Studies to unravel some underlying mechanisms in nanodielectrics
R.C. Smith, C. Liang, M. Landry, J.K. Nelson and L.S. Schadler
Rensselaer Polytechnic Institute, Troy, NY12180, USA
Abstract: The intense interest in nanocomposites in the
last few years results from the documented
enhancements in mechanical, thermal and electrical
properties that are possible when particulates of
nanometric scale are embedded in a polymer matrix. It
has become clear that the primary reason for these
improvements is the dominance of the internal
interfacial surfaces which characterizes nanodielectrics
of this form. This contribution seeks to employ a
plurality of experimental methods (dielectric
spectroscopy, thermally-stimulated currents, divergentfield spectrally-resolved electroluminescence and
absorption current measurements) to shed some light on
the relevant mechanisms responsible for the electrical
characteristics
observed.
A
cross-linked
polyethylene/SiO2 system is used, where material
formulation has allowed alteration of both the interfacial
area (through particle size adjustment) and the chemical
nature of the interface. The conclusion is reached, after
assessment of the interrelationships between these
techniques, that the internal surfaces at the interfaces
have a profound effect on both the injection of charge
and its subsequent migration through the structure.
Material preparation and testing methods
Commercially-available low-density polyethylene beads
(LDPE) and silica powder (of nominal particle size 6
m and 12 nm) were each vacuum-dried (the
polyethylene at 80oC and the silica at 165oC) for 24
hours. Two different batches of the 12 nm nanosilica
were dried, one containing untreated particles, the other
having undergone a surface treatment with triethoxy
vinyl silane vapor. The polyethylene and the powders
were removed from the vacuum ovens just prior to
mixing to ensure that water ingress was minimal, since
it has been demonstrated that the presence of moisture
in the blending process causes increased particle
agglomeration [1], which would reduce the desired
polymer/silica interaction zone surface area.
For
composite mixing, the LDPE and appropriate silica
particles were first dry-mixed using a dual asymmetric
centrifuge to help break up macroscopic powder
agglomerations and to pre-coat the LDPE beads with
micro or nano particles. Next, a high-shear melt mixer
was used to thoroughly blend the materials at 130oC;
blending effectiveness has been verified using electron
microscopy. 2% (by weight) dicumyl peroxide was
added as a cross linking agent. Laminar samples were
pressed at 165oC, and gold electrodes were applied by
sputter deposition. Highly-divergent field samples were
pressed in a cylindrical mold and electrolytically-etched
tungsten electrodes inserted to produce the high field
required for an electroluminescence experiment. Postpressing vacuum oven treatment at 80oC was performed
on the samples for 72 hours, to remove residual
moisture and cross linking byproducts.
As a preliminary investigative tool to reveal
behavioral differences between conventional (micro)
composites and nanocomposites, low-field dielectric
spectroscopy was used to evaluate the materials’
relative dependence on the removal of moisture and
cross linking byproducts, which are generally known to
affect electrical properties [2]. The laminar samples
were tested both before and after the byproducts were
removed, over a range of frequencies and temperatures.
The same laminar samples were later placed in a
guarded test cell under 50 kV/mm applied field and
absorption current was measured using an electrometer
to investigate charge mobility and the related charge
trapping. Next, the samples were used as electrets in a
thermally-stimulated discharge current (TSC) study.
After poling at 30 kV/mm at room temperature for 10
minutes, cooling to -160oC under the field, then
removing the field and shorting each sample through an
electrometer, the relaxation current was measured as
each sample was slowly (3oC per minute) warmed to
90oC. The distribution of space charge was also
measured in each laminar sample using pulsed
electroacoustic (PEA) analysis, in a test cell under a
high electric field to ensure charge injection. This
charge was spatially resolved and calibrated using
deconvolution software.
Finally, an electroluminescence experiment, which
uses the highly-divergent electric field sample
mentioned above, was performed to study the presence
and behavior of both trapped and mobile space charges
by recording the relative amount of light emitted, and its
energy (wavelength) during electroluminescence, under
excitation at high field. A series of discrete optical
filters and a sensitive photomultiplier/photon counter
were used to resolve the energy levels for study.
permittivity and loss in the nanocomposite materials can
be an indicator of reduced charge mobility due to
trapping and/or scattering mechanisms occurring at the
polymer/nanofiller interface [6].
