Influences of fabrication process on dielectric properties of porous silicon nitride ceramics

Two different series of porous silicon nitride (Si₃N₄) ceramics with different porosities from 30% to 60% were fabricated using two different preparation routes: the addition of pore-forming agent and freeze-drying. The microstructures of the samples were observed in detail. The results show that there is a big difference in the porous structure of the samples prepared by the different fabrication routes. The samples prepared using the pore-forming agent have a dense matrix containing close pores and cavities with some needle-shaped and flaky β-Si₃N₄ grains, while those prepared by the freeze-drying process contain open and connective pores with a flat shape scattered almost uniformly in the Si₃N₄ matrix. The porosity, pore structure and dielectric properties were measured separately. The results show that with the increase of porosity, the electric constant and dielectric loss of the ceramic decrease, and the pore distribution of the ceramic is also a major factor that influences its dielectric properties. The ε of the samples prepared by the addition of pore-forming agent is lower than that by freeze-drying, while its tan δ is higher under the same porosity. The ε and tan δ of the porous Si₃N₄ ceramics typically are in the range of 5.21-2.91 and 9.6 × 10^-3-2.92 × 10^-3, respectively.