Ion-cathode bombardment for the creation of tightly bound deuterium clusters in palladium

A complementary approach of experimental and computational methods was used in pursuit of determining optimal ion bombardment parameters for the creation of deuterium clusters with high binding energies. The incident ions create damage cascades leading to the production of defects such as vacancies, dislocations, and voids. These defects are known to trap interstitial deuterium with binding energies dependent on the trap’s geometry and volume. To simultaneously obtain high concentrations of defects and hydrogen, a simple DC glow discharge method was employed. Deuterium ions bombarded a palladium cathode at varying fluences (1 x 〖10〗^18 ions/cm^3,1 x〖 10〗^19 ions/cm^3,and 1 x 〖10〗^20 ions/cm^3) and incident energies dependent on cathode bias (-0.75 kV,-0.875 kV,and -1.0 kV). Langmuir probe measurements of the Electron Energy Distribution Function (EEDF) validated a COMSOL simulation’s accuracy which confirmed the proper methodology for reproducing discharge dynamics. The simulation was a sequential coupling between COMSOL's Plasma module and Boltzmann Equation, Two-Term Approximation module allowing for more exact calculations of the Townsend coefficients. Properly accounting for the Townsend coefficients is necessary to represent the kinetics of DC discharges with low ionization fractions and species mobility highly dependent on the electric field. With these corroborating results, the model was expanded to conditions where measuring plasma properties became no longer feasible. The model produced Ion Angular Energy Distribution Functions (IAENDF) at the cathode which allowed for finding trends in Thermal Desorption Spectroscopy (TDS) curves. The desorption peaks centered around 600-800 K. There appeared to be a deuterium trapping limit dependent on defect concentration where once a distinct defect density was met any further damage was counterproductive to deuterium trapping. The condition that produced the most trapped deuterium was -0.75 kV cathode bias in 1 Torr deuterium with a fluence of 1 x 〖10〗^18 ions/cm^3. To further investigate these TDS trends some samples were observed under SEM and TEM. The results showed surface pit and blister formations which grew in concentration as the fluence increased. Beneath the surface formations, cross-section images showed large voids and holes in the material with cracks at grain boundaries. TEM images displayed the resulting damage structure which extended ~250 nm into the cathode for a sample at 1.0 Torr and -1.0 kV. A proposal is that as the damage concentration increased, these voids grew to such an extent that they formed the blisters and eventually ruptured leading to the release of the trapped deuterium