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Silicon carbide foams produced by siliciding carbon foams derived from mixtures of mesophase pitch and nano-SiC particles
Materials Science and Engineering A 488 (2008) 514–518
Silicon carbide foams produced by siliciding carbon foams derived from mixtures of mesophase pitch and nano-SiC particles Yu Yang a,b , Quangui Guo a,∗ , Sizhong Li a,b , Song Zhao a,b , Jingli Shi a , Lang Liu a a
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China b Graduate University of the Chinese Academy of Sciences, Beijing 100039, China Received 21 August 2007; received in revised form 13 November 2007; accepted 13 November 2007
Abstract Silicon carbide foam with high bending strength and well oxidation resistance was prepared by infiltrating liquid silicon into carbon foam derived from mesophase pitch added with nano-SiC particles. Microstructure observation and phase identification of resultant foams were performed by scanning electron microscope and X-ray diffraction, respectively. Bending strength and bulk density of final SiC foams were also measured and calculated. Oxidation resistance of foams was appraised by the analysis of thermogravimetric curves of specimen in flowing air from room temperature to 1473 K. Results showed that addition of nano-SiC particles could significantly reduce the crystal size and compact struts during the infiltration of liquid silicon into carbon foams, which results in the increase of bending strength of silicon carbide foams. As the addition amount of nano-SiC was 15 wt%, silicon carbide foams with low bulk density of 0.54 g/cm3 were obtained, which possessed high bending strength of 11.96 MPa and excellent oxidation resistance. © 2007 Elsevier B.V. All rights reserved. Keywords: Silicon carbide foam; Carbon foam; Infiltrating silicon; Nano-SiC particles
1. Introduction Ceramic foams are high porosity, low-density materials which possess unique three-dimensional skeleton structure [1]. Silicon carbide foams are widely used as catalysis carriers, hightemperature insulation materials and filters for hot gases and molten metals due to high strength in high temperature, well thermal shock resistance, and excellent oxidation resistance [2,3]. Silicon carbide foams with bending strength of 2.87 MPa have been prepared from commercial SiC particles and polyurethane sponge by sintering after a coating process [4]. However, the sintering of SiC is so difficult because of the strong covalent bond [5] that the sintering temperature is above 2300 K even if various additives such as boron, alumina and yttria have been used [6]. Recently, biomorphic porous SiC ceramics prepared by chemical gas-phase infiltration into various charcoal have become a matter of increasing interest. Qian et al. produced a cellular silicon carbide ceramic from oak chip by reaction with silicon monoxide, which retains anisotropy distribution of oak tissue and possesses mean pore size less than 200 m [7–10]. ∗
Corresponding author. Tel.: +86 3514184106; fax: +86 3514083952. E-mail address: [email protected] (Q. Guo).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.11.034
Aoki and McEnaney gained open cell SiC foam from porous carbon preform derived from polymer foams by reaction with Si vapour [11]. Although much work has been done, the preparation of macroporous (above 500 m) silicon carbide foams with high strength and well oxidation resistance is still the aim that researchers persist in. As known, carbon foams derived from mesophase pitch (MP) possess uniform pore distribution and tailorable pore size [12]. If these carbon foams were used as templates, silicon carbide foams would be prepared by infiltration of molten silicon into these carbon foams. The resultant foams would hold uniform and controllable pore size. Furthermore, this method is more economical and feasible because of low reaction temperature and machinable template. In this work, silicon carbide foams with relatively high bending strength and oxidation resistance were prepared from mixtures of MP and nano-SiC particles, followed by foaming, carbonization, and infiltration of a liquid silicon. Nano-SiC particles with high specific surface area and good wetting ability to pitch were used to enhance the strength of silicon carbide foams. Effects of addition amount of nano-SiC on the structure and performance of as-prepared silicon carbide foams were investigated.
