**3. Heat transfer to the wall of reactor**

The investigations of metal and inorganic compounds nanopowders synthesis included experimental studies of heat and mass transfer in a confined plasma jet reactor [16].

Following topics were studied:


**3.** physical and chemical properties of nanopowders deposited on the surface in various zones under various process parameters

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

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

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

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

The investigations of metal and inorganic compounds nanopowders synthesis included

**1.** heat flux density distribution along the reactor's length to the nanoparticle deposition

experimental studies of heat and mass transfer in a confined plasma jet reactor [16].

**2.** mass flux density distribution of deposited nanoparticles along the reactor's length

with given properties.

6 Powder Technology

limiting layer's thickness.

Following topics were studied:

surface

**3. Heat transfer to the wall of reactor**

Following processes were carried out in the reactor:

• Copper nanopowders production via evaporation-condensation of dispersed copper (raw particles less than 40 μm) in a nitrogen plasma;

flow (gas + particles) some increase in the heat flux density at the reactor initial sections was observed. This may be due to the radiation from condensed particles to the wall in the highest temperature zone of the reactor. This zone is located at the initial section of the plasma jet.

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

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9

When aluminum oxide nanopowder was synthesized by oxidation of aluminum powder in air plasma jet, the significant differences in heat fluxes distribution in comparison with other realized processes were observed (**Figure 3**). With an increase in raw aluminum powder feed

**Figure 2.** Normalized heat flux and mass flux distributions at plasma reactor wall.


The reactions underlying these processes differ in thermal effects calculated under standard conditions. Copper evaporation-condensation reaction has a zero thermal effect, the reaction of tungsten trioxide reduction by hydrogen is weakly endogenous (0.5 MJ/kg WO<sup>3</sup> ), and the oxidation of aluminum by oxygen has a strong exogenous character (31 MJ/kg Al).


The experiments were carried out in the following parameters variation range:

It was experimentally observed that when clean (no nanoparticles are present) nitrogen plasma jet enters the reactor, the heat flux density distribution along the reactor's length shows maximum in the attachment region of the high-temperature flow to the reactor wall, which is typical for the separated flows in the channels with sudden expansion. The value of the heat flux density is determined mainly by the plasma flow power at the reactor's inlet and in our experiments it varied in the range 25–45 kW/m<sup>2</sup> . The maximum value of the heat flux density exceeds by 2.5–3 times the values at initial and final sections of the reactor. Distribution of normalized heat flux density, i.e. flux related to the magnitude of the maximum, remains practically unchanged in the whole experimental range of input parameters variation (thermal power, flow rate and enthalpy) (**Figure 2A**). The presence of hydrogen in nitrogen practically did not change the heat fluxes values in the reactor. A decrease of the torch nozzle diameter from 10 mm to 6 mm led to the relocation of flow attachment region further downstream from the reactor inlet, and location of maximum wall heat flux changed accordingly.

In experiments when nanopowders of copper, tungsten, and W-C composition were synthesized, it was found that heat flux density distribution along the reactor length also had extremum (**Figure 2B**), as in the case of the flow containing no dispersed particles. But for a two-phase flow (gas + particles) some increase in the heat flux density at the reactor initial sections was observed. This may be due to the radiation from condensed particles to the wall in the highest temperature zone of the reactor. This zone is located at the initial section of the plasma jet.

Following processes were carried out in the reactor:

particles less than 40 μm) in a nitrogen plasma;

raw particles less than 10 μm) in air plasma;

tion of dispersed tungsten trioxide WO3

experiments it varied in the range 25–45 kW/m<sup>2</sup>

hydrogen-nitrogen plasma.

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(raw particles less than 40 μm) in hydrogen-nitrogen plasma

• Copper nanopowders production via evaporation-condensation of dispersed copper (raw

• Production of tungsten nanopowders by reduction of dispersed tungsten trioxide WO3

• Production of aluminum oxide nanopowders by oxidation of disperse aluminum (ASD-4,

• Production of multicomponent composition in tungsten-carbon system (W-C) via interac-

The reactions underlying these processes differ in thermal effects calculated under standard conditions. Copper evaporation-condensation reaction has a zero thermal effect, the reaction

It was experimentally observed that when clean (no nanoparticles are present) nitrogen plasma jet enters the reactor, the heat flux density distribution along the reactor's length shows maximum in the attachment region of the high-temperature flow to the reactor wall, which is typical for the separated flows in the channels with sudden expansion. The value of the heat flux density is determined mainly by the plasma flow power at the reactor's inlet and in our

exceeds by 2.5–3 times the values at initial and final sections of the reactor. Distribution of normalized heat flux density, i.e. flux related to the magnitude of the maximum, remains practically unchanged in the whole experimental range of input parameters variation (thermal power, flow rate and enthalpy) (**Figure 2A**). The presence of hydrogen in nitrogen practically did not change the heat fluxes values in the reactor. A decrease of the torch nozzle diameter from 10 mm to 6 mm led to the relocation of flow attachment region further downstream from

In experiments when nanopowders of copper, tungsten, and W-C composition were synthesized, it was found that heat flux density distribution along the reactor length also had extremum (**Figure 2B**), as in the case of the flow containing no dispersed particles. But for a two-phase

the reactor inlet, and location of maximum wall heat flux changed accordingly.

of tungsten trioxide reduction by hydrogen is weakly endogenous (0.5 MJ/kg WO<sup>3</sup>

oxidation of aluminum by oxygen has a strong exogenous character (31 MJ/kg Al).

