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Cobalt-containing silicon oxycarbide glasses derived from poly[methyl(phenyl)]siloxane and cobalt phthalate
Journal of Non-Crystalline Solids 352 (2006) 2892–2896 www.elsevier.com/locate/jnoncrysol
Cobalt-containing silicon oxycarbide glasses derived from poly[methyl(phenyl)]siloxane and cobalt phthalate F. Kola´rˇ a
a,*
, V. Machovicˇ
a,b
, J. Svı´tilova´
a
Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, V Holesˇovicˇka´ch 41, CZ-18209 Prague, Czech Republic b Institute of Chemical Technology Prague, Technicka´ 5, Prague 162 08, Czech Republic Received 27 June 2005; received in revised form 3 November 2005 Available online 5 June 2006
Abstract Cobalt-containing silicon oxycarbides have been produced by curing and pyrolysis of polymethyl(phenyl)siloxane resin Lukosil 901 and Co-phthalate blends. The structure of the Si(Co)OC glasses was studied by electron microscopy, back-scattered electron imaging (BSE), Raman and infrared spectroscopies. The cobalt incorporated into SiOC structure increases the polymerization degree of the glass phase. It seems that the free carbon phase is closing into the SiO(Co)C network. This idea also supports the results of combustibility studies – prepared glasses are characterized by excellent oxidation resistance at high temperatures. 2006 Elsevier B.V. All rights reserved. Keywords: Ceramics; Optical spectroscopy; Scanning electron microscopy; FTIR measurements; Raman spectroscopy; Polymers and organics; Sol–gels (xerogels)
1. Introduction Commercially available silicone resins are mostly based on poly[methyl(phenyl)siloxane)]s, which can be cured at temperatures of about 250 C to form silicone polymers. By the pyrolysis of cured polysiloxanes in an inert atmosphere, compact glassy materials are formed which are very resistant to oxidation at high temperature [1–5]. These materials represent modifications of a quartz glass in which some of the pairs of oxygen atoms are substituted by an atom of carbon. In addition, certain portions of the free carbon structures are distributed loosely dissipated in the siliconoxycarbide network [1,4–8]. Strictly speaking, the carbon is not present in elemental form but as structures with both aromatic and aliphatic C–H bonds [4,5]. Implantation of transition metals or other elements into silicon oxycarbides appears to be interesting for various applications [9–11]. For example silicon oxycarbide glasses *
Corresponding author. Tel./fax: +42 266 009 343. E-mail address: [email protected] (F. Kola´rˇ).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.02.087
containing boron were prepared. A structural characterization of the precursor as well as of the SiBOC glass at various stages of the pyrolysis treatment was performed in reference [12]. SiAlOC ceramics produced by pyrolysis of commercial poly(methylsilsesquioxane) precursors is described in [13]. Wootton et al. studied structural properties of silicon oxycarbide glasses derived from metal alkoxide precursors [14]. The aim of this work was to prepare silicon oxycarbide glasses containing cobalt atoms incorporated in SiOC structure. We present some results of our investigation of structure and properties of prepared materials. 2. Experimental 2.1. Sample preparation Commercially available resin Lukosil 9011 was used as a starting material for the silicon oxycarbide glasses. Lukosil 901 is xylene solution of poly[methyl(phenyl)]siloxane. 1
Lucebni Zavody Corp., Czech Republic.
F. Kola´r et al. / Journal of Non-Crystalline Solids 352 (2006) 2892–2896
Composition and properties of Lukosil 901 (starting liquid resin, cured and pyrolyzed ones) are described in papers [1,4,5]. Cobalt precursor was prepared by reaction of saturated benzyl alcohol solution of phthalic anhydride and surplus of CoCO3. After filtration of excess CoCO3 we have obtained violet solution containing 0.0152 g Co per 1 g of the solution. Mixtures of Lukosil 901 and Co-phthalate were prepared. The solvents were evaporated at room temperature first and then at 200 C. The samples in thickness about 0.5 mm were cured at 250 C for 6 h on Teflon plates, and pyrolyzed in nitrogen, heating rate 70 K h1 up to 1000 C, the cooling rate 100 K min1.
heat-treated in nitrogen at 1000 C. In this way were prepared the Si(Co)OC glasses. Co-precursor (benzyl alcohol solution of Co-phthalate) and Lukosil 901 are miscible over a sufficient concentration range. Cobalt mass fractions in cured polymers and derived Si(Co)OC glasses were determined from Cobalt contents in starting mixtures with the relations: ðLÞ
ðCÞ
W Co ¼
2.3. Backscattered electron (BSE) image Samples analyses of Co were obtained using the Cameca SX 100 electron microprobe, employing X-PHI correction. The operating conditions were: accelerating potential 20 kV, beam current 4 nA, Ka lines were used on LiF crystal.
