Research On The Fabrication Of Polymer-derived SiBN Ceramic Fiber

Envisioned for the urgent need of high-temperature resistant continuous ceramic fibers with both good mechanical performance and wave-transparent property in the radomes of ultra-high speed precision-guided missiles, and for the serious property disadvantages of the fibers currently available, the present work is mainly focused on the basic research on the fabrication of a novel mechanical/wave-transparent SiBN fiber by polymer-derived method.Considering the requirements for SiBN fiber precursor and through molecular design, the synthesis strategy was selected by using the carbon-containing polyborosilazane (PBSZ) as the preceramic precursor, and the decarburization was performed in the subsequent pyrolysis process. The desirable polymer should be composed of Si-N, N-H, Si-H, and borazines. Aimed at future pilot plant scaling up, PBSZ synthesis was chosen via a one-pot route by using borontrichloride (BTC), dichloromethylsilane (DCMS), and hexamethyldisilazane (HMDZ) as the starting materials. This synthesis route proves to be cheaper, simplier, and can be scaled up without any in-principle problems.Orthogonal design and single factor alternation method were both used to study the synthesis process. The optimized parameters were found to be the molar ratio (BTC:DCMS:HMDZ) 1.25:2:8, maximum reaction temperature 270℃with a holding time of 14 h. Under such parameters, soluble and meltable polymer can easily be obtained as colorless transparent bulky solid with the softening point of about 118℃. The synthesis yield is about 91 wt%. Such parameters are well replicable and the scale-up synthesis of 500 g PBSZ per batch has been achieved currently.Elemental analysis, XPS, FTIR, NMR, and TGA were used for the characterization of the as-synthesized PBSZ. The polymer was composed of silicon, boron, nitrogen, carbon, and hydrogen. The backbone of PBSZ was Si-N-B with some borazines. Carbon exists as saturated methyl groups that can easily be removed in the pyrolysis process. It contains latent reactive groups such as Si-H and N-H, which account for the high ceramic yield of 63 wt% in N2 atmosphere up to 1000℃. Meanwhile, PBSZ exhibits good hydrolysis stability because of the steric groups -SiMe3 in the polymer. The hydrolysis of PBSZ is the first order reaction with the relative humidity unchanged. PBSZ hydrolyze 6.5 % after exposed for 27 h under ambient conditions with the relative humidity 75 %.Experimental verification, as well as theory computation was used to investigate the synthesis mechanism. The reaction mainly involves the condensation of Si–Cl and B–Cl with N–SiMe3 followed by SiMe3Cl evaporation in the first stage. An intermolecular condensation with the loss of HMDZ and SiMe3Cl takes place in a second stage. And the dehydrogenation between N–H and Si–H also occurs during the formation of polymeric network.Rheological studies suggest that molten PBSZ is a pseudo-plastic fluid. For PBSZ with the softening point of 102℃, the flowing index is 0.83~0.90 and the apparent activation energy about 145 kJ·mol-1 The as-obtained PBSZ exhibits good melt-spinnability and smooth polymer fibers with 12~15μm in diameter and >1000 m in continuous length can easily be obtained. The multifilamental continuous melt-spinning of PBSZ was also preliminarily realized.Curing of the PBSZ green fibers was achieved by chemical vapor curing (CVC). The condensation of–SiMe3, N–H between–Cl groups in the gaseous curing agents such BTC and DCMS occurs in the CVC process. On one hand, such reactions lead to the crosslinking of the fiber surface; on the other hand, the elimination of–SiMe3 groups preliminarily reduce the carbon content of the fiber, which facilitates the carbon removal in the pyrolysis step. The optimum curing process was found to be exposing the green fiber to BTC vapor for 10 min at 85℃, followed by the passage of ammonia, and then heating the sample to 350℃with an additional 1 h-holding. Under such parameters, the obtained cured PBSZ fiber exhibits a gel content of 95 %.The pyrolysis process was investigated by TG, MS, FTIR and 29Si-MAS NMR. The inorganic conversion of the cured PBSZ fiber mainly involves three stages: (I) Below 400℃, besides the desorption of water vapor absorbed from ambient atmosphere, the precursor fiber also undergoes further thermal crosslinking such as transamination reactions during this step. (II) At 400~800℃, the thermolysis of the organic groups occurs, leading to escape of hydrocarbons and hydrogen. The mechanisms are mainly radical and nucleophilic substitution. (III) Above 800℃, the residual hydrogen atoms are gradually removed as H2, the mineralization was completed with radical reactions. The ceramic yield of cured PBSZ fibers in N2 and in NH3/N2 atmosphere up to 1000℃is 92 wt% and 88 wt%, respectively. The corresponding pyrolysate is mainly composed of amorphous SiCxN4-x(x=0,1or 2).Kinetic study of the pyrolysis process indicates that the stage I is dominated by Jander equation-controlled diffusion with the apparent activation energy (Ea) about 15.2 kJ·mol-1. Different kinetic mechanisms were found for cured PBSZ fiber pyrolyzed in NH3/N2 and in N2 atmospheres. Under N2 atmosphere, stage II and III are both the diffusion processes controlled by Zhuralev-Lesokin-TemPelman equation. The corresponding Ea is 210～280 kJ·mol-1 and 390 kJ·mol-1, respectively. Introduction of active ammonia to pyrolysis atmosphere will decrease Ea. At 673～833 K in the NH3/N2 pyrolysis process, Ea is about 110 kJ·mol-1 and the process is nulceation and growth mechanism controlled by Avrami equation I. Likewise, stage III in NH3/N2 pyrolysis process is a diffusion process controlled by Zhuralev-Lesokin-TemPelman equation with the Ea lowered to 164 kJ·mol-1. The decarburization is a Valensi equation-controlled two-dimensional diffusion process with the Ea 83 kJ·mol-1.Studies on the pyrolysis process of SiBN fiber show that the carbon content was decreased while increasing the pyrolysis temperatures. The carbon decrease is most effective at 500～600℃. Extending the holding time, lowering the heating rate, and enhancing the NH3 concentration all favor the carbon elimination. The tensile strength of SiBN fibers follows Weibull statistical rule. The average tensile strength and Weibull modulus, indicative of the uniformity of the mechanical strength, are both increased with the pyrolysis temperature rise. Those fibers pyrolyzed under N2 atmosphere have higher tensile strength than those in NH3/N2 atmospheres. Pyrolysis temperature also has marked effects on the dielectric properties of the obtained SiBN fibers. Increasing the pyrolysis temperature will lower the dielectric constant and loss tangent. Fibers pyrolyzed under N2 atmosphere exhibit inferior dielectric properties to those pyrolyzed in NH3/N2 atmospheres.Typically, the as-obtained SiBN fiber has a near-stoichiometric composition Si1.13BN2.47. It shows good mechanical properties, excellent dielectric performance and high temperature resistance. The room temperature tensile strength and Young's modulus of the as-obtained SiBN fiber is 1.83 GPa and 201 GPa, respectively. The dielectric constant and loss tangent of SiBN fiber at room temperature is about 3.68 and 0.0011, respectively. Besides, SiBN fiber retains its amorphous state and 83 % of the room temperature tensile strength after annealing at 1700℃in N2 atmosphere for 2 h. The combined performances make the SiBN fiber a potential candidate material for the high temperature resistant wave transparent application. The present study has laid a sound technical foundation for the pilot industrial scale-up production and application of continuous SiBN fibers.The present work is innovative in the material system, the polymer synthesis, and the process of carbon elimination.