Sol–gel preparation of low oxygen content, high surface area silicon nitride and imidonitride materials

Abstract

Reactions of Si(NHMe)4 with ammonia are effectively catalysed by small ammonium triflate concentrations, and can be used to produce free-standing silicon imide gels. Firing at various temperatures produces amorphous or partially crystallised silicon imidonitride/nitride samples with high surface areas and low oxygen contents. The crystalline phase is entirely α-Si3N4 and structural similarities are observed between the amorphous and crystallised materials.

Synthesis and methane cracking activity of a silicon nitride supported vanadium nitride nanoparticle composite

“…Ammonolytic gelation of Si(NHMe) 4 can be facilitated by the presence of a catalytic (∼0.4 mol%) amount of ammonium triflate, and the resulting gels can be fired to produce high quality silicon nitride/imido nitride materials with mesoporosity. 5 Ammonolysis of a 1 : 1 molar ratio of Si(NHMe) 4 and V(NMe 2 ) 4 under similar conditions did not result in gelation with or without the added triflate, instead a dark precipitate would separate from a turbid solution within a few hours of allowing the mixtures to warm to ambient temperature. Removal of solvent in vacuo from these mixtures resulted in colourless, low volatility, viscous liquid in the trap, which gelled on exposure to air and was probably the unreacted silicon amide and/or small oligomers.…”

“…2 Silicon nitride is also of interest as a catalyst support due to the high thermal conductivity relative to the more commonly used silicas, and in the α-Si 3 N 4 form is oxidation resistant at typical catalytic process temperatures. 3,4 Gel-based routes to silicon nitrides and imidonitrides utilise solutionphase reactions of precursor molecules, usually amides, with cross-linking groups such as ammonia 5 or isocyanate. 6 These make polymeric species in solution, and when the polymeric material immobilises the liquid phase, the latter can be removed to provide high surface area materials that often contain basic surface groups.…”

Synthesis and processing of silicon nitride and related materials using preceramic polymer and non-oxide sol-gel approaches

“…It is still necessary to measure the quantity of ammonia, but the mixture can be assembled in a single reaction vessel and gelation occurs a period of time after the mixture warms to ambient temperature, with that period determined by the catalyst concentration [207]. This work showed that the triflate is incorporated and does result in some oxygen contamination, although this can be minimised by reducing its concentration to genuinely catalytic quantities (albeit with an increase in reaction time).…”

“…During pyrolysis a number stages will occur and these may or may not involve an active gas environment such as NH3, and Si3N4 is placed in quotation marks above as this is only achieved at very high temperatures under ammonia. There is a clear possibility that carbon will also be incorporated from any alkyl groups present in the precursor, and imide groups persist even at 1400 °C in the amorphous material [207].…”

“…Cheng et al suggested that the triflate anions, rather than the protons introduced from the acid, were important in the catalysis of the condensation process, probably by attacking silicon centres first then providing a good leaving group when they are attacked by amide groups on other silicon centres [212]. Use of ammonium triflate as the catalyst resulted in slower gelation (suggesting the acid may have some role), but allowed the process to be simplified by addition of the catalyst from the start of the reaction [207]:…”