Catalytic For Polymerization Of The Cyclic Silazane And Thermal Decomposition Of Its Polymer Into Si₃N₄-based Materials

Silicon nitride is a kind of excellent materials for use in aerospace. Inrecent years, with the development of aerospace technology, silicon nitridematerials with excellent properties became a hot research. Compared withinorganic methods, silicon nitride was prepared by pyrolysis of precursor withthe advantage of high purity, high strength, low cost and simple processing. Themain method of preparation of precursor was ammonolysis of chlorosilane,however which had lower molecular weight. So the method of catalyzedpolymerization was usually adopted to further improve the molecular weight. Thepolysilazane synthesized by Alkali metal hydride and trifluoromethanesulfonicacid as catalyst had poor process. The process of the polysilazane can be adjustedby controlling the reaction conditions with tetrabutylammonium fluoride ascatalyst. But there are few reports of the research of preparation ofpolymerization of silazane in the presence of tetrabutylammonium fluoride ascatalyst.In this paper polysilazane PHSZ and PSN2with high molecular weight wereprepared via the polymerization of MHSZ and MVSZ with tetrabutylammoniumfluoride as catalyst. MHSZ was synthesized by the ammonolysis ofmethylhydrodichlorosilane and MVSZ was synthesized by the ammonolysis ofmethylhydrodichlorosilane and methylvinyldrodichlorosilane. The effects ofcatalyst concentration and polymerization time on the molecular weight, productstate, ceramic yield were discussed. It was suggested that all of thepolymerization of PHSZ was refractory solid with the catalyst content of0.25%or0.5%; the PHSZ can solve in alkanes, toluene and tetrahydrofuran; the TGA showed that ceramic yield is over80%at1000℃under N2and significantlyhigher than the MHSZ.The effect of pyrolysis atmosphere and temperature (1100℃~1600℃) onstructure and composition of pyrolysis products of PHSZ and PSN2was studied.The results showed the pyrolysis products of PHSZ and PSN2began tocrystallize at1500℃and crystallized completely at1600℃in all threeatmosphere. The pyrolysis products of PHSZ which had very low carbon contentgot the main component of silicon nitride under NH3and N2. And the principalcrystalline phase of pyrolysis products of PHSZ and PSN2was α-Si3N4under N2;the principal crystalline phase was SiC and α-Si3N4under Ar.