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Staff Accomplishments

RESS to the rescue

August 2004
Sometimes good things come in very small packages! Pacific Northwest National Laboratory researchers have created an environmentally friendly process for rapidly creating minute particles. This process is called the rapid expansion of carbon dioxide solutions, or RESS for short. Minute particles, nanoparticles, are valuable to scientists because these tiny materials react differently than larger particles. For example, an iron particle only two to three nanometers in diameter will catalyze reactions differently than a larger iron particle. The distinct properties allow scientists to use nanoparticles in new applications as diverse as waste cleanup and biomedical equipment. In the RESS, carbon dioxide, an environmentally friendly solvent, is converted from a gas into the supercritical phase by the application of heat and pressure. Next, scientists dissolve the solute into the supercritical carbon dioxide. Then, the solution is sprayed through a nozzle. The ability to be sprayed is a unique property of supercritical carbon dioxide. "All the CO2 evaporates and, because the supercritical fluid becomes a gas so quickly in the spraying process, RESS inherently generates nanoparticles," said John Fulton, who leads PNNL's supercritical fluids research. As the solution leaves the nozzle, the carbon dioxide abruptly transfers to a very low density gas. This abrupt transition from a supercritical fluid to a gas results in the nucleation and growth of nanometer-sized particles of the solute. Because the solvent is transformed into the gas phase in RESS, the nanoparticles do not contain any interfering residual liquids or solvents. The solute products generated during RESS can have different forms depending on specific processing parameters. To date, submicrometer particles, thin films, and fine polymer fibers have all been created using RESS. The RESS process was patented by the Laboratory in the late 1980s. Two recent innovations have resulted in new applications for the process. In the past, particles in the range from 10 to 500 nanometers were difficult or impossible to deposit on a surface since their extremely low mass caused them to remain entrained in the gas. Now, scientists have discovered that by charging the nanoparticles as they leave the nozzle and creating an electric field in the spray container, the particles can be easily collected on a nearby surface. In addition, scientists have discovered that by applying a beam of light to the plume of nanoparticles, they can make the particles react to create different substances. For example, scientists working on waste remediation recently discovered that ground iron mixed with contaminated earth would convert chlorinated hydrocarbons, a common part of insecticides, to non-harmful chemicals. But they didn't know how the chemistry worked. Because iron does not dissolve in carbon dioxide, Fulton and his team dissolved a related metal, iron carbonyl, in supercritical carbon dioxide and sprayed it into a bright ultraviolet beam. The light reacted with the iron carbonyl nanoparticles, turning them into iron oxide and iron metal nanoparticles. Waste remediation scientists are now using the iron metal nanoparticles to analyze the chemistry of the iron-chlorinated hydrocarbon reaction. Also, scientists are using supercritical fluids and RESS to apply coatings to materials. The films resulting from RESS are superior to those produced by conventional deposition techniques, said Walter Weimer, product line manager for RESS. In the medical field, RESS is being used to apply a thin coating to vascular stents, used to open arteries in the heart. Typically, the human body rejects the stainless steel stents by growing tissue around the stent. "The body recognizes foreign surfaces and builds up tissue to get rid of them and that reclogs the artery," Fulton said. Collaborating with industry partners, PNNL scientists have successfully used the RESS process to coat the stents with a nanoparticle polymer matrix and a drug that prevents tissue buildup. "By coating the stent with this mixture, we not only have a surface that is compatible with the body, but also a drug that releases over a long time," Fulton said. Contacts: John Fulton, project manager, john.fulton@pnl.gov
Walter Weimer, product line manager, walter.weimer@pnl.gov

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