plots of real and imaginary permittivity as a function of frequency for the samples of PUR/CB with different amount CB showed the same qualitative behavior with higher values for low frequencies and high temperatures. Measurement of imaginary dielectric permittivity showed peaks due to the process of interfacial polarization relaxation between the carbon black and polymeric matrix. However, these peaks are more pronounced when using the electrical modulus, which minimizes the effects of dc conduction. The statistical model reproduced well the experimental data obtained from the measurement of impedance spectroscopy. From theoretical and experimental settings it can be observed that the electrical conductivity for the 99/01 sample is predominantly by electron conduction in the insulating regions, in according at modified model of Miller and Abrahams. However, for samples 95/05 and 90/10 model of Abrahams and Miller dominates at low frequencies (below 103 Hz) and the Drude model is predominant in the higher frequency (above 104 Hz). / Orientador: Darcy Hiroe Fujii Kanda / Coorientador: Haroldo Naoyuki Nagashima / Banca: José Antônio Malmonge / Banca: Rodrigo Fernando Bianchi

Orientador: Cláudio Luiz Carvalho / Banca: Rafael Zadorosny / Banca: Carlos Yujiro Shigue / Resumo: Com a descoberta dos supercondutores de alta temperatura crítica na década de 80, as propriedades elétricas desses materiais passaram a ser intensamente estudadas, principalmente por meio do processo de dopagem. Desta forma, este trabalho teve por objetivo estudar os efeitos da dopagem nas propriedades elétricas do sistema supercondutor BSCCO com fórmula estequiométrica Bi1,6Pb0,4Sr2-xRExCa2Cu3O10+δ, por meio da substituição do elemento terra rara (RE), lantânio (La), em sítios de Sr, onde 0≤x≤2,0, em intervalos de 0,5. As soluções precursoras foram preparadas baseadas no método de Pechini, obtendo-se uma resina polimérica submetida a tratamento térmico de 200oC/10h, resultando em um material na forma de pó, o qual foi levado novamente a tratamento térmico entre 400oC a 810oC. Com o pó, foram preparadas pastilhas, submetendo-as a um novo tratamento térmico de 810oC/+31h. Para se conhecer as principais características das amostras, foram feitas a caracterização estrutural por meio da técnica de Difratometria de Raios X (DRX) para todos os tratamentos térmicos realizados; a caracterização elétrica, pelo método de quatro pontas dc; para a caracterização morfológica e química, a técnica de microscopia eletrônica de varredura (MEV), juntamente com a técnica de Espectroscopia por Energia Dispersiva de Raios X (EDX); e medidas magnéticas ac e dc para a caracterização magnética das amostras. Os resultados apontaram uma degradação das propriedades de supercondução, além de mudanças estruturais, morfológicas e magnéticas com o aumento da concentração de dopante / Abstract: With the discovery of high critical temperature superconductors in the 80, the electrical properties of these materials became intensively studied, mainly through the doping process. Thus, the objective of this work was the study of the effects of doping on the electrical properties of BSCCO superconducting system with stoichiometric formula Bi1,6Pb0,4Sr2-xRExCa2Cu3O10+δ, by the substitution of rare earth element (RE), lanthanum (La) in the sites of Sr, where 0 ≤ x ≤ 2.0, in interval of 0.5. The precursor solutions were prepared based by Pechini method, obtaining a polymer resin subjected to heat treatment at 200° 10h, which results in a material in C/ a powder. In sequence, futher heat treatments between 400oC to 810oC were made. With the powder were prepared bulks by subjecting them to a new heat treatment at 810° C/+31h. To know the main characteristics of the samples, structural characterization were made by the technique of X-Ray Diffraction (XRD) for all heat treated samples; the electrical characterization was made by the dc four probe method; for the morphological and chemical characterization by the technique of Scanning Electron Microscopy (SEM), together with the technique of Energy Dispersive X-ray Spectroscopy (EDX); and ac and dc magnetic measurements to magnetic characterization of the sample. The results shown a degradation of superconducting properties, as a consequence of structural, morphology and magnetic changes with the dopant concentration increase / Mestre

