February 2001

From Washington University in St. Louis

Up in flames: Patented technology makes valuable nanoparicles

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 associate professor of mechanical engineering, calls his technology the sodium/halide flame and encapsulation technology (SFE). With a three-inch long 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 metal and ceramic nanoparticles. The particles are 10 to 100 nanometers in diameter. One nanometer is one one-thousandth of a micron, which is 50,000 times smaller than a human hair.

While flames are used to produce hundreds of millions of tons of materials annually from silica to carbon black, Axelbaum is the first person to patent a flame technique that makes stable nonoxide materials in the nanoparticle range. The SFE technology is licensed to AP Materials, Inc., St. Louis, where three of Axelbaum's former students are employed, two of which are co-inventors of the technology.

In Axelbaum 's laboratory, his flame technique produces 90 grams of nanopowder an hour. His group has produced six metals and four ceramics with the technique, and he estimates that over 30 metals, intermetallics, ceramics and composites can be produced with his technology.

"The beauty of our flame technology is that material production is accomplished in a single step," Axelbaum says. "The key feature of the process is that we're able to produce stable, high-purity particles in large quantities. We're also able to have control of particle size and shape."

Purity and stability have been drawbacks to successful, cost-efficient production of nanoparticles. But Axelbaum surmounts the stability problem with the production of salt in his flame process. The salt encapsulates the nanoparticles, making them stable in the air. Salt encapsulation also isolates the nanoparticles, enabling control of particle size and shape.

The technique creates extremely pure materials because the particles are produced in the gaseous flame environment, and thus the reaction does not occur near the chamber walls. The high temperature inside the reactor, over 1000 degrees Centigrade, also helps to purify the powder by driving off undesirable impurities.

Axelbaum detailed the technique in his paper, "Synthesis of stable metal and non-oxide ceramic nanoparticles in sodium/halide flames," published in the December 2000 issue of Powder Metallurgy (Vol. 43, No. 4). The technology has been developed with support from the National Science Foundation, the Department of Defense, the National Institute of Standards and Technology, the National Aeronautics and Space Administration, and AP Materials, Inc.

In the materials world, smaller is better. Take catalysts and the property of surface area. Surface area is very important to catalyst function. In the past, industry produced 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 400,000 pounds of aluminum powder. Axelbaum's laboratory can make aluminum powder 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, a standard electronic component is the capacitor. Much smaller capacitors can be made with Axelbaum's technology. This increases the capacitors that can be made 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 electronics, 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 lightweight; the smaller particles make for a stronger, stiffer tennis racket with improved strength and fracture resistance.

Currently, Axelbaum is producing vast amounts of tantalum and aluminum nitride powders, key materials for the electronics and computing industries.

"Our immediate goal is to produce nanoparticles for industry to improve existing technologies," Axelbaum says. "But our plans are to develop new materials like transparent ceramics that we hope will create new markets. We feel that our technology can produce the next generation of nano materials."












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