| Metal tritides have received increasing attention in recent years for applications as special functional materials in the experimental research of nuclear fusion technology. The demand for metal tritides is urgent. One of the most important outstanding problems of these materials is the3He release due to the natural decay of tritium, which directly affects the working life and storage life of tritium storage materials or related devices. Since the1950s, people have been actively seeking new tritium storage materials with high3He retention ability, but the progress is slow.Taking into account the highest3He retention ability of Pd among metallic simple substances, this paper puts forvard a new type of tritium storage system called ’Pd-M hybrid tritium storage system’which consists of Pd and the tritium storage material M(M=Ti, Zr, Sc) with low equilibrium tritium pressure from the point of view of the development of new technology. The general design principles of this system are:components in the system maintained its respective properties of tritium absorption and desorption, while the whole system will have the characteristics of both low equilibrium tritium pressure and powerful3He retention ability; and the system can work at near room temperature. This system provides an new idea to solve the "He release of tritium target materials using in neutron generator in the long-term storage.In order to realization of the design of Pd-M(M=Ti, Zr, Sc) hybrid tritium storage system, both of the two studies must be carried out:the study of the tritium absorption properties by M(M=Ti,Zr,Sc) at room temperature and the study of tritium transfer behavior from Pd tritide to metal M(M=Ti, Zr, Sc). In response to this demand, the paper choose deuterium as the simulated material of tritium, several relevant studies were carried out. These studies included: thermal stabilities of the surface passivation layers (SPL) of metal M(M=Ti,Zr,Sc) under ultra-high vacuum condition (UHV); and comparative studies of deuterium absorption properties of M(M=Ti,Zr,Sc) in the form of slices, powders and films at room temperature. On the basis of the above studies, deuterium transfer behavior from Pd deuteride to M(M=Ti, Zr, Sc) were investigated. Results of these related studies provide technical support for the design of this new tritium storage system, and are of positive significance to solve the JHe release problems of the tritium storage materials or related devices during long-term storage.(1) Thermal stabilities of SPL on metal M(M=Ti, Zr, Sc) under UHV were studied using X-ray Photoelectron Spectroscopy (XPS). The results show that the C, N and O species on the surface of M(M=Ti,Zr,Sc) reduce significantly or even disappear when heated to700℃in UHV, thus inducing an appearance of "active" surface with clean metal; while heated to lower than300℃the oxygen content on the surface will further increase and the corresponding carbides or nitrides are formed. Among these species, TiO, ZrO2, SC2O3and TiN decompose at600℃due to the underlying metal. TiC and ZrC decompose at700℃while Sc carbide can’t decompose even at700℃. The active surface of M(M=Ti, Zr, Sc) obtained at700℃are contaminated by oxygen again when cooled down to room temperature in the UHV. The active surface of M(M=Ti, Zr, Sc) with clean metal can be obtained by Ar+ion sputtering or heating to high temperatures over700℃in UHV. It is easier to get the active surface of M(M=Ti, Zr, Sc) at room temperature by Ar+ion sputtering.(2) The deuterium absorption properties of Ti, Zr and Sc slices at room temperature were studied by PVT method and hot-stage microscope technique. The results show that Sc slice deuterated at room temperature present an incubation time before the absorption became obvious. This stage is followed by a surface controlled process that changed gradually to a parabolic stage related to D atom diffusion through the SCD2layer into the bulk. The amount of deuterium absorption (D/Sc) is1.21for4h after the introduction of D2into the reactor and reaches the predetermined value of1.62for7h at the end of the D2absorption process. XRD confirms the formation of the5phase ScD2with the CaF2structure on the surface of the sample. In contrast to Sc, D2absorption rate of Ti or Zr slice at room temperature is very slow; the amount of deuterium absorption is no more than0.2(D/M, M=Ti, Zr) for Ih after the introduction of D2. Ti or Zr slice can continue to absorb D2by heating. During heating, the temperatures of the occurrence of the rapid deuterium absorption are550℃and480℃for Ti and Zr, respectively. The surface morphology of Ti slice changes strangely in the process of the rapid deuterium absorption during heating. The Ti slice chaps first from the boundary on which the stresses concentrate. Then the chapped rings advance to the central part of the sample quickly. The unique change in the sample morphology is related to the multi-step phase transition which happened in the rapid deuterium absorption process of Ti during heating.