Dielectric Relaxation in the Hybrid Epoxy/MWCNT/MnFe2O4 Composites

Abstract

The electrical properties of epoxy/MWCNT (multi-walled carbon nanotubes)/MnFe2O4 hybrid composites loaded with MWCNTs (below, 0.09 vol.%, and above, 0.58 vol.%, percolation threshold) and varying concentrations of MnFe2O4 up to 10 vol.% were studied in a wide frequency range (20 Hz–40 GHz) at different temperatures (20 K–500 K). At low frequencies, the dielectric permittivity and the electrical conductivity of composites with fixed amounts of MWCNT are strongly dependent on MnFe2O4 content. For MWCNT concentrations above the percolation threshold (i.e., 0.58 vol.%), the electrical conductivity highly decreases with the increase of the MnFe2O4 fraction. In contrast, for the epoxy/MWCNT just below the onset of electrical conductivity (0.09 vol.% of MWCNTs), there exists an optimal concentration of MnFe2O4 inclusions (i.e., 0.025 vol.%), leading to a dramatic increase of the electrical conductivity by three orders of magnitude. The electrical transport in composites is mainly governed by electron tunneling at lower temperatures (below 200 K), and it is highly impacted by the matrix conductivity at higher temperatures (above 400 K). The electrical properties were discussed in terms of the Maxwell–Wagner relaxation and distributions of relaxation times. A non-invasive platform based on dielectric relaxation spectroscopy was proposed for enhancing the synergetic effect coursed by using multiple nanoinclusions in polymer composites just below the percolation threshold.

Dielectric Relaxation Spectroscopy and Synergy Effects in Epoxy/MWCNT/Ni@C Composites

“…Both series, mono-filled (Ni@C/epoxy) and hybrid (Ni@C/MWCNTs/epoxy) composites, were prepared according to the standard procedure for filler particles dispersing in an epoxy matrix [ 27 , 28 ]. The producing procedure of Ni@C/epoxy composites with Ni@C concentrations of 10, 15, 25 and 30 vol.% included the dispersion of Ni@C particles in isopropanol using an ultrasonic probe for 1 h, repeated one-hour sonication after addition of epoxy resin to the Ni@C/isopropanol suspension and curing after isopropanol evaporation.…”

“…The techniques described above allowed us to obtain mono-filled Ni@C/epoxy composites with Ni@C concentration of 10, 15, 25 and 30 vol.%, as well as hybrid Ni@C/MWCNTs/epoxy composites with a fixed MWCNTs concentration of 0.09 vol.% (just below the percolation state in corresponding mono-filled composites [ 27 ]) and various Ni@C content of 0, 0.025, 0.2, 0.6, and 1 vol.%. Hybrid composites with relatively low Ni@C concentrations were prepared because composites with higher concentrations demonstrate non-uniform distributions of nanoparticles (will be shown below).…”

Epoxy Nanocomposites Reinforced with Functionalized Carbon Nanotubes

“…Currently, nanocomposites are often used in various industries, due to their unique physicomechanical characteristics, which are due to the fact that nanodispersed particles are not stress concentrators in polymer composites [4,[12][13][14][15].…”

“…Taking into account excellent physical and mechanical properties, as well as high aspect ratio of carbon nanotubes (CNTs), they are believed to be a promising reinforcement phase for polymer composites among inorganic nanofillers [14]. Analysis of scientific literature was shows that often when CNTs were introduced into epoxy composites, there was no significant increase in the strength of epoxy composites, and sometimes this even was leads to its decrease, which is associated with poor adhesion between CNTs and the epoxy matrix and with the fact that CNTs are energetically more favorable to aggregate with each other [16][17][18][19][20].…”

THz Spectroscopy as a Versatile Tool for Filler Distribution Diagnostics in Polymer Nanocomposites

“…Multi-filler composites containing different types of fillers (including non-carbon) tend to form heterogeneous percolation networks [ 30 , 31 , 32 ]. This fact allows making composite materials without expensive fillers through the introduction of low-cost ones into the hybrid percolation network [ 33 , 34 ], or even enhancing the electromagnetic properties due to the synergistic effect [ 35 , 36 , 37 ].…”