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A cost-effective process for large-scale production of submicron SiC by combustion synthesis
Materials Chemistry and Physics 73 (2002) 198–205
A cost-effective process for large-scale production of submicron SiC by combustion synthesis夽 Chien-Chong Chen∗ , Chia-Ling Li, Keng-Yuan Liao Chaoramics Laboratory, Department of Chemical Engineering, National Chung-Cheng University, Chia-Yi 621, Taiwan Received 20 February 2001; received in revised form 18 April 2001; accepted 20 April 2001
Abstract In this paper, a cost-effective process was developed for large-scale production of submicron SiC by the combustion synthesis. Large and thin reactant samples (120 × 120 × 6 mm3 ) were prepared from ground reactant powders of silicon and carbon black. In order to reduce the cost of the expensive silicon powders, different particle sizes were used. Prior to the cold press of the reactant sample, a small amount of PVA (polyvinyl alcohol) slurry, which served as a binder, was added into the mixed reactant powders. The reactant samples were combusted by a custom-built oxy-acetylene torch in air. The averaged reaction yield throughout the product was about 94%. If the molar ratio between carbon black and silicon was increased from 1 to 1.1, the averaged yield raised to about 97%. Further increase of the molar ratio could not raise the yield. The product consisted of -SiC with a trace of ␣-SiC. The grain size of SiC was around 0.2 m and the morphology showed that SiC particles were aggregated. In some cases, the aggregated SiC particles were sintered together. Particle size distribution of SiC powders before and after grinding were both narrow. The averaged particle size of both SiC powders were approximately 0.25 m. Using different particle sizes of starting silicon powders all resulted in the similar combustion results. The questions, why submicron SiC powders were produced and why starting particle sizes of silicon powders did not change the combustion products were discussed. Further reductions of process cost and proposals of continuous production lines were also addressed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Combustion synthesis; SiC; Cold press; SHS
1. Introduction Silicon carbide is an attractive material for its high chemical stability, high electron mobility, high thermal conductivity, low thermal expansion, and high thermal shock resistance. Although various methods such as CVD, plasma, laser and carbon reduction can produce silicon carbide, they are rather expensive processes. Comparing to the above methods, combustion synthesis or self-propagating high-temperature synthesis (SHS) is capable of producing SiC and other advanced materials less expensively. Generally, SHS has proven advantages [1]: lower energy requirement, higher product purity, simpler and cheaper equipment, higher sinterability of product, and possible nonequilibrium phases in the products. The materials synthesized by the SHS method included refractories, intermetallic materials, super alloys, cermets, and superconductors [1–5]. In SHS, a reactant pellet is subjected to an external heat source on one end and is ignited at this end. Once the pellet is ignited, 夽 Part of this work was presented in the 3rd Okinaga Symposium, Tokyo, 1997. ∗ Corresponding author. Fax: +886-5-272-1206. E-mail address: [email protected] (C.-C. Chen).
the external heat source is removed immediately. Due to the highly exothermic reaction, a self-sustained combustion wave propagates from this ignited end to the other and converts the reactant pellet into the final products. Because the SHS method is economical, we utilized this method to synthesize SiC material. It should be noted that a large heat of formation is required such that the combustion wave can be self-sustained. Therefore, two empirical criteria [2] were proposed for the capability of producing the sustained combustion wave: |H|/Cp at 298 K is larger than 2000 K and the adiabatic temperature Tad must exceed 1800 K. However, the heat of formation for the SiC is only −69 kJ mol−1 . Thus, the combustion synthesis of SiC is, in principle, not feasible under the normal reaction condition (1 atm. and 298 K). For this reason, auxiliary techniques are adopted for the combustion synthesis of SiC. The utilized techniques included preheating the reactant pellets to greatly raise the adiabatic temperature [6–8], initiating the Si–N reaction in a high-pressure nitrogen environment to trigger the Si–C reaction [9], increasing the heating surface under high pressure [10,11], using chemical or oxidative agents to alter the reaction routes [12,13], and supplying additional energy via an electric field [14]. To increase the reactivity, the particle sizes of Si and carbon powders used in the above
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studies were all near or much smaller than 1 m. Recently, without additional treatment, Narayan et al. [15] has successfully produced silicon carbide by heating 3-mm thick reactant pellets (diameter is 10 mm). This result is consistent with our finding [16] that direct combustion synthesis of SiC is feasible but with a limitation that the conversion length in the direction of wave propagation is about 5 or 6 mm. In our previous study [16], nearly fully converted SiC products were obtained by combusting reactant pellets with a tungsten coil in vacuum or an oxy-acetylene torch in air. The reactant pellets were consisted of silicon (<44 m) and carbon black (aggregated particle size less than 2 m) powders. If reactant powders were subjected to grinding to increase the contact area between reactant particles, however, the combustion was too violent that the reactant pellet broke apart in the course of combustion. Because our aim in this study is to develop a cost-effective process for mass production of SiC by combustion synthesis, to increase the production quantity and to reduce the process cost are the two most important issues. First, for raising the amount of SiC product, large and thin reactant samples were prepared and combusted. Thin reactant samples (<6 mm) can assure the complete conversion in the direction of combustion wave, while large surface of samples can increase the production rate. Next, two considerations were taken into account to reduce the production cost. First, because SiC could be synthesized both by a tungsten coil heating in a chamber with a controlled atmosphere and by a torch firing in air, it is of economical consideration to select the latter as the heating method. Next, in general, the major cost of the combustion synthesis comes from the expensive reactant materials, which are often the elementary powders with both extremely high purity and small particle size. This severe concern of cost associated with the combustion synthesis is also the main critic raised by the materials community. For production of silicon carbide from the elementary powders, the major process cost comes from the silicon powder, because carbon black powder is not expensive. Moreover, the smaller the particle size of the high-purity powder, the higher the price. For example, the market price in Taiwan for the 99%-purity 325 mesh (<44 m, Cerac) and the 100 mesh (<149 m, Showa Chemicals) silicon powders are $207.6 and 47.34 a pound, respectively. Therefore, different particle sizes of silicon powders were used to determine whether the cheaper and larger silicon powders could be used to produce silicon carbide and to reduce the process cost. This is contrasted to the silicon particle size reported in the literature being 1 m or much smaller. An even cheaper replacement of silicon powders using waste silicon wafer will be addressed in another manuscript.
