Availableonlineatwww.sciencedirect.com SCIENCE DIRECT E噩≈3S ELSEVIER Journal of the European Ceramic Society 25(2005)123-127 www.elsevier.com/locate/jeurceramsoc a new type of precursor for fibers in the system Si-C J. Clade, E Seider, D. Sporn Fraunhofer-Institut fiir Silicatforschnung ISC, Neumerplat: 2, 97082 Wir=burg, Germany Available online 18 September 2004 Abstract Ceramic fibers with compositions in the system SiC have a great potential for high-temperature applications. In recent years, our efforts have been dedicated to the development of polymers consisting of polysilanes suitable to spin fibers and build up matrices for CMC as well The polysilanes are synthesized via disproportionation of the so-called disilane fraction [Richter, R, Roewer, G, Bohme, U, Busch, K Babonneau, F, Martin, H.-P. et al, App. Organomet. Chem., 1997, 11, 71(and references cited therein)). A further thermal treatment yields materials which are soluble in organic solvents, and these solutions can be dry-spun to give fibers which are subsequently pyrolyzed. Solubility and high c yield make this precursor a promising candidate for matrix infiltrations, too. The chemistry and the adjustment of y d solubility to the requirements of the fiber processing as well as the conversion of the dried fibers to pure SiC fibers by thermal O 2004 Elsevier Ltd. All rights reserved Keywords: Polysilanes: Si-C, CMC, Fibers 1. Introduction manufacture of SiC ceramic fibers via melt-spinning; the cur- ing of the green fibers by heating them in air is usually re- SiC fibers have been of great interest for high tempera- quired ture applications for the last 20 years. Yajima et al. -al Many other methods for the preparation of prece- ready obtained polymer-derived SiC fibers in 1975, when ramic polycarbosilanes have been reported, for exam- they heated dodecamethyl cyclohexasilane(already reported ple the Wurtz-like coupling of pheny methyldichlorosilane/ by Burkhard in 1949 )in an autoclave, extracted low molec- dimethyldichlorosilane mixtures> and of r2SiClz/ ular proportions with acetone, dissolved the high molecular RSiCl3 mixtures, 6 the hydrosilylation of vinylsilanes7-19 portion with xylene, dry-spun the solution and pyrolyzed the and the ring-opening polymerization of 1, 3-disilacyclobutane green fibers. A more simple approach, during which the use derivatives with Pt catalysts 20-22 They have been summa of an autoclave could be avoided, was invented by the same rized in several review articles. 23-25 group of researchers, leading to a polycarbosilane which Unfortunately, the fibers commercially available so far was meltable and could thus be spun via the melt-spinning(Nicalon, Hi-Nicalon, Tyranno, etc. )either show insufficient process. 4, 6-8 Since the green fibers obtained in this process stabilities against oxidation or thermal creep due to their were still meltable, they had to be cured by heating in air up rather high oxygen content, or are only available at very high to 200C. This leads to a considerable oxygen content in the costs due to expensive processing steps such as electron beam SiC ceramic fibers Verbeek and Winter applied the synthesis of polycar- sCe g(e.g Hi-Nicalon), which prevents their use at a large bosilanes via a radical polymerization of monosilanes for Disilane mixtures forming as a high-boiling fraction a patent, a reaction which was first described by Fritz et during the Muller-Rochow process- as well as their base- al.0-2 The patent of Verbeek and Winter also describes the catalyzed disproportionation into monosilanes and polysi lanes are well-known. 27-30 Extensive studies of the use off these polysilanes as preceramic polymers were reported by Corresponding author. Baney31-33 as well as roewer and co-workers4(see also 0955-2219/S- see front matter 2004 Elsevier Ltd. All rights reserved doi: 10.1016/j-jeurceramsoc. 2004.07.009