W Co ; W cur
ðCÞ
ðPÞ
W Co ¼
W Co ; W pyr
where ðLÞ
W Co is Co mass fraction of starting (liquid) mixtures Lukosil 901 + Co-phthalate, ðCÞ W Co Co mass fraction of cured mixtures, ðPÞ W Co Co mass fraction of pyrolyzed mixtures Wcur and Wpyr are cure and pyrolysis residues.
2.2. Infrared and Raman spectroscopy Infrared spectra were recorded with an FTIR spectrometer (Bruker IFS 66v) equipped with an IR microscope Hyperion ATR objective containing germanium crystal. Final spectra were adjusted using the ATR correction. Raman spectra were collected using a LabRam system Jobin Yvon model Labram HR equipped with a 532 nm line laser (power on the head of the laser 50 mW, reduced about one order of magnitude with a gray filter) for excitation. The objective (·100) was used to focus the laser beam on the sample placed on an X–Y motorized sample stage. The scattered light was analyzed by spectrograph with a holographic grating (600 g/mm), slit width 100 lm and opened confocal hole (1000 lm). The adjustment of the system was regularly checked using a silicon sample and by measurement in the zero-order position of the grating. The time of acquisition of a particular spectral window was optimized for individual sample measurements. Twenty accumulations were Co-added to obtain a spectrum.
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Cobalt contents in the samples are provided in Table 1. The casting mixtures were homogenous liquids, transformed during curing to the transparent polymers. Co-phthalate molecules were finely dispersed in the polysiloxane network. Only in the case of sample 4 was Co-phthalate content too high. This sample became turbid during evaporation and curing. Spherical particles 0.5–1.5 lm in diameter were produced and the resulting polymer was an opaque product. These were retained after pyrolysis and were visible in electron microscope (see Fig. 1). BSE analysis showed the particles contained a much higher quantity of cobalt than the surrounding material (see Table 2). The structure of cured mixtures Lukosil 901 and Cophtalate (Nos. 0, 1, 5) and derived pyrolyzates were analyzed by infrared spectroscopy using Attenuated Total Reflectance (ATR). The spectra of the cured samples are shown in Fig. 3. In the figure are evident bands at 1268 cm1 corresponding to stretching vibration of Si– CH3, the band of 1130 cm1 belongs to the vibration of Si–C6H5, the band at 1130 cm1 has been assigned to in-plane ring vibrations and the band 1130–1000 cm1 to bending ring vibrations of the aromatic system. The bands between 900 and 600 cm1, belongs to asymmetric rocking vibrations of Si–CH3, and to Si–C stretching vibrations.
2.4. Combustibility test The samples of prepared Si(Co)OC glasses (10 · 7 · 0.5 mm) were placed in porcelain dishes and heated at a rate of 10 C/min in tube furnace in air up to 825 C. The samples were held isothermally for 2 h and spontaneously cooled room temperature. Weight of samples were about 10 g, errors of weighting ±0.1 mg. 3. Results The liquid mixtures of Lukosil 901 and Co-precursor were cured at 250 C to hard, transparent cross-linked polymers of violet color. These polymers were subsequently
Table 1 Cobalt contents in starting and cured blends of Lukosil 901 + Co-phthalate and in derived pyrolyzates ðLÞ
ðCÞ
ðPÞ
No.
W Co
W Co
W Co
Wcur
Wpyr
0 1 2 3 4
0 0.0002 0.0003 0.0006 0.0053
0 0.0004 0.0007 0.0013 0.0118
0 0.0006 0.0008 0.0017 0.0149
0.4501 0.4454 0.4540 0.4499 0.4510
0.8482 0.8121 0.8148 0.8066 0.7884
Number of samples was n =q5.ffiffiffiffiffiffiffiffiffiffiffi Standard deviation (s = ±0.00001) were xÞ 2 i estimated from relation s ¼ ðxn1 , where x are relevant mass fractions. ðLÞ W Co is Co mass fraction of starting (liquid) mixtures Lukosil 901 + ðCÞ ðPÞ Co-phthalate; W Co Co mass fraction of cured mixtures; W Co Co mass fraction of pyrolyzed mixture.
