Preparação e caracterização de fibras e nanotubos de BiFeO3 e FeNbO4

Esta dissertação teve como objetivo a produção e caracterização física de fibras e nanotubos de BiFeO3 e FeNbO4. Para o desenvolvimento destes materiais utilizou-se a técnica de fusão com laser (LFZ), o método sol-gel (Pechini) e o método de poros absorventes. As amostras obtidas foram sujeitas a uma caracterização estrutural por difração de raios-X e espetroscopia de Raman, morfológica por microscopia electrónica de varrimento e elétrica por medidas de constante dielétrica. Os resultados obtidos com a técnica de difração de raios-X mostraram que o gel com tratamento a 750 ºC é polifásico. Para conseguir produzir nanotubos escolheu-se o LaCoO3 como material alternativo. Usando a técnica de fusão de zona com laser (LFZ) obtiveram-se fibras de BiFeO3, FeNbO4 e compósitos de BiFeO3+FeNbO4. Com esta técnica foram crescidas fibras a várias velocidades (5, 10, 25, 50, 100 e 200 mm/h), tendo os resultados obtidos com a difração de raios-X evidenciado que todas as amostras obtidas são polifásicas, sendo a amostra de 10 mm/h para o BiFeO3 e a de 5 mm/h para o FeNbO4 as que apresentam melhores propriedades. As amostras de 5 mm/h de todos os compósitos são aquelas que possuem menor quantidade de segundas fases e portanto foram alvo de estudo mais aprofundado. A caracterização dielétrica permitiu verificar que todas as amostras apresentam fenómenos de relaxação dielétrica. Verifica-se também que para o BiFeO3 a constante dielétrica é superior na amostra crescida à velocidade de 10 mm/h, para o FeNbO4 é superior na amostra crescida a 5 mm/h e nos compósitos a amostra com 75% de BiFeO3 e 25% de FeNbO4 apresenta um comportamento diferente das restantes, eventualmente devido à sua microestrutura singular.

Caracterización espectroscópica de impedancia de polímeros para el uso en electrodos de electrocardiogramas

Un estudio comparativo de la impedancia de un polímero tipo poliolefina termoplástica elastómera con un electrodo comercial gelificado universal ha sido hecho con el fin de determinar si el polímero puede ser usado como electrodo seco para la adquisición de señales cardiacas. Los resultados de la espectrometría de impedancia mostraron características muy parecidas y que ambos se comportan como dispositivos resistores, por ejemplo a 120 Hz el polímero registró una resistencia de 6,3 k y el electrodo comercial 7,2 k . Además, se obtuvo una relación lineal de corriente contra voltaje, determinando resistencias de 7 k y 8,2 k para el polímero y electrodo comercial gelificado respectivamente. Finalmente concluimos que es aplicable el uso de este polímero en electrodos secos.

Determinação eletroanalítica de ácido salicílico em produto dermatológico com eletrodos de nanotubos de carbono e nanopartículas de óxido de ferro

Dissertação (mestrado)—Universidade de Brasília, Instituto de Química, Programa de Pós-Graduação em Tecnologias Química e Biológica, 2016.

Surface Activity and Bulk Defect Chemistry of Solid Oxide Fuel Cell Cathodes

In the first half of this thesis, a new robotic instrument called a scanning impedance probe is presented that can acquire electrochemical impedance spectra in automated fashion from hundreds of thin film microelectrodes with systematically varied properties. Results from this instrument are presented for three catalyst compositions that are commonly considered for use in state-of-the-art solid oxide fuel cell cathodes. For (La_0.8Sr_0.2)_0.95Mn_O3+δ (LSM), the impedance spectra are well fit by a through-the-film reaction pathway. Transport rates are extracted, and the surface activity towards oxygen reduction is found to be correlated with the number of exposed grain boundary sites, suggesting that grain boundaries are more surface-active than grains. For La_0.5Sr_0.5Co_O3-δ (LSC), the surface activity degrades ~50x initially and then stabilizes at a comparable activity to that of previously measured Ba_0.5Sr_0.5Co_0.8Fe_0.2O3-δ films. For Sr_0.06Nb_0.06Bi_1.87O₃ (SNB), an example of a doped bismuth oxide, the activity of the metal-SNB boundary is measured.

