1

PRODUCTION OF NANOPARTICLES BY VAPOUR PHASE

SYNTHESIS USING TRANSFERRED ARC REACTOR A.C. da Cruz1 , T. Addona2, and R.J. Munz2

1 Institute for Technological Research of the State of São Paulo – IPT, Mechanics and Electricity Division, Brazil, acarlos@ipt.br; 2 CRTP, Department of Chemical Engineering, McGill University, Canada

ABSTRACT: A short review of recent work on the development of a transferred arc reactor concept applied to the vapour phase synthesis of nanoparticles is presented. Examples are given of experimental and modelling work carried out on the preparation of nanoscale silicon oxide (fumed silica) and aluminium nitride, and fine aluminium powders.

INTRODUCTION

The preparation of nanoparticles has been the object of growing interest due to the

distinct properties they present when compared with materials in the bulk state1,2,3. Because of

the large surface-area-to-volume ratio, a great portion of the composing atoms reside by the

surface of one such particle. As a result, the physical properties of materials in this form

correspond to neither those of the free atoms or molecules making up the particle nor to those of

bulk materials having the same composition. For example, nanoparticles find application in the

preparation of high performance catalysts and sintering of materials at lower temperatures, both

of which profit from the larger specific surface areas of nanoparticles. Nanostructured materials

are usually prepared by compaction of a powder of nanoparticles4. They are characterised by a

large number of grain boundary interfaces in which the local atomic arrangements are different

from those of the crystal lattice. The range of applications of nanoparticles also includes the

preparation of nanocomposites. These consist of nanoparaticles dispersed in a continuous matrix,

creating a compositional heterogeneity of the final structure. In this way, materials can be

prepared which exhibit interesting properties such as higher critical superconductor transition

temperature and alloying of normally immiscible materials4.

The many methods employed in the preparation of metal and ceramics nanoparticles

include gas-phase processes, laser ablation, sputtering techniques, and chemical methods. In the

work of Kruis et al.4 a comparison is made between two of these methods: gas-phase and liquid

based process. A number of advantages of the first are pointed out. These include higher purity

products, the potential to create complex chemical structures, better process and product control,

economics, and less chemical segregation. Moreover, because the gas-phase method usually

leads to continuous processing, it is better suited to larger scale production.

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The evaporation-condensation technique is one of the gas-phase methods used to produce

nanoparticles. In order to achieve reasonable evaporation rates, this method requires high

temperatures which implies the use of intense energy sources. In particular, the evaporation-

condensation of materials using thermal plasmas has been intensively studied5. For many years, a

variety of different thermal plasma systems have been used to synthesise ultra-fine particles

(Figure 1), including transferred and non-transferred arcs, high frequency induction plasmas, and

combinations of these. Each of these systems has its own distinct advantages and limitations.

Particles produced are characterised by their small size (of the order of few to the hundred of

nanometers) and may include pure metals, alloys, both oxide and covalent ceramics, and even

composite materials.

This paper is focused in the application of the transferred-arc in such syntheses. The main

advantage of transferred-arc reactors is the very high temperature driving force for evaporation.

Temperatures of more than 10000 K can easily be sustained either by striking the arc directly to

the anode work piece, or by feeding raw material in the form of powders directly into the arc.

Because transferred–arcs can be operated over a broad range of gases and gas flow rates,

including very low flow rates, operating costs can be kept low. Also, control over the

concentration of the evaporated species in the exhaust gas (and consequently over particle size)

can be provided simply by adjusting the desired flow rate at a given evaporation rate. An

additional advantage of transferred-arc systems is that the plasma evaporator may be scaled up

over a broad range of powers simply by increasing the arc current (100 – 2000 A). In the

following sections, the recent experimental and modelling work performed both at the CRTP

laboratories of McGill University/Canada6 and at IPT/Brazil in the transferred arc concept are

reported. These include the preparation of fine metal particles as well as nanoscale oxide and

covalent ceramic powders.

