Synthesis And Properties Of Me-B(Me=Cr?V?Fe) Powders Prepared By The Molten Salt Method

Metal borides?Me-B?have been applied widely in many areas due to their high melting point,high hardness,good chemical and thermal stability,excellent corrosion resistance and other characteristics.At present,metal borides is normally synthesized at high temperatures with large particle sizes,single morphology and low purity.In this paper,Cr B₂,Cr B,VB₂,Fe B,and Fe₂B powders were synthesized successfully at low temperatures by introducing molten Na Cl/KCl in the synthesis process of borothermal reduction or elemental reaction.Also,some influencing factors in the synthesis process and properties of as-synthesized powders were investigated.The major works and results are summarized as follows.Firstly,CrB₂ and Cr B powders were synthesized by borothermal reduction in molten salts with Cr₂O₃ and amorphous B powders as raw materials.The lowest synthesis temperature was only 800 ^oC.The amount of molten salt had a great influence on the phase composition of as-synthesized powders.Too small and too much molten salt caused impurity phase in the products.When the weight of the molten salt was 10 times the reactants,the obtained Cr B₂ or Cr B powders were the purest.The highest specific surface area of as-synthesized Cr B₂ powders was 11.09 m²/g and the corresponding particle size was 104 nm.The higherst specific surface area of as-synthesized Cr B powders was 22.18 m²/g and the corresponding particle size was 44 nm.As Cr₂O₃ is more easily reduced to Cr B₂ at higher temperatures,the oxygen content of Cr B₂ powders decreased gradually with increasing temperatures.The lowest oxygen content of as-synthesized Cr B₂ powders was 2.04 wt%.The oxygen content of Cr B powder was gradually increased with increasing temperatures with the lowest value of 3.23 wt%.Secondly,VB₂ powder was synthesized successfully by boro-thermal reduction on molten salt using NH₄VO₃ and amorphous B powder as raw materials.The addition of molten salt provided a flowing circumstance for the reaction,which accelerated the rate of mass transfer transport and reduced the synthesis temperature of the VB₂ powder from1000 ^oC to 900 ^oC.In addition,the grain growth of VB₂ particles was inhibited by the molten salt,which resulted in the particle size of less than 300 nm.Finally,Fe B and Fe₂B powders were synthesized by molten salt method using Fe and B powder as raw materials.Excessive Fe or B powders was beneficial to elemiante the mesophase impurities produced during the synthesis process,while the excess Fe or B could be removed by acid leaching or magnetic separation.The optimum Fe/B molar ratio for the synthesis of Fe B and Fe₂B powders were 1:2 and 2.5:1,respectively.The amount of molten salt has a great influence on the phase composition of as-synthesized Fe B powders.Too small or too much molten salt caused impurities in the products.While,the amount of molten salt has no obvious effect on the purity of as-synthesized Fe₂B powders.The morphology Fe B powder was short rod-like with adiameter and length of30-40 nm and 100-150 nm,respectively.When the morlar ratio of Fe/B was 2.5:1,the as-synthesized Fe₂B powders exhibited a regular plate-like morphology and the particle size was 20-40 nm.When the molar ratio of Fe/B was 4:1,the as-synthesized Fe₂B powders were nearly spherical morphology with an average diameter of less than 100 nm.The as-synthesized FeB and Fe₂B powders showed typical soft magnetic properties.The saturation magnetization and coercivity of Fe B were 36.4 emu/g and 15.5 k A/m,while the values for Fe₂B were 126.9 emu/g and 6.1 k A/m,respectively.Electrochemical tests show that the as-synthesized Fe B powder could be used as an electrode material,which exhibited typical pseudopolecular properties and good cycling properties.