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Navegando por Autor "Marques, Fernando M. B."

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    Artigo
    Assessment of processing route on the performance of ceria-based composites
    (Wiley Online Library, 2021-02-02) Nascimento, Rubens Maribondo do; Grilo, João Paulo de Freitas; Macedo, Daniel Araújo de; Marques, Fernando M. B.
    Gd-doped ceria (GDC)-based composites, including a eutectic mixture of lithium and sodium carbonate (NLC) as second phase, were prepared using several processing routes, namely chemical synthesis, co-firing of both phases, or impregnation of a presintered porous matrix. LiAlO2-based composites were also prepared and used as reference. The structural, microstructural, and electrical properties (impedance spectroscopy in air, up to 700 C) of these materials were assessed in detail. A limited set of compositions was also used in measurements of total electrical conductivity at different oxygen partial pressures (from 0.21 to <10−25 atm), in the 600 C to 700 C range. The results obtained confirmed the enormous impact of the processing route on the percolation of the ceramic phase. In the exploited range of operating conditions, GDC-NLC composites prepared by infiltration of molten carbonates within a mechanically robust ceramic matrix exhibit a high total ionic conductivity strongly influenced by the contribution of the molten salt while the ceramic phase is decisive in the appearance of a significant component of electronic conductivity under reducing conditions
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    Artigo
    Effect of composition on the structural development and electrical conductivity of NiO-GDC composites obtained by one-step synthesis
    (Elsevier, 2017-08) Nascimento, Rubens Maribondo do; Grilo, João Paulo de Freitas; Moura, Caroline Gomes; Macedo, Daniel Araújo de; Rajesh, Surendran; Figueiredo, Filipe M. L.; Marques, Fernando M. B.
    NiO-C0.9Gd0.1O1.95 (NiO-GDC) composites obtained using a chemical route (one-step synthesis) were characterized by thermal analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance spectroscopy (between 300 and 650 °C in air). Rietveld refinement of XRD data indicated that synthesized powders are ultrafine and the crystallite size of the GDC phase decreases with increasing NiO content. The relative density of sintered samples is influenced by the NiO content, but easily brought to values above 95% after sintering at 1450–1500 °C. NiO-GDC composites exhibited homogeneous phase distribution and grain size often lower than 1 μm. With 30–40 wt% NiO this phase dominates the overall electrical conductivity of NiO-GDC. The combination of grain size, conductivity and microstructural characteristics shows the efficacy of the adopted processing route to obtain high quality Ni-GDC cermet precursors
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    Artigo
    Electrical properties of lanthanum silicate apatite electrolytes prepared by an innovative chemical route
    (Electrochemical Society, 2014-09-29) Martinelli, Antonio Eduardo; Macedo, Graziele Lopes; Macedo, Daniel Araújo de; Rajesh, Surendran; Figueiredo, Filipe M. L.; Marques, Fernando M. B.; Nascimento, Rubens Maribondo do
    This work reports the synthesis of apatite-type lanthanum silicate powders with nominal composition La10Si6O27 by an innovative chemical route which combines co-precipitation and sol-gel approaches. The herein proposed synthetic procedure is advantageous in reducing energy consumption and processing time, key elements to decrease the total manufacturing cost. The asprepared powder was calcined between 500 and 900 °C. Structure of the calcined powders, characterized by X-ray diffraction (XRD) and Rietveld refinement, indicated the formation of apatite crystalline lanthanum silicate with minor amount of L2O3 at 900 °C. The apatite powder was further analyzed by scanning electron microscopy (SEM). The effect of sintering temperature, ranging from 1400 to 1450 °C, on the electrical properties was investigated by impedance spectroscopy. Electrical measurements were recorded in the 400 – 800 °C temperature range. The lanthanum silicate electrolyte sintered at 1450 °C exhibited total electrical conductivity (σ700 ºC = 2.12 x 10-3 S.cm-1) comparable with samples from literature prepared by sol-gel and freeze drying methods and sintered at 1500 °C. The results in the present study demonstrate that the co-precipitation method combined with a solgel approach is a promising alternative route to synthesize lanthanum silicate powders
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    Artigo
    Electronic conductivity in Gd-doped ceria with salt additions
    (Elsevier, 2019-09) Nascimento, Rubens Maribondo do; Grilo, João Paulo de Freitas; Macedo, Daniel Araújo de; Marques, Fernando M. B.
