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www.etasr.com Danikas et al.: Some Factors Affecting the Breakdown Strength of Solid Dielectrics: A Short Review
Some Factors Affecting the Breakdown Strength of
Solid Dielectrics: A Short Review
M. G. Danikas
Democritus University of Thrace
School of Engineering
Department of Electrical & Computer
Engineering
Xanthi, Greece
G. E. Vardakis
Democritus University of Thrace
School of Engineering
Department of Electrical & Computer
Engineering
Xanthi, Greece
R. Sarathi
Indian Institute of Technology
Madras
Department of Electrical
Engineering
Chennai, India
Abstract—This paper refers to some factors affecting the
breakdown strength of solid insulating materials. Solid insulating
materials play a most important role in the high voltage industry.
Factors, such as area effect, crystallinity, impregnation with
liquids, temperature, the role of interfaces and mechanical
stressing, are investigated and commented upon.
Keywords-solid dielectrics; solid insulation; breakdown
strength; crystallinity; amorphous regions; temperature; interfaces;
mechanical stressing
I. INTRODUCTION
Solid insulating materials are essential for the good
functioning of high voltage equipment in most applications [1-
3]. Such materials include traditional materials, such as paper,
as well as polymeric materials and – more recently – polymer
nanocomposites. Various factors, such as the shape of
electrodes, their area, the applied electric field, gap spacing, the
structure and type of polymeric materials, their molecular
weight, types of additives, water and gas content, type of
voltage waveform used, pressure of the impregnant (if any),
have an effect on the breakdown strength of solid dielectrics. In
the following sections a short review of some of the
aforementioned factors will be given.
II. AREA EFFECT OF THE ELECTRODES
The influence of the area of the electrodes affecting the
breakdown strength of high voltage insulation in general is well
known, especially from the research with insulating liquids [4,
5]. Such a phenomenon exists also when investigating the
breakdown strength in solid dielectrics. As was shown quite
early, an area effect exists since larger area electrodes include
more defects or weak points. Larger electrode areas result in
lower breakdown strength, as was shown in experiments with
Teflon films [6]. Larger electrodes also produce shorter
average breakdown times [6]. Authors in [7], worked with
spherical and cylindrical electrodes at power frequency and
indicated that a pronounced area effect exists since much lower
breakdown strength results were obtained with cylindrical
depressions [7]. Early breakdown studies were somehow
related with the notion of “intrinsic electric strength”, a notion
prevalent at that time [8]. Later research indicated that this
notion was erroneous and that the so-called intrinsic breakdown
strength was not a property characterizing the insulating solids
[9]. As is written in [9], “this word “intrinsic” reflecting history
in that early theories stimulated belief in the existence of a
physical constant, independent of geometry, electrode material,
voltage waveform, and determined by molecular and/or crystal
structure and temperature. The concept, is not supported by
experiment”. Admittedly, one cannot find much data on the
electrode area effect on the breakdown strength of solid
dielectrics. However, the general lines that are followed in the
other categories of insulating materials, are also followed in the
case of solid dielectrics, namely that the breakdown strength
depends on the configuration of the electrodes, that the physical
shape of the electrodes gives the variation of the breakdown
voltages, and that the above mentioned quantities define the
electric field distribution [10].
III. CRYSTALLINITY
Earlier experiments with various types of organic insulating
materials indicated that the breakdown strength of polyethylene
was not affected by doubling its crystallinity [11]. Polystyrene,
however, a partially aromatic polymer, showed a considerable
reduction in the breakdown strength by increasing the ordered
structure of the polymer (4.9MV/cm was the average
breakdown strength for isotactic polystyrene pressed film
compared to 7.1MV/cm for atactic polystyrene pressed film).
The isotactic character of the polystyrene gives a crystallisable
polymer, which permits a favorable geometry for interaction
between the adjacent benzene rings [11]. Other researchers,
investigating the dielectric properties of thermally aged
biaxially orientated polypropylene (BOPP), observed that
surface degradation was much faster than the bulk degradation
although the total thickness of the films was no more than
25µm. They also showed that the morphological nature of
BOPP films changed during thermal aging. The crystallinity of
BOPP increased by thermal aging since recrystallization of part
of the amorphous phase occurred. The interfacial domain of
crystalline and amorphous phases may play a significant role
for various properties of semi-crystalline polymers including
breakdown strength [12]. During aging, changes in the
chemical structure of polymers may alter processes such as
softening or melting above their glass transition temperature
and consequently, changes in their chemical structure may
Corresponding author: M. G. Danikas
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www.etasr.com Danikas et al.: Some Factors Affecting the Breakdown Strength of Solid Dielectrics: A Short Review
occur. Less obvious physical effects, such as secondary
crystallization in partially crystalline materials may occur and
may cause increased stiffness and structural relaxation in an
amorphous component rendering thus densification and
embrittlement possible [13].
