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 29Si, 13C NMR, and infrared spectroscopy. The reaction involved cross-linking of polycarbosilane through bridging carbon bonds and the formation of Si–OCH2CH3 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 29Si, 13C 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–Si–O and C–Ta–O 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 β–SiC and NaCl structured TaC.
A new family of disordered materials called amorphous covalent ceramics (ACC) has been obtained by pyrolysis of metal-organic polymers. The transformation from polycarbosilane to amorphous and microcrystalline SiC has been studied. New a-SiC:H thin films have also been prepared by the polymer solution. Electrical and optical properties of this new a-SiC have studied. Amorphous binary carbide and oxycarbide have also been prepared by crosslinking polycarbosilane and metal alkoxides. The p-n control of this new a-SiC:H has been achieved, which also lead to the possible device applications.
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