High-Performance Dielectric Ceramic Films for Energy Storage Capacitors: Progress and Outlook

More than fifty years after the publication of the early work on conduction and dielectric breakdown of solids, we are still unable to describe quantitatively the electrical response of these materials. During this period of time, concepts derived from semiconductor physics have been transposed to the case of insulating solids, and among them, to polymers. Alternative descriptions have been proposed as well. In spite of this, there is still no agreement on how to describe charge transport and there is still some controversy as regards the applicability of semiconductors physics to the case of disordered insulating materials and in particular to polymers used in electrical engineering applications. The last twenty years have been marked by the publication of excellent review papers summarizing the physical concepts available to describe charge transport. Enormous steps forward have been achieved as regards computing facilities and our ability to spatially map the space charge, quantitatively, inside dielectric materials. We consider these two factors as fundamental in providing possibilities for developing sound models of charge transport, by using the basis of fundamental knowledge that has been accumulated in the previous years, and by coupling up-to-date techniques in experiments and in simulation. In this paper, which is not a review of either the published work on modeling or of new concepts in dielectric physics, we emphasize recent progress in the field of atomistic and macroscopic modeling and we discuss challenges based on such approaches that, we think, constitute a direction for future research.

Charge transport modeling in insulating polymers: from molecular to macroscopic scale

Charge Transport Modeling in Insulating Polymers : From Molecular to Macroscopic Scale

Dry processes to develop wheat fractions and products with enhanced nutritional quality

We examine various electronic processes that underlie the quenching of the emission from highly efficient phosphorescent and electrophosphorescent organic solid-state molecular systems. As an example, we study the luminescent efficiencies from the phosphorescent iridium (III) complex, fac tris (2-phenylpyridine) iridium $[\mathrm{Ir}{(\mathrm{ppy})}_{3}]$ doped into a diamine derivative doped polycarbonate hole-transporting matrix and in the form of vacuum-evaporated films, as a function of electric field. We demonstrate that the observed decrease in electrophosphorescence efficiencies at high electric fields, and electric-field-induced quenching of phosphorescence from neat $[\mathrm{Ir}{(\mathrm{ppy})}_{3}]$ solid films is due to the field-assisted dissociation of Coulombically correlated electron-hole (e-h) pairs. They are formed in a bimolecular recombination process prior to the formation of emissive triplet excitons, or are charge-transfer (CT) states originating from the localized electronic excited states as a result of the initial charge separation upon photoexcitation, respectively. It is found that the high-field dependence of the quenching efficiency in both cases follows the three-dimensional Onsager theory of geminate recombination, the fit yielding the initial intercarrier distance ${(r}_{0})$ of the carrier pairs. We find ${r}_{e\ensuremath{-}h}g~3.5\mathrm{nm}$ for the triplet exciton precursor pairs in the bimolecular recombination, and ${r}_{\mathrm{CT}}=1.8\ifmmode\pm\else\textpm\fi{}0.1\mathrm{nm}$ for the initial carrier separation from the photo-excited electronic states. Triplet-triplet and triplet-charge carrier annihilation processes are shown to play major roles in the decrease of the electrophosphorescence efficiency within the lower-field regime at lower current densities. Summarizing the results allows us to point out some emitter features important for identifying phosphors useful for practical electroluminescent devices.

Quenching effects in organic electrophosphorescence

We introduce and develop two bipolar transport models which are based on appreciably different physical assumptions regarding the distribution function in the energy levels of trap states. In the first model, conduction is described by an effective mobility of the carriers and the accumulation of stored space charge is taken into account through a single trapping level. In the second model the hypothesis of an exponential distribution function of trap depth is made, with conduction taking place via a hopping process from site to site. The results of simulations of the two models are compared with experimental data for the external current and the space-time evolution of the electrical space charge distribution. The two descriptions are evaluated in a critical way, and the prospects for these models to adequately describe real systems are given.

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Models of bipolar charge transport in polyethylene

The aim of this article is to investigate the extent of space charge accumulation due to a temperature gradient in comparison with other charge-supply mechanisms, particularly injection from the electrodes. For this purpose, space-charge measurements were carried out on HVDC cable models under application of different temperature gradients across the cable insulation, above and close to the threshold field for space-charge accumulation. The main results, consisting of space-charge patterns and extracted quantities, are discussed here.

HVDC Cable Design and Space Charge Accumulation. Part 3: Effect of Temperature Gradient [Feature article]

The electric field distribution of cable insulation systems under HVDC can be affected significantly at interfaces due to space charge build-up. In this article, the second part of a three-article series, face and space charge accumulation are analyzed first in terms of macroscopic physics, then through approximate mathematical models that will be used to fit experimental data obtained for model cables having two insulation layers and constituting cylindrical interfaces.

Feature article - Polymeric HVDC cable design and space charge accumulation. Part 2: insulation interfaces

The intent of this paper is to cross-correlate the information obtained by space charge distribution analysis and electroluminescence (EL) detection in cross-linked polyethylene samples submitted to dc fields, with the objective to make a link between space charge phenomena and energy release as revealed by the detection of visible photons. Space charge measurements carried out at different field levels by the pulsed electro-acoustic method show the presence of a low-field threshold, close to 15-20 kV mm-1, above which considerable space charge begins to accumulate in the insulation. Charges are seen to cross the insulation thickness through a packet-like behaviour at higher fields, starting at about 60-70 kV mm-1. EL measurements show the existence of two distinct thresholds, one related to the continuous excitation of EL under voltage, the other being transient EL detected upon specimen short circuit. The former occurs at values of field corresponding to charge packet formation and the latter to the onset of space charge accumulation. The correspondence between pertinent values of the electric field obtained through space charge and EL analyses provides support for the existence of degradation thresholds in insulating materials. Special emphasis is given to the relationship between charge packet formation and propagation, and EL. Although the two phenomena are observed in the same field range, it is found that the onset of continuous EL follows the formation at the electrodes of positive and negative space charge regions that extend into the bulk prior to the propagation of charge packets. Charge recombination appears to be the excitation process of EL since oppositely charged domains meet in the material bulk. To gain an insight into specific light-excitation processes associated with charge packet propagation, EL has been recorded for several hours under fields at which charge packet dynamics were evidenced. It is shown that current and luminescence oscillations are detected during charge packet propagation, and that they are in phase. The mechanisms underlying EL and charge packets are further considered on the basis of these results.

/pdf/charge-distribution-and-electroluminescence-in-cross-linked-epvwpvfmgk.pdf

Len A. Dissado

Papers

Models of bipolar charge transport in polyethylene

HVDC Cable Design and Space Charge Accumulation. Part 3: Effect of Temperature Gradient [Feature article]

Feature article - Polymeric HVDC cable design and space charge accumulation. Part 2: insulation interfaces

Charge distribution and electroluminescence in cross-linked polyethylene under dc field

Space charge in solid dielectrics