Mechanism of cyclohexene vapor curing polycarbosilane fibers

Cyclohexene vapor, instead of air, is applied to cure polycarbosilane (PCS) fibers. The cured fibers are characterized by infrared (IR), electron spin resonance (ESR), elements analysis (EA) and simulated through the HyperChem^TM program for comparison. The curing process is investigated by thermoanalysis. The results indicate that the Si–H and Si–CH₃ bonds in PCS are induced by cyclohexene to cleavage and form Si-central radicals. A fully developed cross-linking fibers come into being through the combination of these radicals, and the byproducts, some cyclohexyls bonded onto PCS derived from cyclohexene, introduce the variations in IR spectra, weight gain and carbon contents increase of PCS. On the basis of investigation and simulation, a likely mechanism of curing reaction is presented.     

Synthesis of SiC ceramic fibers from nuclear reactor irradiated polycarbosilane ceramic precursor fibers

Polycarbosilane (PCS) ceramic precursor fibers are irradiated in a nuclear reactor and pyrolyzed under inert atmosphere. Bridge structure of Si–CH₂–Si is formed in the irradiated products by the rupture of Si–H bonds and succeeding cross-linking. When irradiated at the neutron fluence of 2.2 × 10¹⁷ cm⁻² under N₂ atmosphere, the gel content and ceramic yield at 1,273 K of PCS fibers are up to 80% and 94.3%, respectively, and their pyrolysis products are still fibrous, which illuminates that the infusibility of PCS fibers has been achieved. FT-IR spectra indicate that the chemical structure of pyrolysis products is very similar to that of pure SiC, while X-ray diffraction curves suggest that β-SiC microcrystals are formed in the fibers, and their mean grain size is about 7.5 nm. The oxygen content (1.69–3.77 wt%) is much lower than that of conventional SiC fibers by oxidation curing method (about 15 wt%). Tensile strength of the SiC fibers is up to 2.72 GPa, which demonstrates that their mechanical properties are excellent. After heat-treated at 1,673 K in air for an hour or at 1,873 K under Ar gas atmosphere for 0.5 h, their external appearance is still undamaged and dense, and their tensile strength decreases to a small extent, which verifies that heat resistance of the SiC fibers is eximious.     

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. This revised version was published online in November 2006 with corrections to the Cover Date.     

Polymer–ceramic conversion of a highly branched liquid polycarbosilane for SiC-based ceramics

Liquid polycarbosilane (LPCS) with a highly branched structure was characterized by fourier-transform infrared spectrometry (FT-IR) and ¹H, ¹³C, ²⁹Si nuclear magnetic resonance spectrometry (NMR). The LPCS was then cured and pyrolysized up to 1,600 °C under flowing argon. The structural evolution process was studied by thermogravimetric analysis and differential scanning calorimetry (TG-DSC), FT-IR, and X-ray diffraction (XRD). Hydrosilylation, dehydrocoupling, and polymerization cross-linking reactions between Si–H and C=C groups occurred at low temperatures, which mainly accounted for the high ceramic yield (70%) up to 1,400 °C. The organic groups gradually decomposed and the structure rearranged at high temperatures. The FT-IR analysis revealed that Si–CH₂–Si chains, the backbone of original polymer, can be retained up to 1,200 °C. At temperatures higher than 1,200 °C, the Si–CH₂–Si chains broke down and crystalline SiC began to form. The final crystalline products were β-SiC and a small amount of carbon.     

自由碳的脱除对SiC纤维微观结构和性能的影响

采用加氢烧成法脱碳, 制备了不同自由碳含量的连续SiC纤维。通过元素分析、红外、X射线衍射和拉伸试验等手段对纤维的脱碳过程、元素组成、微观结构和性能进行了分析。结果表明: 加氢烧成通过抑制脱H₂反应、促进脱CH₄反应而实现有效脱碳, 且氢气浓度越高, 纤维中的碳含量越低。纤维芯部元素分布均匀, 表明该方法可以实现均匀脱碳, 但表面出现很薄的富碳层, 这是纤维经氢气处理后表面吸附氧形成的富氧层在高温烧成时分解所致。自由碳的脱除引起纤维晶粒长大, 密度增加, 孔隙率降低, 电阻率升高, 拉伸强度与拉伸模量提高。近化学计量SiC纤维具有优异的综合性能。     