Des manganites de formule La0.67Ba0.33Mn1-xFexO3 et Pr0.67Ba0.33Mn1-xFexO3 ont étésynthétisés par voie céramique et leurs propriétés structurales et physiques ont été étudiées. Leséchantillons présentent une transition paramagnétique-ferromagnétique pour un taux de Fe £ 10%(resp. 5%) pour la série de La (resp. Pr) alors qu’un comportement type verre de spin est observéau delà de ces taux. Une transition métal-semiconducteur est associée aux transitionsferromagnétiques et un comportement semiconducteur sur toute la gamme de température estobservé lorsqu’il n’y a pas de transition ferromagnétique. Les propriétés physiques peuvents’interpréter par le mécanisme de double échange qui s’affaiblit pour des taux croissants desubstitution. La conduction métallique est due à la diffusion des électrons (par les phonons et lesjoints des grains) alors que le caractère semiconducteur est dû à la présence de polarons.L’étude magnétocalorique sur Pr0.67Ba0.33MnO3, Pr0.67Ba0.33Mn0.95Fe0.05O3, La0.67Ba0.33MnO3et La0.67Ba0.3Mn0.95Fe0.05O3 montre que nos matériaux sont prometteurs pour être utilisés dans latechnologie de la réfrigération magnétique. / Manganites of formula La0.67Ba0.33Mn1-xFexO3 and Pr0.67Ba0.33Mn1-xFexO3 were synthesizedby ceramic route and their structural and physical properties were studied. The samples exhibit aparamagnetic-ferromagnetic transition for Fe rate £ 10% (resp. 5%) for the La (resp. Pr) series,while a typical behavior of spin glass is observed beyond this rate. This ferromagneticparamagnetictransition is associated to a metallic-semiconductor transition and a semiconductorbehavior throughout the whole temperature range is observed when there is no ferromagnetictransition. The physical properties can be interpreted by the double exchange mechanism whichweakens for increasing rates of substitution. The metallic conduction is due to electron scattering(by phonon and grain boundaries), while the semiconductor character is due to the presence ofpolarons.Magnetocaloric study for Pr0.67Ba0.33MnO3, Pr0.67Ba0.33Mn0.95Fe0.05O3, La0.67Ba0.33MnO3 andLa0.67Ba0.3Mn0.95Fe0.05O3 shows that our materials are promising to be used in magneticrefrigeration technology.

Ces travaux de thèse portent sur le développement d’une périphérie innovante de TRIAC exploitant le caractère semiisolant du silicium poreux (PS). L’intégration de caissons PS type P à partir des profils de dopage du TRIAC est en effet accessible. Une revue des propriétés électriques du PS type P réalisée à partir de nos propres échantillons méso voire micro-poreux a donc été entreprise. Des mesures de capacités et des relevés I-V ont ainsi permis de déterminer l’évolution de la constante diélectrique relative du PS ainsi que sa résistivité en fonction de la porosité. Plus cette dernière est élevée et plus les propriétés diélectriques du PS se rapprochent de celles d’un isolant. L’analyse des résultats a également permis de clarifier les mécanismes de transport des porteurs au sein de la couche de PS. Des prototypes de TRIACs avec une terminaison de jonction à base de PS ont ensuite été conçus, fabriqués et étudiés. La localisation du PS et la gestion des contraintes mécaniques résultant de la formation du PS sont apparus comme les principaux verrous technologiques à surmonter. Des solutions ont été proposées, néanmoins les tenues en blocage atteintes se sont avérées insuffisantes. Des courants de fuite supérieurs à la dizaine de milliampères ont en effet été mesurés et ce pour des tensions de polarisation de l’ordre de 100 V. La géométrie des caissons PS et/ou la présence de charges fixes à l’interface PS / Silicium sont jugées responsables des résultats. Enfin, en s’appuyant sur un modèle macroscopique du PS, une nouvelle structure plus optimisée a été suggérée. / This PhD thesis deals with the development of a novel TRIAC periphery, exploiting the semi-insulating nature of porous silicon (PS). It is namely accessible to integrate P type PS wells through the doping profiles encountered in the TRIAC. Thus, a review of the P type PS electrical properties was achieved through dedicated samples. In this context, capacitance measurements and I-V plots were used to determine the evolution of the PS relative dielectric constant and its resistivity with the porosity. Higher the latter is, more insulating the PS is. By analyzing all the results, it was also possible to clarify the carrier transport mechanisms in the PS. Some TRIAC prototypes with a PS based junction termination were then designed, processed and studied. The stress coming from the PS formation and the PS masking were the main technological steps to solve. First solutions were proposed, nevertheless insufficient blocking performances were reached. Leakage currents higher than 10 mA were demonstrated while the bias voltage was only 100 V. The presence of fixed charges at the PS / Silicon interface and/or the geometry of the PS wells may explain these results. Finally, with the help of a macroscopic PS model, a more optimized structure was proposed.

