Basic Research On Preparation Of The Third Generation Continuous SiC Fiber

The polyaluminocarbosilane?PACS?is prepared by introducing aluminum into PCS,which is used as the precursor for the third generation SiC fiber.The approach involves spinning of PACS fibers,air-curing and sintering to give SiC fiber which has a near stoichiometric composition and large crystallite.This strategies is the most cost effective ways for the 3^rd generation fiber,that is named SiC?Al?fiber.Domestic continuous SiC?Al?fiber manufacture suffers several serious drawbacks.For example,the poor spinnability and spinning stability of PACS and the problem of sintering desification.In this paper,we synthesized PACS with stability and excellent spinnability by optimizing the synthesis conditions and the controling of product.As studing on the high-temperature transformation of SiCO?Al?fiber,the dense SiC?Al?fiber was obtained by proper heattreatment and sintering technology.The supercritical fluid method has been applied to prepared low-molecular PCS?S-LPCS?,which has a homogeneous molecular weight distribution and its GPC curve is nearly“single peak”.S-LPCS also has higher Si-H content,that make it more suitable for a raw material to prepare PACS.Different S-LPCSs have been obtained at different reaction temperature,their softenning point ranges from 60 to 110?,number-average molecular weight?Mn?ranges from 780 to 1180.S-LPCS reacts with[Al?acac?₃]to form PACS.The spinnability of PACS is mainly affected by the softening point as well as the residual acetylacetonate in the structure.At lower reaction temperature,the molecular structure of products is mainly hanging structure like Si-O-Al?acac?₂.As reaction temperature increases,the acetylacetonate of hanging structure reacts with other S-LPCS molecular to form bridging structure like-Si-O-Al?acac?-O-Si-.Meanwhile,the molecular weight and softening point of PACS are increasing.The optimizing reaction temperature is 410?,that PACS has neither much residual acetylacetonate nor branched structure.Using appropriate S-LPCS to match various weight ratio of[Al?acac?₃],different PACS that contain Al form 0.3 to0.7wt%have been obtained.All the PACS have softening point range from 190 to200?,which are suitable to melt spinning.The PACS fibers with the diameter of13.0?m¹4.5?m were produced by continuous melt spinning.PACS fiber has more oxygen and less Si-H than PCS fiber.PACS fibers could not turn into fully crosslinked fibers untill the curing temperature increases to 200?²40?.But,the oxygen content of cured PACS fibers is as high as 11.3wt%¹3.8wt%because of much Si-OH residue in the fiber.It leads to the fact that the oxygen content of SiCO?Al?fibers that were obtained by pyrolysizing is too high.In this paper,two methods were applied to promote the formation of the crosslinking structure of Si-O-Si and Si-CH₂-Si,which were named“preoxidattion-CVC”and“preoxidation-thermal crosslinking”,respectively.The oxygen content of cured PACS fibers was reduced effectively through adjusting curing conditions.Forthermore,the oxygen content of SiCO?Al?fiber was reduced to 9wt%¹1wt%.SiCO?Al?fiber undergoes three main processes?SiC_xO_y decomposition,SiC crystal growth and densification?during high-temperature heattreatment,and then it translates to SiC?Al?fiber.The results reveal that the composition of fiber,especially Al and O content,severely affects the transformation process.After being heated at 1900?,SiCO?Al?fibers with more than 10wt%oxygen could obtain SiC?Al?fibers that its C/Si is closer to 1 and SiC crystal is bigger.But,SiCO?Al?fiber with over-high oxygen content tends to produce large pores and coarse grain.The fiber is unable to densification as it loses integrity.When the oxygen of SiCO?Al?fiber is less than 9wt%,the fiber contains much residual free carbon after high-temperature heating that hinders the densification process.With the SiC_xO_y decomposition,the grain growth of SiC is divided into two stages:below 1500?,the mechanism of SiC crystal growth is a slow diffusion process because of the crystal was surround by SiC_xO_y phase and free carbon;above 1500?,SiC crystals become free to coalesce when the obstacles were strongly degraded.At the same time,Al begins to retard the SiC crystal growth that leads to the fiber being loose.The optimizing Al content is 0.6wt%.Except the composition,the heattreatment conditions also affect the sintring process of fiber.The high-temperature transformation process of SiCO?Al?fiber is divided into two stages that SiC_x O_y decomposition and desification by obvious temperature boundary.They are endothermic and exothermic reactions respectively.The SiC_xO_y decomposition could not be completed until the heating temperature up to1650?.Static heating technology is beneficial for the transformation process,that the SiC_xO_y decomposition and the SiC grain growth could carry out thoroughly before sintering.The optimal heating condition is that the SiCO?Al?fiber was heated to 1650?at a rate of 150?/h and held for 1h in Ar atmosphere.The fiber converted from dense amorphous structure to loose polycrystaline structure after the SiC_x O_y decompostion had finished.Meanwhile,the tensile strength of fiber falled to the lowest value.The fiber realized U-shape recovery in tensile strength after sintering at 1900?.SiC?Al?fiber with tensile strength of about 1.5GPa,density of about 2.96g/cm³,average diameter of about 8.95?m,C/Si of about 1.28 and average crystal size of?-SiC about 45nm was obtained.