Light but stable: novel cellulose-silica gel composite aerogels

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 (China) and the University of Tokyo (Japan) have 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."

More information: Jie Cai, Cellulose–Silica Nanocomposite Aerogels by In Situ Formation of Silica in Cellulose Gel, Angewandte Chemie International Edition, http://dx.doi.org/ … ie.201105730

Provided by Wiley (news : web)

New process for manufacturing nanocellulose: Using nanocellulose to create novel composite materials

For some time now nanocellulose has been at the focus of a good deal of industrial and scientific interest as a novel biomaterial. Potential applications range from the creation of new kinds of commercially useful materials and uses in medical technology all the way to the food and pharmaceutical industries. Researchers with Switzerland's Empa research institute have now developed a manufacturing process for nanocellulose powder, the raw material for creating polymer composites which can be used, for example, in lightweight structures for the car industry or as membrane and filter material for biomedicinal applications.

Cellulose is a biopolymer consisting of long chains of glucose with unique structural properties whose supply is practically inexhaustible. It is found in the cell walls of plants where it serves to provide a supporting framework -- a sort of skeleton. Cellulose is extremely strong in tension and can be chemically modified in many ways, thereby changing its characteristics. It is also biodegradable. In the search for novel polymer materials with certain desirable characteristics material scientists have developed such substances as high performance composites in which nanofibers of cellulose are embedded. In the form of lightweight structural material, these composites have similar mechanical properties to steel, while as nanoporous "bio"-foam they provide an alternative to conventional insulating materials.

The ideal lightweight structural material

Classical cellulose chemistry on the industrial scale is primarily used in the wood pulp, paper and fiber industry. Commercial research is currently focused on isolating and characterizing cellulose in the form of nanofibers. So-called nanocellulose consists of fibers or crystals with a diameter of less than 100 nm. Material scientists hope to be able to use nanocellulose to create new lightweight materials boasting high mechanical strength -- in short the ideal material for creating lightweight structures.

The cellulose experts in Empa's Wood Laboratory isolated cellulose nanofibers from wood pulp. These are several micrometers long but only a few nanometers thick and are closely interlinked. The fibers have an extremely large surface area on which chemical-physical reactions with substances such as water, organic and inorganic chemicals and polymer compounds can occur. Cellulose nanofibers can therefore be used as stable, extremely reactive raw materials for technical applications while boasting the additional advantages of being biologically produced and biodegradable. Such applications include reinforcing (bio-)polymers to create very promising, environmentally safe, lightweight construction material for the car industry, as well as membrane or filter materials for applications in packaging and biomedicine.

The solution lies in chemical modification

Nanocellulose isolated from wood pulp is initially in the form of a water-based suspension. If the material dries out the cellulose fibers stick together forming rough clumps and it loses its outstanding mechanical properties. For this reason the Empa researchers sought to develop a process which allowed them to dry nanocellulose without it clumping and becoming rough. To achieve this, the cellulose was treated using a technique which is easily implemented on a large scale and is also completely harmless, even being suitable for applications in the food industry. The method prevents the cellulose fibrils from forming clumps and sticking together

The results are worth looking at: after being re-dispersed in water the dried nanocellulose powder boasts the same outstanding properties as undried, unmodified cellulose. This makes the new product an attractive alternative to conventional cellulose suspensions for the synthesis of bio-nanocomposite materials. Suspensions currently in use consists of over 90% water which causes the transport costs to explode and increases the danger of degradation by bacteria or fungi. In addition aquatic cellulose suspensions are laborious to work with since usually in the course of chemical processing solvents must be exchanged.

Empa Research Prize 2011 goes to Christian Eyholzer

The work on developing the new manufacturing process and identifying applications for nanocellulose in various biopolymers was recently recognized with the award of the Empa Research Prize 2011. In a collaborative project with the "Lulea University of Technology," Sweden, Empa researcher and PhD student Christian Eyholzer and his co-workers used the novel nanocellulose powder to reinforce adhesives, hydrogels and biodegradable synthetics. After completing his doctoral dissertation Eyholzer left Empa and is currently employed by Sika as project leader in the product development department.

Story Source:

The above story is reprinted from materials provided by Empa. The original article was written by Nina Baiker.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Built like the Dreamliner: 2013 debut of carbon composite cars

The revolutionary material used to build the Boeing 787 Dreamliner, the Airbus A350 super-jumbo jet, and the military's stealth jet fighter planes is coming down to Earth in a new generation of energy-saving automobiles expected to hit the roads during the next few years. That ultra-strong carbon fiber composite material — 50% lighter than steel and 30% lighter than aluminum — is the topic of the cover story in the current edition of Chemical & Engineering News, ACS's weekly newsmagazine.

