| Although the ScD2film has a relatively low neutron emission rate, it can reach steady-state yield more quickly than other deuterides, and it also has good thermal stability and does not produce crater formation on the target surface when bombarded by a deuterium beam. So, scandium deuteride is under consideration to be a very important candidate as the D(T) ion source or target material in the neutron generator.In order to fully explore the high hydrogen storage property of scandium, the following scientific problems need to be solved systemically:thermodynamic and kinetic property of deuterium absorption in scandium system, the growth mechanism of Sc film, the influence of microstructure and deuterated processing on the deuterium absorption. However, most of the researches have been stayed on the thermodynamic property of hydrogen absorption in Sc, fundamental researches of the properties of deuterium absorption, the growth mechanism of Sc film, as well as the relationship between the film microstructure and the deuterium absorption are still missing. In this study, the bulk Sc and Sc film were taken as the main research objects to systematically discuss the thermodynamic and kinetic property of bulk Sc, the growth character of Sc film, and the influence of microstructure on the deuterium absorption in Sc film.(1) Thermodynamics and kinetics of deuterium absorption in scandium systemThermodynamic and kinetic characteristics of the Sc-D system are investigated as a complement to the earlier studies of the Sc-H system. A Sieverts apparatus is employed to conduct the measurements. The Sc-D system is characterized by two phase regions:the metal-rich and the deuteride phases. The equilibrium plateau relationships in the two-phase regions are determined from the Van’t Hoff plots and found to be: lnP(Pa)=(-16374.4±188.88)/T+(23.56±0.18)The enthalpy and entropy of reaction are calculated to be (-136.14±1.57) kJ·mol-1D2and (-100.06±1.50) J·mol-1·K-1D2, respectively. From the relationship of In[(P0-Pf)/(P-Pf)] and time t, the reaction of the Sc-D system is confirmed to be a first-order reaction in the temperature range of650-800℃. The temperature has a negative effect on the reaction rate (ka), which decreases from0.0717s-1to0.0130s-1with the temperature increasing from650℃to800℃. In addition, a minus activation energy of (-93.87±6.22) kJ-mol-1is acquired. However, once increasing temperature up to850℃, the relationship of ln[(P0-Pf)/(P-Pf)] and time t firstly satisfies an exponential equation of y=-0.5471exp(-x/9.1879)+0.00272. After50 s. it begins fitting a linear equation again, indicating the various reaction mechanisms.(2) Research on the growth mechanism of Sc film experimentally and theoreticallyThe growth of Sc films has been proved to be Volmer-Weber mode. The microstructural changes of scandium films with the substrate temperatures are consistent with the reported structure-zone models. Such columnars perpendicular to the substrate are caused by both the incidence direction of deposition atoms and the minimization of the surface free energy in [002] direction. Sc films. deposited on Mo substrate with the highest temperature650℃. as well as deposited on Si substrate at the temperature not higher than550℃, show a (002) preferred orientation. For the films on Si substrates, interface reaction has been detected at the temperature as low as200℃. and the product is monosilicide ScSi. While with the temperature increasing to650℃, Sc film reacts with the substrate Si completely, with the product ScSi and ScSi2. In addition, Sc reacts with Si to form ScSi first, then ScSi2-Compared with the effect of substrate temperature, changes in deposition rates make no influence on the phase structure of films and bring pronouced changes in the surface roughness and the peak intensities. For Sc films on Mo substrates, the films show smoother surfaces at the higher deposition rate, while too large deposition rate is detrimental to the growth of (002) texture. The grain sizes first increase with the increasing deposition rate and then decrease with it. The largest grain size (-246nm) is obtained at the deposition rate of5nm/s. For Sc films on Si(111) substrates, the effects of deposition rate on the morphologies and structures of Sc films are weak at lower temperature. While, at the higher temperature of650℃, we find with increasing the deposition rate, the micro cracks appear and the number of the silicide peaks increases.(3) Influence of microstructure of Sc film on the absorption propertyThe vacuum degree in the reaction chamber makes important influence on the absorption capacity of Sc films. The atomic ratios△(D/Sc) in Sc films with fine grains, pores and defects are usually lower than those in compact Sc films with less defects and larger grains. Furthermore, improvement of the vacuum degree is advantage to increase the absorption capacity.At a high vacuum degree of5.4×10-4Pa, the difference in absorption capacity caused by various microstructures is slight. Though the atomic ratios△(D/Sc) are comparable, their microstructure, grain size, growth orientation, and so on show each other’s character. The ScD2films retain the column structure of the original Sc films. The stronger (002) texture of Sc-metal film results in the stronger (111) texture of deuteride film, indicating that the deuteride nuclei might inherit their texture from the initial metal grains’structure. Unexpectedly, an inverse correlation appears that larger Sc-metal grains lead to smaller deuteride grains due to the improved deuterium diffusion kinetics in the more defective and fine-grained metal structure. After deuterium absorption, there are many microcracks in Sc films with thickness of28kA. while films with thickness of8kA are still unbroken.In Sc films. D atoms locate not only in interstitial sites, but also in the pores, grain boundaries and defects. Between the above interstitial mechanism and the capture mechanism, the capture mechanism is dominant in the Sc films, causing thickness increasing obviously. There are three stages for the relation between Pressure and Temperature (P-T) during the deuterium absorption by Sc films:physical adsorption(I), deuterium absorption in Sc films(II), volume shrinkage(III). And the second stage is divided into two stages:to destory the oxide layer(II-1), and fast diffusion of D in Sc films(II-2) acting as the rate-determining step.(4) Influence of substrate temperature on hardness and elastic modulusThe grain size, film densification, grain orientation and degree of oxidation together affect the film hardness and elastic modulus. Among them, the grain size is the key factor. |
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