A cheap way to prepare films of highly porous solids which have diverse applications
Those who handle expensive cameras know
the importance of silica gel - these granules
normally put in a small cloth bag are essential
to protect the lenses from moisture
which could induce formation of fungus.
Silica gel belongs to a class of highly
porous, sponge-like solids called aerogels,
which find diverse applications in industry
and research. Now,' some researchers in the
US have found a cheap way of producing
aerogel films which could give rise to a host
of new applications.
Aerogel films can be used as reflective
and anti-reflective coatings in optics. In electronics, these films can be used as insulating
layers. in the field of architecture, they are
being used as super-insulating glazings (both
transparent and opaque) for energy efficiency. Aerogels are also being used in
detecting high speed elementary particles as
in Cerenkov detectors.
Normally, silica gel is formed by condensation of a chemical compound containing silica, like tetraethoxysilane. The gel consists of
branching chains of minute, nanometre size
(a nanometre is a thousandth-millionth of a
metre) silica beads. Between the chains there
are voids of about 50 nanometres which are
filled with liquid. The problem is that if the
gel is dried in air, the compressive effect
(due to the capillary forces) experienced by the wet gel forces it to collapse
irreversibly.
In 1931, a scientist - S S Kistler - discovered a novel way to preserve the gel network by transforming the liquid within
the pores to a supercritical one by raising
the temperature and pressure above the
critical point (A substance is said to be in a
critical state - critical
temperature, critical
pressure and critical volume - when the density of the liquid phase is
the same as of the
vapour phase). For this,
the drying has to be
done under pressure in
an autoclave (a strong
vessel used for chemical
reactions at high temperature and pressure)
thereby making the
process cumbersome,
hazardous and expensive. This severely restricts the commercial
exploitation of aerogels (Nature, Vol 374, No 6521).
Now, Sai S Prakash and Sudeep M Rao
of the University of New Mexico and their
colleagues have developed a new method
of producing gels which does away with
the drying in the autoclave. Using a simple
method at normal pressure, the group has
been able to generate highly porous aerogel films.
The technique involves adding surface
groups - chemical groups that remain on
the surface - which are different from
those ordinarily used. The inner surface of
these gels is coated with the silyl group
instead of the usual hydroxyl group. In
ordinary gels, the adjacent hydroxyl
groups condense on
compression and form
new bonds which
counter the elasticity of
the silica gel network. In
the modified gels, the
silyl groups being chemically inert, they are
able to detach as the
pressure is released. The
gel contracts on drying
but gradually springs
back towards a porous state.
Aerogels are the
most highly porous
material known to us.
Their porosity could be
upto 99 per cent corresponding to a density of 20 kg per cubic metre (in comparison, water has a density of 1000 kg per cubic
metre). The new technique for the manufacture of aerogels could be revolutionary
not only for producing aerogel films but
also for bulk aerogels which are required
for many applications such as insulating material.
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