CAS OpenIR
有机前驱体法BN-Si3N4复相陶瓷纤维的制备与性能研究
谭竞
Thesis Advisor张伟刚
2020-09-01
Degree Grantor中国科学院大学
Degree Name博士
Degree Discipline材料学
Keyword有机前驱体,氮化硼,氮化硅,复相陶瓷纤维
Abstract

随着航空航天工业的发展,高马赫数飞行器对于高温透波材料,特别是高性能透波陶瓷纤维的需求日益剧烈。BN-Si3N4复相陶瓷纤维结合了BN纤维优异的热稳定性、介电性能和Si3N4纤维出色的力学性能的优点,是一种极具发展潜力的高温透波陶瓷纤维材料。本论文以聚环硼氮烷(PBN)和聚硅氮烷(PSZ)为前驱体,通过前驱体的复合、熔融纺丝、电子束交联和高温热解制备了具有优良力学、介电、抗氧化等综合性能的BN-Si3N4复相陶瓷纤维。论文首先考察了PBN与PSZ前驱体的基本性质与热解过程,并研究了复合前驱体的复合效应与热解机理;研究了复合前驱体的流变性和熔融纺丝工艺,制备了优质的纤维原丝;考察了不同交联工艺对纤维不熔化处理的影响;最后经过高温烧成制备了复相陶瓷纤维,并探讨了纤维的形貌与结构对纤维性能的影响。本论文的主要研究结论如下:1、聚环硼氮烷(PBN)前驱体的热解产物为乱层t-BN,而聚硅氮烷(PSZ)前驱体的热解产物为混合的α-Si3N4与β-Si3N4。二者的热解产物中碳含量均在0.5wt%以下,说明PBN与PSZ适合作为透波陶瓷纤维的前驱体使用。2、PBN/PSZ复合前驱体的热解过程存在复合效应,复合前驱体中PSZ含量的增加能够提高热解产物中BN相的结晶度。当热解产物中Si3N4含量为25wt%时,BN晶体的结晶度达到最大值。复合前驱体热解产物中Si3N4均为无定形态。3、PBN/PSZ复合前驱体的热解过程分为无机化过程和晶体化过程。无机化过程主要通过N-H、-NHCH3、-CH3以及Si-H等基团之间的反应放出CH4、CH3NH2等小分子来完成。复合前驱体的无机化过程在1000℃完成,其热解产物中的BN相从1400℃开始出现结晶行为,在1600℃保温120 min可以确保h-BN结晶过程的完成。4、PBN/PSZ复合前驱体具有优异的纺丝性能,其粘流活化能为14.20 kJ/mol。其熔体为假塑性流体,呈现剪切变稀特性,在120-150℃温度段流动指数为0.812-0.982。5、PBN/PSZ复合前驱体熔融纺丝工艺由纺丝温度、纺丝压力和收丝速度控制,三个工艺参数需要相互协调匹配,才能制备出均匀致密,直径合适的纤维原丝。本实验中合适的纺丝条件为纺丝温度110-135℃,纺丝压力0.3-0.7 MPa,收丝速度>8 m/s。6、采用NH3交联与电子束交联两种方式对纤维原丝进行了不熔化处理。研究结果表明,电子束交联是更合适的不熔化处理工艺。不熔化纤维的凝胶含量为81.8wt%,热解至1000℃后,所得的陶瓷纤维均匀致密,没有缺陷。7、不熔化纤维经过1000℃、NH3处理和1600℃、N2高温热解后,陶瓷收率为40wt%,残余的碳含量约为0.2wt%,所得陶瓷纤维的组成接近化学计量比,为BN(Si3N4)0.05。8、1600℃高温烧成的复相陶瓷纤维呈“壳芯结构”。纤维的边缘由一层结晶度较低的BN构成,而纤维的中心区域,BN晶粒发育十分完善,(002)晶面间距为0.333 nm,符合h-BN晶体的理论值,晶粒尺寸超过15 nm。9、复相陶瓷纤维具有优秀的力学、介电和抗氧化性能。得到的纤维的拉伸强度和杨氏模量分别为1040 MPa和90 GPa,其介电常数和介电损耗分别为3.21和3.11×10-3。在空气中900℃和1000℃下中氧化4 h其增重率分别为2wt%和7wt%。;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 applications.In 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 follows:1. The pyrolysis product of polyborozane (PBN) precursor is turbostratic BN, while the pyrolysis product of polysilazane (PSZ) precursor is the mixture of α-Si3N4 and β- 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 fibers.2. 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 process.4. 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. 

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/49667
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
谭竞. 有机前驱体法BN-Si3N4复相陶瓷纤维的制备与性能研究[D]. 中国科学院大学,2020.
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