Study On The Composition,structure And Performance Of SiCN And SiBN Fibers

High-performance continuous ceramic fibers,which the key raw materials for applications in the defense and military industry such as aerospace propulsion systems,thermal protection systems,and transparent windows,are important parts of advanced ceramic matrix composite materials.Nitride fibers such as SiCN and SiBN fibers,are advanced ceramic fibers combining with excellent structural and functional properties.Their composition,microstructure and performance have not yet been systematically studied,which restricts the design and application of relevant equipment.In this paper,SiCN fibers and SiBN fibers with widely tunable composition and microstructure were prepared via polymer-derived method.The structure-effect relationship of SiCN fibers and SiBN fibers between the composition/microstructure and high temperature properties is discussed via the systematical characterization of composition,microstructure and performance.Meanwhile,the relationship between the composition/microstructure and dielectric properties of the two types of nitride fibers is discussed.These results provide the scientific basis for the performance optimization of SiCN fibers and SiBN fibers and their high-temperature absorbing/transmitting applications.The method of controlling the composition and structure of SiCN fibers is studied,and the characteristic microstructure and formation mechanism of SiCN fibers with different compositions are systematically analyzed.Using electron beam irradiation cross-linked polycarbosilane（EB-PCS）fibers as the starting material,continuous SiCN fibers were prepared by nitrogen decarburization method in which ammonia gas was involved in the organic-inorganic conversion process at high temperature.The carbon content of SiCN fibers could be adjusted from 1.86 wt%to 37.5 wt%by controlling the process parameters such as ammonia gas flow and temperatures.The obtained SiCN fibers show tensile strength of about 2.0 GPa,with elastic modulus of 168-233 GPa and average diameter of 12μm.SiCN fibers have microstructure characteristics that consisted of Si₃N₄,SiC/C,and gradient SiC_xN_yphase.The content of these three phases is related to the composition of SiCN fibers:SiCN fibers with carbon content less than15.7 wt%show a major phase of Si₃N₄;when the carbon content is between 15.7 wt%and 22.8 wt%,large amounts of gradient SiC_xN_yphase are formed;SiCN fibers carbon content with more than 22.8 wt%show a major phase of SiC/C.The formation of gradient SiC_xN_yphase indicates that the nitriding and decarburization process of EB-PCS fibers are the nanochannel diffusion-controlled nitridation process.The high-temperature structural evolution and mechanical properties of SiCN fibers with different compositions after annealing at nitrogen,argon,and air atmospheres were studied.In an inert atmosphere above 1400℃,amorphous Si₃N₄was consumed via carbothermal reduction and high-temperature decomposition reactions,resulting in a rapid degradation of the monofilament tensile strength for low-carbon SiCN fibers（<15.7 wt%）and lost their tensile strength after the heat treatment at1600℃;SiCN fibers with carbon content higher than 22.8 wt%showed a slow degradation of tensile strength when the phase separation of gradient SiC_xN_yphase and the two-dimensional growth of free carbon nanoclusters occurred at high temperatures,and finally showed a tensile strength retention rate higher than 50%after treatment at1600℃.The degradation rate of tensile strength of SiCN fibers in argon atmosphere is significantly faster than that in a nitrogen atmosphere.At the same time,the initial crystallization temperature of amorphous Si₃N₄in argon is about 50℃lower than that in nitrogen atmosphere.After treatment with an air atmosphere above 1200℃,the surface of SiCN fibers is oxidized to form an amorphous SiO₂layer with a thickness of about 200 nm.The thickness of the oxide layer for SiCN fibers with lower carbon content is smaller after the treatment under the same conditions.However,the oxidation interface is obviously rough due to the high porosity and uneven oxygen diffusion rate for SiCN fibers with rather low carbon content,thus the stress concentration is exacerbated,which finally caused a rather lower tensile strength retention.The structural characteristics of SiBN fibers with different compositions,the microstructure evolution and performance of SiBN fibers as well as the effects of boron element under high temperature were studied.The continuous SiBN fibers with boron content of 0.23 wt%-6.81 wt%were prepared by the polymer-derived method.The monofilament tensile strength of SiBN fibers is about 1.40 GPa,with elastic modulus of110-140 GPa,average diameter of about 13μm and the density of 1.9-2.3 g/cm³.The