Direct measurement of space-charge injection from a needle electrode into dielectrics

Costa

et al. 2001

An. Acad. Bras. Ciênc.

Electrostatic phenomena in insulators have been known for the past four centuries, but many related questions are still unanswered, for instance: which are the charge-bearing species in an electrified organic polymer, how are the charges spatially distributed and which is the contribution of the electrically charged domains to the overall polymer properties? New scanning probe microscopies were recently introduced, and these are suitable for the mapping of electric potentials across a solid sample thus providing some answers for the previous questions. In this work, we report results obtained with two of these techniques: scanning electric potential (SEPM) and electric force microscopy (EFM). These results were associated to images acquired by using analytical electron microscopy (energy-loss spectroscopy imaging in the transmission electron microscope, ESI-TEM) for colloid polymer samples. Together, they show domains with excess electric charges (and potentials) extending up to hundreds of nanometers and formed by large clusters of cations or anions, reaching supramolecular dimensions. Domains with excess electric charge were also observed in thermoplastics as well as in silica, polyphosphate and titanium oxide particles. In the case of thermoplastics, the origin of the charges is tentatively assigned to their tribochemistry, oxidation followed by segregation or the Mawell-Wagner-Sillars and Costa Ribeiro effects.

Section: Electrostatic Phenomenasupporting

confidence: 83%

Supramolecular ionics: electric charge partition within polymers and other non-conducting solids

Costa

et al. 2001

An. Acad. Bras. Ciênc.

“…The underlying principle is that the volume of the encapsulant that is exposed to the highest fields is very small, and this reduces the risk of breakdown initiation. However, another possibility suggests itself: Perhaps the 828-Z epoxy has greater high-field conductivity [15] than the 459 epoxy, and the high fields in the near vicinity of sharp electrode features are able to relax on a time scale comparable to the voltage risetime. It is clear that elucidating the responsible physical process would require further research.…”

Section: F Discussionmentioning

confidence: 99%

“…The difference in dielectric strength is apparently caused by the different resin and/or curative. We speculate that a greater high-field conductivity [15] may be present in the older 828-Z formulation, which allows the high electric fields near electrode edges to relax on the time scale of the high-voltage pulse. Further experiments would be needed to confirm this hypothesis.…”

Section: Discussionmentioning

confidence: 99%

Pulsed Dielectric Breakdown of Aluminum Oxide (ALOX) Filled Epoxy Encapsulants: Effects of Formulation and Electric Stress Concentration

Anderson¹,

Lagasse²,

Schroeder³

et al. 2001

Aluminum oxide (ALOX) filled epoxy is the dielectric encapsulant in shock driven highvoltage power supplies. ALOX encapsulants display a high dielectric strength under purely electrical stress, but minimal information is available on the combined effects of high voltage and mechanical shock. We report breakdown results from applying electrical stress in the form of a unipolar high-voltage pulse of the order of 10-µs duration, and our findings may establish a basis for understanding the results from proposed combined-stress experiments. A test specimen geometry giving approximately uniform fields is used to compare three ALOX encapsulant formulations, which include the new-baseline "459 epoxy resin" encapsulant and a variant in which the Alcoa T-64 alumina filler is replaced with Sumitomo AA-10 alumina. None of these encapsulants show a sensitivity to ionizing radiation. We also report results from specimens with sharp-edged electrodes that cause strong, localized field enhancement as might be present near electrically-discharged mechanical fractures in an encapsulant. Under these conditions the 459-epoxy ALOX encapsulant displays approximately 40% lower dielectric strength than the older Z-cured Epon 828 formulation. An investigation of several processing variables did not reveal an explanation for this reduced performance. The 459-epoxy encapsulant appears to suffer electrical breakdown if the peak field anywhere reaches a critical level. The stress-strain characteristics of Z-cured ALOX encapsulant are measured under high triaxial pressure and we find that this stress causes permanent deformation and a network of microscopic fractures. Recommendations are made for future experimental work. 4Acknowledgments

“…considered that tree initiation can be regarded as partial electrical breakdown because of the permanent loss of dielectric insulating properties. And it is found that this partial destruction is due to the collision between electrons and polymer molecular chains [18,19]. When external electrical field was applied to the samples, the electrons in the bulk of PE films or injected from the needle electrode would be accelerated.…”

Section: Investigationsmentioning

confidence: 99%

Influences of Morphology on Tree Initiation in Crystalline Polyethylene Films

1999

It is well known that electrical trees are greatly influenced by the morphological structure in crystalline polymers. In this paper, samples of polyethylene films, including low-density polyethylene, linear lowdensity polyethylene and high-density polyethylene have been crystallized and re-crystallized by annealing. The tree initiation voltages of these samples have been measured. It was found that after re-crystallizing the crystallinity and thickness of lamella increase, while the size of the spherulites hardly changes. It was also found that the tree initiation voltage increases after re-crystallizing. Based on the concept of mean free path in solid dielectric, it was assumed that the increase of tree initiation voltage was inherently related to the decrease of mean free path in the spherulites that was due to the increase of the thickness of lamellae.