Title: 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 Nb2O5B2O3C 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 Nb2O5B2O3C system was about 360 kJ mol−1, which is in good agreement with that for the NbCB2O3C system. These results indicate that the rate-determining step of the carbothermal reduction was the formation of NbB2 from NbCB2O3C mixture. For continuous production of NbB2 particles a vertical tubular reactor was used in which the pulverized Nb2O5B2O3C 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.
Publication Year: 1994
Publication Date: 1994-11-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 33
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