Synthesis of SiC/TaC ceramics from tantalum alkoxide modified polycarbosilane
- PDF / 793,372 Bytes
- 9 Pages / 576 x 792 pts Page_size
- 66 Downloads / 186 Views
Eric Liimatta Ethyl Technical Center, P.O. Box 14799, Baton Rouge, Louisiana 70898
John D. Mackenzie UCLA Department of Materials Science and Engineering, 5731 Boelter Hall, Los Angeles, California 90024 (Received 10 October 1990; accepted 31 May 1991)
The reaction between tantalum ethoxide and an inorganic, silicon-carbon based polymer known as polycarbosilane resulted in a modified polymer that could be thermally converted into a binary ceramic of SiC and TaC. In this report, the initial reaction of the precursors and the high temperature transformations that resulted in the mixed ceramic carbide are discussed. The synthesis of this modified polymer was characterized using 29 Si, 13 C NMR, and infrared spectroscopy. The reaction involved cross-linking of polycarbosilane through bridging carbon bonds and the formation of Si-OCH 2 CH3 ligands. According to these data and to the low-angle x-ray diffraction data, the structure of the reaction product can be described as a network of modified polycarbosilane with intimately dispersed tantalum oxide particles. The structural transformations that occurred during inert atmosphere pyrolysis of the polymer product were determined using 29 Si, 13 C MAS NMR, infrared and x-ray diffraction spectroscopy. Inert atmosphere pyrolysis at temperatures below 500 °C involved continued cross-linking of polycarbosilane through the endothermic formation of bridging carbon bonds. During pyrolysis at 500 °C, an exothermic reaction between the modified polycarbosilane and the intimately dispersed tantalum oxide particles was observed. This reaction involved the formation of an inorganic, amorphous oxycarbide phase that can be described as a continuous network of C - S i - 0 and C - T a - 0 bonds. At pyrolysis temperatures exceeding 1000 °C, carbothermal reduction of the oxide constituents initiated. Further pyrolysis at temperatures exceeding 1200 °C resulted in the crystallization of zinc-blend /?-SiC and NaCl structured TaC. I. INTRODUCTION Today's high technical applications have stringently increased the property requirements demanded of ceramics. As a consequence, new and alternative techniques for the preparation of ceramics are sought. The synthesis and fabrication of advanced ceramic carbides from the pyrolysis of chemically designed polymeric precursors has attracted much attention in recent research studies.1"5 Synthesis by this method offers several unique advantages as compared to traditional powder processing techniques that can provide lower processing temperatures, improved impurity control, and the ability to form complex monolithic shapes, continuous fibers, and thin films. The exploitation of these advantages can enable the formation of many advanced ceramic carbides for a number of new high temperature strength and oxidation resistant applications. The practical advantages and commercial value of this polymer production technique were proven through J. Mater. Res., Vol. 6, No. 10, Oct 1991 http://journals.cambridge.org
Downloaded: 11 Mar 2015
the pioneering work of Y
Data Loading...
