Download Review of Recent Activities on Dielectric Films for Capacitor

Journal of International Council on Electrical Engineering Vol. 4, No. 1, pp.1~6, 2014
http://dx.doi.org/10.5370/JICEE.2014.4.1.001
Review of Recent Activities on Dielectric Films for Capacitor Applications
Lejun Qi †, Linnea Petersson** and Tieliang Liu*
Abstract – Polypropylene (PP) film has been used in capacitors since 1970s. The high breakdown
strength, low dielectric losses, and high availability make PP well suitable for use as capacitor dielectric.
At the moment, the typical energy density achieved with PP film at room temperature is about 1.2 J/cm3.
Recent research on PP films focus on improving the permittivity, breakdown strength, thermal
withstandability, and thermal conductivity, by producing copolymers, blending with other polymers, or
adding inorganic particles. Some results claim energy density higher than double that of commercial biaxially oriented PP capacitor film. Detailed research approaches and dielectric properties improvements
are reviewed in this paper. The major focus is often placed on permittivity and breakdown strength.
Unfortunately, little research shows positive results for maintaining the low dielectric losses.
Keywords: Capacitor, Dielectric film, Polypropylene, Polyester, Polycarbonate, Poly(vinylidene fluoride)
permittivity (most, <10) still limits the energy density [2].
As a result, an improvement of the intrinsic electrical
properties of the most commonly used polymers is highly
requested. It will have a high impact of the size of power
capacitors.
Table 1 shows the main features of the most common
dielectric polymer films used today in different capacitor
applications. The main characteristics of these polymer
films will be reviewed in the following sections of the
paper.
1. Introduction
Capacitors are important passive components for
electrical networks and apparatus. They can either improve
the efficiency of HVAC power networks, or act as energy
storage and stabilize the voltage level in HVDC systems.
Dielectric materials in HV capacitors play a key role for
charge control and energy storage.
Before the 1970s, impregnated kraft paper was the main
capacitor dielectric and it was used in combination with
mineral oil, polychlorinated diphenol (PCB), as impregnated
liquid [1]. However, due to the low dissipation factor, high
insulation resistance, good stability, and high availability,
the polymer films gradually replaced the kraft paper in
capacitors. A switch from paper to polymer film also
shortened the production process for capacitors, by
reducing the drying time needed before impregnation.
In general, the energy density of a dielectric material can
be expressed as:
Table 1. General characteristic of common dielectric
polymer films [3]
-12
Where ε0 is the vacuum permittivity (=8.85 ×10 F/m),
εr is a relative dielectric permittivity of linear dielectric
materials and E is applied electric field. Although polymers
exhibit high breakdown strength, their low dielectric
Energy
Density
(J/cm3)
Dissipation
Factor %
1kHz
640
1-1.2
<0.02
125
570
1-1.5
<0.5
2.8
125
528
0.5-1
<0.15
3
200
550
1-1.5
<0.03
12
125
590
2.4
<1.8
εr
Polypropylene
(PP)
2.2
105
Polyester (PET)
3.3
Polycarbonate
(PC)
Polyphenylene
sulfide (PPS)
Polyvinylidenefluoried (PVDF)
Ue =1/2εrε0E2,
Maximum Breakdown
temperature strength
(℃)
(MV/m)
Polymer film
2. Polypropylene films
†
Corresponding Author: ABB China Corporate Research Center,
Beijing, China ([email protected])
*
ABB China Corporate Research Center, Beijing, China
** ABB AB, Corporate Research, 721 78 Vasteras, Sweden
Received: November 19, 2013; Accepted: January 6, 2014
The most important polymer film used in commercial
capacitors is bi-axially oriented isotactic polypropylene.
The beneficial properties of PP come from the polymers
chain structure which does not contain any polar groups.
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Review of Recent Activities on Dielectric Films for Capacitor Applications
PP has the highest breakdown strength among common
polymer films used in capacitor applications. PP also has a
low dielectric loss and excellent self-healing capability,
which allows operation at high electric field. On the other
hand, PP has relatively low permittivity, resulting in a
typical energy density of 1.2 J/cm3 at room temperature [3].
Much work has been dedicated towards improving the
dielectric properties of PP film. Examples trying to increase
the permittivity and breakdown strength are quite common.
Researchers have tried to blend PP with other polymers and
nanoparticles, or to synthesize PP-based copolymers.
after an applied electric field of E=600 MV/m, without
showing any significant increase of energy loss.
A family of thermally cross-linkable isotactic poly
(propylene-co-p-(3-butenyl)styrene) (PP-BSt) copolymers
have been synthesized [4,5]. The thin films made of these
copolymers have also been prepared. Upon cross-linking,
the x-PP thin film dielectric having 3.65 mol % BSt units
(x-PP-3 as shown in Fig. 1), exhibits a ε~3, which is
independent of temperature ( 20°C - 100°C) and frequency
(100 Hz - 1 MHz). The cross-linked PP also shows high
breakdown strength (E=645 MV/m), narrow breakdown
distribution (β=42), and reliable energy storage capacity >5
J/cm3 (double that of state-of-the-art BOPP capacitors) [6].
