Research On Synthesis,Spinning And Ceramic Properties Of Polysilazane Ceramic Precursor

Silicon nitride(Si3N4)ceramic fibers have many unique characteristics,such as high mechanical properties,excellent thermal shock resistance,high oxidation resistance,high insulation and good elastic modulus.These properties make silicon nitride ceramic fibers a promising candidate material for many applications,such as reinforcement material for ceramics/metals and aerospace field etc.The polymer-to-ceramic conversion route is one of the main methods for the preparation of silicon nitride ceramic fibers,which mainly consists of four furnaces:polysilazane synthesis,spinning,non-melting treatment and high temperature sintering.The paper focuses on the preparation,structure characterization and spinning properties of polysilazane,and discusses the non-melting treatment and ceramicization properties.In this paper,the molecular precursors are synthesized with methylsilane dichlorosilane(CH3SiHCl2)and dimethyldichlorosilane((CH3)2SiCl2)as raw materials by one-step copoly condensation,and then polymerized to polysilazane PSZ-50,PSZ-60,PSZ-63,PSZ-65 and PSZ-67.(PSZ-X:X is the proportion of CH3SiHCl2).The molecular structure,spinning performance,non-melting and high temperature sintering of polysilazane are investigated by a series of characterization methods such as GPC,FT-IR,DSC,TG,XRD,XPS,SEM,etc.The specific research conclusions are as follows:(1)The polysilazane PSZ-65 is the optimal precursor,it exhibits excellent spinning performance in the melt state.The prepared polysilazane PSZ-50,PSZ-60,PSZ-63 and PSZ-65 are colorless transparent brittle solids,PSZ-67 is white fluffy brittle solid.The PSZ-65 with Mw=14231,melting point is 130℃,and it exhibits excellent spinning performance in the melt state.The TG analysis shows that the ceramic yield of PSZ-65 at 800℃ under nitrogen atmosphere is 63%.In the process of polysilazane polymerization,Si-Si dehydrogenation,Si-N dehydrogenation,and ring-forming reactions mainly occur,so the polysilazane prepared is a polymer containing four-membered rings and six-membered rings.(2)The polysilazane PSZ-65 is pseudoplastic fluid,the spinning of polysilazane fibers using melt spinning machine with a continuous fiber length of up to 1000 m.The rheological properties of PSZ-65 polysilazane were tested by rheometer.PSZ-65 is pseudoplastic fluid and exhibits shear thinning behaviors.In the angular velocity measurement range,the loss modulus is always greater than its storage modulus.With the increase of angular velocity,loss modulus increased,storage modulus increased,which proves that PSZ-65 have good spinnability.Through temperature-apparent viscosity curve,determine the best spinning temperature is 200℃.The spinning of polysilazane fibers using melt spinning machine with a continuous fiber length of up to 1000 m,and fiber diameter from 10 to 80 μm.The obtained polysilazane fibers have uniform diameter and smooth surface.(3)After non-melting treatment at 110℃ for 6 h,the final ceramic yield of polysilazane increases from 63%to 80%.When the pyrolysis temperature rises to 1400℃,Polysilazane can be crystallized from amorphous state to obtain α-Si3N4,β-Si3N4 and SiC crystal phases.The PSZ-65 polysilazane precursor and polysilazane fiber are placed in an oven at 90℃,100℃,110℃,115℃ and 120℃ for 1 h by oxygen/thermal non-melting treatment.The surface morphology of the fibers are observed by metallographic microscope.Finally,combined with TG test analysis the optimum non-melting condition is determined to be treated at 110℃ for 6 h.After non-melting treatment,the final ceramic yield of polysilazane increases from 63%to 80%.The mechanism of polysilazane oxygen/thermal non-melting is that Si-N bonds are converted into Si-O-Si bonds under the action of oxygen to achieve the purpose of cross-linking.The non-melted polysilazane precursor is placed in the tube furnace under high temperature sintering at 1200℃,1300℃,1400℃,1500℃ and 1600℃,and the characterization test of the sintered product is carried out by XRD.The sintered product at 1200℃ and 1300℃ is still amorphous,and no crystallization peak appears.When the temperature rises to 1400℃,the α-Si3N4,β-Si3N4 and SiC crystallization peak begins to appear.The pyrolysis temperature higher,the crystallization effect more obvious.Discovered by SEM characterization,polysilazane is thermally cracked at 1400℃ to obtain silicon nitride nanowires,and the temperature raises to 1600℃ to obtain silicon nitride nanoribbons.In summary,the polysilazane PSZ-65 prepared in this paper with Mw=14231,melting point is 130℃,and it exhibits excellent spinning performance in the melt state.The polysilazane fiber has a smooth and glossy surface.The fiber diameter is 10～80 μm and the continuous spinning length is up to 1000 m.After the polysilazane fiber was treated at 110℃ for 6 h in an oxygen atmosphere,the final ceramic yield increases from 63%to 80%.When the pyrolysis temperature rises to 1400℃,polysilazane can be crystallized from amorphous state to obtain α-Si3N4,β-Si3N4 and SiC crystal phases.The research in this paper has reference significance for the spinning of polysilazane fiber.