In the second half of this thesis, SrCo_0.9Nb_0.1O_3-δ is selected as a case study of perovskites containing Sr and Co, which are the most active oxygen reduction catalysts known. Several bulk properties are measured, and synchrotron data are presented that provide strong evidence of substantial cobalt-oxygen covalency at high temperatures. This covalent bonding may be the underlying source of the high surface activity.

Preparação e caracterização de filmes cerâmicos para cátodos de células a combustível de óxido sólido

Alternative and clean energy generation research has been intensified in last decades. Among the alternatives, fuel cells are one of the most important. There are different types of fuel cells, among which stands out intermediate temperature solid oxide fuel cell (IT-SOFC) matter of the present work. For application as cathode on this type of devices, the ceramic Ba0.5Sr0.5C0.8Fe0.2O3-δ doped with rare earth ions (Nd, Sm) have been quite promising because they show good ionic conductivity and operate at relatively low temperatures (500 - 800°C). In this work, Ba0.5Sr0.5Co0.8Fe0.2O3-δ, (BaSr)0.5Sm0.5Co0.8Fe0.2O3-δ and (BaSr)0.5Nd0.5C0.8Fe0.2O3-δ were obtained by modified Pechini method, making use of gelatin as polymerizing agent. The powders were characterized by X-Ray Diffraction (XRD), Temperature Programmed Reduction (TPR) and Scanning Electron Microscopy (SEM). The perovskite phase was observed in all X-ray patterns for the materials Ba0.5Sr0.5C0.8Fe0.2O3-δ doped with rare earth ions (Nd, Sm). The SEM images showed that the materials have a characteristics porous, with very uniform pore distribution, which are favorable for application as cathodes. Subsequently, screen-printed assymmetrical cells were studied by impedance spectroscopy, to assess the kinetics of the cathode for the reduction reaction of oxygen. The best resistance to the specific area was found for the cathode BSSCF sintered at 1050 °C for 4 hours with around 0.15 Ω.cm2 at 750 °C as well as cathodes BSNCF and BSCF obtained resistances specific area of 0.2 and 0.73 Ω.cm2, respectively, for the same conditions. The polarization curves showed similar behavior to the best cathodes BSSCF and BSNCF, such combination of properties indicates that the film potentially depict good performance as IT-SOFC cathodes

Efeito da Gália como aditivo de sinterização em eletrólitos cerâmicos à base de céria sintetizados pelo método de complexação de cátions

Fuel cells are considered one of the most promising ways of converting electrical energy due to its high yield and by using hydrogen (as fuel) which is considered one of the most important source of clean energy for the future. Rare earths doped ceria has been widely investigated as an alternative material for the electrolyte of solid oxide fuel cells (SOFCs) due to its high ionic conductivity at low operating temperatures compared with the traditional electrolytes based on stabilized zirconia. This work investigates the effect of gallium oxide (Gallia) as a sintering aid in Eu doped ceria ceramic electrolytes since this effect has already been investigated for Gd, Sm and Y doped ceria electrolytes. The desired goal with the use of a sintering aid is to reduce the sintering temperature aiming to produce dense ceramics. In this study we investigated the effects on densification, microstructure and ionic conduction caused by different molar fraction of the dopants europium (10, 15 and 20%) and gallium oxide (0.3, 0.6 and 0.9%) in samples sintered at 1300, 1350 and 1450 0 C. Samaria (10 and 20%) doped ceria samples sintered between 1350 and 1450 °C were used as reference. Samples were synthesized using the cation complexation method. The ceramics powders were characterized by XRF, XRD and SEM, while the sintered samples were investigated by its relative density, SEM and impedance spectroscopy. It was showed that gallia contents up to 0.6% act as excellent sintering aids in Eu doped ceria. Above this aid content, gallia addition does not promote significant increase in density of the ceramics. In Ga free samples the larger densification were accomplished with Eu 15% molar, effect expressed in the microstructure with higher grain growth although reduced and surrounded by many open pores. Relative densities greater than 95 % were obtained by sintering between 1300 and 1350 °C against the usual range 1500 - 1600 0 C. Samples containing 10% of Sm and 0.9% of Ga reached 96% of theoretical density by sintering at 1350 0 C for 3h, a gain compared to 97% achieved with 20% of Sm and 1% of Ga co-doped cerias sintered at 1450 0 C for 24 h as described in the literature. It is found that the addition of gallia in the Eu doped ceria has a positive effect on the grain conductivity and a negative one in the grain boundary conductivity resulting in a small decrease in the total conductivity which will not compromise its application as sintering aids in ceria based electrolytes. Typical total conductivity values at 600 and 700 °C, around 10 and 30 mS.cm -1 respectively were reached in this study. Samples with 15% of Eu and 0.9 % of Ga sintered at 1300 and 1350 °C showed relative densities greater than 96% and total conductivity (measured at 700 °C) between 20 and 33 mS.cm -1 . The simultaneous sintering of the electrolyte with the anode is one of the goals of research in materials for SOFCs. The results obtained in this study suggest that dense Eu and Ga co-doped ceria electrolytes with good ionic conductivity can be sintered simultaneously with the anode at temperatures below 1350 °C, the usual temperature for firing porous anode materials

