Preparation Of Spherical AlN Powder By Microemulsion-Carbothermal Reduction Nitridation Method

Spherical AlN powder is an ideal high power integrated circuit substrate material and packaging material.The spherical AlN powder has a better fluidity and a higher bulk density.Compared with the multi-angled AlN powder particles in the preparation of the aluminum nitride ceramic,the AlN ceramic substrate is prepared by casting under the same molding process conditions.At the time,the prepared ceramic body will be more dense.At the same time,the micron-sized spherical aluminum nitride powder also facilitates the preparation of structurally complex parts by an injection molding process.In addition,spherical aluminum nitride powder is an ideal substitute for spherical alumina and spherical silica powder as a high and low thermal conductivity encapsulating filler,which can maintain good fluidity in the polymer and obtain higher filling rate.In summary,it can be seen that the spherical AlN aluminum nitride powder has a broad application prospect.In this paper,aluminum sol,water-soluble starch,sucrose,polyvinyl alcohol,25%glutaraldehyde aqueous solution is used as raw material,aluminum fluoride（AlF₃）and cerium nitrate hexahydrate（Y（NO₃）₃·6H₂O）as additives,NH₃·H₂O is a precipitant.Firstly,the carbon-aluminum precursor microspheres are prepared by microemulsion method,then subjected to carbothermal reduction nitridation（CRN）reaction and 680℃for 3 h for carbon removal treatment,and finally micron-sized spherical AlN powders are prepared.The conditions of cross-linking into spheres during the preparation of precursor microspheres,the composition of microemulsions,and the influence of process conditions on the morphology of the powders were investigated.In the carbothermal reduction nitridation reaction stage,different CRN reaction parameters such as reaction temperature,nitrogen flow rate,carbon-aluminum ratio,holding time,additive type and content were investigated by X-ray diffraction spectroscopy（XRD）and scanning electron microscopy（SEM）.The conversion rate of nitriding reaction and the influence of powder particle morphology.The conclusions drawn from the analysis of the experimental test results are as follows:（1）The cross-linking reaction of the precursor microspheres in this study was carried out by using a monomeric PVA aqueous solution and a 25%aqueous solution of glutaraldehyde as a crosslinking agent.In the experiment,the optimum concentration of the monomer PVA aqueous solution is 6 wt%,the amount of the crosslinking agent is 3 ml,the corresponding 6 wt%PVA aqueous solution is 30 ml,and the optimum temperature of the crosslinking reaction is 50°C,the optimum pH range is 2-4.The experiment determined that the optimal HLB value of the microemulsion system was9.7,the mass fraction of the composite emulsifier was 9 wt%of the aqueous phase,and the volume fraction of the aqueous phase was 13wt%of the oil phase.The amount of n-butanol is 1 wt%of the amount of the emulsifier.（2）The difference in the drying method of the precursor will seriously affect the morphology of the precursor microspheres.When the speed of water loss is too fast during drying,the surface of the precursor microspheres will crack,and more seriously,the interior of the drying When the water is discharged outward,the internal inorganic salt substance is brought to the surface layer of the microsphere,so that the inside of the microsphere is found to be a hollow structure in the subsequent CRN reaction stage.Freeze drying will avoid the above disadvantages.In the subsequent carbonization stage,the carbonization temperature of the precursor is set to 260°C to ensure the activity of the reaction product.In addition,the last step of the cleaning stage in the preparation process of the precursor is applied to the addition of a surfactant such as poly in distilled water for aqueous phase cleaning.Ethylene glycol 400 can alleviate the hard agglomeration of powder particles.In addition,a suitable heating system has a significant impact on the dispersion of the powder.（3）The spherical AlN powder was synthesized in flowing N₂ by CRN process.When AlF₃ was introduced alone,the particles were mostly hexagonal plate and cross-plate structure,and the hexagonal plate structure became more complete with the increase of the addition amount.An increase in the reaction temperature results in an increase in product conversion,complete conversion at 1550°C,and no additive residue.The N₂gas flow rate can change the morphology of the formed AlN crystal within a certain range,because the flow rate of the N₂ gas changes the concentration of the reaction substance.When Y₂O₃ is used as an additive alone,the amount of whiskers in the product gradually decreases with the addition amount,and the particles become smooth,but the agglomeration of the powder is greatly aggravated;at 1500°C,the holding time is extended to 6h.Compared with 3h,the product conversion rate is obviously improved,but when the addition amount of Y2O3 is 2wt%,the strength of the particles is not enough,the particles are easily broken and the surface of the particles is rough;increasing the temperature will lead to an increase of AlN whiskers and a decrease.The holding time will result in a decrease in conversion rate.（4）The morphology of the product particles with 7wt%AlF₃ and 3wt%Y₂O₃ content at 1550°C was smooth,and the holding time was 2h,which was lower than that of 3h and 6h.Although the reaction temperature was increased,it did not cause The large amount of AlN whiskers is generated,and the diffused AlN seeds in the reaction chamber remain in a high concentration range,which is not conducive to the growth of AlN whiskers.At the same time,due to the formation of the AlN plate and the growth of the AlN grains,the total surface area of the AlN crystallites in the particles is somewhat reduced,that is,the surface area of the AlN crystallites alone is increased,while the total surface area is increased.This is greatly reduced,which allows less liquid yttrium aluminate to substantially completely cover the surface of the AlN nucleus in the particles to transport the reactive material and bond the particles to increase their strength.