Research On Synthesis,Spinning,Infusiblity Of Polyaluminumsilanze And Preparation Of Ceramic Fiber

Si₃N₄ ceramic fiber is an ideal reinforcing material for high-performance ceramic matrix composites due to its excellent properties such as high strength,high modulus,high temperature resistance and oxidation resistance.Owing to the wide application prospect in aerospace,military,missile and other fields,it has attracted the attention of researchers.The introduction of a small amount of aluminum in the precursor of silicon nitride fiber polysilazanes can improve the molecular structure of the polysilazanes,which is not only conducive to non-melting treatment,but also can play the role of sintering aid and crystallization inhibition during high temperature cracking.Meanwhile,it is beneficial to the preparation of more high-temperature resistant fiber materials.Herein,we used silazane oligomer（LSZ）and aluminum acetylacetone（Al（Ac Ac）₃）as raw materials,a series of precursor polyaluminosilazanes containing aluminum silicon nitride fibers（PASZ-x:x is the mass percentage of Al（Ac Ac）₃ in the raw material）were prepared by using different raw material ratio:PASZ-0.1,PASZ-0.2,PASZ-0.3,PASZ-0.5 and PASZ-1.Si Al CN（O）fibers were obtained by melting spinning,infusible treatment,high temperature cracking.The elemental composition,structure change and micro-morphology of polyaluminosilazane precursor,non-melting fibers and ceramic fibers were analyzed by the means of gel permeation chromatography,infrared spectroscopy,X-ray photoelectron spectroscopy,thermogravimetric,scanning electron microscope and X-ray diffraction.Meanwhile,the four key problems of the synthesis of polyaluminosilazane,spinning,infusible treatment and ceramics were investigated.The main conclusions are as follows:（1）Precursor polymerization:The adoption of silazane oligomer and Al（Ac Ac）₃thermal polymerization was conducive to the design of the precursor polymer chain structure.The main dehydrogenation reaction between Si-H and Si-N occurred during the polymerization of PASZ to form the Si-N bond.Aluminum mainly existed in the form of Si-O-Al bond in the structure of PASZ,which acted as a cross-linking point in the process of thermal polymerization,making part of the molecular chains cross-linked and improving the yield of ceramics.（2）Melt spinning:The influence of different Al content on the spinning properties of PASZ were studied.It was found that the PASZ fiber prepared with Al（Ac Ac）₃ content below 0.3%had excellent spinning properties.The fiber surface was smooth and flat after melting spinning,and there were no defects such as groove and cracks.The fiber diameter was 20～70μm,and the continuous spinning length could achieve 3000 m.The end breaking phenomenon of PASZ was more and more likely to occur in the spinning process,and the spinning property became worse while the content of Al（Ac Ac）₃ continued to increase.The optimal proportion of precursor was PASZ-0.2,which introduces more aluminum while ensuring excellent spinning property.（3）Oxygen thermal infusible treatment:The influence of cross-linking time and temperature on infusiblity effect was studied.The optimal oxygen thermal infusiblity process for PASZ-0.2fiber was slowly heated to 240℃and for 10 h.The smaller the fiber diameter was,the shorter the oxidation cross-linking time was.The ceramic yield of PASZ-0.2 fiber increased from 54.1%to90.0%（nitrogen atmosphere,800℃）after infusible treatment.The oxygen thermal cross-linking mechanism of PASZ was analyzed,it was considered that,similar to PSZ,the active group Si-H was oxidized under the condition of heating to form Si-O-Si structure.However,PASZ could achieve the effect of infusible due to the action of aluminum could introduce less oxygen.（4）High temperature cracking ceramization:The effect of cracking process on fiber properties was investigated.It was found that the heating rate was not too fast in the low temperature cracking period,otherwise the cracked small molecules would not be able to escape and caused fiber damage.Si Al CN（O）fiber fired via 600℃,1200℃,1300℃,1400℃had smooth surface and partial hollow phenomenon inside.The reason was that the internal cross-linking degree of the fiber was low and it was cracked at high temperature to form hollow.Si Al CN（O）fibers were mainly amorphous,containing a large number of Si-N,Si-O bonds and a small amount of C-C bonds.The introduction of aluminum element mainly existed in the form of Al-O bonds.Si Al CN（O）fiber had a certain mechanical property（0.43 GPa）,and it formed the core structure after high temperature oxidation treatment.In summary,Polyaluminosilazane PASZ-0.2 precursor with excellent spinning performance was prepared by aluminothermic polymerization of silazane oligomer and acetylacetone.The surface of the prepared polyaluminosilazane fibers by melt spinning was smooth and flat,and the diameter was about 20～70μm,and the continuous spinning length could achieve 3000 m.The ceramic yield increased from 54.1%to 90.0%after 240℃for 10 h with infusible treatment.Ceramic fibers cracking at temperature of 600℃,1200℃,1300℃,1400℃were mainly amorphous and hollow in the fibers.The fiber prepared in this paper has a certain mechanical property（0.43 GPa）,which had a certain reference significance for the continuous preparation of high-performance Si₃N₄ fiber containing aluminum.