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碳纤维增强ZrC-SiC复相陶瓷基复合材料具有耐超高温、抗氧化、抗烧蚀、低密度和机械性能良好等优点，是应用于高超声速飞行器耐热结构件的首选材料。通过陶瓷前驱体在多孔C/C复合材料中的浸渍和热解（Precursor Infiltration Pyrolysis, PIP）是制备此类高性能陶瓷基复合材料的有效途径。因此，合成适用于PIP工艺的陶瓷前驱体，是成功制备高性能复合材料的关键。本文针对高超声速飞行器耐热结构件的迫切需求，以四氯化锆、乙酰丙酮和1.4-丁二醇为主要原料，制备了适用于PIP工艺的ZrC陶瓷有机前驱体（Precursor of Zirconium Carbide, PZC），并对该前驱体的合成工艺及热解行为进行了系统的研究。利用PIP法将合成的PZC与聚碳硅烷（polycarbosilane, PCS）混合（质量比为1:1）溶液浸渍热解，制备了致密化的C/C-ZrC-SiC复合材料，分析表征了复合材料的结构和微观形貌。主要的研究内容和结果如下：（1）利用有机合成法，通过ZrCl4的醇化，与烯醇式乙酰丙酮的螯合，以及和1.4-丁二醇的醇交换反应后，合成了线性高分子聚合物-ZrC陶瓷有机前驱体。该前驱体不含氯，化学稳定性好，在有机溶剂中溶解度高（>400 g/100 g 甲苯），可与PCS的甲苯溶液任意比例互溶。通过对PZC二甲苯溶液（50 wt%）的流变性能测试，得知其在40℃-80℃较宽的温度区间内具有非常低的粘度值（<5 cP），该特性有利于降低浸渍阻力，提高浸渍效率。（2）对PZC合成过程中的阶段产物进行了分析表征，通过对添加杂质元素的追踪，研究了杂质元素在工艺流程中的迁移与截留。结果表明，ZrCl4的醇化反应可以去除大部分原料中常含有的金属杂质氯化物，如CaCl2、MgCl2、AlCl3等；但FeCl3会和ZrCl4一起经醇化反应进入下一步工艺单元，因此，需严格控制原料中Fe的含量。另外，通过聚合物相对分子量变化规律分析得知，反应过程中聚合物的相对分子量呈S形增长，反应前期为典型的逐步聚合反应，而反应后期受活性基团有效碰撞率低的影响，聚合物相对分子量的增加变缓。（3）对PZC的热解过程和陶瓷化机理进行了研究。结果表明，PZC经1500℃（Ar气氛）热处理，先后发生了有机结构裂解重排，热解碳和二氧化锆的形成与结晶，二氧化锆的碳热还原等反应阶段，生成纳米尺寸的立方相ZrC陶瓷，陶瓷产率33.45%。同时，由于PZC的桥联结构，热解产物的纳米低熔点效应与低表面自由能特点，以及化学平衡转移的共同影响，致使PZC热解过程中，在远低于热力学温度的条件下完成碳热还原反应。（4）对PZC和PCS的混合前驱体性能进行了分析。研究发现两种前驱体在混合后并未发生明显的交联反应。同时测定了其二甲苯溶液（50 wt%）的粘温曲线，发现溶液在40-80℃具有较低的粘度值，在此温度区间进行浸渍操作，以保证较高的浸渍效率。通过对混合前驱体的热解过程研究得知，两者在热解过程中产生了协同作用，该作用可抑制热解自由碳的生成和SiO的挥发，使混合前驱体的陶瓷产率有所增加。（5）利用PIP工艺，结合化学气相渗透（Chemical Vapor Infiltration, CVI）工艺对编织体进行预处理，以混合前驱体溶液为浸渍液，经12次浸渍裂解循环制备了密度为2.254 g/cm3的致密化C/C-ZrC-SiC复合材料。通过对复合材料的结构与微观形貌分析得知，增韧碳纤维的周围均匀包裹着一层沉积碳，沉积碳及其周边连续致密的ZrC、SiC陶瓷共同构成了复合材料的基体；在基体中，Zr、Si元素均匀分布，同时，纳米尺寸的ZrC颗粒均匀的分散在连续的SiC相中。;Carbon fiber-reinforced ZrC-SiC ceramic matrix composites (CMCs) are one of the candidate materials for use in structural components of hypersonic vehicles due to the advantages of lower density, thermal resistance against ultra-high temperatures, anti-oxidation and anti-ablation performance, etc. Polymeric precursor infiltration pyrolysis (PIP) is an effective process to produce ceramic matrix composites with these excellent properties. Hence, synthesis of ceramic precursors of ZrC which is fully capable of PIP has great significance for preparing CMCs.In this study, aim to the urgent demand of structural components used in hypersonic vehicle and thus, a precursor of zirconium carbide (PZC) for PIP was synthesized via the reaction of zircomiun tetrachloride, ethanol, acetylacetone and 1,4-butylene glycol. At the same time, the synthesis mechanism and pyrolysis behavior of PZC were studied in detail. The structure and microstructure of the composites that prepared by using solution of PZC and PCS (mass ratio 1:1) were also characterized. Major results are summarized as follows:(1) Via an organic polymerization, linear precursor of PZC without chlorine was synthesized via the chelate reaction of alkoxy-substituted ZrCl4 and acetylacetone, transesterification with 1,4-butylene glycol. And PZC shows well chemically stablity, excellent solubility in organic solvents, particularly, it has superior intersolubility with PCS in any mixing proportions. The rheological properties of the PZC xylene solution (50 wt%) were tested, and the results showed that the viscosity of the solution was very low in a wide temperature range (40-80℃), which was beneficial to improve efficiency of impregnation.