**5. Particle size distribution and morphology**

Granulometric composition is one of the most important nanopowders characteristics, which determines the possibility of their use in solving scientific problems and in practical applications. According to the results of electron microscopy, all nanopowders synthesized in plasma reactor are polydisperse and consist of particles of equiaxial shape (**Figure 4**). The presence of nanoobjects with oriented growth forms is not detected. Formation of nanoparticles under the conditions of plasmachemical synthesis occurs through the macro-mechanisms "vapor–liquid-crystal" (VLC), "vapor-crystal" (VC) and mixed mechanism, including a combination of these mechanisms (VLC-VC). Thermodynamic calculations of the nanopowder equilibrium yield as a function of temperature elucidate the mechanism of nanoparticle formation in a particular process.

Suppose that the substance in question exists in the liquid and solid state, and its yield depends on the temperature. Let us determine T\* as the temperature corresponding to the maximum yield of the nanoparticle substance, T<sup>c</sup> as the maximum temperature at which the nanoparticle exists in the condensed state, and Tm as the melting temperature of nanoparticle

**Figure 4.** TEM and SEM micrographs. 1—Al<sup>2</sup>

10—TiCN.

O3 , 2—TiO<sup>2</sup>

, 3—SiC, 4—W-C, 5—Cu, 6—W, 7—W-cu, 8—W-Ni-Fe, 9—TiC,

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Nanopowders Production and Micron-Sized Powders Spheroidization in DC Plasma Reactors http://dx.doi.org/10.5772/intechopen.76262 13

**5. Particle size distribution and morphology**

**No Nanopowder Initial reagents Plasma** 

, H<sup>2</sup>

, H<sup>2</sup> , N<sup>2</sup>

, H<sup>2</sup>

O3 Al, O<sup>2</sup> Air δ – Al<sup>2</sup>

, N<sup>2</sup>

Al, Me, O<sup>2</sup> Air MeAl2

, O<sup>2</sup> O2 + Ar TiO2

, O<sup>2</sup> O2 + Ar ZrO2

, Al, O<sup>2</sup> O2 + Ar ZrO2

, O<sup>2</sup> O2 + Ar Y2

WO3 , CH<sup>4</sup>

7 AlN Al, NH<sup>3</sup>

8 TiC TiCl4

9 TiCN TiCl4

10 SiC SiCl4

O3 – MeO (Me = Mg, Co)

14 AlON Al, NH<sup>3</sup>

15 TiO2 TiCl4

16 SiO2 SiCl4

17 ZrO2 ZrCl4

**Table 1.** Nanoparticles syntheses.

– Al2

O3 ZrCl4

O3 Y(COOH)<sup>3</sup>

11 W – C (Ctotal = 6.2 mass %)

Oxides

12 Al2

12 Powder Technology

13 Al2

18 ZrO2

19 Y2

maximum yield of the nanoparticle substance, T<sup>c</sup>

Granulometric composition is one of the most important nanopowders characteristics, which determines the possibility of their use in solving scientific problems and in practical applications. According to the results of electron microscopy, all nanopowders synthesized in plasma reactor are polydisperse and consist of particles of equiaxial shape (**Figure 4**). The presence of nanoobjects with oriented growth forms is not detected. Formation of nanoparticles under the conditions of plasmachemical synthesis occurs through the macro-mechanisms "vapor–liquid-crystal" (VLC), "vapor-crystal" (VC) and mixed mechanism, including a combination of these mechanisms (VLC-VC). Thermodynamic calculations of the nanopowder equilibrium yield as a function of temperature elucidate the mechanism of nanoparticle formation in a particular process.

**forming gas**

, H<sup>2</sup> H2 + N<sup>2</sup> WC1-x, W<sup>2</sup>

**Properties of nanopowders Phase composition Specific** 

C, W, C 15–25

O3 15–50

(spinel) 12–16

(rutile + anatase) 10–120 [Cl]

(tetragonal) 17 [Cl]

(cubic) 15–25

18–32 [Cl]

, N<sup>2</sup> N2 AlN 75–100 [Al]metal

, CH<sup>4</sup> H2 + N<sup>2</sup> TiN 13–23 [Cl]

O4

, O<sup>2</sup> N2 AlON 20–70 [Al] metal

tetragonal)

O3

(monoclinic +

, O<sup>2</sup> O2 + Ar amorphous 200–300 [Cl]

, CH<sup>4</sup> H2 + Ar TiC 15–45 [Cl]

, CH<sup>4</sup> H2 + Ar Β - SiC 20–75 [Cl]

