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Structural Studies of Anomalous Behavior in the Silica‐Alumina Gel System
Author(s) -
Hietala Susan L.,
Smith Douglas M.,
Brinker C. Jeffrey,
Hurd Alan J.,
Carim Altaf H.,
Dando Neal
Publication year - 1990
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1990.tb06680.x
Subject(s) - crystallite , small angle x ray scattering , high resolution transmission electron microscopy , materials science , magic angle spinning , crystallinity , analytical chemistry (journal) , small angle neutron scattering , crystallography , neutron scattering , scattering , transmission electron microscopy , chemistry , nuclear magnetic resonance spectroscopy , composite material , nanotechnology , organic chemistry , physics , optics
This paper compares the structure ofAl 2 O 3 ‐SiO 2 xerogels prepared with Al 2 O 3 contents ranging from 21 to 75 wt% (13 to 64 mol%). The 47 wt% Al 2 O 3 xerogel (Al/Si ≅ 1) exhibits anomalously low surface area (≅1 m 2 /g) and skeletal density compared with other Al 2 O 3 ‐Si 2 compositions. Based on the results of nitrogen adsorption/condensation, helium displacement, high‐resolution transmission electron microscopy (HRTEM), small‐angle X‐ray scattering (SAXS), and small‐angle neutron scattering (SANS), we attribute the low surface area and skeletal density (density of the solid phase which is inaccessable to helium) to the formation of closed micropores, whereas the higher surface area materials exhibit a slightly coarser texture comprising open pores with radii of ≅1 nm. X‐ray diffraction (XRD) and 29 SI and 27 Al magic‐angle spinning nuclear magnetic resonance (MASNMR) indicate no anomalous behavior in the 47% sample on molecular length scales. HRTEM indicates the presence of a small fraction of crystallites which is supported by the SAXS results, but it is unknown if this crystallinity is related to low surface area. Low‐field 1 H NMR spin‐lattice relaxation measurements show that the physical structure of all of the ‘wet’ gels is similar implying that pore closure occurs during drying. Consistent with this idea, gel surface area and density increased significantly when the pore fluid (water) was replaced with a lower surface tension fluid (formamide, dioxane, ethanol. water/surfactant).