REACTOR CONCEPT

Early workers who used transferred-arc systems (particularly investigating the synthesis

of ultra-fine particles in reacting systems) attempted to carry out both metal evaporation and

reaction inside the plasma chamber using a reactive plasma gas7,8,9. Because of steep temperature

and concentration gradients around the arc, control of particle formation in such an environment

is difficult to control and, moreover, large lumps may form. To minimise these problems, the

reactor concept used at CRTP for the production of nanoparticles (schematically shown in Figure

2, after Moura10) has the following features: (i) separation of evaporation and reaction (or

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quench); and (ii) particle formation in axial flow cylindrical reactor with radial injection of

second reactant and/or quench.

THERMAL PLASMA SYSTEMS

Protectiogas

Plasma

Plasma

c) RADIO FREQUENCY INDUCTION

Work

Plasmaarc

Plasma gas

b) TRANSFERRED ARC

Plasmaarc

Plasma gas

a) NON-TRANSFERRED ARC

( - )

( + )( - )

( + )

Figure 1 – Schematic representation of the three different plasma systems commonly used in ultra-fine

particles synthesis investigations.

This approach has the main advantage of allowing separate optimisation of the two

reactor sections, thus giving better control over conversion and particle size distribution. Because

the only energy loss occurs, in principle, at the cathode (and this can be limited to the order of

five percent), the system is also highly energy efficient. Although this transferred-arc reactor

concept has many advantages over other reactor systems, practical limitations exist mainly

concerning construction materials. The intense arc radiation may lead to melting or other

deterioration of the side wall of the reactor. To allow for high evaporation rates and the high

energy efficiency just mentioned, the anode must be well insulated (mainly to avoid conductive

losses) and thus operate at very high temperatures. This poses not only thermodynamic limits

(reaction of the molten anode with crucible material), but physical limits as well (melting and

dissolution of crucible material in the molten anode).

In an attempt to overcome the above mentioned problems, a new reactor design is being

tested at IPT. In comparison with the previous CRTP/McGill work, the present version

introduces modifications which are aimed at avoiding not only undesirable reaction between

metal and crucible material, but also to promoting larger product powder yields. A schematic

drawing of the experimental apparatus is presented in Figure 3. Unlike the previous version, the

molten anode has been replaced by a circular water cooled anode that is assembled in the mid

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portion of the reactor. The concept of separation of evaporation and reaction (or quench) is still

maintained, in that that metal precursor in the form of a powder is continuously fed into the arc.

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Cathode

Plasmachamber

Arc

Quenching

FilterAnode

Tubular reactor

Figure 2 – CRTP reactor concept, after Moura8.

REACTOR

FilterHeat exchamger

Vaccumpump

ExhaustionModule 1(arc chamber)

Module 2(hot gaschamber)

Module 3(mixing chamber)

Exhaustionchamber

Anodeassembly

Cathodeassembly

Figure 3 – Schematic diagram of the plasma reactor for the vapour phase synthesis

of ultrafine powders developed at IPT.

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In the following, some examples are given of research results using these two reactor

versions in reactive, both oxide (fumed silica) and covalent (aluminium nitride), and non-

reactive systems (production of aluminium ultrafine particles).

NANOPHASE SILICA

Nanophase silica, also known as fumed silica, is a speciality chemical widely used as

filler in silicon rubber and toothpaste as a thixotropic agent, thickening and gelling agent, as

flattening agent in varnish, and also in the preparation of optical fibers preforms11. Fumed silica

is currently produced using aerosol flame reactors. Nanoparticles, includeding fumed silica,

titanium dioxide, and carbon black, are produced at a rate of 100 metric tons per day using this

method12.

