M.T. Colomer

Fe4+ content and ordering of anion vacancies in partially reduced AFexTi1-xO3-y (A = Ca, Sr; x⩽0.6) perovskites. An 57Fe Mössbauer spectroscopy study

CaFexTi1-xO3-y (0.05≤x≤0.60) and SrFexTi1-xO3-y (0.20≤x≤0.60) perovskites were prepared by solid state reaction and subjected to different heat treatments in different atmospheres. Mossbauer spectroscopy of the CaFexTi1-xO3-y could detect Fe4+ in all the samples, even in those heated at 1000 °C in Ar atmosphere for 10 hours. Fe3+ coordinated by six, five and four anions were also identified and the estimated hyperfine parameters were consistent with those previously reported for the CaFexTi1-xO3-x/2 oxides where all the Fe cations were present as Fe3+. The Fe4+/Fe3+ ratios estimated from Mossbauer data are in agreement with coulometric titration data. The absence of tetracoordinated Fe3+ in SrFexTi1-xO3-y suggests that in contrast to CaFexTi1-xO3-y no ordering of anion vacancies takes place even for x = 0.60 and explains the differences in the Fe concentration dependence of the electric transport properties of these materials.

Impedance spectroscopy of Sr0.97Ti1−xFexO3−δ materials with moderate Fe-contents

Abstract Complex impedance spectroscopy was used to characterize the electrical behavior of polycrystalline Sr0.97Ti1−xFexO3−δ materials. The impedance plots obtained for samples with x≥0.07 (7%Fe or higher) did not reveal a resistive grain boundary contribution, thus suggesting a major change in grain boundary dispersion (relative to samples with small Fe contents). The dielectric constant remains nearly independent of the Fe content, and is close to the dielectric constant of strontium titanate single crystals. SEM microstructures reveal a remarkable change in grain growth behaviour. Ceramic samples with x≥0.07 show average grain sizes in the range 0.3–2 μm, which is much smaller than found for samples with up to 5% Fe or lower Fe contents. The bulk conductivity increases rapidly with the Fe content, mainly due to a decrease in activation energy. The bulk conductivity results deviate from a typical Arrhenius law, and suggest a transition towards a variable range hopping mechanism at room temperature or lower.

Electrophoretic deposition of TiO2/Er3+ nanoparticulate sols

TiO(2) and TiO(2)/Er(3+) nanoparticulate sols were obtained by the colloidal sol-gel route. Thanks to the combination of three optical techniques (laser diffraction, LD, dynamic light scattering, DLS, and multiple light scattering, MLS), the peptization time was quantified, demonstrating that erbium(III) ions retard the process. The isoelectric point of TiO(2) shifts up to higher pHs when Er(3+) ions are present, which suggests that they are adsorbed onto the surface of the TiO(2) nanoparticles. Moreover, the viscosity of the sols increases when the erbium(III) amount increases. The xerogels obtained from each sol were characterized by XRD and HRTEM, obtaining in all cases anatase as the major phase, although traces of brookite were also present. In the EPD experiments, the addition of ethanol was necessary to reduce the water hydrolysis and facilitate the drying process. As a result, transparent thin films were obtained at short times and low current densities and opal films for larger current densities and deposition times; in addition, the thickness, measured by ellipsometry, increased gradually, but the refractive index did not change significantly (1.9-2). The topography profile of the films and the particle size were obtained by atomic force microscopy (AFM), giving similar values to those measured by DLS, indicating that the addition of ethanol helps to maintain stabilization without further agglomeration or sedimentation.

Synthesis and characterization of TiO2/Rh3+ nanoparticulate sols, xerogels and cryogels for photocatalytic applications

Nanoparticulate TiO2/Rh3+ sols have been synthesized by the colloidal sol–gel route. The combination of the data measured with optical techniques such as laser diffraction, dynamic light scattering and multiple light scattering with a near-infrared light allows us to follow up the evolution of the peptization process and to establish the effect of the presence of Rh3+ on it. It is observed that the presence of rhodium ions retards the peptization step (t2) and decreases both the average particle size of the nanoparticles and the viscosity of the nanoparticulate sols. In addition, when Rh3+ is present the isoelectric point shifts up to higher pH, which suggests that chemical adsorption of the rhodium (III) cations onto the surface of the TiO2 nanoparticles is produced. The xerogels and cryogels obtained from the sols are constituted by anatase as major phase and traces of brookite. The phase transition is observed at lower temperatures for the xerogels containing rhodium (III) and at higher temperatures in the case of the cryogels. Finally, photocatalytic activity is higher in the case of the TiO2/Rh3+ sols due to the rhodium (III) effect on the electronic transitions from the valence band to the conduction band.

