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Citation: Nelson, J. K., Fothergill, J., Dissado, L. A. and Peasgood, W. (2002). Towards
an understanding of nanometric dielectrics. Conference on Electrical Insulation and
Dielectric Phenomena (CEIDP), Annual Report, pp. 295-298. ISSN 0084-9162
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Towards an understanding of nanometric dielectrics
J. Keith Nelson
§
, John C. Fothergill, L.A. Dissado and W. Peasgood
Department of Engineering, University of Leicester, UK
§
on leave from Rensselaer Polytechnic Institute, Troy, NY, USA
Abstract: Dielectric studies are described aimed at
providing an understanding of the charge storage and
transport of an epoxy resin containing TiO
2
nanoparticles. Comparative results for conventionally
filled composites are given, and the results discussed in
terms of the underlying physics. It is shown that
nanometric fillers mitigate the interfacial polarization
characteristic of conventional materials with a reduction
in the internal field accumulations.
Background and Vision
Nanoparticles are the fundamental building blocks in
the design and creation of assembled nano-grained
larger scale structures with excellent compositional and
interfacial flexibility. However, rather surprisingly, the
current push to develop nanomaterials based on
nanotechnology has not focused much on the
opportunities for dielectric materials, but rather centred
on optical and mechanical applications. Nonetheless, the
few examples in the literature
provide encouragement
that this is likely to be fertile ground. Furthermore there
are good theoretical reasons why the pursuit of
nanomaterials for dielectric applications may have
particular promise. Some of these have been reviewed
by Lewis[1] and by Frechette[2]. While the technology
is in its infancy, one may speculate that it may be
possible to self-assemble nanodielectrics by providing
chemical structures with “hooks” which provide
preferential attachment points for the nanostructured
materials allowing automatic and predictable self
assembly.
Fundamental Issues and Rationale
The use of conventional fillers for polymer materials is
well known and is usually employed to reduce the cost
of a material or to modify one of the material properties
for a particular application. Examples of that would be
discharge resistance, thermal expansion, etc. Often the
use of such fillers will affect dielectric strength and loss
in a negative way. In this context, it is thought that
fundamental to controlling the dielectric strength of
insulating polymers is the cohesive energy[3] density
and the associated free volume[4] of a polymer
structure. This may be gauged by examining the
changes in electric strength (up to a factor of 10)
exhibited by most polymers as they are taken through
their glass transition temperature[5]. In the simplest
situation, the bonding of a polymer to a filler can be
expected to give a layer of “immobilized” polymer. The
size of this layer is critical to the global properties
(electrical, mechanical and thermal) of the composite.
Such a picture is not, however, complete since
the in-filled material will give rise to space-charge
accumulation and the associated Maxwell-Wagner
polarization due to the implanted interfaces.
Furthermore, the macroscopic theories of interfacial
polarization do not incorporate a molecular approach
since the response is given by relaxation equations if the
wavelength is large in comparison with molecular
dimensions. In considering pre-breakdown high-field
conduction in pure materials, the existence of localized
states within the energy band gap (close to the
conduction or valence bands) is usually invoked, so
giving rise to a mobility edge for electron (or hole)
transport[6]. These states are essentially localized on
individual molecules. This is because, unlike the strong
covalent bonds of elemental crystalline solids, the
intermolecular binding arises from weak van der Waals’
forces that do not allow inter-molecular electronic
exchange
System Characterization
In order to provide the basis for engineering
nanodielectrics, this study has provided a
characterization of micro- and nano-particulates of
Titanium Dioxide (TiO
2
) when embedded in a resin
matrix. A Bisphenol-A epoxy (Vantico CY1300 +
HY956) was chosen because it was benign (i.e. without
other fillers or dilutents), with a low initial viscosity,
Material
+ Filler
Size
(nm)
Loading
(%)
Tg
(ºC)
CY1300 Resin N/A N/A 63.8
CY1300 + TiO
2
Micro (1500) 1 76.1
CY1300 + TiO
2
Micro (1500) 10 73.9
CY1300 + TiO
2
Micro (1500) 50 79.9
CY1300 + TiO
2
Nano (38) 1 62.9
CY1300 + TiO
2
Nano (38) 10 52.4
CY1300 + TiO
2
Nano (38) 50 62.1
Table 1 Glass transition of nano- and micro-filled TiO
2
1E+0
1E+1
1E+2
1E+3
1E+4
1E+5
1E+6
1E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6
Frequency (Hz)
Rel. Permittivity (Real)
1E-2
1E-1
1E+0
1E+1
1E+2
1E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6
Frequency (Hz)
Loss Tangent
Figure 1. Permittivity and loss tangent for micro-filled epoxy.