Journal of the European Ceramic Society 25 (2005) 123–127 A new type of precursor for fibers in the system Si–C J. Clade∗, E. Seider, D. Sporn Fraunhofer-Institut f ¨ur Silicatforschung ISC, Neunerplatz 2, 97082 W ¨urzburg, Germany Available online 18 September 2004 Abstract Ceramic fibers with compositions in the system Si–C have a great potential for high-temperature applications. In recent years, our efforts have been dedicated to the development of polymers consisting of polysilanes suitable to spin fibers and build up matrices for CMC as well. The polysilanes are synthesized via disproportionation of the so-called disilane fraction [Richter, R., Roewer, G., Bohme, U., Busch, K., ¨ Babonneau, F., Martin, H.-P. et al., Appl. Organomet. Chem., 1997, 11, 71 (and references cited therein)]. A further thermal treatment yields materials which are soluble in organic solvents, and these solutions can be dry-spun to give fibers which are subsequently pyrolyzed. Solubility and high ceramic yield make this precursor a promising candidate for matrix infiltrations, too. The chemistry and the adjustment of viscosity and solubility to the requirements of the fiber processing as well as the conversion of the dried fibers to pure SiC fibers by thermal treatment will be reported. © 2004 Elsevier Ltd. All rights reserved. Keywords: Polysilanes; Si–C; CMC; Fibers 1. Introduction SiC fibers have been of great interest for high temperature applications for the last 20 years. Yajima et al.1–4 already obtained polymer-derived SiC fibers in 1975, when they heated dodecamethyl cyclohexasilane (already reported by Burkhard in 19495) in an autoclave, extracted low molecular proportions with acetone, dissolved the high molecular portion with xylene, dry-spun the solution and pyrolyzed the green fibers. A more simple approach, during which the use of an autoclave could be avoided, was invented by the same group of researchers, leading to a polycarbosilane which was meltable and could thus be spun via the melt-spinning process.4,6–8 Since the green fibers obtained in this process were still meltable, they had to be cured by heating in air up to 200 ◦C. This leads to a considerable oxygen content in the SiC ceramic fibers. Verbeek and Winter applied the synthesis of polycarbosilanes via a radical polymerization of monosilanes for a patent,9 a reaction which was first described by Fritz et al.10–12 The patent of Verbeek and Winter also describes the ∗ Corresponding author. manufacture of SiC ceramic fibers via melt-spinning; the curing of the green fibers by heating them in air is usually required. Many other methods for the preparation of preceramic polycarbosilanes have been reported, for example the Wurtz-like coupling of phenylmethyldichlorosilane/ dimethyldichlorosilane mixtures13–15 and of R2SiCl2/ RSiCl3 mixtures,16 the hydrosilylation of vinylsilanes17–19 and the ring-opening polymerization of 1,3-disilacyclobutane derivatives with Pt catalysts.20–22 They have been summarized in several review articles.23–25 Unfortunately, the fibers commercially available so far (Nicalon, Hi-Nicalon, Tyranno, etc.) either show insufficient stabilities against oxidation or thermal creep due to their rather high oxygen content, or are only available at very high costs due to expensive processing steps such as electron beam curing (e.g. Hi-Nicalon), which prevents their use at a large scale. Disilane mixtures forming as a high-boiling fraction during the Muller-Rochow process ¨ 26 as well as their basecatalyzed disproportionation into monosilanes and polysilanes are well-known.27–30 Extensive studies of the use of these polysilanes as preceramic polymers were reported by Baney31–33 as well as Roewer and co-workers34 (see also 0955-2219/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2004.07.009
J Clade et al. /ournal of the European Ceramic Sociery 25 (2005 )123-127 Refs. 23-25). The manufacture of SiC ceramic fibers from about 35: 1. The viscosity of the homogeneous solution ob- these polysilanes has also already been described 33-37 Since tained is adjusted to about 2 Pas by partially removing the the polysilanes are meltable solids, green fibers obtained by solvent under vacuum the melt-spinning process must undergo a further curing step, so that they retain their shape and do not stick together during 2.2. Fibe pyrolysis. Although this was successful in some cases. 