Y. Yang et al. / Materials Science and Engineering A 488 (2008) 514–518
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2. Experimental 2.1. Samples preparation In this research, the Mitsubishi naphthalene-based MP was used as the precursor of carbon foams. Nano-SiC particles with special surface area of 99 m2 /g and mean particle diameter of 20 nm were added into the melting MP at 570 K, followed by agitating, cooling. The grinded mixture was put in a pressure vessel and heated to 523 K at 4 K/min, and then heated to 723 K at 2 K/min in a nitrogen atmosphere at pressure up to 3 MPa, finally held at 723 K for 4 h. The resultant foams were carbonized at 1273 K for 5 h with slow heating rates less than 15 K/h in a nitrogen atmosphere. The carbon foam preforms with a dimension of 10 mm × 10 mm × 40 mm were put into graphite crucible and silicided via the infiltration of molten silicon at 1900 K under vacuum. 2.2. Samples characterization The pore volume fraction of the resultant foams was calculated by the Archimedes method. The microstructures of foams were characterized and analyzed by using scanning electron microscope (SEM) (JEOL JSM-6360LV). Three-point bending strength of each foam was measured by using an CMT4303 universal material testing system with a crosshead speed of 0.5 mm/min, and the bending strength value (σ f ) was counted by σ f = (3PL)/(2wt2 ), where P is the fracture stress of the samples, L is the span, w and t were the width and the height of the samples, respectively. Room temperature X-ray diffraction (XRD) measurements were conducted by using a Brucker-AXS D8 Advance vertical θ/2θ goniometer. The diffractometer utilized Cu K␣ radiation (40 kV and 40 mA). The crystallite dimensions of foams were calculated by using Scherrer equation to analyze the 1 1 1 diffraction peak. Kλ B cos θ where t was the average crystallite size in the sample, λ was the X-ray wavelength (0.15406 nm), B was the breadth of the diffraction peak (full width at half-maximum), and 2θ was the diffraction angle, K was 0.89 for given substance. Thermogravimetric analysis (NETZSCH STA 409) was applied to characterize the oxidation resistance of the samples. The resultant foams were tested from room temperature to 1473 K with a heating rate of 5 K/min in flowing air.
t=
3. Results and discussion
Fig. 1. Effects of nano-SiC content on the bulk density and pore volume fraction of SiC foams (b) and their precursors (a).
but pore volume fraction showed a reverse tendency. As nanoSiC content increased, MP content decreased and viscosity of the molten MP increased [13], which restrained the decomposing of MP [14] and made it difficult that bubbles grew and combined [15]. Especially, the agglomeration of nano-SiC particles was aggravated and the molten MP phase was destroyed when the addition amount of nano-SiC was above 20%, carbon foams with uniform pore distribution could not be obtained. So carbon foams with addition amount of nano-SiC above 20% were not discussed in this paper. As shown in Fig. 1b, the variety of bulk density and pore volume fraction of silicon carbide foams presents the same rules as that of their precursors with the increase of nano-SiC content, i.e. the bulk density and pore volume fraction of silicon carbide foams increased and decreased with the increase of nano-SiC content, respectively. At the same time, the bulk density and pore volume fraction of the resultant foams become, respectively higher and lower than that of carbon precursors because of a change of carbon into silicon carbide during the infiltration of the molten silicon.
3.1. Effects of nano-SiC content on the bulk density and pore volume fraction of SiC foams and their precursors
3.2. Microstructure
Fig. 1 shows the bulk density and pore volume fraction of silicon carbide foams and their precursors as a function of nanoSiC content. Fig. 1a indicates the bulk density of carbon foams as templates increased with the increase of nano-SiC content,
The effect of nano-SiC particles addition on the cell of silicon carbide foams is shown in Fig. 2. The structure of the silicon carbide foam without nano-SiC addition presents relatively loose cell walls composed of irregular crystal (Fig. 2a).
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Fig. 2. SEM photomicrographs of SiC foams of SCF1 series (SCF1-0, 5, 10, 15, 20, the addition amount of nano-SiC were 0%, 5, 10, 15 and 20 wt%, respectively).
