Preparation,Formation Mechanism And Regulation Of Ultrafine B,LaB₆ And CeB₆ Powders By Salt-assisted Combustion Synthesis

Superfine or nano powders of boron?B?and metal borides show promising application value in the aerospace,defense,military,chemical and metallurgical fields,and play an increasingly important role,because of their unique properties,such as high specific surface area,high interfacial atom and molecular chemical activity and good adsorption properties and so on.The performances of these powders mainly depend on theirs size and purity.The particle size controlled in small range as far as possible,the purity of powder improved,is of great significance to further increase their performances.Since the powder size,size distribution and the purity are associated with the preparation methods,the preparation methods will inevitably affect the product performances.Self-propagating high temperature synthesis?SHS?technique,because of its high reaction speed,high reaction temperature,quick cooling speed,simple equipment,low energy consumption and so on,attracts much attention in the synthesis of new materials.But adiabatic combustion temperature of SHS is high,the reaction has a long time at high temperature,easy to cause side effects,reduce the product purity,and cause particles sintering and growing up together.To reduce the adiabatic combustion temperature of SHS process,diluent was used in this paper,B powders with high quality were prepared by salt-assissed combustion synthesis on a large scale.Superfine and high purity CeB₆ and LaB₆ powders were obtained by controlling preparation parameters.By scanning electron microscopy?SEM?,energy spectrum?EDS?and transmission electron microscopy?TEM?,X-ray diffraction?XRD?,differential scanning calorimetry?DSC?,specific surface area analyzer and X-ray photoelectron spectroscopy?XPS?,inductively coupled plasma emission spectrometer?ICP?and so on,the powders prepared were characterized,and the formation mechanism of B,CeB₆ and LaB₆ powders was discussed by salt-assissited combustion synthesis.The influence of the raw materials amount,the diluent amount,the molding pressure and Mg excess on product morphology,size,size distribution and purity were studied.Main contents and results are as follows:?1?.Adding diluent NaCl to B₂O₃-Mg system,salt-assissited combustion synthesis was used to prepare B powders with purity more than 96wt.%and average particle size in the sub-micron or micron on a large scale,and the B powders prepared are amorphous.Specific surface area of B powders is 27.62 m/g,which indicates that B powders are highly active.From the DSC curve can be concluded that the reaction mechanism is liquid-liquid reaction.The average particle size and the purity of prepared B powders are 1.13?m and 98.22wt.%with adding 30wt.%NaCl.The purity of 98.22wt.%is the highest among reported results by magnesium thethermal method and is close to purity by physical vapor deposition?PVD?.When the amount of NaCl are 40wt.%and 50wt.%,the average particle size is 0.85?m and 0.76?m,respectively,and the purity of two samples are more than 97wt.%.These indicators are up to America SB Boron 95 standards.The average particle size 0.76?m is the better value of the granularity which can be achieved by magnesium thethermal method.In this paper,adding strong reductant NaBH4 to the system of B₂O₃-Mg,B powders with the average particle size of 0.69?m and purity of 98.12wt.%were prepared.These indicators reached America SB Boron95 requirement for B powders,So B powders prepared by this method can be used as solid rocket propellant component.NaBH4 is a strong reductant.When it was added into the B₂O₃-Mg-NaCl system,it can reduce the temperature of the combustion synthesis and accelerate the reaction speed.It can as well make the reaction fully and improve the purity of B powders.Furthermore,when the reaction rate was accelerated,the time of high temperature state would last short,it avoided the side reaction under the high temperature,and the purity of the product can be further improved.?2?.In this paper,adding NaCl to Mg-B₂O₃-La2O3 system,LaB₆ powdes were prepared by salt-assissited combustion synthesis.With NaCl content increasing,the sample particles are more and more small,the product purity is increased.LaB₆ prepared is cubic,purity is more than 96wt.%,and the grain size is in the sub-micron or micron.The minimum average particle size in all samples is 0.96?m,the purity is 98.24wt.%.Compared with the literature,in the current preparation methods of high purity superfine LaB₆,salt-assisted combustion synthesis has the most advantage.NaCl is an ideal diluent,which can effectively absorb reaction heat by sensible heat and latent heat,and reduce the temperature of the combustion synthesis.In addition,the melted NaCl can be used as a protective layer to prevent agglomeration,sintering and oxidation of the particles.?3?.In order to increase the reaction temperature,to meet the dynamic conditions of self propagating combustion of CeO₂-B₂O₃-Mg system,to improve growing environment of the CeB₆ crystal particles,the KClO₃ heating agent must be added into the system.KClO₃ is a heating agent and a diluent when it is decomposed.In this paper,taking CeO₂-B₂O₃-Mg as raw materials,adding KClO₃ as heating agent and diluent,the superfine CeB₆ powders with high purity were prepared by salt-assissited combustion synthesis.Prepared CeB₆ is cubic,whose purity is more than 96wt.%and particle size is between sub-micron or micron.In all samples,the purity of two samples is more than 99wt.%.In the literature,in addition to the physical vapor deposition?PVD?,this preparation method can achieve the highest purity.