Design And Preparation Of Axial Graded SiC Fibers With Sinusoidal Structure

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.