PAULO SERGIO MARTINS DA SILVA
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Resumo IPEN-doc 26122 Ceria-based ceramic composites for high temperature thermochemical applications2018 - SILVA, PAULO M.; ESPOSITO, VINCENZO; MARANI, DEBORA; FLORIO, DANIEL Z. de; FONSECA, FABIO C.Among thermochemical conversion processes, the production of fuels such as H2 via solar thermochemical cycles is potentially more efficient and more economical than the use of electric energy to electrolyze water. The principle of solar thermochemical cycles is based on the remarkable properties of some oxides, which can be reduced and oxidized cyclically (redox cycles), i.e., releasing and absorbing oxygen under certain temperature (or pressure) regimes. These redox cycles can be efficiently used to convert H2O (or CO2) to H2 (CO). Thermochemical redox cycles avoid the problematic step of fuel / O2 separation and allow operation at more moderate temperatures (~ 1500 K) [1]. In this work, a new material concept for the separation of high temperature H2O based on porous ceramic composites composed of an ultra-high temperature ceramic phase (UHTC) and doped cerium oxide is proposed. UHTC usually exhibit extremely low mass diffusion rates and excellent thermomechanical properties for high temperature applications [2]. Gadolinium-doped ceria (CGO) presents unique processes at low oxygen partial pressure (pO2 < 10-12 atm) and high temperatures (T > 800 °C) such as faster mass diffusion, which are not observed in conventional sintering under ambient air conditions. In CGO/Al2O3 composites the resulting effects driven by such mass diffusion are low viscosity flows and high reactivity between phases, indicated by the formation of CeAlO3[3]. In this work, a comparison is made between sintering CGO/Al2O3 under ambient air and reducing condition, focusing on densification, viscosity and the evolution of the microstructure. The redox process of CGO/Al2O3 is investigated using dilatometry, microscopy, and electrochemical impedance spectroscopy. The preliminary results evidenced that new phases with remarkable microstructure can be obtained at reducing atmosphere depending on the temperature of reoxidation during coolingArtigo IPEN-doc 24797 Thermochemical stability of zirconia-titanium nitride as mixed ionic-electronic composites2018 - SILVA, P.S.M.; ESPOSITO, V.; MARANI, D.; FLORIO, D.Z. de; MACHADO, I.F.; FONSECA, F.C.Dense zirconia (8% molar yttria-stabilized ZrO2)-titanium nitride (TiN) composites are fabricated to obtain mixed ionic-electronic conducting ceramic systems with high degree of electronic and thermal conductivity. The composites are consolidated by spark plasma sintering (SPS), starting from pure powders of the pristine phases mixed in different ratios (TiN = 25, 50, 75 wt%). A careful optimization of the SPS conditions allows producing highly dense samples with no reaction between the phases or degradation by oxidation, thus maintaining the chemical integrity of the two phases. For all the composites, high electrical conductivity is attained. Samples exhibit metallic behavior, showing an unexpected percolation of TiN in the YSZ matrix for volume fraction ≤ 25 wt% (27 vol%). Chemical degradation and electrical properties of the compounds were monitored under oxidative (air) and inert (Ar) atmosphere at high temperatures. The oxidation kinetics of the nitride phase was inhibited by the microstructure of the composite. The electrical properties of such composites were explored at high temperature to evaluate its application in electrochemical devices. As results, it is shown that electrical transport properties of the composite can be tuned by both the relative volume fraction of phases and controlled oxidative treatments. Adjusting such parameters different electric behaviors were observed ranging from predominant electronic conductors, to temperature-independent resistivity, and semiconducting.Dissertação IPEN-doc 21190 Projeto, construção e testes de um sistema de medidas elétricas e estudo de compósitos de zircônia-ítria e nitreto de titânio2015 - SILVA, PAULO S.M. daNeste trabalho de mestrado são descritos o projeto, a montagem e os testes de funcionamento de uma câmara porta amostra para medidas elétricas de quatro pontas de prova dc. A estrutura da câmara porta amostra é em material cerâmico, que garante a estabilidade física e química do sistema, prolongando sua vida útil e melhorando a qualidade das análises realizadas. Este sistema permite a realização de medidas elétricas de diferentes materiais desde a temperatura ambiente até ~1500 °C em ampla faixa de pressões parciais de oxigênio. A funcionalidade da câmara porta amostras foi avaliada por meio da comparação de medidas da dependência da resistividade elétrica com a temperatura de amostras de zircônia estabilizada com ítria. Visando à aferição e a aplicação deste sistema de medidas elétricas, foram fabricados e caracterizados compósitos à base de zircônia estabilizada com ítria e nitreto de titânio, obtidos pela técnica de sinterização por plasma pulsado (\"spark plasma sintering\", SPS). As propriedades gerais destes compósitos foram investigadas por meio de análises térmicas, difratometria de raios X (DRX) e medidas elétricas de quatro pontas de prova dc, usando o sistema construído. As análises das amostras dos compósitos à base de zircônia e TiN mostraram que a técnica SPS produz amostras densas, sem reação entre as fases ou degradação do TiN por oxidação. As amostras com adição de TiN apresentaram comportamento metálico da resistividade elétrica, evidenciando a percolação do nitreto na matriz de zircônia para frações volumétricas ≤ 27 vol.%. Medidas de resistividade elétrica combinadas com análises térmicas e de DRX foram usadas para monitorar a oxidação do TiN nos compósitos em altas temperaturas. As amostras produzidas apresentam propriedades promissoras para aplicações de alta temperatura que requeiram elevada condutividade elétrica.Artigo IPEN-doc 20730 Spark plasma sintering of yttria-stabilized zirconia/titanium nitride composites2015 - SILVA, PAULO S.M.; FONSECA, FABIO C.