Preceramic polyazanes via sol gel route in the ammono system and via molecular single source precursors - a comparison of performance

All steps of the sol gel process in the oxo system can be transferred analogously to the ammono system. For this purpose, element alkyl amides of titanium, zirconium, tantalum and boron, have been co-ammonolised with silicon alkyl amides in aprotic solvents. Pyrolysis of the resulting polyazanes yields multinary nitridic ceramic powders. For the model systems Si(NHMe)(4)/ Ti(NMe2)(4), Si(NHMe)(4)/Zr(NMe2)(4), Si(NHMe)(4)/Ta(NMe2)(5), and Si(NHMe)(4)/B(NMe2)(3) the corresponding polymeric polyboro-, polytitano-, polyzirkono- and polytantalosilazanes were synthesised. Pyrolysis in flowing ammonia at 1000degreesC results in ternary amorphous silicon nitrides; further heating in nitrogen leads to partly crystalline composites. The process of pyrolysis as well as the morphology and composition of the final pyrolysis products were analysed by means of NMR, MAS-NMR, FT-IR, DTA-TG-MS, XRD, SEM, EDX, and elemental analyses. In order to verify this new and convenient access to multinary nitride ceramics, single source precursors were synthesised and, via polymer intermediates, processed to ceramic powders. A specially developed reaction sequence is generally applicable to syntheses of single source precursors for various ternary metal silicon nitrides. Dilithiated silazanes of the type Si(NMe2)(2)(NLiR)(2), with R = t-Butyl, SiMe3, CH3, synthesised and structurally characterised for the first time, are good silicon synthones. While forming nitrido bridges, those compounds react with e.g. two- or four valent transition metal chlorides, to silicon transition metal adducts. Actually, we have analysed the reaction of Si(NMe2)(2)(NLiR)(2) and TiCl4, ZrCl4, TaCl5, CrCl3, MnCl2, and ZnCl2, respectively. The adducts as formed were crosslinked with ammonia and pyrolysed. In the case of the chlorides of the 4.-6. group metals, amorphous ceramics were obtained. Treatment at higher temperatures results in composites of transition metal nitride and an amorphous matrix. MnSiN2 and a new hexagonal modification of ZnSiN2 were obtained in the systems Mn/ Si/ N and Zn/ Si/ N. Surprisingly, during the synthesis of ZnSiN2, ZnCN2 was obtained as a side product.