- Email: [email protected]
Synthesis of hollow SiC microglobules by a combustion method
Microporous and Mesoporous Materials 117 (2009) 368–371
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis of hollow SiC microglobules by a combustion method H.H. Nersisyan a,*, H.I. Won a, C.W. Won a, J.H. Lee b a b
Rapidly Solidified Materials Research Center (RASOM), Chungnam National University, Yuseong-gu, Daejeon 305-764, South Korea Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yuseong-gu, Daejeon 305-353, South Korea
a r t i c l e
i n f o
Article history: Received 15 April 2008 Received in revised form 19 June 2008 Accepted 9 July 2008 Available online 16 July 2008 Keywords: b-SiC Combustion synthesis Hollow microglobule Carburization
a b s t r a c t Hollow b-SiC microglobules have been synthesized successfully by a combustion technique by using a Si + C starting mixture. In this process, a controlled amount of KClO3 + 3Mg is mixed with Si + C to enable a self-sustaining combustion regime and to promote a carburization of the silicon. The structures and morphologies of the products were analyzed by X-ray diffractometry (XRD) and a scanning electron microscopy (SEM). This procedure provides an easy method for the synthesis of hollow microglobular assemblies with SiC composed of submicrometer size particles. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Cubic silicon carbide (b-SiC) is a material used in advanced ceramic applications due to its strength, hardness, corrosion resistance, low thermal expansion coefficient, and high thermal conductivity. Among the various microstructures, hollow spheres and globules of b-SiC reveal a high potential for porosity ceramics, lightweight materials, filters, and functional and structure materials. A variety of chemical and physicochemical methods have been employed to prepare hollow spheres: a sacrificial-core method, a layer-by-layer deposition, an emulsion/sol–gel method, spray and coaxial-nozzle techniques, and reaction-based and other methods [1–10]. Among them, the sacrificial-core technique and a layer-by-layer deposition are frequently used to fabricate hollow spheres with homogeneous and dense shells. In the sacrificial-core technique [1], first a template particle (core) is prepared and then coated in a solution, either by a controlled surface precipitation of inorganic molecule precursors or by a controlled absorption through specific functional groups on the cores, to form a shell. Subsequent removal of the core by a calcination or chemical extraction results in hollow spheres whose inner diameter is dependent on the template size. Layer-by-layer self-assembly technique (LBL), is used to deposit small particles of a coating material on to the cores by a heterocoagulation [2]. The principle of the layer-by-layer self-assembly technique (LBL) is based on a simple alternative adsorption of polyelectrolytes and/or oppositely charged inorganic species onto charged colloidal templates and a further removal of the template leads to the formation of hollow shells. * Corresponding author. Tel.: +82 42 821 6587; fax: +82 42 822 9401. E-mail address: [email protected] (H.H. Nersisyan). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.07.011
Zhang et al. used glass-carbon microspheres as a template to prepare hollow SiC microspheres [11]. Carbon spheres were mixed with micro-grade silicon powder and then heated to 1300 °C in an argon gas flow. The resulting powder was treated with a HF-HNO3 solution, washed by distilled water, dried, and post heat calcined at 700 °C to eliminate the carbon core. The diameter of the hollow SiC spheres prepared by this technique was about 500 nm and the shell thickness was between 50 and 250 nm. Shen et al. synthesized hollow SiC nanospheres by using SiCl4 and C6Cl6 as source materials, and metallic sodium as a reductant [12]. In their experiment a small amount of the initial reagents placed into a stainless steel autoclave was heat treated in a furnace at 600 °C for 5–12 h. The reaction product was acid leached to remove any amorphous silicon and other impurities, then washed by distilled water and dried. Diameter of the hollow SiC spheres prepared by this technique was about 100–200 nm. In the present study, hollow silicon carbide microglobules were synthesized by a combustion synthesis technique, using a micrometer size silicon powder and black carbon as source materials and a KClO3 + 3Mg mixture as an exothermal additive. Studies revealed that micrometer size hollow SiC globules assemblies composed of 200–500 nm particles can be synthesized by a one stage solidstate combustion technique. Compared to the template-synthetic methods, this template free method is very simple, convenient, and scaleable.
