World Journal Of Engineering
Applications of nanotechnology in high voltage power equipment - Nanodielectrics
George Chen
Electronics and Computer Science, University of Southampton, United Kingdom
Introduction
The concept of nanotechnology has been with us for a while and
it represents a group of emerging technologies in which the
structure of matter is controlled at the nanometer scale to
produce novel materials and devices that have useful and unique
properties. Due to extensive research in the last twenty years
nanotechnology has found various applications in medicine,
energy, aerospace, information/communication and construction
etc. This paper covers a less known area of application of
nanotechnology, nanodielectrics – nanotechnology application
in high voltage power equipment. The term of nanodielectrics
was first proposed by Prof. Lewis in his very first paper on
nanometric dielectrics in 1994 [1]. Since then nanodielectric
materials have been a popular subject within the electrical
sector. The number of publications on the topic increases
significantly over the years and some of the key papers will be
reviewed to highlight the progress concerning nanodielectric
materials, including their characterisation and possible
mechanisms of operation. In addition the paper also presents
some of our research achievements in nanodielectric research.
Compared with the other areas of nanotechnology research, it is
useful to understand difficult tasks ahead before achieving the
full potential that nanodielectrics can offer. In the last section of
the paper a list of challenges for nanodielectric research is given.
particle size lessens [2].
Interatomic and intermolecular forces are especially important
on the nanoscale. They vary in strength and behaviour, going
from the strong nuclear forces which are extremely short range
and highly repulsive, to electrostatics which repel and attract
over larger distances. The net charge of particles has a
significant effect on these interactions, and dipole interactions
are also important. Van der Waals forces are well understood
and explain dipole interactions. Based on these interactions,
Prof. Tanaka proposed a multi-core model [3] to explain the
interactions of the interfaces created around nanoparticles in
dielectric materials as shown in Figure 2.
Materials and mechanisms
Nanodielectrics consist of a vast array of different materials.
However, the most commonly used in dielectric research are
polymeric nanocomposites, which consist of a dielectric
polymer that has been infused with particles with dimensions
with at least one axis on the nanoscale - below 100nm in length.
[1-2] These materials have been researched in some depth over
the past decades and whilst progress is slower than
nanotechnology relevant to the other industries, the properties
of nanodielectrics are beginning to be understood, and
mechanisms have been postulated to allow for improved
properties in future materials.
One of the key features of incorporating nanoparticles into
polymeric matrix is the rapid increase in surface area. Therefore,
the most widely accepted mechanism framework suggests that
the interaction zone of nanoparticles is the primary cause of the
properties inherent to nanodielectrics [11]. This differs to the
generally accepted mechanisms for microparticles, which tend
to be material dependant. This would be expected, as the scale
of the interaction zone compared with the particle size is several
times larger for an nanoparticle than a microparticle as shown in
Figure 1. These interaction zones can be described by a number
of interaction processes and properties.
The multi-core model shows the nanoparticles with three
distinct layers surrounding them, representing interactions at
short, medium and long range, i.e. (i) a strong bonded layer, (ii)
a bound layer and (iii) a loose layer. A Gouy-Chapman electric
double layer is then super-imposed over the top. This forms the
basis for the best current model for predicting behaviours of
nanodielectrics. However, there are many observed phenomena
that can not be explained by the existing model, therefore, the
improvement on the model or new models should be pursued in
future research.
Figure 2 A multi-core model account for the interactions at the
interface [3].
Achievements
Despite a lack of understanding into detailed mechanisms,
research results from nanodielectrics so far have shown major
improvements in electrical properties and other properties
important to high voltage power equipment applications as
shown below [4-5]:
(i)
Improved partial discharge resistance
(ii)
Suppressed space charge formation
(iii)
Altered dielectric breakdown strength
(iv)
Enhanced thermal conductivity
(v)
Improved flame retardancy
Southampton research experience
Figure 1 Interface properties become more important as the
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World Journal Of Engineering
Nanodielectric material forms a key area of research in The
Tony Davies High Voltage Laboratory at the Southampton
University. Over the years three major polymers (low density
polyethylene, epoxy resin and polyimide) loaded with various
nanoparticles have been studied. A flavour of some interesting
results is shown here. Figure 3 reveals the relationship between
steady state current and the applied voltage for low density
polyethylene (LDPE) loaded with different amount of alumina
nanoparticles (~13 nm in diameter). The sample has a thickness
of 0.2mm and the number after PE represents percentage of
alumina loading.
alumina. It has been found that the presence of untreated nano
Al2O3 fillers shows no improvement in dielectric breakdown
behavior of epoxy resin composites, whereas pre-treated nano
Al2O3 fillers seems improved both the breakdown strength of
epoxy resin samples and its consistence as well. However, for
both epoxy resin nanocomposites samples with and without
surface treatment, highest breakdown strength has been
observed at a loading concentration of 3wt%.
