Transmission Electron Microscopy of Non-Thickness-Limited Anodic Films on Tantalum

Abstract

Non-thickness-limited (NTL) growth of anodic films on tantalum is a remarkable recently reported phenomenon whereby anodic films can be formed on tantalum to high thickness due to a change in the film growth mechanism from the usual high-field type of process. The present work employs transmission electron microscopy to reveal the structure, composition, and morphology of an NTL-type film formed by anodizing tantalum at 1Am−21Am−2 in 10 wt % dibasic potassium phosphate/glycerol electrolyte at 453 K. A relatively compact, amorphous layer of NTL-type film material, composed of essentially tantala, was formed above an inner layer of usual high-field-type tantala under the selected conditions of anodizing and drying of the electrolyte. The field for growth of NTL-type film material is ⩽10MVm−1,⩽10MVm−1, compared with ∼400MVm−1∼400MVm−1 for high-field growth. Thus, the thickness of the inner layer is approximately equal to the product of the formation ratio for usual anodic tantala and the final anodizing voltage. © 2002 The Electrochemical Society. All rights reserved.