Studv On Preparation And Properties Of BN-Si₃N₄ Composite Fibers Derived From Polvmeric Precursors

With the development of the aerospace industry,high-Mach number aircraft has an increasing demand for high-temperature wave-transparent materials,especially high-performance wave-transparent ceramic fibers.BN-Si3N4 composite fiber combines the advantages of BN fiber's excellent thermal stability,dielectric properties and Si3N4 fiber's excellent mechanical properties,showing great potential in high-temperature wave-transparent applicationsIn this thesis,BN-Si3N4 composite ceramic fiber were prepared by using polyborazine(PBN)and polysilazane(PSZ)as polymeric precursors,after melt spinning,electron beam crosslinking and high-temperature pyrolysis.BN-Si3N4 composite ceramic fiber has excellent comprehensive properties,such as mechanical properties,dielectric properties,and oxidation resistance.The thesis first investigated the basic properties and pyrolysis process of the PBN and PSZ precursors,and studied the combination effect and pyrolysis mechanism of the hybrid precursor;the rheology of the hybrid precursor and the melt spinning process were then studied,and high-quality green fibers were prepared.The influence of different curing method on the fiber cross-linking process was investigated;finally,the composite ceramic fibers were prepared by high-temperature pyrolysis,and the influence of fiber morphology and structure on fiber properties was discussed.The main results of this study are as follows1.The pyrolysis product of polyborozane(PBN)precursor is turbostratic BN,while the pyrolysis product of polysilazane(PSZ)precursor is the mixture of ?-Si3N4 and ?-Si3N4.The carbon content of the pyrolysis products of both is below 0.5wt%,indicating that PBN and PSZ are suitable as precursors for wave-transparent ceramic fibers2.There is a combination effect in the pyrolysis process of hybrid PBN/PSZ precursors.The increase of the PSZ content in hybrid precursors can improve the crystallinity of the BN phase in the pyrolysis product.When the content of Si3N4 in the pyrolysis product is 25wt%,the crystallinity of the BN crystal reaches the maximum.The Si3N4 in the pyrolysis products are all amorphous.3.The pyrolysis process of hybrid PBN/PSZ precursors is divided into ceramization process and crystallization process.The process of ceramization is completed by the reaction between N-H,-NHCH3,-CH3 and Si-H,small molecules such as CH4 and CH3NH2 are eliminated.The ceramization process of the composite precursor was completed at 1000?,and the crystallization behavior of BN phase starts at 1400?.Holding at 1600? for 120 min is sufficient for the h-BN crystallization process4.The hybrid PBN/PSZ precursor has excellent spinning performance,and its activation energy is 14.20 kJ/mol.The polymeric melt is a pseudo-plastic fluid with shear thinning characteristics,and the flow index is 0.812-0.982 in the temperature range of 120-150?5.The melt-spinning process of hybrid PBN/PSZ precursor is controlled by the spinning temperature,spinning pressure and spinning speed.Thees three parameters should be coordinated and matched with each other to prepare uniform and dense green fibers with appropriate diameters.The suitable spinning conditions in this experiment are the spinning temperature of 110-135?,the spinning pressure of 0.3-0.7 MPa,and the spinning speed>8 m/s.6.NH3 and electron beam are used for the curing of green fiber.The results show that electron beam cross-linking is a more suitable curing method.The gel content of the infusible fiber is 81.8wt%.After pyrolysis to 1000?,the obtained ceramic fiber is uniform and dense without defects.7.The yield of the infusible fibers is 40wt%after 1000?,NH3 pyrolysis and 1600?,N2 high temperature treatment,and the residual carbon content is about 0.2wt%.The obtained ceramic has a stoichiometric composition of BN(Si3N4)0.05.8.The composite fibers prepared at 1600? exhibited a "shell-core" structure.The edge of the fiber is composed of a layer of BN with low crystallinity,while at the central area of the fiber,BN grains are very perfect,the(002)crystal plane spacing is 0.333 nm,which is in accordance with the theoretical value of h-BN crystal,and the grain size exceeds 15 nm.9.The composite fibers have excellent mechanical,dielectric properties and oxidation resistance.The tensile strength and Young's modulus of the obtained fiber were 1040 MPa and 90 GPa,respectively,and the dielectric constant and dielectric loss were 3.21 and 3.11 × 10-3,respectively.The weight increase of fibers treated in air at 900? and 1000? for 4 h were 2wt%and 7wt%,respectively.