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January 31st, 2012

Toughened, flexible silica aerogel? Joint Japanese–Chinese group shows how to do it

Published on January 31st, 2012 | By: pwray@ceramics.org

Demonstration of the flexibility of cellulose–silica composite aerogel. Credit: J. Cai et al.; Angewandte Chemie.

This sounds like the type of breakthrough aerogel fans have yearning for.

A newly published paper in Angewandte Chemie reports on an Asian group’s success at using cellulose fibers as a scaffold/template for a resultant silica aerogel that delivers a product that has great mechanical strength and flexibility, while retaining a large surface area and semitransparency.

Aerogel has been something of a tease for many years. It has incredible insulating abilities, but the one enormous problem for silica aerogel is that it is frustratingly brittle and difficult to work into practical applications. Some developers have found limited success via hybridization techniques with support materials such as polyurethane, polystyrene or even nanofibrillar bacterial cellulose and microfibrillated cellulose gel.

However, with support from the Japan Society for the Promotion of Science‘s Foreign Researcher Fund of Japan and the National Basic Research Program of China, researchers at Wuhan University, China, and University of Tokyo, took a different cellulose-based route. They already knew that they could exploit “cellulose II” crystallinity  (dissolution and then regeneration/reassembly of fibrils) to form aerogels with good mechanical strength, light transmittance and high porosity — characteristics that they suspected would make it an effective substrate for silica aerogel.

In brief, the group, led by Lina Zhang, impregnated a sample of nanoporous cellulose gel (with its interconnected nanofibrillar network) with a silica precursor, tetraethyl orthosilicate. According to the paper, “The resulting composite gels were dried with supercritical CO2 to give cellulose–silica aerogels with low density, moderate light transmittance, a large surface area, high mechanical integrity and excellent heat insulation.”

They then went one step farther and used calcination to remove the cellulose matrix, leaving a silica-only aerogel. The key point here is that this silica aerogel’s structure is much different than pure silica aerogel. In the latter, primary silica nanoparticles form and then randomly coagulate resulting in an isotropic 3D network. “In contrast,” again quoting from the paper, the authors say, “the formation of silica nanoparticles in the cellulose gel seems to cause their deposition onto the cellulose fibrils. As a result, removal of cellulose by calcination results in the nanofibrillar silica network.”

The group compared a variety of aerogels, including silica-only and cellulose-only aerogels; cellulose-silica composites, with varying levels of silica; and cellulose-templated silica aerogel.

What they found at the macroscopic level is that the composite aerogels didn’t inherit the fragility of the silica, but instead seem to inherit the flexibility and strength of the cellulose network (see knotted sample of one of the composites, above).

While the tensile modulus and strength of the cellulose–silica aerogel were less than pure cellulose aerogel, “the compression modulus of the composite (7.9MPa) is more than two orders of magnitude higher than that of silica aerogel, and about 50 times higher than that of the aerogel prepared from bacterial cellulose.”

Because of the cellulose content, the composite aerogels break down when used above 300°C. However, below that temperature, the cellulose-silica aerogel retained strong heat insulating properties. Thermal conductivity of the prepared samples ranged from 0.025 W m1 K1 to 0.045 W m1 K1.

These numbers compare favorably with polystyrene foam (0.030 W m1 K-1), however, the researchers note that the ability of the cellulose–silica aerogels to perform up to 300°C give it a leg up on insulation materials made of polymer that soften and breakdown at similar temperatures.

“Thus,” according to the authors,”the cellulose–silica composite is potentially useful as heat insulating material with high mechanical stability, together with processability to form sheets, fibers, or beads. … [They] retained the mechanical strength and flexibility, large surface area, semitransparency, and low thermal conductivity of the cellulose aerogels. The ease of preparation and wide tuneability of composition/properties with this method are expected to form the basis for the development of various advanced nano-porous materials.”

The paper, “Cellulose-silica nanocomposite aerogels by in situ formation of silica in cellulose gel,” (doi:10.1002/ange.201105730) is written by Jie Cai, Shilin Liu, Jiao Feng, Satoshi Kimura, Masahisa Wada, Shigenori Kuga, and Lina Zhang.

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