35,36 the development of a technically running process has met The polymer solution described above is transferred into a considerable difficulties in the past spinning apparatus consisting of a heater block, out of which To overcome these difficulties, we have recently developed the material is extruded by a piston through 1-5 spinning a process starting with the readily available disilane fraction nozzles(o 150 pm; 1=0.45 mm). The polymer fibers were and yielding oxygen-free SiC fibers, during which a curing dried during spinning in a stream of nitrogen at 40C and step is completely superfluous. The key of this procedure is a stretched by winding them up on a rotating spool at veloci further thermal treatment of the initially formed polysilane, ties between 50 and 300 m/min. Fiber bundles with lengths which results in a polymer no longer fusible, but still soluble corresponding to the circumference of the spool were ob- in organic solvents like benzene or toluene Fibers can be tained by cutting the wound-up fibers. These fiber bundles dry-spun from these solutions, and due to the infusibility were then pyrolyzed by pulling them through a vertical tube of the material no further curing step is required before furnace under nitrogen atmosphere. The furnace contained py three heating zones, allowing pyrolysis at a rate of 12 K/min From NMR investigations 38 it is known that polysilanes up to an end temperature of 1200C undergo further crosslinking as well as partial Kumada rearrangement to polysilane-polycarbosilane copolymers 2.3. Fiber characterisation upon heating at temperatures between 350 and 450C. Yet, no investigations about the macroscopic properties of these The oxygen content of the fibers was determined using a products, aiming at their processability, have been reported hot gas extraction analyser( LECO EF-400); fibers prepared under proper conditions contained typically 0.6-0.8 wt% oxygen. The determination of the chlorine content of the 2. Experimental polymers was carried out by Pascher Mikroanalytisches Laboratorium, Remagen/Germany. Tensile strengths and Because of the moisture sensitivity of the compounds used Youngs modulus were measured at room temperature us- in this work, all steps are carried out under inert gas atmo- ing a tensile test machine type Zwicki 1 120(Zwick/roell, sphere(preparation of polymers and polymer solutions under JIm/Germany )and a gauge length of 10 mm. The XRD mea- argon,fiber spinning and pyrolysis under nitrogen) surements were carried out using a Siemens type D 500 diffractometer(Cu Ko radiation, 610 geometry ). For the an 2. 1. Preparation of the polymer solutions nealing and crystallization experiments, a ASTRO furnace with graphite heaters was used; the annealing was carried The disilane fraction used in this work was purchased out under vacuum. Finally, DTA/TG measurements were from Wacker, Burghausen/Germany, and purified by distilla- performed using a thermal analyser(NETZSCH STA 409 tion. The portion boiling at 150-154 C was used. 29Si NMR C). Polymers were analysed under argon atmosphere up to spectra showed that it consisted of a 1: I mixture of 1, 1, 2, 2- 1400C; the heating rate was 10 K/min tetrachlorodimethyldisilane(8(29Si)=180 ppm)and 1, 1,2 trichlorotrimethyldisilane(8(Si)=15.6 ppm, 25.0 ppm) The disproportionation was performed by adding 1-2 wt. 3. Results and discussion of N-methylimidazole as a catalyst and, optionally, 10 wt of phenyltrichlorosilane and by heating to 220C with a 3.1. Preparation and properties of the polymers rate of 0.5 K/min. During heating, a distillate consisting of (CH3 )SiCl3, (CH3)2 SiCl2, traces of (CH3)3 SiCl and higher The polysilanes used in this work were prepared via oligosilanes is obtained. After cooling at room temperature, disproportionation of the disilane fraction purchased from the dark brown residue is heated to 400C with a rate of Wacker, Burghausen/Germany. The disilane fraction was 3 K/min and kept at this temperature for 50 min. During this purified by distillation, and the portion boiling at 150-154C procedure, a small amount of a yellow distillate(consisting was used. 29Si NMR spectra showed that it consisted of a of monosilanes and oligosilanes)is obtained. The product is 1: I mixture of 1, 1, 2, 2-tetrachlorodimethyldisilane(8(Si) then cooled at room temperature, and traces of low molec 180ppm)and 1, 1, 2-trichlorotrimethyldisilane(8(29Si) lar weight components are removed under vacuum. The dark 15.6 ppm, 25.0 ppm). The disproportionation was performed brown material is then dissolved in an excess of toluene under by adding 1-2 wt %of N-methy imidazole, which had turned reflux, and then a solution of l wt. of polystyrene in toluene out to be the preferable catalyst in preliminary experiments, is added so that the weight ratio of copolymer polystyrene is and by heating to 220C. In some experiments, 10 wt%
