Microestrutura e propriedades elétricas da zircônia dopada com óxidos mistos de terras raras para aplicação como eletrólito sólido em sensores de oxigênio

Tese (doutorado)—Universidade de Brasília, Departamento de Engenharia Mecânica, 2013.Neste trabalho são realizadas a síntese e caracterização da zircônia totalmente estabilizada para possível uso em sensores de oxigênio, utilizando-se como métodos de sínteses o método Pechini e a precipitação heterogênea. Como precursor de ítria foi usado um carbonato de terras raras produzido no Brasil, pela NUCLEMON, a partir da monazita brasileira. A vantagem de utilizar este carbonato é o baixo custo em relação às terras raras de alta pureza. O método Pechini permitiu obter pós com tamanho de partícula submicrométrico. Por outro lado, a precipitação heterogênea permitiu obter uma mistura de óxidos homogênea, estruturas do tipo core-shell. Em ambos os casos se garantiu a estabilização da fase cúbica da zircônia após sinterização. Corpos de prova foram confeccionados via prensagem uniaxial a frio e posteriormente sinterizados usando dois cronogramas de aquecimento, S1 e S2. Em todas as cerâmicas em estudo verificou-se que o tamanho de grão foi dependente do cronograma usado na sinterização. Por outro lado, diagramas de impedância evidenciam que as fases presentes, assim como o tamanho de grão, têm influencia direta sobre o comportamento elétrico das cerâmicas. A amostra mais condutora, independente do método de síntese, foi obtida com adição de 10 % mol de ítria, confirmando que o excesso de dopagem em sistemas baseados em zircônia, neste trabalho dopagens de 12 e 14% mol, gera vazios de oxigênio que não participariam de forma efetiva no processo de condução, dando origem a defeitos mais complexos como clusters de vacâncias ou pares ordenados impureza/vazio da forma [Y_Zr^''-V_O^(oo) ]^x, [Y_Zr^'-V_O^(oo) ]^o ou [Y_Zr^'-V_O^(oo)-Y_Zr^' ]^x os quais dificultam a mobilidade dos portadores de carga. O valor de condutividade total à temperatura de 600 °C desta amostra foi de 2,85x10-3 ?-1 cm-1. Ao realizar uma comparação direta destes valores com os reportados na literatura, identifica-se semelhança com o sistema tradicional zircônia-ítria, o mais usado comercialmente como eletrólito em sensores de oxigênio, confirmando o potencial uso deste sistema para esta aplicação especifica. As vantagens de usar os sistemas baseados em zirconia terras raras obtidos neste trabalho são: baixo custo de fabricação, controle estequimetrico e reprodutivilidade do processo por poder controlar todas as variáveis no processo de produção da matéria prima. Finalmente, com a matéria prima que permitiu obter a amostra mais condutora, foi confeccionado um protótipo de sensor de oxigênio tipo sonda lambda (STSL). Este protótipo foi testado de forma preliminar, utilizando-se um dispositivo projetado no laboratório fixado na saída do escapamento de automóvel, na presença de gás veicular. Os resultados destes testes preliminares evidenciaram que o dispositivo é sensível às variações no teor de oxigênio no gás, ao se excitar o carro com pulsos no acelerador. _______________________________________________________________________________________ ABSTRACTIn this work synthesis and characterization of fully stabilized zirconia were carried out aiming possible application as oxygen sensors. Synthesis methods were in this case the Pechini and the heterogeneous precipitation. The rare earth carbonate produced in Brazil by Nuclemom was used as yttria precursor, from Brazilian monazite. The advantage of this carbonate is its low cost compared to high purity rare earth. Pechini method allowed synthesis of powder with sub-micrometric particle size. The heterogeneous precipitation produced a homogeneous mixture of core-shell structures. In both cases it was guaranteed stabilization of zirconia cubic phase after sintering. Samples were cold uniaxial pressureless sintered at resistive furnace. Two sintering schedules were used, S1 and S2, independently of powder synthesis method. For all evaluated ceramics it was observed grain size dependence on the used sintering schedule. Impedance diagrams demonstrated direct influence of electric behavior with existing phases and grain sizes. The most conductive sample, independently of synthesis method, was those with 10% mol of yttria. This confirm theoretical explanation that exceeding doping at zirconia based systems, in this case 12% and 14% mol, causes excess of oxygen vacancies that do not participate at conduction process. It gives rise to complex defects such as clusters of vacancies or ordered pair impurity/vacancy with the form … −… ••_,… − ….. ou … − … •• −…. that impair charge mobility. Total conductivity at 600 0C was 2.85x10-3 Ω -1 cm-1 for Pechini powder. Obtained values for this system are similar to those of the traditional zirconia yttria system, largely used as electrolyte for oxygen sensors. Advantages of the system used in this work are low manufacturing costs, stoichiometric control of synthesis method and process reproductivity caused by control of variables at raw material production. Finally, with the most conductive material, one lambda sensor prototype was manufactured. The prototype was tested preliminarily, using device specially designed at the University and fixed to automotive exhaust pipe. Preliminary results showed that the oxygen sensor prototype is sensitive to oxygen content at exhaust gas during accelerator pulses. These results confirm the potential of the system used in this work, for application at gas sensors

