Browsing by Author "Gomes, A."
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- Corrosão de aços inoxidáveis austeníticos em sais fundidos ternários de cálcioPublication . Cunha Diamantino, Teresa; Gomes, A.; Paiva Luís, Teresa; Figueira, Isabel; Pedrosa, Fátima; Nogueira, Carlos; Guerreiro, L.; Collares-Pereira, M.RESUMO: O armazenamento de energia térmica (TES) é um fator chave para melhorar a eficiência e a despachabilidade dos diferentes sistemas energéticos, devido a um melhor ajustamento entre a procura e a oferta de energia, e ao permitir o desenvolvimento e a implementação de energias renováveis. Um exemplo claro é o TES em instalações de concentração de energia solar (CSP), onde o excesso de energia solar durante o período de luz solar é armazenado para ser utilizado durante os períodos em que a energia solar é necessária, mas não está disponível. Os sais fundidos têm sido amplamente utilizados como fluidos de transferência de calor (HTF) e para TES em instalações CSP. Uma das características fundamentais dos sais fundidos no que diz respeito à sua utilização como HTF e TES é a sua compatibilidade com os materiais de construção. Por esta razão, este trabalho apresenta a resistência à corrosão de dois aços inoxidáveis austeníticos AISI 316L e AISI 321H em contacto com sal ternário fundido com cálcio. Ambos os aços inoxidáveis demonstraram ter velocidades de corrosão aceitáveis para as centrais CSP. Entre os aços austeníticos estudados, o AISI 316L apresenta a menor velocidade de corrosão.
- Da Tectónica de Placas à Teoria da Evolução; dos supercontinentes à dispersão dos seres vivosPublication . Dias, R.; Nascimento, F.; Machado, L.; Pereira, E.; Paiva, A.; Batista, A.; Silva, V.; Gomes, A.; Silva, M Inês; Soares, Alexis; Garcia, Karina LuciaRESUMO: A evolução do conhecimento do interior do nosso planeta tem levado a profundas alterações da forma como a dinâmica da Terra é encarada, evidenciando a existência de ciclo dos supercontinentes. Esta transformação tem permitido uma visão mais integradora, onde os processos tectónicos e de evolução da Vida na Terra surgem como complementares. No entanto, esta nova abordagem ainda não se reflete nos curricula do ensino. A realiza ção de um conjunto de atividades práticas explorando a forma como os grandes blocos continentais foram evoluindo, desde o supercontinente Rodínia até à Atualidade, e a sua influência na dispersão de alguns dos prin cipais grupos de seres vivos, constitui uma abordagem eficiente de alguns aspetos associados ao ciclo dos supercontinentes.
- Durability of CSP materials [Comunicação oral]Publication . Cunha Diamantino, Teresa; Gomes, A.; Paiva Luís, Teresa; Vasques, I. F.; Gonçalves, Rita
- Effect of the oxide loading on the surface characteristics of LaNio3 oxide coated electrodesPublication . Soares, C. O.; Silva, R. A.; Carvalho, M. D.; Jorge, M. E. Melo; Gomes, A.; Rangel, C. M.; Pereira, M. I. da SilvaThe LaNiO3 perovskite-type oxide is one of the most tested anode for the oxygen evolution reaction in alkaline solutions. It is well established that the oxide preparation conditions and the electrode fabrication are key factors to control the electrochemical behaviour of oxide coatings. In a previous work the authors studied the influence of preparation conditions of the oxide and support type on the electrochemical behaviour of Ni foam coated LaNiO3 electrodes. Ni foam was selected as support due to its unique characteristics namely low contact resistance between the oxide and support, possibility of high metal oxide loadings and dimensional stability [1]. No studies were performed, concerning the influence of the oxide loading. Studies performed by Singh et al. on LaNiO3 coatings on Ni foil supports have shown that the electrode roughness factor increased with increase in oxide loading at the beginning and finally attained a constant value around 0.03 g cm-2 [2]. The present work reports on the study of the dependence of roughness factor (Rf) and morphology factor (φ) on the oxide loading for Ni foam coated LaNiO3 electrodes with loadings varying between 0.02 and 0.14 g cm-2. Cyclic voltammetry and electrochemical impedance spectroscopy were used to evaluate the Rf and φ values, complemented by optical microscopy observations. A non-linear increase of both Rf and φ with the oxide loading is observed, showing a level off when the oxide loading is increased. The level off was interpreted as a progressive exclusion of the crystallites from the contact with the solution as the oxide coating thickness increases.
