Thermodynamic Assessment Of MgO-B₂O₃ Pseudo-binary System

The MgO-B₂O₃ system belongs to the alkaline earth borate system,and the intermediate compounds in this system are functional materials that have been widely used as the reinforced material,friction material application because they can effectively reduce the friction coefficient and enhance the mechanical properties of materials.The magnesium borate have potential applications such as catalysts for the conversion of hydrocarbons,thermoluminescence materials for fluorescent discharge lamps,X-ray screen,electroconductive treating agents,wide band gap semiconductors,and reinforcing materials for plastics or aluminum/magnesium matrix alloys.Three thermal stability magnesium borate,MgB₄O₇?diborate?,Mg₂B₂O₅?pyroborate?and Mg₃B₂O₆?orthoborate?exist in the MgO-B₂O₃psuedobinary phase diagram which were constructed by Davis,Knight and Miyagawa.Various synthetic methods for the fabrication of magnesium borates have been reported thus far.In general,nanoscale structures such as tubes,wires,rods,belts,and whiskers have been the focus of intensive research for their exceptional physical properties and potential applications in nanodevice technology,and the synthetic methods contain high temperature molten salt method,high temperature and cosolvent method,microwave method,hydrothermal method,solvent hot method,sol-gel method,electrochemical synthesis method,chemical vapor deposition method and mechanical ball grinding.Magnesium borates have many excellent physics properties,and the raw materials are easy to get,but the shortage and scatter of phase equilibrium information and the thermodynamic data in MgO-B₂O₃ system cause difficulties to the performance study and process design of magnesium borates crystals materials.Therefore,we collected the information about the phase diagram of MgO-B₂O₃system in this work?including the liquidus and solidus,solid solubility ranges,solid transformation temperature,melting point and compositional ranges of the intermediate compounds,etc.?,thermal chemical data?including the formation enthalpy,entropy and gibbs free energy,etc.?and crystallography data?including lattice type,atomic placeholder and lattice constant,etc.?.Experimental datas from different researchers and different experimental methods have been comprehensive evaluated.The MgO-B2O3 pseudobinary system has been thermodynamically assessed by CALculation of PHAse Diagram?CALPHAD?technique using Thermo-Calc software.An ionic two-sublattice?I2SL?model,(Mg²⁺)_p(O^2-,BO₃^3-,B₄O₇^2-,B₃O_4.5)_Q,is proposed to describe the liquid phase in the system,whereas the Redlich-Kister expression is used to formulate its excess Gibbs energy.All the solid intermediate compounds have been treated as stoichiometric compounds and their Gibbs energy is modeled following Neumann-Kopp's rule.A set of self-consistent thermodynamic parameters are then obtained.The calculated phase diagram and thermodynamic properties agree well with the available phase diagram and experimental data in the literature.The critical evaluation and optimization of the MgO-B2O3 system reported in this study is also part of a wide research programaimed at the complete characterization of phase equilibria and thermodynamic properties of the ternary system B₂O₃-MgO-M_mO_n?M=transition metal or rare earth element?.