Effective permittivity of nanocomposite powder compacts

Advances in nanotechnology have led to a variety of new materials with strong potential applications to microwave and millimeter-wave components, e.g. dispersions of nanoscale particles, nanoparticle-filled polymers, self-assembled nanolattices of magnetic particles. More specifically, the properties of nanocomposites can be tailored for operation as insulators, ferro- and ferrimagnetic materials, highly conductive materials as well, for specific applications. In this study, we have investigated the electromagnetic response at microwave frequencies, using frequency domain network analysis, of cold-pressed powder compacts made of Ni, γ-Fe₂O₃, Co, and ZnO nanosized powders. Effective complex permittivities of composites over the frequency range (100 MHz-10 GHz) as a function of composition were studied. Within the frequency range of measurements the real and imaginary parts of the effective permittivity of nanocomposites exhibit spectra which can be analytically well represented by power laws. The associated power law exponents, which are similar for the real and imaginary parts of the permittivity, are in the range 0.05-0.20 in agreement with data in the published literature. The dependences of the effective permittivity vs. composition are compared to those obtained from the effective medium theory of Bruggeman, which is found not to be adequate for all nanocomposites studied.