Solid Oxide Fuel Cells have attracted much attention over the past few decades due to their huge potential for clean power generation in stationary, portable and transport applications and our increasing need for sustainable energy resources. The purpose of this research is to develop an interconnect and cathode material for use in solid oxide fuel cells which demonstrates desired properties of high electrical conductivity, excellent chemical stability at high temperatures, desirable thermal expansion characteristics and which can be easily manufactured by sintering in conditions acceptable with other cell components. The present work was initiated to study the synthesis and properties of five different perovskite oxides comprising of Lanthanum in combination with different mol% of Chromium, Ferrum, Cobalt and Nickel. A polymer complexing route with slight modifications was used to prepare the precursor powders. The powder x-ray diffraction patterns at room temperature show that all samples were formed in single phase. The powders in the form of pellets were sintered at 1400°C. The temperature dependent resistivity data was measured and the conductivity data was calculated. This conductivity data have been fitted with the Arrhenius model for entire studied range of temperature (25-800°C) to calculate the activation energy. La based perovskite oxides were characterized using X-ray diffraction (XRD), and scanning electron microscopy (SEM). Electrical properties and microstructural studies show potential applications of the materials as interconnect and cathode for Solid Oxide Fuel Cell. The material which has the above desired properties was proposed and component modifications for tailoring such properties were shown for SOFCs and other similar applications.

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Orientador: José Alberto Giacometti / Banca: Antonio José Felix de Carvalho / Banca: Nilson Cristino da Cruz / Resumo: O objetivo deste trabalho é a caracterização de polímeros isolantes elétricos para seu uso na fabricação de dispositivos eletrônicos orgânicos. Os polímeros estudados foram: a poli(amida-imida), PAI, poli(fenil-metilsilsesquioxano), PSQ, e o fotoresiste SU8. O processo de fabricação dos filmes poliméricos foi realizado pela técnica de spin-coating e eles foram submetidos a um processo de cura apropriado para o entrecruzamento das cadeias poliméricas. Os filmes foram caracterizados espectroscopia de impedância e microscopia de força atômica (AFM). Curvas de capacitância e tan'delta' também foram analisadas e valores menores de 10-2 foram encontradas para a tn'delta' indicando que os polímeros são bons isolantes elétricos. Curvas características de corrente versus diferença de potencial elétrico (ddp) em diferentes temperaturas foram obtidas para todos os filmes. A condução elétrica foi analisada e conclui-se que o processo de condução predominantes nos filmes poliméricos é por injeção de cargas por efeito Schottky. A altura da barreira de injeção polímero/metal calculada foi da ordem de 1,1 eV para o PSQ e 1,2 eV para o SU8 e PAI / Abstract: This work aims the films characterization of electric insulating polymers for the fabrication of organic electronic devides: the poly(amide-imide), PAI, poly(phenylmethylsilsesquioxane), PSQ, and the SU8 photoresist. The manufacturing process of the polymer films was performed by spin-coating and they were submitted to the appropriated curing process in order to promote the cross-linking of polymeric chains. The thin films were characterized by using electric impedance technique and scanning atomic force microscopy (AFM). Curves of capacitance and tan'delta' were also analyzed and values smaller than 10-2 were found for tan'delta' indicating that the polymers are good electrical insulators. Current versus voltage characteristics curves at different temperatures were obtained for all films. The electrical conduction was analyzed and it was concluded that the conduction process on polymeric films are dominated by Schottky charge injection. The injection barrier height of polymer/metal was found to be of the order of 1,1 eV for PSQ and 1,2 eV for SU8 and PAI / Mestre