2. Experimental Silicon powder and black carbon were used as source materials. Metallic magnesium and potassium chlorate mixture were used as
369
H.H. Nersisyan et al. / Microporous and Mesoporous Materials 117 (2009) 368–371 Table 1 Reactant properties Particle size, lm
Purity, %
Silicon powder Magnesium
Junsei Chemical Co., Ltd. Daejung Chemicals and Metals, Korea Daejung Chemicals and Metals, Korea Daejung Chemicals and Metals, Korea
<10 50–200
99.0 98.5
<100
99.0
<0.01
99.0
Potassium chlorate Black carbon
2000
1600
1200
o
Manufacture
T, C
Reactant
0.05 0.0375 0.025
800
0.02
400 To
0 -4
0
4
8
16
20
Fig. 1. Temperature profiles of the Si + C + a (KClO3 + 3Mg) system versus a.
1- α-SiC(3C) 2- β -SiC(4H) 1 1 1 1
a 2
b
2
2
2
2
2
12
1
c 20
30
2
40
50
60
70
80
2θ( ) o
3. Results and discussion The combustion process in a Si + C + a (KClO3 + 3Mg) system (here, a is moles number of additive) versus an additive concentration was investigated experimentally. Recorded by a thermocouple technique, the temperature profiles of the steadily propagating combustion waves at different a are shown in Fig. 1. As seen, the temperature profiles show a rapid increase of the temperature to 1700–1900 °C in the initial portion of the reaction zone and then a long tail of the after-burning zone follows. The propagation of the combustion wave was stable until 1700 °C (at a = 0.02). Below this temperature a total attenuation of the combustion process took place. According to the X-ray diffraction, the as-received combustion products contained silicon carbide, magnesium oxide, and potassium chloride. After an acid leaching and washing by distilled water, an almost single phase b-SiC was obtained at a = 0.02– 0.025 (Fig. 2a). At a higher a along with b-SiC, a small amount of a-SiC(4 H) was also detected (Fig. 2b and c). SEM images of the initial mixture and the final SiC product are shown in Fig. 3. As may be seen from Fig. 3a the silicon particles are mostly shapeless and display a size of 1–10 lm. As for the final SiC particles, they reveal the shape of hollow microglobules and a size of less than 10 lm (Fig. 3b). These microglobules are composed of submicrometer size SiC particles as shown in the upper left side of
12
Time, sec
Intensity (a.u.)
an exothermic additive. The specifications of the chemicals used in this the study are listed in Table 1. The preparation of the green mixture was performed in a ceramic mortar until a homogenous mixture formation. Then it was hand stamped in a cylindrical mold to 40 mm in diameter and 70–100 mm in height. Tungsten–rhenium thermocouples (W/Re-5 versus W/Re-20, 50 and 100 lm in diameter) were used to determine the combustion velocity (Uc) and to monitor the combustion temperature (Tc). Three thermocouple wells (3 mm in diameter, 15–20 mm deep) were drilled into each specimen perpendicular to the cylinder axis at a spacing of 1 cm. The upper thermocouple was placed 1.0–1.5 cm from the top of a specimen. A computer, at a rate of 10 Hz using a data logger (DASTC, Keithley), continuously recorded the thermocouple time histories. Combustion speeds were determined from the temperature profiles and the known spacing between the thermocouples. All the experiments were conducted in a laboratory scale constant-pressure reactor. At the start of the experiment, the combustion chamber was sealed, evacuated, and purged with argon gas. The chamber was then filled with argon gas to the desired partial pressure. The combustion reaction was initiated by resistively heating the nickel–chromium filament. Power to the filament was immediately discontinued after an ignition of a sample. After the experiments the reaction product was subjected to a hydrochloric acid treatment and washed by distillated water to eliminate the MgO and KCl. The structures of the final products were investigated by an powder X-ray diffractometry (Siemens D5000). A scanning electron microscopy (SEM; JSM 5410) was used to examine the morphologies of the synthesized SiC globules.
T*
Fig. 2. XRD patterns of the reaction products after an acid enrichment: (a) a = 0.025; (b) a = 0.0375; (c) a = 0.05.
