SYNTHESIS OF ULTRADISPERSE POWDERS IN R.F. PLASMA

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SYNTHESIS OF ULTRADISPERSE POWDERS IN R.F. PLASMA

J. Szépvölgyi, I. Tóth, T. Székely

To cite this version:

J. Szépvölgyi, I. Tóth, T. Székely. SYNTHESIS OF ULTRADISPERSE POWDERS IN R.F.

PLASMA. Journal de Physique Colloques, 1990, 51 (C5), pp.C5-35-C5-41. �10.1051/jphyscol:1990505�.

�jpa-00230803�

COLLOQUE DE PHYSIQUE

Colloque C5, supplbment au n018, Tome 51, 15 septembre 1990

SYNTHESIS OF ULTRADISPERSE POWDERS IN R.F. PLASMA J. SZEPVOLGYI, I. T ~ T H and T. S Z ~ K E L Y

Research Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, H-1112 Budapest, Budaorsi ut 45, Bulgarie

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Powders were characterized by making use of AAS, ICP-AES, TG, DTA, SEM, BET, XPS and XRD techniques. Effects of the conditions of synthesis on the properties and yield of powders have been studied.

Ultradisperse powders offer a number of potentials in high tech applications due to their physical properties, chemical composition and phase distribution.

These powders can favourably be synthesized in R.F. inductively coupled plasma torches. where reactions are taking place in clean conditions at high tempera- ture of the plasma and the quenching rates are high [l].

Silicon nitride powder is a basic material for sintered high-temperature-re- sistant ceramic materials. Advanced ceramic materials based on silicon nitride require extremely pure and well characterized starting powders. The starting powder and its synthesis technique determine the sintering behaviour and the subsequent microstructure formation in the ceramic materials.

Plasma silicon nitride powders have mean particle size of less than 100 nm, a very narrow particle size distribution and usually low crystallinity [2-41.

Recently these powders were demonstrated to exhibit improved sintering proper- ties C53.

Titania powders of well defined chemical composition and phase distribution can also be produced in oxygen-argon R.F. plasmas. Ultradisperse, spherical, mainly anatase powders with grain size of 5-500 nm have been gained by the plasma oxidation of Tic14 [6,7]. Anatase has lower nucleation energy barrier compared with rutile, hence the predominance of anatase phase in powders sug- gests that anatase nucleates from liquid droplets [7]. Addition of a slight amount of AlCls to Tic14 and increase of the feed rate of TIC14 resulted in powders of higher rutile content due to transformation of anatase particles to rutile during cooling [6].

In this paper the interdependence between the parameters of synthesis in an R.F. inductively coupled plasma torch and the properties of products, naaely silicon nitride and titania powders will be discussed.

The experimental set-up is to be seen in Fig. 1. High purity Ar was used as plasma gas, while Ar, NZ or Oz were applied as sheath gas.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990505

C5-36 COLLOQUE DE PHYSIQUE

3

R.F.

generator

I

Reactor Absorber Off-gas

g

Fig. 1

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The R.F. generator was manufactured by Linn Elektronik QmbH (FRG). Parameters of the generator are summarized in Table 1. Power from the generator was in- ductively coupled to the torchbeactor made of quartz glass.

Type: LINN FS-7

Power tube: SIEMENS RS3010, air cooling Frequency: 27.17 MHz

Output power: 0-7.2 kW, continuous tuning Reflected power: 10-25 X of the output power

Anode voltage: max. 7 kV Anode current: max. 2.5 A

Table 1

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Powders were characterized in terms of bulk and surface chemical composition, phase conditions, thermal behaviour and morphology (Table 2).

Characteristics Measuring method Thermal behaviour TG, DTA

Bulk chemical composition wet chemical analysis AAS, ICP-AES Specific surface area BET

Morpholcgy SEM

Surface composition ESCA Phase conditions XRD, IR

Table 2

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Silicon nitride powders were produced from SiClr in accordance with the fol- lowing overall reaction:

3 SiClr + 16 NHs = SisNr + 12 N&C1 (1 1

SiClr of pure grade (FLUKA) was fed to the reactor by nitrogen saturated with SiClr.

