Preparation And Properties Of Al, Ti-doped Li₄ SiO₄ 氚 Proliferation Agent

Fusion is a potential source of safe, non-carbon emitting and virtually limitless energy. The tritium breeding blanket, which acts as the breeding zone of tritium fuel and serves as the main thermal power conversion system, is one of the most important components of a fusion reactor. The breeding materials are important as one of the key functional materials in the breeding blanket. Increased versatility in the available properties of tritium breeding materials can be of benefit to the design of TBM. However, the related study in China is very scarce. The purpose of this dissertation is to obtain some new tritium breeding materials which has improved properties based on Li4SiO4.To improve the tritium release properties and mechanical properties, a design based on the point defect to obtain the solid solutions has been developed. Three elements have been chosen as the Silicon substituting element. Aluminum element which has a smaller charge has been used to form interstitial lithium as a result of charge compensation. Vanadium element which has a higher charge has been used to create Lithium vacancy as a result of charge compensation. Titanium element which has a same charge with a longer atomic radius has been used to change the size of ion transport channel.First principles study based on the density functional theory has been used to estimate the feasibility and calculate the properties of Li4SiO4 and doped Li4SiO4. The results show that the Al atom can incorporation into the Li4SiO4 structure as silicon substituting element. The crystalline structure expands while the lithium atom density improves as the Al concentration. The direct band gaps of Li4SiO4 and Li4+xSi1-xAlxO4 (x=1/14) are 5.07 eV and 4.78 eV, respectively. It means that they are insulators and the conductivity or tritium release property may be enhanced by Al addition. The specific heat at constant volume (Cv) of β-Li5AlO4 is bigger than γ-Li4SiO4 at the selected temperature range, so the thermal conductivity of Li5AlO4 doped Li4SiO4 (Li4+xSi1-xAlxO4) may be improved compared with Li4SiO4.Then, the solid state reaction method has been used to prepare the powder of Li4SiO4, Li5AlO4, Li4+xSi1-xAlxO4. The pebbles have been obtained used the freezing drying method which is developed by Institute of Nuclear Physics and Chemstry (one kind of the wet method). The high purity powder can be obtained by LiOH·H2O rather than Li2CO3, especially Li5AlO4. To synthesize the Al-doped Li4SiO4, the direct solid reaction method is better than the indirect solid reaction method. Compared to the melting method, the narrow size distribution of the pebbles prepared by the wet method is the most attractive advantage.The X-ray diffraction (XRD) and neutron diffraction (ND) have been used to identify the structure of Li4+xSi1-xAlxO4. SEM with EDS observations are employed to check the morphology and Si/Al ratio of the sample powders. UV-Vis is used to get the band gap of the samples. The thermal conductivity and heat specific heat are collected by Micro Flash Analyzer. The mechanical property of the pebbles is also obtained using the universal testing machine. The tritium release behavior of the Li4SiO4, lI5ALO4 and Li4+xSi1-xAlxO4 powder which are irradiated in fission reactors is studited by Out-of-pile annealing experiments. The results indicate that:the lithium atom density of Li4+xSi1-xAlxO4 (x=0,0.1,0.2,0.3) is 0.559、 0.569？0.577、0.587 g/cm3, respectively. The band gap of Li4+xSi1-xAlxO4(x=0.2) is 4.84 eV while the band gap of Li4SiO4 is 5.66 eV. Thermal conductivity of Li4+xSi1-xAlxO4 is significantly improved by Al-doped as it is expected.The mechanical property of Li4+xSi1-xAlxO4 is significantly improved by Al-doped as it is expected.The temperature of the main tritium release peak of Li4+xSi1-xAlxO4 is lower than Li4SiO4.Additionally, the Li4TiO4 doped Li4SiO4 has been obtained by solid reaction. The band gap of this materials is lower than Li4SiO4 and LisAlO4 doped Li4SiO4, so the temperature of tritium release of Li4TiO4 doped Li4SiO4 may be lower than Li4SiO4 and Li5AlO4 doped Li4SiO4.Aluminum and vanadium are chosen as silicon substituting elements since they possess different atomic charges than Si4+; aluminum has a smaller charge (3+), while vanadium has a larger charge (5+). Therefore, the lithium ion conductivity can be promoted by the formation of point defects as a result of charge compensation. Additionally, the AGf of the metal oxide of both elements are more negative than that of SiO2 (-856.3 kJ/mol).The aluminum and vanadium oxides have the following AGf values:-1582.3 and-1419.5 kJ/mol, respectively. Titanium is chosen as silicon substituting elements since it can change the ionic migration channel and its △Gf is also more negative than that of SiO2. Then the lithium ionconductivity and tritium performance are improved.