Semiconductor magnéticamente diluido de SnO2- Fe obtenido por precipitación controlada

Fueron obtenidas soluciones solidas de SnO2 dopadas con 5% y 8% mol de Fe usando el método de precipitación controlada. Después del proceso de síntesis las muestras en polvo fueron tratadas térmicamente a 350, 500 e 900°C durante una hora. Mediante DRX se observó la casiterita como única fase, y analizando los datos con refinamiento Rietveld se determinó una estimativa de tamaño de cristalito. Usando espectroscopia Raman se estudió la presencia de hierro en las muestras, observando su banda característica en 727cm-1. Microscopia electrónica de transición permitió conocer el tamaño de partícula. Comparando los tamaños de cristalito y de partícula fue posible concluir que las nanopartículas obtenidas son monocristales. Los polvos fueron caracterizados magnéticamente, encontrando comportamiento ferromagnético a 5K en la muestra dopada con 8% y comportamiento paramagnético con tendencia superparamagnetica a 300K.Solid solutions were synthesized SnO2 doped with 5% and 8 mol% Fe by the controlled precipitation method. The particles size was obtained of ∼12 nm with heath treatment 450 °C. The presence of iron in the structure is evidenced by Raman spectroscopy The crystallite size we obtained with the results of XRD, and particle size by MET, it was concluded that the nanoparticles obtained were monocrystalline. The particles were characterized magnetically, for the powders doped to 8% Fe was determined ferromagnetic behavior at 5K, with a tendency superparamagnetic and paramagnetic at a temperatue of 300

Obtention and characterization of lithium superionic conductors using the glass-ceramic method

