Anodic oxides on  Al-Nb alloys and niobium

de Sá, A.I.

http://hdl.handle.net/10400.9/417

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Authors

de Sá, A.I.

Abstract(s)

Anodic oxide films of aluminium and tantalum have been widely used as dielectric materials in the capacitor industry. Niobium is a metal that can also be anodised and it has been considered as a possible substitute for tantalum because of its similar electrochemical behaviour. However, anodic niobia (Nb2O5) has a higher dielectric constant than tantala (Ta2O5), which is advantageous for use as a dielectric because a higher capacitance can be achieved with the same oxide thickness. Furthermore, niobium is more abundant than tantalum and this usually means a reduced material price. However, niobium has some disadvantages, like semiconducting rather than dielectric properties, higher leakage current and lower potential of dielectric breakdown. The combination of aluminium with niobium may be a route to reduce these factors, and the magnetron sputtering technique is ideal for such a study since the deposited alloys can form a single solid solution over the entire concentration range. In this study, anodising of Al-Nb alloys, containing 21 to 89 at.% niobium and niobium, prepared by magnetron sputtering, has been undertaken at a constant current density of 5 mA cm-2 in 0.1 M ammonium pentaborate electrolyte. The resultant anodic films were analysed by SEM, TEM and RBS to determine the film composition and thickness. Anodic oxides with a wide range of thicknesses were analysed by EIS to verify the dielectric or semiconducting properties. Xenon implantation was carried out to determine the cation and anion transport numbers and relative migration rate of incorporated species. The results have shown that the anodic oxides are featureless, with uniform composition and of aluminium and niobium contents similar to those of the alloys. The Al-21 at.% Nb and Al-31 at.% Nb alloys, anodised to 150 V revealed a very thin alumina layer close to the oxide/electrolyte interface due to the lower migration rate of niobium cations relative to aluminium cations. The dielectric constants of the oxides were determined by EIS using a mercury drop contact for alloys of 21 to 81 at.% niobium, and the results showed a linear trend, proportional to composition, in agreement with the dielectric constants of Al2O3 and Nb2O5. The results obtained for niobia and Al-Nb oxides with niobium content above 74 at.% Nb revealed n-type semiconductor properties when analysed in a borate buffer solution. The carrier concentration was in the range of 1018-1019cm-3. Thus, the results demonstrate that anodic oxides formed on sputtered Al-Nb alloys up to 44 at.% of niobium behave as dielectric materials and represent a valid alternative for capacitors.