**surface area, m2 /g**

**Impurities**

Suppose that the substance in question exists in the liquid and solid state, and its yield depends on the temperature. Let us determine T\* as the temperature corresponding to the

nanoparticle exists in the condensed state, and Tm as the melting temperature of nanoparticle

as the maximum temperature at which the

**Figure 4.** TEM and SEM micrographs. 1—Al<sup>2</sup> O3 , 2—TiO<sup>2</sup> , 3—SiC, 4—W-C, 5—Cu, 6—W, 7—W-cu, 8—W-Ni-Fe, 9—TiC, 10—TiCN.

matter. Taking into account the fact that the plasma process occurs at a decreasing temperature initially exceeding Tc , the temperature conditions for the nanoparticles formation by the above-mentioned macro-mechanisms can be written as:

with [21] can be due to the lognormal distribution of the particles residence time in the growth zone. For the nanoparticles syntheses (**Table 1**) where formation of particles occurs through various macro mechanisms, it has been experimentally established that the average size of the nanoparticles increases with increasing concentration of the gas component precursor [22–30].

The effect of the plasma process parameters, as well as effect of the characteristic dimensions of the reactor, was studied in Ref. [24] in case of tungsten and nickel nanopowders synthesis by

that the average metal nanoparticle size can be affected by the characteristic dimensions of the

and NiO oxides in hydrogen-nitrogen and propane-air plasmas. It is shown

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reduction of WO3

**Figure 6.** Particles size distributions for Al2

O3 , TiO<sup>2</sup>

, W, cu, TiCN and TiN nanopowders.

mechanism VLC, Т<sup>m</sup> < Т \* < Т<sup>c</sup> , all particles have a spherical habit (**Figure 5a**; **4**–**1**; **4**–**5**);

mechanism VC, T\* < T<sup>c</sup> < Tm, all particles have a faceted habit (**Figure 5b**, **4**–**2**);

mechanism of VLC-VC, T\* < T<sup>m</sup> < Tc , particles have both spherical and faceted habit (**Figure 6c**; **4**–**9**; **4**–**10**).

The VLC mechanism is realized if, under conditions of a decreasing process temperature, the maximum yield of nanoparticle matter occurs at temperatures above the melting point temperature (**Figure 5a**). The VC mechanism will determine formation of nanoparticles if the formation occurs at the temperatures below the melting temperature of the nanoparticle matter (**Figure 5b**), or the substance does not exist at all in the liquid state.

If during nanoparticles formation temperature is reduced and the substance undergoes crystallization (solidification) before the maximum yield is reached, then nanoparticles formation mechanism changes from VLC to VC, and product will contain both spherical and faceted particles (**Figure 5c**). As follows from the microphotographs of the obtained nanopowders (**Figure 4**), nanoparticles formation in the realized plasma syntheses can occur through all three of these mechanisms (VLC, VC, and VLC-VC). Under plasma synthesis conditions, all of the above mechanisms took place in the formation of Al2 O3 , TiO<sup>2</sup> , Cu, W, TiN, TiCN and W-C composition nanoparticles. The micrographs of the nanopowders were used to construct the histograms of the particle size distribution, and statistical analysis was carried out (**Figure 6**) [19].

It was established that the lognormal particle size distribution function (PSDF) reliably (with a correlation coefficient of more than 0.95) describes all the objects under investigation over wide range of changes in the granulometric composition of the investigated nanopowders.

In the PSDF formula *d* is the diameter of the particle, *m* is the median of the distribution, and σ is the standard deviation. It should be emphasized, that the validity of lognormal particle size distribution was confirmed earlier for the case of nanopowders obtained in the processes where the formation of particles occurs via coagulation mechanism, i.e. VLC [20]. The experimentally established lognormal particle size distribution in the absence of coagulation growth in accordance

**Figure 5.** Possible characteristic relations between temperatures, when nanoparticles are formed via different mechanisms.

with [21] can be due to the lognormal distribution of the particles residence time in the growth zone. For the nanoparticles syntheses (**Table 1**) where formation of particles occurs through various macro mechanisms, it has been experimentally established that the average size of the nanoparticles increases with increasing concentration of the gas component precursor [22–30].

matter. Taking into account the fact that the plasma process occurs at a decreasing tempera-

The VLC mechanism is realized if, under conditions of a decreasing process temperature, the maximum yield of nanoparticle matter occurs at temperatures above the melting point temperature (**Figure 5a**). The VC mechanism will determine formation of nanoparticles if the formation occurs at the temperatures below the melting temperature of the nanoparticle mat-

If during nanoparticles formation temperature is reduced and the substance undergoes crystallization (solidification) before the maximum yield is reached, then nanoparticles formation mechanism changes from VLC to VC, and product will contain both spherical and faceted particles (**Figure 5c**). As follows from the microphotographs of the obtained nanopowders (**Figure 4**), nanoparticles formation in the realized plasma syntheses can occur through all three of these mechanisms (VLC, VC, and VLC-VC). Under plasma synthesis conditions, all of the

position nanoparticles. The micrographs of the nanopowders were used to construct the histograms of the particle size distribution, and statistical analysis was carried out (**Figure 6**) [19]. It was established that the lognormal particle size distribution function (PSDF) reliably (with a correlation coefficient of more than 0.95) describes all the objects under investigation over wide range of changes in the granulometric composition of the investigated nanopowders.