This paper proposes the glass-ceramics method for obtaining lithium ion (Li+) solid electrolytes. This technique provides high chemical and microstructural homogeneity as well as low porosity. Glass samples were subjected to either single or double heat treatments, between 700 °C and 1000 °C, in order to obtain the glass-ceramics. Differential Scanning Calorimetry – DSC – results evidenced the possibility of fabricating these ceramics from glass in the system Li2O·Al2O3·TiO2·P2O5. Samples observed by Scanning Electron Microscopy – SEM – showed a finely grained microstructure which was homogeneously distributed and non-porous. X-ray Diffraction – XRD – patterns showed the formation of the high conducting phase LiTi2(PO4)3. A high ionic conductivity, in the order of 10-3 S/cm at 1000 °C, was measured by Impedance Spectroscopy – IS. It suggests that the synthesis method used in this research is useful for fabricating lithium ion glass-ceramics and opens up a new alternative for manufacturing different electrical ceramics.Este artículo propone la ruta vitrocerámica para obtener electrolitos sólidos por ion litio (Li+). Esta técnica provee alta homogeneidad química y microestructural, así como baja porosidad. Muestras vítreas fueron sometidas a tratamientos térmicos, simples y dobles, entre 700 °C y 1000 °C, para obtener las vitrocerámicas. Resultados de calorimetría diferencial de barrido – DSC – evidenciaron la posibilidad de fabricar estas cerámicas a partir de vidrios del sistema Li2O·Al2O3·TiO2·P2O5. Muestras observadas por microscopía electrónica de barrido – SEM – mostraron una microestructura de granos finos, homogéneamente distribuidos y sin porosidad. Patrones de difracción de rayos-x – XRD – permitieron verificar la formación de la fase altamente conductora LiTi2(PO4)3. Una conductividad iónica alta, del orden de 10-3 S/cm a 1000°C, fue medida utilizando espectroscopía de impedancia – IS. Lo anterior sugiere que el método de síntesis, utilizado en este trabajo, es útil para fabricar vitrocerámicas de ion litio y abre una nueva alternativa para fabricar diferentes cerámicas eléctricas

Conductividad y evaluación microestructural de Electrolitos sólidos SGDC sintetizados por Pechini y precipitación controlada

In this work, controlled precipitation synthesis methods and polymeric precursors (Pechini) were used to obtain gadolinium-doped cerium oxide solid electrolytes (Ce0.8Gd0.2O1.9) and double-doped cerium oxide with gadolinium and samarium (Ce0.8Gd0.2−xSmxO1.9 x = 0.01; 0.03 and 0.05). The main results indicate that the synthesized powders by the Pechini method present better microstructural characteristics such as larger specific surface area and smaller particle size. Test specimens were compacted (187 MPa) and sintered (1500 °C/6 h) and subsequently characterized. Through the impedance spectroscopy analysis, an increase in the percentage of the samaria caused an increase in the grain boundary and grain conductivity in all the samples under study. In the grain interior, samples without samaria showed heterogeneous regions, where the cation-vacancy pairs are tightly bound, precluding these vacancies from participating in ion conduction. In the samples with samaria, these defect groups are smaller, indicating structural homogeneity, resulting in greater ionic conductivity. Grain boundary specific conductivities show slight increase for samples codoped with samarium oxide. This effect suggests that Sm3+ additions change the charge equilibrium in space-charge layer favoring better dopant distribution.En este trabajo se utilizaron los métodos de síntesis precipitación controlada y precursores poliméricos (Pechini) para obtener electrolitos sólidos de ceria dopada con gadolinio (Ce0.8Gd0.2O1.9) y óxido de ceria doblemente dopada con gadolinio y samario (Ce0.8Gd0.2−xSmxO1.9 x = 0.01; 0.03 and 0.05). Los principales resultados indicaron que los polvos sintetizados por el método Pechini presentaron mejores características microestructurales: mayor área de superficie específica y menor tamaño de partícula. Los polvos fueron compactados (187 MPa), sinterizados (1500 °C/6 h) y, por fin, caracterizados. Con análisis de los espectros de impedancia se observó que el aumento del porcentaje de samario provocó aumento de la conductividad del grano y del contorno de grano en todas las muestras en estudio. En el interior del grano las muestras sin samaria mostraron regiones heterogéneas (grupos de defectos) donde pares catión-vacancia están estrechamente unidos, lo que impide su participación en la conducción iónica. En las muestras con samaria, estos grupos de defectos son más pequeños, indicando homogeneidad estructural, obteniendo una conductividad iónica mas alta. La conductividad específica del límite de grano muestra un ligero aumento para las muestras codopadas con óxido de samario. Este efecto sugiere que las adiciones de Sm3+ cambian el equilibrio de carga en la capa de carga espacial favoreciendo una mejor distribución del dopante

