Polyethylene/silica nanocomposites: absorption current and the interpretation of SCLC

Abstract

The topic of nanodielectrics continues to receive significant attention from today's dielectrics community, due to the property enhancements that can stem from the unique interfacial features within such material systems. Nevertheless, understanding of the interfacial phenomena that occur in nanodielectrics and which determine their electrical behaviour is challenging. In this paper, we report on an investigation into the absorption current behaviour of two nanocomposite systems, one containing an untreated nanosilica and the other containing the same nanofiller chemically modified using trimethoxy(propyl)silane.The results indicate that the absorption current behaviour of all the nanocomposites is very different from that of the reference, unfilled polymer; while the current flowing through the unfilled polyethylene decreased monotonically with time in a conventional manner, all nanocomposites revealed an initial decrease followed by a period in which the current increased with increasing time of electric field application. Possible mechanisms leading to the observed absorption current behaviour in the nanocomposites are discussed with the aid of space charge measurements. The presence of space charge limited conduction (SCLC) and its trap-filled limit is proposed.Keywords: nanocomposites, nanosilica, interface, absorption current, space charge limited conduction. 2 IntroductionThe addition of a nanofiller to a polymer matrix has been shown to be a flexible means of tailoring the dielectric properties of materials [1][2][3] and Chen et al. [19], for example, have emphasized the unsatisfactory nature of our understanding of charge transport mechanisms. In conventional polymeric insulation, charge transport mechanisms are extremely complicated, when compared with many conducting and semiconducting materials [20,21]. In semicrystalline polyethylene, for example, chainfolded lamellar crystals are surrounded by amorphous conformations and there is likely to be a high concentration of traps relating to each of these structural motifs. On the addition of a nanofiller, charge transport mechanisms will become yet more complicated, since the inclusion of nanoparticles will introduce extensive interfacial surfaces between the nanofiller and the polymer. The presence of such interfaces may directly introduce additional nanofiller/polymer trapping sites and/or modify the surrounding polymer morphology, thereby affecting the local density of states within the system.The current flow caused by the action of an applied electric field on charge carriers within a dielectric material can broadly be categorized into three types [22]: the initial current that flows through the material is the capacitive charging current, which causes a dramatic rise at the very beginning of the voltage application. This is followed by a gradual decrease of current, known as the absorption current or the anomalous current. Conventionally, the absorption current decreases slowly until it reaches a quasi-steady state, providing a conduction c...

The effect of resin/hardener stoichiometry on the electrical properties of silicon nitride/epoxy nanocomposites

“…6, it is evident that the behaviour of E/100-H/2-SiN is very different from this in that a clear increase in conductivity with time is evident beyond ~60 min. Comparable behaviour has been seen elsewhere and discussed in terms of two processes: trap filling effects [15]; absorption of water influencing charge injection from electrodes [16]. The origin of the process shown here is currently unclear, but an investigation into the influence of water on charge transport dynamics in epoxy/silicon nitride nanocomposites is currently underway.…”

Electrical conductivity of waxes as model systems for polyethylene: Role of water

“…Initially, a high displacement current flows as polarization charges are built‐up adjacent to the electrodes, these then serve to limit further charge injection and hence the measured current in the external circuit falls. An equilibrium is eventually reached between injected charge and recombination of charge carriers in the bulk and the measured current then reflects the true conduction current . The short‐term fluctuations in some of the traces are due to variations in oven temperature (typical accuracy ±2 °C).…”

“…An equilibrium is eventually reached between injected charge and recombination of charge carriers in the bulk and the measured current then reflects the true conduction current. 15,16 The short-term fluctuations in some of the traces are due to variations in oven temperature (typical accuracy AE2 C). Despite this, it is evident that the conductivity tends toward a steady-state value for times, t > 2000 s at the lowest temperatures, and that this point is more quickly reached at higher temperatures.…”

“…In the work reported here, we sought to work in a regime where the measurements are not unduly influenced by the applied electric field, which suggests that: (1), a constant applied field is used and (2) all work is performed with fields <20 kV mm −1 . However, another complication is evident in working under constant field conditions; that is, the current is observed to fall over a period of hours . Therefore, sufficient measurement time must be allowed if a reliable steady‐state current is to be assured.…”

“…However, another complication is evident in working under constant field conditions; that is, the current is observed to fall over a period of hours. 15,16 Therefore, sufficient measurement time must be allowed if a reliable steady-state current is to be assured. Additional complications are posed by the influences of both water 9,13 and impurities 17 and so these factors must either be eliminated or controlled.…”