Fabrication And Properties Of 3D-SiC(rGO)PDCs Based On Graphene Oxide/Polycarbosilane Precursor

Silicon carbide(SiC)polymer-derived ceramics(PDCs),as a kind of advanced structural materials,possess excellent properties including low density,high strength,high modulus,high temperature resistance and good thermal shock resistance,etc.They are widely used in aerospace,nuclear energy and other high-tech fields.PDCs offer a unique route with designable molecular structure,adjustable ceramic composition,good processing property and additive free by pyrolysis of precursors,which exactly suggests a significant breakthrough in low-dimensional forms such as fibers,coatings,films or membranes.However,their popularity is hindered by limitations in fabrication techniques of three-dimensional(3D)bulk PDCs.The main reason is that the release of a large amount of gaseous species during pyrolysis can lead to generation of cracks within the ceramic,and hence destroying its integrity and mechanical properties.Therefore,there has been a considerable amount of interest in the development of PDC route for producing 3D-SiC PDCs with low shrinkage,high ceramic yield,high hardness,high fracture toughness,few pores or cracks.This research would help to explore the integration of satisfying structural and functional features of PDCs for versatile applications.The main subjects and results of this thesis are summarized as follows:(1)A technology that re-pyrolysis ceramic/precursor mixture to prepare 3D-SiC PDCs was proposed.First,different proportion of SiC(rGO)px/PVG mixture were obtained by ball milling.The SiC(rGO)p particles here act as inert filler while its precursor polycarbosilane-vinyltriethoxysilane-graphene oxide(PCS-VTES-GO,PVG)powder act as binder.The mixtures were respectively molded into green compacts,followed by re-pyrolysis at 1200 ? to successfully obtain series of 3D-SiC(rGO)px nanocomposite PDCs.The ceramics consist of ?-SiC,SiOxCy,SiO2,rGO and free-carbon phase.The ?-SiC nanocrystals are dispersed in the SiOxCy/Cfree amorphous phase with compound rGO,and SiO2 grains are embedded in the matrix of ?-SiC/SiOxCy/Cfree.As the optimum mass ratio of filler SiC(rGO)p particles to binder PVG powder is 6:4,3D-SiC(rGO)p0.6 ceramics show higher ceramic yield(94.49%)and lower linear shrinkage(5.00%)than others,and exhibit excellent mechanical properties with a hardness of 3.96 GPa and a fracture toughness of 3.21 MPa·m1/2.The surface of 3D-SiC(rGO)p0.6 is smooth and compact,and can be further densified by using polymer infiltration pyrolysis(PIP)method to fill micro-pores and cracks.(2)The precursor PACS was synthesized by introducing A1 element into PCS,and PACS was composited with GO to obtain a novel Al-containing precursor PACS-VTES-GO(PAVG).Series of high-quality 3D-SiC(Al,rGO)ceramics were prepared by ball milling and molding with 60 wt.%SiC(Al,rGO)p particles and its precursor PAVG powder,followed by re-pyrolysis at different temperatures.The effects of pyrolysis temperature on the surface morphology,chemical composition,microstructure and mechanical properties of the ceramics were studied in detail.3D-SiC(Al,rGO)ceramics contain SiO2 grains and ?-SiC nanocrystals embedded in SiOxCy/Cfree amorphous phase with compound rGO and Al atoms.With the increase of re-pyrolysis temperature,SiOxCy would further decompose with the generation of ?-SiC grains and free-carbon,while SiO2 grains would also decompose and completely disappear at 1500 ?.The 3D-SiC(Al,rGO)ceramics prepared under optimum re-pyrolysis temperature 1300 ? possess the highest hardness value(2.57 GPa)and fracture toughness value(3.32 MPa m112)in the series of ceramics.Even when the re-pyrolysis temperature increases to 1500 ?,the ceramics can still maintain relatively good mechanical properties.The surface of 3D-SiC(Al,rGO)ceramics sintered at 1300 ? is the densest,and could be modified by PIP method to fill micro-pores,cracks and interface,further improving their durability and reliability.These results reveal that the presence of A1 element not only inhibits ?-SiC crystallization and transfers crystallization temperature of PDCs to a higher level,but also improves thermostability and fracture toughness of the ceramics.