Preparation And Performance Of Fine-diameter SiC-based Ceramic Fiber

Fine-diameter silicon carbide(SiC)fiber is promising reinforcing fiber for ceramic matrix composites at high temperature applications,because of its high tensile strength and high temperature oxidation resistance.The titanium and boron-containing SiC fiber was prepared from the boron-modified polytitanocarbosilane to improve the performance of SiC fiber prepared by polymer precursor method.The structure and melt-spinning ability of boron-modified polytitanocarbosilane,the curing process of polymer fiber,the mechanical properties and wave-absorbing properties were studied systematically.Firstly,the titanium and boron-containing polymer was synthesized by the reaction of liquid polysilane,tetrabutyl titanate and tributyl borate.Then the titanium and boron-containing polymer was mixed with polycarbosilane(PCS)to prepare boron-modified polytitanium carbosilane with different titanium boron content.The preparation process of SiC fiber was explored.Results show that the structure of as-synthetic boron-modified polytitanocarbosilane was similar with polytitanocarbosilane.The molecular weight of polymer increased and the melt-spinning ability deteriorated with the increasing of titanium and boron contents.The oxidation curing of green fiber was carried out by the reaction of Si-H and O2.In the range of 156? to 210?,the oxygen content increased with the curing temperature increasing.At the same curing temperature,the increments of oxygen content decreased with the titanium and boron content increasing.After the pyrolysis and sintering of curing fiber,the titanium and boron were incorporated in the SiC fiber.Secondly,the mechanical properties of SiC fiber with differnent preparation process as well as titanium and boron content were studied.The tensile strength the SiC fiber prepared from the polymer fiber with titanium content of 1.64%and boron content of 1.01%increased with the curing temperature increasing.The crystallization trend appeared easily in the SiC fiber cured at lower temperature.The SiC fiber pyrolyzed at 900?1300? maintained the amorphous.The tensile strength of SiC fiber pyrolyzed at 1200? was highest,while the tensile strength of SiC fiber pyrolyzed at 1300?decreased because of the appearance of pore defect.The SiC fiber with different titanium and boron contents cured at 210? and pyrolyzed at 1200? was amorphous.The presence of titanium and boron in fiber can inhibit crystallization of SiC and decomposition of Si-O-C.Therefore,the tensile strength of SiC fiber increased with the titanium and boron contents increasing.In addition,the crystallization degree of SiC fiber sintered at 1400?1800? was gradually improved.The tensile strength and the boron element of SiC fiber sintered at 1800? almost vanish.Finally,the dielectric and wave-absorbing properties of SiC fiber with different preparation processes as well as titanium and boron contents were studied.Result shows that the resistivity was the main factor that influenced the dielectric and wave-absorbing propeties.The titanium and boron-containing SiC fiber cured at different temperatures and pyrolyzed at 1200? had the similar resistivity.Therefore,they had similar dielectric loss and wave-absorbing properties.The SiC fiber pyrolyzed at 1200? had the lowest resistivity as well as dielectric loss and wave-absorbing properties among the curing fiber pyrolyzed at 900?1300?.The resistivity of titanium and boron-containing SiC fiber decreased when the titanium and boron content are in the range of 0.83 to 3.05 and 0.38 to 1.60.Because of the good electrical conductivity,electromagnetic wave will be reflected.Therefore,the wave-absorbing properties of fiber with highest titanium and boron contents reduce greatly.