Study On Preparation And Properties Of H-BN Ceramics

Boron nitride(h-BN)has the characteristics of high temperature resistance,thermal shock resistance,thermal stability,corrosion resistance,high thermal conductivity,good insulation performance and so on.In recent years,especially as the candidate materials of ultra-high temperature materials,it has been widely used in the fields of metallurgy,electronics,aerospace,atomic energy and so on.Although h-BN has many excellent properties,h-BN is a highly covalent compound with high melting point and low self-diffusion coefficient,which makes it difficult for h-BN to obtain high density sinter.In this paper,sub-micron BN powder was prepared by optimizing the grinding parameters by means of high speed mechanical lapping.H-BN ceramics with high sintering activity and sub-micron boron nitride powder were prepared by hot pressing and non-pressure sintering,and the effects of the preparation process on the properties of h-BN ceramics were studied.In this paper,the effect of high speed grinding method,grinding medium,dispersant type,grinding ball diameter,ball ratio and other parameters on the particle size of boron nitride powder was studied in this paper.The submicron h-BN powder was prepared by mechanical grinding method.The research process is based on the ratio of the ball ratio of 10:1 to 0.2mm,0.4mm,0.8mm mixed particle size of zirconia ball grinding medium.The efficiency of grinding is studied with twelve alkyl sodium sulfate as dispersant and deionized water as abrasive.The results show that the particle size of boron nitride powder decreases with the increase of grinding time,but when the grinding time exceeds 8 h,the average particle size increases.When grinding 8h,the average particle size of BN powder is about 700nm.In this paper,h-BN ceramics were prepared by hot pressing sintering.The effects of sintering parameters and particle size on densification and mechanical properties of h-BN ceramics were investigated.The results show that the sintering pressure and sintering temperature are the main factors determining the densification and mechanical properties of h-BN ceramics with lamellar structure.The smaller the size of the original h-BN powder,the higher the sintering activity,the better the density and mechanical properties of the prepared h-BN ceramics.The mechanical properties of the h-BN ceramics prepared by the Nano powder are 1.of the micron boron nitride powder.5 times.With Nano boron nitride powder as raw material,1750℃ is selected as sintering temperature,heat preservation 60min,under 30MPa pressure,the prepared h-BN ceramics have the best properties,the bending strength is 105MPa,the fracture toughness can reach 1.37 MPa·m1/2.Because the hot-pressing sintering can only make the simple structure of ceramic devices,it is not good for h-BN to play its unique performance advantage in many fields.This paper made use of the pressure free sintering to prepare h-BN ceramics.The effect of the pressure free sintering process and the grain size of the powder on the densification of h-BN ceramics is studied.The results show that the addition of sintering aids and sintering temperature can promote the mass transfer of h-BN grains and increase the density of h-BN.The liquid phase sintering with Al2O3-Y2O3 as sintering aid is helpful to the diffusion and migration of grain boundary,and the mechanical and thermal properties of ceramics are enhanced more significantly than MgO-Y2O3 as sintering aids.The sintering activity of Nano particle particles is high,which promotes the Sintering Densification of h-BN ceramics.Compared with the micron grade powder,the relative density and bending strength of h-BN ceramics by Nano scale powder are increased by about 10%.With Nano powder as raw material and adding 3vol%Al2O3-Y2O3 as sintering aids,1900℃ is the sintering temperature and 60min,the properties of h-BN ceramics are best,the relative density is 65.7%,the bending strength is 45.9MPa,and the thermal conductivity is 17.6W/(m K).