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Melt-spinnable polyaluminocarbosilane (PACS) is of importance as the precursor to prepare the Si–C–Al–O ceramic fibers, which can be employed for the preparation of SiC fibers with high tensile strength and good thermal stability. In this work, low-softening-point polycarbosilane (LPCS) was synthesized by pyrolysis of polydimethylsilane and applied to prepare PACS precursors with variable aluminum content by the reaction with aluminum(III) acetylacetonate. GPC, 1H NMR, UV–Vis, FT-IR, 29Si NMR, 27Al MAS NMR, TGA, and elemental analysis were used to analyze the composition and structure of the PACS precursors. Finally, Si–C–Al–O fibers were obtained successfully by melt-spinning, curing, and final pyrolysis of the precursors. The method demonstrated in this work can be further extended to synthesize other melt-spinnable metal-containing polycarbosilane (PMCS, M: Zr, Ti, Fe, Co, et.) of high ceramic yield and adjustable M content by reacting LPCS with other corresponding metal-containing compounds.

Aluminum-containing silicon carbide (SiC) fiber, such as Tyranno SA fiber, is the typical 3rd generation SiC fiber. Polyaluminocarbosilane (PACS) is the precursor of Al-containing SiC fiber. PACS was synthesized via elimination of HCl during the reaction between dimethyaluminium chloride ((CH 3 ) 2 AlCl) and polycarbosilane (PCS) in this work. Composition and structure of the as-synthesized PACS were characterized by elemental analysis, FT IR, GPC, and TG, respectively. The results showed that, the yield of the reaction was 72.5% and the reaction degree of Si–H bonds was 16.5%. The obtained PACS had melting point of about 218.8 °C, experimental formula as SiC 2.003 Al 0.023 O 0.048 H 3.713 , number average molecular weight M n of 1859, weight average molecular weight M w of 4895, polydispersity index ( M w / M n ) of 2.632. The molecular structure of PACS was similar to that of PCS. Four stages could be divided during pyrolysis process of PACS according to TG spectrum. The ceramic yield was found to be around 67.3% at 1000 °C in N 2 atmosphere. The PACS precursor is melt spinnable into flexible and uniform green fibers with the diameter about 12.6 μm.

Wave-transparent components to protect radar equipment from aerodynamic load and heat requires highly mechanical reliability at high temperatures. This study reports the microstructure, high-temperature mechanical properties and oxidation behavior of a new 2.5DSi3N4f/BN wave-transparent composite prepared by a borazine infiltration and pyrolysis route. The obtained composite had a relative density of 91.1% and the derived BN matrix was turbostratic in microstructure with a low interlayer spacing of 3.44 Ǻ. The bending strength of the composite at room temperature was 132.6 MPa, which decreased to 120.8 MPa upon in-situ testing at 1000 °C in air, and further declined to 101.2 and 73.4 MPa when tested at 1200 and 1300 °C without evidence for brittle fracture. The strength degradation was related to the oxidation of the composite at high temperatures. The oxidation of BN matrix was the predominant oxidation of the composite below 1200 °C while liquid B2O3 became less protective as the temperature increased. A more oxygen-resistant viscous SiO2-rich glass was formed beyond 1200 °C due to the oxidation of the reinforcing fibers, which enlarged the temperature range of passive oxidation up to 1400 °C and prevented the composite from excessive oxidation. Keywords: Ceramic matrix composites, High-temperature properties, Mechanical properties, Interface, Oxidation

The structural evolution of a silicon oxynitride fiber reinforced boron nitride matrix (Si-N-Of/BN) wave-transparent composite at high temperatures was investigated. When heat treated at 1600 °C, the composite retained a favorable bending strength of 55.3 MPa while partially crystallizing to Si2N2O and h-BN from the as-received amorphous structure. The Si-N-O fibers still performed as effective reinforcements despite the presence of small pores due to fiber decomposition. Upon heat treatment at 1800 °C, the Si-N-O fibers already lost their reinforcing function and rough hollow microstructure formed within the fibers because of the accelerated decomposition. Further heating to 2000 °C led to the complete decomposition of the reinforcing fibers and only h-BN particles survived. The crystallization and decomposition behaviors of the composite at high temperatures are discussed.

