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The effect of environment pressure on high temperature stability of silicon oxycarbide glasses derived from polysiloxane
Materials Letters 65 (2011) 1538–1541
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
The effect of environment pressure on high temperature stability of silicon oxycarbide glasses derived from polysiloxane Tianheng Xu, Qingsong Ma ⁎, Zhaohui Chen Key Laboratory of Advanced Ceramic Fibres and Composites, College of Aerospace and Materials Engineering, National University of Defense Technology, Changsha 410073, PR China
a r t i c l e
i n f o
Article history: Received 10 December 2010 Accepted 20 February 2011 Available online 25 February 2011 Keywords: Silicon oxycarbide Atmosphere materials Thermal properties Degradation
a b s t r a c t Silicon oxycarbide glasses (SiOC) have been produced by siloxane resin under flowing argon atmosphere at 1000 °C. Those glasses were further annealed at 1200, 1300, 1400, and 1500 °C under the pressure of 0.01 KPa and 101 KPa, respectively, to investigate the effect of environment pressure on their high temperature 29 stability. The two series of glasses were characterized by X-ray diffraction, SiMASNMR, and chemical element analysis. The environment pressure plays a crucial role on high temperature stability of SiOC glasses by promoting or hindering bonds redistribution and carbothermal reductions. It can be found that SiOC glasses exhibit enhanced thermal stability in high pressure environment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Silicon oxycarbide glasses can be considered as anionic modification of silica glass in which Si–O bonds are partially replaced by Si–C bonds. This modification is expected to improve the thermal and mechanical properties suitable for applications at high temperatures [1,2]. These glasses have potential applications as catalyst supports [3], bloodcontacting device [4], matrices or fibers for ceramic matrix composites (CMCs) [5], and anodes in lithium ion rechargeable batteries [6]. SiOC glasses can be prepared by pyrolysis of polysiloxanes or sol-gel derived precursors. Its composition is usually reported in the following equation: SiCxO2(1 − x) + yCfree, where SiCxO2(1 − x) describes the amorphous silicon oxycarbide network and Cfree is free carbon. At high temperatures, SiOC glasses undergo a redistribution of Si–O bonds and Si–C bonds leading to the formation of SiC and SiO2; in the same temperature range, carbothermal reductions may be active leading to the weight loss and to a subsequent deterioration of the mechanical properties [7–9]. Because some applications (such as components for advanced space launch vehicles) concern high temperature utilization, there is a strong need to understand their high temperature behavior in various environments. Many factors have been shown to affect the high temperature stability such as composition of SiOC glasses and particularly of the content of extra free carbon [10], samples size [10], type of precursor [11], microstructure (porosity and pore size distribu-
⁎ Corresponding author. Tel.: +86 731 84573169; fax: +86 731 84573165. E-mail address: [email protected] (Q. Ma). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.065
tion) [12], pyrolysis atmosphere (materials of crucible and furnace) [10], and CO partial pressure [13]. The last two are environmental factors. In this paper, we report the study of an environmental factor influencing the high temperature stability: the environment pressure. To reduce fluctuation of pressure and composition of inert gas in furnace during heat treatment, small amount of SiOC glasses were heat treated in a big furnace in each run. 2. Experimental Commercial available polymethyl(phenyl)siloxane resin (Dow Corning 249 flake resin) was cross-linked at 250 °C in air for 8 h, ball milled, and finally sieved to particle size b75 μm. Green bodies were obtained by cold pressing of powder at 130 MPa. SiOC glasses were obtained through pyrolysis of green bodies in flowing argon with a heat rate of 20 °C/min up to 1000 °C held for 1 h. Then, these glasses were heat treated, loading 2 g of bulk in graphite crucibles, at 1200, 1300, 1400, and 1500 °C under Ar atmosphere in a graphite furnace (diameter, 60 cm; depth, 70 cm) for 1 h at a heating rate of 15 °C/min. The pressures in furnace were 0.01 KPa and 101 KPa, respectively. Quantitative elemental analysis (EA) of the samples was performed on LECO CS600 for C and TCH600 for O. The perchloric acid dehydration gravimetric method was adopted for the determination of Si content. X-ray diffraction (Bruker D8 advance) with Cu Kα radiation was used 29 to verify the crystal phases. Si MAS NMR spectra of the two serials samples (heat treated in 0.01 KPa and 101 KPa) were measured with a Varian Infinity Plus 300 NMR spectrometer, 59.56 MHz. The reference materials for chemical shift were tetramethysilane (TMS), and its chemical shift was adjusted to 0 ppm. Spinning rates of the samples at a magic angle was 4.0 KHz and recycle time was 30 s.
