Caracterización estructural y espectroscópica de fibras cristalinas de Ce0.4Zr0.6O2 crecidas mediante el método de fusión zonal asistida por láser

[EN] A structural and spectroscopic characterization of crystalline rods of Ce0.4Zr0.6O2 grown by the laser floating zone (LFZ) method is presented. A precursor rod of Ce0.4Zr0.6O2 composition was sintered at 1500 ºC in air atmosphere and then processed by the LFZ technique with a CO2 laser. The processed material was characterized by XRD, SEM and Raman spectroscopy. In the as-grown, dark-color processed rod, the Raman spectrum evolves radially from a t’-like one, corresponding to Ce0.37Zr0.63O2 composition, at the edge of the rod, to a very broad and weak, cubic-like one, at the center. The degree of cerium reduction and oxygen non-stoichiometry were determined through measurements of the electronic Raman spectrum of Ce3+. A strong Ce3+ signal was found at the core of the rod, indicating strong reduction, whereas no Ce3+ signal was detected at the edge. To restore oxygen and Ce4+ content a section of the fiber was reoxidized at 620 ºC for 24 h. A very homogeneous spectrum was found in the reoxidized sample, that was assigned to a t’-phase of composition Ce0.42Zr0.58O2.[ES] Presentamos una caracterización estructural y espectroscópica de fibras cristalinas de Ce0.4Zr0.6O2 crecidas mediante fusión zonal asistida por láser (LFZ). Una barra del material precursor, de composición Ce0.4Zr0.6O2, fue sinterizada a 1500 ºC en atmósfera de aire y después procesada por LFZ con un láser de CO2. El material procesado fue caracterizado por difracción de rayos X, microscopía electrónica de barrido y espectroscopía Raman. En la fibra recién procesada, de color gris oscuro, el espectro Raman varía radialmente desde un espectro de tipo t’, correspondiente a una composición Ce0.37Zr0.63O2, en el borde de la fibra, hasta un espectro muy ensanchado y débil, de aspecto cúbico, en el centro. El grado de reducción del cerio y la no estequiometría del oxígeno se determinaron a través de las medidas del Raman electrónico del Ce3+. La señal de Ce3+ era muy intensa en el centro de la fibra, lo que indica una fuerte reducción, mientras que no se detectó en el borde. Para restablecer el contenido de oxígeno y de Ce4+ una parte de la fibra fue reoxidada a 620 ºC durante 24 h. El espectro de la muestra reoxidada, de color amarillo pálido y muy homogénea, fue asignado a una fase t’ de composición Ce0.42Zr0.58O2.Financial support from Spanish project MAT2004-03070-C05-03 is acknowledged.Peer reviewe

Microtubular SOFC based on an extruded support

Trabajo presentado al "IV Iberian Symposium on Hydrogen, Fuel Cells and Advanced Batteries" celebrado en Estoril (Portugal) del 26 al 28 de junio de 2013.In this work the processing route for an anode supported microtubular solid oxide fuel cell is adjusted. The fuel cell composition and microstructure design is based on previous work. In this work we have developed microtubular cells based on an extruded support. Firstly, Ni-YSZ anode was manufactured by Powder Extrusion Moulding (PEM). Feedstock composition and extruding parameters were adjusted to obtain tubular green bodies. An YSZ layer was then deposited as the electrolyte and the sintering parameters were optimized to obtain a dense layer. An active area of ~1 cm2 LSMYSZ was deposited as the cathode and its electrochemical performance was measured using pure hydrogen as fuel, yielding a power output at 0.5V of 0.7Wcm-2 at 8500C.Authors would like to thank financial support received from MICINN and Feder program of the European Community (MAT2010-19837-C06 and MAT2012-30763 projects), Madrid regional government (MATERYENER S2009 PPQ-1626 program), and also grant GA-LC-035/2012, financed by the Aragón Government and La Caixa Foundation.Peer Reviewe

High-performance Ni-YSZ thin-walled microtubes for anode-supported solid oxide fuel cells obtained by powder extrusion moulding

Aiming at the fabrication of microtubular anode-supports for Solid Oxide Fuel Cell (SOFC) applications, this contribution deals with the production of Ni-YSZ thin-walled tubes (<1 mm thickness) via Powder Extrusion Moulding (PEM). The overall method has been optimized with an emphasis on the effect of NiO particle size using two commercial NiO powders with mean sizes of 0.7 and 8 μm. A thermoplastic binder system based on polypropylene (PP), paraffin wax (PW) and stearic acid (SA) in volume ratios of 50, 46 and 4, respectively, was used along with corn starch as a pore forming agent. Different feedstocks with solid loadings varying from 45 to 65 vol% were processed and characterized to determine the optimal formulation. Typically, the mixtures exhibited a pseudoplastic behaviour from 100 to 1000 s. Feedstocks with finer NiO particles had the most balanced properties for PEM purposes and an optimal powder volume content of 65 vol% was established. After extrusion and debinding steps, defect-free and constant cross-section tubes with 15 mm of length and 4 mm of nominal diameter were obtained. The final microstructure and DC conductivity were found to be closely linked to the NiO particle size, yielding a higher degree of open porosity and a better performance when using finer NiO powder. Based on this study, the packing mechanism was found to be likely limited by the contribution of steric hindrances when dissimilar and coarse powders are mixed, which may play a decisive role in order to set tailored formulations.Authors would like to thank financial support received from MICINN and Feder program of the European Community (MAT2013-46452-C4-3R and MAT2012-30763 projects), and Madrid regional government (MATERYENER3CM S2013/MIT-2753 Program).Peer Reviewe