(Diluted) Magnetic semiconductor of SnO2-Fe obtained by controlled precipitation

Solid solutions were synthesized SnO2 doped with 5% and 8 mol% Fe by the controlled precipitation method. The particles size was obtainedof ∼12 nm with heath treatment 450 °C. The presence of iron in the structure is evidenced by Raman spectroscopy The crystallite size we obtained with the results of XRD, and particle size by MET, it was concluded that the nanoparticles obtained were monocrystalline. The particles were characterized magnetically, for the powders doped to 8% Fe was determined ferromagnetic behavior at 5K, with a tendency superparamagnetic and paramagnetic at a temperatue of 300K.Fueron obtenidas soluciones solidas de SnO2 dopadas con 5% y 8% mol de Fe usando el método de precipitación controlada. Después del proceso de síntesis las muestras en polvo fueron tratadas térmicamente a 350, 500 e 900°C durante una hora. Mediante DRX se observó la casiterita como única fase, y analizando los datos con refinamiento Rietveld se determinó una estimativa de tamaño de cristalito. Usando espectroscopia Raman se estudió la presencia de hierro en las muestras, observando su banda característica en 727cm-1. Microscopia electrónica de transición permitió conocer el tamaño de partícula. Comparando los tamaños de cristalito y de partícula fue posible concluir que las nanopartículas obtenidas son monocristales. Los polvos fueron caracterizados magnéticamente, encontrando comportamiento ferromagnético a 5K en la muestra dopada con 8% y comportamiento paramagnético con tendencia superparamagnetica a 300K

Semiconductor magnéticamente diluido de SnO2- Fe obtenido por precipitación controlada

Fueron obtenidas soluciones solidas de SnO2 dopadas con 5% y 8% mol de Fe usando el método de precipitación controlada. Después del proceso de síntesis las muestras en polvo fueron tratadas térmicamente a 350, 500 e 900°C durante una hora. Mediante DRX se observó la casiterita como única fase, y analizando los datos con refinamiento Rietveld se determinó una estimativa de tamaño de cristalito. Usando espectroscopia Raman se estudió la presencia de hierro en las muestras, observando su banda característica en 727cm-1. Microscopia electrónica de transición permitió conocer el tamaño de partícula. Comparando los tamaños de cristalito y de partícula fue posible concluir que las nanopartículas obtenidas son monocristales. Los polvos fueron caracterizados magnéticamente, encontrando comportamiento ferromagnético a 5K en la muestra dopada con 8% y comportamiento paramagnético con tendencia superparamagnetica a 300K

Obtention and characterization of lithium superionic conductors using the glass-ceramic method

This paper proposes the glass-ceramics method for obtaining lithium ion (Li+) solid electrolytes. This technique provides high chemical and microstructural homogeneity as well as low porosity. Glass samples were subjected to either single or double heat treatments, between 700 °C and 1000 °C, in order to obtain the glass-ceramics.Differential Scanning Calorimetry –DSC–results evidenced the possibility of fabricating these ceramics from glass in the system Li2O·Al2O3·TiO2·P2O5. Samples observed by Scanning Electron Microscopy –SEM–showed a finely grained microstructure which was homogeneously distributed and non-porous. X-ray Diffraction –XRD–patterns showed the formation of the high conducting phase LiTi2(PO4)3. A high ionic conductivity, in the order of 10-3S/cm at 1000 °C, was measured by Impedance Spectroscopy –IS. Itsuggests that the synthesis method used in this research is useful for fabricating lithium ion glass-ceramics and opens up a new alternative for manufacturing different electrical ceramics.Este artículo propone la ruta vitrocerámica para obtener electrolitos sólidos por ion litio (Li+). Esta técnica provee alta homogeneidad química y microestructural, así como baja porosidad. Muestras vítreas fueron sometidas a tratamientos térmicos, simplesy dobles, entre 700 °C y 1000 °C, para obtener las vitrocerámicas. Resultados de calorimetría diferencial de barrido –DSC–evidenciaron la posibilidad de fabricar estas cerámicas a partir de vidrios del sistema Li2O·Al2O3·TiO2·P2O5. Muestras observadas por microscopía electrónica de barrido –SEM–mostraron una microestructura de granos finos, homogéneamente distribuidos y sin porosidad. Patrones de difracción de rayos-x –XRD–permitieron verificar la formación de la fase altamente conductora LiTi2(PO4)3. Una conductividad iónica alta, del orden de 10-3S/cm a 1000 °C, fue medida utilizando espectroscopía de impedancia –IS. Lo anterior sugiere que el método de síntesis, utilizado en este trabajo, es útil para fabricar vitrocerámicas de ion litio y abre una nueva alternativa para fabricar diferentes cerámicas eléctricas