An important point is made in [14], where it is remarked
that “a polymer melt has structural viscosity, i.e the
macromolecules are orientated according to the shear velocity.
[Since] non-Newtonian polymer melt displays a velocity
distribution that is more uniform in the central section of the
flow [and] the velocity gradient close to the boundary surface is
higher … salient orientations and stretchings of the
macromolecules occur in these zones …[consequently]
orientations [that] are initially retained in the insulation and are
frozen during the cooling process by reduction of the chain
mobility and crystallization” may cause inhomogeneities with a
subsequent effect on breakdown strength. On the other hand,
swelling of a polymer – in case of impregnation with liquid –
may be reduced by some factors such as crystallinity. Authors
in [15] reported that the breakdown strength of both polythene
and polypropylene depends mainly on their semi-crystalline
structure and the presence of defects rather than their chemical
structure, since the two polymeric materials have similar values
of breakdown strength. The preponderant role of defects was
also emphasized in [16, 17], since defects can vary from those
of molecular dimensions up to gross defects arising from
inclusions. In a more recent publication, performing tests on
polyethylene terephthalate (PET) with AC voltage, it was
remarked that the breakdown strength is reciprocal to the
longest free path [18]. Although the authors of [18] did not
specifically study the relation between material crystallinity
and breakdown strength, they tacitly imply the relation between
the two aforementioned quantities. Furthermore, they noted
that in thicker films, the boundaries between amorphous and
crystalline parts can be a cause of inhomogeneities, thus
reducing breakdown strength. More recently, authors in [19]
investigated both ceramic (Al2O3, TiO2, SrTiO3,and BaTiO3)
and polymeric materials (PMMA, polystyrene and n-butyl
acetate) and used the so-called dielectric breakdown toughness
(Gbd) which is related to the critical energy release necessary
to initiate the unstable growth of the longest conducting
filament in the sample surface [19]. Evidently, such a quantity
is related also to the very structure of a polymer.
Since a polymer consists of amorphous and crystalline
regions, some sort of cause that will modify the relation
between these two regions, will necessarily have an influence
on the breakdown strength. The microhardness (MH) of a
polymeric material can be expressed as
MHc = αH + (1-α)Ηa (1)
where Hc and Ha are the hardness values of the crystalline and
amorphous regions respectively and α is the volume fraction of
the crystalline region [20]. Authors in [20] showed that the
microhardness of a polymer decreases with temperature, with
the decrease following an exponential law of the type:
H = H0·e
-βT
(2)
where H0 is a constant, T is the temperature (
o
C), and β is the
coefficient of thermal softening. According to [20], a decrease
in microhardness is mainly associated to the softening of the
crystalline region. An increase in the thickness of amorphous
region – and therefore a decrease in crystallinity – will result to
a decrease of the breakdown strength of the insulating material
[21, 22]. On the other hand, working with poly (ether ether
ketone) – PEEK, which is a high temperature insulating
thermoplastic, it was observed that the breakdown strength
decreased by increasing the crystalline/amorphous ratio of the
polymer [23, 24]. The author attributed this to the fact that
increased crystallinity introduces breakdown pathways not
present in the amorphous material.
Investigating BOPP samples, researchers relatively recently
remarked that the breakdown strength increased substantially
when the initially spherulitic morphology was transformed into
a highly ordered fibrillar network structure of densely packed
lamellar crystallites aligned parallel to the film surface [25]. In
accordance with the above, it was reported by other researchers
that with DC high voltage, the breakdown strength increases
with increasing crystallinity because the free volume of
polyethylene becomes smaller as the crystallinity increases
[26]. Thus the free path of electrons decreases and it is more
difficult for them to accelerate. Consequently, there is an
increase in breakdown strength. It is evident from the above
that the effect of crystallinity on the breakdown strength of
polymers depends on the very chemical structure of the
materials as well as on the defects that may be included.
