254
S. X. Drakopoulos et al.
4 Polyolefin Composites for Electrical Energy Storage
and Insulation
4.1 Background
In recent years, renewed effort has been invested in the investigation of the storage,
efficient recovery and distribution of electrical energy, affecting directly an interdisciplinary range of scientific fields, from fundamental and applied natural sciences to
economics and sociology. The need for new advancements in the energy materials
goes in parallel with the drawbacks of oil as a source of fuel, such as its impact
on climate change and variable cost. In addition, other sources of electrochemical
energy such as batteries are characterized by long recharging hours, thus limiting
the applicability for future fast-responsive applications, [41] and technologies that
are hazardous for the environment, generating a demand for more effective and safer
storage materials.
Insulating materials used as dielectric mediums to store capacitive electrical
energy find application in many modern electronic systems, ranging from electronic
devices to hybrid electric cars [42]. For such applications, a specific combination of
properties is required including [43–46]: (i) High values of dielectric permittivity,
which is a measure of the ability of the material to store energy; (ii) low dielectric
loss (tanδ) values, to maximize efficiency; (iii) low electrical conductivity, in order
to reduce leakage currents; and (iv) high dielectric breakdown strength so higher
electrical fields can be physically endured by the dielectric medium. Polymers, and
especially non-polar ones such as polyolefins, are characterized by low values of
dielectric permittivity and high breakdown strengths. In contrast, ceramic materials
exhibit the exact reciprocal behavior, presenting high permittivity and low breakdown
strength. Therefore, composite materials composed of a polymer matrix and ceramic
nanofillers are a priori great candidates for electrical energy storage materials [47].
In heterogeneous dielectric nanomaterials, understanding the role of interface/interphase properties and the possibility to tailor them at will is of great scientific and technological importance [48, 49]. In this respect, semicrystalline polymers
often are considered as heterogeneous materials, with the amorphous regions of the
polymer chains forming the continuous matrix phase and the rigid crystallites in the
role of the filler [50]. The crystallinity and crystal morphology also affect the dielectric properties by enhancing interfacial polarization phenomena [51], and increase the
dielectric breakdown strength due to higher resistance to electrical treeing (current
propagation to failure) [52]. Toward this direction, biaxial orientation of semicrystalline polymers, like polypropylene, has found use as thin dielectric membranes for
electrical energy storage applications [53]. In addition, fillers of various shapes, electrical characteristics and orientation, have been employed to enhance the dielectric
behavior of such polymers [43].
The addition of electrically conducting fillers within an insulating polymer matrix
can enhance their electrical and electromagnetic properties for applications in electromagnetic interference shielding and conductive adhesives in circuit elements in the
S. X. Drakopoulos et al.
4 Polyolefin Composites for Electrical Energy Storage
and Insulation
4.1 Background
In recent years, renewed effort has been invested in the investigation of the storage,
efficient recovery and distribution of electrical energy, affecting directly an interdisciplinary range of scientific fields, from fundamental and applied natural sciences to
economics and sociology. The need for new advancements in the energy materials
goes in parallel with the drawbacks of oil as a source of fuel, such as its impact
on climate change and variable cost. In addition, other sources of electrochemical
energy such as batteries are characterized by long recharging hours, thus limiting
the applicability for future fast-responsive applications, [41] and technologies that
are hazardous for the environment, generating a demand for more effective and safer
storage materials.
Insulating materials used as dielectric mediums to store capacitive electrical
energy find application in many modern electronic systems, ranging from electronic
devices to hybrid electric cars [42]. For such applications, a specific combination of
properties is required including [43–46]: (i) High values of dielectric permittivity,
which is a measure of the ability of the material to store energy; (ii) low dielectric
loss (tanδ) values, to maximize efficiency; (iii) low electrical conductivity, in order
to reduce leakage currents; and (iv) high dielectric breakdown strength so higher
electrical fields can be physically endured by the dielectric medium. Polymers, and
especially non-polar ones such as polyolefins, are characterized by low values of
dielectric permittivity and high breakdown strengths. In contrast, ceramic materials
exhibit the exact reciprocal behavior, presenting high permittivity and low breakdown
strength. Therefore, composite materials composed of a polymer matrix and ceramic
nanofillers are a priori great candidates for electrical energy storage materials [47].
In heterogeneous dielectric nanomaterials, understanding the role of interface/interphase properties and the possibility to tailor them at will is of great scientific and technological importance [48, 49]. In this respect, semicrystalline polymers
often are considered as heterogeneous materials, with the amorphous regions of the
polymer chains forming the continuous matrix phase and the rigid crystallites in the
role of the filler [50]. The crystallinity and crystal morphology also affect the dielectric properties by enhancing interfacial polarization phenomena [51], and increase the
dielectric breakdown strength due to higher resistance to electrical treeing (current
propagation to failure) [52]. Toward this direction, biaxial orientation of semicrystalline polymers, like polypropylene, has found use as thin dielectric membranes for
electrical energy storage applications [53]. In addition, fillers of various shapes, electrical characteristics and orientation, have been employed to enhance the dielectric
behavior of such polymers [43].
The addition of electrically conducting fillers within an insulating polymer matrix
can enhance their electrical and electromagnetic properties for applications in electromagnetic interference shielding and conductive adhesives in circuit elements in the