study of dielectric behavior of pen (polyethylene- naphtalate) by … · 2015-08-03 · v....
TRANSCRIPT
F. Benabed, T. Seghier
Laboratoire d’étude et de développement des matériaux
semi conducteur et diélectriques (LED MASD),
Université Amar Telidji
Laghouat, Algérie
Abstract—The method of dielectric spectroscopy is an
instrument of choice for the diagnosis of insulation used in high
voltage and also to assess the quality of the insulation of HV
equipment such as transformers, cables, capacitors, etc..
This method allows to estimating the state and the quality of
the insulation using the dielectric response of the frequency
range. In this article, we have presented results of dielectric
studies in polyethylene naphtalate by means of dielectric
relaxation spectroscopy (DRS) in frequency range 10-2 - 106 Hz
and temperature between -60 and 140 °C, we will invest this
method on solid insulation PEN “Polyethylene Naphtalate” to
measure the dielectric properties and evaluate the performance
of this insulator.
Keywords—dielectric relaxation; polyethylene naphtalate;
dielectric spectroscopy.
I. INTRODUCTION
Solid dielectrics, namely polymers are widely used in
insulation of HV systems, they are present in almost all units
of production or transmission of electrical energy, which
results in a steady development in the design of these
dielectrics, therefore we must follow this development and
improve the methods of diagnosing the condition of insulation
systems for both understanding how they react vis-à-vis the
implementation of a electric field in the long term and also
find the parameters that come into play in the failure of these
systems[1].
In this paper, we present the dielectric proprieties of :
poly (ethylene naphthalate) PEN, which is widely employed in
electrical Engineering, polyethylene naphthalate is a
thermoplastic polyester, it is a semi-crystalline material used
mainly in the electrical and electronic fields[2] .The chemical
structure of PEN is shown in Fig.1.
S.Boudrâa, M.Belkheiri
Laboratoire d’étude et de développement des matériaux
semi conducteur et diélectriques (LED MASD),
Université Amar Telidji
Laghouat, Algérie
The study of dielectric properties such as permittivity ε’,
conductivity σ and loss factor Tanδ for this solid material is
made by dielectric spectroscopy.
II. DIELECTRIC SPECTROSCOPY
Dielectric spectroscopy, which is based on the measurement
of current and voltage (amplitude and phase AC system) is a
method widely used to study the dielectric properties of
polymers such as (ε ', tanδ …)[3,4].
Its scope is very high frequencies (~ THz) are used to
characterize all phenomena type atomic and electronic
polarization, down to very low frequencies (~ MHz) to
characterize the different interfaces which may exist between
constituents of the material [5].
Under an alternative (ac) sinusoidal supplied voltage, the
real part of permittivity and ε’ and loss factor Tanδ is
computed using the following equations:
Where ɛ0 is the vacuum permittivity, d the thickness of
the sample polymer, A the electrode area and ω the angular
frequency.
The static (dc) conductivity has been derived from the (ac)
conductivity measurements at low frequency:
Study of Dielectric Behavior of PEN (Polyethylene-
Naphtalate) by Dielectric Spectroscopy
Fig.2. Principle of dielectric spectroscopy measurement
Fig.1. Chemical structure of PEN
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ISBN: 978-1-61804-325-2 110
Where K is an empiric parameter and n represents the
high frequency slope of the (ac )conductivity (from 0 to 1) [6].
III. EXPERIMENTAL METHOD
Measurements of the real part of the permittivity ε', the
loss factor Tanδ and the conductivity for polymer samples
were performed under AC voltage 1V in the frequency range
10-2
Hz to 1 MHz using a Broad Band Dielectric Controller
(Novocontrol), Alpha, Beta Analyzer with temperature range
was varied between -60 °C to 140°C.
The experimental setup is shown in Fig. 3.
The sample of the solid insulation used in this study is
shown in Fig.3; circular gold electrodes were sputtered onto
the free surface of the samples for this operation we used
Scancoat 6 Sputter Coater for sputtering thin, high quality
gold films.
The electrodes have a circular shape with a diameter of
16mm. The thickness of the samples is 0.027mm.