Influence of polymer infiltration and pyrolysis process on mechanical strength of polycarbosilane-derived silicon carbide ceramics

In this article, strength evaluation of silicon carbide (Si–C) ceramics fabricated from polycarbosilane (PCS) precursor is described. Si–C ceramics was prepared by firing a green body made of the mixture of Si–C nano-powders and a PCS solution at 1,273 K in N₂ gas for an hour. To obtain dense Si–C, the solution was infiltrated into the produced body, and then it was fired again. The polymer infiltration and pyrolysis (PIP) process was conducted up to 12 cycles. Si–C ceramics was diced to be rectangle shape measuring 1.0 mm × 3.0 mm × 0.5 mm, and was subjected to the three-point bending test for measurement of the Young’s modulus and bending strength. Si–C specimens fabricated through PIP processes less than 2 cycles showed non-linear force–displacement curves like a polymer, whereas those through the processes more than 3 cycles showed linear relations and fractured in a brittle manner. The Young’s modulus of 12-cycles-PIPs specimen was found to be 56 GPa on average, which was approximately 22-fold of non-PIP specimen. The bending strength was also increased with an increase in the number of PIP process. The maximum value was found to be 157 MPa. The cause of the influence of PIP process on the mechanical characteristics is discussed using a PCS-derived Si–C model.     

Evolution of crystallization and its effects on properties during pyrolysis of Si–Al–C–(O) precursor fibers

The high-temperature resistant Si–Al–C–(O) fibers were prepared through polymer-derived method using continuous polyaluminocarbosilane (PACS) fibers. Evolutions of the crystallization during the pyrolysis of the Si–Al–C–(O) precursor fibers were investigated by a series analysis. The structure of the fibers transforms from organic state to inorganic state and the crystalline phases appear during the pyrolysis. The β-SiC crystallite size increases when the temperature is higher than 1,300 °C. At the same time, the α-SiC appears. At 1,600 and 1,800 °C, the grain size of β-SiC of the fibers is 15.4 and 22.1 nm, respectively. The growth of β-SiC and the appearing of α-SiC have a great influence on the properties of the fibers. The change of the tensile strength of the pyrolysis products is divided into three stages with the growth of the crystal. The tensile strength of the Si-Al-C fibers is higher than 1.9 GPa.     

Synthesis and properties of ceramic fibers from polycarbosilane/polymethylphenylsiloxane polymer blends

We synthesized ceramic fibers based on silicon carbide (SiC) from polymer blends of polycarbosilane (PCS) and polymethylphenylsiloxane (PMPhS) by melt-spinning and radiation curing. PMPhS was compatible with PCS up to 30 mass%, and formed a transparent melt at temperatures higher than 513 K. The softening point was also lowered by adding PMPhS and 15 mass% of PMPhS to PCS was the most suitable condition for obtaining thin fibers with an average diameter of 14.4 μm. Due to the lowered softening point of the PCS–PMPhS fibers, γ-ray curing in air was adopted. The ceramic yield of the cured fiber was 85.5% after pyrolysis at 1273 K. In spite of the small diameter, the resulting tensile strength at 1273 K was rather limited at 0.78 GPa. Blooming of the PMPhS component during pyrolysis may have caused surface defects. After high-temperature pyrolysis at 1673–1773 K, a porous nanocrystalline SiC fiber with a unique microstructure was obtained with surface area of 70–150 m²/g. When the fiber was pyrolyzed at the same temperature under a highly reductive atmosphere, wire bundle-shaped fibers were obtained by gas evolution and reactions.     

