Anisotropic particle geometry effect on magnetism and microwave absorption of carbonyl iron/polyimide composites

Two carbonyl iron (CI)/polyimide (PI) composites (65:35 wt) with different CI particle geometry (flake like carbonyl iron FCI against spherical carbonyl iron SCI) were prepared. Due to the difference of demagnetization factor from the geometry anisotropy for SCI and FCI, the saturation magnetization Ms suppresses from 126 emu/g to 116 emu/g and the coercivity Hc from 25 Oe to 4.6 Oe. The geometry anisotropy of CI particles from spherical one to flake like one enhances the real part of permittivity and permeability loss of the CI/PI composites. The microwave absorption peak is shifted to lower frequency by 5.2 GHz for FCI/PI composite comparing to SCI/PI. A whole X and Ku bands (from 8.2 to 18 GHz) reflection loss (RL) < −5 dB absorption bandwidth is achieved in FCI/PI composite with thickness ≥1.2 mm and the minimum RL value of −19.28 dB appears at 11.62 GHz at thickness of 1.5 mm. The composites show excellent thermal stability with change of complex permittivity and permeability less than 7% after heated at 230 °C for 10 h.