THE DIRECT OBSERVATION OF ANODIC BI2O3 FORMED ON BISMUTH IN AMMONIUM DIHYDROGEN PHOSPHATE SOLUTION

Using a relatively thin bismuth layer, sputter-deposited on to an oxid e-coated, microscopically flat aluminium substrate, ultramicrotomed se ctions of anodic Bi2O3 have been prepared successfully, allowing direc t observation of the film section in the transmission electron microsc ope. Various important features are observed for the anodic oxide film formed in ammonium dihydrogen phosphate solution. The film is essenti ally amorphous and consists of two distinct layers of different compos itions, comprising an inner, pure oxide layer located adjacent to the metal and an outer layer doped with phosphate anions. The ionic conduc tivity of the outer, electrolyte-doped layer is considerably lower tha n the inner, pure oxide layer. Further, the effect of incorporated pho sphate anions on the ionic conductivity of the oxide is considerably g reater than that reported previously for amorphous anodic oxides on ta ntalum and niobium.