Mechanism of synthesizing dense Si–SiC matrix C/C composites

The following technique is known to synthesize C/C (carbon fiber-reinforced carbon) composites. The organic matter in the preformed yarn (plastic straw covered yarn including bundles of long carbon fibers, carbon powder, and organic binder) is pyrolyzed at 500 °C and concurrently hot-pressed. Then, the carbon ingredient is graphitized in an atmosphere of nitrogen at 2000 °C. The authors used the above mentioned C/C composites as a starting material and developed a dense Si–SiC matrix C/C composites in which most long carbon fibers remain without reacting with Si which is infiltrated in argon at 1600 °C and 100 Pa. As a result, production of 1 × 2 m large size plates free from warps and cracks was attained in NGK Insulators, Ltd. This mechanism consists of three steps. First, a trunk-shaped Si–SiC matrix is synthesized between yarn and yarn. Then a trunk-shaped Si–SiC matrix extends a yarn by force. Only differential gap is made in a yarn surface. Finally, branch-shaped Si–SiC matrix is synthesized so that a trunk-shaped Si–SiC matrix leads to the yarn inside.     

Evaluation of oxidation resistance of thin continuous silicon oxycarbide fiber derived from silicone resin with low carbon content

Si–O–C ceramic fiber was synthesized from a kind of silicone resin with low carbon content. The melt-spun resin fiber was exposed to SiCl₄ vapor under a nitrogen gas flow, and the fiber was heated at 373 K for 2 h to complete the curing process. The cured fiber was pyrolyzed at 1273 K in an inert atmosphere to be converted to Si–O–C fiber. The entire chemical composition of the pyrolyzed fiber was almost identical to that of a previously reported resin which was pyrolyzed without curing. Auger spectrum analysis indicated an increase in silicon content near the fiber surface. The Si–O–C fiber thus obtained was heat-treated at 1511 or 1603 K in an air flow to evaluate oxidation resistance. Elemental analysis, XRD measurement, and SEM image observations were carried out on the oxidized Si–O–C fibers. Even with such thin fiber diameters, the oxidation process under these conditions was slow and the formation of a thin oxide layer on the fiber surface was confirmed. The existence of a residual Si–O–C core surrounded by a crystallized silica layer was observed in fractured fiber cross-sections after severe treatment conditions of 24 h oxidation at 1511 K or 3 h oxidation at 1603 K.     

Refinement of primary Si and modification of eutectic Si for enhanced ductility of hypereutectic Al–20Si–4.5Cu alloy with addition of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles

It is very difficult to simultaneously refine and modify Si particles in hypereutectic Al–Si–Cu alloys to enhance their ductility. This study investigates how nanoparticles affect Si particles during solidification in hypereutectic Al–Si–Cu alloys. 0.5 wt% γ-Al₂O₃ nanoparticles were added in hypereutectic Al–20Si–4.5Cu alloy melt and further dispersed through an ultrasonic-cavitation-based technique. The as-cast Al–20Si–4.5Cu–Al₂O₃ nanocomposites showed marked enhancements in both ductility and strength. The ductility of Al₂O₃ nanocomposite was more than two times higher than that of the monolithic alloy without the nanoparticles. Microstructural analysis with optical and scanning electron microscopy revealed that both the primary and eutectic Si particles were significantly refined. The primary Si particles were refined from star shapes to polygon or blocky shapes, and their edges and corners were much smoother. The large plate eutectic Si particles were also modified into the fine coralline-like ones. The porosity of alloy was also reduced with the addition of γ-Al₂O₃ nanoparticles. Study suggests that γ-Al₂O₃ nanoparticles simultaneously refine and modify Si particles as well as reduce porosity in cast Al–20Si–4.5Cu, resulting in unusual ductility enhancement that could have great potential for numerous applications.     

Effect of dilution gases in methane on the deposition of diamond-like carbon in a microwave discharge II: Effect of hydrogen

The effect of hydrogen as a dilution gas on the deposition of diamond-like carbon by the decomposition of methane in a microwave discharge was studied from surface analysis of the substrate and from plasma diagnostics. When carbon deposited from a CH₄-Ar plasma and consisting of large amounts of graphite and small amounts of diamond, was placed in the hydrogen plasma chemical sputtering of carbon to form hydrocarbons and adsorption of hydrogen on the carbon substrate were observed. The reaction occured only on graphite and not on diamond. The effects of hydrogen as a dilution gas on the deposition of diamond-like carbon from CH₄-H₂ plasma are to cause the formation of CH₃ radicals in the plasma, the removal of graphite from the deposit and the adsorption of atomic hydrogen on the deposit as an active participant in the diamond crystallization process.     