Fig. 3b. These hollow globules were not strong enough during the acid treatment and washing and they were partially damaged (Fig. 3c). The shell of a globule is not uniform, it has a grained structure, and the average thickness of the shell is 0.5 lm, as estimated from the micrograph in Fig. 3c. The external surface of the globules has two characteristic morphologies, smooth (Fig. 3b), and covered by whiskers (Fig. 3c). In the Si + C combustion process, the KClO3 + 3Mg mixture acts as an additional heat source to enable the Si carburization process. It is known that the reaction of KClO3 with Mg is highly exothermic and can produce 443 kcal/mol heats [13]:
KClO3 þ 3Mg ! 3MgO þ KCl
ð1Þ
To estimate the heat energy contribution of Eq. (1) to the total process of a silicon carburization, the thermal profiles were collected for the Si + a (KClO3 + 3Mg) system at a = 0.02 and 0.025, and are shown in Fig. 4. When the black carbon was eliminated from the reaction mixtures, the maximum combustion temperature, which is lower than the Si melting point (1410 °C) was recorded: 1240 °C at a = 0.02 and 1320 °C at a = 0.025. Consequently, based
370
H.H. Nersisyan et al. / Microporous and Mesoporous Materials 117 (2009) 368–371
Fig. 3. SEM images of the initial mixture and the hollow SiC globules prepared by a combustion method: (a) initial mixture; (b) typical SEM image of the SiC globules; (c) hollow globules with a inside morphology; (d) globules covered by whiskers.
o
Temperature, C
1500
o
Tc1=1320 C
1200
o
Tc2=1240 C
900
Uc1=0.3 cm/s
600
Uc1=0.15 cm/s
2
1
300
1. α = 0.025 2. α = 0.02
0 10
20
30
40
50
Time, sec Fig. 4. Temperature profiles of the Si + a(KClO3 + 3Mg) system:1. a = 0.025; 2. a = 0.02.
on the research data it can be concluded that the carburization process of silicon in the Si + C + a (KClO + 3Mg) system starts at a temperature interval between 1200 and 1300 °C. However, the microstructure analysis of the final SiC powder (Fig. 3) suggests that the basic process for the carburization of silicon occurs after the actual melting of the silicon. Consequently, the melting of the silicon precedes a globule formation. We think that, first the black carbon is dissolved into the surface layer of the molten silicon particle and then a precipitation of the SiC submicrometer size particles occurs:
Si ðliq:Þ þ C ! SiC
ð2Þ
At the same time, a shortage of silicon on the surface layer generates a concentration gradient, and the liquid silicon, under the resultant tension forces, continuously moves from the core to the
shell, resulting a hollow globule formation as shown in Fig. 3c. A probable mechanism for the formation of such hollow microglobules is presented in Fig. 5. The key factor for the formation of microglobules is the melting process of Si. However, to form mechanically strong microglobular assemblies the melting of Si only is not sufficient yet. This is why the combustion synthesized SiC, in most cases, was solid particles instead of hollow globules [14,15]. In the Si + C + (KClO3 + 3Mg) system, a low melting point KCl (760 °C) is formed after the decomposition of KClO3. The melt of KCl formed in the combustion wave makes it easier for the final SiC particles to slide and rotate, provides opportunities for particle– particle contacts and merging, thus increasing the strength of the microglubules shell. The salt concentration can be critical for the strength of the hollow microglobules and that related research is underway. As for the mechanism for the SiC whiskers formation, it is believed to be attributed to the vapor–liquid–solid (VLS) process [16,17]. The whiskers formation process is complex and its fundamental mechanisms remain to be ascertained. However some assumptions can be offered here. Generally, SiC whiskers formation on a globule shell involves gas-transport reactions with the participation of silicon and carbon gaseous oxides: SiO, CO and CO2. In our system, a sufficient quantity of silicon and carbon oxides can be formed by the following reactions:
KClO3 þ 2C ! COðgÞ þ CO2ðgÞ þ KCl
ð3Þ
2KClO3 þ 3Si ! 3SiO2 þ KCl
ð4Þ
SiO2 þ COðgÞ ! SiOðgÞ þ CO2ðgÞ
ð5Þ
After these gaseous components are formed, a whisker’s growth on a globule’s surface becomes available by the following reactions:
SiOðgÞ þ 2CðsÞ ! SiCðsÞ þ COðgÞ SiOðgÞ þ 3COðgÞ ! SiCðsÞ þ 2CO2ðgÞ
ð6Þ ð7Þ
H.H. Nersisyan et al. / Microporous and Mesoporous Materials 117 (2009) 368–371
>1400 oC
371
HCl
Si C
Mg KClO3
Si
SiC
SiC hollow microglobules
Initial mixture Fig. 5. Schematic mechanism for the formation of SiC hollow microglobules.