T. Xu et al. / Materials Letters 65 (2011) 1538–1541 Table 1 Elemental analysis and char yield measured on samples anneal for 1 h at given temperature. Sample
Raw sample SiOC(101 KPa)
SiOC(0.01 KPa)
Temperature (°C)
Char yield (%)
Composition Si (wt.%)
O (wt.%)
C (wt.%)
1000 1200 1300 1400 1500 1200 1300 1400 1500
– 99.90 99.92 99.55 96.26 98.67 82.44 55.34 47.40
36.81 36.52 35.48 35.99 36.69 36.47 41.73 62.71 66.12
27.14 27.37 27.01 26.53 26.49 27.13 22.85 6.41 0.53
35.10 34.95 35.15 36.13 35.36 35.32 34.56 30.26 32.37
3. Results and discussion The char yield and elemental analysis (EA) results for the two series of samples heat treated in 0.01 KPa and 101 KPa (called SiOC (101 KPa) and SiOC(0.01 KPa)) are reported in Table 1. It shows clearly that the high temperature stability is related to the environment pressure. Indeed, the SiOC(101 KPa) displays a lower value of weight loss (≈4%) and indiscernible oxygen content changes from 1000 to 1500 °C. On the other hand, the SiOC(0.01 KPa) show a higher value of weight loss (≈53%), and the oxygen has been almost removed from 1000 to 1500 °C. It is worth noting that char yield and chemical composition values of SiOC(0.01 KPa) are almost constant up to 1200 °C. Once temperature is higher than 1300 °C, obvious changes of char yield and chemical composition can be observed. The degradation process is directly related to carbothermal reductions occurring between SiO2 and free carbon. The overall reaction usually proposed for carbothermal process is as follows:
1539
(Fig. 2). The β-SiC phase and amorphous SiO2 (broad halo centered at ~ 22°) is present at 1300 °C. Up to 1400 °C, only β-SiC phase can be observed. On the other hand, the SiOC(101 KPa) are amorphous oxycarbide glasses from 1000 to 1400 °C, and a slight crystallization of β-SiC can be observed at 1500 °C. 29 The Si MAS NMR spectra obtained from the two series of samples are reported in Fig. 3. Depending on the annealing temperature, SiOC exhibit various amounts of the SiOxC4 − x tetrahedral sites (SiO4 ~−108 ppm, SiCO3 ~−74 ppm, SiC2O2 ~−34 ppm, SiC4 ~−16 ppm) [14]. At 1000 °C, as-received SiOC samples consist primarily of SiO4, SiO3C, and SiO2C2, with a smaller amount of, or without, SiOC3 and SiC4 units. On increasing the annealing temperature, the NMR data show a redistribution of Si–C and Si–O bonds as seen by a decrease in SiCxO4 − x (where 1 ≤x ≤ 2) with respect to SiC4 and SiO4 units. For SiOC(101 KPa), the evolution is gradual starting from 1000 °C and going up to 1500 °C. SiOC(0.01 KPa) exhibits different peak evolutions. (i) Bonds redistribution are starting from 1000 °C and almost completing at 1400 °C. (ii) At temperature above 1300 °C, a narrow peak can be observed at around −16 ppm. It is even possible to distinguish several peaks at −16, −20, and −25 ppm, which are due to the presence of crystalline β-SiC, with some α-SiC. 29 The compositions of the silicon oxycarbide network detected by Si MAS NMR experiments are reported in Table 2. The oxygen content within silicon oxycarbide network seems overestimated in the NMR spectra compared with the EA results. However, comparison between samples can be made to extract general trends. (i) For SiOC(0.01 KPa), the decreasing contents of SiO4 units can be observed at temperature higher than 1300 °C. This evolution is related to the carbothermal reductions occurring between SiO2 and free carbon. (ii) For SiOC (101 KPa), The carbothermal reductions are indiscernible at temperature below 1500 °C. It accords with the results of EA discussed above. Carbothermal process depicted in reaction (1) is endothermic and should proceed in the following steps [15]:
ð1Þ
SiO2 ðsÞ þ CðsÞ→SiOðgÞ þ COðgÞ
ð2Þ
The XRD patterns obtained from the two series of samples after heat-treated at various temperatures are reported in Fig. 1. The SiOC (0.01 KPa) samples are amorphous oxycarbide glasses below 1200 °C
SiOðgÞ þ 2CðsÞ→SiCðsÞ þ COðgÞ
ð3Þ
SiO2 ðsÞ þ 3CðsÞ→SiCðsÞ þ 2COðgÞ
Fig. 1. XRD patterns for SiOC samples treated with various pressures. (a) 0.01 KPa and (b) 101 Kpa SEM micrographs examined from the two series of samples are reported in Fig. 2. Both SiOC(101 KPa) and SiOC(0.01 KPa) showed smooth fracture surfaces at lower temperature (1200 °C). For SiOC(0.01 KPa), rough fracture surfaces composed of “particles” with a diameter bellow 100 nm can be observed at temperature above 1300 °C. These observations indicated that samples treated at 1200 °C are dense materials whereas samples treated above 1300 °C are porous materials. This evolution is resulted from the weight loss during carbothermal process. For SiOC(101 KPa), changes in fracture surfaces morphology are indiscernible from 1200 °C to 1500 °C.
