Mechanically Flexible Butyl Rubber–SrTiO ₃ Composite Dielectrics for Microwave Applications

Mechanically flexible butyl rubber–SrTiO ₃ composites were prepared through sigma mixing followed by hot pressing. Tensile tests established the mechanical flexibility of the composites. Swelling tests revealed the degree of matrix–filler interactions. The effect of SrTiO ₃ ceramic filler on the dielectric properties of the composites was studied at 1 MHz and 5 GHz using an LCR meter and a Split Post Dielectric Resonator, respectively. The relative permittivity of the composites increased with filler loading while maintaining low dielectric loss. For the maximum filler content of 0.42 volume fraction, the composite showed a relative permittivity of 13.2 and a loss tangent of 2.8 × 10 ⁻³ at 5 GHz. Theoretical modeling of effective relative permittivity of the composites was performed and the results were correlated with the experimental data. Effect of repeated bending on the microwave dielectric properties of the composite was also studied. The coefficient of thermal expansion of the composites was observed to decrease with the increased ceramic filler dispersion and reached a minimum value of 26.2 ppm/°C.