F. Kola´r et al. / Journal of Non-Crystalline Solids 352 (2006) 2892–2896
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Fig. 3. ATR spectra of cobalt-containing polysiloxanes, A – Sample 4 ðCÞ ðW Co ¼ 0:0118Þ, B – Sample 1 ðW CCo ¼ 0:0004Þ, C – Sample 0 ðW CCo ¼ 0Þ. Fig. 1. BSE image of sample 4P.
Fig. 2. SEM photo of surface (sample 4P). Table 2 Results of BSE measurements, s = ±0.001% (see Table 1) No.
0 1 2 3 4 4 a b
Co [wt%] Cured
Pyrolyzed
0 0.04 0.07 0.06 7.47 0.29
0 0.02 0.06 0.06 6.10a 0.14b
in Fig. 4. There was intensity growth of bands at 1200 cm1. This region appears to exhibit an increase in both Q4 and Q3 units. The structure of the free carbon phase was characterized with the help of Raman spectroscopy. Raman spectra are shown in Fig. 5. Two major characteristic bands can be found in Raman spectra of these carbon-rich materials. First, the so-called G-band (graphite band) is the fundamental Raman mode of sp2 binding structures caused by stretching vibrations of the C–C bonds. This band is located at 1575 cm1, with decreasing orientation of the carbon structure its position slightly changes and its halfwidth increases. Second, the so-called D-band (disordered band) lying usually between 1300 and 1400 cm1 can be found in amorphous materials. This band assigned to sp2-bound carbon appears in the Raman spectrum when the structure of the carbon material there is sp3 carbon linking of graphene units, and there are asymmetries in the graphitic lattice [18] (E2g1) which is not resolvable from the second-order Raman bands are assigned both to overtone and combination scattering. The structure parameters of these carbon phases were calculated. The spectra were separated to the four bands at 1190, 1335, 1535 and
Measured in center of particles. Measured in surroundings of particles.
Besides those bands of out-of-plane vibrations of phenyl C–H bonds, vibration of Si–C6H5 system are also in this region. An ATR spectrum of cured polysiloxane Lukosil 901 without Co is shown in Fig. 3 (curve C). Large intensity growth of bands in high-frequency areas of Si–O bonds in infrared spectra was found in case of silicon oxycarbides prepared by pyrolysis at 1000 C, shown
ðPÞ
Fig. 4. ATR spectra of Si(Co)OC glasses, A – Sample 4 ðW Co ¼ 0:0149Þ, ðPÞ ðPÞ B – Sample 1 ðW Co ¼ 0:0004Þ, C – Sample 0 ðW Co ¼ 0Þ.
F. Kola´r et al. / Journal of Non-Crystalline Solids 352 (2006) 2892–2896
Fig. 5. Raman spectra of Si(Co)OC glasses, A – Sample 4 ðPÞ ðPÞ ðPÞ ðW Co ¼ 0:0149Þ, B – Sample 1 ðW Co ¼ 0:0004Þ, C – Sample 0 ðW Co ¼ 0Þ.
1603 cm1. Band separation employing a mixed Gauss– Lorentz function was performed. In-plane graphite crystallite size was determined from band areas following the relation La = 4.4 Æ (AG/AD) [15]. The value of La was 1.79 nm for SiOC glass without Co addition. In the case of Co-containing SiOC glasses we obtained values of 1.73 nm (sample 4) and 2.10 nm (sample 1). Differences in La values are low and we suppose the cobalt atoms are incorporated only into SiOC structures while carbon phase thus is not affected. The influence of Co content on thermal oxidation resistance of polysiloxanes and derived Si(Co)OC glasses was studied by firing samples at 825 C. The results are shown in Table 3. From table, it is evident that the changes associated with cured Co-containing polysiloxanes resulting from firing are marked but that the weight changes after combustion are very similar. On the other hand, weight losses and changes in color and state of Si(Co)OC glasses expressly depend upon Co concentration (see Table 3). 4. Discussion It seems that the polymerization of polysiloxane resin Lukosil 901 and consequent pyrolysis is affected by the
