Papers by Author: María Isabel Nieto

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: 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.