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SiC powders prepared from fly ash
Journal of Materials Processing Technology 117 (2001) 52±55
SiC powders prepared from ¯y ash Wang Hongjie*, Wang Yonglan, Jin Zhihao School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, China Received 28 April 2000
Abstract Fly ash is a naturally occurring byproduct of the burning process of coal-®red powder plant. In this paper, sub-micrometer SiC powders were synthesized successfully from ¯y ash and carbon. It is shown that the best synthesizing temperature is about 14008C with a holding time of 2 h. The average diameter of the powders is about 8 mm, but the powder size will increase at higher temperature or longer holding time. # 2001 Published by Elsevier Science B.V. Keywords: Fly ash; Carbon-thermal reduction; SiC powder
1. Introduction Silicon carbide has been long known for having many superior properties such as high hardness and strength, excellent resistance to oxidation and corrosion, a low coef®cient of thermal expansion and high heat-transfer capability. The industrial manufacture of SiC is fabricated by the Acheon process, carbon-thermal reduction of silica sand with green petroleum coke; however, this process needs a very high temperature of around 24008C, and the product has a large grain size (>1 mm) which requires extensive grinding to reduce the powder to sinter size [1±3]. In order to optimize the production of sub-micrometer SiC particles, several authors have used ®ne starting materials: reaction mixtures prepared by sol±gel techniques or plasma-driven chemical vapor deposition [4±6]. As the most easily scalable process is the carbon-thermal reduction of silica, a large effort has been made to improve the control of SiC particle size by the appropriate control of the size of carbon and silica particles in the reaction mixture. Because the main problem of the carbon-thermal reduction process is the particle size of the reactants and the need for intimate contact between them, it is not surprising that a considerable amount of attention has been focused on the use of ¯y ash as a starting material for the production of SiC. Fly ash is a naturally occurring byproduct of the burning process at coal-®red power plant. It is a lightweight, inert, hollow sphere composed largely of silica and alumina and
* Corresponding author. Fax: 86-29-3237910. E-mail address: [email protected] (W. Hongjie).
0924-0136/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 1 1 5 1 - 7
®lled with air or other gases. Despite the great production of ¯y ash in China (about 79 million tons in 1992) or in the whole world (about 400 million tons in 1992), only 20±30% is employed as a valuable resource; so, the storage of ¯y ash has become a serious environmental pollution problem. Consequently, both private companies and the government are supporting investigation projects to use ¯y ash in construction and other applications. This paper supports a new method in the preparation of sub-micrometer size SiC powder from ¯y ash. Little or no information with regard to the use of ¯y ash in the carbonthermal reduction synthesis of SiC powders is available. The formation of SiC powder from ¯y ash provides a potential application of ¯y ash in environmental waste disposal. 2. Experimental procedure Fly ash from Baqiao coal-®re powder plant in Xi'an was used. The composition of the ash used is provided in Table 1. The starting materials were ¯y ash and carbon black with an average particle size of 8.21 mm. They were blended in the correct proportions (incorporating 10% excess carbon) and ball-milled in ethanol for 24 h using an agent media. The mixture was dried. All synthesis reactions were carried out in a vacuum furnace under a ¯ow of nitrogen at 1200± 15008C, held at high temperature for 0.5±3 h and then allowed to cool naturally. After the reaction, the products were examined by Xray powder diffraction (XRD). The amount of SiC in each sample was estimated by comparing the relative height of the SiC diffraction peak with that of all diffraction peaks.
W. Hongjie et al. / Journal of Materials Processing Technology 117 (2001) 52±55
53
Table 1 Average fly ash properties Size (mm)
8.21
Chemical composition (wt.%)
Color
SiO2
Al2O3
Fe2O3
CaO
MgO
57.07
31.30
3.30
1.93
5.04
Off-white to gray
The particle size distribution of the products was examined using a Shimadzu SA-CPS. The morphology of the product was explored by the use of a scanning electron microscopy ®tted with a dispersive X-ray analyzer. 3. Results 3.1. Effect of sintering temperature The effect of temperature on the synthesis of b-SiC from the physical mixtures can be seen in Fig. 1. After 2 h from the beginning of the reaction, the SiC phase content increases with temperature, but when at 14008C or higher, the SiC phase content decreases. Fig. 2 shows the X-ray diffractograms of the reaction products obtained at different temperatures (1300, 1400,
Fig. 1. Effect of the reaction temperature on the yield of SiC in the system.