    Samples of Gd-doped ceria (GDC, Ce0.9Gd0.1O1.95) with eutectic mixtures of Na2CO3 and Li2CO3 (NLC), or K2CO3, Na2CO3 and Li2CO3 (KNLC) as sintering aids (5 mol%) were studied against pure GDC and cobalt doped GDC. The former electrolytes reached densifications in excess of 95% when sintered at 1100 C, values comparable to pure GDC sintered at 1500 C, and more than 20% higher than for GDC sintered at 1100 C. X-ray diffraction powder patterns showed only typical GDC peaks without any noticeable lattice modification and microstructural analysis suggested a homogeneous distribution of residual amounts of sintering aids. The electrical properties of all materials were studied by impedance spectroscopy in air from 200 to 750 C. Materials with NLC and KNLC showed a total ionic conductivity matching pure GDC at 600 C. Hebb-Wagner polarization measurements, used to assess the impact of sintering aids on the nand p-type electronic conductivity of GDC (600e750 C) showed that NLC and KNLC additions lower the p-type conductivity of GDC while Co additions have the opposite effect. The efficacy of alkali metal salts to produce ceria-based electrolytes with competitive ionic and electronic transport properties is exposed
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    Artigo
    One-step synthesis of composite electrolytes of Eu-doped ceria and alkali metal carbonates
    (Elsevier, 2013-12-13) Nascimento, Rubens Maribondo do; Rajesh, Surendran; Macedo, Daniel Araújo de; Souza, Graziele Lopes de; Figueiredo, Filipe Miguel Henriques Lebre Ramos; Marques, Fernando M. B.
    Europium-doped ceria (EDC, Ce0.9Eu0.1O2 d)/alkaline carbonate (LNC, (Li,Na)2CO3) composite ceramics prepared through a one-step citrate-based route were analyzed by powder X-ray diffraction, infrared and laser Raman spectroscopies as well as scanning and transmission electron microscopy. The electrochemical behavior of the electrolyte material was studied by impedance spectroscopy in air, CO2 and N2 þ H2 (90/10 vol%, respectively) gas mixtures, in the temperature range 300e600 C. The sub micrometric and even nanosized ceramic particles appeared as merged inside the mixed carbonates, with modest grain to grain necking. The EDC/LNC composite electrolytes showed a conductivity of 0.27 S cm 1 at 600 C in air, amongst the best ever reported, exceeding the usual requirements for fuel cell applications
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    Artigo
    Performance of GDC with alkali metal carbonates as sintering aids
    (Elsevier, 2020-03) Nascimento, Rubens Maribondo do; Grilo, João Paulo de Freitas; Macedo, Daniel Araújo de; Marques, Fernando M. B.
    Gd-doped ceria electrolytes (GDC, 10 at.% Gd) were prepared using a eutectic mixture of Li and Na carbonates as sintering aid (NLC - 0, 1, 3 and 5 mol%), using two processing routes (ceramic and chemical). Structural and microstructural characterization provided no evidence for significant solubility of alkali metal cations in GDC. Materials sintered at 1100 °C/4 h were similar in densification (> 95%) and microstructure to those obtained from a commercial powder sintered at 1500 °C/4 h. Impedance spectroscopy in air (200–600 °C) and as a function of oxygen partial pressure (from 0.21 to about 10−25 atm, 600–750 °C) showed that the low temperature conductivity of materials prepared using chemical synthesis surpassed the conductivity of a commercial GDC due to higher grain boundary conductivity. The estimated n-type conductivity of materials with NLC was also slightly higher than for pure GDC. The role of sintering aid and processing routes is discussed taking into consideration decomposition and volatilization of alkali salts
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    Artigo
    A versatile route for the preparation of Ni–CGO cermets from nanocomposite powders
    (Elsevier, 2013-05) Macedo, Daniel Araújo de; Souza, Graziele L.; Cela, Beatriz; Paskocimas, Carlos Alberto; Martinelli, Antonio Eduardo; Figueiredo, Filipe Miguel Henriques Lebre Ramos; Marques, Fernando M. B.; Nascimento, Rubens Maribondo do
    A comparative microstructural study between Ni–Ce0.9Gd0.1O1.95 (Ni–CGO) anodes obtained from NiO–CGO nanocomposite powders prepared by in situ one-step synthesis and by mechanical mixture (two-step synthesis) of NiO and CGO powders is reported. The open porosity and microstructure of sintered and reduced pellets were investigated as a function of the citric acid content used as pore forming agent. Nanosized crystallites for the one-step and two-step routes were around 18 nm and 24 nm against 16 nm and 37 nm, for CGO and NiO, respectively. Overall results show that both routes provided suitable microstructures either for anode-support, or for functional anodes for solid oxide fuel cells (SOFCs), with more versatile characteristics in the case of the one-step route. The electrical characterization of selected NiO–CGO samples, carried out between 90 and 260 °C by impedance spectroscopy, confirms electrical percolation of both phases in the composites. However, based on combined microstructural and impedance data, it seems clear that the one-step processing route is the best approach to make SOFC anodes with improved performance
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