The flame synthesis of fumed silica uses silicon tetrachloride fed into an

oxygen/hydrogen flame. Although well developed, this process is expensive and

environmentally problematic since it produces HCl. The proposed plasma process using the

CRTP reactor concept shown in Figure 4 uses quartz raw material which is thermally

decomposed at the anode to form SiO(g) and oxygen. In the tubular reactor outside the plasma

chamber, a steam quench reforms the silica in the desired particle size and amorphous form13.

Fumed Silica Reactor

Figure 4 – Transferred-arc silica evaporator from Addona14.

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The experimental study included the investigation of the effect of both plasma gas flow

rate and arc current on the rate of silica decomposition at the anode11. It was verified that the

decomposition rate was practically independent of gas flow rate but increased with arc current as

expected (higher energy input and temperatures at the anode). Use of hydrogen was also

investigated14. Hydrogen acts not only as a reducing gas but also provides much better heat

transfer properties. The introduction of 3% hydrogen mixed with argon resulted in an increase in

the decomposition rate of 7.5 times showing that both the production rate and efficiency of the

process can be greatly increased with the use of hydrogen. The quality of the product was found

to be almost identical to commercially available material (Aerosil® 200) in terms of surface area,

appearance, hydroxylation, etc., but lacked the thixotropic properties of that material. This was

attributed to the lack in the plasma process of the agglomerator which is included in the flow

sheet of the commercial process.

Modelling work was carried out in this system aimed at examining the flow within the

transferred arc evaporator and predicting the evaporation rates of silicon monoxide as a function

of arc current and plasma gas flow rate. The model allows computation of the temperature,

velocity and current density fields in both gas and condensed phases. Figure 5(a) shows the

temperature distribution in the gas and anode (argon flow rate of 15 lpm and arc current of 200

A). The highest temperature (about 20000 K) is reached near the cathode tip. At the arc root, the

gas temperature is of the order of 12000 K. A more detailed temperature distribution of the

condensed phase (anode) is shown in Figure 5(b). This figure shows that vaporisation (~ 3000 K)

is limited to a diameter of about 10 mm below the arc root.

Figure 5 Ð Temperature distribution (a) in the gas and molten silica and (b) molten silica detailed; after Addona14.

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ALUMINUM NITRIDE

Plasma synthesis of aluminium nitride (AlN) has been studied by a number of workers15.

AlN has potential application as a substrate support for electronic circuits where its high thermal

conductivity and high electrical resistivity are important properties. The early work of Moura8

used the side discharge reactor shown in Figure 2, similar to that of Addona11,12, with injection of

ammonia in the form of radial cross flow into a hot stream of argon carrying aluminium vapour.

Da Cruz developed a co-axial reactor (Figure 6), which allowed both radial and axial injection of

the ammonia, using both pure argon and argon containing 10% hydrogen.

b) Axial injection or reagent (at A or B).

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NH3/Ar(B)

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water in

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metal filter

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T4

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NH3/Ar

T5

T6

a) Radial injection of reagent.

NH3/Ar(A)

gassampling (NH )3

T7

Figure 6 - Transferred-arc plasma reactor used for AlN

ultrafine powder synthesis from Da Cruz17.

In the radial injection experiments using the apparatus shown in Figure 6, conversion to

AlN was incomplete, possibly due to condensation of the aluminium before ammonia injection.

The experiments using axial injection, on the other hand, were successful in producing a 100%

converted AlN powder. Control of particle size, evaluated in terms of powder surface area, was

possible over a broad interval (38-282 m2/g). Overall, particle size control depended on several

operating parameters, including quenching gas flow rate, gas temperature (1800K and 2000K),

gas composition (pure Ar, Ar/H2), and quenching gas injection geometry (axial or radial).