(2) The products of each stage in the process of synthesis were characterized, furthermore, the migration and residual of impurity elements in the intermediates and PZC were discussed through tracking of these elements. There was a little impurity such as Al, Ca, Mg but more Fe remained in zirconium ethoxide which was prepared by the ethanol and ZrCl4, therefore, the content of Fe in ZrCl4 should be as low as possible. In addition, the variation of relative molecular weight in S-type indicates that it is a typical step polymerization, however, the low effective collision rate of active groups leads to the slow growth of molecular weight in the later stage of the reaction.(3) Pyrolysis process and conversion mechanism of the precursor to ceramics was studied as well, the results showed that c-ZrC crystalline in nano sizes was formed from PZC as it was heat treated at 1500℃ in an inert atmosphere, in which the reactions that decomposition of the organic molecules and carbothermal reduction between carbon and ZrO2 were completed successively, with a yield of ceramic of 33.45 wt% ultimately. The carbothermal reduction reaction was completed at a temperature lower than that from thermodynamic calculations by possible reasons of the bridging structure of PZC, nanometer effect of pyrolysis products, low free energy of surface, and the co-effect of chemical equilibrium transfer.(4) The mixed precursors were analyzed and characterized, which was found that there was no obvious crosslinking between them. Moreover, the optimum temperature range used for impregnation is 40-80℃. The investigation of mixed precursor to ceramics indicated that the ceramic yield of mixed precursors was higher than the theoretical value because of the synergistic effect of them in the pyrolysis process, which inhibited the formation of pyrolysis carbon and the volatilization of SiO.(5) Dense C/C-ZrC-SiC composites with a bulk density of 2.254 g/cm3 were prepared by CVI pretreatment and 12 times PIP process, using this mixed precursor solution as impregnating. Through the analysis of the structure and micro-morphology of the composites, it was found that the carbon fibers were coated with approximately 1 μm thick layers of pyrolytic carbon, which was deposited as an interfacial layer between carbon fibers and continuous dense carbide (ZrC, SiC) around it; the elements of Zr and Si were evenly distributed in the matrix composed of pyrolytic carbon and carbide, and especially the ZrC particles of nano sized were evenly dispersed in the continuous SiC substrate.
|孔玮佳. 碳化锆陶瓷前驱体的合成与应用研究[D]. 中国科学院大学,2018.|
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