2nd International Conference on Ultrafine Grained & Nanostructured Materials (UFGNSM) International Journal of Modern Physics: Conference Series Vol. 5 (2012) 263 269 World Scientific Publishing Company DOI: 10.1142/S2010194512002115 SYNTHESIS OF NANOSIZE SILICON CARBIDE POWDER BY CARBOTHERMAL REDUCTION OF SiO 2 M. DEHGHANZADEH School of Metallurgy and Materials Engineering, University of Tehran Tehran, Iran dehghanzadeh@ut.ac.ir A. ATAIE School of Metallurgy and Materials Engineering, University of Tehran Tehran, Iran aataie@ut.ac.ir S. HESHMATI-MANESH School of Metallurgy and Materials Engineering, University of Tehran Tehran, Iran sheshmat@ut.ac.ir A mixture of silicon carbide nano-particles and nano-whiskers has been synthesized through solid state reduction of silica by graphite employing high energy planetary ball milling for 25 h and subsequent heat treatment at 1300-1700 C in dynamic argon atmosphere. Effects of process conditions on the thermal behavior, phase composition and morphology of the samples were investigated using DTA/TGA, XRD and SEM, technique, respectively. DTA/TGA analysis shows that silicon carbide starts to form at ~ 1250 C. Analysis of the XRD patterns indicates that the phase composition of the sample heat treated at 1300 C for 2 h mainly consists of SiO 2 together with small amount of β-sic. Nano-crystalline silicon carbide phase with a mean crystallite size of 38 nm was found to be dominate phase on heat treatment temperature at ~ 1500 C. Substantial SiO 2 was still remained in the above sample. SEM studies reveal that the sample heat treated at 1500 C for 2 h contains nano-particles and nano-whisker of β-sic with a mean diameter of almost ~ 85 nm. The results obtained were also showed that the characteristics of the synthesized SiC particles strongly depend on the mechanical activation and heat treatment conditions. Keywords: Nano-structured materials; Silicon carbide; Mechanical activation 263
264 M. Dehghanzadh, A. Ataie & S. Heshmati-Manesh 1. Introduction Silicon carbide is a commonly used ceramic material with a tetrahedral structural unit similar to diamond 1, 2 and with attractive properties such as high strength, stiffness, good wear and corrosion resistance and semi-conductivity, which are mainly characteristic of covalent bonded ceramic materials. It is a promising material for thermo-mechanical and electronic applications 3. To obtain high-performance SiC ceramics, fine powder with a narrow particles-size distribution are required 4. The industrial production of SiC is widely performed by the Acheson process, which includes a carbo-thermal reduction of sand by petrol coke at about 2400 C. In the carbo-thermal reduction of SiO 2, the commonly accepted mechanism of SiC formation is the gas solid reaction between SiO (g) and graphite 5. The carbo-thermal reduction of SiO 2 is controlled by atmosphere, pressure, temperature and time. The produced SiC possess large grains and batches which demand grinding and purification processes for further application 3. As advanced ceramic materials based on SiC require extremely fine starting powders, a great deal of effort has centered on the synthesis of ultrafine SiC powders 6. The carbo-thermal reduction of SiO 2 by graphite is the most widely used method to synthesize SiC materials because of its innocuous source materials, safe operation conditions, and low cost. However, the carbo-thermal reduction follows a solid-solid (S- S) mechanism between the particles in the initial stage, which makes the reaction very difficult at a lower temperature for a high conversion 7. In this method, the mixing condition of the two reactants, SiO 2 and graphite, greatly influences the properties of the powder produced. Homogeneous mixing condition is expected to accelerate reaction rate and lower the reaction temperature. Lower reaction temperature is preferable for producing fine powders 4. This research is aimed to synthesize nano-size particles of SiC from mechanically activated mixture of SiO 2 and graphite by carbo-thermal route at a relatively low temperature. The effects of the milling and heat treatment conditions on the powder particle characteristics have been investigated. 2. Experimental procedure Starting materials were commercially pure silica powder (98%, 50-100µm) and graphite (99.2%, 0.05-5 µm). They were mixed together according to the reaction (1) and then subjected to intense mechanical milling in a high-energy planetary ball mill in argon atmosphere for 25 h. SiO 2 + xc SiC + 2CO + (x-3)c x=3.9 (1)
Synthesis of Nanosize Silicon 265 A 30 %mol excess carbon was added to compensate the probable losses during handling and milling. The rotation speed and ball to powder weight ratio were selected to be 290 rpm and 35:1, respectively. The vial and balls were both made of hardened steel. The milling products were heat treated at 1300, 1400, 1500 and 1700 C for 2 h in dynamic argon flow. The morphology and phase identification of the products were examined by SEM (Cam Scan MV2300) equipped with an Energy Dispersive Spectrometer (EDS) (Oxford Instrument) and XRD (Philips Xpert Pro) using a Co-k α radiation (λ= 1.78892 Å), respectively. The mean crystallite size was calculated according to the Scherrer formula 8. NETZSCH DTA/TGA unit under the protection of high-purity argon from room temperature to 1450 C at a heating rate of 10 C/min was used to study the thermal behavior. 3. Results and Discussion The DTA/TGA traces of the sample milled for 25 h are shown in Fig. 1. The DTA curve exhibits one exothermic peak at 1250 C. The peak is associated with significant weight losses, as shown in the corresponding TGA curve. The sharp weight loss corresponds to the following reduction reaction: SiO 2 (s) + 3C(s) SiC(s) + 2CO(g) (2) Fig. 1. DTA/TGA curves of a mixture of SiO 2 and graphite milled for 25 h.
