J. Vanderschueren

Thermally stimulated current study of space charge formation and contact effects in metal-Polyethylene Terephthalate film-metal systems. I. Generalities and theoretical model

The Thermally Stimulated Depolarization Current (TSDC) method has been used to emphasize the important role played by the nature of the aluminum-polyester contact on the electrical properties of Metal-Insulator-Metal systems involving thin Polyethylene Terephthalate films (PET) used in capacitors. A progressive change of initially ohmic contacts into blocking ones upon heat treatment has been shown to be responsible for current reversals observed in the space charge region of the TSDC profiles. The use of an electronic band model allowed us to account for the nature and properties of the space charge accumulated in films ranging in thickness from 1.5 to 12 μm. Heterogeneity and retrapping effects have been put forward to explain the difference in behavior observed from ultrathin (1.5 μm thick) to thin films (12 μm thick). It has been shown that the existence of a significant gradient in density of localized states along the thickness of the film gives rise to a significant potential difference between the two vacuum deposited electrodes.

Thermally stimulated current study of space charge formation and contact effects in metal-Polyethylene Terephthalate film-metal systems. II. Influence of polarization conditions

The influence of the polarization conditions on the space charge formation in Metal-Insulator-Metal systems obtained by vacuum deposition of aluminum electrodes on both faces of thin Polyethylene Terephthalate (PET) films has been investigated by using the Thermally Stimulated Depolarization Current (TSDC) method in a wide range of poling temperatures, poling times and applied field strengths, emphasizing the effects of the presence of Schottky barriers at the Al-PET interfaces after heat treatment under helium. TSDC spectra of PET films ranging in thickness from 1.5 to 12 μm have been interpreted on the basis of the extended electronic band model described in a previous paper. Different values of activation energies, corresponding to distinct processes occurring during the space charge relaxation, have been determined, allowing a clear relationship between dipolar orientation and electronic detrapping to be evidenced. The height of the interfacial potential barriers between aluminum and PET has also been estimated.

Study of the thermal relaxation effects in polymers by a conjugated use of thermally stimulated depolarization and polarization current methods

Abstract Among the various thermally stimulated current methods used for analyzing the relaxation behavior of polymers, the thermally stimulated polarization current (TSPC) technique is rarely used because the classical theory provides for obtaining spectra having relaxational characteristics similar to those obtained from the more familiar thermally stimulated depolarization current (TSDC) technique. In fact, a number of factors such as the temperature dependence of equilibrium polarization, the effect of thermal expansion, the annealing process necessarily associated with TSDC experiments, or the existence of a temperature-dependent structure, can lead to very dissimilar results. This is especially the case in polymers, where the use of TSDC and TSPC methods in conjunction is very useful, e.g., for differentiating between various polarization processes, for studying the physical aging phenomenon or the behavior of thermally unstable structures, and for revealing non-equilibrium intermediate relaxations.

Temperature dependence of the viscosity of dilute solutions of poly(2-chlorophenyl methacrylate) and poly(2,4,5,-trichlorophenyl methacrylate) in various solvents

Abstract According to Moores treatment, an Arrhenius expression was used to describe the temperature dependence of the actual viscosity of dilute solutions of poly(2-chlorophenyl methacrylate) and poly(2,4,5-trichlorophenyl methacrylate) in various solvents. The effect of the polymer on the activation energy for viscous flow was found to be negative in poor and moderate solvents (toluene, benzene and dioxane) and positive in a good solvent (THF). Irrespective of the nature of the solvent, the presence of polymer chains in solution led to an increase of the pre-exponential factor of the Arrhenius expression. The parameters of Moores treatment are discussed in comparison with the Fox-Flory and the Kuhn-Mark-Houwink relations. The observed behaviours can be primarily related to variations in coil expansion with temperature.

Thermally stimulated current study of space-charge formation and contact effects in metal-polyethylene terephthalate films-metal systems. III. Influence of heat treatments

The influence exercised by heat treatments on the charge exchange properties of the interfacial barriers existing in metal-insulator-metal (MIM) systems obtained by vacuum deposition of aluminum electrodes on both faces of thin polyethylene terephthalate (PET) films has been investigated by using the thermally stimulated depolarization current (TSDC) method. A number of parameters have been systematically varied, including annealing atmospheres (helium, helium + oxygen, helium + nitrogen, vacuum, or air), annealing temperature, annealing time, sample fixing mode (free or taut annealing) and order adopted for applying the two procedures of annealing and metallization. The variations observed have been interpreted in terms of the electronic band model developed in the first paper of this series. X-ray photoelectron spectroscopy, thermomechanical analysis, density, aluminum adhesion and oxygen permeation measurements, were used to specify both volume and interfacial modifications undergone by the MIM structures and to support the TSDC data.

Conductivity and Thermally Stimulated Processes in Ionomers Based on SIS Block Copolymers

The relaxation and conductivity properties of ionomers based on styrene-isoprene-styrene triblock copolymers modified by maleic anhydride were investigated from 100 to 400 K by means of thermally stimulated depolarization and polarization methods, stressing the comparison of materials containing alkaline and alkaline-earth cations. It was shown that the behavior is largely determined by the thermal stability of ionic associations and their ability to trap charge carriers. With alkaline-earth cations, the associations are thermally stable at least up to 400 K, which gives rise to a conductivity level smaller than that of the starting material by several orders of magnitude. An opposite result is observed with alkaline cations as a consequence of the thermal breaking of ionic associations, which occurs in the glass transition range and results in a rapidly growing number of mobile ions.

Electrical Properties of Ultra-Thin Polyester Films: Method of Determination and Interpretation of Transient and Steady State Conductivity

Abstract The determination of transient and/or steady-state conductivity of PET ultra-thin films by means of electrode configuration involving any load or stress imposed to the measuring area during measurement usually leads to anomalous behaviors of the charging current and thus prevents a good evaluation of the electrical properties of the sample. This can be avoided by the use of a two-electrode system with lateral contacts, allowing to obtain reproducible results without any requirement for previous mechanical, thermal or electrical treatments (as is often recommended in the literature), which permits to characterize industrial films down to 1. 5 μm thick in true storage conditions after production.