A model SiC-based fiber with a low oxygen content prepared from a vinyl-containing polycarbosilane precursor

Abstract

In a previous study [1], we prepared a kind of vinylcontaining precursor by introducing cyclomethylvinylsilazane (MVCSZ) to the polycarbosilanes (PCSs). The results showed that vinyl can enhance the irradiation curing of PCSs and a vinyl content of 2.33 wt% in PCS can lower its gel dose (curing dose) from 10 MGy (15 MGy) to 0.5 MGy (5 MGy), respectively. In order to observe the mechanical properties of soobtained low-oxygen-content SiC fiber from the vinylcontaining polycarbosilanes (Vi-PCSs), this paper studied the pyrolysis of uncured, EB-cured and aircured Vi-PCS precursors. Bundles of the green Vi-PCS fibers with a vinyl content of 2.33 weight percent were irradiated in helium atmosphere in a quartz vessel under an electron accelerator with a rate close to 0.8 MGy per min. The absorbed dose was 5.0 MGy. Afterwards, the samples were annealed in helium gas flow at 130 ◦C for 1 h and irradiation cross-linked Vi-PCS fibers were finally obtained. Another batch of the same green Vi-PCS fibers was air-cured in a furnace with air flowing through, up to 190 ◦C for 2 h with a weight gain of 10 wt%. The EB-cured or air-cured fibers were then pyrolyzed under a flow of high-purity nitrogen (Q = 10−5 m3/ min, P = 100 kPa) up to 1200 ◦C for 30 min with a relatively slow heating rate (100 ◦C/h). After this treatment, the fibers that were no longer sensitive to oxidation could be handled in air. The high temperature treatment was then achieved with a pyrolysis equipment comprising a graphite crucible with a radio frequency coil. The samples were heated (30 ◦C/min) and maintained at the pyrolysis temperature Tp under a high-purity nitrogen atmosphere (100 kPa) for tp = 30 min. The fibers were characterized by the following ways. Thermogravimetric analysis (TGA) (Rigaku, Japan) was performed, up to 1200 ◦C, in platinum crucibles on about 10 mg samples under a high-purity nitrogen flow (flow rate: 4 × 10−5 m3/min) with a temperature increase rate of 10 ◦C/min. The weight loss before and after high temperature treatment was measured and calculated with an analytical balance. Elemental (Si,C,N) analyses were performed on the samples by a chemical-analysis process. Oxygen contents were determined by a nitrogen/oxygen analyzer (TC-436, LECO, America). X-ray diffraction (XRD) spectra (Cu-Ka/Siemens D500 diffractometer, Germany) were recorded from tows of fibers for Tp = 1600 ◦C. The apparent mean grain size (L) of the β-SiC crystalline phase present in the samples was calculated from the width (D) of the (111) diffraction peak at mid-height, according to the following Equation 1: