L. Testa

Stochastic resonance in magnetic systems described by Preisach hysteresis model

We present a numerical study of stochastic resonance in magnetic systems described by Preisach hysteresis model. It is shown that stochastic resonance occurs in these systems. Specifically, the signal-to-noise ratio (SNR) and the signal amplification (SA) present a maximum as a function of noise intensity. We also found that the hysteresis loops, dynamically described by the system, are strongly modified near the maxima of SNR and of SA.

Effect of aspect ratio and water contamination on the electric properties of nanostructured insulating materials

Organically-modified nanofiller clays can have significantly different aspect ratios as well as accumulate a relatively large amount of water in the composite bulk due to the contribution of the filler itself and the interaction between filler and polymer matrix. This paper investigates the effect of water absorption in a nanostructured thermoplastic polymer, namely ethylene-vinyl-acetate (EVA), on electrical property modifications considering the contribution of aspect ratio. The change of electrical properties (particularly space charge accumulation, electric strength, bulk conductivity and permittivity/losses) is studied as a function of water content absorbed by nanofillers having different aspect ratio, i.e. fluorohectorite and bohemite. An increase of the space charge build up and of the conductivity (and decrease of the electric strength) is observed as a function of the water content for specimens containing layered silicates as nanofillers (fluorohectorite), while the electric properties of bohemite specimens do not show significant variations with the absorbed water content. This behavior can be associated with the different aspect ratio of the nanofillers. A filler having higher aspect ratio, in fact, seems to be more effective in worsening the electrical properties of the final nanocomposite. This could be explained considering that the higher the aspect ratio of the nanoparticles, the larger the percolation probability. To quantify how water content may affect electric properties of the final nanocomposite a simplified model providing the percolation probability as a function of water content and geometry of the particles is developed in this paper.

Advanced modeling of electron avalanche process in polymeric dielectric voids: Simulations and experimental validation

This paper deals with aging phenomena in polymers under electric stress. In particular, we focus our efforts on the development of a novel theoretical method accounting for the discharge process (partial discharge) in well known defects present in polymers, which are essentially tiny air gaps embedded in a polymeric matrix. Such defects are believed to act as trigger points for the partial discharges and their induced aging process. The model accounts for the amplitude as well as the energy distribution of the electrons during their motion, particularly at the time in which they impact on the polymer surface. The knowledge of the number of generated electrons and of their energy distributions is fundamental to evaluate the amount of damage caused by an avalanche on the polymer-void interface and get novel insights of the basic phenomena underlying the relevant aging processes. The calculation of such quantities would require generally the combined solution of the Boltzmann equation in the energy and space...

Space Charge Dynamics in Nanostructured Epoxy Resin

The results of a comparative analysis performed on specimens of nanostructured epoxy resins obtained by different filler concentrations are reported in this paper. The specimens were prepared by dispersion of bohemite nanoparticles into a cycloaliphatic epoxy resin cross linked under UV. Dielectric properties, in particular space charge accumulation, are measured as a function of nanofiller content. Results indicate that space charge build up and charge mobility are affected largely by filler content. Concentrations of nanofiller of 5-7 wt% show a significant decrease of the space charge with respect to the base epoxy-resin and an increased mobility of negative carriers.

Evolution of pd patterns in polyethylene insulation cavities under AC voltage

In the framework of developing defect-based life models, in which breakdown is explicitly associated with partial discharge (PD)-induced damage growth from a defect, ageing tests and PD measurements were carried out in the laboratory on polyethylene (PE) layered specimens containing artificial cavities. PD activity was monitored continuously during aging. A quasi-deterministic series of stages can be observed in the behavior of the main PD parameters (i.e. discharge repetition rate and amplitude). The evolution of the parameters reflects the physicalchemical changes taking place at the dielectric/cavity interface during the aging process. PD activity shows similar time behavior under constant cavity gas volume and constant cavity gas pressure conditions, suggesting that the variation of PD parameters may not be attributed to the variation of the gas pressure. It is speculated that the change of PD activity is related to the composition of the cavity gas, as well as to the properties of dielectric/cavity interface.

