Dynamic behavior of hydrogen isotopes in tungsten–carbon composite materials

Abstract

The thermal re-emission of hydrogen isotopes from WC layers on graphite, implanted with 5 keV H + 2 and D + 2 ion beams up to saturation at room temperature, has been studied by means of the elastic recoil detection (ERD) technique. It is found on isochronal annealing for 10 min that the re-emission takes place in three stages: the first stage occurs for retained fraction of 1.0–0.6, the second stage for retained fractions of 0.6–0.2 and the third stage below the retained fraction of 0.2, which is ascribed to re-emission from carbon segregated in the grain boundaries of the WC layers. It is also found on isothermal annealing that the concentration of retained hydrogen decreases rapidly in the beginning and hereafter very gradually with increasing the annealing time. The isothermal re-emission curves have been analyzed by taking into account the two trapping sites and the mass balance equations with two kinds of thermal detrapping ( Σ d 1 , Σ d 2 ), retrapping (Σ t) and local molecular recombination ( K 1) between free hydrogen atoms. The isotope difference of the rate constants for elementary processes determined by best fitting of the solution to the experimental re-emission profiles is discussed with relevance to the nature of two trapping sites.