María Isabel Nieto

Thermogelling polysaccharides for aqueous gelcasting: Part III: mechanical and microstructural characterization of green alumina bodies

Abstract The use of gelling additives, such as polysaccharides, in colloidal processing provides adequate mechanical properties to the green bodies to be handled. In this work, the green density and the mechanical behaviour (stress–strain relationships, elastic modulus, bend strength and fracture mechanism) at room temperature of gelcast alumina bodies are studied, in order to establish the influence of the type and the concentration of additive. Furthermore, the previous concentration of polysaccharide solutions is also taken into account as an important variable. Agar, agarose and carrageenan were used as gelling additives. Values of the bend strength up to 4 MPa are obtained, significantly higher than those corresponding to slip cast alumina without gelling additives, and they increase with the final concentration of polysaccharide, while Youngs modulus values are mainly influenced by the concentration of additive in the precursor solution. For bodies with a large final concentration of additive, extensive plastic deformation during fracture is observed.

Synthesis of Gamma-Alumina Nanoparticles by Freeze Drying

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.

Thermogelation of composite films produced by electrophoretic codeposition

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.

Forming of Ceramic Coatings by Dipping and EPD Through Carrageenan Gelation

The use of ceramic coatings and tailored composite materials has gained interest for many structural applications. Dipping and electrophoretic deposition (EPD) have demonstrated to be economic and simple methods to obtain coatings and self-supported microlaminates, while gelcasting with biopolymers is a powerful route for manufacturing complex-shaped bulk parts. The combination of these two forming processes has led to the development of a new method for manufacturing ceramic coatings from aqueous suspensions. Graphite and ceramic substrates were coated with Al2O3/ZrO2 films by combining either dipping or EPD with carrageenan gelation. The rheological behaviour of Al2O3/ZrO2 aqueous suspensions was studied as a function of temperature focusing the effect of the concentration of gelling additive and the volume fraction of particles. Its effect on the EPD conditions was also studied in order to obtain uniform and reliable films. After sintering at 1550°C/2h dense films with a thickness ranging from 10 to 300 μm were obtained.