From Precursors to Non-Oxide Ceramics: Pyrolytic Mechanisms Studied by NMR
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70569 Stuttgart,
Germany **Max-Planck-Institute for Polymer Research, 55128 Mainz, Germany
ABSTRACT The pyrolysis of a poly ethylsilazane was studied using a C0 2-laser beam heated solid state MAS-NMR probe head. Chemical structures of the intermediate stages could be identified. The analogy of the pyrolysis evolution for the following two different methods could be shown: A) in an inert gas furnace conventionally prepared and B) laser irradiated in the NMR probe head under magic-angle spinning (MAS) conditions. Samples prepared by method A were studied by 29Si MAS-NMR and samples prepared by method B were studied with an appropriate cross polarization time by 29 Si CP-MAS-NMR. Both experiments showed the same mechanisms for the pyrolysis as the polymer is transformed into a Si 3N 4/C ceramic. INTRODUCTION New non-oxide materials can be prepared by pyrolyzing ceramic precursors, e.g. poly silazanes. These form ceramics which are amorphous up to 1000'C and partially or fully crystallize at higher temperatures. This way novel nitride and carbide ceramics can be prepared without containing an oxide glass phase. Therefore new materials with increased thermal stability can be achieved as their high temperature properties are independent from the instability of this glass phase.' During pyrolysis the preceramic compound is transformed in a series of many chemical reactions into a ceramic. It has been shown by several authors that the chemical structure and the reaction control has a great influence on the final ceramic. 2' 3' 4 But still little is known about the mechanisms which play an important role in the control of the whole process. So far NMR has prooved to be a useful tool for the elucidation of the chemical structures that occur in the pyrolytic process. These structural investigations then allow possible reaction mechanisms to be concluded. Our intention was to further elucidate these structural changes and the on-going reactions during the formation of a ceramic out of a polymer precursor. Therefore, a specially constructed solid state NMR probe head (high-temperature-solid-state-CP-MAS-NMR) was set up, which allows investigations at temperatures up to and above 800'C. The precursor, a poly silazane, can be transformed into a non-oxide ceramic in the NMR-spectrometer. In this paper we will report results obtained with this novel technique and compare them with results of quenched samples that were prepared in an inert gas furnace.
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Mat. Res. Soc. Symp. Proc. Vol. 410 ©1996 Materials Research Society
EXPERIMENTAL Compounds: The polymer used in this study is a poly ethylsilazane with the commercial name ET70 (Hoechst AG, Frankfurt, Germany). It is synthesized by ammonolysis of trichloro ethylsilane. The polymer is supplied as a 70 weight % solution in toluene. The average molecular weight is 1600 g/mol measured by osmometry and GPC. The chemical structure is [Si(C 2Hs)(NH) 3], containing some dimethylamines as end groups. For NMR studies the polymer was heated at 400'C in a flask to remove the toluene.
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