Nanometer-Size Fiber Composite Synthesis by Laser-Induced Reactions

A new approach in nanocomposite synthesis involves rapid condensation of metallic and non-metallic species, produced by laser-induced reactions. These composite surface layers form by codeposition of fine (25-120 nm. dia.) amorphous silica fibers and a metal matrix, at rates of ~ Ijim /sec, and are strongly adherent to a nickel substrate material. The synthesis involves rapid condensation of amorphous silica from an intense plume, formed by the interaction of a continuous C02 laser beam with a ceramic target. This is either accompanied by codeposition of: (i) tungsten from a heated filament via a chemical vapor transport mechanism, (ii) aluminum formed by laser evaporation. These reactions occur in 98.5 % hydrogen, 1.5 % methane atmosphere using a continuous flow reaction chamber, with access to an incident continuous C02 laser beam. In the composite layer thus produced, the silica fibers exist in the form of a random weave structure. Experimental evidence does not support catalytic growth, as in the Vapor-Liquid-Solid growth mechanism. An alternative theoretical model involving the fiber nucleation and growth has been developed. Basically, ultrafine silica particles (10-25 nm. dia.) form by rapid condensation from the laser plume. These, by virtue of a strong dipole moment coagulate to form clearly defined linear arrays, as in colloidal aggregation. Experimental observations provide strong evidence that these arrays transform into cylindrical fibers. In the theoretical model it is proposed that small linear particle clusters circulate within natural convection cells (above the substrate surface). Fiber growth continues by the progressive aggregation of the ultrafine particles provided by laser rapid condensation. Eventually, the fibers adhere to the growing metal matrix either by chemical adsorption or electrostatic interaction. Effects of radiation-vapor interactions, diffusion-controlled coagulation are also discussed.