Temp: (bottom to top) 293, 318, 343, 368, 393 K
1E+0
1E+1
1E+2
1E+3
1E+4
1E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6
Frequency (Hz)
Rel. Permittivity (Real)
1E-2
1E-1
1E+0
1E+1
1E+2
1E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6
Frequency (Hz)
Loss Tangent
Figure 2. As Figure 1, but for nano-filled material
and with a glass transition below 100 ºC. Test samples
were formed by molding between polished surfaces held
apart by spacers in the manner previously described[7].
The molded films ranged in thickness between 500 and
750 µm. The weighed resin and hardener were degassed
at 35 ºC and the relevant dried particulate fill was
incorporated into the resin by mechanical stirring. Due
to their small size, surface interactions for nanoparticles,
such as hydrogen bonding, become magnified. This
means that the particles tend to agglomerate and
dispersion in resins is quite difficult, even in polymers
that should be relatively compatible. Hence, in the case
of nano-particles, large shear forces are needed in the
mixing process to obviate unwanted clustering of the
particles. For most electrical characterization, the cast
film was provided with evaporated 100 nm aluminium
electrodes.
Differential Scanning Calorimetry (DSC)
A Stanton Redcroft DSC 1500 calorimeter was used to
thermally characterize the materials. Results on the
determination of glass transition temperatures are
provided in Table 1 for post-cured samples from which
it is evident that the nano-material reduces T
g
in contrast
to the larger size particles that have the opposite effect.
This suggests that particles of nanometric dimensions
behave in a similar way to infiltered plasticizers[8],
rather than as “foreign” materials creating a
macroscopic interface.
Dielectric Spectroscopy
Some insight into the way that the incorporation of
materials on nanometric dimensions affect the dielectric
properties may be obtained by examining the variation
of the real and imaginary components of relative
permittivity as a function of temperature and frequency.
This has been done for the TiO
2
material using a
Solartron H.F. frequency response analyzer (Type 1255)
in combination with a Solatron Dielectric Interface,
Type 1296. Examples for the micro- and nano-filled
materials are shown in Figs. 1 and 2 respectively. At a
nominal 10% (weight percent) particulate loading, the
spectra of the resin when filled with particles of micron
size (1.5 µm) are virtually indistinguishable from the
base resin. This suggests that the low frequency process
is probably associated with charges at the electrodes and
not due to particulates in the bulk. With the filler
replaced with 10% of nanometric size TiO
2
(38 nm
average diameter measured by TEM), the main
differences seen relate to a marked modification of the
process seen in the base resin at low frequencies and
high temperatures. For the nanometric material the
process exhibits a flat tan δ response at low frequencies
in marked contrast to the micron-sized filler. This
suggests that a percolation conduction process is
operative. In the presence of the nano-filler, the mid
frequency dispersion is noticeably reduced. The nano
materials are clearly inhibiting motion (see PEA results
below). The mid-frequency process shows a small
change in estimated activation energy from 1.7 eV to
1.4 eV. The magnitude of this process is reduced in the
case of nanoparticles since the side chains responsible
for the mid-frequency dispersion bind to the particle
surface.
Reduction of the particulate loading from 10 to
1% (by weight) did not have any very obvious
fundamental changes, but the nano-filled material then
does start to exhibit a low frequency response more
typical of the base resin and micro-filled material,
suggesting that changes engineered by the
nanomaterials do require loadings greater than a few
percent.
Space charge assessment
In order to determine whether nanomaterials function
cooperatively as opposed to providing sites for
interfacial polarization, a Pulse ElectroAcoustic (PEA)
study has also been conducted to assess the field
distortions in the bulk. The method has been described
elsewhere[9]. The initial distribution of stress shows
little deviation from the nominal 4.3 kVmm
-1
uniform
level across the bulk (see Figure 3). However,
characteristic results are shown in Figs. 4 and 5 for the
micro- and nano-materials (10% loading) respectively
after several hours of stressing. These plots show the
charge, potential and field distributions, for a 3 kV
steady DC field applied. The 1.5µm filler generates
substantial internal charge, in marked contrast to the
nano-material which behaves in a similar way to the
base resin. Fig 4 shows several distinctive features:
(a) heterocharge accumulation of both signs
leading to steep internal charge gradients
(b) a cathode field augmented to over 40
kVmm
-1
(10x the nominal value)
(c) field reversal yielding a point of zero
stress which will greatly complicate
charge transport.
