ELSEVIER

April 1995

Materials Letters 23 (1995) 153-156

Fine silica powder preparation by use of a transferred arc thermal plasma reactor

P.K. Mishra, B.C. Mohanty, B.B. Nayak Special Materials Division, Regional Research Laboratory, Bhubaneswar 751013, Orissa, India

Received 25 October 1994; in final form 16 January 1995; accepted 20 January 1995

Abstract

Highly dispersed fine silica powder has been prepared from the quartzite mineral and petroleum coke by reactive evaporation in the thermal plasma and condensation from the vapour phase. The process made use of an indigenously developed dc plasma reactor in transferred arc mode. Silicon monoxide gas produced by heating the above mixture was condensed in the presence of steam. The product obtained was in the form of very fine agglomerates and characterized by scanning electron microscopy (SEM) and laser particle size analysis. BET surface area and viscosity measurements were also carried out.

1. Introduction

The use of thermal plasmas has emerged as a proven high-temperature technology in recent times for pro- ducing fine and ultrafine ceramic powders and coatings [ l-41. Some of the advantages of thermal plasmas are: ( 1) availability of high temperature and high energy density which makes it easier to process high-temper- ature materials; e.g., ceramic oxides and refractories; (2) independent control of crucial process parameters like oxygen content or oxygen potential in any material; (3) attainability of very high cooling rate (as high as million deg/s is possible) ; (4) availability of high temperature in plasmas increases the reaction kinetics by several orders of magnitude.

Fine silica powder, with large surface area, has attained considerable importance in recent years. Con- ventionally fine silica powder is prepared by flame pyrolysis of sitane/silane-derivatives in oxy-hydrogen flames [ 5,6], It is also prepared by a sol-gel method using sodium silicate with mineral acid and by super- critical hydrolysis of ethylsilicate esters [ 7,8]. Some

reports mention the synthesis of nanocrystalline silica powders by controlled hydrolysis of ethylsilicate esters by reverse micelles [9]. But all these processes use costly raw materials. The present paper presents the results of preparing highly dispersed silica from the mineral quartzite in a dc transferred arc thermal plasma reactor. When a mixture of quartz and coke is heated, silicon monoxide gas is formed around 2100 K, which is much lower than the boiling point (3000 K) of sili- con-dioxide. Post-oxidation of SiO to form SiOa at low temperature helps to achieve a high cooling rate of the vapour product. Here, the controlled environment such as an inert atmosphere, combined with the fast quench- ing rate helps to produce smaller particles with clean surfaces. Details of the preparation and the characteri- zation of the products by IR, SEM and laser particle size analysis are discussed in this paper.

2. Experimental procedure

Quartzite (size 5-10 mm with 99% purity) and petroleum coke (5-10 mm) were used as the raw mate-

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154 P.K. Mishra et al. /Materials Letters 23 (199.5) 153-156

Plasmagen gas 4

Graphite clcctrode (-)

Reactor tube

Graphite crucible1

Bubble Alumina

Mild SteelChamber

IL Graphite electrode (+)

L Collection Chamber

-4 r Suc+t ion

Fig. 1. Schematic diagram of the plasma reactor.

rials for preparation of fine silica powder. A mixture of silica and carbon in the molar ratio of 1: 1 was fed into a graphite crucible, kept in a plasma reactor (Fig. 1) where a plasma was formed in a transferred arc mode. A 20 kw dc power source (Memco, model MTW 500) and argon as the plasmagen gas were used. The current density, gas flow rate and the arc length were 50 A/ cm*, 1 l/min and 6 cm, respectively. The reactor was connected to another reactor tube having provision for supply of steam. The silicon monoxide vapour (pro- duced from the reaction of SiO, and carbon at a tem- perature of around 2100 K) reacts with the incoming steam in the reactor tube producing highly dispersed silica. These products were characterised by use of a

JASCO FTJR spectrometer, model 5300, using KBr mixed pellets of the sample. The pellet technique involves mixing the finely ground sample with AR grade KBr powder and pressing the mixture in an eva- cuable die at sufficient pressure to produce a transparent disc. Particle size analysis was carried out by use of a model 3600 He-Ne laser particle size analyzer. The specific surface area was determined by the single point BET (Braunauer, Emmett and Teller) nitrogen adsorp- tion method using a high-speed surface area analyzer (Micromerities, USA; model 2200). Viscosity meas- urements were carried out using a Haake model RV 100 viscometer at 298 K. One percent (w/w) solution was made using tricresylphosphate as the solvent (vis-

P.K. Mishra et al. /Materials Letters 23 (1995) IS3-156 155

cosity = 45 cP) . The surface morphology of the product was studied by scanning electron microscopy (JEOL, model JSM 35 CF) .

