Preparation of superfine tungsten carbide

In recent years, the methods of preparing ultrafine or nanometer WC powder mainly include: mechanical alloying, direct reduction carbonization, sol-gel, gas carbonization, fixed-bed chemical vapor method, plasma method, etc.

Mechanical alloying
Grahame, etc by mechanical alloying method, first the W powder mixes with C powder by atomic ratio 1:1, in the steel tube and argon gas is piped in, and then choose a diameter of 12 mm WC grinding ball, the ball material ratio of 18:1, finally in high-energy ball mill on the planet type ball mill, the method obtain the average grain size of 7.2 nm WC powder. Ma Xueming et al. used mechanical alloying technology to mix W powder and C powder with a particle size of about 75μm at an atomic ratio of 1:1, and the selected ball to material ratio of 30:1. The WC powder with an average grain size of 11.3nm was obtained by ball milling on a QM-1F planetary high-energy ball mill for 100 hours.

Direct reduction carbonization
The reduction and carbonization methods for preparing ultra-fine WC powder can be divided into two categories: (1) Two-step reduction and carbonization method: the first step is to decompose and reduce W from the precursor containing W to prepare W powder; The second step is to mix W powder with C containing substance and heat to high temperature, carbonization through chemical reaction to generate WC powder. In this method, WC powder is formed by mixing W powder and C powder at high temperature (1400-1600℃). (2) The one-step method of C-reduction is the direct reduction and carbonization method: the precursor containing W is mixed with the C-containing substance and directly reduced and carbonized at high temperature to generate WC powder. This method not only improves the production efficiency of WC powder, but also makes the distribution of WC powder more uniform and the grain size smaller.

Luo Chongling et al. obtained WC powder with grain size of 15-30nm by direct reduction carbonization method. The preparation method is as follows: With WO3 and C as raw materials, the mixture of WO3 and C is first wet ground, where the atomic ratio of C and W is greater than 1. Then the slurry after wet grinding is spray dried, and N2 is used as a protective gas to reduce and carbonize WC powder and intermediate products of excess C at high temperature (1000-1100℃). Finally, the carbon content was adjusted to (6.13±0.05) %.

Sun Peng obtained nanometer tungsten carbide powder by embedding – direct reduction carbonization. The reduction and combination reactions are carried out in the A12O3 embedding device, which can provide a reducing atmosphere at high temperature to avoid the oxidation of WC. Firstly, the raw materials WO3 and C were pretreated by high-energy ball milling, and then the synthesis reaction was carried out by holding at 1300℃ for 3 hours. Finally, the reaction products were reprocessed by high-energy ball milling, and the WC powder with grain size of 26nm was prepared after 40h of ball milling.

Sol-gel method
Xiong Renjin et al. prepared nanometer tungsten carbide powder by sol-gel/in-situ carbonization method, and the preparation steps were as follows: To hydrogen peroxide (H202 mass fraction 30%) join the W powder (200 mesh), with glacial acetic acid and ethyl alcohol as stabilizer, tungsten sol was yellow first, and by evaporation to remove excess water, then add soluble phenolic resin of anhydrous ethanol, after ultrasonic mixing get sol containing W and C sources, will get the gel after its Chen, Finally, WC powder with grain size 10.2nm was prepared by carbonization at 900℃ with H2 and AR as protective gases.

Gas phase carbonization
Akari Mitsui from Japan obtained nanometer tungsten carbide powder by gas phase carbonization method. He took WCl6 as the W source and CH4 as the gas phase C source, and prepared WC powder with grain size of 20-30nm by chemical reaction at high temperature (1300-1400℃). The relationship of grain size between reactants and products and the reaction temperature are discussed in detail. Tokyo Tungsten Company has applied for the patent of super fine WC powder prepared by direct carbonization method with WO3 as the W source and CO as the carbonization gas. The particle size and C content of WC powder prepared by it can be controlled.

Stationary bed chemical vapor method
Li Jigang et al. successfully prepared WC powder of about 15nm using the fixed-bed chemical vapor method. Nano WO3 was used as W source and acetylene as C source. The preparation steps were as follows: nano WO3 was put into a quartz reaction boat, and then the boat was put into a high temperature stainless steel tubular reactor; After vacuuming, H2 is added, and the nano-wo3 powder is completely reduced to nano-a-w powder after holding at 660℃ for 1.5h. Then, the flow rate of H2 is reduced, and acetylene is added, and the nano-a-w powder is transformed into WC powder when the temperature is raised to 800℃ for holding for 4h.

Plasma method
There is another commonly used method to prepare ultrafine/nanometer WC powder: plasma method. Plasma is used as the heat source, and its temperature can reach 4000 ~ 5000℃. At such high temperature, powder raw materials will decompose and react to generate the required products. This method generally uses WO3, WC or W as the W source and CH4 as the C source, and mainly generates β-WC or W2C after the reaction. According to the research of Tetsuya Kameyama et al., when the molar ratio of CH4 to WC is greater than 15, the mass fraction of β-WC is 90%-95%, and the particle size of the powder is about 10nm. The grain size of β-WC is 5-20nm, which has good fractional property.

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