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addition of cobalt phthalate. During pyrolysis of cured samples in inert atmosphere, Co-containing glasses are obtained. If excess of Co-precursor is added, spherical particles 0.5–1.5 lm in diameter were formed in the polymer. These particles are also retained after pyrolysis. BSE analysis show that the particles contained a much higher quantity of cobalt than the surrounding material (see Table 2, sample 4). Interesting formations appeared on the surface of the pyrolyzed sample 4 (see Fig. 2). This can be due to crystallization of cobalt phthalate during the evaporation of solvents with the resulting patterns retained after curing and pyrolysis. The structure of amorphous silica glasses is mainly created from tetrahedral SiO4 units, where all oxygen atoms are bridging ones (BO), but some part of non-bridging oxygen atoms (NBO) can be present [15,16]. The number of BO is described in terms of Q0, . . . , Q4 units corresponding to frequencies 850–880 cm1 (Q0), 900–920 cm1 (Q1), 950 cm1 (Q2), 1100–1150 cm1 (Q3) and 1200 cm1 (Q4) in infrared spectra [17]. The incorporation of cobalt atoms into silicon oxycarbide structures increases the intensities of bands at 1185, 1158, 1130 and 1200 cm1. Hence it is possible to come to the conclusion, that content Q3 units grows. On the basis of these enhancements in band intensities, we propose that the presence of cobalt increases the polymerization degree of the glass phase. A certain portion of the phenyl groups transform during pyrolysis to a highly condensed polyaromatic system close to carbon, a so-called ‘free carbon phase’ [1–3]. It was published previously that the resistance of silicon oxycarbide glasses to oxidation at high temperatures is related to the content of free carbon [5]. Silicon oxycarbide glass prepared by pyrolysis of Lukosil 901 resin contains about 40% of free carbon which moreover forms a continuous network. For this reason, the resistance to oxidation at high temperatures is quite low. It was mentioned above the cobalt addition increases the polymerization degree of the glass phase and that the content of Q3 units grows during curing and pyrolysis. It seems that the polyaromatic
Table 3 Weight loss and changes in color and state of cured (0–4) and pyrolyzed (0–4P) samples, s = ±0.001% (see Table 1) No.
0 1 2 3 4 0P 1P 2P 3P 4P
Color
After burning Weight loss [%]
State
Color
Colorless/transparent Slightly violet/transparent Slightly violet/transparent Violet/transparent Violet/opaque
38.52 38.16 39.37 39.45 40.25
Powder Compact Compact Compact Compact
Disintegrated Partial disintegrated Partial disintegrated Partial disintegrated Intensive disintegrated
White Grey Grey Grey Black
Black Black Black Black Black
30.20 15.26 3.71 0.75 0.07
Compact Compact Compact Compact Compact
Low disintegrated Low disintegrated Minor changes Minor changes Minimum changes
Black Black Black Black Black
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F. Kola´r et al. / Journal of Non-Crystalline Solids 352 (2006) 2892–2896
phase is closed into SiO(Co)C network and that oxygen access to combustible substances is therefore limited. 5. Conclusion Cobalt containing silicon oxycarbide glasses were prepared via Co-pyrolysis of silicone resin and a Co-precursor. The structures of prepared Si(Co)OC glasses were studied by electron microscopy, BSE, Raman and infrared spectroscopies. It was found that the cobalt incorporated into SiOC structure affects the polymerization process. The polymerization degree increases and the free carbon phase is probably closing into SiO(Co)C network. This idea also supports the results of the combustibility studies. Spherical particles of about 1 lm in diameter were observed in the case of high Co-precursor concentration in the BSE image analysis. Different formations were found on the surfaces of the samples. These formations are caused by the crystallization of Co-precursor during evaporation of solvents. After curing and pyrolysis the pattern of formations remained fixed in the material. Acknowledgements This work has been supported by the Institute research plan of IRSM CAS CZ No. A VOZ30460519. The authors are grateful to Ing. A. Langrova´ (Institute of Geology CAS) for the SEM and BSE measurements.
