High temperature diffusion characteristics of the alkali metals in Zircaloy-2, 304 stainless steel, and tantalum

The diffusion rates of the alkali metals in zircaloy-2, 304 stainless steel, and tantalum, were determined using a mass spectrometer technique. The mass spectrometer also provided a means for determining an allotropic phase change for the stainless steel at 9550C, and a phase transition region between 7720C and 10720C exists for the zircaloy-2. Diffusion rates were measured in thin (0.001'' and 0.0005'') ribbons which were used as high temperature thermal ion sources, and the alkali metals were naturally occurring impurities. In the temperature and pressure regime in which diffusion measurements were made, the solute atoms evaporated from the ribbon filaments as soon as the impurities diffused to the surface, and a fraction of these impurity atoms were ionized according to the Langmuir-Saha relation. These ions were then accelerated, mass analyzed, and detected by the double focusing spectrometer. Diffusion coefficients were then calculated from the time variation of the ion currents from the filaments, using Fick's law for impurity evaporation from a thin plate. By evaporating the solute atoms from the filaments, at successively higher temperatures, diffusion coefficients were determined, and an attempt was made to correlate diffusion data to the ionic radius of the impurity