IV. IMPREGNATION EFFECT
Impregnation of solid insulation with appropriate liquids is
necessary in order to avoid the risk of partial discharges and/or
early breakdown. The impregnating liquids must have a
suitable viscosity, so that they can impregnate micro-cavities
that may exist in the solid insulation [1]. It has been established
that an impregnated system prepared without sufficient care
against contamination and/or possible micro-cavities may be
subject to several mechanisms, which may cause its ultimate
deterioration [16]. Quite early it was shown that the nature of
the impregnating liquid is most important for the electrical
behavior of a composite insulating system. The breakdown
strength of low density polyethylene (LDPE) extruded film was
reported to be lower when the film was immersed into o-
Dichlobenzene than when it was immersed into transformer oil
or chlorinated biphenyls. Many pre-breakdown discharges were
evident in the o-Dichlorobenzene in contrast to either the
transformer oil or the chlorinated biphenyls [11]. Silicone oil or
diethyleneglycol were shown to impregnate XLPE cable
insulation better since they both indicated considerable
improvement in breakdown strength. Such liquids exhibited a
50% or even better increase in impulse and AC breakdown
strength. This improvement was attributed to the moderation of
the fatigue mechanism when liquid replaced gas in the
microspaces and consequently to a reduction in the energy of
ionic or electronic deteriorative process [27]. In further studies,
other researchers continued working on XLPE cables
considering filling the microporosities in XLPE with
appropriate gases (such as SF6). They showed that the AC
breakdown strength as well as the partial discharge behavior
improved [28]. In [15], DC and impulse breakdown strength of
polypropylene increased by impregnation with suitable dipolar
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www.etasr.com Danikas et al.: Some Factors Affecting the Breakdown Strength of Solid Dielectrics: A Short Review
liquids (hexachlorodiphenyl –HCP, trichlorodiphenyl – TCP,
dibutyl phthalate-DP) at both room and liquid-air temperatures.
The breakdown strength of polythene impregnated with
chlorinated diphenyl was also found to increase at room
temperature under impulse conditions.
High stability is required from the combination of
polymeric insulation and the impregnant since they have to
resistant to thermal degradation and to oxidation [29, 30]. Low
viscosity oils are preferable because of their better
impregnation when in contact with polymers. Moreover, low
viscosity oils absorb gas more easily than heavily viscous oils
[31]. As was pointed out more recently, “the gas absorption
property of an impregnating liquid together with its viscosity
determines to a very great extent the partial discharge
performance of the liquid impregnated system” [32]. A low
viscosity and high gas absorption capacity oil is always
desirable in order to have low discharge activity and
consequently a high breakdown strength. A combination of
polymer and of a low viscosity absorbing liquid was proposed
several years ago. That was a combination of uniaxially
orientated polyethylene (UOPE) and dodecylbenzene (DDB)
oil [33-35]. Experimental results indicated that since collision
impact of electrons accelerated in the free volume is the
dominant cause for electrical tree initiation, liquid
impregnation is a promising way to improve the performance
of polymeric materials and to rise the breakdown strength [36].
Swelling of the polymer is another parameter determining the
behavior of a composite insulation and must be avoided since
the breakdown strength of the composite insulation can be
seriously deteriorated. Previous work on this has shown that the
AC breakdown strength of a polypropylene laminated paper/oil
combination decreases as swelling worsens [37]. Similar
conclusions were reached with another composite insulating
system consisting of UOPE/DDB oil [33-35]. In [38], the
problem of solid/liquid combination was attacked from another
angle, namely that of the interface and also in terms of the
water content. It was concluded that the breakdown voltage is
reduced more dramatically when temperature is low and water
content is also low. Swelling of polypropylene (PP) films with
rapeseed oil was not that significant and, moreover, the oil
impregnation rose the breakdown strength from 640V/µm to
810V/µm. This rise was attributed to the diffusion of the
rapeseed oil into the PP film. With oil diffusion, the PP foil
structure becomes more regular as the free volume of the
dielectric is filled with the impregnant. Therefore the electric
field distribution in the case of applied voltage becomes more
homogeneous and thus the breakdown strength of the PP foils
increases [39]. The advantages of rapeseed oil (good
impregnation, good resistance to aging) were also reported in
[40]. It must be noted, however, that rapeseed oil lacks w.r.t.
aging vegetable oil which has excellent resistance to aging
[41].
