SABRINA GONCALVES DE MACEDO CARVALHO

SABRINA GONCALVES DE MACEDO CARVALHO

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W-doped Lanthanum Molybdenum Oxide/Lithium-Sodium-Potassium Carbonate Composite Membranes for Carbon Dioxide Permeation
2023 - MEDINA, MIDILANE S.; CARVALHO, SABRINA G.M.; TABUTI, FRANCISCO N.; MUCCILLO, ELIANA N.S.; FONSECA, FABIO C.; MUCCILLO, REGINALDO
Single-phase tungsten-doped lanthanum molybdenum oxide (La2MoWO9) ceramic powders were synthesized using the complex polymerization technique. Porous ceramic pellets were obtained by thermally removing graphite, which served as a pore former. The porous pellets were then impregnated with molten eutectic lithium-sodium-potassium carbonates. The energy dispersive X-ray analysis and scanning electron microscopy (FEG-SEM) images of the external and fracture surfaces of the La2MoWO9-(Li,Na,K)2CO3 composite dual-phase membrane revealed the percolation of the carbonate mixture through the pores. Electrochemical impedance spectroscopy measurements conducted at temperatures below and above the melting point of the eutectic carbonate composition demonstrated the contributions of oxygen and carbonate ions to the ionic conductivity of the dual membrane. The electrical conductivity of the carbonate ions within the membrane was continuously monitored for over 1300 h with negligible degradation, implying that the membrane could be used for long-term monitoring of CO2 without aging effects. A comparison of FEG-SEM images taken before and after this endurance test suggested minimal fouling, indicating that the membrane could potentially replace similar zirconia- and ceria-based composite membranes.
Electric field-assisted sintering (gadolinia-doped ceria/alkali salts) composite membranes
2020 - CARVALHO, S.G.M.; MUCCILLO, E.N.S.; MARQUES, F.M.B.; MUCCILLO, R.
Composite ceramic membranes were prepared according to two routes: (i) vacuum impregnation of molten eutec- tic sodium-lithium carbonates (NLC) into porous ceria-20 mol% gadolinia (20GDC) solid electrolytes; (ii) electric field-assisted sintering of a 25 wt% NLC/75 wt% 20GDC mixture. Porous 20GDC ceramics were obtained by controlled thermal removal of 40 vol% KCl added as pore former. Electric field-assisted (flash) sintering was car- ried out monitoring thickness during application of 200 V cm − 1 to the specimen positioned in a sample chamber inserted in a vertical dilatometer. The surfaces of the sintered membranes were observed in a scanning electron microscope. Electrochemical impedance spectroscopy measurements were performed in the 5 Hz to 13 MHz fre- quency range in the 280–580°C range. Arrhenius plots showed the transition from oxide ion conduction (due to the solid electrolyte) to carbonate ion conduction (due to the molten NLC). Membranes flash sintered at 420°C in 2 min showed electrical conductivity similar to membranes conventionally sintered at 690°C for 2 h.
Enhancement of the ionic conductivity in electric field-assisted pressureless sintered BITIVOX solid electrolytes
2019 - MEDINA, MIDILANE S.; CARVALHO, SABRINA G.M.; MUCCILLO, ELIANA N.S.; MUCCILLO, REGINALDO
Bi4V1.8Ti0.2O11 (BITIVOX) ceramic pellets, prepared with powders obtained by a sol gel technique, were sintered either conventionally at 800 C/8 h or by applying an AC electric voltage, limiting the electric current through the pellets. Electric voltages were applied isothermally at 700 C and 800 C during 5 min in the green pellet positioned in the sample holder of a dilatometer for monitoring thickness variation. The BITIVOX pellets shrank 13.6% after applying 200 V cm􀀀1 at 800 C and 10.4% heating to 800 C for 8 h. Thermal analysis and X-ray di raction of the powders were performed to evaluate the crystallization temperature and the structural phase, respectively. The electrical behavior of the sintered BITIVOX pellets was analyzed by the impedance spectroscopy technique, showing that the sample flash sintered at 800 C/5 min had lower bulk resistivity than the sample conventionally sintered at 800 C/8 h. The surfaces of the sintered pellets were observed in a scanning electron microscope showing similar grain sizes and pore content in all sintered samples.