1540
T. Xu et al. / Materials Letters 65 (2011) 1538–1541
Fig. 2. SEM micrographs of SiOC samples treated with various pressures. (a) 0.01 KPa and (b) 101 KPa.
The temperature, at which the reaction (2) is initiated, is strongly dependent on pressure. It is governed by Eq. (4). 0 0 ΔG = ΔG + ΔnRTln p = p
ð4Þ
where ΔG 0 = GSiO + G CO − G SiO2 − G C , Δn = 2, R = gas constant, T = temperature, p = pressure. According to Eq. (4), a higher pressure will shift the start of reaction (2) to higher temperatures. For example, when ΔG = 0, the temperature is 1110 °C in pressure of 0.01 KPa whereas the temperature will shift to 1749 °C in pressure of 101 KPa. In higher pressure environment, when reaction (2) is hindered, the carbothermal reduction will be also inhibited. For SiCxO2(1 − x) network, at high temperatures, both reaction (5) [7] and reaction (6) [16] will be active. SiCx O2ð1−xÞ →SiO2 þ SiC
ð5Þ
SiCx O2ð1−xÞ →SiC þ SiOðgÞ þ COðgÞ
ð6Þ
Similar with reaction (2), a high pressure will also shift the start of reaction (6) to higher temperatures. In higher pressure environment, reaction (2) is inhabited and thus minimizes the decomposition of
29
SiCxO2(1 − x) network. As discussions above, high pressure will hinder carbothermal process and minimize the decomposition of SiCxO2(1 − x) network. Thus, SiOC glasses exhibit enhanced thermal stability in higher pressure environment. 4. Conclusion In summary, the effect of environment pressure on high temperature stability of SiOC glasses has been studied. The environment pressure plays a crucial role on high temperature stability of SiOC glasses by promoting or hindering bonds redistribution and carbothermal reductions. In lower pressure environment (0.01 KPa), bonds redistribution is starting from 1000 °C and going up to 1400 °C. Carbothermal reductions are active at 1300 °C and almost completed at 1500 °C. In higher pressure environment (101 KPa), bonds redistribution occurs between 1000 °C and 1500 °C; in same temperature range, carbothermal reductions are not active. Acknowledgments This study was supported by the Hunan Provincial Natural Science Foundation of China (no. S2010J504B), National Defense Preliminary
Fig. 3. SiMAS NMR spectra for SiOC samples treated with various pressures. (a) 0.01 KPa and (b) 101 Kpa.
T. Xu et al. / Materials Letters 65 (2011) 1538–1541
1541
Table 2 Quantitative analysis determined for Si species. Sample
Raw sample SiOC(101 KPa)
SiOC(0.01 KPa)
Temperature (°C)
1000 1200 1300 1400 1500 1200 1300 1400 1500
Si species (%)
SiOC composition
SiO4
SiO3C
SiO2C2
SiC4
From NMR
From EA
52 58 64 74 87 66 54 15 NA
36 35 20 6 NA 28 22 NA NA
12 7 13 7 NA 6 5 NA NA
NA NA 3 13 13 NA 19 85 100
SiO1.70C0.15 SiO1.76C0.12 SiO1.71C0.14 SiO1.64C0.18 SiO1.74C0.13 SiO1.80C0.10 SiO1.46C0.27 SiO0.30C0.85 SiO0.00C1.00
SiO1.29C0.35 SiO1.31C0.34 SiO1.33C0.33 SiO1.29C0.35 SiO1.26C0.37 SiO1.30C0.35 SiO0.96C0.52 SiO0.18C0.91 SiO0.01C0.99
NA indicates cannot be detected by NMR.
