Dielectric Behavior of Nonpolar Polymers and Their Composites …
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microelectronics industry [54]. Due to the vastly different electrical conductivities
between the constituents of such system, the direct-current conductivity is strongly
dependent on the conducting filler concentration, thus leading to two basic charge
transport mechanisms:
(i) At low filler concentrations, the mean distance between conducting fillers is
sufficiently high and so the electrical properties of the composite are dominated
by the insulating matrix. In such cases, the charge carriers (in most cases electrons, ions, and holes) hop to a nearby state that can be quantum-mechanically
described with higher or lower energetic jumps of the potential barrier, the latter
achieved through quantum tunnelling [55].
(ii) At concentrations equal or higher than the percolation threshold, the conducting
fillers are in contact and the charge carriers can move with low resistance (current
flow), exhibiting the behavior of a conductor [56]. This can also be determined
by the temperature dependence of electrical conductivity. In dielectric materials, conductivity increases with temperature due to higher mobility of charge
carriers, whereas in conductors conductivity decreases with temperature due to
polaron scattering effects [57].
The movement of the charge carriers from the conductive fillers can cause a dipolar
response from the insulating polymer matrix and the corresponding interphase as
well, leading to a Maxwell–Wagner–Sillars interfacial polarization and increasing
the capacitive storage ability of the resulting composite [58]. Surface modification
processes to customize the interfaces between the matrix and the filler have been
extensively studied in the past decade to enhance dielectric performance. These
modifications affect the polarizability of the polyolefin matrix by introducing polar
groups and enhancing the hydrophobicity, resulting into better-performing insulating
materials [59].
4.2 Polyethylene Composites
Polyethylene is traditionally employed as an insulating material for cable manufacturing, due to its extremely weak conductivity and high dielectric breakdown strength.
The two are interconnected, as the dielectric breakdown strength is dependent on electrical conductivity; the addition of high permittivity or high conductivity particles
will enhance or decrease the dielectric breakdown strength, respectively. Moreover,
the presence of agglomerates forming a percolating network can highly improve the
thermal conductivity of the nanocomposite but decrease the dielectric breakdown
strength [60]. In polyethylene/montmorillonite composites, Li et al. observed that
the inclusion of aligned fillers results in an enhancement of the dielectric breakdown
response that adds up to that provided by the oriented polymer crystals [61]. This
effect could be exploited in materials that already present a drastic improvement of
thermal conductivity when stretched, such as UHMWPE, [62] to provide a suitable
material for electrical energy storage.
255
microelectronics industry [54]. Due to the vastly different electrical conductivities
between the constituents of such system, the direct-current conductivity is strongly
dependent on the conducting filler concentration, thus leading to two basic charge
transport mechanisms:
(i) At low filler concentrations, the mean distance between conducting fillers is
sufficiently high and so the electrical properties of the composite are dominated
by the insulating matrix. In such cases, the charge carriers (in most cases electrons, ions, and holes) hop to a nearby state that can be quantum-mechanically
described with higher or lower energetic jumps of the potential barrier, the latter
achieved through quantum tunnelling [55].
(ii) At concentrations equal or higher than the percolation threshold, the conducting
fillers are in contact and the charge carriers can move with low resistance (current
flow), exhibiting the behavior of a conductor [56]. This can also be determined
by the temperature dependence of electrical conductivity. In dielectric materials, conductivity increases with temperature due to higher mobility of charge
carriers, whereas in conductors conductivity decreases with temperature due to
polaron scattering effects [57].
The movement of the charge carriers from the conductive fillers can cause a dipolar
response from the insulating polymer matrix and the corresponding interphase as
well, leading to a Maxwell–Wagner–Sillars interfacial polarization and increasing
the capacitive storage ability of the resulting composite [58]. Surface modification
processes to customize the interfaces between the matrix and the filler have been
extensively studied in the past decade to enhance dielectric performance. These
modifications affect the polarizability of the polyolefin matrix by introducing polar
groups and enhancing the hydrophobicity, resulting into better-performing insulating
materials [59].
4.2 Polyethylene Composites
Polyethylene is traditionally employed as an insulating material for cable manufacturing, due to its extremely weak conductivity and high dielectric breakdown strength.
The two are interconnected, as the dielectric breakdown strength is dependent on electrical conductivity; the addition of high permittivity or high conductivity particles
will enhance or decrease the dielectric breakdown strength, respectively. Moreover,
the presence of agglomerates forming a percolating network can highly improve the
thermal conductivity of the nanocomposite but decrease the dielectric breakdown
strength [60]. In polyethylene/montmorillonite composites, Li et al. observed that
the inclusion of aligned fillers results in an enhancement of the dielectric breakdown
response that adds up to that provided by the oriented polymer crystals [61]. This
effect could be exploited in materials that already present a drastic improvement of
thermal conductivity when stretched, such as UHMWPE, [62] to provide a suitable
material for electrical energy storage.