Mar. 1, 2001 A mechanical engineer at Washington University in St. Louis has developed a patented technology that makes nanoparticles smaller, faster, cleaner, and cheaper than existing commercial processes.
Richard L. Axelbaum, Ph.D., Washington University professor of mechanical engineering, calls his technology the sodium/halide flame and encapsulation technology (SFE). With A 3-inch flame inside a four- foot- long tubular flow reactor, Axelbaum uses sodium reduction of metal halides such as boron trichloride and titanium tetrachloride, to produce metals and ceramic nanoparticles. The particles are on the order of 10 to 60 nanometers. One nanometer is one one-thousandth of a micron, which is 10,000 times smaller than a human hair.
While flames are used to produce millions of tons of materials annually from silica to carbon black, Axelbaum is the first person to patent a flame technique that makes materials in the nanoparticle range. The SFE technology is licensed to AP Materials, Inc., St. Louis.
In Axelbaum’s laboratory, his flame technique produces 40 grams of materials in an hour. He has produced seven elements and four ceramics with the technique, and he estimates that 35 elements, intermetallics and ceramics can be produced with his technology.
"The beauty of our flame technology is that materials production is all done in the flame in one step," Axelbaum says. "The key to the process is that we're able to produce stable, high-purity particles in large quantities. We're also able to control particle size."
Purity and stability have been drawbacks to successful, cost-efficient nanoparticles production. But Axelbaum surmounts the stability problem with the production of salt in his flame process. The salt encapsulates the nanoparticles produced, making them stable in the air. Salt also plays a role in the ability to produce large quantities at desirable sizes.
The technique creates pure particles because the reactants and products are clustered about the flame, and thus don't take on any heat from the walls. The high temperature inside the reactor —1000 degrees Centigrade — is self-purifying.
Axelbaum detailed his technique in his paper, "Synthesis of stable metal and non-oxide ceramic nanoparticles in sodium/halide flames," published in the February 2001 issue of Powder and Metallurgy. His work is supported by the National Science Foundation and the Department of Defense.
In the materials world, smaller is better. Take catalysts and the property of surface area. Surface area is very important to catalyst function. Industry today produces particles for catalysts in the micrometer size range, which is 1000 times larger than nanoparticles. By going to the nanometer size range, the surface area of a catalyst can be hundreds of thousands of times larger. Similarly, every time the space shuttle launches, it uses 440,000 pounds of aluminum powder. Axelbaum can make this aluminum powder in his laboratory in the nanometer size. Such powder in that small range will burn faster and more completely, and thus enhance the function of the shuttle launch.
In cell phones and computers, the standard electronic component is the capacitor. Much smaller capacitors can be made with Axelbaum's technology. This increases the capacitors per- unit- mass, which results in smaller, less expensive electronics.
There are a host of applications for nanoparticles and nanocomposites. They can be used for a variety of industrial uses, most notably in the aerospace, defense, medical and sports and recreation industries. For instance, Axelbaum can make titanium nanoparticles for golf clubs and tennis racket. The titanium makes these items strong and light weight; the smaller particles make for a stronger, stiffer tennis racket with improved fracture resistance and ductility.
Semiconductor nanoclusters have been touted as potential components of optical switching, conversion and signal processing devices, and as integral parts of computers that move ultra high-speed packets of light called photons. These materials are of interest for their optical and electro-optical properties.
Currently, Axelbaum is producing large amounts of tamtalum and aluminum nitride powders, key materials for the electronics and computing industries.
"Our approach is to make these particles for industrial use in existing technologies," Axelbaum says. "But our plans are to develop new materials like metal matrix composites that we hope will create new markets. We feel that our technology can produce the next generation of nano materials."
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