【作者】 胡天娇；

【导师】 李效东；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2012， 博士

【摘要】 先驱体转化法制备的SiC纤维具有高强度、高模量、耐高温、抗氧化、耐腐蚀、低密度等优异性能，是高温高强复合材料的关键增强材料。近年来，开发新型功能SiC纤维成为SiC纤维研制的另一个热点。功能型SiC纤维不仅拥有普通SiC纤维的优良性能，同时还兼具电磁吸收、气体吸附等其他特殊性能。以这些功能型SiC纤维为基础，可以开发出适用于高温、高腐蚀环境下的结构功能材料，极具应用前景。功能梯度材料是一种组成和微观结构梯度变化的材料，这种组成和结构变化可以满足在单一材料器件内部的不同部位实现不同功能的需要，从而优化器件的整体性能。实际上，先驱体法制备的SiC纤维本就是一种径向组成与结构渐变的梯度材料，但真正意义上的梯度SiC纤维应是在纤维轴向存在组成与结构的梯度变化。这种轴向梯度SiC纤维的结构与普通SiC纤维存在很大差异，这必然会给其性能带来众多独特之处，然而目前国内外均未见与之相关的报道。本课题将功能梯度材料的设计思想与结构控制理论引入到SiC纤维的制备中，研制了一种结构和电阻率沿纤维轴向周期性渐变的功能梯度SiC纤维。文中首先采用两种常见的烧成模式进行恒速烧成实验，系统研究了烧成工艺、纤维电阻率及纤维结构三者间的关系。在此基础上，设计了一种用于制备轴向梯度SiC纤维的变速烧成系统，并结合实验与模拟结果优化了烧成平台参数，最终成功制备出电阻率的变化周期与变化幅度可调的轴向梯度SiC纤维。最后，对轴向梯度SiC纤维的电磁性能及耐高温、抗氧化性能进行表征，并采用该轴向梯度纤维制备出宽频吸波性能优异的吸波材料。一步恒速烧成所得I-SiC纤维的表面存在约5~50nm厚的碳层，纤维电阻率随碳层增厚而降低。该碳层是I-SiC纤维的主要导电相，其形成过程可分为先驱体裂解、小分子炭化、热解炭沉积三个阶段。采用具有针对性的工艺技术对上述三个阶段加以控制，就可以控制I-SiC纤维表面的碳层厚度，进而调整I-SiC纤维电阻率。I-SiC纤维的导电行为符合“壳-芯”结构的特点，根据此模型计算出I-SiC纤维表面碳层的电阻率约为10-2cm。两步恒速烧成模式下制备的II-SiC纤维由于缺少表面碳层作为导电相，因此纤维电阻率比I-SiC纤维要高。游离碳是II-SiC纤维中的主要导电相，它通过一种逾渗作用决定了纤维的导电性能。烧成温度升高可促进纤维内游离碳含量与尺寸的增大，从而有效降低II-SiC纤维电阻率。但1300℃以上的热处理会使纤维内的SiCxOy结构发生分解，在纤维内部形成孔缺陷，导致纤维导电性能变差。II-SiC纤维的导电行为可用综合有效介质理论来描述，根据此模型计算得到的II-SiC纤维内部游离碳的电阻率约为10-1cm，其逾渗阀值在11.3%左右。文中采用椭圆形收丝筒取代传统的圆形收丝筒进行连续烧成，使SiC纤维烧成时间出现周期性梯度变化，最终实现对SiC纤维轴向结构与性能的周期性梯度控制。结合实验与模拟结果优化了烧成炉恒温区长度、椭圆收丝筒形状等烧成平台参数。此外，还采用一种新的套管烧成技术对SiC纤维表面的碳层厚度进行有效控制，制备出表面碳层厚度介于I-SiC和II-SiC纤维之间的IDR-SiC纤维。通过调整烧成时间，可使其电阻率在101~105cm之间变化，且实验重复性好。使用套管技术进行一步变速烧成，得到表面富碳的IDR-G-SiC纤维。该纤维电阻率在100~102cm之间梯度变化。纤维电阻率变化的周期和范围可简单地通过改变收丝筒尺寸和烧成温度来调控。结构分析表明该纤维内部以无定形结构为主，纤维轴向电阻率发生周期性梯度变化主要是由其表面碳层的波动变化所引起的。采用两步烧成模式进行变速烧成，制备了电阻率可在102~106cm之间梯度变化的表面富氧的II-G-SiC纤维。与预烧条件相比，终烧的温度和时间对II-G-SiC纤维电阻率的影响更大。此外，不熔化纤维的Si-H反应程度越低，所得II-G-SiC纤维的电阻率也越低，但变化幅度基本不变。结构分析表明，II-G-SiC纤维的结晶程度优于IDR-G-SiC纤维，纤维的电阻率变化主要由其内部游离碳含量变化引起。对不同条件下制备的轴向梯度SiC纤维的电磁参数进行了测试。结果表明，由于IDR-G-SiCf纤维表面存在碳层，因此其介电损耗正切值较高，适合用作吸波材料的吸收体；而II-G-SiC纤维的表面无碳层，tgδ值较低，适合用作透波材料或是吸波材料中的介质层。以II-G-SiCf作为的介质层材料，IDR-G-SiCf作为内部吸收层材料，采用不同的叠加方式，制备出两种具有优良吸波性能的吸波材料。其最大反射衰减分别为-26.3dB和-23.5dB，反射衰减低于-10dB的频宽分别为7.7GHz和11.6GHz。针对轴向梯度SiC纤维在未来可能的高温、氧化等苛刻环境中的应用，研究了梯度SiC纤维的高温稳定性和抗氧化性能。由于表面碳层在高温处理过程中被氧化，IDR-G-SiC梯度纤维的电阻率在热处理后变化明显，强度略有降低。II-G-SiC纤维常温力学性能比IDR-G-SiC纤维略低，但其耐高温、抗氧化性能整体优于IDR-G-SiC纤维。更多还原

【Abstract】 The silicon carbide fibers (SiCf) are typically used as reinforcement for hightemperature structural composites due to their excellent tensile strength, stiffness andhigh temperature resistance in oxidizing atmosphere. In recent years, development offunctional SiCfhas become another hotspot of the SiCfpreparation. The functional SiCf,which combine excellent specific strength, high temperature stability and other uniqueproperties, should find applications in the fields of high temperature, high humidity andcausticity.Functionally graded materials (FGMs), which integrate two or more constituentphases with a continuously variable composition, are recognized as the most promisingcandidates to result remarkable functional properties. Owing to the unique features ofthe fabrication