The experiments were carried out in the following parameters variation range:

Plasma forming gas flow rate 0.85–2 st. m3

Plasma flow power 6.6–12.3 kW, Flow rate of dispersive raw material 1.0–7.0 g/min, Duration of experiments 5–80 min

Plasma torch nozzle diameter 6–12 mm, Plasma flow enthalpy at the reactor inlet 13–29 MJ/st. m<sup>3</sup>

(raw particles less than 40 μm) with methane in

/hour,

,

. The maximum value of the heat flux density

), and the

When aluminum oxide nanopowder was synthesized by oxidation of aluminum powder in air plasma jet, the significant differences in heat fluxes distribution in comparison with other realized processes were observed (**Figure 3**). With an increase in raw aluminum powder feed

**Figure 2.** Normalized heat flux and mass flux distributions at plasma reactor wall.

mass of synthesized nanopowder. Consequently, the final properties of the produced nanopowders are largely determined by the properties of the product that is precipitated exactly in the reactor. The degree of nanoparticles deposition decreases with increasing of the process duration (thickness of the deposited layer of nanoparticles) and with an increase in the raw materials feed rate (processing rate). It follows from analysis of experimentally found heat flux density distributions and mass fluxes density distributions along the reactor wall that the maximum heat flux position coincides with location of maximum mass flux, where growth of the nanoparticle layer occurs at the maximum rate. The effect of heat flux makes possible nanoparticles transformations such as sintering, chemical interaction with the active gaseous medium, and phase transformations most probable exactly in this region of the reactor internal surface.

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

The layers of deposited nanoparticles had an extremely low bulk density, equal to 3–8% of the theoretical density. The deposited layers thickness varied from 0.05 to 2.7 mm in the experiments. Sintering of deposited nanoparticles near the maximum heat flux density location was noted only for copper nanopowder, where the melting point of the metal is 1360 K. A slight change in the average nanoparticle size inside the deposited layer along the reactor length

ing point. General list of nanopowder syntheses, performed in confined jet reactor, is given in **Table 1**. Some syntheses (AlN, AlON) were carried out in a combined reactor with disperse raw materials pre-evaporation in a heat-insulated channel followed by gas chemical

**forming gas**

,

O N2 Cu, Cu<sup>2</sup>

, H<sup>2</sup> H2 + N<sup>2</sup> W, Ni-Fe 5–12 [O]

, CuO H2 + N<sup>2</sup> W, Cu 4–8 [O]

), N<sup>2</sup> N2 TiN 10–20 [Ti]metal

, N<sup>2</sup> H2 + N<sup>2</sup> TiN 11–39 [Cl]

H8 + air H2 + N<sup>2</sup>

C3 H8 + air

Cu(HCOO)<sup>2</sup> N2 Cu, Cu<sup>2</sup>

Cu(re-condensation) N2 Cu, Cu<sup>2</sup>

5 Ag-SnO2 Ag, SnO<sup>2</sup> air Ag, SnO<sup>2</sup> 4–25

, (W-C)), whose materials have much higher melt-

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11

**Properties of nanopowders Phase composition Specific** 

Me 2–30 [O]

**surface area, m2 /g**

O, CuO, CuCl 2–5 [Cl], [O]

O, CuO 2–7 [C]

O, CuO 5–35 [C]

O, CuO 20–36 [O]

**Impurities**

O3

is noted for other nanopowders (W, Al<sup>2</sup>

**No Nanopowder Initial reagents Plasma** 

Mex Oy , H<sup>2</sup> , C<sup>3</sup>

Cu(CH<sup>3</sup>

WO3

WO3

TiCl4 , H<sup>2</sup>

2 Cu CuCl H2 + N<sup>2</sup> Cu, Cu<sup>2</sup>

COO)<sup>2</sup> ∙ H2

, NiO, Fe<sup>2</sup> O3

quenching.

Metals

1 W, Mo, Ni, Co, Re

Metal composites

3 W-Ni-Fe (W – 95 mass %)

4 W-Cu (W - 80 mass %)

Nitride, carbides, carbonitrides

6 TiN Ti, (TiH<sup>2</sup>

**Figure 3.** Heat flux density distribution in Al<sup>2</sup> O3 nanopowder synthesis for various Al feed rates.

rate, the maximum heat flux density shifts toward the beginning of the reactor, while its magnitude increases due to additional heat release as a result of the highly exogenous reaction of aluminum oxidation by air oxygen. Depending on the raw material feed rate, the additional power released as a result of this reaction was equal to 10–40% of the plasma jet power. The local heat flux density on the reactor wall in the studied nanopowder syntheses varied in the range 10–40 kW/m<sup>2</sup> . It follows from experiments that non-uniform wall heat flux density distribution exists in plasma reactor with a confined jet flow, and the exogenous reactions with a pronounced thermal effect can exert a significant influence on the wall heat flux distribution. Some nanoparticles degradation might occur inside the deposited nanoparticles layer in the area of maximal heat flux.

