Effect of magnesium on the phase equilibria in magnesio-thermic reduction of Nb2O5

•Molar fraction of Mg influences the magnesio-thermic reduction kinetics of Nb2O5.•Intermediate Mg-Nb-O oxides can form depending on the Mg content.•Higher temperatures enhanced formation kinetics of intermediate Mg-Nb-O oxides.•Fully reduced metallic Nb was immiscible with the surrounding MgO.•FCC Mg-Nb-O and hexagonal Mg3Nb6O11 phases were identified.

Nb powders were synthesized by magnesio-thermic reduction. Nb2O5 is reduced by Mg via an oxygen-metal cation exchange in a mixed powder bed. Although thermodynamics favor reduction of Nb2O5 with Mg, the final phase distributions can be affected by the Mg content and interaction between products and reactants. Mg/Nb2O5 molar ratio was changed from 5 to 7.5 in order to evaluate the reaction pathway and relevant phases. Due to premature Mg depletion, samples with Mg/Nb2O5=5 resulted in intermediate Mg-Nb-O oxides according to the phase equilibria. A time-series comparison of Mg/Nb2O5=5 at 700 °C and 800 °C resulted in faster Mg-Nb-O intermediate phase formation at higher temperatures. Depending on the average oxidation state, the face-centered cubic Mg-Nb-O and hexagonal Mg3Nb6O11 phase were identified.

Graphical abstractMagnesio-thermic reduction of Nb2O5 and the phase evolution verified by XRD, SEM, and TEM. Note the formation of a cubic and hexagonal Mg-Nb-O intermediate phase.Figure optionsDownload full-size imageDownload as PowerPoint slide