Hydrogen trapping in thoria-dispersed nickel

Abstract

Trapping of hydrogen in thoria-dispersed nickel has been investigated by the hydrogen thermal desorption technique. It has been found that voids are formed at the particle-matrix interface during cold-working. Annealing at high temperature removes these extra trapping sites, allowing an intrinsic trapping effect due to the thoria particle-matrix interface to be measured. The trapbinding energy and trap-activation energy of hydrogen at the ThO2-nickel matrix interface are estimated as 35 kJ mole-1 and 48.7 kJ mole-1, respectively, using the mathematical models derived from the existing trap theory. The energy level of hydrogen around the ThO2-lattice interface is derived from the above values. The saddle-point energy of hydrogen at the ThO2-lattice interface, 13.7 kJ mole-1, is lower than the activation energy for hydrogen diffusion through a normal lattice, 40.2 kJ mole-1. It is suggested that during thermal desorption from thoria-dispersed nickel, some of the hydrogen atoms in normal lattice interstitial sites are retrapped at ThO2-matrix lattice interfaces which are not occupied with hydrogen at the charging temperature. Trap sites at ThO2-matrix interfaces are dilutely occupied under 1-atm hydrogen pressure in the temperature range of 598 to 773 K. The fractional occupancy of traps ranged from 0.14 to 0.05.