Nanometer-size fiber composite synthesis by laser-induced reactions

Abstract

A new approach in ultrafine composite synthesis involves rapid condensation of metallic and nonmetallic species, produced by laser-induced evaporation. A heated tungsten filament is simultaneously employed for codeposition of W via a chemical transport mechanism. This process occurs in a reducing environment of hydrogen gas, where evaporated species were produced by a laser-induced plume. Composite layers were formed on a Ni alloy substrate surface, at a rate of about 1 μm/s. The matrix of the composite films was either Al or W, and the dispersed phase was amorphous silica fibers. The diameter of the fibers was between 25 and 120 nm, depending on the laser-beam materials-interaction time. Various analytical techniques have been employed to characterize the as-synthesized layers. Experimental evidence does not support the vapor-liquid-solid model for fiber growth. An alternative fiber growth mechanism is proposed: ultrafine silica particles (10–25 nm diam) form by rapid condensation from the laser plume, and by virtue of a strong dipole moment coagulate to form clearly defined linear arrays which eventually transform into fibers.