Fabrication Of AlN,Si₃N₄ Thermal Fillers With Various Morphologies And Study On Thermal Grease With High Thermal Conductive Properties

Because of excellent properties of thermal conductivity and electrical resistance, AlN and Si3N4 ceramics have shown great application potentials as thermal conductive substrate. Besides, the usage as thermal fillers in high thermal conductive composite materials is attractive as well. In addition to the high thermal conductivity, grain morphology of the thermal fillers is of great importance, too. This dissertation deals with the synthesis of AlN and Si3N4 ceramics powders with different grain morphologies by using various improved combustion synthesis methods and the preparation of thermal grease with high thermal conductivity by using the synthesized AlN and Si3N4 ceramics powders as thermal fillers.By using mechanical-activation-assistant combustion synthesis method, single phase AlN andβ-Si3N4 ceramics powders with high purity were synthesized. Influence of nitrogen pressure, mechanical dealing time and content of diluents on the results was invetigated. The experiment results showed that, single-phase AlN orβ-Si3N4 ceramics powders could be achieved by chosing appropriate diluents content, nitrogen pressure and mechanical dealing time and effective decrease of the activation energy of Al or Si could be achieved after treating by mechanical-activation process.With high-temperature-liquid-assisted sphericizing method, Spherical AlN powders with low oxygen content could be acquired by choosing proper additive, the temperature and the dwell time at chosen temperature. It is revealed that the surface tention, which was causing by eutectic liquid around the AlN particles formed at high temperature, is the driven force to realize the formation of spherical AlN. Oxygen could be removed from the AlN surface because of the reaction between CaO and the oxide surface layer.AlN whiskers were prepared with fluidized combustion synthesis method. SEM and TEM analysis results showed that majority of the AlN whiskers grew along <0001> direction. By using quenching method, VLS mechanism was confirmed to be the main growth mechanism of the AlN whiskers.Assisted by foaming process, single phaseβ-Si3N4 ceramics powders were synthesized. Results showed that"micro-sinter"phenomenon in the product was alleviated effectively and thus the synthesized product could be easily smashed. The presence of C made the oxygen consumed and, as a result, made theβ-Si3N4 crystal shorter and thicker. However, the presence of Ca madeβ-Si3N4 crystal longer and thiner.Thermal grease is prepared by using the synthesized AlN andβ-Si3N4 powders as thermal fillers and combining with silicone oil (polydimethyl siloxane fluid). Results showed that higher thermal conductivity of filler are employed, higher thermal conductive property could be achieved. With addition of proper and appropriate amount of coupling agent and fine particle size fillers, higher thermal conductive properties of thermal grease could be obtained.