Plasma is considered to be the fourth basic form of solid, liquid, gas and other substances. It is a collection of charged particles and neutral particles that are freely moving and interact with each other.
Macroscopically, it is electrically neutral. When there is enough energy added to the gas, the gas molecules and high-energy electrons will occur between the collision or absorption of electromagnetic control and ionization process occurred, and then the formation of plasma state.
The basic principle of plasma preparation of ultrafine tungsten powder is in an inert atmosphere or reactive atmosphere, the DC discharge (or otherwise) to produce high temperature gas ionization plasma, so that the molten metal evaporation, metal vapor, quenching or chemical reaction to form ultrafine particles.
At present, the main methods to obtain plasma are thermal ionization method, shock wave method, photoionization method, ray radiation method, and DC, low-frequency, radio frequency, microwave gas discharge method and so on. A tungsten alloy powder with an average grain size of 10nm can be prepared by mixing plasma with DC plasma and DC arc.
The tungsten powder prepared by plasma method has the advantages of good quality, cleanness, less pollution, small size and narrow size distribution range. In addition, plasma technology is considered to be the most effective way to prepare spherical powder.
The disadvantage of the plasma method is that it is not easy to maintain the continuous stability of the plasma flow, so that the evaporation and reaction of the raw material can not be ensured. Although the method of preparing the nanometer spherical tungsten powder and tungsten carbide powder has been reported recently, there is still a great distance from industrial production.
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