L. Berquez

3D High-resolution Mapping of Polarization Profiles in Thin Poly(vinylidenefluoride-trifluoroethylene) (PVDF-TrFE) Films Using Two Thermal Techniques

In this paper, two non-destructive thermal methods are used in order to determine, with a high degree of accuracy, three-dimensional polarization distributions in thin films (12 mum) of poly(vinylidenefluoride-trifluoroethylene) (PVDF-TrFE). The techniques are the frequency-domain Focused Laser Intensity Modulation Method (FLIMM) and time-domain Thermal-Pulse Tomography (TPT). Samples were first metalized with grid-shaped electrode and poled. 3D polarization mapping yielded profiles which reproduce the electrode-grid shape. The polarization is not uniform across the sample thickness. Significant polarization values are found only at depths beyond 0.5 mum from the sample surface. Both methods provide similar results, TPT method being faster, whereas the FLIMM technique has a better lateral resolution.

Study of signal treatment for a pulsed electro-acoustic measurement cell: a way of improving the transfer matrix condition number

This work is focused on the improvement of the condition number of the transfer function matrix in a pulsed electro-acoustic (PEA) cell. A numerical electro-acoustic model is developed with the software COMSOL. This model is one-dimensional and the system of equations with partial differential functions is solved using a finite element method in non-stationary situations. Using this model, we can establish the output voltage of the piezoelectric sensor and acoustic pressure at each point of the calculation domain. Our approach consists in recovering the charge distribution within the sample using a deconvolution method between the simulated output voltage and the transfer function of the PEA cell. Results show why some changes of the PEA cell such as the nature of materials or sensor geometry involve an ill-posed problem or ill-conditioned problem, and why other arrangements lead to a well-conditioned problem, more amenable to giving the appropriate solution.

Deconvolution techniques for space charge recovery using pulsed electroacoustic method in coaxial geometry

The Pulsed Electro-Acoustic method (PEA) is a powerful tool for assessing space charge accumulation in a large variety of polymeric materials having various thicknesses. However, the acoustic attenuation and dispersion phenomena, especially when dealing with relatively thick samples, affect the space charge resolution at positions far away from the sensor. Moreover, for cable systems, the PEA signals are affected by the electrostrictive effect, due to the divergent nature of the electric stress, which introduces extra-distortion on the PEA signal. In this work, we report on two different deconvolution techniques being temporal and frequential. Both methods correct, with a similar figure of merit, the wave attenuation, dispersion and electrostrictive effects. Application of such deconvolution algorithms on experimental signals from a model power cables enables to show injection of negative charges at the inner electrode which results in a built-up of negative heterocharges adjacent to the outer electrode during volt-on.

Charge build-up and transport in electron beam irradiated polymers in a new irradiation chamber

In order to study the behavior of dielectric materials under an electronic bombardment, a new irradiation chamber has been designed and lately constructed in the laboratory. In a near future, this chamber will be equipped with various experimental set-up allowing measurements in-situ. The aim is to combine complementary information to get a better knowledge on charge transport, storage and release in insulating materials in relation to the electron beam energy and flux. In this paper, we briefly describe the irradiation chamber, and we present our first results obtained by Pulse Electro-Acoustic (PEA) method and by Focussed Laser Intensity Modulated Method (FLIMM) on Polytetrafluoroethylene (PTFE) films after irradiation under a low energetic electron beam.

Surface temperature measurement for space charge distribution measurements with thermal methods

The implementation of space charge measurements based on thermal perturbation on thin films requires an improvement of the temperature distribution estimation at the surface and in the depth of dielectric materials for getting reliable space charge profile measurements. Absolute temperature variations are needed, both in time and space. The present contribution addresses surface temperature measurements based on either thermoelectric or bolometric effects. Both responses have been measured on copper-coated silicon nitride layers and gold-coated polypropylene films heated with a Nd:YAG laser pulse. It is shown that high temporal resolution thermal response can be obtained through the bolometric response and that the information appears nearly independent on the nature of the coating electrode. The setup developed provides good signal to noise ratio for heated electrodes of a few ohm resistance. Strategies are still to develop to get the temperature profile in the insulating sample layer.

