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Navegando por Autor "Grilo, João Paulo de Freitas"

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    Artigo
    Assessment of NiO-CGO composites as cermet precursors
    (Elsevier, 2018-08) Nascimento, Rubens Maribondo do; Grilo, João Paulo de Freitas; Macedo, Daniel Araújo de; Marques, Fernando Manuel Bico
    NiO-Ce0.9Gd0.1O1.95 (NiO-CGO) composite powders produced by a one-step chemical route were assessed as precursors of fuel cell anode cermets. Porous NiO-CGO composites were prepared by uniaxial pressing followed by firing at 1400 °C for 4 h. Ni-CGO cermets were subsequently obtained by reduction in 10 vol% H2 + 90 vol% N2 at 750 °C. Similar materials obtained by conventional mechanical mixing of commercial powders were also prepared and tested. The structure and particle size of the calcined material were investigated by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). The electrical properties of all materials were duly studied by impedance spectroscopy (NiO-CGO) (in air and several N2 + O2 gas mixtures) or dc conductivity (Ni-CGO). The total and relative magnitude of impedance arcs observed in the low temperature impedance spectra can be used to obtain significant insight on microstructural characteristics, due to a variable role of NiO when shifting from a well percolated electronic conducting pathway to a dispersed ion-blocking phase. The confirmed superior conductivity of one-step materials was interpreted as a consequence of the presence of a network of well-connected and homogeneously distributed NiO(/Ni) grains, as shown by SEM
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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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    Battery-like behavior of Ni-ceria based systems: Synthesis, surface defects and electrochemical assessment
    (Elsevier, 2019-04-15) Paskocimas, Carlos Alberto; Araújo, Allan Jedson Menezes de; Silva, Vinícius Dias; Sousa, Angel Roberta Oliveira de; Grilo, João Paulo de Freitas; Simões, Thiago Araújo; Macedo, Daniel Araújo de; Nascimento, Rubens Maribondo do
    NiO, CeO2 and respective composites are extensively used in energy storage devices due to mostly their high electrochemical activity. However, the assessment of battery-like behavior of Ni-ceria based systems comprising (Ni or Gd)-doped ceria combined with NiO seems to be neglected in the literature. In this work, NiO and ceria-based solid solutions composite powders were obtained by a co-precipitation synthesis method. The structure and particle size of the calcined powders were investigated by X-ray diffractometry (XRD) and field emission scanning electron microscopy (FESEM), respectively. Oxidative states of composites were inspected by X-ray photoelectron spectroscopy (XPS). The electrochemical performance of powders was evaluated by cyclic voltammetry, galvanostatic charge-discharge and impedance spectroscopy. Refinement of the XRD patterns showed that powders have nanosized crystallites and mean size of particles within 20 – 70 nm were revealed by FESEM. The improved specific capacity of the NiO-CeO2 electrode material (about 2.5 times higher than that of NiO-CGO at 5 mV s−1) is due to an increase in Faradic reactions taken place on its surface with a higher fraction of defects (namely Ni3+, Ce3+ and oxygen vacancies), as determined by XPS. The superior electrochemical performance of the NiO-CeO2 electrode is also confirmed by electrochemical impedance spectroscopy
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    Artigo
    Cathodic polarisation of composite LSCF-SDC IT-SOFC electrode synthesised by one-step microwave self-assisted combustion
    (Elsevier, 2019-05) Nascimento, Rubens Maribondo do; Loureiro, Francisco J. A.; Macedo, Daniel Araújo de; Cesário, Moises Rômolos; Grilo, João Paulo de Freitas; Yaremchenko, Aleksey A.; Fagg, Duncan Paul
    Nano-sized composite La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF)-Ce0.8Sm0.2O1.9 (SDC) powder was synthesised by one-step microwave self-assisted combustion synthesis and characterized by X-ray diffraction and transmission electron microscopy. Electrochemical performance of composite LSCF-SDC cathode in contact with Ce0.8Gd0.2O1.9 (GDC) solid electrolyte under steady state polarisation was studied employing electrochemical impedance spectroscopy in 3-electrode cell configuration. The evolution of elementary steps of oxygen reduction reaction was analysed under cathodic polarisation in potentiostatic mode (up to −0.8 V) in the temperature range 650–750 °C. The composite LSCF-SDC cathode prepared by one-step microwave combustion route demonstrated a lower overall polarisation resistance when compared to literature reports of similar cathodes produced by other synthesis routes