The Processing of Nanocomposites

Manufacturing techniques can have a significant impact on the dielectric properties of nanostructured materials, sometimes even larger than the effect of the nanofiller itself. Indeed, the choice of the best nanofiller to improve the electric behavior of the final nanocomposite is often frustrated by residual contaminants of the matrix-nanofiller compatibilization process, moisture absorption, nanofiller aggregation, etc. Therefore manufacturing techniques should be tuned to eliminate or limit the spurious effect of defects/contaminants to obtain the desired material properties. In the following, after a comprehensive presentation of the most common nanofillers and compatibilization treatment, attention is paid to the main processing techniques and purification procedures to minimize the effect of contaminants.

Partial Discharge Tests Using CIGRE Method II Upon Nanocomposite Epoxy Resins

Nanocomposite polymers are becoming a new family of dielectric materials with promising insulating properties. In order to apply this technology to insulation systems, a study about the material longterm endurance (life) is needed. In this paper results from comparative life tests performed upon base and nanostructured epoxy resin samples are presented. The nanofiller is a organophillic-modified ultra-clean montmorillonite. Specimens were subjected to ageing under surface discharge phenomena using a standard electrode configuration, that is the CIGRE method II. Life data and partial discharge phase resolved patterns, acquired during time under stress, are presented and commented, in order to characterize the ageing processes that leads to the specimen breakdown and to explain the longer life of the nanocomposite materials. A further insight on the breakdown mechanism is obtained through microscopic observations (using optical and SEM techniques) of the surfaces of the samples before and after ageing and breakdown. A different morphology of the discharge channel walls is observable for base and nanostructured resins.

Patterns of partial discharge activity in XLPE: From inception to breakdown

With the aim of developing defect-based life models (i.e., life models were breakdown is explicitly associated with the partial discharge-induced growth from a defect beyond a critical level), measurements were carried out in the lab on XLPE sandwich test models containing artificial defects. During aging, PD activity was continuously monitored, while the test was stopped shortly at regular times to measure the evolution of PD inception voltage. The results obtained so forth allow to highlight some features of the PD processes prior breakdown. In particular, a quasi-deterministic series of stages can be observed in the trend lines of statistical markers associated with PD magnitude and repetition rate.

Model of ageing inception and growth from microvoids in polyethylene-based materials under AC voltage

Several degradation mechanisms may affect polymeric insulation system reliability. Some authors postulate that, even in a perfect dielectric, nanoscale cavities can enlarge due to various mechanisms (from mechanical fatigue to lowering of the degradation reaction energy barrier) up to a point where highly energetic phenomena, which bring about breakdown, can be incepted. Other authors are more focused on the inherent limits of manufacturing processes, which leave cavities within the insulation system whose size is large enough to cause electron avalanches, thus a measurable partial discharge (PD) activity, from the time the system is put in service or as a function of external factors (e.g. mechanical damage, thermal shrinking, overvoltages). Given the time scale of polymeric system failures, this latter mechanism seems to be more plausible. It is therefore worthwhile to investigate in depth the degradation rates associated with PD in micrometric cavities in polymeric insulation systems subjected to AC voltage. The proposed model is based on damage accumulation on cavity surfaces caused by PD phenomena. The main degradation mechanism associated with PD is considered to be the hot-electron induced bond-breaking process. This process accumulates with time, leading to the creation of a damaged region of critical size and, ultimately, to breakdown. The proposed model describes the defect induction and growth phase until treeing phenomena start, which is, normally, the largely prevailing component of breakdown time.

Electrical treeing in EVA-Boehmite and EVA-Montmorillonite nanocomposites

The present experimental work focuses on the growth of electrical treeing inside different Ethylene-vinyl acetate (EVA) nanocomposites containing Bohemite (an aluminum oxide hydroxide) and Montmorillonite (a phyllosilicate clay mineral) nanoparticles. Bohemite and Montmorillonite particles have different aspect ratios: the first one has a nanometric cube-like symmetry, while the latter has a typical layered structure. The results evidence that the growth of the electrical treeing inside the original polymer can be altered significantly by the dispersion of inorganic nanoparticles.