256
S. X. Drakopoulos et al.
Another very important factor for cable insulation applications is the desired
hydrophobic character of the dielectrics in use; the unwanted water molecules
induce charge transport resulting into poor dielectric breakdown strengths and failure.
Studies of hydrophilic/hydrophobic nanofillers at different humidity environments
have shown that the drawbacks from the moisture sorption can be greater than the
benefit of having the ceramic nanofiller in some cases [63]. As it can be observed
in Table 1, composites with oxide fillers perform much worse in ambient and wet
environments when compared with dry conditions, whereas composites using nitride
fillers had a much more consistent response under ambient humidity. Ayoob et al.
introduced hexagonal boron nitride in polyethylene to impart good great thermal
conductivity and hydrophobicity to the polymer matrix, resulting in minimal charge
transport at even very high filler concentrations (30% w/w) [60]. To reduce the
drawbacks from the diffusion and adsorption of water molecules intended for such
applications, chemical modification of the surface between the fillers and the polymer
matrix can be implemented [64].
Other key features to consider when designing new composite insulators are the
geometry and orientation of the fillers. Montmorillonite (MMT) nanosheets characterized by a high aspect ratio, have been proved to diminish the electrical treeing,
through the encapsulation of mobile charge carriers in the interfaces between the
matrix and the nanosheets [43, 49]. Further enhancement on the dielectric properties was observed by inducing filler orientation, which yields higher dielectric
breakdown strength and higher energy efficiency when compared to an isotropic
system of the same composition [43]. The anisotropic character can be enhanced
further by orienting the crystalline and amorphous domains of the polymer matrix
Table 1 DC breakdown values for polyethylene composites with oxide and nitride fillers in
different humidity environments [63]. Adapted from Hosier et al. The effects of hydration on
the DC breakdown strength of polyethylene composites employing oxide and nitride fillers. IEEE
Transactions on Dielectrics and Electrical Insulation 2017, 24, 3073–3082
Nominal filler content
and type
Ambient conditioning
(MV/m)
Dry conditioning
(MV/m)
Wet conditioning
MV/m
–
416 ± 30
425 ± 32
408 ± 32
5 wt. % Si 3 N 4
409 ± 30
431 ± 34
172 ± 14
10 wt. % Si 3 N 4
389 ± 30
450 ± 27
150 ± 12
5 wt. % SiO 2
228 ± 14
431 ± 30
135 ± 12
10 wt. % SiO 2
194 ± 12
414 ± 30
102 ± 7
5 wt. % AlN
366 ± 19
362 ± 18
241 ± 14
10 wt. % AlN
362 ± 18
373 ± 15
350 ± 26
5 wt. % Al 2 O 3
349 ± 17
393 ± 24
241 ± 14
10 wt. % Al 2 O 3
259 ± 15
360 ± 25
154 ± 12
5 wt. % SiO 2 ©
425 ± 42
442 ± 34
295 ± 26
10 wt. % SiO 2 ©
463 ± 29
436 ± 20
207 ± 23
S. X. Drakopoulos et al.
Another very important factor for cable insulation applications is the desired
hydrophobic character of the dielectrics in use; the unwanted water molecules
induce charge transport resulting into poor dielectric breakdown strengths and failure.
Studies of hydrophilic/hydrophobic nanofillers at different humidity environments
have shown that the drawbacks from the moisture sorption can be greater than the
benefit of having the ceramic nanofiller in some cases [63]. As it can be observed
in Table 1, composites with oxide fillers perform much worse in ambient and wet
environments when compared with dry conditions, whereas composites using nitride
fillers had a much more consistent response under ambient humidity. Ayoob et al.
introduced hexagonal boron nitride in polyethylene to impart good great thermal
conductivity and hydrophobicity to the polymer matrix, resulting in minimal charge
transport at even very high filler concentrations (30% w/w) [60]. To reduce the
drawbacks from the diffusion and adsorption of water molecules intended for such
applications, chemical modification of the surface between the fillers and the polymer
matrix can be implemented [64].
Other key features to consider when designing new composite insulators are the
geometry and orientation of the fillers. Montmorillonite (MMT) nanosheets characterized by a high aspect ratio, have been proved to diminish the electrical treeing,
through the encapsulation of mobile charge carriers in the interfaces between the
matrix and the nanosheets [43, 49]. Further enhancement on the dielectric properties was observed by inducing filler orientation, which yields higher dielectric
breakdown strength and higher energy efficiency when compared to an isotropic
system of the same composition [43]. The anisotropic character can be enhanced
further by orienting the crystalline and amorphous domains of the polymer matrix
Table 1 DC breakdown values for polyethylene composites with oxide and nitride fillers in
different humidity environments [63]. Adapted from Hosier et al. The effects of hydration on
the DC breakdown strength of polyethylene composites employing oxide and nitride fillers. IEEE
Transactions on Dielectrics and Electrical Insulation 2017, 24, 3073–3082
Nominal filler content
and type
Ambient conditioning
(MV/m)
Dry conditioning
(MV/m)
Wet conditioning
MV/m
–
416 ± 30
425 ± 32
408 ± 32
5 wt. % Si 3 N 4
409 ± 30
431 ± 34
172 ± 14
10 wt. % Si 3 N 4
389 ± 30
450 ± 27
150 ± 12
5 wt. % SiO 2
228 ± 14
431 ± 30
135 ± 12
10 wt. % SiO 2
194 ± 12
414 ± 30
102 ± 7
5 wt. % AlN
366 ± 19
362 ± 18
241 ± 14
10 wt. % AlN
362 ± 18
373 ± 15
350 ± 26
5 wt. % Al 2 O 3
349 ± 17
393 ± 24
241 ± 14
10 wt. % Al 2 O 3
259 ± 15
360 ± 25
154 ± 12
5 wt. % SiO 2 ©
425 ± 42
442 ± 34
295 ± 26
10 wt. % SiO 2 ©
463 ± 29
436 ± 20
207 ± 23