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

Production of WC-Co Composite Powder

A mixture of WCl^sub 6^ and CoCl^sub 2^ powders was fed to produce WC-Co composite powder. The main product obtained by this system was W^sub 2^C and even Co^sub 3^W^sub 3^C was produced, as shown in Figure 6a. Thus, the volatilizers for WCl^sub 6^ and CoCl^sub 2^ were placed where the temperatures were 440°C and 1,400°C, respectively. With this arrangement, the main product was a mixture of WC and cobalt with some free carbon, as shown in Figure 6b. The grain size of WC was 24±1 nm.

The particle size obtained with an input C/W molar ratio of 2.3 was examined using ZetaPALS and TEM. ZetaPALS gave an average particle size of 380 nm but TEM examination showed that the size of individual particles was 25±5 nm, although this measurement is based on a rather limited number of particles. Figure 7 shows the TEM photograph of the WC and cobalt powders. The difference in the particle sizes obtained by the two methods can be explained by the agglomeration of the particles as well as their motion in liquid media that contributed to errors in measurement by ZetaPALS. In addition, excess carbon was coated on the surface of produced particles, as shown in the TEM photograph. The TEM examination of powders showed the same particle size as the grain size calculated by the Scherrer equation applied to the XRD result. Thus, the produced particles seem to be single crystals and it is reasonable to use the calculated grain size from the XRD result as the particle size. The excess carbon can be removed by post-treatment such as reaction with hydrogen.29 The uniformity of mixing of the WC and cobalt nanoparticles can be seen from the EDX mapping of the powder shown in Figure 8. This presents an example of the significant feature of the CVS process to produce very uniformly mixed powders, as mentioned earlier.

Since excess CH^sub 4^ was used to synthesize WC-Co powder, free carbon was always present in the produced powder. Thus, hydrogen was used as a reducing gas as well as to reduce the amount of free carbon. Figure 9 shows the XRD patterns of the product obtained at three different H^sub 2^ flow rates: 0 L/min., 0.01 L/min., and 0.05 L/min. (25°C, 86.1 kPa), under otherwise identical conditions (reaction temperature of 1,400°C, total flow rate of 1.1 L/min. [25°C, 86.1 kPa], CH^sub 4^ feeding rate of 0.01 L/min. [25°C, 86.1 kPa], WCl^sub 6^ feeding rate of 0.06 g/min., and CoCl^sub 2^ feeding rate of 0.02 g/min.). To compare the degree of carburization, the WC^sub 0.5^/WC molar ratio was calculated as shown in Figure 10. Without H^sub 2^, the main product was WC and cobalt with a small amount of W^sub 2^C. However, as the concentration of H^sub 2^ increased, the WC^sub 0.5^/WC ratio increased. The WC^sub 0.5^/WC molar ratio was 0.126 when the input H^sub 2^/CH^sub 4^ molar ratio was unity and the value increased to 0.187 as the H^sub 2^/CH^sub 4^ ratio increased to 5. In addition, Co^sub 3^W^sub 3^C was produced and its amount also increased with increasing H^sub 2^ concentration. Hydrogen addition inhibited the decomposition of CH^sub 4^ into carbon and H^sub 2^, although it decreases the formation of free carbon. The grain size of WC was 25±1 nm and it was not affected by hydrogen concentration within the range tested.