As the nano-SiC mass fraction increases, SCF1-5 and SCF1-10 possess more compact struts and cell walls stacked by more uniform and smaller crystal (Fig. 2b and c). However, the structure of SCF1-20 with nano-SiC addition amount of 20 wt% (Fig. 2e) was looser than that of SCF1-15 with nano-SiC addition amount of 15 wt% (Fig. 2d). Ness and Page figured out the process of reaction sintering of C–Si is a resolving and secondary deposition process [16]. As it could be seen in Fig. 3, a little carbon can dissolve in liquid silicon above 1687 K, this endothermal process lowers the temperature in reaction interface, which accelerates the saturation of carbon and the deposition of SiC crystallite. On the other hand, the exothermic effect of SiC depositing facilitates the resolving of carbon. The silicon carbide foams were prepared by the interaction of resolving and secondary deposition. With addition of nano-SiC, SCF1-15 possessed more uniform and smaller crystal size than SCF1-0, which attributed to the pre-determined nucleation of nano-SiC particles as the crystal seeds. Nevertheless, excessive addition of nano-SiC would lead to particles’ agglomeration, which prevented the heterogeneous nucleation function of nano-SiC. So the structure of the SiC foam with nano-SiC addition amount of 20 wt% was looser than that of the SiC foam with nano-SiC addition amount of 15 wt%.
XRD revealed -SiC as the major phase with minor fraction of C, the amount of which increased with the increase of nanoSiC (Fig. 4). With addition of nano-SiC particles, growth and combination of bubbles became more difficult during molten MP foaming process, and close pore was formed in local area, which block up the infiltration and reaction of the liquid silicon. Accordingly, a small quantity of carbon without reaction with
Fig. 3. C/Si binary phase diagram [17].
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Fig. 4. XRD spectra of different reaction products.
silicon remained in resultant foams. The crystallite dimensions of foams were calculated by using Scherrer equation to analyze the 1 1 1 diffraction peak. The values of the average crystallite size (t) in different samples were calculated in Table 1. As it is shown, the value of the average crystallite size in SCF1-15 was smallest among this series, nano-SiC particles effectively reduced the average crystallite size during the reaction. 3.3. Influence of microstructure on the performances of silicon carbide foams 3.3.1. Bending strength The mechanical properties of open cell ceramic foams were complicated, which might depend on both bulk density and struts strength of foams through previous studies [18,19]. The struts strength of polycrystal SiC foams lied on mean crystal size and their microstructural arrangement in matrix, basing on the polycrystal materials fracture rule. Fig. 5 shows bulk density and bending strength of silicon carbide foams as a function of nano-SiC content. Bulk density increased with the increase of nano-SiC addition amount. However, bending strength of silicon carbide foams firstly increased and then decreased with the increase of nano-SiC addition amount.
The mechanical strength equation derived for a model cubic open cell foam was put forward by Gibson and Ashby [19]: 3/2 ρb σ = Kσs ρs where σ is the mechanical strength of the open cell foams, σ s is the theory strength of materials constituting foams, K is a constant, ρb and ρs were bulk density and strut density in foams, respectively. It was found bending strength of silicon carbide foams (σ) increased with the increase of the nano-SiC addition amount, which attributed to the increase of the relative density of foams (ρb /ρs ). On the other hand, the effects of the nano-SiC addition amount on strut strength in foams showed complicated rule. As the nano-SiC mass fraction increases, the resultant foams
Table 1 The value of the average crystallite size (t) in different samples SiC foams
t (nm)
SCF-0 SCF-5 SCF-15 SCF-20
57.76 58.57 32.19 54.32 Fig. 5. Bulk density and bending strength of SCF1 series.
518
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4. Conclusions
Fig. 6. Thermogravimetric curves of SCF1-0 and SCF1-15 specimens.
possess more compact struts and cell walls stacked by more uniform and smaller crystal, which would reduce flaws in foams and strengthen struts illustrated by the Hall–Petch equation. For polycrystal materials, the smaller the crystal size the higher the mechanical strength. However, excessive addition of nanoSiC would make against the foaming of the molten MP, which might lead to the increase of close pore and residual carbon. It increased flaws in foams and lowered the struts strength. So bending strength of silicon carbide foams first increased and then decreased with the increase of nano-SiC addition amount. 3.3.2. Oxidation resistance Fig. 6 shows weight changement of SCF1-0 and SCF1-15 specimens heated from room temperature to 1500 K with a heating rate of 5 K/min in flowing air, as a feature to test the oxidation resistance of the resultant foams. As shown in Fig. 6, weight gain of SCF1-0 in flowing air started at about 673 K and reached maximum at 1060 K, which could attribute to the oxidation of residual silicon. And then amorphous carbon embedded in residual silicon emerged and reacted with oxygen, which resulted in a weight loss from 1060 to 1370 K. Finally a few silicon carbide foam matrix was oxidated when the temperature was above 1370 K. It was shown that SCF1-15 exhibited higher oxidation resistance in the same condition. The oxidated matrix was so little that weight gain could hardly be observed, which should attribute to more compact struts preventing oxygen from infiltrating. The oxidation resistance of silicon carbide foam with nano-SiC addition amount of 15 wt% was excellent below 1500 K.
