Caracterização metalúrgica e do tempo de vida em contactos eléctricos de relés

Dissertação para obtenção do Grau de Mestre em Engenharia de Materiais

Growth and Characterization of ZnO based Heterojunction diodes and ZnO Nanostructuresby Pulsed Laser Ablation

Transparent conducting oxides (TCO’s) have been known and used for technologically important applications for more than 50 years. The oxide materials such as In2O3, SnO2 and impurity doped SnO2: Sb, SnO2: F and In2O3: Sn (indium tin oxide) were primarily used as TCO’s. Indium based oxides had been widely used as TCO’s for the past few decades. But the current increase in the cost of indium and scarcity of this material created the difficulty in obtaining low cost TCO’s. Hence the search for alternative TCO material has been a topic of active research for the last few decades. This resulted in the development of various binary and ternary compounds. But the advantages of using binary oxides are the easiness to control the composition and deposition parameters. ZnO has been identified as the one of the promising candidate for transparent electronic applications owing to its exciting optoelectronic properties. Some optoelectronics applications of ZnO overlap with that of GaN, another wide band gap semiconductor which is widely used for the production of green, blue-violet and white light emitting devices. However ZnO has some advantages over GaN among which are the availability of fairly high quality ZnO bulk single crystals and large excitonic binding energy. ZnO also has much simpler crystal-growth technology, resulting in a potentially lower cost for ZnO based devices. Most of the TCO’s are n-type semiconductors and are utilized as transparent electrodes in variety of commercial applications such as photovoltaics, electrochromic windows, flat panel displays. TCO’s provide a great potential for realizing diverse range of active functions, novel functions can be integrated into the materials according to the requirement. However the application of TCO’s has been restricted to transparent electrodes, ii notwithstanding the fact that TCO’s are n-type semiconductors. The basic reason is the lack of p-type TCO, many of the active functions in semiconductor originate from the nature of pn-junction. In 1997, H. Kawazoe et al reported the CuAlO2 as the first p-type TCO along with the chemical design concept for the exploration of other p-type TCO’s. This has led to the fabrication of all transparent diode and transistors. Fabrication of nanostructures of TCO has been a focus of an ever-increasing number of researchers world wide, mainly due to their unique optical and electronic properties which makes them ideal for a wide spectrum of applications ranging from flexible displays, quantum well lasers to in vivo biological imaging and therapeutic agents. ZnO is a highly multifunctional material system with highly promising application potential for UV light emitting diodes, diode lasers, sensors, etc. ZnO nanocrystals and nanorods doped with transition metal impurities have also attracted great interest, recently, for their spin-electronic applications This thesis summarizes the results on the growth and characterization of ZnO based diodes and nanostructures by pulsed laser ablation. Various ZnO based heterojunction diodes have been fabricated using pulsed laser deposition (PLD) and their electrical characteristics were interpreted using existing models. Pulsed laser ablation has been employed to fabricate ZnO quantum dots, ZnO nanorods and ZnMgO/ZnO multiple quantum well structures with the aim of studying the luminescent properties.

Development of p-type transparent semiconducting oxides for thin film transistor applications

Semiconductor physics has developed significantly in the field of re- search and industry in the past few decades due to it’s numerous practical applications. One of the relevant fields of current interest in material science is the fundamental aspects and applications of semi- conducting transparent thin films. Transparent conductors show the properties of transparency and conductivity simultaneously. As far as the band structure is concerned, the combination of the these two properties in the same material is contradictory. Generally a trans- parent material is an insulator having completely filled valence and empty conduction bands. Metallic conductivity come out when the Fermi level lies within a band with a large density of states to provide high carrier concentration. Effective transparent conductors must nec- essarily represent a compromise between a better transmission within the visible spectral range and a controlled but useful electrical con- ductivity [1–6]. Generally oxides like In2O3, SnO2, ZnO, CdO etc, show such a combination. These materials without any doping are insulators with optical band gap of about 3 eV. To become a trans- parent conductor, these materials must be degenerately doped to lift the Fermi level up into the conduction band. Degenerate doping pro- vides high mobility of extra carriers and low optical absorption. The increase in conductivity involves an increase in either carrier concen- tration or mobility. Increase in carrier concentration will enhance the absorption in the visible region while increase in mobility has no re- verse effect on optical properties. Therefore the focus of research for new transparent conducting oxide (TCO) materials is on developing materials with higher carrier mobilities.