References [1] J. Brus, F. Kola´rˇ, V. Machovicˇ, J. Svı´tilova´, J. Non-Cryst. Solids 289 (2001) 62. [2] R.W. Rice, Bull. Am. Ceram. Soc. 62 (1983) 889. [3] M. Peuckert, T. Vaahs, M. Brueck, Adv. Mater. 2 (1990) 398. [4] V. Machovicˇ, F. Kola´rˇ, J. Svı´tilova´, P. Mateˇjka, J. Mizera, J. Maixner, Acta Montana B12 (2002) 126. [5] F. Kola´rˇ, V. Machovicˇ, J. Svı´tilova´, L. Borecka´, Mater. Chem. Phys. 86 (2004) 88. [6] J. Parmentier, G.D. Soraru, F. Babonneau, J. Eur. Ceram. Soc. 21 (2001) 817. [7] G.D. Soraru, Q. Liu, L.V. Interrante, T. Apple, Chem. Mater. 10 (1998) 4047. [8] E. Radovanovic, M.F. Gozzi, M.C. Goncalves, I.V.P. Yoshida, J. Non-Cryst. Solids 248 (1999) 37. [9] P. Bergo, W.M. Pontushca, J.M. Prison, C.C. Motta, J.R. Martinelli, J. Non-Cryst. Solids 348 (2004) 84. [10] E. Cattaruzza, F. D’ Acapito, C. de Julian Fernandez, A. de Lorenzi, F. Gonella, G. Mattei, C. Maurizio, P. Mazzoldi, S. Padovani, B.F. Scremin, F. Zontone, Nucl. Instrum. Methods Phys. Res. B 191 (2002) 406. [11] S. Hazra, A. Ghosh, Solid State Commun. 96 (7) (1995) 507. [12] Gian Domenico Soraru, Florence Babonneau, Stefano Maurina, Jean Vicens, J. Non-Cryst. Solids 224 (1998) 173. [13] Rahul Harshe, Corneliu Balan, Ralf Riedel, J. Eur. Ceram. Soc. 24 (2004) 3471. [14] A.M. Wootton et al., J. Non-Cryst. Solids 204 (1996) 217. [15] Z. Wang, T.F. Cooney, S.K. Sharma, Geochimica et Cosmochimica Acta 59 (8) (1995) 1571. [16] P. Colomban, J. Non-Cryst. Solids 323 (2003) 180. [17] S.A. MacDonald, C.R. Schardt, D.J. Masiello, J.H. Simmons, J. Non-Cryst. Solids 275 (2000) 72. [18] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126.
Cobalt-containing silicon oxycarbide glasses derived from poly[methyl(phenyl)]siloxane and cobalt phthalate F. Kola´rˇ a
a,*
, V. Machovicˇ
a,b
, J. Svı´tilova´
a
Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, V Holesˇovicˇka´ch 41, CZ-18209 Prague, Czech Republic b Institute of Chemical Technology Prague, Technicka´ 5, Prague 162 08, Czech Republic Received 27 June 2005; received in revised form 3 November 2005 Available online 5 June 2006
Abstract Cobalt-containing silicon oxycarbides have been produced by curing and pyrolysis of polymethyl(phenyl)siloxane resin Lukosil 901 and Co-phthalate blends. The structure of the Si(Co)OC glasses was studied by electron microscopy, back-scattered electron imaging (BSE), Raman and infrared spectroscopies. The cobalt incorporated into SiOC structure increases the polymerization degree of the glass phase. It seems that the free carbon phase is closing into the SiO(Co)C network. This idea also supports the results of combustibility studies – prepared glasses are characterized by excellent oxidation resistance at high temperatures. 2006 Elsevier B.V. All rights reserved. Keywords: Ceramics; Optical spectroscopy; Scanning electron microscopy; FTIR measurements; Raman spectroscopy; Polymers and organics; Sol–gels (xerogels)
1. Introduction Commercially available silicone resins are mostly based on poly[methyl(phenyl)siloxane)]s, which can be cured at temperatures of about 250 C to form silicone polymers. By the pyrolysis of cured polysiloxanes in an inert atmosphere, compact glassy materials are formed which are very resistant to oxidation at high temperature [1–5]. These materials represent modifications of a quartz glass in which some of the pairs of oxygen atoms are substituted by an atom of carbon. In addition, certain portions of the free carbon structures are distributed loosely dissipated in the siliconoxycarbide network [1,4–8]. Strictly speaking, the carbon is not present in elemental form but as structures with both aromatic and aliphatic C–H bonds [4,5]. Implantation of transition metals or other elements into silicon oxycarbides appears to be interesting for various applications [9–11]. For example silicon oxycarbide glasses *
Corresponding author. Tel./fax: +42 266 009 343. E-mail address: [email protected] (F. Kola´rˇ).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.02.087
containing boron were prepared. A structural characterization of the precursor as well as of the SiBOC glass at various stages of the pyrolysis treatment was performed in reference [12]. SiAlOC ceramics produced by pyrolysis of commercial poly(methylsilsesquioxane) precursors is described in [13]. Wootton et al. studied structural properties of silicon oxycarbide glasses derived from metal alkoxide precursors [14]. The aim of this work was to prepare silicon oxycarbide glasses containing cobalt atoms incorporated in SiOC structure. We present some results of our investigation of structure and properties of prepared materials. 2. Experimental 2.1. Sample preparation Commercially available resin Lukosil 9011 was used as a starting material for the silicon oxycarbide glasses. Lukosil 901 is xylene solution of poly[methyl(phenyl)]siloxane. 1
Lucebni Zavody Corp., Czech Republic.
