Regenerated cellulose gel prepared from an aqueous alkali–urea solution serves as scaffold/template for the in situ preparation of cellulose–silica composite aerogels (see picture) by a sol–gel process from organic silicates, and drying with supercritical CO2. The resulting composite aerogels have the mechanical strength and flexibility, large surface area, semi-transparency, and low thermal conductivity of the cellulose aerogels. |
Delicate
and translucent as a puff of air, yet mechanically stable, flexible,
and possessing amazing heat-insulation properties—these are the
properties of a new aerogel made of cellulose and silica gel.
Researchers led by Jie Cai at Wuhan University in China have introduced
this novel material, which consists almost completely of air, in the
journal Angewandte Chemie.
Gels
are familiar to us in forms like Jell-O or hair gel. A gel is a loose
molecular network that holds liquids within its cavities. Unlike a
sponge, it is not possible to squeeze the liquid out of a gel. An
aerogel is a gel that holds air instead of a liquid. For example,
aerogels made from silicon dioxide may consist of 99.98% air-filled
pores. This type of material is nearly as light as air and is
translucent like solidified smoke. In addition, it is not flammable and
is a very good insulator—even at high temperatures. One prominent
application for aerogels was the insulation used on space shuttles.
Because of their extremely high inner surface area, aerogels are also
potential supports for catalysts or pharmaceuticals. Silica-based
aerogels are also nontoxic and environmentally friendly.
One
drawback, however, has limited the broader application of these airy
materials: silica-based aerogels are very fragile, and thus require some
reinforcement. In addition to reinforcement with synthetic polymers,
biocompatible materials like cellulose are also under consideration.
The
researchers at Wuhan University and the University of Tokyohave now
developed a special composite aerogel from cellulose and silicon
dioxide. They begin by producing a cellulose gel from an alkaline urea
solution. This causes the cellulose to dissolve, and to regenerate to
form a nanofibrillar gel. The cellulose gel then acts as a scaffold for
the silica gel prepared by a standard sol–gel process, in which a
dissolved organosilicate precursor is cross-linked, gelled, and
deposited onto the cellulose nanofibers. The resulting liquid-containing
composite gel is then dried with supercritical carbon dioxide to make
an aerogel.
The
novel composite aerogel demonstrates an interesting combination of
advantageous properties: mechanical stability, flexibility, very low
thermal conductivity, semitransparency, and biocompatibility. If
required, the cellulose part can be removed through combustion, leaving
behind a silicon dioxide aerogel. The researchers are optimistic: “Our
new method could be a starting point for the synthesis of many new
porous materials with superior properties, because it is simple and the
properties of the resulting aerogels can be varied widely.”
Cellulose–Silica Nanocomposite Aerogels by In Situ Formation of Silica in Cellulose Gel