8

Examples of particles produced under different operating conditions are given in the

micrographs shown in Figure 7. The dependence of particle size on the various operating

parameters mentioned are more systematically shown in the graphs of Figure 8. It is observed the

higher temperature invariably lead to larger particle sizes. This is seen to result from the greater

concentrations of metal vapour (more intense evaporation) which are achieved at higher

temperatures. Depending on the plasma gas composition, two apparently contradictory trends

were observed for the particle size change as a function of quenching intensity in case of the

radial injection [Figure 8(a)]. For a plasma gas formed by Ar/10%H2, as expected the surface

area per unit mass increases (particle size decreases) as the quench intensity is increased. In the

case of a pure Ar plasma gas, unexpectedly the particle size increased as quench intensity

increased. In the case of axial injection of NH3, it was found that particle sizes were greater when

the injection was further upstream [Figure 8(b), z = -215 mm] because residence time was longer

and particles had more time to grow.

In the case of the most recent reactor version presented in Figure 3, two types of

experiments have been carried so far: (i) Al evaporation/condensation; (ii) AlN synthesis. As in

the case of the axial injection experiments previously presented, the AlN synthesis experiments

also resulted in fully converted aluminium nitride powders, with specific surface area of about 60

m2/g (approximately 30 nm average particle diameter). The x-ray diffraction pattern of these

powders, with are typical of the powders also produced in the previous version is shown in

Figure 9. Based on overall mass balances for the many experiments carried out, a production rate

in the 72 to 163 g/h range is estimated. A summary of main results obtained with the pure Al

production experiments is presented in the next section.

It was previously mentioned the practical limitations observed with respect to

construction materials in the case of the reactor version in which the arc is transferred to a

molten bath. The same problem was not observed with the use of a circular water cooled anode

with the metal precursor fed directly into the arc. On the other hand, using this configuration in

the AlN synthesis resulted in energy losses to the anode ranging from 20 to 50%, depending on

the specific operating conditions.

Considering the reaction between NH3 and Al(v), a 2-D model based on the moments of

particle size distribution was developed also for this system. This development includes a new

nucleation model that accounts for the effect of surface reaction on the surface of cluster of the

new condensed phase16. A cluster of x atoms of Al and y “molecules” of AlN was considered and

allowed to interact with the gas phase according to the elementary reactions shown in Figure 10.

The model allowed the prediction of the intensity of nucleation rate, gas phase reaction rate,

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condensation rate (Al and AlN), as well as surface reaction rate. A summary of the relative

magnitude of these different gas-to-condensed phase mechanisms is shown in Figure 11. Is may

be seen that while the amount of nucleation (in kg/m3s) is small, it is very important in

determining the final conversion and size distribution of the product powder. The gas phase

reaction rate and subsequent aluminium nitride condensation rate are both quite small. The larger

terms are the aluminium condensation rate and surface reaction rate, which are roughly equal in

magnitude. By integrating over any chosen reactor cross-section, the model can be used to

predict product composition and particle size distribution, and thus used for reactor design and

optimisation.

METAL FINE PARTICLES

A systematic study of the production and characterisation of metal nanoparticles ranging

from about 3 to 100 nm using an evaporation/condensation ultrafine particles generator was

presented by Granqvist and Buhrman1. This early work demonstrated the ability to producing

quantities of particles of controlled size, on the interest of fundamental studies of the physics of

minute metal particles. Nowadays, nanostructured metal particles are used in a variety of

applications such as catalysis, superalloys, and thin film coatings in the chemical and electronics

industries.

Production of fine metal particles can be carried out in any of the reactor versions

previously presented by quenching the metal vapour laden plasma chamber off gases using inert

gas. Figure 12 shows spherical sub-micrometer aluminium particles which were produced in the

reactor version shown in Figure 3; the starting powder particles exhibiting irregular shape and

much larger particle sizes are shown as well. Particle sizes of starting and prepared powders

compared, it is observed that the average size is reduced by approximately 80 times. The particle

size analysis of starting (Höganäs MG 300) and produced powder in one of the early experiments

(Exp. 4) is shown in Figure 13. The two peaks observed in case of the fines particles produced in

the reactor indicate that approximately 15% by mass of the air-atomised aluminium powder used

as raw material did not completely evaporate under the conditions of this particular experiment.