15008C; 2 h). Both diffraction patterns at 1300 and 14008C show all the main peaks corresponding to b-SiC, but in the case of 15008C, there is an AlN peak near to that of SiC, which makes the SiC phase content to decrease. The
Fig. 2. XRD patterns of the products obtained at different temperatures: (a) 1300; (b) 1400; (c) 15008C.
54
W. Hongjie et al. / Journal of Materials Processing Technology 117 (2001) 52±55
There are some possible stages with respect to the following reactions [6]:
Fig. 3. Effect of reaction time on the yield of SiC in the system at 14008C.
phenomenon shows that it is easy to form AlN at higher temperature. 3.2. Effect of holding time Fig. 3 shows the evolution of phase content as a function of reaction time at 14008C. From this ®gure, it can be seen that the amount of b-SiC produced at the time of holding of 2 h is the largest, whilst if the holding time is longer or shorter than 2 h, the SiC phase content will decrease. An X-ray diffraction pattern for the powder held at 14008C for 3 h is shown in Fig. 4. As with Fig. 2, despite the b-SiC peaks, there is an AlN peak near to the SiC peak. Compared with Fig. 2(b), it can be seen that holding time is another important factor in¯uencing the formation of AlN. Fig. 5 shows the SiC grain morphology. The morphology of SiC is almost spherical and the particle size of grain increases with the increase of the holding time. 4. Reaction mechanism Several mechanisms have been proposed for the synthesis of SiC, but in all of them there is an initial stage with the formation of silicon monoxide SiO2 s C s SiO g CO g
(1)
SiO g 3CO g SiC s 2CO2 g
(2)
SiO g 2C s SiC s CO g
(3)
SiO g C s Si s CO g
(4)
Si s C s SiC s
(5)
Generally, the accepted mechanism is the reaction of silicon monoxide with carbon (reaction 3). According to the thermodynamics, the standard free energy charge (DG) of reaction (3) is DG 609:023
0:351T kJ=mol
where T is the temperature in Kelvin. Reaction (3) is highly endothermic until 15008C, but in this study, there is a clear SiC phase peak at 13008C which is about 2008C lower than the temperature for the traditional condition (Fig. 2(a)). The reason for this phenomenon is that there exists Fe2O3 in the system that is introduced by the fly ash raw material. There are two possible explanations of the role of the catalyst, Fe2O3: 1. The catalyst (Fe2O3) produces a local decrease of the CO produced because the metal oxide favors the disproportionation of CO to give CO2 [7]. The CO2 can react again with carbon to produce CO, improving reaction (2) to form SiC. 2. The consequence of the above is a reaction mechanism based on the formation of SiC through the formation of Fe as an intermediate stage. Fe2O3 is reduced by CO to produce Fe. Then, the intermediate phase Fe±Si is formed [8], which, at the reaction temperature, is liquid and where the carbon is dissolved to form a supersaturated solution from which the solid-phase SiC is removed: Fe Sim ; Cn SiC Fe Sim1 ; Cn1 where m1 < m and n1 < n.
Fig. 4. X-ray pattern for powder holding at 14008C for 3 h.
W. Hongjie et al. / Journal of Materials Processing Technology 117 (2001) 52±55
55
Fig. 5. The SiC grain morphology at: (a) 0.5; (b) 2; (c) 3 h.
5. Conclusions
References
1. It is possible to produce sub-micrometer size b-SiC powders from fly ash using the carbon-thermal reduction synthesis method. 2. Iron or its oxides can act as good catalysts for the synthesis of SiC. 3. A suitable carbon-thermal reduction synthesis condition is about 14008C and 2 h of holding time with a synthesis rate of 82% and a mean grain size of 7.16 mm.
[1] N. Murakawa, et al., High Tech. Ceramics, Elsevier, Amsterdam, 1987, p. 50. [2] Y. Sugahara, et al., J. Mater. Sci. Lett. 8 (8) (1989) 944±946. [3] G.C.-T. Wei, J. Am. Ceram. Soc. 66 (7) (1983) C-111±C-113. [4] F. Hatakegama, et al., J. Am. Ceram. Soc. 73 (7) (1990) 2107±2110. [5] K. Ishizaki, et al., J. Mater. Sci. 1204 (10) (1989) 3553±3559. [6] J.A.O. Neill, et al., J. Am. Ceram. Soc. 72 (7) (1989) 1130±1135. [7] J.P. Hindermann, G.J. Hutchings, A. Kiennmann, Catal. Rev. Sci. Eng. 35 (1993) 1. [8] J.V. Milewoki, F.D. Gac, et al., J. Mater. Sci. 20 (1985) 160±165.