266 M. Dehghanzadh, A. Ataie & S. Heshmati-Manesh Generally, the accepted mechanism of carbo-thermal reduction of SiO 2 consists of three stages that often occur simultaneously. In the initial stage, silicon monoxide (SiO) and carbon monoxide (CO) are formed owing to the direct contact between graphite and SiO 2 particles in the original mixtures. C(s)+SiO 2 (s) SiO(g)+CO(g) (3) Secondly, the synthesized SiO reacts with graphite and CO to produce SiC 7. SiO(g)+2C(s) SiC(s)+CO(g) (4) SiO(g)+3CO(g) SiC(s)+2CO 2 (g) (5) Finally, the emerged CO 2 can be changed to CO according to the following reaction, which forms a feedback with reaction (4) to produce SiC until SiO is consumed completely. CO 2 (g)+c(s) 2CO(g) (6) In the overall process, the initial stage is an evident restrictive step, which is hardly stimulated below 1749 C as suggested by Fig. 2. In the second stage, once large amounts of SiO and CO gases are produced, reactions (3)-(5) can progress with little difficulty 7. Fig. 2. Free energy changes versus temperature under the pressure of 1.01 10 5 Pa 7. XRD patterns of the products are shown in Fig. 3. XRD pattern of the milled powder after annealing at 1300 C shows that small fraction of SiO 2 and graphite was converted
Synthesis of Nanosize Silicon 267 to SiC. XRD analysis of the milled powder annealed at 1400 C, 1500 C and 1700 C, revealed that the product powder was mainly β-sic together with a small fraction of SiO 2. In multiphase systems, milling causes intermixing of phases, which gives increase in the interfacial surface area while shortening the diffusion pass. These enhancements lead to the occurrence of reactions at substantially lower temperatures and/or at greater rates than those of the physically mixed powders, resulting in a more rapid attainment of equilibrium within a milled system 9. Fig. 3. XRD patterns of the milled samples for 25 h after heat treatment at various temperatures for 2 h. The average crystallite sizes of the heat treated samples calculated from (111) diffraction peak are listed in Table 1. Table 1: Average crystallite sizes calculated from the (111) diffraction peak. Heat treatment temperature Mean crystallite size (nm) 1400 C 38 1500 C 41 1700 C 49 Nano-particles and nano-whiskers of SiC are observed in the typical SEM images of the sample milled for 25 h after heat treatment at 1500 C for 2 h (Fig. 4). A high temperature annealing leads to a faster conversion rate and a higher driving force for sintering. Thus, high temperatures in short period of times may result in the finest powder products. Furthermore, higher temperatures support the endothermic reaction so that the gas formation is more vigorous. A rapid gas formation is believed to promote the development of small crystals 3, 10.
268 M. Dehghanzadh, A. Ataie & S. Heshmati-Manesh Formation of SiC whiskers observed in the SEM images (Fig. 4), could not be explained by the gas solid reaction of SiO(g) and C(s). It is more likely that the whiskers formed via SiO(g) and CO(g) gas-gas reaction 11. The reactions could proceed as follows: SiO 2 (s)+ CO(g) SiO(g)+ CO 2 (g) (7) SiO(g)+ 3CO(g) SiC(s)+ 2CO 2 (g) (8) Therefore, both gas solid and gas gas reactions occur in the synthesis of SiC via carbo-thermal reduction of SiO 5 2. Fig. 4. SEM images of the synthesized nano-crystaline SiC at 1500 C a) whisker b) powder. 4. Conclusions β-silicon carbide nano-particles and nano-whiskers were prepared by milling and further annealing of SiO 2 and graphite powder mixture. First a mixture of SiO 2 and graphite with 1-3.9 molar ratio was milled for 25 h. β-sic nanometer sized powders and whiskers were prepared by carbo-thermal reduction at 1500 C in dynamic argon atmosphere. In addition, the mixture heat treated at various temperatures. The minimum temperature that the mixture was almost converted to silicon carbide was 1500 C. Acknowledgements The authors would like to acknowledge the Iranian Nanotechnology Initiative for financial support of this work.
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