Research Program of China (no. 9140C8203040904), and Natural Science Foundation of China (no. 90916002 and no. 50802113). References [1] [2] [3] [4] [5] [6]
Renlund G, Prochazka S, Doremus RH. J Mater Res 1991;6:2723–34. Pantano C, Singh A, Zhang H. J Sol-Gel Sci Tech 1999;14:7–25. Singh AK, Pantano CG. J Am Ceram Soc 1996;79:2696–704. Zhuo R, Colombo P, Pantano C. Acta Biomater 2005;1:583–9. Adler PN. 41st International SAMPE Symposium; March 24–28 1996. Xing W, Wilson AM, Eguchi K, Zank GA, Dahn JR. J Electrochem Soc 1997;144:2410–6.
[7] Brequel H, Parmentier J, Walter S, Badheka R, Trimmel G, Masse S, et al. Chem Mater 2004;16:2585–98. [8] Latournerie J, Dempsey P, Bahloul DH. J Am Ceram Soc 2006;89(5):1485–91. [9] Wei Q, Pippel E, Woltersdorf J, Scheffler M, Greil P. Mater Chem Phys 2002;73: 281–9. [10] Sorarù GD, Suttor D. J Sol-Gel Sci Tech 1999;14:69–74. [11] Sorarù GD, Liu Q, Interrante LV, Apple T. Chem Mater 1998;10:4047–54. [12] Parmentier J, Sorarù GD, Babonneau F. J Eur Ceram Soc 2001;21:817–24. [13] Bibbo GS, Benson PM, Pantano CG. J Mater Sci 1991;26:5075. [14] Lipowitz J, Freeman HA, Chen RT, Prack ER. Adv Ceram Mater 1987;2(2):121–8. [15] Martin H-P, Ecke R, Muller E. J Eur Ceram Soc 1998;18:1737–42. [16] Shimoo T, Okamura K, Takeuchi H. J Mater Sci 2003;38:4973–9.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
The effect of environment pressure on high temperature stability of silicon oxycarbide glasses derived from polysiloxane Tianheng Xu, Qingsong Ma ⁎, Zhaohui Chen Key Laboratory of Advanced Ceramic Fibres and Composites, College of Aerospace and Materials Engineering, National University of Defense Technology, Changsha 410073, PR China
a r t i c l e
i n f o
Article history: Received 10 December 2010 Accepted 20 February 2011 Available online 25 February 2011 Keywords: Silicon oxycarbide Atmosphere materials Thermal properties Degradation
a b s t r a c t Silicon oxycarbide glasses (SiOC) have been produced by siloxane resin under flowing argon atmosphere at 1000 °C. Those glasses were further annealed at 1200, 1300, 1400, and 1500 °C under the pressure of 0.01 KPa and 101 KPa, respectively, to investigate the effect of environment pressure on their high temperature 29 stability. The two series of glasses were characterized by X-ray diffraction, SiMASNMR, and chemical element analysis. The environment pressure plays a crucial role on high temperature stability of SiOC glasses by promoting or hindering bonds redistribution and carbothermal reductions. It can be found that SiOC glasses exhibit enhanced thermal stability in high pressure environment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Silicon oxycarbide glasses can be considered as anionic modification of silica glass in which Si–O bonds are partially replaced by Si–C bonds. This modification is expected to improve the thermal and mechanical properties suitable for applications at high temperatures [1,2]. These glasses have potential applications as catalyst supports [3], bloodcontacting device [4], matrices or fibers for ceramic matrix composites (CMCs) [5], and anodes in lithium ion rechargeable batteries [6]. SiOC glasses can be prepared by pyrolysis of polysiloxanes or sol-gel derived precursors. Its composition is usually reported in the following equation: SiCxO2(1 − x) + yCfree, where SiCxO2(1 − x) describes the amorphous silicon oxycarbide network and Cfree is free carbon. At high temperatures, SiOC glasses undergo a redistribution of Si–O bonds and Si–C bonds leading to the formation of SiC and SiO2; in the same temperature range, carbothermal reductions may be active leading to the weight loss and to a subsequent deterioration of the mechanical properties [7–9]. Because some applications (such as components for advanced space launch vehicles) concern high temperature utilization, there is a strong need to understand their high temperature behavior in various environments. Many factors have been shown to affect the high temperature stability such as composition of SiOC glasses and particularly of the content of extra free carbon [10], samples size [10], type of precursor [11], microstructure (porosity and pore size distribu-