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    Cobalt-free perovskite Pr0.5Sr0.5Fe1 xCuxO3 d(PSFC) as a cathodematerial for intermediate temperature solid oxide fuel cells
    (Elsevier, 2016-09-01) Nascimento, Rubens Maribondo do; Moura, Caroline Gomes; Fagg, Duncan Paul; Grilo, João Paulo de Freitas; Macedo, Daniel Araújo de; Cesário, Moises Rômolos
    SFC (Pr0.5Sr0.5Fe1 xCuxO3 d) is a new perovskite-type oxide that has gained considerable attention ascathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs), due to its high mixedionic-electronic conductivity below 800 C. In this work, PSFC (Pr0.5Sr0.5Fe1 xCuxO3 d,x¼0.2 and 0.4)powders were synthesized by the citrate method and structurally characterized by X-ray diffractometry.Screen-printed cathodes were sintered at 1050 C and electrochemically characterized by impedancespectroscopy at 600e800 C in pure oxygen. The area specific resistances (ASR) of thePr0.5Sr0.5Fe0.8Cu0.2O3 dmaterial are shown to be competitive with typical values reported for cobalt-based cathodes in the measured temperature range, while, importantly, offering a significantly loweractivation energy, 0.62 eV. The thermal expansion coefficients of these Co-free cathodes are in the rangeof 13e15 10 6 C 1, in a temperature range 200e650 C, demonstrating a good thermal compatibilitywith gadolinia doped ceria (CGO) electrolytes
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    Dissertação
    Compósitos NiO-CGO obtidos pelo método de síntese em uma etapa
    (Universidade Federal do Rio Grande do Norte, 2015-06-25) Grilo, João Paulo de Freitas; Nascimento, Rubens Maribondo do; Macedo, Daniel Araújo de; ; http://lattes.cnpq.br/1027496814443777; ; http://lattes.cnpq.br/8671649752936793; ; http://lattes.cnpq.br/6106065386252349; Aquino, Flávia de Medeiros; ; http://lattes.cnpq.br/5822416945469021; Pergher, Sibele Berenice Castella; ; http://lattes.cnpq.br/5249001430287414
    Compósitos NiO-C0.9Gd0.1O1.95 (NiO-CGO), um dos materiais mais utilizado para fabricação de anodos de Células a Combustíveis de Óxido Sólido (CCOS) atualmente, foram obtidos por uma rota química que consiste na mistura das soluções precursoras das fases NiO e CGO previamente obtidas pelo método Pechini, com percentual mássico da fase NiO variando entre 0 e 100 %. Os nanopós como obtidos foram caracterizados por técnicas de análise térmica (termogravimetria e Calorimetria Diferencial Exploratória) e os materiais calcinados foram estudados por difratometria de raios X (DRX). Amostras sinterizadas entre 1400 e 1500 ºC por 4 h foram caracterizadas pelo método de Arquimedes. Os efeitos da composição relativa na microestrutura e propriedades elétricas (condutividade e energia de ativação) de compósitos sinterizados a 1500 °C foram investigados por microscopia eletrônica e espectroscopia de impedância (entre 300 e 650 °C em ar). O refinamento dos dados de DRX indicaram que os pós são ultrafinos e que o tamanho do cristalito da fase CGO tende a diminuir com o aumento de teor de NiO. De maneira semelhante, o cristalito da fase NiO tende a diminuir com o aumento da concentração de CGO, principalmente acima de 50% em massa de CGO. A análise por Arquimedes evidencia uma variação na densidade relativa em função do teor de NiO. Densidades relativas acima de 95% foram obtidas em compósitos contendo a partir de 50 % em massa de NiO e sinterizados entre 1450 e 1500 ºC. Os resultados de microscopia e espectroscopia de impedância indicam que a partir de 30 - 40 % de NiO há aumento do número de contatos NiO-NiO, ativando o mecanismo de condução eletrônica que governa o processo de condução a baixas temperaturas (300-500 °C). Por outro lado, com o aumento da temperatura de medida a mobilidade das vacâncias de oxigênio torna-se maior que a dos buracos eletrônicos do NiO, como resultado, a condutividade de alta temperatura (550 - 650 °C) em compósitos contendo até 30-40% de NiO é inferior à do CGO. Variações na energia de ativação confirmam mudança do mecanismo de condução com o aumento do teor de NiO. O compósito contendo 50 % em massa de cada fase apresenta condutividade de 19 mS/cm a 650 °C (pouco superior a 13 mS/cm encontrada para o CGO) e energia de ativação 0,49 eV.
