Chemische Zusammensetzung und Mikrostruktur polymerabgeleiteter Glaeser und Keramiken im System Si―C―O Teil 2: Charakterisierung der Gefuegeausbildung mittels hochaufloesender Durchstrahlungselek-tronenmikroskopie und Feinbereichsbeugung

摘要

Liquid or solid silicone resins represent the economically most interesting class of organic precursors for the pyrolytic production of glass and ceramics materials on silicon basis. As dense, dimen-sionally stable components can be cost-effectively achieved by admixing reactive filler powders, chemical composition and micro-structure development of the polymer-derived residues must be exactly known during thermal decomposition. Thus, in the present work, glasses and ceramics produced by pyrolysis of the model precursor polymethylsiloxane at temperatures from 525 to 1550℃ are investigated. In part 1, by means of analytical electron microscopy, the bonding state of silicon was determined on a nanometre scale and the phase separation of the metastable Si―C―O matrix into SiO_2, C and SiC was proved. The in-situ crystallization could be considerably accelerated by adding fine-grained powder of inert fillers, such as Al_2O_3 or SiC, which permits effective process control. In part 2, the microstructure is characterized by high-resolution transmission electron microscopy and selected area diffraction. Turbostratic carbon and cubic β-SiC precipitate as crystallization products. Theses phases are embedded in an amorphous matrix. Inert fillers reduce the crystallization temperature by several hundred ℃. In this case, the polymer-derived Si―C―O material acts as a binding agent between the powder particles. Reaction layer formation does not occur. On the investigated pyrolysis conditions, no crystallization of SiO_2 was observed.