In Situ Electron Microscopy of Helium Bubble Implantation in Metal Hydrides

Here we investigated the microstructural response of various Pd physically vapor deposited films and Er and ErD2 samples prepared from neutron Tube targets to implanted He via in situ ion irradiation transmission electron microscopy and subsequent in situ annealing experiments. Small bubbles formed in both systems during implantation, but did not grow with increasing fluence or a short duration room temperature aging (weeks). Annealing produced large cavities with different densities in the two systems. The ErD2 showed increased cavity nucleation compared to Er. The spherical bubbles formed from high fluence implantation and rapid annealing in both Er and ErD2 cases differed from microstructures of naturally aged tritiated samples. Further work is still underway to determine the transition in bubble shape in the Er samples, as well as the mechanism for evolution in Pd films. Introduction Pd and Er metal hydrides are used for the storage of H and its isotopes. Radioactive decay of tritium produces He, the accumulation of which eventually forms gas bubbles, which in turn can have detrimental effects on mechanical properties [1]. The decay of tritium occurs with a half-life of 12.3 years, so experiments studying naturally aged tritiated specimens must span years in order to accumulate sufficient amounts of He for bubble formation [2]. Furthermore, the added difficulty of safely examining radioactive specimens increases the desirability of alternative testing methods. Ex situ ion implantation of He has been used to simulate the effects of He accumulation in Er and ErD2 films. Prior studies noted lattice expansion from He implantation, with ErD2 expanding more than pure Er [3]. Previous annealing studies showed sensitive temperature and dose dependence on blister formation [3-5]. While both natural aging and ex situ implantation have been used successfully to study cavity formation in both Pd and Er, neither approach can monitor cavity formation and evolution in real time. Here we use in situ ion irradiation transmission electron microscopy techniques in an effort to provide a more fundamental understanding of the cavity formation and growth mechanisms in physical vapor deposited (PVD) Pd films and Er and ErD2 samples prepared from neutron tube targets. Approach Er and ErD2 samples were prepared from neutron tube by a FIB lift out technique using an FEI DB-235 dual beam focused ion beam/scanning electron microscope (FIB/SEM). The samples were finished with a low-kV polish [6], and placed on the same carbon coated Cu TEM grid. The Pd films investigated were either PLD or sputter deposited to a thickness ranging from nominally 25 nm to 100 nm onto a NaCl substrate. The Pd films were then made fee-standing by removing desolving the NaCl substrate in water and placing the film on a 3 mm Cu or Mo TEM grid. In addition, the Pd films were also directl deposited on SiN membranes associated with the window of the TEM gas heating cell. In situ ion implantation with 10 keV He was performed at room temperature at a rate of 2.9 × 10 ions cms using the ITEM facility at Sandia National Laboratories [7]. For the Er and ErD2 samples, implantation was paused at 1, 10, and 100 minutes, corresponding to fluences of 1.7 × 10, 10, and 10 He ions cm, and still images were collected. After implantation, in situ annealing to a nominal temperature of 790 °C was performed using a Philips CM30 TEM and a Gatan heating stage. Finally, post-annealing selected area SAND2014-17919R