Fyzikální ústav Akademie věd ČR

The ultra-broadband dielectric spectroscopy is particularly useful for studying dielectric–conductor nanocomposites, particularly near the electrical percolation threshold. Theoretically, we have studied the spectra of effective dielectric response of such composites using several models within the effective medium approximation [1]. It was shown that the divergence of the static permittivity at the percolation threshold is caused by a strong dielectric relaxation, which softens on approaching the threshold with changing the composition, passing through the whole frequency range from infrared down to zero.

Experimentally, we have studied e.g. nanocomposites of polymers with carbon nanotubes (CNTs) [2]. A well electrically conducting CNT, due to its specific shape, enables processing composites with extremely low percolation threshold. Within our collaboration with Queen’s University in Belfast, who prepared well-dispersed composites of polyethylene terephthalate (PET) polymer with CNTs, we characterized them by broadband conductivity and dielectric spectroscopy, covering up to 17 orders of magnitude in frequency (10⁻⁴–10¹³ Hz) from room temperature down to 5 K. An extremely low percolation threshold of 0.07 vol% CNT was revealed as appearance of the low-frequency AC conductivity plateau corresponding to the DC conductivity, whose value increased with CNT concentration according to the power law with a high critical exponent of 4.3. Its semiconductor temperature dependence obeying the tunnelling law confirmed the model that each CNT in the percolating clusters is covered by a thin (~ 1 nm) layer of PET [2].

Our first results on other nanocomposites of similar type were reviewed in [3]. Successful modelling of porosity in high-permittivity ceramics in infrared as well as low-frequency range using the Lichtenecker model was achieved for a classical relaxor PbMg_1/3Nb_2/3O₃ (PMN) [4].

[1] J. Petzelt et al., Ferroelectrics 526, 171 (2012)
[2] D. Nuzhnyy et al., Nanotechnology 24, 055707 (2013)
[3] J. Petzelt et al., Phys. Status Solidi A 210, 2259 (2013)
[4] D. Nuzhnyy et al., Phys. Rev. B 89, 214307 (2014)

Fig. 1: Room-temperature AC conductivity spectra dependent on the CNT concentration show that from the concentration 0.114 vol.% up, CNT plateaus of low-frequency conductivity appear, which correspond to DC conductivities and increase and broaden with the CNT concentration. The inset shows the critical power dependence of the conductivity at 1 Hz above the percolation threshold. At high frequencies the conductivity increases up to the THz range, which corresponds to the increase in localized conductivity due to the contributions from non-percolated CNT clusters.