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High‐temperature and low‐temperature supercritical drying of aerogels – structural investigations with SAXS
Author(s) -
Wang P.,
Emmerling A.,
Tappert W.,
Spormann O.,
Fricke J.,
Haubold H.G.
Publication year - 1991
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s0021889891002327
Subject(s) - small angle x ray scattering , supercritical fluid , materials science , scattering , particle (ecology) , chemical engineering , particle size , methanol , autoclave , solvent , porosity , crystallography , analytical chemistry (journal) , chemistry , composite material , chromatography , organic chemistry , optics , physics , oceanography , engineering , metallurgy , geology
Base‐catalyzed silica aerogels are composed of particles with a mean size of about 5 nm, which form a chain‐like porous network. Up to now it has been assumed that supercritical drying (SCD) of highly porous gels in autoclaves leaves the structure nearly unchanged. Small‐angle X‐ray scattering (SAXS) measurements provide evidence that this is only true for the low‐temperature CO 2 drying process with a critical temperature T c = 304.2 K and a critical pressure p c = 73.9 × 10 5 Pa. In the high‐temperature methanol process ( T c = 512.5 K and p c = 80.9 × 10 5 Pa) with remaining water and catalyst, however, structural changes are introduced. The SAXS measurements can be explained by a narrowing of the particle‐size distribution during the heating period of the autoclave process. In a double‐logarithmic representation of the scattered intensity I versus the scattering vector q , intermediate slopes smaller than −4 in the Porod region as well as oscillations in an Iq 4 vs q plot appear. On the contrary, SCD hardly affects the particle structure of acid‐catalyzed gels and, if the basic solvent is exchanged for pure methanol, of base‐catalyzed gels.