124 J. Clade et al. / Journal of the European Ceramic Society 25 (2005) 123–127 Refs. 23–25). The manufacture of SiC ceramic fibers from these polysilanes has also already been described.35–37 Since the polysilanes are meltable solids, green fibers obtained by the melt-spinning process must undergo a further curing step, so that they retain their shape and do not stick together during pyrolysis. Although this was successful in some cases,35,36 the development of a technically running process has met considerable difficulties in the past. To overcome these difficulties, we have recently developed a process starting with the readily available disilane fraction and yielding oxygen-free SiC fibers, during which a curing step is completely superfluous. The key of this procedure is a further thermal treatment of the initially formed polysilane, which results in a polymer no longer fusible, but still soluble in organic solvents like benzene or toluene. Fibers can be dry-spun from these solutions, and due to the infusibility of the material no further curing step is required before pyrolysis. From NMR investigations38 it is known that polysilanes undergo further crosslinking as well as partial Kumada rearrangement to polysilane–polycarbosilane copolymers upon heating at temperatures between 350 and 450 ◦C. Yet, no investigations about the macroscopic properties of these products, aiming at their processability, have been reported. 2. Experimental Because of the moisture sensitivity of the compounds used in this work, all steps are carried out under inert gas atmosphere (preparation of polymers and polymer solutions under argon, fiber spinning and pyrolysis under nitrogen). 2.1. Preparation of the polymer solutions The disilane fraction used in this work was purchased from Wacker, Burghausen/Germany, and purified by distillation. The portion boiling at 150–154 ◦C was used. 29Si NMR spectra showed that it consisted of a 1:1 mixture of 1,1,2,2- tetrachlorodimethyldisilane (δ( 29Si) = 18.0 ppm) and 1,1,2- trichlorotrimethyldisilane (δ( 29Si) = 15.6 ppm, 25.0 ppm). The disproportionation was performed by adding 1–2 wt.% of N-methylimidazole as a catalyst and, optionally, 10 wt.% of phenyltrichlorosilane and by heating to 220 ◦C with a rate of 0.5 K/min. During heating, a distillate consisting of (CH3)SiCl3, (CH3)2SiCl2, traces of (CH3)3SiCl and higher oligosilanes is obtained. After cooling at room temperature, the dark brown residue is heated to 400 ◦C with a rate of 3 K/min and kept at this temperature for 50 min. During this procedure, a small amount of a yellow distillate (consisting of monosilanes and oligosilanes) is obtained. The product is then cooled at room temperature, and traces of low molecular weight components are removed under vacuum. The dark brown material is then dissolved in an excess of toluene under reflux, and then a solution of 1 wt.% of polystyrene in toluene is added so that the weight ratio of copolymer:polystyrene is about 35:1. The viscosity of the homogeneous solution obtained is adjusted to about 2 Pa s by partially removing the solvent under vacuum. 2.2. Fiber spinning and pyrolysis The polymer solution described above is transferred into a spinning apparatus consisting of a heater block, out of which the material is extruded by a piston through 1–5 spinning nozzles (Ø 150 m; l = 0.45 mm). The polymer fibers were dried during spinning in a stream of nitrogen at 40 ◦C and stretched by winding them up on a rotating spool at velocities between 50 and 300 m/min. Fiber bundles with lengths corresponding to the circumference of the spool were obtained by cutting the wound-up fibers. These fiber bundles were then pyrolyzed by pulling them through a vertical tube furnace under nitrogen atmosphere. The furnace contained three heating zones, allowing pyrolysis at a rate of 12 K/min up to an end temperature of 1200 ◦C. 2.3. Fiber characterization The oxygen content of the fibers was determined using a hot gas extraction analyser (LECO EF-400); fibers prepared under proper conditions contained typically 0.6–0.8 wt.