The preparation and performances of silicon carbide foam with high bending strength and well oxidation resistance were reported. It was illustrated that the addition amount of nanoSiC particles in MP foam template could significantly affect the structure and performances of silicon carbide foams such as bulk density, pore volume fraction, cell microstructure, bending strength and oxidation resistance. Pre-determined nucleation of nano-SiC particles as the crystal seeds during the infiltration of the molten silicon reduced crystal size and compacted the struts in silicon carbide foams, which would enhance bending strength of resultant foams. However, excessive addition of nano-SiC would lead to particles agglomeration, which made against the improvement of mechanical strength of foams. It was considered an attractive method that silicon carbide foams with high mechanical properties and good oxidation resistance were produced by siliciding carbon foams doped with nano-SiC particles. References [1] Y.H. Zhang, Mater. Res. Bull. 39 (2004) 755–761. [2] J.M. G´omez de Salazar, M.I. Barrena, G. Morales, L. Matesanz, N. Merino, Mater. Lett. 60 (2006) 1687–1692. [3] S.M. Zhu, S.Q. Ding, H.A. Xi, R.D. Wang, Mater. Lett. 59 (2005) 595–597. [4] Y. Liu, X.M. Yao, Z.R. Huang, S.M. Dong, D.L. Jiang, J. Chin. Ceram. Soc. 32 (2004) 107–112. [5] Y.X. Wang, S.H. Tan, D.L. Jiang, Carbon 42 (2004) 1833–1839. [6] A. Gubernat, L. Stobierski, P. Labaj, J. Eur. Ceram. Soc. 27 (2007) 781–789. [7] J.M. Qian, J.P. Wang, Z.H. Jin, G.J. Qiao, Mater. Sci. Eng. A 358 (2003) 304–309. [8] P. Greil, E. Vogli, T. Fey, A. Bezold, N. Popovska, H. Gerhard, et al., J. Eur. Ceram. Soc. 22 (2002) 2697–2707. [9] E. Vogli, J. Mukerji, C. Hoffman, R. Kladny, H. Sieber, P. Greil, J. Am. Ceram. Soc. 84 (2001) 1236–1240. [10] D.W. Shin, S.S. Park, Y.H. Choa, K. Niihara, J. Am. Ceram. Soc. 82 (1999) 3251–3253. [11] Y. Aoki, B. McEnaney, Br. Ceram. Trans. 94 (1995) 133–137. [12] J.W. Klett, R. Hardy, E. Romine, C. Walls, T. Burchell, Carbon 40 (2000) 953–973. [13] Q.H. Wu, J.A. Wu, Introduction to Polymer Rheology, China Chemical Industry Press, Beijing China, 1994. [14] Kearns, Process for preparing pitch foams, US Patent 5,868,974, (1999). [15] S.J. Neethling, H.T. Lee, P. Grassia, Colloids Surf. A 263 (2005) 184–196. [16] J.N. Ness, T.F. Page, J. Mater. Sci. 21 (1986) 1377–1397. [17] S. Er, Semiconductor in High Temperature: Silicon Carbide, vol. 2, Shanghai Science and Technology Press, Shanghai, China, 1962. [18] R. Brezny, D.J. Green, C.Q. Dam, J. Am. Ceram. Soc. 72 (1989) 885–889. [19] L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, 2nd ed., Cambridge University Press, Cambridge, UK, 1997.