Experimental results
Dielectric spectroscopy of pre and post vacuum
oven-treated materials
Absorption current testing
Figure 1 illustrates the spectroscopy results for the real
electric permittivity ’ at room temperature. As is
commonly seen in conventional (i.e. micro) composite
materials, there is an increase in the low frequency
permittivity, presumably due to moisture (and perhaps
derivatives of the cross linking peroxide, such as
cumene and cumyl alcohol), which can introduce
polarization at the polymer/microparticle interface,
raising the bulk ’ in this frequency region [3,4]. The
interfacial polarization comes about due to the
separation of space charges at the interface, modifying
the local electric field condition [5]. As the figure
shows, vacuum treatment substantially mitigates the
increase. Further evidence of interfacial polarization
was noted in the before and after plots of ” (not
shown), in that the low-frequency loss peak was
5
4
3
2
On the subject of charge mobility, absorption current
tests were performed on the materials to evaluate the
relative ease with which charge fronts propagate
through the material bulk. This transit time, tt through a
dielectric’s thickness, d, under an applied voltage, V,
has been used to make such mobility calculations [7]
0.79  d 2

tt  V
Absorption data, at an applied field of 50 kV/mm
(above the charge injection threshold for both
materials), are illustrated in Figure 2. Initially, both the
XLPE and the nanocomposite display the classic
μ 
I(t)  t  n shape, presumably until the above-mentioned
charge front arrives at the electrode, at which time there
is a demonstrable change in the slope. For XLPE, this
takes place at approximately 20 seconds, while for the
composite it occurs at 100 seconds, indicating that
charge mobility has been reduced by a factor of 5. It
may be inferred that the combined effects of trapping
(by the vast increase in impurity sites) and scattering
(by the greatly increased particle surface area) are
responsible for the change.

10000
1
0
1
2
3
4
5
6
log f (Hz)
Micro After
1000
Nano Before
Nano After
Figure 1: ’ at 25oC before and after vacuum treatment
pA
Micro Before
100
considerably reduced in the microcomposite after the
vacuum heat treatment.
In contrast to the microcomposite plots, the real
permittivity of the treated nanocomposite was nearly
independent of the presence – or subsequent removal –
of moisture and the cross linking byproducts, in the
frequency range studied. In fact, the non-surface treated
nanocomposite (curves not shown here), displayed
nearly identical behavior to that of the treated
nanocomposite: no substantial difference in permittivity
before and after vacuum treatment. This would suggest
that the large increase in the polymer/filler interface
surface area (on the order of 1000 times more for the
nanocomposite if we make the simplifying assumption
of no particle agglomeration) is the overriding factor in
low-field dielectric behavior, rather than impurity
chemistry (see the Conclusions section). Reduced
10
10
100
1000
t (s)
XLPE
Nano
Figure 2: Absorption current for XLPE and the treated
nanocomposite at 50 kV/mm applied field
Thermally-stimulated discharge currents (TSC)
Figure 3 presents the data gathered during the
thermally-stimulated current experiment. While the
interpretation of TSC relaxation data for semicrystalline polymers like polyethylene is generally
understood to be more complicated than that for purely
amorphous materials, one feature does stand out here.
There is an enhancement of the relaxation peak above
room temperature, (the so-called α- peak). It is known
that enhancement in the number of charge trap sites, and
subsequent measurement of increased thermally-assisted
relaxation currents can occur, especially for
heterogeneous materials like the nanocomposite, where
the myriad polymer/particle interfaces provide such
surfaces [8]. The demonstrated result supports this idea,
because the released charges produce a current 3 to 5
times more intense in the nanocomposite than in the
base XLPE.
1.E+06
-peaks
Cathode
I (pA)
1.E+05
XLPE
Micro
VS-Nano
Anode
Figure 4: Space charge profiles from the PEA
experiment 10 seconds after power-off
1.E+04
Electroluminescence (EL)
1.E+03
1.E+02
1.E+01
-160
homocharge, indicating its lack of mobility for injected
space charge under the influence of the electric field.
-120
-80
-40
0
40
80
o
C
XLPE
Nano
Figure 3: TSC results (poled at 30 kV/mm)
Pulsed electroacoustic (PEA) analysis
Figure 4 illustrates the space charge condition for the
base XLPE, microcomposite, and surface-treated
nanocomposite 10 seconds after power-off following a
2-hour poling period.
A series of PEA experiments
had previously been performed to determine the
threshold electric field intensity for charge injection,
and each curve in the figure was obtained under the
same multiple of that threshold. While the base XLPE
displays a region of cathode-shielding homocharge
extending nearly to the anode, and the microcomposite
contains some field-strengthening heterocharge near
both electrodes, the plot indicates the presence of
shielding homocharge at both electrodes for the
nanocomposite material. This may be further evidence
of increased trapping of charge and/or scattering.
