Microstructure And Evolution During The Preparation Of Silicon Carbide Based Precursor-derived Ceramics

As is known to all,the microstructure of a material determines its macroscopic properties.Precursor-derived route is an advanced method to obtain high performance silicon carbide(SiC)based ceramics,which mainly involves crosslinking,organic-to-inorganic transformation,and high-temperature crystallization processes.Polycarbosilane(PCS)oligomer is an important precursor for the precursor-derived SiC based ceramics whose microstructures and performances will be significantly determined by the microstructure of PCS and its evolution during the preparation processes.However,owing to the complex molecular structure of PCS and the lack of appropriate characterization methods for PCS,the microstructure of PCS and its evolution during the crosslinking,organic-to-inorganic transformation,and high-temperature crystallization processes remain unclear.Understanding the microstructure of PCS precursor and studying the microstructural evolution during the preparation processes of precursor-derived SiC based ceramics not only benefit to the development of research methods and the improvement of oligomer theory,but also provide a scientific basis for the design and control of precursor-derived SiC based ceramics with the specific microstructures and excellent properties.In this work,the as-prepared PCS samples,and the crosslinking PCS samples before and after different heat treatments without and with the introduction of aluminum were systematically investigated.The aggregation structure model of PCS,and characterization methods suitable for the PCS oligomer were established by combining molecular simulation with several experimental techniques including elemental analysis,infrared spectroscopy,nuclear magnetic resonance spectroscopy,thermogravimetric-(chromatography)mass spectrometry,scanning electron microscopy,transmission electron microscopy,nitrogen absorption-desorption isotherm and wide angle X-ray scattering spectroscopy.Accordingly,the quantitative analysis methods to evaluate the organic particle sizes of PCS and crosslinked PCS samples undergoing the organic-to-inorganic transformation were developed.Furthermore,the microstructure and its evolution in the process of high-temperature crystallization,as well as the effect of aluminum were discussed.The granular microstructures were suggested for both the PCS precursor and the precursor-derived SiC based ceramics.The results revealed that the PCS with the number average molecular weight about 1000 g/mol had a granular microstructure forming by single molecule particles with particle size of 1-2 nm and exhibited the pore fractal characteristic.Each particle is formed by a single organic molecule.Minor reduction in the size of single molecule particles was found after the oxidative crosslinking PCS due to the intramolecular crosslinking.The heat-treated PCS samples with 350-900℃ showed the granular microstructure,which was gradually evolved from the organic single molecule particles to the inorganic SiOxCy nanocluster particles resulted from the characteristic reactions such as ring-opening and chain-breaking with increasing temperature.Both the organic single molecule particles and inorganic SiOxCy nanocluster particles contained Si-C short-range ordered domains with the particle size of 2 nm.The granular microstructure composed of the SiOxCy nanoclusters was stabilized upon the heat treatment between 350℃ and 900℃.The size of nanocluster changed slightly,while the Si-C ordered domains increased appreciably with increasing temperature.The higher ordered Si-C domains became SiC fine grains at 1300℃,and the large area crystallization occurred at 1500℃ with the grain size of 22.3 nm.Furthermore,with a small amount of aluminum the reaction temperature for the characteristic ring-opening and chain-breaking was lowered by 100℃.The size of nanoclusters was similar to that of Si-C ordered domains in nanoclusters at 1300℃.In addition,the reduced temperatures were observed for the SiC fine grains in higher ordered Si-C domains at 1100℃ and the large area crystallization at 1500℃,led to the smaller SiC grain size of 14.2 nm as compared with that in the absence of aluminum(22.3 nm).