F. Kola´r et al. / Journal of Non-Crystalline Solids 352 (2006) 2892–2896
Composition and properties of Lukosil 901 (starting liquid resin, cured and pyrolyzed ones) are described in papers [1,4,5]. Cobalt precursor was prepared by reaction of saturated benzyl alcohol solution of phthalic anhydride and surplus of CoCO3. After filtration of excess CoCO3 we have obtained violet solution containing 0.0152 g Co per 1 g of the solution. Mixtures of Lukosil 901 and Co-phthalate were prepared. The solvents were evaporated at room temperature first and then at 200 C. The samples in thickness about 0.5 mm were cured at 250 C for 6 h on Teflon plates, and pyrolyzed in nitrogen, heating rate 70 K h1 up to 1000 C, the cooling rate 100 K min1.
heat-treated in nitrogen at 1000 C. In this way were prepared the Si(Co)OC glasses. Co-precursor (benzyl alcohol solution of Co-phthalate) and Lukosil 901 are miscible over a sufficient concentration range. Cobalt mass fractions in cured polymers and derived Si(Co)OC glasses were determined from Cobalt contents in starting mixtures with the relations: ðLÞ
ðCÞ
W Co ¼
2.3. Backscattered electron (BSE) image Samples analyses of Co were obtained using the Cameca SX 100 electron microprobe, employing X-PHI correction. The operating conditions were: accelerating potential 20 kV, beam current 4 nA, Ka lines were used on LiF crystal.
W Co ; W cur
ðCÞ
ðPÞ
W Co ¼
W Co ; W pyr
where ðLÞ
W Co is Co mass fraction of starting (liquid) mixtures Lukosil 901 + Co-phthalate, ðCÞ W Co Co mass fraction of cured mixtures, ðPÞ W Co Co mass fraction of pyrolyzed mixtures Wcur and Wpyr are cure and pyrolysis residues.
2.2. Infrared and Raman spectroscopy Infrared spectra were recorded with an FTIR spectrometer (Bruker IFS 66v) equipped with an IR microscope Hyperion ATR objective containing germanium crystal. Final spectra were adjusted using the ATR correction. Raman spectra were collected using a LabRam system Jobin Yvon model Labram HR equipped with a 532 nm line laser (power on the head of the laser 50 mW, reduced about one order of magnitude with a gray filter) for excitation. The objective (·100) was used to focus the laser beam on the sample placed on an X–Y motorized sample stage. The scattered light was analyzed by spectrograph with a holographic grating (600 g/mm), slit width 100 lm and opened confocal hole (1000 lm). The adjustment of the system was regularly checked using a silicon sample and by measurement in the zero-order position of the grating. The time of acquisition of a particular spectral window was optimized for individual sample measurements. Twenty accumulations were Co-added to obtain a spectrum.
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Cobalt contents in the samples are provided in Table 1. The casting mixtures were homogenous liquids, transformed during curing to the transparent polymers. Co-phthalate molecules were finely dispersed in the polysiloxane network. Only in the case of sample 4 was Co-phthalate content too high. This sample became turbid during evaporation and curing. Spherical particles 0.5–1.5 lm in diameter were produced and the resulting polymer was an opaque product. These were retained after pyrolysis and were visible in electron microscope (see Fig. 1). BSE analysis showed the particles contained a much higher quantity of cobalt than the surrounding material (see Table 2). The structure of cured mixtures Lukosil 901 and Cophtalate (Nos. 0, 1, 5) and derived pyrolyzates were analyzed by infrared spectroscopy using Attenuated Total Reflectance (ATR). The spectra of the cured samples are shown in Fig. 3. In the figure are evident bands at 1268 cm1 corresponding to stretching vibration of Si– CH3, the band of 1130 cm1 belongs to the vibration of Si–C6H5, the band at 1130 cm1 has been assigned to in-plane ring vibrations and the band 1130–1000 cm1 to bending ring vibrations of the aromatic system. The bands between 900 and 600 cm1, belongs to asymmetric rocking vibrations of Si–CH3, and to Si–C stretching vibrations.
2.4. Combustibility test The samples of prepared Si(Co)OC glasses (10 · 7 · 0.5 mm) were placed in porcelain dishes and heated at a rate of 10 C/min in tube furnace in air up to 825 C. The samples were held isothermally for 2 h and spontaneously cooled room temperature. Weight of samples were about 10 g, errors of weighting ±0.1 mg. 3. Results The liquid mixtures of Lukosil 901 and Co-precursor were cured at 250 C to hard, transparent cross-linked polymers of violet color. These polymers were subsequently
Table 1 Cobalt contents in starting and cured blends of Lukosil 901 + Co-phthalate and in derived pyrolyzates ðLÞ
ðCÞ
ðPÞ
No.