2. Experimental procedures In this study, powders of Si (99.5% pure, <44 m, Cerac; 99% pure, 150 m, Merck; 99% pure, <149 m, Showa
199
Fig. 1. Schematic drawing of the custom-build torch used to ignite the reactant sample.
Chemicals) and carbon black (99.9% pure, <2 m, China Synthetic Rubber) were used. In certain arrangements, powders were subjected to grinding (Struler, DAP-7) for 60 min to reduce the particle sizes. The desired ratios of powder reactants were first mixed, then dispersed in n-hexane and ultrasonically shaken to achieve a good and thorough mixing. The resulting slurry was air-dried and the dried mixture was cold pressed into a rectangular reactant sample by a uniaxial single acting press at 5000 psi. The length, width and height of the reactant sample were 120, 120 and 6 mm, respectively. To reinforce the structural strength of the reactant sample to endure the torch firing, PVA slurry was added into the mixed reactant powder, where concentration of PVA water slurry was 8 wt.%. PVA (98% pure, Merck) is a highly adhesive polymer, serving as a binder to hold the silicon and carbon black powders together. The volume of the PVA slurry used was 65 cm3 for 80 g of reactant powders. A custom-built comb-shape torch with nine nozzles lined up in series (Fig. 1), which served as a pseudo-linear heat source, burned one edge of the reactant sample. Once the ignition started, we continually moved the torch forward until the whole reactant sample was completely burned. A high-speed CCD (Toshiba, ik-c40) focused on the reactant sample was used to monitor the progress of the combustion, and to record the reaction sequences.
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The obtained products were analyzed by XRD (MAC, MXP3) and SEM (Jeol, JSM-5410). The reaction yield was computed as the weight ratio, w1 /w0 , where w0 was the weight of the product and w1 the weight of the product after removing the residual silicon in 1 M NaOH solution for 24 h and burning the remaining carbon in an oven at 800 ◦ C for 3 h. A particle size analyzer (Malvern, Zetasizer 2000 HSA) was used to characterize the particle size distributions of product powders.
3. Results and discussions 3.1. Preparation of sturdy green samples The process used in this study was to burn the large reactant sample by a torch in air, a certain degree of structural strength was required for the reactant sample to endure the torch flame. If the reactant sample is too fragile, it will be easily blown apart by the torch so that the combustion wave automatically stopped. However, it is not easy to prepare such a large and sturdy reactant sample from the starting powders. Several methods had been attempted without any success. They included repeatedly cold pressed reactant powders with different compaction pressures, applied BN as a lubricant, used high-strength springs to increase the com-
paction pressure, as well as added paraffin wax to increase the powders mobility. The difficulty of forming compact and sturdy reactant samples does not result only from the large sample geometry, but also from the carbon black powders. Carbon black is not easily cold pressed because of its poly-chain microstructure, which acts like a spring. Because this spring-like property of carbon black tends to bounce back once the compaction pressure is released, it renders the structure of compacted sample not sturdy enough. Therefore, PVA slurry was introduced to the reactant powders, where PVA served as a binder to hold the powders together. The added PVA will not contaminate the final products, since it is burned and evaporated at the elevated temperature. The TGA thermal analysis of PVA was shown in Fig. 2, indicating most of the PVA burned away near 500 ◦ C. 3.2. Combustion using large silicon powders (<44 µm) It was observed that for a reactant sample consisted of Si (<44 m), carbon black and PVA, the combustion took place only at the burned surface and the propagation length of the combustion wave was less than 1 mm. The converted length was much smaller than the 5 or 6 mm in our previous study. This result should be due to the PVA, which was completely burned away in the early stage of combustion. When PVA was removed, the space originally occupied by the PVA
Fig. 2. TGA analysis of PVA powders.