4. Conclusion In summary, we have demonstrated a novel and facile approach for a simple fabrication of hollow SiC globules from silicon and black carbon, when more than 0.02 mol KClO3 + 3Mg is added to the initial mixture. The exothermic interaction of KClO3 with Mg increased the system temperature to 1200–1300 °C, and then a carburization of the silicon by the black carbon followed. Hollow b-SiC globules obtained in this study displayed a micrometer size and were created from submicrometer size SiC particles. Method described in this paper is promising for a large-scale fabrication of hollow silicon carbide globules.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Acknowledgements
[13]
This research was supported by the Program for the Training of Graduate Students in Regional Innovation which was conducted by the Ministry of Commerce Industry and Energy of Korean Government.
[14] [15] [16] [17]
P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 453. G. Decher, Science 277 (1997) 1232. M. Iida, T. Sasaki, M. Watanabe, Chem. Mater. 10 (1998) 3780. T. Nakashima, N. Kimizuka, J. Am. Chem. Soc. 125 (2003) 6386. Q. Zhou, S. Wang, X. Fan, R.C. Advincula, Langmuir 18 (2002) 3324. P.J. Bruinsma, A.Y. Kim, J. Liu, S. Baskaran, Chem. Mater. 19 (1997) 2507. M. Zhao, L. Sun, R.M. Crooks, J. Am. Chem. Soc. 120 (1998) 4877. R.K. Rana, Y. Mastai, A. Gedanken, Adv. Mater. 14 (2002) 1414. Y. Lu, H. Fan, A. Stump, T.L. Ward, T. Rieker, C.J. Brinker, Nature 398 (1999) 223. M.S. Wendland, S.C. Zimmerman, J. Am. Chem. Soc. 121 (1999) 1389. Y. Zhang, E.W. Shi, Z.Z. Chen, X.B. Li, B. Xiao, J. Mater. Chem. 16 (2006) 4141. G. Shen, D. Chen, K. Tang, Y. Qian, S. Zhang, Chem. Phys. Lett. 375 (2003) 177. R.C. Weast (Ed.), Handbook of Chemistry and Physics, CRC Press, Inc., FL, 1987. Y. Yang, K. Yang, Z.M. Lin, J.T. Li, Mater. Lett. 61 (2007) 671. C.C. Chen, C.L. Li, K.Y. Liao, Mater. Chem. Phys. 73 (2002) 198. J.V. Milewski, F.D. Gac, J.J. Petrovic, S.R. Skaggs, J. Mater. Sci. 20 (1985) 1160. G. Urretavizcaya, Porto J.M. López, J. Mater. Res. 9 (1994) 2981.
