Sol-gel synthesis develops polymeric or particulate structures in solution, then links them into a gel as a means of controlling the shape or internal structure of materials, typically oxides. The technique is poorly developed in non-oxide materials, but developing new routes has the potential to deliver nitride materials in morphologies that cannot otherwise be accessed. We have developed routes based on ammonia or primary amines as the cross-linking agent to control the gelation process and applied these to a number of systems.
We have shown that sol-gel synthesis of nitride materials can lead to the controlled formation of transition metal nitrides as nanostructured materials, aerogels and monolithic objects. We have developed methods to deposit films and inverse opal films of titanium and silicon nitrides by dip coating. We have doped lanthanide elements into silicon nitride to produce high stability phosphors. We can also produce monolithic gels and have used supercritical drying to make silicon nitride aerogels (very low density spongy materials).
The same chemistry as used in sol-gel processing is also effective for production of nanocrystalline metal nitride powders, giving access to many of the materials we use in charge storage applications. It also has the advantage that the metal oxidation state can often be maintained from the precursor to the nitride material, we have used this to prepare novel nitrogen-rich Hf3N4 and Ta3N5 high pressure phases.