Complex permittivity and microwave absorbing properties of SiC fiber woven fabrics

A comparative investigation of electric conductivity, complex permittivity, and microwave absorbing properties of KD-1 and Nicalon-202 fibers in the form of fabrics within the range of 8.2–12.4 GHz (X band) has been carried out. The electric conductivity value of KD-1 filaments is two orders larger than Nicalon-202. Both the values of real part (ε′) and imaginary part (ε″) of KD-1 fabrics are larger than their counterparts of Nicalon-202 especially the imaginary part, which is in agreement with larger DC conductivity (σ_d). The surface morphology and chemical component were characterized by SEM, EDS, Raman spectroscopy and XRD, which shows that both the KD-1 and Nicalon-202 SiC fibers are rich in carbon, while there is rich carbon layer on the surface of the former and the degree of order in the free carbon phase is higher compared with the latter. In addition, the amount of amorphous Si–C–O phase of KD-1 fibers is higher while the SiC crystal is smaller than Nicalon-202. The free carbon on the surface of KD-1 fibers can establish electric conductivity network. The larger ε″ and ε′ of KD-1 fabrics are believed to be mainly caused by conductive network established by rich carbon outer layer and relaxation polarization enhanced by more Si–C–O phase. The reflection loss of KD-1 and Nicalon-202 fabrics is −3.5 to 0.7 and −5.1 to −4.3 dB, calculated according to tested complex permittivity.     

Microstructure characteristics of products in Ti–Si system via combustion synthesis reaction

The products of combustion synthesis reaction in Ti–Si system with molar ratios of Ti:Si = 3:1, 5:3, 5:4, 1:1, and 1:2 were investigated. The phase composition of products and degree of completion in the reaction considerably depend on the initial stoichiometric ratios of reactants. During the SHS reaction of Ti–Si system, the degree of completion follows the order of 5Ti + 3Si > Ti + Si > Ti + 2Si > 5Ti + 4Si > 3Ti + Si. Besides, the micro-structural morphology of Ti–Si compounds, i.e., TiSi₂, TiSi, Ti₅Si₄, and Ti₅Si₃ were also characterized in this study.     

Electrical and structural properties of LiNbO<Subscript>3</Subscript> films,grown by RF magnetron sputtering

Nanocrystalline films of LiNbO₃ on substrates (001)Si and (001)Si–SiO₂ were synthesized by the method of RF magnetron sputtering. The elemental composition, structure of the LiNbO₃ films, and also—electrical properties of heterostructures (001)Si–LiNbO₃ and (001)Si–SiO₂–LiNbO₃ were studied. The dielectric constant of the LiNbO₃ films calculated from the capacitance at the accumulation region was about 28. The resistivity was 1·10⁹ ohm cm for films on (100)Si and 1.6·10¹¹ ohm cm for films on (001)Si–SiO₂. It has been determined that transmission of the current in the studied structures during direct biases is defined by hopping conduction, and, during reverse biases—by the Poole–Frenkel effect.     

Curing and pyrolysis of polysiloxane/divinylbenzene and its derived carbon fiber reinforced Si—O—C composites

The curing and pyrolysis of hydrogen-containing polysiloxane (PSO) and divinylbenzene (DVB) were investigated in this paper. It was found that H₂PtCl₆ was an effective catalyst for the curing of DVB/PSO. The mass ratio of DVB/PSO had great effect on ceramic yield. The cured DVB/PSO with a mass ratio of 0.5:1 had the highest ceramic yield (76%) at temperature up to 1000°C, and its pyrolysates consisted of 38.3 wt% silicon, 27.4 wt% oxygen, and 34.3 wt% carbon of which 26.3 wt% was free carbon. The composition and structure of pyrolysates of DVB/PSO were changed with increasing pyrolysis temperature. The pyrolysis behavior of DVB/PSO was characterized by thermal analysis. DVB/PSO-derived Si–O–C composites reinforced with carbon fiber cloth (C_f/Si–O–C) were fabricated. The results showed that the flexural strength of C_f/Si–O–C composites could be increased from 118.00 ± 5.00 MPa to 139.78 ± 7.68 MPa if the pyrolysis temperature was elevated from 1000 to 1400°C, which was ascribed to the weakened interfacial bonding.     