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis of hollow SiC microglobules by a combustion method H.H. Nersisyan a,*, H.I. Won a, C.W. Won a, J.H. Lee b a b
Rapidly Solidified Materials Research Center (RASOM), Chungnam National University, Yuseong-gu, Daejeon 305-764, South Korea Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yuseong-gu, Daejeon 305-353, South Korea
a r t i c l e
i n f o
Article history: Received 15 April 2008 Received in revised form 19 June 2008 Accepted 9 July 2008 Available online 16 July 2008 Keywords: b-SiC Combustion synthesis Hollow microglobule Carburization
a b s t r a c t Hollow b-SiC microglobules have been synthesized successfully by a combustion technique by using a Si + C starting mixture. In this process, a controlled amount of KClO3 + 3Mg is mixed with Si + C to enable a self-sustaining combustion regime and to promote a carburization of the silicon. The structures and morphologies of the products were analyzed by X-ray diffractometry (XRD) and a scanning electron microscopy (SEM). This procedure provides an easy method for the synthesis of hollow microglobular assemblies with SiC composed of submicrometer size particles. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Cubic silicon carbide (b-SiC) is a material used in advanced ceramic applications due to its strength, hardness, corrosion resistance, low thermal expansion coefficient, and high thermal conductivity. Among the various microstructures, hollow spheres and globules of b-SiC reveal a high potential for porosity ceramics, lightweight materials, filters, and functional and structure materials. A variety of chemical and physicochemical methods have been employed to prepare hollow spheres: a sacrificial-core method, a layer-by-layer deposition, an emulsion/sol–gel method, spray and coaxial-nozzle techniques, and reaction-based and other methods [1–10]. Among them, the sacrificial-core technique and a layer-by-layer deposition are frequently used to fabricate hollow spheres with homogeneous and dense shells. In the sacrificial-core technique [1], first a template particle (core) is prepared and then coated in a solution, either by a controlled surface precipitation of inorganic molecule precursors or by a controlled absorption through specific functional groups on the cores, to form a shell. Subsequent removal of the core by a calcination or chemical extraction results in hollow spheres whose inner diameter is dependent on the template size. Layer-by-layer self-assembly technique (LBL), is used to deposit small particles of a coating material on to the cores by a heterocoagulation [2]. The principle of the layer-by-layer self-assembly technique (LBL) is based on a simple alternative adsorption of polyelectrolytes and/or oppositely charged inorganic species onto charged colloidal templates and a further removal of the template leads to the formation of hollow shells. * Corresponding author. Tel.: +82 42 821 6587; fax: +82 42 822 9401. E-mail address: [email protected] (H.H. Nersisyan). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.07.011
Zhang et al. used glass-carbon microspheres as a template to prepare hollow SiC microspheres [11]. Carbon spheres were mixed with micro-grade silicon powder and then heated to 1300 °C in an argon gas flow. The resulting powder was treated with a HF-HNO3 solution, washed by distilled water, dried, and post heat calcined at 700 °C to eliminate the carbon core. The diameter of the hollow SiC spheres prepared by this technique was about 500 nm and the shell thickness was between 50 and 250 nm. Shen et al. synthesized hollow SiC nanospheres by using SiCl4 and C6Cl6 as source materials, and metallic sodium as a reductant [12]. In their experiment a small amount of the initial reagents placed into a stainless steel autoclave was heat treated in a furnace at 600 °C for 5–12 h. The reaction product was acid leached to remove any amorphous silicon and other impurities, then washed by distilled water and dried. Diameter of the hollow SiC spheres prepared by this technique was about 100–200 nm. In the present study, hollow silicon carbide microglobules were synthesized by a combustion synthesis technique, using a micrometer size silicon powder and black carbon as source materials and a KClO3 + 3Mg mixture as an exothermal additive. Studies revealed that micrometer size hollow SiC globules assemblies composed of 200–500 nm particles can be synthesized by a one stage solidstate combustion technique. Compared to the template-synthetic methods, this template free method is very simple, convenient, and scaleable.
2. Experimental Silicon powder and black carbon were used as source materials. Metallic magnesium and potassium chlorate mixture were used as
369
H.H. Nersisyan et al. / Microporous and Mesoporous Materials 117 (2009) 368–371 Table 1 Reactant properties Particle size, lm
Purity, %
Silicon powder Magnesium
Junsei Chemical Co., Ltd. Daejung Chemicals and Metals, Korea Daejung Chemicals and Metals, Korea Daejung Chemicals and Metals, Korea
<10 50–200
99.0 98.5
<100
99.0
<0.01
99.0
Potassium chlorate Black carbon
2000
1600
1200
o
Manufacture
T, C
Reactant
0.05 0.0375 0.025
800
0.02
400 To
0 -4
0
4
8
16
20
Fig. 1. Temperature profiles of the Si + C + a (KClO3 + 3Mg) system versus a.