Influencia del enfriamiento en las propiedades de titanatos de lantano y litio

Se ha estudiado el efecto de los tratamientos a alta temperatura sobre la estructura y la movilidad del litio para la solución sÛlida Li3xLa2/3-xTiO3 (0.03<x<0.167) con difracciÛn de rayos X a alta temperatura (DRXAT), espectroscopia Raman, RMN y de impedancias. Los patrones de difracción de rayos X (DRX) a temperatura ambiente de muestras enfriadas lentamente muestran una estructura tipo perovskita doblada en el eje c con simetrÌas tetragonales u ortorrómbicas, mientras que las muestras con un enfriamiento r·pido muestran una estructura tipo perovskita c ̇bica simple. Sin embargo, el espectro Raman de las muestras analizadas se interpreta, en todos los casos, con una simetrÌa tetragonal en la que el desorden catiónico se incrementa con el contenido de litio y el tratamiento de enfriamiento. La existencia de microdominios de maclado, orientados a lo largo de las tres direcciones de la perovski- ta, favorece la detecciÛn de la fase c ̇bica en los patrones de DRX. A partir de la espectroscopia de RMN del 7Li, se ha detectado un movimiento bidi- mensional del litio en las muestras ordenadas, el cual se convierte progresivamente en un movimiento tridimensional conforme se incrementa el desorden catiÛnico. Asimismo, la presencia de microdominios hace disminuir la conductividad dc de muestras con contenidos bajos de litio

Microstructure, magnetic and mechanical properties of Ni-Zn ferrites prepared by Powder Injection Moulding

Nowadays, the electronic industry demands small and complex parts as a consequence of the miniaturization of electronic devices. Powder injection moulding (PIM) is an emerging technique for the manufacturing of magnetic ceramics. In this paper, we analyze the sintering process, between 900 °C and 1300 °C, of Ni–Zn ferrites prepared by PIM. In particular, the densification behaviour, microstructure and mechanical properties of samples with toroidal and bar geometry were analyzed at different temperatures. Additionally, the magnetic behaviour (complex permeability and magnetic losses factor) of these compacts was compared with that of samples prepared by conventional powder compaction. Finally, the mechanical behaviour (elastic modulus, flexure strength and fracture toughness) was analyzed as a function of the powder loading of feedstock. The final microstructure of prepared samples was correlated with the macroscopic behaviour. A good agreement was established between the densities and population of defects found in the materials depending on the sintering conditions. In general, the final mechanical and magnetic properties of PIM samples were enhanced relative those obtained by uniaxial compaction

The role of Ce reduction in the segregation of metastable phases in the ZrO2–CeO2 system

The complexity of the ZrO2–CeO2 phase diagram arises from several factors: the low solubility of each compound into the other one, the slow kinetics of cation diffusion, the occurrence of Ce reduction at high temperatures, and the existence of several metastable phases whose appearance and evolution depend on synthesis method and thermal history of the sample. Identification of phase content is moreover complicated because the X-ray diffractograms of some ZrO2–CeO2 phases are very close or even indistinguishable, which imposes the use of other techniques more sensitive to small oxygen displacements. In this work we present a Raman spectroscopic study of phase segregation in the ZrO2–CeO2 system between 1300 and 1650 °C, focusing on the effect of Ce reduction at high temperatures and its relation with the appearance of metastable phases upon cooling. The nature of the high-temperature defective cubic phase is discussed.This work was supported by Spanish CICYT through MAT2010-19837-C06-06 grant and Feder funds.Peer reviewe

Fabrication of Ni-YSZ tubes by powder extrusion moulding for SOFC anodes

Resumen del trabajo presentado al "IV Iberian Symposium on Hydrogen, Fuel Cells and Advanced Batteries" celebrado en Estoril (Portugal) del 26 al 28 de junio de 2013.Peer Reviewe

Origin of constant loss in ionic conductors

We have analyzed the constant loss contribution to the ac conductivity in the frequency range 10 Hz–1 MHz and temperatures down to 8 K, for two Li ionic conductors, one crystalline (Li_(0.18)La_(0.61)TiO_(3)) and the other glassy (61SiO_(2) . 35Li_(2)O . 3Al_(2)O_(3) . P_(2)O_85)). As temperature is increased a crossover is observed from a nearly constant loss to a fractional power law frequency dependence of the ac conductivity. At any fixed frequency ω, this crossover occurs at a temperature T such that ω = v_(0) exp(-E_(m)/k_(B)T), where v_(0) is the attempt frequency and E_(m) is identified with the barrier for Li^(+) ions to leave their wells

Electrical conductivity relaxation and nuclear magnetic resonance of Li conducting Li_(0.5)La_(0.5)TiO_(3)

Lithium ionic conductivity of Li_(0.5)La_(0.5)TiO_(3) has been studied using nuclear magnetic resonance (NMR) and admittance spectroscopy (AS) techniques. Spin-lattice relaxation and electrical conductivity relaxation are well described in terms of stretched-exponential correlation functions in the time domain of the form φ(t) = exp(-(t/τ) (β), but showing different relaxation times scales (τ_(0) = 1.4 x 10^(-11) s from NMR and τ_(0) = 10^(-14) s from AS), and activation energies (0.15 and 0.4 eV, respectively). Different β exponents, 1 from spin lattice relaxation and 0.4 from electric-field relaxation have been also deduced. A microscopic activation energy for lithium motion of 0.15 eV is deduced from both techniques. Discrepancies between both techniques are analyzed and discussed in terms of frequency-dependent correlation effects