Fabricación y caracterización de películas delgadas de óxidos transparentes con aplicaciones ópticas.

Los óxidos transparentes conductores (TCO′ s) son materiales compuestos conformados por oxígeno y un metal, que presentan una combinación única de alta estabilidad química, alta concentración electrónica y alta transparencia óptica. Por esta razón, el procesamiento de TCO′ s en película delgada va orientado hacia aplicaciones específicas tales como ventanas ópticas en celdas solares, sensores de gases, electrodos en dispositivos de pantallas planas, ventanas inteligentes. En este proyecto se trabajó en la síntesis experimental de dos TCO′ s relevantes tanto en investigación fundamental como en aplicaciones tecnológicas: el óxido de indio (In2O3) y el óxido de estaño (SnO2). Ambos TCO′ s se depositaron por la técnica de erosión iónica reactiva por corriente directa (DC). Para el análisis de las películas se utilizaron varias técnicas de caracterización: difracción de rayos X, espectroscopia UV-Visible, resistividad eléctrica, efecto Hall, así como microscopías electrónica de barrido y de fuerza atómica. Se fabricó también una bicapa de In2O3/SnO2, la cual se caracterizó además con espectroscopia de fotoemisión de rayos X (XPS).En esta tesis se reporta por primera vez la síntesis y caracterización de esta bicapa, la cual abre una línea de investigación en el área de interfaces. Asimismo, se desarrolló e implementó un procedimiento, basado en los modelos ópticos, tal que permite obtener parámetros que se utilizan para evaluar a cualquier película delgada TCO como potencial metamaterial. Las propiedades de las muestras se analizaron en función de la temperatura aplicada post-depósito: temperatura ambiente (TA), 100oC, 200oC, 300oC, bajo una atmósfera de argón o argón-oxígeno. Los resultados confirman que las películas presentan un crecimiento de tipo poli cristalino. Además, la calidad cristalina tiende a incrementarse como función del incremento de la temperatura. El In2O3 creció con estructura cúbica bcc (a=10.11 ˚A, ICDD #71-2195). A partir de 200C, se detectaron trazas de la fase romboédrica (a=5.490 ˚A, c=14.520 ˚A, ICDD #73-1809). Asimismo, el SnO2 creció con estructura tetragonal (a = 4.737 ˚A, c = 3.186 ˚A, ICDD #88-0287). Las películas de In2O3 poseen una transparencia promedio del 90 % en una ventana de 500 nm a 1100 nm. El borde de absorción se recorre al azul como función de la temperatura, de Eg=3.3 eV a Eg=3.7 eV por el efecto Burstein-Moss. Por otra parte, la bicapa presentó una interfaz claramente definida, sin difusión de especies metálicas. Al incrementarse la temperatura, de TA a 400oC, se detectaron dos fases de óxido de estaño: SnO2 y SnO, en un porcentaje atómico de ≈70 %:30 %, respectivamente. Se concluye que los parámetros y valores obtenidos de las películas como son el texturizado y espesor homogéneo, alta transparencia, crecimiento preferencial, ancho prohibido y resistividad eléctrica, son comparables a los que se requieren del In2O3 y SnO2 en película delgada para aplicaciones optoelectrónicas.

Síntese e caracterização do compósito SnO2/TiO2 nanoestruturado

Pós-graduação em Ciência e Tecnologia de Materiais - FC

Desenvolvimento, caracterização e aplicação eletroanalítica de um compósito fluido de adesivo epóxi, grafite e ciclo-hexanona

A fluid conducting composite material prepared from graphite powder, commercial epoxy resin Araldite®, and cyclohexanone has been developed. The composition was optimized considering the mechanical properties as conductivity and adhesiveness using response surface methodology. This work employed cyclic voltammetry and amperometry to investigate the characteristics of such composite electrodes without and with the insertion of Prussian blue in the electrode body (bulk modified electrode). The composite electrodes were also successfully used for the amperometric detection of hydrogen peroxide at 0.0V vs Ag/AgCl.