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the edge of the reactant sample, it took just about 10 s to ignite the sample. Because the propagation of the combustion wave in the longitudinal direction, i.e. the burning direction, easily exceeded 6-mm thickness of the reactant sample, the reaction in the longitudinal direction was completed. The combustion wave also traveled transversely about 15 mm ahead. The torch continually moved forward to the unburned part of the reactant sample at a speed of 3 mm s−1 . The burning process continued until the entire reactant sample was converted into the final product. It is noted that when the ignition starts, an extremely bright glowing combustion wave appears, which clearly identifies the initiation of SHS reaction. Because the reactant sample was relatively large, the conversion of the reaction may not be uniform throughout the entire sample and the product morphology could be spatially dependent. In order to avoid the possible misinterpretation, small amounts of samples taken from different representative locations (indicated in Fig. 3) were all subjected to analysis. Table 1 shows that the reaction yield ranged from Fig. 3. Six representative sample regions subjected to property analysis.
became voids and the decrease of contact areas between Si and carbon black particles prohibited the propagation of the combustion wave. In order to increase the reactivity, the reactant powders were subjected to grinding to lower the particle sizes and to increase the contact areas. The combustion of this reactant sample made from ground reactant powders was much better than the one without grinding. However, the averaged reaction yield was only around 65%. In order to improve the reaction yield, samples were subjected to preheating. The reactant sample was first preheated for 1 min by moving the torch back and forth at a speed of 6 cm s−1 . Next, when the torch was immediately applied to
Table 1 Measured reaction yields at the different locations of products with different molar ratios between carbon black and silicon Spatial location
Molar ratio (C/Si) 1.00
1.05
1.10
A B C D E F
88.7 92.0 97.9 90.2 98.3 94.4
98.8 90.5 94.4 90.9 99.5 95.7
93.6 94.3 99.7 97.4 96.8 98.7
Average yield Standard deviation
93.6 4.0
95.0 3.8
96.8 2.4
Fig. 4. XRD patterns of SiC product.
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88.7 to 98.3%, with an averaged value of 93.6%. The reason that the yield failed to reach 100% should be first due to the torch. Because it is not a complete linear heat source, reactions taking place between torch nozzles were slightly deteriorated. Next, it was observed from Table 1 that reaction yields at the sample edges (A, B and D) were smaller than the other regions. This indicates that large heat loss at the sample edges reduced the combustion temperature and the reaction yield as well. Another influence is that in the open-air environment, carbon black can be readily oxidized at the elevated temperature. This carbon oxidation is severe at the oxygen-rich sample edges and surfaces. In order to compensate the oxidized carbon, an excess amount of carbon black was used. If the starting molar ratio
between carbon black and silicon was increased from 1.0 to 1.05 and 1.10, the averaged reaction yield was raised from 93.6 to 95.0% and 96.8%, respectively (Table 1). When the molar ratio increased, reaction yields at six regions were simultaneously increased, especially significant at the sample edges. However, further increase of the molar ratio resulted in decrease of reaction yield, indicating that extra carbon black served as a diluent to deteriorate the combustion reaction. Fig. 4 shows the XRD pattern of the product. The product was mainly consisted of the -SiC phase with a trace of ␣-SiC. Depending on the spatial location, the morphology of the product powders was aggregated particles or particles sintered together. For example, the morphology of product powders at the sample edges, where the reaction
Fig. 5. Microstructure of submicron SiC: (a) aggregated particles at sample location A and (b) sintered particles at sample location F.
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Fig. 6. Particle size distribution of SiC powders (a) without grinding and (b) after grinding in an agate grinder for 10 min.
yield and the combustion temperature were both slightly smaller, were aggregated particles as shown in Fig. 5a. On the other hand, Fig. 5b shows that sintered particles were produced at the sample location where the combustion temperature was higher. The particle size of product SiC was around 0.2 m. Because the SiC particles in the combusted sample were aggregated or sintering together, it is of interest to examine the particle size distribution of SiC powders to determine the degree of aggregation and sintering. The combusted SiC sample was very hard. However, the SiC powders could be easily scratched off from the sample edge. Fig. 6 shows the particle size distribution of SiC powders, indicating that there were two broad peaks centered at 0.2 and 1 m, respectively. If these powders were ground in an agate grinder for 10 min, the previous two broad peaks were narrowed and the most populated particle sizes were reduced to around 0.2 and 0.5 m (Fig. 6). Therefore, the degree of particle aggregation or sintering is not serious and one can obtain submicron SiC powders easily. 3.3. Combustion using small silicon powders (150 and <149 µm) It is noted that the above results were obtained for silicon powders with a particle size less than 44 m. In order to study the feasibility of lowering the process cost, same
Fig. 7. Scanning micrographs of submicron SiC produced by large Si powders (150 m).
experimental procedures were carried out for two different silicon powders, whose particle sizes were, respectively, 150 and <149 m. Combustion results given in Table 2 indicated that combustion process parameters such as preheat time, torch speed, overall process time, as well as the averaged reaction yield were all similar to the previous values using <44 m silicon powders. Moreover, Fig. 7 shows for larger starting Si powders, the particle size and morphology of product powders were extremely close to those in Fig. 5.