In this work, the preparation of polymer-derived Fe/Si/C ceramics was achieved by synthesis and pyrolysis of the preceramic hyperbranched polyferrocenylsilane. The Fe/Si/C ceramics obtained under various experimental conditions such as different pyrolytic temperatures, atmospheres and heating rates, were investigated by TG–MS, XRD, and XPS. Moreover, the magnetic properties of the ceramics were investigated by the vibrating sample magnetometer (VSM). Experimental results indicated that the pyrolysis of the preceramic polymer started above 250 °C and completed among 800–1000 °C. The crystallization of the pyrolytic product started at 500 °C. Besides the heat treatment temperature, the atmosphere and heating rate also played important roles in the composition and magnetic properties of the corresponding Fe/Si/C ceramics. Fe existed as α -Fe in ceramics sintered at 900 °C under N 2 and NH 3 atmospheres. However, the main components in ceramic product sintered under air were γ -Fe 2 O 3 and carbon. Although all of the three ceramic products sintered under different atmosphere were ferromagnetic with low remanent magnetization and coercivity, the ceramics sintered under NH 3 had the largest saturation magnetization. The saturation magnetization of the Fe/Si/C ceramics further ascended when fabricated under NH 3 with a slower heating rate, in which Fe existed as α -Fe, Fe 3 O 4 and γ ′-Fe 4 N.

In the present study, ZrB 2 precursor which met the requirements of polymer infiltration-pyrolysis process, was synthesized and characterized. ZrB 2 precursor was synthesized with bis(cyclopentadienyl)zirconium dihydride (Cp 2 ZrH 2 ), borane-dimethyl sulfide complex (BH 3 ·S(CH 3 ) 2 ), and vinyltrimethylsilane ((CH 3 ) 3 Si(CH CH 2 ), VTMS). The molecular structure of the precursor, pyrolysis behavior, and the composition of the derived ceramics were investigated. The results showed that, the as-synthesized precursor was an oligomer based on Zr–H–B polycentric bridge bonds. The precursor was stable in an inert atmosphere. The ceramic yield of the precursor at 1200 °C was around 65.5% under N 2 atmosphere. The derived ceramics obtained under 1200 °C were composed of h-ZrB 2 , m-ZrO 2 and t-ZrO 2 . When the temperature was up to 1400 °C, peaks of ZrC emerged due to carbothermic reduction. m-ZrO 2 and t-ZrO 2 disappeared with the pyrolysis temperature above 1600 °C. The derived ceramics pyrolyzed at 1800 °C were composed of h-ZrB 2 and ZrC, and the former was the predominant phase.

Zirconium diboride (ZrB2) is one of the most important ultrahigh temperature ceramics (UHTCs). ZrB2 precursor was synthesized with bis(cyclopentadienyl)zirconium dihydride (Cp2ZrH2) and borane-dimethyl sulfide complex (BH3·S(CH3)2). The influences of molar ratio of reactants and reaction temperature on the solubility of the as-synthesized precursors were investigated. The molecular structure of the precursor, pyrolysis behavior, and the composition of the derived ceramics were investigated by X-ray photoelectron spectroscopy (XPS), Fourier Transformed Infrared Spectroscopy (FT IR), Raman Spectroscopy (RMS), 1H Nuclear Magnetic Resonance Spectroscopy (1H NMR), 11B Nuclear Magnetic Resonance Spectroscopy (11B NMR), Thermogravimetric-Mass Spectroscopy (TG-MS), X-ray Diffraction (XRD), and Scanning Electron Microscopy (SEM), respectively. The results showed that, the precursor was an oligomer based on Zr–H–B polycentric bridge bonds with molecular weight of 750 and formula as (Cp2Zr(BH4)2)3. The precursor would probably further polymerize under vacuum or at high temperature and lead to an insoluble polymer. The ceramic yield of the precursor at 1000 °C was around 66% under N2 atmosphere. After pyrolyzed at 1800 °C, the derived ceramics were composed of h-ZrB2, ZrC, and free carbon with a formula as ZrB1.38C2.18.

