Study On Preparation And Growth Mechanism Of Zirconium Diboride Powder With Controllable Granularity And Morphology

Zirconium diboride(ZrB₂)ultra-high temperature ceramics have become the most promising candidate material for components such as hypersonic aircraft thermal protection systems and rocket propulsion systems due to their high melting point,high thermal conductivity,excellent thermal shock performance,good oxidation and corrosion resistance.However,the performance of ZrB₂ ceramics largely depends on the quality of the raw material ZrB₂ powder.The powder synthesized by traditional methods,such as solid-phase method,cannot meet the performance requirements of precision equipment due to the disadvantages of large particle size and uneven morphology.Therefore,it is a practical need for the development of high-temperature ceramic materials to prepare ZrB₂ powder with controllable particle size and morphology that can be used in industrial production.In order to promote the application and research of ZrB₂ ceramic powder,this topic adopted the sol-gel cooperative carbothermic reduction method to prepare ZrB₂ powder.The controllability of the particle size and morphology of ZrB₂ ceramic powder was studied by controlling factors such as the ratio of raw materials,gel temperature,pyrolysis temperature,holding time and the use of dispersant.The purpose is to study the specific reaction process in the process of gel-sol and carbothermal reduction,and to master the control mechanism of the key factors of particle size and morphology of ZrB₂ ceramic powder,so as to provide ZrB₂ powder with high purity,small particle size and uniform morphology for the actual production of ZrB₂ultra-high temperature ceramics.The main experimental results of this subject are as follows:(1)Using boric acid,sorbitol and zirconium n-propanol as the experimental boron,carbon and zirconium sources,respectively.ZrB₂ powder with single crystal phase and fine particle size was prepared by the sol-gel coordinated carbothermic reduction method.The research results showed that the ratio of raw materials has a great influence on the purity and morphology of the product,when the molar ratio of boron to zirconium is 3,the molar ratio of carbon to zirconium is 11,the volume ratio of ethanol to acetic acid is 2:5,and the gel temperature is 70?,ZrB₂ powder can be obtained by pyrolysis at a relatively low temperature(1300?).With the increase of pyrolysis temperature and the extension of holding time,the purity of ZrB₂ powder became higher.When the pyrolysis temperature is 1550?and the pyrolysis time is 30 min,the comprehensive properties of ZrB₂ powder,such as appearance and granularity are the best.The particle size of ZrB₂ powder is about 300?500 nm,the oxygen content is 1.29wt%,the specific surface area is 27.6 m²g^-1.However,the powder still has some agglomeration phenomena.Further,we studied the formation principle of the ZrB₂ precursor gel network and the specific process of carbothermal reduction.The precursor gel may contain a three-dimensional gel network in which Zr—O—C—B and Zr—O—Zr coexist.It could induce two different reaction pathways during carbothermal reduction,which not only reduced the synthesis temperature,but also made the reaction more uniform and thorough.(2)Due to the high surface energy of ultrafine powder,it is easy to lead to agglomeration during the growth process,which will reduce the sintering activity of the ceramic powder.For the agglomeration of ZrB₂ powder,we added different dispersants(polyethylene glycol(PEG),polyvinylpyrrolidone(PVP)and oleic acid(OA))to the precursor solution to improve its dispersibility.The results showed that PVP and PEG had no obvious effect on the dispersion of ZrB₂ powder,while OA significantly improved the dispersion state of ZrB₂ powder.OA coated the surface of the ZrB₂ precursor,reducing the surface energy and inhibiting the aggregation and growth of particles in the subsequent heat treatment process.The addition of OA not only reduced the particle size of the product,but also has a significant effect on the reduction of carbon and oxygen content.When 3 wt%of OA was added,a spherical powder with the best dispersibility and clearest particle edges is obtained.The particle size of ZrB₂ powder is about 200?300 nm,the oxygen content is less than 1 wt%,and the specific surface area could reach up to 119.2 m²g^-1.