W Co
W Co
W Co
Wcur
Wpyr
0 1 2 3 4
0 0.0002 0.0003 0.0006 0.0053
0 0.0004 0.0007 0.0013 0.0118
0 0.0006 0.0008 0.0017 0.0149
0.4501 0.4454 0.4540 0.4499 0.4510
0.8482 0.8121 0.8148 0.8066 0.7884
Number of samples was n =q5.ffiffiffiffiffiffiffiffiffiffiffi Standard deviation (s = ±0.00001) were xÞ 2 i estimated from relation s ¼ ðxn1 , where x are relevant mass fractions. ðLÞ W Co is Co mass fraction of starting (liquid) mixtures Lukosil 901 + ðCÞ ðPÞ Co-phthalate; W Co Co mass fraction of cured mixtures; W Co Co mass fraction of pyrolyzed mixture.
F. Kola´r et al. / Journal of Non-Crystalline Solids 352 (2006) 2892–2896
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Fig. 3. ATR spectra of cobalt-containing polysiloxanes, A – Sample 4 ðCÞ ðW Co ¼ 0:0118Þ, B – Sample 1 ðW CCo ¼ 0:0004Þ, C – Sample 0 ðW CCo ¼ 0Þ. Fig. 1. BSE image of sample 4P.
Fig. 2. SEM photo of surface (sample 4P). Table 2 Results of BSE measurements, s = ±0.001% (see Table 1) No.
0 1 2 3 4 4 a b
Co [wt%] Cured
Pyrolyzed
0 0.04 0.07 0.06 7.47 0.29
0 0.02 0.06 0.06 6.10a 0.14b
in Fig. 4. There was intensity growth of bands at 1200 cm1. This region appears to exhibit an increase in both Q4 and Q3 units. The structure of the free carbon phase was characterized with the help of Raman spectroscopy. Raman spectra are shown in Fig. 5. Two major characteristic bands can be found in Raman spectra of these carbon-rich materials. First, the so-called G-band (graphite band) is the fundamental Raman mode of sp2 binding structures caused by stretching vibrations of the C–C bonds. This band is located at 1575 cm1, with decreasing orientation of the carbon structure its position slightly changes and its halfwidth increases. Second, the so-called D-band (disordered band) lying usually between 1300 and 1400 cm1 can be found in amorphous materials. This band assigned to sp2-bound carbon appears in the Raman spectrum when the structure of the carbon material there is sp3 carbon linking of graphene units, and there are asymmetries in the graphitic lattice [18] (E2g1) which is not resolvable from the second-order Raman bands are assigned both to overtone and combination scattering. The structure parameters of these carbon phases were calculated. The spectra were separated to the four bands at 1190, 1335, 1535 and
Measured in center of particles. Measured in surroundings of particles.
Besides those bands of out-of-plane vibrations of phenyl C–H bonds, vibration of Si–C6H5 system are also in this region. An ATR spectrum of cured polysiloxane Lukosil 901 without Co is shown in Fig. 3 (curve C). Large intensity growth of bands in high-frequency areas of Si–O bonds in infrared spectra was found in case of silicon oxycarbides prepared by pyrolysis at 1000 C, shown
ðPÞ
Fig. 4. ATR spectra of Si(Co)OC glasses, A – Sample 4 ðW Co ¼ 0:0149Þ, ðPÞ ðPÞ B – Sample 1 ðW Co ¼ 0:0004Þ, C – Sample 0 ðW Co ¼ 0Þ.
F. Kola´r et al. / Journal of Non-Crystalline Solids 352 (2006) 2892–2896
Fig. 5. Raman spectra of Si(Co)OC glasses, A – Sample 4 ðPÞ ðPÞ ðPÞ ðW Co ¼ 0:0149Þ, B – Sample 1 ðW Co ¼ 0:0004Þ, C – Sample 0 ðW Co ¼ 0Þ.
