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- Synthesis of Non-Cubic Nitride Phases of Va-Group Metals (V, Nb, and Ta) from Metal Powders in Stream of NH3 Gas under Concentrated Solar RadiationPublication . Shohoji, Nobumitsu; Oliveira, Fernando Almeida Costa; Galindo, José; Rodriguez, Jose; Cañadas, Inmaculada; Fernandes, Jorge Cruz; Rosa, Luís GuerraABSTRACT: Using a high-flux solar furnace, loosely compacted powders of Va-group transition metal (V, Nb, and Ta) were reacted with stream of NH3 gas (uncracked NH3 gas) being heated by concentrated solar beam to a temperature (T) range between 600 and 1000 degrees C. From V, sub-nitride V2N (gamma phase) and hypo-stoichiometric mono-nitride VN possessing fcc (face-centered cubic) crystal lattice structure (delta phase) were synthesized. On the other hand, in the reaction product from Nb and Ta, hexagonal mono-nitride phase with N/M atom ratio close to 1 (epsilon phase) was detected. The reaction duration was normalized to be 60 min. In a conventional industrial or laboratory electric furnace, the synthesis of mono-nitride phase with high degree of crystallinity that yield sharp XRD peaks for Va-group metal might take a quite long duration even at T exceeding 1000 degrees C. In contrast, mono-nitride phase MN of Va-group metal was synthesized for a relatively short duration of 60 min at T lower than 1000 degrees C being co-existed with lower nitride phases.
- Homogeneização do fluxo em concentradores solares de elevada intensidadePublication . Pereira, J.C.G.; Fernandes, Jorge Cruz; Rodríguez, J.; Rosa, Luís GuerraRESUMO: Quer as medições experimentais realizadas com radiómetros, quer as simulações ópticas já efectuadas, evidenciam a dificuldade de obtenção de fluxos homogéneos de radiação solar, nos atuais fornos solares de elevada concentração. Apesar de se recorrer a dispositivos ópticos denominados homogeneizadores do fluxo da radiação, os atuais fornos solares não conseguem contornar satisfatoriamente este problema. Neste trabalho propõe-se uma solução baseada na utilização de concentradores solares com dois parabolóides, um grande e um pequeno, com o mesmo ponto focal. De acordo com as nossas simulações de óptica geométrica, a dupla reflexão pelos dois parabolóides permite obter radiação homogénea numa região central circular com dimensões adequadas.
- Thermal Shock Resistance of Commercial Oxide-Bonded Silicon Carbide Reticulated Foams under Concentrated Solar Radiation at PSA: A Feasibility StudyPublication . Oliveira, Fernando Almeida Costa; Galindo, José; Rodriguez, Jose; Cañadas, Inmaculada; Fernandes, Jorge CruzABSTRACT: Volumetric ceramic receivers can be regarded as a promising technology to heat air above 1000 ◦C for solar thermal electricity production. In this study, the thermal shock behavior of commercial 10 ppi (A) and 20 ppi (B) oxide-bonded silicon carbide (ob-SiC) reticulated porous ceramic (RPC) foams was evaluated using the SF60 solar furnace at Plataforma Solar de Almería. The foams were subjected to well-controlled temperature cycles ranging from 800 to 1000, 1200, 1300 or 1400 ◦C, for 25, 100, and 150 cycles. The extent of the damage after thermal shock was determined by crushing tests. The damage was found to be critically dependent on both the bulk density and cell size. Decreasing both the bulk density and cell size resulted in better thermal shock resistance. The B foam exhibited approximately half the stress degradation compared to the A foam when exposed to a temperature difference of 600 K (in the range of 800 to 1400 ◦C) and subjected to 150 cycles.
- A New Insight into the Role of Silicate-Type Binders on the Crushing Strength of Alumina FoamsPublication . Oliveira, Fernando Almeida Costa; Pommier, Stéphane; Fernandes, Jorge Cruz; Dias, DiamantinoABSTRACT: Semi-closed cell macroporous alumina foams with relative densities ranging from 0.26 to 0.35 have been produced by the well-established replication method based on the coating of a polyurethane (PU) template foam by a ceramic slurry, followed by burnout of the PU template, and sintering of the ceramic skeleton. Collapse of the three-dimensional structure upon the volatilisation of the PU sponge can only be prevented using appropriate binders. Scarce data are available on the slurry formulations of commercial alumina foams. The aim of this study was to investigate the influence of silicate-type binders, namely kaolin and bentonite additives, on the crushing strength of alumina foams. The highest crushing strength of around 10 MPa was observed at a porosity of 66 +/- 2%. The open-cell model is inadequate to fit the crushing strength data of such semi-closed cell type structures. Both microscopic and macroscopic flaws resulting from the foam processing method contribute to the wide scatter of the strength, thereby explaining the Weibull modulus ranging from 4 to 7. Both flaw populations require further improvement to maximise the crushing strength of these foams with high potential for the design of structured catalyst carriers and molten aluminium filters.
- Manufacturing and Thermal Shock Resistance of 3D-Printed Porous Black Zirconia for Concentrated Solar ApplicationsPublication . Oliveira, Fernando Almeida Costa; Sardinha, Manuel; Galindo, José; Rodríguez, José; Cañadas, Inmaculada; Leite, Marco; Fernandes, Jorge CruzABSTRACT: A novel approach for manufacturing porous materials, foreseen as solar receivers for concentrated sun radiation, used in the power tower technology is presented. In such applications, materials are subjected to steep thermal gradients and thousands of cycles. Yet, materials consisting of honeycombs and ceramic foams showed insufficient thermal performance. By using the fused filament fabrication process, one can design printed parts meeting the requirements for solar receivers, namely dark color and high solar absorptance. This exploratory study unveils data on the retained crushing strength of newly developed 3D-printed porous Black Zirconia cubes after thermal cycling under similar conditions to those experienced by volumetric receivers and catalyst substrates for solar fuels (H-2 and/or CO) production via the thermochemical cycle. Unlike dense ceramics, the resistance to thermal shock of 3D-printed cubes underwent a gradual decrease with the increase in the thermal gradient. The thermal shock cycles were performed between 800 degrees C and 1100, 1200, and 1300 degrees C, corresponding to a Delta T of 300, 400, and 500 K, respectively. Additionally, water quenching tests were performed at Delta T = 300 K up to 400 K. Crushing strength measurements carried out to evaluate the retained mechanical strength after exposure up to 100 cycles showed that the Black Zirconia cubes can withstand thermal gradients up to at least 400 K.