Figure 5 illustrates the effect of nanoparticles of TiO 2 on
surface potential decay. It clearly shows the fast decay rate of
charge with a high percentage of nanoparticle loading. This may
relate to the conductivity increase caused by addition of nano
TiO2 particles [7].
Figure 3 I – V characteristics in alumina nanopartilces filled
LDPE samples at 20oC [6].
The conduction current increases with the applied voltage as
expected. However, the addition of alumina nanoparticles
changes the magnitude of conduction current. There are several
features which can be seen in this diagram. The first one is the
conduction current in the PE-1 sample is lower than the sample
without any alumina nanoparticles. This means that the addition
of small amounts of alumina hinders the movement of charge in
the bulk of the material. As the amount of alumina increases the
conduction current in the PE-5 increases significantly compared
to the PE-0. The second feature is the conduction current in the
PE-10 sample showing significant reduction at lower voltages
but becoming very high once the applied voltage exceeds 5 kV.
This is unexpected. Similar observations were found at higher
temperatures but the threshold voltage for PE-10 moves to
higher voltages. The activation energy calculated shows
complex pattern with percentage of alumina loading.
(a) untreated
Figure 5 Surface potential decay curves for various initial
potential for one-layer samples (a) pure PI film, (b) PI/TiO2-15
wt.% (c) concentration effect and (d) decay rate [7].
Challenges ahead
Nanotechnology is still very much an emerging technology and
investigations into nanodielectric materials have had its share of
challenges inherent within cutting edge research.
(i) Sample preparation method - finding ways of incorporating
nanoparticles successfully into polymers are a huge problem at
the moment. Inconsistence in sample preparation in literature
often results in conflict conclusions.
(ii) Characterisation – understanding the effect of
nanodielectrics needs a thorough characterisation of both
nanoparticles and sample loaded with nanoparticles. New
technologies should be employed to unlock the mystery of the
interface between nanoparticle and polymer matrix.
(iii) New applications – the benefits nanodielectrics can offer
are significant and potentially encompass all areas of the
electrical industry.
References
[1] T. J. Lewis, Nanometric dielectrics, IEEE Trans. Dielectr.
Electr. Insul., Vol. 1, pp. 812-825, 1994.
[2] T. J. Lewis, Interfaces are the dominant feature of dielectrics
at the nanometric level, IEEE Trans. Dielectr. Electr. Insul, Vol.
11, pp. 739 - 753, 2004.
[3] T. Tanaka, M. Kozako, N. Fuse and Y. Ohki, Proposal of a
multi-core model for polymer nanocomposite dielectrics, IEEE
(b) treated
Figure 4 Influence of nano Al2O3 surface treatments on
breakdown characteristics in epoxy resin.
Figure 4 shows the influence of nanoparticle surface treatment
on breakdown characteristics of epoxy resin loaded with
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Trans. Dielectr. Electr. Insul, Vol. 12, pp. 669-681, 2005.
[4] R. C. Smith, C. Liang, M. Landry, J. K. Nelson and L. S.
Schadler, The mechanisms leading to the useful electrical
properties of polymer nanodielectrics, IEEE Trans. Dielectr.
Electr. nsul, Vol. 15, pp. 187- 196, 2008.
[5] Y. Cao, P. C. Irwin and K. Younsi, The future of
nanodielectrics in the electrical power industry, IEEE Trans.
Dielect. Elect. Insul., Vol. 11, pp. 797-807, 2004.
[6] G. Chen, J. T. Sadipe, Y. Zhuang, C. Zhang and G. C.
Stebens, Conduction in Linear Low Density Polyethylene
Nanodielectric Materials, ICPADM, pp. 845 – 848, 2009.
[7] J. Zha, G. Chen, Z. Dang and Y. Yin, The influence of TiO2
nanoparticle incorporation on surface potential decay of coronaresistant polyimide nanocomposite films, J. Electrostatics,
Vol.69, pp. 255 – 260, 2011.
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