Table 2 Combustion process parameters for different starting silicon particle sizes (the molar ratio between carbon black and silicon is 1.0) Particle size (m)
Preheat time (s)
Torch speed (mm s−1 )
Overall time (s)
Averaged yield (%)
<44 150 <149
60 70 60
3.0 3.2 2.5
110 118 118
93.6 93.9 94.3
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ferent from the studies in literature where particle sizes of reactant powders were near or smaller than 1 m. The underlying cause why submicron silicon carbide was produced needs a further study. The combustion process developed in this study to produce the submicron SiC can be easily implemented in practice and a fully automatic continuous process can be readily incorporated in a kiln. In order to control the powder morphology, the degree of particle sintering can be controlled by adjusting the temperature gradient in the kiln. Moreover, the reaction yield can be greatly enhanced by designing the torch to be a completely linear heat source. The daily production rate of SiC in a single kiln is around 35 kg and the estimated production cost is very competitive with the currently available commercial SiC powders.
4. Conclusions In this study, a cost-effective process was successfully developed for large-scale production of submicron SiC by the combustion synthesis. The averaged yield throughout the product was about 94%. If the molar ratio between carbon black and silicon was increased from 1 to 1.1, the averaged yield raised to around 97%. The product consisted of -SiC with a trace of ␣-SiC. The grain size of SiC was around 0.2 m and the morphology showed that SiC particles were aggregated or sintered together. Particle size distribution of SiC powders before and after grinding were both narrow. The averaged particle size of both SiC powders were approximately 0.25 m. Because different particle sizes of starting silicon powders all resulted in the similar results, the cheaper silicon powders with a large particle size can be used to reduce the production cost. Fig. 8. Scanning micrographs of Si powders (<44 m) (a) before and (b) after grinding for 60 min.
Therefore, starting silicon powders with large particle sizes resulted in similar results and, hence, cheaper silicon powders can be used to lower the process cost. The reason why silicon powders with different particle sizes produced similar combustion results is due to the grinding of reactant powders prior to combustion. Scanning micrographs of silicon powders before and after grinding were shown in Fig. 8. The particle size of silicon powders after 60 min of grinding was in the order of 10 m, which was almost identical for all three starting silicon powders (<44, <149 and 150 m). This indicated that the actual silicon particles participating the combustion reactions were of a similar particle size for all three silicon powders. Therefore, the similar combustion results and product properties were of no surprise. It is noted that the product particle size of SiC was around 0.2 m obtaining from reaction between 2-m carbon black and 10-m silicon. This is significantly dif-
Acknowledgements This work was supported by the National Science Council of the Republic of China and the China Petroleum Company under grant number NSC 88-CPC-E-194-006. References [1] A.G. Merzhanov, in: Z.A. Munir, J.B. Holt (Eds.), Combustion and Plasma Synthesis of High-temperature Materials, VCH, New York, 1990. [2] Z.A. Munir, Am. Ceram. Soc. Bull. 67 (1988) 342. [3] A.G. Merzhanov, A.S. Rogachev, Pure Appl. Chem. 64 (1992) 941. [4] V. Hlavacek, Am. Ceram. Soc. Bull. 70 (1991) 240. [5] A. Varma, J.-P. Lebrat, Chem. Eng. Sci. 47 (1992) 2179. [6] V.M. Martinenko, I.P. Borovinskaya, in: Proceedings of the Third All-Union Conference on Tech. Combs., Chernogolovka, 1990, p. 180. [7] O. Yamada, Y. Miyamoto, M. Koizumi, J. Mater. Res. 1 (1986) 275. [8] R. Pampuch, L. Stobierski, J. Lis, J. Am. Ceram. Soc. 72 (1989) 1434.
C.-C. Chen et al. / Materials Chemistry and Physics 73 (2002) 198–205 [9] O. Yamada, K. Hirao, M. Koizumi, Y. Miyamoto, J. Am. Ceram. Soc. 72 (1989) 1734. [10] H.H. Nersisyan, V.N. Nikogosov, S.L. Kharatyan, A.G. Merzhanov, Fiz. Gor. I Vzryva 27 (1991) 77. [11] S.K. Bhaumik, C. Divakar, S.U. Devi, A.K. Singh, J. Mater. Res. 14 (1999) 906. [12] S.L. Kharatyan, H.H. Nersisyan, Int. J. SHS 3 (1994) 17.
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[13] O. Yamada, Y. Miyamoto, M. Koizumi, Am. Ceram. Soc. Bull. 64 (1985) 319. [14] A. Feng, Z.A. Munir, Metal. Mater. Trans. B 26 (1995) 587. [15] J. Narayan, R. Raghunathan, R. Chowdhury, K. Jagannadham, J. Appl. Phys. 75 (1994) 7252. [16] C.-C. Wu, C.-C. Chen, J. Mater. Sci. 34 (1999) 4357.
