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Development of advance obscurants materials using synthesis of metallic hollow nanoparticles


OBJECTIVE: Develop a low cost obscurant material utilizing metallic hollow nanostructures. With novel synthesis and chemical reactions, these nanostructures will take on strong surface plasmon resonance (SPR) effects that exhibit negative index material. Most common plasmonic materials are gold and silver, however, there are many other materials that show metal like optical properties can be explored to lower costs. Researchers are encourage to look for cost saving alternatives to gold and silver, with the goal to drive costs lower than silver processing costs. Processes such as Sequential Galvanic Exchange and Kirkendal growth at room temperatures have shown to produce very unique metallic structures with intriguing optical and mechanical properties. Unique forms of intermetallic coupling and formation of cavities between layers generate surface Plasmon resonance peaks that can be tunable from the visible to the near-infrared region. This method has been successfully applied to prepare gold, platinum and palladium based hollow nanostructures with a wide range of different morphologies, including cubic nanoboxes, cubic nanocages, triangular nanorings, prism-shaped nanoboxes, single-walled nanotubes, and multiple-walled nanoshells or nanotubes. Morphological transformation of refluxing silver with HAUCl4 creates gold/silver alloyed shell structures could offer a cost savings approach. The optical effects of impurities in the system are unknown to the author for these metallic structures. Size distribution for traditional materials plays an important role in obscurants. Based on the index of refraction, specific sizes and shapes are most efficient at obscuring targeted specific areas of interest (reference Janon Embury's report). Since plasmonic materials have unusual behaviors for the index of refraction, these properties would need to be researched to see if similar optimizing effects are present. DESCRIPTION: Smoke and obscurant materials are used by the Army to protect both the individual soldier as well as military assets such as tanks and aircraft. Recent advances in sensors, seekers and trackers are demanding for more efficient and better performing obscurant counter measures in both the visible and infrared regions. With the advancement of portable electronic systems, the soldiers in small units are overburdened, having less available space for carry tradition smoke devices. More efficient devices are needed to provide adequate protection to the soldier. Metal nanoparticles are being investigated in considerable detail due to their exciting potential for application in catalysis, biological and chemical sensing, optoelectronics and magnetic memory. Nanoparticle shape plays a crucial role in determining optical and electronic properties, and the ability to control the nanocrystals has lead to the first observations of plasmon resonance modes in silver nanoprisms. In addition to anisotropic nanostructures, there is also considerable interest in the synthesis of nanoscale structures consisting of a core of one chemical composition covered with a concentric shell of another material in what is commonly known as the"coreshell"configuration. These configurations have shown very unique optical properties without having the high conductivities associated with traditional metal flakes that are used to reduce infrared electromagnetic signatures. In addition to plasmonic optical properties of these metallic nanostructures, hollow cores will aid in the buoyancy of these particles. Current obscurant materials tend to have high deposition rates. Lighter, hollow core metallic have the potential for staying airborne for much longer periods of time. PHASE I: Develop and synthesize various metallic nanostructures, both an anisotropic and core-shell structures and measure the ability to obscure electromagnetic energies. Most of this research has been based on solution based chemistry. Traditional characterization of these types of materials have been in wet cell spectroscopy. For the infrared (IR), nujol measurements have been used. Novel characterization methods are encouraged to determine effectiveness of engineered shapes on various areas of the electromagnetic spectrum. The primary focus of this effort is to provide proof of concept that these metallic nanostructure materials can provide an effective obscurant and determine which shapes best attenuate the wavelengths of military interest. With current threat systems, four basic areas of interests are identified. For visual; 0.4 to 0.7um, for near IR 0.9 to 1.5um , for mid IR; 3 to 5um, for long IR; 8 to 12um. For a material to warrant continued investigation for a Phase II effort, this material must be able to efficiently absorb or scatter electromagnetic energy better than our current standard materials. For infrared screening materials, brass powders are used having an efficiency extinction of approximately 1.4 m2/g. For visible screening materials, Titanium Dioxide powers are used having an efficiency extinction of approximately 4.5 m2/g. PHASE II: Continue with cost effective scale up material development and fabrication. The ultimate goal for this material is to make an aerosolized cloud that can be deployed quickly between an incoming threat and the equipment and personnel being protected. That generally involves some type of dissemination system involving dry materials. During phase II, processes and methodologies will need to be investigated to make dry powders from the materials that were developed in Phase I. In addition, developing novel ways to aerosolize these materials by minimizing particle to particle agglomeration. Perhaps some kind of microturbluance systems could be employed, either pyrotechnically or pneumatically. Several concepts should be investigated to effectively pack these materials and fabricate device to disseminate this packed system. PHASE III: The techniques developed in this program can be integrated into current and future military obscurant applications. Improved grenades and other munitions are needed to reduce current logistics burden of countermeasures to protect the soldier and his equipment. Improved dissemination techniques can be beneficial for all powdered materials in the metallurgy, ceramic, pharmaceutical and fuel industries. Industrial applications include electronics, fuel cells, batteries and solar energy. REFERENCES: 1. Bohren, C.F.; Huffman, D.R. Absorption and Scattering of Light by Small Particles; Wiley-Interscience: New York, 1983. 2. Embury, Janon; Maximizing Infrared Extinction Coefficients for Metal Discs, Rods, and Spheres, ECBC-TR-226, Feb 2002, ADA400404, 77 Pages 3. Sastry, Murali'New approaches to the synthesis of anisotropi, core-shell and hollow metal nanostructures; Journal of Materials Chemistry, April 2005 4. Young, Kayle; Synthesis and Galvanic Replacement Reaction of Silver Nanocubes in Organic Medium, 5. Chen et al."Gold Nanocages:Engineering Their Structure for Biomedical Applications": Advance Materials, 17, 2005(2255-2261). 6. Sun ec al."Mechanistic Study on the Replacement Reaction between Silver Nanostructures and Chloroauric Acid in Aqueous Medium"; JACS, 126, 2004 (3892-3901) 7. Wiley, Sun, Chen."Shaped-Controlled Synthesis of Silver and Gold Nanostructures"; Cambridge Online
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