Silicon carbide foams produced by siliciding carbon foams derived from mixtures of mesophase pitch and nano-SiC particles Yu Yang a,b , Quangui Guo a,∗ , Sizhong Li a,b , Song Zhao a,b , Jingli Shi a , Lang Liu a a
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China b Graduate University of the Chinese Academy of Sciences, Beijing 100039, China Received 21 August 2007; received in revised form 13 November 2007; accepted 13 November 2007
Abstract Silicon carbide foam with high bending strength and well oxidation resistance was prepared by infiltrating liquid silicon into carbon foam derived from mesophase pitch added with nano-SiC particles. Microstructure observation and phase identification of resultant foams were performed by scanning electron microscope and X-ray diffraction, respectively. Bending strength and bulk density of final SiC foams were also measured and calculated. Oxidation resistance of foams was appraised by the analysis of thermogravimetric curves of specimen in flowing air from room temperature to 1473 K. Results showed that addition of nano-SiC particles could significantly reduce the crystal size and compact struts during the infiltration of liquid silicon into carbon foams, which results in the increase of bending strength of silicon carbide foams. As the addition amount of nano-SiC was 15 wt%, silicon carbide foams with low bulk density of 0.54 g/cm3 were obtained, which possessed high bending strength of 11.96 MPa and excellent oxidation resistance. © 2007 Elsevier B.V. All rights reserved. Keywords: Silicon carbide foam; Carbon foam; Infiltrating silicon; Nano-SiC particles
1. Introduction Ceramic foams are high porosity, low-density materials which possess unique three-dimensional skeleton structure [1]. Silicon carbide foams are widely used as catalysis carriers, hightemperature insulation materials and filters for hot gases and molten metals due to high strength in high temperature, well thermal shock resistance, and excellent oxidation resistance [2,3]. Silicon carbide foams with bending strength of 2.87 MPa have been prepared from commercial SiC particles and polyurethane sponge by sintering after a coating process [4]. However, the sintering of SiC is so difficult because of the strong covalent bond [5] that the sintering temperature is above 2300 K even if various additives such as boron, alumina and yttria have been used [6]. Recently, biomorphic porous SiC ceramics prepared by chemical gas-phase infiltration into various charcoal have become a matter of increasing interest. Qian et al. produced a cellular silicon carbide ceramic from oak chip by reaction with silicon monoxide, which retains anisotropy distribution of oak tissue and possesses mean pore size less than 200 m [7–10]. ∗
Corresponding author. Tel.: +86 3514184106; fax: +86 3514083952. E-mail address: [email protected] (Q. Guo).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.11.034
Aoki and McEnaney gained open cell SiC foam from porous carbon preform derived from polymer foams by reaction with Si vapour [11]. Although much work has been done, the preparation of macroporous (above 500 m) silicon carbide foams with high strength and well oxidation resistance is still the aim that researchers persist in. As known, carbon foams derived from mesophase pitch (MP) possess uniform pore distribution and tailorable pore size [12]. If these carbon foams were used as templates, silicon carbide foams would be prepared by infiltration of molten silicon into these carbon foams. The resultant foams would hold uniform and controllable pore size. Furthermore, this method is more economical and feasible because of low reaction temperature and machinable template. In this work, silicon carbide foams with relatively high bending strength and oxidation resistance were prepared from mixtures of MP and nano-SiC particles, followed by foaming, carbonization, and infiltration of a liquid silicon. Nano-SiC particles with high specific surface area and good wetting ability to pitch were used to enhance the strength of silicon carbide foams. Effects of addition amount of nano-SiC on the structure and performance of as-prepared silicon carbide foams were investigated.
