Preparation of Aerogel and Xerogel Nanocomposite Materials
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ABSTRACT Sol-gel method was used to prepare nickel oxide-silica and iron oxide-silica nanocomposites materials in form of aerogels and xerogels. The samples were characterized by thermal analysis, X-ray diffraction, N 2 physisorption and transmission electron microscopy techniques. The variation of the supercritical solvent extraction conditions gives rise to differences in the morphological characteristics of the aerogels. These differences influence the size of the nickel oxide nanoparticles in nickel containing aerogels. On the other hand they do not affect the structure and size of the iron oxide nanoparticles in iron containing aerogels. The differences between the xerogel and aerogel nanocomposites are discussed. INTRODUCTION The sol-gel process has shown to be a valid method for the preparation of nanocomposite materials constituted of nanometric metal or metal oxide particles embedded in amorphous silica.'- 3 These materials present a variety of interesting magnetic, electric and catalytic properties which depend on the dimensions, distribution and concentration of the nanoparticles.4-6 It is well known that the sol-gel process allows a good control of composition, texture and structural characteristics of the final products. 7 In particular, the drying step plays an important role in determining the final properties of the materials. In fact, if the drying step is performed under supercritical conditions, aerogels with a high surface areas and pore volumes can be obtained, while xerogels, in which the original pore structure of the alcogels is lost, are obtained by slowly removing the solvent. NiO-SiO 2 nanocomposites in form of aerogel seems to be particularly attractive since they can be easily converted into the corresponding Ni-SiO 2 nanocomposites, useful materials for catalytic applications.8 Fe 20 3 -SiO 2 nanocomposites have shown to be able to stabilize the yFe 20 3 phase (maghemite), which has attractive magnetic properties. In this case it is of interest to study the influence of the porous structure on maghemite formation and stabilization. In this paper the effects of different drying procedures on the structural and morphological properties of NiO-SiO 2 and Fe20 3-SiO 2 nanocomposites in form of aerogels and xerogels are studied using TGA/DTA, XRD, N2 physisorption and TEM techniques. EXPERIMENT Nickel oxide-silica (17% wt of nickel oxide) and iron oxide-silica (23% and 33% wt of iron oxide) nanocomposite materials were prepared by sol-gel method using TEOS and either nickel nitrate or iron nitrate as precursors.3' The obtained alcogels were submitted to two different procedures of solvent drying. Xerogels were obtained by slow heat treatments in static air while aerogels were obtained by high temperature supercritical drying performed in an autoclave. Four different conditions of supercritical drying were used to obtain the NiO-SiO 2 aerogels (Al, A2, A3, A4) by varying the heating ramp, the solvent used to fill the autoclave and the 363 Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Re
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