SiC powders prepared from ¯y ash Wang Hongjie*, Wang Yonglan, Jin Zhihao School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, China Received 28 April 2000
Abstract Fly ash is a naturally occurring byproduct of the burning process of coal-®red powder plant. In this paper, sub-micrometer SiC powders were synthesized successfully from ¯y ash and carbon. It is shown that the best synthesizing temperature is about 14008C with a holding time of 2 h. The average diameter of the powders is about 8 mm, but the powder size will increase at higher temperature or longer holding time. # 2001 Published by Elsevier Science B.V. Keywords: Fly ash; Carbon-thermal reduction; SiC powder
1. Introduction Silicon carbide has been long known for having many superior properties such as high hardness and strength, excellent resistance to oxidation and corrosion, a low coef®cient of thermal expansion and high heat-transfer capability. The industrial manufacture of SiC is fabricated by the Acheon process, carbon-thermal reduction of silica sand with green petroleum coke; however, this process needs a very high temperature of around 24008C, and the product has a large grain size (>1 mm) which requires extensive grinding to reduce the powder to sinter size [1±3]. In order to optimize the production of sub-micrometer SiC particles, several authors have used ®ne starting materials: reaction mixtures prepared by sol±gel techniques or plasma-driven chemical vapor deposition [4±6]. As the most easily scalable process is the carbon-thermal reduction of silica, a large effort has been made to improve the control of SiC particle size by the appropriate control of the size of carbon and silica particles in the reaction mixture. Because the main problem of the carbon-thermal reduction process is the particle size of the reactants and the need for intimate contact between them, it is not surprising that a considerable amount of attention has been focused on the use of ¯y ash as a starting material for the production of SiC. Fly ash is a naturally occurring byproduct of the burning process at coal-®red power plant. It is a lightweight, inert, hollow sphere composed largely of silica and alumina and
* Corresponding author. Fax: 86-29-3237910. E-mail address: [email protected] (W. Hongjie).
0924-0136/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 1 1 5 1 - 7
®lled with air or other gases. Despite the great production of ¯y ash in China (about 79 million tons in 1992) or in the whole world (about 400 million tons in 1992), only 20±30% is employed as a valuable resource; so, the storage of ¯y ash has become a serious environmental pollution problem. Consequently, both private companies and the government are supporting investigation projects to use ¯y ash in construction and other applications. This paper supports a new method in the preparation of sub-micrometer size SiC powder from ¯y ash. Little or no information with regard to the use of ¯y ash in the carbonthermal reduction synthesis of SiC powders is available. The formation of SiC powder from ¯y ash provides a potential application of ¯y ash in environmental waste disposal. 2. Experimental procedure Fly ash from Baqiao coal-®re powder plant in Xi'an was used. The composition of the ash used is provided in Table 1. The starting materials were ¯y ash and carbon black with an average particle size of 8.21 mm. They were blended in the correct proportions (incorporating 10% excess carbon) and ball-milled in ethanol for 24 h using an agent media. The mixture was dried. All synthesis reactions were carried out in a vacuum furnace under a ¯ow of nitrogen at 1200± 15008C, held at high temperature for 0.5±3 h and then allowed to cool naturally. After the reaction, the products were examined by Xray powder diffraction (XRD). The amount of SiC in each sample was estimated by comparing the relative height of the SiC diffraction peak with that of all diffraction peaks.
W. Hongjie et al. / Journal of Materials Processing Technology 117 (2001) 52±55
53
Table 1 Average fly ash properties Size (mm)
8.21
Chemical composition (wt.%)
Color
SiO2
Al2O3
Fe2O3
CaO
MgO
57.07
31.30
3.30
1.93
5.04
Off-white to gray
The particle size distribution of the products was examined using a Shimadzu SA-CPS. The morphology of the product was explored by the use of a scanning electron microscopy ®tted with a dispersive X-ray analyzer. 3. Results 3.1. Effect of sintering temperature The effect of temperature on the synthesis of b-SiC from the physical mixtures can be seen in Fig. 1. After 2 h from the beginning of the reaction, the SiC phase content increases with temperature, but when at 14008C or higher, the SiC phase content decreases. Fig. 2 shows the X-ray diffractograms of the reaction products obtained at different temperatures (1300, 1400,
Fig. 1. Effect of the reaction temperature on the yield of SiC in the system.
