| Tritium is an essential element for future thermonuclear energy production and generally chosen considers a storage technology based on metal hydrides with high hydrogen storage density and low equilibrium pressure. Titanium was wiedly used due to its high hydrogen storage and low equilibrium pressure. During the early storage in metal tritides, the 3He atoms generated by tritium decay are trapped in the lattice and form bubbles. However, whenever a critical 3He concentration is reached in the material,3He atoms are released in the storage vessel at a rate equal to or even higher than the generation rate. This phenomenon is known as the"accelerated release stage". To understand the evolution of 3He atoms in metal tritides during aging, vacuum thermal helium desorption method was used to investigate the He behavior in titanium deuteride-tritide films in whole life period in this paper.Experiments were carried out for thermal desorption measurement of He from metal material with linear temperature increase in vacuum condition, and He measurement were performed on line by a Quadrupole Mass Spectrometer (QMS). Thermal helium desorption spectra (THDS) was obtained by He partial pressure vs temperature. THDS can be used to research evolutive behavior of He in aging metal tritides due to its information of He atoms state in materials.The heating rate. which is the important parameter of vacuum thermal desorption method was discussed in this paper. According to the influence of THDS by different heating rate, the optimized heating rate (60 K·min-1～120K·min-1) was determined. This work provided a technical guarantee for obtainment of actual states of He atoms in metal possibly.After obtain, THDS require to be explained accurately. By connecting the THDS with Positron Annihilation Spectra (PAS), X Ray Diffraction (XRD) and Scanning Electron Microscope (SEM), the relation between THDS and bondage states of He atoms was established. The results indicated that the process of tritium desorption and titanium convert to titanium oxide does not result in He's rapid desorption. And He desorption at low temperature slowly can be attributed to mobile free He atoms in lattice, and He desorption at moderate temperature relate to the He bubble in large size defects such as grain boundary and He desorption at high temperature relate to the He bubble in grain was determined preliminarily. Moreover, two mathematics models were used to invetisgated free He atoms diffusion and He desorption from He cluster and He bubble respectively due to its different desorption mechanism. The William's flat diffusion model was accord with free He atoms diffusion. But for He desorption from He cluster and He bubble, we deduced a new model accordance with fist-order desorption mechanism and He partial pressure.Based on the optimization of vacuum thermal desorption method and the relevance between THDS and PAS, XRD, SEM, the thermal He desorption behavior of titanium deuteride-tritide films was investigated. The results indicated that there are four He states in titanium deuteride-tritide films:free He atoms near surface, free He atoms in lattice, He bubbles at grain boundary and He bubbles in grain. The concentration of free He in deuteride-tritide titanium films keeps constant after nucleation. More than 90% He atoms stay in grain in the form of He bubbles before 3Hegen/Ti=0.15, however, the He bubble network formed gradually after 3Hegen/Ti=0.15.Due to above work, we understood the evolution of He atoms in aged titanium deuteride-tritide films, and pointed out the aging discipline of titanium tritide films. All the results supplied science cognize for the studies and application of metal tritieds. |
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