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前驱体转化法制备的SiC纤维是陶瓷基复合材料重要的高性能增强纤维，不仅强度和模量高、耐高温氧化性好还具有优异的耐烧蚀性能。SiC纤维的热稳定性和高温抗氧化性可以通过添加异质元素Ti、Zr、B和Al等进行改善。根据前驱体转化法制备陶瓷纤维的研究思路，本论文利用实验室合成的新型无氧前驱体-聚锆碳硅烷（PZCS），以及聚环硼氮烷（PBN）作为原料，经过常压高温处理制得具有良好结构、组成与可纺性的前驱体-PZCS和聚锆碳硅硼烷（PZBCS）。将制得的前驱体PZCS和PZBCS通过熔融纺丝、交联以及高温热解制得ZrC-SiC和ZrB2-ZrC-SiC两种新型复相陶瓷纤维。本论文在研究前驱体的处理过程、热解过程以及流变和纺丝性能的基础上，着重研究了原纤维的制备过程、交联过程、热解过程以及所得陶瓷纤维的组成、结构及其力学性能和抗氧化性能，主要研究内容和结论如下：（1）研究了热处理工艺条件对前驱体分子基团结构、分子量、粘温特性和陶瓷收率的影响。研究结果表明，前驱体PZCS和PZBCS经过高温热处理后分子量均增大，粘度和陶瓷收率均增加。其中，PZBCS的分子量、粘度和陶瓷收率要高于PZCS，主要是由于PBN含有活性基团N-H，与Si-H键可以反应生成更多的交联结构。前驱体的处理温度优选为260 oC，该条件下既可以得到直径满足要求的连续原纤维，同时具有较高的陶瓷收率。（2）研究了前驱体无机化过程中各组成的变化以及热处理气氛对陶瓷产物收率、抗氧化性能的影响。结果表明，前驱体高于750 oC时可实现无机化过程，1000 oC时产物中出现陶瓷相，且ZrC相较SiC相更早出现。PZBCS经过高温陶瓷化，其晶粒明显小于PZCS热解陶瓷，表明B、N元素的存在可以抑制ZrC和SiC晶粒的生长。前驱体陶瓷化后ZrC、ZrB2和SiC晶粒被无定形结构的C相包围。无机化过程中，不同气氛对前驱体的陶瓷收率有很大影响，PZCS和PZBCS氩气气氛下的陶瓷收率分别为67.21 %和69.18 %，明显高于氢气气氛下的54.59 %和56.05 %，氢气气氛下所得陶瓷产物的抗氧化性能优于氩气气氛下所得陶瓷产物，主要是由于前驱体在氢气中热解减少了产物中的自由碳含量。（3）研究了前驱体的稳态和动态流变特性以及纺丝工艺对原纤维形貌的影响。研究发现，在各自的纺丝温度范围前驱体PZCS和PZBCS熔体的粘性模量高于弹性模量，表现出粘性特征。前驱体熔体为剪切变稀的非牛顿流体，而粘度对可纺性有明显的影响。利用实验室制备的单孔纺丝设备对PZCS和PZBCS熔融纺丝工艺进行研究。结果表明，PZCS和PZBCS适宜的纺丝条件分别为：130~160 oC和140~170 oC，0.3~0.6 MPa，收丝速率高于7.54 m/s。（4）研究了由原纤维转变为交联纤维的结构变化以及所得陶瓷纤维的微结构与组成。结果表明，原纤维在交联处理过程中Si-H键被氧化生成Si-O-Si 键，从而使原纤维交联，达到交联的目的。交联纤维经过热解可以制得ZrC-SiC和ZrB2-ZrC-SiC复相陶瓷纤维。透射电镜分析表明，PZCS和PZBCS所得纤维经过1400 oC和1600 oC热解后分别出现了纳米级均匀分散的ZrC、SiC和ZrB2、ZrC、SiC晶粒。陶瓷纤维中Zr含量在12~15 %之间。（5）研究了原纤维电子束交联制备交联纤维的结构变化以及所得陶瓷纤维的微结构与组成。结果表明，原纤维经电子束交联处理时主要是Si-H键反应生成Si-Si和Si-CH2-Si键，达到交联目的。交联纤维在氩气和氢气气氛中热解，均可以制得致密均匀的ZrC-SiC和ZrB2-ZrC-SiC复相陶瓷纤维。纤维在氢气气氛中热解能够脱除更多的自由碳，制得近化学计量比的复相陶瓷纤维。（6）研究了不同气氛热解所得纤维经过不同高温恒温处理后力学性能的变化。结果表明，在1100~1200 oC高温处理后，纤维的拉伸强度略微的上升；在1200~1400 oC高温处理后，纤维拉伸强度有较小的下降趋势；在1400~1600 oC高温处理后，纤维拉伸强度明显下降。在氩气气氛中热解制得的复相陶瓷纤维强度较高，这是因为交联纤维在氩气中热解比氢气中热解具有更致密化的结构。（7）研究了ZrC-SiC和ZrB2-ZrC-SiC复相陶瓷纤维的氧化机理。结果表明，在氩气气氛中热解得到的陶瓷纤维氧化失重，而在氢气气氛中热解得到的陶瓷纤维氧化增重。这是因为在氢气中热解可以减少纤维中的自由碳，自由碳在空气中氧化会出现明显的失重。其中，ZrC-SiC纤维的氧化失重和增重都要高于ZrB2-ZrC-SiC纤维。ZrB2-ZrC-SiC复相陶瓷纤维抗氧化性更好，这主要是因为纤维中的B、Si元素高温氧化生成玻璃态B2O3-SiO2，阻止了氧的进一步扩散。;Continuous silicon carbide (SiC) fiber synthesized from precursor is one of the most important reinforcements for ceramics composites. The thermal stability and oxidation resistance of SiC can be improved greatly by doping with metals such as Ti, Zr, and Al. This work is based on the idea of producing SiC fiber through precursor transformation. This dissertation proposes a thermal polymerization method of synthesizing polyzirconocenencarbosilane (PZCS) and a low molecular weight polyborazine (PBN) for synthesizing PZBCS which contains Zr and B. Polyzirconocenencarbosilane (PZCS) and Polyborazine-zirconocenencarbosilane (PZBCS) with improved structure, compositions and spinnability were synthesized. Through melt-spinning, the green fibers with fine diameter were prepared. Two types of composite ceramic fibers, namely ZrC-SiC and ZrB2-ZrC-SiC, were produced from the two cross-linked PZCS and PZBCS fibers after heat treatment at high temperature. In this work, the thermal polymerization and pyrolysis processes of PZCS and PZBCS was revealed. The rheological property of the two precursors and its influence on melt-spinning were systematically studied. In addition, the preparation processes of composite ceramic fibers, including curing and pyrolysis, were studied. Finally, the microstructure, compositions and properties of ZrC-SiC and ZrB2-ZrC-SiC were investigated. The obtained main conclusions as follows:(1) The effects of heat treatment on the molecular structure, number average molecular weight, viscosity-temperature characteristics and ceramic yield of precursors were studied. It was found that the number average molecular weight, viscosity and ceramic yields of PZCS and PZBCS increased after thermal polymerization, in which the PZBCS was higher. The reactive N-H in PBN reacted with Si-H to generate more cross-linked structure.To achieve the higher ceramic yield and the fine green fibers, the final processing temperature was 260 °C.