Microwave dielectric properties of polybenzoxazine based composites for microwave substrate application

Abstract

The composite of polybenzoxazine and barium strontium titanate (BST or Ba0.3Sr0.7 TiO3 with different BST loadings up to 80 wt.%) were synthesized and fabricated as a substrate. The effects of volume fraction of ceramic filler on the microstructure and microwave dielectric properties of the composite were studied for frequency and temperature dependence. A dielectric constant of 6.12 and loss tangent of 0.00065 at 1 GHz (room temperature) were obtained for aniline-based polybenzoxazine (PBA-a), which was higher than 4.49 and 0.00089 of the fluorinebased polybenzoxazine (PBA-f), respectively. Subsequently, PBA-a was used as a polymer matrix in the composite. The dielectric constant at 1 GHz could be increased from 6.12 up to 22 (without fillers), by adding BST 80 wt.% (48.8 vol.%). PBA/BST composites exhibited nearly zero temperature coefficient of dielectric constant in the range of -50°C to 150°C. The frequency dependence of the dielectric constant of the composite was found at 100 MHz - 1 GHz range. Additionally, the effects of surface modification on the dispersion of BST and microwave dielectric properties were studied using 3 chemical agents (3-aminopropyl-trimethoxysilane, phthalocyanine, and a benzoxazine monomer). The surface modification on BST by a silane compound agent enhanced the adhesion force between the matrix and filler resulted in the increase of the dielectric constant. Surface modification by phthalocyanine and the benzoxazine monomer could fabricate BST fillers dispersion better than the silane compound that leads to the lowering of loss tangent of the composite.