Orientador: Prof. Dr. José Antonio Souza / Tese (doutorado) - Universidade Federal do ABC, Programa de Pós-Graduação em Física, 2014. / Neste trabalho, foram estudados tres sistemas de metais de oxidos de transicao os quais apresentam propriedades fisicas fortemente correlacionadas. O primeiro sistema foi uma serie da familia das manganitas La1-xCaxMnO3, com x = 0.20, 0.25, 0.30, 0.34, 0.40 e 0.45. Um estudo sistematico das propriedades magneticas e de transporte eletrico foi realizado nessa serie. O mecanismo de transporte eletrico usando o modelo de hopping de pequenos polarons no regime nao adiabatico foi revisitado considerando termos de ordem superior na equacao que descreve a probabilidade de hopping. Foi obtida uma equacao mais generica para descrever satisfatoriamente a dependencia com a temperatura da resistividade eletrica dos compostos com maior dopagem (x = 0.40 e 0.45). A analise dos parametros fisicos obtidos do ajuste indica que o regime nao adiabatico considerando termos de ordem superior e necessario para descrever o mecanismo de transporte eletrico em compostos com resistividade eletrica elevada. Por outro lado, um desvio da lei de Curie-Weiss foi observado no estado paramagnetico nesta serie indicando a presenca de interacoes magneticas de curto alcance. Um estudo sistematico do momento magnetico efetivo em funcao da temperatura e dos portadores de carga indica que tal desvio nao pode ser causado pela formacao de clusters ferromagneticos, onde os ions de Mn3+ e Mn4+ interagem via o mecanismo de dupla troca, como comumente sugerido na literatura. Os resultados revelam que os eletrons eg estao localizados no ion de Mn3+ independentemente da introducao de buracos no sistema sugerindo a presenca de interacoes magneticas do tipo supertroca. O segundo sistema estudado e a ferrita de bismuto com propriedades multiferroicas Bi2Fe4O9. A influencia da introducao de desordem quimica no composto Bi2Fe4O9, atraves da substituicao parcial do Fe pelo Mn [Bi2Fe4-xMnxO9+¿Ã, com x = 0, 1.0, 2.0, 3.0 e 4.0], sobre as propriedades fisicas foi investigada. Foi encontrado que a desordem quimica causa alteracoes em varias propriedades: (1) surgimento de uma coexistencia de duas fases cristalograficas tipo-Bi2Mn4O10 e tipo-Bi2Fe4O9 para x = 1.0 e 2.0; (2) inducao de uma transicao antiferromagnetica em temperaturas muito baixas onde a temperatura de Neel (TN) varia com x; (3) diminuicao do valor da resistividade eletrica alcancando o seu minimo valor em x = 3.0. Foi observado que o mecanismo de transporte eletrico de todas as amostras obedece ao regime adiabatico do modelo de hopping de pequenos polarons. A energia de ativacao e a frequencia de hopping de algumas amostras exibem uma alteracao em altas temperaturas. Foi sugerido que tais alteracoes sao induzidas devido a variacoes nas posicoes de equilibrio dos ions de oxigenio, caracterizadas atraves de medidas locais usando a tecnica de correlacao angular perturbada. Alem disso, um campo magnetico hiperfino foi encontrado, o qual surge devido a transicao antiferromagnetica de longo alcance das amostras Bi2Fe4O9 (250 K) e Bi2Mn4O10 (39 K). Evidencias do acoplamento magnetoeletrico nas amostras Bi2Fe4O9 e Bi2Mn4O10 foram reveladas atraves de uma variacao anomala na dependencia com a temperatura da frequencia quadrupolar (¿ËQ) e do parametro de assimetria (¿Å). O terceiro composto estudado e a ferrita de bismuto BiFeO3 a qual tambem exibe propriedades multiferroicas. Um estudo sistematico das propriedades magneticas e eletricas de nanoparticulas de BiFeO3 com dois tamanhos levemente diferentes (S1 e S2) foi realizado. Medidas de susceptibilidade magnetica em funcao da temperatura sugerem a presenca de um estado do tipo spin-glass em baixas temperaturas nas duas amostras. Foi mostrado que a presenca de fase magnetica espuria em porcentagens muito pequenas (nao detectaveis por difracao de raios x) pode afetar fortemente as propriedades magneticas e eletrica de nanoparticulas de BiFeO3. Mostrou-se que as propriedades eletricas (resistividade eletrica, impedancia, polarizacao e constate dieletrica) desse composto podem ser influenciadas pela adsorcao quimica e fisica de moleculas de agua na superficie da amostra. Medidas de resistividade eletrica obtidas em atmosfera ambiente revelaram que a principal contribuicao ao mecanismo de conducao e a delocalizacao de vacancias de oxigenio. Ja as medidas realizadas com fluxo de argonio mostraram uma queda abrupta em forma de cascata da resistividade eletrica em funcao da temperatura. Esse processo e reversivel com a diminuicao da temperatura sugerindo uma transicao de fase. Acredita-se que nas medidas em atmosfera ambiente, esse efeito e suprimido devido ao aumento da resistividade eletrica