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  • alumina aerogels  (2)
  • alumina gels  (1)
  • 1
    Electronic Resource
    Electronic Resource
    Preparation of Alumina Aerogel Films by Low Temperature CO2 Supercritical Drying Process (1997)
    Springer
    Journal of sol gel science and technology 8 (1997), S. 825-829 
    Details
    ISSN: 1573-4846
    Keywords: alumina aerogels ; supercritical drying ; sol gel ; aerogel films
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: 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 (∼0.1 μm thick), and pores (0.1–0.5 μm 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1018354612026
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Effect of Aging on Alumina Gels Rheology and Aerogels Surface Area (1999)
    Springer
    Journal of sol gel science and technology 14 (1999), S. 131-136 
    Details
    ISSN: 1573-4846
    Keywords: alumina aerogels ; gels ; aging ; rheology
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Alumina gels were synthesized by catalyzed gelation of aluminum sec-butoxide (ASB) via the Yoldas process. The gels were aged for up to 6 months and then supercritically dried (SCD) with CO2. The molar ratio of acid to ASB was in the range of 0.01–0.6. Viscosity measurements of the gels showed a shear thinning and plastic behavior with no response up to a limiting yield stress. The gel rheology obeys the Casson model. Analysis of the viscosity as a function of the acid to alkoxide molar ratio, showed that the average molecular weight of the gels is inversely proportional to the acid to alkoxide molar ratio. The viscosity of all the gels increased with aging time for a period of about 6 months reaching an asymptotic value after 1–2 weeks. The viscosity is shown to correlate with the microstructure of these nanomaterials during aging. Aging gives rise to a nearly constant surface area of ∼350 m2/g regardless of acid to alkoxide ratio in an aging period of about 6 months.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008777731237
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Microstructure Evolution of Nonhydrolytic Alumina Gels (1997)
    Springer
    Journal of sol gel science and technology 10 (1997), S. 5-12 
    Details
    ISSN: 1573-4846
    Keywords: alumina gels ; nonhydrolytic sol-gel ; fractal structures
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Nonhydrolitic sol-gel processes of aluminum chloride and aluminum bromide with isopropyl ether and aluminum sec-butoxide were performed at various temperatures. Based on the Arrhenius type variation of the gelation time with temperature, activation energies for the gelation were found to be in the range 19–25 Kcal/mol range. The energies were found to be sensitive to the nature of the aluminum ligands and the chemical scheme. Due to the large activation energy, it is possible to stop the reaction at any time before gelation by cooling the sol to room temperature. Small angle X-ray scattering (SAXS) of sols from the AlClAlCl3/Pr $$_2^i $$ O system shows unique development of a fractal like structure with nanometer scale order, demonstrated by discrete peaks in the SAXS data. A fractal dimension D = 1.64 was found. An aggregation scheme is proposed to explain this phenomenon. A fractal dimension of 2.4 without small scale ordering found for xerogels prepared from the AlCl3/ASB system reflects the effect of the different precursors on the microstructure of nonhydrolytic gels.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1018306019878
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    Paper (German National Licenses)
    Fulltext
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