**1. Introduction**

Nanosized powders of elements and their inorganic compounds are the basis for development of various nanostructured materials. These materials include nanostructured functional ceramics, hard alloys with increased wear resistance and toughness, dispersion hardened and modified structural alloys with enhanced performance characteristics, nanostructured protective

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

thermo-, corrosion and wear-resistant coatings, polymer composites with fillers and inorganic nanoparticle modifiers for alloys [1–9].

the power of modern DC arc plasma torches reaches 3–5 MW with a service life of up to 103 h [13, 14], while the power of existing RF plasmatrons does not exceed 1 MW. The usage of V-shaped DC arc plasmatrons where tungsten electrodes operate in an argon inert gas medium [15] allows to minimize the presence of impurities of the electrode material in the thermal plasma flow and to ensure the production of high-purity target products. Westinghouse Plasma Corporation developed plasmatrons with a power of 300–2400 kW and thermal efficiency of 70–85%. Such devices are used in waste materials processing and metallurgical furnaces operations [14]. DC plasma torches have high energy efficiency and can be used in the realization of high-temperature processes on an industrial scale. This paper provides a review of the research in the field of nanopowders synthesis and processing (spheroidization) of micron sized powders in thermal plasma flows generated by DC arc plasma torches. The research was carried out at the Institute of Metallurgy and Materials Science (IMET RAS) in

Nanopowders Production and Micron-Sized Powders Spheroidization in DC Plasma Reactors

http://dx.doi.org/10.5772/intechopen.76262

5

IMET RAS developed DC arc plasma torches with a nominal power of 30–150 kW with self-setting arc length and gas discharge stabilization, as well as plasmatrons with an interelectrode insert. The torches were used for the generation of thermal plasma in IMET laboratories and pilot plants. The plasma torches operated with reducing, oxidizing and inert gases and their mixtures and provided stable generation of plasma jets with an equilibrium temperature of up to 4000–8000 K (for molecular gases) and up to 12,000 K (for monatomic gases). The torches were used for both nanopowder production processes and for spheroidization processes. Plasma synthesis of nanopowders includes a complex set of physicochemical processes occurring in turbulent gas-dispersed non-isothermal flows. At present, plasma reactors with confined jet are widely used for nanopowders production. In confined jet reactor, the plasma jet flows into the volume of the reactor, which is confined by the cooled cylindrical surface. The ratio of the torch nozzle diameter to the reactor's diameter is of the order of 10. The plasma jet can be generated by any type of plasma generator (DC arc discharge, high frequency discharge, microwave discharge). When a plasma jet outflows from plasma torch into reactor's volume, a rapid temperature drop occurs, resulting in supersaturated vapors formation. Vapors condensation leads to the nanoscale particles formation. Evolution of nanoparticles granulometric composition occurs in the reactor's volume because of their condensation and coagulation growth. Phase and chemical compositions of nanoparticles can also change. Control of nanoparticles formation is achieved by variation of such operational parameters as plasma jet chemical composition, enthalpy and flow rate; concentrations of reagents in the reactor; and parameters of the reagents injection into the plasma jet. If solid powder is used as raw material, the initial size of the solid particles has significant effect of nanoparticles

During nanoparticles formation in the volume of plasma reactor, they move toward internal cooled surfaces of the reactor. The layer of nanoparticles is formed at these surfaces. The deposited layer evolution is affected by heat flux from the high temperature gas flow inside

collaboration with partners over recent years.

**2. Plasmachemical reactor**

formation.

Various techniques are used for nanopowders synthesis, including processing in gas, liquid and solid phases. Such methods employ physical and chemical deposition from gas phase, precipitation from solutions, mechanical grinding, etc. The formation of nanoparticles by homogeneous nucleation in supersaturated vapors followed by nanoparticles growth via condensation and coagulation is the basis of any gas phase nanoparticles manufacturing process. Fast cooling of saturated vapors or gas phase chemical reactions produce supersaturated vapors. Depending on the method used, processes for nanopowders manufacturing in gas phase include flame synthesis, evaporation in high-energy beams (laser radiation, accelerated electrons, focused microwave radiation), and plasmachemical synthesis in DC arc plasma.

Plasmachemical synthesis is the most versatile method for manufacturing of metal and inorganic compounds nanopowders, or nanopowders mixtures using inert, reducing and oxidizing atmospheres with controlled composition. Main advantages of nanopowders plasmachemical synthesis are:


High efficiency and other technological characteristics of nanopowder production in plasma testify to the competitiveness of the plasma method and wide possibilities for its application. As estimates show, the cost of nanopowders produced using plasma technologies at mass production level should slightly differ from the cost of "traditional" powders of this nomenclature. Plasma technologies can be considered as an effective way of obtaining a wide range of nanopowders. Thermal plasma can be generated using various types of electric discharges [10]. They include DC arc discharge, high-frequency (radio frequency) induction plasma discharge (RF), microwave plasma (UHF), as well as combined discharges.

At present, RF plasma reactors developed and manufactured by TEKNA [11] are widely used for nanopowders production. When operating electrodeless RF and microwave plasmatrons, the impurities (such as electrode's erosion products) in the nanopowders are absent, but it might appear when using DC arc plasma generators. However, it should be borne in mind that the present value of 1 kW of power generated by RF and microwave plasma torches is up to 3 times higher than the cost of plasma generation in DC arc plasma torches [12]. Besides, the power of modern DC arc plasma torches reaches 3–5 MW with a service life of up to 103 h [13, 14], while the power of existing RF plasmatrons does not exceed 1 MW. The usage of V-shaped DC arc plasmatrons where tungsten electrodes operate in an argon inert gas medium [15] allows to minimize the presence of impurities of the electrode material in the thermal plasma flow and to ensure the production of high-purity target products. Westinghouse Plasma Corporation developed plasmatrons with a power of 300–2400 kW and thermal efficiency of 70–85%. Such devices are used in waste materials processing and metallurgical furnaces operations [14]. DC plasma torches have high energy efficiency and can be used in the realization of high-temperature processes on an industrial scale. This paper provides a review of the research in the field of nanopowders synthesis and processing (spheroidization) of micron sized powders in thermal plasma flows generated by DC arc plasma torches. The research was carried out at the Institute of Metallurgy and Materials Science (IMET RAS) in collaboration with partners over recent years.
