Figure 1 shows the XRD pattern and morphology of Ag/PANI composites. The broad double-peak at circa 19–23° indicated the amorphous nature of PANI (Fig. 1a). The diffraction peaks corresponding to Ag can be confirmed in pure Ag, while the characteristic peaks were covered in the composites (Fig. 1a). Figure 1b showed the morphology of Ag nanowires with a large aspect ratio. However, after adding the prepared Ag nanowires to synthesize PANI, the nanowires can hardly be observed in the SEM image of Ag/PANI composites. It indicated that the nanowires were destructed by the violent stir during the synthesis of PANI, then the shortened silver were wrapped by the PANI matrix. Hence, it is difficult to observe the silver nanowires in the composites. When the content of Ag reached to 10 wt%, the neighboring Ag particles cannot completely contact with each other to establish a conductive network (shown in Fig. 1c). After the Ag content reached to 30 wt%, Ag particles interconnected and gradually constructed a percolation network (Fig. 1d).
Frequency dependence of the real part permittivity (εr′) is shown in Fig. 2a. The largest values of εr′ of PANI fabricated in this work reached up to 104, but εr′ gradually decreased as frequency increased. When the content of Ag was 10 wt%, εr′ of the Ag/PANI composite was greatly enhanced due to the increasing interfacial polarizations between Ag and PANI matrix. When the composites were put into an electric field, electric charges aggregated at the heterogeneous interfaces, contributing to the enhanced permittivity. However, the decreasing tendency of εr′ versus to frequency was more evident. The variation of εr′ of PANI and PANI-10% Ag composites showed relaxation features and can be well fitted by Debye equation [9]:
where ε∞ is the permittivity at nearly optical frequency, εs is the static permittivity, ω is the angular frequency and τ is the relaxation time. Dielectric relaxation occurs along with a significant decrease of permittivity, when polarization cannot keep up with the change of frequency. Further increasing the content of Ag caused a negative permittivity in Ag/PANI composites. This is attributed to the fact that electrons in the interconnected Ag paricles became delocalized in the percolating networks. Thus, the negative permittivity originated from the plasmonic state of free electrons within the composites. Furthermore, negative permittivity of free electron gas was depicted by Drude model [16]:
ωp=(neffe2/meffε0)1/2 is the plasma frequency determined by the effective mass (meff) and concentration (neff) of electrons. The experimental results agreed well with the Drude model, suggesting that the plasma-like negative permittivity is related to plasma oscillations of electrons introduced by Ag particels. Meanwhile, the magnitude of the negative permittivity was proportional to the content of Ag owing to the increase of effective electron concentration.
Figure 2b shows the reactance (Z″) spectra of Ag/PANI composites. When the content of Ag was no more than 10 wt%, Z″ was negative, indicating that the composites was electrical capacitive. When the content of Ag reached up to 15 wt%, Z″ became positive and the composites turned into electrical inductive. The spectra of Z″ were also fitted by the inserted equivalent circuit models in Fig. 2b and the fitting results also demonstrated that inductive components were employed to analyze the samples with negative permittivity. In addition, there was a relationship between εr′ and Z″ [17]:
It suggested that the sign of εr′ is closely related to that of Z″. Thus, the appearance of negative permittivity in the composites with high content of Ag is corresponding to the inductive character. Meanwhile, the phase relationships between electric voltage and currents were different in the composites with positive and negative permittivity. When εr′ is positive and Z″ is negative, the current phase lags the voltage phase, while that is just reversed when εr′ is negative and Z″ is positive.
Figure 3a shows the ac conductivity (σac) of Ag/PANI composites with different content of Ag. σac was almost frequency independent in low frequency region but slightly increased in high frequency region for the composites with low content of Ag. This phenomenon was related to the hopping conduction mechanism, which depicted that the conductivity was contributed by carriers’ hopping among the isolated Ag particles. When the content of Ag increased from 30–40%, σac was greatly enhanced under the action of conductive silver networks within PANI matrix. In this case, the increased conductivity was ascribed to electrons’ movements in the formed percolating pathways. Thus, as the content of Ag increased, more electrons in Ag fillers contributed to the electrical conductivity. The inserted figure in Fig. 3a shows the variation of σac at 5 kHz versus to the mass fraction of Ag. It indicated that the conductivity of Ag/PANI composites evidently improved after incorporating Ag particles, which agreed well with the percolation theory: σ∝(f-fc)t [11]. Thus, the conduction mechanism of hopping conduction changed to metal-like conduction as the conductivity increased dramatically. Imaginary part of the complex permittivity (εr″) usually indicates dielectric loss in the materials. Frequency dependence of εr″ is shown in Fig. 3b. εr″ of all samples was almost inversely proportional to frequency. Taking into the measured data of σac into the formula for conduction loss calculation: εr″=σac/ωε0 [18], the calculated results were almost consistent with the measured εr″, which suggested that the dielectric loss of Ag/ PANI composites was mainly caused by conduction loss.