Influence Of Impurities And Vacancies On Structure And Properties Of Tungsten Borides

In recent years,the transition mental borides?TMBs?have attracted extensive attention of the researchers,due to its outstanding physical and chemical properties.In addition to its simple synthetic conditions and can be synthesized under ambient pressure.So there has provoked a lot of theoretical and experimental research.The study shows that new type of superhard material is related to the covalent bond formed of light elements and unconcerned about transition metals.Therefore,the paper mainly studies that light elements and a variety of transition metals doped the transition metal borides thermodynamic and mechanical properties,which are investigated by first-principle based on density functional theory.The main research contents are as follows:?1?The thermodynamic and mechanical properties of transition metal triborides WB_3OsB₃?WB_3OsB₃ represents WB₃ in OsB₃ structure?and WB₃₁66₆u with doping light elements carbon or nitrogen are investigated by CASTEP code.The concentration is1.5%?2%?3%?4.25%?6.25%?8.25%?12.5%and 25%.Thermodynamic stability of compounds is predicted by the formation enthalpy.With the increase of doping concentration,the trend of formation enthalpy is gradually increasing.When mixed with concentration of 25%,leading to the structure of the thermodynamic instability,It may be caused by high concentration and lattice distortion.Among the considered compounds,WC_0.5B_2.5OsB₃-6 has the maximum bulk modulus 355GPa,WC_0.08B_2.926u-3has maximum shear modulus 258GPa and hardness 42GPa,Indicating that they may be high incompressibility materials,or potential superhard materials along a certain crystal.Then,we calculated thermodynamic and mechanical properties that W atoms in the center of WB₃-6u structure replaced by transition metal elements Mo?Tc?Hf?Ta?Re?Os and Ir.The concentration of 0.52%?1%?2%?4.2%?8.3%and 12.5%.Among them,the addition of Ta formed three-element compound W_0.8333Ta_0.1667B₃ of hardness greater than the undoped structure,and the hardness value is 39GPa,which is a potential superhard material.?2?Thethermodynamics and mechanical properties of WB₄-2u with doping?Mo,Tc,Hf,Ta,Re,Os,Ir,Pt?were calculated based on the first principle of density functional theory.The concentration is 1.25%,2.5%and 10%.Indicating the trend of data is increase and then decrease.Among then,with 1.25%of Re elements,the shear modulus increases more,which is 252GPa,and the hardness reach35GPa.When adding 2.5%of Ta elements,the shear modulus is 248GPa,the hardness is up to 35GPa,and is followed by Re elements.When the concentration is 10%,the thermodynamic properties of W_0.5Ta_0.5B₄-2u structure are most stable,which may be a good antioxidant material.?3?The lattice parameters,formation and elastic constants of the TMBs containing boron vacancy were calculated by the first principle.The thermodynamic and mechanical properties of the structure are analyzed.The main research structures areWB₃₆u?WB_3OsB₃ and WB₄-2u,The supercells of 2?4?8?9 multiples are set up.It is indicated that with B vacancy structures of thermodynamicproperties are greatly linear reduced,The enthalpy formation of WB_2.5OsB₃?WB_2OsB₃ and WB_3.5-2u are greater than zero,and thermodynamic is unstable.There are small increase in mechanical properties of WB₄-2u with vacancy,althoughthe bulk modulus and shear modulus are gradually reduced of WB_3.945-2u?WB_3.875-2u,according to the relationship between bulk modulus and shear modulus,the hardness is improved slightly.The hardness is about 30GPa.For WB_3OsB₃with vacancy,the C₄₄<0,C₅₅<0 for WB_2.5OsB₃ and WB_2OsB₃do not meet the criterion of mechanical stability,which is the same as that WB_3.75-2u and WB_3.5-2u of WB₄-2u do not meet the criterion of mechanical stability,indicating that they are thermodynamically unstable.