Preparation, characterization and microwave absorption properties of NiFe₂O₄ and its composites with conductive polymer

Spinel ferrite NiFe₂O₄ was synthesized by polyacrylamide sol–gel method, and its composites with polyaniline (PANI) and polypyrrole (PPY) were prepared via in situ emulsion polymerization. The morphology of NiFe₂O₄ had a flake structure, while that of NiFe₂O₄/PANI and NiFe₂O₄/PPY composites existed a flake-grain structure. The magnetic properties (Ms, Mr and Hc) of NiFe₂O₄/PPY were lower than those of NiFe₂O₄/PANI, and they were all lower than those of NiFe₂O₄. NiFe₂O₄/PPY composite exhibited a higher conductivity value of 3.76 S/cm than that of NiFe₂O₄ and NiFe₂O₄/PANI. Moreover, the imaginary part of permittivity also enhanced with the increase of conductivity, contributing to the increase of dielectric loss. The electromagnetic characteristics of NiFe₂O₄, NiFe₂O₄/PANI and NiFe₂O₄/PPY were also investigated with a vector network analyzer in 1–18 GHz. The absorbing peak position moved to lower frequency with the increasing thickness of NiFe₂O₄/PANI or NiFe₂O₄/PPY composites. Under the same condition, the minimum reflection loss of NiFe₂O₄/PPY was much lower than that of NiFe₂O₄ and NiFe₂O₄/PANI, indicating the incorporation between NiFe₂O₄ and PPY could reach a better matching in terms of electromagnetic parameters. The value of the minimum reflection loss was −42 dB at 8 GHz for NiFe₂O₄/PPY with a thickness of 3.5 mm, and a broad peak with a bandwidth lower than −10 dB was about 4.5 GHz.