The composition often is not precisely the desired composition, El-Shall stresses, so researchers adjust it through trial and error. "It really depends on the physical properties of the metal atoms and the diffusion coefficient," he notes. "Sometimes if you have two different metals with very different properties, one diffuses several ," or 100 ," times faster than the other. In this case, you have a problem ," they will not be together at the same place at the same time. You can compensate for that in the composition. Sometimes we have to start with something like 80 percent of A and 20 percent of B to get the nanoparticles 50/50," he adds.
Catalysis is one potential candidate application for these nanopowders, notes El-Shall. A silica platinum composed of nanoparticles showed "very strong catalytic activity for hydrolyzation reaction, he adds.
Such nanoparticle-based catalysts are heterogeneous catalysts, says El-Shall, but work much like homogeneous catalysts ," sometimes even better ," because the particles are so small. "A homogenous catalyst is very well dispersed because it's contained in the reaction medium," he explains.
El-Shall and other VCU researchers recently announced a nanoparticle breakthrough using the laser vaporization process. They assembled a new class of metallic and related types of nanoparticle fibers and filaments by suspending nanosized pieces of metal and other materials in vacuum chambers filled with electrically charged vapor.
"Using a process that produces ultra-pure nanoparticles with laser vaporization and controlled condensation and then applying an electric field, we have shown that several classes of metallic and semiconductor nanoparticles can be assembled into chains and filaments that retain their unique properties," says El-Shall. "This holds great promise for the development of novel functional materials and for the engineering of a wide variety of nanodevices and sensors."
The discovery could result in the development of stronger plastics that incorporate nanoparticle filaments within polymer chains, added El-Shall. Researchers now are evaluating the nanoparticles for use as catalysts to remove carbon monoxide from air, as well as for use as a "smart dust" that detects chemical and biological warfare agents and environmental pollutants.
Understanding nanoparticle fluidization
Current nanopowder research is not limited to innovative production routes. Researchers at the Illinois Institute of Technology (IIT) in Chicago and the Newark, N.J.-based New Jersey Institute of Technology have been working to understand the physics of fluidization and transport of nanoparticles under a National Science Foundation (NSF) grant. A better understanding will advance the production of nanocomposites with tailored properties for catalyst and other applications.
Dr. Hamid Arastoopour, Max McGraw professor and chairman of IIT's chemical and environmental engineering department, said researchers want to get a handle on nanoparticle behavior within conventional, rotating and circulating fluidized beds. Fluidization is used widely in many powder processes within the chemical, petrochemical and pharmaceutical industries, but on micron-scale particles. These applications could benefit significantly from the unique properties of nanoparticles. Figure 3 shows a rotating fluidized bed in a nanoparticle operation.
"With a nanoparticle, the size is very small, and with a little bit of force or if you blow the air through or fluid through, the particles are airborne, and they will leave the reactor," Arastoopour explains. "It's difficult to keep them because they're extremely small and very light. We are increasing the gravitational force ," in that way, we keep the particles inside the reactor. Then we can fix them and put them in the catalyst and so on."
Researchers increase the gravitational force on the reactor and then rotate the reactor, says Arastoopour. "We call it rotation of a fluidized bed," he says. "The centrifugal forces will create this kind of force pushing them toward the wall. Therefore, we can go up to 50 G.' The particles, although they're very small and light, [stay] in the reactor."
By changing the flow of the gas or fluid, researchers can alter the size of the aggregate of the smaller nanopowders, says Arastoopour. They then can control the surface area. "We can have a much higher surface area, enhanced conversion," he maintains. "Therefore, we can have much smaller reactors and more efficient reactions." Operation costs also are reduced, he adds.
During the four-year NSF project, researchers hope to come up with a computer simulation that uses a governing equation for the design of systems using nanoparticles. Also planned, says Arastoopour, is a very detailed experimental and theoretical analysis of the high-gravitational-force rotating fluidized bed.
"At the end of this project, we hope that scientists and engineers who want to use reactors based on nanoparticles [can benefit from] tools for design and calculations," Arastoopour stresses.
Figure 3. Rotating Fluidized Bed
A rotating fluidized bed is used to determine the effect of varying gravitational forces on fluidization behavior.
Source: Dr. Hamid Arastoopour, Illinois Institute of Technology.
Outside academia are numerous small companies actively involved in nanopowder research. Two such companies are Nanopowder Enterprises Inc. of Piscataway, N.J., and Tetronics Ltd. of Faringdon, Oxfordshire, England.
Nanopowder Enterprises currently is performing research in the coatings area, working with nanopowders to produce harder, scratch-resistant products.
"We provide a mechanism [that imparts] multiple functionality to the coating," says Dr. Ganesh Skandan, chief operating officer of Nanopowder Enterprises. For example, he says, a coating could be transparent, hard, scratch-resistant and conductive.
The 12-person company also is developing "nano-additives" for coolants. "If you put nano-additives in," says Skandan, "they alter the thermal properties of the fluid." If the thermal conductivity is increased, he adds, the size of the cooling equipment potentially could be reduced.
Nanopowder Enterprises also has its hands in other nanopowder applications. However, Skandan maintains that the company's mission ultimately is "to be a nanomaterials provider" ," a manufacturer providing products directly to the end user.
At Tetronics Ltd., the principal focus is the application of thermal plasmas either as a clean heat source or in material processing. According to Dr. Michael Wise, a Tetronics process engineer, the company has been producing nanopowders and other novel materials for more than 15 years.
"The production of nanopowders involves the vaporization of the feedstock material in a plasma flame," explains Wise, "then recondensing the vapor, with or without the use of a quench gas, and then collecting the resultant powder."
This route results in contaminant-free spherical particles that can be formed from solids and faceted particles. The powders have very high specific surface area/mass ratios and potential uses in electronics, catalysis and other applications.
Plasma flame temperatures typically range between 5,000 Degrees F to 14,000 Degrees F for powder production, says Wise. "At these temperatures, even refractory oxides can be dissociated into their constituent elements," he notes, "which results in the feedstock being transformed into a homogeneous gas phase with the potential for rapid chemical reaction."
The company has commercialized two nanopowder-related processes, says Wise. The first is used for the production of oxides, including mixed oxides and doped oxides. The second process, says Wise, is used to produce an "energetic" aluminum powder for rapid-burn applications such as rocket fuel he adds.
In the future, Wise expects nanopowders to become more widely used as catalysts and says plasma-induced powders offer great potential in this area. "Plasma-produced powders tend to have higher surface activities," he maintains.