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

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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

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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