Transient studies (not shown here) indicate that
subsequent increase of applied voltage increases the size
of the charge peaks with little change to the complex
internal distribution. The stable stationary positioning of
these peaks may be due the interaction of space charge
with local polarization to create a self-compensating
situation.
Appraisal
Very marked differences in charge accumulation are
seen in filled materials depending on whether the filler
has micron or nanometric dimensions. Furthermore, the
characteristics suggest that, for the micron-sized filler,
carriers are blocked at the anode yielding a heterocharge
Figure 3 Initial distribution of electric field . Electroacoustic
study of nano-filled material. Voltage, V (kV); charge,
ρ
(C.m
-3
),
and electric field, E (kV.mm
-1
). The double headed arrow
indicates the 726 µm thickness of the sample (cathode on left).
8
0
-8
V
ρ
E
Figure 4. Pulsed electroacoustic study of micron-sized filler.
80
0
-70
E
ρ
V
Figure 5. Pulsed electroacoustic study of nano-sized filler
6
0
-7
E
V
ρ
situation, and giving rise to the large anomalous field
distortions seen in Figure 4. This behavior clearly has
substantial implications for the subsequent migration of
charges and probably accounts for the fact that temporal
studies (not given here) show that the image charge in
the cathode at first decreases and then recovers. Again
in contrast to the micro-filled material, the decay of
charge in the nano-filled TiO
2
is very rapid; with
insignificant homocharge remaining after just 2
minutes. Although there is some injection of negative
charge at the cathode, the nano-filled material is
characterized by much less transport perhaps brought
about by the larger density of shallower traps.
The PEA results taken in conjunction with the
Dielectric Spectroscopy and DSC studies suggest that
significant interfacial polarization is implied for
conventional fillers which is mitigated in the case of
particulates of nanometric size, where a short-range
highly immobilized layer develops near the surface of
the nanofiller (1-2 nm). This bound layer, however,
influences a much larger region surrounding the particle
in which conformational behavior and chain kinetics are
significantly altered. This interaction zone is
responsible for the material property modifications
especially as the curvature of the particles approaches
the chain conformation length of the polymer. Evidence
suggests that the local chain conformation and
configuration play major roles in determining the
interactions of a polymer with nanofillers[10], as is
evidenced here by the DSC results of Table 1. The
polymer binding to the nanoparticles replaces some of
the cross-linking and thus loosens the structure. In
contrast, the micron scale case produces significant
Maxwell-Wagner polarization giving rise to the
characteristics of Fig.4.
In the case of nanofillers, there is evidence that
a grafted layer is formed by the absorption of end-
functionalised polymers onto the surface especially
when the functional groups are distributed uniformly
along the polymer backbone. Hence the local chain
conformation is critical to determining the way in which
bonding takes place (and thus the cohesive energy
density). The defective nature of nanoscale particles can
be expected to enhance the bonding if chemical
coupling agents (CVD coatings on nanoparticles or
triblock copolymers) are employed.
The large interaction zone in nanofilled
polymers with reduced mobility (free volume) should be
accompanied by a significant change in electrical
properties. Studies of electrical behavior thus provide an
opportunity both for a fundamental study of this
interaction zone, and also an opportunity for optimizing
performance for specific and critical applications.
The finding that conventional fillers are
accompanied by substantial bulk charge accumulation is
clearly a factor in the common experience of the lower
electric strengths exhibited for filled materials. The
mitigating effects of nanoparticles provides
encouragement that nanocomposites can be engineered
with strengths that are commensurate with the base
polymer. Such studies are ongoing.
Acknowledgements
The authors are indebted to the UK EPSRC under
whose auspices this study has been started, and for the
loan of the DSC equipment.. Thanks are also due to
Rensselaer for sabbatical leave, and the provision of
nanometric powders through the Nanotechnology
Center.
References
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[3] Sabuni H. and Nelson J.K., “Factors determining
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