Solid-State Anodic Oxidation of Tantalum

Abstract

When an anodic potential is applied to a tantalum point contact to MnO2 , the current‐voltage behavior is analogous to that observed during the anodic oxidation of a valve metal in an aqueous electrolyte. It is proposed that this represents a solid‐state anodic oxidation whereby tantalum oxide is formed at the expense of the local oxygen content of the MnO2 , and that this process represents an important “healing” mechanism in the normalTa‐Ta2O5‐MnO2 solid electrolyte capacitor. This process requires that the conductivity of the reduced manganese oxide phase, which must be formed adjacent to the Ta2O5 , have a finite ionic portion, and experimental support for this is described. The current efficiency of the solid‐state anodization is very low and is dependent on ambient moisture. Similar behavior has been found for aluminum and niobium point anodes and with PbO2 as the cathode.

Electron Injection into Anodic Tantalum Oxide Assisted by Ionic Interface Polarization

“…The activation process is time dependent, and the rate of increase of the electronic conductivity is a function of the applied field. Second, in cases where MeO shows some small degree of ionic conductivity, as for example in the case of MnO2, one can detect some additional growth of Ta20~ underneath MeO (21). Third, after activation has proceeded to a certain conductivity level one finds this condition to persist in the absence of the field for extended periods of time, but with a general tendency to decline over longer periods of time.…”

Review of Anodic Tantalum Oxide Nanostructures: From Morphological Design to Emerging Applications

“…The 20th century research on ATO structures was primarily dedicated to the understanding optical interference effects (coloring effect) and capacitance–voltage relations with respect to the formed oxide thickness. ,− In most cases, a two-component anodizing solution, comprising various concentrations of H 2 SO 4 , Na 2 SO 4 , H 3 PO 4 , HCl, NaBF 4 , citric acid, etc., in water was utilized for the formation of ATO structures. ,,− ,,,,,− The anodizing potential was maintained below the dielectric breakdown potential, typically up to 300 V, resulting in the formation of a uniform, compact, and amorphous ATO layer. In 2005, I. Sieber et al reported, for the first time, the formation of porous ATO layer by anodizing Ta foil in a 1 M H 2 SO 4 electrolyte containing a small amount of HF (0.1–3 wt %). , The anodizing potential was ramped from E ocp to 20 V at 10 mV s –1 , resulting in an observed ATO layer thickness of about 130 nm (Figure a) .…”

“…The composition and concentration of anodizing electrolytes significantly affect the morphology of ATO layers. − Electrolytes used for anodization of Ta can be broadly classified into three categories: (i) inorganic including, e.g., H 2 SO 4 , Na 2 SO 4 , H 3 PO 4 , HCl, NaBF 4 , H 2 CrO 4 , etc., (ii) organic acids, such as citric acid, oxalic acid, acetic acid, and (iii) mixtures of inorganic and organic electrolytes. − Earlier studies on anodization of Ta in two component electrolytes, e.g., H 2 SO 4 , Na 2 SO 4 , H 3 PO 4 , HCl, NaBF 4 , chromic acid, citric acid, etc. in water, resulted in the formation of a compact or barrier type oxide layer with the thicknesses of few nm. ,,−…”

“…In general, the morphology of anodic tantalum oxide layers can be categorized into four types: (a) a compact layer with a microstructural morphology, (b) nanodimple, (c) disordered porous, and (d) ordered nanoporous/nanotubular structure. − As discussed earlier, anodizing conditions like appropriate F – ions containing electrolytes, voltage range, and anodizing time are necessary to shape the metal oxide layer into a nanotube/nanopore geometry, and this can be correlated to an equilibrium condition between the oxide formation and its dissolution. In this section, we will discuss all possible growth mechanisms of ATO in electrolytes containing F – ions or free of F – ions.…”

“… 87 , 92 − 94 In most cases, a two-component anodizing solution, comprising various concentrations of H 2 SO 4 , Na 2 SO 4 , H 3 PO 4 , HCl, NaBF 4 , citric acid, etc., in water was utilized for the formation of ATO structures. 61 , 66 , 70 − 73 , 75 , 76 , 87 , 88 , 93 − 95 The anodizing potential was maintained below the dielectric breakdown potential, typically up to 300 V, resulting in the formation of a uniform, compact, and amorphous ATO layer. In 2005, I. Sieber et al reported, for the first time, the formation of porous ATO layer by anodizing Ta foil in a 1 M H 2 SO 4 electrolyte containing a small amount of HF (0.1–3 wt %).…”