Mechanism of formation and growth of sunflower-shaped imperfections in anodic oxide films on niobium

Control of morphology and surface wettability of anodic niobium oxide microcones formed in hot phosphate–glycerol electrolytes

“…In the field-induced crystallization of barrier-type anodic niobium oxide, the crystalline niobium oxide thickened faster than the amorphous niobium oxide under the high electric field [34], 14 suggesting lower ionic resistivity of the crystalline oxide. A current peak appeared when field-induced crystallization occurred during growth of barrier-type anodic oxide films [36,37]. Since similar current peak is observed in the present study (Fig.…”

“…Crystallization of amorphous anodic oxides was reported to occur also during growth of barrier-type anodic oxide films on niobium and tantalum in aqueous electrolytes [36,37]. In the field-induced crystallization of barrier-type anodic niobium oxide, the crystalline niobium oxide thickened faster than the amorphous niobium oxide under the high electric field [34], 14 suggesting lower ionic resistivity of the crystalline oxide.…”

Rapid and controlled electrochemical synthesis of crystalline niobium oxide microcones

“…[1][2][3] Anodization is an attractive method for fabricating oxides, including those of niobium, because of the possibility of creating self-organized structures with a high degree of order. While far less studied than aluminum, anodization of niobium has been reported to result in structures ranging from random porous films [4] and sunflower formations [5] to granular forms [6] and membranes with smooth, linear pores [7] or with rough, vein-like pores. [8] A particularly intriguing morphology is that of microcones, which appears to be unique to anodized niobium.…”

Formation–structure–properties of niobium-oxide nanocolumn arrays via self-organized anodization of sputter-deposited aluminum-on-niobium layers

“…26 It should be noted that, during the higher-potential 95 reanodizing experiments, the potential was closely approaching the value of 550 V, this being however accompanied by increasingly occurring potential fluctuations (current overshoots), which were likely due to the combined effect of stress generation during the film growth and the beginning of NO field 100 crystallization, resulting in destructive oxygen evolution. 25,27 The smooth growth of a defect-free anodic film on niobium metal in the range of formation potentials up to 450 V was achieved for the first time due to great flexibility offered by the porousalumina-assisted anodizing approach and the appropriate 105 combination of technological, electrical, and electrolytic formation conditions. Digital optical photos and SEM images confirming the defect-free formation of large-surface-area NO nanostructures are available in ESI) Figure 2 shows SEM images of the surface and sectional views of bands, up to about 800 nm long, extend at regular intervals from the band outward into the pores in the alumina film.…”