(3) The deuterium absorption rate of M(M=Ti, Zr, Sc) powder decreased with increasing temperature anomaly. Under the low initial pressure (9-25kPa), the fully activated Ti powder can quickly absorb deuterium near room temperature and the Zr powder and Sc powder can quickly absorb deuterium at high temperatures (>400℃). The negative temperature effects of deuterium absorption rate constants maybe caused by the shift of the corresponding reaction equilibrium.(4) The deuterium absorption behaviors of M(M=Ti, Zr, Sc) film at room temperature were studied using the PVT method. The Ti, Zr and Sc films on the Mo substrates were prepared by DC magnetron sputtering. The thicknesses of Ti, Zr and Sc films are6.94μm,3.69μm and4.20μm, respectively. Under low initial deuterium pressure(100to300Pa), Ti and Zr films can react with D2at room temperature whereas the Sc film deuterated at room temperature presented an incubation time like the Sc slice. The deuterium contents of M(M=Ti, Zr, Sc) films are0.62,0.23and0.68(DM, M=Ti, Zr, Sc) for2h after after the introduction of D2, respectively. Grazing incidence X-ray diffraction (GIXRD) patterns show that8-TiD2and5-ScD2were obviously formed on the surface layers of the Ti and Sc films at the end of the D2absorption experiments, whereas no obvious Zr deuteride was formed on the surface layer of the Zr film. The thicknesses of the deuteride layers formed on the surfaces of M(M=Ti,Zr,Sc) film are4.40μm,0.51μm and3.04μm, respectively, estimating’from the shell shrinking core model. The thicknesses of the deuteride layers on the surface of Ti and Sc films can meet the needs of the depth of tritium target, which indicates that the deuterium absorption by Ti and Sc films at room temperature are successful.(5) The kinetics curves of deuterium desorption from palladium deuteride were experimentally measured at temperature range of25～50℃. The results show that deuterium can be desorbed fast from palladium deuteride near room temperature. The desorption rate increased with the increasing temperature. The desorption kinetics are determined by the recombination of chemisorbed deuterium atoms at the surface of Pd. The activation energy of the reaction is30.02kJ·mol-1(6) Deuterium transfer behavior from Pd deuteride to the M(M=Ti,Zr,Sc) powder was investigated. The deuterium in Pd deuteride can be nearly completely absorbed by Ti powder in1.5h and by Sc powder in2h, respectively; and the vacuum pressure decreases to0.1Pa. The deuterium contents in Ti and Sc powder reach the desired value of1.7(D/M, M=Ti, Sc) after deuterium transfer experiments. The deuterium transfer rate from Pd deuteride to Zr powder is very slow. The system pressure is maintained at4.0kPa which is the equilibrium pressure of deuterium desorption from Pd deuteride at25℃. The deuterium transfer rate from palladium deuteride to M(M=Ti, Zr, Sc) powder at room temperature is controlled by the deuterium absorption rate of M(M=Ti, Zr, Sc) powder.(7) The deuterium transfer rate from Pd deuteride to the M(M=Ti, Zr, Sc) film is very slow at room temperature. The deuterium contents of Ti, Zr and Sc films are0.01D/Ti for lOh,0.30D/Zr for14h and0.55D/Sc for40h after the beginning of each experiment respectively. The GIXRD patterns show that SCD2was obviously formed on the surface layers of the Sc film at the end of the deuterium transfer experiments. Weak peaks of Zr deuteride appear on the XRD patterns of Zr film, while peaks of δ-TiD2are easily evident on the surface of Ti film. The thicknesses of the deuteride layers formed on the surface of M(M=Ti, Zr, Sc) film are0.03μm,0.57μm and0.89μm respectively, estimating from the shell shrinking core model. The deuterium absorption rate of M(M=Ti, Zr, Sc) film in the deuterium transfer experiment is slower than that of the separate system, and this phenomenon may be related to the surface condition of M(M=Ti, Zr, Sc). |
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