Study On The Iodine-curing And Pyrolysis Of Polysilazane Fibers

Silica-based ceramic fibers prepared by precursor conversion method are widely used as high temperature structural materials in aerospace,military and nuclear energy fields.It has excellent properties such as high strength and high temperature oxidation resistance.Among them,Si CN fibers have the characteristics of light weight,good thermal stability and oxidation resistance at high temperature,excellent dielectric properties and so on,which has been widely concerned by researchers.Curing treatment is one of the key steps to prepare Si CN fibers.In view of the problems of long processing time,high processing temperature and easy bonding of traditional air curing methods,this paper uses iodine-curing method to cross-linking and curing polysilazane fibers and prepare Si CN fibers by pyrolysis.The iodine-curing mechanism and pyrolysis process of polysilazane fibers,the mechanical properties and high temperature oxidation resistance of Si CN fibers were investigated by SEM,FT-IR,TGA,PY-GC-MS,XPS,XRD and electronic single fiber strength analyzer.The main results are as follows:（1）Continuous polysilazane fibers were prepared by ammonolysis,thermal polymerization and melt spinning.The iodine-curing treatment process of polysilazane fibers was studied,and the effects of fiber diameter,iodine-curing temperature and time on the curing effect were discussed.The results show that when the treatment temperature is 80℃,for polysilazane fibers with different diameters（6.5～38.3μm）,the smaller the diameter,the shorter the initial curing time.When the diameter is 6.5μm,the required time is 1 h,and when the diameter is 38.3μm,the required time is 4h.Different treatment temperature（50～90℃）and treatment time（1～5 h）will affect the curing treatment effect of the fibers.When the treatment time is 1 h,the curing degree of the fibers increases with the increase of temperature,and reaches the best at80℃.At 80℃,with the prolongation of treatment time,the curing degree of the fibers increases,and reaches the relative optimum at 3 h.By optimizing the process,it is determined that the relatively best iodine-curing treatment process is 80℃,3h,and the Gel content of the cured fibers is 96%.（2）The mechanism of iodine-curing of polysilazane fibers was investigated.By studying the process of iodine-curing and air curing of polysilazane fibers,the similarities and differences of the two curing methods were compared.The results show that air curing needs to be heated slowly to 110°C and kept for 8 hours to achieve curing of fibers,while iodine-curing can be achieved by curing at 80°C for 1hour,and the best curing effect can be achieved in 3 hours.At the same time,under the same conditions,the Si-H reaction degree of iodine-curing is also better than that of air curing.Air curing mainly uses the oxidation of Si-H to form the Si-O-Si cross-linked structure to achieve fiber curing,while the mechanism of iodine-curing is different from air curing and can be carried out at a lower temperature.Due to the absence of oxide layer,iodine vapor can continue to spread to the fiber core and carry out cross-linking reaction after deposition on the fiber surface:iodine promotes the breaking of active chemical bonds such as Si-H,C-H,N-H,forming corresponding hanging bonds,and recombing into Si-C,Si-N,Cmurn,Si-O-Si and N-I groups.Polysilazane fibers were cured by forming network cross-linking structures such as Si-C-Si,Si-O-Si,Si-N-Si and so on.（3）The pyrolysis properties of green fibers and cured fibers were compared.the pyrolysis mechanism of polysilazane fibers was studied.The results indicate that the weight loss of green fibers mainly comes from the loss of medium and low molecular weight polymers,while the weight loss of cured fibers comes from the release of iodine and the decomposition and inorganization of cross-linked structures.Cured fibers contain a large amount of elemental iodine and combined iodine,which release a large amount of iodine vapor during pyrolysis.The pyrolysis gas products mainly include I₂,CO,NH₄,CH₄,and various silanes.（4）The effects of pyrolysis temperature and holding time on the tensile strength and morphology of Si CN fiber were studied.When the heat treatment time is 60min,the mechanical properties of the sintered fiber are improved with the increase of temperature in the temperature range of 700～1100℃.When the heat treatment temperature is 1100℃,in the time range of 60～120min,with the extension of time,the tensile strength of the Si CN fiber changes little,generally maintained at about 1.5GPa,and 90 min is selected as the best holding time.The best pyrolysis process is1100℃,90 min.The amorphous Si CN fibers with smooth surface was obtained by iodine-curing method.The internal of the fibers are compact and the tensile strength is up to 1.5 GPa,which is much higher than that of air curing method（0.1 GPa）.After oxidizing 30 min at 1400℃,Si CN fibers formed the typical skin-core structure,and the tensile strength decreased to 0.6 GPa,and the strength retention rate was 40%.In summary,polysilazane fibers were prepared by ammonolysis,thermal polymerization and melt spinning.The surface of green fibers was smooth,the average diameter was 6.5μm,and the continuous spinning length was more than 3000 m.After iodine-curing treatment at 80℃for 3 hours,the Gel content of the cured fibers reached 96%.The iodine-curing process of polysilazane fibers was systematically studied,and the possible iodine-curing mechanism was proposed.The pyrolysis process was studied and discussed.The Si CN fibers obtained by pyrolysis at 1100°C for 90 min are amorphous,with smooth surfaces,dense interiors,and high mechanical properties（1.5 GPa）.The development of fiber has certain reference value.