REVUE DE PHYSIQUE APPLIQUÉE
Colloque C4, Supplément au n°4, Tome 24, Avril 1989 C4-23
FREEZE DRYING OF SILICA GELS PREPARED FROM SILICIUMETHOXID
E. DEGN EGEBERG and J. ENGELL
Industry, Technical University of Denmark, DK-2800
Résumé - Les gels étudiés ont été prépares par une hydrolyse en cata-
lyse acide de Si(0Et>4 dilué.; dans Bu^OH. Les acides employés sont H3PO4 et
acide oxalique. L'acide oxalique fonctionne aussi comme DCCA. Le séchage a
froid des gels sans échange du solvant resuite en une désintégration en
morceaux de l'ordre du millimètre. La détérioration du gel devient moins
importante quand la concentration de EtOH dans la phase liquide est réduite par
l'échange du solvant. Une série d'expériences indiquent que le Bu^-OH sature en
air montre une expansion (5 vol%) durant la congélation â l'opposé du Bu^-OH pur
qui ne montre qu'un petit changement de volume. Cette expansion du volume est
probablement responsable de la fracture des pièces monolithiques.,
Abstract - The freeze dried gels were prepared by acid catalysed hydrolysis of
Si(0Et)4 diluted by Bu^H. The acids used include H3PO4 and oxalic acid.
Oxalic acid has DCCA-properties. Freeze drying without prior solvent exchange
results in disintegration of the gels into mm size flakes. The fracturing of
the gels becomes less severe as the concentration of EtOH in the liquid phase
is reduced by solvent exchange. Pure Bu*-OH shows only a very small volume
change upon crystallization, but air saturated Bu*-OH expand about 5 vol% upon
freezing. This volume expansion is presumably the cause of the crack-pattern
observed in the solvent exchanged monolithic pieces.
The drying process is a critical step in the preparation of sol-gel derived monoliths (15,
23). During drying stress caused by capillary forces in the pore structure of the gel results
in shrinkage and fracturing unless special precautions are taken.
Monolithic silica xero-gels (ca. 10 cm-^J, with bulk densities in the range 1.5-1.8 g/crrH,
can be produced by controlled conventional drying in about 5 days using acid-catalysed hydro-
lysis of Si(0Et)4 and a relative high casting density (9). The use of Drying Controlling
Chemical Additives (DCCA) like formamide, oxalic acid or glycol permit a relatively rapid
drying (5, 18). Thus, monoliths up to >100 cirH in size of silica xero-gel with bulk densities
in the range 1.2-1.4 g/cm^ have been made within 2 days using formamide and acid catalysed
hydrolysis of Si(0Me)4 (5). The use of DCCA results in the formation of a narrow pore size
distribution by elimination of the smallest pore (1, 5, 18). Hereby the capillary forces are
reduced and the drying process facilitated. Mizuno et al. (12) have recently succeeded in
producing small monoliths of silica xero-gels with bulk densities as low as 0.73 g/cm^ by
conventional drying of Si(0Me>4 derived gels. Contrary to conventional wisdom these results
were obtained by aqueous solvent exchange before drying. This was done in order to strengthen
the gel by eliminating organic groups and thus, favour crosslinking.
Monolithic silica aero-gels with bulk densities in the range 0.02-0.3 g/cm^ can at present
only be produced by supercritical drying (6, 16, 19) where the liquid-vapour interface is
eliminated and capillary stresses thereby avoided.
The present work concerns the preparation of monolithic silica gels by freeze drying, i.e.
drying by sublimation of a frozen solvent. Freeze drying is used on a large scale in food and
medical technology (21). The technique has also proved very useful for laboratory preparation
of high quality ceramic powders (3, 14, 22). In principle the process makes it possible to
conserve the structure of the material being dried. However, monoliths can not be made from
aqua-gels by this method. The freezing process results in the formation of small flakes, or if
directional freezing is used fibres (11). Important factors are here, firstly the large volume
expansion upon freezing of water (10 vol%) and secondly the surface tension between the phases
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989404
Monolithic silica gels have been prepared from Si(OEt)4 diluted by BU~OH, H3PO4, oxalic
acid and H20. After several solvent exchange and long aging time freeze drying of such gels
gave large translucent pieces (6~3~0.5
cm) with bulk densities in the range 0.3-0.5 9/cm3.
In order to facilitate the freezing process, and to avoid boiling during the freeze drying,
it is necessary to reduce the EtOH concentration below 0 . 5 vt%. Using a volume of BU~OH equal
to the volume of the gel, 5 solvent exchanges are sufficient in order to reduce the EtOH
content to an acceptable level. It is necessary to use high purity, air-free BU~OH for the
solvent exchange, since impurities lower the freezing point and air saturated BU~OH expands
With further improvements it should be possible to produce monolithic aero-gels by freeze
This work was financed by the Danish Energy Research Program through contract no. EFP-87-
1213-701-01-01 and the Danish Technical Research Council through contract no. 5 . 1 7 . 1 . 6 . 1 9 .
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