% oxygen. The determination of the chlorine content of the polymers was carried out by Pascher Mikroanalytisches Laboratorium, Remagen/Germany. Tensile strengths and Young’s modulus were measured at room temperature using a tensile test machine type Zwicki 1120 (Zwick/Roell, Ulm/Germany) and a gauge length of 10 mm. The XRD measurements were carried out using a Siemens type D 5005 diffractometer (Cu K radiation, θ/θ geometry). For the annealing and crystallization experiments, a ASTRO furnace with graphite heaters was used; the annealing was carried out under vacuum. Finally, DTA/TG measurements were performed using a thermal analyser (NETZSCH STA 409 C). Polymers were analysed under argon atmosphere up to 1400 ◦C; the heating rate was 10 K/min. 3. Results and discussion 3.1. Preparation and properties of the polymers The polysilanes used in this work were prepared via disproportionation of the disilane fraction purchased from Wacker, Burghausen/Germany. The disilane fraction was purified by distillation, and the portion boiling at 150–154 ◦C was used. 29Si NMR spectra showed that it consisted of a 1:1 mixture of 1,1,2,2-tetrachlorodimethyldisilane (δ( 29Si) = 18.0 ppm) and 1,1,2-trichlorotrimethyldisilane (δ( 29Si) = 15.6 ppm, 25.0 ppm). The disproportionation was performed by adding 1–2 wt.% of N-methylimidazole, which had turned out to be the preferable catalyst in preliminary experiments, and by heating to 220 ◦C. In some experiments, 10 wt.%
J Clade et al /Journal of the European Ceramic Sociery 25 (2005)123-12 Table I Behaviour of polysilane-polycarbosilane copolymers obtained after a thermal treatment of polysilanes Sample Themal treatment (C/min) Solubility(solvent) Cl content(wt % 330/60 soluble(CH? CI) Melting range 160-180oC(bulk) 16.7 400/60 soluble(CH CI No melting below 370C(bulk) 12.2 Readily soluble(tolt Melting range 90-120.C (bulk) 21.7 4678 400/60 Readily soluble(CH2Cl2 No melting below 370 C(bulk) 10.0 Readily soluble(benzene) Green fibers partially fused after pyrolysis Not determined Readily soluble(benzene) Green fibers partially fused after pyrolys Not determined eadily soluble(benzene, toluene) Green fibers not fused after pyrolysis Disproportionation was carried out after addition of 10 wt. of phenyltrichlorosilane to the disilane mixture. of phenyltrichlorosilane were added before heating. In The TG curve shows a strikingly low mass loss, which preliminary experiments, the latter was shown to modify means a strikingly high ceramic yield of approximately the crosslinking behaviour of the polysilane, leading to a 85 wt % Since the material is readily soluble in solvents like monomodal molecular weight distribution. Additionally, toluene and since the viscosity of these solutions can the addition of phenyltrichlorosilane before the dispro- ied continuously by their polymer content, this high ce of the polysilane-polycarbosilane copolymers obtained infiltrations, too material a promising candidate for m portionation seems to have an influence on the solubility the after a further thermal treatment of the polysilanes(see 3. 2. Fiber spinning In a series of experiments, the fusibility and solubility of copolymers, which resulted from the thermal treatment of the Several preliminary experiments were performed to find polysilanes described above, were investigated. Table I gives out a solvent suitable for the dry-spinning process. The an overview of the results copolymers were readily soluble in methy lene chloride, but From the data in the table it can be seen that the fusibility this solvent evaporated too fast during the spinning process, of the material is drastically reduced by thermal treatment, so that no fibers could be obtained. Benzene showed similar whereas its solubility in organic solvents is not significantly results. although in some cases a few fibers could be collected influenced in the temperature regime examined. Some sam- Toluene showed even better results, allowing continuous fiber ples(not mentioned in the table)were only partially solu- spinning during several hours A