Anodic oxidation and depth-distribution studies with V, Mo, and W

Abstract

<p>This investigation is primarily concerned with the development of a suitable high-precision sectioning technique to be employed in studies of depth distributions of energetic ions in Mo and V. It consists of two main parts.</p> <p>In the first part, uniform anodic oxide films of controlled thicknesses have been grown on poiycrystalline V, Mo, and W (the latter, primarily for comparison with earlier works on W) in acetic acid-sodium tetraborate solutions containing small quantities of water. These anodic films which show bright characteristic interference colours when formed at >10 volts, are shown to be rapidly soluble in a dilute solution of KOH whereas the underlying metal is attacked at a rate of less than ~100Å per day. The thickness of the anodic films per volt has been established using ellipsometry and conventional weight-loss measurements.</p> <p>Kinetics of anodization have been studied at constant-current and constant-voltage. An analysis of these data shows the composition of the films on Mo and W to be close to Mo0₃ and W0₃ respectively while those on V were intermediate between V₂0₄ and V₂0₅. The density of the films on Mo and the differential field strength, E_d, for films on Mo and W have also been determined.</p> <p>In the second part, a theoretical background consisting of an outline of the theory of Lindhard, Scharff and Schiøtt, has been provided; in the absence of experimental data, this theory is generally used to estimate the depth distribution of ions in amorphous targets. Experimental range profiles have been determined for 5-30 keV Kr⁸⁵ in polycrystalline Mo using the technique developed in the first part and the results obtained show a large discrepancy when compared with Lindhard theory. The extent of disagreement with theory is similar to that obtained by previous workers with targets such as Al or W.</p> <p>Evidence is presented to show that this large discrepancy may be attributted to crystal-lattice effects (i.e. channelling). For example, it is shown that the discrpency is not due to the anodizing-stripping sequence being sensitive to bombardment. Also, it is demonstrated that the Mo used had a pronounced preferred orientation such that the open directiors <100>, <211>, and <111> were normal to the surface.</p> <p>An important part of future work will be to extend techniques such as those described here to the metal Be, for Be is the lightest target material that can be conveniently worked with.</p>