#### **4. Mass transfer to the wall of reactor and formation of particle's layer**

In the near-wall region of the plasma reactor, the nanoparticles transfer occurs under conditions when the average size of the nanoparticles is smaller or commensurable with the mean free path of the gas molecules, so the deposition of nanoparticles onto reactor wall from turbulent non-isothermal flow will be determined by the resulting effect of thermophoresis and Brownian diffusion [18]. The performed experiments have demonstrated that the deposited particles distribution along the reactor length has single extremum (**Figure 2C**), while the location of the maximum particle mass flux density coincides with the location of the maximum heat flux density. A similar particle flux density distribution is observed for all the studied processes, including aluminum oxide synthesis, where heat flux density distribution could have a bimodal character. The mass flow density is determined by the condensed phase mass concentration in the solid–gas flow, with the maximum value of the mass flow density exceeding by up to 2–3 times the mass flow density at the initial and final sections of the reactor.

For all the processes in the studied parameter variation ranges a high degree of nanoparticles deposition on the reactor surface is observed. The deposited mass is equal to 40–80% of the total mass of synthesized nanopowder. Consequently, the final properties of the produced nanopowders are largely determined by the properties of the product that is precipitated exactly in the reactor. The degree of nanoparticles deposition decreases with increasing of the process duration (thickness of the deposited layer of nanoparticles) and with an increase in the raw materials feed rate (processing rate). It follows from analysis of experimentally found heat flux density distributions and mass fluxes density distributions along the reactor wall that the maximum heat flux position coincides with location of maximum mass flux, where growth of the nanoparticle layer occurs at the maximum rate. The effect of heat flux makes possible nanoparticles transformations such as sintering, chemical interaction with the active gaseous medium, and phase transformations most probable exactly in this region of the reactor internal surface.

The layers of deposited nanoparticles had an extremely low bulk density, equal to 3–8% of the theoretical density. The deposited layers thickness varied from 0.05 to 2.7 mm in the experiments. Sintering of deposited nanoparticles near the maximum heat flux density location was noted only for copper nanopowder, where the melting point of the metal is 1360 K. A slight change in the average nanoparticle size inside the deposited layer along the reactor length is noted for other nanopowders (W, Al<sup>2</sup> O3 , (W-C)), whose materials have much higher melting point. General list of nanopowder syntheses, performed in confined jet reactor, is given in **Table 1**. Some syntheses (AlN, AlON) were carried out in a combined reactor with disperse raw materials pre-evaporation in a heat-insulated channel followed by gas chemical quenching.

rate, the maximum heat flux density shifts toward the beginning of the reactor, while its magnitude increases due to additional heat release as a result of the highly exogenous reaction of aluminum oxidation by air oxygen. Depending on the raw material feed rate, the additional power released as a result of this reaction was equal to 10–40% of the plasma jet power. The local heat flux density on the reactor wall in the studied nanopowder syntheses varied in the

O3

tribution exists in plasma reactor with a confined jet flow, and the exogenous reactions with a pronounced thermal effect can exert a significant influence on the wall heat flux distribution. Some nanoparticles degradation might occur inside the deposited nanoparticles layer in the

**4. Mass transfer to the wall of reactor and formation of particle's layer**

In the near-wall region of the plasma reactor, the nanoparticles transfer occurs under conditions when the average size of the nanoparticles is smaller or commensurable with the mean free path of the gas molecules, so the deposition of nanoparticles onto reactor wall from turbulent non-isothermal flow will be determined by the resulting effect of thermophoresis and Brownian diffusion [18]. The performed experiments have demonstrated that the deposited particles distribution along the reactor length has single extremum (**Figure 2C**), while the location of the maximum particle mass flux density coincides with the location of the maximum heat flux density. A similar particle flux density distribution is observed for all the studied processes, including aluminum oxide synthesis, where heat flux density distribution could have a bimodal character. The mass flow density is determined by the condensed phase mass concentration in the solid–gas flow, with the maximum value of the mass flow density exceeding by up to 2–3 times the mass flow density at the initial and final

For all the processes in the studied parameter variation ranges a high degree of nanoparticles deposition on the reactor surface is observed. The deposited mass is equal to 40–80% of the total

. It follows from experiments that non-uniform wall heat flux density dis-

nanopowder synthesis for various Al feed rates.

range 10–40 kW/m<sup>2</sup>

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area of maximal heat flux.

**Figure 3.** Heat flux density distribution in Al<sup>2</sup>

sections of the reactor.



**Table 1.** Nanoparticles syntheses.