Y. Yang et al. / Materials Science and Engineering A 488 (2008) 514–518
515
2. Experimental 2.1. Samples preparation In this research, the Mitsubishi naphthalene-based MP was used as the precursor of carbon foams. Nano-SiC particles with special surface area of 99 m2 /g and mean particle diameter of 20 nm were added into the melting MP at 570 K, followed by agitating, cooling. The grinded mixture was put in a pressure vessel and heated to 523 K at 4 K/min, and then heated to 723 K at 2 K/min in a nitrogen atmosphere at pressure up to 3 MPa, finally held at 723 K for 4 h. The resultant foams were carbonized at 1273 K for 5 h with slow heating rates less than 15 K/h in a nitrogen atmosphere. The carbon foam preforms with a dimension of 10 mm × 10 mm × 40 mm were put into graphite crucible and silicided via the infiltration of molten silicon at 1900 K under vacuum. 2.2. Samples characterization The pore volume fraction of the resultant foams was calculated by the Archimedes method. The microstructures of foams were characterized and analyzed by using scanning electron microscope (SEM) (JEOL JSM-6360LV). Three-point bending strength of each foam was measured by using an CMT4303 universal material testing system with a crosshead speed of 0.5 mm/min, and the bending strength value (σ f ) was counted by σ f = (3PL)/(2wt2 ), where P is the fracture stress of the samples, L is the span, w and t were the width and the height of the samples, respectively. Room temperature X-ray diffraction (XRD) measurements were conducted by using a Brucker-AXS D8 Advance vertical θ/2θ goniometer. The diffractometer utilized Cu K␣ radiation (40 kV and 40 mA). The crystallite dimensions of foams were calculated by using Scherrer equation to analyze the 1 1 1 diffraction peak. Kλ B cos θ where t was the average crystallite size in the sample, λ was the X-ray wavelength (0.15406 nm), B was the breadth of the diffraction peak (full width at half-maximum), and 2θ was the diffraction angle, K was 0.89 for given substance. Thermogravimetric analysis (NETZSCH STA 409) was applied to characterize the oxidation resistance of the samples. The resultant foams were tested from room temperature to 1473 K with a heating rate of 5 K/min in flowing air.
t=
3. Results and discussion
Fig. 1. Effects of nano-SiC content on the bulk density and pore volume fraction of SiC foams (b) and their precursors (a).
but pore volume fraction showed a reverse tendency. As nanoSiC content increased, MP content decreased and viscosity of the molten MP increased [13], which restrained the decomposing of MP [14] and made it difficult that bubbles grew and combined [15]. Especially, the agglomeration of nano-SiC particles was aggravated and the molten MP phase was destroyed when the addition amount of nano-SiC was above 20%, carbon foams with uniform pore distribution could not be obtained. So carbon foams with addition amount of nano-SiC above 20% were not discussed in this paper. As shown in Fig. 1b, the variety of bulk density and pore volume fraction of silicon carbide foams presents the same rules as that of their precursors with the increase of nano-SiC content, i.e. the bulk density and pore volume fraction of silicon carbide foams increased and decreased with the increase of nano-SiC content, respectively. At the same time, the bulk density and pore volume fraction of the resultant foams become, respectively higher and lower than that of carbon precursors because of a change of carbon into silicon carbide during the infiltration of the molten silicon.
3.1. Effects of nano-SiC content on the bulk density and pore volume fraction of SiC foams and their precursors
3.2. Microstructure
Fig. 1 shows the bulk density and pore volume fraction of silicon carbide foams and their precursors as a function of nanoSiC content. Fig. 1a indicates the bulk density of carbon foams as templates increased with the increase of nano-SiC content,
The effect of nano-SiC particles addition on the cell of silicon carbide foams is shown in Fig. 2. The structure of the silicon carbide foam without nano-SiC addition presents relatively loose cell walls composed of irregular crystal (Fig. 2a).
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Fig. 2. SEM photomicrographs of SiC foams of SCF1 series (SCF1-0, 5, 10, 15, 20, the addition amount of nano-SiC were 0%, 5, 10, 15 and 20 wt%, respectively).
As the nano-SiC mass fraction increases, SCF1-5 and SCF1-10 possess more compact struts and cell walls stacked by more uniform and smaller crystal (Fig. 2b and c). However, the structure of SCF1-20 with nano-SiC addition amount of 20 wt% (Fig. 2e) was looser than that of SCF1-15 with nano-SiC addition amount of 15 wt% (Fig. 2d). Ness and Page figured out the process of reaction sintering of C–Si is a resolving and secondary deposition process [16]. As it could be seen in Fig. 3, a little carbon can dissolve in liquid silicon above 1687 K, this endothermal process lowers the temperature in reaction interface, which accelerates the saturation of carbon and the deposition of SiC crystallite. On the other hand, the exothermic effect of SiC depositing facilitates the resolving of carbon. The silicon carbide foams were prepared by the interaction of resolving and secondary deposition. With addition of nano-SiC, SCF1-15 possessed more uniform and smaller crystal size than SCF1-0, which attributed to the pre-determined nucleation of nano-SiC particles as the crystal seeds. Nevertheless, excessive addition of nano-SiC would lead to particles’ agglomeration, which prevented the heterogeneous nucleation function of nano-SiC. So the structure of the SiC foam with nano-SiC addition amount of 20 wt% was looser than that of the SiC foam with nano-SiC addition amount of 15 wt%.