crucial point for fiber spinning via the dry-spinning pro- lanes which resulted from disproportion of the disilane mix- cess is the rheological properties of the copolymer solutions ture in the presence of pheny trichlorosilane always remaine solutions with viscosities of about 2-5 Pas turned out to be completely soluble in benzene or toluene. Furthermore, a de- appropriate. Furthermore, the addition of an organic crease of the chlorine content from about 20 to 25 wt. in the polysilane to about 10 wt. in the copolymer can be ob- could not be spun under tensile force, resulting in unwanted high diameters and a corresponding brittleness of the green by Babonneau et al. 38 fibers. Thus, the addition of 2 wt. of polystyrene to solutions The ceramic yield of the copolymers obtained by thermal treating of the polysilanes was examined by DTA/TG mea- (Fg.1) DTA/PVImg /% 14.53 700°c 1400°C 200400600800100012 emperature/C Fig. 2. X-ray powder diagrams of Sic yrolyzed(bot Fig 1. DTA/TG measurement of a polysilane-polycarbosilane copolymer tom curve ), and heated in a vacuum up to 1400C, 1700C, and 1900C, obtained by the thermal treatment of methylchloropolysilane respectively(upper curves)
J. Clade et al. / Journal of the European Ceramic Society 25 (2005) 123–127 125 Table 1 Behaviour of polysilane–polycarbosilane copolymers obtained after a thermal treatment of polysilanes Sample Thermal treatment (◦C/min) Solubility (solvent) Fusibility Cl content (wt.%) 1 – Readily soluble (toluene) Melting range 85–110 ◦C (bulk) 24.4 2 330/60 Readily soluble (CH2Cl2) Melting range 160–180 ◦C (bulk) 16.7 3 400/60 Readily soluble (CH2Cl2) No melting below 370 ◦C (bulk) 12.2 4a – Readily soluble (toluene) Melting range 90–120 ◦C (bulk) 21.7 5a 400/60 Readily soluble (CH2Cl2) No melting below 370 ◦C (bulk) 10.0 6 350/40 Readily soluble (benzene) Green fibers partially fused after pyrolysis Not determined 7 390/3 Readily soluble (benzene) Green fibers partially fused after pyrolysis Not determined 8a 400/50 Readily soluble (benzene, toluene) Green fibers not fused after pyrolysis 10.3 a Disproportionation was carried out after addition of 10 wt.% of phenyltrichlorosilane to the disilane mixture. of phenyltrichlorosilane were added before heating. In preliminary experiments, the latter was shown to modify the crosslinking behaviour of the polysilane, leading to a monomodal molecular weight distribution. Additionally, the addition of phenyltrichlorosilane before the disproportionation seems to have an influence on the solubility of the polysilane–polycarbosilane copolymers obtained after a further thermal treatment of the polysilanes (see below). In a series of experiments, the fusibility and solubility of copolymers, which resulted from the thermal treatment of the polysilanes described above, were investigated. Table 1 gives an overview of the results. From the data in the table it can be seen that the fusibility of the material is drastically reduced by thermal treatment, whereas its solubility in organic solvents is not significantly influenced in the temperature regime examined. Some samples (not mentioned in the table) were only partially soluble after thermal treatment, but those obtained from polysilanes which resulted from disproportion of the disilane mixture in the presence of phenyltrichlorosilane always remained completely soluble in benzene or toluene. Furthermore, a decrease of the chlorine content from about 20 to 25 wt.% in the polysilane to about 10 wt.% in the copolymer can be observed during thermal treatment, as has already been reported by Babonneau et al.38 The ceramic yield of the copolymers obtained by thermal treating of the polysilanes was examined by DTA/TG measurements (Fig. 1). Fig. 1. DTA/TG measurement of a polysilane–polycarbosilane copolymer obtained by the thermal treatment of methylchloropolysilane. The TG curve shows a strikingly low mass loss, which means a strikingly high ceramic yield of approximately 85 wt.