Microstructure of SiC-Si-Al 2 O 3. composites derived from silicone resin - metal aluminum filler compounds by low temperature reduction process

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1 IOP Conference Series: Materials Science and Engineering Microstructure of SiC-Si-Al 2 O 3 composites derived from silicone resin - metal aluminum filler compounds by low temperature reduction process To cite this article: M Narisawa and Y Abe 2011 IOP Conf. Ser.: Mater. Sci. Eng Related content - Processing of polysiloxane-derived porous ceramics: a review B V Manoj Kumar and Young-Wook Kim - Sol-Gel Synthesis of Au-Nanoparticle Dispersed Bicontinuous Macroporous Siloxane Gel Y Hamada, M Nishi, Y Shimotsuma et al. - A novel precursor composed of polycarbosilane and palladium(ii) acetate for a SiC-based gas separation membrane Akira Idesaki, Masaki Sugimoto and Masahito Yoshikawa View the article online for updates and enhancements. This content was downloaded from IP address on 15/11/2018 at 20:03

2 Microstructure of SiC-Si-Al 2 O 3 composites derived from silicone resin - metal aluminum filler compounds by low temperature reduction process M Narisawa and Y Abe Graduate School of Engineering, Osaka Prefecture University, 1-1, Gakuen-Cho, Naka-Ku, Sakai , Japan nar@mtr.osakafu-u.ac.jp Abstract. Concentrated slurry of a silicone resin with low carbon content, 3 µm aluminum particles and ethanol were prepared. After casting, addition of cross-linking agent and drying, silicone resin-aluminum composite with thick sheet form was obtained. The prepared sheet was heat-treated at 933 or 1073K with various holding times to characterize formed phases during the heat treatments. XRD patterns and FT-IR spectra revealed free Si formation and existence of Si-O-Si bond at 933K. The Si-O-Si bond, however, disappeared and silicon carbide was formed at 1073K. SEM observation indicated formation of cracks bridged with a number of tiny struts at 933K and conversion to wholly porous structure at 1073K. 1. Introduction Silicon oxycarbide (Si-O-C) materials derived from silicone resins attract wide interest as ceramic precursors in recent decades [1,2]. Such resins are highly useful to prepare heat resistant porous ceramics by using the viscoelastic nature [3-5]. Even synthesis of continuous Si-O-C fiber is possible by adjusting melt spinning and curing conditions [6]. By the way, the starting silicone resins are often used with additional inorganic fillers. Unique interface reactions between the silicone resin matrix and the dispersed inorganic fillers are becoming apparent [7,8]. For example, Greil reported efficiency of metal silicide addition in the starting silicon resin with high carbon content [9]. Metal in the filler trapped carbon from the decomposition gas and yielded metal carbide, while silicon in the filler trapped nitrogen in the pyrolysis atmosphere and yielded Si 3 N 4 after pyrolysis. The process was named active filler method, which efficiently reduced volume shrinkage during pyrolysis. By a glance of such developments, it is easily predictable that trap of oxygen from silicone resin matrix will be possible, if selected filler, like aluminum, acts as an effective reductant. It remembers classic thermite process. In spite of the simple idea, however, there are very few previous studies on reductant filler application on ceramic precursors. Colombo et al. reported joining of SiC/SiC composites by mixtures of silicone resin and Al-Si eutectic alloy [10]. A role of SiC-Al 2 O 3 phase formation during pyrolysis on high joint strength was suggested. There were, however, no practical data about characterization of formed phases and resulting microstructures. In a previous study, we reported ceramization process of silicone resin-metal Al particle composites with the particle size of 30 µm or 60 nm. Free silicon, aluminum carbide, silicon carbide and alumina were formed suddenly at K [11]. In this c 2011 Ceramic Society of Japan. 1 Published under licence by Ltd

