Evaluation of Radio Wave Reflection/Transmission Characteristics of Metal Granular Composite Materials Containing Anisotropic Shape Particles

We evaluated the reflection and transmission characteristics in the microwave region of metal granular composite
materials containing flaky Cu particles, flaky Ag-coated Cu particles, and acicular FeCo nanoparticles as anisotropic
shape particles. In the flaky Cu particle composite and the flaky Ag-coated Cu particle composite, a significant jump of the ac conductivity σ with the particle content was observed at the percolation threshold φc at 4 and 3 vol.%, respectively; the σ of composites with particle content above φc showed a value above 10-1 S/cm. The 6 vol.% flaky Ag-coated Cu particle composite exhibited metallic reflection and transmission properties. On the other hand, the 6 vol.% flaky Cu particle composite showed a tendency for the transmission coefficient to decrease and the reflection coefficient to increase with increasing frequency. For the acicular FeCo nanoparticle composites, the electrical conductivity was low and the permeability was not affected by eddy currents even in the GHz range. The 49.9 vol.% acicular FeCo nanoparticle composite exhibited the frequency dispersion of permeability by magnetic resonance at several GHz, and the 10 vol.% composite showed slight absorption above 10 GHz.