Polycarbosilane Research Articles

Synthesis and Properties of Chemically Modified Polycarbosilane Containing Organofluoric Groups

The chemically modified polycarbosilane (PC) containing organofluoric groups, PCOCF, was synthesized through chemical reaction of PC and fluoric alcohol. Pyrolysis of the polymer precursor occurred in three stages under 1000 °C. Weight loss due to evaporation of low molecular compounds was observed at the first stage below 300 °C, followed by the largest weight loss from 300 to 600 °C at the second stage. At this stage, side chains formed during the chemical reaction were removed. Finally, side chains such as Si–CH3 decomposed at the third stage above 600 °C. At 1000 °C, the chemical modification of PC resulted in high yield of solid product. After coating PCOCF on silicon carbide (SiC) powders, the conversion yield of pyrolyzed PCOCF was further improved due to the interaction between PCOCF and SiC powders. Coating of PCOCF on SiC powders was found to be effective in increasing the green density of uniaxially pressed bodies.

Use of Blended Precursors of Poly(vinylsilane) in Polycarbosilane for Silicon Carbide Fiber Synthesis with Radiation Curing

Blended organosilicon precursors containing 10 or 20% of poly(vinylsilane) (PVS) in polycarbosilane (PCS) were prepared and shaped into fiber form by melt‐spinning. The influence of PVS addition on the spinning, radiation‐curing, and pyrolysis processes was investigated. The addition of PVS increased the spinability of the precursor melt and increased the oxidation sensitivity of the precursor. By adjusting the precursor compositions and the radiation conditions, highly heat‐resistant silicon carbide fibers were obtained.

Development of silicon nitride fiber from Si-containing polymer by radiation curing and its application

Abstract A silicon nitride fiber (Si3N4) was synthesized from polycarbosilane (PCS) fiber by radiation application. PCS fibers were cured by electron beam (EB) irradiation in a helium gas atmosphere prior to the pyrolysis. The cured PCS fiber was converted to Si3N4 ceramic fiber with flexibility by nitridation in ammonia gas at a high temperature of 500–1000°C. The obtained Si3N4 fibre showed a high heat resistance up to 1300°C, a high tensile strength of 2 GPa and excellent electrical resistivity of 1013 Ω cm. The ceramic fiber was fabricated to cloth and applied for electric wire insulator. The wire cable is flexible and can be applied at a high temperature atmosphere of around 1000°C.

Processing of silicon carbide ceramics using chemically modified polycarbosilanes

Chemically modified polycarbosilane (PC) which contains Si–Al–C–O component, PCOAl, was synthesized using PC and aluminum triisopropoxide. Ceramic yield was greatly improved through the modification of PC with a metal alkoxide. The phase transformation behavior and microstructure development of silicon carbide (SiC) were studied on β–SiC powders coated with chemically modified PC. The β-α phase transformation of SiC was enhanced by the coating of chemically modified PC on β–SiC powder. A unique microstructure with submicron-sized plate-like grains was developed, since the fine a phase produced at low temperature served as a nucleation site for the β-α phase transformation of SiC.

Effect of High-Yield Polycarbosilane Addition on Microstructure and Mechanical Properties of Alumina.

Effect of polycarbosilane (PC), with high ceramic yield of 85%, added to Al2O3 was investigated. Green density of the Al2O3 powder compact increases by the PC coating. Al2O3 ceramics containing 5 to 10vol% SiC derived from PC were fabricated by the hot-press sintering at temperatures between 1600°C and 1800°C. After hot-pressing at 1600°C, almost all PC-derived SiC-phase exists at grain boundaries as fine crystalline grains, which results in a significant decrease in Al2O3 grain-growth rate and in a low fraction of porosity as well. Some of the PC-derived SiC-phase may be drawn inside the Al2O3 grains during grain growth at 1800°C to form intragranular SiC nanoparticles. Four-point bending strength of Al2O3 was increased, and the scatter in bending strength values significantly decreased by the PC coating. Intragranular fracture mode was observed in the PC-coated Al2O3 composite, while Al2O3+SiC powder composite exhibited intergranular fracture.

Fabrication of polycarbosilane-derived SiC bulk ceramics by carbothermic reduction: Effect of green density on crystallinity of pyrolyzed compacts

We have recently demonstrated that the fabrication of SiC bulk ceramics with various atomic ratio of Si:C:O is achieved by pyrolysis and subsequent heat treatment of shaped polycarbosilane (PCS) compacts after controlling oxygen content. The procedure is a novel processing route of the combination of polymer precursor techniques, powder technological process and high-temperature material chemistry. This paper deals with the investigation of the effect of the green density and specimen size on the crystallization and densification behavior during the pyrolysis conversion from polymer into ceramic grade. It is reported that the pyrolyzed PCS compact with a higher green density shows a finer microstructure and a smaller pore size.

