Kevin Thorne

Synthesis of SiC/TaC ceramics from tantalum alkoxide modified polycarbosilane

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 CH 3 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–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.

Silicon Carbide Via the Hydrolysis-Condensation Process of Dimethyldiethoxysilane/Tetraethoxysilane Copolymers.

Transparent monolithic pieces of gel have been prepared from the hydrolysis of a mixture of dimethyldiethoxysilane (DEDMS) and tetraethoxysilane (TEOS). The characterization of this gel has been done by infrared and shows that a copolymerization between the two precursors has occurred during the hydrolysis-condensation process. Pyrolysis of this gel lead to the formation of β-SiC at 1550°C. The characterization of the pyrolysing process has been investigated. The Si-CH 3 bonds are broken during the heat treatment, leading to a mixture of silica and carbon. The formation of β-SiC is therefore due to a carbothermal reduction of this silica matrix.

Chemically Designed, UV Curable Polycarbosilane Polymers

To prepare new polycarbosilane polymer precursors with high solubility and the capability of UV cross-linking, commercial polycarbosilane was modified by a chemical route. These modifications involved AlCl 3 catalyzed chlorination reactions of polycarbosilanes Si-H bonds. The resultant Si-Cl bonds were substituted by a reaction with sodium acetylyde to form Si-C=CH ligands. These ligands are suitable for controlled, free radical initiated cross-linking of the polycarbosilane polymers. The increase in molecular weight should allow for increased Tgs and the retention of polymer pre-forms. In this report, the chlorination of the polycarbosilane polymer and the substitution reactions of polycarbosilane were examined with IR, 29 Si and 13 C NMR spectroscopy. In addition, the retention of polymer pre-forms were analyzed after UV exposure and inert atmosphere pyrolysis.

Boron Carbide-Based Ceramics Via Polymer Route Synthesis

Boron carbide is a ceramic material with excellent high temperature physical properties. As compared to conventional techniques, the preparation of boron carbide from polymeric precursors is attractive as this technique offers a number of unique advantages. In this paper, the screening of polymeric precursors to boron carbide will be discussed. Two promising boron carbide, carborane containing polymeric precursors have resulted in 60-70 wt.% ceramic yields. The chemistry of polymer synthesis and the transformations from the polymer to amorphous and crystalline boron carbide were investigated with infrared spectroscopy, NMR spectroscopy, thermal analysis, and x ray diffraction.

Alkynyl Substituted Carboranes as Precursors to Boron Carbide Thin Films, Fibers and Composites

The use of alkynyl substituted derivatives of o-carborane as precursors to boron containing ceramics is described. These compounds undergo a thermally or photochemically induced polymerization to afford cross linked polyakynyl-o-carborane derivatives. The increase in molecular weight should allow for increased Tgs and the retention of modelled polymer preforms. In this report, these modification reactions are described. In addition, the retention of molded polymer preforms were analyzed after UV exposure and inert atmosphere pyrolysis.