A cost-effective process for large-scale production of submicron SiC by combustion synthesis夽 Chien-Chong Chen∗ , Chia-Ling Li, Keng-Yuan Liao Chaoramics Laboratory, Department of Chemical Engineering, National Chung-Cheng University, Chia-Yi 621, Taiwan Received 20 February 2001; received in revised form 18 April 2001; accepted 20 April 2001
Abstract In this paper, a cost-effective process was developed for large-scale production of submicron SiC by the combustion synthesis. Large and thin reactant samples (120 × 120 × 6 mm3 ) were prepared from ground reactant powders of silicon and carbon black. In order to reduce the cost of the expensive silicon powders, different particle sizes were used. Prior to the cold press of the reactant sample, a small amount of PVA (polyvinyl alcohol) slurry, which served as a binder, was added into the mixed reactant powders. The reactant samples were combusted by a custom-built oxy-acetylene torch in air. The averaged reaction yield throughout the product was about 94%. If the molar ratio between carbon black and silicon was increased from 1 to 1.1, the averaged yield raised to about 97%. Further increase of the molar ratio could not raise the yield. The product consisted of -SiC with a trace of ␣-SiC. The grain size of SiC was around 0.2 m and the morphology showed that SiC particles were aggregated. In some cases, the aggregated SiC particles were sintered together. Particle size distribution of SiC powders before and after grinding were both narrow. The averaged particle size of both SiC powders were approximately 0.25 m. Using different particle sizes of starting silicon powders all resulted in the similar combustion results. The questions, why submicron SiC powders were produced and why starting particle sizes of silicon powders did not change the combustion products were discussed. Further reductions of process cost and proposals of continuous production lines were also addressed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Combustion synthesis; SiC; Cold press; SHS
1. Introduction Silicon carbide is an attractive material for its high chemical stability, high electron mobility, high thermal conductivity, low thermal expansion, and high thermal shock resistance. Although various methods such as CVD, plasma, laser and carbon reduction can produce silicon carbide, they are rather expensive processes. Comparing to the above methods, combustion synthesis or self-propagating high-temperature synthesis (SHS) is capable of producing SiC and other advanced materials less expensively. Generally, SHS has proven advantages [1]: lower energy requirement, higher product purity, simpler and cheaper equipment, higher sinterability of product, and possible nonequilibrium phases in the products. The materials synthesized by the SHS method included refractories, intermetallic materials, super alloys, cermets, and superconductors [1–5]. In SHS, a reactant pellet is subjected to an external heat source on one end and is ignited at this end. Once the pellet is ignited, 夽 Part of this work was presented in the 3rd Okinaga Symposium, Tokyo, 1997. ∗ Corresponding author. Fax: +886-5-272-1206. E-mail address: [email protected] (C.-C. Chen).
the external heat source is removed immediately. Due to the highly exothermic reaction, a self-sustained combustion wave propagates from this ignited end to the other and converts the reactant pellet into the final products. Because the SHS method is economical, we utilized this method to synthesize SiC material. It should be noted that a large heat of formation is required such that the combustion wave can be self-sustained. Therefore, two empirical criteria [2] were proposed for the capability of producing the sustained combustion wave: |H|/Cp at 298 K is larger than 2000 K and the adiabatic temperature Tad must exceed 1800 K. However, the heat of formation for the SiC is only −69 kJ mol−1 . Thus, the combustion synthesis of SiC is, in principle, not feasible under the normal reaction condition (1 atm. and 298 K). For this reason, auxiliary techniques are adopted for the combustion synthesis of SiC. The utilized techniques included preheating the reactant pellets to greatly raise the adiabatic temperature [6–8], initiating the Si–N reaction in a high-pressure nitrogen environment to trigger the Si–C reaction [9], increasing the heating surface under high pressure [10,11], using chemical or oxidative agents to alter the reaction routes [12,13], and supplying additional energy via an electric field [14]. To increase the reactivity, the particle sizes of Si and carbon powders used in the above
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 3 7 7 - 7
C.-C. Chen et al. / Materials Chemistry and Physics 73 (2002) 198–205
studies were all near or much smaller than 1 m. Recently, without additional treatment, Narayan et al. [15] has successfully produced silicon carbide by heating 3-mm thick reactant pellets (diameter is 10 mm). This result is consistent with our finding [16] that direct combustion synthesis of SiC is feasible but with a limitation that the conversion length in the direction of wave propagation is about 5 or 6 mm. In our previous study [16], nearly fully converted SiC products were obtained by combusting reactant pellets with a tungsten coil in vacuum or an oxy-acetylene torch in air. The reactant pellets were consisted of silicon (<44 m) and carbon black (aggregated particle size less than 2 m) powders. If reactant powders were subjected to grinding to increase the contact area between reactant particles, however, the combustion was too violent that the reactant pellet broke apart in the course of combustion. Because our aim in this study is to develop a cost-effective process for mass production of SiC by combustion synthesis, to increase the production quantity and to reduce the process cost are the two most important issues. First, for raising the amount of SiC product, large and thin reactant samples were prepared and combusted. Thin reactant samples (<6 mm) can assure the complete conversion in the direction of combustion wave, while large surface of samples can increase the production rate. Next, two considerations were taken into account to reduce the production cost. First, because SiC could be synthesized both by a tungsten coil heating in a chamber with a controlled atmosphere and by a torch firing in air, it is of economical consideration to select the latter as the heating method. Next, in general, the major cost of the combustion synthesis comes from the expensive reactant materials, which are often the elementary powders with both extremely high purity and small particle size. This severe concern of cost associated with the combustion synthesis is also the main critic raised by the materials community. For production of silicon carbide from the elementary powders, the major process cost comes from the silicon powder, because carbon black powder is not expensive. Moreover, the smaller the particle size of the high-purity powder, the higher the price. For example, the market price in Taiwan for the 99%-purity 325 mesh (<44 m, Cerac) and the 100 mesh (<149 m, Showa Chemicals) silicon powders are $207.6 and 47.34 a pound, respectively. Therefore, different particle sizes of silicon powders were used to determine whether the cheaper and larger silicon powders could be used to produce silicon carbide and to reduce the process cost. This is contrasted to the silicon particle size reported in the literature being 1 m or much smaller. An even cheaper replacement of silicon powders using waste silicon wafer will be addressed in another manuscript.
