THE INCIPIENT KINETICS OF HYDRIDE GROWTH ON CERIUM SURFACES

The incipient hydriding stage of cerium is characterized by nucleation and growth of the hydride phase on the surface of the metal. The kine tics of this initial growth process were studied utilizing a hot-stage microscope (HSM) equipped with a video camera. Growth velocities of n uclei were determined as a function of reaction temperature and hydrog en pressure over a grid of temperatures and pressures which enabled th e maintenance of near isothermal conditions (25-100 degrees C, 10-1000 mbar H-2). A linear pressure dependence of the growth velocities was obtained, with temperature-dependent slopes. An Arrehenius plot of the se slopes yielded a straight line with apparent activation energy of 6 .3 kcal (g atomic H)(-1) (0.27 eV). The results were compared with tho se of the previously studied massive hydriding stage, where the reacti on is characterized by a contracting envelope morphology with a hydrid e-metal boundary moving at a constant (temperature-pressure dependent) velocity. Under similar experimental conditions? the velocities of th e initial nuclei growth process are significantly higher than the corr esponding boundary velocities at the massive stage. Different types of controlling mechanism are proposed to account for these observed diff erences between the incipient and the massive hydriding stages.