Synthesis parameters having effect on the reaction and their ranges selected in this work are summarized in Table 3.

Reactor

internal diameter 27 mm

total length 530 mm

Volumetric feed rates:

plasma gas (Ar) 350-750 l/h (STP) sheath gas (Ar or NZ ) 300-1500 l/h (STP)

ammonia 9.4-290 l/h (STP)

SiClr feed rate: 9.5-50 g/h

NHs /Sic14 molar ratio : 33-112 Output power : 2.9-3.8 kW

Table 3

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The powder was mainly collected from the lower part of the reactor and in a smaller amount from the heat exchanger and the dust separator. NH4Cl formed as by-product was condensed together with the SisN4 powder. Composition of the products was changed against the distance from the coil as it is shown in Fig.

2. However, NH4Cl can be decomposed to gaseous products and hence is removed by thermal treatment in nitrogen at 400°C for 1 hour.

0 I I

20 60 100 140 180 220 Distance from the coil (cm)

Fig. 2

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Table 4

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Ar-Ar

Ar-NZ

Effects of the operating conditions (Table 3) on the powder characteristics were studied in Ar-Ar plasma. Neither the NHs/SiClr molar ratio nor the output power of the generator have significant effect on the composition and yield of powders. At the same time, the N-content and the yield varied in a complex way against the total feed rates of gases (plasma+sheath) and the Sic14 input (Fig. 3).

Optimum parameters determined on the basis of above results are presented in Table 5.

O (g/h)

15.9 16.0 28.2 30.5 9.5 15.0

Powders synthesized under these conditions possess very fine microstructure as proved by SEM measurements (Fig. 4). Uniform, spherical grains have been formed which are susceptible to agglomeration and electrostatic charging. Size of the agglomerates, however did not exceed 400 nm.

Composition, wt%

N Si 0* CCa,Fe,Al 36-5 62.1 <1.4

38.1 60.6 (1.3 0.08 37.9 60.2 <l.9 0.06 34.7 62.8 (2.5

34.9 63.1 <2.0 0.12 32.3 61.8 ( 5 . 9

C5-38 COLLOQUE DE PHYSIQUE

Fig. 3

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Sheath gas (Ar), feed rate 790 l/h (STP)

Sic11 feed rate 23 g/h

NH3 /Sic14 molar ratio 3 0 Generator, output power 3 kW Characteristics of the powder

N-content 38.1 %(m/m)

Yield related to SiClr input 66.0-%

Specific surface area 33 m2/g Mean particle size 55 nm

Table 5

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Fig. 4

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ESCA studies revealed considerable oxidation in the uppermost surface layers.

In addition to the oxygen some carbon and chlorine were detected, as well (Table 6). N/Si atomic ratios support presence of SiOz both in the bulk and near to the surface. Concentration of SiOz, however, seems to be higher in the

bulk compared to the surface. Both the surface N/Si atomic ratio and the form and position of Si 2p line in the XPS spectrum indicate formation of some Sick Ny

,

Table 6

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Si3N4 powders synthesized were mainly of amorphous structure with an a-phase content below 5%. Extensive studies have been performed on the crystallization of these powders. Without going into details: thermal treatment at 1480°C for 6 hours resulted in highly crystalline powder of 80 % a-SisN4 content. Pre- treatment of the powders covering mixing with ethanol, pelletization and ther- mal treatment in nitrogen at 400°C for 1 hour improved the extent of crystal- lization.

Titanium tetrachloride of "puriss" grade and high purity oxygen were used as starting materials. The experimental conditions were changed between limits given in Table 7.

