SYNTHESIS OF ULTRAFINE NBB2 POWDER BY RAPID CARBOTHERMAL REDUCTION IN A VERTICAL TUBULAR REACTOR

Niobium diboride (NbB2) powder was synthesized by carbothermal reduction of Nb2O5-B2O3-C precursor prepared with Nb2O5, H3BO3 and cornstarch. The reaction was consecutive and Nb2O5 was formed while NbC was consumed. The apparent activation energy of NbB2 formation for the Nb2O5-B2O3-C system was about 360 kJ mol(-1), which is in good agreement with that for the NbC-B2O3-C system. These results indicate that the rate-determining step of the carbothermal reduction was the formation of NbB2 from NbC-B2O3-C mixture. For continuous production of NbB2 particles a vertical tubular reactor was used in which the pulverized Nb2O5-B2O3-C precursor particles were entrained downwards with an argon gas flow. The size and purity of the NbB2 formed were strongly dependent on the calcination temperature of the precursor, precursor size and temperature of carbothermal reduction. The carbonaceous material in the precursor particles was quite reactive when cornstarch was calcined at 400 degrees C rather than 700 degrees C. The smaller the precursor size, the longer the precursor particles could stay in the reaction zone, giving a better yield of NbB2. Sintering of NbB2 particles occurred at 1800 degrees C. Under optimum conditions the NbB2 crystallites were as small as 40-50 nm and the residual carbon content was 2.65% by mass.