2. Experimental procedures In this study, powders of Si (99.5% pure, <44 m, Cerac; 99% pure, 150 m, Merck; 99% pure, <149 m, Showa
199
Fig. 1. Schematic drawing of the custom-build torch used to ignite the reactant sample.
Chemicals) and carbon black (99.9% pure, <2 m, China Synthetic Rubber) were used. In certain arrangements, powders were subjected to grinding (Struler, DAP-7) for 60 min to reduce the particle sizes. The desired ratios of powder reactants were first mixed, then dispersed in n-hexane and ultrasonically shaken to achieve a good and thorough mixing. The resulting slurry was air-dried and the dried mixture was cold pressed into a rectangular reactant sample by a uniaxial single acting press at 5000 psi. The length, width and height of the reactant sample were 120, 120 and 6 mm, respectively. To reinforce the structural strength of the reactant sample to endure the torch firing, PVA slurry was added into the mixed reactant powder, where concentration of PVA water slurry was 8 wt.%. PVA (98% pure, Merck) is a highly adhesive polymer, serving as a binder to hold the silicon and carbon black powders together. The volume of the PVA slurry used was 65 cm3 for 80 g of reactant powders. A custom-built comb-shape torch with nine nozzles lined up in series (Fig. 1), which served as a pseudo-linear heat source, burned one edge of the reactant sample. Once the ignition started, we continually moved the torch forward until the whole reactant sample was completely burned. A high-speed CCD (Toshiba, ik-c40) focused on the reactant sample was used to monitor the progress of the combustion, and to record the reaction sequences.
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The obtained products were analyzed by XRD (MAC, MXP3) and SEM (Jeol, JSM-5410). The reaction yield was computed as the weight ratio, w1 /w0 , where w0 was the weight of the product and w1 the weight of the product after removing the residual silicon in 1 M NaOH solution for 24 h and burning the remaining carbon in an oven at 800 ◦ C for 3 h. A particle size analyzer (Malvern, Zetasizer 2000 HSA) was used to characterize the particle size distributions of product powders.
3. Results and discussions 3.1. Preparation of sturdy green samples The process used in this study was to burn the large reactant sample by a torch in air, a certain degree of structural strength was required for the reactant sample to endure the torch flame. If the reactant sample is too fragile, it will be easily blown apart by the torch so that the combustion wave automatically stopped. However, it is not easy to prepare such a large and sturdy reactant sample from the starting powders. Several methods had been attempted without any success. They included repeatedly cold pressed reactant powders with different compaction pressures, applied BN as a lubricant, used high-strength springs to increase the com-
paction pressure, as well as added paraffin wax to increase the powders mobility. The difficulty of forming compact and sturdy reactant samples does not result only from the large sample geometry, but also from the carbon black powders. Carbon black is not easily cold pressed because of its poly-chain microstructure, which acts like a spring. Because this spring-like property of carbon black tends to bounce back once the compaction pressure is released, it renders the structure of compacted sample not sturdy enough. Therefore, PVA slurry was introduced to the reactant powders, where PVA served as a binder to hold the powders together. The added PVA will not contaminate the final products, since it is burned and evaporated at the elevated temperature. The TGA thermal analysis of PVA was shown in Fig. 2, indicating most of the PVA burned away near 500 ◦ C. 3.2. Combustion using large silicon powders (<44 µm) It was observed that for a reactant sample consisted of Si (<44 m), carbon black and PVA, the combustion took place only at the burned surface and the propagation length of the combustion wave was less than 1 mm. The converted length was much smaller than the 5 or 6 mm in our previous study. This result should be due to the PVA, which was completely burned away in the early stage of combustion. When PVA was removed, the space originally occupied by the PVA
Fig. 2. TGA analysis of PVA powders.
