Controlling the synthesis of TaC nanopowders by injecting liquid precursor into RF induction plasma

Thermal plasma processing has been used to synthesize nano-size powders through the condensation of reactant species from a vapor phase. Further development of this synthesis method will require the careful selection of an appropriate precursor and precise control of products species and their particle sizes. Direct introduction of liquid mist into thermal plasma gives us a wider choice of precursors than does vapor-phase precursor injection and lets us inject the precursors in larger amounts. In the present work, nano-size tantalum carbide powder was prepared from a liquid precursor, tantalum ethoxide Ta(OC2H5)5, by using r.f. thermal plasma. The liquid precursor was atomized to generate micron-sized mist droplets, and the mist was introduced into plasma. This atomized precursor evaporated quickly in the high-temperature plasma flame, and nanoparticles were formed as temperature decreased. The process was controlled by changing the hydrogen addition, process pressure, carrier gas flow rate for mist injection, and quenching condition. Adding hydrogen improved the powder quality by removing solid carbon, but excess hydrogen suppressed the formation of tantalum carbide. The quenching conditions gave significant effects on the reduction of particles size by two thirds and yielded average particle sizes as small as 8 nm.