Papers by Author: Rodrigo Moreno

Abstract: 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.

Abstract: Previous works studied the colloidal processing of nickel-silica and nickel-titania nanocomposites fabricated via slip casting. A rheological approach was used to characterize and optimize a 30 vol.% nickel aqueous suspension with up to 10 vol.% SiO₂ and 5 vol.% TiO₂ nanoparticles. In this work, the effect of mechanical activation of Ni-SiO₂ and Ni-TiO₂ nanocomposites on microstructural and mechanical properties was studied. For this aim Ni-SiO₂ and Ni TiO₂ slip-cast compacts were attrition milled for up to 12 hours. Green bodies of the mechanical-alloyed powders were obtained by cold pressing (300 MPa). Next, those green bodies were annealed at 700°C for 1 h, repressed at 700 MPa and sintered at 900°C for 1 h in flowing Ar/5%H₂ atmosphere. Porous and dense materials were characterized by SEM/FESEM, Archimedes densities and Vickers microhardness measurements. Mechanical alloying produces a remarkable improvement on microstructure homogenization, sintering densification and hardness comparing with slip-cast composites.

Abstract: Alumina-aluminium titanate (A-AT) composites and laminates have been recently investigated because they can provide improved flaw tolerance and toughness associated to a microcracking mechanism. A-AT composites have been produced by slip casting and reaction sintering of submicron sized alumina and titania powders. This work deals with the preparation of thick self-sustained A-AT films from mixtures of submicron sized alumina and nanosized titania. Suspensions were prepared in water to high solids loadings ranging from 30 to 50 vol.%. The stability of diluted suspensions was studied through zeta potential measurements as a function of pH and deflocculant type and concentration. The stability of the concentrated suspensions as a function of deflocculant content, sonication time and solids loadings was studied from rheological measurements. Self-sustained films were obtained by aqueous EPD using graphite substrates under constant current density conditions. The evolution of mass per unit area with current density and deposition time was recorded. The films were characterized in the green state and after debinding and sintering by density measurements, and electron microscopy observations.

Abstract: 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.

Abstract: In this work the rheological behaviour of aqueous alumina suspensions with solids loadings ranging from 37 to 59 vol% is studied. A complete rheological characterisation is performed by measuring under both controlled rate and controlled stress conditions to obtain expanded flow curves, and by oscillatory tests to determine the elastic moduli. A commercial polyelectrolyte was used as dispersant. The interparticle potentials are calculated considering the electrostatic and the steric contributions. The electrostatic pair potential is estimated through the determination of the elastic modulus, G’, the double layer thickness and the surface potential by using a spatial particle distribution model. The consideration of the steric potential, by using a soft spheres model, produces significant changes in the shape and values of the potential energy curves and allows predicting the stability of suspensions that should coagulate according to the DLVO equation. The colloidal stability and the rheological behaviour are correlated with the green and sintered densities of Al2O3 bodies produced by slip casting.

Abstract: Synthesis by freeze-drying basically consists in rapidly freezing an aqueous salt solution containing the desired cation and the further sublimation of ice under vacuum conditions. After a conventional thermal treatment, the oxide nanoparticles are obtained. The influence of the parameters involved in the synthesis of γ-Al2O3 nanoparticles by a freeze-drying method have been studied: type of precursor salt (chloride and sulphate), salt solution concentrations (0.76-1.40 M), freezing rate and thermal treatment. Amorphous aluminium sulphate spherical granules with diameters ranging from 1 to 100 μm have been obtained. This compound decomposes at 825°C, leading to the formation of γ-Al2O3. These porous granules are constituted by soft agglomerates of nanoparticles with primary particle size lower than 20 nm and values of the specific surface area of 120-180 m2/g. By optimizing the dispersing conditions a minimum particle size < 30 nm is measured, thus confirming that granules can be easily dispersed into nanoparticles. The thermal treatment determines the crystalline degree of the γ-Al2O3 and the surface area. A multimodal porosity is always obtained. The mesoporosity with mean size of 10 nm, attributed to the interparticulate porosity is practically constant, and the interagglomerate porosity (100-600 nm) strongly depends on the salt concentration, freezing rate and thermal treatment of the powders.

Abstract: Electrophoretic deposition (EPD) is a powerful method for obtaining particulate layers in a broad range of thicknesses if an adequate control of the growing kinetics is reached. Existing models of EPD kinetics consider that the growth of the deposit increases linearly with deposition time and deviations are due to a reduction of powder concentration and/or a decrease of electric field when EPD is performed at constant voltage conditions. Experimental observations show that long time tests lead to a S-shaped growing kinetics. This work presents a resistivity model that predicts a S-shape variation of mass per unit area with deposition time, with a first step in which the deposition rate increases, as a consequence of resistivity changes, followed by a decreasing slope associated to the lose of powder concentration. Currently available EPD models, such as the Hamaker and Sarkar & Nicholson models are particular cases of the generalized resistivity model proposed in this work.

Abstract: There is a growing interest to develop reliable, economic and environmentally-friendly methods for manufacturing thick coatings and layered systems. For thick films, the adhesion to the substrate and the shape retention become important problems that lead to low density, cracking, and heterogeneous surfaces. To overcome these problems, a processing route is proposed consisting in the formation of a thick deposit by EPD in water and its immediate consolidation by thermogelation of polysaccharides. The process is based on the use of aqueous suspensions containing low concentration of biopolymers (i.e. carrageenan) and the formation of the film by dipping and/or EPD on suspensions heated at 60°C. On withdrawal at constant rate a film is obtained after cooling at room conditions. Maintaining the stability of the hot suspensions and during gelation is a key parameter to produce controlled deposits with good adhesion to the substrate and uniform microstructure. In this work, ceramic suspensions or Ni-containing suspensions have been stabilized with acrylic-based polyelectrolytes at controlled pH. Self-supported films of Al2O3/Y-TZP have been produced by dipping and EPD on graphite substrates that are burnt out on heating. On the other hand, Ni/YSZ composites have been deposited on graphite to produce either monolithic or laminated films. The optimization of the rheological behaviour of the suspensions and the EPD kinetics allows manufacturing a variety of complex structures with layers having controlled thickness ranging from 10 to 200 6m.

Abstract: Nickel-reinforced alumina composites have been manufactured by aqueous slip casting and pressureless sintered under flowing atmosphere of argon with 0,36 and 1% of oxygen in order to force interfacial reactions leading to the formation of a nickel-aluminum spinel. Colloidal stability of concentrated suspensions of alumina with 5, 10 and 15 vol% of nickel has been studied in terms of zeta potential, rheometry and packing density. The processed composites show a high dispersion of the nickel into the alumina matrix and green densities of 60-70 %th. The effect of sintering temperature and atmosphere on the mechanical behaviour of the composites has been investigated through Vickers indentation and fractographic SEM observations.