Transmission electron microscopy of non-thickness-limited anodic films on tantalum

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 1 A m(-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 less than or equal to10 MV m(-1), compared with similar to400 MV m(-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. (C) 2002 The Electrochemical Society.