na_PLIF

Investigation of Sodium/Metal Halide Reactions
for Production of Unagglomerated Nanoparticles
and Thin Films

no TiCl4 with TiCl4 added

                                                                                    Na2/Ar                                                                Na2/Ar
                          2-D Laser Induced Fluorescence Imaging of Na2 and MIE scattering, shown above
                                    in a counter flow diffusion reactor.

This study focuses on the characterization of a gas-phase method for the formation of nanoscale titanium and boron particles and thin films. This versatile method, which can be used to form a variety of metals and ceramic powders, proceeds according to the reaction (mn)Na + (n)MCl_m -> M_n +(mn)NaCl. When TiCl₄ or BCl₃ reacts with Na vapor in a counterflow diffusion flame reactor, the Cl is stripped from the metal chloride by the Na vapor. Nanosize Ti or B particles form and, under certain thermodynamic circumstances, become encased in NaCl. The 2-D spatial distribution of Na₂ has been optically interrogated using planar laser-induced fluorescence (PLIF) under various conditions to clarify the influence of concentration and transport on particle formation.

The images above include Na₂ PLIF and 488 nm scattered light. The image on the left shows the Na₂ distribution with no chloride added to the upper stream. In the image on the right, BCl₃ has be added to the upper flow stream, causing the appearance of a thin horizontal band of light resulting from laser light scattered from particles being formed by the reaction of Na and BCl₃. Reactant concentration and time available for reaction were found to dramatically influence the reactive flow. Simulations using a counterflow diffusion flame model show that formation of TiB₂ likely occurs by gas phase clustering reactions involving both precursors. The model indicates that experimental results are consistent with the proposed chlorine abstraction mechanism with near-collisional reaction rates.

We have recently demonstrated that this method can also be used to deposit thin films, whereby the chemistry is conducted such that the salt remains in the vapor.

In the figure below we show the formation of an iron nanoparticle coated with salt as an example of the formation of temporary encapsulation to preserve microstructure. We have also made other metals, semiconductors and ceramics using this method.

Selected References

Kristen L. Steffens, Michael R. Zachariah, Douglas P. DuFaux and Richard L. Axelbaum, In-Situ Studies of a Novel Sodium Flame Process for Synthesis of Fine Particles, to appear in Mat. Res. Soc. Symp. Proc. 400, 1996.
K. Steffens, M.R. Zachariah, D. DuFaux and R. Axelbaum, “ Optical and Modeling Studies of Sodium/Metal Halide Reactions for the Formation of Titanium and Boron Nanoparticles” Chemistry of Materials 8, 1871 (1996)
J.H. Hendricks, M.I. Aquino, J. Maslar, and M.R. Zachariah "Metal and Ceramic Thin Film Growth by Reaction of Alkali Metal with Metal Halides: A New Route for Low Temperature CVD"Chemistry of Materials 10, 2221 (1998)
S. H. Ehrman, M.I Aquino and M.R. Zachariah “ Effect of Temperature and Vapor-phase Encapsulation on Particle Growth” J. Mat. Research 14: (4) 1664-1671 (1999)