Research on composites and their effective medium modelling


In the past we have achieved a large progress in investigating complex dielectric response of various composites. We combined our experimental studies in a broad spectral range (10 mHz - 1014 Hz) with theoretical modelling based on the effective medium approximation. Various models, based on different microstructures, topology and connectivity of the composites are used for the modelling. Some of the models were derived in our Departement by I. Rychetský (from the Theory group). For example, we have determined and interpreted the dielectric spectra (100 - 1014 Hz) of the conducting polymer polyaniline (PANI) as a composite of metallic emeraldine salt and dielectric base. AC conductivity spectra of the core-shell composites of PANI with the non-conducting poly(p-phenylenediamine), modelled by using various EMA models, enabled us to estimate the percolated fraction of the PANI-core through the shells to ~0.1 vol%. In collaboration with CSIC Llanera, Spain, we have studied the temperature dependent (10 - 450 K) dielectric spectra (102 – 1013 Hz) of new ceramic composites of barium titanate BaTiO3 with NiO of a normal and core-shell topology and have modelled them with various EMA models, using also our knowledge of the dielectric spectra of pure BaTiO3 ceramics. In collaboration with Brno Technical University, THz-IR spectra measurements and their EMA modelling were also successfully used in the case of several alumina ceramics with highly anisotropic IR-phonon grain response. In collaboration with Brock University, Ontario, Canada, we studied in detail the new giant-permittivity ceramics of (Nb+In) co-doped rutile TiO2. It was shown that the origin of giant permittivity in these slightly semiconducting ceramics can be fully explained by thermally activated inhomogeneous conductivity, where the nm-thin grain boundaries and ~100 nm thick near-electrode depletion layers have the dc conductivities by ~5-orders of magnitude smaller than those of the grain bulk. The known dielectric spectra up to the THz range were then used to calculate the microwave shielding efficiency of our composites, shown to have some potential for applications. EMA modeling was also succesfully used for explanation of anisotropic dielectric properties of polar nanoclusters in relaxor ferroelectrics (see above).

Broad-band ac conductivity spectra of composites
Broad-band ac conductivity spectra of PPDA (shell) and their composites with PANI and PPy (core). Spectra of neat PANI are shown for comparison. Experimental data are depicted by the symbols and lines show the fits and modelling. CS stands for coated spheres.

[1] D. Nuzhnyy et al., Broad-band conductivity and dielectric spectroscopy of composites of multiwalled carbon nanotubes and poly(ethylene terephthalate) around their low percolation threshold, Nanotechnology 24, 055707 (2013).

[2] J. Petzelt et al., Broadband dielectric and conductivity spectroscopy of inhomogeneous and composite conductors, Phys. Stat. Solidi A 210, 2259 (2013).  

[3] V. Bovtun et al., Wide Range Dielectric and Infrared Spectroscopy of (Nb+In) co-doped rutile ceramics, Phys. Rev. Mat. 2, 075002 (2018).

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