Synthesis Of Boron Nitride Powder In Molten Salt

Boron nitride?BN?is an extraordinary topic in the area of materials science due to its unique physical and chemical properties,such as high temperature stability,high thermal conductivity,high melting point and low dielectric constant.Owing to these properties,boron nitride can be widely used as lubricants,electrical insulators and refractories.However,the synthesis temperatures were high and same of the starting materials were environmentally unfriendly.So far a facile method to achieve large-scale production of h-BN is still desired.In the present work,an improved molten salt synthesis method was designed and used to synthesize hexagonal boron nitride?h-BN?and rhombohedral boron nitride?r-BN?powders.The effects of different kind of reactions,salt,moler ratio of the raw materials and reaction temperature on the synthesis of BN were investigated.The phase compositions and morphology of the final products were characterized by XRD,SEM,TEM,FTIR and XPS.The formation mechanism of BN in molten salt was proposed,and the connection between sodium and r-BN salt was studied.The conclusions can be drawn as follows:?1?H-BN powders can be synthesized at 800-1300 °C for 3 h by heating Na₂B₄O₇ and Mg in MgCl₂ molten salt.H-BN nanoplates with a diameter of 200-600 nm and a thickness of less than 30 nm were generated at 1200 °C when the mass ratio of salt?MgCl₂?/reactants?Na₂B₄O₇ and Mg?is 200-400 wt%.Stromatolite BN crystal can be synthesized at 1300 °C when the mass ratio of salt/reactants is 100-200 wt%.?2?A mixture of r-BN and t-BN could be obtained when heating a mixture of Mg and Na₂B₄O₇ at 1000 °C for 3h in KCl molten salt.At 1200 °C,some MgNB9 could be generated in the powders.A mixture of r-BN and t-BN could be obtained when heating a mixture of Mg and Na₂B₄O₇ at 800-1000 °C for 3 h in NaCl molten salt.At 1200 °C,when the moler ratio of Mg/Na₂B₄O₇ is 5-6:1 boron nitride powders were well dispersed.When the moler ratio of Mg/Na₂B₄O₇ is 7-8:1 boron nitride powders synthesized at 1200 °C were bad dispersed.The BN with a diameter of over 1 ?m were generated at 1300 °C in the sodium chloride and potassium chloride molten salt.?3?Only h-BN powders can be synthesized at 1000 °C for 3 h by heating B₂O₃ and Mg in NaCl molten salt.A mixture of r-BN and h-BN powders can be synthesized at 1000 °C for 3 h by heating Na₂B₄O₇ and Mg in NaCl molten salt.Thermodynamic software confirmed Na could be generated when heating Na₂B₄O₇ and Mg in NaCl molten salt.R-BN phase could be generated when heating t-BN?obtained in MgCl₂ molten salt?and Na at 800-1200 °C for 3 h.Results showed that the sodium played an important role in formation of r-BN.?4?Lamellar structure t-BN powders can be synthesized at 1000 °C for 3 h in NaCl-MgCl₂ and KCl-MgCl₂ molten salt medium.The low viscosity of molten salt can promoted the formation of lamellar structure BN.