Electrical behavior of electric field-assisted pressureless sintered ceria-20 mol% samaria
2019 - REIS, SHIRLEY L.; CARVALHO, SABRINA G.M.; MUCCILLO, ELIANA N.S.; MUCCILLO, REGINALDO
CeO2:20 mol% Sm2O3 green ceramic pellets were sintered conventionally at 1500 C/2 h and flash sintered by applying a 200 V cm􀀀1 electric field at 800 C, 1000 C and 1200 C. The thickness shrinkage of the pellets was followed bythe specimen being positioned inside a dilatometer adapted with platinum electrodes and terminal leads connected to a power supply for application of the electric voltage. The microstructure of the surfaces of the sintered samples were observed in a scanning electron microscope. The electrical properties were evaluated by the impedance spectroscopy technique in the 5 Hz–13 MHz frequency range from 210 C to 280 C. The main results show that (i) the final shrinkage level is nearly independent of the temperature when the electric field is applied and slightly better than that of the 1500 C sintered pellet, and (ii) the bulk conductivity of the sample flash sintered at 1200 C is similar to that of the sample sintered at 1500 C. The availability of a pathway for the electric current pulse derived from the applied electric field is proposed as the reason for the achieved shrinkages. Scavenging of the grain boundaries by Joule heating is proposed as the reason for the improved oxide ion bulk conductivity.
Electrical behavior and microstructural features of electric field-assisted and conventionally sintered 3 mol % yttria-stabilized zirconia
2018 - CARVALHO, SABRINA G.M.; MUCCILLO, ELIANA N.S.; MUCCILLO, REGINALDO
ZrO2: 3 mol % Y2O3 (3YSZ) polycrystalline pellets were sintered at 1400 C and by applying an alternating current (AC) electric field at 1000 C. An alumina sample holder with platinum wires for connecting the sample to a power supply was designed for the electric field-assisted sintering experiments. The apparent density was evaluated with the Archimedes technique, the grain size distribution by analysis of scanning electron microscopy images, and the electrical behavior by the impedance spectroscopy technique. Sintering with the application of AC electric fields to 3YSZ enhances its ionic conductivity. An explanation is proposed, based on the dissolution back to the bulk of chemical species, which are depleted at the grain boundaries, leading to an increase in the oxygen vacancy concentration. For the enhancement of the grain boundary conductivity, an explanation is given based on the diminution of the concentration of depleted chemical species, which migrate to the bulk. This migration leads to a decrease of the potential barrier of the space charge region, known to be responsible for blocking the oxide ions through the intergranular region. Moreover, the heterogeneity of the distribution of the grain sizes is ascribed to the skin effect, the tendency of the AC current density to be largest near the surface, decreasing towards the bulk.
Inclusão e remoção térmica de NaCl, KI e grafite para obtenção de cerâmicas porosas de zircônia estabilizada com ítria
2013 - CARVALHO, SABRINA G. de M.
Cerâmicas de zircônia estabilizada com ítria são utilizadas na forma densa como eletrólito e na forma porosa como ânodo em células a combustível de óxido sólido. Neste trabalho cerâmicas porosas de zircônia estabilizada com 8 mol% de ítria foram preparadas por meio da adição de diferentes teores de KI, NaCl e grafite como aditivo sacrificial. A remoção térmica do aditivo foi avaliada por meio de análises termogravimétrica, térmica diferencial e dilatométrica. As amostras foram preparadas por meio de mistura, compactação e sinterização a 1400 ºC/2 h. As amostras foram caracterizadas por difração de raios X(DRX) e análise topográfica em microscópio de varredura por sonda e microscópio eletrônico de varredura de superfícies polidas e atacadas para avaliação da distribuição do teor de poros e tamanho médio de grãos. O teor do aditivo residual foi avaliado por fluorescência de raios X (FRX). O comportamento elétrico foi analisado por espectroscopia de impedância (EI) na faixa de frequências 5 Hz-10 MHz entre 300 ºC e 450 ºC. Os resultados de FRX mostram que não há resíduo do aditivo após sinterização. A análise de DRX indica que todas as amostras têm fase única, cúbica tipo fluorita. Os diagramas de impedância mostram que há aumento i) das resistividades elétricas intergranular e intragranular, evidenciando a formação de poros em ambas as regiões, ii) do ângulo de descentralização do semicírculo a baixas frequências, devido ao aumento do grau de heterogeneidade pela presença de poros, e iii) do produto do fator de bloqueio R pelo fator de frequência f, consequência do aumento do teor de poros. Esses resultados estão em concordância com os resultados das análises de microscopia de varredura por sonda e de microscopia eletrônica de varredura.