Development of polyphenylene oxide-based microwave composite substrate and its application in microstrip antenna

Polyphenylene oxide (PPO)-based microwave composite substrates filled with different volume fractions (10, 20, 30, 40, 50 vol%) of MgTiO 3 -Ca 0.7 La 0.2 TiO 3 (MTCLT) ceramics were prepared by injection-molding process. The microstructure, thermal, mechanical, and dielectric properties of the PPO/MTCLT substrates were fully studied. The results showed that PPO/MTCLT substrates with dense microstructure and uniform filler distribution were obtained when the filler fraction didn’t exceed 30vol%. Further increase of the filler fraction brought about agglomeration of ceramics and weakened interfacial bonding, producing more pores in the substrates. With increasing volume fraction of ceramics, the coefficient of thermal expansion (CTE) of the substrates decreased significantly, while the bending strength increased first and then decreased, reaching a maximum at 30 vol%. Moreover, the dielectric constant of the substrates increased remarkably, while the dissipation factor decreased first and then increased. Increasing the testing frequency had little effect on the dielectric constant but increased the dissipation factor gradually. PPO/MTCLT composite substrate with filler fraction near 30 vol% exhibited the best performance and was used to fabricate a microstrip antenna. The antenna showed good performance with antenna gain over 3.54 dB and low axial ratio below 1.24 dB. The PPO/MTCLT microwave composite substrates prepared by injection-molding process exhibited good comprehensive properties, being important candidates for microwave and millimeter-wave applications.