Lingjie Li,Zhonghao Ning,Wenyan Huang,Liang Liao,Yinong Zheng,Kun Zhuang,Siqi Lan,Yinggan Zhang,Rongqian Yao
An ingenious strategy to prepare in-situ formed SiC(Al, rGO) bulk polymer-derived ceramics (PDCs) from re-pyrolyzing SiC(Al, rGO) p /polyaluminocarbosilane-vinyltriethoxysilane-graphene oxide (PACS-VTES-GO, PAVG) blends is highlighted. Flexible PAVG modified by Al with highly cross-linked structure effectively improves molecular weight and formability of precursors. Rigid SiC(Al, rGO) p fillers provide high mechanical properties owing to their unique microstructure which is consistent with SiC(Al, rGO) bulk PDCs. β-SiC nanocrystals are well dispersed in amorphous SiO x C y /C free , and rGO exhibits good compatibility with SiO x C y /C free . Besides, possible Al 2 O 3 is beneficial to relaxing thermal stress at β-SiC nanocrystalline boundaries. Interestingly, introduction of Al could avoid coarsening of β-SiC grains to enhance elevated temperature resistance. Furthermore, effect of re-pyrolysis temperature on the formation and microstructure of SiC(Al, rGO) bulk PDCs was also investigated. Particularly, lightweight samples re-pyrolyzed at 1300 °C display optimal fracture toughness (3.32 MPa m 1/2 ), compressive strength (177.16 MPa) and flexural strength (51.25 MPa), high ceramic yield (94.46%) and low linear shrinkage (4.38%), by far superior to those of most PDCs. As treated via precursor infiltration and pyrolysis (PIP) route and heated by butane gun flame in air, such products possess stable structures under elevated temperatures and have potential emerging uses in aerospace components.
SiC fiber is an important structural material in high-temperature extreme environments and nuclear energy applications. Oxidation and electron irradiation are main curing methods for the fabrication of industrial SiC fibers. In this work, a photo-sensitive polyvinylaluminocarbosilane (PVACS) is facilely prepared by blending polyaluminocarbosilane (PACS) and 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane (D4Vi), which can be melt spun, cured by ultraviolet irradiation at room temperature. After thermal pyrolysis and sintering, the SiC(OAl) fibers with oxygen content of 8.84 wt% and C/Si ratio of 1.34, the SiC(Al) fibers with oxygen content of 0.29 wt% and C/Si ratio of 1.03 are achieved. The cured mechanism and the structural evolution from the precursor fiber to the final ceramic fibers is investigated. The results show that homopolymerization of D4Vi is the main curing mechanism in addition to the β-addition of hydrosilylation. Different from the typical oxidative curing, the oxygen in the fibers mainly comes from D4Vi. At about 800 ℃, the SiAl x O y phase, the SiC x O y phase and free carbon are formed. At 1200 ℃, the Al-C bonds possibly in Al 4 C 3 are formed. At around 1600 ℃, Al may exist mainly in the form of Al 4 O 4 C, and the amorphous carbon phase changes into crystalline graphitic structure. Meanwhile, a lot of mesopores and macropores in the SiC(OAl) fibers are formed due to thermal decomposition of the SiC x O y phase. However, at 1800 ℃, the SiC(Al) fibers densify again due to the formation of Al 2 OC and Al 4 SiC 4 phases, which also inhibit the growth of β-SiC and favor the formation of α-SiC.
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