SABRINA GONCALVES DE MACEDO CARVALHO

SABRINA GONCALVES DE MACEDO CARVALHO

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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.
Sintering under AC electric field of samarium doped ceria
2018 - REIS, SHIRLEY L. dos; CARVALHO, SABRINA G. de M.; MUCCILLO, ELIANA N. dos S.; MUCCILLO, REGINALDO
Samarium doped ceria (SDC) is a high oxygen-ion conductor compound with application in solid oxide fuel cells operating at intermediate temperatures (500-700ºC). Polycrystalline SDC exhibits low sinterability, requiring high temperatures to achieve full densification under conventional sintering. Flash sintering is a recently developed method in which ceramic compacts may be sintered at lower temperatures and shorter times than those under conventional sintering. It consists in applying an electric field to a green compact, either during heating or at a fixed temperature up to the occurrence of an electric current pulse. In this study, samarium doped ceria (Ce0.8Sm0.2O1.9) was sintered by applying AC electric field at a fixed frequency of 1.0 kHz to the specimen during heating (dynamic sintering) or during a fixed temperature (isothermal). The influence of sintering on densification, microstructure and ionic conductivity was investigated. Similar density values were obtained for specimens sintered at 800ºC (dynamic) and 1100ºC (isothermal).
Flash sintering of 3 mol% yttria-stabilized zirconia with AC and DC electric fields
2018 - CARVALHO, SABRINA G. de M.; MUCCILLO, REGINALDO; MUCCILLO, ELIANA N. dos S.
Experiments on sintering ZrO2: 3 mol% Y2O3 polycrystalline ceramics (Y-TZP), pressed to cylindrical pellets, were carried out by heating to 1000 oC for application of a DC and AC (1 kHz) electric field (isothermal electric field-assisted sintering) of 200 V.cm-1 for 30 seconds, and 2 A as current limit. The experiments were carried out positioning cylindrical specimens inside a vertical dilatometer, with platinum meshes at the parallel surfaces connected with platinum wires to a power supply. Both 3YSZ sintered samples, besides having their apparent densities determined, had their surfaces observed in a scanning electron microscope to evaluate average grain size and distribution of grain sizes. Moreover, impedance spectroscopy analyses were carried out to evaluate the intergranular (mainly grain boundary) and intragranular (bulk) contributions to the electrical resistivity. The results show that besides both samples present similar densities and electrical resistivity, the one submitted to AC electric fieldassisted sintering presented smaller grains sizes and a discrete grain size distribution when compared to the one sintered with DC electric field.
Electrical behavior and microstructural features of conventionally and electric field-assisted sintered 3 mol% yttria-stabilized zirconia
2018 - CARVALHO, SABRINA G. de M.; MUCCILLO, REGINALDO; MUCCILLO, ELIANA N. dos S.
ZrO2: 3 mol% Y2O3 ceramic powders, pressed to cylindrical pellets, were sintered conventionally at 1400oC (CS) and by applying an AC electric field in green (FS) and in pre-sintered (CS-FS) pellets. The electric field (200 V.cm-1, 1 kHz frequency, 3 A limiting current) was applied at 1000oC. The experiments were carried out positioning cylindrical specimens inside a vertical dilatometer furnace, with platinum meshes at the parallel surfaces connected with platinum wires to a power supply. The density of CS, FS and CS-FS were 98.9, 98.6 and 99.1% of theoretical density respectively. The average grain size were 282±73, 340±108 and 387±138 nm for conventionally sintering, conventionally follow by flash sintering and only flash sintered samples. Although the samples have close density values, the bulk and grain boundary conductivities of samples submitted to electric field assisted sintering are higher than conventional sintered sample. The intragranular and intergranular conductivities obtained by electrochemical impedance spectroscopy of CS, FS and CS-FS samples were 8.7 and 9.8, 9.4 and 10.2 and 13.7 and 29.6 kΩ.cm, respectively. These results show that the application of an AC electric field to a green or pre-sintered solid electrolyte enhances its ionic conductivity due to the welding of the grains and the increased in the oxide ion concentration due to diffusion to the grains of the chemical species depleted at the space charge region, enhancing the oxide ion concentration, both phenomena are provoked by the Joule heating produced by the electric current flow.
Densificação de céria-samária por meio de sinterização sem pressão assistida por campo elétrico AC
2019 - REIS, S.L.; CARVALHO, S.G.M.; MUCCILLO, E.N.S.; MUCCILLO, R.