Avaliação da espécie vegetal Croton cajucara Benth como inibidor de biocorrosão em aço carbono Aisi 1020

Actually in the oil industry biotechnological approaches represent a challenge. In that, attention to metal structures affected by electrochemical corrosive processes, as well as by the interference of microorganisms (biocorrosion) which affect the kinetics of the environment / metal interface. Regarding to economical and environmental impacts reduction let to the use of natural products as an alternative to toxic synthetic inhibitors. This study aims the employment of green chemistry by evaluating the stem bark extracts (EHC, hydroalcoholic extract) and leaves (ECF, chloroform extract) of plant species Croton cajucara Benth as a corrosion inhibitor. In addition the effectiveness of corrosion inhibition of bioactive trans-clerodane dehydrocrotonin (DCTN) isolated from the stem bark of this Croton was also evaluated. For this purpose, carbon steel AISI 1020 was immersed in saline media (3,5 % NaCl) in the presence and absence of a microorganism recovered from a pipeline oil sample. Corrosion inhibition efficiency and its mechanisms were investigated by linear sweep voltammetry and electrochemical impedance. Culture-dependent and molecular biology techniques were used to characterize and identify bacterial species present in oil samples. The tested natural products EHC, ECF and DCTN (DMSO as solvent) in abiotic environment presented respectively, corrosion inhibition efficiencies of 57.6% (500 ppm), 86.1% (500 ppm) and 54.5% (62.5 ppm). Adsorption phenomena showed that EHC best fit Frumkin isotherm and ECF to Temkin isotherm. EHC extract (250 ppm) dissolved in a polar microemulsion system (MES-EHC) showed significant maximum inhibition efficiency (93.8%) fitting Langmuir isotherm. In the presence of the isolated Pseudomonas sp, EHC and ECF were able to form eco-compatible organic films with anti-corrosive properties

Synthesis of Sodium Poly[4-styrenesulfonyl(trifluoromethylsulfonyl)imide]-co-ethylacrylate] Solid Polymer Electrolytes

Sodium-based batteries are being considered to replace Li-based batteries for the fabrication of large-scale energy storage devices. One of the main obstacles is the lack of safe and conductive solid Na-ion electrolytes. A Na-ion polymer based on the (4-styrenesulfonyl(trifluromethylsulfonyl) imide anion, Na[STFSI], has been prepared by a radical polymerization process and its conductive properties determined. In addition, a number of multi-component polymers were synthetized by co-reaction of two monomers: Na[STFSI] and ethyl acrylate (EA) at different ratios. The structural and phase characterizations of the polymers were probed by various techniques (DSC, TGA, NMR, GPC, Raman, FTIR and Impedance spectroscopy). Comparative studies with blends of the homopolymers Na[PSTFSI] and poly(ethylacrylate) (PEA) have also been performed. The polymers are all thermally stable up to 300°C and the ionic conductivity of EA copolymers and EA blends are about 1-3 orders of magnitude higher than that of Na[PSTFSI]. The highest conductivity measured at 100°C was found for Na[PSTFSI-blend-5EA] at 7.9 × 10^-9 S cm^-1, despite being well below its Tg. Vibrational spectroscopy indicates interaction between Na⁺ and the EA carbonyl groups, with a concomitant decrease in the sulfonyl interaction, facilitating Na⁺ motion, as well as lowering Tg.

