Non-thickness-limited growth of anodic oxide films on tantalum

The non-thickness-limited (NTL) growth phenomenon reported by Melody et al. in which anodic oxide films on tantalum continue to grow in dry glycerol s olutions of dibasic potassium phosphate to large thicknesses (tens of micro meters) at low applied voltages (tens of volts) was investigated using most ly constant current growth. The field strengths in the oxide for growth of anodic oxide films at 180 degreesC with constant current density in wet gly cerol solutions (1 vol % water) were in the MV/cm range and close to those calculated from the equations fitted to data from growth in aqueous solutio ns at lower temperatures. At constant current density and 180 degreesC in d ry glycerol solutions, the field fell with time and eventually the voltage decreased as the NTL state developed. The current efficiency for the growth of oxide was estimated by comparing thickness increase measured by ellipso metry with that calculated from the charge passed. It was lower in the NTL state. This is consistent with increased electronic conductivity in the oxi de. The dielectric losses of the NTL films were much higher than those of f ilms grown in wet glycerol solutions. Those grown in wet glycerol showed th e normal, almost frequency independent, tan delta as found for ordinary ano dic oxide growth on tantalum. Those grown in dry glycerol solutions showed increasing tan delta as the frequency decreased, consistent with increased electronic conductance through the oxide. It is suggested that the increase d electronic and ionic conductivity in the NTL state may be caused by nonst oichiometry induced by the change in hydroxyl ion availability to the oxide suggested by Melody et al. when dry glycerol solutions are used. (C) 2001 The Electrochemical Society.