Download The use of Electron Paramagnetic Resonance (EPR) in the probing

The use of Electron Paramagnetic Resonance (EPR) in the probing
of the nanodielectric interface
Robert K. MacCrone,1 J. Keith Nelson,1* Linda S. Schadler1 , Robert Smith1 and Robert J. Keefe2
1
Rensselaer Polytechnic Institute
Troy, NY 12180-3590, USA
2
EPRI
Palo Alto, CA 94304, USA
* E-mail : [email protected]
Abstract: EPR (sometimes annotated as ESR) has been
carried out on functionalized nanoscale silica embedded
in crosslinked polyethylene (XLPE) in comparison with
the base resin. In situ EPR measurements under an
applied electric field have been carried out, and the
technique is outlined. The preliminary measurements
show that EPR can detect trap sites in the bulk and
interfacial polymer. In addition, EPR can detect texture
in the polymer due to its processing. At the same time,
measurements to determine the dynamics and spatial
distribution of the space charge have been carried out
under similar conditions using the pulsed electroacoustic analysis technique. The results are discussed in
terms of the way in which the nanoparticles affect the
local polymer environment.
INTRODUCTION AND RATIONALE
The finding that enhanced properties can be
demonstrated in composite materials formulated using
nano particulates has spurred considerable interest in
means to optimize performance. It is now becoming
clear that the regions associated with the interfaces are
pivotal, and, as the nanoparticle size is reduced, these
regions dominate in determining the dielectric
properties. In this context, Lewis has recently advanced
the premise [1] that the interface is characterized by a
Stern-Gouy-Chapman layer not unlike the diffuse
double layer known to occur in liquid dielectrics. Such
layers may affect the local conductivity and contribute
to polarization phenomena which give these materials
some of their unique properties. Indeed, it is known that
the space charges in nanodielectrics behave very
differently from the Maxwell-Wagner interfacial
process known in conventional dielectric composites
[2]. The understanding of these phenomena is
undoubtedly the key to being able to tailor the
properties of these new dielectrics. For example, the
introduction of nanoparticles is shown to have a very
significant positive effect on the voltage endurance
exhibited. Over two orders of magnitude improvement
are seen in widely differing materials – both epoxy
thermosets and polyolefins [3].
Often, various chemical treatments (functionalization)
are used to modify the interfacial zones seeking to
enhance performance. The electrical behavior of such
inhomogeneous material depends upon the electrical
parameters of the various components (polymer,
nanoparticles, interfacial regions) which in turn are
determined by their respective resident traps (donors,
acceptors). The approach taken here is to identify and
characterize the singly occupied traps by electron
paramagnetic resonance (EPR), which arises from the
unpaired spin. By comparing subsequent measurements
of electrical behavior with the corresponding trap
content, the goal is to obtain new insights into the
mechanisms involved.
In addition, knowledge of the macroscopic spatial
distribution of the injected or induced charge under an
applied electric field is another key consideration. The
possibility exists that the time dependencies of the two
behaviors may enable a correlation to be made between
the corresponding changes, and thus determine where
particular processes occur. Such knowledge is important
to the ultimate goal to tailor the electrical properties of
this new class of materials particularly as particle size is
reduced and the interfacial regions start to dominate the
properties.
THE USE OF EPR AS A PROBE
Electron transport in polymer dielectrics takes place by
electrons moving between traps or localized states. The
term impurity conduction, used here, denotes the
tunneling or hopping of electrons from filled donors to
empty acceptors without thermal ionization into the
conduction band. Despite the recognition that impurity
conduction is important in insulator dielectrics, and
extensive literature discussion of the trapping of charge
in localized traps, the actual donor and acceptor trap
sites have eluded direct spectral and structural
identification. For example, in the case of thermally
stimulated currents, the estimation of the trap site
activation energy is deduced from the results, but the
actual nature of the trap site is not.
One method for probing these electrically active sites is
EPR. In this technique, the magnetic field on the
sample, placed in an X-band microwave cavity, is
increased until the energy difference between the spinup and spin-down orientations, which are also affected
by the local environment, match the microwave
frequency of the instrument. Strong absorption is then
detected. The instrument plots the derivative of the
imaginary part of the magnetic susceptibility as a
function of field.
In the simplest case of spin only, a very sharp peak and
trough line would be observed. On the scale of the
figures used here, the peak and trough (only a few mT
wide) would not be resolved. Fortunately, in the cases
studied here, the signal arises from oxygen radicals in a
tetragonal orthorhombic environment. The EPR
parameters depend on the details of the chemical
bonding. A random orientation of such a set of moieties
gives rise rather to a very broad (25 – 50 mT)
asymmetrical peak. In this work, such a set is identified
by the location of the low field maximum on the
magnetic field axis.
The SiO2/XLPE polymer nanocomposite used in this
work has previously [4] been shown to be rich in
oxygen radicals. It is important to note that:
•
•
•
oriented vertically in the web due to flow during
molding. Thus when the magnetic field is parallel to the
electric field, it will be across the chain direction and
when the magnetic field is perpendicular to the electric
E
there can be several different structural types of
oxygen radical species present,
each type of oxygen radical species is associated
with a unique donor or acceptor energy, and
there were few organic radicals, or defects, that
could be detected that are associated with the
carbon atoms of the organic polymer itself.
