Fabrication of SiC fibres from yttrium-containing polycarbosilane

The yttrium as a sintering aid was introduced into polycarbosilane（PCS） to prepare yttrium-containing PCS（PYCS）.Two types of yttrium-containing SiC fibres,the SiC（OY） fibres and the SiC（Y） fibres,were fabricated with PYCS.The structural evolution and the associated properties on changing from SiC（OY） to SiC（Y） fibres during the sintering process were studied.The chemical composition of the SiC（OY） fibres is SiC1.53O0.22Y0.005 with an amorphous structure.The composition of SiC（Y） fibres is SiC1.23O0.05Y0.005.The fibres are composed of a large number of β-SiC crystallites with a size of 50 nm and a small amount of α-SiC crystalline.The tensile strength and fracture toughness of the SiC（OY） fibres are 2.25 GPa and 2.37 MPa·m1/2,respectively,and 1.61 GPa,1.91 MPa·m1/2,respectively for SiC（Y） fibres.The SiC（Y） fibres have a higher thermal stability than the SiC（OY） fibres.

Influence of pyrolysis temperature on structure and dielectric properties of polycarbosilane derived silicon carbide ceramic

β-SiC ceramic powders were obtained by pyrolyzing polycarbosilane in vacuum at 800-1200 °C. The β-SiC ceramic powders were characterized by TGA/DSC, XRD and Raman spectroscopy. The dielectric properties of β-SiC ceramic powders were investigated by measuring their complex permittivity by rectangle wave guide method in the frequency range of 8.2-18 GHz. The results show that both real part ε′ and imaginary part ε″ of complex permittivity increase with increasing pyrolysis temperature. The mechanism was proposed that order carbon formed at high temperature resulted in electron relaxation polarization and conductance loss, which contributes to the increase in complex permittivity.

One-pot synthesis and characterization of a new, branched polycarbosilane bearing allyl groups

A highly branched polycarbosilane bearing allyl groups has been prepared by a one-pot synthesis with chloromethyltrichlorosilane, chloromethylmethyldichlorosilane and allyl chloride as the starting materials. The resultant polymer, with the approximate formula [SiH1.2(CH3)0.71(CH2CHCH2)0.09CH2]n, has been characterized by 1H, 13C, and 29Si NMR, GPC, TG and elemental analysis. It could be cross-linked thermally at 170 °C in the absence of oxygen. Pyrolysis of the polymer gave a ceramic with a yield of about 70%.

Synthesis and ceramization of polycarbosilane containing beryllium

Polycarbosilane containing beryllium(BPCS) precursors was prepared by the reaction of polycarbosilane(PCS) with beryllium acetylacetone(Be(acac)2).The analysis of structures and components of BPCS demonstrates that their main structures are basically the same as PCS.Ceramization of BPCS precursors shows that BPCS precursors are organic below 600 °C and inorganic at 800 °C.At 1400 °C,BPCS precursors convert into silicon carbide ceramics.The ceramization of different beryllium content precursors were studied,which show that beryllium plays an important role in the inhibition of crystalline grain growth of β-SiC at high temperature and it can adjust the dielectric constant of silicon carbide ceramics.

PREPARATION AND PROPERTIES OF BULK CERAMICS VIA PYROLYSIS OF POLYCARBOSILANE WITH ACTIVE FILLERS I. EVOLUTION PROCESS IN ACTIVE FILLER CONTROLLED PYROLYSIS

The pyrolytic process of polycarbosilane (PCS) with active fillers (Ti, TiH 2, TiB 2, Cr, CrSi 2) is investigated. The ceramic yields with active fillers reach over 100% in N 2 atmosphere, and no shrinkage occurs when 12vol% Ti, or 4vol% Cr, or 3vol% CrSi 2 is added into PCS. XRD patterns indicate that metal carbides and nitrides form by reacting with gaseous evolution and N 2.

Study on the Pyrolysis Behavior of Polycarbosilane

The pyrolysis behavior of polycarbosilane （PCS） and chemical reaction mechanism during the pyrolysis process were studied by thermogravimetric-mass spectrometry （TG-MS） combined with X-ray diffraction and infrared spectroscopic analysis methods. The experimental results indicate that the main gas phase products generated during pyrolysis of PCS in nitrogen atmosphere include H2, -CH3 and CH4. The heating rate has a large effect on the pyrolysis process of PCS, the lower heating rate releases more small molecule gases and gets bigger rate of pyrolysis mass loss, demonstrating that the lower heating rate is beneficial to fully pyrolysis of PCS and obtain ceramics products with better microstructure.

PREPARATION AND PROPERTIES OF BULK CERAMICS VIA PYROLYSIS OF POLYCARBOSILANE WITH ACTIVE FILLERS II.FLEXURAL STRENGTH AND ANTI-OXIDATIVE PROPERTIES OF ACTIVE FILLER/PCS DERIVED BULK CERAMICS

The flexural strengths and oxidative resistant properties of the ceramics derived from polycarbosilane and active fillers (Ti, TiH 2, TiB 2, Cr, and CrSi 2) were measured and characterized. The introduction of active fillers enhances slightly the flexural strengths, and further densification is required to obtain higher strengths. The oxidative resistant behaviors of the specimens with active fillers are, by means of weight gain in air, poorer compared with those without active fillers.