IV. RESULTS AND DISCUSSION
The experimental results of the study of frequency and
temperature dependencies of the overall loss factor Tanδ , real
part of the permittivity ε’ for the samples of PEN are illustrated
in Fig.5 and 6.
From Fig.5and 6, we can see an important increasing in
value of ε’ and Tanδ at low frequencies and high temperature
which can be explained by the presence of a relaxation
mechanism in this domain of frequency called α-relaxation
associated with the glass-rubber transition in the amorphous
regions[7]. Also, the contribution of the conductivity effects
due the bulk interfacial polarization between the amorphous
and crystalline regions possibly causes this increase [8, 9].
Fig.7, show the variation of the real part of conductivity '
versus frequency for different temperatures, it is clear that '
increases with frequency with small effect of temperature.
For more details, we presented the variations of the real
part of the permittivity ε’, the Loss factor Tan(δ) and the real
part Conductivity ' as a function of frequency and
temperature in 3D curves, on figures 8, 9 and 10 respectively.
Fig.6. The real part of the permittivity ε’of PEN as function of frequency for
different temperature
Fig.5. Loss factor Tan(δ) of PEN as function of frequency for different
temperature
Fig.4. Picture of PEN the solid insulation sample.
Fig.3. Scheme of the experimental setup :
1 – PC, 2 – Control System (temperature and frequency),
3 – System, 4 – Measurement Cell.
1
2 4
3
Recent Advances in Mechanics, Mechatronics and Civil, Chemical and Industrial Engineering
ISBN: 978-1-61804-325-2 111
In Fig.8 and 10, the values of the real part of the
permittivity ε’ rises with temperature and decreases with
frequency, in contrast to the actual values of the conductivity
', which increases with the frequency with a slight increase
when the temperature goes up to the glass transition
temperature Tg ≈122°C ,that can be explained by the role of Tg
on the motion of charge carriers[8].
In addition to the α-relaxation described above, from Fig.9,
we note the presence of β* and β processes (in order of
decreasing temperature) : The two sub-Tg,, relaxations β and
β* are associated to local motions of ester groups (like for
PET) and to partially cooperative movements of naphthalene
aggregates respectively[10]. The β*-relaxation appears at
temperatures at ≈ 80°C to 100°C and β-relaxation of PEN
shows up at low temperatures similar to those in the case of
PET [11].
Since the naphthalene group present in the repeat unit of
PEN is not symmetric with respect to the main chain axis, the
motions of this group about the main chain would imply
changes in the dipolar moment giving rise to the β*-
process[11].
Fig.7. The real part Conductivity '(-1
cm-1) of PEN as function of
frequency for different temperature
Fig.10. The real part Conductivity '(-1cm
-1) of PEN as function of
frequency and temperature
Fig.8. The real part of the permittivity ε’of PEN as function of frequency
and temperature
Fig.9. Loss factor Tan(δ)of PEN as function of frequency and temperature
Conductivity
β*
β
α
Recent Advances in Mechanics, Mechatronics and Civil, Chemical and Industrial Engineering
ISBN: 978-1-61804-325-2 112
V. CONCLUSION
In this work we have presented results of electrical and
dielectric studies in Polyethylene Naphtalate PEN, by using of
dielectric relaxation spectroscopy (DRS in frequency range
10-2
- 106 Hz and temperature between -60 °C and 140 °C.
The obtained results show three relaxations α, β* and β,
which contribute to the increasing values of the loss factor
Tanδ, the real part of permittivity ε’ and the real part of
conductivity ', when temperature goes up to the glass
transition temperature Tg ,that can be explained by the role of
Tg on the motion of charge carriers.
We find that the dielectric permittivity takes higher value
at low frequency this fact is attributed to the contribution of
the conductivity effects due the bulk interfacial polarization
between the amorphous and crystalline regions
ACKNOWLEDGEMENTS
This work was performed in the laboratory OF LAPLACE at
the Paul Sabatier University of Toulouse, The authors would
like to thank the personal for their support in materials as well
as all the collaborators of this work.
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Recent Advances in Mechanics, Mechatronics and Civil, Chemical and Industrial Engineering
ISBN: 978-1-61804-325-2 113