%. Since the material is readily soluble in solvents like toluene and since the viscosity of these solutions can be varied continuously by their polymer content, this high ceramic yield makes the material a promising candidate for matrix infiltrations, too. 3.2. Fiber spinning Several preliminary experiments were performed to find out a solvent suitable for the dry-spinning process. The copolymers were readily soluble in methylene chloride, but this solvent evaporated too fast during the spinning process, so that no fibers could be obtained. Benzene showed similar results, although in some cases a few fibers could be collected. Toluene showed even better results, allowing continuous fiber spinning during several hours. A crucial point for fiber spinning via the dry-spinning process is the rheological properties of the copolymer solutions; solutions with viscosities of about 2–5 Pa s turned out to be appropriate. Furthermore, the addition of an organic polymer as a spinning aid was necessary, since otherwise fibers could not be spun under tensile force, resulting in unwanted high diameters and a corresponding brittleness of the green fibers. Thus, the addition of 2 wt.% of polystyrene to solutions Fig. 2. X-ray powder diagrams of SiC ceramic fibers as pyrolyzed (bottom curve), and heated in a vacuum up to 1400 ◦C, 1700 ◦C, and 1900 ◦C, respectively (upper curves)
J Clade et al. /Journal of the European Ceramic Sociery 25(2005)123-127 Fig 3.(Left) SEM image of SiC fibers obtained from a polysilane-polycarbosilane copolymer (Right)SEM image of a fracture surface of an SiC fiber containing 60-70 wt. of copolymer in toluene led to a mate- leading to an infusible copolymer which is still soluble in rial which could be spun continuously into green fibers with organic solvents and thus suitable for e.g. dry-spinning. No diameters of about 30 um. The further optimization of the curing of the green fibers before pyrolysis is required. Fur spinning conditions, aiming at even lower green-fiber diam- ther work, aiming at an optimization of the fiber properties eters, is in progress. shape, mechanical strength, microstructure), is in progress 3.3. Fiber properties Acknowledgements After pyrolysis at 1200C under nitrogen, the fibers show room-temperature tensile strengths of up to 800 MPa(diam- This work is based upon investigations performed in close eters of about 25 um)and Youngs moduli of up to 120 GPa; cooperation with the Institut fur Anorganische Chemie of the their oxygen content is less than I wt. %. Annealing up to Te hnische Universitat Bergakademie Freiberg, Germany 1900C leads to an increase of the Young's moduli to val- Therefore, Professor Dr. Gerhard Roewer, Dr. Dorit Mein- ues of up to 170 GPa at room temperature without a signif- hold and Thomas lange are gratefully acknowledged for this icant alteration of the tensile strength. In comparison, SiC cooperation. Professor Dr Martin Jansen and Dr. Thomas fibers obtained by Roewer et al. via the melt-spinning of a Jaschke of the Max Planck-Institut fur Festkorperforschung methylchloropolysilane, the curing of the green fibers un- in Stuttgart, Germany, are also gratefully acknowledged for der ammonia and the subsequent pyrolysis under argon up to the dTA/Tg measurements 1200C showed tensile strengths of up to 1000 MPa at room temperature; the Youngs modulus was estimated to be about 150GPa35 References During the heating process, the crystallization of p-SIC takes place, as is indicated by the X-ray powder diagrams in 1. Yajima, S, Hayashi, J. and Omori, M, Chem. Lett, 1975, 931 Fig 2 SEM photographs of the fibers(Fig 3)show that their 2. Yajima, S, Okamura, K. and Hayashi, J, Chem. Lett., 1975, 1209 cross-section is not perfectly circular, but oval or kidney 3. Yajima, S, Liaw, C, Omon, M. and Hayashi, J, Chem. Letf, 1976, shaped. Additionally, in most of the fracture surfaces little voids can be seen, which explains the relatively poor ten- 4. Yajima, S, Hayashi, J and Omori, M, US 4, 100, 233(1978) Burkhard. C. A.J. Am. Chem. Soc.. 1949.71.963 sile strengths. To overcome these disadvantages further work S, Omori, M, Hayashi, J. and Okamura, K, Chem. Lett. aiming at an optimization of the spinning, drying step, and 976.551 the pyrolysis process is in progress 7. Yajima, S, Hasegawa, Y, Hayashi, J and himura, M, J. Mater Sci. 978,13,2569 8. Hasegawa, Y, limura, M. and Yajima, S.,J. Mater: Sci., 1980, 15, 4. Conclusions 9. Verbeek, W. and Winter, G, DE 2, 236,078(197. 10. Fritz, G. and Raabe, B, Z. Norg. Allg. Chem., 1956, 286, 149 We have developed a new process which leads to Sic ce- IL. Fritz, G and Raabe, B, Z Anorg. Alg. Chem., 1959, 299, 232 ramic fibers with a low oxygen content, starting from readily 12. Fritz, G, Habel, D, Kummer, D and Teichmann, G, Z Norg. Allg available raw materials. The key step in this process consists 13. West, R, David, L. D, Djurovich, P. 1 and Yu, H, Bull. Am. Ceram of a thermal treatment of the initially synthesized polysilane Soc,1983,62,899