Reactor

internal diameter 46 mm

total length 716 mm

Volumetric feed rates:

plasma gas (Ar) 315-500 l/h (STP) sheath gas (02 ) 400-550 l/h (STP) oxygen as reagent 50-200 l / h (STP) TiClr feed rate: 3.6-155 g/h

0 2 /TIC14 molar ratio: 10-63

Output power : 3.1-4.5 kW

Table 7

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Under these conditions high purity titania powders were produced (Table 8).

The plasma oxidation of Tic14 was far less "sensitive" to the changes in the operating parameters than the synthesis of SisN4. Both the total gas flow rate and the 0z/TiC14 molar ratio can be regarded as indifferent variables (at least within the limits given in Table 7) as far as the titanium content and the yield of titania powder are concerned. Only TIC14 feed rate has a de- tectable effect on these characteristics (Fig. 5).

Table 8

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(g/h) 22.5 43.0 54.2 108.7 136.4 154.9

Composition, wt%

Ti Fe A1 Si

58.8 0.051 0.30 0.07 56.1 0.057 0.24 0.18 59.2 0.056 0.29 0.13 60.5 0.024 0.10 0.03 59.3 0.027 0.11 0.25 60.4 0.028 0.12 0.12

CS-40 COLLOQUE DE PHYSIQUE

54

C - 7 - d

10 50 90 130 $70 G*

Fig. 5

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10 50 90 130 170 G*

(b) against Tic14 feed rate A maximum Tic14 feed rate of about 130 g/h is recommended for the given exper- imental set-up.

Similarly to SiaN4 powder, the plasmathermal TiOz powder is also characterized by spherical grain morphology (Fig. 6), and rather uniform, narrow particle size distribution. The mean particle size varied from 50 to 270 nm along with specific surface area of 5 to 29 mz/p.

Some carbon and chlorine were present on the surface of powders. Increase of Tic14 feed rate led to the decrease of surface C/Ti and Cl/Ti ratios (Table 9).

Fig. 6

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Regarding the phase distribution, powders consisting mainly of anatase (in an amount of 85-95%) were formed. Further thermal treatment of these powders led to considerable phase transformations: TiOz of 99% rutile and less than 1% of anatase content was produced. The phase transformation was accompanied by a threefold increase of the mean grain size.

Table 9

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R.F. inductively coupled plasma torches proved to be proper tools for the syn- thesis of ultradisperse powders using chlorides as starting materials.

In the case of Si3Nr synthesis the volumetric gas feed rat5 and the Sic14 feed rate were the operating parameters which determined the SisN4 content and the yield of the mainly amorphous powders formed. These powders were transformed to-crystalline products of about 80% a-SiaNr content by thermal treatment.

High purity anatase powders were formed when TiClr was oxidized. The only pa- rameter having significant effect on the composition and yield of TiOz powder was the feed rate of TiC14. After thermal treatment powders of 99% rutile con- tent have been produced.

SYMBOLS

N N-content of the SisN4 powder, w t %

G, G* feed rate of Sic14 and TiClr, respectively,g/h H yield as related to the SiClr input, %

K yield as related to the Tic14 input, %

V feed rate of gases (plasma+sheath), l/h (STP) Ti Ti-content of the TiOz powder, wt%

REFERENCES

[l] Fauchais, P. et al., High Pressure Plasmas and their Application to Ce- ramic Technology (in: Topics in Current Chemistry, Vol. 107), Springer, Berlin, (1983) p. 164.

C23 S. F. Exell et al., Fine Particles, 2nd Conference Electrochem. Soc.

(1974) 165.

C33 Y. Chang and E. Pfender, Plasma Chemistry and Plasma Processing 1 (1987) ( 3 ) 299.

[4] T. Hussain and V. J. Iberson, Proc. 7th Internat. Symp. on Plasma Chem.

(Eindhoven) 1985, Vol. 1. p. 692.

C53 Futaki, S. et al., J. Mat. Sci. 22 (1987) 4331.

[6] Gani, M.S., McPherson, R., S . Mat. Sci. (1980) 1915.

[7] Wassenaar, P. et al., Proc. ISPC-9 Bari (1989) Vol. 2. p. 919.