15008C; 2 h). Both diffraction patterns at 1300 and 14008C show all the main peaks corresponding to b-SiC, but in the case of 15008C, there is an AlN peak near to that of SiC, which makes the SiC phase content to decrease. The
Fig. 2. XRD patterns of the products obtained at different temperatures: (a) 1300; (b) 1400; (c) 15008C.
54
W. Hongjie et al. / Journal of Materials Processing Technology 117 (2001) 52±55
There are some possible stages with respect to the following reactions [6]:
Fig. 3. Effect of reaction time on the yield of SiC in the system at 14008C.
phenomenon shows that it is easy to form AlN at higher temperature. 3.2. Effect of holding time Fig. 3 shows the evolution of phase content as a function of reaction time at 14008C. From this ®gure, it can be seen that the amount of b-SiC produced at the time of holding of 2 h is the largest, whilst if the holding time is longer or shorter than 2 h, the SiC phase content will decrease. An X-ray diffraction pattern for the powder held at 14008C for 3 h is shown in Fig. 4. As with Fig. 2, despite the b-SiC peaks, there is an AlN peak near to the SiC peak. Compared with Fig. 2(b), it can be seen that holding time is another important factor in¯uencing the formation of AlN. Fig. 5 shows the SiC grain morphology. The morphology of SiC is almost spherical and the particle size of grain increases with the increase of the holding time. 4. Reaction mechanism Several mechanisms have been proposed for the synthesis of SiC, but in all of them there is an initial stage with the formation of silicon monoxide SiO2 s C s SiO g CO g
(1)
SiO g 3CO g SiC s 2CO2 g
(2)
SiO g 2C s SiC s CO g
(3)
SiO g C s Si s CO g
(4)
Si s C s SiC s
(5)
Generally, the accepted mechanism is the reaction of silicon monoxide with carbon (reaction 3). According to the thermodynamics, the standard free energy charge (DG) of reaction (3) is DG 609:023
0:351T kJ=mol
where T is the temperature in Kelvin. Reaction (3) is highly endothermic until 15008C, but in this study, there is a clear SiC phase peak at 13008C which is about 2008C lower than the temperature for the traditional condition (Fig. 2(a)). The reason for this phenomenon is that there exists Fe2O3 in the system that is introduced by the fly ash raw material. There are two possible explanations of the role of the catalyst, Fe2O3: 1. The catalyst (Fe2O3) produces a local decrease of the CO produced because the metal oxide favors the disproportionation of CO to give CO2 [7]. The CO2 can react again with carbon to produce CO, improving reaction (2) to form SiC. 2. The consequence of the above is a reaction mechanism based on the formation of SiC through the formation of Fe as an intermediate stage. Fe2O3 is reduced by CO to produce Fe. Then, the intermediate phase Fe±Si is formed [8], which, at the reaction temperature, is liquid and where the carbon is dissolved to form a supersaturated solution from which the solid-phase SiC is removed: Fe Sim ; Cn SiC Fe Sim1 ; Cn1 where m1 < m and n1 < n.
Fig. 4. X-ray pattern for powder holding at 14008C for 3 h.
W. Hongjie et al. / Journal of Materials Processing Technology 117 (2001) 52±55
55
Fig. 5. The SiC grain morphology at: (a) 0.5; (b) 2; (c) 3 h.
5. Conclusions
References
1. It is possible to produce sub-micrometer size b-SiC powders from fly ash using the carbon-thermal reduction synthesis method. 2. Iron or its oxides can act as good catalysts for the synthesis of SiC. 3. A suitable carbon-thermal reduction synthesis condition is about 14008C and 2 h of holding time with a synthesis rate of 82% and a mean grain size of 7.16 mm.
[1] N. Murakawa, et al., High Tech. Ceramics, Elsevier, Amsterdam, 1987, p. 50. [2] Y. Sugahara, et al., J. Mater. Sci. Lett. 8 (8) (1989) 944±946. [3] G.C.-T. Wei, J. Am. Ceram. Soc. 66 (7) (1983) C-111±C-113. [4] F. Hatakegama, et al., J. Am. Ceram. Soc. 73 (7) (1990) 2107±2110. [5] K. Ishizaki, et al., J. Mater. Sci. 1204 (10) (1989) 3553±3559. [6] J.A.O. Neill, et al., J. Am. Ceram. Soc. 72 (7) (1989) 1130±1135. [7] J.P. Hindermann, G.J. Hutchings, A. Kiennmann, Catal. Rev. Sci. Eng. 35 (1993) 1. [8] J.V. Milewoki, F.D. Gac, et al., J. Mater. Sci. 20 (1985) 160±165.