In the story, C&EN Senior Correspondent Marc S. Reisch describes how carmakers such as BMW, Mercedes, and Audi are turning to carbon fiber composites to reduce the weight and improve the mileage of their next-generation of electric and hybrid vehicles. Carbon fiber composites are plastics containing fine strands of carbon that are spun into fibers and woven into a fabric. Manufacturers lay the fabric into a mold with the shape of the final part, and soak it with epoxy or other resin to produce parts for aircraft and other products.

Despite concerns about the high cost of carbon fiber composites, automakers are embracing this energy-saving material, even though it may increase the cost of small electric or hybrid cars by $5,000 or more, the article notes. It describes major auto manufacturers' plans for marketing vehicles made with carbon fiber composites, and research underway to reduce the cost of the material.

More information: “Getting the Steel Out” http://pubs.acs.or … 39cover.html

Provided by American Chemical Society (news : web)

Chemistry: Separation a thousand-fold faster may lead to new composite materials

 Numerous industrial processes make use of blends. Researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences have studied how the external electric field affects the rate of component separation in blends composed of polymers and liquid crystals and those composed of various types of polymers. The observations gathered open interesting opportunities, e.g., for the development of new composite materials.

Inhomogeneous blends of polymers with other polymers or liquid crystals are widely used in industrial applications -- in LCD displays, gas-flow sensors, optical memories and other devices. Researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw analysed the behaviour of such blends in alternating external electric field. „We managed to determine precisely the conditions permitting even a thousand-fold acceleration of component separation process in the blends under study," says Prof. Robert Hołyst.

With time, many blends separate spontaneously into their components, usually at a very low rate. It has been known since long that the separation can be accelerated when an inhomogeneous liquid is placed in an external alternating electric field with adequately tuned frequency. It is generally accepted that the acceleration of separation is due to ions -- natural constituents of such mixtures.

The researchers from the Institute of Physical Chemistry of the PAS studied blends of a polymer with another polymer or liquid crystal. In the presence of an alternating electric field with the strength of several million volts per meter the ions of the component with higher conductivity start to move freely towards the electrode with the opposite charge. Having reached the phase interface with a non-conductive material on the other side they are strongly hampered. „Under these conditions, an additional force appears at the interface. With electric field alternating at appropriate frequency the ions start to yank the interface. Due to the yanking, the droplets of a component merge with each other significantly more efficiently than in the normal case, thus leading to a faster separation of both phases," says Natalia Ziębacz, a PhD student at the IPC PAS.

The separation efficiency of studied blends into their components is strongly dependent on the frequency of the applied electric field. Optical measurements carried out at the IPC PAS have shown that under optimal conditions, at frequencies up to the kilohertz range, the separation takes place even thousand-fold faster. Too low or too high frequencies of the electric field do not result in significant movements of ions and the separation occurs at a regular, low rate. The physical mechanism of the phenomenon suggests that similar effect can be expected in all blends contaminated with ions and containing components with different charge conductivities.

Controlling the rate of separation process over so long time range, extending over three orders of magnitude, opens the way to interesting applications. The separation process can be carried out very quickly, and then virtually stopped at a precisely selected stage. The structure of the blend so obtained can be then fixed, for instance by changing the temperature. Thus, the method to control separations of blends of polymers and liquid crystals using electric field turned out to be an excellent tool for the development of new materials. A patent application for the method has been filed.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Institute of Physical Chemistry of the Polish Academy of Sciences, via AlphaGalileo.

Journal Reference:

Natalia Zie?bacz, Stefan A. Wieczorek, Tomasz Szymborski, Piotr Garstecki, Robert Hołyst. Thousand-Fold Acceleration of Phase Decomposition in Polymer/Liquid Crystal Blends. ChemPhysChem, 2009; 10 (15): 2620 DOI: 10.1002/cphc.200900505

Silver-diamond composite offers cooling capabilities for electronics

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A microscope image of diamond particles. (Credit: Jason Nadler)

(PhysOrg.com) -- Researchers at the Georgia Tech Research Institute (GTRI) are developing a solid composite material to help cool small, powerful microelectronics used in defense systems. The material, composed of silver and diamond, promises an exceptional degree of thermal conductivity compared to materials currently used for this application.