causado pela dessorcao de moleculas de agua em altas temperaturas. Alem disso, a polarizacao eletrica e constante dieletrica assumem valores colossais em altas temperaturas e baixas frequencias. / In this work, we have studied three systems of transition metal oxides which exhibit strongly correlated physical properties. The first system was the magnetoresistive series La1-xCaxMnO3, with x = 0.20, 0.25, 0.30, 0.34, 0.40 and 0.45. A systematic study of the magnetic and electrical transport properties has been done in this series. The non-adiabatic regime of the small polaron model was revisited considering higher order terms in the hopping probability equation. We have obtained a more general equation in order to describe the temperature dependence of the electrical resistivity for compounds with higher doping (x = 0.40 to 0:45). The analysis of the physical parameters obtained from the fitting indicates that the non-adiabatic regime with higher order terms is needed to describe the electric transport mechanism in compounds with high electrical resistivity. On the other hand, it was observed a deviation from the Curie-Weiss law in the paramagnetic state indicating the presence of short-range magnetic interactions in this series. A systematic study of the effective magnetic moment as a function of temperature and charge carriers indicates that such deviation may not be caused by the formation of ferromagnetic clusters, where the ions Mn3+ and Mn4+ interact via double exchange mechanism, as commonly suggested in the literature. The results reveal that electrons eg are localized in the Mn3+ ion regardless of the introduction of holes in the system suggesting the presence of super-exchange like magnetic interactions. The second studied system was the bismuth ferrite Bi2Fe4O9 with multiferroic properties. The influence of the chemical disorder, produced by partial substitution of Fe in the Mn-site [Bi2Fe4-xMnxO9+å, with x = 0, 1.0, 2.0, 3.0 and 4.0], on the physical properties of Bi2Fe4O9 was studied. We have found that several physical properties are altered by the chemical disorder: (1) appearance of coexistence of two crystallographic phases Bi2Fe4O9-type and Bi2Mn4O10-type for x = 1.0 and 2.0; (2) presence of an antiferromagnetic transition at very low temperatures where the Neel temperature (TN) depends on x; (3) reduction of electrical resistivity value which reaches its minimum value at x = 3.0. It has been observed that the electric transport mechanism for all samples follows the adiabatic regime of the small polaron model. Interesting, the values of the activation energy and hopping frequency of some samples are not constant. We suggested that this result is caused by variation in the equilibrium positions of oxygen ions. Such variations were characterized by using the perturbed angular correlation local technique. Furthermore, we have observed that a hyperfine magnetic field arises simultaneously with the long-range antiferromagnetic transition in the samples Bi2Fe4O9 (TN = 250 K) and Bi2Mn4O10 (TN = 39 K). An anomalous behavior in the temperature dependence of the quadrupolar frequency (íQ) and asymmetry parameter (ç) provided evidence about the existence of magnetoelectric coupling in the compounds Bi2Fe4O9 and Bi2Mn4O10. The third studied system was the bismuth ferrite BiFeO3 which also exhibits multiferroic properties. We have performed an electrical and magnetic properties comprehensive study on two sets of BiFeO3 nanoparticles with slightly different sizes (S1 and S2). It was observed that the presence of spurious magnetic phase in very small amounts (not detectable by x-ray diffraction) can strongly affect the magnetic and electrical properties of BiFeO3 nanoparticles. We have also observed that the electrical properties (resistivity, impedance, polarization, and dielectric constant) of this compound can be influenced by chemical and physical adsorption of water molecules on the sample surface. Furthermore, the analysis of the electrical resistivity measurements obtained at ambient atmosphere suggests that the delocalization of oxygen vacancies is the main contribution to the transport mechanism. On the other hand, the measurements obtained with argon flow revealed that the electrical resistivity undergoes an abrupt decrease such as cascade-like behavior. This process is reversible on warming and cooling curves suggesting a phase transition. We believe that in the measurements obtained at ambient atmosphere, this effect is suppressed due to increase of the electrical resistivity caused by desorption of water molecules at high temperatures. The electrical polarization and dielectric constant exhibit colossal values at high temperatures and low frequencies.