(2) The change of compositions in the conversion process of precursors into inorganic substance and the effects of pyrolysis atmosphere on ceramic yield were studied. It was found that the conversion process was completed at 750 °C. The crystalline phases showed up at 1000 °C and ZrC phase appeared earlier than SiC phase in the process of ceramization. After pyrolysis at high temperature, the grain sizes from PZBCS were much smaller. Therefore, the existence of B and N elements restrained the growth of the ZrC and SiC grains surrounded by amorphous carbon. In the conversion process, the different atmosphere had a great influence on the ceramic yield of precursor, which the ceramic yield is higher in argon atmosphere (67.21 % and 69.18 %) than the hydrogen atmosphere (54.59 % and 56.05 %). The oxidation resistance of ceramic products from hydrogen atmosphere was better. The results showed that the free carbon content can be significantly reduced when the precursors pyrolyzed in the hydrogen.(3) The rheological properties of the precursors and the effect of spinning conditions on the morphology of the green fibers were studied. It is found that the viscous modulus of PZCS and PZBCS melt is higher than elastic modulus in the respective spinning temperature. The melts of precursors are non-Newtonian fluids and the viscosity decreases with shear rate increasing. Viscosity has a significant effect on the spinnability of precursors. The melt spinning process of PZCS and PZBCS was studied by a single hole spinning equipment. The experimental results showed that the proper spinning conditions for PZCS and PZBCS were as follows: spinning temperature 130~160 °C and 140~170 °C, pressure 0.3~0.6 MPa, speed rate above 7.54 m/s. (4) The structure change from the green fibers to the cross-linked fibers and the microstructure and compositions of the ceramic fibers were studied. The results showed that the Si-H bond is oxidized to form Si-O-Si bond in the cross-linking process to achieve the purpose of infusible. ZrC-SiC and ZrB2-ZrC-SiC composite ceramic fibers can be obtained by pyrolysis of the cross-linked fibers. The HR-TEM showed that the nanodispersed ZrC, SiC and ZrB2, ZrC, SiC grains after pyrolysis at 1400 °C and 1600 °C respectively. The contents of element Zr in ceramic fibers is between 12~15%.(5) In this paper, the structural changes of the fibers prepared by electron beam cross-linking and the microstructure and compositions of the obtained ceramic fibers were studied. The results show that the Si-Si and Si-CH2-Si bonds are generated in the cross-linking process to achieve the purpose of infusible. The compact and homogeneous ZrC-SiC or ZrB2-ZrC-SiC composite ceramic fibers can be obtained by the pyrolysis of the fibers in argon and hydrogen atmosphere. The near-stoichiometric composite ceramic fibers were prepared through the pyrolysis in the hydrogen atmosphere to remove free carbon.(6) The tensile strength of the fibers obtained by pyrolysis at different temperatures was studied. The results showed that from 1100 to 1200 °C, the tensile strength of the fibers increased slightly; from 1200 to 1400 °C, tensile strength of fibers had a smaller downward trend; the tensile strength of fibers from 1400 to 1600°C fibers decreased significantly. The composite ceramic fibers pyrolyzed in the argon atmosphere had a higher tensile strength, because the cured fibers pyrolyzed in argon atmosphere have a more compact structure.(7) The oxidation mechanism of ZrC-SiC and ZrB2-ZrC-SiC composite ceramic fibers was studied. The results showed that there is a slightly weight loss during the oxidation process of ceramic fibers obtained in argon atmosphere. However, the weight ceramic fibers obtained in hydrogen atmosphere increased slightly. The near-stoichiometric ceramic fibers were obtained through the pyrolysis in hydrogen. The weight changes of ZrC-SiC fibers were higher than that of ZrB2-ZrC-SiC fibers. ZrB2-ZrC-SiC composite ceramic fibers had better oxidation resistance, which is mainly due to the high temperature oxidation of elements B and Si in the fibers to form the glass state B2O3-SiO2.The oxide film prevents the further diffusion of oxygen from air.
|吕晓旭. ZrB2/ZrC-SiC复相陶瓷纤维的制备研究[D]. 中国科学院大学,2017.|
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