process, the polymer-derived SiCfare more or less innate gradient alongthe radial direction (so-called core-shell structure). But, as real FGMs, the continuousSiCfwith gradient structure and properties in the axial direction will be more attractive.In this work, we study the relationships among the preparation condition, structureand electrical resistivity of the polymer-derived SiCf. Base on the mechanism study, wedesign a simple process by which graded fibers with sinusoidal electrical resistivity inthe axial direction can be directly prepared through the precursor polymer pyrolysisroute. Two kinds of axial graded SiC fibers, IDR-G-SiCfand II-G-SiCf, are preparedaccording to this approach. The electrical resistivity, high temperature resistance andoxidation resistance of the graded fibers is investigated along the axial direction. Radarabsorbing materials (RAMs) comprising this novel functional SiCfshows excellentmicrowave absorbability.SiC fibers prepared by one step continuous pyrolysis using circular rotating spoolare denoted as I-SiCf. A carbon layer with thickness of5~50nm appears on the surfaceof I-SiCf. The electrical resistivity of I-SiCfdecreases as the thickness of the carbonlayer increases. This carbon layer is the main conductive phase of I-SiCf. Its formationprocess is discussed in three steps: the decomposition of the cured preceramic to releaseorganic gaseous species, the carbonization of the gaseous species to generate pyrolyticcarbon, and the deposition of the pyrolytic carbon to form a carbon layer on the surfaceof the fibers. The electrical resistivity of I-SiCfcan be adjusted by accurate control ofthe three steps. A “core-shell” model is proposed to explain the the electrical conductivebehavior of I-SiCf. Electrical resistivity of the carbon layer calculated from this model isaround10-2cm.II-SiCfis the fibers obtained from two step continuous pyrolysis using circularrotating spool. The electrical resistivity of II-SiCfis much higher than I-SiCfdue to thelack of carbon layer on its surface. Free carbon is the main conductive phase in the II-SiCf. The amount of the free carbon but mainly, its texture, govern the electricalconductivity of the II-SiCfaccording to a percolation effect. Increase of the pyrolysistemperature promotes the growth of free carbon and thus decreases the electricalresistivity of II-SiCf. But heat treatment over1300℃may cause decompose of SiCxOyphase, which will release SiO and CO gases and leave pores in the fibers. The increaseof the pore volume leads to a102order of increase in the fiber electrical resistivity. Theelectrical behavior of II-SiCfin the percolation regime can be described using generaleffective media theory. Electrical resistivity and percolation threshold of the free carboncalculated from this model is10-1cm and11.3%, respectively.A rotating spool having the shape of an ellipse is employed in the pyrolysis systemto periodically adjust the pyrolysis time of SiCf. The periodic variation of