Continuous silicon oxynitride fiber reinforced boron nitride matrix (Si–N–Of/BN) composite has been recognized as a promising microwave transparent radome material. In the present study, the ablation behavior and properties of the Si–N–Of/BN composites under an oxyacetylene torch ablation environment were investigated. The average linear and mass ablation rates of the composites decreased with the increasing of the fabrication temperature and were 0.132 mm s−1 and 0.057 g s−1 for the 1200 °C fabricated composites. The ablation products mainly consisted of glassy SiO2 and B2O3, and highly crystallized BN. The ablation center involved three different actions: decomposition of the Si–N–O fibers and sublimation of the BN matrix, oxidations of the matrix and the reinforcing fibers, and thermal treatment of the BN matrix beneath the covering SiO2 film.

The fabrication of a 2.5 dimensional silicon oxynitride fiber fabric reinforced boron nitride matrix (2.5 D Si–N–Of/BN) composites through borazine infiltration and pyrolysis routine is reported. The Si–N–O fiber exhibited an amorphous structure with high composition uniformity. The borazine-to-ceramic conversion completed at 1200 °C and generated hydrogen-free turbostratic BN ceramic. Both the density and crystalline degree of the derived BN products increased with the elevating of pyrolysis temperature. The average bending and tensile strength of the 1200 °C fabricated 2.5 D Si–N–Of/BN composites were 127.4 MPa and 78.3 MPa, respectively. Considerable fiber pull-out was observed throughout the fractured surfaces, which was mainly attributed to the appropriate bonded fiber/matrix interfaces and the relative low elastic modulus of the matrix. Meanwhile, the composites displayed low dielectric constant and loss tangent values of 3.37 and 0.0031, as well as a low thermal conductivity of 1.04 W m−1 K−1.

Silicon carbide nanotubes (SiCNTs) were directly synthesized by chemical vapor deposition (CVD) in the paper. Methyltrichlorosilane (MTS) was selected as the SiC gaseous source and, ferrocence and thiophene as the catalyst and the cocatalyst, respectively. The influences of reaction temperature, contents of catalyst and cocatalyst, and content of gaseous source on the morphologies of the products were investigated, respectively. The products were identified by high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray (EDX), respectively. The synthesis of SiCNTs by CVD suggested a condition-dependent process. Novel SiCNTs, with 20 approximately 80 nm in outer diameter and 15 approximately 35 nm in inner diameter, respectively, were observed. The wall structure similar to that of carbon nanotubes was not found for the SiCNTs.

Polycarbosilane (PCS) synthesized at high temperature under high pressure from liquid polysilane was shown to be an organosilicon polymer with a Si–C backbone. MW of the PCS with a softening point of 209°C was about 2861. Elemental analysis gave an empirical formula of SiC1.86H7.50O0.02. IR and 29Si-NMR showed the presence of SiC4 and SiC3H structural units. The H mole ratio for C–H bond to Si–H bond was about 9.83, and the Si mole ratio for SiC3H to SiC4was about 0.94. TG analysis showed the ceramic yield of the PCS at 1200°C in N2 to be about 78.9%. β-SiC microcrystal was obtained when the PCS was pyrolyzed at 1250°C in N2 with a grain size of about 35.9 A.

A polycarbosilane precursor of SiC fiber was synthesized at high temperature under high pressure from liquid polysilane (LPS), which was obtained by thermal decomposition of poly(dimethylsilane). The effect of reaction temperature on the Si–H bond content, degree of linearity, Si–Si bond content, molecular weight, molecular weight distribution, elemental composition, softening point, and yield of the polycarbosilane (PCS) was studied spectroscopically (FTIR, UV, 1H-NMR, and 29Si-NMR) and by gel permeation chromatography (GPC). The results showed that the molecular weight, yield and softening point of the PCS increased, the molecular weight distribution broadened, and the Si–Si bond content and degree of linearity decreased when the reaction temperature increased. The Si–H bond content increased when the thermolysis reaction temperature was less than 450°C and decreased when the temperature was over 460°C. Increasing of the reaction temperature affected the composition with a general decrease in the amount of carbon, hydrogen and oxygen in the product. A middle molecular weight region of PCS in the GPC appears when the reaction temperature approaches 450°C; and, the as-synthesized PCS was stable with low Si–Si bond content. Synthetically, the conversion process is initially the formation of PCS by thermal decomposition of LPS, which is followed by an increase in molecular weight via condensation of PCS molecules.