Further increase of arc power to powder feed rate ratio resulted in complete evaporation.

Although not shown, for the same quenching conditions applied in the study of

production of AlN [Figure 8(a)], little effect was observed of quenching intensities on the

average particle size of powders produced17. Producing powders with finer particles requires

either the use of more intensive quenching, lower system pressure, or lower metal

concentrations18.

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a)

b)

c)

d)

Figura 7 – TEM micrographs of AlN powders produced under various operating conditions: a) Exp. 17/6.1, radial injection, Ar, 1800 K, 282 m2/g, dBET= 7 nm; b) Exp. 15/1.1, radial injection, Ar/H2, 1800K, 111.6 m2/g, dBET=16 nm; c) Exp. 10, radial injection, Ar/H2 , 2000 K, 93.5 m2/g, dBET= 20 nm; and d) Exp. 21.3, axial injection, Ar, 2000 K, 46.5 m2/g, dBET=40 nm.

Temperature (K)

1750 1800 1850 1900 1950 2000 2050

Spec

ific

surfa

ce a

rea

(m2 /g

)

0

20

40

60

80

100

120

140

160

180

200

z = - 175 mm

z = - 215 mm

Quenching intensity (Ar flow rate, lpm)

1 2 3 4 5 6 7 8 9

Spec

ific

surfa

ce a

rea

(m2 /g

)

0

50

100

150

200

250 Ar

Ar / H2

- 1800 K- 2000 K

z = 0 mm

AlN - Effect of Operating Conditions on Particle Size

a) Radial injection, Ar and Ar/H2 b) Axial injection, Ar

Figure 8 – Surface area of powders produced under radial and axial quenching gas injection, showing also the influence of quenching intensity, gas composition, temperature, and residence time. The parameter z represents the distance of the NH3 injection (z = 0 mm refers to the axial distance of radial injection; z = - 175 and - 215 mm are the axial injection positioned up stream the radial injector).

50 nm 100 nm

50 nm 100 nm

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Figure 9 – Typical x-ray diffraction pattern of fully converted powders produced.

Figure 11 – Predictions of mass rates corresponding to the various phase transition mechanisms for AlN; after Da Cruz17.

CONCLUSIONS

An overview of recent experimental and modelling work carried out in the development

and application of transferred arc reactors to the synthesis of fine particles was presented. A

summary of preparation of oxide and nitride nanoparticles, as well as metal fine particles was

given. Raw materials feeding into the system has been accomplished using either a molten anode

and or direct feeding of powder metal into the arc (with water cooled, non-consumable anode).

With respect to previous work using transferred arcs, the separation of evaporation and reaction

(or quenching) into two distinct sections has proven effective in providing a better control over

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conversion and particle size distribution. At the same time, this approach allows separate

optimisation of the two reactor sections. The use of a water cooled anode resulted in a better

performance with respect to the problem of crucible material deterioration observed in the

molten anode version. On the other hand, this design resulted in relatively high energy losses to

the anode ranging from 20 to 50%, depending on the specific operating conditions. Based on

overall mass balances for the many experiments carried out, a production rate in the 72 to 163

g/h range is estimated in case of the water cooled anode version. Modelling work has been used

for better understanding the process and reactor optimisation as well.

(a)

(b)

(c)

Figure 12 – Preparation of fine metal (b) and ceramic powders (c) from air-atomised metal powder [(a), Höganäs MG 300], using non-reacting and reacting atmosphere, respectively.

Non-reacting

Reacting

200 nm

100 nm

13

0

10

20

30

40

50

60

70

80

90

100

0.1 1.0 10.0 100.0 1000.0

Particle diameter (µm)

% a

ccum

ulat

ed

0

1

2

3

4

5

6

7

8

9

10

% in

crem

enta

l

MG 300Exp. #4

Figure 13 – Particle size distribution of commercial aluminium powder (Höganäs MG 300) and powder

produced in the evaporation/condensation process (Exp. 4). ACKNOWLEDGEMENTS

The financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC), the Government of Quebec through FCAR, Baskatong Quartz Inc. (Canada), and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, Brazil) are gratefully acknowledged. The PhD work of A.C. da Cruz was supported by a scholarship from Conselho Nacional de Pesquisa Científica (CNPq), Brazil.