3 article, effect of holding period on such low temperature ceramization process was investigated in detail by XRD, FT-IR and FE-SEM equipped with EDX in order to shed light on the mechanism. 2. Experimental procedure Commercialized silicone resin (YR3370, Momentive Performance Materials Japan, Japan) was prepared for synthesizing the resin-metal Al composites. The analyzed chemical composition of the resin was SiO 1.78 C 1.22 H The molecular structure, which was spectroscopically characterized in previous studies, consists of major -Si(CH 3 )O units, additional silanol and possible methoxyl groups [6]. The 35 mass% resin ethanol slurry was prepared, and metal aluminum powder (3µm diameter, Kojundo Chemical Lab. Co. Ltd., Japan) was dispersed in the slurry. An amount of aluminum was controlled at 40% of the solute resin in the slurry. It corresponds to the O (in the silicone)/al (in the filler) molar ratio of 1.6. The stirred slurry was cast in plastic cylinder (30mm diameter) in a depth of 2mm. Cross-linking agent (Silquest A-1100, Momentive Performance Materials Japan, Japan), aminopropyl triethoxysilane, was added in the slurry. During standing for 24h, moderate geletion proceeded and disk shape composites in thickness of 1mm were obtained. The obtained composite was heat-treated at 933 or 1073K with hold for 0-10h in an Ar flow. Since the furnace did not equip a cooling system, 0h roughly corresponded to the hold for 5-10min at the appointed temperatures. The heating rate from room temperature to 473 K was 60 K h 1. After a hold at 473 K for 2 h, the sample was heated up to the appointed temperatures with a heating rate of 300 K h 1. The XRD patterns of the pyrolyzed materials were obtained by using an X-ray diffractometor (RINT-1100, Rigaku, Japan). IR spectra were obtained by an FT-IR spectrometer (Spectrum GX, Perkin Elmer, Japan) using the KBr pellet method. The fractured cross section of the pyrolyzed samples was observed by FE-SEM (S-4500, Hitachi, Japan) with EDX (Sigma, Kevex, USA). 3. Result and discussion Isothermal mass loss curves of the synthesized composites in an Ar gas flow were observed by a tubetype vertical furnace system equipped with an automatic balance. The sample weight was g, and the system sensitivity was 0.1mg. At 933K, 3.3% mass loss proceeded during the heating, and moderate mass loss of 4.8% occurred during the hold. At 973K, most of the mass loss (8.0%) proceeded during the temperature rising, and only 1.2% mass loss occurred during the hold. At 1073K, all the mass loss occurred during the temperature rising (9.4%), and no mass loss continued during the hold. The pyrolyzed materials held disk-like shape up to 973K. The material pyrolyzed at 1073K, however, was sometimes suffered from large cracks propagating from thick rim area in the sample. Figure 1 shows XRD patterns of the composites after the holds at 933K. Free Si and Al 4 C 3 phases were observed even after the 0h hold, while the pattern of metal Al was substantially reduced. There was no indication of SiC and Al 2 O 3 in all the XRD patterns, although these phases are thermodynamically stable as compared with free Si, SiO 2 and Al 4 C 3. Even after the 10h hold, main phases were free Si and Al 4 C 3, while the pattern of metal Al disappeared. Figure 2 shows IR spectra of the same samples held at 933K. Si-CH 3 and C-H bonds were reduced during the heating, and completely disappear after the 3h hold. The absorption band of Si-O-Si, however, was remained even after the 10h hold. Reduction of the shoulder at 1120cm -1 probably corresponded to densification of Si-O-Si network during the heat treatment. On the other hand, formed phases after holds at 1073K were completely different. SiC and γ-al 2 O 3 simultaneously formed even after the 0h hold, while a part of metal Al remained (Figure 3). Existence of free Si was certain. Intensity of the free Si pattern was, however, weak as compared with those observed after the 933K holds. In addition, there was no indication of Al 4 C 3 in all the XRD patterns at 1073K. Figure 4 shows IR spectra of the same samples held at 1073K. Absorption bands assigned to organic groups such as C-H, O-H and Si-CH 3 disappeared during the heating. In addition, the Si-O-Si band also showed remarkable decrease and completely disappeared beyond 1h. Only a broad band assigned to Si-C remained after the 10h hold. It means that reduction of the silicone matrix by metal Al filler requires 1073K, while 933K is not sufficient in order to reduce all the Si-O-Si bonds. 2

