The phase conversions and morphological changes of alumina aerogel films and monoliths during heat treatment were investigated by TGA/DTA, BET, XRD, and image analysis of SEM micrographs. A preliminary model explaining the structural evolution of the film during heat treatment is proposed.

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Alumina aerogels were synthesized by low temperature CO 2 supercritical drying (SCD) of gels via the Yoldas process. The aerogels have a surface area of ϳ425 m 2 ͞gr, similar to that obtained under high pressure/temperature SCD. The surface area and the cluster size of the aerogels are strongly influenced by the amount of acid during gelation. Gels and aerogels were studied by small angle x-rays scattering (SAXS), and the data were analyzed using the Fisher-Burford equation. The SAXS results along with TEM observations support the existence of a hierarchical aggregation at the gelation stage, having a mass fractal dimension of D m 2.6 -2.8. During the SCD the morphology collapses to form a structure with surface fractal dimension D s 2.6 -2.9. 430

Artificial calcite lining represents a novel technique for the rehabilitation of water mains. Calcite linings, similar to the commonly used cement-mortar linings, are of porous nature. The corrosion protection mechanisms of calcite linings were investigated by polarization techniques, complemented by scanning electron microscopy observations and energy dispersive spectrometry analyses. The effect of time on lining durability was examined by exposure of coated mild steel (1020) coupons in a controlled flowing water system. Iron ion distributions in calcite sections, before and after exposure to water flow, indicate that the calcite corrosion protection mechanism is based mainly on accumulation of corrosion products inside the lining. At the calcite−metal interface, a protective film is formed. At the calcite−water interface, the calcite structure, being alkaline with respect to the water, promotes precipitation of the iron ions and blockage of the pores near that interface. The formation of this protective oxide film was evident from the polarization data which showed that both the anodic Tafel constant and the polarization resistance increase with time. This paper also examines the applicability of potentiodynamic and linear polarization techniques for the characterization of the corrosion protection of a porous lining, such as calcite. It is shown that these convenient electrochemical techniques provide reliable and meaningful corrosion protection information.

Abstract. Alumina aerogel thin films were formed by a new synthesis route. Sols were prepared by the Yoldas process. Gels were formed by sol evaporation in a few hours. Films were prepared by dip coating glass or alumina substrates into both the sols and the gels. Aerogel films with special morphology were produced for the first time by exchanging the film solvent with acetone after the dip coating, followed by supercritical drying. The morphology of the films, studied by SEM and TEM, consists of fiber-like network of round chains (N0.1/zm thick), and pores (0.1-0.5/zm in diameter). It is shown that the fibers contain a homogeneous arrangement of sol particles, 2-4 nm in size. Formation of this microstructure can be attributed to phase separation in the alumina-water-acetone system in a 2D film geometry. A conceptual model for the film development is proposed.