The polycarbosilane (PCS), which is the precursor of SiC fiber, was synthesized under high pressure by thermal decomposition of polydimethylsilane. The composition, structure, and properties of the PCS were characterized by the measurements of softening point, elemental analysis, IR, GPC, NMR, TG-DTG-DTA, XRD, and oxidative reaction activity, respectively. Structure model of the PCS was therefore inferred. The results showed that the PCS was the polymer with a SiC backbone with Mn about 1587. IR and NMR showed the presence of SiC4 and SiC3H structure units containing SiCH3, SiCH2Si, and SiH groups. The ratio between H in CH bond and H in SiH bond was about 8.84 with SiC3H/SiC4 and about 0.51 from 1H NMR and 29Si NMR, respectively. Elemental analysis gave an empirical formula of SiC1.87H7.13O0.03. TG analysis showed that the ceramic yield of the PCS at 1200°C in a N2 flow was about 78.9%. β-SiC microcrystal could be obtained when PCS was pyrolyzed at 1250°C with the crystal size about 37.5 A. Compared with the PCS with similar softening point synthesized under normal pressure, the PCS synthesized under high pressure had approximate elemental composition, higher SiH bond content and reaction activity, higher molecular weight, and higher ceramic yield, but lower ratio of SiC3H and SiC4. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1188–1194, 2006

Hollow silicon boron nitride (Si-B-N) ceramic fibers with composition of Si 0.3 BN 1.4 were prepared by melt-spinning, partial curing with trichlorosilane (HSiCl 3 ), and pyrolysis of a novel polyborosilazane under NH 3 up to 1000 °C. The polyborosilazane fibers with low ceramic yield were partially cured to make sure the hollow Si-B-N ceramic fibers could be fabricated after pyrolysis. The hollow Si-B-N ceramic fibers were ~ 16 μm in diameter with inner hollow pore diameter of ~ 4 μm, and showed good average tensile strength of 1.03 GPa and elastic modulus of 106 GPa. Moreover, the hollow Si-B-N ceramic fibers also exhibited excellent dielectric properties with the average dielectric constant real part and loss tangent about 3.06 and 0.0032 at 2–18 GHz, respectively, making them to be promising microwave-transparent materials.

Boron nitride (BN) ceramics were prepared via the preceramic polymer route from two (alkylamino)borazine (AAB) monomers. The effect of monomer structure on the chemical composition, microstructure, oxidation resistance and high-temperature stability of BN was investigated. The two ceramics were characterized by elemental analysis (EA), IR, XPS, XRD, HRTEM and TGA. With similar composition, BN derived from symmetric monomer had a higher crystallinity and exhibited better oxidation resistance/high temperature stability. This is due to that the structure of symmetric molecular derived polymeric precursor is close to that of hexagonal BN ( h -BN).

Nearly stoichiometric boron nitride (BN) fiber with low dielectric constant was prepared by curing, pyrolysis of a novel poly[(alkylamino)borazine] (PAAB) under NH3 up to 1200 °C in a ceramic yield of 60 wt.%. Based on IR, XRD, XPS and elemental analysis (EA), the fiber with a composition of BN1.09 shows a characteristic of turbostratic BN (t-BN). The tensile strength is approximately 0.6 GPa with 13 μm in diameter. Moreover, the fiber possesses very low real part (e′) of 2.66 and loss tangent (tan δ) of 0.0012 at 10 GHz, respectively, making it possible to be used in microwave-transparent material.