The research is focused on producing a silver-diamond thermal shim of unprecedented thinness – 250 microns or less. The ratio of silver to diamond in the material can be tailored to allow the shim to be bonded with low thermal-expansion stress to the high-power wide-bandgap semiconductors planned for next generation phased-array radars.

Thermal shims are needed to pull heat from these high-power semiconductors and transfer it to heat-dissipating devices such as fins, fans or heat pipes. Since the semiconductors work in very confined operating spaces, it is necessary that the shims be made from a material that packs high thermal conductivity into a tiny structure.

Diamonds provide the bulk of thermal conductivity, while silver suspends the diamond particles within the composite and contributes to high thermal conductivity that is 25 percent better than copper. To date, tests indicate that the silver-diamond composite performs extremely well in two key areas -- thermal conductivity and thermal expansion.

'We have already observed clear performance benefits -- an estimated temperature decrease from 285 degrees Celsius to 181 degrees Celsius -- using a material of 50 percent diamond in a 250-micron shim,' said Jason Nadler, a GTRI research engineer who is leading the project.

The researchers are approaching diamond percentages that can be as high as 85 percent, in a shim less than 250 microns in thickness. These increased percentages of diamond are yielding even better performance results in prototype testing.

Nadler added that this novel approach to silver-diamond composites holds definite technology-transfer promise. No material currently available offers this combination of performance and thinness.

Natural Thermal Conductors

Diamond is the most thermally conductive natural material, with a rating of approximately 2,000 watts per meter Kelvin, which is a measure of thermal efficiency. Silver, which is among the most thermally conductive metals, has a significantly lower rating -- 400 watts per meter K.

Nadler explained that adding silver is necessary to:

- bond the loose diamond particles into a stable matrix;
- allow precise cutting of the material to form components of exact sizes;
- match thermal expansion to that of the semiconductor device being cooled;
- create a more thermally effective interface between the diamonds.

Nadler and his team use diamond particles, resembling grains of sand, that can be molded into a planar form.

The problem is, a sand-like material doesn't hold together well. A matrix of silver -- soft, ductile and sticky -- is needed to keep the diamond particles together and achieve a robust composite material.

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This image shows different preparations of diamonds, ready for integration into a silver matrix. (Credit: Gary Meek)

In addition, because the malleable silver matrix completely surrounds the diamond particles, it supports cutting the composite to the precise dimensions needed to form components like thermal shims. And silver allows those components to bond readily to other surfaces, such as semiconductors.

Tailoring Thermal Expansion

As any material heats up, it expands at its own individual rate, a behavior known as its coefficient of thermal expansion (CTE).

When structures made from different materials -- such as a wide-bandgap semiconductor and a thermal shim -- are joined, it is vital that their thermal-expansion coefficients be identical. Bonded materials that expand at different rates separate readily.

Diamond has a very low coefficient of thermal expansion of about two parts per million/Kelvin (ppm/K). But the materials used to make wide-bandgap semiconductors -- such as silicon carbide or gallium nitride – have higher CTEs, generally in the range of three to five ppm/K.

By adding in just the right percentage of silver, which has a CTE of about 20 ppm/K, the GTRI team can tailor the silver-diamond composite to expand at the same rate as the semiconductor material. By matching thermal-expansion rates during heating and cooling, the researchers have enabled the two materials to maintain a strong bond.

Unlike metals, which conduct heat by moving electrons, diamond conducts heat by means of phonons, which are vibrational wave packets that travel through crystalline and other materials. Introducing silver between the diamond-particle interfaces helps phonons move from particle to particle and supports thermal efficiency.

"It's a challenge to use diamond particles to fill space in a plane with high efficiency and stability," Nadler said. "In recent years we've built image-analysis and other tools that let us perform structural morphological analyses on the material we've created. That data helps us understand what's actually happening within the composite -- including how the diamond-particle sizes are distributed and how the silver actually surrounds the diamonds."

A remaining hurdle involves the need to move beyond performance testing to an in-depth analysis of the silver-diamond material's functionality. Nadler's aim is to explain the thermal conductivity of the composite from a fundamental materials standpoint, rather than relying solely on performance results.

The extremely small size of the thermal shims makes such in-depth testing difficult, because existing testing methods require larger amounts of material. However, Nadler and his team are evaluating several testbed technologies that hold promise for detailed thermal-conductivity analysis.