The work on identifying qualitatively and quantitatively
the acceptor and donor traps around the nano-particles
strongly suggests that the Gouy-Chapman layer so often
invoked in discussion, may, in fact, involve these
oxygen and organic radicals. More importantly, they
may be involved not only as ions, but also as acceptor
and donor states responsible for impurity conduction as
defined above. To prove this conclusively requires that
the filled donor and empty acceptor concentration
changes induced by the electric field be monitored. This
implies in situ measurements. Consider a simple model
shown below of 5 oxygen radicals.
O(.) – O(.) – O(.) – O(.) – O(.)
Here the (.) represents an unpaired (EPR active)
electron spin on the oxygen. The EPR spectrum of this
structure would have a relative amplitude of 5. On
polarization the electron arrangement might be
O( ) – O(.) – O(.) – O(.) – O(..)
now having a relative amplitude of 3. This predicted
decrease in EPR intensity is considerably more general
than the very specific model used above might imply.
EXPERIMENTAL ARRANGEMENTS
The principle of the EPR technique has been previously
described by Wolter et al. [5] and the configuration
used here is depicted in Fig.1. The innovation involves
the application of electric fields to the polymer
specimen while being scanned in the microwave cavity.
A special ‘I’-shaped sample is molded to achieve large
creep surfaces to accommodate the high-voltage (See
insert of Fig.1). Note that the polymer chains will be
Figure 1: A schematic of the EPR arrangement – sample
configuration shown in the insert
field, it will be along the chain direction.
RESULTS AND INTERPRETATION
To investigate the nature of the trap, donors, unpaired
electrons and holes (hereafter called EPR active
moieties) in the polymer/matrix materials, EPR was
measured on thin slabs of XLPE, and a 12½ wt %
functionalized XLPE/SiO2 polymer. The formulation of
the cross-linked polyethylene (XLPE) materials and the
functionalized nanocomposite have been described
earlier [4]. EPR spectra for a XLPE sample is shown in
Fig.2 without any applied electric field on the specimen.
The two plots represent two orthogonal orientations of
the sample, the directions are designated B⊥ and B║.
Tests conducted at intermediate angles (not shown here)
showed a systematical variation from the two extremes
shown in Fig. 2, and, presumably, also for Fig 3 (see
later).
The immediate salient features of note are:
● The spectra extend over a wide magnetic field, more
than 0.2T as evidenced by the significant signal on
either side of the peak.
● For both specimen types, the B║ spectra are very
different from the B⊥ spectra. This is due to the
preferred orientation (texture) of the polymer chains.
● The B║ spectrum of the XLPE shows only one low
field maximum at ~ 0.30 T, while that for the 12½%
nanocomposite material shows two low field maxima at
~ 0.30 and ~0.26 T respectively (see arrows in Fig.3).
These values will be used to label the different sets of
oxygen radicals [4] which in turn reflect the different
respective polymer structures.
An assumption is being made here that these unpaired
spins are associated with oxygen radicals, even though
the pure polymer contains no extrinsic silica. It is well
known that, during normal polymer processing, oxygen
radical formation occurs and in this case peroxides are
added to aid in crosslinking which certainly introduce
oxygen to the system.
Interestingly, in-house
characterization of the starting functionalized SiO2
powders used in the preparation of the nanocomposite
also showed no detectable oxygen radical species.
Differential of Imaginary Mag. Susceptibility
2.15
2.1
2.05
2
1.95
1.9
1.85
1.8
From these features several important observations can
be made:
1.75
1.7
0.24
0.26
0.28
0.3
0.32
0.34
0.36
0.38
0.4
Applied Flux Density (T)
Differential of Imaginary Mag. Susceptibility
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
0.24
0.26
0.28
0.3
0.32
0.34
0.36
0.38
0.4
Applied Flux Density (T)
Figure 2. EPR Spectra of XLPE. Magnetic field (a)
perpendicular, B⊥, (b) parallel. B║.
Differential of Imaginary Mag. Susceptibility
2.65
2.55
(a)
2.45
2.35
2.25
2.15
2.05
0.22
0.27
0.32
0.37
0.42
Applied Flux Density (T)
Differential of Imaginary Mag. Susceptibility
2.3
2.2
(b)
2
1.9
1.8
1.7
1.6
0.27
0.32
0.37
To investigate the motion of the carriers along the
chains, selected EPR spectra were measured under the
action of an electric field in the E direction (see Fig.1).
First, in principle, without charge injection, it would be
expected that many of the distances between the carriers
would be changed due to the charge motion and
subsequent pile up at internal barriers. Unfortunately,
with the electric fields up to 15 kVmm-1 and limited
signal averaging, the expected changes were not seen.
Second, with charge injection, it would be expected that
the Fermi Energy would increase to accommodate the
injected charge, and the number of unpaired spins
would decrease. This being the case, the magnitude of
the absorption would also be expected to decrease. This
was also not observed.