A processable N-methylpolyborosilazane was synthesised by an original method by using boron trichloride (BTC), trichlorosilane (TCS) and heptamethyldisilazane (HpMDZ) as the starting materials. The reaction mainly involves the condensation between B–Cl, Si–Cl and –SiMe3 with ClSiMe3 evaporation. The steric N–CH3 in the HpMDZ plays an important role in improving the processing properties of the polymer. The obtained polymer has a relatively high softening point and can easily be melt-spun into polymer fibres and then be converted into high performances SiBNC fibres. This route is suitable for the continuous preparation of the preceramic polymer for SiBNC fibres under mild conditions.

A novel molybdenum-containing polycarbosilane, polymolybdenocarbosilane (PMoCS), for SiC(Mo) fiber has been synthesized from polysilacarbosilane (PSCS) and MoCl5 as raw materials. The synthesis conditions is investigated. Characterization performed on the as-synthesized PMoCS includes Fourier Transformed Infrared Spectroscopy (FT IR), gel-permeation chromatography (GPC), X-ray photoelectron spectroscopy (XPS), 1H NMR, and 29Si NMR, respectively. The polymer-to-ceramic conversion is studied with thermogravimetric analysis (TGA) and X-ray diffraction (XRD). Preliminary research on the melt spinnability of PMoCS is carried out. The results show that the synthesis reaction is temperature-dependent. Mo exists mainly as SiMo bonds. The reaction mechanism is believed to involve HCl elimination between SiH and MoCl, and followed by the Kumada rearrangement. The ceramic yield of PMoCS, prepared with 10 wt % MoCl5, is approximately 78.5% at 1200°C in a N2 flow. The XRD results suggest that ceramics pyrolyzed in the range of 800–1000°C are all amorphous, while β-SiC and β-MoSi2 crystals are observed at 1200 and 1400°C, respectively. The conversion from β-MoSi2 to α-MoSi2 is almost completed at 1600°C. PMoCS is melt spinnable into flexible and uniform green fibers with the diameters about 13.4 μm to 12.2 μm. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

A novel variety of silicon-based ceramic fibers has been prepared from a preceramic organosilicon polymers called polytantalocarbosilane (PTaCS). This melt-spinnable polymer has been synthesized by thermally induced reactions between tantalum (V) tetraethoxyacetylacetonate (Ta(Acac)(OEt) 4 ) and polysilacarbosilane (PSCS). The polymer in which [–Si–C–] n chains are crosslinked via Ta-containing bridges as identified by infrared spectroscopy, XPS and NMR, is decomposed in high ceramic yield (76%) and can be spun in the molten state into fibers to be cured in air then pyrolyzed in flowing nitrogen at 1200 °C into amorphous SiTaC(O) fibers. Complete characterization of this new generation of silicon-based ceramic fibers was made based on mechanical tests, XRD and SEM. These fibers exhibit relatively good mechanical properties and excellent high-temperature stability with good oxidation resistance.

The processing and reaction mechanism of metallic aluminum (Al) powder as an active filler that controls polycarbosilane (PCS) precursor pyrolysis are investigated. Since Al can react with N2 to produce aluminum nitride (AlN), the linear shrinkage upon pyrolysis decreases and the ceramic yield of PCS increases. The linear shrinkage is zero when the volume ratio between Al and PCS, νAl/PCS, is about 56%. Aluminum also enhances the three-point bending strength of the ceramics with a flexural strength of 212 MPa when νAl/PCS is 60%. The relationship among pyrolysis temperature, T p, linear shrinkage and flexural strength of the derived ceramics has been investigated. The results showed that, for the Al-containing PCS-derived ceramics, a linear expansion occurred as the flexural strength was enhanced when T p increased from 400 to 1000°C. The reaction mechanism of Al-controlled PCS pyrolysis was investigated by thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), and elemental line-scanning electron microscopy (ELSEM). The results showed that the SiC powder took on the role of a catalyst, which decreased the nitridation temperature of aluminum and increased the conversion yield from Al to AlN.