⁎ Corresponding author. Tel.: +86 731 84573169; fax: +86 731 84573165. E-mail address: [email protected] (Q. Ma). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.065
tion) [12], pyrolysis atmosphere (materials of crucible and furnace) [10], and CO partial pressure [13]. The last two are environmental factors. In this paper, we report the study of an environmental factor influencing the high temperature stability: the environment pressure. To reduce fluctuation of pressure and composition of inert gas in furnace during heat treatment, small amount of SiOC glasses were heat treated in a big furnace in each run. 2. Experimental Commercial available polymethyl(phenyl)siloxane resin (Dow Corning 249 flake resin) was cross-linked at 250 °C in air for 8 h, ball milled, and finally sieved to particle size b75 μm. Green bodies were obtained by cold pressing of powder at 130 MPa. SiOC glasses were obtained through pyrolysis of green bodies in flowing argon with a heat rate of 20 °C/min up to 1000 °C held for 1 h. Then, these glasses were heat treated, loading 2 g of bulk in graphite crucibles, at 1200, 1300, 1400, and 1500 °C under Ar atmosphere in a graphite furnace (diameter, 60 cm; depth, 70 cm) for 1 h at a heating rate of 15 °C/min. The pressures in furnace were 0.01 KPa and 101 KPa, respectively. Quantitative elemental analysis (EA) of the samples was performed on LECO CS600 for C and TCH600 for O. The perchloric acid dehydration gravimetric method was adopted for the determination of Si content. X-ray diffraction (Bruker D8 advance) with Cu Kα radiation was used 29 to verify the crystal phases. Si MAS NMR spectra of the two serials samples (heat treated in 0.01 KPa and 101 KPa) were measured with a Varian Infinity Plus 300 NMR spectrometer, 59.56 MHz. The reference materials for chemical shift were tetramethysilane (TMS), and its chemical shift was adjusted to 0 ppm. Spinning rates of the samples at a magic angle was 4.0 KHz and recycle time was 30 s.
T. Xu et al. / Materials Letters 65 (2011) 1538–1541 Table 1 Elemental analysis and char yield measured on samples anneal for 1 h at given temperature. Sample
Raw sample SiOC(101 KPa)
SiOC(0.01 KPa)
Temperature (°C)
Char yield (%)
Composition Si (wt.%)
O (wt.%)
C (wt.%)
1000 1200 1300 1400 1500 1200 1300 1400 1500
– 99.90 99.92 99.55 96.26 98.67 82.44 55.34 47.40
36.81 36.52 35.48 35.99 36.69 36.47 41.73 62.71 66.12
27.14 27.37 27.01 26.53 26.49 27.13 22.85 6.41 0.53
35.10 34.95 35.15 36.13 35.36 35.32 34.56 30.26 32.37
3. Results and discussion The char yield and elemental analysis (EA) results for the two series of samples heat treated in 0.01 KPa and 101 KPa (called SiOC (101 KPa) and SiOC(0.01 KPa)) are reported in Table 1. It shows clearly that the high temperature stability is related to the environment pressure. Indeed, the SiOC(101 KPa) displays a lower value of weight loss (≈4%) and indiscernible oxygen content changes from 1000 to 1500 °C. On the other hand, the SiOC(0.01 KPa) show a higher value of weight loss (≈53%), and the oxygen has been almost removed from 1000 to 1500 °C. It is worth noting that char yield and chemical composition values of SiOC(0.01 KPa) are almost constant up to 1200 °C. Once temperature is higher than 1300 °C, obvious changes of char yield and chemical composition can be observed. The degradation process is directly related to carbothermal reductions occurring between SiO2 and free carbon. The overall reaction usually proposed for carbothermal process is as follows:
