Millimeter-Wave Driven Polyol Processing of Nanocrystalline Metals
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Millimeter-Wave Driven Polyol Processing of Nanocrystalline Metals L. K. Kurihara1, 3*, D. Lewis1, A. M. Jung2, A. W. Fliflet2 and R. W. Bruce4 Naval Research Laboratory, Washington, DC 20375 *Email: [email protected] 1. Multifunctional Materials Branch, Code 6350 2. Beam Physics Branch, Code 6790 3. Also at Potomac Research Int., Fairfax, VA 4. LET Corp., Washington, DC ABSTRACT Nanocrystalline metallic powders and coatings have been synthesized using a millimeter wave driven polyol process. We have been able to prepare powders of single elements, alloys, metastable alloys, composites and coatings. Examples of a few of the metals processed in this study include Fe, Co, Ni, Cu, Ru, Rh, Pt, Au, FePt, FexCo100-x, NiAg and Cu-Ni. The polyol experiment was set up in the millimeter wave processing chamber, the beam was directed into the center of the solution and it was brought to reflux, using the millimeter wave beam as a heat source. Varying the power input easily controlled the rate of reflux. INTRODUCTION Fine metal particles have many uses in pigments, magnetic materials, catalysts, electromagnetic shielding, ferrofluids, sensors, biomedical, electronics and advanced-engineered materials. Among the various preparative techniques used to prepare nanoscale particles, chemical routes provide a practical route for preparing fine and ultrafine powders. The polyol method, in which the polyol acts as a solvent and as the reducing agent, and also acts as a surfactant, is a suitable method for preparing nanophase and micron size particles with welldefined shapes and controlled particle sizes (1-10). The high intensity millimeter-wave beams (102–105 W/cm2) that can be generated by powerful gyrotron oscillators have unique capabilities for rapid, selective heating of many materials to high temperatures. Previous work by the authors has demonstrated the efficacy of such rapid heating for both production of nanophase materials and processing of such materials into useful components (11-13). The advantages of using a millimeter wave beam as the heat source includes rapid heating and cooling, volumetric heating, elimination of thermal inertia effects, and spatial control of heating. Using the millimeter wave as the heating source in the polyol process allows for very short processing times; high heating rates due to bulk heating of the solution, which results from direct coupling of the beam energy to the solution elements; the ability to selectively coat substrates where the beam has been focused; as well as working in a superheated liquid region. By working in this regime, it is possible to suppress the growth process and to promote nucleation. Millimeter wave driven polyol processing has been used to prepare nanocrystalline powders of Fe, FexCo100-x, FePt, Co, Ni, NiAg, Cu, CuxNi100-x, Ru, Rh, Ag, Pt and Au. Nanostructured coatings of copper have been deposited on AlN.
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EXPERIMENTAL A new continuous-wave (CW) gyrotron-based material processing system has been set up at the Naval Research Laboratory (NRL). T
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