Chemical Vapor Synthesis of Inorganic Nanopowders, The

JOM, Dec 2007 by Sohn, Hong Yong, Ryu, Taegong, Choi, Jin Won, Hwang, Kyu Sup, Han, Gilsoo, Choi, Young Joon, Fang, Zhigang Zak

Chemical vapor synthesis (CVS) is a process for making fine solid particles by the vapor-phase chemical reactions of precursors. At the University of Utah, this process has been applied to the synthesis of the aluminides of titanium and nickel, other metallic and intermetallic powders, and subsequently aluminum nanopowder and WC-Co nanocomposite powder. This work has demonstrated that it is possible to prepare fine particles of 5-200 nm size by CVS. Further, it has been shown that this technique has a unique capability to produce uniformly mixed powders of different solids. This is possible because the reactants are perfectly mixed in the gas phase. More recently, the CVS process has been carried out in a plasma reactor. This system has shown considerable promise for many applications as a method of producing nanosized powders.

Author's Note: This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

INTRODUCTION

Many materials in the form of powder, especially ultrafine or nanosized powder, display useful physical properties in such areas as light absorption,1 magnetism.2 and superconductivity.3 Chemical vapor synthesis (CVS) is a process for making fine inorganic particles by the reaction of vaporized precursors. Early examples of CVS include the preparation of metallic powders by the hydrogen reduction of single-metal chlorides.4-6 A reaction between a metal halide and hydrogen can in general be written as follows:

where M and X represent the metal and the halogen, respectively. Tungsten carbide particles have also been prepared by the vapor-phase reaction of WCl^sub 6^, hydrocarbon gas, and H^sub 2^.7-9 In recent years, H.Y. Sohn and coworkers10-14 at the University of Utah have applied the CVS process to prepare fine powders of intermetallic compounds and tungsten carbide as well as composite nanopowders of WC-Co. This work has demonstrated that it is possible to prepare fine particles of 5-200 nm size by CVS. It has also been shown that this technique has a unique capability to produce uniformly mixed powders of different solids. An example of this feature will be presented in this paper in conjunction with the CVS of WC-Co composite powder. Additionally, the process easily allows the incorporation of minor amounts of additives or doping agents, again uniformly distributed throughout the product powder. These unique features are made possible because the reactants are perfectly mixed in the gas phase. This article will summarize this work and present new results obtained by performing the CVS reaction in an electrically heated tubular reactor and a non-transferred plasma reactor.

APPLICATION TO ALLOYS AND INTERMETALLIC COMPOUNDS

Several methods have been practiced in the production of intermetallic powders. Sohn and coworkers10-14 applied the basic concepts of the previously mentioned chloride reduction method to the CVS of intermetallic and metal alloy powders. These reactions can, in general, be written as follows, when hydrogen is used:

where M and N represent two different metals, with x and y being the valences and M^sub m^ N^sub n^ the intermetallic compound formed.

H.Y. Sohn and S. PalDey10,11 synthesized fine powder (100-200 nm) of Ni^sub 4^Mo at 900°C to 1,100°C and nickel aluminide (Ni^sub 3^, Al) particles (50-100 nm) at 900°C to 1,150°C, according to Equation 2. These authors10 also prepared a coating of Ni^sub 4^Mo of 0.7 µm thickness on a nickel substrate. The fact that aluminum chloride is reduced by this reaction scheme is very significant because the reduction of AlCl^sub 3^ alone by hydrogen is thermodynamically unfavorable at moderate temperatures. The negative free energy of formation of the intermetallic compound makes the overall reaction feasible. Using the same chemical vapor synthesis process. Sohn et al.14 prepared ultraline particles of Fe-Co alloys by the hydrogen reduction of FeCl^sub 2^-CoCl^sub 2^ mixtures.

Magnesium is a much stronger reducing agent for chlorides than hydrogen. Thus, the reduction of titanium and aluminum chlorides by magnesium is feasible, whereas the reduction of these chlorides by hydrogen is not feasible up to 2,500 K. Sohn and PalDey12,13 synthesized ultrafine powders of the aluminides of titanium and nickel using magnesium as the reducing agent.