RAPID SOLID-STATE SYNTHESIS OF REFRACTORY NITRIDES

Rapid solid-state metathesis (SSM) reactions are used to synthesize a series of refractory transition-metal nitrides. Exothermic reactions of metal halides with Li3N and/or NaN3 produce the mononitrides (TiN, ZrN, HfN, NbN; and TaN) as crystalline powders. The partial substitution of sodium azide for lithium nitride favors the formation of nitrogen-rich phases. in some cases, achieving a high reaction temperature and large internally generated nitrogen gas pressure enables the formation of nitride phases normally only synthesized at high temperatures and pressures (e.g. cubic NbN and TaN). Powder X-ray diffraction, magnetic susceptibility, electron microscopy, thermal and elemental analysis are used to characterize the compounds. All of the mononitrides are cubic with an NaCl structure (except TaN which also contains a hexagonal phase) and have lattice parameters that are consistent with nearly stoichiometric products. The superconducting transition temperatures for ZrN (8.5 K) and NbN (15.5 K) are also indicative of metal to nitrogen ratios near unity. These rapid reactions produce crystallites with average sizes near 500 Angstrom (as determined from X-ray line broadening) with size variation possible through changes in the reaction conditions. Differential scanning calorimetry and in situ reaction temperature measurements on the ZrN system indicate that initiation occurs when one precursor undergoes a phase change or decomposes. The result is a rapid self-propagating reaction which reaches temperatures of nearly 1370 degrees C within half of a second of initiation followed by rapid cooling to near room temperature within 30 s. The measured maximum reaction temperature agrees well with a theoretical value (1408 degrees C) calculated assuming an adiabatic system.