Negative Permittivity Behaviors Derived from Dielectric Resonance and Plasma Oscillation in Percolative Bismuth Ferrite/Silver Composites

Negative permittivity of materials can be obtained by plasma oscillation or dielectric resonance, but the relationship between these two negative permittivity behaviors has been neglected. Combining the advantages of two negative permittivity behaviors, the negative permittivity can be more tunable and is expected to realize epsilon-near-zero behavior. In this work, percolative bismuth ferrite/silver composites with different contents of Ag were successfully fabricated by a simple solid-state sintering method. With the addition of Ag, the Lorentz-like and Drude-like negative permittivity behaviors were successively observed in bismuth ferrite/silver composites. The reasons for the two types of negative permittivity behaviors are dielectric resonance and plasma oscillation, respectively. Further investigation revealed that the synergistic effect of dipole resonance and free electrons led to a change of resonance frequency, a decrease of magnitude, and a transition of two negative permittivity behaviors. Moreover, with the addition of Ag in percolative bismuth ferrite/silver composites, the electrical conductivity and thermal conductivity changed abruptly at a certain volume fraction, suggesting a percolation behavior.