First-principles Study On Fluorination Behavior Of Cerium-based Rare Earth Polishing Powder

The importance of rare earth as a strategic resource is comparable to oil. There is the world's largest Bayan Obo light rare earth ore in Baotou, Inner Mongolia.The mine contains about 50% cerium. As raw materials and processing base of cerium, a number of cerium-based rare earth polishing powder enterprises have appeared in recent years in Baotou, which opens up a new field of light rare earth materials application. Due to its high surface activity, easy agglomeration and irregular particle morphology, CeO₂ can not be directly used for precision polishing. At present, most of the commercially available cerium-based polishing powder is treated with fluoride. The process is that the cerium oxide precursor is added a fluorinating agent, such as hydrofluoric acid, fluorosilicic acid, ammonium fluoride or sodium fluoride. Experiments show that the morphology of fluorinated cerium-based polishing powder is changed from rod-like or sheet-like to spherical shape, the agglomeration is significantly weakened, and the surface finish of the polished part is improved. In order to study the mechanism of fluorine in cerium-based rare earth polishing powder,the first-principles method was used to study the steady-state structure offluorinated cerium oxide and its surface, the effect of fluorinated cerium crystal on the mechanical polishing performance, the influence of fluorine on the morphology of cerium oxide polishing powder, the controlling factors of fluorination process of cerium oxide, and the role of fluorine in chemical polishing of silica glass. From the perspective of electronic structure, the intrinsic reason for the influence of fluorination on macroscopic mechanical properties of the bulk and the surface energy of the surface were analyzed. The optimum fluorine content of cerium-based composite polishing powder is given.The purpose of this paper is to provide the theoretical basis for the development of high-level cerium-based polishing powder. The main work are as follows:1. Analysis of mechanical properties and electronic structure of CeO₂ under atmospheric pressure and external pressure. The mechanical stability of CeO₂ under normal pressure was determined by calculating the binding energy and elastic constant. The bulk modulus, shear modulus, Young's modulus and Vickers hardness of CeO₂ were calculated and compared with the experimental results. The mechanical and dynamic stability of fluorite structure CeO₂ was analyzed when the external pressure is lower than 30 GPa. The changes of elastic modulus, hardness, toughness and anisotropy with pressure were analyzed. The Hubbard U parameter is introduced to modify the strong correlation effect of the Ce4f layer electrons. The state density and charge density distribution of CeO₂ were analyzed. The change of electronic density of states distribution, and bonding of Ce ions and O ions was investigated under external pressure. The reason for the change of mechanical properties was deeply understood.2. Effect of fluorination on mechanical properties and electronic structure of CeO₂. The total energies of fluorine-doped CeO₂ with three possible positions were investigated, and the doping formation energies were calculated to determine the steady-state structure of fluorine modified CeO₂. The elastic properties of CeO_2-xF_x with six kinds of fluorine content were calculated. The variation of elastic modulus, hardness, toughness and anisotropy with the fluorine content was analyzed to determine the fluorine content to maintain mechanical polishing performance. The density of States and charge density distribution of four fluorine-doped systems (CeO_2-xF_x) were calculated. The interaction of partial density of states of F ions with Ce and O ions was analyzed.The changes of elastic mechanical properties and electronic structure of CeO0.75F0.25 were studied under the external pressure of polishing.3. Fluorination of CeO₂ and its influence on crystal morphology. The CeO₂?111? surface, CeO₂ ?100? and ?110? reconstructed surfaces were constructed,and their total energies were calculated by relaxation. The surface energies of three surfaces were compared, and compared with the calculated values and experimental values of other literatures. The potential energy surfaces of F atom adsorbed on three surfaces were calculated to determine the stable adsorption configurations. The surface energies of three surfaces after adsorbing F atom were compared to predict the morphology of the polishing grains. The effect of surface energy difference between ?111? surface and ?100? surface on the crystal shape was studied under different F atoms coverage. The oxygen vacancy formation energies of surface and sub-surface were calculated. The migration activation energies of F atom from the ?100? surface into the surface oxygen vacancy, and further into the sub-surface oxygen vacancy were studied to determine the control factors of F atom migration. The electronic structure of CeO0.75F0.25 ?111? and ?100? surfaces were studied to determine the effect of fluorine on the Ce-O bonding, total energy and surface energy.4. Polishing reaction process of fluorinated cerium-based polishing powder.Two grinding surfaces of CeO₂ ?111? and CeO1.75F0.25 ?100? were constructed.The adsorption energies of SiO₂ grain on the two surfaces were calculated to determine the stable initial state configuration. The reaction process of two surfaces grinding SiO₂ grain was studied in detail. The evolution of bond formation and bond breaking of Si atoms was analyzed. The role of fluorine was judged in chemical polishing.