1539
(Fig. 2). The β-SiC phase and amorphous SiO2 (broad halo centered at ~ 22°) is present at 1300 °C. Up to 1400 °C, only β-SiC phase can be observed. On the other hand, the SiOC(101 KPa) are amorphous oxycarbide glasses from 1000 to 1400 °C, and a slight crystallization of β-SiC can be observed at 1500 °C. 29 The Si MAS NMR spectra obtained from the two series of samples are reported in Fig. 3. Depending on the annealing temperature, SiOC exhibit various amounts of the SiOxC4 − x tetrahedral sites (SiO4 ~−108 ppm, SiCO3 ~−74 ppm, SiC2O2 ~−34 ppm, SiC4 ~−16 ppm) [14]. At 1000 °C, as-received SiOC samples consist primarily of SiO4, SiO3C, and SiO2C2, with a smaller amount of, or without, SiOC3 and SiC4 units. On increasing the annealing temperature, the NMR data show a redistribution of Si–C and Si–O bonds as seen by a decrease in SiCxO4 − x (where 1 ≤x ≤ 2) with respect to SiC4 and SiO4 units. For SiOC(101 KPa), the evolution is gradual starting from 1000 °C and going up to 1500 °C. SiOC(0.01 KPa) exhibits different peak evolutions. (i) Bonds redistribution are starting from 1000 °C and almost completing at 1400 °C. (ii) At temperature above 1300 °C, a narrow peak can be observed at around −16 ppm. It is even possible to distinguish several peaks at −16, −20, and −25 ppm, which are due to the presence of crystalline β-SiC, with some α-SiC. 29 The compositions of the silicon oxycarbide network detected by Si MAS NMR experiments are reported in Table 2. The oxygen content within silicon oxycarbide network seems overestimated in the NMR spectra compared with the EA results. However, comparison between samples can be made to extract general trends. (i) For SiOC(0.01 KPa), the decreasing contents of SiO4 units can be observed at temperature higher than 1300 °C. This evolution is related to the carbothermal reductions occurring between SiO2 and free carbon. (ii) For SiOC (101 KPa), The carbothermal reductions are indiscernible at temperature below 1500 °C. It accords with the results of EA discussed above. Carbothermal process depicted in reaction (1) is endothermic and should proceed in the following steps [15]:
ð1Þ
SiO2 ðsÞ þ CðsÞ→SiOðgÞ þ COðgÞ
ð2Þ
The XRD patterns obtained from the two series of samples after heat-treated at various temperatures are reported in Fig. 1. The SiOC (0.01 KPa) samples are amorphous oxycarbide glasses below 1200 °C
SiOðgÞ þ 2CðsÞ→SiCðsÞ þ COðgÞ
ð3Þ
SiO2 ðsÞ þ 3CðsÞ→SiCðsÞ þ 2COðgÞ
Fig. 1. XRD patterns for SiOC samples treated with various pressures. (a) 0.01 KPa and (b) 101 Kpa SEM micrographs examined from the two series of samples are reported in Fig. 2. Both SiOC(101 KPa) and SiOC(0.01 KPa) showed smooth fracture surfaces at lower temperature (1200 °C). For SiOC(0.01 KPa), rough fracture surfaces composed of “particles” with a diameter bellow 100 nm can be observed at temperature above 1300 °C. These observations indicated that samples treated at 1200 °C are dense materials whereas samples treated above 1300 °C are porous materials. This evolution is resulted from the weight loss during carbothermal process. For SiOC(101 KPa), changes in fracture surfaces morphology are indiscernible from 1200 °C to 1500 °C.
1540
T. Xu et al. / Materials Letters 65 (2011) 1538–1541
Fig. 2. SEM micrographs of SiOC samples treated with various pressures. (a) 0.01 KPa and (b) 101 KPa.