V. TEMPERATURE EFFECT
Temperature has long been recognized of having a
significant effect on the breakdown process of solid insulating
materials. Quite early it was reported, among other types of
deterioration, the thermal depolymerization of polymeric
insulating materials. At elevated temperatures, polymer chains
break into smaller units and this can happen even when oxygen
is not present [42]. More recently, some other researchers,
investigating oil filled kraft-paper cable at high temperature for
a long time, reported that there will eventually be a failure
because of saturation of the impregnant by gas formed as a
result of thermal degradation of the cellulose [43]. Experiments
with polyethylene films indicated that the breakdown strength
decreases with increasing temperature. As an explanation, it
was put forward the electron thermal runaway in localized
electron system caused by the interaction with conduction
electrons [21].
Other researchers looked at the temperature effect from
another interesting viewpoint, namely that of the temperature
influence during sample preparation. In this case, temperature
may greatly influence the morphology of a polymer and also
the field necessary for tree initiation. As ambient temperature
increases, both tree initiation and breakdown occur at lower
stresses [44]. Experiments with low-density polyethylene
(LDPE) samples of various thicknesses showed that in the
range of 80
o
to 100
o
C, breakdown strength increased. Above
and below this temperature range, the slope of temperature
dependence of breakdown strength was observed to be
negative. Such a behavior could be explained by suggesting
that the input energy rate from impulse voltage is partly
devoted to melt the crystalline lamellae increasing thus the
amorphous parts and decreasing the electron mobility.
Consequently, an increase of the electric stress is needed to
trigger a breakdown [45].
Orientated polymers seem to be more stable than non-
orientated polymers. However, as mentioned in [29], orientated
polymers may be subjected to a rapid shrinkage above a critical
temperature. Degradation of UOPE films has been observed
several years ago, especially when these are combined with
DDB oil [33-35]. Besides the fact that such a composite system
(UOPE/DDB oil) starts having problems at about 110
o
C, oils
with aromatic constituents can cause crazing to polymers much
more easily than polybutene oil [46, 47]. On the other hand,
polybutene oil has poorer gas absorbing characteristics, its
absorption rate of hydrogen being equal to 1/9 of that of the
DDB oil [48]. Regarding composite solid/liquid insulating
systems, it is worth noting that, in earlier decades some
researchers investigating the gassing characteristics of mineral
oils, reported that an increase in temperature leads to an
increase in gas absorption as well as in gas evolution [49, 50].
Elevated temperature tests turn out to give lower breakdown
strength, which may be due to an increase of the thickness of
the amorphous region of the polymer and to the greater
solubility and diffusivity of the gaseous byproducts. Greater
solubility and diffusivity in turn will sustain discharges of a
certain magnitude and will possibly cause discharges of an
even larger magnitude, which will erode the polymer with
accelerating rate and will eventually lead to failure [33-35].
Measurements with DC voltages showed that the
breakdown strength of fluorinated parylene (PA-F) decreased
with increasing temperature from ambient up to 350
o
C. The
authors attributed the decrease to the fact that the thermal
energy induces changes leading to the acceleration of free
electrons. Moreover, as the temperature increases, the trapped
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www.etasr.com Danikas et al.: Some Factors Affecting the Breakdown Strength of Solid Dielectrics: A Short Review
electrons are freed more easily with less electric field needed.
A further contributing factor to the decrease of the breakdown
strength is the decrease of the material density due to the free
volume increase [51]. Such an interpretation is in agreement
with previous publications [52, 53]. The interpretations of [51-
53] are not fundamentally different from the interpretations
given earlier [45]. The free volume increase, the accumulation
of internal heat and of the space charges cause an increasing
electric field distortion with increasing temperature and thus
they become the contributing factors for the decrease of
breakdown strength of XLPE with DC voltages [54]. In [55],
polyethylene terephthalate (PET) showed a decrease of
breakdown strength going up from 33
o
C to 73
o
C because of –
as the authors claimed – the activation of thermal breakdown
pathways [55]. Such an approach is not radically different from
the interpretation given in [21].
The work performed with thin and thick dielectric films
introduced yet another parameter. It was observed that while
the breakdown strength decreased with increasing temperature,
such a decrease was less noticed with thicker samples than with
thinner samples. It was suggested that the breakdown strength
is less sensitive to temperature for thicker samples because the
electromechanical mechanism is the dominant mechanism for
the dielectric breakdown [56, 57]. On the other hand, their
interpretation for the dependence of the thinner samples was
similar to the interpretation put forward in [51].
