Synthesis of SiC/TaC ceramics from tantalum alkoxide modified polycarbosilane

Abstract

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.

Advanced Ceramics from Preceramic Polymers Modified at the Nano-Scale: A Review

“…An alternative way to polymer-derived nano-structured ceramics, mainly for functional applications, is provided by the direct chemical modification of preceramic polymers using alkoxides or other chemical precursors (liquid or gaseous). This was proposed long ago for polycarbosilane, which has been modified with Ti [ 40 – 47 ], Al [ 48 , 49 ], Zr [ 50 – 52 ], Fe [ 53 ], Ta [ 54 ] or B [ 55 , 56 ], and has also been exploited for silazanes, for the realization of silicon carbonitrides containing boron (of particular interest for their exceptional high temperature properties) [ 57 – 66 ], early transition metals (e.g., Ti, Zr, Hf) [ 67 – 70 ], other transition metals (e.g., Ni, Fe, Co, Pd), aluminum [ 71 ] or others [ 72 ]. Silicone resins have also been modified using chemical precursors for metals [ 73 – 78 ], although the work has been devoted mainly to the fabrication of SiCO-based nanocomposites, heat treating the samples in inert atmosphere and not in air.…”

Method for preparing polyaluminocarbosilane

“…These unsaturated C¼ ¼C groups must also originate from the fragments of the ligand in Al(acac) 3 . In addition, the disappearance of most of the signals (115-145 ppm) in the 13 C distortionless enhancement by polarization transfer (DEPT) 135 spectrum of PACS-M [ Fig. 3(C)] indicates that most of the carbon atoms in unsaturated C¼ ¼C groups should exist as quaternary carbons.…”

“…The small free-radical fragments cannot escape the closed reaction system. Some of them are incorporated into the molecular structure of PACS [detectable by 13 results in reduced oxygen content in the final product. That is why the measured content of oxygen in PACS is less than the calculated value.…”

“…[1][2][3][4] Moreover, various kinds of polymetallocarbosilanes have been developed by the incorporation of metal elements such as titanium, [5][6][7][8] zirconium, [9][10][11][12] tantalum, 13 and aluminum, [14][15][16][17][18][19][20] into PCS to improve the thermal resistance of SiC fibers and ceramics. Polymetallocarbosilanes have been also developed as important preceramic polymers for functional gradient materials [21][22][23] and gas-separation membranes.…”

Preparation and magnetic properties of Fe/Si/C/N ceramics derived from a polymeric precursor

“…Another important advantage about this route is to prepare silicon‐based multicomponent ceramics through chemical modification of silicon‐based polymeric precursor. In this progress, desired elements (such as B, Ti, Zr, Hf, Ta, and Mg) for functional modified aids can be introduced to the precursors and the chemical composition including the desired elements can be regulated at the molecular or atomic level, which is expected to produce new types of high‐performance silicon‐based ceramic composites 3–7…”