Dominant role of ceramic connectivity in microwave dielectric properties of porous ceramics

The connectivity of constituting phases plays an important role in the physical properties of composites and porous materials, while it is still overlooked in many cases. In the present work, CaTiO3 is used as the model material to clarify the effect of ceramic connectivity on the microwave dielectric properties of porous ceramics. CaTiO3-A and CaTiO3-B porous ceramics are prepared by partial sintering at 700-1200 & DEG;C and sintering at the optimum temperature of 1350 & DEG;C with the aid of porogen, respectively. With increasing the sintering temperature from 700 to 1000 & DEG;C, the relative density of CaTiO3-A ceramics maintains around 58%, while their & epsilon;r, Qf, and & tau;f increase dramatically (20.4-48.2, 968-4,237 GHz, and 443.9-724.2 ppm/ & DEG;C). The three parameters are even higher in CaTiO3-B ceramic with a similar relative density of 54.1% (69.1, 5,658 GHz, and 782.9 ppm/ & DEG;C). The huge variation in microwave dielectric properties is attributed to the significantly improved ceramic connectivity with sintering temperature observed from microstructure analysis. Finite element analysis further reveals that the ceramic connectivity plays the dominant role in the microwave dielectric properties by deciding the electric field distribution in CaTiO3 phase and pores and hence their contribution to dielectric response. The exponent & alpha; in exponential dielectric mixing rule is employed to describe the ceramic connectivity, and the trinary relationship among the microwave dielectric properties, constitution, and microstructure of porous ceramics is well constructed. This relationship can be extended to ceramic-polymer and ceramic-ceramic composites, which brings new perspectives for designing such microwave dielectric materials.