1- α-SiC(3C) 2- β -SiC(4H) 1 1 1 1
a 2
b
2
2
2
2
2
12
1
c 20
30
2
40
50
60
70
80
2θ( ) o
3. Results and discussion The combustion process in a Si + C + a (KClO3 + 3Mg) system (here, a is moles number of additive) versus an additive concentration was investigated experimentally. Recorded by a thermocouple technique, the temperature profiles of the steadily propagating combustion waves at different a are shown in Fig. 1. As seen, the temperature profiles show a rapid increase of the temperature to 1700–1900 °C in the initial portion of the reaction zone and then a long tail of the after-burning zone follows. The propagation of the combustion wave was stable until 1700 °C (at a = 0.02). Below this temperature a total attenuation of the combustion process took place. According to the X-ray diffraction, the as-received combustion products contained silicon carbide, magnesium oxide, and potassium chloride. After an acid leaching and washing by distilled water, an almost single phase b-SiC was obtained at a = 0.02– 0.025 (Fig. 2a). At a higher a along with b-SiC, a small amount of a-SiC(4 H) was also detected (Fig. 2b and c). SEM images of the initial mixture and the final SiC product are shown in Fig. 3. As may be seen from Fig. 3a the silicon particles are mostly shapeless and display a size of 1–10 lm. As for the final SiC particles, they reveal the shape of hollow microglobules and a size of less than 10 lm (Fig. 3b). These microglobules are composed of submicrometer size SiC particles as shown in the upper left side of
12
Time, sec
Intensity (a.u.)
an exothermic additive. The specifications of the chemicals used in this the study are listed in Table 1. The preparation of the green mixture was performed in a ceramic mortar until a homogenous mixture formation. Then it was hand stamped in a cylindrical mold to 40 mm in diameter and 70–100 mm in height. Tungsten–rhenium thermocouples (W/Re-5 versus W/Re-20, 50 and 100 lm in diameter) were used to determine the combustion velocity (Uc) and to monitor the combustion temperature (Tc). Three thermocouple wells (3 mm in diameter, 15–20 mm deep) were drilled into each specimen perpendicular to the cylinder axis at a spacing of 1 cm. The upper thermocouple was placed 1.0–1.5 cm from the top of a specimen. A computer, at a rate of 10 Hz using a data logger (DASTC, Keithley), continuously recorded the thermocouple time histories. Combustion speeds were determined from the temperature profiles and the known spacing between the thermocouples. All the experiments were conducted in a laboratory scale constant-pressure reactor. At the start of the experiment, the combustion chamber was sealed, evacuated, and purged with argon gas. The chamber was then filled with argon gas to the desired partial pressure. The combustion reaction was initiated by resistively heating the nickel–chromium filament. Power to the filament was immediately discontinued after an ignition of a sample. After the experiments the reaction product was subjected to a hydrochloric acid treatment and washed by distillated water to eliminate the MgO and KCl. The structures of the final products were investigated by an powder X-ray diffractometry (Siemens D5000). A scanning electron microscopy (SEM; JSM 5410) was used to examine the morphologies of the synthesized SiC globules.
T*
Fig. 2. XRD patterns of the reaction products after an acid enrichment: (a) a = 0.025; (b) a = 0.0375; (c) a = 0.05.
Fig. 3b. These hollow globules were not strong enough during the acid treatment and washing and they were partially damaged (Fig. 3c). The shell of a globule is not uniform, it has a grained structure, and the average thickness of the shell is 0.5 lm, as estimated from the micrograph in Fig. 3c. The external surface of the globules has two characteristic morphologies, smooth (Fig. 3b), and covered by whiskers (Fig. 3c). In the Si + C combustion process, the KClO3 + 3Mg mixture acts as an additional heat source to enable the Si carburization process. It is known that the reaction of KClO3 with Mg is highly exothermic and can produce 443 kcal/mol heats [13]:
KClO3 þ 3Mg ! 3MgO þ KCl
ð1Þ
To estimate the heat energy contribution of Eq. (1) to the total process of a silicon carburization, the thermal profiles were collected for the Si + a (KClO3 + 3Mg) system at a = 0.02 and 0.025, and are shown in Fig. 4. When the black carbon was eliminated from the reaction mixtures, the maximum combustion temperature, which is lower than the Si melting point (1410 °C) was recorded: 1240 °C at a = 0.02 and 1320 °C at a = 0.025. Consequently, based
370
H.H. Nersisyan et al. / Microporous and Mesoporous Materials 117 (2009) 368–371
Fig. 3. SEM images of the initial mixture and the hollow SiC globules prepared by a combustion method: (a) initial mixture; (b) typical SEM image of the SiC globules; (c) hollow globules with a inside morphology; (d) globules covered by whiskers.
