ニオブアノード酸化皮膜の結晶化挙動と誘電特性の評価

抄録

The anodizing behavior and crystallization of anodic oxide films formed on niobium in sulfuric acid were investigated by measurement of current density transient during voltage holding, voltage-time curves on re-anodizing, leakage current, capacitance and X-ray diffraction, and by observation of film structure at different stages. Current density transient during voltage holding, which was reflected in crystallization of anodic niobium oxide films, was divided into three regions : first current decay region (nucleation of crystalline oxide), current increasing region (development of crystallization), and second current decay region (repair of defects in the film). From the detailed observation of the fracture sections of oxide films by SEM, it was confirmed that the crystallization occurred during first current decay region. The features of the cracks, which were produced by volume expansion of crystalline oxide particles forming within the amorphous oxide layer, were quite similar to those associated with tantalum oxide films. With increasing anodizing time, the crystalline areas grew outwards radially, pushing up the amorphous film like petals. The available experimental data on various electrochemical measurements was in good agreement with the directly observed transformation of crystalline areas in the oxide film during anodizing. The crystallization of the film during anodizing of niobium was accelerated by employing a low acid concentration at high temperature and high anodizing voltage. The dielectric properties of the crystallized film deteriorated except for capacitance, in comparison with the amorphous film formed on niobium.