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Abstract:
This work dealt with the development of a mild chemical route for making silicon nitride and silicon diimide based materials, i.e. Si3N4 and Si(NH)2. The strategy we used was to synthesise partially ammonolysed silicon amides, i.e. amino silylamines such as (Me2N)3SiNH2 and (Morpholine)3SiNH2, in order to achieve a degree of control, on their further ammonolysis. In particular we found that while (Me2N)4Si and (Me2N)3SiNH2 are inert towards ammonia, they react in the presence of acid catalysts affording Si(NH)2 powders. However when the ammonolysis reaction is carried out on the cyclic trisilazane [(Me2N)2SiNH]3, obtained by selective self-condensation of the silylamine (Me2N)3SiNH2 with triflic acid as catalyst, a Si(NH)2 gel is obtained. In analogy with the parent SiO2 gels, the Si(NH)2 gels have surface area up to 1000 m2/g and a pore size distribution comparable to that of mesoporous silica. This result represents the first example of high surface area non-oxide gel.
The special reactivity of the silylamine (Me2N)3SiNH2 has been used to synthesise a new class of multielement single source precursors. By reaction with LiBu the silylamine has been deprotonated affording the corresponding lithium salt (Me2N)3SiNH2 (structurally characterised), that by methatetic reaction with AlCl3 afforded LiAl[NHSi(NMe2)3]4 (structurally characterised). By reaction with metal amides, using the SiNH2 functionality, a series of mixed amides has been synthesised: Ti[NHSi(NMe2)3]4, [((Me2N)3SiNH)2Ti(NSi(NMe2)3)2]2, (Me2N)B[NHSi(NMe2)3]2. By reaction with organometallic compounds like AlEt3 and (MeAlO)n, the corresponding substituted product Al[NHSi(NMe2)3]3 and [(Me2N)3SiNHAlO]n has been obtained.
Some of these single source precursors, i.e. Al[NHSi(NMe2)3]3 and (Me2N)B[NHSi(NMe2)3]2, have been ammonolysed with acid catalyst to give mixed silicon diimide based high surface area gels, while the polyalumionosilazane (Me2N)3SiNHAlO]n has been converted in a microcrystalline Sialon ceramic.
