Energetic Cluster Studies May Lead to Improved Rocket Propellants

  • Published
  • By William J. Sharp
  • AFOSR Public Affairs
ARLINGTON, Va. --  The Air Force Office of Scientific Research here completed a thorough periodic review of its molecular dynamics research program June 7 during an annual conference held this year in Arlington.

Two of the 90-plus researchers and scientists in attendance included Drs. A. Welford "Will" Castleman Jr, chemistry and physics professor, Pennsylvania State University, University Park, Pa., and Shiv N. Khanna, physics professor, Virginia Commonweatlh University, Richmond, Va. The two are collaborating on an AFOSR-funded project entitled, "Laying the Foundation for Developing Nanoscale Energetic Materials."

During the conference, Castleman showed certain clusters have unique properties that can mimic atomic elements of the periodic table. These mimics are referred to as "super atoms." He provided a strategy for employing cluster assembly to produce new nanoscale materials with tailored properties. Castleman's experimental group and Khanna's theoretical group share a major research goal - design and produce energetic materials of value as additives, for example, to rocket fuels. The goal is to have the energetic materials retain their stability until the moment of use in the combustion processes. The researchers want to bring stability to finely divided reactive solids, such as aluminum, solids that have traditionally been prone to oxidation. Other systems under study include aluminum, boron, and early transition metals bound to carbon, nitrogen, silicon, and oxygen.

"Most things seen in nature are atom based," Khanna said. "Atoms are generally lined up in periodic patterns, one after another." Sometimes, he explained, nature does make materials where the building blocks are molecules and the properties of these materials are different from those built of constituent elements alone. For example, the properties of water molecules - H2O - are different from those of liquid hydrogen or oxygen. Khanna and Castleman hope to build upon nature but in a controlled fashion.

"We are trying to design synthetic solids where the building blocks will not be atoms or molecules but clusters of atoms," Khanna said. "Because you can change the properties of clusters by changing size or composition, you can manipulate clusters into materials with desirable properties.

Castleman added, "We are looking at new ways of making nanoscale material - a sort of super atom. The idea is to use the concept of clusters. We are looking to produce clusters with certain desirable chemical or physical properties such as high energy content."

There are many potential applications for cluster based nanoscale materials, Castleman said. One benefit could be improvements made to thrust, engine efficiency, and weight ratios. Additionally, the clusters may have value in the areas of energy storage, oxygen transfer, and oxidation reactions. Other features of interest include clusters having novel catalytic, electronic and optical properties.

Castleman and Khanna's program has two thrusts. The first is to clarify the dynamics and reactivities of cluster building blocks. The second is to develop strategies for the synthesis and assembly of clusters. Both men have considerable experience in this field. Each has studied cluster assemblies for more than 20 years and energized clusters for the past several years.

Making cluster-assembled materials is a complex process since clusters are frequently unstable.

"If it were easy," Khanna said, "nature would have made clusters a long time ago." Therefore, one has to find more stable species and develop creative ways in which the building units could be assembled, maintaining their identity upon assembly. Our research is precisely directed towards this objective."

"What we've discovered is that the formation of stable cluster building blocks, and their assembly, is, in fact, possible," Castleman said. "This is likely to become a large field of work far beyond that pertaining to energetic materials we are currently studying."

"In clustering, one atom makes a difference," Castleman explained. "We can change properties by making a very trivial change in the basic building blocks. In doing so, we can make materials with characters that have no reactivity research results of Castleman and Khanna may be far reaching.

"Nature's periodic table has slightly more than 100 elements," Khanna said. "What we are doing is designing clusters that mimic atoms and hence, we are extending the periodic table into a third dimension."

More than 90 researchers and scientists attended the AFOSR 2006 Contractors' Meeting in Molecular Dynamics. Organizations represented included the National Science Foundation and the Office of Naval Research, Arlington, Va; the Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio; and BioTools Inc. Dozens of researchers from universities worldwide included the Universität Göttingen, Germany, and the University of Sao Paulo, Sao Paulo, Brazil.

Topics presented during the three-day conference included origins and failure in thermal barrier coatings, an update on plasma kinetics, and new electric hybrid lasers.