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the edge of the reactant sample, it took just about 10 s to ignite the sample. Because the propagation of the combustion wave in the longitudinal direction, i.e. the burning direction, easily exceeded 6-mm thickness of the reactant sample, the reaction in the longitudinal direction was completed. The combustion wave also traveled transversely about 15 mm ahead. The torch continually moved forward to the unburned part of the reactant sample at a speed of 3 mm s−1 . The burning process continued until the entire reactant sample was converted into the final product. It is noted that when the ignition starts, an extremely bright glowing combustion wave appears, which clearly identifies the initiation of SHS reaction. Because the reactant sample was relatively large, the conversion of the reaction may not be uniform throughout the entire sample and the product morphology could be spatially dependent. In order to avoid the possible misinterpretation, small amounts of samples taken from different representative locations (indicated in Fig. 3) were all subjected to analysis. Table 1 shows that the reaction yield ranged from Fig. 3. Six representative sample regions subjected to property analysis.
became voids and the decrease of contact areas between Si and carbon black particles prohibited the propagation of the combustion wave. In order to increase the reactivity, the reactant powders were subjected to grinding to lower the particle sizes and to increase the contact areas. The combustion of this reactant sample made from ground reactant powders was much better than the one without grinding. However, the averaged reaction yield was only around 65%. In order to improve the reaction yield, samples were subjected to preheating. The reactant sample was first preheated for 1 min by moving the torch back and forth at a speed of 6 cm s−1 . Next, when the torch was immediately applied to
Table 1 Measured reaction yields at the different locations of products with different molar ratios between carbon black and silicon Spatial location
Molar ratio (C/Si) 1.00
1.05
1.10
A B C D E F
88.7 92.0 97.9 90.2 98.3 94.4
98.8 90.5 94.4 90.9 99.5 95.7
93.6 94.3 99.7 97.4 96.8 98.7
Average yield Standard deviation
93.6 4.0
95.0 3.8
96.8 2.4
Fig. 4. XRD patterns of SiC product.
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88.7 to 98.3%, with an averaged value of 93.6%. The reason that the yield failed to reach 100% should be first due to the torch. Because it is not a complete linear heat source, reactions taking place between torch nozzles were slightly deteriorated. Next, it was observed from Table 1 that reaction yields at the sample edges (A, B and D) were smaller than the other regions. This indicates that large heat loss at the sample edges reduced the combustion temperature and the reaction yield as well. Another influence is that in the open-air environment, carbon black can be readily oxidized at the elevated temperature. This carbon oxidation is severe at the oxygen-rich sample edges and surfaces. In order to compensate the oxidized carbon, an excess amount of carbon black was used. If the starting molar ratio
between carbon black and silicon was increased from 1.0 to 1.05 and 1.10, the averaged reaction yield was raised from 93.6 to 95.0% and 96.8%, respectively (Table 1). When the molar ratio increased, reaction yields at six regions were simultaneously increased, especially significant at the sample edges. However, further increase of the molar ratio resulted in decrease of reaction yield, indicating that extra carbon black served as a diluent to deteriorate the combustion reaction. Fig. 4 shows the XRD pattern of the product. The product was mainly consisted of the -SiC phase with a trace of ␣-SiC. Depending on the spatial location, the morphology of the product powders was aggregated particles or particles sintered together. For example, the morphology of product powders at the sample edges, where the reaction
Fig. 5. Microstructure of submicron SiC: (a) aggregated particles at sample location A and (b) sintered particles at sample location F.
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Fig. 6. Particle size distribution of SiC powders (a) without grinding and (b) after grinding in an agate grinder for 10 min.
yield and the combustion temperature were both slightly smaller, were aggregated particles as shown in Fig. 5a. On the other hand, Fig. 5b shows that sintered particles were produced at the sample location where the combustion temperature was higher. The particle size of product SiC was around 0.2 m. Because the SiC particles in the combusted sample were aggregated or sintering together, it is of interest to examine the particle size distribution of SiC powders to determine the degree of aggregation and sintering. The combusted SiC sample was very hard. However, the SiC powders could be easily scratched off from the sample edge. Fig. 6 shows the particle size distribution of SiC powders, indicating that there were two broad peaks centered at 0.2 and 1 m, respectively. If these powders were ground in an agate grinder for 10 min, the previous two broad peaks were narrowed and the most populated particle sizes were reduced to around 0.2 and 0.5 m (Fig. 6). Therefore, the degree of particle aggregation or sintering is not serious and one can obtain submicron SiC powders easily. 3.3. Combustion using small silicon powders (150 and <149 µm) It is noted that the above results were obtained for silicon powders with a particle size less than 44 m. In order to study the feasibility of lowering the process cost, same
Fig. 7. Scanning micrographs of submicron SiC produced by large Si powders (150 m).
experimental procedures were carried out for two different silicon powders, whose particle sizes were, respectively, 150 and <149 m. Combustion results given in Table 2 indicated that combustion process parameters such as preheat time, torch speed, overall process time, as well as the averaged reaction yield were all similar to the previous values using <44 m silicon powders. Moreover, Fig. 7 shows for larger starting Si powders, the particle size and morphology of product powders were extremely close to those in Fig. 5.