3. Results and discussion

Very fine agglomerates of the silica powder (99.5% purity) were obtained. The samples were collected from three places: (a) in the vicinity of the arc, (b) from the walls of the plasma reactor, and (c) from the reactor tube as well as from the collection chamber. A detailed study of the vibrational modes of the silanol group on the surface of silica collected from the differ- ent locations of the reactor (Fig. 1) was carried out. The products collected from (a) in the vicinity of the arc and (b) from the wall of the reactor were found to be similar and show IR absorption peaks at 500, 800, 1120 and 3436 cm-’ (Fig. 2) which are due to silica. The powder collected from both the reactor tube and collection chamber shows an absorption peak at 944.6 cm-‘, typical of an isolated &OH group, along with absorption peaks at 1015, 1620,350O and 3720 cm-’ corresponding to Si-0-Si, OH group, geminal group of Si ( OH) *, and the OH (of the silanol group) respec- tively (Fig. 3). It also shows a small peak at 480 cm- ’ which is due to silica.. The product showing the above characteristic IR spectra is expected to be formed from the intermediates condensed from the reaction of sili- con monoxide vapour with steam. Hydroxylated silica may be considered as an agglomeration of chains of ultrafine particles of amorphous silica each of which has a surface coverage of silanol (Si-OH) groups. These silanol bonds are of three distinct types: ( 1) there are the isolated silanol groups, which are sufficiently far apart that they are unable to interact with each other; (2) similar groups with neighbouring silanol groups which are close enough so as to permit hydrogen bonds to form between the two, and (3) hydroxyl groups resulting from water :dsorbed on the silica surface [ 91. However, our results show the presence of all three types of surface silanol groups in the product collected from the tube reactor. But the percentage is very low compared to the non-hydroxylated silica. Related stud- ies on silica surfaces have been reported by different workers [ 10-121. The viscosity measurements of our product and that of the commercially available Aerosil were carried out and have been found to be 57 and 87

WI ,

f--------fl I I I 1

LOO0 3000 2000 1000 400

WAVE riUMEER KM-‘)

Fig. 2. IR spectra of the silica sample collected from the vicinity of the arc aad from the walls of the plasma reactor.

2 I I 4600 LOW 3000 2000 IO00 0

WAVE NUMBER [CM-‘)

Fig. 3. IR spectra of the silica collected from the reactor tube.

Fig. 4. Particle size distribution of the silica product.

CP respectively. The viscous fluid formed by the silica powder showed newtonian behaviour.

The particle size distribution of the product, shown in Fig. 4, was determined using a model 3600 Malvern He-Ne laser particle size analyzer. It shows that 11% of the silica particles were below 1.94 pm in diameter, 68% were below 16.7 pm and the remainder were within 188 pm. The BET surface areas of the samples were found to be 60 m*/g on the average (of five batches).

Surface morphological studies by SEM showed agglomerated products. The surface morphology of the silica (obtained from the reactor tube) is shown in Fig. 5 which reveals a web-like structure indicating the pres- ence of hydroxylated silica of the first kind ( vide infra) .

156 P.K. Mishra et al. /Materials Letters 23 (1995) 153-156

Fig. 5. Surface morphology of the hydroxylated silica obtained from the reactor tube. Magnification: 1250 X . (The bar has a length of 10 urn.)

4. Conclusion

It was possible to prepare fine silica powders with particle sizes within the l-150 l.r.rn range from quartzite and coke in a transferred arc thermal plasma reactor. Characterization of the product by IR, SEM, BET sur- face area and particle size analysis as well as viscosity studies revealed that the fine silica product is mostly non-hydroxylated using the operating conditions reported here, although surface morphological studies of the samples collected from the reactor tube revealed the presence of hydroxylated silica.

Acknowledgement

The authors are thankful to the Director, RRL, Bhu- baneswar, for giving permission for publication of this work.

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