XRD revealed -SiC as the major phase with minor fraction of C, the amount of which increased with the increase of nanoSiC (Fig. 4). With addition of nano-SiC particles, growth and combination of bubbles became more difficult during molten MP foaming process, and close pore was formed in local area, which block up the infiltration and reaction of the liquid silicon. Accordingly, a small quantity of carbon without reaction with
Fig. 3. C/Si binary phase diagram [17].
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Fig. 4. XRD spectra of different reaction products.
silicon remained in resultant foams. The crystallite dimensions of foams were calculated by using Scherrer equation to analyze the 1 1 1 diffraction peak. The values of the average crystallite size (t) in different samples were calculated in Table 1. As it is shown, the value of the average crystallite size in SCF1-15 was smallest among this series, nano-SiC particles effectively reduced the average crystallite size during the reaction. 3.3. Influence of microstructure on the performances of silicon carbide foams 3.3.1. Bending strength The mechanical properties of open cell ceramic foams were complicated, which might depend on both bulk density and struts strength of foams through previous studies [18,19]. The struts strength of polycrystal SiC foams lied on mean crystal size and their microstructural arrangement in matrix, basing on the polycrystal materials fracture rule. Fig. 5 shows bulk density and bending strength of silicon carbide foams as a function of nano-SiC content. Bulk density increased with the increase of nano-SiC addition amount. However, bending strength of silicon carbide foams firstly increased and then decreased with the increase of nano-SiC addition amount.
The mechanical strength equation derived for a model cubic open cell foam was put forward by Gibson and Ashby [19]: 3/2 ρb σ = Kσs ρs where σ is the mechanical strength of the open cell foams, σ s is the theory strength of materials constituting foams, K is a constant, ρb and ρs were bulk density and strut density in foams, respectively. It was found bending strength of silicon carbide foams (σ) increased with the increase of the nano-SiC addition amount, which attributed to the increase of the relative density of foams (ρb /ρs ). On the other hand, the effects of the nano-SiC addition amount on strut strength in foams showed complicated rule. As the nano-SiC mass fraction increases, the resultant foams
Table 1 The value of the average crystallite size (t) in different samples SiC foams
t (nm)
SCF-0 SCF-5 SCF-15 SCF-20
57.76 58.57 32.19 54.32 Fig. 5. Bulk density and bending strength of SCF1 series.
518
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4. Conclusions
Fig. 6. Thermogravimetric curves of SCF1-0 and SCF1-15 specimens.
possess more compact struts and cell walls stacked by more uniform and smaller crystal, which would reduce flaws in foams and strengthen struts illustrated by the Hall–Petch equation. For polycrystal materials, the smaller the crystal size the higher the mechanical strength. However, excessive addition of nanoSiC would make against the foaming of the molten MP, which might lead to the increase of close pore and residual carbon. It increased flaws in foams and lowered the struts strength. So bending strength of silicon carbide foams first increased and then decreased with the increase of nano-SiC addition amount. 3.3.2. Oxidation resistance Fig. 6 shows weight changement of SCF1-0 and SCF1-15 specimens heated from room temperature to 1500 K with a heating rate of 5 K/min in flowing air, as a feature to test the oxidation resistance of the resultant foams. As shown in Fig. 6, weight gain of SCF1-0 in flowing air started at about 673 K and reached maximum at 1060 K, which could attribute to the oxidation of residual silicon. And then amorphous carbon embedded in residual silicon emerged and reacted with oxygen, which resulted in a weight loss from 1060 to 1370 K. Finally a few silicon carbide foam matrix was oxidated when the temperature was above 1370 K. It was shown that SCF1-15 exhibited higher oxidation resistance in the same condition. The oxidated matrix was so little that weight gain could hardly be observed, which should attribute to more compact struts preventing oxygen from infiltrating. The oxidation resistance of silicon carbide foam with nano-SiC addition amount of 15 wt% was excellent below 1500 K.
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