Soft X-ray fluorescence measurements of irradiated polymer films

Fluorescent soft X-ray carbon Kα emission spectra (XES) have been used to characterize the bonding of carbon atoms in polyimide (PI) and polycarbosilane (PCS) films. The PI films have been irradiated with 40 keV nitrogen or argon ions, at fluences ranging from 1 × 10 14 to 1 × 10 16 cm −2. The PCS films have been irradiated with 5 × 10 15 carbon ions cm −2 of 500 keV and/or annealed at 1000°C. We find that the fine structure of the carbon XES of the PI films changes with implanted ion fluence above 1 × 10 14 cm −2 which we believe is due to the degradation of the PI into amorphous C:N:O. The width of the forbidden band as determined from the high-energy cut-off of the C Kα X-ray excitation decreases with the ion fluence. The bonding configuration of free carbon precipitates embedded in amorphous SiC which are formed in PCS after irradiation with C ions or combined treatments (irradiation and subsequent annealing) is close to either to that in diamond-like films or in silicidated graphite, respectively.

98/03286 Industrial applications of SiBN(C) ceramics made from polymeric precursors: Baldus, H. P. et al. Fortschrittsberg. Dtsch. Keram. Ges., 1997, 12, (1), 117–131. (In German)

A polycarbosilane (PCS) blended with tetraethyl orthosilicate (TEOS) in xylene solvent was used as a precursor for developing anti-oxidation coatings on a needle-punched carbon fiber felt with pyrolytic carbon deposition. Elemental analysis, Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TG) were employed to characterize curing products of the precursor. The converted ceramic products of the precursor under Ar flow and in vacuum to a maximum temperature of 1000 °C and 1200 °C, respectively, were investigated by FT-IR, X-ray diffraction (XRD), TG and transmission electron microscope (TEM) with energy-dispersive X-ray spectroscopy (EDX). All pyrolysis products were found to contain β-SiC and amorphous SiO2 with more acute β-SiC XRD peaks obtained for the higher pyrolysis temperature. Two representative TEM regions were shown to have different empirical chemical compositions. Needled carbon fiber felts with a pyrolytic carbon inner deposition layer impregnated in the precursors were cured in air and then pyrolyzed at high temperatures to form duplex SiC-based outer coating layers. Comparative study of a series of processing conditions through scanning electron microscope (SEM) observation indicated that, carbon fiber preforms, which were coated by using the 10% PCS and 5 wt% TEOS precursor in xylene and repeating the impregnation–pyrolysis cycle twice, witnessed relatively complete coverage of the coating in the thickness range of 35–120 nm with a small amount of pyrolysis product lumping.

Thermal oxidation crosslinking in the blended precursors of organosilicon polymers containing polyvinylsilane with polycarbosilane

The polymer precursors containing polyvinylsilane (PVS) with polycarbosilane (PCS) in coating form were prepared. As the content of PVS increases, the onset temperature of the precursor oxidation decreases and the surfaces formed after the oxidation become smooth and hard. The role of Si–H bonds in the PCS–PVS systems during the oxidation is quantitatively analysed on the basis of the absorption of the Si–H stretching band in the precursor spectra. In the blended systems, the oxidation of the Si–H groups forms the dense crosslinked polymer layer which limits the oxygen diffusion into the precursor interior. © 1998 Chapman & Hall

Silicon-Based Interpenetrating Polymer Networks (IPNs): Synthesis and Properties

Both “condensation” and “hydrosilylation” reactions have been successfully applied for the synthesis of silicon-based interpenetrating polymer networks (IPNs) consisting of a stable Si−O and Si−C linkage. We adopted a ladder silsesquioxane oligomer (LDS 1; commercial name, Glass Resin GR100) and a polycarbosilane (PCS) by hydrosilylation polymerization of bifunctional Si−H and Si−vinyl monomers as the two components for the IPN. The combined LDS/PCS curing system was studied to try to obtain homogeneously cured material, which may lead to IPN formation, by using both reactions selectively in one pot for chain-extending and cross-linking of both components, respectively. We synthesized the cured LDS/PCS; LDS/PCS = 8/2, 5/5, and 2/8 [wt/wt]. Tuning the rates of the two reactions to each other was important for obtaining a transparent cured product. The morphology of the cured LDS/PCS (8/2) was examined by TEM and AFM. TEM analysis showed a good miscibility with an nm level of LDS/PCS (8/2). The thermal and mechanical properties of the LDS/PCS (8/2) cured sample were better than the calculated values from each component's value. The experimental data imply the IPN structure of the cured LDS/PCS.