o
Temperature, C
1500
o
Tc1=1320 C
1200
o
Tc2=1240 C
900
Uc1=0.3 cm/s
600
Uc1=0.15 cm/s
2
1
300
1. α = 0.025 2. α = 0.02
0 10
20
30
40
50
Time, sec Fig. 4. Temperature profiles of the Si + a(KClO3 + 3Mg) system:1. a = 0.025; 2. a = 0.02.
on the research data it can be concluded that the carburization process of silicon in the Si + C + a (KClO + 3Mg) system starts at a temperature interval between 1200 and 1300 °C. However, the microstructure analysis of the final SiC powder (Fig. 3) suggests that the basic process for the carburization of silicon occurs after the actual melting of the silicon. Consequently, the melting of the silicon precedes a globule formation. We think that, first the black carbon is dissolved into the surface layer of the molten silicon particle and then a precipitation of the SiC submicrometer size particles occurs:
Si ðliq:Þ þ C ! SiC
ð2Þ
At the same time, a shortage of silicon on the surface layer generates a concentration gradient, and the liquid silicon, under the resultant tension forces, continuously moves from the core to the
shell, resulting a hollow globule formation as shown in Fig. 3c. A probable mechanism for the formation of such hollow microglobules is presented in Fig. 5. The key factor for the formation of microglobules is the melting process of Si. However, to form mechanically strong microglobular assemblies the melting of Si only is not sufficient yet. This is why the combustion synthesized SiC, in most cases, was solid particles instead of hollow globules [14,15]. In the Si + C + (KClO3 + 3Mg) system, a low melting point KCl (760 °C) is formed after the decomposition of KClO3. The melt of KCl formed in the combustion wave makes it easier for the final SiC particles to slide and rotate, provides opportunities for particle– particle contacts and merging, thus increasing the strength of the microglubules shell. The salt concentration can be critical for the strength of the hollow microglobules and that related research is underway. As for the mechanism for the SiC whiskers formation, it is believed to be attributed to the vapor–liquid–solid (VLS) process [16,17]. The whiskers formation process is complex and its fundamental mechanisms remain to be ascertained. However some assumptions can be offered here. Generally, SiC whiskers formation on a globule shell involves gas-transport reactions with the participation of silicon and carbon gaseous oxides: SiO, CO and CO2. In our system, a sufficient quantity of silicon and carbon oxides can be formed by the following reactions:
KClO3 þ 2C ! COðgÞ þ CO2ðgÞ þ KCl
ð3Þ
2KClO3 þ 3Si ! 3SiO2 þ KCl
ð4Þ
SiO2 þ COðgÞ ! SiOðgÞ þ CO2ðgÞ
ð5Þ
After these gaseous components are formed, a whisker’s growth on a globule’s surface becomes available by the following reactions:
SiOðgÞ þ 2CðsÞ ! SiCðsÞ þ COðgÞ SiOðgÞ þ 3COðgÞ ! SiCðsÞ þ 2CO2ðgÞ
ð6Þ ð7Þ
H.H. Nersisyan et al. / Microporous and Mesoporous Materials 117 (2009) 368–371
>1400 oC
371
HCl
Si C
Mg KClO3
Si
SiC
SiC hollow microglobules
Initial mixture Fig. 5. Schematic mechanism for the formation of SiC hollow microglobules.
4. Conclusion In summary, we have demonstrated a novel and facile approach for a simple fabrication of hollow SiC globules from silicon and black carbon, when more than 0.02 mol KClO3 + 3Mg is added to the initial mixture. The exothermic interaction of KClO3 with Mg increased the system temperature to 1200–1300 °C, and then a carburization of the silicon by the black carbon followed. Hollow b-SiC globules obtained in this study displayed a micrometer size and were created from submicrometer size SiC particles. Method described in this paper is promising for a large-scale fabrication of hollow silicon carbide globules.
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Acknowledgements
[13]
This research was supported by the Program for the Training of Graduate Students in Regional Innovation which was conducted by the Ministry of Commerce Industry and Energy of Korean Government.
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