**2. Plasmachemical reactor**

thermo-, corrosion and wear-resistant coatings, polymer composites with fillers and inor-

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 plas-

**1.** Various types of nanopowders (individual elements, compounds and mixtures) can be

**2.** Physical and chemical characteristics of the nanopowders can be controlled and nanopowders with required parameters (purity, chemical and phase compositions, specific surface)

**5.** Process can be easily scaled-up from laboratory setup to the level of industrial equipment

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 dis-

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,

**3.** Plasma reactors have small dimensions and high production rate;

charge (RF), microwave plasma (UHF), as well as combined discharges.

**4.** Traditional commonly applied raw materials can be used;

ganic nanoparticle modifiers for alloys [1–9].

machemical synthesis are:

can be produced;

with high productivity.

produced;

4 Powder Technology

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 formation.

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 the reactor. The evolution of nanoparticles in the layer is determined by the temperature distribution and lifetime of the layer, and the temperature distribution depends in turn on the temperature of the cooled surface, the density of the mass flux of the deposited nanoparticles, and the density of the heat flux passing through the layer. Under plasmachemical synthesis conditions the layer thickness, as well as its thermal resistance, are increased in time. The unsteady temperature field in the layer can lead to the time changes of the layer's structure, phase and chemical composition. These changes are due to chemical reactions, phase transformations, and particles sintering. All these changes occur when the temperature in the growing layer increases. To obtain the nanopowder with required specifications, where nanoparticles retain the properties determined by the conditions of their formation in the gas stream, it is necessary to exclude or minimize the possibility of physicochemical transformations in the layer of precipitated particles. It is necessary to prevent the layer's temperature rise above certain threshold values. These values are the temperatures of nanoparticles characteristic chemical and phase transformations, and temperatures related to nanoparticles growth due to their contacts in the layer. Nanoparticles are formed in the plasma process inside the reaction zone, but possible nanoparticles transformations in the growing layer on the reactor's surfaces might change the properties of nanoparticles and become a problem. This problem is important for the realization of controlled plasma synthesis of nanopowders with given properties.

**3.** physical and chemical properties of nanopowders deposited on the surface in various

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

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

A cylindrical sectioned plasma reactor with confined jet stream was used. Reactor had diameter and length of 200 and 600 mm correspondently (**Figure 1**) [17]. The length of the sections varied in the range 70–130 mm. DC arc plasma torch with a rated power of 25 kW was used

were used as plasma-forming gases. The synthesized nanoparticles were deposited on the reactor's walls and partially removed with the exhaust gases into the filtration apparatus.

), and air

7

for thermal plasma generation. Nitrogen, hydrogen-nitrogen mixture (22 vol. % H<sup>2</sup>

zones under various process parameters

**Figure 1.** General view of 30 kW plasma setup.

An unlimited growth of the nanoparticles layer thickness will inevitably lead to an increase in the layer's temperature resulting in the particles sintering and coarsening, as well as possible change in their phase and chemical composition. These effects will be most pronounced for nanoparticles with a low temperature of possible physicochemical transformations, especially for particles with low melting point. Thus, to obtain the nanopowder with required specifications, the nanoparticles physical and chemical transformations in the deposited layer have to be blocked. To achieve this, the thickness of this layer, formed on the stationary cooled reactor's surface, must be limited to a certain value. For particular target nanoproducts, the size of the precipitating nanoparticles, the initial temperature of the deposition surface and the heat flux density from the high temperature stream to the deposition surface will determine this limiting layer's thickness.