Effect of treated filler loading on the photopolymerization inhibition and shrinkage of a dimethacrylate matrix

This study shows how treated filler loading influences the photopolymerization of a dimethacrylate comonomer mixture, regarding, in particular, shrinkage and inhibition under atmospheric oxygen, present in oral environment. Bis-GMA/TEGDMA (75/25 wt.%) resins were loaded with hybrid filler (Ba aluminosilicate glass and pyrogenic silica), treated with γ-methacryloxy(propyl)trimethoxysilane, at 0–50 wt.% and light cured over a total of 30 s (45 mW/cm²). Degree of double-bond conversion (DC), obtained using FTIR, decreased with filler content. ¹H MAS spectra (293–340 K) and STRAFI images (293 K) were obtained as a function of irradiation time and filler concentration. ¹H signals of unreacted methacrylate groups were more intense for higher loaded resins and resonances from –CH₂SiO₂(OH) (T²) and –CH₂SiO₃– (T³) units, also observed by ²⁹Si NMR, were resolved and suggest the presence of T²–resin bonds. 1D images show a reduction on polymerization contraction and reaction inhibition at the composite resin surface with filler loading. 2D resin images present a highly mobile surface layer, hence with lower DC.     

Influence of melt treatments on sliding wear behavior of Al–7Si and Al–7Si–2.5Cu cast alloys

The microstructures and dry sliding wear behavior of Al–7Si and Al–7Si–2.5Cu cast alloys were studied after various melt treatments like grain refinement and modification. Results indicate that combined grain refined and modified Al–7Si–2.5Cu cast alloys have microstructures consisting of uniformly distributed α-Al grains, eutectic Al– silicon and fine CuAl₂ particles in the interdendritic region. These alloys exhibited better wear resistance in the cast condition compared with the same alloy subjected to only grain refinement or modification. The improved wear resistances of Al–7Si–2.5Cu cast alloys are related to the refinement of the aluminum grain size, uniform distribution of eutectic Al-silicon and fine CuAl₂ particles in the interdendritic region resulting from combined refinement and modification. This paper attempts to investigate the influence of the microstructural changes in the Al–7Si and Al–7Si–2.5Cu cast alloys by grain refinement, modification and combined action of both on the sliding wear behavior.     

Possibility of methane decomposition in a gas discharge

The possibility of using a gas discharge such as a corona discharge or a barrier discharge for decomposition of methane in different gaseous mixtures is investigated theoretically. The effect of preheating of the gas to a temperature of 1200 K on the degree of methane conversion in the discharge is studied. A kinetic model that describes the processes of methane decomposition and oxidation in CH₄/CO₂, CH₄/H₂O, and CH₄/O₂ mixtures is developed. The effect of the discharge parameters and gas additives on the efficiency of methane decomposition is investigated. The optimum temperature of the mixture, particle lifetime, and initial concentration of oxygen for the production of hydrogen molecules are found. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 71, No. 6, pp. 1016–1023, November–December, 1998.     

Preparation of Y-TZP ceramic fibers by electrolysis-sol-gel method

Polycrystalline yttria stabilized tetragonal Zirconia (T-ZrO₂) fibers were obtained by pyrolysis of gel fibers using zirconium oxychloride octahydrate as raw material. The spinnable zirconia sol was prepared by electrolyzing the zirconium oxychloride octahydrate solution in the presence of acetic acid and sugar (sucrose, glucose or fructose), in which the molar ratio of CH₃COOH/ZrOCl₂ · 8H₂O and sugar/ZrOCl₂ · 8H₂O was in the range of 1.0–4.0 and 0.2–0.4, respectively. The relation of spinnability to the shape of colloidal particle was discussed. The as-prepared zirconia fibers sintered at different temperatures show smooth and crack-free surface with the diameter of 5–10 μm. Slow heating rate below 600 °C and then sintering at 1,400 °C for 30 min were necessary to obtain the dense tetragonal zirconia ceramic fibers, the particles composed the fibers had the size of ∼150 nm.