Synthesis of Fibrous Shape Ceramics from Blended Precursors of Organosilicon Polymers Containing Polymethylsilane

Viscous solutions of polymethylsilane (PMS) and polycarbosilane (PCS) mixed at various ratios were prepared and shaped into fiber form by the dry-spinning method using benzene as solvent. The addition of PMS to PCS lowered onset temperature of the precursor fiber oxidation, and slightly increased the residual mass after pyrolysis. The fibers from the precursors containing PMS yielded the ceramic fibers after heat treatment at 1273 K without degradation and fusing, but they changed into crystalline fibers having thin amorphous layer on the surface after the heat treatment at 1673 K.

Synthesis of Silicon Carbide Thin Films with Polycarbosilane (PCS)

Polycarbosilane (PCS) thin films were deposited on silicon (and other) substrates and heat treated under vacuum (∼10‐‐6>torr)at temperatures in the range of 200°–1200°C. At temperatures in the range of 1000°–1200°C, the initially amorphous PCS films transformed to polycrystalline ß‐silicon carbide (ß‐SiC). Although PCS films could be deposited at thickness up to 2 μm, the films with thicknesses >1 μm could not be transformed to SiC without extensive cracking. The resulting SiC coatings were characterized using Fourier transform infrared spectroscopy, glancing‐angle X‐ray diffractometry, secondary‐ion mass spectroscopy, Raman spectoscopy, transmission electron microscopy, and scanning electron microscopy. The temperature and time dependence of the amorphous‐to‐crystalline transition could be associated with the evolution of free carbon, oxygen, and hydrogen in the films.

Dissolution of oxidation-cured poly(carbosilane) fiber into polar organic solvents

The solubility of oxidation-cured polycarbosilane (PCS) fiber into benzene, tetrahydrofuran (THF), acetone, and ethanol was investigated. The solubility of the fiber in benzene and THF decreases as the curing temperature increases. The fiber cured at 433–463 K, however, shows higher solubility in acetone and ethanol than the original fiber and the fiber cured at 473 K. The remaining gels in benzene and THF keep their fibrous shape, while in acetone and ethanol often appear as swollen gels. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:261–265, 1997

Dissolution of oxidation‐cured poly(carbosilane) fiber into polar organic solvents

The solubility of oxidation-cured polycarbosilane (PCS) fiber into benzene, tetrahydrofuran (THF), acetone, and ethanol was investigated. The solubility of the fiber in benzene and THF decreases as the curing temperature increases. The fiber cured at 433–463 K, however, shows higher solubility in acetone and ethanol than the original fiber and the fiber cured at 473 K. The remaining gels in benzene and THF keep their fibrous shape, while in acetone and ethanol often appear as swollen gels. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:261–265, 1997

Pore Structure and Permeance of Amorphous Si-C-O Membranes with High Durability at Elevated Temperature

A porous α-alumina tube of 2.4 mm o.d. with an average pore size of ca. 150 nm was coated with p-xylene solutions of polycarbosilane (PC) and polystyrene (PS). The weight fraction of PS to total solid mass (PC + PS) was varied as 0, 1, 3 and 5 wt% (denoted as PC-PSx for a solution of x wt%). Then the film was cured in air at 473 K and pyrolyzed in argon at 1223 K. The procedure of dipping, drying, and pyrolysis was repeated three or four times in total. An Si-C-O membrane that was prepared with the PC-PS1 solution exhibited an H2 permeance of 4 × 10−8mol·m−2·s−1·Pa−1 and an H2/N2 selectivity of 20 at 773 K. The PS was concentrated into ellipsoidal domains due to phase separation during the curing step, and it was depolymerized and dissipated during the pyrolysis step. All membranes were resistant to heat treatment at 1223 K in argon for 20 hours. Except for the PC-PS5 membrane, voids that were formed by the decomposition of PS did not perforate the top layer. When the PC-PS1 membrane was exposed to a mixture of steam and helium at 773 K, the permeance was decreased to half in the first day, but was not greatly changed during the next 2 days.

Continuous SiC-based model monofilaments with a low free carbon content: Part I: From the pyrolysis of a polycarbosilane precursor under an atmosphere of hydrogen

Pyrolysis of a Yajima-type polycarbosilane (PCS) has been performed under an atmosphere of hydrogen on both bulk samples and model monofilaments up to 1000°C, in order to reduce the free carbon content of the resulting ceramics. The organic/inorganic transition occurs within the 400–800°C temperature range, with mainly an evolution of CH4. At 1000°C, it yields an hydrogenated amorphous ceramic with a C/Si atomic ratio and a free carbon content significantly lower than for its counterpart obtained under inert atmosphere (namely, 1.18 and 9 at% versus 1.72 and 27 at%). Hydrogen is thought to favour the release of the pendent methyl groups of the PCS via demethanation radical reactions. Continuous model filaments were produced via the melt spinning of the PCS, electron beam curing, pyrolysis under hydrogen up to 1000°C, and a final heat treatment under argon up to 1600°C. The ceramic fibres exhibit a C/Si atomic ratio of 1.10, a free carbon content of ≈8 at%, a Young's modulus of 260–300 GPa and a tensile failure stress of 2100 MPa. Their thermal stability is limited to 1400°C due to some oxygen contamination during the process.