Homocharge injected by the cathode in XLPE (which,
from the absorption current experiment, was seen to be
relatively free to move) appears to have migrated all the
way to the anode, where some recombination could
have occurred with space charge there. Similarly, the
microcomposite’s heterocharge regions may have
resulted from electron migration from the cathode
leaving behind positive ionic space charge. However,
the nanocomposite maintains two distinct regions of
More information on the charge trapping condition was
sought using an electroluminescence experiment, where
the transition of charge carriers to lower-energy states
below the conduction band emits light. The wavelength
emitted is related by Planck’s constant to the energy
released, and by extension of this idea, to the trap depth
[9]. The electroluminescence concept may be further
divided into intrinsic EL, where carriers internal to the
dielectric enter the conduction band through tunneling
or as a result of impact ionization, and extrinsic
(injection) EL where the external electrodes supply the
carriers, which then either become bound by trap sites in
the dielectric or recombine with other species,
producing luminescence. Figure 5 presents the results
of the experiment for XLPE and the nanocomposite
material. It is seen that, compared to the XLPE, there is
a shift toward the lower energy (higher wavelength)
bands in the nanocomposite One might envision that
charge carriers encounter a vastly-increased number of
interfacial sites introduced by the nanoparticles, and
thus the carriers’ kinetic energies do not attain the same
(relatively) high values as in the base XLPE, This
results in either a change in the recombination levels or
the excitation of lower energy luminescent centers.
Therefore, we see the movement of luminescence
energy toward the higher wavelengths in the
nanocomposite.
As an aside, it needs to be noted that the XLPE
emission at 400 nm is not necessarily maximum, since
apparatus limitations do not permit testing at
wavelengths below 400 nm at the present time;
however, there is evidence in the literature that
polyethylene’s emission spectrum below about 400 nm
drops off sharply [10], supporting the notion that the
apparent peak at 400 nm is in fact a real one.
a.u.
XLPE
400
300
200
100
0
400
450
500
550
600
550
600
wavelength (nm)
a.u.
12-1/2% VS-Nano
400
300
200
100
0
400
450
500
wavelength (nm)
Figure 5: Electroluminescence
electrode field of 500 kV/mm
results
using
an
Conclusions
This effort set out to examine the relationships between
data collected via several test methods to obtain a
clearer picture of some of the mechanisms involved in
nanocomposite electrical behavior. As a result, a better
understanding of interfacial polarization, charge
trapping, and charge movement in the materials tested
has been realized.
Dielectric spectroscopy over a limited frequency
range revealed a certain insensitivity of the
nanocomposite material to the removal of moisture and
the remnants of the cross linking chemical. It is
speculated that the great increase in filler particle
numbers (that is, compared to an equal loading of micro
filler) produces a relative homogeneity in the region
immediately surrounding molecules of water and the
cross linking byproducts, such that there is significant
cancellation of the impurity/particle dipole moments. In
this way, the net effect of interfacial polarization would
actually be reduced over a microcomposite before its
vacuum treatment. It was also stated that there was
essentially no difference in the dielectric properties
between
the
surface-treated
and
untreated
nanocomposite, providing further evidence that the
presence of the nanoparticle/polymer interfaces is of
more importance in this test than is the specific surface
chemistry.
Evidence of increased charge trapping and/or
charge carrier scattering was seen in the absorption
current data. The experiment was conducted for the
base XLPE and the nanocomposite above the charge
injection threshold for both materials, and evidence of
reduced mobility was seen by the higher charge front
transit time in the nanocomposite. In addition, the
pulsed electroacoustic analysis experiment revealed that
regions of injected screening homocharge were built up
in front of both electrodes in the nanocomposite, an
indicator of reduced charge mobility, while the base
XLPE showed evidence of space charge migration from
cathode to anode, for the same applied field and test
duration.
The TSC and EL results both suggested that the
number of trap sites is increased in the nanocomposite
material, resulting in an increase of stored energy during
the poling phase of TSC while evidenced as a shift of
electroluminescence emission toward lower energy.
It is thus clear that the nanoparticle surfaces have
an important effect upon the behavior and movement of
charge in the nanocomposite. The complementary tests
used allowed for a study of charge injection, migration,
and mobility, and provided qualitative information
about trap depth and scattering. Further work will allow
refinement and better quantification of these important
parameters.
Acknowledgement: The authors are indebted to EPRI
for their support of this work.
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[7]
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Author address: Rob Smith, Rensselaer Polytechnic
Inst., Troy, NY 12180-3590, USA
Email:
smithr8@rpi.edu