Charge Transport in LDPE Nanocomposites Part I—Experimental Approach

Abstract

This work presents results of bulk conductivity and surface potential decay measurements on low-density polyethylene and its nanocomposites filled with uncoated MgO and Al 2 O 3 , with the aim to highlight the effect of the nanofillers on charge transport processes. Material samples at various filler contents, up to 9 wt %, were prepared in the form of thin films. The performed measurements show a significant impact of the nanofillers on reduction of material's direct current (dc) conductivity. The investigations thus focused on the nanocomposites having the lowest dc conductivity. Various mechanisms of charge generation and transport in solids, including space charge limited current, Poole-Frenkel effect and Schottky injection, were utilized for examining the experimental results. The mobilities of charge carriers were deduced from the measured surface potential decay characteristics and were found to be at least two times lower for the nanocomposites. The temperature dependencies of the mobilities were compared for different materials.

Trap Modulated Charge Carrier Transport in Polyethylene/Graphene Nanocomposites

“…This is because the charging current is composed of a polarization current and a conduction current. With the lapse of polarization time, the polarization comes into stable state gradually and the conduction current becomes dominant 34 . However, for the 0.5 wt% LDPE/graphene composites, the charging current decays slowly during the measuring time and still does not reach a steady state up to 2000 s, which is ascribed to the injection and accumulation of space charge in the bulk rather to a slow polarization process 35 .

Figure 2( a ) DC conductivity of LDPE/graphene NCs as a function of polarization time under 10 kV/mm.

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The Role of Interfaces in Polyethylene/Metal‐Oxide Nanocomposites for Ultrahigh‐Voltage Insulating Materials

“…c) Surface potential decay at different temperatures after corona charging on LDPE (“Ref”) and LDPE/Al 2 O 3 nanocomposite (“NC”) films. Reproduced under the terms of the CC‐BY 4.0 license . Copyright 2016, The Authors; published by MDPI, Basel, Switzerland.…”

“…The macroscopic evaluation of nanocomposites after corona charging shows a surface potential decay (SPD) with time (Figure c), whereas the nanoscale surface potential measurement by intermodulation electrostatic force microscopy (ImEFM, Figure b) illustrates with higher resolution that a nonuniform local charge injection/extraction occurs at the polymer/nanoparticle interface, with a significant potential imbalance in the vicinity of the nanoparticles. The macroscale SPD characterization reveals that in a high electric field (30–40 kV mm −1 ), the overall charge mobility of a low‐density polyethylene (LDPE) sample is reduced from 4 × 10 −14 to 5 × 10 −15 m 2 V −1 s −1 at 60 °C by the addition of 3 wt% Al 2 O 3 nanoparticles . It has been suggested that reduction in charge mobility in LDPE/Al 2 O 3 nanocomposites, referred to as “charge suppression,” is due to an increased barrier against tunneling/hopping of charges, which also leads to a reduction in DC conductivity of the polyethylene matrix (from 2 × 10 −14 to 10 −15 S m −1 ) .…”

Determination of Volume Conductivity of Polyethylene Using Positive Corona, When the Current through the Sample Depends Linearly on Grid Potential

“…A metallic grid connected at different DC potential (Model 240 A, Keithley Instruments) of the same polarity as that of the corona electrode was inserted between the point and the sample surface. Thus, a better control over the potential to which the surface was charged, and over the charge uniformity could be achieved [5] [11] [12].…”

“…Consequently, the surface density of charges embedded, and the potential ( ) 0, V t that they cause will be considered uniform [5] [11] [12].…”