IMPACT FOR INTERNAL CHARGE
2.1
1.5
0.22
● The orientation of the different oxygen radical species
configurations is not isotropic as evidenced by the
difference between the B║ spectra with the B⊥ spectra.
Since the polymer segments in such polymers are
known to be textured, the EPR active oxygen moieties
almost certainly lie along the polymer segments and are
also ultimately responsible for the texturing.
● The 12½ wt % specimen not only contains the oxygen
radical structure present in the XLPE only polymer,
namely the 0.3 T structure, but also contains another
different radical structure as well, namely, the 0.26 T
structure. It is reasonable to associate this latter
structure (0.26 T) with the interfacial polymer.
Interestingly, the texturing process has not
discriminated between the two material samples.
0.42
Applied Flux Density (T)
Figure 3. EPR Spectra of functionalized SiO2/XLPE
nanocomposite. Magnetic field (a) perpendicular, B⊥, (b)
parallel. B║.
Figure 4 provides the time-resolved spatial distribution
of the internal charge in the two materials with the
polarization charge subtracted out. It is evident that
homocharge is injected both from the anode (A) and
cathode (C) in both cases. However, although the
applied nominal field (30 kVmm-1) is the same in both
instances, the base XLPE is characterized by the
injection of copious charge (see arrows) not seen to the
same extent in Fig. 4(b). This is consistent with Fig. 5
which shows that the onset of charge injection (where
the charge magnitudes depart from linearity) for the
nanocomposite is about 31 kVmm-1 in contrast with a
stress of about 20 kVmm-1 for the XLPE alone. Fig. 5
relates to an applied field which has been applied for 2
hr. Calculations of the associated field show that the
behavior results in a 32% increase in the peak field over
the nominal value for the XLPE which is reduced to
only 7% for the nanocomposite under the same
conditions. This may, in part, account for the improved
Charge Density (C/m 3 )
3
2
(a)
Material
Anode
Cathode
XLPE
54.5 ks
16.6 ks
SiO2/XLPE
nanocomposite
0.8 ks
11.1 ks
Maxwell-Wagner polarization which has been seen to
be characteristic of nanocomposites [2].
1
0
-1
C
CONCLUSION
A
-2
0.00
0.20
0.40
0.60
0.80
1.00
Normalized Position
1 min
30 min
1 hr
2 hr
Charge Density (C/m 3 )
3
2
(b)
1
0
-1
C
ACKNOWLEGEMENTS
0.20
0.40
0.60
0.80
1.00
Normalized Position
1 min
30 min
1 hr
2 hr
Figure 4. Distribution and dynamics of internal
calibrated charge for (a) XLPE and (b) SiO2/XLPE
nanocomposites.
breakdown properties seen in nanocomposites.
However, perhaps of greater importance in the context
of this study is the charge decay characteristic (curves
not shown here). The decay time constants are estimated
from the charge dynamics when the samples are short
circuited and depicted in Table 1. This suggests that the
environment changes associated with the particles
reflected in the EPR results involves mobile charges
which would act to mitigate the
60
Charge Density (C/m )
The authors are grateful for funding received from EPRI
during the conduct of this work. Pulsed electroacoustic
tests were conducted by Mrs. C. Liang whose efforts are
also much appreciated.
REFERENCES
[1] T.J. Lewis, “Interfaces are the dominant feature of
dielectrics at the nanometric level”, IEEE Trans EI.
Vol. 11, 2004, pp 739-53.
[2] J.K. Nelson and J.C. Fothergill, “Internal charge
behaviour in nanocomposites”, Nanotechnology,
Vol. 15, 2004, pp 586-9
[3] J.K. Nelson, “The promise of dielectric
nanocomposites”, IEEE Int. Symposium on Elect.
Ins.., Toronto, Canada, June 2006, pp 452-57
[4] Roy M., Nelson J.K., MacCrone R.K., Schadler
L.S., Reed C.W., Keefe R. and Zenger W.,
“Polymer nanocomposite dielectrics – the role of
the interface”, Trans. IEEE, Vol. DEI-12, 2005, pp
629-43 and p 1273
80
40
20
0
-20 0
The EPR results show that the oxygen radicals
associated with the interfacial polymer have lower
ligand field splittings compared to their counter-part
oxygen radicals in the pure polymer: specifically
0.26T:0.33T. These energy differences, resulting
from structural changes which are induced by
proximity to nanoparticles may play a crucial role in
determining the conductivities of the various regions.
These, in turn, are reflected in the behavior of the
measured internal charge.
A
-2
0.00
3
Table 1. Charge decay time constants in XLPE
and SiO2/XLPE nanocomposite
10
20
30
40
-40
-60
-80
-100
Electric Field (kV/mm)
Figure 5. Determination of injection threshold.
12.5% SiO2/XLPE nanocomposite. 2 hr stressing.
[5] K.D. Wolter, J.F. Johnson and J. Tanaka in
“Engineering Dielectrics”, Vol 2B, ASTM, 19