モリブデンとNa₂O・2B₂O₃ガラス融液との分極下における反応について

抄録

In the previous papers, the authors investigated the electrochemical phenomena of some electric conductive materials such as Pt, Ni, Co, graphite etc. in molten sodium diborate glass. In this paper, molybdenum was chosen as the object of study since molybdenum is a particularly interesting electrode material used in electric melting of glass. Anodic dissolution, passivation and catholic deposition of molybdenum were investigated by linear potential sweep voltammetry in Na₂O·2B₂O₃ at 900°C in N₂ gas atmosphere. Moreover, the metal-glass interface after polarization was analyzed by EPMA.
On cathodic polarization of Mo electrode, a relatively narrow residual current region appeared first and then a steep current increase followed at higher cathodic potential than ca. -0.7V (vs. Pt reference). Borate anions were easily reduced to boron element and the molybdenum boride layer having close texture covered the electrode surface. Sodium oxide layer was formed outside the boride layer at ca. -2.5V, and this layer moved toward molybdenum metal at higher cathodic polarization. Sodium ions were not seemed to be reduced readily till -5V. Ni electrode also formed nickel boride before sodium deposition like in the case of Mo.
The anodic voltammogram of Mo electrode showed three current peaks due to the formation of passive films made of molybdenum oxides. At a lower anodic potential than ca. 1V, MoO₂ and Mo₂O₅ films were likely to be formed, but both films re-dissolved at higher anodic polarization. Anodic current increased up to ca. 4V due to dissolution of Mo oxides and Mo metal. Passivation phenomenon at ca. 4V could be attributed to the formation of MoO₃ layer. The MoO₃ layer looked remarkably porous and was readily sublimed at 900°C, so that Mo electrode was consumed acceleratedly. Ni electrode was revealed to form passive NiO layer.