| The fundamental goal of the study was to develop a sintering additive system for silicon nitride based ceramics in radome application use. Cordierite and spodumene were fabricated and mixed with silicon nitride powders. The reactions in sintering process were discussed, and the dielectric properties and mechanical properties indicated that a composite sintering additive had a great effect in decreasing sintering temperature.The cordierite powders that were prepared by both sol-gel method and solid-state reaction method were used as sintering additives. The main phase was identified as Si2N2O in high sintering temperatures, which was believed as a resultant of complex reaction of Si3N4, SiO2 and Si2N2O. Evaporating-condensation transferring took place in the sintering process for the samples using sol-gel method cordierite, but high strength couldn't be achieved in this way. The samples using solid-state reaction cordierite sintered at 1490°C achieved low dielectric properties (ε=5.174, tgδ=1.340×10-2) with an appropriate strength (147.09MPa), which could be explained by solution-precipitation transferring.The spodumene powders were fabricated at 1200°C by both sol-gel method and solid-state reaction method. Spodumene and cordierite mixed as composite additive were used in sintering silicon nitride material. The samples using sol-gel method spodumene could decrease the sintering temperature, but didn't improve the dielectric properties and strength. The samples added solid-state reaction spodumene showed lower sintering temperature than using cordierite as additive only, as the lower forming temperature of liquid phase. Composite material sintered at 1390°C using cordierite and spodumene with the ratio of 2:1 showed the most appropriate properties (ε=4.103, tgδ=2.641×10-2, 129.77MPa), which could fulfill the application use.
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