2.1 Copolymers
2.2 Blends
Fig. 2. Temperature dependence of the dielectric permittivity
of PVDF/PP blend composite in a 1/1 volume
fraction at selected frequencies as indicated. [8].
Besides synthesizing copolymers, many efforts have
been made to blend other polymers directly with PP base.
For instance, low density polyethylene (LDPE), poly-vinyl
chloride (PVC), ethylene propylene rubber (EPR), and
ethylene–vinyl acetate (EVA) are easy to process and are
excellent electrical insulators in electrical dielectric designs
[7]. Furthermore, poly(vinylidene fluoride) (PVDF) / polypropylene (PP) blend composites [8] and polypropylene
(PP) / acrylonitrile-butadiene-styrene (ABS) blends [9]
have been investigated. Dang et al. prepared PVDF/PP
blends using a Haake mixer close to the melting
temperature [8]. The blends had high permittivity values
over wide frequency ranges. The permittivity increases with
the temperature and the amount of PVDF added into the
blend. The dielectric permittivity was about 5.4 for a
composite with 50 vol % PVDF at room temperature. It was
Fig. 1. (a) Dielectric constants (vs frequency and temperature) and (b) Weibull distribution for breakdown
strength of PP, x-PP-1, x-PP-2, and x-PP-3 thin film
dielectrics, which respectively have 0, 0.64%,
2.97%, 3.65% BSt units. [6].
In order to increase the permittivity, poly(propylene-coundecen-11-ol) copolymer has been synthesized, containing
flexible OH polar groups [4]. This polar group structure
shows a significant increase in the dielectric constant to
about 4.6. A linear reversible charge storage behavior was
also observed with a high releasing energy density (7 J/cm3)
2
Lejun Qi , Linnea Petersson and Tieliang Liu
also revealed that PP-g-MAH could improve the interface
interaction between two polymer matrices [10].
double that of pure PC. The 32-layered films showed
comparable results for high PC concentrations but much
lower Ud at high PVDF concentrations.
2.3 Nanofillers
There are many investigations on dielectric and electrical
properties of composites with inorganic fillers in isotactic
(iPP) or syndiotactic (sPP) polypropylene matrices,
especially nanofillers [11], such as multiwalled carbon
nanotube [12], layered silicate [13, 14, 15] , Fe2O3, ZrO2
and Al2O3 as core in shell nanoparticles (NPs) [16], carbon
nanofiber [17], carbon black [18], and glass beads [19].
Increased permittivity and loss factor were often seen
compared with unfilled iPP [20-23], It has been shown that
trapping properties are highly modified by the presence of
nanofillers [24]. The composites also exhibit improved
thermal stability, and higher crystallinity due to that the
nanofillers hinders the mobility of the polymer chains [10].
A novel two-phase composite system using polypropylene
(PP) as matrix and semiconductor bismuth sulfide (Bi2S3)
as filler has been reported [25]. The results reveal a
percolative behavior of the Bi2S3/PP composites at the
threshold of 8 vol%. The composite demonstrates high
thermal stability and breakdown strength, which show their
potential in film capacitors applications.
Fig. 3. Energy density of 32-layer (solid circles) and 256layer (open circle) PC/PVDF films [29].
3.2 Polyethylene terephthalate (PET)
Next to PP, PET is the second most commonly used
dielectric polymer in high-performance capacitor
applications. PET offers slightly lower breakdown strength
than PP films, a reasonable dielectric constant (ε=3.3), and
a high operating temperature (125 °C) [30]. When only
dielectric constant counts, the energy density of a capacitor
is 50% higher if PET films are used to replace PP films.
The disadvantage of PET is the relatively high dissipation
factor, which increases with temperature and frequency
with a minimum at around 50 Hz. In case of a high
repetition rate, polyester is not suitable for high pulse
power applications [31].
Hybrid coatings, based on poly (ethylene oxide) (PEO)
or polycaprolactone (PCL), and silica (SiO2) have been
used atop the PET films [32]. The electrical strength of the
coated films increases up to 10-15% of the uncoated ones
regardless of polymer type (PEO/PCL) and amount of
inorganic phase, as far as the thickness of the coating is
below 5 µm. However, the loss factor together with the
permittivity also increases particularly at low frequencies
(< 10 Hz). To reduce the dielectric loss of PET film,
acrylate/barium titanate nanocomposites have been
prepared and coated on PET [33]. The composite film had
higher permittivity, higher dielectric strength and lower
dielectric loss. This was a new method of producing high
stored energy density capacitor with low cost industrially.
3. Other dielectric films
3.1 Laminates
The aim of producing laminates is to ensure both high
energy density and high electrical breakdown strength.
Multilayer structures are produced by combing two (or
more) materials that are commercially available and used in
capacitor applications. The multilayer structure can be
produced either directly co-extruded during film manufacturing, or by melting together films that are already
prepared.
Baer et al. [26-29] produced multilayer dielectric films
through the process of multilayer coextrusion. Layermultiplying coextrusion is an advanced polymer processing
technique for combing two or more polymers in a layered
configuration with controlled architecture. It is a continuous
processing technique, capable of economically producing
films with up to thousands of layers with individual layer
thickness from micro- to nano-meter scale.