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    Designing experiments for the preparation of Ni- GDC cermets with controlled porosity as SOFC anode materials: effects on the electrical properties
    (Elsevier, 2018-12-15) Paskocimas, Carlos Alberto; Araújo, Allan Jedson Menezes de; Grilo, João Paulo de Freitas; Loureiro, Francisco J.A.; Campos, Liszandra Fernanda Araújo; Nascimento, Rubens Maribondo do; Macedo, Daniel Araújo de
    NiO-Ce0.9Gd0.1O1.95 (NiO-GDC) composites (30, 40 and 50 wt% NiO) were obtained by a “one-step” synthesis method. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses were applied to the calcined compositions, revealing powders in the nanometer range. NiO-GDC composites were sintered at 1450-1550 °C and the respective Ni-GDC cermets were produced in a 10%H2-N2 atmosphere at 800 °C. A factorial design tool allowed to establish a regression equation that correlates the effects of Ni content and sintering temperature on the porosity of Ni-GDC cermets. Impedance spectroscopy was used to analyse the electrical properties of the solid oxide composites in air (at 500-650 °C) and of the Ni-GDC cermets in 10%H2-N2 (at 650 °C). The impedance analysis in reducing conditions revealed an improved in the electrical properties for the Ni-GDC cermet derived from the composite with 50 wt% NiO and sintered at 1450 °C. The electrical resistivity values are among the best ones reported for Ni-based cermets produced by different conditions
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    Tese
    Efeitos da adição de resíduos de cinza da casca de café na microestrutura e propriedades de cerâmicas à base de cordierita
    (Universidade Federal do Rio Grande do Norte, 2021-12-15) Ferreira, Pedro Alighiery Silva de Araújo; Nascimento, Rubens Maribondo do; https://orcid.org/0000-0001-9094-0044; http://lattes.cnpq.br/8671649752936793; http://lattes.cnpq.br/5600597533105290; Moura, Caroline Gomes; Macedo, Daniel Araújo de; Grilo, João Paulo de Freitas; Dutra, Ricardo Peixoto Suassuna
    A cordierita é um dos compostos do sistema MgO-Al2O3-SiO2 que apresenta o menor coeficiente de expansão térmica dentre os óxidos inorgânicos. Apesar desta vantagem, que resulta em elevada resistência ao choque térmico, a cordierita monofásica é difícil de ser preparada devido à sua estrteita faixa de sinterização e a existência de fases secundárias que eleva o seu coeficiente de expansão térmica. Os fundentes são geralmente utilizados para estimular a sinterização de pós precursores de cordierita e para modificar as microestruturas e propriedades dessas cerâmicas. Neste contexto, este trabalho propõe estudar os efeitos da adição dos diferentes teores de resíduo da cinza da casca de café, como agente fundente, na eliminação de fases secundárias, analisando o comportamento durante os processos de sinterização e os efeitos nas microestruturas obtidas e nas propriedades de cerâmicas à base de cordierita. Uma formulação padrão foi preparada usando argila caulinítica, talco e alumina como fonte dos elementos: alumínio, silício e magnésio. Amostras da formulação padrão pura e desta formulação contendo diferentes teores de RCCC foram preparadas por sinterização reativa (em forno convencional) na faixa de temperatura entre 1250 e 1450 °C. A análise de fase quantitativa de amostras sinterizadas foi determinada pelo refinamento de Rietveld de dados de DRX. As propriedades físicas foram estimadas pela metodologia de arquimedes, a microestrutura foi inspecionada por Microscopia Eletrônica de Varredura. O coeficiente de expansão térmica foi determinado por meio de um dilatômetro térmico e as propriedades dielétricas foram determinadas pela espectroscopia de impedância. A adição de até 1% de resíduo da cinza da casca de café (RCCC) estimulou a reação entre cristobalita e espinélio para formar cordierita, contribuindo para a diminuição da densidade aparente e aumento de porosidade aparente, porém com a adição acima de 1% de resíduo, o teor da fase cordierita reduz e ocorre um aumento de espinélio praticamente na mesma proporção, contribuindo com o aumento do coeficiente de expansão térmica. As amostras com 2% e 4% em peso de teor de resíduo de cinza da casca de café apresentaram uma descontinuidade de comportamento elétrico sem precedentes para materiais à base de aluminosilicatos.
