The Preparation Of Refractory And Rare Earth Metal Boride Powders

Most metallic elements can combine with elemental boron to form borides.Among many borides,refractory metal borides and rare earth metal borides have been concerned widely since their high melting point,high hardness,good electrical conductivity,stable chemical properties and special electron emission characteristics.Refractory metal borides have been processed into thermalprotective material,ceramic armor,nuclear ceramic,special refractory material,high temperature electric heating element,and high temperature electrode.And these materials have entered into industrialized popularization application sting.Rare earth metal borides are used in many instruments with electron beam as high brightness electron cathode.In addition,the applications of both of these borides in the fields of photothermal conversion,energy storage and catalysis are also the hot study point.Metallic boride materials are often processed through sintering,impregnation,spraying,which makes boride powders become the key raw material for manufacturing such materials.Using high purity and ultra-fine boride powder will be beneficial to obtain high performance materials.At present,the synthesis methods of metallic boride powders are often unable to take into account low cost of manufacture,controllability of synthetic process,and quality of products,which severely restricts the development and application of metallic boride materials.The objectives of this dissertation were to prepare high quality refractory metal(Ti,Zr,Hf,V,Nb and Ta)and rare earth(La and Ce)metal boride powders,and considered both raw material cost and process feasibility.In this work,by comparing many synthesis methods proposed in the past and summarizing the thermodynamic properties of borides,the raw material combination of B4C(boron source)and metallic Ca/Al(decarbonizer)was innovatively proposed.By using this combination,pure borides powders could be prepared under mild reaction conditions.This work focused on the process of high temperature reaction and the mechanism of particle size control.In addition,the pilot test was carried out,and refractory metal boride powders were prepared by "carbothermic reduction +boronizing in Ca melt" in kilogram scale on the basis of the researches of laboratory.The research of this dissertation provided a set of reasonable technology for the industrial preparation of high-quality metal boride powder,which would provide a guarantee of raw material for manufacturing metal boride materials.The research results are as follows:1.After summarizing the thermodynamic data of many borides,the standard Gibbs free energies of formation of different borides were plotted in a same diagram,and the boron-potential diagram was obtained.The boron-potential diagram showed the chemical binding capability of every element with boron.To analysis oxygen-potential diagram,carbon-potential diagram and boron-potential diagram,thermodynamic basis of designing new synthesis method was provided.2.A method of preparing metal borides by reducing refractory metal oxides with CaB6 was proposed.CaB6 was a reductive substance with high boron potential.However,commercial CaB6 had a high carbon content and was not suitable for the preparing refractory metal borides.In this study,low carbon and pure phase CaB6 powder was synthesized by reducing B4C with metallic Ca.At the same time,it was found that the size of CaB6 particles was inherited from that of B4C.Micron pure phase diboride powders were synthesized by reducing refractory metal(Ti,Zr,Hf,V,Nb,Ta and Cr)oxides with CaB6.Because the by-product CaO-B2O3 molten salt could promote the growth of diboride product,the particle size of diboride prepared by this method would not be further refined.This method was characterized by simple reaction and mild conditions,but it was only suitable for preparing of refractory metal diborides in micron scale.3.A new method for preparing refractory metal diborides by carbothermal reduction and boronizing in Ca melt was proposed.Firstly,the excess carbon black was used to reduce refractory metal oxides to complete the deoxidation,and the refractory metal carbide intermediate product was prepared.Then,the intermediate product was boronized in the Ca melt with B4C as boron source.At the same time,the Ca melt could convert C into CaC2(removed by leaching),so as to diborides were obtained.This method solves the shortcoming of traditional carbothermal reduction mothed that reactant and boron source could not be accurately dosing and the product had high carbon content.The diborides of Ti,Zr,Hf,V,Nb and Ta were prepared by this method.And a series of ZrB2-ZrC composite powders were obtained by controlling the addition amount of B4C,which realized the precise composition control of carbide and boride composite powders.It was found that the particle size of borides prepared by "carbothermal reduction+boronizing in Ca"was similar to that of raw materials oxides,and the nano ZrB2 powder was prepared by using this rule.4.The pilot production of "carbothermal reduction+boronizing in Ca"method was carried out,and TiB2 and ZrB2 powder was prepared in kilogram scale.The products of TiiB2 and ZrB2 were single-phase,and the particle morphologies of the products was uniform.The pilot production results showed that the process of" carbothermal reduction+boronizing in Ca " was easy to implement,mild and controllable,and this method had the conditions of large-scale production.5.Using Al as reducing agent and decarburizing agent,micron LaB6,CeB6 and La1-xCexB6 solid solution powders was prepared by rare earth oxides and B4C reacting with Al.The study of high temperature reaction showed that hexaborides were formed in large quantities at 1100～1300 K.but in order to ensure that the generated α-Al2O3 and Al4C3 further combine to form easily leachable Al4O4C,the reduction temperature should be increased to 1773 K.In addition,La1-xCexB6 solid solution powders was dissolved in high temperature Al flux,and a series of millimeter-sized hexaboride single crystals were obtained by slow cooling.The compositions of solid solution single crystals were uniform.This method is characterized by easy process control,low cost of raw materials and simple equipment requirements.The preparation cost of rare earth hexaborides could be significantly reduced by this process.