The temperature, at which the reaction (2) is initiated, is strongly dependent on pressure. It is governed by Eq. (4). 0 0 ΔG = ΔG + ΔnRTln p = p
ð4Þ
where ΔG 0 = GSiO + G CO − G SiO2 − G C , Δn = 2, R = gas constant, T = temperature, p = pressure. According to Eq. (4), a higher pressure will shift the start of reaction (2) to higher temperatures. For example, when ΔG = 0, the temperature is 1110 °C in pressure of 0.01 KPa whereas the temperature will shift to 1749 °C in pressure of 101 KPa. In higher pressure environment, when reaction (2) is hindered, the carbothermal reduction will be also inhibited. For SiCxO2(1 − x) network, at high temperatures, both reaction (5) [7] and reaction (6) [16] will be active. SiCx O2ð1−xÞ →SiO2 þ SiC
ð5Þ
SiCx O2ð1−xÞ →SiC þ SiOðgÞ þ COðgÞ
ð6Þ
Similar with reaction (2), a high pressure will also shift the start of reaction (6) to higher temperatures. In higher pressure environment, reaction (2) is inhabited and thus minimizes the decomposition of
29
SiCxO2(1 − x) network. As discussions above, high pressure will hinder carbothermal process and minimize the decomposition of SiCxO2(1 − x) network. Thus, SiOC glasses exhibit enhanced thermal stability in higher pressure environment. 4. Conclusion In summary, the effect of environment pressure on high temperature stability of SiOC glasses has been studied. The environment pressure plays a crucial role on high temperature stability of SiOC glasses by promoting or hindering bonds redistribution and carbothermal reductions. In lower pressure environment (0.01 KPa), bonds redistribution is starting from 1000 °C and going up to 1400 °C. Carbothermal reductions are active at 1300 °C and almost completed at 1500 °C. In higher pressure environment (101 KPa), bonds redistribution occurs between 1000 °C and 1500 °C; in same temperature range, carbothermal reductions are not active. Acknowledgments This study was supported by the Hunan Provincial Natural Science Foundation of China (no. S2010J504B), National Defense Preliminary
Fig. 3. SiMAS NMR spectra for SiOC samples treated with various pressures. (a) 0.01 KPa and (b) 101 Kpa.
T. Xu et al. / Materials Letters 65 (2011) 1538–1541
1541
Table 2 Quantitative analysis determined for Si species. Sample
Raw sample SiOC(101 KPa)
SiOC(0.01 KPa)
Temperature (°C)
1000 1200 1300 1400 1500 1200 1300 1400 1500
Si species (%)
SiOC composition
SiO4
SiO3C
SiO2C2
SiC4
From NMR
From EA
52 58 64 74 87 66 54 15 NA
36 35 20 6 NA 28 22 NA NA
12 7 13 7 NA 6 5 NA NA
NA NA 3 13 13 NA 19 85 100
SiO1.70C0.15 SiO1.76C0.12 SiO1.71C0.14 SiO1.64C0.18 SiO1.74C0.13 SiO1.80C0.10 SiO1.46C0.27 SiO0.30C0.85 SiO0.00C1.00
SiO1.29C0.35 SiO1.31C0.34 SiO1.33C0.33 SiO1.29C0.35 SiO1.26C0.37 SiO1.30C0.35 SiO0.96C0.52 SiO0.18C0.91 SiO0.01C0.99
NA indicates cannot be detected by NMR.
Research Program of China (no. 9140C8203040904), and Natural Science Foundation of China (no. 90916002 and no. 50802113). References [1] [2] [3] [4] [5] [6]
Renlund G, Prochazka S, Doremus RH. J Mater Res 1991;6:2723–34. Pantano C, Singh A, Zhang H. J Sol-Gel Sci Tech 1999;14:7–25. Singh AK, Pantano CG. J Am Ceram Soc 1996;79:2696–704. Zhuo R, Colombo P, Pantano C. Acta Biomater 2005;1:583–9. Adler PN. 41st International SAMPE Symposium; March 24–28 1996. Xing W, Wilson AM, Eguchi K, Zank GA, Dahn JR. J Electrochem Soc 1997;144:2410–6.
[7] Brequel H, Parmentier J, Walter S, Badheka R, Trimmel G, Masse S, et al. Chem Mater 2004;16:2585–98. [8] Latournerie J, Dempsey P, Bahloul DH. J Am Ceram Soc 2006;89(5):1485–91. [9] Wei Q, Pippel E, Woltersdorf J, Scheffler M, Greil P. Mater Chem Phys 2002;73: 281–9. [10] Sorarù GD, Suttor D. J Sol-Gel Sci Tech 1999;14:69–74. [11] Sorarù GD, Liu Q, Interrante LV, Apple T. Chem Mater 1998;10:4047–54. [12] Parmentier J, Sorarù GD, Babonneau F. J Eur Ceram Soc 2001;21:817–24. [13] Bibbo GS, Benson PM, Pantano CG. J Mater Sci 1991;26:5075. [14] Lipowitz J, Freeman HA, Chen RT, Prack ER. Adv Ceram Mater 1987;2(2):121–8. [15] Martin H-P, Ecke R, Muller E. J Eur Ceram Soc 1998;18:1737–42. [16] Shimoo T, Okamura K, Takeuchi H. J Mater Sci 2003;38:4973–9.