Synthesis and characterization of high ceramic yield polycarbosilane precursor for SiC

The polycarbosilane(PCS)precursor for SiC with high molecular weight and medium molecular weight distribution was synthesized from polydimethylsilane at normal pressure.The chemical formula,the number average molecular weight,and the polydispersivity index of the synthesized PCS are SiC_(1.94)H_(5.01)O_(0.028),1135,and 1.66,respectively,which can be attributed to the higher reaction temperature used for polymerization.The polymer to ceramic conversion of PCS was completed at 900℃with a ceramic yield of 85%.The crystallization started at 1100℃,and at 1200℃,well resolved peaks ofβ-SiC were formed with small amount ofα-cristobalite.The X-ray diffraction(XRD)and transmission electron microscopy(TEM)studies indicated the presence of nanocrystallineβ-SiC.

Effect of heating rate on the properties of silicon carbide fiber with chemical-vapor-cured polycarbosilane fiber

Silicon carbide (SiC) fiber has recently received considerable attention as promising next-generation fiber because of its high strength at temperatures greater than 1300 ℃ in air.High-quality SiC fiber is primarily made through a curing and heat treatment process.In this study,the chemical vapor curing method,instead of the thermal oxidation curing method,was used to prepare cured polycarbosilane (PCS) fiber.During the high temperature heat treatment of the cured PCS fiber,varied heating rates of 10,20,30,and 40 ℃/min were applied.Throughout the process,the fiber remained in the amorphous silicon carbide phase,and the measured tensile strength was the greatest when the oxygen content in the heat-treated fiber was low,due to the rapid heating rate.The fiber produced through this method was also found to have excellent internal oxidation properties.This fast,continuous process shows a great promise for the production of SiC fiber and the development of high-quality products.

STUDIES ON THE POLYSILANE CONSISTING CYCLODISILAZANE UNITS IN THE MAIN CHAIN

Since the reactivity of Me_2SiCl_2(Ⅰ) and ■(Ⅱ) are very different. it is difficult to cocondensate the above two monomers with sodium in general Wurtz reaction. We have proposed a modified method to prepare copolymer containing permethylpolysilane segments and cyclodisilazane units by reaction of (Ⅱ)with Na at first, then adding (Ⅰ) into the mixture to proceed with further reaction.The obtained copolysilanes are pale yellow elastic substance and soluble in common solvents. The amounts of N in the copolymers are influenced by the operating conditions of the synthetic method. The resultant products were characterized by IR, UV, NMR, and elementary analysis. The highest molecular weight determined by gel permeation chromatography is 60000.

Synthesis of ceramic precursor polycarbosi-lane (PCS) under supercritical fluids (SCFs) state

Using xylene as supercritical fluids (SCFs) media and polydimethylsilane (PDMS) as raw material, the ceramic precursor polycarbosilane (PCS) with moderate molecular weight and narrow molecular weights distribution was synthesized under high pressure and temperature in autoclave. Softening-point test, Fourier transform infrared spectroscopy (FT-IR), gel permeation chromatography (GPC), element analysis and the yield test of PCS were adopted to characterize the SCFs-method and compared with two other PCS synthesis methods named normal-pressure high-temperature (NP) method and high-pressure hith-temperature (HP) method. The results indicate that the PCS maintains a high Si-H content and its chemical formula is SiC1.91H7.27O0.05. The yield is 63.5 wt% which has been improved by about 20 wt%, the softening-point is 180–220°C, the number average molecular weight is 1477 and the molecular weights distribution index is 1.61. The synthesis method via supercritical fluids with good diathermancy and reaction uniformity is a new feasible method of synthesizing PCS with good qualities.

In situ formation of nanometer-scale TiO_2/SiC functional compositional film on carbon fiber

In the toluene solution of the precursor Polycarbosilane (PCS) containing low- molecular-mass additive Ti(OC4H9)4, TiO2/SiC nanometer-scale functional compositional film with the surface TiO2 layer on CF was formed in situ by means of polymer-derived precursors. The effects of Ti (OC4H9)4 concentrations and the maturating time were studied on the densification and TiO2 particle size of surface layer. The compositions of film were TiO2 and SiC crystal by XRD. According to the results of ESCA analysis, Ti(OC4H9)4 compound oozed gradiently from the pre-ceramic PCS to the surface layer after maturating time of 100 h. In the conditions of 45wt% Ti (OC4H9)4 and 100 h maturation, the nanometer-scale TiO2 particles on continuous surface layer were formed by SEM photographs. The nanometer-scale TiO2/SiC functional compositional film can modify the resistance to oxidation of carbon fiber.

Effects of Heating Rate on Properties and Structures of 3D-Cf/SiC Composites by PIP Process

The properties and structures of 3D-Cf/ SiC composites prepared by rapid precursor infiltration pyrolysis process were studied.PCS solution was infiltrated into 3D-carbon fiber perform in vacuum,then these performs were dried at room temperature and pyrolyzed with three kinds of heating rate,3 900 min,1 500 min and 1 000 min respectively.The distinction of 3D-Cf/SiC samples fabricated at three kinds of heating rate was not obvious.Bending strength of sample at 1 000 min heating rate was 762.5 MPa,and a little better than the others.Three samples at various heating rate had similar micro-graph of fracture surface and fracture toughness.The research results show that mechanics performances of Cf/SiC composites did not decrease when heating rate was properly increased.