1603 cm1. Band separation employing a mixed Gauss– Lorentz function was performed. In-plane graphite crystallite size was determined from band areas following the relation La = 4.4 Æ (AG/AD) [15]. The value of La was 1.79 nm for SiOC glass without Co addition. In the case of Co-containing SiOC glasses we obtained values of 1.73 nm (sample 4) and 2.10 nm (sample 1). Differences in La values are low and we suppose the cobalt atoms are incorporated only into SiOC structures while carbon phase thus is not affected. The influence of Co content on thermal oxidation resistance of polysiloxanes and derived Si(Co)OC glasses was studied by firing samples at 825 C. The results are shown in Table 3. From table, it is evident that the changes associated with cured Co-containing polysiloxanes resulting from firing are marked but that the weight changes after combustion are very similar. On the other hand, weight losses and changes in color and state of Si(Co)OC glasses expressly depend upon Co concentration (see Table 3). 4. Discussion It seems that the polymerization of polysiloxane resin Lukosil 901 and consequent pyrolysis is affected by the
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addition of cobalt phthalate. During pyrolysis of cured samples in inert atmosphere, Co-containing glasses are obtained. If excess of Co-precursor is added, spherical particles 0.5–1.5 lm in diameter were formed in the polymer. These particles are also retained after pyrolysis. BSE analysis show that the particles contained a much higher quantity of cobalt than the surrounding material (see Table 2, sample 4). Interesting formations appeared on the surface of the pyrolyzed sample 4 (see Fig. 2). This can be due to crystallization of cobalt phthalate during the evaporation of solvents with the resulting patterns retained after curing and pyrolysis. The structure of amorphous silica glasses is mainly created from tetrahedral SiO4 units, where all oxygen atoms are bridging ones (BO), but some part of non-bridging oxygen atoms (NBO) can be present [15,16]. The number of BO is described in terms of Q0, . . . , Q4 units corresponding to frequencies 850–880 cm1 (Q0), 900–920 cm1 (Q1), 950 cm1 (Q2), 1100–1150 cm1 (Q3) and 1200 cm1 (Q4) in infrared spectra [17]. The incorporation of cobalt atoms into silicon oxycarbide structures increases the intensities of bands at 1185, 1158, 1130 and 1200 cm1. Hence it is possible to come to the conclusion, that content Q3 units grows. On the basis of these enhancements in band intensities, we propose that the presence of cobalt increases the polymerization degree of the glass phase. A certain portion of the phenyl groups transform during pyrolysis to a highly condensed polyaromatic system close to carbon, a so-called ‘free carbon phase’ [1–3]. It was published previously that the resistance of silicon oxycarbide glasses to oxidation at high temperatures is related to the content of free carbon [5]. Silicon oxycarbide glass prepared by pyrolysis of Lukosil 901 resin contains about 40% of free carbon which moreover forms a continuous network. For this reason, the resistance to oxidation at high temperatures is quite low. It was mentioned above the cobalt addition increases the polymerization degree of the glass phase and that the content of Q3 units grows during curing and pyrolysis. It seems that the polyaromatic
Table 3 Weight loss and changes in color and state of cured (0–4) and pyrolyzed (0–4P) samples, s = ±0.001% (see Table 1) No.
0 1 2 3 4 0P 1P 2P 3P 4P
Color
After burning Weight loss [%]
State
Color
Colorless/transparent Slightly violet/transparent Slightly violet/transparent Violet/transparent Violet/opaque
38.52 38.16 39.37 39.45 40.25
Powder Compact Compact Compact Compact
Disintegrated Partial disintegrated Partial disintegrated Partial disintegrated Intensive disintegrated
White Grey Grey Grey Black
Black Black Black Black Black
30.20 15.26 3.71 0.75 0.07
Compact Compact Compact Compact Compact
Low disintegrated Low disintegrated Minor changes Minor changes Minimum changes
Black Black Black Black Black
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phase is closed into SiO(Co)C network and that oxygen access to combustible substances is therefore limited. 5. Conclusion Cobalt containing silicon oxycarbide glasses were prepared via Co-pyrolysis of silicone resin and a Co-precursor. The structures of prepared Si(Co)OC glasses were studied by electron microscopy, BSE, Raman and infrared spectroscopies. It was found that the cobalt incorporated into SiOC structure affects the polymerization process. The polymerization degree increases and the free carbon phase is probably closing into SiO(Co)C network. This idea also supports the results of the combustibility studies. Spherical particles of about 1 lm in diameter were observed in the case of high Co-precursor concentration in the BSE image analysis. Different formations were found on the surfaces of the samples. These formations are caused by the crystallization of Co-precursor during evaporation of solvents. After curing and pyrolysis the pattern of formations remained fixed in the material. Acknowledgements This work has been supported by the Institute research plan of IRSM CAS CZ No. A VOZ30460519. The authors are grateful to Ing. A. Langrova´ (Institute of Geology CAS) for the SEM and BSE measurements.
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