Preparation And Characterization Of AlN Powders For Thermally Conductive Substrates

Aluminum nitride(AIN)is a new type of functional ceramic material.Due to its high thermal conductivity,reliable insulation properties and thermal expansion coefficient similar to silicon,AIN is an ideal material for heat sink substrate.The industrial production of aluminum nitride is mainly realized with three methods:carbothermal reduction,direct nitriding of aluminum powder and self-propagating high-temperature synthesis.It is known that AIN particles prepared by the carbothermal reduction method are smaller in size,with high purity and good dispersion.In addition,the raw material sources are rich,the technical process is simple,then carbothermal reduction method it is considered as most suitable one to be developed for AIN production.In this work,the determination on the technical parameters in AIN production with the carbothermal reduction method,the selection of equipment,and the technical problems in the production process are explored.(1)Determination on the technical parameters in AIN production with the carbothermal reduction is conducted.The technical parameters include heating temperature,carbon to aluminum ratio,additives and mixing methods.Heating temperature is one of the most important technical parameters affecting the nitridation reaction.The reaction rate and the purity of the prepared AIN increases with the heating temperature.In the case where the particle size of the raw material is under 1 ?m,1500? is critical to the reaction.Nitridation reaction at 1500?,or higher is accelerated,pure phase AIN can be prepared,while the reaction at temperatures lower than 1500? result in Al2O3 impurity.However,higher reaction temperature lead to the increased AIN particle size.The addition of CaF2 will form intermediate liquid phase at low temperature and this can favorable for lowering down the heating temperature,but the particle size of AIN powders increases sharply with the addition of CaF2.The addition of AIN seed crystal as additive introduces more uniform particle size.The effect of the carbon-aluminum ratio on the reaction is mainly due to the fact that the amount of carbon powder participating in the reaction should not be too less or excessive.The nitridation reaction is incomplete with less amount of carbon,which leaves residual Al2O3 unreacted.Excessive carbon powder increases the time for subsequent decarburization,resulting in an increase in oxygen content and an increase in cost.The optimized carbon-aluminum mass ratio is found to be 0.5.The mixing method has a huge influence on the nitride powders.When Al2O3 is in full contact with carbon powder,the reaction temperature,the particle size and the oxygen content of the prepared AIN powders are reduced to some extents.manual mixing,machine mixing and wet mixing are studied to provide reference data for production.(2)Experiments and tests of the preparation of-AIN powders were carried out on four different equipment:tube furnace,box furnace,carbon tube furnace,pusher kiln.Pure phase AIN powders can be prepared under different experimental conditions.But carbon tube furnaces have the best results.(3)The problems occurred in practical production are studied in detail,including the effect of the flow rate on the reaction,the effect of the feeding height on the reaction,and the effect of the gas flow pattern.