VI. THE ROLE OF INTERFACES
Interfaces play a significant role in determining the
breakdown strength of solid dielectrics. Differing phases may
also play a role. The interfacial domain between crystalline and
amorphous phases may well determine the various properties
of semi-crystalline polymeric materials [58]. Interfacial
breakdown was studied with a variety of electrode systems.
Paper and transformer oil were used as insulation. When the
paper was not carefully dried, interfacial breakdown would
occur [59]. In [59], however, was also noted that with a
carefully prepared paper-oil interface, breakdown did not
necessarily took place at the interface. Such observations were
also made later with silicone rubber interfaces [60]. Modern
insulating systems, such as those with solid dielectrics, did not
avoid problems of interfaces, since there were interfaces of
semi-conducting sheaths with the main insulation or the
inclusion of microcavities and/or impurities [61, 62]. The role
of microcavities at interfaces between solid and solid dielectric
was treated in [63]. Microcavities between solid dielectrics,
especially with the applied field parallel to the interface,
enhance the risk of low breakdown strength. Such risk may be
avoided if a mechanical pressure is applied to the two solids.
Mechanical pressure (i.e. contact pressure) implies a better
contact between the two surfaces and consequently, a better
interface [64, 65]. In [66], the same research group claimed that
lubricated interfaces offer higher breakdown strength than non-
lubricated interfaces. Moreover, in silicone rubber-silicone
rubber interfaces, despite the fact that air voids are the limiting
factor, the injection of liquids/gels to air voids is of vital
importance for high breakdown strength. Authors in [64-66]
confirmed the observations and conclusions of earlier works
[67, 68]. In a further work, one of the authors of [66]
distinguished between vented and enclosed cavities at an
interface between solid-solid dielectrics, and he proposed the
idea that discharges inside such cavities may develop to
streamers, which in turn will lead to breakdown. The gas
pressure inside the enclosed cavities is likely to increase as a
function of the change in the size of the cavity, the extent of
which is dependent on the contact pressure and elastic
modulus. Increased gas pressure, in turn increases the discharge
field strength of the cavities according to Paschen’s law [69].
The threshold electric field for partial discharge initiation
depends primarily on the solid dielectric under stress, when the
surrounding medium is air or liquid. This threshold field was
shown to be much lower in the case of solid-air interfaces than
in the case of solid-liquid interfaces. Such experimental data
indicated the importance of impregnation with liquids [70].
Quite early research pointed out that prolonged heating in a
composite solid-liquid insulating system affects mainly the
solid component, and in the case of paper-oil system, a
significant increase of acidity was observed. Evidently, the
increase of acidity of the oil after prolonged heating in the
presence of paper was due to the degradation of the paper and
not to the alteration of the oil [71]. Furthering and extending
the thoughts of [71], it was shown later that – with
polycarbonate impregnated with low viscosity mineral oil –
ionizable impurities in mineral oil enhance space charge losses.
The latter in turn causes the lowering of breakdown strength
[72]. The importance of ionic carriers for the dielectric
behavior of a polypropylene/oil system was also noticed in
[73]. Considering the role of interfaces on lapped taped
polymers (such as HDPE, polypropylene, and nylon
impregnated with supercritical helium) authors in [74]
indicated that there is no tendency for the discharges and
breakdown to concentrate in butt gaps adjacent to the metal
conductor. The butt gaps next to the conductor were no more
frequently the seat of breakdown than the butt gaps in the
interior of the insulation. In this respect, the authors in [74]
reached similar conclusions with the authors in [59, 60]. More
recently, the importance of the difference between the
permittivities of the solid dielectric and the liquid insulant was
emphasized w.r.t the electric field applied parallel to the
interface. As the solid dielectric has – as usually is the case – a
higher permittivity than the liquid, the initial streamer will
convert to breakdown as the strong attractive force causes the
streamer to settle on the interface. Parallel high permittivity
liquid immersed dielectrics accelerate the charge transport
between the electrodes [75]. The conclusions of [75] are in
accordance with those published earlier, where it was remarked
that the flashover mechanism is initiated in the liquid
immediately adjacent to the spacer/liquid interface as a
consequence of electric-field intensification induced by
permittivity mismatching between the solid and the liquid [76].
As noted elsewhere, breakdown strength depends on the
quality of the interfaces. Due attention must be paid to the
choosing of the materials for specific applications and to the
construction of the composite insulating system. Interfaces in
the conventional insulating systems, can be considered as the
weak aspects of such systems. Without the need of an
exaggeration, one can conclude that the problems arising in
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high voltage applications are for the most part problems of
interfaces [77].
