Synthesis, Structures, and Properties of Bulk Si(O)C Ceramics from Polycarbosilane

Authors

  • Ya-Li Li,

    Corresponding author
    1. Key Lab of Advanced Ceramic and Machining Technology, Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China
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  • Hua Fan,

    1. Key Lab of Advanced Ceramic and Machining Technology, Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China
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  • Dong Su,

    1. Key Lab of Advanced Ceramic and Machining Technology, Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China
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  • Claudia Fasel,

    1. Fachgebiet Disperse Feststoffe, Fachbereich Material- und Geowissenschaften, Technische Universität Darmstadt, Petersenstraße 23, Darmstadt 64287, Germany
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  • Ralf Riedel

    1. Fachgebiet Disperse Feststoffe, Fachbereich Material- und Geowissenschaften, Technische Universität Darmstadt, Petersenstraße 23, Darmstadt 64287, Germany
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  • P. Columbo—contributing editor

  • This work is supported by the National Science Foundation of China (50552001), National Science Foundation for Distinguished Young Scholars (50625207), and Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and State Key Laboratory of Precise Measurement and Equipment, Tianjin University, China. Fonds der Chemischen Industrie, Frankfurt, Germany financially supported R. R.

†Author to whom correspondence should be addressed. e-mail: liyali@tju.edu.cn

Abstract

Bulk Si(O)C ceramics are fabricated from polycarbosilane (PCS) by bulk pyrolysis along the route of cross linking, warm-pressing, and pyrolysis. The PCS was thermally oxidized for cross linking at various temperatures as a critical step toward the bulk transformation of PCS into bulk Si(O)C ceramics. The degree of cross linking of PSC affects the densities and bonding qualities of the warm-pressed powder compacts and, hence, the resultant ceramics. Under optimized processing conditions, crack-free bulk Si(O)C ceramics are obtained with a bulk density attaining 2.2 g/cm3. Despite the existence of a considerable amount of oxygen in the ceramics (16.08 wt%), resulting from the thermal oxidation processing, the ceramics show the characteristics of structures and properties of SiC-based ceramics. 29Si-solid state nuclear magnetic resonance spectra (NMR) reveal that the as-pyrolyzed X-ray amorphous Si(O)C phase consists mainly of SiC4 coordination units in the ceramic network, with the remainder being silicon-coordinated carbon and oxygen. Microhardness tests show that the as-pyrolyzed amorphous Si(O)C ceramics have a high hardness, attaining 24.91 GPa at a load of 2 N, and 19.82 GPa at a load of 10 N. Upon annealing at 1300°C in argon, the amorphous ceramics crystallized into nanophase β-SiC ceramics, and the ceramics kept the bulk nature of the amorphous ceramics with an increased density. The 29Si-solid state NMR spectrum indicates that the nanophase β-SiC ceramics consist of SiC4 units together with some mixed coordination units, namely SiO2C2 and SiOC3. The hardness of the crystallized nanophase Si(O)C ceramics attains 23.20 GPa at a load of 10 N. The present study demonstrates the possibility of fabricating bulk Si(O)C ceramics via the polymer-processing route, resulting in ceramics with promising structural and mechanical properties.

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