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REFERENCES 1 Granqvist, C.G. and Buhrman, R.A., Ultrafine metal particles, J. App. Physics, Vol. 47(5), pp. 2200-2219, 1976. 2 Rao, C.N.R., Chemical Approaches of the Synthesis of Inorganic Materials, John Wiley & Sons, 1994. 3 Wegner, K. and Pratsinis, S.E., Aerosol flame reactors for the synthesis of nanoparticles, KONA Powder and Particles No. 18, pp. 170 – 182, 2000. 4 Kruis, F.E., Fissan, H., and Peled, A., Synthesis of nanoparticles in the gas phase for electronic, optical and magnetic applications – a review, J. Aerosol Sci., Vol. 29(5/6), pp. 511-535, 1998. 5 Fauchais et al., High Pressure Plasmas and Their Application to Ceramic Technology, Springer-Verlag, 1983. 6 Munz, R.J., Addona, T., and Da Cruz, A.C., Application of transferred arcs to the production of nanoparticles, Pure and Appl. Chem., Vol. 71(10), pp. 1889-1897, 1999. 7 Etemadi, K., Formation of aluminum nitrides in thermal plasmas, Plasma Chemistry and Plasma Processing, Vol. 11, No. 1, p. 41-56, 1991. 8 Godin, M. F., Chevallier, F., Amouroux, J., and Morvan, D., Synthesis of aluminum nitride powders by thermal plasma, International Symposium on Plasma Chemistry 10, Brochum, p. 1.4-4 p.1, 1991. 9 Ageorges, H., Megy, S., Chang, K., Baronnet, J. M., Williams, J. K., and Chapman, C., Synthesis of aluminum nitride in transferred arc plasma furnaces, Plasma Chemistry and Plasma Processing, Vol. 13, No. 4, p. 613-632, 1993. 10 Moura, F.J., Vapour phase synthesis of AlN using a transferred arc plasma system, PhD thesis, McGill University, 1993. 11 Kammler, H.K. and Pratsinis, S.E., Scaling-up the production of nanosized SiO2-particles in a double diffusion flame aerosol reactor, Journal of Nanoparticle Research, Vol. 1, pp. 467-477, 1999. 12 Wegner, K. and Pratsinis, S.E., Aerosol flame reactor for the synthesis of nanoparticles, KONA Powder and Particle No. 18, pp. 170-182, 2000. 13 Addona, T. and Munz, R.J., Silica decomposition using a transferred arc process, Ind. Eng. Chem. Res., Vol. 38, pp. 2299-2309, 1999. 14 Addona, T., Study of a novel thermal plasma process for the production of fumed silica. PhD thesis, McGill University, Montreal, Canada, 1998. 15 Da Cruz, A.C. and Munz, R.J., Review of the vapour-phase synthesis of aluminum nitride powder using thermal plasmas, KONA Powder and Particle No. 17, pp. 85-94, 1999. 16 Da Cruz, A.C. and Munz, R.J., Nucleation with simultaneous chemical reaction in the vapour phase synthesis of AlN ultrafine particles, Aerosol Science and Technology, Vol. 34(6), pp. 499-511, 2001. 17 Da Cruz, A.C., Experimental and modelling study of the plasma vapour-synthesis of ultrafine aluminum nitride powders, PhD thesis, McGill University, Montreal, 1997. 18 Panda, S. and Pratsinis, S.E., Modeling of the synthesis of aluminum particles by evaporation-condensation in an aerosol flow reactor, NanoStructured Materials, Vol. 5(7/8), pp. 755-767, 1995.