The effect of mechanical relaxation on ultra-fast charge pulses in flexible epoxy resin nanocomposites

Abstract

Previously we have reported the existence of small-amplitude charge pulses in crosslinked Polyethylene (XLPE) and epoxy resin with a mobility several orders of magnitude higher than that found for the incoherent charge transport relevant to the steady state current. Here the relationship of this phenomenon to mechanical relaxation in the material is investigated by using a series of epoxy resin nanocomposites based on a resin that has its flexibility increased above that of the fully cured glassy epoxy network by the addition of a suitable flexibilizing chemical. Differential Scanning Calorimetry (DSC) measurements show that the stiffness of the nanocomposite is progressively increased as the nanoparticle concentration increases. Pulsed Electro-Acoustic (PEA) measurements reveal that both positive and negative fast charge pulses exist in the unfilled epoxy at 45 and 70°C under a field of 10 kV/mm with mobility 5 × 10 −10 to 9 × 10 −10 m 2 V −1 s −1 , amplitude between 2 × 10 −5 and 3.6 × 10 −5 C m −2 and repetition rates between 8 and 12 s −1 . These values are reduced progressively as the nanoparticle concentration is increased from 0% in the unfilled epoxy. A β-mode mechanical relaxation is identified in the loss modulus by Dynamical Mechanical Analysis (DMA), whose activation energy moves to higher values with increasing nanoparticle concentration. It is shown that the repetition rates of both positive and negative pulses have similar values and are correlated with the β-mode activation energy; a similar correlation is found for the activation energy of the mobility of positive pulses. The correlation of the activation energy of the mobility of negative pulses and that of the β-mode is weaker although both show a progressive increase with nanoparticle concentration. The modification of the fast charge pulse properties by the mechanical stiffness of the epoxy nanocomposite is discussed in terms of the theory presented previously for their formation and transport.

Observation of current spikes caused by possible fast carrier transport through a low-density polyethylene sheet under high DC fields

“…[10][11][12][13][14][15][16][17][18] In most of these studies, temporal changes in space charge distribution were observed on long time scales of the order of 10 min. On the other hand, there are only few reports on space charge dynamics on a shorter time scale of the order below 10 −1 s. [19][20][21][22][23] To observe the transient phenomena of space charge transport in a millisecond range, high-speed sampling techniques are required. Several years ago, Fabiani et al observed fast charge packets crossing an insulation layer in a cross-linked polyethylene (XLPE) cable at a very high velocity.…”

“…Montanari et al reported that the velocity of a fast charge packet increased with temperature and it was up to 0.1 mm=s at 70 °C. 23) Recently, they have experimentally found the presence of repetitive current pulses for a XLPE sample under dc stresses from 0.1 to 0.5 MV=cm. 24) The superimposed pulses on a dc current component were regarded as evidence of fast charge packets.…”