amorphous SiBN fibers are mainly composed of SiN₄and BN₃structural units,which constructs the microstructure that including amorphous Si₃N₄,amorphous BN,and amorphous Si-N-B network structure.The effects of boron element on the high-temperature evolution are obviously different for SiBN fibers with different boron contents.When the annealing temperature is below 1600℃,boron element can significantly inhibit the high-temperature crystallization of SiBN fibers.Generally,SiBN fibers with higher boron content showed higher initial crystallization temperature,and could present tensile strength retention higher than 50%after annealing at 1600℃.Extending the treatment time and increasing the treatment temperature above 1600℃could both cause the crystallization of amorphous SiBN fibers.The boron element with a content range of 3.56 wt%-5.14 wt%combining with a small amount of oxygen element could form liquid phase at high temperature,which then could play a role of sintering additives by promoting the crystallization ofβ-Si₃N₄crystallization;while boron element with content higher than 6.81 wt%mainly exists in the form of solid BN phase,which can hinder the migration of silicon atoms,and shows inhibition effects on the high-temperature crystallization of SiBN fibers.The high-temperature microstructural evolution and performance of SiBN fibers after treated in nitrogen,argon and air atmosphere were studied.SiBN fibers started to crystallize obviously with tensile strength decreased significantly to about 0.5 GPa after being treated at 1700℃in nitrogen.At the same time,nitrogen gas at high temperature is reactive,which could bond with boron to form BN on the fiber surface,resulting in a radial gradient distribution of boron element with high content on the fiber surface and low content at the internal fiber.In argon atmosphere,SiBN fibers began to crystallize obviously after being treated at 1600℃,when the crystalline phases such asα-Si₃N₄,β-Si₃N₄and Si₂N₂O were formed,with tensile strength decreased to 0.5-1.0 GPa.SiBN fibers exhibit the diffusion oxidation process after high-temperature treatment in air,when BN and Si₃N₄phases are oxidized to form B₂O₃and SiO₂,respectively.Due to the vaporing and escape of B₂O₃at high temperature,oxide layer with a complex fine structure could form,which contains four zones such as SiO₂,SiO₂/BN,SiBNO and diffusion transition zone.The relationship between the composition/microstructure of SiCN fibers and dielectric properties was studied.The resistivity and dielectric constant of SiCN fibers are closely related to their carbon content and microstructure.SiCN fibers with carbon content of 37.5 wt%presented the real and imaginary parts of permittivity both higher than 20,thus the incident electromagnetic wave（EMW）mainly reflects back on the fiber surface;SiCN fibers with carbon content of 28.2 wt%-15.7 wt%showed the dielectric constants that can meet the requirements for absorbers with excellent EMW absorbing properties.Among them,SiCN fibers with carbon content of 28.2 wt%presented the minimum reflection coefficient of-46.1 d B and effective absorption bandwidth of 3.79 GHz.According to the analysis of their microstructural characteristics,continuous SiC/C phase in SiCN fibers could supply the conductivity loss by absorbing EMW energy into current flow.Meanwhile,the gradient SiC_xN_yphase could improve the impedance matching and form heterojunctions such as SiC_xN_y-SiC and SiC_xN_y-Si₃N₄that can enhance the absorption performance through the polarization loss mechanism.After the heat treatment at 1400℃,the minimum reflection coefficient can be as low as-63.7 d B,with effective absorption bandwidth enlarged to 4.20 GHz,showing excellent high-temperature electromagnetic wave absorption performance.The relationship between the composition and dielectric properties of SiBN fibers was studied.SiBN fibers contains only trace amounts of carbon,with dielectric constant and dielectric loss values both decrease significantly with the boron content increasing,which can be explained by the Lichtenecker’s logarithmic mixing law.The boron element is beneficial to improve the electromagnetic wave transparent properties of SiBN fibers,mainly due to the presence of some boron element in the form of BN phase with low dielectric loss.Meanwhile,the Si-N-B network structure can inhibit the flow of electron clouds in the microstructure and further reduce the observed dielectric constant and dielectric loss of SiBN fibers.At the same time,the changes of the dielectric properties of SiBN fibers at different temperatures showed that the boron element could also effectively stabilize the dielectric parameters in a wide temperature range,which makes SiBN fibers to meet the application requirements of high-temperature wave-transmitting composite materials.