pyrolysis timesubsequently resulted in a sinusoidal electrical resistivity of the SiCf. The apparatusparameters, such as the length of the temperature constant area of the pyrolysis furnaceand the size of the elliptic rotating spool, are optimized according to simulation andexperiment results. A novel pyrolysis technology, namely deposition retarded pyrolysistechnology, is adopted to control the thickness of the carbon layer on the surface of SiCf.The IDR-SiCfprepared by using this technology shows adjustable electrical resistivity inthe range of101~105cm.Axial graded SiC fibers with carbon-enriched surface are prepared by one stepcontinuous pyrolysis using elliptic rotating spool and deposition retarded pyrolysistechnology. This kind of axial graded SiCf, denoted as IDR-G-SiCf, shows sinusoidalelectrical resistivity in the range of100~105cm. Desired period and range of theelectrical resistivity can be facilely controlled by adjusting the size of the rotating spooland the pyrolysis temperature. Structure analyses reveal that the IDR-G-SiCfis mainlycomposed of amorphous phase. The fluctuating carbon layer thickness on fiber surfaceis responsible for the electrical resistivity variation of the fibers.Direct pyrolysis under two step continuous pyrolysis using elliptic rotating spoolgives axial graded SiCfhaving oxygen-enriched surface. This kind of axial graded SiCf,denoted as II-G-SiCf, exhibits sinusoidal electrical resistivity in the range of102~106cm. In comprison with pre-pyrolysis conditions, the final-pyrolysis temperature andtimes are more efficient in adjusting the electrical conductivity of the II-G-SiCf. Inaddition, decrease in oxygen content will lower the electrical resistivity of the II-G-SiCf,but the extent of the electrical resistivity variation is independent of the oxygen content.The II-G-SiCfshows better crystallizability than IDR-G-SiCf. The sinusoidal electricalresistivity of II-G-SiCfis result from the variation of free carbon content in the fibers.Electromagnetic parameters of axial graded SiCfprepared at different condition aretested. The loss tangent of IDR-G-SiCfis higher than II-G-SiCf, because the carbon layerof the former is effective in consuming electromagnetic wave. The measurement results of electromagnetic parameters reveal that the IDR-G-SiCfis applicable to use asmicrowave absorber and the II-G-SiCfare good permeable materials. Radar absorbingmaterials comprising the graded SiCfshow excellent wide frequency absorbingproperties.For the potential applications in high temperature and oxidation environment, thethermal stability and the anti-oxidation properties of graded SiC fibers areinvestigated.The oxidation of carbon layer leads to a drastic increase in the electricalresistivity of the IDR-G-SiCf. The tensile strength of the II-G-SiCfat room temperature isslightly lower than that of the IDR-G-SiCf. But the II-G-SiCfexhibits betterhigh-temperature and oxidation resistance due to the protection of oxygen-enrichedlayer.更多还原