Aluminium nitride(ALN)crystal is a kind of Covalent bond compound with Warzite structure based on A1N4 tetrahedron.The density is 3.26g/cm.It belongs to Hexagonal crystal family.The decomposition temperature at atmospheric pressure is 2480℃.Aluminum nitride ceramic material has a large thermal conductivity coefficient(about 320W/m·K,close to BeO and SiC,more than 5 times that of Al2O3),high mechanical strength,high hardness,good high-temperature electrical insulation,high melting point,strong resistance to molten metal and salt corrosion,and is non-toxic.It is a very promising material.Today's topic will briefly talk about this material,from three aspects:the preparation and synthesis of ALN powder,the factors affecting aluminum nitride cooperation in carbon reduction method,and future prospects.

　　1;Synthesis method of aluminum nitride powder:
　　1.1 Direct nitriding method of metal aluminum powder:
　　The direct nitriding process is simple and suitable for large-scale industrial production.It is a mature industrial method for producing aluminum nitride.Highly finely dispersed high-purity aluminum powder is nitrided in a nitrogen gas stream at high temperature to produce AlN powder.The reaction between aluminum and nitrogen begins at 600℃,and as the temperature increases,the nitriding rate significantly increases
　　The production practice shows that:strong Exothermic reaction,the released heat will cause the product to easily condense into blocks at high temperature.In addition,because of the fast reaction speed,a layer of very tightly adhered AIN film was formed on the surface of Al metal at the initial stage of the reaction.Under a nitrogen pressure,the protective film on the metal surface was damaged,making it difficult for nitrogen to penetrate further,and the nitriding rate was slowed down,so it was best to conduct secondary nitriding,This can prepare uniform AlN powder.
　　This method has drawbacks such as low yield,easy agglomeration of powders,high reaction temperature,and the need for careful grinding and repeated nitriding of the synthesized product.In order to improve the conversion rate and prevent powder agglomeration,aluminum powder with finer particle size and higher activity is used.Low bond energy gases such as ammonia and hydrogen are used to partially replace nitrogen,which can effectively reduce the nitriding temperature and improve the conversion degree of aluminum powder.
　　1.2 Carbon thermal reduction method:
　　This method involves mixing ultrafine aluminum oxide powder and high-purity carbon powder,and reducing AI and O with carbon in a flowing nitrogen atmosphere.The reduced Al and N react to produce AlN in a flowing state.The synthesis temperature range is 1600-1750℃,with a holding time of 4-10 hours.Then,it is cooled in N2 atmosphere to obtain black powdery nitrides.Then,it is held in air at 600-700-C for 1-16 hours for decarburization treatment to obtain gray white and well flowing AlN powder.
　　In the process of preparing AIN powder by carbothermal reduction,alkaline earth metals and rare earth metal oxides such as CaO,CaF2,Y2O3,etc.are often added as catalysts.Different calcination temperatures,holding times,aluminum sources,and carbon sources all have important effects on the carbothermal reduction products.
　　The carbon thermal reduction method has the characteristics of rich raw materials,simple process,high purity of synthesized powder,small particle size,and uniform distribution;Its main disadvantages lie in the longer synthesis time,higher nitriding temperature,and the need to decarbonize the excess carbon after the reaction,resulting in higher production costs.
　　1.3 Chemical vapor deposition method:
　　Chemical vapor deposition or pyrolysis method is a method of depositing AlN from the gas phase based on the chemical reaction of aluminum volatile compounds with ammonia or N.
　　According to different heating methods,chemical vapor deposition can be divided into plasma chemical vapor deposition,laser chemical vapor deposition and Thermochemistry vapor deposition;According to the different raw materials used,it can be divided into the ammonia complex pyrolysis method of aluminum and the ammonia decomposition method of hexafluoroaluminate.This method has the characteristics of high product purity,controllable particle size and crystal structure,precise control and adjustment of the process,and continuous process.It is one of the relatively developed technologies,but its significant drawback is that the process is relatively complex.
　　1.4 High temperature self propagating synthesis method:
　　This method is to convert A