Fabrication of closed-pore inclusive low-permittivity substrates insensitive to ambient humidity

With the aim to reduce the permittivity of a dielectric substrate, closed pores have been introduced into alumina based ceramicsby our innovated superplastisically foaming method. Affect of the ambient humidity on the dielectric constant was compared to those for conventional porous ceramics as well as full densified one. In the dry atmosphere, the dielectric constant (k) decreased with the porosity similar to the conventional porous ceramics. Affect of the ambient humidity on the dielectric prop-erty was almost identical to that of fully densified one. The degradation of mechanical strength depended on the pore size introduced.