Anodic film growth on Al layers and Ta–Al metal bilayers in citric acid electrolytes

A systematic investigation has been undertaken of anodising behaviour of aluminium in citric acid solutions under changing the current density, electrolyte temperature, concentration and stirring conditions. Porous films of steady-state, controlled and extraordinary large morphologies have been obtained at the formation voltages of 260–450 V. The barrier layer thickness, cell and pore diameters appeared to be directly proportional to the formation voltage. For the first time, a porous anodic film with cell diameter of 1.1 μm, barrier layer thickness of 0.5 μm and pore diameter of 0.23 μm was naturally grown on aluminium in 0.125 mol dm−3 citric acid electrolyte. The selected electrical and electrolytic conditions have been applied to sputter-deposited Ta–Al bilayers. The tantalum underlayer was anodised through the initially grown at 450 V porous alumina film with the formation of tantalum oxide ‘columns’ penetrating into the pores and mixing with the outer part of the alumina cell walls. During subsequent constant current reanodising of the Ta–Al bilayers in 0.5 mol dm−3 H3BO3, the voltage rose up to 1100 V, until voltage fluctuations terminated normal oxide growth. The columns thickened uniformly up to 1.42 μm with the nm V−1 ratio of 1.3, exhibiting nearly ideal dielectric behaviour up to almost 1000 V of the formation voltage. The outer column material is composed mainly of stoichiometric Ta2O5 mixed with small amount of Al2O3, while the innermost column material is tantalum monoxide with little content of boron species. Implications of these findings for the formation of composite dielectrics for high-voltage electrolytic capacitors are discussed.