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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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    Effects of laser fluence and liquid media on preparation of small Ag nanoparticles by laser ablation in liquid
    (Elsevier, 2017-12-01) Nascimento, Rubens Maribondo do; Moura, Caroline Gomes; Pereira, Rafael Santiago Floriani; Andritschky, Martin; Lopes, Augusto Luís Barros; Grilo, João Paulo de Freitas; Silva, Filipe Samuel
    This study aims to assess a method for preparation of small and highly stable Ag nanoparticles by nanosecond laser ablation in liquid. Effect of liquid medium and laser fluence on the size, morphology and structure of produced nanoparticles has been studied experimentally. Pulses of a Nd:YAG laser of 1064 nm wavelength at 35 ns pulse width at different fluences were employed to irradiate the silver target in different environments (water, ethanol and acetone). The UV-Visible absorption spectra of nanoparticles exhibit surface plasmon resonance absorption peak in the UV region. STEM and TEM micrographs were used to evaluate the size and shape of nanoparticles. The stability of silver colloids in terms of oxidation at different liquid media was analyzed by SAED patterns. The results showed that characteristics of Ag nanoparticles and their production rate were strongly influenced by varying laser fluence and liquid medium. Particles from 2 to 80 nm of diameter were produced using different conditions and no oxidation was found in ethanol and acetone media. This work puts in evidence a promising approach to produce small nanoparticles by using high laser fluence energy
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    Electrical properties of Ca-doped ceria electrolytes prepared by proteic sol-gel route and by solid-state reaction using mollusk shells
    (Elsevier, 2021-05-13) Nascimento, Rubens Maribondo do; Garcia, Maxwell F. L.; Araújo, Allan Jedson Menezes de; Raimundo, Rafael Alexandre; Grilo, João Paulo de Freitas; Macedo, Daniel Araújo de
    Ca-doped ceria (CCO) electrolytes were prepared using two processing approaches, proteic sol-gel synthesis using gelatin (SG) and solid-state reaction (SSR). Mollusk shell powder was used in the SSR method as the main source of Ca. Materials sintered at 1350–1450 °C exhibited relative density above 95% and different microstructural features regarding grain size and morphology. Impedance spectroscopy in air revealed a comparatively better total electrical response of materials prepared by the proteic sol-gel route. CCO ceramics prepared by the SG route have bulk conductivity four times higher than the highest conductivity of samples processed by the SSR route. Electrical properties of the grain boundaries of both series of materials reveal distinct oxygen vacancy concentration profiles and space charge potentials. The role of processing route and sintering schedules was discussed taking into consideration the distinct impact of Ca interaction in the grain boundaries
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    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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    Environmentally friendly synthesis methods to obtain the misfit [Ca2CoO3-d]0.62[CoO2] thermoelectric material
    (Elsevier, 2019-11-01) Paskocimas, Carlos Alberto; Emerenciano, Aline Alencar; Araújo, Allan Jedson Medeiros de; Grilo, João Paulo de Freitas; Macedo, Daniel Araújo de; Rasekh, Shahed; Kovalevsky, Andrei V.; Nascimento, Rubens Maribondo do
    This work reports the microstructural and thermoelectric characterization of the misfit [Ca2CoO3-d]0.62[CoO2] compound obtained by a solid-state synthesis using mollusk shells and a proteic sol-gel method, which uses gelatin as a polymerizing agent. The results clearly demonstrate the capability of these routes to produce pure Ca3Co4O9 with plate-like morphology. Sintered ceramic samples show randomly oriented grains and relative densities in the range of 63–67%. The obtained microstructures provide reasonable electrical properties and result in competitive thermoelectric performance for the material prepared by the proteic sol-gel synthesis (P.F. of 205 lW/K2 m at 700 C)
