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

CVS IN A PLASMA REACTOR

The thermal plasma process provides many advantages on the synthesis of nanosized powder such as a high processing temperature to vaporize all the reactants completely and rapidly, a high quench rate to form fine powders, and the versatility of a wider range of reactants to choose from. Numerous reports on the thermal plasma synthesis of metals, ceramics, and composites in recent years have shown that thermal plasma synthesis has given a new direction and impetus to many industrial applications as one of the most promising methods for producing nanosized powders. Thus, a plasma reactor has been used to prepare WC-Co composite powders in this laboratory.

The plasma system is equipped with a plasma generator with a downward plasma torch, a power supply unit, a cylindrical reactor, a quenching chamber, a cooling system, a precursor feeding system, a powder collector, a gas delivery system, an off-gas scrubber containing 5% NaOH solution, and an off-gas burner, as shown Figure 11. The plasma torch consists of a water-cooled tungsten cathode and a copper anode nozzle operating at atmospheric pressure. The reactor consists of a vertical water-cooled stainless steel tube of 15 cm inner diameter and 60 cm length and a graphite cylinder of 7.6 cm inner diameter and 60 cm length. Graphite felt is placed between the graphite tube and the inner wall of the water-cooled stainless-steel tube to insulate the reactor. The quenching chamber connected to the bottom of the reactor is a water-cooled two-layer stainless-steel box to cool the outgoing gas to a temperature lower than 150°C. A data acquisition system records the temperatures at the reactor exit, the input and output cooling water, and outgoing gas from the quenching chamber. The precursor feeding system for this plasma reactor is the same as that for the tubular reactor except for a water-cooled delivery line through which the precursor is fed toward the outside boundary of the visible plasma flame (7 mm diameter) from a distance of 15 mm near the exit of the torch. Argon and H^sub 2^ gases are used as the primary and secondary plasma gases, respectively. Before delivering precursor into the plasma flame, the reactor is heated by the plasma flame generated until its temperature reaches a steady level. The same reactants as in the previous section are used. The mixture of argon and CH^sub 4^ flows through the precursor feeding system to carry tungsten chloride powder into the plasma flame. The powder produced is collected using a Teflon-coated polyester filter with a pore size of 1 µm. The powder feeding system consists of an entrained-flow powder feeder, a vibrator, a carrier gas line, a sample container, and a powder delivery line. Argon gas is passed through the powder feeder as the carrier gas as well as an inert gas to keep the atmosphere in the sample container inert.

Experiments were conducted to evaluate the effect of the plasma power by varying it from 11 kW to 32 kW. The pressure of the primary plasma gas (argon) to generate plasma flame was 345 kPa, which resulted in a flow rate of 57 L/min. (25°C, 86.1 kPa). The feeding rate of WCl^sub 6^ was 3.5 g/min. and the flow rate of CH^sub 4^ was fixed to give an input C/W molar ratio of 6.3. The flow rate of argon to carry the WCl^sub 6^ powder was 1 L/min. (25°C. 86.1 kPa). The pressure in the reaction chamber was always 86.1 kPa. Figure 12 shows the XRD patterns of the products obtained with different power levels of the plasma torch. The main product was WC^sub 1-x^ mixed with small amounts of WC or W^sub 2^C. This result is consistent with the phase diagram available in the literature,30 which indicates that WC^sub 1-x^ is the stable tungsten carbide phase at a temperature higher than 2,530°C. WC and W^sub 2^C phases are stable below 2,530°C.