First Principles Calculations Of The Ground State And Defect Properties Of Lithium-based Ceramics And Experimental Study Of The Deuterium Behavior In The Lithium Silicate

All human beings worldwide are confronting with so great challenges of environmental pollution and energy source crisis that almost all countries are focusing upon developing the novel technologies of energy sources. Numerous people have accepted the viewpoint that fusion energy is the main future energy type due to its'special natural characters of safety, clean, abundant sources, few radiation wastes and so forth. China, as a member of the International Thermo-nuclear Experimental Reactor (ITER) program, the scientific researches and technological developments related to ITER program must be conducted as soon as possible. Among theses R&D, the tritium, one of the most important fuels in D-T type fusion reactor, must be self-sufficient within fusion reactor to reach the balance of "supplement and consumption". Accordingly, the tritium-breeding blanket becomes as one essential functional part. The helium cooled solid pebble bed (HCSPB) is the first choice in China test blanket module (TBM) designs. In order to extract tritium efficiently from the TBM and reduce the retentions of tritium in it, the tritium diffusion and release behaviors in the key functional material of solid tritium breeder, i.e. lithium-based ceramics, naturally become as the very important basic science problem. Present thesis investigated these key scientific problems within density functional theory (DFT) frame and analogy experiments. The ground state properties of four solid tritium-breeding candidates, i.e. Li2O, Li2SiO3, LiSiO4 and Li2TiO3, were studied using the first principle calculations with pseudopotential plane wave (PPW) method based on the DFT. We obtained the ground state electronic properties, defects properties and the microscopic behaviors of tritium in these lithium-based ceramic bulks. The pure Li2SiO3 and Li4SiO4 powders were prepared using solid-state reaction method, and characterized by thermal-gravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electronic microscope (SEM). The optimal synthesis conditions for Li2SiO3 and Li4SiO4 were figured out. The behaviors of deuterium introduced in Li4SiO4 by ion implantation technology were studied using XRD, SEM and thermal desorption spectrum (TDS). It was briefly discussed that the effects of deuterium ion implantation on the phase structures and microstructures of Li4SiO4. The deuterium location sites and diffusion mechanism were also discussed. In the ground state properties of lithium based ceramic bulks, the calculated electronic energy band structures of these four ceramics show that they are typical insulators with the band gap of 4.988 eV,5.185 eV,5.529 eV and 3.371 eV for Li2O,Li2SiO3,Li4SiO4 and Li2TiO3, respectively. In Li2SiO3 and Li4SiO4, their crystalline frames are in the form of SiO4 tetrahedra with Si-O covalent bond; In Li2TiO3, the TiO6 octahedra form the crystal frame through sharing one edge of the octahedra, and the Ti-0 is covalent bonding. These covalent bonds within above three crystals result in higher lattice binding energy than in Li2O. Specially, there are two types of oxygen atoms which are bridging oxygen atoms (BOs) and non-bridging oxygen atoms (NBOs) existing in Li2Si03 crystal. The effective ion valence states, Si-O bond length, and the overlap populations between Si-O bonds have obvious differences between the BOs and NBOs. The overlap populations between Li and O atoms in these four ceramics show that it is typical ionic bonding for Li-O bonds.In the ground state properties of lithium ionic vacancies (VLi) of Li2O, Li2SiO3, Li4SiO4 and Li2Ti03, the formation energy of single VLi is 35.711 eV,7.664 eV,8.635 eV and 6.89 eV, respectively. It shows that it is not favorable to form the lithium ionic vacancy defects in view of the fact that the formation energy has positive value. The calculations show that the serious lattice distortion happens in Li2O while a single VLi existing; however, just trifling distortion phenomenon is observed in the other three ceramics. The VLi exhibits so weak ability of trapping electrons in crystal that it can hardly change the electronic band structures and the distributions of charge density.In the ground state properties of oxygen ionic vacancies (VO) in Li2O and Li2SiO3 crystals, the calculated formation energy of Vo is 10.708 eV and 22.665 eV for Li2O and NBO of Li2Si03, respectively. This demonstrates that it is difficult to form a single oxygen ionic vacancy in Li2O and Li2SiO3. Significant lattice distortions occur when a single Vo exists in Li2O and Li2Si03 crystals, and especially for the locations and bond length of the ions neighboring with the site of VO.The oxygen ionic vacancy has relative strong ability of