Electron spin resonance studies of amorphous and crystallized residues derived from pyrolysis of pre-ceramic polymers

ESR spectra are presented for pyrolysis residues derived from 5 pre-ceramic polymers: polycarbosilane (PCS), hydridomethylcyclosilazane (HMCS), hydridomethylcyclopolysilazane (HMCPS), polydiphenylsilazane (PDPS) and polymethylsilylethylenediamine (PMSED). The amorphous black and white residues derived from 1100°C pyrolysis in N2 and NH3, respectively, exhibit symmetric peaks with g values of 2.003. This is compatible with values obtained for unpaired spins existing in carbon impurities that remain in the ceramic residues after pyrolysis of the organosilicon precursor polymers. The spin concentrations of black residues obtained from pyrolysis of pre-ceramic polymers at 1100°C in N2 are invariably higher in magnitude than those measured for white-grey residues generated by initial 1100°C-NH3 pyrolysis of pre-ceramic polymers showing that the NH3 pyrolysis removes polymer-associated carbon more efficiently from ceramic residues than 1100°C-N2 pyrolysis. The Si2N2O/Si3N4 powders produced are purer and have lower ESR activity than commercial Si3N4. When the black residues are crystallized at 1550°C under Ar, carbon is removed and the ESR activity reduced. The ESR signal becomes asymmetric and exhibits some partially resolved structure. Crystallization of white-grey residues is accompanied by an overall increase in ESR activity caused in part by the uptake of carbon from the graphite resistance furnace. For the PCS- and PDPS-derived white-grey residues, additional ESR activity probably arises from the creation of defects in the crystalline Si2N2O formed as a result of the 1550°C firing. In general, ESR properties of the pyrolytic residues correlate well with the relative conducting properties of the corresponding ceramic coatings.

Structure and composition of interlayers in joints between SiC bodies

Joints between SiC bodies (sintered α-SiC and SiC SiC f composites) which are intended for thermonuclear fusion reactor applications were produced by coating their surfaces with polycarbosilane (PCS) or a silicone resin (GE SR350) alone, or mixed with SiC or Ti powders, followed by heating in an argon flux at temperatures between 900 and 1200 °C applying no pressure or a very moderate one (< 0.1 MPa). The joint quality was determined by microstructural and microchemical analyses (TEM, AEM) and by fracture shear tests for comparative evaluations. For SR350, the joining material was a silicon oxycarbide glass, with no oxygen diffusion occurring between this and the SiC joined bodies. The flat interface structure and the lack of any reaction layer suggest that the joining mechanism involves the direct formation of chemical bonds between the SiC bodies and the joining material. Adding SiC or Ti powders does not improve the joining strength. Contrary to silicone resin, polycarbosilane does not serve as an effective joining material, possibly because of its different decomposition behaviour upon pyrolysls.

Conversion of organic-inorganic polymers to ceramics by ion implantation

Thin films of polycarbosilane (PCS) and polysiloxane (SR350 resin) deposited on Si substrates were irradiated with He, C, or Au ions in order to convert them into SiC and SiOC ceramic coatings. The transformation kinetics was assessed by means of ion beam analysis (RBS, NRA, ERDA) and of FTIR, while the hardening was measured by nanoindentation tests. The yield of H release increased exponentially with the amount of electronic excitations up to a saturation value, and correlated properties of compaction and hardness varied in proportion to this yield (SR350) or to its square (PCS). PCS films became as hard as amorphous SiC and SR350 films 60% harder than silica. A relative softening was observed in the case of Au irradiations with respect to lighter ions, due to the atomic disordering by collision cascades. The irradiations improved also the oxidation resistance during subsequent annealings.

Oxidation behaviours of carbon/carbon composite with multi-coatings of LaB6-Si/polycarbosilane/SiO2

The influence of multi-coatings of LaB6-Si/polycarbosilane/SiO2 on the oxidation behaviour of carbon/carbon composite materials was investigated in the temperature range from 500 to 1400 °C. The additives of LaB6-Si offered lower oxidation rates and accelerated increases in oxidation rates at temperatures below 900 °C. The coating of polycarbosilane (PCS) improved the compatibility of the coating on the carbon/carbon composite and lowered the oxidation rates of the LaB6-Si coated composite below the transition temperature. With the SiO2 coating, the cracks of the LaB6-Si/PCS coating was sealed and a good oxidation resistance of the LaB6-Si/PCS/SiO2 coated composite was found at temperatures up to 1300 °C.