Solid Phase Preparation Of MgSiN₂ Ceramic Powders

Agricultural unmanned aerial vehicle has become a new agricultural production tool because of its simple operation and high efficiency.Especially for mountainous areas,hills,paddy fields and other ground,the advantages of unmanned aerial vehicles are more significant.Future avionics and modules will develop towards intelligence,miniaturization and high reliability.With the improvement of circuit integration,heat dissipation of substrate has become an important problem.The results show that 55%of the failure rate of electronic equipment is caused by temperature exceeding the specified value.At present,AlN is the ceramic material with the best thermal conductivity,but due to its high preparation cost,it is urgent to develop new materials.The crystal structure of MgSiN₂ is similar to AlN.Thus,MgSiN₂ has attracted extensive attention owing to its high fracture toughness(3 MPa m^1/2),high stress intensity（280 MPa）,high hardness（20 GPa）,high-temperature electrical insulation,high-temperature oxidation resistance,excellent thermal conductivity,and the mechanical properties are much better than AlN,and the preparation cost is lower than AlN,it is highly valued by researchers at home and abroad.MgSiN₂ can also be used as sintering aid of non-oxide ceramics with high thermal conductivity,and as growth additive in the combustion synthesis ofβ-Si₃N₄ rod crystals,it can also be used to improve the thermal conductivity and insulation of composites.It is considered a very promising engineering and functional ceramic material,and expected to replace AlN materials to become a new generation of high thermal conductive ceramic substrate materials and ideal packaging materials.In this paper,we use magnesium powder and silica powder as raw materials through changing process parameters for preparation of high purity magnesium silicon nitride powder.Magnesium silicon nitride powder were prepared by adding salt additives with magnesium powder and silica powder.Based on Mg₂Si/Si₃N₄,Mg₂Si/Si₃N₄/Mg,Si₃N₄/Mg three kinds of reaction system for preparing magnesium silicon nitride powder.Using X-ray diffractometer（XRD）,field emission scanning electron microscopy（FESEM）and energy spectrometer（EDS）,and other means to study the effects of the types and content of raw materials and additives and preparation parameters on the purity and morphology of the MgSiN₂ products.The main content and results are as follows:（1）Mg-Si system for preparing MgSiN₂ powder.In Mg and Si as raw materials,the single phase MgSiN₂ was obtained by high temperature solid phase method at 1250°C for 11h and 1350°C for 1 h.When the temperature reached 1450°C,MgSiN₂ decomposed to give rise to Si₃N₄,with the increase of holding time,but also Si₃N₄ decomposed,produce Si impurities.At 1250°C,the crystals of product mainly grew into two types,i.e.,lump and columnar.The block morphology is mainly due to the large amount of small MgSiN₂ grain sintering agglomeration at high temperature.The formation of columnar products is caused by temperature gradient and concentration gradient during crystal growth.The columnar products grow toward the surface and grow toward the inner void.The formation of columnar products may be due to the surface of block products possesses a certain temperature gradient.Thus,the farther away from the surface of the block product,the lower is the temperature,the more easily adsorbed is the gas;this promotes the growth of MgSiN₂ on the surface of massive products along the tip.When the growth of MgSiN₂,magnesium and silicon atoms and consumption due to diffusion or evaporation,it is easy to form holes.Within the voids of the bulk products,it is possible that the evaporation of Mg and Si results in a concentration gradient of Mg and Si vapors in the void,resulting in the columnar growth of MgSiN₂.However,it is possible that some of the surfaces of Mg and Si still have adsorption properties,which can adsorb N₂,Mg and Si atoms,so that some MgSiN₂ grows along the surface of the block products.（2）Salt additive assisted synthesis of MgSiN₂ powder.Using Mg powder and Si powder as the main raw material,adding solid nitrogen source urea to prepare MgSiN₂powder can effectively promote the nitridation reaction,reducing Si and MgO impurities in the product,the product refined particles,the grain size becomes more uniform.With the increase of urea content,the diffraction peak intensity of Si impurity and MgO is gradually weakened,urea addition has a certain influence on the purity of the product,but can not completely eliminate the impurities.Adding NH₄Cl can make the product appear loose,the product color becomes white,produce Si₃N₄,Si,MgO impurity,and the addition of NH₄Cl makes the morphology of some products change to form the columnar morphology of magnesium silicon nitride.The addition of NaCl makes the products produce Si₃N₄,Si,MgO impurities,and a small amount of needle like magnesium silicon nitride in the product.The addition of KCl makes the products produce Si₃N₄,Si,and MgO impurities,and has no obvious influence on the morphology of the products.（3）Si₃N₄/Mg₂Si,Si₃N₄/Mg₂Si/Mg,Si₃N₄/Mg three kinds of reaction material system for preparing silicon nitride powder.Taking Mg₂Si and Si₃N₄ as raw materials,preparation of magnesium silicon nitride powder by high temperature solid phase method,the single phase magnesium silicon nitride cannot be obtained by changing the ratio of raw material,there are a large number of unreactive Si₃N₄ and Si impurities in the product.Using Mg₂Si/Si₃N₄ as the raw material to add the right amount of Mg to prepare the magnesium nitride powders,With the increase of Mg content,the content of silicon nitride decreased gradually,and when Mg increased to 20 wt%,magnesium nitride powders containing a small amount of MgO were obtained.Using magnesium Mg/Si₃N₄ as raw material to the preparation of silicon nitride powders.When the Mg/Si₃N₄ mole ratio reaches 6:1,although silicon nitride falling with the increase of Mg content,but still cannot completely eliminate.Further increase of Mg content in the raw material proportion,Mg consumption is too large.MgSiN₂ powders were difficult to be obtained by high temperature solid phase method using Mg/Si₃N₄ as raw materials.