Unexpected effect of tetraglyme plasticizer on lithium ion dynamics in PAMPS based ionomers

Li(+) cation conducting ionomers based on poly(2-acrylamido-2-methyl-1-propane sulphonic acid) (PAMPS) incorporating a low molecular weight plasticizer have been characterized. Previously we have observed an apparent decoupling of ionic conductivity and lithium ion dynamics from the Tg of this ionomer along with an increase in ionic conductivity obtained by incorporating a quaternary ammonium co-cation. The incorporation of tetraglyme as a coordinating plasticizer was investigated in order to further improve the ion dissociation and dynamics. Solid-state NMR, thermal analysis, impedance spectroscopy and infrared spectroscopy were used to characterize these systems. As expected, the glass transition temperature Tg decreased upon the addition of the plasticizer. However, in contrast to the previously reported Na-conducting systems, the ionic conductivity was also decreased by several orders of magnitude, indicating that the tetraglyme recouples the conductivity back to the polymer dynamics. Temperature dependent (7)Li NMR line width and T1 measurements were used to probe the Li(+) dynamics, which were found to be dependent on the Li(+) concentration, the nature of the co-cation and the presence or absence of tetraglyme.

Real-time Biosensor for the Assessment of Nanotoxicity and Cancer Electrotherapy

Knowledge of cell electronics has led to their integration to medicine either by physically interfacing electronic devices with biological systems or by using electronics for both detection and characterization of biological materials. In this dissertation, an electrical impedance sensor (EIS) was used to measure the electrode surface impedance changes from cell samples of human and environmental toxicity of nanoscale materials in 2D and 3D cell culture models. The impedimetric response of human lung fibroblasts and rainbow trout gill epithelial cells when exposed to various nanomaterials was tested to determine their kinetic effects towards the cells and to demonstrate the biosensor’s ability to monitor nanotoxicity in real-time. Further, the EIS allowed rapid, real-time and multi-sample analysis creating a versatile, noninvasive tool that is able to provide quantitative information with respect to alteration in cellular function. We then extended the application of the unique capabilities of the EIS to do real-time analysis of cancer cell response to externally applied alternating electric fields at different intermediate frequencies and low-intensity. Decreases in the growth profiles of the ovarian and breast cancer cells were observed with the application of 200 and 100 kHz, respectively, indicating specific inhibitory effects on dividing cells in culture in contrast to the non-cancerous HUVECs and mammary epithelial cells. We then sought to enhance the effects of the electric field by altering the cancer cell’s electronegative membrane properties with HER2 antibody functionalized nanoparticles. An Annexin V/EthD-III assay and zeta potential were performed to determine the cell death mechanism indicating apoptosis and a decrease in zeta potential with the incorporation of the nanoparticles. With more negatively charged HER2-AuNPs attached to the cancer cell membrane, the decrease in membrane potential would thus leave the cells more vulnerable to the detrimental effects of the applied electric field due to the decrease in surface charge. Therefore, by altering the cell membrane potential, one could possibly control the fate of the cell. This whole cell-based biosensor will enhance our understanding of the responsiveness of cancer cells to electric field therapy and demonstrate potential therapeutic opportunities for electric field therapy in the treatment of cancer.

Vector Fitting based Automatic Circuit Identification

This paper describes a method to automatically obtain, from a set of impedance measurements at different frequencies, an equivalent circuit composed of lumped elements based on the vector fitting algorithm. The method starts from the impedance measurement of the circuit and then, through the recursive use of vector fitting, identifies the circuit topology and the component values of lumped elements. The method can be expanded to include other components usually used in impedance spectroscopy. The method is firstly described and then two examples highlight the robustness of the method and showcase its applicability.