VII. MECHANICAL STRESSING
In the context of the present paper we will not deal with
mechanical stressing and its effect on electrical trees in solid
dielectrics (since this topic has been dealt before [78]) nor we
will deal with internal residual mechanical stresses. The aim
here is to somehow mention several studies in relation to the
mechanical stressing to the breakdown strength of soli
dielectrics. About four decades ago, it was shown that both DC
and impulse breakdown strength of polythene increased by
compression. The effect of changing from an uncompressed to
a compressed sample was an increase of 38% in the mean
breakdown strength (from 0.61 to 0.84GV/m) [15]. Indications
of the influence of mechanical stressing on the breakdown
strength of polythene were given even earlier, when it was
noted that its impulse strength at 80
o
C was much higher than
the breakdown strength of the same material tested under the
same temperature with a slowly applied electric stress [79]. The
beneficial effect of compressive stresses can be understood on
the basis of an initial increase of density, which results in
closing up voids and to increase molecular packing. On the
other hand, the compressive stresses may cause the breakage of
tie-bonds and create defects. This is the reason for which,
experiments with PET and epoxy resin showed that, although
initially there was an increase of breakdown strength,
subsequently it decreased. Tensile stresses may cause crack
initiation and growth. Such stresses may cause defects at the
amorphous-lamellae interfaces of semi-crystalline polymers
(such as polyethylene and polypropylene) [80]. The
observations of [80] confirmed previous research, where it was
indicated that elongation causes a decrease of breakdown
strength for both LDPE and HDPE, no matter what sort of
voltage was applied (AC, DC, and impulse). The authors
attributed the decrease to defects produced by mechanical
stretching [81]. Confirmation of [80], as to the perplexing
effect of compressive stresses on polymers, was reported
recently [82], where the authors investigated the breakdown
strength of an acrylic dielectric elastomer. The authors
especially emphasized on the relation between the electrode
configuration and the applied compressive stresses. Tensile
stresses may have a beneficial effect with some polymeric
materials since they tend to decrease thickness and
consequently to increase breakdown strength. Such data were
given with transparent silicone polymers in [83].
A recent report tackled the problems of both tensile and
compressive stresses. It stated that when the silicone rubber is
subjected to a tensile stress, the molecular chains are stretched
in the stress direction leading to fracturing. On the contrary,
when a compressive stress is applied, the molecular chains
come closer and they generate more attraction bonds and
consequently more stabilization in the silicone rubber structure
[84]. In yet another recent publication, it was reported that the
accumulated damage is higher in the case of tensile stress than
in the case of compressive stress, whereas the space charge,
free volume and partial discharges increase in the presence of
tensile stresses and decrease in the presence of compressive
stresses [85].
VIII. SOME GENERAL REMARKS
In this paper, a brief review on some factors affecting the
breakdown strength of polymeric materials is given. Although
effects such the area effect are more or less clear by now, other
factors, such as the role of crystallinity, need further
clarification. As was noted recently, the effect of polymer
crystallinity on breakdown strength is a problem of carrier
mobility in general. As carrier mobility increases, breakdown
strength decreases. Therefore polymers with higher
crystallinity have lower breakdown strength than those with
lower crystallinity. On the other hand, if polymers have a lot of
small crystallites (even if crystallinity is large), inter-crystal
boundaries may reduce mobility, and therefore increase
breakdown strength. Detailed states of polymers (morphology,
impurities, crystal-amorphous boundary conditions, density of
boundaries, length of crystals and amorphous regions) play a
crucial role [86]. Carrier mobility also increases as temperature
increases in both crystalline and amorphous regions and drastic
increase in mobility is anticipated especially in the amorphous
regions [86]. The influence of mechanical stresses is also a
subject in need of further exploration since with tensile stresses
there is a critical value for crack formation, whereas with
compressive stresses there is the possibility of breakage of tie-
bonds and of creation of defects.
IX. CONCLUSIONS
The present paper deals with some factors affecting the
breakdown strength of polymeric materials. Factors such as the
area of the electrodes, the polymer crystallinity, the
impregnation of polymers, the temperature effect, the quality of
interfaces, and the effect of mechanical stresses are
investigated and commented upon. In the context of this brief
review, only conventional polymers were considered.
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