- High surface area LaNiO3 electrodes for oxygen electrocatalysis in alkaline mediaPublication . Rangel, C. M.; Soares, C. O.; Silva, R. A.; Carvalho, M. D.; Jorge, M. E. Melo; Gomes, A.; Pereira, M. I. da Silva
- High-temperature corrosion performance of austenitic stainless steels type AISI 316L and AISI 321H, in molten solar saltPublication . Gomes, A.; Navas, M.; Uranga, N.; Paiva Luís, Teresa; Vasques, I. F.; Cunha Diamantino, TeresaABSTRACT: The corrosion rates of AISI 316L and AISI 321H austenitic stainless steel, immersed in a stagnant isothermal mixture of 60% NaNO3 and 40% KNO3 molten salt at 550 degrees C in atmospheric air are 8.6 and 9.0 mu m/yr, respectively. The corrosion mechanism was proposed by recording the weight changes of the steel coupons at different time intervals up to 3000 h, and by the characterization of multilayer oxide scales formed on the steel surface. Multilayers made of different oxides, mainly Fe2O3 and Fe3O4, are the principal scale products. At 3000 h, the thickness of the scale layer formed on AISI 321H (7.5 +/- 2.9 mu m) is slightly higher than the one formed at the AISI 316L (6.9 +/- 2.1 mu m). This small difference might reflect the partial spallation of the corrosion layer on AISI 321H, which is seen for times longer than 1000 h. A minimal change of the composition of the molten nitrate salt is observed in time and is predominantly due to the appearance of soluble chromate products and nitrite compounds (0.004 wt% and 1.4 wt% at 3000 h, respectively). The observed corrosion behaviour of these alloys shows that they are good candidate for usage as containers of molten nitrate salts in the thermal energy storage (TES) system for a CSP plant.
- Improving electrocatalityc activity of LaNiO3 coatings by deposition on foam nickel substratesPublication . Soares, C. O.; Carvalho, M. D.; Jorge, M. E Melo.; Gomes, A.; Silva, R. A.; Rangel, C. M.; Pereira, M. I. da Silva
- Improving electrocatalityc activity of LaNiO3 films by deposition on foam nickel substratesPublication . Soares, C. O.; Carvalho, M. D.; Jorge, M. E. Melo; Gomes, A.; Silva, R. A.; Rangel, C. M.; Pereira, M. I. da SilvaThe electrodes were obtained by coating a nickel foam support with the oxide suspension. Optical microscopy and cyclic voltammetry were used on the electrodes characterization. The evaluation of the electrodes electrocatalytic activity, towards the oxygen evolution reaction in alkaline medium, was performed by means of steady state measurements. The reaction follows a first order kinetics, with respect to OH- concentration, with Tafel slopes close to 40 mV, for low overpotentials. Based on the apparent and real current densities it was possible to conclude that the increase on the electrode activity, when compared with the published data, is mostly related to geometric factors. This fact has been associated with the high electrode/electrolyte contact area provided by the foam nickel substrate. Synergetic effects between the Ni foam and the perovskite oxide cannot be discarded.
- LaNiO3-based catalyst in gas diffusion electrodes: activity and stability for oxigen reactionsPublication . Silva, R. A.; Soares, C. O.; Carvalho, M. D.; Jorge, M. E. Melo; Gomes, A.; Rangel, C. M.; Pereira, M. I. da SilvaPerovskite-type oxides are potential catalysts for next generation of regenerative fuel cells. In particular, LaNiO3 has been recognised as one of the most promising oxygen electrodes. In this work LaNiO3 perovskite-type oxides, prepared by a self-combustion method [1, 2], have been used for the preparation of porous gas-diffusion electrodes (GDE). Electrodes were prepared on Toray carbon paper (CP) substrates, consisting of a diffusion layer, a catalyst layer and a Nafion® layer. The gas diffusion layers were prepared using Vulcan XC-72R. The catalyst ink was prepared by suspending the material in isopropanol, stirring the mixture in an ultrasonic bath to thoroughly disperse it. Ink slurries were also pasted onto glassy carbon discs and used as working electrodes for full kinetic studies at potential domains for the oxygen reduction (ORR) and oxygen evolution (OER) reactions. A systematic study on the effect of the oxide loading (OL) on the electrodes surface area was done by cyclic voltammetry. It was found a quasi linear variation between the electrodes surface area and the oxide loading. Roughness values varying from 106±3 to 307±6 were obtained for OL between 1 and 5 mg cm-2 respectively. The results show that the peak current density increases with the increasing on oxide loading as shown in Fig. 1. Higher current densities for ORR were obtained for the electrodes prepared using LaNiO3-based perovskite with partial substitution of Ni by Cu. Stability studies of the GDEs, performed using a pre-defined cycling protocol in 1M KOH, will be discussed together with catalytic activity parameters relevant for their potential use as bifunctional oxygen electrodes.
- Molten nitrate salts containing lithium as thermal energy storage media : a short reviewPublication . Pedrosa, Fátima; Marcelo, Teresa; Nogueira, Carlos; Gomes, A.; Cunha Diamantino, TeresaABSTRACT: Energy storage plays a crucial role in the development and progress of renewable energy sources. In the case of thermal energy, storage has been a step ahead since storage solutions already exist, unlike other conversion energy technologies. In this sense, the utilization of molten salts, mainly composed of mixtures of alkali/alkaline earth metals, in nitrate forms (sometimes also with nitrite), have been proposed and successfully applied. Molten salts have recognized advantages over other solutions namely in what concerns simultaneous use as heat transfer fluids and as storage media. Commercial utilization of molten salts has been tested essentially with the Na/K salt mixtures. Introduction of lithium nitrate in these formulation has been proposed, since can allow decreasing the melting point of the mixtures, thus providing safety concerning the risks of freezing in the tubes or tanks of the storage plant. This paper provides a short review on the utilization of lithium in molten salt mixtures used for thermal energy storage. Physical, chemical and thermal properties, as well as stability and decomposition issues are evaluated. Corrosion of stainless steels in contact with the salts is another crucial question that is also addressed.