Novel Methods For The Synthesis Of PCS And PMCS And Production Of High Temperature Resistant SiC Fibers

High temperature resistant SiC fiber is a crucial raw material for producing advanced high temperature composite materials. This work is based on the idea of producing high temperature resistant SiC fiber through introducing densification promoting elements into the precursor, controlling oxygen content with a low oxygen curing method and sintering densification. This dissertation proposes a method of synthesizing polycarbosilane(PCS) in a supercritical fluid state and a low molecular weight polycarbosilane (LPCS) thermal polymerization method for synthesizing PCS which contains Al and Y through. Polyyttriocarbosilane (PYCS), polyaluminocarbosilane (PACS) and Polyyttrio- aluminocarbosilane (PYACS) of improved composition, structure and spinnability were synthesized. Cross-linked PYCS, PACS and PYACS fine diameter fibers with controllable oxygen content were produced through air oxidation and chemical vapor curing(CVC). Three types of high temperature resistant fibers, namely SiC(Y), SiC(Al) and SiC(Y/Al), were produced from the three cross-linked fibers after heat treatment at 1000℃and sintering at 1800℃.For the first time, a supercritical fluid method (SCF method) has been applied to the synthesis of PCS through the pyrolytic decomposition of polydimethylsilane(PDMS). The SCF method has solved the problems with heat transmissibility and reaction uniformity in the PCS synthesizing process. Xylene was used as the supercritical fluid. An autoclave controls the supercritical fluid state by temperature and pressure. Compared to normal pressure, high temperature method (NP method) and high pressure method (HP method), SCFs method is a new feasible method for synthesizing PCS. The chemical composition and molecular structure of SCF-PCS is similar to HP-PCS and NP-PCS. All three of them with Si-C main chain are composed of SiC3H and SiC4. However, the SiC3H structure ratio in SCF-PCS is higher than that in others. The SCF method, which has improved heat transmissibility and reaction uniformity, features a high PCS yield, high synthesis efficiency and a short process. Therefore, in SCF-PCS, Si-H content is higher and the molecule structure has better linearity; meanwhile, the Pi vaule is lower, the molecular weight distribution is more homogeneous and its GPC curve has a'double peak'distribution. The molecular structure and molecular weight distribution of SCF-PCS results in good spinnability.The methods of sythesizing PCS which contains Y and Al are also studied in this dissertation. The SCF method has the advantage of effeciently introducing Al into the precursor. However, a large amount of high molecular weight SCF-PACS with branching and cross-linked structure was produced in SCF-PACS due to the rapid and complete reaction at high temperatures (420~470℃) . The LPCS thermal polymerization method has been applied in order to avoid this problem. The generation of the high cross-linked molecular structure was restrained by the control of LPCS molecular weight and reaction conditions. With LPCS thermal polymerization method, the desification promoting elements Y, Al and Y/Al were introduced into PACS, PYCS and PYACS effectively. Regarding the typical product PACS, its softening point is 189.9~218.3℃, the chemical formula of PACS is SiC1.97H5.20O0.07Al0.015, and its molecule weight is 1476. In the GPC curve, the high molecular weight which takes the form of'lug boss'and'tailed peak'distributions did not occur and the distribution appeared homogeneous. Part of the AcAc bases remained in the molecular structure of PACS. Al exists in PACS as Al-O-C and Al-O-Si. The properties of PYCS and PYACS are similar to those of PACS. They both feature homogeneous molecular weight distribution and quality spinnability.This dissertation researched the melt spinning conditions of PACS, PYCS and PYACS. The PACS, PYCS and PYACS fibers with the diameter of 10~11μm were produced by stable continuous spinning. With air oxidation and CVC treatment, the cross-linked PYCS, PACS and PYACS fibers with controllable oxygen content were obtained. The Si-C-O-M fibers were produced by heat treatment of low-oxygen cross-linked fibers at 1000℃. SiC(Y), SiC(Al) and SiC(Y/Al) were produced by sintering the three fibers at 1800℃.The three types of SiC(M) fibers all have good mechanical properties, high temperature and oxidation resistance. The tensile strength is 1.61~2.10GPa, the modulus is 330~350GPa and the fracture toughness is 1.91~2.35 MPa.m1/2. After SiC(M) fibers were heated to 1800℃in argon for 1h, the tensile strength became 1.12~1.48GPa and remained above 70% of the initial strength. After oxidation tests at 1500℃for 1h, the tensile strength was 1.35~1.64GPa and remained above 76% of the initial strength. The chemical formulae of SiC(Y), SiC(Al) and SiC(Al/Y) fibers were confirmed as SiC1.23O0.05Y0.05, SiC1.20O0.057Al0.014 and SiC1.14O0.067Y0.003Al0.011 respectively. During the high temperature sintering process, deoxidation and decarbonization reactions occurred with increased the fiber purity. After high temperature sintering, there was a thin carbon rich layer around 20 nm thick on the surface of SiC(M) fibers. The carbon content decreased from the surface towards the interior. The fibers have a stable interior composition. The fibers were composed of a large number ofβ-SiC grains (20~50 nm), a small number ofα-SiC grains and amorphous carbon with partial graphitization.In the process of producing SiC(M) fibers from Si-C-O-M fibers, the tensile strength appeared in a special'saddle'shape, which varied with the sintering temperature. The decreased oxygen content in Si-C-O-M fibers reduced the pyrolytic decomposition of SiCxOy at 1400~1600℃. The introduction of Y and Al into SiC fibers restrained the growth ofβ-SiC. These two effects reduced the damage as well as the decrease of tensile strength of SiC fibers. The sintering densification of Al and Y started at 1600℃and was enhanced with increasing temperature. This resulted a recovery in tensile strength at 1600~1800℃. Above 1800℃, the fast coarsening ofβ-SiC led to increasing defects of fibers, which resulted in the rapid decrease of tensile strength. 1800℃was an ideal sintering temperature for producing high temperature resistant SiC fiber. The control of oxygen content and the introduction of densification promoting elements were two key controlling factors for the production of high temperature resistant SiC fibers.The analysis of the microstructure of SiC(Y) and SiC(Al) fibers indicated the apparent sintering densification and showed that the SiC grains sintered together. HRTEM analysis indicated that at the SiC grain boundary, there were some amorphous phases which had a bonding function. In the sintering process, Y and Al gradually combined with O and C, and diffused towards the grain boundary, where they improved the sintering. Hence, Y and Al are important in improving the sintering of SiC(M) fibers.