4 Figure 1. XRD patterns of silicone resin-al composites after the holding at 933K in an Ar gas flow. Figure 2. IR spectra of silicone resin-al composites after the holding at 933K in an Ar gas flow. Figure 3. XRD patterns of silicone resin-al composites after the holding at 1073K in an Ar gas flow. Figure 4. IR spectra of silicone resin-al composites after the holding at 1073K in an Ar gas flow. Figure 5 shows fractured cross sections of the composites held at 933K for 10h. A number of cracks were observed in the cross sections. The cracks were, however, bridged with struts in a diameter of 2-3 µm (Figure 5 (b)). Spherical domains originated from Al particles in the resin matrix existed. The center of the domain was often vacant, and porous shells around the central vacancy were observed (Figure 5 (c)). EDX analysis on the strut region revealed the averaged Si/(Al+Si) molar ratio of 0.21 and the averaged O/(Al+Si) molar ratio of 0.15 (Figure 5 (d)). On the other hand, EDX analysis on the matrix area bridged with the struts revealed the Si/(Al+Si) molar ratio of 0.97 and the O/(Al+Si) molar ratio of The values thus estimated by the EDX analysis are often not completely precise in particular about oxygen. It is, however, apparent that the matrix area is oxygen-rich and does not contain a considerable amount of aluminum. It is known that surface of metal Al is protected by aluminum oxide in ordinary conditions. Such oxide layer may inhibit direct reduction process of the resin matrix by Al. It is, however, remembered that Greil proposed that dispersed filler in the resin usually traps methane evolved from the silicone matrix, and metal carbides will be formed [9]. In the 3

5 cases of metal Al filler, the filler also may trap methane transferred through the surface oxide layer. At the same time, such filler carbonization process yields hydrogen as shown in the following equation. 4Al(s)+3CH 4 (g) Al 4 C 3 (s)+6h 2 (g) (1) Evolved hydrogen can diffuse out from the filler through the oxide layer. Thus silicone resin will be pyrolyzed in a hydrogen-rich atmosphere. It was reported that pyrolysis of polycarbosilane in the H 2 -rich atmosphere, decomposition of methyl groups was accelerated. Such process is now available to produce stoichiometric silicon carbide fiber industrially [12]. Even the silicon-rich composition beyond the stoichiometry was possible when hydrogen potential in the atmosphere was sufficiently high [13]. In the case of silicone resin with the main units of SiO 1.5 (CH 3 )-, the accelerated removal of methyl groups can yield silicon- rich chemical composition as shown in the following equation. -Si(CH 3 )O H 2 (g) 0.75SiO 2 (s)+0.25si(s)+ch 4 (g) (2) Eqs. (1) and (2) can form a cycle. Thus Si and Al 4 C 3 formation are accelerated. It is, however, known that crystallization of excess Si during precursor pyrolysis requires high heat treatment temperature beyond 1273K in general [14,15]. In the case of metal Al filler, however, Al liquid phase can extract the excess Si trapped in the pyrolyzed matrix during the holding. Such Al-Si liquid phase may be an origin of the tiny struts bridging the cracks. Maximum dissolution rate of Si in Al liquid is, however, limited in such a low temperature region. Therefore free Si concentration mechanism is still delicate problem in the proposed simple treatment. The methane - hydrogen cycle model is, however, promising because it can cover very characteristic phenomena observed during the 933K ceramization, which are difficult to be explained by other models. Figure 5. SEM images of fractured cross section of the composites pyrolyzed at 933K; (a) overview, (b) a crack bridged with struts, (c) vacant in Al filler, (d) an individual strut analyzed by EDX. 4

6 On the other hand, Fig. 6 shows fractured cross-sections of the composites pyrolyzed at 1073K after the 10h hold. Whole structure became porous, and matrix area was converted to aggregates of small particles (Figure 6 (b)). EDX analysis on the aggregated area revealed the Si/(Al+Si) molar ratio of 0.43 and the O/(Al+Si) molar ratio of At 1073K, direct reduction of partly pyrolyzed resin matrix by metal aluminum probably proceeded in success. Al (l) +SiO x C y aal 2 O 3 (s) + bsic(s) +csi(s) (3) Dispersed Al particles completely disappeared and pores in a diameter of 3-5 µm appeared at the locations of the dispersed fillers. The pore area was filled by hexagonal plates (Figure 6 (c)). In glance, these hexagonal plates can be assigned to Al 2 O 3 after remarkable grain growth, which may have an origin in the protective oxide layer on the starting Al particles. EDX analysis on these grains, however, revealed a relatively large amount of Si content (the Si/(Al+Si) molar ratio of 0.12) and a relatively small amount of O content (the O/(Al+Si) molar ratio of 0.09). On the other hand, it is difficult to find free Si domains in the fractured surface. In our try, however, we found two or three domains assigned to free Si (Figure 6 (d)). They located at the pore area in the shape of cubic or rectangular. The EDX analysis revealed the Si/(Al+Si) molar ratio of 0.91 and the O/(Al+Si) molar ratio of 0.04, indicating almost free Si. At this temperature range, it is quite probable that direct reduction of the surrounding resin matrix by molten aluminum proceeds. Surface oxide layer on the particles is rapidly destroyed by severe grain growth, and reduction process is triggered. Off course, proposed mechanism is extreme in expression. Chemical formulas shown in eq. (1)-(3) do not proceed in 100%. Dependent on the pyrolysis condition, some reaction should occur incompletely and overlapping of several reactions is also considerable. Figure 6. SEM images of fractured cross section of the composites pyrolyzed at 1073K; (a) overview, (b) matrix area attacked by Al, (c) pores with hexagonal plates embedded in matrix, (d) free Si domain identified by EDX. 5

7 In the previous study, we observed that the open porosity of the Al-resin materials started to increase ca. at 933K, which may have a relationship with the observed large vacancy formation at the filler particular location [6]. On the other hand, the further increase in open porosity at higher heat treatment temperature possibly corresponds to aggregate formation at the matrix area. The mechanical property of the materials with increasing the porosity under different heat treatment history is interesting issue. In order to access the issue, large gel sheet synthesis without cracks is necessary by adjusting composite preparation conditions. Investigation of the composite preparation process is now going on, which includes not only wet process (polymer gel method) but also dry process like hot press by using thermosetting resin nature. 4. Conclusion At low temperature region up to 933K, Al 4 C 3 and free Si formations are accelerated. Trap of methane evolved from the resin by the metal Al particles and decarbonization reaction of the silicone resin accelerated by hydrogen recycled from the filler are probable mechanism to explain the formation of Al 4 C 3 and free Si. Free Si concentration mechanism is possibly extraction of restricted Si from the pyrolyzed matrix by residual metal liquid Al. The identified microstructure is the Si-O-C matrix (may be low carbon content) with cracks bridged with tiny struts rich in Al. At high temperature region beyond the melting point of aluminum, SiC and Al 2 O 3 formations are accelerated rapidly. Direct reduction of the silicone resin by the molten aluminum is probable. Identified microstructure is porous SiC-Al 2 O 3 in chemical composition. The pore area is, however, usually filled with large Al 2 O 3 grains. In rare cases, independent free Si domains are found at the pore area. The role of oxide barrier dividing the acceleration stages by temperature is proposed. The large hexagonal grains growing from the pore walls observed at 1073K may be a trace of severe deconstruction of the surface oxide on the Al particles just before the direct reduction. 5. References [1] Renlund GM, Prochazka S and Doremus RH 1991 J. Mater. Res [2] Burns GT, Taylor RB, Xu Y, Zangvil A and Zank GA 1992 Chem. Mater [3] Colombo P and Modesti M 1999 J. Am. Ceram. Soc [4] Kim YW, Kim SH, Wang C and Park CB 2003 J. Am. Ceram. Soc [5] Zeschky J, Höfner T, Arnold C, Weiβmann R, Bahloul-Hourlier D, Scheffler M and Greil P 2005 Acta Materialia [6] Narisawa M, Sumimoto R, Kita K, Kado H, Mabuchi H and Kim YW 2009 J. Appl. Polym. Sci [7] Riedel R, Toma L, Fasel C and Miehe G 2009 J. Eur. Ceram. Soc [8] Ionescu E, Linck C, Fasel C, Muller M, Kleebe HJ and Riedel R 2010 J. Am. Ceram. Soc [9] Greil P 1995 J. Am. Ceram. Soc. 78: 835 [10] Colombo P, Riccardi B, Donato A and Scarinci G 2000 J. Nuclear Mater [11] Narisawa M, Kado H, Mabuchi H and Kim YW 2010 Appl. Organomet. Chem [12] Takeda M, Sakamoto J, Imai Y and Ichikawa H 1999 Composite. Sci. Technol [13] Takeda M, Saeki A, Sakamoto J, Imai Y and Ichikawa H 2000 J. Am. Ceram. Soc [14] Kobayashi T, Sakakura T, Hayashi T, Yumura M and Tanaka M 1992 Chem. Lett [15] Laine RM and Babonneau F 1993 Chem. Mater Acknowledgments We thank to Professor Young-Wook Kim (The University of Seoul) for information about physical mechanical properties of YR 3370 resin. This work is partly supported by a Grant-in Aid for Scientific Research C (No ) from Japan Society of Promotion Science and a FY2009 Research for Promoting Technological Seeds A (discovery type ) from Japan Science and Technology Agency. 6