Ultrasonically Improved Sol-Gel Reactions - Hielscher Ultrasonics0 pages

Sonochemical Effects on Sol-Gel Processes
by Kathrin Hielscher, Hielscher Ultrasonics GmbH, www.hielscher.com

Introduction
Ultrafine nano-sized particles and spherical shaped particles, thin film coatings, fibers, porous and
dense materials, as well extremely porous aerogels and xerogels are highly potential additives for the
development and production of high performance materials. Advanced materials, including e.g.

ceramics, highly porous, ultralight aerogels and organic-inorganic hybrids can be synthesized
from colloidal suspensions or polymers in a liquid via the sol-gel method. The material shows
unique characteristics, since the generated sol particles range in the nanometer size.
Thereby, the sol-gel process is counted to the nanochemistry.
In the following, the synthesis of nano-sized material via ultrasonically assisted sol-gel routes
is reviewed.
Sol-Gel Process
Sol-gel and related processing includes the following steps:
(1)
making sol or precipitating powder, gelling the sol in a mold or on a substrate
(in case of films), or making a second sol from the precipitated powder and its
gelation, or shaping the powder into a body by non-gel routes;
(2)
drying;
(3)
firing and sintering. [Rabinovich 1994]
Precursor
Suspension

Gelation

Drying

Sintering

Sol-Gel
Table 1: Steps of Sol-Gel synthesis

Sol-gel processes are a wet-chemical technique of synthesis for the fabrication of an
integrated network (so-called gel) of metal oxides or hybrid polymers. As precursors,
commonly inorganic metal salts such as metal chlorides and organic metal compounds such
as metal alkoxides are used. The sol - consisting in a suspension of the precursors transforms to a gel-like diphasic system, which consists in both a liquid and a solid phase.
The chemical reactions that occur during a sol-gel process are hydrolysis, poly-condensation,
and gelation.
During hydrolysis and poly-condensation, a colloid (sol), which consists in nanoparticles
dispersed in a solvent, is formed. The existing sol phase transforms to the gel.
The resulting gel-phase is formed by particles which size and formation can vary greatly from
discrete colloidal particles to continuous chain-like polymers. The form and size depends on
the chemical conditions. From observations on SiO2 alcogels can be generally concluded that
a base-catalyzed sol results in a discrete species formed by aggregation of monomer-