trapping electrons and these trapped electrons exhibit some local properties. The system electronic band structures have been changed that partial Si 3p orbital of Li2SiO3 and partial Li 2s orbital of Li2O pass through the Fermi level. The original top of valence bands in perfect crystals have been moved about 3 eV and 2 eV to the deeper valence band.The ground state properties of the interstitial tritium defects (Tin) have been studied using first principle calculations within DFT. The results show that the Tin preferentially occupy the center sites of Li2O and Li4SiO4 and the lithium atoms layer of Li2TiO3, however, in Li2Si03 it will be favorable to occupy the interstitial site of (0.500,0.003,0.658) near the (010) plane. The calculated formation energy of interstitial tritium defect EF(Tin) is-0.606 eV,-26.269 eV and -0.402 eV for that in Li2O, Li2SiO3 and the lithium atoms layer of Li2TiO3, respectively. It is favorable to form Tin defects due to their negative formation energy. In Li4SiO4 and the Ti atoms layer of Li2TiO3, nevertheless, the Tin defects are difficult to form because the formation energy of Tin defects is positive values of 6.250 eV and 1.098 eV, respectively. The calculated electronic energy band gaps are 0.614 eV,3.731 eV,3.497 eV and 2.682 eV for the Li2O,LiSiO3,Li4SiO4 and Li2TiO3 system with a single interstitial tritium defect. The band gaps of these four lithium ceramics reduce and the Li2O has the tendency to transform into semi-conductor when a Tin defect exists. The interstitial tritium exhibits the ability of attraction of electrons.As for the ground state properties of another important defect of substitutional trittium (TLi), the calculated formation energy of TLi is 6.383 eV,6.503 eV,-6.646 eV,-5.917 eV,-6.077 eV and -6.043 eV for Li2O, Li2SiO3, oxygen atoms player of Li4SiO4, silicon atoms layer of Li4SiO4, lithium atoms layer of Li2TiO3 and titanium atoms layer of Li2TiO3, respectively. It shows that it is difficult to form TLi in Li2O and Li2SiO3 crystals, but favorable in Li4SiO4 and Li2TiO3. The initial lithium ionic vacancy is not the stable site for tritium occupation in these four lithium-based ceramics. The tritium will always bond to the neighboring oxygen ion to form one T-O hydroxyl with bond length of about 1.0 (?). In the Li4SiO4 super structure, the substitutional tritium is favorable to occupy the lithium atoms layer. However, it has no obvious choice of occupation site between the lithium atoms layer and titanium atoms layer for the substitutional tritium defect in Li2TiO3. There are less effects of TLi on the band structures of these four ceramics and the band gaps are similar to those of perfect system.The lithium silicates were synthesized using the solid-state reaction (SSR) between Li2CO3 and amorphous silica. The solid-state reaction process was studied by the TGA technology and the as-prepared lithium silicates powders were characterized by XRD, SEM and BET specific surface area analysis. It is demonstrated that the SSR process can be divided into two steps of (Ⅰ) first formation of Li2SiO3 at the temperature range of 515℃-565℃and (Ⅱ) continuing formation of Li2SiO3 and accompanying with the transformation of Li2SiO3 into Li4Si04 at the temperature range of 565℃-754℃. In present thesis, the best synthesis condition of Li4SiO4 is that the Li/Si molar ratio is 4:1 and the calcinations temperature is 800℃, while for Li2SiO3 the best one is that the Li/Si molar ratio is 2:1 and the calcinations temperature is 900℃. However, Li4SiO4 will slowly decomposed into Li2SiO3 and Li2O when the calcinations temperature is high as 900℃. The tritium release behavior in lithium silicates was studied with analogy experiments of deuterium introduced by the ion implanting technology. It was obtained that the effects of the deuterium implantation on the surface microstructure and the phase composition of lithium silicates from the SEM and XRD results, and the implanted deuterium release characters using the thermal desorption spectrum method. The possible occupation sites of deuterium and the effects of deuterium ion irradiation on its' release behavior were initially discussed. The results show that the distribution depth of deuterium with implanting energy of 400 keV is about 3.0μm, and many complicated defects will produce in the Li4SiO4 bulk. The phase structure changes significantly. The original preferable orientation in (013) plane existed in sintering samples before ion implantation will be eliminated and it alters to the (-231) plane preferring orientation after ion implantation. The main occupation sites are still the substitutional deuterium that are bonding to the oxygen atoms and forming the D-O hydroxyl in system. The deuterium release process can be divided into five steps and the major deuterium release at the temperature range of 300℃-500℃.