| A systematic study of the effect of surface titanium nitride layers on the equilibrium deuterium concentration-depth profiles in titanium and on the deuterium absorption kinetics was performed. We have measured the concentration depth profiles for nitrogen and deuterium in titanium samples exposed to 1 Torr nitrogen gas at 650°C for 0, 11, 45, 90 and 180 minutes, and subsequently exposed to 1 Torr deuterium at 500°C. The measurements were performed by using the secondary ion mass spectroscopy, the Auger electron spectroscopy, the nuclear reaction analysis and the Rutherford backscattering spectrometry.;A three-region nitrogen concentration near-surface profile is obtained. The first region spans from 100 to 1000 A depending on the gas exposure time, and contains about 33 at. % (Ti2N). The second region contains solid solution of nitrogen in close packed hexagonal (cph) titanium TiN x, with x decreasing from 25 at. % to 1 at. % over a depth of 3000--7000 A. In the third region, the nitrogen concentration is below 1 at. %. The deuterium concentration in the first two regions is in the range of 0.1--1.0 at. %, which is low as compared to about 4.5 at. %, the bulk deuterium concentration. In the third region the deuterium concentration increases slowly, over 10 000--20 000 A depth, toward the bulk concentration. Such a slowly increasing region is explained by the reduced capacity of the titanium layers from this region to expand as required by the deuterium absorption, due to the rigidity of the titanium nitride layer. Three regimes of absorption kinetics were identified as generated by the nitrogen presence on the sample surface and by the surface pre-exposure to nitrogen. The absorption kinetics is shown to be driven by the deuterium chemical potential gradient and not by the concentration gradient as currently used. An empirical expression for the chemical potential and an analytical model for the deuterium absorption process are proposed. |
