Particle aggregation in alumina aerogels
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Particle aggregation in alumina aerogels S. Keysar, Y. De Hazan, and Y. Cohen Chemical Engineering Department, Technion, Haifa 32000, Israel
T. Aboud Materials Engineering Department, Technion, Haifa 32000, Israel
G. S. Gradera) Chemical Engineering Department, Technion, Haifa 32000, Israel (Received 28 November 1995; accepted 11 July 1996)
Alumina aerogels were synthesized by low temperature CO2 supercritical drying (SCD) of gels via the Yoldas process. The aerogels have a surface area of ,425 m2ygr, 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 Dm 2.6 –2.8. During the SCD the morphology collapses to form a structure with surface fractal dimension Ds 2.6 –2.9.
I. INTRODUCTION
II. EXPERIMENTAL
The work on alumina aerogels is motivated by the intrinsic stability of alumina, which enables operation at high temperatures, under reducing and corrosive environments. Alumina aerogels therefore form the basis for many advanced technologies such as catalyst supports, filters in combustion gas atmospheres, as well as membrane reactors.1,2 Although several investigations have been carried out on the nanostructure of alumina gels,2 the corresponding data on alumina aerogels are scarce. In the present work small angle x-rays (SAXS) and transmission electron microscopy (TEM) analysis are used to study the nanostructures in alumina aerogels. Following the work of Kistler,3 alumina aerogels were prepared from aluminum sec-butoxide dissolved in sec-butanol using a high temperature and pressure supercritical drying (SCD) procedure.4 Alumina aerogels are currently still made under high pressure, high temperature conditions.5–8 In this work, aerogels were prepared at low temperature by a modification of the Yoldas process presently used to prepare xerogels.9 After the gelation step (at 80–90 ±C), the solvent is exchanged with acetone followed by CO2 -SCD at 34 ±C. Over the last decade it has been shown that aerogels are branched polymers having a fractal nature.10–16 The fractal dimension of aerogels was mainly studied by SAXS, small angles neutron scattering (SANS), and TEM. In this work SAXS and TEM analysis were used to show the connection between the microstructure of alumina aerogel and the fractal dimension of the system.
The sols were prepared by dissolving aluminum secbutoxide (ASB) (Aldrich, purity 97%) in hot distilled water (80 –90 ±C). The molar ratio of water to ASB was 100 : 1; this ratio is typical of the Yoldas process.9 After hydrolysis (80–90±) for 0.5 h in a shaking bath, nitric acid (97% purity) was added to catalyze the condensation. Sols with a
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