A solução sólida céria-samária é uma das principais candidatas para aplicação como eletrólito sólido em células a combustível de óxido sólido, devido sua alta condutividade iônica em temperaturas intermediárias de operação (500-750 °C). Um dos problemas ainda não solucionados é sua relativamente baixa sinterabilidade. Altas temperaturas de sinterização, em geral acima de 1500 °C, são necessárias para obter densificação com impermeabilização a gases (> 92% da densidade teórica). Em altas temperaturas pode ocorrer redução de Ce4+ para Ce3+, com consequente aumento na condutividade eletrônica. No entanto, a utilização de métodos de sinterização não convencionais como a sinterização sem pressão assistida por campo elétrico é uma das estratégias para diminuir a temperatura de sinterização e otimizar as propriedades. Este trabalho tem como objetivo avaliar os efeitos do método de sinterização isotérmica assistida por campo elétrico AC durante o patamar em uma temperatura fixa. Foram realizados experimentos variando campo elétrico, tempo de aplicação do campo e limite de corrente elétrica. Os resultados mostraram que é possível densificar a céria-samária em temperaturas inferiores às convencionais por meio de sinterização sem pressão assistida por campo elétrico. O aumento do valor do campo elétrico promoveu diminuição nos valores de resistividade intragranular e intergranular. O aumento na temperatura de aplicação do campo elétrico resultou em amostras com condutividade elétrica maior em relação à amostra sinterizada convencionalmente em 1500 °C/2 h.
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.
AC electric field assisted pressureless sintering zirconia
2018 - CARVALHO, SABRINA G.M.; MUCCILLO, ELIANA N.S.; MUCCILLO, REGINALDO
3mol% yttria ionic conductors are sintered by applying AC electric fields with frequencies in the 0.5-1.0kHz range at 1100 degrees C. The sintering experiments are conducted in pressed pellets positioned between platinum electrodes inside a dilatometer furnace. The dilatometer is modified in order to allow for the simultaneous monitoring of thickness shrinkage, electric voltage, and current across the pellet. The results show that the higher the frequency of the electric field, the higher the attained shrinkage and the apparent density of the pellets. Increasing the frequency of the applied electric field leads to an increase in the Joule heating promoted by the electric current pulse through the polycrystalline ceramic sample. A higher frequency therefore leads to higher amount of thermal energy delivered to the sample, favoring enhanced densification. The ionic resistivity decreases in pellets sintered with increasing frequency of the applied electric field. We suggest that Joule heating favors pore elimination and the removal of chemical species at the space charge region, inhibiting the blocking of oxide ions at the interfaces.
Electric field-assisted pressureless sintering of zirconia–scandia–ceria solid electrolytes
2018 - MUCCILLO, ELIANA N.S.; CARVALHO, SABRINA G.M.; MUCCILLO, REGINALDO
Electric field-assisted (flash) pressureless sintering experiments were carried out in ZrO2 ceramics doped with 10 mol% Sc2O3 and 1 mol% CeO2 (10Sc1CeSZ). All experiments were conducted isothermally at 1050 °C for 2–5 min with the application of a 100–150 V cm−1 AC electric field at 1 kHz with 1–4 A limiting current in green compacts and in samples pre-sintered at different temperatures. Shrinkage level, structural phases and grain morphology data were collected by dilatometry, X-ray diffraction and scanning electron microscopy analyses, respectively. The results showed that for the same delivered electric power, the final shrinkage was higher for higher temperature applications of the electric field and for higher electric current pulses. Moreover, the higher the porosity, the higher the final densification of the flash-sintered 10Sc1CeSZ samples, showing that pores play a role as a preferential path in the flash sintering mechanism.
AC electric field-assisted sintering of stabilized zirconias
2017 - CARVALHO, S.G.M.; MUCCILLO, E.N.S.; MUCCILLO, R.
Green pellets of ZrO2:3 mol% Y2O3 and ZrO2: 10 mol% Sc2O3:1 mol% CeO2 were sintered by applying AC (500 Hz – 1.1 kHz) electric fields (typically 100 V.cm-1) during the first stage sintering stage (T < 1200oC). The experiments were carried out positioning the specimens inside a vertical dilatometer with platinum disks acting as electrodes in a capacitor-like setup. The shrinkage level was controlled by monitoring the dilatometer gauge. Under the same conditions of temperature and magnitude of the applied AC voltage, the results show that the higher is the frequency of the electric field, the higher are the attained shrinkage and apparent density. Microstructural analyses of surfaces of the flash sintered specimens show that the average grain size also depends on the frequency of the electric field for the same sintering temperature, sintering time, applied electric field and electric current limit. We propose that increasing the frequency of the electric current pulse (resulting from the applied electric field) leads to an increase of charge carriers collisions, therefore increasing the amount of Joule heating delivered to the specimen, which is the primary phenomenon responsible for the densification of the specimens.