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    Innovative improvement of sintered ceramic electrolytes by salt infiltration
    (Elsevier, 2021-02-15) Nascimento, Rubens Maribondo do; Grilo, João Paulo de Freitas; Macedo, Daniel Araújo de; Marques, Fernando Manuel Bico
    Previously sintered (1500 °C, 4 h) dense pellets of Ce0.9Gd0.1O1.95 (GDC) were covered and heat treated with eutectic mixtures of Na2CO3 and Li2CO3 (NLC), and their electrical performance was assessed against pure GDC and chemically synthesized GDC+NLC. Microstructural analysis of NLC impregnated samples confirmed slight migration of the molten phase to the interior of the GDC pellets via grain boundaries, resulting in a significant improvement of the grain boundary conductivity, increasing with duration of heat treatment (0.5-2 h) and temperature (600 to 800 oC range). The observed total conductivity exceeded in almost 20% the corresponding values obtained for standard GDC samples. Cells tested before and after direct current polarization (0.5 V, 500 °C) showed the same electrical performance, discarding the possibility of parallel contributions of salt ions to the total conductivity. Grain boundary engineering using salt infiltration is an effective tool to improve the electrical performance of ceramic electrolytes
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    Laser induced synthesis of nickel alloy (Alumel) nanoparticles in liquids
    (Scientific Array, 2020) Nascimento, Rubens Maribondo do; Moura, C. G.; Cano-Raya, C.; Grilo, João Paulo de Freitas; Viana, J. C.; Denchev, Z. Z.; Silva, F. S.
    Pulsed laser ablation in liquids (PLAL) has been growing as a powerful technique for the synthesis of multielement nanoparticles (NPs) such as metal alloys with complex composition. In consequence, there is a great necessity in expanding the current knowledge about alloy NPs formation during this method, aiming to control the chemical composition of produced NPs. In this work, a nanosecond Nd:YAG laser was used to synthesize NPs of a novel nickel (Ni) alloy, and a brief description regarding the influence of liquid media in size and stability of produced nanoparticles is presented. The results showed great stability in the colloids produced in isopropanol medium without using a surfactant, whereas this environment allowed the production of smaller nanoparticles
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    Microwave-assisted hydrothermal synthesis of ceria (CeO2): microstructure, sinterability and electrical properties
    (Elsevier, 2020-10) Nascimento, Rubens Maribondo Do; Carregosa, João Domingos Covello; Grilo, João Paulo de Freitas; Godoi, Glauber Silva; Macedo, Daniel Araújo de; Oliveira, Rosane Maria Pessoa Betânio
    This work reports the microstructural (XRD/SEM), dilatometric and electrical characterization (impedance spectroscopy) of ceria (CeO2) obtained by microwave-assisted hydrothermal synthesis under mild conditions (T = 120 °C and short reaction times of 10 and 20 min). The as-prepared and calcined powders show characteristic diffraction peaks of the cubic fluorite-type structure of cerium oxide. A remarkable sinterability allows obtaining dense ceramics (relative densities exceeding 95%) with enhanced electrical properties (σt = 2.14 × 10−5 S cm−1 at 500 °C) after sintering at a temperature as low as 1200 °C
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    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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    Preparação de compósitos alumina-mulita de baixo custo via sinterização reativa entre uma argila caulinítica da Paraíba e hidróxido de alumínio
    (ABCERAM, 2016-09) Martinelli, Antonio Eduardo; Medeiros, S. G.; Dutra, Ricardo Peixoto Suassuna; Grilo, João Paulo de Freitas; Paskocimas, Carlos Alberto; Macedo, Daniel Araújo de