Microstructure and electrical properties of solid electrolytes of fully stabilized zirconia with rare earth mixed oxides

In this work was obtained solid electrolytes of fully stabilized zircônia with doped of 10 and 12 mol% of Re2O3 (mixed oxides rare earth), for use in oxygen sensors and or fuel cells. The specimens were prepared by uniaxial pressing and sintered using two heating schedules, S1 and S2. Impedance diagrams show that the crystalline phases and the grain size change the electrical behavior of the ceramics. The sample with the best electrical performance was obtained with 10 mol% doped and was sintered with the curve S1. The value of the total conductivity of this sample was 2,85x10-3 W-1.cm-1 (taken at 600 ° C). When making a comparison between this values of conductivity with the reported in the literature is identified similarity with or traditional system zirconia-yttria the most widely used commercially as electrolyte oxygen sensors and fuel cells, confirming the potential use of the oxides mixed rare earth for these specific applications

Microstructure and electrical properties of solid electrolytes of fully stabilized zirconia with rare earth mixed oxides

In this work was obtained solid electrolytes of fully stabilized zircônia with doped of 10 and 12 mol% of Re2O3 (mixed oxides rare earth), for use in oxygen sensors and or fuel cells. The specimens were prepared by uniaxial pressing and sintered using two heating schedules, S1 and S2. Impedance diagrams show that the crystalline phases and the grain size change the electrical behavior of the ceramics. The sample with the best electrical performance was obtained with 10 mol% doped and was sintered with the curve S1. The value of the total conductivity of this sample was 2,85x10-3 W-1.cm-1 (taken at 600 ° C). When making a comparison between this values of conductivity with the reported in the literature is identified similarity with or traditional system zirconia-yttria the most widely used commercially as electrolyte oxygen sensors and fuel cells, confirming the potential use of the oxides mixed rare earth for these specific applications

Synthesis and electrical properties of strontium-doped lanthanum ferrite with perovskite-type structure

In this study, we prepared strontium-doped lanthanum ferrites with the perovskite-type structure for application as solid oxide fuel cell (SOFC) cathodes. We used the Pechini method to prepare strontium-doped lanthanum ferrites with the strontium:lanthanum molar ratios of 20:80 and 40:60. The resulting doped materials were characterized using various analytical tools. The calcination process was conducted at 450 °C because above this temperature, the stabilization of mass loss occurred and no phase transformation was observed. The X-ray diffraction results confirmed the mixing of the powder phases after the calcination process and the presence of a single powder phase in the air-sintered samples. The high-resolution transmission electron microscopy results revealed the presence of agglomerated nanoparticles smaller than 20 nm in size in the samples. The electrochemical impedance spectroscopy results revealed that the sample with 20% strontium exhibited a conductivity of 3.9 × 10−3 S cm−1 at 95 °C and activation energy of 0.37 eV. In contrast, the sample with 40% strontium exhibited a conductivity of 3.5 × 10−2 S cm−1 and activation energy of 0.29 eV. These results suggest that with an increase in the strontium content, the conductivity of the samples increased, where as the activation energy of the conduction process decreased. Therefore, the ferrites synthesized in this work are potential catalysts for SOFC cathodes