Ceramic conductors for electrochemical cell applications: new perspectives in materials and synthesis

Abstract A great number of compounds with electronic and ionic conductivity and new synthesis routes are being explored for electrochemical applications. In this work, selected examples are described. (a) CaO–TiO 2 –Fe 2 O 3 materials prepared by combustion synthesis are low cost products with interesting mixed conduction, the electrical behavior of which indicates the existence of a transition, where the grain interior electrical contribution is the dominant effect from 7% mol iron content. (b) Mixed electronic–ionic conducting SmCoO 3 perovskites with dispersed Pt particles were prepared by combustion synthesis. The as-prepared powders showed very small metallic Pt particles and exhibited acceptable catalytic performance in methanol oxidation. (c) Materials belonging to the Ti 1− x Ru x O 2 system, 0≤ x ≤1 (mol), were synthesized by a polymeric sol-gel route. Two types of electrical behavior were detected in sintered materials of this system: semiconductor and metallic. A wide semiconductor/metal transition was observed. The metal/electrical percolation could be located at a composition of x =0.26. (d) On the other hand, amorphous nanopore materials with protonic conduction obtained by appropriate synthesis can be used as electrolytes in PEMFCs. SiO 2 nanopore ceramic membranes were prepared using a colloidal sol-gel technique. The chips were mesoporous with an average pore size of 3.5 nm.

Effects of humidity on the electrical behaviour of Sr0·97Ti0·97Fe0·03O3-δ

Abstract It is widely recognised that the electrical behaviour of acceptor-doped strontium titanate ceramics is often controlled by resistive grain boundaries. This work shows that the electrical behaviour of Sr 0·97 Ti 1-x Fe x O 3−δ materials may also be affected by other factors such as the humidity in the atmosphere, especially for materials with open porosity. For example, the impedance of dense samples with large grain size may exceed the impedance of samples with residual porosity. The bulk and grain boundary resistivities both increase with the humidity in the atmosphere.

Manufacture of YSZ-LSM Semi-Cell by Colloidal Processing

Yttria stabilized zirconia (YSZ) is the most common electrolyte in solid oxide fuel cells (SOFC). The planar configuration is widely used for designing single cells, in which a thick cathode layer can be used as the supporting electrolyte film. The manufacture of the semi-cell formed by anode and electrolyte has been widely studied and there are many works dealing with its colloidal processing. However, the semi-cell formed by cathode and electrolyte has received much lower attention. This work deals with the manufacture of a semi-cell consisting of YSZ as electrolyte and strontium-doped lanthanum manganite (LSM) as a cathode through a colloidal processing route. The colloidal behavior of diluted suspensions of YSZ and the rheology of their concentrated suspensions were studied as a function of deflocculant content, mixing time by using ultrasounds probe and ageing time. The colloidal stability of aqueous suspensions of LSM was studied by measuring the zeta potential as a function of pH and deflocculant content. These concentrated suspensions were used to obtain thick self-sustained substrates by casting methods. The YSZ electrolyte and LSM cathode were prepared by tape casting in water medium.

Thin Films of Europium (III) Doped-TiO2 Prepared by Electrophoretic Deposition from Nanoparticulate Sols

Colloidal sol-gel is a common method used for the preparation of stable and homogeneous sols and thin films. The nanoparticulate sols can be easily deposited by EPD, which is a versatile technique for producing denser and thicker coatings than those produced by other techniques like dipping. A complete characterization of the sols, such as colloidal stability and electrophoretic mobility, which can be determined through zeta potential measurements, as well as the influence of deflocculants in the surface properties, is needed before using electrophoretic deposition. In this work, we have prepared sols of TiO2 with an alkoxide:water molar ratio of 50:1 and Eu (III) doped-TiO2 (2 mole % Eu (III)) using as precursors titanium (IV) isopropoxide and europium (III) acetate hydrate, respectively. The stability of the particulate sols was studied in terms of conductivity, zeta potential and viscosity evolution. Anatase stable sols, after peptization and without the use of any additive, were deposited on stainless steel substrates by electrophoretic deposition under both constant current and constant voltage conditions. Using different intensities and deposition times we have obtained thin films with different features (thicknesses and morphology) and different optical properties. The presence of europium (III) increases particle size, viscosity and peptization time and decreases the band gap of TiO2.