Table 2 Combustion process parameters for different starting silicon particle sizes (the molar ratio between carbon black and silicon is 1.0) Particle size (m)
Preheat time (s)
Torch speed (mm s−1 )
Overall time (s)
Averaged yield (%)
<44 150 <149
60 70 60
3.0 3.2 2.5
110 118 118
93.6 93.9 94.3
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ferent from the studies in literature where particle sizes of reactant powders were near or smaller than 1 m. The underlying cause why submicron silicon carbide was produced needs a further study. The combustion process developed in this study to produce the submicron SiC can be easily implemented in practice and a fully automatic continuous process can be readily incorporated in a kiln. In order to control the powder morphology, the degree of particle sintering can be controlled by adjusting the temperature gradient in the kiln. Moreover, the reaction yield can be greatly enhanced by designing the torch to be a completely linear heat source. The daily production rate of SiC in a single kiln is around 35 kg and the estimated production cost is very competitive with the currently available commercial SiC powders.
4. Conclusions In this study, a cost-effective process was successfully developed for large-scale production of submicron SiC by the combustion synthesis. The averaged yield throughout the product was about 94%. If the molar ratio between carbon black and silicon was increased from 1 to 1.1, the averaged yield raised to around 97%. The product consisted of -SiC with a trace of ␣-SiC. The grain size of SiC was around 0.2 m and the morphology showed that SiC particles were aggregated or sintered together. Particle size distribution of SiC powders before and after grinding were both narrow. The averaged particle size of both SiC powders were approximately 0.25 m. Because different particle sizes of starting silicon powders all resulted in the similar results, the cheaper silicon powders with a large particle size can be used to reduce the production cost. Fig. 8. Scanning micrographs of Si powders (<44 m) (a) before and (b) after grinding for 60 min.
Therefore, starting silicon powders with large particle sizes resulted in similar results and, hence, cheaper silicon powders can be used to lower the process cost. The reason why silicon powders with different particle sizes produced similar combustion results is due to the grinding of reactant powders prior to combustion. Scanning micrographs of silicon powders before and after grinding were shown in Fig. 8. The particle size of silicon powders after 60 min of grinding was in the order of 10 m, which was almost identical for all three starting silicon powders (<44, <149 and 150 m). This indicated that the actual silicon particles participating the combustion reactions were of a similar particle size for all three silicon powders. Therefore, the similar combustion results and product properties were of no surprise. It is noted that the product particle size of SiC was around 0.2 m obtaining from reaction between 2-m carbon black and 10-m silicon. This is significantly dif-
Acknowledgements This work was supported by the National Science Council of the Republic of China and the China Petroleum Company under grant number NSC 88-CPC-E-194-006. References [1] A.G. Merzhanov, in: Z.A. Munir, J.B. Holt (Eds.), Combustion and Plasma Synthesis of High-temperature Materials, VCH, New York, 1990. [2] Z.A. Munir, Am. Ceram. Soc. Bull. 67 (1988) 342. [3] A.G. Merzhanov, A.S. Rogachev, Pure Appl. Chem. 64 (1992) 941. [4] V. Hlavacek, Am. Ceram. Soc. Bull. 70 (1991) 240. [5] A. Varma, J.-P. Lebrat, Chem. Eng. Sci. 47 (1992) 2179. [6] V.M. Martinenko, I.P. Borovinskaya, in: Proceedings of the Third All-Union Conference on Tech. Combs., Chernogolovka, 1990, p. 180. [7] O. Yamada, Y. Miyamoto, M. Koizumi, J. Mater. Res. 1 (1986) 275. [8] R. Pampuch, L. Stobierski, J. Lis, J. Am. Ceram. Soc. 72 (1989) 1434.
C.-C. Chen et al. / Materials Chemistry and Physics 73 (2002) 198–205 [9] O. Yamada, K. Hirao, M. Koizumi, Y. Miyamoto, J. Am. Ceram. Soc. 72 (1989) 1734. [10] H.H. Nersisyan, V.N. Nikogosov, S.L. Kharatyan, A.G. Merzhanov, Fiz. Gor. I Vzryva 27 (1991) 77. [11] S.K. Bhaumik, C. Divakar, S.U. Devi, A.K. Singh, J. Mater. Res. 14 (1999) 906. [12] S.L. Kharatyan, H.H. Nersisyan, Int. J. SHS 3 (1994) 17.
205
[13] O. Yamada, Y. Miyamoto, M. Koizumi, Am. Ceram. Soc. Bull. 64 (1985) 319. [14] A. Feng, Z.A. Munir, Metal. Mater. Trans. B 26 (1995) 587. [15] J. Narayan, R. Raghunathan, R. Chowdhury, K. Jagannadham, J. Appl. Phys. 75 (1994) 7252. [16] C.-C. Wu, C.-C. Chen, J. Mater. Sci. 34 (1999) 4357.