126 J. Clade et al. / Journal of the European Ceramic Society 25 (2005) 123–127 Fig. 3. (Left) SEM image of SiC fibers obtained from a polysilane–polycarbosilane copolymer. (Right) SEM image of a fracture surface of an SiC fiber. containing 60–70 wt.% of copolymer in toluene led to a material which could be spun continuously into green fibers with diameters of about 30 m. The further optimization of the spinning conditions, aiming at even lower green-fiber diameters, is in progress. 3.3. Fiber properties After pyrolysis at 1200 ◦C under nitrogen, the fibers show room-temperature tensile strengths of up to 800 MPa (diameters of about 25m) and Young’s moduli of up to 120 GPa; their oxygen content is less than 1 wt.%. Annealing up to 1900 ◦C leads to an increase of the Young’s moduli to values of up to 170 GPa at room temperature without a significant alteration of the tensile strength. In comparison, SiC fibers obtained by Roewer et al. via the melt-spinning of a methylchloropolysilane, the curing of the green fibers under ammonia and the subsequent pyrolysis under argon up to 1200 ◦C showed tensile strengths of up to 1000 MPa at room temperature; the Young’s modulus was estimated to be about 150 GPa.35 During the heating process, the crystallization of -SiC takes place, as is indicated by the X-ray powder diagrams in Fig. 2. SEM photographs of the fibers (Fig. 3) show that their cross-section is not perfectly circular, but oval or kidneyshaped. Additionally, in most of the fracture surfaces little voids can be seen, which explains the relatively poor tensile strengths. To overcome these disadvantages, further work aiming at an optimization of the spinning, drying step, and the pyrolysis process is in progress. 4. Conclusions We have developed a new process which leads to SiC ceramic fibers with a low oxygen content, starting from readily available raw materials. The key step in this process consists of a thermal treatment of the initially synthesized polysilane, leading to an infusible copolymer which is still soluble in organic solvents and thus suitable for e.g. dry-spinning. No curing of the green fibers before pyrolysis is required. Further work, aiming at an optimization of the fiber properties (shape, mechanical strength, microstructure), is in progress. Acknowledgements This work is based upon investigations performed in close cooperation with the Institut fur Anorganische Chemie of the ¨ Technische Universitat Bergakademie Freiberg, Germany. ¨ Therefore, Professor Dr. Gerhard Roewer, Dr. Dorit Meinhold and Thomas Lange are gratefully acknowledged for this cooperation. Professor Dr. Martin Jansen and Dr. Thomas Jaschke of the Max Planck-Institut f ¨ ur Festk ¨ orperforschung ¨ in Stuttgart, Germany, are also gratefully acknowledged for the DTA/TG measurements. References 1. Yajima, S., Hayashi, J. and Omori, M., Chem. Lett., 1975, 931. 2. Yajima, S., Okamura, K. and Hayashi, J., Chem. Lett., 1975, 1209. 3. Yajima, S., Liaw, C., Omori, M. and Hayashi, J., Chem. Lett., 1976, 435. 4. Yajima, S., Hayashi, J. and Omori, M., US 4,100,233 (1978). 5. Burkhard, C. A., J. Am. Chem. Soc., 1949, 71, 963. 6. Yajima, S., Omori, M., Hayashi, J. and Okamura, K., Chem. Lett., 1976, 551. 7. Yajima, S., Hasegawa, Y., Hayashi, J. and Iimura, M., J. Mater. Sci., 1978, 13, 2569. 8. Hasegawa, Y., Iimura, M. and Yajima, S., J. Mater. Sci., 1980, 15, 720. 9. Verbeek, W. and Winter, G., DE 2,236,078 (1972). 10. Fritz, G. and Raabe, B., Z. Anorg. Allg. Chem., 1956, 286, 149. 11. Fritz, G. and Raabe, B., Z. Anorg. Allg. Chem., 1959, 299, 232. 12. Fritz, G., Habel, D., Kummer, D. and Teichmann, G., Z. Anorg. Allg. Chem., 1959, 302, 60. 13. West, R., David, L. D., Djurovich, P. I. and Yu, H., Bull. Am. Ceram. Soc., 1983, 62, 899.
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