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The operating temperature of energy conversion and electronic devices affects their efficiency and efficacy. In many devices, however, the reference values of the thermal properties of the materials used are no longer applicable due to processing techniques performed. This leads to challenges in thermal management and thermal engineering that demand accurate predictive tools and high fidelity measurements. The thermal conductivity of strained, nanostructured, and ultra-thin dielectrics are predicted computationally using solutions to the Boltzmann transport equation. Experimental measurements of thermal diffusivity are performed using transient grating spectroscopy. The thermal conductivities of argon, modeled using the Lennard-Jones potential, and silicon, modeled using density functional theory, are predicted under compressive and tensile strain from lattice dynamics calculations. The thermal conductivity of silicon is found to be invariant with compression, a result that is in disagreement with previous computational efforts. This difference is attributed to the more accurate force constants calculated from density functional theory. The invariance is found to be a result of competing effects of increased phonon group velocities and decreased phonon lifetimes, demonstrating how the anharmonic contribution of the atomic potential can scale differently than the harmonic contribution. Using three Monte Carlo techniques, the phonon-boundary scattering and the subsequent thermal conductivity reduction are predicted for nanoporous silicon thin films. The Monte Carlo techniques used are free path sampling, isotropic ray-tracing, and a new technique, modal ray-tracing. The thermal conductivity predictions from all three techniques are observed to be comparable to previous experimental measurements on nanoporous silicon films. The phonon mean free paths predicted from isotropic ray-tracing, however, are unphysical as compared to those predicted by free path sampling. Removing the isotropic assumption, leading to the formulation of modal ray-tracing, corrects the mean free path distribution. The effect of phonon line-of-sight is investigated in nanoporous silicon films using free path sampling. When the line-of-sight is cut off there is a distinct change in thermal conductivity versus porosity. By analyzing the free paths of an obstructed phonon mode, it is concluded that the trend change is due to a hard upper limit on the free paths that can exist due to the nanopore geometry in the material. The transient grating technique is an optical contact-less laser based experiment for measuring the in-plane thermal diffusivity of thin films and membranes. The theory of operation and physical setup of a transient grating experiment is detailed. The procedure for extracting the thermal diffusivity from the raw experimental signal is improved upon by removing arbitrary user choice in the fitting parameters used and constructing a parameterless error minimizing procedure. The thermal conductivity of ultra-thin argon films modeled with the Lennard-Jones potential is calculated from both the Monte Carlo free path sampling technique and from explicit reduced dimensionality lattice dynamics calculations. In these ultra-thin films, the phonon properties are altered in more than a perturbative manner, referred to as the confinement regime. The free path sampling technique, which is a perturbative method, is compared to a reduced dimensionality lattice dynamics calculation where the entire film thickness is taken as the unit cell. Divergence in thermal conductivity magnitude and trend is found at few unit cell thick argon films. Although the phonon group velocities and lifetimes are affected, it is found that alterations to the phonon density of states are the primary cause of the deviation in thermal conductivity in the confinement regime.

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The principal aim of this work was the development of a novel conductivity probe for measuring solids concentrations in slurries. The relevance of the thesis to this probe is that it requires rapid transport of aqueous electrolyte through a porous disc to an internal conductivity cell.

The thesis describes novel glass micropipette thermal sensor fabricated in cost-effective manner and thermal conductivity measurement of carbon nanotubes (CNT) thin film using the developed sensor. Various micrometer-sized sensors, which range from 2 µm to 30 µm, were produced and tested. The capability of the sensor in measuring thermal fluctuation at micro level with an estimated resolution of ±0.002oC is demonstrated. The sensitivity of sensors was recorded from 3.34 to 8.86 µV/oC, which is independent of tip size and dependent on the coating of Nickel. The detailed experimental setup for thermal conductivity measurement of CNT film is discussed and 73.418 W/moC was determined as the thermal conductivity of the CNT film at room temperature.

Advancements in Li-ion batteries are needed especially for the development of electric vehicles and stationary energy storage. Prior research has shown mesoscale variations in electrode electronic conductive properties, which can cause capacity loss and uneven electrochemical behavior of Li-ion batteries. A micro-four-line probe (μ4LP) was used to measure electronic conductivity and contact resistance over mm-length scales in that prior work. This work describes improvements to overcome the challenge of unreliable surface contact between theμ4LP and the sample. Ultimately a second generation flexible probe called the micro-radial-surface probe (μ4LP) was designed and produced. The test fixture was also optimized to obtain consistent contact with the new measurement probe and to perform measurements at a lower force. The μ4LP was then used to study the effect of heterogeneity on calendering, which is the compression of electrode films to obtain a uniform thickness and desired porosity. The thickness, electronic conductivity and contact resistance of two cathodes and one anode were measured before and after calendering. The the spatial standard deviation divided by the mean was used as a measure of heterogeneity. The results show variability in conductive properties increased for two of the three samples after calendering, despite the increased uniformity in thickness of the electrodes. This suggests that additional quality control metrics are needed besides thickness to be able to identify uneven degradation and produce longer lasting batteries.