Oxidation State of Anodic Tantalum Oxide after Heat-Treatment

Abstract

During heat-treatment of anodic Ta205 in vacuum (10 -5 Torr) at temperatures in the range from 200 ~ to 700~ oxygen exchange between the anodic oxide, the Ta substrate, and the atmosphere was observed. Galvanostatic reanodization experiments resulted in potential-time (charge) curves which could be used to determine the oxidation state of heat-treated TaOn as a function of the distance from the electrolyte/oxide interface into the oxide. Profiles of the oxidation state across the oxide were measured for tantalum oxide anodized in 0.1M H3PO4 and 0.1M H2SO4 after heat-treatment at 10 -5 Torr and 500~ Of several experimental variables, the time of heat-treatment and the oxide thickness were chosen to illustrate the method of determining oxidation state profiles for the case of heat-treatment in vacuum. In all cases * Electrochemical Society Active Member.

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) .…”

“…Subsequent efforts were made to understand the rate controlling step and the movement/migration of metal ions during the anodic oxide growth on Ta. , For example, B. Verkerk et al investigated the mechanism of tantalum oxide growth utilizing radioactive labeling techniques. In 1972, E. Gunzel examined the anodization of Ta in electrolytes, such as chromic acid, phosphoric acid, sulfuric acid, and citric acid with variations in concentrations. This study suggested, for the first time, the possibility of incorporation of sulfur and phosphorus from electrolytes into the anodic oxide layer.…”

“…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.…”

“…radioactive labeling techniques. In 1972, E. Gunzel95 examined the anodization of Ta in electrolytes, such as chromic acid, phosphoric acid, sulfuric acid, and citric acid with variations in concentrations. This study suggested, for the first time, the possibility of incorporation of sulfur and phosphorus from electrolytes into the anodic oxide layer.…”