Conference on Electrical Insulation and Dielectric Phenomena Effects of Nano-sized MgO-filler on Electrical Phenomena under DC Voltage Application in LDPE

A nanocomposite material, composed of nano-sized MgO-filler added to a low-density polyethylene (LDPE), was subjected to electrical property investigations under DC voltage application. The volume resistivity of LDPE, under 40-80kV/mm DC field at 90 °C, increases by the power of 10 due to the addition of only a few percent of nano-filler. The amount of space charge in LDPE under high electric field is reduced and the ratio of stress enhancement by space charge becomes less due to the addition of nanosized MgO-filler. The DC breakdown strength ofLDPE is increased by adding nano-sized MgO-filler. Based on these results, the mechanism where by the nano-sized MgO filler had improved DC characteristics of LDPE was discussed. Introduction Polymer nanocomposite materials have been given much attention as a new insulation material because the properties of the original material can be drastically improved by adding a few percent of nano-sized filler [1]. We have studied the electrical properties of a nanocomposite material of nano-sized magnesium oxide (MgO) added to a low-density polyethylene (LDPE) [2]. In this paper, the preparation process of MgO/LDPE nanocomposite, dispersion state of MgO in LDPE, and the investigation results on volume resistivity, space charge behavior and DC breakdown strength ofMgO/LDPE nanocomposite are described. Materials MgO nano-filler, which has an average diameter of 50nm was mixed with LDPE by using a twin screw extruder. Before kneading, MgO was subjected to surface treatment with a silane coupling agent and to jet grinding treatment. A master batch was firstly prepared using the twin screw extruder in a high concentration of MgO. The master batch was then diluted to the prescribed concentration of 1, 5 and 10 phr. (phr: per hundred parts of resin, for example 1 phr means 1 g of MgO was mixed to 100 g of LDPE.) LDPE was also prepared as 0 phr sample for reference. The dispersion state of MgO in the MgO/LDPE composite material was evaluated by using a transmission electron microscope (TEM) as shown in Figure 1. Figure 1 shows TEM photograph of 5 phr sample. In this photograph, the lamella structure can be seen as a white stripes pattern. Other points, much darker in color than the surrounding area, can be seen. It was confirmed that these darker points were MgO by using energy dispersive X-ray fluorescence analysis (EDX) [2]. From the results of TEM investigation and EDX analysis, MgO of diameter 200nm or less is dispersed in LDPE. In addition, a similar dispersion of MgO was confirmed from several TEM photographs that were taken from different parts of the specimen. Therefore it seems that nano-sized MgO is homogeneously dispersed in LDPE. Figure 1: TEM photograph of MgO/LDPE nanocomposite material. MgO content is 5 phr. 0-7803-9257-4/05/$20.00 @2005 IEEE 158 Experiments The volume resistivity of MgO/LDPE nanocomposite materials, which are about 200 gm thickness in the sheet specimen, were investigated. A three-terminal electrode system was employed and the diameter of the main electrode was 65 mm. Measurements were made at up to an electrical field of 80 ky/mm, and at temperatures of 90 'C. The volume resistivity p was evaluated from the leakage current value ten minutes after the measurement was began. The space charge behavior in MgO/LDPE nanocomposite materials was measured with the PEA method (the pulsed electro-acoustic method). A sheet specimen of about 100 ,um thickness was used. Both high voltage and ground electrode are formed by gold evaporation with a diameter of 10 mm. The space charge measurement was carried out at a temperature of 30 °C. The DC breakdown strength of MgO/LDPE nanocomposite materials was measured applying a linearly rising voltage with a rising rate of 500 V/s. A spheresphere electrode system (Mckeown electrode system) was employed for the breakdown strength measurement. The breakdown test was carried out at a temperature of30 'C.