Fig. 3 shows that the highest measured energy density,
Ud ~14 J/cm3, for the 50/50 layered material was ~60%
greater than that measured for pure PVDF and was nearly
3
Review of Recent Activities on Dielectric Films for Capacitor Applications
Meanwhile, REF also has a high dielectric constant, which
is attractive for development of high-density energy storage
capacitors and electronic packaging. As shown in Table 2,
PVDF and P(VDF-TrFE) could also be converted into
relaxor ferroelectrics, e.g. P(VDF-CTFE)[10], P(VDFHFP)Tomer, and P(VDF-TrFE-CFE)[11], by importing
bulky groups CTFE, HFP and CFE as defects in molecular
structures to obtain high electric energy density. The novel
Ag-BaTiO3/PVDF (polyvinylidene fluoride) three-component
nanocomposites have been prepared, which exhibit high
energy density of nearly 10 J/cm3 with ε=28 at room
temperature[39].
3.3 Polycarbonate (PC)
When polycarbonate (PC) film was first introduced as
capacitor dielectric in the 1960s, many electronic systems
relied on PC film capacitors for operation at temperatures
not exceeding 125°C. The use of PC in capacitors was
successful for decades [34]. However, because of the large
shrinkage, small Young's modulus, and easy formation of
defects and weak spots, the amount of PC used for
capacitors has been significantly reduced. It is today only
available in small quantities. Today, other commercially
available plastic dielectric films are used to replace PC
dielectrics, such as polyimide (PI, “Kapton” as trademark
name by Dupont) and polyphenylene sulfide (PPS).
Because the KaptonTM is only available in thick films, it is
impractical for capacitors.
PPS is a good candidate for PC replacement. PPS is a
crystalline polymer, a high quality engineering plastics,
with outstanding chemical resistance, low water absorption
(about 0.02%), excellent thermal stability (heat deflection
temperature of greater than 260 °C), high elastic modulus,
and it is flame-retardant. The comparison by Mark A.
Crater of constructed capacitors showed that PPS exhibits
lower dissipation factor and equivalent series resistance
versus frequency than PC [35]. PPS films have gradually
been employed in both AC and DC capacitors.
Aromatic polyurea thin films fabricated through vapor
deposition polymerization also possesses a high thermal
stability, above 200 °C [36]. This high quality polymer film
exhibits a high breakdown field of 800 MV/m and a highenergy density of 12J/cm3 with ε = 4.2 at room temperature,
which make it attractive for high-energy density capacitors,
especially under high temperatures. However, the
production routine of such film does not allow for large
scale applications.
Table 2. Comparison of some dielectric polymers based on
PVDF and P(VDF-TrFE) [39]
Polymer film
εr
P(VDF-CTFE)
P(VDF-HFP)
P(VDF-TrFE- CFE)
Ag-BaTiO3/PVDF
13
15
52
28
Breakdown strength
(MV/m)
620
700
400
280
Energy Density
(J/cm3)
25
25
10
10
4. Conclusion
The most commonly used dielectric material in power
capacitors today is isotactic polypropylene due to its high
breakdown strength, low losses, good self-healing properties,
and its excellent ageing behavior. Capacitors are becoming
a larger part of many types of power system applications
and as a result it is becoming increasingly important to
reduce the size of capacitor. The development of new
dielectric materials with high breakdown strength and high
energy density is of high focus for many research groups.
Improvements using different methods have been reviewed
in this paper. Some improvements in dielectric strength,
permittivity, as well as energy density, have been claimed.
However, the losses are difficult to maintain at the low
level of pure iPP.
3.4 Polyvinylidene–fluoride (PVDF)
PVDF and its copolymer P(VDF-TrFE) seem to be the
best known ferroelectric polymer with a very high dielectric
constant. They appeared to be promising in a variety of
applications.. However, they also show high dissipation,
due to the polar vinylidene fluoride group, leading to high
losses [37].
It has been found that P(VDF-TrFE) can be changed
from a normal ferroelectrics to a relaxor ferroelectrics
(RFE) as small defect concentration introduced by highenergy electron irradiation [38]. The RFE developed here
exhibits a high electrostrictive strain which is greatly
attractive for many actuator and transducer applications.
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Lejun Qi is a Scientist at the ABB Corporate Research
Center in Beijing, China. He holds a Ph.D. degree in
material science and engineering from the University of
Minnesota, USA. Prior to joining ABB, he worked for
Formulated Systems R&D of Dow Chemical, developing
epoxy formulations for electrical insulation and fiberreinforced composites. He has been involved in research
and development projects for insulation materials for
outdoor applications and dielectric films for power
capacitor applications.
Linnea Petersson is a Principal Scientist at the ABB
Corporate Research Center in Västerås, Sweden. She holds
a Ph.D. in material science and engineering from the
Norwegian University of Science and Technology, Norway.
Prior to joining ABB she worked as Materials R&D
Engineer at Zarlink Semiconductor. At ABB she has been
leading research and development projects on insulation
materials for high voltage applications, with special focus
on metalized film power capacitors.
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