    Este estudo aborda os efeitos do processamento na microestrutura e nas propriedades de compósitos alumina-mulita via sinterização reativa de uma mistura argila caulinítica e hidróxido de alumínio. Amostras prensadas a 40 MPa foram sinterizadas entre 1300 e 1600 °C. As propriedades tecnológicas foram determinadas em função da temperatura de sinterização. A avaliação das fases cristalinas e da microestrutura foi realizada por difração de raios X e microscopia eletrônica. Os resultados indicaram a obtenção de mulita e alumina a partir de 1300 °C. Para sinterizações acima de 1450 °C foram obtidos valores de porosidade aparente inferiores a 0,8% e tensão de ruptura à flexão superior a 40 MPa. Amostras com morfologia bimodal (grãos equiaxiais e alongados) e resistência mecânica de 70 MPa foram obtidas após sinterização a 1600 °C. Este estudo demonstra a possibilidade de preparar compósitos alumina-mulita de baixo custo e com propriedades tecnológicas compatíveis com as de cerâmicas produzidas por companhias com grande atuação no mercado internacional de produtos refratários
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    Proteic sol–gel synthesis of Gd-doped ceria: a comprehensive structural, chemical, microstructural and electrical analysis
    (Springer, 2020-09-21) Paskocimas, Carlos Alberto; Araújo, Allan Jedson Menezes de; Grilo, João Paulo de Freitas; Loureiro, Francisco J. A.; Holz, Laura I. V.; Macedo, Daniel Araújo de; Fagg, Duncan Paul
    Gd-doped ceria (Ce1-xGdxO2-d; x = 0, 0.1 and 0.2) is one of the best ceramic electrolytes for application in solid oxide electrochemical devices. This work reports an innovative and environmentally friendly route known as ‘‘proteic sol–gel synthesis’’ using gelatin for their preparation. Materials are characterized via hermogravimetric analysis, X-ray diffractometry, scanning electron microscopy, Raman and X-ray photoelectron spectroscopies, and electrochemical impedance spectroscopy. The proposed method allows to produce pure phase nanosized powders at 400 C that exhibit excellent sinterability at 1350 C. Significant differences are found in the bulk at lower temperatures, with estimated defect association enthalpies of 0.45 and 0.56 eV for the compositions containing 10 and 20 mol% Gd, respectively, leading to an increased bulk conductivity in the first case. In contrast, identical grain boundary Schottky barrier height values of around 0.2 V are a possible result of the relatively low sintering temperature, decreasing the extent of the acceptor dopant segregation to the grain boundaries due to insufficient cation mobility. This leads to similar specific grain boundary characteristics in both doped compositions. Overall, this work provides a rational understanding of a novel route for the synthesis of CGO ceramics with competitive performance and decreased sintering temperature
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    Synthesis and properties of CuO-doped Ce0.9Gd0.1O2 δ electrolytes for SOFCs
    (Elsevier, 2015-04) Lima, Chrystian G.M.; Santos, Thamyscira H.; Grilo, João Paulo de Freitas; Dutra, Ricardo P.S.; Nascimento, Rubens Maribondo do; Rajesh, Surendran; Fonseca, Fábio C.; Macedo, Daniel Araújo de
    The effects of copper oxide (CuO) addition on the crystal structure, densification and microstructure of Gd-doped ceria (Ce0.9Gd0.1O2 δ, CGO) synthesized by the polymeric precursor method have been studied. Ce0.9 xGd0.1CuxO2 δ (0rxr0.01) precursor powders were calcined at 600 1C for 1 h and the phase formation was studied by powder X-ray diffraction combined with Rietveld refinement analysis. Relative density measurements and microstructural analysis were performed on pellets sintered in the temperature range 1000–1100 1C in air. The effect of CuO as a sintering aid becomes more visible when its content is in the 0.5–1 mol% range where the relative densities are found to be 98% for sintering temperature as low as 1000 1C. A remarkable reduction (up to 500 1C) in the sintering temperature for 1 mol% CuO-doped CGO ceramics is observed together with the formation of a dissolved phase of Gd2O3–CeO2–CuO. Ce0.89Gd0.1Cu0.01 yO2 δ sintered at 1000 1C shows total electrical conductivity of 15.5 mS cm 1 at 600 1C slightly higher than Ce0.9Gd0.1O2 δ sintered at 1500 1C (12.4 mS cm 1).
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