Charge Accumulation In Bulk Research Articles

Nanodielectrics: opportunities and challenges

Nanodielectrics, a nowadays popular term in the dielectrics community, has been the subject of intensive research over the past 10 years. In fact, the term stems out of the field of polymer nanocomposites—an emerging field of nanotechnology. Whereas polymer concern polymers within which nanometer-sized fillers are homogeneously dispersed at just a few weight percentage (wt%), the term nanometric dielectrics, [1] or [2], [3], refers to of specific interest in connection with their dielectric characteristics. Nevertheless, for the scope of HV electrical insulation research, the terms nanocomposites and are used interchangeably to refer to polymer/ nanoparticle mixtures of dielectric interest. The year 2015 marked the 21st anniversary of nanodielectrics from their first conceptual introduction in the 1990s [1]. Over this period, progress in nanodielectric research can be divided into two main phases [4], [5], i.e., the one before 2002 and the one thereafter. Initially, experimental studies on nanodielectrics were scarce and countable [6], [7]. Researchers only began to shift their attention to these materials when promising applications of nanodielectrics in electrical insulation were experimentally demonstrated by Nelson and his coworkers in 2002 [8]. Whereas conventional microfilled materials come with reduced dielectric strength due to bulk charge accumulation, Nelson et al. [8] discovered mitigated space charge accumulation and enhanced charge decay in nanofilled materials as compared with their microfilled counterparts, which could lead to improved dielectric strength of nanofilled materials over microfilled materials. Since then, research interest in nanodielectrics has increased tremendously, and various electrical insulation properties such as partial discharge resistance, treeing progression, space charge formation, and dielectric breakdown strength of nanodielectrics have been compared with their unfilled and microfilled counterparts. Promising improvements in these properties have been reported with nanofiller addition [9]–[17].

Effects of bulk and interfacial charge accumulation on fill factor in organic solar cells

The effects of charge accumulation on fill factor (FF) are studied by using light intensity-dependent current-voltage and capacitance-voltage measurements based on poly-{bi(dodecyl)thiophene-thieno[3,4-c]pyrrole-4,6-dione}:1(3-methyloxycarbony1)propyl-1-phenyl [6,6] (PBTTPD:PCBM) solar cells. We find that the FF of un-annealed device shows a non-monotonic dependence of light intensity: first an increase and then a decrease with increasing light intensity. This suggests that both bulk charge accumulation and recombination determine the FF before thermal annealing. Furthermore, the morphological enhancement of PBTTPD:PCBM film leads to a monotonic decrease of FF as increasing light intensity. This indicates that only surface charge accumulation plays an important role in the determination of the FF after thermal annealing.

Influence of oxyfluorination time on space charge behavior in polyethylene

Linear low density polyethylene (LLDPE) samples were surface oxyfluorinated for different times to investigate the influences of oxyfluorination time on charge injection from carbon black loaded poly(ethylene-co-vinyl acetate) (EVA) electrode and charge accumulation in bulk. Oxyfluorination led to the substantial variations in chemical composition, forming large numbers of various polar groups in the surface layers, depending on oxyfluorination time, as indicated by attenuated total reflection infrared (ATR-IR) analyses. Space charge measurements based on the pressure wave propagation method revealed obvious dependence of the charge injection and accumulation on oxyfluorination time when the samples were submitted to an average direct current field of about 50 kV/mm at 40°C. Suppression of the charge injection and accumulation by the oxyfluorinated surface layer was enhanced with extending oxyfluorination time. The total amounts of the positive and negative charges within the samples oxyfluorinated for 1, 6, and 13 h were calculated to be around 67, 50, and 29% of the total amount in the original sample, respectively. Diffusion of the volatile byproducts within the EVA electrode into LLDPE was evidenced by the ATR-IR analyses or indirectly by open-circuit thermally stimulated discharge current measurements. The diffusion influenced the charge injection and accumulation by changing charge traps in the surface layer and the field assisted ionization of the diffused volatile byproducts, and was influenced by the surface oxyfluorination depending on the treatment time. The influences of oxyfluorination on charge injection and accumulation were attributed to the remarkable increase in surface layer permittivity and the changes in its charge trap and barrier property to the diffusion of volatile by-products. The change in surface layer permittivity was indirectly evaluated by surface energy calculations.

Convective and absolute electrokinetic instability with conductivity gradients

Electrokinetic flow instabilities occur under high electric fields in the presence of electrical conductivity gradients. Such instabilities are a key factor limiting the robust performance of complex electrokinetic bio-analytical systems, but can also be exploited for rapid mixing and flow control for microscale devices. This paper reports a representative flow instability phenomenon studied using a microfluidic T-junction with a cross-section of 11 µm by 155 µm. In this system, aqueous electrolytes of 10:1 conductivity ratio were electrokinetically driven into a common mixing channel by a steady electric field. Convectively unstable waves were observed with a nominal threshold field of 0. 5k V cm −1 , and upstream propagating waves were observed at 1. 5k V cm −1 . A physical model has been developed for this instability which captures the coupling between electric and flow fields. A linear stability analysis was performed on the governing equations in the thin-layer limit, and Briggs–Bers criteria were applied to select physically unstable modes and determine the nature of instability. The model predicts both qualitative trends and quantitative features that agree very well with experimental data, and shows that conductivity gradients and their associated bulk charge accumulation are crucial for such instabilities. Comparison between theory and experiments suggests the convective role of electro-osmotic flow. Scaling analysis and numerical results show that the instability is governed by two key controlling parameters: the ratio of dynamic to dissipative forces which governs the onset of instability, and the ratio of electroviscous to electro-osmotic velocities which governs the convective versus absolute nature of instability.