Contribution to improving the spatial resolution of a pulsed electro acoustic cell measurement: An analysis of acoustics waves propagation

This work aims at improving the spatial resolution for the pulsed electro-acoustic method — PEA. Whatever the measurement principle considered, the best spatial resolution achieved so far is of the order of several micrometers, being at most 10µm in case of the PEA method. This limit constitutes a drawback when considering rather thin insulating layers (order of tens of µms), as the case in some capacitors or in films layed on outer parts of satellites. Also, a better resolution is expected to provide a better description of charge generation in insulations at metal dielectric interfaces or under low energy electron beam. This work is focused on acoustic waves propagation inside the PEA cell and the analysis of influence of piezoelectric transducer geometrys of the quality of output voltage signal.

FLIMM and TSM: two thermal methods for space charges investigation

For the measurement of space charges profiles, various experimental techniques are available, the most known being the thermal and the acoustic methods. The methods we use for the determination of the space charges belong to the group of non-destructive thermal methods, called FLIMM (Focused Laser Intensity Modulation Method) and TSM (Thermal Step Method). These thermal methods are based on the interaction between the thermal wave and the electric charges present in the insulator, which generates a pyroelectric current. Their thermal excitation process are different: the FLIMM method uses a laser beam modulated in intensity, on the other hand, the TSM method rests on the creation of a thermal step on one side of the sample. Knowing the pyroelectric current and the temperature profile, and using an appropriate mathematical deconvolution, the space charges distribution can be determined. In this paper, after detailing the principle of these two techniques, one will present the space charges profiles obtained for PET (polyethylene terephthalate) samples conditioned at different temperatures and electric fields.

Space charge and polarization profiles determination in thin polymer insulators by FLIMM

In this work, the Focused Laser-Intensity Modulation Method (FLIMM) is used as a technique for space charge and polarization profiles determination in thin (up to 100 /spl mu/m) polymer layers. This well-known technique is here applied to the study of an electrode-charged polyethylene (PE) and a corona-poled Poly(vinylidene Fluoride) (PVDF) sample. The experimental set-up is first described, and theoretical aspects of LIMM fundamentals are reminded. Then, difficulties encountered during LIMM pyroelectric current deconvolution are discussed, and space charge and polarization profiles are shown.

Adaptation of the LIMM technique data treatment to perform measurements in vacuum

Dielectric materials are used as thermal regulator on satellites, as they offer for the embarked electronic devices the required protection. Their behavior in space environment must be studied in order to prevent electrostatic discharges that can be harmful for satellites. An irradiation chamber called MATSPACE has been built to study these materials. It is equipped with various material characterization devices. We will focus this paper on the Laser Intensity Modulation Method (LIMM) and the Split Faraday Cup (SFC) arrangement in the chamber. In a first approach it was necessary to reconsider the thermal aspect of the LIMM data treatment as it was now used under vacuum. In a second step, a new deconvolution treatment has been developed in order to recover the charge profile near the surface and further in the bulk. The influence of frequency range in which the signal is recorded will be also presented.

Preliminary measurements on dielectric materials by the pulsed electro-acoustic method using a ring electrode

In the last past thirteen years many Pulsed Electro Acoustic (PEA) set-ups have been developed to study space charge behavior in irradiated material mainly used for space applications. In this paper we will present our first measurements with a ring electrode. In these preliminary studies measurements are realized in the air. A dielectric film is polarized under a positive or a negative voltage then PEA measurements are realized using the open PEA set up and the new Open Ring Electrode (ORE). After the record of a reference signal and a deconvolution process, the space charge profile is obtained. We will compare and discuss the results recorded from both configurations and will explain which improvement still need to be brought.