Synthesis of ultrafine NbB2 powder by rapid carbothermal reduction in a vertical tubular reactor

Abstract

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°C rather than 700°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°C. Under optimum conditions the NbB₂ crystallites were as small as 40–50 nm and the residual carbon content was 2.65% by mass.