In the PSDF formula *d* is the diameter of the particle, *m* is the median of the distribution, and σ is the standard deviation. It should be emphasized, that the validity of lognormal particle size distribution was confirmed earlier for the case of nanopowders obtained in the processes where the formation of particles occurs via coagulation mechanism, i.e. VLC [20]. The experimentally established lognormal particle size distribution in the absence of coagulation growth in accordance

**Figure 5.** Possible characteristic relations between temperatures, when nanoparticles are formed via different mechanisms.

O3 , TiO<sup>2</sup>

mechanism VC, T\* < T<sup>c</sup> < Tm, all particles have a faceted habit (**Figure 5b**, **4**–**2**);

ter (**Figure 5b**), or the substance does not exist at all in the liquid state.

, the temperature conditions for the nanoparticles formation by the

, particles have both spherical and faceted habit (**Figure 6c**;

, Cu, W, TiN, TiCN and W-C com-

, all particles have a spherical habit (**Figure 5a**; **4**–**1**; **4**–**5**);

ture initially exceeding Tc

**4**–**9**; **4**–**10**).

14 Powder Technology

mechanism VLC, Т<sup>m</sup> < Т \* < Т<sup>c</sup>

mechanism of VLC-VC, T\* < T<sup>m</sup> < Tc

above-mentioned macro-mechanisms can be written as:

above mechanisms took place in the formation of Al2

The effect of the plasma process parameters, as well as effect of the characteristic dimensions of the reactor, was studied in Ref. [24] in case of tungsten and nickel nanopowders synthesis by reduction of WO3 and NiO oxides in hydrogen-nitrogen and propane-air plasmas. It is shown that the average metal nanoparticle size can be affected by the characteristic dimensions of the

**Figure 6.** Particles size distributions for Al2 O3 , TiO<sup>2</sup> , W, cu, TiCN and TiN nanopowders.

plasma apparatus, such as reactor diameter and plasma torch nozzle diameter. These parameters determine the dimensions of the high-temperature zone where the nanoparticles formation takes place. The chemical processes, occurring at nanoparticle surface, also could influence the regularities of nanoparticle growth. The results of studies of various nanopowders production in the plasma reactor indicate that the influence of the process parameters on the average particle size is a multifactor problem, where the physicochemical features of the process play significant role.

The process consisted of the following stages: microgranulation of ultrafine powder, heat treatment of microgranules (drying at 100°C, removal of organic binder at 300°C, thermo-

microgranules with separation of microgranules fraction in the range 25 to 50 μm, spheroidization of the isolated fraction of microgranules in the thermal plasma flow, separation of

at 1000°C, vacuum treatment at 1200°C), classification of heat-treated

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

chemical treatment in H2

**Figure 7.** Micrographs of spheroidized titan powder.

**Figure 8.** Micrographs of granules. (А) – Initial alloy components, (В) – Spheroidized in plasma.

It was found that the average nanoparticle size depends on the synthesis parameters such as the initial precursor concentration, plasma jet enthalpy and velocity. The individual features of the specific process determine the degree of influence of these parameters. Production of nanoparticles of extremely small size in the confined jet reactor can be achieved only if the initial vapor concentration is significantly reduced or the jet velocity is increased. Reducing the initial concentration results in a decrease in the synthesis productivity, and the velocity increase has certain physical and technical limitations. Controlled change of nanoparticles coagulation growth time in the thermal plasma flow manipulates the size of nanoparticles, formed by the VLC mechanism. Additional channel to control the nanoparticle growth time is fast quenching by cold gas injection. Cold gas injection forces cessation of the coagulation growth after completion of vapor–liquid phase transition.

Distributed radial injection of quenching gas was organized at the periphery of the hightemperature flow in the synthesis of alumina nanopowder by oxidation of a metal powder in air plasma flow [25]. Quenching was carried out at the different distances from the reactor inlet, thus varying the particles residence time in the coagulation growth zone. The change of the injection gas flow rate and the injection position allowed the variation of the average particle size in the range of 35 to 75 nm. The obtained results indicate that confined DC plasma jet reactor is capable to produce wide range of individual elements nanopowders as well as nanopowders of inorganic compounds and composites.
