SHIRLEY LEITE DOS REIS

SHIRLEY LEITE DOS REIS

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Ionic conductivity of doped lanthanum gallate and strontium gallate composites
2017 - REIS, S.L.; MUCCILLO, E.N.S.
Polycrystalline ceramic solid electrolytes based on strontium- and magnesium-doped lanthanum gallate exhibit high ionic conductivity and find potential application in solid oxide fuel cells operating at intermediate temperatures. Sintering of this solid electrolyte is usually carried out at high temperatures, being responsible for loss of Ga and consequent formation of impurity phases. In this work, composites consisting of La0.9Sr0.1Ga0.8Mg0.2O3-δ with additions of strontium gallate were prepared by solid state reaction, and the influence of the additiveon electrical conductivity and phase composition of the composite electrolyte was investigated. The sintered density of composites is higher than 95% of the theoretical value after sintering at 1350°C. The contents of impurity phases decreased with increasing additions of strontium gallate. The overall ionic conductivity of the composites is higher than that of LSGM. This effect is attributed to the decrease in the fraction of free MgO at grain boundaries in the composites.
Influence of yttria-stabilized zirconia on microstructure and electrical properties of doped lanthanum gallate
2019 - FUJIMOTO, TALITA G.; REIS, SHIRLEY L.; MUCCILLO, ELIANA N. dos S.
Lanthanum gallate with partial substitutions for strontium and magnesium shows higher ionic conductivity than traditional ionic conductors such as yttria-stabilized zirconia (8YSZ), and has been considered for application as solid electrolyte and electrode in Solid Oxide Fuel Cells operating at intermediate temperatures (500-700ºC). Parallel to the development of new solid electrolytes, improvement of the already known electrolytes has been one goal. In this work, the effects of adding 8YSZ to La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM), on the microstructure and electrical performance were investigated. Compound electrolytes consisting of up to 20 wt.% 8YSZ were prepared by solid state reactions. Sintered pellets with up to 10 wt.% 8YSZ achieved near full density after sintering at 1450ºC and reduction in the fraction of impurity phases. Addition of 8YSZ promoted grain growth to LSGM. The higher ionic conductivity was obtained for 1 wt.% 8YSZ addition with slight decrease of the apparent activation energy.
Influence of small amounts of gallium oxide addition on ionic conductivity of La0.9Sr0.1Ga0.8Mg0.2O3-δ solid electrolyte
2018 - REIS, S.L.; MUCCILLO, E.N.S.
The effects of small amounts of gallium oxide on intragrain and intergrain conductivity of La0.9Sr0.1Ga0.8Mg0.2O3-δ are investigated by impedance spectroscopy in the 280–420 °C range. Bulk specimens with 0.5, 1.0 and 1.5 mol% gallium oxide are prepared by solid state reaction at 1350 °C. All specimens achieved relative density values higher than 95%. The additive promotes grain growth indicating solid solution formation. A small fraction of the additive remains at grain boundaries and increases the fraction of the gallium-rich, LaSrGa3O7, impurity phase. The intragrain conductivity of gallium oxide containing specimens is higher than that of the parent solid electrolyte. Similar effect is found for the intergrain conductivity, which is maximum for 1 mol% gallium oxide addition.
Preparation of dense La0.9Sr0.1Ga0.8Mg0.2O3-δ with high ionic conductivity by solid-state synthesis
2018 - REIS, S.L.; MUCCILLO, E.N.S.
The results of a systematic investigation on the effects of processing steps, via solid-state reactions, on structural phase characteristics and ionic conductivity of La0.9Sr0.1Ga0.8Mg0.2O3-δ solid electrolyte are reported. The main purpose of this work is to establish an optimized route for obtaining good densification and high ionic conductivity of this solid electrolyte. Processing routes with three successive calcinations at 1250 °C followed by attrition milling (R1), and with two sequences of calcination at 1350 °C with intermediary attrition milling (R2) give rise to near full density at 1450 °C sintering temperature. The rate of grain growth is fast when the relative density reaches 95%. Elemental mapping reveals uniform distribution of the constituents in the matrix along with La4Ga2O9, LaSrGa3O7 and sub-micrometer MgO grains at grain boundaries. The ionic conductivity of grains remains unchanged with the processing route and sintering profile. The blocking effect of charge carriers at grain boundaries decreases with increasing the dwell temperature.
Improved ionic conductivity of zirconia-scandia with niobia addition
2017 - GROSSO, ROBSON L.; REIS, S.L.; MUCCILLO, E.N.S.
The ionic conductivity and the crystalline structure of ZrO2−10 mol% Sc2O3- x mol% Nb2O5 solid electrolytes were investigated for x=0.25, 0.5 and 1. Dense specimens with relative densities higher than 95% were prepared by solid state reaction and sintered at 1500 °C for 5 h. Full stabilization of the cubic structure at room temperature was obtained for compounds with x=0.5 and 1, whereas the cubic and rhombohedric structures coexist for x=0.25. The highest ionic conductivity in codoped system was found for specimen containing 0.5 mol % niobium pentoxide, with the same order of magnitude as that of the parent solid electrolyte (zirconia-10 mol % scandia) in the high temperature range (above 600 °C). Preliminary investigation on phase stability shows that the isothermal conductivity of the new solid electrolyte remained constant up to 100 h at 600 °C. Niobium pentoxide addition was found to improve the overall ionic conductivity of zirconia-scandia solid electrolyte.
Influence of additives on phase stabilization of scandia-doped zirconia
2017 - MUCCILLO, ELIANA N. dos S.; GROSSO, ROBSON L.; REIS, SHIRLEY L. dos; MUCCILLO, REGINALDO
The effects of small additions of tin, zinc, calcium and boron oxides on phase composition and electrical conductivity of zirconia-10 mol% scandia were investigated. Compounds containing 1 mol% zinc, tin and calcium oxides and 1, 3 and 5 wt.% boron oxide were prepared by solid state reaction and characterized by X-ray diffraction, density measurements, scanning electron microscopy and impedance spectroscopy. Full stabilization of the cubic structure at room temperature was obtained with additions of 1 mol% calcium oxide and 2 wt.% boron oxide. Partially stabilized compounds exhibit herringbone structure, characteristic of the β- rhombohedric phase. Specimens with calcium as additive show total conductivity of 23.8 mS.cm-1 at 750ºC with activation energy of 1.13 eV. Liquid phase sintering by boron oxide addition is effective to enhance the densification of the solid electrolyte.
Ionic conductivity of chemically synthesized Lasub(0.9)Srsub(0.1)Gasub(0.8)Mgsub(0.2)Osub(3-'delta') solid electrolyte
2014 - REIS, S.L.; MUCCILLO, E.N.S.
Strontium and magnesium-doped lanthanum gallate is a solid electrolyte with higher ionic conductivity than yttria-stabilized zirconia. This perovskite-type ionic conductor has been investigated for application as solid electrolyte in intermediate-temperature solid oxide fuel cells. In this work the La0.9Sr0.1Ga0.8Mg0.2O3-δ composition was synthesized by a soft chemistry route seeking for a more homogeneous microstructure. Densification was carried out by the conventional method and by fast firing. The ionic conductivity of sintered specimens were evaluated by impedance spectroscopy. The apparent density was about 90% and 94% for fast fired and conventionally sintered specimens, respectively. No significant difference was found for the grain conductivity of specimens fast fired at 1500 ºC for 5 min and sintered by the conventional method at 1450 ºC for 4 h.