Compósitos de Al2O3/ZrO2 recobertos com hidroxiapatita dopada com íons Ag

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

'Ti''O IND.2'/'Sn''O IND. 2 modificado com 'Ag IND. 2''O', para obtenção de fotocatalisadores

Pós-graduação em Química - IQ

Microcompósitos fotocatalíticos a base de 'Ti' 'O IND.2' modificados com 'Ag'

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Avaliação da ação bactericida e bacteriostática das nanopartículas TiO2 e Ag aplicadas em pinos intraradiculares

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Study of the oxygen vacancy influence on magnetic properties of Fe- and Co-doped SnO2 diluted alloys

Transition-metal (TM)-doped diluted magnetic oxides (DMOs) have attracted attention from both experimental and theoretical points of view due to their potential use in spintronics towards new nanostructured devices and new technologies. In the present work, we study the magnetic properties of Sn0.96TM0.04O2 and Sn0.96TM0.04O1.98(V (O))(0.02), where TM = Fe and Co, focusing in particular in the role played by the presence of O vacancies nearby the TM. The calculated total energy as a function of the total magnetic moment per cell shows a magnetic metastability, corresponding to a ground state, respectively, with 2 and 1 mu(B)/cell, for Fe and Co. Two metastable states, with 0 and 4 mu(B)/cell were found for Fe, and a single value, 3 mu(B)/cell, for Co. The spin-crossover energies (E (S)) were calculated. The values are E (S) (0/2) = 107 meV and E (S) (4/2) = 25 meV for Fe. For Co, E (S) (3/1) = 36 meV. By creating O vacancies close to the TM site, we show that the metastablity and E (S) change. For iron, a new state appears, and the state with zero magnetic moment disappears. The ground state is 4 mu(B)/cell instead of 2 mu(B)/cell, and the energy E (S) (2/4) is 30 meV. For cobalt, the ground state is then found with 3 mu(B)/cell and the metastable state with 1 mu(B)/cell. The spin-crossover energy E (S) (1/3) is 21 meV. Our results suggest that these materials may be used in devices for spintronic applications that require different magnetization states.

Produção e análise do desempenho óptico e elétrico de filmes finos de SnO2:F para aplicações fotovoltaicas

Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq

Compact Ag@fe3o4 Core-shell Nanoparticles By Means Of Single-step Thermal Decomposition Reaction.

A temperature pause introduced in a simple single-step thermal decomposition of iron, with the presence of silver seeds formed in the same reaction mixture, gives rise to novel compact heterostructures: brick-like Ag@Fe3O4 core-shell nanoparticles. This novel method is relatively easy to implement, and could contribute to overcome the challenge of obtaining a multifunctional heteroparticle in which a noble metal is surrounded by magnetite. Structural analyses of the samples show 4 nm silver nanoparticles wrapped within compact cubic external structures of Fe oxide, with curious rectangular shape. The magnetic properties indicate a near superparamagnetic like behavior with a weak hysteresis at room temperature. The value of the anisotropy involved makes these particles candidates to potential applications in nanomedicine.

Biogenic Silver Nanoparticles Associated With Silver Chloride Nanoparticles (ag@agcl) Produced By Laccase From Trametes Versicolor.

In the present study, semi-purified laccase from Trametes versicolor was applied for the synthesis of silver nanoparticles, and the properties of the produced nanoparticles were characterized. All of the analyses of the spectra indicated silver nanoparticle formation. A complete characterization of the silver nanoparticles showed that a complex of silver nanoparticles and silver ions was produced, with the majority of the particles having a Ag(2+) chemical structure. A hypothetical mechanistic scheme was proposed, suggesting that the main pathway that was used was the interaction of silver ions with the T1 site of laccase, producing silver nanoparticles with the concomitant inactivation of laccase activity and posterior complexing with silver ions.

Durabilidade de compósito biomassa vegetal-cimento modificado por polímero

The durability of the cellulose-cement composites is a decisive factor to introduce such material in the market. Polymers have been used in concrete and mortar production to increase its durability. The goal of this work was the physical and mechanical characterization of cellulose-cement composites modified by a polymer and the subsequent durability evaluation. The work also evaluated the dispersion of acrylic polymer in composites made of Pinus caribaea residues. The physical properties observed were water absorption by immersion and bulk density. Rupture modulus and toughness were determined by flexural test. The specimens were obtained from pads, produced by pressing and wet curing. Samples were subjected to accelerated aging tests by repeated wetting and drying cycles and hot-water bath and natural aging. The scanning electron microscopy (SEM) allowed verifying the fiber and composite characteristics along the time. For the composite range analyzed, it was observed the polymer improved the mechanical properties of composites besides a significant decreasing in water absorption. The use of polymer improved the performance of vegetable fiber-cement composites when compared to the conventional mortar, due to water absorption decreasing.